Supported C-Scorpionate Vanadium(IV) Complexes as Reusable Catalysts for Xylene Oxidation

Article abstract of DOI:10.1002/asia.201700499

C-Scorpionate vanadium(IV) [VO_xCl_3?x{κ³-RC(pz)₃}] [pz=pyrazol-1-yl; x=0, R=SO₃ (1); x=1, R=CH₂OH (2) or CH₂OSO₂Me (3)] complexes supported on functionalized carbon nanotubes (CNTs) are the first V-scorpionate catalysts used so far for the neat oxidation of o-, m- or p-xylene, with TBHP (70 % aqueous solution), to the corresponding toluic acids (main products), tolualdehydes and methylbenzyl alcohols. Remarkably, a p-toluic acid yield of 43 % (73 % selectivity, TON=1.34×10³) was obtained with 2@CNT in a simple microwave-assisted mild oxidation procedure, using a very low catalyst charge (3.2×10^?2 mol ^% vs. substrate). Further, this occurred in the absence of any bromine source, what is significant towards the development of a greener and more sustainable process for oxidation of xylenes. Moreover, reuse of catalysts with preservation of their activity was found for up to six consecutive cycles. The effects of reaction parameters, such as reaction time, temperature, amount of catalyst or type of heating source, on the performance of the above catalytic systems are reported and discussed.

Full text of DOI:10.1002/asia.201700499

CHEMISTRY
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Accepted Article
Title: Supported C-scorpionate vanadium (IV) complexes as reusable
catalysts for xylene oxidation
Authors: Jiawei Wang, Luisa Martins, Ana P. C. Ribeiro, Sónia A. C.
Carabineiro, José L. Figueiredo, and Armando J. L. Pombeiro
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Supported C-scorpionate vanadium(IV) complexes as reusable
catalysts for xylene oxidation
Jiawei Wang,[a] Luísa M. D. R. S. Martins,*[a,b] Ana P. C. Ribeiro,*[a] Sónia A. C. Carabineiro,[c] José L.
Figueiredo,[c] and Armando J.L. Pombeiro[a]
Dedication ((optional))
Abstract: C-scorpionate vanadium(IV) [VOxCl3-x{κ3-RC(pz)3}] [pz =
pyrazol-1-yl; x = 0, R = SO3 (1); x = 1, R = CH2OH (2) or
CH2OSO2Me (3)] complexes supported on functionalized carbon
nanotubes (CNT) are the first V-scorpionate catalysts used so far for
the neat oxidation of o-, m- or p-xylene, with TBHP (70% aqueous
solution), to the corresponding toluic acids (main products),
tolualdehydes and methylbenzyl alcohols. Remarkably, a p-toluic
acid yield of 43% (73% selectivity, TON = 1.34 × 103) was obtained
reported[2] homogeneous greener alternatives (e.g., a spray
[5]
process,[3] the use of water[4] or supercritical CO2 as solvents
or of additives such as N-hydroxyimides and guanidines[6]), as
well as heterogeneous ones (e.g., Co/Mn cluster complexes
encapsulated in zeolite Y,[7] Pd, Sb and Mo supported on TiO2,[8]
or AuPd alloy nanoparticles[9]), the development of a halogen
free, low cost, sustainable and still highly efficient catalytic
system to produce terephthalic acid is needed. Moreover, all
2a]
with 2@CNT in
a
simple microwave-assisted mild oxidation
heterogeneous systems described so far[1a,
lack activity
procedure, using a very low catalyst charge (3.2 × 10-2 mol% vs.
substrate). Further, this occurred in the absence of any bromine
source, what is significant towards the development of a greener and
more sustainable process for xylenes oxidation. Moreover, catalysts
reuse with preservation of their activity was found for up to six
consecutive cycles. The effects of reaction parameters, such as
reaction time, temperature, amount of catalyst or type of heating
source, on the performance of the above catalytic systems are
reported and discussed.
compared to the homogeneous initiator system and the active
metal compounds are leached in the reaction medium.
Although benzylic oxidations can be catalyzed by vanadium
species,[10] the oxidation of xylenes using vanadium complexes
as catalysts, either in homogeneous environment or
heterogeneized, has not been previously reported, what
prompted us to investigate the catalytic activity of such metal in
the above oxidation reaction.
