Structure of active sites

Cp Ti(OMe)3, reacted mixtures of Cp Ti(OMe)3 and TIBA, and reacted mixtures of Cp Ti(OMe)3, TIBA, and MAO are examined by X-ray absorption near-edge structure (XANES) and extended X-ray absorption line structure (EXAFS) analysis. Rgure 4.11 shows the results of these measurements [23]. 

A sharp peak was observed in XANES for Cp Ti(OMe)3, but it disappeared by the addition of TIBA, MAO, or borate. TIBA, MAO, or borate changes the coordination structure of the titanium. The position of the edge is shifted to the left, showing that the alkylaluminum reduces the valence of titanium from +4 to +3. 

EXAFS shows that the electron density of the titanium has been reduced by TIBA and MAO or borate and that the cyclopentadienyl ligand came closer to the titanium. Therefore, the structures of the active sites formed using either MAO or borate are probably almost the same. The signal inside the cyclopentadienyl ligand decreased by the addition of cocatalysts. The -OMe group may change to hydrogen. 

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The Phillips catalyst has attracted a great deal of academic and industrial research over the last 50 years. Despite continuous efforts, however, the structure of active sites on the Phillips-type polymerization systems remains controversial and the same questions have been asked since their discovery. In the 1950s, Hogan and Banks [2] claimed that the Phillips catalyst is one of the most studied and yet controversial systems . In 1985 McDaniel, in a review entitled Chromium catalysts for ethylene polymerization [4], stated we seem to be debating the same questions posed over 30 years ago, being no nearer to a common view . Nowadays, it is interesting to underline that, despite the efforts of two decades of continuous research, no unifying picture has yet been achieved. 

J. M. Thomas, C. R. Catlow and G. Sankar, Determining the structure of active sites, transition states and intermediates in heterogeneously catalysed reactions, Chem. Commun., 2002, 2921. 

Structures of actives sites are given either in schematic chemical form or with explicit reference to a crystal structure (pdb). 

From these results, it may be concluded that the reactions that proceed through covalently bound intermediates are controlled precisely by the structures of active sites, but the reactions that occur via ionic intermediates are less affected by the structures of active sites. 

A catalytic cycle is composed of a series of elementary processes involving either ionic or nonionic intermediates. Formation of covalently bound species in the reaction with surface atoms may be a demanding process. In contrast to this, the formation of ionic species on the surface is a facile process. In fact, the isomerization reaction, the hydrogenation reaction, and the H2-D2 equilibration reaction via ionic intermediates such as alkyl cation, alkylallyl anion, and (H2D)+ or (HD2)+ are structure-nonrequirement type reactions, while these reactions via covalently bound intermediates are catalyzed by specific sites that fulfill the prerequisites for the formation of covalently bound species. Accordingly, the reactions via ionic intermediates are controlled by the thermodynamic activity of the protons on the surface and the proton affinity of the reactant molecules. On the other hand, the reactions via covalently bound intermediates are regulated by the structures of active sites. 

Macedo-Ribeiro S, Hemrika W, Renirie R, Wever R, Messerschmidt A (1999) X-Ray Crystal Structures of Active Site Mutants of the Vanadium-Containing Chloroperoxidase from the Fungus Curvularia inaequalis. J Biol Inorg Chem 4 209... 

Ab inito chemical shielding calculation, combined with solid-state NMR experiment, is a powerful tool to elucidate the structures of active sites in proteins and enzymes. 

The heat effect is one of the main characteristics of every chemical process. The heat effects of the reactions occurring at the solid surface and involving gas-phase molecules can be directly measured. To do this, one must know the amount of heat release during the reaction (microcalorimetry) and the number of absorbed gaseous molecules (volumo-metry). The heat effects of some reactions proceeding at the surfaces of activated silicon and germanium oxides and accompanied by the modification of the chemical structure of active sites are given in Table 7.4. 

Bums, R. G. and Solberg, T. C. (1990) Crystal structure trends in Mossbauer spectra of 57Fe-bearing oxide, silicate, and aluminosilicate minerals. In Structures of Active Sites in Minerals. (L. M. Coyne, S. W. S. McKeever D. F. Drake eds Amer. Chem. Soc. Publ.), ACS Symp. Ser., 415, 262-83. 

The current general understanding of the mechanism operating in cycloolefin metathesis polymerisation leads us towards the acceptance of the structure of active sites in systems with catalysts belonging to the aforesaid three major groups as one that alternates between metal carbene complexes and metallacyclobutanes. 

