Cage-like hierarchically mesoporous hollow silica microspheres templated by mesomorphous polyelectrolyte-surfactant complexes for noble metal nanoparticles immobilization

Abstract

In this paper, well-defined CHMSs were synthesized through co-hydrolysis of tetraethylsiloxane (TEOS) and 3-aminopropyltriethoxysilane (APTS) template by mesomorphous complexes of hexadecylpyridinium chloride (CPC) and poly(acrylic acid) (PAA). It was demonstrated that the adding amount of APTS in the reaction system played a key role in the formation of the hollow cage-like structure and a possible formation mechanism of CHMSs was proposed. The synthesized CHMSs presented as uniform microsphers with average diameter of 600 nm and possessed tri-model pore structure that were the ordered mesopores (˜3 nm) originated from CPC micelle, the secondary nanopores (˜55 nm) templated by phase separated PAA chain in the silica shell as well as the hollow cavity (several hundred nanometer). At the same time, the co-hydrolysis of APTS resulted in amino groups functionalized silica framework of the CHMSs. Ascribed to the unique structure and the inherent amino groups, the CHMSs were utilized as scaffold for preparation of heterogeneous noble metal nanoparticle catalysts. By choosing the palladium (Pd) as a typical example, 1-Pd@CHSMs catalysts containing 1.0 wt% Pd and uniform distributed small size Pd nanoparticles (˜2.9 nm) were obtained. The catalytic reduction of 4-nitrophenol demonstrated that the heterogeneous 1-Pd@CHSMs catalyst was efficient and reusable.

Introduction

The microstructure of inorganic materials plays an important role in their performance [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]]. Specially, inorganic materials with hollow structure have aroused significant interests because of their unique properties, such as large surface area, good dispersibility, and excellent loading capacity [[14], [15], [16], [17], [18], [19], [20], [21]]. So far, various hollow inorganic materials including metal oxides [22,23], rare-earth doped NaYF₄ [[24], [25], [26]], carbonaceous materials [[27], [28], [29]], and silica [30] have been prepared. Among of them, silica materials have advantages of nontoxic, highly biocompatible, mechanically stable material with tunable pore structure, which are significantly important for practical applications in the areas such as sensors, drug delivery systems, energy storage, optics, catalysts, and so on [[31], [32], [33]].

Most fabrication approaches for synthesis of hollow silica microspheres involves the template-assisted method, using either hard templates [[34], [35], [36], [37]] (organic and inorganic particles) or soft templates [[38], [39], [40], [41]] (emulsion droplets, surfactant/polymer micelles, etc.), whose sizes determine the dimension of hollows. In recent years, hollow silica spheres with mesoporous shells have received much attention, as they facilitated the applications, especially in drug delivery and catalysis [[42], [43], [44], [45], [46], [47], [48]]. Generally, mesoporous hollow silica spheres are prepared with dual template such as surfactant/polymer microspheres [43], surfactant/emulsion [45], surfactant/carbon spheres [46], surfactant/ammonium metatungstate hydrate [47], surfactant/trivinyl cyclohexane [48]. In our recent work, we demonstrated the synthesis of mesoporous hollow hydridosilica nanoparticles employing dual template of nonionic triblock copolymer P123 and fluorocarbon surfactant [49]. The synthesized mesoporous hollow hydridosilica nanoparticles were utilized as scaffold for preparation of heterogeneous noble metal catalysts. However, diffusion through these mesoporous silica shells (pores size <10 nm) is often a slow process, which seriously affects mass transfer in the delivery and catalytic process.

To solve this problem, many strategies have been proposed to the synthesis of hollow silica microspheres with macropores (>50 nm) in the silica shell [[50], [51], [52], [53], [54]]. For instance, Sakaguchi et al. first prepared hybrid hollow particles of lysozyme-silica (L-SHHs) through hydrolysis of TEOS in the present of lysozyme under sonication. After calcination of the L-SHHs composite, cage-like hollow spherical silica with through-holes (50–250 nm) in the silica shell were obtained [50]. Fujiwara et al. reported the synthesis of novel hollow silica spheres with nano/macro holes sized up to 120 nm to 1 μm through a water/oil/water double emulsion system [51]. Li et al. reported the preparation of cage-like silica hollow spheres with macroporous shells (hole diameter: 50–300 nm) through a toluene-in-diethylene glycol microemulsion method [49]. As hollow silica microspheres with large pores in silica shell can greatly promote the matter exchange between hollow silica microspheres and external environment [52,54], however the pores in the silica shell are too large to immobilize nanospecies such as noble metal nanoparticles.

Very recently, hierarchically porous materials have been highlighted due to their improved performances in mass transfer and adsorption/desorption process [[55], [56], [57], [58]]. Thus hollow silica microspheres with hierarchically porous shell are such a good material that may be superior to the normal hollow silica microspheres and hierarchically porous silica due to their unique structure. However, the synthesis is still a challenge. Liu et al. have reported the synthesis of hierarchically porous cage-like hollow silica microspheres employing latex and surfactant as dual templates. In their synthetic system, hierarchically porous cage-like hollow silica microspheres can be prepared simply by controlling the surfactant concentration [59]. Zhou et al. demonstrated the construction of hierarchical silica capsules via a tri-template method, where templates of different properties including hexadecyltrimethylammonium bromide surfactant, CaCO₃ rods, and emulsion droplets were utilized to simultaneously fabricate hierarchical pore structure in a single particle [60].

Inspired by the formation processes of the natural biominerals such as sea urchin spines, our group have demonstrated a “dynamic template” method of employing organic mesomorphous complexes formed by oppositely charged polyelectrolyte and surfactant as template for the preparation of hierarchically porous silica materials [[61], [62], [63], [64], [65], [66], [67]]. Herein, cage-like hierarchically mesoporous hollow silica microspheres (CHMSs) were synthesized by the “dynamic template” method simply by adding of 3-aminopropyltriethoxysilane (APTS) in the reaction system. The effect of APTS on the morphology and pore structure of the synthesized silica microspheres was explored and the mechanism of the formation of CHMSs was discussed. Moreover, the synthesized CHMSs were utilized as scaffold for immobilization of palladium nanoparticles and the catalytic properties of the resultant heterogeneous Pd catalyst were evaluated.