Hierarchically porous monolithic silica with varying porosity using bis(trimethoxysilyl)arenes as precursors

Article abstract of DOI:10.1039/c6nj00758a

The ortho-, meta- and para-isomers of bis(trimethoxysilyl)benzene and bis((trimethoxysilyl)phenyl)dimethoxysilane were synthesized from their corresponding diarylbromides. Mixtures of these bis(trimethoxysilyl)arenes and tetramethoxysilane (TMOS) were used as organosilica precursors for the preparation of macro-mesoporous silica monoliths using different amounts of the porogen poly(ethylene glycol) to investigate the effect on the porosity of the final materials. The prepared samples were characterized by nitrogen and dibromomethane sorption measurements, mercury intrusion porosimetry and scanning electron microscopy. The analysis of dibromomethane isotherms indicated a significantly lower polarity of the mesopore surface compared to that of pure SiO₂ monoliths, proving the presence of arene units at the mesopore surfaces.

Full text of DOI:10.1039/c6nj00758a

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Hierarchically porous monolithic silica with
varying porosity using bis(trimethoxysilyl)arenes
Cite this:DOI: 10.1039/c6nj00758a
as precursors†
Martin von der Lehr, Rudiger Ellinghaus and Bernd M. Smarsly*
¨
The ortho-, meta- and para-isomers of bis(trimethoxysilyl)benzene and bis((trimethoxysilyl)phenyl)dimethoxy-
silane were synthesized from their corresponding diarylbromides. Mixtures of these bis(trimethoxysilyl)arenes
and tetramethoxysilane (TMOS) were used as organosilica precursors for the preparation of macro-
mesoporous silica monoliths using different amounts of the porogen poly(ethylene glycol) to investigate
the effect on the porosity of the final materials. The prepared samples were characterized by nitrogen
and dibromomethane sorption measurements, mercury intrusion porosimetry and scanning electron
microscopy. The analysis of dibromomethane isotherms indicated a significantly lower polarity of the
mesopore surface compared to that of pure SiO2 monoliths, proving the presence of arene units at the
mesopore surfaces.
Received (in Montpellier, France)
9th March 2016,
Accepted 15th April 2016
DOI: 10.1039/c6nj00758a
www.rsc.org/njc
the utilization of an organically modified precursor in the
starting sol–gel mixture offers the advantage of a convenient
Introduction
Inorganic–organic hybrids have received considerable attention one-pot strategy.
during the last decades as they combine the specific demands
The integration of such compounds into the siliceous
of each species in a single material. They are of peculiar interest matrices of periodic mesoporous organosilicas (PMOs) was
due to the possibility of adjusting their final physical and chemical intensively described and reviewed by Froeba et al.4 Moreover,
characteristics such as porosity, hydrophobicity and functionality PMOs made from silsesquioxanes with bridging phenyl groups
depending on the desired application. Such materials can be were synthesized by Kuschel and Polarz and studied with
prepared by sol–gel chemistry routes, known for mild and versatile respect to their catalytic applications.5 Nakanishi et al. reported
reaction conditions. In addition, this pathway offers the benefit the integration of alkyltrialkoxysilanes and alkylene-bridged
of straightforward control of the composition of the matrix. For alkoxysilanes as organosilica precursors for the synthesis of silica
obtaining a hybrid material, in the simplest case a single methyl monoliths.6 Huesing and co-workers utilized different ethylene-
group is incorporated by using a methyl-substituted alkoxysilane glycol-modified derivatives of organoalkoxysilanes such as methyl-,
or attaching it to the support.1 Incorporating bifunctionalized/ phenyl-, ethylene-, and phenylene-bridged silanes in the presence
disubstituted organic moieties as an integral component of of a surfactant to achieve monolithic hybrid materials with hier-
the inorganic support is an alternative approach. The utilization archically ordered porosity.7 Inorganic–organic hybrid materials
of molecular building blocks that contain a variable organic assembled by sol–gel polymerization of polyfunctional molecular
fragment attached to two or more trialkoxysilane groups offers a building blocks were also described by Corriu8,9 and studied in
higher degree of linkage per monomer unit and leads to bridged detail by Shea and Loy, i.e. by incorporating bridged hydrocarbons
