Sustainable pathway to furanics from biomass: Via heterogeneous organo-catalysis

Article abstract of DOI:10.1039/c6gc02551j

An organic sulfonated graphitic carbon nitride is synthesized and its application has been demonstrated in the conversion of carbohydrates into furanics and related value-added products. The most important feature of the material is the stability and acidity, which could be utilized at elevated temperatures for cleaving carbohydrates and converting them into biologically important scaffolds and platform chemicals.

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Sustainable pathway to furanics from biomass via heterogeneous
organo-catalysis
Received 00th January 20xx,
Accepted 00th January 20xx
a†
a†
b
c
Sanny Verma , R. B. Nasir Baig , Mallikarjuna N. Nadagouda , Christophe Len , Rajender S.
a*
Varma
DOI: 10.1039/x0xx00000x
An organic sulfonated graphitic carbon nitride is synthesized and its application has been demonstrated in the conversion
of carbohydrates into furanics and related value added products. The most important feature of the material is the
stability and acidity which could be utilized at elevated temperatures for cleaving carbohydrates and converting them into
biologically important scaffolds and platform chemicals.
www.rsc.org/
into 5-hydroxymethylfural (HMF), levulinic acid, etc.
Introduction
notwithstanding some limitations in terms of product yields
7
1
and experimental conditons. Researchers are incessantly
investigating pathways to achieve higher yields by
The threat of climate change is at the doorstep of our planet
and its symptoms are increasingly visible every passing year in
terms of unpredicted weather conditions and patterns.
Although technological advancements are catering to the need
of increased energy demand, its environmental consequences
are rampant. In the very near future, we have to substitute
the petroleum-based feedstocks with bio-derived secondary
sources to minimize the ongoing environmental changes and
reconnoitring
a
wide range of homogenous and
8
heterogeneous catalysts; using ionic liquids and biphasic
9
1
0
8
systems. Heterogeneous and homogeneous catalysis have
shown limited success in terms of productivity, yield and cost-
efficiency. The quest for identification of highly acidic and
relatively benign material which could accomplish the
conversion of carbohydrates to furanics efficiently with
2
its effect on living species. The scientific community has
1
1
recyclability attributes has been a challenge. Heterogeneous
organo catalysis is probably an ideal choice as it does not
started seeking sustainable solutions for the conversion of
biomass to value added platform chemicals; which can be used
1
involve the use of any metals. Cao et al. reported the use of
2
3
as fuels substitutes and feedstocks for industrial applications.
1
3
ammonium resin to convert glucose in to HMF. However,
this reaction required high temperature (140 °C) along with
aqueous-organic solvent (70% DMSO in water) with a
maximum yield of 56%. Bhaumik et al. used sulfonated
Plants are the bio-renewable source of energy which
ultimately can generate most of the chemicals while
maintaining an ecological balance. The popular and general
consensus is that the carbohydrates obtained from plants
1
4
triphenylaniline polymer as acidic organo-catalyst wherein
the catalyst preparation required toxic triphenylaniline,
polymerization under anhydrous conditions followed by a
sulfonation step. Further, the high temperature and
requirement of anhydrous DMSO has limited the wider scope
of this reaction. In continuation of our focus on the
could be
a benign substitutes which can meet the
requirements of human beings without compromising the
4
environment.
Glucose and fructose are among the most abundant plant
5
derived carbohydrates. They have been converted into
building blocks for the synthesis of value added chemicals,
used in drug discovery and in the synthesis of polymers,
6
among other industrial applications. In general, these
1
5
development of sustainable methods, herein, we report the
heterogeneous organo acid catalyst derived from the electron
carbohydrates are heated with minerals acids to convert them
Scheme 1. Synthesis of sulfonated graphitic carbon nitride (Sg-CN)
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rich graphitic carbon nitride for the conversion of B.E.T. surface analysis of sulfonated graphitic carbon nitride
2
carbohydrates into value added platform chemicals (Scheme (10.04 m²/g, ESI) and graphitic carbon nitride (35.42 m /g, ESI).
1).
3
After the immobilization -SO H ion goes and sit inside the
pores via ionic interaction with nitrogen lone pair. A
comparison of adsorption and desorption curves for g-CN (ESI,
S7 and S8) and Sg-CN (ESI, S5 and S6) clearly indicates the
Synthesis and characterization
Invariably, carbohydrates can be converted into furanics under
3
SO H ions are sitting in the pores and binding with the
1
6
acidic conditions. The scarcity of an active heterogeneous
acidic organic framework which is obtainable from a readily
available starting material enthused us to design and develop
an inexpensive solid acid organo-catalyst. Accordingly, we
designed and synthesized sulfonated graphitic carbon nitride
nitrogen in Sg-CN. Due to this, the nature of g-CN changes
from amorphous to crystalline in Sg-CN which results in better
packing and consequently decrease in the total surface area
although pore distribution remains uneven.
