Study on Synthesis of TPA-Silicalite-1 from Initially Clear Solutions of Various Base Concentrations by in Situ Calorimetry, Potentiometry, and SAXS

Abstract

The synthesis process of TPA-silicalite-1 from a series of initially clear solutions (with a general formula based on mole ratios of the components x:25:480:100 TPAOH:SiO2:H2O:C2H5OH, where TPA = tetrapropylammonium, x = 3−13) was investigated using small-angle X-ray scattering (SAXS), in situ calorimetry, and in situ pH measurement. The size of the nanoparticles detected in the initially clear solutions decreases from 8.5 to 2.5 nm as x increases from 3 to 13. Crystal growth from these solutions at 95 °C is first exothermic and then endothermic. The exo−endo thermal switch coincides with a jump in the solution alkalinity. With increasing x the integral heat of exothermic crystal growth decreases monotonically from −0.61 kJ/mol (per mole of Si in the mixture) at x = 4 and approaches zero at x = 12. The integral heat of endothermic crystal growth increases to approach a plateau (0.26 kJ/mol of Si) at x = 7. Crystal growth is fastest at x = 6−7 and relatively slower at x either higher than 7 or lower than 6...