Zeolites basic, aldol condensations

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. 

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... 

The control of zeolite acidity is of special importance when catalyzing reactions involving strong bases such as NH3 or pyridines. For such reactions a zeolite catalyst with excessive acidity will be rapidly poisoned by adsorption of the reactant or the basic products. For instance, in the aldol condensation of aldehydes and ketones with ammonia for the production of pyridine and 3-methylpyridine, an important intermediate in the synthesis of vitamin B3, milder acidities are preferred [11]. 

The aldol condensation of acetone to form diacetone alcohol is a well known reaction catalyzed by basic catalysts. This is also regarded as one important reason for the deactivation of MBOH reaction over alkali-exchanged X and Y zeolites ° Thus the pure acetone vapour was introduced into the infrared cell which contains fresh zeolite samples. It is found that the spectra after acetone adsorption are nearly completely the same as those obtained after MBOH reactions ° except for the region around 3400 cm where the spectra after MBOH reaction at 180°C still showed some bands belonging to the residual adsorbed MBOH. Figure 2 shows the IR spectra of alkali exchanged X zeolites and other Na exchanged zeolites... 

As an example of base-catalyzed reactions over zeolites of different basicity (Cs, Na-Y, Cs, Na-X, CsOH/Cs, Na-Y), the aldol condensation of n-butyraldehyde was explored by Weitkamp et al. [910] using in-situ FTIR to monitor the conversion. During the reaction a band emerged at 1580 cm, which could be ascribed to a COO stretching vibration of a carboxylate anion. These anions were suspected to poison the active basic sites and cause the catalyst deactivation. 

Basic catalysts also show very different behaviour from acid catalysts for the alkylation of aromatics. Whereas acid catalysts promote alkylation of the aromatic ring, with high shape selectivity in the important case of ZSM-5 (Chapter 8), alkali metal zeolites catalyse side chain alkylation. In the case of the reaction of toluene with methanol over Cs-X, for example, the products include ethylbenzene and styrene. The side chain alkylation proceeds by the following base-catalysed steps, (i) formation of formaldehyde from methanol, (ii) activation of the toluene by polarisation of the methyl group (tending towards carbanion formation) and (iii) nucleophilic attack of the carbanion of toluene on the carboxyl group of formaldehyde. Side chain alkylation of aromatics is therefore a special case of aldol condensation. Reactions of this... 

Knoevenagel condensation, Michael additions, and aldol condensations Homogeneous base catalysts Basic zeolites — [58]... 

Numerous types of basic heterogeneous catalysts, such as alkahne earth metal oxide, anion exchange resins and alkali metal compounds supported on alumina or zeolite can catalyze various chemical reactions such as isomerization, aldol, Michael, and Knoevenagel condensation, oxidation and transesterification [1], Today considerable attention is devoted to the production of biodiesel (FAMEs) as an alternative for petroleum-derived diesel fuel. Biodiesel is synthesized by direct transesterification of vegetable oil or animal fat with a short-chain alcohol, viz. methanol, ethanol, and isopropanol in presence of an acid, base or enzymatic catalyst [2], Considering the advantages of solid base catalysts, for easy separation and recovery, reduced corrosion and environmental acceptance [1], many studies have been conducted on basic heterogeneous catalysts development for biodiesel production [3-13],... 

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