Selective hydrogenation of cinnamaldehyde on nickel nanoparticles supported on titania: role of catalyst preparation methods

With nickel acetate as the precursor, 15%(w/w) Ni/TiO2 (P-25) catalysts have been prepared by using four different methods, namely, direct impregnation (IM), deposition-precipitation (DP) with urea and chemical reduction using hydrazine hydrate (HH) and glucose (GL) as reducing agents. XRD, hydrogen chemisorption, TEM and TPR analyses reveal that smaller Ni crystallites, 8-12 nm in size, are obtained by adopting HH & GL methods compared to 15-20 nm crystallites obtained by using IM and DP methods. The nature of metal-support interactions (MSI) varies depending on the method of preparation. XPS studies reveal the presence of residual Ni2+ ions along with the Ni metal. All the catalysts exhibit good conversion of cinnamaldehyde (CAL) (in methanol, at 20 Kg cm(-2) H-2 pressure and 80-140 degrees C) and selectivity to cinnamyl alcohol (COL)/hydrocinnamaldehyde (HCAL) up to 1 h, beyond which further hydrogenation of COL to hydrocinnamyl alcohol (HCOL) occurs. The catalysts prepared by HH & GL display higher CAL conversion and selectivity to COL. Conversion of HCAL to HCOL proceeds at a slower rate compared to that of COL to HCOL. Introduction of HCAL along with CAL as a feed increases selectivity to HCOL, while introduction of COL in the same manner decreases selectivity to HCAL. Infrared spectra of CAL adsorbed on the catalysts reveal surface bonding through C=C and C=O groups. The steric hindrance due to adsorption of COL and the presence of small amounts of Ni2+ favour adsorption of CAL through the C=O group leading to higher selectivity to COL.