Surfactant chain length effect on the hexagonal-to-cubic phase transition in mesoporous silica synthesis

doi:10.1016/j.micromeso.2011.06.021

Microporous and Mesoporous Materials

Volume 147, Issue 1, January 2012, Pages 242-251

https://doi.org/10.1016/j.micromeso.2011.06.021 Get rights and content

Abstract

In this study, silica-based mesoporous materials (the M41S family mesoporous molecular sieves) are synthesized using alkyltrimethylammonium bromide with different chain lengths (C_nH₂_n₊₁N(CH₃)₃Br, n = 10, 12, 14, 16) as templates. The resulting silica structures are characterized by X-ray diffraction and are found to exhibit the phase transformation from the hexagonal mesophase MCM-41 to the cubic mesophase MCM-48 (with the space group of Ia3d). The structural phase transition in our study is controlled by the alkyl chain length of the surfactant: with an increase in the surfactant chain length (from C10 to C16), the structure goes from MCM-41 (synthesized by C10), through an intermediate structure (synthesized by C12), to MCM-48 (synthesized by C14 and C16). The amount of ethanol, which is used as a cosolvent, affects the pore size of the structured mesoporous silica, but only to a small extent. In the mean time, the autoclaving time has some effect, though not distinctively, on the structure integrity as well. With increased surfactant to silica ratio, the phase transformation can be shifted to longer chain template.

Graphical abstract

Highlights

► Surfactant chain-length affects hexagonal to cubic transformation in M41S materials. ► A mechanism for chain-length phase transformation is hypothesized. ► Chain-length phase transformation can be shifted with surfactant/silica ratio.

Introduction

Disclosed in the early 1990s by Mobil researchers, the M41S family of mesoporous molecular sieves is a series of ordered silica materials that have large and uniform pore structures, including MCM-41 (hexagonal), MCM-48 (cubic), and MCM-50 (lamellar) [1], [2]. Considerable effort has been devoted to the study of MCM-41, which is a 2-D mesoporous material constructed by hexagonal-packing of cylindrical pores with space group of P6m, since its high (above 1000 m²/g) surface area, thermal stability, and tunable pore sizes are ideal for use as a catalyst support. Despite these useful properties of MCM-41, there may be advantages for the three-dimensional MCM-48 that has two networks of pores interwoven together to make a cubic (Ia3d) configuration [2], [3], [4], [5]. With the increase in dimension, MCM-48 may be superior to MCM-41 due to the elimination of pore blockage which produces mass transfer limitations [6]. However, the number of publications on MCM-48 is much smaller than may be expected, mainly because of the difficulty posed in its synthesis.

Mostly synthesized using the surfactant-silicate and a self-assembly strategy [7], the surfactant (alkyltrimethylammonium hydroxides or halides) self-organizes into the template that directs the nucleation and growth of the silicate while maintaining its geometrical structure. The aqueous solution phase diagram of cetyltrimethylammonium bromide (C₁₆TAB), the most commonly used surfactant in mesoporous material synthesis, was reported in 1989 by Auvray et al. [8] and the authors indicated possible phase transformations among the hexagonal (P6m), cubic (Ia3d), lamellar mesophases in the conventional surfactant-water system. In silicate-including systems the final siliceous phases resemble those found for surfactant in water (though not always) [9], assuming that a liquid–crystal-like phase with hexagonal/cubic/lamellar morphology results from the self-organization of the surfactant. However, the introduction of the silicate species does complicate the templating mechanism. The silicate species may participate in the mesophase ordering process (instead of simply condensing onto the liquid–crystal phase) and influence the morphology, which is supported by the fact that hexagonal, cubic, and lamellar structures can be obtained by simply changing the concentration of the silica source [10]. Either way, the transformation of the mesophases is believed to be associated with changes in the interface curvature, surfactant molecular packing, and energy given by interface bending and charge separation [11]. Sometimes the existence of co-surfactant in the synthesis mixture also embodies complex phase behavior [12]. Therefore, the size, shape, and charge of the surfactants (and co-surfactants) may play an important role in determining the phases and their transformation. Such information is summarized in the effective surfactant ion pair packing parameter [13], [14], g = V/a₀l, which is proposed to describe the hydrophilic-hydrophobic and electrostatic interactions in the surfactant solution and, as a first approximation, can be used to index the geometry of the mesophase formed.