3
The immobilization of -SO H is confirmed by the presence of
(
Scheme 1). Synthesis of the catalyst involves calcination of
sulfur in energy-dispersive X-ray spectroscopy (EDX) (ESI, S9)
and further supported by the X-ray photoelectron
1
7
urea at 500 °C for 2 hours followed by stirring with
chlorosulfonic acid leading to the formation of sulfonated
graphitic carbon nitride (Sg-CN).
The synthesized catalyst was characterized using scanning
electron microscopy (SEM), transmission electron microscopy
(
TEM), X-ray diffraction (XRD), Fourier transform infrared
1
3
spectroscopy (FTIR) and solid state CNMR. The SEM (Figure
), TEM (Figure 1) and XRD (Figure 2) of the graphitic carbon
1
nitride (g-CN) and sulfonated graphitic carbon nitride (Sg-CN)
Figure 2. XRD spectra of g-CN and Sg-CN
Figure 1. SEM image of a) g-CN and b) Sg-CN; TEM image of c) g-CN and d) Sg-CN
show a visible difference in morphology and crystalline nature.
3
The SO H immobilized Sg-CN becomes crystalline whereas
before immobilization g-CN was amorphous in nature.
1
3
The FTIR (Figure 3), and solid state CNMR (Figure 4) confirms
the functionalization of graphitic carbon nitride surface. The
Figure 3. FTIR spectra of g-CN and Sg-CN
-1
presence of strong absorption between 1190- 1170 cm and
-
1
1
020-1040 cm confirms the presence of -SO
3
H moiety in the
Sg-CN. The different pattern is due to a change in the
1
3
electronic environment after the sulfonation. For the CNMR
spectra of graphitic carbon nitride and sulfonated graphitic
carbon nitride, the peak at 140 ppm is the characteristic signal
for sulfonated graphitic carbon nitride (Sg-CN) which is
affirmed by FTIR of g-CN and Sg-CN. The stability of Sg-CN was
examined using thermo gravimetric analysis (TGA). The TGA
analysis curve of sulfonated graphitic carbon nitride (Sg-CN)
~
confirmed that the catalyst is stable up to 250 °C as negligible
weight loss (< 3 %) was observed in the TGA (Figure 5). The
immobilization of sulfonic group was further confirmed by
13
Figure 4. Solid state CNMR of g-CN and Sg-CN
2
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After synthesizing the sulfonated graphitic carbon nitride (Sg-
CN), it was imperative to find ideal conditions for the
conversion of carbohydrates into furanics. Accordingly, xylose
was treated with Sg-CN in DMSO under ambient conditions
and reaction was monitored at regular time intervals. DMSO
was chosen as a solvent as it was the most effective media
1
4
known for the reaction.
~
Apparently, 3% formation of
furfural was isolated after 24 hours stirring at room
temperature. Raising the reaction temperature to 50 °C to
accelerate the rate of reaction was resulted in a 60% yield of
furfural after 8 hours. Increasing the reaction temperature to
1
00 °C, afforded a quantitative yield of desired aldehyde
within 25 min. After finding the ideal conditions, we screened
low boiling solvents to accomplish the transformation.
Consequently, the reaction was run in dimethyl formamide
(DMF), dioxane, acetonitrile, ethanol, methanol and water. We
observed selective conversion of xylose into furfural in pure
water, which was not demonstrated earlier. Other solvents
were not as effective as water and DMSO (Table 1). The
appropriate weight percentage of the catalyst requirement
was evaluated to accomplish the speedy conversion of xylose
to furfural. With this goal in mind reactions were performed
using 50 mg, 25 mg and 12.5 mg of the catalyst for 2.0 mmol of
the substrate. The reaction with 50 mg and 25 mg of the Sg-
CN were complete within 30 min, whereas, the reaction with
12.5 mg of Sg-CN required longer reaction time of 4 hours.
Finally, studies were conducted with 25 mg of the catalyst to
maintain the expeditious conversion of carbohydrates to the
corresponding furan derivatives.
Figure 5. TGA analysis of Sg-CN
6
5
4
3
2
1
0
0
0
0
0
0
C 1s/5
FWHM L.Sh.
O 1s/6
1
6
Name
Pos.
Area
%Area
Name
P
o
s
.
FWHM L.Sh.
Area
%
A
r
e
a
C-C, C-H
284.7590 1.3473 GL(30) 6687.46 51.97
286.1214 1.5382 GL(30) 1581.72 12.29
O
O
1s
1s
531.0801 1.3731 GL(30) 19359.57 76.87
C-N
+
C-O
532.4866 1.8000 GL(30)
5827.49 23.13
N-C-N ou C=O 288.3260 1.3111 GL(30) 4598.31 35.73
14
1
1
2
0
8
6
4
2
After optimizing the reaction conditions, we embarked on the
applications of sulfonated graphitic carbon nitride (Sg-CN). In
order to make broader use of this methodology, we added 1,
2-phenylenediamine to the reaction mixture which directly
gave the pharmaceutically important benzimidazole derivative
in 84% yield (Scheme 2).
