An entangled flexible MOF for capture of CO2 and store/ separation of (C1-C3) hydrocarbons

Globally the swift growth of industrial sectors has immensely shuddered the environmental balance, particularly due to large quantity of greenhouse gas emission from the fossil fuel combustion. Therefore the capture of greenhouse gas, such as CO2 and also its separation from the gas mixtures at ambient conditions is important. Metal-organic frameworks, a class of crystalline permanently porous material has been found to be a promising candidate to address this issue. By virtue of its periodic well defined structure, it is possible to simulate its porous properties and thus design of suitable porous structure for CO2 capture and separation is viable. To embark upon we sought for an entangled framework with pore surface carved with polar functional groups and aromatic π clouds. {[Zn2(bdc)2(bpNDI)]⋅4DMF}n (bdc = 1,4-benzenedicarboxylate, bpNDI = N,N′-bis-(4-pyridyl)-1,4,5,8-naphthalenediimide), is such a twofold entangled framework with imide (R2NC=O) functionalized pore surface (Fig. 1). Most importantly, the 3D framework contains two different adsorption sites created by the entanglement of two different nets. The framework also shows guest specific dynamic structural transformation based on the mutual sliding of the two 3D entangled nets and structural flexibility is comprehended from the powder X-ray diffraction (PXRD) and stepwise adsorption of gases like CO2 and C2H2 at 195 K. At 273 and 298 K, CO2 uptakmounts are 14.7 wt% and 12.7 wt%, respectively. In contrast, framework reveals little uptake of N2 and CH4 at 298 K. Thus it is an ideal system to separate CO2 from N2 and CH4. It is worth mentioning that flue gas (emission from coal-fired power plant) contains ~76% N2 and ~13 % CO2 and rests are water vapor (~6%) and oxides of N2 and sulphur. Similarly natural gas (mainly CH4) also contains certain amount of non-calorific CO2. Thus separation of CO2 from N2 and CH4 is of paramount importance. Thus we have tested this framework for CO2 separation from a binary mixture of CO2/N2 (15:85 v/v) and CO2 /CH4 (50:50 V/V) by breakthrough column experiments (Fig. 1). Indeed, this dynamic framework showed as an efficient CO2 separation material at ambient conditions.

Fig. 1. Two-fold interpenetrated dynamic porous metal-organic frameowork. The framewok showed selective CO2 capture and potential for separation of CO2 from N2 and CH4 gas mixture. The framework also adsorb significant amount of different C2-C3 hydrocarbons and poetentail for separation of C2H6from CH4 as realized by breakthrough column xperiments.

In addition to the above described applications, this framework can also adsorb different C2-C3 hydrocarbons. As mentioned before, the rapid growth of industrialization also demands continuous supply of energy and hence searches for alternative fuel source and their storage is in high demand. Light hydrocarbons such as CH4 (methane), C2H2 (acetylene), C2H4 (ethylene), C2H6 (ethane), C3H6 (propene), and C3H8 (propane) are known to be used as feedstock for numerous industrially important materials and also can serve as resource of energy as a fuel. At 298 K, the activated framework can adsorb 81, 44, 49 and 43 cc/g of C2H2, C2H4, C2H6 and C3H8, respectively. The adsorption capacity is in accordance to the kinetic diameter and adsorbent-adsorbate interactions in the hydrophobic pore surface. Using IAST calculations and breakthrough measurements of the gas mixtures (C2H6/CH4, 50:50 V/V) we could also confirm the selective capture of ethane (C2H6) in this porous functionalized framework (Fig. 1).

In brief, the entangled functionalized porous framework showed as a potential material for CO2 capture and also revealed its potential to separate hydrocarbons and CO2 from mixture of gases at ambient condition. In absence of unsaturated metal sites, such prolific efficiency to selectively capture and separate gases is rarely achieved.

Tapas K. Maji
Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research,
Bangalore, India



Dynamic Entangled Porous Framework for Hydrocarbon (C2-C3) Storage, CO2 Capture, and Separation.
Sikdar N, Bonakala S, Haldar R, Balasubramanian S, Maji TK
Chemistry. 2016 Apr 18


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