inorganics
Article
Direct Catalytic Conversion of Cellulose to
5-Hydroxymethylfurfural Using Ionic Liquids
Sanan Eminov 1,2 , Paraskevi Filippousi 2 , Agnieszka Brandt 2 , James D. E. T. Wilton-Ely 1, *
and Jason P. Hallett 2, *
1
2
*
Department of Chemistry, Imperial College London, London SW7 2AZ, UK; [email protected]
Department of Chemical Engineering, Imperial College London, London SW7 2AZ, UK;
[email protected] (P.F.); [email protected] (A.B.)
Correspondences: [email protected] (J.D.E.T.W.-E.); [email protected] (J.P.H.)
Academic Editor: Andreas Taubert
Received: 21 June 2016; Accepted: 11 October 2016; Published: 20 October 2016
Abstract: Cellulose is the single largest component of lignocellulosic biomass and is an attractive
feedstock for a wide variety of renewable platform chemicals and biofuels, providing an alternative
to petrochemicals and petrofuels. This potential is currently limited by the existing methods
of transforming this poorly soluble polymer into useful chemical building blocks, such as
5-hydroxymethylfurfural (HMF). Ionic liquids have been used successfully to separate cellulose
from the other components of lignocellulosic biomass and so the use of the same medium for the
challenging transformation of cellulose into HMF would be highly attractive for the development
of the biorefinery concept. In this report, ionic liquids based on 1-butyl-3-methylimidazolium
cations [C4 C1 im]+ with Lewis basic (X = Cl− ) and Brønsted acidic (X = HSO4 − ) anions were used
to investigate the direct catalytic transformation of cellulose to HMF. Variables probed included the
composition of the ionic liquid medium, the metal catalyst, and the reaction conditions (temperature,
substrate concentration). Lowering the cellulose loading and optimising the temperature achieved
a 58% HMF yield after only one hour at 150 ◦ C using a 7 mol % loading of the CrCl3 catalyst.
This compares favourably with current literature procedures requiring much longer reactions times
or approaches that are difficult to scale such as microwave irradiation.
Keywords: biorenewables; platform chemicals; catalysis; biorefinery; lignocellulose
1. Introduction
The biorefinery concept requires routes that are efficient in terms of both materials and energy
for the conversion of biomass to useful platform chemicals [1,2]. Lignocellulosic biomass represents
a promising renewable feedstock for commercial large-scale biorefining, as it is diverse, widely
distributed, and can be grown on a billion ton scale [1]. Agricultural byproducts (e.g., wheat straw,
sugarcane bagasse, corn stover) or bioenergy crops, which do not rely on the use of arable land
(e.g., Miscanthus, switchgrass, tree wood), avoid competition with food production (the food vs. fuel
debate) and result in a higher reduction of net CO2 emissions. Unfortunately, the processing
of lignocellulosics requires energetically demanding pretreatment routes to separate lignin from
cellulose [3], and subsequent cellulosic sugar and biofuel production relies on slow biocatalytic
transformations (enzymatic saccharification and microbial fermentation) [4].
Cellulose and hemicellulose make up the largest part of lignocellulosic biomass and account for
50%–80% of the total mass [5]. Many recent papers have reviewed the use of ionic liquids for accessing
the cellulose in lignocellulosic biomass; however, these articles focus mainly on the advantages of ionic
liquids in the isolation of cellulose compared with traditional methods [6–12]. For example, it has
been shown that ionic liquids can decrystallise the cellulose portion of biomass and disrupt linkages
Inorganics 2016, 4, 32; doi:10.3390/inorganics4040032
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For example, it has been shown that ionic liquids can decrystallise the cellulose portion of biomass
to the hemicellulose and lignin portion [6], while George et al. studied the possibility of removing
and disrupt linkages to the hemicellulose and lignin portion [6], while George et al. studied the
ligninpossibility
from theof
other
biomass constituents using ionic liquids and selectively extracting the lignin and
removing lignin from the other biomass constituents using ionic liquids and selectively
hemicellulose
two aspects
could
combine
withaspects
the present
to enable
extracting fractions
the lignin[13].
and These
hemicellulose
fractions
[13].
These two
could research
combine with
the the
directpresent
synthesis
of
platform
chemicals,
such
as
5-hydroxymethylfurfural
(HMF),
from
biomass.
research to enable the direct synthesis of platform chemicals, such as
The
hydrolysis of cellulose
andfrom
hemicellulose
produces different sugar monomers, from which
5-hydroxymethylfurfural
(HMF),
biomass.
The hydrolysis
of cellulose
andcould
hemicellulose
produces
different
sugarhave
monomers,
from which
a wide range
of important
chemicals
be produced.
Several
papers
been published
on the
a wide
range of important
chemicals couldweight
be produced.
Several papers
have beenwith
published
the
chemical
transformation
of low-molecular
carbohydrates
to chemicals
usefulon
industrial
chemical
transformation
weightand
carbohydrates
tostudied
chemicals
usefultransformation
industrial
application
profiles
[14–23]. of
Forlow-molecular
example, Dumesic
co-workers
thewith
catalytic
application profiles [14–23]. For example, Dumesic and co-workers studied the catalytic
of biomass-derived oxygenated feedstocks into useful fuels and chemicals. These processes
transformation of biomass-derived oxygenated feedstocks into useful fuels and chemicals. These
included hydrolysis, dehydration, isomerisation, aldol condensation, reforming, hydrogenation,
processes included hydrolysis, dehydration, isomerisation, aldol condensation, reforming,
and oxidation
[24]. Aand
summary
of the
methods
to catalytically
transform
lignocellulose
hydrogenation,
oxidation
[24].alternative
A summary
of the used
alternative
methods used
to catalytically
biomass
is
shown
in
Figure
1
[25].
transform lignocellulose biomass is shown in Figure 1 [25].
Figure
1. Lignocellulosicpathways.
pathways. A
A scheme
scheme for
biorefinery
[25].[25].
Figure
1. Lignocellulosic
foraachemo-catalytic
chemo-catalytic
biorefinery
Various review articles have provided an overview of key areas, such as the synthesis and use
Various
articles
providedand
an cellulose
overview[26]
of key
such as
thefor
synthesis
andofuse of
of sugar review
derivatives
fromhave
hemicellulose
and areas,
the methods
used
conversion
sugarbiomass
derivatives
and
cellulose[22,27].
[26] and
the methods
used for
conversion
of biomass
wastefrom
intohemicellulose
useful platform
chemicals
Among
the important
bio-derived
building
wasteblocks,
into useful
platform chemicals
[22,27].
Among
the as
important
bio-derived
building
blocks,
5-hydroxymethylfurfural
(HMF)
has been
reported
one of the
most promising
[28,29],
after the U.S. Department
of Energy
it as a major
chemical
that could
be derived
5-hydroxymethylfurfural
(HMF)
has identified
been reported
as oneplatform
of the most
promising
[28,29],
after the
from lignocellulosic
biomass
on a large
[30].
