Novel alkoxysilane pentacoordinate O=V(IV) complexes as supported catalysts for cyclohexane oxidation with dioxygen

Article abstract of DOI:10.1016/j.apcata.2010.06.018

A variety of newly synthesized and well characterized alkoxysilane pentacoordinate Oxovanadium(IV) complexes, VO[Sal(PMeOSi)DPTA] 3[a], VO[Cl-Sal(PMeOSi)DPTA] 3[b], VO[Sal(PMeOSi)DETA] 6[a] and VO[Cl-Sal(PMeOSi)DETA] 6[b], (Sal = salicylaldehyde, DPTA = bis(aminopropyl)amine, DETA = diaminoethylamine), have been anchored by covalent bond into the surface of SiO₂ and/or Al₂O₃ via silicon-alkoxide route by a condensation process as supported catalysts. These solid supported catalysts (abbreviated as catalysts A to H) showed high catalytic efficiency in the selective oxidation reaction of cyclohexane using molecular oxygen under relatively mild condition in a micro-batch reactor. The Catalyst C (SiO ₂/3[a] complex) system exhibits best activity, overall yield 38.5% (TONs, ca. 5.0 × 10³) as well as high selectivity 98% (cyclohexanol 74%, cyclohexanone 24%). Notably, cyclohexane shows significantly improved yield 44.0%, by the addition of pyrazinecarboxylic acid as a co-catalyst. The TGA indicates these catalysts are stable up to maximum reaction temperature, ca. 473 K and ICP analysis shows there is negligible vanadium loss from the supported catalyst after the reaction, allowing further use of the V-catalyst. The various factors influences (i.e. temperature, O₂ pressure, reaction time, catalyst amount) were also investigated in the systematic way, to optimize the reaction processes. The impact of radical traps and detection of intermediate peroxy radical were also investigated to establish a radical mechanism.

Full text of DOI:10.1016/j.apcata.2010.06.018

Applied Catalysis A: General 384 (2010) 136–146
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Novel alkoxysilane pentacoordinate O V(IV) complexes as supported catalysts
for cyclohexane oxidation with dioxygen
Gopal S. Mishraa,∗, Anil Kumarb, Suman Mukhopadhyayc, Pedro B. Tavaresa
a Centro de Química-Vila Real, Departamento de Química, Universidade de Trás-os Montes e Alto Douro, 5001-801 Vila Real, Portugal
b Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India
c Indian Institute of Technology Indore, IET-DAVV Campus, Khandwa Road, Indore 452017, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A
variety of newly synthesized and well characterized alkoxysilane pentacoordinate Oxovana-
Received 10 March 2010
Received in revised form 2 June 2010
Accepted 11 June 2010
dium(IV) complexes, VO[Sal(PMeOSi)DPTA] 3[a], VO[Cl-Sal(PMeOSi)DPTA] 3[b], VO[Sal(PMeOSi)DETA]
6[a] and VO[Cl-Sal(PMeOSi)DETA] 6[b], (Sal = salicylaldehyde, DPTA = bis(aminopropyl)amine,
DETA = diaminoethylamine), have been anchored by covalent bond into the surface of SiO2 and/or
Al2O3 via silicon-alkoxide route by a condensation process as supported catalysts. These solid supported
catalysts (abbreviated as catalysts A to H) showed high catalytic efficiency in the selective oxidation
reaction of cyclohexane using molecular oxygen under relatively mild condition in a micro-batch
reactor. The Catalyst C (SiO2/3[a] complex) system exhibits best activity, overall yield 38.5% (TONs, ca.
5.0 × 103) as well as high selectivity 98% (cyclohexanol 74%, cyclohexanone 24%). Notably, cyclohexane
shows significantly improved yield 44.0%, by the addition of pyrazinecarboxylic acid as a co-catalyst.
The TGA indicates these catalysts are stable up to maximum reaction temperature, ca. 473 K and ICP
analysis shows there is negligible vanadium loss from the supported catalyst after the reaction, allowing
further use of the V-catalyst. The various factors influences (i.e. temperature, O2 pressure, reaction time,
catalyst amount) were also investigated in the systematic way, to optimize the reaction processes. The
impact of radical traps and detection of intermediate peroxy radical were also investigated to establish
a radical mechanism.
Available online 19 June 2010
Keywords:
V-complexes
Covalent bond anchoring
Supported catalysts
Cyclohexane
Oxygen
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
[12–15]. The main drawback of these processes are (i) leaching of
metal because it is added at the last step of reaction and (ii) possibly
The vanadium complexes have attracted interest owing to their
potential roles in several types of catalytic reactions and biological
activities with industrial significance [1–6]. In particular, the penta-
coordinate vanadium complexes have been well known for a long
time to undergo reversible coordination of the N3O2-ligands in a
solution [7,8]. On the other hand, heterogenized V-complexes have
been found to act as efficient catalysts for various types of reactions,
as well [9,10]. Previously, we have also successfully applied some
heterogenized Schiff-base V-complexes for the oxidation reactions
of alkanes [11] but, although promising, the use of these vanadium
metal complexes as supported catalyst still constitutes an under-
developed field of research. Several types of supporting materials
(i.e. SiO2, MCM-41, SBA-15) have been used for the immobiliza-
tion purpose of the metal complexes. This was mainly achieved by
the formation of covalent bond between support material matrixes
with –OH and alkoxysilanes via –Cl, –NH2, –CN and –SH groups
the breaking of original structure and (iii) difficulties to character-
ize the original structure of the metal complex on the supported
matrix.
In view of relevance, we have selected cyclohexane as substrate
and its partially oxidized products (cyclohexanol and cyclohex-
anone) for the production of adipic acid and caprolactum, which are
further used in the manufacturing of nylon-6,6ꢀ, nylon-6, urethane
foams, polyamide-6 and lubricating additives [16–18]. Normally,
cobalt naphthenate or cobalt acetate have been used for the indus-
trial oxidation reaction, with O2, at above 150 ◦C, and only a low
conversion (ca. 4%) is achieved to obtain a high selectivity (ca.
85%) towards a mixture of cyclohexanone and cyclohexanol [19,20].
Thus, commercial cyclohexane oxidation uses inherently inefficient
methodology that necessitates repeated recycling of feedstock. The
alternative use of peroxides is expensive and is also accompa-
nied by the formation of by-products [21]. The establishment of
a greener, more effective and selective cyclohexane oxidation sys-
tem with atmospheric oxygen is therefore a current need. Many
heterogeneous catalysts have been developed for this reaction and
generally these catalysts are either oxides or metal cations [22].
∗
Corresponding author. Tel.: +351 960380888; fax: +351 259350480.
E-mail address: mishrags@utad.pt (G.S. Mishra).
0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.06.018

G.S. Mishra et al. / Applied Catalysis A: General 384 (2010) 136–146
137
The NMR spectra (1H, 13C and 15N) were recorded on a Bruker
ARX400 NMR spectrometer at ambient temperature; ı values are
in ppm relative to SiMe4. Coupling constants are in Hz; abbrevi-
ations: s = singlet, d = doublet, t = triplet, q = quartet, m = complex
multiplets; br = broad, dd = doublet or doublets, dm = doublet of
multiplets. Infrared spectra (4000–400 cm−1) were recorded on
a Unicam (Research Series) FT-IR spectrometer in transmission
mode using KBr pellets; wavenumbers are in cm−1; abbreviations:
vs = very strong, s = strong, m = medium, w = weak, br = broad. The
C, H, N elemental analyses were carried out by Fisons EA-1108.
