Room temperature, near-quantitative conversion of glucose into formic acid

Article abstract of DOI:10.1039/c9gc02201e

Herein, a facile and efficient method was developed to selectively transform glucose into formic acid at room temperature. After parameter optimization, formic acid was obtained in an unprecedented 91.3% yield with a reaction time of 8 h in lithium hydroxide aqueous solution with hydrogen peroxide as the oxidant. The synergistic effects of the base and the oxidant promoted the glucose conversion at room temperature and enhanced the selectivity towards FA. Besides, the employed mild conditions have suppressed FA decomposition that often occurred under harsh conditions, which further improved the FA selectivity. A series of model compound tests were conducted to probe the possible intermediates based on which a plausible reaction pathway was proposed. In addition, the process is applicable to various carbohydrates such as cellobiose, starch, xylan, etc. This work opens up a simple, mild but effective method to produce FA from renewable biomass resources, which would remarkably alleviate the energy consumption, capital costs, handling risks, etc.

Full text of DOI:10.1039/c9gc02201e

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DOI: 10.1039/C9GC02201E
ARTICLE
Room Temperature, Near-Quantitative Conversion of Glucose into
Formic Acid
a
a
a
b
c
c
bde
Can Wang, Xi Chen,* Man Qi, Jianeng Wu, Gökalp Gözaydın, Ning Yan, Heng Zhong and
Received 00th January 20xx,
Accepted 00th January 20xx
abde
Fangming Jin*
DOI: 10.1039/x0xx00000x
Herein, a facile and efficient method was developed to selectively transform glucose into formic acid at room temperature.
After parameter optimization, formic acid was obtained at an unprecedented 91.3% yield with a reaction time of 8 h in
lithium hydroxide aqueous solution with hydrogen peroxide as the oxidant. The synergistic effects of the base and the
oxidant promoted the glucose conversion at room temperature and enhanced the selectivity towards FA. Besides, the
employed mild conditions have suppressed FA decomposition that often occurred under harsh conditions, which further
improved the FA selectivity. A series of model compound tests were conducted to probe possible intermediates based on
which a plausible reaction pathway was proposed. In addition, the process is applicable to various carbohydrates such as
cellobiose, starch, xylan, etc. This work opens up a simple, mild but effective way to produce FA from renewable biomass
resources, which would remarkably alleviate the energy consumption, capital costs, handling risks, etc.
biodegradable. Besides, it is also utilized as a building block in
organic synthesis, a reducing agent and/or solvent in diverse
chemical reactions. Moreover, FA is an efficient and eco-
Introduction
The inevitable depletion of fossil oils with drastically elevated
carbon dioxide (CO ) level in the atmosphere have become a
friendly energy carrier for hydrogen storage (with a high
volumetric capacity of 53.4 g/L) and thus a value-added medium
2
vital, global problem and gained constantly growing attentions.
In the past decades, enormous efforts were made to seek for
alternative renewable resources to supply the society with
energy and chemicals. Biomass represents the largest carbon
resource on Earth, and is also an inherently suitable feedstock
for chemical production due to the rich functional groups in its
structure. Among various biomass resources, cellulosic biomass
is abundant, widely available and low cost whose valorization is
the most prevalently studied and reported. Hitherto, a diversity
of chemicals has been obtained from glucose and cellulose via
1
9
in hydrogen energy utilization. The global market of FA in 2019
is estimated to be around US$ 619 million and predicted to keep
a steady increase in the forthcoming years.
1
-8
At present, the commercial production of FA is primarily
based on non-renewable fossil feedstock via a two-step route:
the carbonylation of methanol with carbon monoxide (CO)
under ~40 bar pressure and the subsequent hydrolysis of the
methyl formate into FA. In the past decades, persistent efforts
have been made to explore alternative renewable resources
2
0
such as biomass for FA production. So far, there were three
general chemical strategies to obtain FA from cellulosic biomass
including hydrolysis, wet oxidation and catalytic oxidation. The
hydrolysis method normally employs an acid catalyst to
transform glucose or cellulose with simultaneous production of
levulinic acid and FA at around 413-473 K, in which FA is usually
9
-18
various transformation strategies.
Formic acid (FA), with a
systematic name methanoic acid is a broadly-used versatile
chemical. It has a variety of applications in agricultural,
pharmaceutical, rubbery, leather, textile, food industries, etc.,
because it is relatively noncorrosive, nontoxic and readily
2
1-25
a by-product with relatively low yield.
