Au/CNTs catalyst for highly selective hydrodeoxygenation of vanillin at the water/oil interface

Article abstract of DOI:10.1039/c4ra04692g

Au/CNTs assembled at the interfaces of a Pickering emulsion are reported, for the first time, exhibiting good catalytic activity and 100% selectivity for the hydrodeoxygenation of vanillin to p-creosol under mild reaction conditions. Simultaneous reaction and separation of the target products are achieved, which leads to substantial simplification of the separation and purification process for bio-oil upgrading.

Full text of DOI:10.1039/c4ra04692g

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Au/CNTs catalyst for highly selective
hydrodeoxygenation of vanillin at the water/oil
Cite this: RSC Adv., 2014, 4, 31932
interface†
a
a
a
a
b
ad
Received 19th May 2014
Accepted 10th July 2014
*
*
Xiaomin Yang, Yu Liang, Xu Zhao, Yifan Song, Lianghai Hu, Xiaofeng Wang,
Zichen Wanga and Jieshan Qiuc
DOI: 10.1039/c4ra04692g
www.rsc.org/advances
Au/CNTs assembled at the interfaces of a Pickering emulsion are efficiency. Therefore, there is a strong incentive to develop
reported, for the first time, exhibiting good catalytic activity and 100% catalysts that operate under mild reaction conditions while still
selectivity for the hydrodeoxygenation of vanillin to p-creosol under producing a modied bio-oil with improved properties. Owing
mild reaction conditions. Simultaneous reaction and separation of the to the high catalytic activities of noble metal catalysts, a series of
target products are achieved, which leads to substantial simplification noble metal catalysts have been employed recently to investi-
of the separation and purification process for bio-oil upgrading.
gate their hydrogenation effects on bio-oil upgrading, such as
Pt,21–23 Ru,24–28 and Pd29–33 catalysts. However, the catalytic
properties of gold nanoparticles for the hydrodeoxygenation of
bio-oil compounds have been somewhat neglected. Bio-oil is a
complex liquid formed by two immiscible phases (aqueous and
organic) and diffusion limitations between aqueous phase and
organic phase reduce the catalytic activity and selectivity of the
employed catalysts. On the other hand, the separation and
recycling issues of products and reagents increase the cost of
operation. Therefore, the development of methods that allow a
convenient separation and recycling of catalysts, products and
reagents is a key issue in this research area.
In emulsion catalysis, reactions are carried out in a biphasic
mixture of two immiscible solvents, and emulsiers were added
to enhance the interfacial surface area by emulsication, which
could enhance the mass transfer of molecules between different
phases. Surfactants were conventionally used as emulsiers,
which are difficult to separate and recover from the nal
mixtures. It has been reported that solid particles can be used to
stabilize aqueous-oil emulsions, which are called Pickering
emulsions.34–40 The catalytic properties of Pickering emulsions
catch the eyes of researchers in recent years.41–52 Carbon nano-
tube supported metal nanohybrids have been proved that they
could act as both recoverable emulsiers and catalysts in our
previous work.43,44 The novel catalytic applications of carbon
nanotube supported metal nanohybrids are waiting for
exploring.
With increasing environmental concerns, the development of
highly selective reaction systems using heterogeneous catalysts
is greatly desired. In selective hydrogenation reactions, gold
nanoparticle catalysts have recently drawn considerable interest
owing to their fascinating unique properties compared with
those of conventional hydrogenation catalysts based on Pt
group metals.1 These reactions involve hydrogenations of
unsaturated carbonyl compounds to unsaturated alcohols,2–6
nitroaromatics to amines,7,8 alkynes to alkenes,9–11 deoxygen-
ation of epoxides to alkenes12–15 and so on. Therefore, it is
important to explore new applications of gold catalysts for high
efficiency liquid-phase organic transformations.
