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Ind. Eng. Chem. Res. 2010, 49, 602–608
Synthesis of Uniform Porous Silica Microspheres with Hydrophilic Polymer as Stabilizing Agent Adham Ahmed,† Rob Clowes,‡ Elizabeth Willneff,‡ Harald Ritchie,§ Peter Myers,† and Haifei Zhang*,† Department of Chemistry, UniVersity of LiVerpool, Crown Street, LiVerpool, L69 7ZD, Centre for Materials DiscoVery, UniVersity of LiVerpool, Crown Street, LiVerpool, L69 7ZD, Thermo Fisher Scientific, 112 Chadwick Rd, Runcorn, U.K.
Porous silica microspheres have wide applications in various areas. It has been a challenge to produce uniform silica microspheres with tunable pore size. In this study, uniform porous silica microspheres were synthesized using a modified Sto¨ber method. Cetyltrimethylammonium bromide (CTAB) was used as a cationic surfactant to introduce ordered mesoporosity into silica spheres. Hydrophilic polymers poly(vinyl alcohol) (PVA), polyvinylpyrrolidone (PVP), and poly(ethylene glycol) (PEG) were introduced into the reaction system. It was found that the use of PVA led to the formation of uniform and well-dispersed porous silica microspheres. The effect of the polymers on pore size and particles morphology was also studied. Another cationic surfactant dihexadecyldimethylammonium bromide (DiCTAB) and a nonionic surfactant (Brij-35) were further investigated by replacing CTAB. Uniform nonporous silica nanospheres were formed for Brij-35 and Brij35/PVA systems. The use of DiCTAB resulted in submicrometer silica spheres with high surface area and larger pore size. The addition of PVA into the reaction system dramatically reduced the surface area and pore volume. The mesopores in the silica microspheres formed from the CTAB-PVA template were expanded using 1,3,5-trimethylbenzene and N,N-dimethyldecylamine as swelling agents by a hydrothermal method. The pore size was increased from 2.7 to 4.5 nm and 6.5 nm, respectively. Introduction Porous silica has been widely and intensively investigated since the discovery of ordered mesoporous silica MCM-41 by Kresge et al.1 due to their broad applications in areas such as catalysis, controlled release, and separation science. Spheres are one of the most investigated morphologies. Nonporous monodispersed silica spheres were first synthesized by Sto¨ber et al. as such monodispersed particulate suspensions could offer many experimental and theoretical advantages.2 To prepare silica particles, it was essential to use a base catalyst such as ammonia in a system also including water, alcohol, and tetraalkoxysilane. The Sto¨ber method was modified to prepare micro/mesoporous silica spheres by introducing a cationic surfactant as the directing agent. CTAB and n-hexadecylpyridinium chloride were used as cationic surfactants, whereas ethanol was chosen as the solvent and tetraorthosilicate (TEOS) as the silica source.3,4 Aqueous ammonia was used as a base catalyst. After the reaction, the surfactant was removed by calcination at 823 K to produce ordered mesoporous MCM-41.3,4 This method was also used to prepare spherical silica with MCM-48 structure5 and also with heteroatoms incorporated into the framework.6 Indeed, by varying the amount of ethanol in the system, a succession of different mesophases on the order of MCM41MCM48-lamellar phase-spherical particles were obtained at room temperature.7,8 It was established that the lower concentration of ethanol had a limited effect on the external morphology. When the ethanol concentration was increased further, it acted mainly as a cosolvent producing spherical particles. The seed growth method was used to obtain large-sized monodisperse silica spheres.9 The formation of secondary * To whom correspondence should be addressed. E-mail:
[email protected]. † Department of Chemistry, University of Liverpool. ‡ Centre for Materials Discovery, University of Liverpool. § Thermo Fisher Scientific.
particles during seed particle growth causing a multimodal distribution of particle sizes was suppressed via fine adjustment of the reaction conditions (such as the amount of ammonia, water concentrations, feeding time, and rigorous agitation). The seed particles had an average diameter 300 nm, whereas most particles were growing to 1-1.6 µm.9 By attaching a fluorescent dye to a silane coupling agent to prepare seed particles, monodisperse fluorescent core-shell silica particles with a diameter of 1.5 µm were successfully synthesized.10 However, these silica particles were nonporous. By adding a (TEOS)/ octadecyltrimethoxysilane (porogen) mixture to a suspension of prepared nonporous silica spheres, submicrometer-sized solid core/mesoporous shell silica spheres were produced after removal of the porogen by calcination. The specific surface area increased with increasing amounts of porogen up to a maximum value of 350 m2/g.11 Cn-TAB (n ) 12, 14, 16, 18) were also used as the structure-directing agents for the mesoporous shell on solid silica spheres with the mesopore channels perpendicular to the surface.12 In a recent study, core silica mesoporous microspheres were made using an aerosol-assisted process in the presence of nonionic surfactant Pluronic P123. The mesoporous shell encapsulating the core microsphere was realized by means of a modified Sto¨ber process with CTAB as a structure-directing agent. Thus, mesoporous silica microspheres with doubly ordered core-shell structure were produced.13 For many applications, controlling the particle size and the pore size of silica spheres is very important. For example, silica spheres are widely used as packing materials for chromatographic applications. The column efficiency could be improved by using smaller uniform silica microspheres (e.g.,