HMF ischemical
a versatile
intermediate
between from
U.S. Department
of Energy
identified
it as ascale
major
platform
that
could be derived
biomass-based
carbohydrate
the
industrial
chemistry,
with
lignocellulosic
biomass
on a largechemistry
scale [30].and
HMF
is apetroleum-based
versatile intermediate
between
biomass-based
widespread potential use in a variety of chemical manufacturing applications and industrial
carbohydrate chemistry and the petroleum-based industrial chemistry, with widespread potential use
products [31,32]. Most prominent among these is 2,5-furandicarboxylic acid (FDCA) [33], which can
in a variety of chemical manufacturing applications and industrial products [31,32]. Most prominent
be used to make renewable polymers [34] and liquid transport fuels, such as 2,5-dimethylfuran [35].
among
these is 2,5-furandicarboxylic acid (FDCA) [33], which can be used to make renewable
The potential of HMF as a synthetic building block is now widely recognised, which has led to a
polymers
and
liquidoftransport
fuels, such
[35].HMF
Theispotential
of HMF
surge [34]
in the
amount
research utilising
HMFasas2,5-dimethylfuran
a feedstock [36]. While
an attractive
as a synthetic
building
block
is
now
widely
recognised,
which
has
led
to
a
surge
in
the
amount
of
renewable building block, several byproducts from degradation reactions occur during or after HMF
research
utilising HMF
as atofeedstock
[36].ofWhile
HMF
is an
attractive
renewable
building
transformation,
leading
the formation
levulinic
acid,
formic
acid, and
humins [37,38],
andblock,
reducing
the overall
yield; this
is particularly
prevalent
acidic transformation,
aqueous conditions,
several
byproducts
fromHMF
degradation
reactions
occur during
orunder
after HMF
leading
the HMF
product combines
with unreacted
to produce
humins
[39].HMF
to thewhere
formation
of levulinic
acid, formic
acid, andsugars
humins
[37,38], water-insoluble
and reducing the
overall
HMF can be
producedunder
with sufficient
selectivity
and yieldwhere
from food-grade
fructose for
it
yield;However,
this is particularly
prevalent
acidic aqueous
conditions,
the HMF product
combines
to become a nascent industrial process [40]. Production from more prevalent, lower cost sugars (such
with unreacted sugars to produce water-insoluble humins [39]. However, HMF can be produced
as glucose) requires a two-step catalytic mechanism involving isomerisation and dehydration, under
with sufficient selectivity and yield from food-grade fructose for it to become a nascent industrial
Lewis basic and Brønsted acidic conditions (Figure 2) [41]. The solubility of HMF can also be an issue
process
[40]. sugar
Production
from
more prevalent,
sugars (such
as glucose)
requires
a two-step
at high
loadings
in aqueous
systems, lower
leadingcost
to byproducts
formed
from sugars
with
the
catalytic mechanism involving isomerisation and dehydration, under Lewis basic and Brønsted acidic
conditions (Figure 2) [41]. The solubility of HMF can also be an issue at high sugar loadings in aqueous
systems, leading to byproducts formed from sugars with the HMF. This has led some researchers to
explore removing HMF from the aqueous phase (which contains the sugars) using immiscible organic
solvents, thereby reducing humin formation [42].
Inorganics 2016, 4, 32
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HMF. This
Inorganics
2016,has
4, 32 led
some researchers to explore removing HMF from the aqueous phase (which
3 of 15
contains the sugars) using immiscible organic solvents, thereby reducing humin formation [42].
Figure
to 5-hydroxymethylfurfural
5-hydroxymethylfurfural (HMF)
(HMF) via
via fructose.
fructose.
Figure 2.
2. Transformation
Transformation of
of glucose
glucose to
While HMF production from sugar monomers is largely straightforward, the most desirable
While HMF production from sugar monomers is largely straightforward, the most desirable and
and challenging route to produce HMF involves widely available renewable sources such as
challenging route to produce HMF involves widely available renewable sources such as cellulose [43].
cellulose [43]. However, a high-yield, low-cost, energy-efficient, and direct conversion of cellulose
However, a high-yield, low-cost, energy-efficient, and direct conversion of cellulose into HMF is
into HMF is still a challenge and the subject of substantial research. Many different routes and
still a challenge and the subject of substantial research. Many different routes and methods have
methods have been proposed but the routes published to date are low-yielding, complex, and hence
been proposed but the routes published to date are low-yielding, complex, and hence high in cost or
high in cost or energy inefficient.
energy inefficient.
The proposed synthetic route from cellulose to HMF is a multistep approach consisting of the
The proposed synthetic route from cellulose to HMF is a multistep approach consisting of the
production of glucose from cellulose, followed by glucose isomerisation to fructose, and finally HMF
production of glucose from cellulose, followed by glucose isomerisation to fructose, and finally
formation from the dehydration of fructose [44]. Interestingly (in the context of this work), the
HMF formation from the dehydration of fructose [44]. Interestingly (in the context of this work),
conversion of cellulose to HMF has also been shown to occur in the absence of a catalyst using ionic
the conversion of cellulose to HMF has also been shown to occur in the absence of a catalyst using
liquids, albeit in low yields [33].
ionic liquids, albeit in low yields [33].
The key step in the conversion of cellulose to HMF (Figure 3) is the hydrolysis of cellulose to
The key step in the conversion of cellulose to HMF (Figure 3) is the hydrolysis of cellulose to
sugar molecules, which is hampered by the poor solubility of polymeric cellulose in most solvents.
sugar molecules, which is hampered by the poor solubility of polymeric cellulose in most solvents.
Acid-catalysed hydrolysis of cellulose is one method that has been studied widely, in which the
Acid-catalysed hydrolysis of cellulose is one method that has been studied widely, in which the
hydroxonium cations interact with the cellulose network, leading to breakdown of the polymer.
hydroxonium cations interact with the cellulose network, leading to breakdown of the polymer.
The most common acids used in cellulose hydrolysis are mineral acids (H2SO4, HCl) and
The most common acids used in cellulose hydrolysis are mineral acids (H SO4 , HCl) and organic
organic acids, such as p-toluenesulfonic or carboxylic acids. Sulfuric acid2 has
been used most
acids, such as p-toluenesulfonic or carboxylic acids. Sulfuric acid has been used most frequently
frequently (at elevated temperatures), which is unsurprising due to its low cost. While higher
(at elevated temperatures), which is unsurprising due to its low cost. While higher temperatures and
temperatures and longer reaction times are required with dilute acid [45], the treatment of cellulose
longer reaction times are required with dilute acid [45], the treatment of cellulose with concentrated
with concentrated acid accelerates the hydrolysis markedly, such that the polymer molecules
acid accelerates the hydrolysis markedly, such that the polymer molecules depolymerise to monomers
depolymerise to monomers and this substantially increases dehydration of the monomeric sugars to
and this substantially increases dehydration of the monomeric sugars to HMF [46]. However, due to
HMF [46]. However, due to low selectivity and corrosion concerns, dilute acid is preferable for the
low selectivity and corrosion concerns, dilute acid is preferable for the controlled hydrolysis of cellulose.
controlled hydrolysis of cellulose. Heterogeneous solid acid catalysts have also been used for the
Heterogeneous solid acid catalysts have also been used for the depolymerisation of cellulose [47,48],
depolymerisation of cellulose [47,48], though the catalytic activity and selectivity of solid acids are
though
the
activity
andcellulose
selectivity
of so
solid
acidsreaction
are verytimes
low for
poorlyfor
soluble
very low
forcatalytic
the poorly
soluble
and
longer
arethe
required
thesecellulose
systems
and
so
longer
reaction
times
are
required
for
these
systems
compared
with
liquid
acid
catalysts.
compared with liquid acid catalysts. Overall, while offering advantages for removal of the catalyst
Overall,
offering
advantages
for removal
of the catalyst
medium,
the
catalytic
from thewhile
medium,
the catalytic
activity
and selectivity
of suchfrom
solidthe
acids
are very
low
and soactivity
longer
and
selectivity
of
such
solid
acids
are
very
low
and
so
longer
reaction
times
are
required
for these
reaction times are required for these systems compared with liquid acid catalysts.
systems
compared
with
liquid acid
catalysts.
Besides
the acid
catalysed
depolymerisation
of cellulose, there exist other methods to achieve
Besidessuch
the as
acid
catalysed[49]
depolymerisation
of cellulose, there
exist other
to none
achieve
this effect,
solvolysis
and catalytic hydrothermal
liquefaction
[50].methods
However,
of
this
effect,
such
as
solvolysis
[49]
and
catalytic
hydrothermal
liquefaction
[50].
