Structural (XRD) and thermal (DSC, TGA) and BET analysis of materials derived from non-metal cation pentaborate salts

Abstract

The synthesis, structural characterization (XRD), and thermal properties of nine non-metal cation (NMC) pentaborate anion salts, [NMC][B₅O₆(OH)₄] (1a–1i) is described (NMC = [NH₃CMe₂(CH₂OH)]⁺ (a), [O(CH₂CH₂)₂NH₂]⁺ (b), [NH₃CMe(CH₂OH)₂]⁺ (c), [2-(2-CH₂CH₂OH)PyH]⁺ (d), [(CH₂)₄NH(CH₂CH₂OH)]⁺ (e), [(CH₂)₅NH(CH₂CH₂OH)]⁺ (f), [2-MeImid]⁺ (g), [Me₃NCMe₂(CH₂OH)]⁺ (h), [O(CH₂CH₂)₂NMe₂]⁺ (i). Single-crystal X-ray diffraction studies on all compounds show that they contain isolated pentaborate anions, H-bonded together in a supramolecular array, with the cations occupying the cavities within the network. Compound 1c was obtained as a partial hydrate (0.16H₂O). TGA and DSC analysis (in air, 25–1000 °C) indicate that compounds 1a–1i thermally decompose via a 2 stage process to B₂O₃. The first stage (<250 °C) is dehydration to condensed polymeric pentaborates {approximate composition: [NMC][B₅O₈] (2a–2i)}. Five condensed pentaborates (2a–c, 2e, 2g) were synthesised and characterized by powder XRD and BET analysis. These condensed pentaborates were amorphous. The isolated pentaborates intumesced at ∼600 °C (occupying ∼10 times their original volume), and then contracted back to black glassy B₂O₃ solids at 1000 °C. The intumescent materials (3a), (3b), (3e), (3g), and a final B₂O₃ sample (4b) were synthesised and isolated and their porosities determined. BET surface area analysis on the isolated pentaborates (1a–c, 1e, 1g), the condensed pentaborates (2a–c, 2e, 2g), intumesced materials (3a, 3b, 3e, 3g), and B₂O₃ (4b) showed that they were all ‘non-porous’ (<1.59 m² g⁻¹).