Conversion of glucose into levulinic acid in continuous segmented turbulent flow with enhanced chemical reaction

Article abstract of DOI:10.1016/j.tetlet.2021.153330

Fine chemicals derived from renewable carbon feedstocks can be potentially used as various platform chemicals in the production of fuel additives, polymers, green solvents, foods, and cosmetics. Batch reactions are widely used for the conversion of lignocellulosic biomass into fine chemicals, which possess several drawbacks. Therefore, we investigated the conversion of glucose into various fine chemicals using a segmented (plug) flow reaction system with different segment lengths. The highest conversion of glucose (67.3%) and reaction yield of formic acid (75.1%), levulinic acid (68.9%), and 5-hydroxyl methylfurfural (0.49%) were obtained using reaction conditions of 1 g/L glucose solution and 0.25 M H₂SO₄ at 150 °C for a short segment reaction. Higher glucose conversion was observed with shorter segments as the result of internal mixing, which enhanced the chemical reactions.

Full text of DOI:10.1016/j.tetlet.2021.153330

Tetrahedron Letters 80 (2021) 153330
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Conversion of glucose into levulinic acid in continuous segmented
turbulent flow with enhanced chemical reaction
Kam Sheng Lau , Siew Xian Chin b, , Sharifah Nabihah Syed Jaafar , Chin Hua Chia
a
⇑
a
a,
⇑
a
Materials Science Program, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia
ASASIpintar Program, Pusat GENIUS@Pintar Negara, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Fine chemicals derived from renewable carbon feedstocks can be potentially used as various platform
chemicals in the production of fuel additives, polymers, green solvents, foods, and cosmetics. Batch reac-
tions are widely used for the conversion of lignocellulosic biomass into fine chemicals, which possess sev-
eral drawbacks. Therefore, we investigated the conversion of glucose into various fine chemicals using a
segmented (plug) flow reaction system with different segment lengths. The highest conversion of glucose
Received 30 June 2021
Revised 9 August 2021
Accepted 10 August 2021
Available online 14 August 2021
(
(
67.3%) and reaction yield of formic acid (75.1%), levulinic acid (68.9%), and 5-hydroxyl methylfurfural
0.49%) were obtained using reaction conditions of 1 g/L glucose solution and 0.25 M H SO at 150 °C
Keywords:
2
4
Fine chemicals
Flow synthesis
Levulinic acid
Segmented flow
for a short segment reaction. Higher glucose conversion was observed with shorter segments as the result
of internal mixing, which enhanced the chemical reactions.
Ó 2021 Elsevier Ltd. All rights reserved.
Introduction
reaction systems have several drawbacks, such as poor mixing,
limited reaction volume, and poor heat transfer [15–17].
To migrate towards a greener future, alternative renewable
resources are crucial to reduce the usage of fossil fuels and slow
down global warming, thus minimizing the negative impact on
the environment [1,2]. Among all renewable feedstocks, lignocellu-
losic biomass resources, which consist of lignin, hemicellulose, and
cellulose, can be converted into various valuable chemicals, includ-
ing fuel additives, biopolymers, resins, and solvents [3,4]. Levulinic
acid (LA) can be obtained by the conversion of sugar precursors
and further used as fuel additives, polymers, green solvents, foods,
and cosmetics [5–9]. Acid-catalyzed reactions by homogenous or
heterogeneous acid catalysts in the conversion of cellulosic feed-
stock increase the selectivity in yielding 5-hydroxymethylfurfural
Recently, much attention has been given to continuous flow
chemistry reactions, especially for the synthesis of various chemi-
cal compounds [18,19]. A continuous flow chemistry system with a
low Reynold number will direct the laminar flow towards one
direction, minimizing internal mixing of the reactants in the flow
system [20,21]. A segmented (plug) flow system, either gas–liquid
phase or liquid–liquid phase, results in turbulent mixing of the
reactant mixture, which allows rapid molecular diffusion and col-
lision between molecules, subsequently enhancing chemical reac-
tions [22–24]. Therefore, in the present study acid hydrolysis of
glucose into various fine chemicals were conducted in a continu-
ous flow reaction approach. The effect of the formation of segment
in the flow on the hydrolysis product yield was investigated.
(
5-HMF), which can undergo further rehydration to yield equiva-
lent moles of LA and formic acid (FA) [10,11].
Conventionally, the conversion of polysaccharides into fine
chemicals and organic acids has been conducted using batch reac-
tion systems, such as autoclaves and closed system microwave
reactors [12–14]. These synthesis methods allow water to reach a
hydrothermal state in a pressurized closed system, allowing the
reaction to occur at elevated temperatures. However, these batch
Experimental
Ò
Calcium carbonate (CaCO
acid (purity > 98%, for synthesis), and sulfuric acid (H SO ) (pu-
2 4
rity > 95–97%, EMSURE ) were purchased from Merck. 5-hydrox-
ymethylfurfural (purity > 98%, analytical grade), formic acid
3
) (purity > 99%, EMSURE ), levulinic
Ò
(
purity > 95%, reagent-grade), and D-(+)-glucose (purity > 99.5%,
T
M
T
M
Hybri-Max ) were purchased from Sigma Aldrich. Fluorinert
Electronic Liquid FC-70 was purchased from 3 M .
TM
⇑
Corresponding authors.
E-mail addresses: chinsiewxian@ukm.edu.my (S.X. Chin), chia@ukm.edu.my
Acid hydrolysis of glucose was performed using a flow reactor
setup (Uniqsis), as shown in Fig. 1, which consisted of a binary
(
C.H. Chia).
https://doi.org/10.1016/j.tetlet.2021.153330
040-4039/Ó 2021 Elsevier Ltd. All rights reserved.
0

