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

Abstract

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 nanofiber (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 2 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.

Comparison of alkene hydrogenation in carbon nanoreactors of different diameters: probing the effects of nanoscale confinement on ruthenium nanoparticle catalysis

“…These results are in agreement with previous observations that performing reactions in confinement has a number of important effects on catalysis and alters the outcome of reactions in a complex fashion; 15 by enhancing the activity of nanaoparticle cataltysts, 47 increasing the local concentration of reagents and thus increasing the rate of reactions, 48 and by imposing restritions on both the transition states of intermediates 41 and the flow of reactants in and products out of the nanoreactor. 27 To investigate the 3D structure of the materials the experimentally measured active surface area of RuNPs@SWNT, RuNPs@GNF and commercial Ru/C catalysts were compared with theoretically calculated surface areas based on ideal models of the materials, i.e.…”

Magnetically Recyclable Catalytic Carbon Nanoreactors

“…Carbon nanotubes are mechanically robust, thermally and chemically stable cylinders of sp 2 ‐carbon that can be used to immobilize both molecules and nanoparticles which efficiently adsorb onto the nanotube walls and/or are encapsulated within the internal cavity of the nanotube via noncovalent interactions such as van der Waals forces . Once the catalyst is immobilized in the hollow structure, catalytic chemical reactions which occur within the accessible nanoscale space of the nanoreactor interior can benefit from enhanced rates of reactions and selectivity …”

“…PtNPs confined within GNF were also investigated in the oxygen reduction reaction (ORR) by Gimenez‐Lopez et al and outstanding electrochemical stability was observed over 50 000 cycles of ORR, with the PtNPs stabilized by the step edges significantly more strongly than commercial PtNPs on carbon black . In addition to NP based catalysts, Lebedeva et al synthesized fullerene containing and fullerene free Pd(II)Salen metal complexes and encapsulated both species on the step edges of the internal GNF surface to form catalysts which displayed significantly higher activity and selectivity in several Heck reactions compared to the reactions in solution …”

“…Therefore, carbon nanotubes are of great interest for use as nanoreactors in a variety of different catalytic chemical reactions as they not only template the formation of catalytically active metallic nanoparticles but also influence the subsequent pathway of reactions . However, despite the fact that carbon nanotubes are excellent support materials for heterogeneous catalyst systems, the inherent properties of carbon nanotubes, including their low density and hydrophobisity, make their separation from the reaction solution using conventional separation techniques, such as filtration and centrifugation challenging, meaning that currently expensive equipment and secondary processes are required .…”

“…Furthermore, GNFs have differently structured internal and external surfaces and wide, continuous internal channels, with an average internal diameter of ≈50 nm. Finally, unlike carbon nanotubes, the internal surface has a succession of step edges which can act as anchoring points for guest species making GNF a highly effective nanoreactor for immobilization of catalytic nanoparticles and to perform catalytic reactions at the nanoscale . With this aim, we developed two different procedures for forming magnetically recyclable GNF based carbon nanoreactors: (1) in situ formation of Fe@C n inside the GNF channels and (2) attachment of commercially available Co@C n to GNF through noncovalent interactions.…”