NMR studies of the sorption of p-xylene, toluene, and benzene on ZSM5 zeolite

Deuterium NMR results of p-xylene-d{sub 6}, toluene-d{sub 3}, and benzene-d{sub 6} as well as proton MASS of p-xylene-d{sub 6} and -d{sub 4} sorbed in H-ZSM5 are reported. The analysis of the results shows that below room temperature the sorbed p-xylene molecules undergo at most reorientation about their para axis. When samples are heated above room temperature, an increasing fraction of the molecules become mobile and unlike in Na-ZSM5 a significant fraction undergoes rapid three-site jumps between intersecting channel segments. This reflects the much weaker hindering effect that H{sup +} ions have on the diffusion of the guest molecules compared with the larger sodium ions. Toluene behaves similar to p-xylene, but the appearance of the dynamic species starts at significantly lower temperatures. Sorbed benzene molecules in both Na- and H-ZSM5 undergo fast reorientation about their C{sub 6} axes even at the lowest temperature studied ({minus}120{degree}C). Above {minus}60{degree}C, new species appear in which the benzene C{sub 6} axes undergo fast but restricted local reorientation. No direct evidence for fast three-dimensional diffusion of the guest molecules within the zeolite channels was obtained, but this process cannot be ruled out for the species undergoing fast three-site jumps. Heating a sample of sorbed p-xylene in H-ZSM5 to above 100{degree}C results in a structural change in the zeolite framework and concomitantly a change in the distribution of the guest molecules toward more mobile species. Above room temperature, exchange between the aromatic hydrogens of p-xylene and the hydrogens in the zeolite framework takes place.