Synthesis and Comparative Catalytic Study of Zirconia–MnCO3 or –Mn2O3 for the Oxidation of Benzylic Alcohols

Article abstract of DOI:10.1002/open.201600116

We report on the synthesis of the zirconia–manganese carbonate ZrO_x(x %)–MnCO₃ catalyst (where x=1–7) that, upon calcination at 500 °C, is converted to zirconia–manganese oxide ZrO_x(x %)–Mn₂O₃. We also present a comparative study of the catalytic performance of the both catalysts for the oxidation of benzylic alcohol to corresponding aldehydes by using molecular oxygen as the oxidizing agent. ZrO_x(x %)–MnCO₃ was prepared through co-precipitation by varying the amounts of Zr(NO₃)₄ (w/w %) in Mn(NO₃)₂. The morphology, composition, and crystallinity of the as-synthesized product and the catalysts prepared upon calcination were studied by using scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and powder X-ray diffraction. The surface areas of the catalysts [133.58 m² g^?1 for ZrO_x(1 %)–MnCO₃ and 17.48 m² g^?1 for ZrO_x(1 %)–Mn₂O₃] were determined by using the Brunauer–Emmett–Teller method, and the thermal stability was assessed by using thermal gravimetric analysis. The catalyst with composition ZrO_x(1 %)–MnCO₃ pre-calcined at 300 °C exhibited excellent specific activity (48.00 mmolg^?1 h^?1) with complete conversion within approximately 5 min and catalyst cyclability up to six times without any significant loss in activity. The specific activity, turnover number and turnover frequency achieved is the highest so far (to the best of our knowledge) compared to the previously reported catalysts used for the oxidation of benzyl alcohol. The catalyst showed selectivity for aromatic alcohols over aliphatic alcohols.

Full text of DOI:10.1002/open.201600116

DOI: 10.1002/open.201600116
Synthesis and Comparative Catalytic Study of Zirconia–
MnCO3 or –Mn2O3 for the Oxidation of Benzylic Alcohols
Mohamed E. Assal,[a] Mufsir Kuniyil,[a] Mujeeb Khan,[a] Abdulrahman Al-Warthan,[a]
Mohammed Rafiq H. Siddiqui,[a] Wolfgang Tremel,[b] Muhammad Nawaz Tahir,*[b] and
Syed Farooq Adil*[a]
We report on the synthesis of the zirconia–manganese carbon-
ate ZrOx(x%)–MnCO3 catalyst (where x=1–7) that, upon calci-
nation at 5008C, is converted to zirconia–manganese oxide
ZrOx(x%)–Mn2O3. We also present a comparative study of the
catalytic performance of the both catalysts for the oxidation of
benzylic alcohol to corresponding aldehydes by using molecu-
lar oxygen as the oxidizing agent. ZrOx(x%)–MnCO3 was pre-
pared through co-precipitation by varying the amounts of
Zr(NO3)4 (w/w%) in Mn(NO3)2. The morphology, composition,
and crystallinity of the as-synthesized product and the catalysts
prepared upon calcination were studied by using scanning
electron microscopy, transmission electron microscopy, energy-
dispersive X-ray spectroscopy, and powder X-ray diffraction.
The surface areas of the catalysts [133.58 m2 gÀ1 for ZrOx(1%)–
MnCO3 and 17.48 m2 gÀ1 for ZrOx(1%)–Mn2O3] were deter-
mined by using the Brunauer–Emmett–Teller method, and the
thermal stability was assessed by using thermal gravimetric
analysis. The catalyst with composition ZrOx(1%)–MnCO3 pre-
calcined at 3008C exhibited excellent specific activity
(48.00 mmolgÀ1 hÀ1) with complete conversion within approxi-
mately 5 min and catalyst cyclability up to six times without
any significant loss in activity. The specific activity, turnover
number and turnover frequency achieved is the highest so far
(to the best of our knowledge) compared to the previously re-
ported catalysts used for the oxidation of benzyl alcohol. The
catalyst showed selectivity for aromatic alcohols over aliphatic
alcohols.
1. Introduction
One of the global challenges is to reduce the emission of
greenhouse gases while increasing the industrial production/
efficiency to meet the society needs. Especially, the petrochem-
ical industry is at the focus of environmental pollution. There-
fore, the development and synthesis of ecofriendly, low-cost,
and efficient catalysts for the petrochemical industry are inevi-
table.
