Simultaneous Conversion of C5 and C6 Sugars into Methyl Levulinate with the Addition of 1,3,5-Trioxane

Article abstract of DOI:10.1002/cssc.201902096

The simultaneous conversion of C₅ and C₆ mixed sugars into methyl levulinate (MLE) has emerged as a versatile strategy to eliminate costly separation steps. However, the traditional upgrading of C₅ sugars into MLE is very complex as it requires both acid-catalyzed and hydrogenation processes. This study concerns the development of a one-pot, hydrogenation-free conversion of C₅ sugars into MLE over different acid catalysts at near-critical methanol conditions with the help of 1,3,5-trioxane. For the conversion of C₅ sugars over zeolites without the addition of 1,3,5-trioxane, the MLE yield is quite low, owing to low hydrogenation activity. The addition of 1,3,5-trioxane significantly boosts the MLE yield by providing an alternative conversion pathway that does not include the hydrogenation step. A direct comparison of the catalytic performance of five different zeolites reveals that Hβ zeolite, which has high densities of both Lewis and Br?nsted acid sites, affords the highest MLE yield. With the addition of 1,3,5-trioxane, the hydroxymethylation of furfural derivative and formaldehyde is a key step. Notably, the simultaneous conversion of C₅ and C₆ sugars catalyzed by Hβ zeolite can attain an MLE yield as high as 50.4 % when the reaction conditions are fully optimized. Moreover, the Hβ zeolite catalyst can be reused at least five times without significant change in performance.

Full text of DOI:10.1002/cssc.201902096

Accepted Article
Title: Simultaneous Conversion of C5 and C6 Sugars into Methyl
Levulinate with the Addition of 1, 3, 5-Trioxane
Authors: Xilei Lyu, Zihao Zhang, Francis Okejiri, Hao Chen, Mai Xu,
Xujie Chen, Shuguang Deng, and Xiuyang Lu
This manuscript has been accepted after peer review and appears as an
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the VoR from the journal website shown below when it is published
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To be cited as: ChemSusChem 10.1002/cssc.201902096
Link to VoR: http://dx.doi.org/10.1002/cssc.201902096
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10.1002/cssc.201902096
ChemSusChem
DOI: 10.1002/anie.201((will be filled in by the editorial staff))
Biomass Conversion
Simultaneous Conversion of C5 and C6 Sugars into Methyl Levulinate with the
Addition of 1, 3, 5-Trioxane
Xilei Lyu#, Zihao Zhang#, Francis Okejiri, Hao Chen, Mai Xu, Xujie Chen, Shuguang Deng*, and Xiuyang Lu*
Abstract: The simultaneous conversion of C5 and C6 mixed sugars
into methyl levulinate (MLE) has emerged as a versatile strategy to
eliminate the costly separation steps. However, the traditional
upgrading of C5 sugars into MLE is very complex as it requires
both acid-catalyzed and hydrogenation processes. Herein, we report
for the first time, a one-pot, hydrogenation-free conversion of C5
sugars into MLE over different acid catalysts at near-critical
methanol conditions with the help of 1, 3, 5-trioxane. For the
conversion of C5 sugars over zeolites without the addition of 1, 3, 5-
trioxane, the MLE yield is quite low due to low hydrogenation
activity. Interestingly, the addition of 1, 3, 5-trioxane significantly
boosts the MLE yield by providing an alternative conversion
pathway that does not include the hydrogenation step. A direct
comparison of the catalytic performance of the five different zeolites
reveals that Hβ zeolite, which has both high Lewis and Brønsted
acid sites density contributes to the highest MLE yield. With the
addition of 1, 3, 5-trioxane, the hydroxymethylation of furfural
derivative and formaldehyde is a key step. Notably, the simultaneous
conversion of C5 and C6 sugars using Hβ zeolite as the catalyst can
attain an MLE yield as high as 50.4% when the reaction conditions
are fully optimized. More importantly, the Hβ zeolite catalyst can be
reused for at least five times without significant change in
performance.
