Studies on the Formation and Characteristics of Two Types of p-Xylene/Silicalite-1 Associates

High resolution 29Si MAS NMR, 13C CPMAS NMR, and TG/DTG/DTA were used to study the interaction between theframework of siliceous MFI type zeolite (silicalite) and the adsorbedp-xylene. The zeolite sample used in this study possesses a perfectframework. A series of high resolution 29Si MAS NMR and 13CCP MAS NMR spectra of the samples with various loadings of p-xylene weremeasured. Experimental results of XRD, NMR and thermal analysis confirm theformation of two types of p-xylene/silicalite associate. The properties ofthe 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 lessthan four molecules per unit cell, the adsorbed molecules disperse inthe sinusoidal channels and interact with the framework O2-forming associate-I (p-xylene/silicalite), and inducing themonoclinic/orthorhombic(I) transition. When the p-xylene loadingis more than 5.2 molecules/u.c., the adsorbed p-xylene moleculesare located in the channel intersections and combined into ap-xylene/p-xylene complex via hydrogen bonding, formingassociate-II (p-xylene/p-xylene/silicalite) and inducingthe orthorhombic(I)/orthorhombic(II) transition. The desorptionof p-xylene from associate-II occurs at about 90 °Cwith an evident endothermic effect, whereas the desorption fromassociate-I occurs at about 140 °C without any visibleheat effect, implying that two types of sorption and desorptionprocesses exist in this host/guest system.