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

Article abstract of DOI:10.1021/acs.nanolett.9b04579

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.

Full text of DOI:10.1021/acs.nanolett.9b04579

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Communication
Cerium oxide nanoparticles inside carbon nanoreactors
for selective allylic oxidation of cyclohexene
Nityananda Agasti, Maxwell A. Astle, Graham A. Rance, Jesum
Alves Fernandes, Jairton Dupont, and Andrei N. Khlobystov
Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b04579 • Publication Date (Web): 24 Jan 2020
Downloaded from pubs.acs.org on January 25, 2020
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Cerium oxide nanoparticles inside carbon
nanoreactors for selective allylic oxidation of
cyclohexene
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a†
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a,b
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Nityananda Agasti , Maxwell A. Astle *, Graham A. Rance , Jesum Alves Fernandes , Jairton
Dupont a,c Andrei N. Khlobystov a,b*
a
School of Chemistry, University of Nottingham, University Park, Nottingham, NG7 2RD, United
Kingdom.
b
Nanoscale and Microscale Research Centre (nmRC), University of Nottingham, University Park,
Nottingham, NG7 2RD, United Kingdom.
c
Univ Fed Rio Grande do Sul, Inst Chem, Ave Bento Goncalves 9500, BR-91501970 Porto
Alegre, RS, Brazil.
KEYWORDS
CeO nanoparticles, carbon nanotubes, nanoreactors, alkene oxidation, nanocatalysis
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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
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(
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 CeO @GNF,
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where nanoparticles are preferentially immobilised at the internal graphitic step-edges of the GNF.
Control over the size of the CeO nanoparticles has been demonstrated within the range c.a. 4 to 9
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nm by varying the mass ratio of the Ce(tmhd) precursor to GNF during the synthesis. CeO @GNF
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were effective in promoting the allylic oxidation of cyclohexene, in high yield, with time-
dependent control of product selectivity, at a comparatively low loading of CeO of 0.13 mol%.
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Unlike many of the reports to date where ceria catalyses such organic transformations, we found
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the encapsulated CeO to play the key role of radical initiator due to the presence of Ce included
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in the structure, with the nanotube acting as both a host, preserving the high performance of the
CeO nanoparticles, anchored at the GNF step-edges, over multiple uses, and an electron reservoir,
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maintaining the balance of Ce and Ce centers. Spatial confinement effects ensure excellent
stability and recyclability of CeO @GNF nanoreactors.
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INTRODUCTION
Lanthanide compounds continue to attract significant attention owing to their unusual magnetic,
redox and optical properties, different to d-block elements, which is attributed to the nature of the
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f orbitals, buried deep inside the atom and thus shielded from the external environment by the
overlying 5s and 5p orbitals. This contracted nature of the 4f orbitals gives lanthanides their unique
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physicochemical properties and underpins a wide range of applications in catalysis, magnetism
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and pharmacology. Among these, catalysis alone represents nearly 75% of the total applications
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of the lanthanides. As a catalyst, cerium (Ce), unlike most of the other lanthanides, is utilised in
the form of its oxide (CeO ), also known as ceria, with a wide range of ceria-based nanocatalysts,
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including spheres, rods, tubes, wires, shuttle-shaped and flower-like nanoparticles, reported
over the last decade. The remarkable catalytic behaviour of ceria observed ubiquitously throughout
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these nanostructures is due to the interconversion between Ce and Ce , controllable by the
reaction conditions, with CeO and Ce O observed under oxidising and reducing conditions,
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respectively. As a consequence of this interconversion, CeO can reversibly release oxygen from
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its structure at moderate temperatures (<600 C) leading to the formation of oxygen vacancies in
the crystal lattice and simultaneous reduction of Ce4+ to Ce . This behaviour makes ceria
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extremely useful for oxygen storage and release applications, as well as in other important
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chemical reactions, such as the preparation of imines, the oxidation of CO to CO and the
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hydrogenation of CO to methanol. However, a less explored and often disregarded application
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of ceria is its ability to initiate radical-mediated organic transformations, which often commence
in the homolytic cleavage of organic peroxides by Ce3+ as demonstrated in the oxidation of
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cyclohexene, the products of which represent important intermediates in the chemical industry.
Akin to the structure-function relationships observed in ceria nanocatalysis, the shape and size of
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ceria nanoparticles are expected to modulate its efficiency as an initiator; 17, 18 yet, further research
is critically required to probe how the physical and chemical properties of CeO influence the
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Despite the breadth of ceria-based nanostructures synthesised to date, the controllable
preparation of monodisperse ultra-small CeO nanoparticles remains a significant challenge.
