Synthesis of cinnamyl ethyl ether in the hydrogenation of cinnamaldehyde on Au/TiO2 catalysts

In this paper we report the results of a study on the hydrogenation of cinnamaldehyde carried out under mild conditions on the Au/Ti02 reference catalyst supplied by the World Gold Council. The reaction was carried out on the as received and on the catalyst reduced al 473K (l TR) and 773 K (HTR). From the hydrogenation of cinnamaldehyde three main products were obtained: cinnamylalcohol (UA) and hydrocinnamaldehyde (HCA), formed from the hydrogenation of the conjugated C=C and C=O bond, respectively, and the asymmetric ether 3-ethoxyprop-1-enylbenzene (CsHsCH=CH-CH2-0-C2H5) or cinnamyl ethyl ether (CEE). The behavior of Au/Ti02 catalyst in the hydrogenation of cinnamaldehyde is of particular interest because it is the first time, as far as we know, that the formation of the allyl ether is observed during this reaction. EXPERIMENTAL: Gold catalyst used in the reaction is .a "reference" Au/Ti02 (TYPE A, Lot No #02-5) supplied by the World Gold Council. The gold load is 1.5 ~% and the metal particle size of the as received sample is 3.7 ± 1.5 nm. The catalyst was reduced at 473 K (L TR) and at 773 K (HTR) for 1 h, under H2 flow. The metal particle size after the reduction treatment were measured by TEM. The catalytic experiments were carried out at atmospheric pressure under H2 flow, at 333 K, in a 100 ml four-necked batch reactor The catalyst (weight 0.5+1g; particle size = 160-200mesh ) was added to 25 ml of ethanol, used as solvent, and treated at 343 K for 1 hour under gaseous H2. The cinnamaldehyde (6 ·10-4 mol) was injected through one arm of the flask. The reaction mixture was stirred at 700 rpm. The progress of the reaction was followed by sampling a sufficient number of microsamples and analyzing them by means of GC-MS (Shimadzu GC-QP5000 Mod.) equipped with a CP-WAX Capillary column, (1.20 ?m, 60 m, 0.32 mm). RESULTS: On the as received Au/Ti02 catalyst the conversion of cinnamaldehyde (CA) leads to the formation of three main products. Cinnamylalcohol (UA) and hydrocinnamaldehyde (HCA) are formed from the hydrogenation of the conjugated C=C and C=O bond, respectively. The third product, identified by means of GC-MS, correspond to the asymmetric ether 3-ethoxyprop-1-enylbenzene (CsHsCH=CH-CH2-0- C2H5) or cinnamyl ethyl ether (CEE). The formation of the saturateci alcohol (SA), 3-phenyl-propan-1-ol, is observed only at high conversion of cinnamaldehyde. The main reaction products UA, HCA and CEE are directly forrned from the conversion of cinnamaldehyde through parallel reactions. The formation of the SA mainly occurs from the further hydrogenation of the C=O bond of HCA. Upon reduction at 473 K (l TR) the selectivity (conv = 50%) towards the formation of HCA and UA decreases whereas the selectivity towards the formation of CEE increases from 19% up to 33%. No change in the product distribution has been observed on HTR with respect to the L TR catalyst. lt has been found that the initial rate of formation of UA and HCA Vf(mol gcat-1 min-\ i.e the rate of hydrogenation of the conjugated C=O bond and C=C bond respectively is not influenced by the reduction of the catalyst remaining almost constant on ali the catalysts investigateci. Conversely, the rate of formation of CEE increases from the as received to L TR catalysts than it remains constant upon a further increase of the reduction temperature up to 773 K. These results suggests that the number of active sites responsible for the hydrogenation of the C=C and C=O bond does not change from the as received to the L TR and HTR catalysts whereas the number of active sites for the synthesis of allyl ether increases already upon low temperature reduction of the catalyst. In a recent study concerning the characterization of the as received Au/Ti02 reference catalysts, used in this work, by means of FTIR-CO adsorplion, Beccuzzi et al .. [1] have found thai afler CO adsorption FTIR speclra show two main bands at 2177 cm-1 and 2150 cm-1 due to the interaction of CO with Ti4+ and OH groups. The intensity of the 2177 cm-1 band is higher with respect to the intensity of the 2150 cm-1 band thus indicating that on the surface of the catalysts the Lewis acid sites are present in larger amount with respect to the Bronsted acid sites. Upon reduction of Au/Ti02 catalyst, in H2 at 573 K, FTIR-CO adsorption spectra show a dramatic increases of the intensity of the band at 2177 cm-1 and the band at 2150 cm-1 almost disappears thus indicating that the reduction of the catalyst leads to an increase of the Lewis acid sites with respect to the as received one. Wilh respect to gold, FTIR-CO spectra pointed out that gold is in metallic state. TEM analysis have shown that the reduction of the catalysts, even at high temperature, do not lead to a significant variation of the gold particle size with respect to the as received sample thus evidencing the strong interaction of the metal with the support. On the basis of the characterization data the catalytic results obtained in the hydrogenation of cinnamaldehyde can be explained considering that the formation of the HCA and UA occurs through the hydrogenation of the C=C and C=O bond adsorbed on the metallic sites whereas the formation of the CEE involves the Lewis acid site of the support. The mechanism of the formalion of the cinnamyl elhyl ether is proposed.