Valorization of monosaccharides towards fructopyrazines in a new sustainable and efficient eutectic medium

Article abstract of DOI:10.1039/c9gc02176k

In this paper, we propose a new approach for valorization of monosaccharides derived from biomass into value added chemicals in a green and economically efficient manner by forming eutectic medium exclusively between reactants. Eutectic mixtures are emerging as an advantageous alternative to ionic liquids and have already found broad applications in biomass treatment; however they are preferably used only as low vapor pressure solvents. Increasing interest in eutectic solvents originates from their flexibility to adjust their properties to different biorefinery needs by simple variation of the compositions of eutectic mixtures. In this work, we made a first attempt to use a low molecular weight component, ammonium formate, to significantly change the physical properties of the eutectic mixtures and simultaneously increase their chemical reactivity. Using this approach, we valorised simple monosaccharides by turning them into fructopyrazines with high yields in their eutectic mixtures with ammonium formate. The research revealed a high sensitivity of the product yields to the temperature of the reaction and composition of the eutectic medium, as well as an important role of formate anions in product stabilization.

Full text of DOI:10.1039/c9gc02176k

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Valorization of monosaccharides towards
fructopyrazines in a new sustainable and efficient
eutectic medium†
Cite this: DOI: 10.1039/c9gc02176k
Svitlana Filonenko, * Antje Voelkel and Markus Antonietti
In this paper, we propose a new approach for valorization of monosaccharides derived from biomass into
value added chemicals in a green and economically efficient manner by forming eutectic medium exclu-
sively between reactants. Eutectic mixtures are emerging as an advantageous alternative to ionic liquids
and have already found broad applications in biomass treatment; however they are preferably used only as
low vapor pressure solvents. Increasing interest in eutectic solvents originates from their flexibility to
adjust their properties to different biorefinery needs by simple variation of the compositions of eutectic
mixtures. In this work, we made a first attempt to use a low molecular weight component, ammonium
formate, to significantly change the physical properties of the eutectic mixtures and simultaneously
increase their chemical reactivity. Using this approach, we valorised simple monosaccharides by turning
them into fructopyrazines with high yields in their eutectic mixtures with ammonium formate. The
research revealed a high sensitivity of the product yields to the temperature of the reaction and compo-
sition of the eutectic medium, as well as an important role of formate anions in product stabilization.
Received 27th June 2019,
Accepted 21st August 2019
DOI: 10.1039/c9gc02176k
rsc.li/greenchem
forming a liquid with melting point lower than those of their
individual components. For practical utilization, DES with
Introduction
The depletion of petroleum resources and the demand for a melting points below 100 °C are favoured. Urea as a hydrogen
greener and more sustainable chemistry dictates economically bond donor combined with choline chloride (hydrogen bond
efficient processes for the conversion of biomass into fine acceptor) constitutes the most common DES.10,11 Different
chemicals or liquid biofuels. A wide variety of products can be hydrogen bond donors were tested to form eutectic mixtures
generated from biomass conversion.1,2 Particularly, cellulose, with choline chloride or other quaternary ammonium or phos-
starch, sucrose and other carbohydrates are the most contri- phonium salts. From this variety of combinations, a great
buting components of biomass and thus are regarded as a advantage of DES as solvents arises, particularly that their
promising renewable source for the production of physical properties (density, viscosity, hydrophobicity, and
chemicals.3,4 Simultaneously, the high complexity of carbo- melting point) can be adjusted to fulfil desired requirements
hydrates requires complex and energy demanding synthetic by changing the composition of DES in a wide range.9 For
techniques, more selective solvents and catalysts, best avoiding instance, in cellulose treatment, DES can act not only as the
precious metals, for valorisation into value-added reaction medium, but also as the reagent and/or catalyst to
chemicals.5,6
drive the conversion.12,13 This can drastically increase the
Important research in this area is aimed at expanding the efficiency of biomass treatment processes and make it more
solvent space for non-aqueous conversions, and includes util- selective towards the targeted products.
