Acid properties of organosiliceous hybrid materials based on pendant (fluoro)aryl-sulfonic groups through a spectroscopic study with probe molecules

Article abstract of DOI:10.1039/c9cy01609k

Two different heterogeneous catalysts carrying aryl-sulfonic moieties, in which the aromatic ring was either fluorinated or not, were successfully synthesized. The multi-step synthetic approaches implemented involved the synthesis of the silyl-derivative, template-free one-pot co-condensation (at low temperature and neutral pH) and tethering reaction. A multi-technique approach was implemented to characterize the hybrid organic-inorganic catalysts involving TGA, N₂ physisorption analysis, FTIR spectroscopy, and ss MAS NMR (¹H, ¹³C, ²⁹Si) spectroscopy. Specifically, the acidity of the organosiliceous hybrid materials was studied through the adsorption of probe molecules (i.e. CO at 77 K and NH₃ and TMPO at room temperature) and a combination of FTIR and ss MAS NMR spectroscopy. The catalytic activity of the two hybrids was tested in the acetal formation reaction between benzaldehyde and ethylene glycol. Preliminary results indicated superior performances for the fluoro-aryl-sulfonic acid, compared to the non-fluorinated sample. The findings hereby reported open new avenues for the design of heterogeneous sulfonic acids with superior reactivity in acid-catalyzed reactions. Moreover, through the implementation of spectroscopic studies, using probe molecules, it was possible to investigate in detail the acidic properties of hybrid organosiliceous materials.

Full text of DOI:10.1039/c9cy01609k

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Acid properties of organosiliceous hybrid
materials based on pendant (fluoro)aryl-sulfonic
groups through a spectroscopic study with probe
Cite this: Catal. Sci. Technol., 2019,
9, 6308
molecules†
c
Andrea Erigoni,a Geo Paul,b Marta Meazza,
María Consuelo Hernández-Soto,a
Ivana Miletto, b Ramon Rios, c Candela Segarra,
Leonardo Marchese,
a
b
c
a
a
b and Urbano Díaz
*
*
Robert Raja,
Fernando Rey,
Enrica Gianotti
Two different heterogeneous catalysts carrying aryl-sulfonic moieties, in which the aromatic ring was either
fluorinated or not, were successfully synthesized. The multi-step synthetic approaches implemented in-
volved the synthesis of the silyl-derivative, template-free one-pot co-condensation (at low temperature
and neutral pH) and tethering reaction. A multi-technique approach was implemented to characterize the
hybrid organic–inorganic catalysts involving TGA, N2 physisorption analysis, FTIR spectroscopy, and ss MAS
NMR (1H, 13C, 29Si) spectroscopy. Specifically, the acidity of the organosiliceous hybrid materials was
studied through the adsorption of probe molecules (i.e. CO at 77 K and NH3 and TMPO at room tempera-
ture) and a combination of FTIR and ss MAS NMR spectroscopy. The catalytic activity of the two hybrids
was tested in the acetal formation reaction between benzaldehyde and ethylene glycol. Preliminary results
indicated superior performances for the fluoro-aryl-sulfonic acid, compared to the non-fluorinated sample.
The findings hereby reported open new avenues for the design of heterogeneous sulfonic acids with
superior reactivity in acid-catalyzed reactions. Moreover, through the implementation of spectroscopic
studies, using probe molecules, it was possible to investigate in detail the acidic properties of hybrid
organosiliceous materials.
Received 11th August 2019,
Accepted 9th October 2019
DOI: 10.1039/c9cy01609k
rsc.li/catalysis
(flexibility and functionality), they are suitable for a wide
range of applications, including catalysis,4 adsorption,5 sepa-
Introduction
Since the early days of heterogeneous catalysis, scientists have
tried to mimic nature in designing catalysts with ever higher
conversion and selectivity. Hybrid materials, formed by the
combination of inorganic and organic building units, are at-
tractive for the purpose of creating high-performance or
highly functionalized structures.1–3 Combining the advan-
tages of inorganic solids (high mechanical, thermal, and
structural stability) and organic molecules or macromolecules
ration,6
microelectronics,7
and
photoluminescence.8
Organosilica-based materials are by far the most used sup-
ports to develop type II9 hybrid catalysts,2 due to the simplic-
ity of their synthesis, their tuneable textural properties and
their highly reactive surface, which make them ideal platform
supports for grafting or tethering additional active sites.10
Among the different approaches to incorporate organic moie-
ties into inorganic frameworks,2 sol–gel chemistry, in particu-
lar the co-condensation of a mixture of siloxanes together
with organo-silanes, is one of the most commonly used.11–13
Interestingly, one-pot synthesis methods of several hybrid or-
ganic–inorganic materials by fluoride-catalysed synthesis have
been described in the literature.12 This strategy offers numer-
ous advantages such as low synthesis temperatures, nearly
neutral pH and the absence of templating agents, at the same
time providing final porous materials with a high surface area
and tuneable porosity.14 The purely siliceous solids obtained
present a high surface area (up to 1000 m2 g−1) and a non-
ordered system of mesopores (so-called NOS-type materials).
