Aromatic alkylation para-selective

To reveal factors which influence activities of acid-base catalysts in alkylation and isomerization is the challenge to activity in this field. Q he greatest amount of work has been done in connection with the effect of para-selectivity, which is observed in alkylation of aromatic hydrocarbons on ZSM-5 type zeolites [1]. This effect has been explained by a number of authors either by the influence of diffusion factors [2,3] or by the isomerizing activity of the external surface of zeolite crystals [4]. In refs. [5,6] and especially in ref.[7] the para-selective effect of ZSM-5 type zeolites is shown to be due to decreasing their isomerizing activity becaiase of the decrease in the concentration of strong protic centres as a result of modifiers introduced. Para-selective effect is related to the action of chemical factors. However, in... 

Ethanol instead of ethylene can also be used in alkylation of toluene334 with para selectivities up to 90%. Anhydrous ethanol was shown to undergo dehydration to ethylene, which, in turn, alkylated the aromatic hydrocarbon. The alkylation step... 

The only aromatic components that appeared at reaction temperatures below 300 °C were toluene and p-xylene. In the case of the small-crystalline H-ZSM-5(M), however, some m- and o-xylene were present in the product mixture even at 245 °C (WHSV = 6 h- ). This can be explained by xylene isomerization at the outer zeolite surface. At conditions where the para-selectivity was high (more than 90% para), the amount of p-ethyl-toluene (PET) in the product were one order of magnitude greater than that of any other Cg-component, but when it was low, the ratio 1,2,4-trimethyl-benzene (124TMB) PET was found to be about 10 1. These experimental facts indicate that 124TMB is mainly formed by secondary xylene alkylation with methanol. Toluene, p-xylene, PET and perhaps ethyl-benzene are more likely to be the primary aromatic products formed in the MTG-reaction. To confirm this suggestion the molar product ratios EB/PX,... 

Without modifying the catalyst, p-xylene can be produced in great excess of the thermodynamic equilibrium distribution. The para-selectivity was enhanced when large H-ZSM-5 crystals were employed. Reaction parameters that reduced the degree of reactions taking place at the outer zeolite surface led to an increased para-yield. M- and o-xylene are formed mainly from p-xylene, which seems to be a primary aromatic product, as do other mono- or para-alkyl arenes. 

Aromatic alkylation reactions which led to a number of para-selective products have been duplicated on phenols and thiophenes (64-65-66-67). So have halogenation and nitration reactions (68—69). 

The results on the effect of temperature, contact time and methanol to toluene ratio on the isomer composition of xylenes on K2.5 salt are given in Table 3. It is seen that selectivity of p-xylene decreases with increase in the temperature whereas the selectivity of m-xylene increases, obviously, due to the isomerization. As contact time increases p-xylene selectivity increases. It is also found that the p-xylene selectivity increases with increasing methanol to toluene ratio. As methanol to toluene ratio increases the catalyst surface will be saturated with more of alkylating species which offers hindrance to the approach of the aromatic substrate and thereby resulting in the preferential alkylation at para position. In conclusion,it may be suggested that high Bronsted acidity is responsible for high para selectivity found in heteropolyoxometallates. 

One of the unique features of zeolites in alkylation reactions is their shape selectivity. In many zeolite-catalysed reactions, however, shape-selective catalysis occurring on the inside of the zeolite can be affected by non-selective catalysis on the external surfaces. Paparatto et al. have reported that during aromatic alkylation, the para isomer is formed selectively within the zeolite, whereas isomerisation occurred only on the external surfaces, decreasing para product selectivity.17... 

Although Schafer s approach at first appears equivalent to a straight forward oxidative coupling, he successfully avoided his previously encountered problems71 of ortho-para selectivity by utilizing a judiciously functionalized, non-aromatic A-ring moiety, Scheme 18. a-Lithiation of formamidine 160 followed by alkylation with the... 

