A used battery supported Ag catalyst for efficient oxidation of alcohols and carbon oxide

Article abstract of DOI:10.1039/c4ra01786b

Green-recycling of existing waste is significant in green chemistry. Herein, we show that used batteries can be applied as cheap and viable supports for silver catalysts and used for gas-phase alcohol and CO oxidation. The catalyst Ag-9/Mn₃O₄ (Ag-loading: 9 wt%) is efficient for oxidizing acyclic, benzylic and polynary alcohols. For instance, Ag-9/Mn ₃O₄ can oxidize cyclohexanol into cyclohexanone with 86% conversion and 88% selectivity. In addition, a special synergistic effect between the Ag and the Mn₃O₄ was revealed, which is particularly valuable for the design of composite catalysts. The facile strategy employed in this study can be expanded to the fabrication of other catalysts with the green reuse of used batteries as supports.

Full text of DOI:10.1039/c4ra01786b

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A used battery supported Ag catalyst for efficient
oxidation of alcohols and carbon oxide†
Cite this: RSC Adv., 2014, 4, 25384
*
Junjie Mao, Guofeng Zhao, Dingsheng Wang and Yadong Li
Green-recycling of existing waste is significant in green chemistry. Herein, we show that used batteries can
be applied as cheap and viable supports for silver catalysts and used for gas-phase alcohol and CO
oxidation. The catalyst Ag-9/Mn3O4 (Ag-loading: 9 wt%) is efficient for oxidizing acyclic, benzylic and
polynary alcohols. For instance, Ag-9/Mn3O4 can oxidize cyclohexanol into cyclohexanone with 86%
conversion and 88% selectivity. In addition, a special synergistic effect between the Ag and the Mn3O4
was revealed, which is particularly valuable for the design of composite catalysts. The facile strategy
employed in this study can be expanded to the fabrication of other catalysts with the green reuse of
used batteries as supports.
Received 28th February 2014
Accepted 22nd April 2014
DOI: 10.1039/c4ra01786b
www.rsc.org/advances
alcohols owing to their higher selectivity, resistance to
leaching, and lower cost.11,12 Moreover, it is well recognized
Introduction
that the selection of a suitable scaffold to support the Ag NPs
has a critical role in determining the catalytic activity and
selectivity. Thus, developing novel recycled materials as
supports for Ag NPs for highly efficient and selective alcohol
oxidation could be important.
Among the various metal oxides that can support Ag NPs
as catalysts, manganese oxide has been widely used as a
stoichiometric oxidant for hydrocarbons or alcohols as well
as a support or promoter for metal or metal oxide cata-
lysts.13–15 For example, Li et al. reported a low-temperature
carbon monoxide (CO) oxidation reaction using Ag/MnO2 as
a catalyst,16 in which the MnO2 enhanced the catalytic ability
of the Ag NPs signicantly. However, to the best of our
knowledge, manganese oxide supported Ag NPs for gas-phase
catalytic alcohol oxidation hasn't been thoroughly
investigated.
Here, we have developed a novel catalyst using a used
battery to support Ag NPs. The in situ reduction results in an
even dispersion of Ag NPs on the MnO2 support. The as-
prepared Ag/Mn3O4 catalyst (transformation from Ag/MnO2,
using weight hourly space velocity (WHSV) ¼ 20 hꢀ1 and
molar ratio of O2 to alcoholic hydroxyl (O2/ol) ¼ 0.6 at 280 ꢁC)
is an efficient catalyst for the transformation of a diverse
range of alcohols to the corresponding carbonyl compounds
under solvent-free conditions. For cyclohexanol, the
catalyst Ag-9/Mn3O4 (Ag-loading: 9 wt%) delivered a high
conversion of 86% and a cyclohexanone selectivity of 88% at
a temperature of 280 ꢁC. Moreover, the results based on
comparable catalysts and characterization indicate that the
formation of a Ag–support interface and the interactions
therein contribute to the high activity for alcohol and CO
oxidation.
