Acidic zeolites, deactivation rates

Traditionally, solid acidic catalysts are applied in industry for the oligomerization of butenes and are still studied. MTS-type aluminosilicates,522 a NiCsNaY zeolite,523 and a silica-alumina containing 13% alumina524 proved to be active and selective catalysts. Moreover, deactivation rates of these catalysts are also favorable. Sulfated zirconia promoted with Fe and Mn was active and selective to yield primarily dimethylbutene isomers under supercritical conditions.525 A small amount of water improved productivity and decreased deactivation. A study showed that the blending octane number of Cg hydrocarbons is directly linked to the number of allylic hydrogens in the molecules.526... 

The disproportionation of ethylbenzene (EB) to benzene (B) and diethylbenzenes (DEB) is an acid-catalysed reaction that occurs readily on a great variety of acid zeolites [1-6]. Under appropriate conditions, the reaction proceeds without deactivation with a rate which is proportional to the number of Bronsted sites it can be performed at atmospheric pressure in simple fixed-bed flow reactors. Thus, the reaction offers itself as a test reaction for assessing the catalytic activity of zeolites [3-4]. At low temperatures, i.e., below 535 K, excellent stoichiometry to equimolar quantities of benzene and diethylbenzene is observed ... 

In a previous paper [5] the conversion of sec-butylbenzene (sBB) was studied under atmospheric pressure over some acidic zeolite catalysts. The reaction turned out to be rather complex, involving many parallel and/or successive steps. Among the reaction products, the iso-butylbenzene is the most interesting one, being an important intermediate for die Ibuprofen preparation [6]. Those results showed that the overall conversion decreased for all the catalysts, with different deactivation rates. Dealkylation was the prevailing reaction and the isomerization product, the iso-butylbenzene was formed in a larger amount on the HY catalyst. 

This paper deals with the hydrothermal deactivation, under an air + 10 vol. % H2O mixture between 923 and 1173 K, of Cu-MFI solids, catalysts for the selective reduction of NO by propane. Fresh and aged solids were characterized by various techniques and compared with a parent H-ZSM-5 solid. The catalytic activities were measured in the absence and in the presence of water. The differences between fresh and aged Cu-ZSM-5 catalysts (destruction of the framework, extent of dealumination...) were shown to be small in spite of the strong decreases in activity. Cu-ZSM-5 is more resistant to dealumination than the parent H-ZSM-5 zeolite. The rate of NO reduction into N2 increases with the number of isolated Cu VCu ions. These isolated ions partially migrate to inaccessible sites upon hydrothermal treatments. At very high aging temperatures a part of the copper ions agglomerates into CuO particles accessible to CO, but these bulk oxides are inactive. Under catalytic conditions and in the presence of water, dealumination is observed at a lower temperature (873 K) than under the (air + 10 % H2O) mixture, because of nitric acid formation linked to NO2 which is either formed in the pipes of the apparatus or on the catalyst itself... 

ZSM-5 zeolite catalysts are well known for their shape selective and acidic properties, and low deactivation rates in efficient transformation of a number of hydrocarbon molecules[3-5]. Xylene isomerization. Toluene disproportionation. Methanol to gasoline and olefins, M-2 forming are some of the important ZSM-5 based processes[6-l 1]. These catalysts are also known to increase LPG range products when they are used as FCC additives. These considerations lead us to the development of ZSM-5 based catalysts such, that optimization of LPG or gasoline can be made by suitable choice of modifying procedure such as acid modification or metal modification[12-17j. 

Influence of the addition of silica, as a binder at a concentration of 10 or 50 wt%, to H-gallosilicate (MFI) zeolite on its inter- and intracrystalline acidity, initial activity, product selectivity and distribution of aromatics formed in the propane amortization (at 550°C) and also on its deactivation due to coking in the aromatization process has been thoroughly investigated. Silica binder caused an appreciable decrease in the zeolitic acidity (both external and intracrystalline acid sites) and also in the propane conversion/aromatization activity. Because of it, the deactivation due to coking of the zeolite in the propane aromatization is, however, decreased. The deactivation rate constant for the initial fast deactivation is decreased but that for the later slow deactivation is increased because of the binder. The aromatics selectivity for aromatics and para shape selectivity of the zeolite, particularly at lower conversions, are increased but the propylene selectivity and dehydrogenation/cracking activity ratio are decreased due to the presence of binder in the zeolite catalyst. 

Both the intracrystalline and intercrystalline (or external) acid sites of the zeolite are decreased by the silica binder. The changes in the intracrystalline acidity of the zeolite are reflected in its propane aromatization activity the activity is reduced significantly by the silica binder. The aromatics selectivity and the dehydrogenation / cracking and aromatization / cracking activity ratios and aromatization/(methane + ethane) mass ratio are also affected appreciably by the silica binder. The shape selectivity of the zeolite is increased markedly by the silica binder. Also because of the binder, the deactivation rate constant for initial fast deactivation is decreased but for the later slow deactivation is increased. 

In situ poisoning experiments were carried out on MgY zeolite by doping the toluene/methanol mbdure with either an acid (acetic acid) or a lase (3,5-dimethyl pyridine). Fresh catalysts were initially tested for about 3 h using a pure toluene/methanol mixture before introducing the doped feed. The activity was defined as a = ro/ro, where r and r(t) are the reaction rates at zero time and time t, respectively. The MgY activity diminished with time on stream when using undoped reactants because of the formation of carbonaceous deposits. Hence, when a teic confound is added into the feed, a simultaneous deactivation process by coke and poison takes place. The activity decay caused by 3,5-dimethyl pyridine (3,5-DMP) alone can not be obtained directly fiom the experimental data. To estimate the poison intrinsic effect, it can be assumed that both effects are additive, which implies that the overall deactivation rate is a simple sum of each individual rates (hypothesis of independence) [19]. According to mechanistic deactivation models [20], the overall deactivation rate is expressed as follows ... 

The introduction of ultrastable Y zeolites as the acid component (74) even if producing lower middle-distillates than amorphous catalysts, they show a better temperature performance, i.e. zeolite catalysts exhibits higher start-ofr run activity and lower deactivation rates (Fig. 16). 

The effect of surface acidity on the behavior of Fe-MFI zeolite catalysts with different Si/Al and Si/Fe ratios for benzene hydroxylation to phenol has been studied by Selli et al. [190] using FTIR and microcalorimetric analysis of adsorbed pyridine. The behavior of the catalysts in terms of activity and deactivation rate is discussed in relation to the nature, concentration and strength of the surface acid sites. Surface acidity, though not involved directly in the hydroxylation reaction, plays a major role in determining the lifetime of the catalyst. 

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. 

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