Basic Zeolite Catalysts

Base catalysis is another area which has received a recent stimulus from developments in materials science and microporous solids in particular. The Merk company, for example, has developed a basic catalyst by supporting clusters of cesium oxide in a zeolite matrix [13]. This catalyst system has been developed to manufacture 4-methylthiazole from acetone and methylamine. 

Alkali metal ion-exchanged zeolites and occluded alkali metal oxide zeolites have been investigated extensively and applied as basic catalysts for a variety of organic transformations (1,41,221,222). Zeolites modified with alkaline earth compounds have been applied much less frequently as base catalysts for organic reactions. 

Side-Chain Alkylation. There is continued interest in the alkylation of toluene with methanol because of the potential of the process in practical application to produce styrene.430 Basic catalysts, specifically, alkali cation-exchanged zeolites, were tested in the transformation. The alkali cation acts as weak Lewis acid site, and the basic sites are the framework oxygen atoms. The base strength and catalytic activity of these materials can be significantly increased by incorporating alkali metal or alkali metal oxide clusters in the zeolite supercages. Results up to 1995 are summarized in a review.430... 

Whereas solid acid catalysts (zeolites) have been intensively studied, only recently has the use of basic solid catalysts received substantial interest for production of intermediate and fine chemicals [1], Heterogeneous catalytic transformations are environmentally friendly compared with processes requiring neutralization and... 

Since acidic or basic sites may interact with each other, the site density ought to be determined. Moreover, an attempt should be made to describe the site location, particularly in the case of microporous acid catalysts (zeolites). 

If a zeolite with a low Si/AI ratio is exchanged with a large monovalent cation, e.g. Cs+ in zeolite X, one obtains a weakly basic catalyst (CsX). The basic sites in this case are the oxygen framework atoms of the zeolite itself. Much stronger basic sites are formed when a zeolite or MCM-41 support is overexchanged or impregnated with, e.g. Cs acetate, and subsequently calcined (34). This results in formation of Cs20 oxidic particles. 

Heterogeneous catalysts, in particular zeolites, with their various properties, contribute extensively to the environmental protection in the synthesis of fine chemicals. For that a broad and very impressive range of acidic and basic catalysts is already available, having all levels of properties between super acidity and super basicity. Also the possibility preparing bifunctional catalysts will gain in importance. 

EF material free, alkali exchanged zeolites are used as quite mild basic catalysts. Light alkali and alkaline earth metal zeolites, such as Na-X, Na-Y [165], alkali-MOR, Na-A and Ca-A [166], have a mild Lewis acid behavior and do not appear to have strong basic character. The same occurs for Na-silica-alumina [167]. However, heavy alkali metal zeolites such as Cs-Y actually act as base catalysts, or rather as acid-base catalysts, for example for toluene side-chain alkylation. Stronger basic character arises from impregnation of alkali zeolites with alkali salts, later... 

Cation-exchanged aluminosilicates can also act as bases, particularly when the extra lattice cation is large. A low Si Al ratio produces a more basic catalyst. A Cs+ exchanged amorphous aluminosilicate can catalyze the liquid phase aldol condensation of benzaldehyde with cyclooctanone (Eqn. lO.lb). Both mono-and di-benzylidene products are formed over this amorphous basic catalyst. As discussed later, with the crystalline aluminosilicates, the zeolites, selectivity for raono-benzylidene product formation is increased. 

In this paper, we will focus on the zeolite basic catalysts, other solid superbases will be considered in detail elsewhere [6]. 

Coke deposition and aging are basic catalyst constraints, constraints lifted in ceitain applications by the discovery of ZSM-5 and related zeolite compositions and pore Structures. It was recognized a number of years ago that coke formation within tiie pores of a zeoUte can be a shape selective reaction (ref. 1). This area has now been excellently reviewed by M. Guisnet and P. Magnoux (ref, 2). 

In the past decade, beta zeolite (given the universal BEA) has rapidly become the catalyst of choice for commercial production of EB and cumene. Mobil invented the basic beta zeolite composition of matter in 1967. Since that time, catalysts utilizing beta have undergone a series of evolutionary steps leading to the development of the state-of-the-art catalysts such as QZ-2000 catalyst and QZ-2001 catalyst for cumene alkylation. 

Zeolites function as Bronsted or Lewis acid catalysts or (less frequently) as basic catalysts. Examples of the former are alkylation and isomerization of aromatics, such as the isomerization of xylenes an example of the latter is the use of cesium zeolite in the synthesis of the key intermediate, 4-methylthiazole, used in the preparation of the anthelmintic, thiabendazole. 

Ethylbenzene can also be formed by side-chain alkylation of toluene with methanol over basic catalysts, such as alkali-exchanged X and Y zeolites [229,230]. The cross-coupling of toluene and methane to produce F.B/styrene at high temperatures (> 700 °C) in the presence of oxygen has also been attempted by using basic zeolites, such as CsX and BaO/NaX [231, 232,233],... 

The basicity of zeolite catalysts can be increased even further by occluding... 

It should be remarked that for very strong zeolite basic catalysts the presence of CO2 and H2O should be avoided in any stage of preparation and reaction, because these lead to rapid catalyst deactivation. 

Despite the existence of basic sites in zeolites, the possibility of using zeolites as basic catalysts was forgotten for many years and it was realized only recently that they can also be successful in this field [56,57]. Two approaches have been used to prepare basic zeolites ion-exchange with alkali metal ions and generation within the pores of small clusters of alkali metals or oxides and alkaline earth oxides. Whereas simple ion-exchange with alkali metal ions produces relatively weakly basic sites, the presence of such clusters results in strongly basic sites. 

Figure 1 compares the progress of the Knoevenagel condensation between ethyl cyanoacetate and benzaldehyde on different exchanged X zeolites, and in Table 1 are summarized the rates calculated for the Knoevenagel condensation of different methylenic compounds on basic catalysts. 

Alkaline or alkaline earth metal oxides are well known basic catalysts and much of work has been devoted to their characterization and to the study of their catalytic activity [64,65]. Taking into account the strong basic character of these oxides it is possible to enhance the basicity of zeolites by over-exchanging them with alkali [66-69] or alkaline earth metals [70-72] and producing after thermal de-... 

Besides the general advantages associated with zeolites, the use of exchanged zeolites as heterogeneous basic catalysts has the additional advantage that they can be exposed to the atmosphere, because carbon dioxide and water are not absorbed too strongly and can be removed by thermal treatment. They can be used even in systems in which water or CO2 are involved as reactants or products. 

Despite all these advantages, however, exchanged zeolites have rarely been used as basic catalysts in the production of fine chemicals. This is probably because of their weak basic character and because bulky reactants are involved in many chemical processes. Their basicity is, nevertheless, sometimes sufficient to catalyze Knoevenagel condensations and Michael additions. 

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