Ind. Eng. Chem. Res. 2004, 43, 2039-2048
2039
Liquid-Phase Hydrogenation of Cinnamaldehyde over a Ru-Sn Sol-Gel Catalyst. 2. Kinetic Modeling Jan Ha´ jek, Johan Wa1 rnå, and Dmitry Yu. Murzin* Laboratory of Industrial Chemistry, Process Chemistry Centre, Åbo Akademi University, Biskopsgatan 8, FIN-20500 Turku/Åbo, Finland
Selective liquid-phase hydrogenation of cinnamaldehyde to cinnamyl alcohol over a sol-gel 5% Ru-5% Sn/SiO2 catalyst has been studied in a stirred batch reactor (373-483 K, 25-70 bar). Experiments provided information about the influence of temperature and pressure on the catalytic performance. The catalyst deactivation in different solvents during repeated experiments has been studied, too. Kinetic modeling based on several reaction mechanisms allowing discrimination between rival models was carried out. Introduction Unsaturated alcohols are commercially valuable intermediates used in the synthesis of industrial commodities.1,2 Unsaturated alcohols can be prepared by chemical3,4 or catalytic hydrogenation of available raw materials, such as R,β-unsaturated aldehydes. Catalytic reduction is clearly preferred; however, thermodynamic constraints favor the formation of saturated aldehydes.5,6 Among the catalytically active metals, the best selectivities toward unsaturated alcohols were obtained over Os and Ir catalysts. Ru, Pt, and Co exhibited fairly good selectivities, while Rh, Pd, and Ni were nonselective.7-9 In recent studies, a modified ruthenium catalyst showed very clearly the positive influence of modification, which considerably increased the yields of unsaturated alcohols.10,11 Promoted bimetallic ruthenium catalysts were highly selective; the most effective promoters were metals from group IVa. Particularly, the bimetallic RuSn catalyst exhibited superior selectivity with respect to CdO group hydrogenation.12,13 Bimetallic catalysts can be prepared by conventional methods (impregnation, precipitation, etc.). Alternatively, there is a possibility of utilizing the sol-gel procedure as a relatively new method adopted for catalyst processing from the fields of glass and ceramics.14-16 The main advantage of bimetallic solgel catalysts is the homogeneous distribution of finely dispersed metal particles.17 Other advantages are improved thermal stability, tunable support microstructure, and high surface areas.18-21 Catalytic reactions are often accompanied by deactivation. Substantial knowledge was gained over the years with respect to deactivation in gas-phase catalytic reactions. There are, however, few papers where the phenomenon is addressed quantitatively for the liquidphase reactions. Liquid-phase catalytic reactions are often performed in stirred tank reactors in a batch mode. The catalyst deactivation in such reactors has been scarcely studied because of difficulties in the separation of kinetics and catalyst deactivation. The decrease of the catalytic * To whom correspondence should be addressed. Tel.: +3582-215-4985. Fax: +358-2-215-4479. E-mail: Dmitry.Murzin@ abo.fi.
activity with time can be interpreted, for instance, by deposition of carbonaceous species on the catalyst surface as well as by purely kinetic phenomena (resembling a first-order dependence of the reaction rate on the substrate concentration). Kinetic interpretations are often found in the literature, and it remains unknown whether deactivation is present in the system or not. According to the common definition, catalysis is a kinetic process. Therefore, reliable kinetic models are of vital importance for solving applied problems in mathematical modeling, design, and intensification of chemical processes. Any design starts from reaction kinetics, i.e., from the reaction mechanism. Reaction kinetics is then translation of our understanding of the chemical process on a molecular level into a mathematical rate expression. In paper presented, deactivation and the reaction behavior of liquid-phase hydrogenation of cinnamaldehyde in 2-propanol over a prepared 5% Ru-5% Sn/SiO2 sol-gel catalyst has been studied. Hydrogenation experiments were performed in the temperature window of 110 K (373-483 K) at total pressures (in the following text, typically the values of partial hydrogen pressure will be given) that ranged from 25 to 70 bar. Catalyst properties will be discussed in terms of selectivity and activity within aforesaid reaction conditions, kinetic models for the reaction behavior were proposed and tested. Experimental Section Preparation of a Sol-Gel 5% Ru-5% Sn/SiO2 Catalyst. Calculated amounts of ruthenium chloride [RuCl3‚xH2O (x e 1), Aldrich] and tin chloride (SnCl2, Aldrich) were dissolved in 18 mL of ethanediol. The solution obtained was stirred for 30 min at 343 K and cooled. The support precursor [98% Si(OC2H5)4, Aldrich] was added to the cooled solution of the metal precursor, and the mixture was heated to 343 K. After 3 h of stirring, 90 mL of distilled water was added, and stirring followed at 343 K until a gel was formed. The gel obtained was left for 12 h and then dried. The first drying stage was performed in a vacuum water-rotary evaporator in order to remove low boiling point residues. At this stage, the bath temperature was slowly increased (20 K/h, 1.9 kPa) up to 363 K. The temperature of 363 K was kept for 12 h. A second drying stage was
10.1021/ie034081u CCC: $27.50 © 2004 American Chemical Society Published on Web 04/03/2004
2040 Ind. Eng. Chem. Res., Vol. 43, No. 9, 2004 Table 1. Selectivity and Activity at Different Temperatures and Pressures
Figure 1. Typical cinnamaldehyde hydrogenation network.
