Vapor-phase methanol and ethanol coupling reactions on CuMgAl mixed metal oxides

摘要

The effects of catalyst composition on methanol and ethanol coupling reactions were studied on CuMgAlO_x mixed metal oxides (MMOs). CuMgAlO_x samples with Cu contents between 4 and 38 at.% were prepared by thermal decomposition of layered double hydroxides. These MMOs contained highly dispersed CuO consisting of isolated and oligomeric CuO (number of CuO nearest neighbors between 2 and 4.5) species as determined by ultraviolet-visible spectroscopy and temperature programmed reduction techniques. The catalysts were tested at 448-523 K, 0.1 MPa, alcohols gas-hourly space velocities (GHSVs) of 1000-3000 std cm~3 g_(cat)~(-1) h~(-1), and a feed MeOH/EtOH molar ratio of 4. Incorporation of Cu in MgAlO_x solid solutions drastically changed product selectivity, formation rates, and catalyst stability. The main products on CuMgAlO_x included C—C coupling (e.g., C~(3+) alcohols, aldehydes, and esters), non-C—C coupling (e.g., acetaldehyde, methyl formate, methyl acetate), and methanol decomposition (i.e., CO_x) products as a result of C—C coupling reactions on M—Al (M = Mg, Cu) and dehydrogenation, esterification, reverse methanol synthesis, and the water-gas shift reactions on Cu, respectively. The highest space-time yield of C—C coupling products was 300 gkg_(cat)~(-1) h~(-1), obtained with CuMgAlO_x containing 21 at.% Cu at 523 K and an alcohols GHSV of 3000 std cm~3 g_(cat)~(-1) h~(-1). In situ Fourier transform infrared spectroscopy during MeOH + EtOH, EtOH, and MeOH reactions on CuMgAlO_x and MgAlO_x suggested that surface car-boxylates (formate and acetate) are spectator species whereas oligomeric forms of formaldehyde and acetaldehyde may be responsible for the deactivation of MgAlO_x.