Highly efficient transformation of alcohol to carbonyl compounds under a hybrid bifunctional catalyst originated from metalloporphyrins and hydrotalcite

Article abstract of DOI:10.1016/j.jcat.2015.11.017

The development of a highly active and selective catalytic system that is economical, environmentally benign, and easily recoverable is highly desirable. Bifunctional hybrid catalysts originated from metalloporphyrins (MTSPP; M = Co, Fe, and Mn), and hydrotalcite have been synthesized, characterized, and investigated in the aerobic oxidation of alcohols in the presence of isobutyraldehyde. The designed catalysts exhibited excellent activity, broad applicable scope, and good stability in the oxidation. The effect of surface basicity on the catalytic performance has been studied in detail. The research results showed that as well as protecting the metalloporphyrin molecule, the surface basicity of hydrotalcite also contributed to improving the catalytic activity and the selectivity of aldehyde, and a synergistic effect was observed in the catalytic system. A proposed mechanism for the reaction involving the formation of high-valence cobalt-oxo porphyrin intermediate was postulated based on catalytic results and Hammett and H₂¹⁸O experiments.

Full text of DOI:10.1016/j.jcat.2015.11.017

Journal of Catalysis 335 (2016) 105–116
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Highly efficient transformation of alcohol to carbonyl compounds
under a hybrid bifunctional catalyst originated from metalloporphyrins
and hydrotalcite
⇑
Wei-You Zhou, Peng Tian, Fu’an Sun, Ming-Yang He , Qun Chen
Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, Changzhou University, 213164 Changzhou, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The development of a highly active and selective catalytic system that is economical, environmentally
benign, and easily recoverable is highly desirable. Bifunctional hybrid catalysts originated from metallo-
porphyrins (MTSPP; M = Co, Fe, and Mn), and hydrotalcite have been synthesized, characterized, and
investigated in the aerobic oxidation of alcohols in the presence of isobutyraldehyde. The designed cat-
alysts exhibited excellent activity, broad applicable scope, and good stability in the oxidation. The effect
of surface basicity on the catalytic performance has been studied in detail. The research results showed
that as well as protecting the metalloporphyrin molecule, the surface basicity of hydrotalcite also con-
tributed to improving the catalytic activity and the selectivity of aldehyde, and a synergistic effect was
observed in the catalytic system. A proposed mechanism for the reaction involving the formation of
high-valence cobalt–oxo porphyrin intermediate was postulated based on catalytic results and
Hammett and H128O experiments.
Received 4 October 2015
Revised 24 November 2015
Accepted 25 November 2015
Keywords:
Bifunctional catalyst
Alcohol oxidation
Metalloporphyrin
Hydrotalcite
Synergistic effect
Ó 2015 Elsevier Inc. All rights reserved.
1. Introduction
always exhibited the most efficient performance. However, a large
excess of homogeneous base additives has always been employed
Selective oxidation of alcohols to carbonyl compounds, espe-
cially from primary alcohols to aldehydes using molecular oxygen,
is one of the most challenging transformations in the synthesis of
fine chemicals, for the low cost and environmentally friendly nat-
ure of oxygen compared with the stoichiometric oxidant [1,2].
However, it is difficult to control this reaction because of overoxi-
dation or side reactions under conventional severe reaction condi-
tions, such as a high pressure of oxygen or a high reaction
temperature. Thus, it is desired to search for milder reaction condi-
tions to suppress these reactions in order to develop an efficient
oxidation method.
in the reactions. The base in alcohol oxidation has been verified to
be beneficial to the catalytic transformation and the selectivity of
aldehyde [10,24–28]. Therefore, to avoid the use of a soluble base,
some common solid bases, such as MgO, Mg(OH)2, and hydrotal-
cite, have been widely applied as supports, such as Pd(II) acet-
ate–pyridine complex/hydrotalcite [29], Ru/hydrotalcites [30],
PtxAuy-starch/hydrotalcites [31], AuAPd nanoalloys supported on
MgAl mixed metal oxides [32], and Au nanoparticles supported
on NiAl layered double hydroxides (LDHs) [33]. It has been demon-
strated that the Au/LDH catalysts can be more active in the absence
of base additives than other commonly used gold catalysts such as
Au/TiO2, Au/CeO2, and Au/Al2O3, suggesting synergy between
AuNPs and basic sites of the LDHs [34,35]. However, from the
viewpoint of economy and sustainable chemistry, it is especially
important to develop economical catalysts for selective oxidation,
because noble metal catalysts are expensive and scarce. In addi-
tion, most of these catalysts are only applicable to a very limited
range of substrates under base-free conditions. For example, Wang
et al. [36] have reported Au nanoparticles supported on LDH cata-
lysts, but the conversion of aliphatic alcohols and the selectivity of
the corresponding carbonyl compounds were low.
The utilization of efficient catalysts (heterogeneous or homoge-
neous) may provide
a highly desirable approach. Obviously,
heterogeneous catalysts in the liquid phase will offer several
advantages over homogeneous ones, such as ease of recovery and
recycling, atom utility, and enhanced stability in the oxidation
reactions. For the purpose, a range of heterogeneous catalysts
based on Cu [3–6], Ni [7], Co [8,9], Mn [10], active carbon [11],
polymers [12], noble metals [13–23], etc. have been extensively
utilized in alcohol oxidation. Among these, the noble metals
Metalloporphyrins, as model catalysts of cytochrome P-450,
have been widely used as catalysts for the oxidation of various
⇑
Corresponding author.
E-mail addresses: hemy_cczu@126.com, hmy@cczu.edu.cn (M.-Y. He).
http://dx.doi.org/10.1016/j.jcat.2015.11.017
0021-9517/Ó 2015 Elsevier Inc. All rights reserved.

