Understanding the Anion-Templated, OSDA-Free, Interzeolite Conversion Synthesis of High Silica Zeolite ZK-5

The Current Understanding of Mechanistic Pathways in Zeolite Crystallization

Zeolite catalysts and adsorbents have been an integral part of many commercial processes and are projected to play a significant role in emerging technologies to address the changing energy and environmental landscapes. The ability to rationally design zeolites with tailored properties relies on a fundamental understanding of crystallization pathways to strategically manipulate processes of nucleation and growth. The complexity of zeolite growth media engenders a diversity of crystallization mechanisms that can manifest at different synthesis stages. In this review, we discuss the current understanding of classical and nonclassical pathways associated with the formation of (alumino)silicate zeolites. We begin with a brief overview of zeolite history and seminal advancements, followed by a comprehensive discussion of different classes of zeolite precursors with respect to their methods of assembly and physicochemical properties. The following two sections provide detailed discussions of nucleation and growth pathways wherein we emphasize general trends and highlight specific observations for select zeolite framework types. We then close with conclusions and future outlook to summarize key hypotheses, current knowledge gaps, and potential opportunities to guide zeolite synthesis toward a more exact science.

Structural Aspects Affecting Phase Selection in Inorganic Zeolite Synthesis

A guideline for zeolite phase selection in inorganic synthesis media is proposed, based on a systematic exploration of synthesis from inorganic media using liquid Na⁺, K⁺, and Cs⁺ aluminosilicate. Although the Si/Al ratio of the zeolites is a continuous function of the synthesis conditions, boundaries between topologies are sharp. The here-derived phase selection criterion relates the obtained zeolite topology to the Si/Al ratio imposed by the synthesis medium. For a given Si/Al ratio, the framework with the highest occupation of topologically available cation sites is favored. The large number of published zeolite syntheses supporting the observation provides strong indication that the concept is applicable in a larger context. The proposed criterion explains how minor variations in the composition of inorganic synthesis media induce the commonly occurring, abrupt changes in topology. It highlights underlying reasons causing the strict demarcation of stability fields of the as-synthesized zeolites experimentally observed in inorganic synthesis.