However,
none
these methods has provided to be an effective and selective direct (“one-pot”) chemical
of
these methods
has provided
to be
an effective
selective
(“one-pot”)
chemical
transformation
of cellulose
to useful
chemicals
such and
as HMF
and, direct
as a result,
they are
either
transformation
of
cellulose
to
useful
chemicals
such
as
HMF
and,
as
a
result,
they
are
either
uneconomical or environmentally unfriendly (or both). For this reason, the challenge remains to
find
uneconomical
or
environmentally
unfriendly
(or
both).
For
this
reason,
the
challenge
remains
to
a potentially economically viable method for cellulose conversion to useful products without
find
a potentially
economically
viable method for cellulose conversion to useful products without
significant
environmental
impact.
significant environmental impact.
Inorganics 2016, 4, 32
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Figure 3. Pathway to HMF from cellulose.
Figure 3. Pathway to HMF from cellulose.
Ionic liquids (ILs) [51] are being developed as alternative, potentially more commercially
Ionic liquids
(ILs) [51] aremedia
being developed
as alternative,
potentially
more
commercially
attractive
attractive
and low-emission
for separating
cellulose from
lignin due
to their
unique chemical
and
low-emission
media for separating
cellulose
from lignin
due to here
their combines
unique chemical
and solvent
and
solvent characteristics
[6–12,52–57].
The research
reported
ionic liquids
as
characteristics
The(relatively
research easily
reported
here combines
liquids
as effective
low with
impact
effective low[6–12,52–57].
impact solvents
handled,
negligibleionic
vapour
pressure,
recyclable)
solvents
(relatively
handled,
vapourofpressure,
recyclable)
with
low-toxicity
metal
low-toxicity
metal easily
catalysts
for the negligible
direct conversion
cellulose to
value-added
chemicals.
For this
purpose,
liquids
(Figureof4)cellulose
have been
studied under
conditions
in
catalysts
fortwo
theionic
direct
conversion
tosystematically
value-added chemicals.
Fordifferent
this purpose,
two ionic
order(Figure
to arrive4)athave
an effective
system for cellulose
The potential
for selectively
liquids
been systematically
studieddissolution.
under different
conditions
in order tomaking
arrive at
fromsystem
fructose
glucose using
ionic liquids
(ILs) as solvents
was firstmaking
demonstrated
by Zhao
et
anHMF
effective
fororcellulose
dissolution.
The potential
for selectively
HMF from
fructose
al.
[57],
who
used
catalytic
amounts
of
metal
salts
to
convert
sugars
to
HMF
in
[C
4
C
1
im]Cl,
with
or glucose using ionic liquids (ILs) as solvents was first demonstrated by Zhao et al. [57], who used
CrCl2 providing
HMF salts
yieldto
at convert
100 °C insugars
3 h. The
were
thanwith
thoseCrCl
achievable
in
catalytic
amounts 70%
of metal
to yields
HMF in
[C4higher
C1 im]Cl,
2 providing
◦
most
aqueous
systems,
due
to
the
suppression
of
HMF
hydrolysis
to
levulinic
and
formic
acids.
70% HMF yield at 100 C in 3 h. The yields were higher than those achievable in most aqueous
Chromium
salts
subsequently
verified
by other
groups as
theformic
preferred
[52–56].
systems,
due to
thewere
suppression
of HMF
hydrolysis
to levulinic
and
acids.catalyst
Chromium
salts
However, the perceived toxicity of chromium is a key issue for commercial application, although
were subsequently verified by other groups as the preferred catalyst [52–56]. However, the perceived
this is largely based on Cr(VI) compounds, whereas the Cr(III) used here is an essential trace metal
toxicity of chromium is a key issue for commercial application, although this is largely based on Cr(VI)
required for the formation of glucose tolerance factor and for insulin metabolism, and is therefore
compounds, whereas the Cr(III) used here is an essential trace metal required for the formation of
Inorganics 2016, 4, 32
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Inorganics 2016, 4, 32
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glucose tolerance factor and for insulin metabolism, and is therefore widely used as a nutritional
supplement
and animals
[58]. Additionally,
risk
of exposure
to metal salts dissolved
in
widely
usedfor
ashumans
a nutritional
supplement
for humansthe
and
animals
[58]. Additionally,
the risk of
ionic liquids
lowered
they are retained
the solvent
medium
extraction
of
exposure
to is
metal
saltsasdissolved
in ionic in
liquids
is lowered
as during
they are
retainedorinseparation
the solvent
the product.
medium
during extraction or separation of the product.
Figure
Figure 4.
4. Ionic
Ionic liquids
liquids used
used for
for the
the dissolution
dissolution and
and breakdown
breakdown of
of biomass
biomass in
in this
this work.
work.
Since ILs can be tuned or designed to suit a certain application, and Brønsted acids have
Since ILs can be tuned or designed to suit a certain application, and Brønsted acids
proven effective catalysts for fructose dehydration [55,59], the mildly acidic ionic liquid
have proven effective catalysts for fructose dehydration [55,59], the mildly acidic ionic liquid
1-butyl-3-methylimidazolium hydrogen sulphate, [C4C1im][HSO4] (Figure 4), was chosen as a
1-butyl-3-methylimidazolium hydrogen sulphate, [C4 C1 im][HSO4 ] (Figure 4), was chosen as a solvent
solvent system for the conversion of fructose to HMF
[52] with a low loading of chromium(III)
system for the conversion of fructose to HMF [52] with a low loading of chromium(III) chloride.
chloride. An excellent HMF yield of 96% was reported using this system after 3 h at◦100 °C without
An excellent HMF yield of 96% was reported using this system after 3 h at 100 C without any
any detectable trace of the common byproducts, levulinic acid, formic acid, or humins
detectable trace of the common byproducts, levulinic acid, formic acid, or humins (water-insoluble
(water-insoluble polymers formed from the reaction of HMF with the monosaccharide or a
polymers formed from the reaction of HMF with the monosaccharide or a reactive intermediate).
reactive intermediate). This imidazolium-based ionic liquid shares the
same anion ([HSO4]−) with a
This imidazolium-based ionic liquid shares the same anion ([HSO4 ]− ) with a family of ionic liquids
family of ionic liquids that have recently been identified as true low-cost options ([HNEt3][HSO4]
that have recently been identified as true low-cost options ([HNEt3 ][HSO4 ] and [C1 Him][HSO4 ]),
and [C1Him][HSO4]), with a similar cost to traditional organic solvents
such as acetone [60].
with a similar cost to traditional organic solvents such as acetone [60]. However, when using cellulose
However, when using cellulose as the feedstock for HMF formation, it appears that the key step is
as the feedstock for HMF formation, it appears that the key step is the dissolution and depolymerisation
the dissolution and depolymerisation of cellulose to glucose monomers rather than the glucose to
of cellulose to glucose monomers rather than the glucose to fructose isomerisation and subsequent
fructose isomerisation and subsequent fructose dehydration. Ionic liquids appear to overcome
fructose dehydration. Ionic liquids appear to overcome these problems as they can play the role of both
these problems as they can play the role of both solvent and catalyst. Therefore, our attention
solvent and catalyst. Therefore, our attention turned principally to Lewis basic ILs, such as those with
turned principally to Lewis basic ILs, such as those with halide anions. This approach is similar to
halide anions. This approach is similar to the interplay between acidity and basicity during glucose
the interplay between acidity and basicity during glucose to HMF transformation in all solvent
to HMF transformation in all solvent systems, including ILs, with the coordinating ability of the IL
systems, including ILs, with the coordinating ability of the IL anion used to promote glucose
anion used to promote glucose isomerisation. For example, the Lewis acidic metal catalyst SnCl4 has
isomerisation. For example, the+ Lewis acidic metal catalyst SnCl4 has been used in ILs with
a
been used
in ILs with a [C4 C1 im] cation and a range of anions [53] where the− non-coordinating BF4 −
+
[C4C1im] cation and a range of anions [53] where the non-coordinating BF4 anion gave the highest
anion gave the highest yield from glucose. This is in contrast to the acid-catalysed mechanism for
yield from glucose. This is in contrast to the acid-catalysed mechanism for HMF production from
HMF production from fructose reported previously. It was therefore decided that the solvent system
fructose reported previously. It was therefore decided that the solvent system best suited to direct
best suited to direct cellulose conversion to HMF would draw inspiration from those used to achieve
cellulose conversion to HMF would draw inspiration from those used to achieve high yields from
high yields from glucose and capable of some cellulose dissolution, such as chloride-based systems.
glucose and capable of some cellulose dissolution, such as chloride-based systems.
