Zeolites aromatic ethers

The acetylation over protonic zeolites of aromatic substrates with acetic anhydride was widely investigated. Essentially HFAU, HBEA, and HMFI were used as catalysts, most of the reactions being carried out in batch reactors, often in the presence of solvent. Owing to the deactivation effect of the acetyl group, acetylation is limited to monoacetylated products. As could be expected in electrophilic substitution, the reactivity of the aromatic substrates is strongly influenced by the substituents, for example, anisole > m-xylene > toluene > fluorobenzene. Moreover, with the poorly activated substrates (m-xylene, toluene, and fluoroben-zene) there is a quasi-immediate inhibition of the reaction. It is not the case with activated substrates such as anisole and more generally aromatic ethers. It is why we have chosen the acetylation of anisole and 2-methoxynaphtalene as an example. 

In zeolite-catalysed acylation of aromatic ethers, zeolites have two advantages with respect to benzene and alkylbenzene acylation ... 

Friedel-Crafts Acylation of Aromatic Ethers Using Zeolites... 

An impressive number of papers and books has been published and numerous patents have been registered on the aq lation of aromatic compounds over solid catalysts. Recently Sartori and Maggi [1] have written an excellent review with 267 references on the use of solid catalysts in Friedel-Crafts acylation. In one section of this review, namely acylation of aromatic ethers or thioethers, the authors report work on acylation by solid catalysts such as zeolites, clays, metal oxides, acid-treated metal oxides, heteropolyacids or Nafion. When examining in details these results, it appeared very difficult for us to build upon these experimental results as the reaction conditions differ drastically from one paper to the next. This prompted us to reinvestigate the scope and limitations of the Friedel-Crafts acylation using heterogeneous solids as catalysts, trying as much as we could to rationalize the observed effects. 

Schiff base complexes to aromatic silyl enol ethers/ " olefins/ sugars,Mn ° Schiff base complexes, " and rhenium(III) complexes " has been reported. Similar reactions are observed with the Mn (N)porphyrin complexes. The reaction products are dependant on the nature of the Schiff base ligand, both the yield and the enantiomeric excess being affected. " Salen nitridomanganese(V) complexes have been incorporated into Zeolite... 

Fig. 1.6 Solid-state - C NMR reveals the successive steps in the conversion of methanol lo gasoline over zeolite ZSM-5. The methanol, resonating at 50 ppm, is first dehydrated to dimethyl ether (60 ppm). Subsequent carbon-carbon bond formation leads lo a hosi of aliphatic (-10 to 30 ppm) and aromatic (not shown) compounds. [Modified from Anderson, M W - Kltnowski. J. J. /1 i. Chem Soc. 1990, 112, 10-16. Reproduced with permission.]...

Although zeolites have been known for their adsorption properties for over a century, it was not until 1952, when the first synthetic zeolite was prepared, that their utility in chemical transformations was explored. Since that time, zeolites have been used for a multitude of purposes, and to this day, they are essential catalysts in the petroleum industry, converting large and small hydrocarbons into high-octane compounds. As an outgrowth of this work, zeolites have found utility in industrial fine chemical synthesis for the construction of aromatics, heterocycles, aliphatic amines, and ethers, and the photochemistry within zeolites has already grown out of its infancy. 

This paper is focused on commercially important syntheses of aromatic ketones and ethers with application in pharmaceuticals, agrochemicals, perfumery, etc. It will be demonstrated using several examples that clean processes using zeolite catalysts can be substituted economically for existing polluting processes. Limitations in the use of zeolite catalysts will also be emphasized, other solid catalysts leading, in certain cases, to much better results in terms of selectivity and stability. 

The formation of hydrocarbons from methanol catalyzed by zeolite H-MFI has been investigated extensively 60,61). As with many hydrocarbon conversions, the catalytic activity of the methanol-to-hydrocarbons reaction decreases over time as a result of the buildup of high-molecular-weight carbonaceous deposits (coke). UV Raman spectroscopy was employed to characterize the accumulation and chemical nature of deposited hydrocarbons as a function of time and reaction temperature with both methanol and dimethyl ether as reactants and with zeolite MFI of various Si/Al atomic ratios as catalysts the first account of this work reported results for a zeolite MFI with low acid content (Si/Al = 90) (62). Both polyolefin and a cyclopentadienyl species were observed as intermediates during the formation of polyaromatic retained hydrocarbons. These observations strongly confirm the mechanism of coke formation proposed by Schulz and Wei (63) involving aromatic formation via a five-membered ring... 

Armengol et al. [227] used protonated Al-MCM-41 molecular sieve for alkylation of bulky aromatic compounds such as 2,4-di-rerr-butylphenol with a bulky alcohol (cinnamyl alcohol). This reaction did not occur in the presence of large pore HY zeolite indicating the importance of the mesoporous structure of the H-MCM-41 catalyst and the accessibility of active sites. Kloetstra et aL [228] obtained excellent results during the tetrahydropyranylation of alcohols and phenols over Al-MCM-41 (Scheme 3). Bulky alcohols including cholesterol, adamantan-l-ol and 2-naphthol were converted into the corresponding tetrahydropyranyl ethers in relatively short periods of time. 

