Hydrolysis behaviors of sugarcane bagasse pith in subcritical carbon dioxide-water

Article abstract of DOI:10.1039/c6ra18436g

The aim of this study was to describe the hydrolysis behavior of sugarcane bagasse pith (SCBP) in subcritical CO₂-water. The hydrolysis was carried out in a batch reactor using different temperatures (160 to 260 °C), liquid to solid ratios (20:1 to 100:1), CO₂ pressures (0 to 7.3 MPa), stirring speeds (0 to 500 rpm) and reaction times (0 to 40 min). The highest total reducing sugar yield (43.6%) was obtained at 200 °C, liquid to solid ratio 30:1, 2 MPa CO₂, 500 rpm and 50 min. Two-dimensional heteronuclear single quantum coherence (2D HSQC) nuclear magnetic resonance (NMR), scanning electron microscopy (SEM) and Fourier transform infrared spectrometry (FT-IR) were used to help elucidate the physical and chemical characteristics of the raw material and residual solid particles, with results consistent with the removal of hemicellulose during hydrolysis. The changes in the concentration of products with time were analyzed to understand product distribution through high-performance liquid chromatography (HPLC) and to infer the reaction mechanism.

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DOI: 10.1039/C6RA18436G
ARTICLE
Hydrolysis behaviors of sugarcane bagasse pith in subcritical
carbon dioxide-water
Jiezhen Liangab, Xiaopeng Chen*ab, Linlin Wangab, Xiaojie Weiab, Feifei Qiua, and Chaochao Lua
The aim of this study was to describe the hydrolysis behaviors of sugarcane bagasse pith (SCBP) in subcritical CO2-water.
The hydrolysis was carried out in a batch reactor using different temperature (160 to 260°C ), liquid to solid ratio (20:1 to
100:1), CO2 pressure (0 to 7.3 MPa), stirring speed (0 to 500 r·min−1) and reaction time (0 to 40 min). The highest total
reducing sugar yield (43.6%) was obtained at 200 °C , liquid to solid ratio 30:1, 2 MPa CO2, 500 r·min−1 and 50 min. Two-
dimensional heteronuclear single quantum coherence (2D HSQC) nuclear magnetic resonance (NMR), scanning electron
microscope (SEM) and a Fourier transform infrared spectrometry (FT-IR) were used to help elucidate the physical and
chemical characteristics of the raw material and residual solid particles, with results consistent with the removal of
hemicellulose during hydrolysis. The changes in the concentration of products with time were analyzed to understand
product distribution through high-performance liquid chromatography (HPLC) and to infer the reaction mechanism.
sugar factories and comminuted during the sugar production process.
The bioconversion of lignocellulose to fuel and added-value
chemicals requires an efficient hydrolysis process to produce
1 INTRODUCTION
Lignocellulose biomass is a complex polymer composed of three
main fractions: cellulose, hemicellulose and lignin1. Lignocellulose
biomass, an agriculture waste, is considered as a potential feedstock
for producing fuels, cellulosic pulp, biomaterials and a variety of
chemicals because it is available in large quantities at low cost. One
major lignocellulosic wastes product is sugarcane bagasse.
fermentable sugars (reducing sugars) which should be free of
inhibitors to provide the efficient fermentation5. Pretreated
lignocellulosic materials can be hydrolysed using acids or enzymes.
