Single site anchored novel Cu(II) catalysts for selective liquid-gas phase O2 oxidation of n-alkanes

Article abstract of DOI:10.1016/j.molcata.2012.01.030

The pentacoordinate schiff-base trialkoxysilane Cu(II) complexes, i.e. Cu[Sal(PMeOSi)DPTA], (III-a) and Cu[Cl-Sal(PMeOSi)DPTA], (III-b) were synthesized and covalently anchored on SiO₂ and Al₂O ₃ matrixes as supported hybrid catalysts (i.e. III-a/SiO₂ as Catal.-1, III-b/SiO₂ as Catal.-2, III-a/Al₂O ₃ as Catal.-3 and III-b/Al₂O₃ as Catal.-4). The characterization of supported Cu(II) complexes were performed with SEM-EDX, TGA, ICP, FT-IR and EPR analysis. Catalytic tests were conducted in the oxidation (O₂) of n-alkanes under relatively mild conditions, in a batch rocking type reactor. Remarkable high catalytic TONs, from 1468 up to 2422, were observed. Catal.-2 provided the best overall yield, 25.2% with 92% selectivity for n-hexane and 20.1% with 75% selectivity for n-heptane. A 20% improvement in the yields was obtained with PCA as co-catalyst. The impact of both C- and O- centred radical traps were also assessed in order to establish a radical mechanism.

Full text of DOI:10.1016/j.molcata.2012.01.030

Journal of Molecular Catalysis A: Chemical 357 (2012) 125–132
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Journal of Molecular Catalysis A: Chemical
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Single site anchored novel Cu(II) catalysts for selective liquid–gas phase O2
oxidation of n-alkanes
Gopal S. Mishraa,∗, Anil Kumarb, Pedro B. Tavaresa
a Centor de Qimica (CQ-VR), Universidade de Trás-os Montes e Alto Douro (UTAD), Vila Real 5001801, Portugal
b Department of Chemical Engineering, Indian Institute of Technology Kanpur (IITK), Kanpur 208016, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The pentacoordinate schiff-base trialkoxysilane Cu(II) complexes, i.e. Cu[Sal(PMeOSi)DPTA], (III-a) and
Cu[Cl-Sal(PMeOSi)DPTA], (III-b) were synthesized and covalently anchored on SiO2 and Al2O3 matrixes
as supported hybrid catalysts (i.e. III-a/SiO2 as Catal.-1, III-b/SiO2 as Catal.-2, III-a/Al2O3 as Catal.-3 and
III-b/Al2O3 as Catal.-4). The characterization of supported Cu(II) complexes were performed with SEM-
EDX, TGA, ICP, FT-IR and EPR analysis. Catalytic tests were conducted in the oxidation (O2) of n-alkanes
under relatively mild conditions, in a batch rocking type reactor. Remarkable high catalytic TONs, from
1468 up to 2422, were observed. Catal.-2 provided the best overall yield, 25.2% with 92% selectivity for
n-hexane and 20.1% with 75% selectivity for n-heptane. A 20% improvement in the yields was obtained
with PCA as co-catalyst. The impact of both C- and O- centred radical traps were also assessed in order
to establish a radical mechanism.
Received 29 September 2011
Received in revised form 27 January 2012
Accepted 30 January 2012
Available online 7 February 2012
Keywords:
Cu catalyst
Oxidation
n-alkanes
Batch reactor
Turnover number
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
–OCH3) [15]. In particular, pentacoordinate schiff-base ligands are
known for reversible, –N3 and –O2 metal coordination [13,14]. The
The transformation of hydrocarbons into valuable oxygenated
products, e.g. alcohols, ketones and carboxylic acids, catalyzed
by supported metal catalysts, has attracted great attention. Sup-
ported metal catalysts have several typical advantages over the
homogenous ones such as easy of removal, selectivity, low toxic and
reusable [1–6]. Nevertheless, hydrocarbons (alkanes) are rather
inert and usually show typically low convertibility and selectivi-
ties with various oxidants [7,8]. Oxidation using molecular O2 is
much attractive route because of its low cost, ready availability in
air and environmentally benign nature compared to the other oxi-
dants [9]. Generally, oxidation of alkanes requires a suitable catalyst
along with high temperatures and pressures. These alkanes usually
undergo competitive reactions, such as cracking [10a], isomer-
ization [10b], dehydrogenation [10c], cyclization [10d], alkylation
[10e], metathesis [10f], oligomerization [10g] and polymerization
[10a,g].
application of these anchored ligands with Cu, as supported cata-
lyst, is still an open field of research. Only recently their catalytic
properties were tested with Vanadium catalyst [15].
