Purification of carbon tetrachloride from petroleum products by distillation. Preparation of anhydrous pure organic solvents. Methods for the rapid determination of peroxides in liquids
Methods for cleaning organic solvents depend on the nature and purpose of the solvent. In most cases, organic solvents are individual compounds and can be characterized by their physicochemical properties. The most elementary solvent purification operation is simple or fractional distillation. However, distillation often fails to get rid of a number of impurities, including small amounts of water.
By conventional purification methods, a solvent of approximately 100% purity can be obtained. With the help of adsorbents, in particular molecular sieves (zeolites), this problem is solved more efficiently and with less time. In laboratory conditions, ion exchangers are most often used for this purpose - zeolites of the NaA or KA brands.
When preparing pure anhydrous solvents, precautions should be especially strictly observed, since most organic solvents are flammable substances, the vapors of which form explosive mixtures with air, and in some of them (ethers) explosive peroxide compounds are formed during long-term storage. Many organic solvents are highly toxic, both when their vapors are inhaled and when they come into contact with the skin.
All operations with flammable and combustible organic solvents must be carried out in a fume hood with ventilation running, gas burners and electric heaters turned off. Liquids should be heated and distilled in a fume hood on preheated baths filled with an appropriate heat transfer medium. When distilling an organic liquid, it is necessary to constantly monitor the operation of the refrigerator.
If flammable solvents (gasoline, diethyl ether, carbon disulfide, etc.) are accidentally spilled, it is necessary to immediately extinguish all sources of open fire and turn off electric heaters (de-energize the working room during the day). The place where the liquid is spilled should be covered with sand, the contaminated sand should be collected with a wooden scoop and poured into a garbage container installed in the open air.
When drying solvents, active drying agents should not be used until pre-coarse drying with conventional drying agents has been carried out. Thus, it is forbidden to dry crude diethyl ether with sodium metal without first drying it with calcined CaCl2.
When working with ethers and other substances (diethyl ether, dioxane, tetrahydrofuran), during the storage of which peroxide compounds can be formed, peroxides are first removed from them, and then distilled and dried. Anhydrous organic solvents should be distilled carefully. All elements of the distillation unit (distillation flask, reflux condenser, refrigerator, alonge, distillate receiver) are preliminarily dried in an oven. The distillation is carried out without access to air, and the alonge is supplied with a calcium chloride tube filled with ascarite and fused CaCl2 to absorb CO2 and H2O. It is advisable to discard the first portion of the distillate, which is used for washing all the equipment.
Methods for purification and dehydration of the most commonly used solvents are discussed below.
Acetone
Acetone CH3COCH3 - colorless liquid; d25-4 = 0.7899; tboil = 56.24 °С; n20-D = 1.3591. Easily ignited. Vapors form explosive mixtures with air. Technical acetone usually contains water, with which it mixes in any ratio. Sometimes acetone is contaminated with methanol, acetic acid and reducing agents.
A test for the presence of reducing substances in acetone is carried out as follows. To 10 ml of acetone add 1 drop of 0.1% aqueous solution of KMnO4; after 15 minutes at room temperature, the solution should not be colorless.
For purification, acetone is heated for several hours with anhydrous K2CO3 (5% (wt.)) in a flask with a reflux condenser, then the liquid is poured into another flask with a reflux condenser 25-30 cm high and distilled over anhydrous K2CO3 (about 2% (wt.) ) and crystalline KMnO4, which is added to acetone until a stable violet color appears in a water bath. In the resulting acetone, there is no longer methyl alcohol, but there is a small amount of water.
To completely remove water, acetone is repeatedly distilled over anhydrous CaCl2. To do this, 1 liter of acetone is poured into a 2-liter round-bottom flask equipped with an efficient reflux condenser closed with a calcium chloride tube with CaCl2, 120 g of CaCl2 are added and boiled in a water bath with closed electric heating for 5-6 hours. Then the reaction flask is cooled and acetone is poured into another similar flask with a fresh portion of CaCl2 and boil again for 5-6 hours. After that, the reflux condenser is replaced with a descending one, to which, using an alonge connected to a calcium chloride tube filled with CaCl2, a receiving flask cooled with ice is attached and acetone is distilled over CaCl2.
Instead of such a long and laborious operation, which often leads to the condensation of acetone, it is better to use NaA zeolite. With prolonged exposure of acetone over this zeolite (5% (mass.)) is achieved absolute acetone.
In small quantities, very pure acetone can be obtained from the adduct (addition product) of acetone and NaI, which decomposes even at low heating, releasing acetone. To do this, when heated in a water bath, dissolve 100 g of NaI in 440 ml of dry, freshly distilled acetone. The resulting solution is rapidly cooled to -3° C. by immersing the vessel in a mixture of ice and NaCl. The separated solid NaI-C3H6O adduct is separated on a Buchner funnel, transferred to a distillation flask and heated on a water bath. On gentle heating, the adduct decomposes, and the liberated acetone is distilled off. The distillate is dried over anhydrous CaCl2 and redistilled with a reflux condenser over CaCl2. Regenerated NaI can be reused for the same reaction.
An express method for purifying acetone from methyl alcohol and reducing substances is as follows: a solution of 3 g of AgNO3 is added to 700 ml of acetone in a 1-liter flask. in 20 ml of distilled water and 20 ml of 1 N. NaOH solution. The mixture is shaken for 10 min, after which the precipitate is filtered off on a funnel with a glass filter, and the filtrate is dried with CaSO4 and refluxed over CaCl2.
Acetonitrile
Acetonitrile CH3CN is a colorless liquid with a characteristic ethereal odor; d20-4 = 0.7828; tboil = 81.6°С; n20-D = 1.3442. It is miscible with water in all respects and forms an azeotropic mixture (16% (wt.) H2O) with tboil = 76°C. A good solvent for a number of organic substances, in particular amine hydrochlorides. It is also used as a medium for carrying out some reactions, which it accelerates catalytically.
Acetonitrile is a strong inhalation poison and can be absorbed through the skin.
For absoluteization, acetonitrile is distilled twice over P4O10, followed by distillation over anhydrous K2CO3 to remove traces of P4O10.
You can pre-dry the acetonitrile over Na2SO4 or MgSO4, then mix with CaH2 until the evolution of gas (hydrogen) stops and distill over P4O10 (4-5 g/l). The distillate is refluxed over CaH2 (5 g/l) for at least 1 h, then slowly distilled, discarding the first 5 and the last 10% of the distillate.
Benzene
Benzene C6H6 - colorless liquid; d20-4 = 0.8790; tmelt = 5.54 °С; tboil = 80 10°С; n20-D = 1.5011. Benzene and its homologues, toluene and xylenes, are widely used as solvents and azeotropic drying media. Benzene should be handled with care due to its flammability and toxicity, as well as the formation of explosive mixtures with air.
Benzene vapors with repeated exposure disrupt the normal function of the hematopoietic organs; in the liquid state, benzene is strongly absorbed through the skin and irritates it.
Technical benzene contains up to 0.02% (wt.) of water, some thiophene and some other impurities.
Benzene forms an azeotropic mixture with water (8.83% (wt.) H2O) with tboil = 69.25°C. Therefore, during the distillation of wet benzene, water is almost completely distilled off with the first portions of the distillate (cloudy liquid), which are discarded. As soon as the clear distillate begins to distill, the drying process can be considered complete. Additional drying of distilled benzene is usually carried out with calcined CaCl2 (for 2-3 days) and sodium wire.
In the cold season, care must be taken to ensure that the distilled benzene does not crystallize in the refrigerator tube, washed with cold water (4-5 ° C).
Benzene and other hydrocarbons dried with metallic sodium are hygroscopic, that is, they can absorb moisture.
Commercial commercial benzene contains up to 0.05% (wt.) of C4H4S thiophene (tboil = 84.12°C; tmelt = 38.3°C), which cannot be separated from benzene either by fractional distillation or crystallization (freezing). Thiophene in benzene is detected as follows: a solution of 10 mg of isatin in 10 ml of conc. H2SO4 is shaken with 3 ml of benzene. In the presence of thiophene, the sulfuric acid layer turns blue-green.
Benzene is purified from thiophene by repeated shaking with conc. H2SO4 at room temperature. Under these conditions, thiophene is predominantly sulfonated rather than benzene. For 1 liter of benzene take 80 ml of acid. The first portion of H2SO4 turns blue-green. The lower layer is separated, and benzene is shaken with a new portion of acid. Purification is carried out until a faint yellow color of the acid is achieved. After separation of the acid layer, the benzene is washed with water, then with 10% Na2CO3 solution and again with water, after which the benzene is distilled.
A more efficient and simpler method for removing thiophene from benzene is boiling 1 liter of benzene with 100 g of Raney nickel in a flask under reflux for 15-30 minutes.
Another way to purify benzene from thiophene is to fractionally crystallize it from ethyl alcohol. A saturated solution of benzene in alcohol is cooled to about -15°C, solid benzene is quickly filtered off and distilled.
Dimethyl sulfoxide
Dimethyl sulfoxide (CH3) 2SO - colorless syrupy liquid without pronounced odor; d25-4 = 1.1014; tboil = 189°С (with decomposition); tm = 18.45 °С; n25-D = 1.4770. Miscible with water, alcohols, acetone, ethylacetone, dioxane, pyridine and aromatic hydrocarbons, but not miscible with aliphatic hydrocarbons. Universal solvent for organic compounds: ethylene oxide, heterocyclic compounds, camphor, resins, sugars, fats, etc. It also dissolves many inorganic compounds, for example, at 60 ° C it dissolves 10.6% (wt.) KNO3 and 21.8% CaCl2. Dimethyl sulfoxide is practically non-toxic.
For purification, dimethyl sulfoxide is kept for a day over active Al2O3, after which it is distilled twice at a pressure of 267–400 Pa (2–3 mm Hg) over fused KOH (or BaO) and stored over NaA zeolite.
Under the action of reducing agents, dimethyl sulfoxide turns into sulfide (CH3) 2S, and under the action of oxidizing agents - into sulfone (CH3) 2SO2, incompatible with acid chlorides of inorganic and organic acids.
N,N-Dimethylformamide
N,N-Dimethylformamide HCON(CH3)2 - a colorless, mobile liquid with a slight specific odor; d25-4 = 0.9445; tboil = 153°С; n24-D = 1.4269. Miscible in any ratio with water, alcohol, acetone, ether, chloroform, carbon disulfide, halogenated and aromatic compounds; aliphatic hydrocarbons dissolves only when heated.
Dimethylformamide distills at atmospheric pressure without decomposition; decomposes under the influence of ultraviolet rays with the formation of dimethylamine and formaldehyde. The dimethylformamide reagent, in addition to methylamine and formaldehyde, may contain methylformamide, ammonia and water as impurities.
The dimethylformamide is purified as follows: 10 g of benzene and 4 ml of water are added to 85 g of dimethylformamide, and the mixture is distilled. First, benzene with water and other impurities is distilled off, and then the pure product.
diethyl ether
Diethyl ether (C2H5) 2O is a colorless, easily mobile, volatile liquid with a peculiar odor; d20-4 = 0.7135; tboil = 35.6°С; n20-D = 1.3526. Extremely flammable; Vapors form explosive mixtures with air. Vapors are approximately 2.6 times heavier than air and can spread over the surface of the work table. Therefore, it is necessary to ensure that all gas burners are extinguished nearby (up to 2-3 m) from the place of work with ether, and electric stoves with an open spiral are disconnected from the mains.
When diethyl ether is stored under the action of light and atmospheric oxygen, explosive peroxide compounds and acetaldehyde are formed in it. Peroxy compounds are the cause of extremely violent explosions, especially when attempting to distill ether to dryness. Therefore, when determining the boiling point and non-volatile residue, the ether should first be checked for the content of peroxides. In the presence of peroxides, these determinations cannot be made.
Many reactions have been proposed for the detection of peroxide in diethyl ether.
1. Reaction with potassium iodide KI. A few milliliters of ether are shaken with an equal volume of 2% aqueous KI acidified with 1-2 drops of HCl. The appearance of a brown color indicates the presence of peroxides.
