Xylenes, catalyzed isomerization

MVPI [Mobil vapor phase isomerization] A process for converting mixed xylene streams to />xylene, catalyzed by the zeolite ZSM-5. Invented by Mobil Corporation in 1973, later superseded by MHTI. See also LTI, MHTI, MLPI. 

Effect of Zeolite Crystallite Size on the Selectivity Kinetics of the Heterogeneous Catalyzed Isomerization of Xylenes... 

Catalytic activity measurements and correlations with surface acidity have been obtained by numerous investigators. The reactions studied most frequently are cracking of cumene or normal paraffins and isomerization reactions both types of reactions proceed by carbonium ion mechanisms. Venuto et al. (219) investigated alkylation reactions over rare earth ion-exchanged X zeolite catalysts (REX). On the basis of product distributions, patterns of substrate reactivity, and deuterium tracer experiments, they concluded that zeolite-catalyzed alkylation proceeded via carbonium ion mechanisms. The reactions that occurred over REX catalysts such as alkylation of benzene/phenol with ethylene, isomerization of o-xylene, and isomerization of paraffins, resulted in product distribu-... 

Xylene inter-isomerization is a well-known acid catalyzed reaction. Over acid catalysts disproportionation is generally observed as a side reaction. Over the bifunctional catalysts used in the commercial processes xylenes can also undergo two other primary reactions ... 

The expert system we are developing concerns several aromatic reactions of great industrial interest [12-23], e.g., alkylation and isomerization of aromatics such as benzene, toluene, and xylene, catalyzed by ZSM-5, mordenite, and Y... 

Friedel-Crafts alkylation reactions are, in general, accompanied by isomerization processes. Olah et a/. reported the results of the water-promoted, AlCb-catalyzed isomerization of o-, m- and p-di-f-bu-tylbenzene. No ortho isomer was present in the equilibrium mixture. The isomerization of o-di-r-bu-tylbenzene was very rapid largely due to relief of steric strain. In these and other related sterically hindered arenes, intramolecular isomerization and not dealkylation was observed. Isomerization of di-and mono-methylnaphthalenes, catalyzed by HF-BF3, was also reported. Isomerization of /i-alkyl-toluenes and -xylenes, catalyzed by AICI3 at room temperature, afforded chiefly /n-/i-alkyltoluenes and /n-/i-alkylxylenes, respectively. The process leading to the meta isomer has a lower energy than the other processes. 

During the early 1970s, Clarence D. Chang, Anthony J. Silvestri, and William Lang, at Mobil Research and Development, discovered Zeolite ZSM-5, one of the most important catalysts ever produced. Zeolite ZSM-5 is a superacidic substance that catalyzes isomerization of ortho-and meta-xylenes to para-xylene the latter s cylindrical shape helps it to diffuse much more rapidly through the zeolite matrix. Quite by accident, they discovered that ZSM-5 catalyzes the conversion of methanol to... 

When bifunctional Pt-zeolite catalysts are used, besides the two Brdnsted catalyzed mechanisms presented above, xylenes can isomerize through partially hydrogenated intermediates (170) (Fig. 34). In this case cracking of the intermediates can also occur diminishing the selectivity to xylenes. 

Consiunption of acid OH groups was also proven by the decrease in the activity of the heat-treated In203/H-BETA samples in the acid-catalyzed isomerization of m-xylene in comparison with equally treated piue zeolite H-BETA. 

Frequently, regioselectivity is compromised by side reactions catalyzed by the HCl byproduct. For example, acetylation of p-xylene by treatment with AICI3 followed by AC2O (CS2, A, 1 h) produces a 69 31 mixture of 2,5-dimethylacetophenone and 2,4-dimethylacetophenone, formation of the latter being indicative of competitive acid-catalyzed isomerization of p-xylene to OT-xylene. Also, although acetylation of anthracene affords 9-acetylanthracene regioselectively, if the reaction mixture is allowed to stand for a prolonged time prior to work-up (rt, 20 h) isomerization to a mixture of C-1, C-2, and C-9 acetyl derivatives occurs. ... 

The isomerization of xylenes catalyzed by SAPO-5 type crystals with compositions consisting of aluminum, phosphorus, and, in several cases, additional elements, including Ti, Mg, Zn and Fe has been described [4]. The data show that these large-pore materials have catalytic activity for the isomerization of xylene mixtures. 

Mass transport selectivity is Ulustrated by a process for disproportionation of toluene catalyzed by HZSM-5 (86). The desired product is -xylene the other isomers are less valuable. The ortho and meta isomers are bulkier than the para isomer and diffuse less readily in the zeoHte pores. This transport restriction favors their conversion to the desired product in the catalyst pores the desired para isomer is formed in excess of the equUibrium concentration. Xylene isomerization is another reaction catalyzed by HZSM-5, and the catalyst is preferred because of restricted transition state selectivity (86). An undesired side reaction, the xylene disproportionation to give toluene and trimethylbenzenes, is suppressed because it is bimolecular and the bulky transition state caimot readily form. 

A unique isomerization has been observed on heating 5-acctyl-2-(cycloalkylamino)-3//-azepines, e.g. 24, in solution in their respective amines at 170 C.119 Rearrangement, via a 3-azabicyclo[4.1.0]hepta-2,4-diene intermediate, affords the 6-acetyI-3//-azepine, e. g. 26. The process appears to be base-catalyzed as there is no reaction in xylene at 170 C. 

Xylene isomerization reactions can be accomplished by contacting a hot gas stream with a solid catalyst. Under these conditions the isomerization reactions may be regarded as reversible and first-order. Unfortunately, the catalyst also catalyzes disproportionation reactions. These reactions may be regarded as essentially second-order and irreversible. If one desires to achieve an equilibrium mixture of isomers with minimal material losses due to disproportionation, what do you recommend concerning the mode in which one should operate a continuous flow reactor ... 