With ever-increasing serious environmental and energy issues,
sustainable harvesting that begins with extracting resources
from earth using cost-effective techniques which minimize
negative impacts is becoming more and more important.1
Taking battery recycling as an example, the global per capita per
annum battery consumption of 330 g can leave a massive
residue of untreated battery waste, which not only pollutes the
environment but also leads to great energy waste.2,3 Thus,
establishing a battery-recycle-system is of signicance to
sustainable development.
The selective oxidation of alcohols to the corresponding
carbonyl compounds is of interest due to its fundamental
importance for laboratory and industrial manufacturing.4–8
Traditionally, alcohols are oxidized in solution using a variety of
metal based homogeneous catalysts. However, these methods
are usually associated with low reaction rates, difficult catalyst–
solvent separation, and serious toxicity issues, which hamper
their wide applications. Thus, a highly selective and recyclable
“green” catalyst for solvent free alcohol oxidation is required for
the large scale synthesis of carbonyl compounds.9,10 Moreover,
the development of reusable catalysts for the gas-phase aerobic
oxidation of alcohols is also valuable to the elds of organic
synthesis and green chemistry.
Recently, silver nanoparticle (Ag NP) catalysts supported
on activated carbon, oxides, or polymers have been proved to
be advantageous over noble metal NPs for the oxidation of
Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China. E-mail:
wangdingsheng@tsinghua.edu.cn
† Electronic supplementary information (ESI) available: Experimental details,
XRD, TEM, N2 adsorption–desorption isotherms and pore size distribution
curves of samples; oxidation of various alcohols over the catalysts. See DOI:
10.1039/c4ra01786b
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Results and discussion
The original powder (MnOx) mixture from used batteries was
rstly characterized by using XRD (Fig. S1†). The XRD patterns
of the MnOx indicated the presence of manganese oxide, zinc
hydroxide, and iron compounds, which was further conrmed
by ICP analysis. Aer the purication of MnOx, pure MnO2 was
successfully obtained (JCPDS 24-0735). Moreover, other crys-
tallized impurities, such as zinc and iron compounds in the
original powder, were not observed. The structure of the recy-
cled MnO2 was characterized by TEM (Fig. S2†).
Fig. 1A shows the XRD patterns of the Ag/MnO2, which
indicate that the MnO2 phase was unchanged aer the Ag
deposition. Moreover, metallic silver diffraction peaks were not
observed for the catalysts with low Ag-loading (3–5%), but then
appeared in the catalysts with high Ag-loading from 7% to 11%.
The Ag/MnO2 catalysts were then characterized by TEM
(Fig. S3†), and the Ag NPs were found to be highly dispersed on
the surface of the MnO2 support. Taking the Ag-9/MnO2 catalyst
as an example, the Ag particle size is between 6–8 nm according
to the particle size distribution (PSD) histogram, with a mean
size of 6.99 nm (Fig. 2A inset). The d-spacing for adjacent lattice
Fig. 2 (A) TEM image of Ag-9/MnO2 (inset is the Ag particle size
distribution), (B) HRTEM images of Ag-9/MnO2 (inset is the enlarged
HRTEM image of Ag nanocrystals), (C) HAADF-STEM image of Ag-9/
MnO2 nanocrystals, and (D) HAADF-STEM image together with the
EDX mapping of Ag-9/MnO2 nanocrystals.
while the deposition of Ag NPs onto the MnO2 surface slightly
reduced the surface area of MnO2 to 35.53 m2 gꢀ1
.