Gas chromatography was carried out with a FISONS GC-8000 series
gas chromatograph equipped with a FID detector and a DB-WAX
capillary column (length: 60 m; internal diameter: 0.32 mm). FAB-
mass spectra were obtained on a Trio-2000 Fisons spectrometer
by bombarding 3-nitrobenzyl alcohol (NBA) matrices of the sam-
ples with 8 keV (ca. 1.28 × 1015 J) Xe atoms. Mass calibration for
data system acquisition was achieved using CsI, GC–MS analyses
were performed by using this spectrometer with a coupled gas
chromatograph Carlo Erba Instruments, Auto/HRGC/MS. Thermo-
gravimetric analysis of the solid catalyst was performed by TA
Instruments Q50. Vanadium concentrations of fresh and filtrates
after the catalytic experiments were determined by AAS analysis
on a Perkin-Elmer 41000ZL spectrometer. Morphology of the solid
supported catalysts was analyzed by scanning electron microscopy
(FEI Quanta 400), equipped with an EDS detector (EDAX).
The system activity is mainly dependent on the correct choice of
the solvent, which is determined by the polarity of the medium
and the size of the substrate that needs to be adsorbed at the cat-
alytic surface [23,24]. Sometimes promoters or co-reactants like
acetaldehyde, cyclohexanone, cyclohexanol and azo-bis isobuty-
ronitrile, pyrazinecarboxylic acid, etc. are also added to reduce the
induction period and to increase the conversion [25]. Among other
oxidation catalysts, heteropolyoxometalates compounds also show
high catalytic activity towards the alkanes oxidation [26].
Hence, the main purpose of this current study is to provide
newly synthesized and well characterized alkoxysilane of N3O2 lig-
ands (i.e. 2[b], 5[a], 5[b]) with Oxovanadium(IV) complexes (i.e.
3[a], 3[b], 6[a] and 6[b]). These silica and/or alumina anchored V-
complexes catalysts have been applied for the selective gas–liquid
phase oxidation reaction of cyclohexane (with molecular oxygen)
without adding any additives and solvents in batch process under
optimized conditions.
2. Experimental
2.1. General materials and procedure
All the reactions and manipulations were performed under an
atmosphere of dinitrogen using standard Schlenk techniques. All
the solvents were purified by standard procedures and freshly
distilled immediately prior to use. The chemicals, i.e. bis(amino-
propyl)amine, potassium carbonate, 2,2-diaminoethylamine (all
from Acros), salicylaldehyde, 5-chlorosalicylaldehyde, 3-iodopro-
pyl trimethoxysilane, pyrizine carboxylic acid, silica gel, alu-
mina, acetonitrile, bromotrichloromethane, diphenylamine, triph-
enylphosphane (all Sigma), bis(acetylacetonate)oxovanadium,
tetra hydro furan, cyclohexane (all Mark), ethanol, n-pentane,
picolinic acid, 2-pyrazinecarboxylic acid, 3-amino-2-pyrazine-
carboxylic acid and 2,6-pyrizinedicarboxylic acid, 5-hydroxy-
2-pyrazinecarboxylic acid, 5-methyl-2-pyrazinecarboxylic acid,
toluene (all from Janssen) were used as received from supplier.
2.2. Metal complex synthesis and characterization
2.2.1. Synthesis of ligand, 2[a]
The ligand, bis(salicylidenimino-3-propyl)amine with 3-
iodopropyl trimethoxysilane 2[a] (Scheme 1), was prepared
and characterized as described earlier [27]. Anal. calcd. for
C
26H39N3O5Si: C, 62.15; H, 7.77; N, 8.37. Found: C, 62.11; H, 7.72;
N, 8.15. FT-IR (KBr pellet, cm−1): ꢀ = 3045 and 2940 [s, ꢀ(C–H)];
1629 and 1542 [s, ꢀ (C C), s, ꢀ (N C)]; 1160 [s, ꢀ (Si–O)]; 1092 [s,
ꢀ (C–N)]. 1H NMR (ı, CDCl3): ı = 0.62 (m, 2H, Si–CH2), 1.54 (m, 2H,
CH2), 1.81 (m, 4H, CH2), 2.40 (m, 6H, CH2N), 3.55 (m, 4H, CH2N;
Scheme 1. Synthesis of trialkoxysilane pentacoordinate Oxovanadium(IV) complexes.

138
G.S. Mishra et al. / Applied Catalysis A: General 384 (2010) 136–146
9H, CH3O), 6.56–7.35 (m, CH, ring), 8.22 (s, 2H, HC N), 9.82 (s, 2H,
C–OH). 13−C NMR (ı, CDCl3): ı = 7.4, 20.6, 29.0 and 50.9 (s, C–H2),
51.8, 57.3, 57.8 (s, O–CH3), 117.3, 118.8, 119.2, 131.6 and 132.4 (s,
C–H, ring), 161.7 and 165.3 (s, N C). 15N NMR (ı, CDCl3): ı = 82.7
(s, imines) and 343.2 (s, amine). FAB+-MS: m/z = 502 [(M+H)+, 25],
364 [M−(CH2Si(OCH3)3)+, 13], 120 [(Si(O CH3)3), 100].
2H, CH2N), 2.63 (m, 4H, CH2N), 3.59 (m, 4H,CH2N; s, 9H, CH3O),
6.01–7.53 (m, Ar + 2OH), 8.2 (m, HC N). 13−C NMR (ı, CDCl3):
ı = 7.5, 23.4, 53.5 and 59.7 (s, C–H2), 51.4, 56.5 (s, O–CH3), 117.2,
118.5, 120.9, 124.6 and 129.6 (s, C–H, ring), 159.6 (–C N) and 164.1
(–C–OH). 15N NMR (ı, CDCl3): ı = 82.4 (s, imines), 344.8 (s, amine).
FAB+-MS: m/z = 547 [(M+H)+, 27], 371 [M−(CH2Si(OCH3)3)+, 14],
121 [(Si(OCH3)3), 99].
2.2.2. Synthesis ligand, 2[b]
The ligand 2[b] (Scheme 1) was prepared with 5-
chlorosalicylaldehyde instead of salicylaldehyde as similar
procedure 2[a] ligand, a solution of 1 (2.0 g, 3.5 × 10−3 mol)
and equimolar amount of 3-iodoprpyle-trimethoxysilane (1.0 g,
3.5 × 10−3 mol) with K2CO3 (0.97 g, 7.0 × 10−3 mol) in MeCN
(50 mL). The reaction mixture was heated and refluxed for 22 h.