However, using concentrated acids, such as H2SO4 and HCl6,
requires corrosion-resistant reactors. These acids are also toxic,
corrosive and hazardous in nature. The major bottleneck to make
enzymatic hydrolysis economically feasible is the long process time
Sugarcane bagasse, the fibrous residue remaining after
extracting the juice from sugarcane during the sugar production
process, is mainly used in papermaking. About 40% of this residue is
a smaller fibre, sugarcane bagasse pith (SCBP), which is removed
during the process of producing pulp for papermaking to reduce
paper brittleness. According to the Food and Agriculture
Organization of the United Nations, about 523 million tons of
bagasse was produced in 2014 throughout the world, given that one
ton of sugarcane produces 280 kg of bagasse2. Only 40% of the
bagasse is used in the paper industry, so the rest (including bagasse
pith) is burnt to produce steam and electricity4. Therefore, because of
the importance of SCBP as an industrial waste, there is great interest
in developing methods for the biological production of fuel and
chemicals that offer economic, environmental, and strategic
advantages. Compared with other agro-based residues such as rice
straw, paddy straw, and wheat straw, SCBP also offers many
advantages: it is available in relatively large quantities, centralised at
7
and the high cost of producing enzyme . Another optional process,
subcritical water treatment, has been developed to be highly
environmentally friendly. Some chemical reactions can occur in
supercritical and subcritical water without causing environment
pollution.
Subcritical water has been extensively investigated for the
hydrolysis and conversion of lignocellulosics to produce useful
compounds8-12. Subcritical water can dissolve hemicellulose
completely and partially remove lignin at high temperatures,
such as 220 °C , within 2 min without the use of chemicals13.
During subcritical water hydrolysis, more than 80% of
lignocellulosic biomass is hydrolysed to low-molecular-weight
components, such as glucose, xylose and several phenolic
compounds. Cellulose and hemicellulose are easily hydrolysed
to monomeric sugars by subcritical water14. Subcritical water
hydrolysis has been used to produce reducing sugars from
15
lignocellulosic biomass, such as Miscanthus giganteus
,
municipal solid waste 16, cornstalks17, wheat straw17, rice bran18,
and other solid wastes. Pressurised carbon dioxide has been
used to acidify the medium through the formation of carbonic
acid and to increase the efficiency of subcritical-water-mediated
hydrolysis of biomass 19, 20. However, scanty information is
available on the hydrolysis of SCBP in subcritical water,
particularly in combining with the use of CO2. In the present
a. School of Chemistry and Chemical Engineering, Guangxi University, Nanning
530004, P. R. China.
b.Key Laboratory for the Petrochemical Resources Processing and Process
Intensification Technology of Guangxi, Nanning, 530004, P. R. China
* Corresponding author, Email: lilm@gxu.edu.cn (Xiaopeng Chen)
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study, subcritical CO2-water hydrolysis will be used to produce water. The samples were withdrawn at predeterminedVrieeawcAtritoicnletOimnlienes
DOI: 10.1039/C6RA18436G
reducing sugars from SCBP. The treated and untreated SCBP for DNS and HPLC analysis. The power controller was switched off
samples will be characterized using two-dimensional and the mixtures were naturally cooled to room temperature. The
heteronuclear single quantum coherence nuclear magnetic hydrolysis mixtures were separated in a vacuum into solid and liquid
resonance (2D HSQC NMR), fourier transform infrared (FTIR) solutions through filtration. Solid residues were placed in an oven at
spectroscopy, and scanning electron microscopy (SEM) 80 °C for 8 h to dry the samples. The samples were then analysed
techniques to help elucidate the physical and chemical through scanning electron microscopy (SEM) and Fourier-transform
characteristics of the raw material and the residual solid infrared spectrometry (FTIR) to figure out their properties.
particles. The possible reaction process will also be investigated.
Analysis methods
The components of SCBP were measured according to the National
Renewable Energy Laboratory (NREL) procedure21. Briefly, SCBP
(200 mg) was homogenized with 72% sulphuric acid (2 mL) at
2 EXPERIMENTAL
Materials
30 °C for
2 h then heated for 1 h at 120 °C after
The SCBP used as a raw material in this study was obtained from a
local mill (Nan-Ning Sugar Mill, Guang-Xi, China). The HPLC
standards, xylose (>P98%), glucose (P99.5%), arabinose (P99%),
cellobiose (P98%), cellotriose (P95%), cellotetraose (P94%),
cellopentaose (>P95%), xylobiose (P98%), glyceraldehyde(>P90%),
acetic acid(P99.9%), lactic acid(P90%), 5 - (hydroxymethyl) furfural
(5-HMF, P99%), furfural (P99.5%), and ethanol (P99.8%) were
purchased from Toronto Research Chemicals (Toronto, Canada).
dilution to 4% with distilled water. The monomeric sugars released
from sulphuric acid hydrolysis of the raw SCBP were analyzed
using HPLC (LC1220, Agilent, Santa Clara, CA, USA) equipped
with a SUGAR KS-801 column (Shodex, Tokyo, Japan) and an RI
detector (RID1200, Agilent).
