Synthesis and hydrogen adsorption properties of internally polarized 2,6-azulenedicarboxylate based metal–organic frameworks

Abstract

To improve the binding energy of hydrogen, incorporation of internally polarized organic units into metal–organic frameworks (MOFs) should be a promising strategy. In this study, two novel MOFs composed of internally polarized 2,6-azulenedicarboxylate (2,6-azd), termed MOF-649 [Zn₂(2,6-azd)₂(dabco), where dabco = 1,4-diazabicyclo[2.2.2]octane] and MOF-650 [Zn₄O(2,6-azd)₃], have been synthesized, and their crystal structures were determined by single-crystal X-ray diffraction analyses. Both materials displayed permanent microporosity, and the Brunauer–Emmett–Teller (BET) surface areas of MOF-649 and MOF-650 are estimated to be 910 and 2630 m² g⁻¹, respectively. The H₂ adsorption measurements showed that MOF-650 adsorbs 14.8 mg g⁻¹ of hydrogen at 77 K and 1 bar. The polarization effect of the azulene unit in the framework is supported by high initial isosteric heat of adsorption of 6.8 kJ mol⁻¹ for MOF-650. A detailed computational analysis using density functional theory was carried out in order to investigate the structure and electronic properties of MOF-650 and subsequently to understand its site-specific interactions with hydrogen.