In wet oxidation, an
a. China-UK Low Carbon College, Research Center for Sustainable Technologies and
oxidant is often utilized in basic solution to promote the
2
6-28
Waste Resource Utilization, Shanghai Jiao Tong University, 3 Yinlian Rd, 201306,
Shanghai, China.
School of Environmental Science and Engineering, State Key Lab of Metal Matrix
decomposition of glucose into FA at elevated temperature.
For example, glucose was transformed in the presence of
hydrogen peroxide (H )in sodium hydroxide (NaOH) aqueous
solution at 523 K under about 75 bar pressure, producing FA
b.
Composites, Shanghai Jiao Tong University, Shanghai 200240, China.
Department of Chemical and Biomolecular Engineering, National University of
2 2
O
c.
Singapore, 117585, Singapore.
Center of Hydrogen Science, Shanghai Jiao Tong University, No. 800, Dongchuan
Road, Shanghai, 200240 China
Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092,
China
2
7
d.
e.
with about 75% yield. For catalytic oxidation, both a catalyst
(such as heteropoly acids, metal oxides, etc.) and an oxidant are
2
9-31
employed.
3 2 4
In a previous work, with NaVO and H SO as the
†
Corresponding emails: chenxi-lcc@sjtu.edu.cn; fmjin@sjtu.edu.cn.
catalysts, about 68.2% yield of FA was obtained from glucose at
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
32
4
33 K under 30 bar oxygen (O
2
) gas. Besides, about 85% FA
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ARTICLE
Journal Name
yield (the current benchmark) was achieved from glucose under
63 K and 20 bar O gas using the homogeneous Keggin-type
polyoxometalate catalyst (H
View Article Online
DOI: 10.1039/C9GC02201E
3
2
3
3
8
5 7
PV Mo O40). However, a biphasic
solvent system was required and the reaction time was
relatively long of around two days.
Overall, the previous reports of FA production from glucose
were carried out under high pressure and high temperature
(363-573 K, 20-75 bar), in which considerable energy inputs and
expensive equipment were needed. Besides, high-pressure
handling has increased the safety risks especially for large-scale
manufacture. Very recently, room-temperature conversion of
3
4
glucose into lactic acid (LA) with high yields was disclosed,
which exemplifies the distinctive merits of biomass utilization
via a mild process to overcome the aforementioned problems.
Inspired by the work, we tested for the first time the potential
to obtain FA from glucose at room temperature and ambient
condition, by a simple and low-cost method using a common
oxidant and base. In the paper, the reaction system was
established by oxidant screening, base selection and parameter
optimization. Under optimized conditions, near-quantitative
glucose conversion was realized within a relatively short period
of 8 h, and the highest FA yield was obtained at 91.3% (with the
selectivity > 90%) which to the best of our knowledge is a new
record for FA formation from biomass resources. The lithium
hydroxide (LiOH) has played a major role in promoting glucose
2 2
conversion at room temperature, while the H O as well as the
mild conditions were crucial for the high FA selectivity. Control
experiments and model compound tests were conducted to
investigate the oxidation mechanism and the reaction pathway.
Lastly, the developed process was applied to diverse biomass
feedstock including different monosaccharaides, disaccharides
and polysaccharides.
Fig. 1 (a) The conversion rates and FA yields using different oxidants. Reaction conditions:
0
.1 M glucose, 0.6 M LiOH, 30% oxidant, 10 mL water, 308 K, 8 h; (b) the conversion rates
and FA yields using different bases. Reaction conditions: 0.1 M glucose, 0.6 M OH-
concentration, 100% H O , 10 mL water, 308 K, 8h.
2
2
2 2 2 2 8 4 2 2
order: H O > TBHP > Oxone > K S O > LiClO . The H O was
Results and discussion
Oxidant screening
frequently used in biomass pretreatments and transformations
and its excellent oxidation ability has been previously
3
5-37
-
described.
