Hydrotalcite basic sites

Mg/Me (Me=Al, Fe) mixed oxides prepared from hydrotalcite precursors were compared in the gas-phase m-cresol methylation in order to find out a relationship between catalytic activity and physico-chemical properties. It was found that the regio-selectivity in the methylation is considerably affected by the surface acid-basic properties of the catalysts. The co-existence of Lewis acid sites and basic sites leads to an enhancement of the selectivity to the product of ortho-C-alkylation with respect to the sole presence of basic sites. This derives from the combination of two effects, (i) The H+-abstraction properties of the basic site lead to the generation of the phenolate anion, (ii) The coordinative properties of Lewis acid sites, through their interaction with the aromatic ring, make the mesomeric effect less efficient, with predominance of the inductive effect of the -O species in directing the regio-selectivity of the C-methylation into the ortho position. 

Figure 2 shows illustrated mechanism for acetone self-condensation over calcined hydrotalcites, where the enolate ion is formed in a first step followed by two possible kinetic pathways 1) In the first case the subtracted proton is attracted by the basic sites and transferred to the oxygen of the enolate ion to form an enol in equilibrium. 2) In the second case the enolate ion reacts with an acetone molecule in the carbonyl group, to produce the aldol (diacetone alcohol). Finally, the p carbon is deprotonated to form a ternary carbon and then loses an OH group to obtain the final products. 

While much effort has been put into obtaining materials with basic sites of different strength, the exact nature of the basic sites acting on those catalysts is not always known. For instance, reactions like aldolization, hydrogen transfer or double bond isomerization require either Brpnsted or Lewis type sites or even the association of strong or medium basic sites with weak acid sites/13-161 Many of these requirements (acid-base associations, versatility of the basic strength) can be accomplished by hydrotalcites and this would explain their significant development as catalysts precursors/13-15,171... 

The Tishchenko reaction of furfural has been found to be difficult when carried out by traditional homogeneous catalysis, but excellent results for the Tishchenko reaction of furfural and 3-furaldehyde[90,91] using CaO and SrO as catalysts have been obtained. The use of other solid base catalysts such as La203, Zr02, ZnO, 7-alum in a, hydrotalcite and KOH/alumina, was unsuccessful. An investigation of the influence of the pretreatment temperature of the MgO and CaO catalysts showed that the active basic sites for this transformation are not OH groups, but rather O2 ions on the MgO surface. 

It has previously been reported that hydrotalcite catalyzes the aldol condensation of acetone (25). Polyoxometalates are known to dehydrate alcohols due to their acidic nature (IS ). In order to compare the relative basicity of polyoxometalate-pillared hydrotalcites to that of hydrotalcite itself, a variety of hydrotalcites were screened for 2-propanol conversion (Table II). This reaction is known to give propylene when the catalyst contains acidic sites (such as alumina) and acetone when the catalyst contains basic sites (such as magnesium oxide). 

The results in Table II suggest that molybdate- and vanadate-pillared hydrotalcites contain both acidic and basic sites, the basic sites located on the metal hydroxide sheets, and the acidic sites located on the polyoxometalate pillars. 

Layered double hydroxides with the hydrotalcite stmcture were synthesized with varying Mg A1 atomic ratios and with different contents of exchangeable CT and CO3 anions. The CO2 adsorption isotherms showed an increase of the uptake and consequently of the basicity with initial CO content and calcination temperature up to 800 K. Increasing the Mg A1 ratio of the hydrotalcites from 2.33 to 3 resulted in an increase of the total number of basic sites [94]. 

The strength and accessibihty of the basic sites of hydrotalcites with an Mg Al ratio of 2, prepared via co-precipitahon of the respective nitrates using carbonate or oxalate as the compensating anions, were assessed by calorimetry of CO2 adsorption. Two different methods were used to activate the Mg-Al hydrotalcites and impart Br0nsted basicity. The initial enthalpies of CO2 adsorption at 303 K on the activated hydrotalcites presented very similar values of-108 kj moT [96]. 

Figure 2.43 Reaction mechanism for epoxidation catalyzed by basic sites of hydrotalcite in the presence of H2O2 and nitriles. Sources adapted from Kaneda et al. [28].

The basic sites of hydrotalcites can also activate H2O2 to give HOO anion species. In the presence of nitriles, the HOO can react v ith the nitrile to form peroxycar-boximidic acid, which is able to transfer the oxygen into an olefin to produce an epoxide together with a carboxyamide [28] (Figure 2.43). Yields over 90-95% for a large variety of substrates and under mild reaction conditions (60 °C) could be obtained. 

