Heterogeneous catalyst Bronsted acid sites

Like the AVADA and the AlkyClean processes, these two processes also replace the liquid acid/base catalysts with solid acids and bases [192]. Although the reaction mechanism for the heterogeneous acid-catalyzed esterification is similar to the homogeneously catalyzed one [207,208], there is an important difference concerning the relationship between the surface hydrophobicity and the catalyst s activity. This is especially true for fatty acids, which are very lipophilic compounds. One can envisage three cases First, if there are isolated Bronsted acid sites surrounded by a... 

Applications of the technique to heterogeneous catalysts have been few, but they have demonstrated that the method is useful for catalyst characterization. For example, the lifetime of the orthopositronium species is inversely proportional to the number of Bronsted acid sites present in alumina-silica cracking catalysts. This interpretation was derived from a correlation between the activity for the alkylation of cumene and the lifetime of the orthopositronium species. 

Furan itself can be used as the starting material for the synthesis of 1-methylpyrrole <2002MI179>. 7-AI2O3 was found to be an effective catalyst for the dehydration reaction between furan and methylamine to afford 1-methylpyrrole. A yield of 57.6% was achieved under the experimental conditions of a reaction temperature of 400 °C, a methylamine/ furan molar ratio of 1.5, and the molar flow rate of furan approximately 3-3.5 mmol/h. Furan was adsorbed onto Bronsted acid sites on the catalyst, while the methylamine was adsorbed onto Lewis acid sites. With this heterogeneous catalyst, the rate determining step of the mechanism was suggested to be the adsorption of furan on the Bronsted acid sites to form a ring-opened species, which is followed by the insertion of the adsorbed methylamine to form secondary amine intermediates. Further dehydration at the Lewis acid sites would yield 1-methylpyrrole. 

The mesoporous solids developed by Mobil group in 1991 were found to be catalytically inactive and have attracted a considerable interest from researchers throughout the world to introduce catalytically active sites within these materials. For example, doping of aluminium into the silica generates Bronsted acid sites and the resulting materials can be used as solid acids in acid-catalysed reactions. It is also possible to deposit metal particles within the pores and to use these materials as redox catalysts in many chemical reactions. Another avenue for catalytic functionalisation is to tether metal complexes within pores in order to prepare heterogeneous catalysts. It has been observed in the process of functionalisation that MCM materials can lose mesoporosity, surface area and pore volume as shown by nitrogen... 

In the same way that replacement of Si4+ by Al3+ in zeolite structures leads to the formation of Bronsted acid sites, so does the replacement of Al3+ by divalent metals in ALPOs. Consequently there has been much recent interest in these materials as potential heterogenous catalysts.8 Generally, however, these materials lack the add strength and stability of zeolites, and important commercial applications have not yet emerged. 

Synthetic layered hydroxides with hydrotalcite-like structures are used as heterogeneous catalysts for base-catalyzed reactions. One of their major drawbacks for technological application is their low stability during hydrothermal and/or thermal treatments. Recent attempts to incorporate large, preferably multicharged anions into their interlayer spaces introduced new routes for preparation of more stable materials. The effect of intercalated polymetalates or transition metal complex anions in the fi amework of layered hydroxides is twofold They increase the gallery heights and thermal stability on one hand and introduce additional Lewis or Bronsted acidic sites in a basic framework on the other hand. 

In this chapter, the use of solid acids as heterogeneous catalysts for the Friedel-Crafts acylahon reaction is described. Our review is split up into seven sechons, describing the application of zeolites, clays, metal oxides, sulfated zirconia, heteropoly acids. Nation, and other less-utilized solid catalysts (i.e., graphite). When possible, the relationship between the acid properhes of the solids (namely, Bronsted and Lewis types) and the catalytic efficiency is shown, as well as the role of the active site location on the catalyst surface. ... 

The solid/gas interface was traditionally studied with respect to adsorption and catalysis. Here the assertion that the Bronsted definition of acidity is a particular case of the Lewis definition is neither obvious nor even helpful. It suffices to say that many reactions in heterogeneous catalysis require specifically the presence of either Bronsted or Lewis acidic (or basic) sites, and the reaction mechanisms depend on the nature of the surface site. A long-term goal of surface studies for the characterization of solid catalysts was to distinguish and quantify the number of Bronsted or Lewis sites with potential catalytic activity for gas-phase reactants. For that reason, when discussing the acid-base behavior of solid surfaces, it is no longer possible, nor desirable, to adopt the viewpoint that subsumes Bronsted acid-base properties in the more general Lewis definition. 

The definition of acidity in terms of the Lewis theory has a broad meaning that encompasses the Bronsted definition as a particular case. In practice, especially in the field of heterogeneous catalysis, it is important to differentiate between Bronsted or Lewis acid sites on solid catalysts, since they may show completely different catalytic behavior. 

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