Solvent effects in liquid-phase hydrogenation reactions

The significance of solvent effects in liquid-phase hydrogenation reactions has been investigated in detail by studying both the kinetic behavior and mass-transfer aspects related to citral hydrogenation on a Pt/SiO 2 catalyst. The possible effect of the solvent on the diffusion characteristics inside the porous silica catalyst was analyzed thoroughly using a Weisz-Prater (W-P) criterion. The conventional W-P criterion was suitably adapted for this citral hydrogenation reaction and, based on the procedure developed in this study, several other hydrogenation systems from the literature were critically analyzed to underscore the importance of diffusional limitations in porous catalysts.; The initial turnover frequency (TOF) for citral hydrogenation exhibited a nearly three-fold variation among the different solvents at 373 K. This variation in initial TOFs did not correlate with either the solvent dielectric constant or its permanent dipole moment. The kinetics of citral hydrogenation based on initial TOFs at different reaction conditions and in different solvents were modeled using a standard Langmuir-Hinshelwood (L-H) mechanism involving one type of active site. The model parameters representing the various kinetic constants involved in the reaction mechanism showed significant variations among the different solvents which could not be explained by either the variation of H2 solubility in these different media or the solution-phase non-ideality existing with different citral-solvent mixtures. The inclusion of a solvent co-adsorption step in the proposed L-H kinetic mechanism, with an objective of explaining the observed solvent effects, also could not account for this behavior because it failed to yield solvent-independent rate constants. Finally, the traditional choice of an ideal catalyst surface for the L-H mechanism was questioned, and consequently, non-ideal surface sites were introduced into the existing reaction model to formulate a modified rate expression. By modeling the kinetic data with this modified rate expression, some knowledge was gained about the relative non-idealities of adsorbed citral and hydrogen in different solvents. Non-idealities associated with surface phenomena that were inherently lumped into the adsorption equilibrium constants were isolated and, as a result, the terms representing these non-idealities in the catalyst system appear to influence the outcome of the kinetic rate constants with different solvents.