Sn-Beta catalysed conversion of hemicellulosic sugars

Article abstract of DOI:10.1039/c2gc16202d

Conversions of various pentoses and hexoses into methyl lactate has been demonstrated for the Sn-Beta catalyst. It is found that pentoses are converted to methyl lactate in slightly lower yields (～40%) than what is obtained for hexoses (～50%), but higher yields of glycolaldehyde dimethyl acetal are observed for the pentoses. This finding is in accordance to a reaction pathway that involves the retro aldol condensation of the sugars to form a triose and glycolaldehyde for the pentoses, and two trioses for hexoses. When reacting glycolaldehyde (formally a C₂-sugar) in the presence of Sn-Beta, aldol condensation occurs, leading to the formation of methyl lactate, methyl vinylglycolate and methyl 2-hydroxy-4-methoxybutanoate. In contrast, when converting the sugars in water at low temperatures (100 °C), Sn-Beta catalyses the isomerisation of sugars (ketose-aldose epimers), rather than the formation of lactates. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2gc16202d

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PAPER
Sn-Beta catalysed conversion of hemicellulosic sugars†
a,b
c
b
b
Martin S. Holm, Yomaira J. Pagán-Torres, Shunmugavel Saravanamurugan, Anders Riisager,
c
a
James A. Dumesic and Esben Taarning*
Received 27th September 2011, Accepted 30th November 2011
DOI: 10.1039/c2gc16202d
Conversions of various pentoses and hexoses into methyl lactate has been demonstrated for the Sn-Beta
catalyst. It is found that pentoses are converted to methyl lactate in slightly lower yields (∼40%) than
what is obtained for hexoses (∼50%), but higher yields of glycolaldehyde dimethyl acetal are observed
for the pentoses. This finding is in accordance to a reaction pathway that involves the retro aldol
condensation of the sugars to form a triose and glycolaldehyde for the pentoses, and two trioses for
hexoses. When reacting glycolaldehyde (formally a C -sugar) in the presence of Sn-Beta, aldol
2
condensation occurs, leading to the formation of methyl lactate, methyl vinylglycolate and methyl
2
-hydroxy-4-methoxybutanoate. In contrast, when converting the sugars in water at low temperatures
(100 °C), Sn-Beta catalyses the isomerisation of sugars (ketose–aldose epimers), rather than the formation
of lactates.
8
Introduction
sugars to lactate. The Sn-Beta catalyst has previously been
reported to be active in a number of reactions including Meer-
wein–Pondorf–Verley–Oppenauer redox reactions, etherification
The development of new and efficient methods to make fuels
and chemicals from lignocellulosic biomass is important if we
are to reduce our dependence on fossil feedstocks. Several
methods already exist or are under development and are based
9
and Baeyer–Villiger oxidation.
1
Here we report a study of the Sn-Beta catalysed transformation
of hemicellulosic pentose and hexose sugars into methyl lactate
and related products. The yields of obtained products are found
to be slightly lower for the pentoses compared to the hexoses
53–55% vs. 55–65%). This finding corresponds well to a pro-
posed reaction pathway involving a retro aldol reaction of the
pentose leading to a C and C fragment, leaving only 3/5 of the
2
3
on different strategies such as gasification, pyrolysis, catalytic
4
5
conversion, and fermentation. Within the production of chemi-
cals, highly versatile platform molecules that can be converted to
(
6
other chemicals are often targeted. Here the goal is to produce a
platform molecule as efficiently and selectively as possible from
a lignocellulosic precursor. Lactic acid is one such platform mol-
ecule which in itself is a useful building block for poly(lactic
acid) plastics but it could also become a useful C precursor for
acrylic acid and propylene glycol. Lactic acid is currently made
by fermentation of corn-derived glucose. Pure glucose constitu-
tes an expensive and edible feedstock, and it would be preferable
if a more crude sugar feedstock could be used instead. We
recently reported that Lewis acidic zeotype materials, such as
Sn-Beta, are capable of converting glucose, fructose and sucrose
in methanol directly to racemic methyl lactate in a reaction that
represents a catalytic version of the base mediated degradation of
2
3
carbon in the C moiety (Scheme 1). We also observed that the
3
monosaccharides readily undergo isomerisation within their
respective families (aldose–ketose–aldose) under the reaction
conditions (Scheme S1†). Consequently, no major differences in
the product distribution are observed for the monosaccharides. A
multi-component monosaccharide mixture was found to produce
the same levels of methyl lactate and related products as could
be predicted from a linear combination of the independent
sugars.
