Selective Production of Glycolic Acid from Cellulose Promoted by Acidic/Redox Polyoxometalates via Oxidative Hydrolysis
Zonghang Li, Xiaohu Yi, Qiwen Wang, Yiming Li, Diantao Li, Regina Palkovits, Anna Katharina Beine, Chun‐Guang Liu, Xiaohong Wang
Abstract
The direct conversion of cellulose to glycolic acid (GA) with a high yield of up to 75% is realized using acidic/redox polyoxometalates (POMs) as catalysts in a one-pot reaction. Analysis of the reaction pathway and mechanism for the three POMs H 3 PMo 12 O 40 (H 3 PMo), H 3 PW 12 O 40 (H 3 PW), and H 5 PMo 10 V 2 O 40 (H 5 PMoV 2 ) by density functional theory calculations and experiments shows that H 3 PMo is especially promising. Activation of O 2 to • O 2 – and 1 O 2 via one-electron transfer assists the depolymerization process of cellulose by acidic/redox H 3 PMo. The reduced form [PMo 10 VI Mo 2 V O 39 ] 5– plays a crucial role in GA production due to its high activity and ability to stabilize the intermediates of the retro-aldol reaction. H 3 PMo was furthermore complexed by the ionic liquid 1-(3-sulfonic group) propyl-3-methyl imidazolium (MIMPS), which enables easy recovery from the reaction solution due to temperature-responsive properties of the complexes. [MIMPS]H 2 PMo provides an outstanding GA selectivity of 61% under aerobic conditions and is comparable to the homogeneous H 3 PMo. Activity and selectivity to GA could be improved to 100 and 75%, respectively, by performing the reaction in the microwave at 190 °C for 2 min. The work deepens the insight on cellulosic biomass transformation over POMs by acidic/oxidative synergetic catalysis and contributes to the effort of designing highly active, selective, and multifunctional catalysts.