A solvent-free, one-step synthesis of sulfonic acid group-functionalized mesoporous organosilica with ultra-high acid concentrations and excellent catalytic activities

Article abstract of DOI:10.1039/c8gc00002f

We demonstrate herein a novel solvent-free technique for the synthesis of sulfonic acid group-functionalized mesoporous organosilica, which was achieved from the self-assembly of a block copolymer template using mercaptopropyltrimethoxysilane (MPTS) and tetramethoxysilane (TMOS) under melting conditions without using additional solvents and subsequent condensation at high temperature (up to 140 °C). The resultant samples were designated as SMS-xs, where x stands for molar ratio of MPTS/(MPTS + TMOS). SMS-xs have relatively large BET surface areas, highly cross-linked frameworks, and abundant and uniform mesopores with wormhole-like characteristics. Interestingly, MPTS could be used as the solo precursor to prepare mesoporous organosilica (SMS-1.0), which had controllable acidity and an ultra-high concentration of sulfur (5.51 mmol g^-1, the highest acid density to date), which was even higher than those of commercial Amberlyst 15 (4.7 mmol g^-1), HS-JLU-20-0.8 (4.61 mmol g^-1) and sulfonated carbon (CH_0.30O_0.33S_0.16, 4.90 mmol g^-1). These structural characteristics give SMS-xs excellent activities and good reusability in biomass conversions and fine chemicals synthesis, which are much better than various solid acids, such as Amberlyst 15, H-form USY zeolite, and sulfonic acid group-functionalized ordered mesoporous silica.

Full text of DOI:10.1039/c8gc00002f

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Solvent-free, one-step to the synthesis of sulfonic group
functionalized mesoporous organosilica with ultra-high acid
concentrations and excellent catalytic activities
Qin Wu a,b, Fujian Liu*a,b, Xianfeng Yi c, Yongcun Zoud and Lilong Jiang*a
a National Engineering Research Center of Chemical Fertilizer Catalyst (NERC-CFC),
School of Chemical Engineering, Fuzhou University, Gongye Road No.523, Fuzhou
350002, Fujian, PR China. E-mail: fjliu1982@gmail.com,jll@fzu.edu.cn
b
College of Chemistry and Chemical Engineering, Shaoxing University, Shaoxing,
312000, China.
c State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics,
National Center for Magnetic Resonance in Wuhan, Key Laboratory of Magnetic
Resonance in Biological Systems, Wuhan Institute of Physics and Mathematics,
Chinese Academy of Sciences, Wuhan 430071, P. R. China.
d State Key Laboratory of Inorganic Synthesis & Preparative Chemistry, Department of
Chemistry, Jilin University, Changchun 130012, China.
Abstract
We demonstrate here a novel solvent-free technique to the synthesis of sulfonic
group functionalized mesoporous organosilica, which was achieved from
self-assembly of block copolymer template with mercaptopropyltrimethoxysilane
1

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(MPTS) and tetramethoxysilane (TMOS) under melting condition without using
additional solvents, and further condensation at high temperature (up to 140 °C).
The resultant samples were designated as SMS-xs, where x stands for molar ratio of
MPTS/(MPTS+TMOS). SMS-xs have relatively large BET surface areas, highly
cross-linked framework, abundant and uniform mesopores with wormhole-like
characteristics. Interestingly, MPTS could be used as the solo precursor to prepare
mesoporous organosilica of SMS-1.0, which has controllable acidity and ultra-high
concentration of sulfur (5.51 mmol g-1, the highest acid densities to data), even
higher than those of commercial Amberlyst 15 (4.7 mmol g-1), HS-JLU-20-0.8 (4.61
mmol g-1) and sulfonated carbon (CH0.30O0.33S0.16, 4.90 mmol g-1). The above
structural characteristics make SMS-xs show excellent activities and good reusability
in biomass conversions and fine chemicals synthesis, much better than various solid
acids such as Amberlyst 15, H-form USY zeolite, sulfonic group functionalized ordered
mesoporous silica.
Keywords: Solvent-free synthesis; Self-assembly; Solid acids; Biomass conversions;
Mesoporous organosilica.
Introduction
Acid catalysis acts as one of the most important reaction has received extensive
research interest in these years, which been widely used in the areas of oil refining,
organic synthesis, biomass conversion and fine chemicals synthesis 1-10. Within this
regard, traditional acids such as sulfuric acid, hydrochloric acid, AlCl3, and
2

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HSO3CF3have been widely used in industry due to their advantages of low cost and
excellent catalytic activities 4, 11-14. However, the drawbacks such as volatile property,
high toxicity, strong corrosion, intensive energy consumption for regeneration
strongly constrain their widely industrial applications 5-14. Therefore, the exploring of
suitable alternatives to liquid acids as the catalysts in various acid-catalyzed reactions
is highly desired in these years.
