Biomass-Derived N-doped Carbon Materials with Silica-Supported Ultrasmall ZnO Nanoparticles: Robust Catalysts for the Green Synthesis of Benzimidazoles

Article abstract of DOI:10.1002/chem.201704823

Ultrasmall ZnO nanoparticles anchored on N-doped carbon materials with a silica support (ZnO/SiO₂-NC) were fabricated from chitosan and metal ions by using a one-pot self-assembly strategy and were successfully applied to the synthesis of 2-arylbenzimidazoles under mild conditions. These catalysts showed excellent stability and could be used six times without any loss of conversion and selectivity. The use of silica gel and the biomass chitosan as a source of hydrophilic N-doped carbon materials facilitated the uniform dispersion of the ZnO nanoparticles in methanol and therefore the contact of these nanoparticles with reactants, thus contributing to a high catalytic performance. TEM analysis showed that the ZnO nanoparticles were around 2.55 nm in diameter and uniformly distributed on the support surface. The binding behavior of ZnO and N-doped carbon materials affected the catalytic activity. Interestingly, temperature-programmed NH₃ desorption indicated that the interactions between ZnO and N-doped carbon materials might induce the presence of more acidic sites in these catalysts, thus resulting in enhanced activity and hence promoting this transformation.

Full text of DOI:10.1002/chem.201704823

A Journal of
Accepted Article
Title: Biomass-Derived N-Doped Carbons with Silica Supported
Ultrasmall ZnO Nanoparticles: Robust Catalysts for the Green
Synthesis of Benzimidazoles
Authors: Bo Chen, Chan Zhang, Libo Niu, Xiaozhen Shi, Huiling
Zhang, Xingwang Lan, and Guoyi Bai
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To be cited as: Chem. Eur. J. 10.1002/chem.201704823
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Biomass-Derived N-Doped Carbons with Silica Supported
Ultrasmall ZnO Nanoparticles: Robust Catalysts for the Green
Synthesis of Benzimidazoles
Bo Chen, Chan Zhang, Libo Niu, Xiaozhen Shi, Huiling Zhang, Xingwang Lan,* and Guoyi Bai*[a]
Dedication ((optional))
Abstract: Ultrasmall ZnO nanoparticles anchored on N-doped
nanoparticles, which can maintain their stability and even
enhance their catalytic activities.[5-8] For catalytic applications,
carbons with silica support (ZnO/SiO2-NC) were fabricated from
chitosan and metal ions using a one-pot self-assembly strategy, and developing highly efficient, supported ultrasmall nanoparticles on
were successfully applied to the synthesis of 2-arylbenzimidazoles
under mild conditions. These catalysts showed excellent stability and
N-doped carbons continues to have much potential.
Nitrogen-doped carbon materials have been widely prepared
could be used six times without loss of conversion and selectivity. by pyrolysis of differing precursors such as nitrogen-containing
Using silica gel and biomass chitosan as a source of hydrophilic N- ligands (dicyandiamide, 1,10-phenanthroline, and melamine),[9-11]
doped carbon facilitated the uniform dispersion of the ZnO nitrogen-containing
polymers
(polyacrylonitrile
and
nanoparticles in methanol and therefore their contact with reactants, polyaniline),[12,13] and metal-organic-frameworks (MOFs)[14]
.
contributing to a high catalytic performance. Transmission electron However, the complicated syntheses and high cost of these
microscope showed that the ZnO nanoparticles were around 2.55 precursors or the requirement for excess reagents or harsh
nm in diameter and uniformly distributed on the support surface; the conditions has hindered further development. In recent years,
binding behavior of ZnO and N-doped carbons affected the catalytic the conversion of biomass, including seaweed,[15] camellia,[16]
activity. Interestingly, NH3-temperature programmed desorption and xylose,[17] into N-doped carbons has attracted considerable
indicated that the interactions between ZnO and N-doped carbons interest, because these materials are widespread, low-cost,
might induce the presence of more acidic sites in these catalysts, abundant, and renewable. For instance, Yao’s group[18] showed
resulting in enhanced activity and hence promoting this
transformation.
that starch can be converted into N- and Fe-doped nanoporous
carbon microspheres under an ammonia atmosphere, and that
the synthesized catalyst could be applied to redox reactions and
exhibited excellent electrochemical catalytic activity. Despite
such excellent applications of biomass as precursors, it is clearly
highly desirable to develop additional, novel supported catalysts
using these green materials.
