3554
Ind. Eng. Chem. Res. 1997, 36, 3554-3562
Kinetic Analysis of Cinnamaldehyde Hydrogenation over Alumina-Supported Ruthenium Catalysts G. Neri*,† Facolta` di Ingegneria, Universita` di Reggio Calabria, Via E. Cuzzocrea 48, I-89100 Reggio Calabria, Italy
L. Bonaccorsi, L. Mercadante, and S. Galvagno Dipartimento di Chimica Industriale, Universita` di Messina, Salita Sperone, I-98166 Messina, Italy
The kinetics of liquid-phase hydrogenation of cinnamaldehyde (CALD) were studied over Ru/ Al2O3 catalysts of different metal dispersion in a slurry reactor, using ethanol as solvent, in the temperature range between 283 and 333 K. The reaction pathway has been described by using Langmuir-Hinshelwood type rate expressions. A two-site model was used to describe the kinetic experiments. The adsorption and hydrogenation of the CdC and CdO groups are suggested to occur on different sites with competitive adsorption of substrate and products. The kinetic parameters of the individual reaction steps have been determined by a nonlinear regression of the experimental data. The specific rate constants of the CdC double-bond hydrogenation are smaller at the lowest Ru dispersion as a consequence of the weaker adsorption of the olefinic bond on the flat surface of the larger Ru crystallites. The specific activity of CdO bond hydrogenation increases as the dispersion decreases. The adsorption strength of the carbonyl group remains constant regardless of the Ru particle size. The results have been explained by assuming that on the ruthenium metal particles having different size, the substrate is chemisorbed with a different geometry. Hydrogenation of CALD over a Ru-Sn/Al2O3 catalyst has also been investigated. The variation of the kinetic parameters on addition of tin is discussed. The high selectivity of the Sn-doped ruthenium catalysts has been attributed to an increase of the reactivity of the conjugated CdO group. The reactivity of the conjugated CdC double bond is not influenced by the presence of tin. Introduction The products of the selective hydrogenation of R,βunsaturated aldehydes are important intermediates in the synthesis of fine chemicals (Rylander, 1979). Among the noble metals used as catalysts, ruthenium is known to have a better intrinsic selectivity to unsaturated alcohols compared to Pt or Pd. In our laboratory, the liquid-phase hydrogenation of cinnamaldehyde on supported Ru catalysts has been widely investigated (Galvagno et al., 1991a; Mercadante et al., 1996; Neri et al., 1996). Monometallic Ru catalysts have been found to display a low selectivity to cinnamyl alcohol (CALC), with the saturated aldehyde, hydrocinnamaldehyde (HCALD), being the main product. However, the product distribution was found to depend on the metal particle size, with the larger particles showing the highest selectivity toward the formation of cinnamyl alcohol (Gallezot et al., 1991; Nitta et al., 1989; Galvagno et al., 1991b). The selectivity to unsaturated alcohol can be improved by addition to the noble metal of suitable promoters. For example, the addition of Sn has been reported to increase greatly the formation of unsaturated alcohol (Coq et al., 1993a; Marinelli et al., 1993; Didillon et al., 1991; Galvagno et al., 1991b; Neri et al., 1996). Despite the great interest for the hydrogenation of R,β-unsaturated aldehydes in the liquid phase, up to now, only a few kinetic studies have been reported in the literature. The CALD hydrogenation shows a complex sequence of consecutive/parallel reactions, due * Author to whom correspondence is addressed. Telephone: +39-393134. Fax: +39-391518. E-mail:
[email protected]. † Present address: Dipartimento di Chimica Industriale, Universita` di Messina, Salita Sperone, I-98166 Messina, Italy. S0888-5885(96)00745-2 CCC: $14.00
to the presence in the molecule of two reducible groups. Recently, Tronconi et al. (1990) have developed for the hydrogenation of cinnamaldehyde over a Pt-Sn catalyst a kinetic model involving two different sites for the hydrogenation of the CdC and CdO groups. This approach has also been used by Hotta and Kubomatsu (1972) in the hydrogenation of 2-methyl-2-pentenal over a Raney cobalt catalyst modified with Co, Mn, Ni, and Pd chloride and by Niklasson and Smedler (1987) in the vapor phase reduction of 2-ethyl-2-hexenal on Ni/SiO2. These results can be interpreted as an indication that two different types of adsorbed R,β-unsaturated aldehyde are involved in the hydrogenation reaction. However, other authors have suggested that a common adsorbed intermediate can be responsible for the hydrogenation of R,β-unsaturated aldehydes (Augustine, 1976; Simonik and Beranek, 1972). To the best of our knowledge, no paper dealing with the modeling of the title reaction over Ru catalysts has been reported in the literature. In this paper we present a kinetic study