Revisiting Silicalite-1 Nucleation in Clear Solution by Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy (EIS) was used to detect and investigate nucleation in silicalite-1 clear solutions. Although zeolite nucleation was previously assumed to be a step event, inducing a sharp discontinuity around a Si/OH^- ratio of 1, complex bulk conductivity measurements at elevated temperatures reveal a gradual decay of conductivity with increased silicon concentrations. Inverse Laplace transformation of the complex conductivity allows the observation of the chemical exchange phenomena governing nanoaggregate formation. At low temperatures, the fast exchange between dissociated ions and ion pairs leads to a gradual decay of conductivity with an increasing silicon content. Upon heating, the exchange rate is slower and the residence time of ion pairs inside of the nanoaggregates is increasing, facilitating the crystallization process. This results in a bilinear chemical exchange and gives rise to the discontinuity at the Si/OH^- ratio of 1, as observed by Fedeyko et al. EIS allows the observation of slow chemical exchange processes occurring in zeolite precursors. Until now, such processes could be observed only using techniques such as nuclear magnetic or electron paramagnetic resonance spectroscopy. In addition, EIS enables the quantification of interfacial processes via the double layer (DL) capacitance. The electrical DL thickness, derived from the DL capacitance, shows a similar gradual decay and confirms that the onset of nanoaggregate formation is indeed not narrowly defined.