Water-addition timing switches MOF-808 formation in DES: Mechanistic insights from ab initio molecular dynamics and spectroscopic evidence

Abstract

Deep eutectic solvents (DESs) have recently emerged as a sustainable, biodegradable alternative to volatile organic solvents for metal–organic framework (MOF) fabrication. Here we address this knowledge gap by asking a simple question: does the timing of water introduction—before or after DES assembly—dictate the fate of MOF-808 formation? The vibrational spectroscopy (Fourier-transform infrared and Raman), differential scanning calorimetry (DSC), and ab initio molecular dynamics (AIMD) simulations were coupled to construct a time-resolved picture and the first atomistic explanation for this “timing” effect. Our data reveal that water added after DES preparation occupies pre-existing nanoscopic voids without perturbing the parent H-bond topology of DES. Consequently, 1,3,5-benzenetricarboxylic acid (BTC) linkers remain “dry”, are rapidly deprotonated by the quaternary ammonium cation, and coordinate Zr clusters generated by free-water-driven hydrolysis of the metal salt, enabling condensation into MOF-808. Conversely, water present during DES assembly integrates into the H-bond network, immobilizes BTC linkers within an extended hydrogen-bond web and traps the water in an inactive state. The water-added-after protocol offers a green, clean route to defect-rich MOFs without post-synthetic acid/base etching, aligning with circular-economy principles. Beyond the Zr-BTC system, these insights establish a general conceptual framework for managing water activity in DES-based MOF synthesis. The method can be extendable to the synthesis of other framework materials.