Model for the Synthesis of Self-Assembling Template-Free Porous

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Model for the Synthesis of Self Assembling Template-Free Porous Organosilicas Brian K. Peterson, Mobae Afeworki, David C. Calabro, Quanchang Li, and Simon C Weston Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b04482 • Publication Date (Web): 09 Mar 2018 Downloaded from http://pubs.acs.org on March 9, 2018

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Chemistry of Materials

Model for the Synthesis of Self Assembling Template-Free Porous Organosilicas

Brian K. Peterson*, Mobae Afeworki, David C. Calabro, Quanchang Li, and Simon C. Weston Corporate Strategic Research, ExxonMobil Research and Engineering Company, Annandale, NJ 08801 ABSTRACT:

High surface area solids are important materials in science and in many industrial applications, but often are produced from expensive and inefficient combinations of materials and processes. New principles for the selection of molecular precursors that yield high surface area solids in simple and efficient solgel processes would be useful. Focusing on organosilicas, we show that an index based on rigidity theory can be used to quantify the relative strength of the gel and the level of condensation at which it is able to withstand the capillary stresses imposed by drying thereby preventing loss of surface area. This index correctly orders precursors according to the surface area of the solid materials produced from them and provides, when correlated to a few data points, a predictive relationship between the index and surface area. Precursor features leading to early formation of a highly-connected rigid network include high ratios of non-hydrolyzing (e.g. methylene) to hydrolyzing (e.g. oxy) groups bridging silicate moieties, large SiOH/Si ratios in the hydrolyzed precursors, and low numbers of non-condensing terminal groups (e.g. methyl). These features explain the extremely high surface areas obtained from 1,1,3,3,5,5-hexaethoxy-1,3,5-trisilacyclohexane and high surface areas obtained by similar materials in aqueous, non-templated syntheses, as shown in a related publication.

Introduction Materials with high porosity and surface area are critical components of adsorbents, catalysts and catalyst supports, coatings, filters, insulators, controlled release agents, bioceramics, optical elements, and low-dielectric films (1, 2). The production of very high surface-area materials, including hybrid organic/inorganic materials, via solgel chemistry has traditionally used syntheses that require expensive components that do not appear in the final product and/or that require significant time and complicated processing (3). Examples include aerogels (4) which are produced with supercritical fluids, xerogels (3) which require very slow drying stages or multiple solventreplacement steps, and periodic mesoporous organosilicas (PMO) (5) and other templated solgel materials (e.g., M41S) (6) that require surfactants or other template molecules which are later removed, often destructively.

When the above methods are not used, the solids produced in solgel syntheses typically collapse under the capillary forces generated during drying, leading to products of modest porosity and surface area (3). Much is known about the influence of solvents, additives, and processing conditions on the final solid texture (3), but new principles for rationally selecting precursors that could produce high surface-area solids in more efficient processes are desired. We combine several ideas in a model of the solgel chemistry and physics in order to explain how precursor structure determines the texture of the resulting dried solid. The exemplary precursor of this work is 1,1,3,3,5,5hexaethoxy-1,3,5-trisilacyclohexane, hereafter called 3R (three Si in a ring, per zeolite nomenclature; see Figure 1), which was previously shown to produce high surface-area periodic mesoporous organosilicas in syntheses using an organic template (7). In a related publication (8) we show that similar high surface area mesoporous organosilicas

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(called EMA-2) can be produced with this precursor using straightforward, template-free solgel synthesis. Extensive discussion of the synthesis and properties of these materials appear in that work, but brief descriptions of the synthesis of EMA-2 and comparative materials are given in the Experimental section. The synthesis uses a solution containing only the precursor, water, acid or base as catalyst, and a procedure consisting of room temperature hydrolysis, higher temperature gelation, and higher temperature drying under vacuum. We provide a novel theoretical framework to describe the key precursor attributes needed to produce stable, high porosity, high surface area products in the absence of a pore-templating agent.