Introduction
The downscaling and control of supported nanoparticle sizes
have attracted considerable attention in the catalysis area
because of the relationship between size and catalytic activity:[1,2]
downsizing the size of nanoparticles to nanometers or sub-
nanometers can significantly increase the ratio of surface to bulk
atoms and thus may promote their catalytic performance.[3,4]
When using supported ultrasmall catalysts, major challenges are
to prevent leaching and aggregating of the nanoparticles from
the supports during reactions. To simultaneously solve these
problems, N-doped carbon materials are frequently employed as
supports due to their interactions with such ultrasmall
Benzimidazoles are significant aromatic nitrogen heterocycles
with many examples having good bioactivity and reactivity and
have been widely applied as pharmaceuticals, for example as
antiulcer agents, antihypertensives, antivirals and antifungals, as
well as intermediates in organic synthesis.[19,20] For their
synthesis, conventional approaches are mainly focused on the
use of homogeneous catalysis. However, many such catalysts
are often difficult to recycle and reuse, and thus developing
more efficient and green heterogeneous catalysis has gradually
become an emphasis. In recent years, a few of heterogeneous
catalysts have been developed for use in the synthesis of
benzimidazoles,[21,22] but there is still a need to develop efficient,
cheap and recyclable catalysts in this area. It is still highly
significant and challenging to design novel heterogeneous
catalysis for triggering such syntheses.
Currently, zinc oxide (ZnO) nanomaterials are widely used in
many electrochemical, photochemical and catalytic processes,
due to their wide bandgap and excellent chemical stability.[23-25]
However, for most purposes, the activities and selectivities of
these ZnO catalysts with different morphologies are still far from
those required for practical applications. Thus, the development
of easy and reliable control methods of the nucleation and
growth of high performance ZnO nanocatalysts is vital in this
area. In continuation of our previous work,[21] we herein report
the incorporation of biomass-derived chitosan as a precursor,
together with silica gel, to obtain N-doped carbons on a silica
[a]
B. Chen, C. Zhang, X.Z. Shi, H.L. Zhang, Dr. L.B. Niu
Key Laboratory of Chemical Biology of Hebei Province, College of
Chemistry and Environmental Science, Hebei University
Baoding, Hebei, 071002, P.R.China
Dr. X.W. Lan
Key Laboratory of Chemical Biology of Hebei Province, College of
Chemistry and Environmental Science, Hebei University
Baoding, Hebei, 071002, P.R.China
Email: hxlxw@sina.cn
Prof. G.Y. Bai
Key Laboratory of Chemical Biology of Hebei Province, College of
Chemistry and Environmental Science, Hebei University
Baoding, Hebei, 071002, P.R.China
Email: baiguoyi@hotmail.com
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
1
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10.1002/chem.201704823
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support, which can homogeneously immobilize and stabilize
ultrasmall ZnO nanoparticles by thermal pyrolysis. The
synthesized catalyst shows several attractive features: 1)
hydroxy and amino groups on the surface of chitosan can
promote the complexation of a zinc salt and increase zinc ion
dispersion; 2) SiO2 can stabilize the formation of N-doped
carbons and increase the surface area of the catalyst; 3)
nitrogen provides both chelation sites and inter-atom spaces in
the formation of the hybrid carbons, which can restrict ZnO
nanoparticles size to sub-nanometer levels and promote the
high dispersity of ZnO nanoparticles. These characteristics
enhance both the performance and the dispersibility of the
catalyst in methanol, which we have demonstrated by its use in
an efficient new preparation of 2-arylbenzimidazoles. The green
and renewable biomass-chitosan was used to prepare the
catalysts, and no harmful byproducts were generated during the
reaction. To the best of our knowledge, this is the first example
of an application of such ultrasmall ZnO nanoparticles supported
on the N-doped carbons with silica gel in the catalysis of an
organic reaction.
