Heteropoly Acid/Nitrogen Functionalized Onion-like Carbon Hybrid Catalyst for Ester Hydrolysis Reactions

Article abstract of DOI:10.1002/asia.201500944

A novel heteropoly acid (HPA)/nitrogen functionalized onion-like carbon (NOLC) hybrid catalyst was synthesized through supramolecular (electrostatic and hydrogen bond) interactions between the two components. The chemical structure and acid strength of the HPA/NOLC hybrid have been fully characterized by thermogravimetric analysis, IR spectroscopy, X-ray photoelectron spectroscopy, NH₃ temperature-programmed desorption and acid-base titration measurements. The proposed method for the fabrication of the HPA/NOLC hybrid catalyst is a universal strategy for different types of HPAs to meet various requirements of acidic or redox catalysis. The hydrophobic environment of NOLC effectively prevents the deactivation of HPA in an aqueous system, and the combination of uniformly dispersed HPA clusters and the synergistic effect between NOLC and HPA significantly promotes its activity in ester hydrolysis reactions, which is higher than that of bare PWA as homogeneous catalyst. The kinetics of the hydrolysis reactions indicate that the aggregation status of the catalyst particles has great influence on the apparent activity. Know your onion-like carbon: A heteropoly acid (HPA)/nitrogen functionalized onion-like carbon (NOLC) hybrid catalyst was successfully synthesized. The hydrophobic environment of NOLC effectively prevents the deactivation of HPA in an aqueous system, and the synergistic effect between NOLC and HPA significantly promotes its activity in hydrolysis reactions.

Full text of DOI:10.1002/asia.201500944

DOI: 10.1002/asia.201500944
Full Paper
Heterogeneous Catalysis
Heteropoly Acid/Nitrogen Functionalized Onion-like Carbon
Hybrid Catalyst for Ester Hydrolysis Reactions
Wei Liu,[a] Wei Qi,*[a] Xiaoling Guo,[a, b] and Dangsheng Su*[a]
Abstract: A novel heteropoly acid (HPA)/nitrogen functional-
ized onion-like carbon (NOLC) hybrid catalyst was synthe-
sized through supramolecular (electrostatic and hydrogen
bond) interactions between the two components. The chem-
ical structure and acid strength of the HPA/NOLC hybrid
have been fully characterized by thermogravimetric analysis,
IR spectroscopy, X-ray photoelectron spectroscopy, NH3 tem-
perature-programmed desorption and acid–base titration
measurements. The proposed method for the fabrication of
the HPA/NOLC hybrid catalyst is a universal strategy for dif-
ferent types of HPAs to meet various requirements of acidic
or redox catalysis. The hydrophobic environment of NOLC
effectively prevents the deactivation of HPA in an aqueous
system, and the combination of uniformly dispersed HPA
clusters and the synergistic effect between NOLC and HPA
significantly promotes its activity in ester hydrolysis reac-
tions, which is higher than that of bare PWA as homogene-
ous catalyst. The kinetics of the hydrolysis reactions indicate
that the aggregation status of the catalyst particles has
great influence on the apparent activity.
Introduction
are restricted. For example, the effective combination of HPA
and organically modified SBA-15 or amphiphilic surfactants
could limit the contact between HPAs and water molecules,
and thus prominently improve their acidic catalytic activity in
an aqueous system.[4b,c]
Heteropoly acid (HPA) is a kind of nanostructured transition-
metal oxide cluster with protons as counter cations, which
have a stronger acidity than conventional acids such as g-Al2O3
and H2SO4.[1] HPA has shown a high catalytic activity in many
acid-catalyzed reactions and related chemical industrial pro-
cesses such as the synthesis of isopropyl alcohol and the poly-
merization of tetrahydrofuran.[2] The significance of HPA cata-
lyst originates from its strong acidity and controllable composi-
tion and structure.[3] However, most HPAs lose their catalytic
activity in aqueous phase because of the formation of HPA sec-
ondary aggregates and also their poor organic compatibility as
inorganic clusters.[4] In addition, the relatively high solubility of
HPAs in water makes them also disadvantageous with regard
to recyclability as heterogeneous catalyst.
