Starbon/High-Amylose Corn Starch-Supported N-Heterocyclic Carbene–Iron(III) Catalyst for Conversion of Fructose into 5-Hydroxymethylfurfural

Article abstract of DOI:10.1002/cssc.201702207

Iron–N-heterocyclic carbene complexes (Fe-NHCs) have come to prominence because of their applicability in diverse catalytic reactions, ranging from C?C cross-coupling and C?X bond formation to substitution, reduction, polymerization, and dehydration reactions. The detailed synthesis, characterization, and application of novel heterogeneous Fe-NHC catalysts immobilized on mesoporous expanded high-amylose corn starch (HACS) and Starbon 350 (S350) for facile fructose conversion into 5-hydroxymethylfurfural (HMF) is reported. Both catalyst types showed good performance for the dehydration of fructose to HMF when the reaction was tested at 100 °C with varying time (10 min, 20 min, 0.5 h, 1 h, 3 h and 6 h). For Fe-NHC/S350, the highest HMF yield was 81.7 % (t=0.5 h), with a TOF of 169 h^?1, fructose conversion of 95 %, and HMF selectivity of 85.7 %, whereas for Fe-NHC/expanded HACS, the highest yield was 86 % (t=0.5 h), with a TOF of 206 h^?1, fructose conversion of 87 %, and HMF selectivity of 99 %. Iron loadings of 0.26 and 0.30 mmol g^?1 were achieved for Fe-NHC/expanded starch and Fe-NHC/S350, respectively.

Full text of DOI:10.1002/cssc.201702207

Accepted Article
Title: Novel Starbon/HACS-supported N-heterocyclic carbene-iron(III)
catalyst for efficient conversion of fructose to HMF
Authors: Avtar Singh Matharu, Suleiman Ahmed, Badriya Al-Monthery,
Duncan Macquarrie, Yoon-Sik Lee, and Yohan Kim
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To be cited as: ChemSusChem 10.1002/cssc.201702207
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10.1002/cssc.201702207
ChemSusChem
Novel Starbon™/HACS-supported N-heterocyclic carbene-iron(III)
catalyst for efficient conversion of fructose to HMF
Avtar S. Matharu*[a], Suleiman Ahmed[a], Badriya Almonthery[a], Duncan J. Macquarrie[a], Yoon-Sik
Lee[b] and Yohan Kim[b].
[a]
[b]
Green Chemistry Centre of Excellence, Department of Chemistry, University of York, UK. YO10 5DD.
School of Chemical and Biological Engineering, Seoul National University, 1 Kwanak-Ro, Kwanak-Gu, Seoul 151-742, Republic of Korea.
to 500 m2 g-1 obtained via controlled pyrolysis of expanded poly-
ABSTRACT: Iron-nitrogen heterocyclic carbenes (Fe-NHCs) have
saccharide precursors (expanded starch). [10] The large surface
come to the fore because of their ability to be employed in diverse
area and mesoporous nature of Starbon™ makes it an important
catalytic applications ranging from C-C cross-coupling and C-X bond
and efficient support for heterogeneous catalysis. [11]
formation to substitution, reduction, polymerization, and dehydration.
The detailed synthesis, characterisation and application of novel het-
As summarised in Scheme 1, herein, the first synthesis of a novel
erogeneous Fe-NHC catalysts immobilised on mesoporous ex-
panded starch and Starbon™ 350 for facile fructose to HMF conver-
sion is reported. Both catalyst types showed good performance for
the dehydration of fructose to HMF when the reaction was explored
at 100 ◦C and varying time (10 min, 20 min, 0.5 h, 1 h, 3 h and 6 h):
Fe-NHC S350, highest HMF yield, 81.7 % (t=0.5 h), TOF=169 h-1,
fructose conversion of 95 % and HMF selectivity of 85.7 %, and; Fe-
NHC expanded HACS, highest yield, 86 % (t=0.5 h), TOF=206 h-1,
fructose conversion of 87 % and HMF selectivity of 99 %. An iron-
loading of 0.26 and 0.30 mmol/g was achieved for the Fe-NHC ex-
panded starch and Fe-NHC Starbon™ 350, respectively.
