Transfer hydrogenation of bio-fuel with formic acid over biomass-derived N-doped carbon supported acid-resistant Pd catalyst

Article abstract of DOI:10.1039/c6cy02461k

Nitrogen-enriched highly mesoporous carbons (NMCs) are synthesized via the one-pot carbonization of biomass-derived glucose and harmless melamine with ZnCl₂ as the porogen agent and catalyst. NMC contains an N content of 15.4 wt% and specific surface area of 1017 m² g^-1 with the mesopore volume proportion of 92.1%. Owing to its rich N species and high mesoporosity, NMC can be adapted as a proper support for the fabrication of well-dispersed Pd catalysts for the transfer hydrogenation of vanillin in the water phase with formic acid (FA) as the hydrogen donor. For example, Pd/NMC exhibits 2.9 times higher activity in comparison to nitrogen-free catalysts, and affords 100% vanillin conversion with 2-methoxy-4-methylphenol (MMP) as the sole product. The Pd/NMC catalyst demonstrates enhanced acid-resistance in acid media and adsorption of substrates. It is found that the electron-deficient Pd (Pd^δ+) percentage is affected by the N species, and the strong Pd-N interaction generates the co-existence of Pd^δ+ and metallic Pd (Pd⁰), which results in Pd/NMC as a novel bifunctional nanocatalyst for both FA dehydrogenation and vanillin hydrogenation.

Full text of DOI:10.1039/c6cy02461k

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Transfer hydrogenation of bio-fuel with formic acid over biomass-
derived N-doped carbon supported acid-resistant Pd catalyst
Received 00th January 20xx,
Accepted 00th January 20xx
Renfeng Nie, Xiaolun Peng, Haifu Zhang, Xiaolong Yu, Xinhuan Lu, Dan Zhou and Qinghua Xia*
Nitrogen-enriched highly mesoporous carbons (NMCs) are synthesized through one-pot carbonization of biomass-derived
glucose and harmfulless melamine with ZnCl2 as porogant agent and catalyst. NMC contains N content of 15.4 wt% and
specific surface area of 1017 m2g-1 with mesopore volume proportion of 92.1%. Owing to the rich N species and high
mesostructure, NMC can be adapted as proper support for fabricating well-dispersed Pd catalyst for transfer
hydrogenation of vanillin in water phase with formic acid (FA) as hydrogen donor. For example, Pd/NMC exhibits 2.9 times
higher activity in comparison to nitrogen-free catalyst, and affords 100% vanillin conversion with 2-methoxy-4-
methylphenol (MMP) as sole product. The Pd/NMC demonstrates enhanced acid-resistance in acid medium and
adsorption for substrate. It is found that the electron-deficient Pd (Pdδ+) percentage is affected by the N species, and the
strong Pd-N interaction generates co-existence of Pdδ+ and metallic Pd (Pd0), leading to Pd/NMC as novel bifunctional
nanocatalyst for both FA dehydrogenation and vanillin hydrogenation.
DOI: 10.1039/x0xx00000x
www.rsc.org/
dehydrogenation reaction (HCOOH → H2+CO2). However, the
accompanying dehydration pathway (HCOOH→H2O+CO) should be
Introduction
strictly controlled, as CO impurities are not well tolerated by
catalysts.11 Many homogeneous and heterogeneous catalytic
systems have been shown to be effective for H2 evolution by FA
decomposition.10, 12, 13 However, despite high efficiencies of FA as
hydrogen source have been obtained in combination with a high
amount of organic bases,14-17 these homogeneous bases result in
increase of complexity and cost in practice. It is strongly desirable if
we can find catalysts that can selectively decompose formic acid to
H2 and CO2 (instead of H2O and CO) and hydrogenate bio-oil with
high selectivity under base-free, heterogeneously-catalysed, cost-
effective and environmentally-benign conditions.