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For most preparations, the precursor was added to a pH=12.55 solution of 30% NH4OH and deionized (DI) water. This mixture was stirred for 1 day at room temperature (20-25 °C), then transferred to an autoclave and aged at 60 or 90 °C for 1 day, or 120 °C for 4 hours to produce a gel. The gel was dried at 120 °C in a vacuum oven overnight to remove water, ethanol and ammonia. In most cases, this produced a clear solid gel product, which was converted to white powder after grinding. The time was shortened for the 120 °C gelation in order to prevent cleavage of Si-C bonds which occurs when extended temperatures and time are used. For 1,1,3,3,5,5-hexaethoxy-1,3,5-trisilacyclohexane (3R: [(EtO)2SiCH2]3), 3.0 g of the precursor was added to 18.6 g NH4OH solution and 23.76 g DI water producing a mixture having the composition: 1.0 [(EtO)2SiCH2]3 : 21 OH : 270 H2O. For bis(triethoxysilyl)methane (MB: [(EtO)3Si]2CH2), 4.1 g of the precursor was added to 12.4 g NH4OH solution and 15.8 g DI water producing a mixture having the molar composition: 1.0 [(EtO)3Si]2CH2: 8.75 OH : 112.5 H2O. For 1,4-bis(triethoxysilyl)benzene (PhB: [(EtO)3Si]2C6H4), 1.21 g of the precursor was added to 6.2 g NH4OH solution and 7.9 g DI water producing a mixture having the molar composition: 1.0 [(EtO)3Si]2CH2: 17.5 OH : 225 H2O. In these cases, a solid or precipitate formed during gelation, the supernatant solution remained fluid, and no gel was observed. When aged at 120 °C for 4 hours, a gel formed and the sample was treated as for the other precursors.

Figure 1: Precursor structures and abbreviations. Precursors used in solgel synthesis of porous solids: 1,1,3,3,5,5hexaethoxy-1,3,5-trisilacyclohexane (3R); 1,3,5-trimethyl1,3,5-triethoxy-trisilacyclohexane (3Me3R); 1,4bis(triethoxysilyl)benzene (PhB; phenyl bridge); bis(triethoxysilyl)methane (MB; methylene bridge); tetraethyl orthosilicate (TEOS), and methyltriethoxysilane (MTES). Experimental Materials 1,1,3,3,5,5-hexaethoxy,1,3,5-trisilacyclohexane (3R), bis(triethoxysilyl)methane (MB) and 1,4bis(triethoxysilyl)benzene (PhB) were purchased from Gelest Inc., and used without further purification. Ammonium hydroxide solution (28-30 wt.%) was obtained from J. T. Baker. Surfactant-free synthesis of MOS in Basic Aqueous Medium

For tetraethyl-orthosilicate (TEOS), 1.56 g of the precursor was added to 6.2 g NH4OH solution and 7.9 g DI water producing a mixture having the molar composition: 1.0 (EtO)4Si: 7 OH : 90 H2O. For these cases, some white suspension formed, but no gel was observed before drying at 120 °C. For methyltriethoxysilane (MTES), 1.34 g of the precursor was added to 6.2 g NH4OH solution and 7.9 g DI water producing a mixture having the molar composition: 1.0 (EtO)3SiCH3: 7 OH : 90 H2O. For this case, some white solids formed, but no gel was observed before drying at 120 °C. For 1,3,5-trimethyl-1,3,5-triethoxy-1,3,5-trisilacyclohexane (3R3Me: [(EtO)CH3SiCH2]3), 0.77 g of the precursor was added to 6.2 g NH4OH solution and 7.9 g DI water producing a mixture having the molar composition: 1.0 [(EtO)CH3SiCH2]3: 21 OH : 270 H2O. For these cases, some sticky solids formed, but no gel was observed before drying at 120 °C. Characterization