polysilsesquioxanes. In these highly cross-linked three-dimensional of different lengths as well as arylenes, such as 1,4-phenylene,
networks the organic moiety is covalently attached to trialkoxy- 1,3,5-phenylene, 4,40-biphenylene and 4,40-terphenylene bridging
silane groups by non-hydrolyzable silicon carbon bonds and groups.2,10 The polycondensation of the disubstituted silanetriols,
can be varied in functionality, length and torsional rigidity by 4,40-bis(trihydroxysilyl)stilbene and 4,40-bis(trihydroxysilyl)diphenyl-
its substitution geometry.2 In contrast to a post-synthetic buta-1,3-diyne was described by Cerveau and co-workers.11
modification of the silica surface (i.e. by classical grafting3),
We recently reported the synthesis of meso-macroporous
monolithic silica based on the hydrolysis and condensation of
1,4-bis(trimethoxysilyl)arenes in a modified sol–gel route.12
This previous study showed the proof-of-principle concept of
incorporating 1,4-bis(trimethoxysilyl)benzene (para-BTMB) and
Institute of Physical Chemistry, Justus Liebig University Giessen, Heinrich-Buff-Ring 17,
D-35392 Giessen, Germany. E-mail: bernd.smarsly@phys.chemie.uni-giessen.de;
Tel: +49 641 9934590
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6nj00758a 1,4-bis((trimethoxysilyl)phenyl)dimethoxysilane (para-BTPMS).
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Based on the previous study, we demonstrate herein how
systematic variations of the porogen used in the synthesis affects
the porosity of the final materials, starting from the ortho-, meta-
and para-isomers of bis(trimethoxysilyl)arenes. The choice of an
arylene bridging group is based on the thermal stability upon
annealing being required with respect to template removal. Even
if the use of triethoxysilyl analogues is widely more common due
to their lower toxicity, the trimethoxy variety was chosen in this
study, as it offers a similar hydrolysis behavior to that of TMOS,
required for co-condensation.
One major goal and challenge of the present study was the
synthesis of the respective precursors, especially the gram-scale
synthesis and purification of the ortho-, meta- and para-isomers
of BTMB and BTPMS. Since elevated temperatures can result in
polymerization reactions, suitable vacuum distillation conditions
had to be applied in order to obtain the pure compounds.
A further main objective of this study lies with the assessment of
the influence of the precursor on the average pore size, the pore size
distribution and pore volume (both meso- and macroporosity) in
the monolith materials obtained by the procedure developed by K.
Nakanishi.13,14 Such insights are of significant relevance with
respect to the crucial role of the combined meso- and macroporosity
of this type of monolithic silica, which endows the material with
both a high surface area (mesoporosity) and a high pore volume
(macroporosity). Evidently, the prepared hybrid materials may have
potential applications as chromatographic supports in separation
sciences (monolithic HPLC columns), as they provide, compared
to pure SiO2 materials, high specific surface areas and a high
permeability in combination with a hydrophobic surface resulting
from the chemical functionality.15 Moreover, they offer the possibi-
lity for further postsynthetic functionalization, i.e. by sulfonating the
aromatic moieties which could be useful in acidic catalysis or in
solid state proton conductivity applications.16
Since the modification of the interaction between the arylene
moieties and species in the liquid and/or gaseous phase lies at the
heart of such effects, we performed in-depth physisorption analysis
using dibromomethane (DBM), performed at T = 290 K, i.e. at room
temperature. Physisorption measurements utilizing DBM provide
relevant information on such hybrid materials. First, in contrast
to N2 or Ar, DBM is a polar fluid and can be expected to show
a different behavior at small p/p0, i.e. at conditions for layer
formation, if pure SiO2 materials and the hybrid materials under
study are compared. Second, the desorption of DBM fluid should
be different for pure SiO2 materials and the hybrid materials, as the
contact angle of liquid DBM on the mesopore surface should
depend on the hydrophilic/hydrophobic character of the mesopore
surface. Hence, we performed an extensive study of DBM sorption
in the prepared hybrid materials, in comparison to corresponding
monoliths of pure SiO2 as reference.
