Dichloroethanes, dehydrochlorination

The mechanism of the ethane oxidative chlorination process is distinguished by the fact that the catalyst accelerates primarily the reactions of hydrogen chloride oxidation and dichloroethane dehydrochlorination. This necessitates the modeling of cement catalytic system with the surface carrying active sites capable of catalyzing both reactions mentioned. 

It is known that the chlorination of ethane with chlorine formed in the oxidation of hydrogen chloride proceeds by a heterogeneous—homogeneous mechanism [3]. This is why the effieiency of cement catalysts was studied separately by the examples of Deacon reaction and dichloroethane dehydrochlorination reaction. 

Systematic investigations on the performance of cement-containing catalytic systems with various chemical and phase compositions in the reaction of 1,2-dichloroethane dehydrochlorination with forming vinyl chloride... 

We can suppose on the strength of the data listed in Table 2 that at the short times-on-stream, the major contribution to the formation of deep oxidation products is made by saturated chlorinated hydrocarbons 1,2 dichloroethane and ethyl chloride. On increasing time-on-stream to more than 6 s, we observed a sharp increase in the yield of deep oxidation products together with the decrease in the yield of vinyl chloride. It is likely that at the longer times-on-stream, the rate of deep oxidation of vinyl chloride would increase and become higher than the rate of dichloroethane dehydrochlorination. Taking into account this fact, we believe that the optimum time-on-stream assuring the best total yield of ethylene and vinyl chloride would be 3—5 s. 

Dichloroethane, an important intermediate for vinyl chloride production, is produced by catalytic chlorination of ethylene in either vapor or Hquid phase or by oxychlorination of ethylene. Thermal dehydrochlorination of 1,2-dichloroethane produces vinyl chloride and coproduct hydrogen chloride. Hydrogen chloride is commonly recycled to an oxychlorination unit to produce 1,2-dichloroethane or is processed into sales-grade anhydrous or aqueous hydrogen chloride. 

Dehydrochlorination of 1,1,2-trichloroethane [25323-89-1] produces vinyHdene chloride (1,1-dichloroethylene). Addition of hydrogen chloride to vinyHdene chloride in the presence of a Lewis acid, such as ferric chloride, generates 1,1,1-trichloroethane. Thermal chlorination of 1,2-dichloroethane is one route to commercial production of trichloroethylene and tetrachloroethylene. 

Dehydrochlorination of 1,1,2-trichloroethane at 500°C in the presence of a copper catalyst gives a different product, ie, cis- and /n7 j -l,2-dichloroethylene. Addition of small amounts of a chlorinating agent, such as chlorine, promotes radical dehydrochlorination in the gas phase through a disproportionation mechanism that results in loss of hydrogen chloride and formation of a double bond. The dehydrochlorination of 1,2-dichloroethane in the presence of chlorine, as shown in equations 19 and 20, is a typical example. 

Oxychlorination. This is an important process for the production of 1,2-dichloroethane which is mainly produced as an intermediate for the production of vinyl chloride. The reaction consists of combining hydrogen chloride, ethylene, and oxygen (air) in the presence of a cupric chloride catalyst to produce 1,2-dichloroethane (eq. 24). The hydrogen chloride produced from thermal dehydrochlorination of 1,2-dichloroethane to produce vinyl chloride (eq. 25) is usually recycled back to the oxychlorination reactor. The oxychlorination process has been reviewed (31). 

Pyrolysis. Pyrolysis of 1,2-dichloroethane in the temperature range of 340—515°C gives vinyl chloride, hydrogen chloride, and traces of acetylene (1,18) and 2-chlorobutadiene. Reaction rate is accelerated by chlorine (19), bromine, bromotrichloromethane, carbon tetrachloride (20), and other free-radical generators. Catalytic dehydrochlorination of 1,2-dichloroethane on activated alumina (3), metal carbonate, and sulfate salts (5) has been reported, and lasers have been used to initiate the cracking reaction, although not at a low enough temperature to show economic benefits. 