106
W.-Y. Zhou et al. / Journal of Catalysis 335 (2016) 105–116
substrates, including alcohols [37–40]. For example, Woo and co-
authors reported the aerobic homogeneous oxidation of benzyl
alcohol with oxotitanium porphyrin (TTP)Ti@O, which gave ben-
zaldehyde in a modest yield (48%) after 94 h [41]. Kato et al.
reported heterogeneous aerobic oxidation of benzyl alcohol using
[RuI2I;III (H2TCPP)]BF4, in which high selectivity of benzaldehyde
(95%) was obtained after 24 h with a low turnover number (TON)
[42]. Some other metalloporphyrins have also been used as cata-
lysts in alcohol oxidation by molecular oxygen in the presence of
isobutyraldehyde (IBA), and Ru(TPP)Cl showed excellent activity
and selectivity for oxidation of various alcohols only when ben-
zotrifluoride was used as the solvent [43]. A similar example was
found in a reaction catalyzed by Ru(TPFPP)(CO), where tetrabuty-
lammonium hydroxide and the solvent bromotrichloromethane
were needed [44]. In addition, the method was not efficient for
alcohol substrates containing heteroatoms and some aliphatic
alcohols containing phenyl. The same drawback was also found
in the metallodeuteroporphyrins-catalyzed system reported by
Sun et al. [45]. Liu and co-workers have reported that (TSPP)Rh
(III) could catalyze the selective oxidation of various alcohols with
a buffer solution, but the TOF of the reaction was only 80 hÀ1 in the
case of benzyl alcohol [40].
selectivity of objective products. With this knowledge and back-
ground, in the present study, tetra(4-sulfonatophenyl)porphi
nato-metal anions (MTSPP, M = Co, Fe, and Mn) intercalated into
ZnAl hydrotalcites have been prepared as heterogeneous bifunc-
tional catalysts for the aerobic oxidation of alcohol in the presence
of IBA. The structures of these hybrids have been characterized,
and the substrate scope has been studied. Also, the synergistic
effect between metalloporphyrin molecules and hydrotalcite and
the reaction mechanism have been discussed.
2. Experimental
2.1. Preparation of MTSPP-ZnxAl-LDHs
A mixture of Zn(NO3)2Á6H2O (1.67 g, 0.0056–0.014 mol) and Al
(NO3)3Á9H2O (1.05 g, 0.0028 mol) was dissolved in 50 mL of deion-
ized water. The aqueous solution was slowly added with mechan-
ical stirring to 10 mL of distillated water at 25 °C with sodium
MTSPP (M = Co, Fe, Mn, 1.54 Â 10À3 mol), synthesized according
to Refs. [56,57]; pH 8.5 was maintained by the continuous addition
of 0.1 mol/L NaOH. The resulting suspension was stirred for 24 h at
80 °C. The product was filtered, washed with distilled water at
80 °C, and finally dried at room temperature.
In addition, these noble metal catalysts were costly, seriously
minimizing their potential for commercial applications. Although
some non-noble metals, such as Co, Mn, Fe, and Cu, have been
introduced into the metalloporphyrin-catalyzed system [43,46],
the catalytic activities and selectivities of benzaldehyde were
rather low or the scope was limited. Another common drawback
for these metalloporphyrin catalytic systems for the oxidation of
alcohol is that they were all homogenous and the active sites were
2.2. Characterization of catalysts
Powder X-ray diffraction (XRD) patterns of the as-synthesized
samples were obtained using a Rigaku D/max 2500 PC X-ray
diffractometer with Cu Ka
(1.5402 Å) radiation at 10 minÀ1. Diffuse
reflectance ultraviolet and visible spectra (DR UV–vis) were
recorded in a Perkin-Elmer Lambda 35 spectrophotometer, using
BaSO4 as a reference. For the scanning electron microscopy (SEM)
of the samples, a JEOL JSM-6360LA scanning electron microscope
was used. The compositions of samples were analyzed by induc-
tively coupled plasma analysis (ICP) using a Varian Vista-AX
device. The basic properties were determined by titration with
0.01 M benzoic acid solution in toluene using 0.15 g of vacuum-
dried solid sample suspended in 2 mL of phenolphthalein indicator
solution [58].
always deactivated by aggregation of porphyrin rings by p–p inter-
action as the reaction proceeded. Therefore, the porphyrin mole-
cule should be supported to be a heterogeneous catalyst to
prolong its lifetime and avoid some inherent drawbacks in homo-
geneous reaction systems.
LDHs having a hydrotalcite-like structure are composed of pos-
itively charged trioctahedral hydroxide layers with interlayer
anions, represented by the general formula [M(II)(1Àx)M(III)x
(OH)2]x+[AnÀ
]
x/nÁmH2O [47,48]. The particular property of LDHs that
their properties can be fine-tuned because of the adjustability of
the cations and anions in the brucite layer and interlayer [49,50]
should provide a new method of designing multifunctional cata-
lysts possessing both basicity and redox functionality. It has been
demonstrated that LDH meets the requirement for an inorganic
support for immobilizing anionic metal complexes and may pro-
vide an alternative to homogeneous metal complexes [51,52].
On the basis of this analysis, we envisioned that if the metallo-
porphyrin molecule with substituted anionic groups was interca-
lated into the interlayers of LDHs, a multifunctional catalyst
should be produced. The intercalated material will possess some
positive functions, including (I) a high capacity for catalytic oxida-
tion of alcohol based on metalloporphyrin, (II) basic sites on the
surface that may act as a co-catalyst in alcohol oxidation, and
(III) conversion of the metalloporphyrin to the heterogeneous cat-
alyst, which can increase the durability of the complex in reactions.
The new material would afford us an efficient, heterogeneous,
inexpensive, and recyclable catalyst for the selective oxidation of
alcohols.
2.3. Catalytic oxidation of alcohol
Liquid-phase catalytic oxidation of benzyl alcohol was carried
out in a 25 mL two-neck flask with reflux condenser and magnet-
ically stirred autoclave heated in an oil bath under atmospheric
pressure. Dioxygen was bubbled (10 mL/min) through a solution
of acetonitrile (ACN, 8 mL), benzyl alcohol (108 mg, 1 mmol), IBA
(216 mg, 3 mmol), and catalyst (1.9 Â 10À6 mmol, calculated on
the intercalated metlloporphyrin) at 60 °C. The product samples
were drawn at regular time intervals and analyzed with a gas chro-
matograph (Shimadzu GC-2010AF) having a Chromopak capillary
column and FID detector. The products were further confirmed
using GC–MS (Shimadzu GCMS-2010). The conversions of the reac-
tions were calculated with no allowance for the background. After
the reaction, the resulting mixture was cooled and the catalyst was
separated by centrifugation and washed with solvent. After drying
in vacuum, the recycled catalyst can be reused in the next run
under the same conditions. The conversion, yield of benzaldehyde,
and selectivity presented here are based on the GC calculations
using chlorobenzene as the internal standard reference compound.
