Palladium, platinum, and ruthenium supported on activated carbon were used as catalysts for the selective hydrogenation of 1-heptyne, a terminal alkyne. All catalysts were characterized by temperature programmed reduction, X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. TPR and XPS suggest that the metal in all catalysts is reduced after the pretreatment with H2 at 673 K. The TPR trace of the PdNRX catalyst shows that the support surface groups are greatly modified as a consequence of the use of HNO3 during the catalyst preparation. During the hydrogenation of 1-heptyne, both palladium catalysts were more active and selective than the platinum and ruthenium catalysts. The activity order of the catalysts is as follows: PdClRX > PdNRX > PtClRX ≫ RuClRX. This superior performance of PdClRX was attributed in part to the total occupancy of the d electronic levels of the Pd metal that is supposed to promote the rupture of the H2 bond during the hydrogenation reaction. The activity differences between PdClRX and PdNRX catalysts could be attributed to a better accessibility of the substrate to the active sites, as a consequence of steric and electronic effects of the superficial support groups. The order for the selectivity to 1-heptene is as follows: PdClRX = PdNRX > RuClRX > PtClRX, and it can be mainly attributed to thermodynamic effects.
BACKGROUND: Heterogeneous hydrogenation catalysts for fine chemical synthesis are a convenient alternative to homogeneous catalysts because of the ease of separation and reuse. In order to be good catalysts they must have high activity and selectivity and good mechanical properties. Appropriate kinetic models should also be available for reactor design.Novel composite supported Pd catalysts were synthesized and tested in the liquid-phase selective hydrogenation of 2,3butanedione to 3-hydroxy-2-butanone (acetoin). The composite support comprised a mixture of an organic polymer and γ -Al 2 O 3 . The support and the Pd catalyst were further characterized by XRD, SEM, EMPA and XPS spectroscopy. Catalytic tests at various conditions were performed in order to elucidate the kinetics of the system.
RESULTS:The composite had better mechanical properties (resistance to radial and axial compression) in comparison with other commercial supports. Good activity and high selectivity to acetoin, a product of partial hydrogenation, were obtained at different reaction conditions. A Langmuir-Hinshelwood chemical rate expression useful for reactor design was regressed from the kinetic data.
CONCLUSIONS:The experimental results could be explained by a Horiuti-Polanyi mechanism in which the addition of an H atom to the carbonyl group in the adsorbed state is the rate limiting step. N Carrara et al.2,3-butanedione to 3-hydroxy-2-butanone has been carried out over composite supported palladium catalysts. Values of conversion to different products as a function of time, temperature, pressure and reactant concentration were used to assess different kinetic models. The models were developed by means of the Langmuir-Hinshelwood-Hougen-Watson (LHHW) methodology and kinetic and adsorption parameters were estimated by regressing experimental data. While the tested models gave insight into the underlying reaction mechanism, the quantitative information on reaction rate parameters is deemed useful for the simulation and optimization of industrial reactors.
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