We developed ceramic core-shell materials based on abundant halloysite clay nanotubes with enhanced heavy metal ions loading through Schiff base binding. These clay tubes are formed by rolling alumosilicate sheets and have diameter of c.50 nm, a lumen of 15 nm and length ~1 μm. This allowed for synthesis of metal nanoparticles at the selected position: (1) on the outer surface seeding 3–5 nm metal particles on the tubes; (2) inside the tube’s central lumen resulting in 10–12 nm diameter metal cores shelled with ceramic wall; and (3) smaller metal nanoparticles intercalated in the tube’s wall allowing up to 9 wt% of Ru, and Ag loading. These composite materials have high surface area providing a good support for catalytic nanoparticles, and can also be used for sorption of metal ions from aqueous solutions.

Halloysite is a natural tubular aluminosilicate clay of ca. 50 nm diameter and 0.5–1.5 micrometers in length. The nanoarchitectural modification of halloysite inner/outer surfaces can be achieved through supramolecular and covalent interactions exploiting its different inside/outside chemistry (Al2O3/SiO2). The tubular morphology makes halloysite a prospective nanotemplate for core-shell structured mesoporous catalysts. Catalytic metals can be incorporated on the nanotubes’ outer surface or in the inner lumens with selective metal binding. 2–5 nm diameter Au, Ag, Pt, Pd, Co, Ru, Cu-Ni, Fe2O3, CoxBy, CdS, and CdxZn1−xS particles were templated on halloysite. In this work, CdS and Ru-containing halloysite based nanocatalysts were synthesized via modification with organic ligands and microwave-assisted wetness ion impregnation. The catalytic hydrogenation of benzene and its homologues as well as phenol was performed. The impacts of the core-shell architecture, the metal particle size and seeding density were optimized for high reaction efficiency. An efficient Co-halloysite catalyst was formed using azines as ligands, and it contained 16 wt. % of cobalt with hydrogen evolution rate of 3.0 L/min × g(cat). The mesocatalysts produced are based on a safe and cheap natural clay nanomaterial and may be scaled-up for industrial applications.

Halloysite tubular nanoclay was applied as a template for synthesis of ruthenium core−shell composite catalysts for the first time; 50 nm diameter ceramic tubular systems with metal seeded interiors were produced. The procedure for the metal deposition and prior halloysite modification had a significant influence on properties of the catalyst and, as a consequence, on its activity in hydrogenation of phenol. Cyclohexanol was the main reaction product, but its yield depended on the substrate conversion and nanoarchitectural composition of the catalysts used. The maximum catalytic activity (turnover frequency, TOF) achieved was 17 282 h −1 in terms of hydrogen uptake per surface Ru atoms. The substrate selectivity of halloysite-based catalysts was also demonstrated at the comparative hydrogenation of phenol and various cresols.

Natural halloysite clay nanotubes were used as a template for clay/Ru core-shell nanostructure synthesis. Ru-nanoparticles were produced via a ligand-assisted metal ion intercalation technique. Schiff bases formed from different organic compounds proved to be effective ligands for the metal interfacial complexation which then was converted to Ru particles. This produces a high amount of intercalated metal nanoparticles in the tube’s interior with more that 90% of the sample loaded with noble metal. Depending on the selection of organic linkers, we filled the tube’s lumen with 2 or 3.5-nm diameter Ru particles, or even larger metal clusters. Produced nanocomposites are very efficient in reactions of hydrogenation of aromatic compounds, as tested for phenol and cresols hydrogenation.