For this purpose, extensive research works have been paid on the development
of solid acids, and use them in a variety of acid-catalyzed reactions. The solid acids
show the advantages including negligible volatility, environmentally friendliness,
reduced corrosion, good activities and easy regeneration 5-16. From the point of view
of supports, the solid acids could be divided into inorganic and organic frameworks 4,
13, 14, 17-20. Compared with inorganic solid acids, organic solid acids such as sulfonated
porous organic polymers (POPs), metal organic frameworks (MOFs) based solid acids,
acidic resins and sulfonated carbons exhibit unique characteristics of easy
functionality, good chemical stability, diverse in building blocks, which show high
17,
concentrations of acid sites, further resulting in their enhanced catalytic activities
21-25. For instance, Bhaumik and coworkers successfully synthesized polymeric solid
acids with enhanced acidity up to 4.4 mmol g-1, which have been used in Hantzsch
condensation and C-H activation 26, 27. However, the low thermal stabilities of
polymeric network strongly constrain them used as long-lived catalysts in various
reactions. The solid acids with inorganic frameworks such as silica, zeolites,
sulfonated metal oxides show much enhanced thermal stability in comparison with
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organic solid acids 13, 28-31, and the most reported inorganic solid acids are
mesoporous organosilica such as sulfonic group functionalized mesoporous silica,
which exhibit homogeneous dispersion and strong acid sites, large BET surface areas,
abundant and controllable mesopores 20, 30-32. The mentioned structural properties
strongly enhance the accessibility of active site, and thus provide enhanced pathways
for the fast diffusion of reactants and products in variously acid-catalyzed reactions.
Notably, the acid concentrations and accessibility of acid sites play key factors for
their catalytic performances 21, 22, 33. Therefore, the development of mesoporous
organosilica with very high acid concentrations, abundant and controllable
mesopores are highly desirable, which will largely enhance their catalytic
performances. However, the reported mesoporous silica usually show limited
concentrations of sulfonic group because of the large collapse of mesopores after
introducing high contents of sulfonic groups 20, 30-32. Xiao and coworkers successfully
synthesized highly sulfonic group functionalized mesoporous silica by employing
32
fluorocarbon-hydrocarbon surfactants as mixed templates at high temperature
.
However, inorganic silica precursor (TEOS) still needs to be introduced to prevent the
collapse of mesopores. Moreover, nearly all of mesoporous organosilicas were
synthesized in presence of abundant solvents such as water and alcohol, which
generates large amount of wastes, results in complicated synthetic processes and
high synthetic cost 34, 35. The above disadvantages strongly constrain the wide
applications of mesoporous silica based solid acids in industry 36-40
.
Recently, solvent free technique has been successfully realized for preparation of
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zeolites and porous organic polymers, which strongly promotes their widely
industrial applications 37-41. However, there were still few reports on preparation of
mesoporous silica especially for mesoporous organosilica based solid acids by using
this state-of-the-art technique. In addition, it remains a great challenge to synthesize
porous inorganic solid acids with ultra-high acid concentrations, which will largely
improve their activities in various acid-catalyzed reactions. In this work, we
innovatively expanded the solvent-free technique for preparation of sulfonic group
functionalized mesoporous silica, where MPTS could be used as solo precursor. It is
noteworthy that SMS-xs synthesized in this work show the highest acid density to
data (See Table S1), controllable and strong acidity. Interestingly, SMS-xs show
excellent activities in
a
variety of acid-catalyzed reactions including
transesterification, esterification, and depolymerization of crystalline cellulose,
higher than various solid acids including HS-JLU-20-0.8, H3PW12O40, SBA-15-SO3H-0.2,
Amberlyst 15, carbonaceous solid acids of CH0.30O0.33S0.16 and H-USY. The preparation
of SMS-xs open a cost-effective and sustainable approach for preparation of
mesoporous organosilica based solid acids, which offers great opportunity for their
wide applications in the industry.
Experimental section
Chemicals and reagents
All reagents were of analytical grade and used as purchased without further
purification.
Amberlyst
15,
5-hydroxymethylfurfural
(HMF)
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1-n-butyl-3-methylimidazolium
([C4mim]Cl), 3-mercaptopropyltrimethoxysilane
(MPTS), tetramethoxysilane (TMOS), crystalline cellulose of Avicel, tripalmitin,
nonionic block copolymer surfactant (Pluronic F127, Mw=12600) were purchased
from Sigma-Aldrich Company, Ltd. (USA). Ethanol, methanol, n-dodecane, palmitic
acid, HCl, H2SO4, sunflower oil were purchased from Sinopharm Chemical Reagent
Co., Ltd, Shanghai, China. H-form of Beta zeolite and ultra-stable Y zeolite (USY) were
supplied by Sinopec Catalyst Co.
Synthesis of SMS-xs
As a typical run for the synthesis of SMS-1.0, 10.0 mmol of MPTS, 1.3 g of
Pluronic F127 and 0.8 mL of HCl were mixed together, and stirring at room
temperature for 2 h. The mixture was then transferred into an autoclave and cured at
140-180 °C for 24h, which gave the pristine products with yellow color. SMS-0.8 could
be synthesized with similar procedures: 8.0 mmol of MPTS, 2.0 mmol of TMOS, 1.3 g
of Pluronic F127 and 0.8 mL of HCl were mixed together, after stirring of the mixture
at room temperature for 2 h, which was then transferred into an autoclave and cured
at 140-180°C for 24 h, giving the pristine SMS-0.8 with yellow color. To obtain SMS-xs
with opened mesopores, the F127templatecould be removed by extraction with the
mixed solvent of ethanol and HCl. The extraction process was performed at 85 °C
under refluxing condition for 48 h, and the volume ratio of ethanol to HCl was 7:1.