Scheme 1 ortho-, meta- and para-substituted bis(trimethoxysilyl)arenes by
silylation of dibromobenzene with chlorotrimethoxysilane. (1) Bis(trimethoxy-
silyl)benzene (BTMB), (2) bis((trimethoxysilyl)phenyl)dimethoxysilane (BTPMS).
muriatic and acetic acid were purchased from Sigma-Aldrich
(Steinheim, Germany), and tetramethyl orthosilicate (TMOS),
urea and poly(ethylene glycol) (PEG, 10 000 g molꢀ1) were
obtained from Merck (Darmstadt, Germany). All reagents were
used as received without further purification.
Synthesis procedure
The general synthetic procedure is outlined in Scheme 1.
According to a literature protocol, chlorotrimethoxysilane was
synthesized by the dropwise addition of 88 mL of anhydrous
methanol to 80 mL of tetrachlorosilane while a flow of argon
was applied and the mixture was stirred for 30 minutes. At a
boiling point of 112 1C the crude product was then obtained by
distillation.17 To 44 g of chlorotrimethoxysilane and 16 g of
magnesium turnings submerged in 300 mL of anhydrous THF,
a solution of 32 g of the corresponding dibromobenzene in
400 mL of THF was added with a dropping funnel. Cooling with
an ice bath was applied when the exothermic reaction became
too intense. The mixture was stirred for 15 hours at room
temperature after which the reactants were filtered and most
of the solvent removed. Extraction of the residue using dry
pentane was carried out under inert conditions using the
Schlenk technique. Products were obtained after removal of the
solvent and fractional distillation in vacuum. While BTMB was
obtained at 121 1C (0.1 mbar) as a clear liquid, the byproduct
BTPMS was distilled at 198 1C (0.4 mbar) and was collected as a
yellow liquid. In the case of para-BTMP the liquid solidified
below 52 1C. Yields were 30% for the compound BTMB and
18% for BTPMS.8 The main reason for such poor yields is the
relatively high temperature at which distillation is carried out,
forcing an easy polymerization of the silica species and thus
adequate vacuum conditions have to be ensured. The purity of
the compounds was shown by NMR (see the ESI†).
Preparation of monolithic hybrid silica rods
The preparation conditions of the monolithic silica rods were
similar to those reported elsewhere in the literature.18 Typical
conditions are as follows. A mixture (2.5 mL) of TMOS and the
corresponding bis(trimethoxysilyl)arene in weight ratios of 95/5
Experimental details
Chemicals
1,2- and 1,3-dibromobenzene were purchased from abcr chemicals and 90/10, respectively, was added to a solution of acetic acid
(Karlsruhe, Germany), 1,4-dibromobenzene, tetrachlorosilane, (0.01 M, 5 mL), urea (0.45 g) and PEG. The amount of PEG was
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varied in 0.02 g steps in the range of 0.355 g to 0.475 g. As a solid
compound, 1,4-bis(trimethoxysilyl)benzene was firstly liquefied
at 70 1C and, after the addition of TMOS, stirred at this tempera-
ture for 1 hour. After mixing, the solutions were stirred at room
temperature for 25 minutes, filtered with a 0.45 mm PTFE filter
and charged into a 10 mL plastic vial which was brought into a
water bath (40 1C) for approximately 16 hours. The resultant gel
was further hydrothermally treated by raising the temperature
slowly over a period of 12 hours up to 95 1C and then kept for
15 hours to form mesopores by ammonia generated through the
hydrolysis of urea. The obtained alcogels were then washed with
methanol for 3 days, replacing the solvent daily. A final calcina-
tion was carried out by raising the temperature over 15 hours up
to 330 1C, then keeping at this temperature for 10 hours and
cooling down afterwards over 15 hours.
Measurements/characterization
Physisorption measurements were performed in an automated
gas sorption station (AS-6, Quantachrome Corporation, Boynton
Beach, USA) utilized for standard characterization measurements
of nanostructured matter by nitrogen sorption isotherms. The
instrument software supported the standard data reduction
algorithms, such as Brunauer–Emmett–Teller (BET), as well as
NLDFT kernels for typical pore geometries. The monolithic silica
rods were placed in standard glass tubes and stabilized at the
measurement temperature with T = 77 K maintained by liquid
nitrogen in standard cryostats. Isotherms were measured up
to 0.95 of the equilibrium nitrogen pressures p0. Samples were
evacuated at W = 120 1C for 12 h before the measurements.
