Catalytic performance of benzoxazine derived ordered mesoporous carbon materials for the selective oxidation of 2-methylnaphthalene

Article abstract of DOI:10.1039/d0nj03226c

2-Methylnaphthoquinone is an important chemical product. The catalyst used in the catalytic oxidation of 2-methylnaphthalene to 2-methylnaphthoquinone has been a research hotspot. In this paper, benzoxazine derived ordered mesoporous carbon materials were used as catalysts, and their catalytic performance in acetic acid/hydrogen peroxides was investigated. The N/S co-doped carbon material (NSCM) was prepared with thiourea-type benzoxazine as a carbon precursor and SBA-15 as a template. The nitrogen-doped carbon material (NCM) was prepared using the same method using aniline-type benzoxazine as a precursor. The carbon materials were characterized by N2 adsorption/desorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The results have shown that both carbon materials exhibit worm-like structure and have graphitized amorphous structure. Nitrogen in the carbon materials existed in the form of pyridine nitrogen and pyrrole nitrogen, which caused the surface defects of carbon materials. The surface defects are critical for the enhancement of the catalytic activity of carbon materials. With NSCM and NCM as catalysts, the yields of 2-methylnaphthoquinone were 78.8% and 65.7%, respectively. By comparison, NSCM has higher catalytic activity, and the catalytic activity remains stable after five cycles of use. These results indicate that NSCM has great potential as a highly efficient catalyst for the selective oxidation of 2-methylnaphthalene. This journal is

Full text of DOI:10.1039/d0nj03226c

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Catalytic performance of benzoxazine derived
ordered mesoporous carbon materials for the
selective oxidation of 2-methylnaphthalene
Cite this:DOI: 10.1039/d0nj03226c
Zhouxuan Zang, Yingge Chang, Yi Yu, Haihan Zhang, Li Xu * and Guoji Liu
2-Methylnaphthoquinone is an important chemical product. The catalyst used in the catalytic oxidation
of 2-methylnaphthalene to 2-methylnaphthoquinone has been a research hotspot. In this paper,
benzoxazine derived ordered mesoporous carbon materials were used as catalysts, and their catalytic
performance in acetic acid/hydrogen peroxides was investigated. The N/S co-doped carbon material
(
NSCM) was prepared with thiourea-type benzoxazine as a carbon precursor and SBA-15 as a template.
The nitrogen-doped carbon material (NCM) was prepared using the same method using aniline-type
benzoxazine as a precursor. The carbon materials were characterized by N adsorption/desorption,
2
X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM),
X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The results have shown that both
carbon materials exhibit worm-like structure and have graphitized amorphous structure. Nitrogen in the
carbon materials existed in the form of pyridine nitrogen and pyrrole nitrogen, which caused the surface
defects of carbon materials. The surface defects are critical for the enhancement of the catalytic activity
of carbon materials. With NSCM and NCM as catalysts, the yields of 2-methylnaphthoquinone were
Received 2nd July 2020,
Accepted 16th September 2020
7
8.8% and 65.7%, respectively. By comparison, NSCM has higher catalytic activity, and the catalytic
DOI: 10.1039/d0nj03226c
activity remains stable after five cycles of use. These results indicate that NSCM has great potential as a
highly efficient catalyst for the selective oxidation of 2-methylnaphthalene.
rsc.li/njc
phase oxidation of 2-methylnaphthalene. The MCM-41 catalyst
exhibited excellent catalytic performance under mild conditions,
1
. Introduction
2
-Methylnaphthoquinone is an important intermediate for and the yield of 2-methyl-1,4-naphthoquinone can reach 65.6%.
5
2+
2+
K-type vitamins, and it is also known as 2-methyl-1,4-naphtho- Serhan Urus- had used silica-supported metal ions (Ru , Pd ,
quinone or b-methyl-1,4-naphthoquinone.
