Zeolites hydrocarbon diffusion

In the experimental systems considered here, the controlling resistance was in each case zeolitic diffusion, but systems in which macropore resistance is dominant are equally common. As examples one may cite the sorption of light hydrocarbons in the Davison 5A molecular sieves which contain much smaller zeolite crystals and correspondingly smaller macropores than the equivalent Linde products (18). 

Thus, it can be concluded that the diffuslvities are dependent on the pore size of HZSM-5 zeolites and the weight, length and minimum diameter of the diffusing hydrocarbon molecule. However, there is no available method to predict the diffuslvities within the IIZSM-5 zeolite crystallite. Hence, a estimation method is developed in the next section. 

Active matrix contributes significantly to the overall performance of the FCC catalyst. The zeolite pores are not suitable for cracking of large hydrocarbon molecules generally having an end point > d00 [-(482°C) they are too small to allow diffusion of the large molecules to the cracking sites. An effective matrix must have a porous structure to allow diffusion of hydrocarbons into and out of the catalyst. 

Reactants and reagents can be conveniently loaded into the dry zeolite by adsorption. This can be accomplished by intimately mixing the solid or liquid reactant and the powdered zeolite, by absorption from the gas phase, or by diffusion in a solvent slurry containing the zeolite and dissolved reactant. The choice of solvent for the slurry method is critical. It must be volatile enough to be removable at a pressure and temperature that does not result in evacuation of the reactant or its decomposition. In addition, the reactant must have a greater affinity for the interior of the zeolite than for the slurry solvent itself. The lack of affinity for the interior of the zeolite is an acute problem for non-polar hydrocarbons that lack binding sites for the intrazeolitic cations. The use of fluorocarbons such as perfluorohexane as slurry solvents takes advantage of the fluorophobicity of many hydrocarbons and has alleviated this problem to some extent.29... 

Only large-pore zeolites exhibit sufficient activity and selectivity for the alkylation reaction. Chu and Chester (119) found ZSM-5, a typical medium-pore zeolite, to be inactive under typical alkylation conditions. This observation was explained by diffusion limitations in the pores. Corma et al. (126) tested HZSM-5 and HMCM-22 samples at 323 K, finding that the ZSM-5 exhibited a very low activity with a rapid and complete deactivation and produced mainly dimethyl-hexanes and dimethylhexenes. The authors claimed that alkylation takes place mainly at the external surface of the zeolite, whereas dimerization, which is less sterically demanding, proceeds within the pore system. Weitkamp and Jacobs (170) found ZSM-5 and ZSM-11 to be active at temperatures above 423 K. The product distribution was very different from that of a typical alkylate it contained much more cracked products trimethylpentanes were absent and considerable amounts of monomethyl isomers, n-alkanes, and cyclic hydrocarbons were present. This behavior was explained by steric restrictions that prevented the formation of highly branched carbenium ions. Reactions with the less branched or non-branched carbenium ions require higher activation energies, so that higher temperatures are necessary. 

Understanding the adsorption, diffusivities and transport limitations of hydrocarbons inside zeolites is important for tailoring zeolites for desired applications. Knowledge about diffusion coefficients of hydrocarbons inside the micropores of zeolites is important in discriminating whether the transport process is micropore or macropore controlled. For example, if the diffusion rate is slow inside zeolite micropores, one can modify the post-synthesis treatment of zeolites such as calcination, steaming or acid leaching to create mesopores to enhance intracrystalline diffusion rates [223]. The connectivity of micro- and mesopores then becomes an... 

Chimie, C.R. (2005) Infrared spectroscopic investigation of diffusion, co-diffusion and counter-diffusion of hydrocarbon molecules in zeolites. Elsevier, Comptes Rendus Chimie, 8, 303-319. 