3
7
Results and discussion
a
Haldor Topsøe A/S, R&D Division, Nymøllevej 55, Kongens Lyngby,
800-DK, Denmark. E-mail: E.Taarning.esta@topsoe.dk,tlf; Fax: +45
2754291; Tel: +45 22754291www.topsoe.dk
Table 1 lists the results for the investigated sugars in this study.
The hemicellulosic sugars constitute both pentoses (xylose and
arabinose) and hexoses (glucose, galactose and mannose). In
addition to these, different disaccharides were investigated. The
sugars were reacted in methanol using naphthalene as an internal
standard at 160 °C using a Sn-Beta catalyst with a very low tin
content (Si/Sn = 400) giving a typical pentose : tin ratio of
2
2
b
Department of Chemistry, Technical University of Denmark, Anker
Engelundsvej 12800-DK, Denmark
c
University of Wisconsin-Madison, Department of Chemical and
Biological Engineering, Madison, WI 53706, USA
†
analysis procedure, sugar isomerization distributions, carbon balances,
NMR spectra and MS fragmentation patterns of key components. See
DOI: 10.1039/c2gc16202d
Electronic supplementary information (ESI) available: Detailed HPLC
4
80 : 1. XRPD and N sorption analyses were performed on the
2
calcined catalyst to ascertain its crystallinity. FT-IR using
702 | Green Chem., 2012, 14, 702–706
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Scheme 1 Proposed reaction pathway involving a retro aldol condensation reaction for the conversion of pentoses to methyl lactate and similar
products.
a
Table 1 Conversion of various sugars in methanol using Sn-Beta
methyl lactate were observed (≤2%). When changing substrate
from monosaccharides to disaccharides, substantially lower con-
versions are observed. Sucrose is the only disaccharide we found
to be easily converted, leading to a high yield of methyl lactate
Reaction
time (h)
Unconverted
sugars (%)
ML
(%)
MVG
(%)
GADMA
(%)
MMHB
(%)
b
Substrate
8
Xylose
Ribose
Arabinose 16
16
16
1
4
4
4
2
2
4
5
42
38
39
39
51
54
47
45
57
10
11
13
26
20
0
7
8
8
7
10
11
9
5
5
2
2
2
1
3
0
5
5
6
6
2
2
2
1
≤2
<2
<2
<2
<2
<2
<2
<2
0
as previously reported. However, sucrose is converted slower
than the monosaccharides (77% conversion after 4 h compared
to >90% conversion for monosaccharides), which indicates that
the hydrolysis/methanolysis is a bottleneck even for sucrose con-
version. This hydrolysis/methanolysis reaction is not likely to be
catalysed by the tin sites as we observed monosaccharide for-
mation when using Si-Beta for sucrose conversion. All other dis-
accharides resisted conversion and were not transformed to
methyl lactate in substantial yields, which we attribute to their
reluctance to undergo hydrolysis/methanolysis under the reaction
conditions.
Lyxose
16
16
16
16
16
16
16
16
Glucose
Fructose
Mannose
Galactose
Sucrose
Lactose
Maltose
Cellobiose 44
Turanose 44
Melibiose 44
Trehalose 44
8
1
39
39
38
28
28
76
<0.5
<0.5
<0.5
<0.5
3
0
0
0
0
0
0
0
We found in all cases that monosaccharide conversion to
methyl lactate is accompanied by monosaccharide isomerisation,
which is most visible at intermediate reaction times or at low
Yields (carbon%) of methyl lactate (ML), methyl vinylglycolate (MVG),
glycolaldehyde dimethylacetal (GADMA) and methyl 4-methoxy-2-
hydroxybutanoate (MMHB) from the conversion of various sugars. Yields are
a
determined by GC and sugar conversion is determined by HPLC. Conditions:
temperatures. The Sn-Beta catalyzed isomerisation of trioses and
b
1
60 °C, 300 mg substrate, 10 g methanol, 100 mg Sn-Beta. Sum of unconverted
8,10
glucose has been mentioned previously
and studied in detail
mono- and disaccharides.
