Influence of surface acid and base sites on the Guerbet coupling of ethanol to butanol over metal phosphate catalysts

Hydroxyapatite (HAP; Ca-10(PO4)(6)(OH)(2)) is a well-recognized catalyst for the Guerbet coupling of ethanol to butanol. In an effort to explore the role of the anion components of the catalyst, steady-state, gas phase catalytic coupling of ethanol to butanol was investigated at 633 K and atmospheric pressure over beta tricalcium phosphate (beta-TCP; beta-Ca-3(PO4)(2)) and fluorine-substituted hydroxyapatite (FAP; Ca-10(PO4)(6)F-2). Both beta-TCP and FAP catalysts were catalytically active for butanol formation, leading to similar to 35% selectivity at low conversion, suggesting that the PO43- group contributes to the active acid-base site pair for butanol formation during ethanol coupling over HAP. Co-feeding water, a product of ethanol coupling, revealed weaker inhibition of the rate over HAP relative to MgO, confirming the potential negative influence of strong base sites on coupling catalysts. Catalytic reactions of ethanol over Mg-3(PO4)(2), beta-TCP, and Sr-3(PO4)(2) catalysts demonstrated the importance of Lewis acidity of the metal phosphates on the reaction. Relatively strong Lewis acid sites on the Mg-3(PO4)(2) surface (Mg2+ cations) favored undesired ethanol dehydration to ethene (36% selectivity) and diethyl ether (52% selectivity) whereas the Sr-3(PO4)(2) catalyst predominantly catalyzed ethanol dehydrogenation to acetaldehyde (91% selectivity) at a rate significantly higher than that observed over the other catalysts. Evidently, the beta-TCP exposes intermediate-strength Lewis acid sites provided by surface Ca2+ cations that enable the material to effectively convert ethanol to butanol with 35% selectivity. (C) 2017 Elsevier Inc. All rights reserved.