Bronsted acidity montmorillonite

Montmorillonite K10 was also used for aldol the reaction in water.280 Hydrates of aldehydes such as glyoxylic acid can be used directly. Thermal treatment of K10 increased the catalytic activity. The catalytic activity is attributed to the structural features of K10 and its inherent Bronsted acidity. The aldol reactions of more reactive ketene silyl acetals with reactive aldehydes proceed smoothly in water to afford the corresponding aldol products in good yields (Eq. 8.104).281... 

This methodology has also been extended [57] to high-valent metal cations such as Al3+ and Fe3+ a simple ball-grinding with the corresponding metal nitrate at ambient temperature in air yields the Al- or Fe-exchanged montmorillonite. Such products are interesting acid-clay catalysts, their Bronsted acidity arising mainly from the dissociation of adsorbed water ... 

From Table 5 and Table 6 it can be seen that all the samples of different calcination time and aging time possess nearly the same acidity at a particular calcination temperature, revealing the negligible effect of the calcination time and aging time on the surface total acidity. However, Table 7 shows that the total number of acid sites and the number of the BrOnsted acid sites on the catalysts decrease with an increase in calcination temperature. It is known that the surface acid sites of hydroxyl-Zr bentonite mainly stem from surface hydroxyl and exposed metal cation [6], and the Bronsted acid sites result from protons on the surface of the octahedral layers. When the calcination temperature increases, the migration of protons to the octahedral layers of montmorillonite will become easier, leading to the decrease of the number of BrGnsted acid sites. 

Z. H. Ge, D. Y. Li, T. J. Pinnavaia, Preparation of alumina-pillared montmorillonites with high thermal stability, regular microporosity and Lewis/Bronsted acidity. Microporous Mater., 3(1994), pp. 165-175. 

Montmorillonite supported zinc chloride, called Clayzic, has become an important solid acid catalyst. This material has both the Bronsted acidity associated with the clay as well as Lewis acidity from the zinc ions. Thermal activation at 275°C gave a catalyst that promoted the benzylation of cumene at 40°C in near quantitative yield. ... 

Chemical heterogeneities present in soils, sediments, and aquifers undoubtedly have an effect on rates of pollutant degradation. Other sources of surface catalysis not discussed here include Bronsted acidity of surface sites, that become apparent as surfaces become dehydrated (El-Amamy and Mill, 1984). Surface and pore structure may play a role in the catalysis of phosmet hydrolysis by montmorillonite (Sanchez-Camazano and Sanchez-Martin, 1983) and in the catalysis of ethyl acetate hydrolysis by zeolites (Nam-ba et al., 1981). 

Special attention has been called to environmentally friendly catalysis because of the increasing demand for cleaner industrial processes in recent years. The use of solid catalysts is likely to be especially important in the future in the manufacture of fine chemicals and intermediates . Clays are effective catalysts for a wide variety of organic reactions. Recently the prowess of KIO montmorillonite,as a strong Bronsted acidic catalyst, has been shown in cyclocondensation reactions. This presentation will focus on the assets and the usefulness of KIO montmorillonite for catalysis of the reactions of imidazole and benzimidazoles with propiolic esters. 

The higher the electronegativity of the interlayer metal cation M"+ the stronger the acid sites. When montmorillonite is ion-exchanged with Al3+ ions, it displays a very high activity towards several Bronsted acid catalysed reactions, comparable to that of 98% H2S04.9... 

The diastereoselectivity of the reaction was independent of the catalyst but was affected by the nature of the solvent. The threo isomer was preferentially formed in toluene, while the erythro isomer was formed in 1,2-dimethoxy-ethane. The proton-exchanged montmorillonite (H+-mont) showed similar activity and diasteroselectivity to Al3+-mont. This fact suggests that the exchangeable Al3+ cations in the montmorillonite do not function as Lewis acid sites and it is the Bronsted acid sites that are essential for catalysis of the aldol reaction. 

Montmorillonites are more frequently used as Bronsted acid catalysts even if Lewis acidity plays a role in their catalytic activity. The origin of Bronsted acidity in metal-exchanged montmorillonites is ascribed to the polarizing influence of the cation on the water molecules in spatially restricted interlayers. The exchangeable cations are either protons or polarizing cations [e.g., aluminum, chromium(III), or iron(III)]. 

Except for Ce(IV)-K10 montmorillonite, this abnormal behaviour disappears when calcined clays are used as catalysts. Given that calcination eliminates internal water and, consequently, most of Bronsted acid sites [5], it can be concluded that Bronsted acidity greatly favours the polymerization of the diene. However, in the case of Ce(IV) clay there must be an additional mechanism for diene polymerization. It has been reported [6] that the formation of radical cations accelerates this iateral reaction. In fact, EPR spectra of Ce(IV)-clays in the presence of cyclopentadiene show a narrow signal at g = 2.004 0.002 which could be characteristic of organic radicals. 

