Self-Organization of Ionic Liquid-Modified Organosilica Hollow Nanospheres and Heteropolyacids: Efficient Preparation of 5-HMF Under Mild Conditions

Article abstract of DOI:10.1002/cctc.201900285

As a biomass-derived platform molecule, 5-hydroxymethylfurfural (5-HMF) is a highly desirable feedstock for manufacturing of high value-added chemicals ranging from starting materials for polyesters to biofuels. In this work, we reported the fabrication of a series of multicomponent solid acid catalysts based on heteropolyacids immobilized ILs-modified organosilica hollow nanospheres (denoted as PW₁₂-ILs-Cn-HNS), in which PW₁₂ (PW₁₂=H₃PW₁₂O₄₀ ? xH₂O) provides Br?nsted acid site, ILs show strong electrostatic interactions with PW₁₂, Cn (Cn=alkyl chain) is attached for hydrophobicity and HNS represents organosilica hollow nanospheres. When applied for catalytic dehydration of fructose to 5-HMF, the PW₁₂-ILs-C4-HNS catalyst with 15.2 % PW₁₂ loading exhibited the best dehydration activity to 5-HMF with 93.7 % yield in DMSO at 100 °C in 2 h. Compared with 2D hexagonal and 3D interconnected structures, the excellent porosity properties of hollow nanospherical structure can provide a high population of the PW₁₂ sites and enough confined nanospace for the dehydration of fructose. Moreover, the PW₁₂-ILs-Cn-HNS catalyst showed excellent stability over six catalytic cycles without obvious loss of activity. Most importantly, careful identification of the observed intermediates revealed crucial information for the dehydration process of fructose to 5-HMF. As such, the proposed heterogeneous catalysts show great potential in biomass conversion processes.

Full text of DOI:10.1002/cctc.201900285

Accepted Article
Title: Self-organization of ionic liquid-modified organosilica hollow
nanospheres and heteropolyacids: efficient preparation of 5-HMF
under mild conditions
Authors: Sai An, Zelin Wang, Huaiying Zhang, Haralampos N. Miras,
and Yu-Fei Song
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemCatChem 10.1002/cctc.201900285
Link to VoR: http://dx.doi.org/10.1002/cctc.201900285
A Journal of
www.chemcatchem.org

ChemCatChem
10.1002/cctc.201900285
Self-organization of ionic liquid-modified organosilica hollow nanospheres
and heteropolyacids: efficient preparation of 5-HMF under mild conditions
a
a
a
b
Dr. Sai An , Zelin Wang , Huaiying Zhang , Dr. Haralampos N. Miras* and Prof. Dr. Yu-Fei
Song*a
aState Key Laboratory of Chemical Resource Engineering, Beijing Advanced Innovation Center for Soft
Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029 P. R. China.
*E-mail: songyufei@hotmail.com or songyf@mail.buct.edu.cn; Tel/Fax: +86 10-64431832.
bWestCHEM, School of Chemistry, University of Glasgow, Glasgow G12 8QQ, U.K. *E-mail:
charalampos.moiras@glasgow.ac.uk
Abstract: As a biomass-derived platform molecule, 5-hydroxymethylfurfural (5-HMF) is a
highly desirable feedstock for manufacturing of high value-added chemicals ranging from
starting materials for polyesters to biofuels. In this work, we reported the fabrication of a series
of multicomponent solid acid catalysts based on heteropolyacids immobilized ILs-modified
organosilica hollow nanospheres (denoted as PW12-ILs-Cn-HNS), in which PW12 (PW12
O) provides Brönsted acid site, ILs show strong electrostatic interactions with
PW12, Cn (Cn = alkyl chain) is attached for hydrophobicity and HNS represents organosilica
hollow nanospheres. When applied for catalytic dehydration of fructose to 5-HMF, the PW12
=
H
3
PW12
O40•xH
2
-
ILs-C4-HNS catalyst with 15.2% PW12 loading exhibited the best dehydration activity to 5-
o
HMF with 93.7 % yield in DMSO at 100 C in 2 h. Compared with 2D hexagonal and 3D
interconnected structures, the excellent porosity properties of hollow nanospherical structure
can provide a high population of the PW12 sites and enough confined nanospace for the
dehydration of fructose. Moreover, the PW12-ILs-Cn-HNS catalyst showed excellent stability
over six catalytic cycles without obvious loss of activity. Most importantly, careful
identification of the observed intermediates revealed crucial information for the dehydration
process of fructose to 5-HMF. As such, the proposed heterogeneous catalysts show great
potential in biomass conversion processes.
