First Macro-Mesocellular Silica SBA-15-Si(HIPE) Monoliths

Article Cite This: Chem. Mater. 2018, 30, 864−873

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First Macro-Mesocellular Silica SBA-15-Si(HIPE) Monoliths: Conditions for Obtaining Self-Standing Materials Armand Roucher,† Ahmed Bentaleb,† Eric Laurichesse,† Marie-Anne Dourges,‡ Mélanie Emo,§ Véronique Schmitt,† Jean-Luc Blin,*,§ and Rénal Backov*,†,# †

CNRS, Univ. Bordeaux, CRPP, UPR8641, 115 Avenue Albert Schweitzer, 33600 Pessac, France Institute of Molecular Sciences, University of Bordeaux, UMR 5255 CNRS, F-33400 Talence, France § Faculté des sciences et technologies, Institut Jean Barriol, UMR CNRS 7565 SRSMC, Université de Lorraine, BP 70239, 54506 Vandoeuvre lès Nancy cedex, France ‡

S Supporting Information *

ABSTRACT: Combining the emulsion and the cooperative templating mechanisms, self-standing macro-mesoporous SBA-15-Si(HIPE) monoliths (90−95% of porosity) have been synthesized for the first time. Materials have been prepared from the Pluronic (P123)/dodecane/water system in the presence of salt and TEOS as silica scaffold precursor. As increasing the ionic strength was shown to be ineffective toward obtaining self-standing monoliths, we induced a kosmotrope “order maker” effect within the starting silica gel making the use of (NH4)2SO4 salt to enhance silica polycondensation. Beyond its effect over the silica polycondensation, we show that the (NH4)2SO4 kosmotrope character has also a strong impact over the micelles organization at the mesoscale, with an input over the voids diameter and connections at the macroscopic length scale. The obtained SBA-15xSi(HIPE) self-standing foams exhibit thereby highly ordered mesopores, high specific surface area, and tunable open macroporosity while offering mechanical strength (Young’s modulus) of 0.15−1 MPa without collapsing at high strain.

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INTRODUCTION Ordered mesoporous silicates1,2 have generated fascination in materials science and heterogeneous catalysis, as witnessed by thousands of publications in this domain.3 These mesoporous materials were stated as outstanding candidates for extending heterogeneous catalysis or adsorbents toward domains where pore sizes larger than those of zeolites were expected. Among the synthesized materials the most renown are certainly MCM41 (Mobil Crystalline Materials)4 using cetyltrimethylammonium as templating agent, SBA-15 (Santa Barbara Amorphous)5 using triblock copolymer Pluronic 123 as mesoscale template, MSU (Michigan State University)6 obtained from nonionic polyoxyethylene alkyl ethers, and HMS (Hexagonal Molecular Sieves)7,8 where dodecylamine is employed as a mesoscale templating agent. Despite academic success, we must confess that few real applications, or even none, have been extended toward the high scale industrial production. The first reason is certainly the use of sacrificial expensive templating agents. The second penalty is a consequence of the high degree of mesopore monodisperse character leading to materials with a unique pore size: 4.0 nm, 9.0 nm, 2.5 nm, and between 2.5 and 5.0 nm for MCM-41, SBA-15, HMS, and MSU, respectively. The third issue is the fact that those mesoporous materials are generally obtained as powders and not as a monolith. Even if obtained as monolith, the high surface area will hinder good diffusion and accessibility through the whole monolith core © 2017 American Chemical Society

while not avoiding high pressure drops. Those drawbacks are certainly the reasons why materials bearing dual or hierarchical porosities have today attracted widespread interest for industrial applications.9 Indeed the intrinsic advantages of assembling hierarchical porosity in one single material rely on the fluid hydrodynamic behaviors occurring at various length scales, from which the IUPAC porous classification has been constructed. If dealing with a Newtonian fluid (viscosity remains constant whatever the applied shear) as a solvent, at the microscopic length scale (1.5−2 nm) the fluid behavior will be driven by diffusion where molecular reactivity occurs and convection is negligible. At the mesoscopic length scale (2−50 nm) the convection is not negligible anymore, and thereby the fluid hydrodynamics is a low convective fluid in which molecules are diffusing; this hydrodynamic behavior is called dispersion. This is also one of the reasons why mesoporous materials were so important within the last 20 years, by offering both molecular reactivity and mass transport pseudo-optimized. At the macroscopic length scale (50 nm and above) the mass transport is driven by convection where diffusion becomes negligible (sometimes the term advection is also employed to signify that molecular diffusion is present, even if negligible).10 Received: October 25, 2017 Revised: December 22, 2017 Published: December 29, 2017 864

DOI: 10.1021/acs.chemmater.7b04483 Chem. Mater. 2018, 30, 864−873

Article

Chemistry of Materials

Figure 1. Overall appearance of the as synthesized materials when varying the (NH4)2SO4 salt concentration. SEM visualization at different magnifications: (a, b) SBA-150.1Si(HIPE), (c, d) SBA-150.5Si(HIPE), (e, f) SBA-151.0Si(HIPE), and (g, h) SBA-152.0Si(HIPE). Insets are optical images of the materials’ self-standing characteristics.

