A structure-activity relationship study using DFT analysis of Bronsted-Lewis acidic ionic liquids and synergistic effect of dual acidity in one-pot conversion of glucose to value-added chemicals

Article abstract of DOI:10.1039/c7nj02364b

The catalytic conversion of biomass-derived carbohydrates to value-added chemicals, such as 5-hydroxymethylfurfural, levulinic acid, and formic acid, is a commercially important reaction and requires the use of both Lewis and Bronsted acids. Multifunctional ionic liquids (ILs) with both types of acidity show promise as useful catalysts as well as solvents for one-pot glucose conversion to value-added chemicals. In this study, a series of chlorometallate ILs were synthesized and characterized, and their catalytic activity towards glucose conversion was investigated. The structure-activity relationship of chlorometallate ILs was investigated by qualitative comparison of their structural parameters obtained using density functional theory and catalytic activity studies. The acidity and basicity of these ILs were correlated with theoretically estimated parameters such as electrostatic surface potential maxima and minima, average local surface ionization energy, Fukui functions, and stabilization energy. Metal atoms showed a significant effect in controlling both the Lewis and Bronsted acidity of these ILs by withdrawing electron density from the cationic counterpart. Fukui function values showed the presence of significant amount of Lewis basicity in the metal halide-containing anionic part of the ILs. The ZnCl₃-containing IL with highest electrostatic surface potential maxima, representing more electron-deficient region in the cationic part of the IL, was found to be highly Lewis acidic. Furthermore, it showed maximum catalytic activity in glucose conversion resulting in yields of 13.4%, 23.8%, and 67.1% of 5-hydroxymethyl furfural (HMF), levulinic acid (LA), and formic acid (FA), respectively. Hence, Lewis acidity was found to be the controlling parameter for glucose conversion to value-added chemicals. These multifunctional chlorometallate ILs were found to be recyclable with no loss of metal chloride from the anion of the ILs.

Full text of DOI:10.1039/c7nj02364b

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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A structure-activity relationship study using DFT analysis of
Bronsted-Lewis acidic ionic liquids and synergistic effect of dual
acidity in one-pot conversion of glucose to value added chemicals
Received 00th January 20xx,
Accepted 00th January 20xx
Firdaus Parveen†, Tanmoy Patra† and Sreedevi Upadhyayula*
DOI: 10.1039/x0xx00000x
The catalytic conversion of biomass-derived carbohydrates to value added chemicals like 5-hydroxymethylfurfural,
levulinic acid, and formic acid is an important class of commercially important reaction which requires the use of both
Lewis and Bronsted acidity. Multifunctional ionic liquids (ILs) with both the acidity, show promise as useful catalysts as well
as solvents for one pot glucose conversion to value-added chemicals. In this study, a series of chlorometallate ILs were
synthesized, characterized, and their catalytic activity was investigated on glucose conversion. The structure-activity
relationship of chlorometallate ILs was investigated by qualitative comparison of the structural parameters of these ILs
obtained using Density Functional Theory and catalytic activity studies. The acidity and basicity of these ILs were
correlated with theoretically estimated parameters like electrostatic surface potential maxima and minima, average local
surface ionization energy, Fukui functions, and stabilization energy. Metal atoms showed a significant effect in controlling
both the Lewis and Bronsted acidity of these ILs by withdrawing electron density from the cationic counterpart. Fukui
function values showed the presence of significant amount of Lewis basicity in the metal halide containing anionic part of
the ILs. ZnCl3-containing IL with highest electrostatic surface potential maxima representing more electron deficient region
in cationic part of the IL was obtained to be highly Lewis acidic. Furthermore, it also showed maximum catalytic activity in
glucose conversion resulting in 13.4%, 23.8% and 67.1% yield of 5-hydroxymethyl furfural (HMF), Levulinic acid (LA), and
Formic acid (FA) respectively. Hence, Lewis acidity was found to be the controlling parameter for glucose conversion to
value added chemicals. These multifunctional chlorometallate ILs found to be recyclable with no loss of metal chloride
www.rsc.org/
from
the
anion
of
the
ILs.
