Ta2O5–ZrO2 catalysts for vapour phase selective hydrogenation of crotonaldehyde

Applied Catalysis A: General 349 (2008) 165–169

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Pt/Ta2O5–ZrO2 catalysts for vapour phase selective hydrogenation of crotonaldehyde E.V. Ramos-Ferna´ndez a, B. Samaranch b, P. Ramı´rez de la Piscina b, N. Homs b, J.L.G. Fierro c, F. Rodrı´guez-Reinoso a, A. Sepu´lveda-Escribano a,* a b c

Departamento de Quı´mica Inorga´nica, Universidad de Alicante, Apartado 99, E-03080 Alicante, Spain Departament de Quı´mica Inorga`nica, Universitat de Barcelona, C/Martı´ i Franque´s, 1-11E-08028 Barcelona, Spain Instituto de Cata´lisis y Petroleoquı´mica, CSIC, C/Marie Curie, s/n. Cantoblanco, E-28049 Madrid, Spain

A R T I C L E I N F O

A B S T R A C T

Article history: Received 15 January 2008 Received in revised form 20 May 2008 Accepted 22 July 2008 Available online 30 July 2008

This paper reports a study on the effect of the support composition in Pt/Ta2O5–ZrO2 catalysts in regard to their catalytic behaviour in the vapour phase selective hydrogenation of crotonaldehyde (2-butenol). Two Ta2O5–ZrO2 supports with different Ta/Zr ratios and a pure Ta2O5 were prepared by a sol–gel route. The platinum catalysts were characterized by CO chemisorption, infrared spectroscopy of adsorbed CO and X-ray photoelectron spectroscopy (XPS). The best catalyst in terms of activity and selectivity to crotyl alcohol (2-butenol) was Pt/Ta2O5, and this behaviour is tentatively related to the metal–support interaction. On the other hand, under the experimental conditions used, the acidic properties of the supports could not be correlated to the catalytic behaviour of these materials. ß 2008 Elsevier B.V. All rights reserved.

Keywords: Crotonaldehyde hydrogenation Ta2O5–ZrO2 Pt SMSI

1. Introduction The search for chemoselective catalysts is of great interest for the production of many pharmaceutical, agrochemical, and fragrance compounds [1–3]. Thus, the hydrogenation of organic substrates containing a number of unsaturated functional groups is of interest for fundamental research in catalysis. The selective hydrogenation of the carbonyl bond in a,b-unsaturated aldehydes is still a challenge in heterogeneous catalysis by metals. Selective reduction can be achieved by means of properly designed organometallic catalysts, through the Merwein–Ponndorf reaction with alcohols as reducing agents and solid Lewis acids as catalysts [4–6]. However, is not an easy task to hydrogenate a carbonyl bond in the presence of an olefinic bond by using metal-based catalysts, due to the fact that the C C bond is preferentially reduced by both thermodynamic and kinetics considerations [7,8]. Therefore, the promotion of the metal is necessary in order to increase the selectivity towards the formation of the unsaturated alcohol. This promotion effect can be managed through the polarization of the C O bond, which would weaken it, and/or by hindering the adsorption through the C C bond. The desired effect can be

* Corresponding author. Tel.: +34 965 90 39 74; fax: +34 965 90 34 54. E-mail address: [email protected] (A. Sepu´lveda-Escribano). 0926-860X/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2008.07.024

achieved, among other methods, by using a support that can interact with the metal after a reduction treatment, TiO2 [9–12], ZnO [13,14], CeO2 [15–18], MgO [19] or SnO2 [20,21], being the most studied. Another important way to get the promotion effect is by the addition of a second, more electropositive metal like iron, zinc [22], or tin [12,23]. The onset of electronic effects and/or the formation of alloy phases, together with the induction of strong metal–support interaction (SMSI), have been proposed to be responsible for the improvement in selectivity. Recently, a systematic study on materials based on Ta2O5–ZrO2 has been performed where these materials have been characterized and their potential use as catalyst supports has been stated [24]. Special interest has been devoted to the analysis of the surface acidity of these materials. It has been concluded that only Bro¨nsted acid sites are present in Ta2O5 [24]. In principle, the presence of this kind of acid sites is not beneficial in the title reaction. However, the formation of Ta2O5–ZrO2 mixed oxides may modify the acid properties by generating Lewis acid sites [24], which can act as promoters to increase the selectivity towards the desired product (the unsaturated alcohol) [25,26]. Furthermore, Tauster and Fung [27] have demonstrated that the SMSI effect can take place when Ta2O5 is used as catalyst support. This effect, which is produced upon reduction treatments at relatively high temperature (around 773 K), produces an important decrease in the CO or H2 adsorption capacity on the supported metal and drastic changes in their

