X. Yang et al. / Catalysis Communications 47 (2014) 28–31
29
fraction, was appealing for study because of its three different types of
Eq. (1). Hydrodeoxygenation of vanillin over Ru/CNTs.
oxygenated functional groups (aldehyde, ether, and hydroxyl) and its
partial solubility in both the organic and aqueous phases as bio-oil.
Therefore, the vanillin was commonly used as a model compound to
explore the catalytic application of catalysts to bio-oil upgrading. Herein
we report that CNT-supported ruthenium catalysts, assembled at the
interfaces of emulsion droplets, show excellent activity and selectivity
for the hydrodeoxygenation of vanillin under mild conditions. Simulta-
neous reaction and separation of the target products are achieved,
which leads to a substantial simplification of the separation and purifi-
cation process.
3. Results and discussion
2. Experimental
The oxidation of CNTs by mixed acid aims to remove amorphous car-
bon and to increase the number of surface oxygen-containing functional
groups that are useful for the metal deposition and dispersion. A typical
TEM image and the particle size distribution of the Ru/CNT sample show
that Ru particles with an average size of 3.5 nm are highly dispersed on
the surface of CNTs, as shown in Fig. 1.
2.1. Materials
CNTs with a diameter of 10–20 nm, a length of 1–2 μm, and a N2 sur-
face area of 151 m2/g were purchased from Shenzhen Nanotech Port Co.,
Ltd. (Shenzhen, China). Before use, pristine CNTs were refluxed in a mix-
ture of HNO3, H2SO4, and deionized water (volume ratio of 2:1:1) at
120 °C for 4 h, then filtered and washed with deionized water until the
pH value of the filtrate reached 7, then dried under vacuum. All reagents
used in this work were of analytical grade.
The hydrodeoxygenation of vanillin was used to explore the catalytic
application of the Ru/CNT nanohybrids to phenolic hydrodeoxygenation.
The effect of solvents on the catalytic activity of Ru/CNTs for the
hydrodeoxygenation of vanillin was studied, as shown in Table 1. Under
identical reaction conditions, the catalytic activity of Ru/CNTs in water is
much higher than in decalin. With decalin as solvent, the conversion of
vanillin is 24%, and the selectivity of the target product of p-creosol is
11%. When some amount of water is added, substantial increases in
the conversion of vanillin and the selectivity of the target product
of p-creosol are obtained. When 20 mL of decalin and 20 mL of
H2O are used as bi-solvents, the highest yield of the target product
of p-creosol is achieved. With the further increase in the water pro-
portion in the bi-solvents, the yield of p-creosol begins to decrease,
which implies that there is a proper decalin/water proportion to ob-
tain the highest yield of the target product.
The effect of reaction pressure on the catalytic activity of Ru/CNTs for
the hydrodeoxygenation of vanillin is shown in Table 2. The Ru/CNT cat-
alysts exhibit good catalytic activities for the hydrodeoxygenation of
vanillin even under very mild reaction conditions (100 °C, 0.4 MPa).
With the increase of the hydrogen pressure from 0.4 to 1 MPa, the con-
version of vanillin increases from 50% to 97%. The selectivity of vanillyl
alcohol decreases slightly and meanwhile the selectivity of p-creosol in-
creases correspondingly, which imply that the hydrogenolysis of
vanillyl alcohol is promoted at higher reaction pressure.
The effect of reaction temperature on the catalytic activity of Ru/CNTs
for the hydrodeoxygenation of vanillin is shown in Table 3. Results show
that the selectivity of products changes significantly with the increasing of
reaction temperature. At 50 °C, the dominant product is vanillyl alcohol.
At 100 °C, the selectivity of p-creosol increases to 26%, which might be
due to the hydrogenolysis of vanillyl alcohol. At 150 °C, p-creosol be-
comes the dominant product, and the decarbonylation of the aldehyde
group is detected which gives the selectivity of 2-methoxyphenol of 2%.
The turnover number measured at 150 °C after the first 0.5 h of reaction
was about 455 h−1. At 200 °C, the selectivity of 2-methoxyphenol in-
creases to 7%, which implies that the decarbonylation of the aldehyde
group is promoted at high temperature. According to experimental
results, it can be found that high efficient hydrodeoxygenation of vanil-
lin over Ru/CNTs is achieved with a full conversion of vanillin and the
p-creosol selectivity of 96% under mild conditions (1 MPa, 150 °C).
The variation of products and reactant distribution with reaction
time at 150 °C is shown in Fig. 2a. Vanillyl alcohol has a concentration
peaking after about 0.5 h of reaction, and 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 the various
products in the organic and aqueous phases after 0.5 h of reaction
is given in Fig. 2b. The GC analysis results show that vanillin alcohol is
2.2. Catalyst preparation and characterization
The Ru/CNT catalysts were prepared by a wetness impregnation
method. The oxidized CNTs were impregnated in an aqueous solution
of RuCl3 under ultrasonic conditions for 30 min, and then the mixture
was incubated at room temperature for 12 h. The CNT-supported
RuCl3 samples were dried at 110 °C for 12 h under vacuum, and re-
duced at 400 °C in flowing hydrogen for 2 h, and then cooled to room
temperature in Ar, obtaining Ru/CNT catalysts. Transmission electron
microscopy (TEM) examination was conducted using a JEM-2100F
equipped with a CCD camera operated at 200 kV.
2.3. Preparation of fluorescently labeled probes (Ru/CNTs-P)
Procedures for the preparation of FITC-labeled Ru/CNTs are as follow-
ing. The Ru/CNT catalysts were reacted with hexamethylenediamine in
the presence of EDC (1-ethyl-3-(3-dimethylaminopropyl) carbodiimide
hydrochloride) and NHS (N-hydroxysuccinimide) to afford a linker
between the CNTs and the subsequent fluorescent probe. Briefly, 2 mg
Ru/CNTs was mixed with 5 mg hexamethylenediamine and 10 mg
NHS/100 mg EDC in 100 mM 2-(N-morpholino)ethanesulfonic acid
(MES buffer, pH = 5.5). The mixed solution was stirred at room temper-
ature for 2 h. The Ru/CNTs conjugated with hexamethylenediamine
(Ru/CNTs–CONH(CH2)6NH2) were obtained by centrifuging at
25,000 rpm to remove the excess reagents. Subsequent reaction of
the Ru/CNTs–CONH(CH2)6NH2 with fluorescein isothiocyanate (FITC)
resulted in the formation of fluorescently labeled Ru/CNTs. Excess
FITC was removed by rinsing with 500 μL phosphate-buffered saline
(PBS) and centrifuging three times at 25,000 rpm.
2.4. Hydrodeoxygenation of vanillin
The vanillin hydrodeoxygenation reactions were performed in a
100 mL autoclave under stirring. Hydrodeoxygenation of vanillin was
used to probe the catalytic hydrogenation activities of the Ru/CNT cata-
lysts, as illustrated in Eq. (1). For a typical run, the reaction was carried
out with 5.9 mmol vanillin, 0.2 mol% Ru/CNTs, 20 mL decalin and
20 mL water as bi-solvents, and 1 MPa H2 at 100 °C for 3 h. After each
reaction, the emulsion was broken by filtering out the catalyst particles.
The two liquid phases were separated and analyzed individually
by means of GC (2014C, SHIMADZU) and GC-MS (QP 2010 Plus,
SHIMADZU).
Yang, Xiaomin