4
Among the oxidants, the ClO species has the
Initially, various common oxidants were examined to transform
glucose at room temperature in basic aqueous solutions (see
Fig. 1a). Encouragingly, most of the oxidants led to the
formation of FA under such mild conditions. Note that the
product existed in the form of formate instead of formic acid,
herein FA was used as an abbreviation for the product (similar
cases for other organic acid products). The glucose conversions
were high for all the oxidants (65%~85%), whereas the FA yields
were quite different depending on the oxidative species. Among
lowest standard electrode potential, but herein the ability to
produce FA from glucose was not correspondingly related to the
standard potentials of these oxidants (see Table S2 for the
-
respective standard potentials) otherwise the S
outperform the rest. Hence, the effectiveness of H
2
O
8
would
2
O
2
was not
merely associated with its oxidative capacity. In addition, the
combined uses of H with other metal oxides were attempted
e.g. H + V , etc.) in an effort to enhance the oxidation
2 2
O
(
O
2 2
2 5
O
efficiency, but no improvement was observed and using H
2 2
O
2 2
them, H O was the most effective one to promote FA
generation with 29.5% yield, followed by the organic peroxide
alone resulted in the best performance (see Fig. S3).
tert-butyl hydroperoxide (TBHP) producing FA with slightly
lower 23.6% yield which is reasonable since TBHP is more Base screening
sterically hindered compared to H . The commercial oxidant
Oxone also led to a satisfactory FA yield of 19.1%, whereas the
potassium peroxydisulfate (K ) and lithium perchlorate
LiClO ) resulted in low FA yields of 7.2% and 0.8% (The yields of
2 2
O
The type of the base could also be an influential factor for FA
production from glucose. Several typical bases including LiOH,
NaOH, potassium hydroxide (KOH) and barium hydroxide
2 2 8
S O
(
4
(
Ba(OH)
in the presence of 100% H
was not stable in the reaction system because it reacted with
2
) were studied at room temperature to convert glucose
by-products were provided in Table S1). According to the data,
the abilities of various oxidizing agents to generate FA were in
the
O
2 2
(see Fig. 1b). Note that Ba(OH)
2
the oxidant H
O
2 2
to form white precipitates barium peroxide
2
| J. Name., 2012, 00, 1-3
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ARTICLE
(
BaO ), leading to relatively low FA yield of about 22.2%. The nearly unchanged in the experiments. Thus, more cVioewncAertinclterOantleinde
2
other bases did not cause any precipitations, and the FA yields base solution principally stimulated thDeOgI:l1u0c.1o0s3e9/Cco9GnCve02rs2i0o1nE
were 61.0%, 58.2% and 57.0% respectively by using LiOH, NaOH leading to concurrent, non-selective increases of the products
and KOH. Thus, the three alkaline bases produced FA in almost (including FA, fructose and GA). A contrast experiment was
comparable yields (slightly higher FA yield when using LiOH), undertaken (see Fig. S2) in which only the oxidant was supplied
which infers that the different types of cations in these bases without the use of any base, the glucose was barely converted
have negligible influences on FA generation. Namely, unlike the and no FA was formed. The result supports that the essential
oxidants, the effects of base type were insignificant in the function of the base is to activate glucose and advance its
reaction. The results did not quite coincide with previous works transformation. The optimal LiOH concentration was selected
where NaOH was regarded as the best type of base for glucose at 0.6 M because satisfactory conversion was readily achieved
2
7
conversion into FA. The inconsistency might signify the and the base usage was relatively small.
different behaviors of bases interacting with glucose at room
In addition, different glucose concentrations were examined
temperature compared to those under harsh conditions. It was in the reaction system (see Table S3). Starting from a dilute
-
likely that the anion OH of the base mainly involved in glucose glucose solution of 0.05 M, a slightly higher FA yield was realized
+
+
2 2
transformation at room temperature while the cations (Na , K , at 91.3% in the presence of sufficient H O . Besides, using 0.2 M
etc.) came into play when temperature and pressure elevated. glucose solution, a remarkable FA yield of 80.6% was still
obtained which demonstrates the potential of the reaction
system to process more concentrated feedstock solutions.
