HKUST-1@etched halloysite nanotube composites for enhanced cryogenic hydrogen adsorption

Halloysite nanotubes (HNTs) represent a unique class of naturally occurring, environmentally friendly nanomaterials, characterized by distinct charge distributions between their inner and outer surfaces. In this study, we engineered an advanced nanomaterial by strategically modifying HNTs and incorporating HKUST-1 (Hong-Kong University of science and Technology-1, Cu 3 (BTC) 2 ) metal-organic frameworks (MOFs) within their tubular structure. To optimize the material's performance, we developed an innovative sulfuric acid etching protocol that significantly expanded the inner lumen of HNTs— creating etched HNTs (EHNTs) with enhanced loading capacity. This etching process proved crucial for achieving effective precursor solution incorporation and subsequent MOF deposition. Comprehensive material characterization was conducted using X-ray diffraction (XRD) for structural analysis, along with scanning electron microscopy (SEM) and transmission electron microscopy (TEM) for morphological evaluation. Gas adsorption performance was quantitatively assessed through Brunauer-Emmett-Teller (BET) measurements, demonstrated remarkable improvements in both hydrogen and nitrogen adsorption capacities. Notably, the HKUST-1@EHNT (5 M) composite exhibited superior gas uptake compared to its HKUST-1@EHNT (1 M) counterpart — directly correlating with the increased internal space and higher MOF loading achieved through concentrated acid etching. These findings not only validate the effectiveness of our etching approach but also establish halloysite-MOF hybrids as a promising platform for gas storage applications. The developed methodology opens new possibilities for creating diverse functional nanomaterials that leveraging the unique properties of natural nanotubes. • 5 M etching expands halloysite lumen 65%, enabling 3× MOF loading • HKUST-1@EHNT-5M: 352 m²/g surface area (vs. 80 m²/g non-etched) • 3352.7 cm³/g H₂ uptake at 20 K for HKUST-1@EHNT-5M (3× >1 M) • MOF-halloysite synergy enhances adsorption kinetics for storage • Vacuum confinement synthesis ensures precise MOF deposition in nanotubes