diagram of the products of 2-methylnaphthalene oxidation is naphthalene. Oxana A. Kholdeeva had used silica supported iron
given in Scheme 1. Compound A is 2-methyl-1,4-naphtho- phthalocyanine as a catalyst to catalyze the oxidation of
quinone which is the major product and B and C are 6-methyl- 2-methylnaphthalene. However, the metal-based catalysts also
1
,2
2+
The schematic Co ) as catalysts for the catalytic oxidation reaction of 2-methyl-
6
1,4-naphthoquinone and phthalic anhydride, respectively, which have significant disadvantages, including high cost, poorer
are the supernumerary products. durability, lower selectivity, prone to poisoning, and environ-
7
In industry, sulfuric acid/chromic anhydride (or sodium mental pollution. Therefore, it is of great practical significance
dichromate) is used to oxidize 2-methylnaphthalene to synthe- to search for a mild, clean and efficient 2-methylnaphthalene
size 2-methyl-1,4-naphthoquinone, but this method produces a liquid–solid phase oxidation catalyst.
lot of waste water containing chromium, which causes serious
Due to the unique physical and chemical properties of
3
environmental pollution. So, in recent years, a large number of carbon materials, their application in the field of catalysis
studies have been carried out on non-chromium heterogeneous has attracted the attention of many scholars in recent years.
catalytic systems. And various catalysts have been developed for An increasing number of carbon materials had been developed
8
9
the liquid–solid phase method, for example, molecular sieves, and applied as catalysts. Hiroyuki Watanabe had used acti-
4
ionic liquids, and metal oxides. Zi had studied the catalytic vated carbon as a raw material to prepare nitrogen-doped
performance of lanthanum doped MCM-41 as a catalyst for the carbon materials and tested their catalytic performance
preparation of 2-methyl-1,4-naphthoquinone by the liquid–solid through the catalytic oxidation of several alcohols. The results
showed that nitrogen-doped activated carbon was catalytically
1
0
School of Chemical Engineering, Zhengzhou University, Zhengzhou, China
active in partial alcohol oxidation. Eldar Zeynalov had used
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Scheme 1 Schematic diagram of catalytic oxidation of 2-methylnaphthalene.
CNTs as catalysts to catalyze the oxidation of hydrocarbons and reagents purchased from Comeo reagents. SBA-15 is purchased
1
1
2 2
explored the effects of carbon nanotube impurities. Song had from Xfnano company, and H O (mass score 30%) is purchased
suggested that the surface defects of carbon materials may be from Luoyang Reagent Factory. The chemicals were used as
active sites. H O molecules can be adsorbed on the defect sites received without any further purification.
2
2
on the surface of carbon tubes and decompose into active
oxygen molecules. The presence of heteroatoms can cause 2.2. Preparation of carbon materials
surface defects in carbon materials, which determines them to
2
.2.1. Synthesis of the thiourea-type benzoxazine. The
be multifunctional carbon-based metal-free catalysts. Doping of
heteroatoms is an effective way to enhance the catalytic perfor-
benzoxazine is synthesized using the solvent method as follows:
thiourea and toluene were added together to the three-necked
flask and the three-necked flask was stirred in an oil bath, while
formaldehyde was added. After stirring for 1 h, the phenol
dissolved in toluene was added. After half an hour, the
temperature of the oil bath was raised to 115 1C, and the
condensation was refluxed for 6 h. When the reaction was
12–14
15
mance of carbon materials.
Su had notably improved the
catalytic activity of carbon materials through doped heteroatoms.
And a large number of reports have indicated that heteroatom-
doped carbon materials can exhibit excellent catalytic performance
16–21
in many liquid-phase reactions.
So far, most of the carbon-based metal-free catalysts are
used in the field of clean and sustainable energy, but few
studies have applied them in the chemical synthesis area.
In view of the status quo of the oxidation of 2-methylnaphthalene
to a 2-methyl-1,4-naphthoquinone catalytic system described above,
in combination with the good performance of carbon materials
in different oxidation reactions, we attempted to apply cheap
and readily available carbon materials to the oxidation of
ꢀ1
completed, the washing with 1 mol L NaOH solution and
suction filtration were performed. Finally, the filtrate was dis-
tilled off. The product was poured into a beaker and dried under
vacuum at 65 1C for 24 h. The formula of the thiourea-type
benzoxazine is shown in Fig. 1.
2.2.2. Synthesis of the aniline-type benzoxazine. The synth-
esis of the aniline-type benzoxazine is the same as 2.2.1, except
that thiourea has been replaced by aniline. The formula of the
aniline-type benzoxazine is shown in Fig. 2.
2-methylnaphthalene to form a 2-methyl-1,4-naphthoquinone
catalytic system, to explore the application of carbon material
catalysts in traditional chemical synthesis.
