Studies On PH-Regulated Selective Hydrogenation Of Carbohydrate Derivatives By Using Cp~*Ir Complexes

Nowaday,energy and chemical industry that relies heavily on fossil resources is facing a serious predicament.To pursue sustainable development,research on exploring clean energy and renewable chemical building blocks has attracted global attention.Biomass is the sole renewable organic carbon source and has large exploration potential.The hydrolysis product of lignocellulosic biomass is 5-hydroxymethylfuraldehyde(5-HMF),levulinic acid(hexose-based platform molecules)and furfural(pentose-based platform molecule),which can be upgraded into 1-hydroxy-2,5-hexanedione(HHD),?-valerolactone(GVL)and furfuryl alcohol resins respectively.Recently,various aqueous hydrogenation reaction systems used in biomass carbohydrate derivatives conversion process.Therefore,it is necessary to develop the selective hydrogenation with high compatibility with the acidic hydrolysis system.Herein,we developed the pH-regulated aqueous hydrogenation of carbohydrate derivatives over semisandwich iridium complexes.5-HMF was catalyzed to produce HHD and furfural was catalyzed to produce furfuryl alcohol and levulinic acid.We can improve the electronic density of the substituent on the ligand and catalytic efficiency and control the product distribution by adjusting the pH value.The major product was HHD at pH 1.5-3.5 and the major product was 2,5-bis-(hydroxymethyl)furan(BHMF)at pH 4.0-6.5 in hydrogenation/hydrolytic ring-opening reaction of 5-HMF.Full conversion of 5-HMF to HHD with 99%yield was achieved at pH 2.5 in aqueous formate buffer solution.A mechanistic study revealed that the hydrolysis/ring-opening reaction of BHMF is the intermediate reaction step.In addition,an isolated yield of 85%for HHD was obtained in a 10 g-scale experiment,and the reaction with fructose as the starting material also led to a 98%GC yield(71.9%to fructose)of HHD,which showed a potential for a large-scale production.The turnover frequencies(TOF)of hydrogenation of furfural up to 13877 h-1 and TOF of hydrogenation of levulinic acid up to 12200 h-1 were achieved under identical conditions(0.0083 mol%catalyst,1.0 MPa H2,120 °C).We found that furfural can be converted into GVL(55%yield)directly in a one-pot reaction without any intermediate product separation step in a strong acidic phosphate buffer system with pH of 1.0.Our system is compatible with a pure water system,which shows a robustness of semisandwich Ir complexes for acidic aqueous system and a remarkable viability for practical hydrolysis system.