On the other hand, the use of metallic complexes with C-
scorpionate poly(pyrazol-1-yl)methane ligands as homo- or
heterogeneous oxidation catalysts has experienced significant
advances in the last years.[11] The tripodal C-scorpionate ligand,
bearing three pyrazolyl moieties (via their N atoms) can assist
proton-transfer steps that are believed to be involved in key
catalytic processes.[12]
Introduction
The selective catalytic oxidation of hydrocarbons is an important
route for preparing commodities in chemical industry worldwide.
Large scale terephthalic acid production from the oxidation of p-
xylene is a very important process in polyester industry[1] as it is
manly used in polyethylene terephthalate (PET) manufacturing,
a polymer that finds huge use in fibers, films and plastic
products. Currently, oxidation of p-xylene is conducted in a
homogeneous Co/Mn catalytic system in the presence of the
highly corrosive bromide dissolved in aqueous acetic acid
(AMOCO process).[1a] Despite the very high conversions of the
reactant attained in the AMOCO process, and some recently
In this work, we aim to use the tripodal C-scorpionate
vanadium(IV) [VOxCl3-x{κ3-RC(pz)3}] [pz = pyrazol-1-yl; x = 0, R =
SO3 (1);[13] x = 1, R = CH2OH (2)[14] or CH2OSO2Me (3)[15]
]
complexes (Fig. 1) that have proven their efficiency as catalysts
for the partial oxidation of cycloalkanes to the corresponding
cyclic alcohols and ketones, but were not used for other
substrates.
[a]
M. Sc. J. Wang, Prof. L.M.D.R.S. Martins*, Dr. A.P.C. Ribeiro*, Prof.
A.J.L. Pombeiro
Centro de Química Estrutural, Instituto Superior Técnico,
Universidade de Lisboa, Av. Rovisco Pais 1049-001 Lisboa,
Portugal
Email: lmartins@deq.isel.ipl.pt; ana.paula.ribeiro@ist.utl.pt
Prof. L.M.D.R.S. Martins*
Chemical Engineering Departament, Instituto Superior de
Engenharia de Lisboa, Instituto Politécnico de Lisboa, Rua
Conselheiro Emídio Navarro, 1959-007 Lisboa, Portugal
Dr. S.A.C. Carabineiro, Prof. J.L. Figueiredo
Laboratório de Catálise e Materiais, Laboratório Associado LSRE-
LCM, Faculdade de Engenharia, Universidade do Porto, Rua Dr.
Roberto Frias 4200-465 Porto, Portugal
Figure 1. Tripodal C-scorpionate vanadium(IV) [VCl3{κ3-SO3C(pz)3}] (1) (pz =
pyrazol-1-yl) and [VOCl2{κ3-RC(pz)3}] [R = CH2OH (2) or CH2OSO2Me (3)]
catalysts.
[b]
[c]
Moreover, in order to combine the useful properties of these
homogeneous catalysts with the advantages of heterogeneous
systems (e.g., recyclability and practical, easy workups) we
have chosen to support them on functionalized carbon
nanotubes, according to the already established protocols.[16]
In addition, the use of microwave (MW) radiation as an
alternative energy source, eventually enhancing product yield
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and selectivity as well as being more energy efficient and
economical in comparison to conventional heating methods,[17] is
appealing for the implementation of sustainable chemical
processes. However, the application of MW irradiation in alkyl
substituted aromatics functionalization is still an unexplored
field.[18]
To our knowledge, this is the first time that the successful use of
a C-scorpionate vanadium catalyst for the oxidation of xylenes is
reported.
oxygenated functional groups (carboxylate or phenolate) of the
CNT, with possible formation of a covalent VO bond, upon
displacement of a chloride ligand[16b,19] or by replacement of one
of the coordinated pyrazolyl arms of the C-scorpionate ligands,
which thus would change from the trihapto to a dihapto
coordination mode. This tri-/bi- interchange is a known[11] feature
of these ligands, believed to be the core of the chemical
versatility of metal complexes of this kind and essential for their
catalytic applications.
