Bi-Functional Magnesium Silicate Catalyzed Glucose and Furfural Transformations to Renewable Chemicals

Article abstract of DOI:10.1002/cctc.202000711

Bio-refinery is attracting significant interest to produce a wide range of renewable chemicals and fuels from biomass that are alternative to fossil fuel derived petrochemicals. Similar to petrochemical industries, bio-refinery also depends on solid zeolite catalysts. Acid-base catalysis plays pivotal role in producing a wide range of chemicals from biomass. Herein, the Mg framework substituted MTW zeolite is synthesized and explored in the valorisation of glucose and furfural. Bi-functional (acidic and basic) characteristics are confirmed using pyridine adsorbed FT?IR analysis and NH₃ and CO₂ temperature-programmed desorption techniques. Textural properties and morphological information are retrieved from N₂-sorption, X-ray photoelectron spectroscopy, and electron microscopy. The activity of the catalyst is demonstrated in the selective isomerisation of glucose to fructose in ethanol. Glucose is converted to methyl lactate in high yield using the same catalyst. Further, the bi-functional activity of this catalyst is demonstrated in the production of fuel precursor by the reaction of furfural and isopropanol. Mg?MTW zeolite exhibits excellent activity in the production of all these chemicals and fuel derivative. The catalyst exhibits no significant loss in the activity even after five recycles. One simple catalyst affording three renewable synthetic intermediates from glucose and furfural will attract significant attention to catalysis researchers and industrialists.

Full text of DOI:10.1002/cctc.202000711

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: Bi-Functional Magnesium Silicate Catalyzed Glucose and
Furfural Transformations to Renewable Chemicals
Authors: Rajendra Srivastava and Abhinav Kumar
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Link to VoR: https://doi.org/10.1002/cctc.202000711
01/2020

10.1002/cctc.202000711
ChemCatChem
FULL PAPER
Bi-Functional Magnesium Silicate Catalyzed Glucose and Furfural
Transformations to Renewable Chemicals
Abhinav Kumar[a] and Rajendra Srivastava*[a]
[a]
Mr. Abhinav Kumar, Dr. Rajendra Srivastava
Catalysis Research Laboratory, Department of Chemistry,
Indian Institute of Technology Ropar,
Rupnagar-140001, Punjab, India.
E-mail: rajendra@iitrpr.ac.in
Supporting information for this article is given via a link at the end of the document. ((Please delete this text if not appropriate))
Abstract: Bio-refinery is attracting significant interest to produce a
wide range of renewable chemicals and fuels from biomass that are
alternative to fossil fuel derived petrochemicals. Similar to
petrochemical industries, bio-refinery also depends on solid zeolite
catalysts. Acid-base catalysis plays pivotal role in producing a wide
range of chemicals from biomass. Herein, the Mg framework
substituted MTW zeolite is synthesized and explored in the
valorization of glucose and furfural. Bi-functional (acidic and basic)
characteristics are confirmed using pyridine adsorbed FT-IR analysis
and NH3 and CO2 temperature-programmed desorption techniques.
Textural properties and morphological information are retrieved from
N2-sorption, X-ray photoelectron spectroscopy, and electron
microscopy. The activity of the catalyst is demonstrated in the
selective isomerisation of glucose to fructose in ethanol. Glucose is
converted to methyl lactate in high yield using the same catalyst.
Further, the bi-functional activity of this catalyst is demonstrated in
the production of fuel precursor by the reaction of furfural and
isopropanol. Mg-MTW zeolite exhibits excellent activity in the
production of all these chemicals and fuel derivative. The catalyst
exhibits no significant loss in the activity even after five recycles.
One simple catalyst affording three renewable synthetic
intermediates from glucose and furfural will attract significant
attention to catalysis researchers and industrialists.
the production of large numbers of chemicals and fuels via an
integrated bio-refinery approach discussed above. Fructose is
not only interesting in food industries because of its sweetening
property ( 2 folds sweeter than sucrose)[6] but also serves as a
precursor for the production of furfural, lactic acid (and its
esters), levulinic acid, and other value added chemicals.[7-12] The
production of fructose by the isomerisation of glucose is limited
by the thermodynamic equilibrium between the two sugar
monomers at any operating temperature.[13] The equilibrium can
be shifted toward the fructose with the help of base (via Lobry de
Bruyn−Alberda van Ekenstein (LdB-AvE) mechanism[14]) or
Lewis acid. Lactic acid, another feedstock derived from
fructose/glucose, is widely used in the polymer industries,
production of bio-degradable plastics, and fine-chemical
industries.[15-17] Similar to fructose, the conventional bio-catalytic
production of lactic acid is not attractive due to waste generation
(salt) and the deployment of non-regenerable costly bio-
catalyst.[15] Thus the efforts are in progress to develop
heterogeneous catalyst mediated process for the production of
alkyl lactate. This transformation can also be accomplished
using basic or Lewis acidic catalyst. Similarly, furan derived
chemicals such as furfural (FAL) and 5-hydroxymethylfurfural
(HMF), which are the precursors for a wide range of fine-
chemicals and fuels are derived from the mono-saccharides.
Several research groups, including our group, have developed a
wide range of catalytic processes for the valorisation of
FAL/HMF.[18-24] In an interesting approach, the application of
basic and Lewis acid catalysts can be explored in the cross
Introduction
condensation
reaction
of biomass derived
carbonyl
Syntheses of renewable chemicals using heterogeneous
catalytic processes in eco-friendly solvents are attractive but
challenging areas of contemporary research. Due to the limited
reserve of fossil fuels and their negative impact on environment,
it has become essential to develop sustainable catalytic
processes for the production of fuels and chemicals derived from
biomass or their derivatives (carbohydrates, lignin, and their
derived chemicals).[1-2] Glucose can be easily produced from the
cellulose and hemicelluloses by the hydrolysis process using a
Brönsted acid catalyst.[3] The production of bio-ethanol from the
glucose is a well established process.[4] Similarly, efforts are in
progress to develop catalytic processes for the valorization of
glucose. Valorization can be made more attractive for industrial
prospective, if the integrated bio-refinery approach is adopted.[5]
In this approach a large numbers of renewable chemicals can be
produced from the same raw material, for example glucose, so
that it can serve the purpose of several chemical, polymer, food,
and petrochemical industries. This article provides the
conversion of glucose to fructose, glucose to alkyl lactate, and
the synthesis of furfural-acetone adducts using a simple catalyst.
