In situ calorimetric study of the growth of silica TPA-MFI crystals from an initially clear solution

Direct evidence from in situ calorimetric experiments demonstrates that the crystal growth of silica TPA-MFI zeolite (TPA = tetrapropylammonium, MFI = framework structure code of ZSM-5) at 95°C from an initially clear solution (9.00TPAOH-25.0SiO₂-480.0H₂O- 100.0C₂H₅OH) is first exothermic and then endothermic. The distinct exo → endo thermal switch coincides with a sharp pH rise of the mother liquor from 12.6 to 13.3. The crystal growth in the exothermic stage is driven by a constant enthalpy release of -0.514 ± 0.014 kJ/mol of SiO₂. The accumulated crystal mass is consistent with a rate of crystal growth that is linear with time and with the integral enthalpy change. In the endothermic stage of growth, the energetic driving force diminishes quickly and an increasing energetic hindering force ensues, about 2.5 kJ/mol of SiO₂ near the end of the reaction. Crystal growth under an energetic hindering force implies an entropic driving force, which is mainly attributed to the liberation of small chemical species such as OH^- and/or H₂O into solution from the eliminated interface. The results are discussed on the basis of a crystal growth mechanism of orderly aggregation of the preassembled primary TPA-MFI particles about 3 nm in diameter formed in the initial solution. Because of the small size and large specific surface area of the particles involved in the crystal growth, the crystal growth results in significant reduction of the overall interface between the solid particles and mother liquor. In the exothermic stage of growth, the constant solution pH indicates that the surface charge on the eliminated interface is compressed onto the remaining interface via deprotonation of the silanols there. However, in the endothermic stage, the rapid increase of the solution pH suggests that further compression of the surface charge by crystal growth apparently becomes energetically prohibited so that releasing OH^- into solution prevails. In much of the crystal growth period, the reaction rate is controlled by surface reaction kinetics. Only when approaching the end of crystal growth does the energetic hindering force become so significant that the thermodynamics gradually dominates the reaction process and finally terminates the reaction.