˚
fringes measured from several different regions was 2.36 A
(Fig. 2B), which is close to that of the (111) planes of a face-
centered cubic (fcc) Ag crystal. The high-angle annular dark-eld
scanning TEM (HAADF-STEM) image (Fig. 2C) further conrmed
that the Ag NPs are well-dispersed on the MnO2 surface. The
energy-dispersive X-ray spectroscopy (EDS) mapping prole
(Fig. 2D) further manifested the homogeneous distribution of
Ag, Mn, and O in the Ag-9/MnO2 nanocrystals. The above results
indicate that the employed in situ reduction method is simple
and facile for the fabrication of the Ag/MnO2 catalyst. N2
adsorption–desorption isotherms (Fig. S4†) indicated the mes-
oporous structure of all the samples, with Barrett–Joyner–
Halenda (BJH) pore sizes of 12.88 nm for MnOx, 32.60 nm for
MnO2, and 21.67 nm for Ag-9/MnO2 (Fig. S4†, insets). BET
measurements indicated that the MnOx and MnO2 have BET
surface areas of 22.98 m2 gꢀ1 and 41.56 m2 gꢀ1, respectively,
Cyclohexanone is an important raw chemical for the organic
syntheses of caprolactam (for nylon 6) and adipic acid, widely
used in the leather industry, and in degreasing agents and
detergents.17,18 Many efforts have been devoted to the conver-
sion of cyclohexanol to cyclohexanone in liquid phase.19,20
However, gas-phase oxidation as a promising route for further
industrial application has been less investigated.21 Here, the
selective gas-phase oxidation of cyclohexanol over the Ag/Mn3O4
catalyst was thoroughly investigated.
In recent years, silver based heterogeneous catalysts have
been developed for alcohol oxidation. Therefore, an initial
experiment was carried out over the catalyst Ag/ZrO2, and the
cyclohexanol conversion was 53% with a high WHSV of 20 hꢀ1
at 280 ꢁC (entry 3 in Table 1), but with a selectivity of 62%, which
has restricted the manufacture of highly pure cyclohexanone
using this route. The relatively efficient alcohol oxidation using
Ag-based catalysts prompted us to develop new systems to
further enhance the catalytic performance of alcohol oxidation.
Manganese oxide has a superior ability to activate and supply
oxygen, and has been extensively investigated as an excellent
support. A series of MnO2 catalysts with different silver loadings
were synthesized and employed for the oxidation of alcohols.
Table 1 Oxidation of cyclohexanol over the various pre-activated
catalysts. Calcination temperature: 400 ꢁ
C
Entry Catalysts
TReaction (ꢁC) Conversion (%) Selectivity (%)
1
2
3
4
5
6
7
Ag-9/Al2O3
Ag-9/ZnO
Ag-9/ZrO2
Ag-9/SiO2
Ag-9/TiO2
Ag-9/Fe2O3
280
280
280
280
280
280
27
45
53
22
58
75
86
74
58
62
23
52
60
88
Fig. 1 (A) XRD patterns of the samples before pre-activation treat-
ment: (a) Ag-3/MnO2, (b) Ag-5/MnO2, (c) Ag-7/MnO2, (d) Ag-9/MnO2,
(e) Ag-11/MnO2; (B) XRD patterns of the catalysts after pre-activation:
(a) Mn3O4, (b) Ag-3/Mn3O4, (c) Ag-5/Mn3O4, (d) Ag-7/Mn3O4, (e) Ag-9/
Mn3O4, (f) Ag-11/Mn3O4. Pre-activation conditions: catalysts for the
gas-phase oxidation of cyclohexanol were pre-activated using WHSV
¼ 20 hꢀ1 and O2/ol ¼ 0.6 at 280 ꢁC in advance.
Ag-9/Mn3O4 280
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We rst studied the efficiency of the MnO2-supported silver (HCl : HNO3 of 3 : 1 (vol : vol)). The ICP results of the obtained
catalysts with varying silver loading for solvent-free alcohol organic-product and the acid-washing liquid solution showed
oxidation by molecular oxygen. Of particular signicance is the the absence of Ag and Mn, indicating the thermal stability of the
observation that the catalytic activity of the Ag/MnO2 catalysts Ag-9/Mn3O4 catalyst for alcohol oxidation. Additionally, many
for the gas-phase oxidation of cyclohexanol was enhanced aer reported results show that the catalyst support plays an
pre-activation using WHSV ¼ 20 hꢀ1 and O2/ol ¼ 0.6 at 280 ꢁC. important, sometimes crucial role in determining the catalyst
The XRD results indicated that aer pre-activation, Ag/MnO2 performance. In order to investigate the effect of the support on
was transformed into Ag/Mn3O4 (Fig. 1B). Fig. 3A and B show catalytic performance, we prepared a series of comparable
the cyclohexanol conversion and cyclohexanone selectivity of catalysts on several other supports, including ZrO2, ZnO, Al2O3,
the reaction over the pre-activated Ag/Mn3O4 catalysts with SiO2 TiO2 and Fe2O3. All of the catalysts were evaluated for the
various Ag-loadings at 280 ꢁC using a WHSV of 20 hꢀ1 and O2/ol oxidation of cyclohexanol, with the results listed in Table 1.