For measuring dibromomethane (DBM) sorption, samples were
evacuated for 60 h and stabilized at T = 290 K (liquid density
2.477 g cmꢀ3). In this case, a Quadrasorb Evo instrument
was used. Mercury intrusion measurements were carried out
using Pascal 140/440 porosimeters (Thermo Fisher Scientific,
Rodano, Italy) in a pressure range from 0 MPa to 400 MPa.
Results were calculated applying the instrument software Sol.I.D.
using Washburn’s equation with a mercury surface tension of
0.48 N mꢀ1 and a contact angle of intruded mercury of y = 1401.
The morphology of monolithic silica rods was examined with a
high resolution scanning electron microscope (HSEM Merlin/
Software Smart SEM V05.03, Zeiss, Oberkochen, Germany).
Fig. 1 Physisorption isotherms (left) and pore size distributions (right) of
TMOS/BTMB hybrids with 5% of organosilica precursor and increasing
amounts of PEG (from top to bottom). The mesopore diameter distribu-
tions were calculated using NLDFT adsorption branch analysis assuming a
cylindrical pore shape.
Results and discussion
The characterization of the obtained meso-macroporous
materials is exemplified for para-BTMB and para-BTPMS test
series containing 5% of the organosilica precursor, shown in
the following figures. N2 physisorption measurements revealed
that an increased PEG amount results in an only moderate
change in the shape of the isotherm (Fig. 1 and 2). The mean
diameter of the mesopores and the mesopore volume were
Fig. 2 Physisorption isotherms (left) and pore size distributions (right) of
TMOS/BTPMS hybrids with 5% organosilica precursor and increasing
amounts of PEG (from top to bottom).
similar to the values typically observed for such SiO2-based broadening of the mesopore size distributions observed,
monoliths. Within the given data series, the average mesopore i.e. the formation of a well-defined porosity was aggravated.
diameters stayed almost constant. Only at significantly higher Statistical deviations constitute inconsistent pressure tightness
PEG amounts in the starting solution was a substantial of the used plastic vials during the hydrothermal treatment of
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the wet gels. A comparison of the different isomers’ porosity using the para-isomers, while the meta-isomers lead to meso-
data of both compounds can be seen in Table 1 and Fig. 3. The pore sizes in the range of 9.5–9.7 nm. The biggest mesopore
smallest average mesopore diameters of 7.7 nm are generated size of 10.4 nm was achieved using the ortho-isomer. Since the
Table 1 Porosity data of the ortho-, meta- and para-isomers of TMOS/BTMB and TMOS/BTPMS hybrids with different amounts of PEG in the starting mixture
PEG
content [g]
BET surf. area
Mesopore volume
Mesopore
diam. [nm]
Macropore
Macropore
diam. [mm]
(SBET) [m2 gꢀ1
]
[cm3 gꢀ1
]
volume [cm3 gꢀ1
]
p-BTMB (5%)
p-BTPMS (5%)
m-BTMB (5%)
m-BTPMS (5%)
o-BTMB (5%)
o-BTPMS (5%)
p-BTMB (10%)
p-BTPMS (10%)
0.355
0.375
0.395
0.415
0.435
0.455
0.475
560
580
570
530
530
520
590
0.95
1.06
1.04
1.01
0.97
1.53
2.52
7.5
7.5
7.5
8.1
7.5
9.4
7.0
2.12
2.16
2.22
2.29
2.37
2.33
2.09
2.9
1.0
1.0
1.1
0.3
0.3
0.1
0.355
0.375
0.395
0.415
0.435
0.455
0.475
550
550
530
470
550
570
570
0.97
1.02
1.04
1.06
1.40
1.31
2.43
7.0
8.4
8.1
9.4
7.5
7.0
7.0
2.07
2.19
2.34
2.31
2.33