These properties, together with a highly reactive surface,
a Instituto de Tecnología Química, Universitat Politècnica de València-Consejo Su-
perior de Investigaciones Científicas, Avenida de los Naranjos s/n, E-46022 Valen-
cia, Spain. E-mail: udiaz@itq.upv.es
b Department of Science and Technological Innovation, Universitá del Piemonte
Orientale, Viale T. Michel 11, 15121 Alessandria, Italy.
E-mail: enrica.gianotti@uniupo.it
c School of Chemistry, University of Southampton, Highfield Campus, SO17 1BJ,
Southampton, UK
† Electronic supplementary information (ESI) available: 1H and 13C NMR spectra
of silyl derivative compounds. FTIR difference spectra in the OH and CO
stretching regions of CO adsorption at 80 K on HSO3–NOS–H and HSO3–NOS–F
hybrid materials. See DOI: 10.1039/c9cy01609k
6308 | Catal. Sci. Technol., 2019, 9, 6308–6317
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make them ideal platform systems for developing novel
supported hybrid organic–inorganic catalysts. In order to co-
valently bond the organic molecule with the silica framework,
the former needs to be functionalized with an alkoxy-silyl
group that will eventually undergo hydrolysis/condensation,
together with the silica precursors.15–17
sis of the two hybrid catalysts containing aryl-sulfonic groups
was carried out following two different approaches
(Scheme 1). In the case of the hybrid with a protonated aro-
matic ring named HSO3-NOS-H (A), sulfanilic acid was firstly
reacted with NaOH to achieve its sodium form (1) and en-
hance its solubility in acetonitrile. The silyl derivative carry-
ing the sodium sulfonate group (compound 2) was obtained
through the reaction of the primary amine present on the
phenyl ring of 1 with 3-(triethoxysilyl)propyl isocyanate (aceto-
nitrile, 65 °C, N2), which led to the formation of the urea
group as indicated in the liquid (1H and 13C) NMR spectra
(Fig. S1 and S2†). The peaks observed were consistent with
the desired product; specifically, the 13C signal centred at 160
ppm is typical of the carbonyl carbon in the urea group. The
synthesis of the heterogeneous catalyst was achieved by co-
condensation of 2 and tetramethyl orthosilicate (TMOS), car-
ried out in methanol and water (1 : 1). Hydrolysis and con-
densation of the alkoxy groups were catalysed by F− ions,
which acted as mineralizing agents.12
As mentioned above, this approach presents several ad-
vantages, such as the synthesis was carried out at 36 °C, for
24 h, under mild pH conditions, using low amounts of sol-
vent and catalyst. Moreover, the textural properties (specific
surface area and mean pore size) could be tuned by varying
the F− concentration and the chain length of the alcohol used
as a co-solvent.12 The synthesis of the hybrid with a fluori-
nated aromatic ring named HSO3-NOS-F (B) was carried out
using a slightly different approach (Scheme 1). Substrate 5
was found insoluble in any solvent besides H2O and DMSO.
Implementing the former would have been detrimental to
the alkoxy groups of the silyl-derivative. Using the latter
would have required a very high temperature for its distilla-
tion. Such a high temperature might promote the spontane-
ous condensation of the alkoxy groups of the organosilane
compounds and their partial polymerization. To avoid this
problem, a Br-propyl functionalized NOS was first synthe-
sized. Afterwards, the solid was reacted with 5, previously
mixed with DBU (1,8-diazabicycloij5.4.0]undec-7-ene) to
Sulfonic acids are one of the many functional groups that
have been used to prepare silica-based heterogeneous
organocatalysts bearing strong acid sites18 for carrying out es-
terification of fatty acids,19 acetalization reactions,20 and ace-
tal hydrolysis.21 The effect of nearby atoms on the catalytic
activity of sulfonic acids has been studied and industrially
exploited for several decades. For example, propyl- and aryl-
sulfonic acids were supported on amorphous silica and their
activity was compared in the reaction of anisol with acetic an-
hydride,22 showing a beneficial effect of having the sulfonic
group bound to an aromatic ring. Fluorinated alkylsulfonic
acids show an even more representative example of the effect
of the chemical makeup of the sulfonic acid on the catalytic
activity.23–26 The inductive effect provided by having fluorine
as a substituent on the carbon in the α position greatly en-
hances the acidity of the proton. Nevertheless, the synergistic
effect of having the sulfonic group bound to a fluorinated ar-
omatic ring has not been deeply explored taking into account
the interaction of acid active groups with the silica surface.
In this work, we report the synthesis of supported aryl-
sulfonic acid organocatalysts on NOS-type materials. To in-
vestigate the inductive effect of fluorine on the aryl-sulfonic
groups and, ultimately, on acidity, two different catalysts
were designed and compared. In one of them the aromatic
ring was fluorinated, while in the other it was not. The two
hybrid materials underwent in-depth physico-chemical char-
acterization aimed to study their morphology, textural prop-
erties, thermal stability and spectroscopic properties. Specifi-
cally, a combination of FTIR and multinuclear solid state
NMR methods was implemented. Additionally, probe mole-
cules of different basic strengths were adsorbed/desorbed
onto the hybrids. Gaseous or highly volatile bases such as
carbon monoxide, ammonia, pyridine and pyridine deriva-
tives have been widely used as probe molecules to study the
acidic properties of purely inorganic catalysts.27–29 Neverthe-
less, within the field of organosiliceous hybrid catalysts, this
insightful characterization approach remains unexplored. In
this study, for the first time, carbon monoxide and ammonia
were adsorbed on the surface of the hybrids and their inter-
action was followed both by FTIR and ss NMR spectroscopy
in order to assess the acidity of the hybrid catalysts. The re-
sults obtained from this characterization study were decisive
to better explain the catalytic activity in solid-acid-catalysed
reactions.