chemicals proceeds via isomerization catalyzed by Lewis
acids/bases, followed by further dehydration and rehydration
reactions in the presence of Bronsted acids.11 A number of
studies were reported in recent years on glucose isomerization
to fructose using Lewis acidic catalysts like metal halides,12,13
bases,14 etc. However, the use of metal halides leads to loss of
metals in each cycle due to the formation of metal oxides and
reduced metals in aqueous media.15 Furthermore, the glucose
Introduction
The increasing demand for energy, depletion of fossil fuels and
global warming has become a major concern in recent times.1–
3 Utilization of renewable resources like lignocellulosic biomass
to produce fuels and value-added chemicals via green
pathways is widely considered as one of the sustainable
solutions to the above problems.4 Value added chemicals such
as 5-hydroxymethyl furfural (5-HMF), levulinic acid (LA), and
formic acid (FA) are obtained by hydrolysis of the main
components of lignocellulose to glucose followed by
isomerization and dehydration reactions as shown in Scheme 1
(a) Reaction pathway of glucose conversion to value added
dehydration to LA and FA was reported using mineral acids,16
18
ion-exchange resins,17 and Bronsted acidic ILs
etc. The
requirement of multiple catalysts for each step leads to
separation problems, solvent, and reagent losses.15,19 To avoid
the loss of metal ions by reacting with the water in the
reaction system, the halometallate ILs can be explored,
wherein the metal ions are in the ion pair form and bound to
the cationic moiety. Again, the need for a catalyst with
multiple functionalities for this reaction can also be
accomplished by incorporating different functionalities on the
different components of the ILs.
chemicals and (b) used multifunctional IL catalysts.a 5–8
These
.
molecules have numerous applications in various sector such
as biofuel, polymer industries, solvent, and aromatics (BTX)
synthesis.9,10 The conversion of glucose to value-added
Till date, several classes of ILs were reported including
Bronsted acidic ILs, Lewis acidic ILs, basic ILs, task-specific
ILs.20–24 Recently, new families of ILs were reported such as
bio-ILs, chiral ILs, amphiphilic ILs, and Bronsted-Lewis acidic
ILs.25–28 Different computational descriptors were defined and
estimated to compare the acidity and basicity of ILs, which are
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in correlation with the experimental values.29–32 The descriptor Geometry optimization of the metal containing IL structures
like Fukui function was used to explain the acidic and basic was performed using previously studied B3LYP hybrid
sites in imidazolium-based ILs.33 Again, several experimental functional and Dunning’s correlation-consistent aug-ccPVDZ
studies were reported such as Kamlet-Taft solvatochromic basis set36 used to study the similar chlorometallate, ILs
parameters and FTIR spectroscopic studies using pyridine or employed in density functional theory (DFT) for cation-anion
acetonitrile as a probe to determine the acidity or basicity of interactions in ILs, to study the structural and electrostatic
imidazolium or pyridinium based ionic liquid.32,34 Several properties of the ILs.37 The optimized structures were
attempts were made to predict and correlate the confirmed to be at the minima of the potential energy surface
acidity/basicity trends of the ILs from both theoretical and by performing the vibrational frequency analysis at the same
experimental studies.29,31,32,35 Very recently, the Bronsted and theory level with no negative values. The stabilization energy
Lewis acidity as well as basicity of the imidazole based ILs were of the ILs was estimated using the following equation
investigated from the theoretically determined parameters.36 described in the literature.35,38
∆ꢀꢁꢂꢃꢄ (ꢅꢆꢇꢈ ꢉꢊꢈꢋꢌ) = ꢀꢎꢏ − ꢀꢐ + ꢀꢑ ∗ 627.503
)ꢒ
ꢍ
(
However, detailed studies of these ILs at the molecular level
are still scarce in reported literature. A molecular-level Where, EIL, EC and EA are the free energy values of the
understanding of this new class of multifunctional ILs having optimized structure of the ILs, a cationic and anionic
both Bronsted and Lewis acidity is necessary to enhance their component of the ILs in Hartree obtained from the frequency
catalytic and solvent functions in multistep reactions. The calculation. Zero point energy (ZPE) correction factor
correlation of acidity-basicity properties of these type of ILs responsible for basis set superposition error of 0.9804 for the
with the catalytic activity was yet to be explored in reactions B3LYP functional was used for the estimation of stabilization
where multiple functionalities were required.
