Synergistic approaches to the acidic activation of halloysite nanotubes for unlocking their potential in sustainable catalysis

The acidic activation of halloysite nanotubes (HNTs) surface was investigated by a synergistic approach based on density functional theory (DFT) calculations and experimental methods. In particular, the protonation of halloysite inner lumen was enlightened at the atomic level by considering both the reactions energy and the geometrical properties. The mono-, bi- and tri-protonations are indeed exothermic processes. Similarly, the aluminum leaching was investigated and the energy required for the dealumination (ΔE_TOT = +882.9 kJ.mol⁻¹) can be supplied by the refilling of the active vacancy (ΔE_TOT = −883.5 kJ.mol⁻¹). Hence, a detailed experimental study of acidic-treated HNTs was carried out by considering their chemical and morphological properties. Most importantly, the nanotubular shape was maintained and the acidic treatment leads to an increase in the total concentration of acid sites and to the formation of new Brønsted sites. The catalytic properties were tested through the isomerization of o-xylene and a clear acidity–activity relationship was demonstrated. The synergy between in silico and in lab studies are of utmost importance for the development of efficient acid catalysts, thus paving the way for the development of multifunctional systems for the one-pot conversion of biomass into value-added molecules.