Diffusion xylenes

Selective Toluene Disproportionation. Toluene disproportionates over ZSM-5 to benzene and a mixture of xylenes. Unlike this reaction over amorphous sihca—alumina catalyst, ZSM-5 produces a xylene mixture with increased -isomer content compared with the thermodynamic equihbtium. Chemical modification of the zeohte causing the pore diameter to be reduced produces catalysts that achieve almost 100% selectivity to -xylene. This favorable result is explained by the greatly reduced diffusivity of 0- and / -xylene compared with that of the less bulky -isomer. For the same reason, large crystals (3 llm) of ZSM-5 produce a higher ratio of -xyleneitotal xylenes than smaller crystahites (28,57). 

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. 

Since their development in 1974 ZSM-5 zeolites have had considerable commercial success. ZSM-5 has a 10-membered ring-pore aperture of 0.55 nm (hence the 5 in ZSM-5), which is an ideal dimension for carrying out selective transformations on small aromatic substrates. Being the feedstock for PET, / -xylene is the most useful of the xylene isomers. The Bronsted acid form of ZSM-5, H-ZSM-5, is used to produce p-xylene selectively through toluene alkylation with methanol, xylene isomerization and toluene disproportionation (Figure 4.4). This is an example of a product selective reaction in which the reactant (toluene) is small enough to enter the pore but some of the initial products formed (o and w-xylene) are too large to diffuse rapidly out of the pore. /7-Xylene can, however. 

An example of this is the commercial process for preparing puru-xylene, the precursor to terephthalic acid, which is polymerised to give polyjethy-lene terephthalate) (PET). In this case, the mixture of xylenes obtained from crude oil is reacted in a zeolite (known as HZSM5). The relative rates of diffusion in and out of the pores are sufficiently different (by a factor of about ten thousand) to allow the extremely efficient and selective conversion of all the isomers to the desired paia isomer, which is the narrowest and can thus move through the structure most rapidly (Figure 4.3). 

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. 

The correlation between selectivity and intracrystalline free space can be readily accounted for in terms of the mechanisms of the reactions involved. The acid-catalyzed xylene isomerization occurs via 1,2-methyl shifts in protonated xylenes (Figure 3). A mechanism via two transalkylation steps as proposed for synthetic faujasite (8) can be ruled out in view of the strictly consecutive nature of the isomerization sequence o m p and the low activity for disproportionation. Disproportionation involves a large diphenylmethane-type intermediate (Figure 4). It is suggested that this intermediate can form readily in the large intracrystalline cavity (diameter. 1.3 nm) of faujasite, but is sterically inhibited in the smaller pores of mordenite and ZSM-4 (d -0.8 nm) and especially of ZSM-5 (d -0.6 nm). Thus, transition state selectivity rather than shape selective diffusion are responsible for the high xylene isomerization selectivity of ZSM-5. 

This may be partly the result of increased steric crowding in the transition state of transalkylation. Another contributory factor to the increased selectivity in ZSM-5 is the higher diffusion rate of ethylbenzene vs m-/o-xylene in ZSM-5 and hence a higher steady state concentration ratio [EB]/[xyl] in the zeolite interior than in the outside phase. Diffusional restriction for xylenes vs ethylbenzene may also be indicated by the better selectivity of synthetic mordenite vs ZSM-4, since the former had a larger crystal size. 

The reaction scheme to be considered is shown in Figure 9. Toluene diffuses into the zeolite with a diffusivity DT. It undergoes disproportionation to benzene and either p-, m-, or o-xylene with a total rate constant kD. The initial product distribution (P, o ) is not known. In the absence of steric... 

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). 

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. 

In order to compare a number of different zeolite preparations we have found it convenient to determine not the diffusivity of o-xylene per se, but to characterize the samples by measuring the time (tQ 3) it takes to sorb 30% of the quantity sorbed at infinite time. The characteristic diffusion time, t0 3, is a direct measure of the critical mass transfer property r2/D ... 

Figure 11. Effect of diffusivity on p-xylene selectivity. Toluene disproportionation at 550°C, 20% conversion o-xylene diffusivity at 120°C.

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. 

Figure 12 Relationship between the diffusion parameter, to.3 and p-xylene selectivity in toluene disproportionation. Temperature 550°C. Pressure 41 bar. Conversion 20%. tQ 3 time to reach 30% of amount sorbed at infinite time.

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. 

Likewise, the value for hexane cracking at 538°c, the a-value, is proportional to the intrinsic value for toluene disproportionation and xylene isomerization, and is much easier to obtain since it is not affected by crystal size or diffusive alteration (5 ). 

Figure 16. Effect of pore filling on diffusivity of o-xylene at 120°C in HZSM-5.

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... 

A quantitative model requires knowledge of the diffusivity under reaction conditions and of the intrinsic activities for toluene disproportionation and xylene isomerization. While these are not easily obtained, the methodology has been worked out for the case of paraffin and olefin cracking (5). So far, we have obtained an approximate value for the diffusivity, D, of o-xylene at operation conditions from the rate of sorptive o-xylene uptake at lower temperature and extrapolation to 482°C (Table V). 

The above discussion is most nearly correct when only one contaminating chemical is involved. When contaminants such as gasoline are introduced into the subsurface, the setting becomes more complex. Gasohne is a mixture of mosdy small molecules, such as benzene, xylene, toluene, hexane, and other molecules that are mostly less than C14. Diffusion of a mixture of dissolved chemicals involves several factors. 

The influence of the CD content in the membrane and the n-PrOH respectively p-xylene content in the feed mixture on the separation factors and sorption and diffusion selectivities of the CD/PVA membranes for the n-PrOH/I-PrOH and p-xylene and o-xylene mixtures by evapomeation are presented in tables 12 and 13. 

Coughlin and Canevari (1%9) have published experimental data on two systems at a variety of operating conditions the extraction of xylene from polypropylene and the extraction of methanol from polypropylene. These studies were conducted in a single screw extruder at low pressures and w was assumed to be small in comparison with w. Coughlin and Canevari developed a model which they used in conjunction with their experimental data to obtain a value for the diffusion coefficient. The values that they computed were of the order of 10 mVsec, which obviously means that the model is incorrect. Coughlin and Canevari also computed values for the mass transfer coefficient and found it to be independent of screw speed. This observation is particularly noteworthy since they saw no evidence of bubble formation. 

Zeolite membranes have also been employed for organic-organic separations where selectivity is based on adsorption and diffusion differences of non-aqueous mixtures. NaX and NaY zeolite were used in the separation of methanol from MTBE and benzene (800 < a< 10000) exploiting the more polar nature of methanol which is attracted to the electrostatic poles of the high A1 content zeolites [38]. Other separations include (i) separation of n-hexane from 2,2-DMB using ZSM5, (ii) benzene from p-xylene using MOR/FER and (iii) xylene isomers [34]. 

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