A green approach for the preparation of a surfactant embedded sulfonated carbon catalyst towards glycerol acetalization reactions

Article abstract of DOI:10.1039/d0cy00336k

The green synthesis of heterogeneous catalysts often requires a solid-state reaction pathway. In this work, a cationic surfactant (CTAB) embedded sulfonated carbon catalyst was prepared via a sustainable route with the aim of having controlled surface hydrophobicity and acidity for glycerol acetalization reactions. The main objective of this study was to tune the hydrophobicity and acidic site density, either via adding a cationic surfactant or changing the carbon to sulphur ratio. The as-synthesized catalyst was characterized via XRD, N2 adsorption/desorption, SAXS, FESEM, FTIR, pyridine-IR, high-temperature DR-FTIR, TGA, 13C-NMR, Raman, and XPS techniques. The incorporation of a cationic surfactant (CTAB) reduces the surface area but increases the acidic site density to a greater extent. The bonding between the surfactant (CTAB) and surface hydroxyl groups was elucidated via XPS analysis. DR-FTIR studies implied that the -SO3H groups are strongly bonded to the carbon network, while the lower amount of water mass loss seen from TGA studies showed the substantial improvement in surface hydrophobicity after modification with the surfactant. Moreover, the combination of acidic site density and hydrophobicity played a key role in attaining around 90% glycerol conversion and 98% solketal selectivity under ambient conditions. Notably, characterization of the used catalyst revealed that the loss of activity is mainly related to a drop in hydrophobicity, which occurs due to the loss of surfactant during washing with methanol.

Full text of DOI:10.1039/d0cy00336k

View Article Online
View Journal
Catalysis
Science &
Technology
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: A. Ghosh, A.
Singha, A. Auroux, A. DAS, D. Sen and B. Chowdhury, Catal. Sci. Technol., 2020, DOI:
1
0.1039/D0CY00336K.
Volume
Number
7
2
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
21
January 2017
Pages 313-534
Catalysis
Science &
Technology
rsc.li/catalysis
Accepted Manuscripts are published online shortly after acceptance,
before technical editing, formatting and proof reading. Using this free
service, authors can make their results available to the community, in
citable form, before we publish the edited article. We will replace this
Accepted Manuscript with the edited and formatted Advance Article as
soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes to the
text and/or graphics, which may alter content. The journal’s standard
Terms & Conditions and the Ethical guidelines still apply. In no event
shall the Royal Society of Chemistry be held responsible for any errors
or omissions in this Accepted Manuscript or any consequences arising
from the use of any information it contains.
ISSN 2044-4761
PAPER
Chaoqiu Chen, Yong Qin et al.
Highly dispersed Pt nanoparticles supported on carbon
nanotubes produced by atomic layer deposition for hydrogen
generation from hydrolysis of ammonia borane
rsc.li/catalysis

Page 1 of 47
Catalysis Science & Technology
View Article Online
DOI: 10.1039/D0CY00336K
A green approach for preparation of surfactant embedded sulfonated carbon
catalyst towards glycerol acetalization reaction†
Anindya Ghosh, Aniruddha Singha, Aline Auroux, Avik Das, Debasis Sen,c,d and Biswajit
a
a
b
c,d
Chowdhury*,a
a
Department of Chemistry, Indian Institute of Technology (Indian School of Mines), Dhanbad,
Jharkhand-826004, India.
b
Institut de recherches sur la catalyse et l'environnement de Lyon (IRCELYON), UMR 5256
CNRS- Université Lyon1, 2 avenue Albert Einstein, 69626, Villeurbanne Cedex, France.
c
Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai-400085, India.
d
Homi Bhabha National Institute, Mumbai-400094, India.
*
Corresponding author. Tel.: (+91)-326-2235663; Fax.:(+91)-326-2296563; E-mail:
biswajit72@iitism.ac.in.
Abstract: The green synthesis of heterogeneous catalysts often required a solid-state reaction
pathway. In this work, the cationic surfactant (CTAB) embedded sulfonated carbon catalyst was
prepared by a sustainable route aiming to have a controlled surface hydrophobicity and acidity for
glycerol acetalization reaction. The main objective of this study was to tune of hydrophobicity and
acidic site density either by adding a cationic surfactant or by changing the carbon sulphur ratio.
The as-synthesized catalysts were characterized by XRD, N adsorption/desorption, SAXS,
2
1
3
FESEM, FTIR, Pyridine-IR, High-temperature DR-FTIR, TGA, C-NMR, Raman, and XPS
techniques. The incorporation of cationic surfactant (CTAB) reduces surface area but induced
acidic site density to a greater extent. The bonding between surfactant (CTAB) and surface
hydroxyl groups was elucidated by XPS analysis. The DR- FTIR implied that the -SO H groups
3
1

Catalysis Science & Technology
Page 2 of 47
View Article Online
DOI: 10.1039/D0CY00336K
are strongly bonded to the carbon network while the less water mass loss in TGA showed the
substantial improvement of surface hydrophobicity after modification with the surfactant.
Moreover, the combination of acidic site density and hydrophobicity are playing a key role in
attaining around 90 % glycerol conversion and 98% solketal selectivity under ambient conditions.
Notably, the characterizations of the used catalyst revealed that the loss of activity is mainly related
to the drop of hydrophobicity which occurs due to the loss of surfactant during washing with
methanol.
Keywords: Carbon, solid acid, hydrophobicity, glycerol, acetalization, solketal.
1
. Introduction
Nanomaterials have been extensively used for the catalytic transformation of various reactions
since the last decade. However, some inherent limitations such as, limited reserves, high cost,
modification in structural integrity with time, leaching of active species, etc. restrict their
applications on an industrial scale. In moving forward on green and sustainable chemistry,
preparation of carbonaceous material through thermal treatment (pyrolysis) of residual biomass
has attracted extensive attention worldwide to decrease the “carbon footprint” of a biomass
1
transformation process. The carbon materials have indeed recognized as a great deal of interest in
numerous research field because a wide variety of materials (amorphous carbon, graphene oxide.
carbon fiber, activated carbon, carbon nanotube, carbon dots, fullerene, carbon black, etc.) can be
prepared with versatile physicochemical properties.2, 3, 4 The carbon materials can be used as
heterogeneous catalysts for a wide range of reactions such as biomass conversion, electro-
catalysis, organic transformations, photochemical reaction because of their diverse porous
structure, chemical inertness, tunable electrical properties, thermal conductivity, easy accessibility,
low density, surface hydrophobicity, and easy surface functionalization method.5, 6
2

