In situ MnOx/N-doped carbon aerogels from cellulose as monolithic and highly efficient catalysts for the upgrading of bioderived aldehydes

Article abstract of DOI:10.1039/c8gc01413b

Herein, we report a sustainable route to in situ synthesize a monolithic MnO_x/N-doped carbon aerogel catalyst (Mn-NCA) by pyrolysing MnO(OH)₂-cellulose aerogel precursors based on an alkali-urea aqueous system. The as-obtained Mn-NCA showed highly efficient catalytic activity for the transfer hydrogenation of a broad range of biomass-derived aldehydes, yielding 90-100% conversion and 64-100% selectivity to the corresponding alcohols under mild conditions in an oven without agitation. A combination of controlled experiments and detailed characterization studies indicated that the superior performance of Mn-NCA is attributed to the monolithic three-dimensional (3D) hierarchical porous architecture and the synergistic effects between homogeneously dispersed MnO_x nanoparticles (NPs) and urea-derived basic sites. The monolithic feature of Mn-NCA exhibits superior dispersibility and separability compared to conventional centrifugation and filtration techniques in a powdery catalytic system. Moreover, a possible reaction mechanism is proposed. Our work provides a new method for developing highly efficient monolithic catalysts from renewable biopolymers for biomass valorization.

Full text of DOI:10.1039/c8gc01413b

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Heterogeneous catalytic oxidation for lignin valorization into valuable
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In situ MnOx/N-doped carbon aerogels from cellulose as
monolithic and highly efficient catalysts for upgrading of
bioderived aldehydes
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Shenghui Zhou a, Guixian Chen a, Xiao Feng a, Ming Wang a, Tao Song a, Detao Liu a, Fachuang Lu a,b
and Haisong Qi *a,b
www.rsc.org/
Herein, we report a sustainable route to in-situ synthesize a monolithic MnOx/N-doped carbon aerogel catalyst (Mn-NCA)
by pyrolyzing MnO(OH)2-cellulose aerogel precursor based on an alkali–urea aqueous system. The as-obtained Mn-NCA
furnish highly efficient catalytic activity for transfer hydrogenation of a broad scope of biomass-derived aldehydes, yielding
90–100% conversion and 64–100% selectivity to corresponding alcohols under mild conditions in an oven without
agitation. Combination of controlled experiments and detailed characterizations indicates that the superior performance
of Mn-NCA is attributed to the monolithic three-dimensional (3D) hierarchical porous architecture and the synergistic
effects between homogeneously dispersed MnOx nanoparticles (NPs) and urea-derived basic sites. The monolithic feature
of Mn-NCA exhibits more excellent dispersibility and separability compared to conventional centrifugation and filtration
techniques in powdery catalytic system. Moreover, a possible reaction mechanism is proposed. Our work provides a new
approach for developing highly efficient monolithic catalysts from renewable biopolymers for biomass valorization.
fibers, crown ethers, resins, drugs, adhesives or as
intermediates for further transformation into liquid biofuel.8,9
There were many efforts reported about the catalytic
Introduction
The excessive exploitation and utilization of depleted fossil
resources have been causing serious energy crisis, increasing
emission of greenhouse gases and severe environmental
pollution.1 Upgrading of optimal inexhaustible biomass-
derived platform chemicals into bio-fuels and value-added
chemicals has received significant attention, as a means to link
raw biomass to the sustainable chemical industry and alleviate
heavily energy dependence on non-renewable fossil
transformations of biomass platform compounds via
hydrogenation or hydrodeoxygenation using hydrogen gas as
the H-donor catalyzed by transition metals.10,11 However, the
use of flammable and explosive H2 with high pressure and
temperature, uncontrollable and unsatisfactory selectivity
toward targeted products owing to overhydrogenation and
C−C cleavage impels us to explore alternative process.12,13
More recently, catalytic transfer hydrogenation (CTH) using
organic hydrogen-donors, such as alcohols and formic acid, to
replace dangerous gaseous hydrogen has emerged as an
attractive strategy for the reductive upgrading of biomass-
derived aldehydes,8,14 Particularly, the utilization of alcohols as
both H-donor and solvent offers significant advantages such as
cheap, abundance, easy storage and easy to remove when
compared with corrosive formic acid.15,16 In recent years,
several homogeneous Mn catalysts (e.g. manganese pincer
complexes) have been employed in CTH reaction of aldehydes,
but it is difficult to recycle and reuse the homogeneous
catalysts.17, 18 Various powdery catalysts based on Fe,19 Co, 8
Ni,20 Cu,21, 22 Al,23 Zr,24 Hf,25 Pd,26 Ru27 and others have been
applied for CTH of bioderived aldehydes with alcohols as
hydrogen donor. However, the recovery of catalysts via
filtration or centrifugation techniques was time-consuming
and less energy-saving. Meanwhile, loss of solid catalysts was
inevitable in the separation process. These issues limited its
application.28,29 To overcome these disadvantages, lots of
resources.2
For
instance,
furfural
(FF)
and
5-
hydroxymethylfurfural (5-HMF), which can be obtained by the
acid-catalyzed dehydration of hemicellulose and cellulose-
derived carbohydrates, respectively, are identified as
appealing versatile feedstocks to manufacture high-value
products through various reactions.3-5 Especially, selective
transformation of FF to furfuryl alcohol (FA) and 5-HMF to 2,5-
bis-(hydroxymethyl)-furan (BHMF) exhibits great practical
application value in biorefinery.6,7 Because, FA and BHMF can
be used as important precursors for production of synthetic
a.State Key Laboratory of Pulp and Paper Engineering, South China University of
Technology, Guangzhou 510640, China. E-mail: qihs@scut.edu.cn.
b.Guangdong Engineering Research Center for Green Fine Chemicals, Guangzhou
510640, China.
