Para selectivity, toluene methylation

Figure 34 shows how the para selectivity of the methylation of toluene depends on the content of Sb203 in the catalyst. The para selectivity reaches the maximum as the content of Sb203 in the catalyst approaches... 

Fig. 34. Para selectivity in toluene methylation with methanol (a) and amount of undispersed or crystalline antimony oxide in terms of XRD intensity (b) versus the total amount of Sb203 added to HZSM-5 zeolite.

Xu, H.-S., Pu, S.-P. and T. Inui. Improvement of para-Selectivity in Methylation of Toluene on Various MFI-Type Metallosilicate Catalysts. Catal. Letter, 1996,41, 83-87. 

Many other catalyst systems have been investigated for toluene alkylation with the aim of reducing the loss in catalyst activity that results upon modification of the HZSM-5 zeolites with Mg, B, or P. The catalysts studied are aU acid and have the potential for molecular sieving to obtain high para selectivity. Examples include pillared clays [51], aluminophosphates [52], and carbon-exchanged NaY zeolites [53], as well as variously modified ZSM zeolites [5459] and nonzeolite molecular sieves [60]. Table 3 summarizes the results of some of these studies, from which it is concluded that no significant improvement in the performance of the catalyst reported by Kaeding et al. [49] has been made to date. An additional difficulty for this potential route to /i-xylene has been demonstrated by the comparative study of the performance of ZSM zeolites toward isomerization of w-xylene and toluene methylation [61]. For both reactions the activity was shown to decrease in the order ZSM-5 > ZSM-22 > ZSM-23, whereas... 

The conclusion of Brown et a/.346,347 viz. that the increased reactivity along a series of alkyl halides is due to the increased polarisation of the carbon-halogen bond has been challenged by Allen and Yats348, who found constant meta para isomer ratios for methylation, ethylation, and isopropylation of toluene, and since this ratio reflects the selectivity and hence reactivity of the electrophile they con-... 

The catalytic activity of Mg/Al/O sample in m-cresol gas-phase methylation is summarized in Figure 1, where the conversion of m-cresol, and the selectivity to the products are reported as a function of the reaction temperature. Products were 3-methylanisole (3-MA, the product of O-methylation), 2,3-dimethylphenol and 2,5-dimethylphenol (2,3-DMP and 2,5-DMP, the products of ortho-C-methylation), 3,4-dimethylphenol (3,4-DMP, the product of para-C-methylation), and poly-C-methylated compounds. Other by-products which formed in minor amounts were dimethylanisoles, toluene, benzene and anisole (not reported in the Figure). 

Potential Uses. Because much toluene is demethylated for use as benzene, considerable effort has been expended on developing processes in which toluene can be used in place of benzene to make directly from toluene the same products that arc derived from benzene. Such processes both save the cost of demethylation and utilize the methyl group already on toluene. Most of this effort has been directed toward manufacture of styrene. A11 alternative approach is the manufacture of para-methylstyrene by selective ethylation of toluene, followed by dehydrogenation. Resins from this monomer are expected to displace polystyrene because of price and performance advantages. 

The proportionality of S and p made possible the adoption of a slight modification of the approach involving the reversal of the conventional Hammett procedure. The Selectivity Factor, 8, rather than p was chosen as the reference parameter. This procedure is the application of the p approach with a single substituent, the methyl group, defining the reaction constant. This scheme is useful because 8f is an experimental quantity determined by the para/meta product ratio in toluene (10). 

The Selectivity Relationship was shown to be applicable for substitution in the meta and para positions of toluene (Section II). The fine adherence of the -methyl group to a linear free-energy relationship (Fig. 37) is apparently typical of the behavior of the other alkyl substituents, as illustrated for the p-ethyl, p-i-propyl, and p-t-butyl groups (Figs. 38-40). Indeed, the data for electrophilic substitution in toluene are better correlated by a linear relationship than are the data for ordinary side-chain reactions of p-tolyl derivatives (Stock and Brown, 1959a). In the Extended Selectivity Treatment (Fig. 25) the side-chain reactions show a slightly greater scatter from the correlation line than the aromatic substitution reactions. 

Figure 5.33 presents Friedel-Crafts acylations, taking benzoylations of toluene (top line) and para-tert-butyl toluene (Figure 5.33, bottom) as an example. The methyl group of toluene preferentially directs the benzoyl residue into the para-position. The ortho-benzoylated toluene occurs only as a by-product. In para-tert-butyl toluene both the methyl- and the tert-butyl substituent direct the electrophile towards the ortho-position, since both para-positions are occupied and could at best react with de-ferf-butylation, i.e., in a—sterically hindered — ipso-substitution (cf. Figure 5.5). Indeed, we see reaction ortho to the methyl group and not ortho to the ferf-butyl group. This selectivity can be ascribed to minimized steric interactions in the preferred sigma complex intermediate.

Benzene, toluene and / -xylene are the most industrially important aromatic hydrocarbons. Their relative proportions must be adjusted according to the needs of the market, which presently requires more benzene and xylene. Dealkylation of toluene can be carried out by hydrogenolysis with H2 using Cr, Mo or Pt oxides as catalyst precursors (Equation 4). Methyl redistribution in toluene is best carried out around 420°C on ZSM-5 zeolite. Under these conditions, xylene is mainly present as the para isomer (90% selectivity. Equation 5) due to the shape-selectivity of the zeolite. 

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... 

The sulfonic acid resins such as Dowex-50 and Amberlyst-15 have been used to promote the alkylation of the more active aromatic rings but attempts to increase their acidity generally resulted in the degradation of the solid. 2 The more strongly acidic perfluorinated resin sulfonic acid, Nafion-H,2>3 has, however, been used to promote the alkylation of benzene and other aromatic compounds. Nafion-H catalyzed the vapor phase reaction between toluene and methanol. When nm at 185°C a 12% yield of the isomeric xylenes was obtained with the ortho isomer the major product. 0 Methylation of phenol at 205°C over this catalyst gave, at 63% conversion. 37% anisole and 10% of a mixture of the ortho and para cresols in a 2 1 ratio. Reaction of anisole with methanol under these conditions resulted in a 14% selectivity to the methyl anisoles at 40% conversion, with the ortho and para isomers formed in nearly equal amounts. ... 

Benzene can be formylated very easily. Fluorobenzene gives fluorobenzaldehyde with a total selectivity in para isomer. On the contrary chloro and bromobenzene are transformed in poor yield, and chloro and bromo toluenes 17 are obtained next to halobenzaldehydes 16, showing that methyl formate can act as an alkylating agent. 

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. 

The acylation of anisole with C2 - C12 acids was carried out under the same conditions as that of toluene, except a shorter reaction time (5 h). The acylated anisole formed as the major product para/ortho = 59 1 - 96 1 and no meta isomers) together with esterification products - methyl esters of carboxylic acids and phenol. No phenyl esters formed. The selectivity to esters increases from acetic to dodecanoic acid, reaching 40% for the latter. The acylation of anisole, in contrast to that of toluene, is most efficient with C2 - C6 acids, giving a 62 - 65% yield of acylated products and only 2 - 6% of methyl esters. 

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