The use of renewable biomass to replace the limited fossil fuels
has been recognised as an potential solution for the sustainable
production of liquid fuels and valuable chemicals.1 For instance,
sustainable fuels obtained by the hydrodeoxygenation (HDO) of
phenolic bio-oils derived from pyrolysis of lignin or cellulose offer
good prospects for industrial applications.2 In conventional
methods, HDO of bio-oil was mainly conducted in the liquid phase
with an excess amount of hydrogen gas (>25 times) and the high
selectivity is generally achieved on harsh conditions.3, 4 Gaseous
hydrogen is difficult and dangerous to handle due to its hazardous
properties. Therefore, there is a strong incentive to develop
environmentally sustainable and chemoselective methods for HDO
of bio-oil without using hydrogen gas.
Nitrogen-doping carbon has been proven to be a fascinating
catalytic material.18 Doping with electron-rich nitrogen modifies the
surface structure of carbons, endows enhanced π-binding ability
A
alternative route to remove excess oxygen-containing
and improved basicity,19 which enables the applications of carbon
functional groups in bio-oil is via catalytic transfer hydrogenation
(CTH), which employs renewable organic compounds such as
alcohols and organic acids as the hydrogen donor.5, 6 However, the
use of various alcohols can lead to side reactions (esterification or
condensation with the intermediate products) and reduce the
selectivity of desired products, and the separation of
dehydrogenized byproducts makes CTH process complicated.7-9
Formic acid (FA), a byproduct from the acidic hydrolysis of biomass,
has attracted a great deal of attention as a suitable liquid for
hydrogen production as an energy source and for hydrogenation
reactions.10 Hydrogen stored in FA can be released via a catalytic
20
with improved performance in catalysis.3,
Meanwhile, in
traditional CTH process, FA-triethylamine system is frequently used
for improving dehydrogenation of FA.14-16 Here, similar chemical
environment of nitrogen in triethylamine and nitrogen-doped
carbon may make N-doped carbon as ‘heterogeneous N-type
species’ for N-mediated FA dehydrogenation, affording some
attractive characters (high stability, recyclability, etc.). As expected,
carbon nitrides have been reported as efficient supports for Pd-
involved CTH process.3 The role of N-doping in modifing the
electron density of the supported metals (Mott-Schottky effect) and
the catalytic activity has been recently highlighted.21 Bulusheva et
al. 22, 23 found that Pd or Pt supported on a N-doped carbon support
has demonstrated 3-10 times higher catalytic activity in FA
dehydrogenation than that of undoped catalysts, assigning to the
activity of electron-deficient Pd or Pt stabilized by pyridinic type
24
nitrogen atoms located on vacancy sites. Xiao’s group deposited
Pd NPs on a composite support (TiO2@N-C) for the HDO of vanillin.
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N-incorporation, the pore volume and pore size increase
obviously. Moreover, the NMC contains mesopore volume
proportion of 92.1 %, higher than that of MC (78.4 %). We
interpret the formation of such a nanostructure seems to be
related to the promotion from ZnCl2 and melamine. It was
reported that ZnCl2 could catalyze the biomass conversion by
stripping hydrogen and oxygen species away from precursor as
water rather than as hydrocarbons or as oxygenated organic
compounds.26 In this work, the Lewis acid ZnCl2 might
coordinate with melamine and catalyze the formation of large-
sized mesopore.27
Scheme 1 Preparation of the Pd/NMC nanocatalysts.
XRD analyses (Fig. S4) of all carbons show C(002) diffraction
They found that the Pd0 and Pd2+ species could be active for the FA
peak with large widths and low intensities at 2θ
≈
23o,
25
ascribing to amorphous carbon.28 Raman spectra in Fig. S5a
show two overlapping bands at 1350 cm-1 and 1570 cm-1,
representing disordered and sp2 bonded graphitic carbons,
respectively.29 Compared with MC (Table S2), the G peak of
NMC obviously broadens and downshifts from 1590 to 1579
cm-1, indicating higher disorders of carbon skeleton in NMC.30
The FT-IR spectra are shown in Fig. S5b, where contains three
bands at 1363, 1616 and 3140 cm−1 that ascribed to aromatic
CN, C=N and N-H stretching bonds, respectively. Clearly those
bands for NMC seem to be strengthened in comparison to MC,
indicating more N atoms exist in carbon.
