Bio-based chemicals: Selective aerobic oxidation of tetrahydrofuran-2,5-dimethanol to tetrahydrofuran-2,5-dicarboxylic acid using hydrotalcite-supported gold catalysts

A new, sustainable catalytic route for the synthesis of tetrahydrofuran-2,5-dicarboxylic acid (THFDCA), a compound with potential application in polymer industry, is presented starting from the bio-based platform chemical 5-(hydroxymethyl)furfural (HMF). This conversion was successfully achieved via oxidation of tetrahydrofuran-2,5-dimethanol (THFDM) over hydrotalcite (HT)-supported gold nanoparticle catalysts (∼2 wt %) in water. THFDM was readily obtained with high yield (>99%) from HMF at a demonstrated 20 g scale by catalytic hydrogenation. The highest yield of THFDCA (91%) was achieved after 7 h at 110 °C under 30 bar air pressure and without addition of a homogeneous base. Additionally, Au-Cu bimetallic catalysts supported on HT were prepared and showed enhanced activity at lower temperature compared to the monometallic gold catalysts. In addition to THFDCA, the intermediate oxidation product with one alcohol and one carboxylic acid group (5-hydroxymethyl tetrahydrofuran-2-carboxylic acid, THFCA) was identified and isolated from the reactions. Further investigations indicated that the gold nanoparticle size and basicity of HT supports significantly influence the performance of the catalyst and that sintering of gold nanoparticles was the main pathway for catalyst deactivation. Operation in a continuous setup using one of the Au-Cu catalysts revealed that product adsorption and deposition also contributes to a decrease in catalyst performance.