Keywords: Heterogeneous catalysis, Solid acid, Biomass, 5-HMF, Confined nanospace
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ChemCatChem
10.1002/cctc.201900285
1
. Introduction
The observed exponential growth of worldwide energy demands and concurrent depleting of
fossil fuel reserves has spurred worldwide research efforts towards the energy effective and
environmentally friendly production of chemicals. According to the U.S. Department of
Energy[1‒3], 5-hydroxymethylfurfural (5-HMF) is one of the top 10 most valuable biomass-
derived feedstocks, and is used as a versatile intermediate for the synthesis of a wide range of
value-added compounds such as 2,5-furandicarboxylic acid, 2,5-dimethylfuran, 2,5-
hydroxymethyl-furan, levulinic acid, caprolactam and caprolactone[4‒10]. Generally, various
polysaccharides and monosaccharides are used as the starting materials[11,12] for the synthesis
of 5-HMF. Environmental and safety considerations highlight the requirement of replacing the
mineral acids by environmentally-benign solid acid catalysts. To date, considerable research
efforts were focused on the modification of various solid acids such as zeolites[13‒15]
,
heteropolyacids[15,16], sulfonated zirconia[17,18], phosphates[19], metal oxides[20‒23] and ion
exchange resins[15,24,25]. For the above-mentioned catalysts, the dehydration reaction required
high temperature and pressure regimes or involved microwave-assisted treatment, leading to
low stability, poor selectivity and leaching of the catalytically active component. As a matter
of fact, an ideal catalytic system should possess a series of optimum parameters such as suitable
acidic centers, hydrophobic surface and excellent catalytic stability etc. In addition, the effects
of side reactions can be limited if the catalytic conversions take place in the confined spaces.
Heteropolyacids are widely used in catalysis[26,27], energy storage[28,29], and materials
chemistry.[30,31] Additionally, the heteropolyacids represent a class of green alternatives to
traditional mineral acids due to their strong Brönsted acidic nature, high proton mobility and
stability[32]. However, the broad utilization of heteropolyacids is still hampered due to their low
2
‒1
BET surface areas (1‒10 m g ) and tendency to dissolve in the reaction mixture which makes
the separation process highly challenging[33]. A simple strategy to overcome the above
limitation is the immobilization of the catalytically active component on various supports, such
as silica-based ones that show diverse compositions, unique structures and high surface area[34‒
36].
Herein, we reported the preparation of a series of heterogeneous catalysts by immobilizing
heteropolyacids within the confined space of the ILs-modified organosilica hollow nanospheres
(denoted as PW12-ILs-Cn-HNS). The as-prepared PW12-ILs-Cn-HNS exhibited much higher
catalytic activity compared with commercially available acid catalysts, such as Amberlyst-15
etc, and in some cases even higher than the homogeneous system based on H PW12O40 under
3
the same reaction conditions.
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ChemCatChem
10.1002/cctc.201900285
Scheme 1. Illustration of the employed synthetic procedure for the preparation of heteropolyacids
immobilized on ILs-modified organosilica hollow nanospheres by taking PW12-ILs-C4-HNS as an example.
2
2
. Experimental
.1 Chemicals and materials
All chemicals were of analytical grade and were used as received without any further
purification. Pluronic P123 (EO20PO70EO20, M
,2-bis(trimethoxysilyl)ethane (BTMSE, 97%), fructose and 5-HMF were purchased from
Energy Chemicals; HCl, 1,3,5-trimethylbenzene (TMB), phosphotungstic acid (H
silicotungstic acid (H 40), phosphomolybdic acid (H 40) and silicomolybdic
acid (H 40) were purchased from Beijing Chemical Company. 1-butyl-3-(3-triethoxy-
silylpropyl)-4,5-dihydroimidazolium hexafluorophosphate ((EtO) Si-ILs-C4), 1-octyl-3-(3-
triethoxy-silylpropyl)-4,5-dihydroimidazolium hexafluorophosphate ((EtO) Si-ILs-C8), and 1-
dodelyl-3-(3-triethoxy-silylpropyl)-4,5-dihydroimidazolium hexafluorophosphate ((EtO) Si -
ILs-C12) were synthesized and characterized according to literature method[37]
.2 Catalyst synthesis
PW12-ILs-Cn-HNS. Typically, P123 (0.5 g, 0.086 mmol) was dissolved in a mixture of
w
= 5800) was purchased from Sigma-Aldrich;
1
3
PW12O40),
4
SiW12
O
5
PMo12O
4
SiMo12O
3
3
3
.
2
‒
1
water (12.7 mL), HCl (12 mol L , 2.4 mL, 28.8 mmol) and TMB (1.3 mL, 9.02 mmol) under
stirring at room temperature for 1 h. Subsequently, BTMSE (0.6 mL, 2.34 mmol), (EtO) Si-
ILs-Cn (n = 4, 8 and 12, 0.21‒0.27 g) and H 40 (0.75 g, 0.26 mmol) were added
successively to the above solution at the interval of every 1 h. The molar ratio of the starting
materials was P123 : H O : HCl : TMB : BTMSE : ILs-Cn : H 40 = 0.86 : 7056 : 248 :
3
3
PW12O
2
3
PW12O
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ChemCatChem
10.1002/cctc.201900285
o
9
2
0.23 : 23.37 : 5.12 (or 5.15 or 5.17) : 2.60. The resulting suspension was stirred at 40 C for
o
4 h, and then aged at 100 C for additional 24 h. Subsequently, the formed products were dried
o
at 100 C overnight and washed with boiling ethanol to remove the P123. Finally, these products
o
were dried at 100 C in air for 12 h (denoted as PW12-ILs-Cn-HNS).
SiW12-ILs-Cn-HNS, PMo12-ILs-Cn-HNS, SiMo12-ILs-Cn-HNS were prepared in a
similar procedure as in the case of PW12-ILs-Cn-HNS except that H
4
SiW12
5
O40, H PMo12
40
O ,
H
4
SiMo12
O
40 were used instead of H
3
PW12
40
O .
