Brønsted Acid Strength Does Not Change for Bulk and External Sites of MFI Except for Al Substitution Where Silanol Groups Form

Abstract

Zeolite synthesis efforts have recently focused on tuning diffusion path lengths by modifying the crystal habit, incorporating mesopores, and using pillared and finned materials to decrease diffusion path lengths, increase rates, and limit zeolite deactivation. These modifications also increase the surface-area-to-volume ratio, increasing the relevance of external Brønsted acid sites. The behaviors of such sites, however, are difficult to study by conventional kinetic studies. Here, we study the acid strength of sites that form on the surfaces of MFI zeolites using density functional theory and compare them to sites within the crystal. We determine acid strength by calculating heterolytic and homolytic O–H bond cleavage energies [deprotonation energy (DPE) and dehydrogenation energy (DHE), respectively] and the adsorption energy of ammonia (Δ E NH 3 ), a common base titrant. These metrics indicate that most sites on the outer (010) surfaces of MFI have similar acid strength to those at internal or bulk positions. Ensemble average DPE values on external surface sites (1486–1543 kJ mol –1 ) are mostly similar to those at internal sites (1477–1506 kJ mol –1 ) on the MFI surface model, and ensemble averaged DHE values (457–483 kJ mol –1 ) are also similar to their bulk counterparts (457–476 kJ mol –1 ). Al substitution at terminal silanol groups (SiOH) on the outer zeolite surface yields Lewis acidic Al, resulting in different acid strengths at those sites, but such sites represent a minority of surface T-sites. Unlike DPE and DHE, Δ E NH 3 probes a combination of acid strength and confinement because a gas-phase species enters the zeolite pores. Ensemble average Δ E NH 3 values indicate that most sites bind NH 3 as strongly on the surface (−160 to −113 kJ mol –1 ) as bulk sites (−160 to −133 kJ mol –1 ), despite the reduced confinement on surface sites. These Δ E NH 3 change very little on the MFI surface because most surface sites are within pockets that retain enough of the zeolite pore to solvate the NH 4 + cations that form when NH 3 binds. Some sites, however, bind NH 3 more weakly if the solvating pore is absent or if NH 3 binds and deprotonates the H 2 O on a Lewis acidic Al. Together, these data suggest that fully coordinated Brønsted acidic Al sites on the outside of zeolite crystals possess similar acid strength to those within the crystal and that many surface sites effectively confine an NH 4 + cation. Such partial confinement on external sites indicates that NH 3 adsorption experiments are ineffective for broadly distinguishing between internal and external sites in MFI materials.

Topics & Concepts