Glucose isomerisation to fructose is the largest bio-catalytic
process because fructose serves as an important feedstock for
(furfural/HMF/DFF etc.) with another carbonyl (linear aldehyde
or ketone, levulinic acid etc.), to produce C8-C15 liquid fuel
intermediates, which can be further transformed into linear-chain
alkanes by hydro-deoxygenation process.[25-26]
Various biomass derived synthetic intermediates
discussed above can be synthesized by acid or base catalysts.
Since these renewable feedstocks need to produce on a large
scale, thus the catalysts should also be easily prepared on a
large scale in economical price and should be recyclable many
times. Zeolites rule the petrochemicals and fine-chemicals
industries due to their tunable acidity, large scale production,
and efficient recyclability.[27] In recent time emphasis has also
been made for the development of mesoporous zeolites for
improving the catalytic activity of zeolites in petrochemical and
fine chemicals industries.[28-29] The catalytic conversion of
glucose does not require mesopores but it highly depends on
the active sites. The transformations of glucose to fructose and
glucose to lactic acid/alkyl lactate are catalyzed using zeolite
based catalysts, especially using mild Lewis acid framework
substituted metals such as Sn and Zr.[30-36] Several solid base
catalysts such as MgO, MgO/silica, hydrotalcite, and other mild
1
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10.1002/cctc.202000711
ChemCatChem
FULL PAPER
basic catalysts have also been explored for the similar
transformations.[37-42] The incorporation of Mg in zeolite
framework is less explored in catalytic reactions, especially in
the biomass conversion. Thus, the novelty of this study is the
synthesis and catalytic applications of bi-functional, magnesium
silicates with MTW framework having optimum acidity and
basicity for the glucose and furfural valorization. Zeolite Beta
(BEA framework) is the most suited for the glucose and furfural
transformations discussed in this study due to its micropore size
of 0.67 nm when compared to the ZSM-5 (MFI framework)
having a micropore size of 0.55 nm. Beta is hydrothermally
synthesized in high Al content. For the incorporation of another
metal ion in the framework (like Mg2+ in this study), first Al needs
to be removed from the BEA framework by the dealumination
process, followed by the framework Mg2+ substitution. The
motivation of selecting MTW in this study is that the Mg2+ can be
directly incorporated in the MTW framework because the MTW
framework is thermodynamically favoured under high-silica (or
even Al-free) synthesis conditions.[43] Moreover, MTW is
prepared with the same micropore directing agent to that of
zeolite Beta, thus, its micropore size is somewhat similar to that
of Beta but relatively larger than MFI and would be better for this
study.
are the characteristic of MTW (ZSM-12) framework structure
(Figure 1a).[44] All the three materials exhibit a similar XRD
pattern and no appreciable change is observed by varying Si/Mg
ratio. In the absence of Mg salt, amorphous silica was formed.
This suggests the importance of Mg2+ in the synthesis of Mg-Si-
MTW silicate. The examination of XRD analysis suggests that
only pure crystalline phase of Mg-Si-MTW is present and no
impurity phase is present in these materials. Moreover, the
absence of characteristic peak at 2θ of 42.8° for MgO, confirms
that no MgO phase exists in the synthesized magnesium
silicates with different Si/Mg.
0.040
(b)
(a)
0.035
0.030
0.025
0.020
0.015
0.010
0.005
0.000
Mg-Si-MTW-25
Mg-Si-MTW-50
Mg-Si-MTW-75
250
200
150
100
50
Mg-Si-MTW-25
Mg-Si-MTW-50
BJH
2
4
6
8
10 12 14
Pore size (nm)
0.5
0.4
0.3
DFT
0.2
0.1
0.0
0
Mg-Si-MTW-75
In the present study, Al-free MTW type magnesium silicate
(Mg-Si-MTW) is prepared via hydrothermal route and
investigated in the biomass conversion in a sustainable alcohol
medium. The objective of this study is to develop a simple, non-
functionalized, recyclable heterogeneous catalyst for the
isomerzation of glucose to fructose in ethanol with a good yield.
Glucose is successfully converted to methyl lactate in methanol
using the same catalyst. Moreover, the versatility of this simple
catalyst is demonstrated for the synthesis of furfural-acetone
adduct (4-(2-furyl)-3-buten-2-one), which acts as a precursor for
the production of liquid hydrocarbon fuel, by the reaction of
furfural and 2-propanol. The use of less explored, stable, and
highly efficient Mg-Si-MTW for the glucose valorization in the
synthesis of three different types of synthetic intermediates will
certainly motivate the researchers working in the field of
biomass conversion for the production of value-added chemicals
and fuels.
1
2
3
4
5
Pore size (nm)
10
20
30
40
50
0.0
0.2
0.4
0.6
0.8
1.0
2 (Degree)
Relative pressure (P/P )
0
Figure 1. (a) XRD patterns and (b) N2-adsorption isotherms of
Mg-Si-MTW-25, Mg-Si-MTW-50, and Mg-Si-MTW-75 (Inset of
Figure b shows the BJH and DFT pore size distributions).
Table 1. Physico-chemical properties of the various synthesized
catalysts.
Catalyst
Mg
SBET
(m2/g)
SExt
(m2/
g)
Vtotal
Davg
Tacidity
(m
ol/g )
Tbasicity
(mol
/g )
content
(wt %)
(cc/g)
(nm)
Mg-Si-
MTW-25
Mg-Si-
MTW-50
Mg-Si-
MTW-75
MgO
3.58
1.82
1.26
270
262
257
89
49
0.237
0.237
0.323
3.81
3.78
3.62
62.2
52.1
29.2
66.2
51.7
31.9
Results and Discussion
In the present study, magnesium silicates having MTW
framework topology with different Si/Mg ratio are synthesized.[44]
For comparative study MgO[38] and MgO/SBA-15[45] are also
prepared. The Mg incorporation has introduced the acid-base
properties in the magnesium silicate as confirmed from
NH3/CO2-TPD and pyridine FT-IR investigations. The catalytic
activities of the acid-base bi-functional magnesium silicates are
investigated in three important catalytic reactions relevant to the
biomass conversion to produce valuable and renewable
chemicals in environmental friendly alcohols solvent. The first
reaction is the glucose to fructose isomerization in ethanol. In
the second reaction, glucose is converted to methyl lactate in
methanol. Third reaction is the aldolization of furfural in 2-
propanol.
