Solid mineral supports

In chemical syntheses under the action of microwave irradiation the most successful applications are necessarily found to be the use of solvent-free systems [6], In these systems, microwaves interact directly with the reagents and can, therefore, drive chemical reactions more efficiently. The possible acceleration of such reactions might be optimum, because they are not moderated or impeded by solvents. Reactions on solid mineral supports and, in turn, the interaction of microwaves with the reagents on the solid phase boundary, which can substantially increase the rate of the reactions, are of particular interest [7]. 

The kinetics of the acid-catalyzed esterification reaction of 2,4,6-trimethylbenzoic acid in i-PrOH under microwave irradiation have been investigated [84], A simple and practical technique for MW-assisted synthesis of esters has been reported wherein the reactions are conducted either on solid mineral supports or by using a phase transfer catalyst (PTC) in the absence of organic solvents [85], The esterification of enols with acetic anhydride and iodine has also been recorded [86],... 

Loupy and Soufiaoui described a comparative study of the reactivity of diphenylnitri-limine 200 with several dipolarophiles under microwave irradiation in the absence of solvent using a solid mineral support or phase-transfer catalysis (PTC) conditions (Scheme 9.62) [30b]. The results showed that the best yields of adducts were achieved upon impregnating KF-alumina with a mixture of the hydrazynoyl chloride 199 and the dipolarophile followed by irradiation of the mixture in a focused oven. Reaction of this mixture under solid-liquid PTC conditions with KF-Aliquat under microwaves afforded lower yields of cycloadducts, perhaps owing to the partial decomposition of Aliquat at the reaction temperature (140 °C). In all cases, worse yields were obtained by classical heating under comparable reaction conditions (time and temperature). 

Almost all solid mineral supports are oxides. They behave like ion exchangers when their surface bears electric charges. Two categories may be distinguished (i) natural exchangers and (ii) amphoteric oxides. 

For the development of a sustainable chemistry based on clean technologies, the best solvent would be no solvent at all. For this reason, considerable efforts have recently been made to design reactions that proceed under solvent-free conditions, using modern techniques such as reactions on solid mineral supports (alumina, silica, clays), solid-state reactions without any solvent, support, or catalyst between neat reactants, solid-liquid phase-transfer catalysed and microwave-activated reactions, as well as gas-phase reactions [37-42]. However, not all organic reactions can be carried out in the absence of a solvent some organic reactions even proceed explosively in the solid state Therefore, solvents will still be useful in mediating and moderating chemical reactions and this book on solvent effects will certainly not become superfluous in the foreseeable future. 

No solvent at all [i.e. solvent-free reactions) All solvents Many types of reactions, using the following techniques Reactions on solid mineral supports (alumina, silica, clays) without any solvent, support, or catalyst between neat reactants solid-liquid phase-transfer-catalyzed and microwave-activated reactions ... 

Reaction on solid mineral support. Reaction between supported reagents on solid mineral supports in dry media by impregnation of compounds on alumina, silica or clay takes place. The reactants are impregnated on solid... 

When arylenediamines are treated with jS-ketoesters by heating in neutral solvents the products arc benzimidazolones. In acidic media, 2-substituted benzimidazoles are formed predominantly [139-142]. Generally, in view of the yields reported and the mixtures which may result in these reactions, they do not offer much potential as alternatives to the more. standard procedures. A recent modification appears more promising. Here, 2-substituted benzimidazoles can be made in 75-92% yields by reaction with a )8-kctoester using solid mineral supports under microwave irradiation. The solid supports are aigiles with surface acidities equivalent to those of concentrated nitric or sulfuric acid [140]. 

For reasons of economy and pollution, solvent-free methods are of great interest in order to modernize classical procedures making them more clean, safe and easy to perform. Reactions on solid mineral supports, reactions without any solvent/support or catalyst, and solid-liquid phase transfer catalysis can be thus employed with noticeable increases in reactivity and selectivity. A comprehensive review of these techniques is presented here. These methodologies can moreover be improved to take advantage of microwave activation as a beneficial alternative to conventional heating under safe and efficient conditions with large enhancements in yields and savings in time. 

A simple and practical technique for MW-assisted synthesis of esters has been reported wherein the reactions are conducted either on solid mineral supports or by using a phase-transfer catalyst (PTC) in the absence of organic solvents [95],... 

