In situ molecular spectroscopic evidence for CO2 intercalation into montmorillonite in supercritical carbon dioxide

CO2 Mineralization by MgO Nanocubes in Nanometric Water Films

Water films formed by the adhesion and condensation of air moisture on minerals can trigger the formation of secondary minerals of great importance to nature and technology. Magnesium carbonate growth on Mg-bearing minerals is not only of great interest for CO2 capture under enhanced weathering scenarios but is also a prime system for advancing key ideas on mineral formation under nanoconfinement. To help advance ideas on water film-mediated CO2 capture, we tracked the growth of amorphous magnesium carbonate (AMC) on MgO nanocubes exposed to moist CO2 gas. AMC was identified by its characteristic vibrational spectral signature and by its lack of long-range structure by X-ray diffraction. We find that AMC (MgCO3·2.3-2.5H2O) grew in sub-monolayer (ML) to 4 ML thick water films, with formation rates and yields scaling with humidity. AMC growth was however slowed down as AMC nanocoatings blocked water films access to the reactive MgO core. Films could however be partially dissolved by exposure to thicker water films, driving AMC growth for several more hours until nanocoatings blocked the reactions again. These findings shed new light on a potentially important bottleneck for the efficient mineralization of CO2 using MgO-bearing products. Notably, this study shows how variations in the air humidity affect CO2 capture by controlling water film coverages on reactive minerals. This process is also of great interest in the study of mineral growth in nanometrically thick water films.

Chemistry and Dynamics of Supercritical Carbon Dioxide and Methane in the Slit Pores of Layered Silicates

ConspectusIn the mid 2010s, high-pressure diffraction and spectroscopic tools opened a window into the molecular-scale behavior of fluids under the conditions of many CO₂ sequestration and shale/tight gas reservoirs, conditions where CO₂ and CH₄ are present as variably wet supercritical fluids. Integrating high-pressure spectroscopy and diffraction with molecular modeling has revealed much about the ways that supercritical CO₂ and CH₄ behave in reservoir components, particularly in the slit-shaped micro- and mesopores of layered silicates (phyllosilicates) abundant in caprocks and shales. This Account summarizes how supercritical CO₂ and CH₄ behave in the slit pores of swelling phyllosilicates as functions of the H₂O activity, framework structural features, and charge-balancing cation properties at 90 bar and 323 K, conditions similar to a reservoir at ∼1 km depth. Slit pores containing cations with large radii, low hydration energy, and large polarizability readily interact with CO₂, allowing CO₂ and H₂O to adsorb and coexist in these interlayer pores over a wide range of fluid humidities. In contrast, cations with small radii, high hydration energy, and low polarizability weakly interact with CO₂, leading to reduced CO₂ uptake and a tendency to exclude CO₂ from interlayers when H₂O is abundant. The reorientation dynamics of confined CO₂ depends on the interlayer pore height, which is strongly influenced by the cation properties, framework properties, and fluid humidity. The silicate structural framework also influences CO₂ uptake and behavior; for example, smectites with increasing F-for-OH substitution in the framework take up greater quantities of CO₂. Reactions that trap CO₂ in carbonate phases have been observed in thin H₂O films near smectite surfaces, including a dissolution-reprecipitation mechanism when the edge surface area is large and an ion exchange-precipitation mechanism when the interlayer cation can form a highly insoluble carbonate. In contrast, supercritical CH₄ does not readily associate with cations, does not react with smectites, and is only incorporated into interlayer slit mesopores when (i) the pore has a z-dimension large enough to accommodate CH₄, (ii) the smectite has low charge, and (iii) the H₂O activity is low. The adsorption and displacement of CH₄ by CO₂ and vice versa have been studied on the molecular scale in one shale, but opportunities remain to examine behavioral details in this more complicated, slit-pore inclusive system.

Intercalation of CO2 Selected by Type of Interlayer Cation in Dried Synthetic Hectorite

Clay minerals are abundant in caprock formations for anthropogenic storage sites for CO₂, and they are potential capture materials for CO₂ postcombustion sequestration. We investigate the response to CO₂ exposure of dried fluorohectorite clay intercalated with Li⁺, Na⁺, Cs⁺, Ca²⁺, and Ba²⁺. By in situ powder X-ray diffraction, we demonstrate that fluorohectorite with Na⁺, Cs⁺, Ca²⁺, or Ba²⁺ does not swell in response to CO₂ and that Li-fluorohectorite does swell. A linear uptake response is observed for Li-fluorohectorite by gravimetric adsorption, and we relate the adsorption to tightly bound residual water, which exposes adsorption sites within the interlayer. The experimental results are supported by DFT calculations.

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.

