Template-Free Self-Assembly of Mesoporous Organosilicas

Article pubs.acs.org/cm

Cite This: Chem. Mater. 2018, 30, 2218−2228

Template-Free Self-Assembly of Mesoporous Organosilicas Quanchang Li, Mobae Afeworki, Nicole M. Callen, Robert J. Colby, Manesh Gopinadhan, Meghan L. Nines Kochersperger, Brian K. Peterson, Michael Sansone, Simon C. Weston, and David C. Calabro* Corporate Strategic Research, ExxonMobil Research and Engineering Company, Annandale, New Jersey 08801, United States

ABSTRACT: Using a known organosilane building block (1,1,3,3,5,5-hexaethoxy-1,3,5-trisilacyclohexane), materials with unexpectedly high surface area (>1200 m2/g) and porosity with narrow pore-size distributions were synthesized in the total absence of pore-templating agents. The properties of these mesoporous organosilicas (MOS), closely resembling those of surfactant-templated mesoporous silicas, are found to vary over a wide range depending on synthesis gel pH and gelation temperature and time. Considerable microporosity is observed at short 90 °C gelation times at pH 12.5, which diminishes to a very minor component of the porosity at times > 23 h. Under the gelation conditions used here (90 °C, pH 12.5), pore volume and pore diameter rise sharply with time, ultimately leveling off at 1.2 cm3/g and 70 Å at times > 70 h. This building block is shown to be stable in acidic gels but undergoes gradual ring opening at gelation temperatures and times in excess of “standard” conditions employed for alkaline gels.The synthesis and properties of these materials, called EMA-2, are compared with materials made from the same precursor in a templated synthesis and from other organosilane precursors in nontemplated syntheses. Comparative TEM and SAXS analysis of products from identical preparations using tetraethylorthosilicate and the 1,1,3,3,5,5hexaethoxy-1,3,5-trisilacyclohexane precursors indicates that the latter forms a highly interconnected network gel lacking discrete particle aggregates.

■

materials.6,7 Whereas the ordering effect of structure directing agents in zeolite synthesis is known but poorly understood, M41S employed surfactants at concentrations above the critical threshold where well-characterized, ordered micelle structures were known to form. The replication of the micelle structure in the uniform pore structure of the still amorphous silica network, combined with the ability to expand the silica pore diameter using standard micelle swelling techniques,7 left little doubt of the templating effect of the surfactant. The impact of surfactant templating on sol−gel chemistry has been extensive. Using essentially the same precursors, reaction conditions, and sol−gel chemistry employed to make xerogels, the paradigm shift of surfactant templating introduced a host of new strategies for designing mesoporous silicas routinely having high surface area and pore volumes typically only achieved using low surface tension solvent exchange or supercritical drying in the absence of templating. Surfactant templating thus provided a facile path to creating amorphous materials distinguished by uniformly sized mesopores with pore systems having the same topology as previously catalogued

INTRODUCTION Materials with high porosity and high surface area are important in material science and in many industrial applications as adsorbents, catalysts and catalyst supports, coatings, filters, insulators, controlled release agents, bioceramics, optical elements, and low-dielectric films. Sol−gel chemistry is a common synthetic route to these materials, producing a broad array of structure types of variable composition. This paper describes the straightforward synthesis of a family of high porosity, high surface area mesoporous organosilicas, all prepared from a single organosilane precursor. The extensive sol−gel chemistry of silicon alkoxides has long included the use of organosilane precursors as a way of incorporating a variety of organic functional groups into silicon oxides.1−5 Organosilanes containing both terminal and bridging R groups, both rigid and flexible, have been used often with the goal of inducing some degree of ordering in these amorphous polyorganosilanes, known as hybrid xerogels. The first reports of the self-assembly of organosilane precursors to form high surface area, mesoporous, hybrid organic−inorganic materials appeared in the late 1980s and early 1990s.2−4 In 1992 Mobil introduced surfactant templating to sol−gel chemistry, effectively creating an intermediate class of porous materials between amorphous silica and crystalline zeolites, collectively known as the M41S family of mesoporous © 2018 American Chemical Society

Received: October 30, 2017 Revised: March 5, 2018 Published: March 9, 2018 2218

DOI: 10.1021/acs.chemmater.7b04480 Chem. Mater. 2018, 30, 2218−2228

Article

Chemistry of Materials

Similarly the six-membered ring precursor, namely, 1,1,3,3,5,5-hexaethoxy-1,3,5-trisilacyclohexane [(EtO)2SiCH2]3 (referred to herein as “3R”), was surfactant-templated from both acidic and alkaline gels followed by extractive surfactant removal to produce a templated, “high organic” periodic mesoporous organosilica (PMO).29,30 The larger six-membered precursor yields a surface area > 1500 m2/g and a templated narrow pore size distribution centered at 22 Å diameter. TEM images of the PMO products obtained using solely this organosilicon precursor in both acid and base media showed a high degree of mesopore ordering. More recently the 3R precursor, in combination with a minor amount of 3R in which one of the ethoxy groups is replaced by a methylene bridge to a −Si(OR)3 group, was co-condensed at room temperature in an acidic gel in the absence of a surfactant template.31 With sufficient aging, this gel was spun-coated as a mesoporous low-k thin film sealant to prevent the diffusion of metal ions in interlayer dielectric devices. The authors did not report the bulk solid product(s) of the synthesis. In this paper we compare surfactant-templated and nontemplated self-assembly of the 3R precursor. We will describe the conditions under which this precursor forms rigid and stable network structures and conditions in which it undergoes ring opening via Si−C bond cleavage. We also show simplified and effective nontemplated pathways to high surface area (>1000m2/g) mesoporous organosilicas with a narrow pore size distribution. In related work,32 we describe a model that explains the high surface area and porosity of these materials in terms of the structural features of the precursor.

