Conversion of saccharides in enteromorpha prolifera to furfurals in the presence of FeCl3

Article abstract of DOI:10.1016/j.mcat.2019.110729

In this work, the conversion of saccharides in Enteromorpha prolifera (E. prolifera) in the presence of a series of metal chloride was studied. FeCl₃, was found the best precursor of catalyst for obtaining furfural products, with 8.9 wt% 5-hydroxymethylfurfural (HMF), 14.9 wt% 5-methylfurfural (5-MF), and 4.2 wt% 2-formylfuran (FF) without levulinic acid (LA) formation, under the optimized reaction conditions (190 °C, 60 min and 0.0125 mol/L FeCl₃ loading). In H₂O-THF-NaCl bi-solvent system catalyzed by FeCl₃ derived species, the highest yields of furfurals (20.0 wt% HMF, 19.8 wt% 5-MF, and 5.2 wt% FF) could be obtained when 2 wt% NaCl was added. With the help of THF by in situ extraction, nearly 90 % of the furfural products was enriched in the THF phase, while H₂O had a great contribution to the dissolution and hydrolysis of polysaccharides. The catalytic activity and product selectivity of FeCl₃ for the conversion of other saccharide model compounds were also studied comparatively. In the presence of FeCl₃, the hexose model compounds were converted to LA, whereas those in E. prolifera to HMF. It was found that FeCl₃ might interact with the saccharides from E. prolifera to form new species, which could catalyze the conversion of hexose to HMF without LA generation. The new species was considered as a kind of complexes containing Fe(II), which suppressed the rehydration decomposition of HMF. This work provided an effective method for the production of value-added chemicals from E. prolifera.

Full text of DOI:10.1016/j.mcat.2019.110729

Molecular Catalysis xxx (xxxx) xxxx
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Conversion of saccharides in enteromorpha prolifera to furfurals in the
presence of FeCl3
Yaguang Chen, Yingdong Zhou, Rui Zhang, Changwei Hu*
Key Laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu, Sichuan 610064, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
In this work, the conversion of saccharides in Enteromorpha prolifera (E. prolifera) in the presence of a series of
Enteromorpha prolifera
metal chloride was studied. FeCl , was found the best precursor of catalyst for obtaining furfural products, with
3
FeCl
3
8
.9 wt% 5-hydroxymethylfurfural (HMF), 14.9 wt% 5-methylfurfural (5-MF), and 4.2 wt% 2-formylfuran (FF)
without levulinic acid (LA) formation, under the optimized reaction conditions (190 °C, 60 min and 0.0125 mol/
L FeCl loading). In H O-THF-NaCl bi-solvent system catalyzed by FeCl derived species, the highest yields of
Hydrothermal conversion
Furfurals
Selectivity
3
2
3
furfurals (20.0 wt% HMF, 19.8 wt% 5-MF, and 5.2 wt% FF) could be obtained when 2 wt% NaCl was added. With
the help of THF by in situ extraction, nearly 90 % of the furfural products was enriched in the THF phase, while
H
2
O had a great contribution to the dissolution and hydrolysis of polysaccharides. The catalytic activity and
product selectivity of FeCl for the conversion of other saccharide model compounds were also studied com-
paratively. In the presence of FeCl , the hexose model compounds were converted to LA, whereas those in E.
prolifera to HMF. It was found that FeCl might interact with the saccharides from E. prolifera to form new
3
3
3
species, which could catalyze the conversion of hexose to HMF without LA generation. The new species was
considered as a kind of complexes containing Fe(II), which suppressed the rehydration decomposition of HMF.
This work provided an effective method for the production of value-added chemicals from E. prolifera.
1. Introduction
conducted in a free-fall reactor at different temperatures ranging from
100 to 750 °C, and the higher heating value (HHV) of the bio-oils ob-
With the rapid depletion of fossil resources and the worsening of
tained was 25.3 MJ/kg, with a higher oxygen content of 30.27 wt% [9],
whereas the pyrolysis characteristics and kinetics of E. prolifera were
also studied [10,11]. Hydrothermal liquefaction did not require the
energy intensive drying steps and was an attractive approach for the
conversion of algae having high moisture content [12]. Under the
condition of 300 °C, 30 min., and 5 wt% Na CO loading, the highest
2 3
yield of bio-oil (23.0 wt%) with about 28−30 MJ/kg HHVs was ob-
tained from hydrothermal conversion of E. prolifera [13]. Relevant re-
environment, much attention has been paid to the utilization of re-
newable resources [1]. Algae as a kind of renewable feedstock, which
had the advantages of no occupation for land resources, no request for
fresh water, short growth cycle, and high energy accumulation poten-
tial compared with the terrestrial biomass, were considered to be pro-
mising alternative to replace the fossil resources [2]. Marine macro-
algae E. prolifera, one of the main algae generated from green tide, had
burst out at the Yellow Sea near Qingdao in recent years, which did
harm not only to the coastal ecological environment but also the sea-
water quality and human health [3]. E. prolifera had high utilization
value for its high contents of saccharides (near 45 % weight percent of
dry weight), which was good for the production of value-added che-
micals or bio-crude [4,5]. Therefore, finding a proper way to efficiently
use E. prolifera was significantly important for solving the problems of
both green tide pollution and resource shortages [6].
