Cu nanoclusters supported on nanocrystalline SiO2-MnO 2: A bifunctional catalyst for the one-step conversion of glycerol to acrylic acid

Article abstract of DOI:10.1039/c4cc03842h

A material consisting of highly dispersed Cu nanoclusters anchored on nanocrystalline SiO₂-MnO₂ has been prepared, and was found to act as a bifunctional catalyst for the one- step conversion of glycerol to acrylic acid using H₂O₂. Under optimized conditions a glycerol conversion of 77.1%, with 74.7% selectivity for acrylic acid, was achieved after 30 h reaction time. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc03842h

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Cu nanoclusters supported on nanocrystalline
SiO2–MnO2: a bifunctional catalyst for the
one-step conversion of glycerol to acrylic acid†
Cite this: Chem. Commun., 2014,
50, 9707
Received 20th May 2014,
Accepted 13th June 2014
Bipul Sarkar,a Chandrashekar Pendem,a L. N. Sivakumar Konathala,a Ritesh Tiwari,a
Takehiko Sasakib and Rajaram Bal*a
DOI: 10.1039/c4cc03842h
www.rsc.org/chemcomm
A material consisting of highly dispersed Cu nanoclusters anchored more than one billion kilograms are produced annually.4 The
on nanocrystalline SiO2–MnO2 has been prepared, and was found entire commercial quantity of acrylic acid is produced via a two-
to act as a bifunctional catalyst for the one- step conversion of stage gas phase oxidation of propylene monomers in air. Here,
glycerol to acrylic acid using H2O2. Under optimized conditions a acrolein is initially formed, followed by the oxidation of the
glycerol conversion of 77.1%, with 74.7% selectivity for acrylic acid, acrolein to acrylic acid. Since the propylene market price is
was achieved after 30 h reaction time.
closely tied to crude oil prices due to the use of crude oil
derivatives as the feedstock for making propylene, use of alter-
As petroleum reserves diminish, researchers are trying to native feedstocks for making acrylic acid is considered to be a
develop new ways to utilize renewable resources as feedstocks viable option. Various catalysts like g-Al2O3, SiO2 and TiO2 have
for the generation of energy and production of chemicals, with been applied5 for the direct conversion of glycerol to acrolein,
the aim of lowering CO2 emissions and fossil-fuel dependency.1 but in most cases the product yield is very low. Metal oxides
Much attention has been focused on developing green catalytic from the Group II and Group III transition series, e.g. Nb2O5,
processes, in order to convert bio-renewable feedstocks into WO3/ZrO2, metal phosphates, molecular sieves like SAPOs and
commodity chemicals and clean fuels. Glycerol is recognized as zeolites have also been used for the conversion of glycerol to
one of the most promising building blocks for the synthesis of acrolein.6 There are few reports7 where the two-step production
fine chemicals from renewable sources.2 It is obtained as a of acrylic acid from glycerol has been presented. Shima et al.7a
by-product of different processes, like soap manufacture, fatty described a two-step process, which includes the dehydration of
acid production, fatty ester production, microbial fermenta- an aqueous solution of glycerol over an alumina base catalyst, to
tion3 and transesterification, or the production of biodiesel. get an acrylic acid yield of B60%. Although a single oxidation
In order to improve the biofuel economy and put this waste step conversion of glycerol to acrylic acid was reported by Jean-
stream to good use, it is necessary to develop new catalytic Luc Dubois et al.7c using molecular oxygen over a plate heat
routes to convert glycerol into value-added chemicals. There- exchanger at 250 1C to 350 1C, the yield was very low. Recently,
fore, the development of new applications of glycerol will boost Trevisanut et al. and Ueda et al. reported the single step conver-
the entire glycerol industry. The single-step production of sion of glycerol to acrylic acid, again with a very poor yield.8
acrylic acid from glycerol is one of the most promising options. Herein, we present the single-step conversion of glycerol to
Acrylic acid is a highly important chemical, and almost acrylic acid over Cu nanoclusters supported on nanocrystalline
4.4 million metric tons per annum are produced as part of the SiO2–MnO2. The hydrothermally prepared Cu/SiO2–MnO2
global petrochemical business, with an average revenue of nearly catalyst achieves a glycerol conversion of 77.1% with 74.7%
$7 billion per year as of 2011. Acrylic acid is largely employed by acrylic acid selectivity, and a TOF (turnover frequency) value of
the chemical industry for the production of super-absorbers, 32.9 hÀ1 was achieved. To the best of our knowledge, there is no
polymers, adhesives, paints, plastics, rubber, detergents, etc. and report on the single-step conversion of glycerol to acrylic acid
with such high conversion and selectivity.
