Toluene para-xylene from

Based on fundamental studies that revealed the important variables in controlling the selectivity, we have been able to modify the ZSM-5 catalyst to produce para-xylene from toluene with much greater selectivity. Actually, what we ve done is partially plug the pore openings in ZSM-5 to make it more difficult for the bulkier meta-xylene and ortho-xylene molecules to come out. 

As illustrated in Figure 10.6, the high para-selectivity in the toluene disproportionation is caused by the selective removal of p-xylene from the silica-alumina particles, which leads to an apparent equilibrium shift between the xylene isomers. 

The excellent high para-selectivity can be explained by the selective escape of p-xylene from the H-ZSM-5 catalyst and inhibition of isomerization on the external surface of catalysts by silicalite-1 coating. In addition to the high para-selectivity, toluene conversion was still high even after the silicalite-1 coating because the silicalite-1 layers on H-ZSM-5 crystals were very thin. 

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. 

Benzene and para-xylene are the most sought after components from reformate and pygas, followed by ortho-xylene and meta-xylene. While there is petrochemical demand for toluene and ethylbenzene, the consumption of these carmot be discussed in the same way as the other four. Toluene is used in such a large quantity in gasoline blending that its demand as a petrochemical pales in comparison. Fthylbenzene from reformate and pygas is typically dealkylated to make benzene or isomerized to make xylenes. On-purpose production of petrochemical ethylbenzene (via ethylene alkylation of benzene) is primarily for use as an intermediate in the production of another petrochemical, styrene monomer. Ethylbenzene plants are typically built close coupled with styrene plants. 

Oxidation of ortho-xylene. The spectra of the adsorbed species arising from interaction of ortho-xylene with the surface of the vanadia-titania catalyst in the presence of oxygen are shown in Figure 4. The spectra show some parallel features with respect to those discussed above concerning the oxidation of toluene and meta- and para-xylene. Also in this case the o-methyl-benzyl species begins to transform above 373 K, with production of adsorbed o-tolualdehyde (band at 1635 cm 0 and of a quinone derivative (band at 1670 cm. Successively bands likely due to o-toluate species (1530,1420 cm 0 grow first and decrease later with production of CO2 gas. 

When toluene is alkylated by methanol with a ZSM-5 catalyst, increase in the ciystallite size from 0.5 to 3 pm approximately doubles the amount of para-xylene produced. Suggest a possible explanation. 

The alkyl groups can also be made to pass from one hydrocarbon to another, toluene in the presence of alnminium chloride yielding both benzene and meta- and para-xylene. 

Lee et al. (65) reported that the selectivity in connection with the formation of p-xylene from methanol and toluene can be improved significantly by adding Sb203 to the HZSM-5 zeolite catalyst. Without this additive, the xylene produced in the catalytic reaction of methanol and toluene over HZSM-5 at 400°C is an equilibrium mixture containing 23.1% p-xylene. However, the para selectivity approaches 100% if the reaction proceeds over the modified HZSM-5 catalyst prepared by calcining a mixture of HZSM-5 AND Sb203 in air at 500°C for 2 h. 

Application To produce high yields of benzene, toluene, xylenes and hydrogen from naphthas via the CCR Aromizing process coupled with RegenC continuous catalyst regeneration technology. Benzene and toluene cuts are fed directly to an aromatics extraction unit. The xylenes fraction, obtained by fractionation and subsequent treatment by the Arofining process for diolefins and olefins removal, is ideal for para-xylene and orthoxylene production. 

Product selectivity results from differences in the size of the products produced from a given reaction. In a homogeneous reactions methylation of toluene gives a mixture of ortho, meta and para xylenes but when H-ZSM-5 is used as the acid, p-xylene is the almost exclusive product (Eqn. 10.21) because the passage of this less bulky isomer through the pores of the zeolite is not restricted while the more bulky o- and m-isomers are too large to easily go through them. 2... 

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... 

Fig. 10. Scatter plot of minimum disappearance rate (Aconcentration/distance) versus the difference between equilibrium concentrations of alkylbenzenes (ABs) and those measured in ground water immediately downgradient from the oil body (modified from Eganhouse et al, 1996). Note B, benzene T, toluene E, ethylbenzene o-X, orr/io-xylene m,p-X, meta- + para-xylenes.

The Cg alkylaromatics fraction is formed by ethylbenzene and the three xylene isomers. Ethylbenzene is used as a raw material to produce styrene by dehydrogenation, or oxidative dehydrogenation. Para-xylene and ortho-xylene are catalytically oxidized to give terephthalic and phthalic acid. The meta-xylene isomer can also be oxidized to give isophthalic acid. The major industrial source of these products is the catalytic reforming of naphthas. The Cyclar process, can also produce xylenes from propane and butane. However, using this process, xylenes are formed less selectively than toluene or benzene in the BTX. 

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. 

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. 

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