Zeolite-catalysed reactions

The molecular size pore system of zeolites in which the catalytic reactions occur. Therefore, zeolite catalysts can be considered as a succession of nano or molecular reactors (their channels, cages or channel intersections). The consequence is that the rate, selectivity and stability of all zeolite catalysed reactions are affected by the shape and size of their nanoreactors and of their apertures. This effect has two main origins spatial constraints on the diffusion of reactant/ product molecules or on the formation of intermediates or transition states (shape selective catalysis14,51), reactant confinement with a positive effect on the rate of the reactions, especially of the bimolecular ones.16 x ... 

An additional difficulty in the determination of actual TOF values for zeolite catalysed reactions deals with the accessibility by reactant molecules to the narrow micropores in which most of the potential active sites are located. The didactic presentation in Khabtou et al.[37] of the characterization of the protonic sites of FAU zeolites by pyridine adsorption followed by IR spectroscopy shows that the concentration of protonic sites located in the hexagonal prisms (not accessible to organic molecules) and in the supercages (accessible) can be estimated by this method. Base probe molecules with different sizes can also be used for estimating the concentrations of protonic sites located within the different types of micropores, which are presented by many zeolites (e.g. large channels and side pockets of mordenite1381). The concentration of acid sites located on the external surface of the... 

A key step is the formation of a stable hydronium ion upon formation of dimethylether. The concept of Bronsted acid-Lewis base catalysis also allows us to understand the formation of ethylene from methanol, as formed in zeolite-catalysed reactions. A possible mechanism is sketched in Fig. 4.68. 

Infrared spectroscopy has been used for many years to probe acid sites in zeolites. Typically, strong bases such as ammonia or pyridine are adsorbed, and the relative or absolute intensities of bands due to Lewis acid adducts or protonated Bronsted acid adducts are measured. The basicity of ammonia or pyridine is however much stronger than that of most hydrocarbon reactants in zeolite catalysed reactions. Such probe molecules therefore detect all of the acid sites in a zeolite, including those weaker acid sites which do not participate in the catalytic reaction. Interest has recently grown in using much more weakly basic probe molecules which will be more sensitive to variations in acid strength. It is also important in studying smaller pore zeolites to use probe molecules which can easily access all of the available pore volume. 

This paper is devoted to the acylation in liquid phase of phenol and anisole by aromatic acetates. Reaction mechanisms are presented and the effect of the solvent polarity which is known to play an important role on the rate and on the selectivity of zeolite catalysed reactions... 

One of the unique features of zeolites in alkylation reactions is their shape selectivity. In many zeolite-catalysed reactions, however, shape-selective catalysis occurring on the inside of the zeolite can be affected by non-selective catalysis on the external surfaces. Paparatto et al. have reported that during aromatic alkylation, the para isomer is formed selectively within the zeolite, whereas isomerisation occurred only on the external surfaces, decreasing para product selectivity.17... 

Last but not least, the versatility of zeolites is demonstrated by exchanging the acidic proton with deuteron which enables investigation of interesting mechanisms related to catalysis and by exchanging the proton with transition metal cations, such as Cu(I), and opens new areas of enviromnentaUy friendly organic chemistry. For these reasons, we are including in this chapter acidic-zeolite catalysed reactions from our own work which can be best understood as examples of confinement effects superelectrophilic, Cu(I) catalysed Click chemistry, and specific H/D exchange reactions. 

Derouane EG, Gilson J-P, Nagy JB. In situ characterizatin of carbonaceous residues from zeolite-catalysed reactions using high resolutin solid state NMR spetroscopy. Zeolites 1982 2 42-6. 

Baur R and Krishna R, 2005, The effectiveness factor for zeolite catalysed reactions , Catal Today, 105,173-179. 

Figure C2.7.13. Schematic representation of diffusion and reaction in pores of HZSM-5 zeolite-catalysed toluene disproportionation the numbers are approximate relative diffusion coefficients in the pores 1131.

