Thermochemistry of the simplest metal organic frameworks: Formates [M(HCOO)2]·xH2O (M = Li, Mg, Mn, Co, Ni, and Zn)
Introduction
In the past decade, thousands of metal-organic frameworks (MOFs) with intriguing topologies have been explored and documented in the literature [1], [2], [3], [4]. The metal formates (MF) constitute the simplest group of MOFs, having the smallest organic moiety derived from the simplest carboxylic acid, formic acid, HCOOH [5]. Thus they can be synthesized readily without expensive organic ligands. They appear stable in humid conditions and can be activated (dehydrated) easily by mild heating to ∼150 °C. The magnetic coupling arising due to the geometrical orientation of formate anions linked between the magnetic metal sites in different bridging modes and through single O atom are very intriguing [6]. Indeed, these bridging modes mediate the ferro- or antiferromagnetic coupling between metal ions at different temperatures and pressures [7], [8], [9]. Thus metal formates with general formula M(HCOO)2·xH2O have been studied as magnetic materials for several decades. Most of the metal formates are isostructural or similar in structure. They have three-dimensional frameworks comprised of layers of M(HCOO)2 linked by trans-M(H2O)4(HCOO)2 units [10] (see Fig. 1), where the metal atoms are octahedrally coordinated with O atoms of formate. They exhibit predominantly long-range antiferromagnetic ordering. They are suggested for applications in gas adsorption [11], [12], [13], [14], as lithium ion battery anode materials [15], [16], and in the storage of small molecules such as hydrogen, carbon dioxide and acetylene [11]. Forster and co-workers recently reported that nanoporous metal formates exhibit selective adsorption of noble gasses [11]. Saravanan et al. [15] investigated the electrochemical performance of metal formates with diamondoid structure—zinc formate [Zn3(HCOO)6], cobalt formate Co3(HCOO)6 and mixed zinc-cobalt formates Zn1.5Co1.5(HCOO)6.
The following mechanism was proposed for the charge-discharge process.
Given their importance in several areas, it is worth investigating the thermodynamic properties of metal formates in order to understand their ease of synthesis and their stability during use. In this work, we carried out the synthesis of a series of metal formates: Li(HCOO), Mg(HCOO)2, Mn(HCOO)2, Co(HCOO)2, Ni(HCOO)2 and Zn(HCOO)2 based on work by other authors [17], [18] and characterized them. We report their formation enthalpies based on direct measurement using room temperature acid solution calorimetry. We compare the results with our recently published work on formate hybrid perovskites [19].These hybrid perovskites are alkali templated metal formates which have gained considerable interest owing to their weak ferromagnetism and other interesting properties [20], [21], [22].
Section snippets
Experimental methods
Formates of Li, Mn, Ni, Co and Zn were synthesized by precipitation by formic acid from a dissolved transition-metal carbonate solution followed by drying. Magnesium oxide was used for magnesium formate [17], [18]. Powder X-ray diffraction patterns (PXRD) were recorded using a Bruker-AXS D8 Advance diffractometer. Thermogravimetric (TG) analysis was performed using a Netzsch 449 TG/DSC instrument. A Calorimetry Sciences Corporation (CSC) 4400 isothermal microcalorimeter operated with IMC data
Results and discussion
PXRD patterns of all the as-synthesized MFs (M = Li, Mg, Mn, Co, Ni, and Zn) are shown in Fig. 2. They match the available ICSD data for hydrated forms of metal formates (PDF No – Li: 00-011-0826, Mg: 00-001-0252, Mn: 00-018-0823, Co: 00-021-0257, Ni: 00-010-0715, Zn: 00-014-0761).
Thermogravimetric (TG) curves of all the samples are shown in Fig. 3. All the MFs show two-step mass losses. For M = Mn, Co, Ni and Zn, the two mass losses occur at ∼100 and ∼250 °C. The first mass loss around 100 °C
Conclusion
We determined the enthalpy of formation of the simplest metal organic frameworks – metal formates from respective oxides/chlorides and formic acid using acid solution calorimetry. Enthalpy of formation for the four isostructural transition metal formates shows that the Zn analog is most stable, followed by Mn, Co, Ni and among the alkali metal formates Mg-analog is more stable than Li. For all the metal formates the formation enthalpy calculated from oxides is exothermic, indicating that they
Acknowledgments
This work was supported by the U.S. Department of Energy, grant DEFG0203ER46053.
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