Hydrogenation of Crotonaldehyde by Nanoparticles of Gold Supported on TiO2

Gold 2003
Rodolfo Zanella, Suzanne Giorgio, Raymonde Touroude,
Abstract Hydrogenation of Crotonaldehyde by Nanoparticles of Gold
Supported on TiO2
Rodolfo Zanella1, Suzanne Giorgio2, Catherine Louis1, Raymonde Touroude3

1 Laboratoire de Réactivité de Surface, UMR 7609 CNRS, Université Pierre et Marie Curie,
4 place Jussieu, 75252 Paris Cedex 5, France
2 CRMC2 CNRS, Campus de Luminy, case 913, 13288 Marseille Cedex, France
3 LMSPC, UMR 7515 CNRS, ECPM, Université Louis Pasteur, 25 rue Becquerel, 67087 Strasbourg Cedex 2, France



Introduction
It has been shown in the last decade that gold is an active catalyst for oxidation reactions providing that gold metal particles are small and supported on oxides. Recently, gold supported on oxides has been tested in hydrogenation reactions and there are very promising examples of possible applications, especially in the partial hydrogenation of a,b-unsaturated aldehydes.1-3 The problem of this reaction is that the desired unsaturated alcohol is difficult to achieve because of thermodynamics that favors hydrogenation of C=C over C=O bond. Moreover for kinetical reasons, the reactivity of the C=C bond is higher than that of the C=O bond.
The influence of the particle size on the activity and selectivity for the hydrogenation of a,b-unsaturated aldehydes is not clear. Claus et al.1 studied the effect of particle size in the selective hydrogenation of acrolein, and concluded that activity (per g of Au) and selectivity in allyl alcohol increased with increasing gold particle size from 1 to 2 nm and were constant for particles between 2 and 5 nm. However, they used three different preparation methods to obtain three different particle sizes. To clarify this point, we studied the dependence of the catalytic properties with gold particle size, using one preparation method only. We also studied the influence of the activation temperature and the capacity of catalyst regeneration after deactivation. The reaction used was the hydrogenation of crotonaldehyde (CH3-CH=CH-CH=O).

Experimental
Three Au/TiO2 catalysts with 7 wt % of Au were prepared by deposition-precipitation with urea (DPU) with various deposition times (1, 2 and 16 h) so as to vary the particle size.4 Gold was reduced in situ in the catalytic reactor under a flow of 50 mL.min-1 of H2 for 1 h at temperatures between 120 and 500 °C. Catalytic reactions were carried out at 120 °C and atmospheric pressure, under a flow of 1% of crotonaldehyde in H2 (50 mL.min-1). The selectivities and activities reported in the Table are values measured at pseudo-stationary state and for conversions * 10%.

Results and discussion
The average gold particle size after reduction under H2 at 300 °C decreased from 4 to 1.6 nm when DP time increased from 1 to 16 h (Table). According to UV-Visible spectroscopy, gold is already mostly as Au0 after reduction at 100 °C. All the catalysts undergo a loss of about 50% of their initial activity during the first 20 minutes of reaction, then they are rather stable. After 45 minutes of reaction, catalyst DPU 16h was again submitted to H2 treatment for 1 h at 300 °C and to hydrogenation reaction. The initial activity was fully restored and the catalyst behaved exactly as during the first run. This indicates that (i) deactivation is due to adsorbed species and not to metal sintering; (ii) the size of the gold particles does not change under reaction conditions.
The influence of the reduction temperature on the catalytic properties of DPU 16h was investigated. The catalytic activity (per g of Au) varied as follows with reduction temperature: 300 °C >200=120 °C >400 °C >500 °C (Table), and was therefore maximum after a reduction treatment at 300 °C. The average particle sizes were almost the same for all reduction temperatures (=1.6 nm), except after reduction at 500 °C, which led to larger particles (2.3 nm).
For the DPU catalyst series reduced at 300 °C, the activity per g of gold or per surface Au atom increased when gold particle size decreased (Table).
The selectivities to crotyl alcohol (CH3-CH=CH-CH2-OH) were always between 65 and 70% whatever the catalyst studied and the reduction temperature. It may be pointed out that these selectivities are high compared to that of Pt/TiO2 catalysts, i.e., 25 to 55% depending on the reduction temperature.
Our results are therefore different from Claus’s 1 since the observed trend regarding the activities are different. In addition, the TOF values do not vary within a large range with particle size, 3 to 5.5 10-3 s-1, which rather suggests a“structure insensitive” character for this reaction catalyzed by gold. Moreover, the selectivities are rather high compared to Pt/TiO2 catalysts, and are almost unaffected by the particle size and the reduction temperature.

Table: Catalytic activities of DPU catalysts versus reduction temperature and particle size
Catalysts Reduction
T (°C) Average particle size (nm) Activity
(mmole.gAu-1.s-1) TOF
(s-1)
DPU 1h 300 4 5.1 3.1 10-3
DPU 2h 300 2.3 14.7 5.0 10-3
DPU 16h 300 1.6 23.2 5.5 10-3
DPU 16h 120 1.5 19.6 4.3 10-3
200 1.4 20.3 4.3 10-3
300 1.6 23.2 5.5 10-3
400 1.8 16.6 4.4 10-3
500 2.3 8.7 3.0 10-3

References
(1) Claus, P.; Brückner, A.; Mohr, C.; Hofmeister, H. J. Am. Chem. Soc. 2000, 122, 11430.
(2) Mohr, C.; Claus, P. Sc. Prog. 2001, 84, 311.
(3) Okumura, M.; Akita, T.; Haruta, M. Catal. Today 2002, 74, 265.
(4) Zanella, R.; Giorgio, S.; Henry, C. R.; Louis, C. J. Phys Chem. B 2002, 103, 7634.

Keywords: catalyst, hydrogentaion of crotonaldehyde, Au/TiO2, deposition-precipitation with urea, Particle size
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