PW12-ILs-Cn-2Dhex (2Dhex = two dimensional hexagonal nanostructure), PW12-ILs-Cn-
Dint (3Dint = three dimensional interconnected nanostructure) were prepared following a
3
similar procedure as in the case of PW12-ILs-Cn-HNS apart from the relevant initial ratios of
the reagents. The initial molar ratio used in the case of PW12-ILs-Cn 2Dhex was P123 : H O :
HCl : BTMSE : ILs-Cn : H 40 = 0.86 : 7056 : 83.76 : 23.37 : 5.12 : 2.60. The initial
molar ratio of PW12-ILs-Cn 3Dint was P123 : H O : HCl : BTMSE : ILs-Cn : H 40 = 0.43 :
2
3
PW12O
2
3
PW12O
7
056 : 164.73 : 23.37 : 5.12 : 2.60.
2
.3 Catalyst characterization
The N adsorption-desorption measurements were performed on a Micromeritics ASAP
2
2
3
020M surface area and porosity analyzer after the samples were degassed under vacuum at
63 K for 1 h and at 373 K for additional 12 h. High-resolution transmission electron
microscopy (HRTEM) observations were performed on a JEM-2100 at an accelerating voltage
1
3
31
29
of 400 kV. C cross polarization-magic angle spinning (CP-MAS) NMR, P and Si MAS
NMR spectra were recorded on a Bruker AV400 NMR spectrometer equipped with a 4 mm
standard bore CP MAS probe head. The dried and finely powdered samples were packed in the
31
ZrO
MAS NMR, C CP-MAS NMR and Si MAS NMR spectra were referenced to the signal of
NH PO (δ = 0.00), C10 = 38.5) and 3-(trimethylsilyl)-1-propanesulfonic acid
2
rotor closed with Ke–F cap which were spun at 12 KHz rate. Chemical shifts for all P
1
3
29
4
H
2
4
H
16 (δCH
2
sodium salt (δ = 0.0), internal standard respectively. PW12 loading was determined using a
Shimadzu ICPS-7500 instrument. X-ray photoelectron spectroscopy (XPS) measurements were
performed using monochromatized ALK exciting X-radiation (PHI Quantera SXM).
Thermogravimetric (TG) analysis was conducted on a STA-449C Jupiter equipment (HCT-2
o
‒1
o
Corporation, China) using a heating rate of 10 C min in the range of 25 to 800 C under air
atmosphere. Fourier transform infrared (FTIR) spectra were recorded on a Bruker Vector 22
infrared spectrometer using the KBr pellet method.
The Brönsted acid density of the prepared catalysts was measured by acid-base titration.
o
The fresh sample (60 mg) was suspended in deionized water (15 mL) stirred at 30 C for 24 h.
Subsequently, the suspension was cooled down to room temperature, and then it was titrated
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ChemCatChem
10.1002/cctc.201900285
‒
1
with NaOH solution (4.0 mmol L ) that was titrated with standard potassium hydrogen
‒
1
phthalate solution (4.5 mmol L ). The acid densities of the prepared catalysts were determined
based on the amount of consumed sodium hydroxide, and they were expressed by the number
+
+
‒1
of equivalents of H (Atitration, μeq(H ) g ). The Brönsted acid strength was measured by an
automatic potentiometric titration system based on the literature method[38,39]. The sample
(0.050 g) was dispersed in acetonitrile and stirred for 12 h, and the resulting suspension was
−
1
titrated with a solution of n-butylamine in acetonitrile (0.1 mol L ).
.4 Catalytic tests
The catalysts were dried at 120 C for 2 h before the catalytic tests. Typically, dehydration
2
o
−
1
of fructose to produce 5-HMF was performed under the conditions of 0.25 mol L fructose in
o
dimethylsulfoxide (DMSO), 60 mg catalysts at 100 C. After completion of the reaction, 50 μL
of the reaction mixture was withdrawn, diluted with 6 mL methanol, filtered using PTEE 0.2
µm filters and analyzed using an Agilent Technologies 1200HPLC. The concentration of
fructose and the resultant 5-HMF were quantified using an Agilent Hi-Plex Ca column (film
o
thickness 8 m, i.d. 7.7 mm, length 300 mm) at 80 C, refractive index detector and Agilent
o
C18 column (film thickness 1.8 m, i.d. 4.6 mm, length 50 mm) at 35 C and UV detector,
respectively. The catalytic activity was evaluated by determining the yield of 5-HMF (Y, %),
where Y (%) = (M
and theoretically calculated. The conversion of fructose (C, %) was calculated using the formula
represented the number of moles of
D
/M
T D
) × 100 where M , M
T
represent the number of moles of 5-HMF produced
C (%) = (Minitial-M
t
/Minitial) × 100, where Minitial and M
t
fructose at initial time (t = 0) and time t, respectively. The possible intermediates and
byproducts yielded during the process of fructose dehydration were identified by Waters
Quattro Premier XE mass spectrometry.
3
3
Results and discussion
.1 Preparation and characterization of the PW12-ILs-Cn-HNS nanospheres
The PW12-ILs-Cn-HNS nanospheres were prepared using a P123-directed sol-gel route.