2. Reaction with titanyl sulfate TiOSO4. The reagent is prepared by dissolving 0.05 g of TiOSO4 in 100 ml of water, acidified with 5 ml of diluted H2SO4 (1:5). On shaking 2-3 ml of this reagent with 5 ml of the test ester containing peroxide compounds, a yellow color appears.
3. Reaction with sodium dichromate Na2Cr2O7. To 3 ml of ether add 2-3 ml of 0.01% Na2Cr2O7 aqueous solution and one drop of diluted H2SO4 (1:5). The mixture is shaken vigorously. The blue color of the ether layer indicates the presence of peroxides.
4. Reaction with ferrothiocyanate Fe(SCN)2. A colorless solution of Fe(SCN)2 under the action of a drop of a liquid containing peroxide turns red due to the formation of ferrithiocyanate (Fe2+ > Fe3+). This reaction makes it possible to detect peroxides in concentrations up to 0.001% (mass.). The reagent is prepared as follows: 9 g of FeSO4-7H2O are dissolved in 50 ml of 18% HCl. Add granulated zinc and 5 g of sodium thiocyanate NaSCN to the solution in an open vessel; after the disappearance of the red color add another 12 g of NaSCN, shake gently and the solution is separated by decantation.
Iron(II) sulfate is used to remove peroxides. When shaking 1 l of ether, usually take 20 ml of a solution prepared from 30 g of FeSO4-7H2O, 55 ml of H2O and 2 ml of conc. H2SO4. After washing, the ether is shaken with a 0.5% KMnO4 solution to oxidize the acetaldehyde to acetic acid. Then the ether is washed with 5% NaOH solution and water, dried for 24 h over CaCl2 (150-200 g CaCl2 per 1 l of ether). The CaCl2 is then filtered off on a large pleated filter paper and the ether is collected in a dark glass bottle. The bottle is tightly closed with a cork stopper with a calcium chloride tube, bent at an acute angle, filled with CaCl2 and glass wool swabs inserted into it. Then, having opened the flask, sodium wire is quickly introduced into the ether, at the rate of 5 g per 1 liter of ether.
After 24 hours, when no more hydrogen bubbles are emitted, another 3 g of sodium wire per 1 liter of ether is added and after 12 hours the ether is poured into a distillation flask and distilled over sodium wire. The receiver must be protected by a calcium chloride tube with CaCl2. The distillate is collected in a dark glass bottle, which, after adding 1 g of sodium wire per 1 liter of ether, is closed with a cork stopper with a calcium chloride tube and stored in a cold and dark place.
If the surface of the wire has changed greatly and hydrogen bubbles are released again when the wire is added, then the ether should be filtered into another flask and another portion of sodium wire should be added.
A convenient and very effective way to purify diethyl ether from peroxides and at the same time from moisture is to pass the ether through a column with active Al2O3. Columns with a height of 60-80 cm and a diameter of 2-4 cm, filled with 82 g of Al2O3, are sufficient to purify 700 ml of ether containing a significant amount of peroxide compounds. Spent Al2O3 can be easily regenerated if a 50% acidified aqueous solution of FeSO4-7H2O is passed through the column, washed with water, dried, and thermally activated at 400-450°C.
Absolute ether is a highly hygroscopic liquid. The degree of moisture absorption by ether during its storage can be judged by the blueness of the anhydrous white powder CuSO4 when it is introduced into ether (a colored hydrate CuSO4-5H2O is formed).
dioxane
Dioxane (CH2) 4O is a colorless flammable liquid with a slight odor; d20-4 = 1.03375; tboil = 101.32 °С; tmelt = 11.80°C; n20-D = 1.4224. Miscible with water, alcohol and ether in any ratio. Forms azeotropic mixtures with water and alcohol.
Technical dioxane contains ethylene glycol acetal, water, acetaldehyde and peroxides as impurities. The method of purification of dioxane should be chosen depending on the degree of contamination, which is determined by adding sodium metal to dioxane. If a brown precipitate is formed, then the dioxane is heavily contaminated; if the surface of sodium changes slightly, then dioxane contains few impurities and is purified by distillation over sodium wire.
Heavily contaminated dioxane is purified as follows: 0.5 l of dioxane, 6 ml of conc. HCl and 50 ml of H2O are heated in a silicone (oil) bath under a stream of nitrogen in a flask with a reflux condenser at 115–120°C for 12 h.
After cooling, the liquid is shaken with small portions of molten KOH to remove water and acid. Dioxane forms the top layer, it is separated and dried with a fresh portion of KOH. Then the dioxane is transferred to a clean distillation flask and heated under reflux over 3-4 g of sodium wire for 12 hours. Purification is considered complete if the sodium surface remains unchanged. If all the sodium has reacted, then it is necessary to add a fresh portion and continue drying. Dioxane, which does not contain peroxide compounds, is distilled on a column or with an effective reflux condenser at normal pressure. Purification of dioxane from peroxides is carried out in the same way as the purification of diethyl ether.
Methyl alcohol (methanol)
Methyl alcohol (methanol) CH3OH is a colorless, easily mobile, flammable liquid, with an odor similar to that of ethyl alcohol; d20-4 = 0.7928; tboil = 64.51 °С; n20-D = 1.3288. Miscible in all respects with water, alcohols, acetone and other organic solvents; not miscible with aliphatic hydrocarbons. It forms azeotropic mixtures with acetone (tbp = 55.7°C), benzene (tbp = 57.5°C), carbon disulfide (tbp = 37.65°C), and also with many other compounds. With water, methyl alcohol does not form azeotropic mixtures, so most of the water can be removed by distillation of the alcohol.
Methyl alcohol is a strong poison that primarily affects the nervous system and blood vessels. It can enter the human body through the respiratory tract and skin. Especially dangerous when taken orally. The use of methyl alcohol in laboratory practice is allowed only in cases where it cannot be replaced by other, less toxic substances.
Synthetic absolute methyl alcohol, produced by the industry, contains only traces of acetone and up to 0.1% (mass.) of water. Under laboratory conditions, it can be prepared from technical CH3OH, in which the content of these impurities can reach 0.6 and even 1.0%. In a flask with a capacity of 1.5 l with a reflux condenser, protected by a calcium chloride tube with CaCl2, 5 g of magnesium chips are placed, they are poured with 60-70 ml of methyl alcohol containing no more than 1% water, an initiator is added - 0.5 g of iodine (or the corresponding the amount of methyl iodide, ethyl bromide) and heated until the latter dissolves. When all the magnesium passes into methylate (a white precipitate forms at the bottom of the flask), 800-900 ml of technical CH3OH are added to the resulting solution, boiled in a flask under reflux for 30 minutes, after which alcohol is distilled off from a flask with a reflux condenser 50 cm high, collecting a fraction with a boiling point of 64.5-64.7°C (at normal pressure). The receiver is provided with a calcium chloride tube with CaCl2. The water content of the alcohol obtained in this way does not exceed 0.05% (mass.). Absolute methyl alcohol is stored in a vessel protected from moisture in the air.
Additional drying of methyl alcohol containing 0.5-1% water can be carried out with magnesium metal without initiating the reaction. To do this, 10 g of magnesium chips are added to 1 liter of CH3OH, and the mixture is left in a reflux flask protected by a calcium chloride tube with CaCl2. The reaction starts spontaneously, and soon the alcohol boils. When all the magnesium has dissolved, boiling is maintained by heating in a water bath for some more time, after which the alcohol is distilled, discarding the first portion of the distillate.
Anhydrous methyl alcohol is also obtained by holding it over NaA or KA zeolite or by passing it through a column filled with these molecular sieves. To do this, you can use a laboratory-type column.
The presence of acetone in methyl alcohol is determined by a test with sodium nitroprusside. The alcohol is diluted with water, alkalized and a few drops of a freshly prepared saturated aqueous solution of sodium nitroprusside are added. In the presence of acetone, a red color appears, which intensifies upon acidification with acetic acid.
To remove acetone, the following method is proposed: 500 ml of CH3OH are boiled for several hours with 25 ml of furfural and 60 ml of 10% NaOH solution in a flask with a reflux condenser, and then the alcohol is distilled off on an efficient column. Resin remains in the flask - the product of the interaction of furfural with acetone.
Petroleum ether, gasoline and naphtha
During the distillation of light gasoline, a number of low-boiling hydrocarbon fractions are obtained, which are used as solvents. Vapors of these hydrocarbons have a narcotic effect.
The industry produces the following reagents:
The high volatility of petroleum ether, gasoline and naphtha, their easy flammability and the formation of explosive mixtures with air require special care when working with them.
Petroleum ether, gasoline and naphtha must not contain impurities of unsaturated and aromatic hydrocarbons.
The presence of unsaturated hydrocarbons is usually established with two reagents: a 2% solution of Br2 in CCl4 and a 2% aqueous solution of KMnO4 in acetone. To do this, a reagent solution is added dropwise to 0.2 ml of hydrocarbon in 2 ml of CCl4 and the color change is observed. The sample is considered negative if no more than 2-3 drops of bromine solution or KMnO4 solution become discolored.
Unsaturated hydrocarbons can be removed by repeated 30-minute shaking on a mechanical shaker of a portion of hydrocarbons with 10% (v/v) conc. H2SO4. After shaking with each portion of the acid, the mixture is allowed to settle, then the lower layer is separated. When the acid layer stops coloring, the hydrocarbon layer is shaken vigorously with several portions of a 2% KMnO4 solution in a 10% H2SO4 solution until the color of the KMnO4 solution no longer changes. At the same time, unsaturated hydrocarbons and partially aromatic hydrocarbons are almost completely removed. To completely remove aromatic hydrocarbons, it is necessary to shake hydrocarbons (petroleum ether, etc.) with oleum containing 8-10% (mass.) SO3 on a rocking chair. A bottle with a ground stopper, in which shaking is performed, is wrapped in a towel. After separation of the acid layer, the hydrocarbon fraction is washed with water, 10% Na2CO3 solution, again with water, dried over anhydrous CaCl2 and distilled over sodium wire. It is recommended to store petroleum ether over CaSO4 and distill before use.
The traditional chemical method of purifying saturated hydrocarbons from unsaturated hydrocarbons is very time consuming and can be replaced by adsorption. Impurities of many unsaturated compounds are removed by passing the solvent through a glass column with active Al2O3 and especially on zeolites, such as NaA.
Tetrahydrofuran
Tetrahydrofuran (CH2) 4O is a colorless mobile liquid with an ethereal odor; d20-4 = 0.8892; tboil = 66°С; n20-D = 1.4050. Soluble in water and most organic solvents. Forms an azeotropic mixture with water (6% (wt.) H2O), tboil = 64°C. Tetrahydrofuran is prone to the formation of peroxide compounds, so be sure to check for the presence of peroxides in it (see Diethyl ether). Peroxides can be removed by boiling with a 0.5% suspension of Cu2Cl2 for 30 min, after which the solvent is distilled and shaken with melted KOH. The upper layer of tetrahydrofuran is separated, 16% (wt.) KOH is again added to it and the mixture is refluxed for 1 hour in a flask under reflux. Then tetrahydrofuran is distilled over CaH2 or LiAlH4, 10-15% of the head fraction is discarded and about 10% of the residue is left in the cube. The head fraction and the bottom fraction are added to the technical products intended for purification, and the collected middle fraction is dried over a sodium wire. The purified product is stored without access to air and moisture.
Chloroform
Chloroform CHCl3 is a colorless mobile liquid with a characteristic sweet smell; d20-4 = 1.4880; tboil = 61.15°С; n20-D = 1.4455. Soluble in most organic solvents; practically insoluble in water. Forms an azeotropic mixture with water (2.2% (wt.) H2O), tboil = 56.1 °C. It is non-flammable and does not form explosive mixtures with air, but it is toxic - it acts on internal organs, especially the liver.
Chloroform almost always contains up to 1% (wt.) ethyl alcohol, which is added to it as a stabilizer. Another impurity of chloroform may be phosgene, which is formed during the oxidation of chloroform in the light.