Ferreira et al. produced a high-activity metallocene catalyst (1457 kg PE/(mol Zr h atm)) by supporting Cp2ZrCl2 (II) on a pillared clay. Calcium montmorillonite was pillared with Keggin ions, [Ali304(0H)24 12H20] +, to increase its Lewis acidity. The clay was pretreated at 200°C to remove adsorbed water, then exposed to a toluene solution of MAO at 50°C to attenuate its Bronsted acidity. High activity was maintained over a period of 1 h at 50°C. The apparent absence of the usual rapid metallocenium deactivation processes was attributed to the suppression of bimolecular interactions in the supported catalyst. 

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... 

As shown in Figure 6, the polymerization rate is dependent on the strength of the electric field around the interlayer cations. An EXAFS study on the Cu type montmorillonite and its intercalated compound with e-caprolactam [11] suggests that Cu " in these compounds coordinated with water molecules even after vacuum drying at room temperature. Besides, the polymerization rate was increased by the addition of water molecules. These facts suggest that the polarized water molecules in the interlayer region of montmorillonite act as a Bronsted acid and opens the e-caprolactam rings to initiate the polymerization. 

The results of TG analysis of montmorillonite and its hexylammonium derivative support this concept the weight loss in the latter case in the temperature range 550-710 C, (where the former undergoes dehydroxylation), reaches only 52% of that of the former, however, the expelled H2O was not detected in GC analysis. As a consequence, the Bronsted acid sites which might catalyze the interlayer reaction of -hexylamine diffusing out from the silicate lattice cannot be formed on the surface. 

The aza-Diels-Alder reaction of imines with diene of Danishefsky is an important route to 2,3-dihydro-4-pyridones. A number of Lewis acids have been used to catalyze the reaction in organic solvents. In water the reaction was realized by acid catalysis via iminium salts or by Bronsted acids. The montmorillonite K-10 catalyzed this cycloaddition in water or in aqueous acetonitrile in excellent yield. Recently Kobayashi has performed the reaction in water at room temperature under neutral conditions in two (imine - - diene) or three (aldehyde -b amine -b diene) component versions by using sodium triflate as catalyst. Imine intermediates from the indium-mediated reaction, in aqueous medium at 50° C, between aromatic nitro compounds and 2,3-dihydrofuran undergo aza-Diels-Alder cycloadditions to give tetrahydroquinoline derivatives in good overall yields. ... 

FIGURE 7 IR spectra of pyridine chemisorption on synthetic mica-montmorillonite showing the transformation of pyridine to pyridinium ion by interaction with water. L, B = Lewis and Bronsted acid sites (63). (Reproduced with permission from the Mineralogi-cal Society of Great Britain and Ireland.)... 

In Fig. 3.58 is shown the integrated intensities of the band at 1540 cm" for pillared beidellite and montmorillonite against the outgassing temperature. Increasing the calcination temperature prior to pyridine adsorption results in a steep drop in the proton content in the case of pillared montmorillonite, while pillared beidellite keeps its acidity. The steep drop of the Bronsted acid sites observed for pillared montmorillonite was attributed to the fact that, upon thermal activation, the protons migrate into the octahedral layer of the clay, where they induce a premature dehydroxylation. Thus, the acidity is mainly of the Lewis type for samples treated at higher temperatures. A similar result was reported also for bentonites pillared with alumina clusters. ... 

Acid type catalysts catalyze cross aldol reaction of silyl ketene acetals with carbonyl compounds and acetals. Aluminum cation and proton exchange montmorillonites are effective catalysts. Although the detailed reaction mechanism is not clear, Bronsted acid sites are considered to be the catalytic sites. 

Both organic and inorganic solid acids are used. As organic acids, Bronsted acids such as condensed compounds of /Mira-substituted phenol with formaldehyde,and Lewis acids such as zinc salts of the above compounds and those of aromatic carboxyl acids are used. As inorganic acids, montmorillonite clays such as bentonite, fuller s earth, kaoline, chinaclay, and their chemically modified materials are used. 

The condensation of EAA with phenol for the preparation of hymecromone using a catalyst of Bronsted acidity, 12-tungstophosphoric acid supported on montmorillonite KIO (PWA/montmoriUonite-KlO) in solventless system in 69% yield was reported by B. Vijayakumar and G. Ranga Rao (12MI233). 

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