Introduction 724 1.1. Carbon Dioxide Emission from Anthropogenic Sources 725 1.2. CO 2 Capture at Stationary Point Sources 726 1.3. Options for CO 2 Sequestration 727 1.4. Current CO 2 Capture Materials 727 1.4.1. Aqueous Alkanolamine Absorbents 728 1.4.2. Solid Porous Adsorbent Materials 729 1.5. MetalÀOrganic Frameworks 731 1.5.1. Synthesis and Structural Features 731 1.5.2. Physical Properties 732 2. CO 2 Adsorption in MetalÀOrganic Frameworks 733 2.1. Capacity for CO 2 733 2.2. Enthalpy of Adsorption 733 2.3. Selectivity for CO 2 739 2.3.1. Estimation from Single-Component Isotherms 741 2.3.2. Ideal Adsorbed Solution Theory (IAST) 742 2.3.3. Gas Mixtures and Breakthrough Experiments 742 2.4. In Situ Characterization of Adsorbed CO 2 742 2.4.1. Structural Observations 743 2.4.2. Infrared Spectroscopy 744 2.5. Computational Modeling of CO 2 Capture 745 3. Post-combustion Capture 746 3.1. MetalÀOrganic Frameworks for CO 2 /N 2 Separation 746 3.2. Enhancing CO 2 /N 2 Selectivity via Surface Functionalization 746 3.2.1. Pores Functionalized by Nitrogen Bases 746 3.2.2. Other Strongly Polarizing Organic Functional Groups 749 3.2.3. Exposed Metal Cation Sites 750 3.3. Considerations for Application 752 3.3.1. Stability to Water Vapor 752 3.3.2. Other Minor Components of Flue Gas 754 4. Pre-combustion Capture 754 4.1. Considerations for Pre-combustion CO 2 Capture 755 4.1.1. Advantages of Pre-combustion Capture 755 4.1.2. Hydrogen Purification 755 4.1.3. Metrics for Evaluating Adsorbents 755 4.1.4. Non-CO 2 Impurities in CO 2 /H 2 Streams 756 4.1.5. MetalÀOrganic Framework-Containing Membranes for Pre-combustion CO 2 Capture 756 4.2. MetalÀOrganic Frameworks as Adsorbents 756 4.2.1. Investigations Based on Single-Component Isotherms 757 4.2.2. Computational Studies 757 5. Oxy-fuel Combustion 760 5.1.

No abstract

Metal-organic frameworks have received significant attention as a new class of adsorbents for natural gas storage; however, inconsistencies in reporting high-pressure adsorption data and a lack of comparative studies have made it challenging to evaluate both new and existing materials. Here, we briefly discuss high-pressure adsorption measurements and review efforts to develop metal-organic frameworks with high methane storage capacities. To illustrate the most important properties for evaluating adsorbents for natural gas storage and for designing a next generation of improved materials, six metal-organic frameworks and an activated carbon, with a range of surface areas, pore structures, and surface chemistries representative of the most promising adsorbents for methane storage, are evaluated in detail. High-pressure methane adsorption isotherms are used to compare gravimetric and volumetric capacities, isosteric heats of adsorption, and usable storage capacities. Additionally, the relative importance of increasing volumetric capacity, rather than gravimetric capacity, for extending the driving range of natural gas vehicles is highlighted. Other important systems-level factors, such as thermal management, mechanical properties, and the effects of impurities, are also considered, and potential materials synthesis contributions to improving performance in a complete adsorbed natural gas system are discussed. Polarizability 9 2.6Å 3 Volumetric density (1 bar, 25 C) 8 0.9 v/v Volumetric density (250 bar, 25 C) 8 263 v/v Volumetric density (1 bar, À162 C) 8 591 v/v

Two new metal-organic frameworks, M(2)(dobpdc) (M = Zn (1), Mg (2); dobpdc(4-) = 4,4'-dioxido-3,3'-biphenyldicarboxylate), adopting an expanded MOF-74 structure type, were synthesized via solvothermal and microwave methods. Coordinatively unsaturated Mg(2+) cations lining the 18.4-Å-diameter channels of 2 were functionalized with N,N'-dimethylethylenediamine (mmen) to afford Mg(2)(dobpdc)(mmen)(1.6)(H(2)O)(0.4) (mmen-Mg(2)(dobpdc)). This compound displays an exceptional capacity for CO(2) adsorption at low pressures, taking up 2.0 mmol/g (8.1 wt %) at 0.39 mbar and 25 °C, conditions relevant to removal of CO(2) from air, and 3.14 mmol/g (12.1 wt %) at 0.15 bar and 40 °C, conditions relevant to CO(2) capture from flue gas. Dynamic gas adsorption/desorption cycling experiments demonstrate that mmen-Mg(2)(dobpdc) can be regenerated upon repeated exposures to simulated air and flue gas mixtures, with cycling capacities of 1.05 mmol/g (4.4 wt %) after 1 h of exposure to flowing 390 ppm CO(2) in simulated air at 25 °C and 2.52 mmol/g (9.9 wt %) after 15 min of exposure to flowing 15% CO(2) in N(2) at 40 °C. The purity of the CO(2) removed from dry air and flue gas in these processes was estimated to be 96% and 98%, respectively. As a flue gas adsorbent, the regeneration energy was estimated through differential scanning calorimetry experiments to be 2.34 MJ/kg CO(2) adsorbed. Overall, the performance characteristics of mmen-Mg(2)(dobpdc) indicate it to be an exceptional new adsorbent for CO(2) capture, comparing favorably with both amine-grafted silicas and aqueous amine solutions.

As a cleaner, cheaper, and more globally evenly distributed fuel, natural gas has considerable environmental, economic, and political advantages over petroleum as a source of energy for the transportation sector. Despite these benefits, its low volumetric energy density at ambient temperature and pressure presents substantial challenges, particularly for light-duty vehicles with little space available for on-board fuel storage. Adsorbed natural gas systems have the potential to store high densities of methane (CH4, the principal component of natural gas) within a porous material at ambient temperature and moderate pressures. Although activated carbons, zeolites, and metal-organic frameworks have been investigated extensively for CH4 storage, there are practical challenges involved in designing systems with high capacities and in managing the thermal fluctuations associated with adsorbing and desorbing gas from the adsorbent. Here, we use a reversible phase transition in a metal-organic framework to maximize the deliverable capacity of CH4 while also providing internal heat management during adsorption and desorption. In particular, the flexible compounds Fe(bdp) and Co(bdp) (bdp(2-) = 1,4-benzenedipyrazolate) are shown to undergo a structural phase transition in response to specific CH4 pressures, resulting in adsorption and desorption isotherms that feature a sharp 'step'. Such behaviour enables greater storage capacities than have been achieved for classical adsorbents, while also reducing the amount of heat released during adsorption and the impact of cooling during desorption. The pressure and energy associated with the phase transition can be tuned either chemically or by application of mechanical pressure.