Zeolite-Supported Rh Catalysts for Methanol Carbonylation: X-Ray Absorption Measurements

Abstract

The possible use of Rh catalysts supported by zeolites for the carbonylation of methanol (and related reactions) has attracted considerable attention 1–4. This reaction takes place in solution with a CH₃I promotor at moderate temperatures. In the case of the homogeneous reaction, the Rh complexes involved form one of the best characterized catalytic systems ^5–6: However, far fewer answers are available to the questions that arise regarding the nature of the heterogeneous system. Infra-red spectroscopy and XPS report Rh valencies of the active catalyst that include Rh^O, Rh^I and more highly oxidized species ^7–11. However, IR is not a very sensitive probe of cation valence state. At the same time, XPS operates under a disadvantage unique to the zeolite supported catalysts since the exposed surface external to the catalyst grains is generally only a few percent of the combined external surface and internal surface. In this case, the well-known sensitivity of XPS to exposed (external) surfaces may act to obscure the chemical state of the majority of the catalytic metal. With this in mind, we have performed measurements of the extended X-ray absorption fine structure (EXAFS) on a series of zeolite supported Rh catalysts preparations.