Our group has been investigating metalloporphyrins as a biomi-
metic catalyst, and modified hydrotalcites and intercalated materi-
als as efficient catalysts in the selective oxidation of many
substrates [38,53–55]. The design and develop of multifunctional
and economical catalysts for oxidation are significantly important
in the industrial and academic fields. Therefore, our ongoing inter-
est has been drawn to synergistic catalysis between the metallo-
porphyrin catalyst and a second functional site for the high
2.4. 18O-labeled H128O experiments
Through
a mixture of benzyl alcohol (11 mg, 0.1 mmol),
CoTSPP-Zn2Al-LDH (1.0 Â 10À6 mmol), IBA (21 mg, 0.3 mmol),

W.-Y. Zhou et al. / Journal of Catalysis 335 (2016) 105–116
107
by Demel [59] and Barbosa [60]. The high affinity of anionic por-
phyrins for the hydroxide layers is evidenced by the fact that all
diffractions can be assigned to pure phases of the hybrid with no
impurities [58], showing that coprecipitation may be better than
the exfoliation/restacking approach [61].
The basal spacing distances of LDHs are actually dependent on
the size of anions and their orientation between the layers, and
the value can be calculated from the reflection (003). Considering
the same crystalline system used to assign the LDH peaks, one
could calculate the prepared hybrid LDHs’ interlayer distance as
shown in Table 1. It is obvious that the distances are much greater
than that obtained for the general inorganic anions, such as CO32–
with a value of 7.58 Å [62], indicating that the interlayer distance
of intercalated material undergoes a large increase following the
intercalation process. The replacement of CO23– by metallopor-
phyrins also resulted in a decrease of the Zn/Al ratio as to the the-
oretical value (examined by ICP, Table 1). For different
metalloporphyrins, the interlayer distance varied from 22.63 to
23.60 Å, which are absolutely ascribed to the complex metals,
which might slightly distort the metalloporphyrin structures.
The interlayer space available for intercalated porphyrin anions
can be estimated similarly to hybrid LDH by subtracting the thick-
ness of the hydroxide layer (i.e., 4.80 Å) from the basal spacing
[63]. The obtained gallery height of these hybrids are between
17.83 and 18.80 Å, which is in excellent agreement with previous
reported values in the literature [64,65], and is comparable to
the dimensions of the metalloporphyrin molecular plane (ca.
18 Â 18 Å) [66], indicating that MTSPP (M = Co, Fe, and Mn) anions
have been successfully intercalated into LDHs and the orientation
of porphyrins in the interlamellar space of LDH is nearly perpen-
dicular. The SEM images in Fig. 2 shows that all of the MTSPP-
Zn2Al-LDH formed platelike agglomerated crystals, representing
the character of layered materials [67,68].
It is possible to demonstrate the presence of the immobilized
MTSPP in the MTSPP-Zn2Al-LDH (M = Fe, Co, and Mn) hybrids by
solid DR UV–vis spectroscopy. The spectra of all the pure dissolved
MTSPP and solid MTSPP-Zn2Al-LDH display the characteristic Soret
band and Q bands (Fig. 3). Intercalation caused appreciable blue
shift of the Soret band for all the samples, possibly indicating that
demetallation occurred to some extent [69,70]. The results could
also explain the different content of metals in the prepared
hybrids, so the amounts of catalysts were all calculated based on
the active metals in the catalytic reaction. The UV–vis spectrum
of CO23–-Zn2Al-LDH is also presented, where MTSPP absorbance
cannot be detected, thereby confirming that the bands observed
in the UV–vis spectra of the MTSPP-Zn2Al-LDH are really due to
the presence of the metalloporphyrins.
Fig. 1. XRD patterns of MTSPP-Zn2Al-LDHs (M = Co, Fe and Mn) and CO23–-Zn2Al-
LDH.
and H128O [90 mg (5 mmol) and 180 mg (10 mmol), 97% 18O
enriched, Aladdin Chemical Co.) in a dried solvent mixture (4 mL)
of CH3CN was bubbled dioxygen (10 mL/min). The reaction mix-
ture was stirred for 60 min at 60 °C and then directly analyzed
by GC–MS. The 16O and 18O composition in benzaldehyde were
determined by the relative abundance of mass peaks at m/z = 106
for 16O and m/z = 108 for 18O.
2.5. Competitive reactions of benzyl alcohol and para-substituted
benzyl alcohols for the Hammett plot
The reaction was carried out in the presence of an excess of sub-
strate. Through a mixture of benzyl alcohol (5 mmol) and para-(X)-
substituted benzyl alcohol (5 mmol, X = AOCH3, ACH3, ACl, and
ANO2), IBA (3 mmol), chlorobenzene (10 mmol, as the internal
standard reference compound), CoTSPP-Zn2Al-LDH (1.9 Â 10À6 mmol),
and solvent (ACN 8 mL) was bubbled dioxygen (10 mL/min). The
mixture was stirred at 30 °C and samples were drawn at regular
time intervals and analyzed by GC. The relative reactivities
were determined using the following equation: kx/kH = CxXi/CHHi,
where Cx and Xi are the conversion and the initial concentration
of benzyl alcohol, CH and Hi are the conversions and the initial
concentrations of substituted benzyl alcohols.
3. Results and discussion
FTIR spectroscopy is very helpful in studying the LDHs contain-
ing interlayer organic anions, as it is very sensitive to the symme-
try of the organic anion and to the interactions of the anion with its
environment, such as hydrogen bonding and coordinate bonding.
When compared with the results of CoTSPP-Zn2Al-LDH reported
in our previous paper [38], the FTIR spectra (Figs. S1 and S2) fur-
ther demonstrate the presence of immobilized MTSPP in the inter-
layer of hydrotalcites. Thermogravimetric analysis (Fig. S3) also
indicated the successful coassembly of LDH sheets with metallo-
porphyrin molecules.