the laboratory scale.[1] One very important application of the
oxidation reactions in petrochemicals is the synthesis of alde-
hydes by oxidation of the corresponding alcohols. Aldehydes
play a vital role as the starting material for many industrial and
pharmaceutical formulations.[2] Among the different catalysts
reported for the oxidation reactions, metal-supported metal
oxides or mixed metal oxides (MMO) have gained much atten-
tion, such as Au NPsÀCeÀSnO for the preparation of benzalde-
hyde from benzyl alcohol,[3] PtÀCeO2/C and PtÀTiO2/C as elec-
trocatalysts for methanol oxidation, or TiO2 (001) nanosheets
for the oxidation of benzyl alcohol.[4] CoCuO MMO micro-
spheres have been used for the oxidation of ethylbenzene,[5]
the electrochemical oxidation of glucose, and the selective oxi-
dation of 5-hydroxymethyl furfural, to name only a few.[6]
MMOs containing manganese received significant attention as
catalysts for various applications including oxidation reactions,
manganese titanium oxide hydroxide-supported palladium
nanoparticles being an example for the electro-oxidation of
methanol,[7] and for the removal of CrVI from aqueous solu-
tions.[8] Fe3O4 and MnO2 assembled on halloysite nanotubes
have been used for the electrochemical detection of mercu-
ry(II) ions,[9] MnO2 films and MnO2 nanosheets decorated with
NiO nanoparticles as efficient supercapacitor electrode materi-
als,[10] and MnCo2O4@C nanoparticles as a catalyst for water
splitting.[11] Manganese-promoted CeÀAlÀSi is a catalyst for the
oxidation of acetone,[12] PdmMn1-mOx nanoparticle-decorated
graphene or graphene-based composite photocatalysts have
Among the chemical transformations, selective oxidation is
an important chemical reaction, which plays an essential role
in the preparation of many chemicals on an industrial and at
[a] M. E. Assal, M. Kuniyil, Dr. M. Khan, Prof. A. Al-Warthan,
Prof. M. R. H. Siddiqui, Dr. S. F. Adil
Department of Chemistry, College of Science
King Saud University
P.O. Box 2455, Riyadh 11451 (Kingdom of Saudi Arabia)
E-mail: sfadil@ksu.edu.sa
[b] Prof. W. Tremel, Dr. M. Nawaz Tahir
Institute of Inorganic and Analytical Chemistry
Johannes Gutenberg-University of Mainz
55128 Mainz (Germany)
E-mail: tahir@uni-mainz.de
The ORCID identification number(s) for the author(s) of this article can
be found under http://dx.doi.org/10.1002/open.201600116.
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This is an open access article under the terms of the Creative Commons
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catalyzed the oxidation of alcohol,[13] and copper manganese
oxide nanoparticles have been used for the oxidation of
carbon monoxide,[14] alcohols,[15] and various hydrocarbons.[14,16]
Similarly, MMOs containing manganese oxide along with those
that are noble-metal doped/supported[16i,17] are also used for
the oxidation of benzyl alcohol.
for the oxidation of various aromatic alcohols was evaluated
by using gas chromatography.
2. Results and Discussion
2.1. Characterization of the Catalysts
2.1.1. Morphology and Phase Structure
Similarly, zirconia-based catalysts are used for the oxidation
[18]
reaction, as an example ZrO2ÀCeO2/SiO2 or gold supported
on Zr doped ceria catalysts have been employed for the oxida-
tion of CO,[19] RuÀCu/ZrO2 as glycerol hydrogenolysis cata-
lysts,[20] and CuÀNi/ZrO2 for the synthesis of diethyl carbonate
from CO2 and ethanol. A polyoxometalate–zirconia (POM/ZrO2)
nanocomposite[21] has been reported for the selective oxida-
tion of alcohols in the liquid phase.[19,22] Zirconia-supported
The catalyst obtained by co-precipitation was calcined at 300
and 5008C. The SEM images of the as-synthesized catalyst
ZrOx(1%)–MnCO3, and the products calcined at temperatures
3008C [ZrOx(1%)–MnCO3] or 5008C [ZrOx(1%)–Mn2O3] are
shown in Figure 1. The SEM images show particles with a well-
[22b]
[22c]
CrO3
or ruthenium-supported on a CaOÀZrO2
have been
used for the oxidation of alcohols to aldehydes, whereas zirco-
nium-based metal–organic frameworks have been employed
for the photocatalytic oxidation of alcohol under visible
light.[22d] Zirconia-supported sodium decatungstate (Na4W10O32/
ZrO2)[22a] led to the formation of carboxylic acids.
However, there are few reports for manganese (Mn)- and zir-
conium (Zr)-based MMOs as catalysts. Also, MnCO3, which is
low cost, environmentally friendly, and transforms to manga-
nese oxides[18] upon heating, has been used as a raw material
to prepare manganese-oxide-based catalysts. However, MnCO3
as a pure phase or mixed with other metal oxides is not stud-
ied as an oxidation catalyst. In this contribution, we report the
synthesis of ZrOx(x%)–MnCO3 (where x=1–7), followed by cal-
cination at elevated temperature to prepare ZrOx(x%)–Mn2O3.
