Selectively Tuned Pore Condensation and Hysteresis Behavior in

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Selectively Tuned Pore Condensation and Hysteresis Behavior in Mesoporous SBA-15 Silica: Correlating Material Synthesis to Advanced Gas Adsorption Analysis Remy Guillet-Nicolas, François Bérubé, Matthias Thommes, Michael T. Janicke, and Freddy Kleitz J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06745 • Publication Date (Web): 28 Sep 2017 Downloaded from http://pubs.acs.org on September 29, 2017

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The Journal of Physical Chemistry

Selectively Tuned Pore Condensation and Hysteresis Behavior in Mesoporous SBA-15 Silica: Correlating Material Synthesis to Advanced Gas Adsorption Analysis Rémy Guillet-Nicolas,a,b,c François Bérubé,†c Matthias Thommes,b Michael T. Janicke,d and Freddy Kleitz*a,c Department of Inorganic Chemistry – Functional Materials, Faculty of Chemistry, University of Vienna, Währinger Straße 42, 1090 Vienna, Austria b Quantachrome Instruments, 1900 Corporate Drive, Boynton Beach, Florida, 33426, USA c Department of Chemistry, Université Laval, Quebec City, Quebec G1V 0A6, Canada d Chemistry Division, Los Alamos National Laboratory, Mail Stop J514, Los Alamos, New Mexico, 87545, USA a

E-mail: [email protected] ABSTRACT: As regard to the design of SBA-15 silica, substantial efforts were deployed in the last decade in order to understand the mechanism of formation and the effects of the different synthesis conditions on the structure and porosity of the resulting materials. However, better insights into both the tailoring and the characterization of the pore structure of such mesoporous materials are still needed in order to enable the accurate control of adsorption and pore condensation properties in SBA-15. For this, the influence of the synthesis parameters on the properties of SBA-15 silica must be rationalized in terms of their implications for pore architecture, i.e., pore structure and network interconnectivity. Herein, it is demonstrated that pore condensation and hysteresis behavior of inert gases in subcritical conditions confined in ordered mesoporous SBA-15 silica can precisely be modulated as function of the synthesis parameters. Synthesis conditions were found for generating SBA-15 samples that can be described entirely as a pseudo one-dimensional (1-D) pore system (i.e., pore condensation and hysteresis behavior is an intrinsic property of the liquid-vapor transition in a finite volume in agreement with the independent open pore model). However, the data also revealed that distinct synthesis conditions allow for the preparation of SBA-15 with pore condensation properties affected by cooperative pore network effects, mimicking the behavior observed for materials with a pristine three-dimensional (3-D) interconnected pore network topology, such as KIT-6 silica. Therefore, this comprehensive study shows that SBA-15 should be best regarded as a family of solids with easily adjustable porosity, ranging from corrugated and/or distorted pore systems to highly interconnected networks of channels. The effect of each different synthesis parameters on the final pore size of SBA-15 was carefully monitored and a threshold acid concentration range for optimal pore size variation was found. In addition to substantial progress in the SBA-15 synthesis, such in-depth characterization of a “model” ordered mesoporous material coupled with advanced application of state-of-the-art NLDFT methods is of prime importance both for the development of fundamental research on the topic and the applications requiring tailored high surface area materials with selectively tuned pore structure.

INTRODUCTION Current research efforts in the field of nanoporous materials aim at the design of new functional materials, which exhibit easily adjustable pore structure and desired functionalities.1-7 Among nanostructured porous materials, block copolymer-directed ordered mesoporous silica materials, e.g., SBA-15-type silica and related materials,8-11 have become increasingly important owing to wide perspectives of applications, such as selective sorbents,12,13 catalyst supports,14,15 hosts for nano-objects,16 biomedical devices17-19 or advanced systems for optoelectronics and sensing.20 In addition, these large pore silicas are now routinely used as solid templates to generate varieties of non-siliceous mesostructured materials (i.e., the “nanocasting” process).5,21-26 Mesoporous SBA-15 silicas are commonly synthe-

sized under aqueous acidic conditions using triblock copolymer, e.g., Pluronic P123, as a structure-directing agent (SDA), in combination with a silica source, e.g., tetramethoxysilane, tetraethoxysilane, or sodium silicates. Under these conditions, cooperative self-assembly of polymerizing silica species and the block-copolymer micelles leads to the formation of a material consisting of a hexagonal arrangement of open cylindricallike pores. The resulting mesoporous material is thus relatively similar to MCM-41 silica,27 however, the pore dimensions and the wall thickness are usually much larger. Pristine SBA-15 materials exhibit large pores ranging from 5 up to 15 nm, which allow inclusion of bulky guest species, such as macromolecules and polymers, nanoparticles, enzymes, high molecular weight drugs, and ideally provide enhanced adsorption and diffusion properties. Furthermore, the thick silica walls of SBA-15 pro-

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vide improved thermal, hydrothermal and mechanical stability.28-31 In addition to these distinctions, SBA-15 exhibits another major difference compared to MCM-41 originating from the interaction of the polyethylene oxide (EO) chains of the block copolymer with the polymerizing silica species during the material formation, which ultimately results in the occlusion of some EO groups inside the silica walls (see scheme 1). Therefore, after removal of all organics from the hybrid material, the resulting mesoporous silica exhibits a secondary pore system in its framework wall (i.e., intra-wall pores). These intra-wall pores are usually in the micropore/small mesopore range.28-39 Nevertheless, the properties of the hybrid block copolymer-silica SBA-15 system in its mother liquor are strongly temperature-dependent. After precipitation of the mesophase, subsequent aging under hydrothermal conditions is often performed to consolidate the mesostructured framework. During this process, the mesopore size and the mesopore volume can be substantially enlarged by applying increasing aging temperature from 40 up to 130 °C.36,37 This evolution of the SBA-15 pore structure upon hydrothermal treatment was substantiated using various methods, such as advanced gas adsorption measurements, X-ray diffraction (XRD)modeling, high resolution transmission electron microscopy (TEM), nanocasting methods, etc. From these studies, it was concluded that SBA-15 material prepared at low aging temperature, e.g., 35-60 °C, consists mainly in independent cylindrical mesopores surrounded by a microporous corona.37 With higher aging temperatures, larger intra-wall pores form, which leads to connections between adjacent main mesopore channels. At the highest aging temperatures, e.g., > 120 °C, segregation between denser silica domains and large hydrophobic block copolymer ones is maximized, yielding a fully interconnected 3-D network of large mesopores, with no apparent microporosity remaining. Therefore, three main regimes of pore structure may be distinguished.36,37 In terms of N2 or Ar physisorption at liquid nitrogen (-196 °C) or argon temperature (-186 °C), respectively, high quality SBA15 materials exhibit a type IV isotherm with a perfectly welldefined H1 hysteresis loop.40-42 Steep capillary condensation occurs at high relative pressure indicating large and well-defined mesopores (> ∼ 4 nm).42-47 As summarized in the 2015 IUPAC recommendations regarding the application of physisorption,42 important advances in understanding the adsorption and phase behavior of wetting fluids confined in ordered nanoporous materials have been achieved over the last decades, leading to major progress in the physisorption characterization methodology. As a result, using this affordable and non-destructive characterization tool, one can now easily access a much more realistic Scheme 1: Schematic representation of the interpenetrated hybrid P123-silica mesophase which is the precursor of SBA-15 (a) and TEM image of typical SBA-15 material after block copolymer removal by calcination (b).

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and accurate description of the porous features of nanoporous materials, and especially, ordered mesoporous ones with uniform, tailor-made pore structure, such as SBA-15.42,43,46-51 In such mesoporous materials, two principal factors usually determine the hysteretic behavior: (i) hysteresis on the level of a single pore of a given shape (i.e., pore model), and (ii) cooperative effects due to the specifics of connectivity of the pore network. On the pore level, standard SBA-15 isotherms can be reasonably well described by the so-called independent pore model, i.e., the experimental isotherm is seen as a weighted sum of isolated (not connected) single pore isotherms in which multilayer formation occurs, followed by pore condensation and hysteresis. According to this independent pore model, hysteresis is solely caused by the fact that capillary condensation in large open cylindrical pores is delayed as compared to the pressure of the equilibrium gas-liquid transition.52-54 Indeed, a classical scenario of capillary condensation implies that the vapor-liquid transition is delayed due to the existence of metastable adsorption films and hindered nucleation of liquid bridges. Therefore, energy barriers need to be overcome in order to nucleate droplets in the pores.42,46-48 On the other hand, in an open cylindrical pore filled by liquid-like condensate, the liquid-vapor interface is already present and capillary evaporation/desorption occurs without nucleation, via a receding meniscus.40-42,44,45,48-50,55,56 Consequently, the desorption process is associated with the equilibrium vapor-liquid transition, being the underlying reason for the routine selection of the desorption branch for pore size analysis. However, modern theoretical and computational methods which are based on the statistical mechanics of nanophases, such as the non-local density functional theory (NLDFT), are capable of qualitatively and quantitatively predicting, at the molecular level, the adsorption metastabilities as well as the resulting pore condensation and hysteresis behavior of confined fluids in pores.42,45,48,55-65 Therefore, one may now access reliable pore size analysis from both branches of the hysteresis. Importantly, the validity and accuracy of NLDFT methods for pore size analysis of various nanoporous materials, including MCM-41 and SBA-15, have been thoroughly verified against other methods (for recent reviews, see references 45 and 48-51). For the physisorption characterization of ordered mesoporous materials, an experimental isotherm measured for SBA-15, for instance, may be compared to sets of theoretical NLDFT isotherms in order to generate the best re-calculated isotherm, matching the experimental data. From this “best-fit” simulated isotherm, a cumulative pore volume plot is calculated. Pore size distribution (PSD) is subsequently obtained as the derivative plot of the NLDFT cumulative pore volume curve. Practically, the sets of theoretical isotherms, also designated as kernels, correspond to a series of theoretical isotherms calculated over a predefined pore size range taking into account the nature of the adsorptive (e.g., N2, Ar, etc.), the temperature of the analysis (e.g., 77.4 K, 87.3 K, etc.), the chemical composition of the adsorbent (silica, carbon) and an idealized shape of both the micro- and mesopores (e.g., cylindrical, spherical, slit-like, and combinations of them). Various NLDFT kernels are now commercially available, accurately describing both the equilibrium liquid-vapor transition and the metastable adsorption process for many systems. It must be mentioned that the application of state-of-the-art NLDFT methods provides an accurate evaluation of the pore size distribution only if the nanoporous system under investigation is realistically compatible with the chosen kernel. If it is not consistent with the physicochemical features