Illustrative cases of n-hexane oxidation (and cyclohexane)
with air and dioxygen mixtures in the presence of metal oxides
(Ni2O3, MoO3, CuCl2, and Fe2O3) over ZSM-5 were used in a
bubble plasma reactor, leading to a diversity of hydroxylated
and carbonyl compounds [16]. Another example of partial n-
hexane oxidation with O2 was reported with Pt-Rh catalyst in
single gauze auto-thermal reactor at 200 ◦C [17], producing var-
ious oxygenated products (70% overall selectivity, including 35%
2,5-dimethyltetrahydrofuran). The Mn-exchanged zeolites were
used for n-hexane with oxygen at 130 ◦C and it produced a diverse
range of oxidation products [18]. The Zr complexes over SiO2
were also produced a broad product distribution (cyclohexanone,
methyl-2-pentane, methyl-cyclopentane, 4-methyl-1-pentane and
toluene). However, the cyclohexanone was the only oxygenated
product obtained at 160–200 ◦C [19]. In the partial O2 oxi-
dation of n-heptane with immobilized V-complexes several
oxidised products were created (heptane-2-one, heptan-4-one,
heptan-2-ol, heptan-ol, trans-3-hepten-2-one, octan-2-ol, octanoic
acid, heptaldehyde and heptanoic acid) [20]. Some molecular
sieves were used for n-heptane oxidation at 300–400 ◦C lead-
ing to the formation of mixtures of heterocyclic compounds
[21]. We have also reported 58% selectivity of hexan-2-one
as main product for n-hexane oxidation with molecular O2
In fact, several supports such as silica, alumina, MCM-41, SBA-
15 and clays, have been used for the immobilization of metal
complexes as supported catalysts [11,12]. The anchoring of the
metal complexes have been achieved mainly through the interac-
tion between the silanol groups of the supporting matrices and the
terminal groups of the metal complexes (e.g. –Cl, –NH2, –CN and
∗
Corresponding author. Fax: +351 259350480.
E-mail addresses: mishrags@utad.pt, mishrags@gmail.com (G.S. Mishra).
1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2012.01.030

126
G.S. Mishra et al. / Journal of Molecular Catalysis A: Chemical 357 (2012) 125–132
(at 160 ◦C), which was catalysed by modified SiO2/V catalysts
[22].
gets precipitated. Solid III-b (Yield: 91%) was isolated by filtra-
tion and washed with n-pentane. It is soluble in most of the polar
solvents and quite insoluble in non-polar solvents (Scheme 1:
Step. A-III). Characterization: FT-IR: 3095 cm−1 (s, C–H, ring);
2926 cm−1 and 2944 (w, C–H); 1590 cm−1 (s, C N); 1605 cm−1 (s,
The current article deals with the synthesis of schiff-
base trialkoxysilane pentacooardinate Cu(II) complexes,
Cu[Sal(PMeOSi)DPTA], (III-a) and Cu[Cl-Sal(PMeOSi)DPTA],
(III-b) and they were covalently anchored as supported hybrid
catalysts (Catal.-1 to Catal.-4). These catalysts were tested in the
O2 oxidation of n-hexane and n-heptane in order to obtain high
turnover numbers (TONs) and high selectivity of corresponding
functionalized alcohol and ketone under relatively mild conditions.
C
C); 1159 cm−1 (s, Si–O); 1110 cm−1 (s, C–N). Elemental anal-
ysis (calcd. for C26H37N3O5SiCu = 563.2 mol. wt.): C, 55.39%; H,
6.59%; N, 7.45%, Found: C, 55.12%; H, 6.11%; N, 7.52%. FAB+-
Mass: m/z = 564 [(M + H)+, 100], 442 [(M + (Si(–OCH3)3))+, 18], 399
+
[(M − {(CH2)3Si(–OCH3)3} , 14], 120 [(Si(–OCH3)3)+, 44]. EPR anal-
ysis: g = 2.0921, g = 1.9834.
||
⊥
2. Experimental
2.3. Synthesis of supported catalysts
2.1. General materials and methods
The wet-impregnation method [24] was used for the immobi-
lization of complexes III-a and III-b with silanol groups of Al2O3
or SiO2 [25]. Each of the Cu complexes, III-a and III-b (50 mg) was
dissolved in dry toluene (50 mL), in presence of K2CO3. Then Al2O3
and SiO2 (1.0 g) were separately added and the resulting mixture
heated under reflux overnight with the elimination of MeOH. After
that, the solid supported catalysts were washed several times with
toluene and dried at 40 ◦C for 12 h. Finally, the complex III-a, 44 mg
on SiO2 (Catal.-1), III-b, 43 mg on SiO2 (Catal.-2), III-a, 41 mg on
Al2O3 (Catal.-3) and III-b, 43 mg on Al2O3 (Catal.-4) were anchored
(Scheme 1: Step. B).
The reagents i.e. bis(aminopropyl)amine (DETA), potassium
carbonate,
2-pyrazinecarboxylic
acid
(PCA),
3-amino-2-
pyrazinecarboxylic acid (APCA), 2,6-pyrizinedicarboxylic acid
(DPCA), silica gel (SiO2), (all form Acros), salicylaldehyde (Sal),
3-idopropyl trimethoxysilane, picolinic acid (PA), 5-hydroxy-2-
pyrazinecarboxylic acid (HPCA), alumina (Al2O3), acetonitrile
(all from Sigma), 5-chlorosalicylaldehyde, Cu(II) acetylaceto-
nate [Cu(acac)2], tetrahydrofuran, (all from Merck) were used as
received. The solvents i.e. ethanol, n-hexane, n-heptane, n-pentane,
toluene (all from Janssen) were purified and dried by standard
methods.