iron-nitrogen heterocyclic carbene (Fe-NHC) supported on ex-
panded starch (1a) and Starbon™ 350 (1b) as potential catalysts
for the dehydration of fructose (2) to 5-(hydroxymethyl) furfural
(HMF) (3) is reported. Expanded high amylose corn starch (4)
was used as a support, and precursor for Starbon™ 350, be-
cause it is (bio)renewable, and readily available from a variety of
waste biomass types. HMF (3) is considered a renewable ma-
terial and an excellent platform molecule because it can be ac-
quired from biomass through chemical dehydration using homo-
geneous and/or heterogeneous acid catalysts. [12,13] The pres-
ence of both hydroxymethyl (-CH2OH) and aldehyde (-CHO)
moieties within HMF makes it a reactive molecule affording fur-
ther downstream products. HMF is a precursor for: 2,5-dimethyl-
furan (DMF), a biofuel and which itself leads to levulinic acid; 2,5-
diformylfuran (DFF);5-hydroxy-4-keto-2-pentenoicacid; 2,5-fu-
ran dicarboxylic acid (FDA), and; 2,5-dihydroxymethylfuran. [14]
Introduction
The dehydration of various carbohydrate precursors to 5-HMF in
an acidic biphasic system under microwave heating was re-
ported by Delbecq et al. [15] More recently the conversion of car-
bohydrate to HMF using hydrothermally stable Nb-SBA-15 cata-
lysts in a biphasic system has been reported by Peng et al. show-
ing good yield of HMF (61.8%). [16] Another investigation on fruc-
tose to HMF conversion was by Garcꢀs et al. in which they in-
vestigate the aqueous phase conversion of hexoses into HMF
and levulinic acid in the presence of hydrochloric acid. [17]
Iron-nitrogen heterocyclic carbenes (Fe-NHC’s) have come to
the fore because of their ability to be employed in diverse cata-
lytic applications ranging from cross coupling and C-X bond for-
mation to substitution, reduction, polymerization, dehydration
and recently their unusual application as promising photosensi-
tizers.[1,2] NHCs are special among other carbenes because they
are stable, directly synthesized, can attach to metals with
different oxidation states and can form organometallic com-
plexes that are stable as catalysts. [3], [4] Moreover, iron is abun-
dant, of low toxicity and benign, widely recycled and, thus, con-
forms to elemental sustainability. The chemical transformation of
biomass to platform molecules can be extremely challenging due
to complex chemistries and/or is tediously slow. Fe-NHC’s have
been used for fructose to HMF conversions, affording good
yields. However, most of the systems are homogeneous and
therefore present challenges in product separation and/or cata-
lyst recovery and recycling.
On the other hand, functional porous materials have been used
in catalysis providing many opportunities. [5] Mesoporous solids
present advantages with respect to conventional porous solids
when they are used as catalyst supports, due to their high sur-
face area, large pore size and thermal and hydrothermal stabili-
ties.[5-8] Our approach combined the catalytic advantages of Fe-
NHC’s and the large surface area, pore size and stability of mes-
oporous supports producing a perfect catalyst for heterogeneous
catalysis. Carbon catalysts and carbon supported catalysts can
facilitate a variety of reactions for the catalytic conversion of bio-
mass into chemicals. [9] The Green Chemistry Centre of Excel-
lence has developed a range of biobased mesoporous carbona-
ceous materials derived from waste polysaccharides termed
Starbon™, possessing large surface areas ranging from 150 up
This article is protected by copyright. All rights reserved.

10.1002/cssc.201702207
ChemSusChem
performed using a slight excess of (5), i.e., 3.16 molar equiva-
lents or 0.16 molar excess, to afford a succinimidyl carbonate
degree of substitution (DS) of 0.33±0.11 corresponding to
20.92±4.88% of succinimidyl carbonate activation on the ex-
panded HACS. The low DS may be due to inaccessibility of the
hydroxyl moieties within starch despite using expanded HACS
(4a) and due to steric hindrance associated with the bulky suc-
cinimdyl carbonate group. It is envisaged that substitution has
most likely taken place preferentially on the C6-OH because it is
least hindered.
Characterization of Fe-NHC bio-based catalyst (1a-b)
FT-IR
The FT-IR spectra of expanded HACS (4a) and its correspond-
ing surface modified forms are given in ESI (Figure S1). Several
absorbance bands arising from starch in the region 1000-1200
cm-1, corresponding to C-O, C-C, C-O-H bond stretching and C-
O-H bending, [19] are evident along with a broad band centred at
3341 cm-1 associated with O-H stretching synonymous with hy-
droxyl groups in expanded HACS (4a). [20] The spectra of DSC-
modified HACS (6a) showed new absorbance bands at 1813,
1738, 1653 cm-1 assigned to carbonyl (C=O str), and 1239 cm-1
assigned to imide (C-N) vibration corresponding to newly at-
tached carbonyl groups on HACS surface. The absorbance band
patterns of IL based ligand grafted HACS (13a) showed newly
grafted imidazolium (1257, 1192 cm-1) and formed carbonyl
(C=O) in carbamate group (1695 cm-1).
For Starbon™ 350 (ESI, Figure S2), shows C=C at 1600 cm-1
and one carbonyl stretching band 1702 cm-1. However, in the
DSC activated Starbon™ 350 (6b), two additional bands appear
at 1566 cm-1 and 1678 cm-1 assigned to carbonyl of the DSC
group attached during activation. This argument is also sup-
ported by a decrease in the intensity of the OH stretching at 3353
cm-1 from Starbon™-350 (4b) to the DSC activated Starbon™
350 (6b), signifying that many OH groups on the Starbon™ 350
were activated by the succinimidyl carbonate group.
Scheme 1. Synthetic route to iron-NHC catalysts 1a and 1b
Moreover, the IR spectra of the ligand grafted Starbon™ 350
(13b) indicates a disappearance of a band at 1567 cm-1 and a
corresponding decrease of the band at 1647 cm-1 assigned to
the C=O stretching of the carbamate. This however, indicates
that the ligand is successfully grafted on the DSC group as pro-
posed in the reaction equation.
Results and Discussion
The desired Fe-NHC catalyst (eg. 1a-b) was successfully ac-
quired from the reaction between (13), in the presence of potas-
sium-tert-butoxide as base and propylene carbonate as solvent,
and iron(III) chloride in good yield (61.5%).
