Saman Alavi

Direct Observation and Quantification of CO2 Binding Within an Amine-Functionalized Nanoporous Solid

Designing Carbon Dioxide Traps One widely discussed means of stemming the rise in atmospheric carbon dioxide concentration is to capture the gas prior to its emission and then bury it. The materials currently known to best adsorb CO2 for this purpose tend to involve amine groups; however, the precise molecular details of adsorption often remain murky, and rational improvement of sorbent properties by structural modification has been challenging. Vaidhyanathan et al. (p. 650; see the Perspective by Lastoskie) have crystallographically resolved the binding motifs of CO2 in an amine-bearing metal-organic framework solid. Accompanying theoretical simulations matched the experimental observations. Crystallographic resolution of bound carbon dioxide in a porous solid validates methods of theoretically predicting binding behavior. Understanding the molecular details of CO2-sorbent interactions is critical for the design of better carbon-capture systems. Here we report crystallographic resolution of CO2 molecules and their binding domains in a metal-organic framework functionalized with amine groups. Accompanying computational studies that modeled the gas sorption isotherms, high heat of adsorption, and CO2 lattice positions showed high agreement on all three fronts. The modeling apportioned specific binding interactions for each CO2 molecule, including substantial cooperative binding effects among the guest molecules. The validation of the capacity of such simulations to accurately model molecular-scale binding bodes well for the theory-aided development of amine-based CO2 sorbents. The analysis shows that the combination of appropriate pore size, strongly interacting amine functional groups, and the cooperative binding of CO2 guest molecules is responsible for the low-pressure binding and large uptake of CO2 in this sorbent material.

Molecular-dynamics study of structure II hydrogen clathrates

Molecular-dynamics simulations are used to study the stability of structure II hydrogen clathrates with different H2 guest occupancies. Simulations are done at pressures of 2.5 kbars and 1.013 bars and for temperatures ranging from 100 to 250 K. For a structure II unit cell with 136 water molecules, H2 guest molecule occupancies of 0-64 are studied with uniform occupancies among each type of cage. The simulations show that at 100 K and 2.5 kbars, the most stable configurations have single occupancy in the small cages and quadruple occupancy in the large cages. The optimum occupancy for the large cages decreases as the temperature is raised. Double occupancy in the small cages increases the energy of the structures and causes tetragonal distortion in the unit cell. The spatial distribution of the hydrogen guest molecules in the cages is determined by studying the guest-water and guest-guest radial distribution functions at various temperatures.

Competition and Cooperativity in Carbon Dioxide Sorption by Amine-Functionalized Metal–Organic Frameworks†

