Direct transformation of carbohydrates to the biofuel 5-ethoxymethylfurfural by solid acid catalysts

Article abstract of DOI:10.1039/c5gc01043h

The direct conversion of glucose to 5-ethoxymethylfurfural (EMF) is a promising biomass transformation due to the potential application of the product as a biofuel. Here, the conversion of glucose to EMF was examined over several solid acid catalysts in ethanol between 96 and 125°C. Among the catalysts employed, dealuminated beta zeolites [DeAl-H-beta-12.5 (700)] gave a moderate yield of EMF (37%) in a single step catalytic process. A combined catalytic system consisting of H-form zeolite and Amberlyst-15 was found to be more efficient for the transformation of glucose to EMF (46%) via a one-pot, two-step reaction protocol. Alternative biomass-based mono-, di- and polysaccharides also gave moderate to good yields of EMF with the catalytic systems, including fructose which yielded 67% of EMF and 4% of ethyl levulinate (ELevu) along with 10% 5-hydroxymethyl furfural (HMF) in the combined reaction protocol. A significant amount of ELevu (1-16%), a rehydrated product of EMF and a promising fuel additive, was observed in this study. Recyclability studies suggested that it was possible to reuse the DeAl-H-beta-12.5 (700) catalyst in consecutive reactions without significant changes in product yields due to its easy recovery and thermal stability during regeneration.

Full text of DOI:10.1039/c5gc01043h

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Journal Name
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Direct transformation of carbohydrates to the biofuel
-ethoxymethylfurfural by solid acid catalysts
5
Cite this: DOI: 10.1039/x0xx00000x
a,b
a
b
a
Hu Li, Shunmugavel Saravanamurugan, Song Yang and Anders Riisager*
Received 00th xx 2014,
Accepted 00th xx 2014
DOI: 10.1039/x0xx00000x
www.rsc.org/
The direct conversion of glucose to 5-ethoxymethylfurfural terphthalic acid in polyester ꢀ as well as 2,5ꢀdimethylfuran (DMF),
EMF) is a promising biomass transformation due to the
(
2
,5ꢀdiformylfuran (DFF), 2,5ꢀbis(hydroxylmethyl)furan (BHMF), γꢀ
products potential application as a biofuel. Here, the
conversion of glucose to EMF was examined over several
solid acid catalysts in ethanol between 96 and 125 C.
valerolactone (GVL) and maleic anhydride (MA). However,
commonly these products have rarely been reported to be efficiently
produced from hexoses in a single oneꢀpot reaction.
ο
4
Among the catalysts employed, dealuminated beta zeolites
[
(
DeAl-H-beta-12.5 (700)] gave a moderate yield of EMF
37%) in a single step catalytic process. A combined In addition to the above mentioned furanic compounds, 5ꢀ
catalytic system consisting of H-form zeolite and
Amberlyst-15 was found to be more efficient for the
transformation of glucose to EMF (46%) via an one-pot,
two-step reaction protocol. Alternative biomass-based
mono-, di- and polysaccharides formed also moderate to
good yields of EMF with the catalytic systems, including
ethoxymethylfurfural (EMF) with a high energy density of 30.3 MJ
−1
−1
−1
L , close to that of diesel (33.6 MJ L ), gasoline (31.1 MJ L ) and
−1
5
ethanol (23.5 MJ L ), is identified to be a promising liquid biofuel.
The synthesis of EMF from HMF can be achieved in high yields (>
0%) by etherification with various acidic catalysts including
Cl, AlCl and FeCl ), dissolved acids (e.g.,
levulinate (ELevu) along with 10 % 5-hydroxymethyl H PW O , H SO4 and pꢀTSA), and solid acids (e.g.,
9
fructose which yielded 67 % of EMF and 4% of ethyl chlorides (e.g., NH
4
3
3
3
12 40
2
furfural (HMF) in the combined reaction protocol. A
significant amount of ELevu (1-16 %), a rehydrated
product of EMF and promising fuel additive, was observed
in this study. Recyclability studies suggested that it was
possible to reuse the DeAl-H-beta-12.5 (700) catalyst in
H SiW O /MCMꢀ41, sulfonated graphene oxide, aluminumꢀ
4
12 40
6
exchanged Kꢀ10 clay and AlꢀTUDꢀ1). However, the cost of HMF is
relatively high (price of 100 g HMF is 900 EUR at SigmaꢀAldrich)
and it is rather instable even at room temperature. Thus, from a
consecutive reactions without significant changes in product processing viewpoint the development of a highly efficient catalytic
yields due to its easy recovery and thermal stability during system providing oneꢀpot conversion of sugars into EMF would be
regeneration.
desirable.