The test for the presence of phosgene is performed as follows: 1 ml of a 1% solution of n-dimethylaminobenzaldehyde and diphenylamine in acetone is shaken with chloroform. In the presence of phosgene (up to 0.005%), an intense yellow color appears after 15 minutes. Chloroform is purified by shaking three times with separate portions of conc. H2SO4. For 100 ml of chloroform, each time take 5 ml of acid. Chloroform is separated, washed 3-4 times with water, dried over CaCl2 and distilled.
Purification of chloroform is also achieved by slowly passing the preparation through a column filled with active Al2O3 in the amount of 50 g per 1 liter of chloroform.
Chloroform should be stored in dark glass bottles.
carbon tetrachloride
Carbon tetrachloride CCl4 is a colorless non-flammable liquid with a sweet smell; d20-4 = 1.5950; tboil = 76.7°С; n25-D = 1.4631. Practically insoluble in water. Forms an azeotropic mixture with water (4.1% (wt.) H2O), tboil = 66°C. Dissolves various organic compounds. It has a less narcotic effect than chloroform, but surpasses it in toxicity, causing severe liver damage.
Carbon tetrachloride is sometimes contaminated with carbon disulfide, which is removed by stirring CCl4 at 60°C in a flask under reflux with 10% (v/v) of a concentrated alcoholic solution of KOH. This procedure is repeated 2-3 times, after which the solvent is washed with water, stirred at room temperature with small portions of conc. H2SO4 until it stops coloring. The solvent is then washed again with water, dried over CaCl2 and distilled over P4O10.
Drying of CCl4 is achieved by azeotropic distillation. The water is removed with the first cloudy portions of the distillate. Once a clear liquid begins to distill, it can be considered anhydrous.
ethyl acetate
Ethyl acetate CH3COOC2H5 is a colorless liquid with a pleasant fruity odor; d20-4 = 0.901; tboil = 77.15°С; n20-D = 1.3728. Forms an azeotropic mixture with water (8.2% (wt.) H2O), tboil = 70.4 °C.
Technical ethyl acetate contains water, acetic acid and ethyl alcohol. Many methods have been proposed for purifying ethyl acetate. One by one, ethyl acetate is shaken with an equal volume of 5% NaHCO3 solution and then with saturated CaCl2 solution. The ethyl acetate is then dried with K2CO3 and distilled on a water bath. For final drying, 5% P4O10 is added to the distillate and shaken vigorously, then filtered and distilled over sodium wire.
Ethanol
Ethyl alcohol С2Н5ОН is a colorless liquid with a characteristic odor; d20-4 = 0.7893; tboil = 78.39 °С; n20-D = 1.3611. Forms an azeotropic mixture with water (4.4% (wt.) H2O). It has a high dissolving power with respect to a wide variety of compounds and is miscible with water and all common organic solvents. Technical alcohol contains impurities, the qualitative and quantitative composition of which depends on the conditions of its production.
The produced absolute alcohol, which is obtained by azeotropic distillation of 95% industrial alcohol with benzene, may contain small amounts of water and benzene (up to 0.5% (wt.)).
Dehydration of 95% alcohol can be carried out by prolonged boiling with calcined CaO. For 1 liter of alcohol take 250 g of CaO. The mixture is refluxed in a 2-liter flask, closed with a CaO tube, for 6-10 hours. After cooling, the flask is attached to an atmospheric distillation apparatus and the alcohol is distilled off. Yield 99-99.5% alcohol 65-70%.
Barium oxide BaO has higher dehydrating properties. In addition, BaO is able to somewhat dissolve in almost absolute alcohol, turning it yellow. On this basis, it is determined when the process of absolutization is completed.
Further dehydration of 99-99.5% alcohol can be carried out by several methods: using magnesium (ethyl alcohol is obtained with a water content of not more than 0.05%), sodium and oxalic acid diethyl ester.
1 liter is poured into a 1.5-liter round-bottom flask with a reflux condenser and a calcium chloride tube with CaCl2. 99% ethanol, after which 7 g of sodium wire are added in small portions. After dissolution of sodium, 25 g of oxalic acid diethyl ester is added to the mixture, boiled for 2 hours, and the alcohol is distilled off.
Similarly, absolute alcohol is obtained using orthophthalic acid diethyl ester. 1 l of 95% alcohol is placed in a flask equipped with a reflux condenser and a calcium chloride tube with CaCl2, and 7 g of sodium wire are dissolved in it, after which 27.5 g of phthalic acid diethyl ester are added, the mixture is boiled for about 1 hour and the alcohol is distilled off. If a small amount of precipitate forms in the flask, this proves that the original alcohol was of fairly good quality. And vice versa, if a large amount of precipitate falls out and boiling is accompanied by shocks, then the initial alcohol was not dried enough.
Drying of ethyl alcohol is currently carried out in column-type apparatuses with NaA zeolite as a packing. Ethyl alcohol containing 4.43% water is fed for drying into a column 18 mm in diameter with a packing bed height of 650 mm at a rate of 175 ml/h. Under these conditions, in one cycle it is possible to obtain 300 ml of alcohol with a water content of not more than 0.1-0.12%. Zeolite regeneration is carried out in a column in a stream of nitrogen at 320 °C for 2 hours. When distilling ethyl alcohol, it is recommended to use devices on thin sections; at the same time, the sections are thoroughly cleaned and not lubricated. It is advisable to discard the first part of the distillate and complete the distillation when a little alcohol remains in the distillation flask.
Distill substances at a temperature much lower than their boiling point. The essence of steam distillation lies in the fact that high-boiling, non-miscible or slightly miscible, i.e. Substances that are poorly soluble in water evaporate when water vapor is passed through them; they are then condensed together with steam in a refrigerator. In order to establish whether a substance is volatile with water vapor, a small amount of it must be heated in a test tube with 2 ml of water. Above this tube, the bottom of the second test tube is held, in which ice is placed. If the drops condensing on the cold bottom of the second tube are cloudy, then the substance is volatile with water vapor. Table 6 Data on some substances distilled with water vapor Substance Boiling point, 0С Content of pure substance of mixture of substance with substance in steam distillate, % Aniline 184.4 98.5 23 Bromobenzene 156.2 95.5 61 Naphthalene 218.2 99 .3 14 Phenol 182.0 98.6 21 Nitrobenzene 210.9 99.3 15 o-Cresol 190.1 98.8 19 The sequence of work is as follows. It is recommended to first heat the flask with liquid and water almost to a boil. This preheating is intended to prevent the volume of the mixture in the flask from expanding too much due to the condensation of water vapor during the distillation. In the future, the distillation flask can not be heated. When a strong stream of steam comes out of the steam generator, close the rubber tube put on the tee with a clamp and start distillation with steam. A fairly strong jet of steam must pass through the liquid in the flask. A sign of the end of distillation is the appearance of a clear distillate (pure water). If the substance to be distilled has appreciable solubility in water (e.g. aniline), a small amount of clear distillate should be collected. At the end of the distillation, the clamp is opened and only after that the burners are extinguished (thereby eliminating the danger of liquid being drawn from the distillation flask into the steam generator). In the receiver, after distillation, two layers are obtained: water and organic matter. The latter is separated from the water in a separating funnel, dried in the usual way and distilled for final purification. Sometimes salting out and extraction are used to reduce the loss of a substance due to its partial solubility in water. High-boiling substances which are difficult to distill with steam having a temperature of 100°C may be distilled with superheated steam, unless there is a danger of decomposition of the substance at a higher temperature. Superheaters of various devices are used to form superheated steam. Typically, the steam from the steam generator enters a metal coil, which has a pipe for measuring the temperature and is heated by the flame of a strong burner. It is necessary to maintain a certain temperature of superheated steam in order to control the rate of distillation and avoid decomposition of the substance. The distillation flask should be immersed in an oil or metal bath heated to the required temperature, and the neck of the flask should be tightly wrapped with asbestos cord. If the distillation is carried out at temperatures above 120-130 ° C, it is necessary to connect successively first air and then water coolers to the distillation flask. The use of superheated steam makes it possible to increase the rate of distillation of hardly volatile substances many times over (Fig. 39). In contrast to conventional, simple distillation, during which steam and condensate pass through the apparatus once in a direction, in countercurrent distillation, or rectification, part of the condensate constantly flows towards the steam. This principle is implemented in distillation distillation columns. Rectification is a method of separating or purifying liquids with fairly close boiling points by distillation using special columns in which rising vapors interact with the liquid flowing towards them (phlegm), which is formed as a result of partial condensation of vapors. As a result of repeated repetition of the processes of evaporation and condensation, the vapor is enriched with a low-boiling component, and the phlegm, enriched with a high-boiling component, flows into a distillation flask. Efficient columns used in industry or research can separate liquids that differ in boiling point by less than 1°C. Conventional laboratory columns allow the separation of liquids with a boiling point difference of at least 10°C. The distillation column must be thermally insulated so that the processes occurring in it proceed under conditions as close as possible to adiabatic. With significant external cooling or overheating of the column walls, its correct operation is impossible. To ensure close contact of the vapors with the liquid, distillation columns are filled with a packing. As nozzles, glass beads, glass or porcelain rings, short pieces of glass tubes or stainless steel wires, glass spirals are used. Distillation columns are also used with a Christmas-tree tattoo of the “star” type. The efficiency of the column depends on the amount of phlegm supplied for irrigation. To obtain sufficient reflux, the distillation column must be connected to a condenser. The role of a condenser with partial condensation of vapors can be performed by a conventional dephlegmator. A simple setup for separating a mixture of liquids is shown in fig. 38.52 Condensers are widely used, in which complete condensation of all vapors passing through the column occurs. Such condensers are equipped with a distillate tap. Rectification can be carried out both at atmospheric pressure and in vacuum. As a rule, vacuum distillation is carried out for high-boiling or thermally unstable mixtures. Questions for control: 1. Tell the types and methods of distillation. 2. In what cases is distillation used at atmospheric pressure, at reduced pressure (in vacuum) and with steam. Why? 3. Tell the principle of operation and the device of a distillation device at atmospheric pressure. 4. Tell the principle of operation and the device of a distillation device with water vapor. Practical part 4.1.4.1. Distillation at atmospheric pressure Reagents: Purified substance. Equipment: simple distillation apparatus. Assemble the instrument for simple distillation at atmospheric pressure as shown in fig. 38. Fig. 38. Device for simple distillation: 1 - Wurtz flask; 2 - thermometer; 3 - descending Liebig refrigerator; 4 - allonge; 5 - receiving flask The distillation flask 1 is filled with a funnel to no more than two-thirds of the distilled liquid. Before filling the device, measure the volume or weight of the liquid. The distillation apparatus is assembled from dry, clean parts and fixed on tripods. Turn on water for cooling. A bath (water, oil) or a mantle heater is used as a heater. Controlling the temperature of the bath with the help of a second thermometer 2 fixed on a tripod, the heating is set to ensure uniform, slow boiling of the contents of the flask. No more than two drops of pure and transparent distillate per second should fall into the receiver. Only under such conditions does the thermometer in the flask show the temperature corresponding to the point of equilibrium between vapor and liquid; if the distillation is too fast, the vapors are easily overheated. The distillation temperature is recorded in a log. The distillation must not be continued to dryness! They finish it at the moment when the boiling point is 2-3 degrees higher than the one at which the main fraction passed. At the end of the distillation, the volume or weight of the distillate and the residue in the distillation flask are determined. Exercise. Purify one of the proposed solvents as directed by the instructor. In organic synthesis, the "purity" of the solvents used is very important. Often, even small impurities prevent the reaction from proceeding, so the purification of solvents is an urgent task for a synthetic chemist. Chloroform 0 20 Tboil = 61.2 C; nd=1.4455; d415=1.4985 Azeotropic mixture (chloroform-water-ethanol) contains 3.5% water and 4% alcohol, it boils at 55.5°C. Sales chloroform contains alcohol as a stabilizer that binds the phosgene formed during decomposition. Cleaning. Shake with concentrated sulfuric acid, wash with water, dry over calcium chloride and distill. Attention! Due to the risk of explosion, chloroform must not be brought into contact with sodium. Carbon tetrachloride 0 20 Tbp. = 76.8 C; nd =1.4603 Azeotropic mixture with water boils at 66°C and contains 95.9% carbon tetrachloride. A ternary azeotropic mixture with water (4.3%) and ethanol (9.7%) boils at 61.8°C. Cleaning and drying. A distillation is usually sufficient. The water is then removed as an azeotropic mixture (the first parts of the distillate are discarded). If high demands are placed on drying and purification, then carbon tetrachloride is refluxed for 18 hours with phosphorus oxide (V), distilled with a reflux condenser. Carbon tetrachloride must not be dried with sodium (risk of explosion!). Ethanol 0 Tbp.