(689b) Controlling Intrinsic Defects in Hierarchical Zeolite ZSM-5 Catalysts and Their Impact on Methanol to Hydrocarbons

Zeolite ZSM-5 is a commonly used catalyst in many (petro)chemical processes such as methanol-to-hydrocarbons (MTH) and fluidized catalytic cracking (FCC); however, the synthesis of ZSM-5 is highly prone to the generation of defects, which can range from residual amorphous material or crystallographic dislocations to non-framework or extra-framework Al species. Minimizing these defects is a subject of interest owing to their correlation with the onset of catalyst deactivation. Conventional strategies to produce defect-free ZSM-5 crystals involve bottom-up syntheses with fluoride ions (using HF or NH₄F), or post-synthesis treatment with mild acids. Here, we will discuss how defects are more prevalent in hierarchical ZSM-5 (MFI-type) materials than previously reported in literature. For this study, we prepared five classes of nanosized/hierarchical zeolites that include nanosheets (ca. 3 nm thickness), self-pillared pentasils (ca. 3 nm thickness), nanoparticles (ca. 20 nm), finned zeolites (ca. 30 nm fins), and coreshells (ca. 10 nm shell thickness). Using MTH as a benchmark reaction, our studies reveal that catalyst performance can be dramatically improved via post-synthesis treatments to reduce defects, which include so-called annealing wherein crystals are subjected to a saturated siliceous solution at high-temperature. The impact of post-synthesis treatment on ZSM-5 catalyst performance is highly dependent on the composition (i.e. Si/Al ratio) and the nature of defects in each sample. Our findings reveal that the removal of defects from ZSM-5 catalysts result in as low as 3-fold to as high as 10-fold increases in turnover number compared to their as-synthesized counterparts. To our knowledge, this study is the first comprehensive comparison of state-of-the-art hierarchical ZSM-5 catalysts, highlighting (i) syntheses among the most highly cited in literature that are more prone to defect formation and (ii) the class of ZSM-5 hierarchical/nano-sized materials yielding the best MTH performance.