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of the adsorbent including the pore shape, the nature and temperature of the adsorptive, the subsequently calculated NLDFT pore size distribution will not be pertinent. This latter point is of tremendous interest for our present study as a major part of the recent progress in physical adsorption characterization of nanoporous materials has been achieved thanks to the practical use of highly ordered and tailor-made mesoporous materials with simple geometries of known pore size, such as SBA-15. Therefore, if the pore system of the investigated SBA-15 is really described within the independent pore model (in terms of pore condensation, i.e., no network effect, see ref. 48), both the NLDFT PSD calculated from the adsorption branch using a kernel of metastable adsorption isotherms and the NLDFT PSD from the desorption branch using a kernel of equilibrium transition isotherms should overlap affording same pore size value, as illustrated on Scheme 2, and it is indeed the case for many SBA-15 samples reported in the litterature,35,42-44,48-50,66,67 Here, pore condensation and hysteresis behavior is solely caused by metastability of the adsorbed film occurring during the adsorption and is thus an intrinsic property of the liquid-vapor transition in a finite volume, in agreement with the independent open pore model.42,43,46-49 In this particular case, this behavior suggests that although microporosity and/or intra-wall porosity could be present in the sample, these additional smaller pores do not seem to influence the process of pore condensation. SBA-15 silica can be prepared under quite different synthesis conditions, which may ultimately lead to noticeably different pore topology.36,37 In addition to aging time and temperature, it has also been demonstrated that parameters, such as choice of the surfactant (e.g., Pluronic P104, P123, F127), solution pH, synthesis temperature and time, nature of silicon source, reagent ratio, additives (e.g., salts, co-surfactants, co-solvents, swelling agents), stirring speed, etc., could all be used to influence the mesophase formation and tailor the final material properties, especially regarding mesoscopic ordering, pore size, intra-wall porosity, and particle morphology.28-39,68-109 As such, it is not clear whether the independent pore model used in physisorption should remain valid in all instances for precise characterization of SBA-15 materials, or, in other words, if the NLDFT models are always proven accurate for pore size analysis of any SBA15-type materials, regardless of the synthesis conditions employed. For the synthesis of SBA-15 silicas, one of the key parameter is the acidity of the synthesis medium. Indeed, hydrolysis and condensation rates of silica precursor species are known to be strongly pH dependent110,111 and to vary in the presence of P123 micelles.108 The HCl concentration (≤ 2 M) has a minor impact on the micelle shapes in the solution109 but as expected, as soon as the inorganic precursor is added, faster precipitation of the mesophase will occur in highly HCl concentrated medium.100,102,104,108 SBA-15 materials were synthesized reproducibly with various acid concentrations (mostly HCl) ranging from 0.2 M to 2 M.36,37,100,108,112 However, some noticeable differences in the structural order, particle morphology and porosity of the final material were observed.88,98-100,108,112 The classical synthesis performed at 1.6 M HCl8 led to a very fast precipitation and may not be optimal for mesophase control and optimization, especially if the synthesis temperature is high.90,98,103,104 On the other hand, if the acidity is lowered, slower kinetics are observed and mesostructure can be tailored in more efficient ways.98,100,108,112 Control on the precipitation and mesophase condensation steps enables, for example, to tune the degree of interconnectivity and wall thickness of the

Scheme 2: a) Typical N2 adsorption-desorption isotherm of SBA-15 silica measured at -196°C and b) Corresponding NLDFT pore size distributions in line with the independent pore model (in terms of pore condensation), inset shows a schematic representation of independent open cylindrical pores.

calcined material.100,112 Ionic strength of the media and addition of salts, usually added in order to obtain a more pronounced “salting in” or “salting out” effect, have been shown to affect as well mesophase formation kinetics, intra-wall porosity and final particle shapes.75-77,79,106,108,113 For example, addition of NaI at specific timing during the synthesis (i.e., optimal time was 4 h into the synthesis) was also found to affect the final intra-wall porosity of SBA-15 while preserving the main mesoporosity.106 Despite the impressive amount of work dedicated to this topic over the recent years, better insights and correlation into both the tailoring of the mesophase and the characterization of the final pore structure of SBA-15 materials are still needed. For this, the influence of synthesis parameters on the properties of the final mesoporous silica must be rationalized, especially regarding the impact on the pore network architecture, i.e., pore structure and network interconnectivity. As explained above, such modifications in the pore topology can be conveniently and precisely monitored with gas adsorption measurements combined with state-of-the-art NLDFT pore size analysis methods. In the following, it will thus be demonstrated that pore condensation and hysteresis behavior of inert gases confined in SBA-15 silicas can precisely be modulated as function of synthesis parameters without the need for any additives, such as salts, swelling agents, polymers or co-surfactants. Therefore, we synthesized SBA-15 silicas under various conditions and the



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obtained as-made and calcined materials were scrutinized using dynamic light scattering (DLS), solid state 29Si MAS NMR, thermogravimetric analysis (TGA), scanning electron microscopy (SEM),TEM and powder XRD. State-of-the-art physisorption experiments were also performed, coupled with advanced gas adsorption analysis, in order to allow in depth investigation of pore condensation and hysteresis behavior of nitrogen at 77.4 K and argon at 87.3 K in the calcined silica samples. Our results bring new insights into the interplay between synthesis parameters and material porosity and structure. This in turn offers a more thorough and comprehensive understanding of the SBA-15 family, ranging from the choice of the synthesis parameters to the final control of the porous architecture. It also opens vast opportunities regarding simple and reproducible “on-demand” synthesis of SBA-15 materials with porous features specifically designed and tailored for a targeted need. Finally, the control of adsorption and pore condensation properties in SBA-15 provides an efficient tool to further test the validity and accuracy of most recent NLDFT methods for pore size analysis. EXPERIMENTAL SECTION Materials. Series of high quality mesostructured SBA-15 materials were synthesized following the procedure reported by Choi et al. in 2003.100 SBA-15 samples were prepared under various synthetic conditions using aqueous solution of Pluronic P123 triblock copolymer (EO20PO70EO20, MW = 5800, SigmaAldrich) and hydrochloric acid (HCl, Fisher Scientific, 37.5%). In all cases, tetraethoxysilane (TEOS, Sigma-Aldrich 98%) was used as the silicon source. The molar composition of the starting reaction mixture was varied in the range of 0.022 P123/x TEOS/y HCl(37.5%)/z H2O, with x = 0.67 − 2.0, y = 0.23 − 4.7, and z = 132 − 111. y and z were always varied accordingly to keep the synthesis volume constant. The reaction temperature (RT) was varied from 30 °C to 40 °C and hydrothermal temperatures (HT) were varied from 60 °C to 140 °C. A typical synthesis (x = 1.0, y = 0.7 and z = 130) can be briefed as follows: 8.0 g of Pluronic P123 was dissolved in 146.25 g of distilled water and 4.5 g of HCl (37.5 %) under vigorous stirring. After complete dissolution, 13.0 g of TEOS was added at once to the homogeneous clear solution. This mixture was further left under stirring at RT = 35 °C for 24 h. Then, the synthesis mixture was placed in an oven at HT = 80 °C for another 24 h under static conditions. Afterward, silica products were filtered hot, dried at 100 °C for 2 h and then overnight at 140 °C. For template removal, the as-synthesized silica powders were first shortly slurred in an ethanol-HCl mixture and were subsequently calcined at 550 °C (5 h under air flow ramp followed by a 3 h isothermal segment). SBA-15 samples are referred as to “w-x-y-z” where w refers to the acid concentration of the mother liquor (0.1 – 2.0 M), x refers to the TEOS/P123 molar ratio (r = 30 – 90), y refers to the reaction temperature (RT = 30 – 40 °C) and z refers to the hydrothermal treatment temperature (HT = 60 – 140 °C). For comparison purpose, KIT-6 silica materials were synthesized following the procedure reported by Kleitz et al.114,115 Typically, 9.0 g of P123 was dissolved in 325 g of distilled water and 17.5 g of HCl (37.5%) under vigorous stirring. After complete dissolution, 9 g of n-butanol (99.9%, Fluka) was added to the mixture and after one hour, 19.4 g of TEOS was added at once to the homogeneous solution. The next synthesis and thermal treatment steps were identical to those of the SBA15 samples. Nanocast cobalt oxide samples were synthesized according to a previously published procedure.116 Typically, 0.3

