Evaluating the effects of carbon nanoreactor diameter and internal structure on the pathways of the catalytic hydrosilylation reaction.

Three different types of carbon nanoreactors, double-walled nanotubes (DWNT), multi-walled nanotubes (MWNT) and graphitised carbon nanofibers (GNF) have been appraised for the first time as containers for the reactions of phenylacetylene hydrosilylation catalysed by a confined molecular catalyst [Rh4(CO)12]. Interactions of [Rh4(CO)12] with carbon nanoreactors determining the ratio of ß-addition products are unchanged for all nanoreactors and are virtually unaffected by the confinement of [Rh4(CO)12] inside carbon nanostructures. Conversely, the relative concentrations of reactants affecting the ratio of addition and dehydrogenative silylation products is very sensitive to nanoscale confinement, with all nanoreactors demonstrating significant effects on the distribution of reaction products as compared to control experiments with the catalyst in bulk solution or adsorbed on the outer surface of nanoreactors. Surprisingly, the widest nanoreactors (GNF) change the reaction pathway most significantly, which is attributed to the graphitic step-edges inside GNF providing effective anchoring points for the catalyst and creating local environments with greatly altered concentrations of reactants as compared to bulk solution. Possessing diameters significantly wider than molecules, GNF impose no restrictions on the transfer of reactants while providing the strongest confinement effects for the reaction. Furthermore, GNF facilitate the effective recyclability of the catalyst and thus represents a superior nanoreactor system to carbon nanotubes.

Competitive hydrosilylation in carbon nanoreactors: probing the effect of nanoscale confinement on selectivity.

Platinum nanoparticles (PtNP) either imbedded within (PtNP@GNF) or adsorbed on the surface (PtNP/GNF) of hollow graphitised carbon nanofibres catalyse hydrosilylation reactions inside or outside the nanoreactor respectively. Comparison of the products formed using PtNP@GNF and PtNP/GNF reveals that nanoreactors create an environment promoting the formation of aromatic over aliphatic products in the competitive hydrosilylation of phenylacetylene with a mixture of triethylsilane and dimethylphenylsilane reactants. Quantification of the distribution of reaction products indicates a three- to four-fold increase in the concentration of aromatic reactants within GNF depending on the dimensions of the carbon nanoreactor. The altered local concentrations of reactants in PtNP@GNF combined with stabilisation of the reaction intermediates by interactions with the nanoreactor interior cause significant changes in the pathways of chemical transformations. The effects of nanoscale confinement on the reactivity of molecules can be harnessed for preparative synthesis in carbon nanoreactors.

Click chemistry in carbon nanoreactors.

Copper-nanoparticle catalytic centres anchored at the graphitic step-edges within hollow carbon nanoreactors exhibit superior activity and stability in cycloaddition reactions as compared to catalytic centres outside the nanoreactors. Nanoscale confinement enables efficient recycling of the catalyst in preparative-scale synthesis without significant changes in activity.

Controlling the regioselectivity of the hydrosilylation reaction in carbon nanoreactors.

Hollow graphitized carbon nanofibres (GNF) are employed as nanoscale reaction vessels for the hydrosilylation of alkynes. The effects of confinement in GNF on the regioselectivity of addition to triple carbon-carbon bonds are explored. A systematic comparison of the catalytic activities of Rh and RhPt nanoparticles embedded in a nanoreactor with free-standing and surface-adsorbed nanoparticles reveals key mechanisms governing the regioselectivity. Directions of reactions inside GNF are largely controlled by the non-covalent interactions between reactant molecules and the nanofibre channel. The specific π-π interactions increase the local concentration of the aromatic reactant and thus promote the formation of the E isomer of the ß-addition product. In contrast, the presence of aromatic groups on both reactants (silane and alkyne) reverses the effect of confinement and favours the formation of the Z isomer due to enhanced interactions between aromatic groups in the cis-orientation with the internal graphitic step-edges of GNF. The importance of π-π interactions is confirmed by studying transformations of aliphatic reactants that show no measurable changes in regioselectivity upon confinement in carbon nanoreactors.

Interactions of gold nanoparticles with the interior of hollow graphitized carbon nanofibers.

Interactions of free-standing gold nanoparticles and hollow graphitized nanofibers in colloidal suspension are investigated, revealing the first example of the controlled arrangement of nanoparticles inside nano-containers, as directed by their internal structure. The ordering is highly effective for small gold nanoparticles whose sizes are commensurate with the height of graphitic step-edges in the graphitized carbon nanofibers and is less effective for larger gold nanoparticles. Studies aimed at understanding the role of the organic-solvent surface tension, employed for the filling experiments, demonstrate that gold nanoparticles become preferentially anchored into the hollow graphitized carbon nanofibers under a mixture of pentane/CO(2) in supercritical conditions. It is shown that a three-step cleaning procedure enables effective removal of gold nanoparticles adsorbed on the exterior surface of graphitized carbon nanofibers, while ordered arrays of encapsulated nanoparticles are retained.

Assembly, growth, and catalytic activity of gold nanoparticles in hollow carbon nanofibers.

Graphitized carbon nanofibers (GNFs) act as efficient templates for the growth of gold nanoparticles (AuNPs) adsorbed on the interior (and exterior) of the tubular nanostructures. Encapsulated AuNPs are stabilized by interactions with the step-edges of the individual graphitic nanocones, of which GNFs are composed, and their size is limited to approximately 6 nm, while AuNPs adsorbed on the atomically flat graphitic surfaces of the GNF exterior continue their growth to 13 nm and beyond under the same heat treatment conditions. The corrugated structure of the GNF interior imposes a significant barrier for the migration of AuNPs, so that their growth mechanism is restricted to Ostwald ripening. Conversely, nanoparticles adsorbed on smooth GNF exterior surfaces are more likely to migrate and coalesce into larger nanoparticles, as revealed by in situ transmission electron microscopy imaging. The presence of alkyl thiol surfactant within the GNF channels changes the dynamics of the AuNP transformations, as surfactant molecules adsorbed on the surface of the AuNPs diminished the stabilization effect of the step-edges, thus allowing nanoparticles to grow until their diameters reach the internal diameter of the host nanofiber. Nanoparticles thermally evolved within the GNF channel exhibit alignment, perpendicular to the GNF axis due to interactions with the step-edges and parallel to the axis because of graphitic facets of the nanocones. Despite their small size, AuNPs in GNF possess high stability and remain unchanged at temperatures up to 300 °C in ambient atmosphere. Nanoparticles immobilized at the step-edges within GNF are shown to act as effective catalysts promoting the transformation of dimethylphenylsilane to bis(dimethylphenyl)disiloxane with a greater than 10-fold enhancement of selectivity as compared to free-standing or surface-adsorbed nanoparticles.