Langmuir, Vol.23, No.8, 4532-4540, 2007
Initial stages of self-organization of silica-alumina gels in zeolite synthesis
We report the effect of aluminum on the formation and structure of silica nanoparticles formed in basic solutions of small organic and inorganic cations using a combination of small-angle X-ray scattering, conductivity, pH, and Al-27 NMR spectroscopy methods. At low silica concentrations, our observations agree with previous reports and show the formation of small oligomers ((HO)(4-n)Al(OSi(OH)(3))(n))(-) that can be modeled qualitatively using a simple aqueous speciation model. We also find that aluminum drastically reduces the concentration of silica at which nanoparticles are formed. Using organic cations, the particles are smaller than the ones observed in pure silica systems (1.5 nm vs similar to 3 nm diameter), but in the case of sodium cations, larger particles are detected (similar to 10 nm in diameter). The data suggests that sodium cations are incorporated within the inorganic silica/alumina core, as opposed to organic cations that appear to be excluded from such clusters. Important insights are gained by making analogies to the behavior of mixed surfactant systems using regular solution theory and noting that the formation of Si-O-Al bonds (as measured by the Delta G degrees(rnx) of condensation) is much more favorable than the formation of Si-O-Si bonds.