Enhanced CO2 adsorption on Al-MIL-53 by introducing hydroxyl groups into the framework

Abstract

A series of hydroxyl functionalized Al-MIL-53 materials (Al-MIL-53-OH_x) containing varying hydroxyl molar ratios (x = 25%, 50%, 75%, and 100%) were synthesized via a mixed-linker approach, wherein x denotes the molar ratio of 2,5-dihydroxy terephthalic acid:(2,5-dihydroxy terephthalic acid + terephthalic acid). All Al-MIL-53-OHs exhibited an identical structure to that of Al-MIL-53. The thermal stability of Al-MIL-53 decreased after introducing hydroxyl groups. The hydroxyl functionalized Al-MIL-53 containing 25 mol% and 50 mol% of hydroxyl group showed higher surface areas (S_BET = 1270 and 1260 m² g⁻¹ for Al-MIL-53-OH₂₅ and Al-MIL-53-OH₅₀, respectively) than that of Al-MIL-53 (S_BET = 819 m² g⁻¹). A further increase in the OH groups (75 mol% and 100 mol%) led to dramatical compromise of the framework. The presence of hydroxyl groups affected not only the CO₂ adsorption capability but also the ‘breathing effect’ of MIL-53 resulting from the intraframework interaction. The CO₂ adsorption capacities of Al-MIL-53-OH₂₅ and Al-MIL-53-OH₅₀ at 1 bar at 25 °C were 8.5 and 8.3 wt%, respectively, which are about 19% higher than that of Al-MIL-53 under the identical conditions. Moreover, pronounced improvement in CO₂ adsorption was observed below 0.2 bar, especially for Al-MIL-53-OH₂₅ (5.5 wt% for Al-MIL-53-OH₂₅ vs. 1.7 wt% for Al-MIL-53). This behavior is due likely to the enhanced isosteric heat of CO₂ adsorption. The hydroxyl group plays a positive role in the CO₂ adsorption performance of Al-MIL-53, which is comparable to amino groups. Al-MIL-53-OHx (x = 75 and 100) displayed lower CO₂ adsorption capacities despite the higher isosteric heat of CO₂ adsorption, which might be due to the blocked pores in the presence of dense hydroxyl groups.