Liquid phase hydrogenation of methyl-N-Boc-pyrrole-2-carboxylate over tailored Ru nanoparticles

Ru nanoparticles were prepared in ethylene glycol at different pH, temperature and RuCl3·nH2O concentration as well as with and without poly(N-vinyl-2-pyrrolidone) (PVP) as protective agent. They were characterized in the synthesis solution by small-angle X-ray scattering (SAXS) using Guinier analysis and indirect Fourier transformation (IFT) technique and by TEM, XRD, XPS and ATR-IR. From SAXS data evaluation, it was derived that size of the formed Ru nanoparticles depends mainly on the pH of the synthesis solution and only to a minor extent on Ru precursor concentration and reduction temperature. The impact of PVP on particle size was also only very low. The Ru nanoparticles were active for hydrogenation of methyl-N-Boc-pyrrole-2-carboxylate in ethanol solution at 25 °C and p(H2) = 5 bar. Particle size was found to be an important parameter determining hydrogenation activity. A maximum in the activity (TOF = 1000 h−1) was obtained over Ru nanoparticles with a mean particle diameter of about 1.6 nm. However, differently prepared nanoparticles of nearly identical particle size diverge strongly in their catalytic activity. These differences might be due to different surface sites and a blocking of active metal surface sites by surface intermediates formed during nanoparticle formation. Both surface structure and the extent of such blocking strongly depend on the synthesis conditions. If the nanoparticles were protected by PVP, their hydrogenation activity clearly drops in comparison to similarly sized nanoparticles without PVP protection.