After the reaction, the product was filtered and washed three times
with n-pentane and dried under vacuo obtained, 2.7 g, 92% yield
of ligand 2[b] (orange colour). Anal. calcd. for Cl2C26H37N3O5Si:
C, 54.64; H, 6.48; N, 7.63. Found: C, 53.08; H, 6.12; N, 7.15. FT-IR
(KBr pellet, cm−1): ꢀ = 3040 [s, ꢀ(C–H)]; 2933 [s, ꢀ(C–H)]; 1620 and
1541 [s, ꢀ(C C), s, ꢀ(N C)]; 1162 [s, ꢀ(Si–OCH3)]; 1082 [s, ꢀ(C–N)];
746 [s, ꢀ(–Cl)]. 1H NMR (ı, CDCl3): ı = 0.51 (m, 2H, Si–CH2), 1.52
(m, 2H, CH2), 1.88 (m, 4H, CH2), 2.55 (m, 6H, N CH2), 3.61 (m, 4H,
2.2.5. Synthesis complex, 3[a]
To a solution of VO[(acac)2] (1.16 g, 4.38 × 10−3 mol) in the THF
(30 mL) was added, with constant starring, an equimolar amount
of ligand 2[a] (2.20 g, 4.38 × 10−3 mol) in THF (20 mL). The green
mixture was heated and refluxed for 4 h (Scheme 1). The final solu-
tion was concentrated and upon addition of n-pentane a green
solid precipitated. The solid was collected by the filtration, washed
with n-pentane (3× 100 mL) and dried under vacuo to obtained,
a new 3[a] complex of vanadium (2.9 g, 87% yield, Scheme 1). It
is soluble in methanol, ethanol, tetrahydrofuran but insoluble in
ether. Anal. calcd. for VC26H37N3O6Si: C, 55.07; H, 6.53; N, 5.38.
Found: C, 55.26; H, 6.32; N, 5.26. FT-IR (KBr pellet, cm−1): ꢀ = 3190
[s, ꢀ(C–H)]; 2921 [s, ꢀ(C–H)]; 1623 and 1534 [s, ꢀ(C N) and s,
ꢀ(C C)]; 1160 [s, ꢀ(Si–OCH3)]; 1076 [s, ꢀ(C–N)], 933 and 635 [s,
ꢀ(V O) and s, ꢀ(V· · ·O)]. EPR (CH2Cl2, at 25 ◦C): g = 1.9982, A|| and
A⊥ (in ×10−4 cm−1) = 1.62 and 59.8. FAB+-MS: m/z = 567 [(M+H)+,
100], 405 [M−(CH2Si(OCH3)3)+, 14], 1 [(Si(OCH3)3), 99].
N
CH2; 9H, CH3O), 6.54–7.51 (m, 2H, –OH), 8.02 (s, 2H, HC N),
9.82 (s, C–OH, ring). 13−C NMR (ı, CDCl3): ı = 7.6, 20.3, 23.1 and
60.2 (s, C–H2), 51.4, 56.5, 57.0 (s, O–CH3), 119.1, 118.3, 121.2, 128.6
and 130.8 (s, C–H, ring), 162.6 and 166.0 (s, N C). 15N NMR (ı,
CDCl3): ı = 82.8 (s, imines), 345.6 (s, amine). FAB+-MS: m/z = 571
[(M+H)+, 27], 371 [M−(CH2Si(OCH3)3)+, 14], 121 [(Si(OCH3)3), 99].
2.2.6. Synthesis complex, 3[b]
To a solution of VO[(acac)2] (0.70 g, 2.63 × 10−3 mol) in THF
(30 mL) was added, with constant starring, an equimolar amount of
ligand 2[b] (1.5 g, 2.63 × 10−3 mol) in THF (20 mL). An olive green
mixture was obtained which was refluxed for 4 h. The final solu-
tion was concentrated and upon addition of n-pentane a green
solid precipitated (Scheme 1). The solid was collected by the filtra-
tion, washed with n-pentane (3× 100 mL) and dried under vacuo
to obtain a new 3[b] complex of vanadium (1.91 g, 86% yield,
Scheme 1). It is soluble in methanol, ethanol, tetrahydrofuran but
insoluble in ether. Anal. calcd. for VCl2C26H35N3O6Si: C, 49.09; H,
5.51; N, 6.61. Found: C, 49.21; H, 5.02; N, 6.26. FT-IR (KBr pel-
let, cm−1): ꢀ = 3145 [s, ꢀ(C–H)]; 2929 [s, ꢀ(C–H)]; 1626 and 1540
[s, ꢀ(C N) and s, ꢀ(C C)]; 1153 [s, ꢀ(Si–OCH3)]; 1059 [s, ꢀ(C–N)],
941 and 630 [s, ꢀ(V O) and s, ꢀ(V· · ·O)]. EPR (CH2Cl2, at 25 ◦C):
g = 1.9892, A|| and A⊥ (in ×10−4 cm−1) = 164.1 and 58.5. FAB+-
MS: m/z = 636 [(M+H)+, 100], 405 [M−(CH2Si(OCH3)3)+, 14], 121
[(Si(OCH3)3), 99].
2.2.3. Synthesis ligands, 5[a]
The ligand 5[a] (Scheme 1) was prepared with 2,2-
diaminoethylamine instead of bis(aminopropyl)amine as
similar procedure 2[a] ligand. A solution of ligand 4[a] (3.24 g,
1.07 × 10−3 mol), and equimolar amount of 3-iodoprpyle-
trimethoxysilane (3.02 g, 1.07 × 10−3 mol) in MeCN (50 mL)
with K2CO3 (2.8 g, 2.03 × 10−3 mol) was heated and refluxed for
22 h. After the reaction, the solution was filtered and washed
several times with n-pentane and dried under vacuo obtained,
5.5 g, 88% yield of ligand 5[a] (orange colour product). Anal. calcd.
for C25H37N3O5Si: C, 61.52; H, 7.59; N, 8.61. Found: C, 60.08;
H, 7.32; N, 7.44. FT-IR (KBr pellet, cm−1): ꢀ = 3038 [s, ꢀ(C–H)];
2929 [s, ꢀ(C–H)]; 1618 and 1534 [s, ꢀ(C C), s, ꢀ(N C)]; 1160 [s,
ꢀ(Si–OCH3)]; 1076 [s, ꢀ(C–N)]. 1H NMR (ı, CDCl3): ı = 0.43 (m,
2H, CH2Si), 1.62 (m, 2H, CH2), 2.21 (m, 2H, CH2N), 2.63 (m, 4H,
CH2N), 3.59 (m, 4H, CH2N; 9H, CH3O), 6.01–7.53 (m, Ar + 2OH),
8.3 (m, HC N). 13−C NMR (ı, CDCl3): ı = 7.5, 23.4, 53.5 and 59.7 (s,
CH2), 51.4, 56.5 (s, O–CH3), 117.2, 118.5, 120.9, 124.6 and 129.6 (s,
C–H, ring), 159.6 (–C N) and 164.1 (–C–OH). 15N NMR (ı, CDCl3):
ı = 82.4 (s, imines), 344.8 (s, amine). FAB+-MS: m/z = 488 [(M+H)+,
27], 371 [M–(CH2Si(OCH3)3)+, 14], 121 [(Si(OCH3)3), 99].