118
NE
NE
86
86
0.523
0.851
0.882
31.0
5.64
6.5
NE
NE
NE
210.5
NE
SBA-15
MgO(2%)
@SBA-
15
698
659
459
585
1.95
42.8
SBET was calculated from the adsorption branch in the relative
pressure range of 0.05 < P/P0 ≤ 0.3. Bulk Mg content was
determined from MP-AES. SBET = total surface area, SExt = external
surface area, Vtotal = total pore volume, Davg = average pore diameter,
Tacidity = total acidity, and Tbasicity = total basicity; NE = Not evaluated.
The N2-sorption measurements of all the synthesized
materials were carried out to understand the influence of varied
Mg contents on the textural properties of resultant materials
(Figure 1b, Table 1). All the three synthesized materials exhibit
type IV isotherm with H3 hysteresis loop (Figure 1b). The
presence of hysteresis loop above P/P0 > 0.4 is due to the
3.1. Physicochemical Characterization
The X-ray diffraction (XRD) patterns of the synthesized materials
were recorded to confirm their crystalline structure. The XRD
patterns exhibit peaks at 2θ = 7.5°, 8.8°, 20.7°, and 23.1°, which
2
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10.1002/cctc.202000711
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capillary condensation into mesopores present in these
materials. These features are the characteristics of the co-
existence of both micropores and mesopores of these materials.
A narrow BJH pore size distribution in the range of 3.6 – 3.8 nm
further reflects the presence of mesopores in the materials
(Figure 1b, inset). DFT pore size distribution shows the pore size
below 1 nm suggesting the presence of uniform micropores
present in the materials (Figure 1b, inset). With increase in the
Mg contents in these materials, only a marginal change in the
surface area and pore volume is obtained (Table 1). For
comparative study, N2-sorption experiments were conducted for
MgO, SBA-15, and MgO(2%)@SBA-15 and the determined
textural properties are summarized in Table 1. MgO has a lower
surface area than Mg-Si-MTW materials but with a larger pore
volume and a wider pore diameter. SBA-15 exhibits large
surface area, pore volume, and pore size of 698 m2/g, 0.851
cc/g, and 5.64 nm, respectively (Table 1). Only a marginal
change in the textural properties is obtained for MgO(2%)/SBA-
15 when compared to SBA-15, which are much larger than Mg-
Si-MTW materials (Table 1). The amount of Mg incorporation in
Mg-Si-MTW-x determined from MP-AES suggests that
somewhat lower amount of Mg is incorporated in the materials
when compared to the input ratio (Table 1). This observation
suggests that during the hydrothermal treatment some amount
of Mg2+ is not incorporated in the zeolite framework.
2f - h) from the HADDF image (Figure 2e), and EDX spectrum
(Figure 2i) obtained from TEM images of Mg-Si-MTW-50.
X-ray photoelectron spectroscopy (XPS) surface survey
spectrum of Mg-Si-MTW-50 confirms the presence of various
constituent elements in the framework structure (Figure S2a).
The high-resolution spectrum of Si 2p exhibits a broad peak,
which can be deconvoluted into two peaks centred at binding
energies of 103.1 eV and 104.0 eV corresponding to Si—O—Mg
and Si—O—Si, respectively.[46] Moreover, a distinguish peak
centred at 1305.0 eV corresponds to Mg2+ is present in Si—O—
Mg.[47] The high-resolution spectrum of
O
1s can be
deconvoluted into two peaks with binding energies of 533.2 eV
and 534.2 eV associated with Si—O—Si/Mg and Si—O—H,
respectively.[48]
MgO is known to exhibit basicity and extremely weak Lewis
acidity.[49] The acidity and basicity are highly dependent on the
calcinations temperature after the sol-gel synthesis.[49,38]
MgO/silica also known to exhibit basic property.[45] Sn or Zr
framework substituted silicate framework exhibits Lewis
acidity.[31-36] When Mg2+ is incorporated in the silicate framework,
it can form following species as depicted in Scheme 1a, which
clearly suggests that the magnesium silicate would demonstrate
acidic and basic properties. Si-O- can acts as a basic site,
whereas Mg2+ has ability to accept electron pairs and thus it acts
as a Lewis acid site. The formation of Mg-Si-MTW-x silicates
and the incorporation of Mg2+ into the silicate framework is
confirmed by FT-IR analysis. Figure S3 shows the FT-IR spectra
of the synthesized Mg-Si-MTW-x silicates. A sharp distinguished
peak at 578 cm-1 is the characteristic of the stretching vibrations
(c)
(a)
(b)
of double five-membered rings in MTW framework structure.[50]
A
5 m
2 m
2 m
broad band at 1074 cm-1 and 800 cm-1 are ascribed to the
presence of Si—O stretching vibration in the silicate structure.[51]
The characteristic band of Mg—O stretching vibration appears at
459 cm-1.[52] Two small bands at 547 cm-1 and 647 cm-1 are
attributed to the symmetric stretching vibration of Si—O—Mg in
Mg-Si-MTW-x silicates.[53] Thus, the FT-IR analysis evidences
the successful formation of MTW framework and Mg
incorporation in the synthesized silicates, which is consistent
with the XRD results.
(d)
(f)
(g)
(e)
Si
Mg
100 nm
100 nm
100 nm
20 nm
O
(i)
30000
(h)
O
Si
20000
10000
Mg
100 nm
(a)
(b)
Mg-Si-MTW-25-Pyridine
Mg-Si-MTW-25-Parent
Figure 2. (a, b) SEM images, (c) TEM image, (d) HR-TEM image,
(e) HAADF image, (f) elemental mapping for Si, (g) elemental
mapping for Mg, (h) elemental mapping for O, and (i) EDX spectrum
of Mg-Si-MTW-50.