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g of calcined mesoporous silica (pore volume = 1.2 cm3/g) was dispersed in 10 mL of ethanol for about 10 minutes followed by the quick addition of 582 mg (2 mmol) of Co(NO3)2∙6H2O (Alfa Aesar). The mixture was stirred for 2 h at room temperature and subsequently dried overnight at 40 °C. The dry material was further calcined at 350 °C for 5 h under air flow. The material was impregnated a second time in 10 mL ethanol with 291 mg (1 mmol) of Co(NO3)2∙6H2O and the solution was allowed to stir for 3 h. After overnight drying at 40 °C, the material was calcined at 450 °C for 5 h under air flow. For silica removal, the cobalt oxide-silica composite was dissolved in 10 mL of 2 M NaOH for 12 h under stirring followed by a 5 minutes centrifugation at 5000 rpm. These steps were repeated twice with fresh basic solution to insure efficient silica removal. Finally, nanocast cobalt oxide powders were washed with water and dried overnight at 80 °C under air. Characterization. The SBA-15 samples shown in Figure 1 were analyzed by powder X-ray diffraction (XRD) recorded on a Stoe STADI P θ-θ X-ray diffractometer in reflection geometry (Bragg-Brentano) using Cu-K1+2 radiation with secondary monochromator and a scintillation detector (MPI für Kohlenforschung, Mulheim an der Ruhr, Germany). XRD patterns of Co3O4 replicas shown in Figure 13 were recorded on a Rigaku Multiplex instrument operated at 2 kW, using Cu-Kα radiation (KAIST, Daejeon, Republic of Korea). The XRD scanning was performed under ambient conditions in 2θ steps of 0.01°, with an accumulation time of 0.5 s. 29Si solid-state NMR measurements were recorded on a Bruker Avance 400 MHz spectrometer with a wide bore superconducting magnet for solid-state magic angle spinning (MAS) NMR (4 mm zirconia rotors spinning at 10000 Hz). The data were collected with a π/2 pulse of 6.0 microseconds and high-power 1H decoupling. There was a delay time of 60 seconds between each acquisition for T1 relaxation and the number of acquisitions was in general at least 3000 scans. Scanning electron microscopy (SEM) images were obtained using a JEOL JSM-840-A microscope operated at 15 kV. Prior to imaging, solid samples were deposited on metallic support and covered with a layer of gold and palladium. High-Resolution SEM (HRSEM) images were obtained with a FEI Magellan 400 at a low landing energy (1.0 kV), without metal coating (KAIST, Daejeon, Republic of Korea). Transmission electron microscopy (TEM) images of mesoporous cobalt replicas were obtained using a JEOL JEM-1230 electron microscope operated at 80 kV. The solid samples were first dispersed in methanol, then deposited on a carbon-coated copper grid. Evolution of the mesophase formation was monitored by dynamic light scattering (DLS) using a Malvern DTS Nano zetasizer 173° (equilibration time set to 10 s; 3 measurements taken on each sample). TGA-differential thermal analysis (DTA) measurements of the as-made SBA-15 materials were performed using a Netzsch STA 449C thermogravimetric analyzer. The analyses were carried out under air flow (20 ml/min) with a heating rate of 10 °C/min. In all analyses, a nitrogen flow (20 ml/min) was used as a protective gas. Advanced porosity analysis. The calcined silica powders were characterized by N2 and Ar adsorption/desorption isotherm measurements at -196 °C and -186 °C, respectively, using an Autosorb-1C adsorption instrument. Prior to the measurements, the inorganic samples were outgassed under vacuum at 200 °C for at least 20 h. Specific surface area, SBET, was determined using the BET equation117 in the range 0.05 ≤ P/P0 ≤ 0.30, and total pore volume was obtained at P/P0 = 0.95 using the Gurvitch rule. Relevant pore size distribution were obtained both from the adsorption and the desorption branch of SBA-15 isotherms


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by applying the kernel of (metastable) NLDFT adsorption isotherms and the kernel of (equilibrium) NLDFT desorption isotherms, respectively, considering an amorphous SiO2 surface and a cylindrical pore model.35,43,45 Total pore volume, specific surface area and micropore volume were also determined using the kernel of (metastable) NLDFT adsorption. The NLDFT calculations were carried out using the Autosorb 1.55 software provided by Quantachrome Instruments, Boynton Beach, FL, USA. For comparison purpose, the micropore volumes were additionally determined using the t-plot method in the range 0.15≤ P/P0 ≤ 0.4. RESULTS Acid concentration and mesophase formation. The synthesis of SBA-15 silica is performed under various concentrations of HCl in aqueous solution, using P123 as the SDA and TEOS as the silicon source. After addition of TEOS to the synthesis mixture, the solution is held at a given temperature (ranging from 30 to 40 °C). Synthesis and aging times were kept constant as well as the stirring speed. No additives were used. This mixture is left under stirring 24 h in total during which a white precipitate forms. Then, the mixture is aged for 24 h in static conditions at temperatures ranging from 60 to 140 °C, after which the solid is recovered by vacuum filtration, without washing, and dried. The block copolymer template is removed by a brief ethanol/HCl extraction followed by calcination under air at 550 °C. This synthesis method offers some advantages over other procedures reported for the preparation of SBA-15, as these conditions afford excellent reproducibility, very high structural order, mesophase purity, and easy scale-up.100,112,114,115,118 The resulting calcined silica materials were first characterized by powder XRD in order to confirm the two-dimensional (2-D) hexagonal nature of the pore arrangement. Figure 1 shows representative low-angle XRD patterns measured for some of the typical samples synthesized under different conditions. Although some marked differences can be immediately noticed on the diffraction patterns depending on the synthesis conditions (i.e., variation in the peaks position, intensity and width), all the diffractograms of the template-free materials indicate excellent structural order with the symmetry of the mesophase being commensurate with the 2-D hexagonal p6mm symmetry. As expected, materials aged at higher temperatures exhibit sharper and thinner peaks, especially for the (100) one (Figure 1b).36,37 One of the key parameters in the synthesis of SBA-15 is the acid concentration of the reaction medium. In the first report of SBA-15 synthesis, a 1.6 M HCl concentration was used.8 In the present study, we selected a modified synthesis using various acid concentrations, i.e., 0.1 - 2 M hydrochloric acid.100 The pronounced influence of acid concentration is well-demonstrated by comparing the different rates of precipitate formation observed as a function of the HCl concentration. “Precipitation time” as it will be discussed in this report reflects the overall kinetics of the material formation which starts with the pH-induced hydrolysis of the inorganic precursor (TEOS) in a micellar solution, immediately and concurrently followed by the socalled cooperative self-assembly process, that includes three main steps that should not be seen as successive reactions but rather as a dynamic association: The micellar sphere-to-rod transformation due to silica oligomers/polymers adsorption on Pluronic micelles, the nucleation or colloidal aggregation of hybrid micelles/flocs and finally, further growth and polycondensation of the hybrid mesophase.75-77,79,91-97,103,104,113,119-127

Figure 1. Low-angle powder XRD patterns obtained for calcined SBA-15 samples synthesized under various conditions and aged at low (a) and high (b) temperatures.

Over the last decade, an substantial amount of work was dedicated to the use of in situ and ex situ techniques such as small angle X-ray scattering (SAXS), small angle neutron scattering (SANS), NMR, EPR, electron microscopy (HRTEM and cryoTEM), or photon correlation spectroscopy (PCS or DLS), to elucidate the early stages of SBA-15 formation (for recent reviews, see references 103, 104, 113 and 127). Even though the general formation mechanism of SBA-15 is fairly well understood, there are still some open questions. The role of the “flocs” is especially discussed. According to recent reports, where in most cases P104 was used as a SDA, combined with a 1.6 M acidity, the nucleation of the mesophase was triggered by the association of siliceous spherical micelles into flocs, that were unambiguously observed using cryo-TEM.105 While these flocs are growing, the micelles within will rearrange and grow into cylindrical ones. Once the associated flocs containing cylindrical micelles reached a certain size (around 400 nm), they will transform into anisotropic particles with 2-D internal hexagonal symmetry.103 Oppositely, these flocs were not observed using SAXS in other studies where Pluronic P123 and different acid concentrations were used for the synthesis of the SBA-15 materials at 20-60 °C.108 Here, the proposed mechanism involved the reorganization of spherical micelles into bundles of threadlike micelles. This transition was driven by the adsorbed silica oligomers/polymers and the condensation of silica ultimately causing the nucleation of the 2-D hexagonal phase followed by the growth and the precipitation of the material.108,113 A recent study performed in collaboration by the authors of the above cited reports seems to unify these two mechanisms and shed some interesting light on the role of the flocs:104 The vast majority of the micelles in solution undergo a sphere-to-rod transition owing to the presence of adsorbed silica oligomers/polymers, forming hybrid cylindrical micelles that remain free in solution. However, in the same time, some local areas in the mother liquor contain (hybrid) micelles and silica species at high volume fraction, i.e., the flocs. These “spots” of higher micellar density may then act as nucleation centers which trigger the large-scale formation of the material. In the present study, we do not investigate the formation mechanism of SBA-15 materials in situ and precipitation is simply defined as the time where the synthesis mixture shifts from transparent to an opaque white colored powder due to the phase separation and precipitation of the silica-based hybrid



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mesophase. The results of the visual inspection of the onset of precipitation time (± 5 min.) estimated for different SBA-15 synthesis conditions, e.g., synthesis temperature (RT) and reagent molar ratio (TEOS/P123), are plotted in Figure 2 as a function of the acid concentration. Obviously, the precipitation rate is strongly dependent on the synthesis conditions. As expected, increasing the acid concentration led to a faster mesophase formation.68,108,119,124 This phenomenon is also well known in solgel chemistry.128,129 In the same way, increasing synthesis temperature will also accelerate the overall precipitation kinetics,90,93,101,108 as shown in Figure 2b. Furthermore, the evolution of the precipitation times is very similar for the two different synthesis parameters studied, that is, an increase in the synthesis temperature (Figure 2a) resulted in a faster precipitation, and so did an increase of the TEOS/P123 ratio added to the system (Figure 2b). Increasing concentration of available alkoxysilanes in the medium will lead to a faster polymerization rate and a quicker formation of adsorbed oligomers, large enough to trigger the micellar aggregation.90,108,111,113,121-123 On the other hand, a too large addition of TEOS can also lead to a small fraction of the material being not ordered.108 However, it should be noted that if one smartly tunes the other synthesis parameters, it is still possible to obtain a highly-ordered SBA-15 material even with a high r ratio of 90, as shown in Figure 1. The lower HCl concentration range emphasized by the dotted areas in Figure 2a and 2b is of particular interest as it represents a “synthesis zone” where the mesophase formation is substantially slower. One may therefore use this larger timeframe window to further alter and control the mesophase formation process. To supplement the visual observations, dynamic light scattering (DLS) measurements were performed. As aqueous colloidal solutions or suspensions of P123 spherical micelles are initially stable,109 they are suitable for DLS measurements. Information regarding precipitation time can be obtained by analyzing the correlogram function of the system. After silica precursor addition, the time at which the decay in the correlation starts to dramatically fluctuate and lose its steepness (due to nucleating, growing or flocculating particles, increased polydispersity, absorbance, and so on) can be related to the precipitation of the hybrid mesophase. Figure 2c presents the precipitation time determined by DLS ( 3 min due to start-up routine and equilibration conditions) as a function of the HCl concentration in the medium. Similar results were obtained as compared to visual observations, even if, as expected, DLS analyses detected sooner the onset of precipitation. For 2.0 M and 1.5 M, for both TEOS/P123 equal to 45 or 60, which correspond to original SBA-15 synthesis conditions essentially, precipitation time given by DLS is in good agreement with previously reported results.90,91-95,108 From the kinetic point of view, we observed that an increase of the HCl concentration by a factor 2 is similar to about a +10 °C increase of the synthesis temperature, both effects inducing a faster silica condensation. In contrast, complete condensation of systems containing less acid (0.1 - 0.3 M) lasted much longer due to a more important competition between hydrolysis, nucleation/micellar aggregation and condensation kinetics leading to a general deceleration of the whole formation process.102,103,108,113,128,129 DLS analyses also confirmed the apparent HCl concentration threshold, located at ca. 1.0 M, beyond which the other synthesis parameters did not affect noticeably the precipitation rate and, hence, the overall mesophase formation. Indeed, in addition to clearly accelerate the hydrolysis of TEOS molecules, very high acid concentration will drastically increase the ionic strength of the mother liquor. Salting out effect of the Cl- anion will then be more pronounced

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Figure 2. Estimated precipitation time as a function of the acidity of the mother liquor investigated by visual inspection (a,b) and by DLS analyses (c) for different SBA-15 synthesis conditions.


