Shape-selective catalysis with zeolites

Shape-Selective Catalysis with Zeolites and Molecular Sieves... 

ABSTRACT. The amount of published work on molecular shape-selective catalysis with zeolites is vast. In this paper, a brief overview of the general principles involved in molecular shape-selectivity is provided. The recently proposed distinction between primary and secondary shape-selectivity is discussed. Whereas primary shape-selectivity is the result of the interaction of a reactant with a micropore system, secondary shape-selectivity is caused by mutual interactions of reactant molecules in micropores. The potential of diffusion/reaction kinetic analysis and molecular graphics for rationalizing molecular shape-selectivity is illustrated, and an alternative explanation for the cage and window effect in cracking and hydrocracking is proposed. Pore mouth catalysis is a speculative mechanism advanced for some systems (a combination of a specific zeolite and a reactant), which exhibit peculiar selectivities and for which the intracrystalline diffusion rates of reactants are very low. 

Several new types of anionic tetrahedron complexes were discovered during the last years an Isolated complex, built of 48 Sl,0 tetrahedra, pyrogroups [P2O7] In a phosphate mineral, a 18-membered [SiOs] rings, a [PO3] chain with 16 tetrahedra In the period, an interrupted [81205] framework, etc. The new scientific ideas on the structures of the siiicates and their analogues stimulate their appiications in different technologies (ex. shape selective catalysis with zeolites). 

Figure 4.20 MTG/MTO reaction path and aromatics distribution with different zeolites as catalysts. (Reprinted from C.D. Chang, W.H. Lang, W.K. Bell, Catalysis in Organic Reactions, Molecular Shape-Selective Catalysis in Zeolites, pp. 93-94. Copyright 1981. With permission from Marcel Dekker.)...

Transalkylation of alkylbenzenes, polyalkylbenzenes and other arenes can be brought about by a variety of catalysts including Lewis acids, Brpnsted acids and various zeolites and silicates with or without being doped with various transition metals or their oxides. There has been a particularly explosive growth in the volume of literature pertaining to the use of various natural and modified zeolites. Recent developments include the study and applications of shape-selective catalysis by zeolites. Much of the work is patented, and largely applies to industrial processes. 

Shape selective catalysis with molecular sieve zeolites has progressed in its first thirty years to become an established branch of catalytic science. Since the first demonstration of selective n-paraffin conversion over 5A molecular sieves, increased insight into how these catalysts function has created opportunities for the development of a number of new industrial processes. 

Source P. B. Weise and V. J. Frilette, Intracrystalline and Molecular-Shape-Selective Catalysis by Zeolite Salts, Journal of Physical Chemistry 64 382 (1960). With permission. 

Some aspects of molecular shape-selective catalysis with hydrocarbons in zeolites J.A. Rabo... 

SOME ASPECTS OF MOLECULAR SHAPE-SELECTIVE CATALYSIS WITH HYDROCARBONS IN ZEOLITES... 

The first examples of molecular shape-selective catalysis in zeolites were given by Weisz and Frilette in 1960 [1]. In those early days of zeolite catalysis, the applications were limited by the availability of 8-N and 12-MR zeolites only. An example of reactant selectivity on an 8-MR zeolite is the hydrocracking of a mixture of linear and branched alkanes on erionite [4]. n-Alkanes can diffuse through the 8-MR windows and are cracked inside the erionite cages, while isoalkanes have no access to the intracrystalline catalytic sites. A boom in molecular shape-selective catalysis occurred in the early eighties, with the application of medium-pore zeolites, especially of ZSM-5, in hydrocarbon conversion reactions involving alkylaromatics [5-7]. A typical example of product selectivity is found in the toluene all lation reaction with methanol on H-ZSM-5. Meta-, para- and ortho-xylene are made inside the ZSM-5 chaimels, but the product is enriched in para-xylene since this isomer has the smallest kinetic diameter and diffuses out most rapidly. Xylene isomerisation in H-ZSM-5 is an often cited example of tranSition-state shape selectivity. The diaryl type transition state complexes leading to trimethylbenzenes and coke cannot be accommodated in the pores of the ZSM-5 structure. 