Today far most carboneous chemicals and fuels are derived from the
Sulfonated catalysts have generally been found to be efficient
fossilꢀbased resources oil, coal and natural gas. However, increasing
concern about, e.g. resource availability and diminution, pollution
catalysts for the conversion of fructose into EMF. For instance,
SO Hꢀfunctionalized ionic liquids afforded 54 % yield of EMF and
3
and global warming has sparked significant interest into utilization
of biomass as a future renewable carbon source for fuels and valueꢀ
added chemicals. In terrestrial biomass, carbohydrates account for up
7
6
% ELevu from fructose in hexaneꢀethanol (100 ºC, 80 min). Also,
a high EMF yield of 63 % could be obtained from fructose in ethanol
100 ºC, 24 h) using sulfonic acid supported on silica as a
heterogeneous catalyst. Similarily, sulfonic acid functionalised
cellulose, graphene oxide, and silicaꢀencapsulated Fe O catalysts
(
1
to 75% of the renewable carbon, and several catalytic routes have
8
been developed for transformation of carbohydrates to useful
3
4
2
products via heterogeneous, homogeneous and enzymatic processes.
were all highly efficient for conversion of fructose to EMF in
This includes in particular compounds like 5ꢀhydroxymethylfurfural
9
ethanol, consistently giving >70 % yield of EMF. Likewise,
(
HMF) which has been demonstrated to serve as a platform molecule
phosphotungstic acid (PA) based heterogeneous catalysts such as
silica coated magnetic Fe O nanoparticles and Kꢀ10 clay supported
3
for the production of various biofuels and chemicals such as, e.g.
,5ꢀfurandicarboxylic acid (FDCA) ꢀ a promising substitute for
3
4
2
PA have also been reported to enable efficient synthesis of EMF
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/
o
C
u
5GrnC0
a
1
l
0
N
43
a
H
me
from fructose in ethanol yielding 55 and 62
% EMF, These findings have now prompted us to extrapolate a reaction
10a,b
respectively.
Recently, we have further reported an EMF yield of protocol for the direct conversion of glucose to EMF.
7
7 % from fructose in ethanol/DMSO solvent mixture with a biꢀ
1
0c
functional amino acidꢀheteropoly acid catalyst. Notably, the In the present work, we have examined a series of solid acid
highest EMF yield (91 %) achieved so far from fructose was catalysts ꢀ including commercial zeolites as well as modified zeolites
obtained with an organicꢀinorganic hybrid solid catalyst methyl ꢀ for the direct conversion of glucose into EMF in an oneꢀpot oneꢀ
imidazolebutylsulfate phosphotungstate ([MImBS] PW O ) in step reaction protocol. Control of the formation of EMF is quite
3
12 40
1
1
ethanol (90 °C, 24 h).
challenging as it gets rehydrated to form ELevu in the presence of
strong acid catalysts. The study has thus also simultaneously focused
2
1
In comparison to the number of studies performed with fructose as on ELevu formation, as it is also an important fuel additive. Oneꢀ
substrate, significant less work has focused on the catalytic pot two step reactions were also performed with a series of
conversion of glucose to EMF via fructose as intermediate. This is combined solid acids, for example zeolites and Amberlystꢀ15, in
surprising since glucose is by far the major carbohydrate source in order to enhance the catalytic performance towards EMF formation.
terrestrial biomass as cellulose. Thus, only a few catalytic systems For comparison, the reactivity of both catalytic systems were further
have been reported to be suitable for the conversion of glucose to studied with different carbohydrate substrates under optimised
5b,6a,12
EMF,
and a maximum EMF yield of 38 % was obtained in a reaction conditions. Lastly, catalyst recycling experiments were
homogeneous ethanolic systems with the Lewis acid AlCl as performed in order to demonstrate the versatility of the catalytic
3
12
catalyst.
system.
In the direct conversion of glucose to EMF, the isomerisation of Results and Discussion
glucose to fructose (Lobry–de Bruyn–van Ekenstein transformation)
is an important – and often decisive ꢀ reaction step which is Catalytic conversion of glucose to EMF with zeolites in one-pot,
1
3
one-step process
predominantly catalysed by base or Lewis acid. Tandem reaction
systems combining glucose isomerisation promoted by Lewis
acids/bases with subsequent dehydration of the formed fructose by
Brønsted acids have been extensively investigated. A combination
of such two types of catalysts, for example hydrotalcite and sulfonic
acidꢀresin, promoting each reaction step were reported to yield >60 %
Intrigued by the previous studies described above, we have
examined whether commercial zeolites ꢀ which are industrially
common materials ꢀ could be used as efficient heterogeneous
catalyst for the direct transformation of glucose into EMF. Initially, a
series of preliminary experiments were carried out to evaluate the
catalytic activity of commercial zeolites as well as sulfonated solid
acids for the conversion of fructose to EMF in ethanol at 96 ºC
1
4b
of HMF from glucose. On the other hand, zeolites containing both
Lewis and Brønsted acid sites have shown to be efficient single
1
5
standing catalysts for such conversions.
Most recently, a
(Table S1, ESI). The obtained EMF yields were between 8 and 18 %
commercial Hꢀbeta zeolite with a moderate Si/Al ratio of 25 was
shown to form HMF from glucose in 50 % yield at 81 % glucose
with the zeolite catalysts. In contrast, sulfonated solid acids gave
much higher yields of EMF, suggesting the presence of strong
Brønsted acid sites promoted the dehydration of fructose to HMF
and etherification of HMF to EMF but also rehydration of
HMF/EMF to ELevu. Among the catalyst employed, Amberlystꢀ15
was found to be the most efficient catalyst for conversion of fructose
to EMF yielding 61 % product along with 13 % ELevu under
ο
conversion (62 % selectivity) at 150 C after 50 min in the ionic
16
liquid 1ꢀbutylꢀ3ꢀmethylimidazolium chloride ([BMIm]Cl).
