Unlocking the green chemistry potential of multilayer nano-minerals: A comprehensive evaluation of HNT’s acid-base properties through theoretical and experimental studies

This paper presents a multidisciplinary approach in which computational chemistry has been combined with experimental data in order to shed light on the acidification mechanism and reaction pathway of acid-modified halloysite, a non-toxic and environmental-friendly mineral widely distributed in nature. Catalysts are synthesized starting from a commercial halloysite and treating it with different concentrations (0-5-10-15-20-30%) of hydrochloric acid. The chemical-physical characterization of the materials is performed through nitrogen physisorption, XRD, XRF and TGA-DSC measurements. Pronation mechanism is assessed by computational analysis and experimentally proved by characterization of the acidic properties using both spectrophotometric measurements and ZPC determination. Moreover, catalytic hydro-isomerization of o-xylene has been used as model reaction to determine the acidity functionalities of the materials and correlating them with the reactivity in a model acid-catalysed reaction. In agreement with DFT studies, the acid activation effectively enhances the acidity of halloysites nanotubes, even at low HCl concentration, with a sharp increase of acid sites up to a maximum value of 78.6 µmol/g for the HNT_10 sample. Then, the higher concentration of acid sites on the surface, the higher activity in o-xylene isomerization reaction, reaching a maximum value for the HNT_10 sample, thus reflecting its superior acidic character. On this account, the activation carried out with medium-low concentrations of hydrochloric acid (max. 10%) proved to be particularly efficient, allowing the best balance between acidity and chemical stability, thus making the acid-treated halloysite nanotubes a promising eco-friendly catalytic material for different applications.