Exploiting Natural Surface Heterogeneity of Halloysite Nanotubes for Site-Selective Ru Nanoparticle Immobilization and Chemoselective Hydrogenation

Natural clay minerals attract significant interest as heterogeneous catalysts due to their surface complexity. In this study, ruthenium nanocatalysts (Ru NCs) were selectively grafted onto the inner and outer surfaces of halloysite nanotubes (HNTs) through acid- or alkali-etching, respectively, followed by functionalization with N-π-conjugated 1,1,3,3-tetramethylguanidine (TMG). The resulting HNTs-supported Ru NCs exhibited high catalytic selectivity in aromatic hydrogenation. HCl-etching preferentially corroded the inner AlO6-rich walls, facilitating Ru NC confinement within the HNT lumens. Conversely, NaOH-etched HNTs, aided by TMG, predominantly anchored Ru NCs (3.0 and 6.0 nm) on the exterior surface. XPS analysis confirmed Ru NCs existed primarily as metallic Ru(0), with minor electron-deficient Ruδ+ species enabling donor–acceptor catalysis. The composite HCl-Ru/TMG/HNTs, with lumen-loaded Ru NCs, delivered high nitrobenzene hydrogenation performance (99.48% aniline yield; nitrobenzene/Ru = 10,000; 90 °C, 5.0 MPa H2, 2 h) but showed negligible benzene conversion. In contrast, exterior-modified NaOH-Ru/TMG/HNTs proved to be highly effective for benzene hydrogenation. This selectivity is attributed to reactant donicity differences interacting with distinct catalyst surfaces, as corroborated by ξ-potential measurements in water, nitrobenzene, and benzene. Surface hydrophobicity/hydrophilicity, validated by static contact angles (NaOH-Ru/TMG/HNTs: 65°; HCl-Ru/TMG/HNTs: 10°), further governed the catalytic efficiency. This work demonstrates that leveraging the intrinsic surface heterogeneity of HNTs provides a strategic route to designing tailored catalysts for polar or nonpolar transformations.