Xylene isomerization acid catalyzed

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. 

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-... 

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. 

It is generally admitted that skeletal transformations of hydrocarbons are catalyzed by protonic sites only. Indeed good correlations were obtained between the concentration of Bronsted acid sites and the rate of various reactions, e g. cumene dealkylation, xylene isomerization, toluene and ethylbenzene disproportionation and n-hexane cracking10 12 On the other hand, it was never demonstrated that isolated Lewis acid sites could be active for these reactions. However, it is well known that Lewis acid sites located in the vicinity of protonic sites can increase the strength (hence the activity) of these latter sites, this effect being comparable to the one observed in the formation of superacid solutions. Protonic sites are also active for non skeletal transformations of hydrocarbons e g. cis trans and double bond shift isomerization of alkenes and for many transformations of functional compounds e.g. rearrangement of functionalized saturated systems, of arenes, electrophilic substitution of arenes and heteroarenes (alkylation, acylation, nitration, etc ), hydration and dehydration etc. However, many of these transformations are more complex with simultaneously reactions on the acid and on the base sites of the solid... 

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 ... 

Medium pore aluminophosphate based molecular sieves with the -11, -31 and -41 crystal structures are active and selective catalysts for 1-hexene isomerization, hexane dehydrocyclization and Cg aromatic reactions. With olefin feeds, they promote isomerization with little loss to competing hydride transfer and cracking reactions. With Cg aromatics, they effectively catalyze xylene isomerization and ethylbenzene disproportionation at very low xylene loss. As acid components in bifunctional catalysts, they are selective for paraffin and cycloparaffin isomerization with low cracking activity. In these reactions the medium pore aluminophosphate based sieves are generally less active but significantly more selective than the medium pore zeolites. Similarity with medium pore zeolites is displayed by an outstanding resistance to coke induced deactivation and by a variety of shape selective actions in catalysis. The excellent selectivities observed with medium pore aluminophosphate based sieves is attributed to a unique combination of mild acidity and shape selectivity. Selectivity is also enhanced by the presence of transition metal framework constituents such as cobalt and manganese which may exert a chemical influence on reaction intermediates. 

For 1-hexene isomerization and for acid catalyzed Cg aromatic reactions all molecular sieves were evaluated in their calcined, powdered state. For the study of Cg aromatics, selected SAPO molecular sieves were aluminum exchanged or steam treated as noted in Table IV. For bifunctional catalysts used in paraffin cyclization/isomerization and ethylbenzene-xylene interconversions, the calcined molecular sieve powder was mixed with platinum-loaded chlorided gamma alumina powder. These mixtures were then bound using silica sol and extruded to form 1/16" extrudates which were dried and calcined at 500°C. The bifunctional catalysts were prepared to contain about 0.54 platinum and about 40 to 504 SAPO molecular sieve in the finished catalysts. 

Conceptually, this route offers the possibility of enhanced xylene yields, but since naphthenic intermediates are present, significant acid catalyzed ring opening and yield loss of aromatics are also possible. A review of available patent literature data(22,23b) suggests that because of these undesirable side reactions, the bifunctional approach to isomerization yields about the same amount of xylene as does the monofunctional route. 

Cg Aromatic Reactions with Hydrogen. The mild acid nature of the family of aluminophosphate based sieves renders them selective for a number of rearrangements as observed in the reactions of olefins and paraffins described above. This property as well as their apparent low disproportionation activity observed in the alkylation of toluene suggests that they be evaluated as the acid function in bifunctional Cg aromatic isomerization. As described above, cyclo-olefins are most likely involved in the conversion of ethylbenzene to xylenes. Strong acid functions, such as in mordenite, actively isomerize cyclo-olefinic intermediates but also catalyze ring-opening reactions which lead to loss of aromatics. A more selective acid function must still effectively interconvert ethyl cyclohexene to dimethylcyclohexenes but must leave the cyclohexene rings intact. 

Even though the synthesis of many medium pore SAPO molecular sieves are well documented, only SAPO-11 has been studied in detail with respect to its shape selectivity and catalytic activity in acid catalyzed reactions. The reaction of m-xylene on zeolites, besides its industrial importance, is abundantly described in literature not only because it provides information on the geometry of the zeolite channels, but also because it is considered as an appropriate reaction to give information on the acidic properties of solid catalysts. Both isomerization and disproportionation are catalyzed by Bronsted acid sites , the disproportionation reactions requiring stronger acid sites than isomerization reactions. Hence SAPO molecular sieves with medium acidity should give better selectivity for m-xylene isomerization than zeolites. 

Figure 10.3 Simplified reaction network for the acid-catalyzed alkylation of toluene with methanol, and the isomerization reactions of o-, m-, and p-xylene.

There arc numerous examples of product selectivity, many of which involve mono-and disubslituted aromatics formed over ZSM-5 (MFI) catalysts (22-24). One of the early examples was xylene isomerization. Xylene can be formed over MFI catalysts via the acid-catalyzed reaction between methanol and toluene (see Fig. 10.3). According to thermodynamics the equilibrium distribution of o-, m-, and p-isomers is 26 51 23, which is different from the industrial demand as the p-isomer is a feedstock for terephthalic acid, a monomer for PET. However, very high selectivities of p-xylene can be obtained over MFT materials primarily because the diffusion of p-xylene is faster in the MFI pores compared to the other two isomers. 

In addition to the variety of isomerization and rearrangement reactions leading to an abundance of different flavor and fragrance chemicals, a- and P-pinene also undergo carbocationic polymerization to yield polyterpene resins (Figure 3B.10).The reaction can be conducted as a batch or continuous polymerization of P-pinene and a smaller fraction of the less reactive a-pinene in a solvent such as xylene and is catalyzed by Lewis acid metal halides, such as aluminum trichloride (AICI3) in the presence of some parts per million of water to create a strong proton donor (the vast majority of pinene polymerization involves with the use of P-pinene, not a-pinene, mostly due to intrinsic... 

Choudhary and Akolekar [90] studied the conversion of cumene, isooctane, and o-xylene over various low-silica and high-silica zeohtes. They defined strong acid sites as sites from which pyridine desorbs above 400 °C and found that isooctane cracking required essentially these very strong acid sites, while o-xylene isomerization and cumene cracking were also catalyzed by weaker acid sites. 

Silica-alumina catalyzes a wide variety of reactions, and good correlations between the number of acid sites and catalytic activities are observed in many cases including propene polymerization, cumene cracking," and o-xylene isomerization. ... 

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. ... 

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