A starch-to-sorbitol biorefinery: Impact of oxygenated impurities on the catalytic transfer hydrogenation of starch hydrolysate.

Compared with traditional hydrogenation operating in high-pressure H2, catalytic transfer hydrogenation (CTH) uses non-H2 organic hydrogen donors, which can be sourced from biomass fermentation to enable a bio-based supply chain. Besides the activation of organic H donors, oxygenated impurities in bio-feedstocks may complicate surface reactions and thus challenge CTH. To address such a knowledge gap, this study investigates the CTH of glucose in starch hydrolysate to sorbitol using isopropanol (IPA) as the H donor over Ru/C catalyst. The results show negligible sorbitol from the CTH of glucose-rich hydrolysate (26.5 g/L), which also contains byproducts in trivial amounts - formic acid (FA; 0.25 g/L) and hydroxymethylfurfural (HMF; 2.59 g/L). Controlled CTH experiments using standard glucose show decreases in sorbitol by ≥60% upon the addition of FA or HMF. Their inhibition is reaffirmed as their removal from starch hydrolysate restores the CTH activity to the expected level. While protons from FA are unlikely to play a role, the anionic formate (HCOO-) could strongly adsorb on and thus block Ru sites from glucose; sorbitol production can increase as HCOO- is suppressed (leftward shift in equilibrium at lowered pH following Le Chatelier's principle). HMF also strongly binds to Ru according to the simulated adsorption energies; furthermore, it consumes hydrogen and forms 2,5-bis(hydroxymethyl)furan, highlighting that HMF does not compete with glucose as an adsorbate only but also as a co-reactant. Our findings elucidate how byproducts formed upstream impact the activation of target reactants during CTH, which should be taken into consideration during catalyst design.