Elsevier

Applied Surface Science

Volume 256, Issue 17, 15 June 2010, Pages 5334-5342
Applied Surface Science

A metal-ion-assisted assembly approach to synthesize disulfide-bridged periodical mesoporous organosilicas with high sulfide contents and efficient adsorption

https://doi.org/10.1016/j.apsusc.2009.12.073Get rights and content

Abstract

Well-ordered two-dimensional (2D) hexagonal periodic mesoporous organosilicas (PMOs) with a high content of disulfide groups have been prepared by a simple metal-ions-assisted amphiphilic surfactant templating process under a strong acidic condition. Long-chain organic bridge silane, bis(triethoxysilylpropyl)disulfide (BTSPDS) was used as a precursor which can be co-condensed with tetraethoxysilane (TEOS) to assemble with the triblock copolymer Pluronic P123 template and to construct the mesostructured organic–inorganic frameworks. The content of disulfide functional groups is up to 20% (BTSPDS molar content in the initial silane mixture) incorporated into the framework. The obtained ordered mesoporous DS-PMO materials have relatively high BET surface area (∼580 m2/g), large uniform pore size (up to 6.3 nm) and thick pore walls (thickness up to 7.1 nm), because of the long-chain disulfide bridges. The metal ions such as Zn2+ formed the four-coordination complex with two sulfides of BTSPDS and ethylene oxide moieties of P123 template, which could enhance the interaction between the “soft” long disulfide groups and P123 template, thus improving the mesostructural regularity correspondingly. The disulfide-bridged PMO materials exhibit excellent hydrothermal stability in boiling water for 5 days, probably due to the thick pore walls. SEM images show that after the hydrothermal treatment, the morphology of the ordered disulfide-bridged PMO materials is retained, as that of the wheat-like SBA-15. Excellent adsorption efficiency (∼716 mg/g) for Hg2+ ions is observed, suggesting a potential application in removal of heavy metal ions in wastewater.

Introduction

Since the first reports in 1999 [1], [2], [3], periodic mesoporous organosilica (PMO) materials have attracted increasing attention because of their unique organic functional frameworks and potential applications in catalysis [4], [5], [6], [7], [8], high-performance liquid chromatography [9], adsorption [10], [11], [12], [13], [14], low-k dielectrics and optical sensors [15], [16], [17], [18]. It has been known that ordered PMOs can be synthesized by using silesquioxane precursors with organic bridging groups as a silica precursor and amphiphilic surfactants as a template via the self-assembly method, similar to that for mesoporous silicates [1], [2], [3]. However, compared with the mesoporous silicas or/and functional mesoporous silicas with terminally bonded organic groups, many advantages of PMOs are evident, such as (a) up to 100% of Si atoms can be connected to organic functional groups; (b) the organic groups are homogeneously distributed in the frameworks to form organic–inorganic composites; (c) the organic groups inside the silicate frameworks impart characteristics or various surface polarity, thus endowing PMOs a wide range of opportunities for designing materials with unique properties. So far, “rigid” organic chain bridging groups originating from methane, ethane, ethylene, as well as small organic aromatic ones such as thiophene, xylene, benzene and biphenylene have successfully been incorporated into the ordered PMO frameworks by amphiphilic-surfactant-assembly synthesis approaches [17], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]. However, PMOs with long chain silesquioxane functional moieties are not easily synthesized, because the large molecular size or complexity of bridging organic groups could not benefit the self-assembly of mesostructures with surfactant to be immobilized in a three-dimensional (3D) continuous framework [31], [32], [33]. The utilization of 100% “soft” bridging organic silanes as a silica precursor usually leads to disordered or worm-like mesostructures. Much effort has been made by previous researchers but without success. A possible but undesirable method is to incorporate a certain amount of fully hydrolysible silanes such as tetramethylorthosilicate (TMOS), tetraethylorthosilicate (TEOS) in the co-condensation process to help construction of the ordered mesostructural framework [12]. However, the incorporated amount of “soft” bridging organic silane precursors is quite low (normally <5%). Therefore, it remains a challenge to enhance the content of the functional organic moieties with ordered mesostructures and open frameworks.

