A facile post-synthetic modification of ordered mesoporous carbon to get efficient catalysts for the formation of acetins

Article abstract of DOI:10.1016/j.cattod.2019.02.049

Highly active and stable solid acid catalysts based on ordered mesoporous carbons were synthesized via hard template method and further functionalization with sulfonic (SO₃H) groups. Two post-synthetic modification strategies were applied. In the first one, the pristine carbon was modified with concentrated sulfuric acid. The second approach included the reaction of carbon with aryl diazonium cation generated in situ from sulfanilic acid. The morphology, structure, composition, and surface functionalities of the resulting carbons were determined by scanning and transmission electron microscopy, X-ray diffraction, thermogravimetric analysis, N₂ adsorption-desorption measurement, elemental analysis, potentiometric titration, X-ray photoelectron spectroscopy and FT-IR method. The obtained samples were further tested in acetylation of glycerol with acetic acid under various conditions, and were found to efficiently produce acetins, valuable fuel additives. An excellent catalytic activity was shown especially by the SO₃H-bearing mesoporous carbon prepared by a facile method with the use of 4-benzenediazonium sulfonate under ambient conditions. Furthermore, very good recycling properties of this catalyst was demonstrated by four consecutive runs. It is supposed that acidic catalytic activity of the synthesized materials can be further extended to other acid-catalyzed reactions.

Full text of DOI:10.1016/j.cattod.2019.02.049

Catalysis Today xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
A facile post-synthetic modification of ordered mesoporous carbon to get
efficient catalysts for the formation of acetins
⁎
Joanna Goscianska , Anna Malaika
Faculty of Chemistry, Adam Mickiewicz University in Poznań, Umultowska 89b, 61-614, Poznań, Poland
A R T I C L E I N F O
A B S T R A C T
Keywords:
Highly active and stable solid acid catalysts based on ordered mesoporous carbons were synthesized via hard
Sulfonated carbons
Sulfuric acid
template method and further functionalization with sulfonic (SO H) groups. Two post-synthetic modification
3
strategies were applied. In the first one, the pristine carbon was modified with concentrated sulfuric acid. The
Diazonium salt
second approach included the reaction of carbon with aryl diazonium cation generated in situ from sulfanilic
Hard template method
Glycerol valorization
Catalytic activity
acid. The morphology, structure, composition, and surface functionalities of the resulting carbons were de-
termined by scanning and transmission electron microscopy, X-ray diffraction, thermogravimetric analysis, N
2
adsorption-desorption measurement, elemental analysis, potentiometric titration, X-ray photoelectron spectro-
scopy and FT-IR method. The obtained samples were further tested in acetylation of glycerol with acetic acid
under various conditions, and were found to efficiently produce acetins, valuable fuel additives. An excellent
catalytic activity was shown especially by the SO H-bearing mesoporous carbon prepared by a facile method
3
with the use of 4-benzenediazonium sulfonate under ambient conditions. Furthermore, very good recycling
properties of this catalyst was demonstrated by four consecutive runs. It is supposed that acidic catalytic activity
of the synthesized materials can be further extended to other acid-catalyzed reactions.
1
. Introduction
produce carbonaceous material functionalized with SO
17] heated naphthalene, perylene and coronene in concentrated
SO at 250 °C for the one-step formation and incomplete carboniza-
tion of sulfoaromatic hydrocarbons. The SO H acid density of the ob-
tained solid was 4.9 mmol/g [17]. Although, the build-in method is
3
H. Hara et al.
[
Ordered mesoporous carbons have proved to be useful in the im-
H
2
4
plementation of various applications, including water and air pur-
ification, gas separation, nanotechnology, catalysis, chromatography,
and energy storage [1–6]. The widespread interest in these materials
results from their unique physicochemical properties such as tunable
pore structure, well-developed surface area, electric conductivity, che-
mical inertness, and high thermal and mechanical stabilities [7,8].
However, the inert and hydrophobic nature of pristine mesoporous
carbons is unfavorable from the point of view of catalytic processes,
which require a functionalized surface with specific affinity and/or
reactivity [9]. Significant efforts have been devoted to the surface
modification of carbon materials in recent years [10–14]. A very in-
teresting direction of studies is functionalization of mesoporous carbons
3
straightforward and the amount of SO H can be tuned by adjusting the
3
quantity of sulfur precursor, this route of modification is difficult to
couple with soft-template synthesis of mesoporous carbons, not only
due to the limited thermal stability of C-SO H moieties, but also due to
3
the influence of the S precursor in the construction of the micellar
mesophase [16]. Therefore, other technologies of surface functionali-
zation of carbons with SO H groups have been developed in recent
3
years. The post-synthetic grafting is one of the most convenient and
simplest methods for modifying the carbon surface with sulfonic
groups. This approach involves the use of various sulfonating reagents,
with sulfonic groups to become SO
carbon solid catalysts [15,16]. These systems are characterized by
strong acidity, well-ordered structure and H O tolerant nature, which is
ascribed to the presence of carbon moieties in the framework. The in-
3
H-bearing (sulfonated) mesoporous
e.g. concentrated or fumed H
2
SO
4
, ClSO H, 4-benzenediazonium sul-
3
fonate, SH or SO H terminated organosilanes, phenyl vinylsulfonate
3
2
[18–25]. The key point in post-synthetic modification is the preserva-
tion of the mesostructure of carbon materials, while incorporating as
corporation of chemically bound SO
3
H groups can be divided into built-
many SO H groups as possible.
