Rate-Determining Step in the NO_<i>x</i> Reduction Mechanism on BaY Zeolites and the Importance of Long-Range Lattice Effects

The mechanism of the NO_<i>x</i> reduction reaction on BaNaY zeolite using acetic acid/acetate as a reductant has been explored using density functional theory. The elementary steps, the reaction intermediates, and the transition states were identified on a zeolite cluster consisting of 10 tetrahedral atoms (10T). The hydrogen abstraction reaction of acetic acid/acetate was identified as the rate-determining elementary step at 473 K. The long-range electrostatic effect of the lattice on the rate-determining step was studied on expanded 24T, 30T, 34T, 40T, 44T, and 50T zeolite clusters. It was found that while acetate may be greatly stabilized on the expanded clusters with additional Na⁺ cations and Al atoms, the stabilization of acetic acid is much less affected by the long-range lattice effect. The reaction barrier of the hydrogen abstraction reaction, on the other hand, is less sensitive to the long-range lattice effect. The results of this paper highlight the importance of long-range electrostatic effects on the modeling of “single-site” catalysts involving ionic species

Group Additivity Determination for Oxygenates, Oxonium Ions, and Oxygen-Containing Carbenium Ions

Bio-oil produced from biomass fast pyrolysis often requires catalytic upgrading to remove oxygen and acidic species over zeolite catalysts. The elementary reactions in the mechanism for this process involve carbenium and oxonium ions. In order to develop a detailed kinetic model for the catalytic upgrading of biomass, rate constants are required for these elementary reactions. The parameters in the Arrhenius equation can be related to thermodynamic properties through structure–reactivity relationships, such as the Evans–Polanyi relationship. For this relationship, enthalpies of formation of each species are required, which can be reasonably estimated using group additivity. However, the literature previously lacked group additivity values for oxygenates, oxonium ions, and oxygen-containing carbenium ions. In this work, 71 group additivity values for these types of groups were regressed, 65 of which had not been reported previously and six of which were newly estimated based on regression in the context of the 65 new groups. Heats of formation based on atomization enthalpy calculations for a set of reference molecules and isodesmic reactions for a small set of larger species for which experimental data was available were used to demonstrate the accuracy of the Gaussian-4 quantum mechanical method in estimating enthalpies of formation for species involving the moieties of interest. Isodesmic reactions for a total of 195 species were constructed from the reference molecules to calculate enthalpies of formation that were used to regress the group additivity values. The results showed an average deviation of 1.95 kcal/mol between the values calculated from Gaussian-4 and isodesmic reactions versus those calculated from the group additivity values that were newly regressed. Importantly, the new groups enhance the database for group additivity values, especially those involving oxonium ions

Density Functional Theory Study on the Adsorption of H₂S and Other Claus Process Tail Gas Components on Copper- and Silver-Exchanged Y Zeolites

The potential use of Cu- and Ag-exchanged Y zeolites as selective adsorbents for hydrogen sulfide (H₂S) from Claus process tail gas was investigated with density functional theory (DFT). The adsorption energies of H₂S and other Claus tail gas components (CO, H₂O, N₂, and CO₂) were computed for these zeolites as well as for Li–Y, Na–Y, and K–Y on a cluster model. Comparison of adsorption energies for H₂S versus the other components indicated that Ag–Y has potential for selective adsorption of H₂S, whereas Cu–Y is subject to strong adsorption of CO, and alkali metal-exchanged Y zeolites are subject to H₂O adsorption. Comparison with alkali metal-exchanged Y zeolites was performed to clarify the role of d electrons, while the influence of the zeolite framework was assessed by comparing adsorption energies on the cluster model with those on bare cations. Absolutely localized molecular orbital energy decomposition analysis (ALMO EDA) revealed that for Cu- and Ag-containing systems, transfer of electrons between the cation and the adsorbate, i.e., the donation of d electrons and the acceptance of electrons in the unoccupied orbitals of the cation, plays an important role in determining the adsorption energy. On the other hand, for alkali metals-containing systems, charge transfer is negligible and adsorption energies are dominated by interactions due to electrostatics, polarization, and structural distortions