Nanoparticle catalyst : two-phase catalysis and easy recovery

In the field of biofuel upgrade catalysis, it is highly desirable to have catalyst that can be easily recovered after the reaction is complete and which can stabilize emulsions.

Surfactant molecules have been widely used to increase the interfacial surface area and aid the transfer of molecules between the two phases. However, these surfactants can also be difficult to separate from the final product mixtures.Oxide nanoparticles have previously been used to stabilize oil-in-water emulsions because their hydrophilicity preferentially orients them toward the aqueous phase at the interface. Carbon nanotubes have also been shown to produce emulsions, but of the water-in-oil variety because they are hydrophobic. However, both the type of nanoparticles have application for only limited set of reactions.

Recently, a team of scientists at University of Oklahoma have develop a family of of solid catalysts that can stabilize water-oil emulsions and catalyze reactions at the liquid/liquid interface. By depositing palladium onto carbon nanotube–inorganic oxide hybrid nanoparticles, biphasic hydrodeoxygenation and condensation catalysis in three substrate classes of interest in biomass refining has been demonstrated. Microscopic characterization of the emulsions supports localization of the hybrid particles at the interface.

Ref: Science, Vol. 327, 5961, 68 - 72

Authors: Steven Crossley, Jimmy Faria, Min Shen, Daniel E. Resasco

Effective catalysis using supported Au25 cluster

It's been almost a decade since first report on successful catalysis by supported gold cluster came out. Since then, it has been a hot area of research and scientist are working hard to explore new strategies for catalysis using similar systems.

Recently, an interesting report published in Chem Commun. by Tsukuda and his coworkers wh talks about efficient and highly selective epoxidation of styrene by HAP (hydroxyapatite: Ca₁₀(PO₄)₆(OH)₂ ) supported Au25 cluster using TBHP as oxidant. Synthesis of supported Au25 on HAP involves deposition of Au₂₅ clusters protected by 18 glutathionate (GS) ligands, Au₂₅(SG)₁₈, on an HAP support and then calcine the composite to remove the GS ligands.

It is expected that Au₂₅(SG)₁₈ get adsorbed on HAP by electrostatic interaction between Ca²⁺/PO₄³⁻ moieties and the GS ligands, and that HAP could stabilize bare Au₂₅ clusters against sintering due to strong interaction with the PO₄^3– moiety.

High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) is used to confirm the size and distribution of resulting Au clusters. Further, TG analysis is used to confirm the loss of thiol group from the surface after calcination at 300 degree C in vaccum.
Oxidation of styrene in toluene at 80 °C using anhydrous TBHP as an oxidant yielded styrene oxide as a major product in every case ( slectivity almost 100%) when Au25 support on HAP system is used as catalyst. Catayst also show excellent reusability without any significant loss in either the catalytic activity or the selectivity.

Reference : Chem. Commun., 2010, 46, 550 - 552