3
in and post-grafting strategies [16]. The direct treatment of carbon
precursor with concentrated sulfuric acid (the built-in approach) can
The application of solid acid catalysts in industrial processes has
been attracted a lot of attention in the last years, and it seems to be one
⁎
Corresponding author.
E-mail address: asiagosc@amu.edu.pl (J. Goscianska).
https://doi.org/10.1016/j.cattod.2019.02.049
Received 31 October 2018; Received in revised form 6 February 2019; Accepted 17 February 2019
0920-5861/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Joanna Goscianska and Anna Malaika, Catalysis Today, https://doi.org/10.1016/j.cattod.2019.02.049

J. Goscianska and A. Malaika
Catalysis Today xxx (xxxx) xxx–xxx
of the hottest research topics in chemistry nowadays. One of the in-
dustrial reactions of great importance for which new solid catalytic
systems are sought is the process of glycerol valorization. Glycerol is
generated as a byproduct in biodiesel synthesis. Taking into account the
scale of biodiesel production, there is a surplus of glycerol in the market
3.5 g of mesoporous carbon was mixed with distilled water
(175 mL), an equimolar amount of 4-aminobenzenesulfonic acid
(Merck) and sodium nitrite (Avantor) (5 g and 2 g, respectively), as
well as concentrated hydrochloric acid (35 mL). The reaction was
carried out at ambient temperature for 20 h under continuous stir-
ring. Finally, the suspension was filtered, and the carbon sample was
thoroughly washed with distilled water, next with methanol
(Avantor), N,N-dimethylformamide (Chempur), and then acetone
(Avantor). The resulting material was dried overnight (110 °C). The
obtained carbon was denoted as CSBA-15-BDS.
•
•
[
26]. It is logical that this must affect the overall economics of the
biodiesel formation technology. There a number of possible ways to
utilize a glycerol glut and thus make biodiesel synthesis more viable
[
27]. Among various options, acetylation of glycerol with acetic acid is
particularly interesting. The products formed in the process – glycerol
acetates, so-called acetins (monoacetin, diacetin and triacetin, MA, DA
and TA, respectively) are considered as important value added chemi-
cals [28]. These compounds find numerous industrial applications,
ranging from cosmetics to biodegradable polymers [28,29]. Also the
fuel market uses acetins. In particular, more substituted esters (namely
DA and TA and their mixtures) are promising fuel blending compounds
3.5 g of carbon sample was mixed with concentrated sulfuric acid
(90 mL, 95%, Chempur) and heated at 140 °C under helium flow
upon continuous stirring. After 20 h, the modified carbon material
was washed with hot distilled water and dried overnight (110 °C).
The sample prepared in this way was denoted as CSBA-15-H
2
4
SO .
[
30].
Heterogeneous catalysts used in the glycerol valorization with
2.2. Catalyst characterization
acetic acid include ion-exchange resins [29,31,32], heteropolyacids
over various supports [33,34], sulfated zirconia catalysts [35] and si-
lica-based systems [36] among others. There are only scarce data on the
application in this reaction of carbon materials [37–40]. Taking into
account the great industrial importance of the glycerol acetylation
process, the current study is focused on the facile preparation of effi-
cient carbon catalysts for this reaction. Ordered mesoporous carbons
have been chosen for catalytic experiments, considering numerous ad-
vantages of this kind of materials.
All the mesoporous carbons obtained were characterized by powder
X-ray diffraction using a D8 Advance Diffractometer (Bruker) with the
copper Kα1 radiation (λ = 1.5406 Å). The XRD patterns were recorded
at room temperature with a step size 0.02° in the small-angle range.
For TEM measurements, powdered samples were deposited on a
grid with a perforated carbon film and transferred to a JEOL 2000
electron microscope operating at 80 kV.
Morphological appearances of the samples were examined with the
use of a scanning electron microscope ZEISS EVO 40.
Textural characteristic of the mesoporous carbons was performed
with the use of a Quantachrome Autosorb IQ apparatus by nitrogen
adsorption/desorption measurements at −196 °C. The Brunauer-
Emmett-Teller (BET) method was employed for the determination of
2
. Experimental section
2.1. Catalyst preparation
surface areas (SBET) of carbon materials. Micropore volumes (Vμpore
)
2
.1.1. Synthesis of SBA-15 template
were found using the t-plot method [42], whereas the total pore volume
Ordered mesoporous silica SBA-15 was hydrothermally synthesized
(VTot) was obtained from the amount of N adsorbed at a relative
2
according to the procedure described previously [41].
pressure close to unity. The average pore size was estimated from the
adsorption branch of isotherm using Barret-Joyner-Halenda (BJH)
method.
Briefly, 0.5 g of Pluronic P123 triblock copolymer (EO20PO70EO20
,
Aldrich) was dissolved in 19 mL of 1.6 mol/L HCl (37%, Chempur) and
stirred at 35 °C for 2 h. Then, 1.1 g of tetraethyl orthosilicate (98%,
Aldrich) was added to the solution, which was maintained at 35 °C for
further 6 h under stirring. The resulting sol-gel slurries were hydro-
thermally aged for 24 h at 35 °C and subsequently for 6 h at 100 °C.
Next, the solid product was filtered without washing and dried at 100 °C
for 24 h in an air oven. Finally, the sample was calcined at 550 °C in air
to remove the triblock copolymer.
CHNS/O elemental analysis was performed using Thermo Scientific
FLASH 2000 Elemental Analyzer (OEA).
