A COMPARATIVE-STUDY OF THE KINETICS OF ADSORPTION AND DESORPTION OF BENZENE AND O-XYLENE IN H-MCM-22-ZEOLITES, H-ZSM-5-ZEOLITES 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 i n situ FTIR. The adsorbents were passed over the activated zeolite waf er, while the spectrum was continuously scanned for about 13 min. Duri ng this period adsorption equilibrium was attained for benzene on all three zeolites and for o-xylene on H-beta, whereas only about one-four th of the expected capacity, or 0.23 mmol g(-1) of o-xylene had been a dsorbed on H-MCM-22 within 3 min, increasing negligibly in the followi ng 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 co uld thus be resolved into a rapid and a slow process, with the rapid p rocess having a diffusion coefficient typical of small molecules in wi de 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 t he rapid process represented interaction with hydroxyls of the externa l surface, and it was thus quantitatively important only because the M CM-22 sample consisted of very small crystals. Since small crystals ar e typical of MCM-22, the results present an explanation of literature reports indicating the existence of a 12-ring pore system. It also exp lains why MCM-22 cannot be used for the separation of xylene isomers l ike the 10-ring MFI-zeolites. Desorption experiments showed that the o -xylene adsorbed on the external silanols was also the first to be des orbed at 298 K, whereas o-xylene adsorbed on internal bridged hydroxyl s was quantitatively desorbed in the range 350-460 K. (C) 1997 Elsevie r Science B.V.