Hierarchically porous Br?nsted acidic ionic liquid functionalized nitrogen-doped carbons for pyrolysis biofuel upgrading via esterification of acetic acid with high boiling point alcohols

Article abstract of DOI:10.1016/j.apcata.2019.01.021

Br?nsted acidic ionic liquid (BAIL) functionalized hierarchically porous nitrogen-doped carbons (NHPCs) with interesting 3D interconnected macro-meso-microporous structure are successfully prepared by chemical bonding ?[C₃][SO₃CF₃^?] (C₃ = PrSO₃H) groups throughout the NHPC supports that are obtained via a single step CaCO₃ nanoparticle-directed nanocasting approach combined with K₂C₂O₄ chemical activation and urea/melamine self-activation during the process of carbonization of glucose and urea/melamine. By changing initial urea-to-glucose weight ratio in the preparation systems, the macro/meso/micropore proportion of the resulting GU?[C₃N][SO₃CF₃] catalysts are well-adjusted. The morphological characteristics, porosity properties and chemical structure of the prepared catalysts are well-characterized so that the contribution of the synergistic effect of CaCO₃ nanoparticle and K₂C₂O₄ to the formation of hierarchically porous structure is revealed. Owing to super strong Br?nsted acidity and unique hierarchically porous structure of the catalysts, they exhibit excellent heterogeneous acid catalytic activity in raw pyrolysis biofuel upgrading via esterification of acetic acid with high boiling point benzyl alcohol or 4-methoxybenzyl alcohol in toluene media, and therefore high acetic acid removal efficiency and efficient production of value-added benzyl acetate or 4-methoxybenzyl acetate are obtained simultaneously. The catalysts also show good reusability in target reactions, attributing to chemical interactions between BAILs and NHPC supports.

Full text of DOI:10.1016/j.apcata.2019.01.021

AppliedCatalysisA,General574(2019)10–24
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Hierarchically porous Brønsted acidic ionic liquid functionalized nitrogen-
doped carbons for pyrolysis biofuel upgrading via esterification of acetic
acid with high boiling point alcohols
Qingqing Zhang, Chaoyue Zhang, Yingnan Sun, Yihang Guo⁎, Daiyu Song⁎
School of Environment, Northeast Normal University, Changchun, 130117, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Brønsted acidic ionic liquid (BAIL) functionalized hierarchically porous nitrogen-doped carbons (NHPCs) with
interesting 3D interconnected macro-meso-microporous structure are successfully prepared by chemical bonding
‒[C3][SO3CF3−] (C3 = PrSO3H) groups throughout the NHPC supports that are obtained via a single step CaCO3
nanoparticle-directed nanocasting approach combined with K2C2O4 chemical activation and urea/melamine
self-activation during the process of carbonization of glucose and urea/melamine. By changing initial urea-to-
glucose weight ratio in the preparation systems, the macro/meso/micropore proportion of the resulting
GU‒[C3N][SO3CF3] catalysts are well-adjusted. The morphological characteristics, porosity properties and
chemical structure of the prepared catalysts are well-characterized so that the contribution of the synergistic
effect of CaCO3 nanoparticle and K2C2O4 to the formation of hierarchically porous structure is revealed. Owing
to super strong Brønsted acidity and unique hierarchically porous structure of the catalysts, they exhibit ex-
cellent heterogeneous acid catalytic activity in raw pyrolysis biofuel upgrading via esterification of acetic acid
with high boiling point benzyl alcohol or 4-methoxybenzyl alcohol in toluene media, and therefore high acetic
acid removal efficiency and efficient production of value-added benzyl acetate or 4-methoxybenzyl acetate are
obtained simultaneously. The catalysts also show good reusability in target reactions, attributing to chemical
interactions between BAILs and NHPC supports.
Hierarchically porous nitrogen-doped carbon
Ionic liquid
Pyrolysis biofuel upgrading
Esterification
1. Introduction
presence of soft (e.g. block-copolymers [10]) and/or hard template (e.g.
CaCO3 [11,12] or ZnO nanoparticles [13]) or combined template and
Carbon materials, prepared from inexpensive and abundant bio-
mass/derivates, are attracting vast interest because of their tunable
physiochemical properties along with the possibility of doping and
functionalization, and they are the promising candidates for applica-
tions in catalysis [1–3], energy storage and production [4,5] and se-
paration [6]. Among versatile carbon materials, hierarchically porous
carbons (HPCs) with 3D interconnected macro-, meso- and micropores
have drawn particular attention [7–9]. In heterogeneous catalysis,
carbon frameworks of HPCs with hierarchical porosity offer minimized
diffusion resistance and shortened diffusion pathway to mass transport
by macropores, while their high surface areas ensure to highly disperse
active sites throughout the micro and/or mesopores. The unique
structural features strongly suggest HPCs as the promising and sus-
tainable catalyst supports. Biomass/derivate-derived HPCs are com-
monly prepared by incomplete carbonization of carbon precursors (e.g.
chitosan, cellulose and carbohydrates) at elevated temperature in the
activation reagent [14]. The template is applied to create macropores of
HPCs, while the activation reagent such as KOH [15], KHCO3 [16],
K2C2O4 [17] and (NH4)2C2O4 [18] can introduce meso/micropores in
macropore wall, adjust meso/micropore proportion and increase the
surface area of HPCs.
In the present work, we devote to develop novel supported Brønsted
acidic ionic liquids (BAILs) as efficient and recoverable solid acid cat-
alysts for upgrading raw pyrolysis biofuel via esterification of acetic
acid with high boiling point benzyl alcohol (205 °C) or 4-methox-
ybenzyl alcohol (259 °C) in a toluene media. BAILs with strong Brønsted
acidity are more preferred over mineral acids in catalytic conversion of
biomass to new generation biofuels and biorenewable platform che-
micals, and the processes are significant to reduce fossil fuel energy
consumption and noxious gas emission [19–22]. However, the separa-
tion of BAILs from the reaction media is a challenge because they ex-
hibit high miscibility with organic solvents, and chemical
⁎ Corresponding authors.
E-mail addresses: guoyh@nenu.edu.cn (Y. Guo), songdy251@nenu.edu.cn (D. Song).
https://doi.org/10.1016/j.apcata.2019.01.021
Received 1 August 2018; Received in revised form 9 January 2019; Accepted 26 January 2019
Availableonline28January2019
0926-860X/©2019ElsevierB.V.Allrightsreserved.