Beckmann rearrangement of oxime is an acid catalysed reaction. The environmental problems associated with the use of sulphuric acid instigated interest to use number of solid acid catalysts [1], There are only scanty references about Lewis acid ion-exchanged MeAlPOs. Beyer et al. [2], Mihalyi et al. [3] and Mavrodinova et al. [4] already suggested the presence of Lewis acid metal ions as MO+ species in zeolites. The present study focussed the synthesis and characterisation of Fe3+, La3+ and Ce3+ ion-exchanged MAPO-36. The catalytic results of Beckmann rearrangement of cyclohexanone oxime over ion-exchanged catalysts are delineated in this article. 

As the Beckmann rearrangement is believed to be a typical acid-catalysed reaction, many researchers have reported the relationship between the vapour phase reaction catalysis and the acidity of the catalysts tested on non-zeolitic catalysts - i2s- i3i. 318-334 and on zeolitic catalysts Another interesting point for the heterogeneous gas-phase Beckmann rearrangement is the location of the reaction on the catalyst and different studies have been published ° . The outer surface of the catalyst particle seems to be the most probable place for the Beckmann rearrangement supported by the traces of reagents, and notable amounts of by-products found only in the outer layers of the zeolite crystal. Development of new and more efficient catalysts have also been reported " . ... 

Zeolite catalysed alkylation of polynuclear aromatics is considered to be simultaneously governed by several mechanisms. To achieve highly shape-selective catalysis, it is essential that the pore size precisely corresponds to the molecular dimensions of reactants and products, and to the transition state of the reaction intermediates. 

The product distribution in the zeolite-catalysed alkylation of polynuclear aromatics depends on the structure of zeolite pores. High regioselectivities were observed in the HM catalysed isopropylation of polynuclear aromatics, such as biphenyl, naphthalene, p-terphenyl, and dibenzofuran, to yield predominantly the least bulky products e.g., 4,4 -DIPB for biphenyl, and 2,6-DIPN for naphthalene. These reactions are controlled by steric restriction at the transition state inside the pores and by the entrance of intermediate products molecules into the pores. On the other hand, the catalysts with large-pore HY and HL zeolite are controlled at low temperature by the electron density of the reactant molecule and at higher temperature by the stability of the product molecules because their pores have enough space for a transition state, which allow the formation of all corresponding isomers. 

TBH-HNaX system. In other cases zeolites have catalysed reactions but with little benefit on 12... 

Nitrogen physisorption of the Ge-ZSM-5 sample revealed a considerable contribution of mesopores to the total pore volume, accompanied by a drop in micropore volume of 20%. In a study of the catalytic activity of these materials it was found that the increased mesoporosity of Ge-ZSM-5 had a beneficial effect on the catalytic activity in a series of acid-catalysed reactions.1771 It was observed that the presence of germanium in the framework does not change the strength of the acid sites but, instead, decreases the extent of deactivation from coke residues formed upon reaction. The microporous domains only have short diffusional lengths, but the shape selectivity ascribed to the zeolitic channels is still fully... 

Since the early works on zeolite catalysed organic reactions (reviewed in 1968 by Venuto and Landis[1]), an incredibly large number of organic reactions was shown to be catalysed by microporous and mesoporous molecular sieves. Three main factors contribute to this broad applicability of zeolite catalysts ... 

Table 2.1 Fundamental and practical differences between homogeneous and zeolite catalysed organic reactions...

Large differences were shown to exist between zeolite catalysed and homogeneous organic reactions. These differences can generate problems and pitfalls in the... 

A rough estimation of the concentration of potential active sites is often possible from the composition of the catalyst. Thus the maximum value of the concentration of the protonic sites of zeolites, which are active in most of the acid catalysed reactions, can be estimated from the complete unit cell formula of the zeolite (with distinction between framework and extra-framework A1 species). The actual concentration of these sites is often lower (two times or more). 

Ti-Beta was also applied in the selective transformation of a-pinene oxide to camphenolenic aldehyde.78 A selectivity of over 98% was observed in the gas-phase reaction that was explained as a combination of a Lewis catalysed reaction in the absence of Brpnsted acid sites. Furthermore the concentration of a-pinene oxide in the zeolite pores was found to be an important factor not ruling out additional transition state selectivity as well. 

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