HMF from the hydrolysis of sugars; HMF can be easily converted to
methyl levulinate (MLE).[5]
The traditional upgrading of hemicellulose-derived C5 sugars to
MLE is very complex.[3e] This multi-step conversion process include
the hydrogenation of FAL into furfuryl alcohol (FOL) in addition to
acid-catalyzed alcoholysis as seen in Scheme 1 (blue arrows).[6] The
hydrogenation process, in particular, requires high-pressure H2,
hydrogenation active site, and other harsh reaction conditions that
hinders the effectiveness of the conversion process.[7] The available
one-pot production strategies for the conversion of C5 sugars into
MLE has shown only limited success. Hu et al., for example,
attempted a one-pot conversion of MLE from xylose at 7 MPa H2
pressure over Amberlyst 70 (acid) and Pd/Al2O3 (hydrogenation)
catalysts, with less than 30% MLE yield.[8] Nevertheless, the
simultaneous conversion of C5 and C6 sugars into MLE still possess
a difficult challenge due to the existence of hydrogenation step. The
provision of an alternative pathway that does not include this
hydrogenation step seems highly promising and very therefore
desirable.
Based on Scheme 1, the reaction routes that include the
conversion of glucose to HMF, xylose to FAL and HMF to MLE
(black arrows) can all be carried out over acid catalysts.[9] In 1999,
Lecomte et al. reported the use of formaldehyde over Mordenite
catalyst for hydroxymethylation of FAL derivatives to produce HMF
derivatives.[10] Following this, the upgrading of C5 sugars through
HMF derivatives route seem more promising. For the preparation of
MLE, however, the aqueous system of formaldehyde renders this
conversion strategy unsuitable since MLE would be further
converted to the hydrolyzed counterpart - levulinic acid.[11] Hence,
L
ignocellulosic biomass as the most abundant carbon-containing
organic resources can be depolymerized and hydrolyzed into C5 and
C6 sugars.[1] These mixed sugars are difficult to be separated due to
their many similarities and having no fixed boiling points.[2] An
efficient approach is to convert them to volatile chemicals such as
the
development
of
non-aqueous-based,
acid-catalyzed
C5-derived
furfural
(FAL)
and
C6-derived
5-
hydroxymethylation of FAL derivatives route holds great promises
for simultaneous conversion of C5 and C6 sugars into MLE.
(hydroxymethyl)furfural (HMF).[3] However, the resultant products
of these conversion processes are usually different, leading to the
occurrence of some costly separation processes.[1a] To avoid this
situation, the simultaneous conversion of C5 and C6 sugars into the
same platform chemical by further converting FAL and HMF has
become a promising solution.[3e,4] Brønsted acid (B acid) has been
identified as the main active site for the production of FAL and
[]
X. Lyu, Z. Zhang, H. Chen, X. Chen, Dr. X. Lu. Key Laboratory of
Biomass Chemical Engineering of Ministry of Education, College
of Chemical and Biological Engineering, Zhejiang University,
Hangzhou 310027, China. E-mail: luxiuyang@zju.edu.cn
Scheme 1. The traditional process and new 1,3,5-trioxane-added strategy for
X. Lyu, M. Xu, Dr. S. Deng. School for Engineering of Matter,
Transport and Energy, Arizona State University, 551 E. Tyler Mall,
Tempe, AZ 85287, USA. E-mail: shuguang.deng@asu.edu
the preparation of MLE from the conversion of C5 and C6 sugars.
F. Okejiri, Department of Chemistry, The University of Tennessee,
Knoxville, TN, USA
In the present study, therefore, a simultaneous conversion of C5
and C6 sugars into MLE over a series of zeolite catalysts with the
addition of 1, 3, 5-trioxane was attempted. Without the addition of 1,
3, 5-trioxane, almost no MLE was obtained from the conversion of
xylose; surprisingly, a “volcano” type increase in MLE yield after
the addition 1, 3, 5-trioxane was observed under the same reaction
#X. Lyu and Z. Zhang contributed equally to this work.
