Tailoring Lewis Acid Properties of Metal–Organic Framework Nodes via Anion Post-Replacement for Gas Adsorption and Separation

Abstract

Metal–organic frameworks (MOFs) with open metal sites (OMSs) have many applications from sensing, catalysis, to gas adsorption and separation. The OMSs have Lewis acid nature, which are capable of accepting electron density from guest molecules, thus enabling selective gas adsorption. The Lewis acid strength of OMSs plays a pivotal role in the adsorption properties of MOFs. In this work, we report a general method capable of adjusting the Lewis acid strength of OMSs in a cationic framework MIL-101. Relying on a simple anion post-replacement, the chemical environment around the metal centers can be regulated effectively, thus achieving the purpose of the Lewis acid strength adjustment. The Lewis acid properties of OMSs have been studied using different characterization techniques, including Fourier transform infrared, X-ray photoelectron spectroscopy, ultraviolet–visible, and in situ IR spectroscopic analysis, combined with a molecular electrostatic potential calculation. The effect of the Lewis acid strength of the OMSs for gas adsorption and separation was validated by adsorption experiments. Gas sorption isotherms reveal that the modified materials show reverse N2, CH4 selectivity with pristine MIL-101-NO3, which was mainly attributed to the different sensitivity of N2 and CH4 to the open Cr sites, as evidenced by density functional theory calculations. Furthermore, the modified material, MIL-101-Cl, exhibited by far the highest N2O adsorption capacity (136 cm3 g–1 at 1.0 bar and 298 K) and benchmark CO2 uptake. Our method will facilitate further deliberate postsynthetic modifications of other MOFs with OMSs to improve them for applications in chemical separation processes.

Non-CO2 greenhouse gas separation using advanced porous materials

“…22a). 246 Operando IR spectroscopic analysis using CO as a probe revealed that the Lewis acidity of MIL-101-X samples was in the sequence MIL-101-Cl > MIL-101-F > MIL-101-COOH > MIL-101-NO 3 , which is the same as the order of N 2 O adsorption capacity (Fig. 22c).…”

Highly Efficient and Reusable Denitrogenation Adsorbent Obtained by the Fluorination of PMA-MIL-101

“…From Figure b, we can find that the peak intensities of free carboxylic acids at the positions of 1666 cm –1 (stretching vibration of CO in the carboxyl group –COOH) and 1444 cm –1 (deformation vibration of –OH in –COOH) disappear after NH 4 F soaking, indicating that the uncoordinated carboxylic acids in M101/P-M101 pores have been washed away by NH 4 F. The two strong bands of the samples at 1620 and 1404 cm –1 can be attributed to typical νas (–COO −) and νs (–COO – ) of carboxylate, respectively. The positions of 662 and 584 cm –1 belong to the O–Cr–O absorption peaks of BDC connected with Cr. , It is worth noting that one of the two vibrational strengths of O–Cr–O was weakened after NH 4 F modification. This result suggests that coordination around the Cr(III) centers may change.…”

“…The positions of 662 and 584 cm –1 belong to the O–Cr–O absorption peaks of BDC connected with Cr. 31 , 32 It is worth noting that one of the two vibrational strengths of O–Cr–O was weakened after NH 4 F modification. This result suggests that coordination around the Cr(III) centers may change.…”

Effect of Desorption Damage on the Kinetic Characteristics of Coal Particle Gas Desorption

“…Taking P / P 0 = 0.5 as the dividing point, when P / P 0 < 0.5, the adsorption of gas in the coal depends on the van der Waals force, and the adsorption amount has a strong correlation with the surface roughness of the coal, so the fractal dimension D 1 can be used to reflect the pore surface roughness. When P / P 0 > 0.5, the gas adsorption is mainly controlled by multilayer adsorption and depends on the volume structure of the pores. − The more complex the pore volume structure is, the stronger the adsorption capacity is, so the fractal dimension D 2 can reflect the complexity of the pore volume structure. − The fractal dimensions D 1 and D 2 should be between 2 and 3. The closer D 1 and D 2 are to 3, the greater the pore surface roughness and pore volume structure complexity are.…”

“…When P / P 0 > 0.5, the gas adsorption is mainly controlled by multilayer adsorption and depends on the volume structure of the pores. 39 − 43 The more complex the pore volume structure is, the stronger the adsorption capacity is, so the fractal dimension D 2 can reflect the complexity of the pore volume structure. 44 − 47 The fractal dimensions D 1 and D 2 should be between 2 and 3.…”