Catalytic conversion of cellulose to hexitols with mesoporous carbon supported Ni-based bimetallic catalysts

Article abstract of DOI:10.1039/c2gc16364k

Robust and highly active Ni-based bimetallic catalysts supported on mesoporous carbon have been developed for catalytic conversion of cellulose to hexitols, over which the maximum hexitol yield reached 59.8%. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2gc16364k

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
Green Chemistry
Cite this: Green Chem., 2012, 14, 614
www.rsc.org/greenchem
COMMUNICATION
Catalytic conversion of cellulose to hexitols with mesoporous carbon
supported Ni-based bimetallic catalysts†
Jifeng Pang,a,b Aiqin Wang,a Mingyuan Zheng,a Yanhua Zhang,a,b Yanqiang Huang,a Xiaowei Chenc and
Tao Zhang*a
Received 31st October 2011, Accepted 20th December 2011
DOI: 10.1039/c2gc16364k
cellulose to hexitols,8a there have been great efforts towards the
Robust and highly active Ni-based bimetallic catalysts sup-
ported on mesoporous carbon have been developed for cata-
enhancement of sorbitol yield.11 Nevertheless, there are few
solid catalysts able to arrive at a hexitol yield exceeding 50%
when microcrystalline cellulose is used as the feedstock. More-
over, the harsh reaction conditions required for the conversion of
cellulose exerts a great challenge on the stability of a solid
catalyst.12
lytic conversion of cellulose to hexitols, over which the
maximum hexitol yield reached 59.8%.
Fossil energy depletion and global warming have stimulated
great interest in the study of biomass conversion due to its
renewable property and low net CO2 emission level. Cellulose is
one of the most abundant sources of biomass, which accounts
for 40–60 wt% of dry lignocellulosic biomass. The plentiful
functional groups (e.g., OH groups) of cellulose make it an ideal
feedstock for sustainable production of chemicals.1 Nevertheless,
the extensive hydrogen-bonding networks and the high crystalli-
nity of cellulose make it resistant to chemical and enzymatic
degradation.2 Great efforts have been devoted to the cellulose
transformation in various ways, such as fermentation with
enzyme,3 photochemical conversion,4 hydrolysis with acid cata-
lysts,5 pyrolysis at high temperatures,6 and etherification with
alcohols.7
One-pot conversion of cellulose with solid catalysts under
hydrothermal conditions is a green and efficient process for the
sustainable production of chemicals, in which several consecu-
tive reactions proceed in one reactor and cooperate well. For
example, when the hydrolysis of cellulose is coupled with the
hydrogenation of glucose over a noble metal catalyst, hexitols
(containing sorbitol and mannitol) are produced as the main
product.8 On the other hand, when the hydrolysis of cellulose is
coupled with hydrogenolysis of sugars over a tungsten-based cat-
alyst, ethylene glycols are yielded predominantly.9
Carbon material is one of the best choices as the catalyst
support due to its excellent stability under hydrothermal con-
ditions and large surface area for dispersing active components.
In particular, mesoporous carbon (MC) materials offer great
advantages over conventional activated carbon (AC) owing to
their well-controlled pore structures in the mesopores which are
favorable to transportation of large molecules.13 Recently, we
found that the MC supported tungsten carbide catalysts exhibited
much superior performance for cellulose conversion into ethy-
lene glycol compared to the AC supported counterparts.9d More-
over, sulfonated MC was more active than the sulfonated AC for
the hydrolysis of cellulose to glucose.5c Similar to our findings,
Fukuoka et al. reported that Ru catalysts supported on mesopor-
ous CMK-3 were water-tolerant and reusable for the catalytic
degradation of cellulose to sugars.14 All these studies demon-
strate that MC is a superior support in the conversion of cellulose
under hydrothermal conditions.
Herein, we report that a series of Ni-based bimetallic catalysts
supported on MC are very active and highly stable for the con-
version of cellulose to hexitols. Typically, Rh-Ni/MC and Ir-Ni/
MC catalysts give a yield of hexitols close to 60% with complete
conversion of cellulose. The modification of nickel with noble
metals improves greatly the stability of catalysts, and 4%Ir-4%
Ni/MC catalysts can be recycled 4 times without any loss of per-
formance. To our knowledge, this is the best performance of het-
erogeneous catalysts ever reported for the cellulose conversion to
hexitols.
