Selective glucose conversion to 5-hydroxymethylfurfural (5-HMF) instead of levulinic acid with MIL-101Cr MOF-derivatives

Article abstract of DOI:10.1039/c6nj01399f

The catalytic conversion of glucose to 5-hydroxymethylfurfural (5-HMF) is highly desirable, but the 5-HMF yield obtained using heterogeneous catalysts is still low compared to homogeneous catalysts, and the mechanism is not elucidated completely. In addition, the isolation and purification of 5-HMF still present a challenge as degradation reactions take place and side products form. The formation of 5-HMF from glucose has been reported using several solid acid catalysts; still metal-organic framework (MOF) catalysts could, so far, catalyze the cascade reaction of glucose to 5-HMF only in low yields of less than 16%. Glucose conversion using MOFs is little investigated and here sulfonated MIL-101Cr (MIL-SO₃H) was found to achieve 29% conversion of glucose to 5-HMF after 24 h in a THF:H₂O (v:v 39:1) mixture. The conversion of maltose resulted in 50% 5-HMF yield (saccharide solutions were 5 wt%). When the reaction was carried out in pure THF using MIL catalysts no product was formed, revealing the indispensability of water for the glucose-to-5-HMF conversion. Importantly, MIL-SO₃H preferentially leads to 5-HMF over levulinic acid (molar ratio 1:0.3), while the catalysts Amberlyst-15 and sulfuric acid form mostly levulinic acid in 5-HMF to levulinic acids ratios of 1:3 and 1:10, respectively. At the same time, MIL-NO₂ is the most selective, yielding only 5-HMF and showing no formation of levulinic acid. Using 5-HMF as a substrate did not result in any conversion to levulinic acid in the presence of MIL-SO₃H, thereby ruling out the catalytic formation of levulinic acid from 5-HMF. Catalyst recycle experiments showed that MIL-SO₃H stays porous and crystalline, but becomes deactivated through fouling by humin formation. With the use of ethanol as an alternative reaction medium, the formation of insoluble humins could be prevented, but the yield of 5-HMF and 5-ethyl-HMF decreased.

Full text of DOI:10.1039/c6nj01399f

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Selective glucose conversion to 5-hydroxymethylfurfural (5-HMF)
instead of levulinic acid with MIL-101Cr MOF-derivatives†
Received 00th January 20xx,
Accepted 00th January 20xx
Annika Herbst and Christoph Janiak*
The catalytic conversion of glucose to 5-hydroxymethylfurfural (5-HMF) is highly desirable, but the 5-HMF yield with
hetereogenous catalysts is still low compared to homogenenous catalysts, and the mechanism is not elucidated
completely. In addition isolation and purification of 5-HMF still presents a challenge as degradation reactions take place
and side products form. The formation of 5-HMF from glucose was reported with several solid acid catalyst, still metal-
organic framework (MOF) catalysts could, so far, catalyze the cascade reaction of glucose to 5-HMF only in low yields of
less than 16%. Glucose conversion with MOFs is little investigated and here sulfonated MIL-101Cr (MIL-SO3H) was found to
achieve 29% conversion of glucose to 5-HMF after 24 h in a THF:H2O (v:v 39:1) mixture. Conversion of maltose resulted in
50 % 5-HMF yield (saccharide solutions were 5 wt%). When the reaction with MIL catalysts was carried out in pure THF no
product was formed, revealing the indispensability of water for the glucose-to-5-HMF conversion. Importantly, MIL-SO3H
preferentially leads to 5-HMF over levulinic acid (molar ratio 1:0.3) while the catalysts Amberlyst-15 and sulfuric acid form
mostly levulinic acid in 5-HMF to levulinic acids ratios of 1:3 and 1:10, respectively. At the same time MIL-NO2 is most
selective yielding only 5-HMF and showing no formation of levulinic acid. Using 5-HMF as substrate did not result in any
conversion to levulinic acid with MIL-SO3H, thereby ruling out the catalytic formation of levulinic acid from 5-HMF. Catalyst
recycle experiments showed that MIL-SO3H stays porous and crystalline, but becomes deactivated through fouling by
humin formation. With the use of ethanol as alternative reaction medium the formation of insoluble humins could be
prevented, but the yield of 5-HMF and 5-ethyl-HMF decreased.
DOI: 10.1039/x0xx00000x
www.rsc.org/
monosaccharides, 5-HMF, furfural, lactic acid, levulinic acid
and formic acid) (cf. Fig. 2).2
Introduction
Biomass is highly attractive as a source for valuable chemicals
and fuels, to save fossil fuels and reduce CO2 emissions
,2
therefrom.1 Especially “waste” biomass from by-product
streams (e.g. in agroindustry, food sector, paper
manufacturing and recycling
) is investigated as feedstock for
platform chemicals, e.g., 5-hydroxymethylfurfural (5-HMF)
(Fig. 1), which can replace the petro-based chemicals and
create new valuable products.3 Biomass of plants or algae
consists mainly of lignocelluloses, which in turn contains
cellulose, lignin and hemicelluloses. Cellulose represents the
largest part with up to 50 weight%.4 Therefore the conversion
of cellulose, respectively its glucose subunits is of broad
interest. The chemical conversion of glucose can proceed via
acid-catalyzed dehydration, which presents challenging
requirements to the catalyst, since a variety of products can be
obtained (e.g. fructose, levoglucosan, other C5 or C6
Fig. 1 5-HMF as precursor platform molecule for different products relevant to
various industries.
