P-Xylene, formation

Various methods have been developed to maximize p-xylene formation. A mild hydrothermal treatment of H-ZSM-5 and the incorporation of Pt greatly reduce disproportionation activities and, consequently, increase para selectivity.355 Suppressing the activity of the external surface can also lead to enhanced para selectivity. By judicious choice of reaction conditions and organometallic reagents... 

Since the shape selectivity is clearly related to reactant and/or product diffusion through the pores and cavities of the zeolite, the selectivity should increase as the size of the zeolite particles and, thus, the extent of diffusion, increases. This has been established using the H-ZSM-5 alkylation of toluene with methanol as the probe reaction. Three catalysts with particles ranging in size from 0.025 j.m to 4.5 im were used. The results listed in Table 10.1 for reactions run at various temperatures show that with the largest catalyst particles selectivity toward p-xylene formation was 100% at all temperatures. As the particle size decreased so did the reaction selectivity. Increasing the temperature increased the reaction selectivity with the smaller particle sized catalysts. 3... 

The alkylation of toluene with methtmol at 400°C over H-ZSM-5 gave, at 36% conversion, a 69% selectivity for xylene formation, of which 27% was the para isomer. 2 Aluminum phosphate based molecular sieve catalysts such as CoAPO gave a lower conversion but higher selectivities for p-xylene formation. 2 Metallosilicates such as As-silicate having a ZSM-5 structure produced an 82% selectivity for p-xylene at 21% conversion. 

From the thermodynamic standpoint, since the reaction shift in favor of p-xylene formation is slightly exothermic, a change in temperature only has a limited effect On the composition of the C aromatics mixture at equilibrium. This is shown by Fig. 4.21. 

Figure 4 shows the evolution of the toluene conversion and para-selectivity over the silicalite/H-ZSM-5 catalyst for the alkylation of toluene with reaction time. Reaction temperature was 673 K and the space time W/F) was 0.06 kg-catalyst h mof . The toluene conversion and para-selectivity over the silicalite/H-ZSM-5 catalyst after the reaction time of 2 h were higher than 35 % and 98 %, respectively. However, the toluene conversion decreased with time similarly to the results of the uncoated H-ZSM-5 catalysts. The toluene conversion went down to lower than 10 % after 6 h. The para-selectivity also slightly decreased to approximately 88 % after the reaction time of 8 h. A possible reason is that the rate of isomerization on only a few acid sites near the external surface did not change with reaction time although the rate of p-xylene formation was decreased with reaction time by coking inside the pores of H-ZSM-5. 

The first compound containing a telluroazepine ring, ll-(4-methylphenyl) dibenzo[d,/][l,4]telluroazepine 62 was obtained in 21% yield upon heating p-xylene solution of 9-azido-9-(4-methylphenyl)telluroxanthene at 130-140°C (87KGS279). Other products of this pyrolitic process are the imine 63 (32% yield) and phenanthridine 64 (21% yield). Formation of the latter implies extrusion of the tellurium atom from dibenzotelluroazepine 62. 

We examined the possibility of a direct formation of two C-C bonds by reaction of a carbanion with [Fe(arene)2]2+ in which the arene bears methyl groups. We could indeed repeat Hellings s experiments but found that mesitylene was the only aromatic allowing this possibility in reasonable yields. With p-xylene, a low yield of an unstable complex was obtained corresponding to double nucleophilic attack of phenyllithium on the same ring in spite of the bulk of the methyl groups [23]. Eq. (4) ... 

In Figure 3 the merits of the two processes for p-xylene oxidation are compared. The main disadvantages of the Eastman Kodak/Toray cooxidation method are the need for a cosubstrate (acetaldehyde of methylethylketone) with concomitant formation of a coproduct (0.21 ton of acetic acid per ton product) and high catalyst concentration. The Amoco MC process, on the other hand, has no coproduct and much lower catalyst concentrations but has the disadvantage that the bromide-containing reaction mixture is highly corrosive, necessitating the use of a titanium-lined reactor. 