The total reducing sugar concentration was estimated by the
3,5-dinitro salicylic acid (DNS) method22, with xylose used as a
standard. The hydrolysis liquor was diluted 10 times. For each 1 mL
of the diluted liquor, 1 mL DNS reagent and 1 mL deionized water
were added. The mixture was then heated in boiling
water for 10 min until the red brown color was developed. Then,
the mixture was cooled to room temperature in a water bath and
diluted to 25 mL. The absorbance was then measured using an
ultraviolet-visible spectrophotometer (UV-2100, Unico, Dayton, NJ,
USA) at 540 nm. The concentration of total reducing sugars was
calculated based on a standard curve obtained with glucose.
Apparatus
The hydrolysis of SCBP was carried out in high pressure
reactor (design pressure up to 20.0 MPa and temperatures up to
350 °C , Dalian Jingyi Autoclave Vessel Manufacturing Co. Ltd.,
Dalian, China ) equipped with a magnetic driven paddle agitator
at the bottom, a temperature controller, a cooling coil, a heat
exchanger,
a CO2 inlet, and a sampling device. The reactor was
heated with the help of 1.5 kW electric heaters. Considering that
the hydrolysis liquor tends to crystallise, we used specific sampling
device to overcome problems in a system at a high pressure and
temperature. In particular, the samples were withdrawn by using a
sampling tube extending almost to the bottom of the reactor. A
stainless steel filter with apertures of less than 10 μm was fixed onto
the bottom of the sampling tube prevented the residual solid particles
from being withdrawn with the liquid sample. The stainless steel
sampling tube (diameter = 3 mm) outside the reactor was insulated
to prevent cooling and the solidification of the hydrolysis liquor. For
sampling, the T-valve in the sample cell was opened, and the
sampling cell was connected to the reactor. The samples for 3,5-
dinitrosalicylic acid (DNS) and HPLC analysis were withdrawn at
predetermined reaction times. The apparatus employed in the
experiment is shown schematically in Figure 1.
The sugars, organic acids and other degradation products in the
liquid samples were quantified by HPLC (LC1220, Agilent). Each
sample was diluted 15-fold and then filtered through a 0.45μm
syringe filter. The filtered sample was injected into a SUGAR KS-
801 (Shodex) column and eluted with water at a flow rate of 1.0
mL·min-1 and a constant temperature of 80°C . An RI detector
(RID1200, Agilent) was used to provide a quantitative analysis of
Hydrolysis procedure
In each experimental run, a set amount of SCBP was mixed with
1000 mL of water in the reactor. The reactor was then closed and
sealed tightly. The air was pumped out of the reactor to an absolute
pressure of approximately 0.003 MPa then pressurised with carbon
dioxide for 10 min. After pressurization, the temperature was
increased to the desired level and maintained for a specified time.
The influence of temperature (160, 180, 200, 220, 240 and 260°C ),
liquid to solid ratio (20:1, 30:1, 60:1, 80:1 and 100:1), CO2 initial
pressure (0, 1, 4, and 7.3 MPa), stirring speed of (100, 200, 400 and
500 r·min−1), and reaction time (1, 3, 5, 10, 20, 30, 40, 50 and 60
min) on the performance of hydrolysis of SCBP in subcritical CO2-
Figure 1. A schematic of hydrolysis treatment system of SCBP: (1)
carbon dioxide tank, (2) reactor, (3) temperature and stir controller,
(4) thermocouple, (5) Sampling cell, (6) thermal retardation
equipment of sampling, (7) sampler, (8) pressure stabilization flask,
(9) vacuum pump, and (10) mercury U-tube manometer
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the products obtained from the hydrolysis process. The HPLC peaks assigned to C2,6-H2,6 correlation in p-hydroxybenzoate substructures
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were identified by comparing the sample peak retention time with or p-coumaric acid substructures. The signalDaOt δI:C1/0δ.H104309./1C36/R2A.51804m36aGy
the help of calibration plots for pure compounds.
be attributed to the solvent DMSO.
