Adsorption of carbon dioxide on TEPA-modified TiO₂/titanate composite nano-rods

A titanate-TiO₂ composite was obtained through hydrothermal treatment of TiO₂ in KOH solution. The presence of a titanate phase was confirmed by X-ray diffraction (XRD), whereas scanning electron microscopy (SEM) measurements showed the porous nano-rod structure of the material. The obtained nano-rods were treated with tetra-ethylene-pentamine (TEPA). Such synthesized sorbents were applied for CO₂ removal. The CO₂ capacity under a pressure of 1 bar and at 80 C was 0.47, 0.34, and 3.11 mmol.g^-1 for the starting TiO₂, the titanate-TiO₂ composite and the TEPA-titanate-TiO₂ composite (27.4 wt% of TEPA), respectively. The experimental isotherms of CO₂ were analysed using the Langmuir, Freundlich, Sips, Toth, Unilan, Redlich-Peterson, Radke-Prausnitz, Dubinin-Radushkevich, Temkin and Pyzhev, and Jovanovich models. The error sums of squares (SSR) function was used to test the fit of the models. The analysis revealed that the Sips isotherm is the best-fitting model for the CO₂ adsorption on the starting TiO₂, whereas the Freundlich equation should be used to describe the CO₂ adsorption isotherm on the titanate-TiO₂ composite. The CO₂ adsorption on the TEPA-modified sorbents was proposed to be described using the Sips isotherm for physical sorption and the modified Sips model for chemical sorption. The calculated isosteric heat of adsorption was found to be E46 kJ mol^-1, which is about two times higher than the heat of CO₂ absorption in liquid TEPA reported in the literature (i.e. E85 kJ.mol^-1). Therefore, it was concluded that the TEPA-titanate-TiO₂ composite is an attractive alternative for liquid amines due to the lower energy of regeneration in the sorption-desorption process. The material was proved to be stable during multiple sorption-desorption cycles. Moreover, its thermal stability up to 150 C was confirmed by thermogravimetric analysis (TGA). All these features make it a promising alternative for sorbents based on liquid amines. (authors)