Superhydrophilic mesoporous sulfonated melamine-formaldehyde resin supported palladium nanoparticles as an efficient catalyst for biofuel upgrade

Article abstract of DOI:10.1039/c3ta10916j

Mesoporous sulfonated melamine-formaldehyde resin (MSMF) has been successfully synthesized from the self-assembly of a copolymer surfactant (F127) with a mixture of melamine, formaldehyde and NaHSO₃ in an aqueous solution. After removal of the copolymer surfactant (F127) by ethanol extraction, the MSMF exhibits uniform mesopore sizes centered at 10.2 nm. The contact angle (0°) of water on the sample shows that the MSMF is superhydrophilic. After impregnating with a Pd species, a MSMF supported Pd catalyst (Pd/MSMF) is obtained, which shows high activity and excellent recyclability in an important model reaction for biofuel upgrade (hydrodeoxygenation of vanillin), compared with a conventional carbon supported Pd catalyst (Pd/C). The superior catalytic properties of the Pd/MSMF catalyst should be potentially important for biofuel upgrades in the future.

Full text of DOI:10.1039/c3ta10916j

Journal of
Materials Chemistry A
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Superhydrophilic mesoporous sulfonated melamine–
formaldehyde resin supported palladium nanoparticles
as an efficient catalyst for biofuel upgrade†
Cite this: J. Mater. Chem. A, 2013, 1,
8630
a
a
a
ab
*
*
Zhongfei Lv, Qi Sun, Xiangju Meng and Feng-Shou Xiao
Mesoporous sulfonated melamine–formaldehyde resin (MSMF) has been successfully synthesized from the
self-assembly of a copolymer surfactant (F127) with a mixture of melamine, formaldehyde and NaHSO3 in
an aqueous solution. After removal of the copolymer surfactant (F127) by ethanol extraction, the MSMF
exhibits uniform mesopore sizes centered at 10.2 nm. The contact angle (0ꢀ) of water on the sample
shows that the MSMF is superhydrophilic. After impregnating with a Pd species, a MSMF supported Pd
catalyst (Pd/MSMF) is obtained, which shows high activity and excellent recyclability in an important
model reaction for biofuel upgrade (hydrodeoxygenation of vanillin), compared with a conventional
carbon supported Pd catalyst (Pd/C). The superior catalytic properties of the Pd/MSMF catalyst should
be potentially important for biofuel upgrades in the future.
Received 5th March 2013
Accepted 24th May 2013
DOI: 10.1039/c3ta10916j
www.rsc.org/MaterialsA
via self-assembly of a copolymer surfactant (F127) with a
mixture of melamine, formaldehyde and NaHSO3 in an aqueous
Introduction
As a renewable energy source, biofuel produced from the
pyrolysis of biomass, has attracted considerable attention as a
substitute for petroleum fuels.1–23 However, biofuel is not suitable
for direct combustion in modern diesel engines due to the
presence of a large amount of oxygen components, which require
upgrading.15–23 As an effective approach, hydrodeoxygenation
plays an important role to upgrade the biofuel.24–43 For example,
large surface carbon supported Pd nanoparticles (Pd/C) is a good
catalyst for biofuel hydrodeoxygenation.33–39 However, due to the
presence of hydrophilic groups such as oxygen components in
the raw biofuels, the relatively hydrophobic Pd/C catalyst has a
relatively poor wettability to the biofuel in some cases. To solve
this problem, scientists have used N-doped carbon as a support
to increase the reactant wettability.40–43
Recently mesoporous phenol–formaldehyde resins with a very
high surface area (as high as 652 m2 gꢁ1) have been successfully
synthesized,44–46 but unfortunately their hydrophobicity signi-
cantly inuences the wettability47 of the biofuel as a reactant in
biofuel upgrading.