13C CPMAS NMR
The 13CPMAS NMR spectra for conversion to 1a from 4a and
1b from 4b are given in ESI, Figures S3 (expanded starch) and
S4 (Starbon™ 350), respectively. For conversion of 4a from 1a,
the NMR signals observed at 61.8, 82.1 and 101 ppm were as-
signed to C-6, C-4 and C-1 in expanded HACS (4a), respec-
tively. [21,22] The major signal at 72.5 ppm is assigned to C-2, -3,
-5. With expanded HACS activation process (see spectra for
DCS activated HACS, 6a), DSC (N,N’-disuccinimidyl carbonate)
groups were attached to hydroxyl groups on HACS surface and
caused some signal shifts on the starch backbone. Carbonyl
signal from succinimidyl carbonate group appeared in 162, and
180 ppm. However, on the starch support, C-1 was affected by
the succinimidyl carbonate groups and shifted from 106 ppm to
102 ppm. After ligand (12) binding to afford 13a, the carbonyl
signal at 180 ppm assigned to the imide of the succinimidyl group
almost disappears leaving the signal at 166 ppm assigned to the
carbamate which also shifted from 162 ppm due to ligand bind-
ing. On the starch support C-2, -3, -5 shifted from 75.5 ppm to
79.4 ppm, with C6 also shifted from 61.8 ppm to 66.2 ppm due
to ligand binding.
Expanded HACS (4a) or Starbon™ 350 (4b) suspended in pro-
pylene carbonate was successfully converted in to succinimidyl
carbonate derivative (6), in the presence of N,N'-disuccinimidyl
carbonate (5) and catalyzed by DMAP, in good yield (60%). In
certain cases, this conversion has been carried in dimethyl
formamide (DMF), [18] a toxic dipolar aprotic solvent, which is re-
moved by washing with copious amounts of water post reaction
resulting in considerable amounts of aqueous waste. The latter
is usually incinerated but because DMF is a nitrogen containing
solvent, NOx emissions are produced. On the other hand, pro-
pylene carbonate (used in this case, see Scheme 1) is a
green(er) alternative produced from (waste) carbon dioxide and
propylene oxide. Unlike DMF, incineration of propylene car-
bonate affords carbon dioxide and water only.
The theoretical degree of substitution (DS) of starch (4a) is 3 be-
cause it comprises three hydroxyl moieties per each anhydroglu-
cose unit, i.e., at C2, C3 and C6. As the stoichiometry of starch
(4) with respect to N,N'-disuccimidyl carbonate (5) is 1:3, initially
the conversion to its succinimidyl carbonate derivative (6) was
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10.1002/cssc.201702207
ChemSusChem
Successful ligand binding was further confirmed by the appear-
ance of new signals at 23, 40-51, 127-137 ppm and 142 ppm
corresponding to aromatic CH3, imidazole -CH2-, carbons of im-
idazole ring and aromatic carbon of the attached imidazole lig-
and respectively. ]23-25] The IL grafted HACS (13a) has one car-
from 150 to 310 oC after which a major weight loss temperature
(550 oC), corresponding to the breakdown of the Starbon™ back-
bone is reached. However, in the modified Starbon™ sample
(6b) the major weight loss temperature is decreased by the
o
chemical modification; 528 C for succinimidyl carbonate acti-
bonyl group connected with HACS and ligand. Similarly, the 13
C
vated Starbon™-350 (6b), 506 oC for imidazolium ligand grafted
Starbon™-350 (13b), and 528 oC for Fe coordinated Starbon™
350 (1b). This indicates that the functionality attached on the
Starbon™ surface affects the intermolecular interaction and
hence decreases the decomposition temperature. However, an
increase in the residual mass of about 2.41% (a characteristic
reddish-brown residue) was obtained in the Fe coordinated Star-
bon™ 350. This entails that Fe is contained in the sample.
CPMAS spectra of IL grafted HACS (13a) also shows one car-
bonyl peak at 160 ppm. Moreover, due to iron coordination reac-
tion, peaks of iron coordinated starch were slightly shifted, ex-
cept the resonance in 51 ppm corresponding to next carbon of
imidazole group.
The 13C CPMAS spectra of Starbon™ 350 and its modified forms
(ES, Figure S4) show similar resonance changes as described
for the HACS materials. However, the ligand grafted Starbon™
350 (13b) shows new signals at 23 ppm and between 40-51
ppm, assigned to aromatic CH3, imidazole -CH2-, and carbons of
imidazole ring, respectively. It was difficult to assign all the aro-
matic carbons, and most importantly the carbonyls of the imide
and carbamate group of the DSC were masked within the signals
for Starbon™ itself.
CHN and ICP-MS Elemental analysis
The succinimidyl carbonate and NHC ligand loadings were de-
termined from nitrogen content measurement by CHN elemental
analysis. The Fe loading on the other hand was determined from
ICP-MS elemental analysis. Table 1 below give a summary of
the loadings in mmol/g of support material.
Thermogravimetry
STA was used to examine the thermal decomposition of ex-
panded HACS (4a) and its modified compounds (6a), (13a) and
(1a) as shown in ESI, Figure S5. The thermal decomposition of
expanded starch (4a) was as per literature [245-333 ◦C] [26]; Initial
loss of moisture (approx. 9% water) both physi- and chemi-
sorbed from 25 ◦C to 135◦C is observed followed by main degra-
Table 1. Loading levels (mmol/g) of succinimidyl carbonate, NHC
ligand and Fe on expanded HACS and Starbon 350.