dehydrogenation and the biofuel HDO. Xu et al. established that
oxidation of Ru/C created a bifunctional catalyst, consisting of
coexisting metal (Ru) and Lewis acid (RuOx) sites, and this
bifunctionality was key to achieving high performance without
hydrogenating the furan ring or cracking the C-C bonds. These
results indicate that the electronic property of metal could be tuned
by N-doped carbon and plays an important role in the CTH of bio-
oil, so that the careful design is necessary for obtaining desirable
catalytic performance.
Herein, nitrogen-enriched highly mesoporous carbons (NMCs)
are prepared through one-pot carbonization of biomass-derived
glucose and melamine, with ZnCl2 as the porogen agent and
catalyst, which can be used as basic support to construct Pd/NMC
catalysts (Scheme 1) for the HDO of vanillin with FA as hydrogen
donor. The influences of nitrogen source on the textural structure
of carbons, the electronic structure of Pd and the catalytic
performance of Pd/NMC are also investigated. Finally, based on the
results, a plausible mechanism for the CTH of vanillin to 2-methoxy-
4-methylphenol (MMP) with FA over Pd/NMC was proposed.
The C1s peak of NMC (Fig. S7), locates at 284.5 eV with a tail
peak at high binding energy of 285-290 eV, is assigned to
electron-deficient carbons bound to nitrogen/oxygen (C=N/C-
N and C-O).31 For NMC, the C=N/C-N signals are significantly
strengthened, suggesting that more N atoms have been doped
into the carbon skeleton,26 well agreed with the FT-IR result
(Fig. S5b). Elemental analysis (Table S2) shows that the N
content of NMC is 15.37 wt%. The N content of NMC
calculated by XPS measurement is about 14.85 wt %, which is
very similar to the elemental analysis result. The elemental
maps (Fig. S8) show the homogeneously dispersed N atoms in
carbon skeleton. Moreover, rich N doping would endow NMC
with better hydrophilicity in water (Fig. S9).
Results and discussion
Characterizations of nanostructured N-doped carbon
materials.
With ZnCl2 as porogen, the effect of melamine amount on
the textural property of NMCs is investigated (Table S3).
Elemental analysis shows that, with increasing the ratio of
melamine/glucose to 7:1, the N content rises quickly to 21.43
wt %, much higher than most reported N-doped carbons.
Meanwhile, with increasing the melamine/glucose ratio from
0:8 to 4:4, the surface area of carbons reduce slightly from
1228 m2/g to 1017 m2/g, indicating that porous structure can
well retained in the range. When the melamine/glucose ratio
is further increased to 7:1, the surface area and pore volume
drop to 540 m2/g and 0.67 cm3/g, revealing that excess
melamine would destroy its porous structure. Notably, the
ZnCl2-catalyzed carbonization of pure melamine gives no
product under the same condition, indicating that co-existence
of glucose and melamine is indispensable for the formation of
highly mesoporous N-doped carbon.
The morphology of NMC is observed by SEM and TEM
images (Fig. S1 and S2). MC appears to be composed of
primary rough carbon particles with the size around 100 nm,
which are aggregated to large networks structure with obvious
porous morphology. By contrast, NMC displays smaller
particles (10-50 nm) with porous structure.
N2 sorption isotherms (Fig. S3) show that MC and NMC
display obvious N2 adsorption at relative pressure of 0.4–1.0
(Fig. S3a), indicating the existence of mesoporous structure.