Page 3 of 47
Catalysis Science & Technology
View Article Online
DOI: 10.1039/D0CY00336K
The unique surface chemistry of the carbon materials control their catalytic activity towards
various reaction. Till now, several methodologies have been adopted to modify the surface. The
oxidation via chemical route is one of the most convenient ways for the introduction of oxygen
functionalities on the carbon surface. In these cases, carboxyl, lactone, phenol carbonyl, quinone
6
as well as aromatic C-H can be observed at the edge or defect sites. Such functional groups have
a positive impact on their ability to form dispersion in polar solvent through reduction of
hydrophobicity but an adverse effect is observed in the biomass transformation process where
water is commonly used. Furthermore, oxidation requires extremely harsh chemical conditions
(
HNO ) which suffers due to the sustainable issue and change in textural and structural integrity
3
of carbon materials. Rodigues et al. reported that the catalytic activity of Au/AC catalyst towards
glycerol oxidation is highly dependent on the amount of oxygenated groups on the surface of the
7
carbon support. Besides, incorporation of heteroatoms (N, P, O, S) influence the electrochemical
and catalytic performances either through generation of acid-base or redox sites or through
hydrophilic character.8, 9
In recent years, sulfonated porous carbon materials are fulfilling several promises as a versatile
acid catalysis and thus bear significant attention in the production of fuels and chemicals.10-17
Kitano et al. reported a high surface area sulfonated carbon catalyst prepared by the addition of
1
8
ZnCl with cellulosic precursor before activation and sulfonation. However, Wang et al.
2
proposed a templated assisted route where a microporous hollow sphere structure is the reason
1
9
behind the high surface area of the carbon material. Usually, carbonization and sulphonation
under harsh oxidative condition i,e in presence of fuming H SO are the two steps involved in the
2
4
preparation of sulfonated carbonaceous materials.20, 21 In this case carbonization can be achieved
either low temperature (473-573 K) or high temperature (573-1073 K) hydrothermal treatment but
3

Catalysis Science & Technology
Page 4 of 47
View Article Online
DOI: 10.1039/D0CY00336K
the low-temperature hydrothermal treatment is more advantageous from a green chemistry
perspective.5, 22 In the sulfonation step, a large number of -SO H groups are incorporated along
3
with the generation of -COOH and -OH groups through oxidation of aliphatic CH /CH . Therefore,
3
2
sulphonated carbonaceous material possesses predominantly Brønsted acidic sites with Hammett
1
acid strength (H ) in the range of -8 < H ≤ -11. In addition, sulphonation provides stability through
0
0
2
3
electron-withdrawing attraction between the aromatic ring and sulphonic acids. The physical
properties of carbonized materials are mostly unaffected by hydrolytic effects in aqueous medium
as compared to sulphonated carbon materials because they mainly consist of randomly arranged
polycyclic aromatic carbon sheets whereas later bears a larger number of hydrophilic functional
groups. Thus, the sulfonated carbonaceous catalyst suffers severely in an aqueous medium because
the hydrophilic character favours the diffusion of water into the pores, which weakens the acid
2
4
strength and finally forces the reaction to deactivate. Souza et al. reported that the incorporation
of surfactant leads to increase in surface hydrophobicity but there is a chance to decrease acidity
2
5
due to the bonding of surfactant with surface hydroxyl groups. Therefore, a simple, sustainable
preparation method for sulfonated carbon catalysts with controlled surface hydrophobicity and
high acidic sites remains as a “holy grail” for the acid-catalyzed transformation, producing water
as a co-product.
Glycerol is obtained as a by-product (10 % w/w mass) during the production of biodiesel, hence a
lot of biodiesel refineries are economically suffering due to the overabundance of glycerol. Thus,
searching and executing a new feasible process for the conversion of glycerol into value-added
chemicals is the key point of interest to build up the economic benefit from biodiesel. A large
number of catalytic processes have been adopted for the up-gradation of glycerol to fine chemicals
such as dehydration, oxidation, etherification, acetalization, hydrogenolysis, esterification, trans-
4

Page 5 of 47
Catalysis Science & Technology
View Article Online
DOI: 10.1039/D0CY00336K
esterification, pyrolysis, gasification, carboxylation, carbonylation steam reforming,
oligomerization and polymerization.26, 27 Acetalization of glycerol with aldehyde or ketone has
gained considerable attention in catalysis research as they have various potential applications in
several industries such as bases, scents, solvents, surfactants, flavours, protecting agents,
fragrance, detergent, fuel additive, and pharmaceutical.28, 29 Particularly, glycerol acetalization
with acetone yields cyclic isomeric ketals (Scheme 1). The five-membered solketal ((2,2-
dimethyl-1,3-dioxolan-4-yl)methanol) is the major one and six-membered compound is the minor
one (2,2-dimethyl-1,3-dioxan-5-ol). Solketal is extensively used as an additive in the formulation
of gasoline, diesel, and biodiesel because it reduces gum formation and increases octane rating by
increasing anti-knocking property.25, 30 Generally, these oxygenated constituents blended with
standard diesel fuel to reduce the uncontrolled emission by the combustion of hydrocarbons,
carbon monoxide, particles and aldehydes.28, 31 It can also be used for improving viscosity and cold
flow properties of biodiesel and achieving require flash point and oxidation stability for long term
biodiesel storage.28
Scheme 1. Acetalization of glycerol with acetone to solketal (2,2-dimethyl-1,3-dioxolan-4-
yl)methanol and 2,2-dimethyl-1,3-dioxan-5-ol.
5