The goal is overall to obtain materials bearing opened and hierarchical porosities and as far as possible to obtain monolithic materials and not powders as it is almost always the case. The reason is that, when dealing with monolith-based heterogeneous catalysis where both surface area and mass transport are optimized, the catalyst and the catalyzed species do not have to be separated in a final step anymore. The drawback is that obtaining self-standing monolith and not only powders bearing hierarchical porosity is everything but an easy task to reach, and in that vein we have to underline Nakanishi and co-workers’ highly efficient path to address monolith generation obtained by coupling sol−gel process, phase separation and swelling lyotrope mesophases.11

An alternative way of obtaining such materials is to employ both lyotropic mesophases to create porosity at the mesoscale and concentrated emulsions to induce open porosity at the macroscale. Those materials were labeled poly-HIPE (polymerized High Internal Phase Emulsion) and discovered by the industry. Such materials were most often obtained from the polymerization of a reverse concentrated emulsion, the continuous phase being composed of the organic polymerizable species.12 Contrary to what is often admitted, the first inorganic-HIPE has also been pioneered by the industry.13 Eight years later Imhof and Pine14 published a paper exhibiting the advantage of working with monodisperse nonaqueous emulsions. Since then, our group has shown how the macroscopic voids can be tuned on demand either by playing 865


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Star detector from Bruker, and spectra were integrated with the Bruker software. For the mechanical properties, we cut the monoliths in small cylinder slices. A DHR-2 rheometer from TA Instruments has been employed for the compression test with plate−plate geometry with a diameter of 40 mm. The compression is applied with a constant velocity of 5 μm·s−1. Micelle Structure in the Presence of Salt. SAXS experiments were performed on a SAXSess instrument (Anton Paar), using a linecollimation system. This instrument is attached to ab ID 3003 laboratory X-ray generator (General Electric) equipped with a sealed X-ray tube (PANalytical, λCu Kα = 0.1542 nm) operating at 40 kV and 50 mA. A multilayer mirror and a block collimator provide a monochromatic primary beam. A translucent beam stop allows the measurement of an attenuated primary beam at q = 0. Samples were put in a special glass capillary (WJM Glas) before being placed at 25 °C inside an evacuated sample chamber and exposed to an X-ray beam for about 90 min. Scattering of the X-ray beam was registered by a CCD detector (Princeton Instruments, 2084 × 2084 pixels array with 24 × 24 μm2 pixel size), placed at 309 mm distance from the sample. Using SAXSQuant software (Anton Paar), the 2D image was integrated into one-dimensional scattering intensities I(q) as a function of the magnitude of the scattering vector q = (4π/λ) sin(θ), where 2θ is the scattering angle. All data were calibrated by normalizing the attenuated primary beam, before being corrected for the background scattering from the cell and the solvent and for slitsmearing effects by a desmearing procedure from SAXSQuant software using the Lake method. After correction, the obtained intensities were scaled into absolute units using water as a reference material.

with the starting emulsion oil volume fraction, for which we labeled those materials Si(HIPE)15 for silicic HIPE, or by playing with Pickering based-emulsions, for which we labeled those materials Si(PHIPE) (Pickering-based High Internal Phase Emulsion).16 The Si(HIPE) series can either be hybridized to generate Organo-Si(HIPE)17 series reaching applications in photonics,18 metallics,19 or enzymatic20 heterogeneous catalysis or be employed as hard exotemplate to generate Carbon(HIPE) with emphases toward hydrogen storage,21 energy storage,22 or enzyme-based energy conversion.23 Up to now, the link between the monolithic Si(HIPE) and the SBA-15 mesoporous materials was missing. Here we bridge the competences of these two sets of materials while generating the first SBA-15-Si(HIPE) foams. The materials are thus bearing a monolithic character and open macroporosity while offering at the mesoscale a highly ordered mesoporous network addressing high surface area.