2 3
sults were found by other researchers using K CO , acids, or hetero-
geneous catalysts to enhance yields and HHVs further, and the content
of bio-crude products was also changed to some extent [14–16]. These
bio-oils still had complex components and high oxygen content, leading
to difficulties to replace fossil fuel, thus further separation and refining
of bio-oil were needed [14].
However, fewer studies had focused on the hydrothermal conver-
sion of algae to produce value-added chemicals, taking advantages of
the high oxygen content of the material itself. Hydrothermal conversion
of some terrestrial biomass or saccharide model compounds to produce
Pyrolysis and hydrothermal liquefaction were two common ther-
mochemical conversion methods applied for the conversion of E. pro-
lifera [7,8]. Fast pyrolysis of E. prolifera for bio-oil production was
−
value-added chemicals was explored [17–20]. Cl played a major role
⁎
Corresponding author.
E-mail address: changweihu@scu.edu.cn (C. Hu).
https://doi.org/10.1016/j.mcat.2019.110729
Received 13 November 2019; Received in revised form 25 November 2019; Accepted 27 November 2019
2468-8231/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Yaguang Chen, et al., Molecular Catalysis, https://doi.org/10.1016/j.mcat.2019.110729

Y. Chen, et al.
Molecular Catalysis xxx (xxxx) xxxx
in breaking the hydrogen bonds in cellulose to enhance cellulose so-
determined by using benzyl alcohol as internal standard.
lubilisation in NaCl-H
tracting solvent for isolating HMF, but also suppressed side-reactions of
HMF from rehydration, degradation, and polymerization to unwanted
2
O system [21], and THF was not only an ex-
The total amount of products in bi-solvent system was the sum of
products in two separate phases. The yields of liquid products con-
verted from saccharides were based on the weight and calculated as
follows:Yield = W(product)/W(saccharides) × 100 %
2 3
byproducts in H O-THF co-solvent system [22]. FeCl was used for the
pretreatment of lignocellulosic biomass to remove almost the whole
hemicellulose before enzymatic hydrolysis [23,24] and showed a great
effect of dissolution of xylose and xylan, as well as the transformation of
carbohydrates to furfural [25]. When the conversion of hexose [26]
2
.3.3. UV–vis
The UV–vis spectra were carried out on a Hewlett-Packard 8453
diode array spectrophotometer at room temperature using 1.00 cm
cells. The aqueous solution of each reaction sample was diluted 500-
fold with water.
(
(
glucose and fructose) or hexose containing lignocellulosic biomass
such as corn stalk [27], corncob [28] and xylose residues [29]) was
3
catalyzed by FeCl , the main product was LA. Different catalytic activity
of FeCl between the saccharides of E. prolifera and other saccharide
3
substrates needed to be further clarified. Here in, several metal chlor-
ides were adopted to form catalysts for the conversion of saccharides in
2
2
.4. Characterization of the solid samples
E. prolifera to furfurals, and the action mechanism of FeCl
3
, which had
.4.1. Biochemical composition of E. prolifera
The analysis of E. prolifera was performed based on our previous
better promotion effect for this reaction, was also studied.
work [30]. The total content of lipids was determined based on the
Bligh and Dyer method. A DNS colorimetric analysis with UV 4100-
spectrophotometer was employed to analyze the content of carbohy-
drates based on the reference [31]. The content of proteins was calcu-
lated by the content of N element multiplied by the conversion factor of
2
. Material and methods
2.1. Material
The E. prolifera was collected in Qingdao (China) in 2015, washed
by water and dried by sunshine. The sample was ground into powder
80 mesh) and dried in an oven at 100 °C for 24 h before use.
NaCl, MgCl ·6H O, AlCl ·6H O, InCl ·4H O, SbCl , BiCl
SnCl ·5H O, CrCl ·6H O, FeCl ·6H O, CuCl ·2H O, ZnCl were pur-
chased from J&K Chemicals, AR.
