Shape-selective catalysis development

Reglospeclflc functionalization of biphenyl is drawing attention as one of key steps in developing advanced materials such as liquid crystals and liquid crystal polymers [1-5]. Catalysis using zeolites is the most promising way to prepare sterlcally small molecules by differentiating between reactants, products, and/or intermediates according to their size and shape. Sterlc restrictions by zeolites Increase the formation of preferred products and prevent the formation of undesirable products [6]. We describe herein shape selective catalysis of 12-membered zeolites, H-mordenite (HM), HY and HL In the alkylation of biphenyl. 

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. 

Beck J., New Developments in Shape Selective Catalysis, Mobil Catalysts Corporation of Japan, 15th anniversary Symposium, Tokyo, Japan, November, (2000). 

The principle of molecular shape-selective catalytic reaction was established by Weisz and associates in 1960 ( 70), and molecular engineering aspects have been reviewed recently by Chen and Weisz (12). The selective sorption of molecules differing only slightly in their critical dimensions has been applied in the development of industrial separation processes to remove straight chain hydrocarbons from a mixed hydrocarbon stream. In shape-selective catalysis, on the other hand, molecules of proper dimensions are continuously entering and leaving the intracrystalline cavities of the molecular sieve, thus allowing specific selectivi-ties to occur. Reactant selectivity (Scheme 7A) occurs when I of 2... 

Shape selective catalysis was first recognized and described by Weisz and coworkers [1] of Mobil Research and Development in 1960. References 2-20 review shape selective catalysis. 

As stated some years ago [2, 4], shape selective catalysis involving bulky molecules continues to be a thrust area in zeolite catalysis. Consequently, test reactions have been developed which are particularly suited to characterize large and super-large pore molecular sieves [34]. In view of possible commercial applications, recent work focussed on the shape selective synthesis of substituted dinuclear aromatics, i.e., 4,4 -diisopropylbiphenyl and 2,6-dialkylnaphthalenes, due to their potential as components in high-temperature resistent polyesters or as liquid crystals. Recent advances in this field are covered in two excellent review articles [35, 36]. 

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. 

Finally, three different sorts of compound which can insert and exchange cations and ions in their structures will be discussed. Of these, the zeolites have been developed from minerals, and are used as ion exchangers, catalysts and sorbents. The use of their framework structures for shape-selective catalysis will be discussed. The unique properties of both layered and three-dimensional compounds which can accept extra ions, such as graphite and will also be examined. 

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. 

Attention in fundamental and applied research on shape-selective catalysis has been largely focused on open-chain and monocyclic compounds. However, we have observed the rapid developments in polymer materials containing multi-ring aromatic units and the need to develop the monomers and other specialty chemicals from polyaromatic hydrocarbons that are rich in coal-derived liquids [Song and Schobert, 1993, 1996]. Scheme 1 shows the structures of some advanced polymer materials containing aromatic ring in the main-chain. 

The engineering of porosity in silica has been emerging as a new area of interest, particularly since the development of the MCM-type of materials [2], Indeed, tailor-made pore sizes and shapes are particularly important in applications where molecular recognition is needed, such as shape-selective catalysis, molecular sieving, chemical sensing and selective adsorption [3],... 

The pore opening of pillared clays, which plays an important role in shape selective catalysis, is determined both by the interlayer distance and by the density of pillar or the number of pillars. The interlayer distance depends on the dimension of intercalating species. Considerable efforts have been undertaken to develop pillared clays with different interlayer distances. Indeed, pillared clays having the interlayer distance from 0.4 to 2.0 nm were prepared using various intercalating species(refs. 3-7). In contrast, there have been few studies concerning the control of pillar density or the number of pillars. 

Even emerging concepts such as those related to the shape selectivity of the external surface of the crystallites (nest effect, pore mouth catalysis,. ..) have already been applied to develop new catalytic processes isodewaxing, selective... 

N. Y. Chen (Mobil Research Development Corp., Princeton, N. J. 08540) It might be of interest to the audience, particularly to those who are not familiar with the application of zeolites in industrial catalytic processes, to mention that since the discovery of catalysis over shape-selective zeolite first published by Weisz and Frilette in I960, a commercial process based on selective hydrocracking reactions similar to that reported in this paper has been in operation on a large scale in more than four of our refineries since 1967. A technical paper describing this process, known as the Selectoforming process, was published in 1968. 

Heterogeneous catalysts which are active for the catalysis of the MPVO reactions include amorphous metal oxides and zeolites. Their activity is related to their surface basicity or Lewis acidity. Zeolites are only recently being developed as catalysts in the MPVO reactions. Their potential is related to the possibility of shape-selectivity as illustrated by an example showing absolute stereoselectivity as a result of restricted transition-state selectivity. In case of alkali or alkaline earth exchanged zeolites with a high aluminium content (X-type) the catalytic activity is most likely related to basic properties. For zeolite BEA (Si/Al=12), however, the dynamic character of those aluminium atoms which are only partially connected to the framework appear to play a role in the catalytic activity. Similarly, the Lewis acid character of the titanium atoms in aluminium free [Ti]-BEA explains its activity in the MPVO reactions. 

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