2.2.3. Preparation of carbon materials. In this experiment,
both carbon materials were prepared using a hard template
method. Certain amount of the benzoxazine monomer was
placed in a vial and dissolved with an appropriate amount of
tetrahydrofuran. And an equal amount of SBA-15 was added
and immersed for about 5 h. Throughout the immersing
process, the temperature was controlled at 30–35 1C. Then,
the obtained solid mixture was cured at 120 1C, 140 1C, 160 1C,
80 1C, 200 1C and 220 1C for 2 h, respectively. The cured
mixture was carbonized under the N
00 1C for 2 h, respectively with the heating rate 2 1C min
Polybenzoxazines (PBZs), a new type of heterocyclic poly-
mers, are known as promising carbon precursors for high-
performance nitrogen doped carbon materials, because of their
nitrogen content, molecular design flexibility, high char yield
and excellent thermal stability. Due to the good molecular
design flexibility, the incorporation of heteroatoms into the
carbon framework can be achieved by selecting phenols or
primary amines containing heteroatoms to synthesize benz-
1
2
atmosphere at 400 1C and
2
2
oxazine monomers.
ꢀ1
8
.
In this study, the different types of benzoxazine (BOZ) were
used as the precursor, and the nitrogen-doped and N/S co-
doped carbon materials were prepared using the hard template
method. Two carbon materials were used as catalysts in the
oxidation of 2-methylnaphthalene and the main influencing
factors were investigated.
After carbonization, 50% HF was used to etch away the tem-
plate agent and washed with deionized water until it becomes
neutral. The carbon materials were dried at 80 1C. Carbon
materials with thiourea-type benzoxazine and aniline-type
benzoxazine as precursors are, respectively, named the N/S
2. Experimental
2.1. Materials
Thiourea, aniline, phenol, formaldehyde solution (mass score
3
7%), toluene, acetic acid, and tetrahydrofuran were all analytical Fig. 1 Formula of the thiourea-type benzoxazine.
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Paper
3
. Results and discussion
3.1. FTIR spectra of benzoxazines
Fig. 3(a) shows the FTIR spectra of thiourea-type benzoxazine.
ꢀ1
It can be seen from Fig. 3(a) that 757 cm is the out-of-plane
bending vibration absorption peak of the benzene ring C–H,
Fig. 2 Formula of the aniline-type benzoxazine.
ꢀ
1
and 933 cm
is the characteristic absorption peak of the
ꢀ
1
ꢀ1
oxazine ring. The peaks at 1234 cm and 1365 cm are the
stretching vibration absorption peaks of the C–O–C and C–N–C
co-doped carbon material (NSCM) and nitrogen-doped carbon
material (NCM) in this paper.
ꢀ
1
bonds on the oxazine ring, respectively. 1595 cm
is the
characteristic absorption peak of the benzene ring skeleton.
Fig. 3(b) shows the FTIR spectra of aniline-type benzoxazine.
The out-of-plane bending vibration absorption peak of benzene
2
.3. Characterization of materials
The two types of benzoxazines were characterized by FTIR (FT-
IR200, America). The carbon materials were characterized using
the N adsorption/desorption test (Autosorb-iQ, America), XRD (D8
Advance, Germany), SEM (JSM-7500F, Japan), TEM (TalosF200S,
America), and XPS techniques (AXIS Supra, Japan).
ꢀ
1
ꢀ1
ring C–H is 754 cm . 945 cm is the characteristic absorption
2
ꢀ1
ꢀ1
peak of the oxazine ring. The peaks at 1156 cm and 1227 cm
are the symmetric and asymmetric stretching vibration absorp-
ꢀ1
tion peaks of C–O–C on the oxazine ring, respectively. 1371 cm
is the C–N–C stretching vibration absorption peak on the oxazine
ring. 1496 cm and 1597 cm are the characteristic absorption
peaks of the benzene ring skeleton. The characterization
results have confirmed the successful synthesis of two types of
benzoxazines.