6.25 (National Standard in China, GB 5009.5-2010). The moisture of
raw materials was measured by a NETZSCH TG 209F1 instrument,
determined by the weight loss at 105 °C. Ash content was determined
by the weight difference of sample before and after calcination in a
muffle furnace at 800 °C for 4 h in air.
(
2
2
3
2
3
2
3
3 5
, WCl ,
4
2
3
2
3
2
2
2
2
2
2
2
.2. Methods
2
.4.2. Element analysis
For the determination of carbon, hydrogen and nitrogen contents in
.2.1. Solvothermal conversion of E. prolifera
The solvothermal conversion of E. prolifera was conducted in a
50 ml stainless-steel autoclave reactor equipped with a stirrer. The
the raw material and residue, a FLASH 1112SERIES Element Analyzer
was employed. The samples were all dried at 80 °C under vacuum
condition before test.
dried E. prolifera sample (2 g) and a certain amount of catalyst precursor
were placed in the reactor along with 100 ml solvent. Nitrogen was
purged in the reactor to replace air, and the initial pressure was
2.4.3. XRD
2.0 MPa. The stirring rate was set to be 400 r/min. The experiments
The X-Ray Diffraction (XRD) patterns were obtained in
a
were performed at designed temperature and lasted for desired time.
The counting of the reaction time was started when the temperature
reached the target temperature (t = 0), that is to say, the period of
temperature rising (40–50 min) has not been included. Once the reac-
tion was completed, the reactor was cooled down to room temperature
in cold water. The liquid products in the reactor were separated by
filtration and diluted for further analysis. The solid residue obtained
was washed three times with distilled water, and then dried in an oven
at 110 °C overnight for further characterization.
PANalytical-Empyrean diffractometer, equipped with CuK
α
(λ
=
1.5406 Å) radiation source. The data were collected over the scat-
tering angle of 2θ from 10° to 40° with a step of 0.02°/s.
2.4.4. FTIR
The FTIR spectra of solid samples were recorded by using a Nicolet
−1
6
700 FTIR spectrometer in the range of 4000–400 cm
with a re-
−1
solution of 4 cm
.
2
2
.3. Characterization of the liquid products
2.5. Extraction of polysaccharides in E. prolifera
.3.1. HPLC
The products in the aqueous phase filtrate were quantitatively
The E. prolifera was treated by hot water to extract the poly-
analyzed by HPLC (waters e2695), equipped with a Refractive Index
detector (RI-101, Shodex) and an aminex column (HPX-87H,
saccharides, and then Savage deproteinization [32] and ethanol pre-
cipitation were carried out. The mixture solution of chloroform and n-
butyl alcohol (v:v=4:1) was added into hydrothermal reaction solution
of E. prolifera by volume ratio of 1:5. After fully concussion (30 min.)
and centrifugation, the protein was separated and seated at the inter-
face between aqueous and organic phase. The aqueous phase solution
was separated to repeat the above steps until no white substance ap-
peared in the stratification place. After removing a small amount of
miscible organic solvent and concentrating the sugar solution by re-
duced pressure distillation, a large amount of anhydrous ethanol was
added into polysaccharide extract with volume ratio of 4:1. After 12 h
of rest, the precipitate was the polysaccharides of E. prolifera and was
filtered out of the solution. Then, the polysaccharides were washed by
ethanol and dried by air before test.
3
00 mm × 7.8 mm, Bio-Rad). The eluent was dilute H
2
SO
4
(5 mmol/L)
−
1
flowing at a rate of 0.60 ml·min
with the column temperature
maintained at 50 °C. The concentrations of all components were de-
termined by comparison to standard calibration curves.
2.3.2. GC
The products in the organic phase filtrate were quantitatively ana-
lyzed by GC (FuLi 9790), equipped with a HP-INNOWAX capilary
column (30 m × 0.23 mm ×0.25 μm). Nitrogen was used as carrier gas
with a flow rate of 0.8 ml/min. The temperature program was
100 °C–220 °C at 2 °C/min and hold at 220 °C for 10 min. The injector
and detector temperatures were both 240 °C. All components were
2

Y. Chen, et al.
Molecular Catalysis xxx (xxxx) xxxx
3. Results and discussion
obvious corrosion phenomenon was observed in the reaction. Thus,
FeCl
The effects of reaction temperature, reaction time, and the loading
of FeCl on the hydrothermal conversion of E. prolifera were optimized
3
was selected for further study of the conversion of E. prolifera.
