Novel model explaining toluene diffusion in HZSM-5 after surface modification

The diffusivities of toluene in HZSM-5 zeolites before and after silylation by chemical liquid deposition of tetraethoxysilane were studied using the frequency response method. Silylation did not lead to variations in the equilibrium amount of toluene adsorbed in the zeolite. Two diffusion processes are identified in both materials and tentatively assigned to the diffusion of toluene in the straight and sinusoidal channels of the zeolites. The difference in toluene diffusivities is associated with the tortuosity of the diffusion pathways in the sinusoidal channels and connected transport barriers. Toluene diffusivities in both channels depend on the toluene loading. This is attributed to variations in the strength of the bonding induced by varying degrees of the interaction of toluene with the acid sites of the zeolite. Silylation of the zeolite leads to a decrease in toluene diffusivities in the straight and sinusoidal channels of the zeolites. In addition, the relative importance of transport in the sinusoidal channels increases after the modification, while the total uptake remains unaffected. In contrast to earlier interpretations invoking narrowing pores, the effect of surface modification could be associated for the first time with arbitrary blocking of pore entrances, thus enhancing the tortuosity of possible diffusion pathways.