Nanocarbon materials, such as carbon nanotubes (CNTs) or
graphenes, have been considered as promising catalyst sup-
ports owing to their advantages such as high purity, high ac-
cessibility, and the possibility of tuning the specific interactions
with the active phases.[5] Several examples for the combina-
tions of HPAs and nanocarbon materials have already been re-
ported.[6] The hydrophobic graphitic structure of carbon in
these hybrid catalysts could effectively prevent the aggrega-
tion of HPAs and further enrich the organic reactants on the
carbon surface.[6] The hybrid catalysts exhibit extraordinary ac-
tivity in acid or redox catalytic reactions. However, there is still
great potential for the further improvement of fabrication
methods for HPA/nanocarbon catalysts, such as obtaining a uni-
form dispersion and increasing the loading amount of HPAs.
Onion-like carbon (OLC) is a kind of spherical nanocarbon
particle with a few nanometers in diameter, which is com-
posed of layers of enclosed graphitic shells.[7] OLC exhibits
a similar graphitic structure and thermal stability as multi-
walled CNTs (MWCNTs), but it has a larger specific surface area,
which should be beneficial to mass transfer and loading of the
active components.[8] Herein, we report a novel HPA/nitrogen-
functionalized OLC (NOLC) hybrid as a solid acid catalyst for
hydrolysis reactions. The introduction of nitrogen species on
the OLC surface could establish electrostatic interactions be-
tween the NOLC support and HPA clusters for an improvement
of the dispersion and loading amount. The synergistic effects
between the two components ensure an optimal acid catalytic
One promising solution to prevent the deactivation of HPA
in an aqueous system is the confinement and immobilization
of HPAs in nanosized hydrophobic pores or channels of the
support where the contacts between HPA and water molecules
[a] W. Liu, Dr. W. Qi, X. Guo, Prof. D. Su
Shenyang National Laboratory for Materials Science
Institute of Metal Research
Chinese Academy of Sciences
72 Wenhua road, Shenyang 110016 (P. R. China)
E-mail: wqi@imr.ac.cn
dssu@imr.ac.cn
[b] X. Guo
Lab of Advanced Materials & Catalytic Engineering
Dalian University of Technology
Dalian 116024 (P. R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/asia.201500944.
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activity and water tolerance of HPAs in ethyl acetate (EA) hy-
drolysis reactions. Furthermore, the proposed method for the
fabrication of HPA/NOLC hybrid catalyst has been proven to be
a universal strategy for both nanocarbons and HPAs, and it is
applicable for other types of HPAs to meet various require-
ments of acidic or redox catalysis.
porting Information). The nitrogen atoms are incorporated into
the graphitic lattice in the form of pyridinic, pyrrolic, and
graphitic nitrogen species and so on, and the introduction of
these species could effectively adjust the surface charge of the
carbon supports.[11]
The surface charge measurements at different pH values
suggest that the point of zero charge (PZC) of NOLC is at 5.7
(Figure S1, Supporting Information), indicating that the NOLC
support is negatively charged under neutral conditions and
further suggesting the success of the nitrogen functionaliza-
tion. Keggin-type phosphotungstic acid (PWA) was firstly ap-
plied as a typical HPA for the hybrid catalyst fabrication. The
pH of the PWA solution for fabrication is below 1, which
means that the surface of NOLC has a positive charge under
the combination conditions, which is favorable for the immobi-
lization of the negatively charged PWA anions on the NOLC
support through electrostatic interactions.
Results and Discussion
Scheme 1 illustrates the concept and preparation process of
HPA/NOLC hybrids. The OLC samples are firstly obtained
through thermal treatment with commercial ultra-dispersed di-
amond (UDD) at 13008C in an argon atmosphere.[9] After sub-
sequent liquid phase oxidation[10] and gas phase nitrogen func-
The morphology of PWA/NOLC was examined by TEM (Fig-
ure 2A). The graphitic structure of OLC can be clearly ob-
Figure 2. (A) TEM image of PWA/NOLC; (B) STEM image of PWA/NOLC.