† Footnotes relaꢀng to the ꢀtle and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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magnetic nanocatalysts were developed and employed in the
fields of biorefinery.30,31 However, tedious functionalization
procedure of magnetic nanoparticles was essential and an
external magnetic field was still necessary for the recovery of
catalysts.28 Therefore, it is still meaningful and imperative to
develop easily recoverable and recyclable, more efficient and
low-cost heterogeneous catalysts to increase the feasibility of
biomass transformation on an industrial scale.
Cellulose which can be obtained from woods and
agricultural waste, is environmentally benign, biodegradable
and the most abundant part of lignocellulosic biomass. Due to
the strong intra- and intermolecular hydrogen bonds
interactions, cellulose is insoluble in water and conventional
solvents, making it difficult for material manufacture.32 In
recent years,
a
variety of solvents including N,N-
(DMAc)/LiCl,33
dimethylsulfoxide Scheme 1 Schematic illustration of synthesis of Mn-NCA
dimethylacetamide
(DMSO)/LiCl,34 ionic liquid,35 N-methylmorpholine-N-oxide catalyst.
(NMMO)36 and NaOH-urea aqueous solution37 have been
fibers were rapidly dissolved into 100g pre-cooled NaOH-urea
developed for dissolving cellulose. Among these solvents, the
alkali-urea solution is the most powerful aqueous solvent and
is economic and environment friendly based on our previous
work.32,38,39
aqueous solution (7wt.%:12wt.%) to obtain a transparent and
sticky cellulose solution. Subsequently, manganese acetate
solution (7.3mmol in 12g water) was added and the sol was
then heated for further gelation. During these characteristic
dissolving and gelling processes, strong intra and inter
molecular hydrogen bonds in compact cellulose fibers with
high crystallinity were broken and restructured. The cellulose
chains could be aggregated into interconnected nanofibrils
with uniformly dispersion of MnO(OH)2, NaOH and urea.
Afterwards, the obtained hydrogel was freeze-dried followed
by carbonization at different temperatures (600, 700, 800 °C
for 2 h). During this pyrolysis process, nitrogen species derived
from urea in cellulose solution were reserved partially and
doped into the carbon skeleton. Carbon aerogel was activated
through the reaction between NaOH and carbon. MnO(OH)2
Herein, we report a novel and sustainable strategy to in-situ
synthesize MnOx NPs supported N-doped carbon aerogel (Mn-
NCA) by dissolving cellulose and manganese acetate in NaOH–
urea aqueous system, followed by a carbonization process.
The dissolving process of cellulose is crucial for the formation
highly active Mn-NCA catalysts in many aspects, including (1)
Transformation of powdery cellulose to carbon aerogel
through dissolving, gelling and carbonization process endowed
the catalysts with monolithic property; (2) NaOH as activating
agent promoted the formation of 3D hierarchically porous
structure with large specific surface area; Basic sites derived
from urea provided excellent synergistic catalytic effects with
MnOx NPs; (3) MnO(OH)2, NaOH and urea could be uniformly
dispersed in cellulose solution, subsequently, resulting in the
homogeneous doping of nitrogen and highly dispersion of
MnOx NPs. Owing to these unique characters, the Mn-NCA
exhibited remarkable catalytic activity for CTH of bioderived
aldehydes to alcohols in an oven without agitation. In detail,
the influence of chemical composition and particularly the
pyrolysis temperature of MnO(OH)2-cellulose aerogel
precursor on catalytic performance of Mn-NCA were
comprehensively investigated. Additionally, the reaction
kinetics and plausible mechanism were also studied.
were decomposed into MnOx NPs (MnO(OH)2
→MnOx + H2O).
Finally, monolithic Mn-NCA catalysts was obtained by washing
with deionized water to remove residual chemicals.
The structure, composition, morphology and physical-
chemical properties about Mn-NCA-T (T stands for the
carbonization temperature) materials were characterized by
several techniques. N2 adsorption/desorption isotherms
showed that all the samples display a type-I plus type-IV curve
Results and discussion
Preparation and characterization of monolithic carbon aerogel
catalysts
The designed procedure for the in-situ fabrication of Mn-NCA is
illustrated in Scheme 1. Herein, abundant cellulose and
Mn(CH3COO)2·4H2O were used as carbon source and Mn source.
NaOH-urea aqueous solution was selected as the dispersion
medium and the solution of cellulose. Firstly, 4g powdery cellulose
Fig. 1 Nitrogen adsorption–desorption isotherms of Mn-NCA-T.
2 | J. Name., 2012, 00, 1-3
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and an obvious H3-type hysteresis loop in 0.4 < P/P0 < 1.0 (Fig.
).40 These results indicated the presence of micro/mesopore
1
structures, which was also confirmed by their corresponding
pore-size distribution curves (Fig. S1). The texture properties in
Table S1 showed that Mn-NCA-700 had medium surface area
(676.95 m2/g), pore volume (0.41 cm3/g) and maximum
average pore diameter (4.89 nm) in Mn-NCA-T. The chemical
composition of all the catalysts were measured by ICP-OES and
elemental analyzer. As shown in Table S2, the metal and N
contents of all catalysts were varied in the range of 13−30
wt.% and 0.5−7wt.%, respectively. Therefore, the pyrolysis
temperature significantly influenced the physical structure and
chemical composition of catalysts. The large specific surface
area and pore size in the Mn-NCA-700 would be helpful to
expose more active sites and improve the mass transfer of
reaction molecules.
