Effect of Solvent Polarity on the Spectral Characteristics of 5,10,15,20-Tetrakis(p-hydroxyphenyl)porphyrin

The electronic absorption and vibrational spectra of deprotonated 5,10,15,20-tetrakis(p-hydroxyphenyl)porphyrin (THPP) are studied as a function of solvent polarity in H₂O-DMF, H₂O-acetone, H₂O-methanol, and DMF-acetone mixtures. The maximum absorption wavelength (λ_max) of the lowest energy electronic absorption band of deprotonated THPP shows an unusual solvatochromism-a bathochromic followed by a hypsochromic shift with reduced polarity. According to the correlation analysis, both specific interactions (H-bonds) and nonspecific interactions affect the spectral changes of this porphyrin. Furthermore, the solvent polarity scale E_T(30) can explain both shifts very well. At higher polarity (E_T(30) > 48), THPP exists as a hyperporphyrin. The E_T(30) is linear with λ_max and a decrease in solvent polarity is accompanied by a bathochromic shift of λ_max. These results can be rationalized in terms of the cooperative effects of H-bonds and nonspecific interactions on the spectra of hyperporphyrin. At relatively low polarity (45.5 < E_T(30) < 48), hyperporphyrin gradually becomes Na₂P as E_T(30) reaches the critical value of 45.5. The spectrum of the hyperporphyrin turns into the three-band spectrum of the metalloporphyrin, which is accompanied by a hypsochromic shift of λ_max.

Control of the Structural Charge Distribution and Hydration State upon Intercalation of CO2 into Expansive Clay Interlayers

Numerous experimental investigations indicated that expansive clays such as montmorillonite can intercalate CO₂ preferentially into their interlayers and therefore potentially act as a material for CO₂ separation, capture, and storage. However, an understanding of the energy-structure relationship during the intercalation of CO₂ into clay interlayers remains elusive. Here, we use metadynamics molecular dynamics simulations to elucidate the energy landscape associated with CO₂ intercalation. Our free energy calculations indicate that CO₂ favorably partitions into nanoconfined water in clay interlayers from a gas phase, leading to an increase in the CO₂/H₂O ratio in clay interlayers as compared to that in bulk water. CO₂ molecules prefer to be located at the centers of charge-neutral hydrophobic siloxane rings, whereas interlayer spaces close to structural charges tend to avoid CO₂ intercalation. The structural charge distribution significantly affects the amount of CO₂ intercalated in the interlayers. These results provide a mechanistic understanding of CO₂ intercalation in clays for CO₂ separation, capture, and storage.

Interaction of low salinity surfactant nanofluids with carbonate surfaces and molecular level dynamics at fluid-fluid interface at ScCO2 loading

HYPOTHESIS

The advanced low salinity aqueous formulations are yet to be validated as an injection fluid for enhanced oil recovery (EOR) from the carbonate reservoirs and CO₂ geosequestration. Interaction of various ionic species present in the novel low salinity surfactant nanofluids with scCO₂/CO₂ saturated aqueous phase interface and at the interface of CO₂ saturated aqueous phase/mixed wet (with CO₂ and Decane) limestone surface at the conditions of low salinity at reservoir conditions are to yet to be understood.

EXPERIMENTS

This study, carried out for the first time in low salinity at scCO₂ loading conditions at 20 MPa pressure and 343 K temperature, comprises of wettability study of the limestone surface by aqueous phase contact angle measurements using ZrO₂ nanoparticles (in the concentration range of 100-2000 mg/L) and 0.82 mM Hexadecyltrimethylammonium bromide (CTAB) surfactant. Molecular dynamics simulations results were used to understand the underlying mechanism of wettability alteration and interfacial tension (IFT) change.

FINDINGS

This study reveals that a low dosage (100 mg/L) of ZrO₂ nanoparticles forming ZrO₂-CTAB nano-complexes helps in wettability alteration of the rock surface to more water-wetting state; certain ionic species augment this effect when used in appropriate concentration. Also, these nano-complexes helps in scCO₂/CO₂ saturated aqueous phase IFT reduction. This study can be used to design advanced low salinity injection fluids for water alternating gas injection for EOR and CO₂ geosequestration projects.

