Berend Smit

Carbon Dioxide Capture: Prospects for New Materials

The escalating level of atmospheric carbon dioxide is one of the most pressing environmental concerns of our age. Carbon capture and storage (CCS) from large point sources such as power plants is one option for reducing anthropogenic CO(2) emissions; however, currently the capture alone will increase the energy requirements of a plant by 25-40%. This Review highlights the challenges for capture technologies which have the greatest likelihood of reducing CO(2) emissions to the atmosphere, namely postcombustion (predominantly CO(2)/N(2) separation), precombustion (CO(2)/H(2)) capture, and natural gas sweetening (CO(2)/CH(4)). The key factor which underlies significant advancements lies in improved materials that perform the separations. In this regard, the most recent developments and emerging concepts in CO(2) separations by solvent absorption, chemical and physical adsorption, and membranes, amongst others, will be discussed, with particular attention on progress in the burgeoning field of metal-organic frameworks.

Molecular Dynamics Simulations

Molecular Dynamics (MD) simulations are in many respects very similar to real experiments. In MD, first, sample is prepared, a model system consisting of N particles is selected, and then Newtons equations of motion are solved for the system until the properties of the system no longer change with time. To measure an observable quantity in a MD simulation, one must first of all be able to express this observable as a function of the positions and momenta of the particles in the system. The best introduction to MD simulations is to consider a simple program. To start the simulation, one should assign initial positions and velocities to all particles in the system. The particle positions should be chosen compatible with the structure that one is aiming to simulate. A good MD program requires a good algorithm to integrate Newtons equations of motion. Accuracy for large time steps is more important because the longer the time step that one can use, the fewer evaluations of the forces are needed per unit of simulation time. For most MD applications, Verlet-like algorithms are perfectly adequate. However, sometimes it is convenient to employ a higher-order algorithm.

Molecular Simulations of Zeolites: Adsorption, Diffusion, and Shape Selectivity

Note: Chevron, Energy Technology Company, 100 Chevron Way, Richmond, California 94802-0627 Reference EPFL-ARTICLE-200588doi:10.1021/cr8002642 URL: http://pubs3.acs.org/acs/journals/doilookup?in_doi=10.1021/cr8002642 URL: http://pubs.acs.org/doi/pdfplus/10.1021/cr8002642 Record created on 2014-08-14, modified on 2017-05-12

Computer simulations of vapor-liquid phase equilibria of n-alkanes

For petrochemical applications knowledge of the critical properties of the n‐alkanes is of interest even at temperatures where these molecules are thermally unstable. Computer simulations can determine the vapor–liquid coexistence curve of a large number of n‐alkanes ranging from pentane (C5) through octatetracontane (C48). We have compared the predicted phase diagrams of various models with experimental data. Models which give nearly identical properties of liquid alkanes at standard conditions may have critical temperatures that differ by more than 100 K. A new n‐alkane model has been developed by us that gives a good description of the phase behavior over a large temperature range. For modeling vapor–liquid coexistence a relatively simple united atom model was sufficient to obtain a very good agreement with experimental data; thus it appears not necessary to take the hydrogen atoms explicitly into account. The model developed in this work has been used to determine the critical properties of the long‐chain alkanes for which experiments turned out to be difficult and contradictory. We found that for the long‐chain alkanes (C8–C48) the critical density decreases as a function of the carbon number. These simulations were made possible by the use of a recently developed simulation technique, which is a combination of the Gibbs‐ensemble technique and the configurational‐bias Monte Carlo method. Compared with the conventional Gibbs‐ensemble technique, this method is several orders of magnitude more efficient for pentane and up to a hundred orders of magnitude for octatetracontane. This recent development makes it possible to perform routinely phase equilibrium calculations of complex molecules.

Accelerating Monte Carlo Sampling

This chapter discusses various advanced Monte Carlo (MC) techniques. The method of parallel tempering provides good sampling of systems that have a free energy landscape with many local minima. It resembles the technique of simulating annealing and is related to several other schemes such as the extended-ensemble method, simulated tempering, and J-walking. The idea of parallel tempering is to include MC trial moves that attempt to “swap” systems that belong to different thermodynamic states. The basic idea behind the hybrid MC scheme is that one can use Molecular Dynamics (MD) to generate MC trial moves. For every trial move, the particle velocities are chosen at random from a Maxwell distribution. It is often advantageous to construct a trial move that consists of a sequence of MD steps. Yet, one cannot make the time step for a single hybrid MC move too long because then the acceptance would become very small. It is also interesting to use hybrid MC on models that have an expensive (many-body) potential energy function that may, to a first approximation, be modeled using a cheap (pair) potential. One of the differences of simulations of models with continuous interactions compared to those of models with hard-core potentials is the way in which MC moves are optimized.