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and could contribute to an extremely fast mesophase formation.79,108 Moreover, no synthesis benefits were reported when working at very high acid concentration (up to 2.5 M).108,109 It therefore seems plausible that a range of “moderate” acid concentration, i.e. below 1 M, could be the most appropriate to enable detailed tuning of the structural and textural properties during the synthesis of SBA-15 materials. Low acid concentrations were indeed successfully proposed for the synthesis of various high-quality large pore ordered mesostructures such as SBA-15, KIT-6, SBA-16 and KIT-5.100,112,114,115,118,130-134 Particularly interesting here is the role of low acidity on the particle morphology. Figure 3 shows the evolution of the particle morphologies obtained for SBA-15 silicas as function of synthesis temperature and r (TEOS/P123) ratio. The HCl concentration was fixed to 0.3M and the aging temperature was set to 80 °C. Magnetic stirring speed was kept constant for all synthesis conditions (about 350-400 rpm). As evidenced, both the synthesis temperature and the amount of silicon source added to the system affected the morphology of the powder aggregates. At 30 °C, large clusters consisting of gyroidal/spheroidal particles were formed whereas larger fiber/rod-like particles were clearly the major shapes obtained at 35 °C and 40 °C. One can note that the length of the elementary blocks constituting the fiber aggregates tends to increase with the temperature.88,90,98 Also, a major impact on the particle morphology was obtained by varying the amount of silicon source introduced during the synthesis step (carried out at 35 °C) as shown in Figure 3a,b,c. When a low r ratio was used (r = 45), aggregates were composed of small (1 - 3 μm) gyroidal/spheroidal particles. With an r ratio of 60, the usual fiber-like morphology was observed, and finally when r ratio was further increased, a mixture between large gyroidal and platelet particles were the two dominant morphologies obtained.88,98

However, one should keep in mind that with the present synthesis conditions (see Experimental section), in all cases the materials exhibited, to some extent, morphological heterogeneities. None of the syntheses yielded perfectly monodisperse and uniform particles on a very large scale. A precise control of particle morphology of SBA-15 silica is, however, not the aim of the present report, as this topic has already been well-documented. For instance, stirring speed and time, which were not investigated in this study, are well-known parameters used for the macroscopic control of particle morphology of mesoporous materials.104 From a general point of view, differences in particle shapes and sizes may be related with the kinetics of the SBA15 formation that have been shown to greatly depend on the synthesis conditions.75-77,81-88,90,98,99,104-106,108,113 SBA-15 particle morphologies were recently rationalized using a simple energetic model, based on the assumption that the final shape and size of the particles are only reached once the thermodynamic equilibrium conditions are achieved. In this model, the two main terms governing the particle shape are the interfacial energies (surface tensions) and the bending energy (in the form of a bending constant).104,107,135 To determine the extent of the silica condensation process, 29Si solid state MAS NMR studies were conducted on SBA-15 materials prepared at 35 °C. Figure 4a shows NMR spectra of asmade, non-aged SBA-15 materials for different HCl concentrations in the mother liquor. All spectra consist mainly of three signals, at δ = -93, -103 and -113 ppm, corresponding to the Q2, Q3 and Q4 units (Qx = Si(OSi)x(OH)4-x), respectively. The appearance of Q4 signal indicates the formation of the fully condensed silica species (Si(OSi)4). One can note that Q4 is, as expected, not the dominant peak in all the spectra but it represents a noticeable amount, which is understandable since materials were synthesized for 24 h. However, as the SBA-15 materials were not hydrothermally treated, silica condensation is incomplete and many silanol groups are still remaining. The relative intensity ratio Q3/Q4 is not dramatically varying in the 0.2 M – 0.6 M range, whereas a marked decrease takes place between 0.6 M and 1.0 M indicating amore condensed material (Figure 4c, blue symbols). Most interestingly, the ratio is almost constant after 1.0 M confirming the impact of high HCl concentration on the silica condensation during the early synthesis stage. Moreover, the Q3/Q4 ratio found for as-made SBA-15 materials synthesized with high HCl content (1.0 – 2.0 M) is consistent with data from the litterature.90 Figure 4b shows NMR spectra of same SBA-15 samples after an aging step performed at 80 °C for 24 h. A substantial decrease in all Q3/Q4 ratios confirmed the further silanol condensation occurring during the hydrothermal treatment in the mother liquor. With SBA-15 materials hydrothermally aged for 24 h, the decrease in the Q3/Q4 ratios is not significantly varying above 1.0 M, following the same trend observed for as-made materials, as well as corroborating the results obtained by DLS. Major changes are however found in the 0.2 – 0.6 M range (Figure 4c, red symbols). Nevertheless, in both cases, the presence of small Q2 signal is indicative of a partial cleavage of the Si-O-Si bonds and re-hydroxylation. After calcination, the differences between the 29Si MAS NMR spectra of SBA-15 materials vanished and only broad signals corresponding to the Q3 and Q4 units are present (see Figure S1), confirming the extensive condensation of the framework at elevated temperatures.90

Figure 3. SEM images of 0.3-x-y-80 samples with x = 45, y = 35 (a), x = 60, y = 35 (b), x = 75, y = 35 (c), x = 60, y = 30 (d), x = 60, y = 35 (e) and x = 60, y = 40 (f).



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Figure 4. 29Si MAS NMR spectra of non-aged (a) and hydrothermally treated (b) SBA-15 materials synthesized with different HCl concentrations at 35°C, and Q3/Q4 ratio evolution as a function of the HCl concentration for non-aged (circles) and aged (squares) samples (c).

Figures 5a and 5b show TGA measurements of a series of SBA15 materials synthesized with different r ratios and HCl concentrations, respectively. An increase in the TEOS quantity, keeping all other synthesis parameters constant, leads to a diminution in the total mass loss of the as-made material. This observation is consistent with the higher amount of silica being condensed during the synthesis and aging steps. A denser material, at least from a bulk point of view, is thus-obtained after filtration since the relative quantity of P123 per gram of as-made silica is lowered. It should be noted that samples described in Figure 5 all led to high quality SBA-15 materials with very wellordered pore structure. It was shown previously that very high r ratios, e.g., twice more as compared to the classical synthesis (r = 45), while keeping the other synthetic parameters constant, could result in a small fraction of the material being disordered.108 Figure 5b shows that a similar trend in the mass loss is observed for materials synthesized with increasing HCl concentration in the synthesis medium. A higher quantity of HCl will favor a denser material containing less P123 per gram of ‘asmade powder’. Furthermore, increase in both TEOS quantity and HCl concentration did not affect the TGA curve profile. Degradation and thermal removal of the copolymer occurred at similar temperature and followed the same pathway as evidenced by the corresponding DTA curves (see Figure S2). Porosity and NLDFT pore size analysis. Representative examples of typical N2 physisorption isotherms, measured at 196°C, of calcined SBA-15 silicas synthesized within the lower range of acidity (0.2 - 0.6 M) and with varying TEOS/P123 molar ratios (r = 30 – 90) are shown in Figure 6a and 6b, respectively. Physicochemical parameters extracted from the isotherms are presented in Table S1. As immediately visible, all the materials, excepted the ones synthesized with r =30 and 90 (Figure 6b) that will be discussed in more details below, exhibit type IV isotherms with H1 hysteresis loop showing steep capillary condensation and evaporation steps at high relative pressures, being characteristic of high quality large pore ordered SBA-15 silicas. All these isotherms are very similar, with only slight variations in adsorption capacities and position of the capillary condensation/evaporation steps, the latter being related to the main pore size of the material. Some subtle differences

are also visible when comparing the details of the hysteresis loops. One can observe variations in hysteresis widths and shapes, some being narrower and/or noticeably steeper. Undeniably, such features are related with the pore architecture of the

Figure 5. Themogravimetric analyses (TGA) of 0.3-x-45-100 (a) and w-60-35-80 (b) SBA-15 materials.


