- Email: [email protected]
Hydrogenation of crotonaldehyde and butyraldehyde on silica supported Pt and PtSn catalysts: A drifts study
Catalysis Communications 7 (2006) 204–208 www.elsevier.com/locate/catcom
Hydrogenation of crotonaldehyde and butyraldehyde on silica supported Pt and PtSn catalysts: A drifts study Andrea B. Merlo a, Gerardo F. Santori a,b, Jorge Sambeth a, Guillermo J. Siri Mo´nica L. Casella a, Osmar A. Ferretti a,b,*
a,b
,
a
b
Centro de Investigacio´n y Desarrollo en Ciencias Aplicadas ‘‘Dr. Jorge Ronco’’ (CINDECA), Facultad de Ciencias Exactas, Universidad Nacional de La Plata. 47 No. 257 – C.C. 59 – (1900) La Plata, Argentina Departamento de Ingenierı´a Quı´mica, Facultad de Ingenierı´a, Universidad Nacional de La Plata. 47 No. 257 – C.C. 59 – (1900) La Plata, Argentina Received 8 September 2005; received in revised form 31 October 2005; accepted 4 November 2005 Available online 27 December 2005
Abstract A DRIFTS ‘‘in situ’’ study on Pt and PtSn catalysts used in the crotonaldehyde and butyraldehyde hydrogenation was carried out in this work. Results show that, on Pt/SiO2 catalyst a decarbonylation process takes place during the initial reaction minutes, causing an instantaneous and irreversible deactivation. The tin addition improves the stability of the resulting catalyst due to the inhibition of CO formation. Bands assigned to the presence of superficial olefinic species allow to explain the deactivation observed, which is more important in the case of butyraldehyde hydrogenation. In the crotonaldehyde hydrogenation with PtSn/SiO2 catalyst, the band at 1651 cm 1 is assigned to C@O strongly coordinated, explaining the high selectivity to UOL of this catalytic system. Ó 2005 Elsevier B.V. All rights reserved. Keywords: DRIFTS; Hydrogenation; Crotonaldehyde; Butyraldehyde; PtSn
1. Introduction Unsaturated alcohols (UOL) are usually valuable intermediates in the preparation of fine chemicals [1,2] and can be obtained from the hydrogenation of unsaturated aldehydes (UAL). Transition metals are very active catalysts for this reaction; however, it is well known that with these metals the hydrogenation of the C@C bond is easier than that of the C@O bond [3,4]. For this reason, the addition of a second metal, such as Ge, Sn, Pb is one of the strategies that have been developed in order to improve the selectivity to UOL [5–8]. In previous works, we have studied the hydrogenation of UAL compounds (crotonaldehyde, cinnamaldehyde) with PtSn/SiO2 catalysts obtained via Surface Organometallic Chemistry on Metals (SOMC/M) techniques *
Corresponding author. E-mail addresses: [email protected] (G.F. Santori), [email protected] (O.A. Ferretti). 1566-7367/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2005.11.002
[7,9,10]. Selectivity to UOL is improved by the tin presence, results that have been usually explained through geometric and electronic effects [8,11–14]. For the hydrogenation of UAL and saturated aldehydes (SAL) (butyraldehyde), we found that the rate of C@O bond hydrogenation is higher for bimetallic catalysts when compared with monometallic Pt/SiO2 catalyst. In Fig. 1 we introduce some conversion versus time results obtained for the hydrogenation of butyraldehyde (SAL) and crotonaldehyde (UAL) on Pt and PtSn catalysts, this information was taken from Santori et al. [10]. In this figure a ‘‘flattening’’ of the curves is observed, being more important for butyraldehyde than for crotonaldehyde hydrogenation, and when comparing both catalysts for each reaction, it is much more pronounced for Pt/SiO2 than for PtSn/SiO2. According to the authors, this fact indicates a more important deactivation process for the monometallic catalyst than for the bimetallic one, and more important for SAL than for UAL hydrogenation.
A.B. Merlo et al. / Catalysis Communications 7 (2006) 204–208
100 90 80 Conversion %
70 60 50 40 30 20 10 0 0
100
200
300
Time (min) Fig. 1. Conversion of butyraldehyde (empty symbols) and crotonaldehyde (full symbols) as a function of time for Pt/SiO2 (h) and PtSn/SiO2 (s) catalysts. (taken from [10]).
In order to improve the understanding of the hydrogenation behavior of crotonaldehyde and butyraldehyde on Pt/SiO2 and PtSn/SiO2 catalysts, we present in this communication a diffuse reflectance IR Fourier transform spectroscopy (DRIFTS) study of these reactions. This technique will be used to explain the origin of the deactivation and the selectivity of the catalysts employed in the hydrogenation of UAL and SAL. 2. Experimental Pt/SiO2 and PtSn/SiO2 catalysts were prepared as it is described in a previously published paper [7], with a content of 1% w/w Pt and 0.25% w/w Sn ((Sn/Pt)at = 0.4), respectively. Platinum and tin contents were determined by atomic absorption. Hydrogen chemisorption was measured in a static volumetric apparatus at room temperature. The size distribution of metallic particles was determined by transmission electron microscopy (TEM) using a Jeol 2010 instrument. To estimate the mean particle size (dTEM), the particles were considered spherical and the second moment of the distribution was employed. X-ray photoelectron spectroscopy (XPS) analyses were obtained with an ESCA 750 Shimadzu spectrometer equipped with a hemispherical electron analyzer and a Mg Ka (1253.6 eV) X-ray source. The binding energy (BE) of the C 1s peak at 284.6 eV was taken as an internal standard.