ization of deep eutectic solvents (DES) in different stages of
It is the aim of the present work to develop new eutectic
biomass treatment.7,8 DES may have an ionic character and media based on sugars derived from biomass hydrolysis and a
thus are considered to be an alternative to ionic liquids, but secondary compound, having high reactivity towards the pro-
with much lower toxicity, easier preparation procedures, and duction of valuable products, as it is promising within the
lower costs.9 DES are mixtures of two or more components concept of sustainable and energy efficient processes. As such
a sugar dissolving component, we decided to use ammonium
formate, a low melting salt (melting point: 116 °C) composed
Max Planck Institute of Colloids and Interfaces, Research Campus Golm,
of two of the cheapest mass chemicals, ammonia and formic
14424 Potsdam, Germany. E-mail: svitlana.filonenko@mpikg.mpg.de
acid. Due to the back reaction into the two components, this
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
salt can even be removed by distillation, and due to its rather
c9gc02176k
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low molecular weight, its viscosity is low and mixing entropy is different types of tumours in both humans and animals.28
very high, thus driving the dissolution of many polar com- DOF also showed activity against immunological and anti-
pounds in this molten salt. The salt can indeed be “distilled”, inflammatory diseases.29
i.e. it has to be handled in autoclaves under autogenous
pressure to explore its full potential.
These high-value heterocyclic compounds are usually
formed in the reaction between reductive sugars and ammonia
In analogy to DES, we used glucose as a hydrogen bond or ammonium salts in aqueous solutions under neutral or
donor and extended it to another saccharide extracted from slightly acidic pH forming the so-called “ammonia
biomass or obtained from biomass hydrolysis. We aimed to caramel”.30,31 However, because of the complexity of the
synthesize nitrogen containing derivatives, namely the Maillard reaction, species with different molecular weights,
Maillard reaction products of glucose with ammonium com- such as furfural, as well as volatile and semi-volatile pyrazine
pounds, which can be transformed by Amadori and Heyns derivatives are usually observed as by-products in such reac-
rearrangements into several (polyhydroxyalkyl)pyrazines tions when conducted in aqueous media.34,35 Usually, the pro-
(Fig. 1).14–16
ducts of these processes, which can be a mixture of DOF and
Due to their distinctive, appealing smell, deoxyfructosazine FZ with other heterocyclic compounds, strongly depend on
(DOF) and fructosazine (FZ) (Fig. 1) are used as flavouring reaction conditions, particularly pH, which advocates the use
agents, and thus can be commercially important for the food of buffers or diluted solutions. The presence of water leads to
flavour industry.17–20 They have been identified in many food rather low product yields and complex product mixtures,
products and beverages including caramel, peanuts, soy sauce which are very difficult to separate. In the established manu-
and carbonated soft drinks.21,22 An additional advantage for facturing processes, the reaction with ammonium formate is
the food applications is the recently discovered antimicrobial conducted in dilute solutions with a glucose concentration of
activity of DOF and FZ to Gram-positive and Gram-negative ca. 18 wt% and yields below 10%.32 Since the reaction requires
bacteria.23,24 Special attention here is attracted to use these elevated temperatures, use of water-free media gains a higher
pyrazine derivatives against Escherichia coli (E. coli), a major level of energy economy in accordance with the 12 principles
contaminant and trouble maker in the meat industry. Several of green engineering.33 Another pathway to obtain DOF is by
E. coli strains have developed high heat resistance and self-condensation of aminosugars, fructosamine (1-amino-1-
acquired specific virulence attributes, which can cause a broad deoxyketose) or glucosamine (2-amino-2-deoxyaldose), in
spectrum of diseases and restrict the usual thermal treatment aqueous acidic reaction media, very often catalysed by metal
methods.23 Utilization of antimicrobial compounds is among cations.36
the novel and alternative strategies that are available for food
safety.24
Alternatively, (polyhydroxyalkyl)pyrazines can be obtained
from plant sources. They can be found in Eugenia jambolana
Their low odour threshold has made these pyrazine deriva- Lamarck in sufficiently large amounts. Extracts isolated from
tives also attractive for the tobacco industry, where they are grains of Eugenia jambolana Lamarck are used for the prepa-
used to increase or modify the flavour of tobacco smoke.25–27
ration of antidiabetic medicaments.37,38 However, preparation
Customarily, (polyhydroxyalkyl)pyrazines are used to treat of the extracts requires tedious processes of seed drying, grind-
and prevent diabetes.28 In addition, the interest in these com- ing, washing and extraction, involving large amounts of energy
pounds in the pharmaceutical industry has massively and solvents. Specifically, ethanol is used to remove the unde-
increased recently, as numerous studies have reported their sirable polyphenol and sterol complex from the plant raw
high biological activity, especially an inhibitory effect on material.