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For measurements of porosity, nitrogen isotherms are collected at 77 K on Quantachrome Autosorb AS-1C and AS-iQC2 instruments and the properties calculated using standard literature techniques. Samples of approximately 100 mg are outgassed under vacuum for 4 hours at 120 °C prior to data collection. The BET surface area is 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 are derived from the t-plot using at least 5 data points from P/P0 0.3 to 0.5. Micropore surface area is estimated by subtracting the external surface area from the BET surface area. Total pore volume is calculated at P/P0 0.95. Based on several triplicate syntheses with characterization of the products, the standard error of the BET surface area results was +/- 18 m2/g and for pore volumes, +/- 0.05 cc/g. Results and Discussion As described in detail elsewhere (8), and briefly in the Experimental and Characterization sections, we produced porous solids, via aqueous template-free solgel processes, from the 6 precursors shown in Figure 1. We first produced solids from 3R and 3Me3R and found surprisingly high surface area for the former and essentially no surface area for the latter (see Table 1). We then developed an explanation for this behavior and used the resulting model to predict the surface area of solids produced from the other precursors included in Figure 1 under similar conditions. Synthesis parameters and the surface area and pore volume (as well as the “solvent index” to be introduced later) for these materials are presented in Table 1. Table 1: Solvent index and experimental properties of porous solgel solids. Reagent W

CommentTgel

SBET 2

Smicropore 2

Vpore

°C

m /g

m /g

cc/g

3Me3R

0

no gel

60

0

0

0

MTES

0

no gel

60

7.5

0

0

TEOS

1

particles 60

321

15

0.32

PhB

1.66

cloudy

60

350

0

0.53

3R

9.26

gel

60

1288

888

0.63

3Me3R

0

no gel

90

1.8

0.4

0.01

TEOS

1

particles 90

226

0

0.24

PhB

1.66

solids

612

0

0.74

MB

3.04

gel

90

862

0

0.79

3R

9.26

gel

90

1206

142

0.87

3Me3R

0

no gel

120

0

0

0.05

TEOS

1

particles 120

226

0

0.30

PhB

1.66

gel

120

664

0

0.96

3R

9.26

gel

120

1160

0

1.21

90

During the hydrolysis step in solgel preparations, alkoxycontaining precursors release alcohol (ethanol in this work) to form reactive Si-OH groups, which begin to condense to form Si-O-Si bond connections. Condensation reactions continue during the gelation and drying periods. Void spaces in the gel are squeezed out during drying due to strong capillary stresses (3). As the precursor molecules are increasingly linked together via condensation reactions, the strength of the network increases. If, at some point during this process, the molecular network achieves sufficient strength to resist the capillary stresses, the structure of the network, and hence its incipient porosity and surface area, will largely be maintained during subsequent solvent removal. Our focus is on the role the number of connections between precursor molecules plays in strengthening the network. Constraint counting (9) or rigidity theory (10) provides a set of conceptual tools for determining the mechanical properties of the network. Balancing the rigidity of the network with the forces that cause collapse, determines the level of linkage between the precursor molecules necessary for the porosity to be maintained. A simple treatment of the mathematical theory of generic or mean-field rigidity, following Gupta (11), and applied to silicate tetrahedra, is given in the Supporting Information. In summary, a collection of rigid objects has degrees of freedom (d.o.f.) of movement equal to the sum of the degrees of freedom of its components. Each connection between objects imposes constraints (e.g., bonding and bond bending constraints) which restrict the d.o.f. When the collection of objects has more d.o.f. than constraints, it has net internal motions and is “floppy”. When the constraints are greater than or equal to the d.o.f., the collection of objects is rigid. When d.o.f. is equal to the constraints, the system is at a transition between floppy and rigid states. The rigidity transition can be attributed to a threshold connectivity of the network of objects where connectivity is defined as the average number of objects joined per possible connection between them. The connectivity at the transition can be determined from the number and type of the constraint-inducing connections between the objects. The materials considered here are organosilicas composed of bridging groups (B; e.g. –O– or –CH2–) and terminal groups (T; e.g. -OH or –CH3) in solids of composition SiB2-y/2Ty. In the Supporting Information we show that a simple form of rigidity theory predicts the transition to occur (Equation S14) for these materials at nB/nSi = 1.5, where nB is the number of bridging groups and nSi the number of silicon atoms. This corresponds to each silicate tetrahedron sharing three bridges with other tetrahedra. This level of connectivity should divide systems which cannot support any stress (nB/nSi < 1.5) from those that can (nB/nSi > 1.5) and should be equal to the minimum



