Studies on the formation and characteristics of two types of p-xylene/silicalite-1 associates

High resolution Si-29 MAS NMR, C-13 CP MAS NMR, and TG/DTG/DTA were used to study the interaction between the framework of siliceous MFI type zeolite (silicalite) and the adsorbed p-xylene. The zeolite sample used in this study possesses a perfect framework. A series of high resolution Si-29 MAS NMR and C-13 CP MAS NMR spectra of the samples with various loadings of p-xylene were measured. Experimental results of XRD, NMR and thermal analysis confirm the formation of two types of p-xylene/silicalite associate. The properties of the two types of associates 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 unit cell, the adsorbed molecules disperse in the sinusoidal channels and interact with the framework O2- forming associate-I (p-xylene/silicalite), and inducing the monoclinic/orthorhombic(I) transition. When the p-xylene loading is more than 5.2 molecules/u.c., the adsorbed p-xylene molecules are located in the channel intersections and combined into a p-xylene/p-xylene complex via hydrogen bonding, forming associate-II (p-xylene/p-xylene/silicalite) and inducing the orthorhombic(I)/orthorhombic(II) transition. The desorption of p-xylene from associate-II occurs at about 90 degrees C with an evident endothermic effect, whereas the desorption from associate-I occurs at about 140 degrees C without any visible heat effect, implying that two types of sorption and desorption processes exist in this host/guest system.