Jointly published by Akadémiai Kiadó, Budapest and Springer, Dordrecht
React.Kinet.Catal.Lett. Vol. 84, No. 2, 351-358 (2005)
RKCL4557 CROTONALDEHYDE HYDROGENATION OVER BIMETALLIC Rh-Sn/SiO2 CATALYSTS María del Carmen Aguirrea, José Luis G. Fierrob and Patricio Reyesa* a
Departamento de Físico-Química, Facultad de Ciencias Químicas, Universidad de Concepción, Casilla 160-C, Concepción, Chile, b Instituto de Catálisis y Petroleoquímica, CSIC, Campus Universidad Autónoma, Madrid, Spain Received April 20, 2004 In revised form October 4, 2004 Accepted October 11, 2004
Abstract The hydrogenation of crotonaldehyde in liquid phase over co-impregnated RhSn/SiO2 catalysts with different Rh contents and a constant bulk atomic ratio Sn/(Sn+Rh) = 0.25 has been studied. The effect of reactant concentrations and the surface properties on the selectivity in the mentioned reaction was investigated. The acid properties of the catalysts, mainly due to tin oxide species and intermetallic phases, played an important role in the activity and selectivity for the unsaturated alcohol. Keywords: Rhodium, tin, crotonaldehyde, hydrogenation, characterization
INTRODUCTION Catalytic hydrogenation of α,β unsaturated aldehydes are of great interest in the preparation of various fine chemicals. The selective hydrogenation of the C=O group in the presence of the olefinic C=C bond is important in the preparation of several chemical agents used for flavoring and pharmaceuticals products [1-3]. The principal difficulty of this reaction is that the C=C bond is preferentially reduced due to both kinetics and thermodynamic factors. Several investigations have sought to increase the selectivity for unsaturated alcohol, where the use of reducible supports [4], cationic promoters [5] and the effect of the metal particle size [6] among others have received special attention. __________________________ * Corresponding author. E-mail:
[email protected] 0133-1736/2005/US$ 20.00. © Akadémiai Kiadó, Budapest. All rights reserved.
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The aim of this work was to add more evidence on the effect of parameters, such as the nature of active sites in Rh-Sn/SiO2 catalysts during the liquid phase hydrogenation of crotonaldehyde. Rh-Sn catalysts were prepared by coimpregnation on silica sol-gel using different metal content. Catalyst characterization was aimed at discerning new active sites created in the presence of Rh and Sn, responsible for selectivity and activity. EXPERIMENTAL The support, SiO2, was obtained from a sol-gel procedure [7]. The monometallic Rh/SiO2 was prepared by impregnation on the silica with an aqueous solution at 35ºC. The RhSn/SiO2 catalysts were prepared by coimpregnation of silica with alcoholic/aqueous solutions containing the desired among of SnCl2 (Merck) and RhCl3 (Merck) at 35ºC. They were kept at 35ºC under stirring and reflux for 3 h. Then, the solids were dried at 110ºC for 12 h and calcined in air at 400ºC for 4 h. Prior to their characterization or catalytic studies, the solids were reduced in situ in flowing H2 at 500ºC for 2 h. The loading of metal was adjusted to obtain catalysts with 0.5, 0.75 and 1.0 wt.% of Rh and a fixed Sn/(Rh+ Sn) ratio of 0.25 in all samples. The catalysts were labeled as Rh(x)Sn/SiO2, where (x) is the weight percent of Rh. The catalysts were characterized by H2 chemisorption, temperature programmed reduction (TPR), transmission electron microscopy (TEM), electron diffraction (ED), and X- ray photoelectron spectroscopy (XPS). In order to study the effect of substrate concentration on the selectivity to crotyl alcohol, the catalyst Rh(0.5)Sn/SiO2 was chosen. Thus, different reactant concentrations (0.9, 0.45, 0.3 and 0.2 M) were used. The catalytic reactions were carried out in a stirred batch reactor at 35ºC at atmospheric pressure of hydrogen. A weight of 0.46 g of the catalyst was added to the reactor and reduced in situ under H2 flow (25 cm3/min) at 500ºC for 2 h. Then, the samples were cooled down to reaction temperature, and 40 mL mixture of a solution of crotonaldehyde in ethanol at 35ºC was fed. Aliquots of the reaction mixture were taken periodically and analyzed by Gas Chromatograph using a GC Varian Star 3400 CX machine and a 6.6 wt.% Carbowax 20 M in a Carbopack B 80/120 m column. The products of the reaction mixture were also analyzed using a Hewlett-Packard Series 5972 mass spectrometer joined to a GC model Hewlett-Packard 5890 series II with a capillary column HP-5MS, 30 m length.
