Pd Nanoparticles Supported on Cellulose as a Catalyst for Vanillin Conversion in Aqueous Media

Article abstract of DOI:10.1021/acs.joc.8b00246

Palladium nanoparticles were first anchored on modified biopolymer as an efficient catalyst for a biofuel upgrade. Fluorinated compounds was grafted onto cellulose to obtain amphiphilic supports for on water reactions. Pd catalyst was prepared by straightforward deposition of metal nanoparticles on modified cellulose. The catalyst exhibited excellent catalytic activity and selectivity in hydrodeoxygenation of vanillin (a typical model compound of lignin) to 2-methoxy-4-methylphenol under atmospheric hydrogen pressure in neat water without any other additives under mild conditions.

Full text of DOI:10.1021/acs.joc.8b00246

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Note
Pd nanoparticles Supported on Cellulose as a
catalyst for vanillin conversion in aqueous media
Dan-dan Li, Jia-wei Zhang, and Chun Cai
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b00246 • Publication Date (Web): 17 May 2018
Downloaded from http://pubs.acs.org on May 17, 2018
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Pd nanoparticles Supported on Cellulose as a catalyst for
vanillin conversion in aqueous media
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Dan-dan Li, Jia-wei Zhang and Chun Cai*,
Chemical Engineering College, Nanjing University of Science and Technology, Nanjing, Jiangsu
210094, P. R. China.
E-mail: c.cai@mail.njust.edu.cn.
RECEIVED DATE (will be automatically inserted after manuscript is accepted).
ABSTRACT: Palladium nanoparticles were firstly anchored on modified biopolymer as an efficient
catalyst for biofuel upgradation. Fluorinated compounds was grafted onto cellulose to obtain
amphiphilic supports for on water reactions. Pd catalyst was prepared by straightforward deposition
of metal nanoparticles on modified cellulose. The catalyst exhibited excellent catalytic activity and
selectivity in hydrodeoxygenation of vanillin (a typical model compound of lignin) to 2-methoxy-
4-methylphenol under atmospheric hydrogen pressure in neat water without any other additives
under mild conditions.
Development of renewable and sustainable fuel sources to reduce reliance on fossil fuel sources
become more and more urgent due to the growing energy demand and global climate change caused
by application of fossil-fuel1-3. Biomass has long been regarded as an ideal energy source in the
future due to its renewable nature and large reservation on the earth. During the last decades, lots of
systems for biomass conversion have been established, biomass could be selectively converted into
high-value chemical products and biofuels. High-value chemical products such as phenols can be
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directly used in the chemical and pharmaceutical industries, and biofuels seems to be an propitious
candidate for fossil-fuel due to their availability, low cost2 and lower greenhouse gas emissions than
conventional petroleum-based gasoline4-6. However, owing to the large amounts of oxygen
components involved in biofuels, calorific value of biofuels is relatively low and biofuel is
immiscible with conventional fuels, what’s more, biofuels is unstable over time due to the high
content of oxygen, which limits its practical application7. Therefore, biofuels must be upgraded to
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lower the oxygen content for future application . Lignin, which constitutes ∼30 wt% of woody
biomass, is one of the most abundant bio-sources on earth, derive biofuels form lignin (typical
pyrolysis or hydrogenolysis) meets the requirements of green and sustainable society. Nevertheless,
upgradation of lignin-derived pyrolysis oil is more challenge due to its highly complex structure 9-
13
.
