Li-Chiang Lin

In silico screening of carbon-capture materials

One of the main bottlenecks to deploying large-scale carbon dioxide capture and storage (CCS) in power plants is the energy required to separate the CO(2) from flue gas. For example, near-term CCS technology applied to coal-fired power plants is projected to reduce the net output of the plant by some 30% and to increase the cost of electricity by 60-80%. Developing capture materials and processes that reduce the parasitic energy imposed by CCS is therefore an important area of research. We have developed a computational approach to rank adsorbents for their performance in CCS. Using this analysis, we have screened hundreds of thousands of zeolite and zeolitic imidazolate framework structures and identified many different structures that have the potential to reduce the parasitic energy of CCS by 30-40% compared with near-term technologies.

Understanding CO2 Dynamics in Metal–Organic Frameworks with Open Metal Sites

Hopping along: Metal-organic frameworks such as Mg-MOF-74 possess open metal sites that interact strongly with CO2. Molecular simulations reveal detailed CO2 dynamics (hops between metal sites and localized fluctuations), which can be used to accurately explain the experimentally measured 13C NMR chemical shift anisotropy pattern. Copyright

Multilayer Nanoporous Graphene Membranes for Water Desalination

While single-layer nanoporous graphene (NPG) has shown promise as a reverse osmosis (RO) desalination membrane, multilayer graphene membranes can be synthesized more economically than the single-layer material. In this work, we build upon the knowledge gained to date toward single-layer graphene to explore how multilayer NPG might serve as a RO membrane in water desalination using classical molecular dynamic simulations. We show that, while multilayer NPG exhibits similarly promising desalination properties to single-layer membranes, their separation performance can be designed by manipulating various configurational variables in the multilayer case. This work establishes an atomic-level understanding of the effects of additional NPG layers, layer separation, and pore alignment on desalination performance, providing useful guidelines for the design of multilayer NPG membranes.

Reversible CO Binding Enables Tunable CO/H2 and CO/N2 Separations in Metal-Organic Frameworks with Exposed Divalent Metal Cations

Six metal-organic frameworks of the M2(dobdc) (M = Mg, Mn, Fe, Co, Ni, Zn; dobdc(4-) = 2,5-dioxido-1,4-benzenedicarboxylate) structure type are demonstrated to bind carbon monoxide reversibly and at high capacity. Infrared spectra indicate that, upon coordination of CO to the divalent metal cations lining the pores within these frameworks, the C-O stretching frequency is blue-shifted, consistent with nonclassical metal-CO interactions. Structure determinations reveal M-CO distances ranging from 2.09(2) Å for M = Ni to 2.49(1) Å for M = Zn and M-C-O angles ranging from 161.2(7)° for M = Mg to 176.9(6)° for M = Fe. Electronic structure calculations employing density functional theory (DFT) resulted in good agreement with the trends apparent in the infrared spectra and crystal structures. These results represent the first crystallographically characterized magnesium and zinc carbonyl compounds and the first high-spin manganese(II), iron(II), cobalt(II), and nickel(II) carbonyl species. Adsorption isotherms indicate reversible adsorption, with capacities for the Fe, Co, and Ni frameworks approaching one CO per metal cation site at 1 bar, corresponding to loadings as high as 6.0 mmol/g and 157 cm(3)/cm(3). The six frameworks display (negative) isosteric heats of CO adsorption ranging from 52.7 to 27.2 kJ/mol along the series Ni > Co > Fe > Mg > Mn > Zn, following the Irving-Williams stability order. The reversible CO binding suggests that these frameworks may be of utility for the separation of CO from various industrial gas mixtures, including CO/H2 and CO/N2. Selectivities determined from gas adsorption isotherm data using ideal adsorbed solution theory (IAST) over a range of gas compositions at 1 bar and 298 K indicate that all six M2(dobdc) frameworks could potentially be used as solid adsorbents to replace current cryogenic distillation technologies, with the choice of M dictating adsorbent regeneration energy and the level of purity of the resulting gases.

Predicting Large CO2 Adsorption in Aluminosilicate Zeolites for Postcombustion Carbon Dioxide Capture

Large-scale simulations of aluminosilicate zeolites were conducted to identify structures that possess large CO(2) uptake for postcombustion carbon dioxide capture. In this study, we discovered that the aluminosilicate zeolite structures with the highest CO(2) uptake values have an idealized silica lattice with a large free volume and a framework topology that maximizes the regions with nearest-neighbor framework atom distances from 3 to 4.5 Å. These predictors extend well to different Si:Al ratios and for both Na(+) and Ca(2+) cations, demonstrating their universal applicability in identifying the best-performing aluminosilicate zeolite structures.

