Silica Self‐Assembly and Synthesis of Microporous and Mesoporous Silicates

Abstract

The microstructure of silica in basic aqueous solutions containing organic cations and prepared from monomeric precursors is reviewed and interpreted within the context of classical ideas of self-assembly of molecular aggregates. The solution properties can be understood by using the pseudo-phase separation approach coupled to the acid-base chemistry of silanol groups and the Poisson-Boltzmann equation. The silica nanoparticles frequently observed in these systems have a core-shell structure with silica in the core and the organic cations at the shell. Individual particles are observed when the forces between particles are repulsive-as is the case for small cations such as tetramethylammonium or tetrapropylammonium-and extended structures such as M41S materials are formed when the forces are attractive--as is the case for surfactants such as cetyltrimethylammonium. These ideas are useful to understand the evolution of zeolite synthesis gels from nucleation to crystal growth. Although at room temperature the silica and the organic cations are segregated, upon heating the organic cations are embedded within the particles. This transformation signals the onset of structure direction whereby the size and geometry of the organic cation induce changes in the structure of silica that may lead to zeolite nuclei.

Vibrational spectra and dissociation of aqueous Na2SiO3 solutions

“…Despite many decades of commercial production experience and research, there is still much empirical knack in these syntheses since the transition of dissolved silicate molecules into desired solids is poorly understood. Research efforts for better understanding have intensified recently [1][2][3][4][5][6][7][8][9][10][11], but especially the initial assembly of molecules into nanoparticles is still quite an obscure process.…”

Synthesis of Porous α-Fe2O3 Nanorods as Catalyst Support and A Novel Method to Deposit Small Gold Colloids on Them

“…Therefore, both the TGA-DTA and the FTIR results reveal that the TEAOH molecules existed in the FeO(OH) product. Similar to the silica case [11], creation of the pores in the nanorods is expected by removal of the TEAOH molecules. This is achieved by the calcination of the FeO(OH) product at 300°C for 0.5 h. Figure 1 presents the isotherms of N 2 adsorption/desorption and pore volume changes before and after creating the pores.…”

“…Generating some shallow pores in the catalyst support to house and fix the Au particles is therefore a possible straightforward solution to improve the stability of the catalysts and their catalytic performances. Inspired by the fact that microporous and mesoporus silicalites were prepared successfully by making use of tetrapropylammonium hydroxide (TPAOH) as the structure director [11], we adopted this method to prepare porous a-Fe 2 O 3 nanorods. However, instead of using TPAOH as the structure director, in our synthesis, tetraethylammonium hydroxide (TEAOH) was used as it could produce pores in the range of 1-5 nm in the a-Fe 2 O 3 nanorods.…”

Monitoring the structure of water soluble silicates

“…A major driving force for these studies is the great technical importance but poor understanding of the synthesis of zeolites and related microporous materials [8,[23][24][25][26]. Nanometer to micrometer sized zeolite-precursor colloid silicate particles have been characterized by small angle X-ray and neutron scattering (SAXS and SANS) [27][28][29], wide angle Xray scattering (WAXS) [29], energy dispersive X-ray dispersion (EDXRD) [30], pulsed electron-nuclear double resonance (ENDOR) spectroscopy [31], dynamic light scattering [32], and other methods many of which were also used for in situ studies.…”

“…The solubility of solid silicates in H 2 O is important in biology, geology, and a variety of technical applications including synthesis of assorted catalysts and adsorbents [1][2][3][4][5][6][7][8]. It is usually assumed that the hydrolytic depolymerization of crystalline or amorphous silicates at elevated temperatures and pressures is a sequential surface process releasing monomers into the aqueous phase [3,5,[9][10][11][12][13]:…”