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Moreover, preservation of their functional properties by preventing deactivation under harsh
reaction conditions is essential for their further development. One recent approach to address the
latter concerns the encapsulation of CeO nanoparticles in core-shell and yolk-shell structures,
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where CeO is protected by an outer shell, which was shown as an efficient way to stabilise CeO2
nanoparticles and explore the effect of spatial confinement at the nanoscale.19,20 However, carbon
nanotubes (CNTs) as a host matrix for the confinement of CeO nanoparticles have the potential
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to solve both of these challenges. Besides being a container with two dimensions on the nanoscale
and thus the capacity to template the formation of particles with nanoscopic sizes, the remarkable
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mechanical (tensile strength much higher than that of steel), thermal (stable up to ca. 600 C in air
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and up to 2800 C in vacuum or inert atmosphere) and chemical stability of CNTs makes them an
excellent candidate support material for nanoparticles under a wide variety of reaction
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conditions. In addition, the curvature of the CNT walls causes re-distribution of the π electron
density of graphene layers from the concave inner to the convex outer surface resulting in an
electron density gradient. As a consequence, encapsulated metal nanoparticles inside carbon
nanotubes often exhibit properties distinct from those adsorbed on the CNT outside surface.22
Therefore, the encapsulation of metal or metal oxide nanoparticles inside CNTs opens a pathway
towards active nanoreactors with tuneable properties, such as those reported by our group23,24 and
others previously.25,26
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While several examples of the successful deposition of CeO nanoparticles onto the outer surface
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biosensing29,30
of carbon nanotubes (CeO -CNT) have been reported and applied in catalysis,
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and as a fuel additive, the preparation of ceria nanoparticles inside carbon nanotubes remains
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largely unexplored, with only one example to date reported for CeO nanoparticles inside carbon
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nanotubes of inner diameter 4-8 nm. Such narrow nanotubes serve as excellent model systems
for catalyst supports, but their applications for preparative scale reactions are limited. In this study,
we utilised graphitised carbon nanofibres (GNF) that have many of the attractive properties of
carbon nanotubes, but with the added benefits of (i) large internal diameters (typically larger than
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0 nm) enabling the efficient mass transport of reactants and products to and from the internal
cavity and (ii) corrugated internal surfaces resulting in the effective stabilisation of nanoparticles,
as demonstrated in our previous studies (Figure 1).33-36 Herein, successfully prepared ceria
nanoparticles within GNF (CeO @GNF) were analysed by a variety of structural and chemical
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characterisation techniques, and applied in the preparative-scale reaction of cyclohexene
oxidation, revealing key features of nanoscale confinement in carbon nanoreactors important for
sustainable organic synthesis.
Figure 1. (a) Low and high magnification (inset) high resolution transmission electron microscopy (HRTEM) images of empty
hollow graphitised carbon nanofibres (GNF). As shown in both the electron micrographs in (a) and the schematic representation
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in (b), GNF are topologically complex nanoscale structures comprising a series of stacked cups encased within concentric tubes.
The internal diameters are much larger than the dimensions of simple molecular species (typically by more than an order of
magnitude) and the termini are always open, thus facilitating highly efficient transport of molecules through the internal volume.
Furthermore, the 3-4 nm high folds on the interior surfaces of GNF formed by rolled-up sheets of graphene provide exemplary
anchoring points for guests, such as molecules and nanoparticles, as shown in (b), due to maximised van der Waals interactions.
This creates localised nanoscale reaction environments, different to the bulk phase, which impart spatial restrictions and thus can
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significantly impact the yields and distributions of products afforded across a range of preparative chemical transformations, as
demonstrated by us and others.37-48
EXPERIMENTAL SECTION
General
All reagents and solvents were purchased from Sigma-Aldrich, including cyclohexene (>99%) and
were used as received. Graphitised carbon nanofibres (GNF) and tetrakis(2,2,6,6-tetramethyl-3,5-
heptanedionato) cerium (IV) (99.9% Ce) were purchased from Pyrograf Products, USA and Strem
Chemicals, USA, respectively. Graphite flakes (Code G/0900/60, Batch 043775) and activated
carbon (Activated Charcoal, DARCO) were purchased from Fisher Scientific. All glassware was
cleaned with a mixture of hydrochloric and nitric acid (3:1 v/v, “aqua regia”) and rinsed thoroughly
with deionised water, cleaned with potassium hydroxide in isopropyl alcohol and finally rinsed
thoroughly with deionised water before use. Prior to use, GNF was ball milled using a Retsch
MM40 Ball Miller with 5 mL jars and 12 mm stainless steel balls at a frequency of 10 per second
for 90 minutes.
