The oxidation of complex, water-insoluble biomass to formic acid is reported using a Keggin-type polyoxometalate (H₅PV₂Mo₁₀O₄₀) as the homogeneous catalyst, oxygen as the oxidant, water as the solvent and p-toluenesulfonic acid as the best additive. The reaction proceeds at 90 °C and 30 bar O₂ and transforms feedstock like wood, waste paper, or even cyanobacteria to formic acid and CO₂ as the sole products. The reaction obtains up to 53% yield in formic acid for xylan as the feedstock within 24 h. Besides the role of the additive as a reaction promoter, the formic acid isolation and the recycling of catalyst and additive are demonstrated.

Herein, we report a remarkable finding that biomass oxidation to formic acid (FA) in water–organic biphasic reaction systems is far more selective than the same reaction in a monophasic aqueous media. While literature claims that the yield of FA from carbohydrates and biomass is limited to less than 68%, even for simple substrates such as glucose or glycerol, we demonstrate in this study that FA yields of up to 85% can be obtained from glucose. Using our biphasic reaction protocol, even raw lignocellulosic biomass, such as beech wood, leads to FA yields of 61%. This is realized by applying polyoxometalate H₈PV₅Mo₇O₄₀ as a homogeneous catalyst, oxygen as the oxidant and water as the solvent in the presence of a long-chain primary alcohol as an in-situ extracting agent. The new, liquid–liquid biphasic operation opens a highly effective way to produce pure FA, a liquid syngas equivalent, from wood in a robust, integrated, and low-temperature process.

Different Keggin-type polyoxometalates have been synthesized and characterized in order to identify optimized homogeneous catalysts for the selective oxidation of biomass to formic acid (FA) using oxygen as an oxidant and p-toluenesulfonic acid as an additive. Applying the optimized polyoxometalate catalyst system H₈[PV₅Mo₇O₄₀] (HPA-5), a total FA-yield (with respect to carbon in the biogenic feedstock) of 60% for glucose within 8 h reaction time and 28% for cellulose within 24 h reaction time could be achieved. The transformation is characterized by its mild reaction temperature, its excellent selectivity to FA in the liquid product phase and its applicability to a very wide range of biogenic raw materials including non-edible biopolymers and complex biogenic mixtures.

The selective oxidation of complex, water-insoluble and wet biomass from second and third generation to formic acid including effective catalyst recycling is reported. Additionally, the relevance and limits of potential contaminants are illustrated by different experimental approaches.