Spatial Distribution and Phase Transition Characteristics of Methane Hydrate in the Water-Excess and Gas-Excess Deposits

Methane hydrate is considered as a new environmentally friendly energy to meet future society development. To achieve safe and efficient hydrate production, it is critical to understand the hydrate accumulation characteristics in different deposits, considering the geological feature differences. In this study, the hydrate distribution characteristics in the gas-excess and water-excess deposits were visually investigated by using magnetic resonance imaging technology. Moreover, the effect of the initial gas pressure on hydrate formation behaviors and stability was analyzed. The results showed that methane hydrate first formed in the water–gas interface for the water-excess deposit, and then, the hydrate formation front gradually expanded into the water phase accumulation area. Moreover, the methane hydrate distribution was mainly determined by the initial distribution of water and gas in the porous media. For the water-excess deposit, the spatial distribution of methane hydrate showed an obvious heterogeneity, and the mass hydrate accumulated at the bottom of the deposit. However, a uniform distribution of methane hydrate in the gas-excess deposit was observed. Furthermore, methane hydrate that formed in the higher initial gas pressure and the water-excess environment had good stability during the water flow process, which prolonged the duration of the hydrate decomposition process. The findings attempt to provide valuable information and guidelines for understanding the gas hydrate system

Organic Oxidations Using Geomimicry

Oxidations of phenylacetic acid to benzaldehyde, benzyl alcohol to benzaldehyde, and benzaldehyde to benzoic acid have been observed, in water as the solvent and using only copper­(II) chloride as the oxidant. The reactions are performed at 250 °C and 40 bar, conditions that mimic hydrothermal reactions that are geochemically relevant. Speciation calculations show that the oxidizing agent is not freely solvated copper­(II) ions, but complexes of copper­(II) with chloride and carboxylate anions. Measurements of the reaction stoichiometries and also of substituent effects on reactivity allow plausible mechanisms to be proposed. These oxidation reactions are relevant to green chemistry in that they proceed in high chemical yield in water as the solvent and avoid the use of toxic heavy metal oxidizing reagents

Modeling and upscaling analysis of gas diffusion electrode-based electrochemical carbon dioxide reduction systems

As an emerging technology for CO2 utilization, electrochemical CO2 reduction reaction (ECO2RR) systems incorporating gas diffusion electrodes (GDE) have the potential to transform CO2 to valuable products efficiently and environment-friendly. In this work, a two-dimensional multiphase model capturing the details of the catalyst layer in a GDE that produces formate with byproducts is established and quantitatively validated against experimental data. This model is capable of describing the mixture gas and aqueous species transportation, electron conduction processes, and a series of interrelated chemical and electrochemical reactions. Specific electrical energy consumption (SEEC) and product yield (PY) have been introduced and used to examine the GDE scalability and evaluate the system performance. The results predict the optimal values for applied cathode potential and catalyst loading and porosity. The effect of inlet gas composition and velocity is also evaluated. Moreover, this study predicts that the GDE is scalable as it retains a stable performance as its geometrical surface area varies. This model together with the simulation findings contributes to the improved understanding of GDE-based CO2 conversion as needed for the future development toward successful industrial applications

Hydrothermal Photochemistry as a Mechanistic Tool in Organic Geochemistry: The Chemistry of Dibenzyl Ketone

Hydrothermal organic transformations under geochemically relevant conditions can result in complex product mixtures that form via multiple reaction pathways. The hydrothermal decomposition reactions of the model ketone dibenzyl ketone form a mixture of reduction, dehydration, fragmentation, and coupling products that suggest simultaneous and competitive radical and ionic reaction pathways. Here we show how Norrish Type I photocleavage of dibenzyl ketone can be used to independently generate the benzyl radicals previously proposed as the primary intermediates for the pure hydrothermal reaction. Under hydrothermal conditions, the benzyl radicals undergo hydrogen atom abstraction from dibenzyl ketone and <i>para</i>-coupling reactions that are not observed under ambient conditions. The photochemical method allows the primary radical coupling products to be identified, and because these products are generated rapidly, the method also allows the kinetics of the subsequent dehydration and Paal–Knorr cyclization reactions to be measured. In this way, the radical and ionic thermal and hydrothermal reaction pathways can be studied separately

Molecular Insights into Arctic Soil Organic Matter Degradation under Warming

Molecular composition of the Arctic soil organic carbon (SOC) and its susceptibility to microbial degradation are uncertain due to heterogeneity and unknown SOC compositions. Using ultrahigh-resolution mass spectrometry, we determined the susceptibility and compositional changes of extractable dissolved organic matter (EDOM) in an anoxic warming incubation experiment (up to 122 days) with a tundra soil from Alaska (United States). EDOM was extracted with 10 mM NH₄HCO₃ from both the organic- and mineral-layer soils during incubation at both −2 and 8 °C. Based on their O:C and H:C ratios, EDOM molecular formulas were qualitatively grouped into nine biochemical classes of compounds, among which lignin-like compounds dominated both the organic and the mineral soils and were the most stable, whereas amino sugars, peptides, and carbohydrate-like compounds were the most biologically labile. These results corresponded with shifts in EDOM elemental composition in which the ratios of O:C and N:C decreased, while the average C content in EDOM, molecular mass, and aromaticity increased after 122 days of incubation. This research demonstrates that certain EDOM components, such as amino sugars, peptides, and carbohydrate-like compounds, are disproportionately more susceptible to microbial degradation than others in the soil, and these results should be considered in SOC degradation models to improve predictions of Arctic climate feedbacks

Anaerobic Mercury Methylation and Demethylation by <i>Geobacter bemidjiensis</i> Bem

Microbial methylation and demethylation are two competing processes controlling the net production and bioaccumulation of neurotoxic methylmercury (MeHg) in natural ecosystems. Although mercury (Hg) methylation by anaerobic microorganisms and demethylation by aerobic Hg-resistant bacteria have both been extensively studied, little attention has been given to MeHg degradation by anaerobic bacteria, particularly the iron-reducing bacterium <i>Geobacter bemidjiensis</i> Bem. Here we report, for the first time, that the strain <i>G. bemidjiensis</i> Bem can mediate a suite of Hg transformations, including Hg­(II) reduction, Hg(0) oxidation, MeHg production and degradation under anoxic conditions. Results suggest that <i>G. bemidjiensis</i> utilizes a reductive demethylation pathway to degrade MeHg, with elemental Hg(0) as the major reaction product, possibly due to the presence of genes encoding homologues of an organomercurial lyase (MerB) and a mercuric reductase (MerA). In addition, the cells can strongly sorb Hg­(II) and MeHg, reduce or oxidize Hg, resulting in both time and concentration-dependent Hg species transformations. Moderate concentrations (10–500 μM) of Hg-binding ligands such as cysteine enhance Hg­(II) methylation but inhibit MeHg degradation. These findings indicate a cycle of Hg methylation and demethylation among anaerobic bacteria, thereby influencing net MeHg production in anoxic water and sediments