Thermodynamic properties of silicalite SiO 2

Abstract

Silicalite, a polymorph of SiO 2, is a new and interesting molecular-sieve material. Low-temperature (5 to 350 K) heat-capacity, flourine-combustion, high-temperature (350 to 1500 K) drop-calorimetric, and reaction-calorimetric with HF(aq) measurements have been performed on this material. At T′ = 298.15 K, Δ f H m o( T′) = −(905.20±0.84) kJ·mol −1; C p, m o( T′) = (46.34±0.23) J·K −1·mol −1; { Δ 0 T′ S m o( T′)} = (46.29±0.23) J·K −1·mol −1; and Δ f G m o( T′) = −(852.23±0.84) kJ·mol −1. The enthalpies of reaction with 24.4 mass per cent hydrofluoric acid of silicalite at mass concentrations of 0.5, 1, 2, and 4 g·dm −3 were determined to be −(144.91±0.23), −(144.80±0.10), −(145.07±0.12), and −(145.06±0.14) kJ·mol −1, respectively. The enthalpies of reaction of silicalite with 20.1 and 15.0 mass per cent hydrofluoric acid at a silicalite mass concentration of 1 g·dm −3 were determined to be −(143.85±0.19) and −(142.68±0.08) kJ·mol −1, respectively. Because silicalite reacts rapidly with HF(aq) at 298.15 K, it has potential as a reference material for thermochemical measurements on inorganic silicates and silicate minerals. Silicalite is less stable than quartz, cristobalite, or tridymite, but more stable than SiO 2(glass) or the high-pressure forms coesite or stishovite.