One-dimensional zeolites have distinct pore topologies for shape-selective catalysis, yet their performance in conventional reactions is frequently compromised by severe diffusion limitations. In this presentation, we will discuss how post-synthesis modification of zeolite ZSM-23 (MTT) dramatically improves its mass transport properties. This is accomplished by a facile secondary growth process that introduces surface roughness on the exterior surfaces of ZSM-23 crystals. High resolution electron microscopy images show that roughened interfaces are step bunches of unfinished layers with ultra-small dimensions that present a series of short one-dimensional (1D) channels with significantly reduced diffusion path lengths (10 – 50 nm) compared to the 1D pores of the parent seed crystal (1 μm). This procedure also removes intrinsic defects of as-synthesized MTT crystals, resulting in reduced mass transport limitations. This is reflected in an approximate 45% increase in micropore volume, reaching a value of 0.10 cm3/g that is markedly higher than most ZSM-23 catalysts reported in literature. Comparison of structure-performance relationships for these materials against conventional and nano-sized ZSM-23, as well as ZSM-5 catalysts were established using the methanol to hydrocarbon (MTH) reaction as a benchmark for assessing differences in catalyst activity and selectivity. Modified ZSM-23 catalysts exhibit a dramatic enhancement in turnover number and product selectivity.
In this presentation we will also discuss preliminary data where we have extended analyses of secondary growth to large-pore 1D zeolites, such as ZSM-12 (MTW) and mordenite (MOR). Our findings reveal comparable increases in micropore volume following a post-synthesis treatment across a broad range of Si/Al ratios. Diffusion measurements using various hydrocarbons indicate the presence of intrinsic defects in these structures. We show that these defects are either partially or completely eliminated after secondary growth, thereby enhancing micropore accessibility. Collectively, these findings highlight potential opportunities for exploring modified 1D zeolites as catalysts for commercial applications.
