In xylene isomerization

In shape-selective catalysis, the pore size of the zeoHte is important. For example, the ZSM-5 framework contains 10-membered rings with 0.6-nm pore size. This material is used in xylene isomerization, ethylbenzene synthesis, dewaxing of lubricatius oils and light fuel oil, ie, diesel and jet fuel, and the conversion of methanol to Hquid hydrocarbon fuels (21). 

Many chemical reactions, especially those involving the combination of two molecules, pass through bulky transition states on their way from reactants to products. Carrying out such reactions in the confines of the small tubular pores of zeolites can markedly influence their reaction pathways. This is called transition-state selectivity. Transition-state selectivity is the critical phenomenon in the enhanced selectivity observed for ZSM-5 catalysts in xylene isomerization, a process practiced commercially on a large scale. 

Structure-Selectivity Relationship in Xylene Isomerization and Selective Toluene Disproportionation... 

As a result of steric constraints imposed by the channel structure of ZSM-5, new or improved aromatics conversion processes have emerged. They show greater product selectivities and reaction paths that are shifted significantly from those obtained with constraint-free catalysts. In xylene isomerization, a high selectivity for isomerization versus disproportionation is shown to be related to zeolite structure rather than composition. The disproportionation of toluene to benzene and xylene can be directed to produce para-xylene in high selectivity by proper catalyst modification. The para-xylene selectivity can be quantitatively described in terms of three key catalyst properties, i.e., activity, crystal size, and diffusivity, supporting the diffusion model of para-selectivity. 

Table I. Selectivity in Xylene Isomerization Feed 70% m-/30% o-Xylene, 316°C Pressure 28 bar...

Figure 2. Effect of intracrystalline cavity diameter of several zeolites on selectivity in xylene isomerization.

Table ill. Selectivity in Xylene Isomerization Feed 15% Ethylbenzene, 85% Xylene (63% m, 22% o)... 

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. 

A reaction which involves both ring atoms and substituent is of considerable interest because the substrate is of a naturally occurring class of compounds. 3-Arylidene-chromanones (585) or homoisoflavanones, when heated with nickel in xylene isomerize to the 3-benzylchromone (586) (74IJC281) a number of homoisoflavanones occur in the bulbs of Eucomis bicolor, for example, eucomin (585 R = OH, Ar = 4-MeOC6H4). The stereochemistry about the double bond of compound (585 R = MeO) is altered from (E) to (Z) by irradiation at 300-400 nm (8lFOR(40)l05>. 

With bifunctional Pt acid catalysts, a third mechanism can participate in xylene isomerization, involving the same intermediates as ethylbenzene isomerization (2,11). 

The participation of protonic acid sites in xylene isomerization is clearly demonstrated by correlations between the isomerization rate and the concentration of protonic sites of silica alumina with various alumina contents (13), alkaline-earth and rare earth FAU zeolites (14, 15), MFI zeolites (16), etc. Evidence is also provided by the fact that protonic sites participate in alkylbenzene disproportionation. On the other hand, it seems most unlikely that Lewis acid sites play a direct role in xylene isomerization and disproportionation (8). 

This effect is presumably responsible of the low yield in by-products in xylene isomerization (near-absence of transalkylation) and is rather beneficial in industrial processes. 

A third type of control, called spatiospectffeity, occurs when both reactants and products pass the opening but reaction intermediates or transition states are restricted by the size of the cavity. In xylene isomerization processes, selectivity is lost through disproportionation to toluene and trimethylbenzene. Diphenylroethane intermediates are too large for ZSM-5... 

Guisnet, M. and Gnep, N.S. (1984), Zeolites as catalysts in xylene isomerization processes, in F.R. Ribeiro et al. (eds.), Zeolites Science and Technology, NATO ASI Series E80, Martinus Nijhoff Publishers, The Hague, Boston, Lancaster, pp. 571-582. 

Xylene Isomerization. After separation of the preferred xylenes, ie, PX or OX, using the adsorption or crystallization processes discussed herein, the remaining raffinate stream, which tends to be rich in MX, is typically fed to a xylenes isomerization unit in order to further produce the preferred xylenes. Isomerization units are fixed-bed catalytic processes that are used to produce a close-to-equiUbrium mixture of the xylenes. To prevent the buildup of EB in the recycle loop, the catalysts are also designed to convert EB to either xylenes, benzene and lights, or benzene and diethylbenzene. 

Amorphous Silica—Alumina Based Processes. Amorphous siHca—alumina catalysts had been used for many years for xylene isomerization. Examples ate the Chevron (130), Mamzen (131), and ICI (132—135). The primary advantage of these processes was their simpHcity. No hydrogen was requited and the only side reaction of significance was disproportionation. However, in the absence of H2, catalyst deactivation via coking... 

To this point the presence of ethylbenzene in the mixed xylenes has been ignored. The amount can vary widely, but normally about 15% is present. The isomerization process must remove the ethylbenzene in some way to ensure that it does not build up in the isomerization loop of Figure 8. The ethylbenzene may be selectively cracked (40) or isomerized to xylenes (41) using a platinum catalyst. In rare cases the ethylbenzene is recovered in high purity by superfractionation. 

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. 