of the liquid phase hydrogenation of CALD over monometallic Ru and bimetallic Ru-Sn supported catalysts. The aim of this work is the development of a kinetic model and the evaluation of the kinetic parameters of the individual reaction steps. This would allow one to obtain valuable information for a better understanding of the reaction mechanism over Ru catalysts and of the key factors responsible for the higher selectivity observed on some of the investigated catalysts. Experimental Section Materials. Cinnamaldehyde (Fluka; purity > 98%), cinnamyl alcohol (Riedel; >98%), ethanol (Fluka; ana© 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3555 Table 1. Chemical Composition and H/Ru Ratio of the Ru/Al2O3 and Ru-Sn/Al2O3 Catalysts catalyst code
Ru (wt %)
RuECI RuECII RuECIII RuCl RuECIII/Sn
0.97 2.5 4.0 2.0 4.0
Sn (wt %)
H/Ru
0.5
0.88 0.44 0.25 0.23 0.12
lytical grade), and ultrahigh-purity hydrogen (Multigas; >99.9%) were used. Before use cinnamaldehyde was further purified by distillation under vacuum. Monometallic ruthenium catalysts of the RuEC series were prepared by contacting a toluene solution of ruthenium acetylacetonate, Ru(acac)3, with γ-Al2O3 (Rhone-Poulenc; SBET ) 220 m2 g-1). The sample RuCl was prepared by incipient wetness impregnation of γ-Al2O3 with an aqueous solution of RuCl3. The metal loading was varied between 1 and 4 wt %. The bimetallic catalyst, RuECIIISn, has been prepared by contacting, under a H2 atmosphere at 353 K, the prereduced monometallic sample RuECIII with tetra-n-butyltin dissolved in n-heptane. All samples, after drying for 2 h, were reduced under H2 at 623 K. Ruthenium dispersion was calculated from hydrogen chemisorption measurements carried out in a static volumetric apparatus. Table 1 reports the chemical compositions and the H/Ru ratios of the investigated samples. Details on the procedures of preparation and characterization of these catalysts are reported elsewhere (Coq et al., 1994). Kinetic Experiments. The kinetic experiments were performed in the liquid phase in a 100-mL fivenecked flask reactor, equipped with a reflux condenser and a thermocouple, in the temperature range 283-333 K. Constant temperature ((0.5 °C) was maintained by circulation of silicon oil in an external jacket connected with a thermostat. The catalyst (0.1-1 g) was added to the required amount of solvent (25 mL of ethanol) and reduced at 343 K for 1 h under H2 flow. The reaction mixture was stirred with a stirrer head with permanent magnetic coupling at a stirring rate of 500 rpm. The reaction was carried out at atmospheric pressure under H2 flow. The progress of the reaction was followed by analyzing a sufficient number of samples withdrawn from the reaction mixture. Products analysis was performed with a gas chromatograph (HP Model 5890), equipped with a wide-bore column (Supelcowax, 30 m, 0.53 mm) and a flame ionization detector. Quantitative analysis was carried out by calculating the area of the chromatographic peaks with an electronic integrator (HP 3396 Series II). The analytical error associated with the detection of each reaction component is not greater than 2%. Before starting the kinetic experiments, the absence of mass-transfer limitations was tested. This was verified experimentally by varying the amount of catalyst, stirring rate, and catalyst grain size. Experiments with different amounts of catalyst have also shown that a fraction of the catalyst is poisoned, likely by impurities present in the substrate and/or solvents. Selectivity, S, was calculated by the expression Si ) (Ci/∑Cp) × 100 where Ci is the concentration of product i and Cp the total concentration of the products. Results and Discussion Reaction Pathway. Figures 1-3 report typical plots showing the course of reaction during the catalytic
Figure 1. Hydrogenation of CALD over the RuECIII sample, Tr ) 333 K. Comparison between experimental and calculated concentrations with model II: (b) CALD; (3) HCALD; (]) CALC; (0) HCALC; (1) PHE.
Figure 2. Hydrogenation of CALD over the RuECII sample, Tr ) 333 K. Comparison between experimental and calculated concentrations with model II: (b) CALD; (3) HCALD; (]) CALC; (0) HCALC; (1) PHE.
Figure 3. Hydrogenation of CALD over the RuECII sample, Tr ) 283 K. Comparison between experimental and calculated concentrations with model II: (b) CALD; (3) HCALD; (]) CALC; (0) HCALC; (1) PHE.
hydrogenation of CALD over the RuEC catalysts. In the first step of the reaction the main product of the reduction of CALD is hydrocinnamaldehyde (HCALD). Cinnamyl alcohol (CALC) is also formed with a selectivity ranging from 8 to 15%. The highest amount of CALC is obtained over the RuECIII catalyst. Hydrocinnamyl alcohol (HCALC) was obtained from the consecutive hydrogenation of the half-hydrogenated intermediates CALC and HCALD. Small amounts (3-5%) of phenylethane (PHE) and (