a Catalytic Conversion & Processes Division, CSIR-Indian Institute of Petroleum,
Dehradun 248005, India. E-mail: raja@iip.res.in; Fax: +91 135 2660202;
Tel: +91 135 2525797
The Cu nanoclusters supported on nanocrystalline SiO2–MnO2
catalyst was prepared by modifying our own preparation method
(in ESI†).9 The amount of Cu present in the catalyst was estimated
by inductively coupled plasma atomic emission spectra (ICP-AES).
It was found that 0.9 wt% Cu was present in the catalyst, and so
the catalyst was denoted as 0.9%Cu/SiO2–MnO2. The powder X-ray
b Department of Complexity Science and Engineering, The University of Tokyo,
Kashiwanoha, Kashiwa-shi, Chiba 277-8561, Japan
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cc03842h
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nanoclusters of Cu2O. Furthermore, the 2p3/2 binding energy value
of Mn was observed to be 642.2 eV, along with a separation of
11.4 eV between 2p3/2 and 2p1/2, which indicates the presence of
Mn4+ within the catalyst.12,13 Fig. S4 (ESI†) shows the TPR profile
com
of 1%Cu/SiO2–MnO2
(prepared by an impregnation method)
and the 0.9%Cu/SiO2–MnO2 catalyst. Three reduction peaks were
observed at 236 1C, 338 1C and 426 1C for 0.9%Cu/SiO2–MnO2.
However, the reduction profile of 1%Cu/SiO2–MnO2com also shows
three characteristic reduction peaks at 406 1C, 376 1C and 443 1C.
We believe that these three peaks are due to the following:
(i) reduction of the Cu species (green line), (ii) reduction of
Mn4+ to Mn3+ (red line), and (iii) reduction of Mn3+ to Mn2+
(blue line), as suggested by the literature.14 It was also observed
that the peaks shifted to lower values for the 0.9%Cu/SiO2–MnO2
catalyst compared to 1%Cu/SiO2–MnO2com. It has to be noted
that, due to the presence of nanocrystalline MnO2 particles in
0.9%Cu/SiO2–MnO2, the peaks for Mn4+ and Mn2+ appeared at
lower values (338 1C and 426 1C) compared to the values for
commercial MnO2 (376 1C and 443 1C). The peak due to Cu
Fig. 1 (A) X-ray diffraction patterns for commercial CuO, Cu2O, MnO2
and the prepared 0.9%Cu/SiO2–MnO2. (B) TEM images of the prepared
0.9%Cu–SiO2–MnO2 at (a) low magnification and (b) high magnification.