Hexitols are not only used as a sweetener in food industry but
also currently considered as an important platform chemical for
the sustainable production of fuels and chemicals.10 Following
the pioneering work of Fukuoka for the one-pot conversion of
aState Key Laboratory of Catalysis, Dalian Institute of Chemical
Physics, Chinese Academy of Sciences, Dalian, 116023, P. R. China.
E-mail: taozhang@dicp.ac.cn; Fax: +86-411-84691570;
Tel: +86-411-84379015
The MC support was prepared by a nanocasting method with
sucrose as the carbon precursor and a commercial silica fume as
the hard template as reported previously.9d,15 As expected, the
MC support presents a high surface area (983 m2 g−1) and a
large pore size (9.4 nm). Interestingly, a loading of 5%Ni or Ni-
based bimetallic components leads to a further increase of the
surface area, up to 1106 m2 g−1 (Table S1†). The preferred tex-
tural properties of the MC support provide a better dispersion of
metal component than AC. As shown in Fig. 1, the Ni particles
are uniformly dispersed on the 20%Ni/MC, and most of the
bGraduate School of Chinese Academy of Sciences, Beijing, 100049,
P. R. China
cDepartamento de Ciencia de los Materiales e Ingeniería Metalúrgica y
Química Inorgánica, Facultad de Ciencias, Universidad de Cádiz,
11510 Puerto Real (Cádiz), Spain
†Electronic supplementary information (ESI) available: General pro-
cedures, catalyst characterization and reaction data are provided. See
DOI: 10.1039/c2gc16364k
614 | Green Chem., 2012, 14, 614–617
This journal is © The Royal Society of Chemistry 2012

View Article Online
many individual particles shows that the molar ratio of Ni to Ir is
not uniform from one particle to another, but most of the par-
ticles are composed of two metals (Fig. S2†), in agreement with
the XRD result (Fig. S1†) that Ni and Ir form alloys in the bi-
metallic catalyst.
The catalytic performances of catalysts were evaluated for
microcrystalline cellulose conversion at 245 °C and 6 MPa H2
for 30 min. As shown in Table 1, the MC supported nickel cata-
lyst gave very good performance. On the 20%Ni/MC catalyst,
the cellulose conversion reached 84.5% and the hexitol yield
was up to 42.1% (Entry 2). This result is even better than that
over the supported Ru catalyst reported in the literature, where
the best yield of hexitols was 39.3% and the cellulose conversion
was 85.5%.8b In contrast, for 20%Ni/AC catalyst, the cellulose
conversion was 61.9% and the hexitol yield was merely 19.7%
(Entry 1). Clearly, the MC supported nickel catalyst is much
more efficient than the AC supported counterpart for cellulose
conversion. Considering the nickel dispersion on the AC is
inferior to that on the MC, we supplemented RANEY® Ni cata-
lyst to the 20%Ni/AC to improve the hydrogenation ability.
However, the mixed catalyst did not show notable improvement
either in the cellulose conversion or in the hexitol yield (Entry
3). Accordingly, one can conclude that the different performance
of Ni/MC and Ni/AC mainly originates from the different nature
of carbon materials.
Fig. 1 TEM images of 20%Ni/AC (A), 20%Ni/MC (B), 1%Ir-5%Ni/
AC (C), and 1%Ir-5%Ni/MC (D).
The promising activity of the Ni/MC catalyst encouraged us to
further investigate its stability during repetitive runs. To facilitate
the catalyst recycling after the reaction, ball-milled cellulose was
used as the feedstock. As shown in Fig. 2A, the hexitol yield
reached 60.8% in the first run with the 20%Ni/MC catalyst. The
catalyst became even more active for the formation of hexitols in
the second run. However, the hexitol yield began to decline
remarkably in the subsequent runs until 22.3% in the fifth run.
