(625b) Reducing Diffusion-Limitations in One-Dimensional Zeolites By Facile Secondary Treatments

Reducing internal molecular diffusion barriers in zeolites is critical to improving their catalyst activity and stability for various applications to meet the needs of the evolving energy landscape. The intrinsic confined pore networks in zeolite structures give rise to well-defined shape-selectivity, but oftentimes at the expense of reduced activity and lifetime. For example, zeolites with one-dimensional pores, such as mordenite (MOR), are highly selective to propylene in naptha cracking, but they deactivate faster than their 2D or 3D counterparts due to severe mass transport limitations leading to extensive coking and pore mouth blockage. Such limitations can be overcome via a facile post-synthesis treatment to produce fins, which are small protrusions grown epitaxially on the external surfaces of seed crystals. This secondary growth technique has been demonstrated for zeolites with both 3D (MFI, MEL) and 2D (FER) pore networks. In this presentation, we will discuss how this approach can be applied to design finned 1D zeolites (e.g., MOR, MTW, MTT, and LTL). When compared to their original seeds, the finned 1D zeolites show higher catalytic turnovers, higher olefin selectivity, and reduced deactivation rates during methanol to hydrocarbons (MTH) reaction. The finned materials also show superior performance in n-hexane cracking with comparable olefin selectivity to commercial zeolites. Furthermore, we observe differences in the intrinsic size and shape of fins depending on the morphology of the original seed. For example, highly anisotropic rod-like MTT crystals exhibit fins that manifest as rough surfaces (i.e., unit-cell thick ‘surface terminations’) due to the high energetic barriers for surface nucleation. Conversely, materials that are more isotropic in morphology (e.g., MTW and MOR) exhibit more traditional fin-like protrusions. These morphological differences in finned 1D zeolites result in different catalyst performance as well as distinct mechanisms of coke formation that we explore through multiple characterization techniques.