Scheme 1. Schematic illustration of the synthesis of the PWA/NOLC catalyst.
served, and the overall morphology of PWA/NOLC is similar to
that of OLC (Figure S2, Supporting Information), which means
that the damage to the surface after immobilization is negligi-
ble. The fact that no PWA aggregates can be observed in the
TEM images suggests a good dispersion of PWA on NOLC. In
the scanning transmission electron microscopy (STEM) image,
the existence of PWA clusters can be clearly confirmed as
white spots (Figure 2B). The supported PWA clusters have a di-
ameter of about 1 nm, which is consistent with the size of
a single PWA cluster (1.2 nm), confirming that the PWA is mon-
odispersed on the NOLC. In addition, the lack of the PWA
signal in the X-ray diffraction (XRD) pattern of PWA/NOLC also
confirms the uniform dispersion of PWA clusters (Figure S3).[14]
The high dispersion of PWA clusters is beneficial for their per-
formance in catalytic reactions.
tionalization[11] with concentrated nitric acid and ammonia, re-
spectively, the NOLC support is eventually obtained. The vibra-
tion modes at 1577 cmÀ1 and 1234 cmÀ1 in the attenuated
total reflection Fourier transform IR (ATR-FTIR) spectra of NOLC
(Figure 1) are assigned as stretching vibrations of graphitic
structure and carbon–nitrogen bond, respectively, confirming
the success of the carbon graphitic shell formation[12] and the
nitrogen functionalization.[13] The X-ray photoelectron spectros-
copy (XPS) measurement shows that the surface atomic con-
tent of nitrogen reaches 4.5% for NOLC sample (Table S1, Sup-
The nature of the interactions between PWAs and NOLC
supports was revealed by using XPS (Figure 3). Unlike nitro-
gen-doped nanocabons synthesized through chemical vapor
deposition (CVD) methods, which contain little or negligible
oxygen on the surface, a considerable content of oxygen func-
tional groups is present on the surface of NOLC (oxygen con-
tent of about 4 at.%) owing to the HNO3 treatment process.[15]
Therefore, the NOLC support may interact with PWA clusters
through both oxygen and nitrogen functional groups. In the N
1s XPS spectrum of the PWA/NOLC hybrid, it is found that the
Figure 1. ATR-FTIR spectra of NOLC and PWA/NOLC.
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Figure 3. (A) N 1s XPS of PWA/NOLC; inset: content of graphitic N (area per-
centage of overall N content) as a function of PWA loading; (B) O 1s XPS of
NOLC before and after PWA immobilization.
Figure 4. (A) TG curves of NOLC and PWA/NOLC with different loading;
(B) N2 adsorption isotherms of (a) NOLC and (b) PWA/NOLC.
graphitic nitrogen content (401.0 eV) increases along with PWA
loading, which agrees with previously reported observations
that HPA anions could connect with graphitic nitrogen through
electrostatic interactions.[6] A decrease of the intensity at about
534 eV in O 1s XPS after immobilization suggests the interac-
tions between PWAs and oxygen functionalities, while the new
peak at about 531 eV could be assigned to the contribution of
oxygen species in PWA clusters.[16] This phenomenon is consis-
tent with other independent studies, which suggest that HPAs
would interact strongly with oxygen functional groups on
carbon materials through hydrogen bonding.[17] These two
kinds of supramolecular interactions (electrostatic interactions
and hydrogen bonding) existing between PWAs and oxygen/
nitrogen functionalities strengthen the connections between
the two components effectively, further leading to a robust
hybrid catalyst.
brids with different PWA loading amounts all exhibit a relatively
high stability below 6008C (Figure 4). A dramatic weight loss
occurs above 6008C because of the combustion of carbon. The
NOLC support shows a similar stability under the same test
conditions, which indicates that the PWAs have a negligible in-
fluence on the structure and stability of NOLC supports. The
burned ash of PWA/NOLC is WO3, while the ash of NOLC is
negligible. The loading amount of PWA in PWA/NOLC is about
15 wt.%, which means that there are about 12 PWA clusters
on average on each NOLC sphere. This loading amount is
much higher than that in a previous report using CNTs as sup-
port (about 5 wt.%).[6] It is also the maximum PWA loading in
current research. In addition, the Brunauer–Emmett–Teller (BET)
surface area of the hybrid decreases by 15% after immobiliza-
tion (NOLC: 377.4 m2gÀ1 vs. PWA/NOLC: 322.5 m2gÀ1) because
of the high molecular weight and loading amount of PWA
(Figure 4).
