(445a) C2 Oxygenates from Syngas: Understanding and Improving Methanol Carbonylation Using Modified Mordenite Catalysts

Ethanol and acetic acid are valuable fuels and chemicals today, and will also be increasingly important as renewably sourced commodities advance. Functionalizing C₁ intermediates via formation of C-C bonds through carbonylation provides a path to selectively producing these C₂ oxygenated products. The circuitous process route of generating syngas, converting it to methanol, separating and cooling methanol from the process stream, carbonylating methanol using pure carbon monoxide and homogeneous catalysts in highly corrosive solutions to make acetic acid, and subsequently hydrogenation the acetic acid to ethanol in a separate step has been practiced¹; however, associated process complexity and costs leave room for improvement. More recent advancements in carbonylation catalyzed by acidic zeolites provide an alternative mechanistic pathway for the carbonylation step through DME or methanol carbonylation with pure carbon monoxide². We present a single reactor configuration operating at a single temperature and pressure with three catalyst beds without interstage separation or heat transfer going from syngas to ethanol. The process first produces methanol which is carbonylated over mordenite in the same process stream to produce acetic acid. When a third hydrogenation catalyst is used, ethanol is produced.

The carbonylation of methanol suffers from deactivation and hydrocarbon formation occurring concurrently on acid zeolites. Mordenites contain acid sites in 12 membered rings which catalyze carbonylation, but also catalyze hydrocarbon formation through trimethyl oxonium cation intermediates³. Acid sites in the 8 membered rings of the zeolite structure also catalyze the carbonylation reaction, however do not contribute to hydrocarbon formation. Selectively poisoning the sites in the 12 membered rings over the 8 membered rings improves selectivity and stability. The contribution of this work is a selective cation-free poison that is air-calcination stable and selectively blocks sites that produce hydrocarbons over carbonylation sites. Reactivity studies, variation in synthetic conditions and precursors, and catalyst characterization are used to understand and improve the performance in the conversion of syngas to C₂ oxygenates with nearly 90% selectivity.

1 Subramani, V. & Gangwal, S. K. A Review of Recent Literature to Search for an Efficient Catalytic Process for the Conversion of Syngas to Ethanol. Energy & Fuels 22, 814-839, doi:10.1021/ef700411x (2008).

2 Cheung, P., Bhan, A., Sunley, G. J., Law, D. J. & Iglesia, E. Site requirements and elementary steps in dimethyl ether carbonylation catalyzed by acidic zeolites. Journal of Catalysis 245, 110-123, doi:https://doi.org/10.1016/j.jcat.2006.09.020 (2007).

3 Boronat, M., Martinez, C. & Corma, A. Mechanistic differences between methanol and dimethyl ether carbonylation in side pockets and large channels of mordenite. Physical Chemistry Chemical Physics 13, 2603-2612, doi:10.1039/C0CP01996H (2011).