Similarly, SMS-0.5 could also be synthesized with similar procedures as that of
SMS-1.0 and SMS-0.8, where MPTS and TMOS with different molar ratio were
employed as the mixed precursor. To transform thiol into sulfonic group in SMS-xs,
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1.0 g of extracted SMS-xs was dispersed into 20 mL of H2O2, the suspension was
stirred at 80 °C in a N2 atmosphere for 24 h, which was then treated with 2M HNO3 at
60 °C for another 24 h. The resultant samples could be obtained from centrifugation,
washing with abundant water and drying at 80 °C for 12 h. Before catalytic
applications, the SMS-xs was activated with following procedures: 0.5 g of extracted
SMS-xs was dispersed into 50 mL of 1 M sulfuric acid. After stirring of the mixture for
24 h at room temperature, the sample was washed with abundant water to remove
the residual sulfuric acid, and the ion exchange process was repeated for two times.
Finally, the sample was dried at 80 °C for 12 h under vacuum condition before use.
For comparison, SBA-15-SO3H-0.2, HS-JLU-20-0.8and CH0.30O0.33S0.16 were synthesized
as the literatures 30, 32, 42
.
Characterizations
Nitrogen adsorption-desorption isotherms were measured using a Micromeritics
ASAP 3020M system. The samples were outgassed for 10 h at 180 °C before the
measurements. The pore size distribution for mesopores was calculated using
Barrett-Joyner-Halenda (BJH) model. FT-IR spectra were recorded by using a Bruker
66V FT-IR spectrometer. Thermogravimetric analysis (TGA) curves were determined
on a Mettler-Toledo TGA/SDTA851e system under flowing air with a ramping rate of
20 °C min-1. TEM images were performed on a JEM-2100F electron microscope (JEOL,
Japan) with an acceleration voltage of 200 kV. XPS spectra were performed on a
Thermo ESCALAB 250 with Al Kα radiation at y=901 for the X-ray sources, the binding
energies were calibrated using the C1s peak at 284.9 eV. 13C solid state NMR
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experiments were carried out on a Varian Infinityplus-300 spectrometer at resonance
frequencies of 75.38 MHz. The experiments were recorded using a 4 mm
double-resonance MAS probe at a spinning rate of 10 kHz. The pulse width (π/2) for
13C was measured to be 4.1 μs. A contact time of 4 ms and a recycle delay of 3 s were
used for the 1H-13C CP/MAS measurement. 29Si solid state NMR spectra were
recorded on a Bruker MSL-400WB spectrometer, fitting the samples in a 7 mm ZrO2
rotor, spinning at 8 kHz.
The solid-state 31P NMR spectra over SMS-xs catalysts were performed as the
following procedures: prior to the sorption of trimethylphosphine oxide (TMPO)
probe molecule, the samples were placed in glass tubes, which were connected to a
vacuum line for dehydration. The samples were kept at a final temperature at 453 K
for 24 h, with the pressure below 10-3 Pa. Then, they were cooled. The preparation of
TMPO adsorbed samples was performed according to the method proposed by
Zheng and coworkers 43. Prior to the NMR experiments, the sealed sample tubes
were opened and the samples were transferred into NMR rotors with a Kel-F end cap
under a dry nitrogen atmosphere in a glove box. 31P NMR experiments were
performed on a Bruker Ascend-500 spectrometer at a resonance frequency of 202.34
MHz for 31P nucleus, with a 4 mm triple-resonance MAS probe at a spinning rate of
12 kHz. Pulse width (π/2) for 31P was measured to be 4.5 μs. 31P MAS NMR spectra
with high power proton decoupling were recorded with a recycle delay of 30 s. The
chemical shift of 31P resonance was externally referenced to 1 M aqueous H3PO4.
Catalytic reactions
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Esterification of palmitic acid with methanol
As a typical run for esterification of the palmitic acid with methanol: 1.282 g of
palmitic acid (5 mmol), 2.5 mL of methanol and 0.1 g of catalyst were mixed together
into a flask equipped with a condenser and a magnetic stirrer. Then, the reaction was
performed at 65 °C for 5 h under vigorous stirring. At different time intervals,
samples were withdrawn and analyzed by using gas chromatography. The product in
the reaction was methyl palmitate with selectivity near 100 %, which was analyzed
by using Agilent 7890A gas chromatography equipped with a flame ionization
detector (FID). The column was HP-INNOWax capillary column (30 m); The initial
temperature was 100 °C, the ramping rate was 20 °C min−1, and the final temperature
was 280 °C; the temperature of the FID detector was 300 °C. In this reaction, the
yield of methyl palmitate was calculated by using the internal standard (dodecane)
method and established the corresponding correction factor.