In this study, the structure-activity relationship of a series of estimated using the “prop” keyword from the electrostatic
laboratory prepared quaternary ammonium-based charges obtained from a full natural bond orbital (NBO)
energy values.39 Electrostatic potential of an atom was
multifunctional ILs was investigated. The computationally and analysis. All these calculations were carried out using the
experimentally determined Bronsted-Lewis acidity and basicity Gaussian 09 program.40 Structural parameters like electrostatic
of the ILs were correlated with their catalytic activity towards surface potential maxima (Vs,max) and minima (Vs,min), the
̅
one pot conversion of glucose to value-added chemicals. From average local ionization energy minima (ꢓs,min) and amount of
theoretical approach, several structural parameters were charge transfer (QCT) of the different components of ILs, were
estimated to compare both Lewis and Bronsted acidity as well estimated using the Multiwfn 3.3.8 program.41 Descriptors for
as Lewis basicity of these ILs. Furthermore, the theoretically electron acceptance and donation, regional electrophilic Fukui
obtained parameters were correlated with experimental function (F+) and nucleophilic Fukui function (F-), were
pyridine-FTIR and Raman spectra. The combined effect of both calculated using the same program. For the visualization of
Lewis and Bronsted acidity of ILs was studied on glucose isosurface of electrostatic potential and average local surface
conversion to the value added chemicals.
ionization energy obtained from Multiwfn program, VMD 1.9.2
package was used.42
Experimental
Materials and methods
Triethylamine, sodium chloride, dimethyl Sulfoxide and
hydrochloric acid were purchased from Merck. Glucose was
purchased from Fischer scientific. Methyl isobutyl ketone was
purchased from Spectrochem. The metal halides, ZnCl2, FeCl3,
AlCl3, CrCl3 and 1, 4-butane sultone were purchased from Loba-
Chemie, Switzerland. All the reagents were analytical grade
and used without any further purification unless or until
mentioned.
Synthetic procedure
Multifunctional ILs were synthesized following the reported
literature 43. In a typical synthesis, triethylamine was reacted
with 1, 4-butane sultone in toluene under vigorous stirring at
80°C for 8 h. After completion of the reaction, the obtained
solid mass of the formed zwitterion was washed with diethyl
ether and toluene respectively. The solid mass was then dried
under reduced pressure of 10 mm of Hg at 120°C. Hydrochloric
acid was then added to the solid and stirred at 60°C for 4 h.
Scheme 1 (a) Reaction pathway of glucose conversion to value added chemicals and (b)
used multifunctional IL catalysts.
Theoretical methods
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Figure 1. Optimized structure of the ionic liquids (a) Cation of ILs, (b) ZnCl3ˉ, (c) FeCl4ˉ, (d) AlCl4ˉ, (e) CrCl4ˉ, (f) IL-Cl, (g) IL-ZnCl3, (h) IL-FeCl4, (i) IL-AlCl4 and (j) IL-CrCl4.
Finally, the metal halides like ZnCl2, FeCl3, CrCl3 and AlCl3 were catalyst, and 8 mL methyl isobutyl ketone (MIBK) were charged
added to the chloride based ILs at the required mole ratio to into a reactor vessel and heated at a designated temperature
1
obtain neutral multifunctional ILs as shown in Scheme 1b. H for different reaction time 44. The reaction mixture was
NMR(400 MHz; D2O): δ: 0.926 (t, 9H), 1.47-1.89 (m, 4H), 2.65 centrifuged, and the filtered organic layer was analyzed with
(t, 2H), 2.91-2.93 (m, 2H), 2.95-2.97 (m, 6H). 13C NMR: (100 HPLC using refractive index detector, HIPLEX column, and
MHz; D2O): δ: 6.67, 19.97, 21.26, 50.07, 52.62, 56.04.
0.5mM H2SO4 as a mobile phase. Experimental yields of HMF,
LA, and FA were calculated from the calibration curves
obtained by running pure samples in HPLC.
FTIR characterization
The FTIR spectral data of the synthesized ILs was recorded
using a Thermoscientific Nicolet-6700 spectrometer in the
range of 1400 to 1600 cm-1. Pyridine was used as a probe for
qualitative analysis of the Lewis as well as the Bronsted acidity
of the ILs 43. Prior to the analysis, pyridine was dried by
treating with KOH and finally distilled over a molecular sieve of
4.0 Å size. Pyridine and ILs were mixed at 1:5 ratio by volume,
stirred for 4 h at a temperature of 60°C and FTIR transmission
spectra was recorded.