Q. Zhang et al.
AppliedCatalysisA,General574(2019)10–24
immobilization of BAILs is therefore the foremost strategy to solve the
separation and recovery problems. Owing to fascinating morphological
and textural properties, hierarchically porous nitrogen-doped carbons
(NHPCs) are selected as the supports for the immobilization of BAILs.
The NHPCs are prepared by a single step CaCO3 nanoparticle-directed
nanocasting approach combined with K2C2O4 chemical activation
during carbonization of glucose and urea/melamine. Functionalization
of the NHPCs via quaternary ammonization by 1,3-propanesultone
followed by anion exchange with HSO3CF3 results in the GU‒[C3N]
[SO3CF3] and GM‒[C3N][SO3CF3] (C3 = PrSO3H); moreover, by
changing initial urea-to-glucose weight ratio in the preparation sys-
tems, nitrogen element contents and thereby Brønsted acid site den-
sities, BET surface areas and meso/micropore proportions of the NHPCs
can be well-adjusted, which may obviously influence the catalytic ac-
tivity of the resulting GU‒[C3N][SO3CF3].
respectively, and the process followed by modified literature method
[11]. Typically, glucose (8.9, 6.7 and 3.4 g, respectively) and urea (1.1,
3.3 and 6.6 g, respectively) were thoroughly ground into powder with
K2C2O4 and then with CaCO3 nanoparticles (particle size of 50–70 nm,
Fig. S1a, purchased from Zhejiang Tianshi Nano Technology Co. LTD,
China) at a weight ratio of (glucose + urea)-to-K2C2O4-to-CaCO3 of
1:1:1. The mixture was firstly heated to 250 °C at a ramp of 5 °C min−1
and held for 10 min. Afterwards, the mixture was continuously heated
to 750 °C at the same ramp heating rate and held for 1 h. Finally, the
furnace was cooled down to 200 °C in 2 h. The above process was car-
ried out under flowing nitrogen gas. After cooling to room temperature,
the black product was washed with diluted HCl for several times to
remove inorganic impurities such as calcium oxide nanoparticles and
potassium compounds generated during the calcination process, and
then it was further washed with water until the filtrate was neutral in
acidity. NHPCs were finally obtained after air-drying at 100 °C for 12 h,
and it was denoted as GU‒x (G and U represent glucose and urea, re-
spectively). Here, x = 1, 2 and 3, respectively, representing GU with
different nitrogen element contents (Table S1 of supplementary in-
formation). Additionally, the other NHPC support (denoted as GM‒2)
was also prepared following the above process and using melamine as a
nitrogen precursor, and the weight ratio of (glucose + melamine)-to-
K2C2O4-to-CaCO3 was 1:1:1 (in which weight ratio of glucose-to-mel-
amine was 2:1). The absence of residual potassium and calcium in as-
prepared various GU/GM supports were confirmed by a Leeman
Prodigy Spec (I) ICP-AES determination after the samples were calcined
in flowing air gas at 600 °C followed by dissolving with dilute HNO3.
Functionalization of GU/GM supports with BAILs was carried out by
the following process. Typically, GU/GM support (0.6 g) was dispersed
in 10 mL toluene solution containing 1,3-propane sultone (0.15 g or
1.26 mmol), and the mixture was continuously stirred at toluene re-
fluxing temperature (110 °C) for 12 h. Afterwards, the mixture was
cooled to room temperature, and HSO3CF3 (0.1 mL or 1.13 mmol) was
added. The resulting mixture was stirred vigorously at room tempera-
ture for 12 h, and then it was rested at room temperature for another
12 h. The solid was obtained by centrifugation, and then it was washed
with abundant dichloromethane at room temperature until the ad-
sorbed BAILs were removed completely. Finally, the product was dried
at 80 °C for 12 h, and it was denoted as GU‒[C3N][SO3CF3]‒x or
GM‒[C3N][SO3CF3]‒2 (C3 = PrSO3H; x = 1, 2 and 3).
Pyrolysis biofuels are produced from fast pyrolysis of waste bio-
mass, and they are promising candidates to replace petroleum fuels
[23–25]. However, raw pyrolysis biofuels are a mixture of acids, alco-
hols, furans, aldehydes, esters, ketones, sugars and multifunctional
compounds [26,27], which lead to them unfavorable properties in-
cluding low heating value, immiscibility with petroleum fuels, thermal
and chemical instability as well as corrosiveness because of their high
oxygen content (20–50 wt.%) and high acidity (pH = 2.5–3) [28–31].
Accordingly, the direct use of fast pyrolysis biofuels is limited, and they
need to be upgraded, e.g., catalytic hydrotreating, to reduce oxygen
content and therefore to satisfy the requirements of boiler fuels or the
additive for the petroleum fuels. Prior to the catalytic hydrotreating, it
is crucial to reduce acidity and oxygen content to a large extent, which
can increase the stability and decrease hydrogen consumption in sub-
sequent upgrading steps. As one of the small oxygenates, acetic acid is
abundant in raw pyrolysis biofuels (ca. 1–10 wt.%) [31], and removal
of acetic acid can remarkably decrease the acidity and improve the
stability of fast pyrolysis biofuels. Ketonization and esterification of
acetic acid are two promising catalytic processes for raw pyrolysis
biofuel pretreatment; moreover, esterification affords a simpler and
more effective process. Acid-catalytic esterification of acetic acid with
methanol or ethanol is a common strategy to reduce the acidity and
viscosity of raw pyrolysis biofuels [32,33]. However, oxygen content of
raw pyrolysis biofuels is unchanged. Both the yielded water and ester is
expected to be eliminated continuously if a continuous process is to
operate via reactive distillation, as a consequence, oxygen content is
decreased, together with high yield of the ester because of the complete
esterification equilibria. For this purpose, alcohols with boiling points
higher than water are chosen to avoid alcohol loss during distillation.
PrSO3H/SBA-15-, resin- or zeolite-catalyzed esterifications of acetic
acid with high boiling point alcohols such as benzyl alcohol, o-cresol,
m-cresol, p-cresol or anisyl alcohol have been studied [31,34–36].
Here the GU/GM‒[C3N][SO3CF3] are successfully applied in cata-
lytic esterification of acetic acid with benzyl alcohol or 4-methox-
ybenzyl alcohol to produce benzyl acetate or 4-methoxybenzyl acetate
in a simulated pyrolysis biofuel environment by using toluene as the
representative of non-polar lignin derivative, and the yielded benzyl
acetate or 4-methoxybenzyl acetate are value-added chemicals
[37–39]. It shows that esterification activity of the GU/GM‒[C3N]
[SO3CF3] outperforms Amberlyst-15 resin and HY zeolite. The excellent
esterification activity of the prepared catalysts is explained in terms of
super strong Brønsted acidity and unique hierarchically porous struc-
ture; additionally, macro/meso/micropore proportion of the catalysts
affects the esterification activity.
2.2. Characterization of BAIL-functionalized NHPCs
Nitrogen (N, %) and sulfur (S, %) element contents in as-prepared
catalysts were determined by a EuroVector CHNS EA3000 elemental
analyzer. FESEM observations were performed on a XL-30 ESEMFEG
field emission scanning electron microscope. Nitrogen gas adsorption/
desorption isotherms were measured on a Micromeritics ASAP 2020
PLUS HD88 surface area and porosity analyzer, and the samples were
outgassed under vacuum at 363 K for 1 h and 373 K for another 12 h
before the measurement. BET surface area (SBET) was calculated using
Brunauer–Emmett–Teller (BET) equation, pore diameter (Dp) was esti-
mated from BJH desorption determination, and pore volume (Vp) was
estimated from the pore volume determination using the adsorption
branch of the N2 isotherm curve at P/P0 = 0.99 single point. Mercury
porosimetry measurement was performed on a Micromeritics Autopore
V mercury porosimeter. Raman scattering spectra were recorded on a
Jobin-Yvon HR 800 instrument with an Ar+ laser source at 488 nm in a
macroscopic configuration. XPS was performed on an Axis Ultra DLD
instrument with a monochromated Al-Kα source at a residual gas
pressure of below 10−8 Pa.
2. Experimental
The Brønsted acid strength of the catalysts was measured by a
WDDY-2008 J microcomputer automatic potentiometric titration in-
strument [40]. The sample (50 mg) was suspended in acetonitrile under
stirring for 24 h, and then the suspension was titrated with 0.1 mol L−1
n-butylamine/acetonitrile solution. The Brønsted acid site density (A,
2.1. Preparation of BAIL functionalized NHPCs
Firstly, NHPCs with different nitrogen contents were prepared by
using glucose anhydrous and urea as carbon and nitrogen precursors,
11

Q. Zhang et al.
AppliedCatalysisA,General574(2019)10–24
μeq(H+) g−1) of the catalysts was determined by acid-base titration
[41]. Typically, fresh catalyst powder (50 mg) was dispersed in a NaCl
solution (2 mol L−1, 10 mL) under stirring at 30 °C for 24 h, and then
the suspension was titrated with standard NaOH solution (9.5 mmol
prepared by following two steps. At first, NHPC supports are derived
from calcination of glucose and urea/melamine at 750 °C under N2 at-
mosphere in the presence of CaCO3 nanoparticles as a hard template
and K2C2O4 as an activating agent. After removing all yielded inorganic
components by dilute HCl and then water washing, NHPCs are created.
Purpose of introduction of nitrogen atoms in NHPC supports is sub-
sequent quaternary ammonization with 1,3-propanesultone, which
gives rise to GU/GM‒[C3−N+]. After anion exchange of C3− from GU/
L
−1). The Brønsted acid site density was then calculated from the
consumed NaOH.
2.3. Catalytic tests
GM‒[C3−N+] with SO3CF3 from HSO3CF3, GU/GM‒[C3N][SO3CF3]
−
is finally formed. In order to adjust nitrogen element contents in the
NHPCs and thereby BET surface areas and micro/mesopore proportions
of the NHPCs as well as Brønsted acid site densities of the resulting GU/
GM‒[C3N][SO3CF3], three NHPC supports with urea-to-glucose weight
ratio of 0.12 (GU‒1), 0.49 (GU‒2) and 1.94 (GU‒3) are prepared at a
weight ratio of (glucose + urea)-to-K2C2O4-to-CaCO3 of 1:1:1. After
functionalization of these NHPC supports by the same amount of 1,3-
propanesultone and HSO3CF3, the GU‒[C3N][SO3CF3]‒1, GU‒[C3N]
[SO3CF3]‒2, and GU‒[C3N][SO3CF3]‒3 are prepared with nitrogen
element contents of 5.2, 6.8 and 8.0%, respectively (Table 1). In the
case of the GM‒[C3N][SO3CF3]‒2, its nitrogen content is 5.6%
(Table 1). Here, the adding amount of 1,3-propanesultone and HSO3CF3
is excessive with respect to nitrogen element contents in the NHPCs so
that various NHPC supports with different nitrogen element contents
can be functionalized by BAILs completely.
Typically, 24 mg of catalyst was added to a mixture of benzyl al-
cohol/4-methoxybenzyl alcohol (3.33 mmol), acetic acid (10 mmol)
and toluene (10 mL), and the reaction was carried out at 100 or 50 °C.
The concentrations of the produced benzyl acetate/4-methoxybenzyl
acetate and residual benzyl alcohol/4-methoxybenzyl alcohol were
analyzed periodically by a Shimadzu 2014C GC-FID fitted with a HP-
INNOWAX capillary column, and ethyl laurate was used as an internal
standard. The catalytic performance of the tested catalysts was eval-
uated quantitatively by the conversion of benzyl alcohol/4-methox-
ybenzyl alcohol, the yield of benzyl acetate/4-methoxybenzyl acetate
and the selectivity to benzyl acetate/4-methoxybenzyl acetate, respec-
tively. Additionally, the esterification processes were also monitored by
a HP6890GC-5973MSD GC–MS.
3. Results and discussion
3.1. Catalyst preparation and characterization
3.1.2. Morphological characteristics
Morphological characteristics of four NHPC supports and the cor-
responding GU/GM‒[C3N][SO3CF3] catalysts are revealed by both
FESEM (Fig. 1) and TEM (Fig. 2) observations. From the images shown
3.1.1. Catalyst preparation
As illustrated in Scheme 1, the GU/GM‒[C3N][SO3CF3] catalysts are
Scheme 1. Illustration of the preparation of hierarchically porous carbons functionalized by BAILs.
12

