Para-xylene, production

Werner Haag was both a scientist and a technologist. This is somewhat reflected by more than 70 U.S. patents that bear his name. A better illustration may be his essential contributions to the currently practiced para-xylene production processes. They provide 30% or more of worldwide production of this raw material for polyester. Probably a majority of readers are wearing clothing made from these molecules. 

In the present study, silicon and transition metal substituted aluminophosphate molecular sieves have also been evaluated for activity and selectivity for para-xylene production via Cg aromatic isomerization. In commercial practice, Cg aromatic cuts are obtained from reformate gasoline and from pyrolysis naphtha streams. Both feeds contain a significant fraction of ethylbenzene which is difficult to separate from xylenes by physical techniques,... 

Daramola M. O., Burger A. J., Giroir-Fendler A., Miachon S., Lorenzen L. 2010. Extractor-type catalytic membrane reactor with nanocomposite MFI-alumina membrane tube as separation unit Prospect for ultra-pure para-Xylene production from m-Xylene isomerization over Pt-HZSM-5 catalyst. Applied Catalysis A General 386(1-2) 109-115. 

While a simple matter of academic curiosity in past years, isomorphously substitued zeolites, gained very recently a major practical significance with the advent of T-substituted MFI structures such as boralite, ferri-silite which established themselves as efficient catalysts in para-xylene production. Not only can the substituting T atoms influence the acid strength and shape selective properties, they are also likely to act on their own as catalytic centres. Typical of such a behaviour is TS-1 brought to public interest by ENI researchers. Titanium as the substituting element in a silicalite structure is active in various mild oxidation reactions. 

Fong, Y. Y., Abdullah, A. Z., Ahmad, A. L., Bhatia, S. (2008). Review—development of functionalized zeolite membrane and its potential role as reactor combined separator for para-xylene production from xylene isomers. Chemical Engineering Journal, 139, 172-193. 

Xylene is also used in the manufacture of insecticides, dyes, vitamins, and pharmaceuticals. Most orffto-xylene is used to produce phthalic anhydride, which is mostly used to make phthalate ester plasticizers for use with vinyl plastics (with only a relatively small portion being used by the rubber industry). On the other hand, most of the para-xylene production goes into four areas of use unsaturated polyester thermosets for plastics, alkyd resins for coatings, polyester molding resins for plastics, and polyester textiles (of which only a small percentage is used as tire cord). 

The fully side-chaia chloriaated products, 1,3-his(trichioromethy1)henzene [881-99-1] and 1,4-his(trichioromethy1)henzene [68-36-0] are manufactured by exhaustive chlorination of meta and para xylenes. For the meta compounds, ring chlorination cannot be completely eliminated ia the early stages of the reactioa. The xyleae hexachlorides are iatermediates ia the manufacture of the xylene hexafluorides and of iso- and terephthaloyl chloride [100-20-9] (see Phthalic acids). 

Distill a small quantity each day to obtain relatively pure o-xylene from a mixture of ortho and para xylene, having boiling points of 142.7°C and 138.4°C, respectively. The feed is 15 Ib-mols (about 225 gallons) per batch, at 0.20 mol fraction para. The desired residue product is 0.020 in the kettle, while the distillate is to be 0.400 mol fraction para. A distillation column equivalent to 50 theoretical plate is to be used. 

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. 

As is apparent from the previous discussion on toluene disproportionation,the observation of high p-selectivity in STDP requires a dramatic change in selectivity. First, the primary product must be directed to be highly para-selective. Secondly, the subsequent isomerization of the primary p-xylene product must be selectively inhibited ... 

The effect of crystal size, 2r, in STOP is demonstrated in Figure 10. These data for three zeolites having similar activity, but with crystal sizes differing by nearly two orders of magnitude, show a significant increase in para-xylene selectivity with increasing crystal size. The primary product selectivity is enhanced and secondary isomerization is retarded. 