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2. Experimental

treatment chamber. The residual pressure in the ion-pumped analysis chamber was maintained below 4.2  10 9 mbar (1 mbar = 101.33 Pa) during data acquisition. The binding energies (BE) were referred to the C 1s peak of adsorbed paraffinic hydrocarbons at 284.9 eV. Peak intensity was calculated as the integral of each peak after smoothing and subtraction of a Shirley background [28] and fitting the experimental curve to Gaussian/ Lorentzian lines. The atomic ratios among different elements were calculated using the sensibility factors [29]. The CO chemisorption experiments were carried out in a manometric apparatus at room temperature (298 K). Before CO chemisorption, the samples were reduced at 673 K under flowing hydrogen and then cleaned with helium at the same temperature. Chemisorption data were collected by sequentially introducing small doses (1–10 mmol) of CO (99.5%, with further purification, Air Liquid) onto the sample until it became saturated. The amount of gas adsorbed (mmol) was determined manometrically from the dose, equilibrium pressures, and the system’s volumes and temperatures. The time required for the pressure to equilibrate after each dose was approximately 40 min.

2.1. Catalyst preparation

2.3. Catalytic behaviour

The supports were prepared by sol–gel method. Propanol solutions of tantalum ethoxide and zirconium propoxide, prepared under argon atmosphere, were added at room temperature to a vessel containing water–propanol solution, with vigorous external mechanical stirring. For the Ta2O5 support, only the tantalum ethoxide solution was used. The suspensions were aged at 343 K for 6 h and then at 298 K for 16 h. The solids obtained were filtered, dried at 383 K, and subsequently calcined in air at 873 K for 4 h. Samples were labelled as xTaZr, where x designates the nominal Ta2O5 wt.% in the solid. The catalysts were prepared by impregnation in inert argon atmosphere. Before the impregnation, the supports were pretreated at 473 K under vacuum during 12 h. H2PtCl6
6H2O was used as platinum precursor and dimethylketone as solvent. After the impregnation, the solvent was removed by vacuum at room temperature and the solid was dried in vacuum at 373 K. The catalysts were then calcined at 673 K and reduced under flowing hydrogen at 673 K.

The catalytic behaviour in the vapour phase hydrogenation of crotonaldehyde (2-butenal) was studied in a microflow reactor at atmospheric pressure under differential conditions. Before the determination of the catalytic behaviour, the catalysts (100 mg) were reduced in situ at 673 K under flowing hydrogen (50 cm3 min 1), and then cooled under hydrogen to the reaction temperature, 353 K. Catalysts were contacted thereafter with a reaction mixture (total flow: 50 cm3 min 1 H2/2-butenal ratio of 26) containing purified hydrogen and crotonaldehyde (from Fluka, >99%), prepared by passing a hydrogen flow through a thermostabilized saturator (293 K) containing crotonaldehyde. The concentration of reactants and that of the products at the outlet of the reactor was determined by on-line gas chromatography with a Carbowax column and a FID detector.

catalytic behaviour. This behaviour has been explained by both electronic and structural factors. In the first case, a charge transfer between the partially reduced support and the metal is assumed. The structural approach assumes that entities of partially reduced support would cover the metal particles, thus decreasing the amount of active metal sites but at the same time creating new active sites at the interface between the metal and the support, sites that could be especially active for some reactions, one of which is the selective hydrogenation of a,b-unsaturated aldehydes. However, very few studies have been reported on the use of metal active phases supported on Ta2O5. The present article reports, for the first time, a preliminary study on the use of Pt/Ta2O5–ZrO2 systems in selective hydrogenation reactions. The aim in studying these systems was to investigate the effect of the presence of Ta2O5 in the support on the catalytic behaviour of Pt in the selective hydrogenation of crotonaldehyde. In addition to Ta2O5–ZrO2 materials with different Ta/Zr ratios and surface acidities, we used Ta2O5 as support.