1485 (L)
1442 (L)
The morphological and microstructural information was
obtained using scanning electron microscope (SEM). Figure 2
presents the SEM and TEM images of Mg-Si-MTW-50 silicate. A
non-uniform rice shaped micrometer size morphology is visible
in the SEM images of Mg-Si-MTW-50 catalyst (Figure 2 a & b).
Mg-Si-MTW materials with different Mg contents exhibit almost
similar morphologies (Figure S1), which are consistence with the
results obtained from N2-sorption analysis. The rice shaped
micrometer size morphology of Mg-Si-MTW-50 silicate is also
confirmed from TEM image (Figure 2c). High resolution TEM
image (HR-TEM) confirms the rod like morphology of a single
particle (Figure 2d). Further, the presence of all elements is
confirmed from the elemental mapping of Si, Mg, and O (Figure
1540 (B)
1595 (L)
1442 (L)
1485 (L)
1540 (B)
1578 (L)
1595 (L)
1578 (L)
Mg-Si-MTW-25
Mg-Si-MTW-50
Mg-Si-MTW-75
1700 1650 1600 1550 1500 1450 1400
1700 1650 1600 1550 1500 1450 1400
-1
-1
Wavenumber (cm )
Wavenumber (cm )
Figure 3. Pyridine adsorbed FT-IR spectra of (a) Mg-Si-MTW-25,
Mg-Si-MTW-50, and Mg-Si-MTW-75, and (b) parent Mg-Si-
MTW-25 and pyridine adsorbed Mg-Si-MTW-25. L = Lewis acid,
B = Brönsted acid.
3
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The pyridine adsorbed FT-IR analysis was carried out to
investigate the types of acidity present in Mg-Si-MTW-x. Figure
3 presents the pyridine adsorbed FT-IR spectra of all the
synthesized magnesium silicates. Weak Lewis acid sites (1578
cm-1 and 1485 cm-1) and H-bonded pyridine (1595 cm-1 and 1442
cm-1) are present in these materials (Figure 3a). Moreover, the
exceptionally weak Brönsted acid sites are present (designated
as 1540 cm-1) in these materials (Figure 3). These findings are in
good agreement with the generation of Lewis acidity with the
Mg-incorporation into the silicate framework (Scheme 1a). For a
better presentation, the FT-IR spectra of parent Mg-Si-MTW-25
and pyridine adsorbed Mg-Si-MTW-25 are plotted together
(Figure 3b) to demonstrate the existence of Lewis acid sites in
the material.
glucose to fructose isomerisation is conducted in ethanol solvent.
No isomerisation takes place in the absence of catalyst (Table 2,
entry 1). Progress of the reaction was monitored using HPLC. All
Mg-Si-MTW materials prepared in this study are found to be
active and selectively produce fructose (Table 2, entries 2-4).
With increase in the Mg contents in the Mg-Si-MTW, the glucose
conversion increases. However, with further increase in the Mg
content, the glucose conversion decreases. The best glucose
conversion is obtained using Mg-Si-MTW-50 (Table 2, entries 2-
4). Under the identical reaction condition, pure MgO exhibits the
lower glucose conversion (Table 2, entry 5). Since the surface
area of MgO is significantly lower than Mg-Si-MTW-50, thus to
obtain a fair comparison, 2 wt% of MgO is loaded on the high
surface area SBA-15. SBA-15 is found to be inactive for this
reaction (Table 2, entry 6). Interestingly, the activity of
MgO(2%)/SBA-15, having higher surface area than Mg-Si-MTW-
50 and almost similar Mg content, is lower than Mg-Si-MTW-50
(Table 2, entry 7). The reasons for the higher activity of Mg-Si-
MTW-50 when compared to MgO and MgO(2wt%)@SBA-15 will
be discussed at the end of this section while discussing the
mechanism of the reaction.
40
30
(a)
(b)
35
30
25
20
15
10
5
Mg-Si-MTW-25
Mg-Si-MTW-50
Mg-Si-MTW-75
Mg-Si-MTW-25
Mg-Si-MTW-50
Mg-Si-MTW-75
25
20
15
10
5
0
0
360 450 540 630 720 810 900 990
Temperature (K)
360 450 540 630 720 810 900 990
Temperature (K)
Table 2. Catalytic activity data for glucose isomerization over
various synthesized catalysts.
Figure 4. (a) NH3-TPD and (b) CO2-TPD profiles of Mg-Si-MTW-
25, Mg-Si-MTW-50, and Mg-Si-MTW-75.
E.
No.
Catalyst
Glucose
Conv. (%)a
None
19.8
Fructose
Sele. (%)a
-
Fructose
yield (%)
-
The total acidity and basicity of the synthesized magnesium
silicates were determined from the temperature-programmed
desorption (TPD) technique, using NH3 and CO2 as the probe
molecules, respectively. Both, the acidity and basicity have a
direct relationship with the Mg2+ content in the synthesized
catalysts. In the NH3-TPD profiles, all the three magnesium
1
2
3
4
5
6
7
No catalyst
Mg-Si-MTW-75
Mg-Si-MTW-50
Mg-Si-MTW-25
MgO
96.2
96.0
93.5
92.4
-
19.0
33.6
28.2
18.8
-
35.0
30.2
20.4
SBA-15
None
9.3
silicates exhibit
a
similar desorption behaviour in the
MgO(2%)@SBA-15
93.2
8.7
temperature range of 325 – 520 K, which can be ascribed to the
desorption of NH3 from weak acid sites (Figure 4a), however,
their concentration varies as it is judged from the area under the
peaks. Further, the desorption profiles vary in the temperature
range of 550-1000 K. The higher concentrations of medium
strength acid sites are present in Mg-Si-MTW-50 whereas higher
concentrations of stronger acid sites are present in Mg-Si-MTW-
25. The distribution of acid strength is provided in Table S1.The
total acidity determined from the NH3-TPD is presented in Table
1. With increase in the Mg content, the acidity increases, and it
follows the order: Mg-Si-MTW-25> Mg-Si-MTW-50> Mg-Si-
MTW-75. Figure 4b presents the CO2-TPD profiles of the
synthesized magnesium silicates. CO2-TPD profiles of these
materials suggest that they have basicity of different strength
(weak, medium, and/or strong). Mg-Si-MTW-25 catalyst has
weak and strong basic sites whereas Mg-Si-MTW-50 and Mg-Si-
MTW-75 have weak and medium strength basic sites. The
distribution of basic strength is provided in Table S1. The total
basicity determined from the CO2-TPD is presented in Table 1.
