A hierarchically ordered porous novel vanado-silicate catalyst for highly efficient oxidation of bulky organic molecules

Article abstract of DOI:10.1039/c2cc30934c

A novel hierarchically ordered porous vanado-silicate nanocomposite with interconnecting macroporous windows and meso-microporous walls containing well dispersed vanadyl species has been fabricated and used as a heterogeneous catalyst for the oxidation of a bulky organic molecule, namely cyclooctene.

Full text of DOI:10.1039/c2cc30934c

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A hierarchically ordered porous novel vanado-silicate catalyst for highly
efficient oxidation of bulky organic moleculesw
Tapas Sen,*ab Jacob Whittlea and Matthew Howarda
Received 9th February 2012, Accepted 6th March 2012
DOI: 10.1039/c2cc30934c
A novel hierarchically ordered porous vanado-silicate nanocomposite
with interconnecting macroporous windows and meso–microporous
walls containing well dispersed vanadyl species has been fabricated
and used as a heterogeneous catalyst for the oxidation of a bulky
organic molecule, namely cyclooctene.
for the incorporation of transition metal cations i.e. vanadium ions
for the oxidation of bulky organic molecules. Herein, we report the
fabrication of hierarchically ordered porous vanado-silicate catalyst
with highly dispersed vanadyl species (supported by electron
paramagnetic resonance spectroscopy) for potential application
in oxidation of bulky organic molecules using hydrogen peroxide
as an oxidising agent under heterogeneous conditions.
Oxidation catalysis plays a large role in industry whether it is
used in the production of fine chemicals or in the removal of
hazardous contaminants.1–4 Vanadyl ions under homogeneous
conditions serve as a powerful oxidation catalyst in the
presence of hydrogen peroxide.4 However the vanadyl catalyst
under homogeneous conditions suffers from (i) the difficulty in
separation of catalyst from the reaction mixture and (ii) lack
of product selectivity. Various heterogeneous catalytic systems
containing vanadium ions have been developed using clays,5,6
microporous,7–10 mesoporous materials11 and macroporous12,13
supports for oxidation of both organic and inorganic molecules.
Diffusional restriction of bulkier molecules through the micro/
mesoporous solids and the poor surface area of macroporous
materials has led to the development of high surface area
materials containing pore sizes of three different length scales
i.e. micropores (o2 nm), mesopores (2 nm to 50 nm) and
macropores (450 nm) with hierarchical ordering. Sen et al.
have reported14,15 the fabrication of large surface area
hierarchically ordered porous silicate materials with pore-
interconnectivity in three length scales in a controlled fashion.
Such materials have great potential as a support for industrial
catalysis or for the separation of biomolecules as they have the
ability to allow such bulky molecules to diffuse through the
macroporous windows (B85 nm) while the incorporation of
meso and micropores into the materials increases the surface
area. Recently, we have reported16,17 a novel hierarchically ordered
porous silicate with superparamagnetic property (i) for magnetic
bioseparation and (ii) as a support for the immobilisation of lipase
for bio-catalysis. However, such materials have never been used
A novel vanado-silicate catalyst was synthesized (see ESIw)
by filling the interstice sites of polystyrene latex monolith of
uniform diameter 423 nm (see Fig. S1 in the ESIw) using silica
gel in an acidic medium (aqueous hydrochloric acid) containing
a tri-block copolymer (F127: EO107PO70EO107) as a surfactant,
n-butanol as a co-solvent and hydrated vanadyl sulfate as the
vanadium source. The vanadium containing silica gel filled
polystyrene monolith composite was dried overnight at 60 1C
(uncalcined form). The dried nanocomposite was calcined at
500 1C in air for a period of 12 h at a heating rate of 1 1C minÀ1
.
The formation of macropores was monitored on a range of
materials calcined at various temperatures (100 1C, 200 1C,
300 1C, 400 1C and 500 1C) under identical conditions.
The vanado-silicate catalyst (MH0.01) was observed to be
ordered macroporous structure consisting of macropore voids
of diameter 380 nm with interconnecting windows of diameter
84 nm (see Fig. 1a and b). TEM micrographs (see Fig. 1c and d)
exhibited a mesoporous wall of disordered structure. The mesopore
size was measured to be around 7 nm with a mesopore wall
thickness of around 2 nm.
The formation of macropores and their interconnectivity
was monitored via mercury intrusion through the materials
under very high pressure (upto 60 000 psi). The total intrusion
of mercury through a polystyrene template was measured to
be 0.624 mL gÀ1 (see Fig. 2a) due to the presence of interstice
sites with a total macropore area of 36 m2 gÀ1 and
total porosity of 30% with a bimodal pore size distribution
(see Fig. S2 in the ESIw) centred at 122 nm (octahedral
interstice sites) and 67 nm (tetrahedral interstice sties). Upon
calcination at different temperatures, the intrusion of mercury
a Centre for Materials Science, School of Forensic and Investigative
Sciences, Preston, PR1 2HE, UK. Fax: +44(0) 1772894981;
Tel: +44(0) 1772894371
and total porosity initially reduced (see Fig. 2a) to 0.1 mL gÀ1
,
10% (at À1100 and 200 1C), followed by a gradual increase to
0.2 mL g , 59% (300 1C), 1.635 mL gÀ1, 75.4% (400 1C), and
7.11 mL gÀ1, 92.7% (500 1C). Calcination below 300 1C
resulted in the melting of polystyrene nanoparticles and hence
in the loss of porosity of the composites. The exceptionally
high value of porosity (92.7%) with interconnecting windows
b Institute of Nanotechnology and Bioengineering,
University of Central Lancashire, Preston, PR1 2HE, UK.
E-mail: tsen@uclan.ac.uk
w Electronic supplementary information (ESI) available: Detailed
experimental protocol, Fig. S1 to S5 and Reaction Scheme S1. See
DOI: 10.1039/c2cc30934c
c
4232 Chem. Commun., 2012, 48, 4232–4234
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Fig. 1 Scanning electron micrographs (a and b) and transmission
electron micrographs (c and d) of MH0.01 calcined at 500 1C.
Fig. 3 X-band electron paramagnetic resonance spectra of uncalcined
form of MH0.01 (a), and V-impregnated hierarchically ordered porous
silica HOPS (b).
The BET surface area of the novel vanado-silica catalyst was
measured to be 372.5 m2 gÀ1 with the hysteresis loop (see Fig. 2b)
indicating the presence of mesopores. The mesopore diameter
using BJH pore size distribution (both adsorption and desorption)
was measured to be 6.1 nm which is close to the value obtained
from TEM data. t-plot analysis (see Fig. 2c) using the Harkins and
Jara equation was measured to be 211.9 m2 gÀ1, indicating that the
material has a large number of micropores due to the removal of
the EO chain of the surfactant molecules similarly to our previous
report.14 The disordered structure of micro and mesoporosity was
further confirmed by small angle X-ray scattering (a broad peak,
see Fig. S3 in ESIw) and powder diffraction pattern (no peaks) in
the high angle region.
The eight-line hyperfine pattern with anisotropy in the electron
paramagnetic resonance spectrum (see Fig. 3a) is an indication of
the presence of well dispersed paramagnetic vanadium ions (V+4
)
in the uncalcined form of MH0.01. Upon calcination, all
paramagnetic V+4 species have been converted to diamagnetic
V-species (see Fig. S4 in ESIw). The nature of V-species was
further confirmed by comparing the EPR parameters with
literature data. The gJ and g> values of the uncalcined
MH0.01 were calculated to be 1.9342 and 1.974. Similarly AJ
and A> values were calculated to be 195.2 G and 74.9 G. The g
and A values corresponded to vanadyl (VQO) species connected to
silica (V–O–Si) in a distorted square pyramidal environment.15,18
The V-impregnated sample exhibited a single line without any
hyperfine splitting (Fig. 3b), indicating the paramagnetic vanadium
species as the aggregated state. A characteristic peak at 960 cmÀ1 in
the FT-IR spectrum (see Fig. S5 in ESIw) is an indication of
Si–O–V bonding in the materials.19
Fig. 2 Change of pore volume due to the formation of macropores
on MH0.01 by mercury porosimetry (a); nitrogen adsorption isotherm
of MH0.01/calcined (b) and t-plot analysis (c).
The silica to vanadium molar ratio of MH0.01/calcined was
calculated to be 581 using X-ray fluorescence spectroscopy
by comparing with a series of standard samples of known
vanadium concentration.
of sizes 87 nm and macropores of size 375 nm (see Fig. S2 in
ESIw) support the electron microscopy data.
c
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Chem. Commun., 2012, 48, 4232–4234 4233

