Molecular shape selectivity

Weisz P B 1981 Molecular shape selective catalysis Proc. 7th Int. Congr. on Catalysis (Tokyo) 1 1... 

Derouane, E.G., New aspects of molecular shape-selectivity catalysis by zeolite ZSM-5 Imelik, B. Naccache, C. ... 

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.)...

Takafuji, M., Fukui, M., Ansarian, H.R., Derakhshan, M., Shundo, A., and Ihara, H., Conformational effect of silica-supported poly(octadecyl acrylate) on molecular-shape selectivity of polycyclic aromatic hydrocarbons in RP-HPLC, Anal. Sci., 20, 1681, 2004. 

Gabelica, Z., and Vedrine, J.C. (1981) Molecular shape selectivity of ZSM-5, modified ZSM-5 and ZSM-11 type zeolites. Faraday Discuss. Chem. Soc,... 

Molecular Shape-Selective Hydrocarbon Conversion over Erionite... 

In principle, all the kinetic concepts of intercalation introduced for layer-structured silicates hold for zeolites as well. Swelling, of course, is not found because of the rigidity of the three dimensional frame. The practical importance of zeolites as molecular sieves, cation exchangers, and catalysts (cracking and hydrocracking in petroleum industry) is enormous. Molecular shape-selective transport (large differences in diffusivities) and micro-environmental catalysis (in cages and channels)... 

The unique properties of zeolites and other micro- or mesoporous solids that may favour their application to fine chemical synthesis are (1) the compatibility between the size and shape of their channels or cavities with the size of the reactants and/or products (generally referred to as molecular shape selectivity) that may direct the reaction away from the thermodynamically favoured route (2) the occurrence of confinement effects increasing the concentration of reactants near the catalytic sites and (3) the ability to tune their catalytic properties (acidic, basic, or other) via various treatments as described in this Volume. 

The catalytic properties associated with the molecular shape-selectivity exhibited by ZSM-5 are now well known. Recent work by Martens et al. (1995) has revealed that the external surfaces of zeolite crystals have also to be considered as potential shape-selective environments. Thus, strong evidence has been obtained for a lock-and-key model, which involves a form of pore mouth catalysis with bulky long-chain molecules that cannot penetrate into the intracrystalline micropores. The proposed lock-and-key model for n-alkane isomerization over ZSM-22 zeolite (with tubular pore openings of 0.55 x 0.45 nm) seems likely to be valid for other catalytic reactions. 

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... 

Zeolite ZSM-S has a three-dimensional pore structure with interconnected channels that have dimensions of 0.53 x 0.57 and 0.55 nm (162). Y zeolite and mordenite have wider openings of their principal channel networks, and these zeolites thus show less molecular shape selectivity than ZSM-5 zeolite. 

In addition to practical applications, metal cluster-derived catalysts, particularly intrazeolite metal cluster compounds, may aid in the identification of catalytically important bonding and structural patterns and thereby further our molecular understanding of surface science and heterogeneous catalysis. The ship-in-bottle technique for the synthesis of bulky metal-mixed metal cluster compounds inside zeolites and/or interlayered minerals has gained growing attention for the purpose of obtaining catalytic precursors surrounded by the interior constraint, imposing molecular shape selectivity. Such approaches may pave the way to offer the molecular architecture of hybrid (multifunctional) tailored catalysts to achieve the desired selectivity and stability for industrial processes. 

In the case of a soft large pore HY zeolite (with high Si/Al ratio), where the molecular shape selective effect does not take place, the a naphthalene derivatives can be synthesized (see Table 1) according to the molecular orbital theory indicated. 

N.Y. Chen and W.E. Garwood, Molecular Shape-Selective Hydrocarbon Conversion over Erionite. In Molecular Sieves , ed. W.M. Meier and J.B. Uytlerhoeven, ACS Symp. Ser 121, 1973, 575-582. 

P.B. Weisz and V.J. Frilette, Intracrystalline And Molecular-Shape-Selective Catalysis by Zeolite Salts. J. Phys. Chem., 1960, 64, 382-383. 

M. Niwa and Y. Murakami, CVD Zeolite Preparation, Characterization and Molecular Shape-selectivity. Mater. Chem. Phys., 1987, 17, 73-85. 

In 1960, Weisz, Frilette, and co-workers first reported molecular-shape selective cracking, alcohol dehydration, and hydration with small pore zeolites (6,7), and a comparison of sodium and calcium X zeolites in cracking of paraffins, olefins, and alkylaromatics (8). In 1961, Rabo and associates (9) presented data on the hydroisomerization of paraffins over various zeolites loaded with small amounts of noble metals. Since then, the field of zeolite catalysis has rapidly expanded,... 

Few detailed studies of olefinic dehydration products have been reported. Rapoport ei al. (105) found cis- and major products from n-pentanol dehydration over NaX only traces of 2-methyl-1-butene and 2-methyl-2-butene were reported. Thus, it is evident that double bond isomerization has accompanied or followed the dehydration reaction. Several authors (99,101,106) have suggested that diffusion processes may be rate controlling, or at least of some significance, in zeolite-catalyzed dehydration reactions. Molecular-shape selective alcohol dehydration (7) was discussed earlier. 

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. 

Molecular-Shape Selective Hydrogenations in Faujasite Catalyst Systems... 

The concept of molecular shape-selective catalysis is based on the action of catalytically active sites internal to the zeolitic framework, to diffusivity resistance either to reactant molecules or to product molecules or to both and to void limitation to reaction intermediates.This implies an intimate interaction between the shape, size and configuration of the molecules and the dimension, geometry and tortuosity of the channels and cages of the zeolite. Several types of effects exist ... 

The relative distribution of para + meta aromatics in methanol conversion was increased over Mo exchanged ZSM-5 but not on the pyridine poisoned sample. The same increase trend was also observed in the disproportionation reaction over Mo exchanged zeolites (Fig. 3), thus a reasonable explanation is the presence of internal Mo. Both methanol and toluene conversions performed on zeolites are molecular shape selective processes. Internal Mo will create diffusional hindrances which will favour the formation of para aromatics (product selectivity). 

Weisz, P.B. V.J. Frilette, R.W. Maatman and E. B. Mower. Catalysis by Crystalline Aluminosilicates II. Molecular-Shape Selective Reactions. J. Catal., 1962, 1, 307-312. 

Silica-occluded H3PW12O40 is a microporous material with a relatively sharp pore-size distribution with a peak at 0.55 nm (Figure 2). This microporous property seems to be favorable for molecular shape-selective reactions. Indeed, silica-occluded H3PW12O40 acts as a shape-selective solid-acid catalyst in the solvent-free alkylation of phenol with formaldehyde (Scheme 1). 

A large amount of information has been published on catalytic cracking with molecular shape-selective catalysts, such as natural or synthetic crystalline aluminosilicates. Several excellent reviews have appeared on the chemistry of catalysis with zeolites (52, 53). Literature on hydrocracking of pure hydrocarbons and simple mixtures of hydrocarbons with zeolite-containing catalysts is limited. However, numerous patents on the use of zeolites in hydrocracking catalysts and published... 

---