11
by Davis and co-workers. Here it was found that glucose iso-
merises to fructose and mannose, ultimately resulting in an equi-
librium mixture of these three sugars. In order to gain insight in
the reaction pathway that takes place when pentoses are con-
verted to methyl lactate, we studied the time-correlated isomeri-
sation of these at 100 °C in water using the Sn-Beta catalyst. The
sugar distributions were determined by HPLC analysis and are
shown in Fig. 1 for the pentoses (A and B) and hexoses (C and
D). In all cases, the extent of sugar isomerisation is limited to its
respective aldose–ketose–aldose family containing two pairs of
aldose epimers (β-hydroxyl group isomers) and their correspond-
ing ketose (Scheme S1†). Thus, equilibrium mixtures of xylose–
xylulose–lyxose, ribose–ribulose–arabinose, glucose–fructose–
mannose and galactose–tagatose–talose are obtained at suffi-
ciently long reaction times, independently of what starting sugar
used within the respective family. Sn-Beta is therefore an inor-
ganic heterogeneous isomerisation catalyst that can produce
equilibrium mixtures containing expensive and/or rare sugars
such as xylulose, tagatose, talose and ribulose starting from the
corresponding inexpensive sugars. However, it should be empha-
sized that at long reaction times, decomposition of the sugars
become the dominating pathway, and we observe the concurrent
formation of lactic acid, 5-hydroxymethylfurfural (HMF) and
d -acetonitrile as a probe molecule showed two red-shifted
3
−1
−1
absorption bands at 2307 cm and 2317 cm , which are
indicative of isomorphically substituted tin and monohydrolysed
tin sites, respectively (Fig. S4†).
From Table 1 it can be seen that comparable yields of methyl
lactate are produced from fructose and glucose, while slightly
lower yields are reproducibly seen for mannose and galactose.
The pentoses were all found to give lower yields of methyl
lactate relative to the hexoses (38–42% versus 45–54%). Interest-
ingly, the Sn-Beta catalyst is capable of producing methyl lactate
from any of the hexose and pentose monosaccharides illustrating
that no significant discrepancy exists regarding which isomer is
used. The differences in the methyl lactate yields obtained from
the different monosaccharides could reflect their relative stability
and tendency to undergo decomposition reactions before the
desired catalytic conversion to methyl lactate occurs. The mono-
saccharide conversion is in all cases greater than 90% after 4 h
of reaction at 160 °C and ample amounts of methyl lactate are
formed when using Sn-Beta. In contrast, when a purely silicious
Si-Beta is used in place of Sn-Beta, only minor amounts of
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In contrast, the fragments formed from a hexose depends on
whether the reaction occurs from the ketose or the aldose form.
The ketohexose fragmentation results in two C fragments (dihy-
3
droxyacetone and glyceraldehyde) while fragmentation of the
aldohexose leads to a C fragment (aldotetrose) and a C frag-
4
2
ment (glycolaldehyde) (Scheme S2†). We believe that the lower
yields of methyl lactate obtained from pentoses relative to
hexoses are related to less triose being formed in the retro aldol
reaction and consequently more carbon ending up as glycolalde-
hyde. We have shown previously that D-erythrose (aldotetrose) is
transformed to methyl vinylglycolate in high yields by the action
8
of Sn-Beta. It is clear that some glycolaldehyde is inactivated
through the formation of glycolaldehyde dimethylacetal.
However, the yield of this is far too low to account for all of the
glycolaldehyde as 2/5 of the carbon in pentoses goes into this
molecule in the retro aldol reaction. In order to investigate the
fate of glycolaldehyde, we studied its conversion using the Sn-
Beta catalyst in methanol at various temperatures (Table 2). Here
it is seen that the major product formed from glycolaldehyde at
1
60 °C is methyl vinylglycolate (27%) and methyl lactate (16%)
together with smaller amounts of methyl 2-hydroxy-4-methoxy-
butanoate and glycolaldehyde dimethylacetal (combined yield of
all components is 56%). At lower reaction temperatures lower
yields of methyl lactate are obtained and methyl 2-hydroxy-4-
methoxybutanote becomes the major C product. The formation
4
of C4 products from glycolaldehyde reveals that C–C bond
forming aldol reactions also occur in the presence of Sn-Beta.
The formation of C products (methyl lactate) further compli-
3
cates the reaction picture, and suggests that both aldol and retro-
aldol reactions are taking place simultaneously. This intermedi-
ary formation of hexoses from glycolaldehyde trimerisation pro-
vides an easy explanation for the formation of methyl lactate
Fig. 1 Monosaccharide distributions when isomerising different pen-
toses and hexoses using the Sn-Beta catalyst in water at 100 °C. See
Table S1† for carbon balances.
