Zeolites transport

ZSM-5 zeolite. Transport D values were Consequently, the diffusion coefficient calculated from FR results assuming that particles are spheres of lO-pm diameter (0) Data determined by the MEMBRANE (A), TAP( ), and QENS (H) methods were taken from refs. 13 and 14 and are given for comparison. 

The ortho- and meto-isomers are bulkier than the para-iaomer and diffuse less readily in the zeolite pores. The transport restriction favours their conversion into the /lara-isomer, which is fonned in excess of the equilibrium concentration. Because the selectivity is transport influenced, it is dependent on the path length for transport, which is the length of the zeolite crystallites. 

Haag W O, Lago R M and Weisz P B 1982 Transport and reactivity of hydrocarbon molecules in a shape-selective zeolite Faraday Disouss. Chem. Soo. 72 317-30... 

The aim of the newly introduced mesoporosity is to enhance the utilization of the microporous network by improved accessibihty of the active sites that are mostly present in the micropores. Although numerous papers have reported on the improved catalytic performance of desihcated zeolites in catalysis (details in Section 2.4.5), only few works are available that reaUy tackle the hierarchical nature of the desihcated zeohtes and demonstrate that selective sihcon removal leads to an enhanced physical transport in the zeohte crystals. 

In order to design a zeoHte membrane-based process a good model description of the multicomponent mass transport properties is required. Moreover, this will reduce the amount of practical work required in the development of zeolite membranes and MRs. Concerning intracrystaUine mass transport, a decent continuum approach is available within a Maxwell-Stefan framework for mass transport [98-100]. The well-defined geometry of zeoHtes, however, gives rise to microscopic effects, like specific adsorption sites and nonisotropic diffusion, which become manifested at the macroscale. It remains challenging to incorporate these microscopic effects into a generalized model and to obtain an accurate multicomponent prediction of a real membrane. 

This paper describes the morphological and transport properties of a composite zeolite (silicalite) - alumina membrane. Some advantages obtained in combining the membrane with a conventional fixed-bed catalyst are also reported. 

The zeolite-alumina tube is no more gas-tight after the thermal treatment. The presence or absence of the V-AI2O3 toplayer in the starting support does not influence the gas transport properties of the final zeolite-alumina tube. 

N. Y. Chen, T. F. Degnan, C. M. Smith 1994, Molecular Transport and Reaction in Zeolites, VCH Publishers, New York. 

The high temperature XRPD technique can be used to investigate the dynamics of solid-state ion exchange of zeolites. Data suggest that the rate of Cd2+ ion transport in the zeolite Y micropores controls the rate of the solid-state ion exchange. 

Toluene alkylation with isopropyl alcohol was chosen as the test reaction as we can follow in a detail the effect of zeolite structural parameters on the toluene conversion, selectivity to cymenes, selectivity to para-cymene, and isopropyl/n-propyl ratio. It should be stressed that toluene/isopropyl alcohol molar ratio used in the feed was 9.6, which indicates the theoretical toluene conversion around 10.4 %. As you can see from Fig. 2 conversion of toluene over SSZ-33 after 15 min of T-O-S is 21 %, which is almost two times higher than the theoretical toluene conversion for alkylation reaction. The value of toluene conversion over SSZ-33 is influenced by a high rate of toluene disproportionation. About 50 % of toluene converted is transformed into benzene and xylenes. Toluene conversion over zeolites Beta and SSZ-35 is around 12 %, which is due to a much smaller contribution of toluene disproportionation to the overall toluene conversion. A slight increase in toluene conversion over ZSM-5 zeolite is connected with the fact that desorption and transport of products in toluene alkylation with isopropyl alcohol is the rate controlling step of this reaction [9]... 

Selectivity to p-isopropyl toluene being close to 30 % was achieved with SSZ-33, SSZ-35 and Beta zeolites. This is connected with the 12-MR channels in SSZ-33 and Beta. In the case of SSZ-35 the presence of 18-MR cavities decreased the differences in the rate of transport of individual isopropyl toluene isomers. In contrast, ZSM-5 zeolite behaves as para-selective catalyst in this alkylation reaction, the selectivity to p-isopropyl toluene reached 76 % after 180 min of T-O-S. 

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