Carlos Andrés Vallejo

The modification of bentonite with Al–Fe species from different concentrated precursors at both stages: (i) the preparation of the (Al/Fe)-mixed pillaring solution and (ii) intercalation itself, was studied at lab scale. The final solids were characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), Cationic Exchange Capacity (CEC), textural analyses by nitrogen adsorption–desorption at 77 K, and hydrogen-temperature programmed reduction (H₂-TPR). Finally, the modified clays were assessed as active materials in the Catalytic Wet Peroxide Oxidation (CWPO) of phenol under very mild conditions through 1.0 h of reaction: T = 25.0 °C ± 0.1 °C, pH = 3.7, ambient pressure (76 kPa), and 0.5 g catalyst per dm³. Metal hydrolysis by the dissolution of elemental aluminium (final Total Metal Concentration TMC = 4.62 mol dm⁻³) achieved the best results, decreasing the volume of solution per mass unit of clay required to successfully expand the layered starting mineral by a factor of close to 75, in comparison with the widespread conventional preparation using highly diluted Al-based pillaring solutions. Even in the absence of any solvent for the clay dispersion, the intercalating/pillaring method was shown to be favourable, as a novel strategy promoting the process intensification and subsequent preparation of Al/Fe- and other Al-based pillared clays at larger scales. The best catalyst prepared from concentrated precursors exhibited 79.1% phenol conversion, 19.3% TOC mineralization, and pretty low iron leaching (0.037 mg Fe dm⁻³; ∼0.12% w/w) in such a short catalytic assessment; all these results were quite comparable or even exceeded those exhibited by the catalyst prepared from dilute precursors.

In this work, the significant intensification of a bentonite pillaring process was achieved by using a novel methodological approach, leading to an intercalating Al/Fe mixed oligomeric precursor, around 100 times more concentrated than usually reported. In addition, the intercalating step was achieved directly on the clay with no previous swelling of the mineral being required; this allowed the successful scaled-up preparation of the Al/Fe-PILC, by a factor of one thousand, from the lab (10 g) to the pilot scale (10 kg). Intercalating solutions prepared under either concentrated (13 cm³) or diluted (widely reported, 2.0 dm³) conditions for lab-scale preparations were both translucent, displaying similar final pH values (close to 4.0) typical of highly oligomerized Al-pillaring solutions. The clay modified from concentrated precursors at the 10 g scale reached a high basal spacing (18.3 Å) and specific surface area (198 m² g⁻¹) with very comparable fractions of Fe forming truly mixed Al/Fe pillars in comparison to a reference material (H₂-TPR analyses). This promoted high performance in the catalytic wet peroxide oxidation of phenol in aqueous solution as a toxic model organic molecule at very mild temperature (25.0 °C ± 1.0 °C) and pressure (76 kPa), exhibiting the highest catalytic efficiency as a function of both parameters (full conversion of phenol together with 45.2% of TOC mineralization) with low iron leaching using a very low catalyst concentration (0.25 g dm⁻³). Particle size refining of the starting clay, the speed of stirring and conditions for the final washing of the interlayered precursor are the main factors influencing successful pillaring at scales higher than 1.0 kg.