=78.33 C; nd20=1.3616;d415=0.789 Ethanol is miscible with water, ether, chloroform, benzene in any ratio. The azeotropic mixture with water boils at 78.17°C and contains 96% ethanol. A ternary azeotropic mixture with water (7.4%) and benzene (74.1%) boils at 64.85°C. 54 Impurities. Synthetic alcohol is contaminated with acetaldehyde and acetone, ethyl alcohol obtained during fermentation is contaminated with higher alcohols (fusel oils). Pyridine, methanol and gasoline are added for denaturation. Drying. In 1 liter of commercial "absolute" alcohol, 7 g of sodium are dissolved, 27.5 g of phthalic acid diethyl ester are added, and the mixture is refluxed for 1 hour. Then distilled with a small column. Distilled alcohol contains less than 0.05 water. Traces of water can be removed from commercial "absolute" alcohol in another way: 5 g of magnesium is boiled for 2-3 hours with 50 ml of "absolute" alcohol, to which 1 ml of carbon tetrachloride is added, then 950 ml of "absolute" alcohol is added, another 5 is boiled h with reflux. In conclusion, they distill. Water detection. Alcohol containing more than 0.05% water precipitates a voluminous white precipitate from a benzene solution of aluminum triethylate. 4.1.4.2. Steam distillation Reagents: Purified substance. Equipment: simple distillation apparatus. Assemble the steam distillation apparatus as shown in fig. 39. Fig. 39. Device for distillation with water vapor: 1- steam generator; 2 - tee with clamp; 3 - distillation flask; 4 - refrigerator; 5 - allonge; 6 - receiving flask; 7 - safety tube; 8 - inlet tube; 9 - a tube that removes steam Steam is formed in the steamer 1 (a flask is also suitable instead of it). The safety tube 7 serves to equalize the pressure, the connecting link - to drain the condensate. The steam enters the distillation flask 3 through the supply tube 8, in which the mixture to be separated is located. Usually this flask is also heated. The distillate enters the refrigerator 4, condenses and flows through the allonge 5 into the receiver 6. Small amounts of the substance can be distilled without using a steamer, but by adding a certain amount of water directly into the distillation flask. Task 1. Conduct a steam distillation of natural raw materials (rose petals, spruce needles) in order to obtain an aqueous extract of essential oil. For this, natural raw materials are loaded into the flask, filled with water, and steam distillation is carried out. Task 2. Obtain anhydrous oxalic acid from its mixture with water by azeotropic distillation of water. The distillation of a mixture of two liquids insoluble in each other is also used for drying organic substances by the so-called azeotropic distillation of water. To this end, the substance to be dried is mixed with an organic solvent, such as benzene or carbon tetrachloride, and the mixture is subjected to heating in a distillation apparatus. In this case, water is distilled off with a vapor of organic matter (at a temperature that is lower than the boiling point of the lowest boiling component of the mixture, for example, benzene or CCl4). With a sufficiently large amount of organic solvent, complete dehydration of the dried substance can be achieved. 4.1.4.3. Rectification Reagents: Purified substance. Equipment: Apparatus for fractional distillation. Rectification at atmospheric pressure Assemble the distillation apparatus as shown in fig. 40. Fig. 40. Device for fractional distillation: 1 - distillation flask; 2 - dephlegmator; 3 - thermometer; 4 - refrigerator; 5 - allonge; 6 - receiving flask Task. Separate the mixture of ethanol and butanol into components by distillation at atmospheric pressure. Collect the following fractions: a) up to 82°C ("pure ethanol"); b) from 83 to 110°C (intermediate fraction); c) remainder. Measure the fraction and residue volume. 4.1.4.4. Distillation under vacuum Reagents: Purified substance. Equipment: Apparatus for distillation under reduced pressure. 56 Fig. 41. Device for distillation under reduced pressure: 1 - Claisen flask or round-bottomed flask with a Claisen nozzle; 2 - capillary connected to a rubber hose with a clamp; 3 - thermometer; 4 - refrigerator; 5 - allonge; 6 - receiving flask; 7 - safety bottle; 8 - manometer Task. Distill quinoline under reduced pressure. T bale quinoline at atmospheric pressure -237.7 ° C, and at 17 mm Hg. Art. -114°C. Questions for the colloquium: 1. Why is a reflux condenser used in fractional distillation? 2. What are azeotropic mixtures? What are the methods of separating them? 3. At what temperature (above or below 100°C) will water boil in the mountains? Explain the answer. 4. Where do impurities remain during the purification of organic compounds by distillation? 4.1.5. Thin layer chromatography (TLC) Chromatography refers to a whole group of physicochemical separation methods based on the work of Tsveta (1903) and Kuhn (1931). Distinguish chromatography in columns, thin-layer, on paper, gas. The separation of substances in these cases occurs either as a result of distribution between two liquid phases (partition chromatography), or due to the different adsorbability of the substance by any adsorbent (adsorption chromatography). Thin layer chromatography consists in using, for example, aluminum oxide as a sorbent. In this case, both distribution and adsorption play a role in separation. The mobile phase, in the flow of which the mixture being separated moves, is called the eluent, and the solution leaving the stationary phase layer and containing the dissolved components of the mixture is called the eluate. Depending on the direction in which the eluent moves across the plate, there are: ascending thin layer chromatography 57 descending thin layer chromatography horizontal thin layer chromatography radial thin layer chromatography. Ascending thin layer chromatography This type of chromatography is the most common and is based on the fact that the front of the chromatographic system rises along the plate under the action of capillary forces, i.e. the front of the chromatographic system moves from bottom to top. For this method, the simplest equipment is used, since any container with a flat bottom and a tight-fitting lid, into which a chromatographic plate can be freely placed, can be used as a chromatographic chamber. The method of ascending thin layer chromatography has a number of disadvantages. For example, the speed of the rise of the front along the plate occurs unevenly, i.e. in the lower part it is the highest, and as the front rises it decreases. This is due to the fact that in the upper part of the chamber the saturation with solvent vapors is less, therefore the solvent from the chromatographic plate evaporates more intensively, therefore, its concentration decreases and the speed of movement slows down. To eliminate this shortcoming, strips of filter paper are attached along the walls of the chromatographic chamber, along which the ascending chromatographic system saturates the chamber with vapor throughout the entire volume. Some chromatographic chambers have a division into two trays at the bottom. This improvement allows not only to reduce the consumption of the chromatographic system (less volume is required to obtain the required height of the chromatographic system), but also to use an additional cuvette for the solvent, which increases the saturation vapor pressure in the chamber. The need to follow the solvent front can also be considered a disadvantage, since the solvent front line can “run away” to the upper edge. In this case, it is no longer possible to determine the actual value of Rf. Descending thin layer chromatography This chromatography method is based on the fact that the front of the chromatographic system descends along the plate mainly under the influence of gravity, i.e. the mobile phase front moves from top to bottom. For this method, a cuvette with a chromatographic system is attached to the upper part of the chromatographic chamber, from which a solvent enters the chromatographic plate using a wick, which flows down and the sample is chromatographed. The disadvantages of this method include the complexity of the equipment. This method is mainly used in paper chromatography. 58 Horizontal thin layer chromatography This method is the most complex in terms of instrumentation, but the most convenient. So, in the chromatographic chamber, the plate is placed horizontally and the system is fed to one edge of the plate using a wick. The solvent front moves in the opposite direction. There is another trick to simplify the camera as much as possible. To do this, an aluminum-based chromatographic plate is slightly bent and placed in the chamber. In this case, the system will act from two sides at the same time. Only aluminum-backed plates are suitable for this purpose, since the plastic and glass base is "inflexible", i.e. does not retain its shape. The advantages of this method include the fact that in a horizontal cell the vapor saturation of the system occurs much faster, the front velocity is constant. And when chromatographing from both sides, the front does not "run away". Radial thin-layer chromatography Radial thin-layer chromatography consists in the fact that the test substance is applied to the center of the plate and the eluent is fed there, which moves from the center to the edge of the plate. The distribution of the components of the mixture occurs between the water absorbed by the carrier1 and the solvent moving through this stationary phase (mobile phase). In this case, Nernst's law applies. The component of the mixture that is more readily soluble in water moves more slowly than the one that is more readily soluble in the mobile phase. Adsorption lies in the fact that adsorption equilibria are established between the carrier and the components of the mixture - each component has its own, resulting in a different speed of movement of the components. A quantitative measure of the rate of transfer of a substance when using a specific adsorbent and solvent is the value of Rf (deceleration factor or mobility coefficient). The value of Rf is defined as the quotient of the distance from the spot to the start line divided by the distance of the solvent (front line) from the start line: Distance from the spot to the start line Rf = Distance from the solvent front to the start The value of Rf is always less than unity, it does not depend on the length chromatograms, but depends on the nature of the selected solvent and adsorbent, temperature, concentration of the substance, the presence of impurities. So at low temperatures, substances move more slowly than at higher temperatures. Contaminants contained in the mixture of solvents used, the inhomogeneity of the adsorbent, foreign ions in the analyzed solution can change the value of Rf. 1 The carrier is an adsorbent, eg alumina, starch, cellulose, and water form the stationary phase. 59 The factor Rs is sometimes used: Distance traveled by the substance from the line to the start Rs= Distance traveled by the substance taken as a standard from the line to the start Unlike Rf, the value of Rs can be greater or less than 1. The value of Rf is determined by three main factors. FIRST FACTOR - the degree of affinity of the chromatographed organic compound to the sorbent, which increases in the following series: alkanes< алкены < простые эфиры < нитросоединения < альдегиды < нитрилы < амиды < спирты < тиофенолы < карбоновые кислоты По мере увеличения числа функциональных групп энергия адсорбции возрастает (Rf уменьшается). Наличие внутримолекулярных взаимодействий, например водородных связей, наоборот уменьшает ее способность к адсорбции (Rf увеличивается). Так, о-нитрофенолы и о-нитроанилины имеют большее значение Rf , чем м- и п-изомеры. Плоские молекулы адсорбируются лучше, чем неплоские. ВТОРОЙ ФАКТОР - свойства самого сорбента, которые определяются не только химической природой вещества, но и микроструктурой его активной поверхности. В качестве сорбентов чаще всего используются оксид алюминия, силикагель, гипс с размером гранул 5-50 мкм. Оксид алюминия обладает удельной поверхностью 100- 200 м2/г, имеет несколько адсорбционных центров. Одни из них избирательно сорбируют кислоты, другие - основания. При этом для кислот c рКа <5 и оснований c рКа >9 is characterized by chemisorption. Aluminum oxide is also effective for separating acyclic hydrocarbons with various numbers of double and triple bonds. Silica gel (SiO2×H2O) has a much higher sorption capacity than aluminum oxide. Large-pore grades of silica gel with a pore size of 10–20 nm and a specific surface area of 50–500 m2/g are used in TLC. Silica gel is chemically inert to most active organic compounds, however, due to its acidic properties (pH 3-5), it sorbs bases with pKa>9 quite strongly. Gypsum is a sorbent with a small sorption capacity and low activity. It is used for chromatography of polar compounds, as well as compounds containing a large number of different functional groups. THE THIRD FACTOR is the nature of the eluent that displaces the molecules of the studied substances adsorbed on the active centers. In order of increasing eluting power, the eluents can be arranged in the following row: 60
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INTRODUCTION
Solvent Purity
The requirements for the degree of purity of the solvent, of course, depend on how this solvent will then be used. Therefore, there are no adequate experimental criteria for the ideal purity of solvents; using conventional purification methods, only about 100% pure solvent can be obtained. From a practical point of view, purity is defined as follows: "A material is considered to be sufficiently pure if it does not contain impurities of such a nature and in such quantities as to interfere with its use for the purposes for which it is intended"Basic Precautions
Listed below are some rules that should be observed when cleaning and handling solvents;
A) Under no circumstances should sodium or other active metals or metal hydrides be used to dry liquids or acidic compounds (or halogenated compounds) which may act as oxidizing agents.