and 2). It is noteworthy that only mono-condensation product
was obtained. We have not detected the formation of other
products[26] except for the Schiff base intermediate [21]. Similarly,
ZnO/SiO2-1 and ZnO/SiO2-2 also delivered low conversions
during the same reaction times (Entries 3 and 4). To our delight,
both the conversion and selectivity were dramatically increased
when using ZnO/SiO2-NC-X (X = 500 oC, 600 oC, and 700 oC)
(Entries 5-7), especially in the case of ZnO/SiO2-NC-600. We
speculate that the interactions between ZnO nanoparticles and
N-doped carbons promote the catalyst by generating more
active sites. Additionally, the synthesized SiO2-NC also had a
reasonable catalytic activity (Entry 8), indicating that SiO2-NC
and ZnO have a synergic catalytic effect.
Table 1. Results of the synthesis of 2-phenylbenzimidazole by different
catalysts.[a]
Entry
Catalyst
Conversion(%)[b]
Selectivity(%)[b]
1
2
3
4
5
6
7
8
Catalyst-free
SiO2
29.1
28.9
30.6
33.4
96.8
98.5
99.0
59.0
70.9
71.3
77.2
77.3
84.9
92.2
88.3
85.2
Results and Discussion
The synthetic procedure for preparation of the ZnO/SiO2-NC
nanocatalyst is depicted in Scheme 1. Firstly, the combination of
chitosan and silica gel resulted in formation of a uniformly
colloidal support under ultrasonic dispersion. Then the
Zn(II)/SiO2-chitosan colloidal solution, obtained by the chelation
between zinc ions and the -NH2 and –OH groups on the chitosan,
was evaporated to generate a Zn(II)/SiO2-chitosan solid, which
finally was calcined at different temperatures (500 oC, 600 oC,
and 700 oC) under a nitrogen flow, and the ZnO/SiO2-NC
nanocatalyst was obtained.
ZnO/SiO2-1
ZnO/SiO2-2
ZnO/SiO2-NC-500
ZnO/SiO2-NC-600
ZnO/SiO2-NC-700
SiO2-NC
[a] Reaction conditions: benzaldehyde (1.0 mmol), o-phenylenediamine (1.1
mmol), methanol (5.0 mL), catalyst (0.22 g, containing 15.1% ZnO), H2O2
(30% in water, 0.2 mL) for 1 h at 40 oC. [b] Determined by Agilent HPLC
1220 based on the starting material. Conversion = (intial mmol of substrate -
mmol of substrate left) / initial mmol of substrate×100%; Selectivity = (mmol
of product in the reaction mixture) / (inital mmol of substrate - mmol of
substrate left)×100%.
Following this preliminary study, the stability of the optimal
ZnO/SiO2-NC-600 was examined. It was recovered from the
reaction mixture by centrifugation, and subsequently reused in
six identical runs without any appreciable loss of its catalytic
activity (Figure 1a). Furthermore, inductively coupled plasma
(ICP) analysis of fresh and recycled catalyst found that the
amount of ZnO did not alter during this recycling (Table S1).
These results demonstrated its outstanding stability as a catalyst
for the synthesis of 2-phenylbenzimidazole. Additionally, the
hydrophilic N-doped carbons could induce the catalyst to better
disperse in methanol (Figure 1b), contributing to its high catalytic
performance.
Scheme 1. Preparation of ZnO/SiO2-NC catalyst.
For comparison, we also prepared other supported
nanocatalysts and tested their catalytic performance in the
model reaction between benzaldehyde and o-phenylenediamine;
these results are shown in Table 1. We found that low
conversions were obtained under catalyst-free conditions and
using SiO2, obtained by the calcination of silica gel (Entries 1
2
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indicating that more active sites were exposed on the surface of
ZnO/SiO2-NC-600, thereby enhancing its reactivity. Furthermore,
all the samples showed two desorption peaks at 155 and 368 oC,
indicating that weak and moderately strong acidic sites were
present, which could be attributed to the presence of ZnO
nanoparticles.[28] Notably, for ZnO/SiO2-NC-600, a desorption
peak at 381 oC was observed, suggesting that strongly acidic
sites exist. In contrast, ZnO/SiO2-1 and ZnO/SiO2-2 had no such
desorption peak. We speculated that N-doping carbons could
promote the uniform distribution of such acidic sites, thereby
improving the catalytic activity.[29]
Figure 1. (a) Stability of the hybrid ZnO/SiO2-NC-600 catalyst. Reaction
conditions: benzaldehyde (1.0 mmol), o-phenylenediamine (1.1 mmol),
methanol (5.0 mL), ZnO/SiO2-NC-600 (0.22 g, including 15.1% ZnO), H2O2
(30% in water, 0.2 mL), 1 h at 40 oC; (b) The catalysts were dispersed in
methanol.
