Influence of the ionic liquid presence on the selective oxidation of glucose over molybdenum based catalysts

Article abstract of DOI:10.1016/j.cattod.2016.06.040

Two different approaches are proposed in this work in order to study the influence of the ionic liquid presence in the reaction of glucose oxidation by H₂O₂ in mild conditions. The ionic liquids are applied either as a solvent by using homogeneous Mo based catalyst, [Mo(O)(O₂)₂(H₂O)_n] complex, or by using it as an integral part of a heterogeneous catalyst, organic inorganic hybrids based on Mo Keggin structure. Both catalytic strategies resulted in acceptable glucose transformation degrees but lead to different oxidation products depending on the role of the ionic liquid. The hybrid approach restrains the number of the received products being the most selective one. A detailed study of the effect of the hybrid nature and reaction conditions is proposed in the second part of this study.

Full text of DOI:10.1016/j.cattod.2016.06.040

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Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Influence of the ionic liquid presence on the selective oxidation of
glucose over molybdenum based catalysts
a,b,∗
a
a,b
a
a
C. Megías-Sayago
J.A. Odriozola
a
, C.J. Carrasco , S. Ivanova , F.J. Montilla , A. Galindo ,
a,b
Departamento de Química Inorgánica, Universidad de Sevilla, C/Profesor García González, S/N, 41012, Sevilla, Spain
Instituto de Ciencia de Materiales de Sevilla, Centro mixto CSIC-US, Avda. Américo Vespucio 49, 41092, Sevilla, Spain
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Two different approaches are proposed in this work in order to study the influence of the ionic liquid
presence in the reaction of glucose oxidation by H2O2 in mild conditions. The ionic liquids are applied
either as a solvent by using homogeneous Mo based catalyst, [Mo(O)(O2)2(H2O)n] complex, or by using
it as an integral part of a heterogeneous catalyst, organic inorganic hybrids based on Mo Keggin struc-
ture. Both catalytic strategies resulted in acceptable glucose transformation degrees but lead to different
oxidation products depending on the role of the ionic liquid. The hybrid approach restrains the number
of the received products being the most selective one. A detailed study of the effect of the hybrid nature
and reaction conditions is proposed in the second part of this study.
Received 14 December 2015
Received in revised form 14 June 2016
Accepted 15 June 2016
Available online xxx
Keywords:
Glucose oxidation
Ionic liquids
Molybdenum catalysts
Polioxometalates
©
2016 Elsevier B.V. All rights reserved.
1
. Introduction
acceptable yields [12]. All this disadvantages stimulates the pursuit
for new efficient, environmental friendly catalysts and processes to
improve carbohydrate conversion and, therefore, biomass exploita-
tion.
The efficient transformations of polysaccharides into valuable
compounds, such as fuels and important intermediate chemicals is
a relevant topic nowadays and has attracted the attention of the
scientific community [1–4]. The glucose, the monomer of cellulose,
constitutes the most abundant and available monosaccharide and
its transformations into furan derivatives like HMF (hydroxymethyl
furfural) [5], FDCA (Furan dicarboxylic acid) [6], DFF (2,5 diformyl-
furan) [7] or sugar acids (aldonic and aldaric acids) is being widely
investigated in the last years [8–10]. Within added-value chemi-
cals issued from glucose, the gluconic acid, obtained via oxidation
of the glucose aldehyde group is in versatile use in pharmaceutical,
chemical, food, beverage and textile industries, either as additive,
as chelating agent for cleaning purposes or for the extraction of
metal traces in solutions [11]. Its current production is carried
out via enzymatic process, in presence of glucose oxidase. How-
ever, this methods presents several drawbacks such as need of
high amounts of expensive enzyme, its irreversible deactivation,
the exhaustive control of process parameters (such as pH and tem-
perature), the need of several pre-reaction purification processes
in order to remove impurities and the long reaction times to obtain
The production of gluconic acid has been studied over catalysts
of different nature, including enzymatic [13] and heterogeneous,
based on platinum [14,15] and, more recently, on gold [16,17].