To study the upgradation process of lignin-derived pyrolysis oil, vanillin was selected as a
model molecule. Vanillin, a common component of lignin derived pyrolysis oil, is expected to
hydrogenated into 2-methoxy-4-methyl-phenol (MMP), which is a potential future biofuel in an
ideal upgradation process. Recently, special efforts have been focused on water phase-involved
hydrodeoxygenation (HDO) reactions to avoid the use of organic solvents. Some novel catalysts
have been developed to be effective7,14 , El-Shall and co-workers reported Pd nanoparticles
immobilized on a mesoporous sulfonic acid-functionalized metal–organic framework SO3H-MIL-
101(Cr) as an efficient bifunctional catalyst for HDO15. However, these catalysts catalyze the
reaction under harsh reaction conditions, such as high temperature (over 90 °C) and high pressure
(over 0.5 MPa). Besides, the by-product (4-hydroxymethyl-2-methoxy-phenol, HMP) was always
formed as a result of incomplete hydrogenation due to the low catalytic activity of the catalyst. A
big challenge exist in water phase-involved organic reactions is the contact between the substrates
and the active metal sites, while dispersion of the heterogeneous catalysts in the aqueous medium
is the foundation for excellent catalytic performance 16,17. Along this line, we thought that
amphiphilic catalysts may guarantee a good dispersion as well as a well contact between the
substrates and the active sites in water.
Several approaches for upgradation of bio-fuels to a liquid transportation fuel have been
established, among which catalytic HDO is considered to be the most important and feasible strategy.
Different noble metal nanoparticles supported on various supports have been demonstrated to be
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effective heterogeneous catalysts in the HDO reactions, such as Ru18,19, Pd7,20, Pt21. In those
established catalytic systems, activated carbon, metal oxides (e.g. TiO2 , MgO, CeO2 , and g-Al2O3 ),
zeolites, SiO2, and carbon nanotubes are frequently used as catalyst matrixes21,22. However, those
supports may suffer from poor dispersion of nanoparticle catalysts, weak catalyst–support
interactions, or poor water dispersibility.
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Recently, hybrid materials have been developed as catalyst supports with enhanced catalytic
performances.23 Cellulose, an earth-abundant environmentally benign biomass-derived material24
has been widely used as support for its biodegradability25-28. Here we focus on the use of cellulose,
which has 3 distinct advantages as a catalyst support: i) it is insoluble in common solvents, ii) it
bears many hydroxyl groups that are expected to stabilize metal NPs effectively, and iii) it has great
water wettability in aqueous phase. Using cellulose as catalyst support has attracted much attention
due to these outstanding properities. Various metal nanoparticles supported on cellulose have been
reported to catalyze different reactions29-34, for example, cellulose-supported chiral Rh nanoparticle
(NP) catalysts have been developed by Kobayashi and co-workers for asymmetric 1,4-addition of
arylboronic acids to enones and enoates. Interestingly, cellulose nanocrystals also have been
demonstrated to have chiral properties and could work as chiral introducer in heterogeneous
reactions35. Various surface modifications have been made to cellulose to improve its ability of
stabilizing NPs36 or enhance catalyst activity by grafting functional groups37. In our previous works,
we report a perfluorobutyl modified cellulose supported Pd catalyst for hydrogenation of nitroarenes
in water at room temperature, in which fluorinated compounds were demonstrated to be able to
stabilize metal nanoparticles38. What’s more, researchers also found that modified the catalyst
support with fluorinated ligand can enhance its lipophilicity39, which may improve the local
concertation of the substrate around the catalyst, thus greatly boost the reaction process. Other
hydrophobic compounds combined with catalyst support have been proved to be able to enhance
catalytic organic reaction in water by creating a hydrophobic environment around the active sites40,41
Herein, an amphiphilic catalyst was fabricated by introducing a trifluoromethyl group in
cellulose support (CF3-Cell), Pd nanoparticles (NPs) were directly deposited on the modified
cellulose. The Pd/CF3-Cell catalyst exhibited excellent catalytic activity and selectivity in
hydrodeoxygenation of vanillin to MMP, the reaction proceed smoothly under atmospheric
hydrogen pressure in neat water without any additives at low reaction temperature (50oC). Excellent
.
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performance of the catalyst may be due to the good dispersion of the catalyst in the reaction medium
(water), what’s more, introducing trifluoromethyl group on cellulose improve the lipophilicity of
the support, which accelerate the absorption of the substrates on the catalyst, thereby accelerating
the reaction process. To the best of our knowledge, this is the first catalytic system based on the
biomass-derived material for the upgradation of bio- fuels.