[]
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
1
This article is protected by copyright. All rights reserved.

10.1002/cssc.201902096
ChemSusChem
conditions. Following this, the roles of 1, 3, 5-trioxane and the
possible reaction pathway was studied. Additionally, important
reaction parameters such as the 1, 3, 5-trioxane loading, the catalyst
types, catalyst loading, different reactants, reaction temperature and
time were all optimized. The acidity of the zeolite catalysts was
characterized by FTIR spectra of pyridine adsorption (Py-FTIR).
The effect of 1, 3, 5-trioxane loadings on the conversion of xylose
in near-critical methanol was first studied and the results presented
in Figure 1a. In the absence of 1, 3, 5-trioxane, the MLE yield was
only 4.9% over screened Hβ catalyst in methanol. The generation of
a small amount of MLE (4.9% yield) could be ascribed to weak
critical methanol solutions are favorable for the conversion of
sugars.[4] In the absence of any catalyst, methyl β-D-xylopyranoside
(MDX, Scheme 2) was found as the major product, which indicates
that hydrolysis of xylose and MDX is difficult to proceed without
the acid catalysts.[13] For SAPO-11 and MCM-41, FAL can be
produced with almost no traces of MLE formation, which indicates
that hydrolysis reaction occurred but further conversion of FAL is
difficult over these two catalysts. The Mordenite catalyst, on the
other hand, gave an MLE yield of approximately 5%. The Py-FTIR
spectroscopic measurement was used to investigate the acidity of the
different zeolite samples in order to rationalize the MLE formation
transfer hydrogenation activity of FAL in methanol over Hβ catalyst. efficiency trend observed in Figure 1b.[14] The FTIR spectra are
The major intermediates were FAL and β-Methoxy-2-furanethanol
(MFE). FAL is the hydrolysis product of xylose, and MFE on the
other hand, is obtained from the acetalization and etherification of
FAL and methanol.[6a] The total carbon balance was quite low,
which could be attributed to the formation of humins from the
shown in Figure S1. When compared to SAPO-11 and MCM-41, the
Mordenite catalyst exhibited higher B acid sites density from the
quantitative results in Table 1. We believe that the higher B acid
sites density exhibited by Mordenite over SAPO-11 and MCM-41 is
directly responsible for the higher production of MLE. In a similar
polymerization of FAL and xylose.[12] Surprisingly, the addition of 1, trend, the MCM-22 catalyst, which has even higher L and B acid
3, 5-trioxane significantly boosted the MLE yield up to 47.4%
(Figure 1a). Additionally, the carbon balance increased dramatically
due to fast transformation of FAL. Considering the fact that FOL
was never obtained, we believe that a new conversion pathway may
be developed after the addition of 1, 3, 5-trioxane.
sites densities, contributed even more MLE yield.
Table 1. The surface area and acidity of different samples determined by BET
and Py-FTIR spectra.
Surface area
(m2/g)
77.8
B acid sites
(μmol/g)
70.0
21.4
182.1
L acid sites
(μmol/g)
38.2
38.5
34.1
56.7
112.8
B/L
SAPO-11
MCM-41
Mordenite
MCM-22
Hβ
1.8
0.6
5.4
4.4
1.9
a)
100
90
740.8
496.2
549.6
634.5
50
248.7
218.9
40
80
70
60
50
40
30
20
MLE
FAL
MFE
As seen in Figure 1b, the highest MLE yield was found over Hβ
catalyst, presumably due to the presence of both high L and B acid
sites densities (Table 1). It can be deduced from the above results
that strong B acid sites are favorable for hydroxymethylation
reaction, which could be further accelerated in conjunction with
strong L acid sites. Based on the N2 adsorption-desorption isotherms
(Figure S2a) and the corresponding pore size distributions (Figure
S2b), the Mordenite, MCM-22, and Hβ are mainly dominated by
micropores.[15] SAPO-11 has much lower surface area (77.8 m2/g) in
comparison to the other four zeolites (Table 1). From the combined
analysis of the observations of the results from Mordenite, MCM-22,
and Hβ catalysts, the MLE yield appears more sensitive to acidity
than pore structure. To further improve the MLE yield, the Hβ
loading, reaction temperature and time were optimized for the
conversion of xylose with the addition of 1, 3, 5-trioxane.
