Hydrolysis of cellulose to glucose over carbon catalysts sulfonated via a plasma process in dilute acids

Article abstract of DOI:10.1039/c7gc02143g

Herein, we reported a novel plasma-sulfonation process for carbon materials in dilute sulfuric acid within a few tens of minutes. The total acidic and -SO₃H densities of the sulfonated carbon were 4.4 mmol g^-1 and 2.2 mmol g^-1, respectively. The glucose selectivity during cellulose hydrolysis using a sulfonated carbon catalyst was 83.9% and retained 98% of its performance after recycling.

Full text of DOI:10.1039/c7gc02143g

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Hydrolysis of cellulose to glucose over carbon catalysts sulfonated
via a plasma process in dilute acid
Received 00th January 20xx,
Accepted 00th January 20xx
ab
c
acd
Oi Lun Li, Ryuhei Ikura , and Takahiro Ishizaki
DOI: 10.1039/x0xx00000x
www.rsc.org/
7
Herein, we reported a novel plasma-sulfonation process of carbon including steam explosion as well as chemical and biological
8
materials under dilute sulfuric acid within a few tens of minutes. treatments . Although various types of acids including HCl,
The total acidic and –SO
3
H densities of the sulfonated carbon were
H
2
SO
.4 mmol g and 2.2 mmol g , respectively. The glucose selectivity hydrolysing cellulose, the systems still suffer from major
during cellulose hydrolysis using the sufonated carbon catalyst drawbacks such as reactor corrosion, catalysts recycling and
4
, HF and organic acids are effective for directly
-1
-1
4
9
was 83.9% and the performance remained 98% after recycling.
post-treatment of waste effluent. In order to minimize the
environmental impact, enzymatic hydrolysis (e.g. cellulase) has
been applied for conversion of cellulose or lignocellulosic
biomass into saccharides, including cellobiose and glucose
under mild conditions. However, the relatively high cost of
cellulase and its low tolerance of environmental factors (e.g.
temperatures and pH values) as well as limited reusability
restricted further application of the process. Alternative routes
for cellulose hydrolysis have been explored, in particular,
utilization of heterogeneous catalysts have attracted many
attentions. Compared to direct acid and enzymatic hydrolysis,
heterogeneous catalysis-based hydrolysis have various
advantages in terms of ease of product separation, less reactor
Current energy systems are mainly based on fossil resources
such as coal and petroleum. However, the rapid consumption
of these resources as well as emerging concerns of green
house gases emissions has urged scientists to develop and
utilize renewable resources in next generation energy
1
systems. Cellulose is the primary component of non-edible
biomass such as grass, wood and agricultural crops, and being
considered as
polysaccharide consisting of repeating units of glucose
monomers linked by -1,4 glycosidic bonds, and is highly
a sustainable resource. Cellulose is a
β
insoluble in common solvents since large amount of intra- and
intermolecular hydrogen bonds form a robust crystal structure
1
0
corrosion, good recyclability and low capital cost. Different
types of solid acid catalysts have been proposed recently,
2
-3
with high chemical stability. In order to utilize cellulose, the
polymer is firstly converted into soluble glucose, and then
subsequently fermented into bioethanol or dehydrated into
1
1
12
including
metal
1
oxides,
ion
exchange
resins,
14
3
heteropolyacids
and sulfonated carbon materials.
In
4
platform molecules. Thus, the selective hydrolysis of cellulose
particular, carbonaceous matrix with superior stability and
recyclability under hydrolysis conditions makes the material a
better candidate among others. Apart from conventional solid
into glucose is the most challenging process.
Conventional processes for cellulose conversion to glucose
include direct hydrolysis by dilute or concentrated mineral
5
acids , enzymatic hydrolysis and lignocellulose pre-treatments
acids with merely -SO H as single functional groups, carbon
3
6
materials bearing –SO H with other hydrophilic groups of –
3
COOH and phenolic-OH enhanced the interaction between the
solid acid catalyst and β-1,4 glucan in cellulose, which
1
5
eventually promote the hydrolysis process.
