(228a) Designing Zeolite Catalysts with Enhanced Diffusion Properties

Designing zeolites with reduced diffusion limitations is critical to improving their catalytic activity and stability in commercial applications such as methanol-to-hydrocarbons (MTH) reactions. Hierarchical and nano-sized zeolites have been explored in this regard, but preparation of such materials is non-trivial and successful syntheses have only been demonstrated for relatively few frameworks. Here, we present alternate approaches to achieve zeolites with improved mass transport properties using two different techniques. The first is the preparation of finned zeolites as a new class of materials where secondary growth of small protrusions (or fins) on the external surfaces of zeolite seeds transforms these catalysts into pseudo nano-sized materials with catalytic performance that far exceeds conventional materials. We will also present side-by-side comparisons that show how finned zeolites with tailored properties (e.g. Si/Al ratio) impact catalyst performance relative to a broader set of well-known hierarchical materials, including self-pillared pentasils (SPP), nanocrystals (e.g. 3Dom), and two-dimensional nanosheets.

A second class of zeolitic materials that will be discussed is core-shells, wherein a thin siliceous shell is epitaxially grown on the surfaces of aluminosilicate seed crystals. These core-shell configurations were realized for zeolites ZSM-5 (MFI) and ZSM-11 (MEL) where we have shown that the incorporation of siliceous shells via facile secondary growth enhances mass transport, leading to MTH catalysts with markedly improved lifetime and total turnover number. We will present these results along with a mechanistic interpretation wherein we have shown that the improved catalytic activity of core-shells is attributed to enhanced diffusion through the siliceous shell and surface passivation (i.e. reduced external coking). Our findings also reveal that a reverse configuration, the eggshell, produce pseudo nanosheets with a five-fold increase in catalyst lifetime and total turnovers compared to homogenous counterparts.