Selective Production of Glycolic Acid from Cellulose Promoted by Acidic/Redox Polyoxometalates via Oxidative Hydrolysis

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 H3PMo12O40 (H3PMo), H3PW12O40 (H3PW), and H5PMo10V2O40 (H5PMoV2) by density functional theory calculations and experiments shows that H3PMo is especially promising. Activation of O2 to •O2 – and 1O2 via one-electron transfer assists the depolymerization process of cellulose by acidic/redox H3PMo. The reduced form [PMo10 VIMo2 VO39]5– plays a crucial role in GA production due to its high activity and ability to stabilize the intermediates of the retro-aldol reaction. H3PMo 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]H2PMo provides an outstanding GA selectivity of 61% under aerobic conditions and is comparable to the homogeneous H3PMo. 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.

Ionic Liquid/Polyoxometalate Hybrid Catalyst with Tunable Functionalities for Boosting the Selectively Oxidative Transformation of Biobased Carbohydrates

“…These species act as homogeneous catalysts to promote the isomerization of glucose into fructose, eventually facilitating the formation of malonic acid via the pathway of [3 + 3] retro-aldol (as seen in Figure ). The slight leaching behavior was also mentioned by Wang et al…”

“…44 No diverged or additional resonances corresponding to other species were observed over the pH range of 1−4. Similarly, the 51 4 PMoV, indicating that its POM anion functioned in an analogue manner. According to the abundance and chemical shift defined by Pettersson et al, 45 the characteristic peak was assigned to the existence of the isomer α-1,2 which generally dominates in the catalytic system.…”

“…According to the abundance and chemical shift defined by Pettersson et al, 45 the characteristic peak was assigned to the existence of the isomer α-1,2 which generally dominates in the catalytic system. Figure S4 showed the 51 V NMR spectrum of H 5 PV 2 Mo 10 O 40 and H 4 PVMo 11 O 40 that reveal only the resonance ascribed to isomer α-1,2, indicating that the isomeric structure of POM remained constant even after its hybridization with IL. Furthermore, the 31P-NMR spectrum shown in Figure S5 revealed a chemical shift at ca.…”

“…This is indicative of the slow deactivation of the catalyst and its durability after repeated usage. The FTIR (Figure 2a) and 51 V NMR characterization (Figure 2d) for the reused [C 2 COOHMIM] 4 PMoV showed that the functional groups in IL cation and the isomer α-1,2 of the polyhedral structure in POM anion still remained, demonstrating the durable structure of IL-POM hybrid catalyst and the vanadium-containing active species are available for the catalytic transformation of glucose after 5-run recyclability. In fact, there is a trace amount of malonic acid during recycling runs 1−3.…”

“…Catalyst Characterization. The IL-POM was characterized by Fourier Transform Infrared Spectroscopy (FT-IR, Thermofisher Nicolet 6700) which was recorded in the range of 4000 to 400 cm −1 , X-ray Diffraction (XRD, Bruker D8 ADVANCE) using Cu Kα radiation source(λ = 1.54 Å) at 50 kV and 60 mA, X-ray photoelectron spectroscopy (XPS, Thermo Fisher Escalab 250Xi) using an Al radiation source and 51 V nuclear magnetic resonance ( 51 V-NMR, 600 MHz Burker AVANCE III HD), 1 H nuclear magnetic resonance ( 1 H NMR, 600 MHz Burker AVANCE III HD), pyridine adsorption infrared spectroscopy (Py-IR, PE FT-IR Frontier) and thermogravimetry (TG, Mettler Toledo TGA/DSC 3+/ 1600HT). The scanning range for XRD spectrogram was 5°∼ 40°at the rate of 5°/min.…”