Alkylamines, such as monoethanolamine, are used to scrub CO2 molecules from flue gas streams, however, as they form strong chemical bonds (85–105 kJmol ), the post-capture recovery of the amine is energy-intensive (130–150 8C including heating the entire aqueous solution). Alternatively, the use of less-basic amines, such as aryl amines, could favor strong physisorption (30–50 kJmol ) with CO2, rather than chemisorption. This would mean a porous compound with such amine groups could give easy-on/easy-off reversible CO2 capture balanced with selectivity. To obtain high efficiency at lower partial pressures, the material, along with having strong CO2 binding sites, needs to have reasonable surface area for capacity. Metal–organic frameworks (MOFs) are widely studied for gas sorption owing to the ability to modify pore sizes, shapes, and surfaces. Functionalizing with specific interaction sites is being actively studied as a route to selective gas capture. Computational modeling can give tremendous insight to the sorption properties of a MOF. We recently reported a zinc aminotriazolato oxalate MOF, {Zn2(Atz)2(ox)} (2), exhibiting amine-lined pores and a high heat of adsorption for CO2 (ca. 40 kJmol ). Further studies showed that the CO2 binding sites could be located crystallographically. These data offered an exceptional opportunity to validate a suite of computational methods to model not only the CO2 isotherm, but also the locations of binding sites and role of specific interactions to the overall CO2 binding enthalpy. The present study applies these methods to understanding CO2 uptake in another MOF, {Zn3(Atz)3(PO4)} (1), that intuitively should give better CO2 capture properties. In comparison to {Zn2(Atz)2(ox)}, only two-thirds of the number of trianionic phosphate groups are required to charge compensate [Zn(Atz)] layers, so larger, amine-lined pores were anticipated and observed. Despite this, the CO2 uptake (at 273 K) and heat of adsorption do not exceed those of 2. The computational methods provide crucial insight to understanding these phenomena and demonstrate the wide spread applicability of such techniques to ascertain binding details in MOFs not directly accessible by experiment. Although the role of the amine functionalities in 1 is surprisingly diminished, the cooperative interactions between CO2 molecules are found to augment overall binding by over 7 kJmol , a significant result for CO2 capture in any porous material. Solvothermal reaction of basic ZnCO3 with 3-amino-1,2,4triazole, H3PO4, and NH4OH gave {Zn3Atz3(PO4)(H2O)3.5}, 1·(H2O)3.5, in both single-crystal and bulk phases (Supporting Information, Figure S1). The aminotriazole ligand has been employed to construct otherMOFs, including with Zn ions, but has not been extensively studied for CO2 capture excepting 2. 1·(H2O)3.5 is made up of cationic Zn–Atz layers pillared by PO4 anions to form a 3D porous network (Figure 1). The Zn(Atz) layers lie in the ac plane and contain three independent Zn ions and Atz ligands. No amine groups coordinate to Zn ions; ligation is exclusively through triazole nitrogen atoms. Pillaring of these layers by the phosphate ions results in a 3D network of pores (accounting for van der

Molecular dynamics simulation of imidazolium-based ionic liquids. I. Dynamics and diffusion coefficient

Molecular dynamics simulations are used to study the dynamics and transport properties of 12 room-temperature ionic liquids of the 1-alkyl-3-methylimidazolium [amim](+) (alkyl = methyl, ethyl, propyl, and butyl) family with PF(6)(-), NO(3)(-), and Cl(-) counterions. The explicit atom transferable force field of Canongia Lopes et al. [J. Phys. Chem. B 108, 2038 (2004)] is used in the simulations. In this first part, the dynamics of the ionic liquids are characterized by studying the mean-square displacement (MSD) and the velocity autocorrelation function (VACF) for the centers of mass of the ions at 400 K. Trajectory averaging was employed to evaluate the diffusion coefficients at two temperatures from the linear slope of MSD(t) functions in the range of 150-300 ps and from the integration of the VACF(t) functions at 400 K. Detailed comparisons are made between the diffusion results from the MSD and VACF methods. The diffusion coefficients from the integration of the VACFs are closer to experimental values than the diffusion coefficients calculated from the slope of MSDs. Both methods can show good agreement with experiment in predicting relative trends in the diffusion coefficients and determining the role of the cation and anion structures on the dynamical behavior of this family of ionic liquids. The MSD and self-diffusion of relatively heavier imidazolium cations are larger than those of the lighter anions from the Einstein results, except for the case of [bmim][Cl]. The cationic transference number generally decreases with temperature, in good agreement with experiments. For the same anion, the cationic transference numbers decrease with increasing length of the alkyl chain, and for the same cation, the trends in the cationic transference numbers are [NO(3)](-) < [Cl](-) < [PF(6)](-). The trends in the diffusion coefficient in the series of cations with identical anions are [emim](+) > [pmim](+) > [bmim](+) and those for anions with identical cations are [NO(3)](-) > [PF(6)](-) > [Cl](-). The [dmim](+) has a relatively low diffusion coefficient due to its symmetric structure and good packing in the liquid phase. The major factor for determining the magnitude of the self-diffusion is the geometric shape of the anion of the ionic liquid. Other important factors are the ion size and the charge delocalization in the anion.

Effect of guest-host hydrogen bonding on the structures and properties of clathrate hydrates.

