Silica/alumina-montmorillonite

The synthesis of imidazoles is another reaction where the assistance of microwaves has been intensely investigated. Apart from the first synthesis described since 1995 [40-42], recently a combinatorial synthesis of 2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted imidazoles has been described on inorganic solid support imder solvent-free conditions [43]. Different aldehydes and 1,2 dicarbonyl compounds 42 (mainly benzil and analogues) were reacted in the presence of ammonium acetate to give the trisubstituted ring 43. When a primary amine was added to the mixture, the tetrasubstituted imidazoles were obtained (Scheme 13). The reaction was done by adsorption of the reagent on a solid support, such as silica gel, alumina, montmorillonite KIO, bentonite or alumina followed by microwave irradiation for 20 min in an open vial (multimode reactor). The authors observed that when a non-acid support was used, addition of acetic acid was necessary to obtain good yields of the products. 

Experiments were carried out using isotopically labelled methanol (97% 0) and ethanol (98% purchased from MSD Isotopes. Anhydrous isobutanol was purchased from Aldrich Chemical Co., Inc. and contained the natural abimdances of orygen isotopes, i.e. 99.8% and 0.2% O. Nafion-H was obtained fi om C. G. Processing, Inc. and Amberlyst resins were provided by Rohm and Haas. The 2SM-5 zeolite was provided by Mobil Research Development Corp. H-Mordenite, montmorillonite K-10, and silica-alumina 980 were obtained firom Norton, Aldrich, and Davison, respectively. y-AIumina was prepared from Catapal-B fi om Vista. 

Reaction of pyrones under dassical conditions requires the use of high temperatures to obtain low to moderate yields. The Diels-Alder reaction of pyrones has been performed in a commercial microwave oven under solvent-free conditions on solid supports such as silica gel, montmorillonite, fitrol clay and alumina. The reaction time was dramatically reduced - from 4 h to 4 min (Scheme 9.9) [50],... 

Rare-earth exchanged [Ce ", La ", Sm"" and RE (RE = La/Ce/Pr/Nd)] Na-Y zeolites, K-10 montmorillonite clay and amorphous silica-alumina have also been employed as solid acid catalysts for the vapour-phase Beckmann rearrangement of salicylaldoxime 245 to benzoxazole 248 (equation 74) and of cinnamaldoxime to isoquinoline . Under appropriate reaction conditions on zeolites, salicyl aldoxime 245 undergoes E-Z isomerization followed by Beckmann rearrangement and leads to the formation of benzoxazole 248 as the major product. Fragmentation product 247 and primary amide 246 are formed as minor compounds. When catalysts with both Br0nsted and Lewis acidity were used, a correlation between the amount of Br0nsted acid sites and benzoxazole 248 yields was observed. 

There has been an enormous technological interest in tertfa/j-butanol (tBA) dehydration during the past thirty years, first as a primary route to methyl te/f-butyl ether (MTBE) (1) and more recently for the production of isooctane and polyisobutylene (2). A number of commercializable processes have been developed for isobutylene manufacture (eq 1) in both the USA and Japan (3,4). These processes typically involve either vapor-phase tBA dehydration over a silica-alumina catalyst at 260-370°C, or liquid-phase processing utilizing either homogenous (sulfonic acid), or solid acid catalysis (e.g. acidic cationic resins). More recently, tBA dehydration has been examined using silica-supported heteropoly acids (5), montmorillonite clays (6), titanosilicates (7), as well as the use of compressed liquid water (8). 

Thenaldehyde (thiophene-2-carbaldehyde) is readily available via the Vilsmeier-Haack reaction of DMF with thiophene catalyzed by phosphorus oxychloride. The Sommelet reaction with 2-chloromethylthiophene also gives reasonable yields (63AHC(l)l). Likewise, thiophene is readily acylated with acyl anhydrides or acid chlorides (equation 14), using mild Friedel-Crafts catalysts, such as tin(IV) chloride, zinc chloride, boron trifluoride, titanium tetrachloride, mercury(II) chloride, iodine and even silica-alumina gels or low-calcium-content montmorillonite clays (52HC(3)l). 

Keywords hydrazide, phenacyl bromide, alumina, montmorillonite, silica gel, microwave irradiation, pyrazine... 

