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Adsorption isotherms and zero length column (ZLC) desorption curves of alkylbenzene (toluene and isopropylbenzene) in a series of microspherical ZSM-5 zeolites with inter- and intracrystalline mesopores were measured for revealing structural and surface characteristics of the hierarchical zeolites. The isotherms showed that adsorption capacities of toluene and isopropylbenzene increase with increase of mesoporosities in the range of higher relative pressure. Henry’s constants (KH) and initial heats of adsorption (Qst) turned out that introduction of mesoporous structure into the zeolites weakens interaction between alkylbenzene molecules and zeolite surface. Moreover, isopropylbenzene and toluene have different adsorption and diffusion behavior in the zeolites. The adsorption capacity, KH and Qst for isopropylbenzene to microporous and mesoporous ZSM-5 zeolites are lower than these for toluene. The effective diffusion time constants (Deff/R2) of alkylbenzene from the ZLC desorption curves increase with increase of mesoporosities in the ZSM-5 zeolites, but diffusion activation energy (Ea) expresses a contrary tendency. Due to low interaction between isopropylbenzene and zeolite surface, Deff/R2 values of isopropylbenzene with lower activation energy in all ZSM-5 samples are higher than these of toluene. The molecular sizes of two alkylbenzenes are not the key factor for their adsorption and diffusion in the mesozeolites. [ABSTRACT FROM AUTHOR]

Probing the mesopore architecture in mesoporous zeolites is of importance for large scale applications of the materials. In this work, the adsorption and diffusion of mesitylene with larger molecule size in mesoporous ZSM-5 zeolites were carried out, in order to acquaint the availability and interconnectivity of mesopores in zeolite crystals. The comparisons of the shape of adsorption isotherms and the mesopore volume calculated from mesitylene and N adsorption in mesoporous ZSM-5 zeolites with different mesoporosities can be used to discriminate two cases of mesopores: accessible mesopores connected to external surface of the zeolite crystals and non-accessible meso-voids that are occluded in the microporous matrix. Furthermore, the effective diffusivity and activation energy of mesitylene in mesoporous ZSM-5 extracted from ZLC desorption curves as a function of mesopore volume calculated from mesitylene adsorption reveal the enhancement of mesopore interconnectivity to molecule diffusion in zeolite crystals. [ABSTRACT FROM AUTHOR]

In order to understand the effects of the surface acid sites in mesoporous zeolites on the adsorption and diffusion of hydrocarbons, the adsorption and diffusion of n -heptane and toluene (two C 7 hydrocarbons) over the mesoporous HZSM-5 zeolites (SiO 2 /Al 2 O 3 = 36) were investigated. The adsorption isotherms of two C 7 hydrocarbons over the mesoporous HZSM-5 zeolites showed the experimental data good fitting to dual-site Langmuir-Freundlich model (DSLF) similar to mesoporous NaZSM-5. Henry's constants and the initial heats of adsorption displayed stronger interactions in HZSM-5 than in NaZSM-5, especially for n -heptane, due to the presence of surface acid sites. The influence of acidity to the adsorbate-adsorbent interactions decreases with increasing of the mesoporosity. The investigation on the diffusion of n -heptane and toluene in mesoporous HZSM-5 samples by the zero-length column (ZLC) method showed the strong interaction of adsorbates and HZSM-5 samples by effective diffusivity and activation energy. Moreover, the diffusions of adsorbates in zeolites were advanced with the introduction of mesopores in the zeolitic samples, though the dramatic influence of mesopore to diffusion is not arisen in mesoporous NaZSM-5, reflecting larger influence of acid sites on diffusion in mesoporous HZSM-5 samples. [ABSTRACT FROM AUTHOR]

Diffusion properties of mesostructured zeolite 5A were investigated by employing n-alkanes as probe molecules using the zero length column (ZLC) method. The mesopores were found to enhance molecule diffusion. Moreover, the effective diffusion time constant ( D/ R) increased with mesoporosity in the zeolites between 308 K and 393 K, whereas the activation energy decreased with increasing mesopore volume. The effective diffusivity values of n-alkanes in mesoporous zeolite 5A were generally higher than that the microporous zeolite 5A sample. This clearly implied the important role of the mesopore in zeolites crystals in facilitating the transport of reaction molecules due to shorter average diffusion path length and less steric hindrance. [ABSTRACT FROM AUTHOR]

Adsorption and diffusion properties of n-octane in meso-structured HZSM-5 zeolites were studied by high precision intelligent gravimetric analysis (IGA) and ZLC technology between 293 K and 393 K. As expected, great increase in adsorption capacity and diffusion efficient of n-octane in the mesostructured HZSM-5 zeolites was observed compared with conventional HZSM-5. At the same time, the adsorption activation energy of n-octane in the mesostructured HZSM-5 zeolites was significantly decreased. The adsorption heats with low n-octane loading showed a clear decline with increase of mesoporosity in the zeolite samples. These results clearly indicate that introduction of mesopores into the zeolites offered a short diffusion path and high diffusion rate for reactants and products, which resulted in a high yield of fuel oil and an enhanced resistance against the catalyst deactivation in the reaction of methanol to gasoline. [ABSTRACT FROM AUTHOR]

Abstract: A series of zeolite composites with hierarchically porous structures and tunable acidities were prepared. The structural, crystalline, and textural properties of the as-synthesized materials, as well as the reference Y and β zeolite samples, were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM)/transmission electron microscopy (TEM) analyses, Fourier transform infrared spectroscopy (FTIR), and nitrogen adsorption/desorption techniques. The acidic properties of the surface were determined by in situ FTIR of adsorbed pyridine. The hierarchy factor is an appropriate tool to classify hierarchically structured materials, and the hierarchical pores of the as-synthesized zeolite composite samples were investigated during the catalytic cracking of isopropylbenzene. The results show that the catalytic activity of isopropylbenzene cracking over the catalysts has a linear dependence with their hierarchy factor (HF). The tunable acidities were tested by methanol dehydration to dimethyl ether (DME), and the dehydration activity of methanol over the catalysts shows a linear decrease with the increased Lewis/Brönsted ratio, while the selectivity of DME shows a linear increase with the enhanced Lewis/Brönsted ratio. [Copyright &y& Elsevier]