Although sulfonated carbon catalysts have showed
promising results as solid acid catalysts for cellulose hydrolysis,
the sulfonation process requires high concentration of sulfuric
o
acid at elevated temperature (100 – 150 C) and hours of
1
6
treatment time (10 – 24 hours). In order to promote the
application of carbon acid catalysts, development of new
environmentally friendly route to sulfonate carbon catalysts is
highly desirable. Herein, we reported a novel and fast
sulfonation process of carbon material via plasma under dilute
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sulfuric acid solution. The cellulose conversion rate
Table 1 lists the total acid densities, sulfonated acid
significantly enhanced from 10 to 40% when the acid densities, specific surface areas, pore sizes and total pore
concentration increased from 0.1 to 1 M within 30 minutes of volumes of the original carbon black (CB), sulfonated CB under
processing time. Also, the selectivity of glucose was over 80% 0.1 M, 0.5 M and 1 M sulfuric acid by the plasma process. For
for carbon catalyst sulfonated in 1 M sulfuric acid, and the comparison purpose, the surface properties of commercial ion
rd
performance remained over 98% after 3 cycling. Compared exchange resin (amberlyst 15) is also presented as well. The
to conventional sulfonation processes, plasma process could original CB exhibited specific surface area, pore size and total
2
-1
2
-1
sulfonate carbon materials at much lower sulfuric acid pore volume of 35.44 m g , 5.1 nm and 0.23 cm g ,
concentration. Without applying any aggressive and hazardous respectively. The total acid density was determined as 1.0
-
1
chemicals, including concentrated sulfuric acid or fuming SO
3
,
mmol g , which was contributed by weak acidic groups of -
H group was
significantly reduces the environmental impact and eliminates COOH and phenolic –OH. Apparently, no –SO
3
post-treatment of the toxic effluent. Plasma process is thus present in the original CB. After plasma process in dilute acid
considered as a “greener” synthesis route for carbon solutions, the physical properties showed no significant change
sulfonation.
while the acidic densities increased proportionally to sulfuric
Sulfonation of the carbon materials was carried out in a acid concentration. The –SO H group and total acidic sites of
3
-
1
1
00 ml dilute sulfuric acid solution of 0.1, 0.5 and 1M for 30 0.1 M CB was 0.5 and 2.3 mmol g , where both densities
-1
minutes. Commercial carbon black (CB) (Meijo carbon, corp.) increased to 2.2 and 4.4 mmol g , respectively, when sulfuric
with particle size ranged between 30 - 40 nm, was chosen as a acid concentration enhanced to 1 M. Since higher intensity of
precursor for carbon catalysts. A pair of tungsten electrode radicals was observed with increasing sulfuric concentration,
was connected to a bipolar power supply and immerged into thus higher amount of acidic sites could be attached to the
the solution in order to generate plasma discharge in the carbon surface in 1 M sulfuric acid. Thermal stabilities of –
solution. An optical probe was applied to investigate active SO H in 1M CB was evaluated under hydrothermal conditions
3
o
radicals during plasma discharge through optical emission between 120 -210 C. The functional group showed relatively
spectrum. A detail schematic image of plasma process is high stability in elevated temperature (Table S1).
demonstrated in Fig. S1. The optical emission spectrum during
The chemical bonding structure of carbon catalysts was
plasma process was shown in Fig. S2. Specific peaks further confirmed by Fourier Transform Infrared Spectroscopy
corresponding to OH, H , H , O were observed at 308nm, (FTIR), as shown in Fig. S4. FTIR spectra shows the major
β
α
-1
4
86nm, 656 nm and 777nm respectively. Regardless of adsorptions are centred at 1350 – 1770 cm and 3200 – 3500
-1
concentrations of the acid solution, the dominant radicals was cm . The peaks at 1390 cm and 1631 cm can be ascribed to
H, followed by O and OH. These radicals were generated C-O stretching, and C=O stretching, respectively, where the
-1
-1
-1
during dissociation of water and sulfuric acid within plasma broad peak at 3200 – 3500cm is contributed by the C-OH
1
zone. Since the most dominant radical was H , the mechanism stretching. Distinct peak at 1040 cm corresponding to
7
-1
α
of carbon sulfonation was expected to be initiated by the symmetric O=S=O stretching vibrations of sulfonic groups
following reactions: (1) active SO3 was generated when H appears in the spectra of 0.5M CB and 1M CB, which confirms
radicals reacted with sulfuric acid, and (2) active SO groups that the -SO H groups were successfully anchored to the
3
3
1
8
aggressively attacked the surface of carbon and formed -HSO3 carbon matrix.