2.2.7. Synthesis complex, 6[a]
To a solution of VO[(acac)2] (0.81 g, 3.07 × 10−3 mol) in THF
(30 mL) was added, with constant starring, an equimolar amount
of ligand 5[a] (1.65 g, 3.07 × 10−3 mol) in THF (20 mL). A dark green
mixture was produced which was refluxed for 4 h (Scheme 1). The
final solution was concentrated and upon addition of n-pentane
a green solid was precipitated. It was collected by the filtra-
tion, washed with n-pentane (3× 100 mL) and dried under vacuo
to obtain a new 6[a] complex of vanadium (2.14 g, 85% yield,
Scheme 1). It is soluble in methanol, ethanol, tetrahydrofuran but
insoluble in ether. Anal. calcd. for VC24H33N3O6Si: C, 53.48; H, 6.13;
N, 7.80 Found: C, 54.03; H, 5.99; N, 7.16. FT-IR (KBr pellet, cm−1):
ꢀ = 3054 [s, ꢀ(C–H)]; 2933 [s, ꢀ(C–H)]; 1600 and 1564 [s, ꢀ(C N) and
s, ꢀ(C C)]; 1145 [s, ꢀ(Si–OCH3)]; 1055 [s, ꢀ(C–N)], 936 and 633 [s,
ꢀ(V O) and s, ꢀ(V· · ·O)]. EPR (CH2Cl2, at 25 ◦C): g = 1.9863, A|| and
A⊥ (in ×10−4 cm−1) = 162.2 and 57.6. FAB+-MS: m/z = 538 [(M+H)+,
100], 405 [M−(CH2Si(OCH3)3)+, 14], 121 [(Si(OCH3)3), 99].
2.2.4. Synthesis ligands, 5[b]
The ligand 5[b] (Scheme 1) was prepared with 5-
chlorosalicylaldehyde and 2,2-diaminoethylamine instead of
salicylaldehyde and bis(aminopropyl)amine as similar proce-
dure 2[a] ligand. A solution of 4[b] (2.0 g, 3.65 × 10−3 mol) and
equimolar amount of 3-iodoprpyle-trimethoxysilane (1.06 g,
3.65 × 10−3 mol) with K2CO3 (1.01 g, 7.32 × 10−3 mol) in MeCN
(50 mL) was heated and refluxed for 22 h. After the reaction, the
mixture was filtered and washed several times with n-pentane and
dried under vacuo obtained 2.75 g, 90% yield of ligand 5[b] (orange
colour product). Anal. calcd. for Cl2C25H25N3O5Si: C, 54.89; H,
6.40; N, 7.68. Found: C, 54.40; H, 6.62; N, 7.25. FT-IR (KBr pellet,
cm−1): ꢀ = 3038 [s, ꢀ(C–H)]; 2929 [s, ꢀ(C–H)]; 1618 and 1534 [s,
ꢀ(C C), s, ꢀ(N C)]; 1160 [s, ꢀ(Si–OCH3)]; 1076 [s, ꢀ(C–N)]. 1H
NMR (ı, CDCl3): ı = 0.43 (m, 2H, CH2Si), 1.62 (m, 2H, CH2), 2.21 (m,
2.2.8. Synthesis complex, 6[b]
To a solution of VO[(acac)2] (0.73 g, 2.74 × 10−3 mol) in THF
(30 mL) was added, with constant starring, an equimolar amount of

G.S. Mishra et al. / Applied Catalysis A: General 384 (2010) 136–146
139
ligand 5[b] (1.66 g, 2.74 × 10−3 mol) in THF (20 mL) to give a olive
green mixture. It was refluxed for 4 h (Scheme 1). The solution was
concentrated and upon addition of n-pentane yielded a green solid
precipitate. The solid was collected by the filtration, washed with
n-pentane (3× 100 mL) and dried under vacuo to obtained, a new
6[b] complex of vanadium (2.1 g, 88% yield, Scheme 1). It is sol-
uble in methanol, ethanol, tetrahydrofuran but insoluble in ether.
Anal. calcd. for VCl2C24H31N3O6Si: C, 47.41; H, 5.10; N, 6.91. Found:
C, 46.99; H, 5.02; N, 6.65. FT-IR (KBr pellet, cm−1): ꢀ = 3050 [s,
ꢀ(C–H)]; 2940 [s, ꢀ(C–H)]; 1643 and 1551 [s, ꢀ(C N) and s, ꢀ(C C)];
1121 [s, ꢀ(Si–OCH3)]; 1044 [s, ꢀ(C–N)], 945 and 629 [s, ꢀ(V O)
and s, ꢀ(V· · ·O)]. EPR (CH2Cl2, at 25 ◦C): g = 1.8999, A|| and A⊥ (in
×10−4 cm−1) = 161.0 and 59.2. FAB+-MS: m/z = 608 [(M+H)+, 100],
405 [M−(CH2Si(OCH3)3)+, 14], 121 [(Si(OCH3)3), 99].
6[a] and 6[b] are buried under those of the supports, one can
observe a broad shoulder (at ca. 3300–3150 cm−1) of the silica and
alumina and strong [s, ꢀ(V O)] bands of V-complexes was also
observed between 945 and 926 cm−1. The other related bands of
complexes are observed at ꢀ = 3050–2935 [s, ꢀ(C–H)]; 1655–1637
[s, ꢀ(C N)]; 1552–1533 [s, ꢀ(C C)]; 1080 [s, ꢀ(Si–O)] (not clear).
TGA: all the supported catalysts A–H, complexes are started to
decompose between ca. 488 and 588 K. SEM analysis: morphological
analysis (20,000× magnification) of the Catalyst C picture shows,
bright spotted portion as V-complex and the presence of vanadium
content (0.37%) was also confirmed by SEM/EDS detector. EPR anal-
ysis: for the catalysts samples A to H (at room temperature) eight
line spectrum with range of g = 1.8999–1.9821 and range of A|| and
A⊥ (in ×10−4 cm−1) = 158.0–166.3 and 58.1–62.2, were obtained,
respectively. ICP analysis: the catalysts A to H was measured in the
range of 0.28–0.34 wt.% of vanadium on the surface of support.
2.3. Supported catalysts preparation and characterization
In a surface modification process, we have used the wet-
impregnation method [28] for immobilization of the vanadium
Schiff-base complexes 3[a], 3[b], 6[a] and 6[b], which are cova-
lently bound on the surface (silanol group, –OH) of silica and or
alumina [13,29]. Each of the above vanadium complexes 3[a], 3[b],
6[a] and 6[b] (50 mg) were separately dissolved in dry toluene
(50 mL), the solution separately added to the SiO2 (1000 mg) and
or Al2O3 (1000 mg) for the preparation of supported catalysts. The
mixtures were heated and refluxed for 12 h with the elimination of
CH3OH (Scheme 2). The functionalized SiO2 and or Al2O3 with cova-
lently anchored vanadium (IV) complexes were filtered-off, washed
several times with methanol, and then dried at 40 ◦C for 4 h, in an
oven. After drying we found that 45 mg of Catalyst A, 41 mg of Cat-
alyst B, 44 mg of Catalyst C, 42 mg of Catalyst D, 45 mg of Catalyst
E, 40 mg of Catalyst F, 43 mg of Catalyst G and 41 mg of Catalyst
H, were separately loaded, per gram of functionalized SiO2 and or
Al2O3 as supported inorganic–organic catalysts. All the supported
catalysts are olive green in colour.