Compared with MC, NMC exhibits type-IV curves with H3
hysteresis loop and a sharp steep at the relative high pressure
(~0.8), reflecting a large external surface area and the
presence of excessive mesopore. Fig. S3b gives the pore-size
distribution curves of MC and NMC. Both the samples contain
a similar fraction of mesopores in the range of 2-40 nm, but
the mesopore pore volume of NMC enlarges markedly. The
calculated BET surface area, pore volume and pore size of MC
and NMC are 1228 m2/g and 1017 m2/g, 1.28 cm3/g and 1.41
cm3/g, 4.3 nm and 9.1 nm, respectively (Table S1). The results
show that, although the surface area declines slightly after the
Pd-based catalysts are synthesized by a non-surfactant strategy
using H2 as the reductant. As shown in Fig. 1, both Pd/MC and
Pd/NMC show broad (111) peak with low intensity. The crystallite
sizes of Pd particles, estimated from Scherrer’s equation by (111)
2 | J. Name., 2012, 00, 1-3
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Table 1 The structure properties of the various catalysts
Percentage
(%)
Percentage (%)
Pd loading
(wt %)a
davg
DPd
Catalysts
Pd/MC
(nm)b
(%)c
Pd0
Pdδ+
Pyridinic
N
N-
Pd
Pyrrolic
N
Quaternary
Pyridine
oxide N
N
-
19.8
22.6
-
-
-
-
71.4
33.9
28.6
66.1
2.14
2.21
4.9
4.3
Pd/NMC
a Determined by ICP.
b Determined by TEM.
36.7
12.5
50.7
0.1
0
c Dispersion of Pd calculated as D=0.97/particle size in nm.
the immobilization and dispersion of Pd NPs on carbons could be
improved by N doping.
The synthetic strategy of Pd/NMC includes ion adsorption and
reduction (Scheme 1). Pd2+ is firstly coordinated using NMC as
heterogeneous ligand, and then mildly reduced by H2.32 It is found
that NMC supported Pd has higher percentage of electron-deficient
Pd (Pdδ+) than that of N-free counterpart (Fig. 3a), which can be
ascribed to the strong interaction between N groups and Pd NPs.33
For example, Pd/MC contains 28.6 % Pdδ+, attributed to the
oxidation of Pd due to the formation of Pd-O bond by air
exposure,34 while this value increases obviously to 66.1 % for
Pd/NMC. It can be assigned those rich Pdδ+ to the formation of
a covalent bond of Pd with pyridinic nitrogen with partial ionic
♦
C(002)
Pd(111)
ꢀ
(b)
(a)
20
30
40
50
60
70
80
2 Theta (degree)
character. Arrigo et al. also notice
to N-doped carbon makes it reduction difficult. Stonkus et al.
d
33 that coordination of Pd2+
Fig. 1 XRD patterns of (a) Pd/MC and (b) Pd/NMC.
35
confirme
d
that the deposition of Pd on N-doped CNFs
peak, are about 4.8 and 4.5 nm for Pd/MC and Pd/NMC,
respectively. The Pd dispersion on carbons is observed by TEM (Fig.
2). Comparatively, Pd/NMC exhibits relatively dispersed Pd NPs
than those on N-free carbon (MC). Moreover, the average Pd sizes
with standard deviation (Table 1) in Pd/MC and Pd/NMC are
4.9±1.23 and 4.3±0.85 nm, respectively. These results reflect that
changes the redox properties of Pd in comparison to that of Pd
Pd0
Pdδ+
(a)
25
a
davg=4.9±1.23 nm
Pd/NMC
20
15
10
5
Pd/MC
330
332
334
336
338
340
342
344
346
Binding Energy (eV)
0
0
2
4
6
8
10
100 nm
Diameter / nm
25
(b)
davg=4.3±0.85 nm
pyridinic N
N-Pd
pyrrolic N
b
20
15
10
5
quaternary N
0
Pd/NMC
396
0
2
4
6
8
10
100 nm
Diameter / nm
394
398
400
402
404
406
408
Fig. 2 TEM images and particle size distributions of (a) Pd/MC and (b)
Binding Energy (eV)
Pd/NMC.
Fig. 3 XPS spectra of (a) Pd3d and (b) N1s in NMC.
