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

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.

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. (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 (Q_st) of C₂H₆ and C₂H₄ at various loading amounts (near-zero coverage Q_st 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. (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 (Q_st) of C₂H₂ and CO₂ at various loading amounts (Q_st 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. (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.