Acetylenedicarboxylate and In Situ Generated Chlorofumarate-Based Hafnium(IV)–Metal–Organic Frameworks: Synthesis, Structure, and Sorption Properties

Abstract

New acetylenedicarboxylate (ADC) and chlorofumarate (Fum-Cl) based hafnium-metal-organic frameworks have been synthesized by alternatively reacting acetylenedicarboxylic acid in DMF or water with appropriate hafnium salt, in the presence of acetic acid modulator. The two materials of respective ideal formulas [Hf₆O₄(OH)₄(ADC)₆] (Hf-HHU-1) and [Hf₆O₄(OH)₄(Fum-Cl)₆] (Hf-HHU-2) have been structurally characterized by powder X-ray diffraction to be UiO-66 isostructural, consisting of octahedral [Hf₆O₄(OH)₄]¹²⁺ secondary building units each connected to other units by 12 ADC or Fum-Cl linkers into a microporous network with <b>fcu</b> topology. This structure was confirmed by Rietveld refinement. Hf-HHU-2 is formed by <i>in situ</i> hydrochlorination of acetylenedicarboxylic acid to chlorofumarate. Its presence has been determined by combined Raman spectroscopy, solid-state NMR, scanning electron microscopy, energy dispersive X-ray and X-ray photoelectron spectroscopies. Hf-HHU-1 and Hf-HHU-2 exhibit very high hydrophilicity as revealed by their water sorption profiles, meanwhile Hf-HHU-2 adsorbs CO₂ with an isosteric heat of 39 kJ mol^-1. Hf-HHU-2 also adsorbs molecular iodine vapor exclusively as polyiodide anions due to grafted chloro-functions on the pores surface. It has been observed that defective nanodomains with <b>reo</b> tolopology can be introduced in the structure of Hf-HHU-2 by variation of the linker to metal-salt molar ratio.