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MATERIALS AND METHODS

High purity dodecane (≥99%), 37 wt % hydrochloric acid (HCl), tetraethylorthosilicate (≥99%, TEOS), (NH4)2SO4, and the nonionic surfactant triblock copolymer, EO20PO70EO20 Pluronic P123, were purchased from Sigma-Aldrich. All the chemicals were used as received without any further purification. Material Synthesis. In a typical synthesis, varied amounts of (NH4)2SO4 (corresponding to 0.1, 0.5, 1.0, and 2.0 M with regard to the total final aqueous phase) were added to 3.20 g (3.17 cm3) of the aqueous solution of P123 (5 wt %) and were stirred to obtain a homogeneous solution. Then, 1.00 g (0.833 cm3) of 37% HCl aq. and 1.00 g (1.06 cm3) of TEOS were added in the previous solution and stirred by hand in a mortar until the prehydrolysis was complete. The emulsion is generated within a mortar by the addition of 7 g (9.33 cm3) of dodecane drop by drop under manual stirring. The final disperse phase volume fraction is therefore equal to 64.8 vol %. The emulsion was then transferred into a Teflon autoclave for hydrothermal treatment at 100 °C for 24 h. The material was washed with ethanol under Soxhlet for 48 h and dried at room temperature for 3 days. Finally, the materials were thermally treated under atmospheric conditions as follows: a first temperature increase is applied at 2 °C/ min until 180 °C with a 2 h plateau followed by a second temperature ramp at 1 °C/min to reach 350 °C. Temperature was held for 2 h, and then, a final temperature ramp at 1 °C/min is imposed to reach 650 °C with a 6 h plateau. The cooling process was uncontrolled and directed by the oven inertia. Final expected meso-macroporous materials are labeled hereafter SBA-15xSi(HIPE), where x refers to (NH4)2SO4 concentration (0.1, 0.5, 1, or 2 M). Images of the final materials are reported on Figure 1. Characterization. SBA-15Si(HIPE) Materials. Scanning electron microscopy (SEM) observations were performed with a HITACHI TM-1000 apparatus operating at 15 kV. Intrusion/extrusion mercury porosimetry was measured with a Micromeritics Autopore IV 9500 porosimeter. The mesoporous structure was observed with a transmission electron microscope (TEM) HITACHI-H600 operating at 75 kV. The materials were crushed into powder, dispersed in ethanol, and deposed over a Formvar/carbon grid and left to dry in air before observations. Nitrogen sorption isotherms were measured by using a Micromeritics ASAP 2010 apparatus. Samples were outgassed at 200 °C overnight and weighed before sorption experiments. The surface area was determined by applying the multipoint Brunauer− Emmett−Teller (BET) model to describe the adsorption isotherm. The mesopore and the pore size distribution were determined by the Barrett-Joyner-Halenda (BJH) method.24 X-ray measurements were performed on a Bruker Nanostar with a Cu anode working at 40 kV/ 35 mA coupled to a Göbel mirror system producing a beam with a wavelength of 0.15418 nm or an energy of 8 keV. A small-angle X-ray scattering (SAXS) configuration (sample−detector distance, D = 105 cm) with transmission geometry was chosen providing access to wave vectors q in the 0.1−2.3 nm−1 range. Images were collected on a Hi-

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RESULTS AND DISCUSSION Kosmotrope−Chaotrope Balance over Ionic Strength Input. The syntheses of both Si(HIPE) monoliths and SBA-15 powders are occurring under strong acidic conditions where the organic−inorganic skeleton growth is promoted through ionic interactions, with the difference that the former makes the use of tetradecyltrimethylammonium bromide cationic surfactant (TTAB) while the latter employs a nonionic triblock copolymer one. When we tried to simply transpose the SBA15 synthesis toward the Si(HIPE) one, we never obtained selfstanding monolith materials but did obtain powdered ones. In the same trend, rather fragile monolithic silica foams bearing a cubic organization of the mesopores, obtained under alkaline condition with nonionic surfactant, have been proposed by Esquena et al.25 It was thought15 that the good resistance of monoliths obtained with the cationic TTAB arises because of two effects: (i) it influences the ionic strength of the initial sol at the origin of the Si(HIPE), and (ii) due to the ammonium headgroup, it promotes the organization of the hybrid organic− inorganic growing clusters. We thus first focused on the ionic strength alone and replaced the cationic surfactant by NH4Cl without success whatever the salt concentration showing that electrostatics is not relevant here. Indeed whatever the NH4Cl concentration, the Debye length, that is to say the range of electrostatic repulsion, is always of the order of 0.3 nm due to presence of electrolytes (pH is adjusted at about 0.05 with HCl, well below the isoelectric point of silica around 2.1).26 Apart from electrostatic screening, ions are known for their ability to provoke order (kosmotrope) or disorder (chaotrope) following their position in the Hofmeister anion series [CO32− > SO42− > S2O32− > H2PO4− > F− > Cl− > Br− ≈ NO3− > I− > SCN−.27 As electrostatics is not relevant, the inefficiency of Cl− to lead to the formation of monolith likely results from its position in the Hofmeister series. To check this hypothesis, we chose the sulfate anion to induce a kosmotrope effect within the starting silica gel.28−30 As evidenced in Figure 1 (embedded figures), 866