The X-ray diffraction (XRD) patterns of the as-obtained NCA-
700 and Mn-NCA-T were presented in Fig. 2. A broad
diffraction peak observed at 24° in NCA-700 can be attributed
to the (002) planes of amorphous graphitic carbon, indicating
partial crystallisation of amorphous carbon to graphite.41 It can
be observed that the Mn-NCA-700 and Mn-NCA-800 displayed
the similar diffraction peaks. The characteristic peaks at 18.0°,
28.9°, 32.3°, 36.1°, 44.4°, 50.7°, 60.6°, and 64.7° belong to the
Fig. 3 SEM images of NCA (a) and Mn-NCA-700 (b). TEM images
of Mn-NCA-700 (c-f) and corresponding element mapping
images (g) of (e).
(101), (200), (103), (202), (220), (105), (215), and (400) lattice macropores. As clearly observed in Fig. 3b that, furthermore,
planes of the Mn3O4 (JCPDS card No. 24-0734). The peaks at MnOx particles with sizes of 100–200 nm were uniformly
34.9°, 40.6°, 58.7°, 70.2°, and 73.8° belong to the (111), (200), anchored onto the channel of 3D carbon aerogel in Mn-NCA-
(220), (311), and (222) lattice planes of the MnO (JCPDS card 700 which was consistent with particles observed in low
No. 78-0424). At the same time, it is quite interesting to note magnification TEM images (Fig. 3c). These relatively large
that Mn3O4 phase cannot be indexed in the sample of Mn- particles were unfavourable for the dispersion of active sites
NCA-600. The possible reason is that the most of formed and the research on inhibiting the agglomeration of
Mn3O4 was reduced into MnO in the presence of sufficient nanoparticles are now in progress. High magnification TEM
carbon due to few carbons were consumed by activating agent image (Fig. 3e) showed that a great quantity of ultra-small
NaOH under relatively low temperatures.
MnOx NPs with an average particle size of about 20 nm were
The morphology and microstructure of the NCA-700 and highly dispersed on the carbon aerogel support. In accordant
Mn-NCA-700 were then examined by SEM and TEM. It can be with the XRD result, the lattice fringes in Fig. 3f were clearly
found in SEM image (Fig. 3a and 3b) that, both NCA-700 and observed with the interplanar spacings of 0.307 nm and 0.256
Mn-NCA-700 had a typical three-dimensional (3D) hierarchical nm, corresponding to the (211) planes of Mn3O4 crystals and
porous frameworks with plentiful continuous interconnected
the (111) planes of the MnO phases, respectively. The EDS
mapping images from representative high-angle annular dark-
field scanning TEM (HAADF-STEM) were shown in Fig. 3g
,
which confirmed that Mn, N, C and O elements were
homogeneously distributed in Mn-NCA-700.
For comparison, the SEM and TEM images of Mn-NCA-600
and Mn-NCA-800 were presented in Fig. S2 and S3
,
respectively. Mn-NCA-600 showed monolithic block structure
with very few crosslinked macropores and dispersed
manganese oxide particles in the surface due to incomplete
activation of carbon aerogel under low pyrolysis temperature.
When the pyrolysis temperature rose to 800 °C, both of 3D
porous frameworks with a large number of macropores and
some very large aggregated particles (about 1μm) could be
observed. In our cases, the temperature of 700 °C was
relatively favourable for the formation of porous carbon
aerogel structure with highly dispersed MnOx NPs.
The Raman spectra in Fig. 4 showed that all samples
exhibited two wide peaks at 1596 and 1354 cm−1
Fig. 2 XRD patterns of NCA-700 and Mn-NCA-T.
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Fig. 4 Raman spectra of NCA-700 and Mn-NCA-T.
corresponding to G and D bands of graphitization carbon,
respectively.42 In addition, a broader 2D peak appeared at
around 2750 cm-1, which was consistent with that of the
nitrogen-doped carbon material reported. The 2D band is a
prominent feature of higher graphitic degree.43 It can be found
that Mn-NCA-700 had the lowest ID/IG (1.006) and the highest
I2D/IG (0.6690) in Mn-NCA-T (Table S1), indicating the lower
defects and higher graphitic degree of Mn-NCA-700.
Additionally, the scatting peak located at 655 cm-1 in Mn-NCA-
700 and Mn-NCA-800 could be attributed to characteristic
peaks of high crystallized MnOx NPs. This peak shifted to 515
cm-1 and the peak shape became broader in Mn-NCA-700,
indicating a higher dispersion of the MnOx NPs.44 Fig. S4
presented the FT-IR spectra of the NCA and Mn-NCA-T
samples. For all the samples, the peaks around 1200 cm-1,
1600 cm-1 were ascribe to the stretches of the C-OH stretching
vibration, and the aromatic C=C stretching vibration,
respectively.45 These two peaks became stronger under low
pyrolysis temperature, indicating the existence of large
numbers of residual hydroxyl groups and aromatic C=C due to
incomplete carbonization.46 In addition, two new absorption
bands in Mn-NCA-T were observed at 624 cm-1 and 518 cm-1,
which were associated with the coupling of Mn-O stretching
modes of tetrahedral and octahedral sites.47 These results
further confirm the existence of manganese oxide in Mn-NCA-
T, well corresponding to the results of other characterizations.
X-ray photoelectron spectroscopy (XPS) tests were further
carried out to gain the electronic states of all elements and
estimate the proportion of Mn2+ and Mn3+ in the Mn-NCA-T
Fig. 5 XPS spectrum of Mn 2p3/2 peaks (a) and N 1s peak (b) of
Mn-NCA-T.
