Kinetics of the liquid phase selective hydrogenation of 2,3‐butanedione over new composite supported Pd catalysts

Abstract

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.

Kinetic study of the selective hydrogenation of styrene over a Pd egg-shell composite catalyst

“…For this reason only the most relevant data necessary for interpreting the kinetic results will be repeated here. For a more detailed description of the characterization of these composite catalysts the reader is referred to our recent reports [11,20].…”

“…According to the theory of slurry reactors the interplay of the different resistances would be described by equations (19)(20)(21) for the reaction rate of a system with pseudo first order dependency with respect to the concentrations of dissolved hydrogen [25]. In equation (19) k L is the liquid film transfer coefficient, k c is the liquid-solid film transfer coefficient, a c is the external catalyst surface, a g is the gas-liquid interface area, m c is the catalyst concentration, k is the pseudo first order global kinetic constant and ξ is the intraparticle reaction efficiency.…”

“…As it can be seen from equations (19)(20)(21)) the intercept of the line of (C H2(liq) /r) as a function of 1/m c , should be equal to R G-L , the gas-liquid mass transfer resistance. A plot of this kind was made using initial reaction rate data.…”

“…(27) In equation (27) the concentration of the intermediate half hydrogenated species is considered to be negligible. This is because the half hydrogenated species is not stable and should rapidly accept another hydrogen while it is still in the adsorbed state [20].…”

“…Singh and Vannice [42] noted that hydrogen adsorption is weak over Pd δ+ due to the depletion of d electrons by adjacent electronegative species. The electrodeficiency of Pd would also reduce the equilibrium concentration of available surface atomic hydrogen and would slow down the hydrogenation steps, probably favoring in this way the existence of the half hydrogenated intermediate [20]. Electrodeficiency would also enhance the adsorption of styrene and ethylbenzene.…”