Transition-state selectivity , zeolite catalysis

Figure C2.12.10. Different manifestations of shape-selectivity in zeolite catalysis. Reactant selectivity (top), product selectivity (middle) and transition state selectivity (bottom).

Supported non-framework elements, as well as substituted or doped framework atoms, have been important for zeolite catalyst regeneration. By incorporating metal atoms into a microporous crystalline framework, a local transition state selectivity can be built into the active site of a catalytic process that is not readily attainable in homogeneous catalysis. The use of zeolites for carrying out catalysis with supported transition metal atoms as active sites is just beginning. The local environment of transition metal elements as a function of reaction parameters is being defined by in situ Mossbauer spectroscopy, electron spin echo measurements, EXAFS, and other novel spectroscopic techniques. This research is described in the second part of this text. 

H. van Bekkum et al. (72) studied a number of catalysts in the Fischer synthesis starting from l-phenyl-2-butanone 40 (with R, = Ph, R2 = CH3) and phenylhydrazine. The isomeric products are the bulky 2-ethyl-3-phenylindole 45 (with R, = Ph, R2 = CH3) and the linear 2-benzyl-3-methylindole 46 (with R, = Ph, R2 = CH3). Catalysis of the inolization of 40 by soluble as well as solid (e.g. Amberlyst 15) catalysts typically yielded a mixture of the two isomers in a bulky/linear ratio of about 75/25. Zeolite BEA reverses this bulky/linear ratio giving 75% of the linear isomer 46, a result interpreted in terms of restricted transition-state selectivity. Although in zeolite BEA the intraporous formation of 45 is largely suppressed, it is in fact probably not completely inhibited. 

We also mentioned stereospecificity of metal-catalyzed reactions inside zeolite cavities. In acid catalysis by zeolites it is well known that shape selectivity can be imposed by (1) selective admission of reactants fitting into zeolite pores, (2) selective release of products able to diffuse through zeolite channels, while larger molecules are retained, and (3) transition state selectivity, favoring, e.g., a monomolecular transition state over a bimolecular state in a narrow cavity. New tools that have conceptually been added to this arsenal include the collimation of molecules diffusing through well-defined pores, which then hit an active site preferentially via one particular atom or group. 

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. 

The sequence is exactly opposite to that of conventional acid catalysis The reactants that are best able to form carbenium ions in solution are the least reactive with zeolite catalysis. The restricted transition state selectivity suppresses cracking of the more highly branched hydrocarbons in the cavities [T25]. 

Zeolites have led to a new phenomenon in heterogeneous catalysis, shape selectivity. It has two aspects (a) formation of an otherwise possible product is blocked because it cannot fit into the pores, and (b) formation of the product is blocked not by (a) but because the transition state in the bimolecular process leading to it cannot fit into the pores. For example, (a) is involved in zeolite catalyzed reactions which favor a para-disubstituted benzene over the ortho and meso. The low rate of deactivation observed in some reactions of hydrocarbons on some zeoUtes has been ascribed to (b) inhibition of bimolecular steps forming coke. 

The reason for the high selectivity of zeohte catalysts is the fact that the catalytic reaction typically takes place inside the pore systems of the zeohtes. The selectivity in zeohte catalysis is therefore closely associated to the unique pore properties of zeohtes. Their micropores have a defined pore diameter, which is different from all other porous materials showing generally a more or less broad pore size distribution. Therefore, minute differences in the sizes of molecules are sufficient to exclude one molecule and allow access of another one that is just a little smaller to the pore system. The high selectivity of zeolite catalysts can be explained by three major effects [14] reactant selectivity, product selectivity, and selectivity owing to restricted size of a transition state (see Figure 4.11). 