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Figure 6. N2 (at -196 °C) adsorption-desorption isotherms of w-6030-90 (a) and 0.3-x-35-80 (b) SBA-15 materials.

of the SBA-15 silicas. It thus becomes interesting to implement state-of-the-art NLDFT methods for pore size analysis of the materials prepared under these different conditions and to com-

pare the results obtained using the kernel of metastable adsorption isotherms (adsorption branch) with the ones obtained using the kernel of equilibrium isotherms (desorption branch). In both cases, the above-mentioned kernels consider isolated open cylindrical siliceous pores, which is clearly the best available assumption for SBA-15 materials. Therefore, the selected kernels should provide the most accurate pore size analysis. As explained in the introduction, on the pore level, existence of the hysteresis loop is here related to the behavior of the vapor-liquid phase transition in the finite volume system represented by the porous network.40-50,52-54,56,59 In open isolated and uniform cylindrical pores of finite length, like in classical SBA-15, metastabilities occur only on the adsorption branch of the isotherm. Consequently, the desorption process is associated with the thermodynamic equilibrium vapor-liquid transition. However, in more complex and realistic pore structures, deviating from ideal cases, the desorption process is often affected by so-called network/structure effects such as pore blocking/percolation or cavitation.42,48 These phenomena occur if larger pores have access to the external surface only through narrower necks. The pore bodies remain filled during desorption until the pore entrances or necks empty at their corresponding (i.e., lower) relative pressures. The desorption pressures are thus dependent on the size and spatial distribution of these necks and may not be representative of the pore structure/size of the probed material. Indeed, if the neck diameter is smaller than a critical size, estimated to be around 5–6 nm for N2 at 77 K, desorption from the pore bodies involves a cavitation mechanism and no valuable pore size information can be obtained from the desorption branch.42-50 In such cases, the only way to obtain an accurate pore size analysis is to use the adsorption branch data applying a theoretical model that consider the adsorption metastabilities (e.g., NLDFT techniques). Such network/structure effects were observed in selectively plugged SBA-15 samples synthesized using different conditions and additives.136-138 From a physisorption point of view, it results in materials exhibiting different hysteresis loops. Instead of classical H1 loops, one may observe H2 (a and b), H3, H4 or H5 ones.42 Such hysteresis loops were also observed in our case as shown in Figure 6b. Sample 0.3-90-35-80, prepared with twice the amount of TEOS (r = 90) as compared to the original Choi et al. procedure (r = 45)100 clearly exhibits an H2a hysteresis, being characteristic of a pore architecture in which network/structure effects are important. The very steep desorption branch can be attributed either to pore-blocking/percolation or to cavitation-induced evaporation. The acid concentration and the synthesis temperature were kept relatively low (0.3 M and 35 °C, respectively) for the synthesis of this material, affecting the TEOS hydrolysis rate as well as the overall kinetics of the nucleation and growth of the SBA-15 structure. As a result, the addition of a large amount of silica precursor led to the formation of a material containing much more silica species into the hybrid mesophase.108 After calcination, it ultimately resulted in a mainly ordered material with corrugated pores and/or heavily distorted pore structure. The presence of small amount of disordered parts can also not be fully excluded. On the contrary, sample 0.3-35-35-80 prepared with very low amount of TEOS (r = 30) displays a H4 hysteresis loop, reflecting a combination of kinetic and thermodynamic effects spanning the complete disordered micro- and mesopore system.42,48 Here, too few silica species were incorporated in the hybrid mesophase and yielded a poorly ordered. In both cases, these two samples cannot be labelled as “true” SBA-15 and demonstrate the limit of synthesis tailoring if only a single pa-



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rameter is modified. However, as it is shown in the XRD patterns in Figure 1b and will be discussed in the following sections, it is possible to obtain a very well-ordered material even with a high r ratio, but several parameters must be modified synergistically. Figure 7 reports the NLDFT mode pore size values obtained for four series of calcined SBA-15 silica samples synthesized under different conditions. The synthesis temperature was fixed at 35 °C. Both pore sizes extracted from the adsorption branch (metastable kernel) and pore sizes obtained from the desorption branch (equilibrium kernel) are plotted. In absence of network effect, one may use proper NLDFT kernel on both branches to obtain accurate and reliable pore size analysis. Synthesis conditions were indeed found for SBA-15 where mode pore size values from adsorption and desorption agree very well, i.e., a perfect overlap of the NLDFT PSD curves was observed, indicative of a pseudo 1-D pore system, in agreement with the independent pore model considered in the NLDFT calculation.. This is for instance the case for SBA-15 samples prepared with a r = 60 at 35 °C and with an aging step performed at 100 °C within the range of HCl concentration of 0.3 – 2 M. Similarly, SBA15 samples obtained with r = 45 and aged at 80 °C showed this behavior within the range of HCl concentration of 0.6 - 1.5 M. Such consistency has also been reported previously for MCM41 and SBA-15 ordered mesoporous silicas.46,52-54 The perfect agreement between the pore size distribution curves obtained from the adsorption and desorption branches also suggests that the intra-wall microporosity28-39 does not affect significantly the pore condensation and hysteresis behavior. On the other hand, there are several conditions leading to inconsistencies between mode pore size values calculated from the adsorption and desorption branches. NLDFT pore sizes obtained from the desorption branches of SBA-15 materials prepared with r = 60, synthesized at 35 °C and aged at 80 °C (Figure 7a, red symbols) were clearly smaller than those obtained from the corresponding adsorption branches, almost over the whole acidity range. In the same way, NLDFT pore size values calculated for samples aged at 100 °C with r = 45 (Figure 7b, blue symbols) did not agree over the entire probed HCl concentration range, with the values derived from the adsorption branches being always noticeably smaller. Such effects were also observed for other SBA-15 series synthesized with lower HCl concentrations (< 0.3 M, not shown). In addition, variations in the absolute mode pore size values were quite remarkable when the HCl concentration was maintained below 0.6 M, in perfect agreement with the assumption of a threshold HCl concentration in the SBA-15 synthesis system, as discussed above. Also, we observed that TEOS:P123 = 1:0.022, in molar ratio (r = 45), afforded a more flexible material in terms of pore network tailoring, irrespective of the aging temperature. In a general way, a systematic shift of either the adsorption or desorption PSD curve was observed upon a given variation of synthesis parameters, e.g., acid concentration, r molar ratio, synthesis and aging temperatures. Such differences are correlated with the nature of the mesopore network topology of the SBA15 materials. Figure 7 provides a rather comprehensive map of different pore condensation and hysteresis behavior in SBA-15 silica. From these plots, one can isolate three principal hysteresis regimes: the first one corresponds to the pseudo independent pore model (Scheme 2) where both NLDFT mode pore sizes given by the adsorption and the desorption branch of the isotherm agree. In the second case, the NLDFT pore size given by the adsorption branch is larger than the one obtained from the

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Figure 7. NLDFT maximum adsorption (solid symbols) and desorption (hollow symbols) mode pore size as a function of the acid concentration in the mother liquor for different SBA-15 materials aged at 80 °C (a) and 100 °C (b).

desorption branch. This case corresponds to a specific physisorption behavior where capillary evaporation (desorption) is delayed relative to the capillary condensation step (adsorption). Such situation is illustrated in Figure 8a and reflects a pore system being affected by network/structure effects such as pore blocking/percolation or cavitation.42-48,139-143 As this effect is expected to occur only if a mesopore has access to the external surface solely through a narrower neck, as in an ink-bottle pore, it is therefore reasonable to associate SBA-15 materials showing such capillary evaporation and hysteresis behavior with a pore network as schematized in the inset of Figure 8a, i.e., presence in the main mesopores of smaller pore openings/necks or distorted pores with constrictions. The isotherm curve of such SBA-15 silica typically exhibits a relatively wide hysteresis loop and a less vertical capillary condensation/evaporation step, characteristic of a less homogeneous pore structure. Moreover, the two branches of the hysteresis are not perfectly parallel, in contrast to classical H1 loops. This kind of isotherms have also been observed when (nano)particles or plugs are confined inside the mesopores.136-138,144 It is important to note that the hysteresis loops of these SBA-15 are still described as H1 ones.


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Figure 8. Two limit condensation and hysteresis behaviors observed for SBA-15 silicas with their corresponding NLDFT pore size distribution and schematic pore topology representation. a) Delayed capillary evaporation and b) Initiated capillary condensation.

In the most extreme cases, as discussed in the following section, they may be considered hybrid H1/H2(b). Therefore, it means that in contrast to materials showing a typical H2(a) hysteresis loop,42 where one must differentiate between pore-blocking/percolation and cavitation induced desorption, here the capillary evaporation behavior can only be rationalized using a pore-blocking/percolation mechanism, i.e., the mesopores remain filled during the desorption until the constricted/distorted portion of the pores and/or “pore necks” desorbing pressures are reached. As corrugations are not homogenous plugs of well-defined sizes, it also results in more heterogeneous pore channels, ultimately causing the broader desorption range in terms of relative pressures. However, since all the constrictions are above the critical size (5-6 nm for N2 at 77K), the desorption branch gives information about the extent of corrugation. One should keep in mind that the so-called “pore blocking” behavior described here is quite subtle and could also be partially affected by local texture/surface roughness effects in contrast to a classical ink-bottle pore situation. Finally, the last case where the NLDFT mode pore size calculated from the desorption branch is larger than the one obtained from the adsorption branch, suggests that adsorption in mesopores could occur at relative pressures lower than predicted using the NLDFT kernel of metastable isotherms. This case is schematized in Figure 8b. Interestingly, such behavior is most frequently observed for materials exhibiting a true interconnected 3-D pore system, such as KIT-667 and SBA-15 materials synthesized under specific conditions,145 which are known to

yield well-structured non-siliceous replicas following hard templating procedures,5,26,66,116,146 owing to extensive and large interconnectivity within the main mesopore system. Large intrawall pores in specific SBA-15 materials could therefore also have a noticeable influence on the capillary condensation process. This phenomenon can be correlated with the initiated or advancing capillary condensation effect.67,139-147 Pore connectivity in open pore networks may thus help to reduce the range over which metastable pore fluid exists, ultimately leading to condensation at slightly lower relative pressure.42,45,48,67 To substantiate the hypothesis of pore interconnectivity affecting capillary condensation in some ordered mesoporous SBA-15 silicas, hysteresis behavior in such samples ought to be compared with pore condensation/evaporation process in an equivalent material possessing an intrinsic 3-D mesopore architecture. In this way, ordered mesoporous KIT-6 silica which exhibits similar features to SBA-15 but with a pristine and well-resolved 3D cubic Ia3d pore symmetry represents the best candidate. From a pore size analysis model point of view, the assumption of cylindrical pore geometry for the 3-D cubic Ia3d pore structure has already been validated as a reliable and accurate first approximation in previous studies.54,67,148 Therefore, one may realistically compare the hysteresis behavior of a SBA-15 described within the independent pore model and a standard KIT6 of similar physicochemical features using dedicated NLDFT methods. In one of our previous investigations,67 such comparison was performed and it was concluded that pore condensation and hysteresis behavior are very similar in both structures,