The Williamson reaction, discovered in 1850, is still the best general method for the preparation of unsymmetrical or symmetrical ethers.The reaction can also be carried out with aromatic R, although C-alkylation is sometimes a side reaction (see p. 515). The normal method involves treatment of the halide with alkoxide or aroxide ion prepared from an alcohol or phenol, although methylation using dimethyl carbonate has been reported. It is also possible to mix the halide and alcohol or phenol directly with CS2CO3 in acetonitrile, or with solid KOH in Me2SO. The reaction can also be carried out in a dry medium,on zeolite-or neat or in solvents using microwave irradiation. Williamson ether synthesis in ionic liquids has also been reported. The reaction is not successful for tertiary R (because of elimination), and low yields are often obtained with secondary R. Mono-ethers can be formed from diols and alkyl halides. Many other... 

All workers agree that the products are formed by an initial dehydration step and Derouane et al used n.m.r. and g.l.c. to study the products obtained from methanol and ethanol interaction with ZSM-5 zeolite in the range 250-400 °C. At temperatures <300 C the products were the usual respective ether and olefin, but when temperatures > 300 °C were used Ca-Cs-olefins and aromatics were obtained, the overall reaction being expressed as (Scheme 12) ... 

Various substituted benzimidazoles have been synthesized in very good yields in solvent-free conditions from 1,2-diaminobenzene and aldehydes in the presence of titanium(IV) chloride as a catalyst. The method is applicable to most aromatic, unsaturated and aliphatic aldehydes and to substituted 1,2-diaminobenzenes without significant differences [14]. Several other catalysts, namely iodine [15], hydrogen peroxide [16], zirconyl(IV) chloride [17], boron trifluoride diethyl etherate [18], ytterbium perfluorooctane sulfonates [19,20], zeolite [11,21], and L-proline [22], have been effectively used for the synthesis of benzimidazole derivatives. 

INITIAL STEPS IN METHANOL CONVERSION AND AN ALTERNATIVE HOMOLOGATION MECHANISM A small amount of methane (ca. 1C%) is formed in methanol conversion, and appears to be one of the first products formed (ref. 11). When a small amount of methanol is sorbed onto ZSM-5 zeolite, the lattice is methylated (ref. 5). Subsequent temperature-programmed desorption gives dimethyl ether and desorbed methanol first, then (at 250-300°C) methane (stable) and formaldehyde (unstable), and finally aromatic products (ref. 22-23). 

Among other zeolitic phases 6—(Ni11 r Col , )-(4-methylpyridine)4 (SCN)2 sorbs both non-polar and polar species (permanent and inert gases alkyl halides n-, iso- and cycloparaffins and aromatic hydrocarbons alcohols and ethers). The host lattice can scmetimes change at critical loadings of sorbate from one porous structure to another. This causes the steps seen in Fig. 2 for sorption isotherms of Xe at various temperatures [15]. 

The reaction of ethanol with ammonia on zeolite catalysts leads to ethylamine. If, however, the reaction is carried out in the presence of oxygen, then pyridine is formed [53]. MFI type catalysts H-ZSM-5 and B-MFI are particularly suitable for this purpose. Thus, a mixture of ethanol, NH3, H2O and O2 (molar ratio 3 1 6 9) reacts on B-MFI at 330 °C and WHSV 0.17 h 1 to yield pyridine with 48 % selectivity at 24 % conversion. At 360 °C the conversion is 81% but there is increased ethylene formation at the expense of pyridine. Further by-products include diethyl ether, acetaldehyde, ethylamine, picolines, acetonitrile and CO2. When applying H-mordenite, HY or silica-alumina under similar conditions pyridine yields are very low and ethylene is the main product. The one-dimensional zeolite H-Nu-10 (TON) turned out to be another pyridine-forming catalyst 54]. A mechanism starting with partial oxidation of ethanol to acetaldehyde followed by aldolization, reaction with ammonia, cyclization and aromatization can be envisaged. An intriguing question is why pyridine is the main product and not methylpyridines (picolines). It has been suggested in this connection that zeolite radical sites induced Ci-species formation. 

An efficient method for the continuous-flow selective acylation of aromatics, including ethers, over BEA zeolite, was patented. The process... 

Smith, K., Zhenhua, Z., and Hodgson, P. K. G. 1998. Synthesis of aromatic ketones by acylation of aryl ethers with carboxylic anhydrides in the presence of zeolite H- 3 (H-BEA) in the absence of solvent. /. Mol. Catal. A Chem. 134 121-128. 

H2 to aromatic molecules or to high-octane-number gasoline. First, methanol and olefins are produced by the catalytic reactions of CO and H2, as discussed above. Then, using a zeolite shape-selective catalyst that is introduced along with the ruthenium or other metal catalyst in the same reaction chamber, methanol and the olefins are converted to aromatic molecules, cycloparaffins, and paraffins. The mechanism involves the dehydration of methanol to dimethyl ether. The light olefins that also form are alkylated by methanol and by the dimethyl ether [134] to produce higher-molecular-weight olefins and then the final cyclic and aromatic products. 

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