Regarding the low solubility of starting educts and (poly-
hydroxyalkyl)pyrazines in non-polar solvents, one of the poss-
ible solutions to increase the reaction yields is the use of DES
as reaction media. Recently, Wu et al. developed a method to
convert glucosamine into DOF and FZ in choline chloride–
urea DES,39 showing that this reaction medium can be success-
fully used in an environmentally friendly process. The reaction
in eutectic medium results in high substrate conversion with
yields reaching up to 30% with amino acids used as catalysts.
However, difficulties of separation of the polar products from
the ionic components of the eutectic mixture can drastically
decrease the commercial value of this process.
We will show here that our new ammonium formate–
glucose system results in complete substrate conversion with
higher yields and higher simplicity. In addition, the possibility
of forming a reaction medium from reactants and products is
Fig. 1 Possible Maillard products of symmetric self-condensation of
glucose with ammonium compounds: 2,6-fructosazine (2,6-FZ), 2,5-
in accordance with the atom economy principle of green
chemistry,40,41 thus representing indeed the first use of this
fructosazine (2,5-FZ), 2,5-deoxyfructosazine (2,5-DOF) and 2,6-deoxy-
fructosazine (2,6-DOF).
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new, economical solvent medium for producing a high value interactions and liquefaction of the mixture, but also chemical
industrial product in a more green and economical fashion reaction evidenced by the gradual change of the colour from
which was not accessible before.
white to dark brown over time: slowly at room temperature
(Fig. 2B and S3†) and in several hours at elevated temperatures
(Fig. 2D). In both cases, DOF and FZ are observed as the pro-
ducts of this reaction (Fig. S4†).
Results and discussion
In order to reveal the physical or chemical precedence of
Eutectic mixtures or mixtures with low transition temperatures the interaction between ammonium formate and saccharides,
are rather common, but the nature of the interactions that take we took glucose (EM-G, Table S1†) as a simple model and com-
place depends on the type of the two components, and hydro- bined it with ammonium formate. For this mixture, we con-
gen bonds or even van der Waals forces interfere with the ducted thermal analysis coupled with mass spectrometry.
ability of the initial compounds to crystallize.42 The substantial According to the data from TGA, decomposition of the mixture
lowering of the melting point observed for the DES is an attrac- occurs at a temperature considerably lower than that for both
tive feature of these systems that allows their use as a liquid components (ca. 105 °C), forming one distinct maximum and
reaction medium with ionic character. Since melting tempera- evidencing formation of one phase (Fig. 2A). In the MS spectra
ture depletion is usually connected to the formation of hydro- of the mixture, ions with m/z = 45 are observed, similar to the
gen bonds between components,43,44 the compounds able to thermal decomposition of ammonium formate (Fig. S9†). This
form a eutectic mixture are required to be hydrogen bond peak is attributed to the formation of formamide. The masses
acceptors, such as chloride counterions in choline chloride, m/z = 2 and 44, detected in the MS spectra of the sample,
and hydrogen bond donors, such as urea and its derivatives, reveal the decomposition of HCOONH4 into hydrogen and
phenols, carboxylic acids or saccharides.
carbon dioxide. However, both processes occur at lower temp-
Substitution of choline chloride for lower molecular weight eratures (Fig. S7†).
ammonium formate, which is also active in hydrogen bond
Fast interaction of the components at high temperatures
formation,45 and combining it with saccharides result in the obstructs the phase transition measurements. For this reason,
formation of eutectic mixtures with unusual properties. That a specific procedure was developed for DSC investigation. The
is, combining these two components leads to not only physical eutectic mixture was heated up to 60 °C to activate the inter-
Fig. 2 Thermal analysis of a glucose/ammonium formate mixture under a He atmosphere, with a heating rate of 2.5 °C min−1 (A); physical appear-
ance of the glucose/ammonium formate mixture after one week at room temperature (B); DSC measurement of the glucose/ammonium formate
mixture in a temperature range from −90 to 60 °C, with a heating/cooling rate of 10.0 °C min−1; the sample was heated from room temperature to
60 °C and cooled down immediately to avoid the reaction between components (C). Physical appearance of the mixture after reaction for 4 hours at
90 °C (D).