The identity of the supported Cu complexes was confirmed by
analytical methods i.e. SEM-EDX, TGA, FT-IR and ICP analysis.
FT-IR spectra (4000–400 cm−1) were recorded on a Unicam
Research Series spectrophotometer in transmission mode, using
KBr pellets; wave numbers in cm−1 (vs = very strong, s = strong,
m = medium, w = weak, br = broad). Elemental analyses were car-
ried out on a Fisons EA-1108 analyzer. TGA was performed on a
TA Instruments Q50 thermogravimetric analyzer. FAB mass spectra
were obtained on a Trio-2000 Fisons spectrophotometer. Mass cal-
ibration for data system acquisition was achieved using CsI. ICP
was used for metal content detection in Perkin Elmer Plasma-400
(sample was digested in HF + HNO3). GC analysis was carried in the
FISONS GC-8000 series equipped with FID detector (DB-WAX col-
umn 60 m; internal diameter: 0.32 mm). GC-MS were performed
in the Carlo-Erba Auto/HRGC/MS spectrophotometer. Morphology of
catalysts was performed in Scanning Electron Microscopy (SEM) in
the FEI Quanta 400, equipped with an EDS detector (EDAX). EPR spec-
tra were recorded in the X band Bruker ESP 300E spectrophotometer,
at room temperature and calibrated with diphenylpicrylhydrazyl
(dpph).
2.4. Oxidation procedure and product analysis
Teflon layered rocking type SS batch reactor (52 cm3) was used
for conducting the oxidation reaction. The inner temperature of the
reactor was monitored by a thermocouple. For each experiment, the
reactor was charged with the n-alkane and the catalyst, closed, the
air was removed and O2 was introduced. At the end of the reaction,
the solid catalyst was separated from the liquid product by filtra-
tion, washed several times with acetonitrile and oven dried at 60 ◦C
for over night, for further recycling studies.
The products were quantitatively analyzed by GC (30 ␮L of
pentanone added as internal standard to 1.0 mL of the reaction
solution) and further identified by GC–MS. He gas was used as the
carrier gas. The yield was calculated as the mole of product formed
per mole of reactant feed and the product selectivity was calculated
as mole of single product per mole of total formed products. TONs
were calculated as mole of products (alcohols + ketones) per mole
of anchored Cu complex on support. For alkyl-hydroperoxide tests,
the excess of triphenylphosphine was added in the product mix-
ture (20 min. before GC analysis) and for radical traps tests, CBrCl3
and Ph2NH (8 mg) were used for added separately each reaction.
2.2. Synthesis of Cu(II) complexes and characterization
2.2.1. Synthesis of Cu[Sal(PMeOSi)DPTA], (III-a)
The trialkoxysilane pentacoordinate schiff-base ligand and
Cu(II) complex III-a were prepared according to published method
[23] (Scheme 1: Step. A). It is soluble in most of the polar solvents
and quite insoluble in non-polar solvents. Characterization: FT-IR:
3100 cm−1 (s, C–H, ring); 2938 cm−1 and 2943 (w, C–H); 1622 cm−1
(s, C N); 1567 cm−1 (s, C C); 1165 cm−1 (s, Si–O); 1092 cm−1 (s,
C–N). Elemental analysis (calcd. for C26H37N3O5SiCu): C, 55.45%;
H, 6.62%; N, 7.46%, Found: C, 55.32%; H, 6.51%; N, 7.60%. FAB+-
Mass: m/z = 565 [(M + H)+, 100], 438 [(M + (Si(–OCH3)3))+, 17], 397
3. Results and discussion
3.1. Synthesis of complexes and immobilisation
The trimethoxysilyl pentacoordinate schiff-base ligands (II-
a and II-b) have five-coordination sites [15,22]. These ligands
with donor sets of two phenolate oxygen, two schiff base nitro-
gens and another tertiary amine nitrogen have been reacted
with the terminal iodo- group of 3-iodopropyl-trimethoxysilane.
Two different aldehydes such as salicylaldehyde (Sal) and 5-
chlorosalicylaldehyde (Cl-Sal) were used for complex synthesis.
The Cu(C5H7O2)2 was added in the next reaction step to obtain
Cu(II) complexes, i.e. III-a and new III-b (Scheme 1). The proposed
structures of Cu(II) complexes were supported by FAB-mass, FT-IR,
EPR and elemental analysis.