Interlayer Cation Polarizability Affects Supercritical Carbon Dioxide Adsorption by Swelling Clays

Several strategies for mitigating the build-up of atmospheric carbon dioxide (CO₂) bring wet supercritical CO₂ (scCO₂) in contact with phyllosilicates such as illites and smectites. While some work has examined the role of the charge-balancing cation and smectite framework features on CO₂/smectite interactions, to our knowledge no one has examined how the polarizability of the charge-balancing cation influences these behaviors. In this paper, the scCO₂ adsorption properties of Pb²⁺, Rb⁺, and NH₄⁺ saturated smectite clays at variable relative humidity are studied by integrating in situ high-pressure X-ray diffraction (XRD), infrared spectroscopic titrations, and magic angle spinning nuclear magnetic resonance (MAS NMR) methods. The results are combined with previously published data for Na⁺, Cs⁺, and Ca²⁺ saturated versions of the same smectites to isolate the roles of the charge-balancing cations and perform two independent tests of the role of charge-balancing cation polarizability in determining the interlayer fluid properties and smectite expansion. Independent correlations developed for (i) San Bernardino hectorite (SHCa-1) and (ii) Wyoming montmorillonite (SWy-2) both show that cation polarizability is important in predicting the interlayer composition (mol% CO₂ in the interlayer fluid and CO₂/cation ratio in interlayer) and the expansion behavior for smectites in contact with wet and dry scCO₂. In particular, this study shows that the charge-balancing cation polarizability is the most important cation-associated parameter in determining the expansion of the trioctahedral smectite, hectorite, when in contact with dry scCO₂. While both independent tests show that cation polarizability is an important factor in smectite-scCO₂ systems, the correlations for hectorite are different from those determined for montmorillonite. The root of these differences is likely associated with the roles of the smectite framework on adsorption, warranting follow-up studies with a larger number of unique smectite frameworks. Overall, the results show that the polarizability of the charge-balancing cation should be considered when preparing smectite clays (or industrial processes involving smectites) to capture CO₂ and in predicting the behavior of caprocks over time.

Enhanced Sorption of Supercritical CO2 and CH4 in the Hydrated Interlayer Pores of Smectite

Understanding the long-term confinement of supercritical fluids in the clay pores of subsurface rocks is important for many geo-energy technologies, including geological CO₂ storage. However, the adsorption properties of hydrated clay minerals remain largely uncertain because competitive adsorption experiments of supercritical fluids in the presence of water are difficult. Here, we report on the sorption properties of four source clay minerals-Ca-rich montmorillonite (STx-1b), Na-rich montmorillonite (SWy-2), illite-smectite mixed layer (ISCz-1), and illite (IMt-2)-for water at 20 °C up to relative humidity of 0.9. The measurements unveil the unsuitability of physisorption analysis by N₂ (at 77 K) and Ar (at 87 K) gases to quantify the textural properties of clays because of their inability to probe the interlayers. We further measure the sorption of CO₂ and CH₄ on swelling STx-1b and nonswelling IMt-2, both in the absence (dehydrated at 200 °C) and the presence of sub-1W preadsorbed water (following dehydration) up to 170 bar at 50 °C. We observe enhanced sorption of CO₂ and CH₄ in STx-1b (50 and 65% increase at 30 bar relative to dry STx-1b, respectively), while their adsorption on IMt-2 remains unchanged, indicating the absence of competition with water. By describing the supercritical adsorption isotherms on hydrated STx-1b with the lattice density functional theory model, we estimate that the pore volume has expanded by approximately 6% through the formation of sub-nanometer pore space. By presenting a systematic approach of quantifying the smectite clay mineral's hydrated state, this study provides an explanation for the conflicting literature observations of gas uptake capacities in the presence of water.

Thermodynamic data of adsorption reveal the entry of CH4 and CO2 in a smectite clay interlayer

The ability of smectite clays to incorporate gases in their interlayers is shown to be conditioned by interlayer spacing, depending, in turn, on phyllosilicate layer composition and effective size of the charge-balancing cations. As illustrated by earlier in situ X-ray diffraction and spectroscopic characterization of the gas/clay interface, most smectites with small-size charge-balancing cations, such as Na⁺ or Ca²⁺, accommodate CO₂ and CH₄ in their interlayers only in a partially hydrated state resulting in the opening of the basal spacing, above a certain critical value. In the present study CH₄ and CO₂ adsorption isotherms were measured for Na- and Mg-exchanged montmorillonite up to 9 MPa using a manometric technique. The process of dehydration of these clays was thoroughly characterized by thermogravimetric analysis and powder X-ray diffraction. A dramatic decrease in specific surface area and methane and carbon dioxide adsorption capacities for fully dehydrated samples in comparison to partially dehydrated ones is assigned to the shrinkage of interlayer spacing resulting in its inaccessibility for the entry of CH₄ and CO₂ molecules. This observation is direct evidence of CH₄ and CO₂ adsorption capacity variation depending on the opening of smectite clay interlayer spacing.