Both catalysts were studied for the oxidation of benzylic alco-
hols to the corresponding aldehydes. The catalysts were pre-
pared by co-precipitation with varying the amounts of Zr(NO3)4
(w/w%) and Mn(NO3)2 (Scheme 1). The morphology, composi-
tion, and phase composition of the as-synthesized catalyst and
those of the catalyst after calcination were monitored by using
scanning electron microscopy (SEM), transmission electron mi-
croscopy (TEM), energy-dispersive X-ray (EDX) spectroscopy,
powder X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET),
and thermal gravimetric analysis (TGA). The catalytic efficiency
Figure 1. SEM analysis of the synthesized catalysts calcined at A–C) 3008C;
D–F) 5008C. A) Overview image for as-synthesized ZrOx(1%)–MnCO3 and
B) ZrOx(1%)–MnCO3. C) Size distribution of ZrOx(1%)–MnCO3, D) overview
image for as synthesized ZrOx(1%)–Mn2O3 and E) ZrOx(1%)–Mn2O3, and
F) size distribution of ZrOx(1%)–Mn2O3.
defined cuboidal morphology. The particle-size distribution
(Figures 1C and 1F, obtained with the Image J program)
shows only slight variations in the particle sizes with changes
in calcination temperature. The composition of the catalyst is
determined by using EDX and stays within experimental error
to the theoretical composition (Table 1).
Table 1. Elemental composition of the catalyst ZrOx(1%)–MnO calcined
at different temperatures.
Calcination
temperature [8C]
Mass [%]
Element
C
Compound
O
Mn
Zr
C
MnO ZrO2
300
500
29.97
22.72
15.84 53.24 0.95
17.5 58.36 1.43
29.97 68.75 1.28
22.72 75.36 1.93
The XRD patterns for the as-synthesized catalysts (uncal-
cined) (Figure 2a) show the presence of rhodochrosite, syn-
manganese carbonate (JCPDS No. 00-007-0268) with space
group R-3c (167), which, upon calcination at 3008C, trans-
formed to rhodochrosites manganese carbonate oxides (JCPDS
No. 00-001-0981) [space group R-3c (167)] (Figure 2b).
Calcination at 5008C led to the formation of bixbyite Mn2O3
(JCPDS No.:00-002-0909) (Figure 2c). The reflections marked
Scheme 1. Synthetic representation of ZrOx–MnCO3 followed by calcination
at 5008C to prepare ZrOx–Mn2O3.
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2.1.2. Thermal Stability
The thermal stability of ZrOx(1%)–MnCO3 calcined at 3008C or
ZrOx(1%)–Mn2O3 obtained after calcining at 5008C catalysts
was determined by using TGA (Figures 4a and 4b). The cata-
lyst calcined at 3008C was stable up to 4108C with a small
weight loss of <8% (attributed to physisorbed moisture). In-
creasing the temperature led to an additional weight loss of
15% in the temperature range between 410 and 6108C, corre-
sponding to the decarboxylation of MnCO3 to form MnO2 +
CO2 and further oxidation of MnO2 to Mn2O3 (O2:4MnO2/
2Mn2O3 +O2), as reported by Zhu et al.[1] These findings are in
agreement with the results (5% weight loss) obtained for heat-
ing the catalysts calcined at 5008C.
Figure 2. XRD pattern of the catalyst at different temperatures a) uncalcined
rhodochrosite, syn-manganese carbonate (JCPDS No. 00-007-0268), b) at
3008C ZrOx(1%)–MnCO3 transformed to rhodochrosites manganese carbon-
ate oxides (JCPDS No. 00-001-0981), and c) at 5008C converted to bixbyite
Mn2O3 (JCPDS No.:00-002-0909). The reflections marked with an asterisk (*)
could be attributed to the presence of ZrOx (monoclinic phase for uncal-
cined and at 3008C calcined and tetragonal for sample calcined at 5008C).
with * could be attributed to the presence of ZrOx (monoclinic
phase for uncalcined and 3008C-calcined samples, and tetrago-
nal for 5008C-calcined sample).
The HRTEM images of ZrOx(1%)–MnCO3 nanoparticles ob-
tained after calcination at 300 and 5008C (Figures 3a and 3b)
show polycrystalline particles with clear lattice fringes. The in-
terplanar distance was calculated from the HRTEM image of
the sample calcined at 3008C (Figure 3a), which revealed d
spacings of 0.29 and 0.23 nm, corresponding to the (104) and
the (110) planes of rhombohedral MnCO3. For the sample cal-
cined at 5008C (Figure 3b), the most prominent lattice spacing
corresponds to the (200) plane of a-Mn2O3.
Figure 4. TGA curve of ZrOx(1%)–MnCO3 calcined at 3008C and 5008C:
a) ZrOx(1%)–MnCO3 and b) ZrOx(1%)–Mn2O3.
2.1.3. Surface-Area Measurements
The surface area of the prepared catalysts was determined by
using BET sorption measurements. The catalyst calcined at
3008C [ZrOx(1%)–MnCO3] has the highest surface area
(133.58 m2 gÀ1), whereas the surface are of the catalyst calcined
at 5008C [ZrOx(1%)–Mn2O3] was severely reduced
(17.48 m2 gÀ1), owing to sintering. It is worth mentioning that
sintering not only results inter-particles, but it also minimizes
the porosity within a single particle. The results are compiled
in Table 2.[23] The catalytic activity of ZrOx(1%)–MnCO3 may be
correlated to the activity surface area (vide infra).