2.2. Characterization The BET surface area was determined by nitrogen adsorption at 77 K with a Micromeritics ASAP9000 instrument. The chemical composition of the samples was determined using inductively coupled plasma with atomic emission spectroscopy (ICP-AES) in an Optima Perkin-Elmer 3200RL apparatus. X-ray diffraction (XRD) patterns were recorded with Siemens D-500 powder X-ray diffractometer equipped with a graphite monochromator and a Cu target. Diffractograms were recorded at a step width of 0.02. The signal was accumulated for 10 s at each step. FTIR analysis of adsorbed CO was carried out with a Nicolet 520 FTIR spectrophotometer equipped with a purge unit, at a resolution of 2 cm 1, by collecting 128 scans. Pellets of 13 mm of diameter were prepared at 400 kPa. The pellet was placed in a specially prepared infrared cell. CO adsorption takes place at room temperature (P = 27 kPa) and, subsequently, thermodesorption of adsorbed CO is achieved by increasing the temperature (298, 323, 373, 423, 473 K). Spectra are collected after each stage. X-ray photoelectron spectra (XPS) were acquired with a VG ESCALAB 200R spectrometer equipped with MgKa (hn = 1253.6 eV,) X-ray exciting source, a hemispherical electron analyzer, and a pre-

3. Results and discussion Table 1 reports the nominal composition of the catalysts prepared and their BET surface areas. The TaZr supports present a surface area higher than that of Ta2O5, the highest surface area corresponding to the 39TaZr support. Fig. 1 shows the XRD patterns of the supports. It shows the interval of 2u angles in which it is possible to detect the crystalline phases of ZrO2 and Ta2O5. The XRD patterns for the TaZr supports present peaks that which can be assigned to the monoclinic and tetragonal phases of ZrO2 [24,30]. The XRD pattern for Ta2O5 is also shown in Fig. 1. The most intense diffraction peak (0 0 1 plane), corresponding to crystalline Ta2O5, which should appear at 2u = 22.858 is not detected. This can be explained on the basis of the calcination temperature (873 K), which is lower than that necessary for the crystallization of Ta2O5 (1023 K) [31,32]. Thus, Ta2O5 calcined at 873 K seems to be nearly amorphous. The support acidity was characterized for two techniques, FTIR analysis of the adsorption of lutidine and 2-propanol dehydration. Table 1 Nominal composition of catalysts and BET surface area Catalysts

B.E.T. (m2/g)

Ta2O5 (%)

Pt (%)

Pt/Ta2O5 Pt/15TaZr Pt/39TaZr

11 54 86

100 14.97 39.48

0.22 0.27 0.26

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Fig. 1. XRD patterns of supports. Diffraction peaks are assigned to monoclinic (M) and tetragonal (T) phases of ZrO2. Fig. 2. CO chemisorption isotherms at 298 K of catalysts reduced at 673 K.

A detailed study has been reported for these materials elsewhere [24]. Lutidine is a specific probe molecule that is appropriate to detect and quantify both Lewis and Bro¨nsted surface acid sites. The adsorption of lutidine over Ta2O5 indicated the presence of surface Bro¨nsted acid sites. Moreover, a small number of Lewis acid sites were also detected. Supports xTaZr showed larger amount of acid sites. Table 2 indicates the amount of acid sites calculated by lutidine adsorption. Supports xTaZr are more acid than Ta2O5, specially the support 39TaZr. The ZrO2 presence generates Lewis acid sites. 2-Propanol dehydratation corroborated these results. Thus, the acidity of the support varies in this order [24]: Ta2O5 < 15TaZr < 39TaZr. Reduced catalysts at 673 K, have been characterized by CO chemisorption at room temperature (298 K). Fig. 2 shows the adsorption isotherms, it can be seen that catalyst Pt/Ta2O5 shows a very low CO adsorption capacity, whereas catalysts with TaZr supports behave in a similar way among them, with a much larger uptake. At first sight, the lower CO uptake of Pt/Ta2O5 could be related to larger Pt particles in this catalyst. However, as it will be discussed below, FTIR analysis of adsorbed CO indicate that this is not the case. On the other hand, it is necessary to take into account that Ta2O5 can interact with the active phase after the reduction treatment as it was reported by Tauster and Fung [27], and a loss of active sites can occur. The different reducibility of the pure Ta2O5 support with regards to the mixed TaZr supports and, as a consequence, the stronger metal–support interaction, can account for the low chemisorption ability of Pt/Ta2O5. By means of FTIR spectroscopy it can be concluded that CO is linearly adsorbed in all samples. Thus, it is possible to estimate the platinum dispersion from the values of adsorbed CO, assuming a CO/Pts stoichiometry of 1:1. In this way, the obtained platinum dispersion was 15% for Pt/Ta2O5, 66% for Pt/15TaZr and 63% for Pt/ 39TaZr.

