Unusual and Highly Tunable Missing-Linker Defects in Zirconium Metal–Organic Framework UiO-66 and Their Important Effects on Gas Adsorption

TLDR

UiO‑66 is a prototypical zirconium metal‑organic framework renowned for its exceptional stability, with each Zr center fully coordinated by 12 organic linkers in the ideal crystal structure. The study aims to systematically tune missing‑linker defects in UiO‑66 by adjusting acetic acid modulator concentration and synthesis time to enhance porosity. The authors varied acetic acid modulator levels and synthesis duration to control linker vacancies, thereby increasing the framework’s porosity. They found that UiO‑66 contains ~10 % missing‑linker defects, and by tuning these vacancies the authors achieved pore volumes up to 1.0 cm³ g⁻¹ and BET areas up to 1600 m² g⁻¹—about 150 % and 60 % higher than defect‑free UiO‑66—and observed that defect‑rich, hydroxylated samples exhibit markedly improved CO₂ adsorption due to stronger metal‑center binding confirmed by neutron diffraction and first‑principles calculations.

Abstract

UiO-66 is a highly important prototypical zirconium metal–organic framework (MOF) compound because of its excellent stabilities not typically found in common porous MOFs. In its perfect crystal structure, each Zr metal center is fully coordinated by 12 organic linkers to form a highly connected framework. Using high-resolution neutron power diffraction technique, we found the first direct structural evidence showing that real UiO-66 material contains significant amount of missing-linker defects, an unusual phenomenon for MOFs. The concentration of the missing-linker defects is surprisingly high, ∼10% in our sample, effectively reducing the framework connection from 12 to ∼11. We show that by varying the concentration of the acetic acid modulator and the synthesis time, the linker vacancies can be tuned systematically, leading to dramatically enhanced porosity. We obtained samples with pore volumes ranging from 0.44 to 1.0 cm3/g and Brunauer–Emmett–Teller surface areas ranging from 1000 to 1600 m2/g, the largest values of which are ∼150% and ∼60% higher than the theoretical values of defect-free UiO-66 crystal, respectively. The linker vacancies also have profound effects on the gas adsorption behaviors of UiO-66, in particular CO2. Finally, comparing the gas adsorption of hydroxylated and dehydroxylated UiO-66, we found that the former performs systematically better than the latter (particularly for CO2) suggesting the beneficial effect of the −OH groups. This finding is of great importance because hydroxylated UiO-66 is the practically more relevant, non-air-sensitive form of this MOF. The preferred gas adsorption on the metal center was confirmed by neutron diffraction measurements, and the gas binding strength enhancement by the −OH group was further supported by our first-principles calculations.