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

The report is the first broader evaluation of the gas sorption properties of CAU-23 for the adsorptives CO2, H-2, 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 H-2 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, H-ads(0) 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 H-ads 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.