Preparation shape selective reactions

A preparation of designed catalyst is one of the interest subjects to understand the catalysis. Efforts have been paid for the development of unique preparation method[1] those are metal cluster catalysts derived from metal carbonyls, tailored metal catalysts through organometallic processor and ultra-fine metal particle catalysts prepared by metal alkoxides, etc. These preparation methods are mainly concentrated to design the active sites on support surfaces. However, the property of support itself is also a dominant factor in order to conduct smoothly the catalytic reaction. It is known that some supports are valuable for the improvement of selectivity. For example, zeolites are often used as catalysts and supports for their regular pore structures which act effectively for the shape selective reaction[2]. In order to understand the property of support, the following factors can be pointed out besides the pore structure structure, shape, surface area, pore size, acidity, defect, etc. Since these are strongly correlated to the preparation procedure, lots of preparation techniques, therefore, have been proposed, too. Studies have been still continued to discover the preparation method of novel materials as well as zeolites[3]. 

Most of the interesting catalytic properties of zeolites are related to the presence of strong acid sites. Many processes have been developed taking advantage of the zeolites acidity and shape selectivity. At the end of the seventies, we have prepared a silicalite with titanium in lattice positions (ref.s 1,2). The discovery of TS-1 and its peculiar properties in oxidative processes with hydrogen peroxide started a completely new field of shape selective reactions (ref. 3). 

The alkylation of phenol investigated over H-MCM-22, H-ITQ-2 and H-MCM-36 showed that the delamelation and pillaring did not improve the catalytic activity and this was explained on the secondary processes taking place during the preparation of the corresponding materials, and which strongly affect the total acidity and the acidity on the external surface. Also, the composition of the reaction products is not influenced to a considerable extent by product shape selectivity effects. This seems to show that the tert-butylation reaction preferentially proceed at (or close to) the external surface of the zeolite layers. 

Zeolites ZSM-5 and ZSM-11 are the most commercially important end-members of a continuous series of intermediate structures belonging to the so-called pentasil family (4,5). The first preparation of ZSM-5 was described in 1972 (6) and since thbn, a number of elaborate synthesis recipes have been reported in the patent literature. Because of the unique and fascinating activity and (shape) selectivity of this material for a variety of catalytic reactions currently processed in chemical industries, increasing attention has been devoted to a better understanding of the various mechanisms that govern the synthesis of ZSM-5 (7-33). 

Elements such as B, Ga, P and Ge can substitute for Si and A1 in zeolitic frameworks. In naturally-occurring borosilicates B is usually present in trigonal coordination, but four-coordinated (tetrahedral) B is found in some minerals and in synthetic boro- and boroaluminosilicates. Boron can be incorporated into zeolitic frameworks during synthesis, provided that the concentration of aluminium species, favoured by the solid, is very low. (B,Si)-zeolites cannot be prepared from synthesis mixtures which are rich in aluminium. Protonic forms of borosilicate zeolites are less acidic than their aluminosilicate counterparts (1-4). but are active in catalyzing a variety of organic reactions, such as cracking, isomerization of xylene, dealkylation of arylbenzenes, alkylation and disproportionation of toluene and the conversion of methanol to hydrocarbons (5-11). It is now clear that the catalytic activity of borosilicates is actually due to traces of aluminium in the framework (6). However, controlled substitution of boron allows fine tuning of channel apertures and is useful for shape-selective sorption and catalysis. 

Most crystalline aluminosilicates have little intrinsic catalytic activity for hydrogenation reactions. However, a considerable amount of data has recently accumulated on the use of zero-valent metal-containing zeolites in many hydrocarbon transformations. Thus noble and transition metal molecular sieve catalysts active in hydrogenation (7,256-760), hydroisomerization (161-165), hydrodealkylation (157, 158,165-167), hydrocracking (168,169), and related processes have been prepared. Since a detailed discussion of this class of reactions is beyond the scope of this review, only a few comments on preparation and molecular-shape selectivity will be made. 

The reaction could not be carried out with the usual sulfonic acid ion-exchange resin, because its maximum use temperature was 120°C. The product cannot be made directly from acetone and aniline owing to the formation of a dihydroquinoline by-product (from two molecules of acetone and one of aniline). A shape-selective zeolite might allow the reaction to take place without formation of this byproduct. An inexpensive way of making this diamine from acetone and aniline, similar to the preparation of bisphenol A from acetone and phenol, could lead to new families of polyamides, polyimides, polyureas, polyurethanes, and epoxy resins. A palladium catalyst supported on Nafion dimerized ethylene much faster in water than in organic solvents. The butene was easy to separate.18... 

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