Preparation of CeO nanoparticles encapsulated within graphitised carbon nanofibres
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CeO @GNF) and deposited on graphite (CeO @graphite) and activated carbon
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Graphitised carbon nanofibres (10 mg, PR19-XT-HHT, ball-milled) and Ce(tmhd) (1, 2 or 3 mg)
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were sealed in a Pyrex glass ampoule under vacuum (2.9 x 10 mbar) and heated at 150 C in an
oil bath for 72 hours to allow for sublimation of the cerium complex and diffusion inside the
channels of GNF. After 72 hours, the ampoule was quickly cooled by placing into ice cold water
for 10 minutes before opening. The obtained bulk solid was then placed in another Pyrex glass
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ampoule, evacuated at 1.6 x 10 mbar and backfilled with argon (this procedure was repeated three
times to ensure removal of molecular oxygen and atmospheric moisture), and the ampoule was
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then sealed and heated at 600 C for 2 hours to decompose the cerium complex to CeO , followed
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by slow cooling for 8 hours to a final temperature of 20 C to yield the resultant CeO @GNF-X
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material (where X = 1, 2 or 3 corresponding to the mass of cerium complex used in the initial step).
The CeO @graphite and CeO @AC nanocomposite was prepared using an analogous method with
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graphite powder or activated carbon (AC) and Ce(tmhd) in a mass ratio of 10:2 (mg), to serve as
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Characterisation of CeO @GNF, CeO @graphite and CeO @AC
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High resolution transmission electron microscopy (HRTEM) was performed using a JEOL 2100F
transmission electron microscope (field emission electron gun source, information limit 0.19 nm).
Energy dispersive X-ray (EDX) analysis was performed using an Oxford Instruments INCA 560
X-ray microanalysis system. TEM samples were prepared by dispersing small quantities in
methanol with bath sonication for 30 seconds to 1 minute, before drop-casting onto a copper TEM
grid coated with a lacey carbon film (Agar Scientific UK) and drying in air. Particle size and d-
spacing were determined using Gatan Digital Micrograph software. Thermogravimetric analysis
(TGA) of samples was carried out using a TA Q500 Thermogravimetric Analyser. All samples
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were deposited onto platinum pans and analysed in air at a heating rate of 10 C/min from 20 to
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000 C with an isotherm for 10 min at 1000 C, and airflow of 60mL/min. Raman spectroscopy
was performed using a Horiba-Jobin-Yvon LabRAM HR spectrometer. Spectra were acquired
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conferring a spectral resolution of better than 1.8 cm . Spectra were baseline corrected using a
linear fitting model. The powder X-ray diffraction (PXRD) patterns were recorded using a
PANalytical X’Pert Pro diffractometer equipped with a Cu K(α) radiation source (λ = 1.5432 Å,
at 40kV and 40mA) in Bragg-Brentano geometry using a Si zero background holder. A few drops
of isopropyl alcohol were added to all samples for good adhesion to the sample holder. The
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parameters for a typical measurement were as follows: start angle 5 , stop angle 120 , step size
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0.0525 , time/step 6080s, scan speed 0.002200/s. X-ray photoelectron spectroscopy (XPS)
measurements were performed using a Kratos AXIS Ultra DLD instrument. The chamber pressure
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during the measurements was 5×10 Torr. Wide energy range survey scans were collected at a
pass energy of 80 eV in hybrid slot lens mode and a step size of 0.5 eV over 20 min. The charge
neutraliser filament was used to prevent the sample from charging over the irradiated area. The X-
ray source was a monochromated Al Kα emission, run at 10 mA and 12 kV (120 W). The energy
range for each ‘pass energy’ (resolution) was calibrated using the Kratos Cu 2p , Ag 3d and
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Au 4f7/2 three-point calibration method. The transmission function was calibrated using a clean
gold sample method for all lens modes and the Kratos transmission generator software within
Vision II. The data were processed with CASAXPS (Version 2.3.17). The high resolution data was
charge corrected to the reference C 1s signal at 284.5 eV.
Oxidation of cyclohexene
The oxidation of cyclohexene with tert-butylhydroperoxide (TBHP, 70 wt% in H O) was carried
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using either CeO @GNF, CeO @graphite or CeO @AC in a two-necked glass round-bottom flask
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equipped with a magnetic stirrer and a reflux condenser open to air. In a typical reaction, a mixture
of cyclohexene (2.9 mmol), TBHP (5.8 mmol), CeO @Y (10 mg, where Y = GNF, graphite or
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AC, containing 0.09-0.18 mol % CeO ) and 1,4-dichlorobenzene (1.4 mmol) as an internal
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standard were dispersed in acetonitrile (2.5 mL) and stirred at 80 C. The progress of the reaction
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was followed using H NMR spectroscopy. To study the reaction kinetics, aliquots (0.01 mL) were
taken from the reaction mixture at one hour intervals. Confirmation of the final products was
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determined using H NMR spectroscopy. For recycling studies, the catalyst was recovered by
filtration, washed with methanol and dried overnight before repeating the procedure for oxidation
of cyclohexene. The same reaction conditions mentioned above with freshly loaded reactants. The
recycling reaction was conducted five times for CeO @GNF.