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on N-free CNFs. The evidence of N-Pd interaction is further on carbons (Fig. S9).37 It is reported that the more hydrophilic the
provided through the N1s deconvolution of Pd/NMC (Fig. 3b). catalyst, the more active it was for the HDO of vanillin in water.38
Apart from pyridinic (398.4 eV), pyrrolic (399.8 eV), and graphitic N Moreover, the adsorption test (Fig. S10 and S11) shows that the
(401.2 eV), the presence of more types of N species with a adsorption capacity of vanillin over NMC (34 mg/g) is higher than
binding energy of 398.7-398.8 eV could be accepted for
a
that of MC (12 mg/g), exhibiting a favorable interaction between
better fit of N1s spectrum, which can be assigned to N species vanillin and NMC. The strong adsorption leads to an increased
interacting with Pd (N-Pd component).22 This observation is in vanillin concentration and accelerated the diffusion and
36
agreement with the recent report
that N1s spectrum transformation of substrate on catalyst.
The impact of melamine/glucose ratio on vanillin hydrogenation
is also investigated (Table 2, entries 6-8). With increasing the ratio
from 0:8 to 4:4, the vanillin conversion increases from 30.2 to 86.7
% for Pd/NMC. Even higher weight ratio (up to 7:1) would suppress
the activity. The obvious difference on the activities demonstrates
that both the incorporated N and pore structure play essential role
in the formation of catalytically active sites and creation of an
efficient catalyst. Although the increased melamine amount leads
increases obviously in the same region after deposition of Pd-
Au NPs on N-doped CNTs. Here, Pd/NMC contains N-Pd
percentage up to 12.5% (Table 1).
Hydrodeoxygenation of vanillin with FA
The vanillin hydrogenation using FA as hydrogen source in
aqueous phase is chosen to evaluate the catalytic activity of
supported Pd catalysts (Table 2). The products of vanillin
hydrogenation are mainly 2-methoxy-4-methylphenol (MMP) and 4- to an increase of N content, excessive melamine would destroy the
hydroxymethyl-2-methoxyphenol (HMP). The mass balance of
vanillin and detected products is > 95%. In blank test, no catalyst
leads to nearly no vanillin transformation (entry 1). The Pd/MC
shows low vanillin conversion (30.2 %) and 96.2 % MMP selectivity.
As for NMC supported Pd catalyst, significant improved vanillin
conversion (86.7 %) with 100 % MMP selectivity could be achieved.
Further prolonging to 3 h leads to full vanillin conversion and 100 %
MMP selectivity. Comparatively, other frequently-used Pd catalysts
(Pd/SiO2, Pd/Al2O3, Pd/CB and Pd/AC, entries 8-12) achieve poor
catalytic activity or MMP selectivity under the same conditions.
NMC greatly improves the catalytic performance in selective
hydrogenation of vanillin with FA in comparison to N-free carbon.
Firstly, the high content of Pdδ+ species on N-doped support
indicates that part of these species is stabilized as metal complexes
by nitrogen atoms.33 It is reported that N- metal structure is the
ideal active site for the formic acid decomposition with a high rate
and high selectivity of H2.22, 23 Therefore, both the high N content
and rich pore of NMC lead to high dispersion of Pd and high
percentage of Pdδ+, thus predetermining its high catalytic activity
for FA dehydrogenation and vanillin hydrogenation.
porous structure of NMCs, leading to negative effect on the
catalytic activity.
The activity is also affected by reaction temperature (Fig. 4).
When Pd/NMC is used as catalyst, the vanillin conversion is
enhanced evidently from 8.2 to 86.7 % as increasing temperature
from 110 oC to 150 oC, but higher temperature leads to a decrease
100
Conv.
Sele.
80
60
40
20
0
110
130
150
170
T (oC )
Secondly, the rich surface N can serve as defect sites for
increasing the hydrophilicity of carbons and wettability of substrate
Fig. 4 Hydrogenation of vanillin under different temperature.