ordered arrays of the surfactant molecules. This architecture had not been previously possible in pure silicas. The productive use of surfactant templating in traditional silica sol−gel chemistry encouraged its expanded use with silsesquioxane precursors resulting in the creation of periodic mesoporous organosilicas (PMO) as a templated version of hybrid xerogels.8−10 Several reviews have been written summarizing the extensive literature on the synthesis, characterization, and properties of periodic mesoporous organosilicas.5,11 The success of templated mesoporosity for a broad range of materials compositions has been attributed to a key role in the development of nanochemistry.12 Notwithstanding the above impact of surfactant templating, the precedent of nontemplated, high surface area, mesoporous, hybrid organic−inorganic materials using silsesquioxane precursors had already appeared.2−4 These self-assembled mesoporous organosilicas (MOS) thus demonstrated the precursors and synthesis conditions that would later be combined with surfactant templating to introduce PMOs. As PMO surfactant templating gained wide acceptance, so too has a much more modest literature accumulated on efforts to achieve selfassembled, i.e., template-free, mesoporosity (MOS). A rare direct comparison of templated and nontemplated assembly with the same silsesquioxane precursor was reported by Lin et al.13 Hexylene-bridged 1,6-bis(triethoxysilyl)hexane was assembled in both acid and base via both sol gel and surfactant templated polymerization. Not surprisingly, the surfactant templated products exhibited highly ordered mesopores in the TEM and cubic symmetry in the XRD, neither of which was observed in the sol gel derived products. The sol−gel product prepared in acid was nonporous; however, in base, sol−gel polymerization of this silsesquioxane produced a high surface area xerogel with pore size distribution comparable to the surfactant templated PMOs. Self-assembled, surfactant-free porosity has also been achieved using diverse strategies for manipulating precursor assembly including nonsurfactant porogens,14−17 surfactantfunctional building blocks,18,19 reactive assembly,20 controlled particle growth and aggregation,21,22 hard templating with silica nanoparticles,23 and van der Waals and H-bonding interactions.24 In a number of cases high surface areas (>600 m2/g) and narrow pore size distributions with pore diameters averaging 30−35 Å were achieved. Within this family of hybrid xerogels the highest degree of structural order, exhibiting both molecular-scale and mesoscale periodicity, was achieved with the polycondensation of the rigid phenylene-bridged silsesquioxane precursor.3,5 Less studied, but equally rigid, cyclic, methylene-bridged silsesquioxanes have been reported using the Grignard coupling reactions of both (trialkoxysilyl)methyl magnesium chloride and (dialkoxychlorosilyl)methyl magnesium chloride.25−27 These silsequioxane precursors consist of four- and sixmembered rings of alternating −Si(OEt)2− and −CH2− moieties. Analogous to the isoelectronic Si−O−Si motif of silicon oxides, these Si−CH2−Si based precursors have been used for organosilicon assemblies. The four-membered ring precursor, 1,1,3,3-tetraethoxy-1,3disilacyclobutane, undergoes acid-catalyzed self-condensation in an ethanol/water solvent to produce a gel product which, after solvent removal, yields solids with surface areas of 730−917 m2/g.28 Interestingly this self-assembled hybrid xerogel exhibits both micropores and a very narrow mesopore size distribution centered at ∼38 Å diameter.

■

EXPERIMENTAL SECTION

Materials. 1,1,3,3,5,5-Hexaethoxy-1,3,5-trisilacyclohexane (henceforth called 3R), bis(triethoxysilyl)methane, and 1,4-bis(triethoxysilyl)benzene were purchased from Gelest Inc. and used without further purification. Cetyltrimethylammonium bromide (CTMABr) was obtained from Aldrich. Ammonium hydroxide solution (28−30 wt %) was obtained from J. T. Baker. pH measurements were done using a Beckman Coulter (pHi 460) pH meter. The final step in the synthesis procedures given below was the manual grinding of the dried gels in a mortar and pestle for a few minutes at room temperature to produce free-flowing powders. Surfactant Templated Synthesis of PMO Using [(EtO)2SiCH2]3 in Basic Aqueous Medium. Repeating the procedure of Landskron et al.,29 cetyltrimethylammonium bromide (CTMABr, 0.9 mmol, 0.32 g, Aldrich) was dissolved in a mixture of 2.16 g of NH4OH (30 wt %) and 3.96 g of DI water at 20 °C. A total of 1.26 mmol, 0.5 g of “3R” was added to this mixture producing a solution having the molar composition 1.0:17:236:0.7 “3R”/OH/H2O/CTMABr which was stirred for 1 day at 20 °C to produce a white precipitate. This mixture was aged for 1 day at 80 °C, and the precipitate was then filtered and washed with water. The solid was then stirred for 48 h in a solution of 12 g of HCl (36 wt %) and 80 g of methanol. The sample was then filtered off again, washed with methanol, and dried at 120 °C under vacuum. Template-Free Synthesis of MOS “EMA-2” Using “3R” in Basic Aqueous Medium. A solution with 18.6 g of 30% NH4OH and 23.76 g of DI water was made. The pH of the solution was 12.55. To the solution was added 3.0 g of “3R”, producing a mixture having the molar composition 1.0:21:270 “3R”/OH/H2O. This mixture was stirred for 1 day at room temperature (20−25 °C), then transferred to an autoclave, and aged at 90 °C for 1 day to produce a gel. The gel was dried at 120 °C in a vacuum oven overnight to remove water, ethanol, and ammonia. This produced a clear solid gel product, which was converted to white powder by grinding. Template-Free Synthesis of MOS “EMA-2” Using “3R” in Acidic Aqueous Medium. To a pH 2 solution consisting of 0.778 mol of DI 2219


Article

Chemistry of Materials Scheme 1. Templated (using CTMABr) vs Template-Free Polymerization Routes of 1,1,3,3,5,5-Hexaethoxy-1,3,5trisilacyclohexane, [(EtO)2SiCH2]3 (3R), Forming Mesoporous Organosilicas, PMO or MOS, Respectively

