A Structure–Activity Study of Aromatic Acid Modulators for the Synthesis of Zirconium-Based Metal–Organic Frameworks

Abstract

Acid modulation is among the most widely employed methods for preparing metal–organic frameworks (MOFs) that are both stable and highly crystalline, yet there exist few guiding principles for selecting the optimal modulator for a given system. Using the Zr-based MOFs UiO-66 and UiO-68-Me2 (UiO = Universitetet i Oslo) as representative materials, here we present for the first time an in-depth structure–activity study of acid modulators and identify key principles of modulation for the synthesis of highly crystalline Zr-MOFs. By applying whole pattern fitting of powder X-ray diffraction patterns as a technique for evaluating modulator efficacy, complemented by scanning electron microscopy, 1H NMR, and thermogravimetric analysis (TGA), we demonstrate that the key to effective modulation is competition between the linker and modulator for coordination to the Zr secondary building units (SBUs). Specifically, we illustrate that a close match in pK a and structure between the linker and modulator favors larger and more well-defined crystallites, particularly with sterically unhindered aromatic acid modulators. Based on our findings, we demonstrate that 5-membered heteroaromatic carboxylic acids are among the most efficient acid modulators identified to date for the synthesis of several representative Zr-MOFs with fcu net topologies. In addition, we find that coordination modulation is superior to exogenous acid modulation at higher modulator concentrations. Finally, we compare 1H NMR and TGA as data-driven methods for quantifying linker deficiencies in modulated MOF syntheses. The guiding principles established herein have critical implications for the scalable and controllable synthesis of highly crystalline and stable MOFs relevant to chemical separations, gas storage, and catalysis.

Enhancing Dynamic Spectral Diffusion in Metal-Organic Frameworks Through Defect Engineering

“…For Form 1, a gradual broadening of the reflections due to a decrease in crystallinity was observed above 50 equiv., which is not expected given the increase in crystallinity that is generally observed when higher equivalents of acid modulators are employed. 25,29,35 In contrast, the crystallinity of Form 2 increased with increasing equivalents of FA, as expected. Because the role of acid modulators is generally to enhance the reversibility of MOF formation, 25 we hypothesize that Form 1 is a kinetic phase whereas Form 2 is the thermodynamically favored phase.…”

“…25,29,35 In contrast, the crystallinity of Form 2 increased with increasing equivalents of FA, as expected. Because the role of acid modulators is generally to enhance the reversibility of MOF formation, 25 we hypothesize that Form 1 is a kinetic phase whereas Form 2 is the thermodynamically favored phase. Consistently, postsynthetic treatment of Form 1 with excess FA converted it to Form 2, as confirmed by PXRD (SI Figure S29).…”

“…This is likely because AA is a better competitor with the linker for the Zr nodes, making MOF formation more reversible. 25,29 Consistently, BA induced the phase change at the lowest concentration (30 equiv.) among tested modulators, reflecting its general superiority as a modulator for the synthesis of Zr-MOFs.…”

“…[25][26][27][28][29][30][31][32][33][34][35] Generally, defect sites are inserted by coordination modulation, in which a monodentate competitor for the linker is added during MOF synthesis, leading to missing linker and/or node defects. 25,26 In particular, Zr-based frameworks exhibit high tolerance towards defects due to their highly interconnected structures. [35][36][37] Although defect engineering is widely employed to improve the catalytic activity, 38 gas binding affinity, 34 and porosity 29 of MOFs, its application for tuning MOF photophysical properties remains relatively unexplored.…”

Strategies for induced defects in metal–organic frameworks for enhancing adsorption and catalytic performance

“…72,74 Later on, the role of modulators was investigated in the synthesis of other MOFs such as the most stable Zr-MOFs (UiO-66, UiO-66–NH 2 , UiO-67, and UiO-68–NH 2 ). 32,75 In these MOFs, strong interactions between six zirconium cations in a square-antiprismatic geometry by bridging μ 3 -O and μ 3 -O –OH with twelve di-carboxylic groups of the ligands make very stable structures. It was indicated that acid modulators ( e.g.…”