The chemical changes in the SCBP before and after hydrolysis
In summary, the results suggested that the hemicelluloses in
were investigated using FT-IR (Nicolet 380, Thermo Scientific, SCBP had a backbone of (1-4)-β-D-xylan and were mainly
Waltham, MA, USA). The SCBP sample was mixed with substituted with 4-O-methyl-D-glucuronic acid residues, and also
spectroscopic grade dried KBr to create a disk. The spectra were arabinose residues and acetyl residues linked to the backbone.
collected in the spectral range 400–4000 cm-1 using 30 scans per
sample. The background spectrum of pure dried KBr was subtracted
from the sample spectrum.
The surface morphology of SCBP before and after hydrolysis
was qualitatively studied using SEM (Hitachi S-3400N, Tokyo,
Japan). The dried samples were fixed using a conducting adhesive
tape to an aluminium stub and then sputter coated with gold for 120
s with a gold sputter coater (Polaron SC 7640, Quorum Technologies,
Ashford, UK). The images of the gold coated samples were obtained
in the SEM operating at 10 kV.
The 2D HSQC - NMR spectra (75 MHz) were recorded on a
Nuclear Magnetic Resonance Spectrometer (AVANCE III HD600,
Bruker, Flatlander, Switzerland) at room temperature. Dried samples
(1 mg) were dissolved in sodium deuteroxide (DMSO-d6, 1.5 mL)
then transferred to an NMR tube (diameter, 5 mm).
Figure 2. 2D HSQC NMR spectra of the SCBP( X: 1,4- linked D-
xylose residues, A: α-L- arabinose residues, U: 4-Omethoxyl-
glucuronic acid residue, G: guaiacyl unit, S: syringyl, pPB: p-
hydroxybenzoate, pCA: p-coumaric acid ubstructures)
RESULTS AND DISCUSSION
Composition of the raw SCBP
The raw SCBP was composed of cellulose (30.16%), hemicellulose
(40.01%), lignin (18.09%), moisture (9.41%), and ash (2.33%). It is
clear that cellulose, hemicellulose and lignin are present in
significant quantities in the raw SCBP while ash is present in low
amounts compared with other agro-industrial residues such as paddy
straw(16% 23), rice straw (14.5%24) and wheat straw (9.2%17).
Because of its lower ash content, SCBP offers many advantages as a
potential source of biomass for the production of reducing sugar.
NMR Results
To obtain information about the structure of hemicelluloses in SCBP,
the HSQC -NMR spectroscopic analysis was performed. Figure 2
showed 2D HSQC - NMR spectra of the SCBP. The assignments of
signals are based on previous publications25,26. As shown in Figure 2,
the signals observed at δC/δH 102.3/4.26, 72.2/3.23, 73.3/3.55, 77.7/
3.67 and 63.6/ 3.37 were related to Cβ-Hβ correlations corresponding
to C-1, C-2, C-3, C-4 and C-5 of the 1,4- linked D- xylose units,
respectively. The signal at δC/δH 61.1/3.55 was related to C-5 of the
α-L- arabinose residue. The signals at δC/δH 98.7/4.68, 71.6/3.44, and
72.9/3.64 were assigned to C-1, C-2 and C-3 of the glucuronic acid
residue, respectively. The prominent signal for methoxyl (δC/δH
56.2/3.72) was probably a result of the 4-Omethoxyl group of
glucuronic acid residues in the hemicelluloses and aromatic
methoxyl groups in the lignin. The weak signal at δC/δH 21.3/2.02
CH3 may be attributed to acetyl in the hemicelluloses.