2. Results and Discussion
2. Results and Discussion
2.1. Catalyst and Ionic Liquid Selection
2.1. Catalyst and Ionic Liquid Selection
Our previous study on monosaccharides achieved a yield of 96% HMF from fructose and 90%
Our
previous
study onsulphate-based
monosaccharides
a yieldILs,
of 96%
HMF from
fructose
and 90%
from glucose
by employing
andachieved
chloride-based
respectively
[52,61].
The next
goal
from
glucose
by
employing
sulphate-based
and
chloride-based
ILs,
respectively
[52,61].
The
next
was to find suitable conditions and ILs for the conversion of cellulose, which is the most important
goal
to find suitable
conditions
ILs abundance
for the conversion
cellulose, of
which
is the
most
targetwas
of renewable
chemicals
due to and
its high
[1]. The of
conversion
cellulose
is more
important
target
renewable
chemicals
due toto
itsbe
high
abundance
The conversion
of cellulose is
challenging
as itof
requires
suitable
conditions
dissolved
and [1].
hydrolysed
to monosaccharides
more
challenging
as
it
requires
suitable
conditions
to
be
dissolved
and
hydrolysed
to
before it can be converted to HMF (Figure 5).
monosaccharides
before
it can
be converted
to HMF
(Figure
The experiments
were
carried
out in ionic
liquids
with 5).
a low water content; however, rigorous
The
experiments
were
carried
out
in
ionic
liquids
with
a
lowofwater
content;used
however,
rigorous
exclusion of moisture during the reactions was employed. Many
the catalysts
were added
in
exclusion
of
moisture
during
the
reactions
was
employed.
Many
of
the
catalysts
used
were
added
in
their hydrated form.
their hydrated form.
Inorganics 2016, 4, 32
6 of 15
Inorganics 2016, 4, 32
6 of 15
Figure 5. Transformation of cellulose to HMF via glucose and fructose.
Figure
5. Transformation of cellulose to HMF via glucose and fructose.
Initially, the conditions and IL used were kept exactly the same as in our previous studies on
Initially,
the conditions
and to
ILcellulose
used were
exactly
the same
as in compare
our previous
studies on
monosaccharides
and applied
as akept
substrate
in order
to directly
HMF yields
from celluloseand
to those
from
Two ionic
liquids
4C1im][HSO
4] and
[C4Cyields
1im]Cl from
monosaccharides
applied
to monosaccharides.
cellulose as a substrate
in order
to [C
directly
compare
HMF
were to
examined
with monosaccharides.
the use of chromium,Two
zinc,ionic
and copper
salts,
have been
reported
good were
cellulose
those from
liquids
[C4which
C1 im][HSO
[C4 C1asim]Cl
4 ] and
catalysts
for
the
dehydration
of
sugars
to
HMF
[52],
and
aliquots
were
collected
after
1.5
and
3
h.
examined with the use of chromium, zinc, and copper salts, which have been reported as good The
catalysts
samples
were
not
collected
before
1
h
because
the
dissolution
of
cellulose
was
slower
than
expected
for the dehydration of sugars to HMF [52], and aliquots were collected after 1.5 and 3 h. The samples
at these conditions. After 1.5 and 3 h, HMF yields were very poor with the use of chromium catalysts
were not collected before 1 h because the dissolution of cellulose was slower than expected at these
and other catalyst mixtures in the [C4C1im][HSO4] ionic liquid (Table 1). Only a 5% HMF yield was
conditions. After 1.5 and 3 h, HMF yields were very poor with the use of chromium catalysts and other
observed using CrCl3·6H2O and CrCl3·6H2O–CuCl2 catalysts after 3 h at 120 °C (Table 1, Entries 1 and
catalyst
mixtures
in the
[C4 C
liquid
(Table
1). Only
a 5%
HMFafter
yield
observed
4 ] ionic
2). In
the presence
of this
IL1 im][HSO
without any
catalyst,
no HMF
formation
was
detected
3 hwas
(Table
1,
◦ C (Table 1, Entries 1 and 2). In the
usingEntry
CrCl33).
·6H
O
and
CrCl
·
6H
O–CuCl
catalysts
after
3
h
at
120
2
3
2
2
Unlike
the sulphate-based
ionic
liquid, the yields were higher in the [C4C1im]Cl ionic
presence
ofwith
this 54%
IL without
catalyst,
no HMF
formation
was
3 h (Table
1, Entry 3).
liquid,
and 11%any
HMF
yields with
the use
of CrCl3·6H
2O detected
and CrCl3after
·6H2O–CuCl
2 catalysts,
respectively
(Table 1, Entries
and 5). the
Only
3% yield
observed
in [C
the4 C
presence
of [C4C
1im]Cl with
Unlike
the sulphate-based
ionic4liquid,
yields
werewas
higher
in the
liquid,
1 im]Cl ionic
without
catalyst
(Table
Entry
After3the
of
the
IL,
ZnCl
2 and CrCl
3·6H2O–ZnCl
2
54% and
11%any
HMF
yields
with1,the
use 6).
of CrCl
·6Hselection
O
and
CrCl
·
6H
O–CuCl
catalysts,
respectively
2
3
2
2
were
also
examined
as
potential
catalysts
for
the
conversion
of
cellulose
to
HMF.
The
maximum
(Table 1, Entries 4 and 5). Only 3% yield was observed in the presence of [C4 C1 im]Cl without any
yields
of 3%1,and
30% 6).
wereAfter
obtained
ZnClof
2 and CrCl3·6H2O–ZnCl2 catalysts (Table 1, Entries 8
catalyst
(Table
Entry
the using
selection
the IL, ZnCl2 and CrCl3 ·6H2 O–ZnCl2 were also
and 9).
examined as potential catalysts for the conversion of cellulose to HMF. The maximum yields of 3%
and 30% were obtained
ZnCl2 of
and
CrCl3to·6H
catalysts
(Table
1,°C.
Entries 8 and 9).
2 O–ZnCl
Tableusing
1. Conversion
cellulose
HMF
(% yield)2 in
ionic liquids
at 120
EntryTable 1. Conversion
Ionic Liquid
Catalyst
of cellulose to HMF (% yield)
in ionic liquids at 1.5
120h◦ C. 3 h
1
[C4C1im][HSO4]
CrCl3·6H2O
<5
<5
2
[C
4C1im][HSO4]
CrCl
3·6H2O + CuCl2 (1:1)
<5
Entry
Ionic Liquid
Catalyst
1.5 h <53 h
3
[C4C1im][HSO4]
No catalyst
0
0
1
[C4 C1 im][HSO4 ]
CrCl3 ·6H2 O
<5
<5
4
[C4C1im]Cl
CrCl3·6H
2O
36
54
2
[C4 C1 im][HSO4 ]
CrCl3 ·6H2 O + CuCl2 (1:1)
<5
<5
5
[C4C1im]Cl
CrCl3·6H2O + CuCl2 (1:1)
11
11
3
[C4 C1 im][HSO4 ]
No catalyst
0
0
4C1im]Cl
No
catalyst
<336
<354
4 6
[C[C
CrCl
4 C1 im]Cl
3 ·6H2 O
[C4C1im]Cl
[C
4C1im][HSO4] (1:1)
3·6H
2O
<311
<511
5 7
[C+4 C
CrCl3 ·CrCl
6H2 O
+ CuCl
1 im]Cl
2 (1:1)
1im]Cl
ZnCl
2
0 <3
3 <3
6 8
[C[C
No
catalyst
4 C41Cim]Cl
CrCl3·6H
2O 3
+·ZnCl
8 <3
30<5
7 9 [C4 C1 im]Cl + [C44C
C11im]Cl
im][HSO4 ] (1:1)
CrCl
6H2 O2 (1:1)
[C4 Cliquid;
0
3in
0.18g cellulose in 0.7 g ionic
yields
1 im]Cl7 mol % catalyst, 120 °C; HMFZnCl
2 shown were determined
1
9
[C
C
im]Cl
CrCl
·
6H
O
+
ZnCl
(1:1)
8
30
4 1
3
2
2
the IL by H NMR spectroscopy.
