Basic zeolite

In contrast with the widespread application of zeolites as solid acid catalysts (see earlier), their use as solid base catalysts received scant attention until fairly recently [121]. This is probably because acid-catalyzed processes are much more common in the oil refining and petrochemical industries. Nonetheless, basic zeolites and related mesoporous molecular sieves can catalyze a variety of reactions, such as Knoevenagel condensations and Michael additions, which are key steps in the manufacture of flavors and fragrances, pharmaceuticals and other specialty chemicals [121]. Indeed, the Knoevenagel reaction of benzaldehyde with ethyl cyanoacetate (Fig. 2.36) has become a standard test reaction for solid base catalysts [121]. 

Cesium-exchanged zeolite X was used as a solid base catalyst in the Knoevenagel condensation of benzaldehyde or benzyl acetone with ethyl cyanoacetate [121]. The latter reaction is a key step in the synthesis of the fragrance molecule, citronitrile (see Fig. 2.37). However, reactivities were substantially lower than those observed with the more strongly basic hydrotalcite (see earlier). Similarly, Na-Y and Na-Beta catalyzed a variety of Michael additions [122] and K-Y and Cs-X were effective catalysts for the methylation of aniline and phenylaceto-nitrile with dimethyl carbonate or methanol, respectively (Fig. 2.37) [123]. These procedures constitute interesting green alternatives to classical alkylations using methyl halides or dimethyl sulfate in the presence of stoichiometric quantities of conventional bases such as caustic soda. 

Alkali-exchanged mesoporous molecular sieves are suitable solid base catalysts for the conversion of bulky molecules which cannot access the pores of zeolites. For example, Na- and Cs-exchanged MCM-41 were active catalysts for the Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate (pKa=10.7) but low conversions were observed with the less acidic diethyl malonate (pKa=13.3) [123]. Similarly, Na-MCM-41 catalyzed the aldol condensation of several bulky ketones with benzaldehyde, including the example depicted in Fig. 2.38, in which a flavonone is obtained by subsequent intramolecular Michael-type addition [123]. 

A serious drawback of these alkali metal oxide loaded zeolites and mesopor-ous molecular sieves is their susceptibility towards deactivation by moisture and/or carbon dioxide, which severely limits their range of applications. 

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Basically, zeolites consist of Si04 and AIO4 tetrahedra (Fig. 5.28), which can be arranged by sharing 0-corner atoms in many different ways to build a crystalline lattice (Fig. 5.29). 

Knozinger, H. and Huber, S. (1998) IR spectroscopy of small and weakly interacting molecular probes for acidic and basic zeolites, J. Chem. Soc., Faraday Trans., 94, 2047. 

Figure 1. Modes of MBOH adsorption on basic zeolites...

The microwave technique has been also found to be the best method for preparing strongly basic zeolites (ZSM-5, L, Beta, etc.) by direct dispersion of MgO and KF. This novel procedure enabled the preparation of shape-selective, solid, strongly base catalysts by a simple, cost-effective, and environmentally friendly process [11, 12]. New solid bases formed were efficient catalysts for dehydrogenation of 2-propanol and isomerization of cis-2-butene. 

One-Pot Multistep Synthesis of Ketones on Bifunctional Zeolite Catalysts. One-pot multistep reactions constitute an elegant and efficient way to decrease the number of chemical and separation steps, hence, to develop greener synthesis processes. Bifunctional metal-acidic or metal-basic zeolite catalysts, which can be prepared easily with the desired properties (e.g., distribution of the... 

The catalyst is made up of platinum dispersed on a basic zeolite, K-L or Ba-K-L [85], The conversion of linear paraffins from to Cg is much faster over Pt-KL or... 

Recently, a combinatorial investigation of alkylation of toluene with methanol to produce styrene with basic zeolites and alkaline earth catalysts was performed (257). The results of tests involving preparation and testing of more than 200 catalysts were modest, and these results emphasize that fine-tuning of the acid and base properties necessary to achieve better alkylation catalysts is not an easy task. 

IX. Alkylation of Aromatic Compounds on Acidic and Basic Zeolites... 

Fig. 25. Proposed mechanism of the side-chain alkylation of toluene by methanol on basic zeolites (a) and the surface species formed during the decomposition of methanol on basic zeolites (b).

To investigate the methylation of aniline by methanol on basic zeolite CsOH/ Cs,NaY, the CF MAS NMR technique was combined with SF protocols (242 ). In the first period, these protocols allowed the observation of adsorbate complexes formed on solid catalysts under steady-state conditions. In subsequent periods, an identification of adsorbates acting as intermediates of the further reaction was carried out. 

The left-hand side of Figs 28a-c shows C CF MAS NMR spectra which were recorded during the conversion of pure C-enriched methanol on zeolite CsOH/ Cs,NaY under CF conditions at reaction temperatures of 473—523 K (242). As was observed previously (234-236), the conversion of methanol (49 ppm) on the basic zeolite catalysts caused the formation of surface formate species, leading to a C MAS NMR signal at 166 ppm. Upon cessation of the methanol flow at the reaction... 

This is a basic reaction for all C02-involved plasma reactions. Some primary investigations have been conducted for the C02 dissociation using corona discharge [8, 14—17]. Some types of C02 adsorbent, such as basic zeolite, can result in a much higher C02 dissociation rate under corona discharge [18],... 

FIGURE 9.2 Schematic bonding model of a basic zeolite. 

To produce basic zeolites, two methods have been developed [18] one line of attack is to ion exchange with alkali metal ions, and the other is to impregnate the zeolite pores with fine particles that can act as bases themselves. The first method, that is, ion exchange, generates moderately weak basic sites, while the latter results in strong basic sites [23],... 

Examples of the application of recyclable solid base catalysts are far fewer than for solid acids [103]. This is probably because acid-catalyzed reactions are much more common in the production of commodity chemicals. The various categories of solid bases that have been reported are analogous to the solid acids described in the preceding sections and include anionic clays, basic zeolites and mesoporous silicas grafted with pendant organic bases. 

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