3.1. Characterization of E. prolifera
3
The contents of elements in E. prolifera obtained were the followings
Table S1): carbon 32.4 %, hydrogen 5.4 %, and nitrogen 0.9 %. A small
(Fig. 2). Temperature had strong impact on the generation of furfurals.
The yield of furfurals increased rapidly when the temperature rose to
190 °C, and declined when the temperature exceeded 200 °C. It was
(
amount of lipids (1.5 %) and a lower content of proteins (5.7 %, cal-
culated by the nitrogen content correspondingly) were contained in the
raw material [33]. The most abundant component in E. prolifera was
carbohydrates (44.5 %) existed in the form of heteroglycan [34], in-
cluding 45 % rhamnose, 20 % xylose, 28 % glucose, and 7 % other
saccharides (sulfate polysaccharide and galactose). While 25.1 % of ash,
and 62 % of volatiles were also detected. These results showed that the
E. prolifera had high potential value by utilizing the high percentage of
carbohydrates.
indicated that the addition of FeCl
temperature of E. prolifera. As for FeCl
increased rapidly with the amount of FeCl
and further addition of FeCl had little effect on enhancing the yield of
3
could also decrease the conversion
loading, the yield of furfurals
added until 12.5 mmol/L,
3
3
3
furfural products. Whereas for reaction time, furfural products in-
creased at first and then decreased with the prolongation of reaction
time. The optimal reaction time for the three main kinds of furfural
products had some differences and similarity, that is, too long reaction
time would lead to the decrease of the yield of the three furfural pro-
ducts and the formation of a small amount of levulinic acid, while the
temperature for the highest yield of each furfural was different. For the
highest total yield of furfurals, the optimal reaction conditions were the
3
.2. The hydrothermal conversion of E. prolifera
3
.2.1. The hydrothermal conversion of saccharides in E. prolifera at
different temperature
3
followings, 190 °C, 12.5 mmol/L FeCl and 60 min. Under these condi-
The hydrothermal conversion of E. prolifera without catalyst was
conducted at 140 °C–240 °C, and the elemental analysis of residue was
shown in Table S2. According to the residue rate and elemental analysis
of the residue, it could be seen that the carbon-containing components
of E. prolifera were easily dissolved into water under hydrothermal
conditions. When the temperature reached 200 °C, the residue rate was
only 34.5 %, and its content of C or H was low, which meant that the
carbon-containing species were almost dissolved. The FT-IR spectra of
raw algae and residue after hydrothermal conversion were presented in
Fig. S1. The changes of the IR peak strength could also well explain the
dissolution of saccharides and protein components in E. prolifera after
hydrothermal reaction.
tions, a high yield of furfural products including 14.9 wt% 5-MF, 8.9 wt
% HMF and 4.2 wt% FF, could be obtained with very little levulic acid
(LA) generated.
3.2.3. Conversion of E. prolifera in H
2
O-THF bi-solvent system
E. prolifera was converted in different volume ratios of H
2
O-THF bi-
solvent under the optimal conditions, and the results were shown in
Table 1. The effect of mixture solvent was better than pure water or
pure THF, especially for obtaining HMF. When the solvent was com-
posed of H
was obtained. THF played an important role in in-situ extraction to
protect HMF, and H O had a great contribution to the dissolution and
2
O and THF at volume ratio of 1:1, a higher yield of furfurals
2
The yield of small molecular liquid products obtained was shown in
Fig. S2. From the result of liquid product analysis, it could be seen that
merely a small amount of rhamnose and xylose was generated at 160 °C.
When the temperature rose to 180 °C, the highest yield of mono-
saccharide (18.8 wt%) was obtained, and some xylose and rhamnose
began to be dehydrated to FF and 5-MF. When the temperature in-
creased to 200 °C, more monosaccharides were converted to furfural
products, and the highest yield of furfural products was obtained. When
the temperature exceeded 220 °C, the yield of furfurals decreased
slightly, which indicated that the products were easy to be polymerized
at higher temperature. Hence 200 °C was the appropriate temperature
for the conversion of saccharides in E. prolifera to furfurals. However
the yield of small molecular liquid products was not high. Combined
with the analysis of the residue after reaction, the results indicated the
solubility of hydrothermal conversion for polysaccharides was high, but
the ability to break the glycoside bonds to form monosaccharide was
limited.
hydrolysis of polysaccharides. [22]
The yield of furfurals could be further improved with increasing
amount of NaCl added to the bi-solvent system (Fig. 3). The highest
yields of furfurals (20.0 wt% HMF, 19.8 wt% 5-MF, and 5.2 wt% FF)
could be obtained with 2 wt% NaCl added, while the further addition of
NaCl had little effect on the furfurals generated but promoted the for-
mation of LA. According to the results obtained from both aqueous and
organic phases, nearly 90 % of the furfural products could be enriched
in the THF phase.
