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

The effects of catalyst composition on methanol and ethanol coupling reactions were studied on CuMgAlOx mixed metal oxides (MMOs). CuMgAlOx 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 cm3 gcat−1 h−1, and a feed MeOH/EtOH molar ratio of 4. Incorporation of Cu in MgAlOx solid solutions drastically changed product selectivity, formation rates, and catalyst stability. The main products on CuMgAlOx included C C coupling (e.g., C3+ alcohols, aldehydes, and esters), non-C C coupling (e.g., acetaldehyde, methyl formate, methyl acetate), and methanol decomposition (i.e., COx) 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 g kgcat−1 h−1, obtained with CuMgAlOx containing 21 at.% Cu at 523 K and an alcohols GHSV of 3000 std cm3 gcat−1 h−1. In situ Fourier transform infrared spectroscopy during MeOH + EtOH, EtOH, and MeOH reactions on CuMgAlOx and MgAlOx suggested that surface carboxylates (formate and acetate) are spectator species whereas oligomeric forms of formaldehyde and acetaldehyde may be responsible for the deactivation of MgAlOx.