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Chemistry of Materials changing Cl− into SO42−anion, we have obtained self-standing monoliths showing the efficiency of the kosmotrop effect on the synthesis. Additionally, we wanted to determine the effect of the cation keeping the sulfate and replacing NH4+ by Na+. No monolith could be obtained with sodium sulfate showing that both the choice of the anion and the cation are of utmost importance. To go into a deeper understanding and determine if the ammonium cation is necessary, materials have been prepared in the same conditions with magnesium sulfate MgSO4, sodium sulfate Na2SO4, and tetramethylammonium sulfate (N(CH3)4)2SO4 (see Supporting Information S1). As it can be seen in the pictures, despite the presence of some blocks with MgSO4, a monolith could only be obtained with the alternate ammonium salt ((N(CH3)4)2SO4). This result shows that additionally to the importance of the anion, the choice of the cation is also a key parameter for the generation of a selfstanding monolith. Therefore, it seems that the ammonium cation brings also cohesion to the material. Kosmotrope−Chaotrope Balance and Salting out Effect at the Macroscopic Length Scale. Under such configuration, (NH4)2SO4 appears as the candidate of choice to obtain monoliths (Figure 1). Their mechanical strength is slightly lower (see below) than the Si(HIPE) ones where TTAB was employed as meso-structurating and emulsion stabilizing agent. Contrary to Esquena et al.25 we did not have to use a cold lyophilization process to dry the sample in order to minimize the collapsing effect induced by the capillarity forces when drying. Beyond, the thermal treatment is not an issue anymore; on the contrary, it sinters the foams rending them more mechanically robust. As a direct consequence, as we will see later in the text, those SBA15-Si(HIPE) foams can endure mercury imbibition at ease when addressing mercury porosimetry investigations. After identifying the salt nature impact, we assessed its concentration effect. Considering the SEM images of Figure 1 we can first observe that the macroscopic cell diameters decrease from 100 to 25 μm while increasing the salt concentration from 0.1 to 1.0 M. The initial emulsion drop size is of the order of 60 μm (Supporting Information S2). It is worth noticing that the emulsion by itself is quite unstable. However, as the polycondensation occurs quickly at the synthesis temperature (100 °C), and the emulsion has no time to phase separate. The size evolution with salt concentration and the fact that a powder is obtained at 0.1 M demonstrate that the kosmotrope effect over the silica polycondensation is not high enough to stop the emulsion droplets coalescence when (NH4)2SO4 is employed at 0.1 M. Then, considering the Figure 1c−h, we can notice an increase of the macroscopic void diameters while increasing the salt concentration. This process can be explained by a “salting-out” effect. For emulsions stabilized by a nonionic surfactant of the polyoxyethylene ether class, we must consider an entropic repulsive force, which originates from the hydrophilic chains in good solvent on close approach.31 The entropic repulsive force can be affected by salts through an effect on the concentration of surfactant at the oil/water interface, on the extension of the chains due to a modification of the solvent quality. As already mentioned above, SO42− is a kosmotrope solute “order maker”, and it decreases the capability of water to act as a solvent, through a “salting-out effect” with nonionic surfactant, where most salting-out electrolytes lower the cloud point of nonionic surfactant in direct proportion to their molar concentration,32,33 with a direct impact over the nonionic surfactant packing density at the oil/water interface of oil-in-water emulsions.34 In