of 0.6. Ag-loading has a strong inuence on the catalytic Generally, most catalysts demonstrated a much lower cyclo-
performance. Increasing the Ag-loading from 0 to 7 wt% hexanol conversion, indicating that the Mn3O4 is an effective
signicantly enhanced the conversion from 42% to 82% and the support of the silver catalyst for cyclohexanol oxidation.
selectivity from 45% to 77%. However, further increasing the
The reaction conditions such as the reaction temperature
Ag-loading to 11 wt% caused a slight decrease of both the and WHSV on the performance of Ag-9/Mn3O4 catalyst for the
conversion and the selectivity, maybe due to the increased size selective gas-phase oxidation of cyclohexanol were elaborately
of the Ag NPs. Among all of the pre-activated catalysts, the most investigated, and the results are shown in Fig. 3 and S5.† As
active was the pre-activated Ag-9/Mn3O4, for which the conver- shown in Fig. 3A and B, the as-obtained Mn3O4 and pre-acti-
sion was further increased to 86% with a selectivity of 88%. The vated Ag-9/Mn3O4 catalyst provided quite different conversion
detectable byproducts included cyclohexene, cyclohexane, behavior on changing the reaction temperature. For Mn3O4 in
benzene and small amounts of COx. These catalytic results the gas-phase oxidation of cyclohexanol, increasing the
clearly revealed that the cyclohexanol conversion and cyclo- temperature from 240 ꢁC to 320 ꢁC gave a small increase in
hexanone selectivity over the Ag-9/Mn3O4 catalyst are about two cyclohexanol conversion, which was accompanied by an
times higher than that of Mn3O4 at the same temperature, obvious reduction in selectivity for the target products from
which demonstrates that the Ag embedment modication of 78% to 30%. When silver NPs were supported on the MnO2 to
the Mn3O4 remarkably enhanced the low-temperature activity. form the Ag-9/Mn3O4 catalyst, the increase of reaction temper-
ꢁ
ꢁ
It should be pointed out that, unless otherwise specied, the ature from 240 C to 320 C caused the conversion to increase
reaction results reported in the following parts are all based on from 76% to 92% while the selectivity decreased slightly to 75%.
the pre-activated catalysts. More interestingly, if the raw mate-
In particular, a high selectivity for the formation of cyclo-
rials from waste batteries were simply washed just with hexanone with a conversion of 86% was obtained using the Ag-
ꢁ
drinking water, satisfactory catalytic performance was also 9/Mn3O4 catalyst at 280 C. However, pure Mn3O4 showed very
obtained, with a conversion of 82% and a selectivity of 83% at poor activity for this reaction under the same conditions
280 ꢁC, thus greatly increasing the potential for industrial (conversion: ꢂ42%, selectivity: ꢂ45%, main by-product: cyclo-
ꢀ1
ꢁ
applications and in other elds.
hexene). At 280 C, on increasing the WHSV from 5 to 20 h ,
A key factor that must be considered for heterogeneous the conversion of cyclohexanol was decreased from 92% to 86%
catalysts is the possibility of the active components vaporizing under O2/ol of 0.6 (Fig. S5†). On increasing the WHSV from 20 to
during the reaction, thereby leading to catalyst deactivation or, 30 hꢀ1, the cyclohexanol conversion sharply decreased to 50%
in the worst case, to the formation of an active gas-phase with the selectivity almost unchanged, which indicated that the
homogeneous catalyst. The reactor below the catalyst bed was resistance time was not sufficient for enough reactant and
washed with strong acid composed of HCl and HNO3 product molecules to react with O2 thereby leading to the
decrease in conversion. Considering a balance between high
product yield and low temperature, the optimal operational
parameters are a WHSV of 20 hꢀ1, with an O2/ol of 0.6 at 280 ꢁC.