Catalysis Science & Technology
Page 6 of 47
View Article Online
DOI: 10.1039/D0CY00336K
Several theoretical and experimental studies were attempted to find out the reasons behind the high
selectivity of solketal. It was observed that the selective solketal formation takes place over
Brønsted acidic sites via short-lived tertiary carbenium ion intermediate. However, Lewis acidic
sites favours Meerwein–Ponndorf–Verley reduction/Oppenauer oxidation reaction pathway for the
formation of the same product.28, 32 Till now, various heterogeneous catalysts have been reported
3
2
30
on glycerol acetalization including mesostructured silica, sulfonic acids functionalize silica,
zeolites,33 supported heteropolyacid,34 Aluminium Triflate Grafted MCM-41,35 niobia and
modified niobium oxyhydroxide,25, 36 promoted SnO catalysts,
28, 37
promoted zirconia, sulfonic
38
2
3
9
31
acid functionalize activated carbon, sulfonated polymer. The abovementioned catalysts are
efficient in terms of activity but they still are not good enough from the catalysis point of view
because of their lengthy catalyst preparation techniques, high reaction temperature, poor
regeneration ability, use of costly, corrosive as well as toxic reagent. Thus, the development of
sustainable, economical, reusable, and efficient catalyst is highly appreciated for glycerol
acetalization reaction.
As we know that the acetalization reaction is reversible and water is formed during
reaction so the synthesis of hydrophobic catalyst is highly desirable to reduce the interaction of
acidic sites with water molecules. Therefore, many processes have been employed to hinder the
reversibility of the reaction. Bruchmann et al. 40 have introduced a technique in which excess
acetone is continuously distilled off to increase the glycerol conversion, whereas Deutsch et al. 41
employed several solvents (benzene, toluene, chloroform, DCM) to increase the glycerol
conversion but these types of processes are not very much efficient and environment friendly.
Hence, scientists are preparing hydrophobic catalysts by modifying chemical composition for
3
3
30
example zeolite with higher Si/Al value, sulfonic acid functionalized mesostructured silica,
6

Page 7 of 47
Catalysis Science & Technology
View Article Online
DOI: 10.1039/D0CY00336K
surfactant modified niobium oxyhydroxide.25 Recently, Gonçalves et al. reported sulfonated
carbonaceous catalysts by carbonization of glycerine with H SO in a single step and they are
2
4
4
2
controlled hydrophobicity by varying the sulphur content. However, not only these processes
involve highly corrosive chemical (H SO ) but also require extended workup procedure to recover
2
4
the final catalyst along with the generation of a large amount of acid waste. Therefore, a simple
sustainable hydrophobic solid acid catalyst will be more convenient for the reduction of the
interaction of acidic sites with water molecules in the acetalization reaction.
Aim at sustainable process, herein we have carried out a solventless synthesis of surfactant
embedded sulfonic acid functionalized carbonaceous catalyst by simple physical mixing of CTAB,
glucose, and p-toluenesulfonic, followed by thermal treatment at a lower temperature. To date,
there is not any report for the preparation of surfactant modified hydrophobic sulfonated carbon
catalyst. The catalyst prepared by the eco-friendly process was tested for glycerol acetalization
reaction with acetone under solvent-free and ambient conditions. Previously, it has been explained
that along with acidity, hydrophobicity is also beneficial for the acetalization reaction.30, 39 Earlier
report also demonstrated that the hydrophobicity of a catalyst is improved by decreasing the
4
2
amount of sulfonating agent. So here also we prepared another two catalysts with two different
carbon sulphur ratios without surfactant. The physicochemical properties of these catalysts were
characterized based on various techniques including XRD, BET, FESEM, FTIR, Pyridine IR, high-
1
3
temperature DR-FTIR, Raman spectroscopy, TGA, CP MAS C-NMR and XPS. In addition, the
bulk structural correlation in the mesoscopic length scale of the catalysts were probed using small-
angle X-ray scattering (SAXS). Synchrotron-based X-ray source was used to map the electron
density fluctuation into reciprocal space to extract important structural parameters. The role of
surface hydrophobicity and acidic sites on catalytic activity was investigated along with the
7

Catalysis Science & Technology
Page 8 of 47
View Article Online
DOI: 10.1039/D0CY00336K
investigation of several reaction parameters. Particularly, the surfactant modification enhances
glycerol conversion due to the increase in hydrophobicity and surface acid density. Moreover, a
special attention was given based on several characterization techniques to find out the reason for
decreasing the activity of the best catalyst in each catalytic cycle.
2
2
. Experimental section
.1. Materials and reagents
Glycerol (99.5%) was purchased from Alfa Aesar. Glucose (AR grade), p-toluenesulfonic acid
AR grade), and acetone (99.8%) were purchased from Sisco Research Laboratories. The cationic
(
surfactant, CTAB(N-Cetyl-N, N, N-Trimethyl Ammonium Bromide, 98%) was bought from Loba
Chemie. All materials were used as received without any further purification.
2
.2.
Catalyst preparation
The surfactant incorporated carbon-based solid acid catalyst was prepared by the physical
mixing of glucose, p-toluenesulfonic acid, and CTAB with a molar ratio of 1:1:0.18. Then the
mixture was transferred in a 50mL Teflon-sealed autoclave and placed at 453 K in a furnace for
2
4 h. The synthesized black material was ground into a fine powder and washed with water (1g/0.5
L) to remove the unreacted sulfonic acid. Finally, the obtained material was dried at 353 K for 12
h and denoted as GS-SO3H. To compare the catalytic activity, two carbon-based solid acid
4
3
catalysts were prepared without surfactant by adopting the procedure reported elsewhere. In this
method, two different molar ratios (1:1& 1:0.5) of glucose and p-toluenesulfonic acid have taken
and applied the same sample preparation procedure. These samples have designated as G-SO3H
th
and GL-SO3H. However, after the 4 recyclability test, GS-SO3H is denoted as GSR-SO3H.
2
.3.
Catalyst characterizations
2
.3.1. Powder X-ray diffraction (PXRD):
8

Page 9 of 47
Catalysis Science & Technology
View Article Online
DOI: 10.1039/D0CY00336K
The structural information of carbon catalysts was investigated using Bruker D8 diffractometer
connected with nickel filtered Cu Kα radiation source under operating voltage 40 kV in the 10-800
region.
2
.3.2. The N adsorption/desorption analysis:
2
The N adsorption/desorption analysis was performed on a Quantachrome NOVA 3200e to
2
calculate the BET surface area in the relative pressure (P/P ) 0.05-0.3 region. Before the
0
experiment, the sample was degassed at 408 K for 4 h. The BJH method was applied to calculate
the pore size and pore volume of these catalysts.
2
.3.3. Field Emission Scanning Electron Microscope (FESEM)
The FESEM images were captured using Supra 55, Carl (Zeiss, Germany) microscope by
spreading the catalysts on carbon tape.
2
.3.4. Small-angle X-ray scattering (SAXS):
The SAXS experiments were performed using a newly developed synchrotron-based SAXS
beamline (BL-18), Indus-2 at RRCAT, Indore, India. Nearly 40-meter-long SAXS beamline has
recently installed on the 5º bending magnet (5T) port of Indus-2. A pair of Si (111) crystals in
non-dispersive mode is used to tune the X-ray energy in the range of 5 to 20 KeV with an energy
-3
resolution of 10 . A toroidal mirror of 1.5m length having 60 nm Pt and 5 nm Rh coating on
silicon substrate was used as focusing optics. The scattering signal as a function of wave-vector
transfer (q) was collected using an online image plate detector kept at ~3m distance from the
-1
sample. The radial averaged scattering intensity in q range of ~0.06 to 2.5 nm was obtained and
transmission and background corrections were performed. The wave-vector transfer (q) is
9