Oxychl orin ation of ethylene has become the second important process for 1,2-dichloroethane. The process is usually incorporated into an integrated vinyl chloride plant in which hydrogen chloride, recovered from the dehydrochlorination or cracking of 1,2-dichloroethane to vinyl chloride, is recycled to an oxychl orin a tion unit. The hydrogen chloride by-product is used as the chlorine source in the chlorination of ethylene in the presence of oxygen and copper chloride catalyst ... 

Chlorohydrin Process. Ethylene oxide is produced from ethylene chlorohydrin by dehydrochlorination using either sodium or calcium hydroxide (160). The by-products include calcium chloride, dichloroethane, bis(2-chloroethyl) ether, and acetaldehyde. Although the chlorohydrin process appears simpler, its capital costs are higher, largely due to material of constmction considerations (197). 

Solvents, Additives, and Extraneous Impurities. The rate of PVC photodegradation under air at wavelengths >250 nm is said to be increased by small amounts of residual tetrahydrofuran (THF) or dichloroethane (56). On the other hand, residual THF has been reported not to enhance the dehydrochlorination of the polymer during irradiation under nitrogen at X >240 nm (41). Nevertheless, under the conditions of the latter study, THF was found to Increase the relative concentrations of the shorter polyene products (41). This effect was attributed to a facile... 

Sotowa, C., Kawabuchi, Y. and Mochida, I., Catalytic dehydrochlorination of 1,2-dichloroethane over pyridine deposited pitch-based active carbon fiber, Chem. Lett., 1996, (11), 967 968. 

Thermal dehydrochlorination of 1,2-dichloroethane188-190 272 273 takes place at temperatures above 450°C and at pressures about 25-30 atm. A gas-phase free-radical chain reaction with chlorine radical as the chain-transfer agent is operative. Careful purification of 1,2-dichloroethane is required to get high-purity vinyl chloride. Numerous byproducts and coke are produced in the process. The amount of these increases with increasing conversion and temperature. Conversion levels, therefore, are kept at about 50-60%. Vinyl chloride selectivities in the range of 93-96% are usually achieved. 

Another modification of the process can be used to meet the growing demand for 1,1,1-trichloroethane (methylchloroform). In this version, the chlorination of dichloroethane can be directed toward maximum production of 1,1,2-trichloroethane (9). This product when dehydrochlorinated yields vinylidene chloride, a widely used monomer. Hydrochlorination of vinylidene chloride yields 1,1,1-trichloroethane, a solvent of increasing importance. 

Schneider M, Wolfrum J. 1986. Mechanisms of by-product formation in the dehydrochlorination of 1,2-dichloroethane Ber Bunsen Ges Phys Chem 90 1058-1062. 

The best method for the preparation of ring-alkylated and -arylated methylenecyclopropanes proves to be the dehydrochlorination of 1-chloro-l-methylcycloprop-anes (Route b). The latter are easily obtained from alkenes and 1,1-dichloroethane in the presence of a unable base6) (Eq. 52). 

In a flow system, at 200-250 C, phosgene combines with ethene over activated charcoal to give 1,2-dichloroethane, with an activation energy calculated to be 29.6 kj mol [1CI88,ICI89,ICI90]. A small quantity of chloroethene is formed at temperatures as low as 100 C from the simultaneous decarbonylation and dehydrochlorination of the intermediate acid chloride [ICI90] ... 

Vinyl chloride monomer, the basic building block of polyvinylchloride (PVC), is commercially manufactured by dehydrochlorination of 1,2-dichloroethane. The modern process for producing 1,2-dichloroethane involves oxychlorination of ethylene in a fluidized bed catalytic reactor ... 

Fig. 2. The conversion of 1,2-dichloroethane in the dehydrochlorination reaction at various catalysts as a function of temperature, t = 8 s 1- galyumin (Sspec = 130 m2/g), 2- galyumin with a dopant of copper (8 wt % in terms of CuO) 3- CsCl/silica gel.

