Niobia-alumina as methanol dehydration component in mixed catalyst systems for dimethyl ether production from syngas

Abstract

Pure or niobia-modified aluminas containing from 5.9 to 20wt% Nb2O5 were used as the dehydration component in the direct synthesis gas conversion to dimethylether (DME), in combination with a conventional CuO-ZnO-Al2O3 methanol synthesis catalyst, modified or not with niobia.A in situ DRIFTS study with the pure and one niobia-modified alumina under a CO/H2 stream at the same temperature used in the catalytic experiments (538K) showed irreversible formation of surface formate on the alumina surface, which occurred reversibly and to a smaller extent on the niobia-modified alumina.Modification with niobia caused a strong decrease on the alumina surface basicity and had little effect on its acidity. With the methanol synthesis catalyst, niobia addition had little effect on crystallite size and reducibility, but caused a decrease in surface basicity and a strong increase in surface acidity.The mixed catalyst systems were tested in the direct syngas to DME conversion in a liquid-phase CSTR, using n-hexadecane as the catalyst suspension medium, at 538K and 5MPa. It was found that the mixed-catalyst system displayed higher overall activity when the methanol dehydration catalyst was a niobia-modified alumina, rather than the pure alumina. The highest activity was obtained for the smallest niobia addition to alumina (5.9wt%niobia). This effect was ascribed to the decrease in the alumina surface basicity due to deposition of niobia, which causes improved tolerance of the methanol dehydration catalyst to large concentrations of carbon monoxide and carbon dioxide in the reaction medium. The opposite effect was found when the methanol synthesis catalyst was modified with niobia.