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.
Acidic zeolites are solid aluminosilicate catalysts whose utility arises from Brønsted sites that predominately reside in their crystalline framework structure. Data described herein indicate that extra-framework aluminum (EFAl) moieties, often proposed as important species in overall catalyst activity via Brønsted−Lewis synergies, can themselves contribute protons that are also reactive Brønsted acid centers. While the MFI family of zeolites is a relatively simple channel-structure type, the quantitative spectroscopic detection of all protons shows that the distribution of reactive Brønsted acid site protons arising from framework and extraframework moieties can be complex. Experiments show that postsynthetic treatments can be used to modify this distribution, in theory enabling routes to HZSM-5 catalysts with only one type of reactive Brønsted site. Quantitative spin-counting NMR experiments combined with chemical washing using ammonium hexafluorosilicate (AHFS) show that the number of framework bridging acid sites (BAS) in typical commercial MFI catalysts (Si/Al equal 15 and 40) is between 50 and 60% of that expected based on the total Al content. Acidic protons from EFAl constitute the major fraction of remaining Brønsted sites. Probe-molecule reactions demonstrate that catalysts with only framework BAS are significantly less reactive than those with both extraframework and framework Brønsted acid sites. Various postsynthetic methods are compared to optimize the desired Brønsted acid site distribution in MFI catalysts, including both removal and re-introduction of acidic protons from EFAl sites.
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