Critical Water Coverage during Forsterite Carbonation in Thin Water Films: Activating Dissolution and Mass Transport

In geologic carbon sequestration, CO₂ is injected into geologic reservoirs as a supercritical fluid (scCO₂). The carbonation of divalent silicates exposed to humidified scCO₂ occurs in angstroms to nanometers thick adsorbed H₂O films. A threshold H₂O film thickness is required for carbonate precipitation, but a mechanistic understanding is lacking. In this study, we investigated carbonation of forsterite (Mg₂SiO₄) in humidified scCO₂ (50 °C and 90 bar), which serves as a model system for understanding subsurface divalent silicate carbonation reactivity. Attenuated total reflection infrared spectroscopy pinpointed that magnesium carbonate precipitation begins at 1.5 monolayers of adsorbed H₂O. At about this same H₂O coverage, transmission infrared spectroscopy showed that forsterite dissolution begins and electrical impedance spectroscopy demonstrated that diffusive transport accelerates. Molecular dynamics simulations indicated that the onset of diffusion is due to an abrupt decrease in the free-energy barriers for lateral mobility of outer-spherically adsorbed Mg²⁺. The dissolution and mass transport controls on divalent silicate reactivity in wet scCO₂ could be advantageous for maximizing permeability near the wellbore and minimize leakage through the caprock.

Supercritical CO2 and CH4 Uptake by Illite-Smectite Clay Minerals

Clay minerals abound in sedimentary formations and the interaction of reservoir gases with their submicron features have direct relevance to many geoenergy applications. The quantification of gas uptake over a broad range of pressures is key toward assessing the significance of these physical interactions on enhancing storage capacity and gas recovery. We report a systematic investigation of the sorption properties of three source clay minerals-Na-rich montmorillonite (SWy-2), illite-smectite mixed layer (ISCz-1), and illite (IMt-2)-using CO₂ and CH₄ up to 30 MPa at 25-115 °C. The textural characterization of the clays by gas physisorption indicates that micropores are only partly accessible to N₂ (77 K) and Ar (87 K), while larger uptakes are measured with CO₂ (273 K) in the presence of illite. The supercritical excess sorption experiments confirm these findings while revealing differences in uptake capacities that originate from the clay-specific pore size distribution. The lattice density functional theory model describes accurately the measured sorption isotherms by using a distribution of properly weighted slit pores and clay-specific solid-fluid interaction energies, which agree with isosteric heats of adsorption obtained experimentally. The model indicates that the maximum degree of pore occupancy is universal to the three clays and the two gases, and it depends solely on temperature, reaching values near unity at the critical temperature. These observations greatly support the model's predictive capability for estimating gas adsorption on clay-bearing rocks and sediments.

CO2 Adsorption of Materials Synthesized from Clay Minerals: A Review

The aim of this work is to make a brief review of the adsorption of CO2 on modified clay minerals. Previous researchers have used different clay modifications, either by making changes in the structure by a reaction with another product or by the addition of a catalyst to improve their CO2 adsorption capacity. In order to obtain high values of CO2 uptake, some researchers have been incorporated amines-speices such as (3-aminopropyl)triethoxysilane (APTES), tetraethylenepentamine (TEPA) and a branched polyethylenimine (PEI) by grafting or impregnation. The synthesis of an adsorbent from mineral clays can generate an increase in its porosity and in its textural properties. These investigations differ in a number of factors such as the kind of clay, the operating conditions, y and the nature of the impregnated compound. The role of these factors in the CO2 adsorption capacity will be considered in detail in this review.

Understanding methane/carbon dioxide partitioning in clay nano- and meso-pores with constant reservoir composition molecular dynamics modeling

CRC-MD simulations show that nanopores in shales bounded by clay minerals have a strong preference for CO₂ relative to CH₄.

Water near its Supercritical Point and at Alkaline pH for the Production of Ferric Oxides and Silicates in Anoxic Conditions. A New Hypothesis for the Synthesis of Minerals Observed in Banded Iron Formations and for the Related Geobiotropic Chemistry inside Fluid Inclusions