The total acidity of the samples was determined by a potentiometric
back titration method using a Cerko Lab System microtitration unit. The
analytical procedure was conducted as follows: carbon material
(100 mg) was mixed with 50 mL of 0.01 mol/L NaOH (Merck) (an ex-
cess of the NaOH solution was used in each case to guarantee complete
neutralization of all surface acidic groups present in the sample). The
suspension obtained was shaken in a sealed flask at room temperature.
After 20 h of the agitation, the suspension was filtered, and 20 mL of the
filtrate was titrated with a 0.05 mol/L HCl solution (Fluka). A blank test
with a NaOH solution and without carbon material was also carried out.
The concentration of sulfonic groups was estimated on the basis of
elemental analysis [43].
2
.1.2. Synthesis of ordered mesoporous carbon
In a typical synthesis of carbon material CSBA-15, 1 g of mesoporous
silica SBA-15 was immersed in a solution of 1.25 g of sucrose (Aldrich),
0
.14 g of concentrated H
2
SO (95%, Chempur) and 5 mL of distilled
4
water. The mixture was heated for 6 h at 100 °C and then the tem-
perature was increased to 160 °C for another 6 h. The resulting solid
®
was again immersed into a solution of 0.8 g of sucrose, 0.09 g of H
SO
2 4
The simultaneous thermal analyzer NETZSCH STA 449F3 Jupiter
and 5 mL of water. The heating process was repeated again by drying
the mixture at 100 °C and 160 °C for 6 h. The carbonization was com-
pleted by pyrolysis with heating to typically 900 °C for 6 h in argon
atmosphere. After carbonization, the template-containing carbon was
washed with 5 wt. % hydrofluoric acid (Sigma-Aldrich) twice at room
temperature to remove the silica template. The template-free product
was washed with water and then ethanol before drying at 105 °C
overnight.
was used to investigate the thermal stability of the samples.
Measurements were carried out under nitrogen flow (10 mL/min) at a
heating rate of 10 °C/min over a temperature range of 30–1000 °C, with
an initial sample weight of approximately 8 mg.
FT-IR spectra were obtained using a Vertex 70 spectrometer
(Bruker). The carbon materials were analyzed in the form of tablets,
made by pressing a mixture of anhydrous KBr (ca. 0.25 g) and 0.3 mg of
the tested substance in a special steel ring, under a pressure of 10 MPa.
−
1
Analysis was performed over a wavenumber range of 4000–400 cm
.
2
.1.3. Modification of ordered mesoporous carbon material
To further elucidate the chemical features of the obtained samples, X-
ray photoelectron spectroscopy (XPS) analyses were performed. The
XPS investigations were carried out using an UHV SPECS spectrometer,
and the spectra were recorded with the use of Al Kα radiation by a
PHOIBOS HSA3500 analyzer, working in the FAT scan mode with a
The main purpose of the modifications used was the creation of
strongly acidic groups on the surface of the initial sample, namely
sulfonic groups. These functionalities were introduced on the carbon
applying two methods:
2

J. Goscianska and A. Malaika
Catalysis Today xxx (xxxx) xxx–xxx
pass energy of 20 eV. The XPS spectra were further calibrated using the
C 1 s peak at 284.5 eV as an internal standard. Core-level peaks were
analyzed using a non-linear Shirley-type background subtraction. The
accuracy of the binding energy (B.E.) values was estimated to
be ± 0.1 eV.
hexagonal pore arrangement.
TEM images of the samples synthesized are depicted in Fig. 2. The
pristine mesoporous carbon shows a highly ordered hexagonal struc-
ture, an inverse replica of SBA-15 silica, consisting of carbon cylindrical
rods separated by ordered arrays of mesoporous channels (Fig. 2 A).
After the modification with concentrated sulfuric acid, the carbon
sample exhibits a high-quality mesostructure (Fig. 2 B). TEM image of
the material CSBA-15-BDS indicates that the mesostructural regularity is
preserved, however, some structural changes are observed (Fig. 2 C).
The arrangement of carbon rods are significantly higher for the sample
2.3. Catalytic tests
The catalytic activity of the prepared materials was subsequently
investigated in the process of glycerol acetylation. Prior to the tests, all
the catalysts were activated at 110 °C for 1.5 h to remove the moisture.
The reaction itself was carried out in a batch reactor equipped with a
reflux condenser and a magnetic stirrer, under argon flow. Glycerol
C
SBA-15-H
2
SO than for CSBA-15-BDS, as previously concluded from XRD.
4
The morphology of catalysts was investigated via scanning electron
microscopy, and corresponding micrographs are presented in Fig. 3. It
can be seen from Fig. 3 A–C that all carbon materials possess a rod-like
structure with a size range 1–3 μm. This suggests that the morphology
of the CSBA-15 sample was maintained well after its modification with
the use of 4-benzenediazonium sulfonate or concentrated sulfuric acid.
The nitrogen adsorption/desorption isotherms of the mesoporous
carbon materials are given in Fig. 4. The pristine and modified samples
display well-defined type IVa isotherms (according to IUPAC classifi-
cation) with H1 hysteresis loops, indicating the presence of uniform
mesopores in their structure [45]. The sharp capillary condensation
step occurs at a relative pressure of p/p0 ˜ 0.4 due to the rapid nitrogen
adsorption within the mesopores.
