A comparative study of the kinetics of adsorption and desorption of benzene and o-xylene in H-MCM-22-, H-ZSM-5- and H-beta-zeolites

The kinetics of adsorption and desorption of benzene and o-xylene in H-MCM-22-, H-ZSM-5- and H-beta-zeolites have been studied by means of in situ FTIR. The adsorbents were passed over the activated zeolite wafer, while the spectrum was continuously scanned for about 13 min. During this period adsorption equilibrium was attained for benzene on all three zeolites and for o-xylene on H-beta, whereas only about one-fourth of the expected capacity, or 0.23 mmol g⁻¹ of o-xylene had been adsorbed on H-MCM-22 within 3 min, increasing negligibly in the following 10 min. After 14 h, the amount of o-xylene adsorbed on H-MCM-22 had increased to 0.39 mmol g⁻¹. The adsorption of o-xylene on MCM-22 could thus be resolved into a rapid and a slow process, with the rapid process having a diffusion coefficient typical of small molecules in wide pore zeolites, and the slow process having a diffusion coefficient in the same range as for o-xylene in MFI-zeolites. It was shown that the rapid process represented interaction with hydroxyls of the external surface, and it was thus quantitatively important only because the MCM-22 sample consisted of very small crystals. Since small crystals are typical of MCM-22, the results present an explanation of literature reports indicating the existence of a 12-ring pore system. It also explains why MCM-22 cannot be used for the separation of xylene isomers like the 10-ring MFI-zeolites. Desorption experiments showed that the o-xylene adsorbed on the external silanols was also the first to be desorbed at 298 K, whereas o-xylene adsorbed on internal bridged hydroxyls was quantitatively desorbed in the range 350–460 K.