Selective Oxidation of Cellulose to Carboxylated Cellulose Catalyzed by a Recyclable Keggin-Type Polyoxometalate H6PMo9V3O40 with O2: An Insight into Mechanism by DFT and Experimental Study

Abstract In this work, we have developed a sustainable and efficient catalytic system for the selective oxidation of the C6 hydroxyl groups of cellulose to carboxyl groups, utilizing a trivanadium-substituted Keggin-type phosphomolybdate (H6PMo9V3O40) as the catalyst and molecular oxygen (O2) as the terminal oxidant. Under the optimal conditions (120 °C, 8 h, 0.3 g catalyst, 1 MPa O2), our H6PMo9V3O40/O2 system achieved a carboxyl content of 1.02 mmol/g, which is comparable to that of the well-established TEMPO-mediated oxidation system (0.8–1.2 mmol/g). The catalyst exhibited outstanding stability and reusability, maintaining essentially unchanged activity over eight consecutive cycles without any regeneration treatment. Density functional theory calculations established a detailed catalytic cycle, revealing that the two proton-coupled-electron-transfer steps with activation energy barriers of 47.57 and 37.93 kcal mol–1 are the rate-determining bottleneck region. The thermodynamic driving force for the whole reaction is calculated to be −96.21 kcal mol–1. Kinetic analysis indicates that the reaction rate exhibits pseudozero-order kinetics. Our catalytic system successfully introduces negatively charged carboxylate groups on the cellulose surface, which generates strong electrostatic repulsion that effectively loosens the tightly bundled elementary fibrils of corn stalks, and effectively promotes the subsequent defibrillation into micro/nanofibrils upon ultrasonication.