Recently, a group of compounds, the as called ionic liquids (ILs),
showed great potential in carbohydrate chemistry either as cata-
lyst or as a solvent. The ILs are compounds consisting in ions with
◦
melting points below 100 C whose principal advantages could be
resumed in high ionic mobility, good electric conductivity, low
vapor pressure and good thermal and chemical stability [18,19].
Most commonly used ILs are composed by quaternary ammonium
salts or cyclic amine salts [20]. They have already found its applica-
tion in catalysis as a solvents [21–24] in electrochemistry [25] and
organic synthesis, however, the use of the ionic liquid as a part of
hybrid catalyst is barely studied [20,26–30].
On the other hand the polyoxometalates, well-known catalyti-
cally active compounds were also considered as a potential glucose
transformation catalysts [20]. In particular, the Keggin-type poly-
oxometalates, classified as anionic metal-oxygen clusters of the
early transition metals, appear as a good candidates for carbohy-
drate conversion [31–33]. Properties like strong Brönsted acidity,
fast multi-electron transfer, high proton mobility, high solubil-
ity in various solvents and resistance to hydrolytic and oxidative
degradations in solutions converts these compounds in versatile
∗ Corresponding author at: Instituto de Ciencia de Materiales de Sevilla, Centro
mixto CSIC-US, Avda. Américo Vespucio 49, 41092, Sevilla, Spain.
E-mail address: cristina.megias@icmse.csic.es (C. Megías-Sayago).
http://dx.doi.org/10.1016/j.cattod.2016.06.040
0
920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: C. Megías-Sayago, et al., Influence of the ionic liquid presence on the selective oxidation of glucose
over molybdenum based catalysts, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.06.040

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Fig. 1. Structure of IL/PMo12O40 hybrids used as catalytic System 2.
Fig. 2. XRDiffractograms of H3PMo12O40·13H2O (JCPDS #01-075-1588) and its hybrids.
materials for using in catalysis [34]. Furthermore, their proper-
ties can be tuned by varying the composition and the counter
cations [31,35,36]. The proton compensated polyoxometalates,
known as heteropoly acids (HPA) however cannot be used in
liquid-phase reactions with polar solvents due to their solubility
and impossibility for recuperation. However, the substitution of
protons with another organic or inorganic cations results in insolu-
ble materials and, therefore, useful in heterogeneously catalyzed
liquid-phase reactions [31]. Additionally, the substitution of all
the protons with organic cations could results in a new class of
organic-inorganic hybrid materials which takes advantages on both
inorganic (strength, thermal stability and chemical resistance) and
organic part (lightness, flexibility and versatility) features. Perfect
example of organic cations which could be applied in the hybrid
are those of ionic liquids (ILs) [20,37–39].
In this context, the main goal of this work is to study the
influence of the IL on the catalytic performance of Mo based
compounds. The ionic liquids are evaluated either as a sol-
vent (System 1) or as integral part of the catalyst (System 2) in
the liquid-phase oxidation of glucose with hydrogen peroxide
as oxidant. In system 1, 1-butyl-3-methylimidazolium hexaflu-
orophosphate (Bmim)PF6 ionic liquid is used as solvent and
2. Experimental
2.1. Synthesis
2.1.1. Synthesis of oxodiperoxomolybdenum complex (System 1)
Solution of the aqua complex of oxodiperoxomolybdenum,
[Mo(O)(O ) (H O)n] (henceforth [MoO5]) was prepared as follows.
2
2
2
A suspension of MoO3 (Sigma Aldrich, 3.617 g, 25 mmol) in 40 mL
30% aqueous hydrogen peroxide (VWR) was heated at 50 C with
continuous stirring overnight after which complete dissolution of
the molybdenum resulting in a clear yellow solution was observed.
At this point the solution was cooled to 0 C and several drops of
hydrogen peroxide were added and the solution was then made up
to 100 mL and stored in a sealed volumetric flask at 4 C. Occasional
venting of this solution is advised upon prolonged storage due to
the accumulation of pressure following catalytic decomposition of
hydrogen peroxide. A solution of [MoO5] with concentration 0.25 M
is thus obtained.
