Mechanochemical Synthesis, Accelerated Aging, and Thermodynamic Stability of the Organic Mineral Paceite and Its Cadmium Analogue

Shaodi Li,Hatem M Titi,Igor Huskić,Novendra Novendra,Alexandra Navrotsky,Tomislav Friščić

doi:10.1021/acsomega.9b00295

Shaodi Li, Hatem M Titi + Show 4 more

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https://doi.org/10.1021/acsomega.9b00295

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Journal: ACS omega	Publication Date: Mar 19, 2019
Citations: 15	License type: cc-by

Affiliation: McGill University, University of California, Davis

Abstract

We demonstrate the use of ball milling mechanochemistry for rapid, simple, and materials-efficient synthesis of the organic mineral paceite CaCu(OAc)4·6H2O (where OAc– is the acetate ion), composed of coordination polymer chains containing alternating Ca2+ and Cu2+ ions, as well as its cadmium-based analogue CaCd(OAc)4·6H2O. While the synthesis of paceite in aqueous solutions requires a high excess of the copper precursor, mechanochemistry permits the use of stoichiometric amounts of reagents, as well as the use of poorly soluble and readily accessible calcium carbonate or hydroxide reactants. As established by thermochemical measurements, enthalpies of formation of both synthetic paceite and its cadmium analogue relevant to the mechanochemical reactions are highly exothermic. Reactions can also be conducted using accelerated aging, a synthetic technique that mimics geological processes of mineral weathering. Accelerated aging reactivity involving copper(II) acetate monohydrate (hoganite) and calcium carbonate (calcite) provides a potential explanation of how complex organic minerals like paceite could form in a geological environment.

Highlights

Over the past 10 years, structural studies of geological samples have identified an increasing number of organic minerals, broadly defined as carbon-bearing mineral species other than carbonates and carbon allotropes.[1,2] Principal representatives of such organic minerals are transition metal and lanthanide oxalates,[3,4] as well as metal formates,[5] mellitates,[6] acetates,[1,2] and organic molecules such as uric acid.[7−9] The search for organic minerals and a deeper understanding of their structures and origins have been promoted by the recent “Carbon Mineral Challenge” initiative, which considers organic minerals, while extremely poorly investigated, as central to understanding mineral evolution and carbon cycle on Earth and other planetary systems.[10,11] The vibrant investigations of organic minerals have recently led to the structural characterization of geoporphyrins,[12] discovery of naturally occurring metal−organic frameworks (MOFs),[13,14] and proposed geological appearance of organic cocrystals[15] in extraterrestrial environments such as Titan.[16]Recent additions to the library of organic minerals are copper(II) acetate monohydrate, Cu(OAc)2·H2O, found as the mineral hoganite in the Potosi Mine in New South Wales (Australia), and paceite, the related double salt of calcium copper(II) acetate hexahydrate, CaCu(OAc)4·6H2O, found as dark blue crystals growing on the surface of hoganite specimens.[17]
Such stoichiometry control was observed for mechanochemistry by neat milling, as well as by liquid-assisted grinding (LAG), a methodology that uses a small amount of a liquid phase to enable and accelerate mechanochemical reactions.[32]
We have demonstrated the use of mechanochemistry for the rapid, simple, and materials-efficient preparation of synthetic samples of the organic mineral paceite and its cadmium analogue, CaCd(OAc)4·6H2O

Summary

Introduction

Over the past 10 years, structural studies of geological samples have identified an increasing number of organic minerals, broadly defined as carbon-bearing mineral species other than carbonates and carbon allotropes.[1,2] Principal representatives of such organic minerals are transition metal and lanthanide oxalates,[3,4] as well as metal formates,[5] mellitates,[6] acetates,[1,2] and organic molecules such as uric acid.[7−9] The search for organic minerals and a deeper understanding of their structures and origins have been promoted by the recent “Carbon Mineral Challenge” initiative, which considers organic minerals, while extremely poorly investigated, as central to understanding mineral evolution and carbon cycle on Earth and other planetary systems.[10,11] The vibrant investigations of organic minerals have recently led to the structural characterization of geoporphyrins,[12] discovery of naturally occurring metal−organic frameworks (MOFs),[13,14] and proposed geological appearance of organic cocrystals[15] in extraterrestrial environments such as Titan.[16]Recent additions to the library of organic minerals are copper(II) acetate monohydrate, Cu(OAc)2·H2O, found as the mineral hoganite in the Potosi Mine in New South Wales (Australia), and paceite, the related double salt of calcium copper(II) acetate hexahydrate, CaCu(OAc)4·6H2O, found as dark blue crystals growing on the surface of hoganite specimens.[17]. Milling 0.5 mmol of Ca(OH)[2] for 30 min with 60 μL of AcOH and 10 μL of water led to the complete disappearance of the solid reactant and formation of hydrated calcium acetate of the formula Ca(OAc)2·H2O (CSD code CEJLIM), identified by powder X-ray diffraction (PXRD) and thermogravimetric analysis (TGA) (Figure 2, see the Supporting Information).

Results

Conclusion