Discovery of Stable Extra-large Pore Zeolites Based on Rational Design of Structure-directing Agents.

Abstract

ConspectusAluminosilicate zeolites possess ordered pore structures and tunable acidities as well as remarkable thermal/hydrothermal stabilities, which render them highly suitable for industrial applications in petroleum refining, in organic chemical synthesis, as catalysts, and in gas separation technologies as adsorbents. Computational predictions indicate that millions of theoretical zeolite framework types may be realizable; however, the International Zeolite Association (IZA) has authenticated only 260 zeolite framework types to date. Among these, approximately 20 types have found industrial applications, and their pore systems are generally confined to not exceeding the 12-membered ring (MR). There is an intense demand in industry to develop novel, stable, three-dimensional (3D) zeolites with extra-large pores, which are essential for heavy oil conversion and macromolecular catalysis.Over the past 30 years, more than 30 extra-large pore zeolites have been synthesized by using bulky organic structure-directing agents (SDAs) as templates. Nevertheless, their utilization has been restricted by various factors, like inferior thermal and hydrothermal stability, unexpected interrupted frameworks, and limitation of 3D connectivity. Research on the exploration of stable 3D extra-large pore zeolites has advanced sluggishly, and there has been a persistent inability to achieve breakthroughs along this direction. Recently, we reported the ZEO series of 3D stable silica-based extra-large pore zeolites, which represents a synthetic breakthrough in this area. This Account focuses on the timeline and comprehensively introduces our decade-long efforts in developing novel 3D stable extra-large pore zeolites, especially for the progressive innovation of designing SDAs. Starting with the semirigid imidazole salts as SDAs, we successfully synthesized NUD-1/2/3, a series of new large and extra-large pore germanosilicate zeolites; later efforts were extended to highly rigid benzimidazole-based SDAs, which led to the successful synthesis of high silica and pure silica extra-large pore zeolites NUD-5/6. Although imidazole salts were found to be much more efficient, their lower stability under alkaline and high temperature conditions limited their application. At the same time, most of the as-synthesized zeolites have only 1D extra-large pores due to the strong molecular interactions between the SDAs caused by their aromatic rings. To address this issue, bulky and stable SDAs derived from cycloalkyl phosphines were subsequently developed. With tricyclohexylmethylphosphonium (TCyMP) as the SDA, we synthesized the first 3D stable extra-large pore aluminosilicate zeolite, ZEO-1, which is a breakthrough in zeolite synthesis. Furthermore, a new 1D to 3D topotactic condensation mechanism stemming from a novel 1D chain silicate, ZEO-2, was discovered, by which the 3D stable extra-large pore zeolites ZEO-3 and ZEO-5 were synthesized, continuously expanding the pore size limits of 3D stable zeolites. The ZEO series, along with the recently reported ZMQ-1, fills the existing gap in 3D stable zeolites between large pore zeolites and mesoporous materials. This achievement paves the way for the selective catalysis of macromolecules in the future. The discovery of these extra-large pore zeolites benefits from not only the synthetic advancements but also the technological advancements in structural characterizations. Furthermore, machine learning as an emerging technique is expected to play a pivotal role in propelling the development of zeolite science.