Studies On Efficient Catalysts For Selective Gas-Phase Dehydrochlorination Of 1,1,2-Trichloroethane

The emission of chlorinated hydrocarbons such as 1,1,2-trichloroethane (TCE) is detrimental to the environment, which may cause acid rain, ozone layer depletion and greenhouse effects. Thus the handling of chlorinated hydrocarbons becomes a very important issue. Currently, there are two approachs to deal with this issue. One approach is the decomposition of these compounds, including direct incineration, catalytic combustion, biodegradation and photo-catalytic decomposition. Another approach is the conversion of these compounds to other valuable chemicals, including catalytic hydrodechlorination and dehydrochlorination. Converting 1,1,2-trichloroethane into valuable products possesses more significance. Hence, developing highly selective, highly active and low-cost catalysts for the transformation of 1,1,2-trichloroethane and exploring the reaction mechanism are the objectives of this thesis. The research contents and results of this thesis are summarized as follows:1. Gas-phase synthesis of 1,1-DCE from TCE has been achieved with pentaethylenehexamine (PEHA) supported on silica as the catalyst. The manufacture of 1,1-dichloroethylene (1,1-DCE) usually employs NaOH liquid phase method by dehydrochlorination of 1,1,2-trichloroethane (TCE), where large amounts of high-concentration salty wastewater is produced inevitably. It is a long-term goal to achieve the gas phase synthesis of 1,1-DCE via supported catalysts. In this work, the pentaethylenehexamine (PEHA) supported on silica was investigated as the catalyst for the gas-phase synthesis of 1,1-DCE from TCE. High and stable selectivity to 1,1-DCE (up to 98%) was obtained, which could be ascribed to the relatively strong basicity of PEHA according to a proposed E2 mechanism. The formation of PEHA chloride from the HCl generated in situ was detected and was considered to be the main reason for the deactivation of the PEHA catalyst.2. Mg catalysts supported on SiO2 was found to be a highly efficient catalyst for the gas-phase synthesis of cis-1,2-DCE from TCE. A series of Mg catalysts supported on SiO2 were prepared by an incipient wetness impregnation method and tested for gas phase dehydrochlorination of 1.1,2-trichloroethane. It was found that these catalysts were very active and highly selective for the production of cis-1,2-DCE. The catalytic performance depended on the Mg loading rather than the Mg precursors as the catalysts using Mg(NO3)2-6H2O and MgCl2·6H2O as the precursors showed the similar performance. A catalyst containing 10 wt.% of Mg showed the best performance with a steady state TCE conversion of 92% and cis-dichloroethene selectivity of 91%. Moreover, characterizations of the catalysts revealed the formation of Cl-containing Mg species on the surface during the reaction. The analyses of the compositions of the stable catalysts under working conditions indicated a C1/Mg ratio of 1:1, which is former porved to be Mg(OH)Cl, suggesting that Mg(OH)Cl could be the active sites for the reaction.