Isomerization selectivity

Chlorohydrins from Epoxides. Traditionally epoxides have been manufactured by the dehydrochlotination of chlorohydrins. However, the reverse reaction may be used as a source of chlorohydrins, especially ia the case of ethyleae chlorohydria from ethyleae oxide [75-21-8] which is aow produced by the direct oxidatioa of the olefia. A study of the reactioa of hydrogea chloride with propyleae oxide [75-56-9] showed that an anhydrous system at low temperatures (<0° C) gives the highest yield of chlorohydria with best isomeric selectivity (16). 

Figure 1. Isomerization selectivity on Pt/WOx-Zr02 (0.3% wt. Pt, 12.7% wt. W, H-atoms formed from H2 or adamantane on Pt, 073 calcination) and Pt/SOx-ZrOj sites are involved in carbocation hydrogen (0.4% wt. Pt, 4.5% wt. S, 723 K...

We compared Pt/silica-alumina, yttria-modified silica-alumina, and fluorided alumina for n-Ci2 isomerization. Not surprisingly, increasing yttria content lowers catalyst activity at a fixed space velocity (Fig. 10). The 9% Y203/Si-Al catalyst compares closely to the 1%F/Al203 catalyst in activity. Of the catalysts evaluated here, the 9%Y203-loaded Si-Al had higher isomerization selectivity at equal conversion (Fig. 11). 

Recent work [4] showed that EU-1, an intermediate pore size (10MR) monodimensional zeolite, leads to very high isomerization selectivity during EB conversion. This results from the blockage by carbonaceous deposits of the access to the inner sites of micropores. 

The objective of this work is to determine the influence of the porous structure (size and shape) and acidity (number and strength of the acid sites) on isomerization selectivity during the conversion of ethylbenzene on bifunctional catalysts PLAI2O3/ 10 MR zeolite. The transformation of EB was carried out on intimate mixtures of Pt/Al203 (PtA) and 10 MR zeolites (ZSM-5, ZSM-22, Ferrierite, EU-1) catalysts and compared to Mordenite reference catalyst activity. 

As previously observed [4] on EU-1 catalyst deactivation leads to isomerization selectivity improvement (table 2) whereas dealkylation and disproportionation selectivity decreases. The same effect is observed for ferrierite and ZSM-22 catalysts to a lesser extent. Isomerization selectivity reach more than 70% for these catalysts after 8... 

Initial inner acid sites isomerization selectivity is low for 10MR zeolites and high for Mordenite catalysts. This suggests that large 12MR channels of Mordenite are favorable to EB isomerization into xylenes in the zeolite microporosity. 

As for 10MR ID zeolites, the isomerization selectivity improvement is correlated with the microporosity plugging, it is proposed that EB isomerization on these coked catalyst mainly occurs on the outer surface acid sites. 

We have examined the rate constants for disproportionation and isomerization for a variety of zeolites, using a commercial-type feed containing 70% m-xylene and 30% o-xylene in a fixed-bed flow reactor. The results, listed in Table I, show the exceptionally low disproportionation/isomerization selectivity of ZSM-5 relative to synthetic faujasite. Synthetic mordenite and ZSM-4 have intermediate selectivities. 

Figure 1. Comparison of the relative disproportionation versus isomerization selectivities of HZSM-5 and synthetic faujasite (8). Feed m-xylene. Temperature 300°C.

The best correlation of the observed isomerization selectivities was found in terms of the diameter of the intracrystalline cavity, determined from the known crystal structure (9) of these zeolites, as shown in Figure 2. While faujasite, mordenite and ZSM-4 all have 12-membered ring ports and hence should be similar in their diffusion properties, they differ considerably in the size of their largest intracrystalline cavity both mordenite and ZSM-4 have essentially straight channels, whereas faujasite has a large cavity at the intersection of the three-dimensional channel system. 

The correlation between selectivity and intracrystalline free space can be readily accounted for in terms of the mechanisms of the reactions involved. The acid-catalyzed xylene isomerization occurs via 1,2-methyl shifts in protonated xylenes (Figure 3). A mechanism via two transalkylation steps as proposed for synthetic faujasite (8) can be ruled out in view of the strictly consecutive nature of the isomerization sequence o m p and the low activity for disproportionation. Disproportionation involves a large diphenylmethane-type intermediate (Figure 4). It is suggested that this intermediate can form readily in the large intracrystalline cavity (diameter. 1.3 nm) of faujasite, but is sterically inhibited in the smaller pores of mordenite and ZSM-4 (d -0.8 nm) and especially of ZSM-5 (d -0.6 nm). Thus, transition state selectivity rather than shape selective diffusion are responsible for the high xylene isomerization selectivity of ZSM-5. 