Therefore, in spite of very good performance of the fresh Ni/MC
catalyst, it can not be recycled. The TEM image of the spent
catalyst after 5 repetitive runs reveals significant aggregation/
particles are ∼ 10 nm in size. In contrast, over the 20%Ni/AC, as
well as Ni particles of ∼10 nm, many large nickel particles are
formed in sizes of several tens of nanometres. This is consistent
with the XRD result that the Ni on the MC has notably higher
dispersion than that on the AC (Fig. S1†). The unique 3-D open
structure and the suitable pore size distributions of the MC might
contribute to the better metal dispersion. The bimetallic catalysts
of 1%Ir-5%Ni/AC and 1%Ir-5%Ni/MC have similar metal par-
ticle size distributions in the range 2–10 nm. EDX analysis of
Table 1 The polyols product distribution from the cellulose conversion over different catalystsa
Yield (%)
Entry
Catalyst
EG
1,2-PG
HA
Gly
Ery
Man
Sor
Hexitol
Gas
Conv. (%)
1
2
3
4
5
6
7
8
20%Ni/AC
20%Ni/MC
11.9
9.9
9.9
7.3
5.3
3.3
5.3
4.7
7.6
8.1
8.3
9.5
8.9
8.0
5.5
11.8
15
4.2
3.7
5.4
3.4
3.0
3.0
3.5
3.4
4.3
3.8
5.7
2.7
2.4
1.9
3.3
5.0
7.5
4.0
3.3
4.3
4.2
3.6
7.7
4.8
5.6
11.6
7.1
1.2
2.8
0
0.9
3.8
2.4
4.9
5.1
5.2
5.4
6.7
1.4
1.6
4.4
2.6
2.4
2.2
1.1
0.7
—
1.7
2.6
1.3
2.8
3.0
2.3
3.4
3.0
0.7
1.0
2.7
3.1
2.9
2.4
0.8
1.4
2.7
4.6
2.7
6.8
7.5
5.6
12.6
8.3
9.6
1.1
2.5
4.5
6.0
7.0
11.5
1.3
1.5
4.7
15.1
39.4
13.5
39.9
47.6
41.6
51.5
47.9
3.6
19.7
42.1
20.3
47.4
53.2
54.2
59.8
57.5
4.7
10.4
37.0
53.5
54.8
58.3
3.5
10.0
6.0
13.2
3.4
2.6
13.8
7.7
2.6
1.3
2.3
1.7
1.2
1.3
2.4
4.1
7.8
3.8
61.9
84.5
—
Mixed catalystb
1%Pt-5%Ni/MC
1%Pd-5%Ni/MC
1%Ru-5%Ni/MC
1%Rh-5%Ni/MC
1%Ir-5%Ni/MC
1%Ir/MC
100
100
100
100
100
90.4
85.7
92.3
99.5
100
100
65.6
58.6
78.9
9
10
11
12
13
14
15
16
17
5%Ni/MC
7.9
Mixed catalystc
2%Ir-5%Ni/MC
3%Ir-5%Ni/MC
4%Ir-4%Ni/MC
1%Ir/AC
32.5
47.5
47.8
46.8
2.2
6.0
8.0
5.7
15.7
2.2
5.4
5%Ni/AC
1%Ir-5%Ni/AC
7.5
12.7
a The reactions were conducted at 245 °C and 6 MPa H2 for 30 min. EG, 1,2-PG, HA, Gly, Ery, Man, Sor represent ethylene glycol, 1,2-propylene
glycol, hydroxyacetone, glycerol, erythritol, mannitol, and sorbitol, respectively. b 0.15 g RANEY® Ni was mixed with 0.15 g 20%Ni/AC. c 0.15 g
1%Ir/MC was mixed with 0.15 g 5%Ni/MC.
This journal is © The Royal Society of Chemistry 2012
Green Chem., 2012, 14, 614–617 | 615

View Article Online
catalysts gave 100% cellulose conversion (Entries 4–8). Mean-
while, the hexitol yields were from 47% to 60% depending on
the noble metals, and following the order of 1%Pt-5%Ni/MC <
1%Pd-5%Ni/MC < 1%Ru-5%Ni/MC < 1%Ir-5%Ni/MC < 1%
Rh-5%Ni/MC. The best result was obtained over the 1%Rh-5%
Ni/MC catalyst with the hexitol yield as high as 59.8%. This is
the highest value obtained so far on heterogeneous catalysts. The
1%Ir-5%Ni/MC catalyst was only slightly less active than the
1%Rh-5%Ni/MC catalyst, but Ir is much cheaper than Rh and
will be a more practical choice in industrial applications. There-
fore, we made a more detailed study of Ir-Ni/MC catalyst
system. Compared with the two monometallic catalysts as well
as their mechanical mixture (Entries 9–11), the MC supported Ir-
Ni bimetallic catalysts presented strong synergistic effects. We
also investigated the effect of the amount of Ir on the perform-
ance of the Ir-Ni/MC bimetallic catalysts. Increasing the Ir
loading from 1% to 4% (Entries 8, 12–14) did not result in pro-
nounced enhancement in the hexitol yield.