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except for the narrower hysteresis loop visible for KIT-6 compared to that of SBA-15.66,67 Moreover, while the corresponding NLDFT PSD curves from adsorption and desorption of the given SBA-15 sample were in perfect agreement, those of KIT6 did not match. The mode value of the PSD curve of KIT-6 calculated using the adsorption branch always resulted in a smaller pore size. To explain this behavior, we proposed that the “delay in condensation”, i.e., metastabilities, is not as pronounced in the 3-D connected pore system of KIT-6 as it would be expected for a system consisting uniquely of independent pores. It was postulated that large interconnectivities in the 3-D pore system have a significant contribution for reducing the nucleation barriers associated with capillary condensation. In other words, pore connectivity in such a pristine 3-D porous material reduces the metastability range of the low-density adsorbed film leading to an initiated capillary condensation. 66,67 Other effects, especially the pore surface roughness, including the presence of pore corrugations at various degrees, may also have an effect on the hysteresis loop width.149-152 However, in our previous study described above,67 network corrugation of the SBA-15 silica was expected to be rather small, if any, as the material was aged at 135°C. Materials hydrothermally aged at such elevated temperature do not exhibit anymore microporosity, the main mesopores are noticeably enlarged (i.e., 10-11 nm) and are connected together via large intra-wall pores. This results in a pore network which is highly interconnected.36,37 As shown in Figure 9, by tuning the synthesis parameters properly, it is possible to obtain SBA-15 silicas with the exact same hysteresis width as compared to a typical KIT-6 sample, clearly indicating identical capillary condensation and evaporation behaviors in gas physisorption. One can easily note the perfect agreement between the NLDFT PSDs calculated from the adsorption branches of both materials, as well as from both desorption branches (Figure 9b). This observation of initiated condensation in some SBA-15 samples confirmed the role of the large interconnections between the main mesoporous channels on the capillary condensation. This latter type of SBA-15 silicas should therefore rather be considered as a 3-D pore network instead of a system of independent cylinders, mimicking the 3-D cubic Ia3d KIT-6 silica, with respect to N2 sorption behavior. Correlating synthesis parameters and network structure. Based on the above observations, one may now rationalize the influence of the synthesis parameters on physicochemical and gas adsorption properties of SBA-15 silicas, especially in the hysteresis region. To do so, the NLDFT pore size analysis method was shown to be a very suitable tool. Figure 10 shows the effect of HCl concentration over the main mesopore network of SBA-15. First, it can be noted that at a reaction temperature of 35 °C and an aging temperature of 80 °C, an acid concentration of 0.1 M will not lead to a perfectly ordered material, in agreement with the material discussed in Figure 6b. Here, the hysteresis loop of SBA-15 sample made with 0.1 M HCl is not properly closing as it would be expected for a typical H1 loop.42 The adsorption and the desorption branches do not overlay immediately after the capillary condensation and evaporation steps, resulting into a “stretched” hysteresis on the two ends. Moreover, NLDFT pore size analysis shows a bimodal distribution, characteristic of a lower quality material. Here again, the limitations of tailoring SBA-15 mesophase while modifying only a single synthesis parameter are clearly evidenced.

Figure 9. Comparison of N2 (at -196 °C) adsorption-desorption isotherms (a) and corresponding NLDFT pore size distributions (b) of a particular SBA-15 (0.2-60-35-100) and a KIT-6 material.

For the 0.2 M material (blue squares), a classical H1 hysteresis loop was obtained and perfect agreement was found for the NLDFT pore size given by the adsorption and desorption branches. Therefore, it can be considered that the pore network of this SBA-15 material is fully described within the independent pore model. When higher HCl concentrations are used (0.3 M and above), the hysteresis becomes wider, capillary condensation and evaporation branches become less sharp and less parallel. For the 1.0 M and 2.0 M SBA-15 materials, one can notice the larger hybrid H1/H2b hysteresis loop. This physisorption behavior indicates a pore-blocking-like desorption behavior, which is confirmed by the NLDFT PSD curves (Figure 10b). Indeed, for such materials, the mode pore size given by the adsorption branch was always larger than the one given by the desorption branch. It can be concluded that increasing HCl concentration during the synthesis step will favor the apparition of corrugations/distortions in the resulting SBA-15 main mesopores. A higher HCl amount in the synthesis will also reduce the pore size of the final calcined SBA-15 silica, even though,


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Figure 10. N2 adsorption-desorption isotherms (at -196 °C) of w60-35-80 SBA-15 materials (a) and corresponding NLDFT pore size distributions (b).

above 0.6 M, the decrease in pore size is clearly less marked. Moreover, NLDFT pore size distributions given by the adsorption and desorption branches were almost identical above 1.0 M. These results on calcined SBA-15 materials are in line with observations made during mesophase formation and aging treatment. They confirmed that optimal mesopore modulation is best achieved using a moderated HCl concentration. It should

also be kept in mind that perfect agreement between adsorption and desorption NLDFT mode pore sizes can be found with different HCl concentrations depending on the modifications applied to the other synthetic parameters, such as aging temperature and r molar ratio, as described in Figure 7. For materials synthesized with a r ratio of 45, perfect agreement between NLDFT pore size given by the adsorption and desorption branch is found for a HCl concentration of 1.0 M, whereas a material following the NLDFT independent pore model is found for a HCl concentration of 0.2 M when a r ratio of 60 is used (Figure 7a). However, with an aging temperature of 100 °C and synthetic parameters described in Figure 7b, no more “delayed evaporation” effects are observed. Moreover, for the x-35-45-100 SBA-15 series with a r ratio of 45 and hydrothermally aged at 100 °C, only “3-D-like” pore network behavior was observed even with the highest HCl concentration (2.0 M). Moderated HCl concentration (0.2 – 0.6 M) is thus highly desirable for SBA-15 material synthesis if one aims at an accurate control and tuning of the main mesoporous channels. An increase in the aging temperature will lead to an increase in the pore size of the SBA-15 material., Furthermore, increase in the hydrothermal temperature will create some larger interconnections between the main mesopores and thus confers a 3-D-like character to the pore system.36,37 Figure 11 confirmed such findings for all tested series. In the case of SBA-15 aged at relatively low aging temperatures, the NLDFT mode pore size given by the desorption branch is always smaller than the one given by the adsorption branch. It is typical of SBA-15 samples affected by delayed evaporation phenomenon during physisorption. In the 0.6-60-30-z series, an almost perfect overlap of both NLDFT pore size distributions is found for the SBA-15 sample aged at 100 °C (Figure 11a) indicating that the main mesopore network has evolved into a pseudo 1-D array of cylindrical-like pores under the influence of the aging temperature. In this case, the intra-wall pores are not important enough (in sizes and/or numbers) to influence the fluid adsorption behavior, at least from a physisorption point of view. On the other hand, above 100 °C, the NLDFT mode pore size given by the desorption branch of the isotherm is always larger than the one given by the adsorption branch indicating a further evolution of the mesopore network into a “3-D-like” behavior. Nevertheless, one can also easily obtain a situation where perfect agreement between adsorption and desorption NLDFT mode pore sizes is found by a smart adjustment of only one of the synthetic parameters, (see Figure 11b). Increasing the silica source content during the synthesis will shift the agreement between the two NLDFT pore sizes to higher aging temperatures. Regardless of the synthetic parameters varied, the typical mesopore architecture evolution shown in Figure 11 was found to always follow the same “physisorption pattern”: corrugated/distorted towards pseudo-1-D towards interconnected “3-D like” cylindrical mesopores. The reverse evolution is of course also possible, i.e., from a 3-D like behavior to pseudo 1-D to a corrugated/distorted one. Moreover, an evolution in the disagreements between the two NLDFT pore size values can be noted. Indeed, the gap is larger with lower temperatures and tends to minimize and then stabilize with higher ones. Above ca. 120 °C, the gap between the two NLDFT pore sizes becomes almost constant denoting a definitive/fixed mesopore topology reached by the system.36,37 It is particularly important to underline that these shifts in NLDFT PSDs were not only observed for nitrogen physisorption at 77.3 K but also for argon measurements at 87.4 K as shown in Figure 12.



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Figure 11. NLDFT adsorption (solid symbols) and desorption (hollow symbols) mode pore size as a function of the aging temperature for various SBA-15 materials with two different HCl concentrations (a) and three different r ratios (b).

For all investigated materials, hysteresis shapes and widths were similar for measurements performed with both adsorptives at their respective boiling temperature. One may note that argon isotherms exhibited a higher adsorbed volume than nitrogen ones, as expected since liquid argon has a higher liquid density as compared to nitrogen. Slight differences were also observed in NLDFT mode pore sizes given for argon and nitrogen data (Figure 12b), originating from the chemical nature of the adsorptives. Indeed, argon is a non-polar monoatomic molecule that does not have any preferential side of adsorption compared to the more “polar” diatomic N2 quadrupole. Due to the absence of quadrupole moment, its higher boiling temperature and the weaker attractive fluid-wall interactions, Ar is noticeably less sensitive to differences in surface chemistry and structure of the adsorbent in comparison to nitrogen. As a result, Ar adsorption at 87 K allows a much more straightforward correlation to be obtained between the pore filling pressure and the corresponding pore size and will lead to more accurate pore size determination, especially for polar surfaces.

Figure 12. Ar (at -186 °C, dashed lines) and N2 (at -196 °C, solid lines) adsorption-desorption isotherms of 0.6-x-35-100 samples (a) and corresponding NLDFT pore sizes distributions (b).