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RESULTS AND DISCUSSION H/Rh atomic ratios obtained from hydrogen chemisorption revealed a change in the Rh dispersion due to the addition of tin, as observed in Table 1. The significant drop in the H/Rh ratio in the bimetallic Rh-Sn when compared with the Rh/SiO2 counterpart is larger than can be expected if only a change in the Rh metal particle size took place. In fact, by TEM, small particles were observed with characteristics of pure Rho that coexists with core-shell type bimetallic particles. This is a pure dark core Rho surrounded by a bright intermetallic phase, probably more externally enriched with tin. XPS results confirm that surface enrichment exists in tin, and also shows that in general, the bimetallic samples have approximately 60% of tin as alloy phases. The electron Table 1 Sn and Rh content, bulk atomic ratio, H/Rh ratio and metal particle size obtained from TEM of RhSn/SiO2 catalysts Catalysts
Rh(0.5)/SiO2 Rh(1.0)/SiO2 Rh(0.5)Sn/SiO2 Rh(0.75)Sn/SiO2 Rh(1.0)Sn/SiO2 Sn(0.88)/SiO2
Sn loading (wt.%)
Rh loading (wt.%)
Sn/(Sn+Rh)b Atomic ratio
H/Rh
d(nm) TEM
------0.20 0.22 0.35 0.88
0.5 1.0 0.5 0.75 1.0 ---
0 0 0.27 0,22 0.23 1
0.47 0.30 0.27 0.17 0.12 -----
1.8 2.0 2.3 2.5 2.9 ----
diffraction pattern of a representative Rh(1.0)/SiO2 catalyst showed a good agreement with Rho (fcc) under the pre-treatment reduction conditions. The electron diffraction of the Rh(1.0)Sn /SiO2 catalyst showed the formation of hexagonal outlines in the samples, characteristic of most fcc metals, such as rhodium. An alloy substitution of RhxSny is suggested where the central face characteristic of the cubic crystal (Rho) is maintained with modifications in the cell parameter. Quantitative XPS data are compiled in Table 2. The BE of the Rh 3d5/2 peak at ca. 306.8 eV corresponding to Rho species is observed in the monometallic Rh/SiO2; whereas, the 3d core level for Sn/SiO2 catalyst exhibit a B.E. of 486.7 eV, corresponding to oxide species. The bimetallic RhSn/SiO2 catalysts do not show the presence of slightly oxidized Rh species; whereas the Sn 3d profiles indicate the presence of different species. In fact, curve fitting of the experimental spectra indicates a reduced tin species, Sno (B.E.= 485.1 eV), and oxide species (ca. 487.0 eV). Unfortunately, the oxide state (Sn2+ and Sn4+) cannot be distinguished by XPS [8]. In general, a surface enrichment in tin
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exists, even in the sample with lower metal content, which is approximately three times higher than the bulk composition. The high proportion of Sn0 is indicative of Sn in an alloyed phase with Rh, present behavior that is different from those catalysts prepared by successive impregnation [5] in which most of the tin was in oxide form on the surface. It can be seen in Table 2 that the Rh/Si surface atomic ratios exhibit virtually constant values, higher than those shown by the monometallic Rh/SiO2 catalysts, whereas Sn/Si ratios decrease with Rh loading. The observed drop in the Sn/Rh ratio in the catalysts at higher Rh loading may be attributed to an increase in the particle size. Table 2 Binding energies of Sn y Rh 3d obtained from XPS and surface atomic of RhSn/SiO2 catalysts Sn 3d 5/2 B.E. (eV)
Rh 3d 5/2 B.E. (eV)
-----
307.2 306.8
0.0036 0.0094
-----
-----
Rh(0.5)Sn/SiO2
487.1 (41) 485.0(59)
306.7
0.0132
0.091
0.87
Rh(0.75)Sn/SiO2
487.2(46) 485.0(54)
306.8
0.0112
0.066
0.85
Sn(1.0)Rh/SiO2
487.2(38) 485.2(62)
306.8
0.0149
0.070
0.82
Sn(0.88)/SiO2
486.7
---
---
0.578
---
Catalyst Rh(0.5)/SiO2 Rh(1.0)/SiO2
(Rh/Si)s
(Sn/Si)s
Sn/(Rh+Sn)s
The TPR profile of the calcined Rh/SiO2 catalyst shows a single peak centered at 127ºC corresponding to the reduction of Rh2O3 to Rho (See Fig.1). The reduction of Sn/SiO2 catalyst starts at 400ºC showing a broader peak centred at 521ºC and a peak at 727ºC. In the bimetallic RhSn samples, two partially overlapping peaks appear. The main peak at 80ºC is attributed to the reduction of Rh2O3 to Rho, and it partially overlaps another peak at 122ºC, which can be attributed to Sn reduction in intimate contact with Rh crystals. A broader, not-well-defined peak at 448ºC is attributed to the partial reduction of isolated patches of SnO2 reduction, hydrogen spill over from metallic sites, and may be the responsible for the observed reduction temperature shift. Crotonaldehyde hydrogenation was studied at 35ºC and atmospheric pressure of H 2 (flow 25 cm3 /min), where the products obtained were crotyl alcohol (CROL), butyraldehyde (BUHO), butanol (BUOH) and some
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H2 consumption, a.u.