Likewise, a modified beta zeolite prepared by calcinating at 750 ºC
gave 43 % yield of HMF at 78 % glucose conversion (55 %
selectivity) in a mixture of solvents consisting of water, DMSO, and
1
7
THF at 180 ºC after 3 h.
optimised reaction conditions. On the other hand, SO HꢀSBAꢀ15, a
3
mesoporous functionalised material, gave a global yield of 71 % (50 %
EMF and 21 % Elevu). It should be noted that the formation of
ELevu was also observed with all the catalysts, and the yield was
between 3 and 21 %.
Recently, we have found that commercially available beta and USY
zeolites are universally promising catalysts for efficient aldoseꢀ
ketose isomerisation in methanol and water in oneꢀpot twoꢀstep or
oneꢀpot oneꢀstep reactions at 100ꢀ120 C. This has been
demonstrated for the isomerisation of glucose to fructose (>50 %
yield), xylose to xylulose (47 % yield) and erythrose to erythrulose
o
18
When glucose was used as substrate the commercial zeolites gave
lower than 10 % combined yields of HMF and EMF in ethanol at 96
and 110 °C after 11 and 6 h, respectively (Table S2 and Fig. S1, ESI).
Similary, Amberlystꢀ15 proved to form predominantly ethyl
glucopyranosides and not EMF under these reaction conditions.
However, significant increased EMF yields were obtained with the
zeolites at reaction temperature of 125 °C as shown by the compiled
results in Table 1. The Hꢀbeta (12.5) and Hꢀbeta (19) catalysts gave
here yields of 18ꢀ20 % of EMF (Table 1, entries 5 and 6).
Modification of the Hꢀbeta catalysts by calcination at high
temperature (i.e., 700ꢀ850 °C) was further performed to induce
(42 % yield). Moreover, we have also shown that glucose can be
directly converted to their corresponding alkyl levulinates in about
1
9
5
0 % yield with the same zeolite catalysts at higher temperatures.
In contrast, sulfonic acid functionalised SBAꢀ15 or ionic liquids
afforded ethylꢀ ꢀglucopyranoside (EDGP) in high yields (70ꢀ80 %)
in ethanol under identical reaction conditions rather than ethyl
D
2
0
levulinate.
Hence, this clearly indicated that the product
distribution could be well adjusted by proper selection of acid types
contained in the used catalysts as well as by the reaction conditions.
2
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thermal dealumination by partial cleavage of SiꢀOꢀAl bonds in the measurements (Fig. S2, ESI). In the NH ꢀTPD profiles, a peak with
3
17
framework. The resulting thermally dealuminated catalysts, DeAlꢀ Tmax at 190 °C corresponding to a weak acid site was found to be
Hꢀbeta (12.5)ꢀ700, DeAlꢀHꢀbeta (19)ꢀ700, DeAlꢀHꢀbeta (12.5)ꢀ800 more intense in the DeAlꢀHꢀbeta (12.5)ꢀ700 catalyst compared to the
and DeAlꢀHꢀbeta (12.5)ꢀ850 (Table 1, entries 7ꢀ10), all had unmodified Hꢀbeta (12.5) material, indicating that weak acid sites
improved catalytic performance yielding 33ꢀ37 % EMF along with 4 could be responsible for the isomerisation of glucose to fructose, as
1
8
%
ELevu. In order to get insight on the acid sites of the parent and also observed in our previous studies.
modified Hꢀbeta (12.5), the catalysts were subjected to NH ꢀTPD
3
a
Table 1 Zeoliteꢀcatalyzed conversion of glucose in oneꢀpot, oneꢀstep reaction.
b
c
Entry
Catalyst
Conversion (%)
Product Yield (%)
BioꢀHEs Yield (%)
HMF
5
5
5
14
7
EMF
13
1
10
1
18
20
37
36
33
34
29
16
ELevu
EMFda
<1
0
<1
1
2
3
4
5
6
7
8
9
HꢀY (2.6)
HꢀUSY (30)
HꢀUSY (6)
Hꢀmordenite (10)
Hꢀbeta (19)
96
84
92
37
85
90
87
84
84
86
88
87
7
2
16
1
9
10
4
4
4
4
1
25
8
31
16
34
38
54
52
52
49
35
0
<0.5
<0.5
<1
<1
<1
<1
<0.5
<1
Hꢀbeta (12.5)
8
DeAlꢀHꢀbeta (12.5)ꢀ700
DeAlꢀHꢀbeta (19)ꢀ700
DeAlꢀHꢀbeta (12.5)ꢀ800
DeAlꢀHꢀbeta (12.5)ꢀ850
Snꢀbeta
13
11
15
11
5
1
1
1
0
1
2
BꢀHꢀbeta (12.5)
11
1
28
a
b
c
Reaction conditions: 0.15 g glucose, 100 mg catalyst, 5 mL ethanol, T = 125 ºC, t = 6 h. Si/Al ratio of the zeolites are denoted in parenthesis. BioꢀHEs denote HMF,
EMF, ELevu and EMFda (5ꢀethoxymethylfurfural diethyl acetal).
A timeꢀcourse study was further conducted with DeAlꢀHꢀbeta thus indicating that the presence of strong Lewis acid sites promoted
(12.5)ꢀ700 to examine the reaction pathway leading to EMF (Fig. S3, the glucose isomerisation. Recently, boric acid modified zeolites
ESI). The results showed that glucose was relatively fast converted (> with extraꢀframework B–OH species and retained silanols were
2
2
7
0 %), but only about 20 % EMF was generated after 2 h of reaction. demonstrated to be highly efficient for dehydration reactions.