◦
◦
◦
2.1.2. Synthesis of heteropolyacid and hybrids (System 2)
Phosphomolybdic acid was prepared as reported in the litera-
ture [40] by using Na MoO ·2H O (Sigma Aldrich), H PO (85%)
2
4
2
3
4
[
Mo(O)(O ) (H O)n] complex as catalyst. In system 2, three dif-
2 2 2
(Panreac), HCl (37%) (VWR), diethyl ether (Panreac), HNO3 (65%)
(VWR) and distilled water.
3−
ferent hybrids based on phosphomolybdic acid (PMo12O40 ) and
three IL cations, 1-ethyl-3-methylimidazolium (Emim), 1-buthyl-
For the Emim [PMo12O40] (from this point forward Emim-Mo)
3
3
-methylimidazolium (Bmim) and 1-hexyl-3-methylimidazolium
synthesis [30], appropriate quantities of prepared H PMo12O40
3
(
Hexmim), are used as catalysts in aqueous solution.
(0.9 g) and 1-ethyl-3-methylimidazolium methanesulfonate (Alfa
Please cite this article in press as: C. Megías-Sayago, et al., Influence of the ionic liquid presence on the selective oxidation of glucose
over molybdenum based catalysts, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.06.040

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Table 1
Identified crystal phases at different temperatures.
◦
Temperature,
C
Phases
H3PMo12O40·13H2O)
Phases
(Emim3[PMo12O40])
Phases
(Bmim3[PMo12O40])
Phases
(Hexmim3[PMo12O40])
(
RT
H3PMo12O40·13H2O
H3PMo12O40·13H2O
H3PMo12O40·6H2O
H3PMo12O40·1.5H2O
H3PMo12O40·1.5H2O
H3PMo12O40·1.5H2O
Emim3[PMo12O40]
Emim3[PMo12O40]
Emim3[PMo12O40]
Emim3[PMo12O40]
Emim3[PMo12O40]
Emim3[PMo12O40]
Bmim3[PMo12O40]
Bmim3[PMo12O40]
Bmim3[PMo12O40]
Bmim3[PMo12O40]
Bmim3[PMo12O40]
H3PMo12O40·13H2O +
Hexmim3[PMo12O40]
Hexmim3[PMo12O40]
Hexmim3[PMo12O40]
Hexmim3[PMo12O40]
Hexmim3[PMo12O40]
Amorphous
5
1
1
2
2
0
00
50
00
50
10MoO3·H3PO4·24H2O
3
3
4
00
50
00
H3PMo12O40·1.5H2O
H3PMo12O40·1.5H2O
H3PMo12O40·1.5H2O
Emim3[PMo12O40]
H3PMo12O40·13H2O +
Amorphous
Amorphous
Amorphous
1
0MoO3·H3PO4·24H2O
H3PMo12O40·13H2O +
H3PMo12O40·13H2O +
1
0MoO3·H3PO4·24H2O
10MoO3·H3PO4·24H2O
H3PMo12O40·13H2O +
H3PMo12O40·13H2O
+
10MoO3·H3PO4·24H2O + MoP2O7
1
0MoO3·H3PO4·24H2O + MoP2O7
450
500
550
600
650
700
750
MoP2O7 + MoO3
MoP2O7 + MoO3
MoO3
MoO3
MoO3
H3PMo12O40 + MoP2O7 + MoO3
10MoO3·H3PO4·24H2O + MoP2O7
Amorphous
Amorphous + MoO3
MoO3
MoO3
MoO3
MoO3
MoO3
MoO3
MoO3
MoO3
MoO3
MoO3
MoO3
MoO3
MoO3
MoO3
MoO3
MoO3
MoO3
MoO3
MoO3
Aesar) were dissolved in distilled water (20 mL) separately. When
mixed, a precipitate was formed, then filtered and dried at
room temperature. Another two hybrids were analogously pre-
pared by using 1-butyl-3-methylimidazolium methanesulfonate
(Varian 360-LC) and MilliQ water as mobile phase. Conversion,
selectivity and C balance were obtained from HPLC measurements.