The industrially important acetoxylation consists of the aerobic oxidation of ethylene into vinyl acetate in the presence of acetic acid and acetate. The catalytic cycle can be closed in the same way as with the homogeneous Wacker acetaldehyde catalyst, at least in the older liquid-phase processes (320). Current gas-phase processes invariably use promoted supported palladium particles. Related fundamental work describes the use of palladium with additional activators on a wide variety of supports, such as silica, alumina, aluminosilicates, or activated carbon (321-324). In the presence of promotors, the catalysts are stable for several years (320), but they deactivate when the palladium particles sinter and gradually lose their metal surface area. To compensate for the loss of acetate, it is continuously added to the feed. The commercially used catalysts are Pd/Cd on acid-treated bentonite (montmorillonite) and Pd/Au on silica (320). 

It has been a long time since mineral clays like montmorillonite were discovered to have strong acidity in the solid state. Moreover, clays had been utilized as catalysts for catalytic cracking to produce gasoline before amorphous silica-alumina and zeolites were invented. At the present time, besides use as catalysts, clay is being utilized in a variety of fields medicine, paint, cosmetics, detergent, and casting. 

Highly acidic natural clays, montmorillonite are complex layers of S1O4 and AIO4 tetrahedra. They also contain small amounts of MgO and Fe20j. These impurities are leached with sulfuric aid, which also adds protons to increase maximum pK values from -3.0 to -8.2. These clays were the first cracking catalysts used with fixed and moving beds. However, they were quickly replaced by the superior synthetic silica-aluminas that were ideal for fluidized beds. Today, they are used as the matrix in zeolite-based cracking catalyst. 

Figure 5. A comparison of catalytic activities using a light cycle oil over deactivated catalysts based on pillared montmorillonite, pillared rectorite, NaY and an amorphous silica-alumina FCC.

Most of the catalysts were commercial SA, a silica-alumina containing 13 wt% aliunina, from Ketjen K 10, an acidified montmorillonite, from Siid-Chemie FAU 2.5, the Linde tsrpe Y molecular sieve SK-41, from Alfa-Products FAU 15, MOR 11 and MOR49, dealuminated HY and Mordenites, were gifts from Zeocat MOR 6.5 was obtained from Norton MFI 25, a H-ZSM-5 zeolite, from Conteka. BEA 27, a beta zeolite, was synthesized according to Wadlinger et... 

The behaviours of the acidified montmorillonite (K 10) and of the silica-alumina (SA) are similar to the one of the dealuminated faujasite, except for the presence of induction. With SA, the best yield is in accordance with the best diffusion of the primary product through the amorphous structure of the catalyst. 

Acylation reactions of thiophene have been also investigated under heterogeneous conditions a montmorillonite clay and silica-alumina [9] and Nafion-H [10] have been shown to be efficient catalysts of the acylation of thiophene with acetyl chloride. 

Interestingly, 2,5-disubstituted 1,3,4-oxadiazoles have been synthesized by oxidation of l-aroyl-2-arylidene hydrazines with potassium permanganate on silica, alumina, or montmorillonite KIO clay surfaces under MW irradiation conditions [133], KIO clay seemed to be the most appropriate support. 

When montmorillonite is heated, water that has been absorbed between the silica layers is evolved first, between 100° and 200°C. About 700°C, the clay mineral breaks down, giving up its water of constitution, and an amorphous mass of silica, alumina and magnesia remains, corresponding to meta-kaolin . On being heated to about 1200°C, mullite, cristobalite, cordierite and spinel form. 

The theory of structural acids is largely due to Pauling (22). In any crystal lattice involving both n ative and positive ions, a net negative charge can be created by the isomorphous substitution of a positive ion of a valence lower than that of the substituted podtive ion. Thus, if an aluminum ion is substituted for a silicon ion in a silica lattice made up of silica tetrahedra, a tiivalent ion has been substituted for a quadrivalent ion and there results a positive valence deficiency of one for each aluminum ion so isomorphously introduced. In many naturally-occurring silica-alumina structures, this type of substitution has taken place. In all these systems the valence deficiency or net n ative charge in the crystal lattice is made up or satisfied by a positive ion at or near the point in the structure at which the substitution has taken place. Materials typical for these structural characteristics are natrolite and other natural zeolites, montmorillonites, and feldspars. 

While a variety of inorganic solid supports such as alumina, clay, silica and montmorillonite K 10 clay surfaces have been explored, but the in situ generated Schiff s bases have been successfully reduced using clay to deliver r idly secondary and tertiary amines 47). Clay not only behaves as a Lewis acid but provides water from its interlays that enhances the reducing ability of NaBH4. 

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