In order to obtain a good wettability of the biofuel on the
support, we have designed and synthesized a superhydrophilic
mesoporous sulfonated melamine–formaldehyde resin (MSMF)
solution. The superhydrophilicity of the support is helpful for
increasing the miscibility of the catalyst with the biofuel reac-
tant. As a typical model for biofuel upgrade, catalytic tests in the
hydrodeoxygenation of vanillin to 2-methoxy-4-methylphenol
show that the MSMF supported Pd catalyst (Pd/MSMF) is highly
active and extremely recyclable, compared with a conventional
Pd/C catalyst.
Experimental
Synthesis of MSMF
MSMF was hydrothermally synthesized at the temperature
of 150 ꢀC from a mixture and molar ratio of 1.0 melamine : 3.5
HCOH : 1.0 NaHSO3 : 0.004 F127 : 0.005 NaOH : 40 H2O. In a
typical run, 10.0 g of melamine, 23 g of formaldehyde solution
(37 wt%) and 0.16 g of NaOH were dissolved in 17 mL
of deionized water and stirred at 80 ꢀC for 30 min. Aer
the addition of 8.3 g of NaHSO3 with stirring at 80 ꢀC for
another 2 h, 25 mL of deionized water containing 4 g of
F127 was added, followed by stirring for 1 h at room temper-
ature. Aer the addition of an aqueous HCl solution (36 wt%)
to adjust the pH value to 2–3 and further stirring at 70 ꢀC
for 30 min, the mixture was transferred into an autoclave
for heating at 150 ꢀC for 24 h. Aer ltrating, washing
and drying, a solid product was nally obtained (Fig. S1 and
S2†). F127 surfactant was removed by the extraction with
ethanol at 60 ꢀC for 3 h. The synthesis route is outlined in
Scheme 1.
aDepartment of Chemistry, Zhejiang University, Hangzhou, 310028, P. R. China.
E-mail: mengxj@zju.edu.cn; fsxiao@zju.edu.cn; Fax: +86 571-8827-8232; Tel: +86
517-8827-3698
bZhejiang Provincial Engineering Research Center of Industrial Boiler & Furnace Flue
Gas Pollution Control, Hangzhou, 310027, P. R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ta10916j
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Preparation of MSMF supported Pd catalyst (Pd/MSMF)
Pd/MSMF catalyst was prepared by a simple impregnation
method. Typically, 1 g of MSMF was added to 50 mL of deion-
ized water in a beaker under stirring, followed by the addition of
85 mg of PdCl2 and this was further stirred for 24 h at room
temperature. Aer adjusting the pH value to 8–9 by a NaOH
solution, 0.1 g of NaBH4 in 50 mL of deionized water was added
followed by further stirring for 30 min. Aer ltrating, washing
and drying, a solid product of Pd/MSMF was nally obtained.
Characterization
X-Ray powder diffraction (XRD) patterns were measured with a
˚
Rigaku X-ray diffractometer using Cu Ka (l ¼ 1.54 A) radiation.
Nitrogen sorption isotherms at the temperature of liquid
nitrogen were measured using a Micromeritics Tristar system.
The samples were outgassed for 10 h at 100 ꢀC before the
measurements. XPS spectra were performed on a Thermo
ESCALAB 250 with Al Ka irradiation at q ¼ 90ꢀ for X-ray sources,
and the binding energies were calibrated using the C1s peak at
284.6 eV. FTIR spectra were performed on an iS5 (Nicolet) IR
spectrometer in the range of 400–4000 cmꢁ1. Transmission
electron microscope (TEM) images were obtained using a
Hitachi HT-7700. 13C MAS spectra were performed on a Varian
Innityplus-400 spectrometer. The static contact angles were
measured with SL200KB at 25 ꢀC. CHNS elemental analyses
were performed on a Perkin-Elmer series II CHNS analyzer
2400.
Scheme 1 Proposed route for synthesizing MSMF.
