| Abstract
| - Aggregation crystal growth of TPA-silicalite-1 from pre-assembled nanoparticles in synthesis solution has two steps: attachment and alignment/merging. The second step may cause a rise either in surface charge density (exothermic route) or in solution pH (endothermic route). Crystal growth often switches from an exothermic route to an endothermic one.
- The synthesis process of TPA-silicalite-1 from a series of initially clear solutions (with ageneral 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 sizeof the nanoparticles detected in the initially clear solutions decreases from 8.5 to 2.5 nm asx increases from 3 to 13. Crystal growth from these solutions at 95 °C is first exothermicand then endothermic. The exo−endo thermal switch coincides with a jump in the solutionalkalinity. With increasing x the integral heat of exothermic crystal growth decreasesmonotonically from −0.61 kJ/mol (per mole of Si in the mixture) at x = 4 and approacheszero at x = 12. The integral heat of endothermic crystal growth increases to approach aplateau (0.26 kJ/mol of Si) at x = 7. Crystal growth is fastest at x = 6−7 and relativelyslower at x either higher than 7 or lower than 6. At x = 6−11 the zeolite particles assumesimilar 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 (onsilica 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. Thethermodynamic and kinetic aspects of the synthesis process are analyzed and discussed interms of total surface area reduction of silicate species during crystal growth, effect of surfacecharge density on kinetics and mechanism of crystal growth, relationship between thermalswitch and solution pH jump, and the change in enthalpy and entropy of the synthesis system.
|
