Rigid Arrangements of Ionic Charge in Zeolite Frameworks Conferred by Specific Aluminum Distributions Preferentially Stabilize Alkanol Dehydration Transition States

Abstract

Zeolite reactivity depends on the solvating environments of their micropores and the proximity of their Brønsted acid sites.T urnover rates (per H +)f or methanol and ethanol dehydration increase with the fraction of H + sites sharing sixmembered rings of chabazite (CHA) zeolites.D ensity functional theory (DFT) shows that activation barriers vary widely with the number and arrangement of Al (1-5 per 36 T-site unit cell), but cannot be described solely by Al-Al distance or density.C ertain Al distributions yield rigid arrangements of anionic charge that stabilizec ationic intermediates and transition states via H-bonding to decrease barriers.T his is ak ey feature of acid catalysis in zeolites olvents,w hichl ack the isotropyo fl iquid solvents.T he sensitivity of polar transition states to specific arrangements of charge in their solvating environments and the ability to position such charges in zeolite lattices with increasing precision herald rich catalytic diversity among zeolites of varying Al arrangement.

Kinetic effects of molecular clustering and solvation by extended networks in zeolite acid catalysis

“…, framework Al, other defect sites) in the zeolite lattice. 18 Unique molecular configurations that arise when multicomponent mixtures fill micropores at high coverage are reported for alkane mixtures 88 and aqueous solutions of alcohols, 89,90 suggesting that unique configurations of other mixtures may prevail at high coverage and provide opportunities to investigate their consequences for catalysis. Such insights will inform new strategies to design catalysts that prefer advantageous clustering motifs (either IS or TS) by modifying the surrounding spatial constraints provided by porous solids 91 and their arrangements of atoms and charges.…”

“…Furthermore, the confining voids of zeolites are a “solid solvent” that enforces the specific positioning of active sites, reacting moieties, and solvent molecules. Thus, specific interactions provided by moieties associated with the zeolite lattice, such as the polarity introduced by Si–OH and Si–O(H)–Al groups and which varies with their density and arrangement, 18,74 can be leveraged to influence the structure and reorganization of solvent networks. Work by Eckstein et al 17 shows that the density of H + (H 2 O) n influences activity coefficients, prompting further questions about whether their local proximity or the arrangement of the lattice Al sites that bind H + (H 2 O) n may also play a role.…”

“…Although networks of solvent molecules can mediate charge interactions that destabilize adsorbates, specific arrangements of charged reactant molecules can stabilize transition states. Hoffman et al 18 recently reported that alkanol dehydration transition states become stabilized when H + active sites are arranged in proximal positions of zeolite lattices, which form rigid networks of charge that facilitate hydrogen bonding between transition states and coadsorbed reactant clusters. Non-ideal charge interactions have similarly been described for polyoxometalate clusters, 79 and solvent-enabled expansion of the length scales over which charge interactions mediate reactions has been demonstrated for other solid Brønsted acids, 80 metals, 81,82 and oxides.…”

Influence of Confinement on Barriers for Alkoxide Formation in Acidic Zeolites

“…[27] Herein, we focus on alkoxide formation as the first step of the stepwise dehydration as it was shown to be typically the rate-determining step, [28,29] while the second step, leading either to ether or olefin formation typically requires a lower barrier. [15,[30][31][32][33][34] We note that dehydration can also occur via the concerted mechanism, which is typically favored at lower temperatures. [28,29] The studied frameworks, already including the Brønsted acid site, with their corresponding pore diameters (and diameters of pore openings when present) are shown in Figure 1.…”

“…Herein, we focus on alkoxide formation as the first step of the stepwise dehydration as it was shown to be typically the rate‐determining step, [28,29] while the second step, leading either to ether or olefin formation typically requires a lower barrier [15,30–34] . We note that dehydration can also occur via the concerted mechanism, which is typically favored at lower temperatures [28,29] .…”