Mechanistic insights into C–O bond cleavage in erythritol during hydrodeoxygenation on an Ir–ReOx catalyst†

Abstract

1,4-Butanediol (1,4-BDO) is a key ingredient in the polymer industry. When derived from renewable erythritol, it can pave the way for sustainable poly(butylene terephthalate), polyurethane and polyester manufacturing. Hydrodeoxygenation (HDO) of erythritol on Brønsted acidic metal–metal oxide catalysts can result in 1,4-BDO, among other alcohols. Selective synthesis of 1,4-BDO requires deep insights into the preference for the cleavage of the different C–O bonds and the energy landscape for the formation of other polyol intermediates. In this work, we used density functional theory (DFT) simulations to investigate HDO of erythritol and other polyol intermediates on an inverse Ir–ReO_x catalyst, where rhenium oxide is dispersed on iridium. While Ir nanoparticles can drive HDO through dehydroxylation, a protonation and dehydration mechanism happening at the Ir–ReO_x interface has greater kinetic relevance. We show the kinetic preference for secondary C–O cleavage in erythritol to explain the predominant formation of 1,2,4-butanetriol (1,2,4-BTO) during erythritol HDO. The kinetic preference for 1,4-BDO formation from 1,2,4-BTO makes it the most prominent butanediol during erythritol HDO. C–O bond cleavage in 1,4-BDO has a high barrier making 1,4-BDO less reactive in a polyol mixture. This indicates potential selective formation of 1,4-BDO, with a possibility of tuning reaction conditions and reaction time to maximise its yield. Our analyses reveal that C–O cleavage is not always the kinetically relevant step and it can be the hydrogenation that follows the C–O cleavage. Hence, reactions at high hydrogen pressure and lower temperatures might suit higher selectivity towards desired alcohols such as 1,4-BDO.