3.1. Characterization of MTSPP-Zn2Al-LDHs
The XRD patterns of the hydrotalcite-like samples containing
different MTSPP (M = Co, Fe, and Mn) intercalated in the layers
are shown in Fig. 1. Compared with that of CO23–-Zn2Al-LDH, new
peaks are observed in the 2h range 2.5–25°. All the samples exhibit
intense common features with reflections located at the angles
typical of a hydrotalcite-like phase: sharp (003), (006), (009),
(0012), (0015), (0018), and broad and asymmetrical for (0021)
and (110), respectively, which are consistent with those reported
Table 1
Compositions and spacing distances of the prepared LDH samples.
Samples
Content of MTSPP (mmol/g)
Content of Zn (wt.%)
Content of Al (wt.%)
Mol ratio of Zn/Al
Spacing distance (Å)
CO23À-Zn2Al-LDH
CoTSPP-Zn2Al-LDH
FeTSPP-Zn2Al-LDH
MnTSPP-Zn2Al-LDH
–
42.3
20.4
21.7
18.9
19.2
4.8
5.2
2.2
1.8
1.7
1.7
7.58
22.63
23.60
22.98
0.38
0.09
0.48
4.6

108
W.-Y. Zhou et al. / Journal of Catalysis 335 (2016) 105–116
Fig. 2. The SEM images of MTSPP-Zn2Al-LDHs: (A) CO23–-Zn2Al-LDH, (B) CoTSPP-Zn2Al-LDH, (C) FeTSPP-Zn2Al-LDH, and (D) MnTSPP-Zn2Al-LDH.
Fig. 3. DR UV–vis spectra of CO23–-Zn2Al-LDH, MTSPP-Zn2Al-LDH, and MTSPP (M = Co, Fe, and Mn).
These results clearly verified that sulfated metalloporphyrins
(M = Co, Fe, and Mn) have been successfully intercalated into the
layer of Zn2Al hydrotalcites and the hybrid materials based on
MTSPP and hydrotalcites formed with good crystalline in our
study.
3.2. Selective oxidation of benzyl alcohol catalyzed by MTSPP-Zn2Al-
LDH with O2
In this present investigation, the oxidation of benzyl alcohol
was examined as a standard substrate under the catalysis of
metalloporphyrin-intercalated hydrotalcites, using molecular O2
as the oxidant in the presence of IBA. The reaction conditions, such
as the compositions of catalyst, the reaction temperature, the sol-
vent, and the ratio of IBA to substrate, have been optimized.
The effect of intercalated metalloporphyrins was first investi-
gated. From the results in Fig. 4 and Table 2, it can be seen that
without the addition of catalysts, the oxidation reaction was slow.
In 150 min, only 38% of benzyl alcohol was converted to aldehyde,
obviously indicating the catalytic activities of the designed hybrid
catalysts. The catalytic activities of these different complexed met-
als are in the order of Co > Fe > Mn, which is in accordance to many
Fig. 4. Time course for the oxidation of benzyl alcohol catalyzed by different
samples. Reaction conditions: benzyl alcohol 1 mmol, ACN 8 mL, catalyst
1.9 Â 10À6 mol, IBA 3 mmol, O2 10 mL/min.

W.-Y. Zhou et al. / Journal of Catalysis 335 (2016) 105–116
109
Table 2
Optimization of reaction conditions.a
Entry
Catalyst
Solvent
T/°C
Rb
Reaction time/min
Amount of catalyst/mol
Conversion/%
Selectivity/%c
TOF/hÀ1d
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
No catalyst
ACN
ACN
ACN
ACN
p-Xyl
DMF
Diox
DCE
ACN
ACN
ACN
ACN
ACN
ACN
ACN
ACN
60
60
60
60
60
60
60
60
40
80
60
60
60
60
60
60
3
3
3
3
3
3
3
3
3
3
1
5
3
3
3
3
150
120
135
135
135
135
135
135
165
75
135
105
150
105
120
120
–
38
90
83
68
79
70
45
39
55
87
45
92
66
100
90
89
98
>99
98
>99
>99
>99
>99
>99
>99
94
>99
97
>99
88
>99
>99
–
CoTSPP-Zn2Al-LDH
FeTSPP-Zn2Al-LDH
MnTSPP-Zn2Al-LDH
CoTSPP-Zn2Al-LDH
CoTSPP-Zn2Al-LDH
CoTSPP-Zn2Al-LDH
CoTSPP-Zn2Al-LDH
CoTSPP-Zn2Al-LDH
CoTSPP-Zn2Al-LDH
CoTSPP-Zn2Al-LDH
CoTSPP-Zn2Al-LDH
CoTSPP-Zn2Al-LDH
CoTSPP-Zn2Al-LDH
CoTSPP-Zn2Al-LDH
1.9 Â 10À6
1.9 Â 10À6
1.9 Â 10À6
1.9 Â 10À6
1.9 Â 10À6
1.9 Â 10À6
1.9 Â 10À6
1.9 Â 10À6
1.9 Â 10À6
1.9 Â 10À6
1.9 Â 10À6
1.0 Â 10À6
4.0 Â 10À6
1.9 Â 10À6
1.9 Â 10À6
237
218
178
208
184
118
103
145
229
118
242
174
263
237
234
3ed
CoTSPP-Zn2Al-LDH 5th
a
b
c
Reaction conditions: benzyl alcohol 1 mmol, solvent 8 mL, O2 10 mL/min.
Ratio of IBA/benzyl alcohol.
Selectivity of aldehyde.
d
Turnover frequency with no allowance for the background.
oxidations using metalloporphyrins as the catalyst and mainly due
to the redox potentials and the stability of different valences of
metal atoms [71,72].
Reutilization is one of the greatest advantages of a heteroge-
neous catalyst, which can also provide useful information about
the intercalation process and catalytic stability during the catalytic
cycles. The recyclability of the CoTSPP-Zn2Al-LDH catalyst was
subsequently examined in the aerobic oxidation of benzyl alcohol.
