Materials science communicationFurther investigations on the modified Stöber method for spherical MCM-41
Introduction
Since Stöber et al. [1] synthesized silica spheres by hydrolyzing alkylsilicates in mixed solutions of ammonia, alcohol and water in 1968, the sol–gel based wet-chemistry route to prepare silica spheres has widely been concerned [2], [3], [4]. About 30 years later, Unger's group [5] successfully prepared ordered mesoporous silica spheres in the same system by introducing C16TAB, which has been initially and continually used as a template for synthesizing ordered mesoporous silica materials [6], [7]. The latter process has been assigned as a “modified Stöber method” [8], [9], [10], [11]. The obtained MCM-41 spheres have shown superior performance to non-spherical MCM-41 particles when used as column filler materials in a high-performance liquid chromatography (HPLC) [12].
We carried out further investigations on the “modified Stöber method” to understand factors influencing the synthesized materials, providing more information for future researchers to get ordered mesoporous silica spheres with desirable properties.
Section snippets
Experimental
The synthesis procedure from ref. [5] was adopted. All chemicals were from ACROS Organic and used as received without further purification. Typically, C16TAB was dissolved in distilled water, aqueous ammonia and ethanol solution. After it was stirred for 15 min, tetraethyl orthosilicate (TEOS) was added. The solution was further stirred 2 h. A gel with a molar composition of 1 TEOS:0.3 C16TAB:11 NH3:58 ethanol:144 H2O was obtained. The resulting solid was collected by filtration and washed with
Influence of the reaction time
The XRD patterns for the samples synthesized with 2 and 16 h are depicted in Fig. 1A. A slight left-shift of the (1 0 0) diffraction peak can be seen upon increasing the reaction time. Consequently, increments of the interplanar space, d100, and the unit cell parameter, a, can be expected according to the formula d100 = λ/2 sin θ and a = 2d100/√3 [7]. Meanwhile, the N2 sorption isotherms (Fig. 1B) show a significant ascending of the plateau for the sample synthesized with 16 h. Calculations indicate that
Conclusions
- (1)
Prolonging the reaction time leads to an increase of the pore volume; increasing the amount of ammonia solution results in increases of the pore diameter and decreases of the pore volume and the specific surface area. However, increasing the amount of C16TAB, the pore volume and the specific surface area increases while the pore diameter keeps intact.
- (2)
Hydrothermal post-synthesis treatment can greatly improve the crystallinity of the mesoporous microspheres without changing the spherical
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