Gas adsorption is the most obvious application that takes advantage of porous properties. The amount of gas adsorbed seems to depend on the surface area and pore size of the materials. Nevertheless, the adsorption effectiveness not only relies on physical properties but also on chemical ability to interact with guest molecules.102
Apart from hydrogen34, 106-110 and methane84, 111-113, which attract much research interest due to their use as novel alternative and clean energy resources, carbon dioxide (CO2) capture is another target had been linked to materials.13, 114, 115 CO2 is a green house gas and is responsible for global warming.116 There are three processes of carbon dioxide capture.13 The first process is the pre-combustion capture dealing with contaminated carbon dioxide in natural gas. There is around 35.5 % CO2 contained in the gas stream. It requires high temperature and pressure durable materials. The second process is the post-combustion capture coping with carbon dioxide in exhaust gas. This kind of gas consists of approximately 15 % CO2 and contains a higher level of water compared to pre-combustion capture so selectivity for CO2 and stability towards water is needed for any potential capture materials. The third process is the CO2 capture from atmosphere to remove CO2 directly from air. As the CO2 is highly diluted, only about 0.04% or 400 ppm117, materials should have CO2 adsorption and selectivity at low CO2 concentration.
At this time, the technologies used in CO2 sorption are aqueous amine and amine solutions that can absorb CO2 and form C-N bonds.118 However, there are problems due to the high energy required for recycling and corrosion.119 Moreover, energy needed for absorption is in principle more than for adsorption. Thus, materials used as adsorbents are expected to be a new technology for CO2 capture, especially porous materials based on carbon, which generally have inert property,
34 | P a g e low cost, and high surface area.13 However, challenges still remain. Materials for CO2 capture should have high capacity for CO2 adsorption, low energy for the release of CO2 from the materials, and low producing cost.120 Therefore, materials have to be carefully designed to solve these problems. There are three strategies to optimise materials for CO2 capture: (1) by changing the composition of the polymer frameworks, (2) by tailoring the surface areas and pore size, and (3) by pore surface modifications.120
Different kind of the frameworks of course affect the physical and chemical structures of the networks. Various materials and their CO2 uptakes, selectivity and heat of adsorption were summarised by Dawson et al.13, 69
Surface area is an important constituent affecting the amount of CO2 uptake and selectivity. The highest surface area MOP, PPN-4 which has surface area of 6460 m2/g exhibited very high CO2 uptake of 48.20 mmol/g at 50 bar and 295 K.43 PAF-1 which is the second highest surface area MOPs also showed high CO2 capacity of 29.55 mmol/g at 40 bar and 298 K.40 Nevertheless, surface area is not the sole factor as high surface area materials did not always show impressive CO2 uptakes, especially at low pressure.
There are other factors that influence the quantity of CO2 sorption. As mentioned before, a pore size that is less than five times that of the gas molecule is considered to be the most effective.121, 122 CO2 molecular size is 0.209 nm so the appropriate pore size should be smaller than 1.0 nm to adsorb CO2 at atmospheric pressure. For higher pressure, pore sizes could be 0.7 to 2.0 nm.66
Moreover, chemical modification of “CO2-philic” on the surface of pores is believed to improve the interaction between CO2 molecules and framework leading to higher CO2 uptakes. Not only is high CO2 capture adsorption important, selectivity over other gases is also needed to be considered. For an ideal CO2 adsorbent, high CO2 uptakes and selectivity required. Normally, the materials with high selectivity exhibit low adsorption. Many attempts have been tried to improve the networks with high the selectivity while CO2 capacity still remain or increase.
Because CO2 is slightly acidic, the basicity of adsorbents is predicted to play a significant part for CO2 capture. Therefore, basic functional groups are introduced
35 | P a g e in many materials especially amine functionality. Polyamine-tethered PPNs obtained by PSM of PPN-6 also showed significant increase in CO2 uptakes even had much lower surface areas. PPN-6-CH2DETA (SA = 555 m2/g) had the CO2 uptakes of 4.3 mmol/g, higher than PPN-6 (SA = 4023 m2/g) which had CO2 uptakes of 1.3 mmol/g.8 The amine group introduced into the PPNs by PSM from anchored aldehydes was also found to have high carbon dioxide heat of adsorption up to 50 kJ/mol. Moreover, the selectivity of CO2 over N2 and CH4 was also enhanced.9
N-containing materials were also investigated. By replacing the benzene node in a series of CMPs with N-containing, triazine node (TCMPs), the surface areas were comparable. However, the CO2 uptakes increased.123 Petal et al. also showed the catalyst-free synthesis of azo-bridged covalent organic polymers (azo-COPs,
Figure 1.7) which provide high BET surface area materials up to 729 m2/g as well as high selectivity of CO2 over N2 of 288 at 55 oC without lowering CO2 capacity (CO2 uptake of 151.3 mg/g).124 This is resulting from the phobicity of N2 to the azo group. Surprisingly, unlike most of other materials, the networks show higher selectivity when temperature was increased. More azo-COPs were further investigated.
Increasing of π–surface area functional groups demonstrated more interaction with CO2 which affect to CO2 than N2.125 Combining azo N2-phobic group with CO2- philic group leads to improvement of CO2/N2 selectivity and CO2 uptakes. This kind of material could have the very high CO2 capacity up to 5.37 mmol/g at 273 K and 1 bar.126
36 | P a g e Apart from N-containing networks, other heteroatom-containing monomers such as S, O and P were also investigated. HCPs synthesised by heterocyclic monomers including thiophene, pyrrole and furan were investigated by Tan group.70 All thiophene (Ty-1), pyrrole (Py-1) and furan (Fu-1) HCPs exhibited high CO2 uptakes of 12.7, 11.9 and 9.7 wt% at 273 K and 1 bar. The capacities are higher than many MOPs reported earlier and also higher than 9.1 wt% in PAF-140 which has ultrahigh surface area under the similar conditions. The phosphorus node was also studied the effect on CO2 sorption ability. Phosphine-containing MOPs were synthesised by Yang et al. The networks demonstrated good binding to CO2 with isosteric heat of adsorption more than 25 kJ/mol as well as high CO2/CH4 selectivity.127
Acid functional groups were also found to influence the CO2 uptakes. In fact, such the functional group exhibited higher CO2 adsorption than basic group in many cases. Various functionalised CMPs were compared in terms of CO2 uptakes and isosteric heats by the Cooper group. (Figure 1.8) Carboxylic acid functionalised CMP, CMP-1-COOH, showed higher heat of adsorption than other functional groups. This study supported that these factors depended on functionalities more than surface areas or pore volumes.103 PPN-6 grafted with sulfonic acid (PPN-6- SO3H) and its lithium salt (PPN-6-SO3Li) also showed higher CO2 uptake and selectivity than the parent networks even though they had lower surface areas.7 The CO2 capacity was also higher than the amine grafted analogue (PPN-6-CH2DETA)8.
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