Cyclohexene hydrogenation catalyst characterization

A new class of heterogeneous catalyst has emerged from the incorporation of mono- and bimetallic nanocolloids in the mesopores of MCM-41 or via the entrapment of pro-prepared colloidal metal in sol-gel materials [170-172], Noble metal nanoparticles containing Mex-MCM-41 were synthesized using surfactant stabilized palladium, iridium, and rhodium nanoparticles in the synthesis gel. The materials were characterized by a number of physical methods, showed that the nanoparticles were present inside the pores of MCM-41. They were found to be active catalysts in the hydrogenation of cyclic olefins such as cyclohexene, cyclooctene, cyclododecene, and... 

Table 1, Characterization data on the catalysts turnover rates (molecules exp. atom V1) in the hydrogenation of cyclohexene (chex) and 1-hexene (1-hex) over these catalysts (Boudart s data under similar conditions, Pt/Si02 7.67-9.16 [11], Pd/Si02 6.47-8.25 [12])...

In this equation the Rate is the molar TOF of the reaction, moles of product formed/mole of metal catalyst/unit time. The terms in [ ] are the STO measured site densities given in moles of site/mole of metal. The specific site TOFs, A, B and C, have units of moles of product/mole of site/unit time. Of these factors, the site densities are available from an STO characterization of the catalyst and the Rate is determined for the specific reaction nm over the STO characterized catalyst. When a series of at least three STO characterized catalysts is used for the same reaction, run under the same conditions, the specific site TOFs can be calculated from the simultaneous equations expressed as in Eqn. 3.6. When this approach was used in the hydrogenation of cyclohexene over a series of seven Pt/CPG catalysts specific site TOF values for the Mr and MH sites were found to be 2.1, 18.2 and 5.2 moles of product/mole of site/second, respectively.21 Not surprisingly, that site with the weakly held hydrogen was the most active and that on which the hydrogen was strongly held was the least active. 

Since the overall turnover frequency (TOP) for a hydrogenation reaction is the sum of the TOFs for each type of saturation site as described in Eqn. 3.6, it should be possible to determine the extent of solvent interaction with different types of sites by calculating the specific site TOFs for the same reaction run in different solvents. These data are listed in Table 5.3 for the hydrogenation of 4-methyl-1-cyclohexene over a series of STO characterized Pt/Si02 catalysts. ... 

Cerveny et al. (69), attempted to apply the Drougard-Decroocq equation (34) to the data obtained in the hydrogenation of cyclohexene and 1-hexene on the catalyst 5% Pt on silica gel in 19 solvents. Since correlation of the reaction rates with the parameters of solvents x, originally obtained for the homogeneous Menschutkin reaction (88) between methyl iodide and tri-propylamine was unsuccessful, an analogous definition was used for the parameters x, which were to characterize the solvent with respect to its effect in heterogeneously catalyzed reactions ... 

Benzyl phenyl sulphide, norbornene, cw-cyclooctene, and 4-vinyl-1-cyclohexene were obtained from Aldrich and (IS)-(-)-a-pinene from Fluka. Phenyl sulphide was prepared from benzene and sulphur chloride following the literature procedure[9]. Reference samples of sulphoxides and sulphones were prepared by oxidation of sulphides with sodium periodate[10] and hydrogen peroxide[ll] respectively. Reference samples of epoxides were made by following Kaneda et al.[ 2 procedure. Metal phthalocyanines[13] were prepared from appropriate metal salt, 1,2-dicyanobenzene with ammonium molybdate as catalyst and were characterized by elemental analysis. 

The catalyst were characterized by inductively coupled plasma spectrometry (Jarrell-Ash ICAP-757), TEM (Hitachi H-800), EDS (Horiba EMAX-3000), XRD (MAC Science MXP 18), XPS (Shimadzu ESCA 850), and adsorption of hydrogen (Bel Japan Belsorp 36), water, benzene and cyclohexene (Microscal Microcalorimeters Mark-3V). 

Bcrkani ct al. [222] proposed the transformation of an alcohol (cyclopenianol) in the presence of a ketone (cyclohexanone) as a model reaction to characterize the acidity and the basicity of oxide catalysts (Figure 3). Actually, it is a Meerwein-Ponndorf-Verley (MPV) reduction which will be discussed in section 5. The authors found a good correlation between basicity of NaCsX zeolites or K impregnated alumina and their activities for hydrogen transfer (monitored by cyclohexanol or cyclohexene production). On the other hand, the more acidic the catalyst, the higher the dehydration extent. 

Step 6 in the catalytic cycle (boxed) represents substrate insertion and is invariably the rate-controlling step. Thus, let us consider the hydrogenation of cyclohexene by Wilkinson s catalyst The catalytic cycle in Hgure 15.4 shows that aU steps other than step 6 are characterized as last or having reached equilibrium. Hence, the rate-determining step of the cycle is step 6. 

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