2.4. Typical oxidation procedure and product analysis
In typical oxidation conditions, 3.0 mL (27.8 mmol) of neat
cyclohexane and 30 mg of supported vanadium catalysts were
charged in a micro-batch reactor (a cylindrical rocking type reactor,
40 mL capacity, provided with a pressure gauge and gas delivery
inlet). The reaction mixture was stirred for 180–1800 min under
oxygen pressure. At the end of the reactions, the supported cata-
lysts were separated from the reaction mixture by filtration, using
a filter paper, and washed three times with acetonitrile. The sup-
ported catalysts could be reactivated for further use by heating in
an air oven at 40 ◦C for 6 h. The reaction products analysis was
performed by GC as follows: 60 ␮L of cyclopentanone as internal
standard and 1000 ␮L of final product were added and injected
(0.5 ␮L), in a GC injector (at 240 ◦C, He used as carrier gas). The ini-
tial temperature was maintained at 100 ◦C for 1 min, then raised
10 ◦C/min up to 180 ◦C, and held at this temperature for 1 min.
They were further confirmed by GC–MS analysis. After the oxida-
tion reaction, the mixture was treated with the excess PPh3 for
the detection of alkyl hydroperoxide (CyOOH) and also analyzed
Characterization: FT-IR (KBr pellet, cm−1): for all the final sup-
ported catalysts (A to H) of vanadium (IV) complexes 3[a], 3[b],
Scheme 2. Immobilization of trialkoxysilane pentacoordinate Oxovanadium(IV) complexes to the silica or alumina matrix as supported hybrid catalysts.

140
G.S. Mishra et al. / Applied Catalysis A: General 384 (2010) 136–146
by GC. The turnover numbers (TONs) (moles of product per mole
of catalyst) and yields (moles of product per mole of alkane) were
estimated. Caution! The oxidation reactions (exothermic reactions)
are potentially explosive and necessary care and safety reactions
should be taken.
was also observed. The presence of main characteristic bands of the
O unit is clearly reflected in the typical IR signals at 926–960 [s,
ꢀ(V O)] and 630–635 [s, ꢀ(V· · ·O)]. It was further confirmed by the
EPR spectra which show typical eight line spectrum of paramag-
netic V O(IV) monomeric species (51V, I = 7/2) with characteristic
g, A|| and A⊥ parameters.
V
3. Results and discussion
3.2. Supported V-catalysts: preparation and characterization
The potentially active novel trimethoxysilyl pentacoordinated
The main target of our study was the preparation of supported
vanadium complex catalysts, which had been obtained from a
grafted reaction technique of the silanol groups (–OH) on surface of
supports matrix (SiO2 and Al2O3) with trimethoxy groups (–OCH3)
of Schiff-base Oxovanadium(IV) complexes 3[a], 3[b], 6[a] and 6[b],
by the condensation reaction (overnight reflux in toluene) and lib-
eration of MeOH (Scheme 2) [13(a)]. These final supported catalysts
are abbreviated as (SiO2/7[a] as Catalyst A, Al2O3/7[a] as Catalyst
B, SiO2/7[b] as Catalyst C, Al2O3/7[b] as Catalyst D, SiO2/8[a] as
Catalyst E, Al2O3/8[a] as Catalyst F, SiO2/8[b] as Catalyst G and
Al2O3/8[b] as Catalyst H). As methoxysilane is sensitive towards
moisture all the manipulations have been carried out under N2. Any
residual and excess of V-complexes in toluene were subsequently
removed by the soxhlet thimble filled by the supported V-catalysts
extraction.
The thermal stability of all the solid catalysts samples were
analyzed by TGA (Fig. 1) showing only a 1% weight loss up to
523 K, below the maximum reaction temperature (448 K in entry
8 of Table 2). In FT-IR spectrum after the immobilization of V-
complexes, a broad shoulder was appeared between; ꢀ = 3620 and
3375 and ꢀ = 1620 and 1010 in all the supported catalysts due to
support effect (silica and alumina). The anchoring of V O(IV) com-
plex on the support was confirmed by the V O bond between
ꢀ = 926 and 960 in all the catalysts. The eight line of EPR results
of solid supported catalysts also confirm the presence of vana-
dium (IV) oxidation state of complexes with typical g values and a
parameters. The presence of vanadium on the supported catalysts
were further confirmed by inductive couple plasma (ICP) analysis
in fresh catalysts (0.28–0.34 wt.%) and after the oxidation reaction
of used catalysts shows very little amount (∼0.2 wt.%) of metal loss.
Schiff-base Oxovanadium(IV) complexes,
methoxysilyl)DPTA], V O[Sal(PMeOSi)DPTA] 3[a], V O[Cl-Sal(n-
propyltrimethoxysilyl)DPTA], O[Cl-Sal(PMeOSi)DPTA] 3[b],
O[Sal(n-propyltrimethoxysilyl)DETA], V O[Sal(PMeOSi)DETA]
V O[Sal(n-propyltri-
V
V
6[a] and
V
O[Cl-Sal(n-propyltrimethoxysilyl)DETA],
V O[Cl-
Sal(PMeOSi)DETA] 6[b] (Scheme 1), have been synthesized and
well characterized by modern spectroscopic techniques. These
are the first examples of the alkoxysilane ligands with Oxovana-
dium(IV) complexes in best of our knowledge. Only one article
has been found with Schiff-base Co and Cu complexes with
2[a] ligand [32]. These vanadium metal complexes have been
covalently anchored into the functionalized silica and/or alumina
surface via silicon-alkoxide route by the condensation process
(elimination of CH3OH) as supported inorganic–organic hybrid
catalysts (supported catalysts A to H, see in Scheme 2). These
supported catalysts with Oxovanadium(IV) complexes have been
tested for the cyclohexane oxidation reaction with molecular
oxygen (cheaper and freely available from air) under relatively
mild condition in micro-batch reactor.
3.1. V-complexes: synthesis and characterization
Trimethoxysilyl pentacoordinate Schiff-base ligand with
five-coordination 2[a] was synthesized as according to Carré
et al. [27] and ligands 2[b], 5[a], 5[b] are included first time
here. Two different amines 2,2ꢀ-bis(aminopropyl)amine(DPTA),
2,2ꢀ-bis(aminoethyl)amine (DPTA) have been used along
with two different aldehydes, viz. salicylaldehyde (Sal), 5-
chlorosalicylaldehyde (Cl-Sal). These ligands with donor sets of
two phenolate oxygen, two Schiff-base nitrogen and another
tertiary amine nitrogen atom have been reacted with the terminal
iodo group of the 3-iodopropyl-trimethoxysilane as ligands 2[a]
yield 90%, 2[b] yield 92%, 5[a] yield 88% and 5[b] yield 90%. The
structures of all the ligands have been confirm by 1H NMR, 13−C
NMR, 15N NMR, FAB-mass, elemental analysis and FT-IR spectra
(see Section 2). The V O(C5H7O2)2 was introduce at the last step
of synthetic process [30], with the ligands 2[a], 2[b], 5[a] and
5[b] for the preparation of four trimethoxysilyl pentacoordinate
Schiff-base V-complexes, 3[a] yield 87%, 3[b] yield 86%, 6[a] yield
85% and 6[b] yield 88% (Scheme 1).
