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| - Early-Stage Reactions in Synthesis of TPA−Silicalite-1: Studies by in Situ Calorimetry, SAXS, and pHMeasurements
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| - Formation of TPA−silicalite-1 precursor nanoparticles is strongly exothermic. The reaction first consumes considerable base and forms mainly silicate oligomers. Then the formation of nanoparticles proceeds rapidly because of the decrease in solution pH and the surface charge effect. Polymerization/condensation of silicate species rather than hydrolysis of TEOS controls the reaction kinetics.
- In situ calorimetry, small-angle X-ray scattering, and pH measurements were used toinvestigate early stages of nanopatticle formation during early stages of synthesis ofsilicalite-1 in an initially clear solution (9TPAOH−25SiO2−480H2O−100C2H5OH, TPA =tetrapropylammonium). The solution was prepared by incremental addition of tetraethylorthosilicate (TEOS) (from 0% to 100% of the required amount) relative to a given amountof TPAOH solution. The reaction is strongly exothermic (ca. −100 kJ/mol-Si) at early TEOSadditions. Part of the observed exothermic effect at this stage is attributed to the mixingenthalpy 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. Withincreased TEOS addition, the reaction becomes less exothermic and then the reactionenthalpy levels off at 50% addition of TEOS at −50 ± 2 kJ/mol-Si. At 50−60% addition ofTEOS, the reaction rate gradually decreases to a minimum and thereafter increases. Withincrease of TEOS addition up to 40%, the mass of the nanoparticles formed in the solutiononly 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 implicationof the experimental results to the evolution of the nanoparticles are discussed.
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