After a reaction time of 10 h, the formation of HMF and EMF Nevertheless, such a prepared catalyst, BꢀHꢀbeta (12.5), showed
reached 20 and 40 %, respectively. At prolonged reaction time EMF poor catalytic activity for formation of EMF in the present reaction
started to get rehydrated to ELevu and converted to undesired compared to the modified beta zeolites (Table 1, entry 12), possibly
products (humins), clearly indicating that careful optimisation of because dehydration of the glucose was difficult.
reaction conditions are important in order to obtain a high yield of
1
3
EMF. In a supplementary experiment with 2ꢀ Cꢀglucose in ethanolꢀ Based on the obtained results, the possible reaction pathways
6
13
d (step 1: 110 °C, 1 h; step 2: 125 °C, 3 h), C NMR analysis of for the catalytic conversion of glucose to HMF, EMF, EMFda
reaction samples after each step (Fig. S4, ESI) confirmed that the and ELevu are compiled in Scheme 1. Initially, glucose was
sugarꢀbased intermediates fructose (δ/ppm
fructofuranose; 102.1, βꢀDꢀfructofuranose;
=
104.6, αꢀDꢀ rapidly converted into fructose as well as EDFF (confirmed by
1
3
98.8,
βꢀDꢀ
C NMR, see above), which were further partially transformed
fructopyranose), ethylꢀDꢀglucopyranoside (EDGP) (δ/ppm = 74.8, into EDGP by Lewis and Brønsted acid sites present in the
ethylꢀβꢀDꢀglucopyranoside; 72.5, ethylꢀαꢀDꢀglucopyranoside) and zeolite. Subsequently, the obtained EDFF could form EMF via
ethylꢀDꢀfructofuranoside (EDFF) (δ/ppm
= 107.6, ethylꢀαꢀDꢀ dehydration with strong acid sites of the catalysts. At the same
fructofuranoside; 103.9, ethylꢀβꢀDꢀfructofuranoside; 100.4, ethylꢀβꢀ time, fructose could also be dehydrated to produce HMF, which
Dꢀfructopyranoside) accounted for the high conversion of glucose would be further etherified to give EMF. At this stage, strong
(
unconverted glucose: δ/ppm = 73.6, βꢀDꢀglucopyranose; 72.1, αꢀDꢀ acid sites at moderate reaction temperature or weaker acid sites
glucopyranose). Likewise, the analysis corroborated that EMF at high reaction temperature could promote to increase the yield
δ/ppm = 151.9) was preferentially formed from glucose via the two of EMF. It should be noted that EDGP could hardly be further
(
step cascade reactions.
converted as shown by the obtained results with EDGP as
substrate (Table 4), while mannose could isomerise to fructose
The other examined zeolites, including HꢀUSY (30) and Hꢀ and successively convert to EMF. It is well known that HMF
mordenite (10), were found to be less active than the dealuminated can be transformed into ELevu and formic acid/ethyl formate
2
0
samples for the catalytic transformation. On the other hand, Snꢀ (FA) in ethanol, and a similar reaction route is expected for
containing beta zeolite yielded 29 % of EMF (Table 1, entry 11) conversion of EMF into ELevu.
which was higher than the yield obtained with the Hꢀbeta zeolites,
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Jou5GrnC0al Name
Catalytic conversion of glucose to EMF with combined solid
acids in one-pot, two-step process
As mentioned above, commercial zeolites as well as Amberlystꢀ15
gave poor yields of HMF and EMF from glucose in ethanol at 96 °C.
The latter catalyst was, however, a highly efficient catalyst for the
conversion of fructose to EMF (Table S2, ESI). Accordingly, a
reaction approach from glucose to EMF using the two different types
of catalyst in cascade was examined. When performing the reaction
with an optimised mass ratio of HꢀUSY (6) and Amberlystꢀ15 of 3:1
(Fig. S5, ESI) the yield of EMF was significantly increased (as
anticipated) compared to using HꢀUSY (6) alone (Table 2, entries 1
and 4), implying that HꢀUSY (6) was responsible for the
isomerisation of glucose to fructose and Amberlyst for the
dehydration of fructose and ethyl fructosides (as discussed earlier,
13
based on C NMR results) to enhance the yield of EMF. Adversely,
13
the formation of ethyl glucosides (confirmed by C NMR) in large
amount in the presence of Amberlystꢀ15, indicated that conversion
of glucose to ethyl glucoside was facile and that EMF was partly
converted to a rehydrated product ELevu (14 %) thus lowering the
yield of EMF. It has previously been reported that the addition of
dimethylsulfoxide (DMSO) into ethanol could efficiently improve
6
f,9b
the yields of the furanic products.
Therefore, an appropriate
amount of DMSO and ethanol was also used as the reaction solvent
for the conversion of glucose to EMF (Figs. S6 and S7A, ESI).
Under optimal reaction conditions (131 °C, 24 h) a combined yield
of HMF and EMF of 46 % could be achieved with this approach
Scheme 1. Suggested reaction pathways for the catalytic conversion
of sugars to HMF, EMF, EMFda and ELevu.
(Table 2, entry 5), however the DMSO lowered considerably the
selectivity to EMF. The combined yield was increased slightly to 54 %
at 114 °C, but the selectivity remained low (results not shown).