Among
these
glucose-dehydration
products
5-
hydroxymethylfurfural (5-HMF) is of special interest because it
can be converted to important polymer precursors such as 2,5-
furan dicarboxylic acid (replacement of terephthalic acid), 1,6-
hexanediol, caprolactam and 2,5-dimethylfuran, which is
interesting as a fossil fuel alternative (Fig. 1).5 Consequently,
an enormous research interest arose with the aim of 5-HMF
synthesis from glucose. During the last two years and also very
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recently, a number of excellent reviews have been published review of Jiang and Yaghi.19 These include encapsulation of
on this topic and provide a thorough overview.4,6
Brønsted acidic molecules in the cavity of a MOF, ligation of
5-HMF can be synthesized from glucose in aqueous media, Brønsted acid groups to metal open sites or (post)synthetic
DMSO, ionic liquids and in biphasic systems. Numerous functionalization of linkers with acidic moieties. There are
catalysts have been tested which can be roughly classified in examples for all three types of Brønsted acid functionalities in
homogeneous Brønsted and Lewis acids (H2SO4, CrCl3 or AlCl3) MOFs with subsequent catalysis.19 At present there are only a
and solid acids (zeolites, resins and metal organic frameworks). handful of studies dealing with the conversion of biomass
A brief reactions sequence of glucose to HMF is depicted in Fig. using MOFs. Li and Hensen et al. demonstrated the Brønsted
2. Generally, 5-HMF is accessible via Lewis-acidic isomerization acidic conversion of fructose and glucose to 5-HMF in DMSO
of glucose to fructose and Brønsted acidic conversion to 5- by encapsulating phosphotungstic acid (PTA) in MIL-101Cr,
HMF.7 Tessonnier et al. also investigated the selective albeit with very low yield of 2% from glucose.20 Hatton
isomerization to fructose via base catalysis, using amines, and reported a MOF/polymer composite material from MIL-101Cr
found the same performance as the state-of-the-art Lewis acid and poly(N-bromomaleimide) which was able to convert
catalysts with a yield of 32% and a selectivity to fructose of fructose to 5-HMF in 87% yield.21 A hybrid material containing
,
63% after 20 min at 100°C.8 9 Huber et al. recently proposed a PTA and ruthenium immobilized on MIL-100Cr was used for
new mechanistic pathway for the formation of 5-HMF from the conversion of cellulose and cellobiose into sorbitol.22
cellulose and glucose which is based on acid catalysis through Matsuda and Kitagawa showed that linker functionalization in
the intermediate levoglucosan instead of fructose (Fig. 2).10
MIL-101Cr with SO3H enabled the hydrolysis of cellulose into
monosaccharides in water.23 The same group studied the
glucose to fructose isomerization ability of MIL-101Cr
derivatives (with bdc-NO2, -NH2, -SO3H; bdc = benzene-1,4-
dicarboxylate) in water. MIL-101Cr can have open metal sites
that could act as catalysts for glucose to fructose
isomerization. The highest activity was found for the NO2- and
the SO3H-functionalized MIL-101Cr (18.4 and 21.6% conversion
to fructose, respectively, after 24 h, 100 °C, 0.2 g catalyst and
25 mg glucose in 2.0 g H2O).24 Chen et al. investigated the
activity of different SO3H-functionalized MOFs (MIL-101Cr-
SO3H, UiO-66Zr-SO3H and MIL-53Al-SO3H) for the conversion of
fructose into 5-HMF in DMSO with a maximum conversion of
90%.25 To the best of our knowledge, until now no conversion
of glucose to 5-HMF using MOF catalysts with yields higher
than 16% has been reported. The glucose conversion of 16%
was achieved with MIL-101Cr-PMAi-Br with a MIL-101Cr
catalyst loading of 6 mol% (or 21 wt%) after 6 h in the solvent
DMSO which made product separation difficult (PMAi-Br =
poly(N-bromomaleimide-co-divinylbenzene)).21
OH
O
OH
HO
HO
Glucose
OH
Lewis acid
Brønsted acid
OH
O
O
HO
O
HO
HO
OH
Fructose
Levoglucosan
OH
HO
OH
Brønsted acid
Brønsted acid
Brønsted
O
O
HO
acid
O
Humins/
degradation
O
H
OH
Levulinic acid
O
5-Hydroxymethylfurfural, 5-HMF
Formic
acid
+
OH
Fig. 2 Reaction scheme for 5-HMF production from glucose.10
The isomerization of glucose to fructose can be carried out
enzymatically but consumes over 10 million tons of glucose
isomerase annually, representing the largest industrial use of
immobilized enzymes.26 Glucose isomerase has a narrow
operating window concerning pH and temperature.26
Therefore, it would be highly desirable to carry out a one-pot
Interestingly, the best 5-HMF yield of 44% from cellulose was
reported for a mixture of THF/water 40:1, using H2SO4 as
catalyst at 190 °C. As postulated by Vlachos et al. the first step
of glucose dehydration to levoglucosan in the Brønsted acidic
mechanism is a protonation step (see section on mechanistic
aspects in ESI†).11
glucose-to-5-HMF transformation without the need of
separate glucose-to-fructose isomerization.
a
It would be a further improvement to use separable and
recyclable solid Brønsted acid instead of homogenous acid
catalysts in terms of green chemistry or green catalysis12 which
requires that catalysts be designed for easy separation from
the reaction products and multi-time efficient reuse/recycl-
ing.13,14,15
Recently, we showed that the catalyst activity of nitro-
modified MIL-101Cr in benzaldehyde acetalization reaction
originates from Brønsted acidity.27 It has been concluded that
inside the pores of MIL-101Cr there exists high proton acidity
through polarized chromium-coordinated water molecules
(aqua ligands). It is a logical extension to investigate the
activity of functionalized MIL-101Cr in the acid catalyzed
glucose-to-5-HMF dehydration. We are aware of the better
catalytic performance of other heterogeneous catalysts like
sulfated ZrO2 or zeolite-beta,28 but our aim here is to
investigate the potential of MOFs for this cascade reaction.
