Pei Xiaoyan, Song Wangyue
College of Chemistry and Chemical Engineering, Xinyang Normal University, Xinyang, Henan 464000, P. R. China.
Langmuir. 2023 Oct 24;39(42):15046-15054. doi: 10.1021/acs.langmuir.3c02011. Epub 2023 Oct 9.
Hierarchical-pore metal-organic frameworks (H-MOFs) are considered to be emerging stabilizers for Pickering emulsion formation because of their hierarchically arranged pores, tailorable structures, and ultrahigh surface areas. However, stimulus-triggered Pickering emulsions built by H-MOFs have been seldom presented to date despite their great significance in diverse applications. Herein, by grafting Pd(OAc) on the hierarchical-pore zirconium MOF UiO-66, namely, H-UiO-66, with the aid of 1-alkyl-3-methylimidazolium 2-cyanopyrrolide salts ([CMIM][2-CN-Pyr], = 4, 6, and 8), a series of Pd(OAc)-[CMIM][2-CN-Pyr]@H-UiO-66 have been developed and utilized as emulsifiers for constructing CO-switching Pickering emulsions. It was found that Pd(OAc)-[CMIM][2-CN-Pyr]@H-UiO-66 was able to stabilize the -hexane-water mixture to form a Pickering emulsion even at an amount of 0.5 wt %. Upon alternate addition of CO and N at normal pressure, Pickering emulsions could be smartly converted between demulsification and re-emulsification. Through combining varieties of spectroscopic techniques, the mechanism of the switchable phase transformation lay in the acid-base reaction of ionic liquids with CO on H-UiO-66 and the creation of more hydrophilic salts, which reduced the wettability of the emulsifier and destabilized the emulsion. As an example of application, the stimulus-triggered Pickering emulsion was employed as a palladium-catalyzed Suzuki-Miyaura cross-coupling microreactor to achieve the combination of chemical reactions, isolation of products, and recovery of catalysts.
分级孔金属有机框架材料（H-MOFs）因其分级排列的孔道、可定制的结构和超高的比表面积，被认为是用于形成皮克林乳液的新兴稳定剂。然而，尽管H-MOFs构建的刺激触发型皮克林乳液在各种应用中具有重要意义，但迄今为止很少有相关报道。在此，借助1-烷基-3-甲基咪唑鎓2-氰基吡咯盐（[CMIM][2-CN-Pyr]，n = 4、6和8），通过将Pd(OAc)接枝到分级孔锆基MOF UiO-66（即H-UiO-66）上，开发了一系列Pd(OAc)-[CMIM][2-CN-Pyr]@H-UiO-66，并将其用作乳化剂来构建CO响应型皮克林乳液。研究发现，即使在用量为0.5 wt%时，Pd(OAc)-[CMIM][2-CN-Pyr]@H-UiO-66也能够稳定正己烷-水混合物以形成皮克林乳液。在常压下交替通入CO和N₂时，皮克林乳液能够在破乳和再乳化之间智能转换。通过结合多种光谱技术，发现这种可切换相变的机制在于离子液体与H-UiO-66上的CO发生酸碱反应，生成了更多亲水性盐，从而降低了乳化剂的润湿性并使乳液失稳。作为应用实例，这种刺激触发型皮克林乳液被用作钯催化的铃木-宫浦交叉偶联微反应器，以实现化学反应、产物分离和催化剂回收的结合。