Transesterification
As a typical run for transesterification of tripalmitin with methanol: 0.84 g (1.04
mmol) of tripalmitin and 0.10 g of catalyst were mixed well in a flask equipped with a
condenser and magnetic stirrer at 70 °C, followed by the addition of 3.71 mL (93.6
mmol) of methanol. The reaction was lasted for 16 h under vigorous stirring and
refluxing. The catalysts could be regenerated from centrifugation, washing with
abundant CH2Cl2 for removing of the absorbed reactants and products, and drying at
60 °C under vacuum condition. The product in this reaction was methyl palmitate
with selectivity near 100 %. In this reaction, the yield of biodiesel was evaluated by
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using internal standard method, where dodecane was used as internal standard.
Transesterification of sunflower oil with methanol was performed as the
following procedure: 1.0 g of sunflower oil (1.16 mmol) and 0.15 g of catalyst were
added into a three-necked round flask equipped with a condenser and a magnetic
stirrer, and the reaction temperature was rapidly increased to 80 °C. Then, 3.5 mL of
methanol (86.3 mmol) was quickly introduced under vigorous stirring, the reaction
was lasted for 18 h. The molar ratio of sunflower oil/methanol was 1/72.1 and the
mass ratio of catalyst/sunflower oil was 0.15. The main products in this reaction were
methyl palmitate (C16:0), methyl stearate (C18:1), methyl oleate (C18:2), methyl linoleate
(C18:3), methyl arachidate (C20:0), 11-eicosenoic methyl (C20:1), methyldocosanoate
(C22:0) and methyltetracosanoate (C24:0). The quantitative analysis of these products
was performed on an Agilent GC/MS instrument (Agilent 6890N/5975I) equipped
with a programmable split/splitless injector. The injector-port temperature was set at
270 °C. The oven-temperature program was initially set at 140 °C and ramped to
270 °C with the ramping rate of 10 °C min−1. The sample could be easily recovered
from filtration, washed with abundant methanol and drying at 60 °C for 12 h under
vacuum condition. Before recycling, the catalyst was activated with 0.1M H2SO4 for
12 h, and washed with abundant water for removing absorbed H2SO4, which was
then used for the next time.
Depolymerization of Avicel cellulose
100 mg of Avicel cellulose was dissolved into 2.0 g of [C4mim]Cl ionic liquid at
100 °C for 5 h under vigorous stirring, which gave a clear cellulose solution. Then, 20
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mg of specific catalyst was introduced into the mixture, after stirring of the mixture
for 40 min, then 600 μL of water was added into the reaction system with a very slow
rate. The reaction temperature was kept at 100 °C. At different time intervals, the
samples were withdrawn, weighed (recorded as M1), quenched immediately with
cold water, and centrifuged at 14,800 rpm for 5 min for removing of catalysts and
unreacted cellulose. The resultant reaction mixtures were subsequently analyzed,
the volume of which was measured and recorded as V1. Unreacted Avicel was
separated, washed, dried and weighed.
Preparation of DNS Reagent
DNS reagent was prepared as following procedures: 182 g of potassium sodium
tartrate was added into 500 mL of hot deionized water at 50 °C, followed by addition
of 6.3 g of 3,5-dinitrosalicylic acid (DNS) and 262 mL of 2 M NaOH. 5 g of phenol and
5 g of sodium sulfite were then introduced into the solution under vigorous stirring
to obtain homogeneous solution. The solution was cooled down to room
temperature and diluted with deionized water to 1000 mL to give the DNS reagent.
Total Reducing Sugar (TRS) tests
TRS value was measured by DNS method as literature 8, 42. 0.5 mL of DNS regent
was mixed with 0.5 mL of the reaction solution and heated at 100 °C for 5 min. The
mixture was then cooled down to room temperature, and 4 mL of deionized water
was added to dilute the solution for five times. The adsorption at 540 nm was
measured in a calibrated NanoDrop2000 UV-spectrophotometer. The value of TRS
was then determined based on a standard curve, which was built from standard
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glucose solution.
Measuring the yields of glucose, cellobiose and HMF
The concentrations of glucose, cellobiose and HMF in the reaction mixture were
measured by a Shimadzu LC-20A high-performance liquid chromatography (HPLC)
system equipped with both refractive index detector (RID) and ultraviolet detector
(UVD) based on the standard curve methods; Separation was carried out with an
SCR-101N column using extra-pure water during the mobile phase at a flow rate of
0.5 mL min−1. The column temperature was set to 50 °C, and the volume of each
injection was 10 µL. Pre-measured glucose, cellobiose and HMF were used to
establish the calibration curves for the HPLC. The concentrations of soluble sugars
from the reactions were then determined from the calibration curves (e.g., glucose
yield %=carbon mass of glucose/mass of cellulose, equation 1; Cellobiose
yield %=carbon mass of cellobiose/carbon mass of cellulose, equation 2).
MGlucose
………………… (1)
YieldGlucose
=
×100%
MCellulose
M Cellobiose
YieldCellobiose
=
×100%
………………… (2)
MCellulose
Results and discussion
Porosity characterizations
Figure S1 shows small angle XRD patterns of various SMS-xs samples, which was
a classical and reliable technique for investigating the regularity of mesoporous
materials. Notably, SMS-0.2, SMS-0.5 and SMS-0.8 show a well resolved diffraction
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peak centered at 0.91° indexed as (100) reflection, indicating the worm-like
mesopores with certain ordering in these samples 34. In contrast, a very weak and
broad peak diffraction peaks could be observed in SMS-1.0, suggesting its much
decreased ordering of mesopores in comparison with other samples 34
.
