Solvent effects in the hydrogenation of 2-butanone

doi:10.1016/j.jcat.2012.01.011

Journal of Catalysis

Volume 289, May 2012, Pages 30-41

https://doi.org/10.1016/j.jcat.2012.01.011 Get rights and content

Abstract

In liquid-phase reaction systems, the role of the solvent is often limited to the simple requirement of dissolving and/or diluting substrates. However, the correct choice, either pure or mixed, can significantly influence both reaction rate and selectivity. For multi-phase heterogeneously catalysed reactions observed variations may be due to changes in mass transfer rates, reaction mechanism, reaction kinetics, adsorption properties and combinations thereof. The liquid-phase hydrogenation of 2-butanone to 2-butanol over a Ru/SiO₂ catalyst, for example, shows such complex rate behaviour when varying water/isopropyl alcohol (IPA) solvent ratios. In this paper, we outline a strategy which combines measured rate data with physical property measurements and molecular simulation in order to gain a more fundamental understanding of mixed solvent effects for this heterogeneously catalysed reaction. By combining these techniques, the observed complex behaviour of rate against water fraction is shown to be a combination of both mass transfer and chemical effects.

Graphical abstract

The complex rate behaviour observed during the hydrogenation of 2-butanone using water/IPA mixtures is attributed to a combination of mass transfer and structural dynamics effects. This work highlights the potential beneficial role of water in both lowering the activation barrier as well as increasing the proton diffusion coefficient.

Highlights

► We investigate mixed solvent effects during the hydrogenation of 2-butanone. ► Complex rate/composition behaviour is observed and correlated to mass transfer and kinetics. ► Strong correlation found between hydrogen bonding structure and increased reaction rates. ► Results show that catalytic performance can be tailored through controlling structural dynamics.

Introduction

The ability of a solvent to influence either the rate or selectivity of a reaction has been known within organic chemistry for 150 years [1]. Over that time there have been a number of attempts to understand the role of the solvent with methods including multiple linear regression analysis, factor analysis and principal component analysis all being used in an attempt to develop some level of understanding and predictability of these effects. Such analysis tends to rely on various free energy relationships, and despite the complicated nature of solvents and solutions, they have shown that they can provide some insight into many chemical processes [1].

Like the organic reactions above, many heterogeneously catalysed processes are carried out in a solvent for the simple purpose of dissolving the reactants and keeping the products in solution. Here too, solvents are known to influence both rate and selectivity although in such systems the multi-phase nature of the reactions increases their complexity. Therefore, in addition to factors such as solvent polarity, dielectric constant and acid/base properties of the reaction medium, factors such as the solvation of reactants and products, gas solubility and other mass transfer effects need to be considered as these can all significantly influence reaction rates and product selectivities [2]. Other important criteria for consideration include the potential for improved heat transfer and the influence on deactivation such as reduced carbon laydown on the catalyst surface [3].

Toukonitty et al. for example investigated solvent effects for the enantioselective hydrogenation of 1-phenyl-1,2-propanedione with a cinchonidine modified Pt/Al₂O₃ catalyst [4]. When using a range of different solvents, including binary solvent mixtures, it was observed that while no correlation between the solvent dielectric constant and hydrogenation rate could be found the enantiomeric excesses decreased non-linearly with an increasing solvent dielectric constant. This dependence was partly attributed to the open (3) cinchonidine conformer, although it was primarily taken into account by applying transition state theory and the Kirkwood treatment with the result that a model was able to predict the behaviour of the system as a function of the solvent dielectric constant. Contrasting with this work, Mukherjee and Vannice later reported on solvent effects during the liquid-phase hydrogenation of citral using Pt/SiO₂ and eight nonreactive solvents [5]. Here it was observed that the rates were affected by the choice of solvent; however, differences in the product distribution were not significant. In this case, it was found that the variation in specific activity did not correlate with either the solvent dielectric constant or the dipole moment. Similarly, Gómez-Quero et al. studied solvent effects during the liquid-phase hydrodehalogenation of haloarenes in methanol, THF, water/methanol and water/THF mixtures using a Pd/Al₂O₃ catalyst [6]. Within this work they noted that higher initial rates were observed with increasing water content in the solvent mixture and attributed this to an increase in the dielectric constant of the medium. Such observations were consistent with an electrophilic mechanism in which the solvent helped to stabilise the arenium intermediate. An overall dependence of rate on solvent for this reaction could then be established with approximately 80% of the contribution being due to the dielectric constant with molar volume being a secondary factor. Mixed alcohol/water solvents, the subject of this work, have also been investigated previously. For example, a minimum in the rate of 2-butene-1,4-diol hydrogenation (the second step in the hydrogenation of 2-butyne-1,4-diol over Pd/Al₂O₃) occurred at 80–90 mol_H2O% in a 2-propanol/water solvent [7]. Elsewhere, an enhancement in the rate of reaction in mixed alcohol/water solvents as compared to the pure alcohol has been observed in 2-butanone hydrogenation over Pd catalysts [8]; acetophenone hydrogenation over Raney-Ni [9]; and o-nitrotoluene hydrogenation over Pd/C [10]. In the latter example, an increase of almost 50% in the reaction rate was reported upon changing solvent from pure methanol to a mixed methanol/water solvent containing 18% water.

