Methanol carbonylation to acetic acid is an important commercialized process. However, there are comparatively few reports on ethanol or higher carbon number alcohol carbonylation. Here we describe recent and ongoing studies aimed at understanding ethanol carbonylation to propionates primarily using Rh-based solid catalysts in the vapor phase. The primary side reactions are dehydration to ethylene and diethyl ether. This work addresses increasing interest in using renewable feedstocks to synthesize important chemical intermediates.
Catalyst supports include heteropolyacids, 13X zeolites, and quaternized poly(vinylpyridine) resins, all with Rh complexes immobilized through incipient wetness impregnation or ion exchange. In several cases, we see significant improvements in propionate selectivity after additional exchange with various alkali salts. Gas phase reactions were carried out in plug flow micro-reactors where the feed consisted of ethanol vaporized into the CO stream. Temperatures were controlled in the range of 160 - 280° C and pressures were varied from atmospheric up to 500 psi. Product concentrations were measured by GC-FID and TCD with good mass balance. This work is among the first to demonstrate vapor-phase ethanol carbonylation with good propionate selectivity (>80%), and with adequately low levels of halide promoters. Catalytic results are supported by physical and chemical characterization techniques.
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