30
20
10
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
1, 3, 5-trioxane loading (g)
b)
60
50
40
30
20
10
0
100
80
60
40
20
0
MDX
MFE
FAL
MLE
a)
130 °C
150 °C
170 °C
140 °C
160 °C
50
40
30
20
10
0
1
e
2
11
O-
AP
it
en
Hβ
-4
-2
ANK
BL
d
r
MCM
MCM
S
Mo
Figure 1. a) Effect of 1, 3, 5-trioxane loadings on the catalytic conversion of
xylose over Hβ catalyst; b) Catalytic conversion of xylose to MLE over different
zeolites with 0.42 g 1, 3, 5-trioxane. Reaction condition: 0.06 g xylose, 0.12 g
catalyst, 6 mL methanol, 160 °C for 18 h.
0
5
10
15
20
25
Time (h)
Having adopted the new conversion pathway engineered by the
addition of 1, 3, 5-trioxane, the effect on the conversion of xylose
over different zeolites was studied and the results presented in
Figure 1b. More than 80% conversion of xylose was achieved for all
catalysts including the blank experiment. This is because the near-
2
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10.1002/cssc.201902096
ChemSusChem
b)
100
80
60
40
20
0
50
MLE
FAL
MFE
MDX
40
30
20
10
0
1st
2nd
3rd
4th
5th
6th
Figure 2. a) Effect of reaction temperature and time on MLE yield from the
conversion of xylose; b) Reusability of Hβ on conversion of xylose at 160 °C for
18 h (recycled Hβ was calcined at 550 °C for 6th reuse experiment). Reaction
conditions: 0.06 g xylose, 0.42 g 1, 3, 5-trioxane, 0.12 g Hβ, 6 mL methanol.
Scheme 2. The proposed reaction pathway for one-pot conversion of xylose to
MLE with 1, 3, 5-trioxane.
As seen in Figure S3, the Hβ loading considerably affected the
product distributions. With 0.03 g Hβ, the yields of FAL, MLE and
MFE significantly increased which underscores the role of acidity in
promoting the hydrolysis of xylose and hydroxymethylation of FAL
derivatives. As the Hβ loading increases from 0.03 to 0.12 g, the
MLE yield increased gradually at the expense of yields of FAL and
MFE, but remained constant with further increase in Hβ loading.
This suggested that the acidity of 0.12 g Hβ is sufficient for
optimum production of MLE from the conversion of xylose and
intermediates. The effect of reaction temperature and time on the
conversion of xylose are shown in Figure 2a and S4. Based on
Figure S4a, temperatures above 150 °C are sufficient for this
conversion process. In general, the MLE yield has a direct
relationship with reaction temperature as increased temperature
enhances the production rate of MLE in Figure 2a. The slight
decrease in MLE yield at prolonged reaction time at 170 °C can be
ascribed to the occurrence of some side reactions.[16]
Thereafter, the reusability of the Hβ catalyst was analyzed after
multiple cycles of usage. The results are presented in Figure 2b. As
the number of cycles increase, (1st to 5th), the MLE yield gradually
decreases continuously as increasing yields of FAL and MFE.