carried out for 2 h in an oil bath (0.5 kPa, 473 K). The dried catalyst was reduced at room temperature by a 10% solution of NaBH4 (97%, Fluka) in distilled water (nNaBH4 ) 10nRu + 10nSn) and then washed with small amounts (10-20 mL) of distilled water and ethanol (ca. 500 mL). The washed catalyst was dried in a nitrogen (5.0) atmosphere (2 h, 473 K). Characterization. The pore-size distribution (Dollimore-Heal) and Brunauer-Emmett-Teller surface area of a prepared 5% Ru-5% Sn/SiO2 catalyst were measured on carefully outgassed samples (2 h, 473 K, 7 mbar) and determined from full nitrogen adsorptiondesorption isotherms at 77 K (Sorptomatic 1900, Carlo Erba Instruments). The catalyst was mesoporous (Figure 4) with a narrow distribution of pores (1-4 nm). The surface area of the catalyst was 297 m2/g. Ruthenium and tin contents were verified by DCP (direct current plasma). Results and Discussion Hydrogenation of cinnamaldehyde was performed in the temperature window of 110 K. Experimental runs were carried out at 373 and 433 K and near the critical point of 2-propanol at 483 K. The total pressures were 25, 48, and 70 bar. More details on the experimental procedure and verification that experiments were performed in the absence of mass transfer are provided in part 1 of this series of papers. The main reaction products were cinnamyl alcohol (B), 3-phenylpropanal (C), and 3-phenylpropanol (D). The presence of compounds resulting from aromatic ring hydrogenation was revealed during reactions at high conversions (90%) at the highest temperature used (483 K). The proposed reaction scheme for a typical reaction is shown in Figure 1. As shown in Figure 2A, hydrogenations provided behavior typical of a consecutive-parallel reaction system.
temperature [K]
pressure [bar]
activity [mol × 10-4/min‚gcat]
selectivity (B) [%]
373 373 373 433 433 433 483 483
23 46 68 14 37 59 17 39
2.6 2.7 3.9 6.4 11.3 12.3 20.1 21.9
79 82 80 63 58 63 53 57
The yield of cinnamyl alcohol increased with conversion at the beginning of the reaction and remained constant after conversion of ca. 30-40% was reached. A selectivity (B) decrease was observed at high degrees of conversion (90-95%). The selectivity toward 3-phenyl-2-propanal (C) decreased with conversion (Figure 2B). A summary of the cinnamaldehyde hydrogenation results is presented in Table 1. The selectivity with respect to unsaturated alcohol is related to 50% cinnamaldehyde conversion; activities were evaluated from the initial reaction rates. Catalyst Deactivation. The phenomenon of catalyst deactivation has been widely studied mainly in gasphase reactions. Limited investigation of batch liquidphase reactions is associated with apparent experimental and theoretical difficulties. Three consecutive experiments with the same catalyst batch were performed to reveal deactivation in the liquid-phase hydrogenation of cinnamaldehyde. Successive experiments demonstrated only a minor deactivation of the catalyst in 2-propanol (Figure 3A). Compared to the first run, the selectivity increased by about 10% in the second run, while no further selectivity enhancement during the last run was observed (Figure 3B). Deactivation of catalysts in the liquid-phase hydrogenation is related mostly to the formation of carbonaceous deposits either on catalyst active sites or in the pores, decreasing the accessible catalyst surface. To elucidate further deactivation, the surface area of the spent catalysts was measured. During three consecutive reactions in 2-propanol, the surface area decreased by about 35% (Table 2). The measured pore-size distribution showed small changes in pore distribution (Table 2 and Figure 4). The catalyst was inactive in apolar solvents: hexane and cyclohexane (Figure 5). Its surface decreased after the first run to below 15 m2/g (Table 2).
Figure 2. Typical reaction behavior (433 K, 59 bar): (A) hydrogenation pathway; (B) selectivity.
Ind. Eng. Chem. Res., Vol. 43, No. 9, 2004 2041
Figure 3. Deactivation of the sol-gel Ru-Sn catalyst (433 K, 37 bar): (A) activity; (B) selectivity. Table 2. Surface Area Measurements pore-size distribution [nm]a solvent
catalyst
none 2-propanol cyclohexane hexane
fresh after 3 runs after 1 run after 1 run
a
297 194
