Gas Sorption and Separation

Lots of porous solids, such as porous carbons, porous silica, zeolites and porous organic frameworks, have been reported and investigated for gas sorption. Compared with these porous materials, PAFs show unique advantages of combining high surface area and excellent physicochemical stability. This is especially true for PAF-1, which has a high surface area of 5640 m g and could survive extreme conditions such as boiling in solvent or addition to cold basic or acidic solutions.  

MOFs with even higher surface area and increased gas adsorption have been synthesized (as shown in Table 8.1), the weak coordination in MOFs makes them rather unstable under harsh conditions and makes them incomparable with PAFs for long time storage and repeated usage. 

Compound Interaction Sbett Pore volume (cm= g-i) Excess H2 uptake (mgg ) Excess CO2 uptake (mg g ) Excess CH4 uptake (mg g- ) 

PAF-3 and PAF-4 did show better selective adsorption of CO2 than PAF-1. At 1 atm and 273 K, PAF-3 exhibits extraordinarily promising selectivity of 87 1 for the adsorption of CO2 over N2, and PAF-4 shows a selectivity of 44 1, while this value is only 38 1 for PAF-1. 

The above results reveal the interesting earbon dioxide eapture and gas storage properties of PAFs. Among all the reported PAFs, PAF-1 exhibits an extremely high surface area, as well as exeellent physieoehemieal and hydrothermal stabilities. Though some metal organie frameworks reported 

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ACs are the most commonly used form of porous carbons for a long time. Typically, they refer to coal and petroleum pitch as well as coconut sheUs-based AC. In most cases, ACs are processed to be filled with rich micropores that increase the surface area available for gas sorption and separation. For this category, to get a definite classification on the basis of pore structure is difficult due to their countless products as well as their complex pore features. Based on the physical characteristics, they can be widely classified into the following types powdered, granular, extruded, bead ACs, etc. For the pore structure of ACs, actually, all the three types of pores (micropore, mesopore, and macropore) are included in one product (Fig. 2.1), with a wide pore size distribution [1, 2]. Up to now, many kinds of ACs have been well commercialized in gas sorption/separation including CO2 capture. For example, the BPL type with specific area of 1,141 m g is able to adsorb 7 mmol g CO2 under the conditions of 25 °C and 35 bar, while under the same conditions MAXSORB-activated carbon with specific area of 3,250 g can capture up to 25 mmol g [3]. 

The calixarene and its related derivative molecules are usually crystallized in a closely packed crystalline arrangement and thus are inherently non porous. However, specific low density forms have shown to possess permanent porosity that are available for the uptake and storage of gaseous guest. This book chapter gives a brief summary of the gas-sorption and separation application involve of calixarene and related derivatives. 

Physisorption based gas-sorption and separation techniques using solid state adsorbents are considered as a viable alternative for energy intensive cryogenic distillation and liquid extraction [3]. The solid state adsorbents are generally porous few... 

The research on guest-inclusion complexes of calixarene based molecular crystals paved the way for different type of gas-sorption studies in these materials under various conditions. Although these materials generally dcai t possess uniform pore nor have high surface area (unlike traditional porous materials such as zeolite, molecular sieves or MOFs), their unique void structures make them attractive candidates for selective gas-sorption and separation applications. It should be noted that no uniform method were used to analyze the gas-sorption capacity (or separation capability) of these... 

The general conclusion to be drawn from the above discussion is that the dual gas sorption and mobility model has very useful predictive potential in the field of membrane gas separation. At the present stage, no comparable predictive capability is discernible in the alternative treatments so far published 33 >34 63). 

Rao, M. B. Sircar, S. (1996). Performance and Pore Charaeterization of Nanoporous Carbon Membranes for Gas Separation. Journal of Membrane Science, 110(1), 109-118. Merkel, T. C., etal. (2QQy). Effect of Nanoparticle son Gas Sorption and Transport in Poly (l-trimethylsllyl-l-propyne). Macromolecules, 36(18), 6844-6855. 

MOF material can selectively capture and separate Kr from Kr/Xe mixtures at moderate temperature via a temperature gating mechanism on a copper-based fluorinated MOF [65], MOFs have shown great application potential in the gas capture and separations area, which will help in evaluating the MOFs for sorption, separation-related applications, and selective single-component sorption/separation applications, respectively. 

Gas capture and separation Heterogeneous catalysis Metal-organic frameworks Oil spill cleanup Sensing applications, sensors Water sorption... 

The main emphasis in this chapter is on the use of membranes for separations in liquid systems. As discussed by Koros and Chern(30) and Kesting and Fritzsche(31), gas mixtures may also be separated by membranes and both porous and non-porous membranes may be used. In the former case, Knudsen flow can result in separation, though the effect is relatively small. Much better separation is achieved with non-porous polymer membranes where the transport mechanism is based on sorption and diffusion. As for reverse osmosis and pervaporation, the transport equations for gas permeation through dense polymer membranes are based on Fick s Law, material transport being a function of the partial pressure difference across the membrane. 

The sorption and diffusion behaviour of gas mixtures is of particular interest from the point of view of membrane gas separation, which is steadily gaining in importance by virtue of its low energy requirements. On the basis of the dual mode sorption model, one may reasonably expect that sorption of a binary gas mixture A, B in the polymer matrix will exhibit little gas-gas interaction and hence will tend to occur essentially additively. In the Langmuir-like mode of sorption, on the other hand, there will be competition between A and B for the limited number of available sites. These considerations led 67) to the following reformulation of Eqs. (8) and (9)... 

The gas-polymer-matrix model for sorption and transport of gases in polymers is consistent with the physical evidence that 1) there is only one population of sorbed gas molecules in polymers at any pressure, 2) the physical properties of polymers are perturbed by the presence of sorbed gas, and 3) the perturbation of the polymer matrix arises from gas-polymer interactions. Rather than treating the gas and polymer separately, as in previous theories, the present model treats sorption and transport as occurring through a gas-polymer matrix whose properties change with composition. Simple expressions for sorption, diffusion, permeation and time lag are developed and used to analyze carbon dioxide sorption and transport in polycarbonate. 

J.H. Petropoulos, Mechanisms and Theories for Sorption and Diffusion of Gases in Polymers, in Polymeric Gas Separation Membranes, D.R. Paul and Y.P. Yampol skii (eds), CRC Press, Boca Raton, FL, pp. 17-82 (1994). 

Separation of individual species and element-specific detection (extraction, sorption, ion exchange, gas permeation, and electrolysis)... 

Most polymers that have been of interest as membrane materials for gas or vapor separations are amorphous and have a single phase structure. Such polymers are converted into membranes that have a very thin dense layer or skin since pores or defects severely compromise selectivity. Permeation through this dense layer, which ideally is defect free, occurs by a solution-diffusion mechanism, which can lead to useful levels of selectivity. Each component in the gas or vapor feed dissolves in the membrane polymer at its upstream surface, much like gases dissolve in liquids, then diffuse through the polymer layer along a concentration gradient to the opposite surface where they evaporate into the downstream gas phase. In ideal cases, the sorption and diffusion process of one gas component does not alter that of another component, that is, the species permeate independently. 

Visser, T., Koops, G.H. and Wessling, M. (2005) On the subtle balance between competitive sorption and plasticization effects in asymmetric hollow fiber gas-separation membranes. Journal of Membrane Science, 252, 265-277. 

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