Photocatalytic Transfer Hydrogenation Reactions Of Aromatic Aldehydes And Their Selectivity Control

The method development of lignocellulosic platform compound conversion has laid the foundation for the preparation of fuels and fine chemicals from renewable resources.Transfer hydrogenation without the use of hydrogen gas provides a convenient and low carbon footprint method for reductive refining of biomass-based platform compounds.However,currently developed thermos-catalytic transfer hydrogenation required high reaction temperature to activate the hydrogen donor and realize the hydrogen transfer process,which not only increased energy consumption,but also tended to cause side reactions.In recent years,emerging photocatalytic reactions,which activate substrates and hydrogen donors under mild conditions,provide new opportunities for transfer hydrogenation.However,current photocatalytic transfer hydrogenation systems still suffer from limited reaction types and limited methods for switching selectivity.Based on these,the photocatalytic transfer hydrogenation of lignocellulosic aromatic aldehydes was developed in this work.Meanwhile,the effect of substrate concentration on hydrogenated products was explored,providing a method for switching selectivity in photocatalytic reduction reaction under mild conditions.The main results are as follows:Photocatalytic hydrodeoxygenation and reductive etherification of aromatic aldehydes were investigated on the same Pd/TiO2 with aliphatic alcohols as hydrogen donor.Selective hydrodeoxygenation of the aromatic aldehydes was achieved with a low concentration of substrates,whilereductive etherification of aromatic aldehydes was achieved with a high concentration of substrates.Aliphatic alcohols act as the hydrogen donor,solvent and etherification reagent.The substrate scopes of aromatic aldehydes and aliphatic alcohols were investigated.Through detecting the reaction processes,it was found that benzyl alcohol,benzyl acetal and benzyl alkyl ether were gradually formed and consumed during the deoxygenation reaction.In the reductive etherification process,only benzyl acetal intermediates were formedinitially,and benzyl alkyl ethers kept stable in the presence of benzyl acetal intermediates.Based on control experiments and photocurrent test of TiO2 at different concentrations,it is found that the density of photo-generated charge on TiO2 may determine whether benzyl alcohol intermediate is formed.By comparing the adsorption energies of benzyl ether and benzyl acetal on the catalyst,it was found that the adsorption of benzyl acetal on the catalyst was stronger than that of benzyl ether,therefore,the presence of excess benzyl acetal inhibited the transformation of benzyl ether.Based on the radical capture reaction and dark reactions,the formation of benzyl acetal may proceed via a radical pathway beyond the acid-catalytic pathway.Overall,substrate concentration controlled the single electron reduction or multielectron reduction under light irradiation,and affected the cooperation of photo reduction with acid-catalyzed reactions,thus determining the selectivity of intermediates and products.In this work,the type of photocatalytic reductive transformation of lignocellulosic aromatic aldehydes is enriched,and a concentration-controlled selectivity method was proposed,which provided a new route for the photocatalytic refining of lignocellulosic platform compounds.