An efficient and reusable "hairy" particle acid catalyst for the synthesis of 5-hydroxymethylfurfural from dehydration of fructose in water

Article abstract of DOI:10.1039/c3cc43127d

Poly(4-styrenesulfonic acid) brush-grafted silica particles, synthesized by surface-initiated atom transfer radical polymerization, were employed as a reusable acid catalyst for dehydration of fructose to 5-hydroxymethylfurfural (HMF) in water. The particles exhibited a high activity with the HMF yield of up to 31%, in contrast to 26% from the corresponding free homopolymer catalyst.

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An efficient and reusable “hairy” particle acid catalyst for the synthesis
of 5-hydroxymethylfurfural from dehydration of fructose in water
a,b
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Chengcheng Tian, Chunhui Bao, Andrew Binder, Zhenqian Zhu, Bin Hu, Yanglong Guo,* Bin
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Zhao,* and Sheng Dai*
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
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Poly(4-styrenesulfonic acid) brush-grafted silica particles,
synthesized by surface-initiated atom transfer radical
catalysts’ properties to promote organic reactions in water.
Because the surface hydrophobicity of inorganic materials can be
polymerization, were employed as a reusable acid catalyst for 50 readily modified by introducing organic moieties, hybrid
organic–inorganic mesoporous materials have attracted
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fructose dehydration to 5-hydroxymethylfurfural (HMF) in
water. The particles exhibited a high activity with the HMF
yield up to 31%, in contrast to 26% from the corresponding
free homopolymer catalyst.
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considerable interest in the past few years.
These materials
combine the advantages of both organic and inorganic
components introduced into the mesoporous solids, exhibiting
improved properties compared with either the wholly organic or
inorganic materials. The observed higher activities of these
catalysts have been attributed to the increased hydrophobicity
near catalytic sites and the enhanced diffusion of organic
reactants and products within the hydrophobic mesopores.
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Renewable biomass resources are promising alternatives for
the sustainable supply of liquid fuels and chemical intermediates.
As one of top buildingꢀblock chemicals derived from biomass, 5ꢀ
hydroxymethylfurfural (HMF) is a versatile intermediate for fine
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However, when a high content of organic catalytic groups is
demanded, an important limitation for mesoporous materials is
the surface saturation by small organic species. To overcome this
constraint, herein we report the synthesis of a “hairy” particle
acid catalyst by growing outward from the surface of silica
particles poly(sodium 4ꢀstyrenesulfonate) brushes, which can be
easily acidified to poly(4ꢀstyrenesulfonic acid) (PSSH) brushes,
and the use of PSSH hairy particles as a recyclable acid catalyst
for the dehydration of fructose to HMF in water (Scheme 2).
Polymer brushes are a dynamic system possessing a certain
chemicals, pharmaceuticals, plastic resins, fuels, etc. HMF is
usually made from fructose by acidꢀcatalyzed dehydration
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reaction. From an environmental and economical point of view,
although soluble small molecule acids are inexpensive and highly
active catalysts for dehydration reactions, heterogeneous solid
acid catalysts are more desired because of their facile recovery
and lower adverse impacts to environment. As shown in previous
studies, high HMF yields can be obtained in some organic
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solvents (e.g., dimethylsulfoxide) and ionic liquids. However,
these systems have some drawbacks in terms of high separation
and recycling costs, and are generally not environmentally
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degree of mobility.
The grafted linear PSSH chains take on
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extended conformations in water, due to their hydrophilic nature,
and hence have increased interactions with fructose. Thus,
polymer brushꢀsupported acid catalysts resemble linear soluble
polymerꢀsupported catalysts. The increased mobility, compared
with insoluble crosslinked polymerꢀsupported catalysts, would
improve the catalyst’s performance. On the other hand, a unique
microenvironment surrounding catalytic sites, i.e., sulfonic acid,
is created by the hydrophobic polymer backbone, which to some
degree mimics the characteristics of organic solvents and thus
would facilitate the diffusion of organic reactants to the
hydrophilic catalytic sites. When fructose molecules diffuse into
the polymer brush layer of hairy particles, they experience a
microenvironment of highly concentrated, hydrated acid groups
attached to the hydrophobic polystyrene backbone, different from
bulk aqueous phase, leading to the catalytic formation of HMF. In
addition, the hairy particle acid catalyst can be separated from
products after reaction, e.g., by centrifugation, and can be reused.
