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][B(5)O(6)(OH)(4)] (1a-1i) is described (NMC = [NH(3)CMe(2)(CH(2)OH)](+) (a), [O(CH(2)CH(2))(2)NH(2)](+) (b), [NH(3)CMe(CH(2)OH)(2)](+) (c), [2-(2-CH(2)CH(2)OH)PyH](+) (d), [(CH(2))(4)NH(CH(2)CH(2)OH)](+) (e), [(CH(2))(5)NH(CH(2)CH(2)OH)](+) (f), [2-MeImid](+) (g), [Me(3)NCMe(2)(CH(2)OH)](+) (h), [O(CH(2)CH(2))(2)NMe(2)](+) (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 (0.16H(2)O). TGA and DSC analysis (in air, 25-1000 degrees C) indicate that compounds 1a-1i thermally decompose via a 2 stage process to B(2)O(3). The first stage (<250 degrees C) is dehydration to condensed polymeric pentaborates {approximate composition: [NMC][B(5)O(8)] (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 approximately 600 degrees C (occupying approximately 10 times their original volume), and then contracted back to black glassy B(2)O(3) solids at 1000 degrees C. The intumescent materials (3a), (3b), (3e), (3g), and a final B(2)O(3) 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 B(2)O(3) (4b) showed that they were all 'non-porous' (<1.59 m(2) g(-1)).