DFT investigation of polyalcohols reforming on palladium cluster

Biomass conversion technologies have recently gained high industrial interest for the production of sustainable fuels and fine chemicals; starting feedstocks for these processes are generally complex mixtures of oxygenated compounds, ranging from lignans, carbohydrates and polyalcohols to carboxylic acids [1]. Framed within this scientific context the entire reforming mechanism of two well-known polyols, namely ethylen glycol (C2) and glycerol (C3), on a small Pd cluster was investigated by means of density functional theory. Among the large amount of reaction pathways that can be followed in the reforming of oxygenates, we discuss here only the route that brings to carbon monoxide and hydrogen as final products, since it is the most relevant in the biomass treatment. It was found that the C-H bond cleavage, where the H atom transfers to the cluster, has an activation energy which is typical of such processes [2] and common to all the mechanism we will deal with. On the other hand, the rate determining step (rds) is the C-C bond breaking, with an activation barrier which exceeds the 160 kJ/mol. The same investigation applied to C3 suggests that the mechanisms of the two studied polyols cross after the C-C reforming step, through a facile 1,2 hydrogen shift in a shared intermediate. In order to obtain information on the reforming of heavier polyols, the rds was calculated for all the stereoisomers of C4 (erythritol), C5 (xilytol, arabitol and ribytol), C6 (mannitol, sorbitol, galactitol and iditol), by including the influence of the position of the breaking C-C bond. Part of the reaction mechanism is in this case affected also by secondary interaction between hydroxiles and the palladium cluster.