The catalyst showed no appreciable reduction of activity even after
five runs (Table 1, entries 15 and 16). The XRD pattern and FTIR
spectrum for the reused catalyst suggested that the layered struc-
ture was completely preserved after several reuses (Figs. S9 and
S10). The SEM image (Fig. S11) also revealed retention of the plate-
like agglomerated crystals, convincingly demonstrating that the
structural integrity of the complex intercalated LDH remained
unaltered after the oxidation reaction.
Then, varied solvents, including ACN, dimethylformamide
(DMF), 1,4-dioxane (Diox), 1,2-dichloroethane (DCE), and p-xylene
(p-Xyl), were investigated in benzyl alcohol oxidation. Using
the prepared catalyst, all the selected solvents exhibited almost
100% selectivity of benzaldehyde, while the ACN showed the
highest conversion and yield (Table 2, entries 2, 5–8). As to
the reaction temperature, it is observed (entries 2, 9, and 10) that
low reaction temperature significantly reduced the catalytic per-
formance, while higher reaction temperature could result in
increased reactivity, as well as overoxidation of benzaldehyde to
benzoic acid, which would decrease the selectivity and yield of
product. The same pattern was found in the influence of the
amount of catalyst (entries 2, 13, and 14). Compared with the
blank experiment, increase of the catalyst amount could signifi-
cantly improve the catalytic activity. The product yield was highest
under 60 °C when 1.9 Â 10À6 mol of catalyst was used (see
Figs. S3–S7 for the time courses for the oxidation of benzyl alcohol
under different conditions). Higher molar ratios of IBA to substrate
gave higher conversions (entries 2, 11, and 12); however, no signif-
icant difference was observed when the IBA/substrate ratio
reached more than 3:1 for the present system, because IBA in the
reaction system could be oxidized into isobutyric acid, which
was clearly observed in the reaction (Fig. S8).
In addition, precise comparison of the activity of each catalyst
must be done for the turnover frequency (TOF) of the active site.
Characteristic data of other catalysts based on cobalt that afford
high yield in the aerobic oxidation of benzyl alcohol to benzalde-
hyde are collected in Table 3. It can be observed that the prepared
CoTSPP-Zn2Al-LDH exhibited high rates (TOF = 237 hÀ1) among the
reported results and was better than most of the reported results.
Although the montmorillonite-immobilized cobalt porphyrin
(CoTM4PyP-MT) exhibited
a
higher rate (TOF = 789 hÀ1), the
method was not efficient for the aliphatic alcohols and electron-
withdrawing substituted benzyl alcohol [75].
In summary, the intercalated hybrid MTSPP-Zn2Al-LDHs
(M = Co, Fe, and Mn) were found to be efficient heterogeneous
Table 3
The main results of other catalysts based on cobalt in the oxidation of benzyl alcohol by O2.a
Entry
Catalyst
Conversion/%
Selectivity/%
TOF/hÀ1b
Ref.
1
3
4
5
6
7
8
9
CoTSPP-Zn2Al-LDH/IBA
Co(NO3)2/DH2/TEMPO
RGO–MnCoO/(21 bar O2)
CoTM4PyP/MT
90
96
78
>99
100
100
237
48
–
789
–
10
33
28
13
27
50
This work
[73]
[74]
[75]
[76]
[77]
[78]
[78]
[6]
c
90d
ꢀ100
–
Co3O4/AC
87.3
–
Co(TPP)NO/BF3ÁEt2O/(50 psi O2)c
SBA-click-Co Schiff base II/IBA
SBA-click-Co Schiff base IA/IBA
CoL2@SMNP nano complex/NHPI
[CoPc/PANI]/IBA
90d
98d
97
10
11
12
>99
80d
97–98d
[79]
[81]
Polymer-bound Co Schiff bases complex/IBA
a
b
c
See the reference for the detailed structure of the catalyst.
Turnover frequency of the catalyst.
The pressure of oxygen.
d
Yield of benzaldehyde.

110
W.-Y. Zhou et al. / Journal of Catalysis 335 (2016) 105–116
catalysts for the selective aerobic oxidation of alcohol to aldehyde
or ketone in the presence of IBA. The conversion reached 90% with
a >99% selectivity to aldehyde under the optimized conditions. The
TOF of the catalytic system reached 237 hÀ1, significantly higher
than for most other heterogeneous reaction systems based on
cobalt. In addition, the prepared hybrid catalysts exhibited excel-
lent stability for the structure and catalytic properties.
3.4. Discussion of synergistic effect and mechanism
In the above sections, we have described an efficient heteroge-
neous catalyst, namely MTSPP-Zn2Al-LDH (M = Fe, Co, and Mn),
originated from non-noble metals, for the selective oxidation of
alcohol to aldehyde or ketones using O2 as the oxidant. While
the active center might be related to the metalloporphyrin mole-
cule, the catalytic properties were markedly better than for the
sole metalloporphyrins [40–44,46] and other supported metallo
complexes [6,75,77–80]. We speculated that the excellent catalytic
activity should be ascribed not only to the metalloporphyrin mole-
cule, but also to the support, i.e., hydrotalcites. In other words, a
synergistic effect might exist between the metalloporphyrin mole-
cule and hydrotalcite in the oxidation of alcohol by O2 in the pres-
3.3. Catalytic activity of CoTSPP-Zn2Al-LDH for other alcohols
With the optimized reaction conditions in hand, a variety of
alcohols were employed as substrates to investigate the scope of
alcohols that can be tolerated in aerobic oxidation using the
CoTSPP-Zn2Al-LDH catalyst (Table 4), which exhibited the best
results among the three samples.
ence of IBA. To explore the effect,
a series of controlled
In general, when benzyl alcohol and its analogs were used as
substrates, excellent yields were obtained (entries 1–8). From the
reaction time needed for these substrates with different sub-
stituents, it is easy to conclude that electronic variation of the aro-
matic substituents has some effects on the activity. Benzyl alcohols
substituted with electron-donating groups such as CH3 and OCH3
(entries 2, 3, and 6) exhibited higher activity than those with
electron-withdrawing groups (entries 4 and 5). On the other hand,
the oxidation of secondary alcohols with different amounts of IBA
could lead to the corresponding ketones in excellent ꢀ99% yields
(entries 9–11). 1-Phenylethanol and diphenylmethanol were
transformed to corresponding ketones quantitatively in the sys-
tem, while the yields were only 37% and 33%, respectively, under
the Ce(III)-complex intercalated LDH [81]. The difference might
be partly related to the small basal spacing value of the hydrid
(7.6 Å), while the net interlayer height of the synthesized
CoTSPP-Zn2Al-LDH was 18.20 Å, which could provide sufficient
space for the substrates to approach the central metal of metallo-
porphyrin [82].
experiments have been conducted and the results are shown in
Table 5 and Fig. 5.