Results and discussion
To modulate the acid strength in sulfonic hybrid catalysts,
two different organic moieties containing protonated or fluo-
rinated aryl-sulfonic acid groups were developed. The synthe-
Scheme 1 Representation of the synthesis of the supported sulfonic
acids: (A) HSO3-NOS-H and (B) HSO3-NOS-F.
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promote the deprotonation of the hydroxyl group, leading to
a non-porous hybrid material with pendant (fluoro)aryl-sul-
fonic groups (6).
that probably the thermal stability of the ether groups is
slightly higher than that of the urea groups, with the HSO3-
NOS-F sample being more stable than HSO3-NOS-H, from a
thermal point of view. After investigating the amount and the
stability of the organic species constituting the hybrid mate-
rials, the focus was shifted to the textural properties. N2
physisorption analysis at 77 K was conducted on the two hy-
brid catalysts and on a pure silica sample obtained with the
same synthetic methodology in order to study the specific
surface area, the free pore volume of the materials and the ef-
fect of the presence of the R–SiIJOR′)3 species in the synthesis
mixture (Fig. 2, Table 1). The isotherms show a type IV behav-
iour (IUPAC), with a hysteresis loop typical of conventional
mesoporous materials. The N2 uptake is also compatible with
mesoporous materials obtained through a F−-catalysed sol–
gel approach. Furthermore, cavitation and an ink-bottle pore
shape could give rise to hysteresis and sharp peaks in the de-
sorption band.12 The Brunauer–Emmett–Teller (BET) method
was used to obtain information regarding the surface area of
the catalysts. Both organo-catalysts show a surface area com-
parable with that of the pure silica reference material (802
m2 g−1). Specifically, HSO3-NOS-F exhibits an area of 816
m2 g−1 whereas HSO3-NOS-H shows a surface area of 683
m2 g−1. This slightly lower value may be related to the usage
of a bulkier organosilane precursor used in the one-pot syn-
thesis of the solid. The adsorption branches of the isotherms
were analysed by means of (i) NLDFT (non-localized density
functional theory) and (ii) Barrett–Joyner–Halenda (BJH)
methods to obtain information on the pore-size distributions.
Both functionalized non-ordered silicas and the reference
pure silica material show a pore distribution spacing between
20 and 60 Å with a peak centred at about 35 Å, typical for
non-ordered silica materials obtained through a F−-catalyzed
sol–gel synthesis process.12 In all three samples a small
amount of micropores is present. This was already observed
in other syntheses carried out in NH4F medium.12,21 After ex-
ploring the amount and stability of the organic moieties and
the textural properties of the hybrid catalysts, ss NMR and
In both synthetic pathways, the exchange of sodium with
a proton was achieved by suspending 3 and 6 in an HCl solu-
tion in EtOH. The total exchange of Na was checked by ICP
analysis. HSO3-NOS-H and HSO3-NOS-F underwent in-depth
physico-chemical characterization. Firstly, thermogravimetric
analysis was conducted on both samples to measure the or-
ganic content and to probe the thermal stability of the mate-
rials. The weight losses (TGA) and the respective DTA curves
of the materials are reported in Fig. 1. The first weight loss
experienced by both materials occurs between 70 and 150 °C
and it's associated with the removal of physisorbed water (I).
Focusing on HSO3-NOS-H (curves a and a′), a second weight
loss is observed between 250 and 380 °C (II). This is probably
due to the breakage of the urea bond and consequent desorp-
tion of the phenyl-sulfonic moieties.21 Finally, the loss be-
tween 500 and 600 °C is due to the decomposition of the
propyl chain anchored to the surface (III). Considering this
as the limit temperature for the total decomposition of the
organic moieties supported on silica, the loss was found to
be about 15% of the overall weight (temperature range be-
tween 200 and 600 °C). Shifting to HSO3-NOS-F (curves b and
b′), after the initial loss due to the desorption of physisorbed
water, a main weight loss due to the organic moieties is ob-
served at about 300–450 °C and it is related to the breakage
of the ether bond linking the fluorinated phenyl ring to the
propyl chain (IV). Lastly, the weight loss between 500 and 600
°C was attributed to the thermal decomposition of the alkyl
chains, still anchored to the silica surface (V). The total
weight loss due to the organic moieties was about 8.4% of
the total weight. In both hybrid solids, a progressive weight
loss at temperatures above 600 °C was detected due to de-
hydroxylation phenomena, i.e. condensation of external
silanol groups with the release of water molecules and forma-
tion of strained siloxane bridges. The TGA results evidenced
Fig. 2 A) N2 adsorption/desorption isotherms at 77 K of HSO3-NOS-H,
HSO3-NOS-F and non-ordered silica (NOS, reference material). B)
Pore-size distribution of the materials in the mesopore range.