+
[(M − {(CH2)3Si(–OCH3)3} , 13], 119 [(Si(–OCH3)3)+, 42]. EPR anal-
ysis: g = 2.1481, g = 2.0522.
||
⊥
2.2.2. Synthesis of Cu[Cl-Sal(PMeOSi)DPTA], (III-b)
The newly synthesized complex III-b was initially obtained
by the ligand II-b [15]. The Cu[(acac)2] (100 mg, 3.8 × 10−3 mol)
was added in the THF 50 mL solution of ligand II-b, (215.2 mg,
3.8 × 10−3 mol) and refluxed for 4 h. The mixture was concen-
trated and diethyl ether was added until no more green solid

G.S. Mishra et al. / Journal of Molecular Catalysis A: Chemical 357 (2012) 125–132
127
Scheme 1. Synthesis of pentacoordinate schiff-base trialkoxysilane Cu(II) complexes and covalently anchored supported hybrid Cu catalysts.
In the FT-IR spectra of all the ligands show a broad shoulder ca.
3400 cm−1 which was due to the –OH; the latter is no longer visi-
ble after the coordination with Cu species. Emergence of a band in
the ligand molecules between 1590 and 1622 cm−1, associated to
the C N, confirms the presence of schiff-base moiety, whereas the
absence of bands in the region between 1590 and 1622 cm−1, asso-
ciated with the N–H bonds, were also observed. Typical Si–O lattice
vibration bonds were observed around 1095–1160 cm−1 [26]. The
mass spectra of Cu(II) complexes were characterized by an intense
molecular ion, which was the most abundant ion in the spectrum,
as well as relatively abundant doubly charged parent molecular
ions. There were no polymeric species found which suggests that
only monomeric species of complexes were formed. The hyperfine
splits were clearly observed in EPR spectra for the paramagnetic
63Cu(I = 3/2) between g = 2.1481–2.0921 and g = 2.0522–1.9834
The FT-IR spectra of Catal.-1 to Catal.-4 show that few changes
occurred in the range, 3620–3290 cm−1 and 1550–1075 cm−1 can
be assigned due to the presence of the support (Al2O3 or SiO2). How-
ever, the main characteristic were observed for C–H bands between,
s, 2920–2946 cm−1
, C N
band between; s, 1582–1610 cm−1
and Si–O bond between: s, 1123–1155 cm−1. The catalysts EPR
results exhibited the values g = 2.1013, g = 2.0205 for Catal.-1,
||
⊥
g = 2.1001, g = 1.9943 for Catal.-2, g = 1.9721, g = 1.9930 for
||
⊥
||
⊥
Catal.-3 and g = 1.9661, g = 1.9711 for Catal.-4. The principal g
||
⊥
values were calculated by the published method [27]. These val-
ues also strongly support the metal complex anchoring on support
matrix.
Thermal analysis results show an initial weight loss, most likely
due to absorbed atmospheric moisture (Fig. 1). These complexes
have melting point, 213 ◦C for III-a and 208 ◦C for III-b. After the
anchoring, they showed improve thermal stability (ca. 245 and
275 ◦C for Catal. 1 to Catal. 4) in compare to unsupported com-
plexes. Beyond it, rapid weight losses were observed in all the
catalysts. An absolute decay was observed above 375 ◦C up to 550 ◦C
(wt. loss ca. 5.2%). Such results suggest that the supported catalysts
were stable in the range of oxidation reaction temperatures.
Morphological study of the most active Catal.-2 was shown
in two different magnifications (Fig. 2). Brighter particles on the
surface of the dark SiO2 indicate the presence of the Cu complex
(Fig. 2b). Same area was analysed in the SEM-EDS detector and
||
⊥
for III-a and III-b, thus confirming the presence of monomeric
species of Cu(II) complex [27].
Supported catalysts Catal.-1 to Catal.-4 were anchored by the
reaction of surface silanol (–OH) of supports (Al2O3 and SiO2) with
methoxysilane (OCH3–Si) of metal complexes by a condensation
process, overnight refluxing in toluene (Scheme 1: Step. B). All the
manipulations were performed under N2, since these complexes
contain sensitive methoxysilane groups. To the best of our knowl-
edge, these supported Cu catalysts are being described for the first
time.

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G.S. Mishra et al. / Journal of Molecular Catalysis A: Chemical 357 (2012) 125–132
1
0
oxidation of n-alkanes, under relatively mild conditions in batch
process without requiring any solvents and additives. First tests
were conducted with soluble derivatives i.e. III-a and III-b
(2 × 10−2 mmol, each) in homogeneous conditions. Low conver-
sions were obtained with III-a (8.3% for n-hexane and 7.1% for
n-heptane) and with III-b (10.2% for n-hexane and 8.4% for n-
heptane), under the analogous condition (Table 1). In second place
we tested the Al2O3 and SiO2 supports, and obtaining low conver-
sions too (for n-hexane ca. 4.6% by Al2O3 and 4.1% by SiO2 and for
n-heptane, ca. 4.3 by Al2O3 and 3.8% by SiO2). The blank experi-
ments (absence of catalyst) provided the lowest conversions ca. 2.2
and 1.6% for n-hexane and n-heptane, respectively.