2.4. Experiment for 2-methylnaphthalene catalytic
ꢀ
1
ꢀ1
oxidation
In this experiment, acetic acid was used as a reaction solvent
and hydrogen peroxide as an oxidant. The reaction was carried
out in a 50 mL three-necked flask by adding a certain amount
of raw materials and carbon material catalysts, and simulta-
neously adding acetic acid. H O was added dropwise after the
reaction mixture was heated to the set temperature. After
the reaction, the temperature was naturally lowered and the
catalyst was filtered under reduced pressure. The products
were analyzed by Agilent 7890B gas chromatography, and the
yield of 2-metaphthoquinone is used as the evaluation index
3
.2. N adsorption/desorption of carbon materials
2
2
2
2
Fig. 4 shows the N adsorption/desorption isotherm and BJH
pore size distribution plots of two carbon materials. The
adsorption/desorption isotherms of both carbon materials are
typical IV adsorption/desorption isotherms, which indicates the
2
3
mesoporous structure of carbon materials. For the nitrogen-
2
ꢀ1
doped carbon material, the specific surface area is 674.04 m g
,
0
of catalyst performance. The n (C11H10) and n(C11H10) are
3
ꢀ1
pore volume is 1.02 cm g , and average pore size is 6.1 nm. For
the N/S co-doped carbon material, the specific surface area is
the amounts of 2-methylnaphthalene before and after the
reaction. The amount of 2-menadione is represented by
n(C H O ). The conversion of 2-methylnaphthalene and the
selectivity of 2-methylnaphthoquinone were calculated as
follows:
2
ꢀ1
3
ꢀ1
8
86.19 m g , the pore volume is 1.6 cm g , and the average
1
1
8 2
pore size is 7.2 nm.
3.3. X-ray diffraction of carbon materials
n0ðC11H10Þ ꢀ nðC11H10Þ
XC11H10 ð%Þ ¼
SC11H O ð%Þ ¼
ꢁ 100%
Fig. 5(a) shows small-angle XRD diffraction patterns of the
NSCM. The small-angle XRD pattern of SBA-15 shows three
well-resolved diffraction peaks associated with six-symmetric
n0ðC11H10Þ
nðC11H8O2Þ
(
100), (110) and (200) reflections, indicating that SBA-15
ꢁ 100%
8
2
2
4
n0ðC11H10Þ ꢀ nðC11H10Þ
has a well-ordered structure. The small-angle XRD pattern
Fig. 3 (a) FTIR spectra of thiourea-type benzoxazine; (b) FTIR spectra of aniline-type benzoxazine.
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Fig. 4 (a) Nitrogen adsorption–desorption isotherms and (b) pore size distribution curves.
Fig. 5 (a) Small angle XRD pattern of SBA-15 and thiourea-NSCM; (b) wide angle XRD pattern of the thiourea-NSCM.
of the NSCM shows a similar (100) diffraction peak around
2
y = 0.81, which indicates that the order of the pores of the
material is reduced compared with SBA-15. The wide-angle
XRD pattern of the NSCM is shown in Fig. 5(b), and two
diffraction peaks were observed near 2y = 251 and 2y = 441,
which belong to the diffraction of the (002) and (100) planes of
the hexagonal carbon, respectively. The (002) diffraction peak
indicates that the lattice spacing of the graphite layer has low
crystallinity, and the (100) diffraction peak indicates that the
2
5
sample has a graphitized amorphous structure.
3.4. SEM and TEM images of carbon materials
The morphology and microstructure of carbon materials were
characterized by SEM and TEM. Fig. 6(a) and (b) are SEM
images of NCM and NSCM. It can be seen from the SEM
images that both carbon materials exhibit a worm-like struc-
ture, which indicates that the carbon material partially
replicates the structure and morphology of the template.
Fig. 6(c) and (d) are TEM images of NCM and NSCM. As can
be seen that both carbon materials are uniform stripe-like
and hexagonal arranged structures, which indicates that the
carbon materials have an ordered structure. This further
Fig. 6 SEM images of carbon materials and TEM images of carbon materials.
3.5. XPS spectra of carbon materials
proves that the carbon material was successfully copied from Fig. 7 shows the elemental composition of NCM obtained by
2
6
the template.
XPS characterization. There are three absorption peaks in NCM: C
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Fig. 7 (a) XPS spectra of NCM; high resolution XPS spectra of N 1s (b) and O 1s (c).
Table 1 Elemental content of the NCM sample
pyridine-N and pyrrole-N, in the carbon materials. Unsaturated
nitrogen will make carbon materials produce surface defects
that make carbon materials have certain catalytic activity.