2.39
2.10
1.5
1.4
1.4
1.1
0.4
0.5
0.7
0.355
0.375
0.395
0.415
0.435
0.455
0.475
510
470
500
470
470
500
470
0.93
0.90
0.94
0.96
0.94
1.03
1.26
9.4
9.4
9.4
9.4
9.4
9.4
11.6
—
—
—
—
—
—
—
—
—
—
—
—
—
—
0.355
0.375
0.395
0.415
0.435
0.455
0.475
520
560
590
560
300
590
560
1.13
1.23
1.19
1.26
0.81
1.55
2.09
10.4
9.0
9.4
9.4
9.4
9.4
9.4
—
—
—
—
—
—
—
—
—
—
—
—
—
—
0.355
0.375
0.395
0.415
0.435
0.455
0.475
450
440
430
460
440
460
440
0.96
0.97
1.01
1.14
1.11
1.41
2.47
10.4
10.4
10.4
9.4
10.4
10.4
11.6
—
—
—
—
—
—
—
—
—
—
—
—
—
—
0.355
0.375
0.395
0.415
0.435
0.455
0.475
470
410
440
400
440
410
420
0.93
0.97
1.02
1.01
1.14
1.16
1.67
9.4
—
—
—
—
—
—
—
—
—
—
—
—
—
—
10.4
10.4
11.6
10.4
10.4
10.4
0.355
0.375
0.395
0.415
0.435
0.455
0.475
560
580
570
530
530
520
590
0.95
1.06
1.04
1.01
0.97
1.53
2.52
7.5
7.5
7.5
8.1
7.5
9.4
7.0
—
—
—
—
—
—
—
—
—
—
—
—
—
—
0.355
0.375
0.395
0.415
0.435
0.455
0.475
550
550
530
470
550
570
570
0.97
1.02
1.04
1.06
1.40
1.31
2.43
7.0
8.4
8.1
9.4
7.5
7.0
7.0
—
—
—
—
—
—
—
—
—
—
—
—
—
—
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Table
2
Mesoporosity parameters obtained by different evaluation
methods for the sample p-BTMB (5%, 0.355 g PEG)
NLDFT NLDFT NLDFT (ads, BJH BJH
(des)
0.94
(ads, cyl)
0.95
cyl/sph)
0.92
(ads) (des)
0.93 0.97
Mesopore volume
[cm3 gꢀ1
]
Average mesopore
diameter [nm]
9.0
7.5
12.9
8.8
7.4
advantage of density functional theory (DFT) methods is the
determination of an accurate and valid pore size distribution
across the complete mesopore (and micropore) size range. In
contrast to classical calculation methods like the BJH method,
which typically underestimates the mesopore size systematically,
the calculation via adsorption branch kernels takes the delayed
condensation of a metastable fluid correctly into account.20
As shown in Fig. 5 and 6 in a direct comparison of the N2
physisorption analysis, the mesoporosity of the monoliths
prepared using para-BTMB and para-BTPMS is almost identical
to the mesoporosity observed in pure SiO2 monoliths, prepared
under standard conditions.18 The pore size distributions and
also the mesopore volumes are thus almost indistinguishable,
Fig. 3 Mesopore diameters of silica hybrids with different isomers and 5%
precursor.
mesoporosity is generated during the hydrothermal treatment
by etching SiO2 with ammonia originating from the decom-
position of urea, we attribute these differences to two possible
mechanisms: slight differences in the surface polarity, caused
by the substitution geometry of the incorporated silica precur-
sor, can affect the diffusion of the ammonia-containing liquid
into the siliceous matrix. In addition, also a different degree of
condensation of the silica’s silanol groups based on the ortho-,
meta- and para-isomers as organosilica precursors can be
discussed as an origin of the different degrees of etching by
ammonia. While no corresponding solid-state NMR data were
available, we exclude this possible mechanism, since all the
prepared materials were subjected to a final treatment at 330 1C
for 10 hours, which can be expected to result in a comparable
condensation. In conclusion, the successful synthesis of meso-
and macroporous monoliths was only possible by using a
weight ratio of 95/5 and 90/10 of both TMOS/BTMB and
TMOS/BTPMS, respectively.
