Kinetics of liquid-phase hydrogenation reactions over supported metal catalysts — a review

Abstract

The kinetics of liquid-phase hydrogenation reactions have been reviewed with a special emphasis on α, β-unsaturated aldehydes as reactants. These reactions can be complex and can be influenced by factors such as metal specificity, side reactions, and metal-support interactions as well as reaction parameters. The importance of results in the absence of heat and mass transfer limitations has been emphasized. Hydrogenation reactions are typically assumed to be structure-sensitive, but dependencies on metal crystallite size have been reported; however, this behavior has been attributed to side reactions which can inhibit activity. Finally, solvent effects can exist, but the effect of H₂ concentration in the liquid phase has infrequently been isolated. Thermodynamic arguments indicate that a solvent effect can enhance the surface coverage of hydrogen on the catalyst surface at a constant H₂ partial pressure, but in the absence of any solvent effects, surface coverage is independent of the liquid-phase H₂ concentration.

Introduction

The application of catalytic techniques for the production of fine chemicals and pharmaceuticals is important, especially with increased environmental awareness. Table 1 displays order of magnitude estimates for the quantity of by-products formed per kg of product, termed the E-factor [1]. An E-factor of 1–5 is common in the bulk and commodity chemicals industry, and some petrochemical processes can exhibit values as low as 0.1. In contrast, significantly higher E-factors are encountered in the specialty chemical and pharmaceutical sectors where values as high as 50–100 can be obtained. Such a large E-factor has been attributed, at least in part, to multi-step syntheses using stoichiometric reagents that result in accumulation of inorganic salts that need to removed from the final product. Admittedly, the use of the E-factor alone is a gross oversimplification, nevertheless, these values highlight the need for development of selective catalytic routes for transformations relevant in fine chemicals and pharmaceuticals. Significant efforts are focused on development of homogeneous catalytic techniques that offer significant potential because they can be molecularly tuned through ligand modification. Molecular tuning of heterogeneous catalysts is more difficult; however, such catalysts have enormous advantages compared to their homogeneous counterparts in terms of ease of handling, separation, catalyst recovery, and regeneration that make them industrially attractive.

Hydrogenation reactions are a class of reactions that are valuable in the pharmaceutical and specialty chemical industry [2], [3], [4], [5], [6]. A review of heterogeneously catalyzed liquid-phase hydrogenation reactions appears to be a daunting task; however, the focus of this review will be restricted to the kinetics of such reactions with a primary emphasis on hydrogenation of α,β-unsaturated aldehydes over supported metal catalysts. A survey of the literature regarding liquid-phase hydrogenation reactions reveals that much of the work performed in this area has been aimed at studying selectivity issues and only scant quantitative kinetic data are reported. This is very apparent for liquid-phase hydrogenation of α,β-unsaturated aldehydes, a field of chemistry which has been extensively studied for over a decade, but only recently has had quantitative kinetics determined. These results with Pt/SiO₂ showed an unusual effect of temperature on reaction rate and product distribution [7], [8]. Augustine reviewed heterogeneously catalyzed hydrogenation reactions from a selectivity standpoint and outlined a number of different hydrogenation reactions; however, reaction kinetics were not addressed [9]. A kinetic approach to a better mechanistic understanding of liquid-phase hydrogenation reactions has already proven to be valuable [7], [8], [10], [11], [12], and the discussion that follows will highlight some of the important aspects describing the kinetic behavior of liquid-phase hydrogenation reactions.