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

Abstract

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 38at.% 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 448–523K, 0.1MPa, alcohols gas-hourly space velocities (GHSVs) of 1000–3000stdcm3gcat−1h−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 CC coupling (e.g., C3+ alcohols, aldehydes, and esters), non-CC coupling (e.g., acetaldehyde, methyl formate, methyl acetate), and methanol decomposition (i.e., COx) products as a result of CC coupling reactions on MAl (M=Mg, Cu) and dehydrogenation, esterification, reverse methanol synthesis, and the water–gas shift reactions on Cu, respectively. The highest space-time yield of CC coupling products was 300gkgcat−1h−1, obtained with CuMgAlOx containing 21at.% Cu at 523K and an alcohols GHSV of 3000stdcm3gcat−1h−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.