With increase in the Mg content, the basicity increases, and it
follows the order: Mg-Si-MTW-25> Mg-Si-MTW-50> Mg-Si-
MTW-75.
Reaction condition: Glucose (90 mg), catalyst (50 mg), ethanol
(8 mL), temperature (393 K), and time (2 h). aObtained from
HPLC analysis.
40
35
30
25
20
15
10
5
40
35
30
25
20
15
10
5
(a)
(b)
Glucose Conversion
Fructose Yield
Glucose Conversion
Fructose Yield
0
0
363
373
383
393
403
20
30
40
50
60
Temperature (K)
Catalystamount (mg)
40
35
30
25
20
15
10
5
40
(c)
(d)
35
30
25
20
15
10
5
Glucose Conversion
Fructose Yield
Glucose Conversion
Fructose Yield
0
0
0.5
1
1.5
2
2.5
Time (h)
Figure 5. Optimization of reaction parameters (a) temperature
[glucose (90 mg), Mg-Si-MTW-50 (50 mg), ethanol (8 mL), and
time (2 h)], (b) catalyst amount [glucose (90 mg), ethanol (8 mL),
time (2 h), and temperature (393 K)] (c) time [glucose (90 mg),
Mg-Si-MTW-50 (50 mg), ethanol (8 mL), and temperature (393
K)], and (d) solvents [glucose (90 mg), Mg-Si-MTW-50 (50 mg),
solvent (8 mL), temperature (393 K), time (2 h)].
3.2. Evaluation of catalytic activity
3.2.1. Selective isomerization of glucose to fructose
One of the challenges present in the carbohydrate conversion is
the glucose to fructose isomerisation, especially in the eco-
friendly solvent medium, such as, alcohol and water. First, the
4
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Having found the best catalyst, the influence of various
reaction parameters is investigated over Mg-Si-MTW-50 catalyst
(Figure 5). No reaction takes place at ambient temperature.
Thus the influence of temperature is investigated in the range of
363 K to 403 K (Figure 5a). With an increase in the reaction
temperature, glucose conversion increases. At all temperatures,
fructose is selectively obtained over Mg-Si-MTW-50. The highest
glucose conversion is obtained at 393 K. With a further increase
in the temperature, no further increase in the glucose conversion
is observed; therefore 393 K is optimized as the reaction
temperature. With increase in the catalyst amount from 20 mg to
50 mg (for 0.5 mmol of glucose), the glucose conversion
increases (Figure 5b). However, no further improvement in the
glucose conversion is obtained using 60 mg of catalyst. Thus, 50
mg of catalyst for 0.5 mmol of glucose is optimized for the
reaction. With increase in the time from 0.5 h to 2 h, the glucose
conversion increases (Figure 5c).
conversion of 28.1 % with fructose yield of 21.2 % is obtained
relative to the higher glucose conversion of 35 % with fructose
yield of 33.6 % in ethanol (Figure 5d). In this case, glycolic acid,
lactic acid, acetic acids, and formic acid are detected as the side
products along with other side-products obtained in the case of
alcohols. When methanol and 1-propanol (PrOH) are used,
lower fructose yields are obtained when compared to ethanol. In
this case, glyceraldehyde, 1, 3-dihydroxyacetone, and
pyruvaldehyde are obtained (Scheme 1b). Moreover, in
methanol, methyl lactate is also obtained as the side product
(Scheme 1b). Further, in other polar solvents, such as N, N-
dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), lower
fructose yields are obtained (Figure 5d). The higher yield of
fructose in ethanol is due to the solvation effect.[37] The solvation
enthalpy of fructose in ethanol is higher than the glucose and
thus favours the equilibrium towards the fructose formation.[37]
Table 1 and Figure 4 suggest that Mg-Si-MTW materials
have different amounts of acidity and basicity. Magnesium
silicates contain Lewis acid (Mg2+) and base (O-sites in Mg-O-Si
linkage) as the active sites to conduct the glucose isomerization
reaction. Literature reports suggest that MgO and
MgO(2%)@SBA-15 have only basic sites.[45,49] It is known in the
literature that the glucose isomerization takes place in the
presence of Lewis acid catalysts.[30-36] Further, the base
catalysts are also known to catalyze this reaction.[37-42] Thus, the
high activity of magnesium silicates is due to the presence of
both, basic sites, and Lewis acid sites. The higher activity of Mg-
Si-MTW-50 to that of Mg-Si-MTW-75 confirms that higher acidity
and basicity are required for this reaction. The lower activity of
Mg-Si-MTW-25 to that of Mg-Si-MTW-50 suggests that medium
strength acidity and basicity are suitable for this reaction. NH3-
TPD and CO2-TPD analyses suggest that medium strength
acidity and basicity are predominating in Mg-Si-MTW-50 than
Mg-Si-MTW-25, which are attributed to the higher activity of Mg-
Si-MTW-50 (Table S1). Furthermore, 5-hydroxymethylfurfural
(HMF) is not detected as a side-product during glucose to
fructose isomerisation, which suggests that exceptionally weak
Brönsted acidity did not promote the formation of HMF, which is
consistent with the pyridine-IR studies discussed above. The
absence of HMF as a side product supports the fact that glucose
isomerization reaction stops at fructose and no further
dehydration occurs, which is one of the reasons for the higher
fructose yield. However, glucose to fructose isomerization
reaction is a reversible reaction, which is possible to occur under
this catalytic system. A base-catalyzed (MgO or MgO@SBA-15)
reactions suggest that glucose to fructose isomerization follows
the Lobry de Bruyn-Alberda van Ekenstein mechanism which
involves the -hydrogen abstraction by the base catalyst.[37]
However, a 1,2-hydride shift mechanistic pathway is reported
with Lewis acid catalysts.[54] This evident that in the present
study, both, basic sites and Lewis acid sites are involved in the
glucose isomerisation (Scheme 1b), however, it is difficult to
accurately determine, among these sites, which is dominating
over other sites in catalyzing glucose isomerization reaction.