The textural properties of synthesized SMS-xs samples were determined by using
N2 as the probing molecule. Figure 1 shows the N2 adsorption-desorption isotherms
at -196 °C and the mesopore size distributions of SMS-xs. Notably, all the three
samples of SMS-1.0, SMS-0.8 and SMS-0.5 show typical type-IV isotherms with
relatively high volume adsorption, which increases steeply at the relative pressure of
P/P0=0.5-0.85, indicating the presence of abundant mesopores in these samples. The
corresponding mesopore size distribution covers a broad range of 4.5-10.0 nm. The
calculated textural parameters of SMS-xs are also illustrated in Table 1. It is found
that SMS-xs have relatively large BET surface areas and total pore volumes (214~589
m2 g-1, 0.29~0.81 cm3 g-1, Table 1), which are higher than variously reported solid
acids 32, 44. It is noteworthy that the BET surface areas and total pore volumes of
SMS-xs decrease with the increasing of sulfur contents. For instance, SMS-0.2 with
lowest sulfur content, has the largest BET surface area and total pore volume at 589
m2 g-1 and 0.81 cm3 g-1 respectively, larger than those of SMS-0.5, SMS-0.8 and
SMS-1.0. This phenomenon should be attributed to the loaded sulfur largely
increases the weight of the framework of SMS-xs, which has negative effect on their
BET surface areas and pore volumes. Similar results could also be found in mesopore
size distribution, the higher sulfur contents results in the decreased mesopore sizes.
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For instance, SMS-0.5 shows the mesopore size at 10.0 nm, larger than those of
SMS-0.8 (6.9 nm) and SMS-1.0 (4.6 nm). The decreased mesopore sizes in SMS-0.8
45
and SMS-1.0 was resulted from the blocking of their mesopores by sulfonic group
.
Therefore, the mesoporosity and BET surface areas of SMS-xs can be easily tuned by
adjusting the ratio of MPTS/TMOS.
Furthermore, SMS-xs show the highest acid concentrations among various solid
acids to data. For instance, the sulfur content of SMS-1.0 was as high as 5.51 mmol
g-1, and its acid capacity was up to 5.59 mmol g-1 (Table 1), much higher than
variously reported solid acids (Table S1). In fact, both hydroxyl and sulfonic groups in
SMS-xs contribute the acidity, and SMS-xs contain large amount of silicon hydroxyl
group besides sulfonic group (31P NMR spectra). However, a high density of sulfonic
acid groups can cause the formation of pairs that after the ion exchange treatment
retains a bridged H+ besides a Na+ ion, apart from the accessibility. Furthermore, the
disulfide bonds should also be formed in the high sulfonated samples 46, 47. Although
the presence of the mentioned two negative effects on the acidity, the presence of
large amount of silicon hydroxyl group still results in relatively higher acidity than the
S content in SMS-xs samples (Table 1). The high acid density plays key factor for the
activities of solid acids in a variety of acid-catalyzed reactions such as biomass
conversions and fine chemicals synthesis.
Figure 2 shows the high-resolved scanning electron microscope (SEM) images of
SMS-0.5 and SMS-0.8 with high magnifications. Both samples show rough surface
characteristics with abundant aggregated and worm-hole like mesoporosity, and the
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packed pore sizes were distributed in the range of 20-50 nm, while the worm-like
mesopores were distributed in the range of 5-20 nm. Compared with SMS-0.5, the
building unit in SMS-0.8 shows more uniform spherical morphology, which was
resulted from its high contents of organosilica 32. In contrast, SMS-0.5 monolith was
formed from the aggregation of the SiO2 nanoparticles with irregular shapes. The
above mentioned phenomenon results in the more uniform mesopores of SMS-0.8 in
comparison with SMS-0.5, which agrees well with N2 adsorption-desorption results
(Figure 1).
Figure 3 shows transmission electron microscopy (TEM) images of SMS-0.5 and
SMS-0.8. Notably, both samples show abundant and uniform mesopores with a
wormhole like characteristics. Compared with SMS-0.8, SMS-0.5 shows enhanced
ordering of mesopores (Figure 2C), suggesting high sulfur content disturbs the
self-assembly and condensation processes, which has negative effect on the ordering
of mesopores. The mesopore sizes of SMS-0.8 and SMS-0.5 estimated from TEM
images, were distributed in the range of 5.0-10.0 nm, which agree well with N2
adsorption-desorption isotherms results. TEM images confirm that abundant and
worm-hole like mesopores could be introduced into SMS-xs via the novel solvent free
technique exploited by us in this work, which was consistent with small angle XRD
results (Figure S1).