This resembles crossꢀlinked polymerꢀsupported catalysts.
friendly. Because of the low cost and the low toxicity, aqueous
systems have attracted increasing attention for HMF preparation.
Unfortunately, previous work reported that the dehydration of
fructose in aqueous media is generally nonselective, leading to
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many byꢀproducts such as insoluble humins. Note that water is a
product in the dehydration of fructose to HMF, and also a
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reactant in the further hydration of HMF (Scheme 1). To
enhance the selectivity and to facilitate the extraction of HMF,
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aqueous/organic biphasic systems were employed recently.
Scheme 1. A typical reaction scheme for HMF production from fructose.
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On the other hand, increasing materials’ hydrophobicity has
been demonstrated to be an effective method for enhancing
The poly(sodium 4ꢀstyrenesulfonate) brushꢀgrafted silica
particles were prepared by surfaceꢀinitiated atom transfer radical
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calculated the grafting density of PSSH brushes, and it was 0.40
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chains/nm . Additionally, the FTIR spectrum of hairy particles
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showed new bands at 1415, 1125 and 1010 cm , which are
assigned to the stretching vibrations of sulfonic acid groups
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(Figure
S2).
Detailed
synthetic
procedures
and
characterization data can be found in the supporting information.
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Scheme 2. Schematic illustration for the synthesis of poly(4ꢀ
styrenesulfonic acid) brushꢀgrafted silica particles and the dehydration of
fructose to HMF catalyzed by the “hairy” particle acid catalyst.
polymerization (ATRP) of sodium 4ꢀstyrenesulfonate from
ATRPꢀinitiatorꢀfunctionalized silica particles. ATRP is a versatile
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“living”/controlled radical polymerization technique. The bare
silica particles with an average size of 175 nm (Figure 1a), made
Figure 1. Transmission electron microscopy images of (a) bare silica
by the Stöber process, were used in the present work. An ATRP 80 particles, (b) initiator particles, (c) poly(4ꢀstyrenesulfonic acid) (PSSH)
initiatorꢀterminated triethoxysilane, 2ꢀ[4ꢀ(chloromethyl)phenyl]ꢀ
ethyltriethoxysilane, was immobilized onto the surface of silica
particles via an ammoniaꢀcatalyzed hydrolysis and condensation
brushꢀgrafted silica particles, and (d) thermogavimetric analysis (TGA) of
initiator particles (red line) and PSSHꢀgrafted particles (blue line). TGA
was performed in air at a heating rate of 20 °C/min.
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process in ethanol. Note that the use of a benzyl chloride
initiatorꢀterminated triethoxysilane yielded linkage that
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The catalytic performance of the hairy particle acid catalyst for
the dehydration of fructose to HMF in water was examined. A
typical condition is as follows. PSSHꢀgrafted particles (0.10 g),
fructose (0.23 g), and distilled water (3.5 mL) were added into a
flask, which was then placed in a 120 °C oil bath. The effects of
contained only carbonꢀcarbon bonds between the surface of silica
particles and the polymer brushes, rendering the grafted polymer
chains stable under the acidic conditions at elevated temperatures.
The particle catalyst thus can be recycled multiple times without
any significant change. The surfaceꢀinitiated ATRP of sodium 4ꢀ 90 reaction time on the fructose conversion and the HMF yield were
styrenesulfonate from the initiator particles was carried out in a
mixed solvent of methanol and water with a weight ratio of 1:3.5
at 75 °C using CuCl/N,N,N′,N′,N′′ꢀpentamethyldiethylenetriamine
investigated by HPLC and the results are shown in Figure 2a. The
conversion of fructose reached 80% at 6 h and the HMF yield
was 27%. With increasing the reaction time, the fructose
conversion and the HMF yield slowly climbed to ~ 91% and 31%,
as catalyst in the presence of a free initiator, benzyl chloride. The
1
polymerization was monitored by H NMR spectroscopy and was 95 respectively, and then levelled off. Generally, a further increase
stopped after the monomer conversion reached 100%. The hairy
particles were isolated and redispersed in acidic water with pH of
~ 1 and centrifugated again. This washing process was repeated
additional five times, followed by drying of the PSSH hairy
particles under high vacuum at 50 °C overnight. The degree of 100 catalytic system, indicating that the further hydration reaction of
polymerization (DP) of the free polymer formed from the free
in reaction time decreases the HMF selectivity due to its
secondary reactions involving the hydration to levulinic acid and
the formation of polymers and humins (Scheme 1). Interestingly,
the reaction time had little effect on the HMF selectivity in this
HMF was restrained (Figure S3). For comparison, the
homopolymer analogue, PSSH obtained from the free polymer
formed in the synthesis of hairy particles by dialysis against pH
1.0 water, was also tested for the dehydration of fructose to HMF
initiator was 309, calculated from the monomer conversion and
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the monomerꢀtoꢀinitiator ratio.