ZnO/SiO2-NC-600 and related catalysts were characterized
using several techniques. In Figure 2a is shown the N2 sorption
isotherms of ZnO/SiO2-NC-600, ZnO/SiO2-1, and ZnO/SiO2-2.
Their corresponding Brunauer-Emmett-Teller (BET) surface
areas and pore volumes are summarized in Table S2. All
samples show type-II curves with a hysteresis loop at lower
relative pressures of 0.8-1.0, which leads to small BET surface
areas, showing the existence of mesopores and/or macropores
in all samples.[27] As shown in Figure 2b, the corresponding pore
size distributions reveal these pores with a large pore diameter
formed in the support or between nanostructures, resulting in
small pore volumes of these samples. Importantly, larger size
distributions of ZnO/SiO2-NC-600 and ZnO/SiO2-1 were
observed, suggesting that ZnO nanoparticles could be highly
distributed in the support owing to the presence of N-doped
carbons during the preparation process. In contrast, ZnO/SiO2-2
Figure 3. NH3-TPD spectra of (a) ZnO/SiO2-NC-600, (b) ZnO/SiO2-1, and (c)
ZnO/SiO2-2.
Figure 4A shows a comparison of X-ray diffraction (XRD)
patterns of ZnO/SiO2-NC-600, ZnO/SiO2-1, ZnO/SiO2-2 and SiO2.
It was found that all these samples had a broad signal at 2θ =
20-30°, which was assigned to the diffraction peak of amorphous
SiO2.[30] For ZnO/SiO2-2, obvious diffraction peaks at 2θ = 31.8°,
34.3°, 36.5°, 47.7°, 56.6°, 62.9° and 68.0° were observed.
These diffraction signals were well defined and assigned to the
hexagonal wurtzite phase of ZnO.[23] ZnO/SiO2-1 also showed
the same diffraction peaks due to ZnO, but these were
significantly suppressed by the strong signal of amorphous SiO2
and not clearly shown in our case. However, for ZnO/SiO2-NC-
600, no obvious diffraction peaks were observed in the XRD
spectrum (Figure 4B), suggesting that ZnO nanoparticles were
uniformly distributed on the support surface with very small
particle sizes. Further, an EDS spectrum of ZnO/SiO2-NC-600
confirmed that the sample contained Zn (Figure 4C). A UV-
visible absorption spectrum of ZnO/SiO2-NC-600 (Figure 4D)
shows a series of absorption peaks characteristic of ZnO under
400 nm wavelength, indicating the existence of ZnO.[31]
Moreover, the spectra showed a blue shift phenomenon, which
resulted in all characteristic peaks moving towards the short
wavelength region, due to the good dispersion of ZnO
nanoparticles in the support, in line with the XRD results.
shows
a narrower range due to the aggregation of ZnO
nanoparticles in the support, in the absence of N-doped carbons.
In addition, the elemental analysis indicates that the ZnO/SiO2-
NC-600 has a nitrogen content of about 1.58 mass%. The
content of N-doped carbons in the catalyst was 14.7 mass%,
determined by thermogravimetric analysis under an oxygen
atmosphere (Figure S1).
Figure 2. (a) N2 adsorption-desorption isotherms and (b) the pore size
distributions of the hybrid ZnO/SiO2-NC-600, ZnO/SiO2-1 and ZnO/SiO2-2.
In Figure 3 is depicted the NH3 temperature programmed
desorption (NH3-TPD) profiles of ZnO/SiO2-NC-600, ZnO/SiO2-1
and ZnO/SiO2-2. Obviously, the peak area of ZnO/SiO2-NC-600
was significantly larger than that of ZnO/SiO2-1 and ZnO/SiO2-2,
3
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Figure 5. (a) TEM image of ZnO/SiO2-NC-600; (b) HRTEM image of
ZnO/SiO2-NC-600; (c) The distribution of particle size of ZnO/SiO2-NC-600; (d)
STEM-HAADF images and corresponding EDS mapping analysis results of
ZnO/SiO2-NC-600.
