Nature and Location of the Active Sites

The Brpnsted acid sites were found to be important for the catalytic 

The mechanism of skeletal isomerization of n-butenes may be rationalized in terms of the steps presented previously the key reaction intermediate is the 5-butyl cation. The predominent structure of the adsorbed intermediate was recently considered to be an alkoxy 50), which cither adds to one butene molecule and cracks into C3, C4, or C5 fragments (the bimolccular mechanism) or rearranges into isobutylene (the monomolecular mechanism) via a primary carbenium ion. 

To overcome the formation of a primary carbenium ion intermediate, it has been proposed for an aged ferricrite catalyst, which is highly selective for isobutylene, that a carbenium ion trapped within the carbonaceous residues formed in or on the zeohte could be the active site. The authors suggested that the structure of such an active site could be 

The monomolecular mechanism would transform n-butenes into isobutylene via the formation of a secondary carbenium ion as foUows  

The proposed pathway will be more favorable kinetically than that suggested for the true monomolecular process, whereby a primary carbenium ion is formed. To further test the idea that carbonaceous residues are the active and selective sites for the skeletal isomerization of n-butenes, the authors reported results showing that the rate of isobutylene formation catalyzed by ferrierite passed through a maximum as the conversion continuously decreased (Fig. 12) (51). 

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The aim of this review is to describe the most interesting results characterizing the skeletal isomerization of n-butenes catalyzed by zeolitic and nonzeolitic molecular sieves and to discuss the state of the art of the isomerization mechanism, the nature and location of the active sites responsible for the selectivity for isobutylene, and the influence of the pore dimensions and pore structures of the molecular sieves. 

Fig. 16. A. Plot of log iNa as a function of T 1 (°K) using the experimental values of the rate constants and the location of the binding sites in Eq. 4. The Gibbs free energy of activation is calculated from Eq. 3 the AS are taken to be zero, and the current is calculated by means of Eq. 4. The purpose is to demonstrate that multibarrier channel transport can be seen as single rate process with average values for the enthalpies of activation. Non-linearity of such a plot is then taken to arise form the dynamic nature of the channel.

Long (81) showed that the complex from biscyclopentadienyltitanium dichloride and methylaluminum chloride or a simply derived product from it, was an active ethylene polymerization catalyst. There have been a number of attempts to determine the exact nature of initiation in polyethylene. However, by any techniques available until now, it has not been possible to determine the actual ionic nature of the active catalyst which polymerizes ethylene. Karapinka and Carrick (82) studied the polymerization of ethylene with biscyclopentadienyltitanium dichloride and various alkylaluminum compounds. They found that the alkyl group exchanged so readily between the aluminum and titanium, that the location of the initiating site could not be determined. All that could be concluded was that an ethyl group initiated the polymerization more easily than the phenyl. 

In most of the reactions discussed the active entity of the zeolite catalysts is introduced via ion exchange. Thus a knowledge of the possible siting of cations is a prerequisite for an understanding of the location and nature of the active sites in zeolites. In this respect the periodicity of the internal surface of the zeolites provides an almost unique opportunity to study the surface composition in considerable detail using powerful analytical methods such as X-ray diffraction. 

The chemistry of the colorful, perplexing, and challenging problem of catalase mechanism has been set forth in numerous reviews. Those by Brill 16), Nicholls and Schonbaum (f ), and most recently, Deisseroth and Bounce (15) summarize the fundamental properties of catalase and its reactions, and their physiological implications. Further, Feinstein 19) and Aebi 20-22) have presented detailed evaluations of acatalasemia, and de Duve 23) and others have discussed catalase biosynthesis 23-26), its intracellular location 23-25) and its turnover 24, 26-28). These facets of the catalase problem will not be reiterated. Instead, a brief synopsis of the enzyme characteristics will be followed by a discussion on the nature of the active site and the chemistry of the catalase reaction mechanism. 

The key to understanding the function of histone deacetylases lies in their three dimensional architecture. As outlined above, the class I, II, and IV enzymes are all metal ion dependent in most cases, a zinc ion is essential for activation and hydrolysis of the amide group, which is located within the active site of the enzyme. However, it has been shown that other metal ions can efficiently adopt the role of the catalytic ion. For instance, the nature of the ion bound to the catalytic site influences the specific activity of HDAC8 in the following order Co2+ > Fe2+ > Zn2+ > Ni2+. These data suggest that Fe2+ rather than Zn2+ may be responsible for the in vivo activity of HDAC8 [33]. 

How do we determine the essential amino acid residues Several questions arise about the events that occur at the active site of an enzyme in the course of a reaction. Some of the most important of these questions address the nature of the critical amino acid residues, their spatial arrangement, and the mechanism of the reaction. The use of labeling reagents and X-ray crystallography allows us to determine the amino acids that are located in the active site and critical to the catalytic mechanism. 

Zeolite ZSM-5, as a member of the family of pentasil zeolites, has aroused tremendous interest after its first discovery by the research group of Mobile Company in the year 1972 [1]. With its adjustable framework A1 content (from 0 to about 8A1 per unit cell), two dimensional micropore channels (0.55 nm x 0.54 nm Fig. la), sinusoidal pore geometry along c axis (Fig.lb) and easy insertion of hetero-T atoms, this material plays an important role in many of crucial catalytic processes such as hydro-cracking, de-waxing, alkylation, etc., [2-5] as well as in separation of organic compounds with different sizes and shapes [6]. In the case when zeolite ZSM-5 was used as catalyst, most of reactions are diffusion-controlled [7]. This means that the product distribution largely depend on the nature and location of active sites in the crystalline framework of catalyst. Thus, the increase of the... 

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