Reaction conditions: vanillin (0.5 mmol), FA (3 mmol), Pd/NMC (20 mg), H2O
(10 mL), 0.5 MPa N2, 2 h.
Table 2 Hydrogenation of vanillin over different catalysts.a
Entry
Catalysts
-
Pd/MC
t (h) Conversion (%) Selectivity (%)
100
1
2
2
2
<1
30.2
-
Conv.
Sele.
96.2
80
3
Pd/NMC
Pd/NMC
2
3
86.7
100
100
4
>99.0
60
5 b
6 b
7 b
8
Pd/NMC(6:2)
Pd/NMC(2:6)
Pd/NMC(1:7)
Pd/Al2O3
Pd/SiO2
Pd/MgO
Pd/CB
Pd/AC
2
2
2
2
2
2
2
2
65.7
81.5
49.3
1.8
100
100
100
83.3
100
53.1
100
94.4
40
0% conv.
20
9
1.3
10
11
12
11.3
13.5
21.7
0
0
-1
1
3
5
7
nFA (mmol)
a Reaction conditions: vanillin (0.5 mmol), FA (3 mmol), catalyst (20 mg), H2O
(10 mL), 0.5 MPa N2, 150 oC.
Fig. 5 The effect of FA amount on vanillin hydrogenation.
Reaction conditions: vanillin (0.5 mmol), Pd/NMC (20 mg), H2O (10 mL), 0.5
MPa N2, 150 oC, 2 h.
b The value in parentheses is the weight ratio of melamine/glucose.
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FA decomposition.22 Here, a large amount of Pdδ+ species on NMC is
probably main factor that is responsible for FA decomposition (as
shown in Fig. 7). Therefore, we infer that the hydrogenation of
vanillin with FA is a tandem process comprising at least two steps:
the FA dehydrogenation on electron-defficient Pd (Pdδ+) and the
vanillin hydrogenation on metallic Pd (Pd0, Fig. 7).
100
80
60
40
20
Conv.
Sele.
Considering the acidic environment due to the presence of FA,
the recycle of catalyst is an important issue for the development of
sustainable chemical. The Pd/NMC catalyst can be easily recovered
and reused for five times without significant loss of efficiency (Fig.
8), and the ICP analysis (Table S6) of fresh and recovered catalysts
0
0
0.5
reveal negligible Pd leaching (0.6 %) in aqueous solution. The
crystalline state and dispersion of Pd NPs in recovered Pd/NMC,
based on the corresponding TEM image (Fig. 9), do not show a
significant change after five runs. In contrast, when Pd/MC is used 5
1
2
3
4
t (h)
Fig. 6 Time-activity profile for vanillin hydrogenation.
Reaction conditions: vanillin (0.5 mmol), FA (3 mmol), Pd/NMC (20 mg), H2O
(10 mL), 0.5 MPa N2, 150 oC.
of activity to 69.2 %. The effect of FA amount on catalytic activity is
also studied under same condition (Fig. 5). In the blank test, almost
no vanillin transformation occurs. Upon increasing FA to 5 mmol,
the vanillin conversion quickly reaches to 95.3 % at 2 h. However,
further increasing FA amount greatly suppresses the activity of
Pd/NMC (59.8 %).
100
Conv.
Sele.
80
60
40
20
0
The evolution of reactant and products with reaction time is
performed over Pd/NMC catalyst (Fig. 6). It can be found that
vanillin is quickly hydrogenated to MMP in the first 1 h and the
conversion further reaches >99.0 % at 3 h. Notably, nearly no HMP
is detected and the MMP selectivity remains >99.5% in the whole
process. This result indicates a different hydrogenation route in
comparison to the earlier observation that using molecular
hydrogen as hydrogen source, in which HMP is firstly generated and
1
2
3
4
5
Reuse
Fig. 8 Re-uses of Pd/NMC for vanillin hydrogenation.
Reaction conditions: vanillin (0.5 mmol), FA (3 mmol), Pd/NMC (20 mg), H2O
(10 mL), 0.5 MPa N2, 150 oC, 2 h.