Ti, B, Ni, Cr, Fe, Co, Mn) has been described, as was the synthesis of nonsiliceous materials such as oxides of W, Fe, Pb, Mo, and Sb [18]. Although these materials do not represent tme zeolites, they are highly interesting materials which are commonly covered in the zeolite literature with great potential for shape-selective catalysis of bulky molecules. 

In this paper, we report on the shape selective isomerization of l methylnaphthalene in suitable zeolite catalysts in which the undesired transalkylation reaction is suppressed. Furthermore, new results concerning the alkylation of 2-methylnaphthalene with methanol are presented in an endeavor to contribute to a critical evaluation of Fraenkel s model. At the same time, the potential of shape selective catalysis for the manufacture of... 

The consecutive formation of o-hydroxybenzophenone (Figure 3) occurred by Fries transposition over phenylbenzoate. In the Fries reaction catalyzed by Lewis-type systems, aimed at the synthesis of hydroxyarylketones starting from aryl esters, the mechanism can be either (i) intermolecular, in which the benzoyl cation acylates phenylbenzoate with formation of benzoylphenylbenzoate, while the Ph-O-AfCL complex generates phenol (in this case, hydroxybenzophenone is a consecutive product of phenylbenzoate transformation), or (ii) intramolecular, in which phenylbenzoate directly transforms into hydroxybenzophenone, or (iii) again intermolecular, in which however the benzoyl cation acylates the Ph-O-AfCL complex, with formation of another complex which then decomposes to yield hydroxybenzophenone (mechanism of monomolecular deacylation-acylation). Mechanisms (i) and (iii) lead preferentially to the formation of p-hydroxybenzophenone (especially at low temperature), while mechanism (ii) to the ortho isomer. In the case of the Bronsted-type catalysis with zeolites, shape-selectivity effects may favor the formation of the para isomer with respect to the ortho one (11,12). 

Zeolites are well known for shape-selective catalysis. Here the shape of the zeolite pores or cavities can control the shape of product. When catalytic reactions take place in channels of zeolites only those products that can be accommodated in the channels advance and emerge. Mobil s ZSM-5 is an example of a shape-selective catalyst. Many more zeolites with different pore sizes or large surface areas are being synthesized, extending the principle of shape-selective catalysis. Such developments are helpful for both existing industrial processes and environmental protection. 

The dehydrated zeolites exchanged with various cations have been of catalytic interest in many reactions, among which cracking (259) and shape-selective catalysis (260) are most important. Other reactions include oxidation, carbonylation, and related reactions (261) as well as other nonacid catalytic reactions (262). 

Porous oxide catalytic materials are commonly subdivided into microporous (pore diameter <2nm) and mesoporous (2-50 nm) materials. Zeolites are aluminosilicates with pore sizes in the range of 0.3-1.2 nm. Their high acidic strength, which is the consequence of the presence of aluminium atoms in the framework, combined with a high surface area and small pore-size distribution, has made them valuable in applications such as shape-selective catalysis and separation technology. The introduction of redox-active heteroatoms has broadened the applicability of crystalline microporous materials towards reactions other than acid-catalysed ones. 

In this paper, we review primary and secondary shape selective acid catalysis with zeolites. Next, we discuss shape selectivity with metal containing zeolites.We conclude with a section that deals with future trends in shape selective catalysis. 

Of particular interest for shape-selective catalysis is the modification of the zeolite by means of cation exchange as well as the modification of the inner and/or outer crystallite structure by a treatment with chemically reacting agents which leads to a deposition of additional functional groups or compounds. This can be done either in the gas phase (i.c. chemical vapor deposition (CVD)) or in the liquid phase. 

The shape of the 10-R pore may be round, oval or even teardrop . Subtle effects on the shape selectivity catalysis could be associated with the uniformity of the pore, its shape and its connectivity to other pores. The later effect, in particular, is important for two- and three-dimensional zeolites, where the intersection of the pore leads to the formation of pockets or cages of slightly larger dimension. 

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