The main cross-signals observed in the aromatic region (δC/δH
100.0-135.0/5.50-8.50) were assigned to the p-hydroxyphenyl,
guaiacyl and syringyl units of lignin, respectively. The signal at
δC/δH 104.2/6.68 was related to C2,6-H2,6 correlations in syringyl
units. The strong signals at δC/δH 116.1/6.77 corresponded to C5-H5
Figure 3. SEM images of (a) raw SCBP, (b) insoluble fraction fibres
obtained after hydrolysis
correlations in the guaiacyl unit. The signal at δC/δH 130.7/7.49 was
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SEM Results
Effect of SCBP: water ratio on the yield of total reducing
View Article Online
DOI: 10.1039/C6RA18436G
Figure 3 shows the SEM images of the raw SCBP and hydrolysis sugars
residues. A well-defined fibre bundle with a smooth surface was The liquid/solid ratio has been recognized as an important factor in
observed in the raw SCBP. The raw pith differed slightly from the reducing sugars yield. Figure 6 illustrates the influence of water:
treated pith. This finding indicates that the structure of a cellulose SCBP ratio on the reducing sugars yield. At first, the yield of the
unit in the solid phase remains essentially unchanged during total reducing sugars increased as the concentration of water
hydrolysis. However, after hydrolysis, the SCBP was damaged to a increased, then it reduced slightly when the amount of the SCBP
certain extent; some materials peeled off from the surface and small decreased. These results may be attributed to the fact that at water:
holes were formed on the surface.
SCBP ratio of 20:1, water was too little to completely hydrolyse the
hemicelluloses and cellulose. However, the ionisation constant of
water increases as the concentration of water increases when water:
SCBP ratio exceeds 30:1, which is beneficial for the hydrolysis. As
the water: SCBP ratio increased further, the concentration of the raw
SCBP decreased. According to the previous report31, as the
concentrations of hemicelluloses drops, the rate of formation of
oligomers slowed enough that they cannot be replenished as fast as
they react to monomers. Thus, reducing sugar yield decreased at
water: SCBP ratio above 30:1.
FT-IR Results
A comparative analysis of the IR spectra of SCBP before and after
hydrolysis treatment is shown in Figure 4. The characteristic peaks
of raw pith at the wave numbers of 897, 1038, 1059, 1112, 1162 and
1262 cm-1 can be identified for cellulose. These peaks are consistent
with the observation of previous publication27. A sharp band
developed at 897 cm-1 attributing to β-glycosidic linkages between
sugarunits was observed, indicating that xylan in hemicellulose of
the SCBP was linked by β-form bonds. This was also in line with the
observations from the 2D HSQC - NMR spectra of SCBP. The peak
of the treated pith at 1262 cm-1 weakened rapidly corresponding to
the discussion in the reaction mechanism that the constituent of
hemicellulose in the SCBP was mostly consumed during the reaction.
The peaks at 1038, 1059, 1112, 1162, 1458, and 1544 cm-1 exhibited
evidently indicating that the treatment was able to remove
hemicellulose so as to enhance the characteristic of cellulose and
lignin contained in the hydrolysis residues. The absorbance peak at
1637cm-1 can be identified for aldehyde or carbonyl group. This
peak of hydrolysis residues wither in a significant way, meaning that
the C=O bonds have mostly been ruptured.