As an example of the selective removal of products, Foley et al. [36] anticipated a selective formation of dimethylamine over a catalyst coated with a carbon molecular sieve layer. Nishiyama et al. [37] demonstrated the concept of the selective removal of products. A silica-alumina catalyst coated with a silicalite membrane was used for disproportionation and alkylation of toluene to produce p-xylene. The product fraction of p-xylene in xylene isomers (para-selectivity) for the silicalite-coated catalyst largely exceeded the equilibrium value of about 22%. 

Selective oxidation of p-xylene to terephthaldehyde (TPAL) on W-Sb oxide catalysts was studied. While WO3 was active in p-xylene conversion but non-selective for TPAL formation, addition of Sb decreased the activity in p-xylene conversion but increased TPAL selectivity significantly. Structure change was also induced by Sb addition. Evidences from various characterization techniques and theoretical calculation suggest that Sb may exist as various forms, which have different p-xylene adsorption property, reactivity toward p-xylene and TPAL selectivity. Relative population of each species depends on Sb content. 

Attempts to sensitize this reaction resulted in no product formation, suggesting participation of a benzene singlet in the direct photolysis. Irradiation of isoprene in excess benzene resulted in products similar to those observed with butadiene/7 Irradiation of toluene and o- and p-xylene in the presence of isoprene yielded products similar to (62).<72)... 

These results suggest that the transition states leading to the formation of the cyclo-adducts (33) and (34) are product-like and that the greater than statistical formation of adducts (34) is due to the increased thermodynamic stability of a trisubstituted double bond. In agreement with this explanation is the fact that in reactions with for example p-xylene and durene (1,2,4,5-tetramethylbenzene) only the adducts (35) and (36) were obtained 54-59). Also as expected, two adducts were obtained with tetralin but only the compound (37) was obtained using 5,8-dimethyltetralin, which we may regard as a 1,2,3,4-tetra-alkylben-zene 54>. 

Neopentane does not undergo isomerization 185) on chromia/alumina (non-acidic) at 537°C, the only significant reaction been hydrogenolysis to methane and iso-C4. However, the reality of isomerization is made clear from, for instance, the formation of xylenes from 2,3,4-trimethylpentane. For o- and p-xylene, the reactions are (24) and (25) 182, 93). These processes are formally quite analogous to those we have described in previous... 

The products for which the cyclo-C4 isomerization intermediate has been suggested, can also be explained by a sequence of vinyl insertions. Thus, two vinyl insertions would be adequate to explain the formation of m-xylene from 2,3,4-trimethylpentane. Although we have seen in previous sections that extensive reaction sequences are possible on platinum, isomerization by a single vinyl insertion process on chromium oxide is relatively difficult, and the chance of two occurring in sequence would therefore be expected to be very low. In fact, the proportion of m-xylene is comparable to that of o- and p-xylene. 

Among the wide variety of organic reactions in which zeolites have been employed as catalysts, may be emphasized the transformations of aromatic hydrocarbons of importance in petrochemistry, and in the synthesis of intermediates for pharmaceutical or fragrance products.5 In particular, Friede 1-Crafts acylation and alkylation over zeolites have been widely used for the synthesis of fine chemicals.6 Insights into the mechanism of aromatic acylation over zeolites have been disclosed.7 The production of ethylbenzene from benzene and ethylene, catalyzed by HZSM-5 zeolite and developed by the Mobil-Badger Company, was the first commercialized industrial process for aromatic alkylation over zeolites.8 Other typical examples of zeolite-mediated Friedel-Crafts reactions are the regioselective formation of p-xylene by alkylation of toluene with methanol over HZSM-5,9 or the regioselective p-acylation of toluene with acetic anhydride over HBEA zeolites.10 In both transformations, the p-isomers are obtained in nearly quantitative yield. 

Carbene Is proved to be photolabile, and long-wavelength irradiation (A. > 515 nm) results in the irreversible formation of the strained cyclopropene 3s. The methyl shift to give p-xylene, which is energetically much more favorable, is not... 