The reusability of the Ir-Ni/MC bimetallic catalyst was tested
and the results are shown in Fig. 2B and Fig. S4.†In the four
repetitive runs, the 1%Ir-4%Ni/MC gives a much improved stab-
ility than monometallic 20%Ni/MC. Further increasing the Ir
loading to 4% results in even more stable performance; no sign
of any decay is observed on 4%Ir-4%Ni/MC. The XRD patterns
(Fig. S5†) and TEM images (Fig. S6†) of the used catalyst show
no aggregation of metal particles occurred on the Ir-Ni/MC
bimetallic catalyst. This result demonstrates that the noble metal
modified nickel catalysts are rather durable in hydrothermal con-
ditions for cellulose conversion.
In contrast to the MC-supported Ir-Ni bimetallic catalysts, the
AC-supported Ir-Ni bimetallic catalysts behaved rather poorly in
the conversion of cellulose to hexitols, and no synergistic effect
occurred (Entries 15–17). This is a surprising result because the
particle sizes as well as structures of the bimetallic particles look
very similar in AC and MC supports.
To uncover the underlying reason for the different behaviors
of AC and MC supported Ir-Ni bimetallic catalysts, we designed
some control experiments. Firstly, we compared their catalytic
performance for the reaction of glucose hydrogenation, which is
an intermediate step in the conversion of cellulose to hexitols.
The results show that the addition of a small amount of noble
metals to the monometallic nickel catalysts could enhance
greatly the activity for glucose hydrogenation, and a strong
Fig. 2 The reusability of Ni-based catalysts in cellulose conversion.
(A) 20%Ni/MC with ball-milled cellulose as the feedstock; (B) 4%Ir-4%
Ni/MC with microcrystalline cellulose as the feedstock.
agglomeration of the Ni particles (Fig. S3†), which is probably
caused by the leaching of the Ni particles out of the catalyst into
hot water and re-deposition on the catalyst surface. On the other
hand, we noticed that the conversions of ball-milled cellulose
were kept at 100% in the recycling tests, which implies that the
activity for hydrolysis of cellulose does not decay by the leach-
ing of Ni. In other words, the MC support may play important
roles in promoting the hydrolysis of cellulose, while the Ni com-
ponent catalyzes the hydrogenation of sugars.
Noble metals have good hydrothermal stability and excellent
hydrogenation activities. Furthermore, when they combine with
nickel, the bimetallic catalysts usually show distinguished hydro-
genation performance thanks to the tunable electronic and
chemical properties superior to those of the parent metals.16
Herein, we introduce the noble metals to Ni catalysts to enhance
the catalytic activity and stability.
Table 1 shows the results of cellulose conversion over various
Ni-based bimetallic catalysts. All the MC supported bimetallic
Table 2 Cellulose hydrolysis over different carbon materialsa
Yield (%)
Cat.
Feedstock
Cello.
Glu.
Fru.
HMF
Con.b(%)
Blank
AC
MC
Blank
AC
MC
Microcrystalline cellulose
Microcrystalline cellulose
Microcrystalline cellulose
Ball-milled cellulose
Ball-milled cellulose
Ball-milled cellulose
1.5
—
0.6
1.4
1.7
0.9
6.7
2.5
14.7
8.2
10.1
41.0
0.5
0.3
1.0
2.1
2.0
2.7
3.8
0.2
0.5
1.5
1.3
2.0
29.3
26.9
38.9
35.7
45.3
71.2
a The reactions were conducted with a rapid temperature ramp: from room temperature to 245 °C in 45 min and then rapidly cooled to room
temperature with water; Cello., Glu. and Fru. represent cellobiose, glucose and fructose, respectively. b The cellulose conversion was calculated based
on the carbon balance: Con.(%) = (moles of total organic carbon in the liquid phase)/(moles of carbon in the cellulose fed into the reactor)×100%.
616 | Green Chem., 2012, 14, 614–617
This journal is © The Royal Society of Chemistry 2012

View Article Online
synergistic effect was observed between the two metals
(Table S2†). The better hydrogenation activity of the MC-sup-
ported bimetallic catalysts would decrease sugar degradation
under harsh reaction conditions and result in higher hexitol yield
in the cellulose conversion. On the AC supported bimetallic cata-
lysts, however, no such synergistic effect was found in the
glucose hydrogenation.