Catalysis Science & Technology
Page 10 of 47
View Article Online
DOI: 10.1039/D0CY00336K
4
휋푆푖푛(휃)
휆
defined as
, where 2 is the scattering angle, and  Å is the X-ray wavelength
corresponding to 12 keV of X-ray energy.
.3.5. FTIR and pyridine FTIR analysis:
The FTIR analysis was performed on a Perkin Elmer GX spectrometer the range of 400–4000
2
−
1
cm using KBr pellet. The Pyridine FTIR was carried out on an Agilent Cary 600 spectrometer
using Praying Mantis accessories at ambient conditions. Before the analysis, approximately 50
mg of sample was dried in a sample cup at 373 K for 1 h then the sample was directly contacted
with 0.1 mL of pyridine and further heated the pyridine adsorbed sample in a vacuum oven at 393
K for 1 h. After completion of heat treatment, the sample was cooled to room temperature and the
−
1
28
spectrum was recorded in the spectral region 1400-1700 cm using KBr background.
2
.3.6. Temperature-dependent DR-FTIR analysis:
The high-temperature DR-FTIR was investigated by Agilent Cary 600 spectrometer Praying
Mantis diffuse reflection accessories fitted with the HARRICK temperature controller. The
spectrum was collected by diluting the sample with KBr from room temperature to 573 K
temperature.
1
3
2
.3.7. Solid-state C-NMR analysis:
1
3
The CP MAS C-NMR was performed at room temperature on JEOL (ECX400) 400 MHz
spectrometer connected with a magic angle spin probe.
2
.3.8. Surface acidity:
The total acidity was calculated using acid-base titration using phenolphthalein indicator.
Where 250 mg sample was taken in 0.05 M 30mL NaOH solution and subsequently stirring it
1
0

Page 11 of 47
Catalysis Science & Technology
View Article Online
DOI: 10.1039/D0CY00336K
for 1 h at room temperature. After filtration of the catalyst, the supernatant solution is titrated
with 0.05 M HCl solution.
2
.3.9. X-ray photoelectron spectroscopy (XPS):
The XPS spectra were collected using a monochromatic Al K X-ray source (150 W) on
α
an Axis Ultra Dld spectrometer.
2
.3.10. CHNS/O analysis:
The CHNS/O analysis was carried out using Thermo Scientific™ FLASH 2000 CHNS/O
analyzer.
.3.11. Thermogravimetric analysis (TGA):
2
The TGA analysis was investigated using NETZSCH STA 449F3 thermal analyzer at a
temperature range from room temperature to 1073 K by flowing nitrogen gas.
2
.3.12. Raman Spectroscopy:
The Raman spectra were obtained using LabRAM HR (HORIBA, France) Spectrometer with
an excitation wavelength of 633 nm.
2
.4.
Catalytic activity testing
The catalytic glycerol acetalization reactions with acetone or other carbonyl derivatives were
performed at room temperature (303 K). Where 0.01 mol glycerol, 0.01-0.05 mol of acetone or
carbonyl compounds, 0.001 mol of 1,4-dioxane as internal standard and 5 wt% (60-80 mesh) pre-
activated catalyst (with respect to glycerol weight) were taken in a 50 mL round-bottom flask.
Then the reaction mixture was stirred through a cylindrical magnetic stirrer bar (size: 20 mm ×
8
mm) at 600 rpm for 4 h. After completion of the reaction, the catalyst was separated by
centrifugation at 7000 rpm and the reaction mixture was analyzed by gas chromatography (Agilent
1
1

Catalysis Science & Technology
Page 12 of 47
View Article Online
DOI: 10.1039/D0CY00336K
7
890 B) using the FID detector attached with capillary FFAP column (30m × 0.32 mm × 0.25 μm).
The glycerol conversion and product selectivity were calculated as per standard procedure.44
During activity study, a diffusion problem can be raised due to catalyst composition, reaction
medium, reaction temperature, particle size. As reported by Xu and Chuang 45 external diffusion
is not a rate-controlling step within the stirrer speeds of 160-760 rpm. A speed of 600 rpm was
chosen to ensure that the reaction rate is free from external diffusion influences. The catalyst has
been ground well to a fine powder in a mortar before the activity study. All the parameters like
temperature, reaction medium, solvent are kept constant during activity studies in order to avoid
internal diffusion resistance.
3
3
. Results and discussion
.1. Physicochemical characterizations
Figure 1. XRD analysis of G-SO3H, GL-SO3H, and GS-SO3H catalyst.
1
2

Page 13 of 47
Catalysis Science & Technology
View Article Online
DOI: 10.1039/D0CY00336K
The Powder XRD pattern of all three catalysts has been shown in Figure 1. The two carbon
catalysts G-SO3H and GL-SO3H, displayed two broad diffraction peaks in a certain 2θ range of
0
0 43, 46, 47
1
0-30 and 35-50 .
These peaks are indexed to (002) and (004) plane for amorphous carbon
framework where mostly the aromatic carbon sheets arranged in a random fashion. On the other
hand, surfactant incorporated carbon catalyst (GS-SO3H) showed a similar pattern with certain
0
0
0
0
crystallinity at around 18 , 20 , 22 , 25 may be due to the formation of semi-crystalline
polysaccharides like structure.48 This observation attributed to the incomplete carbonization,
occurred during hydrothermal heat treatment at 453 K for 24 h.
Figure 2. N adsorption/desorption analysis (a) and BJH pore size distribution (b) of G-SO3H,
2
GL-SO3H, and GS-SO3H catalysts.
The N adsorption/desorption analysis was performed to explore the textural features of these
2
catalysts. From the Figure 2(a), it can be inferred that three isotherms are classified as type-IV
accompanied by H3 type hysteresis. This hysteresis resembles that these materials contain slit-like
4
9
pores, comprised of non-rigid aggregates of plate-like particles. The capillary condensation
obtained at higher P/P (0.85) implies the mesoporous nature of catalysts. However, the lower
0
1
3

