Mesoporous Sulfonated Carbon Materials Prepared by Spray Pyrolysis

Article abstract of DOI:10.1002/cctc.201500483

A one-step approach was developed for the production of mesoporous sulfonated carbon materials by means of an aerosol synthesis. Nebulizing a clear aqueous solution of sucrose and sulfuric acid through a heated oven leads to subsequent dehydration, carbonization and sulfonation of the carbohydrate structure, in less than two seconds residence time. Acid site concentrations ranging from 0.1 to 0.6 mmol g^-1 can be obtained. Porosity can easily be introduced via salt templating, and can be adjusted by varying the loading and type of salt used. The highest surface area was obtained with Li₂SO₄, giving a BET surface area of 506 m² g^-1 and a mesopore size distribution between 2 and 8 nm. Fructose dehydration and inulin hydrolysis showed that the porous materials synthesized by salt templating are more active than the bulk ones, especially for inulin hydrolysis, for which the initial activity is enhanced by a factor of seven, making these materials competitive with the most active commercial resins. A one-step synthesis for the production of mesoporous sulfonated carbon materials is presented, by which sulfuric acid and an organic precursor are converted, in one step, to an active sulfonated carbon material containing high surface area. Fructose dehydration and inulin hydrolysis reveal the competitiveness with commercial acidic resins.

Full text of DOI:10.1002/cctc.201500483

DOI: 10.1002/cctc.201500483
Full Papers
Mesoporous Sulfonated Carbon Materials Prepared by
Spray Pyrolysis
[
a]
[a]
[a]
[a]
Nicolas Duyckaerts, Ioan-Teodor Trotu s¸ , Valentina Nese, Ann-Christin Swertz,
[a]
[b]
[a]
Sebastian Auris, Hartmut Wiggers, and Ferdi Schüth*
A one-step approach was developed for the production of
mesoporous sulfonated carbon materials by means of an aero-
sol synthesis. Nebulizing a clear aqueous solution of sucrose
and sulfuric acid through a heated oven leads to subsequent
dehydration, carbonization and sulfonation of the carbohy-
drate structure, in less than two seconds residence time. Acid
used. The highest surface area was obtained with Li SO ,
2 4
2
À1
giving a BET surface area of 506 m g and a mesopore size
distribution between 2 and 8 nm. Fructose dehydration and
inulin hydrolysis showed that the porous materials synthesized
by salt templating are more active than the bulk ones, espe-
cially for inulin hydrolysis, for which the initial activity is en-
hanced by a factor of seven, making these materials competi-
tive with the most active commercial resins.
À1
site concentrations ranging from 0.1 to 0.6 mmolg can be
obtained. Porosity can easily be introduced via salt templating,
and can be adjusted by varying the loading and type of salt
Introduction
[
12]
Sulfonated carbon materials are known for carrying strong sul-
fonic acid groups, potentially offering an alternative to the use
or soft- templated carbon materials have been sulfonated
under different sulfonation conditions; also ordered mesopo-
[
1]
[13]
of inorganic acids. They were often reported to have similar
or superior activities to regular commercial acid resins, typically
rous carbon materials and carbon composites were used as
[
14]
starting materials. Many of the mentioned systems are ele-
gantly synthesized, and also for some of the systems excellent
catalytic performance has been reported for different reactions.
Therefore a simple, scalable production process for sulfonated
carbon materials appeared to be highly attractive.
[2]
for biomass conversion. This class of materials were shown to
[
3]
be applicable in the field of adsorbents, ion-exchangers, lithi-
[
4]
[5]
um battery anodes, and fuel cells. Moreover, mesoporous
carbon materials treated with acid also show advantages in
metal dispersion after impregnation with metal salt solutions,
Such a process, leading to the formation of sulfonated
carbon materials with controllable porosity, is described herein.
Our technique makes use of sulfuric acid and an organic pre-
cursor, which are converted in one step to a catalytically active
sulfonated carbon, with the added advantage that the surface
area and porosity of the material can easily be tuned.
[6]
due to the increased hydrophilicity of the support. Mesopo-
rous carbon materials have also been used as templates, which
[7]
are preferably low-cost and highly tunable.
