Isomerization para-xylene

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. 

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. 

Licensors offer a variety of catalysts to promote the isomerization— silica alumina by itself or enhanced with a noble metal like platinum or a non-noble metal like chromium. Another uses hydrofluoric acid with boron trifluoride In the case of the noble metal catalytic process, the feed enters a vessel with a fixed catalyst bed at 850°F and 14.5 psi. As is often the case, a small amount of hydrogen is present to reduce the amount of coke laying down on the catalyst. The effluent is processed in a standard fashion to separate the hydrogen, the para- and ortho-xylene, and any unreacted or miscellaneous compounds. Yields of para-xylene are in the 70% range. 

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... 

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]. 

Xylenes. Because of the practical significance of xylenes, isomerization of xylenes over zeolites is frequently studied.348 The aim is to modify zeolite properties to enhance shape selectivity, that is, to increase the selectivity of the formation of the para isomer, which is the starting material to produce terephthalic acid. In addition, m-xylene isomerization is used as a probe reaction to characterize acidic zeolites.349,350... 

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) ... 

The feed to an aromatics complex is normally a C6+ aromatic naphtha from a catalytic reformer. The feed is split into Cg+ for xylene recovery and C7 for solvent extraction. The extraction unit recovers pure benzene as a product and C7+ aromatics for recycling. A by-product of extraction is a non-aromatic C6+ raffinate stream. The complex contains a catalytic process for disproportionation and transalkylation of toluene and C9+ aromatics, and a catalytic process for isomerization of C8 aromatics. Zeolitic catalysts are used in these processes, and catalyst selectivity is a major performance factor for minimizing ring loss and formation of light and heavy ends. The choice of isomerization catalyst is dependent on whether it is desired to isomerize ethylbenzene plus xylenes to equilibrium or to dealkylate ethylbenzene to benzene while isomerizing the xylenes. Para-selectivity may also be a desired... 

Let us now consider the methyl shift isomerizations that lead from para-xylene to meta-xylene (see Figure 15). 

The conversion of ortho- to meta- and para-xylene was carried out on a series of decationated catalysts which were subjected to thermal treatment at various temperatures. 10-/xl pulses of o-xylene were used, the catalyst amount was 300-350 mg, and the flow rate of the helium carrier gas was 50-100 ml/min. The products were analyzed on 7.8-benzoquinoline on Chromosorb W gas-chromatographic column. The m-xylene predominated over the para isomer. A small toluene production seemed to parallel that of isomerization. The conversion of xylene X... 

Sastre et aL [112] studied the isomerization of m-x>iene over OfBretite and observed monotonical increase in m-xylene conversion upon exdumge of the K -cations. This was ascribed to the increase of the concentration of the protons and the increase in accessibility of the pores, which resulted in a higher selectivity for the isomerisation reaction at the e q>aise of the disproportionation reactioiL Only a sHght increase in the p-xyloie in the fraction of oitho-and para-xylene was observed. Over Beta a maximum activity for the xylene isomersation was observed and this was explained by either a pos le existence of a eigistic effect between extra-framework aluminium and the fitunework Bronsted acid sites or a concentration effect [113]. 

Ortho-meta- and para-Xylenes.—The three definite isomeric xylenes have the following physical propertie s which enable us to distinguish them from each other. For the present we may designate them simply as 4, -B, C. 

Pseudo-cumene from Brom para-Xylene and Brom meta-Xylene. Bl om para-xylene, as will be recalled from the discussion of the constitution of para-xylene, exists only in one form as no isomeric compounds are possible. This substance, by means of Fittig s synthesis, yields pseudo-cumene, the constitution of which, therefore, can only be I-3-4-tri-methyl benzene. 

Of the three isomeric xylenes, each of which yields nitro products, it is the meta-xylene or i-3-di-methyl benzene which is most easily nitrated. The number of isomeric nitro xylenes possible has been previously explained (pp. 472 and 482). In the case of meta-xylene three such nitro compounds are possible but only one is readily obtained. It is I-3-di-methyl 4-nitro benzene that is, the nitro group enters the ring ortho to one methyl group and para to the other. This is just what we should expect from the influence of the methyl group upon subsequent substitution (p. 506). The nitro xylenes are not so important as nitro benzene or the nitro toluenes, but have some use in dyestuff manufacture. 

Technical Xylidine.—The para-xylene is also present in the technical product which of course yields only one xylidine, viz., 1-4-dimethyl 2-amino benzene. From the or//fo-xylene present the vicinal or I-2-di-methyl 3-amino benzene is obtained. The technical xylidine contains these three isomeric compounds and is used in the preparation of azo-dyes. Of the amino derivatives of the higher homologues only one will be mentioned. 

When refering to shape selectivity properties related to diffusivity, it seems obvious that the larger the zeolite grain, the higher will be the volume/sur f ace ratios and the shape selectivity, since the reaction will be more diffusion controlled. The external surface area represents different percents of the total zeolite area depending on the size of the grains which could be important if the active sites at the external surface also play a role in the selectivity. For instance in the case of toluene alkylation by methanol, the external surface acid sites will favor the thermodynamical equilibrium due to isomerization reactions (o m p-xylene - 25 50 25 at 400 C) while diffusivity resistance will favor the less bulky isomer namely the para-xylene. It may therefore be useful to neutralize the external surface acidity either by some bulky basic molecules or by terminating the synthesis with some Al free layers of siliceous zeolite. 

Figure 10, for example, shows how the channel size of ZSM-5 retards an undesirable reaction — the transalkylation of xylene to produce the bulky intermediates which would yield trimethyl benzene and toluene. The desirable reaction is the isomerization of a xylene mixture to para-xylene — the raw material for polyester fibers. 

Aromatic Reactions. In addition to reforming, the isomerization of Cg aromatics to produce para-xylene is another area where olefinic intermediates may play a significant mechanistic role. 

The production of para-xylene is of interest to the petrochemical industry because of its use as monomer in polyester production. In addition to Cg aromatic isomerization, there are a number of important routes to para-xylene including the alkylation of toluene with methanol and the disproportionation of toluene. The catalytic properties of the SAPO molecular sieves for toluene methylation reactions have been described(11). While both large and medium pore SAPO s were active for the alkylation reaction, the medium pore materials were distinguished by remarkably high selectivity for methylation reactions, with disproportionation of the toluene feed representing less than 2% of the total conversion. By comparison, large pore SAPO-5 had nearly 60% disproportionation selectivity and the zeolite reference LZ-105 had nearly 80% disproportionation selectivity. The very low disproportionation activity of the medium pore SAPO s, attributed to their mild acid character, resulted in reduced losses of toluene to benzene and increased xylene yields relative to LZ-105 and SAPO-5. 

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,... 

The enhanced selectivities observed with medium pore silico-and metalloaluminophosphates may, to a large extent be attributed to a unique combination of mild acidity and shape selectivity. The lack of hydride shift and cracking activity in olefin-mediated reactions is suggestive of mild acidity. The observed resistance to coke deactivation and the enhanced selectivity to para-xylene in methylation and isomerization reactions is evidence of shape-selective catalysis. 

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