Transition metals sites with Lewis acidic properties

Several properties of transition metals make them useful in catalysis. Metal ions provide a high concentration of positive charge that is especially useful in binding small molecules. Because transition metals act as Lewis acids (electron pair acceptors), they are effective electrophiles. (Amino acid side chains are poor electrophiles because they cannot accept unshared pairs of electrons.) Because their directed valences allow them to interact with two or more ligands, metal ions help orient the substrate within the active site. As a consequence, the substrate-metal ion complex polarizes the substrate and promotes catalysis. For... 

Catalytic Properties. In zeoHtes, catalysis takes place preferentially within the intracrystaUine voids. Catalytic reactions are affected by aperture size and type of channel system, through which reactants and products must diffuse. Modification techniques include ion exchange, variation of Si/A1 ratio, hydrothermal dealumination or stabilization, which produces Lewis acidity, introduction of acidic groups such as bridging Si(OH)Al, which impart Briimsted acidity, and introducing dispersed metal phases such as noble metals. In addition, the zeoHte framework stmcture determines shape-selective effects. Several types have been demonstrated including reactant selectivity, product selectivity, and restricted transition-state selectivity (28). Nonshape-selective surface activity is observed on very small crystals, and it may be desirable to poison these sites selectively, eg, with bulky heterocycHc compounds unable to penetrate the channel apertures, or by surface sdation. 

Thus, transition metal cations in the lower valence state may also act as Lewis bases. Factors that affect the reactions promoted by Lewis acidity are listed in Table I. Lewis acid sites reversibly adsorb water (6s 9, 42), which may thus strongly compete with organic compounds that have weaker Lewis base properties, such as aromatic hydrocarbons. Lewis acidity depends on the degree of hydration and is strongest under desiccating conditions. Examples of reactions that are promoted by Lewis acidity are summarized in Table II. Other examples have been reviewed by Solomon and Howthorne (37). 

Nb monomer catalyst (1) is remarkably suppressed to 1/300 and the dehydration is promoted 4 times on the dimer catalyst (2). It is to be noted that the change of the number of Nb atoms at the active sites from one to two metal atoms gives rise to a complete reverse of basicity/acidity in the catalytic properties. In the dimer catalyst (2) the preferable conformation at the transition state observed with the monomer catalyst (1) is difficult. Furthermore, the Lewis acidity of the Nb atoms in the dimer (2) is increased by the oxygen-bridge. 

More recently, Hattori and Shiba (26) and Ward (68) studied the acidity of X zeolites. For Mg, Mn, and ZnX, Hattori and Shiba (26) report a small amount of Bronsted acidity and Lewis acidity which is too weak to be converted into Bronsted acidity by water. On Ca and SrX, they reported strong Lewis acidity which could be converted into Bronsted acidity. In contrast, Ward (68) observed Bronsted acidity on all Group II A zeolites, the concentration of sites increasing with decreasing cation radius or increasing field. The concentration of Bronsted acid sites was increased by hydration. No Lewis acid sites were detected, although pyridine interaction with the cations was observed. Ignat eva et al. (32) reported the presence of Bronsted acid sites on CaY but not on NaY. Bronsted acidity but no Lewis acidity was observed on the transition metal ions, Mn, Co, Zn, Ag, Cd, but not on Cu (68). The concentration was increased in all cases by hydration. There appeared to be no relationship between the concentration of acid sites and the physical properties of the zeolites. Studies of the same series of transition metal ion Y zeolites yielded similar results (69). 

The oxide catalysts are microporous or mesoporous materials or materials containing both types of pores. In the latter case, the applicability is larger in terms of the molecular size of the reactants. Acid-base properties of these materials depend on the covalent/ionic character of the metal-oxygen bonds. These sites are involved in several steps of the catalytic oxidation reactions. The acid sites participate with the cation redox properties in determining the selective/unselective catalyst behavior [30,31]. Thus, many studies agree that partial oxidation of organic compounds almost exclusively involves redox cycles and acid-base properties of transition metal oxides and some authors have attempted to relate these properties with activity or selectivity in oxidation reactions [31,42]. The presence of both Bronsted and Lewis acid sites was evidenced, for example, in the case of the metal-modified mesoporous sihcas [30,39,43]. For the bimetallic (V-Ti, Nb-Ti) ions-modified MCM-41 mesoporous silica, the incorporation of the second metal led to the increase of the Lewis sites population [44]. This increased concentration of the acid sites was well correlated with the increased conversion in oxidation of unsaturated molecules such as cyclohexene or styrene [26,44] and functionalized compounds such as alcohols [31,42] or phenols [45]. 

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