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...
Template-containing silica-based MCM-41 and MCM-50 materials have been shown to have microporous adsorbent properties, with the characteristic adsorption energy in such micropores being strongly dependent on the molecular mass of the adsorptives employed. The total adsorption volume was found to decrease as the molar volume of the adsorptive increased. A model explaining the peculiarities of the adsorptive properties of the templatecontaining materials is proposed.
We have observed motion of coke in deactivated HZSM-5 with the displacement vector alternately directed toward the outer surface of the zeolite crystals and into the interior of the zeolite structure. The driving force for the oscillations is the higher chemical potential of massive carbon µ m at 500°C than for carbon clusters. The switching mechanism involves an alternating jumpwise change in the value of Dµ.Key words: deactivated HZSM-5 zeolite, micro-oxidation of coke, coke localization, oscillatory behavior of coke, driving force for oscillations, switching mechanism for oscillations.Zeolites are modern catalysts for a number of key carbonium ion processes in petroleum refining and petroleum chemistry. Deactivation of zeolites occurs mainly as a result of the formation of coke, blocking the active sites. Publications in recent years [1-8] confirm the importance of studying coke formation. Study of coke formation, the chemical composition of coke and its behavior in the deactivated sample, and the fine details of oxidation is definitely not a simple problem, since there have been no methods making it possible to differentiate between outer-surface and intraporous coke, not to mention coke localization on individual structural elements of zeolites. The only method allowing us to obtain such information is the method of discontinuous sequential micro-oxidation of coke [9-11], which makes it possible to separate the oxidation process into a number of sequential but easily distinguishable stages, where the first stage is oxidation on the outer surface of the zeolite microcrystals and the final stage is oxidation in the very smallest elements of the zeolite structures [10,11].The aim of this work was to use the method of discontinuous sequential micro-oxidation of coke to study zeolite HZSM-5 that has been deactivated and aged for different time periods, in order to establish the behavior of the coke formed. EXPERIMENTALThe HZSM-5 sample was prepared from NaZSM-5 produced by Sorbent AO (Nizhnii Novgorod, Russia, technical specifications TU 38.102168-85, Si/Al = 20.5, static adsorption capacity for water vapor equal to 0.07 g H 2 O per gram). We used ion exchange to do this, replacing the sodium of the original ZSM-5 with ammonium from a 3 M NH 4 NO 3 solution, followed by calcination of the ammoniated form at 550°C. The powdered HZSM-5 formed was tabletted without binders at a pressure of 2·10 4 kg/cm 2 .The catalyst (4 cm 3 , 1-2 mm fraction) was placed in a stainless steel flow-through reactor and dehydrated (2 h) in a stream of air at 500°C. 256
Three HLaCaNaX samples with approximately the same cation content but differing in their IR spectral characteristics were synthesized under different conditions. Testing these samples in the alkylation of isobutane by butenes showed that the catalyst displaying IR bands at 3540 and 3610 cm -1 have the greatest efficiency.The acidity of zeolite catalysts is the determining factor in activity in carbonium ion processes [1,2]. The alkylation of isobutane by butenes yielding 2,2,4-, 2,2,3-, 2,3,4-, and 2,3,3-trimethylpentanes (TMP), which are high-octane gasoline components, holds considerable practical and theoretical interest [3,4]. The replacement of presently used but outdated and ecologically dangerous alkylation techniques with concentrated sulfuric acid and hydrogen fluoride by modern heterogenous catalytic technologies is an important current problem [5]. In this regard, greatest interest is found in faujasite zeolite catalysts. As a rule, polycation-decationized forms of zeolite type-Y are preferred among such faujasites [6]. However, a significant difference in relative efficiency between types X and Y has not been found.Nevertheless, HLaCaNaY and HLaCaNaX zeolites have nonidentical IR spectra in the OH bond stretching region (3500-3800 cm -1 ). Thus, for example, while the HLaCaNaY samples absorb at 3540 and 3650 cm -1 (depending on the preparation conditions and their cationic occupancy, these bands may shift in the range of 10-15 cm -1 [7]), an additional band is found at 3610 cm -1 in the IR spectrum of HLaCaNaX [8,9].The band at 3650 cm -1 , which disappears after chemisorption of, for example, pyridine, is related to acid hydroxyls in large cavities, while the band at 3540 cm -1 , which remains in the spectrum, is related hydroxyls at sites inaccessible for pyridine molecules [7]. The band at 3610 cm -1 has not yet been assigned.Hence, it was of interest to synthesize HLaCaNaX samples differing in the IR bands at 3550, 3610, and 3650 cm -1 and study these samples as alkylation catalysts. EXPERIMENTALThree portions of synthetic zeolite NaX (provided by Sorbent, Nizhnii Novgorod, Russia, SiO 2 /AlO 2 = 2.3) were subjected to ion exchange modification to give acid forms of HLaCaNaX with optimal cation composition [10]. The ion exchange sequence was the same for all the samples. The procedure entailed exhaustive exchange of starting calcium by 192 0040-5760/05/4103-0192
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