Effect of hydrolysis temperature on the yield of total
reducing sugars
Figure 5 illustrates the influence of hydrolysis temperature on the
reducing sugars yield. The yield of the total reducing sugars initially
increased as temperature increased. These results are consistent with
the observation of previous publication that higher temperature gives
somewhat better yields than lower temperature28,29. This may be
attributed to the increase in the ionisation constant (KW) at a higher
temperature. The concentrations of hydronium and hydroxide ions
increased to break the glycosidic bonds in hemicellulose and
cellulose. Once attacked by electrophilic hydrogen atoms on the
glycosidic bonds, cellulose and hemicellulose can be hydrolysed in
pure water. However, this reaction is very slow at ambient
temperature and pressure30. The rate of hydrolysis can be increased
by increasing temperatures and pressures or by inducing acid
catalysis from carbonic acid which was formed by the addition of
CO2. During the acid hydrolysis of hemicellulose and cellulose, a
proton is initially attached to the oxygen atom of the glycosidic bond,
which slowly breaks down to give a cyclic carbeniumion. Then, free
sugars, such as glucose and xylose, are liberated. The reducing
sugars produced may further decompose into other products.
However, as the temperature further increased, the yield of total
reducing sugars decreased when the temperature exceeded 200 °C .
This may have been caused by the rate of decompose of monomers
becoming more rapid than the rate of hydrolysis of hemicellulose
and cellulose.
Figure 4. IR spectra of (a) raw SCBP, (b) insoluble fraction fibres
obtained after hydrolysis
Figure 5. Effect of hydrolysis temperature on the yield of total
reducing sugars. The hydrolysis was carried out at 500 r·min−1 for 40
min without any CO2 added. The water: SCBP ratio was 30:1.
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Effect of carbon dioxide on the yield of total reducing
sugars
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DOI: 10.1039/C6RA18436G
When dissolved in water, CO2 increases the availability of protons,
thereby catalysing hydrolysis reactions32. The effects of carbon
dioxide on the yield of the total reducing sugars are shown in Figure
7. A model to predict the pH of a binary CO2-H2O system in a
temperature range of 100-250°C and pressures up to a CO2 partial
pressure of 150 atm (∼15.2 MPa) was proposed by Gurgel33:
pH  8.00106T2  0.00209T 0.216ln(P )  3.92
(1)
CO2
where T (°C ) is the temperature and P (atm), CO2 partial pressure.
The pH of the system at 200°C predicted by Eq. (1) is shown in
Table 1. The pH of the system decreased as the CO2 partial pressure
increased. This may be attributed to the fact that the addition of CO2
leads to acid-hydrolysed catalysis caused by the formation of
carbonic acid34. And the acidity was strengthened as the increase of
CO2 partial pressure. As acidity strengthened, the yield of total
reducing sugar was increased indicating that adding of CO2
facilitated the yield of reducing sugars.
Figure 7. Effect of CO2 on the yield of total reducing sugars. The
hydrolysis was carried out at 200 °C and 500 r·min−1 for 40 min. The
water: SCBP ratio was 30:1.
Table 1 Predicted results of pH value of the system at 200°C
CO2 partial pressure (MPa)
Reaction pressure(MPa) pH
1.0
2.0
4.0
7.5
2.0
4.2
7.5
12.6
4.0
3.9
3.7
3.6
Effect of stirring speed on the yield of total reducing sugars
External diffusion is reflected in the rate at which the water reaches
the surface of the SCBP. The resistance effect of external diffusion
focuses on the viscous flow over the pith surface. This effect can be
eliminated by changing the speed of the mechanical stirrer. Figure 8
shows the effect of stirring speed on the yield of the total reducing
sugars, where the yield increased as the stirring speed increased.
Figure 8. Effect of stirring speed on the yield of total reducing
sugars. The hydrolysis was carried out at 200 °C for 40 min with
2MPa CO2 added. The water: SCBP ratio was 30:1.
Effect of reaction time on the yield of total reducing sugars
Figure 9 illustrates the curve of the yield of the total reducing sugars
with reaction time. Initially, the yield increased as the reaction time
extended. As the reaction time was further extended, the yield then
decreased, as the reducing sugars produced decomposed further into
Figure 9. Effect of reaction time on the yield of total reducing
sugars. The hydrolysis was carried out at 200 °C and 500 r·min−1
with 2 MPa CO2 added. The water: SCBP ratio was 30:1.