Results and discussion
Lewis acidity of ILs
Multifunctional ILs have evolved immensely as an efficient
catalyst system for various commercially important
reactions.16,19,45 Here,–SO3H functionalized ILs containing
metal atoms like Zn, Fe, Al and Cr in their halogenated form
were chosen as shown in Scheme 1b, to investigate the acidity
Raman Characterization
and basicity properties of these ILs. Along with these metal
containing ILs, simple Cl-atom containing IL was chosen as a
reference to investigate the effect of the halometallate ions on
their acidity and basicity trends. The optimized geometries of
the ILs, obtained from Density functional theory (DFT)
calculations as shown in Figure 1, reveals that the Cl-atom in
IL-Cl penetrates more towards the H-atoms of C1 atom of the
side alkyl chain forming strong H-bonding as compared to the
Raman spectroscopic data of the ILs was recorded using
Confocal Raman spectrometer (Model: Xplora, Horiba JOBIN
VYON) with a laser of 785 nm wavelength.
Catalytic activity study
Glucose conversion experiments were carried out in stainless
steel teflon lined autoclaves. 1 mmol each of glucose and the
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other halometallate ILs. This effect is also supported by the towards the alkyl chain, resulting in its poor electron
highest ΔEStab value of the IL-Cl amongst all the ILs as shown in withdrawal tendency from the alkyl chain. Consequently,
Table 1. The bond distances between the Cl-atom of the anion electron withdrawal tendency of the CrCl4ˉ anion from the –
and H-atom of alkyl chains follow the order IL-Cl<IL-ZnCl3<IL- SO3H group decreases and electron deficiency as well as Vs,max
FeCl4<IL-CrCl4<IL-AlCl4. Consequently, the bond distances resides near the C2-atom only. Again, in the case of IL-AlCl4, the
between the metals and Cl-atom increases because of the H- AlCl4ˉ anion, with more charge density and more penetration
bonding between the Cl-atom and alkyl chain H-atoms The towards the alkyl chain than the CrCl4ˉ anion, can withdraw
Lewis acidity of a molecule is demonstrated by its tendency to more electron density. In the case of IL-Cl and IL- ZnCl3, the Cl-
accept electron density from an electron rich species.16 In atoms are able to penetrate more towards the protons
terms of electrostatic interactions, more is the interaction of a attached to the alkyl chain and therefore, they are able to
molecule with the electron rich species, more is its Lewis withdraw more electron density resulting in larger Vs,max
acidity. In halometallate containing ILs, the Lewis acidity arises values. However, the charge transfer from the protons of the
due to the
alkyl chain to the metal atoms in the case of IL-ZnCl3, resulted
inherent electron deficiency in the metal atoms. Electrostatic
in larger Vs,max value than the IL-Cl. In contrary, the other
surface potential maxima, Vs,max, is the parameter to quantify tetrachlorometallate anions are not able to penetrate towards
the electron deficiency located in a particular region of a the alkyl chain H-atoms because of their steric constraints and
molecule. Now, more is the value of Vs,max, more is its Lewis hence, resulted in lower Vs,max values. Thus, the Vs,max value of
acidity at the microscopic level. The Vs,max values of the ILs are the ILs follow the order IL-ZnCl3>IL-Cl>IL-FeCl4>IL-AlCl4>IL-CrCl4.
shown in Table 2 and also in Figure 2
In line with the general trend, the Vs,max value as well as Table 1 Structural parameters of the ILs under study.
the Lewis acidity of the ILs are located in the cationic part.
Strong charge delocalization through the O-S bonds of the –
SO3H group, decreases the electron density over the H-atom
and hence, leads to higher value of Vs,max in that region. As
shown in Figure 2, the Vs,max appears near the H atom of the –
SO3H group in all the ILs except in IL-CrCl4. This discrepancy
arises due to the weaker interaction of the CrCl4ˉ anion with
the cation. The CrCl4ˉ anion containing four Cl-atoms is of
larger size as compared to the ZnCl3ˉ anion containing three
Cl-atoms. Thus, the CrCl4ˉ anion is not able to approach closer
ꢕ
ΔEStab
VS, max
VS, min
ꢔS, min
ILs
(kcal/mol)
(kcal/mol)
(kcal/mol)
(kcal/mol)
IL-Cl
-76.52
-56.72
-59.85
-57.06
-52.04
61.63
64.97
56.66
56.01
53.21
-62.23
-45.26
-36.60
-36.91
-34.86
5.85
8.03
3.54
8.76
8.17
IL-ZnCl3
IL-FeCl4
IL-AlCl4
IL-CrCl4
In terms of charge transfer (QCT) from cationic part of the
ILs to chlorometallate group with respect to the IL-Cl, the
Figure 2. Structural parameters of the ILs, (a) electrostatic surface potential maxima (Vs,max), (b) electrostatic surface potential minima (Vs,min), and (c) average local surface
ionization energy (Is,min).