The process involved hydrolysis and condensation of the BTMSE and ionic liquid (EtO)
ILs-Cn (n = 4, 8 and 12) in the presence of TMB under acidic conditions assisted by the
electrostatic interactions developed between ILs-Cn with H 40 (Scheme 1). The molar
ratio of the initial materials (P123 : H O : HCl : TMB : BTMSE : ILs-Cn : H 40) was
3
Si-
3
PW12O
2
3
PW12O
crucial for preparation of the hollow nanospheres. It was identified that the relevant ratio of the
components should be carefully adjusted to the above optimum values since they affect
dramatically the rates of hydrolysis, condensation of silica precursors, the morphologies of
lyotropic liquid crystal phases. The next step involved addition of H PW12O40 anions to the ILs-
3
modified silica/carbon framework, resulting in the formation of the PW12-ILs-Cn-HNS
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ChemCatChem
10.1002/cctc.201900285
o
composite. The aging of the above mixture at 100 C for 24 h was crucial for efficient
interaction of the ILs-Cn and H 40 anions. Finally, the PW12-ILs-Cn-HNS with hollow
3
PW12O
spherical nanospheres morphologies can be achieved after removal of P123 by washing with
boiling ethanol.
In contrast to the post-grafting synthetic method with multiple step process and/or the
impregnation method with poor dispersion of the active sites, the sol-gel strategy not only is
facile and time-saving, but also leads to the PW12-ILs-Cn-HNS with well-defined mesoporosity,
uniform dispersion and controllable loadings of catalytically active sites. Different alkyl chains
can be incorporated into the PW12-ILs-Cn-HNS by simply adjusting the corresponding
(
EtO) Si-ILs-Cn, demonstrating the modularity of this approach. Another noteworthy feature
3
is the ability of the amphiphilic triblock copolymer, i.e., Pluronic P123, can be assembled into
micelles through hydrogen bonding and/or hydrophobic/hydrophilic interactions in acidic
media due to the presence of both hydrophilic −CH
2
CH
2
O (PEO) and hydrophobic
−
CH (CH )CHO (PPO) groups. Initially, the PPO blocks form the core of the spherical micelles,
2
3
while the PEO blocks form a hydrated corona around the core, followed by the expansion of
the micelles driven by the diffusion of the swelling agent (TMB) into the core of the
nanostructure. The surface charge density on micelles gradually decreases as a function of the
TMB molecules that diffuse into the micelles, leading to the formation of well-defined and
stable hollow spherical nanostructures.
Fig. 1 HRTEM images of (a) PW12-ILs-C4-HNS; (c) PW12-ILs-C8-HNS; (d) PW12-ILs-C12-HNS; (b) EDX
of PW12-ILs-C4-HNS.
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Morphological characteristics and textural properties. Fig. 1 presents HRTEM images
of three PW12-ILs-Cn-HNS nanospheres, which are all composed of small and uniform
spherical nanostructures with hollow interior. Close inspection of the images shows that the
prepared PW12-ILs-C4-HNS nanospheres are well-defined with uniformly dispersed black
spots of 1‒1.5 nm in diameter. This is in agreement with the dimensions of the H
3
PW12
O
40
clusters, and suggests that the active sites (i.e., H PW12O40) are uniformly incorporated into the
3
silica/carbon framework.
In terms of PW12-ILs-C8-HNS (Fig. 1c) and PW12-ILs-C12-HNS (Fig. 1d), they both form
nanospheres with uniform morphology. The energy-dispersive X-ray spectrometry of PW12
-
ILs-C4-HNS (Fig. 1b) reveals the presence of C, O, Si, P and W elements, which is consistent
with the composition of the nanospheres.
Fig. 2 The N
2
adsorption-desorption isotherms (a) and BJH pore size distribution profiles (b) of various
PW12-ILs-Cn-HNS (n = 4, 8, and 12; POM loading: 15.2%).
Fig. 2a shows the N adsorption-desorption isotherms, in which the type IV isotherms
2
confirm the mesoporous nature of the PW12-ILs-Cn-HNS. Moreover, they all exhibit one
hysteresis loop that can be attributed to the hollow interior of the spherical nanostructures or
the void space formed between the loosely packed spheres[40,41]. The capillary condensation
steps occur at P/P = 0.45−0.99. In addition, with increasing of the PW12 loading from 5.8, 8.7
0
to 17.6, the hysteresis loop becomes smaller accordingly (Fig. S3a).
As shown in Fig 2b, BJH pore size distribution curves show that the PW12-ILs-C4-HNS,
PW12-ILs-C8-HNS and PW12-ILs-C12-HNS exhibit one peak at 15.7, 15.3 and 15.6 nm,
respectively, which confirms the mesoporous structure of the nanospheres and corresponds to
the uniform hollow interior of the nanospheres. As shown in Table 1, the PW12-ILs-Cn-HNS
2
–1
nanospheres exhibit a large BET surface area (125‒156 m g ) and high pore volume (0.40‒
3
‒1
0
.47 cm g ). Moreover, the length of alkyl-chain does not affect dramatically on the porosity
of the produced catalyst.
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Table 1 The physical-chemical parameters of various heteropolyacids immobilized on ionic liquids-modified
organosilica nanospheres and reference catalysts.
S
BET
D
p
V
p
A
titration
E
i
Catalysts
2
–1 a
(nm)b
15.7
15.3
15.6
n.d.
3
–1 c
‒1 d
(mV)e
596.7
557.4
456.2
470.7g
720.2
(
m g )
(cm g )
0.43
(μeq g )
1179
987
PW12-ILs-C4-HNS
PW12-ILs-C8-HNS
PW12-ILs-C12-HNS
Amberlyst-15
140
156
0.40
125
0.47
860
50f
n.d.