2
RESULTS AND DISCUSSION
Growth of CeO nanoparticles inside GNF (CeO @GNF)
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CeO nanoparticles encapsulated inside graphitised carbon nanofibres (CeO @GNF) have been
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prepared by heating a volatile precursor deposited within GNF (Scheme 1).
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Scheme 1. Schematic illustration depicting the preparation of CeO
initial sublimation under vacuum and subsequent thermal decomposition of the cerium complex, tetrakis(2,2,6,6-tetramethyl-3,5-
heptanedionato)cerium(IV) [Ce(tmhd) ] into cerium oxide, CO and CO in an inert atmosphere.
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nanoparticles inside graphitised carbon nanofibres by
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Prior to filling, graphitised carbon nanofibres were ball milled to reduce their length from more
than 10 µm to around 1.2 µm to reduce the possible impact of length-dependent transport resistance
of reactant molecules from the bulk liquid-phase when later applied as a nanoreactor. The
precursor Ce(tmhd) was mixed with milled GNF, sealed under vacuum in a glass ampoule and
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heated above its sublimation temperature to ensure the diffusion of Ce(tmhd) vapour into the
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channel of GNF, forming the [Ce(tmhd) @GNF] composite. The ampoule was then rapidly cooled
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in a cold water reservoir to facilitate the condensation of the precursor molecules inside GNF.48
The [Ce(tmhd) @GNF] composite was subsequently heated under an argon atmosphere at a
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o
temperature above the decomposition point of the Ce(tmhd) , in this case 600 C, (Figure S1, SI)
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yielding CeO nanoparticles inside GNF. In order to study the effect of precursor concentration on
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the formation of CeO nanoparticles, three different amounts of the cerium complex (1, 2 and 3
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mg) have been used with a fixed mass of GNF (10 mg) under the same reaction conditions.
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Figure 2. (a) TGA, (b) Raman spectroscopy, (c) PXRD and d) XPS analysis of the CeO @GNF nanocomposites. A decrease
in the GNF combustion temperature and increase in the residual mass of CeO
thermograms (a) both correlate with the increasing amount of cerium complex used in the initial step. The Raman spectra (b) and
PXRD patterns (c) of the CeO @GNF nanocomposites both confirm the presence of both GNF and CeO . Traces in (b) and (c)
have been offset on the y-axis for visual clarity. The Raman spectra in (b) have been normalised to the intensity of the G-band.
Annotated labels in (b) and (c) indicate Raman bands and diffraction planes corresponding to CeO (red) and GNF (black). The
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obtained after GNF combustion observed in the
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fitted peak envelope for Ce 3d (d) in the XP spectrum confirms the presence of Ce3+ which can act as an initiator in the reaction
of cyclohexene and TBHP.
To ascertain the loading of ceria in the CeO @GNF nanocomposites and to study the resultant
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GNF thermal stability, thermogravimetric analysis was conducted (Figure 2a). For all three
CeO @GNF samples, a single weight loss, representing ~92-95% of the total weight and
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corresponding to the combustion of GNF, was observed. The residual weight subsequent to GNF
combustion (4.7, 5.9 and 7.7 %) is associated with CeO loading and correlates well with the initial
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amount of cerium precursor added to GNF (1, 2 and 3 mg, respectively). It is interesting to note
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that the GNF combustion temperature for CeO @GNF is much lower (~625-675 C) than that of
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empty GNF (~750 C). This decreases with increasing CeO loading and indicates a catalytic effect
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of CeO on the oxidation of graphitic carbon in air.
This observation supports the notion that
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CeO is an effective catalyst of oxidation due to the reversible loss of oxygen and interconversion
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+
3+
31
of Ce and Ce , thus facilitating the combustion of GNF at a lower temperature. TGA profiles
also indicate a uniform one-step oxidation process, which is consistent with a homogeneous
composition of the total CeO @GNF sample, i.e. negating the independent formation of CeO and
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GNF in isolation.
The Raman spectra of CeO @GNF (Figure 2b) are dominated by two characteristic GNF bands:
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the G (graphitic) band – a carbon-carbon stretching vibration of E symmetry, typically observed
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g
-1
in graphitic nanocarbons – at ~1580 cm and the D (disorder) band – a ring-breathing mode of A
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-1 50
symmetry, requiring a defect for its activation – at ~1340 cm . Interestingly, a small increase in
the intensity ratio of the D and G bands (I :I ) in the CeO @GNF nanocomposites relative to GNF
D
G
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was observed, indicating a change in the structural ordering in the graphitic sidewalls of GNF,
consistent with the shortening/opening and thermal treatments employed during the preparation of
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the nanoreactors. In addition, a very weak band at ~450 cm is observed in some of the Raman
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spectra of CeO @GNF, consistent with the vibration of the Ce-O bond, confirming the formation
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of CeO . It is important to note that the observed low intensity of CeO vibrations is expected
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given the large number of graphitic layers comprising the GNF sidewalls that shield the confined
CeO nanoparticles, indicating that most of the ceria nanoparticles are encapsulated inside GNF.
