MOF-alumina composites for improved methane adsorption under wet conditions

Abstract

HKUST-1 MOF is considered among the most promising materials for methane adsorption. However, the huge decrease in adsorption capacity under humid conditions, due to water hydrolysis of the structure and blockage of open metal sites, decreases its potential. To address this issue, composites of HKUST-1 and γ-Al2O3 particles were synthesized and tested for methane adsorption. MOF impregnation on large particles reduces the pressure drop through the fixed-bed, whereas the known hydrophilicity of the alumina is expected to minimize the water damage to the MOF. Impregnation was performed using two different methods (drip impregnation and solvothermal) with different HKUST-1/Al2O3 initial mass ratios (0.05–1.29 g/g) in a PTFE autoclave. The loading, dispersion and main morphological features were confirmed by PXRD, XPS, SEM, BET, DRIFT and TGA. Water influence was determined by ageing samples in a fixed bed under air with 100% RH for 24 h and 3 consecutive cycles, and methane adsorption capacity was checked before and after the humid treatment. Adsorption capacity of HKUST-1 decreases by about 37%, while composites with low MOF loads (<9%) withstand and even improve the methane adsorption capacity by more than 38%. This effect is more noticeable for a more heterogeneous MOF distribution on the alumina surface, following a similar principle to the methane hydrate formation through a surface promoter, but at milder conditions. Surprisingly, these composites also show a significant improvement in thermal stability.