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

Abstract

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 H₂ pressure and 80–140 °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 CC and CO groups. The steric hindrance due to adsorption of COL and the presence of small amounts of Ni²⁺ favour adsorption of CAL through the CO group leading to higher selectivity to COL.