One pot conversion of glucose to ethyl levulinate over a porous hydrothermal acid catalyst in green solvents

Article abstract of DOI:10.1039/c9ra03902c

The one-pot conversion of glucose to ethyl levulinate over an acid-functionalised hydrothermal catalyst (derived from glucose) provides high initial yields up to 37 mol%, comparable to the homogeneous H₂SO₄ catalyst, whilst catalyst performance is strongly influenced by green solvent choice.

Full text of DOI:10.1039/c9ra03902c

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One pot conversion of glucose to ethyl levulinate
over a porous hydrothermal acid catalyst in green
solvents†
Cite this: RSC Adv., 2019, 9, 20341
ab
e
bcd
and Robin J. White *af
Monika Bosilj,
Johannes Schmidt, Anna Fischer
Received 23rd May 2019
Accepted 17th June 2019
The one-pot conversion of glucose to ethyl levulinate over an acid-functionalised hydrothermal catalyst
derived from glucose) provides high initial yields up to 37 mol%, comparable to the homogeneous
DOI: 10.1039/c9ra03902c
(
rsc.li/rsc-advances
H SO
2 4
catalyst, whilst catalyst performance is strongly influenced by green solvent choice.
1
With regard to further elaboration of biorenery concepts,
Along similar lines to levulinic acid production, the treat-
glucose is a key platform compound for the production of ment of sugar-based biomass (e.g. glucose), under acidic
2
11
a range of fuels and chemicals, including levulinic acid and its conditions generates carbonaceous humin by-products. In the
3
–5
esters. Obtaining these products in high yield from glucose is context of chemicals and fuel production, this solid formation
a challenging task in comparison to fructose conversion due to is undesired as it lowers product yield and oen deactivates the
12
hindering isomerisation of glucose to fructose. Nevertheless, catalyst via pore and active site blocking. Whilst the carbo-
using fructose as a starting compound is more costly than naceous by-product can be of value, efforts have been made to
glucose. Levulinic ester ethyl levulinate (EL) is of interest, as it is suppress the formation of these polymeric carbonaceous
6
a potential bio-derived solvent, avouring agent or plasticizer,
products. It has been reported that humin formation can be
7
as well as an octane fuel additive. Mechanistically, a one-pot inhibited to some extent via non-aqueous solvent use such as
ꢀ
3
4
13
14
synthesis of EL from C6-sugars (at 200 C) could proceed via methanol, EtOH, g-valerolactone (GVL) or ethylene glycol.
two routes. The rst one starts with glucose ethanolysis fol- Different types of solid acid catalysts have been proposed for
lowed by ethyl D-glucoside isomerisation to ethyl fructoside. one-pot conversion of glucose to EL, from sulphonated ionic
This rapidly dehydrates to 5-ethoxymethylfurfural (Scheme 1a). liquids, zeolites, sulphated metal oxides to sulphonated
-Ethoxymethylfurfural is further converted to EL. The second carbons. Carbon-based materials containing sulphonic acid
15
16
4
4,8
17
5
route is possible with glucose isomerisation to fructose, its groups are especially attractive catalysts in biorenery processes
6,9
dehydration to 5-HMF and nal esterication (Scheme 1b).
due to expected high stability and resistance to acidic and
High reaction temperatures and greater equivalents of alcohol chelating media. The advantage of carbon material is to allow
6
could favour intermolecular alcohol dehydration. It was re- catalyst hydrophilicity and acid–base character to be readily
ꢀ
ported that at lower temperatures (140 C) glucose reacts pref- tuned and usually have lower production costs than that of
erentially with ethanol (EtOH) to form ethyl D-glucoside,
10
preventing further glucose isomerisation and dehydration.
a
Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstraße 2, 79110 Freiburg
im Breisgau, Germany. E-mail: robin.white666@googlemail.com
b
Department of Inorganic and Analytical Chemistry, University of Freiburg,
Albertstrasse 21, 79104 Freiburg, Germany
c
Freiburg Materials Research Center (FMF), University of Freiburg, Stefan-Meier-Straße
2
1, 79104 Freiburg im Breisgau, Germany
d
FIT – Freiburg Center for Interactive Materials and Bioinspired Technologies,
University of Freiburg, Georges-K ¨o hler-Allee 105, 79110 Freiburg im Breisgau,
Germany
e
Technische Universit a¨ t Berlin, Fakult a¨ t II Institut f u¨ r Chemie, Sekretariat BA 2,
Hardenbergstr. 40, 10623 Berlin, Germany
f
The Netherlands Organisation for Applied Scientic Research (TNO), High Tech
Campus 25, 5656AE Eindhoven, The Netherlands
Scheme 1 (a) and (b) Proposed reaction pathway for one pot synthesis
of EL from glucose with EtOH, (c) HTC synthesis, (d) post-thermal
carbonisation and (e) introduction of S-containing functional groups
into HTC-400 support.
