Ind. Eng. Chem. Res. 2009, 48, 10335–10342
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Kinetics of Cinnamaldehyde Hydrogenation by Supported Ionic Liquid Catalysts (SILCA) Pasi Virtanen,*,† Tapio Salmi,† and Jyri-Pekka Mikkola†,‡ Laboratory of Industrial Chemistry and Reaction Engineering, Process Chemistry Centre, Åbo Akademi UniVersity, Piispankatu 8, FI-20500 Turku/Åbo, Finland, and Technical Chemistry, Department of Chemistry, Chemical-Biological Center, Umeå UniVersity, SE-90187 Umeå, Sweden
The research of ionic liquids and their applications in catalysis are attracting increasing attention in chemistry and chemical engineering. A supported ionic liquid catalyst (SILCA) consists of immobilized catalytic species (e.g., transition-metal particles, metal complexes, or enzymes) residing in an ionic liquid layer immobilized on a porous solid support. The kinetics of cinnamaldehyde hydrogenation over SILCAs that contained palladium nanoparticles in ionic liquid, which, in turn, was immobilized on active carbon cloth (ACC), was studied and modeled in detail. A mechanistic kinetic model, which describes the differences of the activity and selectivity of the catalysts consisting of different ionic liquids, was developed. The model explained the experimental results well. 1. Introduction Ionic liquids (ILs) are composed of ionic compounds, meaning that they are formed exclusively of charged species, cations and anions, but can also be comprised of, e.g., zwitterions. Typically, ILs are formulated as a combination of a large organic cation with the possibility of charge delocalization and, consequently, a relatively large inorganic or organic anion. They have many exceptional properties compared to conventional ionic compounds or salts. The main difference is their significantly lower melting point. For example, sodium chloride has a melting point of 801 °C, whereas many ILs have a melting point below room temperature. Other special characteristic features that can be attributed to most ILs often include the following: they have a negligible vapor pressure (∼10-8 bar), they are liquid over a wide temperature range, they have unique solvation properties (implying that a wide variety of polar and nonpolar compounds are soluble in ILs), and they have a wide electrochemical window and good ion conductivity.1-3 Still, it should be kept in mind that these features are not common to each ionic liquid, as has been noted by Deetlefs and Seddon.4 Although ILs have exhibited their abilities in various chemical applications,5 the widespread use of ILs in industrial applications is often compromised by the cost aspects, as well as limited knowledge of their physical and chemical properties and concerns about their biodegradability. Research in the field of ILs is continuously attracting more interest in scientific world. Many new applications and improved techniques have already been introduced, such as, e.g., the Basil process by BASF, the Difasol process by IFP, and the alkylation of isobutene by PetroChina.5 Catalysis is one of the areas where ILs have already shown their potential in enhancing the rates of many reactions.6-8 In fact, catalysis is one of the applications where ILs present considerable potential. Thus, supported ionic liquid catalysts (SILCAs) with a catalytic amount of an IL immobilized on a solid surface represent a noteworthy alternative from an industrial point of view.9-16 * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: +358 2 2154479. † Laboratory of Industrial Chemistry and Reaction Engineering, Process Chemistry Centre, Åbo Akademi University. ‡ Technical Chemistry, Department of Chemistry, Chemical-Biological Center, Umeå University.
There are several different techniques and approaches that can be used to immobilize and support ILs, such as simple impregnation, grafting, polymerization, sol-gel method, encapsulation, or pore trapping.9-12,17-19 A novel, straightforward preparation method involves impregnation of the support material with an IL, diluted with a molecular solvent that is easily evaporated, such as acetone. The dilution followed by evaporation of the co-solvent results in a uniform and thin ionic liquid layer on the support material. The IL can be assumed to remain liquid when immobilized on a solid support, because if the IL formed a solid surface on palladium particles, the catalyst would not be as active as it is. As SILCA catalysts prepared in such a way are applied in a liquid-phase process, a bulk solvent that is not miscible with the IL should be selected. In parallel to the ionic liquid immobilization, transition-metal ions or complexes can be dissolved into the IL layer. These organometallic species can then further be reduced to metallic nanoparticles. A picture of such a catalyst used in cinnamaldehyde hydrogenation is presented in Figure 1. Generally, the selective hydrogenation of R,β-unsaturated aldehydes, ketones, and esters is a versatile method to obtain many interesting products that find use in the perfumery industries, hardening of fats, preparation of pharmaceuticals, and synthesis of organic chemical intermediates. Cinnamaldehyde and its hydrogenation products are widely used in the perfumery and fine chemical industry. Selective hydrogenations of R,β-unsaturated aldehydes and ketones are challenging, because these species can contain three different double bonds: isolated and conjugated carbon-carbon double bonds, as well as a carbonyl group. Much
Figure 1. Schematic of a supported ionic liquid catalyst used in hydrogenation.
10.1021/ie901041z CCC: $40.75 2009 American Chemical Society Published on Web 09/17/2009
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Ind. Eng. Chem. Res., Vol. 48, No. 23, 2009
Scheme 1. Cinnamaldehyde Reaction
Scheme 2. Adsorption and Desorption of Reactants and Products
research has been conducted with conventional heterogeneous and homogeneous catalysts, in conventional as well as supercritical solvents and ILs.20-25 The potential of supported IL catalysts has already been illustrated in, e.g., hydrogenation of alkenes, displaying competitive performance in comparison to biphasic systems and conventional solvents.14,15 2. Experimental Section Two different SILCAs were prepared according to the method introduced previously.26,27 IL (∼150 mg) and palladium acetylacetonate (Pd(acac)2) (∼50 mg; Aldrich, 99%) were both dissolved into dry acetone (∼15 mL; Acros Organics,