The proposed formulation for these vanadium complexes have
been supported by FAB-mass spectral data, FT-IR spectroscopy, EPR
spectroscopy and elemental analysis. The mass spectrum of each
V-complex is characterized by an intense molecular ion, which is
the most abundant ion in the spectrum, as well as relatively abun-
dant doubly charged parent molecular ions. No polymeric species
were detected in any spectrum which suggests that either the com-
plexes are monomeric or any polymers present were destroyed
upon sublimation or upon electron impact.
The infrared spectra of all the ligands show a broad shoulder
around 3400 ꢀ(–OH) which was due to the –OH, which has been dis-
appeared after the coordination with vanadium occurs. The typical
Si–O lattice vibrations bonds was observed around 1160 ꢀ(–Si–O).
Appearance of a band at ꢀ(1623–1643) associated to the C N group
confirms the presence of Schiff-base moiety, whereas the absence
of bands in the region ꢀ(1634–1651) associated with the N–H bond
Fig. 1. TGA analysis results of supported catalysts A to H (trialkoxysilane pentaco-
ordinate V O(IV) complexes anchored with inorganic martial of solid supports).

G.S. Mishra et al. / Applied Catalysis A: General 384 (2010) 136–146
141
Fig. 2. Morphological study of Catalyst C complex, analyzed by scanning electron microscopy [image magnification, 5 nm (A), 1 ␮m (B) and by EDS detector (C)].
The above analytical evidences indicate the possible anchoring of
V-complexes on the surface of support matrix. The morphologi-
cal study of Catalyst C is shown by SEM results in Fig. 2A (500 nm
magnification), the bright particles on the dark background on the
surface of supported matrix, i.e. SiO2 being assigned as the pres-
ence of the vanadium complex. The SEM-EDS detector analysis
result clearly indicates the presence of vanadium metal percent-
age (Fig. 2C), as bunch of brighter particles on the SiO2 matrix in
Fig. 2B.
mole of C6H12) of C6H11OH and C6H10O of 28.5 and 9.4% (37.9%
overall for these products), within a total 38.5% of C6H12 yield,
including a little amount of by-products (i.e. alkyl hydroperoxide,
adipic acid, hexanolactone and other esters), for molecular oxy-
gen, p(O2) = 10.1 bar, at 423 K. Conducting a mass balance to this
run, we obtain that 2.340 g of cyclohexene (27.82 mmol) reacts
with 0.484 g of O2 (15.13 mmol) producing 0.794 g of cyclohexanol
(7.92 mmol), 0.256 g of cyclohexanone (2.61 mmol) and 0.019 g of
by-products (0.17 mmol); 1.439 g of cyclohexane (17.10 mmol) and
0.316 g of O2 (9.86 mmol) remain unreacted. The catalytic activity
corresponds to a TON (estimated as number of moles of cyclohex-
anol plus cyclohexanone per mole V-complex loaded on support)
of 5.0 × 103 and is higher than those of the other supported cat-
alysts, A to H (Table 1). The increasing molar ratios of the K/A oil
(under low temperature and pressure, Table 2) were suggested that
the decompose of CyOOH intermediate in the reaction mixture and
constantly alcohol was convert into ketone [16(c)]. This intermedi-
ate was completely decomposed, under deep oxidation condition
(above 423 K or 10.1 bar). It was conformed by GC analysis when
the reaction mixture was treated with the excess of PPh3. We also
observed not a big difference of A/K oil ratio between supported
and unsupported C catalyst (Table 2, run 3[e]). When the oxida-
tion reaction carried out in the presence of the above V-catalyst,
3.3. Typical cyclohexane oxidation with molecular oxygen
A series of SiO2 and/or Al2O3 anchored trimethoxysilane penta-
coordinate Oxovanadium(IV) complexes as supported catalysts (A,
B, D, E, F, G and H) were applied for the selective oxidation reaction
with neat cyclohexane and molecular oxygen (O2) in a rocking type
micro-batch reactor. The cyclohexane was converted by the chem-
ical oxidation process, to cyclohexanol and cyclohexanone under
relatively mild conditions, without adding any solvent or additive
(Table 1 and Scheme 3).
Supported Catalyst C (SiO2/7[b]), provides the most effective
catalytic system, in a typical entry (entry 3, Tables 1 and 2) lead-
ing, after 720 min reaction time, to yields (moles of product per
Table 1
Oxyfunctionalization of cyclohexane with molecular oxygen catalyzed by Oxovanadium(IV) complexes supported on silica or alumina under solvent free conditionsa.
Entry
Catalysts
Yield (%)b
A/K Ratio
TONc
C6H11OH (A)
C6H10O (K)
Overalld
1.
2.
3.
4.
5.
6.
7.
8.
SiO2/7[a] (Catal. A)
Al2O3/7[a] (Catal. B)
SiO2/7[b] (Catal. C)
Al2O3/7[b] (Catal. D)
SiO2/8[a] (Catal. E)
Al2O3/8[a] (Catal. F)
SiO2/8[b] (Catal. G)
Al2O3/8[b] (Catal. H)
20.3
19.5
28.5
23.6
20.8
17.4
23.5
21.9
7.0
6.9
9.4
8.1
8.0
7.1
9.2
8.9
30.3
28.6
38.5
33.7
29.8
26.4
34.1
31.8
2.9
2.8
3.0
2.9
2.6
2.5
2.6
2.5
3192
3499
5023
4494
3192
3057
4291
4245
a
Reaction conditions (unless stated otherwise): cyclohexane = 27.82 mmol, supported catalyst = 30 mg, temperature = 323 K, time = 720 min, p(O2) = 10.1 bar (measured at
25 ◦C; 1 atm = 1.01 bar = 101 kPa).
b
Percentage molar yield (moles of product/mole of alkane).
Turnover number (mole of C6H11OH + C6H10O/mol V-complex immobilized on 30 mg of solid catalyst).
C6H11OH + C6H10O + by-products.
c
d

142
G.S. Mishra et al. / Applied Catalysis A: General 384 (2010) 136–146
Scheme 3. Cyclohexane oxidation with O2 in the presence of supported V-catalyst.
a strong smell at the end of the experiment indicated the forma-
tion of oxyfunctionalized product, which have been analyzed by GC
and further confirmed by GC–MS techniques that allow to quantify
the amounts of cyclohexanol (main product) and cyclohexanone
(in lesser amount). As a result of chemical reaction, the colour of
the supported catalysts changes to brown. After being used, the
supported catalysts can be reactivated upon washing (three times)
with acetone and drying in an air oven at 60 ◦C for 6 h, showing
usually a comparable activity to that of the initial entry.
The unsupported vanadium complexes, 3[a], 3[b], 6[a] and
6[b], do not lead to amounts of products significantly above those
obtained in blank essays, which indicates that the V(IV)-species
(covalently anchored pentacoordinated Oxovanadium(IV) com-
plexes on SiO2 or Al2O3) are more active than their parents. The
effects of various factors such as reaction time, temperature, oxygen
pressure, amount of catalyst, presence of a co-catalyst and radical
traps on the catalytic activity and selectivity, were studied towards
the optimization of the chemical reaction process. All the catalytic
results are summarized in Tables 1–3, as well as in Figs. 3–6 for the
case of the silica supported Catalyst C in Table 1, which provides
the most reactive system.