Catalysis Science & Technology
Page 14 of 47
View Article Online
DOI: 10.1039/D0CY00336K
5
0
limit of the desorption branch is associated with cavitation induced relative pressure. The specific
2
-1
3
-1
surface area of G-SO3H is found to be 471 m g with a pore volume of 0.811 cm g (Table 1).
Whereas GL-SO3H showed a significant decrease in surface area and pore volume. This reduction
might be due to the destruction of internal large pores as obtained in pore size distribution plot
(Figure 2(b)). The G-SO3H catalyst to be rich in larger mesopores. By employing of surfactant in
2
- 1
the G-SO3H catalyst causes reduction of surface area (90 m g ) as well as pore volume to a
greater extent. This could be due to the blocking of large mesopore by the surfactant, also evident
from the average pore size distribution plot (Figure 2(b)).
Figure 3. FESEM images of G-SO3H (a), GL-SO3H (b), and GS-SO3H (c) catalysts.
The FESEM micrographs of G-SO3H (Figure 3(a)), GL-SO3H (Figure 3 (b)), and GS-
SO3H (Figure 3(c)) catalysts showed a heterogeneous mixture of particles with various
morphologies. In these cases, no standard set of sizes is dominant rather they are aggregated and
appeared with rough surface topography. Further, the average particle size obtained from
magnified FESEM images (Figure S1) showed that the surfactant incorporation may increase the
aggregate size for the GS-SO3H catalyst, unlike the case of the G-SO3H and GL-SO3H where
average sizes are more or less same.
1
4

Page 15 of 47
Catalysis Science & Technology
View Article Online
DOI: 10.1039/D0CY00336K
The CHNS/O of each catalyst is tabulated in Table S1. Where carbon, sulphur and oxygen
are present in both G-SO3H and GL-SO3H catalysts but nitrogen along with above constituent
elements are present in case of GS-SO3H catalyst.
Table 1. Textural properties of the catalysts.
a
c
Catalyst
BET surface Pore volume Average pore Total acidity
Surface S Acid density
2
-1
-1
-1
b
1
-2
area (m g )
cc g
diameter
nm)
(mmol g )
contents
(mmol m )
-
(
(mmol g )
G-SO3H
471
352
90
0.811
0.77
20.1
20.1
20.7
5.45
3.87
4.32
3.10
1.81
1.74
0.012
0.010
0.048
GL-SO3H
GS-SO3H
0.229
a
b
c
Total acidity is determined by Boehm titration, From XPS analysis, Acid density = Total
acidity/surface area
Figure 4. (a) SAXS profile of the catalysts. The solid line is the mathematical model fit to the
experimental data. (b) Estimated particle size distribution from the SAXS model fit.
1
5

Catalysis Science & Technology
Page 16 of 47
View Article Online
DOI: 10.1039/D0CY00336K
The SAXS intensity distribution over q is the Fourier transform of the mesoscopic structure
to reciprocal space and hence a feasible mathematical model is required to extract the structural
information. The scattering intensity I(q) can be written as,
r
퐷푚
sin (qr)
qr
∞
0
∞
― ξ
퐼(푞) = 퐶 ∫ P(q,r)D(r)v (r)dr + C [1 + ∫ r퐷푚 ― 3e
2
r2dr] --------(1)
1
2
퐷
푚
0
ro
The first term denotes the scattering contribution of the mesopores, whereas the second term
represents the scattering contribution of the surfactant molecules. The spherical form factor of the
2
푠푖푛(푞푟) ― (푞푟)cos (푞푟)
particles with radius r is given by, 푃(푞,푟) = 3
] . The particles were found to be
[
푞푟)3
(
polydisperse in size and their size distribution (D(r)) was considered as log-normal distribution, 퐷
2
푟
1
―[ln ( 휇)]
(푟,µ,휎) =
2푒푥푝(
). The median size and polydispersity index of log-normal
휋휎2푟
2휎2
2
distribution are denoted by  and , respectively. v(r) is the volume of the spherical particle with
radius r. The surfactant molecules are found to make an aggregate of size ~ 10 nm with a mass
fractal type distribution, as denoted by the second term. D is defined as a mass fractal dimension
m
with an aggregate size of 흃, r represents the radius of each monomer. C and C are intensity scale
0
1
2
factors. The fitted parameters have been tabulated in Table 2. The particles were found to be
significantly polydisperse in size and their size distribution was depicted in Figure 4(b). In the
-1
case of the GS-SO3H catalyst, a very small peak was observed at ~q~2.12 nm , which signifies a
concentrated agglomerate of the surfactant molecules. In this case, a hard-sphere type structure
factor was also considered. The local volume fraction was found to be ~0.55.
1
6

Page 17 of 47
Catalysis Science & Technology
View Article Online
DOI: 10.1039/D0CY00336K
Table 2. Parameters obtained from SAXS model fitting to experimental scattering data.
Catalyst
Median size Polydispersity Average
Aggregate
size (nm)
Monomer
size (nm)
2.2 (fixed)
2.2 (fixed)
Fractal
(nm)
index
size (nm)
7.4 ± 0.1
9.8 ± 0.4
dimension
2.9 ± 0.1
2.9 ± 0.1
G-SO3H
6.20 ± 0.01
7.94 ± 0.08
0.59 ± 0.01
0.65 ± 0.05
10.0 (fixed)
10.0 (fixed)
GL-SO3H
GS-SO3H
25.00 ±0.05
0.60 ± 0.03
30.9 ± 0.6
10.0 (fixed)
2.2 (fixed)
2.0 ± 0.1
However, estimation of the particle size distribution (Figure 4(b)) indeed depends on the nature
of the techniques used to determine it. In principle, the X-ray beam gets scattered at small angles
regime on when there is fluctuation in electron density in the mesoscopic length scale. Such
fluctuations may be either due to the presence of particles or by pores. Babinet’s principle: which
says that equivalent two structures, consisting either voids or materials, generate the same
scattering profile hints to this answer as scattering is proportional to the square of scattering length
density and not the scattering length only. In general, SAXS provides the information about the
minority phase (considering matrix and particle model). If porosity in a material is more than 50
%, the quantitative information as obtained from SAXS is expected to correspond to that about the
particles and vice versa.51, 52 In the present case, we have calculated porosity by using equation 2.
Pore volume
Porosity =
× 100 ― ― ― ― (2)
Total volume
The porosity was obtained around 70% for all three catalysts. Thus, anticipating that the
information from SAXS provides the information about the particle size distribution, we have
drawn histograms from the FESEM images as shown in Figure S1, and compared the results with
SAXS. Besides, we have plotted a correlation between specific surface area and particle size
1
7