B) Vigorous drying agents (such as Na, CaH 2 , LiAlH 4 , H 2 SO 4 , P 2 O 5 ) should not be used until preliminary coarse drying has been carried out using conventional agents (Na 2 SO 4 and etc.) or the substance is not guaranteed to have a low water content.
C) Before distillation and drying of ethers and other solvents, it is imperative to check for the presence of peroxides in them and remove them. To avoid the formation of peroxides, most ethers should not be stored in the light and in the air for a long time.
D) It should be remembered that many solvents (for example, benzene, etc.) are toxic and have the ability to accumulate in the body; therefore, inhalation of the vapors of these solvents must be avoided. It should also be remembered that many solvents, with the exception of, for example, CCl 4 and CHCl 3 , are highly flammable; diethyl ether and CS 2 are especially dangerous in this respect.
E) Carefully purified solvents are recommended to be stored in sealed glassware in an inert atmosphere (usually N 2 , free of O 2 ). If tightness cannot be ensured, an excess pressure of an inert gas should be created above the surface of the liquid. Long-term storage of some solvents is ensured by sealing the closed container with paraffin.
METHODS FOR THE RAPID DETERMINATION OF PEROXIDES IN LIQUIDS
1. The most sensitive method (allows you to determine up to 0.001% peroxide); Under the influence drops liquid containing peroxide, the colorless ferrothiocyanate is converted to red ferrithiocyanate. The reagent is prepared as follows: 9 g of FeSO 4 7H 2 O are dissolved in 50 ml of 18% HCl. Some granular Zn and 5 g of sodium thiocyanate are added; after the disappearance of the red color, another 12 g of sodium thiocyanate is added and the solution is decanted from the unreacted Zn into a clean flask.
2. A few milliliters of liquid are placed in a flask with a glass stopper. Add 1 ml of freshly prepared 10% aqueous KI solution, shake and leave to stand for 1 min. The appearance of a yellow color indicates the presence of peroxide. A faster method is as follows: about 1 ml of liquid is added to an equal volume of glacial acetic acid containing about 100 mg of NaI or KI. The yellow color of the solution indicates the presence of a low concentration, brown - a high concentration of peroxide.
3. The method for the determination of peroxides in liquids insoluble in water is as follows: a few milliliters of liquid are added to a solution containing about 1 mg of sodium dichromate, 1 ml of water and 1 drop of dilute H 2 SO 4 . The blue color of the organic layer (perchromate ion) indicates the presence of peroxide.
4. A certain amount of liquid is “shaken off with a drop of pure mercury; in the presence of peroxide, a black film of mercury oxide is formed.
REMOVAL OF PEROXIDES (PARTICULARLY FROM ETHERS)
1. Large quantities of peroxides are removed by holding liquids over alumina or by passing them through short columns filled with alumina. The use of activated alumina allows the solvent to be dried simultaneously. Precautionary measures: when passing solvents through the column, it is necessary to ensure that the alumina is completely wetted with the solvent; adsorbed peroxides should be eluted or washed out, for example, with a 5% FeSO 4 aqueous solution (see below).
2. From liquids insoluble in water, peroxides are removed by shaking with a concentrated ferrous salt solution (100 g of iron (II) sulfate, 42 ml of concentrated HCl, 85 ml of water). With this treatment, some ethers may form small amounts of aldehydes, which are removed by washing with 1% KMnO 4 solution, then with 5%. NaOH aqueous solution and water.
3. One of the most effective reagents for removing peroxides is an aqueous solution of sodium pyrosulfite (also called Na 2 S 2 O 5 metabisulphite), which reacts rapidly with peroxides in stoichiometric ratios.
4. Peroxides in high concentrations are completely removed from the ethers by washing in the cold with triethylenetetramine (25% by weight of the ether).
5. Stannous dichloride SnCl 2 is the only inorganic reagent that is effective in the solid state.
6. From water-soluble ethers, peroxides are usually removed by boiling the ether under reflux in the presence of 0.5 wt.% Cu 2 Cl 2 and subsequent distillation.
CLEANING METHODS
The use of the following purification methods makes it possible to obtain solvents with a degree of purity that in most cases satisfies the requirements of chemical and physical experiments (synthesis, kinetic studies, spectroscopy, determination of dipole moments, etc.). This assumes that the experimenter uses commercially available solvents with a certain standard purity (see Chapter 1) for cleaning, and not industrial solvents containing a large amount of impurities. Unless otherwise stated, solvent distillation. carried out at atmospheric pressure. If the method of crystallization of the solvent from another liquids, crystallization means the freezing of the solvent to be purified; at the same time, up to 20% of the liquid is drained from the crystalline mass. In addition to the methods outlined here, in many cases so-called "adsorption filtration" using activated alumina can be recommended for the purification of solvents.
aromatic hydrocarbons
Very high purity benzene (bp 80.1°; mp 5.53°) is obtained by fractional crystallization from ethanol or methanol followed by distillation. When using the traditional purification method, benzene is shaken or mixed with concentrated sulfuric acid (100 ml per 1 liter of benzene) and then the acid layer is removed; the operation is repeated until the acid layer has a very faint color. Benzene is decanted and distilled. Purification using sulfuric acid removes thiophene impurities, olefins and water from benzene.
Toluene(bp 110.6°) and xylenes cleaned in the same way; it should be remembered, however, that these hydrocarbons have a higher ability to sulfonate than benzene, therefore, when treating them with sulfuric acid, it is necessary to cool the mixture, maintaining the temperature below 30 ° C. In addition to sulfuric acid, it is also recommended to use CaCl 2 for drying, although, in general, simple distillation may be sufficient, since these hydrocarbons form azeotropic mixtures with water or have a much higher boiling point than water.
Acetone (bp 56.2°)
Acetone is very difficult to dry; the use of many of the commonly used drying agents (even MgSO 4 ) results in acetone condensation. For drying, it is convenient to use a molecular sieve 4A and K 2 CO 3 . Distillation over a small amount of KMnO 4 allows the destruction of impurities contained in acetone, such as aldehydes. Very pure acetone is obtained as follows: saturated with dry NaI at 25-30°C, the solution is decanted and cooled to -10°C; NaI crystals form a complex with acetone, which is filtered off and heated to 30°C; the resulting liquid is distilled.
Acetonitrile (bp 81.6°)
Acetonitrile containing water is pre-dried, then stirred with CaH 2 until gas evolution ceases and distilled over P 2 O 5 (≤5 g/l) in a glass apparatus with a high reflux reflux condenser. The distillate is refluxed over CaH 2 (5 g/l) for at least 1 hour, then slowly distilled, discarding the first 5% and the last 10% of the distillate, in order to reduce the acrylonitrile content. If the acetonitrile contains benzene as an impurity (UV absorption band at 260 nm, strong tail at 220 nm), the latter is removed by azeotropic distillation with water before treatment with P 2 O 5 .
tert-Butyl alcohol (bp 82°)
To obtain alcohol of very high purity (mp. 25.4 °), it is distilled over CaO, followed by multiple crystallization.
Dimethyl sulfoxide [i.e. bale 189° (dec.)]
Dimethyl sulfoxide may contain, in addition to water, impurities of dimethyl sulfide and sulfone. For cleaning, it is kept for 12 hours or more over fresh activated alumina, drierite, BaO or NaOH. Then it is distilled under reduced pressure (~2-3 mm Hg, bp 50°) over NaOH or BaO granules and stored over a 4A molecular sieve.
Dimethylformamide (bp 152°)
N,N-Dimethylformamide may contain impurities of water and formic acid. The solvent is stirred or shaken with KOH and distilled over CaO or BaO.
1,4-Dioxane (bp 102°)
Dioxane can contain a large amount of impurities, so it is difficult to purify it. It is known that many of the described methods are ineffective in the purification of this solvent, as they lead to the decomposition of the liquid. The traditional cleaning method is as follows. A mixture of 300 ml of water, 40 ml of concentrated HCl and 3 l of dioxane is refluxed for 12 hours under a slow stream of nitrogen (to remove acetaldehyde, which is formed by hydrolysis of the glycol acetal impurity). The solution is cooled and KOH pellets are added until they no longer dissolve and the layers separate. The dioxane layer (top layer) is decanted and dried over fresh potassium hydroxide. The dried dioxane is boiled over Na for 12 hours or until the Na retains a shiny surface. The solvent is then distilled over Na and stored in the dark under N 2 atmosphere.
LiAlH 4 should not be used to dry dioxane, as it can decompose at the boiling point of the solvent. In order to ensure the absence of oxygen and peroxides in purified dioxane, it is recommended to use benzophenone ketyl.
Diethyl ether (bp 34.5°)
In all cases, except when a ready-made "absolute" ether is used, the solvent should be checked for the presence of peroxides and treated accordingly. When working with ether, additional precautions must be taken due to the flammability of the solvent. Sufficiently dry ether can be obtained by drying and distillation over sodium wire, however, the most effective method is distillation over LiAlH 4 (or CaH 2).
Methanol (bp 64.5°)
In methanol, in addition to water, impurities of carbonyl and hydroxyl-containing compounds with the number of C atoms from 1 to 4 are found, however, a reagent grade solvent usually contains only traces of such impurities. Acetone is removed from methanol as iodoform after treatment with NaOI. Most of the water can be removed by distillation, since methanol does not form azeotropic mixtures with water. Very dry methanol is obtained by holding the solvent over 3A or 4A molecular sieves or passing through a column filled with these molecular sieves; then the solvent is dried over calcium hydride. Drierite is not recommended as a drying agent for methanol! Residual water can also be removed with magnesium methoxide as follows: a mixture of 50 ml of methanol, 5 g of Mg in the form of chips and 0.5 g of sublimated iodine is refluxed until the solution becomes colorless and the evolution of hydrogen ceases. Then add 1 l of methanol, reflux for about 30 minutes and gently distill.
Nitroalkanes
Commercially available compounds with 1 to 3 carbon atoms can be purified fairly well by drying over calcium chloride or P 2 O 5 followed by gentle distillation. High purity nitromethane is also obtained by fractional crystallization (mp -28.6°).
Nitrobenzene (bp. 211°)
Nitrobenzene, purified by fractional crystallization (mp 5.76°) and distillation over P 2 O 5 , is colorless. A solvent containing impurities quickly stains over P 2 O 5 ; pure solvent remains colorless even after prolonged contact with P 2 O 5 .
Pyridine (bp 115.3°)
Pyridine is dried for a long time over KOH granules, then distilled over BaO. It should be borne in mind that pyridine is very hygroscopic (forms a hydrate, bp 94.5 °), so care must be taken that moisture does not get into the purified solvent.
2-Propanol [iso-propanol] (bp 82.4°)
2-Propanol forms an azeotropic mixture with water (9% water, bp 80.3°); water can be removed by refluxing or by distillation over lime. The solvent is prone to the formation of peroxides, which are usually destroyed by boiling under reflux over SnCl 2 . Sufficiently dry and pure solvent is obtained by distillation over anhydrous calcium sulfate; very dry alcohol is obtained using Mg according to the procedure described for methanol.
Sulfuric acid (b.p. about 305 °)
According to Jolly, 100% acid is usually made by adding fuming sulfuric acid to standard 96% acid until the water it contains turns into sulfuric acid. The end time of this procedure is determined as follows: humid air is blown through the acid with a small rubber syringe; the formation of fog indicates an excess of SO 3; if the acid is not yet 100%, mist will not form. This method allows you to adjust the composition of the acid with an accuracy of 0.02% (!). Sulfuric acid is very hygroscopic, so care must be taken to prevent moisture from getting into it.
Carbon disulfide (bp 46.2°)
Carbon disulfide is a highly flammable and toxic liquid and special precautions must be taken when handling it. The solvent should be distilled very carefully, using a water bath, which is recommended to be heated to a temperature slightly above the boiling point of CS 2 . Sulfur impurities are removed from carbon disulfide by shaking the solvent first with Hg, then with a cold saturated solution of HgCl 2 and then with a cold saturated solution of KMnO 4, after which it is dried over P 2 O 5 and distilled.