Structural and acidity characterizations
Figure 4 shows 13C and 29Si solid state NMR spectra of SMS-0.5 and SMS-1.0,
which is an effective and reliable technique for investigating the chemical structures
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of various solid materials. SMS-0.5 shows two groups of peaks in the range from -50
to -80 ppm and -100 to 130 ppm, which are attributed to the signals of
organosiloxane [Tm, RSi(OSi)m(OH)3-m, m=1-3; T3 at -68 ppm and T2 at -60 ppm] and
siloxane [Qn, Si(OSi)n(OH)4-n, n=2-4; Q4 at -111 ppm and Q3 at -101 ppm], respectively
30, 32. Notably, the signals associated with T group are stronger than that of Q group,
indicating large amount of organic sulfonate group has been successfully introduced
into SMS-0.5. Furthermore, the obviously stronger intensity of T3 and Q4 in SMS-0.5
indicate its high condensation degree of framework, which strongly enhance their
hydrothermal stability in the water-contained reactions 48. Interestingly, only one
peak at around -68 ppm associated with T3 was observed in SMS-1.0, which confirms
its completely organic silica network with ultra-high condensation degree, further
resulting in its ultra-high concentration of sulfonic group and good hydroyhermal
stability 30, 32, 48. Additionally, 13C solid-state NMR spectrum further confirms the
successful and controllable incorporation of sulfonic group in SMS-0.5. For instance,
the resultant extracted sample show the signals centered at around 11, 18, 27, 41, 48,
57, 65, 75 and 177 ppm. The peaks at around 11 (1C), 18 (1C), 27 (1C), 41 (2C) and 48
(2C) ppm should be attributed to the signals of alkyl chain in organo sulfonic groups
30-32; The peaks at around 57 (3C) and 65 (4C) should be attributed to the carbon
connected with sulfonic group and hydroxyl group 30-32, the peak at around 177 ppm
should be attributed to the carbon connected with carboxylic group 49-51. The
presence of hydroxyl group and carboxylic group should be resulted from the partial
oxidation of alkyl chain by HNO3 and H2O2. Notably, a small and broad peak centered
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at around 75 ppm should be attributed to the carbon atoms in the residual F127
template 49-51, which could not be completely removed from SMS-xs samples by
solvent-extraction approach, similar phenomenon has also been reported previously
49-51
.
Figure 5 shows XPS spectra of SMS-0.8. The survey spectrum shows the signals
of Si2p, S2p, C1s and O1s in SMS-0.8. The high resolved peaks of Si2p and S2p were
centered at around 103 and 169 eV 11, 21, 22, 30, which were assigned to the signals of
SiO2 framework and sulfonic group. Correspondingly, the peak of C1s could be fitted
and deconvoluted into three peaks centered at around 284.4, 285.9 and 287.5 eV,
which were assigned to C-C, C-S and C-O bonds. These results further confirmed the
successful introduction of sulfonic group in SMS-0.8, in good agreement with NMR
results.
To further confirm the successful and controllable introduction of sulfonic group
in SMS-xs via the solvent free technique. FT-IR spectra of SMS-0.5, SMS-0.8 and
SMS-1.0 were also investigated in Figure 6. It can be seen that the strong vibration at
around 2900 cm-1 associated with C-H stretch vibrations (of the propyl chains)
increases as the MPTS/(MPTS+TEOS) molar ratio was increased. The vibration at
around 926 cm-1 was assigned to the signal of VS (S-O), the vibration at around 1036
cm-1 was assigned to the signal of C-S bond, and the vibration at around 1370 and
1289 cm-1 were assigned to the signals of VS (S=O) and VAs (S=O), respectively 11, 21, 22,
30, 52. From SMS-0.5 to SMS-1.0, the signals associated with sulfonic group became
more resolved because of the increased concentrations of sulfonic group. The
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combination of NMR, XPS and FT-IR results, we can conclude that mesoporous
organosilica with high contents of sulfonic group and highly cross-linked framework
could be extensively prepared via the novel high temperature, solvent free technique
in this work.
The acidity properties of SMS-xs were further investigated by using 31P
solid-state NMR techniques involving trimethylphosphine oxide (TMPO) as probe
molecule, which could interact with the acid sites on the SMS-xs samples. Notably,
the 31P NMR probe technique is a state-of-the-art approach to determine the acid
type (Brønsted or Lewis acid) and acid strength (strong or weak acid) of various kinds
of acid catalysts with different stabilities 53-58. Figure 7 illustrates the 31P solid-state
MAS NMR spectra of TMPO adsorbed on various SMS-xs samples. SMS-0.5 shows
multiple 31P resonance peaks centered at around 49, 62, 76, and 89 ppm, while
SMS-1.0 exerts that at approximately 49, 64, 81, and 89 ppm, which indicates the
presence of various acid sites with different acid strengths in these samples. The
peaks below 64 ppm may be assigned to the signals of weakly acidic sites such as
silicon hydroxyl group, while the resonances above 76 ppm can be attributed to the
relatively strong ones. Interestingly, the relative amount of 31P peaks for the strong
acid sites (above 76 ppm) is dramatically increased from SMS-0.5 to SMS-1.0, which
should be attributed to the higher sulfonic group content of SMS-1.0 sample. The
grafted sulfonic groups were confined into the nanopores of SMS-1.0 with different
chemical environments, hence directly resulting in the higher degree of exposure of
acid sites. In contrast, the weaker acidity of SMS-0.5 may be attributed to its
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relatively lower sulfonic group contents, since large amount of acid sites were
embedded into the framework and remained inaccessible due to the large usage of
inert silica precursor of TMOS. Whatever, the aforementioned results showed the
comparable strong acidity of SMS-xs with 100 % H2SO4 (89 ppm) 58. The strong
acidity of SMS-xs could also be confirmed by NH3-TPD technique (Figure S2). The
strong and controllable acidity was favorable for the large enhancement of their
catalytic activities in various acid-catalyzed reactions such as biomass conversions.
