Dynamic catalytic studies

Uopis, F.J., Sastre, G., and Corma, A. (2006) Isomerization and disproportionation of m-xylene in a zeolite with 9- and 10-membered ring pores molecular dynamics and catalytic studies. J. Catal., 242, 195-206. 

We describe the development of in situ (dynamic) ETEM for direct imaging of CS defects in dynamic catalytic oxides in chapter 3. These studies have recently led to better insights into the formation of CS planes (leading to further developments in the dislocation model) and their role in oxidation catalysis. By directly probing the formation of CS planes and their growth by in situ ETEM... 

In situ dynamic ETEM studies in controlled environments of oxide catalysts permit direct observations of redox pathways under catalytic reaction conditions and provide a better fundamental understanding of the nucleation, growth and the nature of defects at the catalyst surface and their role in catalysis (Gai 1981-1982 92). The following paragraphs describe the methods of observation and quantitative analyses of the surface and microstmctural changes of the catalyst, and correlation of microstmctural data with measurements of catalytic reactivity. We examine examples of pure shear and crystallographic (CS) shear defects that occur under catalytic conditions. 

Solvent polarity is known to affect catalytic activity, yet consistent correlations between activity and solvent dielectric (e) have not been observed [12,102]. However, a striking correlation was found between the catalytic efficiency of salt-activated subtilisin Carlsberg and the mobility of water molecules (as determined using NMR relaxation techniques) associated with the enzyme in solvents of varying polarities (Figure 3.11) [103]. As the solvent polarity increased, the water mobility of the enzyme increased, yet the catalytic activity of the enzyme decreased. This is consistent with previous EPR and molecular dynamics (MD) studies, which indicated that enzyme flexibility increases with increasing solvent dielectric [104]. 

The dynamic XAFS study of the catalytically active structures under the reaction conditions should enable us to further develop novel catalysts for direct phenol synthesis from benzene and the efficient activation of molecular oxygen. 

Jolly and co-workers thoroughly investigated the Pd-catalyzed reactions of 1,3-dienes and provided much of the basis for the present understanding of the catalytic cycle of Pd-catalyzed telomerization reaction depicted above [47, 49-52], Several intermediates were observed in solution, and their dynamics were studied using NMR spectroscopy and some even isolated, as complexes of type B could, for instance, be studied by single crystal X-ray diffraction. 

Baraldi and co-workers [52] have described a wealth of dynamic XPS studies on surface reactions, including adsorption, dissociation, desorption, and even catalytic reactions, such as the epoxidation of alkenes [60], and the reduction of NO by H2 and CO [61]. 

The further step of monitoring a dynamic catalytic reaction on a surface is barely approachable with existing surface science method by using molecular beam techniques, or by isolating a model catalysts prepared and analysed in UHV in a reactor appended to the UHV system. Although such studies apparently have not been performed for ceria supported model catalysts, it is appropriate to mention reactor studies of CO oxidation and water gas shift d reactions performed... 

Yang, X. Stern, C.L. Marks, T.J. Cationic zirco-nocene olefin polymerization catalysts based on the organo-Lewis acid tris(pentafluorophenyl)-borane. A synthetic, structural, solution dynamic, and polymerization catalytic study. J. Am. Chem. Soc. 1994, 116, 10,015. 

The interaction of nitric oxide (NO) with metal ions in zeolites has been one of the major subjects in catalysis and environmental science and the first topic was concerned with NO adsorbed on zeolites. NO is an odd-electron molecule with one unpaired electron and can be used here as a paramagnetic probe to characterize the catalytic activity. In the first topic focus was on a mono NO-Na" complex formed in a Na -LTA type zeolite. The experimental ESR spectrum was characterized by a large -tensor anisotropy. By means of multi-frequency ESR spectroscopies the g tensor components could be well resolved. The N and Na hyperfine tensor components were accurately evaluated by ENDOR spectroscopy. Based on these experimentally obtained ESR parameters the electronic and geometrical structures of the NO-Na complex were discussed. In addition to the mono NO-Na complex the triplet state (NO)2 bi-radical is formed in the zeolite and dominates the ESR spectrum at higher NO concentration. The structure of the bi-radicai was discussed based on the ESR parameters derived from the X- and Q-band spectra. Furthermore the dynamical ESR studies on nitrogen dioxides (NO2) on various zeolites were briefly presented. 

