Acidic zeolite aromatic compounds alkylation

Shape selective catalysis as typically demonstrated by zeolites is of great interest from scientific as well as industrial viewpoint [17], However, the application of zeolites to organic reactions in a liquid-solid system is very limited, because of insufficient acid strength and slow diffusion of reactant molecules in small pores. We reported preliminarily that the microporous Cs salts of H3PW12O40 exhibit shape selectivity in a liquid-solid system [18]. Here we studied in more detail the acidity, micropore structure and catal3rtic activity of the Cs salts and wish to report that the acidic Cs salts exhibit efficient shape selective catalysis toward decomposition of esters, dehydration of alcohol, and alkylation of aromatic compound in liquid-solid system. The results were discussed in relation to the shape selective adsorption and the acidic properties. 

IX. Alkylation of Aromatic Compounds on Acidic and Basic Zeolites... 

It seems that other acidic sites are the most efficient for the alkylation of aromatic compounds than for the reverse reaction, the cracking of alkylaromatic compounds [361]. For the forward process, a linear correlation was observed between the activity of decationized Y zeolites and the number of acidic sites corresponding to H0 < + 3.3, whereas for the cracking, the sites corresponding to H0 < —3.0 correlated with the activity. 

The conventional resinsulfonic acids such as sulfonated polystyrenes (Dowex-50, Amberlite IR-112, and Permutit Q) are of moderate acidity with limited thermal stability. Therefore, they can be used only to catalyze alkylation of relatively reactive aromatic compounds (like phenol) with alkenes, alcohols, and alkyl halides. Nafion-H, however, has been found to be a suitable superacid catalyst in the 110-190°C temperature range to alkylate benzene with ethylene (vide infra) 16 Furthermore, various solid acid catalysts (ZSM-5, zeolite /3, MCM-22) are applied in industrial ethylbenzene technologies in the vapor phase.177... 

Traditionally, nitration has been performed with a mixture of nitric and sulfuric acids (mixed acid method). However, the method is highly unselective for nitration of substituted aromatic compounds and disposal of the spent acid reagents presents a serious environmental issue. In order to address these problems several alternative methods for aromatic nitration have been developed recently. For example, lanthanide triflates catalyse nitration with nitric acid, which avoids the use of large volumes of sulfuric acid but provides no enhancement of selectivity.6 Selectivity of nitrations with alkyl nitrates,7 acyl nitrates,8 or even nitric acid itself9,10 can, however, be enhanced by zeolites. 

For alkylation of aromatic compounds with olefins, alcohols and alkyl halogenids, acidic zeolite catalysts may also be applied as it shown decads ago [1]. Alkylation of benzene with propene over acid catalysts yields isopropylbenzene (cumene) accompanied by formation of n-propylbenzene, di-isopropylbenzenes and propene oligomers as main by-products. 

Cs" salts catalyse the acylation of toluene, p-xylene and m-xylene with crotonic acid. Some alkylation of aromatic compounds with crotonic acid also takes place. Heteropoly acid was found to be more active than zeolites HY and H-Beta in the acylation. 

Zeolites were used in various processes that convert paraffins or olefins into alkyl monoaromatics containing chiefly from six to nine carbon atoms.There are various catalysts, and they involve a base or an acid zeolite, according to the type of process. The first catalyst, identified at the end of the 1970s. is composed of Pt deposited on the L zeolite (Table 2) exchanged with large alkaline ions such as potassium or alkaline earths, such as Ba. which gives it an alkaline nature. This cataiyst is monofunctional and is conceptually different from the conventional bifunctional acid catalysts based on Pt on chlorinated alumina. It selectively dehydrocyclizes the paraffins into aromatics, particularly hexane, which is the least reactive of them. The reaction takes place on the metal, which develops a special selectivity in contact with the alkaline zeolite. This aromatization process has not been successful so far. partly because of the extreme sensitivity of the catalyst to the slightest trace of sulfur compounds. 

Certain Mobil ZSM-5 type zeolites have pore openings with rings of 10 oxygen atoms (17). This structure permits access to reactant or product molecules with larger dimensions, such as substituted aromatic compounds, which can diffuse in and out of the catalyst. A mixture of toluene and alkylating agents, such as methanol or ethylene can easily enter the pores and react at an acidic site to produce the corresponding xylenes or ethyltoluenes In previously reported work, thermodynamic equilibrium mixtures of Isomers were produced (18). Furthermore, individual Isomers were Isomerized to the equilibrium mixture under alkylation conditions over a zeolite with similar properties (19). 

In the absence of propane, the interaction between methane and zeolite Zn/HBEA yields methylzinc (ZnCHj) and methoxide (ZnOCHj) species and formate fragments, which undergo further conversion into acetaldehyde and acetic acid (Fig. 29D). In the presence of benzene, only the formation of the methoxide ZnOCH is observed, which is apparently not oxidized by oxygen of the defected ZnO structure (Fig. 29E). At 823 K, benzene is methylated by ZnOCHj, yielding methyl-substituted aromatics, namely, toluene and xylenes (Fig. 29F). It was thereby found that methane participated in the methane-propane co-aromatization reaction hy alkylating the aromatic compounds that resulted from propane, as is illustrated hy Scheme 7. 

Coke formed from toluene. By CP/MAS- C-NMR it was shown that on LaY at 350°C coke formed from toluene had a polynuclear aromatic structure with few methyl substituents [6]. Coke formed on USHY at 350°C and 450°C was recovered from the coked zeolite by treatment with a solution of hydrofluoric acid [9]. For T = 350°C (wt % coke = 8.2) all the coke components were soluble in methylene chloride (R = 100 %) while for T = 450°C R was very low 55 % and 10 % for 3 and 14 wt % coke. At 350 C pyrene was the main coke component while at 450 C the main components of the soluble coke were (like from n-heptane, propene or cyclohexene) cyclopentapyrenes and indenopyrenes. A mechanism involving the alkylation of pyrenes by small olefins (C2-C olefins were observed in the reaction products) was proposed to explain the formation of these latter compounds. The number of coke molecules which caused the complete deactivation of the zeolite at 350°G was close to the number of strong acid sites while at 450 C it was about twice smaller which could show a blockage of the access to sites of supercages unoccupied by coke molecules. However, another explanation could be that the bulky insoluble coke molecules formed at 450 C occupy two supercages. 

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