Design Strategies for the Synthesis of Zeolites with Large or Extra-Large Features
ExxonMobil Research and Engineering
Organic structure directing agents (SDAs) are instrumental in guiding the formation of high-silica zeolites. A strategy in our efforts has been to avoid the promotion of common zeolites (e.g. ZSM-5, ZSM-12) by employing diquaternary ammonium molecules (diquats) with bulky end groups that make them unlikely to fit within the channel systems of these "default" phases. Although these prior efforts produced a few novel zeolites phases, we still were unable to avoid known phases like ZSM-50 (EUO) or the SSZ-26/33 family of zeolites. We postulated that the placement of additional bulk within the center of the diquaternary ammonium molecules would make the SDA molecules less likely to form common materials. To be effective SDAs, it is also important that the molecules be relatively stable under the conditions of synthesis. Zeolites are typically prepared under highly basic conditions at high temperatures, and the first appearance of a novel zeolite may require weeks or even months of heating. To this end, we have prepared diquat molecules with phenyl or cyclohexyl cores that have at least 3 methylene carbons between them and the bulky end groups. This family of molecules provides many avenues for design in terms of the o, m, or p-placement of the methylene chains to the phenyl core, the nature of the core, the nature of the ends groups, and the length of the methylene chains. Small changes in this family have dramatic effects on the zeolite phase selectivity. This strategy led to the discovery of zeolites with novel frameworks: EMM-28, EMM-31, EMM-41, and EMM-59.
We continued the theme of bulky SDA molecules by examining quats or diquats synthesized from benzimidaolium derivatives. These efforts led to the discovery of zeolites that either have very large cages or extra-large pores or windows. In one case, we discovered a highly siliceous composition of a zeolite that previously has been prepared as a low-silica zeolite. Our examination of this material led to the realization that the diquaternary ammonium analogues have the ability to intramolecularly pi-stack to form large molecular assemblies. Here we introduce the concept of intramolecular assembly that is akin to folding of biologically-based macromolecules. We subsequently designed other molecules that can intramolecularly pi-stack by using simple 1-step alkylation reactions with inexpensive chemical reagents. We will show how these easily prepared molecules are able to direct zeolites that previously required molecules that are expensive or difficult to prepare. Furthermore, we will show how some of these molecules are able to make "exotic" zeolite phases in compositions not previously attainable.