Acknowledgements
This work is supported by the National Science Foundation of
China (20773124, 20903089, and 21176235) and 973 program
of China (2009CB226102).
Secondly, we conducted the reactions of cellulose hydrolysis
with MC and AC supports without metal loading. As shown in
Table 2, in the absence of carbon materials, 29.3% microcrystal-
line cellulose was degraded in the blank experiment and produced
glucose in a yield of 6.7%. The presence of the AC support does
not bring about any increase either in the cellulose conversion or
in the yield of sugars, suggesting the AC acts only as an inert
support and does not participate in the hydrolysis of cellulose. In
contrast, in the presence of MC, the conversion of microcrystalline
cellulose was increased to 38.9% and the glucose yield was up to
14.7%. When the cellulose was ball milled and used as the feed-
stock, the yield of glucose over MC was further increased to
41.0%. Evidently, the MC support itself presented notable cataly-
tic activity for hydrolysis of cellulose into sugars. This is consist-
ent with the results reported by Fukuoka et al.14
Notes and references
1 (a) G. W. Huber, S. Iborra and A. Corma, Chem. Rev., 2006, 106, 4044;
(b) S. Van de Vyver, J. Geboers, P. A. Jacobs and B. F. Sels, Chem-
CatChem, 2011, 3, 82.
2 (a) L. R. Lynd, P. J. Weimer, W. H. van Zyl and I. S. Pretorius, Microbiol.
Mol. Biol. Rev., 2002, 66, 506; (b) R. Rinaldi, R. Palkovits and F. Schüth,
Angew. Chem., Int. Ed., 2008, 47, 8047.
3 Y. P. Zhang, M. E. Himmel and J. R. Mielenz, Biotechnol. Adv., 2006,
24, 452.
4 J. C. Colmenares, R. Luque, J. M. Campelo, F. Colmenares, Z. Karpiński
and A. A. Romero, Materials, 2009, 2, 2228.
5 (a) S. Suganuma, K. Nakajima, M. Kitano, D. Yamaguchi, H. Kato,
S. Hayashi and M. Hara, J. Am. Chem. Soc., 2008, 130, 12787;
(b) A. Onda, T. Ochi and K. Yanagisawa, Green Chem., 2008, 10, 1033;
(c) J. F. Pang, A. Q. Wang, M. Y. Zheng and T. Zhang, Chem. Commun.,
2010, 46, 6935; (d) R. Rinaldi, N. Meine, J. vom Stein, R Palkovits and
F. Schüth, ChemSusChem, 2010, 3, 266.
6 (a) D. Mohan, C. U. Pittman and P. H. Steele, Energy Fuels, 2006, 20,
848; (b) T. R. Carlson, T. P. Vispute and G. W. Huber, ChemSusChem,
2008, 1, 397.
7 (a) W. P. Deng, M. Liu, Q. H. Zhang, X. S. Tan and Y. Wang, Chem.
Commun., 2010, 46, 2668; (b) N. Villandier and A. Corma, Chem.
Commun., 2010, 46, 4408; (c) M. Mascal and E. B. Nikitin, Chem-
SusChem, 2010, 3, 1349.
8 (a) A. Fukuoka and P. L. Dhepe, Angew. Chem., Int. Ed., 2006, 45, 5161;
(b) C. Luo, S. Wang and H. C. Liu, Angew. Chem., Int. Ed., 2007, 46,
7636.
9 (a) N. Ji, T. Zhang, M. Y. Zheng, A. Q. Wang, H. Wang, X. D. Wang and
J. G. Chen, Angew. Chem., Int. Ed., 2008, 47, 8510; (b) N. Ji, T. Zhang,
M. Y. Zheng, A. Q. Wang, H. Wang, X. D. Wang, Y. Y. Shu,
A. L. Stottlemyer and J. G. Chen, Catal. Today, 2009, 147, 77;
(c) M. Y. Zheng, A. Q. Wang, N. Ji, J. F. Pang, X. D. Wang and T. Zhang,
ChemSusChem, 2010, 3, 63; (d) Y. H. Zhang, A. Q. Wang and T. Zhang,
Chem. Commun., 2010, 46, 862.