Attempts were made not only to find an alternative way to replace dimer and to deposit high-quality poly(tetrafluoro-p-xylylene) film, but also to eliminate the dibromide as the precursor because of the difficulty of synthesis. Therefore, the deposition of poly(tetrafluoro-p-xylylene) film by using hexafluoro-p-xylene as the precursor instead of dibromotetrafluoro-p-xylene was tried. However, no polymer film was deposited on the wafer. Effort was expanded and other metal reagents such as nickel or copper were used to react with l,4-bis(trifluoromethyl)-benzene to generate a,a,a, a -tetrafluoro-p-xylylene to deposit poly(tetrafluoro-p-xylylene) film. However, the result showed that no film was deposited, which was not unexpected, because a C—X bond that is weaker than C—F bonding might be necessary to initiate the formation of the desired intermediate. 

First of all p-xylene is dehydrogenated to obtain its dimer (i.e., di-/ -xylene). This is done by using superheated steam at 950°C. The dimer formed is a crystalline solid at room temperature and it is heated to 600°C at 1 mm pressure when it sublimes and forms and equilibrium mixture of diradical and a quinonoid. This equilibrium mixture when quenched to 50°C over metal surface results in the formation of a linear polymer known as... 

In a laboratory scale dehydrocyclization reaction using a dual-function catalyst, Davis (8) reported that the aromatic products were o-xylene, /w-xylene and ethylbenzene in approximately equal amounts (ca. 20-30% each) and p-xylene (ca, 15%). The formation of these was assumed to be from a direct 1,6-ring closure, as sketched in the following two diagrams ... 

Since there are two different connections possible for n-octane, 1,6 or 3, 8, which could lead eventually to ethylbenzene, there is a statistical entropy factor involved here which is not part of the o-xylene route. Therefore, if both closures were equally possible from an enthalpy perspective, one would predict a 2 1 ethylbenzene to o-xylene ratio. The formation of the m- and p-xylene requires prior isomerization of n-octane to 2- and 3-methylheptane, respectively. 

In our calculations we will first discuss our results starting with both the 2-and 3- octyl cations (the 4- octyl cation cannot form a 1,6-p-H-structure). The n-octane conversion to aromatics, as described by Davis (8), is a good test of our proposed mechanisms, for several reasons (1) his experimental observation would require the formation of approximately equal amounts of 1,2-dimethylcyclohexane (o-xylene) and ethylcyclohexane (ethylbenzene), even though in our mechanism the structure of the needed 1,6-p-H cation intermediates are quite different, and (2) the formation of to- and p-xylene requires a prior isomerization of n-octane to 2- and 3- methylheptane, and this must be a faster reaction than the dehydrocyclization (or at least competitive with it). If our mechanisms are valid, we should be able to reproduce some aspects of the above results. 

Wu, P., Komatsu, T., and Yashima, T. (1998) Selective formation of p-xylene with disproportionation of toluene over MCM-22 catalysts. Micropor. Mesopor. Mater., 22, 343-356. 

The hydrotropes in this era were short chain aromatic sulfonates, with the p-xylene sodium sulfonate as a typical example. Their action is preventing the formation of liquid crystals is easily understood from a direct comparison of their molecular geometry (Fig. 1). 

Chemical/Physical. Under atmospheric conditions, the gas-phase reaction of o-xylene with OH radicals and nitrogen oxides resulted in the formation of o-tolualdehyde, o-methylbenzyl nitrate, nitro-o-xylenes, 2,3-and 3,4-dimethylphenol (Atkinson, 1990). Kanno et al. (1982) studied the aqueous reaction of o-xylene and other aromatic hydrocarbons (benzene, toluene, w and p-xylene, and naphthalene) with hypochlorous acid in the presence of ammonium ion. They reported that the aromatic ring was not chlorinated as expected but was cleaved by chloramine forming cyanogen chloride. The amount of cyanogen chloride formed increased at lower pHs (Kanno et al., 1982). In the gas phase, o-xylene reacted with nitrate radicals in purified air forming the following products 5-nitro-2-methyltoluene and 6-nitro-2-methyltoluene, o-methylbenzaldehyde, and an aryl nitrate (Chiodini et ah, 1993). 

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