The Quest for Electrokinetic Control over Bulk Zeolite Synthesis: Trials, Challenges and Effects

Abstract

While electrostatic stabilization between negatively charged aluminosilicate species and positively charged structure-directing agents is widely recognized as a key driving force in zeolite crystallization, the net interaction energy is often a superposition of Coulomb, steric, Van der Waals, and solvation contributions, which are challenging to deconvolute experimentally. Close energetic competitions and metastability often prioritize kinetic control over thermodynamic stability, which cannot always be described by classical kinetic laws. Here, taking into account the charge-rich media, we hypothesize that electrokinetic control (EKC) can be considered as a reactor-based adaptation for manipulating the crystallization of microporous materials, as was illustrated in the state-of-the-art for proteins. Various crystallizing zeolite systems were experimentally studied under (high-)voltage direct and alternating current (DC and AC) electric fields (EFs) up to 20 kV, using different reactor setups. The conceptual ideas of EKC effects in steering zeolites nucleation and crystallization thermodynamics and kinetics were tested. The potential effects of EKC over hydrothermal synthesis were assessed through product phase selectivity, particle size, and aluminum content. Four different setups were developed to match the studied EF modes (internal and external, with both uniform and non-uniform fields). The setup designs and EF configurations were further complicated by the need to adapt them to the harsh conditions of bulk hydrothermal synthesis. We believe this work offers valuable insights into the effects of charge interactions during intricate zeolite synthesis. While inconclusive in many series of experiments, this experimental work details the journey of our search for EKC across a wide range of conditions. Its results, supported by conceptual theories, can guide future researchers in how to select appropriate EF parameters, build effective reactor setups and target the right types of zeolite recipes to maximize the chances for manipulating zeolite nucleation and growth using EKC.