Xylenes, selective isomerization

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. 

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. 

The effect of crystal size, 2r, in STOP is demonstrated in Figure 10. These data for three zeolites having similar activity, but with crystal sizes differing by nearly two orders of magnitude, show a significant increase in para-xylene selectivity with increasing crystal size. The primary product selectivity is enhanced and secondary isomerization is retarded. 

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

The activity and selectivity of HZSM5 zeolites in the catalytic methylation of toluene at 570 K is found to be constant for small particle samples. While p-xylene can easily be transported in the pores of ZSM5, the bulkier isomers, m- and o-xylene (which are also primary products in toluene methylation) are partly retained in the pore system. Adsorbed at the catalytically acitve sites, these xylene molecules isomerize to the faster diffusing p-xylene and increase in this way the p-selectivity. After reaching a steady state in the surface coverage of the xylenes, the rate of formation of m- and o-xylene equals the rate of isomerization to p-xylene plus the rate of transport out of the pores. 

As many organic compounds may transform simultaneously through mono molecular (intramolecular) and bimolecular (intermolecular) processes, it is easy to understand that the shape and size of the space available near the active sites often determine the selectivity of their transformation. Indeed the transition state of a bimolecular reaction is always bulkier than that of a monomolecular reaction, hence the first type of reaction will be much more sensitive to steric constraints than the second one. This explains the key role played by the pore structure of zeolites on the selectivity of many reactions. A typical example is the selective isomerization of xylenes over HMFI the intermediates leading to disproportionation, the main secondary reaction over non-spatioselective catalysts, cannot be accommodated at its channel intersections (32). Furthermore, if a reaction can occur through mono and bimolecular mechanisms, the significance of the bimolecular path will decrease with the size of the space available near the active sites (41). 

XyMax A selective process for making p-xylene and o-xylene by isomerizing C8 aromatics-rich hydrocarbon streams, using a proprietary zeolite catalyst. Developed by ExxonMobil and first used in 2000 in its refinery on Jurong Island, Singapore. Five more units have subsequently been licensed. Retrofit applications are licensed by ExxonMobil Chemical, grassroots applications by Axens. 

Fig. 10. Schematic representation of the p-xylene selective extraction through a MFI membrane from an isomerization reactor.

The results on the effect of temperature, contact time and methanol to toluene ratio on the isomer composition of xylenes on K2.5 salt are given in Table 3. It is seen that selectivity of p-xylene decreases with increase in the temperature whereas the selectivity of m-xylene increases, obviously, due to the isomerization. As contact time increases p-xylene selectivity increases. It is also found that the p-xylene selectivity increases with increasing methanol to toluene ratio. As methanol to toluene ratio increases the catalyst surface will be saturated with more of alkylating species which offers hindrance to the approach of the aromatic substrate and thereby resulting in the preferential alkylation at para position. In conclusion,it may be suggested that high Bronsted acidity is responsible for high para selectivity found in heteropolyoxometallates. 

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. 

Kaeding et al. [49] proposed that the high / -xylene selectivity obtained with the modified zeolites was a result of steric hindranee effects within the pores of the zeolite. The first step of the reaction mechanism is thought to be methanol protonation that occurs on the Bronsted acid sites of the zeolite. This is followed by transfer of the methyl group to the aromatic ring. Alkylation at the para position is predicted to be less sterically hindered than at the meta or ortho position and is favored in the small pores of the modified zeolite. Furthermore, diffusion out of the pore by /n-xylene and o-xylene would be relatively slow, resulting in isomerization of these components to /i-xylene. Without modification of the zeolite, the pore dimensions are such that o-xylene and M-xylene can be produced within the pores of the zeolite [49]. 

As shown in Figure 1, the equiHbrium concentration is affected slightly by temperature (11). The actual concentration is affected by the reaction rate and the initial concentration of each isomer. Deviations beyond equiHbrium can be achieved when zeoHtes are used, owing to shape selectivity (see Molecularsieves). The thermal isomerization of the three xylenes has been studied at 1000°C (12). Side reactions predominated, and only a small percentage of xylenes was interconverted. 

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). 

Xylenes. The main appHcation of xylene isomers, primarily p- and 0-xylenes, is in the manufacture of plasticizers and polyester fibers and resins. Demands for xylene isomers and other aromatics such as benzene have steadily been increasing over the last two decades. The major source of xylenes is the catalytic reforming of naphtha and the pyrolysis of naphtha and gas oils. A significant amount of toluene and Cg aromatics, which have lower petrochemical value, is also produced by these processes. More valuable p- or 0-xylene isomers can be manufactured from these low value aromatics in a process complex consisting of transalkylation, eg, the Tatoray process and Mobil s toluene disproportionation (M lDP) and selective toluene disproportionation (MSTDP) processes isomerization, eg, the UOP Isomar process (88) and Mobil s high temperature isomerization (MHTI), low pressure isomerization (MLPI), and vapor-phase isomerization (MVPI) processes (89) and xylene isomer separation, eg, the UOP Parex process (90). 

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. 

Careful chromatographic and detailed HNMR spectroscopic analysis of the products from the thermolyses of ethyl azidoformate in o-, m- and p-xylene revealed in all cases a mixture of 1 //-azepines.80 In o-xylene, only two of the four possible isomers were separated and characterized, namely, ethyl 4,5-dimethy 1-1 //-azepine-1 -carboxylate (9 %) and ethyl 3,4-dimethyl-l H-azepine-1-carboxylate (7 %). w-Xylene yielded a 2 3 mixture of ethyl 3,5-dimethyl-l//-azepine-1-carboxylate and ethyl 2,4-dimethyl-l//-azepine-l-carboxylate. The 2,4-dimethyl isomer (20 %) can be isolated from the mixture by removal of the 3,5-dimethyl isomer as its Diels-Alder cycloadduct with ethenetetracarbonitrile. p-Xylene gave a mixture of the two possible isomeric azepines which were partially separated by column chromatography. A pure sample of ethyl 2,5-dimethyl-1//-azepine-1-carboxylate (26%) was obtained from the mixture by selective decomposition of the 3,6-dimethyl isomer with refluxing alcoholic potassium hydroxide. 

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. 

The excellent high para-selectivity can be explained by the selective escape of p-xylene from the H-ZSM-5 catalyst and inhibition of isomerization on the external surface of catalysts by silicalite-1 coating. In addition to the high para-selectivity, toluene conversion was still high even after the silicalite-1 coating because the silicalite-1 layers on H-ZSM-5 crystals were very thin. 

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. 

Among 10MR ID zeolite, EU-1 zeolite exhibits higher performances for EB isomerization (activity and selectivity for xylenes). 

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). 

The meta-selectivity for toluene activation, observed for both systems, is very unusual (Fig. 5). Also remarkable is the switch in selectivity from aryl C-H activation to benzylic activation inp-xylene, just by changing the chelate ligand from the diimine equipped with trifluoromethyl substitutents in the meta-positions of the phenyl residue to the diimine bearing methyl substituents in the ortho-positions (Fig. 5). The authors suggested that the C-H bond activation is reversible and the isomeric a-methane complexes are in equilibrium prior to the substitution of... 

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... 

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

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. 

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