Thermal stability
The thermal stability of solid acids is very important for their applications in the
area of acid catalysis. Figure 8 shows TG curves of SMS-1.0, SMS-0.8 and SMS-0.5,
which was a reliable technique for evaluating thermal stability of different kinds of
materials. The TG curves of SMS-xs could be divided into three temperature ranges of
50-130, 320-490, and 490-700 °C. The weight loss in the temperature range of
50-130 °C should be attributed to desorption of the adsorbed water. The weight loss
in the temperature range of 320-490 °C should be attributed to the destruction of
sulfonic group. The weight loss in the temperature range of 490-700 °C should be
attributed to the decomposition of alkyl chain in SMS-xs samples. The high
decomposition temperatures verify the good thermal stability of sulfonic group
confined in SMS-xs, which was favorable for them used as stable catalysts in various
reactions. Notably, SMS-1.0, SMS-0.8 and SMS-0.5 showed different weight residuals
of 55.7, 41.3 and 38.1 wt% because of their different concentrations of sulfonic
group, which indicates different contents of organic incorporation. The TG analyses
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are in consistent with elemental analysis and solid NMR results.
Catalytic performances
Figure 9 shows the kinetic behaviors of various solid acids in the esterification of
palmitic acid with methanol, the esterification of fatty acids was an important
reaction for the production of high quality biodiesel. Notably, SMS-1.0 and SMS-0.8
show much better catalytic performances than those of HS-JLU-20-0.8,
SBA-15-SO3H-0.2, and Amberlyst 15. For instance, the yields of methyl palmitate
catalyzed by SMS-1.0 and SMS-0.8 exceeded 40 % within 20 min; WhileHS-JLU-20-0.8,
SBA-15-SO3H-0.2, and Amberlyst 15 give relatively lower yields at 30.5, 18.8 and
16.3 %, respectively. Further prolong the reaction time to 120 min, SMS-1.0 and
SMS-0.8 gave the product yields at 90.5 and 86.7 %, which were much higher than
those of HS-JLU-20-0.8 (77.2 %), SBA-15-SO3H-0.2 (63.2 %), and Amberlyst 15
(56.6 %). Compared with HS-JLU-20-0.8, SBA-15-SO3H-0.2 and Amberlyst 15, the
synergistic effects of high concentrations of sulfonic group, strong acidity, and
abundant porosity result in their better activities in esterification.
Except for esterification, the excellent activities of SMS-xs could also be observed
in transesterification, which was an important reaction to transform low cost oil such
as plant or animal oils into biodiesel 59, 60. Figure 10 shows kinetic curves in
transesterification of tripalmitin with methanol catalyzed by SMS-1.0, SMS-0.8,
HS-JLU-20-0.8, SBA-15-SO3H-0.2 and Amberlyst 15, and the yields of methyl
palmitate steadily increase with the reaction time. Notably, the catalytic activities of
SMS-1.0 and SMS-0.8 were much better than those of HS-JLU-20-0.8,
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SBA-15-SO3H-0.2 and Amberlyst 15. For instance, the yields of methyl palmitate
catalyzed by SMS-1.0 and SMS-0.8 were all higher than HS-JLU-20-0.8,
SBA-15-SO3H-0.2 and Amberlyst 15 at every time point. After reacting for 16 h,
SMS-1.0 and SMS-0.8 gave the product yields at 85.6 and 82.1 %, respectively; While
HS-JLU-20-0.8, SBA-15-SO3H-0.2and Amberlyst 15 gave relatively lower product
yields at 70.2, 52.3 and 49.7 %.
