A Strategy for Separating and Recycling Solid Catalysts Based on the pH-Triggered Pickering-Emulsion Inversion

摘要

The efficient separation and recycling of catalysts is one of major objectives of sustainable and green chemistry. For liquid-phase reactions, solid catalysts are usually separated from products through filtration or centrifugation. These separation methods are widely used but bothersome, especially when the catalyst particle sizes are in a submicrometer-to-micrometer range, because of catalyst loss, blocking of filters, high time-and energy-consumption, and risk of air oxidation during the course of catalyst transport. These limitations stimulate development of new methods that facilitate catalyst separation and recycling, for example, the currently extensively investigated magnetic-field-assisted separation. We envision if a solid catalyst could be reversibly transferred between two immiscible phases on command, for example, an organic phase and an aqueous phase, the successive catalyst separation and recycling would be facilely conducted in one vessel without need of catalyst transport. Although this idea has been successfully used to separate and recycle homogeneous and polymer-based catalysts in biphasic systems, there are hardly any reports on its implication for separating and recycling heterogeneous (solid particle) cata-lysts. The major obstacle is that for micrometer-sized particles, the energy required for crossing the oil/water interface is calculated to be as high as 10~7 K_B T (K_B is Boltzmann constant) according to Young's equation, making it thermodynamically impossible.