diffraction (XRD) pattern of the 0.9%Cu/SiO2–MnO2 catalysts is appears for 406 1C in the 1%Cu/SiO2–MnO2com catalyst, whereas
shown in Fig. 1A. The standard diffraction patterns for Cu2O and the value shifted to 236 1C for 0.9%Cu/SiO2–MnO2. We attri-
CuO are also shown for reference. All peaks for 0.9%Cu/SiO2–MnO2 buted this peak to the reduction of very small copper oxide
(Fig. 1A) are attributed to those of the crystalline a-MnO2,10 which is nanoclusters.15 It is easier to reduce small copper oxides rather
in accordance with the JCPDS card, No. 44-0141. No XRD peaks than the larger particles present in the bulk, due to the diffusion
attributed to either metallic Cu or its oxides were observed for the limitation.16 Therefore, the size of the copper oxide particles is
sample of 0.9%Cu/SiO2–MnO2, which indicates the presence of very smaller in 0.9%Cu/SiO2–MnO2 compared to the bulk 1%Cu/SiO2–
small Cu species over the SiO2–MnO2 oxide surface. The morpho- MnO2com. Commercial MnO2 does not show any acidity, but SiO2
logy of the 0.9%Cu/SiO2–MnO2 sample was determined by scan- shows weak acid sites (Fig. S5A–C; ESI†); however, the as-prepared
ning electron microscopy (SEM) and is shown in Fig. S1B, ESI.† The Cu/SiO2–MnO2 catalyst shows a moderate acidity of 149 mmol gÀ1
.
SEM image of the commercial 1%Cu/SiO2–MnO2 is also shown in We believe that the enhancement of surface acidity is mainly due to
Fig. S1A (ESI†). It was found that its morphology is totally different the presence of nanocrystalline SiO2, along with hexagonal MnO2
from that of the hydrothermally prepared 0.9%Cu/SiO2–MnO2 (as confirmed by TEM analysis, shown in Fig. S2, ESI†). Earlier
catalyst (Fig. S1B, ESI†). The SEM image shows that the particle reports also revealed that hexagonal MnO2 is responsible for the
sizes are almost uniform. The presence of Cu on the SiO2–MnO2 generation of acid sites.17 Therefore, the total acidity arises due to
support was confirmed by EDAX (energy dispersive analysis by the presence of SiO2 and hexagonal MnO2 in the catalyst. The NH3-
X-ray) analysis and the result is shown in Fig. S1C (ESI†). Elemental TPD pattern of the Cu/SiO2–MnO2 (Fig. S4, ESI†) shows a broad
mapping of Cu was also performed, and it was found that Cu was peak spanning the temperatures 200–320 1C. Deconvolution of the
homogeneously distributed over the SiO2–MnO2 support (Fig. S1D, peak shows the presence of two peaks. We assume that the first
ESI†). TEM images of the 0.9%Cu/SiO2–MnO2 catalyst are shown in peak is due to the presence of SiO2 and the second peak is due to
Fig. 1B. The average particle sizes of the supported catalyst were the presence of hexagonal MnO2 in the catalyst. It is very clear from
found to be between 25 and 50 nm. No Cu particles were imaged the TPD spectra that the second peak is bigger than the first peak,
by HRTEM, indicating the presence of very small Cu clusters. which clearly indicates that the enhancement of the acidic sites is
However, lattice fringes with a d-spacing of 3.1 Å corresponding mainly due to the formation of hexagonal MnO2. The Cu-XANES
to the (110) lattice plane of hexagonal a-MnO2 are shown in (X-ray absorption near edge structure) spectra of the fresh and
Fig. 1Bb and Fig. S2, (ESI†). The BET surface area as measured spent Cu/SiO2–MnO2 catalysts are shown in Fig. S6 (ESI†); for
by N2 adsorption for the 0.9%Cu/SiO2–MnO2 was 78 m2 gÀ1
.
comparison, the XANES spectra of Cu foil, commercial Cu2O and
Fig. S3B (ESI†) shows the Cu2p binding energy (BE) peaks. CuO are also presented. It is well known that Cu1+ compounds
3/2
The deconvolution of the peak shows the presence of two different exhibit two peaks due to the transition of 1s - 4px,y at 8984 eV and
types of Cu species. The BE peak at 934.3 eV shows the presence of 1s - 4pz at 8995 eV.18 Metallic Cu and Cu1+ have no hole in the 3d
Cu2+ species and the peak at 932.4 eV shows the presence of either orbital, but Cu2+ compounds are in a d9 configuration, showing a
Cu1+ or Cu0, as there is no difference in the Cu1+ and Cu0.11 weak pre-edge peak that represents the quadrupole allowed 1s -
However, from EXAFS (extended X-ray absorption fine structure) 3d transition appearing below the edge. The XANES spectra of the
analysis (discussed later) we could not detect the presence of 0.9%Cu/SiO2–MnO2 catalyst are not typical of Cu2O, but are more
metallic Cu, so the catalyst contains Cu1+ rather than Cu0. We likely to represent a mixture of Cu2O and CuO.