Hydrotalcites, owing to their layered structure, offer also interesting possibilities to develop multifunctional catalysts that combine the basic sites of HT with other catalytic functionalities. Numerous transition metals can be easily introduced into the brucite-like layer, interlayer space or surface by using the following characteristics ... 

The basic sites of HT (type A in Figure 2.44) are active in the Baeyer-Villiger oxidation of various carbonyl compounds, for example, the oxidation of a substrate in the presence of O2 and an aldehyde (e.g., benzaldehyde) through the intermediate formation of a peracid. An example is the oxidation of cyclopentanone to 5-valer-olactone [275]. The base property of hydrotalcites can be tuned by changing the Mg/Al ratio and the content of interlayer anion species, for example, Cl and S04 ... 

The basic sites of hydrotalcites, the amount of which could be changed by modification of the interlayer anion species, promote the step of oxygen transfer... 

Because of their basicity, hydrotalcites stored in air become carbonated, then neutral. They can be decarbonated by treatment above 673 K, and a mixed oxide of MgO structure is then obtained which has basic properties. The basic strength can be estimated from the temperature of decarbonation, and it is then observed that this temperature is affected by the presence of chlorine on the surface, even in trace amounts [11]. This temperature is shifted by ca 30 K towards high temperatures when CP has been fully exchanged by carbonate. Because chlorine is normally present at trace levels only, these anions must have a long-range influence and reduce the basicity of the stronger basic sites. 

In Mg-Al calcined hydrotalcites the weak basic sites are proposed to be the surface OH" groups, while the medium strength sites are related to Mg -0 , but also to Al -O pairs [11-13]. Isolated O anions, common in pure oxides, are also responsible for the strong basic sites in calcined hydrotalcites [14,15]. 

Ru-grafted hydrotalcite is an excellent multifunctional catalyst for a one-pot s3mthesis of quinolines from 2-aminobenzyl alcohol 169 and various carbonyl compounds through aerobic oxidation by Ru, followed by an aldol reaction on basic sites of the hydrotalcite (Table 3) (04TL6029). 

Calcined Hydrotalcite. In most catalytic applications, hydrotalcite materials are used after calcination at around 773 K to form mixed oxides (16,17). The Mg0-Al203 mixed oxides have periclase (MgO) structure and are highly basic with some acidic sites. They are active for aldol condensation, hydrogen-transfer reactions, and many other reactions. From the catalytic activity of a calcined Mg-Al hydrotalcite for the Knoevenagel condensation of benzaldehyde with various activated methylenic compounds with different values, it was concluded that the calcined hydrotalcite had basic sites with H- up to 16.5, most of them being in the range of 10.7 < H < 13.3 (21). 

Mixed Oxides. A variety of mixed oxides shows basic properties especially when they contain Mg or Ca as a component (2,28-30). In a variety of combinations containing Mg, the number of basic sites is higher than that of neat MgO, probably because of increased surface area. Mg0-La203 is highly basic and an active catalyst for transfer esterification and Michael addition (29,30). Mixed oxide precursors can be prepared by a co-precipitation method or by a sol-gel method. Decomposition of hydrotalcite and its analogs at about 673 K is a typical way of preparing Mg-Al oxides, as described above. 

MacLeod and co-workers (320) studied the transesterification of rapeseed oil over LiNOs/CaO, NaNOs/CaO, KNOs/CaO, and LiNOs/MgO and reported conversions of more than 90% in a standard 3-h test. However, metal leaching from catalysts was detected, as reported by others (319,360,361). Xie and co-workers (362) tested Mg-Al hydrotalcites as solid base catalysts in the reaction of transesterification of soybean oil with methanol and found that a sample with a Mg Al ratio of 3.0 calcined at 773K presented the highest basicity and the best catalytic activity for this reaction. Serio and co-workers (363) tested MgO and calcined hydrotalcites as base catalysts for the transesterification of soybean oil with methanol and proposed a correlation of the activity with the basicity of the catalysts. In addition, they reported the existence of a least four different types of basic sites on the surface of these catalysts. The strongest basic sites promote the transesterification reaction at 100° C, whereas the medium strength sites require higher temperatures. All samples showed good stability in the presence of moisture. 

The double oxide, MgO —AI2O3, prepared by calcination of hydrotalcite is active for the polymerization of /3-propiolactone at 323 Strongly basic sites are suggest-... 

---