Zeolite synthesis route

In the search for new zeolite synthesis routes, the topotactic condensation of layered precursors into 3D structures has proven to be a promising method for generating novel zeolite materials with desired framework structures. Based on a solid-state reaction with thermal treatment, two silanol groups on the surface of two neighboring layers dehydrate spontaneously and high-siUca zeolites can be formed while keeping the layered structures. 

The Friedel-Crafts acylation of aromatic compounds is an important synthesis route to aromatic ketones in the production of fine and specialty chemicals. Industrially this is performed by reaction of an aromatic compound with a carboxylic acid or derivative e.g. acid anhydride in the presence of an acid catalyst. Commonly, either Lewis acids e.g. AICI3, strong mineral acids or solid acids e.g. zeolites, clays are used as catalysts however, in many cases this gives rise to substantial waste and corrosion difficulties. High reaction temperatures are often required which may lead to diminished product yields as a result of byproduct formation. Several studies detail the use of zeolites for this reaction (1). 

The conversion of porous glass into the zeolites with MFI- and FER-structure has been performed following various synthesis routes, with or without a template addition. Tetrapropylammoniumbromide (TPABr) or propylamine (PA) have been used as a so-called structure directing template. Starting reaction mixtures expressed in mole ratios of the oxides are given below ... 

The other way to introduce heterometals is their isomorphous substitution for Si in the framework, in a similar manner to the isomorphous substitution of Al. The heteroatoms should be tetrahedral (T) atoms. In hydrothermal synthesis, the type and amount of T atom, other than Si, that may be incorporated into the zeolite framework are restricted due to solubility and specific chemical behavior of the T-atom precursors in the synthesis mixture. Breck has reviewed the early literature where Ga, P and Ge ions were potentially incorporated into a few zeolite structures via a primary synthesis route [9]. However, until the late 1970s, exchangeable cations and other extraframework species had been the primary focus of researchers. 

Breck has reviewed the early literature where Ga3+, P5+, and Ge4 were potentially incorporated into a few zeolite structures via a primary synthesis route (2). Evidence has also been presented to show that the small amounts of Fe3+, typically present in both natural and synthetic zeolites, are located in framework tetrahedral positions (3). A more recent review of "isomorphic substitution" in zeolites, via primary synthesis methods, speculates on the potential Impact of such substitutions on catalysis (4). The vast majority of work has been related to the high silica zeolites, particularly of the ZSM-5 type. Another approach to substitution of metal atoms into the open frameworks of zeolite structures has been to replace the typical silica alumina gel with gels containing other metal atoms. This concept has resulted in numerous unique molecular sieve compositions containing aluminum and phosphorus 5 silicon, aluminum and phosphorus (6) and with... 

The product is a valuable intermediate for the production of fragrances. This compound is hardly obtainable by known synthesis routes (Rosemund reduction). Using Al- or B-pentasil zeolite at 300°C the 4-formyltetrahydropyrans can be synthesized in an easy manner and high yields over 97% (18, 19, 20). This reaction was also carried out in the gas phase in a fixed bed reactor (5) with a WHSV = 2 h 1 and a ratio of substrate/solvent (THF) = 50/50. Using SiC>2 as solid catalyst gave an additional yield of 96.5% (9). 

There are some other methods that might be envisioned that could lead to OMS and OL materials. One obvious direction would be to use structure directors or templates that are similar to those used in zeolite synthesi such as tetraalkylammonium halides. Unfortunately, we have observed that such structure directors and templates react with KMn04 and get oxidized to C02- Another seemingly obvious route would be electrochemical syntheses. Some research has been done in this area, however, it is difficult to synthesize a sizeable amount of material such as with controlled potential electrolysis. In addition, some early work showed the generation of amorphous materials that after inital formation can be heated to form spinel phases without apparently going through the OMS/OL phases. 

All of the reactions discussed till now have been catalyzed by zeolites functioning as Bronsted or Lewis acids. In a major new development, zeolites have also been tailored to act as base catalysts (Hathaway and Davis, 1988a,b,c Tsuji et al., 1991). A superbase catalyst of this type has now found application in the synthesis of the key intermediate, 4-methylthiazole, used in the preparation of the anthelmintic, thiabendazole. The existing industrial route for 4 methylthiazole (18) involves using several hazardous chemicals such as chloroacetone and carbon disulfide. The new zeolite-based route uses the base-catalyzed reaction of SO2 with the imine from acetone (reaction 6.12). 

Combination of several individual reactions into one synthesis step, ie. shortening the synthesis route by using multifunctional catalysis with zeolites. 

Multifunctional catalysis, in which reactions consisting of several reaction steps are carried out by a shorter synthesis route, is becoming increasingly important in organic synthesis. Molecular sieve catalysts, too, help to combine several catalytic steps and tailor them optimally to one another [15, 18, 24], In this respect, molecular sieves like zeolites can be used as carriers for catalytically active components such as transition metals, noble metals. In addition the catalytic behaviour of these components the intrinsic acidic or basic or redox properties of the zeolites combined with shape selective feature are still present. 

Discovering unknown synthesis routes is extremely exciting for the preparative chemist and success brings him considerable satisfaction. Zeolite catalysts make it possible in specific cases. The example comes from the acylation of heteroaromatics. 

Other examples of the discovery of new synthesis routes with zeolite catalysts, including the benzamine rearrangement [75 - 77] and the... 

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