Template-free self-assembly of Mesoporous OrganoSilicas

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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 Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b04480 • Publication Date (Web): 09 Mar 2018 Downloaded from http://pubs.acs.org on March 9, 2018

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Chemistry of Materials

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, NJ 08801 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 surfactanttemplated 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 pH12.5, which diminishes to a very minor component of the porosity at times >¬ 23hours. Under the gelation conditions used here (90°C, pH12.5), pore volume and pore diameter rise sharply with time, ultimately leveling off at 1.2cm3/g and 70 angstroms at times >70hours. 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 non-templated syntheses. Comparative TEM and SAXS analysis of products from identical preparations using tetraethylorthosilicate and the 1,1,3,3,5,5-hexaethoxy-1,3,5-trisilacyclohexane precursors indicates that the latter forms a highly interconnected network gel lacking discrete particle aggregates.

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. Solgel 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 solgel 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 ‘80’s, early ‘90’s.2-4 In 1992 Mobil introduced surfactant templating to solgel chemistry, effectively creating an intermediate class of porous materials between amorphous silica and crystalline zeolites, collectively known as the M41S family of mesoporous materials.6,7 Whereas the ordering effect of structure directing agents in zeolite synthesis is known but poorly understood, M41S employed surfactants at

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concentrations above the critical threshold where wellcharacterized, 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 solgel chemistry has been extensive. Using essentially the same precursors, reaction conditions and solgel 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 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 solgel 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 a key role in the development of nanochemistry.12 Notwithstanding the above impact of surfactant templating, the precedent of non-templated, 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 non-templated assembly with the same silsesquioxane precursor was reported by Lin, et.al.13 Hexylene-bridged 1,6bis(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 solgel product prepared in acid was non-porous, however in base, solgel polymerization of this silsesquioxane produced a high surface area xerogel with pore size distribution comparable to the surfactant templated PMOs.

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Self-assembled, surfactant-free porosity has also been achieved using diverse strategies for manipulating precursor assembly including non-surfactant porogens,14-17 surfactant-functional building blocks,18,19 reactive assembly,20 controlled particle growth and aggregation,21,22 hard templating with silica nanoparticles23 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 Ǻ 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 six-membered 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 selfcondensation 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 selfassembled hybrid xerogel exhibits both micropores and a very narrow mesopore size distribution centered at ~38 Ǻ diameter. 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 surfactanttemplated 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 Ǻ 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-coat 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.


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We also show simplified and effective non-templated pathways to high surface area (>1000m2/g) mesoporous organosilicas with a narrow pore size distribution. In related work32, we describe a model that explains the highsurface area and porosity of these materials in terms of the structural features of the precursor. Experimental Materials 1,1,3,3,5,5-hexaethoxy,1,3,5-trisilacyclohexane (henceforth called 3R), bis(triethoxysilyl)methane and 1,4bis(triethoxysilyl)benzene were purchased from Gelest Inc., and used without further purification. Cetyl trimethylammonium 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. Synthesis of templated PMO using “3R” Surfactant templated synthesis of PMO using [(EtO)2SiCH2]3 in basic aqueous medium. Repeating the procedure of Landskron, et al.(2003): Cetyltrimethylammonium bromide (CTMABr, 0.9 mmol, 0.32 g, Aldrich) was dissolved in a mixture of 2.16 g NH4OH (30 wt.%) and 3.96 g DI water at 20 °C. 1.26 mmol, 0.5 g of “3R” was added to this mixture producing a solution having the molar composition: 1.0 “3R” : 17 OH : 236 H2O : 0.7 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, the precipitate was then filtered and washed with water. The solid was then stirred for 48 hours in a solution of 12 g HCl (36 wt.%) and 80 g of methanol. The sample was then filtered off again and washed with methanol, dried at 120 °C under vacuum. Synthesis of template-free MOS “EMA-2” in basic medium using “3R” Surfactant-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 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 “3R”: 21 OH: 270 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.

Synthesis of template-free MOS “EMA-2” in acidic medium using “3R” Surfactant-free synthesis of MOS (EMA-2) using “3R” in acidic aqueous medium. To a pH 2 solution consisting of 0.778 mol DI water and 0.14 mmol concentrated HCl, was added 1.0 g (2.52 mmol) of “3R” producing a mixture having the molar composition: 18 “3R” : 1 HCl : 5556 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. Synthesis of template-free MOS in basic medium using bridged silsesquioxanes Surfactant-free synthesis of MOS using methylene-bridged precursor, [(EtO)3Si]2CH2 in basic aqueous medium. A solution with 12.4 g of 30% NH4OH and 15.8 g 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 [(EtO)3Si]2CH2: 8.75 OH : 112.5 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 DI water was made. The pH of the solution was 12.55. To the solution was added 1.21 g of 1,4-bis(triethoxysilyl)benzene ([(EtO)3Si]2C6H4), producing a mixture having the molar composition: 1.0 [(EtO)3Si]2CH2: 17.5 OH : 225 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, 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 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 TEOS: 7 OH: 90 H2O. This mixture was stirred for 1 day at room temperature (20-25°C), then transferred to an autoclave and aged at 90ºC for 1 day. The colloidal solution was dried at 120ºC 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 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-1,3,5trisilacyclohexane ([(EtO)2SiCH2]3), producing a mixture having the molar composition: 1.0 [(EtO)2SiCH2]3 : 21 OH : 270 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 length of time (from 0 hour to 144 hours) to produce a solution or gel. The solution or gel thus produced was dried at 120 °C in a vacuum oven for 24 hours 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 [(EtO)2SiCH2]3 : 1 HCl : 5556 H2O. This mixture was stirred for 1 day at room temperature (20-25 °C), then transferred to an autoclave and aged at 90 °C 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, Si = 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 hours 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 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 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