To provide improved understanding of guest-host interactions in clathrate hydrates, we present some correlations between guest chemical structures and observations on the corresponding hydrate properties. From these correlations it is clear that directional interactions such as hydrogen bonding between guest and host are likely, although these have been ignored to greater or lesser degrees because there has been no direct structural evidence for such interactions. For the first time, single-crystal X-ray crystallography has been used to detect guest-host hydrogen bonding in structure II (sII) and structure H (sH) clathrate hydrates. The clathrates studied are the tert-butylamine (tBA) sII clathrate with H(2)S/Xe help gases and the pinacolone + H(2)S binary sH clathrate. X-ray structural analysis shows that the tBA nitrogen atom lies at a distance of 2.64 A from the closest clathrate hydrate water oxygen atom, whereas the pinacolone oxygen atom is determined to lie at a distance of 2.96 A from the closest water oxygen atom. These distances are compatible with guest-water hydrogen bonding. Results of molecular dynamics simulations on these systems are consistent with the X-ray crystallographic observations. The tBA guest shows long-lived guest-host hydrogen bonding with the nitrogen atom tethered to a water HO group that rotates towards the cage center to face the guest nitrogen atom. Pinacolone forms thermally activated guest-host hydrogen bonds with the lattice water molecules; these have been studied for temperatures in the range of 100-250 K. Guest-host hydrogen bonding leads to the formation of Bjerrum L-defects in the clathrate water lattice between two adjacent water molecules, and these are implicated in the stabilities of the hydrate lattices, the water dynamics, and the dielectric properties. The reported stable hydrogen-bonded guest-host structures also tend to blur the longstanding distinction between true clathrates and semiclathrates.

Molecular-dynamics simulations of binary structure II hydrogen and tetrahydrofurane clathrates

The binary structure II hydrogen and tetrahydrofurane (THF) clathrates are studied with molecular-dynamics simulations. Simulations are done at pressures of 120 and 1.013 bars for temperatures ranging from 100 to 273 K. For the small cages of the structure II unit cell, H2 guest molecule occupancies of 0, 16 (single occupancy), and 32 (double occupancy) are considered. THF occupancies of 0-8 in the large cages are studied. For cases in which THF does not occupy all large cages in a unit cell, the remaining large cages can be occupied with sets of four H2 guest molecules. The unit-cell volumes and configurational energies are compared in the different occupancy cases. Increasing the small cage occupancy leads to an increase in the unit-cell volume and thermal-expansion coefficient. Among simulations with the same small cage occupancy, those with the large cages containing 4H2 guests have the largest volumes. The THF guest molecules have a stabilizing effect on the clathrate and the configurational energy of the unit cell decreases linearly as the THF content increases. For binary THF + H2 clathrates, the substitution of the THF molecules in the large cages with sets of 4H2 molecules increases the configurational energy. For the binary clathrates, various combinations of THF and H2 occupancies have similar configurational energies.

Hydrogen-bonding alcohol-water interactions in binary ethanol, 1-propanol, and 2-propanol+methane structure II clathrate hydrates

The small alcohols ethanol, 1-propanol, and 2-propanol are miscible in water, form strong hydrogen bonds with water molecules, and are usually known as inhibitors for clathrate hydrate formation. However, in the presence of methane or other help gases, clathrate hydrates of these substances have been synthesized. In this work, molecular dynamics simulations are used to characterize guest-host hydrogen bonding, microscopic structures, and guest dynamics of binary structure II clathrate hydrates of methane (small cages) with ethanol, 1-propanol, and 2-propanol in the temperature range of 100-250 K to gain insight into the stability of these materials. We observe that these alcohols form structures with dynamic long-lived ( approximately 10 ps) guest-host hydrogen bonds in the hydrate phases while maintaining the general cage structure of the sII clathrate hydrate form. The hydroxyl groups of ethanol, 1-propanol, and 2-propanol act as both proton acceptors and proton donors and there is a considerable probability of simultaneous hydrogen bonding between O and H hydroxyl atoms with different cage water molecules. The presence of the nonpolar methane molecule and the hydrophobic moieties of the alcohols stabilize the hydrate phase, despite the strong and prevalent alcohol-water hydrogen bonding. The effect of the alcohol molecules on the structural properties of the hydrate and the effect of guest-host hydrogen bonding on the guest dynamics are studied.