35
davg=6.46
±1.90 nm
30
25
20
15
10
5
0
0
2
4
6
8
10 12 14
Diameter / nm
Fig. 7 Possible reaction mechanism of transfer hydrogenation of vanillin over
25
20
15
10
5
Pd/NMC.
davg=4.6±1.13 nm
b
then hydrogenated into MMP under high concentration of HMP.39 It
could be concluded that FA plays an important role for the
hydrogenation of vanillin into MMP as sole product through the
possible interaction with vanillin or intermediates that suppresses
the generation of HMP. Our previous study showed that
mesoporous CN supported Pd was active for water-phase
hydrogenation of vanillin at low H2 pressure (<1 MPa) and
0
0
2
4
6
8
10
temperature (<70 C).32 Thus we speculate the FA dehydrogenation
o
50 nm
Diameter / nm
could be the rate-determining step in the transfer hydrogenation of
vanillin over Pd catalyst (Table S4 and S5). It is reported that
isolated Pd2+ cations stabilized by a pair of pyridinic type nitrogen
atoms in N-doped carbon are active for hydrogen production from
Fig. 9 TEM images and particle size distributions of reused (a) Pd/MC and
(b) Pd/NMC after carrying out the hydrogenation reaction of vanillin for 5
times.
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times, about 7.3 % of Pd in Pd/MC is leached into the water, and Characterization
large cluster of multiple Pd particles can be clearly observed in TEM
Powder X-ray diffraction (XRD) was performed on a Bruker D8A25
diffractometer with CuK radiation ( =1.54184 Å) operating at 30
kV and 25 mA. Fourier transform infrared (FTIR) spectra were
analyzed on an OPUS Fourier Transform Infrared
image. These results indicate that rich N species on NMC may
provide more strong Pd-N interaction at the interface, making N-
doped carbon supported Pd as acid-tolerant and highly-active
bifunctional catalyst in water system.
α
λ
Spectrophotometer in the range of 400-4000 cm-1. Raman spectra
were collected at room temperature from 100 to 4000 cm-1 with
514.5-nm argon ion laser (Rhenishaw Instruments, England). N2
adsorption was carried out at -196 oC using an auto-adsorption
analyzer (Micromeritics, TriStar II). N2 adsorption was carried out at
-196 oC using an auto-adsorption analyzer (Micromeritics, TriStar II).
Before adsorption measurements, the samples were degassed at
250 oC under vacuum overnight. The total pore volume was
determined from the aggregation of N2 vapor adsorbed at a relative
pressure of 0.99. The micropore volume (Vmicro) was estimated
using the αs method.40 The mesopore volume was calculated by
subtraction of the micropore volume from the total pore volume
(Vmeso=Vtotal-Vmicro). The specific surface area was calculated by using
the Brunauer−Emmeꢀ–Teller (BET) method, and the pore size
distributions were measured by using Barrett−Joyner−Halenda
(BJH) analysis from the desorption branch of the isotherms. CHNS
elemental analysis for the content of non-metallic elements was
performed using a vario EL III Elementar. Scanning electron images
(SEM) were collected on JEOL (JSM6700F)) at an accelerating
voltage of 15 kV. Transmission electron microscope (TEM) images
were obtained using an accelerating voltage of 200 kV on a JEOL-
135 2010F Transmission Electron Microscope. The particle size
Conclusion
Nitrogen-enriched highly mesoporous carbons (NMCs) are
prepared via one-pot carbonization of biomass-derived glucose,
nitrogen source and chloride salt. Using ZnCl2 as porogen and
melamine as nitrogen source, the obtained NMC possesses high
surface area, high mesopore volume proportion and high
N
content. This material is adapted for anchoring highly dispersed Pd
and endows Pd atoms with two states (electron-deficient Pd (Pdδ+)
and metallic Pd). The as-prepared novel catalyst shows high activity,
stability and selectivity for the HDO of vanillin to MMP (> 99%) with
FA as hydrogen donor, which could be attributed to the high
mesostructure, rich N species and intimate interaction between Pd
and N on surface of NMC. This work offers a simple way to prepare
N-doped carbons from biomass and use them as ‘heterogeneous N-
type ligand’ for metal-mediated transfer hydrogenation of biomass.
