Transition-State-Mimicking Structure-Directing Agents Induced ZSM-5 Zeolite Synthesis toward Boosting Light Olefin Selectivity in n -Heptane Catalytic Cracking

Catalytic cracking of light alkanes represents an important route to produce light olefins with reduced energy input and flexible product selectivity with respect to conventional steam cracking. Herein, we demonstrate the synthesis of ZSM-5 zeolites using a series of n -heptane cracking transition state-mimicking organic structure-directing agents (OSDAs), i.e., N, N, N -trimethylpentylammonium (TMPA), N -ethyl- N, N -dimethylbutylammonium (EDMBA), and N -Ethyl- N, N -dimethylpentylammonium (EDMPA) under inorganic cation-free conditions. MFI zeolites derived from TMPA and EDMBA crystallize as thin nanoplates of 80–130 nm and 70–110 nm, in thickness, respectively, as a result of anisotropic growth along the bc plane, and the intergrowth of nanoplates constructs a house-of-cards architecture with penetrable voids. EDMPA-generated sample forms an alternative intergrowth made up of ship-like crystals of 1.0 to 1.6 μm, without auxiliary porosity. The decrease in crystal dimension, increase in surface area, and integration of macropores can boost the diffusion property and an incremental order of diffusion characteristic time constants: ZSM-5-C < ZSM-5-TMPA < ZSM-5-EDMPA < ZSM-5-EDMBA has been measured using 2-methylhexane as a probe molecule. The site densities for intermediate-to-strong Brønsted acid increase in the order of ZSM-5-TMPA < ZSM-5-EDMPA < ZSM-5-EDMBA, which coincides with their crystallinity. In n -heptane cracking, the light olefin selectivity and catalyst lifetime have been boosted, and ZSM-5-EDMBA outperforms others due to the presence of more catalytically important strong acid sites, favorable center C–C cleavage, and reduced residence time for primary olefin products that suppresses their further conversion into aromatics and coke precursors. The use of transition-state-mimicking OSDAs to generate zeolite provides an effective strategy for cracking catalyst design toward light olefin production.