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following trend was obtained IL-FeCl4>IL-AlCl4>IL-CrCl4>IL-ZnCl3 leads to several positive charge centres around the –SO3H
as shown in Table 2. In the case of IL-FeCl4 and IL-AlCl4, the group as well as in the alkyl chain. Thus, Lewis acidity in these
chlorometallate anions attract more electron density as types of ILs resides over the cationic moiety and site of Lewis
compared to other ILs to complete the octet by filling the acidity depends mainly on the nature of chlorometallate ions.
vacant d- and p-orbital respectively. The IL-CrCl4 consisted of
Lewis basicity of ILs
the tetrahedral CrCl4ˉ anion, has half-filled higher energy t2
level in d-orbital. Thus, it will have little tendency to accept The basicity of the ILs can be explained by their tendency to
electron density from the alkyl chain H-atoms in the higher donate electron density to an electron deficient species. Now,
energy orbital breaking the half-filled electronic configuration. the surface electrostatic potential minima, Vs,min, represents
In the case of IL-ZnCl3, ZnCl3ˉ anion has fulfilled d-orbitals and the strength and orientation of weak electrostatic interactions
thus, it has the lowest charge transfer value. Again, from Table like the hydrogen bonding, halogen bonding, and other Van
2 and Scheme 1b, the charges increase on C2 of ethyl group on der Waals forces. Electrostatic surface of a molecule with a
N atom and C7 of the alkyl chain, whereas the charges large negative value of Vs,min will have more charge density in
decrease on C4/5. The charge increases on H1 (C2/4/5/6/7) and that region. The average local surface ionization energy, ꢓs,min
,
̅
decreases on H2 (C2/4/5/6/7). The charge distribution pattern represents local polarizability and hardness, which in turn
confirms that the charge is dragged from the O-H bond of the predicts the ability to loose electrons. From Figure 2, it is clear
–SO3H group in the cation to the chlorometallate anion that basicity of ILs is located at Cl-atom irrespective of the
through the alkyl chain H-atoms. Overall, in the presence of anion. The addition of metal in anion considerably decreases
̅
s,min value to much larger
the chlorometallate ions the withdrawal of charge density the values of Vs,min and increases the
ꢓ
Table 2. Amount of charge transfer from IL-Cl to metal halides and the NBO charges in different atoms of the ILs.a
ILs
IL-Cl
IL-ZnCl3
IL-CrCl4
IL-FeCl4
IL-AlCl4
QCT
C1
-
-0.1582
-0.667
-0.242
0.233
0.287
-0.366
-0.215
0.248
0.242
-0.455
0.231
0.234
-0.465
0.249
0.247
-0.614
0.26
-0.1912
-0.665
-0.225
0.238
0.264
-0.366
-0.223
0.272
0.234
-0.472
0.253
0.218
-0.465
0.263
0.239
-0.613
0.276
0.267
2.281
-0.921
-0.916
-0.905
0.524
0.338
-0.339
-0.38
-0.4281
-0.663
-0.231
0.229
0.272
-0.365
-0.225
0.266
0.236
-0.473
0.247
0.231
-0.466
0.274
0.241
-0.602
0.273
0.267
2.269
-0.937
-0.919
-0.901
0.523
0.383
-0.388
-0.333
-0.322
-0.275
-0.3584
-0.665
-0.23
-0.669
-0.25
0.211
0.299
-0.365
-0.213
0.243
0.241
-0.454
0.233
0.233
-0.465
0.249
0.245
-0.612
0.259
0.264
2.278
-0.928
-0.903
-0.905
0.525
-
C2
H1(C2)
H2(C2)
N3
0.235
0.271
-0.366
-0.227
0.273
0.234
-0.48
C4
H1(C4)
H2(C4)
C5
H1(C5)
H2(C5)
C6
0.252
0.227
-0.466
0.274
0.242
-0.606
0.273
0.274
2.267
-0.915
-0.941
-0.901
0.524
1.403
-0.592
-0.596
-0.603
-0.551
H1(C6)
H2(C6)
C7
H1(C7)
H2(C7)
S8
0.264
2.28
O9
-0.926
-0.903
-0.904
0.526
0.937
-0.652
-0.583
-0.634
-
O10
O11
H12
Mn+
Cl1
-0.87
-
Cl2
Cl3
-
-0.346
-0.243
Cl4
-
a Values are in a.u.
