Influencing Aluminum Proximity in MFI Zeolites Using Mixtures of Structure-Directing Agents and Consequences for Brønsted Acid-Catalyzed Methanol Dehydration

The specific conditions and structure-directing agents (SDAs) used for zeolite crystallization bias framework Al siting¹, but the underlying mechanistic factors are incompletely understood, precluding efforts to develop synthesis routes that predictably crystallize zeolites with varying Al distributions. Here, we study the dominant framework-SDA interactions that determine the energetics of Al siting in isolated and proximal configurations, for low-symmetry MFI (ZSM-5) zeolites that contain 12 crystallographically-unique tetrahedral sites. We adapt concepts from charge density mismatch theory² by varying the Na⁺/tetrapropylammonium(TPA⁺) ratio to crystallize MFI with different Al content and proximity. Characterizations including Co²⁺ titration of proximal Al sites and NH­₄⁺ titration of all sites were used to quantify isolated and proximal Al sites. The fraction of proximal Al increased with total Al content in samples crystallized using only TPA⁺, and with increasing Na⁺/TPA⁺ ratio in samples of fixed Si/Al. DFT calculations indicate that stability of an TPA⁺-Al arrangement is dictated by electrostatic interactions between Al and the cationic N center of TPA⁺. DFT-computed Co²⁺ exchange energies at various Al-Al configurations in MFI reveal numerous site pairs that bind Co²⁺ favorably. We use catalytic probe reactions to interrogate H⁺-H⁺ site pairs directly. Methanol dehydration to dimethyl ether is a versatile probe reaction of acid strength and confinement in solid Brønsted acids³, calibrated on materials of known structure and composition⁴. Measured zero-order rate constants (per H⁺, 415 K) on MFI do not vary with total or proximal H⁺ content, reflecting constant acid strength for all arrangements. The ability to synthetically manipulate active site proximity in zeolites, aided by theoretical modeling and quantitative characterization of site arrangements and catalytic behavior, allows diversifying the catalytic opportunities for zeolites.

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