Support
Loading level (mmol/g)
Succinimidyl
carbonate
2.09
NHC ligand
Fe
◦
Expanded
HACS
Starbon
350
1.23
0.63
0.26
0.31
dation and decomposition (Td, 326 C) of glycosidic bonds and
the carbohydrate skeleton from 300-350 ◦C corresponding to ap-
proximately 70% mass loss. Heating from 400-600 ◦C afforded a
further, smaller, mass loss (~5%). Approximately, 18% of residue
remained at the end of the analysis. The thermal decomposition
of the subsequent compounds, i.e., succinimidyl carbonate
starch (6a) and ligand grafted starch (13a) showed a similar de-
composition profile to expanded starch (4a). Both show an initial
1.18
Mössbauer Spectroscopy
◦
mass (approx. 5%) from 25-135 C again due to bound water
The Mössbauer spectra for expanded HACS (1a, ESI Figure S7,
top) and S350 fabricated catalysts (1b, ESI Figure S7, bottom),
respectively, confirms the presence of iron in the form of Fe3+. A
characteristic isomer shift of 0.47 coupled with quad. splitting of
0.81 are representative of Fe3+.[27] The presence of iron, as well
as oxygen and nitrogen, was further evidenced by XPS anal-
yses.
within starch but a small and gradual mass loss is observed from
135-200 ◦C, which may be due to residual propylene carbonate
solvent. Interestingly, conversion of (4a) to (6a), lowers the de-
composition temperature from 326 ◦C (4a) to 321 ◦C (6a) indicat-
ing that the structure and packing within starch has been dis-
rupted. In particular, the extensive inter- and intra-hydrogen
bonding network associated with the hydroxyl groups of starch
is disrupted as modification (DS, 0.33±0.11%) reduces the num-
ber of hydroxyl groups. Evidence for substitution may also be
considered by the fact that approx. 24% of residue is left at the
X-Ray Photoelectron spectroscopy
Survey Scan
◦
end of decomposition (600 C) of (6a) compared with approx.
18% residual mass for (4a). The extra mass is from the additional
elemental contribution (mainly carbon) from succinimidyl car-
bonate moiety. Similarly, the greatest residual mass is observed
for decomposition of (13a) which has the greatest proportion of
carbon with respect to compounds (4a) and (6a), which also has
the lowest decomposition temperature (Td, 300 ◦C) due to great-
est structural and chemical modification.
The XPS survey spectrum of expanded HACS (4), (ESI, Figure
S8 A) and Starbon™ 350 (ESI, Figure SB B), have two main ab-
sorption bands typically at 283.45 and 530.87 eV corresponding
to C1s and O1s energy levels, respectively. However, the Fe-
NHC expanded HACS catalyst (1a) (ESI, Figure S9 A) and Fe-
NHC Starbon™ 350 catalysts (1b) (ESI, Figure S9 B) has four
absorption peaks at about 283.45, 399.88, 533.78 and 711.34
eV corresponding to C1s, N1s, O1s and Fe2p, respectively,
providing clear evidence for the presence of iron and nitrogen in
addition to the expected carbon and oxygen already in the un-
modified supports.
The thermogram for the Fe-NHC catalyst (1a) shows slight en-
hancement in thermal stability (Td, 301 ◦C) with respect to its pre-
cursor ((13a), Td 300 ◦C) which may be related to additional en-
ergy associate with iron-carbene complexation, i.e., Fe-NHC. In-
◦
terestingly, slightly less residual mass was observed at 600 C
for (1a) than for (13a) probably due to iron itself triggering or cat-
alyzing decomposition of starch. However, this is to be further
investigated.
Carbon 1s peaks
Comparing the fitted carbon 1s XPS spectra of the unmodified
expanded HACS (4a) and S350 (4b) supports to that of Fe-NHC
modified expanded HACS (1a) and S350 (1b) (ESI, Figure S10-
S11), additional carbon peaks at 283.6, 286.1 and 289.2 as-
signed to C-C(Ar), C-N and C=O are observed.[28] These addi-
tional peaks are believed to come from compound (12) attached
on the expanded HACS.
Similarly, the thermograms for the equivalent Starbon™ 350
tethered materials are shown in ESI, Figure S6. An early mass
loss was observed at approximately 40 oC to 120 oC due to loss
of both physio and chemisorbed water in each of the Starbon™
materials. The unmodified Starbon™ 350 (4b) was fairly stable
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10.1002/cssc.201702207
ChemSusChem
Oxygen 1s peaks
that the extent of pitting and mesh-like network (surrogate for
porosity) decreases on the surface from (4a) to (1a) which may
also be related to the corresponding decrease in BET surface
area discussed earlier. Similarly, figure 1 (bottom images) shows
SEM of Starbon™ 350 (4b), ligand grafted Starbon™ 350 (13b)
and Fe-NHC Starbon™ 350 (1b), indicating also that Starbon™
350 support (4b) also contains a network of pores and that the
porosity is retained during modification process.
