Solid bases zeolites

The traditional definition of a zeolite refers to microporous, crystalline, hydrated aluminosilicates with a tliree-dimensional framework consisting of comer-linked SiO or AlO tetrahedra, although today the definition is used in a much broader sense, comprising microporous crystalline solids containing a variety of elements as tetrahedral building units. The aluminosilicate-based zeolites are represented by the empirical fonmila... 

The ZSM5 (Si/Al=40) as base zeolite was prepared by us. Cu was built in the framework by solid phase ion-exchange, Cr by conventional ion-exchange. Ni-samples were obtained by both methods. Ti-ZSM5 was synthesized by isomorphic substitution [8]. 

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. 

Bedard, R.L, Wilson, S.T., Vail, L.D., Bennett, J.M., and Flanigen, E.M. (1989) The next generation synthesis, characterization, and structure of metal sulfide-based microporous solids, in Zeolites Facts, Figures, Future (eds... 

In addition to transesterification reactions, solid base catalysts, including both simple oxides and zeolitic materials, have been used to carry out esterification reactions of fatty acids. These studies have mainly focused on the... 

The results in Table 2 show that the pyridine is less active than any of the X zeolites and Ge faujasite except the lithium form which shows slightly lower activity, whereas all Y zeolites show lower activity than pyridine. Piperidine, however, is more active than any of the zeolite samples studied here. From this comparison, it appears that, most of the basic sites of the zeolites must have pK<10.3. However, the fact that zeolites are also active for catalyzing the condensation of benzaldehyde with ethyl malonate, indicate that these samples have some basic sites with pK< 13.3. On a quantitative bases, and comparing the activity of zeolites for condensation with ethyl cyanoacetate, ethyl acetoacetate and ethyl malonate (Fig. 2), we can conclude that most of the basic sites of the zeolite have pK<9.0 with a sensible amount with 9.0basic strength of different solid base catalysts. 

Since solid acid catalysts are used extensively in chemical industry, particularly in the petroleum field, a reliable method for measuring the acidity of solids would be extremely useful. The main difficulty to start with is that the activity coefficients for solid species are unknown and thus no thermodynamic acidity function can be properly defined. On the other hand, because the solid by definition is heterogeneous, acidic and basic sites can coexist with variable strength. The surface area available for colorimetric determinations may have widely different acidic properties from the bulk material this is especially true for well-structured solids like zeolites. It is also not possible to establish a true acid-base equilibrium. 

The method of choice for the synthesis of unsaturated arylsulfones is the Knoevenagel condensation of arylsulfonylalkanes with aldehydes. The condensation between aldehydes and phenylsulfonyl acetonitrile, has been performed in the presence of various solid base catalysts (MgO, Cs-exchanged X zeolite, Al-Mg mixed oxide, and aluminophosphate oxynitride) at 373 K in the absence of solvents.1641 The most active catalyst for this transformation was the aluminophosphate oxynitride, but MgO and the Al-Mg mixed oxide also were found to have excellent activity, yielding the Knoevenagel adduct in yields of 86 and 71 %, respectively, after 2 h in a batch reactor. 

Zeolites are aluminosillicate solids based on a framework of comer-sharing Si04 and A104 tetrahedra. 

However, the second step requires stronger basic sites than the first step. This demonstrates the importance of being able to tune the base strength. In this sense, zeolites form a valuable complement to other solid base catalysts, e.g. MgO, or mixed oxides derived from layered double hydroxides. 

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. 

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]. 

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As described above, zeolites can also act as solid base catalysts when the Si Al ratio is low and the extra framework cation is a large one such as The most basic common exchanged zeolite is CsX. With this material the aldol condensation of cyclooctanone with benzaldehyde gave only the monobenzylidene compound (Eqn. 10.23) 5 while reactions using piperidine, m nesium oxide or amorphous cesium aluminosilicate gave both the mono- and di-benzylidene products(Eqn. 10.12). The smaller ketones, cyclohexanone and... 

To set up a more environmentally friendly process, it is mandatory to replace the caustic soda by solid bases. Many catalysts such as zeolites [2], alumina [3,4], sepiolite [5] and hydrotalcites [6,7] have been regarded as promising solid basic materials. Hydrotalcites, which are anionic clays, are the subject of this investigation. They are layered double hydroxides with the general formula [M(II)i.xM(III)x(0H)2].[A" x/n].mH20. The host layers are charged positively by replacement of divalent metal cations usually Mg ", Ni and... 

The aldol condensation of benzaldehyde with acetophenone has been used to compare hydrated hydrotalcites with many solids claimed to be strong solid bases-KF and KNO3 supported on alumina, X zeolites containing excess Cs or Mg, and lanthanum oxide. Only hydrated HDT and KF/AI2O3 could be used to perform this reaction at 273 K-HDT with 100% selectivity for chalcone and KF with lower selectivity, because of secondary Michael addition of the chalcone to acetophenone. The other solids were inactive, but could induce comparable conversions at 423 K. 

The development of environmentally friendly solid catalysts for the production of fine chemicals has known a recent growing interest and recent review articles have been devoted to catalysis by solid bases. Several solid bases have been proposed such as alkali ion-exchanged zeolites [1], alkaline oxides supported on microporous [2] and mesoporous solids [3], sodium metal clusters in zeolites... 

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