To further reduce the cost of biodiesel, the selecting of low cost feedstock such
as plant oil has received considerable attention in these years, which was very
important for the industrial application of biodiesel 59, 60. Therefore, we further
investigate the catalytic activities of SMS-xs solid acids for catalyzing conversion of
sunflower oil into biodiesel, and the main products in this reaction were methyl
palmitate (C16:0), methyl stearate (C18:1), methyl oleate (C18:2), methyl linoleate (C18:3),
methyl arachidate (C20:0), methyl 11-eicosenoate (C20:1), methyldocosanoate (C22:0
)
and methyltetracosanoate (C24:0), which were summarized in Table 2. For comparison,
the activities of a variety of acid catalysts were also summarized in Table 2, which
include H2SO4, HS-JLU-20-0.8, carbonaceous solid acid of CH0.30O0.33S0.16, H3PW12O40,
SBA-15-SO3H-0.2, Amberlyst 15 and H-USY. Interestingly, SMS-xs show much
improved catalytic activities in comparison with these solid acids, and the efficiency
of acid center in SMS-xs was a little lower than H2SO4. For instance, SMS-xs show
relatively high yields of C16:0 (75.7-83.4 %), C18:1 (74.8-82.3 %), C18:2 (76.3-80.1 %), C18:3
(76.2-81.8 %), C20:0 (75.5-81.1 %), C20:1 (78.3-82.3 %), C22:0 (76.7-81.3 %) and C24:0
(75.4-81.8 %, Table 2, run 1-3), which were as comparable as that of H2SO4 with the
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same acid number (C16:0: 83.2 %, C18:1: 82.6 %, C18:2: 80.4 %, C18:3: 82.3 %, C20:0: 80.5 %,
C20:1:81.7 %, C22:0: 80.9 % and C24:0: 80.7 %, Table 2, run 4). The activities of SMS-xs
were higher than various solid acids including HS-JLU-20-0.8, carbonaceous solid acid
of CH0.30O0.33S0.16, H3PW12O40, much higher than those of SBA-15-SO3H-0.2,
Amberlyst 15 and H-USY (Table 2, run 5-10). More importantly, SMS-xs show very
good reusability, and the decreasing of activities cannot be observed even after five
times recycling (Table 2, run 11). The excellent activities and good reusability were
attributed to their ultra-high acid concentrations, strong acid strength and good
hydrothermal stability 30, 32, 45
.
Besides the catalytic production of biodiesel, we also investigate the catalytic
activities of SMS-xs in depolymerization of crystalline cellulose (DCC, Table 3), and
the main products were glucose, cellobiose and HMF. For comparison, the activities
of commercial acid resin of Amberlyst 15 and HCl were also summarized in Table 3.
Notably, DCC has received extensive research interests in the area of biomass
conversion in these years, which transforms low cost and waste biomass into useful
chemicals. Acid catalysts have been widely used in this reaction due to their
advantages such as low cost, mild reaction conditions, good activities and enhanced
reusability 1, 2, 7, 8. Interestingly, SMS-xs were very active in DCC, much better than
that of commercial acid resin of Amberlyst 15, which were as comparable as HCl with
the same acid number. For instance, the yields of glucose, cellobiose and HMF
catalyzed by SMS-xs were in the range of 30.8-40.7, 19.1-22.3 and 5.9-10.5 %, which
were as comparable as HCl (41.5, 22.7 and 9.8 %). In contrast, Amberlyst 15 shows
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very low yields at 14.1, 17.8 and 3.9 %. Correspondingly, the TRS values of SMS-xs
were ranged from 70.5-86.3 %, a little lower than that of HCl (88.2 %), much higher
than that of Amberlyst 15 (39.8 %). Compared with SMS-0.5, SMS-1.0 and SMS-0.8
show higher activities because of their enhanced acid concentrations and acid
strength. Based on the above-mentioned results, SMS-xs could be used as highly
efficient and reusable solid acids for catalyzing production of biofuels and fine
chemicals, much better than variously reported solid acids. Their excellent catalytic
performances were resulted from their structural characteristics such as large BET
surface areas, abundant mesopores, good hydrothermal stabilities, ultra high acid
concentrations and controllable acid strength.
Conclusion
In summary, a series of mesoporous organosilica of SMS-xs with large BET
surface areas, abundant nanopores, highly cross-linked framework, ultra high acid
concentrations and controllable acid strength were prepared via a novel high
temperature, solvent free induced self-assembly technique. The synthesized SMS-xs
could be used as highly efficient and reusable solid acids in esterification of fatty acid
with methanol, transesterification of plant oil with methanol and depolymerization
of crystalline cellulose into fine chemicals, and their catalytic activities were much
better than a variety of reported solid acids. This work develops a sustainable and
low cost approach for preparation of highly efficient and stable mesoporous silica
solid acids, which offers great opportunity for the wide applications of mesoporous
organosilica based solid acids in the areas of biomass conversions and fine chemicals
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synthesis in the industry.
Conflicts of interest
The authors declare no competing financial interest.
Acknowledgements
This work was supported by the National Natural Science Foundation of China
(21573150, 21203122, U1662108, 21473244, 11575117), Natural Science Foundation
of Zhejiang Province (LY15B030002).
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Table 1. Textual properties of variously synthesized SMS-xs.
Samples
S contents (mmol S Acid capacity (mmol SBET (m2g-1) c Vt (cm3g-1) d
Pore
g-1 sample) a
1.97
H+ g-1 sample) b
diameter e
SMS-0.2
SMS-0.5
SMS-0.8
SMS-1.0
2.00
4.02
5.07
5.59
589
421
378
214
0.81
0.80
0.53
0.29
9.1
10.0
6.9
3.98
5.01
5.51
4.6
a
b
Sulfur contents determined by elemental analysis.
Acid–base titration using Na+ as
exchange-cation. c Surface area calculated from the BET equation in the relative pressure range of
d
0.05~0.20. Single point total pore volume calculated at the relative pressure of 0.99. e Pore size
determined using the BJH approach, and calculated from the adsorption branch.
29