Kam Sheng Lau, Siew Xian Chin, Sharifah Nabihah Syed Jaafar et al.
Tetrahedron Letters 80 (2021) 153330
Fig. 1. Experimental setup for the continuous hydrolysis of glucose.
pump module, heating block, perfluoroalkoxy (PFA) reaction coil
with outer diameter and inner diameter of 1.588 mm and
mole of 5 ꢀ HMF
%
Yield of 5 ꢀ HMF ¼
ꢁ 100%
ð4Þ
mole of glucose
1
.0 mm, respectively, and a back-pressure gauge. 2 g/L glucose
The effect of segment length was studied by adjusting the flow
rate of pump A and pump B to control the mixing ratio of the aque-
ous phase (which consisted of glucose and H SO ) and FC-70 at a T-
mixer. For reactions without segments, FC-70 was injected first,
followed by injection of the aqueous phase and lastly FC-70. This
procedure minimized diffusion of the reaction mixture throughout
the flow reaction process. The hydrolysates were treated as above
before being analyzed by HPLC.
solution and 0.5 M H SO solution was mixed at a ratio of 1:1
2
4
and injected into injection loop A, while the immiscible liquid,
FC-70, was injected into injection loop B to allow the formation
of segments in the reaction. The back-pressure regulator could
maintain the pressure in the flow system and prevent the solution
from boiling above 100 °C. Reaction/retention times of 50–200 min
and reaction temperatures of 110–150 °C were studied. The hydro-
lysate that was collected after the reaction was then neutralized to
2
4
The effect of the ratio of glucose and acid concentration was
studied by using 0.5 M and 0.005 M glucose solution and 0.005–
pH 5 with CaCO
3
powder. The hydrolysates were then filtered with
a 0.22- m syringe filter (Agilent) before being analyzed by a high-
performance liquid chromatography (HPLC) instrument (Agilent
Technologies 1260 Infinity), which was equipped with a Hi-Plex
l
0
2 4
.5 M H SO solution. The hydrolysates were treated as above
before being analyzed by HPLC.
H
column, ultraviolet–visible (UV) detector at wavelength
Results and discussion
2
80 nm, and reflective index (RI) detector. All calibration curves
had R2 values > 0.9980. The conversion of glucose was calculated
as follows:
We adapted a plug flow system in this study to investigate the
effect of segment length on the conversion of glucose into FA, LA,
initial mole of glucose ꢀ final mole of glucose
Conversion of glucose ¼
initial mole of glucose
ꢁ
100%
ð1Þ
The percentage yield of FA, LA, and 5-HMF was calculated as
follows:
mole of levulinic acid
%
Yield of le
v
ulinic acid ¼
ꢁ 100%
ð2Þ
ð3Þ
mole of glucose
mole of formic acid
mole of glucose
%
Yield of formic acid ¼
ꢁ 100%
Fig. 2. Formation of segments with water and FC-70 in PFA tube.
2