While part of the electron-releasing effect of alkyl groups toward double bonds and aromatic rings can be attributed to the electronegativity difference between sp and sp carbon, the fact that the (3-carbon of alkenes and the ortho and para positions of aromatic rings are selectively affected indicates a resonance component. 

The substitution of a hydrocarbon side chain into a benzene ring is called aromatic alkylation. Using zeolite ZSM-5, modified by the inclusion of phosphate ions (P04 ), at 500 °C, para-xylene has been synthesized selectively with a purity of up to 97%. A typical aromatic alkylation of monoalkyIbenzenes uses a toxic and hazardous catalyst, such as AICI3 or FeCl3, and results in predominantly ortho-para substitution. Not only that, but rapid secondary alkylation reactions usually generate a mixture of products with two, three or more alkyl substituents (Figure 4.1). 

The bromination of phenyl n-pentyl ether is more para-selective in anionic micelles than it is in water. This contrasts with the lower para-selectivity of nitration of bromobenzene in the cationic micelles formed by dissolving lauric acid in 95% H2S04. It is not clear whether these effects are due to substrate orientation or to micelle-induced changes in the selectivity parameter for electrophilic aromatic substitution. The rates of solvolysis of alkyl p-trimethyl-ammoniumbenzenesulphonate trifluoromethanesulphonates (42) are strongly inhibited by anionic micelles of sodium lauryl sulphate or sodium dodecanoate. In water, homomicelles of (42) or sodium dodecanoate micelles, undergo inversion of stereochemistry, but in sodium lauryl sulphate 22% retention of... 

Chloro-aluminate ionic liquids promote the carbonylation of alkylated aromatic compounds, but fails in the case of oxygenated aromatics. Aldehyde yields of formylation in the acidified neutral ionic liquids were generally similar compared to reactions conducted in HF as solvent/catalyst (cf Table 2.2). The increase in aldehyde yields with the use of extended alkyl chain lengths of the cationic part of the melt, may be due to improved CO solubility. HF/BFs-acidified neutral ionic liquids showed both increases in para-selectivity compared to HF as solvent and catalyst. Formylation of anisole and toluene, but not of phenol in the neutral ionic liquids resulted in increased secondary product formation in comparison with hydrogen fluoride used as solvent/catalyst. This difference in behaviour is not understood at present, but suggests that phenol is a good substrate for formylation in this medium, particularly with the development of a system catalytic with respect to HF/BF3 in mind. 

The efficiency of the developed synthetic approaches was tested in the alkylation reactions of toluene and tert-butylbenzene, as well as large aromatic molecules, naphthalene and biphenyl, to better identify the shape selectivity of the pores. It was found that monoalkylation of biphenyl with tert-BuC had a 100% para-selectivity, which exceeds the selectivity shown by H-MOR and H-BEA taken as model zeolite catalysts. It was concluded that, in the case of dicarboxylate MOF structures, the pore size can be tuned depending on the size of the substrate by an appropriate choice of the bridging hgand. 

C-alkylation of secondary and tertiary aromatic amines by hexafluoroacetone or methyl trifluoropyruvate is performed under mild conditions [172] (equation 147) The reaction of phenylhydrazme with hexafluoroacetone leads selectively to the product of the C-hydroxyalkylation at the ortho position of the aromatic ring The change from the para orientation characteristic for anilines is apparently a consequence of a cyclic transition state arising from the initial N hydroxy alky lation at the primary amino group [173] (equation 148)... 

Co(ni) alkyl peroxides have been prepared and used by Mimoun and coworkers in the hydroxylation of hydrocarbons with this metal a Haber-Weiss type of reactivity is suggested. Square-planar Pt(II) complexes, of the type [(dppe)Pt(CF3)(solv)], used by Strukul in the epoxidation of alkenes and in Baeyer-Villiger oxidations of ketones (Schemes 8 and 9), are effective catalysts also in the direct hydroxylation of aromatics with hydrogen peroxide. The reactivity increases in the presence of electron releasing substituents in the aromatic ring. Ortho and para derivatives are practically the only products observed and interesting selectivity toward the ortho products has been detected (equation 85). 

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