2.3.2. General procedure for System 2
(
Sigma Aldrich) to produce the Bmim [PMo12O40] (Bmim-
3
␣-d-Glucose (1 mmol) and 30% hydrogen peroxide (3 mmol)
Mo) and 1-hexyl-3-methylimidazolium chloride (Alfa Aesar) for
were added to the reactor using water (2 mL) as solvent and each
hybrid (0.25 or 2.5 mol% of glucose) as catalyst, respectively, at the
above mentioned reaction conditions. In this case, no extraction
process was necessary. Samples were analyzed in the same manner
by HPLC.
Hexmim [PMo12O40] (Hexmim-Mo). All the hybrids were used as
3
prepared and their structures are presented in Fig. 1.
2.2. Characterization
3
. Results and discussion
X-ray diffraction (XRD) analysis were registered on a Panalitycal
ꢀ
◦
X Pert Pro diffractometer, equipped with Cu anode with 0,05 step
◦
3.1. Hybrids characterization
size and time acquisition of 80 s at room temperature in the 5 65 2␪
range.
3
.1.1. Ray diffraction study
XRD patterns of the phosphomolybdic acid and its correspond-
Temperature dependent X-ray diffraction (TDRX) analysis was
performed in a high temperature camera Anton Paar HTK 1200 cou-
pled with an X Pert Pro Philips diffractometer equipped with Cu
ꢀ
ing hybrids are presented in Fig. 2. The hybrid patterns are very
similar to that of the parent acid. Nevertheless, a shift and some
new diffractions are detected accounting for the presence of organic
cation around the Keggin anion and the formation of a new struc-
ture. The changes of the XRD profiles appear to be related to the
type of cation being the XRD profile of Emim-Mo more similar to
the parent acid than Bmim-Mo and Hexmim-Mo, in a way that
shorter the aliphatic chain, smaller the modification of the parent
acid diffractogram. For the Hexmim-Mo hybrid even new diffrac-
tion lines appear at low 2␪ angles indicating most probably the
formation of an ordered structure.
◦
anode. The diffractograms were taken every 50 C in the range of
◦
◦
◦
◦
3
0–750 C, from 5 to 65 2␪ (step size 0,05 and step time 50 s) in
◦
air. The heating ramp was fixed to 10 C/min.
Raman spectroscopy analysis were performed in a LabRAM
Horiba Jobin Yvon microscope equipped with confocal microscopy
and green laser (␭ = 532 nm). Samples were studied with 50× lens
and intensity filter (D2).
2.3. Catalytic reaction
2.3.1. General procedure for System 1
3.1.2. X-ray diffraction study as a function of temperature
The reactor (a 50 mL vial equipped with a Young valve and con-
The phase transformations of the samples as a function of tem-
perature are studied and presented. The XRD measurements are
taining a stirrer flea) was charged with 0.25 M aqueous [MoO5]
100 ␮L, 2.5 mol% of glucose), the solvent [41] (BmimPF , 2 mL),
◦
◦
(
performed in flowing air from 30 C to 750 C with isotherms
6
◦
the oxidant (30% aqueous H O , 340 ␮L, 3 mmol) and ␣-d-Glucose
at every 50 C. However, Fig. 3 presents only the tempera-
2
2
(
Sigma Aldrich, 182 mg, 1 mmol), in the aforementioned order. The
tures for which significant changes are detected. The parent
reactor was sealed and the solution reacted with constant stirring
acid H PMo12O40·13H O (JCPDS #01-075-1588) loses hydration
3
2
◦
(
600 rpm) in a thermostated oil bath (60 C) during 18 h. Upon com-
water and transforms to H PMo
O
12 40
·6H O (JCPDS#01-070-1705)
3
2
◦
◦
pletion, the reactor was immediately cooled to 0 C and the mixture
and H PMo12O40·1.5H O (JCPDS#00-046-0482) at 100 and 150 C,
3
2
◦
was extracted with H O (3 × 2 mL) and filtered with 0.45 ␮m nylon
respectively. This phase remains intact till 450 C where trans-
2
syringe filter. The resulting solution was diluted in ultra-pure water
forms into MoP O7 (JCPDS#00-039-0026) and MoO3 (JCPDS
2
◦
(
500 ꢀL of sample + 500 ꢀL of H O) and analyzed by HPLC by using
#00-001–0706 + #00-047-1081). From 550 C till the last measured
2
a Hi-Plex H column (300 × 7.7 mm), a refractive index detector
temperature only MoO3 is detected.