Table S1† compiles the catalytic results for the oxidation
of a range of structurally different alcohols (such as straight-
chain, benzylic and polynary alcohols) over the pre-activated
Ag-9/Mn3O4. The experimental results show that the catalysis
was inuenced by the structure of the alcohol substrates. It is
apparent that the Ag-9/Mn3O4 catalyst showed a higher
activity for aromatic alcohols than for aliphatic ones. The
pre-activated Ag-9/Mn3O4 could selectively oxidize 1-phenyl-
ethanol to acetophenone with a high conversion of 95% at
Fig. 3 Catalytic performance of the catalysts with various Ag loadings.
(A) Cyclohexanol conversion and (B) cyclohexanone selectivity over
250 ꢁC but only converted 23% of 2-phenylethanol, even at
the higher temperature of 280 ꢁC. This difference may be due
to the specic interactions of this substrate with the active
the pre-activated Ag/Mn3O4 catalysts. (The gas-phase oxidation of
cyclohexanol was performed using O2/ol of 0.6 and WHSV of 20 hꢀ1
.
Catalyst: 0.1 g; quartz sands: 0.2 g (100–150 mesh).)
site. In the oxidation of 1-octanol and 2-octanol, the primary
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linear aliphatic alcohol (1-ol) was oxidized with a conversion
From the fact that the cyclohexanol conversion is very
of 40% and an 87% selectivity to caprylic aldehyde at 270 ꢁC, different over the catalysts with various supports (Ag/ZrO2,
which is almost the same as the performance observed for the Ag/ZnO, Ag/Al2O3, Ag/SiO2, Ag/TiO2 and Ag/Fe2O3), it is
secondary alcohol (2-ol). Additionally, the selective oxidation evident the nature of the support plays a vital role in the
of benzyl alcohol to benzaldehyde is one of the most impor- performance of silver for the present reaction. The XRD
tant organic transformations, attracting enormous interest results have showed that MnO2 in the fresh catalyst was
due to wide applications in the perfumery and food indus- transformed into Mn3O4 by pre-activation during the
tries.9,21–23 The catalytic results reported here indicate that process of cyclohexanol oxidation. Similarly, XPS results
the Ag-9/Mn3O4 catalyst was very reactive for the aliphatic showed that the surface composition of the support was
alcohols. Benzyl alcohol was selectively oxidized to benzal- transformed from MnO2 into Mn3O4, and that the Ag surface
dehyde with a conversion of 92% and a selectivity of 98% at remained almost unchanged (Fig. 5A and B). These results
ꢁ
ꢁ
230 C, which is just 25 C higher than the boiling point of indicate that the support is an active participant for the
ꢁ
benzyl alcohol (205 C). Furthermore, 1,2-propanediol (con- cyclohexanol oxidation. However, for the catalyst with no
taining two hydroxyl groups) also could be oxidized to the silveꢁr loading, the cyclohexanol conversion was only 42% at
target product with a selectivity of 74% at a conversion of 280 C. The results of the control experiments showed that,
ꢁ
54% at 270 C.
