Molecular sieve catalysts, preparation

Molecular Sieve Catalyst Preparation. The aluminophosphate based molecular sieves used in the present study were prepared according... 

Trimm, D. L. and B. J. Cooper. 1970. The preparation of selective carbon molecular sieve catalysts. Chem. Commun. No, 8 477-78. 

A1 MAS-NMR spectra of mesoporous, cubic Na-Al-MCM-48 molecular sieve catalyst showed tetrahedrally-coordinated aluminium in both as-prepared and calcined samples.379... 

Most crystalline aluminosilicates have little intrinsic catalytic activity for hydrogenation reactions. However, a considerable amount of data has recently accumulated on the use of zero-valent metal-containing zeolites in many hydrocarbon transformations. Thus noble and transition metal molecular sieve catalysts active in hydrogenation (7,256-760), hydroisomerization (161-165), hydrodealkylation (157, 158,165-167), hydrocracking (168,169), and related processes have been prepared. Since a detailed discussion of this class of reactions is beyond the scope of this review, only a few comments on preparation and molecular-shape selectivity will be made. 

Sterte J, Hedlund J, Creaser D, et al., Application of the seed-film method for the preparation of structured molecular sieve catalysts, Catal. Tod 2001 69(1-4) 323-329. 

Titanosilicates have been synthesized which have the large pore structure of UTD-1. These molecular sieves were prepared using the metal complex Cp 2Co (bis(pentamethylcyclo-pentadienyl)cobalt(III) ion) as the template. Ti-UTD-1 has been found to be an effective catalyst for the oxidation of alkanes, alkenes and phenols using hydrogen peroxide as well as the larger oxidant t-butylhydroperoxide. The channel structure defined by 14 membered rings in Ti-UTD-1 also allows the conversion of larger substrates such as 2,6-di-t-butylphenol. 

Sodium alumiaate is widely used in the preparation of alumina-based catalysts. Aluminosilicate [1327-36-2] can be prepared by impregnating siHca gel with alumiaa obtained from sodium alumiaate and aluminum sulfate (41,42). Reaction of sodium alumiaate with siHca or siHcates has produced porous crystalline alumiaosiHcates which are useful as adsorbents and catalyst support materials, ie, molecular sieves (qv) (43,44). 

The catalyst, 3-benzyl-5-(2-hydroxyethyl )-4-methyl-l, 3-thiazoHum chloride, is supplied by Fluka AG, Buchs, Switzerland, and by Tridom Chemical, Inc., Hauppauge, New York. The thiazolium salt may also be prepared as described below by benzylation of 5-(2-hydroxyethyl)-4-methyl-l,3-thiazole which is commercially available from E. Merck, Darmstadt, West Germany, and Columbia Organic Chemicals Co., Inc., Columbia, SC. The acetonitrile used by the checkers was dried over Linde 3A molecular sieves and distilled under nitrogen, bp 77-78°C. The same yield of thiazolium salt was obtained by the checkers when benzyl chloride and acetonitrile from commercial sources were used without purification. 

Quinoxaline 1,4-dioxides have also been prepared by condensation reactions carried out on the surface of solid catalysts such as silica gel, " molecular sieves, " or alumina. " As a representative example, " BFO 1 and the P-dicarbonyl compound 16 were combined with silica gel in methanol. The excess methanol was removed by evaporation and the silica gel with adsorbed reagents was allowed to stand for two weeks without drying. The quinoxaline 1,4-dioxide 17 was obtained in 90% yield after elution from a silica gel column. 

Chiral salen chromium and cobalt complexes have been shown by Jacobsen et al. to catalyze an enantioselective cycloaddition reaction of carbonyl compounds with dienes [22]. The cycloaddition reaction of different aldehydes 1 containing aromatic, aliphatic, and conjugated substituents with Danishefsky s diene 2a catalyzed by the chiral salen-chromium(III) complexes 14a,b proceeds in up to 98% yield and with moderate to high ee (Scheme 4.14). It was found that the presence of oven-dried powdered 4 A molecular sieves led to increased yield and enantioselectivity. The lowest ee (62% ee, catalyst 14b) was obtained for hexanal and the highest (93% ee, catalyst 14a) was obtained for cyclohexyl aldehyde. The mechanism of the cycloaddition reaction was investigated in terms of a traditional cycloaddition, or formation of the cycloaddition product via a Mukaiyama aldol-reaction path. In the presence of the chiral salen-chromium(III) catalyst system NMR spectroscopy of the crude reaction mixture of the reaction of benzaldehyde with Danishefsky s diene revealed the exclusive presence of the cycloaddition-pathway product. The Mukaiyama aldol condensation product was prepared independently and subjected to the conditions of the chiral salen-chromium(III)-catalyzed reactions. No detectable cycloaddition product could be observed. These results point towards a [2-i-4]-cydoaddition mechanism. 

Allylic alcohols can be converted to epoxy-alcohols with tert-butylhydroperoxide on molecular sieves, or with peroxy acids. Epoxidation of allylic alcohols can also be done with high enantioselectivity. In the Sharpless asymmetric epoxidation,allylic alcohols are converted to optically active epoxides in better than 90% ee, by treatment with r-BuOOH, titanium tetraisopropoxide and optically active diethyl tartrate. The Ti(OCHMe2)4 and diethyl tartrate can be present in catalytic amounts (15-lOmol %) if molecular sieves are present. Polymer-supported catalysts have also been reported. Since both (-t-) and ( —) diethyl tartrate are readily available, and the reaction is stereospecific, either enantiomer of the product can be prepared. The method has been successful for a wide range of primary allylic alcohols, where the double bond is mono-, di-, tri-, and tetrasubstituted. This procedure, in which an optically active catalyst is used to induce asymmetry, has proved to be one of the most important methods of asymmetric synthesis, and has been used to prepare a large number of optically active natural products and other compounds. The mechanism of the Sharpless epoxidation is believed to involve attack on the substrate by a compound formed from the titanium alkoxide and the diethyl tartrate to produce a complex that also contains the substrate and the r-BuOOH. ... 

Thermal treatments can be applied to modify the properties of a material, for example, dealumination and optimization of crystalHne phases. These techniques do not require oxidants. Oxidative thermal treatments are generally employed to activate molecular sieves, by removing the organic templates employed during synthesis. This is one of the key steps when preparing porous catalysts or adsorbents. In air-atmosphere calcination, the templates are typically combusted between 400... 

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