Martin
von der Lehr
,
Rüdiger
Ellinghaus
and
Bernd M.
Smarsly
*
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
First published on 18th April 2016
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.
The integration of such compounds into the siliceous matrices of periodic mesoporous organosilicas (PMOs) was intensively described and reviewed by Froeba et al.4 Moreover, PMOs made from silsesquioxanes with bridging phenyl groups were synthesized by Kuschel and Polarz and studied with respect to their catalytic applications.5 Nakanishi et al. reported the integration of alkyltrialkoxysilanes and alkylene-bridged alkoxysilanes as organosilica precursors for the synthesis of silica monoliths.6 Huesing and co-workers utilized different ethylene-glycol-modified derivatives of organoalkoxysilanes such as methyl-, phenyl-, ethylene-, and phenylene-bridged silanes in the presence of a surfactant to achieve monolithic hybrid materials with hierarchically ordered porosity.7 Inorganic–organic hybrid materials assembled by sol–gel polymerization of polyfunctional molecular building blocks were also described by Corriu8,9 and studied in detail by Shea and Loy, i.e. by incorporating bridged hydrocarbons of different lengths as well as arylenes, such as 1,4-phenylene, 1,3,5-phenylene, 4,4′-biphenylene and 4,4′-terphenylene bridging groups.2,10 The polycondensation of the disubstituted silanetriols, 4,4′-bis(trihydroxysilyl)stilbene and 4,4′-bis(trihydroxysilyl)diphenylbuta-1,3-diyne was described by Cerveau and co-workers.11
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 1,4-bis((trimethoxysilyl)phenyl)dimethoxysilane (para-BTPMS). 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 possibility 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.
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). |
PEG content [g] | BET surf. area (SBET) [m2 g−1] | Mesopore volume [cm3 g−1] | Mesopore diam. [nm] | Macropore volume [cm3 g−1] | Macropore diam. [μm] | |
---|---|---|---|---|---|---|
p-BTMB (5%) | 0.355 | 560 | 0.95 | 7.5 | 2.12 | 2.9 |
0.375 | 580 | 1.06 | 7.5 | 2.16 | 1.0 | |
0.395 | 570 | 1.04 | 7.5 | 2.22 | 1.0 | |
0.415 | 530 | 1.01 | 8.1 | 2.29 | 1.1 | |
0.435 | 530 | 0.97 | 7.5 | 2.37 | 0.3 | |
0.455 | 520 | 1.53 | 9.4 | 2.33 | 0.3 | |
0.475 | 590 | 2.52 | 7.0 | 2.09 | 0.1 | |
p-BTPMS (5%) | 0.355 | 550 | 0.97 | 7.0 | 2.07 | 1.5 |
0.375 | 550 | 1.02 | 8.4 | 2.19 | 1.4 | |
0.395 | 530 | 1.04 | 8.1 | 2.34 | 1.4 | |
0.415 | 470 | 1.06 | 9.4 | 2.31 | 1.1 | |
0.435 | 550 | 1.40 | 7.5 | 2.33 | 0.4 | |
0.455 | 570 | 1.31 | 7.0 | 2.39 | 0.5 | |
0.475 | 570 | 2.43 | 7.0 | 2.10 | 0.7 | |
m-BTMB (5%) | 0.355 | 510 | 0.93 | 9.4 | — | — |
0.375 | 470 | 0.90 | 9.4 | — | — | |
0.395 | 500 | 0.94 | 9.4 | — | — | |
0.415 | 470 | 0.96 | 9.4 | — | — | |
0.435 | 470 | 0.94 | 9.4 | — | — | |
0.455 | 500 | 1.03 | 9.4 | — | — | |