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1 Cell modification
Scheme 1. Modification of cellulose
Scheme 1 briefly illustrates the preparation of the CF3-Cell. The CF3-Cell composite was
prepared by using a modified procedure reported by MacLachlan et al42. Cellulose and
trimethoxy(3,3,3-trifluoropropyl)-silane were dispersed in toluene and stirred under reflux
temperature.
2 Catalyst characterization
Fig. 1. IR spectrum of Cellulose and CF3-Cell
Cellulose and CF3-Cell were characterized by IR spectrum (Fig.1). The strong peak around 3300
cm-1 which is due to the stretching vibration of the O-H bonds has an obvious decrease in the peak
intensity after cellulose was modified, which showed that the modification process has acted. A part
of the O-H bonds on the surface of cellulose has been changed into parts with trifluoromethyl group.
The O-H bending vibration frequencies at 1640 cm-1 demonstrated that the sample combined micro
amount of water, which nearly disappeared after fully dried. Compared to cellulose, new absorbance
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bands at 600 cm−1 corresponding to the symmetric stretching vibrations and the bending mode of
Si−O were detected.
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The amount of the palladium supported on CF3-Cell was 2.833 mg/g determined by ICP, which
was a little lower than the theoretical value (2.99 mg/g).
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Morphology of Pd/ CF3-Cell was directly observed by Scanning electron microscopy (SEM)
(Fig.S1). As shown in Fig.S1, the catalyst support is rod shaped with rough surface. The elemental
mapping was showed on Fig.S2, indicating that element F is successfully grafted on cellulose and
Pd is dispersed uniformly throughout the obtained CF3-Cell Deposition of Pd NPs on CF3-Cell is
examined by TEM (Fig.2a). The TEM image of the catalyst authenticated the formation of
palladium nanoparticles, which was amorphous spherical morphology with an average diameter of
2~3 nm as showed in Fig.2b. However, few particles in the size range of 5–7 nm was also found.
Fig. 2. (a) TEM image of Pd/ CF3-Cell. (b) Size distribution of Pd NPs of Pd/ CF3-Cell
Fig. S4 depicts the XPS spectrum in the Pd 3d region for Pd/CF3-Cell, the intense doublet
centering at 335.7 and 340.8 eV belongs to metallic Pd. And there is no other doublets ascribed to
the +2 oxidation state of Pd, illustrating no metal salts exists on nanoparticle surface.
2 Catalytic hydrodeoxygenation of vanillin
Considering that water is a desirable solvent for chemical reactions due to its environmentally
friendly nature, low cost and easy handling, we first conducted the reaction in water and found that
excellent results for the selective hydrodeoxygenation could be achieved over the cellulose-
supported Pd catalyst including a 100% conversion of vanillin with a 100% selectivity for the 2-
methoxy-4-methylphenol product within 2h (Table 1, entry 1). The rapid reaction might because of
the hydrophobicity of trifluoromethyl compounds. In the initial stage of the reaction, reactants can
be easily adsorbed by trifluoromethyl compounds, which improves the local concertation of the
substrate around the catalyst and then accelerates the reaction. While in the presence of 10 wt%
Pd/C catalyst, a rather low catalytic activity and selectivity were obtained when the reaction was
carried out under the same conditions (Table 1, entry 10). The results clearly show that the Pd/CF3-
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Cell catalyst shows significantly higher activity and 2-methoxy-4-methylphenol selectivity as
compared to the Pd/C catalyst. Over the pure support CF3-Cell, however, no reaction took place,
implying that Pd nanoparticles are inevitable for vanillin hydrodeoxygenation (Table 1, entry 11).
And the conversion and selectivity did not decrease obviously until the amount of catalyst was
decrease to 0.7 mol%, which means only 1 mol% catalyst can catalyze the reaction completely
within 2h (Table 1, entries 1-4). Temperature is also crucial for this reaction. Unreduced vanillin
and byproduct was found when temperature was lower than 50℃.
