Two samples of pentasil hydrogen forms were obtained: an ordinary sample and a sample lacking surface acid sites. The samples were tested in the disproportionation of toluene over short and very short periods of contact of the reaction mixture with the catalyst layer. The primary conversion products were found to be para-xylene and ethylbenzene in addition to benzene.

Along with oxidation of hydrogen-containing coke structures, we have observed oxidation of compact accumulations of hydrogen-free coke and as a result we can quantitatively differentiate between outer-surface and intraporous burned coke, and the intraporous coke in turn can be quantitatively distributed over the different elements of the zeolite structures.Coke formation is a very important reaction accompanying all carbonium-ion conversions of hydrocarbons. In this case, the coke plays a dual role. In small amounts, it promotes the occurrence of the major reaction [1], speeding up the movement of hydrogen, as a primary ingredient in carbonium ion conversions in all its forms [2]. But when accumulating on the surface of the catalyst, coke blocks the active sites, necessitating the process step of oxidative regeneration. Therefore information is needed about the chemical composition of coke and its localization within the crystal structure of the catalyst. Such information is complicated to obtain because of the indefinite composition of coke at a specific instant of time and also difficulties encountered in extracting this substance in unaltered form from the catalyst, as much as the at least equal difficulties involved in studying it without extraction [3,4].Earlier [5], in a study of the kinetics of regeneration of single grains of coked zeolite by the microbalance method in a stream of oxygen-containing gas in the temperature range 350-510°C, some variability (periods of slowdown and acceleration) in combustion over time was observed at 350, 360, and 375°C, which may indicate oxidation of coke of different elemental compositions or coke localized differently in the zeolite structure.The method of discontinuous sequential micro-oxidation of coke [6] was developed recently, which makes it possible to determine some characteristic features of coke composition and coke localization in the structure of acid zeolite catalysts for different purposes: alkylation of isoparaffins by olefins, isomerization of linear paraffins, disproportionation of monoalkyl aromatic hydrocarbons to form benzene and dimethyl-substituted aromatics.Each of these catalysts is prepared differently: by modification of the original forms by replacing the native sodium cations by other cations [7] or by reducing the latter (for example, nickel cations) to the zero-valence state [8]. Methods for such modification have been sufficiently developed.Considerably less attention has been focused on dealumination of the outer surface of zeolite crystals to avoid formation of outer-surface acid sites while grafting acidity into the modified samples. Moreover, these sites, as the most accessible to reactant molecules, may play a dominant role in the occurrence of the corresponding reactions, including coke 198 0040-5760/09/4503-0198

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...