Fig. 1 TGA and DTA curves of HSO3-NOS-H (a and a′) and HSO3-
NOS-F (b and b′).
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Table 1 Textural properties of HSO3-NOS-H and HSO3-NOS-F. Microporous and mesoporous contributions to the surface area and pore volume were
estimated by means of the non-linear DFT method. VBJH was calculated from the adsorption branch of the isotherms
SSABET
(m2 g−1
SSADFT
(m2 g−1
SAMicro
(m2 g−1
SAmeso
VBJH
VDFT
Vmicro
Vmeso
)
)
)
(m2 g−1
)
(cm3 g−1
)
(cm3 g−1
)
(cm3 g−1
)
(cm3 g−1
)
HSO3-NOS-H
HSO3-NOS-F
NOS
683
816
802
549
613
607
91
36
80
458
577
527
0.39
0.49
0.52
0.485
0.6
0.643
0.0301
0.0163
0.027
0.455
0.584
0.616
FTIR spectroscopic characterization was carried out to study
the organic–inorganic interaction and to assess the acidic
properties of the hybrids.30 Prior to recording the FTIR spec-
tra, all the samples have been outgassed at 453 K for 2 h in
order to remove physisorbed water. The HSO3-NOS-H spec-
trum (Fig. 3, curve c) appears quite different compared with
the pure silica reference material, NOS (Fig. 3, curve a). The
presence of the bulky silyl-derivative in the synthesis leads to
a lower amount of isolated silanols in the final material: a
signal centred at 3745 cm−1, due to the O–H stretching mode
of Si–OH groups, is visible but its intensity is much lower
than that of the reference material. Instead, a broad signal re-
lated to silanols interacting with each other through a
H-bond is visible between 3500 and 3200 cm−1. A number of
signals related to the stretching mode of C–H groups present
in the aromatic ring, in the propyl linker and in eventual
uncondensed alkoxy groups, are visible between 3000 and
2800 cm−1. The low frequency region appears quite rich in
signals, due to the presence of the urea group. The signal
centred at 1685 cm−1 is attributed to the stretching modes of
the CO bond of the urea group. The presence of the urea
group is confirmed by the amide II band, centred at 1600
cm−1, which is related to a motion combining the N–H bend-
ing and C–N stretching of the amide group.31 Lastly the sharp
band at 1535 cm−1 is ascribed to the C–C stretching of the ar-
omatic ring. In addition, a weak band at 1380 cm−1 is visible
due to the SO asymmetric stretching mode of sulfonic
groups. Shifting the focus to the HSO3-NOS-F FTIR spectrum
(Fig. 3, curve b), it can be observed that by using a less bulky
silyl-derivative as an organosilane precursor, the amount of
isolated silanols (3745 cm−1) in the resulting material appears
comparable to that of the pure silica one. A composed signal
centred between 3000 and 2800 cm−1 is observed and attrib-
uted to the stretching mode of C–H groups present in the
propyl chains (and in uncondensed alkoxy groups). Due to
the simpler liking point between the propyl chain and the
aryl-sulfonic moieties, in this spectrum, the low frequency re-
gion appears poorer in signals, with a sharp band centred at
1500 cm−1, related to the C–C stretching of the fluorinated ar-
omatic ring. In order to study the acidity of the sulfonic
groups exposed to the inorganic silica surface, CO was
adsorbed at 80 K on the hybrid catalysts (Fig. S3†). Both ma-
terials behave equally upon interaction with CO. Focusing on
the high frequency region, the signal related to the stretching
modes of isolated silanols (3745 cm−1) is shifted downward,
forming a broad signal centred at approximately 3644 cm−1.
The 100 cm−1 frequency shift is consistent with previous
studies reported on purely siliceous materials.28 In the CO
stretching region, at low coverages a signal centred at 2155
cm−1 arose, due to the CO molecules interacting with the
SiOH groups on the surface of the support.28 At higher CO
dosages the typical band related to the stretching mode of
the free CO molecule appears (νCO liquid-like at 2138 cm−1).32
Given a specific site with which CO is interacting, the blue
shift of the stretching mode is expected to be proportional to
the ability of the site to accept electronic density, i.e. its acid-
ity. Nevertheless, despite the presence of highly acidic sul-
fonic groups, no signal is observed at higher frequencies
other than that of CO interacting with silanols. The CO ad-
sorption at 80 K is able to reveal only mild acidity due to the
available Si–OH of the inorganic support. The high concen-
tration of external silanol groups masks the presence of sul-
fonic groups present in the hybrid materials. Then, a more
selective probe molecule has been used, such as ammonia.
NH3 was adsorbed at room temperature on the hybrids and
on the pure non-ordered silica support (Fig. 4). NH3 interac-
tion with NOS (Fig. 4A) produced typical features due to the
formation of a H-bond between Si–OH groups of the silica
and the probe molecule.33 At high NH3 pressure (30 mbar),
bands at 3400 and 3320 cm−1 appear, overlapped with a
Fig. 3 FTIR spectra of non-ordered silica (NOS, curve a), HSO3-NOS-F
(curve b) and HSO3-NOS-H (curve c). The samples have been
outgassed at 453 K prior to recording the spectra.