Catalytic tests
As
a model reaction for biofuel upgrade, the hydro-
deoxygenation of vanillin was chosen. 0.304 g (2 mmol) of
vanillin and 0.021 g of Pd/MSMF catalyst were sealed in an
autoclave containing 20 mL of deionized water, which was
purged with H2 3 times. The pressure of H2 was adjusted to the
desired value and the autoclave was placed in the preheated oil
bath. Aer the reaction at 110 ꢀC for 2 h, the mixture was
extracted with ethyl acetate and the catalyst was taken out from
the system by centrifugation before being analyzed by gas
chromatography. Dodecane was added as an internal standard.
Results and discussion
Catalyst characterizations
Fig. 1 shows XRD patterns, N2 sorption isotherms, and TEM
images of MSMF and Pd/MSMF samples. The small-angle XRD
patterns shows that both MSMF and Pd/MSMF exhibit a broad
peak at ca. 0.8ꢀ (Fig. 1A), which should be assigned to the
presence of a worm-like mesostructure in the samples as
reported in the previous literature.48,49 The Pd/MSMF shows
peaks at 40.0, 46.6 and 68.1ꢀ in the wide-angle XRD patterns
(Fig. 1B), which are assigned to the characteristic (111), (200)
and (220) planes of the Pd nanoparticles. The N2 sorption
isotherms show that both MSMF and Pd/MSMF have typical
type-IV curves with a hysteresis loop at 0.45–0.90, conrming
the presence of mesoporosity in the samples.50 Correspond-
ingly, their pore sizes are distributed at 10.2 and 11.2 nm
Fig. 1 (A) Small-angle XRD patterns, (B) wide-angle XRD patterns, (C) N2 sorp-
tion isotherms and (D) pore size distribution of (a) MSMF and (b) Pd/MSMF
samples. TEM images of (E) MSMF and (F) Pd/MSMF samples. Line a in C and D
have been offset by 200 cm3 gꢁ1 and 0.0005 cm3 gꢁ1 respectively, along the
vertical axis for clarity.
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(CH2–O–) is undetectable, which is consistent with the only
methylene linkage in the sample. The methylene linkage is very
important in the polymer network because this linkage is more
stable than the methylene–ether linkage. The IR spectrum in the
region of 500–2000 cmꢁ1 of the sample gives bands at 609, 781,
812, 1031, 1187, and 1554 cmꢁ1. The bands at 1554, 812 and 781
cmꢁ1 are assigned to characteristic frequencies of triazine, while
the bands at 609, 1031, and 1187 cmꢁ1 are assigned to the
characteristic vibration of sulfonic groups.53,54 In addition,
elemental analysis of MSMF shows that the molar ratio of C/H/N/
S is 1/2.13/1.16/0.12 (Table S2†), indicating the presence of a large
amount of N element in the sample.
Based on the results of XRD patterns, N2 sorption isotherms,
TEM images, NMR and IR spectra, it could be concluded that
MSMF with a large surface area and uniform mesopores has
been successfully synthesized.
Fig. 3 shows the Pd3d XPS spectrum of the Pd/MSMF sample,
giving the peaks of Pd3d5/2 and Pd3d3/2 at 335.7 and 341.0 eV,
respectively. In contrast, the Pd/C sample shows a little higher
values at 336.2 and 341.5 eV (Fig. S5†). This shi is possibly
related to the nitrogen species in the MSMF acting as strong
Lewis base sites, which strongly inuences the electronic state
of Pd/MSMF, compared with Pd/C. Similar phenomena have
been shown in literature.41 Furthermore, a molar ratio of C/N/O/
S in MSMF is also analyzed by XPS, as presented in Table S3.†
Fig. 4 shows the contact angles of water and 2-methoxy-4-
methylphenol (product of vanillin hydrodeoxygenation) on the
surface of Pd/MSMF and Pd/C catalysts. When a water droplet is
contacted with the Pd/MSMF, the water is momentarily adsor-
bed, giving a swollen surface (Fig. 4A), suggesting very good
wettability of water into the Pd/MSMF.55,56 However, when a
water droplet is contacted with the Pd/C, the contact angle is
about 9.5ꢀ (Fig. 4B). These results indicate that the Pd/MSMF
has a much better water wettability than the Pd/C catalyst. In
addition, when a 2-methoxy-4-methylphenol droplet is con-
tacted with the surface of Pd/MSMF, the angle is about 19.5ꢀ
(Fig. 4C). In contrast, when a 2-methoxy-4-methylphenol droplet
is contacted with the surface of the Pd/C catalyst, the catalyst
Fig. 2 (A) 13C MAS NMR and (B) IR spectra of the MSMF sample.
