Zeolitic Paraffin isomerization Catalysis

An optimum isomerization activity versus Si/Al2 ratio was reported for mordenite [5,13,16,17]. At too high an acid density, the acid strength decreases and olefin oligomerization reaction is favored. On the other side, too few acid sites impair activity. Reported optimum Si/Al2 ratios are around 20, which is close to the value 

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]. 

Like in any catalytic process, process variables crucially impact reaction kinetics, conversion efficiency and catalyst stability. Increasing temperature favors cracking, thus decreasing the isomerate yield. It is preferred to have a high-activity catalyst and operate at the lowest possible temperature to achieve the highest RONC. Hydrogen shifts the equihbrium concentrations of olefins and carbenium ions. 

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As in previous conferences, the section on catalysis contains the most papers. A general review of the different reactions which can be catalyzed by zeolites is presented by Kh. M. Minachev. H. W. Kouwenhoven discusses the isomerization of paraffins on zeolites. Cracking, isomerization, and electron transfer reactions are discussed in several papers. Correlations between particular activities and physicochemical properties are covered. Selectivities related to crystal size and molecular shapes are also studied. Most of the work is still done on modified Y zeolites, but mor-denite and erionite also receive attention. 

The molecular sieve zeolites have attained great technological importance for catalysis, gas separation and drying and many other applications. They are now used as industrial catalysts for such reactions as the cracking of paraffins and the isomerization and disproportionation of aromatic compounds (Thomas and Theocharis, 1989 Thomas, 1995, Martens etal., 1997). 

The early use and success of molecular sieve catalysis was spurred by the dramatic improvement in activity selectivity for catalytic cracking of vacuum gas oil achieved by using the faujasite based catalysts in comparison to the previously used amorphous SiOj/AUOj. These catalysts had a factor of about 10 -10 higher catalytic activity than the amorphous SiOj/AfrOj catalysts [42]. Paraffin, C4 to C8 isomerization [43] was one of the first successful non-petroleum processing applications using zeolite catalysts. The complexity of tailoring zeolite catalysts, however, is well illustrated by the fact that is only four years back that Shell has developed the first zeolite based process for isomerization of n-butene to isobutene [44]. 

Realizations of such microcompartments are obtained in normal heterogeneous catalysis by using zeolite crystals as support material, e. g., in the formation of are-nes from cycloparaffins by use of Y-zeolite crystals as catalysts [15] or the hydro-isomerization of light paraffins by Pt-doped Y-zeolite [16]. Concentration effects, resembling channeling in enzyme-catalyzed reactions, are caused by the hindrance of the transport of larger molecules through the apertures between the cavities which form the three-dimensional pore texture of zeolite crystals. 

Medium pore aluminophosphate based molecular sieves with the -11, -31 and -41 crystal structures are active and selective catalysts for 1-hexene isomerization, hexane dehydrocyclization and Cg aromatic reactions. With olefin feeds, they promote isomerization with little loss to competing hydride transfer and cracking reactions. With Cg aromatics, they effectively catalyze xylene isomerization and ethylbenzene disproportionation at very low xylene loss. As acid components in bifunctional catalysts, they are selective for paraffin and cycloparaffin isomerization with low cracking activity. In these reactions the medium pore aluminophosphate based sieves are generally less active but significantly more selective than the medium pore zeolites. Similarity with medium pore zeolites is displayed by an outstanding resistance to coke induced deactivation and by a variety of shape selective actions in catalysis. The excellent selectivities observed with medium pore aluminophosphate based sieves is attributed to a unique combination of mild acidity and shape selectivity. Selectivity is also enhanced by the presence of transition metal framework constituents such as cobalt and manganese which may exert a chemical influence on reaction intermediates. 

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