205
Hydrogenation of crotonaldehyde and butyraldehyde was studied in a DRIFTS equipment. Before DRIFTS experiments, Pt/SiO2 and PtSn/SiO2 catalysts were treated under flowing H2, increasing the temperature from ambient to 773 K, and holding it for 2 h. The catalysts were handled without being exposed to the air. Spectra were obtained using a controlled temperature and environment diffuse reflectance DRIFTS chamber (Spectra Tech 0030-103) with KBr windows coupled to a Bruker IFS66 infrared spectrometer. The samples were placed inside the chamber without packing or dilution and spectra were taken between room temperature and 318 K. Then, the reaction was analyzed for 30 min maintaining the temperature at 318 K. Crotonaldehyde and butyraldehyde were fed in a flow of 20 mL min 1 H2/N2 (10/90). Spectra were obtained by collecting 200 scans at 2 cm 1 resolution. 3. Results and discussion Table 1 shows the characteristics of the studied catalysts. From XPS results, it was determined that in the PtSn/SiO2 system, approximately 70% tin is present as Sn(0) and the remainder as ionic tin (Sn(II, IV)). Results of H2 chemisorption on Pt/SiO2 indicate a high dispersion of the metallic phase on the support (H/Pt = 0.64), in agreement with TEM results (dTEM = 2.4 nm). In the case of the bimetallic catalyst, the H/Pt ratio decreases up to a value of about 0.2 and the average particle size increases in 0.3 nm, this fact indicates a high interaction between Pt and Sn. A series of experiments of ‘‘in situ’’ DRIFT spectroscopy was performed, in which IR spectra were recorded during the substrate hydrogenation (crotonaldehyde and butyraldehyde), employing Pt/SiO2 and PtSn/SiO2 catalysts. Fig. 2(a) and (b) show the IR spectra taken during the butyraldehyde hydrogenation and Fig. 3(a) and (b), the corresponding to the crotonaldehyde hydrogenation, for Pt/SiO2 and PtSn/SiO2 catalysts. In the four spectra, bands assigned to m(C@O) appear at approximately 2000–2050 and 1800–1900 cm 1. These bands are attributed to CO adsorbed on the metallic surface arising from decarbonylation of substrates [15]. Figs. 2(a), (b) and 3(a), (b) show that the characteristic of these bands is presented by the fact that they appear as soon as the catalytic reaction begins and that they do not undergo important changes either under time on stream or varying the temperature; this indicates that they arise from strongly chemisorbed species [16]. In this way, this CO adsorbed would be the cause of an irreversible and instantaneous
Table 1 Characterization by XPS and H2 chemisorption and TEM of Pt/SiO2 and PtSn/SiO2 catalysts Catalyst
Pt/SiO2 PtSn/SiO2
Sn/Pt
0 0.40
H/Pt
0.64 0.20
dTEM 2.4 2.7
Binding energies (eV)
Sn(0)/[Sn(0) + Sn(II, IV)]
Pt 4f7/2
Sn(0) 3d5/2
Sn(II, IV) 3d5/2
71.6 70.9
– 485.0
– 487.0
– 0.67
5 min RT 1min RT
3000
2500
2000 1500 Wavenumber (cm-1)
1000
RT
3000
2500 2000 Wavenumber (cm-1)
1863
1990
2330
Absorbance (UA)
5 minRT
2955 2860 2760
1010
1620
1866
1990
2375
2950 2850
Absorbance (UA)
30 min 318 K
303 K
1430 1348
303 K
a
5 min 318 K
1680 1563
1870
318 K 5 min
1400 1325
a
1500
3000
2500 2000 1500 Wavenumber (cm-1)
1000
318 K 30 min 318 K 5 min 303 K RT
1 minRT
b
318 K 30 min
1690 1651 1558 1465 1372
303 K
Absorbance (UA)
5 min 318 K
2330
2955 2860 2760
30 min 318 K
2050
1010
1383 1290
1710 1640
2375
1860
2029
A.B. Merlo et al. / Catalysis Communications 7 (2006) 204–208
2950 2850
Absorbance (UA)
206
1000
Fig. 2. (a) In situ DRIFT spectra of Pt/SiO2 during butyraldehyde hydrogenation at different temperatures and time on stream. (b) In situ DRIFT spectra of PtSn/SiO2 during butyraldehyde hydrogenation at different temperatures and time on stream.