Remarkably, very high TONs from 1468 up to 2422 with
overall conversions 12% up to 25% were obtained when sup-
ported catalysts (Catal.-1 to Catal.-4) were used (Table 1). The
most effective result was provided by Catal.-2, leading to an
overall conversion 25.2% for n-hexane and 20.1% for n-heptane.
We have obtained hexane-ol selectivity 60.3% (hexane-2-ol plus
hexane-3-ol) as main products and other products in less amounts
(24.6%, hexane-2-one and 7.9% caproic acid). In n-heptane reac-
tion, it gives good selectivity of heptane-ol 74.6% (at 2nd and
3rd positions) with heptane-2-one selectivity of 13.9% and hep-
tanoic acid of 7.0%. Minor amount (less then 1.0%) of by-products
were also detected in the GC/MS analysis (hexanal, heptan-4-one,
hexane-3-one etc.). There were no isomerization and dehydro-
cyclization product detected on both the n-alkanes. Concerning
the mass balance for n-hexane in typical conditions (Table 1,
catalyst 2, 150 ◦C, 12 atm), the n-hexane (2.051 g, 23.8 mmol)
reacted with O2 (0.754 g, 23.56 mmol) producing hexan-2-ol
(0.248 g, 2.43 mmol), hexan-3-ol (0.122 g, 1.19 mmol), hexan-2-
one (0.148 g, 1.48 mmol), caproic acid (0.055 g, 0.48 mmol). Some
by products were also produced i.e. hexanal (0.012 g, 0.12 mmol),
H2O (0.003 g, 0.19 mmol) and unidentified product (≈0.05 mmol).
The unreacted n-hexane (1.534 g, 17.80 mmol) and O2 (0.651 g,
20.34 mmol) are also present in the final mixture. In the case of
n-heptane in typical conditions (Table 1, catalyst 2, 160 ◦C, 10 atm),
the mass balance gives that the n-heptane (1.764 g, 17.6 mmol)
Catal.1
Catal.3
Catal.2
Catal.4
-1
-2
-3
-4
-5
-6
50
150
250
350
450
550
650
750
Temp., ºC
Fig. 1. Thermogravimetric analysis of supported hybrid Cu catalysts Catal.-1 to
Catal.-4.
the presence of Cu (peaks at 8.10 and 8.8 keV) was confirmed on
the surface support (Fig. 2c). The occurrence of Cu was further
confirmed by the ICP analysis (0.19 weight% of fresh Catal.-2 and
one time used catalyst 0.17 and 0.16 weight% for n-hexane and n-
heptane, respectively). It shows only a negligible amount of metal
loss for both the n-alkanes reactions (under the same condition
Table 1).
3.2. Catalytic n-alkanes O2 oxidation
We have applied all the supported catalysts (Catal.-1 to Catal.-
4) and their soluble derivatives (III-a and III-b) for selective O2
Fig. 2. Scanning electron microscopy images 2 and EDS spactra of Catal. 2.

G.S. Mishra et al. / Journal of Molecular Catalysis A: Chemical 357 (2012) 125–132
129
Table 1
Oxyfunctization of n-alkanes catalyzed by supported Schiff-base trialkoxysilane pentacoordinate Cu(II) catalysts.a
Substrate
Catalyst
TONsb
Yield (%)
Overallc
hex-2-ol
hex-3-ol
hex-2-one
hex-acid
n-hexane
Catal.-1
Catal.-2
Catal.-3
Catal.-4
2203
2422
1882
1931
23.3
25.2
17.7
20.6
8.6
10.2
6.8
5.1
5.0
4.0
4.2
6.3
6.2
3.9
5.3
1.6
2.0
1.0
1.2
7.9
Overallc
hept-2-ol
hept-3-ol
hept-2-one
hept-acid
n-heptane
Catal.-1
Catal.-2
Catal.-3
Catal.-4
1753
1968
1468
1597
18.2
20.1
12.0
14.5
10.1
11.0
6.2
3.3
4.0
2.3
2.4
2.3
2.8
2.1
2.0
1.0
1.4
0.6
0.8
8.1
a
Reaction conditions: supported catalyst = 25 mg, n-hexane = 23.8 mmol; 150 ◦C; 12 atm for 8 h and n-heptane = 17.6 mmol; 160 ◦C; 10 atm for 8 h (measured at RT;
1 atm = 1.01 bar = 101 kPa).
b
TONs = alcohols + ketones.
Overall yield = main products + by products.
c
40
30
20
10
n-heptane, other products yields were obtained like heptane-3-ol;
3.6% (11% selectivity), heptane-2-one; 5.8% (18% selectivity) and
hexanoic acid; 3.2% (10.1% selectivity) (Fig. 3b). However, the reac-
tion temperature was not allowed to go beyond this temperature,
since an unidentified black charring material appears under deep
oxidation. Moreover, the best selectivity results towards the main
products (hexane-2-ol; 40.5%, hexane-3-ol; 20.2%, hexane-2-one;
24.6%) were obtained at 150 ◦C. At the same moment, the hex-
anoic acid was observed in a small amount (8.0%). On the other
hand, a best selectivity result of n-heptane was obtained at 160 ◦C,
i.e. heptan-3-ol; 20%, heptan-3-ol; 54.7%, heptan-2-one; 15.0%).