Despite the mild conditions, the FA yield is superior to those
Parameter optimizations
After screening, H
the base in the following experiments. The impact of H
concentration on FA formation was scrutinized by varying the
concentration in a wide range from 30% to 250%. As
shown in Fig. 2a, the H concentration exerted prominent
2 2
O was selected as the oxidant and LiOH as
reported in previous literatures. Apart from the notably
enhanced FA selectivity by concentrated H , the mild
conditions are also beneficial for high FA yielding. FA conversion
2 2
O
2 2
O
2 2
H O
2 2
O
influence on the reaction. First, a gradual and almost linear
growth in glucose conversion rates was noticed (from 75.3% to
9
2 2
9.7%) with increased H O concentration. Meanwhile, the FA
yield was considerably enhanced from 29.5% to a nearly three-
fold value of 88.8%, along with the decrease of yields for side
products such as fructose, glycolic acid (GA), etc. The drastically
improved FA yield was not just because of the increased
conversion efficiency, but more importantly due to the raised
selectivity towards FA. In Fig. 2a, the FA selectivity at each point
was plotted as a gray line, which shows an obvious upward
climb with increased H
selectivity was around 39.2%, which rose to as high as 83.0%
and ~90% when H concentration was 200% and 250%
respectively. Hence, the increase in H concentration has
2 2 2 2
O concentration. With 30% H O , the FA
2 2
O
2 2
O
significantly boosted the FA selectivity for the reaction and was
predominantly responsible for markedly improved FA yield. This
2 2
specific function of H O on changing product selectivity in
glucose transformation has not been unravelled previously.
Besides, another interesting finding is that the increased H
concentration altered the species of the products. At low H
2
O
O
2
2
2
concentration of 30%, fructose, GA and LA were detected as by-
products with yields of 21.5%, 7.4% and 4.6%. Nevertheless,
2 2
further increase in H O concentration led to the disappearance
of LA product.
Next, the base concentration was varied from 0.2 M to 1.0
M. As shown in Fig. 2b, a sharp rise in glucose conversion was
noted from 47.8% to 81.5% when LiOH concentration increased
from 0.2 M to 0.4 M, and then a slower improving rate with
further increase in LiOH concentration reaching ~92% at 1.0 M.
Similar trends were observed for the FA yield which grew
accordingly with the glucose conversion rate. Distinct from the Fig. 2 (a) The influence of H
2 2
influence of H O concentration, the base concentration
showed negligible impact on FA selectivity which remained
2
O
2
concentration and (b) LiOH concentration on glucose
transformation and product selectivity. The corresponding variations are made based on
the original conditions: 0.1 M glucose, 0.6 M LiOH, 100% H , 10 mL water, 308 K, 8 h.
2
O
2
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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ARTICLE
into CO
Journal Name
was often observed in previous literatures,29, 33, 38 and M LiOH). As shown in Table 1, first, the organiVciewacAirdticslewOnelirnee
2
the side reaction normally caused decreased FA selectivity. In insignificantly converted and produced loDwOI:F1A0.1y0ie39ld/Cs.9GGClu0c2o20n1iEc
our reaction system, this problem was obviated since FA was acid was seldom converted generating FA with ~3% yield, and
quite stable under such mild conditions as indicated by the this suggests that glucose oxidation into its corresponding acid
control experiment (see Fig. S2). Less than 1% of FA was is not involved in the reaction pathway to produce FA. When LA
decomposed under employed conditions in a reaction period of was engaged as the substrate, 28.4% LA was converted and AA
8
h, showing that the side reaction was significantly hindered. was formed in 11.4% yield with no FA product detected. This
Therefore, both the mild conditions and the use of sufficient explains the presence of AA as a minor product in Fig. 3a,
were assigned as the main reasons for the high FA furthermore, the result has ruled out the possibility of LA as an
2 2
H O
selectivity. Moreover, engaging mild conditions is beneficial intermediate for FA formation. Meanwhile, the substrates AA
which allows the reaction to be undertaken in a common, low- and oxalic acid were not transformed with no products
cost container with minimal energy inputs and considerably detected. GA was converted with a rate of 24.6% and FA
reduced handling risks.
production with 10.0% yield, which infers that the -OH group
was more reactive than the -COOH group that is almost inactive.
Other alcohol species such as sorbitol and methanol were
examined, and FA was obtained in 15.9% and 7.5% yield which
substantiates the assumption. Nonetheless, the organic acids
and alcohol species resulted in rather low FA yields, and thus
they were unlikely the key intermediates for FA production.
Product evolution with reaction time
The glucose conversion and product profile were monitored
with reaction time ranging from 0.5 h to 10 h both in the
absence or presence of the oxidant (see Fig. 3). Without H
glucose was gradually consumed in LiOH solution (see Fig. 3a).