deactivation of the catalysts. For both, butyraldehyde and crotonaldehyde, bands are more important in the Pt/SiO2 monometallic catalyst, particularly the band around 2000–2050 cm 1 corresponding to the Pts–CO lineal form. This fact indicates a more poisonous effect of the CO formed on monometallic than on bimetallic catalysts and explains our catalytic results. Fig. 1 shows a strong increase in the initial hydrogenation rate when passing from monometallic to the bimetallic catalyst, a result that can be correlated with the intensity decrease of the m(C@O) bands when passing from Pt/SiO2 to PtSn/SiO2 catalyst. Consequently, the lower CO contents observed in the bimetallic PtSn/SiO2 catalyst allow to explain the fact that the tin presence increases noticeably the hydrogenation rate of both UAL and SAL due to a decrease of the instantaneous poisoning of the surface (for example, in the case of the crotonaldehyde hydrogenation, rinitial = 95 and 800 lmol s 1 gPts1 , for Pt/SiO2 and PtSn/SiO2, respectively [10]). In the 1000–1700 cm 1 range, the analysis of DRIFT spectra is also very interesting, when it is related to the catalytic properties of the systems under study. In the case of butyraldehyde, for both monometallic and bimetallic
3000 b
2500 2000 Wavenumber (cm-1)
1500
1000
Fig. 3. (a) In situ DRIFT spectra of Pt/SiO2 during crotonaldehyde hydrogenation at different temperatures and time on stream. (b) In situ DRIFT spectra of PtSn/SiO2 during crotonaldehyde hydrogenation at different temperatures and time on stream.
catalysts (Fig. 2(a) and (b)), a band at 1010 cm 1 is observed, which is a typical indication of the presence of a primary alcohol adsorption [17]. This band does not appear for crotonaldehyde (Fig. 3(a) and (b)) probably because although butanol (SOL) is the product of its total hydrogenation, it is found in a low proportion at the conversion level at which the spectra were taken. The fact that the reaction product presents an important adsorption may constitute a penalization of the activity, and this is in agreement with the hydrogenation behavior of butyraldehyde and crotonaldehyde. In DRIFT spectra obtained during crotonaldehyde hydrogenation (Fig. 3(a) and (b)), the region at approximately 1500–1700 cm 1 is interesting in relation to the possible adsorbed species shown in Fig. 4, described according to well proven structures derived from spectroscopic studies [2]. The band at 1680 cm 1 (Pt/SiO2)/1690 cm 1 (PtSn/ SiO2) corresponds to C@O weakly coordinated on the catalyst (pCO); the band around 1560 cm 1 is assigned to C@C strongly coordinated (rCC). In the spectra corresponding to PtSn/SiO2 catalyst, the development of a band
A.B. Merlo et al. / Catalysis Communications 7 (2006) 204–208
Fig. 4. Possible structures for the adsorption of a,b-unsaturated aldehydes (taken from [2]).
at 1651 cm 1 is observed and can be assigned to C@O strongly coordinated (rCO) to the catalyst surface, which could occur in Lewis acid sites (Sn (II, IV), determined by XPS). This adsorption way corresponds to adsorbed species favorable for the unsaturated alcohol formation from unsaturated aldehyde hydrogenation [3,16], which is in agreement with the high selectivity to UOL presented by this catalytic system. In the case of unsaturated aldehydes, the hydrogenation rate shows a noticeable increase in catalysts modified with tin, with respect to the monometallic catalyst. In the systems modified with tin, the dilution favours the presence of species of the types g1-(O) and g2(C,O) in crotonaldehyde; these species could promote the hydrogenation of the C@O group. This dilution must inhibit other forms of chemisorption competitive with the previous ones, as for example of the types g2-(C,C) and (g2-(C,O) + g4-(C,C,C,O)), favourable for the hydrogenation of the C@C group, this leading to a lower SAL production (see Table 2). The analysis of the bands in the region between 2750 and 3000 cm 1 allows to provide an interpretation of the deactivation mechanism observed during the hydrogenation reaction. Bands appearing for butyraldehyde at 2950 and 2850 cm 1 (Fig. 2) and for crotonaldehyde at 2955, 2860 and 2760 cm 1 (Fig. 3) can be assigned to stretching of CHx groups (superficial olefinic species) associated with bands between 1300–1450 cm 1 corresponding to symmetric and asymmetric CHx vibrations as it is proposed by Dandekar and Vannice [18]. It is reasonable to assign these bands to olefinic species of considerable molecular weight (oligomers). Figs. 2(a), (b) and 3(a), (b) show that these
207
