Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

Kenji Sumida, David L. Rogow, Jarad A. Mason, Thomas M. McDonald, Eric D. Bloch, Zoey R. Herm, Tae-Hyun Bae, Jeffrey R. Long

Carbon Dioxide Capture in Metal–Organic Frameworks

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.

Carbon Dioxide Capture: Prospects for New Materials

Mesoporous organic-inorganic hybrid materials, a new class of materials characterized by large specific surface areas and pore sizes between 2 and 15 nm, have been obtained through the coupling of inorganic and organic components by template synthesis. The incorporation of functionalities can be achieved in three ways: by subsequent attachment of organic components onto a pure silica matrix (grafting), by simultaneous reaction of condensable inorganic silica species and silylated organic compounds (co-condensation, one-pot synthesis), and by the use of bissilylated organic precursors that lead to periodic mesoporous organosilicas (PMOs). This Review gives an overview of the preparation, properties, and potential applications of these materials in the areas of catalysis, sorption, chromatography, and the construction of systems for controlled release of active compounds, as well as molecular switches, with the main focus being on PMOs.

Silica-based mesoporous organic-inorganic hybrid materials.

Since the time of the industrial revolution, the atmospheric CO(2) concentration has risen by nearly 35 % to its current level of 383 ppm. The increased carbon dioxide concentration in the atmosphere has been suggested to be a leading contributor to global climate change. To slow the increase, reductions in anthropogenic CO(2) emissions are necessary. Large emission point sources, such as fossil-fuel-based power generation facilities, are the first targets for these reductions. A benchmark, mature technology for the separation of dilute CO(2) from gas streams is via absorption with aqueous amines. However, the use of solid adsorbents is now being widely considered as an alternative, potentially less-energy-intensive separation technology. This Review describes the CO(2) adsorption behavior of several different classes of solid carbon dioxide adsorbents, including zeolites, activated carbons, calcium oxides, hydrotalcites, organic-inorganic hybrids, and metal-organic frameworks. These adsorbents are evaluated in terms of their equilibrium CO(2) capacities as well as other important parameters such as adsorption-desorption kinetics, operating windows, stability, and regenerability. The scope of currently available CO(2) adsorbents and their critical properties that will ultimately affect their incorporation into large-scale separation processes is presented.

Adsorbent Materials for Carbon Dioxide Capture from Large Anthropogenic Point Sources

Carbon dioxide adsorption from a simulated flue gas stream was successfully performed with a hyperbranched aminosilica (HAS) material. The HAS was synthesized by a one-step reaction, spontaneous aziridine ring-opening polymerization off of surface silanols, to form a 32 wt % organic/inorganic hybrid material. The adsorption measurements were performed in a fixed-bed flow reactor using humidified CO2. The advantage of this adsorbent over previously reported adsorbents is the stability of the organic groups covalently bound to the silica support compared to those made by physisorbed methods. Furthermore, a large CO2 capacity (∼3 mmol CO2/g adsorbent) associated with the high loading of amines was observed.

Designing adsorbents for CO2 capture from flue gas-hyperbranched aminosilicas capable of capturing CO2 reversibly.

CO2 adsorption/desorption on SBA-15 grafted with γ-(aminopropyl)triethoxysilane (APTS) has been studied by infrared spectroscopy coupled with temperature-programmed desorption. SBA-15, a mesoporous silica material with a uniform pore size of 21 nm and a surface area of 200−230 m2/g, provides an OH functional group for grafting of γ-(aminopropyl)triethoxysilane. The amine-grafted SBA-15 adsorbed CO2 as carbonates and bicarbonates with a total capacity of 200−400 μmol/g. The heat of CO2 desorption was determined to be 3.2−4.5 kJ/mol in the presence of H2O and 6.6−11.0 kJ/mol in the absence of H2O during temperature-programmed desorption. Repeated CO2 adsorption/desorption CO2 cycles shifted the desorption peak temperature downward and decreased the heat of CO2 adsorption.