Mesopore size distributions using different evaluation methods
are given in Fig. 4 and the corresponding values are presented in
Table 2. The NLDFT model for cylindrical pores utilizing kernels
for the adsorption branch of the isotherm was chosen in this
study, as it is known to give reasonable results for the pore
size distribution for silica materials of this kind.19 A main
Fig. 5 Nitrogen physisorption isotherms of a pure TMOS based reference
monolith compared to the hybrid samples containing the para-isomer of
BTMB/BTPMS (5%, 0.355 g PEG).
Fig. 4 Mesopore size distributions for the sample p-BTMB (5%, 0.355 g
PEG) obtained by different evaluation methods: NLDFT equilibrium model
for cylindrical pore shapes (’), NLDFT adsorption branch model for
cylindrical pore shapes (m), BJH adsorption branch model (J) and BJH
desorption branch model (&).
Fig. 6 Pore size distribution from nitrogen physisorption (NLDFT adsorp-
tion branch analysis with cylindrical pore shapes) of a pure TMOS based
reference monolith compared to the hybrid samples containing the para
isomer of BTMB/BTPMS (5%, 0.355 g PEG).
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suggesting that our synthetic procedure is sufficiently optimized This behavior is in line with the different contact angle of DBM
to produce such monoliths in a highly reproducible fashion. on a pure SiO2 surface and a surface additionally containing
While the analysis of N2 physisorption experiments (performed non-polar moieties, resulting in different values for the partial
at T = 77 K), combined with the aforementioned evaluation pressure of pore emptying according to the Kelvin equation.
procedure, provides accurate and meaningful structural para-
Since the DBM isotherms feature a well-defined closure upon
meters describing the mesoporosity, other relevant information desorption, the presence of swelling or other effects related to
on the mesopore network cannot be obtained. In particular, kinetically hindered desorption can be excluded. These results are
the modified polarity of the silica surface (resulting from the in contrast to DBM sorption investigations performed recently
incorporated aromatic moiety) cannot be determined from N2 by our group.21 In these studies, porous SiO2 spheres were
physisorption measurements, not even in a qualitative way. post-functionalized by alkyl chains, and the isotherm showed
A further aspect is that sorption experiments performed near pronounced non-closure effects along the desorption branch.
room temperature are helpful to judge the presence of possible While the origin of this peculiar difference in the DBM sorption
swelling effects under those conditions, being relevant for properties has not been studied in detail, we speculate that the
applications in chromatography, e.g. by using polar/non-polar mesopore network of the materials described in ref. 21 exhibits
solvents. Such insights into the polarity of the surface of the a hindrance towards emptying for DBM under the applied
prepared hybrid materials were addressed in the present study conditions. In conclusion, these comparative sorption experi-
by performing additional physisorption experiments utilizing ments using DBM reveal a reduced interaction of DBM with the
dibromomethane (DBM) as a polar adsorbate at T = 290 K.
organically modified surface, which is reasonable taking into
Fig. 7 presents DBM isotherms comparing a pure SiO2 mono- account the unpolar nature of the precursors used. Furthermore,
lith (prepared using TMOS) and monoliths generated using para- the sorption experiments using DBM allow for an estimation of
BTMB and para-BTPMS ((5%, 0.355 g PEG) see Table 1). The latter the contact angle, following the procedure described in litera-
can be classified as type V hysteresis loops according to IUPAC ture.21 This approach is based on the Kelvin equation
recommendations, while N2 isotherms exhibit type IV hysteresis
ln(p/p0) = ꢀ2gVLcos y/RTrm
(1)
behavior (see above). This difference is in accordance with the
fact that DBM generally does not exhibit the formation of layers where p/p0 is the relative pressure, g is the surface tension, VL is
on siliceous surfaces. In general, it should be noted that the the molar volume of the bulk liquid adsorbate, and is its