connectivity for a solid of the type considered here to survive drying with intact void spaces. The connectivity and the rigidity transition as described above are average or mean-field concepts. They apply most directly to homogenous systems where the density of connections is evenly spread throughout the solid portions of the system. If the surface area and pore volume of interest are contained within homogeneous regions, the rigidity transition should provide a useful separation between systems with stable or unstable surface properties under the influence of capillary stresses. The connectivity in many systems of interest is less homogenously distributed, at least during some stages of the synthesis. Particulate systems have relatively homogenous regions (within the particles) interrupted by the contacts or necks which join them. The material of which these contacts are made may be more or less similar to the properties of the intraparticle material. The extent to which the void space of a particulate gel collapses during drying depends on the nature and number of these contacts between the particles. If there is a sufficient number of thick-enough contacts and if the connective material is similar to (has the same density of connections as) the intraparticle material, then the same rigidity transition will govern the collapse of interparticle voids as well as it does intraparticle voids. If the interparticle connective material has different properties than the particulate material, then the rigidity transition based on the average connectivity may no longer govern the collapse of the interparticle void space. Similarly, if a sufficient number of contacts does not exist, then even if the connective material is rigid, the particulate gel will not be rigid and interparticle voids can be removed during drying. A different manifestation of rigidity theory could be applied to particulate-forming precursors in order to understand the number and type of contacts necessary to preserve porosity in those systems (15).

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gested elsewhere (12; see also the Supporting Information) that the mechanical properties of non-porous amorphous solids, with varying degrees of connectivity above the rigidity transition, depend smoothly on the level of connectivity: X = Xfcf(nB/nSi), where X is a mechanical property and Xfc is the mechanical property for the fully connected material. However, the properties of a still-wet gel depend both on the strength of the solid phase from which it is formed (i.e. the struts or lattice) and on the total volume of that solid phase, or equivalently, on the amount of void space or porosity. The elastic deformation and stress of such solids has recently been reviewed by Gor et al. (13) As an illustrative example, consider a Gibson and Ashby (14) expression applied to the yield stress of the porous gel (ignoring effects of the enclosed solvent),

σ gel = σ (1 − φ )n

, where φ is the porosity or void fraction, σ is the yield stress of the partially connected solid, and n is an empirical exponent, usually 1 ≤ n ≤ 3 . If we combine this result with the connectivity expression for the solid, we see that there is a curve in (nB/nSi,φ) where the yield stress of the network equals that imposed by the capillary stresses present at a given time during drying:

φ = 1 − ((σ dry σ fc ) f (n B nSi ))n

1

(Equation S19 derived in the Supporting Information). A solid material with higher connectivity can support a larger porosity under a given imposed stress. The situation is depicted schematically in Figure 2a where porosity is plotted versus nB/nSi.

Based on the above, we expect the calculated rigidity transition to have direct relevance to systems that form homogeneous space-spanning molecular network gels, to systems that form particles with many interparticle contacts which are made of material similar to the intraparticle material, and to systems with (e.g. non-spherical) particles that pack with little interparticle void space. Gels made of spherical particles with few interparticle contacts and/or with connective material significantly different than the intraparticle material may require a different approach. We now qualitatively link the growth of the connectivity during drying and gelation to the mechanical properties of the gel that enable it to prevent collapse during drying to get an overall picture of the process. It has been sug-

Figure 2: Connectivity and porosity in the solgel process. (a) Porosity vs. Connectivity (number of bridging groups per Si) for several solgel systems is shown schematically.