Sample
NCM
C (%)
89.2
N (%)
4.95
O (%)
5.85
3
0
Compared with NCM, NSCM contains more unsaturated nitro-
gen, and the addition of sulfur will form a synergistic effect
with nitrogen atoms. This effect is beneficial to increase the
3
1–33
(285 eV), N 1s (399.3 eV), O 1s (531.3 eV), and the contents of each surface defects of carbon materials.
Therefore, the N/S
element are listed in Table 1. Nitrogen exists in two forms: co-doped carbon materials have higher catalytic activity.
pyridine-N (397.8 eV) and graphite-N (400.3 eV), and oxygen exists
3
.6. Raman analysis of carbon materials
in four forms: quinone oxygen (530.6 eV), CQO (531.3 eV), C–O–C
532.2 eV) and O–CQO (533 eV). The calculated results show that Fig. 9 shows the results of Raman analysis of carbon materials.
pyridine-N in NCM accounted for 51.3% of the total nitrogen The surface defect ratio of carbon materials is expressed by
(
3
4
content.
I
D
/I
G
,
D G
and a higher I /I value suggests the presence of more
Fig. 8 shows the elemental composition of NSCM obtained structural defects. According to the calculation of the analysis
by XPS characterization. There are four absorption peaks in results, the surface defect ratio of NCM is 0.95, and the surface
NSCM: C 1s (285 eV), N 1s (399.3 eV), S 2p (164.7 eV), O 1s defect ratio of NSCM is 1.12. Therefore, NSCM has a higher
(531.3 eV), and the contents of each element are listed in ratio of surface defects and higher catalytic activity, which also
Table 2. Nitrogen exists in three forms: pyridine-N (397.8 eV), corresponds to XPS analysis. The graphite crystallite sizes of
3
5
pyrrole-N (399.1 eV), graphite-N (400.3 eV), sulfur exists in two the NCM and NSCM that were calculated using the formula
forms: C–S bonds (163.4 eV, 164.6 eV) of thiophene-like sulfur are 20.2 nm and 17.2 nm, respectively.
2
7
and C–SOx–C bonds (167.5 eV), oxygen exists in two forms:
quinone oxygen (530.6 eV) and C–O–C (532.2 eV). We also
calculated from Fig. 8(b) that pyridine-N and pyrrole-N together
account for 68.2% of the total nitrogen content of NSCM.
3.7. Catalytic performance
3.7.1. Catalytic performance of NSCM catalysts. In this
section, the catalytic performances of NSCM catalysts were
on carbon catalysts have shown that the surface systematically studied. The effect of reaction temperature
2
8,29
Studies
defects of carbon materials may be their active sites. The XPS on the progress of the reaction is shown in Fig. 10. At lower
analysis has shown that there are two unsaturated nitrogen, temperatures, the reaction proceeds more slowly. With the
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Fig. 8 (a) XPS spectra of NSCM; high resolution XPS spectra of N 1s (b), S 2p (c) and O 1s (d).
Table 2 Elemental content of the NSCM sample
The effect of reaction time on the yield of the target product
was studied. As shown in Fig. 11. As the reaction time increases,
the conversion rate increases, while the selectivity gradually
decreases. This may be due to the increase of supernumerary
products which reduces selectivity. Considering the effect of
reaction time on both conversion and selectivity, the best
Sample
NSCM
C (%)
69.13
N (%)
19.16
O (%)
10.71
S (%)
1
increase of temperature, the reaction has gradually acceler- reaction time of the N/S co-doped carbon material is 4 h.
ated, but the selectivity begins to decrease, due to the increase The effect of acetic acid has been explored experimentally.
of inside reactions. Considering the effect of temperature on Fig. 12 shows the effect of acetic acid on the conversion and
both conversion and selectivity, 85 1C is chosen as the selectivity over NSCM catalysts. As the amount of acetic acid
suitable reaction temperature of the N/S co-doped carbon increases, the conversion rate increased first and then
material.
decreased, while the selectivity increased first, then decreased;
Fig. 9 (a) Raman analysis of NCM, (b) Raman analysis of NSCM.
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Fig. 10 Effect of reaction temperature on the conversion and selectivity
over NSCM. Reaction conditions: reaction time, 4 h; the mass ratio of
Fig. 12 Effect of acetic acid on the conversion and selectivity over NSCM.