The ATR-FTIR spectra of PWA/NOLC (Figure 1) clearly exhibit
the characteristic vibration modes of Keggin-type PWA clusters
(between 800 and 1100 cmÀ1). Among them, the vibrations of
W-Oc-W, W-Od and P-Oa in PWA/NOLC are identical to those of
The superficial acid–base property of the hybrid catalysts
was firstly investigated by ammonia temperature-programmed
desorption (NH3 TPD), as shown in Figure 5A. The desorption
peaks centered below 4008C for bare PWA reflect desorbed
NH3 combined with PWA via hydrogen bonding (NH2ÀH···O),
while the peak centered at 5708C is attributed to the desorp-
tion of chemically adsorbed ammonia on PWA. For NOLC sup-
ports, the peaks below 4008C originate from the desorption of
the weakly chemisorbed NH3. The peak centered above 5008C
is a result of the decomposition of nitrogen species in the
form of NH3, which has been also reported in a previous
work.[6] The NH3 TPD profile of PWA/NOLC exhibits the charac-
bare PWA clusters (820 cmÀ1 vs. 818 cmÀ1
,
978 cmÀ1 vs.
978 cmÀ1, and 1078 cmÀ1 vs. 1076 cmÀ1), but there is a 15 cmÀ1
red-shift of the W-Ob-W vibration band after PWA immobiliza-
tion on NOLC (898 cmÀ1 vs. 913 cmÀ1).[4c,18] This red-shift indi-
cates the interactions of PWA with the functional groups of
NOLC, and it is consistent with previous reports.[17a]
Thermogravimetric analysis (TGA) under oxygenous atmos-
phere was applied to investigate the thermal stability of PWA/
NOLC and also to calculate the content of PWA. PWA/NOLC hy-
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amount of weak acidic sites compared to strong ones (2.28”
10À4 mol H+ gÀ1 vs. 0.64”10À4 mol H+ gÀ1) is consistent with
the NH3 TPD results. Because of the similarity in the size of
PWA and NOLC sphere and the uniform dispersion of PWA, it
is reasonable to assume that one PWA cluster connects not
only to a single NOLC sphere but multiple NOLCs.
Oxidized OLC (oOLC), which has little nitrogen but massive
oxygen functional groups on its surface, was also used as sup-
port to immobilize PWA (PWA/oOLC) as a reference to evaluate
the role of nitrogen functional groups. In the O 1s XPS spectra
of PWA/oOLC and oOLC (Figure S4), a decrease of the intensity
at about 534 eV after immobilization is observed, indicating
the interactions between PWA and the oxygen functionalities.
The PWA loading of PWA/oOLC from TGA (Figure S5) is signifi-
cantly lower than that of PWA/NOLC (6.0 wt.% vs. 14.8 wt.%),
suggesting that the interactions between PWA and oOLC are
weaker than those between PWA and NOLC, which further
suggests the critical role of nitrogen functional groups in im-
mobilizing PWAs.
H3PMo12O40 (PMoA) and H4SiW12O40 (SiWA), which have dif-
ferent central or addenda atoms compared with PWA, were
also used to prepare hybrid catalysts (PMoA/NOLC and SiWA/
NOLC, respectively) following the synthesis strategy shown in
Scheme 1. The clear signals in Mo 3d and W 4f XPS of the cor-
responding hybrid catalysts confirm the successful immobiliza-
tion of different HPA clusters on NOLC (Figure S6). The TG and
XPS characterization results (Figures S7 and S8) confirm that
PMoA/NOLC and SiWA/NOLC catalysts exhibit a structure simi-
lar to PWA/NOLC hybrids, and further suggest the universality
of the proposed fabrication strategy for HPA/NOLC hybrid cata-
lysts for various HPAs.
Figure 5. (A) NH3 TPD profiles of PWA, NOLC and PWA/NOLC; (B) Titration
curve of PWA/NOLC. (a) pH vs. NaOH consumption, (b) The first order deriva-
tive of (a).
teristic desorption peaks of both PWA and NOLC. The integrat-
ed peak area below 4008C increases by 173% (NOLC: 0.60”
10À7 vs. PWA/NOLC: 1.65”10À7), while some strong acid sites
(desorption of NH3 above 5008C) for PWAs are lost after immo-
bilization. This phenomenon is also observed in the case of
PWAs supported on mesoporous molecular sieves,[19] which
may be due to the interactions between PWA clusters and
NOLC supports. The average number of available protons on
each PWA cluster was determined to 1.9 protons per PWA
through careful comparison of the NH3 desorption peak areas
of PWA, NOLC and PWA/NOLC. This value is below the stoi-
chiometric number of PWA clusters (3.0 protons per
PWA), indicating that the interactions between PWAs
and the functional groups hinder some protons,
which means that some protons of PWA cannot be
used in the acid-catalyzed reaction in an aqueous
system.
