2018 , 58 , 1037 - 1052 ), may aid in the development and systematic improvements of classical force fields to model proteins in aqueous and nonaqueous phases accurately.
Experimental observations for ionic hydration free energies are highly debated mainly due to the ambiguous absolute hydration free energy of proton, ΔG(H). Hydration free energies (HFEs) of the 112 singly charged ions in the Minnesota solvation database were predicted by six methods with explicit and implicit solvent models, namely, thermodynamic integration (TI), energy representation module (ERmod), three-dimensional reference interaction site model (3D-RISM), and continuum solvation models based on the quantum mechanical charge density (SMD) and on the Poisson-Boltzmann (PB) and generalized Born (GB) theories. Taking the solvent Galvani potential of water into account, the resulting real HFEs from TI calculations for the generalized Amber force field (GAFF) modeled ions best match the experiments based on ΔG(H) = -262.4 kcal/mol (Randles Trans. Faraday Soc . 1956 , 52 , 1573 - 1581 ), in agreement with our previous work on charged amino acids (Zhang et al. J. Phys. Chem. Lett. 2017 , 8 , 2705 - 2712 ). The examined computational methods show an accuracy of ∼7 kcal/mol for the GAFF-modeled ions, except for SMD with a higher accuracy of ∼4 kcal/mol. A biased deficiency in modeling anionic compounds by GAFF is observed with a larger standard deviation (SD) of 9 kcal/mol than that for cations (SD ∼ 4 kcal/mol). The relatively cheap ERmod and 3D-RISM methods reproduce TI results with good accuracy, although ERmod yields a systematic underestimation for cations by 9 kcal/mol; PB and GB generate relative (but not absolute) HFEs comparable to the TI predictions. Computational accuracy is found to be more limited by the accuracy of force fields rather than the models themselves.
For the purpose of industrial applications,
deactivation behavior
of the hydroxyapatite catalyst was studied in the dehydration of lactic
acid to acrylic acid. The hydroxyapatite catalyst shows stable acrylic
acid selectivity with 140 h on stream. Obvious increases in acetaldehyde
and lactide selectivities were observed from 140 to 250 h, which resulted
in a decrease of sharp acrylic acid selectivity. The fresh and used
hydroxyapatite catalysts were further characterized by techniques
including N2 adsorption–desorption, X-ray diffraction,
transmission electron microscopy, 13C cross-polarization
magic-angle spinning nuclear magnetic resonance, thermal gravity analysis,
and temperature-programmed desorption of CO2 and NH3. These results indicate that the mechanism of coke deactivation
consists of two steps. One is the formation of aliphatic carbon by
the deposition of low carbon products with 140 h; the other is the
formation of aromatic coke by the condensation of lactide. Also, the
best generated time of 140 h was determined.
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