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region, bands at 1465 and 1485 cm−1 due to the asymmetric
+
bending mode of NH4 protonated species, meaning that sul-
fonic acid groups are strong enough to protonate ammonia.34
Ammonia protonation is an irreversible process; in fact upon
+
NH3 desorption at room temperature, the signal due to NH4
ions is still visible on both hybrid catalysts (blue curves). In
the insets are reported the difference FTIR spectra in order to
+
better evidence the formation of the NH4 bands that are par-
tially overlapped with other signals typical of the organic part
of the hybrids.
A series of multi-nuclear magic angle spinning nuclear
magnetic resonance (MAS NMR) spectroscopic experiments
have been conducted to study the hybrid organic–inorganic
mesoporous silica-based catalysts. The successful incorpora-
tion of aryl-sulfonic acids in non-ordered mesoporous silica
was established by 29Si NMR spectroscopy while the integrity
of the organic components on the hybrid catalysts was deter-
mined by 13C NMR spectroscopy. This information gave a
first idea on local arrangements of organosilane species and
1
furthermore was supported by various H MAS NMR methods
which would help to determine the proton environments as
well as the acidic nature of the catalysts. The 29Si population
distribution as well as the organosilane functionalization in
different hybrid catalysts was studied by 29Si CPMAS NMR.
Fig. 5 (left panel) shows the 29Si CPMAS NMR spectra of the
hybrid catalysts, where both Q and T Si sites are present.30
There are three different Q silicon sites, namely Q4 (SiIJOSi)4)
silicon sites (at around −110 ppm), Q3 (SiIJOSi)3OH) silicon
sites (at around −100 ppm) and Q2 (SiIJOSi)2IJOH)2) silicon
sites (at around −90 ppm) seen in the hybrid catalysts origi-
nating from the non-ordered mesoporous silica support. On
the other hand, the signals from T3 (RSiIJOSi)3) silicon sites
(at around −66 ppm) and T2 (RSiIJOSi)2OH) silicon sites (at
around −58 ppm) stemming from the organosilane groups
confirm their successful silica surface functionalization.21
Fig. 4 FTIR spectra of NH3 adsorption at 298 K on NOS (A), HSO3-
NOS-H (B), and HSO3-NOS-F (C). Red curves: Hybrids under vacuum
before NH3 adsorption, black curves: adsorption of NH3 at high
pressure (30 mbar), blue curves: NH3 outgassing at rt. B and C insets:
FTIR difference spectra.
broader band centred at 3020 cm−1 while the band at 3745
cm−1, due to the O–H stretching mode of free silanols (red
curve), almost completely disappears (bold black curve). The
broad band centred at 3020 cm−1 is assigned to the O–H
stretching of the silanol groups H-bonded to NH3 molecules.
The bands at 3400 and 3320 cm−1 are respectively due to the
N–H asymmetric and symmetric stretching modes of NH3
bonded to silanols. At lower frequency, a band at 1635 cm−1
is also present which is assigned to the asymmetric bending
mode of the ammonia molecules adsorbed on Si–OH groups.
Such complexes are very weakly held as demonstrated by the
fact that all their spectroscopic features progressively fade
away when decreasing the NH3 pressure and almost
completely disappear upon outgassing the sample at room
temperature for 30 minutes (blue curve), giving a spectrum
which is similar to that of the bare NOS sample before NH3
adsorption (red curve). Different from the behaviour observed
upon NH3 adsorption of the pure silica mesoporous support,
the ammonia adsorption on both HSO3-NOS-H (Fig. 4B) and
HSO3-NOS-F (Fig. 4C) hybrids produced, in the low frequency
Fig. 5 29Si (left panel) and 13C (right panel) CPMAS NMR spectra of
various hybrid catalysts recorded at a 10 kHz MAS rate. A CP contact
time of 2 ms was employed for 29Si CPMAS while for 13C CPMAS
values of 4 ms and 20 ms were used for samples HSO3-NOS-H (red
curves) and HSO3-NOS-F (black curves), respectively.
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Similarly, Fig. 5 (right panel) shows the 13C CPMAS NMR
spectra of the as-synthesized hybrid catalysts. The assign-
ments of the spectral resonances are based on either 13C
solution-state NMR or on software predicted computed
spectra. In fact, the 13C CPMAS NMR spectra of the hybrid
catalysts show the typical 13C signals associated with
organosilane groups confirming the integrity of the synthesis
process. On the other hand, the incomplete hydrolysis of alk-
oxy groups from the precursors is clearly evident through the
presence of their peaks (at around 15, 50 and 59 ppm). The
weak aromatic resonances (110 to 145 ppm) in the HSO3-
NOS-F catalyst are due to the fact that no protons are directly
attached to the phenyl carbons as the intensity of 13C reso-
nances depends on the proton–carbon dipolar interactions
precursors. It is important to note that peaks due to residual
ammonium fluoride, used as a catalyst in the synthesis of
non-ordered mesoporous silica, were detected at 6.6 ppm. Fi-
nally, the narrow low intensity resonance detected at 9.4 ppm
in the HSO3-NOS-F catalyst is attributed to sulfonic acid pro-
tons (Fig. 6, inset) not in exchange with other compounds.
