Adsorption And Diffusion Of Hydrocarbons In Hierarchical Porous ZSM-5 Zeolite

Owing to its favorable properties, such as, high hydrothermal stability, strong acidic property and good shape selectivity, ZSM-5 zeolite has been extensively used as a heterogeneous catalyst for a variety of industrial catalytic reactions. However, the sole and small pore diameter of ZSM-5 confines the diffusion of products and reactants, especially when bulky molecules are involved, and greatly reduces the accessibility of active sites. Furthermore, a slow mass transport to and away from the catalytic center increase the possibility of secondary reactions, with coke formation and catalyst deactivation as a consequence. In order to overcome above shortages, researchers introduce a secondary mesopore system on the basis of connatural microporous structure into zeolite to form hierarchical zeolite, which is allowing for promoting the rate of molecular transport, improving the effective utilization of the active sites, decreasing in the deactivating effect of coke towards zeolite by the presence of mesopores and maintaining simultaneously the functionality of the material owing to their contents of micropores. And the catalytic activity of hierarchical zeolite is enhanced significantly compared with conventional microporous zeolite.Adsorption and diffusion of hydrocarbons in zeolite are very important for optimizing catalytic reactions and for designing adsorption-based separation processes. So far, the synthesis of hierarchical zeolite has become an important issue. However, the complexity of its structure leads to very special patterns in the studies of adsorption and diffusion within it, which are by far more difficult to be assessed than in purely microporous zeolites. The lack of corresponding kinetic and thermodynamic data causes our failure in the understanding of the adsorption and diffusion mechanism. For this, this paper conducts a systematic and detailed analysis of the adsorption and diffusion properties of various hydrocarbons over two different kinds of hierarchical porous ZSM-5 zeolite. Adsorption isotherms and desorption curves were measured by using gravimetric method and ZLC method respectively to investigate the impacts of pore structure, acidity and the properties of probe molecules on the adsorption and diffusion. The main research contents and conclusions of this paper are as follows:1. Adsorption and diffusion of C7 hydrocarbons on alkali-treated ZSM-5 zeolitesAdsorption isotherms of both n-heptane and toluene over microporous ZSM-5 zeolite display a typical type-I adsorption isotherm which can be successfully described by the Langmuir-Freundlich model. The dual-site Langmuir-Freundlich model fitted the isotherms over mesoporous ZSM-5 zeolites very well, in which the isotherms show a combination adsorption behaviour of type I and type IV isotherms. The introduction of mesopores into zeolite increases the mesopore volume, thus enhances the adsorption amounts of both sorbates. However, the introduction of mesopores by NaOH treatment sacrifice the homogeneity of the samples. As a result, the adsorption environment of adsorbates changes, the sorbent-sorbate interaction decreases consequently. Differences in kinetic diameter, configuration and polarity of n-heptane and toluene result in different adsorption capacities and KH, which are both higher for n-heptane in the micropore of ZSM-5, due to significant steric limitation and stronger repulsion forces between micropore with toluene. Since the steric hindrance imposed by pore size limitation diminished in mesopore, in this case, as the interplay between polar toluene is much stronger than nonpolar n-heptane, therefore, toluene is observed to possess a higher adsorption amounts in mesopore. For diffusion process, as the diffusion path can be shortened and the diffusion resistance can be reduced via introducing mesopores into the ZSM-5 zeolite, the Deff/R2 are enhanced greatly while the activation energies for diffusion are reduced. In alkali-treated samples, the diffusion of n-heptane is considered to be controlled by micropore diffusion and mesopore diffusion is a dominant mechanism for toluene transport.2. Effects of the acid properties on the adsorption and diffusion of C7 hydrocarbons on alkali-treated ZSM-5 zeolitesIn HZSM-5 samples, the adsorption amount and adsorption affinity can be enhanced resulted from the electrostatic forces between the acid sites located on the surface of the zeolite with the adsorbate. The increasing trends of the KH of toluene are not as obvious as those of n-hetpane due to the stronger repulsion forces between micropore with toluene, thus weaken the impact of electrostatic forces on adsorption. The exist of electrostatic forces make it difficult for the diffusion of the adsorbates, epically of toluene, owing to its benzene ring. Further, as the diffusion process of toluene is controlled by mesopores and micropore diffusion is accounted for the transport of n-heptane, the diffusion of toluene is affected seriously by the L acid on the external surface, thus the lower Deff/R2 are obtained for toluene.3. Adsorption and diffusion of aromatic hydrocarbons on hierarchical ZSM-5 microspheresAdsorption isotherms of benzene, toluene and p-xylene on five ZSM-5 samples exhibit inflection behaviour. This phenomena can be explained from two aspects:for one thing, the inflection in the isotherm is caused because these molecules prefer to occupy the intersections of ZSM-5; for another, some kind of phase transition takes place in the system of ZSM-5 during adsorption. The introduction of mesopores into ZSM-5 reduces the spatial resistance from micropore towards mesitylene, the sorption amounts enhanced as a consequence. Adsorption amounts of five sorbates on certain sample at the same relative pressure and temperature are in order:mesitylene< cumene< toluene< benzene< p-xylene, suggesting that the amount that the sorbate can be absorbed is governed by the size and the shape of the adsorbate molecule and the way that they arranged in the channel of zeolite. It is well known that the present of mesopore provides multiple diffusion paths and greatly speed up the transfer process. However, the degree of the improvement depends on the connectivity of the mesopores.