Although rather scant information concerning the structure of active sites in the above copolymerisation systems is available [183-189], cyclic acid anhydrides can be considered as coordinating to metal species via the carbonyl oxygen atom and reacting by nucleophilic attack of the metal substituent on the carbon atom of the coordinated carbonyl group [190,191], Thus, the oxi-rane/cyclic acid anhydride copolymerisation pathway may be presented schematically as follows [82] ... 

Figure 7.17 Structures of active-site directed amylase inhibitors. The K values are for glucoamylase (GA), porcine pancreatic oamylase (PPA) and cyclomaltodextrin glucanosyltransferase (CGTase).

Macedo-Ribeiro, S., Hemrika, W., Renirie, R., Wever, R. and Messerschmidt, A. (1999). X-ray crystal structure of active site mutants of the vanadium-containing chloroperoxidase from the fungus Curvularia inaequalis. Journal of Biological Inorganic Chemistry, 4,209-219. 

Electronic Structures of Active Sites in Copper Proteins and Their Contributions to Reactivity... 

R. Huber, W. Vondersaal, K. Wirthen-SOHN, R.A. Engh, X-ray structure of active site-inhibited clotting factor Xa — implications for drug design and substrate recognition./. Biol. Chem. 1996, 271, 29988-29992. 

Figure 2 Structure of active site of recombinant soybean ascorbate peroxidase (sAPX) showing the heme, Lys30, Cys32, and Argl72 with the substrate ascorbate (This figure was generated from coordinates of lOAF deposited in the Protein Data Bank)...

Figure 17.11 Proposed structure of active site and dormant site for styrene polymerization (a) Active site (b) dormant site by the coordination error of monomer (c) dormant site by the polymer chain rotation...

Deactivation of the copper zeolites under de-NO, conditions was one of the major reasons why the catalyst was never used in a commercial application. Recent environmental legislation intensified the hunt for a water- and sulfur-stable active catalyst One of the most successful preparative methods was reported by HaU and Feng [42, 43]. They reported excellent de-NO, performance based on an iron exchanged ZSM-5 zeolite. The activity was reported to remain constant for extended times, even under high water and sulfur content conditions. The initial catalytic study initiated a whole raft of characterization studies by a number of groups. The interest was significantly increased when it became obvious that there are issues with catalyst preparation reprodudbihty [44, 45]. XAS was crucial in the discussion of the structure of active sites for de-NO, and the site responsible for the high... 

Examination of characteristic IR bands of adsorbed probe molecules such as NO, CO, NH3, CO2, and pyridine is useful for providing information about the structure of active sites and acidity and basicity of (sulfided) catalysts. Some IR investigations of phosphorus-containing catalysts with adsorbed probe molecules have also been reported. Because the acidity measurements with pyridine and NH3 adsorption were considered previously, only the results obtained with NO and CO probe molecules are considered in this section. 

The ESR data shows that both the number of centers and the local structure of active sites associated with Cu isolated ions are not changed noticeably at T < 500°C as a result of cobalt introduction. At the same time, catalytic testing shows a 3-fold rise in oxidative activity of bi-cationic sample (Fig. 1) demonstrating an increase either in the number of sites or in the intrinsic activity of catalytic centers. The effect can be explained only in assumption of the high dispersion of cobalt ions in microporous matrix it is difficult to imagine a considerable contribution from the big particles of cobalt oxide on the outer surface of zeolitic crystals. 

Reaction mechanism and the structure of active site, over Cu-ZnO based catalyst Many experimental results suggest methanol is produced not from CO, but from CO2 directly. As mentioned above, methanol yield from H2/CO2 is higher than that from H2/CO at a wide range of reaction conditions.[5,6,7,20] A study of influence of contact time on product distribution suggested methanol was a primary product[21]. Fujita et al.[7] and Koeppel et al.[22] showed methanol and CO were produced... 

Shimizu, K., Shibata, J., Yoshida, H., Satsuma, A. and Hattori, T. (2001) Silver-alumina catalysts for selective reduction of NO by higher hydrocarbons structure of active sites and reaction mechanism. Appl. Catal. B Environ., 30, 151—162. 

Ample experimental evidence indicates that the habit of crystallites of the catalyst has a profound influence on the activity and selectivity of the reaction, which results in the appearance of structure sensitivity of the selective oxidation reactions at oxide catalysts (39). However, little is known about the origin of this phenomenon and about the differences of the structure of active sites present at various crystal planes. Even less is known about the role of defects present at the surface of an oxide, in determining the catalytic properties. Only recently studies of the properties of (100) surface of a monocrystal of NiO revealed that an ideal surface is chemically inert and the reactivity of the system increases only if defects are introduced in the surface (40). At such a surface, dissociation of molecular water to form hydroxyl groups is observed in contrast to an ideal surface which is inactive in water dissociation. 

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