Q. Zhang et al.
AppliedCatalysisA,General574(2019)10–24
Table 1
Chemical composition, textural parameters and Brønsted acid properties of various NHPCs functionalized by BAILs.
Catalyst
N (%)
S (%)
SBET (m2 g−1
)
Dp (nm)
Micro/Meso
Vp (cm3 g−1
Micro/Meso
)
A (μeq(H+) g−1
)
Ei (mV)
GU‒[C3N][SO3CF3]‒1
GU‒[C3N][SO3CF3]‒2
GU‒[C3N][SO3CF3]‒3
GM‒[C3N][SO3CF3]‒2
5.2
6.8
8.0
5.6
4.7
6.4
7.2
8.5
7.1
6.2
201
452
673
438
388
0.3/3.9
0.4/3.8
0.4/3.8
0.4/3.9
n.d.
0.06/0.23
0.21/0.39
0.51/0.25
0.09/0.32
n.d.
980
612
684
638
683
660
1170
1440
1150
950
C
2
O
4
GUK
‒[C3N][SO3CF3]‒1
2
GUCaCO ‒[C3N][SO3CF3]‒1
Template-free GU‒[C3N][SO3CF3]‒1
Amberlyst-15
3
4.6
5.8
4.7
n.d.
n.d.
900
672
4.1
n.d.
n.d.
4.5
n.d.
n.d.
8.7
42
624
n.d.
n.d.
0.7
n.d.
n.d.
0.28
710
4400
608
654
617
426
HY zeolitea
a
Y zeolite with protonic form (HY) is obtained by calcination of the NH4-Y zeolite at 550 °C for 6 h in air atmosphere.
in Fig. 1a-c it is found that three GU supports exhibit similar 3D in-
terconnected hierarchically porous networks that are full of open and
plentiful macropores ranging from tens nanometers to several micro-
meters, and slightly denser structure and decreased interconnected
cavity are observed as increasing urea-to-glucose weight ratio. As for
GM support, it exhibits similar morphology with that of GU (Fig. 1d).
However, owing to the resolution limitation of the FESEM, meso- and
micropores in the samples are hardly observed. After functionalization
with BAILs, microstructures of the resulting GU/GM‒[C3N][SO3CF3]
catalysts remain intact basically (Fig. 1a’-d’). To explore the influence
of CaCO3 nanoparticles and K2C2O4 on the morphology and porosity
properties of the NHPCs, FESEM images of the GUCaCO3‒1 (Fig. 1e) and
uptakes at P/P0 > 0.8 are related to large meso/macropores, and al-
most plateau nitrogen uptakes at 0.1 < P/P0 < 0.8 are originated from
small mesopores. The presence of small and large mesopores as well as
small macropores in the GU/GM‒[C3N][SO3CF3] is also reflected in
their BJH pore size distribution profiles. As can be seen in Fig. 3b, the
small mesopores of the GU/GM‒[C3N][SO3CF3] are well-distributed
featuring sharp profiles and centering at ca. 4.0 nm (Table 1), while
their large meso/small macropores are uneven-distributed featuring a
continuous distribution in the pore diameter range from 15 to 105 nm.
To further investigate the microporosity of as-prepared materials, ni-
trogen gas adsorption/desorption isotherms and BJH pore size dis-
tribution profiles are determined by microporous program. As shown in
Fig. 3c, fast increased nitrogen uptakes occur at P/P0 < 0.1, char-
acteristics of the materials with microposity. The result is in accordance
with the corresponding pore size distribution profiles, demonstrating
that the materials possess micropores (diameter of ca. 0.4 nm, Fig. 4c).
Because the macropore filling occurs at pressure too close to the
saturated vapor pressure of liquid nitrogen (i.e. P/P0 closes to 1.0), the
evaluation of macroporosity by the nitrogen physisorption measure-
ment is limited. Therefore, mercury porosity measurement is applied to
further investigate the meso/macropore distribution of the re-
presentative GU‒[C3N][SO3CF3]‒1 and GU‒[C3N][SO3CF3]‒2 samples.
As shown in Fig. 3e, both samples exhibit a continuous distribution in
the pore diameter ranging from tens nanometers to several micro-
meters; moreover, at smaller macropore range (50–5000 nm),
GU‒[C3N][SO3CF3]‒1 shows richer macropores than GU‒[C3N]
[SO3CF3]‒2. The mercury-intrusion curves show that more mercury is
imported in GU‒[C3N][SO3CF3]‒1 than GU‒[C3N][SO3CF3]‒2
(Fig. 3f), in line with the result of Fig. 3e.
3D interconnected hierarchically macro-, meso- and microporous
structure of the GU/GM‒[C3N][SO3CF3] materials is originated from
their GU/GM supports, while the hierarchically porous structure of the
supports is induced by the synergetic effect of CaCO3 nanocasting and
K2C2O4 chemical activation [11]; meanwhile, self-activation of urea/
melamine also contributes to some micro- and mesopores of NHPC
supports. On the one hand, during the process of incomplete carbona-
tion of glucose and urea/melamine at 750 °C in the presence of K2C2O4
and CaCO3, CO gas is produced via redox reaction of K2C2O4; mean-
while, the resulting K2CO3 reacts with CaCO3 to form double carbonates
K2Ca(CO3)2, and thermal decomposition of K2Ca(CO3)2 yield CO2 gas.
Additionally, self-activation of urea/melamine produces plentiful NH3
and CO2 gas. By the interactions between the expanding gases including
CO, CO2 and NH3 with as-formed carbon intermediates and graphitic
carbons (see Raman scattering spectra displayed in Fig. S2), the carbon
species are etched, creating microcavity and small mesocavity in the
wall of the supports. On the other hand, large meso/macropores with
wide pore size distribution range that construct cores of the supports
are induced by the multiple template particles derived from K2CO3 and
CaCO3 nanoparticles including K2Ca(CO3)2, K2Ca2(CO3)3 and CaO. The
unique hierarchically porous structure of the NHPC supports renders
GUK C O4‒1 (Fig. 1f) that are prepared by carbonization of glucose
(8.9 g) and urea (1.1 g) at 750 °C under nitrogen atmosphere in the
presence of CaCO3 (10 g) only and K2C2O4 (10 g) only are also pro-
vided. It indicates that the GUCaCO3‒1 mainly exhibits macroporous
structure with pore diameter ranging from 70 to 150 nm; additionally,
compared with its GU‒1 counterpart, pores in the GUCaCO3‒1 is less
2
2
abundant. As for the GUK C O4‒1, it displays sheet-like bulk structure
without any macropore being found; moreover, it is difficult to observe
2
2
meso- and micropores in the GUK C O4‒1 from FESEM image. The
template-free GU‒1 prepared in the absence of both CaCO3 and K2C2O4
exhibits bulk structure (Fig. S1b), while majority macropores and
minority meso/micropores are found in HPC support that is prepared
by carbonization of glucose only in the presence of both CaCO3 and
K2C2O4 (Fig. S1c). BAIL-functionalized GUCaCO3‒[C3N][SO3CF3]‒1
2
2
(Fig. 1 e’) and GUK C O4‒[C3N][SO3CF3]‒1 (Fig. 1f’) exhibit the same
morphologies with their supports.
2
2
In order to observe the morphology of the GU/GM‒[C3N][SO3CF3]
more clearly, their TEM images are recorded. As shown in Fig. 2a-d, 3D
interpenetrating network of the samples are observed; meanwhile, not
only macropores but also meso- and micropores can also be found.
Additionally, the macroporous structure of the GUCaCO3‒[C3N]
[SO3CF3]‒1 is also found in Fig. 2e. In the case of the GUK C O4‒[C3N]
[SO3CF3]‒1, it possesses uniform microporous structure with pore
diameter of lower than 2 nm (Fig. 2f).
2
2
3.1.3. Porosity properties
The formation of macro-, meso- and micropores in the GU/
GM‒[C3N][SO3CF3] materials is further evidenced by nitrogen gas ad-
sorption/desorption and mercury porosimetry measurements. Fig. 3a
and b demonstrate nitrogen gas adsorption/desorption isotherms and
corresponding BJH pore size distribution profiles of four BAIL-func-
tionalized NHPCs determined by micro/mesoporous program. The
isotherms show combined characteristics of type I and IV with con-
siderable nitrogen uptakes occurring at both low (P/P0 < 0.1) and high
(P/P0 > 0.8) relative pressure (P/P0). The result implies that the GU/
GM‒[C3N][SO3CF3] catalysts exhibit hierarchical porosity consisting of
macro-, meso- and micropores. The pronounced nitrogen uptakes at P/
P0 < 0.1 correspond to the micropores of the catalysts, sharp nitrogen
13