on the surface. In addition, the formation of -COOH and The catalytic performance of cellulose hydrolysis was
phenolic –OH can also be simultaneously occurred due to the investigated with various types of catalysts. Prior to hydrolysis,
present of O and OH radicals. The proposed mechanism is microcrystalline cellulose was pre-treated by ball-milling for 48
1
9
hours in order to decrease the crystallinity (Fig. S3). The
hydrolysis of cellulose was prepared under following
conditions: a mixture of 45 mg cellulose and 50 mg catalyst
was added into 5 ml of deionized water in 100 mL autoclave,
o
and heated at 150 C for 24 hours. The concentration of total
soluble carbohydrates was determined by phenol-sulfuric acid
2
0
method. The soluble sugar compounds and by-products were
quantified and qualified by high performance liquid
chromatography
(HPLC).
The
carbon
content
in
microcrystalline cellulose was firstly evaluated by CHNS
analyser, while the total conversion of cellulose (C mol. %) and
glucose yield were determined, respectively, by the ratio of
carbon and glucose in soluble carbohydrates to that of original
carbon content in cellulose. Glucose selectivity was calculated
by the ratio of glucose concentration to that of total soluble
carbohydrates. The definition and calculations of total
conversion of cellulose, glucose yield and glucose selectivity
were presented in ESI.
presented in Fig. 1.
Fig. 1 Proposed mechanism of surface functionalization of
carbon materials during plasma process
2
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Table 1 Surface properties of orginal CB and sulfonated CB under various concentrations of sulfuric acid by plasma process.
d
Catalyst
Total acid
-SO H
-COOH,
Specific
surface
Pore
Total pore
Conversion
C (%)
Glucose
Yield
Glucose
selectivity
(%)
3
a
-1 a
c
c
d
densities
(mmol g )
phenolic –
size
volume
-
1
b
c
2
-1
d
(
mmol g )
OH
area
(nm)
(cm g )
(%)
-1
2
-1
(
mmol g )
m g
No catalyst
CB
-
-
-
-
-
-
2.1
3.3
1.1
0.6
52.3
18.1
53.2
82.8
83.9
81.1
1.0
2.3
3.9
4.4
4.7
0
1.0
1.8
2.0
2.2
-
35.44
36.37
41.20
39.52
45
5.1
5.5
5.6
5.7
3.3
0.23
0.25
0.26
0.24
0.31
0
.1M CB
.5M CB
0.5
1.9
2.2
4.7
10.9
30.5
41.2
9.4
5.8
0
25.3
34.6
7.6
1
M CB
Amberlyst
1
5
a
21 b
The density of acidic densities was determined by titration method. The densities of –COOH and phenolic –OH were
c
determined by subtracting acid density of -SO
3
H by total acidic densities. Specific surface area, pore size and total pore volume
d
were N adsorption-desorption isotherms based on Brunauer-Emmett-Teller (BET) method. The hydrolysis reaction was carried
2
o
out at 150 C in a constant temperature oven (ETTAS ONW-300S) for 24 hours with 45 mg of ball-milled cellulose, 50 mg of
catalysts and 5 mL water in a 100 mL autoclave without agitation.
Table 1 summarizes the percentage of conversion and
The catalytic performance reported in current study was
glucose selectivity via various catalysts. Commercial solid acid also compared to that of previous studies based on sulfonated
1
5-16
catalyst (amberlyst 15) was applied as a reference catalyst of carbon materials under similar hydrolysis conditions.