Figure 3. HRTEM analysis of the catalysts calcined at a) 3008C and b) 5008C.
Table 2. Effect of calcination temperature on the catalytic properties.[a]
Entry
Catalyst
T
[8C]
Surface area
Conversion
[%]
Selectivity
[%]
Specific activity
TON
TOF
[hÀ1
]
[m2 gÀ1
]
[mmolgÀ1 hÀ1
]
1
2
ZrOx(1%)–MnCO3
ZrOx(1%)–Mn2O3
300
500
133.58
17.48
100.00
21.53
<99
<99
13.33
2.87
82.14
17.68
164.27
35.37
[a] Reaction conditions: amount of catalyst 300 mg, reaction temperature 1008C, oxygen flow rate 20 mLminÀ1, benzyl alcohol 2 mmol, toluene 10 mL,
and reaction time 30 min.
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2.2. Catalytic Evaluation
2.2.2. Effect of the Amount of ZrOx Promotor
To investigate the catalytic performance of the prepared cata-
lysts and to compare the catalytic activity of ZrOx(1%)–MnCO3
and ZrOx(1%)–Mn2O3, both catalysts underwent testing for
the oxidation of benzyl alcohol to benzaldehyde as a model re-
action. To check the effect of ZrOx, the activity of phase-pure
MnCO3 and Mn2O3 was tested under identical experimental
conditions for comparison.
To investigate the optimum amount of ZrOx present in MnCO3
to effect the kinetics of the reaction, catalysts prepared by
varying the amount of ZrOx precursor from 1 to 7% were
tested for the oxidation of benzyl alcohol to benzaldehyde. All
catalysts were calcined at 3008C, and the reaction was per-
formed at 1008C. ZrOx(1%)–MnCO3 showed a complete con-
version of benzyl alcohol. The specific activity for this conver-
sion is 13.33 mmolgÀ1 hÀ1 and the TOF was 64.27 hÀ1.
ZrOx(3%)–MnCO3, ZrOx(5%)–MnCO3, and ZrOx(7%)–MnCO3
led to 95.25, 61.49, and 40.25% conversion, respectively. To as-
certain the presence of ZrOx as the promoter, a catalyst pre-
pared without ZrOx was subjected to catalytic evaluation
under a similar set of conditions. A 82.55% benzyl alcohol con-
version along with specific activity of 11.0 mmolgÀ1 hÀ1 was
achieved, which clearly indicates the promotion effect of ZrOx
in the catalyst with 1 and 3% ZrOx. The specific activity, TON,
and TOF were found to decrease with increased percentage
presence of ZrOx in the catalyst. Further increasing the ZrOx
content led to a decrease in the catalytic performance, which
may be attributed to a blocking of active sites or to an ag-
glomeration of zirconia, as indicated by the results of the BET
analysis. The graphical representation is given in Figure 6.
2.2.1. Effect of Calcination Temperature
As the calcination temperature may play an essential role for
the kinetics of the reaction,[24] the obtained catalysts were eval-
uated for the oxidation of benzyl alcohol. The catalyst calcined
at 3008C [ZrOx(1%)–MnCO3] yielded a 100% conversion of
benzaldehyde with 13.33 mmolgÀ1 hÀ1 specific activity and
164.27 hÀ1 turnover frequency (TOF), within 30 min of the reac-
tion, whereas the catalyst calcined at 5008C [ZrOx(1%)–Mn2O3]
yielded a 21.53% conversion within the first 30 min (Figure 5)
Figure 5. Graphical representation of benzyl alcohol oxidation using catalyst
calcined at a) 3008C and b) 5008C.
with 2.87 mmolgÀ1 hÀ1 specific activity and 35.37 hÀ1 TOF
(Table 2). This shows that the presence of ZrOx on MnCO3 plays
an important role during the oxidation reaction, whereas ZrOx-
supported Mn2O3 obtained upon calcination at 5008C displays
the lowest catalytic performance and specific activity. This may
be related to a decomposition of the active catalyst phase or
reduction in surface area (as mentioned above), whereas the
sample calcined at 3008C has MnCO3 oxides with active sites
to promote catalysis. These catalytic activity results also corre-
late with results obtained from the surface area analysis, which
revealed that the Zr(1%)–MnCO3 catalyst synthesized at
3008C possesses the highest surface area, whereas the other
calcination temperature results in the depreciation of surface
area (Table 2). The specific activity, turnover number (TON), and
TOF were found to decrease with increasing calcination tem-
perature. The benzyl alcohol conversion, specific activity, TON,
TOF and selectivity obtained over the catalyst are listed in
Table 2.
Figure 6. Graphical representation of benzyl alcohol oxidation using catalyst
a) MnCO3, b) ZrOx(1%)–MnCO3, c) ZrOx(3%)–MnCO3, d) ZrOx(5%)–MnCO3,
and e) ZrOx(7%)–MnCO3.