3.2.4. Different catalytic activity of FeCl
substrates
3
for the conversion of other
To further investigate the catalytic mechanism of the system with
FeCl added, a series of saccharide model compounds and other cellu-
3
losic biomass materials were tested under the same conditions dis-
cussed above (Table 2). The result indicated that different results were
obtained for these substrates with FeCl added, especially for hexose
3
conversion. The conversion of glucose and microcrystalline was low,
and the preferred product was LA. Similar result was gotten when
pubescens and corncob residue, which have relatively large amount of
cellulose components, were used as substrates. Fructose could be almost
completely converted to LA, and soluble starch also had an obvious
selectivity to LA. A lower yield of 5-MF was obtained from rhamnose
conversion, and a higher yield of FF from xylose than that from E.
prolifera was also obtained.
3
.2.2. The conversion of saccharides in E. prolifera in the presence of
different metal chlorides
A series of metal chlorides, including NaCl, MgCl
2
, AlCl
, CuCl
were adopted to promote the conversion of saccharides in E.
3
, InCl
3
,
SnCl
ZnCl
4
SnCl
2
, SbCl
3
, BiCl
3
, YCl
3
, CrCl
3
, WCl
5
, FeCl
3
, FeCl
2
2
and
2
prolifera (Fig. 1). It could be seen that different metal chlorides showed
different promotion effect for the conversion of saccharides. A high
yield of levulic acid was obtained by using SnCl
large number of small molecular acids were obtained by using InCl
FeCl and CuCl exhibited a higher promotion effect to generate fur-
4
or WCl
5
, whereas a
Then, the conversion of these model compounds was conducted in
the reaction solution obtained from the conversion of E. prolifera with
FeCl added (Table 3). It was interesting to find that a high yield of
3
3
.
3
2
furals among these metal chlorides. The highest yield of furfurals in
liquid products (27.0 wt%) was obtained, with the lowest yield of LA or
furfurals was obtained from the conversion of saccharide model com-
pounds, which was similar to these obtained from the conversion of E.
prolifera. Furthermore, another control experiment using 4 times
amount of hexose to that comprised in E. prolifera saccharides (to re-
duce the impact of products from E. prolifera itself) were added to E.
other small molecules promoted by FeCl
3
. A higher yield of 24.4 wt%
. However, it was
observed that Cu had corroded the reactor container wall during the
furfural products could also be obtained by CuCl
2
2
+
3
+
reaction. Although Fe also had potential corrosion effect for iron, no
prolifera, and the reaction was carried out with the addition of FeCl
3
3

Y. Chen, et al.
Molecular Catalysis xxx (xxxx) xxxx
Fig. 1. The distribution of products from hydrothermal conversion of E. prolifera promoted by different metal chlorides (10 mmol/L) at 200 °C for 60 min.
Fig. 2. The yields of products from the hydrothermal conversion of E. prolifera promoted by FeCl
3
under varying conditions. a. 2.0 g of raw material in 100 ml of
distilled water at different temperature for 60 min. with 10 mmol/L FeCl
for different reaction time.
3
added; b. at 190 °C for 60 min. with different FeCl added; c.190 °C, 10 mmol/L FeCl added
3
3
Table 1
result in a significant change in selectivity.
The effect of H
2
O/THF ratio on the conversion of E. prolifera.
Solvent
Yield (wt%)
3.2.5. Transformation of FeCl
When the FeCl solution was heated in autoclave reactor without
any substrate, a large amount of red-brown precipitate appeared, which
indicated that the FeCl solution was unstable under high temperature
and high pressure, and FeCl was easy to be hydrolyzed to precipitate.
XRD characterization of the precipitate showed that α-Fe and β-
FeOOH formed (Fig. S3). Nearly 70 % of Fe was precipitated by ICP
detection. According to reference [35], the species of Fe remaining in
the solution were a series of complicated incomplete hydrolysates. In
the experiment of the conversion of E. prolifera and carbohydrate model
3
in the Catalytic process
3
FF
5-MF
HMF
LA
H
2
H
2
H
2
H
2
O
4.0
3.7
5.2
5.2
5.1
14.3
10.0
17.4
16.8
9.1
8.5
9.2
12.8
12.8
8.9
0.0
0.6
0.4
0.9
0.0
3
O-THF(3:1, V/V)
O-THF(1:1, V/V)
O-THF(1:3, V/V)
3
2 3
O
3+
THF
3
Reaction conditions: 190 °C, 60 min., 12.5 mmol/L FeCl .