an autoclave, and aged at 90 or 120 °C for different times to produce a gel. The gel was dried in a vacuum oven at 120 °C overnight to produce a clear solid gel product, which was converted to white powder after grinding. Characterization. Solid-State NMR. Solid-state 29Si magic-angle spinning (MAS) NMR data were recorded using an InfinityPlus-500 spectrometer operating at 11.74 T corresponding to Larmor frequencies of 499.2 MHz for 1H and 99.2 MHz for 29Si, respectively. Samples were loaded in 7.5 mm (OD) zirconium dioxide rotors and spun at 5 kHz using air. Quantitative 29Si MAS NMR spectra were recorded using 90°-pulse and recycle delays of 60 s or longer, with 1H decoupling during data acquisition. All of the solids NMR measurements reported were performed at ambient temperatures, and the spectra are referenced against tetramethyl silane (TMS, δSi = 0 ppm). Surface Area (BET). For measurements of porosity, nitrogen isotherms were collected at 77 K on Quantachrome Autosorb AS-1C and AS-iQC2 instruments, and the properties were calculated using standard literature techniques. Samples of approximately 100 mg were outgassed under vacuum for 4 h at 120 °C prior to data collection. The BET surface area was calculated using data points in a linear regime from P/P0 = 0.001 to P/P0 = 0.35. The external (or mesoporous) surface area and micropore volume were derived from the t-plot using at least 5 data points from P/P0 = 0.3 to P/P0 = 0.5. Micropore surface area was estimated by subtracting the external surface area from the BET surface area. Total pore volume was calculated at P/P0 = 0.95. Pore size distribution was determined from the desorption curve using the BJH technique [Figure 3b]. Average pore size distribution and the average “mode” pore diameter for Figures 2b and 3c were obtained using the DFT fit option in the Quantachrome software. Based on triplicate determinations, the standard errors of the BET and pore volume results were better than 1.5 and 3.4% of the mean, respectively. X-ray Scattering. X-ray powder diffraction patterns were collected on a PANalytical X’pert diffractometer equipped with an accessory for low angle measurements. XRD analyses were recorded using the Cu Kλ = 0.1540598 nm line in the 2θ range from 0.5 to 10° with a step size of 0.0167° and a counting time of 1.2 s. Templated and templatefree 3R-MOS samples were prepared in the same way for X-ray analysis in order to get consistent XRD patterns. An in-house Rigaku Cu Kα (λ = 1.542 Å) rotating anode generator was used to produce ultrasmall angle X-ray scattering (USAXS) and small angle X-ray scattering (SAXS) data on 3R-MOS and TEOS samples. X-ray optics include an Osmic multilayer confocal focusing mirror with multiple sets of very low scatter vertical/horizontal slits located between the mirror and the sample. Combined with the slit geometry, the Osmic Kirkpatrick-Baez mirror configuration optimizes X-ray performance parameters including flux, divergence, and spectrum. The SAXS configuration consists of a slit defined pinhole/divergence geometry before the sample. A 1D Mythen1K strip detector was used to collect the data. The USAXS configuration consists of a Bonse-Hart collimator crystal and analyzer crystal based on a channel cut Si(111) 4 bounce geometry. The sample was located between the collimator crystal and the analyzer crystal. The 1D detector configured as 0D detector was used to collect the data while scanning the analyzer crystal. SAXS and USAXS dry gel samples were loaded into a low scatter quartz capillary with a 1.5 mm O.D. and 0.01 mm wall. An empty capillary was used for scaled background subtraction from the raw

water and 0.14 mmol of concentrated HCl, was added 1.0 g (2.52 mmol) of “3R” producing a mixture having the molar composition 18:1:5556 “3R”/HCl/H2O. This mixture was stirred for 1 day at room temperature (20−25 °C), then transferred to an autoclave, and aged at 90 °C for 1 day to produce a gel. The gel was dried in a vacuum oven at 120 °C overnight, and the clear solid gel product was converted to white powder by grinding. Template-Free Synthesis of MOS Using Methylene-Bridged Precursor, [(EtO)3Si]2CH2MOS in Basic Aqueous Medium. A solution with 12.4 g of 30% NH4OH and 15.8 g of DI water was prepared, and the pH of the solution was 12.55. To the solution was added 4.1 g of bis(triethoxysilyl)methane ([(EtO)3Si]2CH2), producing a mixture having the molar composition 1.0:8.75:112.5 [(EtO)3Si]2CH2/OH/ H2O. This mixture was stirred for 1 day at room temperature (20−25 °C), then transferred to an autoclave, and aged at 90 °C for 1 day to produce a gel. The gel was dried at 120 °C in a vacuum oven overnight to remove water, ethanol, and ammonia. This produced a clear solid gel product, which was converted to white powder by grinding. Surfactant-Free Synthesis of MOS Using Phenylene Bridged Precursor, [(EtO)3Si]2C6H4, in Basic Aqueous Medium. A solution with 6.2 g of 30% NH4OH and 7.9 g of DI water was made. The pH of the solution was 12.55. To the solution was added 1.21 g of 1,4bis(triethoxysilyl)benzene ([(EtO)3Si]2C6H4), producing a mixture having the molar composition 1.0:17.5:225 [(EtO)3Si]2CH2/OH/ H2O. This mixture was stirred for 1 day at room temperature (20−25 °C), then transferred to an autoclave, and aged at 90 °C for 1 day, a solid precipitate formed, the supernatant solution remained fluid, and no gelation occurred. The mixture of solid and solution was dried at 120 °C in a vacuum oven overnight to remove water, ethanol, and ammonia. This produced a solid product, which was converted to white powder after grinding. Xerogel Synthesis Using TEOS. A solution with 6.21 g of 30% NH4OH and 7.92 g of deionized water (DI) water was made. The pH of the solution was 12.55. To the solution was added 1.56 g (7.5 mmol) of tetraethyl orthosilicate (TEOS), producing a mixture having the molar composition 1.0:7:90 TEOS/OH/H2O. This mixture was stirred for 1 day at room temperature (20−25 °C), then transferred to an autoclave, and aged at 90 °C for 1 day. The colloidal solution was dried at 120 °C in a vacuum overnight to remove water, ethanol, and ammonia. This produced a clear solid gel product, which was converted to white powder after grinding. Gel Monitoring Experiments of Template-Free MOS from “3R”. Basic. A solution with 63.1 g of 30% NH4OH and 79.2 g of DI water was made. The pH of the solution was 12.55. To the solution was added 10.0 g of 1,1,3,3,5,5-hexaethoxy-1,3,5-trisilacyclohexane ([(EtO)2SiCH2]3), producing a mixture having the molar composition 1.0: 21:270 [(EtO)2SiCH2]3/OH/H2O. This mixture was stirred for 1 day at room temperature (20−25 °C), then the solution was split into 10 portions, and each portion was placed in an autoclave and aged at different temperatures (e.g., 90 and 120 °C) for different lengths of time (from 0 to 144 h) to produce a solution or gel. The solution or gel thus produced was dried at 120 °C in a vacuum oven for 24 h to remove water, ethanol, and ammonia. This produced a clear solid gel product, which was converted to white powder on grinding. Acidic. To 14 g of a pH 1.9 HCl solution was added 1.0 g (2.52 mmol) of [(EtO)2SiCH2]3 producing a mixture having the molar composition 18:1:5556 [(EtO)2SiCH2]3/HCl/H2O. This mixture was stirred for 1 day at room temperature (20−25 °C), then transferred to 2220


Article

Chemistry of Materials sample data; resulting data from both regimes were merged to generate plots. Transmission Electron Microscopy. Samples were prepared for transmission electron microscopy (TEM) by depositing a suspension of ground TEOS or 3R-MOS in ethanol onto a copper grid coated in a layer of thin carbon (∼5 nm) on top of a thicker layer of carbon with micrometer-scale holes. Samples were dried but otherwise untreated. TEOS (Figure 6a,b) and 3R-MOS samples (Figure 6c,d) were examined at 200 kV using a double-aberration-corrected FEI Themis. Lower magnification images (Figure 6a,c) are acquired with a brightfield aperture at significant defocus to highlight feature edges. Higher resolution phase contrast images (Figure 6b,d) were acquired at slight defocus as well. Images were acquired with low enough fluences to preserve the nominal shapes and sizes of features in both samples. In both cases, energy dispersive spectroscopy (EDS) was used to confirm the compositions of the materials were consistent with expectations.

