A pH-switchable Pickering emulsion synergistically stabilized by nanoparticles as an efficient medium for utilizing ammonia borane for organic conversion

Abstract

Tandem reactions are one of the effective measures for efficiently utilizing ammonia borane for organic conversion. However, in conventional reaction systems, mismatched reaction rates or too-long reaction times will cause excessive pressure in the reaction vessel, wastage of the hydrogen source, or reduction of the reaction efficiency. Therefore, higher requirements are proposed for the reaction system to utilize ammonia borane fully and rapidly for organic conversion. Herein, a novel Pickering emulsion stabilized by nanoparticles, which include emulsifying particles and hydrophilic particles, was developed as the reaction system for series-parallel catalysis. The emulsifying particles were carboxyl-modified silica nanoparticles loaded with Pd, and were endowed with pH-responsiveness, and catalysis and emulsification properties, and adsorbed at the oil–water interface of the emulsion. The hydrophilic particles were silica nanoparticles loaded with Pd and dispersed in a continuous phase. Ammonia borane released hydrogen in the water phase, while water-soluble organic matter and oil-soluble organic matter were hydrogenated in the water phase and oil phase, respectively. The reaction rates of dehydrogenation and hydrogenation were very close, resulting in efficient use of ammonia borane. The conversions of styrene and 2,5-dihydrofuran (2,5-DHF) reached as high as 95.42% and 96.08%, respectively. In addition, since the emulsions were endowed with pH-sensitivity, this enabled oil–water separation using a simple strategy. After 5 cycles, there was still a high conversion rate of organic matter.