Enhanced Photocatalytic Oxidation of Isopropanol by HKUST-1@TiO2 Core–Shell Structure with Ultrathin Anatase Porous Shell: Toxic Intermediate Control (Enhanced Photocatalytic Oxidation of Isopropanol by HKUST-1@TiO2 Core–Shell Structure with Ultrathin Anatase Porous Shell: Toxic Intermediate Control)