Exploiting Nonclassical Crystallization to Tailor Molecular Diffusion in Zeolite Catalysts
Confined channels and cages of zeolites have been widely used as shape-selective heterogeneous catalysts in the petro(chemical) industry. To develop commercial-viable zeolite catalysts, it is critical to understand the mechanism of crystallization to guide the rational design of zeolite catalysts with improved mass-transport properties. This dissertation aims to improve the catalytic performance and synthesis efficiency of zeolite catalysts by capitalizing on the fundamental understanding of zeolite crystallization mechanisms. Nonclassical crystallization pathways involve multiple species that consist of (but are not limited to) multi-ion complexes, oligomers (or clusters), nanoparticles or liquid droplets, and nanocrystallites. Engineering zeolite materials with desired properties often requires the controlled assembly and structural evolution of colloidal precursors to tailor nucleation and growth processes. In this dissertation, we demonstrate that several quaternary amines serve as efficient zeolite growth modifiers (ZGMs) to modulate the kinetics of zeolite SSZ-13 (CHA) crystallization. Notably, polydiallyldimethylammonium (PDDA) was found to have the most pronounced impact on the induction period during nucleation, leading to a 4-fold reduction in crystallization time. A combination of light scattering techniques revealed that PDDA can both induce bridging flocculation of amorphous precursors and promote the precipitation of soluble silicates in a certain narrow range of polymer concentration. Thus, the overall efficiency of SSZ-13 synthesis can be improved by selecting the appropriate additives. The small micropores of zeolite catalysts inherently suffer from severe coke formation, which impedes catalytic performance owing to internal diffusion limitations. The advent of two-dimensional, self-pillared, and hierarchical zeolites have led to materials with superior catalytic performance compared to conventional analogues; however, synthesizing zeolite crystals with sizes less than 100 nm is nontrivial, and often requires the use of complex organics with relatively low yield. In this dissertation, we discovered an alternative and versatile approach to improve the internal mass transport properties by the epitaxial growth of fin-like protrusions on seed crystals, inspired by the phenomenon of nonclassical 3-dimensional nucleation and growth on the surfaces of MFI zeolites. We validated this generalizable practice on two common three-dimensional frameworks (MFI and MEL) and extended the concept to a two-dimensional framework (FER) where it was confirmed in all cases that fins are in crystallographic registry with the underlying seeds. Molecular modeling and time-resolved acid site titration experiments of finned zeolite decoded the internal diffusion and provided more in-depth understanding to explain the remarkable improvements in mass transport (e.g., less coke and/or changes in selectivity), consistent with catalytic tests of benchmark methanol-to-hydrocarbon and 1-butene isomerization reactions showing that these unique finned structures behave as pseudo nanocrystals. This approach can be used to upgrade the performance of commercial catalysts and serve as a generalized platform for the rational design of zeolites across a broad range of framework types for diverse applications in the (petro)chemical industry.