Tom K. Woo

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.

Atomic and electronic structure of unreduced and reduced CeO2 surfaces: A first-principles study

The atomic and electronic structure of (111), (110), and (100) surfaces of ceria (CeO2) were studied using density-functional theory within the generalized gradient approximation. Both stoichiometric surfaces and surfaces with oxygen vacancies (unreduced and reduced surfaces, respectively) have been examined. It is found that the (111) surface is the most stable among the considered surfaces, followed by (110) and (100) surfaces, in agreement with experimental observations and previous theoretical results. Different features of relaxation are found for the three surfaces. While the (111) surface undergoes very small relaxation, considerably larger relaxations are found for the (110) and (100) surfaces. The formation of an oxygen vacancy is closely related to the surface structure and occurs more easily for the (110) surface than for (111). The preferred vacancy location is in the surface layer for CeO2(110) and in the subsurface layer (the second O-atomic layer) for CeO2(111). For both surfaces, the O vacancy forms more readily than in the bulk. An interesting oscillatory behavior is found for the vacancy formation energy in the upper three layers of CeO2(111). Analysis of the reduced surfaces suggests that the additional charge resulting from the formation of the oxygen vacancies is localized in the first three layers of the surface. Furthermore, they are not only trapped in the 4f states of cerium.

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

Mechanistic Insights into the Rhodium-Catalyzed Intramolecular Ketone Hydroacylation

[Rh((R)-DTBM-SEGPHOS)]BF(4) catalyzes the intramolecular hydroacylation of ketones to afford seven-membered lactones in large enantiomeric excess. Herein, we present a combined experimental and theoretical study to elucidate the mechanism and origin of selectivity in this C-H bond activation process. Evidence is presented for a mechanistic pathway involving three key steps: (1) rhodium(I) oxidative addition into the aldehyde C-H bond, (2) insertion of the ketone CO double bond into the rhodium hydride, and (3) C-O bond-forming reductive elimination. Kinetic isotope effects and Hammett plot studies support that ketone insertion is the turnover-limiting step. Detailed kinetic experiments were performed using both 1,3-bis(diphenylphosphino)propane (dppp) and (R)-DTBM-SEGPHOS as ligands. With dppp, the keto-aldehyde substrate assists in dissociating a dimeric precatalyst 8 and binds an active monomeric catalyst 9. With [Rh((R)-DTBM-SEGPHOS)]BF(4), there is no induction period and both substrate and product inhibition are observed. In addition, competitive decarbonylation produces a catalytically inactive rhodium carbonyl species that accumulates over the course of the reaction. Both mechanisms were modeled with a kinetics simulation program, and the models were consistent with the experimental data. Density functional theory calculations were performed to understand more elusive details of this transformation. These simulations support that the ketone insertion step has the highest energy transition state and reveal an unexpected interaction between the carbonyl-oxygen lone pair and a Rh d-orbital in this transition state structure. Finally, a model based on the calculated transition-state geometry is proposed to rationalize the absolute sense of enantioinduction observed using (R)-DTBM-SEGPHOS as the chiral ligand.

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.

Phosphonate Monoesters as Carboxylate-like Linkers for Metal Organic Frameworks

Bidentate phosphonate monoesters are analogues of popular dicarboxylate linkers in MOFs, but with an alkoxy tether close to the coordinating site. Herein, we report 3-D MOF materials based upon phosphonate monoester linkers. Cu(1,4-benzenediphosphonate bis(monoalkyl ester), CuBDPR, with an ethyl tether is nonporous; however, the methyl tether generates an isomorphous framework that is porous and captures CO(2) with a high isosteric heat of adsorption of 45 kJ mol(-1). Computational modeling reveals that the CO(2) uptake is extremely sensitive both to the flexing of the structure and to the orientation of the alkyl tether.

Rapid and Accurate Machine Learning Recognition of High Performing Metal Organic Frameworks for CO2 Capture.

In this work, we have developed quantitative structure-property relationship (QSPR) models using advanced machine learning algorithms that can rapidly and accurately recognize high-performing metal organic framework (MOF) materials for CO2 capture. More specifically, QSPR classifiers have been developed that can, in a fraction of a section, identify candidate MOFs with enhanced CO2 adsorption capacity (>1 mmol/g at 0.15 bar and >4 mmol/g at 1 bar). The models were tested on a large set of 292 050 MOFs that were not part of the training set. The QSPR classifier could recover 945 of the top 1000 MOFs in the test set while flagging only 10% of the whole library for compute intensive screening. Thus, using the machine learning classifiers as part of a high-throughput screening protocol would result in an order of magnitude reduction in compute time and allow intractably large structure libraries and search spaces to be screened.