0.475 | 470 | 1.26 | 11.6 | — | — | |
m-BTPMS (5%) | 0.355 | 520 | 1.13 | 10.4 | — | — |
0.375 | 560 | 1.23 | 9.0 | — | — | |
0.395 | 590 | 1.19 | 9.4 | — | — | |
0.415 | 560 | 1.26 | 9.4 | — | — | |
0.435 | 300 | 0.81 | 9.4 | — | — | |
0.455 | 590 | 1.55 | 9.4 | — | — | |
0.475 | 560 | 2.09 | 9.4 | — | — | |
o-BTMB (5%) | 0.355 | 450 | 0.96 | 10.4 | — | — |
0.375 | 440 | 0.97 | 10.4 | — | — | |
0.395 | 430 | 1.01 | 10.4 | — | — | |
0.415 | 460 | 1.14 | 9.4 | — | — | |
0.435 | 440 | 1.11 | 10.4 | — | — | |
0.455 | 460 | 1.41 | 10.4 | — | — | |
0.475 | 440 | 2.47 | 11.6 | — | — | |
o-BTPMS (5%) | 0.355 | 470 | 0.93 | 9.4 | — | — |
0.375 | 410 | 0.97 | 10.4 | — | — | |
0.395 | 440 | 1.02 | 10.4 | — | — | |
0.415 | 400 | 1.01 | 11.6 | — | — | |
0.435 | 440 | 1.14 | 10.4 | — | — | |
0.455 | 410 | 1.16 | 10.4 | — | — | |
0.475 | 420 | 1.67 | 10.4 | — | — | |
p-BTMB (10%) | 0.355 | 560 | 0.95 | 7.5 | — | — |
0.375 | 580 | 1.06 | 7.5 | — | — | |
0.395 | 570 | 1.04 | 7.5 | — | — | |
0.415 | 530 | 1.01 | 8.1 | — | — | |
0.435 | 530 | 0.97 | 7.5 | — | — | |
0.455 | 520 | 1.53 | 9.4 | — | — | |
0.475 | 590 | 2.52 | 7.0 | — | — | |
p-BTPMS (10%) | 0.355 | 550 | 0.97 | 7.0 | — | — |
0.375 | 550 | 1.02 | 8.4 | — | — | |
0.395 | 530 | 1.04 | 8.1 | — | — | |
0.415 | 470 | 1.06 | 9.4 | — | — | |
0.435 | 550 | 1.40 | 7.5 | — | — | |
0.455 | 570 | 1.31 | 7.0 | — | — | |
0.475 | 570 | 2.43 | 7.0 | — | — |
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 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
NLDFT (des) | NLDFT (ads, cyl) | NLDFT (ads, cyl/sph) | BJH (ads) | BJH (des) | |
---|---|---|---|---|---|
Mesopore volume [cm3 g−1] | 0.94 | 0.95 | 0.92 | 0.93 | 0.97 |
Average mesopore diameter [nm] | 9.0 | 7.5 | 12.9 | 8.8 | 7.4 |
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, suggesting that our synthetic procedure is sufficiently optimized to produce such monoliths in a highly reproducible fashion. While the analysis of N2 physisorption experiments (performed at T = 77 K), combined with the aforementioned evaluation procedure, provides accurate and meaningful structural parameters describing the mesoporosity, other relevant information on the mesopore network cannot be obtained. In particular, the modified polarity of the silica surface (resulting from the incorporated aromatic moiety) cannot be determined from N2 physisorption measurements, not even in a qualitative way. A further aspect is that sorption experiments performed near room temperature are helpful to judge the presence of possible swelling effects under those conditions, being relevant for applications in chromatography, e.g. by using polar/non-polar solvents. Such insights into the polarity of the surface of the prepared hybrid materials were addressed in the present study by performing additional physisorption experiments utilizing dibromomethane (DBM) as a polar adsorbate at T = 290 K.