3
compounds (with enough sugar substrate) in the presence of FeCl , no
(
Table 4). HMF was the main product whereas little LA was generated
in the conversion of the mixture substrate, which reflected that the
reaction of FeCl with polysaccharides of E. prolifera could rapidly
iron-containing precipitation was found. According to ICP analysis of
the E. prolifera residue and reaction solution, the content of iron species
in the solution was about 97 %. Therefore, it could be concluded that
sugar substrate or the intermediates or products formed changed the
3
generate a kind of new catalytically active species, which had high
selectivity and activity to HMF for hexose conversion, while the rehy-
dration decomposition of HMF to generate LA was suppressed. The
result of control group, which had the same amount of hexose (the sum
of the amount of model compounds and that contained in E. prolifera)
without E. prolifera used, showed that the increment of substrate did not
3+
existence form of Fe in the reaction, which prevented the hydrolysis
of Fe3 to precipitate.
+
Dropping 1 mol/L NaOH solution into the reaction solution of sac-
charide model compounds in the presence of FeCl
3
, a large amount of
gray-green precipitate was observed and turned red-brown after adding
4

Y. Chen, et al.
Molecular Catalysis xxx (xxxx) xxxx
2 3 3
Fig. 3. The effect of different NaCl loading on the conversion of E. prolifera in H O-THF bi-solvent system (a. without FeCl , b. with 0.0124 mol/L FeCl ).
Table 2
Table 4
The hydrothermal conversion of different hexose model compounds in the re-
action solution of E. prolifera and FeCl with 4 times amount of hexose used or
The products obtained from the hydrothermal conversion of different substrates
with FeCl
3
added.
3
in the presence of FeCl
3
with 5 times amount of hexose used without E. prolifera.
Substrate
Yield(wt%)
FF
Substrate
Yield(wt%)
5-MF
HMF
LA
FA
FF
HMF
LA
Glucose
Fructose
Xylose
0.0
0.1
5.3
–
5.2
0.2
0.1
6.0
0.0
4.2
–
–
–
9.0
9.0
–
–
–
1.3
0.2
–
1.4
7.3
–
3.0
5.8
4.1
4.2
6.7
4.4
9.4
5.1
6.5
8.9
Fructosea
0.0
0.1
0.1
0.3
0.2
0.5
0.2
0.2
9.6
0.3
8.0
1.7
8.5
2.6
9.3
2.1
0.8
8.9
0.6
6.7
0.4
4.8
0.5
5.6
b
Fructose
a
Rhamnose
–
–
Glucose
b
Mixture (glu, xyl, rha)
Microcrystalline cellulose
Soluble starch
Pubescens
Corncob residue
E. prolifera
1.2
1.0
1.7
2.1
2.1
8.9
2.4
2.7
9.0
3.7
6.5
0.0
Glucose
a
b
Microcrystalline cellulose
Microcrystalline cellulose
Soluble starch
a
b
–
14.9
Soluble starch
Reaction conditions: 190 °C, 60 min.
Reaction conditions: 190 °C, 60 min., 12.5 mmol/L FeCl
3
.
a
4
times amount of hexose to that comprised in 2 g E. prolifera saccharides
was added to the reaction solution of 2 g of E. prolifera and 12.5 mmol/L FeCl
3
Table 3
in 100 ml distilled water.
b
The hydrothermal conversion of different saccharide model compounds in the
reaction solution of E. prolifera and FeCl
5 times amount of hexose to that comprised in 2 g E. prolifera saccharides to
under the same conditions.
3
.
the solution of FeCl
3
Substrate
Yield(wt%)
FF
found that more than 85 % iron species existed in the form of Fe(II) in
the reaction solution obtained from the conversion of all saccharides in
5-MF
HMF
LA
3
the presence of FeCl at 190 °C for 1 h (Table S3). Thus, the active
Fructose
0.0
4.3
0.2
0.4
4.2
–
12.2
–
–
14.9
6.9
8.0
6.7
8.7
8.9
0.5
0.6
0.2
0.3
0.0
species in reaction solution of E. prolifera was considered as a complex
of iron(II)-saccharides which was less stable than complex between Fe
(II) and 1,10-phenanthroline. Meanwhile, the pH of the reaction solu-
tion was always stable at 2.4 when E. prolifera was added as substrate,
while the pH of the reaction solution only consisted of saccharide model
compounds was usually around 1.9. Compared with 0.0125 mol/L
Mixture (glu, xyl, rha)
Microcrystalline cellulose
Soluble starch
E. proliferaa
Reaction conditions: 190 °C, 60 min.
a
The control group was the reaction solution of E. prolifera and FeCl
3
, which
3 2
FeCl solution (pH = 2.4) and 0.0125 mol /L FeCl solution (pH = 3.9),
has been deducted in other experimental groups.