The acidity of the hybrid catalysts was determined by
adsorbing a basic probe molecule that would be converted to
a protonated species upon contact with acidic sulfonic pro-
tons. Ammonia was adsorbed on the hybrid catalysts that
were previously treated at 180 °C under vacuum for 90 mi-
1
nutes. H spin-echo NMR spectra were recorded after ammo-
nia adsorption and are shown in Fig. 7. It is clearly evident
that ammonium ions were formed after adsorption as the
intensity of the 6.6 ppm peak has increased. Furthermore, a
small decrease in the peak intensity of sulfonic acid protons
is also observed in sample HSO3-NOS-F. It is important to
note that every single sulfonic acid proton would be
converted into four equivalent ammonium ion protons upon
adsorption. The presence of a 9.4 ppm peak in the spectrum
after ammonia adsorption also confirms the fact that not all
sulfonic acid protons are accessible to ammonia. On the
other hand, a marked increase in the formation of ammo-
nium ions in sample HSO3-NOS-H after ammonia adsorption
is surprising as no free sulfonic acid protons were detected
in
a CPMAS experiment. In addition, the presence of
unreacted 3-bromopropyl silane was also detected in this
sample. The 1H MAS and spin-echo NMR spectra of the
hybrid catalysts, previously treated at 180 °C for 90 minutes
under vacuum, recorded at a MAS rate of 15 kHz are shown
in Fig. 6, left and right panel, respectively. Firstly, the intense
resonance at 1.7 ppm is attributed to isolated silanols origi-
nating from the mesoporous silica support and the broad
shoulder in the range 2–10 ppm is ascribed to hydrogen
bonded silanols.35 However, contributions from water, amine
protons and sulfonic acid protons also fall in this broad
shoulder as they are in mutually exchangeable state. Ex-
changeable protons undergo different binding dynamics and
form spatially structured clusters of variable sizes with chem-
ical shift values determined by exchange between bound wa-
ter, free water, isolated SiOH, hydrogen bonded SiOH and
acidic/basic protons. The resulting 1H chemical shift is the
weighted shift of the compounds which are involved in the
fast chemical exchange.36 Furthermore, proton resonance
peaks associated with organosilanes are clearly detected at
around 1 ppm for aliphatic components and at around 7.5
ppm for aromatic units. No aromatic protons were detected
in the HSO3-NOS-F catalyst as they are substituted by fluo-
rine. The sharp resonance peaks in the range 3.2 to 3.6 ppm
are attributed to the alkoxy groups from the unhydrolysed
1
in the H NMR spectrum of the parent sample. As has been
pointed out earlier, the sulfonic acid protons are in exchange-
able state (either with water or with silanols or both) in this
sample, however, they can easily protonate a stronger base
upon its presence. On the other hand, sulfonic acid proton
signals can also be overlapped with other signals or the reso-
nance can be broadened due to stronger dipolar interactions.
In order to circumvent such issues, we have employed a dif-
ferent probe molecule to determine the acid strength on
these hybrids. It is well established that the 31P MAS NMR
chemical shift scale can be used to determine the acidic
Fig. 6 1H MAS (left panel) and 1H spin-echo (right panel) NMR spectra
of various hybrid catalysts recorded at a 15 kHz MAS rate. An echo de-
lay (tau) of 2 ms was used in the spin-echo experiment. The inset
shows the zoomed spectra.
Fig. 7 1H spin-echo NMR spectra of various hybrid catalysts, before
(black) and after (red) ammonia adsorption, recorded at a 15 kHz MAS
rate and an echo delay (tau) of 2 ms.
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strength of solid acids by adsorbing trimethylphosphine ox-
ide (TMPO).37 The larger the 31P NMR down-field shift of the
adsorbed TMPO molecule, the higher the acidity of the mate-
rial.37,38 Fig. 8A shows the room temperature 31P MAS NMR
spectra of HSO3-NOS-H and HSO3-NOS-F after TMPO adsorp-
tion. Both samples exhibit an intense peak at 47 ppm,
assigned to physisorbed TMPO molecules. Interestingly, the
resonance peaks at 89 and 90 ppm in HSO3-NOS-H and
HSO3-NOS-F, respectively, confirm the very strong acidity
displayed by these hybrids. A recent article has charted the
acid strength of super acids using 31P MAS NMR spectroscopy
and the largest chemical shift for TMPO was reported to be
93 ppm for a sulfonated polymer embedded in montmoril-
lonite.39,40 Therefore, it can be concluded that the aryl-
sulfonic acid-based catalysts presented in this work display
acid strength similar to 100% H2SO4 (Fig. 8B) The higher 31P
MAS NMR chemical shift for catalyst HSO3-NOS-F confirms
the higher acidity of this hybrid catalyst. To summarize,
solid-state NMR methods provided detailed information on
the local structure of organosilanes and dynamics of sulfonic
protons. The presence of acidity of the catalysts was
evidenced, by protonation of ammonia upon adsorption,
Scheme
2
Catalytic reaction used to test the reactivity and
recyclability of HSO3-NOS-H and HSO3-NOS-F catalysts.