10% of the glucose was converted at 0.5 h, which steadily rose
to 57.8% at 4 h and then 80.3% at 10 h. At the initial stage
within 4 h), fructose was accumulated as the dominant product
2 2
O ,
~
(
whereas it decreased slowly after 4 h. At the same time, a
continuous increase of LA yield occurred during the whole
reaction time and LA became the major product in the latter
stage. After the reaction time of 10 h, LA was obtained in a
relatively high yield of 46.7%. Notably, no GA was detected in
the entire period, meanwhile, trace amounts of FA and acetic
acid (AA) were determined (~1% yield). The results corroborate
with our previous assumption that the base was crucial for
glucose conversion (glucose was readily transformed without
the oxidant), while provide further information that the base
was not able to promote FA generation. In fact, the product
distribution was similar with those of previous papers when
3
4, 39-41
glucose was transformed in basic solutions.
Under the
influence of the base, glucose was dominantly isomerized into
fructose and then cleaved into LA via retro-aldol condensation.
When H O (100% amount) was introduced into the system,
2 2
distinct product profile was displayed (see Fig. 3b): FA was
identified as the paramount product during the whole period
with fructose and GA generated as the side products, whereas
no LA was detected. Besides, the glucose conversion was
averagely increased, which was 25.6%, 81.9% and 90.9%
respectively at the reaction time of 0.5 h, 4 h and 10 h, showing
that the oxidant has further promoted glucose conversion.
Herein, the distinct product profiles further confirmed that the
2 2
presence of H O has preferentially favored the FA formation
and inhibited parallel reactions such as glucose isomerization.
Furthermore, the drastic change in product species suggests
2 2
that the H O may fundamentally affect the cleavage patterns
of glucose and shift the reaction pathway.
Fig. 3 The product distributions evolved with reaction time (a) in the absence of H
b) in the presence of 100% H . Reaction conditions: 0.1 M glucose, 0.6 M LiOH, 10 mL
water, 308 K.
2 2
O and
(
2 2
O
Reaction pathways and proposed mechanism
To understand the reaction pathway, a series of model
compound tests were conducted using possible intermediates
as the substrates under employed conditions (100% H O , 0.6
2 2
4
| J. Name., 2012, 00, 1-3
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DOI: 10.1039/C9GC02201E
ARTICLE
Table 1 The reaction of model compounds to produce FA under room temperature a
Product yields%
Entry
Substrate
Gluconic acid
Lactic acid
Glycolic acid
Oxalic acid
Acetic acid
Sorbitol
Methanol
Conversion%
3.5
FA%
3.3
0
10.0
0
0
LA%
0
71.6
0
0
0
0
0
0
0
12.9
54.8
14.9
0
0
0
AA%
0
11.4
0
0
100
0
0
0
0
0
GA%
0
0
75.4
0
0
0.2
0
0
1.2
17.5
0
37.7
0
1
2
3
4
5
6
7
8
9
28.4
24.6
0
0
16.0
26.2
100
100
100
100
100
55.9
91.6
99.1
15.9
7.5
Formaldehyde
Glycolaldehyde (dimer)
Glyceraldehyde
Pyruvaldehyde
1,3-dihydroxyacetone
Erythrose
95.4
95.0
57.7
11.5
30.6
33.5
58.3
89.1
1
1
1
1
1
1
0
1
2
3
4
5
26.6
2.9
0.8
0
Fructose
Xylose
21.0
2.2
0
a
2 2
Reaction conditions: 0.1 M substrate, 0.6 M LiOH, 100% H O , 10 mL water, 308 K, 8 h.
Compared to organic acids and alcohols, aldehyde/ketone glucose producing FA in 58.3% yield, which implies that glucose
species generally displayed superior activity. For example, the isomerization is not a necessary step for FA generation. Besides,
formaldehyde and glycolaldehyde dimer (the C and C no LA product was detected from fructose indicating that the
aldehydes) were quantitatively converted affording FA with degradation of fructose did not follow the retro-aldol
appreciable yields of 95.4% and 95.0%. Hence, these two condensation pathway in the presence of H . Particularly, the
1
2
2 2
O
aldehydes were very active species to form FA. Although the long-carbon-chain substrate xylose has led to an extraordinary
formaldehyde would be structurally easier to produce FA than FA yield of 89.1%, which was considerably higher than those
the glycolaldehyde, comparable performances were obtained from the shorter-chain aldehydes (the C
2
, C
3
and C aldehydes)
2
and C aldehydes.