Q. Zhang et al.
AppliedCatalysisA,General574(2019)10–24
Fig. 1. FESEM images of GU‒1 (a), GU‒2 (b), GU‒3 (c), GM‒2 (d), GUCaCO ‒1 (e), GU
‒1 (f), GU‒[C3N][SO3CF3]‒1 (a’), GU‒[C3N][SO3CF3]‒2 (b’), GU‒[C3N]
K
C O
2 4
2
3
[SO3CF3]‒3 (c’), GM‒[C3N][SO3CF3]‒2 (d’), GUCaCO ‒[C3N][SO3CF3]‒1 (e’) and GU
‒[C3N][SO3CF3]‒1 (f’).
K
2
C O
2 4
3
them combined characteristics of type I and IV isotherms (Fig. S3) and
extremely large BET surface area (Table S1). That is, the BET surface
areas of GU‒1, GU‒2, GU‒3 and GM‒2 are 957, 1425, 1626 and
1515 m2 g−1, respectively, which are related to urea/melamine-to-
glucose weight ratio or nitrogen element content (Table S1). Since more
urea/melamine can introduce more micropores and thereby larger BET
surface area, the GU‒1 and GU‒3 possess the smallest and largest BET
surface area. Additionally, HPC support exhibits much smaller BET
surface area (631 m2 g−1) in comparison of its GU‒1 counterpart. In the
case of single K2C2O4- or CaCO3-directed NHPC, the GUK C O4‒1 mainly
exhibits type I isotherm (Fig. S3) and very large BET surface area
(1455 m2 g−1), whereas the GUCaCO3‒1 shows little nitrogen uptake
(nonporous) and much smaller BET surface area (16 m2 g−1). The result
further evidences that the BET surface areas of the NHPCs are mainly
2
2
14

Q. Zhang et al.
AppliedCatalysisA,General574(2019)10–24
Fig. 2. TEM images of GU‒[C3N][SO3CF3]‒1 (a), GU‒[C3N][SO3CF3]‒2 (b), GU‒[C3N][SO3CF3]‒3 (c), GM‒[C3N][SO3CF3]‒2 (d), GUCaCO ‒[C3N][SO3CF3]‒1 (e)
3
C
2
O
4
and GUK
‒[C3N][SO3CF3]‒1 (f).
2
contributed from their micro and/or mesopores. In the absence of both
K2C2O4 and CaCO3 nanoparticles, the resulting template-free GU‒1 has
a very small BET surface area (36 m2 g−1).
NH3 gas is generated during thermal decomposition of melamine
[42,43]. From Fig. 3b and e it is also observed that the GU‒[C3N]
[SO3CF3] with lower urea-to-glucose weight ratio possesses higher
macroporosity, and they follow macroporosity order: GU‒[C3N]
[SO3CF3]‒1 > GU‒[C3N][SO3CF3]‒2 > GU‒[C3N][SO3CF3]‒3.
After functionalization of the NHPCs with ‒[C3][SO3CF3−] groups,
the BET surface areas of the resulting GU/GM‒[C3N][SO3CF3] mate-
rials decrease significantly, originating from the block of parts of
micro/mesopores in the supports. The BET surface areas and micro/
mesopore proportions of the GU/GM‒[C3N][SO3CF3] catalysts are also
related to the initial urea-to-glucose weight ratio. As shown in Table 1,
increasing the initial urea-to-glucose weight ratio from 0.12, 0.49 to
1.94, the BET surface area of the resulting GU‒[C3N][SO3CF3]‒1,
GU‒[C3N][SO3CF3]‒2 and GU‒[C3N][SO3CF3]‒3 increases from 201,
452 to 673 m2 g−1, respectively; correspondingly, the volume ratio of
micropores to mesopores (Vmicro/Vmeso) increases from 0.26, 0.54 to
2.04. The result suggests that self-activation of urea mainly contributes
microporosity to the hierarchically porous structure of the GU‒[C3N]
[SO3CF3]. As for glucose- and melamine-derived GM‒[C3N]
[SO3CF3]‒2, its Vmicro/Vmeso is 0.28, lower than that of the GU‒[C3N]
[SO3CF3]‒2. The result implies that contribution of micropores due to
self-activation of melamine is negligible, originating from that little
3.1.4. Structural information
The graphitization degree of the NHPC support and GU/GM‒[C3N]
[SO3CF3] catalysts is studied by Raman scattering spectroscopy (Fig.
S2). The NHPC support (represented by GU‒2) exhibits D band at
1352 cm−1 and G band at 1583 cm−1. G band demonstrates the in-
plane vibration of the sp2 carbon atoms, while the D band is a defect-
induced Raman feature representing the non-perfect crystalline struc-
ture of carbon-based material [16]. The intensity ratio of D band to G
band (ID/IG) is ca. 1.0, indicating its defective structure with incomplete
graphitization. The reason is ascribed that doping N atoms in glucose-
derived polycyclic aromatic carbon structure has some influence on the
graphitization degree of carbon. After functionalization of NHPC sup-
port with ‒[C3][SO3CF3−] groups, the position of D band is un-
changeable, and ID/IG value still maintains at ca. 1.0; however, the G
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Q. Zhang et al.
AppliedCatalysisA,General574(2019)10–24
Fig. 3. Nitrogen gas adsorption-desorption isotherms (a, c) and BJH pore size distribution profiles (b, d) of various GU/GM‒[C3N][SO3CF3] determined by micro/
mesoporous and microporous program, respectively. Mercury pore size distribution profiles (e) and mercury-intrusion curves between 0 and 200 MPa (f) of the
representative samples.
band of the GU/GM‒[C3N][SO3CF3] shifts to slightly higher Raman
shift (ca. 1593 cm−1), indicating that functionalization of NHPC sup-
port with ‒[C3][SO3CF3−] groups has some perturbation on the gra-
phitic structure.
To gain deep insight into the interaction between BAILs and NHPC
support as well as surface chemical states and composition of the GU/
GM‒[C3N][SO3CF3], XPS probe technique is conducted (Fig. 4). As
shown in Fig. 4a, high resolution C 1s XP spectrum of the representative
GU‒[C3N][SO3CF3]‒1 and GU‒[C3N][SO3CF3]‒3 samples are fitted
and deconvoluted into four peaks centered at 284.6, 286.1, 289.0 and
292.2 eV, corresponding to three sp2 graphitic carbon species from
CeC, CeN and CeO bonds in N-doped polycyclic aromatic carbon
framework as well as CeF bonds from SO3CF3 anion, respectively
−
[44]. Successful introduction of N atoms into graphitic carbon frame-
work and chemical states of nitrogen species is confirmed by high re-
solution N 1s XP spectrum displayed in Fig. 4b. Three deconvoluted N
1s peaks are attributed to N species from CeN (399.6 eV) and N+–H
(401.6 eV) bonds of N-doped hierarchically porous carbonaceous fra-
mework as well as N+–C (quaternary-N species, 400.6 eV) of the pro-
tonated structure of GU‒[C3N][SO3CF3] [45]. High resolution S 2p XP
spectrum can further evidence successful functionalization of NHPCs
with ‒[C3][SO3CF3−] groups. As can be seen from Fig. 4c, two
16