3+
2+
it could be concluded that Fe was converted to Fe and the acidity
of solution increased by the reaction of saccharides. As the active spe-
cies, Fe2+ and H not only catalyzed glucose isomerization and de-
hydration to HMF, but also led HMF to be rehydralized to a large
amount of LA.
. The results indicated that a large number of Fe3+ were converted
2 2
H O
+
2
+
to Fe
in the conversion process of carbohydrate model compounds,
and the Fe(II) species existed as ions or unstable complexes which could
easily combine with OH to precipitate when the pH rose. However,
precipitate could hardly be generated in reaction solution using E.
prolifera as substrate when NaOH solution dripped in. The results
showed that the iron species in the reaction solution from E. prolifera
−
In order to confirm the assumption of the catalytic activity after the
transition of Fe(III)to Fe(II) further, 0.0125 mol/L FeCl
hydrochloric acid was used to adjust the pH of reaction solution to 1.8
Table 5). The results indicated that a large amount of LA and a small
2
was used and
(
might be a kind of complex which was more stable than Fe(OH)
OH)
via UV–vis spectrophotometry to quantitatively analyze the Fe(II)
species and the total iron content by using 1,10-phenanthroline, it was
2
or Fe
amount of HMF were obtained from the conversion of glucose or
fructose, which were similar to that using these substrates in the pre-
sence of FeCl . However, when the pH of solution was adjust to 2.4, the
3
(
3
.
5

Y. Chen, et al.
Molecular Catalysis xxx (xxxx) xxxx
Table 5
The hydrothermal conversion of different substrates catalyzed by FeCl2.
Substrate
pH
Yield (wt%)
FF
5-MF
HMF
LA
Fructose
Fructose
2.4
1.8
2.4
1.8
2.4
1.8
2.4
1.8
2.4
1.8
2.4
1.8
0.3
0.2
5.5
5.5
0.0
0.0
0.1
0.0
1.6
3.5
1.7
3.0
0
0
8.6
6.9
–
–
–
–
5.7
8.0
4.0
4.9
2.8
0.1
1.5
0.5
0.3
0.2
1.5
0.5
4.5
4.9
2.4
3.6
2.3
9.5
2.7
4.5
0.0
0.5
0.6
1.7
0.3
0.9
0.4
0.7
Mixture(glu,xyl,rha)
Mixture(glu,xyl,rha)
Microcrystalline cellulose
Microcrystalline cellulose
Soluble starch
Soluble starch
E. prolifera
E. prolifera
E. Prolifera + Mixture (glu, xyl, rha)
E. Prolifera + Mixture (glu, xyl, rha)
Reaction conditions: 190 °C, 60 min., 12.5 mmol/L FeCl
added to adjust the pH.
2
and hydrochloric acid
Fig. 4. The variation of UV–vis spectra for FeCl3, the solution of FeCl
polysaccharide of E. prolifera reacted for different time.
3
and
2
catalytic activity of FeCl for model compounds were not the same as
1
90 °C for 10 min. The absorption peaks of new species appeared at low
wavelengths over a period of time, which might be attributed to n→σ*
transition of the complex. The characteristic absorption of FeCl also
disappeared in reaction of glucose, but it was just similar to the mixture
solution of FeCl and glucose without new absorption peaks appeared.
the reaction solution of E. prolifera with pH of 2.4. HMF and LA were
both generated without obvious selectivity in liquid products. Thus, the
acidity of solution had some effect on the distribution of products, but
was not the main cause of different catalytic activity for E. prolifera and
model compounds.
3
2
It was also found that the iron species in iron-polysaccharide com-
plex was reduced to Fe(II) when it reacted with saccharide substrate in
airtight autoclave at high temperature, but oxidized to Fe (III) when it
was exposed to air. The result of XRD indicated that both crude poly-
saccharides of E. prolifera and relevant iron-polysaccharides were in
amorphous states (Fig. S3), and were different from the red-brown
precipitates formed via the hydrolysis of FeCl . ICP detection reflected
3
that iron-polysaccharides contained about 12 wt% Fe and the SEM re-
sult showed 16 wt% Fe was on the surface layer (Fig. S4).
