Adsorption And Diffusion Of Hydrocarbons In Hierarchical Porous Zeolites

Zeolite molecular sieves are widely used in adsorption, separation and catalyzation because of their developed ordered porous structures, high specific surface area, strong acidity, and better hydrothermal stability. However, their use was hampered by their small microporous channels, which are subject to diffusional limitations on reaction rates, because of the similarity between the size of the involved guest molecules and the micropore diameter. Therefore, the researchers create secondary mesopores on the basis of retaining the zeolite’s microporous structure and named hierarchical porous zeolite. This kind of zeolites with a micro/mesostructure are of particular interest because they combine the advantages of both microporous crystalline zeolites and those of mesoporous materials, which largely shorten the diffusion length of the guest molecules, increase the diffusion path of the guest molecules, and improve the effective utilization of the active sites of the zeolite surface, thus highly valued by people.Currently, studies of the hierarchical porous zeolite mainly focus on the preparation, characterization, and evaluation of its catalytic performance. However, based on the analysis of the specific application of the hierarchical porous zeolite, it is not difficult to find that adsorption and diffusion of the adsorbate (diffusion molecules) in the crystal are the main factors affecting the process performance. To this end, the paper conducts a systematic and detailed analysis of the adsorption and diffusion properties of hydrocarbons in the hierarchical porous zeolite. These studies are crucial for adsorption^ separation and catalytic dynamics and will play an important guiding role in developing or improving hierarchical porous zeolite molecular sieve based catalysts and the use conditions of hierarchical porous zeolite molecular sieves.Considering the differences in pore structure, modification mechanism and application scope, this paper respectively selects meso-zeolite 5A, and meso-zeolite ZSM-5 of different pore structure as the research object. First, the research uses adsorption isotherms and desorption curves obtained by IGA (the Intelligent Gravimetric Analyzer) and ZLC (Zero Length Column) to compare the shapes of adsorption isotherms and desorption curves of different samples; after that, based on the adsorption and desorption data, according to the corresponding equations and models, it calculates the thermodynamics and kinetics parameters which mainly include:adsorbed amount, Henry constant, effective diffusion time constant, activation energy, etc.; Analyzes how adsorbance changes with the specific surface area and the outside specific surface area, how Henry constant and the adsorption heat change with changes in the pore structure, the relationship between the adsorption heat and the loading, the impact of mesopore porosity on the effective diffusion time constant and the activation energy and the impact of different pore structures on the diffusion mechanism. The main conclusions of the paper are as follows:1 Adsorption and Diffusion of n-Paraffins in Hierarchical Porous 5A ZeolitesThe adsorption and intra-crystalline diffusion of n-heptane, n-octane and n-decane in 5A zeolites with an intracrystalline mesopores were studied, and were compared with that of microporous 5 A zeolite. It is shown that the adsorption data from three n-paraffins on mesoporous 5A zeolites might be well correlated by using the Langmuir-Fraudlich and Toth models. Duo to the coexistence of micropores and mesopores on mesoporous 5A zeolites, both the higher maximum adsorption capacity and more energetic heterogeneity on the surfaces of them were present, whereas, the affinity to the adsorbent measured_by the Henry’s constants and the heats of adsorption at zero coverage decreased with the introduction of mesopores in zeolite crystals. Furthermore, the isosteric heat of adsorption by caculated by Clausius-Clapeyron equation decreased with the increasing surface coverage of n-paraffins. The change of decane is most obviously. The mesopores were found to enhance molecule diffusion. Moreover, the effective diffusion time constant (Deff/R2) increased with mesoporosity in the zeolites, 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.2 Adsorption and Diffusion of n-heptane and toluene in nano-ZSM-5 ZeolitesMicro and mesoporous ZSM-5 were investigated with regard to their equilibrium and kinetic characteristics using n-heptane and toluene as sorbates by the standard gravimetric technique and ZLC method. The adsorption capacities of nano-ZSM-5 samples were significantly higher than those of microporous ZSM-5, In contrast to that, the isosteric heats of adsorption and Henry constants in the nano-ZSM-5 were found to be much smaller. The Diffusion rates of heptane and toluene depends on the particle size of the nano-ZSM-5 crystals. The diffusion time constants for mesoporous ZSM-5 were found to be higher than for microporous ZSM-5. However, the diffusion activation energies for both C7 hydrocarbons in nano-ZSM-5 zeolite were found to be lower than for microporous ZSM-5, which suggests that the decrease of the diffusion path length within the microporous crystals by reducing the particle size can promote the process of diffusion. Diffusion activation energies of n-heptane were higher in comparison to toluene, which has a larger kinetic diameter.3 Adsorption and Diffusion of n-heptane and toluene in Hierarchical Porous ZSM-5 ZeolitesAdsorption and diffusion properties of n-hetane and toluene in meso-structured ZSM-5 zeolites were studied by high precision intelligent gravimetric analysis (IGA) and ZLC technology. As expected, great increase in adsorption capacity and diffusion efficient of n-heptane and toluene in the mesostructured ZSM-5 zeolites was observed compared with conventional ZSM-5. At the same time, the adsorption activation energy of n-heptane and toluene in the mesostructured ZSM-5 zeolites was significantly decreased. The adsorption heats with low n-heptane and toluene 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. The effect of the acid properties on the adsorption and diffusion of C7 hydrocarbons n-heptane and toluene in mesostructured ZSM-5 zeolites was further studied. As expected, the interaction with the acid sites causes a increase in the heats of adsorption and Henry constants and a decrease in the diffusivities of both sorbates. The values of the activation energy of diffusion for both C7 hydrocarbons in H-form ZSM-5 zeolites are higher than those in Na-forms.4 Adsorption and Diffusion of hydrocarbon in ZSM-5 Zeolite microspheresThe ZSM-5 microspheres were consisted by nanosized zeolite particles and there were intracrystalline mesopores inside the nanosized particles. Thus hierarchical pore system exists in these materials. The first one is the innate micropore of zeolite ZSM-5, the second one is the intercrystalline mesopores formed owing to the aggregation of nano-crystal, and the third one is the intracrystalline mesopores inside the nano-crystal. The zeolite ZSM-5 microsphere showed enhanced catalytic effect In benzylation of benzene by benzyl alcohol (BA) and MTG. Here, mesoporous ZSM-5 zeolites were further investigated with regard to their adsorption and diffusion characteristics. Such adsorption heats and Henry constants were found to be dependent on the framework mesoporosity and showed a clear decline with increase of mesoporosity in the zeolite samples. Adsorption heats for n-heptane, toluene and cumene on the same samples decreased with increase of mesoporosity. The isosteric heats of adsorption estimated by the Clausius-Clapeyron equation are significantly higher for toluene and cumene in comparison with n-heptane, and they are strongly correlated to the sorbate loadings. The sharp initial decrease of isosteric heats of adsorption with respect to coverage, which shows different behavior for the different sorbate/adsorbent systems. The diffusion activation energies for hydrocarbons in ZSM-5 microsphere were found to be lower than for microporous ZSM-5. which suggests that the decrease of the diffusion path length within the microporous crystals by reducing the particle size and introduction of mesopores into the zeolites can promote the process of diffusion. The transport of n-heptane in zeolite ZSM-5 microsphere was found to be mainly controlled by micro and mesopore diffusion in the pore structure, while that of toluene was dominated by the mesopore diffusion process. Diffusion activation energies of n-heptane are higher in comparison to toluene, which has a larger kinetic diameter.