Therefore, 700 °C was more suitable for the formation of high-
valence MnOx in this study. The N 1s spectrum of Mn-NCA-T
catalyst in Fig. 5b could be deconvoluted into three different
types of nitrogen species, corresponding to pyridinic-N (398.5
eV), pyrrolic-N (400.0 eV) and graphitic-N (401.25 eV),
respectively.49 Noticeably, the percentage of pyridinic-N
decreased as the pyrolysis temperature increased and the
graphitic-N became the dominant phase in Mn-NCA-800,
indicating the transformation of pyridinic-N to graphitic-N
under high temperatures. In addition, the N 1s spectrums of
recycled and grinded Mn-NCA-700 catalyst were almost the
same as that of fresh catalyst (Fig. S5d), indicating that high
stability of nitrogen species. As reported, the doped-nitrogen
can reduce the energy barrier for reactants adsorption and
accelerate the electron transfer, which led to a significant
enhancement of the catalytic activity of the carbon surface.50
In this work, we speculated that doped nitrogen in the Mn-
NCA-T catalyst should act as basic sites, which could be
beneficial to enhance the generation and transfer of proton
from 2-propanol. The C 1s spectrum presented in Fig. S5b was
fitted into four parts arising from C–C/C=C (284.0 eV), C–O
(285.4 eV), C–N (285.8 eV), and C=O (289.2 eV) groups,
(Fig.5). The survey spectra confirmed the existence of the Mn,
N, C and O elements in Mn-NCA-T (Fig.S5a). Fig. 5a shows the
high-resolution Mn 2p3/2 peak of Mn-NCA-T, and this peak
could be deconvoluted into two peaks located at 640.9 and
642.4 eV, which were assignable to Mn2+ and Mn3+ species,
respectively.48 XPS results revealed that Mn-NCA-700 exhibited
maximum proportion (70%) of Mn3+ among Mn-NCA-T. It may
be due to that high-valence Mn3+ was mostly reduced into
Mn2+ by sufficient carbon under relatively low temperatures
(Mn3+ + C
→
Mn2+ + CO2). However, high temperature also
enhanced the redox reaction between MnOx NPs and carbon.
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Table 1 Comparison on catalytic performance of different
catalysts for FF transfer hydrogenation to FA a
O
O
2-propanol
Mn-NCA
OH
O
FF
FA
Entry
Catalysts
Conv.
(%)
Sel. (%)
TOF
(h-1) c
FA
Others b
-
1
2 d
3
Blank
100
18
83
34
32
13
15
29
18
16
36
45
19
4
0
MnOx
10
4
82
17
60
68
87
85
71
0.26
-
CA-700
4
NCA-700
16
35
32
98
52
-
5
Mn-CA-700
Mn-NCA-600
Mn-NCA-700
Mn-NCA-800
Mn-NCA-700
Co-NCA-700
Fe-NCA-700
Cu-NCA-700
Ni-NCA-700
0.92
0.84
2.58
1.37
1.58
2.35
0.24
0.37
0.92
Fig. 6 CO2-TPD profiles for Mn-CA-700 and Mn-NCA-T.
6
7
respectively.51 As for the O 1s region in Fig. S5c, three main
peaks located at around 530.2 eV, 532.3 and 533.1 eV in the
deconvolution spectrum, corresponding to manganese oxide
(Mn–O), hydroxyl bonding to Mn (Mn–OH) and C–OH bonds,
respectively.52
8
9 e
10
11
12
13
60
89
82
84
9
64
55
81
Then we further performed CO2-TPD to analyze the surface
basicity of samples. As shown in Fig.
6, three obvious
14
35
desorption peaks centered at around 70 °C, 120 °C and 260 °C
in NCA-700, Mn-NCA-700 and Mn-NCA-800. These peaks were
correspondingly attributed to weak basic sites, medium basic
sites and medium-strength basic sites. As for Mn-CA-600, only
weak basic sites and medium basic sites were formed. It was
worth noting that Mn-CA-700 exhibited no peaks compared
with other samples, implying the inexistence of basic sites.
These results demonstrated that urea in cellulose solution
played a crucial role in the formation of basic sites in Mn-NCA-
T and the pyrolysis temperature also influenced the strength
of the basic site.
a Reaction conditions: 0.5mmol FF in 5ml 2-propanol, 160 °C,
1h, catalyst (40 mol% Mn).
Others include 4-(2-furyl)-3-buten-2-one and isopropyl 2-
b
furoate (the aldol condensation product of FF with acetone).
c TOF is defined as mol(converted FF)/[mol(total metal added)
× h(time)].
d MnOx were prepared by calcination of Mn(CH3COO)2·4H2O at
500 °C in air for 6 h, the reaction autoclave was stirred at 160
°C in oil bath.
e
The monolithic carbon aerogel catalyst was grinded into
powder.
