Brian C. Dunn

Production of diethyl carbonate from ethanol and carbon monoxide over a heterogeneous catalytic flow reactor

Abstract Diethyl carbonate (DEC) is a candidate for use as an oxygen-containing additive in gasoline and diesel fuel to diminish pollutant emissions. The synthesis of DEC by the oxidative carbonylation of ethanol in the gas phase over heterogeneous CuCl 2 /PdCl 2 catalysts supported on activated carbon (AC) has been investigated using a laboratory-scale continuous flow reactor with online GC/MS. Influences of various reaction conditions and catalyst pretreatment on the DEC yield and selectivity have been tested. Yield of DEC at 150 °C reached a maximum of 12.5 wt.% and was found to increase with an increase of residence time, reaction temperature, and reaction pressure as expected. The relationship between CO flow rate and production of DEC showed three distinct regions of DEC production. A by-product, diethoxymethane (DEM), was formed when ethanol was introduced in stoichiometric excess. Catalyst pretreated with KOH presented the best catalytic performance, and all metal hydroxides tested enhanced the yield of and selectivity for DEC simultaneously. However, the CuCl 2 /PdCl 2 /AC–KOH catalyst with a higher OH/Cu mole ratio than 2.0 showed an even lower DEC yield than that found using a CuCl 2 /PdCl 2 /AC catalyst without KOH.

Ferrate(VI) oxidation of aniline

A detailed kinetic and thermodynamic study of ferrate(VI) oxidation of aniline (aminobenzene) has been carried out in isotopic solvents, H2O and D2O, as a function of reductant concentration, solution pH, temperature and pressure by means of conventional stopped-flow and high-pressure stopped-flow spectrophotometric methods. Under pseudo first-order conditions with reductant in at least 10-fold excess over ferrate, these redox processes give rise to a simple exponential change of optical density. The temperature profile reveals relatively low activation enthalpies, and the activation entropies found for these processes are very negative. In addition, the significant negative activation volumes estimated from the pressure dependence of the rate constants indicate a substantial decrease in partial molar volume during the formation of the transition state, suggesting that highly structured transition states are formed in these reactions. An EPR result indicates a free radical reaction mechanism. The kinetic isotopic results for aniline systems measured in H2O and in D2O solvent indicate that the amino hydrogen/deuterium plays a role in the formation of the transition states.

Stopped-flow rapid-scan Fourier transform infrared spectroscopy

A stopped-flow rapid-mixing device interfaced with a rapid-scan FT-IR spectrometer and a diode-array UV-visible spectrophotometer permits the observation of reaction transient intermediates over a broad range of wavelengths at minimal cost. The system has been evaluated for both spectral regions with the use of two different chemical reaction systems. The presence of a transient intermediate is clearly indicated in one case. Unexpected reactivity was observed in the other case. This approach will allow the study of chemical reactions even when no spectral changes occur in the UV-visible region.

Ostwald ripening of cobalt precipitates in silica aerogels? An ultra-small-angle X-ray scattering study

Monolithic silica aerogels with radial symmetry were synthesized by supercritical drying, doped to 2% and 10% with cobalt, and reduced with hydrogen. All samples were investigated with ultra-small-angle X-ray scattering. The non-doped aerogels have three populations of scatterers with radii of gyration of about 10, 40 and 60-70 A. The doped aerogels show an additional structure with a radius of gyration ranging from 1050 to 3000 A. This structure causes intensity oscillations, thus revealing a relatively narrow size distribution. Scattering curves of the 10%-doped aerogels fitted well to a Lifshitz-Slyozov-Wagner particle size distribution, thus revealing that Ostwald ripening might have occurred during aerogel preparation. The same range also shows differences depending on whether the samples were reduced, or in their as-prepared condition. Scattering curves obtained from the cylinder-axis region were different from the scattering curves obtained from the sample boundary, indicating a process-dependent skin effect.

Enhancement in the reducibility of cobalt oxides on a mesoporous silica supported cobalt catalyst

The silylation of SBA-15 enhances the reducibility of cobalt oxides on a SBA-15 supported cobalt catalyst, and consequently increases the catalytic activity for Fischer-Tropsch synthesis of hydrocarbons from syngas and selectivity for longer chain products.