The third and last part of the book (Chapters 12-16) deals with zeolite catalysis. Chapter 12 gives an overview of the various reactions which have been catalyzed by zeolites, serving to set the reader up for in-depth discussions on individual topics in Chapters 13-16. The main focus is on reactions of hydrocarbons catalyzed by zeolites, with some sections on oxidation catalysis. The literature review is drawn from both the patent and open literature and is presented primarily in table format. Brief notes about commonly used zeolites are provided prior to each table for each reaction type. Zeolite catalysis mechanisms are postulated in Chapter 13. The discussion includes the governing principles of performance parameters like adsorption, diffusion, acidity and how these parameters fundamentally influence zeolite catalysis. Brief descriptions of the elementary steps of hydrocarbon conversion over zeolites are also given. The intent is not to have an extensive review of the field of zeolite catalysis, but to select a sufficiently large subset of published literature through which key points can be made about reaction mechanisms and zeolitic requirements. 

To reveal factors which influence activities of acid-base catalysts in alkylation and isomerization is the challenge to activity in this field. Q he greatest amount of work has been done in connection with the effect of para-selectivity, which is observed in alkylation of aromatic hydrocarbons on ZSM-5 type zeolites [1]. This effect has been explained by a number of authors either by the influence of diffusion factors [2,3] or by the isomerizing activity of the external surface of zeolite crystals [4]. In refs. [5,6] and especially in ref.[7] the para-selective effect of ZSM-5 type zeolites is shown to be due to decreasing their isomerizing activity becaiase of the decrease in the concentration of strong protic centres as a result of modifiers introduced. Para-selective effect is related to the action of chemical factors. However, in... 

FIGURE 7.16 Effect of calcium exchange for sodium in zeolite A on hydrocarbon adsorption. Replacement of four sodium ions by two calcium ions permits easy diffusion of n-alkanes into the zeolite channels. 

Auerbach et al. (101) used a variant of the TST model of diffusion to characterize the motion of benzene in NaY zeolite. The computational efficiency of this method, as already discussed for the diffusion of Xe in NaY zeolite (72), means that long-time-scale motions such as intercage jumps can be investigated. Auerbach et al. used a zeolite-hydrocarbon potential energy surface that they recently developed themselves. A Si/Al ratio of 3.0 was assumed and the potential parameters were fitted to reproduce crystallographic and thermodynamic data for the benzene-NaY zeolite system. The functional form of the potential was similar to all others, including a Lennard-Jones function to describe the short-range interactions and a Coulombic repulsion term calculated by Ewald summation. 

Sorption and Diffusion of Light Hydrocarbons and Other Simple Nonpolar Molecules in Type A Zeolites... 

The results of experimental studies of the sorption and diffusion of light hydrocarbons and some other simple nonpolar molecules in type-A zeolites are summarized and compared with reported data for similar molecules in H-chabazite. Henry s law constants and equilibrium isotherms for both zeolites are interpreted in terms of a simple theoretical model. Zeolitic diffusivitiesy measured over small differential concentration steps, show a pronounced increase with sorbate concentration. This effect can be accounted for by the nonlinearity of the isotherms and the intrinsic mobilities are essentially independent of concentration. Activation energies for diffusion, calculated from the temperature dependence of the intrinsic mobilitieSy show a clear correlation with critical diameter. For the simpler moleculeSy transition state theory gives a quantitative prediction of the experimental diffusivity. 

Relaxation times T, and T2 depend on the motion of molecules which contain the nuclei (236) and their measurement often leads to the various kinetic parameters for the adsorbed molecules, the knowledge of which is essential for the understanding of the mechanism of many zeolite-mediated processes. The diffusion coefficient of the reactants and products in a catalytic reaction, which can be determined from NMR, is often rate limiting. Relaxation studies can also determine surface coverage by the sorbed species and provide information about the distribution of adsorption energy between the different sites on the surface of a catalyst. For these reasons a great deal of NMR work has been done with adsorbed species in zeolites in the course of the last twenty years. From the applied viewpoint the emphasis is on water and hydrocarbons as guest molecules from the fundamental viewpoint species such as Xe, SF6, H2, CH4, and NH3 are of special interest. 

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