180–1800 min (under constant conditions: substrate = 27.8 mmol,
temp. = 423 K, p(O2) = 10.1 bar and catalyst = 30 mg) and it was
observed that the maximum yield (41.1%) towards the desired
products, i.e. cyclohexanol (28.4%) and cyclohexanone (9.7%),
reaches after 1800 min, but the best selectivity of alcohol (74%) and
ketone (24%) were obtained after 720 min. Moreover, extending the
reaction period above 720 min results in a detrimental effect on
product selectivity particularly for cyclohexanol and little change
in formation of cyclohexanone was observed (Fig. 3), concomitantly
with an increment of the by-products formation.
We have investigated the best optimized reaction temper-
ature for this system by using the range from 373 to 473 K
(Table 1, entry 3, Table 2, entries 6–9 and Fig. 4), under constant
reactions: substrate = 27.8 mmol, time = 720 min, p(O2) = 10.1 bar,
catalyst = 30 mg. Although the overall yield, ca. 19.6%, of cyclohex-
ane to desired product (KA oil) increases rather slowly up to 398 K,
a rapid change occurs towards the overall yield, ca. 38.5% at 473 K,
with good yield of cyclohexanol 28.5% and low yield of cyclohex-
anone 9.4%. The selectivity dependence on the temperature graph
is also shown in Fig. 4 which indicates that the best selectivity,
74.1% towards cyclohexanol is achieved at ca. 423 K. However,
the reaction temperature was not allowed to go beyond 473 K,
due to the formation of an unidentified polymerized dark brown
material.
The ideal condition for the cyclohexane in respect of reaction
time was investigated by the most active supported Catalyst C
(Table 1, entry 3 and Table 2, entries 1–5, Fig. 3) in the time range of
Table 2
Effect of various factors on oxyfunctionalization of cyclohexane to cyclohexanol and cyclohexanone under solvent free conditions, catalyzed by supported Catalyst C with
molecular oxygena.
b
Entry
Time (min)
Temp. (K)
p(O)2 (bar)
n (catalyst) × 10−5
Yield (%)c
Molar ratio A/K
n (substrate) (mmol)
C6H11OH (A)
C6H10O (K)
Over-alld
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
180
360
720
1080
1800
720
720
720
720
720
720
720
720
720
720
720
720
423
423
423
423
423
373
398
448
473
423
423
423
423
423
423
423
423
10.1
10.1
10.1
10.1
10.1
10.1
10.1
10.1
10.1
5.1
15.2
25.3
10.1
10.1
10.1
10.1
10.1
7.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
2.5
5.0
9.9
3.4
11.2
28.5
28.6
28.4
4.3
13.4
30.8
33.0
9.2
32.7
32.0
7.8
15.4
31.1
31.6
34.8
1.7
4.8
9.4
9.9
9.7
5.1
16.2
38.5
40.4
41.1
6.6
19.6
42.6
46.0
13.4
44.2
46.1
11.9
22.6
42.8
44.5
49.1
2.0
2.3
3.0e
2.9
2.9
1.9
2.3
3.0
3.0
2.2
3.1
3.0
2.0
3.0
3.1
3.2
3.2
2.3
5.9
10.2
11.0
4.2
10.7
10.6
3.9
6.8
10.0
9.8
12.4
7.5 + PCAf
10.8
a
Reaction conditions (unless stated otherwise): supported Catalyst C = 2.1 × 10−3 mmol, cyclohexane = 27.8 mmol, temperature = 323 K, time = 720 min, p(O2) = 10.1 bar
(measured at 25 ◦C; 1 atm = 1.01 bar = 101 kPa), micro-batch reactor = 40 mL capacity.
b
Measured at 25 ◦C (1 atm = 1.01 bar = 101 kPa).
Molar yield in percentage (moles of product per mole of substrate).
c
d
Alcohol + ketone + by-product (total yield).
e
2.2% ratio, A/K obtained by unsupported V-complex catalyst (7.8 × 10−3 mmol).
f
2-Pyrazinecarboxylic acid.

G.S. Mishra et al. / Applied Catalysis A: General 384 (2010) 136–146
143
Table 3
Effect of heteroaromatic carboxylic acids as co-catalysts on the cyclohexane oxidation with molecular oxygen catalyzed by supported Oxovanadium(IV) Catalyst Ca.
Entry
Co-catalyst
Time (min)
Yield (%)
Molar ratio A/K
C6H11OH (A)
C6H10O (K)
Overall
1.
2.
3.
4.
5.
PCA
180
720
4.7
31.2
2.2
10.5
7.2
44.0
2.1
3.0
DPCA
HPCA
MePCA
APCA
PA
180
720
4.6
29.7
2.1
9.4
7.0
42.5
2.2
3.1
180
720
4.6
29.1
2.0
9.4
7.0
42.2
2.3
3.0
180
720
4.2
28.6
2.1
9.3
6.6
41.4
2.0
3.1
180
720
4.0
27.6
1.9
8.6
6.2
40.7
2.1
3.2
6.
180
720
3.7
26.5
1.8
8.4
5.7
39.0
2.0
3.1
a
Reaction conditions same as Table 1, for other conditions (see Section 2).
The effect of molecular oxygen (as oxidant) pressure, p(O2) on
chemical reaction was investigated in the range from 5.1 to 25.3 bar
(Table 1, entry 3, Table 2, entries 3 and 10 to 12, under constant
conditions: substrate = 27.8 mmol, time = 720 min, temp. = 423 K,
and catalyst = 30 mg). The increase of this pressure results in an
enhancement of the yields of cyclohexane, e.g. upon changing p(O2)
from 5.1 to 25.3 bar, the overall yield increases from 13.4 to 46.1%,
what is consistent with the promotion of the solubility of this oxi-
dant (oxygen gas) in the current system (Fig. 5). The selectivity
towards alcohol also increases with p(O2), but only until 10.1 bar,
while that for the ketone decreases with the increase of pressure
due to the promotion of the relative amount of by-products. Hence,
the use of O2 pressures above ca. 25.3 bar is not advantageous in
terms of selectivity.
11.7–44.5% increases with the amount of Catalyst C, 10–50 mg up
to a limit. Beyond 50 mg of catalyst, no appreciable further increase
in the overall yield is observed (Fig. 6). The selectivity towards the
major product, i.e. cyclohexanol, remains nearly invariant, ca. 71%,
but the relative amount of by-products increases.
Some heteroaromatic acids, such as picolinic acid (PA), 2,6-py-
razinedicarboxylic acid (DPCA), 5-hydroxy-2-pyrazinecarboxylic
acid (HPCA), 5-methyl-2-pyrazinecarboxylic acid (MePCA), 3-
aminopyrazine-2-carboxylic acid (APCA) and 2-pyrazinecarboxylic
acid (PCA) have been tested with two time intervals, i.e.
180 and 720 min, as possible co-catalysts (constant amount,
8.1 × 10−2 mmol, Table 3). The PCA was found to be the most effec-
tive one, promoting the yield of cyclohexane oxidation (overall
yield increase from 38.5 to 44.0%, entry 3, Table 2, to entry 1,
Table 3, respectively) to the corresponding alcohol and ketone.
The above results of PCA (as co-catalyst) shows, it is necessary for
this oxidation process to increase the yield of product significantly.