Bio-oil, produced by fast pyrolysis of biomass, is a promising
second-generation renewable energy carrier. However, due to its
high oxygen content and instability, the direct use of bio-oil as a
liquid fuel is currently not feasible. Hydrodeoxygenation is
considered to be the most important and promising method for
bio-oil upgrading.16–19 Conventional hydrogenation catalysts are
sulded NiMo and CoMo catalysts.20 However, the process
conditions are rather severe, which leads to the low energy
aCollege of Chemistry, Jilin University, Changchun 130012, China. E-mail:
wangxf103@jlu.edu.cn
bSchool of Life Sciences, Jilin University, Changchun 130012, China. E-mail:
Vanillin, one typical oxygen-containing phenolic compound
of bio-oil, was selected as the model compound to explore the
catalytic properties of carbon nanotube supported gold catalyst
for bio-oil upgrading because of its three different types of
oxygenated functional groups (aldehyde, ether, and hydroxyl)
and its partial solubility in both the organic and aqueous
lianghaihu@jlu.edu.cn
cCarbon Research Laboratory, State Key Lab of Fine Chemicals, Dalian University of
Technology, Dalian 116023, China
dState Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of
Chemistry, Jilin University, Changchun 130012, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra04692g
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phases just as bio-oil. Herein we report for the rst time that up to 100% and it is independent on the solvent system, indi-
carbon nanotube supported gold catalyst, assembled at the cating that Au/CNTs is a highly selective catalyst for the
interfaces of water-oil biphase, show excellent activity and hydrodeoxygenation of vanillin to p-creosol. Results show that
especially 100% selectivity for the hydrodeoxygenation of the proportion of bi-solvents has effect on the conversion of
vanillin to the target product of p-creosol under mild reaction vanillin. When 20 mL of decalin and 20 mL of H2O are used as
conditions. Simultaneous reaction and separation of the target bi-solvents, the highest yield of the target product of p-creosol is
products are achieved, which leads to a substantial simplica- achieved. With the further increase of the water proportion in
tion of the separation and purication process.
the bi-solvents, the yield of p-creosol begins to decrease, which
The Au/CNT and Au/AC catalysts were prepared by a sol implies that there is a proper solvent proportion to obtain the
immobilization method.53,54 Details of the preparation process highest yield of the target product in the aqueous and organic
can be found in the ESI.† A typical TEM image and the particle bi-phase reaction system. According to our experimental
size distribution of the Au/CNT catalyst (Fig. 1) show that gold results, it can be found that highly efficient hydrodeoxygenation
particles with an average size of 3.8 nm are highly dispersed on of vanillin over Au/CNTs are achieved with a nearly full
the surface of CNTs, which is consistent with the XRD result conversion of vanillin and the p-creosol selectivity of 100%
(Fig. S1†). According to a typical TEM image and the particle under mild reaction conditions (1 MPa, 150 ꢀC). In order to rule
size distribution of the Au/AC catalyst (Fig. S2†), the average size out the possibility that residual metal traces in CNTs catalyze
of gold particles is 5.3 nm and the sintering of gold nano- the reaction itself, the blank reactions were performed, where
particles could be found giving a broad particle size distribution the catalytic activity of the modied CNTs was tested for the
from 2.25 nm to 12.75 nm. ICP analysis shows that the actual hydrodeoxygenation reaction of vanillin. The conversion of
loading of gold in the Au/CNT and Au/AC catalysts are 0.44 wt% vanillin is not detected, which reveals that gold nanoparticles
and 0.48 wt%, respectively.
instead of residual metal traces are active for the hydro-
The hydrodeoxygenation of vanillin to p-creosol was used as deoxygenation reaction.