Heptanoic acid; 7.0% was also observed in as small amount. Tem-
perature study shows that the higher temperatures lead to higher
n-alkanes conversions but lower selectivities to main products,
whereas lower temperatures correspond to lower conversions with
good selectivities.
(a)
(b)
overall
hex2ol
hex3ol
hex2one
hexacid
0
Overall
hept3ol
hept20l
hept2one
heptacid
30
20
10
0
100
120
140
160
180
200
3.2.2. Effect of O2 pressure
Temperature, °C
The study of the effect of molecular O2 pressure on n-alkanes
oxidation was performed between 6 and 20 atm, under constant
conditions: catalyst amount of 25 mg; reaction time of 8 h; tem-
perature of 150 ◦C for n-hexane and 160 ◦C for n-heptane (Fig. 4a
and b). The maximum overall yields values 41.6% and 33.6% were
reached at the highest p(O2) pressure (20 atm), for n-hexane and n-
heptane, respectively. At same point, highest n-hexane products
Fig. 3. Effect of temperature on percentage yield in the course of n-alkanes oxidation
catalyzed by Catal.-2.
reacted with O2 (0.628 g, 19.63 mmol) and produced heptan-2-ol
(0.225 g, 1.94 mmol), heptan-3-ol (0.082 g, 0.70 mmol), heptane-2-
one (0.056 g, 0.49 mmol), heptanoic acid (0.032 g, 0.25 mmol). The
by products were heptan-4-ol (0.012 g, 0.11 mmol), H2O (0.001 g,
0.05 mmol) and unidentified product (≈0.04 mmol). The unre-
acted n-heptane (1.406 g, 14.03 mmol) and unreacted O2 (0.568 g,
17.74 mmol) are also present in the final mixture. Various effecting
factors such as temperature, O2 pressure, time and catalyst amount
affect the oxidation reaction and were investigated towards the
optimization conditions for best activity and selectivity, with most
active Catal.-2. The details of these reactions, including recycle
tests and effect of co-catalysts were summarized in Figs. 3–7,
Tables 1 and 2.
overall
hex2ol
hex3ol
hex2one
hexacid
40
(a)
30
20
10
0
Overall
40
30
20
10
0
(b)
hept3ol
hept20l
hept2one
heptacid
3.2.1. Effect of temperature
The temperature effect was studied between 90 and 200 ◦C for
n-alkanes oxidation under constant conditions: catalyst amount
of 25 mg; reaction time of 8 h; p(O2) = 10 atm for n-hexane and
p(O2) = 12 atm for n-heptane (Fig. 3a and b). An increase in the reac-
tion temperature leads to an enhancement of the overall yields,
which reaches 38.2% yield for n-hexane (main product yield, 13.8%
hexane-2-ol) at 190 ◦C and 32.4% for n-heptane (main product yield,
14.6% heptan-3-ol) at 200 ◦C. The other n-hexane products yields
were obtained in lesser amount i.e. hexane-3-ol; 6.88%, hexane-
2-one; 6.99% and hexanoic acid; 4.0% (Fig. 3a). In the case of
4
8
12
16
20
O2 Pressure, atm.
Fig. 4. Effect of O2 pressure on percentage yield in the course of n-alkanes oxidation
catalyzed by Catal.-2.

130
G.S. Mishra et al. / Journal of Molecular Catalysis A: Chemical 357 (2012) 125–132
50
40
30
20
10
yield were obtained i.e. hexane-2-ol; 15.8%, hexane-3-ol; 7.49%,
overall
hex2ol
hex3ol
hex2one
hexacid
(a)
hexane-2-one; 7.07% and hexanoic acid; 3.7% (Fig. 4a). However,
the best selectivity values for main products of n-hexane were
obtained at 10 atm (50.1% hexan-2-ol and 25.5% hexane-2-one).
For n-heptane (Fig. 4a), the best selectivities result was achieved
at 12 atm (57% heptan-2-ol, 20% heptan-3-ol and 14.1% heptan-3-
one). Above these pressures (10 atm for n-hexane and 12 atm for
n-heptane), the yields of major products were increased but the
selectivities decreased. Hence, the O2 pressure is an essential factor
to maintain a high conversion and best selectivity.
0
Overall
hept3ol
hept20l
hept2one
heptacid
(b)
40
30
20
10
0
3.2.3. Effect of time
The reaction time was studied from 4 to 24 h for both n-alkanes
oxidation, under constant condition: catalyst; 25 mg, p(O2); 10 atm,
temperature;150 ◦C for n-hexane and p(O2) 12 atm, 160 ◦C for n-
heptane (Fig. 5a and b). The overall yield increases with increase
of reaction time in both n-alkanes reaction. The maximum yields
were reaches for n-hexane, 16 h (overall 34.6%) and for n-heptane
24 h (44.2%); beyond these times, the overall yields were signifi-
cantly reduces due to the formation of black charring material. At
maximum time, the main n-alkanes products yields (7.8% hexan-
2-ol, 7.6% hexan-3-ol, 6.7% hexan-2-one, 16.8% heptan-2-ol, 6.0%
heptan-3-ol and 4.4% heptane-2-one) were achieved. However, the
best product selectivities results were observed at 8 h reaction time
for both n-alkanes. Therefore, time is an essential factor to maintain
the best conversion and products selectivities.
