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

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. At x = 6-11 the zeolite particles assume similar spherical shape and size, about 100 nm in diameter. Outside this range (x < 6 or x > 11), particle size increases significantly. The percentage yield of the zeolite product (on silica basis) can be linearly correlated to x (yield % = 119.7 - 6.34x) from x = 5 to x = 11.5. Outside this linear zone the yield is below this trend at both lower and higher x. The thermodynamic and kinetic aspects of the synthesis process are analyzed and discussed in terms of total surface area reduction of silicate species during crystal growth, effect of surface charge density on kinetics and mechanism of crystal growth, relationship between thermal switch and solution pH jump, and the change in enthalpy and entropy of the synthesis system.