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AppliedCatalysisA,General574(2019)10–24
Fig. 4. High resolution XPS spectra of the GU‒[C3N][SO3CF3]‒1 and GU‒[C3N][SO3CF3]‒3 in the C 1s (a), N 1s (b), S 2p (c) and F 1s (d) binding energy regions.
deconvoluted characteristic peaks centered at 169.0 (S 2p1/2) and
and excessive acetic acid or benzyl alcohol can forward the reaction to
the formation of benzyl acetate.
167.9 eV (S 2p3/2) are attributing to sulfur species from the PrSO3H
−
group and SO3CF3 anion [46]. Similarly, Fig. 4d displays high re-
At nBA:nAA value of 1:3, the GU‒[C3N][SO3CF3]‒2 also shows the
highest conversion of benzyl alcohol and the selectivity to benzyl
acetate, and after the esterification reaction proceeds for 60 min, the
conversion of benzyl alcohol and the selectivity to benzyl acetate are
93.2 and 95.6%, respectively (Fig. 5b). Under the same reaction time,
the conversion of benzyl alcohol (85.1 and 80.6%) and the selectivity to
benzyl acetate (88.4 and 87.3%) decrease slightly at nBA:nAA values of
1:2 and 1:1. To confirm the excellent selectivity of the GU‒[C3N]
[SO3CF3]‒2 to benzyl acetate, GC–MS analysis is performed. As shown
in Fig. S4a, after the esterification reaction carries out at nBA:nAA value
of 1:3 for 60 min (yield of benzyl acetate is 89.1%), only trace side
products such as monobenzyltoluene (retention time of 15.06 min),
benzyltoluene (retention time of 15.20 min) and diphenyl ether (re-
tention time of 16.49 min) are identified in the reaction media except
for the product. Monobenzyltoluene and benzyltoluene are yielded
from the GU‒[C3N][SO3CF3]‒2-catalyzed condensation of benzyl al-
cohol with toluene, while diphenyl ether is produced by the GU‒[C3N]
[SO3CF3]‒2-catalyzed etherification of benzyl alcohol (Scheme 2a).
Generally, raw pyrolysis biofuels contain abundant acetic acid; ad-
ditionally, acetic acid is more inexpensive than benzyl alcohol.
Therefore, in subsequent catalytic tests, nBA:nAA value is set at 1:3 so
that the acidity and oxygen content of crude pyrolysis biofuels can be
reduced; meanwhile, the value added benzyl acetate can be produced
with high yield.
solution F 1s XP spectra, and the peak positioned at 688.0 eV is assigned
to F species of CeF bonds from ‒[SO3CF3] groups [22].
3.2. Catalytic performance evaluation
3.2.1. Catalytic activity and selectivity
The catalytic activity and selectivity of the GU/GM‒[C3N][SO3CF3]
are evaluated by esterification of acetic acid with benzyl alcohol or 4-
methoxybenzyl alcohol in toluene media. To obtain the highest acetic
acid removal efficiency and the highest yield of benzyl acetate or 4-
methoxybenzyl acetate simultaneously, the influence of the key ex-
perimental parameters including benzyl alcohol/4-methoxybenzyl al-
cohol-to-acetic acid molar ratio (nBA:nAA or nMA:nAA) and reaction
temperature is studied firstly.
3.2.1.1. Esterification of acetic acid with benzyl alcohol to produce benzyl
acetate. At first, the influence of nBA:nAA value on the esterification
activity is explored by selecting the GU‒[C3N][SO3CF3]‒2 as the
representative catalyst. As shown in Fig. 5a, the esterification activity
of the GU‒[C3N][SO3CF3]‒2 increases gradually as decreasing nBA:nAA
value from 1:1, 1:2 to 1:3. That is, after the esterification reaction
proceeds at 100 °C for 60 min, the yield of benzyl acetate reaches 70.4,
75.2 and 89.1%, respectively. Additionally, at nBA:nAA value of 2:1
(benzyl alcohol is excessive), the yield of benzyl acetate is also the
highest (87.7%, 60 min). This is due to the fact that esterification of
acetic acid with benzyl alcohol is an equilibrium limitation reaction,
Fig. 5c shows the influence of the reaction temperature on the yield
of benzyl acetate catalyzed by the GU‒[C3N][SO3CF3]‒2, finding that
the yield (69.2, 81.8 and 89.1% at 60 min) increases gradually with the
17

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AppliedCatalysisA,General574(2019)10–24
Fig. 5. Influence of benzyl alcohol-to-acetic acid molar ratio (nBA:nAA) (a, b) and reaction temperature (c) on the catalytic activity of the GU‒[C3N][SO3CF3]‒2 in
esterification of acetic acid with benzyl alcohol. Catalytic activity comparison of various NHPCs-supported BAILs (d) and reference acid catalysts (e) in the target
reation revealed by the yield of benzyl acetate and/or conversion of benzyl alcohol. Reaction conditions: 24 mg catalyst, 10 mL toluene, 100 °C (a, b, d and e), 60 min
(b), nBA:nAA = 1:3, benzyl alcohol 3.33 mmol, acetic acid 10 mmol (c, d and e).
temperature changing from 80 to 100 °C, and continuous increasing the
temperature to 110 °C, the increase of the yield (93.8% at 60 min) is
unobvious. The result suggests that higher reaction temperature can
speed the esterification reaction to produce the ester with high yield,
and therefore the reaction temperature is set at 100 °C to obtain high
yield of the ester.
temperature can forward the reaction to the formation of benzyl acetate
and thereby higher benzyl acetate yield, the reaction rate constants of
the GU‒[C3N][SO3CF3]‒2-catalyzed esterification of acetic acid with
benzyl alcohol performed at nBA:nAA values of 1:1 (0.0194 min−1), 1:2
(0.0205 min−1) and 1:3 (0.0329 min−1) are determined on the basis of
the plots of lnc0/ct vs. t (Fig. S5a), showing that the reaction rate in-
creases obviously with changing the nBA:nAA value from 1:1, 1:2 to 1:3;
additionally, the determined rate constants of the GU‒[C3N]
To further confirm that the esterification is an equilibrium-con-
trolled reaction, excessive acetic acid or benzyl alcohol as well as higher
18

Q. Zhang et al.
AppliedCatalysisA,General574(2019)10–24
(30.2%, 60 min) and HY zeolite (2.2%, 60 min). As displayed in Fig. 5e,
lower benzyl acetate yield in Amberlyst-15 resin-catalyzed esterifica-
tion reaction is due to its low selectivity to the product because the
conversion of benzyl alcohol (82.8%, 60 min) is relatively high. GC–MS
analysis can support the above result (Fig. S4b), finding that con-
siderable amount of undesired byproducts including diphenyl ether
(retention time of 15.51 min), monobenzyltoluene (retention time of
14.08 min) and benzyltoluene (retention time of 14.23 min) produce in
the reaction system, but only small amount of benzyl acetate (retention
time of 9.03 min) is yielded. This is due to the fact that Amberlyst-15
resin is swollen by plentiful of toluene at 100 °C, resulting in its high
affinity to toluene and benzyl alcohol rather than acetic acid. Accord-
ingly, condensation of benzyl alcohol with toluene and etherification of
benzyl alcohol are more favorable reactions with respect to esterifica-
tion of acetic acid with benzyl alcohol [47]. The result suggests that use
of Amberlyst-15 resin is limited to raw pyrolysis biofuel upgrading via
esterification of acetic acid.
As for HY zeolite-catalyzed esterification reaction, the conversion of
benzyl alcohol is also very low (15.6%, 60 min), suggesting that HY
zeolite is almost inert to this reaction although it has a very large
specific surface area. The result is further supported by GC–MS analysis
(Fig. S4c), in which only trace benzyl acetate (retention time of
8.97 min) and a small amount of diphenyl ether (retention time of
15.49 min), monobenzyltoluene (retention time of 14.06 min) and
benzyltoluene (retention time of 14.19 min) are identified.
Scheme 2. GU/GM‒[C3N][SO3CF3]-catalyzed esterification of acetic acid with
benzyl alcohol (a) and 4-methoxybenzyl alcohol (b) in a toluene solvent.
3.2.1.2. Esterification of acetic acid with 4-methoxybenzyl alcohol to
produce 4-methoxybenzyl acetate. The catalytic activity of various BAIL
functionalized NHPCs is further evaluated by esterification of acetic
acid with 4-methoxybenzyl alcohol in toluene media, another reaction
for upgrading crude pyrolysis biofuel and the production of value-
added 4-methoxybenzyl acetate (biofuel stabilizer [48]) simultaneously
(Fig. 6). At first, the impact of nMA:nAA value on the catalytic activity of
the representative GU‒[C3N][SO3CF3]‒2 catalyst is explored. As shown
in Fig. 6a, the yield of 4-methoxybenzyl acetate increases gradually
with decreasing nBA:nAA value from 1:1, 1:2 to 1:3, and the
corresponding yield of 4-methoxybenzyl acetate is 20.8, 52.7 and
72.7%, respectively, after the esterification reaction proceeds for
60 min at 50 °C. The result reflects that excessive acetic acid can
speed the esterification reaction with 4-methoxybenzyl alcohol,
similar to the first target reaction, and the nMA:nAA value is therefore
set at 1:3 in subsequent catalytic tests.
The influence of the reaction temperature on the yield of 4-meth-
oxybenzyl acetate over the representative GU‒[C3N][SO3CF3]‒2 cata-
lyst is then studied. As shown in Fig. 6b, the influence of the reaction
temperature on the yield of 4-methoxybenzyl acetate is complicated. At
lower reaction temperature (40 and 50 °C), the yield of 4-methox-
ybenzyl acetate monotonically increases with the reaction time;
meanwhile, increasing the temperature from 40 to 50 °C, the yield of 4-
methoxybenzyl acetate increases sharply. For example, after the reac-
tion proceeds for 60 min, the yield of 4-methoxybenzyl acetate is 25.0%
(40 °C) and 72.7% (50 °C), respectively. Continuous increasing the
temperature to 60 °C or higher, influence of the temperature on the
yield of 4-methoxybenzyl acetate exhibit a volcano trend: the highest
yield of 4-methoxybenzyl acetate occurs at 40 (77.9%, 60 °C), 20
(71.9%, 70 °C) and 10 min (64.9%, 80 °C), respectively, and then the
yield begins to decrease with prolonging the reaction time.
[SO3CF3]‒2-catalyzed esterification of acetic acid with benzyl alcohol
performed at 80 (0.0184 min−1), 90 (0.0282 min−1
) and 100 °C
(0.0329 min−1) continuously increase (Fig. S5b), indicating that higher
reaction temperature has a positive influence on the reaction rate ob-
viously.
At nBA:nAA value of 1:3 and 100 °C, the esterification activity of
various BAIL functionalized NHPCs and reference acid catalysts is
compared (Fig. 5d). Firstly, contribution of the NHPC support (re-
presentated by GU‒1) to the esterification activity is tested, showing
that only 1.0% of benzyl acetate yield is obtained after the GU‒1-cat-
alyzed reaction proceeds for 60 min. Otherwise, three glucose and urea-
derived GU‒[C3N][SO3CF3] catalysts exhibit excellent esterification
activity, and their esterification activity follow the order: GU‒[C3N]
[SO3CF3]‒1 > GU‒[C3N][SO3CF3]‒2 > GU‒[C3N][SO3CF3]‒3. For in-
stance, over period of 60 min, the corresponding benzyl acetate yield is
99.2, 89.1 and 83.4%. As for glucose and melamine-derived GM‒[C3N]
[SO3CF3]‒2, it exhibits similar benzyl acetate yield (88.4%, 60 min)
with its GU‒[C3N][SO3CF3]‒2 counterpart. In the cases of single
K2C2O4- or CaCO3-directed, glucose and urea-derived NHPCs-supported
GUK C O4‒[C3N][SO3CF3]‒1
and
GUCaCO3‒[C3N]
2
2
BAILs,
i.e.,
[SO3CF3]‒1, they both exhibit remarkably lower esterification activity
than their GU‒[C3N][SO3CF3]‒1 counterpart; moreover, the
GUK C O4‒[C3N][SO3CF3]‒1 (79.5%, 60 min) is more active than the
2
2
GUCaCO3‒[C3N][SO3CF3]‒1 (53.7%, 60 min). Template-free GU‒[C3N]
[SO3CF3]‒1 possesses poor activity (20.7%) in this reaction under the
−
identical reaction conditions. Previously prepared ‒[C3][SO3CF3
]
functionalized ethyl-bridged organosilica hollow nanospheres, [C3Im]
[SO3CF3]-Si(Et)Si HNSs with ‒SO3H loading of 8.4% (sulphur element
content of 6.6%) and Brønsted acid site density of 1060 μeq(H+) g−1
[47], also displays good esterification activity in target reaction, and
the yield of benzyl acetate is 74.4% under the same reaction conditions.
Homogeneous p-toluenesulfonic acid with super strong Brønsted
acidity exhibits the highest benzyl acetate yield (99.1%) among all
tested catalysts, however, unavoidable problems such as waste gen-
eration, equipment corrosion and recycling difficulty render it lacking
of competitiveness with respect to heterogeneous GU/GM‒[C3N]
[SO3CF3] catalysts. Additionally, all GU/GM‒[C3N][SO3CF3] catalysts
show significantly higher esterification activity than Amberlyst-15 resin
To explain the above results, the products and byproducts yielded
during the GU‒[C3N][SO3CF3]‒2-catalyzed esterification of acetic acid
with 4-methoxybenzyl alcohol at different reaction temperatures and
times are identified by GC–MS analysis. As shown in Fig. S6b). As
shown in Scheme 2b, 4,4-dimethoxydiphenylmethane is formed due to
the acid-catalyzed self-condensation of 4-methoxybenzyl alcohol, while
bis(4-methoxybenzyl)ether is yielded owing to the acid-catalyzed
etherification of 4-methoxybenzyl alcohol. The formation of these by-
products may compete with the production of 4-methoxybenzyl acetate,
19