order to evidence this effect, we prepared materials at a higher pH to slow down the polycondensation. It can be seen in Supporting Information S3 that, below the monolith, a precipitate has formed evidencing the loss of solubility of the P123 nonionic surfactant by addition of (NH4)2SO4. Moreover, a 5 wt % P123 solution at room temperature (temperature at which the emulsion is prepared) in pure water or in 0.1 M of (NH4)2SO4 remains completely transparent and homogeneous while a precipitate appears for brines at 0.5, 1, and 2 M. As just demonstrated, in the present case, the increase of the (NH4)2SO4 concentration diminishes the solubility of the nonionic surfactant and thus its amount available for covering the oil/water interfaces. This effect enhances the coalescence phenomenon increasing the oil droplet diameters (Figure 1d− h) and modifying the pore volume fraction (Table 1). For 0.5 M of salt, the salting-out effect induces a solvent expulsion leading to an increase of the pore fraction. For higher salt concentrations, the salting-out effect is more important, and the amount of P123 remaining and acting as surfactant decreases so much that the emulsion stability is not high enough to keep the pore volume fraction constant (0.63 and 0.52 for 1 and 2 M, respectively). As a direct consequence of this instability, the final silica foam microscopic morphology evolves with the salt concentration as evidenced in Figure 1. Indeed, the structure, of the SBA-150.5Si(HIPE) is that of a foam with interconnected cells with windows while the SBA151.0Si(HIPE) and SBA-152.0Si(HIPE) looks like fingerprints of adjacent spherical drops. In the case of SBA-151.0Si(HIPE), on Figure 1f, the macrocellular walls are almost free of internal connecting windows (holes within the walls) with a continuous external path of void (called hereafter external windows) created through the juxtaposition of adjacent cells, while SBA152.0Si(HIPE) exhibits both internal and external windows (Figure 1h). This latter morphology has also been observed within the Si(HIPE) series.15 When considering the Figure 1h we can see that the internal connecting windows are obvious, and this characteristic witnesses the fact that when increasing the salt concentration from 1 to 2 M, respectively, for the materials SBA-151.0Si(HIPE) and SBA-152.0Si(HIPE), the coalescence phenomenon is further expressed. Indeed, the patch size df between drops is proportional to the drop size at fixed dispersed phase volume fraction.35 Beyond SEM qualitative investigations we have also performed quantitative investigations through mercury porosimetry experiments. The first important result, as said before, is that these foams can support mercury infiltration being a sign of their good mechanical properties. When employing mercury porosimetry we have assessed the diameters of the voids that limit the mercury infiltration, in the present case it corresponds to the diameters of the connecting windows or the interstices between adjacent cells and not the cells themselves. The pore size distributions are shown within Figure 2. Considering the pore size distribution, we can notice that they are spread and certainly multimodal. Despite this multimodal character two main pore sizes distribution are emerging at 120 and 20 μm for the foam SBA-150.1Si(HIPE) and 68 and 4 μm for the foam SBA-150.5Si(HIPE). It is worth remembering that the initial emulsion drops size distribution was centered around 60 μm. This confirms the better emulsion stability at 0.5 M compared to the other salt concentrations. Considering the materials obtained with the higher salt concentrations we can notice that the pore size distributions are sharper for SBA-151.0Si(HIPE), 867


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Figure 2. Pore sizes distributions obtained through mercury porosimetry. SBA-150.1Si(HIPE) (black curve), SBA-150.5-Si(HIPE) (red curve) (for the sake of clarity the curve has been translated 20 units up on the y axes), SBA-151.0Si(HIPE) (blue curve) (for the sake of clarity the curve has been translated 80 units up on the y axes), and SBA-152.0 Si(HIPE) (green curve) (for the sake of clarity the curve has been translated 100 units up on the y axes).

7.2 6.3 9 5 280 340 210 320 0.74 0.79 0.63 0.52

average pore diameter (nm) BJH surface area (cm3·g−1)

0.78 0.68 0.76 0.98

580 760 450 560

where a bimodal or trimodal characteristic is more expressed than for SBA-152.0Si(HIPE). For the SBA-151.0Si(HIPE) we can see that three main pore diameters are emerging at 28, 52, and 95 μm, while for the SBA-152.0Si(HIPE) foam, the curve expressed a multimodal pore size distribution in a range from 15 to 100 μm. Indeed, the main difference between those last two foams is that the pore sizes distribution of the SBA-151.0Si(HIPE) expresses the presence of both external and internal junction cell, as it was the case for the Si(HIPE) series,15 while for the foam SBA152.0Si(HIPE) the pore size distribution relies only on the internal cell junctions as the external ones are absent (Figure 1h). Information concerning the porosity percentage as well as bulk and skeleton densities are summarized within Table 1. We can see that the porosity percentages fall within 90−95% which is higher than in classical Si(HIPE). As a direct consequence the bulk densities are lower (0.04 to 0.06 g cm−3) for foams obtained with a salt concentration from 0.1 to 1.0 M, while being in the same range, 0.1 g cm−3, when a salt concentration of 2.0 M is employed to generate the silica foams. The same trend is observed when dealing with the skeleton densities that are lower (below 1.0 g cm−3) compared to the traditional Si(HIPE) silica foams where the skeleton densities were higher (above 1.0 g cm−3).15 These evolutions where the bulk and skeleton densities are lowering when compared with Si(HIPE) are indeed expected. Kosmotrope−Chaotrope Balance and Salting out Effect at the Mescoscopic Length Scale. The Si(HIPE) bears a higher surface area (around 700−900 m2 g−1), but this high specific surface is essentially involved within the microporosity where the disorganized vermicular mesoporosity represents only 90 m2 g−1 at best.15 Here we can see that with the SBA-15-Si(HIPE) series, the configuration is not the same where the mesoporosity is now reaching 50% of the total surface area (Table 1), which will induce lower bulk and skeleton densities measured by mercury porosimetry.

a

BJH calculated from the nitrogen desorption curves.

