Early-stage reactions in synthesis of TPA-silicalite-1: Studies by in situ calorimetry, SAXS, and pH measurements

In situ calorimetry, small-angle X-ray scattering, and pH measurements were used to investigate early stages of nanopatticle formation during early stages of synthesis of silicalite-1 in an initially clear solution (9TPAOH-25SiO(2)-480H(2)O-100C(2)H(5)OH, TPA = tetrapropylammonium). The solution was prepared by incremental addition of tetraethyl orthosilicate (TEOS) (from 0% to 100% of the required amount) relative to a given amount of TPAOH solution. The reaction is strongly exothermic (ca. -100 kJ/mol-Si) at early TEOS additions. Part of the observed exothermic effect at this stage is attributed to the mixing enthalpy of ethanol (byproduct from hydrolysis of TEOS) with the solution and the acid-base reaction reflected by the decrease in the net hydroxide concentration in solution. With increased TEOS addition, the reaction becomes less exothermic and then the reaction enthalpy levels off at 50% addition of TEOS at -50 +/- 2 kJ/mol-Si. At 50-60% addition of TEOS, the reaction rate gradually decreases to a minimum and thereafter increases. With increase of TEOS addition up to 40%, the mass of the nanoparticles formed in the solution only increases slightly and their size remains about 2.2 nm in diameter. Not until over 60% addition of TEOS is a rapid growth of their size and mass seen. The analysis and implication of the experimental results to the evolution of the nanoparticles are discussed.