(621ea) Identifying the Mechanism of SSZ-13 Crystallization and Methods to Tailor Zeolite Properties
Zeolite catalysts are used in a wide range of applications that span petrochemicals to biomass and natural gas conversion. Despite their extensive use in commercial processes, an understanding of their growth mechanism(s) remains elusive. The rational design of zeolite catalysts calls for more versatile synthetic approaches capable of tailoring crystal properties, such as crystal size, morphology, and composition. The ability to selectively tune these properties can improve catalyst performance (e.g., on-stream lifetime and conversion). Here, we will discuss the mechanism of zeolite growth involving transformations from amorphous precursors to the crystalline product. To this end, we will focus on the industrially-relevant zeolite SSZ-13, which has a CHA type framework structure with 3-dimensional pores and small-pore aperture. We examined the temporal evolution of precursors and crystals at various stages of growth and observed a complex series of events involving precursor aggregation and structural rearrangements. As crystallization proceeds, there is a bimodal distribution of crystal size wherein smaller crystals are ~100 nm in dimension and larger crystals are of the order of 2 micron. The larger crystals exhibit rough surfaces and features with similar sizes as those in the small crystallites. We postulate that SSZ-13 grows by an aggregation mechanism and that this process can be controlled through the judicious selection of growth modifiers. We have successfully identified modifiers capable of tuning SSZ-13 crystal size by two orders of magnitude, ranging from 180 nm crystals (i.e., smallest size reported in the literature) and large crystals (ca. 25 micron) that are amenable to AFM and diffusion studies. This facile method is unmatched in its ability to tune SSZ-13 crystal size. Given the fact that modifiers are inexpensive and recoverable (post-synthesis), this practical approach to crystal engineering has the potential to be more broadly applicable to a wider range of zeolite framework types.