(207d) Methanol and Ethanol Coupling Reactions to Oxygenates Over Copper Containing Mixed Metal Oxides

Currently, there is a tremendous interest in the development of technologies to produce alcohols from lignocellulosic sources for use as fuels and chemical feedstocks for synthesis of olefins, paraffins, and aromatics in a bio-refinery.  Use of bio-derived  alcohols as fuels is strongly motivated by existing legislation mandating a renewable fuels standard of 36 billion gallons per year by 2022 (EISA 2007, RFS 2). Higher alcohols such as propanol and butanols could be viable candidates to fulfill these mandates as their energy contents are higher than those of methanol and ethanol.

In this work, the synthesis of chemicals and potential fuels (higher alcohols) from methanol and ethanol coupling reactions was studied over copper containing mixed metal oxides. Layered double hydroxides with varying copper contents (prepared by coprecipitation methods and used as parent materials) were treated at high temperatures to yield the corresponding highly dispersed copper mixed metal oxides. Typical products included those from Guerbet reactions (e.g., formaldehyde, acetaldehyde, propanal, propanol, butanals, butanols), esterification of aldehydes with MeOH and EtOH (methyl and ethyl esters), and MeOH decomposition (CO and CO₂).

On CuMgAl mixed metal oxides (Cu/(Cu+Mg+Al)=0-0.45; Mg/Al=1;  250 ^oC, 1 bar, He:MeOH:EtOH=75:20:5, SV=4000 cm³ g_cat^-1 h^-1), for example, it was found that increasing Cu content resulted in significant increase in EtOH conversion (typical EtOH conversion > 90%), at the expense of a decrease in C-C coupling alcohols selectivity but with corresponding gain in selectivities towards C-C coupling (e.g., C3+) and non C-C coupling (HAc, EtOAc, MeOAc, HCOOCH₃) aldehydes and esters products. The result is higher yields of C-C coupling (~150 g kg_cat^-1 h^-1) and non C-C coupling products (~450 g kg_cat^-1 h^-1) than those observed with Cu free MgAl and related mixed oxides, reported in the literature. The C3+ aldehydes and esters could be easily hydrogenated to the corresponding alcohols for use as fuels, while non C-C coupling esters could be used as bio-derived feedstocks for various chemical conversions. The presence of Cu also resulted in more stable catalysts (EtOH conversion drop < 15% after 24 h) in comparison with the MgAl oxide (EtOH conversion drop > 50% after ~5 h). These results suggest that appropriate manipulation of acid/base, dehydrogenation/hydrogenation properties (e.g., by adjusting Cu content) of mixed metal oxides can be used for the fine tuning of product yields relevant for use as fuels and chemical feedstocks from bio-derived methanol and ethanol mixtures.