Based on these results, we speculate that the generation of EDPG in
high yield as well as ELevu, 5ꢀethoxymethylfurfural diethyl acetal
(EMFda) and humins account for the high glucose conversion and
lowering of EMF yields.
a
Table 2. Catalytic conversion of glucose with solid acids via different processes.
b
c
Entry Catalyst
Conversion (%)
Product Yield (%)
BioꢀHEs Yield (%)
HMF
EMF
6
<0.1
2
17
26
46
40
43
36
44
40
ELevu
EMFda
<1
0
<0.5
<1
<0.5
<1
<0.5
<1
<0.5
<1
<1
1
2
3
4
HꢀUSY (6)
Amberlystꢀ15
Snꢀbeta
92
98
76
91
98
87
83
86
84
85
88
2
<1
2
3
20
3
5
1
4
3
1
1
<1
14
1
3
2
1
<1
2
9
2
5
34
47
53
48
46
41
50
HꢀUSY (6) + Amberlystꢀ15
HꢀUSY (6) + Amberlystꢀ15
HꢀUSY (6) + Amberlystꢀ15
HꢀUSY (6) + Amberlystꢀ15
Snꢀbeta + Amberlystꢀ15
Snꢀbeta + Amberlystꢀ15
DeAlꢀHꢀbeta (12.5)ꢀ700 + Amberlystꢀ15
DeAlꢀHꢀbeta (19)ꢀ700 + Amberlystꢀ15
d
5
6
7
8
9
e
f
e
f
e
e
1
1
0
1
3
2
46
a
b
c
Reaction conditions: 0.15 g glucose, 100 mg catalyst, 5 mL ethanol, T = 96 ºC, t = 11 h. Si/Al ratio of the zeolites are denoted in parenthesis. BioꢀHEs denote HMF,
d
e
EMF, ELevu and EMFda. T = 131 ºC, t = 24 h, 5 mL DMSO/ethanol (3:7). Oneꢀpot, twoꢀstep process with successive addition of 75 mg zeolite for 5 h followed by 25
f
mg Amberlystꢀ15 for 6 h. Oneꢀpot, twoꢀstep process where the zeolite was removed after 5 h followed by addition of Amberlystꢀ15 for 6 h.
In order to enhance the yield of EMF and to supress the protocol was developed with successive addition of HꢀUSY (6)
formation of EDGP, an optimised oneꢀpot, twoꢀstep reaction and Amberlystꢀ15 catalysts for 5 and 6 h of reaction,
4
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respectively (Table S3, Figs. S8 and S9, ESI). Interstingly, a Dealuminated Hꢀbeta (12.5) and Hꢀbeta (19) used in
substantial improvement in the yield of EMF (46 %) with a combination with Amberlystꢀ15 also exhibited good catalytic
relatively high selectivity of 53 % was achieved with glucose in performance in the oneꢀpot, twoꢀstep approach (Table 2, entries
ethanol with the twoꢀstep reaction protocol (Table 2, entry 6). 10 and 11), clearly indicating the important role of the
For comparison, Snꢀbeta
ꢀ a wellꢀknown catalyst for Lewis/weak acid sites generated by the dealumination for the
isomerisation and epimerization of glucose into fructose and glucose isomerisation in the first reaction step. To enhance the
2
3
mannose, respectively ꢀ was also applied in the twoꢀstep yield of EMF in the second reaction step, alternative Brønsted
reaction. Here a comparable yield of EMF (43 %) was observed acidꢀcontaining catalysts were also explored instead of
(
Table 2, entry 8), suggesting that similar intermediates might Amberlystꢀ15 in combination with HꢀUSY (6) (Table 3). The
have formed as found with HꢀUSY (6). To elucidate the role of four employed alternative sulfonic acidꢀbased catalysts gave
HꢀUSY (6) and Snꢀbeta in the second reaction step, the zeolites global yields of 31ꢀ42 % EMF and 2ꢀ4 % of ELevu (Table 3,
were separated from the reaction mixture before adding entries 1ꢀ4), which were slightly lower than obtained with
Amberlystꢀ15 for the subsequent reaction step (Table 2, entries Amberlystꢀ15 (Table 2, entry 6). Moreover, sulphated zirconia
7
and 9). In these experiments, the yields of EMF decreased only gave 10 % of EMF under the applied reaction conditions
slightly whereas the yields of HMF increased slightly, thus (Table 3, entry 4).
demonstrating the ability of the zeolites to induce isomerisation
as well as etherification also after the addition of Amberlystꢀ15.
a
Table 3. Catalytic conversion of glucose with alternative Brønsted acid catalysts in oneꢀpot, two step reaction.
b
Entry Catalyst
Conversion (%)
Product Yield (%)
BioꢀHEs Yield (%)
HMF
EMF
39
42
31
33
ELevu
EMFda
<1
<1
<1
1
1
2
3
4
5
HꢀUSY (6) + SBAꢀ15ꢀSO H
HꢀUSY (6) + Dowex 50WX8ꢀ100
HꢀUSY (6) + Nafion NR50
HꢀUSY (6) + Amberlyst 70
88
92
92
85
54
4
4
5
7
2
3
4
3
2
1
47
51
40
43
14
3
HꢀUSY (6) + SO (3.9 wt%)ꢀZrO2
10
<1
4
a
Reaction conditions: 0.15 g glucose, 100 mg catalyst, 5 mL ethanol, T = 96 ºC, t = 11 h. Oneꢀpot, twoꢀstep process with successive addition of 75 mg
b
zeolite for 5 h followed by 25 mg Brønsted acid catalyst for 6 h. BioꢀHEs denote HMF, EMF, ELevu and EMFda.