4
under employed reaction conditions. Meanwhile, some C and comparable with those from the C
3
1
species (including glyceraldehyde, 1,3-dihydroxyacetone and Although the reason behind the phenomenon is still unclear, it
pyruvaldehyde) were completely converted. However, the FA is tentatively assumed that the xylose was highly active to be
yields were obviously lower than the C
were 57.7%, 30.6% and 11.5% respectively. The reason might two glycolaldehyde molecules thus leading to the high FA yield.
be that these species were more prone to be transformed into Xylose displayed unexpectedly high activity to produce FA
1 2
and C aldehydes, which cleaved at multiple sites into one formaldehyde molecule and
other products. Increased yields of GA and LA were determined which indicates that the pentose is probably a crucial
when using glyceraldehyde and 1,3-dihydroxyacetone as the intermediate for FA generation. The conversion and product
substrates, especially, the pyruvaldehyde afforded LA and AA as profiles of xylose conversion with different reaction times were
the major products with high yields of 54.8% and 26.6%. Indeed, examined (see Fig. S4). Much faster reaction kinetics than
the pyruvaldehyde was a well-recognized intermediate to glucose can be observed that ca. 67% of xylose was readily
3
4
generate LA instead of FA. As a result, these C
probably not the key intermediates for FA formation. Besides, conversion rate exceeded 80%. FA was the dominant product
the C aldehyde erythrose was converted with 55.9% rate and a with trace amounts of GA detected. The data suggested that
3
species were transformed within a short period of 10 min. At 15 min, the
4
FA yield of 33.5%. The relatively poor performance of erythrose xylose may exist in very low concentration as the intermediate
as the substrate for FA production was previously reported in a during the reaction because of its remarkably high reactivity
3
1
catalytic oxidation work, whereas the reason for it remained and thus not detected on HPLC. In an effort to probe it, in-situ
unclarified. Herein, the low conversion efficiency has caused the water-phase acetylation was undertaken for the solution after
inferior FA yield.
glucose was converted under employed conditions with a
reaction time of 0.5 h. The acetylated sample was analysed on
5 6
Furthermore, xylose and fructose were tested (the C and C
sugars). The performance of fructose was similar to that of GC-MS. In the GC-MS spectrum (see Fig. S5), there were mainly
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solvent and derivation reagent peaks before the retention time
of 10 min (not shown). At the retention time of about 17~19
min, the peaks were assigned to the acetylated pentoses, while
the two large peaks at about 26 min and 28 min were assigned
to the acetylated hexoses. The presence of multiple peaks was
originated from the acetylation process. Based on the combined
model compound tests, reaction profiles and GC-MS analysis, it
is deduced that the pentose such as xylose is a key intermediate
for glucose conversion into FA.
View Article Online
DOI: 10.1039/C9GC02201E
Based on the information provided by the model compound
tests, the reaction pathways for glucose conversion into FA in
basic aqueous solution at room temperature were tentatively
proposed (see Fig. 4). First, the aldehyde species in the reaction
are more probable to be the intermediates than the organic
acids and alcohols. Besides, the initial cleavage of glucose at
different positions would strongly affect the FA yield. For
instance, the α-scission at the C1-C2 position is supposed to be
Fig. 4 The proposed reaction pathways. The primary cleavage of glucose at C1-C2, C2-C3
the most advantageous pathway to produce FA, by which the and C3-C4 position was depicted by black solid arrows (Route 1), gray solid arrows (Route
2
) and gray dashed arrows (Route 3). The conversion via fructose was also proposed as
1 5
active C and C aldehydes would be generated as the
a side pathway (Route 4).
intermediates (Route 1). The α-scission is more likely to occur
at the aldehyde end of glucose instead of at the C5-C6 position,
because the -CHO group boasts superior activity than the -OH
group in our reaction system. The deduction coincides with
previous reports that α-scission was regarded as the main
For the oxidative cleavage of glucose at room temperature,
one key question remained is about the oxidation mechanism.
During the reaction, partial decomposition of the H
in which O gas was released. Meanwhile, it also generated free
radicals that are strong oxidizing species such as the hydroxyl
radical ( • OH) and the superoxide ion (O
important to figure out which of them is the main active
component. A control experiment was undertaken replacing
2 2
O occurred
2
7
2
pathway for FA production from glucose. On the other hand,
although the erythrose was not a very active intermediate to
afford FA, the cleavage at the C2-C3 position (β-scission) of
glucose could still lead to satisfactory FA yield due to the
formation of the highly active glycolaldehyde intermediate
-
2
• ). Hence, it is
H
O
2 2
with pressurized O
glucose was transformed but the FA yield was merely 12.9%
much lower than the ~60% yield when 100% H was added).