The fitted O1s spectrum of expanded HACS (4a) (ESI, Figure
S12, LHS), shows two peaks at 531.5 and 533.5eV assigned to
C-O and O-H bonds in the expanded HACS. However, the XPS
peak of the Fe-NHC expanded HACS catalyst (1a) ((ESI, Figure
S12, RHS), shows additional peak at 533 eV corresponding to
C=O.29 Another important and striking difference is the decrease
in the O-H peak intensity which confirms the loss of some hy-
droxyl groups from the expanded HACS (4a) due to activation.
This decrease in the intensity of the hydroxyl peak in the ex-
panded HACS and the corresponding appearance of a new C=O
peak in the Fe-NHC expanded HACS, indicates and further
prove the successful attachment of (12) on the expanded HACS
surface leading to the formation of (1a).
The O1s spectrum of the Starbon™ 350 (4b) (ESI, Figure S13,
LHS), shows three peaks at 529.5, 531.5 and 534.2 eV assigned
to C=O, C-O and O-H bonds, respectively. [29] However, the de-
convolution of the XPS peak of the Fe-NHC Starbon™ 350 (1b)
(ESI, Figure S13, RHS) shows another important and striking dif-
ference with the decrease in the O-H peak intensity which con-
firms the loss of some hydroxyl groups from the Starbon™ 350
support (4b). This decrease in the intensity of the hydroxyl peak
and the corresponding increase of the C=O peak in the Fe-NHC
Starbon™ 350, indicates and further prove the successful at-
tachment of 12 on the Starbon™ 350 support (4b).
Figure 1. SEM micrographs of (a) expanded HACS (4a), (b) Ligand grafted
expanded HACS (13a), (c) Fe-NHC expanded HACS (1a), (d) Starbon™ 350
(4b), (e) Ligand grafted Starbon™ 350 (13b) and (f) Fe-NHC Starbon™ 350
(1b).
Nitrogen 1s peaks
Catalytic activity: Fructose (2) to HMF (3) conversion
The appearance of a nitrogen 1s peak at 400eV in the XPS sur-
vey spectra of the Fe-NHC expanded HACS (1a) and Fe-NHC
Starbon™ 350 (1b) which was originally absent in the survey
spectrum of the unmodified expanded HACS (4a) and Starbon™
350 (4b) supports, is a clear indication that a new compound
containing nitrogen has been attached to the expanded HACS
and Starbon™ 350 supports. Further deconvolution of the N1s
peak (ESI, Figures S14 and S15 LHS) indicates three different
nitrogen bonding environments at 398.3, 399.2 and 401.3eV as-
signed to N-H, C-N, and C=N, respectively. This however, is be-
lieved to have come from compound 12 attached to the supports.
Initially, a series of control experiments were undertaken in the
absence of fructose (2), i.e., DMSO-d6 and Fe-NHC catalyst (1a),
in order to confirm the carbohydrate (starch) support was itself
not acting as a sacrificial or competing substrate with respect to
fructose and to investigate iron leaching. However, no significant
change was observed. The filtrate was analysed by ICP-MS to
reveal negligible trace of iron in solution (79.49 ppb) which may
be due to surface trapping of residual iron(III) chloride and/or iron
oxide. Nevertheless, it can be assumed that the Fe-NHC catalyst
(1a) is neither being sacrificed nor does it leach any complexed
iron.
Iron 2p peaks
In addition, an insitu NMR study was conducted to investigate
influence of temperature which revealed temperature depend-
ency for fructose to HMF conversion. No significant conversion
The appearance of a Fe doublet peak with the Fe 2p1/2 at 711eV
in the spectra of the Fe-NHC expanded HACS (1a) and Fe-NHC
Starbon™ 350 (1b) (ESI, Figures S14 and S15 RHS) confirms
◦
[30-
was detected below 80 C. The best temperature that gave
the presence of iron predominantly in the +3 oxidation state.
clearly discernible 1H NMR signals for HMF (3) was from 80-100
◦C. When held at this temperature then increasing time shows a
corresponding increase in signals for HMF (ESI, Figure S16 A
and B). Thus, armed with this knowledge the dehydration reac-
tion was further investigated at 100 ◦C.
32]
Nitrogen Porosimetry and SEM
The BET surface area, pore width and pore volume of expanded
HACS (4a) and its subsequent Fe-NHC modified form, (1a) were
determined using nitrogen adsorption porosimetry (see Table 1
in ESI). On modification of expanded HACS (4a) through to the
desired Fe-NHC (1a) the BET surface area decreased: 186.7
m2/g (4a); and; 135.5 m2/g (1a). The subsequent decrease in
surface area and pore volume may be explained by possible
blocking and filling of the porous structure both by the ligand and
importantly iron. The pore size distribution shows a broad peak
within the mesoporous region (2-50 nm). [33] The BET surface
area of Starbon™ 350 also shows a corresponding decrease
with modification, 332 m2/g (4b) to 138 m2/g (1b). These data
prove that activation of the expanded HACS, grafting of the lig-
and and coordination of iron maintains porosity with significant
retention of mesopores.