An alternative hypothesis for the origin of the banded iron formations and the synthesis of prebiotic molecules is presented here. I show the importance of considering water near its supercritical point and at alkaline pH. It is based on the chemical equation for the anoxic oxidation of ferrous iron into ferric iron at high-subcritical conditions of water and high pH, that I extract from E-pH diagrams drawn for corrosion purposes (Geophysical Research Abstracts Vol 15, EGU2013-22 Bassez 2013, Orig Life Evol Biosph 45(1):5-13, Bassez 2015, Procedia Earth Planet Sci 17, 492-495, Bassez 2017a, Orig Life Evol Biosph 47:453-480, Bassez 2017b). The sudden change in solubility of silica, SiO2, at the critical point of water is also considered. It is shown that under these temperatures and pressures, ferric oxides and ferric silicates can form in anoxic terrains. No FeII oxidation by UV light, neither by oxygen is needed to explain the minerals of the Banded Iron Formations. The intervention of any kind of microorganisms, either sulfate-reducing, or FeII-oxidizing or O2-producing, is not required. The chemical equation for the anoxic oxidation of ferrous iron is applied to the hydrolyses of fayalite, Fe2SiO4 and ferrosilite, FeSiO3. It is shown that the BIF minerals of the Hamersley Group, Western Australia, and the Transvaal Supergroup, South Africa, are those of fayalite and ferrosilite hydrolyses and carbonations. The dissolution of crustal fayalite and ferrosilite during water-rock interaction needs to occur at T&P just below the critical point of water and in a rising water which is undersaturated in SiO2. Minerals of BIFs which can then be ejected at the surface from venting arcs are ferric oxide hydroxides, hematite, FeIII-greenalite, siderite. The greenalite dehydrated product minnesotaite forms when rising water becomes supersaturated in SiO2, as also riebeckite and stilpnomelane. Long lengths of siderite without ferric oxides neither ferric silicates can occur since the exothermic siderite formation is not so much dependent in T&P. It is also shown that the H2 which is released during hydrolysis/oxidation of fayalite/ferrosilite can lead to components of life, such as macromolecules of amino acids which are synthesized from mixtures of (CO, N2, H2O) in Sabatier-Senderens/Fischer-Tropsch & Haber-Bosch reactions or microwave or gamma-ray excitation reactions. I propose that such geobiotropic synthesis may occur inside fluid inclusions of BIFs, in the silica chert, hematite, FeIII-greenalite or siderite. Therefore, the combination of high-subcritical conditions of water, high solubility of SiO2 at these T&P values, formation of CO also at these T&P, high pH and anoxic water, leads to the formation of ferric minerals and prebiotic molecules in the process of geobiotropy.

Supercritical CO2 uptake by nonswelling phyllosilicates

Reliable estimates of geologic carbon storage capacities (needed for policymaking) in both saline aquifers and unconventional gas/oil shales rely on understanding trapping mechanisms. We found that CO₂ uptake by muscovite (a common mineral and a conservative proxy for illite) far exceeds the maximum adsorption capacity of its external surface area. Our measurements using different methods collectively suggest that CO₂ enters muscovite interlayers without bulk interlayer expansion, contrary to the conventional wisdom that only swelling clays take up CO₂ into interlayers. Because the nonswelling illitic clay is the major clay mineral in deep subsurface tight rocks, their excess uptake of CO₂ may significantly contribute to CO₂ storage capacity and warrants further in-depth studies.

Interactions between supercritical (sc) CO₂ and minerals are important when CO₂ is injected into geologic formations for storage and as working fluids for enhanced oil recovery, hydraulic fracturing, and geothermal energy extraction. It has previously been shown that at the elevated pressures and temperatures of the deep subsurface, scCO₂ alters smectites (typical swelling phyllosilicates). However, less is known about the effects of scCO₂ on nonswelling phyllosilicates (illite and muscovite), despite the fact that the latter are the dominant clay minerals in deep subsurface shales and mudstones. Our studies conducted by using single crystals, combining reaction (incubation with scCO₂), visualization [atomic force microscopy (AFM)], and quantifications (AFM, X-ray photoelectron spectroscopy, X-ray diffraction, and off-gassing measurements) revealed unexpectedly high CO₂ uptake that far exceeded its macroscopic surface area. Results from different methods collectively suggest that CO₂ partially entered the muscovite interlayers, although the pathways remain to be determined. We hypothesize that preferential dissolution at weaker surface defects and frayed edges allows CO₂ to enter the interlayers under elevated pressure and temperature, rather than by diffusing solely from edges deeply into interlayers. This unexpected uptake of CO₂, can increase CO₂ storage capacity by up to ∼30% relative to the capacity associated with residual trapping in a 0.2-porosity sandstone reservoir containing up to 18 mass % of illite/muscovite. This excess CO₂ uptake constitutes a previously unrecognized potential trapping mechanism.

Atomistic Structure of Mineral Nano-aggregates from Simulated Compaction and Dewatering

The porosity of clay aggregates is an important property governing chemical reactions and fluid flow in low-permeability geologic formations and clay-based engineered barrier systems. Pore spaces in clays include interlayer and interparticle pores. Under compaction and dewatering, the size and geometry of such pore spaces may vary significantly (sub-nanometer to microns) depending on ambient physical and chemical conditions. Here we report a molecular dynamics simulation method to construct a complex and realistic clay-like nanoparticle aggregate with interparticle pores and grain boundaries. The model structure is then used to investigate the effect of dewatering and water content on micro-porosity of the aggregates. The results suggest that slow dewatering would create more compact aggregates compared to fast dewatering. Furthermore, the amount of water present in the aggregates strongly affects the particle-particle interactions and hence the aggregate structure. Detailed analyses of particle-particle and water-particle interactions provide a molecular-scale view of porosity and texture development of the aggregates. The simulation method developed here may also aid in modeling the synthesis of nanostructured materials through self-assembly of nanoparticles.