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AppliedCatalysisA,General574(2019)10–24
Fig. 6. Influence of 4-methoxybenzyl alcohol-
to-acetic acid molar ratio (nMA:nAA) (a) and
reaction temperature (b) on the catalytic ac-
tivity of the GU‒[C3N][SO3CF3]‒2 in ester-
ification of acetic acid with 4-methoxybenzyl
alcohol. Catalytic activity comparison of var-
ious NHPC-supported BAILs (c) and reference
acid catalysts in the target reation revealed by
the yield of 4-methoxybenzyl acetate and/or
conversion of 4-methoxybenzyl alcohol. (d).
Reaction conditions: (a, c) 50 °C; (b, c and d)
nMA:nAA = 1:3; (d) 4-methoxybenzyl alcohol
3.33 mmol, acetic acid 10 mmol; 24 mg cata-
lyst, 10 mL toluene.
leading to the decreased selectivity of the catalyst to 4-methoxybenzyl
acetate and thereby the limited yield (72.7%, 60 min).
(4-methoxybenzyl acetate yield of 55.2%, 60 min) is more active than
the GUCaCO3‒[C3N][SO3CF3]‒1 (4-methoxybenzyl acetate yield of
47.3%, 60 min). As for template-free GU‒[C3N][SO3CF3]‒1-catalyzed
esterification reaction, only 11.6% of 4-methoxybenzyl acetate is pro-
duced. The yield of 4-methoxybenzyl acetate is 57.1% over the [C3Im]
[SO3CF3]-Si(Et)Si HNSs-catalyzed esterification of acetic acid with 4-
methoxybenzyl alcohol proceeds for 60 min.
As displayed in Fig. 6d, homogeneous p-toluenesulfonic acid-cata-
lyzed esterification of acetic acid with 4-methoxybenzyl alcohol ex-
hibits relatively high conversion of 4-methoxybenzyl alcohol (77.8%,
60 min) but moderate yield of 4-methoxybenzyl acetate (53.4%,
60 min), implying that some side reactions deteriorate the selectivity of
the catalyst to 4-methoxybenzyl acetate. Through GC–MS analysis it is
found that significant amount of byproducts including bis(4-methox-
ybenzyl)ether (retention time of 20.98 min) and 4,4-dimethox-
ydiphenylmethane (retention time of 18.89 min) are formed in the re-
action system (Fig. S8a). Consequently, lower selectivity of p-
toluenesulfonic acid to 4-methoxybenzyl acetate leads to the moderate
yield of 4-methoxybenzyl acetate. However, Amberlyst-15 resin and HY
zeolite are almost inert to the target reaction, and the corresponding
conversion of 4-methoxybenzyl alcohol and yield of 4-methoxybenzyl
acetate is extremely low, i.e., 6.2 and 5.0% (Amberlyst-15 resin) and 4.8
and 1.8% (HY zeolite). Additionally, less side products yielded in both
systems as confirmed by GC–MS shown in Fig. S8b and c.
Considering the difference of the tested acid catalysts in acid site
densities, their esterification activity is further compared by TOF va-
lues. Here, TOF values are calculated from the linear portion of the
initial reaction rate profile for the benzyl acetate yield (Fig. 7a) and 4-
methoxybenzyl acetate yield (Fig. 7b), which are normalized by the
acid site densities (Table 1). It shows that the representative GU‒[C3N]
[SO3CF3]‒2 catalyst with TOF values of 4.95 and 2.52 min−1 exhibits
the highest catalytic activity in both esterification reactions, sig-
nificantly outperforming p-toluenesulfonic acid (0.82 and 0.36 min−1),
Amberlyst-15 resin (0.15 and 0.12 min−1) and HY zeolite (0.11 and
0.11 min−1).
When the reaction performs at 70 °C and 10 min, conversion of 4-
methoxybenzyl alcohol is fast, accompanying with the formation of
considerable 4-methoxybenzyl acetate; additionally, two byproducts
including 4,4-dimethoxydiphenylmethane (retention time of 19.89 min;
trace) and bis(4-methoxybenzyl)ether (retention time of 21.97 min;
small) are found (Fig. S7c). Based on the above discussion it is con-
cluded that increasing the reaction temperature and extending the re-
action time result in the formation of more byproducts with remarkably
large amount during the process of the GU‒[C3N][SO3CF3]‒2-catalyzed
esterification of acetic acid with 4-methoxybenzyl alcohol, which ulti-
mately give rise to poor selectivity to 4-methoxybenzyl acetate and
thereby continuously decreased yield of 4-methoxybenzyl acetate.
Therefore, the reaction temperature of esterification of acetic acid with
4-methoxybenzyl alcohol is chosen at 50 °C.
At nMA:nAA value of 1:3 and 50 °C, the catalytic activity of various
BAIL-functionalized NHPCs as well as the [C3Im][SO3CF3]-Si(Et)Si
HNSs in esterification of acetic acid with 4-methoxybenzyl alcohol in
toluene media is tested. Contribution of the NHPC support (re-
presentated by GU‒1) to the target reaction is negligible, and only 1.2%
of 4-methoxybenzyl acetate yield is obtained after the GU‒1-catalyzed
esterification reaction proceeds for 60 min. However, significant
amount of 4-methoxybenzyl acetate is yielded in the presence of var-
ious acid catalysts (Fig. 6c). Moreover, four tested GU/GM‒[C3N]
[SO3CF3] catalysts follow the same activity order in the production of 4-
methoxybenzyl acetate and benzyl acetate: after the GU‒[C3N]
[SO3CF3]‒1-, GM‒[C3N][SO3CF3]‒2-, GU‒[C3N][SO3CF3]‒2- and
GU‒[C3N][SO3CF3]‒3-catalyzed esterification reaction proceeds for
60 min, the yield of 4-methoxybenzyl acetate is 83.2, 74.3, 72.7 and
64.1%, respectively. Under the same reaction conditions, single
K2C2O4- or CaCO3-directed, glucose and urea-derived BAIL-functiona-
lized NHPCs, i.e., GUK C O4‒[C3N][SO3CF3]‒1 or GUCaCO3‒[C3N]
[SO3CF3]‒1, exhibits lower esterification activity than their GU‒[C3N]
2
2
[SO3CF3]‒1 counterpart; additionally, the GUK C O4‒[C3N][SO3CF3]‒1
2
2
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AppliedCatalysisA,General574(2019)10–24
results in benzyl acetate/4-methoxybenzyl acetate, and the process
accompanies with the regeneration of the GU/GM‒[C3N][SO3CF3]
(step 5). Here, the Brønsted acid strength of the catalysts is measured by
nonaqueous potentiometric titration with n-butylamine (0.1 mol L−1
)
in acetonitrile. In this method, the initial electrode potential (Ei) in-
dicates the maximum acid strength of the surface site, and the sample
with Ei value higher than 100 mV is defined as the super strong solid
acid [49]. From the potentiometric titration curves shown in Fig. S9 it is
found that seven tested samples present similar potentiometric titration
behaviors, and the measured Ei values are in the range of 612‒684 mV
(Table 1). The result clearly confirms the super strong Brønsted acidity
of the GU/GM‒[C3N][SO3CF3] as well as the GUK C O4‒[C3N]
[SO3CF3]‒1 and GUCaCO3‒[C3N][SO3CF3]‒1, and their Brønsted acid
strength has little to do with the nitrogen element contents, morpho-
logical characteristics and porosity properties. Accordingly, the
Brønsted acid strength of the BAIL-functionalized NHPCs are dominated
by the incorporated ‒[C3][SO3CF3−] groups. As for Amberlyst-15 resin
(Ei value of 617 mV) and HY zeolite (Ei value of 426 mV), their Brønsted
acid strength is lower than that of the BAIL-functionalized NHPCs,
which may decelerate the esterification reaction rate and/or decrease
the selectivity to the products owing to side reactions such as con-
densation and etherification. This is one of the reasons that Amberlyst-
15 resin or HY zeolite is less active than as-prepared supported BAILs.
To explore the influence of the Brønsted acid site densities on the
esterification activity of the BAIL-functionalized NHPCs, their Brønsted
acid site densities are determined by acid-base titration with aqueous
NaOH (9.5 mmol L−1) solution, and the results are summarized in
Table 1. It shows that the Brønsted acid site densities of the BAIL-
2
2
functionalized NHPCs are influenced by the nitrogen element contents.
−
Higher nitrogen element content can introduce more ‒[C3][SO3CF3
]
groups and thereby higher acid site densities of the catalysts. As a
consequence, three GU‒[C3N][SO3CF3] catalysts follow the acid site
density order: GU‒[C3N][SO3CF3]‒1 (980 μeq(H+) g−1) < GU‒[C3N]
[SO3CF3]‒2 (1170 μeq(H+) g−1) < GU‒[C3N][SO3CF3]‒3 (1440 μeq
(H+) g−1). In the case of the GM‒[C3N][SO3CF3]‒2, it has the similar
nitrogen element content (5.6%) with that of the GU‒[C3N][SO3CF3]‒2
(5.2%), as expected, they both have the similar acid site density. Si-
milarly, the GUCaCO3‒[C3N][SO3CF3]‒1 (900 μeq(H+
) ) and
g−1
GUK C O4‒[C3N][SO3CF3]‒1 (950 μeq(H+) g−1) possess the similar acid
site density to that of the GU‒[C3N][SO3CF3]‒1. By the combination of
the catalytic testing results and the acid site densities of various BAIL-
functionalized NHPCs it is found that their activity difference is not
mainly originated from the acid site densities.
2
2
Fig. 7. Catalytic activity comparison of the GU‒[C3N][SO3CF3]‒2 with respect
to reference acid catalysts revealed by TOF value in the esterification of acetic
acid with benzyl alcohol (a) and 4-methoxybenzyl alcohol (b). Reaction con-
ditions: (a) 100 °C, nBA:nAA = 1:3, benzyl alcohol 3.33 mmol, acetic acid
10 mmol; (b) 50 °C, nMA:nAA = 1:3, 4-methoxybenzyl alcohol 3.33 mmol, acetic
acid 10 mmol; 24 mg catalyst, 10 mL toluene solvent, 60 min.
On the other hand, the GU/GM‒[C3N][SO3CF3] catalysts with 3D
interconnected hierarchically macro-, meso- and microporous struc-
tures give the important contribution to the enhanced esterification
activity as compared with template-free GU‒[C3N][SO3CF3], single
3.2.2. Explanation of the catalytic activity
The excellent esterification activity of the GU/GM‒[C3N][SO3CF3]
in the above two target reactions is attributed to their super strong
Brønsted acid strength and unique hierarchically porous structure with
3D interconnected macro-, meso- and micropores.
On the one hand, the GU/GM‒[C3N][SO3CF3] catalysts exhibit
super strong Brønsted acid strength, which can ensure the production of
benzyl acetate and 4-methoxybenzyl acetate rapidly and selectively. As
shown in Scheme 3, during the process of esterification of acetic acid
GUK C O4‒[C3N][SO3CF3]‒1
or
2
2
K2C2O4-
or
CaCO3-directed
GUCaCO3‒[C3N][SO3CF3]‒1; meanwhile, the macro/meso/micropore
proportion of the catalysts influence the esterification activity in some
extents. Specifically, hierarchical porosity properties of the GU/
GM‒[C3N][SO3CF3] can offer minimized diffusion resistance and
shortened diffusion pathway to mass transport by open and plentiful
macropores, while their high surface area ensures to highly disperse
acid sites throughout the micro and/or mesopores. This synergistic ef-
fect of macro-meso-micropores in the GU/GM‒[C3N][SO3CF3] catalysts
significantly improve the accessibility of the reactants to the acid sites,
giving rise to the positive influence on their esterification activity.
with benzyl alcohol/4-methoxybenzyl alcohol, carbonyl group of acetic
−
acid molecule is firstly protonated by the incorporated ‒[C3][SO3CF3
]
groups, which gives rise to a protonated acetic acid and increasing the
electrophilicity of its carbonyl carbon atoms (step 1). Afterwards, the
nucleophilic oxygen atom of benzyl alcohol/4-methoxybenzyl alcohol
molecule quickly attacks the protonated carbon atom of acetic acid,
which leads to the oxonium ion I (step 2). Next, the oxonium ion I
undergoes a proton transfer to generate the oxonium ion II (step 3), and
subsequently dehydration from oxonium ion II leads to the protonated
benzyl acetate/4-methoxybenzyl acetate (step 4). Finally, deprotona-
tion from the protonated benzyl acetate/4-methoxybenzyl acetate
Although the GUK C O4‒[C3N][SO3CF3]‒1 has much larger BET surface
area (388 m2 g−1), its lower esterification activity than the GU‒[C3N]
[SO3CF3]‒1 is due to the microporous structure, which may severely
decrease the accessibility of bulky benzyl alcohol or 4-methoxybenzyl
alcohol molecules to the acid sites and cause serious diffusion resistance
of the reactants and products simultaneously. Lower esterification ac-
tivity of macroporous GUCaCO3‒[C3N][SO3CF3]‒1 is ascribed to its very
small BET surface area (4.7 m2 g−1) because of lacking meso- and
2
2
21

