Diffusion Of Hydrocaibons In Mesoprous Zeolites By Zero Length Column Technique

Zeolite is immensely used in catalytic conversion reaction. However, the similar pore size of zeolite seriously restricted the intracrystalline diffusion behavior of many hydrocarbons and the catalytic properties of zeolite catalysts. Especially in the process of adsorption or catalytic reaction, there are often some of the larger reactant molecules, relatively limited aperture of the zeolite, which will increase the resistance of the diffusion process and reduce the activity center utilization. At the same time, the longer the diffusion path will reduce the rate of diffusion of the molecules also. That is all have an affecting on activity, selectivity and stability of zeolite catalyst. At last, these will affect the zeolite in industrial applications. Studies have shown that intracrystalline mesopores of zeolite, which can effectively improve the diffusion behavior of hydrocarbons in the zeolite, thereby enhancing the catalytic properties of zeolites.Diffusion kinetics of hydrocarbons in zeolite was performed by the Zero-length column(ZLC)technology. The method involves a purge of a previously equilibrated sample to obtain a curve of fractional desorption versus time. the diffusion time constant can be obtained by matching the desorption dynamics to the theoretical solution of the diffusion equation. This method can eliminate the intrusion of axial disperation,heat transfer and bed diffusion resistances by the use of low adsorbate concentration and very small adsorbent sample amount as well as high carrier flow rate during desorption. Due to the the ZLC device and method of operation are simple. Therefore,It has been widely used to study the diffusion behavior of hydrocarbons in hierarchical porous zeolite.In view of the importance of of hydrocarbons in the catalysis research and practical significance in the industrial applications. The objective of this study is to investigate the diffusion kinetics of hydrocarbons in different mesoporous zeolite using the zero length column (ZLC) method and evaluate the promotion effects of mesopore on molecules diffusion in zeolite. The specific research content and results include the follows:1. The ZLC technology is studied to the performation of the sorption kinetics. Before each performing run, the experiment including the blank, different carrier gas flows as well as the part-loading equilibrated tests. Then, the feasibility of ZLC technology is to be sure. A simple technique to fit over the complete time range of the desorption curve while maintaining the accuracy of the ZLC method is used. This analysis method of desorption curves is taken to overcome the disadvantage of LT and ST methods. Good agreement of fitting curve and experimental data by matlab software shows the method is feasible. The effective diffusion time constants (Deff/R2) can be obtained in the analysis result, so the corresponding complete ZLC techniques will be established.2. Diffusion properties of hydrocarbons in the mesopores zeolite. 1) The diffusion for heptane, octane, and decane from mesoporous zeolite A.A large number of mesopores structure of mesopores zeolite NaCaA-n. As an example, the mesoporous sample NaCaA-3 which has the larger SBET (519m2/g), Sext, (370m2/g) and developed mesoporous structure. The Vmeso is about 0.18cc/g, and the pore size distribution in 2nm to 3nm of mesoporous structure.Temperature and mesoporous structure play a role in promoting the diffusion for alkanes from zeolite in experimental ZLC. Such as the diffusion fitting results for octane in from NaCaA-n at different temperatures (60℃,90℃ and 120℃). The Deff/R2 of conventional CaNaA-1 corresponds to 0.90*10-4S-1,1.32* 10-4S-1 and 2.02*10-4S-1. But the Deff/R2 of mesoporous sample CaNaA-1 corresponds to 1.34*10-4S-1, 1.87*10-4S-1 and 2.41*10-4S-1. Adsorption process of this paper is physical adsorption, because the diffusion activation energy is less than 40KJ/mol form NaCaA-n. Mesoporous structure can promote the the diffusion, but reduce the diffusion activation energy. For example, the Ea for n-octane from zeolite NaCaA-n corresponds to 14.8 KJ/mol,11.8 KJ/mol and 10.8 KJ/mol. The desorption process is slower, corresponding to the lower Deff/R2 but the higher Ea with the increase in the number of carbon atoms in straight-chain alkanes.2) The diffusion for heptane and toluene from mesoporous nano-zeolite ZSM-5.The mesoporous zeolites have larger specific surface area and lots of mesoporous structure relatively. Take an example, the mesoporous sample ZSM-5-c which has the larger SBEr (523m2/g), Sext (245m2/g) and developed mesoporous structure. The Vmeso is about 0.33cc/g, and the pore size distribution in 2nm to 3nm of mesoporous structure.The mesoporous zeolite has better catalytic performance than conventional zeolite in MTG Mesoporous zeolite catalyst can still get the product of fuel oil for 48 hours, however, the conventional catalyst can not get fuel oil nearly for 10 hours in the catalytic reaction.Temperature and mesoporous structure are conducive to diffusion for alkanes from zeolite in experimental ZLC, The higher the temperature, the more obvious diffusion rate. The mesoporous pore volume has the greater role in promoting the small kinetic diameter molecules, reducing the Ea obviously. Because of benzene ring structure, toluene has bigger steric effects and molecular repulsive in the C7Hy, so there is the smaller Deff/R2 and the bigger Eα in the diffusion process. Such as the Deff/R2 for heptane or toluene from the mesoporous ZSM-5-c, respectively corresponding to 5.64*10-4S-1,14*10-4S-1 and 1.73*10-4S-1,13*10-4S-1 at the diffferent temperature(60℃ and 120℃), Also the Ea corresponding to 16.7KJ/mol and 28.6KJ/mol.3) The diffusion for octane from mesoporous zeolite ZSM-5/H-ZSM-5.There are larger surface area and mesoporous pore volume in alkali-treated HZSM-5, the mesoporous H-ZSM-5-4 with maximum SBET(192m2/g) and Vmeso(0.39cc/g) in the HZSM-5-n, and the mesoporous pore diameter distribution in 3nm or so.The alkali-treated catalyst which has better catalytic performance than conventional catalyst reduces the structure of the acid strength in MTG. The methanol conversion rate is still above 75%, and the fuel oil production rate for the initial 22% for 48 hours in the mesoporous H-ZSM-5-4 catalyst. However, the methanol conversion rate has dropped to about 75%, and the fuel oil production rate only 5% for 12 hours in the conventional H-ZSM-5-1 catalyst.Temperature and mesoporous structure play a positive role in the diffusion for alkanes from zeolite, but acidic of adsorbent is negative in ZLC. There is the smaller Deff/R2 and the bigger Eα in the H-ZSM-5, for example, the Deff/R2 for octane from NaZSM-5-3 or H-ZSM-5-3 respectively correspond to 0.98*10-4S-1, 1.11*10-4S-1,1.32*10-4S-1 and 0.81*10-4S-1,1.04*10-4S-1,1.30*10-4S-1 at the diffferent temperature(35℃, 45℃ and 60℃), The corresponding Eα are 10.1KJ/mol and 15.7KJ/mol.