Investigation of CO hydrogenation catalysts for producing oxygenated hydrocarbons

Abstract

The objective of this study was to test the performance of promoted rhodium catalysts supported on silica for oxygenated hydrocarbon (particularly ethanol) production by CO hydrogenation. Two different catalysts were prepared by incipient wetness impregnation: 1.5wt%Rh-2.6wt%La/SiO2 and 1.5wt%Rh-2.6wt%La/1.5wt%V/SiO2. Activity tests included a parametric study of the effects of reaction temperature, pressure, contact time (W/FCO) on CO conversion as well as on the selectivity for methane, ethanol and carbon dioxide. The microstructures of catalyst surfaces were characterized by ESEM with EDX analysis. The reaction system consisted of a fixed bed micro-reactor integrated within a fully automated high-pressure reactor system equipped with a programmable logic controller and software for the remote control of the system. Two on-line gas chromatographs (GC and GC-MS) were used for feed and product analysis. Experiments were first conducted on 1.5wt%Rh-2.6wt%La-1.5wt%V/SiO2 using a fixed inlet composition of 10 mol% CO and 20 mol% H2 with balance N2 at a W/FCO ratio of 0.98 mg.min.μmol-1. Reaction conditions were varied in a range of 14-35 bar and 240-315oC. Subsequent experiments were performed at 25 bar and 285oC using W/FCO ratios of 0.49- 1.95 mg.min.μmol-1 over doubly-promoted Rh/SiO2. Experiments for singly-promoted catalyst were performed at 270-300oC in the same pressure range at a W/FCO ratio of 0.98 mg.min.μmol-1. Increasing contact times, increasing reaction temperatures and addition of V as a promoter all improved catalytic activity in terms of CO conversion significantly. No significant amount of oxygenated hydrocarbon was produced over Rh-La/SiO2 whereas ethanol, acetic acid and ethyl ether were detected as major oxygenated products over the doubly promoted Rh-La-V/SiO2 catalyst. Higher pressures enhanced the formation of oxygenated products. Regarding ethanol selectivity, the highest value obtained was ca. 19% at a CO conversion of 9% (270oC, 35 bar), followed by a selectivity of 15% at 31% CO conversion (300oC, 35 bar). The activity of the catalyst was mostly controlled by reaction temperature rather than pressure.