Evolution of Nanoscale Pore Structure in Coordination Polymers During Thermal and Chemical Exposure Revealed by Positron Annihilation

Abstract

Over the past decade, nanostructured materials constructed from metal ions/clusters linked by organic groups were demonstrated to have remarkably high porosity and specific surface areas higher than the best activated carbons. [1][2][3][4][5] These microporous coordination polymers (MCPs), are self-assembled, periodic, porous structures that have redefined what is possible with adsorption. [6][7][8][9] Hydrogen storage and CO 2 sequestration are two of the most intensively studied areas with record-setting capacities achieved for several MCPs. [10][11][12] Despite these performance advantages, MCPs suffer from an incomplete understanding of the fundamental mechanisms of adsorption; furthermore problems with structural integrity and poor atmospheric/ temperature stability are compounded by difficulties in characterizing structural damage. Although X-ray diffraction and gas-adsorption techniques have facilitated the development of MCPs, these methods give a structurally averaged picture of the pore structure and are therefore ill-suited to study defects and other nonperiodic phenomena of critical importance for future applications. In particular, their value for monitoring pore structure evolution under conditions directly relevant for sorption applications is limited. Positronium annihilation lifetime spectroscopy (PALS) is an in situ pore-/void-volume characterization technique, [13] in which the shortening of the annihilation lifetime of Ps (Ps ¼ positronium, the hydrogenlike bound state of an electron with its antiparticle, the positron) due to collisions with the pore walls is correlated with the pore size. Ps readily forms by electron capture when positrons are injected into insulators. Moreover, it is energetically favorable for Ps to localize inside voids providing a natural probe from within. The Ps lifetime is correlated with pore size [14][15][16][17] and the relative intensity of this component is related to the porosity of the material. In this first application of PALS to MCPs we demonstrate its unique ability to uncover new phenomena in these materials with nanoscale porosity.MOF-5 [18] (MOF ¼ metal-organic framework) is one of the earliest examples of an exceptionally high surface area MCP and the most thoroughly investigated representative of this class. Therefore, it is an ideal benchmark to test the suitability of PALS and presents an extremely challenging system, in which to reveal new phenomena. The material is an open cubic structure that consists of face-sharing cubic cages that extend in all three dimensions (see Fig. S1 in the Supporting Information). A high-quality sample ($1 cm 3 ) of MOF-5 crystals ($0.5-mm cubic crystals) with a Brunauer-Emmett-Teller theory (BET) surface area of 3500 m 2 g À1 was prepared for the PALS experiments. Ps, formed via electron capture by a positron in this MCP framework, is energetically driven into the pores. Fitting of the PALS time spectrum (a typical spectrum is Fig. S2 in the Supporting Information) reveals, surprisingly in the light of the supposedly u...

Investigation on positron annihilation characteristics of CO2-exposed zeolite

“…3, the CO 2 density in the a-cage was more than 20 amagat. This proves that the CO 2 molecules that formed cluster (liquidlike) were at pressures greater than 5 atm [17,38]. The presence of CO 2 molecules and their clusters in the a-cage induces shielding effect, which increased with CO 2 pressure.…”

“…Except for membrane separation, currently, the absorption technology [4,5,7,8], apart from membrane separation processes, is also used to remove CO 2 from gas mixtures using absorbents such as amine and porous inorganic materials, which do not exhibit CO 2 -induced plasticization, like zeolites and novel materials with high porosities, such as MOF (metal-organic framework) [16,17] being considered for adsorption applications at present. Therefore, it is very important to understand the architecture such as pore size/concentration in materials, especially at gas-exposed conditions.…”

“…There are a lot of literatures on positron annihilation characteristics in different gas-exposed zeolites using positron annihilation lifetime (PAL) [21,[26][27][28]. However, studies on CO 2 sorption behavior in the cavities of zeolite are rare [17].…”

Correlation of Gas Permeability in a Metal-Organic Framework MIL-101(Cr)–Polysulfone Mixed-Matrix Membrane with Free Volume Measurements by Positron Annihilation Lifetime Spectroscopy (PALS)

“…Table 1 lists the diameters, the radii subtracted with the van der Waals radii of 0.14 nm and the expected o-Ps lifetimes due to the Tao-Eldrup model (see Section 2.6). As already indicated in the literature [103], there are longer lifetime contributions (~80 ns) in the spectra, which might arise from delocalized positronium states and, thus, represent an “artefact” of the measurement. We evaluated the corresponding spectra for the pure MOF with five and six components, the corresponding results are summarized in Table S6 and Table S7.…”

Defects and Disorder in MOFs

“…The 3D defects alter the crystal pattern over a finite volume. They might include precipitates, large voids, or inclusions of second-phase particles [78][79][80][81].…”