Several synthesis strategies for sulfonated carbon materials
were reported, leading to a wide variety of such materials. Two
general approaches can be chosen: (1) carbon precursors are
Aerosol-synthesis, or spray-pyrolysis, is a straightforward
[8]
[15]
first sulfonated, followed by a partial carbonization, or (2) the
technique for the production of carbon materials.
To the
[9]
carbonization step precedes the sulfonation step. In the first
case, one-step syntheses have been developed, but making
use of already sulfonated organic compounds. In the latter
best of our knowledge, however, there are no reports on the
preparation of sulfonated carbon materials by this route. With
spray pyrolysis we were able to combine carbon formation,
sulfonation, and salt templating in one step, leading to
a simple and low-cost method for the direct synthesis of sulfo-
nated carbon materials, offering texture and porosity control.
To produce the materials, it is sufficient to nebulize an aqueous
solution of sucrose and sulfuric acid into an aerosol, which is
subsequently sent through an oven at elevated temperature.
Evaporation of the water, dehydration of the carbohydrate
structure, carbonization, and sulfonation take place in less than
two seconds residence time.
[10]
[11]
case, several forms of activated carbon materials and hard-
[
a] N. Duyckaerts, I.-T. Trotu s¸ , V. Nese, A.-C. Swertz, S. Auris, Prof. Dr. F. Schüth
Heterogeneous catalysis
Max-Planck-Institut fuer Kohlenforschung
Kaiser-Wilhelm-Platz 1, 45470 Muelheim an der Ruhr (Germany)
E-mail: Schueth@kofo.mpg.de
[
b] Dr. H. Wiggers
CeNIDE, Center for Nanointegration Duisburg-Essen
University of Duisburg-Essen
Carl-Benz-Str. 199, 47057 Duisburg (Germany)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500483.
This publication is part of a Special Issue on Carbon in the Catalysis
Community. Once the full issue has been assembled, a link to its Table of
Contents will appear here.
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Results and Discussion
Functional groups and acidity
Functional groups of the synthesized carbon materials were
qualitatively determined by attenuated total reflectance (ATR)
infrared spectroscopy (Figure 1). The strongest absorptions
Figure 2. Total acidities determined by NaOH titration, comprising weak and
strong acidic sites (blue). Salt-splitting capacities determined by Na ion ex-
change comprising strong acidic sites (red), for various synthesis tempera-
tures.
after that treatment any residual free sulfuric acid has been re-
moved.
However, sulfonated carbon materials are notorious for
partly hydrolyzing and losing acidic groups under reaction
[
8c,10a,12a,17]
conditions.
For the carbon materials described here
only approximately 45% of the strong acidity is prone to hy-
drolyze under aqueous conditions at high temperatures,
whereas the other 55% shows stable catalytic activity. The hy-
drolyzed strong acidic groups can be more labile sulfonic
groups (CÀSO ÀH) but most probably less stable sulfate
3
Figure 1. IR spectra of sulfonated carbon materials synthesized at various
groups (CÀOSO H), which hydrolyze more easily. The leaching
3
temperatures. Main peaks are qualitatively identified following literature
[16]
behavior was investigated by performing a hydrothermal treat-
ment at 1208C, which revealed that no further hydrolysis
occurs after one hour of treatment, not even after prolonging
the treatment up to seven days. The extent of acidity loss de-
pends on the synthesis temperature, ranging from 40 to 50%
for 400 to 8008C. The sulfonic acid of the material synthesized
at 10008C hydrolyzed to an extent of 82%, which leads to the
conclusions that this temperature is not appropriate for the
synthesis of a stable sulfonated carbon.
values.
[16]
were identified based on literature values; of main impor-
À1
tance is the broad absorption band at 1157 cm attributed to
S=O stretching vibrations of sulfonic acid.