3
00
296
292
288
284
280
540
536
532
528
524
Binding Energy (eV)
Binding Energy (eV)
N 1s/7
S 2p/8
3
0
Name
Pos.
FWHM L.Sh.
Area
%Area
5
4
4
3
3
2
2
1
1
0
5
0
5
0
5
0
5
0
N
N
sp2 398.6988 1.3734 GL(30) 1618.44 19.54
sp3 400.0701 1.3734 GL(30) 5462.57 65.91
NH3+ 401.2949 1.3734 GL(30) 1206.16 14.55
25
2
0
2
H N
O
O
15
HO
HO
O
H
2
N
N
OH
OH
Xylose
N
1
0
5
Sg-CN
Water, 100 oC, 30 min
Benzimidazole derivatives
4%
8
Scheme 2. Conversion of xylose to benzimidazoles
4
08
404
400
396
392
171
168
165
162
Binding Energy (eV)
Binding Energy (eV)
Done at IC2MP
Table 1. Reaction optimization for the conversion to furfurl from xylosea
Figure 6. XPS analysis of Sg-CN
Sg-CN
Xylose
O
spectroscopy (XPS) of Sg-CN (Figure 6). The exact
concentration of sulfur was determined using elemental
analysis and it was found to be 5.47 mmol/g. The higher acidity
strength of Sg-CN catalyst was anticipated due to positive
charge developed on the nitrogenous framework after the -
Solvent,100 C
o
O
Furfural
o
Yieldb
Entry
Solvent
Temperture ( C)
Time
1
2
3
4
5
6
DMSO
DMSO
25
50
24 h
8 h
< 3%
60%
95%
30%
10%
trace
3
SO H immobilization.
DMSO
100
100
100
82
25 min
1 h
DMF
Dioxane
Acetonitrile
1 h
1 h
Result and discussion
7
8
9
Ethanol
Methanol
Water
78
65
1 h
1 h
9%
10%
95%
100
30 min
1
0c
Water
Water
100
30 min
96%
11d
100
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J. Name., 2013, 00, 1-3 | 3
4 h 86%
Reaction conditions: a) Xylose (2 mmol), Sg-CN (50 mg); b) Isolated
yield; c) 25 mg of Sg-CN used; d) 12.5 mg of Sg-CN used
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2
0
intermediate
I
and dehydration of fructose to HMF via enol
1
2
The success of Sg-CN catalyzed conversions of xylose into value tautomerization intermediate.
added platform chemicals motivated us to expand further the
prospects and applications of this catalyst. Consequently, we
O
S
O
H O
treated fructose with sulfonated graphitic carbon nitride under
similar condition which led to the formation of corresponding
O
OH
O
-H O
2
O
O
S
O
O
O
O
Sg-CN
2
HO
OH H O
HO
OH
-H O
HO
5-hydroxymethylfurfural (HMF) in very good yield (Scheme 3).
OH
D-Xylopyranose
Sg-CN
I
-H
2
Utilizing a one pot protocol, KBr, Sg-CN catalyst and fructose
afford a 94% yield of 2, 5-diformylfuran (DFF) via in-situ
oxidation HMF (Scheme 4). KBr serves as an internal oxidant
which oxidizes HMF in-situ leading to the selective formation
of DFF. Replacement of KBr oxidant with DMSO, which may
O
O
Furfural
Scheme 5. Xylose dehydration mechanism via cyclic intermediates
O
O
1
3
H
O
S
O
O
H
also serve as an oxidant,
resulted in no desired product.
OH
OH
OH
O
H O
2
-H O
HO
H
O
HO
H
HO
Interestingly, the use of KBr in conjunction with DMSO
afforded predominantly 5-methylsulfanylmethyl-furan-2-
carbaldehyde with a minor amount of DFF (95:5 ratio; Scheme
HO
OH
HO
OH
HO
Sg-CN
Enol tautomerization
Fructose
-H
O
2
O
3
), the thiolated compound being the major product. In most
H O
S
O
O
H
O
O
HO
HO
O
O
HO
of the documented results under similar conditions, DFF was
the sole product described with occasional traces of 5-
O
O
HO
Sg-CN
O
S
O
2
H O
H O
O H
Dehydration/
Deprotonation
1
3, 18
HMF
Sg-CN
methylsulfanylmethyl-furan-2-carbaldehyde.
