Solids as Catalysts

Many essential transformations of carbohydrates regularly employ toxic and corrosive reagents, such as Lewis acids and strong mineral acids. Hence, the use of zeolites and related porous solids as catalysts in such reactions affords a practical and environmentally compatible alternative to the standard protocols. 

A clear example of the possible use of acid and/or superacid solids as catalysts is the alkylation of isobutane with butenes. Isobutane alkylation with low-molecular-weight olefins is one of the most important refining process for the production of high-octane number (RON and MON), low red vapor pressure (RVP) gasoline. Currently, the reaction is carried out using H2SO4 or HF (Table 13.1), although several catalytic systems have been studied in the last few years. 

The purpose of the present work is to study comparatively the activity of different acidic solids as catalysts in other "classical" type of molecular rearrangement as it is the conversion of oximes to amides (Beckmann rearrangement, egn.l), by adopting "dry media ... 

POTENTIAL OF POST-SYNTHESIS FUNCTIONALIZED MICROPOROUS AND MESOPOROUS SOLIDS AS CATALYSTS FOR FINE CHEMICAL SYNTHESIS... 

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. 

When examining in detail these results it appeared very difficult for us to build upon these experimental results as the reaction condition differ drastically from one paper to the other. This prompted us to reinvestigate the scope and limitation of the Friedel-Crafts acylation using heterogeneous solid as catalysts in trying as much as we could to rationalized the observed effects. 

The diffusion rates of fluids, particularly gases, throagh porous solid materials are of particular interest because of the common practice of using porous solids as catalysts and adsorbents. Diffusions transport rates within the pores may limit the rates by which such procesms occur. 

The intramolecular 5,e-epoxycarbonyl rearrangement of 8a,17-epoxy-14,15-dinorlabdan-13-one I to give stereoselectively two diastereoisomeric ketals II or III was promoted in heterogeneous media using silicic porous solids as catalysts, making possible to choose a convenient selective system through an adequate combination of solvent, catalyst and temperature. 

Veloso, CO Henriques, CA Santos, EN Monteiro, JLF. Basic solids as catalysts for the synthesis of limonene derivatives. Proceedings of the I3th ICC, Paris (France) 2004. Reference P5-062. 

Palladium Supported on Solids as Catalysts for Carbon-Carbon Coupling Reactions 327... 

Solid state FT-IR characterization of sohd substantiated the presence of the aforementioned species. SEM analysis of this sohd manifested spherical morphology of polysiloxane-conjugated Pd nanoclusters in the 40-50 nm size regime. Based on the above results, it can be concluded that the solid is a partially condensed polysiloxane network conjugated with Pd nanoclusters. Furthermore, the possibility of reusability of precipitated solid as catalyst was explored by injecting the substrate into the Schlenk tube with precipitated sohd. After 30 min of stirring, the mixture became homogeneous. Multinuclear spectroscopy characterizations of the reaction mixture confirmed formation of polysilyl ester. 

Multiphase reactions may involve gas-liquid, gas-liquid-solid (solid as catalyst or reactant), liquid-liquid, liquid-liquid-solid reactions, etc. The reactions may vary from very slow to very fast, endothermic to highly exothermic. Based on the reaction characteristics, different types of multiphase reactors are used in industrial practice. A number of texts deaUng with design of multiphase reactors are available (Satterfield 1970 Shah 1979 Ramachandran and Chaudhari 1983 Westerterp et al. 1988 Deckwer 1992). Considerable information on theoretical, hydrodynamics, and mass transfer aspects of different multiphase reactors has become available since the publication of the above texts. This recent information is likely to allow rational, simple and yet reliable designs of many industrially important multiphase reactors. In this book, different types of multiphase reactors falUng under two categories—(1) gas-liquid and (2) gas-liquid-solid— are considered. The basic aim is to provide user-friendly, simple, and reasonably accurate design procedure for each multiphase reactor. 

The process of waste minimisation has many facets. One major goal must be to enhance the intrinsic selectivity of any given process so that less waste is produced. A second is to provide a means of recovering reagents in a form which allows easy collection and regeneration. Yet another is to replace stoichiometric processes by catalytic ones. Solids, as catalysts or as supports for other reagents, offer potential for benefit in all of these areas. 

In reactions using amorphous solids as catalysts, where reaction occurs at the surface of the solid, the mobility of the reactants is reduced from three dimensions to two. In effect this concentrates the reactants in small, localised areas and thus raises the rate of reaction. In lamella or porous solids the limitations may be even greater. One type of catalysis common with porous solids when two species are in competition in a reaction involves biasing the process in favour of one species by use of a solid that allows only one of the competitors to enter the pores. This essentially separates the two species, allowing a selective reaction to occur. It is also possible to control the outcome of a reaction by selecting one of a set of possible products on the basis of its shape. Quite commonly reactions produce more than one product, or isomers of the same material, that have different physical shapes. By restricting the space in which the reaction occurs it is sometimes possible to control which product is formed or which can escape from the pores of the solid. 

Another way to prepare calcium aluminate supports and sorbents is based on the interaction of Ca(OH)2 with Al(OH)3 followed by heat treatment of the resulting Ca3[Al(OH)6]2- This last compound can be thermolyzed to yield Cai2Ali4033, in which the zeolite type structure was clearly established. IR spectroscopic investigation provided evidence for the bifunctional nature of the active sites on the surface of calcium aluminate based supports and sorbents. The presence of acidic and basic sites on the surface makes the use of these solids as catalysts for acid-base tranformations of organic substances promising. 

There could be situations where all the three phases would be present gas, liquid, and solid (as catalyst or reactant). The most common example of this is the slurry reactor used in reactions such as hydrogenation. Here, the rate is sometimes expressed as in the case of catalytic reactions, that is, per unit catalyst volume, weight, or surface, but more commonly in terms of the total reactor volume (liquid -I- gas -i- catalyst). 

Recently, Quevedo et al. evaluated different solids as catalysts— a natural bentonite from Colombia, homogenized calcium bentonite, Al-pillared bentonite, a mixed oxide (Al203-Mg0, M3+/(M2++M +)=0.5), and an Al-Zr pillared vermiculite— in the Pictet-Spengler reaction between dopamine hydrobromide and aromatic aldehydes to synthe-tize the 1,2,3,4-tetrahydroisoquinoline [141]. The best yields, up to total conversion, were obtained when the mixed oxide was used as catalyst. It seems that this catalyst exhibited acid-base pairs due to the presence of sites... 

Nanocatalysis is a rapidly growing field which involves the use of nanostruc-tured materials such as NPs, nanofilms, and nano porous solids, as catalysts. In comparison with their bulk counterparts, the nano-structured catalytic materials possess high surface-to-volume ratio, high surface energy, and different electronic state, which result in unique catalytic activity in many reactions. Conventionally, soHd-supported nano-metal particles (NMPs) have been widely studied in various reactions. Besides the specific function of a soHd base to build the catalytically... 

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