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

Deciphering the Reactive Pathways of Competitive Reactions inside Carbon Nanotubes

Nanoscale control of chemical reactivity, manipulation of reaction pathways, and ultimately driving the outcome of chemical reactions are quickly becoming reality. A variety of tools are concurring to establish such capability. The confinement of guest molecules inside nanoreactors, such as the hollow nanostructures of carbon nanotubes (CNTs), is a straightforward and highly fascinating approach. It mechanically hinders some molecular movements but also decreases the free energy of translation of the system with respect to that of a macroscopic solution. Here, we examined, at the quantum mechanics/molecular mechanics (QM/MM) level, the effect of confinement inside CNTs on nucleophilic substitution (S_N2) and elimination (syn-E2 and anti-E2) using as a model system the reaction between ethyl chloride and chloride. Our results show that the three reaction mechanisms are kinetically and thermodynamically affected by the CNT host. The size of the nanoreactor, i.e., the CNT diameter, represents the key factor to control the energy profiles of the reactions. A careful analysis of the interactions between the CNTs and the reactive system allowed us to identify the driving force of the catalytic process. The electrostatic term controls the reaction kinetics in the S_N2 and syn/anti-E2 reactions. The van der Waals interactions play an important role in the stabilization of the product of the elimination process.

Defect Etching in Carbon Nanotube Walls for Porous Carbon Nanoreactors: Implications for CO2 Sorption and the Hydrosilylation of Phenylacetylene

A method of pore fabrication in the walls of carbon nanotubes has been developed, leading to porous nanotubes that have been filled with catalysts and utilized in liquid- and gas-phase reactions. Chromium oxide nanoparticles have been utilized as highly effective etchants of carbon nanotube sidewalls. Tuning the thermal profile and loading of this nanoscale oxidant, both of which influence the localized oxidation of the carbon, have allowed the controlled formation of defects and holes with openings of 40-60 nm, penetrating through several layers of the graphitic carbon nanotube sidewall, resulting in templated nanopore propagation. The porous carbon nanotubes have been demonstrated as catalytic nanoreactors, effectively stabilizing catalytic nanoparticles against agglomeration and modulating the reaction environment around active centers. CO₂ sorption on ruthenium nanoparticles (RuNPs) inside nanoreactors led to distinctive surface-bound intermediates (such as carbonate species), compared to RuNPs on amorphous carbon. Introducing pores in nanoreactors modulates the strength of absorption of these intermediates, as they bond more strongly on RuNPs in porous nanoreactors as compared to the nanoreactors without pores. In the liquid-phase hydrosilylation of phenylacetylene, the confinement of Rh₄(CO)₁₂ catalyst centers within the porous nanoreactors changes the distribution of the products relative to those observed in the absence of the additional pores. These changes have been attributed to the enhanced local concentration of phenylacetylene and the environment in which the catalytic centers reside within the porous carbon host.

Rh(I)/(III)-N-Heterocyclic Carbene Complexes: Effect of Steric Confinement Upon Immobilization on Regio- and Stereoselectivity in the Hydrosilylation of Alkynes

Rh(I) NHC and Rh(III) Cp* NHC complexes (Cp*=pentamethylcyclopentadienyl, NHC=N-heterocyclic carbene=pyrid-2-ylimidazol-2-ylidene (Py-Im), thiophen-2-ylimidazol-2-ylidene) are presented. Selected catalysts were selectively immobilized inside the mesopores of SBA-15 with average pore diameters of 5.0 and 6.2 nm. Together with their homogenous progenitors, the immobilized catalysts were used in the hydrosilylation of terminal alkynes. For aromatic alkynes, both the neutral and cationic Rh(I) complexes showed excellent reactivity with exclusive formation of the β(E)-isomer. For aliphatic alkynes, however, selectivity of the Rh(I) complexes was low. By contrast, the neutral and cationic Rh(III) Cp* NHC complexes proved to be highly regio- and stereoselective catalysts, allowing for the formation of the thermodynamically less stable β-(Z)-vinylsilane isomers at room temperature. Notably, the SBA-15 immobilized Rh(I) catalysts, in which the pore walls provide an additional confinement, showed excellent β-(Z)-selectivity in the hydrosilylation of aliphatic alkynes, too. Also, in the case of 4-aminophenylacetylene, selective formation of the β(Z)-isomer was observed with a neutral SBA-15 supported Rh(III) Cp* NHC complex but not with its homogenous counterpart. These are the first examples of high β(Z)-selectivity in the hydrosilylation of alkynes by confinement generated upon immobilization inside mesoporous silica.

