P-xylene diffusion

The other isomers (o,m) of xylene isomerize rapidly inside the pores. The p-xylene diffuses out reasonably fast, while other isomers are rather slow and thus these isomers undergo isomerization to p-xylene before escaping from the channel system. 

Shen, D. and Rees, L.V.C., Analysis of bimodal frequency-response behaviour of p-xylene diffusion in silicalite-1, J. Chem. Soc., Faraday Trans., 91, 2027-2033, 1995. 

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. 

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.

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.

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. 

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]. 

When the steps become bigger, or p-xylene concentration is lower, the corner in the steps forms a groove. In this tage, the diffusion effect is increasing in rate determining factors instead of the surface Integration. 

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). 

Similar energy minimization calculations were reported for benzene and p-xylene in silicalite (92). Diffusion coefficients were estimated from minimum energy paths through the pore. The value for benzene, 27.6 kJ/mol, is in good agreement with that of Pickett et al. (91). For the bulkier p-xylene molecule, the activation barrier was predicted to be slightly lower (23 kJ/... 

The calculations of Klein et al. predicted a larger activation barrier for the diffusion of m-xylene than for diffusion of o- and p-xylene. Thus, m-xylene may be expected to diffuse more slowly than o- or p-xylene, which is inconsistent with the diffusion coefficients and activation energies determined experimentally (24,100). Of course, one of the main factors that precludes a truly meaningful comparison is that the calculations simulate the... 

A different product distribution is obtained by shape-selective zeolites.85,86 Since the diffusion coefficient of p-xylene with its preferred shape into the zeolite... 

We have seen previously shape-selective catalysis by ZSM-5 in the conversion of methanol to gasoline (Chapter 15).-7 Other commercial processes include the formation of ethylbenzene from benzene and ethylene and the synthesis of p-xylene. The efficient performance of ZSM-5 catalyst has been attributed to its high acidity and to the peculiar shape, arrangement, and dimensions of the channels. Most of the active sites are within the channel so a branched chain molecule may not be able to diffuse in, and therefore does not react, while a linear one may do so. Of course, once a reactant is in the channel a cavity large enough to house the activated complex must exist or product cannot form. Finally, the product must be able to diffuse out. and in some instances product size and shape exclude this possibility. For example, in the methylu-uon of toluene to form xylene ... 

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... 

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... 

The high isomer selectivity observed for ZSM-5 can be further enhanced by imposing kinetic diffusion-al effects upon the thermodynamic selectivity, thereby magnifying the preference for p-xylene sorption (1 ). ... 

The MFI class of channel zeolites, of which ZSM-5 is a member, are of enormous importance in the petrochemicals industry because of their shape-selective adsorption and transformation properties. The most well-known example is the selective synthesis and diffusion of p-xylene through ZSM-5, in preference to the o- and m-isomers. Calcined zeolites such as ZSM-5 are able to carry out remarkable transformations upon normally unreactive organic molecules because of super-acid sites that exist... 

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],... 

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-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... 

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],... 

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