Para-xylene selectivity

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. 

In view of the difficulty of measuring the diffusivity of o-xylene at the reaction temperature, 482°c, we have used the diffusivity determined at 120°C. For a series of ZSM-5 catalysts, the two D-values should be proportional to each other. Para-xylene selectivities at constant toluene conversion for catalysts prepared from the same zeolite preparation (constant r) with two different modifiers are shown in Figure 11. The large effect of the modifier on diffusivity, and on para-selectivity, is apparent. 

Both para-xylene selectivity and r2/D (tQ 3) increase smoothly with MgO level for a series of large crystal, Mg modified HZSM-5 catalysts, and again para-xylene selectivity increases with tQ 3 (Figure 13, Table IV). However, these catalysts appear to be significantly different from the catalysts just discussed, defining a separate functional dependence on r2/D (tQ 3). These differences will be shown to be attributable to differences in acid activity of this series of catalysts. 

MRs, with 10-MRs the para ortho raho is typically >2, the smaller the pore size, the higher the para-xylene selectivity. More recent molecular dynamics simula-hons verify that the diffusivity ratio for para ortho is much higher for ZSM-5 (7.4) than for Beta (2.3), consistent with the higher para-xylene selectivity obtained over ZSM-5 [76]. 

Figure 12 compares the performance of the modified ZSM-5 with the standard unmodified ZSM-5. A thermodynamic equilibrium mixture of xylenes contains about 24% para-xylene. In laboratory tests with the modified catalyst, we have achieved para-xylene selectivities as high as 98% at low conversions. 

The effect of reaction temperature on the catalytic performance is shown in Fig. 4. With raise in reaction temperature from 773 to 813 K, there is an increase in extent of reactant conversion. The high para xylene selectivity in the xylene fraction was not getting affected due to the change in reaction temperature. 

Fig. 4. Influence of reaction temperature on variation on conversion, and para-xylene selectivity of pore size controlled ZnO-HZSM-5 catalyst. WHSV = 1.1 /h...

Further evidence for diffusion control of para-xylene selectivity in toluene disproportionation over ZSM-5 catalysts has been described by Haag and Olson, who noted a good correlation between the sorption rate of o-xylene and the pora-selectivity... 

As described in the previous section, the silica-alumina catalyst covered with the silicalite membrane showed exceUent p-xylene selectivity in disproportionation of toluene [37] at the expense of activity, because the thickness of the sihcahte-1 membrane was large (40 pm), limiting the diffusion of the products. In addition, the catalytic activity of silica-alumina was not so high. To solve these problems, Miyamoto et al. [41 -43] have developed a novel composite zeohte catalyst consisting of a zeolite crystal with an inactive thin layer. In Miyamoto s study [41], a sihcahte-1 layer was grown on proton-exchanged ZSM-5 crystals (silicalite/H-ZSM-5) [42]. The silicalite/H-ZSM-5 catalysts showed excellent para-selectivity of >99.9%, compared to the 63.1% for the uncoated sample, and independent of the toluene conversion. 

Prior to solving the structure for SSZ-31, the catalytic conversion of hydrocarbons provided information about the pore structure such as the constraint index that was determined to be between 0.9 and 1.0 (45, 46). Additionally, the conversion of m-xylene over SSZ-31 resulted in a para/ortho selectivity of <1 consistent with a ID channel-type zeolite (47). The acidic NCL-1 has also been found to catalyze the Fries rearrangement of phenyl acetate (48). The nature of the acid sites has recently been evaluated using pyridine and ammonia adsorption (49). Both Br0nsted and Lewis acid sites are observed where Fourier transform-infrared (FT IR) spectra show the hydroxyl groups associated with the Brpnsted acid sites are at 3628 and 3598 cm-1. The SSZ-31 structure has also been modified with platinum metal and found to be a good reforming catalyst. 

Para-selectivity for a wide variety of ZSM-5 preparations of comparable activity are shown in Figure 12. These data include results for unmodified HZSM-5 s of varying crystal size as well as chemically modified HZSM-5 s. Since the activity of these catalysts is nearly identical, these data clearly establish the major role of diffusion in the para-xylene content of the xylenes produced in TDP. We have examined in more detail the effect of the concentration of one of these chemical modifiers, MgO. 

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... 

Other examples of systems that are likely to be governed by product shape selectivity effects include toluene disproportionation to para-xylene -i- benzene in favor of other xylenes r- benzene [61]. Toluene alkylation by methanol to give para-xylene in favor of other xylenes is yet another such example [76],... 

In the case of toluene alkylation with methanol an opportunity exists for para selectivity. Para-xylene ortho-xylene ratio was 3.1 over MFl and 0.6 over BEA framework types. 

The catalysts are predominantly modified ZSM-5 zeolite. In general, the modifications are intended to restrict pore mouth size to promote the shape selective production of para-xylene within the microporous structure. The same modifications also serve to remove external acid sites and eliminate the consecutive isomerization of para-xylene. Methods used to modify the zeolite pore openings have included silation [50], incorporation of metal oxides such as MgO, ZnO and P2O5 [51, 52], steaming and the combination of steaming and chemical modification [53]. 

One of the industrial processes using ZSM-5 provides us with an example of product shape-selective catalysis the production of l,4-( ara- xylene. Para-xylene is used in the manufacture of terephthalic acid, the starting material for the production of polyester fibres such as Terylene . 

The selectivity of the reaction over ZSM-5 occurs because of the difference in the rates of diffusion of the different isomers through the channels. This is confirmed by the observation that selectivity increases with increasing temperature, indicating the increasing importance of diffusion limitation. The diffusion rate of para-xylene is... 

C, 10-50 atm). Xylene benzene ratios of 1-10 may be obtained. Metal catalysts were later replaced by zeolites.210,211,326-328 The most recent development is the Mobil selective toluene disproportionation process,329 which takes advantage of the high para shape selectivity of a zeolite catalyst.210 The catalyst activated by a novel procedure ensures a p-xylene content of up to 95%. After the successful com-mercialization at an Enichem refinery in Italy, the process is now licensed. The catalysts and technologies applied in toluene disproportionation may be also used for transalkylation324,325,331 [Eq. (5.74)] ... 

Fig. 1.5 Schematic representation of shape selective effects a) Reactant selectivity Cracking of an n-iso C6 mixture, b) Product selectivity Disproportionation of toluene into para-xylene over a modified HFMI zeolite, c) Spatioselectivity Disproportionation of meta-xylene over HMOR. The diphenylmethane intermediate A in formation of 1,3,5 trimethylbenzene is too bulky to be accommodated in the pores, which is not the case for B...

Application To selectively convert toluene to high-purity (90%+) para-xylene-rich (PX) xylenes and benzene using ExxonMobil Chemical s technologies— PxMax and ASTDP. 

Sastre et aL [112] studied the isomerization of m-x>iene over OfBretite and observed monotonical increase in m-xylene conversion upon exdumge of the K -cations. This was ascribed to the increase of the concentration of the protons and the increase in accessibility of the pores, which resulted in a higher selectivity for the isomerisation reaction at the e q>aise of the disproportionation reactioiL Only a sHght increase in the p-xyloie in the fraction of oitho-and para-xylene was observed. Over Beta a maximum activity for the xylene isomersation was observed and this was explained by either a pos le existence of a eigistic effect between extra-framework aluminium and the fitunework Bronsted acid sites or a concentration effect [113]. 

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