Mn-NCA-700 all resulted in inferior catalytic activity for CTH of
FF to FA. It indicated the nature of the metallic oxide was
crucial for the catalytic performance and MnOx NPs may be the
Catalytic performance evaluation of the various catalysts
Subsequently, the transfer hydrogenation of FF with 2-
propanol was chosen as a model reaction to assess the activity
of various catalysts and the results were shown in Table 1. The
blank run showed that very small amount of FF (4%) was
transformed and the selectivity of FA was zero in the absence
of any catalyst (Table 1, Entry 1). The GC–MS analysis of the
product showed that the aldol condensation product (4-(2-
furyl)-3-buten-2-one and isopropyl 2-furoate) of FF with
acetone were the main by-products (Fig. S6). Thus, the CTH of
FF was unrealistic without catalysts. Gratifyingly, the catalytic
reaction could efficiently proceed when Mn-NCA-700 was
added and 98% conversion of FF with 85% FA selectivity was
obtained with high turnover frequency (TOF) of 2.58 h-1. (Table
active sites in CTH
(Table 1, Entry 10–13). Moreover,
compared to the catalysts without nitrogen doping (CA-700
and Mn-CA-700, Table 1, Entry 3, 5), the N-doped carbon
aerogel catalysts (NCA-700 and Mn-NCA-700, Table 1, Entry 4,
7) acquired remarkably improvement in FF conversion and FA
selectivity, which suggested that nitrogen species played an
important role in enhancing the conversion of FF. The N sites
derived from urea in cellulose solution could be reserved
partially after pyrolysis, which potentially endowed the
catalysts with abundant Lewis basic sites. We speculated that
these basic sites could facilitate the generation of proton and
deliver the free proton to the activated aldehyde molecules,
and thus promote the reaction. In brief, the presence of a
synergistic promoting interaction between MnOx NPs and N-
doped carbon aerogel was of vital importance in affecting the
catalytic performance of Mn-NCA-700 in transfer
hydrogenation of FF.
1, Entry 7). To gain insight into the effects of each component
in Mn-NCA-700, a series of different catalysts were tested
based on CTH of FF. The MnOx was examined and gave a
slightly improved FF conversion (10%) with 82% selectivity to
FA and low TOF of 0.26 h-1
(Table 1, Entry 2). In addition, the
replacement metal center Mn with Co, Fe, Cu and Ni in
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Interestingly, the results also indicated that the pyrolysis
Journal Name
temperature also significantly affected the activity of the Mn-
NCA-T. Among these samples, Mn-NCA-700 afforded the
highest catalytic activity and TOF value (Table 1, Entry 8).
There were mainly two reasons accounting for the better
catalytic performance of Mn-NCA-700 than others combining
with characterization of catalysts: (1) 3D hierarchical porous
structure with large specific surface area and higher
mesoporous and macroporous proportion in Mn-NCA-700
were beneficial for exposure of active sites, diffusion and
transportation of reaction molecules, thus could significantly
improve the reaction rate; (2) The pyrolysis temperature
significantly affected the degree of dispersion of MnOx NPs
and the electronic states of manganese. The most of Mn3O4
was converted into MnO in the presence of sufficient carbon
due to few carbons were consumed by activating agent NaOH
under relatively low temperatures (confirmed by XRD patterns
and XPS spectrum). While pyrolysised at 800 °C, the isolated
active sites in Mn-NCA tended to aggregate (confirmed by SEM Fig.7 Effects of reaction temperature and time on conversion
and TEM images). It was worth noting that, the conversion of of FF (a) and selectivity of FA (b) over the Mn-NCA-700. First-
FF greatly decreased when the Mn-NCA-700 catalyst used was order kinetic fit (c) for the CTH of FF to FA at different
grinded into powder (Table 1, Entry 9). Compared with the temperatures and corresponding Arrhenius plots (d) for Mn-
reported other reported powdery catalysts (Table S3), the NCA-700. Reaction conditions: 0.5mmol FF in 5ml 2-propanol,
monolithic carbon aerogel catalyst could efficiently catalyze 55mg Mn-NCA-700 catalyst.
the reaction in an oven without agitation with comparative
the temperature range 403–433 K. The rate formula of FF
conversion and selectivity under mild conditions. Moreover,
transfer hydrogenation could be expressed by the following
the monolithic catalytic system was energy-efficient and
Eq. (1) since the catalyst amount was a constant and 2-
timesaving and showed great advantage in the operation of
propanol with an extremely higher concentration compared to
reaction and separation of catalyst.
FF. Hypothesizing that the reaction was a first-order reaction
Effects of reaction temperature and time and reaction kinetics (γ = 1), c0 was the initial concentration of FF and ct was the
study
concentration of FF after a reaction time of t, and thus Eq. (1)
could be expressed as Eqs. (2) or (3). Experiment results
demonstrated the ln(c0/ct) had a linear relationship with t. As
The influence of reaction time and temperature on synthesis
of FA from FF catalyzed by Mn-NCA-700 was also investigated.
compiled in Fig.7c
, reaction rate constants (k) were
As shown in Fig. 7a and b, the conversion of FF was remarkably
determined at each temperature from the corresponding slope
of the plots. According to Arrhenius equation and its variable
formula (4), the apparent activation energy Ea was calculated
to be 83 kJ/mol from the corresponding Arrhenius plot
presented in Fig.7d. The low activation energy of the Mn-NCA-
700 should be related to the synergistic effects between MnOx
NPs and N-doped carbon aerogel.
elevated with an increase in reaction temperature from 403 to
473 K. For example, only 8% of FF conversion with 65% of FA
selectivity was obtained if the reaction was conducted at 403 K
for 10 min, whereas a much higher 57% conversion of FF with
87% of FA selectivity could be achieved if the reaction
temperature was increased to 473 K in a reaction time of 10
min. Additionally, the reaction time also had a large influence
on the reaction. It could reach nearly full conversion of FF
(99%) and 85% of FA selectivity by prolonging the reaction
time to 1 h at 433 K. As shown in Fig. 7b, meanwhile, aldol
condensation products as the side product gradually
r = -dct/dt = k · [FF]γ, k = ko · [Cat]α · [2-propanol]β
r = -dct/dt = k · [FF]
(1)
(2)
(3)
(4)
ln(ct/c0) = -k · t
ln k = lnA - Ea/RT
decreased with the increase of the reaction temperature. Effects of the catalyst dosage, heterogeneity and recyclability of
These results indicate that higher temperature promoted the Mn-NCA catalyst
equibrilium shift from the aldol condensation product to FA. In
The effect of catalyst dosage on CTH of FF into FA was also
addition, when the temperature was increased to 473 K, the
examined (Fig. S7). The selectivity toward FA increased from
selectivity of FA increased firstly and then gradually declined
79 to 84% with the elevated catalyst dosage from 30−55 mg.