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action between components, and immediately cooled down to bution of reaction products is influenced by different
−90 °C and subsequently heated back to 60 °C. Heating and ammonium sources and the pH of the reaction mixture,49,50
cooling cycles were repeated, and measurements were con- and larger amounts of water can be identified as the source of
ducted without a pause in between the cycles in order to avoid product diversity.51 Conducting the reaction in eutectic media
the chemical reaction between glucose (fructose) and overcomes these factors and leads to a more single product
ammonium formate. Two cycles of heating and cooling were directed process.31
performed with reproducible glass transition at −6.0 °C
We used this advantage of water-free eutectic media to
(glucose, Fig. 2C) or at −25.3 °C (fructose, Fig. S10†). This con- improve the selectivity of this process and developed a further
firmed the formation of the eutectic mixture prior to the step to use reagents as components of the eutectic media. To
chemical reaction.
increase the reaction rate, we kept the eutectic mixture for
From the chemical point of view, saccharides undergo the 4 hours at 90 °C. The reaction was conducted in autoclaves to
Maillard reaction with basic ammonia or ammonium salts in prevent the evaporation of components. Separation of the pro-
browning caramel solutions. Those reactions are important for ducts was done using liquid chromatography, and their identi-
the preparation of flavours and caramel colors.46 The Maillard fication with MS/MS showed the predominant formation of
reaction of sugars in this case pursues the following mecha- molecular ions ([M + H]+) with m/z ratios 305 and 321 for
nism. The open-chain Amadori rearrangement products EM-G and EM-F mixtures, respectively, (Fig. 3) or corres-
undergo numerous Heyns rearrangements, and can form ponding values for other saccharides (Fig. S6 and S7†). These
α-amino carbonyls with different carbonyl group positions, compounds showed a maximum of absorption at 275.5 nm in
and subsequently give pyrazines via self-condensation. When UV-vis spectra (Fig. S11†), corresponding to the known tran-
conducted in aqueous medium, the rearrangement reaction is sition in di-substituted pyrazines,31 and vibrational modes of
1
sensitive to the presence of hydrogen or hydroxyl ions, and the pyrazine ring in the FTIR spectra (Fig. S12†). The H NMR
depending on the pH in aqueous solution, several pyrazine results show two sets of signals of aromatic compounds corres-
derivatives are found to be formed as products.31,47,48 ponding to disubstituted pyrazines with identical (FZ) and
Similarly, condensation of glucosamine has low selectivity, different (DOF) substituting groups (denoted with arrows in
and despite the high substrate conversion sometimes, distri- Fig. 4) in good agreement with the description from the litera-
Fig. 3 Identification of reaction products by the UHPLC-MS method: a representative UHPLC chromatograms of the reaction mixtures of glucose
with ammonium formate (A) and fructose with ammonium formate (B) incubated at 90 °C for 4 hours, and product ion scans (MS/MS spectra)
corresponding to deoxyfructosazine and fructosazine.
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Fig. 4 Representative 1H NMR spectra of eutectic medium composed of ammonium formate and glucose (A) or fructose (B) after the reaction
showing the formation of DOF and FZ.
ture.39 NMR signals of (polyhydroxyalkyl)pyrazines in the ali- ammonium. This claim is supported by the presence of some
phatic region are difficult to analyse due to the overlap with amount of formic acid in eutectic media, detected by the
numerous signals from –OH protons. The data of 13C NMR TGA-MS analysis (Fig. S9†). Presumably, under the reaction
analysis indicated the formation of 2,6-DOF and 2,6-FZ as the conditions, formate anions form hydrogen bonds with
main products of glucose transformation in the eutectic media glucose, simultaneously forming HCOOH and making
with ammonium formate,35 while fructose under the same ammonium cations more available. Open chain glucose mole-
conditions forms 2,5-DOF and 2,5-FZ. This is in accordance cules react with ammonia to form iso-glucosamine (Amadori
with previous studies in water solutions of saccharides, rearrangement product), which can further undergo oxidative
showing that aldoses predominantly yield the 2,6-isomers deamination to form D-glucosone.8 Furthermore, iso-glucos-
while ketoses predominantly yield the 2,5-isomers.52
amine undergoes the Maillard reaction with glucose, and
Based on this, we assumed that the mechanism of inter- through the Amadori rearrangement cyclizes into 2,6-DOF
actions in eutectic media is similar to that in water solutions. according to Scheme 1.54 Formation of 2,6-FZ in EM-G eutectic
As the first step, a chain opening reaction of saccharides takes medium occurs according to Scheme 2. Since it involves the
place. This reaction is catalysed by OH or H ions.53 Generation interaction of iso-glucosamine with D-glucosone, the yields of
of these ions can be explained by the exchange interactions this reaction are much lower compared to 2,6-DOF formation.