The addition of more PCA (up to a PCA amount: supported Cata-
The effect of supported Catalyst
C amount on oxidation
reaction was also investigated in the range from 10 to 50 mg
(Table 1, entry 3, Table 2, entries 3 and 13 to 16, under constant
conditions: substrate = 27.8 mmol, time = 720 min, temp. = 423 K
and p(O2) = 10.1 bar). We have found that the overall yield, ca.
Fig. 3. Time-dependence of the overall yield and % selectivity in the course of
Fig. 4. Temperature-dependence of the overall yield and % selectivity of the prod-
ucts in the course of cyclohexane oxidation, catalyzed by supported Catalyst C
and O2 as an oxidant (reaction conditions: substrate = 27.82 mmol, catal. = 30 mg,
t = 720 min, p(O2) = 1013 kPa).
catalytic cyclohexane oxidation catalyzed by supported Catalyst
C and O2 as
an oxidant (reaction conditions: substrate = 27.82 mmol, catal. = 30 mg, T = 323 K,
p(O2) = 1013 kPa).

144
G.S. Mishra et al. / Applied Catalysis A: General 384 (2010) 136–146
The catalyst can be reused or recycle for further oxidation
batches by simple admission of more amount of O2, even with-
out isolating or reactivating the catalyst. Nevertheless, after being
used, the catalyst can be reactivated (see Section 2), still displaying
a considerable catalytic activity. Thus, following oxidation reaction
(entry 3, Table 2) for 720 min, the catalyst was separated, reacti-
vated and recycled leading under the same condition, to achieve
overall ca. 23.8% (33.5% less from first run). The used Catalyst C
was further applied for several more oxidation runs (each time for
720 min) and found after second run the overall conversion (see
above) was not decreased significantly (less then ∼3%) from 3rd
to 6th runs. Finally, the used Catalyst C was considerably lost the
catalytic activity after 6th runs (about total 72 h duration). It is sug-
gested that metal loss should not be the main factor responsible for
the observed decrease of the catalytic activity or the deposition on
the catalyst surface of polymerized material of cyclohexane, which
can also act as pore-blocking of support material of V-catalyst,
should probably account for it.
The preservation of some catalytic activity is consistent with
the ICP analysis of the supported catalyst that shows only little
metal loss upon first time use. In fact, the freshly supported Catalyst
C has 0.23 wt.% of vanadium whereas, after the reaction (entry 3,
Table 2), a metal content of 0.21 wt.% (2nd time run) is still present
but change original green into brown colour. Probably, it was due
to contact of oxygen gas during oxidation reaction and also change
the oxidation state of vanadium-IV to V of solid catalyst (confirm
by silent line in EPR for vanadium-V state).
Fig. 5. Pressure-dependence of the overall yield and % selectivity of the products
in the course of cyclohexane oxidation catalyzed by supported Catalyst C and O2
as an oxidant (reaction conditions: substrate = 27.82 mmol, catal. = 30 mg, T = 323 K,
t = 720 min).
Molecular oxygen is a limiting reagent and the catalyst is still
active at the end of oxidation reaction. In fact, after perform-
ing the oxidation of cyclohexane under same condition, entry 3,
Table 2, the reactor was degassed, more oxygen was admitted until
10.1 bar pressure and the reaction was allowed to proceed for fur-
ther 720 min. The overall yield then increased (∼10%), relatively
to the first entry, from 38.5 to 42.1%, showing that the same reac-
tion system still presents a considerable catalytic activity after the
consumption of O2. It can be reused by the simple admission of
more oxygen, even without isolating and reactivating of supported
catalyst.
The involvement of a free-radical mechanism [32] is supported
by the inhibition of the catalytic activity when performing the
reactions in the presence of bromotrichloromethane or dipheny-
lamine, i.e. carbon- and an oxygen-radical trap, respectively. When
the reaction is carried out in the presence of the above liquid rad-
ical traps (under same condition, entry 3, Table 2), the amount of
formed product (cyclohexanol and cyclohexanone) drops consid-
erably. The uses of common solid radical traps were prevented
in the view of their insolubility in the cyclohexane. Moreover,
the addition of a free-radical initiator such as 2,2ꢀ-azo-bis(2-
methylpropionitrile) reduces the induction period of the reaction
and increases the conversion of cyclohexane products.
lyst C molar ratio of 2:1) leads to a considerable yield growth for
both cyclohexanol + cyclohexanone (overall yield reaches, ca. 49%,
Table 2, entry 17). Shul’pin and co-workers have also observed same
promoting effect for alkanes oxidation, when vanadate anions com-
bine together with PCA/H2O2. They produces free radicals HOO•
and create V(IV)-species, which are also responsible to from HO•
radicals. These radicals are playing an important role to initiate the
oxidation reactions of alkanes and cyclohexane [31]. Similar type of
promoting effect with PCA was also observed by us for the oxyfunc-
tionalization of cyclic and linear alkanes with dioxygen, catalyzed
by supported V-species and Re-species [11(a–c),20(a)].
Hence, the cyclohexane (CyH) oxidation is proposed to proceed
via the cyclohexyl radical (Cy•) obtained by reaction of CyH with
molecular O2 upon homolytic C–H bond cleavage (slow induc-
tion period). H-atom abstraction from the CyH to form the alkyl
hydroperoxide (CyOOH), whose homolytic decomposition to alky-
loxy (CyO•) (O–O bond cleavage) and CyOO• (O–H rupture) can be
catalyzed by transition nature of our catalyst, i.e. V(IV) to V(V)–OH
and V(V)–OH to V(IV) and formation of H2O. The CyOH can then
be formed by the H-abstraction from the CyH and CyO• radicals.
The decomposition of the CyOO• to yield also Cy(–H) O, which
also forms further CyOH thus in accord with the higher yield of
the cyclohexanol relatively to the cyclohexanone. This type of rad-
ical mechanism is known to occur for metal catalysts with two
metal oxidation states of comparable stability, like Co(II/III), Cu(I/II),
Mn(IV/V) and Fe(II/III) [18(a),32(a,b),33], and this condition is ful-
filled also in our case involving V(IV/V) oxidation systems. The
Fig. 6. Amount of supported Catalyst
C dependence of the overall yield and
% selectivity of the products in the course of cyclohexane oxidation with O2
as an oxidant (reaction conditions: substrate = 27.82 mmol, T = 323 K, t = 720 min,
p(O2) = 1013 kPa).

G.S. Mishra et al. / Applied Catalysis A: General 384 (2010) 136–146
145
oxidation states of fresh catalyst V(IV) eight line spectrum and used
catalyst V(V), silent spectrum were also confirmed by the EPR.
The formation of intermediate radical (CyOOH) is confirmed by
the increase of the detected amount of CyOH with concomitant
decrease of that of Cy(–H) O, if the final reaction solution, before
the GC analysis, is treated with an excess of PPh3 as indicated by
Shul’pin [34]. The hydroperoxide still present is deoxygenated by
PPh3 to the alcohol CyOH (with formation of the phosphine oxide
Ph3PO) a reaction that replaces the CyOOH decomposition to both
CyOH and Cy(–H) O in the chromatograph. However the amount
of CyOOH at the end of the reaction under our usual experimental
oxidation (entry 3, Table 2) is relatively small since the variations
in the yields of the CyOH and Cy(–H) O upon the above treatment
with PPh3 are typically not higher than 11%.
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