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A detailed mechanism, based on densityfunctional theory calculationsand simulation, is presented outlining the formation of the secondarybuilding unit (SBU) of MIL-101, a chromium terephthalate metal organicframework (MOF). Formation of the metal core and of the SBU is keyto MOF nucleation, the rate-limiting step in the synthesis processof many MOFs. A series of reactions that lead to the formation ofthe SBU of MIL-101 is proposed in this work. The highest barrier (∼35kcal/mol) involves the formation of a dimetal-linker intermediateand high to low spin transition as a third Cr-linker moiety joinsto form a three metal-linker group joined by a central oxygen. Theterephthalate linkers play an important, key mechanistic role withthe carboxylates first joining chromium atoms prior to the formationof bridging oxygens. Subsequent to metal core formation, stepwiselinker addition reactions generate different assembly pathways dueto structural isomers that are limited by the removal of water moleculesin the first chromium coordination shell. A simple kinetic model basedon transition state theory gave a rate of SBU formation similar toa reported rate of MOF nucleation. The least energy path was identifiedwith all linkers on the same face of the metal center added first.These first steps in developing a modeling framework for SBU formationwill hopefully lay the groundwork for future comprehensive predictivemodels of the full MOF framework structure assembly and synthesisconditions required to support the self-assembly process. [ABSTRACT FROM AUTHOR]

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 cm2/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 cm2/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. [ABSTRACT FROM AUTHOR]

We used IR and XRD, with supporting theoretical calculations, to investigate the swelling behavior of Na + -, NH 4 + -, and Cs + -montmorillonites (SWy-2) in supercritical fluid mixtures of H 2 O, CO 2 , and CH 4 . Building on our prior work with Na-clay that demonstrated that H 2 O facilitated CO 2 intercalation at relatively low RH, here we show that increasing CO 2 /CH 4 ratios promote H 2 O intercalation and swelling of the Na-clay at progressively lower RH. In contrast to the Na-clay, CO 2 intercalated and expanded the Cs-clay even in the absence of H 2 O, while increasing fluid CO 2 /CH 4 ratios inhibited H 2 O intercalation. The NH 4 -clay displayed intermediate behavior. By comparing changes in the HOH bending vibration of H 2 O intercalated in the Cs-, NH 4 -, and Na-clays, we posit that CO 2 facilitated expansion of the Na-clay by participating in outer-sphere solvation of Na + and by disrupting the H-bond network of intercalated H 2 O. In no case did the pure CH 4 fluid induce expansion. Our experimental data can benchmark modeling studies aimed at predicting clay expansion in humidified fluids with varying ratios of CO 2 and CH 4 in real reservoir systems with implications for enhanced hydrocarbon recovery and CO 2 storage in subsurface environments.

In 2013, the Pacific Northwest National Laboratory led a geologic carbon sequestration field demonstration where ∼1000 tonnes of CO 2 was injected into several deep Columbia River Basalt zones near Wallula, Washington. Rock core samples extracted from the injection zone two years after CO 2 injection revealed nascent carbonate mineralization that was qualitatively consistent with expectations from laboratory experiments and reactive transport modeling. Here, we report on a new detailed analysis of the 2012 pre-injection and 2015 post-injection hydrologic tests that capitalizes on the difference in fluid properties between scCO 2 and water to assess changes in near-field, wellbore, and reservoir conditions that are apparent approximately two years following the end of injection. This comparative hydrologic test analysis method provides a new way to quantify the amount of injected CO 2 that was mineralized in the field test. Modeling results indicate that approximately 60% of the injected CO 2 was sequestered via mineralization within two years, with the resulting carbonates occupying ∼4% of the available reservoir pore space. The method presented here provides a new monitoring tool to assess the fate of CO 2 injected into chemically reactive basalt formations but could also be adapted for long-term monitoring and verification within more traditional subsurface carbon storage reservoirs.

The rapid growth in the global energy demand for space cooling requires the development of more efficient environmental chillers for which adsorption-based cooling systems can be utilized. Here, in this contribution, we explore sorbents for chiller use via a pore-engineering concept to construct analogs of the 1-dimensional pore metal-organic framework MOF-74 by using elongated organic linkers and stereochemistry control. The prepared pore-engineered MOFs show remarkable equilibrium adsorption of the selected fluorocarbon refrigerant that is translated to a modeled adsorption-based refrigeration cycle. To probe molecular level interactions at the origin of these unique adsorption properties for this series of Ni-MOFs, we combined in situ synchrotron X-ray powder diffraction, neutron powder diffraction, X-ray absorption spectroscopy, calorimetry, Fourier transform infrared techniques, and molecular simulations. Our results reveal the coordination of fluorine (of CH 2 F in R134a) to the nickel(II) open metal centers at low pressures for each Ni-MOF analog and provide insight into the pore filling mechanism for the full range of the adsorption isotherms. The newly designed Ni-TPM demonstrates exceptional R134a adsorption uptake compared to its parent microporous Ni-MOF-74 due to larger engineered pore size/volume. The application of this adsorption performance toward established chiller conditions yields a working capacity increase for Ni-TPM of about 400% from that of Ni-MOF-74, which combined with kinetics directly correlates to both a higher coefficient of performance and a higher average cooling capacity generated in a modeled chiller.

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 Cp 2 MCl L catalyst ( M = Ti, Zr; L = select organic ligands) requires the regeneration of n BuLi from Li(s), butene, and H 2 . 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.