bands increase as a function of reaction time and both for butyraldehyde and crotonaldehyde; their presence is more important in the Pt/SiO2 catalyst than in the PtSn/SiO2 system. Furthermore, if a same catalyst is compared for both substrates, it is observed that these bands are more important in the case of butyraldehyde than in the case of crotonaldehyde. These results are in agreement with previously reported ones [10] and with those presented in Fig. 1, in which, during butyraldehyde hydrogenation, a ‘‘flattening’’ in the conversion vs. time curve is observed which is more pronounced for the monometallic catalyst than for the bimetallic one. As another source of deactivation, a contribution from a strong adsorption of the primary alcohol can not be discarded, as it has been previously mentioned. The deactivation process is less important for the crotonaldehyde hydrogenation resulting almost imperceptible in the case of PtSn/SiO2 catalyst. 4. Conclusions DRIFTS ‘‘in situ’’ studies allow us to explain the behavior of Pt/SiO2 and PtSn/SiO2 catalysts tested in the crotonaldehyde and butyraldehyde hydrogenation. The band around 2000 cm 1 corresponding to the lineal form Pts–CO evidences the existence of adsorbed CO generated by the substrate decarbonylation, which would be responsible for an instantaneous and irreversible deactivation of the catalysts. This band appears as soon as the catalytic reaction starts and, for butyraldehyde as well as for crotonaldehyde, it is more important in the Pt/SiO2 monometallic catalyst. Bands appearing in the region between 2700 and 3000 cm 1 can be assigned to stretching of CHx groups (superficial olefinic species), and this constitutes another deactivation source. This deactivation phenomenon results to be evident in the butyraldehyde hydrogenation and it is less important for the crotonaldehyde, especially when the PtSn/SiO2 bimetallic catalyst is used. This reversible deactivation allows to explain the ‘‘flattening’’ observed in the conversion vs. time curves. During the course of crotonaldehyde hydrogenation using the PtSn/SiO2 catalyst, a band developed at 1651 cm 1 is observed and can be assigned to C@O strongly coordinated to the catalyst surface. This fact allows to explain the high selectivity to UOL of this catalytic system.
Table 2 Hydrogenation of crotonaldehyde and butyraldehyde (taken from [10]) Substrate
Crotonaldehyde Butyraldehyde
Catalysts
Pt/SiO2 PtSn/SiO2 Pt/SiO2 PtSn/SiO2
rco 95 800 – –
rbo – – 53 88
5% conversion
80% conversion
SAL
UOL
SOL
SAL
UOL
SOL
68 36 – –
11 59 – –
21 5 100 100
70 30 – –
7 50 – –
23 20 100 100
rco : overall reaction rate of crotonaldehyde, rbo : overall reaction rate of butyraldehyde. Rate units: lmol seg 1 gPts1 .
208
A.B. Merlo et al. / Catalysis Communications 7 (2006) 204–208
Acknowledgements This work was sponsored by the Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas (CONICET) and the Agencia Nacional de Promocio´n Cientı´fica y Te´cnica (PICT 02 No. 14-11243), Argentina. The XPS experiments were performed at CENACA (Centro Nacional de Cata´lisis), Santa Fe (Argentina) with the equipment donated by JICA (Japan International Cooperation Agency). References [1] [2] [3] [4] [5]
P. Gallezot, D. Richard, Catal. Rev.-Sci. Eng. 40 (1998) 81. P. Claus, Top. Catal. 5 (1998) 51. F. Delbecq, P. Sautet, Catal. Lett. 28 (1994) 89. V. Ponec, Appl. Catal. A: Gen. 149 (1997) 27. B. Didillon, J.P. Candy, F. Lepeltier, O.A. Ferretti, J.M. Basset, Stud. Surf. Sci. Catal. 78 (1993) 147. [6] J.L. Margitfalvi, A. Tompos, I. Kolosova, J. Valyon, J. Catal. 174 (1998) 246.
[7] G.F. Santori, M.L. Casella, G.J. Siri, H.R. Adu´riz, O.A. Ferretti, Appl. Catal. A: Gen. 197 (2000) 141. [8] J.L. Margitfalvi, Gy. Vanko´, I. Borba´th, A. Tompos, A. Ve´rtes, J. Catal. 190 (2000) 474. [9] G.F. Santori, M.L. Casella, G.J. Siri, O.A. Ferretti, Stud. Surf. Sci. Catal. C 130 (2000) 2063. [10] G.F. Santori, M.L. Casella, O.A. Ferretti, J. Mol. Catal. A: Chem. 286 (2002) 223. [11] S. Galvagno, C. Milone, A. Donato, G. Neri, R. Pietropaolo, Catal. Lett. 18 (1993) 349. [12] F.B. Passos, M. Schmal, M.A. Vannice, J. Catal. 160 (1996) 106. [13] F. Coloma, A. Sepu´lveda-Escribano, J.L.G. Fierro, F. Rodrı´guezReinoso, Appl. Catal. A: Gen. 136 (1996) 231. [14] J.M. Ramallo-Lo´pez, G.F. Santori, L. Giovanetti, M.L. Casella, O.A. Ferretti, F.G. Requejo, J. Phys. Chem. B 107 (2003) 11441. [15] F. Coloma, J.M. Coronado, C.H. Rochester, J.A. Anderson, Catal. Lett. 51 (1998) 155. [16] P. Reyes, M.C. Aguirre, I. Melian-Cabrera, M. Lo´pez Granados, J.L.G. Fierro, J. Catal. 208 (2002) 229. [17] C. Li, K. Domen, K. Maruya, T. Ohishi, J. Catal. 125 (1990) 445. [18] A. Dandekar, M. Vannice, J. Catal. 183 (1999) 344.
