Isomerization-cracking selectivity ratio

A variety of solid acids besides zeolites have been tested as alkylation catalysts. Sulfated zirconia and related materials have drawn considerable attention because of what was initially thought to be their superacidic nature and their well-demonstrated ability to isomerize short linear alkanes at temperatures below 423 K. Corma et al. (188) compared sulfated zirconia and zeolite BEA at reaction temperatures of 273 and 323 K in isobutane/2-butene alkylation. While BEA catalyzed mainly dimerization at 273 K, the sulfated zirconia exhibited a high selectivity to TMPs. At 323 K, on the other hand, zeolite BEA produced more TMPs than sulfated zirconia, which under these conditions produced mainly cracked products with 65 wt% selectivity. The TMP/DMH ratio was always higher for the sulfated zirconia sample. These distinctive differences in the product distribution were attributed to the much stronger acid sites in sulfated zirconia than in zeolite BEA, but today one would question this suggestion because of evidence that the sulfated zirconia catalyst is not strongly acidic, being active for alkane isomerization because of a combination of acidic character and redox properties that help initiate hydrocarbon conversions (189). The time-on-stream behavior was more favorable for BEA, which deactivated at a lower rate than sulfated zirconia. Whether differences in the adsorption of the feed and product molecules influenced the performance was not discussed. 

Introduction of Pt significantly enhances zeolite isomerization catalyst stabiUty and alters the reaction pathways. The Pt/acid ratio not only changes the isomeriza-tion/cracking ratio, but also changes the ratio of mono/di-branched isomers in Pt/Y [14]. High Pt dispersion and close proximity to acid sites correlate with high n-hexane conversion as well as high isomerization selectivity [20, 21]. 

From this example it is clear that the selectivity for (a) dehydrogenation, (b) isomerization, and (c) cracking is likely to be related to the relative concentrations of mono-, di-, and tri-adsorbed complexes, etc. More generally, the selectivity of a catalytic reaction will depend on the relative chance for a molecule adsorbed on -surface atoms either to desorb or become adsorbed on (n + 1) surface atoms. This idea easily permits us to understand that dilution of an element A, capable of forming chemisorption bonds with a given molecule, with an inert element B will lower the ratio of poly- to monoadsorbed molecules and have an effect on catalytic selectivity. We will call this concept the primary ensemble effect. 

Very high internal surface area zeolites (lO m /g) can be synthesized with controlled pore sizes of 8-20 A and controlled acidity [(Si/Al) ratio]. These find applications in the cracking and isomerization of hydrocarbons that occur in a shape-selective manner as a result of the uniform pore structure and are the largest-volume catalysts utilized in petroleum refining at present [20]. They are also the first of the high-technology catalysts where the chemical activity is tailored by atomic-scale study and control of the internal surface structure and composition. 

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