Zeolite catalysts shape-selective properties

To improve the yield of mono- and dimethylamines, a shape selective catalyst has been tried. Carhogenic sieves are microporous materials (similar to zeolites), which have catalytic as well as shape selective properties. Comhining the amorphous aluminum silicate catalyst (used for producing the amines) with carhogenic sieves gave higher yeilds of the more valuable MMA and DMA. ... 

For the non-oxidative activation of light alkanes, the direct alkylation of toluene with ethane was chosen as an industrially relevant model reaction. The catalytic performance of ZSM-5 zeolites, which are good catalysts for this model reaction, was compared to the one of zeolite MCM-22, which is used in industry for the alkylation of aromatics with alkenes in the liquid phase. The catalytic experiments were carried out in a fixed-bed reactor and in a batch reactor. The results show that the shape-selective properties of zeolite ZSM-5 are more appropriate to favor the dehydroalkylation reaction, whereas on zeolite MCM-22 with its large cavities in the pore system and half-cavities on the external surface the thermodynamically favored side reaction with its large transition state, the disproportionation of toluene, prevails. 

Selective hydroxylation of phenol with hydrogen peroxide was reported on acid zeolite catalysts [91-92]. Peroxonium ions, formed by H2O2 protonation, are the oxidizing species. When the reaction is carried out on a faujasite catalyst, a mixture of hydroxybenzenes and tars is obtained [91]. In the presence of H-ZSM-5 on the other hand, no tar formation was mentioned (which does not necessarily mean that it was absent) and p-selectivities close to 100% were reported for the hydroxylation [92]. These superior selectivities reflect the shape selective properties of ZSM type zeolites. 

Since the last review by Venuto in 1968,[1] there has been a continuous interest in the application of microporous and mesoporous materials as catalysts in the synthesis of bulk and fine chemicals.[211 Indeed, their acidic and basic properties can be combined with their structural properties in order to take advantage of their adsorption and shape selectivity properties, the latter being an advantageous feature of zeolites compared with other heterogeneous catalysts. Another important aspect... 

The transport and adsorption properties of hydrocarbons on microporous zeolites have been of practical interest due to the important properties of zeolites as shape-selective adsorbents and catalysts. The system of benzene adsorbed on synthetic faujasite-type zeolites has been thoroughly studied because benzene is an ideal probe molecule and the related role of aromatics in zeolitic catalysts for alkylation and cracking reactions. For instance, its mobility and thermodynamic properties have been studied by conventional diffusion 1-6) and adsorption 7-9) techniques. Moreover, the adsorbate-zeolite interactions and related motion and location of the adsorbate molecules within the zeolite cavities have been investigated by theoretical calculations 10-15) and by various spectroscopic methods such as UV (16, 17), IR 17-23), neutron 24-27), Raman 28), and NMR 29-39). 

Furthermore, very high para-selectivities are observed as a result of the shape-selective properties of the zeolite catalyst. 

The announcement in 1972 of the discovery of a new acidic solid with reproducible micropore structure generated a new wave of interest in zeolitic materials (ref. 1). The demonstration of the catalytic efficiency of the ZSM family of zeolites in the conversion of methanol to gasoline-range hydrocarbons was particularly timely in view of the worldwide concern over the continued availability of oil supplies. There followed a burst of activity relating to the structure of the ZSM catalysts, their shape selective properties, their acidic characteristics, their catalytic activities in a variety of reactions and the mechanism of the methanol conversion process (refs. 2-4). Efforts in this area have continued. 

Higher selectivity of zeolite catalyst for the production of more useful lower bases namely pyridine, 2-picoline and 4-picoline is attributed to the unique shape selective property and controlled acidity of ZnO-ZSM-5. 

The use of microporous solid catalysts such as zeolites and related molecular sieves has an additional benefit in organic synthesis. The highly precise organization and discrimination between molecules by molecular sieves endows them with shape-selective properties [12] reminiscent of enzyme catalysis. The scope of molecular sieve catalysis has been considerably extended by the discovery of ordered mesoporous materials of the M41S type by Mobil scientists [13,14]. Furthermore, the incorporation of transition metal ions and complexes into molecular sieves extends their catalytic scope to redox reactions and a variety of other transition metal-catalyzed processes [15,16]. 

In the case of the very versatile zeolite ZSM-5, the silicaialumina ratio is high, so that molecules that penetrate the pores experience a high acidity environment. This gives the interior environment a strong Bronsted activity and the ability to catalyze many reactions. Coupled with its shape-selective properties, ZSM-5 had the ability to become a catalyst of extraordinary breadth and did so. 

Increasing attention is now on solid catalysts possessing both basic and shape selective properties to perform selective base-catalyzed fine organic chemical reactions [1-9]. Exchanged alkali zeolite demonstrated both these characters, however recent results show that these catalysts possess much less activity than sepiolites or hydrotalcites [10,11]. 

A new type of materials has been developed by delaminating the lamellar precursors of some zeolites. These materials show external surface areas > 600 m. g from where active sites can be accessible to very large molecules. If on one hand delamination eliminates geometrical shape selective properties of zeolites, it allows on the other hand to dispose of catalysts with the good reactant accessibility of mesoporous materials, but with the stability and active sites characteristics of zeolites. The very large and well structured external surface area can be specially suited for supporting different catalytic functions, which include, among others, metals, transition metal complexes and enzymes. 

Zeolites first made their appearance as cracking catalysts in the petroleum refining industry and were then quickly taken up by the petrochemicals industry. Chen et al. (1989) and Corma (1991) give excellent reviews of these applications. The extraordinary shape-selective properties of zeolites have since been exploited by many sectors of the chemical industry. The properties of zeolites are so much in tune with the selectivity demands of industry and the environmental regulations of society that one can enthusiastically agree with the statement that zeolite ca talysis and technology will (almost) certainly be the future cornerstone of a clean, environmentally friendly organic chemicals industry (Hoelderich and van Bekkum, 1991). The factual basis for such optimism will be reviewed in this section. 

Incorporation of metals or metal oxides into zeolite cavities leads to the formation of nanosized clusters exhibiting different catalytic properties from the bulk materials. These metal particles are usually introduced into zeolite channels through ion-exchange followed by reduction or oxidation/reduction to get their final dispersions. Metal clusters can also be formed via zeolite impregnation by corresponding azides from methanolic solutions followed by thermal decomposition. " Catalytic activities of the bifunctional or basic catalysts prepared using these methods can be successfully combined with shape-selective properties of parent zeolites. 

While a simple matter of academic curiosity in past years, isomorphously substitued zeolites, gained very recently a major practical significance with the advent of T-substituted MFI structures such as boralite, ferri-silite which established themselves as efficient catalysts in para-xylene production. Not only can the substituting T atoms influence the acid strength and shape selective properties, they are also likely to act on their own as catalytic centres. Typical of such a behaviour is TS-1 brought to public interest by ENI researchers. Titanium as the substituting element in a silicalite structure is active in various mild oxidation reactions. 

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