The specific activity for the conversions decreased from
12.70 to 5.37 mmolgÀ1 hÀ1 with no change in selectivity to-
wards benzaldehyde for all reactions. The benzyl alcohol con-
version, specific activity, and selectivity obtained for all catalyst
are compiled in Table 3.
2.2.3. Effect of Amount of Catalyst
To determine the effect of the amount of catalyst on the cata-
lytic performance, the amount of the ZrOx(1%)–MnCO3 cata-
lyst was varied (100, 200, 300, 400, and 500 mg), while the re-
action conditions [temperature (1008C), molecular oxygen as
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Table 3. Effect on the catalytic properties by weight% of ZrOx in the cat-
alyst.[a]
Entry ZrOx Conversion Selelctivity Specific activity TON
TOF
[hÀ1
]
[%]
[%]
[%]
[mmolgÀ1 hÀ1
]
1
2
3
4
5
0
1
3
5
7
82.55
<99
<99
<99
<99
<99
11.00
13.33
12.70
8.20
–
–
100.00
95.25
61.49
40.25
82.14 164.27
26.08
10.08
4.74
52.16
20.16
9.47
5.37
[a] Reaction conditions: catalyst ZrOx(x%)–MnCO3, amount of catalyst
300 mg, calcination temperature 3008C, reaction temperature 1008C,
oxygen flow rate 20 mLminÀ1, benzyl alcohol 2 mmol, toluene 10 mL,
and reaction time 30 min.
Figure 7. Effect on the conversion of benzyl alcohol to benzaldehyde by
varying amount of the catalyst (mg). Reaction conditions: catalyst
ZrOx(1%)–MnCO3, reaction temperature 1008C, oxygen flow rate
20 mLminÀ1, benzyl alcohol 2 mmol, toluene 10 mL, and reaction time
5 min.
oxidant], and reaction time (5 min) were kept constant. The re-
action carried out using 500 mg catalyst yielded a 100% con-
version product within 5 min (Table 4, entry 5), whereas the
conversion for the remaining samples was <100% within the
same period of time. A 28.47% conversion was obtained with
100 mg of catalyst (Table 4, entry 1). The specific activity for
this conversion was calculated to be 68.32 mmolgÀ1 hÀ1. How-
ever, when the amount of catalyst was increased to 200, 300,
400, and 500 mg, the specific activity decreased to 53.89,
50.24, 48.74, and 48.00 mmolgÀ1 hÀ1, respectively (Table 4, en-
tries 2–5). The selectivity remained unchanged throughout the
studies. The conversion of benzyl alcohol to benzaldehyde in-
creased by increasing the amount of catalyst, whereas the spe-
cific activity decreased. A linear relationship can be deduced
between the amount of catalyst and conversion of benzyl alco-
hol, as shown in Figure 7, and the catalytic results are com-
piled in Table 4.
2.2.4. Effect of Reaction Temperature
Figure 8 shows the effect of reaction temperature on the cata-
lytic activity of the ZrOx(1%)–MnCO3 catalyst for the oxidation
of benzyl alcohol. Upon increasing the reaction temperature
from 20 to 1008C, the conversion of benzyl alcohol with the
synthesized catalyst increased from 49.84 to 100%. The specific
activity, TON, and TOF were found to increase with increased
reaction temperature (Table 5).
Figure 8. Effect of reaction temperature (8C) on the conversion of benzyl al-
cohol. Reaction conditions: catalyst ZrOx(1%)–MnCO3, oxygen flow rate
20 mLminÀ1, benzyl alcohol 2 mmol, toluene 10 mL, and reaction time
5 min.
was formed. To rule out the formation of benzaldehyde by oxi-
dation of the solvent (toluene), a reaction was performed with-
out the substrate (benzyl alcohol), and again no conversion
product (benzaldehyde) was formed.
It is worth mentioning that, when the reaction was carried
out in the absence of a catalyst and otherwise a similar set of
reaction conditions, no conversion product (i.e. benzaldehyde)
Table 4. Influence of amount the catalyst on the conversion product.[a]
Entry
Catalyst
amount [mg]
Time
[min]
Conversion
[%]
Selectivity
[%]
Specific activity
TON
TOF
[hÀ1
]
[mmolgÀ1 hÀ1
]
1
2
3
4
5
100
200
300
400
500
5
5
5
5
5
28.47
44.91
62.8
81.23
100
<99
<99
<99
<99
<99
68.32
53.89
50.24
48.74
48.00
70.12
55.34
51.59
50.05
49.29
841.44
664.08
619.08
600.60
591.48
[a] Reaction conditions: catalyst ZrOx(1%)–MnCO3, reaction temperature1008C, oxygen flow rate 20 mLminÀ1, benzyl alcohol 2 mmol, and toluene 10 mL.
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ZrO2 led to a specific activity of 39.50 mmolgÀ1 hÀ1, and only
23.7% of benzyl alcohol was converted to benzaldehyde with
a selectivity of 92.3%.[25b] In summary, ZrOx(1%)–MnCO3 cal-
cined at 3008C exhibited an excellent catalytic activity com-
pared to all other catalysts listed in Table 6.