Hydrogenation of crotonaldehyde and butyraldehyde on silica supported Pt and PtSn catalysts: A drifts study Andrea B. Merlo a, Gerardo F. Santori a,b, Jorge Sambeth a, Guillermo J. Siri Mo´nica L. Casella a, Osmar A. Ferretti a,b,*
a,b
,
a
b
Centro de Investigacio´n y Desarrollo en Ciencias Aplicadas ‘‘Dr. Jorge Ronco’’ (CINDECA), Facultad de Ciencias Exactas, Universidad Nacional de La Plata. 47 No. 257 – C.C. 59 – (1900) La Plata, Argentina Departamento de Ingenierı´a Quı´mica, Facultad de Ingenierı´a, Universidad Nacional de La Plata. 47 No. 257 – C.C. 59 – (1900) La Plata, Argentina Received 8 September 2005; received in revised form 31 October 2005; accepted 4 November 2005 Available online 27 December 2005
Abstract A DRIFTS ‘‘in situ’’ study on Pt and PtSn catalysts used in the crotonaldehyde and butyraldehyde hydrogenation was carried out in this work. Results show that, on Pt/SiO2 catalyst a decarbonylation process takes place during the initial reaction minutes, causing an instantaneous and irreversible deactivation. The tin addition improves the stability of the resulting catalyst due to the inhibition of CO formation. Bands assigned to the presence of superficial olefinic species allow to explain the deactivation observed, which is more important in the case of butyraldehyde hydrogenation. In the crotonaldehyde hydrogenation with PtSn/SiO2 catalyst, the band at 1651 cm 1 is assigned to C@O strongly coordinated, explaining the high selectivity to UOL of this catalytic system. Ó 2005 Elsevier B.V. All rights reserved. Keywords: DRIFTS; Hydrogenation; Crotonaldehyde; Butyraldehyde; PtSn
1. Introduction Unsaturated alcohols (UOL) are usually valuable intermediates in the preparation of fine chemicals [1,2] and can be obtained from the hydrogenation of unsaturated aldehydes (UAL). Transition metals are very active catalysts for this reaction; however, it is well known that with these metals the hydrogenation of the C@C bond is easier than that of the C@O bond [3,4]. For this reason, the addition of a second metal, such as Ge, Sn, Pb is one of the strategies that have been developed in order to improve the selectivity to UOL [5–8]. In previous works, we have studied the hydrogenation of UAL compounds (crotonaldehyde, cinnamaldehyde) with PtSn/SiO2 catalysts obtained via Surface Organometallic Chemistry on Metals (SOMC/M) techniques *
Corresponding author. E-mail addresses: [email protected] (G.F. Santori), [email protected] (O.A. Ferretti). 1566-7367/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2005.11.002
[7,9,10]. Selectivity to UOL is improved by the tin presence, results that have been usually explained through geometric and electronic effects [8,11–14]. For the hydrogenation of UAL and saturated aldehydes (SAL) (butyraldehyde), we found that the rate of C@O bond hydrogenation is higher for bimetallic catalysts when compared with monometallic Pt/SiO2 catalyst. In Fig. 1 we introduce some conversion versus time results obtained for the hydrogenation of butyraldehyde (SAL) and crotonaldehyde (UAL) on Pt and PtSn catalysts, this information was taken from Santori et al. [10]. In this figure a ‘‘flattening’’ of the curves is observed, being more important for butyraldehyde than for crotonaldehyde hydrogenation, and when comparing both catalysts for each reaction, it is much more pronounced for Pt/SiO2 than for PtSn/SiO2. According to the authors, this fact indicates a more important deactivation process for the monometallic catalyst than for the bimetallic one, and more important for SAL than for UAL hydrogenation.
A.B. Merlo et al. / Catalysis Communications 7 (2006) 204–208
100 90 80 Conversion %
70 60 50 40 30 20 10 0 0
100
200
300
Time (min) Fig. 1. Conversion of butyraldehyde (empty symbols) and crotonaldehyde (full symbols) as a function of time for Pt/SiO2 (h) and PtSn/SiO2 (s) catalysts. (taken from [10]).
In order to improve the understanding of the hydrogenation behavior of crotonaldehyde and butyraldehyde on Pt/SiO2 and PtSn/SiO2 catalysts, we present in this communication a diffuse reflectance IR Fourier transform spectroscopy (DRIFTS) study of these reactions. This technique will be used to explain the origin of the deactivation and the selectivity of the catalysts employed in the hydrogenation of UAL and SAL. 2. Experimental Pt/SiO2 and PtSn/SiO2 catalysts were prepared as it is described in a previously published paper [7], with a content of 1% w/w Pt and 0.25% w/w Sn ((Sn/Pt)at = 0.4), respectively. Platinum and tin contents were determined by atomic absorption. Hydrogen chemisorption was measured in a static volumetric apparatus at room temperature. The size distribution of metallic particles was determined by transmission electron microscopy (TEM) using a Jeol 2010 instrument. To estimate the mean particle size (dTEM), the particles were considered spherical and the second moment of the distribution was employed. X-ray photoelectron spectroscopy (XPS) analyses were obtained with an ESCA 750 Shimadzu spectrometer equipped with a hemispherical electron analyzer and a Mg Ka (1253.6 eV) X-ray source. The binding energy (BE) of the C 1s peak at 284.6 eV was taken as an internal standard.