This shows that the oxidation mainly follows a free radical
mechanism while O gas-assisted oxidation was minor.
Previously, it was reported that the presence of base could
2 2
gas. Under 5 bar O gas, 80.1% of
(Route 2).
Conversely, the rupture at the middle (C3-C4 position) is not
(
2 2
O
a favorable pathway which would result in low FA yield and
more side products such as LA, GA, etc. (Route 3). As
aforementioned, LA was not observed any more in the presence
2
of 50% H
has possibly impeded the reaction pathway of C3-C4 cleavage.
Probably, the increase in H concentration has constantly
shifted the reaction pathway. For example, at 100% and 200%
concentration, the FA yield was ~61% and ~80%
2 2 2 2
O (see Fig. 2a), inferring that the concentrated H O
-
42
2 2 2
accelerate the generation of O • and •OH radicals from H O ,
which are strong oxidative species and probably the crucial
active components promoting FA formation in our system. In
2 2
O
2 2
this way, the oxidative capacity of H O were reinforced in basic
2 2
H O
solutions at room temperature. As a result, the base and the
oxidant are both indispensable for room temperature
transformation of glucose into FA, and the synergy between
them resulted in the high efficiency and selectivity of the
reaction system.
respectively. The increase is likely to be realized by altering the
cleavage pattern from a main β-scission pathway towards the
α-scission pathway. Hence, in the presence of concentrated
H O , the main reaction pathway is proposed as follows: the
2 2
glucose is first transformed into its open-chain form, and then
the α-scission takes place to generate formaldehyde and xylose.
The formaldehyde intermediate is immediately oxidized into FA The scope of the biomass feedstock
upon its formation due to the high activity, and the xylose is
Apart from the monosaccharides (xylose and fructose), various
likely to undertake rapid decomposition into formaldehyde and
glycolaldehyde which are selectively oxidized into FA product.
In addition, glucose isomerization into fructose was a side
reaction observed in the basic solution, and fructose may be
further converted into FA which is one of the possible reaction
pathways as well (as shown in Route 4). In Route 4, the formed
fructose is likely to undertake similar cleavage patterns as
glucose (at C1-C2, C2-C3 and C3-C4 positions) to produce
smaller aldehydes such as formaldehyde which was further
oxidized into formic acid.
disaccharides and polysaccharides were used as the feedstock
for FA production to test the scope of the reaction process (see
Table 2). Except sucrose which generated FA with a very low
yield of 4.8% (Entry 1), the other disaccharides cellobiose,
maltose, and lactose (Entry 2-4) produced FA with considerably
high yields of 73.7%, 68.1% and 75.6% (small amounts of GA,
glucose/fructose as minor products). These data suggest that
the reaction system is applicable to most of the disaccharides
and able to break down the glycosidic linkage to produce FA.
6
| J. Name., 2012, 00, 1-3
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Page 7 of 8
Please Gd roe en no tC ha ed mj u iss tt r my argins
Journal Name
ARTICLE
Cellobiose and maltose are linked by two glucose molecules via
different patterns of β(1→4) or α(1→4) but they produced
comparable amounts of FA, which shows that the two linkage
patterns did not exert distinct influences on FA formation. In
lactose, one glucose molecule is substituted by one galactose
molecule, but no apparent change was found in FA production.
This indicates that the epimer of glucose as the constitute did
not affect the formation pathway of FA neither. For sucrose, it
consists of one glucose unit and one fructose unit connected via
Conclusions
View Article Online
DOI: 10.1039/C9GC02201E
In this work, a simple and efficient method was explored to
convert glucose into FA in dilute basic solutions with H as the
2 2
O
oxidant under extremely mild conditions. The synergy between
the base and oxidant has led to reinforced oxidative capacity of
the reaction system, whereas the base mainly prompts glucose
conversion and the oxidant enhances FA selectivity. Moreover,
the mild conditions obviate the problem of FA decomposition
and further contributes to the improved FA selectivity. These
multiple factors have resulted in highly efficient and selective
FA production, with the highest yield obtained at 91.3%. The
control experiments and model compound tests have shed light
on the reaction pathway and mechanism. The C1-C2 cleavage
was identified as the most advantageous to produce FA
followed by the C2-C3 cleavage, nonetheless the cleavage at C3-
C4 position was the least effective pathway. Besides, the
cleavage mainly follows a free radical mechanism rather than
(1→2) bond between C1 on the glucosyl unit and C2 on the
fructosyl unit. Probably, this type of linkage hampered the
reaction pathway to produce FA and therefore led to the very
low FA yield. In sucrose transformation, glucose and fructose
were detected in 36.3% and 10.9% yields, which suggests that
the hydrolysis of the sucrose occurred very partially. However,
the further degradation of them into FA was somehow inhibited.