Experimental section
Materials
The chemicals used included glucose (99 %, Sigma-Aldrich), PdCl2 distribution was obtained based on randomly measuring more than
(Pd >59 %, Aladdin), melamine (99 %, Sinopharm), ZnCl2 (98 %, 200 primary Pd NPs excluding the agglomerations in TEM images.
Sinopharm). All other chemicals were of analytical purity if not Thermal gravimetric analysis (TGA) was carried out with a Perkin
otherwise noted.
Elmer thermoanalyzer under nitrogen atmosphere. X-ray
photoelectron spectra (XPS) were recorded on a Perkin-Elmer PHI
ESCA system.
Preparation of NMC
In a typical process for NMCs, glucose+melamine (8 g, weight
ratio=1:1) and ZnCl2 (13.63 g) were mixed (~60 min) by grinding. The
mixture was then transferred into a ceramic boat, and moved into a
Vanillin adsorption
150 mg of NMC was added into a 25 mL of vanillin aqueous
tubular furnace, heated to 800 oC at a heating rate of 3.3 oC·min-1 solution with an initial concentration of 500 ppm and stirred at 30
under a flow of nitrogen and kept for 1 h. After natural cooling to oC. At a given interval, 0.5 mL of the suspension was diluted 20
ambient temperature, the sample was crushed into powders and times with water. The supernatant was then obtained by
thoroughly washed with 3M HCl solution and DI water until no Cl− centrifugation and analysed by a Shimadzu UV-visible UV-2550
detected in the supernatant. The product was then collected from spectrometer. The amount of vanillin adsorption was calculated
the dispersion by filtration and finally dried in a vacuum at 60 oC for according to the equation: the amount of vanillin adsorption
12 h, and was denoted as NMC. For the synthesis of MC, no
nitrogen source was added.
ꢀ
ꢅ
ꢁꢂꢃꢁꢄ ×ꢆ
=
ꢂ. Here C0 is the initial concentration of vanillin, Cr is the
ꢇꢈ
residual vanillin concentration after adsorption, V0 is the volume of
aqueous solution, mc is the mass of sample.
Preparation of Pd NPs over NMC
One gram of NMC and 41.7 mg PdCl2 were dispersed in water (50
mL). After stirring for 30 min, the mixture moved into Teflon-lined
Catalytic tests
o
autoclave and was performed at 40 C for 8 h under 2 MPa H2. The
The hydrogenation reactions were performed in a 50 mL
stainless autoclave with a Teflon liner. In a typical run, 20 mg
catalyst, 138 mg FA and 76 mg vanillin were dispersed in 10 mL
DI water. The autoclave was then sealed, purged and
solid was then recovered by filtration and washed thoroughly with
DI water until free of Cl-. The resulting Pd catalyst was dried in
vacuum oven at 80 oC overnight and named as Pd/NMC. MgO-,
Al2O3-, SiO2-, carbon black (CB) and active carbon (AC) supported Pd
catalysts were also synthesized by the same method. The real
amount of loading Pd was detected via ICP analysis.
o
pressurized with N2 to 0.5 MPa, and heated to 150 C under
magnetic stirring (1000 rpm). After reaction, the autoclave was
quickly cooled down within cold water. The organic
6 | J. Name., 2012, 00, 1-3
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ARTICLE
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DOI: 10.1039/C6CY02461K
Graphical Abstract
The strong PdꢀN interaction on biomassꢀderived nitrogenꢀenriched highly
mesoporous carbons (NMCs) generates coꢀexistence of electronꢀdeficient Pd (Pdδ+)
and metallic Pd (Pd0), leading to Pd/NMC as novel bifunctional nanocatalyst for both
FA dehydrogenation and vanillin hydrogenation.