205
Hydrogenation of crotonaldehyde and butyraldehyde was studied in a DRIFTS equipment. Before DRIFTS experiments, Pt/SiO2 and PtSn/SiO2 catalysts were treated under flowing H2, increasing the temperature from ambient to 773 K, and holding it for 2 h. The catalysts were handled without being exposed to the air. Spectra were obtained using a controlled temperature and environment diffuse reflectance DRIFTS chamber (Spectra Tech 0030-103) with KBr windows coupled to a Bruker IFS66 infrared spectrometer. The samples were placed inside the chamber without packing or dilution and spectra were taken between room temperature and 318 K. Then, the reaction was analyzed for 30 min maintaining the temperature at 318 K. Crotonaldehyde and butyraldehyde were fed in a flow of 20 mL min 1 H2/N2 (10/90). Spectra were obtained by collecting 200 scans at 2 cm 1 resolution. 3. Results and discussion Table 1 shows the characteristics of the studied catalysts. From XPS results, it was determined that in the PtSn/SiO2 system, approximately 70% tin is present as Sn(0) and the remainder as ionic tin (Sn(II, IV)). Results of H2 chemisorption on Pt/SiO2 indicate a high dispersion of the metallic phase on the support (H/Pt = 0.64), in agreement with TEM results (dTEM = 2.4 nm). In the case of the bimetallic catalyst, the H/Pt ratio decreases up to a value of about 0.2 and the average particle size increases in 0.3 nm, this fact indicates a high interaction between Pt and Sn. A series of experiments of ‘‘in situ’’ DRIFT spectroscopy was performed, in which IR spectra were recorded during the substrate hydrogenation (crotonaldehyde and butyraldehyde), employing Pt/SiO2 and PtSn/SiO2 catalysts. Fig. 2(a) and (b) show the IR spectra taken during the butyraldehyde hydrogenation and Fig. 3(a) and (b), the corresponding to the crotonaldehyde hydrogenation, for Pt/SiO2 and PtSn/SiO2 catalysts. In the four spectra, bands assigned to m(C@O) appear at approximately 2000–2050 and 1800–1900 cm 1. These bands are attributed to CO adsorbed on the metallic surface arising from decarbonylation of substrates [15]. Figs. 2(a), (b) and 3(a), (b) show that the characteristic of these bands is presented by the fact that they appear as soon as the catalytic reaction begins and that they do not undergo important changes either under time on stream or varying the temperature; this indicates that they arise from strongly chemisorbed species [16]. In this way, this CO adsorbed would be the cause of an irreversible and instantaneous
Table 1 Characterization by XPS and H2 chemisorption and TEM of Pt/SiO2 and PtSn/SiO2 catalysts Catalyst
Pt/SiO2 PtSn/SiO2
Sn/Pt
0 0.40
H/Pt
0.64 0.20
dTEM 2.4 2.7
Binding energies (eV)
Sn(0)/[Sn(0) + Sn(II, IV)]
Pt 4f7/2
Sn(0) 3d5/2
Sn(II, IV) 3d5/2
71.6 70.9
– 485.0
– 487.0
– 0.67
5 min RT 1min RT
3000
2500
2000 1500 Wavenumber (cm-1)
1000
RT
3000
2500 2000 Wavenumber (cm-1)
1863
1990
2330
Absorbance (UA)
5 minRT
2955 2860 2760
1010
1620
1866
1990
2375
2950 2850
Absorbance (UA)
30 min 318 K
303 K
1430 1348
303 K
a
5 min 318 K
1680 1563
1870
318 K 5 min
1400 1325
a
1500
3000
2500 2000 1500 Wavenumber (cm-1)
1000
318 K 30 min 318 K 5 min 303 K RT
1 minRT
b
318 K 30 min
1690 1651 1558 1465 1372
303 K
Absorbance (UA)
5 min 318 K
2330
2955 2860 2760
30 min 318 K
2050
1010
1383 1290
1710 1640
2375
1860
2029
A.B. Merlo et al. / Catalysis Communications 7 (2006) 204–208
2950 2850
Absorbance (UA)
206
1000
Fig. 2. (a) In situ DRIFT spectra of Pt/SiO2 during butyraldehyde hydrogenation at different temperatures and time on stream. (b) In situ DRIFT spectra of PtSn/SiO2 during butyraldehyde hydrogenation at different temperatures and time on stream.