Fig. 3 Pd3d XPS spectrum of the Pd/MSMF sample.
(Fig. 1D), BET surface areas are at 256 and 197 m2 gꢁ1, and pore quickly adsorbs this droplet, forming a swollen surface. These
volumes are at 0.51 and 0.42 cm3 gꢁ1 (Table S1†), respectively. results suggest that the adsorption of 2-methoxy-4-methyl-
TEM images show direct evidence that both samples have the phenol on the Pd/C catalyst is much faster than that on the Pd/
worm-like mesostructure (Fig. 1E and F). In addition, the TEM MSMF. i.e. the desorption of 2-methoxy-4-methylphenol on the
image of Pd/MSMF reveals the dispersion of Pd nanoparticles in Pd/MSMF is much easier than that on the Pd/C catalyst.
MSMF. Particularly, the HRTEM image shows a characteristic
Pd lattice fringe (Fig. S3†). The Pd sizes are estimated at 6–8 nm,
which are consistent with the results obtained from wide-angle
Catalytic activities for the hydrodeoxygenation of vanillin
XRD patterns. Very interestingly, the contact angle of a water To evaluate the catalytic performance of the superhydrophilic Pd/
droplet on the surface of MSMF is zero (Fig. S4†), indicating that MSMF, the hydrodeoxygenation of vanillin as a model for biofuel
the MSMF is superhydrophilic.
upgrade in the presence of a water solvent has been carefully
Fig. 2 shows 13C NMR and IR spectra of the MSMF sample. investigated, as shown in Table 1, Table S4–S6 and Fig. S6 and
ꢀ
The NMR spectrum (Fig. 2A) exhibits peaks at 53, 57, 155, and S7.† Notably, when the temperature is between 110–150 C, the
165 ppm. The strong peaks at 155 and 165 ppm could be assigned Pd/MSMF exhibits both full conversion and selective production
to the triazine rings, while the strong peak at 57 ppm is related to of 2-methoxy-4-methylphenol (entry 1 and 2, Table 1). Particu-
the species of –NH–CH2–SO3–.51,52 The shoulder peak at 53 ppm is larly, the formation of pure 2-methoxy-4-methylphenol product at
ꢀ
proposed to the species of –NH–CH2–NH–. These assignments 150 C takes only 1.0 h (entry 2, Table 1). In contrast, conven-
are in good agreement with those reported previously.51 Notably, tional Pd catalysts exhibit relatively low selectivity for 2-methoxy-
the signal at 70 ppm assigned to the methylene–ether linkage 4-methylphenol (entries 6–9, Table 1), although they have full
8632 | J. Mater. Chem. A, 2013, 1, 8630–8635
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Fig. 5 The dependencies of vanillin conversion (A), vanillin alcohol (B) and 2-
methoxy-4-methylphenol selectivities (C) on reaction time over the Pd/MSMF
catalyst.
Fig. 4 Contact angles of water droplets on (A) Pd/MSMF and (B) Pd/C and 2-
methoxy-4-methylphenol droplets on (C) Pd/MSMF and (D) Pd/C.
adsorption of oxygen-rich reactants (e.g. vanillin) and desorption
of the hydrodeoxygenated 2-methoxy-4-methylphenol product.
