CO2-triggered (3-aminopropyl)triethoxysilane-functionalized M-MOF-74 stabilized Pickering emulsions

Abstract

Pickering emulsions with a stimulus response have been paid extensive concern due to their long-term stability and demulsification on-command. Metal-organic frameworks (MOFs) are expectable emulsifying agents to build Pickering emulsions based on their excellent properties such as high specific surface area, adjustable porosity and pore size. Herein, a novel kind of CO2-triggered Pickering emulsions formed by (3-aminopropyl)triethoxysilane (APTES)-functionalized M-MOF-74 (M-MOF-74-APTES, M = Zn, Cu or Mg) has been developed for the first time. It was found that even at the M-MOF-74-APTES amount of 0.5 wt%, the functionalized MOFs could still stabilize the n-hexane-watermixture to construct Pickering emulsions. The as-generated Pickering emulsions could be efficiently demulsified upon bubbling of CO2, resulting in complete separation of oil and water phases, but could be restored by subsequent N2 bubbling under homogenization. Mechanism study indicated that the acid-base reaction of CO2 with the amino group of M-MOF-74-APTES resulted in the creation of hydrophilic salts, which led to the lowering of particle wettability and broke the stability of the emulsion, causing demulsification of the Pickering emulsion. After removing CO2, the emulsion can be reconstructed by the reversible reaction. As an applied example, the Pickering emulsion has been availed as a micro-reaction system and M-MOF-74-APTES as a carrier of Pd(OAc)2 catalyst to realize the efficient combination of Suzuki-Miyaura reaction, product isolation and reuse of catalysts via alternate bubbling or removing of CO2.