DBM isotherms are of high quality, showing a well-defined contact angle on the adsorbent (here DBM). R is the gas
plateau at large pressures and the absence of non-closure constant, T is the absolute temperature and rm represents the
behavior upon desorption.21
mean radius of curvature. Using the pore radius obtained from
Furthermore, these isotherms feature interesting character- physisorption analysis, it is possible to extract from eqn (1),
istics with respect to the sorption and interaction behavior as a further assuming a surface tension of g = 39.7 mN mꢀ1 and VL
function of composition. First, the DBM uptake of the hybrid calculated based on a density of liquid DBM of 2.477 g cmꢀ3. Here,
materials is lower, in terms of the adsorbed volume, than the we take rm = 7.4 nm assuming a cylindrical pore (see Table 2)
DBM uptake obtained for the monolith made up of pure SiO2, determined from the BJH approach (desorption branch). For a
even for small p/p0. This finding suggests a reduced interaction para-BTMB-derived monolith, we obtained a reasonable agree-
of DBM with the mesopore surfaces within the hybrid materials ment between the average mesopore sizes derived from nitro-
compared to pure SiO2, possessing a certain content of aromatic gen and DBM physisorption if y = 301. By comparison, for the
moieties, which is plausible taking the polar nature of DBM into pure SiO2 monolith a contact angle of ca. 101 (DBM on a SiO2
account. Second, both condensation and desorption branches surface) can be inferred by a corresponding procedure. This
are located at larger p/p0, if compared to pure SiO2 (see Fig. 7). finding is reasonable, as DBM is a polar fluid and thus tends to
spread on a SiO2 surface. In essence, the aforementioned
difference in the contact angles can be seen in the isotherms
themselves by the fact that the shape of the isotherms is almost
identical, but the isotherms are shifted with respect to each
other. Although a significant uncertainty of ca. 5% is present,
this evaluation proves that the mesopore surface in the BTMB-
and BTPMS-derived hybrid monoliths is markedly less polar
than a SiO2 pore surface. At first glance, this finding appears to
be straightforward, but it is important in that the organic
moieties are accessible from the mesopore space, i.e. are not
buried within the siliceous matrix. In ref. 21, porous SiO2 beads
endowed with C18 alkyl chains attached to the walls of the
mesopores were studied.21 Applying the aforementioned analysis
based on DBM sorption and the BJH analysis (desorption
Fig. 7 Dibromomethane sorption isotherms of
a pure TMOS based
branch), i.e. the same procedure as applied in the present
study, a contact angle of y = 451 had been reported, being
reference monolith compared to the hybrid samples containing the
para-isomer of BTMB/BTPMS (5%, 0.355 g PEG).
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Fig. 8 Dibromomethane sorption isotherms of silica hybrids containing
the different isomers of BTMB (5%, 0.355 g PEG).
larger compared to that of the present study using arene-
modified SiO2. The lower polarity of these C18-modified silica
materials is reasonable, as the alkyl groups are attached in a
post-functionalization step. Thus, the surface concentration of
non-polar species on the mesopore walls is markedly higher
than in the here-applied synthesis using organically modified
precursors.
Furthermore, the DBM sorption properties of hybrid mono-
liths prepared from different isomers were studied (Fig. 8 and
Fig. S1 in the ESI†). All of the hybrid materials feature well-
defined isotherms, confirming the well-defined mesopore
structure and a high degree of cross-linking (absence of
swelling). For the hybrids containing the para-isomers the lower
closure point of the hysteresis is observed at ca. p/p0 = 0.3
(BTMB) and p/p0 = 0.2 (BTPMS). The closure point for samples
with the built-in meta-isomer is shifted to higher relative
pressures at ca. p/p0 = 0.35 (BTMB) and p/p0 = 0.3 (BTPMS). For
samples functionalized with the ortho-isomers the closure point
is located at ca. p/p0 = 0.45 (BTMB) and p/p0 = 0.4 (BTPMS). These
difference can be attributed to the differences in the pore-size
distributions (see nitrogen physisorption analysis), and pore-
blocking of cavitation effects can be excluded for DBM sorption
performed at T = 290 K.