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Like the Raman spectra, PXRD patterns of CeO @GNF (Figure 2c) show strong evidence for
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the presence of GNF in the composite. The diffraction peaks of GNF at 2θ = 26.0, 42.4, 44.6, 54.4
o
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and 77.4 correspond to the (002), (100), (101), (104) and (110) planes of the graphitic lattice,
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and remain unchanged in CeO @GNF. In CeO @GNF-1, no significant new peaks corresponding
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to CeO are observed, which can be attributed to a low amount or very small size of CeO
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nanoparticles. However, for CeO @GNF-3, which contains the highest amount of ceria, the PXRD
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o
pattern contains four new peaks at 2θ = 28.3, 32.8, 47.3 and 56.3 corresponding to the (111),
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200), (220) and (311) crystal planes of CeO (JCPDS card no. 43-1002), which are less prominent,
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but still measurable, in CeO @GNF-2. The PXRD patterns confirm the crystalline nature of CeO ,
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exhibiting a face centred cubic fluorite crystal lattice. For CeO @GNF-3, the crystallite size of
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the CeO can be estimated from the full width half maximum (FWHM) of the (111) peak at 2θ =
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o
52
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8.3 using the Scherrer equation as ~9 nm. Therefore, the fact that CeO peaks are more
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prominent in CeO @GNF-3 is related to the highest content of ceria and thus the increase in the
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size of CeO nanoparticles, consistent with previous reports.53
2
Although the TGA, Raman spectroscopy and PXRD results all suggest the formation of a
nanocomposite of CeO and GNF, the location of CeO , i.e. inside or outside GNF, could only be
2
2
ascertained from transmission electron microscopy (TEM). Interestingly, TEM imaging reveals
CeO nanoparticles are well dispersed inside the GNF in all samples (Figure 3). At the lowest
2
amount of cerium precursor (1 mg in CeO @GNF-1), effectively all CeO nanoparticles are
2
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observed to be embedded inside GNF (Figure 3a); however, at higher precursor loadings (3 mg
in CeO @GNF-3), a noticeable, but still minor, number of nanoparticles were found on the outer
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surface of GNF (Figure 3c and S2a and d, SI). This indicates that the best level of control over the
location of nanoparticles is achieved with the smallest amount of the precursor complex.
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Consistent with previous studies, CeO nanoparticles are found to be preferentially adsorbed at the
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internal step-edges of GNF (Figures 1, S2b and c, SI). Furthermore, mean diameters of ceria
nanoparticles of 4.6±0.2, 6.2±0.6 and 8.5±0.3 nm were determined by TEM for CeO @GNF-1, -
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and -3, respectively (insets in Figures 3a-c), the latter of which matching well with the size
estimated from PXRD, clearly showing the positive correlation between nanoparticle size and
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precursor concentration. High resolution TEM images (Figure 3e) of CeO nanoparticles show
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clear atomic fringes with interplanar d-spacings of 0.32 nm corresponding to the (111) planes of
ceria, and EDX spectroscopy analysis identifies Ce and O in the correct stoichiometry for
CeO @GNF (Figure 3d). However, more detailed analysis by XPS (Figure 2d) indicates the
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3+
existence of an oxygen-depleted surface containing both Ce (78.8 at%) and Ce (21.2 at%) based
on deconvolution of the Ce 3d signal (Figure S3 and Tables S1-2, SI).55,56 The presence of Ce3+ is
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+
significant for our reaction, as Ce is known to be key for initiation of radical-mediated organic
transformations.
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Figure 3. HRTEM analysis of CeO
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nanoparticles encapsulated inside GNF (CeO
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@GNF). HRTEM images of CeO
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@GNF-3
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nanoparticles encapsulated inside GNF (nanoparticle diameter indicated by the double-headed red arrow) showing d-spacing of
.32 nm (e).
