Unravelling gas sorption in the aluminum metal-organic framework CAU-23: CO2, H2, CH4, SO2 sorption isotherms, enthalpy of adsorption and mixed-adsorptive calculations

Abstract

The report is the first broader evaluation of the gas sorption properties of CAU-23 for the adsorptives CO2, H2, CH4, and SO2. CAU-23 is of intermediate porosity among Al-MOFs with specific BET surface areas of the order of MIL-100>MIL-53>CAU-23>MIL-160>MIL-53-TDC>Alfum>CAU-10-H and total pore volumes of the order of MIL-100>MIL-53>CAU-23>Alfum=MIL-160>MIL-53-TDC>CAU-10-H. CO2 uptake (3.97 mmol g−1, 293 K) and H2 uptake (10.25 mmol g−1, 77 K) of CAU-23 are second in the series and only slightly smaller than for MIL-160. The CH4 uptake of CAU-23 (0.89 mmol g−1, 293 K) is unremarkable in comparison with the other Al-MOFs. The SO2 uptake (8.4 mmol g−1, 293 K) follows the porosity and higher SO2 uptakes were only observed for MIL-53 and MIL-100. CAU-23 is one of the best Al-MOFs for high-pressure sorption of CO2, with an uptake of 33 wt.-% at 20 bar, 293 K. Gas sorption measurements at two different temperatures gave near zero-coverage enthalpy of adsorptions, ▵Hads0 for CO2 of −22 kJ mol−1 and of SO2 for −38 kJ mol−1 which is at the low end of the other Al-MOFs (−22 to −39 kJ mol−1 for CO2; −41 to −51 kJ mol−1 for SO2), yet ▵Hads increases for CAU-23 with CO2 and SO2 to −25 and −57 kJ mol−1, respectively. For CO2/CH4 and SO2/CO2 separation, ideal adsorbed solution theory (IAST) predicted gas selectivities of 5 and 27–50 (depending on molar ratio and model), respectively, in line with 4.5–6.3 and 17–50, respectively, with most of the other Al-MOFs, where only MIL-53-TDC with 83 and MIL-160 with 126 gave a higher SO2/CO2 selectivity at a molar ratio of 0.5.