Fortunately, the catalytic activity of a used Hβ can be mostly
recovered by mere calcination at 550 °C for 6 h. The N2 adsorption-
desorption isotherms of fresh and used Hβ in Figure S5 suggests
that pore blockages, especially micropores by coke or humins may
have been the major cause of the decrease in catalytic activity;
although the pore structure can almost be restored after regeneration
at 550 °C . Li and co-workers reported a similar system in which
41.6% MLE yield was obtained from the conversion of xylose over
Amberlyst 70 with dimethoxymethane.[3e] However, the acid density
of Amberlyst 70 significantly reduced after the initial use, thereby
affecting the reusability performance of the catalyst. These results
indicate that our 1, 3, 5-trioxane-added strategy with the use of Hβ
catalyst is better suited for the conversion of xylose into MLE.
A series of intermediate products such as FAL, MDX, MFE and
2-dimethoxymethyl-5-methoxymethylfuran (DMMF) were observed
during the conversion process. Their variations with respect to
reaction times and temperatures are crucial for the study of possible
reaction pathway. Based on the results in Figure S4, the possible
reaction routes for the one-pot conversion of xylose to MLE with
the help of 1, 3, 5-trioxane is proposed in Scheme 2. Xylose could
be converted to MDX in near-critical methanol even without
catalysts. They could be further hydrolyzed to FAL with B acid.[8,17]
Also, MFE as main intermediate was detected which possibly
originated from the acetalization and etherification of furfural and
methanol, as shown by previous studies.[6a,18] According to previous
study, a new selective route to HMF from FAL and FAL derivatives
(-CH2OH was grafted onto furan ring) can be achieved over
microporous solid acidic catalysts.[5e] Therefore, we believed that
DMMF as an intermediate may have resulted from the etherification
of hydroxymethylated HDMF at near-critical methanol conditions
despite having not detected HDMF. The undetectable HDMF might
be due to fast HDMF etherification to DMMF in methanol solution.
The production of HDMF from hydroxymethylation reaction is
perhaps the most important step of the overall reaction. As
previously reported, the aldehyde group in FAL would deactivate
the position 5 in the furan ring owing to its electron-withdrawing
character.[3e,9] This can be bypassed by protecting the aldehyde
group with functionalities with electro-donating capacity which
automatically renders the hydroxymethylation of MFE and DOF
more likely to occur as compared to FAL. For the conversion of
MFE to HDMF, it was quite difficult to determine the sequence of
rearrangement and hydroxymethylation reaction, owing to the fact
that DOF and the hydroxymethylated products of MFE (HMFE in
Scheme 2) were not detected. The formaldehyde detected after the
reaction was obtained as a decomposition product of 1, 3, 5-
trioxane,[19] and was used as an electrophile for the production of
HMF derivative from FAL derivative. The catalytic conversion of
the intermediate products - MDX, FAL, and HMF over Hβ were
evaluated as well. The effect of the addition of 1, 3, 5-trioxane on
the conversion of FAL exhibited a similar trend as MLE obtained
from the conversion of xylose in Figure S6. For the conversion of
HMF without the addition 1, 3, 5-trioxane (Figure S7), DMMF was
detected as the main intermediate product from the acetalization and
etherification of HMF; this observation is consistent with previous
study.[5e] Furthermore, MDX could be also converted to MLE with
the help of 1, 3, 5-trioxane (Table 2, entry 9).
3
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10.1002/cssc.201902096
ChemSusChem
This work was supported by the National Natural Science Foundation of China
(No. 21676243, 21476204).
Besides xylose, other C5 monosaccharides such as arabinose can
also be efficiently converted to MLE using our proposed 1, 3, 5-
trioxane-added strategy (Table 2, entry 10). Also, a C5
polysaccharide - xylan (Table 2, entry 11) equally displays high
conversion efficiency to MLE (46.0%), which can be attributed
to fast acid-catalyzed hydrolysis of xylan to xylose.[3f] For C6
monosaccharides and disaccharides (Table 2, entry 3-8), the
high conversion efficiency to MLE exhibited without the
addition of 1, 3, 5-trioxane, additionally collaborates our claim
that this new addition strategy is only favorable for
hydroxymethylation reaction. The mixture of glucose and
xylose afforded high MLE yield of 50.4% (Table 2, entry 12)
which is similar to the calculated average yield of 49.6% based
on their respective entries in Table 2.