for Ag catalysts on inactive supports (ZrO2, ZnO, Al2O3, and
Considerable attention has been paid to CO oxidation over SiO2) as well as the unmodied MnOx catalyst, the catalytic
the past years because of its great signicance in the treatment activity for alcohol oxidation was very low, while for the
of exhaust gas from automobiles and for fuel cell applica- Ag-9/MnOx catalyst, the activity was greatly enhanced. Simi-
tions.16,24 Recently research has focused on precious metal larly for the CO oxidation, Ag-9/MnO2 showed better
catalysts (such as Pt, Au, and Pd), which have high activity and catalytic performance compared to the unmodied MnO2
stability for the oxidation of CO.25–28 Nevertheless, the high cost catalyst. For the Ag–oxide systems, several papers have
and limited availability of these noble metals greatly hamper focused on CO oxidation,16,29 HCHO oxidation30 and gas-
the practical applications and thus activate extensive interest in phase selective oxidation of benzyl alcohol,22 and they all
the development of novel materials with excellent activity for proposed that the Ag–oxide interaction was crucial for the
CO oxidation prepared with inexpensive and more abundant corresponding reactions therein. Based on our control
materials.
experiments and reported results, it is proposed that
Catalytic activity testing for CO oxidation was carried out on strong metal–support interactions (SMSIs) exist between Ag
the as-obtained MnO2, Ag-9/MnOx and Ag-9/MnO2 (Fig. 4). and MnOx. In order to verify this proposal, the used Ag-9/
Clearly, we can see that Ag-9/MnOx and Ag-9/MnO2 are more Mn3O4 was visualized by HRTEM. Interestingly, the
active than the recycled MnO2. The CO conversion reached 80% results showed clear Ag–Mn3O4 interfaces (Fig. 5C and D).
at 106 ꢁC on Ag-9/MnO2, at 159 ꢁC on Ag-MnOx and at 184 ꢁC on Other oxide–metal interfacial or perimeter sites and the
recycled-MnO2, which showed that the presence of the Ag SMSI therein have been known for their high activity in
greatly improved the catalytic activity for CO oxidation. It is various reactions, such as Au–oxide31,32 and Pt–oxide.33–35
noteworthy that, for Ag-9/MnO2 and Ag/MnOx catalysts, the From the above results, it is reasonable to propose that
temperatures required for almost 100% CO conversion are the Ag–Mn3O4 interface and the interactions therein play
150 ꢁC and 180 ꢁC respectively, while for the MnO2 catalyst, the an important or critical role in the oxidation of alcohols
temperature of maximum CO conversion was 240 ꢁC. Our and CO.
results indicate that introducing Ag to MnO2 produces an effi-
cient catalyst for CO oxidation.
Fig. 5 (A) Mn 2p XPS spectra of (a) Ag-9/Mn3O4, (b) Ag-9/MnO2 and
(c) MnO2, (B) Ag 3d XPS spectra of (a) Ag-9/Mn3O4, (b) Ag-9/MnO2 and
Fig. 4 Conversion–temperature curves for CO oxidation over MnO2, (c) Ag NPs, (C) HRTEM image of Ag-9/Mn3O4 catalyst. (D) HRTEM
Ag-9/MnOx and Ag-9/MnO2 catalysts.
image of the region indicated with a box in panel D.
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Conclusions
In summary, the used battery supported silver catalyst was
successfully synthesized and showed a high catalytic perfor-
mance for the gas-phase oxidation of various alcohols and CO.
For cyclohexanol oxidation, Ag-9/Mn3O4 delivered a high
conversion of 86% with a cyclohexanone selectivity of 88% at
280 ꢁC. The high catalytic performance for the oxidation of
alcohols is attributed to the synergistic effects between the Ag
NPs and the Mn3O4 support, while the Ag–MnO2 synergistic
effect is critical for carbon dioxide formation. This study
provides an example of the green-reuse of used batteries for the
oxidation of alcohols and CO. Further investigations into the
industrial application of this catalyst for other oxidation
processes as well as the precise mechanism of the oxidation
process are currently in progress.
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Acknowledgements
This work was supported by the State Key Project of Funda-
mental Research for Nanoscience and Nanotechnology
(2011CB932401, 2011CBA00500, 2012CB224802), the Founda-
tion for the Author of National Excellent Doctoral Dissertation
of P. R. China (201321), Specialized Research Fund for the
Doctoral Program of Higher Education (20130002120013), and
the National Natural Science Foundation of China (21221062,
21131004, 21390393, 21322107).
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