Catalysis of 12-membered zeolites, H-mordenlte (HM), HY, and HL was studied In the alkylation of biphenyl. The para-selectlvltles were up to 70% for Isopropylblphenyl (IPBP), and 80% for dllsopropylblphenyl (DIBP) In HM catalyzed Isopropylatlon. Catalysis of HY and HL zeolites was nonselectlve. These differences depend on differences In pore structure of zeolites. Catalysis of HM to give the least bulky Isomer Is controlled shape-selectlvely by sterlc restriction of the transition state and by the entrance of IPBP Isomers. Alkylation with HY and HL Is controlled by the electron density of reactant molecule and by the stability of product molecules because these zeolites have enough space for the transition state to allow all IPBP and DIBP isomers. Dealuminatlon of HM decreased coke deposition to enhance shape selective alkylation of biphenyl. 

Three types of shape-selective catalysis are distinguished depending on whether pore size limits the entrance of reactant molecules, the departure of product molecules, or the formation of certain transition states [6]. The suitability of zeolite structure for the catalysis is essential for high shape-selectivity. Alkylation of biphenyl is also explained by sterlc control by pore size and shape of zeolite. HY, HL and HM have different pore structures... 

Zeolite catalysed alkylation of polynuclear aromatics is considered to be simultaneously governed by several mechanisms. To achieve highly shape-selective catalysis, it is essential that the pore size precisely corresponds to the molecular dimensions of reactants and products, and to the transition state of the reaction intermediates. 

Although diffusivity is often important in zeolite catalysis, other factors may also be crucial in determining shape selectivity. Recent work by Post 15a), for example, has shown that the shape selectivity behavior observed for the relative cracking rates of hexane isomers over H-ZSM 5 zeolite (see Section VIII) could not be understood on the basis of their measured diffusivities. Spatial restrictions imposed on transition-state species formed within the zeolite pores provide a possible explanation for the observed results. 

As revealed by selected catalytic test reactions, the effective pore width of zeolite MCM-22 is between those of medium pore and the very large pore materials [16-18]. Due to its very large cavities which, on the other hand, can be reached exclusively via 10-membered ring windows, this zeolite offers a remarkable potential for shape selective catalysis involving bulky transition states. 

HL but less selective for pyrrolidine. Other partially exchanged HL zeolites were both less active and less selective than HL itself but de-aluminated HL had enhanced activity and selectivity. The high selectivity for ring transformation was attributed to transition-state shape-selective catalysis in the straight channels of L zeolite. 

However, a very specific sort of catalytic behaviour peculiar to clays and zeolites is shape-selective catalysis. Essentially, owing to the fixed nature of the framework channels and cavities, only molecules of the right geometry can pass through. As only certain molecules will have the right geometry to pass into the zeolite, only these molecules will reach the active sites. The selectivity can occur either in reactant or product, or even the transition state. 

As examples of aluminosilicate materials, it is difficult to draw a clear demarcation between zeolites and other ionic materials. Zeolites are unusual in that the relevant catalysis appears to occur within the well-defined cage and channel structure of the material. Their catalytic activity appears primarily to arise because of their Bronsted acidity, while their control over selectivity arises primarily from geometrical considerations of which molecules (and transition states) can be accommodated within the structure. 

Like enzymes, a zeolite catalyst with a specific composition and structure is veiy selective toward certain reactants and products because only molecules of certain sizes can enter and exit the pores in which catalysis occurs. It is also possible that zeolites derive their selectivity from the ability to bind and to stabilize only transition states that fit properly in the pores. 

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. 

In addition to performing acid/base catalysis, zeolite structures can serve as hosts for small metal particles. Transition metal ions, e.g., platinum, rhodium, can be ion exchanged into zeolites and then reduced to their zero valent state to yield zeolite encapsulated metal particles. Inside the zeolite structure, these particles can perform shape selective catalysis. Joh et al. (16) reported the shape selective hydrogenation of olefins by rhodium encapsulated in zeolite Y (specifically, cyclohexene and cyclododecene). Although both molecules can be hydrogenated by rhodium supported on nonmicroporous carbon, only cyclohexene can be hydrogenated by rhodium encapsulated in zeolite Y since cyclododecene is too large to adsorb into the pores of zeolite Y. 

---