60
50
40
30
20
10
0
60
50
40
30
20
10
0
60
50
40
30
20
10
0
2@CNT
1@CNT
3@CNT
Results and Discussion
p-Xylene and its o- and m- isomers were successfully activated at
one methyl group (Scheme 1) using C-scorpionate vanadium(IV)
[VOxCl3-x{κ3-RC(pz)3}] [pz = pyrazol-1-yl; x = 0, R = SO3 (1); x = 1,
R = CH2OH (2) or CH2OSO2Me (3)] complexes supported on
surface functionalized carbon nanotubes (CNT) as catalysts and
aqueous tert-butyl hydroperoxide (TBHP) as oxidant, leading to
the formation of the corresponding toluic acids (main product),
tolualdehydes and methylbenzyl alcohols. The catalytic results
obtained are displayed in Table S1 (ESI).
o-xylene m-xylene p-xylene
o-xylene m-xylene
p-xylene
o-xylene m-xylene
p-xylene
(a)
(b)
(c)
Figure 3. Products yields obtained for the MW-assisted oxidation of xylenes
(toluic acid,tolualdehyde and methylbenzyl alcohol) with TBHP, at 80 ºC,
catalyzed by (a) 1@CNT, (b) 2@CNT and (c) 3@CNT, in the presence of
HNO3
V-scorpionate@CNT
+
+
O
The reaction can proceed, in the absence of added solvents, at
the desired temperature (typically 80 ºC) using a conventional,
heating mode (oil bath), but the preferred procedure is the
microwave-assisted one (see Table S1 and Fig. 4 for the
formation of m-toluic acid catalyzed by 1@CNT). In fact, the
used low power MW-assisted irradiation, that provides rapid
initial heating and enhanced reaction rates, leads to a much
more efficient synthetic method than conventional heating under
open atmosphere or non-pressurised refluxing, allowing the
attainment of significant higher yields in much shorter times
(e.g., 31% yield of m-toluic acid was obtained after 5 h of MW
reaction catalyzed by 1@CNT, whereas only 7.5% of m-toluic
acid was formed under the same conditions but using an oil
bath, Fig. 4).
OH
OH
TBHP, 80 ºC
Me
Me
Me
Me
Me
O
Scheme 1. Oxidation of o-, m- or p-xylenes to the corresponding methylbenzyl
alcohols, tolualdehydes and toluic acids.
Remarkably, a yield of 43% of p-toluic acid (TON = 1.34 × 103)
was achieved in the presence of the hybrid catalyst 2@CNT,
prepared by heterogeneization of [VOCl2{κ3-HOCH2C(pz)3}] (2,
pz = pyarazolyl) at carbon nanotubes oxidized with HNO3 and
subsequently treated with NaOH (CNT), after 12 h of microwave
irradiation of p-xylene in acidic media (Fig. 2).
100
80
p-toluic acid
60
40
20
0
p-tolualdehyde
4-methylbenzylalcohol
p-xylene
0
6
12
18
24
Time / h
Figure 2. Effect of reaction time on the reaction yields of MW-assisted p-
xylene oxidation with TBHP, at 80 ºC, catalyzed by 2@CNT, in the presence
of HNO3.
The o- and m-xylenes were also mainly converted to the
corresponding acids by the above procedure, although in slightly
lower yields. Under the optimized conditions, catalysts
performance for all the xylenes followed the trend: 2@CNT >
1@CNT > 3@CNT (Table S1, Fig. 3). This follows the amount of
supported V catalyst obtained in each hybrid material (Fig. S2).
The immobilization of C-scorpionate vanadium complexes 1 – 3
on the functionalized carbon nanotubes could occur via with the
Figure 4. Effect of reaction time and heating method, oil bath (•) or microwave
irradiation (), on the yield of m-toluic acid obtained from oxidation of m-
xylene with TBHP at 80 ºC, catalyzed by 1@CNT.
Although the activation at one methyl group of xylenes occurs in
the absence of any additive, the presence of acid enhances it
(Fig. 5). In particular, the use of nitric acid as co-catalyst
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allowed to achieve the highest yields of the desired toluic acids
(Table S1 and Fig. 5 for m-xylene oxidation). Moreover, the
activation at one methyl group of xylenes is significantly
enhanced by the heterogeneization of the vanadium catalysts at
the functionalized carbon nanotubes (compare the results for 1
and 1@CNT, Fig. 5).
50
40
30
20
10
0
1
2
3
4
5
6
Cycle
Figure 6. Effect of the catalyst recycling on the yield of p-toluic acid obtained
by oxidation of p-xylene catalyzed by 2@CNT.
The morphology of catalysts 1-3@CNT before the oxidation
reaction and after the 6th consecutive cycle is different, as
illustrated by SEM (Fig. S3). Moreover, vanadium leaching,
confirmed by EDS (Fig. S3), from the support surface may be
responsible for the observed yield decrease after that cycle. For
example, a drastic decrease of p-toluic acid yield (from 38.5%,
Fig. 6) to values similar to those obtained by 2 (not supported,
12.3%), was observed in the 7th consecutive oxidation cycle
catalyzed by 2@CNT.