Evaluating different classes of porous materials for carbon capture

Carbon Capture and Sequestration (CCS) is one of the promising ways to significantly reduce the CO2 emission from power plants. In particular, amongst several separation strategies, adsorption by nano-porous materials is regarded as a potential means to efficiently capture CO2 at the place of its origin in a post-combustion process. The search for promising materials in such a process not only requires the screening of a multitude of materials but also the development of an adequate evaluation metric. Several evaluation criteria have been introduced in the literature concentrating on a single adsorption or material property at a time. Parasitic energy is a new approach for material evaluation to address the energy load imposed on a power plant while applying CCS. In this work, we evaluate over 60 different materials with respect to their parasitic energy, including experimentally realized and hypothetical materials such as metal–organic frameworks (MOFs), zeolitic imidazolate frameworks (ZIFs), porous polymer networks (PPNs), and zeolites. The results are compared to other proposed evaluation criteria and performance differences are studied regarding the regeneration modes, (i.e. Pressure-Swing (PSA) and Temperature-Swing Adsorption (TSA)) as well as the flue gas composition.

Force-Field Development from Electronic Structure Calculations with Periodic Boundary Conditions: Applications to Gaseous Adsorption and Transport in Metal-Organic Frameworks.

We present a systematic and efficient methodology to derive accurate (nonpolarizable) force fields from periodic density functional theory (DFT) calculations for use in classical molecular simulations. The methodology requires reduced computation cost compared with other conventional ways. Moreover, the whole process is performed self-consistently in a fully periodic system. The force fields derived by using this methodology nicely predict the CO2 and H2O adsorption isotherms inside Mg-MOF-74, and is transferable to Zn-MOF-74; by replacing the Mg-CO2 interactions with the corresponding Zn-CO2 interactions, we obtain an accurate prediction of the corresponding isotherm. We have applied this methodology to address the effect of water on the separation of flue gases in these materials. In general, the mixture isotherms of CO2 and H2O calculated with these derived force fields show a significant reduction in CO2 uptake with the existence of trace amounts of water vapor. The effect of water, however, is found to be quantitatively different between Mg- and Zn-MOF-74.

Atomistic understandings of reduced graphene oxide as an ultrathin-film nanoporous membrane for separations

The intrinsic defects in reduced graphene oxide (rGO) formed during reduction processes can act as nanopores, making rGO a promising ultrathin-film membrane candidate for separations. To assess the potential of rGO for such applications, molecular dynamics techniques are employed to understand the defect formation in rGO and their separation performance in water desalination and natural gas purification. We establish the relationship between rGO synthesis parameters and defect sizes, resulting in a potential means to control the size of nanopores in rGO. Furthermore, our results show that rGO membranes obtained under properly chosen synthesis conditions can achieve effective separations and provide significantly higher permeate fluxes than currently available membranes.

Large-scale Screening of Zeolite Structures for CO2 Membrane Separations

We have conducted large-scale screening of zeolite materials for CO2/CH4 and CO2/N2 membrane separation applications using the free energy landscape of the guest molecules inside these porous materials. We show how advanced molecular simulations can be integrated with the design of a simple separation process to arrive at a metric to rank performance of over 87,000 different zeolite structures, including the known IZA zeolite structures. Our novel, efficient algorithm using graphics processing units can accurately characterize both the adsorption and diffusion properties of a given structure in just a few seconds and accordingly find a set of optimal structures for different desired purity of separated gases from a large database of porous materials in reasonable wall time. Our analysis reveals that the optimal structures for separations usually consist of channels with adsorption sites spread relatively uniformly across the entire channel such that they feature well-balanced CO2 adsorption and diffusion properties. Our screening also shows that the top structures in the predicted zeolite database outperform the best known zeolite by a factor of 4-7. Finally, we have identified a completely different optimal set of zeolite structures that are suitable for an inverse process, in which the CO2 is retained while CH4 or N2 is passed through a membrane.

New materials for methane capture from dilute and medium-concentration sources

Methane (CH4) is an important greenhouse gas, second only to CO2, and is emitted into the atmosphere at different concentrations from a variety of sources. However, unlike CO2, which has a quadrupole moment and can be captured both physically and chemically in a variety of solvents and porous solids, methane is completely non-polar and interacts very weakly with most materials. Thus, methane capture poses a challenge that can only be addressed through extensive material screening and ingenious molecular-level designs. Here we report systematic in silico studies on the methane capture effectiveness of two different materials systems, that is, liquid solvents (including ionic liquids) and nanoporous zeolites. Although none of the liquid solvents appears effective as methane sorbents, systematic screening of over 87,000 zeolite structures led to the discovery of a handful of candidates that have sufficient methane sorption capacity as well as appropriate CH4/CO2 and/or CH4/N2 selectivity to be technologically promising.