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as compared to that of IL-Cl as depicted in Figure 2. Thus, it The values of F± as reported in Table 3, reveal that IL-ZnCl3
can be concluded that metal containing ILs have lesser basicity and IL-AlCl4 have a normal distribution. Whereas, IL-FeCl4 and
as compared to the conventional ILs because of the larger size IL-CrCl4 show bifunctional distribution. The Lewis acidity in IL-
of chlorometallate anions which have more diffused negative ZnCl3 and IL-AlCl4 is located on the H-atom of the –SO3H
̅
charge. Surprisingly, in case of IL-FeCl4, ꢓs,min value is less than group. Lewis basicity is located on the chlorometallate ion.
IL-Cl. The excess charge transfer from the cationic moiety Whereas, in IL-FeCl4 and IL-CrCl4 both Lewis acidity and
results in distortion in FeCl4ˉ anion of the IL, leading to an basicity are located on the anion as Fe and Cr both have
elongated Fe-Cl bond as compared to the free ion. Thus, the partially filled d-orbitals to accommodate and donate the
tendency to donate electron density increases resulting in electron density. The IL-Cl with moderate F+ and Fˉ values fall
̅
s,min in IL-FeCl4.
lower value of
ꢓ
in the borderline distribution category.
Table 3. Electrophilic and Nucleophilic Fukui function values of the cations and anions
of the ILs under study.
Bronsted acidity of ILs
The Bronsted acidity of the ILs was estimated from the
stabilization energy values. The IL structures were optimized to
a minimum, and the structural parameters, as well as
stabilization energies were estimated as shown in Table 1. The
trend of acidity of ILs can be explained using two parameters
(i) the stabilization energy i.e. interaction between the cationic
and anionic species of the IL, and (ii) O-H bond distance in the
SO3H moiety of IL. Here, the bond distance of the Bronsted
acidic O-H bond in –SO3H group of the ILs shows no variation.
The stabilization energy values of the ILs are in the order IL-
Cl>IL-FeCl4>IL-AlCl4>IL-ZnCl3>IL-CrCl4. Hence, upon addition of
the chlorometallate ions to the IL-Cl, the obtained ILs become
less stable and more prone to donate the proton. In contrary,
when the proton is tightly bound in the ILs, more energy will
be required to remove a proton. Thus, more stable ILs in terms
of the ΔEStab values will have the least acidity. Hence, the
acidity order will be the reverse of the ΔEStab as follows: IL-
CrCl4>IL-ZnCl3>IL-AlCl4>IL-FeCl4>IL-Cl.
Cation
Anion
Distribution
ILs
F+
F-
F+
F-
IL-Cl
0.906
0.901
0.134
0.877
0.239
0.307
0.133
0.100
0.187
0.104
0.022
0.034
0.865
0.0329
0.750
0.692
0.867
0.900
0.813
0.896
Borderline
Normal
IL-ZnCl3
IL-FeCl4
IL-AlCl4
IL-CrCl4
Bifunctional
Normal
Bifunctional
Fukui functions
Fukui functions represent the reactivity index of a molecule in
terms of the tendency of donating and accepting electron
density. Fukui function can be used to define the Lewis acidity
and Lewis basicity by calculating electrophilic Fukui function
+) and nucleophilic Fukui function (
equations:
(f
fˉ) using the following
+
(
)
ꢗ = ꢘꢙꢚ ꢗ
ꢖ+
ꢜ
= ꢚꢛ+1 ꢗ − ꢚꢛ ꢗ
(
)
(
)
( )
ꢙꢛ
−
(
)
ꢗ = ꢘꢙꢚ ꢗ
ꢖꢋ
ꢜ
= ꢚꢛ ꢗ − ꢚꢛ−1 ꢗ
(
)
(
)
( )
ꢙꢛ
FTIR characterization
Where, f+(r) and f-(r) are the electrophilic and nucleophilic
Fukui functions respectively; ρN(r), ρN+1(r) and ρN-1(r) are the
electron density values of the uncharged, negatively charged
and positively charged molecules respectively. The estimated
Fukui functions were integrated over the basin P using the
following equation:
The recorded FTIR transmission spectra of the ILs using
pyridine as a probe are summarized in Figure 3. The probe,
pyridine, shows two characteristic peaks, one at 1437 cm-1 and
another at 1581 cm-1. The first one is a characteristic peak for
qualitative analysis of the Lewis acidity, while the latter is
important for characterization of the Bronsted acidity. In the
presence of Lewis acids, the peak at 1437 cm-1 gets shifted
ꢝ±
∑
= ꢞ∈ꢡ ꢖ(ꢗ ) × ꢟꢠ
ꢞ
The regions with the greater F+ will be the more susceptible