0.1 g cellulose in 0.7 g ionic liquid; 7 mol % catalyst, 120 ◦ C; HMF yields shown were determined in the IL by
Based on our previous
1 H-NMR
spectroscopy.
research on glucose and fructose, it was found that [C4C1im]Cl enhances
the isomerisation of glucose to fructose through the improved coordinating ability of the metal
centre when Lewis basic Cl− anions (acting as additional ligands) are present, while [C4C1im][HSO4]
Based
on our
research
on glucose
and fructose,
it was
found
that [Cthat
enhances
4 C1 im]Cl
accelerates
theprevious
conversion
of fructose
to HMF
[61]. It was
also
assumed
the acidity
of the
isomerisation
of glucose
fructosethe
through
the improved
coordinating
ability
of the to
metal
centre
[C4C1im][HSO
4] will to
promote
hydrolysis
of cellulose
to monomers.
In order
probe
this,when
Lewis
basic Clof−ionic
anions
(acting
additional
ligands) are
present, to
while
C
im][HSO
]
accelerates
mixtures
liquids
wereas
tested
for the conversion
of cellulose
HMF[C
but
very
poor
(5%)
HMF
4 1
4
yields were detected
(Table
Entry[61].
7). These
results
thethe
presence
of of
[C4[C
C1im][HSO
4]
the conversion
of fructose
to 1,
HMF
It was
alsosuggested
assumedthat
that
acidity
4 C1 im][HSO
4]
will promote the hydrolysis of cellulose to monomers. In order to probe this, mixtures of ionic
liquids were tested for the conversion of cellulose to HMF but very poor (5%) HMF yields were
detected (Table 1, Entry 7). These results suggested that the presence of [C4 C1 im][HSO4 ] actually
Inorganics 2016, 4, 32
7 of 15
Inorganics 2016, 4, 32
7 of 15
lowers
the conversion of cellulose to HMF, and strongly suggests that the [HSO4 ]− anion
actually
inhibits
the
glucose–fructose
interconversion,
as
was
already
verified
in
our
previous
research
on
actually lowers the conversion of cellulose to HMF, and strongly suggests that the [HSO4]− anion
glucose
to HMF
[61].the
These
results indicate
that, while as
thewas
chromium
is aour
good
choice for
actually
inhibits
glucose–fructose
interconversion,
already catalyst
verified in
previous
cellulose
conversion
to
HMF,
the
[C
C
im][HSO
]
ionic
liquid
inhibits
HMF
formation,
and
ZnCl2 ,
1
research on glucose to HMF [61]. 4These
results4indicate that, while the chromium catalyst is a the
good
·6H2 O–ZnCl
·
CrCl3choice
,
and
CrCl
6H
O–CuCl
catalyst
systems
do
not
provide
a
viable
reaction
system
for cellulose
conversion
to
2
3
2 HMF, the
2 [C4C1im][HSO4] ionic liquid inhibits HMF formation,
◦ Cthe
ZnCl
2, CrCl3·6H
2, and CrCl
at 120and
in the
presence
of2O–ZnCl
[C4 C1 im][HSO
4 ]. 3·6H2O–CuCl2 catalyst systems do not provide a viable
reaction system at 120 °C in the presence of [C4C1im][HSO4].
2.2. Cellulose Concentration
2.2. Cellulose Concentration
The cellulose (substrate) concentration is an important factor in this conversion pathway, as it
cellulose
concentration
is an important
factorthe
in this
conversion
as humin
it
has been The
shown
that (substrate)
reducing glucose
concentration
improves
HMF
yield bypathway,
reducing
has been shown that reducing glucose concentration improves the HMF yield by reducing humin
formation [61] due to a lower concentration of sugarsreacting with 5-HMF to form humins (insoluble
formation [61] due to a lower concentration of sugarsreacting with 5-HMF to form humins (insoluble
carbohydrate degradation products). As a result, improving the reaction kinetics for the conversion
carbohydrate degradation products). As a result, improving the reaction kinetics for the conversion
of glucose
andand
fructose
into
canhave
havea significant
a significant
effect
on supressing
formation.
of glucose
fructose
into5-HMF
5-HMF can
effect
on supressing
huminshumins
formation.
To
To explore
this
hypothesis,
the
amount
of
cellulose
present
in
the
reaction
medium
was
reduced.
explore this hypothesis, the amount of cellulose present in the reaction medium was reduced. The
The results
for this
thisseries
seriesofofexperiments
experiments
illustrated
in Figure
6.
results for
are are
illustrated
in Figure
6.
Figure 6. HMF yield from cellulose at 120 °C in [C4C1im]Cl; 0.7 g ionic liquid; 7 mol % CrCl3·6H2O
Figure 6. HMF yield from cellulose at 120 ◦ C in [C4 C1 im]Cl; 0.7 g ionic liquid; 7 mol % CrCl3 ·6H2 O
catalyst. HMF yields shown were determined in the IL by 1H NMR spectroscopy.
catalyst. HMF yields shown were determined in the IL by 1 H-NMR spectroscopy.
A maximum yield of 54% was obtained after 3 h using a 0.1 g cellulose loading (120 °C).
◦ C).
Lowering
the yield
amount
cellulose
from 0.1after
to 0.05
and then
tog0.02
g led toloading
yields of
HMF
ofLowering
53%
A
maximum
of of
54%
was obtained
3 hg using
a 0.1
cellulose
(120
and
45%,
respectively,
whilst
increasing
the
cellulose
concentration
from
0.1
to
0.2
g
led
to
an
HMF
the amount of cellulose from 0.1 to 0.05 g and then to 0.02 g led to yields of HMF of 53% and 45%,
yield of 25%
afterincreasing
3 h. This could
explainedconcentration
by the limited solubility
in the
respectively,
whilst
thebecellulose
from 0.1oftocellulose
0.2 g led
to IL
ancreating
HMF yield
a
multiphasic
reaction
system.
To
overcome
the
solubility
issue,
the
amount
of
IL
was
tripled
(0.7creating
to
of 25% after 3 h. This could be explained by the limited solubility of cellulose in the IL
2.1 g), while keeping the cellulose amount at 0.2 g and maintaining the catalyst loading at 7 mol %.
a multiphasic reaction system. To overcome the solubility issue, the amount of IL was tripled (0.7 to
The HMF yield after 3 h consequently improved from 25% to 39%. These results suggest that
2.1 g), while keeping the cellulose amount at 0.2 g and maintaining the catalyst loading at 7 mol %.
decreasing the cellulose loading (from the standard conditions of 0.1 g cellulose in 0.7 g IL) has only
The HMF
yield
after
h consequently
improved
25% tothat
39%.