Thirdly, we conducted adsorption experiments using cello-
biose and glucose as the probe molecules, respectively. It is
found that the uptake of cellobiose and glucose on the MC are
0.149 and 0.041 g g−1, respectively, which is almost double that
over the AC support (0.071 and 0.022 g g−1 for cellobiose and
glucose respectively). This result indicates that the sugar inter-
mediates produced during the cellulose degradation are more
readily adsorbed on the MC surface, which will facilitate the
subsequent hydrogenation reaction taking place on the Ni sites
thereon and result in a higher yield of hexitols. More impor-
tantly, it has more than three times the cellobiose uptake than
glucose on the MC support which strongly suggests that the MC
support possesses a unique function to adsorb and activate the
β-1,4-glycosidic bonds of cellulose, which should account for its
higher activity for the hydrolysis of cellulose.5a
The above three control experiments provide strong evidence
that the MC is not only acting as a support for dispersing active
metal components, it also plays important roles in adsorption
and activation of both cellulose and glucose, and thereby pro-
motes greatly the hydrolysis of cellulose as well as the hydrogen-
ation of glucose.
In summary, we have developed highly active and robust Ni-
based bimetallic catalysts for the selective transformation of cel-
lulose into hexitols. The high activity of MC for cellulose
hydrolysis, the mesoporous structure of MC facilitating reactant
transportation, and the enhanced hydrogenation activity of bime-
tallic catalysts are responsible for the excellent performance of
MC supported bimetallic catalysts. By efficiently coupling the
hydrolysis reaction and “in situ” hydrogenation of oligosacchar-
ides, the hexitol yield has reached as high as 59.8%, which is the
best yield ever reported on solid catalysts to date. The strategy in
the present work to develop efficient catalysts might provide gui-
dance for the cellulosic biomass conversion to polyols.
10 (a) G. W. Huber, R. D. Cortright and J. A. Dumesic, Angew. Chem., Int.
Ed., 2004, 43, 1549; (b) J. J. Bozell and G. R. Petersen, Green Chem.,
2010, 12, 539.
11 (a) W. P. Deng, X. S. Tan, W. H. Fang, Q. H. Zhang and Y. Wang, Catal.
Lett., 2009, 133, 167; (b) L. N. Ding, A. Q. Wang, M. Y. Zheng and
T. Zhang, ChemSusChem, 2010, 3, 818; (c) S. Van de Vyver, J. Geboers,
M. Dusselier, H. Schepers, T. Vosch, L. Zhang, G. Van Tendeloo,
P. A. Jacobs and B. F. Sels, ChemSusChem, 2010, 3, 698;
(d) R. Palkovits, K. Tajvidi, A. M. Ruppert and J. Procelewska, Chem.
Commun., 2011, 47, 576; (e) Y. H. Zhu, Z. N. Kong, L. P. Stubbs,
H. Lin, S. C. Shen, E. V. Anslyn and J. A. Maguire, ChemSusChem,
2010, 3, 67; (f) H. Kobayashi, Y. Ito, T. Komanoya, Y. Hosaka,
P. L. Dhepe, K. Kasai, K. Hara and A. Fukuoka, Green Chem., 2011, 13,
326.
12 R. Rinaldi and F. Schüth, Energy Environ. Sci., 2009, 2, 610.
13 (a) C. D. Liang, Z. J. Li and S. Dai, Angew. Chem., Int. Ed., 2008, 47,
3696; (b) S. Kubo, R. Demir-Cakan, L. Zhao, R. J. White and M.
M. Titirici, ChemSusChem, 2010, 3, 188.
14 H. Kobayashi, T. Komanoya, K. Hara and A. Fukuoka, ChemSusChem,
2010, 3, 440.
15 S. Jun, S. Hoon Joo, R. Ryoo, M. Kruk, M. Jaroniec, Z. Liu, T. Ohsuna
and O. Terasaki, J. Am. Chem. Soc., 2000, 122, 10712.
16 (a) W. W. Lonergan, D. G. Vlachos and J. G. Chen, J. Catal., 2010, 271,
239; (b) R. T. Mu, Q. Fu, H. Xu, H. Zhang, Y. Y. Huang, Z. Jiang,
S. Zhang, D. L. Tan and X. H. Bao, J. Am. Chem. Soc., 2011, 133, 1978.
This journal is © The Royal Society of Chemistry 2012
Green Chem., 2012, 14, 614–617 | 617