due to overhydrogenation of FA to 2-methylfuran and
Further increasing the amount of catalyst to 125mg led to
tetrahydrogenfuran with the increase of reaction time.
obvious decrease in the selectivity to FA (80%) with complete
To further determine the apparent activation energy (Ea) of
transformation of FF. Moreover, the heterogeneous nature of
FF transfer hydrogenation, its kinetics was investigated over
Mn-NCA-700 in the catalytic process was assessed by removing
6 | J. Name., 2012, 00, 1-3
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ARTICLE
which is a very important biofuel.54 Furthermore, the Mn-NCA-
700 was also broadly applicable for CTH of other bioderived
Table 2 CTH of different biomass-derived carbonyl compounds to
alcohols. a
Yield b
(%)
En
Time
(h)
Sel.
(%)
Conv.
(%)
Reactant
Product
try
O
O
84
1
2 c
3
1
0.5
1.5
2
99
99
85
89
OH
O
(75)
Fig. 8 Heterogeneity (a) and reusability (b) of the Mn-NCA-700
O
O
88
OH
catalyst.
O
(78)
O
O
the catalyst from the mixture after 20 min at 160
℃, and
91
83 d
64 d
87
HO
O
OH
HO
76
58
87
keeping the autoclave in oven for another 30 min under the
same reaction conditions without solid catalysts. There was no
further obvious reaction occurred (Fig. 8a), verifying that no
active sites were leached into the solution and Mn-NCA-700
was heterogeneous.
O
O
4
90
OH
O
OH
O
5
3
100
The reusability test of Mn-NCA-700 for CTH of FF was
investigated at 160 °C. The monolithic catalyst could be easily
separated from the solution with the assistance of tweezers
after the reaction. As shown in Fig. 8b, there was a slight
decrease in FF conversion observed in the subsequent four
runs and the FA selectivity remained unchanged in all cycles.
The manganese content in reaction mixture of the first and
fifth reaction was extremely low (both <0.02 ppm) detected by
ICP–OES analysis, suggesting that leaching of active species
from Mn-NCA-700 was negligible. In addition, the monolithic
architecture was stable and remained largely unchanged after
the reaction (Fig. S8a). The recovered Mn-NCA-700 catalyst in
the fifth cycle was further characterized by XRD, N2
adsorption-desorption and TG analyses. Compared with the
fresh catalyst, the crystallographic structure and porosity of
spent Mn-NCA-700 all exhibited no clear changes (Fig. S8b, c).
In contrast, the specific surface area of the used catalyst
declined 16 % (to 570.2 m2·g-1) and a slight weight loss in TG
analysis (Fig. S8d) could be observed. This suggested that a
small amount of organic compounds were adsorbed on the
channel of carbon aerogel during reaction, which may partially
block the access of active sites, thus resulting in the slightly
decrease of catalytic activity.
O
HO
6
7
8
6
3
4
99
94
81
96
92
80
90
OH
O
O
O
OH
O
O
O
100
92
81
O
O
O
OH
9
0.7
0.8
2
90
100
99
90
97
96
99
99
99
85
91
O
97
OH
10
11
12
13
14
15
16
(87)
O
OH
95
90
4
91
O
OH
OH
(83)
99
5
100
98
O
(91)
O
OH
97
2
The substrate scope for Mn-NCA-700 catalyst
(88)
Accordingly, the feasibility of the Mn-NCA-700 for CTH with
a series of substrates was also investigated and the results
O
OH
2
100
100
85
91
were summarized in Table
2. Cellulose-derived furanic
aldehydes such as 5-hydroxymethylfurfural (5-HMF) and 5-
methylfurfural (5-MF), are also recognized as important
platform chemicals, therefore, relevant substrate for CTH.53
Clearly, the developed Mn-NCA-700 exhibited also superior
catalytic performance for 5-HMF and 5-MF resulting in both
more than 90% conversion with selectivities to the
corresponding alcohols were 83% and 64%, respectively (Table
10
OH
O
a Reaction conditions: 0.5mmol substrates in 5ml 2-propanol, 160
°C, 55mg catalyst.
b isolated yield
are given in parentheses.
s
c The reaction temperature was 200 °C.
d The main by-product is 2,5-dimethylfuran.
2, Entries 3 and 4). It was worth mentioning that other by-
product of these two reactions was mainly 2,5-dimethylfuran,
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aldehydes. As examples, the conversions of cinnamaldehyde and effects between homogeneously in-situ dispersed MnOx NPs
citral were 99% with selectivities to the corresponding alcohols of and surface basic sites contributed to the superior catalytic
more than 80% (Table 2, Entries 5 and 6). These corresponding
alcohols are important chemical intermediates for
manufacture of pharmaceuticals, flavors and cosmetics in
industry. Besides the reduction of biomass-derived chemicals,
other commercial aldehydes and ketones were also tested to
evaluate its catalytic potential. In these reactions, all
substrates were converted to corresponding alcohols with
excellent conversion (90–100%) and selectivity (85–99%).
However, relatively longer reaction time were needed for
ketones in order to obtain satisfactory yields due to electron-
donating nature of the alkyl group in ketone, for example, 1-
phenylethanol was obtained from acetophenone within 10 h.
