Catalysts active site

As discussed in Chapter 2, nickel, vanadium, and sodium are the metal compounds usually present in the FCC feedstock. These metals deposit on the catalyst, thus poisoning the catalyst active sites. Some of the options available to refiners for reducing the effect of metals on catalyst activity are as follows ... 

The performance of various solvents can be explained with the help of the role of these solvents in the reaction. These solvents help in keeping teth benzene and hydrogen peroxide in one phase. This helps in the easy transport of both the reactants to the active sites of the catalyst. The acetonitrile, and acetone adsorption data on these catalysts (Fig. 6), suggests that acetonitrile has a greater affinity to the catalytic surface than acetone. There by acetonitrile is more effective in transporting the reactants to the catalyst active sites. At the same time, they also help the products in desorbing and vacating the active sites. 

The sterically unencumbered catalyst active site allows the copolymerization of a wide variety of olefins with ethylene. Conventional heterogeneous Ziegler/Natta catalysts as well as most metallocene catalysts are much more reactive to ethylene than higher olefins. With constrained geometry catalysts, a-olefins such as propylene, butene, hexene, and octene are readily incorporated in large amounts. The kinetic reactivity ratio, rl, is approximately... 

The photocatalytic oxidation of alcohols constitutes a novel approach for the synthesis of aldehydes and acid from alcohols. Modification of Ti02 catalyst with Pt and Nafion could block the catalyst active sites for the oxidation of ethanol to CO2. Incorporation of Pt resulted in enhanced selectivity towards formate (HCOO ad)-Blocking of active sites by Nafion resulted in formation of significantly smaller amounts of intermediate species, CO2 and H2O, and accumulation of photogenerated electrons. The IR experimental teclmique has been extended to Attenuated Total Reflectance (ATR), enabling the study of liquid phase photocatalytic systems. 

This preliminary screening gives the following activity order for hydrogenation of Q to DHQ, likely providing evidence for the capacity of the amine to inhibit deactivation by the intermediate(s) on the catalyst active sites ... 

The results obtained with different amines cannot be explained merely on the effects of amine basicity. Thus, to obtain complete hydrogenation of Q to DHQ, the basicity has to be tailored by other factors such as the steric hindrance of the amine and its electronic interaction with the catalyst active sites this seems to be favored by the presence of an electron-rich aromatic ring. Of note, the positive effect of substituted aromatic amines, with a 49% DHQ yield being obtained for ethylanilines, is independent of the substituent position of the alkyl group. 

Cavitation induced turbulence also enhances the rates of the desorption of intermediate products from the catalyst active sites and helps in continuous cleaning of the catalyst surface. 

Based on the experimental data and some speculations on detailed elementary steps taking place over the catalyst, one can propose the dynamic model. The model discriminates between adsorption of carbon monoxide on catalyst inert sites as well as on oxidized and reduced catalyst active sites. Apart from that, the diffusion of the subsurface species in the catalyst and the reoxidation of reduced catalyst sites by subsurface lattice oxygen species is considered in the model. The model allows us to calculate activation energies of all elementary steps considered, as well as the bulk... 

The high values of CH4/total hydrocarbons ratio observed for C-155 and SC-155, suggest that when CO and H2 reach the catalyst active sites and the reactions proceed CH4 is kinetically the first product obtained, and it can leave immediately the catalyst due to the big dimensions of the catalyst pores, denoting a low diffusional resistance for this process. 

Thus, under conditions of our flow experiments, butyl alcohol is present in two forms (i) hydrogen-bonded to the catalyst active site and (ii) nonbonded to the active site. Knowing the pore volumes, we arrive at the concentration of adsorbed n-butyl alcohol at 399 K for HZSM-5 and AAS, respectively, of 4.7 X 1021 mol cm 3 and 6.5 X 1020 mol cm 3. Liquid n-butyl alcohol at 298 K contains 6.6 X 1021 mol cm 3. 

However the sample is prepared, we measure 13C spectra of one or more adsorbates on the catalyst, and then need to interpret the spectra to deduce the structure of adsorption complexes or reactive intermediates formed on the catalyst. In many cases the complexes and intermediates formed are unusual and exotic species for which the interpretation of the spectra may be far less than routine. This is where ab initio chemical shift calculations are essential. In diffraction methods, such as x-ray or neutron diffraction, one can more-or-less easily invert the experimental data to yield molecular structure. There is no straightforward relationship between chemical shift data and structure theoretical calculations provide the bridge between experiment and theory. In a typical study, we model the adsorbates on clusters that represent catalyst active sites, using experience and chemical intuition to create our initial structures. 

Due to their multi-sited nature, Ziegler-Natta and chromium catalysts produce structurally heterogeneous ethylene homo- and copolymers. This means that the polymers have broad MWD and broad composition (short-chain branching) distribution (Fig. 9). Catalyst active sites that produce lower molecular weights also have a tendency to incorporate more comonomer... 

Figure 12.18 Strategies for modifying selectivity in catalysis - the sphere represents the catalyst active site (e.g. metal centre).31...

Figure 12.20 A designer C H oxidation catalyst the positions the reactive CH-bond over the catalyst active site using molecular recognition. The ibuprofen substrate is oxidised to the 2-(4-Isobutyryl-phenyl) -propionic acid product in > 98 % selectivity (reproduced by permission of The Royal Society of Chemistry).

We can also distinguish a few lines of approach regarding catalyst poisons which deactivate the catalyst by coke formation, that is by blocking of pores and the catalyst active sites,... 

A common theme in this catalysis has been the proposal of dinuclear catalyst active sites, often termed bimetallic catalysis. This section will review various dinuclear and dimeric complexes which have shown activity for copolymerization. 

The main elements of chirality possibly present in the intermediates and transition states that can be hypothesised within this framework are as follows [1]. Firstly, a prochiral a-olefin molecule, e.g. propylene, coordinating via its two faces at the catalyst active site gives rise to non-superpo sable re and si diastereoisomeric complexes (Figure 3.24) [362, 363]. According to the considered mechanisms, an isotactic polymer is generated by a long series of... 

The following mesomeric forms, which can be proposed for the complex of the epoxide with the catalyst active site, seem to present the nature of the active species in a more illustrative way [74] ... 

The ruthenium(II) - ethylene episulphide complex [152] may serve as a model for the coordination of tiiranes at the catalyst active site. 

Heterounsaturated monomers that undergo coordination polymerisation or copolymerisation with other monomers can be divided into two classes monomers with a carbene-like structure such as isocyanides and carbon monoxide which are coordinated by n complex formation with the transition metal atom at the catalyst active site, and monomers such as isocyanates, aldehydes, ketones and ketenes which are coordinated via 5-bond formation with the metal atom at the catalyst active site. 

The loss of catalyst activity through metal deposition can be attributed to the interaction of the deposited metals with the original active sites of the catalyst ( active site poisoning ) and the loss of pore volume due to the obstruction of catalyst pores ( pore plugging ) as depicted in Figure 1. 

It is generally accepted that the most valuable information on polymerization mechanism can be obtained by studying the polymer chain unit adjacent to a catalyst active site since the latter determines the structure of polymer chain formed. 

Banares, M.A., Martfnez-Huerta, M.V., Gao, X., Fierro, J.L.G., and Wachs, I.E., in "Metal oxide catalysts active sites, intermediates and reaction mechanisms", Symposium, 220th ACS National Meeting, Washington, USA (2000d). 

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