2 and -3 (a-c) and corresponding size distribution histograms of CeO
2 2
nanoparticles (inset). The EDX spectrum of CeO
(d) confirms the elemental composition and stoichiometry of Ce and O in CeO
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CeO @GNF in the oxidation of cyclohexene
2
The CeO @GNF composites were tested as initiators in the oxidation of cyclohexene by tert-
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butylhydroperoxide. This specific reaction was chosen as it is known to proceed through two
different routes, either via allylic or olefinic oxidation, leading to the formation of five main
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products57-60, with the ratio of these products influenced by the conditions of the reaction, including
the oxidant, temperature, time, pressure and solvent.61,62 Under our experimental conditions of 80
oC for 9 hours in acetonitrile, the control reaction (entry 1, Table 1) gave 18% conversion for
cyclohexene with the formation of products via both allylic (2-cyclohexenylhydroperoxide (1))
and olefinic (cyclohexaneepoxide (4)) oxidation pathways in a 50:50 ratio of (1):(4) indicating low
selectivity of this reaction. In the presence of empty GNF (entry 3, Table 1), the conversion
increased to 45% with the formation of only allylic oxidation products (1), (2) and (3). The
observed higher conversion is consistent with the known effect of decomposition of peroxy
compounds, here TBHP, in the presence of graphitic carbon surfaces generating reactive oxygen
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species, i.e. hydroxyl radicals.63,64 The key effect of the presence of ceria in CeO @GNF, as
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compared to empty GNF, is a significant increase of conversion (up to 94%) with 2-cyclohexenone
(
2) and 2-cyclohexenyl hydroperoxide (1) as major products observed for CeO @GNF-2 (entry 5,
2
Table 1 and Figure 4a). Comparison of the CeO @GNF nanoreactors with different loadings of
2
ceria (CeO @GNF-X where X=1,2,3) did not reveal significant differences in terms of
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cyclohexene conversion (Figure S5 and Table S3, SI). Interestingly, we noted a clear dependence
on the specific distribution of products as a function of reaction time using CeO @GNF-2 (Figure
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4
c). At 7 hours, the yield of (1) is maximal at 89% selectivity (entry 4, Table 1), at a high
conversion of 88%. As the reaction proceeds, (1) is consumed to produce both (2) and (3), with a
conversion of 100% and moderate selectivity of 56% for (2) observed at 24 hours (entry 6, Table
1). This indicates that the preferred product ((1) or (2)) can in principle be obtained in high yield
and moderate-to-good selectivity simply by stopping the reaction at a user-specified time point.
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Table 1. The effects of CeO
2
loading, particle size, reaction time and location in GNF and on graphite and activated carbon on the
conversion of cyclohexene and distribution of the afforded products: 2-cyclohexenyl hydroperoxide (1), 2-cyclohexenone (2), 2-
cyclohexenol (3), cyclohexane epoxide (4) and 1,2-cyclohexanediol (5).
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Entry
Species a
None
CeO
2
loading / %
Particle size nm c
Reaction time
Conversion / % d
Selectivity / %
b
(1):(2):(3):(4):(5) d
/
h
1
2 e
3
-
-
9
9
18
0
50:0:0:50:0
0:0:0:0:0
GNF
-
-
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44
88
94
100
30
41
60:22:18:0:0
89:11:0:0:0
66:34:0:0:0
28:56:16:0:0
93:7:0:0:0
4
CeO
2
@GNF-2
5.9
6.2±0.6
7
5
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6
24
9
7
CeO
2
@graphite
@AC
3.7
4.8
6.7±1.8
4.5±0.8
8
CeO
2
9
84:12:4:0:0
a
Reaction conditions: cyclohexene (2.9 mmol), TBHP oxidant (5.8 mmol), 1,4-dichlorobenzene internal standard (1.44 mmol), acetonitrile (2.5 mL),
0 C. Determined by TGA. c Determined by TEM. d Determined by H NMR spectroscopy. Under anaerobic conditions.
o
b
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Figure 4. The conversion of cyclohexene obtained using CeO @GNF (here CeO @GNF-2) relative to control experiments as a
function of time (a) and repeat use (b). The dotted horizontal line in (b) represents the conversion obtained in control experiments
as reference. The change in selectivity for the products 2-cyclohexenyl hydroperoxide (1) and 2-cyclohexenone (2) with time (c),
2
demonstrating the user-specificity of product selection using CeO @GNF. A proposed pathway for the oxidation of cyclohexene
by ceria in CeO
2
@GNF; Ce3+ reacting with the peroxide reagent triggers a flux of radicals initiating a cycle of reactions with O
2
biradical leading to compound (1) which subsequently transforms to (2) (d).