It can be observed from Fig. 5 that CoTPP accelerated the oxida-
tion of benzyl alcohol, while CO23À-Zn2Al-LDH did not exhibit any
activity in the reaction. When free CoTPP was used as the sole cat-
alyst, the conversion of alcohol reached about 90% after 150 min.
However, under the catalysis of CoTSPP-Zn2Al-LDH, the result
was markedly different from the above controlled catalytic sys-
tems (Table 5, entries 1–3). The reaction rate was a little lower
than with CoTPP, but it could sustainability catalyze the conversion
of benzyl alcohol. These results indicated that intercalation of the
metalloporphyrin molecule into the interlayer of LDHs could stabi-
lize the CoTPP molecules and improve their catalytic performance.
Actually, the CoTPP molecule suffers serious decomposition in
homogeneous systems, which can be verified by the UV–vis spec-
tra (Fig. S12).
To further check the effect on the reaction of replacement of
CO23– by the bigger CoTSPP molecule in the layer of hydrotalcite,
TSPP-Zn2Al-LDH without cobalt was synthesized (the hydrotalcite
structure was verified by XRD; see Fig. S14) and introduced into
the reaction. It can be observed that TSPP-Zn2Al-LDH obviously
exhibits higher catalytic activity than CO23–-Zn2Al-LDH, although
it is significantly lower than CoTSPP-Zn2Al-LDH. The results
showed that without cobalt, TSPP-Zn2Al-LDH could also catalyze
the transformation to some extent, which might be related to the
large conjugated structure of porphyrin.
From the excellent performance of these catalysts, however, we
had reason to believe that LDHs should perform some other func-
tions in the catalytic system. Therefore, a mixture of CoTPP and
was introduced into the reaction system. Interestingly, it showed
a slight lower conversion than that under CoTPP alone, while the
selectivity of benzaldehyde was markedly improved. These results
might show that the existence of hydrotalcite benefited the selec-
tivity of aldehyde, i.e., CO32–-Zn2Al-LDH could stabilize the alde-
hyde molecule to be overoxidized to benzoic acid (see Fig. S13).
We proposed that the effect might be related to the surface basicity
of the LDH materials.
To verify the speculation, intercalated hybrids of CoTSPP in
LDHs with different Zn/Al ratio were prepared by a coprecipitation
method (the hydrotalcite structure was verified by XRD; see
Fig. S14) and introduced into the reaction system, because the
cation ratio can influence the surface basicity of LDHs [48].
The basicity (measured in terms of the pH of the hydrotalcite–
water slurry) and the numbers of basic sites (obtained by nonaque-
ous benzoic acid titration) of the CoTSPP-ZnxAl-LDHs samples
(x = 2, 3, and 5) are shown in Table 6. These samples were different
in basic strength and amount, which decreased as the Zn/Al ratio
increased, and CoTSPP-Zn2Al-LDH exhibited the strongest basicity
and the largest amount. On the other hand, CoTSPP-Zn5Al-LDH,
which had the weakest basicity and the smallest number of basic
sites, exhibited the lowest selectivity, verifying the above
We then turned our attention to aliphatic alcohols, which
usually have a low reactivity in the documented alcohol oxida-
tion. In particular, CoTSPP-Zn2Al-LDH was
a highly versatile
advanced catalyst which could also be employed in the oxida-
tion of aliphatic alcohols including linear and cyclic aliphatic
alcohols, affording the corresponding aldehyde in 75–82% yield
with high >99% selectivity (entries 12–16). In comparison, acids
were afforded as the main product in many reported systems
[45,83].
The oxidation of allylic alcohols has been also introduced
into the catalytic system. For example, a moderate 65% yield
of cyclohexenone was obtained in the oxidation of cyclohexenol,
and epoxide was the main by-product. In addition, cinnamyl
alcohol was mainly epoxided together with a small amount of
benzaldehyde, suggesting that a C@C double bond was attacked
in the present reaction. Low yields were also obtained for the
oxidation of the aromatic alcohols with hetero atoms, such as
pyridine-2-methanol and 2-thiophenemethanol (entries 19 and
20).
Finally, we checked the applicability to the oxidation of diols.
For the 1,4-benzenedimethanol and 1,4-cyclohexanediol, moderate
yields were obtained for the corresponding dicarbonyl products
when sufficient IBA was presented. But the phenyl glycol was
mainly oxidized to benzaldehyde.
In summary, the catalytic system exhibited considerable activ-
ity in the oxidation of many kinds of alcohols, and moderate to
high yields were obtained for most of the studied alcohols.
Although the reaction rate and reaction conditions depend on the
chemical structure of alcohols, it can be concluded that the pre-
pared CoTSPP-Zn2Al-LDH catalysts can oxidize various alcohols to
corresponding aldehydes or ketones under such mild reaction
conditions.

W.-Y. Zhou et al. / Journal of Catalysis 335 (2016) 105–116
111
Table 4
Catalytic oxidation of varied alcohols under CoTSPP-Zn2Al-LDH.a
Entry
1
Substrate
Product
Time/min
120
Conversion/%b
90
Selectivity/%b
>99
2
90
95
90
3
4
90
94
92
94
95
120
5
6
7
8
120
60
94
90
95
>99
>99
>99
180
60
100
85
9c
90
100
100
>99
>99
10c
150
11d
90
100
>99
12
90
90
82
80
63
>99
>99
>99
13
14c
120
15
16
120
120
48
75
>99
>99
(continued on next page)

112
W.-Y. Zhou et al. / Journal of Catalysis 335 (2016) 105–116
Table 4 (continued)
Entry
17
Substrate
Product
Time/min
Conversion/%b
100
Selectivity/%b
4.8
180
7.6
84.6
18
120
97
67
33
19
40
44
89
20
180
150
55
5.5
21c
100
84
16
22c
23c
a
75
100
83
74
180
84.5
Reaction conditions: benzyl alcohol 1 mmol, ACN 8 mL, 60 °C, catalyst 1.9 Â 10À6 mol, IBA 3 mmol, O2 10 mL/min.
b
c
GC yield.