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Pd/CH3- Cell and Pd/C4F9- Cell were also examined in the reaction (Table 1, entries 12-13).
Lower activity and selectivity were obtained. This might because CH3- Cell has less hydrophobicity
and can not adsorb organic reactants efficiently while C4F9- Cell has more hydrophobicity and the
catalyst dispersed poorly in water.
Table 1 Optimization of Hydrodeoxygenation of vanillin Conditions a.
Sel. b (%)
Entry
Cat.(%)
T(℃) Con. b (%)
B
0
C
100
100
36
1
2
Pd/CF3-Cell(2.0)
Pd/CF3- Cell (1.0)
Pd/CF3- Cell (0.7)
Pd/CF3- Cell (0.5)
Pd/CF3- Cell (1.0)
Pd/CF3- Cell (1.0)
Pd/CF3- Cell (1.0)
Pd/CF3- Cell (1.0)
Pd/Cell(1.0)
100
100
100
100
80
100
100
94
0
3
64
96
0
4
93
4
5
100
100
100
94
100
100
100
41
6
60
0
7
50
0
8
40
59
69
78
0
9
50
92
31
10
11
12
13
Pd/C(1.0)
50
45
22
CF3- Cell (1.0)
50
0
0
Pd/CH3- Cell (1.0)
Pd/C4F9- Cell (1.0)
50
75
83
78
17
50
41
22
a Reaction conditions: vanillin (0.05mmol), catalyst, water
(3 mL), H2 (0.1 MPa)
b Conversion and selectivity were analyzed by GC.
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Fig. 3 Trend of reactants and product proportion as a function of reaction time
Fig. 3 shows the evolution of the product concentrations as a function of reaction time. There is
a rapid increase in the content of vanillin accompanied with the decrease of content of vanillin in
first 60 min. In the next 60 min, hydrogenolysis of vanillin alcohol proceeded very rapidly and
almost all of the vanillin alcohol had been converted to 2-methoxy-4-methylphenol via
hydrogenolysis. This trend illustrates that vanillin is mainly hydrogenated to vanillin alcohol in the
first step and then vanillin alcohol hydrogenated to 2-methoxy-4-methylphenol. The primary
reaction mechanism was proposed as follows: at the initial stage of the reaction, H2 is activated by
Pd NPs supported on the catalysts and vanillin is rapidly adsorbed by the lipophilic trifluoromethyl
compounds on the catalyst. And then vanillin was mainly hydrogenated to the vanillin alcohol
(intermedium) first and further converted to the final product by hydrogenolysis43,44
.
3 Catalysts reusability
The catalyst can be easily separated from the reaction solution by simple centrifugation and
the reusability of the catalyst was tested. The recovered catalyst was washed with ethyl acetate
(3×5 ml) and deionized water (3×5 ml). Then the recovered catalyst was used directly in next run
without further drying. The catalyst is highly stable and can be reused for four cycles with only a
litttle loss of its activity and selectivity as demonstrated (Table S1).
The amount of the palladium supported on cellulose after 5 runs was 2.37 mg/g determined by
ICP, indicating metal slight leaching during the reaction procedure, which might be the main reason
for the decrease of catalytic activity. As for every catalytic process, thePd leachingin product solution
after centrifugation was measured by ICP and listed in Table S2. The recovered catalyst was
examined by TEM analysis and a slight increase of palladium particle size was observed (Fig. S5a
and Fig. S5b), which was believed to be another reason for the de-activation of the catalyst.
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Meanwhile, XPS spectrum of Pd 3d region of Pd/ CF3-Cell after 5 runs showed Pd nanoparticles
maintained Pd0 state (Fig. S6). This is might because the reaction proceeded under H2 atmosphere.
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In conclusion, we have developed a heterogeneous tandem catalyst for the hydrodeoxygenation
of vanillin, a common component in lignin-derived bio-oil, under mild reaction conditions. The
developed Pd/CF3-Cell catalyst exhibited high catalytic activity and selectivity in tandem
hydrogenation–deoxygenation reactions and could be reused for four times with only slight loss in
activity and selectivity. The high catalytic performance of Pd/ CF3-Cell is attributed to the unique
characteristics of the CF3-Cell support, which leads not only good dispersion of the catalyst in water
but also increase the local concertation of the substrate around the catalyst. This Pd/ CF3-Cell
catalyst holds promising potential for the biofuel upgrade process in aqueous phase.