also noticed that the atomic ratio of Cu1+ to Cu2+ present in the
surface is 9 : 1. Therefore, the surface contains mostly very small obtained by Cu K-edge EXAFS analysis. K3-weighted Fourier
Detailed structural parameters of the Cu species were
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Transformation of Cu K-edge EXAFS spectra are shown in Fig. S7
(ESI†). The curve fitting results are summarized in Table S1 (ESI†).
The spectra cannot be fitted as either pure CuO crystals or Cu2O
crystals; they are small crystals of Cu2O with dispersed CuO. The
EXAFS spectra of the fresh (Fig. S7A, ESI†) and spent (Fig. S7B, ESI†)
0.9%Cu/SiO2–MnO2 catalyst show the presence of small crystals of
Cu2O with dispersed CuO. For the fresh catalyst, the Cu–O bond
length of 1.954 Æ 0.152 Å with CN 1.9 Æ 0.7 indicates the presence
of Cu2+ species. The copper oxide nanoclusters over SiO2–MnO2
were durable under a glycerol oxydehydration reaction. The struc-
ture parameters determined by Cu K-edge EXAFS for 0.9%Cu/SiO2–
MnO2 after the glycerol oxydehydration indicate that there is no
Scheme 1 Probable mechanistic pathway for the conversion of glycerol
to acrylic acid.
significant change in the structure of the nanoclusters after 30 h Therefore, the presence of Cu nanoclusters and the presence of
reaction time (spent catalyst, Table S1, ESI†). The Cu–O bond acid sites (due to SiO2 and MnO2) are the two key factors for the
length of 1.951 Æ 0.113 Å with CN of 1.7 Æ 0.6 shows that the size oxydehydration of glycerol. A probable mechanistic pathway for
of the nanoclusters is almost the same, even after the reaction. the formation of acrylic acid is shown in Scheme 1. The catalyst
Furthermore, EXAFS analysis confirms that there were no Cu–Cu behaves as a bifunctional catalyst, where the SiO2 and the
bonds present in the catalyst. This clearly indicates that Cu0 hexagonal MnO2 sites act as acidic sites responsible for the
(metallic copper) is not present in our catalyst.
dehydration reaction (glycerol to acrolein), and the Cu+ sites are
The activities of the different Cu-catalysts are shown in responsible for the oxidation reaction (acrolein to acrylic acid).
Table 1. Oxydehydration of glycerol over the 0.9%Cu/SiO2– Based on the results obtained, we believed that the reaction goes
MnO2 catalyst shows acrylic acid and acrolein as the main through at least two intermediates: 3-hydroxypropionaldehyde
products, along with the formation of a small amount of and acrolein. Here, the first step is the dehydration of glycerol
3-hydroxypropionaldehyde. A glycerol conversion of 77.1% with to form 3-hydroxypropionaldehyde over the acid sites of the
74.7% acrylic acid selectivity (entry 4) was achieved over the catalyst. The second step is also a dehydration reaction, this
0.9%Cu/SiO2–MnO2 catalyst after 30 h reaction time, with a time of the 3-hydroxypropionaldehyde to form acrolein. Finally
TOF (turnover frequency) value of 32.9 hÀ1. Commercial MnO2 the acrolein oxidizes to form acrylic acid in the presence of H2O2
(entry 1) gives only 5.5% glycerol conversion, without any (50% in H2O) over the Cu sites. To check the intermediate
formation of acrylic acid. 1%Cu supported on SiO2–MnO2 acrolein formation, we performed the experiment using acrolein
(prepared by physical mixing) also shows negligible selectivity as the substrate, and found that both acrolein conversion and
for acrylic acid (entry 3). MnO2 prepared by a hydrothermal acrylic acid selectivity was 100% within 3 h (entry 6). We believe
method also shows low activity (8.6% conversion and 5.6% that the dehydration step is very slow, but the oxidation step
acrylic acid selectivity, entry 2). Catalysts of Cu supported on is very fast. Initially it takes a long time to form acrylic acid.