Reactions on solid mineral support Reactants are impregnated as neat liquids on solid supports such as alumina, silica, zeolite and clays or using their solutions in adequate organic solvent and for further solvent elimination in the case of sohds, reaction in dry media is performed between individually impregnated reactants, followed by possible heating. At the end of the reaction, organic products are simply removed by elution with an appropriate solvent. 

Microwave heating has proven to be of benefit particularly for reactions under dry media (e.g., solvent-free conditions) in open vessel systems (i.e., in the absence of a solvent, on solid support with or without catalysts) [4]. Reactions under dry conditions were originally developed in the late 1980 s [51], but solventless systems under microwave conditions offer several additional advantages. The absence of solvent reduces the risk of explosions when the reaction takes place in a closed vessel. Moreover, aprotic dipolar solvents with high boiling points are expensive and difficult to remove from the reaction mixtures. During microwave induction of reactions under dry conditions, the reactants adsorbed on the surface of alumina, silica gel, clay, and other mineral supports absorb microwaves whereas the support does not, and transmission of microwaves is not restricted. Moreover, microwaves can interact directly with reagents and, therefore, can more efficiently drive chemical reactions. The possible accelerations of such reactions are expected... 

Some degree of success in supported enantioselective catalysis was accomplished by using functionalisation of mineral support. Due to their unique textural and surface properties, mesoporous micelle-templated silicas are able to bring new interesting properties for the preparation of optically active solids. Many successfully examples have been reported for enantioselective hydrogenation, epoxidation and alkylation. However, the stability of the immobilised catalysts still deserves efforts to allow industrial development of such attractive materials. 

Devic, M. and Shirmann, J.-P. 1991. Cyclodeshydratation de I acide 2-benzoyl-benzoique en anthraquinone, en milieu sec, sur support solide mineral. New J. Chem. 15 949-953. 

Soil is a natural body constituted by solids (minerals and organic matter), liquid and gases, where the soil-forming forces have acted so as to organize it into horizons and to make it able to support the life of rooted plants (Soil Survey Staff, 1999). The soil-forming forces responsible for soil genesis were defined by Jenny (1941), who reported them in the form of the mathematical equation ... 

Mineral acids (H3PO4, H2SO4) on solid porous supports... 

Solid Waste - The steam-reforming process contains up to 8-9 catalytic steps catalysts are replaced after 2-6 years of service. Partial oxidation and coal gasification use 3-4 catalysts. The catalysts contain " hexavalent chromium, nickel, zinc, iron, and mineral supports therefore these materials could not be disposed into landfills, Companies that specialize in producing catalysts or metal-processing companies customarily recycle catalysts. 

A fixed bed or slurry bioreactor incorporates the biocatalyst immobilized on a solid support in an aqueous solution, mineral nutrients and an assimilable source of carbon. 

The utility of such reagents in the oxidation processes is compromised due to their inherent toxicity, cumbersome preparation, potential danger in handling of metal complexes, difficulties encountered in product isolation and waste disposal problems. Immobilization of metallic reagents on solid supports has circumvented some of these drawbacks and provided an attractive alternative in organic synthesis because of the selectivity and associated ease of manipulation. Further, the localization of metals on the mineral oxide surfaces reduces the possibility of their leaching into the environment. 

The practical applications of NaBH4 reductions on mineral surfaces for in situ generated SchifFs bases have been successfully demonstrated. The solid-state reductive amination of carbonyl compounds on various inorganic solid supports such as alumina, clay, silica etc. and especially on K 10 clay surface rapidly afford secondary and tertiary amines [126]. Clay behaves as a Lewis acid and also provides water from its interlayers thus enhancing the reducing ability of NaBH4 [22],... 

It is of primary interest to avoid corrosive mineral acids in synthetic processes. This can easily be achieved by use of acidic solid supports coupled with microwave irradiation. This has been applied to the preparation of quinolines [53] (Scheme 8.35). This procedure is a safe, green alternative to the use of H2S04 at more than 150 °C. In the same way, quinoxaline-2,3-diones were prepared [54] by use of single-mode irradiation. Previous attempts in solution led to explosions, but the authors successfully used solvent-free conditions with acidic supports or catalysts (the best being p-toluenesulfonic acid) and irradiation times of 3 min (Scheme 8.36). 

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