6 mmol of IBA was used.
2 mmol of IBA was used.
d
speculation. But the difference between CoTSPP-Zn2Al-LDH and
in the presence of metal complexes and aldehyde have been inves-
tigated already, especially in the epoxidation of olefins
[41,56,84,85]. Since it is well known that the oxidation reactions
with the O2–aldehyde–metal complex proceed via a radical route,
control experiments were undertaken to confirm the pathway of
aerobic oxidation of benzyl alcohol over CoTSPP-Zn2Al-LDH. A rad-
ical scavenger (BHT) could completely stop the reaction in such an
oxidation system (Table 5, entry 8), indicating the formation of
radicals during the reaction.
The addition of IBA was necessary for the smooth conversion
of benzyl alcohol in the aerobic system under CoTSPP-Zn2Al-
LDH. No benzaldehyde could be obtained by conducting the reac-
tion for 360 min without IBA (Table 5, entry 9), demonstrating
that the metalloporphyrin catalyst hardly activated dioxygen
under ambient pressure in the absence of aldehyde. However, a
38% conversion of benzyl alcohol can be obtained in a blank
experiment (Table, entry 1) when the reaction time was
150 min, which should be attributed to the autooxidation of alde-
hyde [86–88].
CoTSPP-Zn3Al-LDH was slight, which might be due to the small
changes of basicity. Further, when K2CO3 was added into the cat-
alytic system under CoTSPP-Zn3Al-LDH, the profiles of conversions
and selectivities to the reaction time in Fig. S15 indicated that the
basicity of the reaction solution did not affect the catalytic activity
but only the selectivity. The same conclusion could also be
obtained in the CoTSPP-Zn5Al-LDH system (Fig. S16). However,
under the catalysis of CoTSPP-ZnxAl-LDH with different Zn/Al
ratios, the conversion of benzyl alcohol showed a declining trend
with the increasing ratio of Zn/Al (Fig. 6), indicating that the sur-
face basicity of the catalyst was helpful to the catalytic activity.
The above results indicate that only the basic sites in the catalyst
benefit the catalytic activity, which may be ascribed to the fact that
the basic sites on the surface of the catalyst may accelerate the
accessibility of substrates to the active centers.
From the above discussion, it could be concluded that the active
center of the prepared catalysts was CoTSPP rather than CO32À
-
Zn2Al-LDH. And the mechanisms of oxidation by molecular oxygen

W.-Y. Zhou et al. / Journal of Catalysis 335 (2016) 105–116
113
Table 5
Catalytic oxidation of benzyl alcohol under different conditions.a
Entry
Catalyst
Reaction time/min
Conversion/%
Selectivity/%
Aldehyde
TOF/h–1b
Acid
1
2
3
4
5
6
7
8
9
–
150
150
150
150
150
150
150
360
360
38
90
36
89
96
90
85
0
>99
86
>99
98
92
94
92
–
–
14
0
0
8
6
8
–
0
–
79
–
59
237
CoTPPc
CO23–-Zn2Al-LDHd
CoTPP + CO23–-Zn2Al-LDHe
CoTSPP-Zn2Al-LDHf
CoTSPP-Zn3Al-LDHf
CoTSPP-Zn5Al-LDHf
CoTSPP-Zn2Al-LDH + BHTg
CoTSPP-Zn2Al-LDH (without IBA)
0
0
0
0
a
b
c
d
e
f
Reaction conditions: benzyl alcohol 1 mmol, ACN 8 mL, 60 °C, catalyst 1.9 Â 10À6 mol, IBA 3 mmol, O2 10 mL/min.
Turnover frequency with no allowance for the background.
5 mg.
3.8 mg.
CoTPP 5 mg, CO23À-Zn2Al-LDH 3.8 mg.
4 equiv of IBA was used.
The same conditions were used except that the free radical inhibitor BHT (1 mmol) was added.
g
In the reaction catalyzed by metalloporphyrins, the identifica-
decelerating effect of electron-withdrawing substituents indicate
that the dehydrogenation step (b-hydride elimination) is rate-
limiting and the reaction is kinetically controlled [23,95].
tion of reactive species, metal–oxo or metal–hydroperoxo, is the
key problem. Isotopically labeled water (H128O) experiments have
been well established as useful mechanistic probes in testing the
involvement of high-valent metal–oxo intermediates in metal-
mediated oxygen atom transfer reactions [83,89,90]. When labeled
18O is found to be incorporated from H128O into oxygenated prod-
ucts, a high-valent metal–oxo complex is proposed as an oxygenat-
ing species, since the intermediate exchanges its oxygen atom with
labeled water prior to oxygen atom transfer to organic substrates.
Therefore, we have conducted the oxidation of benzyl alcohol by
CoTSPP-Zn2Al-LDH in the presence of 100 and 50 equivalent excess
of H128O to substrate. As a result, 49% and 35%, respectively, of the
oxygen in the benzaldehyde product came from H128O, suggesting
that a high-valent metal–oxo porphyrin intermediate was gener-
ated as a reactive species in the reactions of the metalloporphyrin
and O2 in the presence of IBA, similar to that proposed in cyto-
chrome P-450 enzymes.
To clarify the mechanism, the relative reactivity of p-substi-
tuted benzyl alcohol (X = CH3O, CH3, H, Cl, and NO2) with oxygen
in the presence of CoTSPP-Zn2Al-LDH was determined. In the com-
petitive reaction, a large excess of substrate was used to ensure
that the rate is zero-order in the substrate. The relative rates can
be determined by the conversion of benzyl alcohol when conver-
sion is low (<5%).