As mentioned above, other factors are also important, for example, polar solvents are known to enhance the adsorption of non-polar reactants and non-polar solvents the adsorption of polar reactants [11]. More recently, Vanoye et al. observed an interesting inflection in the initial rates obtained using a mixed ethanol/heptane solvent system for the liquid-phase dehydration of ethanol to diethylether over heterogeneous sulphonic-acid catalysts [12]. In this case, the authors concluded that the observed inflection in non-polar solvents could be explained by the formation of a new liquid phase around the acid site indicating that local structure is also important. It is quite clear from the above discussion that the role of the solvent is less well defined than in normal organic reactions with increases in rate being observed in some systems, and selectivity in others.

In briefly reviewing selective hydrogenation reactions over ruthenium catalysts which relate to the work discussed herein, it is further noted that carbonyl hydrogenations are particularly active in the presence of water [13]. Conversely, in the hydrogenation of benzene over Ru/C, water was found to have an adverse effect on the reaction rate [14]. Similar effects of water have also been observed by Vaidya and Mahajani in the hydrogenation of n-valeraldehyde to n-amyl alcohol over Ru/Al₂O₃; however, no explanation was given for its influence [15]. A possible reason for enhanced activity in water may be due to its ability to dissociate over different metal surfaces. The dissociation of water over Ru is well known in vapour phase studies as well as in aqueous solutions; this leads to the formation of surface hydroxyl intermediates and protons which can subsequently influence the reaction [16], [17].

While such effects are known, to our knowledge there are no studies which extensively examine the role of water in a mixed solvent system covering its effect on mass transfer, diffusion and reaction kinetics. Therefore, a detailed study of the influence of water over diffusion, adsorption, desorption and elementary reaction steps is necessary to improve our current understanding of the role played by the solvent in altering reaction activity and selectivity. In this paper, we attempt to investigate such effects and have probed the role of water on the hydrogenation of 2-butanone (methyl ethyl ketone (MEK)) to 2-butanol using a combination of experiments and density functional theory. These results indicate that the rate of hydrogenation of MEK is strongly correlated to the solvent composition and that water plays an important role in altering energetics and kinetics of the elementary steps involved in the overall hydrogenation.

Section snippets

Materials and methods

Catalysts comprised of 1% and 5% Ru/SiO₂ and were prepared by incipient wetness using an aqueous ruthenium (III) chloride trihydrate salt (density 2250 ± 100 kg m⁻³). They were dried at 393 K in air and subsequently reduced in 5% H₂/95% He at 673 K for 3 h. Kinetic experiments were carried out in a 380 ml Premex stainless steel reactor, equipped with a gas inducing impeller with online hydrogen consumption monitoring. The solvents used were ultra-pure water (distilled, deionised >18 MΩ) and/or

Results and discussion

As shown in Table 1, tests using a range of four different solvents with 5 wt% Ru/SiO₂ at 303.15 K and 1400 rpm demonstrate a significant variation in initial rate with water clearly producing a value over 33 times larger than that observed in methanol. Such results indicate that under the conditions used here no correlation between the dielectric constant and initial rate exists. While it cannot be guaranteed that these experiments were carried out in the kinetic regime, the fact that water has

Conclusions

The results described herein have shown that there is a significant variation in the observed rate of the MEK hydrogenation reaction when using 5% Ru/SiO₂ in different solvents. In particular, a complex behaviour is observed for varying water/IPA mole fractions. Using experimental and estimated mass transfer rates we have shown that while gas–liquid mass transfer also varies significantly with water mole fraction, it, like the liquid–solid mass transfer, is not significant at the scale of the

Acknowledgments

The authors would like to thank the EPSRC under Grant Number GR/S43702/01 and Johnson Matthey for funding this work. The molecular modelling work was done using computational time at the National Centre for Computational Sciences (NCCS) at Oak Ridge National Laboratory and the Environmental Molecular Sciences Laboratory (EMSL) at Pacific Northwest National Laboratory. Both of these are national scientific user facilities sponsored by the Department of Energy’s Office of Science. The authors are

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