An evaluation of least-squares fitting methods in XAFS spectroscopy: Iron-based SBA-15 catalyst formulations

A detailed comparison has been made of determinations by (57)Fe Mössbauer spectroscopy and four different XAFS spectroscopic methods of %Fe as hematite and ferrihydrite in 11 iron-based SBA-15 catalyst formulations. The four XAFS methods consisted of least-squares fitting of iron XANES, d(XANES)/dE, and EXAFS (k(3)chi and k(2)chi) spectra to the corresponding standard spectra of hematite and ferrihydrite. The comparison showed that, for this particular application, the EXAFS methods were superior to the XANES methods in reproducing the results of the benchmark Mössbauer method in large part because the EXAFS spectra of the two iron-oxide standards were much less correlated than the corresponding XANES spectra. Furthermore, the EXAFS and Mössbauer results could be made completely consistent by inclusion of a factor of 1.3+/-0.05 for the ratio of the Mössbauer recoilless fraction of hematite relative to that of ferrihydrite at room temperature (293K). This difference in recoilless fraction is attributed to the nanoparticle nature of the ferrihydrite compared to the bulk nature of the hematite. Also discussed are possible alternative non-least-squares XAFS methods for determining the iron speciation in this application as well as criteria for deciding whether or not least-squares XANES methods should be applied for the determination of element speciation in unknown materials.

Separation of Argon and Oxygen by Adsorption on a Titanosilicate Molecular Sieve

Abstract A titanosilicate molecular sieve adsorbent, Ba-RPZ-3, was synthesized and tested for its use in the separation of O2+Ar mixtures at room temperature. A clean resolution of both gases was achieved in pulse chromatographic experiments using a standard column (0.25″ OD, 3.5 grams of adsorbent). In another experiment, using a column containing 30 grams of adsorbent and a continuous O2+Ar feed at 10 cm3/min, argon breakthrough was detected more than 5 minutes before the oxygen breakthrough, and the separation was sufficiently sensitive to achieve quantitative separation of mixtures with low argon content (5% Ar). Equilibrium adsorption isotherms and isosteric heats of adsorption for oxygen and argon were found to be almost identical at room temperature. The thermodynamic selectivity was found to be mildly in favor of oxygen (∼1.1–1.2). However, the adsorption of oxygen was observed to be much faster than argon, indicating that the separation of the O2+Ar mixtures was based on the sieving properties of the adsorbent and the difference in sizes of O2 molecules and Ar atoms. This indicates that a suitably-oriented oxygen is physically smaller than argon, despite the fact that many references assume that oxygen is larger than argon.

Stopped-Flow FT-IR Spectroscopy of Aqueous Solutions Using Attenuated Total Reflectance

An apparatus for the study of solution phase kinetics using FT-IR spectroscopy has been developed. The observation chamber consists of an integrated tangential mixer-flow cell and a ZnSe element permitting attenuated total reflectance (ATR) measurements. The short optical pathlength afforded by ATR allows mid-IR observation of chemical reactions in aqueous solution, including the spectral region near the water bending vibration (1640 cm−1). High hydraulic backpressures required to force solution rapidly through a thin layer flow cell are not necessary with the ATR flow cell because the optical pathlength and the flow cross-section have been decoupled, allowing for a relativity large flow chamber when compared with instruments incorporating a transmission flow cell. Overall system performance has been evaluated using the hydrolysis of methylchloroacetate as a test reaction. The feasibility of observing reactions with initial half-lives of approximately 250 ms is demonstrated. The system is very robust, with little risk of damaging the optics during routine maintenance.

Interactions between sol-gel encapsulated myoglobin and carbon dioxide in supercritical CO2

The interaction between biomolecules and CO{sub 2} is of crucial importance because CO{sub 2} is produced during mammalian metabolism and must be removed in order for cells to function properly. The ideal solvent for investigations into reactivity of proteins and enzymes towards CO{sub 2} is CO{sub 2} itself since this choice of solvent eliminates any other interactions that would complicate data collection and analysis. The development of an isolated reaction vessel suitable for use with supercritical CO{sub 2} is an important step towards conducting valid spectroscopic experiments because contamination from the high-pressure vessel is avoided. This is crucial when biomolecules are the subject of investigations due to the ease with which contaminants perturb their reactivity. The combination of sol-gel encapsulation of biologically active molecules with supercritical fluid spectroscopy provides an experimental method for studying CO{sub 2} interactions previously considered inaccessible. The molecules retain their biological activity even under conditions required to maintain the CO{sub 2} in the supercritical state.