Tetrahydrofuran (bp 66°)
The solvent must be checked for the presence of peroxides and treated accordingly; traces of peroxides are removed by boiling a 0.5% suspension of Cu 2 Cl 2 in tetrahydrofuran for 30 min, after which the solvent is distilled. The tetrahydrofuran is then dried over KOH granules, refluxed and distilled over lithium aluminum hydride or calcium hydride. This method makes it possible to obtain a very dry solvent.
Acetic acid (bp 118°)
Commercially available glacial acetic acid (~99.5%) contains carbonyl impurities, which are removed by refluxing in the presence of 2 to 5 wt% KMnO 4 or excess CrO 3 , after which the acid is distilled. Traces of water are removed on heating by treatment with a double or triple excess of triacetyl borate, which is prepared by heating at 60° C. a mixture of boric acid and acetic anhydride (in a ratio of 1:5 by weight); the mixture of acetic acid and triacetylborate is cooled and the crystals formed are filtered off. After distillation, an anhydrous acid is obtained. Acetic acid is also dehydrated by distillation over P 2 O 5 .
Carbon tetrachloride (bp 76.5°)
CS 2 impurities from CCl 4 are removed by stirring the hot solvent with 10 vol.% concentrated alcoholic solution of KOH. This procedure is repeated several times, after which the solvent is washed with water, dried over CaCl 2 and distilled over P 2 O 5 .
Chloroform (bp 61.2°)
Commercially available chloroform most often contains about 1% ethanol as a stabilizer that prevents chloroform from being oxidized by atmospheric oxygen to phosgene. One of the following methods is recommended for cleaning the solvent:
A) Chloroform is shaken with concentrated H 2 SO 4 , washed with water, dried over CaCl 2 or K 2 CO 3 and distilled.
B) Chloroform is passed through a column filled with activated alumina (degree of activity 1) (about 25 g per 500 ml CHCI 3).
C) Chloroform is shaken several times with water (about half the solvent volume), dried over CaCl 2 and distilled over P 2 O 5 .
The solvent purified by any of these methods is stored in the dark under N 2 atmosphere.
Ethanol (bp 78.3°)
Incoming in. sale "absolute" ethanol contains about 0.1-0.5% water and, as a rule, 0.5-10% denaturing agent (acetone, benzene, diethyl ether or methanol, etc.). A more accessible and less expensive solvent is usually an azeotropic mixture with water (4.5%) (95% ethanol or rectified alcohol) (b.p. 78.2 °). It is this solvent that is most often used in UV spectrophotometry (reagent grade ethanol or USP does not contain impurities of benzene and other denaturing agents). Pure ethanol is highly hygroscopic and readily absorbs moisture; this circumstance should be read when receiving a dry solvent.
To remove traces of water from absolute ethanol, the following method is recommended. A mixture of 60 ml absolute ethanol, 5 g Mg (shavings) and a few drops of CCl 4 or CHCl 3 (catalyst) is refluxed until all the Mg is converted to ethylate. Add another 900 ml of absolute ethanol, reflux for 1 hour and distill. If it is necessary to ensure the absence of halogen compounds in the absolute solvent, volatile ethyl bromide can be used as a catalyst instead of CCl 4 or CHCl 3 . The formation of a bulk precipitate when a benzene solution of aluminum ethoxide is added to ethanol makes it possible to detect the presence of up to 0.05% water in the solvent. Storage of absolute ethanol over a molecular sieve 3A allows you to save a solvent with a water content of not more than 0.005%.
Most of the water from the 95% alcohol is removed by refluxing over fresh lime (CaO) followed by distillation. As another method, azeotropic distillation is recommended: water is distilled off from a ternary azeotropic mixture, for example, benzene-ethanol-water (b.p. 64.48 °); then benzene is distilled off from the double azeotropic mixture of benzene-ethanol (bp 68.24°).
Ethyl acetate (bp 77.1°)
Commercially available ethyl acetate most often contains water, ethanol and acids as impurities; they are removed by washing the solvent with a 5% aqueous sodium carbonate solution, then with a saturated solution of calcium chloride, after which they are dried over anhydrous potassium carbonate and distilled over P 2 O 5 .
Other solvents
Cellosolves and carbitols are purified by drying over calcium sulfate and distillation. Acid anhydrides are purified by fractional distillation from molten salts of the corresponding acids; high molecular weight anhydrides (6 carbon atoms, etc.) decompose during atmospheric distillation.
Since carbon tetrachloride (CTC) is a banned ozone-depleting substance under the Montreal Protocol, but is inevitably formed as a by-product in the production of chloromethanes, choosing the most efficient method for processing CTC is an urgent task.
Various transformations of CHU have been especially intensively studied in recent years, and there is a large amount of experimental data. Below, an assessment will be made of various options for the transformation of CHU based on our own research and data from other authors.
The papers consider the problem of processing CTC into environmentally friendly products, however, they do not fully cover the possible options for processing, and, in our opinion, the advantages and disadvantages of individual methods for the utilization of CTC are not sufficiently objectively reflected.
There are also some inconsistencies in the articles. .
Thus, the topic of the articles is the processing of CTC into environmentally friendly products, in the text and conclusions, the conversion of CTC into chloromethanes is recommended as promising methods, and in the introduction, chloromethanes are called the main chemical environmental pollutants. In fact, chloromethanes are not included in the Stockholm Convention on Persistent Organic Pollutants, and in terms of toxicity and release volume, chloromethanes are not the main pollutants even among other organochlorine compounds.
The articles talk about the high persistence of chloromethanes. At the same time, it is known that all chloromethanes, except for methyl chloride, are unstable products and require stabilization to preserve their properties. The decomposition of chloromethanes takes place in the boilers of distillation columns, in the evaporator for supplying CCA to the reactor. According to the encyclopedia, chloroform without a stabilizer is unlikely to last without changing its properties during the day if it is in contact with the atmosphere.
CTC processing processes can be classified according to the degree of usefulness of the resulting processed products. This does not mean that the usefulness of the CTC recycling processes themselves will be in the same sequence, since much will depend on the cost of processing and subsequent isolation of the resulting products.
The choice of method is also influenced by the presence of a large amount of other products in the processed waste, in addition to PCC (for example, in still distillation of chloromethane production), when the isolation of PCC from these wastes may require significant costs. The same situation develops during the neutralization of ChChU, which is contained in a small amount in gas emissions. In this case, non-selective complete combustion with the production of CO2 and HCl with practically zero utility due to the low profitability of their isolation may be the most acceptable solution. Therefore, in each specific case, the choice can be made only after a technical and economic comparison.
Burning CHU
When burning ChKhU using air as an oxidizer, a simultaneous supply of hydrocarbon fuel is required to supply heat and bind chlorine to hydrogen chloride. Alternatively, with a small amount of hydrogen chloride, it can be converted to sodium chloride by injecting sodium hydroxide solution into the combustion gases. Otherwise, hydrogen chloride is released from the combustion gases in the form of hydrochloric acid.
Disposal of hydrochloric acid itself can be a problem due to excess supply over demand. The separation of hydrogen chloride from hydrochloric acid by stripping leads to the fact that it becomes more expensive than chlorine. In addition, hydrogen chloride has limited use in oxychlorination and hydrochlorination processes. The conversion of hydrogen chloride to chlorine by electrolysis of hydrochloric acid or oxidation with oxygen (the Deacon process) is a rather expensive and technologically complex operation.
As a method for the complete oxidation of CTC, the authors of the works prefer catalytic oxidation over conventional thermal combustion. According to comparison with combustion, the processes of catalytic oxidation are characterized by a greater depth of destruction of organochlorine wastes and are not accompanied by the formation of dioxins.
These statements are not true and may lead to a misconception about the effectiveness of the compared methods. The article does not provide any evidence to support higher conversions in catalytic oxidation. In the references cited in favor of such a statement, for example, the conversion rates are indeed high 98-99%, but this is not the level that is achieved with thermal combustion. Even if a conversion of 100% or 100.0% is indicated, this only means that the accuracy of these data is 0.1%.
Under the US Resource Conservation and Recovery Act, for major organic hazardous contaminants, destructive removal efficiency must be at least 99.9999%. In Europe, it is also recommended to adhere to this minimum value for the degree of decomposition of obsolete pesticides and polychlorinated biphenyls in combustion plants.
A set of requirements for the combustion process has been developed, called BAT - Best Available Technique (Best Acceptable Technique). One of the requirements, along with the temperature 1200оС and residence time 2 s, is the turbulence of the reaction flow, which allows, in general, to eliminate the problem of the breakthrough of the combustible substance in the near-wall layer and to ensure the ideal displacement regime. Apparently, in a tubular reactor filled with a catalyst, it is more difficult to eliminate the slippage of the combustible substance in the near-wall layer. In addition, there are difficulties in the uniform distribution of the reaction flow through the tubes. At the same time, further progress in eliminating the "wall effect" made it possible to achieve a conversion degree of 99.999999% when burned in a liquid rocket engine.
Another controversial statement of the authors is the absence of PCDD and PCDF in the catalytic oxidation products. No figures are provided to support this. Only two references are given in the work, confirming the absence of dioxins during catalytic oxidation. However, one of the references, apparently due to some kind of error, has nothing to do with catalytic oxidation, since it is devoted to the biotransformation of organic acids. In another work, catalytic oxidation is considered, but no information is reported on the absence of dioxins in this case. On the contrary, data are given on the formation of another persistent organic pollutant, polychlorinated biphenyl, during the catalytic oxidation of dichlorobenzene, which may indirectly indicate the possibility of the formation of dioxins.
It is rightly noted in the work that the temperature range of catalytic processes for the oxidation of organochlorine wastes is favorable for the formation of PCDD and PCDF, however, the absence of PCDD and PCDF may be due to the catalytic destruction of the sources of their formation. At the same time, it is known that the processes of synthesis of macromolecular compounds even from C1 compounds are successfully carried out on catalysts.
In European countries, there are environmental requirements for waste incineration, according to which the limit value for air emissions for dioxins is 0.1 ng TEQ/Nm3.
The environmental indicators of the process of thermal-oxidative (fire) neutralization of liquid organochlorine waste presented above are available in. Finally, it should be noted that in the "Register of existing capacities for the destruction of polychlorinated biphenyls" the most widely used and proven method for the destruction of PCBs is high temperature incineration. Catalytic oxidation is not used for this purpose.
In our opinion, catalytic oxidation, despite the use of supported precious metals as a catalyst, has an advantage in the destruction of residual amounts of toxic substances in gas emissions, since, due to the low process temperature, a significantly lower fuel consumption is required for heating the reaction gas than in thermal combustion. . The same situation occurs when optimal combustion conditions are difficult to achieve, for example in catalytic converters in automobile engines. In addition, pressurized catalytic oxidation of organochlorine wastes (the "cathoxide process") has been used by Goodrich to directly feed combustion gases containing hydrogen chloride into an ethylene oxidative chlorination reactor to produce dichloroethane.
A combination of thermal and catalytic waste gas oxidation has been reported to achieve higher efficiencies than pure catalytic oxidation. Qualified processing of organochlorine waste is also considered in. In our opinion, for the combustion of CTC in the form of a concentrated product, it is more expedient to use conventional thermal combustion.
In concluding this section, it is worth considering one more aspect of the oxidation of CTC. It is a non-combustible substance according to CHU, therefore its combustion can be carried out only in the presence of additional fuel. This is true when air is used as the oxidizing agent. ChKhU is able to burn in oxygen with a slight thermal effect, the calorific value is 242 kcal / kg. According to another reference, the heat of combustion of a liquid is 156.2 kJ/mol (37.3 kcal/mol), and the heat of combustion of steam is 365.5 kJ/mol (87.3 kcal/mol).
Oxidation with oxygen can be one of the ways to process CTC, in which the carbon component is lost, but the chlorine spent to obtain CTC is recovered. This process has an advantage over conventional incineration due to the production of concentrated products.
CCl4 + O2 → CO2 + 2Cl2
The process of oxidative dechlorination of CTC also makes it possible to obtain carbon dioxide, and, if necessary, phosgene.