(
Avantor) and acetic acid (Avantor) were taken to the reactor in the
molar ratio of 1:6 or 1:9 (7.75 g of glycerol and 28.9 mL or 43.4 mL of
acid, respectively). The amount of a catalyst added was 0.175 g, 0.35 g
or 0.7 g. The process itself was performed at 80 or 110 °C for 24 h.
Periodically, small aliquots of the reaction mixture were sampled from
the system and analyzed using a GC chromatograph equipped with a
FID detector and a capillary column (InertCap WAX, GL Sciences,
3
0 m x 0.51 mm I.D, 1 μm film thickness) operating at 150–210 °C. The
conversion of glycerol (XGly) and selectivities to acetins – mono-, di- and
triacetin (SMA, SDA, STA, respectively) were calculated according to the
formulas shown in literature [44]. For the sake of comparison, blank
tests (without a catalyst) were also performed. A commercial ion ex-
change resin (Amberlyst 15) was employed as a reference catalyst. For a
selected carbon (CSBA-15-BDS), also reusability tests were carried out.
After each run, the sample was separated from the reaction mixture by
filtration, washed thoroughly with water and then acetone. After drying
at 110 °C overnight, it was used in the next cycle.
The parameters characterising textural properties of the catalysts
studied are collected in Table 1. These data indicate that pristine me-
2
soporous carbon possesses the best developed surface area of 1203 m /
3
g and large pore volume of 1.32 cm /g. It was observed that the pro-
cesses of functionalization of the CSBA-15 material have a significant
effect on its textural parameters. The smallest changes in the carbon
texture were caused when the modification with concentrated sulfuric
acid was applied, which can be attributed to the relatively low content
of functional groups introduced into the sample structure. The BET
3
. Results and discussion
2
3
surface area and pore volume declined to 1107 m /g and 1.13 cm /g,
respectively. The greatest decrease in surface area and pore volume
with respect to those of the pristine carbon was noted for the sample
3.1. Characterization of catalysts
The XRD patterns of pristine and functionalized carbon materials in
C
SBA-15-BDS. It can be concluded that the sulfonation of the carbon
the small-angle range are shown in Fig. 1. They exhibit three peaks at
surface via diazonium cation generated in situ happens at the micro-
pore/mesopore openings, due to the high potential field in small pores
and the easy susceptibility of attachment at the pore openings. More-
over, the pore diameter was reduced from 4.31 for CSBA-15 to 3.42 nm
for CSBA-15-BDS, which probably follows from the fact of blocking the
access to the pores by the surface functional groups upon the carbon
modification.
2
Θ ≈ 1.1°, 1.8° and 2.1°, corresponding to the planes (100), (110) and
(
200), respectively, typical of mesoporous structure with a 2-dimen-
sional (2-D) hexagonal p6mm symmetry. The intensity of all reflections
decreased after functionalization of carbon material with the use of
diazonium cation generated in situ, indicating deterioration of the
structure ordering. This phenomenon can be caused by a partial
blocking of small mesopores and micropores by generated functional
groups, which will be discussed later. On the other hand, the mod-
ification of CSBA-15 with concentrated sulfuric acid does not affect the
TG analysis of samples was used to estimate an approximate level of
the material functionalization, taking into account the quantity of vo-
latiles evolved during the heating of carbons under nitrogen flow up to
8
00 °C. The TG curves are presented in Fig. 1S (Supplementary
Materials). As shown there, the lowest amount of volatiles was released
from the pristine CSBA-15 sample (4.8%), which suggests the presence of
only small number of different groups on the surface of this catalyst.
This is in line with the results of elemental analysis and acidity mea-
surement obtained for this material (discussed below). On the other
hand, it is clearly visible that the treatment of CSBA-15 with 4-benze-
nediazonium sulfonate generated in situ leads to a significant increase in
volatiles evolved, being more effective method of functionalization
among those applied here. The significant increase in quantity of het-
eroatoms was also observed in elemental analysis of this material (see
also the discussion on EA presented below). For CSBA-15-H
2
SO , con-
4
siderably smaller amount of volatiles was measured, and this suggest
that significantly smaller number of surface functionalities (oxygen and
sulfur groups) was introduced on the carbon surface during functio-
nalization comparing with CSBA-15-BDS.
The DTG patterns of the samples, Fig. 5, show precisely the tem-
perature regions in which maximum weight losses occur. In the profile
of each sample, there are three such regions. First weight decrease, at
Fig. 1. Small-angle X-ray diffraction patterns of ordered mesoporous carbons.
3

J. Goscianska and A. Malaika
Catalysis Today xxx (xxxx) xxx–xxx
Fig. 2. TEM images of CSBA-15 (A), CSBA-15-H
2
4
SO (B), CSBA-15-BDS (C).
Fig. 3. SEM micrographs of CSBA-15 (A), CSBA-15-H
2
4
SO (B), CSBA-15-BDS (C).
