256529 Understanding Acid Strength and Solvation Contributions to Catalytic Turnovers On Solid Acids
Zeolitic acids provide a broad range of confining environments through their structural diversity and a range of Brønsted acid strengths through the substitution of Si T-atoms by elements with lower valence. Confinement and acid strength combine to influence the relative stabilities of relevant precursors and transition states as a result of differences in their size and charge, and have led to contradictory claims about the strength of acid sites and to experimental probes that lead to non-rigorous attribution of reactivity to acid strength differences among zeolites with different Al contents or framework structures. CH3OH dehydration rate constants measured at low and high CH3OH pressures (first-order and zero-order constants, respectively) sense acid strength and solvation by confinement differently because they reflect the energy of the same cationic transition state relative to adsorbed CH3OH monomer and dimer species, which differ in size and charge . Here we present mechanistic interpretations of CH3OH dehydration rate constants, taken together with titrations by organic bases and their infrared spectra during catalysis, that have allowed for the independent measurements of the number, strength and confining environments of acid sites in microporous silicates with different heteroatoms (e.g., Al, Ga, Fe, and B), framework structures (e.g., FAU, BEA, SFH, MTW, MOR, CHA, and MFI) and acid site densities. Zero-order CH3OH dehydration rate constants on Al-, Ga-, Fe- and B-MFI zeolites trend exponentially with rigorous DFT-derived descriptors of acid strength: deprotonation energies (DPE). As DPE increases (acid strength decreases) zero-order rate constants decrease because losses in electrostatic stabilization at the acid site destabilize transition state enthalpies more than enthalpies of less charged protonated CH3OH dimer intermediates. DPE values of zeolites and W-based polyoxometalate clusters affect first- and zero-order rate constants similarly indicating the ubiquitous effects of DPE on catalytic rates. Zero-order rate constants on Al-MFI samples with different Si/Al ratio (Si/Al = 22-118 or per unit cell AlnSi96-nO192 where n = 0.8-3.2) and provenance are comparable (within 1.24) indicating that acid strength is not affected by Al density. Similar zero-order rate constants on various Al-form zeolite frameworks (within 1.3) taken together with acid strength trends in zero-order rate constants on W-based polyoxometalate clusters supported on silica, whose simple structure allow for accurate DFT calculations of DPE, indicate DPE differences no greater than 10 kJ mol-1 among different frameworks. First-order rate constants, however, vary by more than an order of magnitude and follow descriptors of pore environment size (e.g. largest included sphere diameters) because increased confinement preferentially stabilizes larger transition states relative to smaller CH3OH monomer intermediates. Values of first-order rate constants on Al-MFI are similar to those of Al-BEA and Al-MTW and may indicate that Brønsted sites in Al-MFI (Si/Al = 22-118) are located preferentially in intersections of 10-MR channels because dispersive interactions predicted for H+ in 10-MR channels would result in larger measured first-order rate constants. Mechanistic interpretations of CH3OH dehydration rate constants combined with chemical titrations of Brønsted acid sites and their FT-IR spectra have led us to conclude that acid strength differences between various zeolite frameworks with different Al densities are no greater than 10 kJ mol-1 and do not exhibit trends with pore environment.
 Carr, R. T., Neurock, M., and Iglesia, E., J. Catal., 278 (2011) 78-93.
Financial support from Chevron Energy Technology Company and supercomputing time from the Environmental Molecular Science Laboratory at Pacific Northwest National Laboratory are gratefully acknowledged.
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