JOURNAL OF CATALYSIS 124, 376--390 (1990) An Investigation of the Thermodynamic Constraints in Higher Alcohol Synthesis over Cs-Promoted ZnCr-Oxide Catalyst 1 ENRICO TRONCONI, PIO FORZATTI, AND ITALO PASQUON Dipartimento di Chimica Industriale ed Ingegneria Chimica "'G. Natta" del Politecnico, Piazza Leonardo da Vinci 32, 20133 Mitano, Italy Received January 13, 1989; revised January 2, 1990 A thermodynamic analysis of the oxygenate products of the higher alcohol synthesis (HAS) over an alkali promoted high-temperature methanol synthesis catalyst is performed for a wide range of operating conditions. By comparing actual fugacity ratios with equilibrium constants it is found that a number of reactions approach chemical equilibrium under typical synthesis conditions, namely: (a) formation of methanol; (b) water-gas shift reaction; (c) formation of methyl formate and possibly of higher methyl esters; (d) hydrogenation of aldehydes to primary alcohols; (e) hydrogenation of ketones to secondary alcohols; (f) ketonization reactions. The prevailing thermodynamic constraints determine the relative amounts of primary and secondary alcohols, aldehydes, ketones, esters, and acids. They also explain the experimental effects of temperature, pressure, and feed composition on the same products, as well as some of the differences in product composition observed between the HAS over modified high-temperature and low-temperature methanol catalysts. The observed departures from equilibrium provide insight into the reaction network and information on the mechanism of the synthesis. It is also found that the reactions under thermodynamic control are related to some of the major catalytic functions of ZnCr-oxide systems identified by an independent temperature-programmed surface reaction investigation. © 1990 Academic Press, Inc. INTRODUCTION The so-called "modified methanol syn- thesis catalysts" provide a class of potential industrial catalytic systems for the higher alcohol synthesis (HAS), i.e., the synthesis of mixtures of methanol and higher aliphatic alcohols by direct hydrogenation of carbon monoxide. It has been known for many years that addition of alkali promoters to the original catalyst formulation is suitable for steering the behavior of methanol catalysts toward production of C2+ alcohols in addition to CH3OH. For the same purpose, adoption of higher reaction temperatures and lower space velocities is also helpful (1, 2). Both Cu-based low-temperature metha- nol catalysts (3-6) and high-temperature Paper XX in the series "Synthesis of Alcohols from Carbon Oxides and Hydrogen." 0021-9517/90 $3.00 Copyright© 1990 by Academic Press, Inc. All rights of reproductionin any formreserved. methanol catalysts based on mixed oxides of Cr, Zn, and/or Mn (1, 2, 7-12) have been modified successfully in this way. Catalysts belonging to the former class are typically operated in the HAS at temperatures below or around 300°C, whereas the reaction tem- perature is about 400°C for modified high-T methanol catalysts. Modified low-T and high-T methanol cata- lysts yield alcohol products containing mainly methanol, 2-methyl-l-propanol (iso- butanol), and 1-propanol, with smaller amounts of ethanol and linear or mono- methyl branched C5+ alcohols. The forma- tion of carbon dioxide, water, and of a number of side products including hydrocarbons, aldehydes, esters, ketones, and secondary alcohols also occurs over both systems. However, when typical prod- uct distributions generated by catalysts of the two families are compared, differences become apparent in the relative quantities 376