(198a) Effects of Heteroatom Siting in MFI Zeolite Channels on Ion-Pair Transition State Stability for Acid Catalysis

Zeolites voids confine reactions occurring at Brønsted acid sites (H⁺) that compensate trivalent lattice heteroatoms (e.g., Al³⁺, Ga³⁺, Fe³⁺, B³⁺). During zeolite synthesis, Columbic interactions between framework anions introduced by Al substitution and cationic charges in structure directing agents (SDA) strongly influence the thermodynamic preference of Al siting among different tetrahedral-sites (T-sites).¹ For MFI zeolites, quaternary N⁺ centers of tetra-n-propylammonium (TPA⁺) bias Al in T-sites adjacent to channel intersections (~0.7 nm), while co-occlusion of ethylenediamine (EDA) biases Al siting towards smaller (~0.5 nm) channels via H-bonding interactions.^2,3 Acid sites confined in smaller channels have higher selectivity (>70%, 403 K) to para-xylene (p-X) during toluene methylation by C₁ species because larger transition states that form m-X and o-X isomers are preferentially de-stabilized.² Herein, we demonstrate synthetic strategies of biasing active sites in MFI holds generally for other trivalent heteroatoms, which alter acid strength⁴ and, in turn rates of C–C bond formation and cracking reactions (i.e., toluene methylation, alkane cracking), while preserving confinement-induced regioselectivity. MFI synthesized with EDA and TPA⁺ showed marked increases in p-X selectivity (~75%) compared to MFI made with TPA⁺ (~30%), and these selectivity trends hold for different trivalent atoms (Al³⁺, Ga³⁺, Fe³⁺, B³⁺). MFI synthesized with similar SDAs (TPA⁺ only or EDA/TPA⁺) showed xylene formation (per H⁺, 403 K) and propane cracking rate constants (per H⁺, 748 K) that decreased exponentially with deprotonation energy (i.e., acid strength), reflecting the less effective stabilization of carbocationic transition states by less stable conjugate anions of weaker acids. Our findings reveal synthesis-structure relationships governing active site biasing in MFI are generalizable for any heteroatoms with H⁺ sites of varying strength. Conceptual interpretations of this work guide the design of acid catalysts with tailored confinement and acid strength, with predictable consequences on catalytic rates and selectivities.