Q. Zhang et al.
AppliedCatalysisA,General574(2019)10–24
Scheme 3. The proposed reaction mechanism of GU/GM‒[C3N][SO3CF3]-catalyzed synthesis of benzyl acetate/4-methoxybenzyl acetate from esterification of acetic
acid with benzyl alcohol/4-methoxybenzyl alcohol.
micropores. Owing to nonporous properties, template-free GU‒[C3N]
[SO3CF3]‒1 is the most inactive among all tested BAIL-functionalized
NHPCs. Higher esterification activity of the GU‒[C3N][SO3CF3]‒1 than
[C3Im][SO3CF3]-Si(Et)Si HNSs further confirms the unique hier-
archically porous structure is favorable for highly dispersing the acidic
sites, minimizing diffusion resistance and shortening mass transfer
pathway and thereby the excellent esterification activity.
As for three GU‒[C3N][SO3CF3] catalysts, their activity difference is
dominated by the macroporosity properties rather than BET surface
area and acid site densities. The GU‒[C3N][SO3CF3]‒1 with the lowest
nitrogen element content (5.2%) possesses the most abundant macro-
pores (Fig. 3b), which can remarkably diminish the diffusion resistance
and boost the mass transport of the reactants and products. As a result,
the GU‒[C3N][SO3CF3]‒1 is the most active among three GU‒[C3N]
[SO3CF3] catalysts although its BET surface area and acid site density
are the lowest. Based on the above discussion, the lowest esterification
activity of the GU‒[C3N][SO3CF3]‒3 is owing to the enhanced diffusion
and mass transport limitations because it lacks more macropores. The
above explanation is supported by the kinetic studies. In these studies,
three GU‒[C3N][SO3CF3] catalysts with different macroporous pro-
portions and previously prepared mesoporous [C3Im][SO3CF3]-Si(Et)Si
HNSs are tested, and esterification of acetic acid with benzyl alcohol is
selected as the target reaction. As shown in Table S2, the determined
rate constant (k1) at nBA:nAA = 1:3 and 100 °C decreases from 0.0435
macroporous proportion possess the smallest and the largest Ea value
and thereby the fastest and slowest esterification reaction rate.
Similarly, microporous structure of HY zeolite with extremely small
pore dimension of 0.53 nm × 0.56 nm is another reason that is re-
sponsible for the inferior esterification activity owing to poor accessi-
bility of bulky benzyl alcohol or 4-methoxybenzyl alcohol molecules to
the acid sites as well as serious diffusion resistance of the reactants and
products. In addition, because of the similar macroporosity properties,
the GU‒[C3N][SO3CF3]‒2 exhibits similar esterification activity to the
GM‒[C3N][SO3CF3]‒2.
3.2.3. Catalytic reusability and stability
It is an important issue to evaluate the reusability and stability of
the supported catalysts because leaching of the anchored active sites
generally happens during the catalytic process. The reusability and
stability of the representative GU‒[C3N][SO3CF3]‒1 is evaluated by
esterification of acetic acid with benzyl alcohol and 4-methoxybenzyl
alcohol, respectively. After each catalytic cycle, the catalyst is re-
covered by centrifugation and then washed thoroughly with di-
chloromethane. Therefore, after five consecutive cycles the catalytic
activity loss of the GU‒[C3N][SO3CF3]‒1 in two esterification reactions
is ca. 10%, suggesting its good reusability. To find out the reason of
somewhat activity loss, hot filtration experiments are undertaken. After
the reactions proceed for 10 min, the catalyst powders are removed by
hot filtration, and then the reactions continue. As displayed in Fig. 8c,
the reactions stop after 10 min, and then the yields of benzyl acetate
and 4-methoxybenzyl acetate maintain unchangeable with prolonging
the reaction time to 60 min. The results suggest that no catalytically
active ‒[C3][SO3CF3−] units remain in the filtrate, highlighting the
excellent catalytic stability of the GU‒[C3N][SO3CF3] catalysts under
the reaction conditions studied. Additionally, the fifth time used
GU‒[C3N][SO3CF3]‒1 still exhibits 3D interconnected hierarchically
porous structure, as confirmed by TEM observation (Fig. 8d). Therefore,
it is inferred that somewhat activity loss after catalytic runs is owing to
the adsorption of small amount of byproducts yielded from etherifica-
tion and condensation of benzyl alcohol or 4-methoxybenzyl alcohol on
the surface of the GU‒[C3N][SO3CF3]‒1 although dichloromethane
washing is applied, and the speculation is confirmed by FT-IR spectra
shown in Fig. S11. The adsorption of small amount of byproducts also
leads to a slight decrease of the pore volume of the fifth time used
GU‒[C3N][SO3CF3]‒1 catalyst (0.21 cm3 g−1) as compared with the
fresh one (0.23 cm3 g−1). The excellent catalytic stability of the
(GU‒[C3N][SO3CF3]‒1),
0.0329
(GU‒[C3N][SO3CF3]‒2)
to
0.0291 min−1 (GU‒[C3N][SO3CF3]‒3), in line with the macroporous
proportion of the catalysts; additionally, under the same reaction con-
ditions, the rate constant (0.0273 min−1
) of mesoporous [C3Im]
[SO3CF3]-Si(Et)Si HNSs is the smallest among the four tested catalysts.
As for the activation energy, it is determined based on the Arrhenius
equation
lnk = lnA – Ea/(RT)
From the plot of lnk vs. 1/T obtained in the GU‒[C3N][SO3CF3]-
catalyzed esterification of acetic acid with benzyl alcohol performed at
nBA:nAA = 1:3 (Fig. S10), the Ea is determined. As shown in Table S2, Ea
values are 18.3, 34.5 and 41.6 kJ mol−1 for GU‒[C3N][SO3CF3]‒1-,
GU‒[C3N][SO3CF3]‒2- and GU‒[C3N][SO3CF3]‒3-catalyzed esterifica-
tion reaction. The result is also consistent with the macroporous pro-
portion of the GU‒[C3N][SO3CF3] catalysts, and the GU‒[C3N]
[SO3CF3]‒1 and GU‒[C3N][SO3CF3]‒3 with the highest and lowest
22