within the eutectic media. Most frequently, eutectics are
Aldoses react with ammonium species forming glucos-
formed by hydrogen bond interactions in the vicinity of hydro- amine, which undergoes dimerization into 2,5-DOF
gen bond donors shielding the halide anions in quaternary (Scheme 3) and 2,5-FZ (Scheme 4).
ammonium salts, wherein our eutectic media formate anions
Other saccharides undergo similar browning reaction
play the role of hydrogen bond acceptors and can form a tran- forming the products of corresponding pyrazine derivatives
sition state with hydrogen atoms shared between formate and (Fig. S8†), namely hexoses, fructose, mannose and galactose
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form the products with m/z values 305 and 321, while pentoses
result in the formation of two main products with m/z 245 and
261 detected using LC-MS (Fig. S6†). Mixing ammonium
formate with disaccharides more obviously reveals the for-
mation of the eutectic mixture, since chemical interactions in
this case are much slower and colourless viscous liquids are
formed when ammonium formate is heated with sucrose or
maltose. However, after 4 hours at 90 °C, pyrazine derivatives
are detected in these reaction mixtures as well. According to
the LC-MS data, sucrose forms the pyrazine ring after breaking
the glycosidic bonds, and the product with m/z = 305 is
formed. Maltose under these conditions undergoes only
partial breaking of the 1–4 bond, and products with m/z = 305,
467 and 629 are formed (Fig. S7†).
Under the reaction conditions, glucose undergoes numer-
ous Maillard transformations similar to those in cooking pro-
cesses. The main reaction under these conditions is the for-
mation of glucosamine and its dimerization into the pyrazine
derivatives. Additionally, there are side reactions of glucose
and glucosamine degradation products which can produce a
variety of more complex polymeric materials with higher mole-
cular weights, different structures and elemental compo-
sitions.55 Higher temperatures lead to glucose oxidation to
dicarbonyl compounds and inevitable degradation to hetero-
cycles such as furans or pyranes, and their further inclusion
into the polymerization processes.56 These polymers are
usually referred to as caramel or melanoidin and can be identi-
fied by the brown colour and two absorption bands in UV-vis
spectra at around 210 nm and in between 290 and 460 nm,
depending on their molecular weight and composition.44,57
From the absorption bands at around 210 nm and a well dis-
tinguished shoulder at around 295 nm in the UV-vis spectra of
the reaction mixtures, we identified melanoidin as a side
product of this reaction (Fig. S11†).
Optimization of the reaction conditions was conducted at
different temperatures ranging from 80 to 100 °C, substrate
conversion was monitored by UHPLC, and reaction yields were
evaluated based on the NMR measurements (Fig. 5). The
chosen temperatures used are common for molecule activation
in DES formation processes.43,58 Complete glucose conversion
is achieved after 5 hours at 80 °C, and after 4 hours at 90 °C
and 100 °C. For the reactions without the solvent, higher temp-
eratures are indispensable for liquefaction of the eutectic reac-
tion medium in order to achieve homogeneous distribution of
the reactants over the reaction mixture. However, the increase
of temperature results in a decrease in the yields of DOF,
which can be explained by the higher degree of melanoidin
formation. When conducted at 100 °C, the reaction affords the
maximum yield after ca. 1.5 hours while not reaching the com-
plete conversion of glucose. The yields drastically drop at a
longer reaction time, due to further polymerization or degra-
dation of the products.
Scheme 1 Mechanism of 2,6-DOF formation.
Scheme 2 Mechanism of 2,6-FZ formation.
Scheme 3 Mechanism of 2,5-DOF formation.
