Reductive Etherification of Acetaldehyde on Pd-WO x Bifunctional Catalysts

Abstract

The selective conversion of biomass-derived carbonyl compounds into ethers via reductive etherification represents a promising pathway for producing fungible fuel additives and chemical intermediates. Here, we investigated the mechanistic roles of Pd and WOx in bifunctional Pd-WOx/SiO2 catalysts for the reductive etherification of acetaldehyde as a probe system. Characterization with an array of techniques indicated that introduction of Pd increased WOx dispersion compared to WOx/SiO2, while WOx decreased Pd surface site concentration compared to Pd/SiO2. The WOx/SiO2 catalysts exhibited exclusively aldol condensation activity with no etherification, producing 2-butenal as the primary product, while Pd incorporation unlocked a reductive etherification pathway yielding diethyl ether (DEE) as the dominant product at 150 °C. Whereas H2 pressure negligibly affected the rate of aldol condensation, DEE formation was found to be strongly positive-order in H2 and ceased instantaneously when H2 was removed from the reaction mixture. Experiments with mixed feeds of acetaldehyde and (deuterated) ethanol showed that ethanol can directly participate in DEE formation as the primary source of the “ethoxy” component in DEE, while acetaldehyde forms the “ethyl” component. Consistent with this picture, ethoxy intermediates were detected in infrared spectroscopy experiments following reductive etherification. In situ-formed ethanol was thus found to be a kinetically competent coupling intermediate. Therefore, a major role of Pd is the in situ reduction of acetaldehyde to ethanol/ethoxy, which couples with acetaldehyde via a proposed hemiacetal intermediate, 1-ethoxyethanol. Selective poisoning of Brønsted acid sites (BAS) with 2,6-di-tert-butylpyridine (DTBP) almost completely suppressed DEE formation. Furthermore, propylamine temperature-programmed desorption (TPD) showed that Pd modification substantially increased BAS density, but with propene evolution still indicating their strong acid character. Collectively, these results establish a bifunctional mechanism in which Pd activates H2 and generates additional Brønsted acid sites on WOx, while dispersed WOx acid sites facilitate the acid-catalyzed coupling of acetaldehyde with ethoxy intermediates to yield DEE via a hemiacetal-mediated pathway. Kinetic analysis suggests ethoxy formation is likely the kinetically relevant step in the reductive etherification pathway, while hemiacetal dehydration over BAS determines selectivity toward ether products over acetal.