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

EIS was used to detect and investigate nucleation in silicalite-1 clear solutions. Whereas 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 the conductivity with increased silicon concentrations. Inverse Laplace transformation of the complex conductivity allows to observe the chemical exchange phenomena governing nano-aggregate formation. At low temperatures, the fast exchange between dissociated ions and ion pairs leads to a gradual decay of the conductivity with increasing silicon content. Upon heating, the exchange rate is slower and the residence time of ion pairs inside the nano-aggregates 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 to observe slow chemical exchange processes occurring in zeolite precursors. Up to now, such processes could only be observed using techniques such as nuclear magnetic or electron paramagnetic resonance spectroscopy. In addition, EIS enables the quantification of interfacial processes via the double-layer capacitance. The electrical double layer thickness, derived from the double-layer capacitance, shows a similar gradual decay and confirms that the onset of nano-aggregate formation is indeed not narrowly defined.