Reaction conditions: reaction temperature, 85 1C; reaction time, 4 h; the
2 2
acetic acid to the raw materials, 26 : 1; the mass ratio of H O to the raw
mass ratio of H
2 2
O to the raw materials, 8 : 1; the mass ratio of the raw
materials, 8 : 1; the mass ratio of the raw materials to catalysts, 3 : 1.
materials to catalysts, 3 : 1.
Fig. 11 Effect of reaction time on the conversion and selectivity over
NSCM. Reaction conditions: reaction temperature, 85 1C; the mass ratio of
2 2
Fig. 13 Effect of H O on the conversion and selectivity over NSCM.
Reaction conditions: reaction temperature, 85 1C; reaction time, 4 h; the
mass ratio of acetic acid to the raw materials, 26 : 1; the mass ratio of the
raw materials to catalysts, 3 : 1.
acetic acid to the raw materials, 26 : 1; the mass ratio of H
2 2
O to the raw
materials, 8 : 1; the mass ratio of the raw materials to catalysts, 3 : 1.
this may be due to the fact that too much acetic acid reduces was changed. Fig. 14 shows the effect of the mass ratio of raw
the concentration of reactants. Considering the yield for the materials on the catalytic activity of the N/S co-doped carbon
N/S co-doped carbon material, 26 : 1 is selected as the best material. The conversion rate is on the rise, and selectivity
condition for the mass ratio of acetic acid to raw materials.
begins to decline after reaching its peak. And 3 : 1 is the best
Fig. 13 shows the effect of hydrogen peroxide on the catalytic mass ratio of raw materials to the catalysts for the N/S co-doped
performance of the N/S co-doped carbon material. As the carbon material.
quality of hydrogen peroxide increases, the conversion rate
3.7.2. Catalytic performance of the NCM. The catalytic
shows the trend of rising first and then falling, while the performance of the nitrogen-doped carbon material was inves-
selectivity has been decreasing. As the quality of hydrogen tigated through the same scheme as the N/S co-doped carbon
peroxide further increases, the formation of supernumerary material. The best reaction conditions for the nitrogen-doped
products increases the conversion rate. The optimal mass ratio carbon material obtained from experiments are reaction tem-
of H O2 to the raw materials of the N/S co-doped carbon perature: 83 1C, reaction time: 5 h, the mass ratio of acetic acid
2
material is 8 : 1.
2 2
to raw materials: 33 : 1, the mass ratio of H O to raw materials:
Finally, the experiment has also explored the effect of the 10 : 1, and the mass ratio of raw materials to catalysts: 4 : 1. The
mass ratio of raw materials. The quality of the catalyst is kept yield of 2-methylnaphthoquinone under optimal reaction con-
constant during the experiments and the mass of the reactants ditions is 65.8%. Compared to the two carbon materials, it can
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have shown that multi-atom doping is more effective for
improving the catalytic performance of carbon materials than
single-atom doping. The obtained NCSM carbon materials also
have good recycling properties and can maintain high catalytic
activity after repeated use. We can also know from the study of
this paper that compared with some metal catalysts for the
2-methylnaphthalene solid–liquid selective catalytic oxidation
reaction, carbon catalysts can obtain higher product yield
under relatively mild reaction conditions, and they have
relatively steady catalytic performance. We hope that the
carbon materials based on benzoxazine should have great
potential for advanced applications in the traditional field of
chemical heterogeneous catalytic synthesis.
Fig. 14 Effect of the mass ratio on the conversion and selectivity over Conflicts of interest
NSCM. Reaction conditions: reaction temperature, 85 1C; reaction time,
4
h; the mass ratio of acetic acid to the raw materials, 26 : 1; the mass ratio There are no conflicts to declare.
2
of H to the raw materials, 8 : 1.
O
2
Acknowledgements
Table 3 Results of reusability tests
This work was supported by the National Natural Science
Foundation of China (Grant No. 51703205) and the China
Scholarship Council (CSC).
Reusability test
Conversion/%
Selectivity/%
Yield/%
1
2
3
4
5
95.51
94.96
91.97
93.13
92.3
82.11
81.93
84.83
84.65
85.75
78.42
77.80
78.02
78.83
79.15
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