In an atmosphere of air (1H and 13C NMR study)
The results and discussion introduced in this section represent
the dehydration of D-fructose (2) (180 mg, 1mmol) catalysed by
Fe-NHC (1a) (38 mg, 0.01 mmol) Fe-NHC (1b) (32 mg, 0.01
mmol) at 100 ◦C in DMSO-d6 (4 mL) for 10 min, 20 min, 30 min,
1 h, 3 h and 6 h in an air atmosphere. The progress of the dehy-
dration reaction was monitored by nmr spectroscopy (1H and
13C) both insitu and batch. Figure S16A (ESI), shows the stacked
◦
1H NMR spectra at 100 C for the dehydration of fructose (2) in
the presence of Fe-NHC catalyst (1a or 1b) supported on ex-
panded HACS and Starbon™ 350 respectively at t= 10 min, 20
min, 0.5 h, 1 h, 3 h and 6 h. At a purely qualitative level the
dehydration of fructose (2) is characterized by a darkening of the
reaction colour from clear pale yellow (0.5 h) to light brown (1 h
to 3 h) to dark brown (6 h) over time. The onset of the dark brown
colour is characteristic of HMF decomposition to levulinic acid
and humins (see later) as discussed earlier. [34,35]
To further investigate changes in surface structure (topography)
and porosity, SEM (figure 1) was performed on expanded HACS
(4a), ligand immobilized HACS (13a) and Fe-NHC catalyst (1a).
Although, the images reveal limited information it can be seen
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10.1002/cssc.201702207
ChemSusChem
1H NMR (DMSO-d6 ,400 MHz): 9.49 (s, 1 H), 7.44 (d, J=3.2 Hz,
1 H), 6.55 (d, J=3.2 Hz, 1 H), 4.46 ppm (s, 2 H).
The signal at 2.45 ppm is assigned to DMSO- d6, [36] whilst com-
plex multiplets for fructose are in the region 3-5 ppm. [37] As can
be seen from figure S16 A (ESI), corresponding to fructose con-
version using the Fe-NHC expanded HACS (1a), weak signals
at 7.44 ppm, 6.55 ppm and 4.46 ppm starts to appear at 10 min
reaction time which develop in intensity as the reaction proceeds
over time. HMF (3) is clearly evident at 30 min ((1 H, H-C=O,
9.49 ppm), (1H, O=C-C=CH, 7.44 ppm), (1H, H2C-C=CH-, 6.55
ppm) and (2H, HO-CH2 -C=CH, 4.46 ppm)) coupled with
significant reduction in the signals for fructose. The signal at 8.08
ppm was assigned to formic acid which is formed due to the ad-
dition of two water molecules as a consequence of the reverse
hydrolysis reaction that is considered as one of the side reac-
tions of fructose conversion to HMF. [38 39] Interestingly, the inten-
sity of HMF signals starts to decline slightly after 1 h reaction
time which may be due to the re-hydration side reaction to formic
acid and levulinic acid. This observation is confirmed by HPLC
results given later in which the HMF yield calculated after 1 h
reaction time is less than that at t = 1 h.
However, in figure S16 B (ESI) corresponding to fructose con-
version with Fe-NHC Starbon™ 350 (1b), HMF signals were ev-
ident at t= 20 min, coupled with almost the disappearance of the
signals from fructose at 3.3-4.3 ppm. Interestingly, the intensity
of HMF signals also declined after 1 h reaction time which may
be due to the re-hydration side reactions.
The complementary 13C NMR investigation (ESI, Figures S17 A
and B) also revealed a decrease in signals of fructose coupled
with an increase in signals for HMF with respect to time. The
signals for DMSO-d6 are present at 39.5 ppm whilst those for
fructose are in the range 60 - 105 ppm. HMF signals were evi-
dent at t=0.5 h (56.43 ppm, HO-CH2-C=CH-), (110.24 ppm, HO-
CH2-C=CH-), (152.25 ppm, O=HC-CH=C-), (162.66 ppm, HO-
[40]
CH2-C=CH2-) and 178.53 ppm, O=CH-C=CH)).
Signals at
31.19 ppm and 204.9 ppm are attributed to a carbonyl containing
species yet to be fully determined. The signal at 163.45 ppm
1
most likely corresponds to formic acid which agrees with its H
NMR peak observed.
In our situation, conversion occurs at 80-100 ◦C in the presence
of Fe-NHC catalyst (1). Based on the work of Guan et al., [40] we
propose the following mechanism (Figure 2) for conversion of
fructose (2) to HMF (3). The iron (Fe3+) coordinates with the car-
bonyl and adjacent OH within fructose to form a metal fructo-
furanose complex. A series of three dehydrations (-3 H2O) in-
duced by the catalyst attaching and detaching the fructose ring
structure affords the desired HMF (3).
Figure 2. Proposed mechanism for fructose dehydration to HMF with Fe-NHC
catalyst (1a-b)
Quantification via HPLC
The progress of the reaction was also monitored by HPLC to
better investigate yield and selectivity. The results are repre-
sented graphically in Figure 3a and b. In Figure 3a, shows that
at 0.5-1 h, high HMF yield and selectivity (99 %) was obtained
thus indicating effectiveness of our novel Fe-NHC (1a) as a cat-
alyst for fructose (2) dehydration to HMF (3). The high selectivity
at this time could be explained by the low probability of re-hydra-
tion at this point compared to proceeding times. The best HMF
yield with respect to time 86 %, TOF=206 h-1 was obtained at t=
0.5 h. Thereafter, although fructose conversion is the highest
(97 %) for 6 h reaction time both HMF yield and selectivity drop
significantly. As proposed earlier this may be due to re-hydration
to formic acid at 4.74 min retention time which is supported by
the NMR analysis discussed before. Some other side products
are recorded at different retention times. Among them, 3-furoic
acid is identified at 7.88 min with 4.24% area.