(Scheme 2) and indeed we observed trace amounts of HMF
when converting glycolaldehyde, supporting the presence of C6
monosaccharides as intermediates in the reaction mixture.
furfural as well as other decomposition products. Repeating the
isomerization experiments starting from xylose or glucose with
Si-Beta instead of Sn-Beta shows negligible isomerization/degra-
dation of the sugars. The total sum of remaining sugars for
pentose isomerisation at 2 h is 70—80% and after 43 h, when
equilibrium is reached, it is in the range of 30—40% (see
Table S1† for more details). Presumably this loss of sugars is
associated with a pronounced decomposition of the relatively
unstable, newly formed sugars, thus requiring isolation/stabilis-
ation if high yields are to be obtained. Furthermore, we suspect
that some of the formed acid by-products are contributing to the
decomposition/dehydration of the sugars during the isomerisa-
tion reaction.
Similar temperature studies were performed for xylose and
glucose (Table 2). The finding here is that for both substrates
substantially higher yields of methyl lactate are obtained com-
pared to the case for glycolaldehyde. This is expected, since a
more direct pathway for the formation of methyl lactate exists
from the pentoses and hexoses. It can also be seen from the data
that higher yields of glycolaldehyde dimethylacetal are obtained
from xylose than from glucose, which is to be expected as 2/5 of
xylose becomes glycolaldehyde in a retro aldol reaction. At the
same time, the yields of C products are comparable for xylose
4
and glucose reflecting that the erythrose formed directly by retro
aldol reaction from glucose is a higher yielding route to C pro-
4
ducts than that originating from glycolaldehyde condensation.
Three experiments were performed in order to examine the
effect of converting a multi-component sugar mixture consisting
of glucose–xylose, xylose–glycolaldehyde and glucose–galac-
tose–mannose–xylose–ribose–arabinose, respectively (Table 2).
The total mass of sugar was constant in all experiments and the
yields shown in parenthesis are values calculated from a linear
combination of the data obtained from the individual sugars in
Table 1 and Table 2. In all three cases the observed yields are
very close to the calculated linear combination values, indicating
that under these reaction conditions no advantageous synergy or
negative cross reaction occurs.
Based on these isomerisation results, two distinct reaction
pathways for the conversion of pentoses to methyl lactate can be
envisioned: one resulting from a retro aldol reaction of the aldo-
pentoses (xylose–lyxose and ribose–arabinose) leading to glyco-
laldehyde and glyceraldehyde and another resulting from a retro
aldol reaction of the ketopentoses (xylulose and ribulose)
leading to the formation of dihydroxyacetone and glyceralde-
hyde (Scheme 1). Both reaction pathways results in the for-
mation of a triose and glycolaldehyde. Previous studies have
shown that the trioses are quantitatively converted into methyl
10,12
lacatate.
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a
Table 2 Conversion of sugars and glycolaldehyde at various temperatures to methyl lactate and related compounds
Unconverted
sugars (%)
Furfural
dimethylacetal (%)
b
Substrate
T (°C)
ML (%)
MVG (%)
GADMA (%)
MMHB (%)
Glycolaldehyde
160
N/A
N/A
N/A
N/A
N/A
2
16
14
10
4
0.3
42
42
31
20
51
52
46
23
27
30
25
13
1
7
9
3
1
10
9
7
7
8
7
8
3
5
5
5
6
—
—
—
—
1
1
1
8
40
20
00
0 f
12
19
26
11
<2
2
—
Xylose
160
<0.5
2.7
1.1
6.5
—
—
—
—
1
1
1
40
20
00f
2
10
23
2
3
2
2
1.3
0.8
0.3
2.5 (3)
6.5 (6.5)
<2
<2
2
4
5
2 (2)
9 (6)
Glucose
160
1
1
1
40
20
00
7
14
34
7
2
9 (9)
20 (19.5)
c
e
e
e
e
Glucose–xylose
Xylose–
140
140
47 (47)
23 (28)
2.3
2.3
e
e
e
e
4
c
glycolaldehyde
Pseudo hemicellulose
d
e
e
e
160
2
42.5
5 (8)
3.5 (3.5)
<2 (<2)
4.0
e
(43.5)
Yields of methyl lactate (ML), methyl vinylglycolate (MVG), glycolaldehyde dimethylacetal (GADMA) and methyl 2-hydroxy-4-methoxybutanoate
a
(
MMHB) for the conversion of various sugars catalysed by Sn-Beta. 300 mg substrate, 10 g methanol, 100 mg Sn-Beta, in all cases 16 h of reaction
b
c
d
time was used. Sum of unconverted monosaccharides. Substrate is a 50 : 50 weight mixture of the two components. Substrate is a mixture
e
containing 1/6 of each of the six monosaccharides xylose, arabinose, ribose, glucose, mannose and galactose on a weight basis. Yields in parenthesis
are linear combinations of data from experiments using individual sugars. Same data as entry 1 and 5 in Table 1.