In-Situ Infrared Study of CO2 Adsorption on SBA-15 Grafted with γ-(Aminopropyl)triethoxysilane

The adsorption and desorption of CO2 and SO2 on an amine-grafted SBA-15 sorbent has been studied by in situ infrared spectroscopy coupled with mass spectrometry. CO2 adsorbed on an amine-grafted sorbent as carbonates and bicarbonates, while SO2 adsorbed as sulfates and sulfites. The CO2 adsorption capacity of the amine-grafted sorbent was almost twice as much as that of a commercial sorbent. The adsorption of CO2 in the presence of H2O and D2O shows an isotopic shift in the IR frequency of adsorbed carbonate and bicarbonate bands, revealing that water plays a role in the CO2 adsorption on amine-grafted sorbents. Although the rate of adsorption of SO2 was slower than that of CO2, the adsorbed S surface species is capable of blocking the active amine sites for CO2 adsorption. A temperature-programmed degradation study of the amine-grafted sorbent showed that the surface amine species are stable up to 250 °C in air.

Thermal and Chemical Stability of Regenerable Solid Amine Sorbent for CO2 Capture

Hyperbranched aminosilica (HAS) adsorbents are prepared via the ring-opening polymerization ofaziridine in the presence ofmesoporous silica SBA-15 support. The aminopolymers are covalently bound to the silica support and capture CO 2 reversibly in a temperature swing process. Here, a range of HAS materials are prepared with different organic loadings. The effects of organic loading on the structural properties and CO 2 adsorption properties of the resultant hybrid materials are examined. The residual porosity in the HAS adsorbents after organic loading, as well as the molecular weights and degrees of branching for the separated aminopolymers, are determined to draw a relationship between adsorbent structure and performance. Humid adsorption working capacities and apparent adsorption kinetics are determined from experiments in a packed-bed flow system monitored by mass spectrometry. Dry adsorption isotherms are presented for one HAS adsorbent with a high amine loading at 35 and 75 °C. These combined results establish the relationships between adsorbent synthesis, structure, and CO 2 adsorption properties of the family of HAS materials.

Synthesis–Structure–Property Relationships for Hyperbranched Aminosilica CO2 Adsorbents

Silica supported poly(ethyleneimine) (PEI) materials are prepared via impregnation and demonstrated to be promising adsorbents for CO2 capture from ultra-dilute gas streams such as ambient air. A prototypical class 1 adsorbent, containing 45 wt % PEI (PEI/silica), and two new modified PEI-based aminosilica adsorbents, derived from PEI modified with 3-aminopropyltrimethoxysilane (A-PEI/silica) or tetraethyl orthotitanate (T-PEI/silica), are prepared and characterized by using thermogravimetric analysis and FTIR spectroscopy. The modifiers are shown to enhance the thermal stability of the polymer-oxide composites, leading to higher PEI decomposition temperatures. The modified adsorbents present extremely high CO2 adsorption capacities under conditions simulating ambient air (400 ppm CO2 in inert gas), exceeding 2 mol kgsorbent−1, as well as enhanced adsorption kinetics compared to conventional class 1 sorbents. The new adsorbents show excellent stability in cyclic adsorption–desorption operations, even under dry conditions in which aminosilica adsorbents are known to lose capacity due to urea formation. Thus, the adsorbents of this type can be considered promising materials for the direct capture of CO2 from ultra-dilute gas streams such as ambient air.

McMahan L. Gray

Papers

Designing adsorbents for CO2 capture from flue gas-hyperbranched aminosilicas capable of capturing CO2 reversibly.

In-Situ Infrared Study of CO2 Adsorption on SBA-15 Grafted with γ-(Aminopropyl)triethoxysilane

Thermal and Chemical Stability of Regenerable Solid Amine Sorbent for CO2 Capture

Synthesis–Structure–Property Relationships for Hyperbranched Aminosilica CO2 Adsorbents

Amine‐Tethered Solid Adsorbents Coupling High Adsorption Capacity and Regenerability for CO2 Capture From Ambient Air