Thermochemical data for n-alkylmonoamine intercalation into crystalline lamellar zirconium phenylphosphonate

Layered crystalline zirconium phenylphosphonate, Zr(O₃PC₆H₅)₂, changed its interlamellar distance of 1481 pm after intercalation of n-alkylmonoamines, CH₃---(CH₂)_n---NH₂ (n=0–6). The infrared spectra of the precursor host and the corresponding intercalated compounds presented vibrations associated with PO₃ groups in the 1163–1039 cm⁻¹ range and additional bands related to C---H stretching bands in the 2950–2850 cm⁻¹ interval were observed after amine insertion. The thermogravimetric curves showed a mass loss assigned to the phenyl group; however, the amine intercalated fraction was not quantitatively determined. A peak in the ³¹P NMR spectrum centered at −6 ppm for the host was observed. The surface area was 42.0±0.2 m² g⁻¹ and the scanning electron micrograph gave images consistent with lamellar structural features. The layered compound was calorimetrically titrated with amine in ethanol, requiring three independent operations: (i) titration of matrix with amine, (ii) matrix salvation, and (iii) dilution of the amine solution. From those thermal effects the variation in enthalpy was calculated as: −41±1.00,−33.28±0.50,−34.40±0.80,−10.40±0.40,−12.40±0.42,−16.10±0.08 and −7.0±0.04 kJ mol⁻¹, for n=0–6, respectively. The exothermic enthalpic values reflected a favorable energetic process of amine–host intercalation in ethanol. The negative Gibbs free energy results supported the spontaneity of all these intercalation reactions. The positive favorable entropic values, as carbon chain size increased, are in agreement with the free solvent molecules in the medium, as the amines are progressively bonded to the crystalline lamellar inorganic matrix at the solid/liquid interface.