deactivation of the catalysts. For both, butyraldehyde and crotonaldehyde, bands are more important in the Pt/SiO2 monometallic catalyst, particularly the band around 2000–2050 cm 1 corresponding to the Pts–CO lineal form. This fact indicates a more poisonous effect of the CO formed on monometallic than on bimetallic catalysts and explains our catalytic results. Fig. 1 shows a strong increase in the initial hydrogenation rate when passing from monometallic to the bimetallic catalyst, a result that can be correlated with the intensity decrease of the m(C@O) bands when passing from Pt/SiO2 to PtSn/SiO2 catalyst. Consequently, the lower CO contents observed in the bimetallic PtSn/SiO2 catalyst allow to explain the fact that the tin presence increases noticeably the hydrogenation rate of both UAL and SAL due to a decrease of the instantaneous poisoning of the surface (for example, in the case of the crotonaldehyde hydrogenation, rinitial = 95 and 800 lmol s 1 gPts1 , for Pt/SiO2 and PtSn/SiO2, respectively [10]). In the 1000–1700 cm 1 range, the analysis of DRIFT spectra is also very interesting, when it is related to the catalytic properties of the systems under study. In the case of butyraldehyde, for both monometallic and bimetallic
3000 b
2500 2000 Wavenumber (cm-1)
1500
1000
Fig. 3. (a) In situ DRIFT spectra of Pt/SiO2 during crotonaldehyde hydrogenation at different temperatures and time on stream. (b) In situ DRIFT spectra of PtSn/SiO2 during crotonaldehyde hydrogenation at different temperatures and time on stream.
catalysts (Fig. 2(a) and (b)), a band at 1010 cm 1 is observed, which is a typical indication of the presence of a primary alcohol adsorption [17]. This band does not appear for crotonaldehyde (Fig. 3(a) and (b)) probably because although butanol (SOL) is the product of its total hydrogenation, it is found in a low proportion at the conversion level at which the spectra were taken. The fact that the reaction product presents an important adsorption may constitute a penalization of the activity, and this is in agreement with the hydrogenation behavior of butyraldehyde and crotonaldehyde. In DRIFT spectra obtained during crotonaldehyde hydrogenation (Fig. 3(a) and (b)), the region at approximately 1500–1700 cm 1 is interesting in relation to the possible adsorbed species shown in Fig. 4, described according to well proven structures derived from spectroscopic studies [2]. The band at 1680 cm 1 (Pt/SiO2)/1690 cm 1 (PtSn/ SiO2) corresponds to C@O weakly coordinated on the catalyst (pCO); the band around 1560 cm 1 is assigned to C@C strongly coordinated (rCC). In the spectra corresponding to PtSn/SiO2 catalyst, the development of a band
A.B. Merlo et al. / Catalysis Communications 7 (2006) 204–208
Fig. 4. Possible structures for the adsorption of a,b-unsaturated aldehydes (taken from [2]).
at 1651 cm 1 is observed and can be assigned to C@O strongly coordinated (rCO) to the catalyst surface, which could occur in Lewis acid sites (Sn (II, IV), determined by XPS). This adsorption way corresponds to adsorbed species favorable for the unsaturated alcohol formation from unsaturated aldehyde hydrogenation [3,16], which is in agreement with the high selectivity to UOL presented by this catalytic system. In the case of unsaturated aldehydes, the hydrogenation rate shows a noticeable increase in catalysts modified with tin, with respect to the monometallic catalyst. In the systems modified with tin, the dilution favours the presence of species of the types g1-(O) and g2(C,O) in crotonaldehyde; these species could promote the hydrogenation of the C@O group. This dilution must inhibit other forms of chemisorption competitive with the previous ones, as for example of the types g2-(C,C) and (g2-(C,O) + g4-(C,C,C,O)), favourable for the hydrogenation of the C@C group, this leading to a lower SAL production (see Table 2). The analysis of the bands in the region between 2750 and 3000 cm 1 allows to provide an interpretation of the deactivation mechanism observed during the hydrogenation reaction. Bands appearing for butyraldehyde at 2950 and 2850 cm 1 (Fig. 2) and for crotonaldehyde at 2955, 2860 and 2760 cm 1 (Fig. 3) can be assigned to stretching of CHx groups (superficial olefinic species) associated with bands between 1300–1450 cm 1 corresponding to symmetric and asymmetric CHx vibrations as it is proposed by Dandekar and Vannice [18]. It is reasonable to assign these bands to olefinic species of considerable molecular weight (oligomers). Figs. 2(a), (b) and 3(a), (b) show that these
207
bands increase as a function of reaction time and both for butyraldehyde and crotonaldehyde; their presence is more important in the Pt/SiO2 catalyst than in the PtSn/SiO2 system. Furthermore, if a same catalyst is compared for both substrates, it is observed that these bands are more important in the case of butyraldehyde than in the case of crotonaldehyde. These results are in agreement with previously reported ones [10] and with those presented in Fig. 1, in which, during butyraldehyde hydrogenation, a ‘‘flattening’’ in the conversion vs. time curve is observed which is more pronounced for the monometallic catalyst than for the bimetallic one. As another source of deactivation, a contribution from a strong adsorption of the primary alcohol can not be discarded, as it has been previously mentioned. The deactivation process is less important for the crotonaldehyde hydrogenation resulting almost imperceptible in the case of PtSn/SiO2 catalyst. 4. Conclusions DRIFTS ‘‘in situ’’ studies allow us to explain the behavior of Pt/SiO2 and PtSn/SiO2 catalysts tested in the crotonaldehyde and butyraldehyde hydrogenation. The band around 2000 cm 1 corresponding to the lineal form Pts–CO evidences the existence of adsorbed CO generated by the substrate decarbonylation, which would be responsible for an instantaneous and irreversible deactivation of the catalysts. This band appears as soon as the catalytic reaction starts and, for butyraldehyde as well as for crotonaldehyde, it is more important in the Pt/SiO2 monometallic catalyst. Bands appearing in the region between 2700 and 3000 cm 1 can be assigned to stretching of CHx groups (superficial olefinic species), and this constitutes another deactivation source. This deactivation phenomenon results to be evident in the butyraldehyde hydrogenation and it is less important for the crotonaldehyde, especially when the PtSn/SiO2 bimetallic catalyst is used. This reversible deactivation allows to explain the ‘‘flattening’’ observed in the conversion vs. time curves. During the course of crotonaldehyde hydrogenation using the PtSn/SiO2 catalyst, a band developed at 1651 cm 1 is observed and can be assigned to C@O strongly coordinated to the catalyst surface. This fact allows to explain the high selectivity to UOL of this catalytic system.
