Adsorptive separation is very important in industry. Generally, the process uses porous solid materials such as zeolites, activated carbons, or silica gels as adsorbents. With an ever increasing need for a more efficient, energy-saving, and environmentally benign procedure for gas separation, adsorbents with tailored structures and tunable surface properties must be found. Metal–organic frameworks (MOFs), constructed by metal-containing nodes connected by organic bridges, are such a new type of porous materials. They are promising candidates as adsorbents for gas separations due to their large surface areas, adjustable pore sizes and controllable properties, as well as acceptable thermal stability. This critical review starts with a brief introduction to gas separation and purification based on selective adsorption, followed by a review of gas selective adsorption in rigid and flexible MOFs. Based on possible mechanisms, selective adsorptions observed in MOFs are classified, and primary relationships between adsorption properties and framework features are analyzed. As a specific example of tailor-made MOFs, mesh-adjustable molecular sieves are emphasized and the underlying working mechanism elucidated. In addition to the experimental aspect, theoretical investigations from adsorption equilibrium to diffusion dynamics via molecular simulations are also briefly reviewed. Furthermore, gas separations in MOFs, including the molecular sieving effect, kinetic separation, the quantum sieving effect for H2/D2 separation, and MOF-based membranes are also summarized (227 references).

Selective gas adsorption and separation in metal–organic frameworks

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

CO2 storage capacity estimation: Methodology and gaps

M M U Rapid Synthesis of Nitrogen-Doped Porous Carbon Monolith for CO2 Capture N IC By Guang-Ping Hao, Wen-Cui Li, Dan Qian, and An-Hui Lu* A T IO N The development of novel materials and new technologies for CO2 capture and storage has gained great attention over the past decades. The necessity for reducing the concentration of CO2 in the atmosphere is a very important issue, since CO2 is considered as a greenhouse gas causing global warming. In addition, higher concentrations of CO2 are toxic for humans, especially in space-limited chambers like submarines and space ships. Traditional technologies for CO2 capture including absorption and adsorption-coupled membrane separation along with the corresponding sorbents such as aqueous amines, microporous coordination polymers (MCPs), zeolitic imidazolate frameworks (ZIFs), different types of nitrogen-doped zeolite, mesoporous silica, and activated carbons have been widely explored. However, MCPs and ZIFs synthesized with nitrogen-containing organic compounds as the crosslinker often suffer from structural instability and inefficiency for CO2 selectivity in the presence of water and thus are limited in their widespread use. For aminemodified solids, the main drawback is the possible loss of ammonia associated with the temperature needed for regeneration and the energy-intensive nature of the regeneration process. For instance, amine-surface-modified or amineimpregnated porous silica (e.g., MCM-41, MCM-48, SBA-15, SBA-16) and zeolite 13X materials fail to capture CO2 effectively, require high temperature and long regeneration times and lack stability over many cycles. It should be pointed out that amine-modified silica-based solids are usually prepared by a post-treatment, which is a time-consuming and costly procedure and often involves the use of toxic and corrosive reagents. Porous carbon materials have many specific features such as high surface area, thermal and chemical stability, and hydrophobic surface properties. The incorporation of basic nitrogen groups into the carbon framework ensures an improved adsorption/absorption for acidic gases. Up to date, porous carbon materials used for CO2 capture were mostly prepared by post-synthetic amine modification or ammonia treatment, which again leads to materials lacking stability and, in addition, the reagents are corrosive, which brings the same disadvantages as for modified silica and zeolites. Alternatively, nitrogencontaining porous carbons can be prepared directly from

Rapid synthesis of nitrogen-doped porous carbon monolith for CO2 capture.

CBM and CO2-ECBM related sorption processes in coal: A review

This article reviews the storage of captured CO2 in coal seams. Other geologic formations, such as depleted petroleum reservoirs, deep saline aquifers and others have received considerable attention as sites for sequestering CO2. This review focuses on geologic sequestration of CO2 in unmineable coalbeds as the geologic host. Key issues for geologic sequestration include potential storage capacity, the storage integrity of the geologic host, and the chemical and physical processes initiated by the deep underground injection of CO2. The review topics include (i) the estimated CO2 storage capacity of coal, along with the estimated amount and composition of coalbed gas; (ii) an evaluation of the coal seam properties relevant to CO2 sequestration, such as density, surface area, porosity, diffusion, permeability, transport, rank, adsorption/desorption, shrinkage/swelling, and thermochemical reactions; and (iii) a treatment of how coalbed methane (CBM) recovery and CO2-enhanced coalbed methane (ECBM) recovery a...

Stephen B. DuBose

Papers

Sequestration of Carbon Dioxide in Coal with Enhanced Coalbed Methane RecoveryA Review