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. 7 presents DBM isotherms comparing a pure SiO2 monolith (prepared using TMOS) and monoliths generated using para-BTMB and para-BTPMS ((5%, 0.355 g PEG) see Table 1). The latter can be classified as type V hysteresis loops according to IUPAC recommendations, while N2 isotherms exhibit type IV hysteresis behavior (see above). This difference is in accordance with the fact that DBM generally does not exhibit the formation of layers on siliceous surfaces. In general, it should be noted that the DBM isotherms are of high quality, showing a well-defined plateau at large pressures and the absence of non-closure behavior upon desorption.21
Fig. 7 Dibromomethane sorption 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). |
Furthermore, these isotherms feature interesting characteristics with respect to the sorption and interaction behavior as a function of composition. First, the DBM uptake of the hybrid materials is lower, in terms of the adsorbed volume, than the DBM uptake obtained for the monolith made up of pure SiO2, even for small p/p0. This finding suggests a reduced interaction of DBM with the mesopore surfaces within the hybrid materials compared to pure SiO2, possessing a certain content of aromatic moieties, which is plausible taking the polar nature of DBM into account. Second, both condensation and desorption branches are located at larger p/p0, if compared to pure SiO2 (see Fig. 7). This behavior is in line with the different contact angle of DBM on a pure SiO2 surface and a surface additionally containing non-polar moieties, resulting in different values for the partial pressure of pore emptying according to the Kelvin equation.
Since the DBM isotherms feature a well-defined closure upon desorption, the presence of swelling or other effects related to kinetically hindered desorption can be excluded. These results are in contrast to DBM sorption investigations performed recently by our group.21 In these studies, porous SiO2 spheres were post-functionalized by alkyl chains, and the isotherm showed pronounced non-closure effects along the desorption branch. While the origin of this peculiar difference in the DBM sorption properties has not been studied in detail, we speculate that the mesopore network of the materials described in ref. 21 exhibits a hindrance towards emptying for DBM under the applied conditions. In conclusion, these comparative sorption experiments using DBM reveal a reduced interaction of DBM with the organically modified surface, which is reasonable taking into account the unpolar nature of the precursors used. Furthermore, the sorption experiments using DBM allow for an estimation of the contact angle, following the procedure described in literature.21 This approach is based on the Kelvin equation
ln(p/p0) = −2γVLcosθ/RTrm | (1) |
Furthermore, the DBM sorption properties of hybrid monoliths 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.
Fig. 8 Dibromomethane sorption isotherms of silica hybrids containing the different isomers of BTMB (5%, 0.355 g PEG). |
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
Fig. 9 Pore size distributions obtained from mercury intrusion porosimetry (left) and corresponding SEM images (right) of para-TMOS/BTMB (5%) hybrids. |
Fig. 10 Pore size distributions obtained from mercury intrusion porosimetry (left) and corresponding SEM images (right) of para-TMOS/BTPMS (5%) hybrids. |
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 mesopore volumes when using the highest PEG amount and leads to the assumption that two kinds of mesopore species are present. This interpretation is supported by the observation of increasing mesopore volumes when using the highest PEG amount and leads to the assumption that two kinds of mesopore species are present. Whilst one is the result of chemical etching of the silica surface in the hydrothermal treatment, the other species is created via PEG templating on a molecular level.
Furthermore, the reduced macropore volume observed at the highest concentrations of PEG used (Fig. 11) indicates that above a certain amount of PEG the spinodal decomposition mechanism is affected, i.e. an increasing fraction of PEG does not participate in this intricate separation process, but is incorporated into the siliceous matrix and results in mesopores after calcination. Our experiments also showed that the synthesis of silica monoliths with a homogeneous bimodal porosity can only be realized in narrow ranges of the BTMB and BTPMS content.
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. 30° 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 demonstrated that using a weight ratio of 5/95 of phenyltrimethoxysilane (PTMS) and tetramethoxysilane (TMOS) as precursors yielded monolithic silica hybrid capillaries, which exhibited a different interaction towards hydrocarbons compared to capillaries containing pure silica possessing an analogous porosity.15 This substantial impact of small contents of aromatic moieties on the interaction can be attributed to the homogeneous distribution of the organic moieties in combination with the high porosity and high surface area, which enhance this interaction. Thus, the results of this study will be utilized for the analysis of hydrophobicity and separation characteristics as well as tests of column performances in HPLC and the preparation of capillary columns.
Fig. 12 SEM images of TMOS/BTMB and TMOS/BTPMS hybrids with different isomers, each containing 5% of organosilica precursor and 0.355 g of PEG. |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6nj00758a |
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2016 |