Palascak and Shields 1 claim to have derived accurate experimental values for the hydration free energies of H + , OH -, and H 3 O + . The purpose of this Comment is to alert the community that, in fact, their values are less accurate than the values they are meant to replace. In what follows we show the errors Palascak and Shields made and, by example, give practical advice on how to ensure correct assignment of standard states for reactions with water as a reactant or product in gas and solution phases.Palascak and Shields begin the derivation of hydration free energies of OHand H 3 O + by asserting that the most reliable estimate of the experimental value for hydration of a proton is -264 kcal/mol. 2,3 They use this value as if the reference standard states are 1 M for both gas and aqueous phases, i.e., ∆ s G*(H + ). 4,5 This practice is wrong. Tissander et al. 2 and Tuttle et al. 3 derive the hydration free energy of a proton by correlating ion-water cluster data, referenced to standard gas phase conditions (1 bar, 298 K), with free energies of hydration of the anion-cation pairs that are derived from gas phase reaction energies referenced to the 1-bar standard state and aqueous reactions referenced to 1-m standard state. Therefore, the recommended value (-264 kcal/mol) for the hydration of a proton represents the conventional process with standard states essentially equal to 1 atm for gas and 1 M for solution. To convert from the 1-atm gas phase/1-m solution standard state to the 1-M gas/ 1-M solution standard state, one must subtract 1.9 kcal/mol, 6 such that ∆ s G*(H + ) ) -265.9 kcal/mol. 7,8 Bartels and coworkers 9 have recently reproduced this result to within 0.2 kcal/ mol and derived values for temperatures up to 648 K using the SUPCRT92 software package. 10 Solvation energies of ions based on ∆ s G*(H + ) ) -265.9 kcal/mol have been widely adopted. 11 This benchmark experimental value should not be changed unless/until it is superseded by better measurements. [12][13][14] Accordingly, Palascak's and Shields' determination of ∆ s G*(OH -) is too negative by 1.9 kcal/mol. With this correction, the value is ∆ s G*(OH -) ) -104.5 kcal/mol, which is in good agreement with the value previously determined by Pliego and Riveros. 15 Not converting ∆ s G°(H + ) to number density standard states is just one of the problems with Palascak's and Shields' paper. A more serious problem arises in the derivation by Palascak and Shields of the hydration free energy of H 3 O + . Their value is several kcal/mol less negative than the value previously

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The degree of substitution of Si(4+) by Al(3+) in the oxygen-terminated tetrahedra (Al T-sites) of zeolites determines the concentration of ion-exchange and Brønsted acid sites. Because the location of the tetrahedra and the associated subtle variations in bond angles influence the acid strength, quantitative information about Al T-sites in the framework is critical to rationalize catalytic properties and to design new catalysts. A quantitative analysis is reported that uses a combination of extended X-ray absorption fine structure (EXAFS) analysis and (27)Al MAS NMR spectroscopy supported by DFT-based molecular dynamics simulations. To discriminate individual Al atoms, sets of ab initio EXAFS spectra for various T-sites are generated from DFT-based molecular dynamics simulations, allowing quantitative treatment of the EXAFS single- and multiple-photoelectron scattering processes out to 3-4 atom shells surrounding the Al absorption center. It is observed that identical zeolite types show dramatically different Al distributions. A preference of Al for T-sites that are part of one or more 4-member rings in the framework over those T-sites that are part of only 5- and 6-member rings in an HBEA150 zeolite has been determined using this analysis.

Copper oxide clusters synthesized via atomic layer deposition on the nodes of the metal-organic framework (MOF) NU-1000 are active for oxidation of methane to methanol under mild reaction conditions. Analysis of chemical reactivity, in situ X-ray absorption spectroscopy, and density functional theory calculations are used to determine structure/activity relations in the Cu-NU-1000 catalytic system. The Cu-loaded MOF contained Cu-oxo clusters of a few Cu atoms. The Cu was present under ambient conditions as a mixture of ∼15% Cu and ∼85% Cu. The oxidation of methane on Cu-NU-1000 was accompanied by the reduction of 9% of the Cu in the catalyst from Cu to Cu. The products, methanol, dimethyl ether, and CO, were desorbed with the passage of 10% water/He at 135 °C, giving a carbon selectivity for methane to methanol of 45-60%. Cu oxo clusters stabilized in NU-1000 provide an active, first generation MOF-based, selective methane oxidation catalyst.

The catalytic sites of acidic zeolite are profoundly altered by the presence of water changing the nature of the Brønsted acid site. High-resolution solid-state NMR spectroscopy shows water interacting with zeolite Brønsted acid sites, converting them to hydrated hydronium ions over a wide range of temperature and thermodynamic activity of water. A signal at 9 ppm was observed at loadings of 2–9 water molecules per Brønsted acid site and is assigned to hydrated hydronium ions on the basis of the evolution of the signal with increasing water content, chemical shift calculations, and the direct comparison with HClO4 in water. The intensity of 1H–29Si cross-polarization signal first increased and then decreased with increasing water chemical potential. This indicates that hydrogen bonds between water molecules and the tetrahedrally coordinated aluminum in the zeolite lattice weaken with the formation of hydronium ion–water clusters and increase the mobility of protons. DFT-based ab initio molecular dynamics studies at multiple temperatures and water concentrations agree well with this interpretation. Above 140 °C, however, fast proton exchange between bridging hydroxyl groups and water occurs even in the presence of only one water molecule per acid site.

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The dehydration of alcohols is involved in many organic conversions but has to overcome high free-energy barriers in water. Here we demonstrate that hydronium ions confined in the nanopores of zeolite HBEA catalyse aqueous phase dehydration of cyclohexanol at a rate significantly higher than hydronium ions in water. This rate enhancement is not related to a shift in mechanism; for both cases, the dehydration of cyclohexanol occurs via an E1 mechanism with the cleavage of Cβ–H bond being rate determining. The higher activity of hydronium ions in zeolites is caused by the enhanced association between the hydronium ion and the alcohol, as well as a higher intrinsic rate constant in the constrained environments compared with water. The higher rate constant is caused by a greater entropy of activation rather than a lower enthalpy of activation. These insights should allow us to understand and predict similar processes in confined spaces.

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