Cu- and Zr-based metal organic frameworks and their composites with graphene oxide for capture of acid gases at ambient temperature

Abstract

Capture of acid gases (CO2 and H2S) by liquid solvent absorption is the common industrial practice, yet capture relying on solid adsorbents is increasingly gaining interest as potential alternative towards less energy-demanding operations. Herein, we developed and examined various metal organic frameworks (MOFs) bearing Cu- and Zr- metal clusters and their composites with graphene oxide (GO), and evaluated their performance for CO2 and H2S adsorption. Specifically, UiO-66, UiO-66-NH2, HKUST-1, and their GO composites were grown, subjected to structural, morphological, and textural characterization, and subsequently evaluated for their adsorption capacity and selectivity at ambient temperature. The crystallinity of the parent MOFs was preserved upon in-situ growth of the MOF/GO composites, while incorporation of GO yielded uniformly-shaped and well-dispersed MOF crystals resulting in enhanced sorption kinetics, and increased the pore volume compared to pure MOFs due to additional porosity formed in the interstitial spaces between the MOF crystallites and the GO flakes. To this extent, the interplay between additional porosity and pore functionalities in the competitive CO2 and N2 adsorption was investigated. UiO-66-NH2 exhibited enhanced CO2 capacity (3.07 mmol/g at 25 ⁰C and 4 bar) and the highest CO2/N2 selectivity (167 at 1 bar) among the tested MOFs, due to the presence of the amine functional groups in the organic linker that enhanced affinity with CO2. At increased pressures, the UiO-66-NH2/GO composite exhibited higher CO2 capacity compared to pure UiO-66-NH2, which is attributed to activation of the additional porosity between the MOF crystallites and the GO layers. HKUST-1/GO selectivity was also enhanced compared to HKUST-1 in all pressures tested. Under humid environment, the UiO-66 based structures were relatively stable in contrast to the HKUST-1 ones that underwent gradual degradation. These composites offer functionalization tunability due to the presence of both MOF and GO counterparts for targeted gas capture applications.