Scheme 1. Schematic representation of (a) catalytic active sites
in the catalyst and (b) glucose isomerization into fructose
followed by MLA formation over Mg-Si-MTW-50 catalyst.
3.2.2. Selective conversion of glucose into methyl lactate
The developed catalyst is employed in the selective conversion
of glucose to methyl lactate under a different reaction condition.
The conversion of methyl lactate takes place at much higher
temperature than glucose to fructose isomerization. The
occurrence of this reaction is observed during the solvent
studies of glucose to fructose isomerization reaction. Thus, this
However, after 2.5 h no further improvement in the glucose
conversion is obtained when compared to glucose conversion
obtained after 2 h. Thus, 2 h is optimized as the reaction time.
The influence of reaction solvent in the glucose isomerization is
investigated using Mg-Si-MTW-50 at 393 K with 50 mg catalyst
for 2 h and the obtained results in presented in Figure 5d. When
the glucose isomerization is conducted in water, a lower glucose
5
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10.1002/cctc.202000711
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reaction is investigated in methanol. The role of methanol in the
formation of methyl lactate (MLA) will be discussed later at the
end of this section. The reaction is conducted at a high
temperature in pressure controlled liquid phase reactor at the
reaction conditions shown in Figure 6 and progress of the
reaction was monitored using HPLC. A poor glucose conversion
and several degradation products are obtained along with a
small amount of the desired MLA in the absence of a catalyst or
with using SBA-15 as a catalyst. In these cases, the major
product is fructose (~20 %) derived from glucose isomerisation.
In addition, a few of pyruvaldehyde C3 adducts with methanol
and glucose degradation products such as methyl glycolate
(MG) and glycoaldehydedimethylacetal (GADMA) are observed.
It may be noted that during glucose to fructose isomerisation, no
reactivity is shown in Table 2 in the absence of a catalyst.
However, here it produced fructose and other products because
the reaction was conducted at a high temperature and 5 bar N2
pressure. Complete glucose conversion is obtained with various
Mg-Si-MTW-x but the selectivity towards MLA varies by varying
the Si/Mg ratio (Figure 6). Using Mg-Si-MTW-x, small amounts
of pyruvaldehyde-methanol adducts, MG and GADMA are
observed. Comparative study with MgO and MgO(2%)/SBA-15
shows that using Mg-Si-MTW-x, low amounts of side products
are obtained. MgO and MgO(2%)/SBA-15 favours the
isomerisation reaction and produce fructose as the major
product. Among the magnesium silicates, Mg-Si-MTW-25 with
higher amounts and strengths of Lewis acidity and basicity
(Table 1 and Table S1) exhibits the best yield of MLA (76.1 %).
Having obtained the best catalyst, Mg-Si-MTW-25, for this
reaction, various other reaction parameters are optimized.
Pressure condition favours the progress of reaction and also
assists in achieving the higher selectivity towards MLA (Figure
S4). The influence of reaction parameters, such as, temperature,
N2 pressure, time, and catalyst amount suggest that 80 mg
catalyst for 0.5 mmol of glucose, operated at 433 K and 5 bar N2
pressure exhibits the best activity after 12 h of the reaction
(Figure S4).
(GAL) and dihydroxyacetone (DHA), (3) isomerization of GAL to
DHA, (4) dehydration of DHA to pyruvaldehyde, and (5) reaction
with methanol and a 1,2-hydride shift to form MLA (Scheme 1b).
Both, Lewis acid and base catalyst, are known to catalyze the
glucose to methyl lactate transformation, however, the modes of
substrate activation and mechanism for MLA formation over
Lewis acid and base catalyst are different.[49,54] Generally, Lewis
acid sites are more selective than basic sites for carbohydrates
to alkyl lactate conversion. However, solid base catalysts are
also reported to catalyze the MLA formation.[55] The formation of
MG and GADMA takes place via retro-aldol condensation of
glucose at higher temperature and pressure employing Lewis
acidity of the catalyst. It may be noted that the glucose
isomerization to fructose requires moderate reaction
temperature, which is the reason for the absence of retro-aldol
products (MG and GADMA) during glucose isomerization
conducted at 393
K in the previous section. MgO and
MgO@SBA-15 are reported to exhibit basicity. Thus, the
catalytic results suggest that acidity is the dominating factor for
obtaining the good selectivity for MLA. In this case higher Lewis
acidity (amount and strength) and reaction temperature are two
important factors controlling the MLA selectivity. Moreover,
methanol favours the MLA formation when compared to other
alcohols. With an increase in the methanol amount (from 2 – 8
mL), the selectivity towards MLA increases, however, no further
improvement in MLA yield is observed upon using 9 mL MeOH
(Figure S4e). When other alcohols such as ethanol and 2-
propanol are used as the solvent, lower yields of 55.2 % and
36.6 %, respectively, of their corresponding lactates are
obtained over Mg-Si-MTW-25. This confirms the advantage of
using methanol solvent in this reaction. This finding is in good
agreement with the previous literature report employing Sn-Beta
as the catalyst.[15] The boiling points of methanol, ethanol, and 2-
propanol are 337.5 K, 351.5 K, and 355.5 K, whereas, their
corresponding alkyl lactates are 417 K, 427 K, and 440 K. The
lowest boiling points of methanol and methyl lactate when
compared to other alcohols and corresponding lactates are
responsible for the higher yield of MLA under the optimized
reaction condition. It may be noted that water is not chosen in
this study, because in water, various acids are known to form.
The activity of Mg-Si-MTW-25 is found to be better or
comparable to various reported catalysts for the methyl lactate
synthesis (Table S2).
