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. 2022 May 20;13(24):7172-7180.
doi: 10.1039/d2sc01180h. eCollection 2022 Jun 22.

Tailoring a robust Al-MOF for trapping C2H6 and C2H2 towards efficient C2H4 purification from quaternary mixtures

Affiliations

Tailoring a robust Al-MOF for trapping C2H6 and C2H2 towards efficient C2H4 purification from quaternary mixtures

Subhajit Laha et al. Chem Sci. .

Abstract

Light hydrocarbon separation is considered one of the most industrially challenging and desired chemical separation processes and is highly essential in polymer and chemical industries. Among them, separating ethylene (C2H4) from C2 hydrocarbon mixtures such as ethane (C2H6), acetylene (C2H2), and other natural gas elements (CO2, CH4) is of paramount importance and poses significant difficulty. We demonstrate such separations using an Al-MOF synthesised earlier as a non-porous material, but herein endowed with hierarchical porosity created under microwave conditions in an equimolar water/ethanol solution. The material possessing a large surface area (793 m2 g-1) exhibits an excellent uptake capacity for major industrial hydrocarbons in the order of C2H2 > C2H6 > CO2 > C2H4 > CH4 under ambient conditions. It shows an outstanding dynamic breakthrough separation of ethylene (C2H4) not only for a binary mixture (C2H6/C2H4) but also for a quaternary combination (C2H4/C2H6/C2H2/CO2 and C2H4/C2H6/C2H2/CH4) of varying concentrations. The detailed separation/purification mechanism was unveiled by gas adsorption isotherms, mixed-gas adsorption calculations, selectivity estimations, advanced computer simulations such as density functional theory (DFT), grand canonical Monte Carlo (GCMC) and ab initio molecular dynamics (AIMD), and stepwise multicomponent dynamic breakthrough experiments.

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Conflict of interest statement

There are no conflicts to declare.

Figures

Scheme 1
Scheme 1. Schematic representation of the hydrothermal (left) and microwave heating (right) synthesis strategy for Al-MOF and Al-MOFM15, respectively. The corresponding insets are the FESEM images showing the morphologies of Al-MOF and Al-MOF15, respectively (see Fig. S2†). A small and a large pore channel are identified in green and yellow dashed circles, respectively. Ab initio molecular dynamics simulation of the de-solvated MOF at 293 K shows spontaneous closure of the small pore channels (extreme right).
Fig. 1
Fig. 1. (a) Single-component adsorption–desorption isotherms of C2H6 and C2H4 in Al-MOFM15 measured at 273 and 293 K for pressures 0–690 torr. (b) Isosteric heats of adsorption (Qst) of C2H6 and C2H4 at various loading amounts (near-zero coverage Qst values are provided in the inset). (c) Mixed adsorption isotherms and selectivity calculated using IAST for C2H6/C2H4 (50 : 50) in Al-MOFM15 at 293 K. (d) C2H6/C2H4 separation performance in some benchmark porous materials. (e and f) Locations of the highest binding affinity sites determined by DFT optimization for C2H6 (−39.9 kJ mol−1) and C2H4 (−38.7 kJ mol−1). Insets: Corresponding molecule locations, zoomed out, within the large channel. C2H6 participates in two hydrogen-bonding interactions with carboxylate oxygens while C2H4 participates in π–π and hydrogen-bonding interactions with the naphthalene ring and carboxylate oxygen, respectively. Binding sites with lower affinities for C2H4 and C2H6 are shown in Fig. S23 and S24, respectively.
Fig. 2
Fig. 2. (a) Single-component adsorption–desorption isotherms of C2H2 and CO2 in Al-MOFM15 measured at 273 and 293 K for pressures 0–690 torr. (b) Isosteric heats of adsorption (Qst) of C2H2 and CO2 at various loading amounts (Qst values at near-zero coverage area are provided in the inset). (c and d) Mixed adsorption isotherms and selectivity calculated using IAST for (c) C2H2/CO2 (50 : 50) and (d) C2H2/C2H4 (50 : 50) in Al-MOFM15 at 293 K. (e and f) Locations of the highest binding affinity sites determined by DFT optimization for C2H2 (−37.3 kJ mol−1) and CO2 (−36.0 kJ mol−1), respectively. Insets show the positions of the guest molecules within the square cross-section of a large pore, while the corresponding main panels show their neighbourhood. C2H2 participates in π–π and hydrogen-bonding interactions with a naphthalene ring and a carboxylate oxygen, respectively, while CO2 interacts with two naphthalene hydrogens and a carboxylate oxygen via hydrogen-bonding and Lewis acid–base interactions, respectively. Binding sites with lower affinities for CO2 and C2H2 are shown in Fig. S21 and S22, respectively.
Fig. 3
Fig. 3. (a) Experimental stepwise dynamic column breakthrough curves for 0.5 : 0.5 (v/v) C2H6/C2H4 gas mixture and (b) the corresponding maximum release concentration (outlet/feed; C/C0) in the outlet of each component with time for three consecutive cycles. (c) Experimental stepwise dynamic column breakthrough curves for 0.5 : 0.5 (v/v) C2H2/CO2 gas mixture and (d) the corresponding maximum release concentration (outlet/feed; C/C0) in the outlet of each component with time for three consecutive cycles. Quaternary mixture separations of (e) C2H4/C2H6/C2H2/CO2 (0.25 : 0.25 : 0.25 : 0.25) and (f) C2H4/C2H6/C2H2/CH4 (0.75 : 0.12 : 0.01 : 0.12). The continuous flow was regulated by a mass flow controller using helium as the carrier gas with a total flow rate of 2.2–2.9 and 3.2–3.5 mL min−1 for binary and quaternary separations, respectively. The breakthrough experiments were studied in an adsorbed bed packed with ∼1.048 g of Al-MOFM15 at 298 K and 1.05 bar. The packed column dimensions are 16.5 cm in length and 0.3 cm in diameter.

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