Tipping Point for Expansion of Layered Aluminosilicates in Weakly Polar Solvents: Supercritical CO2

Layered aluminosilicates play a dominant role in the mechanical and gas storage properties of the subsurface, are used in diverse industrial applications, and serve as model materials for understanding solvent-ion-support systems. Although expansion in the presence of H₂O is well-known to be systematically correlated with the hydration free energy of the interlayer cation, particularly in environments dominated by nonpolar solvents (i.e., CO₂), uptake into the interlayer is not well-understood. Using novel high-pressure capabilities, we investigated the interaction of dry supercritical CO₂ with Na-, NH₄-, and Cs-saturated montmorillonite, comparing results with predictions from molecular dynamics simulations. Despite the known trend in H₂O and that cation solvation energies in CO₂ suggest a stronger interaction with Na, both the NH₄- and Cs-clays readily absorbed CO₂ and expanded, while the Na-clay did not. The apparent inertness of the Na-clay was not due to kinetics, as experiments seeking a stable expanded state showed that none exists. Molecular dynamics simulations revealed a large endothermicity to CO₂ intercalation in the Na-clay but little or no energy barrier for the NH₄- and Cs-clays. Indeed, the combination of experiment and theory clearly demonstrate that CO₂ intercalation of Na-montmorillonite clays is prohibited in the absence of H₂O. Consequently, we have shown for the first time that in the presence of a low dielectric constant, gas swelling depends more on the strength of the interaction between the interlayer cation and aluminosilicate sheets and less on that with solvent. The finding suggests a distinct regime in layered aluminosilicate swelling behavior triggered by low solvent polarizability, with important implications in geomechanics, storage, and retention of volatile gases, and across industrial uses in gelling, decoloring, heterogeneous catalysis, and semipermeable reactive barriers.

Self-diffusivity, M-S and Fick diffusivity of CO2 in Na-clay: The influences of concentration and temperature

Storing CO₂ in underground saline aquifers is an important way to reduce CO₂ emission in atmosphere, where gas/fluid diffusion in clay plays a key role in CO₂ leakage and migration. Various diffusivities, self-diffusivity, Maxwell–Stefan (M–S) diffusivity and Fick diffusivity, in clay interlayer are investigated by molecular dynamics (MD). Self-diffusivity varies with CO₂ concentration, and reaches the maximum value at 2 molecules/unit-cell. High fluid concentration leads to clay swelling, thereby increasing self-diffusivity. However, the fractional free volume of clay explains the trend of CO₂ self-diffusivity, which does not decrease with CO₂ concentration monotonously but reaches the maximum when CO₂ concentration reaches 2. Displacement distribution of CO₂ molecules is analysed to explore the microscopic diffusion mechanism, which is characterised by logarithmic normal distribution. The mean value of such distribution further explains the self-diffusivity dependence on CO₂ concentration. M–S and Fick diffusivities of CO₂ are calculated by MD for the first time, both of which increase with increasing CO₂ and H₂O concentration and temperature. Based on self-diffusivity and M–S diffusivity, a quantity representing the coupling strength between CO₂ molecules is presented; it increases firstly with CO₂ concentration but begins to decrease when CO₂ concentration is beyond 2.

Effective and Selective Trapping of Volatile Organic Sulfur Derivatives by Montmorillonite Intercalated with a μ-oxo Fe(III)-Phenanthroline Complex

The μ-oxo Fe(III)-phenanthroline complex [(OH₂)₃(Phen)FeOFe(Phen) (OH₂)₃]⁺⁴ intercalated in montmorillonite provides a stable hybrid material. In this study, the ability and efficiency of this material to immobilize thiols in gas phase, acting as a trap at the solid-gas interface, were investigated. Aliphatic thiols containing both hydrophilic and hydrophobic end groups were chosen to test the selectivity of this gas trap. DR-UV-vis, IR, elemental analysis, thermal analysis and evolved gas mass spectrometry, X-ray powder diffraction, and X-ray absorption spectroscopy techniques were employed to characterize the hybrid material before and after thiol exposure and to provide information on the entrapping process. Thiol immobilization is very large, up to 21% w/w for heptanethiol. In addition, evidence was obtained that immobilization occurs through the formation of a covalent bond between the iron of the complex and the sulfur of the thiol. This provides an immobilization process characterized by a higher stability with respect to the methods based on physi-adsorption. Thiol immobilization resulted thermally reversible at least for 20 adsorption/desorption cycles. Unlike standard desulfurization processes like hydrotreating and catalytic oxidation which work at high temperatures and pressures, the present system is able to efficiently trap thiols at room temperature and pressure, thus saving energy. Furthermore, we found that the selectivity of thiol immobilization can be tuned acting on the amount of complex intercalated in montmorillonite. In particular, montmorillonite semisaturated with the complex captures both hydrophobic and hydrophilic thiols, while the saturated montmorillonite shows a strong selectivity toward the hydrophobic molecules.