2CCl4 + O2 → 2COCl2 + 2Cl2
CHU hydrolysis
Another interesting, in our opinion, process of processing CTC into carbon dioxide and hydrogen chloride is hydrolysis.
CCl4 + 2H2O → CO2 + 4HCl
There are few publications in this area. The interaction of OH-groups with chloromethanes in the gas phase is discussed in the article. The catalytic hydrolysis of CCA to HCl and CO2 on magnesium oxide at temperatures above 400°C was studied in. The rate constants of homogeneous hydrolysis of CCA in the liquid phase were obtained in the work.
The process proceeds well, according to our data, at relatively low temperatures of 150-200°C, it uses the most accessible reagent and should not be accompanied by the formation of dioxins and furans. All that is needed is a hydrochloric acid-resistant reactor, for example one lined with PTFE. Perhaps such a cheap and environmentally friendly recycling method can be used for the destruction of other waste.
Interaction of Chu with methanol
Close to hydrolysis and actually flowing through this stage is the process of vapor-phase interaction of CTC with methanol to produce methyl chloride in the presence of a catalyst - zinc chloride on activated carbon. Relatively recently, this process was first patented by Shin-Etsu Chemical (Japan). The process proceeds with high close to 100% conversions of Chu and methanol.
CCl4 + 4CH3OH → 4CH3Cl + CO2 + 2H2O
The authors believe that the reaction of CCA with methanol proceeds in 2 stages: first, CCA is hydrolyzed to carbon dioxide and hydrogen chloride (see above), and then hydrogen chloride reacts with methanol to form methyl chloride and water.
CH3OH + HCl → CH3Cl + H2O
At the same time, a small amount of water that is present in the atmosphere is sufficient to initiate the reaction. It is believed that the first stage limits the rate of the overall process.
At a close to stoichiometric ratio of CCA to methanol (1:3.64), the reaction proceeded stably during the experiment, which lasted 100 hours, with a conversion of CTC 97.0% and methanol 99.2%. The selectivity for methyl chloride formation was close to 100%, as only traces of dimethyl ether were detected. The temperature in the catalyst bed was 200°C.
Then it was proposed to divide the process into two reaction zones: in the first, hydrolysis of CTC takes place, and in the second, the interaction of hydrogen chloride with methanol introduced into this zone. Finally, the same company patented a method for producing chloromethanes without the formation of CTC, which includes the following steps:
. production of chloromethanes by chlorination of methane;
. the interaction of hydrogen chloride, released in the first stage, with methanol to form methyl chloride and dilute hydrochloric acid;
. hydrolysis of Chu with dilute hydrochloric acid in the presence of a catalyst - chlorides or oxides of metals on a carrier.
The disadvantage of the heterogeneously catalytic process of the interaction of CTC with methanol is the relatively short service life of the catalyst due to its carburization. At the same time, high-temperature regeneration for burning out carbon deposits is undesirable due to the volatilization of zinc chloride, and when activated carbon is used as a carrier, it is generally impossible.
In conclusion of this section, it can be mentioned that we have made attempts to get away from the solid catalyst in the process of processing CTC with methanol. In the absence of a catalyst, at a molar ratio of methanol:CCA = 4:1 and with an increase in temperature from 130 to 190°C, the CCA conversion increased from 15 to 65%. For the manufacture of the reactor, materials are required that are stable under these conditions.
Carrying out the catalytic liquid-phase process at relatively low temperatures of 100-130°C and a molar ratio of methanol:NTC = 4:1 without pressure made it possible to achieve only 8% NTC conversion, while it is possible to obtain almost 100% methanol conversion and 100% selectivity for methyl chloride. To increase the conversion of CTC, an increase in temperature and pressure is required, which could not be achieved under laboratory conditions.
A method for the alcoholysis of ChCA has been patented, including the simultaneous supply of ChCA and ³
1 alcohol ROH (R = alkyl C 1 - C 10) into a catalytic system, which is an aqueous solution of metal halides, especially chlorides I B, I I B, V I B and V I I I groups. In the liquid-phase interaction of methanol and CCA (in a ratio of 4:1) in a laboratory reactor with a magnetic stirrer in the presence of a catalytic solution of zinc chloride at a temperature of 180 ° C and a pressure of 3.8 bar, the conversion of CTC and methanol was 77%.
Chlorination with CHU
Chu is a safe chlorinating agent, for example, in the production of metal chlorides from their oxides. In the process of such a reaction, CHU is converted into carbon dioxide.
2Me2O3 + 3CCl4 → 4MeCl3 + 3CO2
Works were carried out to obtain iron chlorides using Chu as a chlorinating agent, the process takes place at a temperature of about 700 ° C. By chlorination with the help of Chu in the industry, their chlorides are obtained from oxides of elements of groups 3-5 of the Periodic system.
Interaction of Chu with methane
The simplest solution to the problem of CTC processing would be the interaction of CTC with methane in a methane chlorination reactor to obtain less chlorinated chloromethanes, since in this case it would be practically only necessary to organize the recycling of unreacted CTC, and the subsequent isolation and separation of reaction products can be carried out on the main system production.
Earlier, when studying the process of oxidative chlorination of methane, both in the laboratory and at the pilot plant, it was noted that when the reaction gas from the direct chlorination of methane containing all chloromethanes, including CTC, is supplied to the reactor, the amount of the latter after the oxychlorination reactor decreases, although it should was with the increase in the amount of all other chloromethanes to increase.
In this regard, it was of particular interest to carry out a thermodynamic analysis of the reactions of the interaction of methane with CCA and other chloromethanes. It turned out that the interaction of CTC with methane is the most thermodynamically probable. At the same time, the equilibrium degree of CCA conversion under conditions of excess methane, which is realized in an industrial chlorinator, is close to 100% even at the highest temperature (the lowest equilibrium constant).
However, the actual course of a thermodynamically probable process depends on kinetic factors. In addition, other reactions can occur in the system of CTC with methane: for example, the pyrolysis of CTC to hexachloroethane and perchlorethylene, the formation of other C2 chlorine derivatives due to the recombination of radicals.
An experimental study of the interaction reaction of CCA with methane was carried out in a flow reactor at temperatures of 450-525°C and atmospheric pressure, with an interaction time of 4.9 s. Processing of the experimental data gave the following equation for the exchange reaction rate of methane with CTC:
r \u003d 1014.94 exp (-49150 / RT). [Сl 4 ]0.5. [CH 4], mol / cm 3 .s.
The obtained data made it possible to estimate the contribution of the exchange interaction of CCA with methane in the process of methane chlorination, to calculate the necessary recycle of CCA for its complete conversion. Table 1 shows the CCA conversion depending on the reaction temperature and CCA concentration at approximately the same concentration of methane, which is implemented in an industrial chlorinator.
The CCA conversion naturally falls with decreasing process temperature. Acceptable CTC conversion is observed only at temperatures of 500-525 o C, which is close to the temperature of bulk chlorination of methane in the existing production of chloromethanes 480-520 o C.
The total transformations of CTC and methane can be characterized by the following total equation and material balance:
CCl 4 + CH 4 → CH 3 Cl + CH 2 Cl 2 + CHCl 3 + 1,1-C 2 H 2 Cl 2 + C2Cl 4 + HCl
100.0 95.6 78.3 14.9 15.2 7.7 35.9 87.2 mol
The second line gives the amounts of methane reacted and products obtained in moles per 100 moles of CTC reacted. The selectivity of the conversion of CCA to chloromethanes is 71.3%.
Since the separation of commercial CTC from distillation stills of chloromethane production was a certain problem, and there were occasional difficulties with the marketing of distillation stills, the processing of CTC in a methane chlorination reactor aroused interest even before the ban on the production of CTC due to its ozone-depleting potential.
Pilot tests of CHC processing in the methane chlorination reactor were carried out at the Cheboksary p.o. "Khimprom". The obtained results basically confirmed the laboratory data. The selectivity of the conversion of CCA to chloromethanes was higher than under laboratory conditions.
The fact that the selectivity of the CCA interaction process in an industrial reactor turned out to be higher than in a laboratory one can be explained by the fact that during methane chlorination in a laboratory reactor, the outer walls heated by a casing with an electrocoil are overheated. Thus, at a temperature in the reaction zone of 500°C, the temperature of the walls of the laboratory chlorinator was 550°C.
In an industrial reactor, heat is accumulated by the central brick column and lining, and the outer walls of the chlorinator, on the contrary, are cooled.
Pilot tests for the return of CTC to the methane chlorination reactor were also carried out earlier at the Volgogradsky p.o. "Khimprom". ChCA was supplied to the industrial chlorinator without isolation as part of still distillation along with all impurities of C 2 chlorinated hydrocarbons. As a result, about 100 m3 of distillation cubic meters were processed per month. However, the processing of the obtained data caused difficulties due to the large number of components in low concentrations and the insufficient accuracy of the analyzes.
To suppress the formation of side chlorohydrocarbons of the ethylene series during the interaction of CCA with methane, it is proposed to introduce chlorine into the reaction mixture at a ratio of chlorine to CCA 0.5.
The preparation of chloromethanes and other products by the interaction of CTC with methane at temperatures of 400-650°C in a hollow reactor is described in a patent. An example is given where the conversion of CCA was in mol %: to chloroform - 10.75, methylene chloride - 2.04, methyl chloride - 9.25, vinylidene chloride - 8.3 and trichlorethylene - 1.28.
Then the same company "Stauffer" patented a method for producing chloroform by the interaction of CCA with C2-C3 hydrocarbons and C1-C3 chlorinated hydrocarbons. According to the examples given, only chloroform is obtained from CCI and methylene chloride at a temperature of 450°C in a hollow reactor, and chloroform and perchlorethylene are obtained at a temperature of 580°C. At a temperature of 490°C, only methylene chloride and chloroform were formed from CTC and methyl chloride at a temperature of 490°C, and trichloethylene also appeared at a temperature of 575°C.
A process was also proposed for the production of methyl chloride and methylene chloride by the interaction of methane with chlorine and CTC in a fluidized contact bed at a temperature of 350-450 o C. The process of chlorination of methane to chloroform in a fluidized bed of contact with the introduction of CCA into the reaction zone to provide heat removal is described. In this case, the reaction of CTC with methane occurs simultaneously.
The exchange reaction between CTC and paraffin results in the formation of chloroform and chlorinated paraffin.
When developing the process of oxidative chlorination of methane, it was found that the oxidative dechlorination of CCA in the presence of methane proceeds more efficiently than the interaction of methane and CCA in the absence of oxygen and a catalyst.
The data obtained indicate that the process of oxidative dechlorination of CTC in the presence of methane and a catalyst based on copper chlorides proceeds at a lower temperature than the interaction of CTC with methane in the absence of oxygen, with the production of only chloromethanes without the formation of side chlorohydrocarbons. Thus, the conversion of CCA at temperatures of 400, 425, and 450°C averaged 25, 34, and 51%, respectively.
An additional advantage of the oxidative processing of CTC is the absence of catalyst carbonization. However, the need for a catalyst and oxygen reduces the advantages of this method.
A method has been patented for the production of chloromethanes by oxidative chlorination of methane without obtaining CTC in the final products due to its complete recycling to the reaction zone. In one of the claims of this application, it is stated that one chloroform can be obtained as an end product by returning methane and all chloromethanes except chloroform to the reaction zone.
CHU processing with hydrogen
The hydrodechlorination of ChC with the help of hydrogen (as well as with methane), in contrast to oxidative transformations with the help of oxygen, makes it possible to use the carbon component of ChC with benefit. Catalysts, kinetics, mechanism and other aspects of hydrodechlorination reactions are considered in reviews.
One of the main problems of CTC hydrodechlorination is selectivity, the reaction often proceeds to the formation of methane, and the yield of chloroform, as the most desirable product, is not high enough. Another problem is the rather rapid deactivation of the catalyst, mainly due to carburization during the decomposition of CTC and reaction products. In this case, the selective production of chloroform can be achieved more easily than the stability of the catalyst. Quite a lot of works have recently appeared, where high selectivity to chloroform is achieved, data on the stability of the catalyst are much less.
