Acidity-dependent mesostructure transformation of highly ordered mesoporous silica materials during a two-step synthesis

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Abstract

Highly ordered mesoporous silica materials have been synthesized under mildly acidic conditions by templating with a nonionic triblock copolymer (Pluronic P104) in a two-step process. It was found that a transformation from the SBA-15 type 2-dimensional (2D) hexagonal channel mesostructure (p6mm symmetry) to the MSU-X type 3-dimensional (3D) worm-like mesostructure could be induced by varying the pH whilst keeping all other conditions constant. The transformation between two types of mesoporous silica materials can be attributed to the effect of varying proton concentration on the interaction between organic micelles and inorganic species. Both types of mesoporous materials have high surface areas, large pore volumes, thick pore walls, large mean pore sizes, and narrow pore size distribution.

Introduction

The first synthesis of M41S-type mesoporous silica was reported in 1992 by researchers in the Mobil Oil Corporation who discovered the remarkable features of this novel type of material and opened up a whole new field of research [1]. Since then, such highly ordered mesoporous silica materials have been intensively studied with regard to synthetic methods, mesostructure, surface modification, and applications as functional materials and in catalysis. In recent years, in addition to M41S molecular sieves, a variety of highly ordered mesoporous silica materials such as SBA, MSU, FDU, JLU, FSM, KIT have also been successfully synthesized using anionic, neutral, or nonionic surfactants, and the various assembly routes have been modified in order to optimize the synthesis process [2], [3], [4], [5], [6], [7], [8], [9], [10]. One of the most studied systems has been SBA-n-type mesoporous silica, synthesized by condensation of alkoxysilanes such as tetraethoxysilane (TEOS) in the presence of commercially available nonionic poly(ethylene oxide–b-propylene oxide–b-ethylene oxide) (EOn–POm–EOn) triblock copolymers as a template. These materials have controllable pore size (ranging from 2 to 30 nm) and thick pore walls (ranging from 1.5 to 8.0 nm), as well as much higher hydrothermal stability than M41S-type materials [2], [11]. In 1998, Zhao et al. reported the synthesis of both SBA-15 having hexagonal symmetry (p6mm) and rod-like morphology and the synthesis of SBA-16 having cubic symmetry by employing Pluronic P123 (EO20–PO70–EO20) and F127 (EO106–PO70–EO106), respectively, as the template [2], [11]. In addition, single crystals of another SBA-16-type mesoporous silica with cubic Im3m structure have been synthesized by using Pluronic F108 (EO132–PO50–EO132) as the template [12]. In contrast to the SBA-n-type mesoporous silica materials, which have either 2-dimensional (2D) ordered hexagonal channels or 3-dimensional (3D) cubic pore architectures, MSU-X-type mesoporous silica materials exhibit a wormhole-like mesostructure and are generally more active as heterogeneous catalysts. There have been several reports of the synthesis of MSU-X-type mesoporous silica materials using polyoxyethylene alkyl ethers as a template under neutral conditions [13], [14], [15].

Unlike M41S molecular sieves, almost all of the well-ordered SBA-n-type mesoporous silica materials (e.g. n = 15 and 16) have been synthesized by using poly(ethylene oxide) based nonionic surfactants under strongly acidic conditions (pH  1). Use of such strongly acidic media is obviously undesirable in large-scale commercial synthesis of such materials, because of corrosion problems as well as environmental and health and safety considerations. It has been suggested that the addition of fluoride (e.g. NaF) to the reaction system can accelerate the condensation of the silica sol and thus allow the ordered porous materials to be formed even when the pH is raised above 2.0 [16], [17]. For example, Prouzet reported a two-step synthesis of the ordered MSU-X family of mesoporous silicas by using nonionic poly(ethylene oxide)-based surfactants in the pH range 2–4 with addition of NaF [18], [19], although this process has the disadvantage that complex manipulations are required in the second step. Brinker has suggested that the rate of hydrolysis of TEOS is higher than that of condensation when pH < 4.0, while the reverse is true at around pH 7.0 [16]. This indicates that it should be possible to control the relative rates of hydrolysis and condensation of TEOS during the synthesis of mesoporous materials by modifying the pH of the reaction mixture.

In our earlier work, we reported a two-step method for synthesis of well-ordered SBA-15-type and SBA-16-type mesoporous silica materials under mildly acidic conditions by separating the hydrolysis and the condensation steps of the silica source in a controlled pH range higher than the isoelectric point of silica (pH 2–5) [20], [21]. It is known that the fluoride ion can serve as a powerful promoter for hydrolytic condensation of alkoxysilanes, such as TEOS, in aqueous media containing amphiphilic surfactants and thus give thermally and hydrothermally stable mesoporous silica. Voegtlin et al. have described the synthesis of mesoporous silica in neutral to basic media by using a nonionic triblock copolymer (Pluronic P123) as template with the addition of fluoride [22] whilst Stucky et al. have reported the synthesis of SBA-15-type mesoporous silica rods with uniform channels at moderate acidity (pH < 2.7), without compromising the long-range symmetry [23]. In this paper, we report an interesting mesostructure transformation from SBA-15 type to MSU-X type during a two-step synthesis of mesoporous silica materials based on a nonionic triblock copolymer (Pluronic P104) template in mildly acidic media.

Section snippets

Experimental

Poly(ethylene oxide–b–propylene oxide–b-ethylene oxide) triblock copolymer (EO27–PO61–EO27, Pluronic P104) was a commercially material obtained from BASF. Tetraethoxysilane (TEOS), sodium fluoride (NaF) and hydrochloric acid (HCl) were supplied by Beijing Chemical Reagents Company.

A typical two-step synthesis was carried out as follows: In the first step, a mixture of P104 (0.50 g) and NaF (0.02 g) were dissolved in deionized water (50.0 g) with stirring in a beaker at 35 °C to form a transparent

Results

A novel two-step synthesis, in which hydrolysis and condensation procedures were separated by means of controlling the acidity of the reaction solution, was employed to prepare the samples in this study. Fig. 2 shows the powder XRD patterns of the samples synthesized under mildly acidic conditions with various pH values through this two-step pathway followed by calcination at 400 °C. The XRD patterns of the samples prepared at pH 1.51, 2.06 and 2.61 (Fig. 2(a)–(c)) show four well-resolved

Discussion

It has been widely reported that ordered SBA-15-type mesoporous silica can be synthesized by templating with triblock copolymers with a low EO/PO ratio through self-assembly between the template and the silica precursor, to form highly ordered 2D hexagonal mesoporous structures with cylindrical channels [2], [11]. The micelle-templating technique involves seeding spherical surfactant aggregates with an SiO2 sol prepared by the hydrolysis of the inorganic precursor, TEOS. There are three

Conclusion

Highly ordered mesoporous silica materials have been synthesized by using nonionic triblock copolymer (Pluronic P104) as template in a two-step pathway under mildly acidic conditions (pH 1.51–4.56). Decreasing the acidity of the medium within this range induced a mesostructure transformation from SBA-15 type to MSU-X type materials. The mesoporous materials synthesized in this study have high surface areas, large pore volumes, thick pore walls, large mean pore sizes, and narrow pore size

Acknowledgement

The authors greatly appreciate financial support from the National Natural Science Foundation of China (Grant No. 50573006) and the Major Project for Polymer Chemistry and Physics Subject Construction from the Beijing Municipal Education Commission (Grant No.: XK100100640).

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