Catalytic conversion of carbohydrates to EMF with two towards hydrolysis to glucose under the applied reaction
different catalytic processes
conditions.
The developed reaction approaches to form EMF were
In order to evaluate the stability of HMF, when formed as an
extended to include other carbohydrate substrates than glucose
with selected catalysts. The results are compiled in Table 4. A
two compounds were further used as substrates. Under the
relatively high total yield of EMF (67 %) and ELevu (4 %) was
intermediate, and the reactivity of EMF as a final product the
optimized reaction conditions, the stability of EMF in ethanol
obtained with fructose using HꢀUSY (6) in combination with
over DeAlꢀHꢀbeta (12.5)ꢀ700 at 125
°
C in the oneꢀpot process
was demonstrated to be higher than when using HꢀUSY (6) +
Amberlystꢀ15 at 96 C in the twoꢀstep reaction process (Table 4,
Amberlystꢀ15 while lower total yield of EMF (55 %) and
ELevu (7 %) was found using DeAlꢀHꢀbeta (12.5)ꢀ700 (Table 4,
entries 2 and 11), thus indicating the former catalyst
combination to be advantageous for the dehydration of
fructose/ethyl fructoside. In contrast, the two catalyst
combinations yielded more comparable global yields (EMF and
ELevu) from mannose (36 and 44 %) and glucose (45 and 49 %)
°
entries 7 and 16). This suggests that EMF was in part
rehydrated to form ELevu (20 %) in the presence of the strong
acid catalyst Amberlystꢀ15. When using HMF as substrate
comparable yields of EMF were formed with the two catalytic
systems (Table 4, entries 8 and 17), demonstrating that the
etherification of HMF were facile with both zeolite catalysts.
(
Table 4, entries 1, 3, 10 and 12), suggesting the reactivity
towards formation of fructofuranose intermediates and HMF to
2
4
be similar in line with ealier observations. Noteworthy,
relatively high yields between 43ꢀ58 % of EMF along with 4ꢀ
1
3 % of ELevu could be obtained from sucrose and inulin over
both catalytic systems (Table 4, entries 4ꢀ5 and 13ꢀ14), as a
result of the higher reactivity of the fructose units of these
sugars. On the other hand, only trace amounts of EMF was
obtained from cellobiose systems (Table 4, entries 6 and 15)
due to low substrate solubility in ethanol and less reactivity
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COMMUNICATION
DOI: 10.1039/C5G 1043H
JournC0al Name
Table 4. Catalytic conversion of carbohydrates via two different processes.
c
Entry Catalyst
Substrate
Product Yield (%)
BioꢀHEs Yield (%)
Glucose
HMF
EMF
46
67
40
50
58
<0.1
48
70
2
41
55
35
43
52
1
ELevu
EMFda
<1
<0.5
0
<0.5
<0.5
0
2
<1
<1
1
<0.5
<0.5
1
1
<0.5
2
a
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
HꢀUSY (6) + Amberlystꢀ15
Glucose
Fructose
Mannose
Sucrose
Inulin
13
0
3
10
1
6
6
3
4
53
81
45
69
71
5
81
90
11
57
68
51
53
61
6
a
HꢀUSY (6) + Amberlystꢀ15
HꢀUSY (6) + Amberlystꢀ15
HꢀUSY (6) + Amberlystꢀ15
HꢀUSY (6) + Amberlystꢀ15
a
a
a
a
a
a
a
0
13
2
<0.5
0
0
3
6
1
1
4
13
7
2
20
5
3
4
7
<0.5
4
HꢀUSY (6) + Amberlystꢀ15 Cellobiose
3
HꢀUSY (6) + Amberlystꢀ15
HꢀUSY (6) + Amberlystꢀ15
HꢀUSY (6) + Amberlystꢀ15
DeAlꢀHꢀbeta (12.5)ꢀ700
DeAlꢀHꢀbeta (12.5)ꢀ700
DeAlꢀHꢀbeta (12.5)ꢀ700
DeAlꢀHꢀbeta (12.5)ꢀ700
DeAlꢀHꢀbeta (12.5)ꢀ700
DeAlꢀHꢀbeta (12.5)ꢀ700
DeAlꢀHꢀbeta (12.5)ꢀ700
DeAlꢀHꢀbeta (12.5)ꢀ700
EMF
HMF
EDGP
Glucose
Fructose
Mannose
Sucrose
Inulin
11
14
5
11
6
16
5
3
b
0
1
2
3
4
5
6
7
b
b
b
b
b
b
b
4
2
<0.1
0
5
Cellobiose
EMF
5
7
13
<0.5
10
8
66
63
85
87
HMF
0
3
a
a
Reaction conditions: 0.15 g glucose, 100 mg catalyst, 5 mL ethanol. Oneꢀpot, twoꢀstep process with successive addition of 75 mg zeolite for 5 h followed
b
c
by 25 mg Amberlystꢀ15 at 6 h at temperature of 96 °C. Oneꢀpot, oneꢀstep process. T = 125 °C, t = 10 h. BioꢀHEs denote HMF, EMF, ELevu and EFMda.