Please cite this article in press as: C. Megías-Sayago, et al., Influence of the ionic liquid presence on the selective oxidation of glucose
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Fig. 3. XRDiffractograms as a function of temperature: A) H3PMo12O40·13H2O, B) Emim-Mo, C) Bmim-Mo and D) Hexmim-Mo.
The phase transformations of the hybrids seem to depend on
the cation nature. Longer the substituted aliphatic chains of the
imidazolium ring (ethyl through butyl to hexyl) lower the tem-
perature of the first transformation. In addition, the transition of
the hybrid structure to the decomposition products of the parent
acid is always accompanied by samples amorphization. Emim-
3.1.3. Raman spectroscopy
The XRD studies demonstrate the structural similarity of the
hybrids to the parent acid and the changes induced by the aliphatic
chain of the substituted imidazolium ring. These changes are also
studied by Raman spectroscopy (Fig. 4). The Keggin structural units
present four types of oxygen [42]: terminal oxygen, Ot (Mo O), two
types of bridging oxygen atoms (Mo O Mo), edge sharing oxygen
Oe and corner sharing oxygen Ob, as well as corner sharing oxy-
gen atoms between the central heteroatom and surrounding metal
◦
Mo is no longer available at temperatures higher than 300 C and
◦
◦
converts into MoO3 at 500 C. Bmim-Mo is not observed at tem-
peratures higher than 200 C, and instead, products issued from
the decomposition of phosphomolybdic acid are detected with
atoms Oa (P
O Mo(x3)). Therefore, four oxygen bonds can be dis-
increasing of the temperature: H PMo12O40·13H O (JCPDS #01-
tinguished. The Raman spectra of the hybrids can be separated in
3
2
−
1
0
75-1588), 10MoO ·H PO ·24H O (JCPDS #00-001-0032), MoP O7
two vibrational zones: the first one, within 100–1100 cm metal-
oxygen bonds range, (Fig. 4A) and the second one corresponding to
3
3
4
2
2
(
1
JCPDS #00-039-0020) and MoO3 (JCPDS #00-001-0706, #00-047-
◦
−1
081). Similar to Emim-Mo, at 500 C only MoO is detected.
organic fraction vibrations, in the 2700–3200 cm range (Fig. 4B).
3
◦
Hexmim-Mo is observed till 200 C then an amorphization occurs
The bands attribution and its comparison with literature data are
presented in Table 2.
◦
and directly the MoO3 appears at 550 C. In all the cases a MoO3
crystallizes as a monocrystal, indicating a probable structure direct-
ing role of the imidazolium cations.
In general, the vibrational bands assigned to the metal-oxygen
bonds do not change significantly in the presence of different imida-
zolium derivatives, the Keggin unit seems preserved, as confirmed
by XRD. The Mo Ob Mo and Mo Oe Mo bands are slightly shifted
for the hybrids: indicating that the organic cations situates near the
bridging oxygens thus affecting their electronic density and causing
the blue shift.
All identified phases for the three hybrids and parent phospho-
molybdic acid at every studied temperature are summarized in
Table 1. The dependence of the hybrid’s stability on the organic
counter cation could be easily observed. Emim-Mo hybrid is the
most stable followed by Bmim-Mo and Hexmim-Mo hybrids. A
possible relationship could be envisaged, less perturbation of the
Keggin-structure higher phase transformation stability.
Please cite this article in press as: C. Megías-Sayago, et al., Influence of the ionic liquid presence on the selective oxidation of glucose
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5
A
B
Hexmim-Mo
Bmim-Mo
Hexmim-Mo
Bmim-Mo
Emim-Mo
H PMo O
Emim-Mo
H PMo O
3
12 40
3
12 40
1
00
200
300
400
500
600
700
800
900 1000 1100
2700
2800
2900
3000
3100
3200
-1
-1
Raman shift, cm
Raman shift, cm
Fig. 4. Raman spectra of H3PMo12O40·13H2O and hybrids.