Table 5. Effect of reaction temperature on the kinetics of the reaction.[a]
Entry
T
Conversion Selectivity Specific activity TON
TOF
[hÀ1
]
[8C] [%]
[%]
[mmolgÀ1 hÀ1
]
1
2
3
4
5
20
40
60
80
49.84
<99
<99
<99
<99
<99
23.92
30.0
36.17
42.25
48.0
11.80 141.60
30.81 369.72
37.14 445.68
43.39 520.68
49.29 591.48
62.51
75.36
88.03
The exact mechanism can be postulated only by using
in situ spectroscopic techniques. However, based on very es-
tablished literature in the field, we propose the following
mechanism for ZrOx–MnCO3 or ZrOx–Mn2O3 (ZrOx =1–7 w/
w%) catalyst-based oxidation of benzyl alcohol. In most of
cases, the dehydrogenation step acts as the rate-limiting step
in the oxidation of alcohols.[26] Moreover, the presence of a pro-
moter (normally a base or catalyst support acting as solid
base) can help to activate the OÀH bond of alcohol to increase
the oxidation. Here, in this catalytic system, ZrOx dispersed on
MnCO3 acts as the promotor (basic sites ZrOx).[27] MnCO3 in the
presence of molecular oxygen oxidized easily to high-valent
Mn; most likely to g-MnOOH,[28] and the organic molecule oxi-
dizes at the expense of these high-valent Mn active sites. As
reported by Tang et al.,[25j] and verified by others; we also be-
lieve that the oxidation mechanism follows the Mar–van Kre-
venlen-type mechanism. The active species is possibly O2À and
likely involves the transfer of two electrons in a single step. As
the mechanism involves the exchange of oxygen from the lat-
tices, the activity is very much dependent on the MnÀO bond
strength in a MnOx catalyst. This is the reason that, in our case,
100 100.00
[a] Reaction conditions: catalyst ZrOx(1%)–MnCO3, amount the catalyst
500 mg, oxygen flow rate 20 mLminÀ1, benzyl alcohol 2 mmol, toluene
10 mL, and reaction time 5 min.
The specific activity and TOF of the catalyst were calculated
and compared (Table 6) with the catalysts reported previously
for the oxidation of benzyl alcohol to benzaldehyde.[16a,21,25]
ZrOx(1%)–MnCO3 showed 48 mmolgÀ1 hÀ1 as the highest spe-
cific activity and a TOF of 591.48 hÀ1 for benzyl alcohol oxida-
tion. The specific activity for the oxidation of benzy alcohol
using a copper/copper oxide nanoparticles supported on SBA-
15 reported (Cruz et al.)[25a] in the aqueous phase
(43.8 mmolgÀ1 hÀ1, 73% conversion and 54% selectivity after
30 min) is significantly lower than the specific activity obtained
for ZrOx(1%)–MnCO3 [48 mmolgÀ1 hÀ1 for ZrOx(1%)–MnCO3
with complete conversion and <99 selectivity in ca. 5 min] in
the present study. Undecamolybdophosphate PMo11-supported
Table 6. A comparison between our result and earlier reported results in the literature for the oxidation of benzyl alcohol to benzaldehyde.
Catalyst
Conversion
[%]
Selectivity
[%]
T
[8C]
Reaction
time [min]
TOF
[hÀ1
Specific activity
[mmolgÀ1 hÀ1
]
Ref.
]
ZrOx(1%)–MnCO3
Cu/IMIL-SBA-15-G1
PMo11/ZrO2. Al2O3
CuZnO
Ru/CaO–ZrO2
2Au/1CuO–ZnO
RHAC–CoPor
CuMn2 oxide
POM/ZrO2
Au(0)–zeolite
CuMn oxide
100
73
23.7
<99
100
94
97.1
100
93
<95
100
53.9
98.1
61
100
100
99
90.98
88.9
18
57.41
93
96
<99
54
92.3
<99
100
<99
97.7
<99
<99
<99
<99
70.6
100
100
80
90
RT
90
5
30
1440
180
45
60
300
80
240
300
35
300
240
1440
90
120
480
480
120
10
1440
240
80
591.48
66.0
48.00
43.80
39.50
33.33
22.22
18.80
24.27
15.00
12.40
0.63
11.43
11.21
1.23
5.08
4.44
zhis work
[25a]
[25b]
[25c]
[22c]
[25d]
[29]
[30]
[21]
[31a]
[15]
[25e]
[32]
[25f]
[17e]
[17d]
[25g]
[25h]
[33]
[34]
[25i]
[25j]
[25k]
[25l]
[35]
492.25
48.31
224.0
[a]
RT
70
–
176.9
2.83
102
25Æ5
80
102
94
110
80
100
100
80
80
120
90
40
100
RT
[a]
–
11.0
1.54
[a]
Mn–Fe–HD
–
MnO2/GO (10/100)
AgNPs/GOSH+NHPI
5%Ag–MnO2–5%Au
1%Ag–MnO2
CuAl2O4
1.17
5.72
58
<99
<99
98
13.55
35.97
3.33
1.25
[a]
–
–
[a]
CoAl2O4
86.95
69.0
<99
93.82
<99
1.14
[a]
AuPd–6% GO/TiO2
Au-100CeO2@SBA-15
2% Au/HT-3
10% Mn/g-Al2O3-p
EBTPPCC
41.0
270.0
9.61
–
27.0
0.96
0.93
0.87
0.65
0.86
0.50
[a]
[a]
–
–
–
[a]
In2S3
41.5
73.0
97
100
94
41.4%
94.3
83.5
82
99
<99
hn
80
50
80
80
80
240
1200
180
900
720
1440
Au/1%CuO–3%MCM
CoPc
cell–Pd(0)
MnOx/MCM-41
Pd(TOP)/MB-H2O2
7.40
194.0
22.22
[a]
–
[36]
0.22
0.026
0.30
[25m]
[16a]
[37]
[a]
–
28.7
36.15
[a] Calculation could not be carried out, owing to insufficient data provided in the literature article.