Further, the chosen reaction media is THF/water in order to
Metal-organic frameworks (MOFs), built from organic ligands
(linkers) and metal nodes, are intensively investigated as
,
heterogeneous
catalysts.16 17
Usually,
metal-organic
frameworks are known for their Lewis acidity, which originates
from open metal sites at the secondary building units (SBU).18
From their porous structure, there are different possibilities to
introduce Brønsted acid functionalities, discussed in a detailed
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avoid high boiling solvents (DMSO) and to facilitate separation Standard conditions: catalyst 5.22·10–5 mol, glucose 223 mg
of 5-HMF. MIL-101Cr and its derivatives are known for their (1.24 mmol), solvent 5 mL (THF:H2O (v:v) 39:1), 130 °C, 24 h.
high water stability and uptake.29,30 MIL-101 is also often used Concentrations of 5-HMF in the organic THF phase were
as a catalyst support, e.g., for metal31 or metal oxide quantified by GC with external standard calibration (R2
=
nanoparticles.32
0.9991, R = 0.9995). For more details see ESI†.
In this work we have examined four functionalized MIL-101Cr
derivatives for the acid catalyzed conversion of glucose to 5-
HMF using a THF/water system. For the first time using a
metal-organic framework catalyst a significant conversion of
29% of glucose to 5-HMF has been achieved. In addition the
selectivity between the formation of 5-HMF and levulinic acid
has been investigated and compared with sulfuric acid and
Amberlyst-15 as catalysts.
Results and discussion
Synthesis and porosity of MOFs
MIL-101Cr36 is a three-dimensional micro- to mesoporous
material based on chromium-terephthalate linkages with the
empirical formula [Cr3(O)X(bdc)3(H2O)2] (bdc = benzene-1,4-
dicarboxylate, X = OH or F, depending on synthesis, here X =
OH).37 MIL-101Cr has inner cages of 29 Å and 34 Å diameters
with pore aperture windows diameter of up to 16 Å (Fig. 3, Fig.
S1 in ESI†).36 Two of the three Cr(III) octahedra in the
trinuclear building unit have terminal aqua ligands, which can
Experimental
Characterization of catalysts
Powder X-ray diffraction, PXRD patterns were recorded on a potentially be Lewis acid sites.38,39
Bruker D2 phaser diffractometer in the Bragg-Brentano
configuration with Cu- Kα1/α2 radiation (λ = 1.54184 Å), nickel
filter and stationary flat-panel low background sample holder
in the range 2θ = 5–50° (step width 0.02° in 2θ). Simulated
powder patterns were based on single-crystal data and
calculated using the STOE WinXPOW software package33 or
Mercury v. 3.3 software34. Scanning electron microscopy, SEM
a)
b)
images were measured on a JSM6510 with LaB6 cathode. The
N2 sorption measurements were performed on
a
Quantachrome NOVA 4200e instrument at 77 K. The samples
were degassed under high vacuum (10–5 Torr) at 150 °C for 3 h,
prior to each measurement. The BET surface area was
calculated from adsorption isotherm data points in the
pressure range p/p0 = 0.05 – 0.2.
c)
3 (a) Trinuclear {Cr3(µ3-O)(OH/F)(H2O)2(O2C-)6} building unit which is
assembed with bridging benzene-1,4-dicarboxylate ligands into (b) vertex-
Fig.
Preparation of catalysts
sharing supertetrahedra whose centers form the corners of (c) mesoporous
All MIL-101Cr materials were synthesized hydrothermally, cages with diameters of 29 or 34 Å shown here in a zeolite-type framework
presentation with the topological connectivity (in green) of the centers of the
utilizing an autoclave and programmable oven according to vertex-sharing supertetrahedra. Not-to-scale drawings (additional ones in Fig. S1,
ESI†) were generated from the deposited X-ray data file at the Cambridge
literature procedures.30,35 MIL-SO3H is described here as an
example: CrO3 (3.0 g, 30 mmol), H2bdc-SO3Na (6.0 g, 30 mmol)
and 12 mol/L HCl (2.2 g, 60 mmol) were dissolved in H2O and
split into four batches of 30 mL each which were transferred
into four Teflon-lined stainless steel autoclaves. The solutions
were heated at 180 °C for 6 days (heating ramp 5 h; cooling
ramp 15 h). For more details see ESI†.
Structure Database (CSD-Refcode OCUNAK)36 using the program DIAMOND.40
MIL-101Cr and its linker-modified derivatives30,41 show
remarkable stability towards water which make them most
suitable
for
applications
in
the
presence
of
moisture/water.29,30 In addition, 2-nitro and 2-sulfo-
terephthalic acid were selected to compare the influence of
different functional groups. The strong electron-withdrawing
effect of 2-nitro-modified MIL-101Cr on the catalyst activity
was already shown in the diacetalization of benzaldehyde.27 All
modified MIL-101Cr materials were synthesized directly from
the linker (2-nitro- or 2-sulfo-terephthalic acid) and CrO3 under
acidic hydrothermal conditions (details in ESI†).30,35 For mixed-
linker MILs a molar ratio of 1:1 of the linkers H2bdc-NO2/
H2bdc-SO3Na and H2bdc/H2bdc-SO3Na was used in the
synthesis. The effective linker ratio was then determined by
Standard procedure for catalytic reactions
In a typical experiment the MIL-101 catalyst (50 mg) was
suspended in a solvent mixture (39:1) of tetrahydrofuran
(without stabilizer) (4.875 mL) and water (125 µL) with a total
volume of 5 mL. The standard glucose loading was 5.0 wt%
(223 mg, 1.24 mmol). The temperature was kept constant at
130 °C through a stirred oil bath around the reaction vial. The
reaction was stopped by cooling the reaction mixture to 0 °C.