Fig. 4. Nitrogen adsorption/desorption isotherms of pristine and functionalized
Fig. 5. DTG curves of mesoporous carbons.
mesoporous carbons.
the temperature with a minimum of about 50–70 °C, is possibly due to
liberation of the physically adsorbed water. Important mass changes,
however, start above 150 °C and are related to the decomposition of
various functional groups present on the surface of the materials. CSBA-
Table 1
Textural parameters determined by nitrogen adsorption-desorption at −196 °C.
a
b
c
d
Sample
S
BET
2
V
Tot
(cm /g)
V
μpore
3
V
μpore/VTot
d
pore
3
15-H
SO shows a relatively intense signal with a minimum at about
(
m /g)
(cm /g)
(-)
(nm)
2
4
2
70 °C corresponding probably to the decomposition of surface sulfonic
C
C
C
SBA-15
1203
1107
547
1.32
1.13
0.47
0.57
0.50
0.19
0.43
0.44
0.40
4.31
4.07
3.42
groups [46,47]. The subsequent weight loss (that starts above 500 °C) is
probably due to the decomposition of oxygen functionalities that can be
also created in the reaction of carbons with concentrated sulfuric acid
[46,47]. This will be further confirmed by elemental and XPS results. In
the DTG profile of CSBA-15-BDS, the weight loss is significantly higher in
2 4
SBA-15-H SO
SBA-15-BDS
a
b
c
BET surface area (SBET).
Total pore volume (VTot).
Micropore volume (Vμpore).
comparison with that of CSBA-15-H
2
SO , suggesting that more sulfonic
4
d
Average pore diameter (dpore).
groups were introduced into the first sample. Interestingly, the
minimum is also shifted to a higher temperature (360 °C). According to
4

J. Goscianska and A. Malaika
Catalysis Today xxx (xxxx) xxx–xxx
Table 2
than that of the pristine material. This indicates, on the one hand, the
efficiency of modifications, on the other hand, suggests differences in
the nature of surface chemistry of the catalysts. CSBA-15 shows total acid
site density of 0.59 mmol/g. A substantial number of new acid sites was
generated on the CSBA-15 surface after the carbon modification with
Bulk (dry basis) elemental composition in wt % and the results of acid surface
properties of the materials (total acid site content and SO
g.
Sample
3
H contents) in mmol/
C
N
H
S
O
SO
3
Ha
Acid sitesb
concentrated sulfuric acid as evidenced by the total acidity of CSBA-15
-
C
C
C
SBA-15
97.0
83.4
71.8
0.0
0.0
1.4
0.8
1.0
2.6
0.0
1.4
7.0
1.7
0.00
0.44
2.18
0.59
1.41
0.96
H
2
SO
4
. According to literature data, SO
3
H groups as well as O-con-
SBA-15-H
2
SO
4
11.5
13.8
taining functionalities of acidic character, such as hydroxyl (phenol),
SBA-15-BDS
lactone, and carboxylic ones, can be formed with H
2
4
SO [47]. The
a
b
on the basis of elemental analysis.
total number of acid sites determined by acid-base titration.
modification with 4-benzenediazonium sulfonate was less effective,
giving the material with the total acid site density of 0.96 mmol/g. This
relatively low total acidity might be a bit surprising taking into account
literature data, this is typical for PhSO
3
H containing materials [48].
the high content of sulfur and oxygen measured for C
SBA-15
-BDS. One of
The type of functional groups formed on the catalyst surface was
determined by infrared spectroscopy. The FT-IR spectra of the materials
are shown in Fig. 2S (Supplementary Materials). The spectrum of
pristine mesoporous carbon CSBA-15 shows the bands assigned to
stretching and bending vibrations of hydroxyl groups (3460 and
the possible explanations of this phenomenon is the occurrence of
certain side reactions resulting in neutralization of some sulfonic
groups, e.g. formation of zwitterions [51,54]. Other reason might be the
limited accessibility of created acidic groups to sodium hydroxide used
in the titration analysis. Thus, in turn, may suggest that some part of
functionalities is located deep in the carbon pores. Table 2 gives also
the results on the number of surface sulfonic groups in the prepared
carbons. The calculation was based on the results of elemental analysis
and conclusions from XPS analysis (see below) [43]. As can be seen, in
−
1
−1
1
480 cm ), stretching vibrations of carbonyl groups (1720 cm ) and
−1
stretching vibrations of carboxylate and epoxy structures (1200 cm
)
[
9]. After the functionalization of the initial sample with diazonium
−1
cation, new bands centered at 1008, 1041 and 1126 cm appear, and
they can be attributed to the symmetric stretching vibrations of S]O in
the case of C
-H SO , the number of SO H functionalities is sig-
SBA-15
2
4
3
−
1
SO
3
H groups. Additionally, the weak band at 880 cm , characteristic
nificantly lower than the sample total acidity. This confirms earlier
conclusions that during the modification with concentrated sulfuric
acid also O-containing groups of acidic character were formed. Inter-
of the stretching vibrations of CeS, is observed [49]. This band is also
noted in the spectrum of the CSBA-15-H SO . The presence of the
SO
2
4
aforementioned signals in the spectra of CSBA-15-BDS and CSBA-15-H
2
4
estingly, the estimated SO H content in the case of C
SBA-15
-BDS is higher
3
materials demonstrates the successful functionalization of samples via
the sulfonation processes performed. Moreover during the modification
of mesoporous carbon with concentrated sulfuric acid also oxygen-
containing groups of acidic character were generated.
than the carbon total acidity. This may indicate the neutralization of
some sulfonic groups, which was suggested above [51,54].