Nevertheless, the main result here, is that SBA-15 network topologies deducted by argon and nitrogen physisorption for a same material were always consistent. For example, in the 0.6x-35-100 SBA-15 series, a r ratio of 45 resulted in the observation of initiated capillary condensation with both gases in physisorption. For r = 60, the Ar and N2 NLDFT PSD analyses ex-


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hibited a perfect overlap between respective adsorption and desorption PSD. Finally, for r = 75, a delayed capillary evaporation behavior (i.e., due to corrugations/distortions) was obtained for both desorption branches. These results validate the ones obtained from N2 physisorption described herein and therefore also confirm that the NLDFT model deviations are neither adsorptive specific nor measurement artifacts. To further “independently” confirm the existence and importance of these pore network variations, nanocast Co3O4 replicas of three SBA-15 exhibiting the three “limit” mesopore topologies according to NLDFT pore size analysis were performed (Figure 13). Nanocasting is a method where a nanoporous mold with desired features (e.g., pore shape and size, particle morphology and size) is filled with precursors of interest. After (thermal) solidification, the mold is then selectively removed via chemical treatment and a negative replica of the pore network of the parent material is usually obtained.5,21-26 While this technique is mainly used to produce structured nanoporous non-silica materials of various compositions, which are difficult to synthesize via other methods, it is also perfectly suited to investigate the extent of interconnectivity in the pore network of the parent mold material (e.g., the mesoporous silica). Regarding this latter point, nanocasting procedures with cobalt salts/oxides have been extensively studied, being sensitive to interconnections in SBA-15 materials, and straightforward protocols leading to reproducible results can be found easily.5,26,116,146,153,154 It is evident that when SBA-15 with a 3-Dlike pore network character (i.e., showing a pronounced initiated condensation behavior in physisorption) is used as a template, a perfectly mesostructured Co3O4 replica is obtained on a large scale as shown on TEM and HRSEM images presented in Figures 13ab and 14ab, respectively. The extent of the replication process can be appreciated on Figures 14a and 14b where no evidence of large bulk Co3O4 particles was detected. Moreover, low angle XRD pattern of the nanocast Co3O4 replica confirmed the excellent overall retention of the mesostructure (Figure 13g). Here, it is proposed that the substantial mesoporous interconnections between the main pores allow a more homogeneous dispersion of the Co precursor solution, ultimately allowing a more efficient replication process. Once the silica is removed, the fairly large interconnections also help maintaining the Co3O4 structure together allowing the longrange replication of the parent material. When a SBA-15 fully described within the physisorption independent pore model was used as the template (Figure 13cd), a poorly mesostructured replica is obtained. The order is somehow maintained locally, owing to the existence of intra-wall porosity, i.e., micro- and small mesopores. However, due to the absence or the insufficient number of large interconnections between the main mesopores, the replication process is solely successful on a short length scale. Indeed, corresponding small angle XRD showed a clearly less defined pattern (Figure 13h) in good agreement with the TEM observations. Only partial replication of the material can be obtained.116 Finally, when a SBA-15 material showing so-called pore-blocking behavior during N2 physisorption was used as a template, no satisfactory mesostructured replica was obtained. Low angle XRD of the corresponding Co3O4 oxide showed only a broad pick characteristic of poorly ordered material (Figure 13i). TEM and SEM analysis of the sample, presented in Figure 13e,f and Figure 14c,d, respectively, confirmed the poor level of ordering of the material. The replicated sample mostly consisted in a mixture of few local replicated bundles of Co3O4 rods, isolated Co3O4 nanorods of various length and bulk particles. Here again, this local and limited replication of the

Figure 13. TEM images of nanocast Co3O4 replicas synthesized using an interconnected (a,b), a pseudo-cylindrical (c,d) and a corrugated/distorted (e,f) SBA-15 as template. Low angle powder XRD patterns of interconnected (g), pseudo-cylindrical (h) and corrugated/distorted (i) pristine SBA-15 (red line) and the corresponding Co3O4 replica (blue line).

parent material could be explained by the absence of sufficiently large connections between the main mesopores combined with the presence of corrugations or distortions in the mesopores ultimately impairing the replication process. Most importantly, at same cobalt precursor loading, normalized as a function of the pore volume of the parent SBA-15 silicas, the replica made from the material exhibiting evidence of pore corrugations or distortions showed numerous bulk Co3O4 particles, as clearly visible on the electron microscopy images (Figures 13ef and 14cd). No evidence of such large bulk particles were found for the two other replicated SBA-15 silicas. The overall shape of the replicated particles, on the macroscopic scale, is also of particular interest. When an interconnected “3D-like” SBA-15 is used as a mold, the replication process mainly produced spheroidal particles (Figure 14a), resulting from a more isotropic diffusion of the metal precursor and growth of Co3O4 inside the mold. On the contrary, when a corrugated/distorded SBA-15 is used as a mold, the replication process is clearly



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Figure 14. SEM images at different magnifications of nanocast Co3O4 replicas synthesized using an interconnected (a and b) and a distorted/corrugated (c and d) SBA-15 as template.

more anisotropic. The metal precursors diffused almost exclusively along the direction of the main cylindrical pore channels resulting in short 2D fibrous structures/bundles (Figure S3). Obviously, diffusion of the metal salts, growth of metal oxide, and ultimately success of the nanocasting process, is directly related with the dimensionality of the SBA-15 pore network, thus demonstrating the importance of an accurate characterization of the parent material. Finally, the results obtained by nanocasting corroborated all the previous observations made by physisorption on the pristine SBA-15 materials. DISCUSSION In this study, we demonstrated that the choice of the synthesis parameters, as well as the quantities introduced during the formation of ordered mesoporous SBA-15 silicas will strongly affect the mesophase development and eventually the pore network architecture. The final mesopore topology as well as the mesopore size can be tailored precisely and can even be anticipated knowing the effects of each reagent. From the different series of SBA-15 materials presented, one may thus conclude that: (i) an increase in the HCl concentration will lead to smaller mode pore sizes in the material and favor the apparition of a corrugated/distorted pore system. In addition, there is a threshold acid concentration located around ca. 0.6 M HCl, above which the fine-tuning of the mesophase is much harder due to fast synthesis kinetics; (ii) an increase in the molar r ratio will

also lead to smaller pore sizes and corrugated/distorted pore architecture in calcined SBA-15 materials. For most synthesis conditions investigated, increase in the amount of silica precursor added to the mother liquor is usually limited to about 75-80. However, it is possible to increase this amount to values exceeding r = 90 and still obtain a high-quality SBA-15 sample if one varies the other parameters accordingly and couples their effects (e.g., acid concentration and aging temperature, see Figure 1b); (iii) an increase in the aging temperature will increase the mode pore size of the resulting material and will favor the formation of a SBA-15 silica with a “3-D-like” or fully interconnected pore topology. Connections may only grow to a maximum size; (iv) an increase in the synthesis temperature will result in a mesoporous silica with smaller mode pore size and will favor the corrugated/distorted-type topology in terms of pore network architecture. Smart variation of these four parameters allows to control the mesopore size over a quite large range, i.e., from about 5 to 11 nm. Obviously, some limitations in the tailoring do exist for each parameter (Figure 6b) and one must stay within favorable and reasonable “cooperative self-assembly process” conditions in order to synthesize a high-quality SBA15 material exhibiting a uniform and well-defined pore structure. As shown in this study, the resulting pore structure of SBA-15 can also be modulated from a distorted pseudo-1-D cylindrical pore system to a highly-interconnected 3D-like network mimicking pristine 3-D porous structures, such as KIT-6, depending on details of the synthesis conditions. The evolution of the pore


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network can be done in two ways, i.e., as a function of the starting synthesis parameters, it is possible to control the occurrence of either large interconnections or distorted/corrugated cylindrical pores in calcined SBA-15. Obviously, it is also possible to synthesize a sample being described within the independent pore model, from a physisorption point of view. Such evolution can be easily followed via a detailed analysis of N2 and Ar sorption isotherms at their boiling temperature, respectively, combined with an advanced analysis of the sorption hysteresis using modern NLDFT data reduction models/methods. Capillary evaporation (desorption) will always be delayed relative to condensation (adsorption) if the material exhibits corrugated/distorted pores (i.e., so-called pore blocking effect). On the other hand, adsorption in mesopores will take place at lower relative pressures than predicted by the NLDFT metastability model (adsorption) when the system resembles a 3-D interconnected pore network. This clearly demonstrates that pore interconnectivity in 3-D cylindrical pore network (i.e., sufficiently large intra-wall pores in SBA-15) reduces the range over which metastable pore fluid exists, ultimately leading to an initiated/advanced capillary condensation behavior, similar to the physisorption behavior observed for materials exhibiting a pristine 3-D pore architecture (e.g., KIT-6). For a standard SBA-15 sample, one should observe a perfect agreement of the two pore size distributions (using modern data reduction model such as the NLDFT method), indicating a sample predominantly consisting of independent cylindrical pores. Mesopores may however still be connected together via the intra-wall porosity, but such porosity is not significant enough (in size and/or frequency) to trigger a deviation from the physisorption independent pore model. The present results also seem to indicate that the microporosity of the different SBA-15 samples does not affect the hysteresis behavior in a noticeable way (Table S1). Intra-wall porosity and/or interconnections must reach a sufficiently large size in order to do so. Moreover, it was possible to obtain corrugated/distorted and pseudo-independent (1-D) pore architecture with samples exhibiting relatively large micropore volume. However, the importance of the microporosity from a physisorption perspective, especially considering the resulting surface roughness of the material, as well as the extent and effect of pore corrugations or distortions, is still under debate. To elucidate these open questions and to confirm the importance of network effects, further advanced in-depth hysteresis scan investigations are mandatory. In this regard, interesting studies were already published over the last decade.42,150-152,155-157 However, detailed understanding of such loops is not straightforward.157 Moreover, as we discussed in this article, SBA-15 is not a standard “calibrated” material and may be synthesized in various ways, ultimately resulting in a final calcined material exhibiting a wide diversity of physicochemical and textural parameters. Rigorous interpretation of hysteresis scans may therefore be quite challenging. Thus, a discussion about such curves for the SBA-15 materials described here would be premature. Further studies are ongoing regarding these aspects. From a fundamental point of view, we used high-quality and tailored SBA15 materials to test the extent of validity of the state-of-the-art NLDFT models. It resulted that kernels being solely based on single pore model structures cannot perfectly capture and describe the adsorption behavior of a wetting fluid confined in an interconnected nanoscale siliceous pore network. However, one may use the imperfections of the present NLDFT models in order to obtain further information regarding the true nature of the SBA-15 silicas under investigation. Therefore, one should keep

