CO2 Guest Molecules Research Articles

Storage and diffusion of CO2 in covalent organic frameworks-A neural network-based molecular dynamics simulation approach.

As a consequence of the accelerated climate change, solutions to capture, store and potentially activate carbon dioxide received increased interest in recent years. Herein, it is demonstrated, that the neural network potential ANI-2x is able to describe nanoporous organic materials at approx. density functional theory accuracy and force field cost, using the example of the recently published two- and three-dimensional covalent organic frameworks HEX-COF1 and 3D-HNU5 and their interaction with CO2 guest molecules. Along with the investigation of the diffusion behaviour, a wide range of properties of interest is analyzed, such as the structure, pore size distribution and host-guest distribution functions. The workflow developed herein facilitates the estimation of the maximum CO2 adsorption capacity and is easily generalizable to other systems. Additionally, this work illustrates, that minimum distance distribution functions can be a highly useful tool in understanding the nature of interactions in host-gas systems at the atomic level.

MUF-16: A Robust Metal-Organic Framework for Pre- and Post-Combustion Carbon Dioxide Capture.

One of the most critical environmental issues of our age is the escalating release of CO2 into the atmosphere. Separation technologies with low energy footprints may be an effective way to capture CO2 and prevent its accumulation. Metal-organic frameworks (MOFs) can meet separation challenges due to their tailored structures and tunable pore surfaces. However, obstacles to their deployment can include the energy consumed by regeneration, a lack of long-term structural stability, and their production on large scales. Herein, we report on MUF-16 ([Co(Haip)2], H2aip = 5-aminoisophthalic acid), a hydrogen-bonded water-stable microporous material that combines high CO2 adsorption with a low affinity for other gases. MUF-16 is built up from inexpensive starting reagents in a scalable process. It can be easily regenerated at room temperature by purging with inert gas, and it maintains its performance over multiple adsorption/desorption cycles. MUF-16 features one-dimensional channels that trap CO2 guest molecules by a raft of attractive electrostatic interactions and size complementarity. It rejects H2 and N2 molecules around room temperature. This was verified by simulated and experimental breakthrough separation measurements on CO2/N2 and CO2/H2 mixtures. MUF-16 can be pelletized by coating with polymeric poly(vinylidene difluoride) (PVDF) to render it compatible with large-scale applications.

Using Molecular Simulations for Elucidation of Thermodynamic Nonidealities in Adsorption of CO2-Containing Mixtures in NaX Zeolite.

Cation-exchanged zeolites are of potential use in pressure swing adsorption (PSA) technologies for CO2 capture applications. Published experimental data for CO2/CH4, CO2/N2, and CO2/C3H8 mixture adsorption in NaX zeolite, also commonly referred to by its trade name 13X, have demonstrated that the ideal adsorbed solution theory (IAST) fails to provide adequately accurate estimates of mixture adsorption equilibrium. In particular, the IAST estimates of CO2/CH4 and CO2/N2 selectivities are significantly higher than those realized in experiments. For CO2/C3H8 mixtures, the IAST fails to anticipate the selectivity reversal phenomena observed in experiments. In this article, configurational-bias Monte Carlo (CBMC) simulations are employed to provide confirmation of the observed thermodynamic nonidealities in adsorption of CO2/CH4, CO2/N2, and CO2/C3H8 mixtures in NaX zeolite. The CBMC simulations provide valuable insights into the root cause of the failure of the IAST, whose applicability mandates a homogeneous distribution of adsorbates within the pore landscape. By sampling 105 equilibrated spatial locations of individual guest molecules within the cages of NaX zeolite, the radial distribution functions (RDFs) of each of the pairs of guest molecules are determined. Examination of the RDFs clearly reveals congregation effects, wherein the CO2 guests occupy positions in close proximity to the Na+ cations. The positioning of the partner molecules (CH4, N2, or C3H8) is further removed from the CO2 guest molecules; consequently, the competition in mixture adsorption faced by the partner molecules is less severe than that anticipated by the IAST. The important message to emerge from this article is the need for quantification of thermodynamic nonideality effects in mixture adsorption.

Insights into high pressure gas adsorption properties of ZIF-67: Experimental and theoretical studies

Cobalt 2-methylimidazolate framework (ZIF-67) of SOD topology has been synthesized by solvothermal method in a 2 L schott duran bottle using methanol or dimethylformamide (DMF) as solvents which yielded ~ 2.5 g of material. The effect of solvent in defining the morphology and surface properties has been studied for methanol and DMF. The Powder X-ray diffraction pattern has been refined by Rietveld method (space group = I-43 m, a = 17.0545(4) Å). Diffuse reflectance UV–Vis and X-ray photoelectron spectroscopic methods, probed the presence of Co(II). Cobalt is tetrahedrally coordinated to N atoms of 2-methylimidazole linker, making metal sites saturated, which do not play a major role in gas adsorption. Gas adsorption properties of ZIF-67 have been studied for H2 and CO2 guest molecules at high pressures. ZIF-67 has revealed H2 adsorption properties at 40 bar owing to storage capacity of 0.28 wt% at 300 K and 2.84 wt% at 77 K. At 300 K and 1 bar the material adsorbs 3.9 wt% of CO2 gas but at high pressure of 50 bar, ZIF-67 shows extraordinary CO2 adsorption capacity of 51.4 wt%, proving itself as potential candidate for CO2 capture. Theoretical calculations to obtain absolute adsorption isotherm curves adopting Unilan model reveals CO2 adsorption capacity of 49.2 wt% and heat of adsorption of −9.9 kJ/mol.