The total acidity was measured by HCl back-titration after
stirring the material in a 0.1m NaOH solution (50 mg in 10 mL),
resulting in quantification of the sulfonic acid (strong acidity)
together with possible carboxylic groups and acidic hydroxyls
Energy-dispersive X-ray spectroscopy (EDX) of the cross sec-
tion of a particle reveals uniform sulfur distribution throughout
the carbon matrix (Figure 3), and elemental analysis of sulfur
corresponds to the measured IEC before and after the hydro-
thermal leaching step (Table 1). The acidity measured through
ion exchange is approximately 15% lower than the sulfur con-
(
weak acidity), which are all deprotonated at the high pH of
the NaOH solution. The amount of strong acidity (sulfonic
groups only) is given in terms of salt-splitting capacity, which
can be seen as a measure of ion-exchange capacity (IEC). The
salt-splitting capacity is measured by ion exchange of the sul-
fonated carbon materials in 1m NaCl solution (50 mg in 10 mL)
after which a known fraction of the supernatant (5 mL) is titrat-
ed with NaOH. In Figure 2 it is illustrated how these acidities
vary with synthesis temperature; higher pyrolysis temperatures
lead to a decrease in strong acidity and an even bigger de-
crease in weak acidity. This result already suggests a higher
carbonization degree of the materials, as evident from the ele-
mental analysis (see below).
During a synthesis of typically 2 h, the particles were
trapped in a water bottle that reaches approximately 608C by
the heat of the carrier gas, bringing the synthesized material
to an average contact time of 1 h with hot water. Afterwards,
À1
the material was rinsed extensively with water (20 Lg ) during
Figure 3. SEM image and corresponding sulfur elemental mapping of a
microtome cross sections showing a homogeneous distribution of sulfur in
a mesoporous carbon particle.
which neutrality of the filtrate is quickly reached as well as
a negative barium sulfate test. This allows the conclusion that
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Table 1. Elemental analysis of carbon materials synthesized at various
temperatures together with acidity content measured by ion-exchange.
À1
(
synthesis conditions: sucrose 100 gL , 1m H
2
SO
4
)
[
a]
À1
[b]
T [8C]
C [wt%]
H
O
S
S
[mmolg
]
Acidity
4
4
6
6
8
8
1
1
00
00HT
00
00HT
00
00HT
000
52.91
55.84
58.77
60.02
63.13
63.96
70.00
71.61
3.70 39.82 3.57 1.12
3.44 38.85 1.87 0.58
2.71 36.06 2.46 0.77
4.54 34.07 1.37 0.43
1.41
0.59
0.63
0.60
0.49
0.30
0.41
0.12
[
[
[
c]
c]
c]
3.11
31.87 1.89 0.59
3.02 32.14 0.88 0.28
3.55 25.05 1.40 0.44
2.75 25.37 0.27 0.08
[
c]
000HT
À1
Figure 4. SEM picture of sulfonated carbon (sucrose 100 gL , 1m aqueous
SO , 8008C).
[
[
a] Determined by elemental analysis. [b] Determined by ion-exchange.
c] Hydrothermal hydrolysis step at 120 8C for 2 h.
H
2
4
tent, except for the materials sprayed at 4008C, for which the
IEC is 25% higher. This could be attributed to the low synthesis
temperature, resulting in a larger amount of weakly acidic
À1
groups (3 mmolg ), which could influence the IEC. As can be
expected, the carbon content per mass increases with increas-
ing pyrolysis temperature, whereas the oxygen and sulfur con-
tent decrease. The elemental analysis results permitted to cal-
culate the overall production yield of this process, because the
volume of liquid sent through the oven was known to be ap-
À1
proximately 5 mLh , and the amount of material obtained is
À1
1
3
00 mgh . The calculated carbonization yield is approximately
0% with respect to the carbon content of sucrose, and the
yield of sulfur incorporation is approximately 1.2% for the sul-
fonated carbon materials synthesized at 8008C. Thermogravi-
metric analysis in air confirmed that the higher carbonization
extent results in an increased thermal stability of the carbon
matrix (see the Supporting Information, Part 1).
Figure 5. TEM images of microtome cross sections showing a homogeneous
À1
distribution of the mesoporous structure. (100 gL sucrose, 3 g Na
1m H SO solution; 6008C left, 8008C right)
2 4
SO in
2
4
Introducing porosity by salt templating
When a sucrose and sulfuric acid solution is sprayed, the drop-
lets are carried through a heated section of a tube in which
water begins to evaporate, sulfuric acid and sucrose become
more and more concentrated and begin reacting. Through
a series of dehydration, carbonization and sulfonation reac-
tions, a bulk carbon material is obtained, with particle sizes
ranging from 1 to 30 mm. Surface areas are approximately 2–
2
À1
Figure 6. Effect of sodium sulfate loading (wt% towards sucrose) on the ob-
tained BET surface (left) and the cumulative pore volume (right) for materials
synthesized at 8008C.