a probe reaction to explore the catalytic application of the Au/
The effect of reaction temperature on the catalytic activity of
CNT catalyst to phenolic hydrodeoxygenation, as illustrated in Au/CNTs for the hydrodeoxygenation of vanillin is shown in
Scheme 1. The effect of catalyst supports, solvents, and bi- Table 2. Results show that reaction temperature has a signi-
solvents on the catalytic activity of Au/CNTs for the hydro- cant impact on chemoselectivity. At 50 ꢀC, the product is vanillyl
deoxygenation of vanillin was studied, as summarized in Table alcohol. At 100 ꢀC, the selectivity of p-creosol increases to 19%,
1. The catalytic activity of CNT supported Au catalyst is higher which is due to the hydrogenolysis of vanillyl alcohol. At 150 ꢀC,
than the AC supported one (Entries 4 and 5), indicating that hydrogenolysis becomes the dominant reaction path, giving the
CNT is a suitable support for the Au catalyst for the vanillin selectivity of p-creosol of 100%. The turnover number measurꢁe1d
hydrodeoxygenation reaction. According to the TEM images and at 150 ꢀC aer the rst 0.5 h of reaction is about 150 h
.
the particle size distributions of the Au/CNT and Au/AC cata- Though the catalytic activity of Au/CNTs is lower than the best-
lysts, a narrow particle size distribution and small average size performing ones reported in the literature,42 but the hydro-
of gold particles are obtained using CNT as catalyst support deoxygenation selectivity of Au/CNTs is excellent. At 200 ꢀC, the
compared with AC, which could be one possible reason for the selectivity of p-creosol decreases to 75%, and the decarbon-
higher catalytic activity of Au/CNTs. Under the identical reac- ylation of the aldehyde group is detected which gives the
tion conditions, the catalytic activity of Au/CNTs in water is selectivity of 2-methoxyphenol of 25%, indicating that the
higher than that in decalin (Entries 1 and 6). To explore the decarbonylation of the aldehyde group is promoted at high
catalytic activity of the Au/CNTs catalyst in the aqueous and temperature.
organic bi-phase reaction system of bio-oil, bi-solvents of dec-
Evolution of reactant and products concentration with
alin and water were employed and the proportion of bi-solvents reaction time at 150 ꢀC is shown in Fig. 2a. Vanillyl alcohol has a
on the effect of the catalytic activity was investigated. It should concentration peaking aer about 0.5 h of reaction, and
be noted that the target product selectivity to p-creosol reaches meanwhile the concentration of p-creosol increases gradually.
The results imply that vanillyl alcohol is an intermediate
product and subsequently it is consumed by hydrogenolysis to
form p-creosol. The partition of reactant and products in the
organic and aqueous phases aer 0.5 h of reaction is given in
Fig. 2b. The GC analysis results show that vanillin alcohol is
highly soluble in the aqueous phase, and the target product of p-
creosol is much more soluble in the oil phase. Therefore, the
formed p-creosol migrates to the organic phase upon formation.
In contrast, vanillin alcohol remains in the aqueous phase and
continues reacting. The results illustrate that the simultaneous
reaction and separation of the products are achieved in the
formed Pickering emulsions, which leads to drastically simplify
the isolation and purication stages.
Fig. 1 TEM image and particle size distribution of the Au/CNT catalyst
via sol immobilization method.
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Scheme 1 Hydrogenation of vanillin over Au/CNTs.
Table 1 Effect of catalyst supports, solvents, and bi-solvents on the
catalytic activity of the supported Au catalysts for the hydro-
deoxygenation of vanillina
recycling experiment was performed to test the stability of the
prepared Au/CNTs catalyst at 150 C for 6 h. The recycled Au/
ꢀ
CNTs catalyst shows that the conversion of vanillin slightly
decreases from 94% to 87%, the selectivity of p-creosol
decreases from 100% to 88%, and the selectivity of vanillyl
alcohol increases to 12%, respectively. With the increase of
reaction time, the conversion of vanillin and the selectivity of p-
creosol will further increase using the recycled Au/CNTs cata-
lyst. Therefore the prepared Au/CNTs catalyst is a convenient
separation and recycling catalyst.
