Diffusion of p-xylene

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... 

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],... 

Fig. 28 Co-diffusion of p-xylene and benzene. Uptake from a mixture of benzene and p-xylene as a function of the square root of time at 395 K but for two different partial pressures of benzene in the mixture. The overshooting of benzene sorption is clearly demonstrated (see text)...

The higher diffusivities of p-xylene compared to those of benzene in silicahte-1 can be ascribed to the combination of enthalpy and entropy effects [60-62]. The sorbed benzene molecules have to lose their rotational freedom around their hexagonal axis, Ce, when they move from an intersection to a channel segment, i.e., there is a large decrease in entropy in this jump step. For p-xylene, the molecules are, however, orientated with their long molecular axis along either channel direction when they are located at an intersection. The diffusion jump step involves, therefore, only a very small... 

There arc numerous examples of product selectivity, many of which involve mono-and disubslituted aromatics formed over ZSM-5 (MFI) catalysts (22-24). One of the early examples was xylene isomerization. Xylene can be formed over MFI catalysts via the acid-catalyzed reaction between methanol and toluene (see Fig. 10.3). According to thermodynamics the equilibrium distribution of o-, m-, and p-isomers is 26 51 23, which is different from the industrial demand as the p-isomer is a feedstock for terephthalic acid, a monomer for PET. However, very high selectivities of p-xylene can be obtained over MFT materials primarily because the diffusion of p-xylene is faster in the MFI pores compared to the other two isomers. 

Treatment with inorganic compounds is considered to reduce the dimensions of pore openings and channels sufficiently to favor outward diffusion of p-xylene, the isomer with the smallest molecular dimension. 

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

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 ... 

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 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],... 

Olson and Haag [80] in 1983 showed that the yield of p-xylene observed during the disproportionation of toluene on various modified and unmodified ZSM-5 catalysts is actually influenced by product shape selectivity. The authors attributed the observed effects to an interaction of diffusion and reaction, characterized by means of a dimensionless modulus similar to the classical Thiele modulus . The mathematical treatment of shape selectivity in zeolite catalysts, which will be applied in this section, is largely based on the theory of Olson and Haag [80], although some modifications and extensions to this are given. 

The difiiisional behavior of p-xylene is complicated. The FR measurements reveal two different diffrisivities corresponding to movement through the straight and sinusoidal channels. The ZLC method increases only the average diffusivity which is similar to the value for benzene but it is possible that the difference between the self and transport diffrjsivity results from the two channel behavior revealed by the FR data [33]. 

For p-xylene, the sorbed molecules diffuse down the direction of both the straight and sinusoidal channels in silicahte-1 at loadings < 4 m./u.c. and low temperatures. At high temperatures, however, only a pure, single diffusion process can be detected. Surprisingly, the sorbed p-xylene molecules diffuse faster down the straight channel direction than benzene, a smaller molecule than p-xylene. The diffusivities of the four aromatics illustrated in Fig. 15 show a decrease in the order of p-xylene > toluene > benzene > ethylbenzene. 

The effect of subtle differences in certain properties of sorbate molecules on the diffusivities can be very intriguingly demonstrated in Fig. 16. With shape and size close to that of p-xylene, the long, rigid molecule, p-dichloro-benzene (p-DCB) displays an FR behaviour in silicalite-1 very similar to that of p-xylene. The flexible saturated cyclic hydrocarbons diffuse much more slowly within the channel framework of silicalite-1 than their rigid aromatic... 

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