In order to produce more paraxylene than is available in catalytic reformate, a xylenes-isomerization plant is sometimes included in the processing scheme. The isomerization step uses the effluent (filtrate) from the paraxylene crystallization step as feed. The filtrate contains about 7-9 percent of paraxylene. The isomerization unit brings the concentration back to its equilibrium value of about 20 percent. 

The first reported example31,117 involved the diethyltetraphenyl-3//-azepines 18 and 19 which were obtained in 85% overall yield by the reaction of2,3-diethyl-2//-azirine with 2,3,4,5-tetraphenylcyclopentadienone (see Section 3.1.1.1.2.). The two isomeric azepines are separable by column chromatography (alumina or silica gel), and each isomer, on warming in xylene for three days, equilibrates to a 3 8 mixture of the 3//-azepines 18 and 19. 

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. 

Table 10.2 Performance of several zeolite membrane reactors in the xylene isomerization reaction. 

Since their development in 1974 ZSM-5 zeolites have had considerable commercial success. ZSM-5 has a 10-membered ring-pore aperture of 0.55 nm (hence the 5 in ZSM-5), which is an ideal dimension for carrying out selective transformations on small aromatic substrates. Being the feedstock for PET, / -xylene is the most useful of the xylene isomers. The Bronsted acid form of ZSM-5, H-ZSM-5, is used to produce p-xylene selectively through toluene alkylation with methanol, xylene isomerization and toluene disproportionation (Figure 4.4). This is an example of a product selective reaction in which the reactant (toluene) is small enough to enter the pore but some of the initial products formed (o and w-xylene) are too large to diffuse rapidly out of the pore. /7-Xylene can, however. 

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 effect of crystal size of these zeolites on the resulted toluene conversion can be ruled out as the crystal sizes are rather comparable, which is particularly valid for ZSM-5 vs. SSZ-35 and Beta vs. SSZ-33. The concentrations of aluminum in the framework of ZSM-5 and SSZ-35 are comparable, Si/Al = 37.5 and 39, respectively. However, the differences in toluene conversion after 15 min of time-on-stream (T-O-S) are considerable being 25 and 48.5 %, respectively. On the other hand, SSZ-35 exhibits a substantially higher concentration of strong Lewis acid sites, which can promote a higher rate of the disproportionation reaction. Two mechanisms of xylene isomerization were proposed on the literature [8] and especially the bimolecular one involving the formation of biphenyl methane intermediate was considered to operate in ZSM-5 zeolites. Molecular modeling provided the evidence that the bimolecular transition state of toluene disproportionation reaction fits in the channel intersections of ZSM-5. With respect to that formation of this transition state should be severely limited in one-dimensional (1-D) channel system of medium pore zeolites. This is in contrast to the results obtained as SSZ-35 with 1-D channels system exhibits a substantially higher... 

Xylol (dimethylbenzene), used as a rubber solvent. Xylene exists in three isomeric forms, commercial xylol being a mixture of all three. Xylyl Mercaptan... 

Parex (1) [Para extraction] A version of the Sorbex process, for selectively extracting p-xylene from mixtures of xylene isomers, ethylbenzene, and aliphatic hydrocarbons. The feedstock is usually a C8 stream from a catalytic reformer, mixed with a xylene stream from a xylene isomerization unit. The process is operated at 177°C the desorbent is usually p-diethylbenzene. The first commercial plant began operation in Germany in 1971 by 1992, 453 plants had been licensed worldwide. Not to be confused with Parex (2). 

XIS [Xylene isomerization] A process for isomerizing /j-xylcne to the equilibrium mixture of C8 aromatic hydrocarbons. Developed by Maruzen Oil in the United States. 

The structure of SSZ-35 (IZA structure code STF) as viewed in the [001] direction is shown in Fig. 17. The dimensions of the 10-MR structures are 5.5 x 6.1 A and the diameter of the 18-MR structures is 12.5 x 9 A. This pore structure is in contrast to the structure of SSZ-44 (IZA structure code SFF) shown in Fig. 18, where the 10-MR structures are nearly spherical (5.8 A) and the 18-MR structures are slightly larger (12.9 x 9 A). These small differences in pore size apparently translate into startling differences in reactivity. A study of m-xylene conversion shows a high degree of isomerization versus disproportionation, which is consistent with a 10-MR pore system (47). The interesting data is the para to ortho selectivity in the isomerization products, where SSZ-44 exhibited a higher para/ortho... 

Intermediate pore zeolites typified by ZSM-5 (1) show unique shape-selectivities. This has led to the development and commercial use of several novel processes in the petroleum and petrochemical industry (2-4). This paper describes the selectivity characteristics of two different aromatics conversion processes Xylene Isomerization and Selective Toluene Disproportionation (STDP). In these two reactions, two different principles (5,j6) are responsible for their high selectivity a restricted transition state in the first, and mass transfer limitation in the second. 

Early attempts to utilize the high acid activity of faujasite zeolite catalysts for direct xylene isomerization suffered from low selectivity. Considerable improvement was obtained first by using a large pore zeolite (7) catalyst and subsequently in several process modifications that use ZSM-5 as catalyst (2). In the following we will show how these selectivity differences can be related to structural differences of the various zeolites. 

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. 

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