Free energies of carbon dioxide sequestration and methane recovery in clathrate hydrates.

Classical molecular dynamics simulations are used to compare the stability of methane, carbon dioxide, nitrogen, and mixed CO(2)N(2) structure I (sI) clathrates under deep ocean seafloor temperature and pressure conditions (275 K and 30 MPa) which were considered suitable for CO(2) sequestration. Substitution of methane guests in both the small and large sI cages by CO(2) and N(2) fluids are considered separately to determine the separate contributions to the overall free energy of substitution. The structure I clathrate with methane in small cages and carbon dioxide in large cages is determined to be the most stable. Substitutions of methane in the small cages with CO(2) and N(2) have positive free energies. Substitution of methane with CO(2) in the large cages has a large negative free energy and substitution of the methane in the large cages with N(2) has a small positive free energy. The calculations show that under conditions where storage is being considered, carbon dioxide spontaneously replaces methane from sI clathrates, causing the release of methane. This process must be considered if there are methane clathrates present where CO(2) sequestration is to be attempted. The calculations also indicate that N(2) does not directly compete with CO(2) during methane substitution or clathrate formation and therefore can be used as a carrier gas or may be present as an impurity. Simulations further reveal that the replacement of methane with CO(2) in structure II (sII) cages also has a negative free energy. In cases where sII CO(2) clathrates are formed, only single occupancy of the large cages will be observed.

Towards more realistic computational modeling of homogenous catalysis by density functional theory: combined QM/MM and ab initio molecular dynamics

Abstract The combined quantum mechanics/molecular mechanics (QM/MM) and the ab initio molecular dynamics methods (AIMD) are fast emerging as viable computational molecular modeling tools. Both methods allow for the incorporation of effects that are often ignored in high level calculations, but may be critical to the real chemistry of the simulated system. In the combined QM/MM method part of the system, say the active site, is treated quantum mechanically whereas the remainder of the system is treated with a faster molecular mechanics force field. This allows high level calculations to be performed where the effects of the environment are incorporated in a computationally tractable manner. With the ab initio molecular dynamics methods, the system is simulated at a finite temperature with no empirical force field. Rather, the forces at each time step are determined with a full electronic structure calculation at the density functional level. Thus, simulations of chemical reactions can be performed where finite temperature effects are realistically represented. In this paper a brief introduction to both methods is given. The methods are further demonstrated with specific applications to modeling homogenous catalytic processes at the molecular level. These applications are our latest efforts to build more realistic computational models of catalytic systems at the density functional level.

A density functional study on the insertion mechanism and chain termination in Kaminsky-type catalysts; comparison of frontside and backside attack☆

Abstract Non-local density functional (DF) calculations have been carried out on the reaction of ethylene with Cp 2 Zr + , which serves as a model for the resting state between two insertions. The β-agostic Cp 2 Zr + Et is 47.0 kJ mol −1 more stable than the α-agostic conformer. Frontside insertion of the olefin can take place after rotation around the ZrC α -bond forming the α-agostic Cp 2 Zr + Et. An α-agostic π-complex is formed with a complexation energy of 81.1 kJ mol −1 and the frontside transition state has an activation energy of 2 kJ mol −1 relative to the π-complex. The reaction is exothermic by 118.9 kJ mol −1 . Without rotation around the ZrC α bond a β-agostic π-complex is formed and H-transfer from the polymer chain end to the olefin takes place. This reaction leads to chain termination with an activation barrier of 29.8 kJ mol −1 . An alternative path for the olefin insertion starts with a backside attack of the olefin. The activation barrier for the backside insertion is 28.9 kJ mol −1 and the reaction is exothermic by 24.9 kJ mol −1 relative to the π-complex. Backside insertion does not involve inversion at the metal centre. The formation of syndiotactic polypropene in the case of the backside insertion can only be explained with chain-end control. Comparison of three chain termination processes (β-hydride elimination, C-H activation and H-exchange) indicates that H-exchange is the most probable reaction. β-Elimination is strongly endothermic and frontside C-H-activation makes a rotation around ZrC α necessary.