Electro-Assisted zeolite Synthesis: where material design meets electrochemistry

Zeolites are crystalline microporous materials with a well-defined porous structure that makes them useful for a wide range of chemical applications on an industrial level, such as adsorption and separation, ion exchange, catalysis, and sensor fabrication. Their three-dimensional (3D) structure is formed by a network of TO4 tetrahedra, which are classically made of aluminum and/or silicon (T-atoms) with four neighboring oxygens. Incorporating other T-atoms into a zeolite framework, such as B, Ti, Sn, and Zn, can improve and/or alter their (Lewis) acidity. The hydro(solvo)thermal nature of zeolite synthesis is of extreme complexity, and the resulting outcome depends on multiple parameter combinations.

The delicate equilibrium during critical crystallization events dictates the final result and its properties. Therefore, it is a long standing challenge to find a universal tool for zeolite synthesis manipulation. In this dissertation, the force of an electric field is chosen to be the suitable candidate for such control. For instance, due to the immense number of Coulomb interactions between charged species, an additional force will restrict possible changes in the molecular interplay and guide a crystallization mixture towards a valuable product, pronounced in zeolite outcome selectivity, morphology, etc. For conducting this interdisciplinary research between the field of microporous material synthesis and electrochemistry, a special reactor was built and this proved to be an essential weapon for exploring hypotheses and potential new modes of control.

Hence, the first investigated aspect considered in this work deals with zeolite synthesis under the influence of external electric fields. By choosing a crystallization recipe with sensitive equilibria (e.g., phase competition), we tried to manipulate phase purity outcome and aluminum distribution. While this subject and the gathered results are debatable, perhaps more than hoped for, we were able to demonstrate that high-voltage electric field can inhibit formation of FAU phase in the competing EAB/FAU synthesis system. Moreover, the influence can have origins in the nucleation stage of the crystallization, which seems reasonable to assume as given the day-by-day synthesis experiments.

Another path taken in search for electrochemical zeolite innovation is metal anodic oxidation, which is widely applied in machining and electroplating. On the other hand, it is well-established that introducing heteroatoms (i.e., other than silicon) into a zeolite structure is a rewarding pathway to superior materials. While incorporation of Al and the resulting Brønsted acidity are widely studied, novel approaches for the metal distribution and improved activity of many other elements (e.g., Ti, Sn, Zn) that create Lewis acidity are less widely encountered. Unfortunately, desired level of Lewis acid sites are not available due to the interference of the elemental composition desired and the growth process.

Therefore, the second major topic of this thesis concerns the introduction of heteroatom elements into zeolite lattices after or via electrochemical release from a metal electrode. This technique, never previously introduced for zeolites, establishes a new era for heteroatom containing material synthesis due to the exceptional flexibility and control. We showcased the applicability of electro-assisted synthesis by creation of 4 different zeolite structures and incorporation of 5 types of metals by electrode dissolution and by offering access to a novel mixed-metal zeolite. Timed and voltage-controlled metal release leads to a 15-fold increased incorporation for the most relevant stannosilicates due to minimizing the interference between tin and the zeolite crystallization.

As a result, this method effectively expands the synthesis space of zeolites and the electro-made materials with record tin content show higher productivities than their classic counterparts in lactate catalysis. Oddly, the most fascinating zeotype – Sn‑BEA – with the highest ever mentioned tin content (Si/Sn 14), was proven by characterization techniques and catalysis to narrow its pore openings due to the size and amount of the inserted heteroatom. This phenomenon is also described for the first time, and opens a new avenue for microporous materials modification.