Until recently, most of the syntheses of MCM-48 have been carried out by the direct hydrothermal process using sodium silicate solution as the silica source and C₁₆TAB as the surfactant, with alcohol added as cosurfactant to control the mesophase [6], [15]. Landry et al. [16] reported the novel method of synthesizing MCM-48 through an in situ phase transformation from MCM-41, which corresponds to the phase diagram of C₁₆TAB solution in that as the temperature of the reactant solution with fixed concentration increases, the structure transforms from hexagonal to cubic (Ia3d). In the report by Wang et al. [17] on MCM-48 synthesis using C₁₆TAB as surfactant and alkylamine (C_nH₂_n₊₁NH₂, n = 8, 10, 12, 14) as co-surfactant, they suggested that by lengthening the co-surfactant chain length from n = 8 to n = 18, the phase transition from cubic MCM-48 to lamellar MCM-50 occurs. However, no study to date has investigated the phase transition from hexagonal MCM-41 to cubic MCM-48 by simply varying the surfactant chain length.

In this paper, we have focused our work primarily on the synthesis of mesoporous silica using alkyltrimethylammonium bromide (C_nH₂_n₊₁N(CH₃)₃Br, n = 10, 12, 14, 16) as templates for the self-assembly process. While in the literature MCM-48 has mainly been synthesized by C₁₆TAB (with 16 carbon atoms in the alkyl chain) as the surfactant, we were able to obtain MCM-48 with very good cubic (Ia3d) structure using surfactants with alkyl chain length other than 16. The X-ray diffraction results substantiate that through variation in the surfactant chain length, the silica mesophase undergoes a structural phase transformation from the hexagonal mesostructure MCM-41 to the cubic (Ia3d) mesostructure MCM-48.

Section snippets

Materials

Surfactants: decyltrimethylammonium bromide (C₁₃H₃₀NBr, TCI-GR) was purchased from Tokyo Chemical Industry Co., Ltd. dodecyltrimethylammonium bromide (C₁₅H₃₄NBr, approx. 99%), tetradecyltrimethylammonium bromide (C₁₇H₃₈NBr, approx. 99%), and hexadecyltrimethylammonium bromide (C₁₉H₄₂NBr, ⩾98%) were all purchased from Sigma Aldrich. Ethyl alcohol was purchased from Pharmco-Aaper (200 Proof, absolute, anhydrous, ACS/USP grade). Colloidal silica Ludox HS40 (40 wt.% suspension in water) were

Phase transformation from hexagonal to cubic

The synthesis method was modified from the paper by Ryoo et al., [15] with the synthesis conditions optimized through a design of experiment (see supplementary data). Several parameters were taken into consideration and compared for optimization purposes: the surfactant alkyl chain length, water to silica ratio (H₂O/SiO₂), surfactant to silica ratio (surfactant/SiO₂), ethanol to surfactant ratio (EtOH/surfactant), and autoclaving time. The same molar compositions (optimized) were then applied

Conclusion

In this study, the effect that the surfactant alkyl chain length has on the phase transition of mesoporous silica materials MCM-41 and MCM-48 was investigated. Controlling parameters, such as the water/silica ratio and surfactant/silica ratio were first fixed (as 130:1 and 1:1, respectively) while both the molar composition of ethanol and autoclaving time were varied, under which conditions the phase transformation from hexagonal MCM-41 (synthesized using C10 surfactant) through an intermediate

Acknowledgement

The authors are grateful to the DOE, Office of Basic Energy Sciences, grant DE-AC02-98CH10886 for financial support.

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Surfactant chain length effect on the hexagonal-to-cubic phase transition in mesoporous silica synthesis

Abstract

Graphical abstract

Highlights

Introduction

Section snippets

Materials

Phase transformation from hexagonal to cubic

Conclusion

Acknowledgement

Biochim. Biophys. Acta

Nature

J. Am. Chem. Soc.

Science

Chem. Mater.

Chem. Commun.

J. Phys. Chem. B

J. Phys. Chem.

Nature