STUDIES ON THE FORMATION AND CHARACTERISTICS OF 2 TYPES OF P-XYLENE SILICALITE-1 ASSOCIATES/

High resolution Si-29 MAS NMR, C-13 CP MAS NMR, and TG/DTG/DTA were us ed to study the interaction between the framework of siliceous MFI typ e zeolite (silicalite) and the adsorbed p-xylene. The zeolite sample u sed in this study possesses a perfect framework. A series of high reso lution Si-29 MAS NMR and C-13 CP MAS NMR spectra of the samples with v arious loadings of p-xylene were measured. Experimental results of XRD , NMR and thermal analysis confirm the formation of two types of p-xyl ene/silicalite associate. The properties of the two types of associate s are given in terms of their XRD patterns, NMR spectra and TG/DTG/DTA curves. When the p-xylene loading is less than four molecules per uni t cell, the adsorbed molecules disperse in the sinusoidal channels and interact with the framework O2- forming associate-I (p-xylene/silical ite), and inducing the monoclinic/orthorhombic(I) transition. When the p-xylene loading is more than 5.2 molecules/u.c., the adsorbed p-xyle ne molecules are located in the channel intersections and combined int o a p-xylene/p-xylene complex via hydrogen bonding, forming associate- II (p-xylene/p-xylene/silicalite) and inducing the orthorhombic(I)/ort horhombic(II) transition. The desorption of p-xylene from associate-II occurs at about 90 degrees C with an evident endothermic effect, wher eas the desorption from associate-I occurs at about 140 degrees C with out any visible heat effect, implying that two types of sorption and d esorption processes exist in this host/guest system.