Acid-Base Pair Sites

The mechanism of phenol methylation on Cul-xCoxFe204 and the participation of Lewis acid-base pair site in the methylation step were recently demonstrated in detail [79]. 

The spectra obtained from the chemisorption of methanol onto catalyst above 100°C indicated the progressive oxidation of methoxy species to formate via dioxymethylene/HCHO and finally to CO, CO2, and H2. Phenol adsorbed on the surface Lewis acid-base pair site and dissociated to phenolate anion and proton. The formation of phenolate anion and proton were discerned from the strong intense C-0 stretching vibration and the disappearence of phenolic 0-H stretching vibration, respectively. Importantly, there were series of definite low intensity bands between 2050 and 1780 cm" that were identified as the out-of-plane aromatic C-H bending vibrations [79, 84-85]. These bending vibrations are possible only if the phenyl ring of phenol is perpendicular to the catalyst surface. 

Adsorption of aniline on Cui xZnxFc204 at <100°C indicates a simple molecular adsorption through N-atom on an acid-base pair site. However, above 100°C, N-H bond dissociates and aniline chemisorbed strongly on the catalyst surface. Chemisorbed aniline... 

It appears to us that the way forward to further industrial application of solid base catalysts may emerge from the discovery of unusual selectivity effects or new reactions that require well-balanced base and acid-base pair sites with relatively inexpensive solid catalysts that last long enough to process large quantities of reactants per unit mass of catalyst. 

The interaction of acetylene with Mg(001) was investigated by LEED (198-200). At 88 K, C2H2 molecules lie almost parallel to the surface, and neither molecule nor substrate distortions have been observed, indicating that only a weak physisorption occurs. Calculations with semiempirical potentials confirm the structure determined by LEED (199). The isomerization of d.v-2-butene on MgO has been reported (201). The dissociation of cyclopen-tadiene on a few active acid-base pair sites of totally dehydrated MgO was followed by IR spectroscopy, and the formation of surface hydroxyl groups and C5H5 species was proposed (202). Methanol decomposition (203) and ethanol decomposition (204, 205) have been reported. 

Until recently, when Peri 155) reported on a model of the silica-alumina surface, there were no detailed models for the surfaces of mixed oxides available. Beside the presence of Br nsted and Lewis acid sites, Peri 156) had proposed the existence of a sites on the Si02—A1203 surface, which he described as acid-base pair sites rather than simple Lewis acid sites. Various molecules, such as acetylene, butene, and HC1, are adsorbed very selectively on these a sites, whereas NH3 and H20 are also held by many other sites 157). To rationalize the formation of these sites, Peri 155) developed a semiquantitative surface model for certain silica-aluminas, which were prepared by reaction of A1C13 with the surface silanol groups of silica and subsequent hydrolysis and dehydration. The model is entirely based on a surface model of silica, which suggests an external surface resembling a (100) face of the cristobalite structure 158). It should be mentioned in this connection that Peri s surface model of silica may... 

A limiting form of this surface species may be the NH3+ form. At high pretreat-ment temperatures (>500°C) and low hydroxyl densities, NH2 groups are formed, probably due to dissociative chemisorption on acid-base pair sites, according to... 

The pyridone surface species has a C=0 stretching band at 1634 cm-1,3 Hydrogen gas has been detected by mass spectrometry (210), and the formation of this surface compound has been established by chemical methods by Boehm (215). This surface reaction points to the existence of strongly basic OH" ions held to certain sites on alumina surfaces, their number being of the order of magnitude of 1013/cm2 (121). Additional evidence for the existence of these reactive and strongly basic OH" ions on aluminas comes from surface reactions observed on adsorption of nitriles and ketones (see Section IV.F) and of carbon dioxide (see Section IV.G). These reactions may, thus, be valuable for the detection of the corresponding sites that most probably have to be considered as acid-base pair sites. 

Carbon dioxide fulfills some of the relevant criteria and contradicts others. Evidently, although C02 exhibits acidic properties, the adsorbed amounts cannot be taken as a measure of surface basicity strong chemisorption of C02 occurs through interaction with acid-base pair sites preferentially. Thus, specific poisoning of basic sites by C02 chemisorption is not possible. Furthermore, a... 

The adsorption of formic acid and acetic acid leads to the formation of car-boxylate groups on aluminas (194, 295-299), titanium dioxides, (134, 135b, 176, 194, 300, 301), chromium oxide (134, 302, 303), zinc oxide (298, 304-306), and magnesium oxide (299, 304, 306). The corresponding dissociative chemisorption step most probably takes place on acid-base pair sites of the type... 