†
Electronic supplementary information (ESI) available: HPLC, and GC-MS of
liquid products, N sorption analysis, XPS and SEM analysis of HTC-400-S. See
2
DOI: 10.1039/c9ra03902c
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other conventional supports, such as alumina or silica.
Regarding the biomass derived catalysts, the sulphonation of
carbons synthesized by incomplete carbonisation of biomass
precursors has been an attractive way due to its simple
synthesis. However, these materials exhibit very low specic
2
ꢁ1
surface area (<10 m g ) and no porosity, which hinder the
18
diffusion of bulky, viscous reactants to active sites.
With regard to biomass derived catalysts, synthesised by
hydrothermal carbonisation (HTC) of sugar-based biomass (e.g.
glucose) with addition of sodium borate (borax) generates
structurally similar to sol–gel monoliths mesoporous xerogels
19
with high specic surface area (Scheme 1c). The synthesis,
characterisation and application of (meso)porous high specic
surface area sulphonated HTC catalysts have thus not been
explored. Typically, large micron sized, low surface, low porosity
HTC spheres have been sulphonated and investigated as
20–22
potential solid acids.
From a catalytic point of view (e.g. site
loading per catalyst bed volume), the introduction of porosity/
high surface area is clearly benecial. In this regard, the use
Fig. 1 (A) X-ray photoelectron spectra of S2p core level for HTC-400-
S, (B) effect of different solvents on EL yield (0.5 wt% HTC-400-S), (C)
of borax in the glucose (aq) solution is partly catalytic, partly EL yields obtained over 0.5 wt% HTC-400-S and 0.05 wt% H SO in
2
4
surface stabilising agent. It binds with glucose via the inherent comparison to non-catalysed reaction and (D) accessible acid sites vs.
EL yield in different solvents.
diol structure allowing access to higher yielding pathway to
HMF and the forming HTC material structure (e.g. via inhibi-
tion of acetalisation of glucose with 5-HMF), whilst also acting
conrming the evidence of two chemically different sulphur
species: a lower energy doublet S2p3/2 at 167.4 eV and 168.3 eV
to direct primary carbon nanoparticle size (via control of the
borax/glucose ratio). Aer the HTC was completed, borax was
24
23
attributed to sulphonic (–SO H) groups and a higher energy
3
doublet S2p1/2 at 168.5 eV and 169.5 eV assignment to sulphate
washed out and the carbonaceous solid product was thermally
2
ꢁ
25
ꢀ
functionalities (–SO4 ). In comparison to other glucose-
carbonised at 400 C under N for 5 h (Scheme 1d). To introduce
2
21
derived sulphonated materials, S-containing functionalities
Brønsted acid functionality (i.e. –SO
was dispersed in 98% w/w H SO
3
H), the carbonised xerogel
26
ꢀ
at lower binding energies such as thiol (C–SH) are not present
in our catalyst.
2
4
at 80 C for 4 h under N
2
(Scheme 1e, catalyst denoted as HTC-400-S). Further synthesis
In this communication, an acid-functionalised “carbona-
ceous” xerogel has been synthesised and applied as a catalyst in
the one pot synthesis of EL from glucose. The inhibition of
humin formation via the use of non-aqueous “green” solvents
such as EtOH, GVL and glycerol (GLY) in tandem with this
functional solid catalyst is also discussed. To our knowledge
GLY has so far not been investigated as a humin formation
inhibiting solvent. Catalytic reactions were performed with
details are reported in the ESI.† Porous HTC-400-S catalyst with
high specic surface area signicantly differs from the previ-
ously reported sulphonated hydrothermal carbons in the liter-
20–22
ature
due to the presence of the structure directive agent
borax in the HTC reaction, which tailors the material porosity
based on the HTC reaction time and its concentration. Further
ꢀ
post-thermal treatment at 400 C opens up interstitial porosity
between the primary particles due to the condensation process.