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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 error 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 2Ɵ range from 0.5 to 10° with a step size of 0.0167° and a counting time of 1.2 s. Templated and template-free 3RMOS samples were prepared in the same way for X-Ray analysis in order to get consistent XRD patterns. An in-house Rigaku CuKα (λ = 1.542 Å) rotating anode generator was used to produce Ultra-Small 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 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 sub-traction from the raw 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 micron-scale holes. Samples were dried, but otherwise untreated. TEOS (a and b) and 3R-MOS samples (c and d) were examined at 200 kV using a doubleaberration-corrected FEI Themis. Lower magnification images (a and c) are acquired with a brightfield aperture at significant defocus to highlight feature edges. Higher resolution phase contrast images (b and d) were acquired at slight defocus as well. Images were acquired with low enough fluences to preserve the nominal shape and size


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of features in both samples. In both cases, energy dispersive spectroscopy (EDS) was used to confirm the composition of the materials was consistent with expectations.

a

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

b 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 used here, a soft gel is obtained in less than 24 hours 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.

c

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 non-templated EMA-2 has a single broad, but asymmetric reflection at ~1.5° 2θ. Figure 1. (a) XRD, (b) N2 isotherm, (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, the nitrogen isotherm and pore size distribution of both materials are comparable.

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. The template-free route is greener and more sustainable. Key physical properties for base prep products are shown.

3R

Mesoporous Organosilica, EMA-2 2

Template free (this work)

Templated (Landskron, 2003)

1410 m /g; 0.92 cc/g; 3.2 nm

Periodic Mesoporous Organosilica 2

1517 m /g; 1.07 cc/g; 3 nm



a 1400 BET

1200 1000 800 600 400 200 0

0

PMO/EMA-2

BET (m2/g)

Pore Diam. (nm)

Pore Volume (cm3/g)

3R PMO (templated)

1517

3.02

1.07

3R EMA-2 (template-free)

1410

3.18

0.92

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.

Microporous

1

2

3

4

5

6

7

8

9 10 11 12 13 14

pH

b

1.5

210

Total Pore Volume

180

Average Pore Diameter

1.2

150 120

0.9

90 0.6

60 30

0.3

0 0.0

Average Pore Diameter (Å)

Table 1. Comparative textural properties of PMO and EMA-2.

lyze the hydrolysis and condensation of the 3R building block. Literature methods using NH4F catalyst for solgel chemistry at or near neutral pH, 33-35 were not attempted.

Surface Area (m2/g)

Comparison of the nitrogen adsorption isotherms and pore size distributions of the PMO and EMA-2 materials in Figures 1b and 1c, 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/Po~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.

Total Pore Volume (cc/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-30

0

1

2

3

4

5

6

7

8

9 10 11 12 13 14

pH

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

pH Effects on the Self-assembly of “3R” The template-free solgel 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 microporous surface area of EMA-2 vary with the synthesis gel pH, keeping all other synthesis conditions the same. Total surface area ranges between 600-800 m2/g at pH 1-3, and 1150-1300 m2/g at pH 11-13.5, respectively. The microporous surface area accounts for about half of the total surface area in acidic media, but 23hours. Under the gelation conditions used here (90°C, pH12.5), pore volume and pore diameter rise sharply with time, ultimately leveling off at 1.2cm3/g and 70 angstrom at times >70hours. 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 (>1000m2/g; ~1.0cm3/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.

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(6) Kresge, C.T.; Leonowicz, M.E.; Roth, W.J.; Vartuli, J.C.; Beck, J.S., Ordered mesoporous molecular sieves synthesized by a liquid-crystal 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.

AUTHOR INFORMATION Corresponding Author * [email protected]

(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.

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

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS 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.

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Insert Table of Contents artwork here

Mesoporous Organosilica 2

3R Template free (this work)

Templated (Landskron, 2003)

1410 m /g; 0.92 cm3/g; 3.2 nm

Periodic Mesoporous Organosilica 2

1517 m /g; 1.07 cm3/g; 3 nm


Template-free self-assembly of Mesoporous OrganoSilicas

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