These results
suggest
decreasing
a minor
effect
on3the
HMF yield. This
is likelyfrom
to indicate
a multiphasic
system
runthat
above
the
the cellulose
loading
(from
the
standard
conditions
of
0.1
g
cellulose
in
0.7
g
IL)
has
only
a
minor
cellulose solubility limit will perform adequately with a steady state (solubility limited)effect
on the
HMF yield. of
This
is likely
to indicate
that a in
multiphasic
systemitrun
above noting
the cellulose
concentration
cellulose
substrate
dissolved
the IL. However,
is worth
that it solubility
was
that all
cellulose had
the(solubility
time the first
samples
of the mediaofwere
takensubstrate
for
limit ensured
will perform
adequately
withdissolved
a steady by
state
limited)
concentration
cellulose
analysis.
dissolved
in the IL. However, it is worth noting that it was ensured that all cellulose had dissolved by
Based
glucose-HMF
conversions
in [C4C
1im]Cl, our previous hypothesis was that the
the time the
firstonsamples
of the media
were taken
for
analysis.
isomerisation of glucose to fructose was the limiting step, rather than the conversion of fructose to
Based on glucose-HMF conversions in [C4 C1 im]Cl, our previous hypothesis was that the
HMF [61]. It was found that decreasing the glucose concentration left less sugar to react with HMF at
isomerisation of glucose to fructose was the limiting step, rather than the conversion of fructose
120 °C, and thus successfully preventing humin formation. However, in the system described here, it
to HMF
[61]. It was found that decreasing the glucose concentration left less sugar to react with HMF
appears that the limiting step is dissolution and hydrolysis of cellulose to glucose monomers rather
◦ C, and thus successfully preventing humin formation. However, in the system described here,
at 120than
the isomerisation of glucose to fructose. This will influence both reaction engineering
it appears
that the
step isconsiderations
dissolution and
hydrolysis
approaches
andlimiting
reactor design
in the
future. of cellulose to glucose monomers rather
than the isomerisation of glucose to fructose. This will influence both reaction engineering approaches
and reactor design considerations in the future.
Inorganics 2016, 4, 32
Inorganics 2016, 4, 32
2.3. Reaction Time
8 of 15
8 of 15
2.3.
Reaction
Timethe effect of time on the yield of HMF from cellulose at 120 and 150 ◦ C at the same
Figure
7 shows
catalyst (CrCl
cellulose
loadings
in [Cof
Prolonging
time
initially
Figure3 ·76H
shows
the effect
of time
on the yield
from
cellulose atthe
120reaction
and 150 °C
at the
2 O) and
4 CHMF
1 im]Cl.
◦
improves
HMF(CrCl
yield,
but
reachingloadings
a maximum,
the
HMFProlonging
yield at 120
declined
same the
catalyst
3·6H
2O) after
and cellulose
in [C4C
1im]Cl.
the C
reaction
timeby 9%
initially
improves
theby
HMF
yield, but 0.5
afterand
reaching
a maximum,
the HMF yield were
at 120not
°C declined
between
3 and
4 h and
6% between
1.5 h at
150 ◦ C. Measurements
taken before
◦ C because
by120
9% between
3 andcellulose
4 h and by
6% betweenwas
0.5 and
150 °C. Measurements
were
taken
1 h at
dissolution
not1.5
yeth at
complete.
The reaction
wasnotvery
fast at
before
1
h
at
120
°C
because
cellulose
dissolution
was
not
yet
complete.
The
reaction
was
very
fast
at
◦
150 C, and the maximum yield was obtained after only 0.5 h (51%). Prolonging the reaction from the
150 °C, and the maximum yield was obtained after only 0.5 h (51%). Prolonging the reaction from the
maximum yield point led to a decrease in the HMF yield, indicating that decomposition of HMF and
maximum yield point led to a decrease in the HMF yield, indicating that decomposition of HMF and
the formation of humins (observed as a precipitate) was occurring at these high temperatures. Not only
the formation of humins (observed as a precipitate) was◦ occurring at these high temperatures. Not
was an
HMF
of 51%
obtained
after 30 min
onlyafter
15 min
only
was yield
an HMF
yield
of 51% obtained
afterat30150
minC,
atbut
150after
°C, but
onlyat15this
mintemperature,
at this
an HMF
yield
of
44%
was
obtained.
These
are
exceptionally
high
HMF
yields
from
cellulose,
achieved
temperature, an HMF yield of 44% was obtained. These are exceptionally high HMF yields from
over cellulose,
a very short
reaction
achieved
overtime.
a very short reaction time.
Figure 7. Yield of HMF from cellulose at 120 °C◦and 150 °C in◦ [C4C1im]Cl; 0.1 g cellulose, 0.7 g ionic
Figure
7. Yield of HMF from cellulose at 120 C and 150 C in [C4 C1 im]Cl; 0.1 g cellulose, 0.7 g
liquid; 7 mol % CrCl3·6H2O catalyst. HMF yields shown were determined in the IL by 1H NMR
ionic liquid; 7 mol % CrCl3 ·6H2 O catalyst. HMF yields shown were determined in the IL by
spectroscopy.
1 H-NMR
spectroscopy.
2.4. Water and Catalyst Loading
2.4. Water and Catalyst Loading
Due to its involvement in all stages of the process, the presence of water is a significant factor in
the
of cellulosein
toall
HMF
(Table
order tothe
explore
this further,
different
amounts offactor
Dueconversion
to its involvement
stages
of 2).
theInprocess,
presence
of water
is a significant
water
were
added
to
the
reaction
mixture
at
150
°C.
However,
increasing
the
water
content
above
in the conversion of cellulose to HMF (Table 2). In order to explore this further, different
amounts
◦
adventitious
levels
resulted
in
no
clear
benefit
(Table
2,
Entries
1
and
2).
This
could
be
attributed
to aabove
of water were added to the reaction mixture at 150 C. However, increasing the water content
dramatic decrease in the solubility of cellulose in [C4C1im]Cl–water mixtures as a result of the
adventitious levels resulted in no clear benefit (Table 2, Entries 1 and 2). This could be attributed
reorganisation of the hydrogen bonding network. The hydrogen bonds between [C4C1im]Cl and the
to a dramatic decrease in the solubility of cellulose in [C4 C1 im]Cl–water mixtures as a result of the
hydroxyl group of cellulose are orientated to favour breakage of the β-1,4-glycosidic bonds [62].
reorganisation
of the hydrogen bonding network. The hydrogen bonds between [C4 C1 im]Cl and the
When substantial amounts of water are present (Table 2, Entry 1), the water molecules preferentially
hydroxyl
group
of cellulose
are orientated
favour
breakage
of the β-1,4-glycosidic
[62]. When
hydrogen bond
with cellulose,
reducingtothe
interaction
between
this substrate andbonds
[C4C1im]Cl,
substantial
amounts
of water
are present
(Table
2, Entry 1),
the
preferentially
hydrogen
resulting
in the lower
solubility
observed
for cellulose
in [C
4Cwater
1im]Cl molecules
[63]. However,
the presence
of
bondsmall
withamounts
cellulose,
the2,interaction
between
this substrate
andof[CHMF
C
im]Cl,
resulting
of reducing
water (Table
Entry 2) appears
to slow
the formation
yet
still
results
inin the
4 1
51%
yield
after
an
hour.
Overall,
it
is
clear
that
the
presence
of
a
small
amount
of
water
is
beneficial
lower solubility observed for cellulose in [C4 C1 im]Cl [63]. However, the presence of small amounts
and is(Table
needed2,for
hydrolysis.
Indeed,torigorous
exclusion
of moisture
has yet
a detrimental
effect
the yield
of water
Entry
2) appears
slow the
formation
of HMF
still results
in on
51%
(see Supplementary
after yield
an hour.