Catalysis Science & Technology
Page 18 of 47
View Article Online
DOI: 10.1039/D0CY00336K
distribution obtained from both the techniques (Figure 5). Where both show an excellent
correlation between surface area and particle size.
Figure 5. Correlations between surface area factors and average particle sizes obtained from
SAXS and FESEM analysis.
The FTIR spectra of carbon catalysts are displayed in Figure 6(a). Where characteristic bands
-1
-1
appeared at 1615 and 1406 cm are for C=C stretching and 1705 cm is due to the C=O bond
stretching vibration of carboxylic acids.53-55 The broadband appeared at 3390 cm is related to the
OH stretching of phenolic groups. Whereas unsaturated C-H stretching vibration located at 3100
-1
-1 31
cm . Furthermore, the presence SO H group is confirmed by the appearance of characteristic
3
-1
vibrations, marked at 1040 and 1194 cm-1 for O=S=O stretching and 1125 cm for SO H
3
stretching.31, 53, 54, 56, 57 The embedment of surfactant molecules within the GS-SO3H catalyst is
-1
+
-1
confirmed from the peaks arising at 812 cm due to C-N stretching modes and 1456 cm due to
5
8
-1
the CH scissoring of CTA molecule. Another two peaks at 2923 and 2854 cm also indicate
2
1
8

Page 19 of 47
Catalysis Science & Technology
View Article Online
DOI: 10.1039/D0CY00336K
that CTA molecules are successfully incorporated within the catalyst. The carbon catalysts also
-1
exhibit another band at around 585 cm which refers to the OH bending vibration of SO H
3
group.31, 56 Thus, the presence of carboxylic phenolic, sulfonic group, and C-N moiety indicate
+
the catalyst is acidic as well as hydrophobic. Notably, the band related to SO H stretching at 1125
3
-1
cm for three carbon catalysts, is also shown in the case of CTAB molecule, which is arising due
5
9
to the N-H bending vibration of CTAB molecule. Besides that, the enlarged view of FTIR spectra
-1
(
Figure 6(b)) showed that the band appeared at 1125 cm for the GS-SO3H catalyst is relatively
more intense than the other two catalysts (G-SO3H and GL-SO3H). That means, an increase in
the amount (per unit volume) of the functional groups associated with the particular band, which
is caused by the overlapping of N-H bending and SO H stretching.
3
Figure 6. FTIR spectra (a) and enlarged view (b) of FTIR spectra of three catalysts and CTAB
molecules.
Further, high-temperature DR-FTIR was done to understand whether the SO H group is
3
simply adsorbed on the surface or attached with a carbon network. It is obtained from Figure 7(a)
that the characteristic O=S=O and SO H stretching are intact throughout the temperature range
3
1
9

Catalysis Science & Technology
Page 20 of 47
View Article Online
DOI: 10.1039/D0CY00336K
from 323 K to 573 K which indicates the strong bonding of the SO H group with carbon surface.
3
-1
Although it may seem that the band related to 1125 cm is not present but the smooth image
(
Figure 7(b)) gives clear evidence about the presence of all bands with a little shifting of SO H
3
stretching.
Figure 7. Temperature-dependent DR-FTIR (a) and smooth spectra (b) of GS-SO3H catalyst.
In order to explore the surface acidity of these catalysts, the degree of pyridine adsorption
was measured by FTIR spectroscopy (Figure 8(a)). The Brønsted acidic sites appeared at 1630,
-1
1
590, 1548, 1531 cm spectral region which are due to the adsorption of pyridine either through
a coordinated bond or by the formation of pyridinium ions.32, 54, 57 The pyridinium ion is formed
by the combination with the proton of SO H, -COOH, phenolic -OH groups. However, two
3
-1
adsorption bands in the region 1445 and 1467 cm are responsible for the adsorption of pyridine
on Lewis acidic sites via a coordinated bond.32, 57 The origination of Lewis sites is due to the
5
7
presence of the S=O bond. Which initiated an electron inductive effect within the S=O bond of
surface SO H group. Hamoudi et al. also showed similar type observation for sulfonic acid
3
2
0

Page 21 of 47
Catalysis Science & Technology
View Article Online
DOI: 10.1039/D0CY00336K
6
0
functionalized material. Tanabe et al. and Hua et al. explained that the dehydration of Brønsted
acidic sites or sulfonation is caused by the generation of Lewis acidic sites.61, 62 The above
mentioned bands are persisted under thermal treatment (573 K) (Figure 7) which imply acidic
sites are thermally stable. Additionally, an overlap band of Brønsted and Lewis acidic sites
observed at 1487 cm-1.32
1
3
Figure 8. Pyridine IR (a) and CP MAS C-NMR (b) spectra of G-SO3H, GL-SO3H, and GS-
SO3H catalysts.
1
3
The solid-state CP MAS C-NMR was carried out to explore the chemical environment
around C nuclei. The chemical shift of GS-SO3H catalyst presented in Figure 8(b) which can be
6
3
labelled with CTAB molecule in solution, described by Dongdong et al.. The sharp resonance
signals in the spectral region at 66, 51, 32, 26, 23, 21 and 18 ppm are due to the surfactant C , C ,
1
N
6
3
C -C , C and C or C respectively. These resonance signals confirm the presence of CTAB
4
13
3,
2
15,
molecules in the GS-SO3H catalysts. A distinguish signal appeared at ca. 139 ppm resembles to
4
6, 64
the resonance of aromatic carbon attached with SO H groups.
In addition, the other four
3
signals at ca. 128, 148, 168, and 185 ppm are attributed to the aromatic carbon atoms, phenolic -
2
1