CO adsorption experiments followed by FTIR were performed with the aim of characterizing the active sites. In order to avoid any dipole–dipole interaction which might modify the n(CO) of adsorbed CO, the singleton frequency (n(CO) at Q(CO) = 0) was determined. For this purpose, we used the method proposed by Primet [33], which consists in recording the spectra of the catalysts exposed to CO from high coverage until the disappearance of the IR band corresponding to CO adsorbed on Pt, and this was achieved by outgassing the sample at increasing temperatures. From a plot of the n(CO) as a function of the CO coverage, the extrapolation of the n(CO) at Q(CO) = 0 provides the singleton value. This value is indicative of interaction between CO and Pt. The values obtained were 2062 cm 1 for Pt/39TaZr, 2058 cm 1 for Pt/15TaZr and 2034 cm 1 for Pt/Ta2O5. Fig. 3 shows the FTIR spectra acquired for Pt/Ta2O5 after CO chemisorption at room temperature and further out-gassing at increasing temperatures. The initial spectra showed a band centred

Table 2 Number of acid sites on the supports determined by lutidine adsorption and further desorption at 423 K [24] Support

Bro¨nsted acidity (mmol/g)

Lewis acidity (mmol/g)

Total acidity (mmol/g)a

Ta2O5 15TaZr 39TaZr

4.6 7.9 21.4

1.5 14.6 16.1

6.1 22.5 37.5

a

Total acidity = Bro¨nsted acidity + Lewis acidity.

Fig. 3. Infrared spectra in the y(CO) region after CO chemisorption at room temperature on Pt/Ta2O5, and thermodesorption at increasing temperatures: T = 298, 323, 373, 423 and 473 K.

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Fig. 4. Values of n(CO) as a function of CO coverage on Pt/Ta2O5.

at 2054 cm 1, which is assigned to CO linearly adsorbed on Pt particles. The presence of this band at low wave numbers could be indicative of an electronic modification of the active phase as a consequence of its interaction with the support. Furthermore, it excludes the possibility of the presence of large platinum particles in this catalyst, as these particles would preferentially expose the Pt(1 1 1), and the IR band of CO adsorbed on that Pt sites would suffer a blue shift towards high wave numbers as a consequence of lateral interactions between adsorbed CO molecules. The values of n(CO) as a function of coverage for Pt/Ta2O5 have been plotted in Fig. 4. By extrapolation, a value of 2034 cm 1 was obtained for n(CO) at zero coverage. It is a low value as compared to those found for platinum on other oxide supports, which could be due to a high interaction between the metal and the Ta2O5 support. The surface composition of the catalysts was analyzed after reduction at 673 K by XPS. The spectra corresponding to the Zr 3d5/ 2, Ta 4f7/2, and Pt 4f7/2 levels were collected. The binding energy value for the Zr 3d5/2 level in both catalysts containing Zr is 182.2 eV, which is typical for the presence of ZrO2. On the other hand, the values of binding energy for the Ta 4f7/2 level are 26.3– 26.4 eV, which corresponds to Ta5+. The Pt 4f7/2 spectra are not so straightforward. Fig. 5 shows the spectra of Pt 4f (4f7/2 and 4f5/2) for the Pt/39TaZr catalyst. The broad band indicates the presence of several platinum species. In any case, the spectra could be deconvoluted into two peaks for each Pt 4f level (4f7/2 and 4f5/ 2). In the case of Pt 4f7/2, two broad bands appeared at 70.8–70.9 eV and 72.3–72.5 eV, respectively. The main band (56–67%) is located at lower binding energies; this band is attributed to metallic platinum (Pt0). The second band, centred at 72.4 eV, is assigned to electron deficient species of Ptn+. The contribution of each band in the 4f7/2 level is reported in Table 3. Complete reduction of Pt was not achieved after the reduction treatment at 673 K, and this can be attributed to strong interaction between the metal species and the support. It is likely that species of the type Pt–O–Ta or Pt–O–Zr can be formed upon the calcination of the just prepared catalysts, which are difficult to reduce. Table 3 shows the XPS atomic ratio Pt/(Zr + Ta) values, which can be used for an estimation of the platinum dispersion on the support. In any case, it is necessary to bear in mind that the XPS analysis can supply information from 2 to 3 nm in depth and, in these multicomponent systems, it may therefore detect species covered by a thin film of other species. The Pt/(Zr + Ta) atomic ratios obtained by XPS are, in all samples, higher than the values found by ICP-AES, as can be seen in

