Rational Design of Zeolite Catalysts for Methanol to Hydrocarbons Reaction

Zeolites are microporous crystalline materials with tunable solid acidity and shape/size selectivity. The unique combination of these properties endows zeolites with extensive applications from separations and ion exchange to catalysis. For zeolite catalysts, understanding structure-performance relationships is crucial for their rational design. The key design variables of zeolite catalysts include framework topology, crystal size/morphology, and solid acidity, among others. This dissertation aims to improve the performance of zeolite catalysts by elucidating and deconvoluting the effects of the aforementioned properties. To this end, a series of ZSM-11 (MEL) and ZSM-5 (MFI) zeolite catalysts of equivalent composition, but differing pore architecture, are prepared with well-defined sizes to deconvolute the effects of framework topology and diffusion path length. For these studies, we selected methanol to hydrocarbons (MTH) to assess the impact of these design variables on the catalyst performance and the hydrocarbon pool (HCP) of MTH chemistry. Our findings reveal that variations in framework topology and crystal size result in similar effects on apparent catalytic performance (e.g., lifetime and selectivity), but different evolutionary behaviors of hydrocarbon species within the zeolite pores. To further improve the diffusion properties and alleviate catalyst deactivation due to coking, a new concept of introducing rough features on crystal surfaces is introduced. It is posited that roughened crystal interfaces can enhance mass transfer by creating more surface micropore openings. Our results from studies of ZSM-11 show that rough crystals can markedly enhance MTH reaction lifetime relative to conventional catalysts. Despite the better diffusion properties of the MEL framework, ZSM-11 synthesis is known to require an organic structure-directing agent (OSDA), which accounts for a large portion of zeolite synthesis cost. In this body of work, a new route to synthesize nano-sized ZSM-11 is developed using 1,8-diaminooctane (DAO) as a low-cost OSDA. The synthesis process is further optimized with a seeded method to dramatically reduce the synthesis temperature, heating time, and OSDA quantity. Based on this optimized synthesis method, another study focuses on controlling the propene/ethene selectivity of MTH reactions by modifying the acidity of ZSM-11 through the incorporation of mixed Al/Ga heteroatoms.