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. 

Figure 3. Acid catalyzed xylene isomerization mechanism.

We have shown that the high selectivity of ZSM-5 in xylene isomerization relative to larger pore acid catalysts is a result of its pore size. It is large enough to admit the three xylenes and to allow their interconversion to an equilibrium mixture it also catalyzes the transalkylation and dealkylation of ethylbenzene (EB), a necessary requirement for commercial feed but it selectively retards transalkylation of xylenes, an undesired side reaction. 

Processes such as UOP s Isomar process are used to carry out isomerization of C8 aromatic species so that p-xylene can be removed selectively from the mixture of xylenes. The reaction is equilibrium controlled, so a continuous isomerization process is used. As is seen below, both aluminophosphate and aluminosilicate zeotypes are capable of catalyzing the reaction (Table 12.13). 

Xylene Isomerization There are several mechanisms by which the three xylene isomers can be interconverted. The one that is of the greatest interest with respect to industrial applications is the so-called monomolecular or direct xylene isomerization route. This reaction is most commonly catalyzed by Bronsted acid sites in zeolitic catalysts. It is believed to occur as a result of individual protonation and methyl shift steps. 

Dealkylation of the ethyl group of EB is a side reaction that is undesirable if isomerization to xylene is desired. This is an acid-catalyzed reaction that is espe-... 

The presence of metal may catalyze demethylation and can occur to some extent in catalysts where the metal function is under-passivated, as by incomplete sulfiding. This would convert valuable xylenes to toluene. The demethylation reaction is usually a small contributor to xylene loss. Metal also catalyzes aromatics saturation reactions. While this is a major and necessary function to facilitate EB isomerization, any aromatics saturation is undesirable for the process in which xylene isomerization and EB dealkylation are combined. Naphthenes can also be ring-opened and cracked, leading to light gas by-products. The zeolitic portion of the catalyst participates in the naphthene cracking reactions. Cracked by-products can be more prevalent over smaller pore zeolite catalysts. 

In the manufacture of terephthalic acid by the oxidation of p-xylene, separation of the xylene from its isomeric mixture is necessary (see Section 2.5.2). An alternative process introduced in Japan uses the oxidation of p-tolualdehyde, which is obtained in good regioselectivity by the HF—BF3 catalyzed carbonylation of toluene without the necessity of separation of the isomers. 

Based upon the organic chemistry of the isomerization of xylenes, the probability of a direct isomerization from o-xylene to p-xylene is quite small. Furthermore, in homogeneously catalyzed liquid-phase reaction the simple series reaction scheme has been demonstrated (7,8,9,10,11,12). 

The reactions of the various xylenes and ethylbenzene have been studied by Pitts and associates (P7). It was found that isomerization reactions among the three xylenes were catalyzed by acidic catalysts, but that interconversions between the xylenes and ethylbenzene required the presence of a hydrogenation-dehydrogenation component. Furthermore, it was found that the conversion of xylenes to ethylbenzene increased with decreasing temperature. Since lower temperatures are more favorable for hydrogenation, it has been suggested that the reaction proceeds by a sequence of steps such as the following (P7, W3) ... 

Migration of Ihe Methyl Groups. Reactions llial involve migration of the methyl groups include isomerization, disproportionation, and dealkylation. The interconversion of the three xylene isomers via isomerization is catalyzed by acids. 

H-[Ti,Al]-MEL and H-[Ti,Fe]-MEL are active catalysts for both oxidation reactions and typical acid-catalyzed reactions such as wi-xylene isomerization. In this reaction, H-[Ti,Al]-MEL deactivates at a lower rate than the H-[A1]-MEL analog (Reddy, J. S., et al., 1994a). 

Rates of model reactions are more commonly used to determine relative rather than absolute surface acidities and a variety of acid-catalyzed reactions have been used for this purpose (1-3). Xylene isomerization is a particularly well-substantiated model reaction, thanks to work by Ward and Hansford (43). They demonstrated that the conversion of o-xylene to p- and /n-xylenes over a series of synthetic silica-alumina catalysts increases as the alumina content is increased from 1 to 7%. The number of strong Brdnsted acids in each member of the catalyst series was measured by means of infrared spectroscopy. Since conversion of o-xylene was found to be a straight-line function of the number of Br0nsted acids (see Fig. 9), rate of xylene isomerization appears to be a valid index of the amount of surface acidity for this catalyst series. This correlation also indicates that the acid strengths of these silica-alumina preparations are roughly equivalent. 

Nafion-H appears to be a very useful catalyst for transalkylation reaction as indicated in these studies. Transalkylation of benzene with diethylbenzenes, as well as with diisopropylbenzene, is efficiently catalyzed by Nafion-H in a flow system. The efficiency of the catalyst is, however, more limited when the transferring group is a methyl group.268 Beltrame and co-workers have also carried out269 detailed mechanistic studies on the isomerization of xylenes over Nafion-H. 

The enhanced diffusivity of polynuclear compounds in sc C02 has been utilized to enhance catalyst lifetimes in both 1-butene/isoparaffin alkylations (Clark and Subramaniam, 1998 Gao et al., 1996). The former may be catalyzed using a number of solid acid catalysts (zeolites, sulfated zeolites, etc.), and the use of sc C02 as a solvent/diluent permits the alkylations to be carried out at relatively mild temperatures, leading to the increased production of valuable trimethylpentanes (which are used as high-octane gasoline blending components). The enhancement of product selectivity in the latter process is believed to result from rapid diffusion of ethylbenzene product away from the Y-type zeolite catalysts, thus preventing product isomerization to xylenes. 

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