Selective hydrogenation of cinnamaldehyde over Pt-supported multi-walled carbon nanotubes: Insights into the tube-size effects

The multi-walled carbon nanotubes (MWCNTs) having different outer diameters but similar inner diameters were loaded with 3.0 wt.% Pt via the reduction of H2PtCl6 by formaldehyde in the solution of ethanol and water. For both the MWCNT supports and catalysts, the BET surface areas and pore size distributions determined by N-2 adsorption and desorption were varied significantly although type 11 adsorption isotherms with closed hysteresis loops were observed. The 3.0 wt.% Pt/MWCNT catalysts were investigated for the selective hydrogenation of cinnamaldehyde (CALD) in a batch reactor under the conditions of 80 degrees C and 2.0 MPa of hydrogen. All the catalysts showed reasonably high catalytic activity but significantly different product selectivities. The highly selective hydrogenation of C = C bonds into hydrocinnamaldehyde (88% selectivity at 100% conversion) occurred over Pt-supported MWCNT4 with the biggest outer diameters (>50 nm). In the case of the other three catalysts, however, C = O bonds were selectively hydrogenated showing about 60-80% selectivities to cinnamyl alcohol (CA) depending on the catalysts and CALD conversions. Irrespective of the catalysts, the results of X-ray diffraction and high-resolution transmission electron microscopy indicated that the metallic Pt particles with an average size of about 11 nm were overwhelmingly deposited on the outer surface of the MWCNTs. The binding energies of Pt and the surface atomic ratios of oxygen to carbon determined by X-ray photoelectron spectroscopy were slightly varied for different catalysts. Based on these characterization results together with the hydrogenation results of CA, the different selective behaviors of the Pt/MWCNTs catalysts were discussed. Besides other factors, the electronic effects induced by the significantly varied tube diameters of the MWCNTs played the major role in determining the selective behaviors of different catalysts. (C) 2008 Elsevier B.V. All rights reserved.