EXPERIMENTAL SECTION
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1 Chemicals
All reagents with AR purity (analytical reagent grade) were purchased and used as received
without further purification.
2 Cellulose modification
In a typical procedure, α-cellulose (1 g) was dispersed in toluene (20ml). Then trimethoxy(3,3,3-
trifluoropropyl)-silane (0.5 g) was added. The mixture was stirred at 110℃ with a condenser for
24h. The final solid was separated by filtration and washed with deionized water (3×15 ml) and
ethanol (3×15 ml) before dried at 60°C for 5 hours.
3 Catalyst preparation
Pd catalyst was prepared by chemical reduction with NaBH4 as reductant. 1 mg PdCl2 was
dissolved in 0.5 ml water and 1 mg NaCl was added to accelerate dissolution. To CF3-Cell (200 mg)
in ethanol (6 ml) in a round-bottom flask (50 mL), the PdCl2 solution was added and the mixture
was stirred at room temperature for 5 h. Then a fresh prepared ethanol solution of NaBH4 (0.8 mg,
in 0.5 mL ethanol) was added into the reactor quickly under vigorous stirring at room temperature
under argon atmosphere. The color of the mixture would turn to black immediately which indicates
that metal salts have been reduced to metal particles. After stirring for another 1 h, the mixture was
filtrated and the solid was dried in vacuum oven at 25°C for 12 h and stored in a desiccator.
4 Catalyst characterization
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Infrared (IR) spectra were collected on a Nicolet NEXUS670 Fourier transform IR
spectrophotometer at room temperature.
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The Pd content in the Pd/CF3-Cell catalyst was measured by using an IRIS Intrepid II XSP
inductively coupled plasma-atomic emission spectrometer (ICP-AES).
Scanning electron microscopy (SEM) was performed using a Hitachi S-4800 apparatus on a
sample powder previously dried and sputter-coated with a thin layer of gold.
Transmission electron microscopy (TEM) was carried out on a JEOL JEM-1200 operating at
300 kV. The sample was diluted in ethanol and sonicated for 10 min to get an suspension. The
ethanol slurry was then added dropwise onto a Cu grid covered with a thin film of carbon.
The surface electronic states were investigated by X-ray photoelectron spectroscopy (XPS,
Thermo VG ESCALAB25 using AlKa radiation). The XPS data were internally calibrated, fixing
the binding energy of C 1s at 284.6 eV.
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5 Catalytic HDO of vanillin
Hydrogen was chosen as the hydrogen donor for the hydrodeoxygenation of vanillin. The
catalytic hydrodeoxygenation of vanillin was carried out in a schlenk tube. Typically, the reactant
and catalysts were dispersed into water, then the schlenk tube was purged with H2 four times to
replace air. Then the mixture was stirred at desired temperature. After reaction, the mixture was
extracted by ethyl acetate. The product and unreacted reactant were analyzed by GC-MS. The
catalysts were separated by centrifugation and washed with ethyl acetate (3×5 ml), deionized water
(3×5 ml) and then directly used in the next cycle.
ASSOCIATED CONTENT
Supporting Information Available: HPLC spectra and MS spectra of synthesized compounds.
This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: c.cai@mail.njust.edu.cn
Notes
The authors declare no competing financial interest.
Acknowledgments
We gratefully acknowledge the Nature Science Foundation of Jiangsu Province (BK
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20140776) for financial support. This work was also supported by the National Natural
Science Foundation of China (21402093 and 21476116) and the Chinese Postdoctoral
Science Foundation (2015 M571761 and 2016 T90465) for financial support. We also
gratefully acknowledge the Priority Academic Program Development of Jiangsu Higher
Education Institutions for financial support.
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