Solid Bronsted basicity

Climent, M. J., Corma, A., Ihorra, S. and Velty, A. Designing the adequate base solid catalyst with Lewis or Bronsted basic sites or with acid-base pairs, J. Mol. Catal., A, 2002, 182-183, 327-342. 

A second example of acid and base catalysts that can be used in a one-pot multireaction is the use of montmorillonite and hydrotalcite. The Ti-exchanged montmorillonite (Ti +-mont) is a solid Bronsted acid catalyst as a result of the presence of Ti + centers in the interlayer spaces. (37)a, (64). In contrast, hydrotalcite (HT) is considered a promising base catalyst because the HT surface displays a tunable basicity. Since large HT particles cannot enter the narrow interlayer of the Ti +-mont, these catalysts can be used in the same reactor, allowing both... 

Adsorption of water is thought to occur mainly at steps and defects and is very common on polycrystalline surfaces, and hence the metal oxides are frequently covered with hydroxyl groups. On prolonged exposure, hydroxide formation may proceed into the bulk of the solid in certain cases as with very basic oxides such as BaO. The adsorption of water may either be a dissociative or nondissociative process and has been investigated on surfaces such as MgO, CaO, TiOz, and SrTi03.16 These studies illustrate the fact that water molecules react dissociatively with defect sites at very low water-vapor pressures (< 10 9 torr) and then with terrace sites at water-vapor pressures that exceed a threshold pressure. Hydroxyl groups will be further discussed in the context of Bronsted acids and Lewis bases. 

Basic molecules such as pyridine and NH3 have been the popular choice as the basic probe molecules since they are stable and one can differentiate and quantify the Bronsted and Lewis sites. Their main drawback is that they are very strong bases and hence adsorb nonspecifically even on the weakest acid sites. Therefore, weaker bases such as CO, NO, and acetonitrile have been used as probe molecules for solid acid catalysts. Adsorption of CO at low temperatures (77 K) is commonly used because CO is a weak base, has a small molecular size, a very intense vc=0 band that is quite sensitive to perturbations, is unreactive at low temperature, and interacts specifically with hydroxyl groups and metal cationic Lewis acid sites.26... 

Some ionic liquids have tunable Lewis acidities and basicities. The tuning can be achieved simply by varying the anion fraction in the overall ionic liquid composition. In some cases, Bronsted acidity can also be introduced into stable ionic liquids. Many publications show the broad applicability of acidic or basic ionic liquid media in catalysis replacing corrosive liquids and solid catalysts. 

Picramide and its N-substituted derivatives introduce the added complication that proton loss may occur from the amino-group to give the Bronsted bases 23 (R = H, alkyl, phenyl). Green and Rowe (1913) found support for this formula from the alkali metal analyses of the solids formed from many polynitroanilines with bases. In fact picramide itself has been frequently used as an indicator for establishing H-acidity scales in basic media on the assumption that its indicator behaviour is due to proton loss (Schaal, 1955 Stewart and O Donnell, 1962 Stewart et al., 1962). The most likely other alternatives for the products of 1 1 interaction of picramides with bases are 24 and 25 (R = H, alkyl, phenyl R =R = H, alkyl). Formula 24, the analogue of Meisenheimer s formula for the adducts of picryl ethers, was suggested by Busch and... 

The solid chosen for their work on reaction (XLVIII) was alumina whose surface possesses several types of Bronsted and Lewis acidic and basic functional groups (-Al+-, -OHa+, -O - H, -0 , and defect sites) [175, 219] which could be potential catalysts for the mutarotation of glucose. Woelm... 

Table 2.11 lists the principal types of solid base catalysts. We should remember, however, that base catalyst is a relative definition and thus the materials listed in Table 2.11 do not necessarily function as a base in all cases. Some of these materials may act as an acid if the reactants are strongly basic. The terms, acid and base, should be used according to the function. The materials may be called solid base catalysts only if acting as a base toward the reactants by abstraction of a proton (Bronsted base) or by donation of an electron pair (Lewis base) to form anionic intermediates that undergo catalytic cycles. 

Mg-Al mixed oxides obtained by thermal decomposition of anionic clays of hydrotalcite structure, present acidic or basic surface properties depending on their chemical composition [1]. These materials contain the metal components in close interaction thereby promoting bifunctional reactions that are catalyzed by Bronsted base-Lewis acid pairs. Among others, hydrotalcite-derived mixed oxides promote aldol condensations [2], alkylations [3] and alcohol eliminations reactions [1]. In particular, we have reported that Mg-Al mixed oxides efficiently catalyze the gas-phase self-condensation of acetone to a,P-unsaturated ketones such as mesityl oxides and isophorone [4]. Unfortunately, in coupling reactions like aldol condensations, basic catalysts are often deactivated either by the presence of byproducts such as water in the gas phase or by coke build up through secondary side reactions. Deactivation has traditionally limited the potential of solid basic catalysts to replace environmentally problematic and corrosive liquid bases. However, few works in the literature deal with the deactivation of solid bases under reaction conditions. Studies relating the concerted and sequential pathways required in the deactivation mechanism with the acid-base properties of the catalyst surface are specially lacking. 

In considering solid surfaces, properties such as superficial area, porosity (number and mean diameter of pores), and the existence of acidic and/or basic (from both types Lewis and Bronsted) sites on the surfaces are of prominent importance to understand and predict the chemical behavior of such solids. 

The acid forms of aluminosilicate zeolites have found wider use as acid catalysts than any other materials. Their outstanding utility derives from their relatively high acid strength, their high hydrothermal stability, their ability to impart shape selectivity to product distributions and the reproducibility with which they can be synthesised and modified. Each of these advantages stems directly from their crystalline structure. The two basic types of acid site types in microporous solids are Bronsted, which are protons located at bridging sites (Si-O-Al in zeolites, M-O-P in aluminophosphates) and Lewis, usually incompletely coordinated metal cations (especially aluminium in zeolites) in... 

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