Figure 4. A) XRD patterns of (a) ZnO/SiO2-NC-600, (b) ZnO/SiO2-1, (c)
ZnO/SiO2-2, (d) SiO2; B) detailed XRD patterns of (a) ZnO/SiO2-NC-600 and
(b) ZnO/SiO2-1; C) EDS spectrum of ZnO/SiO2-NC-600; D) UV-vis absorption
spectra of the prepared samples.
The wide-scan X-ray photoelectron spectroscopy (XPS)
pattern shown in Figure 6a demonstrates the presence of Zn, C,
and N. This indicates that the nitrogen in chitosan was retained
during carbonization, resulting in the formation of an N-doped
carbon support. The chemical states of the Zn, C, and N species
on the surface are shown in Figure 6 b–d. The fine pattern of Zn
2p (Figure 6b) fits well with two spin-or bit-split doublets that
corresponded to Zn 2p3/2 and Zn 2p1/2, confirming that Zn exists
mainly in the form of Zn2+.[24] The high-resolution XPS spectra of
C 1s and N 1s also confirmed the doping of N in the carbon
support. In the C 1s, XPS peaks of ZnO/SiO2-NC-600 (Figure 6c)
showed that the lower energy peak near 284.52 eV is assigned
to graphitic C or C-N bonds, the peak centered at about 285.13
eV is attributed to C=C bonds and the higher binding energy
288.43 eV originates from C-C or C-H bonds in undecomposed
chitosan.[6,33] The deconvolution of N 1s energy level signals
(Figure 6d) suggests that ZnO/SiO2-NC-600 contains several N
types including intact -NH2 groups (397.74 eV), pyridinic N
(398.73 eV), pyrrolic N (399.72 eV), graphitic N (400.70 eV), and
pyridinic N-oxide functions (402.01 eV).[6,34-36] In addition, an N
1s signal also appears at 398.13 eV, which falls in the range of
the binding energies of N-Zn bonds, further verifying that
interactions between ZnO nanoparticles and the N-doped
carbon support exist.[24,37]
In Figure 5 is displayed the TEM images of the ZnO/SiO2-NC-
600. These show that an interconnected, cotton-like support was
formed and that the ZnO nanoparticles were uniformly dispersed
in it (Figure 5a and b). The selected-area electron diffraction
(SAED) pattern of an individual ZnO confirmed the good
crystallinity of these synthesized ZnO nanoparticles (Figure 5a,
inset). The size distribution data shown in Figure 5c indicate an
average size of 2.55 nm for these ZnO nanoparticles, in good
agreement with the result obtained from XRD. We attribute this
phenomenon to two major reasons: 1) the hydroxyl and amino
groups on the surface of chitosan can chelate with zinc ions,
resulting in a good dispersion; 2) abundant N atoms provide
non-chelation sites and inter-atom space in the formation of
hybrid carbons, and these sites can confine the growth of ZnO
nanoparticles and promote their dispersity.[32] In contrast, ZnO
nanoparticles in ZnO/SiO2-1 and ZnO/SiO2-2, prepared in air,
exhibited irregular size and agglomeration (Figure S2 a and b).
Furthermore, scanning transmission electron microscopy (STEM)
was used to investigate the ZnO/SiO2-NC-600 and the result is
depicted in Figure 5d, together with a representative high-angle
annular dark-field (HAADF) image and the elemental mapping of
this material. It is clear that Si, O, C, and N are distributed
throughout the sample with pore nanostructures, while Zn is
distributed throughout the whole sample including pores,
resulting in
a decrease of the pore volume and size, in
accordance with the BET results.
4
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Table 2. Synthesis of 2-arylbenzimidazoles from aldehydes and diamines under
the ZnO/SiO2-NC-600 catalyst.[a]
Entry
1
Diamine
Aldehyde
Product
Time (h)
1
Yield[b]
91
2
2
90
93
2
3
1.5
2
94
90
84
87
86
4
5
6
7
8
Figure 6. XPS spectra of the synthesized ZnO/SiO2-NC-600. (a) Wide-scan
survey, and (b) - (d) fine curves of Zn 2p, C 1s, and N 1s.