The MIL catalysts were separated with a centrifuge at 4800
rpm for 10 min. and the solution was passed through a syringe
filter (0.2 µm diameter).
1
digestive dissolution and H NMR analysis (Table 1) (spectra in
Fig. S ESI†).
The MIL-101 structure of the linker-modified compounds was
verified by positive matching of the simulated and measured
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powder X-ray diffractograms (Fig. S4, ESI†). N2 sorption (1333 m2/g) the lowest surface area. In our earlier contribution
isotherms at 77 K showed the typical MIL-101 curvature with on the diacetal formation with MIL-101Cr derivatives it could
two steps up to p/p0 = 0.25 in the otherwise Type Ib isotherm, be shown that surface area is not the decisive factor for high
characteristic for a wider microporous material (Fig. S5, catalytic activity, but particle size as well as electron
ESI†).42 BET surface areas and pore volumes were calculated withdrawing or donating functionalities have
from these N2 sorption isotherms. Among the four tested influence on conversion.27
catalysts, MIL-NO2 (2058 m2/g) has the highest and MIL-SO3H,
a higher
Table 1 Porous properties of MIL-X materials
Materials
BET surface area
(m2/g)a
Total pore
volume (cm3/g)b
Exp. ratio
NO2:SO3H
or
Number of Brønsted
acid sites, in formula
unitd
Particle diameter ± standard
deviation (σ) (nm)e
bdc:SO3Hc
MIL-101Cr
MIL-NO2
3049
2058
1480
1633
1333
1.50
1.18
0.75
0.93
0.70
--
--
2
2
174 ± 35
168 ± 30
160 ± 41
n.d.f
MIL-NO2/SO3H
MIL-SO3H(0.33)
MIL-SO3H
1:1
2:1
--
3.5
2.99
5
152 ± 43
a
Calculated in the pressure range 0.05 < p/p0 < 0.2 from N2 sorption isotherm at 77 K. The BET error margin is 20–50 m2/g. b Calculated from N2 sorption isotherm at
77 K (p/p0 = 0.95) for pores ≤40 nm. c Determined by digestion of MOF and 1H-NMR (see Fig S2, ESI†). d From molecular formula based on the possible number of protic
H atoms per formula unit (see ESI†) of MIL = [Cr3(O)(OH)(bdc-X)3(H2O)2]. e Obtained from SEM measurements, minimum of 50 particles were counted. Particles are
f
extremely agglomerated, particle size could not be determined (Fig. S3c, ESI†).
(standard conditions, 24 h) (Fig. 4a). Interestingly, MIL-NO2,
Conversion of glucose to 5-HMF
which has not obvious additional Brønsted acid sites than MIL-
Due to the high importance of platform chemicals derived 101Cr, shows a significant higher 5-HMF yield, probably due to
from biomass, several solid acid catalysts have been the stronger polarized aqua ligands through the electron-
investigated for the conversion of glucose to 5-HMF.19,28 withdrawing nitro groups (Fig. 4a). Hence, the electron-
Metal-organic frameworks were investigated for a number of withdrawing effect of the NO2 and additional acidic SO3H
acid-catalyzed reactions,43 including conversion of fructose and groups seem to have a strong influence on the catalytic activity
glucose, albeit in a yield of maximum 16% for the latter.21 of the MIL-101 platform. MIL-SO3H gives the highest yield of 5-
Therefore this work focuses on the conversion of glucose to 5- HMF and, therefore, this catalyst has been investigated in
HMF using MOF catalysts.
more detail.
The conversion of glucose to 5-HMF was performed in the If we assume that Brønsted acid (BA) catalysis is taking place in
presence of each of the four functionalized MIL catalysts (Fig. the glucose to 5-HMF conversion and we normalize for the
4) and conversion was followed by gas chromatography (GC). number of protons per formula unit (cf. Table 1) the activity
In a typical experiment 1.24 mmol glucose (223 mg) in 5 mL becomes highest for MIL-NO2 with just two BA sites (Fig. 4b).
THF/water (v:v 39:1),10 giving a 5 wt% glucose solution, was We have tried to determine the number of accessible acidic
heated in the presence of catalyst (5.22·10–2 mmol) for a sites by potentiometric titration according to literature
chosen time at 130 °C in a Pyrex glass vial. For all experiments procedures.44 Typically, the MIL is stirred in a saturated
4.0 mol% MIL catalyst with respect to glucose was used. In the solution of sodium chloride in order to exchange the protons
literature similar glucose or fructose substrate to MOF-catalyst with sodium. Our experiments revealed that MIL materials are
ratios were used.20,21,25 When glucose was kept in THF/water not stable under these conditions (see ESI†), since two
under the same conditions but in the absence of MIL material inflection points were observed although by the backtitration
no formation of 5-HMF or any other products could be method only one acid strength (HCl/H2O) should be present.
detected which shows that the MIL material is necessary for We also followed a recently published approach from Klet and
the catalytic reaction.
coworkers who used sodium nitrate for proton exchange (see
Comparing the catalytic activity of the four MIL-101Cr ESI†).45 Attempts to quantify the amount of available or
derivatives, the completely SO3H-functionalized MIL has the accessible acid sites remained inconclusive (see ESI†), which is
highest activity. With 29% yield of 5-HMF (determined by why the theoretical number of acid sites was used (Table 1).
calibrated GC after workup), this result is, to the best of our We can only state from titration experiments that the amount
knowledge, the highest 5-HMF yield obtained with MOFs for of accessible protons is much smaller than what is calculated
the formation of 5-HMF from glucose. Interestingly, the other from the formula unit.
