p-Xylene-Selective Metal–Organic Frameworks: A Case of Topology-Directed Selectivity

Para-disubstituted alkylaromatics such as p-xylene are preferentially adsorbed from an isomer mixture on three isostructural metal–organic frameworks: MIL-125(Ti) ([Ti8O8(OH)4(BDC)6]), MIL-125(Ti)-NH2 ([Ti8O8(OH)4(BDC-NH2)6]), and CAU-1(Al)-NH2 ([Al8(OH)4(OCH3)8(BDC-NH2)6]) (BDC = 1,4-benzenedicarboxylate). Their unique structure contains octahedral cages, which can separate molecules on the basis of differences in packing and interaction with the pore walls, as well as smaller tetrahedral cages, which are capable of separating molecules by molecular sieving. These experimental data are in line with predictions by molecular simulations. Additional adsorption and microcalorimetric experiments provide insight in the complementary role of the two cage types in providing the para selectivity.