Overall, it is Materials).
clear that the presence of a small amount of water is beneficial and is
The catalyst loading was also tested in order to confirm that there was sufficient catalyst to
needed for hydrolysis. Indeed, rigorous exclusion of moisture has a detrimental effect on the yield
perform the conversion of cellulose to HMF and that catalyst poisoning was not a significant factor
(see Supplementary Materials).
in the reaction medium. The catalyst loading was increased threefold (to 21%) and the yield was
The catalyst loading was also tested in order to confirm that there was sufficient catalyst to
observed to decrease slightly (Table 2, Entry 4). This is surprising but could be due to the additional
perform
the present
conversion
of cellulose
to HMFwhich
and that
catalyst
poisoning
catalyst
promoting
side reactions,
lower
the HMF
yield. was not a significant factor in
the reaction medium. The catalyst loading was increased threefold (to 21%) and the yield was observed
to decrease slightly (Table 2, Entry 4). This is surprising but could be due to the additional catalyst
present promoting side reactions, which lower the HMF yield.
Inorganics 2016, 4, 32
Table 2. Effect of addition of water and catalyst loading on HMF formation (%) at 150 ◦ C.
Inorganics 2016, 4, 32
9 of 15
9 of 15
Entry2. Effect of addition
Ionic Liquid
Catalyst
(molon%)
0.5 h
1 h at 1503 °C.
h
Table
of water and catalyst
loading
HMF formation
(%)
1
[C4 C1 im]Cl + H2 O (1:1)
7
22
28
Entry
Ionic
Liquid
Catalyst (mol %)
0.5 h
1h
3h
2
[C4 C1 im]Cl + H2 O (3:1)
7
51
44
1
+ HC2Oim]Cl
(1:1)
7
22 45
3 [C4C1im]Cl[C
7
51
30 28
4 1
2
+H
2Oim]Cl
(3:1)
7
51
4 [C4C1im]Cl[C
C
21
44
37
22 44
4 1
3
[C4C1im]Cl
7
51
45
30
◦
0.1 g cellulose in 0.7 g ionic liquid; CrCl3 ·6H2 O catalyst, 150 C. HMF yields shown were determined in the IL
4
[C4CRatios
1im]Cl
21
44
37
22
by 1 H-NMR
spectroscopy.
are by weight.
2.5.
0.1 g cellulose in 0.7 g ionic liquid; CrCl3·6H2O catalyst, 150 °C. HMF yields shown were determined
in
IL by 1H NMR spectroscopy. Ratios are by weight.
Reactionthe
Temperature
2.5. Reaction
The
reactionTemperature
temperature is an important factor in the conversion of cellulose to HMF that directly
affects the
yield
and
reaction rate,
as important
both increase
increases.
After
reaching
The reaction temperature
is an
factorasintemperature
the conversion
of cellulose
to HMF
that the
maximum
yield,
humins
start
to
form
at
elevated
temperatures,
and
lower
the
yield
considerably
directly affects the yield and reaction rate, as both increase as temperature increases. After reaching
(Figure
The reaction
is verystart
fastto
atform
highattemperatures,
with anand
HMF
yield
44%
observed after
the 8).
maximum
yield, humins
elevated temperatures,
lower
the of
yield
considerably
◦ C (andisafter
◦ C). An increase
◦ C (from
very10
fast
at high
temperatures,
with aninHMF
yield of 44%
observed
after100 to
only (Figure
15 min 8).
at The
150 reaction
min
at 170
temperature
of 50
◦
only
15
min
at
150
°C
(and
after
10
min
at
170
°C).
An
increase
in
temperature
of
50
°C
(from
100
150 C) led to a 15-fold higher HMF yield at the 30 min time point. At the three-hour time point,toseven
°C) HMF
led to was
a 15-fold
higher
HMF yield
30 min time
theAlthough
three-hourthe
time
point, rate
times150
more
being
produced
at 120at◦the
C compared
topoint.
at 100At◦ C.
reaction
seven times more HMF was being produced at 120 °C compared to at 100 °C. Although the reaction
increases with temperature, the HMF yield dropped after 30 min at the highest temperatures (150 or
rate increases with temperature, the HMF yield dropped after 30 min at the highest temperatures
170 ◦ C), which can be attributed to the formation of humins. The maximum HMF yield of 54% was
(150 or 170 °C), which◦can be attributed to the formation of humins. The maximum HMF yield of
obtained
afterobtained
3 h at 120
the °C
restriction
on yieldon
at yield
this temperature
was due
the
54% was
after C
3 hbut
at 120
but the restriction
at this temperature
was to
due
to reaction
the
rate, reaction
which was
due
the low
solubility
(and slow
dissolution)
and breakdown
of theofcellulose
rate,slow
which
wastoslow
due to
the low solubility
(and
slow dissolution)
and breakdown
the
◦
polysaccharide
in the ionic liquid.
The liquid.
presence
huminsofwas
alsowas
detected
after 3 hafter
at 120
cellulose polysaccharide
in the ionic
Theof
presence
humins
also detected
3 h atC and
their 120
formation
led to
the subsequent
decrease
HMF yield
after
thisafter
point.
°C and their
formation
led to theobserved
subsequent
observedindecrease
in HMF
yield
this point.
Figure 8. Yield of HMF from cellulose at temperatures from 100 to 170 °C as a function of time; 0.1 g
Figure 8. Yield of HMF from cellulose at temperatures from 100 to 170 ◦ C as a function of time;
cellulose in 0.7 g ionic liquid; 7 mol % CrCl3·6H2O catalyst. HMF yields shown were determined in
0.1 g cellulose1 in 0.7 g ionic liquid; 7 mol % CrCl3 ·6H2 O catalyst. HMF yields shown were determined
the IL by1 H NMR spectroscopy.
in the IL by H-NMR spectroscopy.
At temperatures above 120 °C, the reaction rate increased and moderate yields were obtained
◦ C,be
after
only 10 min.above
This 120
could
to therate
reduced
viscosity
of the reaction
which after
At
temperatures
thedue
reaction
increased
and moderate
yieldsmedium,
were obtained
thecould
mass be
transfer
forthe
cellulose
dissolution
breakdown.
Moreover, which
an HMF
yield of the
only accelerates
10 min. This
due to
reduced
viscosityand
of the
reaction medium,
accelerates
was obtained
fromdissolution
the conversion
cellulose after
only 10 min
170 °C,
which
is one
the
mass44%
transfer
for cellulose
andofbreakdown.
Moreover,
an at
HMF
yield
of 44%
wasofobtained
highest reported yields of HMF directly from cellulose. However,
when the reaction temperature
◦
from the conversion of cellulose after only 10 min at 170 C, which is one of the highest reported
was very high the formation of humins was unavoidable and the yield dropped significantly after
yields of HMF directly from cellulose. However, when the reaction temperature was very high the
only a short period of time (Figure 8), creating a narrow operating window in time. In order to
formation
of humins
was unavoidable
and the yield were
dropped
after only
short at
period
overcome
this problem,
low cellulose concentrations
testedsignificantly
to limit the formation
of a
humins
of time
(Figure
8),
creating
a
narrow
operating
window
in
time.
In
order
to
overcome
this
problem,
high temperatures. This built on the runs conducted at 120 °C (Figure 6), which suggested that
low cellulose concentrations were tested to limit the formation of humins at high temperatures.
This built on the runs conducted at 120 ◦ C (Figure 6), which suggested that changing the standard
Inorganics 2016, 4, 32
10 of 15
0.1 g cellulose in 0.7 g IL had a detrimental effect on HMF yield. It was hypothesised that, as the
reaction rate above 150 ◦ C is very fast, lowering the free sugar concentration in solution will limit the
formation of humins. This follows from the fact that there would be less sugar remaining under these
conditions to react with the HMF formed (to form humins). As an additional benefit, it was considered
that the higher temperatures would also make cellulose dissolution more rapid. Different cellulose
concentrations at elevated temperatures were investigated in order to test this hypothesis (Table 3).
Table 3. Effect of cellulose concentration on HMF yield (%) at elevated temperatures.