Mixed xylenes are used as an octane improver in gasoline and for commercial solvents, particularly in industrial cleaning operations. By far, most of the commercial activity is with the individual isomers. Para-xylene, the most important, is principally used in the manufacture of terephthalic acid and dimethyl terephthalate en route to polyester plastics and fibers (Dacron, films such as Mylar, and fabricated products such as PET plastic bottles). Ortho-xylene is used to make phthalic anhydride, which in turn is used to make polyester, alkyd resins, and PVC plasticizers. Meta-xylene is used to a limited extent to make isophthahc acid, a monomer used in making thermally stable polyimide, polyester, and alkyd resins. 

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. 

Isomerization and transalkylation reactions to redistribute methyl groups on aromatic molecules are important processes in the production of benzene, toluene and xylenes (BTX). In particular, the production of para-xylene is preferred. The interconversion of C8 aromatics is covered in much greater depth in Section 14.3. 

Meta-xylene isomerization to ortho- and para-xylene over 10- and 12-MR zeolites is another illustration of product shape selectivity effects [13]. The two products are essentially equally favorable from the standpoint of thermodynamics. With decreasing pore size, however, kinetics come into play and the selectivity to para-xylene increases, as illustrated in Figure 13.37 for results obtained at 317-318°C, 0.5 kPa meta-xylene pressure (in the presence of He carrier gas) and 10% conversion [64]. While the para ortho ratio is typically 1.0-1.5 with multi-dimensional... 

Other examples of systems that are likely to be governed by product shape selectivity effects include toluene disproportionation to para-xylene -i- benzene in favor of other xylenes r- benzene [61]. Toluene alkylation by methanol to give para-xylene in favor of other xylenes is yet another such example [76],... 

The catalysts are predominantly modified ZSM-5 zeolite. In general, the modifications are intended to restrict pore mouth size to promote the shape selective production of para-xylene within the microporous structure. The same modifications also serve to remove external acid sites and eliminate the consecutive isomerization of para-xylene. Methods used to modify the zeolite pore openings have included silation [50], incorporation of metal oxides such as MgO, ZnO and P2O5 [51, 52], steaming and the combination of steaming and chemical modification [53]. 

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. 

One of the industrial processes using ZSM-5 provides us with an example of product shape-selective catalysis the production of l,4-( ara- xylene. Para-xylene is used in the manufacture of terephthalic acid, the starting material for the production of polyester fibres such as Terylene . 

Because ethylbenzene is used almost exclusively to produce styrene, the product specification on ethylbenzene is set to provide a satisfactory feedstock for styrene production. Levels of cumene, -propylbenzene, ethyltoluenes and xylenes in ethylbenzene are controlled to meet the required styrene purity specification. A typical sales specification is as follows purity, 99.5 wt% min. benzene, 0.1-0.3 wt% toluene, 0.1-0.3wt% ort/io-xylene + cumene, 0.02 wt% max. meto-xylene + para-xylene, 0.2 wt% max. allylbenzene + a-propylbenzene + ethyltoluene, 0.2 wt% max. diethylbenzene, 20 mg/kg max. total chlorides (as chlorine), 1-3 mg/kg max. and total organic sulfur, 4 mg/kg max. (Coty et al., 1987). 

Further complications arise from the fact that the alkylation reactions sometimes are under equilibrium control rather than kinetic control. Products often isomerize and disproportionate, particularly in the presence of large amounts of catalyst. Thus 1,2- and 1,4-dimethylbenzenes (ortho- and para-xylenes) are converted by large amounts of Friedel-Crafts catalysts into 1,3-dimethyl-benzene (meta-xylene) ... 

Reactive crystallization, or precipitation, has been investigated by numerous research groups. Processes of industrial relevance include liquid-phase oxidation of para-xylene to terephthalic acid, the acidic hydrolysis of sodium salicylate to salicylic acid, and the absorption of ammonia in aqueous sulfuric acid to form ammonium sulfate (60). A very special type of reactive crystallization is diastereomeric crystallization, widely applied in the pharmaceutical industry for the resolution of enantiomers (61). Another fine example of reactive precipitation is the earlier-described production of nano-size particles of CaC03 in high-gravity fields (46). 

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