Preparation of mesoporous palladium nanoclusters supported over hematite (α-Fe2O3) for selective catalytic hydrogenation of α,β-unsaturated aldehydes

Abstract

Atomically precise monodisperse palladium nanoclusters protected by N-acetyl-l-cysteine (Pdn(NALC)m) ligand were synthesized. Protected nanoclusters block the mesoporous pores of supports and decrease their specific surface area after loading with 1 wt % as found in our previous work. The new synthesized palladium clusters with special crystal structure did not block the mesoporous pores of iron oxide support (α-Fe2O3). Moreover, the pore volume and pore diameter of the support was increased by synthesizing palladium clusters with 1 wt % dopant. The doped catalyst was irradiated by microwave synthesis labstation 1 h at 80 °C and 500 W to gently remove of the protected N-acetyl-l-cysteine ligands and produce bare palladium clusters supported on iron oxide. The particles size of the synthesized palladium clusters was investigated by high resolution transmission electron microscope (HRTEM). The spectroscopic properties and chemical composition of Pdn(NALC)m clusters were studied by UV-vis spectroscopy and thermogravimetric analysis (TGA), elemental analysis and atomic absorption spectroscopy. FTIR was used for the free ligand (NALC) and the protected palladium cluster, where the disappearance of the S–H vibrational band at 2535–2570 cm−1 in the palladium clusters spectrum confirmed the ligand is anchored to the cluster surface through the sulfur atom. The crystallinity of the bare iron oxide and the doped palladium clusters (1%Pdn(NALC)m/α-Fe2O3) and the microwave treated catalyst (1% Pdn/α-Fe2O3) were determined by powder X-ray diffraction analysis. BET surface area, pore volume and average pore diameter were investigated via nitrogen adsorption at −196 °C. The catalytic properties of atomically precise palladium nanoclusters supported on iron oxide were investigated for reduction of cinnamaldehyde as example for α,β-unsaturated aldehydes. The catalytic activity of synthesized catalysts increased in this order 1% Pdn/α-Fe2O3 > 1% Pdn(NALC)m/α-Fe2O3 > Pdn(NALC)m. The catalytic reduction reaction in toluene was performed at temperature 20–80 °C. The aforementioned synthesized catalysts showed 100% chemoselectivity in the reduction of cinnamaldehyde at room temperature.