O-Xylene diffusion

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). 

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

In the case of H-ZSM-11, the results reported in Table 5.4 indicate that the mechanism for p- and o-xylene diffusion is by hopping between sites or jump diffusion, since the values reported for D0 are within the limits 1 x 10 4 < < 5 x 10" cm2/s. Hence, the values of the pre-exponential... 

Care mustbe used in applying such arguments when estimating practical diffusion coefficients, however, since increases in infinite-dilution hmit can cause significant changes in the effective mobility of the penetrant. Moreover, the (9 In oa/9 In o>a) term can deviate from unity at weight fractions as small as 10-20%, as was indicated in the case of carbon dioxide and ethylene earher. In the case of o-xylene diffusion in polyethylene at 150°C, both the effects of plasticization and of the (9 In oa/9 In self-diffusion coefficient have been analyzed and are shown in Figure 13 (34). 

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... 

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 ... 

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. 

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 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. 

When the molecular diameter approaches the channel opening of a ZSM-5 zeolite, the diffusion coefficient can drop by nine orders of magnitude between normal hexane and 1,3,5-trimethylbenzene, and can drop by three orders of magnitude between p-xylene and o-xylene (table 11.4). 

Using the monomolecular rate theory developed by Wei and Prater, we have analyzed the kinetics of the liquid-phase isomerization of xylene over a zeolitic catalyst. The kinetic analysis is presented primarily in terms of the time-independent selectivity kinetics. With the establishment of the basic kinetics the role of intracrystalline diffusion is demonstrated by analyzing the kinetics for 2 to 4 zeolite catalyst and an essentially diffusion-free 0.2 to 0.4 m zeolite catalyst. Values for intracrystalline diffusivities are presented, and evidence is given that the isomerization is the simple series reaction o-xylene <= m-xylene <= p-xylene. 

Let us now assume that isomerization of xylenes is a simple series reaction. Starting with pure o-xylene feed, and in the absence of any diffusional resistance, most molecules of o-xylene which enter the zeolite crystallites leave after, at most, one reaction step (to m-xylene), yielding primarily a single-step product. When substantial diffusion-resistance exists, most molecules remain in the crystallite long enough for several reaction steps to occur (to m-xylene and p-xylene), and the products are mostly multistep products. Wei (4) has shown that in such a case the apparent kinetics—i.e., those determined from the respective reactant product concentrations in the bulk phase—change from that of a simple series reaction... 

Figure 5. o-Xylene isomerization intrinsic and diffusion-disguised reaction... 

Temperature Dependence of the Activity and Selectivity of Xylene Isomerization over AP Catalyst. Based upon our analysis of the intracrystalline diffusional resistance in AP catalyst, we would expect that when the reaction temperature is increased, the selectivity would shift toward p-xylene since the diffusional effects are increased as the activity increases. A shift in selectivity toward p-xylene as the reaction temperature was increased was observed and is shown in Figure 6. The role of diffusion in changing the selectivity can be seen in the Arrhenius plot of Figure 7. The reaction rate constant for the o-xylene - p-xylene path, fc+3i, goes from an almost negligible value at 300°F to a substantial value at 600°F. Furthermore, the diffusional effects are also demonstrated by the changing... 

Figure 4.11 shows an example of how ZSM-5 is applied as a catalyst for xylene production. The zeolite has two channel types - vertical and horizontal - which form a zigzag 3D connected structure [62,63]. Methanol and toluene react in the presence of the Bronsted acid sites, giving a mixture of xylenes inside the zeolite cages. However, while benzene, toluene, and p-xylene can easily diffuse in and out of the channels, the bulkier m- and o-xylene remain trapped inside the cages, and eventually isomerize (the disproportionation of o-xylene to trimethylbenzene and toluene involves a bulky biaryl transition structure, which does not fit in the zeolite cage). For more information on zeolite studies using computer simulations, see Chapter 6. 

