Higher olefin epoxidation on silver: A combined experimental/theoretical investigation of surface intermediates and reaction mechanisms

An obvious and important goal in catalysis research is the design of new catalytic materials for current and new processes. A major step toward a rational approach is the comprehension of the fundamental mechanisms by which catalysts enhance specific reaction rates. One of the most important classes of reactions where little is understood of the reaction mechanism is heterogeneous olefin epoxidation. To date, only two epoxides can be produced via direct heterogeneous processes: ethylene oxide (EO) and 1-epoxy-3-butene (EpB). Heterogeneous ethylene oxide production has been an important chemical process for over fifty years, but the fundamental reaction mechanism has only recently been discovered. The production of EpB from 1,3-butadiene in this fashion has only recently been commercialized; however, this has invigorated interest in the area of heterogeneous olefin epoxidation, which is undeveloped as a whole.;The primary aim of this research has been to obtain a better molecular-level understanding of surface reactions and intermediate species important to heterogeneous olefin epoxidation chemistry. To gain this understanding, a combined approach of surface science experimentation and ab initio quantum chemical calculations has been utilized to study the intermediate species formed on silver crystal surfaces and their reactions. This approach has proven invaluable in the study of ethylene oxide interacting with the Ag(111) surface. The work in this dissertation focused on the study of other epoxides interacting with silver surfaces to extend the knowledge and understanding of epoxidation chemistry beyond ethylene oxide, the simplest epoxide species. In particular, a link was established in prior work between oxametallacycle structures on silver and ethylene epoxidation. The combined experimental and theoretical approach was used to determine the role of these intermediates in the formation of 1-epoxy-3-butene, styrene oxide, and isobutylene oxide. Oxametallacycle intermediates---cyclic species defined by an -O-C-C- ring structure bonded at both ends to the silver surface---have been suggested as important intermediates in a number of reactions, but have been positively identified in only a few cases. By studying the formation of these species and their reactions, a greater understanding of their importance in olefin epoxidation chemistry has been obtained. Through the understanding of intermediates derived from these epoxide species in relation to ethylene epoxidation, some insight into a generalized oxametallacycle reaction framework on the Ag(111) surface was obtained. (Abstract shortened by UMI.).