Laser Synthesis of Carbonaceous TiO₂ from Metal–Organic Frameworks: Optimum Support for Pd Nanoparticles for C–C Cross-Coupling Reactions

In this work, we report the development of highly active Pd nanoparticle catalysts encapsulated within novel carbonaceous and N-doped carbonaceous TiO2 support nanomaterials for C–C cross-coupling reactions. The support materials are generated, for the first time, via the laser-vaporization-controlled condensation (LVCC) of Ti and amino-functionalized Ti metal–organic frameworks (MOFs): MIL-125(Ti) and NH2-MIL-125(Ti). Ultrasmall crystalline anatase phases with a small fraction of the rutile phase of TiO2 nanoparticles are well dispersed within C and N-doped C spheres, resulting in improved supports for the Pd nanocatalysts. The Pd nanoparticles are generated by the photocatalytic reduction of Pd4+ ions in aqueous solutions containing the MOF-derived carbonaceous or N-doped carbonaceous TiO2 nanostructures using pulsed-laser irradiation without any chemical reducing agents or surface capping materials. The laser photoreduction of the Pd ions is accompanied by oxidative etching of the C spheres, resulting in the formation of hollow C spheres that can encapsulate the Pd nanoparticles to prevent particle aggregation and also to increase the catalyst–support interaction. With 0.3 mol % Pd loading on the TiO2-decorated N-doped carbonaceous support, a 100% conversion to the biphenyl product from the reaction of bromobenzene and benzeneboronic acid is achieved within 5 min under microwave reaction conditions at 80 °C. The catalytic properties of the Pd catalysts supported on the LVCC-generated TiO2-decorated N-doped hollow C spheres far surpass those Pd catalysts supported on the original MIL-125(Ti) and NH2-MIL-125(Ti) MOFs as well as on the TiO2-decorated N-doped carbonaceous supports prepared by thermal pyrolysis of the NH2-MIL-125(Ti) crystals. The results indicate that the TiO2-decorated N-doped hollow C spheres with hierarchical meso/microporous structures provide an optimum support for the Pd nanoparticles for high activity and no significant Pd leaching in the Suzuki C–C cross-coupling reactions. The superior activity and excellent reusability of this nanocatalyst are attributed to the encapsulation of small Pd nanoparticles (2.1 nm) within the high-surface-area TiO2-decorated N-doped C support (216 m2/g) and the unique optimum catalyst–support interaction provided by the Pd/N-doped C/TiO2 interfaces.