in mind that advanced physisorption analyses may provide much more information than traditional pore size distribution curves and should not only be seen as a routine technique but rather as a critical method for the advanced characterization of porous materials. These results are not only limited to the samples discussed in this current study but could also be generally applied to many other materials, as long as one is working with a NLDFT pore size analysis model describing realistically his experimental material. CONCLUSION Our data reveal that different synthesis conditions (acid concentration, TEOS/P123, synthesis and aging temperatures) are suitable for the preparation of ordered mesoporous SBA-15 silicas with selectively tuned textural and pore size properties. Using the results described above, one may conveniently anticipate the mode pore size as well as the final pore architecture of the calcined material, only knowing the quantities introduced in the starting mother liquor. Such results open vast possibilities as they allow the precise design of a siliceous mesoporous material with optimized porous properties taking into account the requirements of the envisioned application. It was found that a moderate HCl concentration, i.e., 0.2 – 0.6 M, allows slower reaction kinetics and thus optimal mesophase tailoring, whereas 1.0 M HCl and above, will quickly lead to a more condensed mesostructured hybrid regardless of other synthetic parameters. The use of NLDFT models combined with advanced analysis of the physisorption hysteresis revealed that distinct synthesis conditions enable the preparation of SBA-15 with selectively tuned pore architecture ranging from a distorted/corrugated to pseudo-1-D cylindrical pore system to a highly-interconnected 3D-like network mimicking pristine 3-D mesostructures such as KIT-6 silica. Therefore, SBA-15 materials should be best regarded as a family of solids with easily adjustable porosity rather than a unique standard mesoporous silica. These advances are not only significant for improving SBA-15 synthesis and accurate porosity assessment, as our methodology can be extended to related siliceous or non-siliceous materials specifically designed for applications in which confinement effects, bimodal porosity, diffusion and mass transport parameters are critical. It will also apply for more complex and/or interconnected pore structures such as the ones found in many industrially relevant oxide- and silicate based hierarchical materials, e.g., mesoporous zeolites, which require selectively-tailored properties. In particular, for many catalytic systems, there is still a tremendous interest in the design of ordered mesoporous transition metal oxides via nanocasting where such knowledge could be of prime importance. Our results provide a deeper understanding of textural features and shed some light on fundamental questions concerning the effect of nanoscale confinement on the phase behavior of confined wetting fluids. However, considering the impressive development of materials synthesized via cooperative self-assembly or templating pathways exhibiting hierarchical and/or multi-scale pore networks, more investigations are mandatory. The development, extensive validation and routine implementation of data reduction methods based on models which better describe the complex reality of pore networks, including potential effect of surface roughness/heterogeneities/texture on confined fluids, is urgently needed. These advanced models would need to go beyond the existing simplified models which are based on the assumption of an array of independent pores of a given geometry, i.e., not taking network effects into account.



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Author Contributions

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ASSOCIATED CONTENT 29Si

MAS NMR spectra of calcined SBA-15 materials, differential thermal analyses of as-made SBA-15 materials, physicochemical parameters obtained from N2 and Ar physisorption experiments for various SBA-15 materials and SEM image of nanocast Co3O4 replica synthesized using a distorted/corrugated SBA-15 as template are presented in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected]

ORCID Freddy Kleitz: 0000-0001-6769-4180

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT The authors thank Dr. Wolfgang Schmidt (MPI für Kohlenforschung, Mülheim an der Ruhr, Germany) for his contribution with the powder XRD measurements of the SBA-15 silica samples, and Dr. Kyoungsoo Kim and Prof. Ryong Ryoo (KAIST and IBS, Daejeon, Republic of Korea) for kindly supplying the high-resolution SEM data. The authors acknowledge the Natural Sciences and Engineering Research Council of Canada (NSERC) for the financial support. NSERC supported this work through a Discovery Grant (Grant N° RGPIN-2014-05821). The authors also acknowledge the University of Vienna (Austria) for additional support.

Present Addresses †François

Bérubé: COREM, 1180, rue de la Minéralogie, Québec, QC, G1N 1X7, Canada.

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Kleitz, F.; Liu, D. N.; Anilkumar, G. M.; Park, I. S.; Solovyov, L. A.; Shmakov, A. N.; Ryoo, R. Large Cage Face-Centered-Cubic Fm3m Mesoporous Silica: Synthesis and Structure. J. Phys. Chem. B 2003, 107, 14296-14300. Yang, C. M.; Schmidt, W.; Kleitz, F. Pore Topology Control of Three-Dimensional Large Pore Cubic Silica Mesophases. J. Mater. Chem. 2005, 15, 5112-5114. Wanka, G.; Hoffmann, H.; Ulbricht, W. Phase Diagrams and Aggregation Behavior of Poly(oxyethylene)-Poly(oxypropylene)-Poly(oxyethylene) Triblock Copolymers in Aqueous Solutions. Macromol. 1994, 27, 4145-4159. Min, B.-H.; Jeong, E.-Y.; Thommes, M.; Park, S.-E. Direct Synthesis of Plugged SBA-15 Type Mesoporous Silica Using Alcoholamines. Chem. Commun. 2011, 47, 4673-4675. Shakeri, M.; Klein Gebbink, R. J. M.; de Jongh, P. E.; de Jong, K. P. Control and Assessment of Plugging of Mesopores in SBA-15 Materials. Micropor. Mesopor. Mater. 2013, 170, 340-345. Wang, J. H.; Tian, G. H.; Li, Z. Z.; Ji, X. H.; Bao, W. R. A Novel and Facile Strategy for Synthesis Plugged SBA-15. Mater. Lett. 2016, 162, 110-113. Coasne, B.; Galarneau, A.; Di Renzo, F.; Pellenq, R. M. Gas Adsorption in Mesoporous Micelle-Templated Silicas: MCM-41, MCM-48, and SBA-15. Langmuir 2006, 22, 11097-11105. Thommes, M.; Smarsly, B.; Groenewolt, M.; Ravikovitch, P. I.; Neimark, A. V. Adsorption Hysteresis of Nitrogen and Argon in Pore Networks and Characterization of Novel Micro- and Mesoporous Silicas. Langmuir 2006, 22, 756-764. Casanova, F.; Chiang, C. E.; Li, C. P.; Schuller, I. K. Direct Observation of Cooperative Effects in Capillary Condensation: The Hysteretic Origin. Appl. Phys. Lett. 2007, 91, 243103-1-243103-3. McBain, J. W. J. An Explanation of Hysteresis in the Hydration and Dehydration of Gels. J. Am. Chem. Soc. 1935, 57, 699-700. Mason, G. The effect of Pore Space Connectivity on the Hysteresis of Capillary Condensation in Adsorption-Desorption Isotherms. J. Coll. Interf. Sci. 1982, 88, 36-46. Krawiec, P.; Weidenthaler, C.; Kaskel, S. SiC/MCM-48 and SiC/SBA-15 Nanocomposite Materials. Chem. Mater. 2004, 16, 2869-2880. Guillet-Nicolas, R., Ahmad, R.; Cychosz, K. A.; Kleitz, F.; Thommes, M. Insights into the Pore Structure of KIT-6 and SBA-15 Ordered Mesoporous Silica – Recent Advances by Combining Physical Adsorption with Mercury Porosimetry. New J. Chem. 2016, 40, 4351-4360. Salabas, E. L.; Rumplecker, A.; Kleitz, F.; Radu, F.; Schüth, F. Exchange Anisotropy in Nanocasted Co3O4 Nanowires. Nano Letter. 2006, 6, 2977-2981. Rojas, F.; Kornhauser, I.; Felipe, C.; Esparza, J. M.; Cordero, S.; Dominguez, A.; Riccardo, J. L. Capillary Condensation in Heterogeneous Mesoporous Networks Consisting of Variable Connectivity and Pore-Size Correlation. Phys. Chem. Chem. Phys. 2002, 4, 2346-2355. Schumacher, K.; Ravikovitch, P. I.; Du Chesne, A.; Neimark, A. V.; Unger, K.K. Characterization of MCM-48 Materials. Langmuir 2000, 16, 4648-4654. Gommes, C. J. Adsorption, Capillary Bridge Formation, and Cavitation in SBA-15 Corrugated Mesopores: A Derjaguin−Broekhoff−de Boer Analysis. Langmuir 2012, 28, 5101-5115. Morishige, K. Nature of Adsorption Hysteresis in Cylindrical Pores: Effect of Pore Corrugation. J. Phys. Chem. C 2016, 120, 2250822514. Morishige, K. Dependent Domain Model of Cylindrical Pores. J. Phys. Chem. C 2017, 121, 5099-5107. Morishige, K. Effects of Carbon Coating and Pore Corrugation on Capillary Condensation of Nitrogen in SBA-15 Mesoporous Silica. Langmuir 2013, 19, 11915-11923. Grewe, T.; Deng, X.; Weidenthaler, C.; Schüth, F.; Tüysüz, H. Design of Ordered Mesoporous Composite Materials and Their Electrocatalytic Activities for Water Oxidation. Chem. Mater. 2013, 25, 4926-4935. Yen, H.; Seo, Y.; Guillet-Nicolas, R.; Kaliaguine, S.; Kleitz, F. One-Step-Impregnation Hard Templating Synthesis of High-Surface-Area Nanostructured Mixed Metal Oxides (NiFe2O4, CuFe2O4 and Cu/CeO2). Chem. Commun. 2011, 47, 10473-10475. Coasne, B.; Gubbins, K. E.; Pellenq, R. J.-L. Domain Theory for Capillary Condensation Hysteresis. Phys. Rev. B 2005, 72, 024304-1024304-9. Grosman, A.; Ortega, C. Nature of Capillary Condensation and Evaporation Processes in Ordered Porous Materials. Langmuir 2005, 21, 10515-10521. Cimino, R.; Cychosz, K. A.; Thommes, M.; Neimark, A. V. Experimental and Theoretical Studies of Scanning Adsorption–Desorption Isotherms. Colloid. Surf. A 2013, 437, 76-89.

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Selectively Tuned Pore Condensation and Hysteresis Behavior in

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