7
6
5
4
30
20
10
0
0
0
0
0
70
60
50
40
30
20
10
0
A
B
Formic acid
Gluconic acid
Glucaric acid
IL + [MoO5]
IL
IL + [MoO5]
IL
Fig. 5. Glucose conversion (%) A) and product yield (%) B) with IL as a solvent reaction conditions: 1 mmol of glucose, 3 mmol of H2O2, [MoO5] (2.5 mol% of glucose), 2 mL of
◦
BmimPF6, 600 rpm, 60 C, 18 h.
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
70
60
50
40
30
20
10
0
A
B
Gluconic acid
Glucaric acid
Bmim-Mo hybrid Without catalyst
Bmim-Mo hybrid Without catalyst
Fig. 6. Glucose conversion (%) A) and product yield (%) B) for Bmim3[PMo12O40] catalyst. Reaction conditions: 1 mmol of glucose, 3 mmol of H2O2, catalyst (2.5 mol% of
◦
glucose), 2 mL of H2O, 600 rpm, 60 C, 18 h.
Fig. 7. Oxidation products in System 1 and System 2.
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Fig. 8. Glucose conversion (%) and product yield (%) as a function of the catalyst’s amount and the ILs nature.
Fig. 9. Glucose conversion (%) A) and yield (%) B) over Emim-Mo 0.25mol% catalyst as a function of reaction time.
Table 2
Vibrational assignments for inorganic and organic fractions.
Experimental Raman shift, cm 1
−
Raman shift, cm [43,44]
−1
Assignment
Experimental Raman shift, cm [45–48]
−1
Assignmenta
9
9
9
6
2
93
69
04,878
02
45, 222
995[43]
981[43]
␯s (Mo = Ot)
2835
2876
2919
2940
2968
␯s CH-CH (imidazolium)
␯s CH R2
␯s CH R1
CH3NHCH
␯as CH R1
␯as (Mo = Ot)
␯as (Mo-Ob-Mo)
␯s (Mo-Oe-Mo)
␯s (Mo-Oa)
909−876 [44]
603[44]
251[44]
3018
3107
3170
␯as CH R2
NC(H)N
␯as CH-CH (imidazolium)
a
R1 (ethyl, butyl or hexyl), R2 (methyl).
For all the hybrids the CH vibration are present confirming the
formation of the hybrids together with the preservation of the Keg-
gin structural unit.
100% is obtained in presence of [MoO5] and only 89% balanced when
IL is used. These results suggest that, although a lower conversion
obtained in presence of [MoO5] the oxidation reactions to glucaric
and gluconic acids are promoted and the total degradation of glu-
cose to formic acid suppressed. On the other hand the deficiency
of the C balance in the case of the IL catalyzed reaction suggests
additional products formation, either probably resulting from poly-
merization reactions or not extracted in the aqueous phase, and not
detected by our HPLC method.
3
.2. Catalytic performance
The oxidation of glucose was carried out following the proce-
dure described above for the two systems. The obtained results for
System 1 are shown in Fig. 5, as glucose conversion and yield in pres-
The obtained results for System 2 are presented in Fig. 6, in pres-
ence or absence of the hybrid Bmim-Mo catalyst. Approximately
a 21% of conversion is obtained in absence of catalyst, and corre-
sponds to the formation of gluconic acid (glucaric acid traces are
also observed). This result changes in presence of Bmim-Mo, where
the conversion of glucose doubles to 40% with 100% selectivity to
gluconic acid. The presence of Bmim-Mo catalyst inhibits the glu-
cose oxidation to glucaric acid. However, in the same way that for
System 1, a C balance of 78% is reached which suggests the forma-
tion of polymerization products, as the most probable cause for C
lost.
