Catherine B. Almquist

The photo-oxidation of cyclohexane on titanium dioxide: an investigation of competitive adsorption and its effects on product formation and selectivity

The photo-oxidation of cyclohexane on titanium dioxide was investigated in neat cyclohexane and in various solvents to determine the effect of the solvent media on the cyclohexane oxidation rate and reaction selectivity to cyclohexanol and cyclohexanone. The solvents that were used in this study include acetone, isopropanol, dichloromethane, chloroform, carbon tetrachloride, benzene, and n-hexane. It was found that the reaction rate and selectivity to the formation of cyclohexanol and cyclohexanone in various solvents depend upon the relative strengths of adsorption of cyclohexane, cyclohexanone, cyclohexanol, the solvent, and the partially oxidized solvent species on the titanium dioxide particles. In non-polar solvents, cyclohexanol preferentially adsorbed onto the titanium dioxide particles and underwent deep oxidation, ultimately to carbon dioxide and water. Therefore, in non-polar solvents, the selectivity of the reaction to cyclohexanol was very low. However, in polar solvents, cyclohexanol adsorbed to the titanium dioxide particles to a lesser extent due to the competition for adsorption sites with the solvent, and the selectivity of the reaction to cyclohexanol significantly increased. Competitive adsorption, in part, determined the overall rate of cyclohexane oxidation and selectivity to the desired products, cyclohexanone and cyclohexanol. The highest product-formation rate in this study was observed in dichloromethane, whereas chloroform and isopropanol significantly inhibited the desired reactions. The ideal solvent for the photo-oxidation of cyclohexane is one that minimizes the strengths of adsorption of the desired products on titanium dioxide and either does not compete with cyclohexane and oxygen for adsorption sites or is strongly adsorbed but is non-reactive with itself upon forming a radical on the illuminated titanium dioxide surface.

A mechanistic approach to modeling the effect of dissolved oxygen in photo-oxidation reactions on titanium dioxide in aqueous systems

Abstract A kinetic model containing three parameters was developed based upon mechanistic considerations to include the effect of dissolved oxygen over a range of conditions in aqueous media. It shows that as organic concentrations increase to concentrations on the order of the solubility of oxygen concentration in water ( ∼1.26 mM at oxygen partial pressures of 1 atm ), the solubility of oxygen in water limits the extent of photo-oxidation of the organic substrate. The model reduces to the commonly used Langmuir–Hinshelwood kinetic rate expression for the photo-oxidation of organic substrates in aqueous media under constant oxygen concentrations, which is an appropriate assumption when oxygen is initially in large excess of the organic substrate or when the reaction mixture is aerated during the reaction. The model was fit to the experimental data generated in this study for the photo-oxidation of phenol to determine the best values for the model parameters. For comparison, the model was also fit to selected published data for the photo-oxidation of phenol.

A Mechanistic Model for Mercury Capture with In Situ–Generated Titania Particles: Role of Water Vapor

Abstract A mechanistic model to predict the capture of gas-phase mercury (Hg) species using in situ-generated titania nanosize particles activated by UV irradiation is developed. The model is an extension of a recently reported model for photochemical reactions by Almquist and Biswas that accounts for the rates of electron-hole pair generation, the adsorption of the compound to be oxidized, and the adsorption of water vapor. The role of water vapor in the removal efficiency of Hg was investigated to evaluate the rates of Hg oxidation at different water vapor concentrations. As the water vapor concentration is increased, more hydroxy radical species are generated on the surface of the titania particle, increasing the number of active sites for the photooxidation and capture of Hg. At very high water vapor concentrations, competitive adsorption is expected to be important and reduce the number of sites available for photooxidation of Hg. The predictions of the developed phenomenological model agreed well with the measured Hg oxidation rates in this study and with the data on oxidation of organic compounds reported in the literature.

The permeation of organophosphorus compounds in silicone rubber membranes

The permeabilities, solubilities, and diffusivities of eight organophosphorus chemicals in silicone rubber were measured at saturation concentration using two different experimental methods: permeation experiments and absorption experiments. All tests were carried out at 25°C (±3°C). The eight organophosphorus chemicals investigated are dimethyl methylphosphonate, diethyl methylphosphonate, dimethyl hydrogenphosphonate, diethyl hydrogenphosphonate, trimethylphosphate, triethylphosphate, trimethylphosphite, and triethylphosphite. These eight chemicals were selected based on their similarities to organophosphorus chemicals used as pesticides and chemical warfare agents. The experimental data were analyzed using solutions of Ficks second law of diffusion and boundary conditions representative of the experimental settings. An unsteady-state diffusion model using boundary conditions that represent uniform initial concentration in the polymer and constant but different surface concentrations was used to interpret the permeation experimental data. In this model, the effective diffusivity calculated from the steady-state permeability and equilibrium solubility of each chemical was used and was assumed to be constant.