3
ꢁ1
2.4 wt% glucose dissolved in 20 g of solvent (i.e. EtOH, EtOH/
Consequently, high pore volume (Vpore ¼ 0.42 cm
g
) and
2
ꢁ1
GVL or EtOH/GLY; mass ratio of mEtOH/mGVL ¼ 1.86/1 or
mEtOH/mGLY ¼ 1.86/1). 0.5 wt% of HTC-400-S was loaded with
the desired reaction solution into a stainless-steel high pressure
specic surface area (SBET ¼ 506 m
g
) determined by N
2
sorption of HTC support were obtained (Table S1 and Fig. S1†),
which improve mass transport and diffusion through the cata-
lyst. The amount of acid groups on the HTC-400-S was initially
ꢀ
autoclave (v ¼ 50 mL) and reactions were performed at 200 C.
ꢁ
1
The data in Fig. 1B show the EL yields and glucose conversions
in different solvents achieved over time starting from 0.5 wt% of
HTC-400-S. It was observed that already aer t ¼ 30 min, 81%
conversion of glucose was reached in the reaction with EtOH as
a solvent. Using EtOH/GVL mixture increases the rate of glucose
conversion to 91% in comparison to the reaction in EtOH.
Glucose was completely converted t > 3 h. Maximum EL yields
determined through back titration. A loading of 2.6 mmol g
acidic groups was recorded for HTC-400-S. CHNS elemental
analysis (Table S1†) conrmed the presence of sulphur
ꢁ1
(
1.1 mmol g /3.6 wt%). As might be anticipated for a carbo-
naceous material of this nature, a higher number of Brønsted
acid sites were determined through titration, attributed to other
surface acidic groups (e.g. –OH, –COOH) in addition to the ex-
(
in mol%) over HTC-400-S were achieved aer t ¼ 6 h. Using
pected sulphonic (–SO H) acidity. Complementary, an analysis
of the X-ray photoelectron spectroscopy (XPS) survey for HTC-
3
GVL or GLY as a co-solvent with EtOH in the reaction obtained
higher EL yields than EtOH alone. The highest EL yield of
400-S indicated a loading of 1.94 at%/4.73 wt% for “S” con-
3
3
7 mol% was achieved in EtOH/GLY (t ¼ 6 h), a slightly lower of
taining surface functionality (Fig. S2a†). The high resolution
S2p core state for HTC-400-S revealed beside the S2p3/2–S2p1/2
spin orbital splitting, the existence of two doublets (Fig. 1A)
5 mol% by using mixture of EtOH/GVL and the lowest EL yield
of 25 mol% was measured by using EtOH as a solvent. This
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suggests that GVL and GLY promote the conversion of glucose for EtOH/GVL mixture aer four cycles. In EtOH/GVL mixture, GVL
to EL. To investigate if they initiate the conversion of glucose, may participate in the reaction and in turn improving EL yields.
the blank reactions without the catalyst were performed. EL Complete loss of catalytic activity in EtOH presumably results from
yield of 10 mol% was reached in EtOH/GVL mixture (Fig. 1C), humin formation and possible sulphur leaching. The EtOH/GLY
whereas in EtOH or EtOH/GLY solvent mixture no EL was mixture prevented visible humin formation (e.g. on the reactor
observed. A control reaction with EtOH/GVL solvent mixture wall), whereas EtOH/GVL only partially inhibited a visible deposi-
without the catalyst and glucose was also performed to explain tion of humins. It has been reported that GVL is capable of sol-
30
if EL could be formed only from GVL and EtOH. 3 mol% yield of ubilising humin but there are currently no reports on humin
EL was obtained (Fig. S7†). solubility in GLY. Despite decreased humin formation in GLY/
th
Most probably GVL undergoes dehydration to a-angelica EtOH and GVL/EtOH solvents, spent HTC-400-S catalyst aer 4
lactone and hydration to levulinic acid, which could further catalytic cycles presented a lower specic surface area, reduced
form EL with EtOH. Another pathway could be through the pore volume and S content (Table S1†). SEM images (Fig. S8†) do
hydration of GVL to g-hydroxyvaleric acid and its dehydroge- not show morphological changes at the micron level in the struc-
27
nation to levulinic acid. This observation warrants more study. ture of HTC-400-S aer usage in different solvents, but at higher
Regarding the enhanced EL yield in EtOH/GLY solvent mixture resolution analysis indicates “smoothing” of surface features. The
with HTC-400-S catalyst, GLY could potentially affect the reac- mass increase of the catalyst was observed aer 4 catalytic cycles as
tivity of the aldehyde group in glucose or in 5-HMF, as He et al. well as the increased amount of carbon presumed to be from
proved that glycerol as a solvent can promote activity of humin deposition on the HTC-400-S surface. The lowest content of
28
aldehydes.
