Unravelling gas sorption in the aluminum metal‐organic framework CAU‐23: CO₂, H₂, CH₄, SO₂sorption isotherms, enthalpy of adsorption and mixed‐adsorptive calculations

Abstract

Abstract The report is the first broader evaluation of the gas sorption properties of CAU‐23 for the adsorptives CO 2 , H 2 , CH 4 , and SO 2 . 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. CO 2 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 CH 4 uptake of CAU‐23 (0.89 mmol g −1 , 293 K) is unremarkable in comparison with the other Al‐MOFs. The SO 2 uptake (8.4 mmol g −1 , 293 K) follows the porosity and higher SO 2 uptakes were only observed for MIL‐53 and MIL‐100. CAU‐23 is one of the best Al‐MOFs for high‐pressure sorption of CO 2 , 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 CO 2 of −22 kJ mol −1 and of SO 2 for −38 kJ mol −1 which is at the low end of the other Al‐MOFs (−22 to −39 kJ mol −1 for CO 2 ; −41 to −51 kJ mol −1 for SO 2 ), yet ▵H ads increases for CAU‐23 with CO 2 and SO 2 to −25 and −57 kJ mol −1 , respectively. For CO 2 /CH 4 and SO 2 /CO 2 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 SO 2 /CO 2 selectivity at a molar ratio of 0.5.