The XPS surveys of the samples are presented in Fig. 6 A. The scans
show qualitative surface composition of the materials. In general, the
Table 2 presents data on the bulk content of C, H, N, S, O. It is
important to note that the sum of the wt % of the above-mentioned
components (and ash) is not equal to 100, and this is due to the pos-
sibility of the presence of other elements in the samples (which will be
discussed later). In general, the results from the elemental analysis are
consistent with those from the TG measurement, and the samples
modified that evolved larger amounts of volatiles in TG depict larger
quantity of heteroatoms, mainly oxygen, but also sulfur and some
amounts of nitrogen. A natural consequence of increasing amount of
heteroatoms is the decrease in the quantity of carbon. Most im-
portantly, the data presented suggest that the modifications applied
spectrum of C
depicts an intense signal from carbon and a small
SBA-15
peak assigned to oxygen, which confirms the earlier results suggesting
that the carbon material possesses only small number of oxygen func-
tionalities on its surface. Further analysis of the scan reveals that on the
surface of C
SBA-15
small amount of fluorine is also present. Apparently,
HF was not completely washed out from the sample during the pre-
paration stage (see Experimental Section) although thorough washing
was applied. It suggests strong adsorption of acid in the carbon pores. It
is also possible that during dissolution of the silica matrix with HF some
carbon-fluorine groups were formed [55]. The surface fluorine con-
centration was found to be about 1.1 wt %. On the other hand, no
signals coming up from silicon were found in the spectrum (about
101.0–103.0 eV) [55], which permits drawing the conclusion that silica
framework was successfully removed from the final sample. The spectra
of modified materials contain some additional signals, proving the
(
aimed at introducing SO H groups on the carbon surface) were suc-
3
cessful as both treated samples show the presence of sulfur. However,
the more effective method of CSBA-15 modification was the carbon re-
action with 4-benzenediazonium sulfonate generated in situ. This
functionalization approach allowed introducing as much as 7.0 wt % of
sulfur in the carbon matrix. Both modification procedures resulted in
the increase in the oxygen content, which can also affirm the successful
presence of sulfur in C
SBA-15
-H SO and sulfur and nitrogen in the C
2
4
SBA-
1
5
-BDS catalyst, which is also consistent with the results of elemental
analysis (Table 2). The comparison of XPS survey scans of the pristine
catalyst and the treated samples (Fig. 6 A) reveals also the increase in
intensity of O1 s peak for the samples after the modifications. This, on
the one hand, can confirm the formation of S-containing functionalities
attachment of SO
3
H groups on the carbon surface. On the other hand,
the content of oxygen in CSBA-15-H
2
SO is disproportionally high in
4
relation to the S content, which can indicate that other surface oxygen
functionalities were created in this case. Not surprisingly, taking into
account that concentrated sulfuric acid is known also for its oxidation
properties [50]. In the case of CSBA-15-BDS, some amounts of nitrogen
were also found. This might suggest that part of the diazonium cations
created in situ did not undergo reduction with the formation of phenyl
radicals [51,52]. On the other hand, it is also possible that some dia-
zonium cations were attached to the carbon surface with the formation
of azo bonds [51,52]. Small amounts of this kind of species were further
revealed by XPS analysis. The presence of residual sulfanilic acid
strongly adsorbed on the carbon surface in spite of the extensive ma-
terial washing step might be also considered [53].
in the form of SO H groups. On the other hand, it can suggest oxidation
3
of the carbon surface, especially in the case of C
SBA-15
-H SO , as sug-
2
4
gested earlier. In general, the XPS survey data confirm previous results
suggesting that the modification via in situ generated diazonium salt is
more efficient method of introducing of SO H moieties into the ordered
3
mesoporous carbon framework comparing to the treatment with con-
centrated H SO .
2
4
To further verify the chemical state of sulfur in the modified sam-
ples, the high-resolution XPS S 2p spectra of C
-BDS and C
SBA-15 SBA-15
-
H SO are presented, Fig. 6 B. A single peak centered at about
2
4
168–169 eV was found in both cases, suggesting that sulfur exists on the
carbon surface mainly as sulfonic groups [48]. No measureable quan-
tities of other forms of S were introduced to the carbons.
The acidity of the samples was measured by a potentiometric back
titration method, and the results obtained are collected in Table 2. As
can be observed, the total acidity of the functionalized carbons is higher
The high-resolution XPS spectra of the O 1 s region of the carbon
5

J. Goscianska and A. Malaika
Catalysis Today xxx (xxxx) xxx–xxx
and potentiometric titration obtained for CSBA-15-H
2
SO , and prove on
4
the carbon surface not only of O-containing groups in the form of SO
3
H
functionalities but also substantial amount of other acidic moieties,
mainly hydroxyl/phenol ones. In the case of CSBA-15-BDS (Fig. 3S C,
Supplementary Materials), the distribution of oxygen-type groups is
different. After the CSBA-15 was modified with 4-benzenediazonium
sulfonate, the dominant peak is that at 531.3 eV (C]O functionalities
or S]O bonds in sulfonic groups). No significant changes in the number
of other oxygen groups were observed after the treatment. Hence,
taking into account also the results of XPS S 1 s analysis, it can be said
that the reaction of carbon with in situ generated diazonium salt forms
surface oxygen groups only in the form of SO H functionalities [51].
3
In CSBA-15-BDS also the presence of nitrogen was confirmed. For the
deconvoluted N 1 s spectrum of the sample (Fig. 4S, Supplementary
Materials), two peaks were found – at the binding energies of about
4
00.2 and 402.1 eV, very similar as in our previous work dedicated to
BDS-modified activated carbons [46]. The first signal can be assigned to
N]N azo groups (very low intensity). Interpretation of the second one,
more intense, is a bit difficult. According to literature data it can be
ascribed to nitrogen atoms, oxidized nitrogen species or quaternary
ammonium salts [46]. However, it should be taken into account that
the content of nitrogen species of different types present in the sample
is not high, which is also in line with the results provided by elemental
analysis (Table 2).
