Disproportionation xylene

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. 

Another example of catalytic isomerization is the Mobil Vapor-Phase Isomerization process, in which -xylene is separated from an equiHbrium mixture of Cg aromatics obtained by isomerization of mixed xylenes and ethylbenzene. To keep xylene losses low, this process uses a ZSM-5-supported noble metal catalyst over which the rate of transalkylation of ethylbenzene is two orders of magnitude larger than that of xylene disproportionation (12). 

Figure 4. Acid catalyzed xylene disproportionation mechanism.

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

As an example for aromatic transformation the mechanism for meta-xylene disproportionation to toluene -i- trimethylbenzene is illustrated in Figure 13.46. In the first step the zeolite extracts a hydride from meta-xylene to form a carbenium ion at one of the methyl groups, presumably the rate-controlling step. This mechanism is likely to involve a Lewis acid site. The carbenium ion then adds to a second... 

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). 

Large differences exist between the xylene disproportionation/isomerization ratios (D/I) found with acid catalysts. With zeolites the size of the space available near the acid sites was shown to play a determining role (2). The smaller the size of the intracrystalline zeolite cavities, the lower the ratio between the rate constants of disproportionation and isomerization 0.05 at 316°C with a FAU zeolite (diameter of the supercage of 1.3 nm), 0.014 and 0.01 with MOR and MAZ (0.08 nm). Steric constraints which affect the formation of the bulky bimolecular transition states and intermediates of disproportionation (Figure 9.4) would be responsible for this observation. However, the very low value of D/I (0.001) obtained with MFI (2), the channel intersection of which has a size of 0.85 nm, is also due to other causes limitations in the desorption of the bulky trimethylbenzene products of disproportionation from the narrow pores of the zeolite ( 0.6 nm) and most likely the low acid site density of the used sample (Si/Al=70 instead of 5-15 with the large pore zeolites). 

The mechanism of ethylbenzene disproportionation depends on the zeolite pore structure (3). With large pore zeolites, this reaction occurs mainly through the carbocation chain mechanism proposed for xylene disproportionation (Figure 9.4) which involves benzylic carbocations and diarylmethane intermediates. With MFI zeolites in the pores of which steric constraints limit the formation of the bulky diarylmethane intermediates, ethylbenzene disproportionation occurs mainly through a successive dealkylation-alkylation process ... 

With mordenite catalysts on which disproportionation occurs through benzylic carbocations and diarylmethane intermediates, the rate of this bimolecular reaction was shown to be dependent on the acid site density the turnover frequency for disproportionation is roughly proportional to the square of the concentration of acid sites (Figure 9.8 (25)). As suggested for xylene disproportionation, this probably means that xylene disproportionation requires two protonic sites for its catalysis (the first one for steps 1, 2, 3 and the second for steps 4, 5 and 6 of Figure 9.4). 

Toluene disproportionation (TDP) is a well-known acid reaction, occurring through the same mechanism as xylene disproportionation (Figure 9.4). Like this latter reaction, toluene disproportionation requires most likely two protonic sites for it catalysis, hence the density of protonic sites has a very positive effect on the catalyst activity. Furthermore, the bimolecular intermediates (methyldiphenyl-... 

Disprop ortionation of m-xylene Al-M Isomerization to other p- and o-xylenes, disproportionation to toluene and trimethyl benzenes were the main reactions. For both reactions, activity increased with increase in the number of pillars. Selectivity for disproportionation increased with decreasing number of pillars due to restricted transition state selectivity. 67 68... 

Bankos et al. measured the heat of adsorption and the amount of ammonia sorbing on a series of Na-H-mordenites in order to determine the strength and number of acid sites, respectivelySteps on the curves for the differential heat of adsorption (see Hgure 5) were attributed to adsorption on sites of different strengths. Furthermore, they assigned acid type, i.e., Lewis versus Brpnsted, to different heats of adsorption. In this way they were able to obtain a complete breakdown of the acid site distribution. Good correlations were obtained between the rate of xylene disproportionation and the number of Lewis sites. A similar approach was used by Stach and Janchen to determine the acid... 

Table 6.18 p-Xylene disproportionation reaction data over Si-HZSM-5 zeolite... 

Localization of stationary points along the reaction path for reactions taking place inside the zeolite pores is one of the greatest challenges in zeolite modeling. The reactions of hydrocarbons are particularly difficult to model since the hydrocarbon...zeolite interaction can be dominated by the dispersion interaction that is not properly accounted at the DFT level. Only one example is presented here. Clark et al. investigated the role of benzenium-lype carbenium ion in the bimolecular w-xylene disproportionation reaction in zeolite faujasite.163] The benzenium-type carbenium ion 1 was identified in zeolite catalyst for the... 

Under these conditions all molecular sieves evaluated give essentially complete isomerization of m-xylene feed to a thermodynamic equilibrium mixture of xylene isomers, while the disproportionation activity to toluene and trimethylbenzenes varies significantly. The results of this study are summarized in Table III and in Figure 3, where xylene disproportionation activity is plotted as a function of molecular sieve pore size. In general, a rough trend can be seen in which the molecular sieves with larger pore sizes are more active for this undesirable side reaction. 

Molecular Sieve a Pore Size b Pore Volume m-Xylene Disproportionation, % Conversion... 

The formation of coke molecules requiring numerous bimolehular reactions an increase with the reactant pressure in the coking rate and in the C/P ratio can be expected. In m-xylene disproportionation on mordenites the coking rate and the coking/disproportionation rate ratio for Pm-xylene were about two times greater than for... 

---