Molecular dynamics studies of melting and some liquid-state properties of 1-ethyl-3-methylimidazolium hexafluorophosphate [emim][PF6].

Molecular dynamics simulations are used to study the liquid-state properties and melting of 1-ethyl-3-methylimidazolium hexafluorosphosphate [emim][PF6] using the force field of Canongia Lopes et al. [J. Phys. Chem. B 108, 2038 (2004)] and geometric constants from crystallographic data. The structures of the solid and liquid states are characterized by carbon-carbon, carbon-phosphorous, and phosphorous-phosphorous radial distribution functions. Spatial correlations among the ions are strong in the liquid state. The cohesive energy density and the temperature dependences of the molar volume and density of the liquid have been computed. The melting point is determined by equilibrating the solid-state supercells in which void defects have been introduced to eliminate the free-energy barrier for the formation of a solid-liquid interface. The computed melting point is 375+/-10 K, which is approximately 10% higher than the experimental value of 333 K.

Methanol incorporation in clathrate hydrates and the implications for oil and gas pipeline flow assurance and icy planetary bodies

One of the best-known uses of methanol is as antifreeze. Methanol is used in large quantities in industrial applications to prevent methane clathrate hydrate blockages from forming in oil and gas pipelines. Methanol is also assigned a major role as antifreeze in giving icy planetary bodies (e.g., Titan) a liquid subsurface ocean and/or an atmosphere containing significant quantities of methane. In this work, we reveal a previously unverified role for methanol as a guest in clathrate hydrate cages. X-ray diffraction (XRD) and NMR experiments showed that at temperatures near 273 K, methanol is incorporated in the hydrate lattice along with other guest molecules. The amount of included methanol depends on the preparative method used. For instance, single-crystal XRD shows that at low temperatures, the methanol molecules are hydrogen-bonded in 4.4% of the small cages of tetrahydrofuran cubic structure II hydrate. At higher temperatures, NMR spectroscopy reveals a number of methanol species incorporated in hydrocarbon hydrate lattices. At temperatures characteristic of icy planetary bodies, vapor deposits of methanol, water, and methane or xenon show that the presence of methanol accelerates hydrate formation on annealing and that there is unusually complex phase behavior as revealed by powder XRD and NMR spectroscopy. The presence of cubic structure I hydrate was confirmed and a unique hydrate phase was postulated to account for the data. Molecular dynamics calculations confirmed the possibility of methanol incorporation into the hydrate lattice and show that methanol can favorably replace a number of methane guests.

Molecular Modeling of the Dissociation of Methane Hydrate in Contact with a Silica Surface

We use constant energy, constant volume (NVE) molecular dynamics simulations to study the dissociation of the fully occupied structure I methane hydrate in a confined geometry between two hydroxylated silica surfaces between 36 and 41 Å apart, at initial temperatures of 283, 293, and 303 K. Simulations of the two-phase hydrate/water system are performed in the presence of silica, with and without a 3 Å thick buffering water layer between the hydrate phase and silica surfaces. Faster decomposition is observed in the presence of silica, where the hydrate phase is prone to decomposition from four surfaces, as compared to only two sides in the case of the hydrate/water simulations. The existence of the water layer between the hydrate phase and the silica surface stabilizes the hydrate phase relative to the case where the hydrate is in direct contact with silica. Hydrates bound between the silica surfaces dissociate layer-by-layer in a shrinking core manner with a curved decomposition front which extends over a 5-8 Å thickness. Labeling water molecules shows that there is exchange of water molecules between the surrounding liquid and intact cages in the methane hydrate phase. In all cases, decomposition of the methane hydrate phase led to the formation of methane nanobubbles in the liquid water phase.