Influence of glucose concentration and catalyst amount on 1). After each reaction run, the catalyst was regenerated by
the catalytic conversion of glucose to EMF
calcination at 550 °C for 6 h to ensure that deposited carbonaceous
products was removed. Through all five consecutive catalytic runs,
the EMF yield remained between 28ꢀ38 % along with a HMF yield
of 10ꢀ12 %. This demonstrated that the catalytic performance of the
DeAlꢀHꢀbeta (12.5)ꢀ700 catalyst was preserved in the consecutive
cycles, suggesting that the catalyst system is highly suitable for reuse.
DeAlꢀHꢀbeta (12.5)ꢀ700 exhibited slightly lower catalytic
a
performance for the production of EMF from glucose compared to
the combined catalyst approach, but the catalyst was easily operated
and separated from the reaction mixture at the end of the reaction.
Accodingly, the influence of glucose concentration and catalyst
amount on reactivity was studied using this catalyst (Table 4). When
increasing the glucose amount from 0.15 g (3.7 wt%) to 0.30 g (7.1
wt%) an unchanged EMF yield was observed (40 %), showing that
the catalyst remained active also at higher concentration of glucose
(Table 5, entries 1 and 2). Also when the amount of glucose was
increased further to 0.50 g (11.3 wt%; Table 5, entry 3) a significant
amount of EMF (31 %) was formed, despite the solubility limit of
glucose in ethanol was surpassed (making it impossible to report a
conversion). As to catalyst dosage, it was found that the yield of
EMF decreased only slightly from 40 to 38 % when the catalyst
loading was reduced from 100 to 75 mg (Table 5, entries 2 and 4),
but more drastically to 30 and 24 % EMF at catalyst loadings of 50
and 25 mg, respectively, as expected (Table 5, entries 5 and 6).
Consequently, 0.30 g glucose in 5 mL of ethanol (7.1 wt%) and 75
mg DeAlꢀHꢀbeta (12.5)ꢀ700 was used as the optimized reaction
conditions to evaluate catalyst recyclability.
Fig. 1 Recycling of DeAlꢀHꢀbeta (12.5)ꢀ700 in the catalytic
conversion of glucose to EMF. Reaction conditions: 1.20 g glucose,
Catalyst recycling in the catalytic conversion of glucose to EMF
3
00 mg DeAlꢀHꢀbeta (12.5)ꢀ700, 20 mL ethanol, T = 125 °C, t = 10
h. Glucose and solvent were adjusted to the amount of recovered
DeAlꢀHꢀbeta (12.5)ꢀ700 in reaction runs number twoꢀfive.
The DeAlꢀHꢀbeta (12.5)ꢀ700 catalyst was evaluated for the
transformation of glucose to EMF (and HMF) in ethanol in five
consecutive reaction cycles under optimized reaction conditions (Fig.
6
| J. Name., 2014, 00, 1-8
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DOI: 10.1039/C5GC01043H
a
Table 5. Effect of glucose concentration and catalyst amount on glucose conversion by DeAlꢀHꢀbeta (12.5)ꢀ700.
b
Entry
Catalyst amount
mg)
Glucose amount
Conversion
Product Yield (%)
BioꢀHEs Yield (%)
(
(g)
(%)
94
95
ꢀ
92
89
69
HMF
11
11
3
13
11
8
EMF
41
40
ELevu
EMFda
1
2
3
4
5
6
100
100
100
75
50
25
0.15
0.30
4
5
1
2
2
1
<0.5
<0.5
57
58
45
54
43
32
c
0.50
31
9
0.30
0.30
38
2
30
1
0.30
24
0.3
a
b
c
Reaction conditions: 5 mL ethanol, T = 125 ºC, t = 10 h. BioꢀHEs denote HMF, EMF, ELevu and EFMda. Glucose amount above the solubility limit.
Conclusions
mg of the sample was placed in a quartz reactor and degassed at
00 C for 1 h in a flow of helium of 50 mL/min. The reactor
was then cooled to 100 C and ammonia (50 mL/min) was
allowed to get adsorbed at the same temperature for 2 h. The
sample was then flushed with helium at a flow of 50 mL/min to
remove physisorbed ammonia, before ammonia desorption was
6
°
In summary, we have demonstrated that a combination of the
solid catalysts HꢀUSY (6) and Amberlystꢀ15 can directly and
efficiently transform biomassꢀderived carbohydrates such as,
e.g. glucose, sucrose and inulin, to the biofuel EMF in high
yields (40ꢀ60%) along with 3ꢀ13% of ELevu in ethanol via an
oneꢀpot, twoꢀstep reaction process. On the other hand,
dealuminated beta zeolites, including DeAlꢀHꢀbeta (12.5)ꢀ700
and DeAlꢀHꢀbeta (19)ꢀ700, can give comparable yields of EMF
°
measured every one second from 100 to 600
ramp of 10 C/min.
°C with a heating
°
Reaction procedure
(
~40%) through a single step process. Moreover, a high yield of
Catalytic batch experiments were all performed in Ace pressure
tubes. In the oneꢀpot, twoꢀstep reaction process zeolite catalyst
EMF (67 %) and ELevu (4 %), a global yield of 71 %, was
obtained from fructose with a combined catalytic system. The
proposed reaction pathway showed that fructose and EDFF
formed from glucose, and both compounds were important
intermediates for obtaining enhanced yields of EMF with
zeoliteꢀbased catalytic systems. Importantly, the dealuminated
beta zeolites seemed to maintain their structural integrity in
ethanol, allowing the catalysts to be reused for at least five
consecutive reaction cycles.
