The hydrogenation of CO and the hydrogenolysis of C/sub 2/H/sub 6/ were studied over the hcp and fcc phases of Mo/sub 2/C, and the fcc phase of Mo/sub 2/N. The CO hydrogenation activity and selectivity of the fcc phases of Mo/sub 2/C and Mo/sub 2/N are identical. The activity of the hcp phase of Mo/sub 2/C is half that of the other two catalysts but the olefin selectivity is higher. Elemental analysis reveals that the catalysts contain a substantial amount of oxygen following extended use for CO hydrogenation. The hydrogenolysis activity of both Mo/sub 2/C phases increases markedly with decreasing content of oxygen in the catalyst. Consistent with the known structure sensitivity of C/sub 2/H/sub 6/ hydrogenolysis, the activity of the hcp phase of Mo/sub 2/C is 200-fold higher than that of the fcc phase. This difference in activity is attributed to differences in the structure of the principal planes exposed by each phase of the carbide. 32 references.
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.
The influence of processing and molecular parameters on the dissolution rate of poly‐(methyl methacrylate), PMMA, thin films (<1 μm) in methyl isobutyl ketone, MIBK, was studied in situ with a single‐element rotating‐polarizer ellipsometer (psi‐meter). Dissolution rates were highly sensitive to the molecular weight distribution, softbake cooling cycle, and dissolution temperature. The apparent activation energy for the dissolution of PMMA in MIBK varied from 25 to 43 kcal/mol depending upon softbake cooling rates and molecular weight distribution. The dissolution rate of air quenched, monodisperse PMMA samples was found to vary with the molecular weight to the −0.98 power. For slowly cooled samples this constant was 85% higher, suggesting improved contrast with slow cooling. Polydisperse samples dissolved about twice as fast as monodisperse ones of the same number average molecular weight. Samples slowly cooled after softbaking and aged for 100h at room temperature or at 60°C showed no change in the dissolution rate with aging. However, the dissolution rate of samples cooled rapidly after softbaking and aged at 60°C decreased by as much as 25%.
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.
Read moreAbstract Endlich mal Dampf ablassen : Die üblichen Flüssigphasen‐ und Hochdruckbedingungen für die Carbonylierung von Formaldehyden werden bei einer neuartigen Dampfphasenreaktion vermieden. Unter Verwendung eines sauren Zeoliths (Faujasit) wird Dimethoxymethan (DMM; das Dimethylacetal von Formaldehyd; siehe Schema) schon nahe bei Atmosphärendruck zu Methylmethoxyacetat (MMAc) carbonyliert. Dies eröffnet einen neuen Zugang zu Ethylenglycol unter milden Bedingungen. magnified image
Read moreWe have developed a united atom (UA) nonpolarizable force field for 1-alkyl-3-methyl-imidazolium chloride ([C<sub><i>n</i></sub>mim][Cl], <i>n</i> = 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) (<i>Phys. Chem. Chem. Phys.</i> <b>2006</b>, <i>8</i>, 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.
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Read morePulsed electrolysis has been demonstrated to improve the faradaic efficiency (FE) to C<sub>2+</sub> products during the electrochemical reduction of CO<sub>2</sub> over a Cu catalyst, but the nature of this enhancement is poorly understood. Herein, we developed a time-dependent continuum model of pulsed CO<sub>2</sub> electrolysis on Cu in 0.1 M CsHCO<sub>3</sub> that faithfully represents the experimentally observed effects of pulsed electrolysis. This work shows that pulsing results in dynamic changes in the pH and CO<sub>2</sub> concentration near the Cu surface, which lead to an enhanced C<sub>2+</sub> FE as a consequence of repeatedly accessing a transient state of heightened pH and CO<sub>2</sub> concentration at high cathodic overpotential. Using these insights, a variety of pulse shapes were explored to establish operating conditions that maximize the rate of C<sub>2+</sub> product formation and minimize the rates of H<sub>2</sub> and C<sub>1</sub> product formation.
Read moreAbstract Synthesis of a pentasil‐type zeolite with ultra‐small few‐unit‐cell crystalline domains, which we call FDP (few‐unit‐cell crystalline domain pentasil), is reported. FDP is made using bis‐1,5(tributyl ammonium) pentamethylene cations as structure directing agent (SDA). This di‐quaternary ammonium SDA combines butyl ammonium, in place of the one commonly used for MFI synthesis, propyl ammonium, and a five‐carbon nitrogen‐connecting chain, in place of the six‐carbon connecting chain SDAs that are known to fit well within the MFI pores. X‐ray diffraction analysis and electron microscopy imaging of FDP indicate ca. 10 nm crystalline domains organized in hierarchical micro‐/meso‐porous aggregates exhibiting mesoscopic order with an aggregate particle size up to ca. 5 μm. Al and Sn can be incorporated into the FDP zeolite framework to produce active and selective methanol‐to‐hydrocarbon and glucose isomerization catalysts, respectively.
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