S on HTC-400-S catalyst aer 4 catalytic cycles was observed when
One pot conversion of glucose performed at longer reaction only EtOH was used as a solvent and higher S contents were
times (t > 6 h) revealed lower EL yields due to the side reactions detected by using GVL/EtOH and GLY/EtOH solvent mixtures
(
Fig. 1B). GC-MS analysis proves different side products (Table S1†). Fig. 1D shows the EL yields in quantitative terms with
2
depending on the used solvent (Fig. S4–S6†). This indicates respect to mmol of accessible acidic sides per m determined by
solvent rearrangement/etherication reactions at longer reac- back titration of the fresh HTC-400-S catalyst and spent catalysts
tion times at 200 C.
ꢀ
nd rd
th
aer 2 , 3 and 4 cycle. The number of accessible acid sites per
2
nd
For comparison and to further ascertain solvent inuence on m of HTC-400-S signicantly decreases aer 2 catalyst reuse in
rd
th
the EL yield over HTC-400-S, control experiments were per- EtOH and therefore no EL in 3 and 4 cycle was obtained. Higher
formed using the homogeneous equivalent H SO (0.05 wt%) catalyst activity aer the 2 cycle in GVL/EtOH and GLY/EtOH
nd
2
4
2 4
(Fig. 1C). As might be expected, H SO performed well with EL mixtures is due to the higher number of accessible acid sites
yield of 34 mol% but no signicant differences in EL yields were which catalyse the conversion of glucose to EL. CHNS elemental
st
observed when using different solvent mixtures. Higher EL analysis and ICP-OES of liquid product solutions aer 1 catalytic
2 4
yields (ca. 48 mol%) obtained by H SO have been reported by run conrmed the leached sulphur in the product solution (Table
Xu et al. aer shorter reaction times (<1 h) however, using S1†). As expected, S loss from HTC-400-S in EtOH was the highest,
29
higher H SO concentrations (1 wt%).
suggesting GLY and GVL partially inhibit S leaching. This conrms
2
4
To demonstrate the advantage aspect of HTC-400-S catalyst our ndings in regard to complete deactivation of HTC-400-S aer
nd
in the one pot conversion of glucose to EL in green solvents, our the 2 recycle in EtOH due to the combination of S-leaching and
catalyst was compared to other carbonaceous acid catalysts as humin deposition on the catalyst. Leached S in the product
well as to commonly applied sulphated metal oxides (Table mixture could be removed by desulphurisation using ion exchange
31
S2†). It is observed that among carbonaceous catalysts, HTC- resins or precipitation methods with salts.
00-S has an outstanding performance in acid catalysed In conclusion, the use of EtOH/GVL and EtOH/GLY solvent
4
conversion of glucose as well as of fructose to EL probably due mixtures in one pot synthesis of EL from glucose was found to
to its high specic surface area, mesoporosity and high acid improve EL yield as well the reusability of HTC-400-S catalyst
density. In regard to the sulphate metal oxides they perform partly by preventing the leaching of S-containing functional
well and offer better recyclability due to the possible calcination groups and visible humin formation. Interestingly, EtOH/GVL
step, which removes the humin deposits from the catalyst solvent mixture was found to initiate the conversion of
surface. EL yields of mesoporous HTC-400-S catalyst were glucose to EL and also yielded very small amounts of EL. EtOH/
compared to the conventional non-porous HTC catalyst (deno- GLY mixture completely inhibited humin deposition on the
ted as HTC-S), which was synthesised without the addition of reactor wall – an advantage with regard to performing the same
borax and post-thermal treatment. Despite its high acidity reaction under continuous ow operation (e.g. avoidance of
ꢁ
1
(
2.9 mmol g ), HTC-S obtained lower EL yields probably as blockages). Further work should focus on the optimisation of
2
ꢁ1
a result of its low specic surface area (43.9 m g ) and non- reaction conditions (e.g. lower reaction temperature) to further
porous structure. inhibit humin formation and consequently improve catalyst
Regarding HTC-400-S catalyst re-use, complete deactivation was recyclability. Likewise, the scope for further modication of the
nd
observed aer the 2 recycle in EtOH (Fig. 1C). The reusability of carbonaceous support chemistry (e.g. via higher carbonisation
HTC-400-S was found to be better in EtOH/GVL and EtOH/GLY temperature) opens opportunities to further manipulate
solvent mixtures. The measured yield decreased from 37 mol% surface–reagent interactions and optimisation of catalyst
to 16 mol% for EtOH/GLY mixture and from 35 mol% to 15 mol% performance.
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