Thiother groups show strong affinity for metal ions and ideal functional sites on mesoporous materials as an efficient adsorbent to remove heavy metal ions in wastewaters. Furthermore, thioether can be oxidized to sulfonic acid groups functioning as an acid catalyst. Much attention has been paid to the synthesis of ordered mesoporous materials with high loading of thioether groups. Shi and co-workers have synthesized thioether functionalized mesoporous silicas by the one-pot co-condensation method of TEOS and tetrasulfidesilane (BTESPTS). However, when the thiother content was higher than 10%, the obtained functional mesoporous silica became a wormlike disordered structure [12]. Recently, Jaroniec and co-workers have reported a synthesis of ordered large-pore PMO materials with disulfide bridging groups via a co-condensation method by using the mixture of BTSPDS and TEOS as silica precursors and triblock copolymers as a template [34]. However, the disulfide content was as low as 3.0% (molar ratio) in the framework. With the increasing disulfide content, disordered mesostructured PMOs are obtained. In order to improve the mesostructural regularity, they demonstrated a microwave-assisted approach and obtained ordered hexagonal disulfide-bridged PMOs. Unfortunately, the disulfide content was as low as 2.5% [35]. Yang and co-workers conducted the synthesis in a buffer solution by using Pluronic P123 as a template, however, the ordered disulfide-bridged PMOs could not be obtained when the disulfide content is higher than ∼5.0% [36].

In this paper, a novel and simple metal-ion-assisted approach to synthesize well-ordered hexagonal disulfide-bridged PMOs by using 1,2-bis(triethoxysily)propane bisulfide (BTSPDS) as a precursor and Pluronic P123 as a template is reported. The BTSPDS content in the 3D open frameworks is as high as 20%, while the ordered mesostructure is maintained. Our results clearly show that the metal ions play an important role in the formation of regular mesostructure and incorporation of the long chain disulfide groups into the framework. Furthermore, the adsorption properties for the removal of heavy metal mercury ions from wastewaters were also investigated.

Section snippets

Chemicals

Triblock poly(ethylene oxide)-b-poly(propylene oxide)-b-poly (ethylene oxide) copolymer Pluronic P123 (EO20PO70EO20) (Mw = 5800), tetraethyl orthosilicate (TEOS) (99%), ethanol, hydrochloride acid, Zn(NO3)2·6H2O and Hg(NO3)2 were purchased from Aldrich. Bis(triethoxysilylpropyl)disulfide (C18H42O6S2Si2, BTSPDS, 90%) was purchased from Gelest Company. All chemicals were used as received without any further purification. Millipore water was used in this study.

Synthesis

The ordered mesoporous organosilicas

Mesostructure

Small-angle XRD patterns (Fig. 1a) of the surfactant-free sulphur-containing mesoporous material DS-PMO-15 prepared by the metal-ion (Zn2+)-assisted triblock-copolymer assembly approach with BTSPDS molar content of 15% exhibits three resolved diffraction peaks with the d-value ratios of 1:3:2, which can be indexed as the 1 0 0, 1 1 0 and 2 0 0 reflections, respectively, for the 2D ordered hexagonal mesostructure with the space group p6mm. The corresponding unit cell parameter a0 is calculated to be

Conclusion

Disulfide-bridged PMOs with the ordered hexagonal mesostructure have been synthesized by the metal-ion-assisted assembly approach with co-condensation of BTSPDS and TEOS. By adding Zn2+ ions, which can coordinate either with the dsulfide-bridged groups or ethylene oxide moieties of P123 template, the interaction between silicate species and P123 template can be enhanced, therefore high disulfide groups incorporation (∼20%) into the framework can be achieved and the mesostructural ordering and

Acknowledgements

This work was supported by Discovery grants from the Australian Research Council (DP0773160 and 0879769).

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