It is important to note that the lack of products from the olefinic oxidation pathway differs
significantly from previous reports on the oxidation of cyclohexene with TBHP using CeO2
nanoparticles, where a mixture of (2), (3) and (4) were identified as major products (Table S4,
SI).65 In addition, Voort et al.66 reported the formation of both (4) and (1) as products of
cyclohexene oxidation by TBHP, suggesting the reaction proceeds through both allylic and olefinic
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oxidation pathways. Moreover, it should be highlighted that our nanoreactors CeO @GNF have
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very small ceria loadings (0.13 mol%) (Table S4, SI) compared to previous reports (by more than
two orders of magnitudes in some instances), with up to 95 wt% of CeO in CeO /VO – more a
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stoichiometric reactant than a catalyst – required to bring about effective oxidation.65
In order to study the stability, and thus recyclability, of CeO @GNF, we have evaluated the
2
cyclohexene conversion using CeO @GNF over five uses. After each use, the catalyst was filtered
2
o
and washed repeatedly with methanol and kept overnight at 65 C in air. The conversion of
cyclohexene was found to subtly increase from the first to the third use, with a very small decrease
observed thereafter (Figure 4b). The increase in conversion after the first use can be attributed to
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the cleaning of the nanoparticle surface under the reaction conditions. TGA profiles of fresh and
recycled CeO @GNF (Figure S4, SI) shows no significant loss of CeO content even after five
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uses, indicating good stability of CeO @GNF to leaching.
2
The enhanced cyclohexene conversion observed with CeO @GNF, and its associated stability
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towards repeat use, can be attributed to the effects of spatial confinement of CeO inside GNF.
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This has been shown to lead to: (i) well dispersed CeO nanoparticles of very small diameter (3-8
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nm), protected at the graphitic step-edges from deactivation mechanisms, yet providing a large
surface area for the adsorption of reactant molecules; (ii) increased local concentration of reactants
inside nanoreactors promoting the effective interaction between reactant molecules and CeO , thus
2
improving conversion and (iii) easy diffusion of reactant and product molecules to and from the
nanoreactor, while maintaining strong local interactions with the CeO nanoparticles at the GNF
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step-edges. These effects are consistent with the spatial confinement observed for other
nanoparticles inside carbon nanoreactors, including GNF, that often result in highly selective
organic transformations use-to-use.25,26
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To validate the importance of step‐edges and nanoscale confinement inside GNF on the
functional properties of ceria, CeO nanoparticles were separately deposited onto activated carbon
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AC) and graphite flakes (Figure S6, SI). AC and graphite both provide anchoring sites for
nanoparticles, akin to GNF, but no spatial restrictions as inside the GNF cavity. TEM analysis
Figure S6c-g, SI) indicates that ceria nanoparticles on AC and graphite are uniformly distributed
on both carbons and are qualitatively similar in size to those observed in CeO @GNF. A
(
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preference for cubic morphology was noted for CeO on graphite. Interestingly, TGA (Figure
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S6a,b) shows clear differences in the combustion temperature of the carbon supports after
o
deposition of ceria: a significant reduction (by more than 200 C) was observed for AC; only a
minimal difference was recorded using graphite. This suggests a higher intimacy of contact
between CeO and AC, like that observed using GNF, relative to graphite where contact is poorer.
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However, H NMR spectroscopy analysis indicates lower conversion in the oxidation of
cyclohexene using both CeO @AC and CeO @graphite relative to CeO @GNF (entries 7, 8 and
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2
2
5, respectively, Table 1). Moreover, whilst cyclohexene conversion is largely retained after the
first use of CeO @AC, this drops significantly when using CeO @graphite (Figure 3b) indicating
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2
low stability, and thus recyclability, of ceria on this carbon support. Thus, whilst we cannot rule
out additional factors that may contribute to the observed reactivity and stability of CeO @GNF,
2
these observations suggest the importance of nanoscale confinement inside GNF nanoreactors,
providing a new way to enhance cyclohexene conversion, use-to-use, whilst simultaneously
improving the selectivity for allylic oxidation at very low loadings.
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Based on our results and previous literature concerning the TBHP-mediated oxidation in the
presence of CeO and transition metals68-70, a plausible mechanism for the oxidation of
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cyclohexene can be proposed (Figure 4d). Since no epoxide is detected during the reaction, the
formation of products through the selective oxidation of the allylic carbon centre by free radicals
appears the most viable option. Our XPS measurement demonstrated that ~20% of cerium cations
3
+
3+
in CeO @GNF are in oxidation state Ce . The redox reaction of Ce centres with t-BuOOH
2
.
71-73
yields t-BuO radicals, similar to the process described for other nanoscale materials.
The free
.
radical t-BuO extracts the allylic hydrogen from cyclohexene to form the cyclohexenyl radical,
74,75
which becomes trapped by atmospheric O to form 2-cyclohexenyl peroxide radical.
The
2
critical role of atmospheric oxygen was ascertained in control experiments, under anaerobic
conditions, where negligible conversion was observed (entry 2, Table 1). The 2-cyclohexenyl
peroxide radical subsequently abstracts hydrogen from the allylic CH of another molecule of
2
cyclohexene to form 2-cyclohexenyl hydroperoxide (1),76,77 the existence of which is confirmed
1
by H NMR spectroscopy. This homolytic abstraction of hydrogen is also energetically favourable:
-1
78
the bond energies of RO-H and the allylic C-H bond are 90 and 85 kcal mol , respectively. In
effect, ceria nanoparticles act as an initiator of the free-radical allylic oxidation cycle which then
.
continues producing (1) which in turn forms (2). The low amount of HO radicals in this process
explain the low yield of product (3), only observed after extensive heating, which forms without
ceria in GNF on shorter timescales.
