Catalytic Transfer Hydrogenation Of Cinnamaldehyde With Alcohol As Hydrogen Source

Chemoselective hydrogenation of cinnamaldehyde into cinnamyl alcohol is a challenging reaction since the hydrogenation of the C=O bond, while leaving the C=C intact, is not favored from a thermodynamic point of view. Cinnamyl alcohol is an important intermediate in the synthesis of fine chemicals, pharmaceuticals and cosmetics. In the traditional industrial process metal hydrides are used to carry out this transformation which is not an environmentally benign process. Noble metal based catalysts have been widely studied for this reaction but in some cases it is also difficult to control the products selectivity. Among various methods such as the use of stoichiometric reducing reagents and hazardous molecular hydrogen ?H2?, transfer hydrogenation has attracted much attention, because the hydrogen donor ?e.g. 2-propanol? is cheap and easy to handle, and no elaborate setups ?e.g. high pressure reactors? are required. In the present study, the selective hydrogenation of cinnamaldehyde was carried out by catalytic transfer hydrogenation method using different alcohols as the hydrogen source and established the plausible reaction mechanism.A series of noble and non-noble metal catalysts were prepared by impregnation method with different supports including MgO, Al₂O₃, Mg-Al hydrotalcite ?HT? and activated carbon ?AC?. Physico-chemical characteristics of these catalysts were investigated by different analytical techniques such as N2 sorption, XRD, XPS, TPD of CO2, H2-TPR, SEM and STEM etc. The catalytic performance was studied in liquid phase by the selective hydrogenation of cinnamaldehyde with alcohol as the in-situ hydrogen source. The effect of catalyst composition on the catalytic performance was studied, including the active site, the support and the loading amount of the active site. It was found that, ?i? among the various supports, HT and ?-Al₂O₃ support ?i.e., with both acidic and basic sites? and MgO support ?i.e., with basic sites? exhibited higher activity than neutral carbon support. The increased catalytic activity and selectivity over the Mg-Al hydrotalcite support may be due to the synergistic effect of both acidic and basic sites; ?ii? the activity of Ni/HT catalyst was higher than those of HT-supported noble metal ?Ag, Pt, Au? and non-noble metal ?Cu? catalysts; ?iii? the amount of metal also has an effect on the catalytic activity and the 1wt% Ni/HT exhibited superior activity and selectivity over the other catalysts.The effect of different reaction parameters on the catalytic performance were studied, including reaction temperature, reaction time and the amount of catalyst. It was revealed that the reaction conditions can affect the conversion of cinnamaldehyde. Under the optimum conditions ?i.e., T=120?, N2, t=6 h, o-xylene solvent,0.1 g catalyst?, the conversion of cinnamaldehyde ?0.5 mmol? was 86%,99%,78% with benzyl alcohol,1-phenyl ethanol and 2-propanol as the hydrogen source ?5 mmol? respectively and the selectivity of cinnamyl alcohol was always more than 99%.The possible reaction mechanism is proposed from the experimental results. It was observed that, firstly, oxidation of the alcohol occurs on the surface of Ni/HT catalyst through abstraction of the alcohol by a basic site of HT to generate [H-HT]+ and alkoxide species at the Ni-HT interface. The alkoxide species then undergoes ?-hydride elimination to give an [H-Ni]- species together with the corresponding carbonyl compound. Next, the cinnamaldehyde is absorbed via the vertical C=O atop geometry though the interaction between the carbonyl group and positively charged center Al3+ and carbonyl group is active. Finally, the active H atoms from [H-HT]+ and [H-Ni]-attack the C=O group of cinnamaldehyde, which subsequently provides cinnamyl alcohol, thereby completing the catalytic cycle.