■

RESULTS AND DISCUSSION We have reproduced the Landskron27 self-condensation of the 3R precursor in base with a surfactant template; we have also prepared a product from the same precursor without the addition of surfactant template (see Scheme 1).30 When surfactant CTMABr is used in the synthesis of PMO using the 3R precursor, the surfactant helps to disperse the precursor in water, and a white precipitate forms during room temperature hydrolysis. For the template-free synthesis, the precursor does not mix with water before hydrolysis. Oil beads are observed in water solution. As the hydrolysis of the precursor proceeds, it becomes more hydrophilic, and with complete hydrolysis, a clear solution forms. At higher precursor concentrations than those used here, a soft gel is obtained in less than 24 h even at room temperature. Following room temperature hydrolysis, exposure to higher temperature (90 °C) accelerates silanol condensation and creates a clear gel that encapsulates the entire liquid phase. The final solid is achieved by simply drying the gel in a vacuum oven at 120 °C, followed by grinding to a free-flowing powder. Figure 1a shows XRD comparison of products from template and template-free syntheses where the presence of a diffraction line at comparably low 2θ is observed for both products. The templated PMO product exhibits a sharp, intense line at ∼2° 2θ with a broad second reflection centered at 3.5°. The nontemplated EMA-2 has a single broad but asymmetric reflection at ∼1.5° 2θ. Comparison of the nitrogen adsorption isotherms and pore size distributions of the PMO and EMA-2 materials in Figure 1b,c, respectively, indicates a broadly similar mesoporosity and breadth of the pore size distribution using “3R” in the presence or absence of surfactant. EMA-2 clearly exhibits the characteristic adsorption/desorption hysteresis loop at P/P0 ∼ 0.5 indicative of mesoporosity and a comparably narrow pore size distribution centered at 3−3.2 nm. Furthermore, textural properties of PMO and EMA-2 shown in Table 1 show very comparable BET surface area, average pore diameter, and pore volume, respectively. EMA-2 forms as a high surface area organosilica having uniformly sized mesopores. As opposed to the PMO prepared from the same precursor using a surfactant template, the absence of any higher angle reflections in the XRD of the surfactant-free EMA-2 product indicates that these mesopores are not periodically ordered; hence, this material is characterized as a mesoporous hybrid xerogel.

Figure 1. (a) XRD, (b) N2 isotherm, and (c) pore size distribution of templated (blue) and template-free (red) mesoporous organosilicas. The XRD indicates disordered pore stacking in EMA-2 relative to PMO, and the nitrogen isotherm and pore size distribution of both materials are comparable.

Table 1. Comparative Textural Properties of PMO and EMA-2 PMO/EMA-2 3R PMO (templated) 3R EMA-2 (templatefree)

BET (m2/g)

pore diam. (nm)

pore volume (cm3/g)

1517 1410

3.02 3.18

1.07 0.92

pH Effects on the Self-Assembly of “3R”. The templatefree sol−gel polymerization of the 3R precursor described above was conducted in both acidic and basic media. The textural properties of EMA-2 are found to be highly sensitive to the pH of the synthesis mixture. Figure 2a shows how both the total BET surface area and the microporous surface area of EMA-2 vary with the synthesis gel pH, keeping all other synthesis conditions the same. Total surface area ranges between 600 and 800 m2/g at pH 1−3 and 1150−1300 m2/g at pH 11−13.5, respectively. The microporous surface area 2221


Article


Figure 2. Dependence of surface area (a) and pore volume and average pore diameter (b) on pH.

accounts for about half of the total surface area in acidic media, but 1 cm3/g. This range of product properties derives from a single rigid precursor via this straightforward change in a single synthesis variable. Since the time-dependent samples discussed above were obtained by manually grinding dried gels, the particle size and morphology of these powders are not intrinsic to the manner in which they were formed. Nonetheless, comparison of their bulk densities should also reflect the trend in porosity seen in Figure 3c. Bulk volumes were determined by placing samples in vials and tapping until the volumes reached steady state. A qualitative trend of the bulk density is obtained from the measurements of mass and volume. While the particle morphology reflects the method and degree of grinding as well as the material properties, bulk density can qualitatively reflect the relative degree of porosity in a related family of samples. This is shown in Figure 4a where 250 mg of each of the samples that yielded the data in Figure 3 are compared. The large increase in the volume occupied for the same weight illustrates the much higher porosity (lower bulk density) of the products at longer gelation times. In Figure 4b are plotted the bulk volume as well as estimates for some of the component volumes for these samples. The bulk volume is measured from the volume of the (tapped) powder in the vials. The material contains residual hydroxyl groups and is hygroscopic, and there is no evidence of static charging effects that would influence the bulk density. The skeletal volume (reciprocal of the skeletal density) was measured for samples at 7, 23, and 144 h gelation time via helium pycnometry, and all samples gave values of 1.56 ± 0.1 (0.64 cm3/g), which was assumed for all of the samples. The volume of micro- plus mesopores was determined via BET analysis. The volume of macropores was determined by difference after estimating a packing fraction of 0.75 (0.25 void fraction of interstitial space between particles) as appropriate for a ground powder with a wide particle size distribution.36 We find that much of the increase in bulk volume with gel time is due to an increase in the volume of pores larger than those detected by BET analysis as micro- and mesopores.

Figure 4. Dependence of porosity with gelation time. (a) Same sample weights (250 mg each) of EMA-2, dried at 120 °C under vacuum for 24 h, synthesized at various gelation times (shown on the vials). (b) The bulk volume, as well as the skeletal, micropore + mesopore, and macropore volumes for selected samples.