The ratio of the reactants plays an important role in the
chemical transformations, and it is especially relevant for the
eutectic media. Screening over the different compositions of
reactive media showed that a decrease of the ammonium
Scheme 4 Mechanism of 2,5-FZ formation.
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Fig. 5 Yields of DOF (A) and FZ (B) at different reaction temperatures. The glucose and fructose conversion was monitored by LC-MS and reactions
were conducted until the complete conversion of saccharides.
Fig. 6 Yields of DOF and FZ from the reaction in the eutectic mixture of glucose with ammonium formate with different compositions (A) and
yields of DOF and FZ from the reaction in eutectic mixtures of corresponding saccharides with ammonium formate (B).
formate amount results in drastic lowering of the yields of pro- charides to nucleophilic attack of ammonium in the Maillard
ducts (Fig. 6A), probably related to the low concentration of reaction.
formate anions in the reaction mixture, which, as we will
To gain deeper insight into the transformation of glucose
discuss further, plays a crucial role in the stabilization of the in the ammonium formate eutectic medium, we performed
products. An excess of ammonium formate slightly increases in situ NMR studies under isobaric and isochoric conditions.
the yields of the main products, which is more pronounced at Formation of DOF is observed after 30 min from the beginning
longer reaction times. We associate this with the fact that, of heating, significantly increasing its amount within the first
unlike glucose, which can undergo a variety of side reactions hour of the reaction (Fig. 7). While moving along the reaction
and thus drastically decrease the yields of the main products coordinates, the position of the formate group in the NMR
when used in excess, ammonium formate is stable in this spectrum gradually shifts from 8.41 ppm in the reaction
temperature range. However, ammonium formate is an alkali mixture before the reaction to 8.25 after the reaction is
salt and increases the pH of the media when used in great completed. This clearly indicates the gradual change in the
excess. This is less desirable for the reaction, usually con- formate anion surroundings from the predominant
ducted in slightly acidic or neutral solutions.21,24,25
Comparing different saccharides shows that fructose the pyrazine derivative after the reaction has ended.
affords considerably higher yields of DOF and FZ, while Under the isobaric conditions, the most notable difference
ammonium cations before the reaction to the vicinity of
aldoses are much less active in the reaction. This can be is the possibility of the formic acid, obtained in the reaction,
related to the lower activity of the aldehyde groups of the sac- to evaporate. This can be clearly observed from the lowering of
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Fig. 7 In situ 1H NMR spectra as a function of reaction time for the glucose transformation in the eutectic medium isochoric process at 90 °C.
the intensity of the NMR signal over the reaction pathway in known to be one of the main products of the thermal decompo-
comparison with the autoclaved procedure (Fig. 8). As an sition of (polyhydroxyalkyl)pyrazines with different lengths of
advantage, this allowed us to reveal the role of formate anions substituting groups, and its relative abundance in the products
in the synthesis process, since formally formate anions are not increases for the decomposition of pyrazines with C4 poly-
included in the reaction. It is worth mentioning here that pyra- hydroxyalkyl chains.60,61 Similarly, pyrazine is found among the
zine and its derivatives contain two nitrogen atoms able to products of thermal decomposition of caramels or tobacco fla-
form hydrogen bonds with acids and form salt-like com- vours, containing other polyhydroxyalkyl derivatives of
pounds with strong ionic bonds. This explains the remaining pyrazine.62,63 Simultaneous depletion of DOF and FZ signals
amount of format anions ionically bonded to pyrazines after supports the assumption of their decomposition.
the end of the reaction despite evaporation under isobaric con-
Another possible origin of pyrazine formation under these
ditions. Similar to this, solutions of (polyhydroxyalkyl)pyra- conditions is related to the cleavage of glucose to smaller bicar-
zines in pharmacies are prepared with acidic pH, preferably bonyl molecules, particularly to glyoxal, able to react with
pH 4, used to stabilize positively charged pyrazines.59 Another ammonium and form nitrogen containing heterocycles.64,65 This
relevant feature of these experiments is the formation of pyra- reaction pathway is used to explain the detection of pyrazine and
zine in the reaction mixture over time. While the autoclaved a variety of its alkyl and hydroxyalkyl derivatives in sugar–
conditions result in predominant formation of two aromatic ammonia systems.66,67 However, usually only low amounts of
heterocycles, DOF and FZ, in the experiment in an open vessel, glyoxal are formed under glucose thermal decomposition.52,54
we observe notable formation of unsubstituted pyrazine over Thus, the idea of DOF thermal decomposition with time is more
the reaction, with an increase of its quantity over time. Since relevant. Both these approaches support the statement about the
pyrazine formation in the experiment occurs with some delay, importance of the autoclaved conditions to remain as the
and its amount increases, while the products of the primary optimal reaction medium and highlight the role of formate
reaction fade away, we assume that degradation of DOF and FZ anions in stabilization of the products as more stable pyrazinium
to pyrazine by elimination takes place due to the decrease of the cations in the proposed eutectic reaction medium, an admittedly
formate anion content in the reaction mixture. Pyrazine is unplanned advantage which however explains the high yields.