Figure 5. Products yields for the MW-assisted net oxidation of m-xylene
catalyzed by different homo- and heterogeneous catalysts. HPCA = pyrazine
carboxylic acid.
The high yields of toluic acids achieved under mild conditions
with very low loadings of V-catalyst (see Fig. S1 for optimization
of reaction conditions) along with the selectivity found for the
activation of one methyl group of xylenes (no hydroxylation of
the aromatic ring was observed) indicate a remarkable activity
and selectivity of these heterogeneous catalytic systems based
on the C-scorpionate vanadium(IV) complexes 1 – 3 supported
on functionalized carbon nanotubes. Note that the use of a
common V(V) oxide did not afford toluic acid or even
tolualdehyde (Fig. 5), suggesting a significant role for the C-
sorpionate ligands at our catalysts (see above). In addition, the
presence of excess of oxidant also did not lead to more oxidized
species under the tested conditions (see Fig. S3).
The catalytic reaction is believed to proceed via a radical
mechanism, since a marked product yield decrease occurs in
the presence of
tetramethylpiperidin-1-oxyl (TEMPO). Methylbenzyl radical
(TolCH2
) can be formed upon oxidation of xylene by VV, and its
reaction with O2 leads to the peroxy radical TolCH2OO , which
a
radical trap, such as 2,2,6,6-
·
·
can account for the formation of the various products, as
reported for other metal catalysts (e.g., of CoIII).[1a, 21, 22]
Conclusions
C-scorpionate vanadium(IV) complexes were supported on
functionalized carbon nanotubes and used for the first time in
the neat oxidation of o-, m- or p-xylene, to the corresponding
toluic acids (which were the main products), tolualdehydes and
methylbenzyl alcohols. A p-toluic acid yield of 43% (73%
selectivity, TON = 1.34 × 103) was obtained with 2@CNT in a
simple microwave-assisted mild oxidation procedure with TBHP,
Therefore, and in contrast to previous studies, a notable catalytic
activity of the C-scorpionate V(IV) complexes
1
–
3
heterogeneized on functionalized CNT is observed under mild
conditions and in the absence of any bromine source, what is
significant towards the development of a safer and more
sustainable process for p-xylene oxidation. In fact, the present
catalytic systems, in comparison with the Amoco process[1a, 2a]
have the advantages of not requiring the use of corrosive HBr
that leads to a hazardous reaction[20] and requires a reactor
costly to build and maintain, and of avoiding the use of acetic
acid solvent.
in the absence of
a bromine source. This is quite an
achievement towards the development of greener and more
sustainable processes for xylenes oxidation. Moreover, catalysts
recycle was possible up to five consecutive cycles without loss
of activity. After the 6th cycle, V leaching is observed and activity
starts to decrease.
Another important advantage of their present systems is the
possibility of the catalyst being recycled and re-used. Recycling
of the best catalyst, 2@CNT, was tested up to six consecutive
cycles. On completion of each stage, the products were
analyzed and the catalyst was recovered by filtration, thoroughly
washed with acetonitrile, and then reused for a new set of
xylene oxidation experiments. Fig. 6 shows the recyclability of
the system 2@CNT.
Experimental Section
Materials and Instrumentation
All the reagents were purchased from commercial sources and used as
received. Solvents were dried when necessary, by refluxing over the
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appropriate drying reagents and distilled under nitrogen prior to use. The
Catalytic tests
scorpionate vanadium(IV) complexes [VCl3{κ3-SO3C(pz)3}] (1),[12]
[VOCl2{κ3-HOCH2C(pz)3}] (2)[13] and
(3)[14]
[VOCl2{κ3-MeSO2OCH2C(pz)3}]
In a typical MW-assisted reaction, 2.5 mmol of xylene, 5 mmol of tert-
butyl hydroperoxide (TBHP, 70% aq. solution) and vanadium catalysts 1-
3@CNT (0.16 – 0.80 mol of V supported at CNT, 6.4 × 10-3 – 3.2 × 10-2
mol% vs. substrate) were introduced into a 10 mL glass vessel. In the
experiments with HNO3 (65%, 1.6 – 8.0 mol) this acid was added
immediately before the addition of the catalyst. The mixture was
subjected to microwave irradiation at the desired temperature for the
selected time at 600 rpm stirring speed. At the end of the reaction, the
mixture was cooled down to r.t., the organics extracted with 2.5 mL of
acetonitrile and identified by GC-MS. The V-catalysts were separated by
filtration, washed with acetonitrile, dried with compressed air and reused
in subsequent oxidation cycles.
were synthesized according to reported procedures and
characterized accordingly.