Sn(0.88)
Rh(0.5)Sn
Rh(0.75) Sn
Rh(1.0)Sn
Rh(0.5) 343
443
543
643
743
843
943
1043
Temperature, K
Fig. 1. Temperature programmed reduction profiles of Rh/SiO2, Sn/SiO2 and Rh(X)Sn/SiO2 catalysts
100
Selectivity, mole %
80
60
40
20
0 0.2
0.3
0.4
0.5
0.6
mole
0.7
0.8
0.9
L-1
Fig. 2. Selectivity to products vs reactant concentration during crotonaldehyde hydrogenation on Rh(0.5) Sn/SiO2 catalyst. &52/ L BuHO, S1, BuOH
N
a
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condensation products (labelled as SI). Figure 2 shows the evolution of product selectivity as a function of the reactant concentration at 10% of conversion on a representative Rh(0.5)Sn/SiO2 catalyst. It is observed that BUHO, product obtained by the hydrogenation of the C=C bond, is the principal product and it decreases at higher crotonaldehyde concentrations. With regard to CROL, the selectivity was close to 20%. Practically no BUOH, the over-hydrogenated product was obtained, which is in line with the decreased hydrogenation ability induced by the addition of tin. Unfortunately, condensation reaction-generated by-products increase significantly as reactant concentration increases. In fact, at lower crotonaldehyde concentrations, almost no condensation products (labeled as S1) were obtained. Due to the observed behavior, a slight enhancement in S1 exists when BUHO decreases, where part of BUHO would be acting as a reactant in side reactions leading to the formation of acetals or esters. Additionally, the reaction between crotonaldehyde and the solvent (ethanol) also occurs to an important extent, principally at higher crotonaldehyde concentrations as was found by mass spectrometry studies, in which the presence of molecular ions due to these by-products were detected at m/z= 144(1,1-dietoxyi-2-butane); 118 (1,1-dietoxyethane) and 116 (ethyl butanoate).
30
Conversion, mole %
25 20 15 10 5 0 0
1
2
3
4
5
6
time, h
N
Fig. 3a. Conversion vs time during crotonaldehyde hydrogenation at 35ºC on 5K6Q6L22, L Rh/SiO2, Rh(X)Sn/SiO2 catalysts. (1.0)Rh/SiO2, Rh(0.75)Sn/SiO2 , Rh(0.5)Sn/SiO2
a
Selectivity to crotyl alcohol, mole %
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50
40
30
20
10
0 0
5
10
15
20
Conversion, mole %
N
Fig. 3b. Selectivity to crotyl alcohol vs conversion on: L Rh(1.0)Sn/SiO2, 5K 6Q 6L22 , Rh(0.5)Sn/SiO2
a
Based on these results, a low crotonaldehyde concentration (0.2 M) was chosen for the standard studies. Figure 3a shows the variation of the activity over time for the studied catalysts. The lower activity showed by the bimetallic samples compared to the Rh counterparts may be attributed to the presence of tin atoms producing a surface rhodium site dilution effect, thus, changing the hydrogenation capability. Figure 3b displays the variation of selectivity to CROL with the conversion for the Rh-Sn catalysts. On supported rhodium, the main products were butyraldehyde (BUHO) with crotyl alcohol selectivity below 3%. The Rh-Sn samples showed CROL selectivity in the range 15-50% with the highest yield exhibited by the bimetallic catalyst with higher Rh loading (1.0 wt.%). Bimetallic Rh-Sn/SiO2 catalysts prepared by successive impregnation have shown different behavior as Rh loading increases, an enhancement in the activity but a decrease in CROL selectivity was found [5]. The observed results clearly indicate that upon the addition of Rh, a change in the nature of the metallic sites is produced. An inhibitor effect on by-product formation was observed for catalysts having lower Rh loading. This fact may be attributed not only to the alloyed phases, which inhibit the hydrogenation of C=C bond, but also to a lesser degree to the lower concentration of SnOx species. In fact, these compounds (mainly acetals and hemiacetals) are produced
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by condensation reactions, favored by the presence of acid sites. Only small quantities of tin are necessary to improve the selectivity, and avoid the side reactions. Figure 4 displays selectivity to products at a constant conversion level (10 mol %) as a function of Rh content in the RhSn catalysts. It is remarkable the increases in crotyl alcohol selectivity at higher Rh content, and that the isomerization reaction of crotyl alcohol to butyraldehyde is highly inhibited in the liquid solvent [9]. 10 0
Selectivity/mol %
80
60
40
20
0 0,5
0,7 5
1 Rh/wt.%
N
Fig. 4. Selectivity to products vs wt.% Rh content in the catalysts at constant conversion level (10%) . &52/ L BuHO , S1
Acknowledgments. The authors thank CONICYT (FONDECYT Grants 1980345 and 2990065) for financial support. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
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