Xylenes Conversion

In 1993, UOP commercialized an improved Pt-based catalyst, 1-210. This catalyst is based on a molecular sieve, but not an alurninosihcate zeoHte. UOP claims that yields ate about 10% better than those for 1-9 catalyst. EB to xylenes conversion is about 22—25% with a Cg aromatics per pass loss of about 1.2—1.5%. As discussed below, UOP s Isomar process can also use zeoHte catalysts which convert EB to benzene rather than to xylenes. UOP has hcensed over 40 Isomar units. 

Selective oxidation of p-xylene to terephthaldehyde (TPAL) on W-Sb oxide catalysts was studied. While WO3 was active in p-xylene conversion but non-selective for TPAL formation, addition of Sb decreased the activity in p-xylene conversion but increased TPAL selectivity significantly. Structure change was also induced by Sb addition. Evidences from various characterization techniques and theoretical calculation suggest that Sb may exist as various forms, which have different p-xylene adsorption property, reactivity toward p-xylene and TPAL selectivity. Relative population of each species depends on Sb content. 

Fig. 3. (Left) p-Xylene conversion and TPAL selectivity at 550 °C as a fimction of Sb content (Right) Product distribution at 550 °C as a function of Sb content.

Molecular modeling showed that WO3 surface has a structure where p-xylene may adsorb well, explaining the high activity of WO3 in p-xylene conversion. Experimental data in this study show that Sb addition modifies significantly the activity and selectivity properties of WO3 as well as the catalyst structure. Fig. 3 shows the minimum and maximum p-xylene... 

WO3 was active in selective oxidation of p-xylene conversion but was not selective for... 

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... 

The product is readily analyzed by vapor phase chromatography. Since the only impurity is o-xylene (conversions range from 80% to 100%), the percentage of reduction product was... 

The kinetic parameters are presented in Table 35. According to these data, the maximum yield of phthalic anhydride (at 100% xylene conversion) is about 45%. 

Fig. 9.5 Transformation of m-xylene over HMORIO and NaHMORlO samples. Disproportionation / Isomerisation rate ratio (D/I) versus m-xylene conversion (X%) on fresh and on deactivated catalysts HMORIO (0), 14NaHMOR ( ), 28NaHMOR (A), 45NaHMOR (+), 63NaHMOR (o)...

Scheme 20.3 Reaction network in o-xylene conversion to phthalimide and phthalonitrile. Adapted from [84].

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]. 

Figure 7. Reactor-selectivity versus p-xylene conversion...

Figure 10. Reactor selectivity versus pH xylene conversion for conversion of an isomer mixture...

Due to the chemical sink inside the zeolite crystals the reaction of para xylene to para products is favored up to high xylene conversions. 

To obtain the total yield in relation to e-xylene, it is necessary to consider that of the phthalic anhydride production step, which barely exceeds 65 to 70 molar per cent in the gas phase oxidation (Von Heyden type reactor). At present no industrial plant employs this process, which is uneconomical in comparison with other methods, given the lack of selectivity of the o-xylene conversion step. 

Figure 6.41 shows the c-Xylene conversion for each of the steady states. The conversion profiles of the three steady states differ slightly in accordance with the small differences between the temperature profiles. At the exit of the reactor the three steady states give almost 100% conversion. 

Figure 6.41 shows that the o-Xylene conversion predicted by the pseudo-homogeneous model is lower than all the three steady states of the heterogeneous model until a length of approximately 0.32 m when the conversion predicted by the pseudo-homogeneous model exceeds that of the heterogeneous model and reaches a value of 1.0. 

Figures 6.50-6.52 show that for this case, the pseudo-homogeneous model predicts temperature, o-Xylene conversion and phthalic anhydride yield profiles which are very close to those of the low temperature steady state (steady state no 1) of the heterogeneous model.

FIGURE 5.41 p-Xylene conversion and phthalic anhydride yield at... 

Figure 6.36 shows that o-Xylene conversion decreases as feed o-Xyiene mole fraction increases, but the rate of o-Xylene consumption increases as the feed mole fraction increases which can be calculated as follows ... 

In Figure 6.37 it is clear that the phthalic anhydride yield Yp decreases as the o-Xylene feed mole fraction increases as a result of o-Xylene conversion decrease. However, it does not mean that the rate of formation of phthalic anhydride is higher for low o-Xylene feed mole fraction than it is for higher o-Xylene mole fraction, where it can be calculated as follows ... 

Figure 6.56 gives the o-Xylene conversion which shows higli exit conversion for the three steady states. For the low steady state the conversion starts increasing at the ignition point where at the same... 

In case of the m-xylene isomerization it should be noted that the m-xylene conversion is restricted to about 48 % for thermodynamical reasons. This equilibrium is practically reached for samples after 24 hcrystallization time. In case of the ethylbenzene it can be seen that the conversion... 

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