In the patent, Ru, Rh, Pd, Os, Ir, Pt, Cu, Ag, or Au are proposed as catalysts for the hydrogenolysis of CTC and chloroform. On a catalyst containing 0.5% platinum on alumina, at temperatures of 70-180 ° C, 97.7-84.8% of chloroform and 2.3-15.2% of methane were obtained from Chu; at higher temperatures methylene chloride is also formed.
In the works, the hydrodechlorination of CCA was carried out on platinum catalysts. The choice of MgO as a support was made based on the higher selectivity to chloroform and the duration of the catalyst compared to other supports: Al2O3, TiO2, ZrO2, SiO2, aluminosilicate and NaY zeolite. It has been shown that for stable operation of the Pt/MgO catalyst with a CCA conversion of more than 90%, it is necessary to maintain a reaction temperature of 140°C, an H2/CTC ratio of more than 9, and a space velocity of 9000 l/kg.h. The effect of the nature of the initial platinum compounds on the activity of the obtained catalyst - 1% Pt/Al2O3 - was found. On catalysts prepared from Pt(NH 3) 4 Cl 2 , Pt(NH3)2(NO3)2, and Pt(NH3)4(NO3)2, the CTC conversion is close to 100%, and the selectivity to chloroform is close to 80%.
Modification of the catalyst - 0.25% Pt/Al2O3 with lanthanum oxide made it possible to obtain a chloroform yield of 88% at a selectivity of 92% at 120°C, a space velocity of 3000 h-1 and a molar ratio of H2:CCl4 = 10.
According to the data, the calcination of the carrier - alumina at temperatures of 800 - 900 ° C reduces the Lewis acidity, thereby increasing the stability and selectivity of the catalyst. On alumina with a specific surface area of 80 m2/g containing 0.5% Pt, a CTC conversion of 92.7% with a selectivity to chloroform of 83% is maintained for 118 hours.
In contrast to the data in the patent, when obtaining methylene chloride and chloroform by hydrodechlorination of Chu, it is recommended to treat the carrier with hydrochloric acid or hydrochloric acid and chlorine, and promote platinum with small amounts of metals, for example, tin. This reduces the formation of by-products and improves the stability of the catalyst.
In the hydrodechlorination of CTC on catalysts containing 0.5–5% Pd on sibunite (coal) or TiO2 at a temperature of 150–200°C, the conversion of CTC was 100%. Non-chlorinated C2-C5 hydrocarbons were formed as by-products. The catalysts worked stably for more than 4 hours, after which regeneration was carried out by purging with argon during heating.
It is reported that when using a bimetallic composition of platinum and iridium, promoted with small amounts of third metals such as tin, titanium, germanium, rhenium, etc., the formation of by-products is reduced, and the duration of the catalyst is increased.
In the study of the non-catalytic interaction of CTC with hydrogen by the method of pulsed compression in a free-piston installation with characteristic process times of 10-3 s, two regions of the reaction were found. At a temperature of 1150K (conversion up to 20%), the process proceeds relatively slowly. By adjusting the composition of the initial mixture and the process temperature, it is possible to obtain a 16% yield of chloroform with a selectivity close to 100%. In a certain temperature range under conditions of self-ignition of the mixture, the reaction can be directed to the predominant formation of perchlorethylene.
Great success in the development of an active, stable and selective catalyst for the gas-phase hydrodechlorination of CTC with hydrogen was achieved by the company "Sud Chemie MT" . The catalyst is noble metals of the V group deposited on microspherical alumina (the composition of the catalyst is not disclosed by the company). The process is carried out in a fluidized catalyst bed at temperatures of 100-150°C, a pressure of 2-4 atm, a contact time of 0.5-2 sec, and a hydrogen:CNC ratio in the reaction zone of 6-8:1 (mol.).
The conversion of ChCA under these conditions reaches 90%, the selectivity for chloroform is 80-85%. The main by-product is methane, methyl chloride and methylene chloride are formed in small quantities.
The hydrodechlorination of CTC on palladium catalysts in the liquid phase was studied in the works. At temperatures of 20-80°C on palladium acetate with the addition of acetic acid and using C7-C12 paraffins, methyl ethyl ketone, dimethylformamide, dioxane and benzyl alcohol as solvents, methane turned out to be the only reaction product. Carrying out the reaction in isopropyl and tert-butyl alcohols as solvents made it possible to obtain chloroform and methyl chloride as the main products, the formation of methane ranged from trace amounts to 5%.
It is noted that the side reaction of hydrochlorination of alcohols used as solvents proceeds with a conversion of 7-12% of the supplied amount and the formation of isomers of chlorine derivatives, which creates the problem of their disposal and makes it difficult to isolate commercial products. Therefore, the implementation of this method is not yet planned.
Apparently, to exclude by-products in the patent, the reaction of hydrodechlorination of CTC to chloroform is proposed to be carried out in a halogenated aliphatic solvent, in particular, in chloroform. The catalyst is a suspension of platinum on a support. The CCA conversion is 98.1% with a chloroform selectivity of 99.3%.
The same process for obtaining chloroform in the presence of Pt and Pd catalysts on a carrier using 1 solvent (pentane, hexane, heptane, benzene, etc.) is described in a patent. The process is said to be carried out continuously or intermittently on an industrial scale.
Supported palladium, platinum, rhodium and ruthenium are the most commonly used catalysts for the hydrodechlorination of CTC to chloroform and other chloromethanes. Such a catalyst is sprayed and suspended in liquid Chu and treated with hydrogen at a pressure of 8000 kPa and a temperature below 250°C. The process is said to be suitable for the production of chloroform on an industrial scale.
When studying the hydrochlorination of CTC in a liquid-phase bubbling reactor, it was shown that palladium supported on activated carbon is the most active and selective catalyst. The advantage of activated carbon as a carrier is due to a more uniform distribution of the metal on its surface compared to such inorganic carriers as alumina and silica gel. According to the activity of metals, catalysts can be arranged in the series Pd/C Pt/C Rh/C Ru/C Ni/C. The main by-product is hexachloroethane.
Subsequently, it was found that the rate of the process is limited by a chemical reaction on the surface.
Transformation of CHU into PCE
Under harsh temperature conditions, the formation of perchlorethylene from CHU occurs. The process of obtaining perchlorethylene from CTC proceeds with the absorption of heat and the release of chlorine, which is fundamentally different from the production of perchlorcarbons (perchlorethylene and CTC) from methane or epichlorohydrin production waste, where the processes proceed with the supply of chlorine and with the release of heat.
At 600°C, H = 45.2 kcal/mol, and the equilibrium degree of conversion at atmospheric pressure is 11.7% 5. It should be noted that the data of various authors on the magnitude of the thermal effect of the reaction differ significantly, which raised doubts about the possibility of complete processing of CTC into perchlorethylene in the production of perchlorcarbons due to the lack of heat for this reaction. However, the complete recycling of CHC is currently carried out in the production of perchlorcarbons at the Sterlitamak CJSC "Kaustik".
The thermal conversion of CTC increases significantly in the presence of chlorine scavengers. Obviously, the acceptor, by binding chlorine, shifts the equilibrium of the reaction:
2CCl 4 → C 2 Cl 4 + 2Cl 2
towards the formation of perchlorethylene.
The conversion of CTC to perchlorethylene in the presence of a chlorine acceptor performs another very important function - it turns an endothermic process into an exothermic one and eliminates the practically unrealistic heat supply through the wall at such temperatures in the presence of chlorine.
The introduction of organic chlorine acceptors (methane, ethylene, 1,2-dichloroethane) in the process of thermal dechlorination of CCA made it possible to increase the yield of PCE to 50 wt%. , however, the amount of by-products (hexachloroethane, hexachlorobutadiene, resins) also increased symbately. Therefore, in the work 53 to implement the process in industry, it is recommended to add an acceptor (methane or ethylene) in an amount of 0.3 of the stoichiometry.
The 54 patent proposes to carry out the process of non-catalytic thermal conversion of CTC to perchlorethylene at a temperature of 500-700°C using hydrogen chlorine as an acceptor, due to which little by-product chlorohydrocarbons are formed.
The conversion of ChC to PCE, if the latter is marketed, has very important advantages over other methods for processing CTC from the production of chloromethanes:
. for processing, it is not required to isolate CHC from still distillation;
. C2 chlorohydrocarbons contained in distillation water are also converted into PCE.
The process of converting CTC to perchlorethylene in the presence of CH4 is accompanied by the formation of a large amount of by-products, some of which (hexachloroethane, hexachlorobutadiene) are processed in the process, others (hexachlorobenzene) are sent to landfill. At the same time, methane, by binding chlorine, turns into Chu, which also needs to be processed, i.e. CTC processing capacity is increasing.
When hydrogen is used as a chlorine acceptor, the amount of by-products decreases, only the yield of hydrogen chloride increases. The process is carried out in a fluidized bed of silica gel. The process temperature is 550-600 o C, the ratio of CHC:H2 = 1:0.8-1.3 (mol.), contact time 10-20 s. CHU conversion reaches 50% 55. The disadvantage of this process is the need to create a separate large technological scheme, as well as the presence of hard-to-recycle waste - hexachlorobenzene.
The formation of heavy by-products can also be minimized in the production of perchlorethylene by chlorination of hydrocarbons and their chlorine derivatives in the presence of Chu and hydrogen.
Other CTC Processing Methods
Some ways to restore CHU are proposed in. For example, chloroform can be obtained by slow reduction of CCl4 with iron with hydrochloric acid, zinc dust with 50% NH4Cl solution at 50-60 o C, ethanol at 200 o C.
In the electrochemical reduction of CTC, mainly chloroform and methylene chloride are obtained. In the presence of aluminum chloride, Chu alkylates aromatic compounds. In free radical reactions and telomerization reactions, CHU serves as a halogen carrier.
findings
1. Since CTC is inevitably formed during the chlorination of methane and chloromethanes, the development of methods for its efficient processing is an urgent task.
2. When destroying CHC by high-temperature incineration, the existing environmental requirements for the efficiency of destructive removal of 99.9999% and the content of dioxins in emissions of no more than 0.1 ng TEQ/Nm3 are achieved. No similar indicators were found in the catalytic oxidation of CTC.
Catalytic oxidation of CTC with oxygen can produce chlorine and/or phosgene.
3. An interesting method of CTC processing from the point of view of a cheap reagent and low process temperature is hydrolysis to carbon dioxide and hydrogen chloride.
4. Combination of the hydrolysis of CCA and the interaction of the formed HCl with methanol also gives a rather interesting process of processing CCA with methanol to obtain methyl chloride and CO 2 .
5. Hydrodechlorination with hydrogen makes it possible to utilize Chu with obtaining the desired less chlorinated chloromethanes. The main disadvantage of this process, as well as interaction with methanol, is the gradual decrease in catalyst activity due to carbonization.
6. The simplest solution to the problem of processing CTC is the interaction of CTC with methane during its return to the methane chlorination reactor. However, in addition to chloromethanes, impurities of C 2 chlorohydrocarbons are formed in this case. The formation of impurities can be avoided by reacting CCA with methane in the presence of a catalyst and oxygen at a lower temperature, but this will require the creation of a separate stage and the presence of oxygen.
7. Pyrolysis of CTC in the presence of methane, hydrogen or other chlorine acceptors makes it possible to obtain perchlorethylene. The process is complicated by the formation of by-products of high molecular weight.
8. Chu is a safe chlorinating agent, for example, in the production of metal chlorides from their oxides.
9. There are a number of other methods for processing CTC, for example, by electrochemical reduction or by using reducing agents. Chu can also be used as an alkylating agent.
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Table 1. Interaction of CTC with methane
| T-ra | Concentrations, % mol. | NTC conversion, % | |||
| p/n | about C | SS l 4 | CH 4 | for chlorine | for carbon |
| 1 | 525 | 22,5 | 53,4 | 27,4 | 25,4 |
| 2 | 525 | 9,7 | 53,0 | 29,4 | 31,9 |
| 3 | 500 | 24,9 | 48,8 | 12,0 | 11,9 |
| 4 | 475 | 23,4 | 47,8 | 6,4 | 5,7 |
| 5 | 450 | 29,5 | 51,1 | 2,9 | 1,9 |
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