4800f
3000
H
3
PW12O
40
5.0h
n.d.
n.d.
Note: POM loading is 15.2%; a Surface area (SBET) was calculated using Brunauer–Emmett–Teller (BET) equation. b Pore
diameter (D
p
) was estimated from BJH adsorption determination. c Pore volume (V
p
) was estimated from the pore volume
d
determination using the adsorption branch of the N
2
isotherm curve at P/P
0
= 0.99 single point. Acid density (A) was measured
e
by acid-base titration. Initial electrode potential (E
i
) was determined by nonaqueous potentiometric titration with n-butylamine
in acetonitrile (0.1 mol L ). The data were cited from Ref. [42]. The data was cited from Ref. [43]. h The data was cited
−
1
f
g
from Ref. [44].
The structural integrity of the as-prepared PW12-ILs-Cn-HNS nanospheres were confirmed
3
1
29
by FT-IR (Fig. 3a), W 4f XPS (Fig. 3b), P (Fig. 3c) and Si MAS NMR (Fig. 3d).
FT-IR of H 40 shows characteristic vibration peaks centered at 1079, 985, 890 and
95 cm , respectively, which can be assigned to the stretching of tetrahedral P‒O bonds,
terminal W=O bonds and two types of bending vibrations originating from the bridging W‒
‒W bonds. The above four characteristic vibrational signals can be observed in the FT-IR
spectra of PW12-ILs-C4-HNS, PW12-ILs-C8-HNS and PW12-ILs-C12-HNS. Furthermore, the
3
PW12O
‒
1
7
t
O
b
‒
1
stretching bands located at 1658 and 2932 cm originate from the presence of ‒CH
2
or ‒CH
3
and C=N groups of ILs, respectively. These results indicate that 1) the as-prepared ionic liquid
and PW12 were successfully incorporated into the hollow nanospheres; 2) the primary Keggin
structure remained intact after immobilization, regardless the length of the carbon-chain.
As shown in Fig. 3b, the W 4f XPS spectrum of the PW12-ILs-C4-HNS hollow nanospheres
is deconvoluted into two signals centered at 38.4 and 36.3 eV due to the W 4f5/2 and W 4f7/2
3
1
spin-orbit components accordingly. As can be seen from the P MAS NMR spectroscopy in
Fig. 3c, the sharp peak centered at -15.2 ppm can be assigned to the resonance of the
3
-
encapsulated PO
4
units within the H PW12O40 cage, indicating the structural integrity of the
3
incorporated H
3
PW12
40
O .
In a similar manner, the 29Si MAS NMR spectra reveal characteristic resonance peaks
centered at −66.0, −63.6 and −64.3 ppm for the PW12-ILs-C4-HNS, PW12-ILs-C8-HNS and
PW12-ILs-C12-HNS, respectively, which can be attributed to the organosiloxane species of T3
[
‒CH
2
CH
2
Si(OSi) ] within the ethyl-bridged organosilica framework and covalently tethered
3
ILs (Fig. 3d). Most importantly, the absence of resonance signals corresponding to the inorganic
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ChemCatChem
10.1002/cctc.201900285
3
4
Si-containing starting materials, such as Q [Si(OSi)
120 ppm][45,46] provides additional evidence for the successful formation of the IL
functionalized ethyl-bridged organosilica structures.
3
(OH), δ = -90 ppm] and Q [Si(OSi) , δ =
4
−
Fig. 3 (a) Characteristic FT-IR fingerprint region of the incorporated heteropolyacid on the ILs-modified
3
1
29
organosilica nanospheres; (b) XPS spectrum of the W 4f core level of PW12-ILs-C4-HNS; (c) P and (d) Si
MAS NMR spectra of the PW12-ILs-C4-HNS, PW12-ILs-C8-HNS and PW12-ILs-C12-HNS.
The Brönsted acid densities of the PW12-ILs-Cn-HNS nanospheres were determined by
‒
1
acid-base titration using NaOH solution (4.0 mmol L ). As shown in Table 1, PW12-ILs-Cn-
HNS (n = 4, 8 and 12) with different alkyl-chain length exhibit comparable Brönsted acid
+
‒1
density of 860, 987 and 1179 μeq(H ) g , respectively.
o
The thermogravimetric studies were carried out in the range of 30 to 800 C. As shown in
Fig. S4, the TGA of PW12-ILs-C4-HNS composite shows three consecutive weight losses. The
o
first weight loss occurs in the range of 30 to 100 C (ca. 1.6%) which can be attributed to the
loss of water molecules adsorbed by the nanospheres. The second weight loss in the range of
o
2
00 to 500 C (ca. 14.1%) can be attributed to the decomposition of the bridging ethyl groups,
o
ionic liquid and residual P123. The third one takes place in the range of 400 to 800 C (ca.
3
.6%), which can be due to the elimination of the silica/carbon framework’s carbon content
before the collapse of the framework and decomposition of the Keggin structure. As a
o
consequence, the prepared PW12-ILs-Cn-HNS nanospheres are thermally stable up to 200 C.