This feature would change the reaction balance, leading to a
signicant enhancement of the catalytic performance. On the
contrary, the Pd/C catalyst with weak hydrophilicity has a low
adsorption rate of the vanillin reactant and a low desorption rate
of 2-methoxy-4-methylphenol product, resulting in a relatively
low catalytic performance in the hydrodeoxygenation of vanillin,
compared with the Pd/MSMF catalyst.
Table 1 Catalytic performance in the hydrodeoxygenation of vanillin over
various catalystsa
Fig. 5 shows dependencies of vanillin conversion and
2-methoxy-4-methylphenol selectivity on reaction time over the
Pd/MSMF catalyst. Notably, in only 5 min, 79.1% of vanillin is
converted, but 2-methoxy-4-methylphenol selectivity is very low
(less than 5%). At this point, the major product is vanillin
alcohol. Therefore, the hydrogenation of vanillin alcohol plays
an important role for the selective formation of the 2-methoxy-4-
methylphenol product.
More importantly, the Pd/MSMF catalyst exhibits excellent
recyclability (entries 4 and 5, Table 1, Table S6†). Aer being
recycled for 6 times, the selectivity for methoxy-4-methylphenol
is still at 87%. At the same time, Pd species in the liquid phase
are still undetectable, indicating that no leaching occurs in the
reaction. The excellent recyclability of this catalyst could be
potentially important for practical applications in upgrading
biofuel in the future.
Sel. (%)b
Entry
Catalyst
Conv. (%)b
B
C
c
c
1
Pd/MSMF
Pd/MSMF
Pd/MSMF
Pd/MSMF
Pd/MSMF
Pd/C
Pd/MMF
Pd/TiO2
Pd/g-Al2O3
>99.5
>99.5
>99.5
>99.5
>99.5
>99.5
>99.5
>99.5
>99.5
—
—
13
—
13
21
29
83
33
>99.5
>99.5
87
>99.5
87
2d
3e
4f
5g
6
c
79
71
17
67
7
8
9
a
Reaction conditions: the reaction was carried out under a hydrogen
pressure of 1 MPa in 20 mL of water containing 2 mmol of vanillin at
b
110 ꢀC for 2 h. The Pd loading is 4.5 wt%. Determined by GC.
c
d
e
Undetectable. Reaction at 150 ꢀC for 1 h. Reaction at 90 ꢀC for 6
Conclusions
h. f Reuse at 110 ꢀC for 2 h. g Recycles for 6 times at 110 ꢀC for 2 h.
Superhydrophilic mesoporous sulfonated melamine–formal-
dehyde resin (MSMF) has been synthesized from a self-assembly
route. Very importantly, MSMF supported Pd (Pd/MSMF) cata-
conversion. Considering the similar Pd loadings between the lyst exhibits high activity, good selectivity and excellent recy-
Pd/MSMF with Pd/C and larger Pd nanoparticles (Fig. 1F) of the clability in the hydrodeoxygenation of vanillin as a model for
Pd/MSMF than those of Pd/C (Fig. S8†), the higher activity and biofuel upgrade, compared with the conventional Pd/C catalyst.
better selectivity for the Pd/MSMF catalyst than those over the The superior catalytic performance is related to the good
Pd/C catalyst are mainly attributed to better the wettability of the wettability of the vanillin reactant to the Pd/MSMF. This feature
reactant to the Pd/MSMF catalyst than that to the Pd/C catalyst. should be important for designing and preparing novel cata-
The superhydrophilic Pd/MSMF catalyst is favorable for lysts for biofuel upgrade in the future.
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Acknowledgements
This work was supported by the NSFC (U1162201 and
21003107), the National High-Tech Research and Development
program of China (2013AA065301), the Science and Technology
Innovative Team of Zhejiang Province (no. 2012R10014-01) and
the Fundamental Research Funds for the Central Universities
(2013XZZX001).
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This journal is ª The Royal Society of Chemistry 2013
J. Mater. Chem. A, 2013, 1, 8630–8635 | 8635