The acid catalytic activity and water tolerance of the hybrid
catalyst were evaluated by its performance in EA hydrolysis re-
actions (Scheme 2). H2O is not only the reactant but also the
solvent for this reaction. Three types of HPA/NOLC (PWA/
NOLC, PMoA/NOLC and SiWA/NOLC) were tested, and the per-
Quantitative acid–base titration was also used to
evaluate the acidity of the PWA/NOLC hybrid catalyst
in aqueous phase. Figure 5B illustrates the depend-
ence of pH on the cumulative consumption of the ti-
trant (NaOH) and its first order derivative. The pH of
the PWA/NOLC dispersion increases with the addition
of NaOH titrant, and two peaks can be observed
from the first order derivative profile of the pH trend,
which means there are two kinds of acid sites in
PWA/NOLC. The first peak, which is located at around
pKa =4.5, represents the free protons (H+), while the
second one locating at pKa around 7.8 could be as-
signed as protons linking with NOLC substrates; this
also indicates that the acid strength of the second
Scheme 2. Schematic illustration outlining the hydrolysis process of EA catalyzed by
type of acid site is very weak. The significantly larger PWA/NOLC.
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formance of PWA/oOLC and bare PWA were also examined as
references. The detected products of EA hydrolysis are only
ethanol and acetic acid. The activity of various acid catalysts in
EA hydrolysis reactions, including PWA/NOLC and some other
classic acid catalysts, are listed in Table 1. All the catalytic ex-
periments were performed at 608C under similar conditions.[4b]
As shown in Table 1, the activity of PWA/oOLC (per catalyst
graphitic structure of NOLC can adsorb nonpolar EA molecules
far more effectively than the hydrophilic PWAs, and the enrich-
ment of EA surrounding the catalytic centers facilitates the cat-
alytic hydrolysis reactions. The release of polar hydrolysis prod-
uct is also easily accomplished on the graphitic surface of
NOLC (Scheme 2). Similar results were also reported using
modified CNTs or SBA-15 as supports.[4b,6] This close coopera-
tion between NOLC and PWA greatly increases the
catalytic efficiency of PWA.
Table 1. Activity of various catalysts for EA hydrolysis in an aqueous system.
The reusability and stability of PWA/NOLC in EA hy-
drolysis reactions was tested as shown in Figure S9.
More than 82% of the apparent activity of PWA/
Catalyst
State of
Catalyst
Catalytic Activity
NOLC remains after the second run, and there is no
further activity drop in the following reaction cycles
(third and fourth reaction runs). The TGA result
shows that the PWA loading amount of PWA/NOLC
deceases by about 20% after the first run (14.8 wt.%
vs. 11.7 wt.%), but there is no further leaking of
PWAs after the second run (Figure S10). In addition,
the N 1s XPS spectrum of the recovered PWA/NOLC
catalysts shows negligible changes (Figure S11), indi-
cating that the interactions between PWA and nitro-
gen functional groups are relatively stable during re-
actions. As a reference, the apparent activity of PWA/
Per Cat. Weight
Per Acidic Protons
(10À3 mmolgÀ1 minÀ1
)
(mmolmolacidÀ1 minÀ1
)
PWA/NOLC
PWA/oOLC
NOLC
PWA
H2SO4
Solid
Solid
Solid
Liquid
Liquid
Solid
Solid
14.9
6.7
0.25
107.7
1980
39
151.0[a,b]
144.6[a]
–
103.4[a]
25[c]
H-ZSM-5 zeolite
SO42À/ZrO2
70[c]
25.5
127[c]
[a] Calculated at the reaction time of 35 h. [b] The acidity value of PWA/NOLC is based
on the acid/base titration result. [c] Ref. [8].