Thus, cement-containing systems provide the conversion of dichloroethane to be increased to more than 70% even at 673K. An important positive factor is that vinyl chloride molecule is stable at this temperature. At 673K, the side reaction of vinyl chloride dehydrochlorination with forming acetylene proceeds slowly, acetylene does not form, and the reaction is not complicated by the formation of a number of by-products, for example, of perchloroethylene. Thus, the above-made supposition about bifunctional character of copper—cement catalytic systems was confirmed in the investigations of their activity in the above-mentioned reactions. 

The dependences shown in Fig. 3 reveal that employing a catalyst with a larger specific surface area with rising temperature would, probably, lead to the deep oxidation of vinyl chloride and, to a lesser extent, of ethylene, resulting in a decrease in the total yield of ethylene and vinyl chloride. A certain increase in the overall yield of COx products, which was observed for catalyst 2, is accompanied with an increase in the total yield of ethylene and vinyl chloride. This suggests that saturated chlorinated hydrocarbons — ethyl chloride and 1,2-dichloroethane — are oxidized predominantly and that the rate of oxidation is lower rate compared to that of the dehydrochlorination of these compounds. 

As in steam cracking, a large number of by-products is produced. Some of them result from the consecutive reactions of the chlorination of vinyl chloride and of its derivatives obtained by dehydrochlorination (tri-, tetra-, pentachloroethane, perchloro-ethane, di-, trichloroethylene. perchloroethyleneX and the others from the hydrochlorination of vinyl chloride il.l-dichloroethane), while others result from decomposition reactions (acetylene, cokei or conversion of impurities initially present (hydrocarbons such as ethylene, butadiene and benzene, chlorinated derivatives such as chloroprene, methyl and ethyl Chlorides, chloroform, carbon tetrachloride, eta, and hydrogen) ... 

A more complex degradation takes place when this process is applied to PVC. The authors propose that PVC depolymerization under supercritical water conditions proceeds in accordance with a mechanism consisting of four different pathways (i) dehydrochlorination and partial oxidation, (ii) dehydrochlorination and chain scission, (iii) dehydrochlorination and total oxidation, and (iv) hydrochlorination. In the reaction products, high yields of vinyl chloride, 1,1-dichloroethane and 1,2-dichloroethane are detected, especially at short reaction times, whereas longer times favour total oxidation products. 

Ramanathan and Spivey observed the formation of vinyl chloride by dehydrochlorination, the removal of HCl from the dichloroethane molecule. They cite similar conclusions by others, on MnC>2-CuO catalysts 02 nickel oxide and cobalt-manganese . Note that the loss of activity of a... 

Thermal dehydrochlorination of 1,1-dichloroethane at about 820 K is generally used for the production of vinylchloride. However, the process suffers from heavy coke deposition on the reactor walls, and catalytic reactions operating at lower temperatures were investigated in industry. Carbons were found to catalyze the dehydrochlorination (DHC) of alkyl chlorides to the corresponding alkenes. This reaction had been studied in 1933 for its suitability in the production of vinyl chloride. A list of early patents is given in ref. 170. Formation of 1-butene from... 

When the addition of chlorine to ethylene is carried out in the vapor phase and in the presence of metal contact agents, higher proportions of 1,2-dichloroethane are obtained. Equimolecular proportions of chlorine and ethylene reacting at 80-100°C in the presence of copper or iron give about 90-95 per cent of the theoretical yield of dichloroethane. The reactions may be carried out in tubes that are packed with shavings or particles of the preferred metal. When small amounts of dichloroethane are recirculated to the reaction zone, the exit gases can be scrubbed more easily. In many plants, the concurrent formation of polychloroethanes is considered desirable, for such compounds are subsequently submitted to dehydrochlorination and yield desired chloroolefins. 

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