Effect of Layer Charge on CO2 and H2O Intercalations in Swelling Clays

The effect of layer charge on the intercalation of supercritical carbon dioxide (scCO₂)-H₂O mixture in Na-montmorillonite clay interlayers under T = 323 K and P = 90 bar geologic sequestration conditions has been further investigated. This effect includes the charge amount and its location (within either octahedral or tetrahedral layers due to isomorphic substitutions). Two clay models with different layer charges are used in this study. Simulation results show that the increase of charge amount shifts the monolayer-to-bilayer (1W-to-2W) hydration transition toward the lower relative humidity (RH), increasing water sorption at the expense of reducing the overall sorption amount of CO₂ in the clay interlayer. However, the combination of the influence of charge amount and charge location leads to insignificant changes in equilibrium basal spacings of the high- and low-charge clays. Molecular dynamics simulations show that the CO₂ dimers, which are frequently seen in low-charge clay interlayers, vanish in high-charge clay interlayers even at low RH of 30%.

Sealed rotors for in situ high temperature high pressure MAS NMR

Here we present the design of reusable and perfectly sealed all-zirconia MAS rotors. The rotors are used to study AlPO4-5 molecular sieve crystallization under hydrothermal conditions, high temperature high pressure cyclohexanol dehydration reaction, and low temperature metabolomics of intact biological tissue.

Molecular dynamics modeling of carbon dioxide, water and natural organic matter in Na-hectorite

Molecular dynamics (MD) modeling of systems containing a Na-exchanged smectite clay (hectorite) and model natural organic matter (NOM) molecules along with pure H2O, pure CO2, or a mixture of H2O and CO2 provides significant new insight into the molecular scale interactions among silicate surfaces, dissolved cations and organic molecules, H2O and CO2 relevant to geological C-sequestration strategies. The simulations for systems containing H2O show the following results; (1) Na(+) does not bridge between NOM molecules and the clay surface at protonation states comparable to near neutral pH conditions. (2) In systems without CO2 the NOM molecules retain charge balancing cations and drift away from the silicate surface. (3) In systems containing both H2O and CO2, the NOM molecules adopt equilibrium positions at the H2O-CO2 interface with the more hydrophilic structural elements facing the H2O and the more hydrophobic ones facing the CO2. In systems with only CO2, NOM and Na(+) ions are pinned to the clay surface with the hydrophilic structural elements of the NOM pointed toward the clay surface. Dynamically, in systems with only CO2, Na(+) diffusion is nearly eliminated, and in systems with a thin water film on the clay surface diffusion perpendicular the surface is greatly reduced relative to the system with bulk water. Energetically, the results for the systems with only H2O show that hydration of the net charge neutral Na-NOM molecule outweighs the sum of its Coulombic and dispersive interactions with the net charge-neutral Na-clay particle and the interactions of the water molecules with the hydrophobic structural elements of the NOM. The aggregation of NOM molecules in solution appears to be driven not by Na(+) bridging between the molecules but by hydrophobic interactions between them. In contrast, for the systems with only CO2 the interaction between the Na-NOM molecules and the CO2 is outweighed by the interaction of NOM with the clay particle. With both H2O and CO2 present, the energetic interactions leading to the hydration of the Na-clay surface and the hydrophilic structural elements of the Na-NOM molecule and the hydrophobic interactions between the CO2 and the hydrophobic aromatic and aliphatic structural elements of the NOM can both be satisfied, leading to the Na-NOM molecules migrating away from the surface and residing at the H2O-CO2 interface. The MD results suggest some alternative explanations for the previously observed (23)Na NMR behavior of Na-hectorite at elevated temperatures and CO2 pressures.

The Nanoscale Basis of CO2 Trapping for Geologic Storage

Carbon capture and storage (CCS) is likely to be a critical technology to achieve large reductions in global carbon emissions over the next century. Research on the subsurface storage of CO2 is aimed at reducing uncertainties in the efficacy of CO2 storage in sedimentary rock formations. Three key parameters that have a nanoscale basis and that contribute uncertainty to predictions of CO2 trapping are the vertical permeability kv of seals, the residual CO2 saturation Sg,r in reservoir rocks, and the reactive surface area ar of silicate minerals. This review summarizes recent progress and identifies outstanding research needs in these areas. Available data suggest that the permeability of shale and mudstone seals is heavily dependent on clay fraction and can be extremely low even in the presence of fractures. Investigations of residual CO2 trapping indicate that CO2-induced alteration in the wettability of mineral surfaces may significantly influence Sg,r. Ultimately, the rate and extent of CO2 conversion to mineral phases are uncertain due to a poor understanding of the kinetics of slow reactions between minerals and fluids. Rapidly improving characterization techniques using X-rays and neutrons, and computing capability for simulating chemical interactions, provide promise for important advances.

