Catalytic oxidation of biomass to formic acid in aqueous solutions using vanadium-containing catalysts

Abstract

Formic acid (FA) is an important material in the chemical industry, and the preparation of FA from biomass could provide a sustainable strategy for the large-scale production of FA. Vanadium (V)-containing catalysts have recently been reported to catalyze the conversion of biomass to FA with a high level of selectivity under aqueous conditions using molecular O 2 as an oxidant. These catalysts mainly include V-containing heteropoly acid, NaVO 3 /H 2 SO 4 and VOSO 4 . In this article, we have provided a detailed review of the performances of these three catalytic systems for the conversion of biomass into FA. This review is mainly focused on four key areas of research, including the mechanisms associated with this transformation, the relationships between various reactions, methods for the separation of FA from the aqueous solution and the recyclability of the catalytic systems. Vanadium-containing catalytic systems can be used to generate FA from numerous bio-based carbohydrates, including sugars, cellulose and hemicellulose, with a high level of selectivity. The active catalytic species in these processes is known to be V 5+ , and these processes can be explained in terms of an electron transfer-oxygen transfer mechanism, where the vanadium species (including V-containing heteropoly acid and VO 2 + ) receives two electrons from the substrate in exchange for one oxygen atom to form FA. Among the various reactions occurring in the catalytic system, the hydrolysis of the biomass and the selective oxidation of the biomass to FA are competitive, and variations in the main reaction parameters can lead to significant changes in the balance between these two reactions. The direct separation of FA from the aqueous reaction mixture by distillation is difficult, and the extraction of FA by organic solvents is by far a relatively feasible method. Among the extraction agents evaluated for this purpose, n-butyl ether and ethyl ether have been reported to be useful for the extraction of FA from catalytic systems using V-containing heteropoly acid and NaVO 3 /H 2 SO 4 as catalysts, respectively. In terms of the recyclability of the NaVO 3 /H 2 SO 4 system, the activity remained largely unchanged after several runs when cellulose and real lignocellulose were employed as substrates. Finally, the problems associated with this process for the conversion of biomass were analyzed and several new strategies have been proposed to further develop the processes involved in the preparation of FA from biomass. Research towards developing a better understanding of the reaction mechanism associated with this process has focused predominantly on the oxidation of potential model compounds and ambiguous inferences. Further studies on the mechanism should focus on the detection of the real coordinated complex formed by the catalyst and substrate through various testing methods. Research on real lignocellulose has so far been inefficient, because of the lack of studies towards lignin conversion and the effect of ash. The separation of FA from an aqueous solution still represents a significant challenge to the industrial application of biomass for production of FA.