MOF-based Dual-Layer Pickering Emulsion: Molecular-Level Gating of Water Delivery at Water-Oil Interface for Efficient Photocatalytic Hydrogenation Using H2O as a Hydrogen Source.

Biphasic system not only presents a promising opportunity for complex catalytic processes, but also is a grand challenge in efficient tandem reactions. As an emerging solar-to-chemical conversion, the visible-light-driven and water-donating hydrogenation combines the sustainability of photocatalysis and economic-value of hydrogenation. However, the key and challenging point is to couple water-soluble photocatalytic hydrogen evolution reaction (HER) with oil-soluble hydrogenation. Herein, we employed metal-organic frameworks (MOFs) and CdS nanorods to construct a MOF-CdS dual-layer Pickering emulsion (water in oil, W/O), which compartmented aqueous phase for photocatalytic HER and oil phase for hydrogenation. The hydrophobic MOF and hydrophilic CdS were isolated at the inner and outer layers of W/O emulsion, respectively. The molecularly regulated hydrophobicity of MOF controlled the water delivery onto CdS photocatalysts, which realized the synergistic regulation of HER and hydrogenation. In the photocatalytic hydrogenation of cinnamaldehyde, the highest yield of MOF-CdS Pickering emulsion reached 187.37 mmol·g-1·h-1, 30 times that of the counterpart without emulsion (6.44 mmol·g-1·h-1). Its apparent quantum yield reached 43.24% without co-catalysts. To our knowledge, this performance is at a top-level so far. Our work realized the precise regulation of water-oil interface to effectively couple two reactions in different phases, providing new perspective for challenging tandem catalysis.