Molecular insight into the formation of adsorption clusters based on the zeta isotherm

This work presents a series of molecular dynamics simulations of argon adsorption on a silicon substrate with different lattice orientations. From the simulation results, the density profiles are discussed and the amount of adsorbed particles is obtained at different pressures. It is found that the solid surface orientation has a great influence on the density distributions and atomic arrangements near the surface. With the collected data, the thermal constants derived from the expression of zeta adsorption isotherms are determined. The calculated isotherms agree well with the simulation results. Also, from a microscopic point of view, the molecular insights show that the structures of the adsorbates are present as clusters with different numbers of particles. The size of the clusters changes with pressure. At a relatively small pressure ratio, most of the clusters consist of a single molecule. As the pressure ratio increases, larger sized clusters appear, forming various cluster-types. The molecular cluster distributions are closely consistent with the basic approximation of the zeta adsorption isotherm. Furthermore, the surface adsorption sites determined from molecular dynamics simulation show good agreement with that predicted by the zeta isotherm model, which reaffirms the effectiveness of the theoretical model. When the isotherm is extended to a pressure ratio greater than unity, a finite amount of adsorption is predicted and the wetting conditions are obtained. Affected by the solid surface orientations, the pressure ratio at wetting for the silicon substrate with the (111) surface plane is larger than those of the (100) and (110) surfaces, indicating that a higher subcooling is required for the wetting transition.

Molecular Simulation of the Catalytic Regeneration of nBuLi through a Hydrometalation Route

Efficient regeneration of organolithium compounds is a challenging aspect in the process of novel organometathetical catalytic cycles. One of these catalytic cycles is a newly suggested method for Mg production from seawater that capitalizes on the rich chemistry of Grignard reagents. The proposed three-step catalytic cycle with Cp₂ MCl L catalyst ( M = Ti, Zr; L = select organic ligands) requires the regeneration of ⁿBuLi from Li(s), butene, and H₂. The potential of this approach is evaluated with density functional theory-based molecular simulations. The results reveal that the high affinity of Li toward Cl and N results in the formation of alkanes, and the strong coupling between the catalyst and BuLi leads to catalyst deactivation. To improve its catalytic performance, we proposed the use of a diamine cocatalyst and a modified catalyst with a ligand that does not contain N, which would help release BuLi from the vicinity of the catalytic center. Ab initio molecular dynamics simulations at 298 K in explicit solvent (THF) were used to estimate the Gibbs free energetics and equilibrium constants obtained from the vibrational density of states using velocity autocorrelation functions. The results show a marked improvement in the free energetics with lower barriers toward the completion of the catalytic cycle and suppression of deactivation channels.

Structure, dynamics and stability of water/scCO2/mineral interfaces from ab initio molecular dynamics simulations

The boundary layer at solid-liquid interfaces is a unique reaction environment that poses significant scientific challenges to characterize and understand by experimentation alone. Using ab initio molecular dynamics (AIMD) methods, we report on the structure and dynamics of boundary layer formation, cation mobilization and carbonation under geologic carbon sequestration scenarios (T = 323 K and P = 90 bar) on a prototypical anorthite (001) surface. At low coverage, water film formation is enthalpically favored, but entropically hindered. Simulated adsorption isotherms show that a water monolayer will form even at the low water concentrations of water-saturated scCO2. Carbonation reactions readily occur at electron-rich terminal Oxygen sites adjacent to cation vacancies that readily form in the presence of a water monolayer. These results point to a carbonation mechanism that does not require prior carbonic acid formation in the bulk liquid. This work also highlights the modern capabilities of theoretical methods to address structure and reactivity at interfaces of high chemical complexity.

Pressure/temperature fluid cell apparatus for the neutron powder diffractometer instrument: probing atomic structure in situ

This contribution describes a new local structure compatible gas/liquid cell apparatus for probing disordered materials at high pressures and variable temperatures in the Neutron Powder Diffraction instrument at the Lujan Neutron Scattering Center, Los Alamos National Laboratory. The new sample environment offers choices for sample canister thickness and canister material type. Finite element modeling is utilized to establish maximum allowable working pressures of 414 MPa at 15 K and 121 MPa at 600 K. High quality atomic pair distribution function data extraction and modeling have been demonstrated for a calibration standard (Si powder) and for supercritical and subcritical CO2 measurements. The new sample environment was designed to specifically target experimental studies of the local atomic structures involved in geologic CO2 sequestration, but will be equally applicable to a wide variety of energy applications, including sorption of fluids on nano/meso-porous solids, clathrate hydrate formation, catalysis, carbon capture, and H2 and natural gas uptake/storage.