Onda
cellulose hydrolysis reaction. The original carbon material (CB) et al. prepared sulfonated-carbon by concentrated sulfuric acid
o
exhibited almost no catalytic activity, where the cellulose and heated under argon flow at 150 C for 16 hours. The total
-1
conversion was lower than 4 %. After carbon sulfonation via acid sites and specific surface area were, 1.63 mmol g and
2
-1
plasma process under 0.1 M sulfuric acid, the cellulose 806 m g , respectively. In hydrolysis reaction, the catalysts
conversion rate was 10.9 %, which was comparable to that of obtained a total cellulose conversion of approximately 43 %
amberlyst 15. However, the glucose selectivity was obviously where the glucose yield was reported as 40.5 %. Up to date,
lower than the commercial solid acid catalyst. The catalytic Pang et al. obtained a higher cellulose conversion percentage
performance was strongly enhanced by increasing the sulfuric of 94.4 % and glucose yield of 74.5 % by increasing the
o
acid concentration, where the conversion percentage sulfonation temperature of carbon catalysts to 250 C and
enhanced from 30.5 % to 41.2 % when sulfuric concentration heated under N flow for 24 hours. The total acidic density and
2
increased from 0.5 M to 1 M. Also, the glucose selectivity of –SO H of the resulted catalyst were correspondingly, 2.39
3
-1
-1
both catalysts were above 80 %. Compared to the mmol g and 0.63 mmol g . On the other hand, our study
performance of amberlyst 15, catalyst of 1M CB has 4 times applied plasma process under dilute acid condition and the
higher cellulose conversion and even higher glucose selectivity. acidic densities of were significantly higher compared to
Beside glucose, major soluble organic was identified as previous studies. The total acidic density of 1M CB has reached
-
1
-1
cellubiose. The total cellulose conversion (mol.%) and the 4.4 mmol g where the density of –SO H was 2.2 mmol g .
3
quantity of each soluble products were present in Fig. 2. The Thus, the inferior catalytic performance of our catalysts should
recovery and recyclability of two carbon catalysts (0.5M CB not be related to the amount of acidic densities. Instead, the
and 1M CB) were also investigated. The carbon catalysts were carbon catalysts showed significant difference in specific
o
first filtered, washed with deionized water and dried at 100 C surface area. Previous studies have applied active carbon as
for 24 hours after each trial. As shown in Fig. S5, the catalytic starting material and the resulted catalysts has a specific
2
performance of 1M CB remained over 98% after the third cycle. surface area in the range from 412 m g to 806 m g , where
-1
2
-1
2
-1
Also, the glucose selectivity remains 75% or above. As a side the surface area of 1M CB was less than 40 m g . It is
note, amberlyst 15 could not be recycled because the catalysts expected that the catalytic performance would significant
o
were decomposed at 150 C. The results confirmed that the enhanced if we performed plasma sulfonation to carbon
sulfonated carbon catalysts by plasma process presented materials with a higher surface area.
excellent stability and reusability.
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Fig. 2 Cellulose conversion percentage in hydrolysis reactions
via various catalysts.
4
3
1
2
3
In summary, we have developed a novel sulfonation
process of carbon materials via plasma process under dilute
sulfuric acid in minutes of process time. The total acidic and –
-1
3
SO H densities were, respectively, 4.4 mmol g and 2.2 mmol
-1
g . The conversion percentage of cellulose hydrolysis by
sulfonated carbon catalysts was 40.1 %, where glucose
selectivity was over 80 %. Although the glucose selectivity was
relatively lower than other reported values, the catalytic
performance is expected to be improved significantly by
applying mesoporous carbon materials with higher surface
area as starting material. Nevertheless, this green sulfonation
process is believed to be highly applicable in preparation of
carbon acid catalysts for renewable energy.
7
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Acknowledgement
This work was partly supported by Grant-in-Aid for Scientific
Research (A) (No. 16H02400) from Japan Society for the
Promotion of Science.
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