H shows a slightly lower activity, under the same reaction
conditions, probably due to its lower acidity. It is highly likely
that the presence of fluorine atoms in the benzene ring of
catalyst HSO3-NOS-F makes it more electron-poor, inducing a
higher degree of acidity in the sulfonic acid moiety, com-
pared to the urea derivative present in catalyst HSO3-NOS-H.
We also investigated the scope of the acetalization reaction
with different aldehydes and ketones using catalyst HSO3-NOS-
H to evaluate the wider potential of this anchored catalyst
containing pendant acid sites (Table 2). Promising results
were obtained when cyclohexanone and other electron-poor
benzaldehydes were employed as substrates. As expected,
electron-rich aldehydes represent a challenge due to their
electronic nature and require longer reaction times or
harsher reaction conditions. To evaluate wider industrial ap-
plicability, we also probed the effectiveness of this anchored
catalyst in the transacetalization reaction achieving promis-
ing results when dimethoxy propane was used (60% yield).
1
through a H peak at 6.6 ppm as well as by TMPO adsorption
and 31P peak observation at around 90 ppm. Moreover, the
structural integrity of the organosilane units and the hybrid
nature of the silica backbone were confirmed by 13C and 29Si
CPMAS NMR experiments, respectively. The solid-acid cata-
lysts outlined in Scheme 1 (HSO3-NOS-H and HSO3-NOS-F)
were evaluated in the acetal formation reaction between
benzaldehyde and ethylene glycol as shown in Scheme 2.
This reaction is of particular interest to synthetic organic
chemists, as it is used to protect aldehydes and ketones,
making them stable under basic reaction conditions, to facili-
tate subsequent reduction and oxidation transformations.41,42
Preliminary catalytic findings as outlined in Fig. 9 show
promising catalytic trends in both reactivity and recyclability,
demonstrating the potential of these type of heterogeneous
catalysts. The anchored catalyst HSO3-NOS-F demonstrates a
slightly higher stability, showing high quantitative yields and
recyclability under the experimental reaction conditions,
thereby proving the robustness of the anchoring strategy for
creating hybrid heterogeneous catalysts. Catalyst HSO3-NOS-
Experimental
Synthesis of the materials
Synthesis of 2. Compound 1 was achieved by reacting
sulfanilic acid (99%, Aldrich) with 1 equivalent of NaOH, in
H2O. 1 was reacted with 3-(triethoxysilyl)propyl isocyanate
(95%, Aldrich) in acetonitrile, at 65 °C, under N2, overnight.
The solvent was then evaporated under vacuum.
Synthesis of 3. A solution of 2, tetramethyl orthosilicate
(98%, Aldrich), MeOH and H2O was prepared and stirred un-
til complete dissolution (solution A). A solution of NH4F (Al-
drich, 98%) in H2O was prepared (solution B). Solution B was
added to solution A and stirred at room temperature until ge-
lation occurred. The gel obtained was aged at 36 °C for 24 h.
Afterwards, the solid was dried overnight at 65 °C, ground
Fig. 8 31P MAS NMR spectra (A) of various hybrid catalysts recorded at
room temperature and a MAS rate of 15 kHz. * denotes spinning
sidebands. A graph (B) showing the acid strength versus 31P MAS NMR
chemical shifts of various super acids catalysts reported in the
literature (see ref. 39).
Fig. 9 Yields obtained with HSO3-NOS-H and HSO3-NOS-F catalysts
in the acetalization reaction.
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Table 2 Evaluating the wider scope of the anchored catalyst, HSO3-
NOS-H, for acetalization and transacetalization reactions
ative to residual proton solvent signals. Nitrogen adsorption
isotherms were measured at 77 K with a Micromeritics ASAP
2010 volumetric adsorption analyser. Before the measure-
ment, the sample was outgassed for 12 hours at 80 °C. The
BET specific surface area was calculated from the nitrogen
adsorption data in the relative pressure range from 0.04 to
0.2. The total pore volume was obtained from the amount of
N2 adsorbed at a relative pressure of about 0.99. The pore di-
ameter and the pore size distribution were obtained using
the Barrett–Joyner–Halenda (BJH) method on the adsorption
branch of the isotherms. The adsorption branch of the N2
physisorption isotherm was analysed by means of the NLDFT
(nonlocal density functional theory) method, to obtain the
pore size distribution of the materials. Thermogravimetric
analyses (TGA/DTA) of materials were performed under argon
flow (100 mL min−1) with a SETSYS Evolution TGA–DTA/DSC
thermobalance, heating from 40 to 1000 °C at 5 °C min−1.
The FTIR spectra of self-supporting pellets were collected un-
der vacuum conditions (residual pressure <10−5 mbar) using
a Bruker Equinox 55 spectrometer equipped with a pyroelec-
tric detector (DTGS type) with a resolution of 4 cm−1. CO was
adsorbed at 77 K and NH3 was adsorbed at room tempera-
ture using specially designed cells permanently connected to
a vacuum line to perform adsorption–desorption in situ mea-
surements. The FTIR spectra were normalized with respect
the pellet weight and, whenever specified, are reported in
difference-mode by subtracting the spectrum of the sample
under vacuum from the spectrum of the adsorbed molecules.
