Thermodynamic and kinetic properties of molecular adsorption and transport in metal–organic frameworks (MOFs) are crucially important for many applications, including gas adsorption, filtration, and remediation of harmful chemicals. Using the in situ 1H nuclear magnetic resonance (NMR) isotherm technique, we measured macroscopic thermodynamic and kinetic properties such as isotherms and rates of mass transfer while simultaneously obtaining microscopic information revealed by adsorbed molecules via NMR. Upon investigating isopropyl alcohol adsorption in MOF UiO-66 by in situ NMR, we obtained separate isotherms for molecules adsorbed at distinct environments exhibiting distinct NMR characteristics. A mechanistic view of the adsorption process is obtained by correlating such resolved isotherms with the cage structure effect on the nucleus-independent chemical shift, molecular dynamics such as the crowding effect at high loading levels, and the loading level dependence of the mass transfer rate as measured by NMR and elucidated by classical Monte Carlo simulations.
Defect engineering leads to an effective manipulation of the physical and chemical properties of metal–organic frameworks (MOFs). Taking the common missing linker defect as an example, the defective MOF generally possesses larger pores and a greater surface area/volume ratio, both of which favor an increased amount of adsorption. When it comes to the self-diffusion of adsorbates in MOFs, however, the missing linker is a double-edged sword: the unsaturated metal sites, due to missing linkers, could interact more strongly with adsorbates and result in a slower self-diffusion. Therefore, it is of fundamental importance to evaluate the two competing factors and reveal which one is dominating, a faster self-diffusion due to larger volume or a slower self-diffusion owing to strong interactions at unsaturated sites. In this work, via Monte Carlo and molecular dynamics simulations, we investigate the behavior of isopropyl alcohol (IPA) in the Zr-based UiO-66 MOFs, with a specific focus on the missing linker effects. The results reveal that unsaturated Zr sites bind strongly with IPA molecules, which in return would significantly reduce the self-diffusion coefficient of IPA. Besides this, for the same level of missing linkers, the location of defective sites also makes a difference. We expect such a theoretical study will provide an in-depth understanding of self-diffusion under confinement, inspire better defect engineering strategics, and promote MOF based materials toward challenging real-life applications.
Reservoir wettability is a parameter of crucial importance for oil recovery1. However, its definition and its measurement are quite complex. For a flat surface, wettability is directly related to the contact angle of a water droplet on a surface controlled by the hydrophilicity of the surface. For an arbitrary surface, the contact angle, however, does not only depend on the hydrophilicity of the surface but also depends on the surface roughness at various length scales. Therefore, wetting is a macroscopic property that is only directly related to the hydrophilicity of the surface if the surface is flat and smooth. The hydrophilicity of the surface is a microscopic property that determined the surface interactions with water and hydrocarbons and plays a crucial role in oil production and recovery. The unknown surface roughness of internal surfaces of rock porous media makes wettability definition and measurement of such internal surfaces extremely challenging. Identifying a wettability index of porous media surfaces with broad applicability is a primary objective in oil and gas industry. Here we demonstrate an NMR-detected isotherm technique for measuring surface wettability of porous media. This technique is not only related directly to the traditional measure of wettability using macroscopic contact angles, it is also directly related to the microscopic surface property of hydrophilicity. It is shown that NMR-detected isotherms of both water and isopropanol (IPA) are needed to obtain the wettability index. Through systematic studies of quartz glass beads and quartz slides that are hydrophilic- or hydrophobic-modified, we established a quantitative relationship between the NMR isotherm-based wettability index and the traditional measure of wettability using contact angle. Therefore, the proposed wettability index derived from NMR isotherms provides a clear link to both macroscopic and microscopic properties of internal surfaces of porous media and can be a reliable measure of interactions between water and hydrocarbon with internal surfaces of rock porous media, as evidenced by preliminary studies of rock samples.
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