Reversible Binding of Nitric Oxide in a Cu(II)-Containing Microporous Metal-Organic Framework

Abstract

We studied the adsorption thermodynamics and mechanism behind the binding of nitric oxide (NO) in the interior surfaces and structural fragments of the high metal center density microporous Metal-Organic Framework (MOF) CPO-27-Cu, by gas sorption, at a series of temperatures. For the purpose of comparison, we also measured the corresponding CO₂ adsorption isotherms, and as a result, the isosteric heats of adsorption for the two studied adsorptives were derived, being in the range of 12-15 kJ/mol for NO at loadings up to 0.5 NO molecules per formula unit (f.u.) of the bare compound (C₄O₃HCu), and 23-25 kJ/mol CO₂ in the range 0-1 CO₂ per f.u. Microscopically, the mode of NO binding near the square pyramid Cu(II) centers was directly accessed with the use of in situ NO gas adsorption X-ray Absorption Spectroscopy (XAS). Additionally, during the vacuum/temperature activation of the material and consequent NO adsorption, the electronic state of the Cu-species was monitored by observing the corresponding X-ray Near Edge Spectra (XANES). Contrary to the previously anticipated chemisorption mechanism for NO binding at Cu(II) species, we found that at slightly elevated temperatures, under ambient, but also cryogenic conditions, only relatively weak physisorption takes place, with no evidence for a particular adsorption preference to the coordinatively unsaturated Cu-centers of the material.

Keywords: EXAFS; adsorption; gas storage; microporosity; nitric oxide; thermodynamics.

Figure 1

The micropore structure of CPO-27-Cu in (a,b) a cluster cut out from the corresponding hexagonal Bravais unit cell-based supercell, representing the chain sequence of Cu(II) coordinatively unsaturated centers as potential adsorption sites that were specifically probed by XAS. Structures visualized with Mercury 4.0 [31].

Figure 2

(a) Nitric oxide adsorption isotherms at a series of temperatures near ambient, along with a CO₂ adsorption isotherm at 283 K for comparison; (b) Carbon dioxide adsorption isotherms in the same temperature range as indicated in the figure legends.

Figure 3

The isosteric heats of adsorption for carbon dioxide and nitric oxide in the CPO-27-Cu materials as derived from the adsorption isotherm data presented in Figure 2.

Figure 4

XANES spectra of the CPO-27-Cu material under different conditions as described in the figure legend. The last spectrum was collected after the adsorbed NO was evacuated at 363 K, during dynamic vacuum and the sample temperature was near the ambient.

Figure 5

The corresponding EXAFS spectra collected during the activation of the material in vacuum and up to 363 K (after 3 h), as shown in the figure legend.

Figure 6

Experimental data and the corresponding fits to the k²-weighted Fourier-transformed EXAFS function for (a) the activated compound, cooled down to 130 K; (b) the same under an equilibrium pressure of 900 mbar; and (c) further cooled to 100 K, with the gas pressure reduced to 5 mbar.

Figure 7

The XANES spectrum of the high temperature 1 bar NO loading—red line, compared to the spectrum of the first 900 mbar NO loading—blue line, after its corresponding first activation in a dynamic vacuum at 363 K—green line, and the final measurement of the spectrum under a dynamic vacuum at 363 K—black line.