Importantly, and unlike potentiometry, voltammetric methods are dynamic and give information on kinetics, that is, rates of electron transfer and coupled (EC) reactions the latter include those in which electron transfer drives a reaction such as ion/proton transfer, or is gated , that is, the case in which the electron-transfer event is controlled by a preceding chemical process. Redox reactions can be quantified in both the potential and time domains, and these may be separated and resolved for example, steady-state catalytic studies of adsorbed enzymes reveal how catalytic electron transport varies as a function of potential, which can be important if the rate is sensitive to the oxidation state of a particular site in the molecule [1]. 

This chapter includes a discussion of the general principles of SSITKA as well as the experimental and theoretical approaches that are used to study the isotopic transient kinetics, reactions of label transfer as applied to the problem to be solved. The dynamic features of the experimental setup that includes the stepwise disturbing system of feed gas flows, catalytic reactors, and mass spectrometric analysis are considered in detail. As a result, the reactor and mass spectrometers that are most convenient for dynamic isotopic studies were selected. The theoretical study of isotopic transient... 

In order to further investigate the catalytic behavior of some transition metal AcAc s, kinetic studies were conducted on EPON 828/HMPA/metal AcAc system. First, a DSC dynamic cure profile at a heating rate of 5 X/min was obtained for each EPON 828/ HMPA/ metal AcAc formuIatiorL Following the dynamic cure studies, isothermal cure behavior of EPON 828/ HMPA/ metal... 

Metal oxos are among the most dynamic, well-studied examples of PCET and continue to attract the attention of the community at large. The oxo basicity and oxidative power of the metal contribute significantly to the BDFEs involved in PCET processes, which is further influenced by the coordination geometry of the 0X0 as well as the ancillary ligation on the metal center. The development of catalytic protocols utilizing such species bears much promise for synthetic chemists for mild substrate modification. 

The coordination chemistry of sumanene (1) reported to date was reviewed here. Stepwise selective benzylic lithiation of 1 was presented. The benzylic anion species exhibits the bowl-to-bowl inversion. Recent study on 1 revealed that the single crystal of 1 shows high electron transport ability with anisotropy [52]. In the prospective view, trapping of such anion is considered to enable various substitutions at the benzylic positions stereoselectively, which is one of the promising approaches to create the functional materials based on 1. Complexation with CpFe+ demonstrated selective formation of the first concave-bound complex, which is expected to lead to the inclusion complexes of n bowls. The inversion behavior observed in the CpRu+ complex may provide the idea of a dynamic catalytic system. Thus, some characteristic features of sumanene complexes are becoming apparent. In the future, n bowls such as 1 are expected to provide novel electrical materials, organometallic catalysts, etc. 

Clusters are intennediates bridging the properties of the atoms and the bulk. They can be viewed as novel molecules, but different from ordinary molecules, in that they can have various compositions and multiple shapes. Bare clusters are usually quite reactive and unstable against aggregation and have to be studied in vacuum or inert matrices. Interest in clusters comes from a wide range of fields. Clusters are used as models to investigate surface and bulk properties [2]. Since most catalysts are dispersed metal particles [3], isolated clusters provide ideal systems to understand catalytic mechanisms. The versatility of their shapes and compositions make clusters novel molecular systems to extend our concept of chemical bonding, stmcture and dynamics. Stable clusters or passivated clusters can be used as building blocks for new materials or new electronic devices [4] and this aspect has now led to a whole new direction of research into nanoparticles and quantum dots (see chapter C2.17). As the size of electronic devices approaches ever smaller dimensions [5], the new chemical and physical properties of clusters will be relevant to the future of the electronics industry. 

We have previously calculated conformational free energy differences for a well-suited model system, the catalytic subunit of cAMP-dependent protein kinase (cAPK), which is the best characterized member of the protein kinase family. It has been crystallized in three different conformations and our main focus was on how ligand binding shifts the equilibrium among these ([Helms and McCammon 1997]). As an example using state-of-the-art computational techniques, we summarize the main conclusions of this study and discuss a variety of methods that may be used to extend this study into the dynamic regime of protein domain motion. 

The Car-Parrinello quantum molecular dynamics technique, introduced by Car and Parrinello in 1985 [1], has been applied to a variety of problems, mainly in physics. The apparent efficiency of the technique, and the fact that it combines a description at the quantum mechanical level with explicit molecular dynamics, suggests that this technique might be ideally suited to study chemical reactions. The bond breaking and formation phenomena characteristic of chemical reactions require a quantum mechanical description, and these phenomena inherently involve molecular dynamics. In 1994 it was shown for the first time that this technique may indeed be applied efficiently to the study of, in that particular application catalytic, chemical reactions [2]. We will discuss the results from this and related studies we have performed. 

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