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Open catalytic active sites

More Approaches to Interrogating Open Catalytic Active Sites 343... [Pg.343]

The development of mesoporous materials with more or less ordered and different connected pore systems has opened new access to large pore high surface area zeotype molecular sieves. These silicate materials could be attractive catalysts and catalyst supports provided that they are stable and can be modified with catalytic active sites [1]. The incorporation of aluminum into framework sites of the walls is necessary for the establishment of Bronsted acidity [2] which is an essential precondition for a variety of catalytic hydrocarbon reactions [3], Furthermore, ion exchange positions allow anchoring of cationic transition metal complexes and catalyst precursors which are attractive redox catalytic systems for fine chemicals [4]. The subject of this paper is the examination of the influence of calcination procedures, of soft hydrothermal treatment and of the Al content on the stability of the framework aluminum in substituted MCM-41. The impact on the Bronsted acidity is studied. [Pg.243]

Another possible role of the additive concerns the peculiar structure of some lipases endowed with a mobile element (named lid) that regulates the entrance to the catalytic active site. It has been suggested that the lower enzyme activity in organic solvent could be due to the fact that, after lyophilization, the enzyme is in a close conformation (less active) [24]. Then, if an additive (e.g. -octyl-(5-glucopyranoside) also favors the open conformation after lyophilization, the... [Pg.74]

The new mechanistic routes opened by the introduction of a catalyst to the reaction mixture typically start with the adsorption of the reactants. Because of the heterogeneous nature of the surface of the catalyst, both the adsorption and the subsequent conversion reactions may take place preferentially at particular ensembles of surface sites, often called active sites or active centers. An atomic ensemble may become active because of a specific structural arrangement of those atoms. Alternatively, the electronic properties of metal atoms may also influence the adsorption and activation of reactants. Typically, the performance of catalytic active sites depends on both structural and electronic effects. [Pg.1495]

Our hypothesis regarding the mechanism for the pH-controlled regulation of LCF activity is that the histidine residues make contacts that stabilize the N-terminal domain and the helix-loop-helix. Disruption of such contacts by protonation in the wild-type, or in his to ala mutants at pH 8, causes the N-terminal domain and the helix-loop-helix to move and open the catalytic active site. This appears to be altogether novel in enzyme chemistry as a mechanism for regulation of activity. [Pg.18]

The subjects of catalytic science include catalysis (cataljAic phenomena and principle) catalyst (composition, structure, performance and manufacturing method and principle) catalytic reaction kinetics (chemical kinetics and mechanism) as well as cataljAic reaction engineering (apparent kinetics inclucing transport process and reaction process and reactor design) etc. The main tasks of catalytic science are to elucidate the nature of catalytic active sites, the function of catalyst and reaction mechanism to explore the main factors influencing activity, selectivity and stabihty of catalyst to accumulate acknowledge for the exploitation and development of chemical catalysis and to open up its relatively new disciplines — bionic catalysis, photo catalysis, electro catalysis and photoelectric catalysis — to indicate... [Pg.67]

Poisoning open sites on metal nanoparticles provides a method to observe how the activity and/or selectivity changes as function of the degree of opeimess of the site. The next few examples illustrate how different poisoning experiments can provide information about the identity of the catalytic active site. [Pg.343]

Figure 4.15 Schematic diagram of the enzyme tyrosyl-tRNA synthetase, which couples tyrosine to its cognate transfer RNA. The central region of the catalytic domain (red and green) is an open twisted a/p stmcture with five parallel p strands. The active site is formed by the loops from the carboxy ends of P strands 2 and S. These two adjacent strands are connected to a helices on opposite sides of the P sheet. Figure 4.15 Schematic diagram of the enzyme tyrosyl-tRNA synthetase, which couples tyrosine to its cognate transfer RNA. The central region of the catalytic domain (red and green) is an open twisted a/p stmcture with five parallel p strands. The active site is formed by the loops from the carboxy ends of P strands 2 and S. These two adjacent strands are connected to a helices on opposite sides of the P sheet.
The discovery of a new heterodinuclear active site in [NiFe] hydro-genases opens the way for the proposal of catalytic cycles based on the available spectroscopic data on the different active site redox states, namely EXAFS studies that reveal that the Ni-edge energy upon reduction of the enzyme supports an increase in the charge density of the nickel (191). [Pg.395]

Another SBU with open metal sites is the tri-p-oxo carboxylate cluster (see Section 4.2.2 and Figure 4.2). The tri-p-oxo Fe " clusters in MIL-100 are able to catalyze Friedel-Crafts benzylation reactions [44]. The tri-p-oxo Cr " clusters of MIL-101 are active for the cyanosilylation of benzaldehyde. This reaction is a popular test reaction in the MOF Hterature as a probe for catalytic activity an example has already been given above for [Cu3(BTC)2] [15]. In fact, the very first demonstration of the catalytic potential of MOFs had aheady been given in 1994 for a two-dimensional Cd bipyridine lattice that catalyzes the cyanosilylation of aldehydes [56]. A continuation of this work in 2004 for reactions with imines showed that the hydrophobic surroundings of the framework enhance the reaction in comparison with homogeneous Cd(pyridine) complexes [57]. The activity of MIL-lOl(Cr) is much higher than that of the Cd lattices, but in subsequent reaction rans the activity decreases [58]. A MOF with two different types of open Mn sites with pores of 7 and 10 A catalyzes the cyanosilylation of aromatic aldehydes and ketones with a remarkable reactant shape selectivity. This MOF also catalyzes the more demanding Mukaiyama-aldol reaction [59]. [Pg.81]

These catalysts are activated by hydrogenation of the cyclooctadiene ligand, which releases cyclooctane and opens two coordination sites at iridium. The mechanism has been probed by computational studies.40 It is suggested that the catalytic cycle involves... [Pg.386]


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