Microstructural response of variably hydrated Ca-rich montmorillonite to supercritical CO2

First-principles molecular dynamics simulations were carried out to explore the mechanistic and thermodynamic ramifications of the exposure of variably hydrated Ca-rich montmorillonites to supercritical CO2 and CO2-SO2 mixtures under geologic storage conditions. In sub- to single-hydrated systems (≤ 1W), CO2 intercalation causes interlamellar expansion of 8-12%, while systems transitioning to 2W may undergo contraction (∼ 7%) or remain almost unchanged. When compared to ∼2W hydration state, structural analysis of the ≤ 1W systems, reveals more Ca-CO2 contacts and partial transition to vertically confined CO2 molecules. Infrared spectra and projected vibrational frequency analysis imply that intercalated Ca-bound CO2 are vibrationally constrained and contribute to the higher frequencies of the asymmetric stretch band. Reduced diffusion coefficients of intercalated H2O and CO2 (10(-6)-10(-7) cm(2)/s) indicate that Ca-montmorillonites in ∼ 1W hydration states can be more efficient in capturing CO2. Simulations including SO2 imply that ∼ 0.66 mmol SO2/g clay can be intercalated without other significant structural changes. SO2 is likely to divert H2O away from the cations, promoting Ca-CO2 interactions and CO2 capture by further reducing CO2 diffusion (10(-8) cm(2)/s). Vibrational bands at ∼ 1267 or 1155 cm(-1) may be used to identify the chemical state (oxidation states +4 or +6, respectively) and the fate of sulfur contaminants.

Partitioning behavior of organic contaminants in carbon storage environments: a critical review

Carbon capture and storage is a promising strategy for mitigating the CO(2) contribution to global climate change. The large scale implementation of the technology mandates better understanding of the risks associated with CO(2) injection into geologic formations and the subsequent interactions with groundwater resources. The injected supercritical CO(2) (sc-CO(2)) is a nonpolar solvent that can potentially mobilize organic compounds that exist at residual saturation in the formation. Here, we review the partitioning behavior of selected organic compounds typically found in depleted oil reservoirs in the residual oil-brine-sc-CO(2) system under carbon storage conditions. The solubility of pure phase organic compounds in sc-CO(2) and partitioning of organic compounds between water and sc-CO(2) follow trends predicted based on thermodynamics. Compounds with high volatility and low aqueous solubility have the highest potential to partition to sc-CO(2). The partitioning of low volatility compounds to sc-CO(2) can be enhanced by cosolvency due to the presence of higher volatility compounds in the sc-CO(2). The effect of temperature, pressure, salinity, pH, and dissolution of water molecules into sc-CO(2) on the partitioning behavior of organic compounds in the residual oil-brine-sc-CO(2) system is discussed. Data gaps and research needs for models to predict the partitioning of organic compounds in brines and from complex mixtures of oils are presented. Models need to be able to better incorporate the effect of salinity and cosolvency, which will require more experimental data from key classes of organic compounds.

Forsterite [Mg2SiO4)] carbonation in wet supercritical CO2: an in situ high-pressure X-ray diffraction study

Mechanisms controlling mineral stabilities in contact with injected supercritical fluids containing water are relatively unknown. In this paper, we discuss carbonation reactions occurring with forsterite (Mg(2)SiO(4)) exposed to variably wet supercritical CO(2) (scCO(2)). Transformation reactions were tracked by in situ high-pressure X-ray diffraction in the presence of scCO(2) containing dissolved water. Under modest pressures (90 bar) and temperatures (50 °C), scCO(2) saturated with water converted >70 wt % forsterite to a hydrated magnesium carbonate, nesquehonite (MgCO(3) · 3H(2)O), and magnesite (MgCO(3)) after 72 h. However, comparable tests with scCO(2) at only partial water saturation showed a faster carbonation rate but significantly less nesquehonite formation and no evidence of the anhydrous form (MgCO(3)). The presence and properties of a thin water film, observed by in situ infrared (IR) spectroscopy and with isotopically labeled oxygen ((18)O), appears to be critical for this silicate mineral to carbonate in low water environments. The carbonation products formed demonstrated by temperature and water-content dependence highlights the importance of these kinds of studies to enable better predictions of the long-term fate of geologically stored CO(2).

Following 18O uptake in scCO2-H2O mixtures with Raman spectroscopy

The uptake of (18)O by scC(16)O(2) in mixtures containing liquid H(2)(18)O was followed with Raman spectroscopy using a specially designed high-pressure optical cell. Characteristic bands from the C(16)O(18)O and C(18)O(2) molecules were identified in the supercritical phase and measured in the spectra as a function of time after introducing the liquid H(2)(18)O into the scC(16)O(2). Temporal dependence indicated the process was diffusion-limited in our cell for both C(16)O(18)O and C(18)O(2). The ratio of concentrations of the (18)O-labeled CO(2) molecules, C(18)O(2)/C(16)O(18)O, was much higher than a random distribution of the isotopes for the system expected at equilibrium. The results are consistent with previous studies showing both rapid kinetics for oxygen exchange in aqueous solutions and the role of CO(2) transport at liquid water interfaces. More importantly, they demonstrate the potential for using Raman spectroscopy with (18)O isotopic labeling in scCO(2) reaction studies with the recently determined frequency and intensity characteristics of the Fermi dyad peaks from (18)O-containing CO(2) molecules.