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Fig. 8 In situ 1H NMR spectra as a function of reaction time for the glucose transformation in the eutectic medium isobaric process at 90 °C.
under constant specified temperature (80 to 100 °C). After
reacting for 4 hours, the autoclave was cooled to room temp-
erature under a stream of water. The products after the reac-
tion were freeze dried to obtain a brown solid with distinctive
smell.
Table 1 Composition of the eutectic mixtures and the yields of DOF
and FZ after the reaction at 90 °C for 4 hours
Molar ratio of
saccharide/
HCOONH4
DOF
yield,
%
FZ
yield,
%
Entry
Sample
Saccharide
For the thermal analysis coupled with mass spectrometry, a
mixture of glucose and ammonium formate was heated from
ambient (25 °C) temperature to 400 °C at a rate of 2.5 °C
under a He atmosphere. Gas evolution occurring during the
TGA measurements was recorded in the MS instrument.
Intensities of the gas products were analysed using evolution
1
2
3
4
5
6
7
8
EM-G
EM-F
EM-X
EM-Man
EM-Gal
EM-A
EM-Mal
EM-S
Glucose
Fructose
Xylose
Mannose
Galactose
Arabinose
Maltose
Sucrose
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 2
1 : 2
30
44
19
23
21
20
8
8
2
6
4
1
a
10
a
a
curves obtained through
a preliminary scan from 1 to
a Not detected by NMR.
200 mass to charge (m/z) values.
DSC measurements were performed in aluminium pans as
the reference. Glucose and ammonium formate were ground
and mixed together, and then immediately transferred for the
DSC measurements. The mixture was heated from ambient
temperature to 60 °C to ensure eutectic formation and cooled
Experimental
The samples were prepared by mixing different hexoses and to −90 °C at a heating/cooling rate of 10 °C min−1. Two more
pentoses, as well as disaccharides with ammonium formate, cycles of heating and cooling were conducted heating the
varying the molar ratios of components (Table 1). After heating sample up to 90 °C.
to 60–80 °C, these formed a liquid mixture at room tempera-
The components of the reaction mixture were separated
ture. To conduct the reaction, two powdered components were using a Dionex UltiMate 3000 UHPLC system, which consisted
mixed in a 10 mL Teflon-lined glass vessel and placed in a of an UltiMate 3000 RS pump, an UltiMate 3000 RS autosam-
stainless steel Parr autoclave. The autoclave was kept pler, an UltiMate 3000 RS column compartment and an
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UltiMate 3000 RS variable wavelength detector (Dionex Sortfon
GmbH, Germany). The mobile phase consisted of 0.1% formic
Conflicts of interest
acid in water (A) and methanol (B). The gradient program was There are no conflicts to declare.
as follows: 100% A (0–5 min), 100–95% A (5–15 min), 95–50%
A (15–25 min), 50–95% A (25–35 min), 95–100% A (35–40 min),
and 100% A (40–45 min). The flow rate was 0.5 mL min−1 and
the injection volume was 5 μL. Elution was performed on a
Acknowledgements
separation column (Thermo Scientific Accucore C18, 2.6 μm, This work was financially supported by Max Planck Society
100 × 3) with a modified method reported by Hrynets et al.36,66 and National Key Research. Open Access funding was provided
The components were detected using an LTQ Orbitrap XL by the Max Planck Society.
(Thermo Scientific) linear ion trap quadrupole mass
spectrometer equipped with an Ion Max electrospray ioniza-
tion (ESI) source (Thermo Fisher Scientific) operated in the
H-ESI mode with acquisition conditions similar to those for
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