MnO2 or SiO2 (entries 7 & 8) also do not show any activity. However, once acrylic acid formation begins, it acts as a
Brønsted acid in the reaction mixture, and the rate of acrylic
acid formation increases. The increase in the H2O2 : glycerol
(Fig. S8, ESI†) molar ratio does not have any substantial effect
on acrylic acid selectivity, as it remains almost the same. A
noticeable increase in glycerol conversion from 49.9% to 77.1%
was observed when the glycerol to H2O2 molar ratio increased
from 2.5 to 5; whereas the conversion decreased from 77.1% to
70.1% with a further increase in the glycerol to H2O2 molar ratio,
to 7.5. It seems that with an increase in the H2O2 molar ratio the
decomposition of H2O2 increases, so the glycerol conversion
decreases. The effect of temperature is shown in Fig. 2A. It was
found that the glycerol conversion increases continuously with
temperature but that the selectivity decreases above 70 1C.
We believe that above 70 1C, the decomposition of H2O2 takes
place, and so the selectivity decreases. The effect of reaction time
Table 1 Activity of the Cu/SiO2–MnO2 catalysta
Selectivity (C %)
d
Sl.
wG
TOF
(hÀ1
AA
3HPA AC OT
no. Catalyst
(%)
)
Eo
com
1
2
3
MnO2
MnO2
5.5
8.6
9.7
0.1 2.8
5.6 2.5
3.4 3.0
3.1 B94.0
1.8 B90.1
—
—
—
B0
0.09
1%Cu/SiO2–
3.5
10.4
11.6
—
90.1
0.06
11.52
10.52
20.0
com
MnO2
4
5
6
0.9%Cu/SiO2– 77.1 74.7 6.8
MnO2
8.1 32.9
10.6 30.7
—
0.9%Cu/SiO2– 72.5 72.0 5.8
b
MnO2
0.9%Cu/SiO2– 100
100
—
—
c
MnO2
7
8
1%Cu/MnO2
1%Cu/SiO2
8.3
7.7
0
0
4.1
3.7
4.8
12.9
91.1
83.4
—
—
—
—
a
Catalyst: 0.046 g; glycerol: 0.92 g in 10 mL CH3CN, temperature: 70 1C; is shown in Fig. 2B. It was observed that both the glycerol
substrate (glycerol): 50% H2O2 (1 : 5, molar ratio). com commercial oxide.
conversion and acrylic acid selectivity continuously increased
when the reaction time was increased from 10 to 30 h, but that
b
c
After 4 successive runs under the same conditions. With acrolein as
d
the substrate after 3 h reaction time. In mol%. TOF: moles of acrylic
acid produced per moles of Cu per unit time; Eo: H2O2 efficiency the selectivity decreased after 30 h.
calculated by (100 Â moles of acrylic acid formed)/total moles of
The reusability of the 0.9%Cu/SiO2–MnO2 catalyst was stu-
died without any regeneration. The catalyst showed almost the
same activity even after four successive runs under the same
H2O2 added; C balance was carried out for most of the experiments
and the value was within 98–102%. wG = conversion of glycerol (C%),
AA = acrylic acid, AC = acrolein and OT = others.
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B.S. acknowledges the University Grant Commission (UGC),
India for the fellowship. R. B. thanks CSIR, New Delhi for the
financial support in the form of the 12 FFYP (CSC-0117 and
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support. The authors thank the Analytical Science Division, Indian
Institute of Petroleum for analytical services. The XAFS measure-
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9710 | Chem. Commun., 2014, 50, 9707--9710
This journal is ©The Royal Society of Chemistry 2014