Rhodium Nanoparticles Stabilized by PEG-Tagged Imidazolium Salts as Recyclable Catalysts for the Hydrosilylation of Internal Alkynes and the Reduction of Nitroarenes

PEGylated imidazolium (bromide and tetrafluoroborate) and tris-imidazolium (bromide) salts containing triazole linkers have been used as stabilizers for the preparation of water-soluble rhodium(0) nanoparticles by reduction of rhodium trichloride with sodium borohydride in water at room temperature. The nanomaterials have been characterized (Transmission Electron Microscopy, Electron Diffraction, X-ray Photoelectron Spectroscopy, Inductively Coupled Plasma-Optical Emission Spectroscopy). They proved to be efficient and recyclable catalysts for the stereoselective hydrosilylation of internal alkynes, in the presence or absence of solvent, and in the reduction of nitroarenes to anilines with ammonia-borane as hydrogen donor in aqueous medium (1:4 tetrahydrofuran/water).

Cerium Oxide Nanoparticles Inside Carbon Nanoreactors for Selective Allylic Oxidation of Cyclohexene

The confinement of cerium oxide (CeO₂) nanoparticles within hollow carbon nanostructures has been achieved and harnessed to control the oxidation of cyclohexene. Graphitized carbon nanofibers (GNF) have been used as the nanoscale tubular host and filled by sublimation of the Ce(tmhd)₄ complex (where tmhd = tetrakis(2,2,6,6-tetramethyl-3,5-heptanedionato)) into the internal cavity, followed by a subsequent thermal decomposition to yield the hybrid nanostructure CeO₂@GNF, where nanoparticles are preferentially immobilized at the internal graphitic step-edges of the GNF. Control over the size of the CeO₂ nanoparticles has been demonstrated within the range of about 4-9 nm by varying the mass ratio of the Ce(tmhd)₄ precursor to GNF during the synthesis. CeO₂@GNF was effective in promoting the allylic oxidation of cyclohexene in high yield with time-dependent control of product selectivity at a comparatively low loading of CeO₂ of 0.13 mol %. Unlike many of the reports to date where ceria catalyzes such organic transformations, we found the encapsulated CeO₂ to play the key role of radical initiator due to the presence of Ce³⁺ included in the structure, with the nanotube acting as both a host, preserving the high performance of the CeO₂ nanoparticles anchored at the GNF step-edges over multiple uses, and an electron reservoir, maintaining the balance of Ce³⁺ and Ce⁴⁺ centers. Spatial confinement effects ensure excellent stability and recyclability of CeO₂@GNF nanoreactors.

Movement of palladium nanoparticles in hollow graphitised nanofibres: the role of migration and coalescence in nanocatalyst sintering during the Suzuki-Miyaura reaction

The evolution of individual palladium nanoparticle (PdNP) catalysts in graphitised nanofibres (GNF) in the liquid-phase Suzuki-Miyaura (SM) reaction has been appraised. The combination of identical location-transmission electron microscopy (IL-TEM) and a nano test tube approach allowed spatiotemporally continuous observations at the single nanoparticle level, revealing that migration and coalescence is the most significant pathway to coarsening of the nanocatalyst, rather than Ostwald ripening. IL-TEM gave unprecedented levels of detail regarding the movement of PdNP on carbon surfaces at the nanoscale, including size-dependent migration and directional movement, opening horizons for the optimisation of future catalysts through surface morphology design.

Chemical reactions at the graphitic step-edge: changes in product distribution of catalytic reactions as a tool to explore the environment within carbon nanoreactors

A series of explorative cross-coupling reactions have been developed to investigate the local nanoscale environment around catalytically active Pd(ii)complexes encapsulated within hollow graphitised nanofibers (GNF). Two new fullerene-containing and fullerene-free Pd(ii)Salen catalysts have been synthesised, and their activity and selectivity towards different substrates has been explored in nanoreactors. The catalysts not only show a significant increase in activity and stability upon heterogenisation at the graphitic step-edges inside the GNF channel, but also exhibit a change in selectivity affected by the confinement which alters the distribution of isomeric products of the reaction. Furthermore, the observed selectivity changes reveal unprecedented details regarding the location and orientation of the catalyst molecules inside the GNF nanoreactor, inaccessible by any spectroscopic or microscopic techniques, thus shedding light on the precise reaction environment inside the molecular catalyst-GNF nanoreactor.

Chemical reactions confined within carbon nanotubes

The confinement of molecules and catalysts inside carbon nanotubes affects the yield and distribution of products of preparative chemical reactions.

Confinement effects on the shape and composition of bimetallic nano-objects in carbon nanotubes

The shape and the composition of bimetallic alloys prepared by in situ reduction of molecular precursors depends on the carbon nanotube diameter.