Hydrolysis-oxidation of cellulose to formic acid in the presence of micellar vanadium-containing molybdophosphoric heteropoly acids

OxFA-processing of inedible cellulose is considered as a perspective way for the production of biogenic formic acid (FA) which appeared to be a promising source for sustainable energy. In our work, the one-pot hydrolysis-oxidation of cellulose to FA was studied in the presence of the micellar acidic catalysts based on Mo–V–P heteropoly acids [Cn-MIM]y/H3+x-yPMo12-xVx (n = 6, 10 and 16; x = 1, 2 and 4; y = 2–6). The influence of amount of V, substitution of H+ by [Cn-MIM] and the length of alkyl group Cn of the cation was revealed. The optimization of the reaction conditions was also performed. To our knowledge, heterogeneous catalysts of such a composition were applied in cellulose hydrolysis-oxidation to FA for the first time. In the presence of C10-MIM/HPMo11V catalyst, the optimal reaction conditions were determined: 10 g/L of cellulose, 1.25–10 g/L of catalyst, 5 MPa of gas mixture (O2/N2 - 20/80 vol/vol) and 170–180 °C. Decreasing the alkyl group length of Cn-MIM+ cation or V content into framework of heteropoly anion were revealed to rise up the yield of the target product. These correlations were caused by the following factors. First, the decreasing acidity of micellar catalysts. Senond, the change in distribution of electron density in heteropoly anion due to the interaction between hydrogen atoms of alkyl group of Cn-MIM cation and oxygen atom of heteropoly anion. Third, blocking of V active sites for reagents. Influence of acidity on catalytic properties of micellar catalysts was also revealed in the course of screening samples with the different degree of protons substitution by C16-MIM+. The reaction rate was demonstrated to rise with increasing degree of protons substitution by C16-MIM + cation in C16-MIM/HyPMo11V and C16-MIM/HyPMo8V4 samples. The yield of FA up to 45 mol% can be reached over micellar HPA catalysts studied.