Fructose
MLA
MG
GADMA
Others
Glucose
120
80
70
60
50
40
30
20
10
0
100
80
60
40
20
0
3.2.3. Reaction of furfural and 2-propanol to produce 4-(2-
furyl)-3-buten-2-one
The first two sections demonstrate the application of Mg-Si-
MTW-x in chemicals synthesis. The importance of bi-functional
Mg-Si-MTW-x catalyst is further explored in the fuel precursor
synthesis. The application of Mg-Si-MTW-x is explored in the
cross condensation reaction of biomass derived carbonyls with
in-situ generated acetone from 2-propanol, to produce liquid fuel
intermediates, which can be easily transformed into linear-chain
alkanes by hydro-deoxygenation process. In the present study,
the reaction of FAL and isopropyl alcohol (IPA) is investigated in
the detail. In this study, IPA is chosen as the reactive partner,
instead of acetone. Here, the acetone is in-situ generated from
IPA. IPA is known to produce acetone via the catalytic transfer
hydrogenation (CTH) process over a suitable acid catalyst while
producing the H2 as the source of hydrogenation during transfer
hydrogenation process.[18] Another advantage of selecting IPA
over acetone as the solvent in this study is that acetone itself
Figure 6. Catalytic activity evaluation for glucose conversion into
methyl lactate over various catalysts synthesized in this study.
[Reaction condition: glucose (90 mg), catalyst (80 mg), methanol
(8 ml), temp. (433 K), time (12 h), N2 (5 bar).]. MLA = methyl
lactate, MG
=
methyl glycolate, and GADMA
=
glycoaldehydedimethylacetal.
The transformation of glucose to MLA includes (1)
isomerization of glucose into fructose over Lewis acid and/or
base catalyst, (2) retro-aldol of fructose into glyceraldehyde
6
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10.1002/cctc.202000711
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FULL PAPER
can give condensation product (mesityl oxide) in higher yield
along with the various side products which will be discuss later.
The aldol condensation reaction is known to be catalyzed by
both, Lewis acid and base catalysts.[35,41] As evident from Py-FT-
IR and NH3/CO2-TPD, the Mg-Si-MTW-50 has optimum amount
of Lewis acid and base sites (medium strength acid and basic
sites), which is the reason for its higher catalytic activity than the
other catalysts investigated in this study. Based on the obtained
catalytic results and above discussion, a schematic reaction
pathway is proposed for the production of FAC by the reaction of
FAL and IPA (Scheme 2). In the first step of the reaction, the
dehydrogenation of IPA takes place over active sites (bi-
functional acid, base sites) on the catalyst, which produces H2
and acetone. The formed acetone reacts with FAL to give
furfural-acetone condensation intermediate (4-hydroxybutan-2-
one, FAC-OH), which upon dehydration, produces the desired
FAC product. The FAC can further reacts with another FAL
molecule to form furfural-acetone-furfural (F2AC) adduct as the
side product. The direct use of acetone generates a self-
condensation product of acetone in higher yield along with small
amounts of FOL, FAC, FALIPE, FAC-OH, and F2AC side
products, thus it is avoided. Therefore, co-generated acetone
pathway, when IPA is used as a hydrogen donor through CTH
reaction, is adopted in this study. The activity of Mg-Si-MTW-50
is found to be better or comparable to various reported catalysts
When FAL is reacted with IPA, transfer hydrogenation can
take place. In the absence of a catalyst at elevated temperature
of 443 K, very low FAL conversion is obtained in pressure
reactor at 5 bar N2 pressure (Figure 7). However, in this case
only transfer hydrogenated product furfural alcohol (FOL) and its
ether derivative (FALIPE) are obtained (Figure 7, Scheme 2).
Moreover, using SBA-15 also very low FAL conversion and FOL
and FALIPE are obtained (Figure 7). Using MgO catalyst, a
higher FAL conversion of 84.1 % is obtained with 35.9 % FAC
and 44.6 % FALIPE yields, respectively (Figure 7). Further,
using MgO(2%)@SBA-15 catalyst, a 33 % FAL conversion and
9.2 %, 9.2 %, and 14.2 % yields of FAC, FOL, and FALIPE are
obtained (Figure 7). This suggests that basic sites are not good
to obtain a higher yield of FAC over MgO. All Mg-Si-MTW-x are
found more active than other catalysts investigated in this study
(Figure 7). Among these silicates, Mg-Si-MTW-50 exhibits higher
yield of the desired FAC aldol condensation product. Having
obtained the best catalyst, Mg-Si-MTW-50, the influence of
reaction parameters is investigated. The influence of reaction
parameters, such as, temperature, N2 pressure, time, and
catalyst amount suggest that 50 mg catalyst for 1.0 mmol of FAL, for the FAC production (Table S3).
operated at 433 K temperature and 5 bar N2 pressure exhibits
the best activity after 12 h of the reaction (Figure S5).
80
70
60
50
40
30
20
10
0
90
80
70
60
50
40
30
20
10
0
FAC
FOL
FALIPE
FAC-OH
F2AC
FAL
Scheme 2. Schematic representation of Mg-Si-MTW-50
catalyzed aldolization of furfural in 2-propanol.
3.2.4. Catalyst recyclability
The catalyst stability and recyclability were carried out for Mg-Si-
MTW-50 catalyst in the isomerization of glucose and FAL
reaction with IPA. After 2 h of the isomerisation reaction of
glucose, the catalyst was separated by centrifugation, washed
several times with ethanol, dried in oven, activated in furnace
and then used in the next cycle. Figure S6a,b shows that the
catalyst exhibits no appreciable change in the activity even after
five recycles during the isomerisation of glucose and the FAL
reaction with IPA to produce FAC. Moreover, hot filtration test
was also conducted for the isomerization of glucose. After 0.5 h
of the reaction, the catalyst was removed and then the reaction
was continued for remaining 1.5 h to evaluate the progress of
the reaction (Figure S6c). HPLC analysis suggests that no
reaction take place after the removal of the catalyst confirming
that the reaction is heterogonous in nature and no active species
are leached in the solution which could catalyze the reaction
after the removal of the catalyst. MP-AES analysis confirms that
no leaching of Mg2+ species is observed during the reaction. The
recycled catalyst was also characterized by XRD, volumetric
analysis, and SEM (Figure S7). No appreciable changes in the
XRD, surface area (SBET = 265 m2/g, VT = 0.238 cc/g), and SEM
Figure 7. Catalytic activity evaluation for the reaction between
furfural and isopropanol over various catalysts synthesized in
this study. Structure of designated products are provided in
Scheme 2. [Reaction condition: furfural (1 mmol), catalyst (50
mg), isopropanol (8 ml), temperature (433 K), time (12 h), N2 (5
bar).]
