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%.

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.

Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation Ting Chen, Berend Smit, Alexis T. Bell; Are pressure fluctuation-based equilibrium methods really worse than nonequilibrium methods for calculating viscosities?. J. Chem. Phys. 28 December 2009; 131 (24): 246101. https://doi.org/10.1063/1.3274802 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAIP Publishing PortfolioThe Journal of Chemical Physics Search Advanced Search |Citation Search

Abstract 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

We have developed a united atom (UA) nonpolarizable force field for 1-alkyl-3-methyl-imidazolium chloride ([C&lt;sub&gt;&lt;i&gt;n&lt;/i&gt;&lt;/sub&gt;mim][Cl], &lt;i&gt;n&lt;/i&gt; = 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) (&lt;i&gt;Phys. Chem. Chem. Phys.&lt;/i&gt; &lt;b&gt;2006&lt;/b&gt;, &lt;i&gt;8&lt;/i&gt;, 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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Abstract Cation exchanged-zeolites are functional materials with a wide range of applications from catalysis to sorbents. They present a challenge for computational studies using density functional theory due to the numerous possible active sites. From Al configuration, to placement of extra framework cation(s), to potentially different oxidation states of the cation, accounting for all these possibilities is not trivial. To make the number of calculations more tractable, most studies focus on a few active sites. We attempt to go beyond these limitations by implementing a workflow for a high throughput screening, designed to systematize the problem and exhaustively search for feasible active sites. We use Pd-exchanged CHA and BEA to illustrate the approach. After conducting thousands of individual calculations, we identify the sites most favorable for the Pd cation and discuss the results in detail. The high throughput screening identifies many energetically favorable sites that are non-trivial. Lastly, we employ these results to examine NO adsorption in Pd-exchanged CHA, which is a promising passive NOx adsorbent (PNA) during the cold start of automobiles. The results shed light on critical active sites for NOx capture that were not previously studied.

Synthesis of high surface-area colloidal assemblies of calixarene-phosphine-capped nanoporous gold with a remarkably high surface-to-volume ratio is reported.

Previous experimental work has demonstrated that variations in the confinement of &lt;em&gt;n&lt;/em&gt;-butane at Brønsted acid sites due to changes in zeolite framework structure strongly affect the apparent and intrinsic enthalpy and entropy of activation for cracking and dehydrogenation. Quantum chemical calculations have provided good estimates of the intrinsic enthalpies and entropies of activation extracted from experimental rate data for MFI, but extending these calculations to less confining zeolites has proven challenging, particularly for activation entropies. Herein, we report our efforts to develop a theoretical model for the cracking and dehydrogenation of n-butane occurring in a series of zeolites containing 10-ring channels and differing in cavity size (TON, FER, -SVR, MFI, MEL, STF, and MWW). Here, we combine a QM/MM approach to calculate intrinsic and apparent activation parameters, with thermal corrections to the apparent barriers obtained from configurational-bias Monte Carlo simulations, to account for configurational contributions due to global motions of the transition state. We obtain good agreement between theory and experiment for all activation parameters for central cracking in all zeolites. For terminal cracking and dehydrogenation, good agreement between theory and experiment is found only at the highest confinements. Experimental activation parameters, especially those for dehydrogenation, tend to increase with decreasing confinement. This trend is not captured by the theoretical calculations, such that deviations between theory and experiment increase as confinement decreases. We propose that, because transition states for dehydrogenation are later than those for cracking, relative movements between the fragments produced in the reaction become increasingly important in the less confining zeolites.