Intercalation and retention of carbon dioxide in a smectite clay promoted by interlayer cations

A good material for CO₂ capture should possess some specific properties: (i) a large effective surface area with good adsorption capacity, (ii) selectivity for CO₂, (iii) regeneration capacity with minimum energy input, allowing reutilization of the material for CO₂ adsorption, and (iv) low cost and high environmental friendliness. Smectite clays are layered nanoporous materials that may be good candidates in this context. Here we report experiments which show that gaseous CO₂ intercalates into the interlayer nano-space of smectite clay (synthetic fluorohectorite) at conditions close to ambient. The rate of intercalation, as well as the retention ability of CO₂ was found to be strongly dependent on the type of the interlayer cation, which in the present case is Li⁺, Na⁺ or Ni²⁺. Interestingly, we observe that the smectite Li-fluorohectorite is able to retain CO₂ up to a temperature of 35°C at ambient pressure, and that the captured CO₂ can be released by heating above this temperature. Our estimates indicate that smectite clays, even with the standard cations analyzed here, can capture an amount of CO₂ comparable to other materials studied in this context.

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.

In situ study of CO₂ and H₂O partitioning between Na-montmorillonite and variably wet supercritical carbon dioxide

Shale formations play fundamental roles in large-scale geologic carbon sequestration (GCS) aimed primarily to mitigate climate change and in smaller-scale GCS targeted mainly for CO2-enhanced gas recovery operations. Reactive components of shales include expandable clays, such as montmorillonites and mixed-layer illite/smectite clays. In this study, in situ X-ray diffraction (XRD) and in situ infrared (IR) spectroscopy were used to investigate the swelling/shrinkage and H2O/CO2 sorption of Na(+)-exchanged montmorillonite, Na-SWy-2, as the clay is exposed to variably hydrated supercritical CO2 (scCO2) at 50 °C and 90 bar. Measured d001 values increased in stepwise fashion and sorbed H2O concentrations increased continuously with increasing percent H2O saturation in scCO2, closely following previously reported values measured in air at ambient pressure over a range of relative humidities. IR spectra show H2O and CO2 intercalation, and variations in peak shapes and positions suggest multiple sorbed types of H2O and CO2 with distinct chemical environments. Based on the absorbance of the asymmetric CO stretching band of the CO2 associated with the Na-SWy-2, the sorbed CO2 concentration increases dramatically at sorbed H2O concentrations from 0 to 4 mmol/g. Sorbed CO2 then sharply decreases as sorbed H2O increases from 4 to 10 mmol/g. With even higher sorbed H2O concentrations as saturation of H2O in scCO2 was approached, the concentration of sorbed CO2 decreased asymptotically. Two models, one involving space filling and the other a heterogeneous distribution of integral hydration states, are discussed as possible mechanisms for H2O and CO2 intercalations in montmorillonite. The swelling/shrinkage of montmorillonite could affect solid volume, porosity, and permeability of shales. Consequently, the results may aid predictions of shale caprock integrity in large-scale GCS as well as methane transmissivity in enhanced gas recovery operations.

Automated high-pressure titration system with in situ infrared spectroscopic detection

A fully automated titration system with infrared detection was developed for investigating interfacial chemistry at high pressures. The apparatus consists of a high-pressure fluid generation and delivery system coupled to a high-pressure cell with infrared optics. A manifold of electronically actuated valves is used to direct pressurized fluids into the cell. Precise reagent additions to the pressurized cell are made with calibrated tubing loops that are filled with reagent and placed in-line with the cell and a syringe pump. The cell's infrared optics facilitate both transmission and attenuated total reflection (ATR) measurements to monitor bulk-fluid composition and solid-surface phenomena such as adsorption, desorption, complexation, dissolution, and precipitation. Switching between the two measurement modes is accomplished with moveable mirrors that direct the light path of a Fourier transform infrared spectrometer into the cell along transmission or ATR light paths. The versatility of the high-pressure IR titration system was demonstrated with three case studies. First, we titrated water into supercritical CO2 (scCO2) to generate an infrared calibration curve and determine the solubility of water in CO2 at 50 °C and 90 bar. Next, we characterized the partitioning of water between a montmorillonite clay and scCO2 at 50 °C and 90 bar. Transmission-mode spectra were used to quantify changes in the clay's sorbed water concentration as a function of scCO2 hydration, and ATR measurements provided insights into competitive residency of water and CO2 on the clay surface and in the interlayer. Finally, we demonstrated how time-dependent studies can be conducted with the system by monitoring the carbonation reaction of forsterite (Mg2SiO4) in water-bearing scCO2 at 50 °C and 90 bar. Immediately after water dissolved in the scCO2, a thin film of adsorbed water formed on the mineral surface, and the film thickness increased with time as the forsterite began to dissolve. However, after approximately 2.5 h, the trend reversed, and a carbonate precipitate began to form on the forsterite surface, exposing dramatic chemical changes in the thin-water film. Collectively, these applications illustrate how the high-pressure IR titration system can provide molecular-level information about the interactions between variably wet scCO2 and minerals relevant to underground storage of CO2 (geologic carbon sequestration). The apparatus could also be utilized to study high-pressure interfacial chemistry in other areas such as catalysis, polymerization, food processing, and oil and gas recovery.

