Solvent effects in liquid-phase reactions II. Kinetic modeling for citral hydrogenation

Abstract The initial liquid-phase reaction to hydrogenate any of the three unsaturated bonds in the citral molecule can be described by a Langmuir–Hinshelwood (LH) mechanism that assumes that molecular citral and H atoms are the two most abundant reaction intermediates. It was applicable with each of the eight solvents studied; however, a wide range of values was obtained for the two adsorption equilibrium constants contained in the optimized rate equation, K cit and K H 2 . Regarding the rate of overall citral disappearance, four possible solvent effects—mass transfer limitations, liquid-phase H 2 solubility, liquid-phase nonideality, and competitive solvent adsorption—were evaluated in detail to see whether one of them could account for the 3-fold variation in turnover frequency and possibly decrease the variation in adsorption equilibrium constants. Using the Weisz–Prater criterion established the absence of mass transfer limitations, using H 2 concentration provided no benefit, and including thermodynamic activity coefficients for citral gave only minimal benefit. However, introducing solvent adsorption into the site balance equation revealed that a narrow range exists for a single apparent rate constant that is applicable for all of the solvents, makes the K H 2 values essentially invariant, and reduces the range of K cit values to a factor of 7. Kinetic studies at three temperatures gave adsorption equilibrium constants that provided consistent, meaningful values for the enthalpy and entropy of adsorption for citral, H 2 , and the solvent. Consequently, this is the best single explanation for the observed kinetic behavior. Finally, individual rates of formation were calculated for the unsaturated alcohol (geraniol and nerol) versus the partially saturated aldehyde (citronellal) and the proposed LH model, either including or excluding competitive solvent adsorption, was able to describe each rate simultaneously using the same optimized adsorption equilibrium constants. Including the solvent in the rate expression again showed that a single set of apparent rate constants can exist that simultaneously describe the three reactions. Thermodynamic consistency of the adsorption equilibrium constants for citral, H 2 , and the solvent was obtained in all cases.