2
4
6
8
10 12 14 16 18 20 22 24 26
Time, h
Fig. 5. Effect of reaction time on percentage yield in the course of n-alkanes oxida-
tion catalyzed by Catal.-2.
overall
hex2ol
hex3ol
hex2one
hexacid
(a)
40
30
20
10
0
3.2.4. Effect of catalysts amount
The amount of catalyst was studied for n-alkanes oxidation
under constant condition: p(O2) 10 atm, 8 h, 150 ◦C for n-hexane
and p(O2) 12 atm, 8 h, 160 ◦C for n-heptane. The increasing amount
Catal.-2 from 5 to 45 mg, leads to a growth of overall yields from 5.2
to 34.2% for n-hexane (Fig. 6a) and from 8.4 to 36.8% for n-heptane.
The yields of main n-hexane products with 45 mg catalyst (i.e. 14.3%
hexan-2-ol, 8.7% hexan-3-ol and 8.4% hexane-2-one) were much
higher than that obtained with 5 mg catalyst (i.e. 2.1% hexan-2-
ol, 1.4% hexan-3-ol and 1.1% hexane-2-one). Same enhance effect
was observed in n-heptane case (Fig. 6b). The best selectivity was
obtained with 25 mg of catalyst amount in both the n-alkanes reac-
tions. Therefore, catalyst amount is also an essential factor for
maintaining the high yield and products selectivities.
Overall
hept3ol
hept20l
hept2one
heptacid
(b)
40
30
20
10
0
0
10
20
30
40
50
Catalyst amount, mg.
Fig. 6. Effect of Catal.-2 amount on percentage yield in the course of n-hexane
oxidation by O2.
3.2.5. Effect of co-catalysts
A significantly improved yield for n-hexane oxidation were
obtained with some heteroaromatic carboxylic acids (as possi-
ble co-catalysts) [22] i.e. 25.7% PA, 27.2% APCA, 28% DPCA, 28.8%
HPCA and 30.6% PCA. For n-heptane, same improvement effect was
observed with these carboxylic acids, i.e. 20.6% PA, 21.0% APCA,
21.6% DPCA, 22.3% HPCA and 23.8% PCA (Table 2). Highest enhance-
ment effect was exhibited with PCA for n-hexane (30.6%) and
for n-heptane (23.8%) under the same conditions (Table 1). Same
enhancement effect with PCA was observed previously, when the
vanadate anions combine with PCA-H2O2 [28–30] and with O2 [31].
3.2.6. Effect of catalyst recycles
The most effective Catal.-2 shows that the reaction system still
presents a significant catalytic activity after recycling. Hence, it
could be reused for further oxidation cycles, after some reactiva-
tion. The overall yields of n-alkanes then decried after 2nd cycle,
relatively to the first runs, from 25.2 to 22.2% for n-hexane and from
20.1 to 16.8% for n-heptane, showing that the reaction system still
presents a significant catalytic activity. Thus, the catalytic experi-
ments were extended further more experiments up to 7th cycles
for 56 h. Until 5th cycle, the catalyst showed some catalytic activity
(14.8% for n-hexane and 12.1% for n-heptane), however after 6th
cycle the catalytic power was completely ceased (Fig. 7).
Fig. 7. Catal.-2 recycle tests for n-alkanes oxidation under analogous conditions as
Table 1.

G.S. Mishra et al. / Journal of Molecular Catalysis A: Chemical 357 (2012) 125–132
131
Table 2
Co-catalysts effects on oxyfunctization of n-alkanes catalyzed by Catal.-2.a
Co-catalyst
Yield (%)
n-hexane
Overall
n-heptane
hex-olb
hex-one
hex-acid
Overall
hept-olb
hept-one
hept-acid
PCA
30.6
28.8
28.0
27.2
25.7
18.4
17.5
17.1
16.6
15.5
7.7
7.2
7.0
6.7
6.3
2.9
2.7
2.5
2.4
2.2
23.8
22.3
21.6
21.0
20.6
17.8
16.4
16.0
15.7
15.5
3.9
3.5
3.3
3.0
2.9
1.9
1.6
1.6
1.5
1.4
HPCA
DPCA
APCA
PA
a
Experimental conditions as those indicated in Table 1.
hex-ol = hex-2-ol + hex-3-ol, hept-ol = hept-2-ol + hept-3-ol.
b
of ROO• form alcohol (ROH) plus can also be the precursor for the
formation of ketone (R(–H) O) and 1/2 O2 (Scheme 2(v)).