sites for the nucleophilic attack to accept the electron pair (or
donate a proton). Whereas, the regions with a greater value of
Fˉ are sites for the electrophilic attack, and have the ability to
donate electronic charge (or accept a proton). ILs can be
classified into the normal, bifunctional and borderline category
in terms of their Fukui function values.33 As per the definition,
in the case of normal distribution, Lewis acidity is mainly
centred on the cation and Lewis basicity is centred on the
anion fragment of the IL (for the cation, F+>0.8 eV and for
anion, F->0.7 eV). Lewis acidity and basicity located on the
same centre of the IL represents bifunctional distribution
(F+>0.7 eV and F->0.8 eV). The remaining ILs follow a
borderline distribution.
1437
1450
1540
IL-CrCl4
IL-AlCl4
IL-FeCl4
IL-ZnCl3
IL-Cl
Pyridine
1600
1575
1550
1525
1500
1475
1450
1425
1400
Wavenumber (cm-1)
Figure 3. FTIR spectra of the mixture of pyridine and multifunctional ILs at a ratio of 1:5
by volume.
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ARTICLE
towards 1450 cm-1 because of the donation of pyridine Glucose conversion to value-added chemicals is a typically two
electron density to the Lewis acid. The more is the shift in the step reaction as shown in Scheme 1 . The first step is the
a
peak from 1437 cm-1, more is the Lewis acidity. The extent of isomerization of glucose to fructose catalysed by Lewis acidic
peak shifting of pyridine from 1437 cm-1 decreases in the order species. The second step is a typical dehydration reaction
IL-ZnCl3>IL-FeCl4>IL-AlCl4>IL-CrCl4>IL-Cl. From the theoretical where Bronsted acidic conditions are required. However, both
results, the Vs,max value follows a similar trend and hence, can these steps can be performed using only Lewis acids or
be confirmed to be the most important parameter for the Bronsted acids as reported in the prior art.44 Detailed
theoretical calculation of Lewis acidity. Again, a peak at 1540 mechanistic studies revealed that the first step is the slowest
cm-1 appears because of the protonation of pyridine with rate determining step.44 At a temperature of 160°C and 90 min
Bronsted acidic protons of the IL. Larger is the shifting of the reaction time, maximum yields of HMF, LA, and FA were
peak from 1540 cm-1, higher is the Bronsted acidity of the IL as obtained using these multifunctional ILs as summarized in
the same ratio of the IL: pyridine was used. The peak shifting Figure . All these ILs showed very high catalytic activity and
around the 1540 cm-1 follows the order IL-CrCl4>IL-AlCl4>IL- more than 99% of glucose conversion was obtained. However,
ZnCl3>IL-FeCl4>IL-Cl which is exactly similar to the theoretically the yield of HMF, LA, and FA varied significantly because of the
obtained ΔEStab order of these ILs.
presence of the different metal ions in the corresponding ILs.
As shown Figure , the IL-ZnCl3 showed 13.4%, 23.8% and 67.1%
yield of HMF, LA, and FA respectively. The yield of these value-
added chemicals decreased in the order IL-ZnCl3>IL-Cl>IL-
Raman Spectroscopy
The Raman spectroscopic data of the multifunctional ILs were FeCl3>IL-AlCl4>IL-CrCl4 which is exactly similar to the Lewis
recorded as shown in Figure . Figure , illustrates the presence acidity order. The activity trend also confirms that the Lewis
of a characteristic peak at 282 cm-1 for ZnCl3ˉ species in IL- acid-catalyzed isomerization of glucose to fructose is the rate-
determining step in this multistep reaction. Non-stoichiometric
IL-AlCl-4
formation of a higher amount of FA over LA can be attributed
327 cm-1
351 cm-1
to more than one possible pathway of glucose conversion as
shown in Scheme 1a. 11,49 In one pathway, glucose is converted
278 cm-1
235 cm-1
334 cm-1
to FA and LA via the intermediate HMF and the other pathway
proceeds via the formation of furfuryl alcohol and FA without
IL-FeCl-4
IL-ZnCl-3
any HMF as intermediate. Further rehydration of furfuryl
alcohol leads to the formation of LA and FA, and thus more
than 67.1% yield of FA is obtained as compared to 23.8% LA in
the case of IL-ZnCl3.