0.04 0.03 0.03 0.11 94 95 95 89 0.1 0.5 1 2

21.2 29.2 27.7 8.4

total pore volume (cm3·g−1) BET surface area (cm3·g−1) skeletal density (cm3·g−1) bulk density (cm3·g−1) intrusion volume (cm3·g−1) porosity (%) x

Table 1. SBA-15xSi(HIPE) Porosity Characteristics at Both the Macro- and the Mesoscopic length Scales Obtained Respectively by Mercury Porosimetry and Nitrogen Physisorption Measurements (italics)a


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Figure 3. Nitrogen physisorption isotherms and pore size distribution (inset) for different SBA-15xSi(HIPE): (a) SBA-150.1Si(HIPE), (b) SBA150.5Si(HIPE), (c) SBA-151.0Si(HIPE), and (d) SBA-152.0Si(HIPE). Black squares represent the adsorption curves and red circles the desorption curves.

All the quantitative values that concern the mesoporosity proposed within the Table 1 have been obtained through nitrogen physisorption measurements depicted within Figure 3. All the isotherms of Figure 3 are class IV, when considering the IUPAC classification.36 We can see N2 adsorption at low relative pressure that reveals the presence of microporosity. Above the relative pressure of 0.4 we can observe the H1-type hysteresis loops between the adsorption and desorption curves, typical of mesoporous materials with homogeneous pore size.33 The pore size distributions have been calculated through the BJH36 desorption curves. These pore sizes distributions (Figure 3 (insets)) reveal a rather well-defined profile of the mesoscopic voids centered at the values provided within Table 1 (ranging between 5 and 9 nm). The BJH surface areas providing the mesoscopic surface areas are provided within Table 1 with values ranging from 250 to 350 m2 g−1. Beyond nitrogen physisorption experiments, SAXS investigations are useful to assess the pores organization at the mesoscale with certainty (Figure 4). According to Figure 4, three more or less well-resolved reflection peaks can be observed on all samples in the range of 0.1−2 nm−1 of wave vectors. The peaks can be indexed to the (100), (110), and (200) reflection lines of a two-dimensional hexagonal p6mm symmetry, indicating a defined SBA-15 mesostructure.5 We can notice that all SAXS patterns exhibited the hexagonal 2D characteristic, but for low salt concentration such as SBA150.1Si(HIPE) and SBA-150.5Si(HIPE), the peaks (110) and

Figure 4. Evolution of the SAXS patterns with the variation of salt concentration: (a) SBA-150.1Si(HIPE), (b) SBA-150.5-Si(HIPE), (c) SBA-151.0Si(HIPE), and (d) SBA-152.0Si(HIPE).

(200) are barely distinguishable. Instead when we increased the salt concentration to 1 M all three peaks can be discretized but at very high concentration like SBA-152.0Si(HIPE), and even if they are still observed, the (110) and (200) reflection lines 869


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Figure 5. TEM observations. (a, b) SBA-150.1Si(HIPE), (c, d) SBA-150.5-Si(HIPE), (e, f) SBA-151.0Si(HIPE), and (g, h) SBA-152.0Si(HIPE).

Figure 6. (a) SAXS spectra of P123 at a concentration of 5 wt %, in water (purple) and in an (NH4)2SO4 solution (0.1 M) (green) at 25 °C in log− log representation, (b) corresponding pair distance distribution, and (c) corresponding excess-electron density profiles.

become less resolved and their intensity decreases, reflecting a less ordered channel arrangement. From the diffraction peaks it is possible to calculate the hexagonal-2D cell parameter “a” (a = 2d100/√3). Cell parameters of 11.2 nm, 11.9 nm, 11.2 nm, and 9.5 nm are obtained for the foams SBA-150.1Si(HIPE), SBA-150.5-Si(HIPE), SBA-151.0Si(HIPE), and SBA-152.0Si(HIPE), respectively. If we consider the average mesoscopic void diameters proposed within the Table 1, and then one can estimate the average silica wall thickness between the mesopores; the wall thickness will be equal to the cell parameter minus the mesopores diameters. As such, the determined silica wall thickness between the mesopores are around 4.0 nm, 5.6 nm, 2.2 nm, and 4.5 nm for the foams SBA150.1Si(HIPE), SBA150.5-Si(HIPE), SBA-151.0Si(HIPE), and SBA-152.0Si(HIPE), respectively. It is also possible to visualize the mesoscopic void organization at the mesoscale with TEM, as depicted within Figure 5. We have to underline the fact that TEM is a very local idea of the mesoscopic voids organization, contrary to SAXS experiments. Thereby we have performed screening of the materials with TEM and proposed images that reflect the materials organization. Overall the trend is in agreement with the SAXS