Table 2 Hydrogenation of crotonaldehyde and butyraldehyde (taken from [10]) Substrate
Crotonaldehyde Butyraldehyde
Catalysts
Pt/SiO2 PtSn/SiO2 Pt/SiO2 PtSn/SiO2
rco 95 800 – –
rbo – – 53 88
5% conversion
80% conversion
SAL
UOL
SOL
SAL
UOL
SOL
68 36 – –
11 59 – –
21 5 100 100
70 30 – –
7 50 – –
23 20 100 100
rco : overall reaction rate of crotonaldehyde, rbo : overall reaction rate of butyraldehyde. Rate units: lmol seg 1 gPts1 .
208
A.B. Merlo et al. / Catalysis Communications 7 (2006) 204–208
Acknowledgements This work was sponsored by the Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas (CONICET) and the Agencia Nacional de Promocio´n Cientı´fica y Te´cnica (PICT 02 No. 14-11243), Argentina. The XPS experiments were performed at CENACA (Centro Nacional de Cata´lisis), Santa Fe (Argentina) with the equipment donated by JICA (Japan International Cooperation Agency). References [1] [2] [3] [4] [5]
P. Gallezot, D. Richard, Catal. Rev.-Sci. Eng. 40 (1998) 81. P. Claus, Top. Catal. 5 (1998) 51. F. Delbecq, P. Sautet, Catal. Lett. 28 (1994) 89. V. Ponec, Appl. Catal. A: Gen. 149 (1997) 27. B. Didillon, J.P. Candy, F. Lepeltier, O.A. Ferretti, J.M. Basset, Stud. Surf. Sci. Catal. 78 (1993) 147. [6] J.L. Margitfalvi, A. Tompos, I. Kolosova, J. Valyon, J. Catal. 174 (1998) 246.
[7] G.F. Santori, M.L. Casella, G.J. Siri, H.R. Adu´riz, O.A. Ferretti, Appl. Catal. A: Gen. 197 (2000) 141. [8] J.L. Margitfalvi, Gy. Vanko´, I. Borba´th, A. Tompos, A. Ve´rtes, J. Catal. 190 (2000) 474. [9] G.F. Santori, M.L. Casella, G.J. Siri, O.A. Ferretti, Stud. Surf. Sci. Catal. C 130 (2000) 2063. [10] G.F. Santori, M.L. Casella, O.A. Ferretti, J. Mol. Catal. A: Chem. 286 (2002) 223. [11] S. Galvagno, C. Milone, A. Donato, G. Neri, R. Pietropaolo, Catal. Lett. 18 (1993) 349. [12] F.B. Passos, M. Schmal, M.A. Vannice, J. Catal. 160 (1996) 106. [13] F. Coloma, A. Sepu´lveda-Escribano, J.L.G. Fierro, F. Rodrı´guezReinoso, Appl. Catal. A: Gen. 136 (1996) 231. [14] J.M. Ramallo-Lo´pez, G.F. Santori, L. Giovanetti, M.L. Casella, O.A. Ferretti, F.G. Requejo, J. Phys. Chem. B 107 (2003) 11441. [15] F. Coloma, J.M. Coronado, C.H. Rochester, J.A. Anderson, Catal. Lett. 51 (1998) 155. [16] P. Reyes, M.C. Aguirre, I. Melian-Cabrera, M. Lo´pez Granados, J.L.G. Fierro, J. Catal. 208 (2002) 229. [17] C. Li, K. Domen, K. Maruya, T. Ohishi, J. Catal. 125 (1990) 445. [18] A. Dandekar, M. Vannice, J. Catal. 183 (1999) 344.