Rates of counterdiffusion are markedly affected not only by the critical molecular diameter of the diffusing molecules but also by other physicochemical properties of the liquids and the nature of the cation in the zeolite. The effective diffusivity in NaY or HY under counterdiffusion conditions is less by a factor of 10-1 to 10-3 than that for unidirectional diffusion into an initially empty pore structure. SCOPEThe factors affecting the rates of unidirectional diffusion and counterdiffusion of hydrocarbons in molecular sieve zeolites are of significant importance in conjunction with the use of zeolitic catalysts in reactors and of zeolitic sorbents in separation processes. Our previous studies of diffusion in zeolites include that of the desorption and counterdiffusion behavior of liquid benzene and cumene in H-mordenite , the single-component (unidirectional) diffusion of selected organic liquids in type Y zeolites (Satterfleld and Cheng, 1972a), equilibria of selected binary organic liquid systems in type Y zeolites (Satterfield and Cheng, 1972b) and the effect on counterdiffusion of liquid hydrocarbons of the nature of the cation present in type Y zeolites .The present study focused on elucidating two aspects of the overall problem: 1. the relationship between the size and physicochemical properties of the diffusant molecule and its rate of diffusion in type Y zeolite in either the Na or H form and 2. the relationship between unidirectional, that is, single component, diffusion and counterdiffusion in binary systems. CONCLUSIONS A N D SIGN I FICANCECounterdiffusion of hydrocarbons in zeolites will not be observed if either adsorption equilibrium does not provide a significant driving force or the size and physicochemical properties of one of the diffusants is such that although it can penetrate an initially empty pore structure its mobility is very slight in the presence of another component in the pores. In the latter case a presorbed compound will not diffuse out even if equilibrium favors its desorption. The diffusion coefficient of any compound under counterdiffusion conditions is less than that under unidirectional diffusion because of molecule-molecule interactions between counterdiffusing species. The decrease in diffusivity for counterdiffusion as compared to unidirectional diffusion markedly affected by the nature of the second substance and by the nature of the zeolitic cation. EXPERIMENTThe NaY and HY and the carefully purified organic species were the same as those used and described previously (Satterfield and Cheng, 1972a, b ) . A description of the apparatus and experimental method has also been published . The HY was prepared by ion exchange of the NaY with a large excess of 10 wt. % ammonium chloride solution at 70°C. By repeated contacting, 97.7% of the Na was exchanged to NH4, as determined by analyses for sodium content by the Central Analytical Facility at M.I.T. The exchanged zeolite was converted to the HY form by spreading it in a thin layer in a container in an oven and heating it ...
On NaY and HY, aromatic compounds are selectively adsorbed in preference to paraffins and naphthenes. Smaller aromatic compounds are adsorbed in preference to larger aromatic compounds. All the compounds studied can be sorbed into the pore structure of the zeolites and the selectivities are primarily a reflection of relative affinity for the zeolite and steric effects rather than a sieving effect.
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