MOFs MIL-NO2, MIL-SO3H(0.33) and MIL-NO2/SO3H give lower,
but among them very similar HMF yields of about 12 to 13%.
Pure MIL-101Cr shows a very low 5-HMF yield of only 2%
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experiments MIL-SO3H was re-used without any intermediate
washing or reactivation steps. In the fourth run the 5-HMF
yield was reduced to 13%. Therefore, the experiment was
repeated such that after each run the catalyst was washed
with water and methanol and dried before re-use. After three
runs, a 5-HMF yield of 16% was measured. After three runs the
PXRD still shows crystalline MIL-SO3H (Fig. 5a). The surface
area was reduced to half after the first run (443 m2/g) and
essentially lost after four runs (91 m2/g) (Fig. 5b). The surface
area increased only negligibly from 91 m2/g to 120 m2/g after
washing the MIL-SO3H material after the third run with HCl
(2 mol/L) at 85 °C for 20 h, followed by water and drying (140
°C, 15 h) .
a)
a)
b)
Figure 4 a) Conversion of glucose to 5-HMF with functionalized MIL-X (catalyst
5.22·10–5 mol, 223 mg glucose, 130 °C, 5 mL THF:H2O (v:v) 39:1, 24 h). b)
Normalized to theoretical number of Brønsted acid (BA) sites (cf. Table 1).
Stability and recycling test
In order to prove that no leaching occurs, MIL-SO3H was tested
in a filtration experiment. The reaction was stopped by cooling
to 0 °C after 5 hours and the catalyst was separated by
centrifugation at 6000 rpm and subsequent filtration of the
filtrate using
a 0.2 µm syringe filter. The reaction was
continued with the filtrate under identical conditions for
different times, followed by the standard workup. No further
formation of 5-HMF takes place over a time of 18:45 h (Table
2). Small differences of 1% are within experimental error of the
reaction procedure, since for every measuring point a new
reaction was started.
b)
Figure 5 a) PXRD of MIL-SO3H before and after one catalytic run (24h),4 runs
without washing, and after 3 runs with washing (see text). (standard conditions).
b) Nitrogen sorption isotherms (Filled symbols for adsorption, empty symbols for
desorption) of MIL-SO3H before and after catalysis with one run (24h), after one
run (24h) using an additional 1mL of H2O in the reaction and after 4 runs without
washing. Standard conditions MIL-SO3H 5.22·10–5 mol, 223 mg glucose, 130 °C,
5 mL THF:H2O (v:v) 39:1, except for THF:H2O (v:v) 4:1
Table 2 Filtration experiment with MIL-SO3H
Time (h)
Yield of 5-HMF (%)a
Effect of water amount on 5-HMF yield
5 (reaction was stopped)
10
9
It was suggested in a molecular simulation study, that MIL-
101Cr is able to adsorb glucose from an aqueous solution.46,47
Since the MIL material is known for its high uptake of water,29
this might be a driving force for glucose to diffuse into the
pores, due to the much better solubility of glucose in water
over THF.
10
12
10
12
11
14
18.45
MIL-SO3H 5.22·10–5 mol, 223 mg glucose, 130 °C, 5 mL THF:H2O (v:v) 39:1; new
batch was started for every entry at t = 0 h..
In order to elucidate the effect of water on the conversion of
glucose, the water content was increased gradually from 39:1
to 1:1. The results in Table 3 show that with increasing water
content the yield of 5-HMF becomes lower.
Multiple run experiments showed that MIL-SO3H can be re-
used up to 3-4 times with a loss in 5-HMF yield from 29% to
13-16% (Table S3 in ESI†).In
a first series of recycle
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If the degradation products are also formed inside the pores,
they block the MOF pores, which lead to deactivation.
Table 3 Conversion of glucose to 5-HMF with different amounts of water
Ratio THF:water (v:v)a Yield 5-HMF [%]b
With increasing amount of water in the reaction solvent the
yield of 5-HMF became lower (see Table 3). Yet, with
increasing water fraction also less insoluble humin products
are formed as evidenced from the color retention of the MIL
and weight of the solid materials after the reaction (Fig. S6 and
Table S2 in ESI†). To determine the amount of insoluble
humins which have formed, the solid material was separated
by centrifugation and dried (120 °C, overnight). The weights
are listed in Table S2. Also, with increasing water fraction the
BET surface area of MIL-SO3H is diminished less after one
catalytic run (Fig. 5b). When the catalytic reaction was carried
out in a THF/water mixture of 4:1, the MIL-SO3H catalyst, still
had a BET surface area of 1161 m2/g after the first run.
39:1
29
9:1
4:1
24
13
7
2.3:1
1:1
5
pure H2O
pure THF
6
0
b
a) Conditions: 50 mg MIIL-SO3H, 5 wt% glucose, 5 mL solvent, 24 h, 130 °C.
