Novel insights into the recovery and recyclability of homogeneous polyoxometalate catalysts applying an efficient nanofiltration process for the selective catalytic oxidation of humins

Abstract

Selective catalytic oxidation (SCO) of humins is a promising strategy to valorize undesired side streams of biomass conversion processes. Keggin-type polyoxometalates are efficient catalysts for the SCO of humins giving platform chemicals like formic acid, acetic acid or their respective esters up to combined yields of 51 %. Moreover, one of the main challenges for establishing continuous processes is the efficient catalyst recycling and product separation. Herein, we combined an optimization study for the process parameters in SCO with an integrated product separation carried out by using nanofiltration membranes. For this purpose, an enhanced reaction system consisting of 5 vol% methanol addition for CO₂ suppression in combination with 1.5 mmol pTSA as solubility promotor was applied using H₅[PV₂Mo₁₀O₄₀] (HPA-2) as a catalyst achieving a maximum humin conversion of 90 % resulting in 57 % carboxylic ester selectivity. In subsequent studies, an appropriate XN 45 nanofiltration membrane was identified allowing for >99 % catalyst and >90 % additive retention efficiently separating the acidic reaction products in the permeate. Furthermore, long-time stability of the membrane and thus of the separation process could be confirmed up to 168 h time on stream. Based on these results, the developed process represents an important milestone in the valorization of humins combined with efficient separation of the molecular polyoxometalate catalyst and could therefore be the basis for future developments.