Catalytic Process for the Conversion of Coal-derived Syngas to Ethanol

Abstract

The catalytic conversion of coal-derived syngas to C2+ alcohols and oxygenates has attracted great attention due to their potential as chemical intermediates and fuel components. This is particularly true of ethanol, which can serve as a transportation fuel blending agent, as well as a hydrogen carrier. A thermodynamic analysis of carbon monoxide (CO) hydrogenation to ethanol that does not allow for byproducts such as methane or methanol shows that the reaction: 2 CO + 4 H2 (yields) C2H5OH + H2O is thermodynamically favorable at conditions of practical interest (for example, 30 bar, (approx)< 250 C). However, when methane is included in the equilibrium analysis, no ethanol is formed at any conditions even approximating those that would be industrially practical. This means that undesired products (primarily methane and/or carbon dioxide (CO2)) must be kinetically limited. This is the job of a catalyst. The mechanism of CO hydrogenation leading to ethanol is complex. The key step is the formation of the initial C-C bond. Catalysts that are selective for EtOH can be divided into four classes: (1) rhodium (Rh)-based catalysts, (2) promoted copper (Cu) catalysts, (3) modified Fischer-Tropsch catalysts, or (4) Mo-sulfides and phosphides. This project focuses on Rh- and Cu-based catalysts. The logic was that Rh-based catalysts are clearly the most selective for EtOH (but these catalysts can be costly), and Cu-based catalysts appear to be the most selective of the non-Rh catalysts (and are less costly).