Overcoming Crystallinity Limitations of Aluminium Metal–Organic Frameworks by Oxalic Acid Modulated Synthesis

Abstract

Am odulateds ynthesis approach based on the chelating properties of oxalic acid (H 2 C 2 O 4 )i sp resented as a robust andv ersatile methodt oa chieveh ighly crystalline Albased metal-organic frameworks. Ac omparatives tudy on this method and the already established modulation by hydrofluoric acid was conducted using MIL-53 as test system. The superior performance of oxalic acid modulation in terms of crystallinity and absence of undesired impurities is ex-plained by assessing the coordination modes of the two modulators and the structural features of the product. The validity of our approach was confirmed for ad iverse set of Al-MOFs,n amely X-MIL-53 (X = OH, CH 3 O, Br,N O 2 ), CAU-10, MIL-69, andA l(OH)ndc (ndc = 1,4-naphtalenedicarboxylate), highlighting the potentialb enefits of extending the use of this modulator to other coordination materials.[a] Dr.[ + + ] These authors contributed equally.[**] Ap revious versiono ft his manuscript has been deposited on ap reprint server (https://doi.org/10.26434/chemrxiv.9999572.v1).Supporting information and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.org/10.Scheme2.The reaction formulas for MIL-53 synthesis modulated by oxalic acid (a) and hydrofluoric acid (b).

Robust and Environmentally Friendly MOFs

“…This is done in particular when the metal-ion is highly oxophilic in order to limit the formation of metal oxides or hydroxides and to maintain a minimal concentration of metal cations needed for the formation of hybrid frameworks in solution. Moreover, the use of monotopic carboxylic acids [28] or more recently oxalates [29] has also shown to be very useful for a better control of the MOF crystallization. These molecules (generally monotopic ligands, but not limited to) are known as modulators (or inhibitors) that are not expected to bridge the IBUs but to compete with the polytopic linker helping in controlling and slowing-down the nucleation-growth processes and yielding to larger crystallites.…”