3
.2 Catalytic studies
In order to optimize the heteropolyacids immobilized on ILs-modified organosilica hollow
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10.1002/cctc.201900285
nanospheres, the influences of the composition and loading of heteropolyacids as well as the
alkyl-chain length on the catalytic activity were carefully investigated under the conditions of
o
‒1
1
00 C and 0.25 mol L fructose in DMSO for 2 h.
Fig. 4 Influence of the (a) heteropolyacids, (b) PW12 loading, (c) ionic liquid with different alkyl-chain length
and (d) morphological nanostructure on the catalytic activity of the catalysts. Reaction conditions: 0.25 mol
‒
1
L fructose in DMSO, 60 mg catalyst.
As shown in Figure 4a, the dehydration activity of a series of catalysts incorporating
different heteropolyacids followed this sequence of PW12-ILs-C4-HNS > PMo12-ILs-C4-HNS >
SiW12-ILs-C4-HNS > SiMo12-ILs-C4-HNS, while the corresponding 5-HMF yield can be
found to be 93.7, 72.5, 63.5 and 58.7%, respectively. Moreover, the PW12-ILs-C4-HNS
exhibited the best activity that can be attributed to the fact that the H PW12O40 possess the
3
strongest Brönsted acid strength among the selective heteropolyacids, which promote the
release of protons during the dehydration of fructose.
The influence of PW12 loading on the dehydration activity was evaluated. Our observations
are summarized in Fig. 4b, where the yield of 5-HMF continuously increases from 47.9, 62.5
to 93.7 after 2 h with increasing the PW12 loading (5.8, 8.7 to 15.2 %). However, the yield drops
abruptly to 62.2% upon further increase of the PW12 loading. Further increase of the PW12
loading leads to undesirable blocking of the pores of the prepared PW12-ILs-C4-HNS
nanospheres, and consequently to decreased 5-HMF yield. Therefore, the optimum PW12
loading of 15.2 was used for subsequent catalytic tests.
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ChemCatChem
10.1002/cctc.201900285
Then, the dehydration activity using catalysts with different alkyl-chains on ionic liquids
were tested (Fig. 4c). The catalytic activity decreases as the following sequence PW12-ILs-C4-
HNS > PW12-ILs-C8-HNS > PW12-ILs-C12-HNS with the 5-HMF yields of 93.7, 77.0 and
7
1.1%, respectively. This result indicates that the dehydration activity is influenced by the
hydrophobicity microenvironment of ILs-modified organosilica particles. The composition of
PW12-ILs-C4-HNS catalyst proved to be a fine balance of the various parameters that affect the
catalytic process and showed the best activity, which leads to efficient mass transfer of fructose
molecules to the active sites and higher dehydration activity.
The influence of the morphological nanostructures on dehydration activity was
investigated (Fig. 4d). The order was as following PW12-ILs-C4-HNS > PW12-ILs-C4-2Dhex
>
PW12-ILs-C4-3Dint, and the corresponding 5-HMF yield was found to be 93.7, 57.8 and 34.9%,
respectively. In comparison with PW12-ILs-C4-2Dhex with hexagonal nanostructure (Fig. S1a)
and PW12-ILs-C4-3Dint (Fig. S1b) with interconnected nanostructure, the PW12-ILs-C4-HNS
with unique hollow spherical nanostructure exhibited higher activity due to the short diffusion
pathway.
In addition, we investigated carefully the influence of temperature, reaction time and
solvents on 5-HMF yield by taking the optimized PW12-ILs-C4-HNS as the representative
catalyst.
Fig. 5 Influence of the (a) temperature, (b) reaction time, and (c) solvents on the catalytic activity of the
‒
1
PW12-ILs-C4-HNS. Reaction conditions: 0.25 mol L fructose in solvent (methanol, ethanol, acetonitrile,
water, 1,4-dioxane or DMSO) or 90 mg fructose in 1 g [BMIM]Cl or [OMIM]BF ; 60 mg catalyst. (d) Kinetic
4
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ChemCatChem
10.1002/cctc.201900285
‒
1
profiles of the dehydration. Reaction conditions: 0.25 mol L fructose in DMSO, 60 mg catalyst.
As shown in Fig. 5a, the yield of 5-HMF continuously increases from 32.7, 53.1 to 93.7%
o
with increasing the reaction temperature from 80, 90 to 100 C. Further increase of the
o
temperature to 110 C leads to the formation of 5-HMF rapidly with a yield of 69.2% in the
first 30 min. After that, the dehydration reaction rate becomes slower gradually, before reaching
the yield of 80.5% after prolonging the reaction time to 2 h. Thus, the optimum reaction
o
temperature for this catalytic system determined to be 100 C.
The reaction time was investigated by proceeding the dehydration reaction for 3 h (Fig. 5b).
The results show that the 5-HMF yield gradually increased in 2 h. With prolonging the reaction
time to 2.5 h and 3 h, no obvious increase of 5-HMF can be observed. Therefore, 2 h proved to
be the optimum reaction time for the efficient production of 5-HMF.
In general, the dehydration of fructose to 5-HMF in acidic media is accompanied by the
formation of oligomeric byproducts or formic acid, which is detrimental on the selectivity. Thus,
the effect of solvents (methanol, ethanol, acetonitrile, water, 1,4-dioxane, [BMIM]Cl,
[
OMIM]BF
methanol, ethanol, acetonitrile and water exhibit similar dehydration activity and a yield of 1.8,
.0, 3.1 and 3.1 %, respectively, after 2 h. Under the same conditions, 1, 4-dioxane shows
slightly increased dehydration activity and a yield of 21.4%. In contrast, the yields of 38.6%
and 70.6% were obtained by using the [OMIM]BF and [BMIM]Cl as solvents, respectively.