This article is protected by copyright. All rights reserved.

10.1002/cssc.201702207
ChemSusChem
without any significant loss in catalytic activity. HPLC results re-
ported 83 %, 82 %, 83%, 81%, and 81% HMF yields over 5 cy-
cles.
Figure 3. Fructose conversion, HMF yields and selectivity over Fe-NHC ex-
panded HACS (1a, Top a) and Fe-NHC Starbon™ 350 (1b, Bottom b) cata-
lysts. Conditions: D-fructose (180 mg, 1mmol), Fe-NHC (1) (32 mg, 0.01 mmol)
at 100 ◦C in DMSO-d6 (4 mL).
Figure 3b, which corresponds to the catalytic conversion using
the Fe-NHC Starbon™ 350 catalyst (1b), shows best catalytic
activity with HMF yield of 81.5 % (TOF=169 h-1), fructose con-
version of 95 % and HMF selectivity of 85.7 % at t=0.5 h. No
significant changes occur on taking the reaction further to t=1 h.
Thereafter, at t=3 h and t=6 h, both HMF yield and selectivity
drops. As earlier stated, may be due to rehydration as a side
reaction.
Figure 4. Reusability data for Fructose conversions, HMF yields and selec-
tivities over Fe-NHC expanded HACS (1a, Top a) and Fe-NHC Starbon™
350 (1b, Bottom b). Conditions: D-fructose (180 mg, 1mmol), Fe-NHC (1a)
(38 mg, 0.01 mmol) Fe-NHC (1b) (32 mg, 0.01 mmol) at 100 ◦C in DMSO-d6
(4 mL), time 1 h.
Fe-NHC catalyst (1) comparison with other heterogeneous
catalysts (Amberlyst-15, Montmorillonite K-10 and ZSM-5
(Si:Al = 30)
Catalyst recycling and reuse
The acid-catalysed dehydration of fructose (2) to HMF (3) has
been well investigated using a variety of heterogeneous cata-
lysts, namely: Amberlyst-15, Montmorillonite K10 and ZSM-5
(SiO2:Al2O3=30). Thus, to compare activity between these cata-
lysts and our catalyst (1), a series of standard reactions were
undertaken using the same amount of fructose and DMSO at
100 ◦C with the appropriate catalyst. Literature data was not used
because lack of knowledge of exactly how the study was per-
formed would add huge uncertainty when trying to compare data.
Our methodology removes variables and introduces consistency
of approach. Conditions: D-fructose (180 mg, 1mmol), catalyst
loading (32 mg) at 100 ◦C in DMSO-d6 (4 mL). The comparison
was based on equal amount by weight of the catalysts employed.
HPLC results for the comparative study are shown in Figure 5a-
c. Over Amberlyst-15, HMF yield occur at t=1 h and thereafter
no significant increase in yield up to t=6 h. The behavior of Mont-
morillonite K-10 is almost a linear increment but with a bit of ab-
normality at t=1 h at which the yield and selectivity seemed to
drop a little before a progressive increase with time up to t=6 h.
Catalyst recycling and re-use was investigated in DMSO-d6 at
100 ◦C with each experiment being monitored by NMR (qualita-
tive) and HPLC (quantitative, Figures 4a and b). Conditions: D-
fructose (180 mg, 1mmol), Fe-NHC (1a) (38 mg, 0.01 mmol), Fe-
NHC (1b) (32 mg, 0.01 mmol) at 100 ◦C in DMSO-d6 (4 mL), time
1 h. Figure 4a shows that the desired Fe-NHC expanded HACS
catalyst (1a) can be re-used up to four times (4x) without
significant loss in performance; HPLC results reported 73.77%,
66.82%, 71.85%, 72.37% and 46.76% HMF yields with lowest
fructose conversion of 88.03% for the second run and lowest se-
lectivity of 52.81% for the last cycle. The NMR spectra reported
show the existence of HMF signals in all 5 cycles of use. Hot
filtration and ICP analysis of the leachate shows no discernible
amounts of Fe (79.49 ppb), thus showing the integrity of our cat-
alytic system.
Figure 4b corresponding to Fe-NHC Starbon™ 350 catalyst (1b)
shows that the catalyst can also be re-used for up to five times
This article is protected by copyright. All rights reserved.

10.1002/cssc.201702207
ChemSusChem
ZSM-5 catalyst attains a good yield at t=6 h but no significant
yield up to t=3 h. Expanded HACS supported Fe-NHC catalyst
(1a) had its highest HMF yield at t= 1 h. However, for S350 Fe-
NHC catalyst (1b) the fructose conversion, HMF yield and HMF
selectivity, starts very high at initial reaction time of t=20 min to
1 h and thereafter the HMF yield start to drop significantly with
increasing reaction time.
TEMPERATURE AND REACTION TIME STUDY ON THE
FRUCTOSE TO HMF CONVERSION WITH Fe-NHC S350.
