HYDROGENATION AND ISOMERIZATION OF α, β-UNSATURATED ALCOHOLS OVER Pd/TiO2 CATALYSTS Maria Grazia Musolino, Paolo De Maio, Andrea Donato, Rosario Pietropaolo Department of Mechanics and Materials, Faculty of Engineering, University of Reggio Calabria, Loc. Feo di Vito, I-89060 Reggio Calabria, Italy - Tel.: 0965 875256; FAX: 0965 875248 e-mail: pietropaolo@ing.unirc.it Introduction The catalytic hydrogenation of olefinic compounds is frequently accompanied by double bond migration and/or cis-trans isomerization [1]. In particular, in the case of α, β-unsaturated alcohols, the double bond isomerization reaction can afford the corresponding saturated aldehydes and ketones trough the following scheme:

Therefore, in these heterogeneously catalysed processes the product selectivity control is of considerable interest in order to address the reaction towards a desired compound. It has been reported in the literature that supported palladium is both the most active and selective catalyst for the hydrogenation and isomerization reactions of alkenes [2]. Studies carried out on the hydrogenation of cis-2-butene-1,4-diol over palladium supported on different metal oxides, such as Al2O3, SiO2, ZnO and TiO2 , have shown that titania is the best support in order to obtain, in high yield, a double bond isomerization product. The peculiar behaviour of TiO2 can be attributed to the presence of Lewis acidic sites on the surface [3]. In this work we report the results on the hydrogenation and isomerization of a series of α, β-unsaturated alcohols, such as 2-propen-1-ol (A2), trans-2-buten-1-ol (TB2), 3-buten-2-ol (B3), trans-2-penten-1-ol (TP2), trans-3-penten-2-ol (TP3), trans–2-hexen-1-ol (TH2), carried out at low partial hydrogen pressure and over 2.5% Pd/TiO2. The major targets of our research are to investigate the influence of the unsaturated alcohol structure on the products distribution and to verify the possibility of a new synthetic route to aldehydes and ketones. Experimental Palladium catalyst was prepared by incipient wetness impregnation of the TiO2 support (anatase phase) with acidic aqueous solution of PdCl2. After impregnation the sample was dried at 393 K and reduced at 423 K for 1h under hydrogen flow. The hydrogenation of different substrates was carried out in liquid phase and in a batch reactor at 303 K and at 0.01 MPa partial pressures of H2, using tetrahydrofuran as

CH CH CH2OHR CH2 CH CHOHR CH2 CH2 CRH

O

CH CH CHR CH2 CHR CH2 CH2R

OH

R' C

OH

R' C

O

R'

isom fast

isom fast

solvent. Before starting the reaction the catalyst was reduced “in situ” at 323 K for 1 h under H2 flow. The progress of the reaction was followed by analyzing samples withdrawn from the reaction mixture by gas chromatography. Results and Discussion The hydrogenation of α, β-unsaturated alcohols was carried out at low partial pressure of hydrogen in order to favour the isomerization reaction versus that of hydrogenation [4]. Under the experimental conditions used, in addition to hydrogenation products (saturated alcohols) we found double bond isomerization and hydrogenolysis compounds. Indeed both double bond isomerization processes from 2 to 1 carbon atom (aldehydes formation) or 3 to 2 (ketones formation) and from 2 to 3 or 3 to 4 were experimentally detected. Subsequent hydrogenation of the carbonyl group of aldehydes or ketones does not occur on palladium catalysts in our experimental conditions (303 K and 0.01 MPA partial pressure of H2). Furthermore, no isomerization reactions occur in absence of hydrogen or on the acidic metal oxide support. Indeed the double bond isomerization is promoted by surface palladium hydride species and, hence, the presence of hydrogen is necessary although its consumption does not take place. Experimental results obtained indicate that the selectivity towards the carbonyl compounds depends on the substrate structure and it decreases as the chain length of the organic substrate increases (Fig.1). Conversely that of other double bond migration products increases. The best yield so far obtained was ~ 80 % when propanal was formed from allylic alcohol. Furthermore, on this substrate, no hydrogenolysis reaction was detected. The behaviour observed can be explained on the basis of possible steric and/or electronic effects of organic moieties involved in the course of the reaction.

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TH2TP2TB2A2

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Full hydr. Aldehyde Isom.

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Figure 1. Selectivity towards hydrogenation (Full hydr.) and double bond isomerization products, calculated at 80 % conversion of the alcoholic substrate. Isom = double bond isomerization compounds other than aldehydes or ketones. References 1. P. N. Rylander, Catalytic Hydrogenation in Organic Syntheses, Academic Press, New York (1979). 2. G. C. Bond, Heterogeneous Catalysis, Clarendon Press, Oxford (1987). 3. M. G. Musolino et al. (unpublished results). 4. M. G. Musolino, C. M. S. Cutrupi, A. Donato, D. Pietropaolo, R. Pietropaolo, Appl. Catal. A:

General (in press).