CO2 capture by coal ash-derived zeolites- roles of the intrinsic basicity and hydrophilic character

Abstract

The affinity towards carbon dioxide (CO2) of coal fly ash-derived zeolites such as 4A, NaP1, zeolite X nanosized and their commercial counterparts was investigated. For this purpose, their surface basicity and hydrophilic character were assessed through thermal programmed desorption measurements (TPD) of CO2 and water retention capacities (CRC and WRC, respectively). X-ray powder diffraction (XRD) and fluorescence (XRF), scanning electron microscopy (SEM), thermal gravimetric analysis (TGA) and measurements on nitrogen adsorption-desorption isotherms demonstrated the key-roles of the zeolite structure and chemical composition. Decreasing CRC induces a WRC increase, indicating a direct proportionality between the zeolite basicity and hydrophobic character, which are essential requirements for potential applications in the capture of acidic gases. TPD patterns deconvolution revealed an enhancement of the hydrophilic character and weak to medium basicity with increasing Si content at the expense of strong basic sites. All zeolites display at least three strengths in retaining CO2 and moisture, whose distribution is governed by the structure and chemical composition. Within specific Si/Al range, the partial negative charge of lattice oxygens was found to strongly influence the zeolite basicity and hydrophobic character. These findings are of great interest, because they allow tailoring the surface properties for optimum and reversible capture of greenhouse gases, acidic gasses and volatile organic compounds.