Figure 6a-c shows the influence of reaction temperature and
time on the fructose conversion, HMF yield and HMF selectivity
respectively, using the fabricated Fe-NHC Starbon™ 350 (1b) as
reaction catalyst. Conditions: D-fructose (180 mg, 1mmol), Fe-
o
NHC (1) (32 mg, 0.01 mmol) at 80 C & 100 ◦C in DMSO-d6 (4
mL). It can be inferred from the results above that increasing the
o
reaction temperature from 80 to 100 C is very much beneficial
if not crucial to achieving higher conversion, yield and selectivity
of 96 %, 82 % and 85 % respectively, within shorter reaction time
of 0.5-1 h as compared to fructose conversion, HMF yield and
HMF selectivity of 53 %, 10 %, and 17 % respectively at reaction
time 0.5-1 h, when explored at a lower reaction temperature of
80 oC. At a lower reaction temperature of 80 oC, longer reaction
time of 3 h is necessary to achieve good HMF yield of 75 %, as
compared to 82 % HMF yield within just 0.5 h when explored at
100 oC.
Figure 5. Comparative study of the fructose to HMF conversion (a), HMF Yield
(b) and selectivity (c) with other catalysts. Conditions: D-fructose (180 mg, 1
mmol), each catalyst (32 mg) at 100 ◦C in DMSO-d6 (4 mL).
This article is protected by copyright. All rights reserved.

10.1002/cssc.201702207
ChemSusChem
ogeneous catalytic conversion of fructose (2) to HMF (3) was in-
vestigated. A good yield of HMF (86 %) was achieved at 100 oC
over 0.5 h reaction time using the expanded HACS immobilised
Fe-NHC catalyst (Ia) . However, the S350 immobilised Fe-NHC
(1b) also gives HMF yield of (81.7 %) at reaction time of 0.5 h.
Further work should be focused on optimization of the reaction
conditions and catalyst/substrate loading to achieve higher HMF
conversions and selectivity over even shorter reaction time. This
however, is part of our future work.
Experimental Section
The synthesis of Fe-NHC (1a-b) using a convergent strategy,
from either expanded high amylose corn starch (HACS) (4a) or
Starbon-350 (4b) and N-(3-aminopropyl) imidazole (7), is de-
picted in Scheme 1. Starbon™ 350 was prepared in-house ac-
cording to literature methods, where, ‘350’ signifies carbonisa-
tion temperature. All intermediates and final products were char-
acterized by a variety of techniques. Instrument details, param-
eters and experimental procedures are detailed in ESI.
Figure 6. Fructose conversion (A), HMF yield (B) and HMF selectivity (C) at 80
oC and 100 oC for Starbon™ 350 catalyst (1b)
Taking reaction time into consideration, figure 6a-c indicates that
at 100 oC, prolonging the reaction after 1 h does not result in any
beneficial effect but actually results in decrease in HMF yield and
Acknowledgements
ASM acknowledges Dr Brian A. Schaefer of Princeton University
for assisting us with Mossbauer spectroscopy of the fabricated
catalysts. SA acknowledges the Nigerian TETFUND and the
Department of Chemistry Wild Fund for PhD study.
o
selectivity. At 80 C reaction temperature however, good fruc-
tose conversion, HMF yield and HMF selectivity were only
achieved after a longer reaction time of 3 h. Prolonging the reac-
tion from 3 h also resulted in decreasing yield and selectivity.
Keywords Starbon™ • High amylose corn starch (HACS) •
5-(hydroxymethyl) furfural • Fructose • Fe-N-heterocyclic car-
bene
Associated content
The experimental section, ATR FT-IR spectra, 13C CP-MASS
NMR, thermalgravity thermograms and XPS deconvolution data
can be obtained from the ESI (electronic supplementary
information).
Corresponding Author
* Email. Avtar.matharu@york.ac.uk
Author Contributions
The manuscript was written through contributions of all authors.
Figure 7. Reaction kinetic curve (yield vs time) for Fe-NHC HACS and Fe-
NHC-StarbonTM-350.
Notes
The authors declare no competing financial interests.
The kinetic profile (figure 7) based on HMF yield efficiency for
two different supports employed, showed a similar HMF yield
with slightly higher yield obtained with the Starbon™ 350 support
at initial reaction time of 20 minutes. Highest yield was obtained
with the HACS support at a later reaction time of 1 h. Both the
two supports show a similar profile with yields falling after 1 h
reaction time.
Abbreviations
HACS, high amylose corn starch; S350, Starbon™ 350; NHC, N-
heterocyclic carbene; HMF, 5-(hydroxymethyl) furfural; IL,
imidazolium ligand; DSC, disuccinimidyl carbonate; TFA,
triflouro acetic acid; CPME, cyclopentyl methyl ether; TEA,
triethylamine; TOF, turnover frequency.
Conclusions
The proposed HACS supported Fe-NHC catalyst (1a) and S350
supported Fe-NHC catalyst (1b) were fabricated successfully as
confirmed by the various characterization techniques employed.
Both mesoporous supports were effective for the immobilization
of the Fe-NHC due to enhanced mesoporosity which offered
functional space and overcome diffusion limitation for bulky re-
actants or products. The performance of the catalysts for heter-
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