Stability of 3R Structure in EMA-2 under Reaction Conditions. To determine whether the “3R” building block remains intact throughout the synthesis conditions, 29Si NMR spectra were collected on the same variable gelation time samples as shown in Figures 3 and 4. Figure 5 shows stacked 29 Si NMR spectra of these samples taken from gels at pH 12.5 and 2, at two gelation temperatures of 90 and 120 °C, with all samples dried in vacuum at 120 °C. The “3R” building block contains exclusively Si D sites, schematically defined in Scheme 2, primarily as two peaks D1 (SiOSi) and D2 (Si(OSi)2) at −15 to −25 ppm in the zero hour 90 °C sample. We also note a species with Si(OH)2, as a shoulder in the alkaline samples and a peak in the acidic samples. With increased time at 90 °C at pH 12.5, the relative intensity of these peaks shifts in favor of D2 reflecting the progressive degree of silanol condensation to form Si−O−Si linkages. At 72 h Si T sites are clearly present, with hints of them even at 23 h. The intensity of this T site resonance increases slightly at 144 h at 90 °C. The changes observed at 90 °C are accelerated and exceeded when gelation is done at 120 °C. In this case both T and Q sites are observed as early as 24 h and indeed become major species at 144 h. These changes are attributed to the known tendency of Si−C bonds to undergo base catalyzed cleavage,37 as confirmed by the exclusive presence of Si D sites at pH 2 under the same temperature/time conditions, even at the most severe condition of 120 °C and 144 h. Thus, the cyclic 3R precursor is fully stable in acid medium at all synthesis conditions explored but undergoes progressive ring opening in base, as illustrated in Scheme 2. All of the results described above in Figures 1 and 2 were obtained on samples gelled for 24 h where there is little evidence of precursor ring opening. Precursor Structure and Self-Assembled Mesoporosity. The textural properties of EMA-2 place the 3R precursor in a small class of precursors yielding self-assembled mesoporous materials having a narrow mesopore size distribution and high surface area formed in the absence of templating. Furthermore, the 3R precursor lacks any functionality that might induce 2223


Article


Figure 5. Quantitative 29Si MAS NMR of samples prepared at pH ∼ 12.5 (left) and pH ∼ 2 (right) at 90 °C (a) and (b) and at 120 °C (c and d). All samples were dried in vacuum at 120 °C prior to the NMR measurement (at ambient conditions). [D, T, and Q are defined below in Scheme 2.]

itself generates a network capable of resisting, to a large extent, the capillary forces of drying that would otherwise result in pore collapse. We suggest (vide infra) that it is the growth toward a critical level of connectivity of the gel network, which is enabled to a greater or lesser extent by different precursors, which plays a strong role in determining the final textural properties of these organosilicas. Table 2 also contains product surface areas and pore diameters of mesoporous organosilicas prepared from the methylene- and phenylene bridged silsesquioxanes and the ethoxylated silacyclobutane precursors mentioned above along with the silacyclohexane (3R) studied here. As indicated by the information in Table 2, synthesis and processing conditions exist in which all of these precursors except TEOS can self-assemble to produce amorphous mesoporous organosilicas with surface areas greater than 500 m2/g. As reported previously, and in agreement with the results in Figure 2a, acid-catalyzed products generally exhibit lower total BET surface area products than base-catalyzed products, at otherwise identical conditions. Using a purely aqueous synthesis mixture and a 90 °C gelation, the conditions used here differ significantly from those literature examples cited in Table 2. To remove any effect(s) these differences might have, we applied the pH 12.5 synthesis procedure employed here for the silacyclohexane precursor to the TEOS and methylene- and phenylene-bridged precursors as well. The product properties obtained are not atypical relative to those reported previously for these precursors; however, a distinct difference was observed in the sol of the phenylene bridged precursor in the aqueous preparation. Whereas the methylene-bridged and 3R precursors fully gelled in water at 90 °C, the phenylene bridged ones never gelled under these conditions. Instead a solid precipitate formed at room temperature with a free liquid supernatant which remained as

Scheme 2. Si−C Bond Cleavage Creates Multiple Types of Si Speciesa

a

Initial cleavage of D sites form a T site and a Si-CH3 species, longer time, or higher temperatures result in further cleavage of Si−C bonds forming Q sites and more Si-CH3 moieties.

electrostatic, lipophilic, π−π, or H-bonding interactions that could generate long-range ordering in the nascent gel network. Lacking any organizing interprecursor forces, we are left with a highly disordered self-assembly that produces high porosity and a narrow pore size distribution. This is not unique to 3R, but it points to the need for a better understanding of the dynamics of sol−gel network formation and drying. Table 2 provides a partial listing of organosilane precursors and the reaction conditions in which they self-assemble into high surface area hybrid xerogels. All of the products in Table 2, except one, are formed via a gel phase from either an aqueous solution (reported here) or alcohol- or THF-based solvents with stoichiometric levels of added water, although initial hydrolysis of the alkoxy groups on the precursors liberates considerable amounts of alcohol (methanol or ethanol) in all cases. All of the mesoporous products are reported to have relatively narrow pore size distributions at the sizes shown. With the possible exception of the phenylene bridged precursor where π−π interactions could be a factor, the selfassembly of these precursors seems to be unaided by direct energetic factors and instead is driven primarily by the kinetics of precursor hydrolysis and condensation. The growth process 2224


Article


Table 2. Selected List of Self-Assembled, Silica-Based Xerogels from Various Precursors, Synthesized at Different Conditions for Comparison to the Work Reported Herea precursorb

solventc

precursor conc. (M)

catalyst

reaction conditions

BET (m2/g)

PD (nm)

TEOS

water

0.5

NH4OH

24 h at RT, 24 h at 90 °C, vacuum-dried at 120 °C

226

4.3

Si−CH2−Si

ETOH

1.1

HCl

551

4.1

Si−CH2−Si

water

0.4

NH4OH

2 weeks at RT, solvent processed in decreasing dielectrics, airdried for 2 days, vacuum-dried at 100 °C overnight 24 h at RT, 24 h at 90 °C, vacuum-dried at 120 °C

862

3.7

Si−phenylene−Si

THF

0.2

HCl

958

2.4

Si−phenylene−Si Si−phenylene−Si Si−phenylene−Si Si−phenylene−Si

MEOH THF THF water

3 0.4 0.4 0.2

NH4F HCl NH4OH NH4OH

72 h at RT, solvent processed, air-dried 24 h, vacuum-dried 24 h at RT, ground, then vacuum-dried 24 h, at RT 48 h at 20 °C, air-dried, ether wash, vacuum-dried at 150 °C ″ ″ 24 h at RT, 24 h at 90 °C, vacuum-dried at 120 °C

1030 129 1262 612

------2.8

1,1,3,3-tetraethoxy-1,3disilacyclobutane 1,1,3,3,5,5-hexaethoxy1,3,5-trisilacyclohexane 1,1,3,3,5,5-hexaethoxy1,3,5-trisilacyclohexane

ETOH/ water water

3.3

HCl

O/N at 50 °C, vacuum-dried 6 h at 60 °C

745−917

---

0.2

NH4OH


1200−1450

2.9

water

0.2

HCl


600−700

2.7

ref this work 38 this work 4 3 3 3 this work 28 this work this work

a

All precursor bridging groups are heteroatom-free; all were self-condensed at the conditions shown. bRemaining alkoxy ligands are omitted from Si for clarity. cAll nonaqueous solvents include approximately stoichiometric levels of water.

