Zeolites deactivation rates

In a series of experimental runs on virgin commercial catalysts and sieves then available and some of our experimental catalysts, we quickly learned that a catalyst impregnated with vanadium, and subjected to high temperatures in steam and air deactivated rapidly. Vanadium, especially in the +5 valence state, rapidly deactivated a catalyst by destroying zeolite crystallinity (Figure 17). In the presence of sodium, the deactivation rate of vanadium was even more severe. (lA-17)... 

ECC catalyst is subject to hydrothermal deactivation. This occurs when the A1 atom in the zeolitic cage is removed in the presence of water vapor and temperature. The result is a loss of activity and unit conversion. The effect of temperature on this process is nonlinear. The deactivation rate increases exponentially with temperature. Units that experience high afterburn have attributed high rates of catalyst deactivation on the higher dilute phase temperatures. This phenomenon is more apparent on units with high combustion air superficial velocities. The high velocity not only increases afterburn, but also increases catalyst entrainment to the cyclones and dilute area. COP is used to decrease afterburn and minimize catalyst deactivation. 

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 gas phase isomerisation of o-dichlorobenzene (odCB) was studied over protonic zeolites HZSM5, HMOR, HMAZ, HOFF, HBETA and a pillared clay HPILC. All the catalysts deactivate. The deactivation rate is the highest when dry air is used as carrier gas, and the lowest with nitrogen... 

Differences between various zeolites and matrix components of the catalyst have been accounted for by fitting coke deactivation rates (exponent n), coking and cracking activities (A and k), and adsorption coefficients (kA and k A). Similar ideas on deactivation of a composite cracking catalyst have been presented by Dean and Dadyburjor (13). Coke on catalyst then becomes the sum of the coke on the matrix and the coke on the zeolite ... 

Estimates of Model Parameters. The reactor models for FFB, MAT and riser include important features for translating the MAT and FFB data to steady state riser performance. A series of key parameters specific to a given zeolite and matrix component are needed for a given catalyst. Such key parameters are intrinsic cracking anc( coking activities (kj, A ), activation energies and heats of reaction (Ej, AHj), coke deactivation rate (exponents nj), and axial dispersion in the FFB unit (DA). Other feedstock dependent parameters include the inhibition constants (kHAj), the coking constants (XAj), and the axial molar expansion factor (a). 

Abstract Alkylation of benzene with ethylene over Y-type zeolite has been carried out under supercritical conditions. Two aspects of the reaction have been paid attention to slowing down the deactivation rate and decreasing the by product selectivity. Experiments have revealed the existence of some coke precursors that are partly removed from the catalyst surface. By product xylenes are decreased and are explained due to high diffiisivity in the supercritical fluid. 

Zeolite Beta has also been studied for isobutane/butene alkylation (65, 66), but it was less selective to the desired TMP than USY, suggesting some diffusional limitations for these highly branched products at the relatively low reaction temperatures used. In fact, an increase of activity was observed when decreasing the crystal size of the Beta zeolite (66). As for USY zeolites, the activity, selectivity and deactivation rate of Beta zeolite were influenced by the presence of EFAL species (67). Medium pore zeolites, such as ZSM-5 and ZSM-11 were also found active for alkylation, but at temperatures above 100°C (68, 69). Moreover, the product obtained on ZSM-5 and ZSM-11 contained more light compounds (C5-C7), and the Os fraction was almost free of trimethylpentanes, indicating serious pore restrictions for the formation of the desired alkylation products. 

Here we refer briefly also to the results of corresponding catalytic measurements performed on the same samples [5,6]. In the toluene disproportionation (770 K, 18.5 vol % toluene in nitrogen) it was found that the overall catalytic activity increases with the iron content (1.8, 3.6 and 5.3 % conversions were determined), as well as the deactivation rate (16.7, 25.0 and 35.8 per cent decreases of the original rates were determined). In selectivity studies the portion of p-xylene among the products decreased with the increase of iron content (32.2, 29.8 and 27.1 per cent selectivities, respectively). (For explanation, it is mentioned that in pure MFI ferrisilicates increased p-selectivity for xylene is expected, due to the comparable size of components and diameter of channels in the zeolite.)... 

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. 

Metal scavengers reported to date have few cracking properties. Thus, their addition to a commercial FCC initially cause a decrease in cracking activity. However, as metals are selectively deposited on the scavenger surface, deactivation rates are significantly reduced and a cross-over point is reached at which the FCC mixture is more active (in the presence of high metals) than the parent (undiluted) FCC (Figure 15). Cross-over points depend on gas oil composition, feed metal levels, and zeolite concentration (and type) in the host FCC and on the properties of the diluents this data must be obtained experimentally. Other important concepts for residuum catalyst development have recently been reviewed by O Connor and coworkers[59]. 

Use of beta zeolite catalyst does not require the benzene feed to be clay treated prior to use in alkylation service. Some of the unsaturated material in the benzene can lead to the formation in the alkylation reactors of polycyclic-aromatic material which will get preferentially trapped in some zeolites having relatively small-sized pores. This can lead to increased deactivation rates in such small-pore zeolites. Beta zeolite s large pore structure makes it possible to more easily handle this polycyclic-aromatic material and as a result does not require further treatment of the benzene feed to remove unsaturated material. In addition, alpha-methylstyrene (AMS) is produced by alkylation of benzene with methylacetylene or propadiene. Some of the AMS alkylates with benzene, forming diphenyl-propane, a heavy aromatic that leaves the unit with the DIPB column bottoms. 

Catalytic performance of copper catalysts based on ZSM5 structure (MFI) were investigated at the exhaust of a lean-bum engine for the NOx reduction. The presence of both Cu and Al resulted indispensable to have catalyst activity in real conditions. While activity remained unchanged for the over-exchanged catalysts, the durability always increased with copper content. Al enhanced both activity and durability. Fast deactivation rate resulted correlated to segregation of small CuO particles dispersed into zeolite channels, as evidenced by characterisation of deactivated catalysts. 

The results of isothermal tests, effected at 400 °C and S.V.=30000 h at the engine exhaust, are presented in Figure 3 in terms of NOx conversions versus time on stream. A positive effect of copper loading on durability properties was observed for both zeolites. In particular, while the initial activity was totally depressed on Z(80)70 and Z(80)221 after about 3 and 12 h, respectively, the deactivation rate of the sample Z(80)536 resulted much lower with respect to the other two catalysts. Similar trend was presented by Z(25)67 and Z(25)166, the last sample resulting the most stable between those considered. 

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... 

Several conclusions may now be drawn from the present work. First of all, the crystallite size plays an important role in the diffusivity of reactant or product molecules, and subsequently in catalytic shape selectivity and deactivation rate. When the crystallite size increases, diffusion rate decreases, while shape selectivity and deactivation rate increase. These features obviously arise from an enhancement in the molecule pathway within the zeolite matrix, and from a decrease in the number of pore openings per unit weight when the crystallite size increases. 

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. 

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