It was also found that the yield of furfurals from E. prolifera in the
presence of FeCl
the presence of FeCl
the interaction of Fe(II) and E. prolifera. The ability of FeCl
coside bond destruction in polysaccharides was very weak, even if
hydrochloric acid was added. Therefore, FeCl played a certain role in
the hydrolysis and fragmentation of polysaccharides, which could not
be accomplished by the combination of FeCl and hydrochloric acid.
2
and hydrochloric acid was lower than that obtained in
, and the inhibition of LA was obvious because of
for gly-
3
2
3
2
From the infrared results, it could be seen that the coordination of
Fe (III) had obvious effect on the polysaccharide functional groups from
E. prolifera, which showed that the characteristic absorption peaks of
3
3
.2.6. The coordination effect of FeCl and polysaccharides of E. prolifera
3
+
Previous studies indicated that Fe
was easy to form stable com-
−
1
−1
the species at 660 cm
and 606 cm
appeared, respectively.
plexes with some biopolymeric polysaccharides, which had a wide
range of applications in biomedicine in treating iron deficiency anemia
According to the description in the reference, it might be β-FeOOH
−1
[
38]. The absorption peak near 1262 cm
ascribed to the stretching
[
36]. However, there was no report on the specific catalytic activity of
and vibrational peak of S]O in polysaccharide sulfate disappeared
obviously in the complex. It was indicated that the coordination of iron
also had a great effect on the structure of polysaccharide sulfate. The
these iron-containing complexes in thermochemical conversion.
Therefore, it could be suspected that the polysaccharides from E. pro-
3
+
lifera formed complex with Fe , which might be the reaction inter-
mediate and showed the catalytic activity after reduction, which cata-
lyzed the conversion of saccharides into furfural products selectively
without LA generation.
In order to prove the above conjecture, the crude polysaccharides of
E. prolifera extracted by hydrothermal method without any catalyst at
different temperature (100 °C∼180 °C) were prepared. The coordina-
−1
peak at 3447 cm , the stretching and vibration peaks of OeH in the
sugars was observed red shifted. Meanwhile, the CeO stretching vi-
−
1
bration peaks of sugars located at 1000∼1100 cm
also changed
greatly, which indicated that the coordination mainly occurred on the
hydroxyl of saccharides, which was consistent with the report in the
reference [39].
3
tion effect of FeCl with polysaccharide was studied. The results showed
that the crude polysaccharide extract from E. prolifera under 140 °C was
4. Conclusions
in colloidal state with almost no monosaccharide. The colloid was easy
to interact with FeCl
3
, and the complex of iron-saccharides was also in
3
In this study, the presence of FeCl was found to promote the con-
colloid state, which had obvious Tyndall effect. The complex was also
stable, and precipitation did not occur when NaOH solution was added
in, which was consistent with the results in reference [37]. Thus, this
complex might be generated in the reaction of E. prolifera in the pre-
version of the saccharides in E. prolifera, one of the main algae gener-
ated from green tide selectively to furfurals. Under the optimal reaction
conditions, 190 °C, 60 min. and 0.0125 mol/L FeCl loading in H O-THF
3 2
bi-solvent system with 2 % NaCl added, high yields of furfurals (20.0 wt
% HMF, 19.8 wt% 5-MF, and 5.2 wt% FF) could be obtained, and near
90 % of furfurals were enriched in THF phase. THF played an important
2
role in the in-situ extraction to protect furfurals and H O had a great
contribution to the dissolution and hydrolysis of polysaccharides, which
enhanced the yield of furfurals especially for HMF.
The specific complex of Fe species formed in situ with poly-
saccharides from E. prolifera acted as the active species to catalyze the
conversion of hexose selectively to HMF with little LA generated. The
above results might enable better design for the preparation of furfurals
3
sence of FeCl .
At the same time, other saccharide model compounds were also
used to prepare the iron-polysaccharides complex with the same
method. The results showed that except soluble starch could coordinate
to a small amount of Fe (III), the coordination ability of glucose, fruc-
tose, xylose, rhamnose and microcrystalline cellulose were weak. The
UV–vis detection results of E. prolifera polysaccharide and FeCl
shown in Fig. 4. The characteristic absorption of FeCl solution at
00 nm disappeared with the addition of E. prolifera polysaccharide at
3
were
3
3
6

Y. Chen, et al.
Molecular Catalysis xxx (xxxx) xxxx
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analysis, Writing - review & editing, Resources. Rui Zhang: Resources,
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the Fundamental Research Funds for the Central Universities. The
characterizations of catalysts and liquid products from the Analytical
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