Secondary gaseous guest-dependent structures of binary neopentyl alcohol hydrates and their tuning behavior for potential application to CO2 capture

Clathrate hydrates, commonly called gas hydrates, are non-stoichiometric inclusion compounds with tunable gas storage and separation capabilities. Various efforts have been made to apply these hydrates to energy and environmental fields including energy storage and greenhouse gas capture and separation technologies. Although the unary use of gaseous guest molecules such as methane, carbon dioxide, and nitrogen is generally expected to achieve maximal storage capacity by filling the guest in both small and large cages of each structure, introducing additional liquid guest molecules into the clathrate hydrates could be a more feasible option from an engineering perspective, since such liquid guest molecules often play a critical role in stabilization of the clathrate hydrate by shifting the thermodynamic phase equilibria to practically implementable milder pressure and temperature conditions. Here, we focus on the binary neopentyl alcohol clathrate hydrates with gaseous guest molecules, including carbon dioxide, methane, and nitrogen, to provide a better understanding of the complex nature of the host–guest interactions occurring in the clathrate hydrates. Thermodynamic P – T phase equilibria were measured, and spectroscopic analyses were performed employing HRPD, Raman, and NMR spectrometers. The results revealed that the NPA guest molecule forms sI or sII hydrate depending on the binary gaseous guest molecules. This paper also reports for the first time that the composition of CO2 guest molecules may be ‘tuned’ within the cages of sI hydrate of NPA.

Structural Evolution of CO2-Filled Pure Silica LTA Zeolite under High-Pressure High-Temperature Conditions

The crystal structure of CO2 filled pure SiO2 LTA zeolite has been studied at high pressures and temperatures using synchrotron based x ray powder diffraction. Its structure consists of 13 CO2 guest molecules, 12 of them accommodated in the large alpha cages and 1 in the beta cages, giving a SiO2:CO2 stoichiometric ratio smaller than 2. The structure remains stable under pressure up to 20 GPa with a slight pressure dependent rhombohedral distortion, indicating that pressure induced amorphization is prevented by the insertion of guest species in this open framework. The ambient-temperature lattice compressibility has been determined. In situ high pressure resistive heating experiments up to 750 K allow us to estimate the thermal expansivity at 5 GPa. Our data confirm that the insertion of CO2 reverses the negative thermal expansion of the empty zeolite structure. No evidence of any chemical reaction was observed. The possibility of synthesizing a silicon carbonate at high temperatures and higher pressures is discussed in terms of the evolution of C-O and Si-O distances between molecular and framework atoms.

Lattice constants and expansivities of gas hydrates from 10 K up to the stability limit.

The lattice constants of hydrogenated and deuterated CH4-, CO2-, Xe- (clathrate structure type I) and N2-hydrates (clathrate structure type II) from 10 K up to the stability limit were established in neutron- and synchrotron diffraction experiments and were used to derive the related thermal expansivities. The following results emerge from this analysis: (1) The differences of expansivities of structure type I and II hydrates are fairly small. (2) Despite the larger guest-size of CO2 as compared to methane, CO2-hydrate has the smaller lattice constants at low temperatures, which is ascribed to the larger attractive guest-host interaction of the CO2-water system. (3) The expansivity of CO2-hydrate is larger than for CH4-hydrate which leads to larger lattice constants for the former at temperatures above ∼150 K; this is likely due to the higher motional degrees of freedom of the CO2 guest molecules. (4) The cage occupancies of Xe- and CO2-hydrates affect significantly the lattice constants. (5) Similar to ice Ih, the deuterated compounds have generally slightly larger lattice constants which can be ascribed to the somewhat weaker H-bonding. (6) Compared to ice Ih, the high temperature expansivities are about 50% larger; in contrast to ice Ih and the empty hydrate, there is no negative thermal expansion at low temperature. (7) A comparison of the experimental results with lattice dynamical work, with models based on an Einstein oscillator model, and results from inelastic neutron scattering suggest that the contribution of the guest atoms' vibrational energy to thermal expansion is important, most prominently for CO2- and Xe-hydrates.

