Mesoporous silica spheres

Encapsulation via the layer-by-layer assembly of multilayered polyelectrolyte (PE) or PE/nanoparticle nanocomposite thin shells of catalase in bimodal mesoporous silica spheres is also described by Wang and Caruso [198]. The use of a bimodal mesoporous structure allows faster immobilization rates and greater enzyme immobilization capacity (20-40 wt%) in comparison with a monomodal structure. The activity of the encapsulated catalase was retained (70 % after 25 successive batch reactions) and its stability enhanced. 

Yamada, Y. Nakamura, T. Ishi, M. Yano, K., Reversible control of light reflection of a colloidal crystal film fabricated from monodisperse mesoporous silica spheres, Langmuir. 2006, 22, 2444 2446... 

Nooney RI, Thirunavukkarasu D, Chen YM, Josephs R, Ostafin AE (2002) Synthesis of nanoscale mesoporous silica spheres with controlled particle size. Chem Mater 14 4721-4728... 

In this paper, we report the synthesis of mesoporous silica and alumina spheres with nanometer size (80 to 900 nm) in the present of organic solvent with aqueous ammonia as the morphological catalyst to control the hydrolysis of tetraethyl orthosilicate (TEOS) and aluminum tri-sec-butoxide.1181 Mesoporous silica spheres show hexagonal arranged pores with monodispersed pore sizes ( 2.4 nm) and high surface areas ( 1020 m2/g) similar to MCM-41. A large pore ( 10 nm) mesoporous alumina sphere templated by triblock copolymer is thermally stable. Calcined alumina sphere shows disordered mesoporous arrays with relatively uniformed pore size distribution and high surface areas ( 360 m2/g). 

Mesoporous silica spheres were synthesized under the catalyst of ammonia in the mixed water-DMF solvent. In typical synthesis, 0.8 g (2.2 mmol) CTAB was heated slightly to allow it dissolved in the mixed solvent of 19.0 g (1.06 mol) water and 19.0 g (0.26 mol) DMF. After cooling to room temperature, 1.0 g (15 mmol) ammonia and 2.08 g (10 mmol) TEOS were added to the mixture with an electromagnetic stirrer and the stirring rate was kept about 480 rpm. After stirring for 16 to 25 h, the white solid product was Filtered on a Buchner funnel and allowed to dry in air at room temperature. The dried precipitate was immersed into highly diluted aqueous ammonia (pH 10) and kept at 100 °C for 2 days, the product was washed with distilled water and dried at room temperature in air. Then the product was calcined at 550 °C for 4h to remove the templates. 

Figure 1. SEM images of as-synthesized mesoporous silica spheres.

Figure 2. X-ray diffraction (XRD) patterns of as-synthesized and calcined mesoporous silica spheres.

Figure 3. TEM images of calcined mesoporous silica spheres with magnification (a) 85000 (b) 212500.

Figure 4. Nitrogen adsorption and desorption isotherm curves and pore size distribution curve (inset) from the adsorption branch of (a) calcined mesoporous silica sphere and (b) calcined mesoporous alumina sphere.

In summary, nanometer-sized mesoporous silica and alumina spheres with tunable diameters (80 - 900 nm) can be synthesized in organic solvent. Mesoporous silica spheres templated by cationic surfactant (CTAB) have hexagonal array with monodispersed pore size (-2.4 nm), high surface areas (-1020 m2/g), and pore volume (1.02 cm3/g). Mesoporous alumina spheres templated by amphiphilic triblock copolymer show a large disordered mesopore (10.0 nm) and high BET surface area (360 m2/g). 

Based on these observations, Wang and Caruso [237] have described an effective method for the fabrication of robust zeolitic membranes with three-dimensional interconnected macroporous (1.2 pm in diameter) stmctures from mesoporous silica spheres previously seeded with silicalite-1 nanoparticles subjected to a conventional hydrothermal treatment. Subsequently, the zeolite membrane modification via the layer-by-layer electrostatic assembly of polyelectrolytes and catalase on the 3D macroporous stmcture results in a biomacromolecule-functionalized macroporous zeolitic membrane bioreactor suitable for biocatalysts investigations. The enzyme-modified membranes exhibit enhanced reaction stability and also display enzyme activities (for H2O2 decomposition) three orders of magnitude higher than their nonporous planar film counterparts assembled on silica substrates. Therefore, the potential of such structures as bioreactors is enormous. 

This paper describes a novel process for the preparation of spherical mesoporous silica spheres in the submicrometer and micrometer size range. Tetra-n-alkoxysilanes are hydrolysed and condensed in the presence of n-alkylamine as nonionic template and ammonia as catalyst. The porosity and the morphology parameters can be independently adjusted in wide ranges. These materials are promising adsorbents in separations techniques and valuable catalyst supports. 

Figure 1. Synthesis concepts for the preparation of mesoporous silica spheres.

Zeolite monlith with macropores can be considered as a micro-macroporous material. Mechanically stable zeolite monoliths[177] containing 3-D, ordered, closed macropores have been fabricated by hydrothermal treatment of nano-zeolite seeded mesoporous silica spheres. The easy speed of sedimentation and digestion renders the whole process suitable for large-scale production of macroporous zeolite materials. 

A.A. Dong, Y.J. Wang, Y. Tang, Y.H. Zhang, N. Ren, and Z. Gao, Mechanically Stable Zeolite Monoliths with Three-dimensional Ordered Macropores by the Transformation of Mesoporous Silica Spheres. Adv. Mater., 2002, 14, 1506-1510. 

Huo Q et al (1997) Preparation of hard mesoporous silica spheres. Chem Mater 9 14... 

In this paper we will describe the preparation and properties of ZSM-5 synthesized by sequential nano-casting using mesoporous silica spheres with a hexagonal pore arrangement analogous to MCM-41 as starting material. 

Mesoporous silica spheres analogous to MCM-41 (S41) were prepared by the Unger method [6], dissolving cetyltrimethylammonium bromide (CTMABr) in a mixture of water (H2O), ethanol (EtOH) and aqueous ammonia (NH3). After stirring for 10 min at room temperature tetraethyl orthosilicate (TEOS) was added to achieve a molar composition of 1 TEOS 0.4 CTMABr ... 

Sonochemistry in synthesis Spaciousness index Sphares, silicalite Spheres, mesoporous silica Spheres, MFI, hollow Spheres, zeolite, hollow Spin echo double resonance Spinel ceramic from zeolites Spiropyrane in zeolite Y Sr exchange Sr removal Sr,K-KFl, synthesis SSZ-33 ( CON)... 

Fig. 19 Entrapment of enzymes into mesoporous silica sphere covered with LbL films of polyelectrolyte and silica nanoparticle...

Wang Y, Caruso F (2005) Mesoporous silica spheres as supports for enzyme immobilization and encapsulation. Chem Mater 17 953-961... 

Figure 2.6 Scheme of the mechanism for the formation of mesoporous silica. Silica polymers formed initially from silica monomers, and associated with surfactant monomers, which form composite self-organised primary particles which can either continue to grow via monomer addition (path 1) or themselves aggregate in a directional fashion (path 2) to form the final mesophase composite. Nondirectional aggregation would cause formation of disordered pore structures. Reprinted with permission from Nooney, R.I. Thirunavukkarasu, D. Chen, Y. Josephs, R. Ostafin, A.E., Synthesis of Nanoscale Mesoporous Silica Spheres with Controlled Particle Size, Chem. Mater., 14, 4721—4728. Copyright (2002) American Chemical Society... 

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