The process and mechanism of the catalytic reactions were
studied in detail. Fluorescence microscopy can be used to
determine the distribution of the amphiphilic catalysts in the
emulsion system.44,55,56 Herein, uorescently labeled probes
(Au/CNTs-P) was prepared to investigate the dispersion of Au/
CNT nanoparticles in the multiphase reaction system. The
preparation procedures are given in the ESI.† Fluorescence
microscope image of the reaction system shows that Au/CNT
nanoparticles are mainly distributed on the surfaces of the
emulsion droplets forming a Pickering emulsion, as shown in
Selectivity (%)
Decalin/H2O
(mL/mL)
Conversion
(%)
Entry
p-Creosol
Vanillyl alcohol
1
2
3
40/0
89
94
98
89
74
97.5
100
100
100
97
100
97
0
0
0
3
0
3
30/10
20/20
10/30
10/30
0/40
4
5b
6
a
Reaction conditions: vanillin (0.1 mmol), Au/CNTs (Au: 0.68 mol%),
b
150 ꢀC, p(H2) ¼ 1 MPa, 8 h. Reaction conditions: vanillin (0.1
mmol), Au/AC (Au: 0.74 mol%), 150 ꢀC, p(H2) ¼ 1 MPa, 8 h.
Table 2 Effect of reaction temperature on the catalytic activity of Au/
CNTs for the hydrodeoxygenation of vanillina
Selectivity (%)
Conversion
T (ꢀC) (%)
Vanillyl alcohol p-Creosol 2-Methoxyphenol Fig. 3a. The formed Pickering emulsion provides higher inter-
facial surface area where the reaction takes place to eliminate
50
41
75
94
100
81
0
0
19
100
75
0
0
0
the limitation of the mass transfer, which has been justied by
the theoretical study of the reference.51 Meanwhile, the formed
Pickering emulsion enhances hydrogen concentration at the
interface because hydrogen has higher solubility in organic
phase than in water. Vanillin and vanillin alcohol are highly
soluble in the aqueous phase and the target product of p-creosol
resulting from hydrogenolysis of vanillin alcohol is much more
soluble in the organic phase. As a result, the oil-soluble
compound, which is valuable as fuel component, can be readily
migrated to the product system upon formation. Schematic
100
150
200
100
0
25
a
Reaction conditions: vanillin (0.1 mmol), Au/CNTs (Au: 0.68 mol%),
solvent (decalin 20 mL, H2O 20 mL), p(H2) ¼ 1 MPa, 6 h.
Aer each reaction period, the emulsion was broken by
ltering out the Au/CNTs catalyst. The ltered Au/CNTs catalyst
was washed with ethanol and dried at 70 ꢀC under vacuum. The
Fig. 2 (a) Evolution of reactant and products concentration with reaction time and (b) partition of reactant and products in the organic and
aqueous phases after 0.5 h of reaction. Reaction conditions: vanillin (0.1 mmol), Au/CNTs (Au: 0.68 mol%), solvent (decalin 20 mL, H2O 20 mL),
150 ꢀC, p(H2) ¼ 1 MPa.
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Fig. 3 (a) Fluorescence microscope image of the Pickering emulsion stabilized by Au/CNT nanoparticles and (b) schematic illustration of the
hydrodeoxygenation of vanillin over Au/CNTs at the water/oil interface of the Pickering emulsion.
illustration of the hydrodeoxygenation of vanillin over Au/CNTs
at the water/oil interface of the Pickering emulsion was shown
in Fig. 3b. Similar studies on the unique properties and func-
tions of Pickering emulsion stabilized by solid catalysts have
been conducted.42–44 The study presented here will provide new
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Acknowledgements
This work was supported by the Key Project of the National
Twelh Five-Year Research Program of China (2011BAE06B06),
National Nature Science Foundation of China (no. 21301066),
Research Fund for the Doctoral Program of Higher Education of
China (no. 20130061120018), China Postdoctoral Science
Foundation (no. 2013M540242), and Postdoctoral Science
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