f
Scheme 2 Proposed reaction pathway for glycolaldehyde conversion using Sn-Beta.
Conclusion
in moderate yields (50–60%). Some of these molecules are inter-
esting in themselves, and others could be interesting if shown to
have similar characteristics as methyl lactate e.g. in polymeris-
ation properties forming biodegradable plastics.
In conclusion, the Sn-Beta material catalyses the conversion of
pentoses in methanol to produce methyl lactate, glycolaldehyde
dimethylacetal and methyl vinylglycolate in a combined yield of
5
3–55% under the reactions conditions used in this study. No
significant discrepancy is seen between the pentoses, and pen-
toses and hexoses are found to transform readily to related
isomers in the presence of Sn-Beta. Since both pentoses and
hexoses are converted to methyl lacatate in moderate yields this
opens up for the use of a crude sugar mixture, e.g. one obtained
by hemicellulose hydrolysis, for the production of lactates using
heterogeneous catalysis. In contrast, microorganisms often have
strict discrepancies towards which sugars they are able to utilise
Experimental
The zeotype catalysts were synthesized according to a previously
published procedure. The catalyst was characterized by XRPD
14
and N -physisorption to confirm crystallinity. Characterization of
2
the catalyst by FT-IR using d -acetonitrile as a probe was done
molecule according to a procedure previously reported. Cataly-
tic experiments were performed in a batch autoclave using
300 mg of substrate (∼0.01 mol carbon), 100 mg of Sn-Beta
(Si/Sn = 400), 50 mg naphthalene as internal standard, in 10 g of
methanol pressurized to 20 bar with argon. Typically, a reaction
3
8
13
as substrates. Furthermore, glycolaldehyde, a major component
of pyrolysis oil, can also be converted into methyl lactate,
methyl vinylglycolate and methyl 4-methoxy-2-hydroxybutanote
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time of 16 h and a temperature of 160 °C were used unless other-
wise specified.
K. Egeblad, ChemSusChem, 2008, 1, 283–289; (c) P. N. R. Vennestrøm,
C. M. Osmundsen, C. H. Christensen and E. Taarning, Angew. Chem. Int.
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Products were identified by GC-MS together with reference
compounds. ML, MVG, GADMA and MMHB were quantified
using GC-FID. Furfural dimethylacetal was quantified by HPLC;
sugar conversion was quantified by HPLC using an Aminex
HPX-87H column (Biorad). It was not possible to obtain a stan-
dard of MMHB so it was therefore isolated by column chromato-
graphy and identified by NMR (see supplementary information
for more details†). All yields are reported as carbon% i.e. the
fraction of carbon ending up in the products divided by the total
carbon present in the substrate.
Isomerisation experiments were performed in 10 ml thick
walled glass reactors (Alltech) containing magnetic stirring bars.
In a typical experiment, 15 mg of Sn-Beta, 30 mg of substrate
and 3 ml of water were added to a reactor. The reactor was
immersed into an oil bath at 100 °C with stirring for a defined
reaction time ranging from 15 min to 43 h. The reaction was
stopped by rapid cooling in an ice bath. Samples were withdrawn
and the conversion and product formation were monitored using
two separate HPLC systems (see supplementary information for
more details†). All monosaccharides were used in their naturally
occurring D-isomeric form and all chemicals were used as pur-
chased without any further purification.
2
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The authors are grateful for financial support from the National
Science Foundation, PIRE Program (Award # 0730277). The
Catalysis for Sustainable Energy Initiative is sponsored by the
Danish Ministry of Science, Technology and Innovation. The
authors thank Prof. R. Madsen (Technical University of
Denmark) for helpful discussions regarding sugars.
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