Q. Zhang et al.
AppliedCatalysisA,General574(2019)10–24
Fig. 8. Catalytic reusability and stability eva-
luation of the GU‒[C3N][SO3CF3]‒1 in ester-
ification of acetic acid with benzyl alcohol (a,
c) and 4-methoxybenzyl alcohol (b, c).
Reaction
conditions:
(a,
c)
100 °C,
nBA:nAA = 1:3, benzyl alcohol 3.33 mmol,
acetic
acid
10 mmol;
(b,
c)
50 °C,
nMA:nAA = 1:3,
4-methoxybenzyl
alcohol
3.33 mmol, acetic acid 10 mmol; 24 mg cata-
lyst, 10 mL toluene solvent. (d) TEM image of
the third time reused catalyst.
GU‒[C3N][SO3CF3] is due to strong chemical interactions between the
‒[C3][SO3CF3−] groups and nitrogen-doped carbon frameworks, which
can ensure the catalysts work as recyclable solid acid catalysts in
practical bio-oil upgrading.
macroporosity properties rather than BET surface area and acid site
densities. The GU‒[C3N][SO3CF3]‒1 catalyst with the lowest nitrogen
element content possesses the most abundant macropores, which sig-
nificantly diminish the diffusion resistance and boost the mass transport
of the reactants and products. As a result, the GU‒[C3N][SO3CF3]‒1 is
the most active among all tested GU/GM‒[C3N][SO3CF3] catalysts.
More importantly, owing to chemical bonding ‒[C3][SO3CF3−] units
throughout the NHPC supports, the GU/GM‒[C3N][SO3CF3] can be
reused for five times without remarkable activity loss and morphology
change, and they are genuinely heterogeneous catalysts and display
potential for practical applications. Further studies exploring con-
tinuous operation process via reactive distillation and catalyst poison in
real pyrolysis biofuel systems are essential milestones to determine the
applications of esterification reactions for real pyrolysis biofuel up-
grading in a large scale.
4. Conclusions
Acid-catalyzed esterification of acetic acid with high boiling point
benzyl alcohol or 4-methoxybenzyl alcohol is one of the effective
strategies to decrease oxygen content and acidity of raw pyrolysis
biofuels prior to their catalytic hydrotreating; meanwhile, the produced
esters are value-added compounds. BAILs with super strong Brønsted
acidity are the promising candidates for the above process, and their
separation and recovery problems can be solved by chemical im-
mobilization of BAILs on suitable supports. In the present work, hier-
archically macro-meso-microporous nitrogen-doped carbons functio-
nalized by the ‒[C3][SO3CF3−] groups are successfully prepared via
quaternary ammonization of the NHPC supports and subsequent anion
exchange. The macro/meso/micropore proportion, BET surface areas
and Brønsted acid site densities of the resulting GU‒[C3N][SO3CF3]
catalysts can be adjusted by changing initial urea-to-glucose weight
ratio in the NHPC preparation systems. The GU/GM‒[C3N][SO3CF3]
catalysts exhibit considerably high activity in esterification of acetic
acid with benzyl alcohol or 4-methoxybenzyl alcohol in toluene media,
and their esterification activity is significantly higher than that of single
K2C2O4- or CaCO3-directed NHPC-supported BAILs as well as template-
free GU‒[C3N][SO3CF3]. The excellent esterification activity of the GU/
GM‒[C3N][SO3CF3] is mainly contributed from the synergistic effect of
macro-meso-micropores within the catalysts, which can significantly
improve the accessibility of the reactants to the acid sites via mini-
mizing the diffusion resistance, shortening mass transport pathway and
increasing acid site population; moreover, the esterification activity of
the GU/GM‒[C3N][SO3CF3] catalysts is dominated by their
Acknowledgement
This work was supported by the Natural Science Fund Council of
China (21573038 and 21703030).
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.apcata.2019.01.021.
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