3
m g . Scanning electron microscopy (SEM) analysis revealed
that these particles mainly consist of broken hollow structures
[15e,18]
(
Figure 4), which is typical for this synthesis method.
The
high initial temperature gradients in the droplets make the car-
bonization start around the interface initiating hollow sphere
formation, while the subsequent water evaporation leads to
a rupture of the hollow structure.
termines porosity. As seen in Figure 6, a plateau is reached at
2
À1
a surface area of 400 m g between 18 and 38 wt% Na SO4
2
loading, between which the pore diameter increases with load-
ing, giving a maximum in measurable pore volume obtained
at 33 wt%.
Porosity can easily be generated by adding inorganic
[15d,19]
salts
to the clear starting solution. If a salt is present in
the sprayed solution, it crystallizes in parallel with the above
mentioned reactions and it therefore serves as a hard tem-
plate, which is readily removed in the water trap. The micro-
graphs in Figure 5 illustrate that the salt templating occurred
uniformly throughout the carbon structure and the plots in
Figure 6 depict to which extent the sodium sulfate loading de-
The salt-templated materials synthesized at 400 and 6008C
had much lower surface areas than those produced at 8008C,
probably as a result of collapse of the pores upon drying (Sup-
porting Information, Part 2). When the salt templating was per-
formed with sulfates having different cations at equal molar
concentrations, different surface areas and pore size distribu-
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tions were obtained (Supporting Information, Part 2). Interest-
ingly, Li SO templating led to a much narrower pore-size dis-
sulfuric acid showed the highest activity, followed by Amber-
lyst70, porous sulfonated carbon, bulk sulfonated carbon and
the blank reaction (turnover frequency per hour: 2.9, 1.9, 1.5,
0.5, 0.1, respectively; calculated after 1 h reaction). Thus, the
porous matrix provided by salt templating leads to an increase
by a factor of three in initial reaction rate compared to the
bulk carbon material, which can be explained by enhanced
transport kinetics or improved accessibility of the active sites.
Yields of 5-hydroxymethylfurfural, levulinic acid, and humins
are shown in the Supporting Information (Supporting Informa-
tion, Part 6).
2
4
tribution (2–8 nm) than Na SO and K SO (2–25 nm) templat-
2
4
2
4
ing, leading to a lowered pore volume and higher surface area
2
À1
(
506 m g ). The use of sulfate salts lowers the sulfonation
degree by approximately 15%, attributable to the lowered
acidity in solution caused by the protonation of the sulfate
ions to bisulfate. This lowered sulfonation, however, could be
compensated by increasing the sulfuric acid concentration of
the starting solution to restore initial acidities, which is consis-
tent with the fact that the salt templating, regardless of the
salt used, does not affect the sulfonation degree, as long as
the pH is kept constant. The weak acidity however, is increased
upon salt templating, by 20% for Na SO and K SO , and even
Inulin hydrolysis to fructose and glucose was performed to
investigate the activity of the materials in the conversion of
bulkier substrates (Figure 8). Interestingly, the porous sulfonat-
2
4
2
4
by 240% for Li SO at a synthesis temperature of 8008C (Sup-
2
4
porting Information, Part 4). This phenomenon could be attrib-
uted to the decreased cross-linking at the interface of the salt
crystals and the carbon matrix during synthesis, resulting in ad-
ditional weakly acidic hydroxyl groups.
Insights into Fructose dehydration and inulin hydrolysis
Fructose dehydration to 5-hydroxymethylfurfural and subse-
quent hydrolysis to levulinic acid and formic acid, was used to
investigate the acitivity of the sulfonated carbon materials in
comparison with sulfuric acid and commercial acid resins
(
Figure 7). The type of commercial resin (Amberlyst70) used
Figure 8. Yield of fructose from inulin hydrolysis, for various catalysts.