In order to understand the action of acid-base pair sites on solids, we can resort to analogies with homogeneous catalysis. Tanabe [288] gives a very illustrative example the mutarotation of tetramethylgiucose catalyzed by 2-hydroxypyridine (291 ]. 

Different authors usually state that must of the heterogeneous reactions which traditionally have been seen as base- or acid-cataly/ed occur, in fact, on acid-base pair sites. In any case, there is no denying the fact that acid-base bifunctional catalysis will be one of the most promising fields of Synthetic, Fine and (ireen (Sustainable) Chemistry in the foreseeable future. 

Further developments in the study of concerted processes. Search for new test reactions for the study of acid-base pair sites. 

Tanabe. K. Are there any probe molecules suitable for measurements of acid-base pair sites Trends in analytical chemistiy. 1994 13. 164-168. 

The base-catalyzed dehydration, hke the aldol condensation reaction, involves a surface acid-base pair site and proceeds through formation of a carbanion intermediate by a-hydrogen abstraction, as presented in Scheme 5. 

However, Lewis acid sites not only modify the Bronsted acid sites. These species will also act themselves in various ways in molecular sieve catalysis. Together with the adjacent framework oxygen atoms, Lewis acid sites will act as Lewis acid-base pairs, and may polarize bonds in reacting molecules, possibly enhancing their chemical reactivity [27,28]. For molecules that are already polar, this polarization could also be sufficient to catalyze a chemical transformation, e.g., in the reaction of alcohols [51]. Lewis acid sites also act as hydride or anion receptors in a variety of reactions. Thus, in most cases the character of an acid-base pair site will be more pronounced in the case of Lewis than in the case of Bronsted acid sites [52,53]. 

Oxides commonly studied as catalytic materials belong to the structural classes of corundum, rocksalt, wurtzite, spinel, perovskite, rutile, and layer structure. These structures are commonly reported for oxides prepared by normal methods under mild conditions [1,5]. Many transition metal ions possess multiple stable oxidation states. The easy oxidation and reduction (redox property), and the existence of cations of different oxidation states in the intermediate oxides have been thought to be important factors for these oxides to possess desirable properties in selective oxidation and related reactions. In general terms, metal oxides are made up of metallic cations and oxygen anions. The ionicity of the lattice, which is often less than that predicted by formal oxidation states, results in the presence of charged adsorbate species and the common heterolytic dissociative adsorption of molecules (i.e., a molecule AB is adsorbed as A+ and B ). Surface exposed cations and anions form acidic and basic sites as well as acid-base pair sites [1]. The fact that the cations often have a number of commonly obtainable oxidation states has resulted in the ability of the oxides to undergo oxidation and reduction, and the possibility of the presence of rather high densities of cationic and anionic vacancies. Some of these aspects are discussed in this chapter. In particular, the participation of redox sites in oxidation and ammoxidation reactions and the role of redox sites in various oxides that are currently pursued in the literature are presented with relevant references. 

Even in reactions which have been recognized to be catalyzed only by acid sites on a catalyst surface, basic sites also act more or less as active sites in cooperation with acid sites. The catalysts having suitable acid-base pair sites sometimes show... 

Thorium oxide and UO2 are rather basic catalysts, though acidic sites seem to participate in the base-catalyzed reactions. Although basicity has not been measured by usual methods, catalytic selectivities and poisoning experiments suggest the existence of basic sites or acid — base pair sites on the surfaces. 

Dehydrohalogenation was also indicated to be catalyzed by acid-base pair sites. Table 3.18 shows the product distribution from dehydrochlorination of 1,1,2-trichloroethane over alumina as well as over silica-alumina (a typical solid acid) and KOH —Si02 (a typical solid base). Silica-alumina and KOH —Si02 showed products typical of acid and base catalysts, respectively. On the other hand, the products from alumina are different from the others and well explained by a concerted mechanism catalyzed by both acid and base. 

For each alumina a linear relationship was found between the logarithm of the rate constant k j ), and the acid strength,( yj, as shown in Fig. 3.37. That is, the Bronsted rule of catalysis holds for each alumina. The fact that three linear plots were obtained means that the acid site having the same acid strength had different catalytic activity from one alumina to another. They explained iurther this difference between the three catalysts by the difference in the basicity of the aluminas shown in Table 3.16. The rate constant for acid strength k(j) becomes greater for alumina with higher basicity-to-acidity ratio. This implies that the acid-base pair sites are the active sites. 

For both reactions, acid-base pair sites operate as the active sites. 

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