Determined by GC
In pure water only very low formation of 5-HMF was detected
(6%) and in pure THF no 5-HMF (and no other products) could
be detected by GC. Therefore, a small amount of water is not
only beneficial, but even necessary for 5-HMF formation. In
this respect, our reaction system differs significantly from the
sulfuric acid study of Huber and coworkers, since for conc.
(water-free) H2SO4 also in water-free THF, formation of 5-HMF
was detected.10 A possible explanation might be that the
development of full acid strength of the MOF catalyst involves
the polarization of water at the chromium centers of the
MOFs. Glucose is more soluble in water than in THF. When
water is adsorbed by the MIL catalyst this might be the driving
force resulting in higher 5-HMF yield, at low amounts of water,
having a higher local concentration of glucose in the MIL
environment. The glucose concentration in the MOF vicinity
would be lowered further with too much or only water as
solvent. Additionally, THF is known to stabilize 5-HMF and
prevent the formation of levulinic acid.48 Abu-Omar et al.
showed that using NaCl in a THF/H2O mixture as phase
separator, the 5-HMF yield from glucose could be increased
from 52% to 61% in comparison to the system without NaCl
(catalyst AlCl3, HCl, 160 °C). Since the reaction takes place in
water a good phase separation and a good continuous transfer
of 5-HMF to the organic THF phase, is necessary.48 In case of
MIL-SO3H catalysis in THF:H2O (v:v 39:1) these requirements
are apparently fulfilled best.
Regeneration attempts were performed by washing the
catalyst after reaction with methanol, THF and H2O as well as
acid (H2SO4) at room temperature and at 80 °C. No significant
improvement of surface area was measured. Since
regeneration attempts failed and calcination is not possible
due to instability of the MOF above 350°°C, the most effective
way would be to prevent the formation of humins during the
catalytic reaction.
A possible strategy to inhibit the formation of insoluble
humans may be to deactivate the polymerizable sites of 5-
HMF. Therefore, experiments were performed using ethanol in
a mixture with THF and water (Table 4). In the presence of
ethanol also the formation of 5-ethoxymethylfurfural (5-Ethyl-
HMF) was observed in addition to 5-HMF. The higher the
amount of ethanol the higher was the yield of 5-Ethyl-HMF.
Table 4 Product distribution in different solvent mixtures.
Solvent mixture
(v:v)a
Yield 5-HMF
[%] b
Yield 5-Ethyl-
HMF [%] b
MIL-SO3H
surface area
SBET [m2/g] after
reaction
THF/H2O (39:1)
THF/EtOH/H2O
(23:16:1)
29
13
0
7
443
1028
Formation of humins and deactivation pathways
EtOH/H2O
(39:1)
5
11
1239
The degradation of carbohydrates to humins plays a crucial
role in glucose conversion and catalyst degradation. This kind
of deactivation can be classified as fouling.49 The formation of
humins is not well investigated. Generally, unidentified soluble
or insoluble degradation products are called humins.5b,d,50
Recently, Vlachos et al. investigated the effect of reaction
conditions, 5-HMF conversion and acid concentration on the
molecular structure and formation of humins, using FTIR
spectroscopy, SEM and in–situ dynamic light scattering, DLS.51
It was concluded that combined effects from aldol
condensation and etherification reactions as well as
nucleophilic attack from 5-HMF and aggregation effects lead to
insoluble polymers, especially at high 5-HMF formation.51
These by-products can block the pores that no substrate can
migrates in and no product can form or migrate out anymore.
a) Conditions: MIIL-SO3H 50 mg, glucose 223 mg, 5 mL solvent, 24 h, 130 °C. b)
Determined by calibrated GC
The amount of insoluble humins seemed to be reduced when
the reaction was carried out in ethanol/H2O, evidenced by the
retention of the MIL porosity, its color and amount of catalyst
(Fig. S6, S7, Table S2 in ESI†). Unfortunately, in EtOH/H2O the
overall yield and product selectivity is much lower than in the
THF:H2O (v:v 39:1) mixture (Table 4).
Time profile
The yield of 5-HMF slowly approaches the determined value
over the maximum allowed reaction time of 24h under our
standard conditions (Fig. 6). The space-time yield is much
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lower than with other catalysts such as sulfuric acid,10 AlCl3,52 It has already been shown by several groups that fructose can
montmorillonite K-10 clay53 or beta zeolites54, where be converted to 5-HMF in good yields by Brønsted acidic MOF
saturation is observed within 40 min to 5 h. This difference in materials in other solvents.20,25 In our study, a maximum yield
reaction rate can give a hint on the reaction mechanism, since of 5-HMF of 47% was obtained after 8 h under standard
it was suggested with homogeneous AlCl3/solvent/H2O that conditions. As expected the conversion of fructose proceeds
the catalysis proceeds by glucose to fructose isomerization much faster than the conversion of glucose. The formation of
(Lewis acid catalyzed) and subsequent dehydration to 5-HMF. humins was also observed with fructose, levoglucosan and
This reaction route seems much faster than only Brønsted acid maltose by color and weight change.
catalyzed conversion by homogeneous sulfuric acid.11,55
Cellulose with MIL-SO3H shows almost no formation of 5-HMF
Glucose signals were still detected after 8 and 15 h when the (1 %). This could indicate that the long polymer chains of
reaction was stopped by cooling after 8, 15 and 24 h and cellulose cannot enter the pores of MIL-SO3H for conversion or
analyzed by NMR in D2O (Fig. S9, ESI†). After 24 h no alternatively, that the chains are not cleaved to a sufficient
saccharide signals were seen anymore in the NMR spectrum extent.