Storage Capacity and Vibration Frequencies of Guest Molecules in CH4 and CO2 Hydrates by First-Principles Calculations

Using first-principle calculations at B97-D/6-311++G(2d,2p) level, we systematically explore the gas capacity of five standard water cavities (5(12), 4(3)5(6)6(3), 5(12)6(2), 5(12)6(4), and 5(12)6(8)) in clathrate hydrate and study the inclusion complexes to infer general trends in vibrational frequencies of guest molecules as a function of cage size and number of guest molecules. In addition, the Raman spectra of hydrates from CO2/CH4 gases are simulated. From our calculations, the maximum cage occupancy of the five considered cages (5(12), 4(3)5(6)6(3), 5(12)6(2), 5(12)6(4), and 5(12)6(8)) is one, one, two, three, and seven for both CH4 and CO2 guest molecules, respectively. Meanwhile, the optimum cage occupancy are one, one, one, two, and four for CO2 molecules and one, one, two, three, and five for CH4 molecules, respectively. Both the C-H stretching frequency of CH4 and the C-O stretching frequency of CO2 gradually decrease as size of the water cages increases. Meanwhile, the C-H stretching frequency gradually increases as the amount of CH4 molecules in the water cavity (e.g., 5(12)6(8)) increases.

Selective Guest Docking in Metal‐Organic Framework Materials

The 1990s saw a quest towards synthesizing modular porous materials from metal-containing linker groups and organic molecule struts. The idea, perhaps in analogy to the spools and sticks of the popular “Tinkertoy” sets, was to use molecular linkers with multiple chelating sites and molecular struts of different lengths to generate porous solid materials with custom-designed pore sizes and shapes. In 1998 Omar M. Yaghi, currently at UCLA, succeeded in developing a systematic route towards synthesizing metal–organic framework (MOF) materials. Since then, MOFs with many metal ion/metal oxide based linkers and a wide variety of bidentate (or multidentate) organic struts have been synthesized. The MOFs are structurally robust, withstanding relatively high temperatures and pressures. They have found many applications in gas-phase separations and gas storage, prominently in H2 storage and CO2 separation from flue gases. MOF-5 (or IRMOF-1) is the prototypical MOF phase with a cubic unit cell, Zn4O metal oxide linkers (with a formal +6 charge) bound to p-benzenedicarboyxlate organic struts in three perpendicular directions. Most MOFs show some degree of selectivity towards guest adsorption from the gas or solution phase. The simplest mechanism of guest separation in MOF materials is based on pore aperture size where large gas or solute molecules are excluded from the solid phase and smaller gas molecules diffuse through the pores to get to the other side of the MOF barrier, much like the action of a sieve. This type of separation is based on considerations of the van der Waals radii of the guests and MOF molecules lining the pore apertures. Geometric selectivity operates in the so-called “sorting domain” and is effective when the pore aperture diameter is close to the molecular diameters of the molecules in the mixture. MOFs with large pore apertures cannot separate components of a mixture based on geometric factors alone. However, many MOFs have organic struts with electron-rich functional groups such as aromatic rings. In these cases the MOFs can separate mixture components based on the different van der Waals interaction strengths between guests and the framework. Guests with greater numbers of electrons overall and stronger total van der Waals interactions, such as CO2, are selectively attracted to electron-rich parts of the framework over smaller guests like CO, N2, CH4, or H2. [8] A similar mechanism can apply to cases where the metal ion linker in the MOF has unsaturated (open) coordinate sites. The representative example of this class of MOF is HKUST-1 where the unsaturated Cu coordination site can act as a centre of H2 [9] or acetylene guest binding. The mechanism of guest adsorption and separation based on different guest–host interaction strengths is called the “coverage domain”. The selectivity in these cases is based on the electron density of the guest molecules and will have limited chemical specificity when gas-phase molecules of the same type are used. The coverage domain mechanism is operative at low and medium external guest pressures. An example of the preferential binding of CO2 molecules to specific electron-rich sites in the ZIF 68 structure at low pressure is shown in Figure 1. Achieving better chemical specificity for guest molecule binding requires custom-designed sites which geometrically and chemically bind to specific adsorbates but at the same time do not disrupt the MOF framework topology. Designing such a system is the important advance achieved by Li, Yahgi, Stoddart and coworkers, who have incorporated large macrocyclic polyether moieties into a cubic isoreticular MOF lattice. Li et al. use a modified form of the electron-rich bis-p-phenylene-34-crown10 (BPP34C10) molecule shown in Figure 2. A 1,5-naphtho-p-phenylene-36crown-10 analogue was also synthesized but is not further discussed. The electron-rich cyclic BPP34C10 forms a pseudorotaxane complex with the electrondeficient N,N’-dimethyl-4,4’-bipyridinium dication (paraquat, PQT , Figure 2) where the two pyridinium rings of PQT are aligned parallel to the two aromatic rings in BPP34C10 by p–p stacking interactions. Hydrogen bonding by the macrocycle ether oxygen atoms with the a-CH atoms of paraquat and electrostatic interactions between the hydroquinone ring oxygen atoms and the paraFigure 1. The selective binding of CO2 guest molecules to specific sites in ZIF-68 pores.