experiment where the mesoscale organization is increased when increasing the salt concentrations from 0.1 to 1 M, while increasing further the salt concentration promotes a decrease of the mesoscale organization, as it was the case with macroscopic voids through the salting-out induced coalescence phenomenon. Indeed, at the mesoscale the salting out effect will be the same, and it will minimize the nonionic surfactant capabilities to be soluble in water. Under salt concentration increases from 0.1 to 1.0 M the salting out effect will induce, through aggregation of the nonionic surfactant, a higher compact configuration while reaching highly organized hexagonal-2D configuration of the lyotrope mesophases. This observation is in good accordance with the results reported by Dag et al.37 Indeed, in a paper dealing with the role of organic and inorganic additives on the properties of mesoporous silica particles prepared from the assembly of CTAB and P123, and the authors have shown that the mesopore ordering is enhanced when the salt concentration is increased. In the absence of SO42−, only one reflection is detected on the XRD pattern, while the (110) and (200) reflections appear in the presence of sulfate anions at a concentration of 0.5 M. The resolution of the peaks also increases with the SO 4 2− 870


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Figure 7. (a) Schematic representation of P123 micelles, (b) representation of the electronic densities of P123 in water, and (c) representation of the electronic densities of P123 in a (NH4)2SO4 solution (0.1 M).

Figure 8. Normal stress σ vs strain ε during compression experiments. (a) Red dots correspond to classic Si(HIPE) with TTAB used as a surfactant, blue dots for SBA-150.5Si(HIPE), and black dots for SBA-151.0Si(HIPE). (b) Focus on the linear domain (yellow background of a).

represented in absolute units and the corresponding pair distance distribution functions (PDDF). The curves presented in Figure 6b exhibit a bell-like shape characteristic of spherical particles. Additional information about the inhomogeneity of the particles (“core−shell” particles) is given by the small bump at around 5 nm (less pronounced for the sample without salt), as expected, given the electronic density difference between the EO groups (ρEO = 371 e/nm3) and the PPO chains (ρPO = 299 e/nm3). The maximum dimension of the particles can be estimated at around 20 nm. This value is in good agreement with the hydrodynamic diameter found with dynamic light scattering (DLS) measurements (19 and 22.3 nm at 25 °C, for the sample without and with salt, respectively). Considering that the length of the extended Pluronic P123 can be estimated at around 38.5 nm, the molecules are folded. The excess-electron density profiles can be obtained from the deconvolution of the pairdistance distribution function (Figure 6c). It confirms the “core−shell” type structure, as expected from the theoretical electronic density profiles (Figure 7b,c). The radius of the hydrophobic core can be defined where Δρ(r) changes sign. Thus, the core radius can be evaluated at 3.9 nm for P123 in water and 4.4 nm for P123 in the (NH4)2SO4 solution. As the extended length of PPO chains in the P123 molecules is about 24.5 nm, the PPO units are bent and disordered in the core of the micelles, as represented in Figure 7a. Moreover, the total radius is estimated at around 9.5 nm, which corresponds to the values obtained with the PDDF. Jansson et al.40 have investigated the structure of P123 micelles in water, and they have obtained results similar to those in this study. The PDDF displayed a distorted bell-shaped distribution, typical of spherical block copolymer micelles, with a maximum

concentration. When increasing further the salt concentration to 2 M, some of the compacted nonionic surfactant will precipitate through the strong salting-out effect and thereby will not contribute anymore to the lyotrope organization at the mesoscale enhancing thus the porosity disorder as witness both with SAXS (Figure 4) and with TEM (Figure 5g,h) experiments. This loss of the mesopore ordering can be also attributed to the shift of the lower consolute boundary (lcb) toward lower temperatures.34 Indeed in a paper dealing with the relation between the position of the lower consolute boundary of various nonionic surfactants in water and the structure of the mesoporous silica materials synthesized from these surfactants-based systems we have shown that the cooperative self-assembly mechanism is not favored if the lcb is not shifted toward high temperatures.38 Moreover, the difference between the phase separation temperature and the temperature at which the silica precursor is added to the surfactant solution is higher, and the mesopore ordering is better.38 Kosmotrope−Chaotrope Balance and Salting out Effect over the P123 Lyotropic Mesophases. To investigate the effect of the presence of (NH4)2SO4 on the structure of the P123 micelles and therefore on the mesopore ordering, SAXS measurements were also carried out on samples of Pluronic P123 at a concentration of 5 wt %, in water or in a (NH4)2SO4 solution at 0.1 M at 25 °C. To perform this study, the obtained scattering data have been analyzed by the generalized indirect Fourier transformation (GIFT),39 taking into account the interparticles interactions. The GIFT method allows determining the pair distance distribution function (PDDF), which corresponds to a histogram of the distances inside the particle. Figure 6a,b shows the SAXS spectra 871