The systematic variation of the PEG ratio used in the starting
mixture exhibited a systematic effect on the macroporosity as
indicated by Hg intrusion porosimetry and SEM observations
(Fig. 9 and 10): with an increasing amount of PEG acting as the
porogen, the average macropore diameter decreases until the
typical macropore morphology finally collapses at the highest
PEG content used. These trends are in accordance with the
results known for pure silica monoliths.21
Interestingly, one can also observe a dependence of the
mesoporosity on the amount of porogen, which suggests that
a molecular templating mechanism of PEG chains occurs. This
interpretation is supported by the observation of increasing
Fig. 9 Pore size distributions obtained from mercury intrusion porosi-
metry (left) and corresponding SEM images (right) of para-TMOS/BTMB
(5%) hybrids.
mesopore volumes when using the highest PEG amount and etching of the silica surface in the hydrothermal treatment, the
leads to the assumption that two kinds of mesopore species are other species is created via PEG templating on a molecular level.
present. This interpretation is supported by the observation of
Furthermore, the reduced macropore volume observed at
increasing mesopore volumes when using the highest PEG the highest concentrations of PEG used (Fig. 11) indicates that
amount and leads to the assumption that two kinds of meso- above a certain amount of PEG the spinodal decomposition
pore species are present. Whilst one is the result of chemical mechanism is affected, i.e. an increasing fraction of PEG does
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Fig. 11 Average macropore diameter of meso-macroporous TMOS/
BTMB- and TMOS/BTPMS-based hybrids (para-isomers with 5% precursor
content) as a function of the varied amount of porogen in the starting
feed solution.
Conclusions
In the present study the ortho-, meta- and para-isomers of
bis(trimethoxysilyl)benzene (BTMB) and bis((trimethoxysilyl)-
phenyl)dimethoxysilane (BTPMS) were synthesized and used
as bifunctionalized organosilica precursors, in mixtures with
tetramethyl orthosilicate (TMOS) as SiO2 precursors. The
obtained compounds were utilized for the preparation of
monolithic silica hybrids in an adapted sol–gel-route resulting
in the formation of a macroporous skeleton by phase separation
induced by poly(ethylene glycol), according to the synthesis
developed by K. Nakanishi.14
The amount of polymer was varied spanning a quite wide
range to adjust defined macropore morphology comparable to
pure silica monoliths. The porosity was characterized by N2 and
dibromomethane sorption, Hg-intrusion and SEM. Similar
trends regarding the macro- and mesoporosity as known from
TMOS-based monoliths were observed over a relatively large
range of PEG concentrations. Hybrids with well-defined and
bicontinuous macroporosity and pronounced mesoporosity
were obtained up to weight ratios of 95/5 and 90/10 of TMOS/
BTMB and TMOS/BTPMS, respectively (Fig. 12). While such
amounts of the bis(trimethoxysilyl)arenes appear to be small,
even such moderate contents of organic moieties are sufficient
to substantially change the materials’ properties, as revealed by
physisorption experiments using dibromomethane. Using a
previously described approach, the hybrid materials possess a
contact angle of the mesopore surface of ca. 301 with respect to
dibromomethane, which significantly exceeds the contact angle
derived for corresponding pure SiO2 monoliths by the same
procedure. Thereby, it is proven that the arene units are
accessible from the mesopore space. We have recently demon-
strated that using a weight ratio of 5/95 of phenyltrimethoxy-
Fig. 10 Pore size distributions obtained from mercury intrusion porosi-
metry (left) and corresponding SEM images (right) of para-TMOS/BTPMS
(5%) hybrids.
not participate in this intricate separation process, but silane (PTMS) and tetramethoxysilane (TMOS) as precursors
is incorporated into the siliceous matrix and results in meso- yielded monolithic silica hybrid capillaries, which exhibited a
pores after calcination. Our experiments also showed that the different interaction towards hydrocarbons compared to capillaries
synthesis of silica monoliths with a homogeneous bimodal containing pure silica possessing an analogous porosity.15 This
porosity can only be realized in narrow ranges of the BTMB substantial impact of small contents of aromatic moieties on the
and BTPMS content.
interaction can be attributed to the homogeneous distribution
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Acknowledgements
We would like to thank Merck KGaA for financial support,
in particular Dr. Karin Cabrera. Moreover, we thank Christoph
Weidmann for performing the physisorption measurements and
Dr. Heike Hausmann for NMR analysis. This project was supported
by the Laboratory of Materials Research (LaMa) of JLU.
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