Mg-Si-MTW-x has Lewis acidity as confirmed from pyridine
IR and NH3-TPD studies. Moreover, it is reported that Sn-MFI
and Sn-Beta zeolite are playing different roles in aldol
condensation of FAL with acetone and exhibited different
selectivity for the desired FAC product.[35] Our finding is
consistent with the literature report and suggests that material
with suitable acidity is good for affording higher yield of FAC. It
may be noted that Mg-Si-MTW-50 exhibits the best activity even
though the acidity of Mg-Si-MTW-50 is lower than that of Mg-Si-
MTW-25. Our findings are in agreement with the literature
reports and confirm the role of framework substituted Mg2+ in
aldol condensation. This signifies the role of catalyst with
suitable active sites for this reaction.
7
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10.1002/cctc.202000711
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FULL PAPER
transferred into a stainless steel autoclave and hydrothermally treated at
413 K for 14 days. In this synthesis procedure, the amount of water
present in the gel is a critical factor. If the excess water is present, then
Mg-Si-MTW silicate would not form and only silica would be obtained.
The obtained mixture was filtered, washed with deionized water, dried at
353 K for 12 h in an oven, and finally calcined at 823 K for 5 h to obtain
Mg-Si-MTW-x. In this study, three materials with Si/Mg (x) ratio of 25, 50
and 75 were synthesized and they are denoted as Mg-Si-MTW-25, Mg-
Si-MTW-50, and Mg-Si-MTW-75, respectively.
image of Mg-Si-MTW-50 catalyst suggest that the Mg-Si-MTW-
50 catalyst was stable during the reaction and it can be used
multiple cycles. Figure S6d shows that the Mg-Si-MTW-25 also
exhibits no considerable change in the activity even after five
recycles during the glucose to MLA transformation. Thus it can
be concluded that Mg-Si-MTW catalysts are stable and
recyclable for all the three reactions discussed in this study.
4. Conclusion
In summary, Mg framework substituted zeolite with MTW
framework structure with rice morphology and rod-like
nanoparticles was successfully synthesized using tetraethyl
ammonium hydroxide as micropore director under hydrothermal
condition. Mg incorporation has generated mild basicity and
Lewis acidity in the material. Bi-functional catalyst Mg-Si-MTW-
50 having medium strength acidity & basicity and ethanol
promoted the selective isomerisation of glucose to produce good
yield of fructose. Comparative higher (strength and amount)
acidity and basicity of Mg-Si-MTW-25 and low boiling
temperature of methanol and methyl lactate were responsible for
the production of high yield of methyl lactate from glucose.
Moreover, medium strength acidity and basicity of Mg-Si-MTW-
50 were found suitable for the furfural aldolization in the
production of fuel precursor. The catalyst exhibited efficient
recyclability and stability after the reactions. The high activity of
Mg-Si-MTW in these reactions was due to the optimum
Lewis/Brönsted acidity & basicity and Si/Mg ratio. A simple and
robust zeolite catalyst demonstrating the production of three
important classes of chemicals derived from the renewable
glucose is highly demanding not only from the academic point of
view but also from the industrial perspective, especially in bio-
refinery.
For comparative study, MgO (final calcinations temperature 973 K),[38]
SBA-15,[45] and MgO/SBA-15[45] were prepared by following the reported
procedure. The details of instruments used for the catalyst
characterization are provided in the Supporting Information Section.
Procedure of the catalytic reactions
The procedures of the catalytic reactions discussed in this study are
provided in the supporting information section.
Acknowledgments
The authors are grateful to the CSIR, New Delhi for funding
through grant number (01(2991)/19/EMR-II). RS acknowledges
the Director of IIT Ropar for providing financial assistance (9-
35/2009/IITRPR/43 dated 09/01/2018). AK thanks MHRD, New
Delhi, for fellowship. We are grateful to Centre for Nano and Soft
Matter Science (Cens), Bangalore funded by
a
TPF
Nanomission for TEM analysis. The authors are also thankful to
the Advanced Material Research Centre (AMRC), IIT Mandi for
XPS analysis.
Conflicts of interest
Experimental Section
The authors declare no competing financial interest.
Catalyst preparation
Keywords: Biomass • Bio-refinery • Glucose derived chemicals
• Isomerization • Zeolite
The details of materials used are provided in the Supporting information
section.
A hydrothermal synthesis strategy was adopted to synthesize Mg-Si-
MTW-x (MTW-type magnesium silicate) by following the molar gel
composition 40 SiO2: 0.8 MgO: 8 TEAOH: 320 H2O.[44] In a typical
procedure, a 50 mL Teflon vessel was charged with the calculated
amount of Mg(NO3)2.6H2O and deionized water. After complete
dissolution of magnesium source, TEOS was added under stirring
condition. The pH of the aqueous gel was maintained at 0.8 pH using
aqueous HCl (36 wt %). This gel was stirred for 30 min at ambient
temperature to obtain a homogeneous mixture. The Teflon vessel was
heated in a pre-heated oil bath at 363 K for 1 h. Then the temperature
was increased to 373 K and the vessel was kept at this temperature for 3
h to adjust the water to silica molar ratio. At lower temperature of 363 K,
sufficient hydrolysis and condensation of silica precursor took place and
then the temperature was ramped to 373 K to yield the complete
hydrolysis and condensation processes. The vessel was cooled to
ambient temperature and then the calculated amount of TEAOH was
dropwise added under stirring conditions. The resultant mixture was aged
for 24 h under stirring condition at ambient temperature. Then pH of the
gel was adjusted to 12.3 by using NaOH. The Teflon vessel was then
Supporting Information
Supporting information includes details of catalysts synthesis, material
characterization, XPS spectra, FT-IR spectra, optimization of reaction
parameters, XRD, SEM, catalyst recyclability, and comparative catalytic
activity data with respect to various catalysts reported in the literature.
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Glucose
Fructose
MethylLactate
Furfural-Acetone
adduct
Furfural
Insert text for Table of Contents here. ((Mg framework substituted zeolite with MTW framework having medium strength acidity & basicity and suitable
alcohol solvents promote the selective isomerization of glucose to fructose, glucose to methyl lactate, and furfural aldolization in the production of fuel precursor.))
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