Sorption and diffusivity characteristics of acid‐leached mordenites

Abstract

The apparent diffusivity for the binary liquid systems benzene-cumene and cyclohexane-cumene in H-mordenite crystals at 25OC to 44OC was increased by an order of magnitude or more after the zeolite was acid-leached to increase the silica-alumina mole ratio from the range of 14 to 18 to the range of 70 to 89. The calculated diffusivities, which range from to 10-'6 cmZ/s, decrease with approach to sorption equilibrium and may be markedly reduced under some circumstances by adsorption of molecules from the bulk liquid, causing blockage of pores.Comparison of sorption capacities as a function of silica-alumina ratio as reported by various investigators indicates that considerable differences exist in the properties of acid-leached mordenites of the same silica-alumina ratio but of different origins. SCOPEH-mordenite and acid-leached mordenites have remarkable catalytic activities for acid-catalyzed reactions, but the rate and selectivity of these reactions can be profoundly affected by the rate of counterdiffusion of product and reactant molecules inside the fine pore structure of these zeolites. Acid leaching removes alumina from the silica-alumina matrix and opens up the pores of mordenites, thereby increasing the rate of diffusion. However, leaching also reduces the concenbation of acid sites, thereby presumably decreasing the intrinsic reactivity of the catalyst. The two effects act in opposite directions on the apparent reactivity of the mordenite under diffusionlimiting conditions. A principle objective of this work was to determine quantitatively the effect of acid leaching on diffusivity in mordenites. Diffusion measurements were made by observing the rate of disappearance of a hydrocarbon species A by adsorption from a dilute solution of A in B into mordenite crystals previously saturated with B-termed here adsorption measurements; or by observing the rate of appearance of B from saturated mordenite into bulk liquid consisting initially of pure A-termed here desorption measurements.Sorption capacities for benzene or cumene vapors were also determined as a function of silica-alumina ratio and compared to previous reports on sorption of these and other hydrocarbons. Sorption capacities are of interest in themselves in connection with adsorption processes, and they also provide some insight into the nature of the fine pore structure.

Liquid diffusion in microporous alumina pellets

“…In all the discussion concerning diffusion in microporous material, it should also be borne in mind that at h >> 0, concept of a continuum within the pore may not be valid, and the use of differential equations in treating diffusion in small pores (that is, for high values of A) may not lie even approximately correct. Satterfield and Cheng (1972), Satterfield and Katzer (1971), Moore and Katzer (1972), and Satterfield and Chiu (1974) reported diffusivity values for organic solutes in zeo!ites several orders of magnitude lower than bulk diffusivitv values and seem to be also influenced by specific interactions between the solutes and the pore walls.…”

Diffusion and partitioning of macromolecules within finely porous glass

“…T h e results of Eagle and Scott ( 1950) qualitatively suggest restricted diffusion. Diffusivities of organic solutes in zeolites (Satterfield and Katzer, 1971;Satterfield and Cheng, 1972;Moore and ICatzer, 1972;Satterfield and Chiu, 1974) are many orders of magnitude lower than bulk values and are greatly influenced by specific interactions between diffusing species and pore walls. In the study of Satterfield e t al.…”

Hydroisomerization of n-C5 and n-C6 mixtures on zeolite catalysts

“…They also state that for n-C5, decreased diffusional resistance associated with the removal of alumina does not affect the catalytic activity. Satterfield and Chiu (1974) also note decreased diffusional resistance with the removal of alumina. They state, however, that considerable differences in pore size distribution and other properties may exist in alumina-deficient mordenites even with the same Si02/Al203 ratio.…”