ence or absence of [MoO5]. As could be expected, BmimPF plays an
6
important role when used as reaction media, a glucose conversion
of 60% is observed (Fig. 5A) in absence of catalyst, which suggests
the participation of the ionic liquid not only as a solvent but also
as a catalyst. The conversion surprisingly drops in presence of the
catalyst [MoO5] to 41% of conversion. As for the products (Fig. 5B),
both reactions result in the same oxidation products, being gluconic
acid the main product. Thus, higher yields are observed in absence
of catalyst (35% of yield vs. 27% in presence of [MoO5]) in the same
way for glucaric acid (17% of yield vs. 13% in presence of [MoO5])
and formic acid (7% in absence of [MoO5] vs. 1.6%). A C balance of
Please cite this article in press as: C. Megías-Sayago, et al., Influence of the ionic liquid presence on the selective oxidation of glucose
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Fig. 10. Glucose conversion (%) A) and products yield (%) B) as a function of the catalyst’s amount and the Keggin anions nature.
When comparing both systems (System 1 vs. 2) higher selectiv-
ity and yield to gluconic acid are observed when the IL hybrid is
used as catalyst, suppressing the degradation of glucose to formic
acid and the formation of glucaric acid as secondary oxidation
product. The higher conversion observed for the System 1 could
be attributed only to higher IL/Glucose ratio, when IL is used as
solvent/co-catalyst. At this point the System 1 was abandoned due
to the lowest selectivity in terms of number of formed products
and only the hybrid materials are used for more detailed study. All
oxidation products obtained in both systems are summarized in
Fig. 7.
The influence of the amount of catalyst was studied by reducing
ten times the quantity of catalyst (0.25 mol%), with the same reac-
tion conditions described above. In addition, to find out how the ILs
nature affects the reaction parameters, the Emim-Mo, Bmim-Mo
and Hexmim-Mo hybrids are prepared and reacted. The obtained
results are presented in Fig. 8.
glucose while about only 80% when the amount of catalyst is ten
times higher.
In order to evaluate the influence of the reaction time, the con-
version and product distribution were also studied as a function of
the time over Emim-Mo 0,25 mol% (Emim [PMo12O40]; 0.25 mol%
3
of glucose amount) catalyst (Fig. 9).
The glucose conversion remains practically unaltered with the
reaction time obtaining a 25% of conversion after 1 h of reaction
and only 30% after 18 h. However, 100% of gluconic acid is reached
at every point.
The results described above suggest that the IL/Keggin hybrid
appears as a very selective catalyst for glucose oxidation. As the
different organic fractions seem not to influence the conversion,
an increase of that could be reached by the change of the Keggin
part hybrid nature. Three different anions were used to obtain the
hybrids with the following molecular formula Emim [PMo12O40],
3
Emim [PMo V O ] [49] and Emim [PW12O40] [30,40], respec-
3
10
2
40
3
It seems that the organic cation nature is hardly important in
terms of glucose conversion. Nevertheless the conversion slightly
increases with the amount of catalyst, as we could expect, obtaining
a 100% of selectivity to gluconic acid in all cases. Regarding the C
balance, a 100% is obtained when the used catalyst is 0.25 mol% of
tively. In the same way, catalysts were used in a 0.25mol% and
2.5mol% of glucose. The results are presented in Fig. 10.
The glucose conversion is slightly higher for the Mo-V contain-
ing Keggin anion hybrid while no significant differences between
the two monoatomic (Mo, W) compounds are detected. The con-
version increases with the amount of used catalyst except for
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Emim [PW12O40] which is the less active one. Regarding the prod-
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study: the direct transformation giving formic acid as a product
and the oxidation reaction with gluconic and glucaric acid as prod-
ucts. The bare ionic liquid used as a solvent in System 1 appears to
catalyze both the glucose transformation into formic acid and the
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[
[
bined with [Mo(O)(O ) (H O)n] catalyst, the direct transformation
2
2
2
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aldonic and aldaric acids. The second approach using the ionic liq-
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Please cite this article in press as: C. Megías-Sayago, et al., Influence of the ionic liquid presence on the selective oxidation of glucose
over molybdenum based catalysts, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.06.040