Catalysis Science & Technology
Page 22 of 47
View Article Online
DOI: 10.1039/D0CY00336K
OH,-COOH, and carboxyl (C=O) groups respectively.46, 47 These individual resonance signals are
similar for G-SO3H and GL-SO3H catalysts and also appeared in a broader form. However, the
chemical shift at ca. 140 ppm vanished for these two catalysts. This may be due to the broader
chemical shifts at Ca. 128 and 150 ppm signals which obscure the resonance peak of the SO H
3
4
6
bonded aromatic carbon. Therefore, this result suggested that the amorphous carbon exists in the
form of a polycyclic aromatic sheet containing phenolic -OH, -COOH and SO H groups. The
3
unusual bandwidth variation of GS-SO3H from the other two is mainly due to the appearance of
the semi-crystalline structure of GS-SO3H catalyst described earlier.65
The amount of surface acidic sites in the catalysts were determined by Boehm titration and
-1
summarized in Table 1. The acid-base titration indicates that G-SO3H (5.45 mmol g ) catalyst
-1
possesses a higher amount of acidity than the GL-SO3H catalyst (3.87 mmol g ). During the
preparation of GL-SO3H, we have used p-toluenesulfonic acid as half of the G-SO3H. Thus, it is
obvious that the amount of acidity increases with an increase in amount of sulfonating agent. XPS
results (Table 1) also suggested that the sulphur content in the G-SO3H catalyst is just double to
that of the GL-SO3H catalyst. If we consider all sulphur elements present in the catalysts that exist
-1
in SO H form then the population of surface acidic sites will be between 3.10-1.74 mmol g
3
depending upon different synthesis procedures (Table 1). It has obtained that the amount of total
acidic sites obtained from acid titration is much higher than the sulphur obtained from XPS
analysis. Such a higher amount is probably due to the generation of -COOH and phenolic -OH
3
6
during carbonization which contributes to the total acidity along with SO H groups. However,
3
the GS-SO3H catalyst is accompanied by a decrease in acidity in comparison with the G-SO3H
catalyst. The surfactant treatment leads to a significant decrease in the population of surface
hydroxyl group. Where CTAB molecules are most likely bonded with free hydroxyl groups,25
2
2

Page 23 of 47
Catalysis Science & Technology
View Article Online
DOI: 10.1039/D0CY00336K
2
-1
while lower surface area (90 m g ) may be the reason behind the increase in areal acid density of
-2
-2
GS-SO3H (0.048 mmol m ) compared to G-SO3H (0.012 mmol m ). Despite containing
surfactant its acid amount is much higher in line with previously reported carbon-based catalysts.31,
3
9, 42, 43
Figure 9. XPS survey spectrum (a) and core-level spectra of C 1s (b), O 1s (c), and S 2p (d) for
GS-SO3H catalyst.
2
3

Catalysis Science & Technology
Page 24 of 47
View Article Online
DOI: 10.1039/D0CY00336K
The XPS analysis was employed to predict the chemical state and surface functionalization
of these carbon catalysts. The full spectrum of each catalyst shows typically three peaks (except
GS-SO3H and GSR-SO3H), analogous to C (1s), O (2s), and S (2p) orbitals. On the other hand,
the unavailability of the Br (3d) peak implies the absence of bromine residue in the catalyst. Herein,
the deconvoluted C (1s) signal of each catalyst can be split into five distinct component peaks
2
3
(
Figures 9(b), S2(b), S3(b), and Table 3) which corresponds to sp C=C or sp C-C carbon (peak
, ca. 284.6 eV), alcohol (C-O) or ether (C-O-C) groups (peak 2, ca. 285.5-286.0 eV), carbonyl
C=O) groups (peak 3, ca. 287.1-287.8 eV), carboxyl (O=C-O) groups (peak 4, ca. 288.7-289.0
1
(
eV) and finally π-π* shake-up features of the aromatic ring (peak 5,6 ca. 290.6-292.1 eV), arising
due to the interaction of emitted photoelectrons with aromatic π electrons.39, 57, 66, 67 It was observed
2
3
from Table 3 that the concentration of sp C=C or sp C-C species is relatively higher than the
other two (G-SO3H and GL-SO3H). This seems to have been originated from surfactant carbon.
The XPS deconvoluted O (1s) spectra have each been consisted into three peaks which are related
to the C=O bond in carbonyl and quinone groups (peak 1, 530.4-531.3 eV), C-OH bond in alcohol
(peak 2, 532.2-533.0 eV) and COOH groups (peak 3, 535.2-535.5 eV) respectively. However, the
absence of the O=C-OH peak and decreasing the concentration of C-OH groups (Figure 9(c),
Table 3) in the GS-SO3H catalyst further confirmed that the carbon surface is modified with the
surfactant (CTAB) through the interaction with surface hydroxyl group. On the other hand, the
appearance of asymmetric S (2p) peaks implied the presence of different sulphur components
(Figures 9(d), S2(d), and S3(d)). The peaks centered at approximately 164.0 and 165 eV are
assigned for the thiophene (C-S-C 2p 3/2, C-S-C 2p 1/2). However, another two peaks obtained at
3
9, 67
around 167.7 and 168.9 eV, for the terminal sulphur groups (C-SO and C-SO ).
In the case
2
3
of GS-SO3H, a small decrease in the concentration of sulphur species are may be due to the partial
2
4

Page 25 of 47
Catalysis Science & Technology
View Article Online
DOI: 10.1039/D0CY00336K
coverage by the surfactant (CTAB). Additionally, the deconvoluted N (1s) spectrum (Figure
+
S4(a)) is shown the main peak at around 402.3 eV ascribed to the presence of quaternary N R4
8
species in GS-SO3H catalyst, which indicates that CTAB molecule is most likely associated with
+
surface hydroxyl groups through electrostatic interaction in between N R4 of CTAB molecule and
oxygen of surface –OH.
Figure 10. Raman spectra (a) and TGA analysis (b) G-SO3H, GL-SO3H, and GS-SO3H catalysts.
The Raman spectra displayed (Figure 10(a)) two distinct peaks at around 1586 and 1322
-1
47, 69
cm corresponding to the D and G vibration modes respectively.
The G band with T2g
2
symmetry is due to the relative motion of sp carbon atoms (graphitic) while D band of A
1
g
6
8
symmetry is mainly due to the presence of disorder within the ring. However, the D band is
forbidden in the case of perfect graphite, which implies the carbon catalysts consist of nonperfect
crystalline aromatic carbon sheets.47, 57 It is also noted that the broad intense D band makes G-
SO3H and GL-SO3H catalysts are more amorphous than GS-SO3H catalyst.57 This result
correlated well with XRD analysis (Figure 1).
2
5

