Brønsted–Brønsted Synergies between Framework and Noncrystalline Protons in Zeolite H-ZSM-5

Zeolite catalysts are solid Brønsted acids whose reactivity is typically associated with the number of protons at crystalline framework bridging acid sites (BAS's). Postsynthetic catalyst modification, titrations with monovalent and divalent cations of varying size, quantitative spin-counting spectroscopy on all protons before and after cation exchange, amine titration, and room-temperature in situ reactions with two different probe molecules reveal that zeolite HZSM-5 reactivity strongly corresponds with the presence of acidic protons from extraframework and/or noncrystalline sites. Significantly, room-temperature hydrogen–deuterium (H/D) exchange reactions between the catalyst and the organic probe molecules reveal that reaction rates are strongly dependent on the total concentration of acidic protons from extraframework and noncrystalline proton sites. The most active catalysts in room-temperature probe reactions contain protons from both BAS's and from noncrystalline species, including reactive extraframework aluminol species that can be removed by solvent treatments. In order to demonstrate the significance of paired framework/extraframework or noncrystalline Brønsted sites to overall catalyst activity, speciation of different protons were quantified after titration with mono- and divalent cations of varying radius (Na+, Ca2+, Cu2+, Ba2+), chemical washing with ammonium hexafluorosilicate (AHFS), and different steaming procedures for HZSM-5 catalysts with Si/Al equal to 15 and 40. Detailed manipulation of reactive Brønsted species in the Si/Al = 15 catalyst enabled direct experimental observation of H/D exchange at both the methine and methyl positions of isobutane, heretofore not reported, clarifying uncertainties surrounding that mechanism. Reaction data indicates that isolated framework BAS's are much less important to overall catalyst reactivity than proximate framework/extraframework or noncrystalline Brønsted sites, and DFT calculations support the importance of proximate proton sites. Potential Brønsted–Brønsted synergies are unique relative to previously proposed Brønsted/Lewis synergies but do not preclude the latter's contribution to increased reactivity.