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the conventional Si(HIPE) materials thanks to the enhanced mesoporosity. At the mesoscale, through a salting out effect of sulfate anions, the porosity can be tuned by varying the salt concentration. Results show that the salt plays a dual role. First, at the macroscale level it decreases the stability of the oil-inwater emulsion, which involves an increase on the cell diameter with the salt concentration. Second, at the mesoscale level, sulfate anions strengthen the hydrophobic interactions leading to a mesopore ordering. Nevertheless, at very high concentration, the kosmotrope effect of sulfate anions disturbs the cooperative templating mechanism, and the SBA-15Si(HIPE) mesopororous network becomes less ordered. The SBA15Si(HIPE) series have potential applications as heterogeneous catalysts as well as thermal and acoustic insulators.

diameter of 20 nm. The excess electron density profile showed a typical core−shell structure with a radius of the PPO-rich core evaluated at 4.2 nm. Therefore, the addition of a small amount of (NH4)2SO4 salt in the medium does not affect the structure of the micelles. However, it disturbs the cooperative templating mechanism, which is responsible for the formation of the hexagonal mesopore ordering of the macropore walls of the SBA-15xSi(HIPE). In fact, the SAXS and TEM analyses of SBA-150.1Si(HIPE) reveal less mesopore ordering than for SBA-15, i.e., for the materials prepared without salt. Increasing the (NH4)2SO4 content, larger aggregates are formed; they cannot be modeled, and no information concerning the P123 aggregates can be obtained from SAXS. Nevertheless, we can assume that, when preparing the SBA-15xSi(HIPE), the sulfate anions are localized close to the oxyethylene units and they make the P123 molecules more hydrophobic.41 The system behaves as if the number of PPO units increased. Therefore, the hydrophobic interactions are enhanced, resulting in a better mesopore ordering. Nevertheless, at concentration higher than 1 M, the presence of SO42− shifts the lower consolute boundary curve of P123 toward lower temperature, which disturbs the cooperative templating mechanism and results in a less ordered mesopore arrangement. SBA-15Si(HIPE) Monolith Mechanical Properties. Finally, we have performed some preliminary mechanical tests over the SBA-15xSi(HIPE) materials (Figure 8). From Figure 8 we can see classical mechanical compression curves bearing the elastic behavior at low strain (below 0.1) and the plastic domain at higher strain. The Young moduli calculated with the slope of the linear elastic domains are, respectively, 1 MPa for the classical Si(HIPE) obtained with TTAB as surfactant and 0.5 and 0.15 MPa for the SBA-150.5Si(HIPE) and SBA-151.0Si(HIPE) macrocellular monolithic foams. What is very interesting is to note that these mineralized foams do not collapse even after a strain of 40%, and the overall behavior while increasing the strain is elastic at low strain, a pseudoplastic domain at intermediary strain, and compaction at high strain. We cannot explain yet this specific mechanical behavior, and further mechanical studies are needed here. Also, The Si(HIPE) series is already offering advantageous “out of the box” catalytic properties,42 and in the near future, it would be very interesting to compare the SBA15-Si(HIPE) catalytic properties when functionalized with metallic nanoparticles or enzyme both with conventional Si(HIPE) and traditional mesoporous materials.43,44

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b04483. Materials monolith versus powdered states while varying the salt concentrations and salt effect on the starting emulsions (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(R.B.) E-mail: [email protected]; [email protected]. fr. *(J.-L.B.) E-mail: [email protected]. ORCID

Jean-Luc Blin: 0000-0002-0947-2552 Rénal Backov: 0000-0001-5946-8917 Present Address

# (R.B.) Department of Civil and Environmental Engineering, Massachusetts Institute of Technology (MIT), 77 Massachusetts Avenue, Office 1-382, Cambridge, Massachusetts 02139, United States.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was financed by the ANR Project No. ANR-15CE07-0023: Intensified & Sustainable Enzymatic Acylation Processes on Innovative Macroporous/Mesoporous Materials. The authors would like to thank Professor Serge Ravaine for fruitful discussions.

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CONCLUSION For the first time, monolithic materials have been obtained exhibiting both the SBA-15 highly ordered mesoporosity and the macroporosity originating from the initial emulsion fingerprints. These materials have been synthesized by combining the sol−gel process, the elaboration of dodecanein-water emulsions, and the ternary phase behavior of Pluronic (P123)/dodecane/water in the presence of (NH4)2SO4. Indeed, we could combine the properties of both kinds of materials. We evidenced the importance of the salt chemical nature. We demonstrated through the use of (NH4)2SO4 that the ammonium gives cohesion to the material and is therefore necessary to obtain a self-standing monolith, while SO42− triggers both the mesoscale organization and the morphology of the macroscopic voids due to its kosmotrope effect. The SBA-15Si(HIPE) series are highly porous (porosity between 90 and 95%) and exhibit lower bulk and skeleton densities than

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REFERENCES

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First Macro-Mesocellular Silica SBA-15-Si(HIPE) Monoliths

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