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The dash-dotted line illustrates the locus of conditions where the porous solid phase is strong enough to withstand the capillary stresses imposed by drying. Its shape is generated by the equation described in the text using exponent n=2. The thin vertical solid line shows the connectivity at the rigidity transition for the solid phase (nB/nSi=1.5). The different arrows show the trajectories, vertically offset for clarity, of different precursors (as explained in the text) in otherwise identical solgel syntheses. (b) Number of bridging groups per Si atom vs. time for the precursors discussed in the text is shown schematically. TEOS and MTES are shown as the black and red lines starting at Bridges/Si = 0. PhB is the green dashed line; 3R, the black dotted line, and 3Me3R the red dotted line. The thin horizontal line at Bridges/Si = 1.5 shows the rigidity transition for the molecular network. Times at which the temperature and environment change are indicated by the thin vertical lines. In schematic Figure 2a, the solid arrows show the path of a system with few available condensing groups. Relatively little increase in nB/nSi occurs during gelation while φ remains constant since gelation occurs at nearly constant volume. During drying, φ decreases until virtually all porosity is squeezed out. The dashed arrows show a system with sufficient potential connectivity so that the gel becomes stiff enough to withstand the capillary stresses at some intermediate point during the drying stage leading to higher retained φ. The dotted arrows show the path of a system containing a larger number of bridging groups per silicon: φ stops decreasing early in the drying stage leading to even higher φ and surface area in the resulting dried solid. For the materials investigated here, 3R follows a path like the dotted arrows, PhB like the dashed arrows, and MTES and 3Me3R like the solid arrows. While the full influence of the precursor structure and processing parameters on the dried solid is complex, the rigidity transition by itself captures significant aspects of the development of the network. First, the transition occurs at the absolute minimum level of connectivity for the entire porous structure to support stress. Second, as depicted in Figure 2a, the extent of gelation or drying at which the transition is reached is related to the eventual connectivity at which φ stops decreasing; the sooner the threshold is crossed, the sooner the combination of nB/nSi and φ are reached such that the system is stable to the capillary stress. Third, when the struts of the solid phase become rigid (as distinct from the rigidity or strength of the fluid-filled porous body), their subsequent transformations may slow or stop. For example, two flexible strands of the network might become stitched together by condensation reactions, thereby removing surface area and modifying the pore size distribution, but this process is hindered when the strands are rigid. For these reasons and because of the simplifications enabled when doing so, we will use the rigidity transition as a proxy for the level

of connectivity necessary for the solid to resist the capillary stresses. The growth of nB/nSi through the hydrolysis, gelation, and drying steps is shown schematically in Figure 2b for the precursors discussed in this work. TEOS and MTES start at nB/nSi = 0, since they have no internal bridging groups. Their connectivity rises slowly during the room temperature hydrolysis step and faster during higher-temperature gelation. TEOS reaches the rigidity transition shortly after the temperature is again raised during drying. MTES never quite reaches the rigidity transition during drying as it can only support a maximum of nB/nSi = 1.5 due to the presence of the non-bridge-forming methyl group. 3Me3R and 3R are shown, respectively, as the red and black dotted lines. They start with nB/nSi = 1 since they have one bridging methylene group per Si atom in the precursor molecule. 3R reaches the rigidity transition early in the gelation step as it only requires 0.5 additional bridges per Si to reach the transition. 3Me3R, like MTES, never reaches the rigidity transition as it has one non-bridging terminal group per Si atom, preventing nB/nSi = 1.5 until complete condensation. PhB is an intermediate case, since it starts with 1 bridge per 2 Si atoms, and it reaches the transition later than does 3R. In general, precursor networks that reach the rigidity transition sooner (Figure 2b), also sooner reach the condition where the porous body can withstand the capillary stresses and hence they retain more surface area and porosity. Next we more quantitatively relate connectivity to the porosity or void fraction; φ. After a sufficiently long time during gelation and/or drying, the solgel system will be at or near equilibrium with respect to hydrolysis and condensation reactions (2 ≡Si-OH ⇄ ≡Si-O-Si≡ + H2O). Assuming equilibrium, the state of the network can be related to the amount of solvent remaining in the gel as solvent participates in the hydrolysis and condensation reactions. The volume of solvent remaining at the point where the network can resist the collapse is directly related to the amount of porosity and surface area in the final dried solid as the solvent fills the space not filled by the solid. In the Supporting Information, we show that the amount of solvent (principally water in our preparations) in a gel at equilibrium, relative to that of a standard material, is approximately:

n H 2O M

(n

H 2O

M

)

std

n

 γ H O ,std = 2  γH O 2 

M

 W  

(1)

where H2O is the number of moles of solvent in the gel per unit mass of the gel, γ is an activity coefficient, and W is the “solvent index” defined by:



(n SiOH W =

(n SiOH

n Si ) n

)

(2)

2

2 Si std

(n SiOSi

n Si )

(n SiOSi

n Si ) std

MW ' std MW '

where MW’ is the mass of a gel (excluding solvent) per mole of silicon. W is essentially the ratio of the equilibrium constant for the hydrolysis and condensation reaction for the system of interest to that of a standard. In what follows, we take TEOS as a representative ‘standard’ precursor and we take the state at which W is defined to be the rigidity transition--both for the materials of interest and for the standard. The rigidity transitions occur at different degrees of condensation for different precursors; they are intrinsic properties of the precursors such that some achieve rigidity at low levels of condensation (they have high W), while some (with very low W) never form a rigid gel network at any level of condensation. In the Experimental section, we present details of gel syntheses performed under high H2O:Si conditions (90:1 except for 112:1 for PhB and 66:1 for MB), high pH, and hydrothermal gel temperatures of 60, 90, and 120 ˚C, from the precursors shown in Figure 1. In Table 1 we show calculated values of W as well as the surface area, micropore surface area, and pore volume measured via N2 adsorption and BET analysis. The BET surface area, SBET, and pore volumes, Vp, are plotted versus W in Figure 3.

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a function of the solvent index, W, for the six precursors described in the text. The reproducibility of the surface areas is +/- 18 m2/g; smaller than the symbols. (b) Pore (Micropore + Mesopore) volume (cc/g) as a function of W. The reproducibility of the pore volumes is +/- 0.05 cc/g. The gelation temperatures used are 60 °C (triangles), 90 °C (open squares), and 120 °C (circles). The dashed line is a guide to the eye in the form of a Langmuir isotherm. The points at W=0 are for MTES and 3Me3R, TEOS is at W=1 (by definition), PhB at W~1.66, MB at W~3.04, and 3R at W~9.26. At each gelation temperature, the different precursors underwent otherwise similar syntheses, with similar molar (per silicon atom) compositions, initial pH, times for hydrolysis, gelation/aging, and drying, and temperatures and other conditions for hydrolysis and drying. Several interesting features are evident in Figure 3. The two precursors with W = 0 (MTES and 3Me3R) yield solids with very small values of surface area and porosity. In the context of the present theory, they lack sufficient connectivity to reach the rigidity transition even at complete condensation. The strands of condensed polymer from these precursors remain flexible throughout the gelation and drying stages and they nearly completely collapse during drying. Experimentally, these two species did not produce a visible gel during the gelation/aging step. The exact level of bridging at the gelation transition is unknown and likely precursor-dependent, but it is necessarily less than or equal to that at the rigidity transition, as the network must be connected before it can be rigid. TEOS, at W = 1 by construction since it is the reference material, falls off of the surface area trend followed by the other materials. This is not surprising since TEOS produces dense spherical particles (see e.g., TEM and SAXS data in Li et al. (8))—the worst case for the theory presented here, as discussed previously. The other materials (W > 1), except for the lowest gelation temperature for PhB, exhibit very high surface area and pore volume with both properties increasing with increasing W. The surface area for these species is almost independent of the gelation temperature, while the pore volume is a strong function of the gelation temperature. All of these species formed a system-spanning gel by the end of the gelation step. For 3R, the gel shows no signs of large primary particles (8); the molecular network homogeneously fills the space. We do not know the nature of the gel for MB at this time. For PhB reacted at 60 ºC, the system formed un-gelled particles. At 90 ºC, it forms gelatinous solids of non-spherical particles, but the gel did not span the entire volume. When the PhB concentration was doubled (details not shown here), it did form a system-spanning particulate gel at 90 ºC.