such even with 90 °C gelation. Under the conditions used here, the methylene and 3R precursors formed clear, translucent, single-phase gel intermediates while the phenylene bridged precursor underwent phase separation. Clearly the nature of the gel structure and its transformation during drying establish the properties of the final product. Gel structures are commonly described as particulate assemblies/ aggregates, or polymeric networks. The former consists of a colloidal suspension of nanosized particles, the latter a single continuous phase of self-condensing precursor molecules and solvent. TEOS gels typically display particle aggregates. The microstructures of xerogels made under identical synthesis conditions using TEOS and 3R precursors were characterized by small-angle X-ray scattering and transmission electron microscopy. Figure 6 compares TEM images of product xerogels made from TEOS and 3R precursors at the same concentrations and using the same synthesis procedures and conditions. As expected, TEM images of TEOS are consistent with a network of spherical particles, with a characteristic size of ∼20 nm, for which there typically appear to be continuous but necked connections between adjacent particles. If the 3R-MOS samples can be said to have a characteristic particle size, it is significantly smaller and less regular than those observed in the TEOS sample. The 3R-MOS images suggest mesopores larger than the primary sizes indicated by SAXS (see below). However, the disordered nanometer-scale pores indicated by SAXS might not be readily visible in a projection image taken from a single orientation. Intensity differences in 3R-MOS reflecting local variations in solid thickness are seen, but evidence of a discrete particle assembly as observed with TEOS is lacking. X-ray scattering data covering ultrasmall angle (USAXS) and small angle regimes (SAXS) of the 120 °C vacuum-dried 3RMOS and TEOS xerogels (without grinding) are shown in Figure 7. Figure 7a overlays the scattering data of both samples. For TEOS, the scattering profile reveals a distinct peak centered on q = 0.034 Å−1, which corresponds to a particle diameter of ∼18 nm. This well-defined scattering peak is a

Figure 6. TEM images of TEOS (a, b) and 3R-MOS (c, d) show distinct morphologies. The TEOS appears to be comprised of larger particles, whereas the 3R-MOS appears to be a more disordered porous mass with smaller features. The pairs of included images are presented at the same scale for ease of comparison, with lower magnification brightfield images (a, c) and higher magnification phase contrast images (b, d).

typical signature of a narrow particle size distribution and thus the scattering data is consistent with the particle size estimates (15−20 nm) inferred from TEM measurements Figure 6a,b. This is followed by the presence of an oscillation with a subsequent power law dependence of intensity, I ∼ q−3.7 (Porod regime), which is characteristic of spherical particle morphology. 2225


Article


removed, and the textural properties of the resulting solid are then largely determined. While this picture has been known for some time, a model for determining the effect of different precursors on gel strength has been lacking. Precursor rigidity has been suggested as an important factor, but it is the entire gel system or network that must resist the stresses. Peterson et al.32 show that the average level of connectivity of a network (as indicated by the number of bridges (e.g., Si− CH2−Si or Si−O−Si) per silicon atom) is a useful way to track the strength of the network during synthesis and drying. As the precursor molecules bond together and hydroxyls are removed by condensation, the connectivity grows from low levels to those characterizing a solid and the mechanical properties progress correspondingly. A form of rigidity theory41 is invoked to show that when the connectivity reaches a critical level (nBridge/nSi = 1.5), the network is incipiently able to support stress. This occurs at a rigidity transition. The strength of the partially solvent-filled gel depends not only on the properties of its solid components but potentially also on the porosity and the solvent. Peterson et al. show that the connectivity of the solid components of the gel at the rigidity transition is a proxy for the connectivity necessary for the gel system to resist collapsing under the capillary stresses. Under the same synthesis conditions, those precursors that reach this transition sooner are found to retain greater amounts of porosity and surface area after drying. For example, the surface areas of materials made in template-free syntheses from the Si−CH2−Si, Si−Phenyl−Si, and 3R precursors, as well as others, smoothly vary with an index that increases when the transition is reached sooner.32 The ability of the 3R precursor and the other precursors shown in Table 2 to retain high surface area and porosity in aqueous, template-free syntheses with rapid drying steps therefore depends on their ability to relatively rapidly achieve a critical level of connectivity. Two factors which aid this ability are (1) a large number of condensable hydroxyl groups (as opposed to noncondensable groups such as −CH3) remaining in the network at the rigidity transition and (2) the presence of “pre-condensed” organic bridging groups (−CH2−, −phenyl−) which are not subject to hydrolysis under the conditions used. All of the precursors which have been shown to produce very high surface-area materials in these simple syntheses contain organic bridging groups. The precursor producing the highest surface-area solids (3R) contains the highest number of such bridges per silicon atom as well as having a large number of hydroxyls, still free to condense, when the network reaches the transition. These concepts may help explain the variation of textural properties with gelation time for the 3R materials as shown in Figure 3. The higher the connectivity that has been achieved when a system begins to be subjected to drying, the stronger is its network. The stronger the network, the sooner it can resist the capillary stresses, and this corresponds to less solvent having been removed and therefore more porosity retained. This may account for the increase of porosity with increasing reaction time. The total and microporous surface areas of the 3R materials first increase and then decrease with increasing gel time as shown in Figure 3. If sufficient condensation has not occurred due to low reaction times, the network does not reach the rigidity transition until the drying stage and both the porosity and the surface area will reflect the time required to do this with more reaction time leading to more porosity as described above

Figure 7. (a) X-ray scattering data of TEOS and 3R-MOS dried gels in the USAXS and SAXS regime. (b) and (c) are individual plots, ln(I) vs ln(q), for TEOS and 3R-MOS, respectively, in the fractal regime. The dotted lines are linear fits to the data. Top axis displays the wave vector, q for comparison.

Conversely, 3R-MOS shows a broad distribution of peak intensity suggestive of a polydispersed particle system, consistent with the TEM observation which reveals a heterogeneous interconnected network of particles, Figure 6c,d. This interpretation is further supported by the absence of scattering oscillations in the Porod regime, while the intensity varies as I ∼ q−3.5.39 At low q-values, we observe a power law dependence of intensity; I(q) ∼ q−d, where the exponent ‘d’ is the slope derived from the ln(I) vs ln(q) plots. TEOS and 3R-MOS exhibit slopes of −1.6 and −2.4, respectively, both suggestive of mass fractal aggregates. Brinker and Scherer40 have reviewed the processes by which a gel can partially or completely collapse due to capillary stresses. Many syntheses avoid the collapse by reducing the stresses via very slow drying, supercritical drying, multiple solvent replacements with solvents of decreasing surface tension, or, for unreduced levels of stress, by propping up the gel structure with template molecules or phases. The gel stops collapsing when the gel system is strong enough to withstand the stresses. At this point the decrease in porosity and surface area largely ceases, even though solvent continues to be 2226


Chemistry of Materials and to more surface area. If more time is allowed and the transition is reached during the gelation step, other factors come into play. The rigidity transition provides the level of connectivity at which the network can just resist a small amount of stress. The strength required to resist the actual stresses present will be somewhat greater. The processes that provide this strength and which occur during further reaction time may include reforming or ripening of small structures into larger ones as thermodynamically unfavorable high curvature regions are removed. Removing a wall between small pores and adding the material to thicken a wall elsewhere can leave the pore volume unchanged while (1) reducing the total surface area and (2) removing micropores and shifting the PSD to larger pores. These are consistent with the observed trends.

