CO2 adsorption on PEHA-functionalized geothermal silica waste: a kinetic study and quantum chemistry approach

Abstract

The use of solid waste and industrial by-products as alternatives to traditional raw materials to develop CO₂ adsorbents has recently gained interest. In the present work, silica waste produced in a geothermal plant was impregnated with pentaethylenehexamine (PEHA) to prepare an amine-modified CO₂ adsorbent. The SiO₂–PEHA adsorbent was characterized, and the effects of temperature and initial CO₂ concentration on the CO₂ adsorption were assessed. Experimental CO₂ adsorption results were fitted to pseudo-first-order (PFO), pseudo-second-order (PSO), Elovich, and intraparticle diffusion (IP diffusion) kinetic models. In addition, using the Gaussian 16 computational chemistry software package with the 6-311+(d,p) basis set coupled with the B3LYP hybrid density functional theory model, an adsorption mechanism between PEHA and CO₂ was proposed. The results showed that the CO₂ adsorption rate increases as the temperature and the initial CO₂ concentration increase. This happens as the resistance to CO₂ diffusion decreases until the mass transfer limitations dominate. The maximum CO₂ adsorption capacity was 1.05 mmol CO₂ per g material at 60 °C with 10 vol% CO₂. The PSO model showed the best fit for the CO₂ adsorption process on the SiO₂–PEHA material. This suggests a chemical reaction between CO₂ and PEHA molecules. Quantum chemical techniques allowed corroborating the formation of ammonium carbamate as a product of the exothermic reaction of CO₂ adsorption on the SiO₂–PEHA material.