Adsorption of aromatic compounds in large MFI zeolite crystals

Very large (180 x 40 x 40 mu m(3)) and well defined crystals have been used ina gravimetric system to investigate the adsorption of aromatic compounds in MFI (silicalite) zeolite. The isotherms and isosteric heats of adsorption (Q(st)) were reported for future comparison with various adsorption models. Three types of phase transition were found. Isotherms of p-xylene showed a sharp step rise from ca. 4 to 8 molecules uc(-1). A clear phase boundary could be outlined. Adsorbed benzene behaved as a dual-phase system in the range between 4.6 and 6 molecules uc(-1). There were more phase transitions above 6 molecules uc(-1) but it was not possible to outline the phase boundary. Molar entropy changes of 210 and 180 J mol(-1) K-1 were found for the observed p-xylene-MFI and benzene-MFI phase transitions. For ethylbenzene and toluene, a dual-phase region was also observed but the transition was less pronounced. Isosteric heats, calculated from isotherms, showed a complex variation with loading and a strong dependence on temperature. It was also found that the adsorption kinetics are strongly influenced by the previous adsorption history. For freshly calcined samples the uptake rate was relatively fast. However, re-calcination after the adsorption of p-xylene created a diffusion barrier inside the crystalline material and subsequent adsorption of other aromatic compounds became much slower and displayed two-step kinetics.