5
5
The scope of applicability of this ZnO/SiO2-NC-600 catalyst
was explored for the synthesis of 2-arylbenzimidazoles using
various aromatic aldehydes and diamines in methanol. As
shown in Table 2, all such reactions proceeded smoothly and
delivered the 2-arylbenzimidazoles in good to excellent isolated
yields (Entries 1-10). Among all of the examples listed, it was
found that the electronic properties of the substituents on the
phenyl ring of the benzaldehydes slightly influenced the product
yields: reactants having electron-withdrawing groups, such as -
Cl and -NO2, afforded the expected products in relatively higher
yields than when electron-donating ones, such as -CH3 and -
OCH3 were present. In contrast, substituted benzaldehydes with
the -Cl group at various positions furnished similar product yields,
suggesting that steric factors had less influence on the reaction
rate. Furthermore, substituted o-phenylenediamines with
benzaldehyde were also examined and good yields were
obtained under the same conditions. Altogether, all these results
indicated good levels of catalytic performance and functional
group tolerance of ZnO/SiO2-NC-600 for the synthesis of 2-
arylbenzimidazoles under mild conditions. Notably, no other
products were found in all of these reactions except for the Schiff
base intermediate.
4
4
4
82
80
9
10
[a] Reaction conditions: aldehyde (1.0 mmol), diamine (1.1 mmol), methanol (5.0
mL), ZnO/SiO2-NC-600 nanocatalyst (0.22 g, containing 15.1% ZnO), H2O2 (30%
in water, 0.2 mL) at 40 oC. [b] Yields of isolated products.
Conclusions
In summary, a novel catalyst based on ZnO nanoparticles
supported on N-doped carbons with silica hybrid material has
been explored; biomass chitosan was used as the precursor of
the N-doped carbon support by a facile one-pot, self-assembly
synthetic strategy. The ultrasmall ZnO nanoparticles (about 2.55
nm) were effectively confined and tightly anchored on the
support with high dispersity, and provided a high abundance of
active surfaces and beneficial metal-support interactions. The
synthesized ZnO/SiO2-NC-600 exhibited excellent catalytic
performance, good stability, and broad scope for the synthesis of
2-arylbenzimidazoles. This one-pot fabrication strategy and the
attractive hybrid material could inspire further nanocomposite
preparations and the design of novel supported heterogeneous
catalysts for other applications.
Experimental Section
Materials
Chitosan, diamines and aldehydes were supplied by Aladdin Chemicals.
Silica gel, zinc nitrate (Zn(NO3)2·6H2O, 99.95% metal basis), acetic acid,
and hydrogen peroxide (H2O2, 30% in water) were purchased from
Baoding Huaxin Reagent and Apparatus Co., Ltd. Unless otherwise
stated, all chemicals were used as received without further purification.
5
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Catalyst preparation
water, 0.2 mL) were added to the solution, which was then stirred at 40
oC for 1 h. The process was monitored by HPLC using a BaseLine C18
column (HPLC-1220) with a mobile phase of methanol–water (6:4 by v/v)
at a 1.0 mL min−1 constant flow. The yields of products were calculated
as follows:
Preparation of ZnO/SiO2-NC: Acetic acid (2.0 g) was added to deionized
water (130 mL), followed by chitosan (1.5 g). The mixture was stirred in
an ultrasonic cleaning bath for 0.5 h. Subsequently, silica gel (5.0 g) was
added to the solution, which was stirred for 0.5 h to uniformly disperse
the soliquid, after which Zn(NO3)2·6H2O (2.0 g) was added and the
suspension stirred for 0.5 h then heated to 80 oC to evaporate the bulk of
the water, which was finally removed completely in an oven at 80 oC.
After drying, the samples were calcined in a tube furnace at various
temperatures (500 oC, 600 oC, and 700 oC) for 4 h under a nitrogen flow.
Finally, a black product was obtained and named as ZnO/SiO2-NC-X (X =
500 oC, 600 oC, and 700 oC).
Pure products were finally isolated by column chromatography on 200–
300 mesh silica gel using petroleum ether–ethyl acetate (6:1 by v/v) as
an eluent.
