Diffusion para-xylene

Thus, evidence has accumulated in support of hydrogen exchange in benzene by a mechanism involving associatively chemisorbed benzene, and without the necessity to postulate the participation of chemisorbed C Hs. One attractive test of these ideas which, so far as we know, has not been made, would be to repeat, for example, the reaction of para-xylene with deuterium using as catalyst a palladium thimble. This system would allow the exchange reaction to proceed either in the presence of molecular deuterium (both reactants on same side of the thimble) or in the presence of atomic deuterium only (xylene and molecular deuterium on opposite sides of the thimble, so that the hydrocarbon reacts only with chemisorbed atomic deuterium that arrives at the surface after diffusion through the metal). 

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. 

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. 

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 7.20 Computer models illustrating how (a) para-xylene fits neatly in the pores of ZSM-5 whereas (h) me/a-xylene is too hig to diffuse through. See colour insert following page 356. 

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... 

Figure 6. Comparison of diffusivities of MFI type between calculated by lattice model and the experimental data. B benzene, T toluene, p-X para-xylene, m-x wt /a-xylene, o-x c r/to-xylene, n-Ci (i=6-8) /-paraffin of carbon number i, i-C8 /.SY -octane.

When refering to shape selectivity properties related to diffusivity, it seems obvious that the larger the zeolite grain, the higher will be the volume/sur f ace ratios and the shape selectivity, since the reaction will be more diffusion controlled. The external surface area represents different percents of the total zeolite area depending on the size of the grains which could be important if the active sites at the external surface also play a role in the selectivity. For instance in the case of toluene alkylation by methanol, the external surface acid sites will favor the thermodynamical equilibrium due to isomerization reactions (o m p-xylene - 25 50 25 at 400 C) while diffusivity resistance will favor the less bulky isomer namely the para-xylene. It may therefore be useful to neutralize the external surface acidity either by some bulky basic molecules or by terminating the synthesis with some Al free layers of siliceous zeolite. 

Figure 11 is a highly simplified diagram of this process. Toluene enters the ZSM-5 crystal and disproportionates at the acid catalyst sites to benzene and the three xylene isomers para-, meta-, and ortho-. Because of their larger size, meta- and ortho-xylenes diffuse slower than para-xylene. As you expect, the longer they stay inside the ZSM-5 crystal, the richer the product will be in para-xylene. 

The relative diffusion rate for para-xylene in this modified ZSM-5 catalyst is at least a thousand times faster than the diffusion rates of the other isomers, and this results in a para-xylene concentration much higher than equilibrium (Ref. 11). 

One of the most important industrial alkylations is the production of 1,4-xylene from toluene and methanol (Reaction 2). ZSM-5, in the proton exchanged form, is used as the catalyst because of its enhanced selectivity for para substituted products. para-Xylene is used in the manufacture of terephtha-lic acid, the starting material for the production of polyester fibres such as Terylene. The selectivity of the reaction over HZSM-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 approximately 1000 times faster than that of the meta and ortho isomers.14... 

The intracrystalline diffusivities of llZSM-5 zeolite were directly measured for several hydrocarbons at higher temperatures (373-773 K) by the constant volume method. High silicious HZSM-5 zeolite, which has no activity for reactions, was used as the adsorbent. Aromatics benzene, xylene-isomers ortho-, meta- and para-xylene) and toluene, and paraffins n-hexane, n-pentane, p-octane and iso-octane, were used as adsorbates. Intracrystalline diffusivities of aromatics markedly depended on the minimum size of the aromatics and that of paraffins depended on the carbon number (molecular weight of the paraffins). A method was developed for predicting diffusivity in terras of pore diameter and molecular properties of hydrocarbons. This method was found to well represent the experimental results. 

Restricting a reaction to produce a specific product, when more than one product is possible, can also occur because of the differences in the relative diffusivities of the various products. In Figure 3.3b, para-xylene can emerge from the pore system much more rapidly than either the meta or the ortho isomer. A representation of para-xylene in a zeolite pore is shown in Figure 3.4. 

During the early 1970s, Clarence D. Chang, Anthony J. Silvestri, and William Lang, at Mobil Research and Development, discovered Zeolite ZSM-5, one of the most important catalysts ever produced. Zeolite ZSM-5 is a superacidic substance that catalyzes isomerization of ortho-and meta-xylenes to para-xylene the latter s cylindrical shape helps it to diffuse much more rapidly through the zeolite matrix. Quite by accident, they discovered that ZSM-5 catalyzes the conversion of methanol to... 