(Fig. 9).
Formation of 5-HMF versus levulinic acid
When 5-HMF is dissolved in THF/water (v:v 39:1) and reacted
at 130 °C for 24 h with the catalyst MIL-SO3H, no reaction is
observed, only the starting material 5-HMF can be detected by
GC or GC-MS. 1H NMR of the reaction solution (without
workup) showed only trace amounts of levulinic acid (molar
ratio 5-HMF to levulinic acid 1:0.04). The catalyst also retained
its green color instead of changing to brown, which indicates
no formation of humins as in the glucose conversion. In
contrast Dumesic and coworkers showed that other catalysts
as for instance H2SO4, Amberlyst-70 or zeolites will give
levulinic acid from furfuryl alcohol in THF/H2O solutions with
levulinic acid yields of over 70%.57
The selectivity of the glucose-to-5-HMF conversion concerning
Figure 6 Time dependent conversion of glucose to 5-HMF. Standard conditions:
catalyst 50 mg, glucose 223 mg, 5mL THF:H2O (v:v) 39:1, 130 °C.
the formation of levulinic acid as by-product was analzyed by
NMR. Since levulinic acid is very good soluble in water, the
complete reaction solution was dried in vacuum, the residue
Assuming that the conversion of glucose takes place inside the
re-disolved in deuterated solvent and then measured. All
pores or at least in the pore mouths of the MOF, diffusion
reactions have been performed under standard conditions for
limitations can be the reason for the slow conversion.
24 h, unless stated otherwise.
When decreasing the temperature to 100 °C under standard
With MIL-SO3H and glucose the selectivity towards 5-HMF over
conditions the formation of 5-HMF is even slower, after 24 h
levulinic acid is 80%, since the molar ratio between 5-HMF and
only 8% can be detected. At 100 °C only formation of 5-HMF
levulinic acid is 1 to 0.25 after 24 h (Table 5, Fig. 7).
and the starting material is visible from NMR (in D2O); no by-
products are formed (Figure S8d).
Conversion of other substrates to 5-HMF
Reaction of maltose (5 wt%, 0.65 mmol) with MIL-SO3H
resulted in a conversion of 50% to 5-HMF. Maltose is a
glucose-dimer with the two monomer units connected through
a glycosyl-bond. The same mass of maltose contains almost
the same amount of glucose monomer units as in 223 mg
glucose used in our standard conditions. Since the 5-HMF yield
from maltose is higher than from glucose, it seems that the
(unknown) intermediate state is easier accessible from the
dimer maltose. Also, the possible intermediates levoglucosan
Figure 7 1H NMR spectrum of the soluble reaction products from glucose (50 mg
MIL-SO3H, 223 mg glucose, 5mL THF:H2O (v:v) 39:1, 130 °C, 24 h) (NMR solvent
D2O). The 5-HMF to levulinic acid ratio was based on the integral ratio of the of
the CH2 groups, that is, the D and the average of the E and F signals.
and fructose were used as starting materials and the reaction
was performed under standard conditions (50 mg MIL-SO3H,
1.24 mmol sugar, THF:H2O (v:v) 39:1, 130 °C). With
After 8 h and 15 h reaction time the selectivity towards 5-HMF
over levulinic acid is even higher (88% and 86%, respectively)
and decreases with time (Table 5 and Fig. S9). This indicates
that 5-HMF also slowly converts to levulinic acid. As pure 5-
HMF with MIL-SO3H shows only a very low conversion over 24
levoglucosan after 8 h a 5-HMF formation of 12% and after
24 h of 28% was measured with GC. It is evident, that 5-HMF
can be formed also from levoglucosan, but only to a similar
extent as from glucose.
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h (see above) other reaction products may assist in this
conversion.
Table 5 Time dependent analysis of reaction mixture by NMR.a
Time (h)
Integral ratio 5-HMF vs.
levulinic acidd
1:0.13
Selectivity of 5-HMF over
levulinic acid
88%
8
15
1:0.16
86%
24
1:0.25
80%
24hb
24hc
1:0.09
92%
1:0.21
83%
a
Conditions: 50 mg MIIL-SO3H, 5 wt% glucose, 5 mL THF:H2O (v:v) 39:1, 130 °C.
Determined by 1H-NMR in D2O.
Levuglucosan as substrate (1.24 mmol). d Integral ratio of CH2 groups: D (5-HMF)
Fructose as substrate (1.24 mmol).c
b
Figure 8 Ratio between 5-HMF and levulinic acid from 1H-NMR. Comparison
between MIL-SO3H, H2SO4 and Amberlyst-15H. Conditions: catalyst MIL-SO3H
5.22·10–5 mol, glucose 223 mg, 5ml THF:H2O (v:v) 39:1, 130 °C. For H2SO4 and
Amberlyst-15 the reaction was conducted with 1.57·10–4 mol of the catalyst
under the same conditions. The estimated amount of acid of MIL-SO3H,
Amberlyst-15 and sulfuric acid is 0.46 mmol H+/g (see Table S4), 4.7 mmol H+/g
(AMBERLYSTTM product information) and 10.2 mmol H+/g, respectively.
and the average of the E and F signal (levulinic acid), see Fig. 9.
