Para-xylene selectivity diffusivity effects

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. 

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. 

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]. 

More recently, methods have been developed for reducing the effective pore and channel dimensions. These techniques employ both physical treatments and chemical reagents. They have provided the basis for para-selective alkylation catalysts (18). These modified zeolites permit discrimination between molecules of slightly different dimensions. As a result, the para-isomers of the xylene or ethyltoluene products with the smallest minimum dimensions (Table 4) are able to diffuse out of the catalyst pores at rates about three orders of magnitude greater than those for the corresponding ortho- and meta-isomers (20). This discrimination capability is schematically represented in Figure 1, where the effective size of a para-selective catalyst pore is shown by the dashed line. 

---