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 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 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.

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► CuMgAlOx mixed oxides were prepared from CuMgAl–CO3 layered double hydroxides.
► CuMgAlOx are the most active oxides for MeOH and EtOH coupling at moderate conditions.
► On CuMgAlOx, CC coupling products are C3+ alcohols, C3+ aldehydes, and C3+ esters.
► Adsorbed alkoxides, carboxylates, and aldehyde oligomers were observed by in situ FTIR.