(
75 mg), sugar (150 mg) and ethanol or DMSO/ethanol (3:7
v/v) (5 mL) were first added and mixed in the pressure tube
using a magnetic stirrer bar. The tube was then heated in a
thermally controlled oil bath to a desired reaction temperature.
The internal reaction temperature was measured and found to
ο
ο
be 96, 110 and 125 C (±1 C) in ethanol and 96, 114 and 131
ο
ο
C (±1 C) in DMSO/ethanol mixture with external oil bath
ο
temperatures of 100, 120 and 140 C, respectively. After a
specific reaction time, the tube was removed from the oil bath
and rapidly cooled down to roomꢀtemperature and the catalyst
filtered off. In the second reaction step, Amberlystꢀ15 (25 mg)
was added to the reaction mixture from the first step, where
after the tube was reꢀimmersed into the oil bath under the
designed reaction conditions. Finally the tube was cooled down
to roomꢀtemperature and the reaction mixture analysed after
removal of the catalyst. In the oneꢀpot, oneꢀstep conversion of
glucose to EMF the tube was charged with catalyst (100 mg),
sugar (150 mg) and ethanol (5 mL) and stirred under heating in
a thermally controlled oil bath set at a desired reaction
temperature (80ꢀ140 ºC). After a specific reaction time the
reaction mixture was quenched with cold water and analysed
after removal of the catalyst.
Experimental Section
Chemicals and materials
5
%
ꢀhydroxymethylfurfural (99 %), 5ꢀethoxymethylfurfural (97
), ethyl levulinate (99 %), glucose (99.5 %), fructose (99 %),
mannose (99 %), sucrose (99.5 %), inulin (from dahlia tubers),
cellobiose (98 %), ethanol (99.9 %) and dimethyl sulfoxide
(
99.9 %, DMSO) were purchased from SigmaꢀAldrich and used
as received.
All commercially available zeolites used in the study were
provided by Zeolyst International as pure compounds in NH4ꢀ
form without content of binder materials. The received zeolites
were calcined at 550 ºC in static air for 6 h with a heating ramp
of 1.2 ºC/min prior to use in order to produce the acidic forms
(
Hꢀforms). The thermally dealuminated zeolites were prepared Reactant and product analysis
by further calcining at 700ꢀ850 ºC in air for another 6 h. Boron
2
3
5ꢀHydroxymethylfurfural
(HMF),
5ꢀethoxymethylfurfural
modified zeolites with boron oxide content of 4 wt% and Snꢀ
2
6
(EMF), ethyl levulinate (ELevu), 5ꢀethoxymethylfurfural
diethyl acetal (EMFda), formaldehyde diethyl acetal (FAda),
formic acid (FA), diethyl ether and the ethylated form of
beta were synthesized according to literature procedures.
NH -TPD measurement
3
intermediates, e.g. ethylꢀ
Dꢀglucopyranoside (EDGP) and ethylꢀ
The number of acid sites present in the zeolites was measured
D
ꢀfructofuranoside (EDFF), were identified by authentic
1
3
by NH ꢀTPD using an AutoChem II 2920 pore analyzer. 100
3
samples, C NMR and GCꢀMS (Agilent 6850 GC system
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COMMUNICATION
DOI: 10.1039/C 1043H
Jou5GrnC0al Name
coupled with an Agilent 5975C mass detector). Sugar
conversions and the yields of HMF, EMF and acetals were
determined by HPLC (Agilent 1200 series, Aminex© HPXꢀ
12, 539–554. (d) D. M. Alonso, J. Q. Bond and J. A. Dumesic, Green
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7H 30 cm column, column temperature of 65 ºC, 0.005 M
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(a) R. J. van Putten, J. C. van der Waal, E. de Jong, C.B. Rasrendra,
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ml/min). HPLC standards were made from commercial
samples. The conversions of sucrose and inulin were calculated
from fructose, glucose and mannose on a carbonꢀbasis since
they invert to the monosaccharides on the acidic HPLCꢀ
column. Likewise, the yields of HMF, EMF and acetals were
calculated assuming each monosaccharide to give equimolar of
product. The yields of ELevu were quantified by GCꢀFID
analysis (Agilent 6890N instrument, HPꢀ5 capillary column
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methanol or ethanol (20 mL) and DeAlꢀHꢀbeta (12.5)ꢀ700
catalyst (300 mg). The tube was heated to 140 °C and the
stirring started once the temperature reached 130 °C (600 rpm).
After 10 h of stirring, the tube was quenched with cold water
and aliquots of the reaction mixture was subjected to GC and
HPLC analysis. The catalyst was regenerated after each cycle
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1
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We thank the Danish Council for Independent Researchꢀ
Technology and Production Sciences (project no.10–081991)
and the Chinese State Scholarship Fund (project no.
2
01306670004) for financial support of the work.
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| J. Name., 2014, 00, 1-8
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J. Name., 2014, 00, 1-8 | 9

Green Chemistry
Page 10 of 10
View Article Online
DOI: 10.1039/C5GC01043H
Graphical Abstract
Biomass-based mono-, di- and polysaccharides are in ethanol directly converted to the biofuel 5-
ethoxymethylfurfural (EMF) in 37-67 % yield at 100-140 °C by solid acid zeolite catalysts or by
combined zeolite-Amberlyst catalyst system using an one-pot, two-step reaction protocol.