In addition, polysaccharides including starch, xylan, inulin
and pretreated cellulose (Entry 5-8) were attempted under
similar conditions. The starch and inulin generated FA with
yields of 14.5% and 5.4%, and the insignificant yields were
possibly due to the lack of hydrolysis ability to cleave the
glycosidic bonds when the feedstock possesses high molecular
weights. Nonetheless, 73.1% FA yield was achieved when using
the polysaccharide xylan (the major constituting unit is xylose).
The result is in agreement with the previous experiment (Table
2
the O gas-promoted mechanism.
The developed process furnishes a desirable solution to
reduce the capital costs, the energy consumption and
associated environmental impacts if conducted under harsh
conditions, making it attractive for scaling up and potential
practical applications. Note that LiOH is used in the paper as a
demonstration, whereas inexpensive NaOH will be preferable
for economic concerns. Besides, since the products were in the
form of organic acid salts, future efforts can be made to develop
highly-dispersed solid base nanocatalysts to ease the product
separation. To be economically viable and compete with the
mature in-market technologies, future directions are to reduce
the oxidant dosage possibly by adding an oxidizing promoter
and to directly convert biopolymers such as cellulose. Although
the reaction process was applicable to transform several other
biomass resources particularly xylan into FA with satisfactory
yields, but currently it is still difficult to transform cellulose
under such mild conditions. Future improvements can be made,
for example, by exploiting an additive/promoter to facilitate the
cleavage of the glycosidic bonds or adopting more effective
pretreatment methods. The work unraveled the unique
chemistry of glucose in basic, oxidative environments for near-
quantitative FA production for the first time, and we anticipate
the work will induce increased academic interests to exploit the
huge potentials in mild biomass utilization processes and
eventually push the processes towards industrialization.
1
, Entry 10) and verified the special effectiveness of the reaction
system for C sugar-based biomass. When transforming
5
cellulose, a previous pretreatment method was used by
grinding cellulose and LiOH in a ball mill to destruct its
robustness and enhance its solubility to some extent (no
4
3
separation was necessary after the pretreatment). Less
concentrated cellulose solution and elevated temperature (323
K) were employed for better dissolution. The conversion of
cellulose under such mild conditions were still challenging with
~3% yield of FA obtained. Nevertheless, it manifests the
potential of cellulose conversion into FA under mild conditions.
Future improvements could be made, for example, by
integrating a hydrolysis catalyst and/or implementing a more
suitable pretreatment protocol.
Table 2 The scope of the biomass feedstock for FA production a
Entry
Feedstock
Sucrose
Cellobiose
Lactose
Maltose
Starch
Glucose%
Fructose%
10.9
0
FA%
4.8
GA%
0.7
0.9
0.8
1.2
0.9
0.9
1.2
0
1
2
3
4
5
6
7
8
36.3
2.2
0
4.7
0.6
0.5
0
73.7
75.6
68.1
14.5
5.4
1.1
0
0
5
0
Conflicts of interest
There are no conflicts to declare.
Inulin
Xylan
Cellulose
73.1
2.5%
b
0
0
Acknowledgements
The authors thank the funding support from the State Key
Program of Natural Science Foundation of China (No.
a
Reaction conditions: 0.1 M feedstock (monomer-based), 0.6 M LiOH, 200% H
0 mL water, 308 K, 8 h.
The concentration of pretreated cellulose was 0.03 M, the reaction temperature
2 2
O ,
1
b
was 323 K and LiOH was increased to 1.0 M.
2
1436007), the Young Scientists Fund of the National Natural
Science Foundation of China (No. 21908145), the Shanghai
Sailing Program (19YF1422100), and the Center of Hydrogen
Science, Shanghai Jiao Tong University, China.
This journal is © The Royal Society of Chemistry 20xx
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Journal Name
3
3
0 M. Niu, Y. Hou, S. Ren, W. Wu and K. N. Marsh, VGierweeArnticClehOenmlin.e,
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