Catalytic chlorocarbon conversion to environmentally benign compounds via supported metal systems.

摘要

Environmentally benign chemical processing has become a common theme for the future of chemical engineering. Heterogeneous catalysis will play an ever-increasing role, enabling mankind to develop technological innovations that promote effective use of limited resources. Accordingly, an understanding of the nature of environmentally significant catalytic processes will provide the foundation for the effective molecular-level design of catalytic systems. This thesis aims at providing a fundamental understanding of the conversion of chlorocarbons into environmentally benign alternatives, specifically the conversion of 1,2-dichloroethane to ethylene and chloromethanes to oligomerization products.; Research addressing the conversion of 1,2-dichlororethane to ethylene catalyzed by Pt-Sn/SiO₂ is presented in Chapters 3–5. For Pt and bimetallic Pt-Sn catalysts with low tin contents (Pt:Sn ≥ 1:1) ethane and ethyl chloride are the major and minor products, respectively. As the tin content is increased (Pt: Sn ≤ 1:2), ethylene selectivity dramatically increases (98% with Pt: Sn = 1:3). By 119Sn Mössbauer spectroscopy, it was shown that Pt-Sn surface alloys rich in tin yield high ethylene selectivity; whereas, platinum rich alloy surfaces yield ethane. Further, application of an alternative (organometallic) preparation technique (exclusive formation of surface Pt-Sn bimetallic entities) showed that the “reaction-induced” selectivity enhancement toward ethylene (Pt: Sn = 1:2), at the expense of ethane, is mainly due to tin surface enrichment.; The role of chlorine in the 1,2-dichloroethane dechlorination was explored with application of bimetallic catalysts (constant Pt loading with various Pt:Sn ratios) prepared with the organometallic method. Tin, coke and chlorine deposition all act to enhance olefin selectivity, supporting the suggestion that “hydrogen-assisted” dechlorination of 1,2-dichloroethane is a structure-sensitive reaction.; Chapter 6 explores catalytic dechlorination of chloromethanes, via Pt-Co/C, resulting in oligomerization products. As a result, the reaction mechanisms associated with the formation of Cu₂₊ products were explored. The elementary steps that follow the formation of the surface carbene species determine their selectivity toward C₁ products and the more desirable C₂₊ products.; In summary, the conversion of chlorinated chemicals to environmentally benign commodity chemicals has been demonstrated. Moreover, a molecular-level understanding is possible enabling an enhanced ability to design effective catalytic systems needed to minimize the release of these chemicals to the environment.