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the ZrOx–MnCO3-based catalyst is more active where the sur-
face Mn is oxidized to g-MnOOH by molecular oxygen and acts
more as a catalytic site than ZrOx–Mn2O3.
Table 7. Selective oxidation of benzyl alcohol and its derivatives into cor-
responding aldehydes catalyzed by the ZrOx(1%)–MnCO3 catalyst.[a]
Reaction Reactant
no.
Product
Time Conversion Selectivity
The catalyst reusability is an important parameter used to
define the significance of the catalyst from commercial point
of view. In order to evaluate the re-usability and the stability,
the ZrOx(1%)–MnCO3 was subjected to the oxidation reaction,
separated from the reaction mixture by centrifugation, washed
with toluene several times and dried at 1008C for 4 h to avoid
contamination with reactant or product of the previous reac-
tion and re-used up to six times, yielding the same conversion
product and selectivity. However, there is an only a slight re-
duction in catalytic performance (Figure 9).
[min] [%]
[%]
1
2
5
5
100
100
>99
>99
>99
3
5
100
4
5
5
5
100
100
>99
>99
6
7
6
6
100
100
>99
>99
Figure 9. Graphical illustration of conversion product and selectivity ob-
tained upon catalyst reuse.
2.3. Catalytic Performance on Substituted Benzyl Alcohol
Detailed catalytic parameters for the conversion of benzyl alco-
hol revealed that the best results obtained were achieved by
using ZrOx(1%)–MnCO3 after calcination at 3008C, 2 mmol
benzyl alcohol, 0.5 g catalyst, 10 mL toluene, 20 mLminÀ1
oxygen flow rate, and 1008C reaction temperature. The oxida-
tion of the other substituted alcohols (benzylic alcohols or cit-
ronellol) with substituents like 4-CH3, 3-CH3, 2-CH3, 4-OCH3, 3-
OCH3, 2,3,4-TriOMe, 4-Br, 4-Ph, 4-Cl, 2-Cl, 2,4-DiCl, 4-NO2, 3-NO2,
2-NO2, 4-OH, 4-C(CH3)3, 4-CF3, 4-F, and 2,3,4,5,6-pentafluoro
groups (Table 7) were tested using the above-mentioned con-
ditions. All benzylic alcohols and citronellol are completely
converted to corresponding aldehydes. A >99% selectivity
toward aldehydes is achieved in all reactions, and no other
products are found. Benzyl alcohols containing electron-donat-
ing groups were observed to be the most reactive and exhibit-
ed shorter oxidation times, whereas benzylic alcohol substitut-
ed with electron-withdrawing groups was less reactive.[38] The
increase in the catalytic performance for with electron-donat-
ing groups is probably caused by the adequate electron densi-
ty at the active center compared to substrates containing elec-
tron-withdrawing groups. For instance, benzyl alcohol with
electron-donating groups in the para positions (4-methoxyl-
benzyl alcohol) completely converted into the corresponding
aldehyde after 5 min (Table 7, entry 3). Conversely, para-nitro-
8
9
6
6
100
100
>99
>99
10
11
6
7
100
100
>99
>99
12
7
100
>99
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hyde within 7 min (Table 7, entry 12), whereas meta- and ortho-
nitrobenzyl alcohol were completely converted into the corre-
sponding aldehydes after 8 and 13 min, respectively (Table 7,
entries 16 and 18). Additionally, steric hindrance is an essential
factor affecting the catalytic activity for the oxidation reactions.
Thus, bulky groups such as 2,3,4-TriOMe, ÀC(CH3)3, ÀCF3, 2,4-
DiCl, and 2,3,4,5,6-pentafluoro attached to the phenyl ring re-
duced the oxidation rate and required longer reaction times.