In the case of PCI/AI2O3 catalysts the morphology of the metal particles is also important because it determines the hydrogenation and isomerization selectivity. On flat metal surfaces isomerization is preferred whereas rough surfaces are more active in hydrogenation129. 

Figure 13.28 Meto-xylene disproportionation isomerization selectivity ratio over various zeolites at 31 7-318°C and 10% conversion [64].

Introduction of Pt significantly enhances zeolite isomerization catalyst stabiUty and alters the reaction pathways. The Pt/acid ratio not only changes the isomeriza-tion/cracking ratio, but also changes the ratio of mono/di-branched isomers in Pt/Y [14]. High Pt dispersion and close proximity to acid sites correlate with high n-hexane conversion as well as high isomerization selectivity [20, 21]. 

Inevitably, the zeolites with a Spaciousness Index between 2 and 16 have a higher Si/Al ratio than zeolites Y or ZSM-20. To separate the effects of the Si/Al ratio and the pore width, two dealuminated samples of zeolite Y, designated YDl and YD2, were tested which resembled in their Si/Al ratios the Beta and EU-1 sample, respectively. Table 1 gives the isomerization selectivities at 40 % conversion. While dealumination of zeolite Y brings about some improvement in selectivity, H-Beta is clearly superior to HYDl, and H-EU-1 is a much better catalyst than HYD2, which indicates that the pore width in the appropriate range is of prime importance. 

Table 1. Conversion of 1-methylnaphthalene on various zeolite catalysts at 300 °C. Influence of the effective pore width, expressed by the Spaciousness Index (SI, [41, 42]), and the Si/Al ratio on the isomerization selectivity (the time on stream was chosen in such a manner that Xi-M-Np amounted to 40 %).

Hydrocarbons higher than C3 can also undergo isomerization on Pt, running in parallel to hydrogenolysis. With butane or isobutane the selectivity for isomerization is rather low, also on Pt, but the higher hydrocarbons show more of isomerization reactions. With higher hydrocarbons some other metals (Ir, Pd) also show some isomerization selectivity. 

The techniques of monomolecular rate theory easily transform measured reaction data into a form where we can analyze apparent kinetics and the effects of intracrystalline diffusion by the use of selectivity data. Time dependency has been eliminated. Since selectivity is extremely reproducible and is independent of short-term aging effects, the number of experimental runs is reduced while data reliability is maintained. For catalyst evaluation at any temperature, it is necessary to determine the equilibrium composition and the straight-line reaction path. With this information any catalyst can be evaluated at this temperature with simply the additional information from a curved-line reaction path. The approach used in the application of monomolecular rate theory to the xylene isomerization selectivity kinetics is as follows. Reference is made to the composition diagram, Figure 1. 

Applicability of Monomolecular Rate Theory to Xylene Isomerization Selectivity Kinetics over Fresh AP Catalyst. The kinetics of liquid-phase xylene isomerization over fresh zeolite containing AP catalyst are effectively interpreted by pseudomonomolecular rate theory. The agreement between the experimental data (data points) and predicted reaction paths (solid lines) for operation at 400° and 600°F is shown in Figure 2. The catalyst used was in the form of extrudates comprised of the zeolite component and an A1203 binder. Since xylene disproportionation to toluene and trimethylbenzenes was low, selectivity data were obtained by mere normalization of the xylene compositions (2 axyienes = 1.0). 

The production of butadiene from butene involves at least three surface intermediates adsorbed butene, 7t-allyl, and butadiene. One or more of these may be particularly vulnerable to attack by gas-phase oxygen on a-Fe203. From the temperature programmed desorption experiments, it was found that the products of isomerization, selective oxidation, and combustion... 

The effect of pressure on the isomerization of n-heptane and n-octane was determined over the Pt//l-zeolite, Mo2C-oxygen-modified and M0O3-carbon-modified catalysts. The weight hour space velocity (WHSV) was changed with the pressure to keep the conversion at a similar level, enabling the effect on the isomerization selectivity and the product distributions to be seen. Other conditions were kept constant. 

Figure 20.8 (a and b) show a comparison of the Pt// -zeolite catalyst which activated molybdenum oxycarbide catalysts, prepared from both Mo2C and Mo03, for the isomerization of n-heptane and n-octane at elevated pressure. For the platinum catalyst the chain length is important, as explained above, with the isomerization selectivity obtained for n-octane dropping more quickly with increasing conversion than that for n-heptane over this plantinum catalyst the isomerization of n-hexane can be... 

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