Xylene isomerization, zeolites

Zeolite and Molecular Sieve-Based Process. Mobil has commercialized several xylene isomerization processes that are based on ZSM-5. Amoco has developed a process based on a medium-pore borosiUcate molecular sieve. 

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. 

Separation of isomers is an application where zeolite membranes could be specifically interesting because of their well-defined pores that lead to molecular sieving effects. An application that is often considered is the xylene isomerization and related reactions. 

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. 

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... 

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. 

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. 

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

Selective xylene isomerization over HZSM5 zeolites (no disproportionation)... 

Lercher and coworkers studied xylene isomerization on surface modified HZSM-5 zeolites [133]. They used time-resolved in situ IR spectroscopy to monitor the concentration of reactants and product inside the pores of the zeolite. Massiani and coworkers studied the conversion of xylene over mordenites [134]. They used in operando IR to characterize the adsorbed surface species and evolution of the active sites as the reaction proceeded. 

Zheng, S., Jentys, A., and Lercher, J.A. (2006) Xylene isomerization with surface-modified HZSM-5 zeolite catalysts an in situ IR study. ]. Catal, 241 (2), 304-311. 

Meta-xylene isomerization to ortho- and para-xylene over 10- and 12-MR zeolites is another illustration of product shape selectivity effects [13]. The two products are essentially equally favorable from the standpoint of thermodynamics. With decreasing pore size, however, kinetics come into play and the selectivity to para-xylene increases, as illustrated in Figure 13.37 for results obtained at 317-318°C, 0.5 kPa meta-xylene pressure (in the presence of He carrier gas) and 10% conversion [64]. While the para ortho ratio is typically 1.0-1.5 with multi-dimensional... 

Uopis, F., Sastre, G., and Corma, A. (2004) Xylene isomerization and aromatic alkylation in zeolites NU-87, SSZ-33, p, and ZSM-5 molecular dynamics and catalytic studies. 

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. 

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. 

Full catalyst formulations consist of zeolite, metal and a binder, which provides a matrix to contain the metal and zeolite, as well as allowing the composite to be shaped and have strength for handling. The catalyst particle shape, size and porosity can impact the diffusion properties. These can be important in facile reactions such as xylene isomerization, where diffusion of reactants and products may become rate-limiting. The binder properties and chemistry are also key features, as the binder may supply sites for metal clusters and affect coke formation during the process. The binders often used for these catalysts include alumina, silica and mixtures of other refractory oxides. 

Coke formation during xylene isomerization has been studied using in situ infrared spectroscopy [78]. A study done on EB isomerization with a bifunctional catalyst containing EUO zeolite indicated that poor initial selectivity of the catalyst improves after a period of fast deactivation, during which micropores are blocked [79]. 

The catalysts for xylene isomerization with EB dealkylahon are dominated by MFI zeolite. The de-ethylation reaction is particularly facile over this zeolite. There have been several generations of catalyst technology developed by Mobil, now ExxonMobil [84]. The features in their patents include selectivation and two-catalyst systems in which the catalysts have been optimized separately for deethylation of EB and xylene isomerization [85-87]. The crystallite size used for de-ethylation is significantly larger than in the second catalyst used for xylene isomerization. Advanced MHAI is one example. The Isolene process is offered by Toray and their catalyst also appears to be MFI zeoUte-based, though some patents claim the use of mordenite [88, 89]. The metal function favored in their patents appears to be rhenium [90]. Bimetallic platinum catalysts have also been claimed on a variety of ZSM-type zeolites [91]. There are also EB dealkylation catalysts for the UOP Isomar process [92]. The zeolite claimed in UOP patents is MFI in combination with aluminophosphate binder [93]. 

ZSM-5 is a Mobil-proprietary, shape-selective zeolite which is used commercially in synthetic fuels (methanol-to-gasoline), petrochemicals (xylene isomerization, toluene disproportionation, benzene alkylation) and in petroleum refining (lube and... 

Xylenes. Because of the practical significance of xylenes, isomerization of xylenes over zeolites is frequently studied.348 The aim is to modify zeolite properties to enhance shape selectivity, that is, to increase the selectivity of the formation of the para isomer, which is the starting material to produce terephthalic acid. In addition, m-xylene isomerization is used as a probe reaction to characterize acidic zeolites.349,350... 

Ward (, 6) determined the acidity of several transition metal X and Y zeolites by infrared spectroscopy but could find no simple relationship between the proton acid concentration and physical parameters of metal ions or catalytic activity for o-xylene isomerization. 

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). 

Ward measured the o-xylene isomerization activities of Na, Mg, RE, and H—Y zeolites and found the rare earth form to be intermediate in activity between the magnesium and hydrogen forms as shown in Table IX (212). The sodium form was essentially inactive. He interpreted the activity relationship RE—Y > Mg—Y to result from the formation of two acidic structural hydroxyl groups per trivalent rare earth cation. The formation of acidic structure hydroxyl groups by exchange of sodium ions with protons in the rare earth solution, as proposed by Bolton (218), may also account for the greater activity of the rare earth-exchanged zeolite. 

---