This clearly suggests the Mn-NCA-700 catalyst to be a highly
versatile catalyst for CTH of both biomass-derived aldehydes
and other carbonyl compounds to alcohols in general.
activity for transfer hydrogenation of biomass-derived
Mechanism study
So far, the transfer hydrogenation of bioderived aldehydes Scheme 2 Possible mechanism for the Mn-NCA-catalyzed
with 2-propanol were generally catalyzed by metal oxide transfer hydrogenation of carbonyl compounds.
catalysts without N doping. However, we demonstrated that
aldehydes/ketones. Particularly, the CTH of FF on Mn-NCA-700
the doped nitrogen in carbon aerogel played a crucial role in
followed first order kinetics and resulted in activation energy
the catalytic activity of the catalyst (Table 2, Entry 3, 4, 5, 7).
of 83 kJ/mol. A plausible mechanism was also proposed.
We speculated that the doped nitrogen in the Mn-NCA catalyst
Compared with the traditional catalytic system using powdery
should act as basic sites (confirmed by CO2-TPD, Fig.6), which
catalyst, the developed monolithic carbon aerogel catalytic
could promote the generation and transfer of proton to
system held multiple advantages such as superior dispersibility
control the conversion of the reaction. According to catalysts
in reactants, energy-efficient reaction process in an oven
characterization, our experimental results and the above
without agitation, more convenient and timesaving separation
analysis, we proposed a plausible reaction mechanism with
process of catalysts. Hence, this new approach for developing
respect to the transfer hydrogenation of carbonyl compounds
highly efficient monolithic catalysts from sustainable cellulose
to corresponding alcohols over Mn-NCA catalyst (Scheme 2).
is expected to be more operational in practical application for
Firstly, the carbonyl compounds and 2-propanol were
catalytic transformation of biomass.
adsorbed on the catalysts and the N-derived basic sites in Mn-
NCA captured the protons from 2-propanol to generate NH+
Experimental
and isopropoxide by dissociation.55 Meanwhile, the carbonyl
groups in the substrate molecules was activated by
electrophilic MnOx NPs via the electron lone pair of oxygen to
Reagents
generate η1(O)-aldehyde species.56 Third, the C=O double bond Cellulose (DP 500) as a model of biomass were supplied by
was attacked by active H atom to form two active Hubei Chemical Fiber Group Ltd. (Xiangfan, China).
intermediates (alkoxide intermediate or hydroxyalkyl Mn(CH3COO)2·4H2O (>99.0%), NaOH (>99.5%), urea (>99.5%)
intermediate).56 These two compounds adsorbed on MnOx NPs were purchased from Shanghai Macklin Biochemical Co., Ltd,
surface then were hydrogenated into corresponding alcohol China. Furfural (FF, >99.5%), 5-hydroxymethylfurfural (HMF,
via the active hydrogen attack in this step. Meanwhile, 2- >99.0%),
5-methylfurfural
(>99%),
citral
(>97%),
propanol was converted into acetone after losing two H atoms. cinnamaldehyde (>99.5%), veratraldehyde (>99%), 2-propanol
Finally, the generated carbonyl compound and acetone was (>99.0%) were purchased from Aladdin Industrial Corporation.
desorbed from the surface of Mn-NCA with the aiding of 2- All chemical reagents were obtained from commercial
propanol.
suppliers and used without further purification.
Conclusions
Preparation of different catalysts
Synthesis of Mn-NCA. The MnOx NPs supported N-doped
carbon aerogel catalysts (Mn-NCA) were prepared according to
the following steps. In a typical procedure, 7 g NaOH and 12 g
urea were dissolved in 81 g distilled water and the mixture
solution was pre-cooled to -12.0 °C. 4 g cellulose powder was
added into the above solution under mechanical agitation to
In summary, we have designed and prepared a highly efficient,
monolithic and easily separated Mn-NCA catalyst by dissolving
cellulose and thermolysis of cellulose-derived aerogels with
uniform MnO(OH)2 doping. A series of experiments and
characterizations revealed that the monolithic hierarchically
porous architecture of carbon aerogel and the synergistic
8 | J. Name., 2012, 00, 1-3
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obtain
ARTICLE
a
transparent cellulose solution. Subsequently, transmission electron microscope (JEM-2100F) with EDX
Mn(CH3COO)2·4H2O (7.3mmol, 1.8g) in aqueous solution (15% analysis (Bruker Xflash 5030T) operated at 200 kV. Raman
wt.%) was added to the above cellulose solution drop by drop spectra were completed on a LabRAM Aramis Raman
under mechanical agitation for 5min. The MnO(OH)2 doped Spectrometer (HORIBA Jobin Yvon) operating with 532 nm
cellulose gel (Mn-CG) was then heated for further gelation at excitation and the wave number range used in the
50 °C for 12 h. Afterwards, the obtained hydrogel was freeze- measurement was from 400 to 2000 cm−1. Fourier transform
dried at -65 °C for 48 h, followed by carbonization in a infrared spectroscopy (FTIR) spectra of all samples in KBr pellet
horizontal tube furnace at different temperatures (600, 700, were conducted on a Bruker Vector33 spectrometer. X-ray
800 °C) for 2 h under Ar flow at a heating rate of 5 °C/min. The photoelectron spectroscopy (XPS) measurements were
obtained samples were immersed in 1000 mL water for 6h and performed on a Kratos Axis Ultra DLD system with a base
then totally washed with deionized water to remove residual pressure of 10−9 Torr. The surface basicity of catalysts was
chemicals. After drying at 50 °C for 12 h, samples were carried out via temperature-programmed desorption of CO2
obtained and are denoted as Mn-NCA-T, where T stands for (CO2-TPD) by a Micromeritics AutoChem II 2920 instrument.