The anaerobic co-digestion of fruit and vegetable waste and horse manure mixtures in a bench-scale, two-phase anaerobic digestion system

In this study, the anaerobic digestion of mixtures of food waste (FW) and horse manure was investigated using a bench-scale two-phase reactor system. Both phases were maintained at 35°C for the duration of the 30-day study period. The first phase reactors were prepared with biomass mixtures in deionized water such that each mixture had an initial total solids (TS) concentration of 6 wt%. The second phase reactors were inoculated with cow manure in water two weeks prior to the study period at 3 wt% TS. The biogas from all second phase reactors contained greater than 60 vol% methane in the biogas before they were used in the study, thus indicating the presence of active methanogens. Filtrate (5 mL) from the first phase was used as feed to the second phase reactor. The chemical oxygen demand (COD), total organic carbon, and volatile solids (VS) of the feed from Phase 1 increased with FW content in the biomass mixture, and so the organic loading rates (OLRs) to the Phase 2 reactors also increased. Accordingly, the volume of biogas and methane generated from Phase 2 also increased with FW content. The low OLR (<0.2 g VS/L/day), the use of a two-phase system, and the use of filtrate from Phase 1as feed to Phase 2 allowed for high utilization of the feed; the observed specific methane yields (mL/g COD) were greater than 80% of the theoretical yields for all mixtures. The methane yields were statistically similar to within a 95% confidence interval.

High Gas Permeability in Open-Structure Membranes

For most polymeric membranes, the gas permeability coefficient (P) is often interpreted as the product of diffusivity (D) and solubility (S) of a penetrant gas in the polymer (P=D S). The basic assumption is that molecular diffusion is primarily responsible for mass transport in the membrane permeation process. However, for some open structure membranes, such as poly(1-trimethylsilyl-1-propyne) [PTMSP] or poly(dimethylsiloxane) [PDMS], the high permeabilities of some gases yield much higher diffusivities when calculated from the above relationship (P=D S) than when calculated by using the direct kinetic measurement of diffusivity. It is hypothesized that this discrepancy is due to the convective transport of gas molecules through such open structured polymers. In most cases, the convective contribution to mass transport through membranes is negligible. However, for polymer membranes with high free volume, such as PTMSP, whose free volume fraction is 20 to 25%, the convective term may dominate the permeation flux. In this study, a non-equilibrium thermodynamic formalism is employed to properly treat the diffusion term and convective term that constitute the Nernst-Planck equation. The current analysis indicates that the total permeation flux, which consists of a diffusion term and a convective term, agrees well with the experimental data for several permeation systems: pure components propane and n-butane/PTMSP, pure gas hydrogen/PTMSP, and mixed gas hydrogen/PTMSP. Also, the permeation systems of a nonporous rubbery membrane, PDMS, and eight organophosphorus compounds were included in the study. It is recommended that the proposed model be validated by using other polymers with high free volumes and high permeabilities of gases and vapors, such as poly(1-trimethylgermyl-1-propyne) [PTMGeP] and poly(4-methyl-2-pentyne) [PMP].

Advanced oxidation degradation kinetics as a function of ultraviolet LED duty cycle

Ultraviolet (UV) light emitting diodes (LEDs) may be a viable option as a UV light source for advanced oxidation processes (AOPs) utilizing photocatalysts or oxidizing agents such as hydrogen peroxide. The effect of UV-LED duty cycle, expressed as the percentage of time the LED is powered, was investigated in an AOP with hydrogen peroxide, using methylene blue (MB) to assess contaminant degradation. The UV-LED AOP degraded the MB at all duty cycles. However, adsorption of MB onto the LED emitting surface caused a linear decline in reactor performance over time. With regard to the effect of duty cycle, the observed rate constant of MB degradation, after being adjusted to account for the duty cycle, was greater for 5 and 10% duty cycles than higher duty cycles, providing a value approximately 160% higher at 5% duty cycle than continuous operation. This increase in adjusted rate constant at low duty cycles, as well as contaminant fouling of the LED surface, may impact design and operational considerations for pulsed UV-LED AOP systems.