4
and DMSO) on the dehydration process was carried out[47,48]. As shown in Fig. 5c,
2
4
The highest dehydration activity was achieved when the catalytic reaction carried out in DMSO
where the yield reached the value of 93.7 % over a period of 2 h. The DMSO, not only inhibits
any side-reactions which lead to the formation of furanoid[49], but also interacts with H to form
+
DMSOH] cations[3,50]. As such, DMSO was selected as solvent for the subsequent catalytic
+
[
tests.
To obtain the kinetic parameters for the PW12-ILs-C4-HNS-catalyzed dehydration of
fructose, conversion of fructose and lnc was plotted against the reaction time (Fig. 5d), in which
represented the fructose concentration at time t. The linear fit of the data revealed that the
t
c
t
2
PW12-ILs-C4-HNS-catalyzed dehydration reaction shows first-order kinetics (R = 0.9953).
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ChemCatChem
10.1002/cctc.201900285
Fig. 6 Reusability of the PW12-ILs-C4-HNS in dehydration reaction (a) and FT-IR spectrum (b), W 4f core
31
29
level (c), TEM image (d), P (e) and Si (f) MAS NMR of the used PW12-ILs-C4-HNS nanospheres.
Regeneration and reusability. Large scale applications and continuous manufacturing
processes require the use of catalytic systems which can be easily recycled and reused for
prolonged period of times. To evaluate the reusability and stability of the catalytic system, the
PW12-ILs-C4-HNS catalyst catalyzed the dehydration of fructose for six consecutive cycles
under the optimized reaction condition. After each catalytic cycle, the catalyst was removed by
filtration, washed with ethanol and acetone before the next cycles. As shown in Fig. 6a, the
yields of 5-HMF are found to be 93.7, 96.9, 91.7, 95.2, 92.6 and 91.2%, respectively, for the
consecutive catalytic cycles. In order to identify potential leaching of the H PW12O40 species
3
into the solution, we conducted ICP-AES analyses to determine the tungsten content in the
reaction mixture after removing the catalyst at the end of the cycle. The results showed that the
concentration of the tungsten in the system is below the detection limit confirming that there is
no detectable leaching of H
the structure of the recycled PW12-ILs-C4-HNS catalyst was further evaluated by FT-IR, XPS,
31P and 29
Si MAS NMR. Fig. 6b shows the FT-IR characteristic peaks which can still be
3
PW12O40 species under the experimental conditions. Subsequently,
detected and assigned to the Keggin and ILs species. The deconvoluted W 4f XP spectrum
reveals again in this case two signals centered at 38.4 and 36.3 eV (Fig. 6c), originating from
the contributions of the W 4f5/2 and W 4f7/2 spin-orbit components, which found to be identical
to the freshly prepared PW12-ILs-C4-HNS catalyst. Additionally, TEM image in Fig. 6d shows
well-dispersed hollow nanospheres, which indicates that the confined nanostructure of as-
prepared catalysts was retained after recycling. Subsequently, the peaks centered at -15.2 and -
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ChemCatChem
10.1002/cctc.201900285
31
29
6
4.5 ppm in P and Si MAS NMR spectra (Figs. 6e and f), confirms that the H
3
PW12
O
40
species retain their structural integrity after the catalytic cycle. Based on the above observations,
we can conclude that the prepared PW12-ILs-Cn-HNS nanospheres can act as efficient and
stable solid acid catalysts for the dehydration reaction of fructose.
Comparison with the reference solid acids. The prepared PW12-ILs-C4-HNS catalyst
was further compared with the H PW12O40 and Amberlyst-15 under the same reaction condition.
3
As shown in Table 2, PW12-ILs-C4-HNS-catalyzed dehydration reaction of fructose exhibits a
yield of 93.7% which is in marked contrast to the yields observed in the case of Amberlyst-15
and H
that the strong Brönsted acidity of H
occurrence of side-reactions, which leads to poor selectivity for 5-HMF. In marked contrast,
3
PW12
O40 catalysts with yields of 63.1 and 58.4%, respectively. This is due to the fact
3
PW12O40 and Amberlyst-15 facilitates simultaneously the
‒
1
the PW12-ILs-C4-HNS exhibits the acid density of 1179 μeq g and E of 596.7 mV which
i
leads ultimately to the efficient dehydration activity without promotion of side-reactions, and a
high yield of 93.7% over 2 h. Moreover, the dehydration activity of PW12-ILs-C4-HNS catalyst
was further compared to the other reported examples. The results in Table 2 confirm that the
prepared PW12-ILs-C4-HNS catalyst could work efficiently in the dehydration of fructose under
mild reaction conditions.
Table 2 Dehydration activity comparison of PW12-ILs-C4-HNS with the reference solid acids.
o
Catalysts
T ( C) Solvent
T (min)
120
Yield (%) Reference
PW12-ILs-C4-HNS
Amberlyst-15
100
100
100
140
130
DMSO
93.7
63.1
58.4
71
This work
This work
This work
Ref 51
DMSO
120
H
3
PW12O
40
DMSO
120
SPAN-11/3
Water/1,4-Dioxane
MIBK/2-Butanol
180
TESAS-SBA-15
141
59.6
Ref 52
Note: TEASA is 3-((3-(trimethoxysilyl)propyl)thio)propane-1-sulfonic acid and SPAN is sulfonated
polyaniline.