Impregnation of ZSM-5 with different compounds produce small changes in the zeolite pore dimensions which increases the selectivity to para-xylene (168,169) as can be seen from the results in Table 12 taken from (169). Iliese results can be explained on the bases of a model which considers the interplay between the relative rates of the toluene disproportionation, product diffusion, and xylene isomerization (170), as it is schematized in Figure 32, taken from (170), in where D is the diffusion coefficient, K is the rate constant, and r is the crystal radius. The reactions considered are Isomerization "(I) and Disproportionation (D), while the reaction products are benzene (Bz), toluene (I and xylenes (P,M,0). Initial products (i), primary products (o), and secondary products (P,M,0) arc considered in this scheme. 

The first examples of molecular shape-selective catalysis in zeolites were given by Weisz and Frilette in 1960 [1]. In those early days of zeolite catalysis, the applications were limited by the availability of 8-N and 12-MR zeolites only. An example of reactant selectivity on an 8-MR zeolite is the hydrocracking of a mixture of linear and branched alkanes on erionite [4]. n-Alkanes can diffuse through the 8-MR windows and are cracked inside the erionite cages, while isoalkanes have no access to the intracrystalline catalytic sites. A boom in molecular shape-selective catalysis occurred in the early eighties, with the application of medium-pore zeolites, especially of ZSM-5, in hydrocarbon conversion reactions involving alkylaromatics [5-7]. A typical example of product selectivity is found in the toluene all lation reaction with methanol on H-ZSM-5. Meta-, para- and ortho-xylene are made inside the ZSM-5 chaimels, but the product is enriched in para-xylene since this isomer has the smallest kinetic diameter and diffuses out most rapidly. Xylene isomerisation in H-ZSM-5 is an often cited example of tranSition-state shape selectivity. The diaryl type transition state complexes leading to trimethylbenzenes and coke cannot be accommodated in the pores of the ZSM-5 structure. 

Young et al. suggested the following kinetic situation under the toluene disproportionation conditions. The transalkylation reaction to form benzene and xylenes within the pores is relatively slow. Benzene diffuses out of the pores rapidly. The xylenes isomerize rapidly within the pores. (Xylene isomerization is about 1000 times faster than toluene disproportionation.) para-Xylene diffuses out moderately fast while the ortho and meta isomers move within the pores relatively slowly and further convert to para isomer before escaping from the channel system. 

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... 

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. 

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. 

The primary product will be rich in the para isomer if initial m-and o-xylene diffuse out of the zeolite crystal at a lower rate (Dm q/t2) than that of their conversion to p-xylene (kj) and the latter s diffusion (Dp/r2). Conversion of the para-rich primary product to secondary product low in p-xylene is minimized when the actual, observed rate of isomerization (kj g is lower than the rate of toluene disproportionation (kD). 

Rigorous treatment of the para-selectivity requires a knowledge of the intrinsic value of the rate constant for all the reactions involved and of the absolute value of the crystal size and of the diffusivity, all under reaction conditions. These values are obtainable only with considerable difficulty and effort. As has been mentioned, the 30 percent sorption time for o-xylene at 120°c, t0.3/ is proportional to the actual values, r2/D. 

It has also been shown that the selectivity features of para-selective catalysts can be readily understood from an interplay of catalytic reaction with mass transfer. This interaction is described by classical diffusion-reaction equations. Two catalyst properties, diffusion time and intrinsic activity, are sufficient to characterize the shape selectivity of a catalyst, both its primary product distribution and products at higher degrees of conversion. In the correlative model, the diffusion time used is that for o-xylene adsorption at... 

In the case of the measurement of the diffusivity in the p-xylene + o-xylene counterdiffusion experiment, the sample was initially saturated with a stream of p-xylene at a partial pressure of 6.7 Pa then, to this stream of carrier gas plus p-xylene, the carrier gas saturated with o-xylene was admitted, to finally obtain the same partial pressure, 6.7 Pa, for both hydrocarbons. The composition of the final hydrocarbon mixture, that is, the gas phase concentration of p-(cp x) and o-(c0.x) xylene, obtained was checked with a gas chromatograph (FISONS 8000) coupled to the gas outlet of the IR cell (see Figure 5.34). The gas phase concentration, for p-(cp x) and o-xylene (c x) in the fed mixture of the counterdiffusion experiment was the same cp x [%] = c0 x [%] = 50 [%] [90], If Figure 5.34, the uptake curves corresponding to the counterdiffusion kinetics of para + ortho xylene in H-ZSM-11 at 375 K and 400 K are shown [90],... 

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