Metal-Organic Frameworks as Adsorbents for Hydrogen Purification and Precombustion Carbon Dioxide Capture

Selected metal-organic frameworks exhibiting representative properties--high surface area, structural flexibility, or the presence of open metal cation sites--were tested for utility in the separation of CO(2) from H(2) via pressure swing adsorption. Single-component CO(2) and H(2) adsorption isotherms were measured at 313 K and pressures up to 40 bar for Zn(4)O(BTB)(2) (MOF-177, BTB(3-) = 1,3,5-benzenetribenzoate), Be(12)(OH)(12)(BTB)(4) (Be-BTB), Co(BDP) (BDP(2-) = 1,4-benzenedipyrazolate), H(3)[(Cu(4)Cl)(3)(BTTri)(8)] (Cu-BTTri, BTTri(3-) = 1,3,5-benzenetristriazolate), and Mg(2)(dobdc) (dobdc(4-) = 1,4-dioxido-2,5-benzenedicarboxylate). Ideal adsorbed solution theory was used to estimate realistic isotherms for the 80:20 and 60:40 H(2)/CO(2) gas mixtures relevant to H(2) purification and precombustion CO(2) capture, respectively. In the former case, the results afford CO(2)/H(2) selectivities between 2 and 860 and mixed-gas working capacities, assuming a 1 bar purge pressure, as high as 8.6 mol/kg and 7.4 mol/L. In particular, metal-organic frameworks with a high concentration of exposed metal cation sites, Mg(2)(dobdc) and Cu-BTTri, offer significant improvements over commonly used adsorbents, indicating the promise of such materials for applications in CO(2)/H(2) separations.

Novel scheme to study structural and thermal properties of continuously deformable molecules.

The authors present a method for calculating the chemical potential of arbitrary chain molecules in a computer simulation. The method is based on a generalization of Siepmanns method for calculating the chemical potential of chain molecules with a finite number of conformations. Next, the authors show that it is also possible to extend the configurational-bias Monte Carlo scheme developed recently by Siepmann and Frenkel (1992) to continuously deformable molecules. The utility of their technique for computing the chemical potential of chain molecules is demonstrated by computing the chemical potential of a fully flexible chain consisting of 10-20 segments in a moderately dense atomic fluid. Under these conditions the conventional particle-insertion schemes fail completely. In addition, they show that their novel configurational-bias Monte Carlo scheme compares favourably with conventional Monte Carlo procedures for chain molecules.

Phase diagrams of Lennard‐Jones fluids

Gibbs ensemble simulations are used to calculate part of the phase diagram of the Lennard‐Jones fluid. The phase diagram turned out to be very sensitive to the details of how the tail of the Lennard‐Jones potential is taken into account in the simulations.

Cooperative insertion of CO2 in diamine-appended metal-organic frameworks

The process of carbon capture and sequestration has been proposed as a method of mitigating the build-up of greenhouse gases in the atmosphere. If implemented, the cost of electricity generated by a fossil fuel-burning power plant would rise substantially, owing to the expense of removing CO2 from the effluent stream. There is therefore an urgent need for more efficient gas separation technologies, such as those potentially offered by advanced solid adsorbents. Here we show that diamine-appended metal-organic frameworks can behave as ‘phase-change’ adsorbents, with unusual step-shaped CO2 adsorption isotherms that shift markedly with temperature. Results from spectroscopic, diffraction and computational studies show that the origin of the sharp adsorption step is an unprecedented cooperative process in which, above a metal-dependent threshold pressure, CO2 molecules insert into metal-amine bonds, inducing a reorganization of the amines into well-ordered chains of ammonium carbamate. As a consequence, large CO2 separation capacities can be achieved with small temperature swings, and regeneration energies appreciably lower than achievable with state-of-the-art aqueous amine solutions become feasible. The results provide a mechanistic framework for designing highly efficient adsorbents for removing CO2 from various gas mixtures, and yield insights into the conservation of Mg2+ within the ribulose-1,5-bisphosphate carboxylase/oxygenase family of enzymes.

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.