3.2. Catalytic performance
The conversion of glycerol as a function of time at a selected tem-
perature (80 °C) was studied and the results are presented in Fig. 7 A.
The data suggest that the prepared materials are active in the process
giving a high glycerol conversion within a relatively short period of
time even if mild reaction conditions are applied. Ordered mesoporous
carbon modified with sulfuric acid works significantly worse than the
catalyst obtained via in situ generated diazonium salt, converting in-
itially 25% of glycerol, while CSBA-15-BDS transforms about 80% of
glycerol within the first hour of the process. In both cases, XGly in-
creases, reaching almost the same value after 24 h (about 98%); how-
ever, more satisfied results were attained when CSBA-15-BDS was used as
a catalyst – a very high glycerol conversion was achieved significantly
much faster. The differences in catalytic activity of the tested samples
must be related to the number of active sites present on the catalyst
surface and their accessibility to the reagents. A blank experiment, with
no catalyst in the mixture, was also carried out and confirmed the
catalytic properties of the obtained carbons. Some glycerol transfor-
mation noticed in the blank reaction is due to the autocatalytic char-
acter of the esterification process [56].
Fig. 6. XPS survey spectrum of the prepared samples (A); the high resolution
spectra of S 2p of the synthesized carbons (B).
catalysts are displayed in Fig. 3S (Supplementary Materials). As men-
tioned before, the pristine sample contains only a small quantity of
oxygen, and the surface oxygen groups are mainly in the form of hy-
droxyl/phenol and ether functionalities as evidenced by the intense
signal at about 532.9 eV in Fig. 3S A (Supplementary Materials).
Carbonyl and carboxylic structures were also found, however, in trace
amounts. The latter could have been formed as a consequence of
spontaneous oxidation of the sample with oxygen from the air during
the carbon storage. There are considerable changes in the oxygen
chemistry for the carbons before and after the modifications. A sub-
stantial number of new O-containing functionalities was formed on the
carbon surface after functionalization. This is indicated by a significant
increase in intensity of all peaks, compared to those present in the
spectrum of the pristine material. The deconvoluted spectrum of CSBA-
The selectivity results for the glycerol acetylation versus time are
depicted in Fig. 7 B–D. As can be seen, monoacetins are predominantly
formed at the beginning of the reaction. However, a gradual transfor-
mation of MA into di- and trisubstituted esters occurs with time, as the
selectivity to MA decreases over time, while the simultaneous increase
in the selectivities to DA and TA is observed. This phenomenon is re-
lated to the mechanism of the glycerol acetylation process, which in-
cludes a series of three consecutive reactions [57]. According to it,
glycerol is first converted into MA, MA is transformed into DA, and DA
is further converted into TA. Water is formed as a byproduct.
A certain regularity can be also noticed, namely that, apart from
other parameters, selectivities depend on the conversion of glycerol.
When the glycerol conversion is low (e.g., the initial conversion mea-
1
5
-H
2
SO (Fig. 3S B, Supplementary Materials) reveals the formation of
4
new hydroxyl/phenol or ether groups (B.E. ≈ 5329 eV). However, the
part of this signal can be also assigned to SeO bond in sulfonic groups
sured for the blank test or CSBA-15-H
2
SO ), monoacetins are the products
4
[
46]. Among newly created groups there are also some carbonyl func-
formed in the vast majority. For the higher glycerol conversion, the
content of di- and triacetins is more satisfied. Thus, generally speaking,
as the conversion of glycerol increases also more efficient transforma-
tion of MA to the most desired acetins (DA and TA) is observed.
To sum up, it can be said that the esterification reaction rate is
generally dependent on the acidity of the catalyst. The data on acidity
tionalities as well as SeO bonds in sulfonic functional groups (the signal
at 531.3 eV) [46]. A certain increase in the intensity of the peak at
5
34.1 ± 0.1 eV assigned to carboxylic groups is observed. It means
that the carbon surface was also oxidized to some extent during the
modification. These results are in line with those of elemental analysis
6

J. Goscianska and A. Malaika
Catalysis Today xxx (xxxx) xxx–xxx
Fig. 7. A–D. Catalytic activity of the SO
3
H-bearing carbon samples in glycerol acetylation under the following reaction conditions: temperature 80 °C; glycerol to
acetic acid molar ratio 1:6; catalyst weight 0.7 g.
measurements of the samples collected in Table 1 suggest that the
number of strong acid sites is crucial for the course of the process. A
commercial catalyst Amberlyst 15 showed superior activity in the re-
weight. At the same time, the dependence of selectivity parameters on
the catalyst mass is also clearly visible.
The use of higher amounts of CSBA-15-BDS favors the conversion of
monoacetins to diacetins and triacetin (so, the most desired products).
As a result, the reaction mixture collected after 1 h of the process per-
formed with 0.7 g of CSBA-15-BDS contains only small amounts of un-
reacted glycerol, and the predominant products are diacetins (se-
lectivity about 59%). Simultaneously, the selectivity to triacetin is
about 8%. In other cases, the combined selectivity to DA and TA is
considerably smaller. The composition of reaction mixture changes
with time, in favor of the DA and TA formation. When 0.7 g of the
carbon is used, the combined selectivity to DA and TA is about 80%
after 6 h, and about 85% after 24 h of the process (Fig. 5S,
Supplementary Materials). However, in the latter case, the TA con-
tribution in the reaction products is higher. As can be seen in Fig. 9,
quite satisfied results after 6 h of the glycerol acetylation were also
attained when 0.35 g of the catalyst was applied (combined selectivity
to DA and TA about 75%). The amount of TA formed was, however, a
bit smaller (16.5% and 22.5% for 0.35 and 0.70 g, respectively).