Keywords: biomass conversion · methyl levulinate · zeolites · C5 &
C6 sugars · 1, 3, 5-Trioxane
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Table 2. Catalytic conversion of different sugars to MLE.
Entry
1
2
Reactant
Xylose
MLE yield/%
47.4
Xylose a
4.9
3
4
5
6
7
8
9
Glucose
48.3
51.8
53.5
60.8
63.9
66.0
43.8
Glucose a
Fructose
Fructose a
Mannose a
Sucrose a
MDX
10
11
12
Arabinose
Xylan
48.3
46.0
50.4
Glucose + xylose b
Reaction condition: 0.06 g reactant, 0.42 g 1, 3, 5-trioxane, 0.12 g Hβ, 6 mL
methanol, 160 oC for 18 h. aIn the absence of the 1, 3, 5-trioxane; b0.03 g
glucose, 0.03 g xylose and 0.21 g 1, 3, 5-trioxane.
Summarily, the present study involved the development of an
alternative pathway in which one-pot conversion of C5 sugars into
MLE can be accomplised over an optimized Hβ catalyst with the
addition of 1, 3, 5-trioxane at near-critical methanol conditions.
Unlike the traditional upgrading of hemicellulose-derived C5 sugars
into MLE, our new conversion strategy is very less complex and
does not require H2 and hydrogenation reaction; instead, it involves
hydroxymethylation reaction of FAL derivative and formaldehyde
to produce HMF derivative, in which formaldehyde is derived from
the consumption of 1, 3, 5-trioxane. The entire reactions can be
catalyzed by acid catalysts with both high Lewis and Brønsted acid
sites densities, such as Hβ. The effect of catalyst types, 1, 3, 5-
trioxane and Hβ loadings, reaction temperature and time on the
conversion of xylose were investigated for optimum MLE yield. The
highest MLE yield realized was 47.4%. The Hβ catalyst can be
regenerated by mere calcination to remove carbon deposition, and
then reused for at least five cycles without significant drop in
activity. In addition to xylose, other C5 monosaccharides and
polysaccharide are also efficiently converted to MLE with the help
of 1, 3, 5-trioxane. More importantly, a high MLE yield of 50.4% is
observed from the simultaneous conversion of C5 and C6 sugars via
this 1, 3, 5-trioxane-added conversion strategy. We believe that this
new conversion strategy holds great promises for simultaneous
conversion of cellulose and hemicellulose into MLE.
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2028.
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[18] W. Li, C. Pan, Q. Zhang, Z. Liu, J. Peng, P. Chen, H. Lou, X. Zheng,
Bioresour. Technol. 2011, 102, 4884-4889.
Published online on ((will be filled in by the editorial staff))
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Acknowledgements
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10.1002/cssc.201902096
ChemSusChem
Entry for the Table of Contents
Biomass Conversion
Xilei Lyu#, Zihao Zhang#, Francis
Okejiri, Hao Chen, Mai Xu, Xujie
Chen, Shuguang Deng*, and
Xiuyang Lu* __Page – Page
Simultaneous Conversion of C5
and C6 Sugars into Methyl
Levulinate with the Addition of 1,
3, 5-Trioxane
Herein, we for the first time report simultaneous conversion of C5 and C6 sugars into
methyl levulinate via 1, 3, 5-trioxane-added strategy. This new strategy can be
accomplished by hydroxymethylation of furfural (or its derivatives) and formaldehyde
derived from the 1, 3, 5-trioxane. Hβ zeolite with both strong L and B acid sites was found
to be the most efficient candidate.
5
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