Solid-state NMR spectra were acquired on a Bruker Advance
III 500 spectrometer and a wide bore 11.7 Tesla magnet with
operational frequencies for 1H, 13C, 31P and 29Si of 500.13,
125.77, 202.47 and 99.35 MHz, respectively. A 4 mm triple
resonance probe with MAS was employed in all the experi-
ments. The samples were packed on a zirconia rotor and
spun at a MAS rate between 10 and 15 kHz. The magnitude
General reaction
Aldehyde/ketone
Alcohol
Yield%
56
R = NO2
R = OMe
100
10
100
60
16
R = NO2
CH3OH
and washed with ethanol and H2O. The final composition of
the synthesis gel was as follows: 0.97TMOS : 0.03RSiIJOR′)3 :
4MeOH : 4H2O : 0.00313NH4F.
Synthesis of 4. The material was obtained using the same
procedure as the synthesis of 3, using (3-bromopropyl)-
trimethoxysilane (97%, Fluorochem) instead of using
compound 2.
Synthesis of 6. A solution of 5 (sodium 2,3,5,6-tetrafluoro-
4-hydroxybenzenesulfonate, 98%, TCI Europe) in DMSO was
prepared under N2. DBU (1,8-diazabicycloij5.4.0]undec-7-ene,
97.5%, Aldrich) was added and the mixture was stirred for 10
minutes. Br-Propyl functionalized silica (4) was added and
the mixture was stirred at 100 °C for 16 h. Afterwards the sus-
pension was cooled down at room temperature and the solid
was filtered and washed abundantly with H2O and ethanol.
Synthesis of HSO3-NOS-H and HSO3-NOS-F. The supported
sodium sulfonates 3 and 6 were exchanged three times with
a 0.05 M HCl solution in EtOH at room temperature.
1
of radio frequency (RF) fields was 100 and 42 kHz for H and
29Si, respectively. The relaxation delay between accumula-
tions was 2 and 2.5 s. For 13C and 29Si cross polarization
(CP) MAS experiments, RF fields of 55 and 28 kHz were used
for initial proton excitation and decoupling, respectively.
1
During the CP period the H RF field was ramped using 100
Catalytic test
increments, whereas the 13C/29Si RF fields were maintained
at a constant level. During the acquisition, the protons are
decoupled from the carbon atoms/silicon atoms by using a
two-pulse phase-modulated (TPPM) decoupling scheme. A
moderate ramped RF field of 62 kHz was used for spin
locking, while the carbon/silicon RF field was matched to ob-
The anchored, solid-acid catalysts, HSO3-NOS-H and HSO3-
NOS-F (4 mol%), were added into a glass-lined reactor,
equipped with a Dean Stark apparatus and a condenser,
followed by subsequent addition of the aldehyde (1 equiv.,
0.5 mmol), dialcohol (30 equiv.) and toluene (5 ml). The reac-
tion mixture was stirred at 120 °C for 20 h and the yield was
tain the optimal signal.
A rotor-synchronised spin-echo
1
quantified by H-NMR using an internal standard. Recycling:
sequence (π/2–τ–π–τ–acquisition) was also applied to record
the 1H NMR spectra with a delay time τ of 2 ms. The delay
time was chosen as an optimised compromise between the
signal decay owing to relaxation and the resolution gain ow-
ing to longer delay times. 31P MAS NMR spectra were
recorded using high power proton decoupling and the RF
field for the 90 degree pulse was 83 kHz. All chemical shifts
are reported using the δ scale and are externally referenced
the crude reaction was filtered, and the catalyst was washed
with dichloromethane, dried under vacuum and reused in
the next reaction.
Characterization
1H and 13C NMR spectra were recorded on a Bruker 300
spectrometer and the chemical shifts are reported in ppm rel-
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to tetramethylsilane (TMS) at 0 ppm for 1H, 13C and 29Si. Conflicts of interest
Ammonium dihydrogen phosphate (1 ppm) was used as a
There are no conflicts to declare.
standard for the 31P nucleus.
Prior to the adsorption of TMPO probe molecules, the cat-
alysts were packed onto a 4 mm zirconia rotor and evacuated
Acknowledgements
at 180 °C for 90 minutes in a high vacuum line. After that,
AE acknowledge “la Caixa” foundation for the PhD scholar-
ship. The authors are grateful for financial support from the
European Union by the MULTY2HYCAT EU-Horizon 2020
funded project under grant agreement no. 720783.
TMPO dissolved in anhydrous dichloromethane was added to
the rotor and rapidly closed with a cap and left for adsorp-
tion for one hour. Later, the rotor was opened and attached
back to the high vacuum line for the complete removal of
dichloromethane at room temperature. Subsequently, the ro-
tor was closed with a zirconia cap and heated to 160 °C for
30 minutes followed by submission to NMR spectral
recording.
Notes and references
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In this work, two novel organosiliceous hybrid catalysts
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NOS-F, has aryl-sulfonic groups in which the aromatic ring is
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