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

Abstract

The confinement of cerium oxide nanoparticles within hollow carbon nanostructures has been achieved and harnessed to control the oxidation of cyclohexene. Graphitised carbon nanofibres (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 immobilised at the internal graphitic step-edges of the GNF.Control over the size of the CeO2 nanoparticles has been demonstrated within the range c.a. 4 to 9 nm by varying the mass ratio of the Ce(tmhd)4 precursor to GNF during the synthesis. CeO2@GNF were effective in promoting the allylic oxidation of cyclohexene, in high yield, with timedependent control of product selectivity, at a comparatively low loading of CeO2 of 0.13 mol%.Unlike many of the reports to date where ceria catalyses such organic transformations, we found the encapsulated CeO2 to play the key role of radical initiator due to the presence of Ce 3+ 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 Ce 3+ and Ce 4+ centers. Spatial confinement effects ensure excellent stability and recyclability of CeO2@GNF nanoreactors.

CuO{001} as the Most Active Exposed Facet for Allylic Oxidation of Cyclohexene via a Greener Route

“…Among the studied samples, CuO FL2 shows the highest catalytic activity in terms of the conversion of cyclohexene with high selectivity towards 2‐cyclohexen‐1‐one, which is ascribed to its high surface charge and active exposed facet. It is worth mentioning that the CuO FL2 exhibits a comparable cyclohexene conversion and a higher 2‐cyclohexen‐1‐one selectivity than that reported recently with CeO 2 @graphifitized carbon nanofibers [51] . A more detailed comparison on the cyclohexene conversion and 2‐cyclohexen‐1‐one selectivity in presence of different catalysts is outlined in Table S1, which clearly establishes the advantages of the present catalyst.…”

“…It is worth mentioning that the CuO FL2 exhibits a comparable cyclohexene conversion and a higher 2-cyclohexen-1-one selectivity than that reported recently with CeO 2 @graphifitized carbon nanofibers. [51] A more detailed comparison on the cyclohexene conversion and 2-cyclohexen-1-one selectivity in presence of different catalysts is outlined in Table S1, which clearly establishes the advantages of the present catalyst. It is interesting to find that despite of having a similar chemical composition and higher surface area in CuO PLs compared to CuO FLs, the catalytic activity is better in the case of CuO FLs.…”

“…2‐cyclohexen‐1‐one emerges as the major product in the reaction because it can be formed via a radical pathway and via a non‐radical pathway through oxidation of 2‐cyclohexen‐1‐ol (Figure 6). [51] At the initiation of the reaction, the selectivity of the 2‐cyclohexen‐1‐ol and 2‐cyclohexen‐1‐one are comparable as both are being formed via the radical pathway. But, as the reaction advances, the selectivity of 2‐cyclohexen‐1‐one increases, and 2‐cyclohexen‐1‐ol selectivity decreases (Figure 5c) because the 2‐cyclohexen‐1‐one can be formed via both the radical pathway and a non‐radical pathway through oxidation of 2‐cyclohexen‐1‐ol from the available molecular oxygen in accordance to our proposed mechanism.…”

Stereochemistry of Simple Molecules inside Nanotubes and Fullerenes: Unusual Behavior of Usual Systems

“…Besides that, due to their wide range of pharmacological action, they may be used for the creation of new drugs [205,206] and also as reagents in fine organic synthesis [207][208][209][210][211]. All this makes it relevant to consider the conformational properties of these compounds inside nanotubes, from the perspectives of nanoreactors [68][69][70][71][72][73][74][75] and drug delivery systems [15,19].…”

“…On the other hand, the calculated energy barrier for Cl − exchange in the S N 2 reaction within nanotubes was higher by 6.6 kcal/mol in comparison with the gas phase [69]. Later, it was shown that SWCNTs may be used as effective nanoreactors for preparative syntheses of inorganic [70] and organic [71][72][73][74][75] products with high yields; it is also possible to achieve enantiomeric excess of the products by using a racemic mixture of P and M enantiomers of (n,m) chiral nanotubes [76].…”