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The authors thank National EPR facility, School of Chemistry,
The University of Manchester, for providing the facility and
Dr J. Hawkett for his time to run the samples. The authors
also thank Dr C. Muryan, School of Chemistry, The
University of Manchester, for helping them to run the samples
in SASX. MH thanks the Centre for Research-informed
Teaching, University of Central Lancashire, for providing
the summer internship in 2009–2010. The authors also thank
Dr Jenny Readman who helped MH to run XRF.
Notes and references
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Fig. 4 A comparative study of oxidation of cyclooctene to various
products using novel vanado-silicate catalyst (MH0.01/calcined) and
V-impregnated HOPS.
When MH0.01/calcined sample was used for the oxidation
of cyclooctene (see the method and reaction scheme,
Scheme S1, in ESIw), the novel hierarchically ordered porous
vanado-silicate (MH0.01/cal) exhibited a high conversion
(38.5%, see Fig. 4) with the product selectivity of cyclooctene
epoxide (24.8%) and cyclooctanone 2-ol (12.7%). The Turn
Over Number (TON) calculated based on the moles of
cyclooctene converted (38.5%) per mole of vanadyl species
present (1.16 Â 10À6 moles in 0.1 g catalyst) in the calcined
MH0.01 was 3019. The TON value for the present system is
exceptionally high compared to the TON value reported7,20
for vanadyl species in MEL and MFI zeolite systems for
oxidation of toluene. A poor conversion (o0.1%) of cyclooctene
was observed when V-impregnated HOPS was used for similar
reaction.
In conclusion, a template assisted hierarchically ordered porous
vanado-silica nanocomposite has been fabricated and used for
the oxidation of a bulky organic molecule (cyclooctene). The
exceptionally high TON of the vanadium species in the
hierarchically ordered porous vanado-silicate for the oxidation
of cyclooctene makes this material a potential catalyst for
many important oxidation catalyses in the near future under
heterogeneous conditions.
20 S. Kannan, T. Sen and S. Sivasanker, J. Catal., 1997, 170, 304.
c
4234 Chem. Commun., 2012, 48, 4232–4234
This journal is The Royal Society of Chemistry 2012