Figure 3: Surface area and pore volume for template-free mesoporous organosilicas. (a) BET Surface area (m2/g) as


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A hypothesis consistent with these observations is that, for the fast condensing species, the ligaments (or struts or walls) of the solid phase of the gel reach the rigidity transition at some time during the gelation step, even at the lowest Tgel attempted, and this nearly fixes the total surface area of the solid. Even at rigidity some of the ligament connections are not strong enough to resist the compressive forces of drying and break (but maintain their surface area), resulting in partial collapse of the pore volume until the network densifies enough to allow the porous (liquid-filled) solid to resist additional capillary stresses. At that point, the solid material resists reconfiguration and the remainder of the solvent leaves without significantly changing the pore volume. Three relevant transitions may occur at different extents of condensation: the gelation transition, the rigidity transition of the network of polymer strands or ligaments of the solid phase, and a transition where the liquid-filled, rigid ligament, solid plus pore system exhibits enough strength to resist collapse under the capillary stress. The W index locates the extent of condensation at which the rigidity transition occurs and for higher W, the rigidity transition occurs with lower extent of condensation. Inspection of the definition of W and the way the extent of condensation depends on precursor features (see Supporting Information) shows that features which lead to higher W are: (1) a higher level of initial bridging with non-hydrolyzing groups such as methylene or 1,4substituted benzene; (2) a low level of hydrolyzing SiOSi/Si bridging groups present at the rigidity transition; (3) a large SiOH/Si ratio at the transition; (4) low numbers of non-condensing terminal groups (exemplified by methyl groups in this work); and (5) more rigid precursor molecules. The W index also captures elements of the kinetics of the process as it is proportional to the ratio of the rates of hydrolysis and condensation at equilibrium and higher W systems are generally faster condensing near the transition. For the 3R precursor: SiCH2Si/Si = 1 and SiCH3 = 0, and at the rigidity transition, SiOH/Si = 1 and SiOSi/Si = 0.5. These are all at the beneficial extremes among the precursors studied here, leading to high W. Consistent with this, 3R has the highest W and the highest experimental surface area amongst the precursors tested, exhibiting

1180≤ SBET ≤ 1260m2/g for the conditions used in this work (the exact values differ from those in Li et al. (8) because of minor differences in the drying conditions for this series of preparations). Landskron (7) found surface areas as high as 1706 m2/g for powders made from this precursor in syntheses incorporating a templating surfactant which was extracted with HCl in ethanol (a type of lower surface tension solvent replacement). To our knowledge, the mesoporous organosilica (MOS) material reported here and in (8) using the 3R precursor gives the highest surface areas ever achieved for solgel materials prepared using water as solvent and with a simple, rapid,