■

ACKNOWLEDGMENTS

■

REFERENCES

We gratefully acknowledge the technical assistance of Kanmi Mao (NMR), David Griffin (NMR), and Maria Martinez (BET). Fruitful discussions with Scott Weigel, Preeti Kamakoti, Andrew Wiersum, Aaron Sattler, and Karl Strohmaier are gratefully acknowledged. We acknowledge the usage of Princeton’s Imaging and Analysis Center which is partially supported by Princeton Center for Complex Materials from National Science Foundation (NSF)-MRSEC program (DMR1420541). We appreciate the support of this research by the management of ExxonMobil Research and Engineering.

■

(1) Pleuddemann, E. P. Silane Coupling Agents; Plenum Press: New York, 1982. (2) Shea, K. J.; Loy, D. A.; Webster, O. Aryl-bridged PolysilsesquioxanesNew Microporous Materials. Chem. Mater. 1989, 1, 572−574. (3) Corriu, R. J. P.; Moreau, J. J. E.; Thepot, P.; Man, M. W. C. New Mixed Organic-Inorganic Polymers: Hydrolysis and Polycondensation of Bis(trimethoxysilyl) Organometallic Precursors. Chem. Mater. 1992, 4, 1217−1224. (4) Shea, K. J.; Loy, D. A.; Webster, O. Arylsilsesquioxane Gels and Related Materials. New Hybrids of Organic and Inorganic Networks. J. Am. Chem. Soc. 1992, 114, 6700−6710. (5) Fujita, S.; Inagaki, S. Self-organization of Organosilica Solids with Molecular-scale and Mesoscale Periodicities. Chem. Mater. 2008, 20, 891−908. (6) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Ordered mesoporous molecular sieves synthesized by a liquidcrystal template mechanism. Nature 1992, 359, 710−712. (7) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C.T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. A new family of mesoporous molecular sieves prepared with liquid crystal templates. J. Am. Chem. Soc. 1992, 114, 10834−10843. (8) Inagaki, S.; Guan, S.; Fukushima, Y.; Ohsuna, T.; Terasaki, O. Novel Mesoporous Materials with a Uniform Distribution of Organic Groups and Inorganic Oxide in Their Frameworks. J. Am. Chem. Soc. 1999, 121, 9611−9614. (9) Asefa, T.; MacLachlan, M. J.; Coombs, N.; Ozin, G. A. Periodic mesoporous organosilicas with organic groups inside the channel walls. Nature 1999, 402, 867−871. (10) Melde, B. J.; Holland, B. T.; Blanford, C. F.; Stein, A. Mesoporous Sieves with Unified Hybrid Inorganic/Organic Frameworks. Chem. Mater. 1999, 11, 3302−3308. (11) Van Der Voort, P.; Esquivel, D.; De Canck, E.; Goethals, F.; Van Driessche, I.; Romero-Salguero, F. J. Periodic Mesoporous Organosilicas: From Simple to Complex Bridges; a Comprehensive Overview of Functions, Morphologies and Applications. Chem. Soc. Rev. 2013, 42, 3913−3955. (12) Ozin, G. D. Nanochemistry on my Mind. Chem. Eng. News 2016, 94 (5), 36−37. (13) Lin, D.; Hu, L.; You, H.; Tolbert, S. H.; Loy, D. A. Comparison of new periodic, mesoporous, hexylene-bridged polysilsesquioxanes with Pm3n symmetry versus sol-gel polymerized hexylene-bridged gels. J. Non-Cryst. Solids 2014, 406, 139−143. (14) Katz, A.; Davis, M. E. Molecular imprinting of bulk, microporous silica. Nature 2000, 403, 286−289. (15) Bellussi, G.; Perego, C.; Carati, A.; Peratello, E.; Massara, E. P.; Perego, G. Amorphous Mesoporous Silica-alumina with Controlled Pore Size as Acid Catalysts. In Zeolites and Related Microporous Materials: State of the Art 1994; Weitkamp, J., Karge, H. G., Pfeifer, H., Holderich, W., Eds.; Studies in Surface Science and Catalysis; Elsevier: 1994; Vol. 84, pp 85−92. (16) Yang, D.; Xu, Y.; Wu, D.; Sun, Y.; Zhu, H.; Deng, F. Super Hydrophobic Mesoporous Silica with Anchored Methyl Groups on the

CONCLUSIONS We have shown that high porosity, high surface area materials can be prepared by the template-free polycondensation of 1,1,3,3,5,5-hexaethoxy-1,3,5-trisilacyclohexane, “3R”, to give products largely the same as templated periodic mesoporous organosilicas (PMOs) made using the same silicon source. The textural properties of these materials are found to vary over a wide range depending on synthesis gel pH and gelation temperature and time. Considerable microporosity is observed at short 90 °C gelation times at pH 12.5, which diminishes to a very minor component of the porosity at times > 23 h. Under the gelation conditions used here (90 °C, pH 12.5), pore volume and pore diameter rise sharply with time, ultimately leveling off at 1.2 cm3/g and 70 Å at times > 70 h. The 3R building block is shown to be stable in acidic gels but undergoes gradual ring opening at gelation temperatures and times in excess of “standard” conditions employed for alkaline gels. Combined TEM and SAXS analysis of a 3R-MOS product prepared under standard conditions indicated a diffuse, disordered porous mass lacking evidence of a discrete particle assembly, as observed in products prepared using TEOS under the same conditions. The surprising ability of this precursor to generate such high surface-area and pore volume materials (>1000 m2/g; ∼1.0 cm3/g) in the absence of a template was explained using concepts from rigidity theory and attributed to the presence of unhydrolyzable bridging groups in combination with a relatively large number of hydroxyls available for condensation. These precursor attributes enable a rapid self-assembly which progressively imparts constraints on the motional degrees of freedom of the nascent assembly. This process generates a rigid network capable of resisting the capillary stresses of drying that would otherwise result in pore collapse.

