Kinetic modelling of the catalytic dehydrogenative coupling of butanol catalyzed by a Ru complex: influence of gas-liquid mass transfer of produced hydrogen
Résumé
A growing interest towards efficient catalytic systems has led over the recent years to the development of a new generation of catalysts for alcohol dehydrogenative coupling (ADC).[1] This green, atom-efficient reaction is capable of turning alcohol derivatives into higher value and chemically more attractive ester molecules. In our project, the main objective is to develop a catalytic process maximizing the ester produced without any solvent starting here from butanol into butyl butyrate. To optimize the process, we investigate in this present work a kinetic model describing the activity of several molecular catalysts (Ru-based pincer complexes) on the catalytic ADC (Figure 1) including the coupling with the H2 gas-liquid mass transfer.
According to the detailed reaction scheme presented in Figure 1, we have developed a kinetic model including 5 parameters. Kinetic parameters values have been obtained by adjustment solving a 7 equation differentials according to the mass balance of the semi-batch reactor used in the study (trust-region reflective algorithm for objective function minimization and ode15s MATLAB function for mass balance integration). A statistical analysis is performed in order to estimate the validity of the model and estimated parameters.
Figure 1 : Mechanism proposed for the catalytic dehydrogenative coupling of alcohol
Influence of gas-liquid mass transfer of H2 has been carefully studied and included in the simulation because the concentration of H2 in the liquid phase is impacting the equilibrium between the active (monohydride-amido) and inactive (dihydride-amino) form of the catalyst. Hydrogen kLa has been estimated from a correlation found in the literature. [2]
Figure 2 illustrates the experimental (maker) and fitted (line) concentration of butanol, butyl butyrate, simulated H2 in liquid and gas phase as a function of time. The equilibrium between the two states of catalyst is also predicted by the model (Figure 2 b).
Figure 2 : a) Experimental and theoretical concentration as a function of time; b) Evolution of the concentration of the active and inactive species of catalyst as a function of time
More than 200 catalytic tests with various catalytic system or temperature were modeled using the model developed. By plotting the experimental concentration versus the predicted values, a parity plot is obtained below. According to the parity plot and the statistical/residuals analysis, we have found that the kinetic model was able to properly describe experimental results. The rate constant predicted for the second step is much higher than the one for the first step (480 000 versus 12 000 h-1) reacts quickly to give the butyl butyrate. A study on the impeller rotation rate (300 rpm up to 1800 rpm) has shown an effect on apparent kinetics due to his effect on the initial value of kLa (0.008 versus 0.6 s-1).
Figure 3 : Parity plot of the butanol and the butyl butyrate, experimental concentration as a function of predicted values
Acknowledgments: This work has been performed, in partnership with the SAS PIVERT, within the frame of the French Institute for the Energy Transition (Institut pour la Transition Energétique (ITE) P.I.V.E.R.T. www.institut-pivert.com) selected as an Investments for the Future (âInvestissements dâAvenirâ). This work was supported, as part of the Investments for the Future, by the French Government under the reference ANR-001-01.
[1] C. Gunanathan and D. Milstein, Science, 341 2013, DOI: 10.1126/science.1229712
[2] V. Meille, N. Pestre, P. Fongarland and C. de Bellefon, Ind. Eng. Chem. Res. 43 2004, 924â927.