This may be attributed to steric hindrance that impedes oxida-
tion of the bulky substituents of benzylic alcohols (Table 7, en-
tries 8, 14, 15, 19, and 20). Usually, the oxidation of aliphatic al-
cohols is much more difficult than that of benzylic alco-
hols.[25d,40] As an example, citronellol, an aliphatic alcohol, ex-
hibited complete conversion in longer reaction time (90 min)
compared to benzylic alcohols, which could be attributed to
the absence of conjugation in the b-position of the hydroxyl
group (Table 7, entry 21).
Table 7. (Continued)
Reaction Reactant
no.
Product
Time Conversion Selectivity
[min] [%]
[%]
13
14
7
8
100
100
>99
>99
15
16
8
8
100
100
>99
>99
3. Conclusions
We have reported the synthesis of ZrOx-(x%)–MnCO3 (where
x=1–7%) and ZrOx-(x%)–Mn2O3 (x=1–7%) catalysts for the
aerobic oxidation of alcohols. To evaluate the catalytic activities
of the as-prepared catalysts, a comparative study of catalytic
efficiency between ZrOx-(x%)–MnCO3 (x=1–7%) and ZrOx-
(x%)–Mn2O3 (x=1–7%) oxide along with the effect of zirconia
on the catalytic performance was carried out. During this
study, it was revealed that ZrOx-(1%)–MnCO3 displayed superi-
or performance as compared to its oxide counterpart. Further-
more, it was ascertained that the presence of zirconia enhan-
ces the catalytic performance of the catalyst, which yielded
100% conversion to benzaldehyde during the oxidation of
benzyl alcohol. The reaction time for this conversion is approxi-
17
18
19
9
13
14
100
100
100
>99
>99
>99
mately 5 min with a specific activity of 48.00 mmolgÀ1 hÀ1
;
these values (best to our knowledge) are among the best for
any catalyst reported for the catalytic oxidation of benzyl alco-
hol. Moreover, the use of molecular O2 as a source of oxygen
makes this catalyst an environmentally friendly catalyst for
liquid-phase oxidation of benzylic alcohols. The ZrOx-(1%)–
MnCO3 catalyst was also tested for various substituted benzylic
alcohols, and it is found that 100% conversion products (i.e.
corresponding aldehydes) are formed; however, the reaction
time varies based on the nature of substituent present on the
aromatic ring. Notably, the oxidation of benzylic alcohols that
contain electron-withdrawing and bulky groups required
longer reaction times than those containing electron-donating
groups to the corresponding aldehydes. Furthermore, catalytic
performance towards the oxidation of aliphatic alcohol
showed that the synthesized catalysts displayed lower conver-
sions for the aliphatic alcohols, which may be attributed to ab-
sence of conjugation. These results suggest that the as-pre-
pared catalysts are selective towards the aromatic alcohols.
Moreover, the prepared catalyst possesses numerous merits in-
cluding high conversions, specific activities, and selectivity to-
wards aromatic alcohols and a very short reaction time.
20
21
17
90
100
100
>99
>99
[a] Reaction conditions: calcination temperature 3008C, amount the cata-
lyst 500 mg, oxygen flow rate 20 mLminÀ, alcohol 2 mmol, toluene
10 mL, and reaction temperature 1008C.
benzyl alcohol, containing a para-electron-withdrawing group,
required a longer reaction time (7 min) (Table 7, entry 12). On
the other hand, it was reported that alcohols with substituents
at the para-position exhibited a higher conversion than alco-
hols substituted in meta or ortho positions, because para sub-
stituents have lower steric hindrance.[39] For example, para-ni-
trobenzyl alcohol was fully oxidized to the corresponding alde-
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[21] S. Farhadi, M. Zaidi, Appl. Catal. A 2009, 354, 119–126.
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Keywords: benzyl alcohol oxidation · manganese carbonate ·
manganese oxide · mixed metal oxides · zirconia
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Published online on && &&, 2016
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These are not the final page numbers! ÞÞ

FULL PAPERS
M. E. Assal, M. Kuniyil, M. Khan,
A. Al-Warthan, M. R. H. Siddiqui,
W. Tremel, M. Nawaz Tahir,* S. F. Adil*
A designed synthesis of catalysts with
compositions ZrOx(x%)–MnCO3 (where
x=1–7) or ZrOx(x%)–Mn2O3, followed
by their comparative catalytic per-
formance for the oxidation of benzylic
alcohols to corresponding aldehydes
using molecular oxygen, is presented.
The catalyst with composition
&& – &&
Synthesis and Comparative Catalytic
Study of Zirconia–MnCO3 or –Mn2O3
for the Oxidation of Benzylic Alcohols
ZrOx(1%)–MnCO3 pre-calcined at 3008C
offers high surface area, cyclability, ther-
mal stability, and an excellent specific
catalytic activity with complete conver-
sion of benzyl alcohol to benzaldehyde
in about 5 min. The catalyst also shows
selectivity for the oxidation of aromatic
over aliphatic alcohols.
&
ChemistryOpen 2016, 00, 0 – 0
www.chemistryopen.org
10
ꢀ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!