In this section, single-component diffusion in zeolites [20,87-92], with the help of the case study of the diffusion of p-xylene and o-xylene in H-ZSM-11 and H-SSZ-24 zeolites, is discussed [90], SSZ-24 is a 12-MR zeolite that was first obtained as the silicon counterpart of AlP04-5, and, later, was obtained as a borosilicate (B-SSZ-24), which could be exchanged with A1 to yield the H-SSZ-24 zeolite [111], This zeolite exhibits the AFI framework, which embodies a one-dimensional channel network without cavities, consisting of parallel 12-MR channels with a free-channel diameter, ow = 7.0A [112], Additionally, ZSM-11 encloses an intersecting two-directional 10-MR channel system, where the two-dimensional channel system is characterized by the free-channel diameter, ow = 5.8 A [112],... 

The probe molecules used in the single diffusion are p-xylene and o-xylene. The kinetic diameter of these probe molecules (o are om = 5.8 A for p-xylene, and om = 7.0 A for o-xylene [11],... 

FIGURE 5.33 Diffusion kinetics uptake curve of (a) /5-xylene and (b) o-xylene in H-SSZ-24 at 400K. Experimental points (-) fitted with the continuous theoretical curve. 

For the systems described in this chapter, the values for om and ow are om = 5.8 and om = 7.0 A for / -xylene and o-xylene, respectively [11,12] and ow = 7 A for the SSZ-24 channel windows and ow = 5.8 A for the ZSM-11 zeolite [112], Therefore, p-xylene and o-xylene relatively, freely move in H-SSZ-24 during single-component diffusion, inasmuch as r p x = 0.83 and T 0, = 1.00. In addition, for H-ZSM-11, the single-component diffusion of o-xylene is hindered by steric factors inasmuch as rp= 1.21, but the single-component diffusion for / -xylene is relatively free since rp, = 1. These facts are reflected on the reported single-component diffusion coefficients (see Table 5.3). Besides, the results reported in Table 5.3 reasonably agree with data previously reported in the literature for the diffusion of xylenes in zeolites with 10- and 12-MR channels [88,116-120],... 

In the case of H-SSZ-24, the values of the pre-exponential factor experimentally obtained (see Table 5.4) do not agree with the values theoretically predicted by the equation for a jump diffusion mechanism of transport in zeolites with linear channels, in the case of mobile adsorption [6,26], Furthermore, the values obtained for the activation energies are not representative of the jump diffusion mechanism. As a result, the jump diffusion mechanism is not established for H-SSZ-24. This affirmation is related to the fact that in the H-SSZ-24 zeolite Bronsted acid sites were not clearly found (see Figure 4.4.) consequently p- and o-xylene do not experience a strong acid-base interaction with acid sites during the diffusion process in the H-SSZ-24 channels, and, therefore, the hopping between sites is not produced. 

Effective Diffusion Coefficients (De x 109 [cm2/s]) in p-Xylene Plus o-Xylene Counterdiffusion in H-ZSM-11 and H-SSZ-24 Zeolites at Different Temperatures at a Gas Phase Concentration,... 

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],... 

In Table 5.5, the effective diffusivity, De, for p-xylene plus o-xylene counterdiffusion in H-SSZ-24 and H-ZSM-11 zeolites at different temperatures and a concentration relation, cp x [%] = c x [%] = 50 [%], are reported [90], It is evident that the kinetics is governed by ordinary diffusion. Additionally, the study of the counterdiffusion of p-xylene + o-xylene and the reverse case o-xylene + p-xylene in a zeolite with a 10 member ring plus 12 member ring interconnected channel-like CIT-1 gives experimental evidence for the existence of molecular traffic control [125],... 

As with HZSM5-la, we attribute the initial deactivation to blocking of catalytically active sites by adsorbed xylene molecules preventing toluene methylation to occur at these sites. The longer residence time of the bulkier xylene isomers in the larger crystals of HZSM5-2 (see Table 1) seems to favour further alkylation of m- and o-xylene to trimethylbenzenes over isomerization to p-xylene Once trimethylbenzene is formed, dealkylation is rather difficult at 573 K and its rate of transport is too low to be able to diffuse out of the zeolite pores. It forms, thus, a dead end product that decreases the availability of active sites and reaction intermediates (leading to slow deactivation). 

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