Distribution of the products from the alkylation of isobutane with butenes at a zeolite catalyst and the reaction mechanism

Abstract

The mechanisms of formation are proposed on the basis of the distribution of the intermediate and final products of zeolite alkylation of isobutane with butenes. The mechanisms are based on activation of the isobutane molecule at the methyl groups, simultaneous intermolecular and intramolecular hydride transfer, and b dissociation during skeletal isomerization of the carbonium ions.The catalytic alkylation of isobutane by butenes constantly attracts interest on account of its great theoretical importance; in the forties of the last century it provided the basis for the fundamental development of Whitmore carbonium-ion theory. The reaction has also been introduced on a large scale in industry in order to utilize the butane-butene fraction from catalytic cracking for the production of a high-octane benzine component consisting mainly of trimethyl-branched pentanes -2,2,3-, 2,2,4-, 2,3,3-, and 2,3,4-TMP. Sulfuric and hydrofluoric acids are used as catalysts. However, in recent decades efforts have made throughout the world to convert the process to solid catalysts, among which the acidic forms of zeolites look the most promising.A special feature of the reaction is the fact that both alkylation itself and the side reactions involving oligomerization of the butenes and secondary alkylation of the finished products take place at the same acid centers, and the oligomerization is realized much more readily than the actual alkylation. The side reactions are suppressed more strongly the higher the isobutane-butene ratio in the transformation zone. The ratios are usually in the range of (5-20) : 1 since it is not easy to maintain them at a higher level by procedure alone. With such ratios the distribution of the reaction products is practically insensitive to the nature of the alkylating butene.At the same time the flow-circulation system, which as far as we know does not have analogs in practical investigation, has been developed [1], making it possible to keep the ratio at a level of several hundreds and even thousands. With such values the alkylating function of the catalyst manifests itself most clearly as a result of maximum suppression of the above-mentioned side reactions. Under these conditions an unequivocal dependence of the distribution of the alkylation products on the nature of the alkylating butene is observed. Thus, for zeolites of the faujasite type with 1-butene and isobutene as alkylating agents the main transformation product is 2,2,4-TMP, whereas for 2-butenes the main products are 2,3,3-and 2,3,4-TMP [1,2]. (For all the butenes under all the alkylation conditions impurity amounts of 2,2,3-TMP are formed.) 0040-5760/11/4704-0205 However, the experimental data on the distribution of the transformation products are not taken into account to a sufficient degree by the existing alkylation mechanisms.Today there are several approaches to the mechanism of formation of TMPs on zeolites.Thus, according to [3,4], at zeolite catalysts the same mechanism is realized as in alkylation in the presence of aluminum chl...

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Obtaining glucose-based 5-hydroxymethylfurfural on large-pore zeolites

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Catalytic Synthesis of Methyl Glycolate from Glyoxal Methanol Solution over Base Catalysts

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