Adsorption-Based Synthesis of Environmentally Friendly Heterogeneous Chromium(III) Catalysts for Oxidation Reactions into Kaolinite, Saponite, and Their Amine-Modified Derivatives

Batch experiments were used to evaluate the Brazilian São Simão kaolinite (Kao), the Spanish Yuncillos saponite (Sa), and their derivatives organofunctionalized with 3-aminopropyltriethoxysilane (Kao-APTES and Sap-APTES) as Cr(III) adsorbents in aqueous medium. Cr(III) concentration and interaction time affected Cr(III) adsorption, which depended strongly on surface charge. The adsorption capacities of Kao and Sap increased after functionalization. Cr(III) adsorption reached equilibrium within 30 min for kaolinite solids and within 120 min for saponite solids. Kinetics tests showed that Cr(III)–adsorbent interactions did not follow a single model, and all the samples fitted the pseudo-second-order model satisfactorily. Equilibrium studies showed that adsorption correctly fits the Langmuir model, but adsorption onto Kao and Sap-APTES fits the Freündlich model better. Desorption experiments showed that Kao-APTES desorbed only 9.7% of Cr(III) cations under different stressing conditions as compared to 47.8% desorption in the case of Sap-APTES. The solids obtained after Cr(III) adsorption were used as heterogeneous catalysts for cis -cyclooctene oxidation reaction, being promising catalysts (conversion higher than 50%) and not showing leaching of Cr(III).