On the basis of these results and analysis, a mechanism for the
CoTSPP-Zn2Al-LDH-catalyzed oxidation of benzyl alcohol by dioxy-
gen in the presence of IBA has been proposed (as shown in
Scheme 1). The cobalt porphyrin initially reacted with the alde-
hyde to generate an acyl radical (a), which could be also generated
from aldehyde autoxidation, although the rate was much lower
[86–88]. The acyl radical then reacted with dioxygen to give an
acylperoxy radical (b), which is known to be extremely fast. The
acylperoxy radical was assumed to play two roles. First, it acted
as a carrier by reacting with another aldehyde molecule to give
peroxyacid and acyl radical; then the peroxyacid could react with
benzyl alcohol to yield benzaldehyde directly, accompanying the
generation of isobutyric acid, which could be verified by the non-
catalyzed reaction. Second, the acylperoxy radical could react with
Co(II) porphyrin to form cobalt(III) acylperoxide species (c) [56].
On the other hand, monomeric cobalt(III)–dioxygen complex
(d) formed with the help of IBA, which has been verified by Iqbal’s
Table 6
The basicity of CoTSPP intercalated ZnAAl hydrotalcites with different Zn/Al ratios.a
Sample
pH
Basicity at H_ = 7.6–15.0 (mmol/g)
CoTPPS-Zn2Al-LDH
CoTPPS-Zn3Al-LDH
CoTPPS-Zn5Al-LDH
8.10
7.99
7.52
0.019
0.015
0.006
The Hammett plot is shown in Fig. 7, and the slope of the Ham-
mett plot (
dation of benzyl alcohol with Fe(TDCPP) (
(TMP) ( = À0.43) [92], Ru/Al2O3
= À0.43) [91], 17 wt.% RuHAp (
= À0.46) [93], and cytochrome P450 enzymes ( = À0.4) [94].
The accelerating effect of electron-donating substituents and the
q
ꢁ À0.47) is similar to the slopes observed in the oxi-
q
= À0.39) [59], Fe
a
q
q
Suspension of 0.3 g hydrotalcite in 20 mL deionized water. 0.1 g hydrotalcite,
suspended in 2 mL phenolphthalein indicator solution, is titrated with 0.01 M
benzoic acid.
(q
q
Fig. 5. The profiles of conversion and selectivity to reaction time under different conditions Reaction conditions: benzyl alcohol 1 mmol, ACN 8 mL, 60 °C, catalyst
1.9 Â 10À6 mol, IBA 3 mmol, O2 10 mL/min. For the selectivity profiles, they were >99% under CO23À-Zn2Al-LDH and TSPP-Zn2Al-LDH.

114
W.-Y. Zhou et al. / Journal of Catalysis 335 (2016) 105–116
Fig. 6. The catalytic results under CoTSPP-intercalated LDHs with different Zn/Al ratio. Reaction conditions: benzyl alcohol 1 mmol, ACN 8 mL, 60 °C, catalyst 1.9 Â 10À6 mol,
IBA 3 mmol, O2 10 mL/min.
the metalloporphyrin, the effect of O2 concentration on the reac-
tion was studied. When air was used as the oxygen source, the
reaction rate was significantly decreased (Fig. S17). Further, chang-
ing the O2 flow rate clearly influenced the reaction. Based on these
results, we supposed that the direct activation of molecular oxygen
by the metalloporphyrin might be primary during the reaction,
because the reaction between acyl radical and dioxygen to give
an acylperoxy radical (b) is known to be extremely fast [56].
After the formation of the monomeric cobalt(III)–dioxygen
complex (d), an intramolecular hydrogen atom transferred from
carbonyl compound to the terminal oxygen atom of bound dioxy-
gen in complex e. Anyway, whatever the reaction pathway leading
to the metal peroxide complex (that is, through the acylperoxyl
radical c and/or superoxometal intermediates e), this transient
metal acylperoxide species could eventually produce an oxometal
species by homolytic OAO bond cleavage of the peroxide group
[56]. That being so, the ultimate active oxidizing agent would be
a high-valent oxometal species (f), which has been verified by
the H128O experiments. There are two possible processes in the oxi-
dation of alcohol by the high-valent iron–oxo species [83]. One is
direct insertion from the metal–oxo species into the CAH bond,
Fig. 7. Hammett plot of p-substituted benzyl alcohols. Reaction conditions: 1 mmol
p-X-benzyl alcohol (X = CH3O, CH3, H, Cl, NO2), 1 mmol benzyl alcohol, CoTSPP-
Zn2Al-LDH 1.9 Â 10À6 mol, ACN 8 mL, 1 mmol chlorobenzene, 60 °C, O2 10 mL/min.
and the other route involves stepwise oxidation of the alcohol to
an a-hydroxy carbinyl radical via hydrogen-atom abstraction, fol-
lowed by oxygen rebound.
[84] and Chang’s [85] studies. In addition, they also found that
hydrogen bonding between the OH proton and the terminal oxy-
gen atom of the bound dioxygen could strongly stabilize the
cobalt(III)–dioxygen adduct and encourage its formation. There-
fore, we supposed that hydroxyl commonly existing on the surface
and in the interlayer of hydrotalcite [47,48] might help the forma-
tion of Co(III)–O–OÅ and accelerate the reaction.
However, the Hammett experiments have indicated that the
reaction pathway was not a direct insertion from the metal–oxo
species into the CAH bond, because hydrogen-atom abstraction
from the carbon atom of the coordinated alcoholate was the
rate-determining step. On the other hand, if the two pathways
were both allowed, in the H128O exchange experiments, the content
of 18O of the product should always be close to 50% irrespective of
To further distinguish the above reaction paths, i.e., the radical
initiation process and the direct activation of molecular oxygen by
Scheme 1. The proposed mechanism of the aerobic oxidation of benzyl alcohol under CoTSPP-Zn2Al-LDH in the presence of IBA.

W.-Y. Zhou et al. / Journal of Catalysis 335 (2016) 105–116
115
the varying amounts of labeled water, because H128O existed at a
large excess compared to the IBA (the maximum of 16O atoms).
Therefore, the evidence described above suggests that active
high-valent Co porphyrin might first abstract a hydrogen atom
from the a-site and form a common gem-diol intermediate. Finally,
the product benzaldehyde formed by losing a molecule of H2O.
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The National Natural Science Foundation of China (21403018) is
acknowledged for financial support.
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