Fig. 5. XP spectrum corresponding to Pt 4f level of reduced Pt/39TaZr catalyst.

Table 3. The atomic ratio Pt/(Zr + Ta) for Pt/Ta2O5 is even higher than in its counterparts, but its capacity for CO adsorption is the lowest (see Fig. 2). These facts can be explained by two factors: (i) the surface area of Pt/Ta2O5 is lower, and (ii) a partial coverage (decoration) of the active phase by a thin film of the support could be taking place [17,22]. Taking into account the CO adsorption followed by FTIR, the XPS results and the CO adsorption isotherms, it is possible to conclude that the active metal (platinum) strongly interacts with the Ta2O5 support. Fig. 6 plots the evolution of the overall catalytic activity (micromoles of crotonaldehyde transformed per second per gram of Pt) as a function of time on stream at 353 K for all samples reduced at 673 K. The catalytic activity for Pt/Ta2O5 is the highest. The characterization results show that this catalyst presents a lower surface than catalysts supported on 15TaZr and 39TaZr. Furthermore, catalyst Pt/Ta2O5 has the lowest CO adsorption capacity. Thus, this catalyst has a lower amount of active sites than catalysts Pt/15TaZr and Pt/39TaZr. In this way, the Pt/Ta2O5 catalyst has a much higher specific activity (activity per number of active sites) than the others. This result shows the exceptional properties of Ta2O5 as support of platinum for this reaction. This enhancement of catalytic performance is ascribed to the SMSI Table 3 Binding energies (BE) (eV) of the Pt 4f7/2 level and atomic ratios determined by XPS and ICP-AES Catalyst

Pt/Ta2O5 Pt/15TaZr Pt/39TaZr a

BE Pt 4f7/2a

70.8 72.3 70.9 72.5 70.8 72.4

(67) (33) (60) (40) (56) (44)

Pt/(Zr + Ta) atomic ratio XPS

ICP-AES

0.0140

0.0025

0.0104

0.0018

0.0122

0.0020

Values in parentheses are peak area percentages.

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metal–support interaction is the main factor affecting the catalytic behaviour. 4. Conclusions

Fig. 6. Catalytic activity at 353 K in crotonaldehyde hydrogenation as a function of time on stream for catalysts reduced at 673 K.

effect, where the active phases are promoted by the support. It is also important to notice the high activity of all catalysts when they are compared with previously reported ones in the same reaction conditions [12,14,17,18,34–36]. The catalytic deactivation in all samples is very low during the reaction time; the catalytic activity remains high after 150 min of reaction. Fig. 7 shows the selectivity of the different catalysts towards the unsaturated alcohol (2-butenol). Catalyst Pt/Ta2O5 presents the highest selectivity towards crotyl alcohol after 140 min on stream. The selectivity increases with the tantalum content. On the other hand, the number of acid sites in the supports (per gram of support) obtained from the analysis of activity in the reaction of 2-propanol dehydration and by 2-lutidine chemisorption followed by FTIR varies in this order [24]: Ta2O5 < 15TaZr < 39TaZr. Thus, the catalytic behaviour in hydrogenation of crotonaldehyde does not correlate with the acidity of the supports. Futhermore and although the support acidity is expected to be modified after the impregnation with the platinum precursor, butenal was the main product found for all the catalysts, and no secondary products related to the acidity of the catalyst surface were detected. All these indicate that acidity does not play an important role in this system. The promoter effect produced by the strong

Fig. 7. Selectivity towards crotyl alcohol at 353 K as a function of time on stream for catalysts reduced at 673 K.

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