Acid catalysis by polyoxometalates: Alkylation chemistry and deactivation.

摘要

Many industrially-relevant hydrocarbon conversion processes are acid-catalyzed, such as cracking, isomerization, oligomerization, and alkylation. Solid acid catalysts are applied industrially in the first three processes. However, the process of isoalkane and alkene alkylation is currently carried out using homogeneous HF and H2SO4 catalysts, which suffer from problems of corrosion, waste disposal, and requirements of catalyst separation. Unfortunately, over 30 years of research has not produced an ideal solid acid catalyst, as all materials examined have showed rapid deactivation. Fundamental studies into the characterization of solid acids and their reactivity are needed to determine the requirements of an active solid acid for alkylation.; Heteropolyacids (HPAs) are active for the alkylation of isobutane with n-butene, but suffer from rapid deactivation. Previous results from this laboratory indicated that treatment with water vapor may partially regenerate a deactivated HPA catalyst. Herein, quantum-chemical methods, specifically density functional theory (DFT) calculations, are used to investigate the energetics of proposed deactivation mechanisms. Reaction energies and activation barriers are determined by DFT for the proposed deactivation processes and further used to establish reaction rates and equilibrium constants to clarify whether a proposed mechanism may explain the deactivation process. Emphasis is placed on understanding how water may affect the deactivation process and on determining how including water in the reaction environment may slow or prevent deactivation. This research also seeks to further our understanding of acid-catalysis mechanisms over heteropolyacids by providing insight into the requirements of an effective solid-acid catalyst for the alkylation of isobutane and n-butene.; Three deactivation modes are proposed to possibly contribute to the loss of catalyst activity: localization of protons in inactive locations, desorption of structural water molecules from the HPA surface, and the build up of heavy hydrocarbons on the catalyst surface. The energetics of each of these processes are determined, and their relative contribution to catalyst deactivation is discussed. The mobility of protons, essential to the first two deactivation mechanisms, is explored in detail. The energetics of the elementary steps in the alkylation mechanism are determined to elucidate the pathways which lead to heavy hydrocarbon build up.