Fig. 10 and Fig. 6S in Supplementary Materials present the variation
in conversion and selectivities to acetins with the changes in glycerol to
acetic acid molar ratio in the process carried out with 0.7 g of the
catalyst. As can be seen, the conversion of glycerol does not alter sig-
nificantly with a change in a molar ratio when the reaction is con-
tinued. The exception is the reaction time < 2 h when notable differ-
ences in XGly are visible. On the other hand, the selectivity parameters
are clearly affected by the glycerol to acetic acid molar ratio. At higher
acetic acid concentration, the results attained at the first stage of the
reaction (1 h) are generally worse – monoacetins dominate, the se-
lectivity to diacetins is relatively low and the amount of triacetin is
unsatisfied. As the reaction proceeds, the abovementioned parameters
vary remarkably, giving finally very good and comparable results.
action, which must be related to the high density of SO H groups on the
3
surface of this material [4.7 meq/g; 53]. However, poor thermal sta-
bility of this system must be taking into account during work under
harsh reaction conditions [58].
As shown above, the conversion and selectivity in glycerol acet-
ylation are dependent on the catalyst nature. Other factors that affect
the catalyst performance are the reaction conditions, such as reaction
temperature and time, glycerol to acetic molar ratio or catalyst weight.
The influence of these parameters on the reaction efficiency was stu-
died. As CSBA-15-BDS showed better activity in the process, this catalyst
was chosen for further tests.
Fig. 8 depicts the catalytic properties of CSBA-15-BDS working at
1
10 °C. Comparing the results obtained here with those shown by this
catalyst at 80 °C (Fig. 7), it can be said that the temperature of the
process strongly affects the activity of the sample. At 110 °C after a
short period of time, above 95% glycerol present in the system is
transformed to acetins. After 6 h of the reaction, selectivity to triacetin
is slightly above 22%, which is twice as much as STA attained after the
same time at the lower temperature used. Because of that, further ex-
periments were carried out at 110 °C.
The activity profile of CSBA-15-BDS after 1 and 6 h as a function of
the catalyst weight/concentration is presented in Fig. 9. The full data on
catalytic activity of the sample versus time of the reaction can be find in
Fig. 5S (Supplementary Materials). As shown in the figures, the initial
glycerol conversion (here after 1 h) increases significantly with an in-
crease in the catalyst weight. The highest value of XGly was obtained
when 0.7 g of the modified mesoporous carbon was applied in the re-
action. This is expected to be related to the number of active sites in-
volved in the glycerol acetylation process, so higher with larger catalyst
7

J. Goscianska and A. Malaika
Catalysis Today xxx (xxxx) xxx–xxx
Fig. 8. Catalytic properties of CSBA-15-BDS in glycerol acetylation under the following reaction conditions: temperature 110 °C; glycerol to acetic acid molar ratio 1:6;
catalyst weight 0.7 g.
Based on the results obtained, the most optimal conditions for
H-bearing mesoporous carbons are: reac-
ordered mesoporous carbons also in other acid-catalyzed processes.
synthesis of acetins over SO
3
tion temperature of 110 °C, a catalyst weight of 0.7 g, a glycerol to
acetic acid molar ratio of 1:9 and the reaction time of 6 h. Under these
conditions, the catalyst gives superior selectivity to the desired di- and
trisubstituted esters together with very high glycerol conversion at a
relatively short time. However, from the economical point of view, the
use of glycerol to acetic acid molar ratio of 1:6 (other parameters the
same as mentioned above) seems to be more viable.
4
. Conclusions
SO H-bearing ordered mesoporous carbons were prepared through
3
modifications with concentrated sulfuric or 4-aminobenzenesulfonic
acid. The characterization data revealed that functionalization brought
about reduction of the carbon porosity, but it simultaneously resulted in
generation of functional groups of acidic nature. The materials showed
excellent catalytic activity in the process of glycerol acetylation, espe-
cially the sample modified via diazonium cation formation. Above 95%
conversion of glycerol and good results of selectivity to di- and triace-
tins (the most desired products) were achieved within a relatively short
To investigate the reusability of prepared materials, a selected cat-
alyst (CSBA-15-BDS) was tested in four subsequent reaction cycles under
the preferred conditions. The obtained results (Fig. 11) show that there
are no significant changes in the activity of the sample. On this basis it
can be concluded that SO H functionalities are strongly attached to the
3
period of time. The number of SO H groups introduced to carbons de-
3
carbon surface and leaching occurs only to a small extent. These find-
pends on the modification procedure, and this parameter was found to
be of great importance to the performed reaction. Besides the sulfonic
ings clearly indicate the possibility of effective usage of SO H-bearing
3
Fig. 9. Catalytic activity of the CSBA15-BDS catalyst used in various amounts after 1 h and 6 h of glycerol acetylation carried out at 110 °C and with glycerol to acetic
acid molar ratio 1:6.
8

J. Goscianska and A. Malaika
Catalysis Today xxx (xxxx) xxx–xxx
Fig. 10. Effect of the acetic acid (Ac) to glycerol (Gly) molar ratio on the catalytic activity of CSBA15-BDS after 1 h and 6 h of the glycerol acetylation process
performed at 110 °C.
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