For comparison MIL-SO3H is also reacted with levoglucosan
(1.24 mmol) and fructose (1.24 mmol) as substrate and similar
ratios between 5-HMF and levulinic acid were obtained (Figure
S8a,b, Table 5). Interestingly, the 1H NMR of the glucose
conversion with MIL-NO2 shows signals of 5-HMF, but no
levulinic acid is found. Only some signals of unidentified
products can be found (Figure S8c). In summary, MIL-SO3H
shows the highest yield of 5-HMF which correlates with the
highest potential number of acid sites of the MIL-materials
(Fig. 4), whereas MIL-NO2 shows a very selective formation of
5-HMF since no levulinic acid was found.
It is evident that the ratio of 5-HMF to levulinic acid inversely
proportional to the acidity of the catalyst. The significantly
lower actual acidity of MIL-SO3H compared to Amberlyst-15
and H2SO4 explains the low high ratio of 5-HMF to levulinic
acid. The even lower acidity of MIL-NO2 then explains the
absence of levulinic acid. At present it is not fully clear if 5-
HMF is converted to levulinic acid. Levulinic acid could also
form through a 5-HMF independent pathway (see section on
mechanisꢀc aspects in ESI†).11
For comparison chromium(III) nitrate, sulfuric acid and
Amberlyst-15 have been tested as catalysts (1.57·10–4 mol) for
the formation of 5-HMF under the same conditions as the MIL-
materials (THF:H2O (v:v) 39:1, 130 °C, 223 mg glucose) for 24 h.
CrCl3 is known to catalyze the isomerization from glucose to
fructose. Also formation of 5-HMF, levulinic acid and formic
acid was observed with CrCl3.7a,56 In contrast to MIL-SO3H the
yield of 5-HMF from glucose catalyzed by Cr(NO3)3 is only 6%
after 24 h. Interestingly. no levulinic acid was detected, but a
high amount of unidentified products can be observed by NMR
and GC-MS. The high formation of by-products in case of Cr3+
salts is supported by a comprehensive study of Vlachos et al.7a
Comparing the product distribution using MIL-SO3H as catalyst
or H2SO4 as homogenous catalyst or the strong acidic resin
Amberlyst-15, starting from glucose a significant change can
be observed (Fig. 8 and S10). For Amberlyst-15 the selectivity
is strongly directed to levulinic acid, with a ratio of 5-HMF to
levulinic acid 1:3 (5-HMF selectivity only 25%). For sulfuric acid
as catalyst the major dominant product is also levulinic acid.
The ratio between 5-HMF and levulinic acid is 1:10 (5-HMF
selectivity 9%).
The 5-HMF yield varies only slightly, essentially within
experimental error, with the amount of MIL-SO3H catalyst
(Fig. 9).
Figure 9 Conversion of glucose to 5-HMF with different amounts of glucose and
MIL-SO3H as catalyst (given is the molar Cr(III) (1.57·10–4 mol) to glucose ratio
(catalyst 5.22·10–5 mol, 130 °C, 5 mL THF:H2O (v:v) 39:1, 24 h).
Increasing the ratio of MIL-SO3H to glucose from the standard
of 1:7.9 (223 mg glucose) to 1:3.5 (100 mg glucose) gives a
yield of 5-HMF of 34% (up from 29%). Decreasing the ratio of
MIL-SO3H to glucose to 1:17.7 (500 mg), a slight decrease in 5-
HMF formation to 26% can be observed (Fig. 9). These results
show that the catalyst remains active also at higher
concentration of glucose and the percentage yield does not
change significantly with a moderate variation of the substrate
ratio.
Recently, Dumesic et al. reported the conversion of furfuryl
alcohol into levulinic acid using different solid acids (including
Amberlyst-70 and H-ZSM-5) as well as sulfuric acid in a similar
THF/water system.57 Huber et al. reported the formation of
levulinic acid as by-product from glucose conversion to 5-HMF
with sulfuric acid (catalyst).10
8 | J. Name., 2012, 00, 1-3
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Parallel to this work, Su and coworkers also investigated the formation of insoluble humins. At present the 5-Ethyl-HMF
activity of MIL-101Cr-SO3H on the glucose to 5-HMF yield is low, awaiting the development of more active MOF-
conversion.58 They used a solvent mixture of γ-valerolactone based acidic catalysts.
(GVL) and H2O (9:1) at 150 °C. The glucose to catalyst ratio can
be calculated as 1:5 or 1:2 (considering 3 Cr ions per formula
Acknowledgements
unit), taking into account that only 60% of linker had a SO3H
moiety. In our study the glucose to MIL-SO3H ratio was 1:24 or
1:8. Su et al.58 synthesized MIL-101Cr-SO3H from monosodium
sulfoterephthalic acid, HF and Cr(NO3)3 instead of CrO3. The
average particle size of MIL-101Cr-SO3H was higher with 400
nm and the BET surface area was 1700 m2/g. Under their
reported conditions a highest 5-HMF yield of 45% and
selectivity of 46% could be achieved. Acidity was calculated
from back-titration experiments with saturated NaCl to 1.61
mmol/g. Unfortunately, no titration curves were given.58
The work was funded in part by the Deutscher Akademischer
Austauschdienst, DAAD through PPP-project no 57053987
(China 2j ab 14).
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Page 11 of 11
New Journal of Chemistry
View Article Online
DOI: 10.1039/C6NJ01399F
Graphical Abstract
5-Hydroxymethylfurfural
X
29%
Glucose
THF/H2O
80% selectivity
7%
MIL-101Cr-SO3H
Levulinic acid
Synopsis
MOFs are making progress also in catalytic biomass transformation, here in the 5-HMF production
with increased yield from glucose in low boiling solvents.