The low-frequency (100-1250 Hz) acoustic properties of metal-organic framework (MOF) materials were examined in impedance tube experiments. The anomalously high sound transmission loss of HKUST-1, FeBTC, and MIL-53(Al) quantitatively demonstrated that these prototypical MOFs are absorptive acoustic metamaterials. To the best of our knowledge, this is the first example of MOFs that have been demonstrated to be acoustic metamaterials. Low-frequency acoustic dampening by subwavelength MOF metamaterials is likely due to sound dissipation and absorption facilitated by multiple internal reflections within the microporous framework structure. Modification of MIL-53(Al) with flexible organic linkers clarified that acoustic signatures of the MOFs may be tailored to add or alter certain diagnostic acoustic signatures. These results may be applied to the rational design of lightweight sound-insulating construction materials and acoustic contrast agents for subsurface mapping and monitoring applications at low frequency (100-1250 Hz).

Alkali-exchanged SSZ-13 adsorbents were investigated for their applicability in separating N 2 from CO 2 in flue gas streams using a dynamic breakthrough method. In contrast to IAST calculations based on equilibrium isotherms, K + exchanged SSZ-13 was found to yield the best N 2 productivity, comparable to Ni-MOF-74, under dynamic conditions where diffusion properties play a significant role. This was attributed to the selective, partial blockage of access to the chabazite cavities, enhancing the separation potential in a 15/85 CO 2 /N 2 binary gas mixture.

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 H 2 O 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., CO 2 ), uptake into the interlayer is not well-understood. Using novel high-pressure capabilities, we investigated the interaction of dry supercritical CO 2 with Na-, NH 4 -, and Cs-saturated montmorillonite, comparing results with predictions from molecular dynamics simulations. Despite the known trend in H 2 O and that cation solvation energies in CO 2 suggest a stronger interaction with Na, both the NH 4 - and Cs-clays readily absorbed CO 2 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 CO 2 intercalation in the Na-clay but little or no energy barrier for the NH 4 - and Cs-clays. Indeed, the combination of experiment and theory clearly demonstrate that CO 2 intercalation of Na-montmorillonite clays is prohibited in the absence of H 2 O. 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.

Computational studies on nanofluids composed of metal organic frameworks were performed using molecular modeling techniques. Grand Canonical Monte Carlo simulations were used to study the adsorption behavior of 1,1,1,3,3-pentafluoropropane (R-245fa) in a MIL-101 metal organic frameworks at various temperatures. To understand the stability of the nanofluid composed of MIL-101 particles, we performed molecular dynamics simulations to compute potentials of mean force between hypothetical MIL-101 fragments terminated with two different kinds of modulators in R-245fa and water. Our computed potentials of mean force results indicate that the metal organic frameworks particles tend to disperse better in water than in R-245fa. The reasons for this difference in dispersion were analyzed and are discussed in the paper. Our results agree with experimental results indicating that the potential models employed and modeling approaches provide good descriptions of molecular interactions and the reliabilities.

Continental flood basalts are attractive formations for geologic sequestration of carbon dioxide because of their reactive divalent-cation containing silicates, such as forsterite (Mg2SiO4), suitable for long-term trapping of CO2 mineralized as metal carbonates. The goal of this study was to investigate at a molecular level the carbonation products formed during the reaction of forsterite with supercritical CO2 (scCO2) as a function of the concentration of H2O adsorbed to the forsterite surface. Experiments were performed at 50 °C and 90 bar using an in situ IR titration capability, and postreaction samples were examined by ex situ techniques, including scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), focused ion beam transmission electron microscopy (FIB-TEM), thermal gravimetric analysis mass spectrometry (TGA-MS), and magic angle spinning nuclear magnetic resonance (MAS NMR). Carbonation products and reaction extents varied greatly with adsorbed H2O. We show for the first time evidence of Mg-carbonate surface complexation under wet scCO2 conditions. Carbonate is found to be coordinated to Mg at the forsterite surface in a predominately bidentate fashion at adsorbed H2O concentrations below 27 μmol/m(2). Above this concentration and up to 76 μmol/m(2), monodentate coordinated complexes become dominant. Beyond a threshold adsorbed H2O concentration of 76 μmol/m(2), crystalline carbonates continuously precipitate as magnesite, and the particles that form are hundreds of times larger than the estimated thicknesses of the adsorbed water films of about 7 to 15 Å. At an applied level, these results suggest that mineral carbonation in scCO2 dominated fluids near the wellbore and adjacent to caprocks will be insignificant and limited to surface complexation, unless adsorbed H2O concentrations are high enough to promote crystalline carbonate formation. At a fundamental level, the surface complexes and their dependence on adsorbed H2O concentration give insights regarding forsterite dissolution processes and magnesite nucleation and growth.

Enhancing the efficiency of the Rankine cycle, which is utilized for multiple renewable energy sources, requires the use of a working fluid with a high latent heat of vaporization. To further enhance its latent heat, a working fluid can be placed in a metal organic heat carrier (MOHC) with a high heat of adsorption. One such material is Ni\DOBDC, in which linear alkanes have a higher heat of adsorption than cyclic alkanes. We carried out molecular dynamics simulations to investigate the structural, diffusive, and adsorption properties of n-hexane and cyclohexane in Ni\DOBDC. The strong binding for both n-hexane and cyclohexane with Ni\DOBDC is attributed to the increase of the heat of adsorption observed in experiments. Our structural results indicate the organic linkers in Ni\DOBDC are the primary binding sites for both n-hexane and cyclohexane molecules. However, at all temperatures and loadings examined in present work, n-hexane clearly showed stronger binding with Ni\DOBDC than cyclohexane. This was found to be the result of the ability of n-hexane to reconfigure its structure to a greater degree than cyclohexane to gain more contacts between adsorbates and adsorbents. The geometry and flexibility of guest molecules were also related to their diffusivity in Ni\DOBDC, with higher diffusion for flexible molecules. Because of the large pore sizes in Ni\DOBDC, energetic effects were the dominant force for alkane adsorption and selectivity.