Catalysis Science & Technology
Page 26 of 47
Table 3. Relative atomic concentration (%) of C, O, and S and their binding energy for G-SO3H, GL-SO3H, GS-SO3H and GSR-SO3H
catalysts.
Carbon (%)
Oxygen (%)
Sulphur (%)
Catalyst
C-OH/C-
O-C
π-π*
transition
2.5
C-S-C
(2p )
C-S-C
C=C/C-C
C=O
O=C-O
C=O
7.1
C-OH CO H
C-SO2 C-SO3
2
(2p )
3
/2
1/2
5
3.5
284.6)
3.9
284.6)
9.4
284.6)
2.0
284.6)
15.6
3.5
4
8.5
1.1
3.5
1.8
0.9
3.5
0.7
G-SO3H
GL-SO3H
GS-SO3H
GSR-SO3H
-
(
(
(
(
(285.5)
15.9
(287.8) (288.9)
3.3 4.0
(287.5) (289.0)
1.5 2.0
(287.1) (288.7)
4.1 3.7
(287.4) (288.8)
(290.6-292.1) (531.3) (533.0) (535.5)
2.1 8.4 9.5 0.5
(290.7-292.1) (530.4) (532.2) (535.3)
(164.0)
(164.1)
(163.1)
(164.0)
(165.0)
(165.3)
(168.9)
5
0.6
-
(285.9)
16.0
(168.3)
5
0.9
(290.8)
1.8
8.5
(531.1) (532.9)
7.7 9.3
7.8
1.4
-
(285.8)
15.2
(165.1)
(165.2)
(167.6) (168.9)
5
1.2
1.1
-
(286.0)
(290.7-292.1) (531.3) (533.0) (535.2)
(167.5)
2
6

Page 27 of 47
Catalysis Science & Technology
View Article Online
DOI: 10.1039/D0CY00336K
Thermogravimetric analysis was performed to check their hydrophobicity and thermal
stability of as-synthesized catalysts. Figure 10(b) showed that all catalysts exhibit a mass loss at
a temperature of 368 K is mainly due to the loss of adsorbed water molecules. For GS-SO3H, a
small plateau at a temperature region 574-647 K indicates the decomposition of sulfonic acid
groups and above 647 K, the mass loss event is related to the decomposition of surfactant
molecules. While a sharp mass loss is obtained for G-SO3H and GL-SO3H in the temperature
range 574-800 K is attributed to the decomposition of sulfonic acid groups along with the
destruction of the carbon framework. So, the TGA revealed that the all catalysts thermally stable
up to 574 K. Furthermore, it has observed that the first mass loss event associated with water
molecules is comparatively higher for G-SO3H and GL-SO3H than GS-SO3H catalyst and
followed the order of G-SO3H> GL-SO3H> GS-SO3H (Figure 10(b)). Hence, it can be concluded
that the GS-SO3H is more hydrophobic than the other two.
3
.2.
Possible formation mechanism of GS-SO3H catalyst
1
3
Based on the result obtained from XRD, FTIR, C-NMR and XPS analysis, one can
conclude that there is an existence of phenolic -OH, -COOH, C=O, C-O-C, SO3H and surfactants
containing oligosaccharides and macromolecules in the GS-SO3H catalyst.69, 70 These functional
groups are generated via a series of complicated reactions including dehydration, decarbonylation,
and aromatization, described in Scheme S1. In combination with the results from previous
literature 69, 70 and our experimental results a possible reaction mechanism for the formation of
GS-SO3H catalyst from glucose, p-toluenesulfonic acid, and surfactant (CTAB) via hydrothermal
route has been illustrated schematically in Scheme 2.
2
7

Catalysis Science & Technology
Page 28 of 47
View Article Online
DOI: 10.1039/D0CY00336K
Scheme 2. Schematic illustration of GS-SO3H catalyst.
Generally, the preparation of sulfonated carbon material from glucose involves the formation of
various macromolecules (1,6-anhydroglucose, furyl, and 1,2,4-benzenetriol) in the first stage
2
8

Page 29 of 47
Catalysis Science & Technology
View Article Online
DOI: 10.1039/D0CY00336K
through glucose condensation reactions such as dehydration, decarbonylation, and aromatization.
Then the free glucose monomers undergo self as well as cross polymerization with the formed
macromolecules followed by intermolecular condensation leading to the formation of carbon
particles having oxygen-containing hydrophilic surface. Once these particles are formed, they
further react with p-toluenesulfonic acid to incorporate -SO H and other oxygen-rich groups. The
3
resulting particles start to grow outward and keep continuing until the final size is reached. The
XRD patterns of this type of material, such as G-SO3H and GL-SO3H, appear as broad diffraction
0
peaks in the range of 2θ=10-30 (Figure 1), indicating the amorphous nature of the carbon
framework, which have randomly arranged aromatic carbon layer. In the case of the GS-SO3H
catalyst, the presence of slight crystallinity is not clear. We believe that the surfactant present in
the system via bonding with surface hydroxyl bond may have slowed down the intermolecular
condensation process which has led some of the polysaccharides to remain in the system and
responsible for the slight crystallinity.
3
.3.
Catalytic activity study
To find out a suitable catalyst, the catalytic experiment of glycerol acetalization reaction with
acetone was carried out under identical reaction conditions using as-synthesized catalysts (Figure
1
1(a)). A negligible glycerol conversion (<2%) is achieved for this reaction without a catalyst and
it is taken as a reference for this catalytic experiment. The catalytic activity of all three catalysts is
found to be in the following order GS-SO3H> GL-SO3H> G-SO3H. It is known that the glycerol
3
1
conversion increases steadily with an increase in acidity and surface acid density. But in our case,
glycerol conversion increases from 68 to 79% when GL-SO3H catalyst, which has a lesser surface
acid density, is used (Figure 11(b)). This observation suggested that the acidity and surface acid
density are not solely responsible for glycerol acetalization reaction but another property i.e.
2
9