Vapor phase ethanol carbonylation over Rh supported on zeolite 13X

• Rh exchanged on zeolite 13X is active for ethanol carbonylation to propionates with added iodide.
• A maximum 62% propionate selectivity is observed, especially with additional alkali deposited.
• The overall mechanism is proposed to be analogous to soluble catalysis.
• The apparent barrier of 135 kJ/mol indicates rate-limiting ethyl iodide formation from ethanol.
• XANES, EXAFS, and XPS show predominantly Rh(I) iodide during reaction.

While methanol carbonylation has been extensively studied, higher alcohol carbonylation has received relatively little attention, even though, for example, ethanol carbonylation could be a useful route for the production of propionates. Here we use Rh/Na13X to investigate the vapor phase carbonylation of ethanol with an ethyl iodide co-feed. In the base case, the catalyst is ∼40% selective to propionates, with the remainder forming ethylene and diethyl ether. Deposition of additional alkali can increase selectivities to ∼60%. Isotopic labeling of ethyl iodide demonstrates reversible formation of ethyl iodide from ethanol, and that preferential incorporation of ethyl iodide initiates the Rh-catalyzed carbonylation cycle. XPS and in situ X-ray absorption spectroscopy are consistent an active anionic RhI iodide species at a zeolite exchange site. This proposed structure and the attendant catalytic reaction network are directly analogous to those of classic solution-phase Rh catalysts and other supported Rh catalysts. However, important differences are noted in the reaction orders and apparent activation barrier, which suggest that the rate of ethyl iodide formation is overall rate limiting under these conditions and for this catalyst.

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