■

Article

AUTHOR INFORMATION

Corresponding Author

*(D.C.C.) E-mail: [email protected]. ORCID

David C. Calabro: 0000-0003-2424-3456 Funding

This research was fully funded by ExxonMobil Research and Engineering Company. Notes

The authors declare no competing financial interest. 2227


Article

Chemistry of Materials Surface by a One-step Synthesis without Surfactant Template. J. Phys. Chem. C 2007, 111, 999−1004. (17) Koo, S. M.; Hah, H. J.; Han, D. I.; Jung, C. Y. A Self-template Method for Mesoporous Organosilica Particles. Chem. Lett. 2007, 36 (7), 906−907. (18) Ruiz-Hitzky, E.; Letaief, S.; Prevot, V. Novel Organic-Inorganic Mesophases: Self-Templating Synthesis and Intratubular Swelling. Adv. Mater. 2002, 14 (6), 439−443. (19) Romeo, H. E.; Fanovich, M. A.; Williams, R. J. J.; Matejka, L.; Plestil, J.; Brus, J. Self-assembly of a Bridged Silsesquioxane Containing a Pendant Hydrophobic Chain in the Organic Bridge. Macromolecules 2007, 40, 1435−1443. (20) Alauzun, J.; Mehdi, A.; Reye, C.; Corriu, R. J. P. CO2 as a Supramolecular Assembly Agent: A Route for Lamellar Materials with a High Content of Amine Groups. J. Am. Chem. Soc. 2005, 127, 11204−11205. (21) Agliullin, M. R.; Grigor’eva, N. G.; Danilova, I. G.; Magaev, O. V.; Vodyankina, O. V. Template-free Sol-gel Synthesis of Catalytically Active Mesoporous Aluminosilicates. Kinet. Catal. 2015, 56 (4), 501− 508. (22) Agliullin, M. R.; Danilova, I. G.; Faizullin, A. V.; Amarantov, S. V.; Bubennov, S. V.; Prosochkina, T. R.; Grigor’eva, N. G.; Paukshtis, E. A.; Kutepov, B. I. Sol-gel Synthesis of Mesoporous Aluminosilicates with a Narrow Pore Size Distribution and Catalytic Activity Thereof in the Oligomerization of Decene. Microporous Mesoporous Mater. 2016, 230, 118−127. (23) Koike, N.; Chaikittisilp, W.; Shimojima, A.; Okubo, T. Surfactant-free Synthesis of Hollow Mesoporous Organosilica Nanoparticles with Controllable Particle Sizes and Diversified Ogranic Moieties. RSC Adv. 2016, 6, 90435−90445. (24) Bantignies, J.-L.; Vellutini, L.; Maurin, D.; Hermet, P.; Dieudonne, P.; Man, M. W. C.; Bartlett, J. R.; Bied, C.; Sauvajol, J.L.; Moreau, J. J. E. Insights into the Self-directed Structuring of Hybrid Organic-Inorganic Silicas through Infrared Studies. J. Phys. Chem. B 2006, 110, 15797−15802. (25) Brondani, D. J.; Corriu, R. J. P.; El Ayoubi, S.; Moreau, J. J. E.; Man, M. W. C. Polyfunctional Carbosilanes and Organosilicon Compounds. Synthesis via Grignard Reactions. Tetrahedron Lett. 1993, 34 (13), 2111−2114. (26) Shen, Q.; Interrante, L. V. Structural characterization of polysilaethylene. Polym. Prepr. 1995, 36, 378. (27) Shi, W. Synthesis of organic/inorganic hybrid polymers and their application in the fabrication of silicon oxycarbide ceramics. Ph.D. Thesis, Rennselaer Polytechnic Institute, 1995. (28) Soraru, G. D.; Liu, Q.; Interrante, L. V.; Apple, T. Role of Precursor Molecular Structure on the Microstructure and High Temperature Stability of Silicon Oxycarbide Glasses Derived from Methylene-bridged Polycarbosilanes. Chem. Mater. 1998, 10, 4047− 4054. (29) Landskron, K.; Hatton, B. D.; Perovic, D. D.; Ozin, G. A. Periodic Mesoporous Organosilicas Containing Interconnected [Si(CH2)]3 Rings. Science 2003, 302, 266−269. (30) Landskron, K.; Ozin, G. A. Periodic Mesoporous Organosilicas: Self-assembly from Bridged Cyclic Silsesquioxane Precursors. Angew. Chem., Int. Ed. 2005, 44, 2107−2109. (31) Goethals, F.; Baklanov, M. R.; Ciofi, I.; Detavernier, C.; Van Der Voort, P.; Van Driessche, I. A New Procedure to Seal the Pores of Mesoporous low-k films with Pre-condensed Organosilica Oligomers. Chem. Commun. 2012, 48, 2797−2799. (32) Peterson, B. K.; Afeworki, M.; Calabro, D. C.; Li, Q.; Weston, S. C. Model for the Synthesis of Self-Assembling Template-Free Porous Organosilicas. Chem. Mat. 2017, 30, DOI: 10.1021/acs.chemmater.7b04482. (33) Cerveau, G.; Corriu, R. J. P.; Framery, E. Sol−gel process: influence of the temperature on the textural properties of organosilsesquioxane materials. J. Mater. Chem. 2000, 10, 1617−1622. (34) Diaz, U.; García, T.; Velty, A.; Corma, A. Hybrid Organicinorganic Catalytic Porous Materials Synthesized at Neutral pH in

absence of Structural Directing Agents. J. Mater. Chem. 2009, 19, 5970−5979. (35) Reale, E.; Leyva, A.; Corma, A.; Martinez, C.; Garcia, H.; Rey, F. A fluoride-catalyzed sol−gel route to catalytically active non-ordered mesoporous silica materials in the absence of surfactants. J. Mater. Chem. 2005, 15, 1742−1754. (36) Sohn, H. Y.; Moreland, C. The Effect of Particle Size Distribution on Packing Density. Can. J. Chem. Eng. 1968, 46, 162− 167. (37) Calabro, D. C.; Schmitt, K. D. Stability of Quaternary Ammonium-functionalized Organosiliconates in Alkaline Hydolysis. J. Colloid Interface Sci. 1995, 174, 40−44. (38) Tran, J. A.; Shea, K. J.; Loy, D. A. Methylene-bridged POSS: Substitution of a Methylene Spacer within a Silicate Matrix. J. Mater. Sci. 2014, 49, 5006−5016. (39) Rieker, T.; Hanprasopwattana, A.; Datye, A.; Hubbard, P. Particle Size Distribution Inferred from Small-Angle X-ray Scattering and Transmission Electron Microscopy. Langmuir 1999, 15, 638−641. (40) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: 1990. (41) Thorpe, M. F., Duxbury, P. M., Eds.; Rigidity Theory and Applications; Springer Science & Business Media: 1999.

2228


Template-Free Self-Assembly of Mesoporous Organosilicas

Recommend Documents