Figure 6. Effect of water: SCBP ratio on the yield of total reducing
sugars. The hydrolysis was carried out at 200 °C and 500 r·min−1 for
40 min without any CO2 added.
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other products. The highest yield of total reducing sugars was 43.6%,
higher than that of reported by Prado (23.1% 35) and Perez (15.5%36),
in which sugarcane bagasse were treated by subcritical water. The
subcritical CO2-water exhibits high capacity for dissolution and
catalysis to promote the hydrolysis of SCBP. The subcritical CO2-
water hydrolysis had a mild reaction condition, improved yield and
was easily controlled compared with that without CO2, so that it was
feasible at larger scale.
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Product distribution
Figure 10 shows HPLC chromatograms of the product obtained at
200 °C for 50min, with a CO2 initial pressure of 1 MPa, a SCBP:
water ratio of 50:1 and a stirring speed of 500 r·min−1. The HPLC
peaks were identified by the comparison of sample peak retention
time with the help of calibration plots for pure compounds. And the
concentration profiles of the compounds in the liquid product as a
function of time are shown in Figure 11. The results showed that the
main products were xylose and arabinose, the soluble hydrolysis
products of hemicellulose. As hemicellulose, a higher content among
the raw material SCBP, is thermally less stable than cellulose,
pentose conversion was much higher than that of hexose conversion.
Since xylan was the major constituent of hemicellulose, xylose was
the major potential product of this process. The amount of observed
glucose derived from cellulose was extremely lower. These were in
agreement with the SEM and FTIR results that hydrolysis of hemi
was the main process in this condition. This was corroborated by the
previous publication37 that little cellulose hydrolysis was occurred
below 230 °C , either in its pure form or when in lignocellulosic
complexes, so that when hemicellulosic sugars recovery is intended,
the process is usually carried out at 150-230 °C . Oligosaccharides
such as cellobiose, cellotriose, cellotetraose and cellopentaose were
observed indicating that cellulose was also partially hydrolysed. As
expected, the content of oligosaccharides initially increased, then
subsequently decreased as time was extended. This was because
cellulose can initially be converted into water-soluble
oligosaccharides and then to glucose. The contents of xylose,
glucose and arabinose in the reaction medium presented the
tendency of increase first and decrease afterwards. This may be
attributed to the fact that xylose, glucose and arabinose generated
decompose rapidly, being converted into 5-HMF, furfural and other
inhibitors or unfermentable products. The high amounts of
glyceraldehyde in the liquid product were likely derived from the
glucuronic acid. It means that major xylan in hemicellulose was
most likely O-acetyl-4-O-methylglucuronoarabinoxylan, a typical
structure existing in grasses38. Its main chain was made up of D-
xylan linked by β-1,4- configuration glycosidic bond. And
substituted 4-O-methyl-α-D-glucuronosyl, α-L-arabinopyranosyl,
and acetyl were linked to the 1,4-linked D-xylopyranoside in the
backbone located on C2 and C3 as side chains, respectively. This is
also in line with the observations from 2D HSQC NMR spectra of
the SCBP. The presence of acetic acid was attributed to the
hydrolysis of acetyl groups linked between hemicellulosic fractions.
The low amounts of ethanol in the liquid product were likely derived
from the further dehydration of acetic acid. This finding was
confirmed by the decrease in acetic acid concentration. The observed
lactic acid was attributed to the decarboxylated of 5-HMF and
furfural.
Figure 10. HPLC chromatograms of hydrolysis liquid sample. (1)
acetic acid; (2) lactic acid; (3) cellopentaose; (4) cellotetraose; (5)
cellotriose; (6) cellobiose; (7) xylobiose; (8) glucose; (9) xylose; (10)
arabinose; (11) ethanol; (12) glyceraldehydes; (13) 5-HMF; (14)
furfural. The hydrolysis was carried out at 200 °C and 500 r·min−1 for
50 min with 1 MPa CO2 added. The water: SCBP ratio was 50:1.
