Orientation of alkyl-substituted aromatic molecules during sorption in the pores of H/ZSM-5 zeolites

Two energetic contributions of the sorption of benzene, toluene, and p-xylene on the MFI zeolites (ZSM-5 and Silicalite-1) process are identified. One is the nonspecific interaction with the pore walls, and the other is the specific directed interaction with the Brønsted acidic SiOHAl groups. Although the energetic contribution of the nonspecific interaction is about 10 times larger than the directed interaction, this minor contribution is sufficient to direct the aromatic molecules at low coverage to sorption sites at intersections containing SiOHAl groups, and only at higher loadings, intersections without bridging hydroxy groups are occupied. The sorption of benzene on the SiOHAl groups is described by a dual-site Langmuir isotherm related to two orientations of the aromatic molecule, differing in the decrease of entropy but with the same heat of adsorption. The two sorption structures are manifested by the appearance of two perturbed hydroxy groups in the IR spectra of the zeolite in contact with the aromatic molecule. At low coverage for toluene, two sorption structures with different steric constraints are differentiated, while at higher coverage only the less constrained structure dominates. In contrast, for p-xylene, the Langmuir model was inadequate to describe the sorption isotherm on the SiOHAl groups, which reflects the strongly confined sorption structure resulting from the two alkyl groups. For all three molecules, the initial preferential adsorption at the intersections causes the molecules to be spatially well-separated, randomly oriented with respect to the SiOHAl groups. With increasing coverage, the molecules arrange into the geometrically less constrained sorption structures.