(54f) Synthesis, Characterization, and Catalytic Applications of MFI/Ton -Type Zeolite Intergrowth

Integrating MFI and TON zeolite frameworks into a single intergrown material provides a strategy to combine the kinetic and diffusion characteristics of both topologies in one medium-pore zeolite structure. MFI zeolites contain three-dimensional channel networks, which facilitate mass transport in propene oligomerization reactions, but heavier hydrocarbon products accumulate within micropores during catalysis and lower effective molecular diffusivities. Conversely, TON zeolites contain one-dimensional channels and show higher selectivity to lighter C₆ dimer products, resulting in less severe intrazeolite diffusion restrictions imposed by the occluded hydrocarbon phase during oligomerization catalysis.

Here, we report MFI/TON intergrowths synthesized via co-templating strategy using dual organic structure directing agents (OSDA). We systematically vary precursor gel composition, pH, crystallization temperature, and time to identify conditions that crystallize solids with X-ray diffraction patterns that exhibit characteristic peaks of both frameworks and scanning electron microscopy images that show needle-like aggregates reminiscent of TON. High-resolution TEM (HAADF-STEM and iDPC) confirms the coherent intergrowth of three- and one-dimensional channel domains.

Catalytic tests included toluene methylation under conditions of strict kinetic control (403 K, <2% conversion; 4 kPa toluene) and propene oligomerization under reaction–diffusion–limited conditions (503 K, <1% conversion; 16 kPa C₃H₆). In toluene methylation, the intergrown material exhibited p‑xylene selectivity comparable to pure TON, indicating that acid sites are predominantly located in unidimensional TON-like channels. During propene oligomerization, the intergrown material showed higher initial rates than a physical mixture of MFI and TON of similar composition and acid site content, while shifting product distribution toward heavier oligomers. These observations suggest that intrazeolite diffusion barriers in the intergrown material are less severe than pure-phase MFI, allowing for the formation and egress of heavier oligomer products. Taken together, these findings demonstrate that intergrown materials allow designing zeolites with tailored pore architectures and altered reaction-diffusion behavior in hydrocarbon conversion processes.