Transmission electron microscopy (TEM) image analysis
provided direct evidence for the successful growth of polymer 105 under the same conditions. The same amount of the
brushes on silica particles (Figure 1aꢀc). The polymer brush layer
in the PSSH hairy particles can be clearly seen from Figure 1c.
Compared with bare and initiator particles, there was a noticeable
increase in the distance between adjacent silica particle cores.
homopolymer catalyst (36.6 mg) was used for the reaction based
on the polymer content of the hairy particle catalyst. Intriguingly,
the PSSH hairy particles always displayed a higher activity than
the free homopolymer at the tested reaction times (Figure 2a).
Thermogravimetric analysis (TGA) revealed that the weight 110 The highest HMF yields for the hairy particle catalyst and the
retentions at 800 °C of initiator particles and PSSH hairy particles
were significantly different (Figure 1d). If the weight retention
difference at 100 °C between initiator particles and hairy particles
is taken into consideration, the polymer content of hairy particles
homopolymer were 31% and 26%, respectively. This difference
is likely due to the favourable microenvironment formed by the
polymer brush architecture for the dehydration reaction.
We then studied the reusability of the hairy particle acid
was 36.6 wt% (see the supporting information). Using the TGA 115 catalyst through five repeated dehydration reactions at 120 °C for
data, the DP of the free polymer, and the size of bare particles, we
6 h under the same conditions. After each round of experiment,
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the HMF yield was measured and the hairy particles were isolated
observation. The catalyst was reused multiple times with little
by centrifugation, purified, and reused. The results, shown in 60 decrease in the HMF yield. These hairy particles represent a
Figure 2b, indicated that this catalyst exhibited an excellent
stability for the fructose conversion into HMF. After being used
five times at 120 °C for 6 h, the HMF yield was still at ~ 24 %,
though a slight decrease was observed. The slight decrease in the
promising approach toward high performance recyclable acid
catalysts for the HMF synthesis in H O from renewable biomass.
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The research was supported financially by the Divisions of
Chemical Sciences, Geosciences, and Biosciences, Office of
HMF yield might be caused by the particle loss in each round of 65 Basic Energy Sciences, US Department of Energy, and NSF
separation and purification or as suggested in the literature by the
possible unstability of Si–O–Si bonds between the grafted
polymer and the particles under the acidic condition.
(DMRꢀ1007986, B.Z.). CCT and YLG also thank the National
Basic Research Program of China (2010CB732300) and 111
Project (B08021).
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Notes and references
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Key Laboratory for Advanced Materials, Research Institute of
Industrial Catalysis, East China University of Science and
Technology, Shanghai 200237, P.R. China. Tel: +86 21-6425
2
923; E-mail: ylguo@ecust.edu.cn (Y. L. Guo)
Chemical Sciences Division, Oak Ridge National Laboratory,
b
Oak Ridge, TN 37831, USA. Fax: +1 865-576 5235; Tel: +1 865-
5
76 7303; E-mail: dais@ornl.gov (S. Dai)
Department of Chemistry, University of Tennessee, Knoxville,
c
TN 37996, USA. Tel: +1 865-946 1066; E mail:
zhao@ion.chem.utk.edu (B. Zhao)
†
Electronic Supplementary Information (ESI) available:
Experimental section and Figures S1 and S2.
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Figure 2. (a) Effect of reaction time on the fructose conversion to HMF
catalyzed by PSSHꢀgrafted silica particles and the homopolymer PSSH at
120 °C. The inset illustrates the effect of reaction time on the fructose
conversion and HMF selectivity catalyzed by PSSH hairy particles at 120
3
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Our hairy particle catalyst was more efficient for the synthesis
of HMF from fructose when compared with commercial solid
acids, such as Nafion NR50 and Amberlystꢀ15 which gave HMF
yields of < 5 % under the same conditions. Unlike commercial
solid acids that usually have rigid, crosslinked microstructures
and are insoluble in water, PSSH brushꢀgrafted particles can be
easily dispersed in water, which maximizes the interactions
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1
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1
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This journal is © The Royal Society of Chemistry [year]
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