However,
here the reaction of fructose with Sg-CN afforded the selective
Scheme 6. Fructose dehydration mechanism via enol tautomerization intermediate
formation of 5-methylsulfanylmethyl-furan-2-carbaldehyde It was imperative to evaluate the stability of the catalyst in
with insertion of methylsulfide (CH S-) in very high yield recycling experiments using fructose as the model substrate.
3
(Scheme 3). This unprecedented formation of 5- The reaction of fructose in water at 100 °C gave the formation
methylsulfanylmethyl-furan-2-carbaldehyde is an important of HMF. After the reaction the catalyst was separated by
finding missing in earlier explorations. Interestingly, heating filtration/centrifugation, washed with acetone and reused for
the aqueous solution of fructose/glucose with Sg-CN in sealed the next cycle with the fresh reactants. The catalyst was found
tube over a longer period of time (8h) at 150 °C provided to be active even after three cycles clearly indicating that it
levulinic acid (Scheme 4). Levulinic acid is an important does not lose its activity during the course of reaction (ESI, S1).
intermediate use in the synthesis of basic building blocks such The SEM, TEM and XRD of the recycled catalyst showed that it
1
9
as γ-valerolactone and many natural products.
retained its morphology and crystalline nature (ESI) which is in
corroboration with the outcome of the recycling experiments.
Sg-CN, 30 min
O
OH OH
Sg-CN, 30 min
OH
OHC
,5-Diformylfuran
4%
CHO
OHC
O
Water, KBr, 100 o
O
Conclusions
C
HO
Water, 100
o
C
2
HO
OH
Fructose
5-(Hydroxymethyl )furfural
96%
9
A heterogenized sulfonated graphitic carbon nitride (Sg-CN)
has been synthesized which showed its ability as a highly acidic
solid organo-catalyst. Several applications have been
demonstrated in the conversion of carbohydrates to value
added furan derivatives. The generated furanics could be used
as a starting material in the synthesis of natural products and
polymers. We have also shown that the furan derivatives could
be used in-situ for the synthesis of biological important
heterocycles in cascade reactions. The most important feature
of the catalyst is its accessibility from benign material,
recyclability and stability.
Sg-CN, 30 min
DMSO, KBr, 100 oC
S
OHC
CHO
+
OHC
O
O
2,5-Diformylfuran
5-Methylsulfanylmethyl-furan-2-carbaldehyde
7%
9
3
%
Scheme 3. Sg-CN catalyzed conversion of fructose to HMF, DFF and thiolated furfural derivatives
O
HO
OH OH
Sg-CN, Water
OH
O
O
o
Sealed tube,150 C, 8h
HO
OH
levulinic acid
Acknowledgements
Glucose
4
1%
S. V. and N. B. R. B. were supported by the Postgraduate
Research Program at the National Risk Management Research
Laboratory administered by the Oak Ridge Institute for Science
and Education through an interagency agreement between the
U.S. Department of Energy and the U. S. Environmental
Protection Agency.
Scheme 4. Sg-CN catalyzed conversion of glucose to levulinic acid
The plausible mechanisms for the dehydration of xylose and
fructose have been proposed in scheme 5 and scheme 6
respectively. The dehydration of xylose to furfural takes place
via formation of 2, 5-anhydroxylose furanose cyclic
4
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and R. S. Varma, Green Chem., 2016, 18, 251-254; e) R. B.
Nasir Baig, M. N. Nadagouda and R. S. Varma, Green Chem.,
2014, 16, 4333-4338.
Disclaimer
The views expressed in this article are those of the authors and
do not necessarily represent the views or policies of the U.S.
Environmental Protection Agency. Any mention of trade
names or commercial products does not constitute
endorsement or recommendation for use.
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Green Chemistry
Page 6 of 6
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DOI: 10.1039/C6GC02551J
Sustainable pathway to furanics from biomass via heterogeneous
organo-catalysis
a
a
b
c
Sanny Verma †, R. B. Nasir Baig †, Mallikarjuna N. Nadagouda , Christophe Len and Rajender
a
S. Varma *
a
Sustainable Technology Division, National Risk Management Research Laboratory, U. S.
Environmental Protection Agency, MS 443, Cincinnati, Ohio 45268, USA. Fax: 513- 569-7677;
Tel: 513-487-2701. E-mail: varma.rajender@epa.gov
b
WQMB,WSWRD, National Risk Management Research Laboratory, U. S. Environmental
Protection Agency, MS 443, Cincinnati, Ohio 45268, USA
c
Sorbonne Universités, Université de Technologie de Compiègne, Compiègne, France
†
Equal contribution
Organic sulfonated graphitic carbon nitride (Sg-CN) is synthesized and deployed in the
conversion of carbohydrates to furanics; enhanced acidity and stability of Sg-CN is exploited for
cleaving and converting carbohydrates to biologically important scaffolds and platform
chemicals.