In order to elucidate the reaction mechanism of the PW12-ILs-C4-HNS catalyzed
dehydration of fructose, we used mass spectrometry to identify the intermediate species
(Scheme 2 and Fig. S8). In the initial stage, the DMSO is protonated firstly via the proton
transfer process[3,50,53], and an intermediate is obtained (A) via hydrogen bonding interactions
between fructose and the protonated DMSO. Then, the enol (B), 2,5-anhydro-D-mannose (C),
and 4-hydroxy-5-(hydroxymethyl)-4,5-dihydrofuran-2-carbaldehyde (D) are successively
formed, accompanied with the release of water molecules. Finally, the 5-HMF can be obtained
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ChemCatChem
10.1002/cctc.201900285
by releasing the water and DMSO molecules.
Importantly, the formation of the 5-HMF can potentially take place in tandem with various
side-reactions. For example, the formation of byproduct of difructose anhydride (F) is promoted
by strong acidic conditions. Consequently, the target product (5-HMF) can further transform to
polymers and humins or formic and levulinic acids via a rehydration reaction[54]. Based on the
above observations it is concluded that the best dehydration activity of PW12-ILs-C4-HNS
‒
1
catalyst can be attributed to its Brönsted acidity (Brönsted acid density of 1179 μeq g and E
i
of 596.7 mV). Finally, it is worth noting that the hydrophobic surface of as-prepared PW12-ILs-
C4-HNS catalyst can promote the efficient removal of the water molecules produced from the
catalyst’s surface and accelerate the dehydration reaction.
Scheme 2 The suggested reaction mechanism of PW12-ILs-C4-HNS-catalyzed dehydration of fructose
towards the formation of 5-HMF.
On the other hand, the morphological characteristics and the porosity properties of the
PW12-ILs-C4-HNS nanospheres influence dramatically the dehydration activity. The hollow
spherical nanostructures of the PW12-ILs-C4-HNS catalyst can serve as nanoreactors providing
well dispersed active sites within an accessible confined space. The morphology and structural
features of the spherical particles provide a short diffusion pathway, which leads to efficient
mass transfer and accessibility of the active sites by the substrate. Simultaneously, the unique
hollow spherical nanostructures of the prepared nanocatalysts exhibit large BET surface areas
2
–1
(
140 m g ) and thereby well-dispersed and controlled population of the heteropolyacids.
3
–1
Additionally, the high pore volume (0.43 cm g ) of the PW12-ILs-C4-HNS renders the
embedded within the nanosphere catalytic sites readily accessible by the reactant molecules.
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ChemCatChem
10.1002/cctc.201900285
Meanwhile, the hydrophobic surface of PW12-ILs-C4-HNS due to the alkyl groups of the ILs-
modified organosilica structures, can inhibit the formation of byproducts such as formic and
levulinic acids by rehydration of the desirable product, 5-HMF, by promoting it’s easy removal
from the catalyst’s surface and ensuring that the dehydration of fructose proceeds at a
considerably high rate. Therefore, the synergistic effects between the PW12-ILs-Cn-HNS
catalyst’s components in the confined space of the nanosphere, suitable Brönsted acidity of the
active species (PW12), excellent porosity and IL-modified organosilica hydrophobic surface,
are all crucial parameters for the enhancement of the dehydration process and production of 5-
HMF at considerably high yield.
4
. Conclusions
A new family of heterogeneous catalysts were successfully fabricated by immobilizing
PW12 clusters within the ILs-modified organosilica hollow nanospheres (PW12-ILs-Cn-HNS)
and fully characterized, in which the PW12 provides Brönsted acid site, ILs bind strongly
through the electrostatic interactions with PW12 and Cn helps to adjust the hydrophobicity.
Among them, the PW12-ILs-C4-HNS with PW12 loading of 15.2% exhibited the best activity
for dehydration of fructose to 5-HMF under mild conditions. Furthermore, the PW12-ILs-Cn-
HNS composites showed excellent stability after six consecutive catalytic cycles without
obvious loss of their catalytic activity. Moreover, it has been demonstrated that compared with
PW12-ILs-Cn-2Dhex and PW12-ILs-Cn-3Dint, the PW12-ILs-Cn-HNS exhibits much improved
catalytic activity, indicating the advantages of the confined nanospherical structure. Careful
modulation of a number of reaction parameters and identification of the intermediate species in
the reaction mixture using mass spectrometry allowed us to shed light upon the mechanistic
details for the dehydration processes. The design strategy presented herein may open a new
pathway for further development of cost-effective and environmentally-benign catalysts for
biomass conversion applications.
Acknowledgements
This research was supported by the National Nature Science Foundation of China (21625101, 21808011,
21521005), the National Key Research and Development Program of China (2017YFB0307303), the
Fundamental Research Funds for the Central Universities (XK1802-6, XK1902, 12060093063), Beijing
Natural Science Foundation (2182047), Postdoctoral Science Foundation of China (2018M631313). H. N.
M would like to thank the University of Glasgow for supporting this work.
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ChemCatChem
10.1002/cctc.201900285
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