We have developed a united atom (UA) nonpolarizable force field for 1-alkyl-3-methyl-imidazolium chloride ([C(n)mim][Cl], n = 1, 2, 4, 6, 8), a potential solvent for the pretreatment of lignocellulosic biomass. The charges were assigned by fitting the electrostatic potential surface (ESP) of the ion pair dimers. The Lennard-Jones parameters of the hydrogen atoms on the imidazolium ring were adjusted to agree with the ab initio optimized geometries of isolated ion pairs. Molecular dynamics (MD) simulations were performed for a wide range of temperatures to validate the force field. Substantial improvements were found in both the dynamical properties and the fluid structures, as compared to those predicted using our previously developed UA force field (UA2006) (Phys. Chem. Chem. Phys. 2006, 8, 1096). Liquid densities were found to lie within 2% experimental data. The simulated heats of vaporization decreased about 30% relative to that predicted using the UA2006 force field. The site-site radial distribution functions between the hydrogen atoms on the imidazolium ring and the chloride anions were in good agreement with those determined by ab initio molecular dynamics. The newly developed force field gives a much better description of the self-diffusion coefficients and shear viscosities, which usually deviate by 1 order of magnitude when determined using other force fields.
The alkylation of benzene by ethene over H-ZSM-5 is analyzed by means of a hybrid MP2:DFT scheme. Density functional calculations applying periodic boundary conditions (PBE functional) are combined with MP2 energy calculations on a series of cluster models of increasing size which allows extrapolation to the periodic MP2 limit. Basis set truncation errors are estimated by extrapolation of the MP2 energy to the complete basis set limit. Contributions from higher-order correlation effects are accounted for by CCSD(T) coupled cluster calculations. The sum of all contributions provides the "final estimates" for adsorption energies and energy barriers. Dispersion contributes significantly to the potential energy surface. As a result, the MP2:DFT potential energy profile is shifted downward compared to the PBE profile. More importantly, this shift is not the same for reactants and transition structures due to different self-interaction correction errors. The final enthalpies for ethene, benzene, and ethylbenzene adsorption on the Brønsted acid site at 298 K are -46, -78, and -110 kJ/mol, respectively. The intrinsic enthalpy barriers at 653 K are 117 and 119/94 kJ/mol for the one- and two-step alkylation, respectively. Intrinsic rate coefficients calculated by means of transition state theory are converted to apparent Arrhenius parameters by means of the multicomponent adsorption equilibrium. The simulated apparent activation energy (66 kJ/mol) agrees with experimental data (58-76 kJ/mol) within the uncertainty limit of the calculations. Adsorption energies obtained by adding a damped dispersion term to the PBE energies (PBE+D), agree within +/-7 kJ/mol, with the "final estimates", except for physisorption (pi-complex formation) and chemisorption of ethene (ethoxide formation) for which the PBE+D energies are 12.4 and 26.0 kJ/mol, respectively larger than the "final estimates". For intrinsic energy barriers, the PBE+D approach does not improve pure PBE results.
Kinetic analysis and isotopic tracer studies were used to identify the elementary steps and their reversibility in the oxidative dehydrogenation of propane over ZrO2-supported MoOx catalysts. Competitive reactions of C3H6 and CH313CH2CH3 showed that propene is the most abundant primary product, and that CO and CO2 are formed via either secondary combustion of propene, or by direct combustion of propane. A mixture of C3H8 and C3D8 undergoes oxidative dehydrogenation without forming C3H8-xDx mixed isotopomers, suggesting that steps involving C−H bond activation are irreversible. Normal kinetic isotopic effects (kC-H/kC-D) were measured for propane dehydrogenation (2.3), propane combustion (1.6) and propene combustion (2.1). These data indicate that the kinetically relevant steps in propane dehydrogenation and propene combustion involve the dissociation of C−H bonds in the respective reactant. H−D exchange occurs readily between C3H6 and D2O or C3D6 and H2O, suggesting that OH recombination steps are reversible and quasi-equilibrated. Reactions of 18O2/C3H8 on supported Mo16Ox species lead to the preferential initial appearance of lattice 16O atoms in H2O, CO, and CO2, indicating that lattice oxygen is required for C−H bond activation and for the ultimate oxidation of the adsorbed products of this reaction. 18O16O was not detected during reactions of C3H8−18O2−16O2 mixtures, consistent with irreversible O2 dissociation steps. These isotopic tracer results are consistent with a Mars−van Krevelen redox mechanism in which two lattice oxygens participate in the irreversible activation of C−H bond in propane. The resulting alkyl species desorb as propene, and the remaining O−H group recombines with neighboring OH groups to form water and reduced Mo centers. The reduced Mo centers finally reoxidize by irreversible dissociative chemisorption of O2. The proposed reaction mechanism leads to a complex kinetic rate expression that accurately describes the observed dependences on the partial pressure of propane, oxygen, and water.
Read moreAbstract High resolution NMR spectroscopy is shown to be qualified to identify the species present in silicate solutions used for zeolite synthesis.
Read moreA single-input double-tuned probe was designed for double resonance nuclear magnetic resonance (NMR) experiments, in which high radio frequency power irradiations were achieved at two closely spaced frequencies (such as 26.1 MHz for Al27 and 28.4 MHz for Cu65). The efficiency of the probe for both nuclei was determined to be about 80%, compared to that obtained with a single-tuned probe. The probe was used successfully for Al–6527Cu SEDOR experiments to derive the internuclear distance between Al and Cu in Cu-ZSM-5 zeolite. The circuit is ideal for SEDOR-type double resonance NMR experiments, when high power irradiation is required for nuclei with closely spaced resonance frequencies.
Read moreA spectroscopic investigation of complexes used to catalyze the oxidative carbonylation of toluene to p-toluic acid was conducted. Rhodium complexes were analyzed by (103)Rh and (13)C NMR, UV-visible spectroscopy, and infrared spectroscopy. In the presence of vanadium and oxygen, the resting state of the Rh-catalyst was found to exist as a Rh(III) complex with carbonyl and trifluoroacetate ligands, consistent with the structure Rh(CO)(2)(TFA)(3). The (13)C NMR spectrum of Rh((13)CO)(2)(TFA)(3) complex exhibited a carbonyl peak with an unusual degree of shielding, which resulted in the appearance of the carbonyl peak at an unprecedented upfield position in the (13)C NMR spectrum. This shielding was caused by interaction of the carbonyl group with the trifluoroacetate ligand. In the absence of oxygen, the Rh(III) complex reduced to Rh(I), and the reduced form exhibited properties resembling the catalyst precursor. Structures and spectroscopic properties calculated using density functional theory agreed closely with the experimental results. The vanadium co-catalyst used to reoxidize Rh(I) to Rh(III) was similarly characterized by (51)V NMR and UV-visible spectroscopy. The oxidized species corresponded to [(VO(2))(TFA)](2), whereas the reduced species corresponded to (VO)(TFA)(2). The spectroscopic results obtained in this study confirm the identity of the species that have been proposed to be involved in the Rh-catalyzed oxidative carbonylation of toluene to toluic acid.
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