weight) is much lower than that of PWA/NOLC, which is consis-
tent with the lower PWA loading in PWA/oOLC than PWA/
NOLC. In addition, NOLC shows a very low activity in the reac-
tion compared with HPA/NOLC hybrid catalysts, and the activi-
ty should originate from its weak surface acidity as illustrated
by NH3 TPD and acid–base titration measurements. Sulfuric
acid and H-ZSM-5 zeolite, which is a traditional homogenous
catalyst and a common solid acid catalyst, respectively, exhibit
a relative low activity because of the deactivation of acid sites
in aqueous phase. SO42À/ZrO2 shows an acceptable activity,
which may come from the unique surface chemical property of
the supports that could protect the active components from
H2O deactivation.
oOLC drastically drops by 41% at its second run, with the PWA
loading amount significantly deceasing from 6.0 wt.% to
3.2 wt.% (Figure S12), indicating that the interactions between
PWA and oxygen functional groups might be vulnerable. The
activity drop of the recovered PWA/NOLC in its second run
may be a result of the leaking of PWA clusters that connected
with oxygen functional groups during the reactions and the
subsequent washing procedures; however, the activity keeps
constant after the leaking of these weakly bonded PWA clus-
ters.
The kinetics of EA hydrolysis catalyzed by HPA/NOLC are
shown in Figure 6. Among all the HPA/NOLC samples, PWA/
NOLC exhibits the highest activity, while PMoA/NOLC shows
the lowest activity, which is consistent with the sequence of
the intrinsic acidity of the unsupported HPAs (acidity: PWA >
SiWA>PMoA).[21] It should be noted that aqueous phase EA
hydrolysis catalyzed by HPAs is normally considered a first-
order kinetic process,[22] while our proposed HPA/NOLC cata-
lysts seem to behave differently as shown in Figure 6. The ap-
The intrinsic activity (TOF, activity per proton) of PWA/NOLC
is higher than that of all other catalysts. Compared with bare
PWAs, the TOF of PWA/NOLC hybrid catalyst is 1.5 times
higher, even though the former one constitutes a homogene-
ous catalytic reaction system, indicating that the problems of
PWA’s secondary aggregates with H2O is well alleviated by the
relatively strong interactions between PWA and NOLC.[4a] The
TOF of PWA/oOLC is similar to that of PWA/NOLC, indicating
that the enhanced PWA activity mainly originates from the hy-
drophobic nature of the support with a graphitic structure.
The reason for the high activity of the immobilized PWAs in
the PWA/NOLC hybrid over unsupported ones may be the uni-
form dispersion of PWA and the synergistic effect between
NOLC and PWAs. HPA clusters could form vesicle-like super-
structure aggregates in many solvents such as water,[20] which
would decrease the surface area of HPA (the surface area of
PWA is about 8 m2 gÀ1 in H2O) and restrict their activity. In our
catalyst, the PWAs are monodispersed on NOLC with a large
surface area of 322.5 m2 gÀ1, which greatly increases the acces-
sibility of PWAs to the reactants. In addition, the hydrophobic
Figure 6. Kinetics of the hydrolysis of EA catalyzed by PWA/NOLC, SiWA/
NOLC and PMoA/NOLC.
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as PWA/NOLC. The fabrication of SiWA/NOLC, PMoA/NOLC and
PWA/NOLC at different loading content followed a similar strategy
with only changing the type and feed ratio of HPA and NOLC.
parent activity for all the hybrid catalysts increases continuous-
ly during the catalytic tests. The reasons for such an abnormal
kinetic behavior might be a change of the HPA/NOLC aggre-
gated status (namely, the number of the accessible active sites)
during the catalytic process. As shown in Figure S13, the aver-
age size of PWA/NOLC clusters decreases with increasing EA
conversion, since the nanosized NOLC spheres can easily ag-
gregate to minimize their surface energy[23] and the polarity of
the reaction system significantly increases at high conversion
(the hydrolysis product ethanol and acetic acid has obviously
a higher polarity than the substrate EA). The change in the ag-
gregation status of HPA/NOLC will lead to a variation of the
quantity of the accessible active sites, which further influences
the apparent activity and the kinetic behavior of the HPA/
NOLC hybrid.