2
(0.56m fructose equivalents, 4 mol% acidic groups, 708C, H O)
ed carbon material shows higher activity than Amberlyst70,
whereas the bulk sulfonated carbon material has inferior activi-
ty. This confirms again the positive influence of the porosity on
mass transfer, which is more pronounced for bulkier reactants.
For inulin, the induced porosity leads to an increase in initial
activity by a factor of seven.
Conclusions
Figure 7. Conversion profile for fructose dehydration with various catalysts,
fructose/acidity 25:1. (0.56m fructose, 4 mol% acidic groups, 1208C, H O).
The spike seen for the bulk material at short times has repeatedly been ob-
served to different extent; see the Supporting Information.
Stable mesoporous sulfonated carbon materials were obtained
by a remarkably easy process. By nebulizing a clear aqueous
solution of sulfuric acid, sucrose, and sodium sulfate through
an oven, a mesoporous sulfonated carbon material was readily
obtained within a remarkably short residence time of 2 s. As
observed for different sulfonated carbon materials at varying
extent, not all sulfonic acid groups were stable. Hydrothermal
treatment at 1208C and autogenous pressure revealed that be-
tween 50 and 60% of the sulfonic groups are stable for several
days under these conditions. The incorporation of porosity by
salt templating had clear benefits in catalysis, increasing the in-
itial reaction rate by 300% for fructose dehydration and by
700% for inulin hydrolysis, assumedly caused by the better ac-
cessibility and enhanced transport kinetics. The mesoporous
sulfonated carbon materials therefore can compete with com-
mercial acidic resins. In addition, the synthesis route is scalable
and makes use of rather cheap starting materials. It also shows
2
for comparison in this paper, is the most active for fructose de-
hydration amongst three other Amberlyst resins tested (Sup-
porting Information, Part 5). The sulfonated carbon materials
synthesized at 8008C were selected for catalysis because of
their high surface areas, combined with a higher carbonization
degree, which leads to a higher observable structural stability
in water (“bulk 8008C” refers to the material synthesized with
À1
1
00 gL sucrose in a 1m H SO solution, and “porous 8008C”
2
4
was synthesized with additional 23.1 wt% Na SO ). The carbon
2
4
materials used for catalysis had all been hydrothermally treated
at 1208C for 2 h and washed before catalysis, to ensure that all
leachable species had been removed. For fructose dehydration,
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that aerosol synthesis techniques are a versatile approach to-
wards functionalized materials, promising further innovations.
images were recorded with a Hitachi H-7100 Transmission Electron
Microscope operated at 100 kV. STEM images and EDX analysis
were performed on a Hitachi HD-2700 dedicated Scanning Trans-
mission Electron Microscope operated at 200 kV equipped with an
EDAX Octane T Ultra W EDX detector. IR spectroscopy was per-
formed on an ATR diamond cell with a Nicolet Magna IR 560 spec-
trometer.
Experimental Section
Material synthesis was performed using an in-house built spray py-
rolyser, depicted in Figure 9. The ultrasonic atomizer nozzle
Nitrogen sorption analysis was performed with a Micromeritics
ASAP 2000 instrument. Thermogravimetric analysis (Air,
(
130 kHz) was purchased from Sonaer Ultrasonics. The nozzle
À1
1
08Cmin ) was performed with a Netzsch Jupiter STA 449 C. Ele-
holder was build in-house together with the trap whereas all re-
maining parts are standard equipment.
mental analysis was performed by: Chemical laboratory H. Kolbe,
Mülheim/Ruhr.
Acknowledgements
The authors kindly thank Silvia Palm for the SEM analysis and
discussion; Heike Hinrichs and Marie Sterling for the HPLC analy-
sis and discussion; together with Andre Pommerin and Layla
Sahraoui. Financial support by the ERC POLYCAT project is grate-
fully acknowledged.
Keywords: aerosol synthesis · fructose dehydration · salt
templating · spray pyrolysis · sulfonated carbon
Figure 9. Schematic representation of the spray-pyrolysis setup.
The sulfonated carbon materials were synthesized from a clear so-
lution of sucrose (50–300 gL ) in an aqueous sulfuric acid solution
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À1
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