Figure 11. Concentration profile of (a) surgars, (b) inhibitors and
acids in the liquid products during hydrolysis of SCBP. The
hydrolysis was carried out at 200 °C and 500 r·min−1 with 1 MPa
CO2 added. The water: SCBP ratio was 50:1.
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DOI: 10.1039/C6RA18436G
Figure 12. Proposed reaction mechanism of SCBP hydrolysis in subcritical CO2-water
SCBP can be hydrolysed in subcritical CO2-water to produce
reducing sugars. 2D HSQC NMR result suggested that the major
xylan in hemicellulose of SCBP was most likely O-acetyl-4-O-
methylglucuronoarabinoxylan. The SEM and FTIR results indicated
that in subcritical CO2-water, hemicellulose was mainly hydrolysed
with cellulose and lignin hardly being affected. A maximum
reducing sugar yield of 43.6% was obtained at 200 °C , a SCBP:
water ratio of 1:30, a CO2 pressure of up to 2 MPa, a reaction time of
50 min and a stirring speed of 500 r·min−1. A reaction mechanism
has been proposed based on the HPLC results. Hydrolysis yielded
oligosaccharides followed by further hydrolysis into xylose and
glucose. Meanwhile, the group on the side chain of hemicellulose
was released, forming arabinose, acetic acid, and glucuronic acid.
Glucuronic acid rapidly decomposed into glyceraldehydes. The
pentoses and hexoses produced were further dehydrated to 5-HMF
and furfural, which were then decarboxylated into lactic acid.
Hydrolysis Mechanism
Based on our studies of these products and cellulose or
hemicellulose model compounds, we propose a reaction mechanism
for the hydrolysis of SCBP in subcritical CO2-water, as depicted in
Figure 12. Cellulose and hemicellulose in SCBP were important bio-
polymers. The reaction of subcritical CO2-water can be of interest to
produce oligomers. The condition of subcritical CO2-water exhibited
a high capacity for dissolution and catalysis to promote the
production of oligomers. In the hydrolysis reaction of cellulose and
hemicellulose, a proton was initially attached to the oxygen atom of
the glycosidic bond which was then broken. As a consequence, a
carbo-cation and a cellulose or hemicellulose fragment was formed.
In the next step, a hydroxide ion from a dissociated water molecule
attached to the carbon-cation and formed another cellulose or
hemicellulose fragment, thereby producing oligosaccharides,
including cellobiose, cellotriose, cellotetraose, cellopentaose, and
xylobiose. Meanwhile, the group on side chain of hemicellulose was
released forming arabinose, acetic acid and glucuronic acid.
Glucuronic acid then rapidly decomposed and was converted into
glyceraldehyde. Oligosaccharides were directly hydrolysed into
pentoses and hexoses to produce predominantly xylose and glucose.
Xylose, glucose and arabinose were further dehydrated to 5-HMF
and furfural. 5-HMF and furfural can be decarboxylated to lactic
acid.
ACKNOWLEDGMENTS
This work was co-financed by the National Natural Science
Foundation of China (Grant No. 31060102, 31560241), the
Guangxi
Natural
Science
Foundation
(Grant
No.
2013GXNSFBA019037), the Key Laboratory of Petrochemical
Resource Processing and Process Intensification Technology
(Grant No. 2013Z010), and Excellent Doctoral Thesis
Cultivation Project of Guangxi (YCBZ2014015).
CONCLUSIONS
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Page 9 of 9
RSC Advances
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DOI: 10.1039/C6RA18436G
The subcritical CO2-water exhibits high capacity for dissolution and catalysis to
promote the hydrolysis of sugarcane bagasse pith.
Sugarcane
bagasse pith
DNS analysis
Effects of process
parameters on the
HPLC analysis
Liquid product
Total
Sugar yield
Reducing
Subcritical CO2-
water reactor
2D HSQC NMR
Residual solid
Proposed reaction
mechanism
SEM
FTIR