Hexan-2-ol, hexan-3-ol, heptan-2-ol and heptan-3-ol could be
formed either by H-abstraction from the n-alkanes (RH) by RO• or
by decomposition of ROO• to yield hexan-2-one and heptan-2-one.
The ROOH involvement is evident by the promotion of the detected
amount of ROH, with a decrease of that of the R(–H) O. When
the final reaction mixture is treated with excess of PPh3 (20 min.
before by the GC analysis) which reduces ROOH to ROH, as a result
eliminating the ROOH [36].
R'
(i)
ROOH
RO
ROO
- R
Cu(II)
(ii)
Cu(III)-OH
Cu(II)
ROOH
ROOH
RO
Cu(III)-OH
-H2O
(iii)
(iv)
ROO
ROH
RH
- R
-1/2 O2
4. Conclusions
(v)
1/2 R'=O
1/2 ROH
ROO
The main idea was to use potentially active Cu complexes as
supported and recyclable catalysts for n-alkanes oxidation under
relatively mild conditions. Therefore, –N3 and –O2 coordinated tri-
alkoxysilane Cu(II) complexes (i.e. III-a and III-b) were synthesized
and well characterized. These novel complexes were covalently
anchored into Al2O3 and SiO2 surface as supported catalysts. Sup-
ported catalysts (Catal.-1 to Catal.-4) and their soluble derivatives
(III-a, III-b) were tested for n-alkanes selective oxidation with O2
(chipper and “green” oxidant) in batch reactor.
Higher TONs from 1468 to 2422 were achieved with the sup-
ported catalysts in comparison with the soluble derivatives. Among
them, Catal.-2 was the best catalyst with an overall yield 25.2% and
TON 2422 for n-hexane and overall yield 20.1% and TON 1968 for
n-heptane. Best selectivity for main products of n-hexane (50.1%
hexan-2-ol and 25.5% hexane-2-one) was obtained under opti-
mized condition (150 ◦C, 10 atm, for 8 h). For n-heptane, the best
selectivity (57% heptan-2-ol, 20% heptan-3-ol and 14.1% heptan-
3-one) were achieved at 160 ◦C, 12 atm, for 8 h. A 20% increase in
the total yields was achieved with PCA as co-catalyst. The radical
traps and PPh3 experiments suggested that these oxidation reac-
tions proceed via free radicals. Recycle studied showed that these
catalysts could be used at least for 40 h reaction time.
Where, R= C6H11 or C6H13, R'= C6H12 or C7H14
Scheme 2. Proposed metal catalyzed reaction mechanism for n-alkanes oxidation.
We believe that metal loss was not the sole responsible factor for
decrease of the catalytic activity but some other factors were also
responsible as well. A possible structural change of the catalyst,
as suggested by the observed change of colour upon use, and the
black charring material deposition on the catalyst surface, which
may also act as pore-blocking of catalyst, should probably account
for it.
3.3. Mechanistic considerations
Generally, three types of mechanisms for the metal catalyzed
alkanes oxidation have been known, viz. oxidation via a free-radical
process, oxidation through substrate coordination and catalytic
oxygen transfer [32–35]. These are mainly depends upon the reac-
tion conditions and available metal oxidation states, e.g. Co(II/III),
Cu(II/III), V(IV/V), Mn(II/III), Fe(II/III). In the current system, it was
conformed by the EPR analysis and observed some splitting for
fresh and no splitting for used catalyst (see Section 3.1). Two liq-
uid radical traps (i.e. CBrCl3 and Ph2NH) were tested for both the
n-alkanes and we obtained relatively low conversions. These lower
conversions indicate that the radicals are presence in the reaction
mixture.
Even though, the mechanistic facts are still unclear, one can con-
sider that the reactions proceed via formation of alkyl radicals (R•),
initially obtained by reaction of the n-alkanes (RH) with molecu-
lar O2 upon homolytic C–H bond cleavage. The R• radicals again
reacts with O2 forming the alkylperoxy radicals (ROO•), which is
further reacting with R• forming and formed alkyl hydroperoxide
(ROOH), Scheme 2(i). H-abstraction from the RH would produce
the ROOH (Scheme 2(i)). Subsequently homolytic decomposition
of ROO• and alkyloxy (RO•) (O–O bond cleavage) and alkyl peroxy
(ROO•) (O–H bond cleavage) can catalyzed by supported Cu cata-
lyst (Scheme 2(ii) and (iii)). The alcohol (ROH) could be derived by
H-abstraction form n-alkane by RO• (Scheme 2(iv)). Decomposition
Acknowledgements
The authors express gratitude to Foundation for Science and
Technology (FCT), Portugal for the financial support under the Ciên-
cia 2007 program, Research projects PTDC/EQE-ERQ/110825/2009
and PEst-C/QUI/U10616/2011. We also thank for the analytical
analysis to Centro de Apoio Científico e Tecnolóxíco á Investi-
gación, Universidade de Vigo, Spain and Unidade de Microscopia
Electrónica at UTAD.
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