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Analysis of CeO @GNF after the reaction offers additional mechanistic insights. XPS shows no
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significant changes in the ratio between Ce (78.1%) and Ce (21.9%), but a shift of ca. 1 eV
towards higher binding energy in all Ce 3d components. The shift is likely to be associated with
the presence of hydroxyl groups on ceria after the reaction (bi-product of peroxide decomposition;
Figure 3d), which corroborates with a large increase of the O 1s peak at ca. 532 eV associated to
hydroxyl groups (Figure S3d, SI). This suggests that GNF acts as an electron reservoir
compensating for the loss of electrons in ceria, as observed previously for other metal compounds
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inside carbon nanotubes,79,80 effectively restoring the Ce /Ce balance.
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CONCLUSIONS
A method for the preparation of very small CeO nanoparticles encapsulated inside graphitised
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carbon nanofibres by gas phase thermal decomposition of cerium complex [Ce(tmhd) ] has been
4
developed. The average size of CeO nanoparticles inside GNF can be tuned from ~4 to 9 nm by
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adjusting the ratio of the precursor complex to GNF. CeO nanoparticles deposited at the graphitic
2
step-edges of GNF promote cyclohexene oxidation, demonstrating a significantly improved
conversion and exclusive selectivity towards allylic oxidation products as compared to previously
reported ceria nanomaterials. Analysis of the product distribution with and without CeO in GNF
2
suggests that the allylic pathway of the reaction is dictated by the carbon nanoreactor itself while
the presence of cerium oxide enhances the efficiency of the nanoreactor CeO @GNF leading to
2
higher conversion, playing the essential role of initiator in the production of cyclohexenyl
hydroperoxide. Furthermore, comparison of CeO @GNF nanoreactors with different sizes of ceria
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nanoparticles demonstrated that ~6 nm nanoparticles are most active and deliver the highest yield
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of 2-cyclohexenone. Investigations of the oxidation reactions with nanoparticles not confined to
nanoscale volumes highlight the ability of the GNF to enhance cyclohexene conversion, whilst
simultaneously improving the selectivity for the allylic oxidation at low loadings, and maintaining
the balance of oxidation states in ceria by effectively acting as a reservoir of electrons. Importantly,
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these nanoreactors CeO @GNF could also be easily separated from the reaction mixture and
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reused, largely retaining their performance over at least five uses. This factor makes these
nanoreactors a suitable candidate for the sustainable use of lanthanide-based materials in
preparative organic synthesis.
ASSOCIATED CONTENT
Supporting Information. This material is available free of charge on the ACS publications
website.
AUTHOR INFORMATION
Corresponding Author
*
School of Chemistry, University of Nottingham, University Park, Nottingham, NG7 2RD, UK.
E-mail: Maxwell.Astle@nottingham.ac.uk; Andrei.Khlobystov@nottingham.ac.uk; Fax: +44 (0)
1
15 9513563; Tel: +44 (0) 115 9513917
Present Addresses: †Department of Chemistry, Deen Dayal Upadhyaya College, University of
Delhi, New Delhi. 1100078, India. Fax: +91-11-25099380, Tel: +91-11-25099381
Author Contributions
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The manuscript was written through the contributions of all authors. All authors have given
approval to the final version of the manuscript.
Funding Sources
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The work was supported by the University of Delhi, India, the Engineering and Physical
Research Council (EPSRC) and the Centre for Sustainable Chemistry, University of Nottingham.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
NA acknowledges the financial support provided by the University of Delhi, India to carry out
the work at the University of Nottingham. We also acknowledge the Nanoscale Microscale
Research Centre (nmRC) for TEM and Raman analysis. MAA, GAR and ANK acknowledge the
Engineering and Physical Research Council (EPSRC) and Advanced Molecular Materials
Research Priority Area (University of Nottingham) for funding, and the Centre for Sustainable
Chemistry (CSC), University of Nottingham, for access to analytical facilities.
ABBREVIATIONS
GNF, graphitised carbon nanofibre; CNT, carbon nanotubes; Ce(tmhd) , tetrakis(2,2,6,6-
4
tetramethyl-3,5-heptanedionato) cerium (IV); HRTEM, high resolution transmission electron
microscopy; EDX, energy dispersive X-ray spectroscopy; TGA, thermogravimetric analysis;
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PXRD, powder x-ray diffraction; XPS, X-ray photoelectron spectroscopy; TBHP, tert-Butyl
hydroperoxide.
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