Kam Sheng Lau, Siew Xian Chin, Sharifah Nabihah Syed Jaafar et al.
Tetrahedron Letters 80 (2021) 153330
and 5-HMF. As shown in Fig. 2, the formation of segments was
introduced by injecting an immiscible liquid (FC-70), which as
plugs, and an aqueous solution (polar phase), known as slugs.
The slugs formed a convex shape due to the hydrophobicity
of the PFA tube, whereas the plugs formed a concave shape due
to the increased wetting effect between FC-70 and the PFA tube
surface.
The formation of long and short segments was performed by
changing the flow rate of both pumps, as illustrated in Fig. 3. After
the formation of segments in the mixer, the long segment length
and short segment length formed were approximately
1
0 ± 0.2 cm and 0.3 ± 0.1 cm, respectively.
Fig. 4 exemplifies the effect of temperature on the conversion of
glucose into FA, LA, and 5-HMF using the short segment reaction.
The conversion of glucose increased to 8.1% as the reaction temper-
ature increased from 110 °C to 150 °C. Further, the yield of FA, LA,
and 5-HMF increased significantly with increasing reaction tem-
perature, as indicated by the successive conversion of glucose into
fine chemicals under non-stoichiometric formation, according to
the proposed pathway (Fig. 5). At higher reaction temperatures,
the hydronium ions become less solvated in the water molecules
which more free hydronium ions are available to catalyze the con-
version of glucose into 5-HMF, thus increasing the yield [25]. Fur-
thermore, the conversion of glucose into LA had higher selectivity
with increasing reaction temperatures, indicating that no further
reaction occurred.
The effect of reaction time on the conversion of glucose into FA,
LA, and 5-HMF is shown in Fig. 6. With an increase in reaction time
from 50 min to 200 min, the conversion of glucose and the yield of
FA and LA increased from 10.7% and 12.6% to 75.1% and 68.9%,
respectively. However, the yield of 5-HMF decreased slightly with
increasing reaction times. This could be due to the reaction tending
to proceed to the consecutive rehydration reaction of 5-HMF to
yield higher amounts of FA and LA. In addition, the increasing reac-
tion time yielded higher amounts of FA than LA, indicating that the
degradation of LA into FA occurred with prolonged reaction times.
The effect of segment length was studied; a fitted plot of the
yield of hydrolysate against segment length is shown in Fig. 7.
With the segment length shortened by 33-fold, the yield of FA
Fig. 4. Effect of reaction temperature on the conversion of glucose into FA, LA, and
5
-HMF under reaction conditions of 1 g/L glucose solution and 0.25 M H
2 4
SO for
50 min in the short-segmented reaction.
and LA increased slightly from 31.7% and 26.5% to 34.5% and
31.3%, respectively. As shown in Fig. 3, convection occurred (ar-
rows) by the turbulent flow of molecules that was created by the
fluid. These enhancing effects of internal mixing in the small pock-
ets that formed allowed the successive collision of molecules,
therefore enhancing the reaction rate [27,28]. However, without
segments, laminar flow occurred, forcing molecules to flow along
the flow direction and interact via Brownian motion. Thus, the
yields of FA and LA were only 4.8% and 8.9%—much lower com-
pared with the segmented flow reactions.
Fig. 8 shows the effects of various glucose-to-acid ratio on the
yield of LA. The yield of LA increased by about nine times from
2.81% to 25.7% with the same glucose concentration (0.005 M)
but increasing H SO concentrations from 0.005 M to 0.5 M,
2
4
respectively. Although the reaction of 0.05 M glucose and 0.05 M
H SO was at a ratio of 1: 1, the yield of LA increased significantly
2
4
Fig. 3. Formation of different segment lengths: (A) long segments and (B) short segments.
3

Kam Sheng Lau, Siew Xian Chin, Sharifah Nabihah Syed Jaafar et al.
Tetrahedron Letters 80 (2021) 153330
Fig. 5. Proposed pathway of conversion of glucose into fine chemicals based on non-stoichiometric formation [26].
Fig. 6. Effect of reaction time on the conversion of glucose into FA, LA, and 5-HMF
under reaction conditions of 1 g/L glucose solution and 0.25 M H SO for the short
2 4
segment reaction at 150 °C.
2 4
Fig. 8. The effects of various ratio of glucose: H SO concentrations on the
conversion of glucose into LA under reaction conditions of 150 °C for 100 min in
short segmented reactions. The right corner diagram shows the formation of
2 4
humins in the reaction of 0.05 M glucose and 0.5 M H SO .
compared with lower glucose and acid concentrations. This is due
to the increasing acidity of H SO in favor of the catalytic conver-
2
4
+
sion of glucose into LA due to more H ions being present in the
reaction medium. However, the reaction of 0.5 M glucose and
0
2 4
.5 M H SO was not reported due to the chaotic overreaction that
occurred, causing random polymerization of pyran and furan
groups, which led to the formation of humins (dark precipitates)
in the flow reaction, causing clogging to the system [11,29,30].
The comparison of reaction yield of LA obtained by various source
and catalyst using flow reactor was listed in Table 1.
Conclusions
The conversion of glucose into various fine chemicals was stud-
ied using a continuous segmented flow reaction system. The high-
est conversion of glucose (67.3%) and reaction yields of FA (75.1%),
LA (68.9%), and 5-HMF (0.49%) were obtained under reaction
Fig. 7. Effect of segment length on the conversion of glucose into FA, LA, and 5-HMF
under reaction conditions of 1 g/L glucose solution and 0.25 M H SO at 150 °C for
00 min.
2
4
1
4

Kam Sheng Lau, Siew Xian Chin, Sharifah Nabihah Syed Jaafar et al.
Tetrahedron Letters 80 (2021) 153330
Table 1
Comparison reaction yield of LA via flow reactor.
Source
Catalyst
HCl
Reaction temperature, °C Residence time, min LA yield
Type
Ref.
Glucose
Sugarcane bagasse
Fructose
Eucalyptus globulus wood
Fructose
149
121
180
220
105
500
100
25
90
20
68 %
2.31%
Plug flow
Plug flow
Plug flow
[31]
[32]
[33]
[34]
H
H
H
2
3
2
SO
PO
SO
4
4
<3 mol %
408 mmol/L Cross-flow
<10%
4
Hexafluoroisopropanol
HFIP)
SO
Continuous fixed bed reactor [35]
(
Glucose
H
2
4
150
200
68.9%
Segmented flow This work
conditions of 1 g/L glucose solution and 0.25 M H
2
SO
4
for short
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The authors acknowledge the financial support provided by the
research grants GGPM-2017-089, Fundamental Research Grant
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