Entry
Ionic Liquid
Cellulose (g)
Temp. (◦ C)
10 min
0.5 h
1h
3h
1
2
3
4
[C4 C1 im]Cl
[C4 C1 im]Cl
[C4 C1 im]Cl
[C4 C1 im]Cl
0.025
0.015
0.1
0.015
150
150
170
170
40
25
44
31
52
45
44
50
58
57
41
50
58
51
-
0.7 g ionic liquid; 7 mol % CrCl3 ·6H2 O catalyst. HMF yields shown were determined in the IL.
In contrast to the results at 120 ◦ C, reducing the cellulose concentration improved the yield at
elevated temperatures. With the cellulose loading decreased to 0.025 g (Table 3, Entry 1), the reaction
was very fast (40% HMF yield after only 10 min) and produced a maximum HMF yield of 58% after
only 1 h. This represents the optimum conditions in our research on direct HMF production from
cellulose in ionic liquids. Decreasing the cellulose concentration further to 0.015 g (Table 3, Entry 2)
did not have any positive effect on the yield, probably because the solution became too dilute. Raising
the temperature still further to 170 ◦ C increased the reaction rate and gave 6% more HMF yield after
10 min (Table 3, Entries 2 and 4) using a 0.015 g loading. However, the overall yield dropped by 7%
after 3 h due to a higher rate of humin formation.
In this work, all conversions to HMF are reported as non-isolated yields that were
obtained by 1 H-NMR spectroscopy using [C4 C1 im]Cl as the internal standard. The isolation of
5-hydroxymethylfurfural from the ionic liquid reaction medium is a significant challenge [33,36] due
to the high boiling point (291 ◦ C at atmospheric pressure) of HMF and its instability on removal from
the system (it is significantly stabilised in the ionic liquid). There have been a number of reports
addressing this [53,64–66], including the use of methyl isobutyl ketone (MIBK) [64] or ethyl acetate [65]
to extract from similar ionic liquid/metal salt systems. These typically employ multiple extractions
(between 3 and 10) to isolate 90%–95% of the HMF present in the ionic liquid media, and introduce
a volatile organic solvent into the system. An alternative approach using an entrainer-intensified
vacuum distillation process has also been employed for the separation of HMF from metal salt–ionic
liquid mixtures, though at a high energy penalty [66]. This isolation issue is common to all ionic liquid
based systems and can either be addressed through improved extraction procedures or, alternatively,
converting the HMF generated into further products without isolation [67].
3. Materials and Methods
3.1. General Comments
All experiments were carried out under aerobic conditions using dried ionic liquids; however,
moisture was not rigorously excluded during the reactions. The ionic liquids used, [C4 C1 im]Cl and
[C4 C1 im]HSO4 were prepared using procedures slightly modified from those detailed in an earlier
report [52]. The metal salts were obtained from Sigma-Aldrich (St. Louis, MO, USA), as were
the cellulose and HMF (to provide data for a pure sample). NMR spectroscopy was performed at
25 ◦ C using a Bruker DRX-400 (400 MHz) spectrometer (Bruker, Billerica, MA, USA). All couplings
are reported in Hertz. A Shimadzu Prominence HPLC instrument (Shimadzu, Kyoto, Japan) with
refractive index (RI) and ultraviolet (UV) detectors equipped with an Aminex HPX-87H column was
used. A comparison of the yield determination using 1 H-NMR spectroscopy and HPLC is reported in
the Supplementary Materials.
Inorganics 2016, 4, 32
11 of 15
3.2. Catalytic Formation of 5-Hydroxymethylfurfural (HMF) from Cellulose
In a typical run, cellulose (100 mg, 0.62 mmol) was added to a round-bottomed flask containing
[C4 C1 im][Cl] (700 mg, 4.01 mmol). The catalyst (0.043 mmol, 7 mol %) under study (CrCl3 ·6H2 O,
CuCl2 , ZnCl2 ) was added to the flask. The flask was equipped with a reflux condenser and the
reaction mixture was stirred at the desired temperature (80, 100, 120, 140, 150 or 170 ◦ C) for 24 h.
Samples were collected after various reaction times (0.5, 1, 3, or 24 h) and analysed by 1 H-NMR
spectroscopy. HMF yields were calculated from the integration of HMF peaks against the ionic liquid
[C4 C1 im][Cl] itself. The ionic liquid was considered to be an internal standard, as it has negligible
vapour pressure. The distinct HMF resonance at 9.55 ppm in the 1 H-NMR spectrum was chosen for
calculation of the yields and measured against resonances for the [C4 C1 im]+ cation, which can be
considered completely non-volatile under the conditions employed. Experiments were performed
three times and an average was taken. A discussion of the accuracy of the 1 H-NMR spectroscopy
method compared to HPLC can be found in the Supplementary Materials. The following NMR
spectroscopic data were recorded for HMF: 1 H-NMR (400 MHz, DMSO) δ 9.55 (s, 1H, CHO), 7.50 (d,
1H, furan-CH, 3 JHH = 3.5 Hz), 6.61 (d, 1H, furan-CH, 3 JHH = 3.5 Hz), 5.57 (t, 1H, OH, 3 JHH = 6.0 Hz),
4.51 (d, 2H, CH2 O, 3 JHH = 6.0 Hz) ppm. All HMF yields reported are therefore non-isolated yields.
4. Conclusions
The conversion of cellulose to HMF was probed using a range of different variables, including the
nature of the ionic liquid medium, the catalyst selection, and modification of the reaction conditions.
The conversion of cellulose to HMF is clearly much more challenging than the generation of HMF
from sugar monomers due to the polymeric nature of cellulose and its resulting low solubility in
the majority of solvent systems. At elevated temperatures (above 150 ◦ C), lowering the cellulose
concentration led to an increase in the yield of HMF but only to a certain level, beyond which the
yield decreased. This suggests that there is a balance to be struck between successful dissolution of
cellulose and sufficiency of material to make the process effective. As has been noted previously [68,69],
the presence of moisture is necessary (see also Supplementary Materials), though a high water content
displays no benefit and slows the formation of HMF (Table 2). More work is needed to probe this
aspect further. Prolonged reaction time led to a decrease in HMF yield due to the formation of humins.
The reaction temperature is one of the most important factors in the conversion of cellulose to HMF,
as it directly affects the yield and reaction rate. At 150 ◦ C, 47% yield of HMF was obtained after only
30 min in contrast to the maximum 54% HMF yield observed at 120 ◦ C, which required 3 h, indicating
that longer reaction times are needed at this lower temperature. Both prolonged reaction times at
lower temperature and shorter reaction times at higher temperatures led to the formation of humins.
A compromise was found to achieve a 58% HMF yield after one hour at 150 ◦ C using a relatively
low cellulose concentration. This result needs to be viewed in the context of leading HMF yields
(as opposed to cellulose conversion) in the literature, which fall in the range of 50%–60% over longer
reaction times (e.g., 5 h [70]) or non-scalable approaches, such as microwave heating [71].
Supplementary Materials: The following are available online at www.mdpi.com/2304-6740/4/4/32/s1,
Figures S1–S3, Tables S1–S4 and discussion of the 1 H-NMR spectroscopy method for HMF yield estimation.
Acknowledgments: We thank the State Oil Company of the Azerbaijan Republic (SOCAR) and the Ministry of
Education of the Azerbaijan Republic for the provision of funding (Sanan Eminov). Paraskevi Filippousi thanks
Climate-KIC (EIT) for funding. Jason P. Hallett and Agnieszka Brandt wish to acknowledge the EPSRC (UK) for
financial support (EP/K014676/1).
Author Contributions: Jason P. Hallett and James D. E. T. Wilton-Ely conceived and designed the experiments;
Sanan Eminov performed the experiments with additional work by Paraskevi Filippousi and Agnieszka Brandt,
and all authors analysed and interpreted the data; Jason P. Hallett and James D. E. T. Wilton-Ely wrote the paper
with contributions by the other authors and the ESI data were prepared by Paraskevi Filippousi.
Conflicts of Interest: The authors declare no conflict of interest.
Inorganics 2016, 4, 32
12 of 15
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