(699g) A Highly Selective Route from Syngas to Ethanol: Tandem Catalysis Unconstrained By Anderson-Schulz-Flory Distribution

Selective higher alcohols synthesis (HAS) from CO/CO₂/H₂ mixtures (syngas) has been motivated by the demand of such products in the chemical and energy sectors, as well as the versatility and economics of syngas production. Research efforts in catalyst development for HAS have spanned decades, and in many cases drew inspiration from methanol synthesis and Fischer-Tropsch technologies. However, unlike the latter processes, insufficient selectivity and activity in HAS from syngas have prevented commercialization of chemocatalytic routes. Thus, further improvement in both metrics is required.

In many cases of HAS catalyzed by single catalysts, the reaction is believed to proceed by concerted hydrogenation and chain growth (C-C coupling) steps, resulting in co-production of aldehydes, alkenes, and alkanes, with chain-length approximately following the Anderson-Schulz-Flory (ASF) distribution. Such a wide spread in products increases separation costs for targeted HAS, and results in lower-value products (such as methane) competing for feed utilization. Advances in the engineering of the catalyst surface composition and structure have led to significant improvements in selectivities, but, to date, the aforementioned limitations have not been completely eliminated. One approach to break away from the constraints of the ASF distribution in HAS is to spatially separate the hydrogenation and chain-growth steps of the process by pairing catalysts that, individually, selectively catalyze only one of the steps, but, in a multi-bed tandem catalysis scheme, enable both.

In this work, we developed a combination of precious-metal-free, inexpensive, heterogeneous catalysts that, under differential conversions of CO/H₂ feed, are able to achieve upwards of 80% selectivity towards ethanol, on a CO₂-free carbon basis, with <1% methane selectivity. We will discuss the response of this system to process variables, and its stability. Additionally, some of the thermodynamic and kinetic considerations that are important in the design of such tandem-catalytic schemes for HAS will be highlighted.