1H and 13C NMR spectra were recorded at ambient temperature on a
Bruker Avance II + 300 (UltraShieldTM Magnet) spectrometer operating at
300.130 and 75.468 MHz for proton and carbon-13, respectively, at
ambient temperature. The chemical shifts are reported in ppm using
tetramethylsilane as an internal reference.
Infrared spectra (4000–400 cm−1) were recorded on a Vertex 70 (Bruker)
instrument in KBr pellets. Far infrared spectra FIR (400-200 cm-1) were
recorded on a Vertex 70 spectrophotometer in CsI pellets.
Elemental analyses were carried out by the Microanalytical Service of the
Instituto Superior Técnico.
For comparison, xylene oxidation reactions were performed in round
bottom flasks under the same reaction conditions but using an oil bath as
heating source (up to 3 mL total volume).
SEM and EDX analyses were carried out on a scanning electron
microscope JEOL 7001F with Oxford light elements EDS detector and
EBSD detector.
Acknowledgements
Reactions under microwave (MW) irradiation were performed in
a
focused Anton Paar Monowave 300 reactor fitted with a rotational system
and an IR temperature detector, using a 10 mL capacity reaction tube
with a 13 mm internal diameter.
Funding from FCT (Fundação para a Ciência e a Tecnologia,
Portugal) (UID/QUI/00100/2013, PTDC/QEQ-ERQ/1648/2014 and
PTDC/QEQ-QIN/3967/201 projects) are acknowledged. APCR
acknowledges FCT for the SFRH/BPD/90883/2012 grant. SACC
GC-MS analyses were performed using a Perkin Elmer Clarus 600 C
instrument (He as the carrier gas), equipped with two capillary columns
(SGE BPX5; 30 m × 0.32 mm × 25 mm), one having an EI-MS (electron
impact) detector and the other one with a FID detector. The temperature
of injection was 330 ºC. The initial temperature (50 ºC) was raised at 5
ºC/min to 300 ºC. Reaction products were identified by comparison of
their retention times with known reference compounds, and by comparing
their mass spectra to fragmentation patterns obtained from the NIST
spectral library stored in the computer software of the mass spectrometer.
acknowledges
FCT
for
Investigator
2013
Program
(IF/01381/2013/CP1160/CT0007), with financing from the European
Social Fund and the Human Potential Operational Program. This
work was financially supported by Project POCI-01-0145-FEDER-
006984 – Associate Laboratory LSRE-LCM funded by FEDER
through COMPETE2020 - Programa Operacional Competitividade e
Internacionalização (POCI) – and by national funds through FCT.
Keywords: xylene • oxidation • C-scorpionate • carbon
nanotubes • vanadium
Support synthesis
Multi-walled carbon nanotubes (CNT) NC3100 from NanocylTM were
treated in reflux with 75 mL of a 5 M nitric acid solution per gram of
carbon material, for 3 h, then separated by filtration and washed with
deionized water until neutral pH, similarly to what is described in
literature.[15, 19] The obtained material was further treated with 75 mL of a
20 mM NaOH aqueous solution (per gram of carbon material) in reflux for
1 h, as in literature.[15, 19b, 20] This material was also separated by filtration
and washed until neutral pH.
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hollow cathode lamp.
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10.1002/asia.201700499
Chemistry - An Asian Journal
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C-scorpionate vanadium(IV)
complexes supported on
Jiawei Wang, Luísa M. D. R. S. Martins,*
Ana P. C. Ribeiro,* Sónia A. C.
Carabineiro, José L. Figueiredo,
Armando J.L. Pombeiro
functionalized carbon nanotubes
(CNT) are the first V-scorpionate
catalysts used so far for the neat
oxidation of o-, m- or p-xylene, with
TBHP (70% aq. solution),
Remarkably, a p-toluic acid yield of
43% (73% selectivity, TON = 1.34 ×
103) was obtained in a simple
microwave-assisted mild oxidation
procedure.
Page No. – Page No.
Supported C-scorpionate
vanadium(IV) complexes as reusable
catalysts for xylene oxidation
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