Protic acids heterogeneous

Since the revised Biginelli mechanism was reported in 1997, numerous papers have appeared addressing improvements and variations of this reaction. The improvements include Lewis acid catalysis, protic acid catalysis, non-catalytic conditions, and heterogeneous catalysis. In addition, microwave irradiation (MWI) has been exploited to increase the reaction rates and yields. 

Toshima and coworkers demonstrated that environmentally friendly heterogeneous catalysts such as montmorillonite K-10, Nafion-H , and S04/Zr02 were very effective for the glycosylations of glycosyl fluorides [74]. This is the first report on the use of a protic acid for activation of glycosyl fluoride. Among them, SO4/Z1O2 was shown to be superior to the... 

While the original Scholl conditions called for both a Lewis and protic acid, research by Kovacic and Kyriakis into the role of oxidizing agents (that facilitate the formation of radical cations) in the Scholl reaction led to the observation that benzene (12) when reacted in the presence of a heterogeneous mixture of anhydrous aluminum chloride and copper chloride afforded poly(para-phenylene) (13). It is important that Ihis reaction was conducted under mild conditions (25-35 °C) and was complete in only 2 h. As discoverers of one of the first Scholl reactions conducted under mild conditions, Kovacic and Kyriakis have greatly increased the synthetic utility of this reaction. 

Thus far the solvent systems we have discussed are typical protic organic media, such as, for example, water-ethanol mixtures containing an added supporting electrolyte. These solvents are presumably quite homogeneous on a microscopic level. However, a number of solvents have been developed in recent years which are heterogeneous on a microscopic scale. Micellar media are one example of such solvents. The electrochemical reduction of nitrobenzene in aqueous solutions containing polyoxyethylene lauryl ether, a substance known to produce neutral micelles, produces azobenzene (4) even at pH somewhat less than 723. This is apparently the first case of formation of a dimeric product from electrolysis of nitrobenzene (1) in acidic media. Another striking example of this phenomenon... 

We extend our imderstanding of the concepts of chemical bonding and reactivity learned in Chapter 3 on metals and Chapter 4 on zeolites to catalysis over metal oxides and metal sulfides in Chapter 5. The featmes that lead to the generation of surface acidity and basicity are described via simple electrostatic bonding theory concepts that were initially introduced by Pauling. The acidity of the material and its application to heterogeneous catalysis are sensitive to the presence of water or other protic solvents. We explicitly examine the effects of the reaction medium in which the reaction is carried out. In addition, we compare and contrast the differences between liquid and solid acids. We subsequently describe the influence of covalent contributions to the bonding in oxides and transition to a discussion on the factors that control selective oxidation. 

Abstract Attention should be devoted to the measurements of the adsorption properties of catalytic surfaces when they have to work in liquid-solid heterogeneous conditions. The mutual characteristics of the surface and the liquid affect the reagent interactions with the surface sites which could be engaged with the liquid interaction and then not-available for the reagent coordination. This leads to observe effective adsorption properties that could be different from the intrinsic properties of the surface. The possibility to quantitatively determine the effective acid properties of catalytic surfaces by base adsorption is here showed. The adsorption can proceed in any type of liquid of various characteristics (apolar, polar, aprotic, protic) with dynamic (pulse liquid chromatographic method) or equilibrium (liquid recirculation chromatographic method) methods. The measurements of effective acidity allows finding more sound relations with the catalytic activity for a better comprehension of the catalyst work and for a more correct determination of the turnover numbers in liquid-solid catalysis. 

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