the carbonization temperature. Replacing Mn(CH3COO)2·4H2O Sample was pretreated under a flow of He (30 mL · min−1) at
with
equimolar
amounts
of
(CH3COO)2·4H2O, 150 °C for 2 h. Subsequently, the sample was cooled to 50 °C
Ni(CH3COO)2·4H2O, Fe(CH3COO)2·4H2O or Cu(CH3COO)2·H2O under He. After adsorption of CO2, the sample was flushed off
synthesized Co-NCA-T, Ni-NCA-T, Fe-NCA-T or Cu-NCA-T, under a helium atmosphere at 50 °C. Finally, the TPD data
respectively.
were collected from 50 °C–800 °C under helium (heating rate
of 20 °C · min−1). Thermogravimetric (TG) analysis was
performed using a Mettler Toledo thermal analyzer under an
Ar flow (30 mL · min−1) from 25–700 °C (heating rate of 15 °C ·
min−1).
Synthesis of NCA-700. The procedure for preparing N-doped
carbon aerogel (NCA-700) was same as that of Mn-NCA-700,
but no Mn(CH3COO)2·4H2O saturated solution was added in
the cellulose solution.
Synthesis of CA-700. The carbon aerogel without N-doping Procedures for the CTH of aldehydes
(CA-700) was prepared according to the following steps. 52 g
The CTH of aldehyde was carried out in a 10 mL stainless-steel
transparent cellulose solution was directly heated at 50 °C for
12 h to obtain cellulose gels. Afterwards, the obtained
hydrogel was totally washed with water to remove NaOH and
urea and then impregnated with 3.5 g NaOH. The following
preparation steps are the same as that of Mn-NCA-700.
autoclave. Typically, aldehyde (0.5 mmol), 2-propanol (5 mL),
and catalyst (55 mg) were loaded into the autoclave, which
was then purged with Ar and sealed. Then the reactor was put
into an oven which has been heated to the desired
temperature without stir. Time zero was recorded when the
inside temperature of autoclave reached the target
temperature. After the reaction, the reactor was rapidly placed
in cold water. The monolithic catalyst was separated by a
tweezers and the liquid mixtures were collected for analysis.
Identification of products in the reaction mixture was
conducted using GC-MS (Agilent 7890B-5977A) equipped with
HP-5MS capillary column (30.0 m × 250 mm × 0.25 mm). The
reactants and products were quantitatively analyzed on the
basis of standard sample using toluene as internal standard on
a GC (Shimadzu Nexis GC-2030) equipped with flame ionization
detector and HP-5 capillary column (30.0 m × 250 mm × 0.25
mm). Product isolation was performed via column
chromatography using silica gel as stationary phase and an n-
pentane/ethyl acetate mixture (4:1) as eluent.
Synthesis of Mn-CA-700. The MnOx NPs supported carbon
aerogel catalysts without N-doping (Mn-CA-700) was prepared
according to the following steps. 58g Mn-CG was then heated
at 50 °C for 12 h for further gelation. Afterwards, the obtained
hydrogel was totally washed with water to remove NaOH and
urea and then impregnated with 3.5 g NaOH. The following
preparation steps are the same as that of Mn-NCA-700.
Catalyst characterization
Low temperature N2 adsorption–desorption isotherms of the
materials were assessed using a Micromeritics ASAP 2020.
Before measurements, the samples were outgassed at 130 °C
for 8 h. The specific surface areas were evaluated using the
Brunauer−Emmeꢁ−Teller (BET) method. The average pore
volume
and
pore
size
were
obtained
by
Catalyst recycling experiments
Barrett−Joyner−Halenda (BJH) method from the desorption
branch of the isotherms. X-ray diffraction (XRD) patterns were
recorded on a Rigaku diffractometer (D/MAX/IIIA, 3 kW) using
Cu Kα radiation (λ = 0.1543 nm, 40 kV, 30 mA). Organic
elemental content was detected by an elemental analyzer
(VarioEL III, Elementar). The manganese content of the
catalysts was determined with Spectro Arcos FHX22
inductively coupled plasma-optical emission spectrometer
(ICP-OES). The structures and morphologies of the materials
After reaction, the monolithic catalyst was separated from
the solution with the assistance of tweezers, washed several
times with 2-propanol and kept for the next run directly under
the same reaction conditions without further treatment.
Conflicts of interest
There are no conflicts to declare.
Acknowledgments
was observed by
a high-resolution scanning electron
microscopy (SEM, MERLIN of ZEISS) and a high-resolution
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The authors are grateful for the financial support for this work 24 Y. Sha, Z. Xiao, H. Zhou, K. Yang, Y. Song, N. Li, R. He, K. Zhi
by the National Natural Science Foundation of China and Q. Liu, Green Chem., 2017, 19, 4829-4837.
(21774036), Thousand Youth Talents Plan, State Key 25 S. Rojas-Buzo, P. García-García, A. Corma, ChemSusChem,
Laboratory of Pulp and Paper Engineering (No. 2017TS01) and 2018, 11, 432-438.
Guangdong Province Science Foundation for Cultivating 26 D. Scholz, C. Aellig and I. Hermans, ChemSusChem, 2014, 7,
National Engineering Research Center for Efficient Utilization 268-275.
of Plant Fibers (2017B090903003).
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Table of contents
Cellulose-derived monolithic hierarchically porous MnOx/N-doped carbon aerogel
exhibited excellent catalytic activity for upgrading of lignocellulose-derived
aldehydes