Catalyst Recycling Test
After the first reaction, the catalyst was collected by centrifugation,
washed three times with methanol and was directly used in a subsequent
reaction without further drying. After the completion of reaction for 1 h,
the mixture was analyzed by HPLC.
Preparation of ZnO/SiO2-1: For comparison, another sample was
prepared by the foregoing procedure but the samples were calcined in a
tube furnace at 600 oC for 4 h under air. The product was obtained as a
white powder and was named ZnO/SiO2-1.
Acknowledgements
Preparation of ZnO/SiO2-2: Silica gel (5.0 g) was added to deionized
water (130 mL) and the mixture stirred for 0.5 h to uniformly disperse the
soliquid. After that, Zn(NO3)2·6H2O (2.0 g) was added and the resulting
suspension stirred for 0.5 h. Then dry as described above. The sample
was calcined in a tube furnace at 600 oC for 4 h under air to give a white
powder, ZnO/SiO2-2.
The authors thank Professor David W. Knight (Cardiff University)
for his kind assistance. The work was supported by the National
Natural Science Foundation of China (21376060 and 21676068),
the Natural Science Foundation of Hebei Province
(B2014201024) and hundred outstanding innovative personnel
support plan of Hebei Universities (SLRC2017020).
Preparation of SiO2-NC: To aqueous acetic acid (2.0 g HOAc in 130 mL
of deionized water) was added chitosan (1.5 g). The mixture was stirred
and dispersed in an ultrasonic cleaning bath for 0.5 h. Subsequently,
silica gel (5.0 g) was added to the mixture, which was then stirred for a
further 0.5 h. After drying, the samples were calcined in a tube furnace at
600 oC for 4 h under a nitrogen flow. Finally, a black powder was
obtained [SiO2-NC].
Conflict of Interest
The authors declare no conflict of interest.
Catalyst characterization
Keywords: biomass • ultrasmall ZnO nanoparticles • N-doped
The Zn content of the respective catalysts was determined using
inductively coupled plasma-mass spectrometry (ICP-MS; Varian Vista
MPX). The BET surface areas and pore volumes of the catalysts were
measured by N2 physisorption at 77 K on a Micromeritics Tristar II 3020
apparatus. Elemental analyses was determined using a Exeter Analytical
ce-440 elemental analyzer. Thermogravimetric (TG) were taken with a
carbons • chitosan • 2-arylbenzimidazoles
[1]
[2]
L.C. Bai, X. Wang, Q. Chen, Y.F. Ye, H.Q. Zheng, J.H. Guo, Y.D. Yin,
C.B. Gao, Angew. Chem. Int. Ed. 2016, 55, 15656-15661.
A. Corma, P. Concepción, M. Boronat, M.J. Sabater, J. Navas, M.J.
Yacaman, E. Larios, A. Posadas, M.A. Lopez-Quintela, D. Buceta,
Nature. Chem. 2013, 5, 775-781.
Netzsch TG 209 F3 thermogravimetric analyzer under
a oxygen
atmosphere. A temperature-programmed desorption of ammonia (NH3-
TPD) test was performed using the Micromeritics Autochem II 2920
instrument. X-Ray photoelectron spectra (XPS) were recorded on a PHI
1600 instrument using an Mg Kα radiation source for excitation; binding
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Entry for the Table of Contents
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Ultrasmall ZnO nanoparticles anchored on
N-doped carbons with silica support
(ZnO/SiO2-NC) has been fabricated by a
self-assembly strategy with chitosan as
precursor. The average size of 2.55 nm for
ZnO nanoparticles was obtained with
assistant of the interaction between zinc
ion and hybrid support (see figure). This
Bo Chen, Chan Zhang, Libo Niu,
Xiaozhen Shi, Huiling Zhang, Xingwang
Lan,* and Guoyi Bai*
Page No. – Page No.
Biomass-Derived N-Doped Carbons
with Silica Supported Ultrasmall ZnO
Nanoparticles: Robust Catalysts for
catalyst
shows
excellent
catalytic
performance and recyclability in the
synthesis of 2-arylbenzimidazoles.
the
Green
Synthesis
of
Benzimidazoles
8
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