Catalytic experiments
The hydrolysis of EA was carried out in a 150 mL thick-wall pres-
sure vessel with mechanical stirring at 608C. In a typical experi-
ment, EA (7.5 g), H2O (140 mL, 7.77 mol) , catalyst (812.5 mg) and
1,4-dioxane (500 mL, 5.90 mmol) as internal standard were added
into the vessel. A small amount of the solution (1 mL) was taken
periodically for gas chromatography (GC) analysis. After the reac-
tion, the catalyst was filtered off, washed with deionized water,
and dried at 1508C for 8 h. The conversion of EA to acetic acid and
ethyl alcohol was determined by the consumption of the substrate
using GC. The GC system used in this work was an Agilent 7890A
system equipped with an FID detector.
Characterizations
Conclusions
TEM and STEM analyses were performed using an FEI Tecnai G2
F20 microscope with an accelerating voltage of 200 kV. XRD data
were obtained using a D/Max-2400 X-ray diffractometer. The size
distribution tests were performed by using a Malvern Nano-ZS90
ZETA sizer. TGA was performed on a NETZSCH STA449F3 instru-
ment under a mixed flow of Ar (20 mLminÀ1) and synthetic air
(30 mLminÀ1) at a heating rate of 108CminÀ1 from 35 to 9508C.
The NH3 TPD was performed on a fixed bed reactor equipped with
a mass spectrometer under helium (50 mLminÀ1) at a heating rate
of 58CminÀ1 from 35 to 7508C. Before heating, samples (50 mg)
were exposed to excess 10% NH3 in Ar (100 mLminÀ1 for 2 h), and
the system was blown with inert gas (He, 50 mLminÀ1 for 8 h) at
358C. The XPS measurements were carried out in an ultra-high
vacuum (UHV) ESCALAB 250 system equipped with a monochro-
matic AlKa X-ray source (1486.6 eV; anode operating at 15 kV and
20 mA). The specific surface area was measured by using the BET
method using nitrogen adsorption–desorption isotherms on a Mi-
crometrics ASAP 2020 system. The ATR-FTIR spectra were recorded
using a Varian spectrometer equipped with a liquid nitrogen-
cooled MCT detector. The titration of the acid strength was done
with a Mettler DL77 titrator with 0.01m NaOH as titrant.
In conclusion, we have prepared a novel HPA/NOLC hybrid
solid acid catalyst, which has shown a promising acid catalytic
activity due to the synergistic effect of the two components.
The structure of HPA clusters remains intact during the immo-
bilization process. The HPA clusters are monodispersed on the
NOLC support, with their content even reaching 15 wt.%. The
HPAs interact with both oxygen and nitrogen functional
groups on the surface of NOLC supports. The strong interac-
tions between HPAs and nitrogen functional groups ensure
the relatively high stability of the hybrid catalysts. The NOLC
support not only enables a uniform dispersion but also pro-
vides a hydrophobic environment for HPAs to enrich organic
substrates, which leads to a higher activity than that of con-
ventional acid catalysts such as H2SO4 and H-ZSM-5 zeolites. To
the best of our knowledge, it is the first time that NOLC has
been used as support to prepare non-noble metal catalysts,
and the kinetic data show that NOLC holds great potential as
catalyst support.
Experimental Section
Acknowledgements
Preparation of NOLC
The authors thank Dr. Bingsen Zhang for TEM and STEM sup-
port. This work was financially supported by NSFC of China
(21303226, 21133010, 51221264, 21261160487), “Strategic Pri-
ority Research Program” of the Chinese Academy of Sciences
and General Financial Grant from the China Postdoctoral Sci-
ence Foundation (2012M520651).
First, ultra-dispersed diamond (UDD) was heated at 13008C in an
argon atmosphere for 6 h to afford OLC. Then, OLC (1 g) was col-
lected and refluxed in 100 mL HNO3 (68%) at 1208C for 2 h. The re-
sulting oxidized OLC, labelled as oOLC, was filtered and washed
with deionized water until the pH of the filtrate reached 7. Subse-
quently, the oOLC was dried at 1208C overnight. Finally, oOLC was
heated in a mixture of 10% NH3 in an argon atmosphere at 4008C
for 4 h to achieve nitrogen functionalization. The resulting sample
is denoted as NOLC.
Keywords: heterogeneous catalysis
hydrolysis · onion-like carbon · support
·
heteropoly acid
·
Preparation of HPA/NOLC
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Manuscript received: September 7, 2015
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