Heat capacity and thermodynamic functions of boehmite (AlOOH) and silica-doped boehmite

Alumina has a variety of technological uses and has been utilized in many fields due to its versatility and abundance. The properties of aluminas, such as pore size, acid site concentration, and the amount of adsorbed water, can be tailored for a specific application using a variety of synthetic techniques. Doping alumina with silica has been shown to improve the thermal stability of the material and allows it to be used in higher temperature applications. Several synthetic routes to γ-Al₂O₃ involves the formation of a boehmite precursor that is later calcined at different temperatures to form various porous transitional aluminas. Silica-doped alumina can be prepared from a silica-doped boehmite precursor. We have measured the constant pressure heat capacities of a pure boehmite and a silica-doped boehmite alumina with the chemical formulas AlOOH·0.202H₂O and AlOOH·0.045SiO₂·0.284H₂O, respectively. Molar heat capacities were measured from 1.8 K to 300 K using a Quantum Design Physical Property Measurement System (PPMS), and the data was fit to a theoretical functions below 15 K, orthogonal polynomials from 10 K to 60 K, and a combination of Debye and Einstein functions above 50 K. These fits were then used to generate C_p,m°, Δ₀^TS_m°, Δ₀^TH_m°, and Φ_m° values at smoothed temperatures from 0 K to 300 K for all samples. The differences in the thermodynamic functions between both samples is attributed to the presence of silica in one of the samples. Results from this study are consistent with the results from a previous study comparing silica-doped γ-Al₂O₃ samples to pure γ-Al₂O₃ samples.