Molecular simulation of carbon dioxide, brine, and clay mineral interactions and determination of contact angles

Capture and subsequent geologic storage of CO2 in deep brine reservoirs plays a significant role in plans to reduce atmospheric carbon emission and resulting global climate change. The interaction of CO2 and brine species with mineral surfaces controls the ultimate fate of injected CO2 at the nanoscale via geochemistry, at the pore-scale via capillary trapping, and at the field-scale via relative permeability. We used large-scale molecular dynamics simulations to study the behavior of supercritical CO2 and aqueous fluids on both the hydrophilic and hydrophobic basal surfaces of kaolinite, a common clay mineral. In the presence of a bulk aqueous phase, supercritical CO2 forms a nonwetting droplet above the hydrophilic surface of kaolinite. This CO2 droplet is separated from the mineral surface by distinct layers of water, which prevent the CO2 droplet from interacting directly with the mineral surface. Conversely, both CO2 and H2O molecules interact directly with the hydrophobic surface of kaolinite. In the presence of bulk supercritical CO2, nonwetting aqueous droplets interact with the hydrophobic surface of kaolinite via a mixture of adsorbed CO2 and H2O molecules. Because nucleation and precipitation of minerals should depend strongly on the local distribution of CO2, H2O, and ion species, these nanoscale surface interactions are expected to influence long-term mineralization of injected carbon dioxide.

CO2 sorption to subsingle hydration layer montmorillonite clay studied by excess sorption and neutron diffraction measurements

Geologic storage of CO(2) requires that the caprock sealing the storage rock is highly impermeable to CO(2). Swelling clays, which are important components of caprocks, may interact with CO(2) leading to volume change and potentially impacting the seal quality. The interactions of supercritical (sc) CO(2) with Na saturated montmorillonite clay containing a subsingle layer of water in the interlayer region have been studied by sorption and neutron diffraction techniques. The excess sorption isotherms show maxima at bulk CO(2) densities of ≈ 0.15 g/cm(3), followed by an approximately linear decrease of excess sorption to zero and negative values with increasing CO(2) bulk density. Neutron diffraction experiments on the same clay sample measured interlayer spacing and composition. The results show that limited amounts of CO(2) are sorbed into the interlayer region, leading to depression of the interlayer peak intensity and an increase of the d(001) spacing by ca. 0.5 Å. The density of CO(2) in the clay pores is relatively stable over a wide range of CO(2) pressures at a given temperature, indicating the formation of a clay-CO(2) phase. At the excess sorption maximum, increasing CO(2) sorption with decreasing temperature is observed while the high-pressure sorption properties exhibit weak temperature dependence.

Biotite dissolution in brine at varied temperatures and CO2 pressures: its activation energy and potential CO2 intercalation

For sustainable geologic CO(2) sequestration (GCS), it is important to understand the effects of temperature and CO(2) pressure on mica's dissolution and surface morphological changes under saline hydrothermal conditions. Batch experiments were conducted with biotite (Fe-end member mica) under conditions relevant to GCS sites (35-95 °C and 75-120 atm CO(2)), and 1 M NaCl solution was used to mimic the brine. With increasing temperature, a transition from incongruent to congruent dissolution of biotite was observed. The dissolution activation energy based on Si release was calculated to be 52 ± 5 kJ mol(-1). By comparison with N(2) experiments, we showed that CO(2) injection greatly enhanced biotite's dissolution and its surface morphology evolutions, such as crack formation and detachment of newly formed fibrous illite. For biotite's dissolution and morphological evolutions, the pH effects of CO(2) were differentiated from the effects of bicarbonate complexation and CO(2) intercalation. Bicarbonate complexation effects on ion release from biotite were found to be minor under our experimental conditions. On the other hand, the CO(2) molecules in brine could get into the biotite interlayer and cause enhanced swelling of the biotite interlayer and hence the observed promotion of biotite surface cracking. The cracking created more reactive surface area in contact with brine and thus enhanced the later ion release from biotite. These results provide new information for understanding CO(2)-brine-mica interactions in saline aquifers with varied temperatures and CO(2) pressures, which can be useful for GCS site selection and operations.