282 cm-1
232 cm-1
70
IL-CrCl-4
IL-Cl
330 cm-1
HMF
LA
FA
60
235 cm-1
276 cm-1
50
40
30
20
10
0
400 375 350 325 300 275 250 225 200
Wavenumber (cm-1)
Figure 4 Raman spectroscopic data of the synthesized multifunctional ILs.
ZnCl3.46 The symmetry stretching of Fe-Cl at 360 cm-1 and 330
cm-1 represents [Fe2Cl7]ˉ and FeCl4ˉ respectively.47 IL-CrCl4
shows three distinct peaks at 330 cm-1, 276 cm-1, and 235 cm-1
which confirm the presence of multiple Cr-Cl complex species.
The peak at 330 cm-1 cornfirms the presence of Cr3+-Cl
expansion and vibration frequency.48 Furthermore, from the
peak intensities, it can be inferred that the FeCl4ˉ is the
predominant species in IL-FeCl4. In the case of IL-AlCl4ˉ, the
peaks at 327 cm-1 and 357 cm-1 shows the presence of both
IL-Cl
IL-ZnCl3
IL-FeCl4
IL-AlCl4
IL-CrCl4
Figure 5 Yield of HMF, LA and FA using different ILs; Reaction conditions: 1mmol
glucose, 1mmol catalyst, 8 mL MIBK, T=160°C, reaction time=90 min.
Conclusions
In this study, the structure-activity relationship of laboratory
prepared multifunctional chlorometallate ILs was studied by
correlating their theoretically and experimetally determined
Bronsted-Lewis acidity and bascity with catalytic activity in
glucose conversion to value-added chemicals. Both Lewis and
Bronsted acidity of the ILs were compared using both
-
the species AlCl4 and [Al2Cl7]ˉ .22
Catalytic activity of multifunctional ILs
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experimental and theoretical studies. The electrostatic surface
potential extrema, average local surface ionization energy and
NBO charges of atoms were used as parameters to compare
the Lewis acidity and basicity of the ILs. In addition, the Fukui
function values were calculated to compare the electrophilic
and nucleophilic tendency of the different atoms of ILs in
correlation with their Lewis acidity and basicity. From the
results of theoretical predictions and experimentation, IL-Cl
was found to be falling in the “borderline distribution”
category, IL-ZnCl3 and IL-AlCl4 were classified in the “normal
Polym., 2017, 159, 146–151.
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distribution” category, and IL-FeCl4 and IL-CrCl4 were found to 10
be falling in the “bifunctional distribution” category. The
introduction of the chlorometallate groups resulted in 11
increased electron deficiency in the triethylammonium
cationic moiety. Especially, the Lewis acidity of the cationic 12
moiety of the ILs increases in the region of the H-atom of the -
SO3H group responsible for glucose isomerization to fructose. 13
However, the Lewis basicity of these ILs was found to be lower
than the conventional ILs because of the delocalized charge 14
distribution in the chlorometallate groups. ZnCl3-containing ILs
being highly electron deficient with maximum electrostatic
surface potential maxima showed maximum efficiency in 15
glucose conversion producing 13.4%, 23.8% and 67.1% yield of
HMF, LA, and FA respectively. The activity of the ILs decreased 16
in order IL-ZnCl3>IL-Cl>IL-FeCl3>IL-AlCl4>IL-CrCl4 which is similar 17
to the Lewis acidity trend as obtained from the theoretical
studies and validated by experimental FTIR spectroscopic 18
studies. This in turn confirms that the glucose isomerization to
fructose catalyzed by Lewis acids is the rate determining step 19
in the multistep conversion of glucose. Furthermore,
comparative study on the acidity and basicity of these 20
multifunctional ILs will help in understanding the structure-
activity relationship in similar complex catalytic systems. This 21
study will also help in the design of multifunctional Bronsted-
Lewis acidic ILs for application in acid-catalyzed multistep 22
reactions.
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