drying step. The PhB precursor also shows high surface area (up to 664 m2/g here, up to 958 m2/g in the original work using organic solvents and slow air drying (16), and 1262 m2/g when using the methoxy version of the starting material and when prepared in organic solvents using slow air drying followed by an ether wash and further drying in vacuum (17)). The MB material gives 862 m2/g with Tgel = 90 °C in this work compared to 551 m2/g when prepared in ethanol followed by multiple solvent replacements before drying (18). The rigidity transition being reached does not preclude all further rearrangement of the material during the gelation step. As described in ref. (3), further “ripening” can occur: hydrolysis and condensation can proceed toward a global minimum energy state by molecules moving from areas of high curvature to areas of lower curvature. In the gel state, this could mean small diameter ligaments being dissolved or reformed in favor of thicker ligaments. While this would change the void volume of the gelled solid little, and decrease the surface area slightly, it would eliminate smaller pores with higher curvature surfaces and would make the resulting network stronger and better able to resist collapse during the drying step. These processes are consistent with the experimental data in Table 1 which shows the higher Tgel syntheses usually giving smaller micropore surface area (eventually reaching zero), somewhat smaller total surface area, and larger pore volume. The high W materials also exhibit very large macropore volumes (8) which also increase with W.

Conclusions Armed with experimental observations of organosilicas produced from 3R and 3Me3R, and comparison to TEOS, we developed a model that explains their behavior and used the resulting model to predict the relative behavior of other precursors under the same conditions. This model should be useful in predicting the behavior of other precursors in similar solgel syntheses. While previous authors have suggested that rigid precursors are helpful in forming high surface-area solids, the main result of this work is that it is the rigidity of the molecular network that matters. From this perspective, precursor rigidity is important mostly because it supports network rigidity; condensation reactions are then not needed to rigidify the precursor and can instead strengthen the network. The key feature of a precursor is that it forms a rigid network during gelation or early during drying. Non-hydrolyzing bridging groups, such as methylene, increase the initial connectivity, increase the equilibrium number of bridging groups, and decrease connectivitycutting hydrolysis reactions. These lead to rapid and extensive connectivity which leads to the rigidity transition being reached early in the synthesis and therefore to a decrease in pore collapse during drying. For the class of



materials considered here and within the approximations of the theory as presented, the rigidity transition always occurs at the same value; nB/nSi = 1.5. A more complete theory would incorporate better descriptions of the rates of the hydrolysis and condensation reactions for different precursors at different extents of reaction, leading to more accurate accounting of the connectivity growth and to a less schematic version of Figure 2b. Similarly, a more complete theory of the relationship between connectivity and the mechanical properties of the solvent-filled gel could lead to a more accurate version of Figure 2a. Such a theory would also be able to accurately predict, a priori, the influence of all of the processing variables such as pH and the temperature of the different stages of synthesis. Until such a theory is developed, the solvent index introduced here can be used to determine the relative ability of different precursors, under similar conditions, to produce high surface area solids in syntheses without templates and in simple aqueous media. Hybrid organic/inorganic porous solids with high surface area offer excellent starting points for designing materials targeted towards a wide variety of applications, from catalysis to separations. The organic moieties and residual hydroxyls provide locations for targeted functionalization and the ability to tune pore sizes and hydrophobicity through precursor selection, synthesis conditions, and post-processing further widen the scope for targeted design. Making explicit the link between molecular precursor structure and the resulting surface area and porosity generated in simple and efficient aqueous syntheses should facilitate an increased number of applications of this important family of materials.

Theory and Calculations: Development of Solvent Index Hydrolysis and Condensation Rigidity Theory and the Rigidity Transition Mechanical Properties Reaction Extent

REFERENCES 1.

2. 3. 4. 5.

6.

7.

8.

9. 10. 11.

12.

AUTHOR INFORMATION Corresponding Author

13.

* [email protected]

current address: [email protected] 14.

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

15.

Notes The authors declare no competing financial interest.

16.

ACKNOWLEDGEMENTS

17.

We gratefully acknowledge the technical assistance of Meghan Nines (BET) and Maria Martinez (BET) and acknowledge Andrew Wiersum for discussions on adsorption. We thank ExxonMobil for permission to publish.

SUPPORTING INFORMATION

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Model for the Synthesis of Self-Assembling Template-Free Porous

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