Bifunctional Heterometallic Metal-Organic Frameworks for Solvent-Free Heterogeneous Cascade Catalysis

Mingming Zheng,Yanxiang Wang,Pingyun Feng

doi:10.3390/catal10030309

Abstract

A family of heterometallic metal-organic frameworks (MOFs) (CPM200s) harmoniously coexisting as Lewis acids and base (azo) sites were prepared. Seven CPM200s were employed as multifunctional heterogeneous cascade catalysts for the one-pot deacetalization-Knoevenagel reaction in a solvent-free system. Benefiting from the cooperation between Lewis acids from the open metal sites and base sites from the ligands, the CPM200s showed high activity and selectivity for the tandem reaction. The heterometallic 3D porous framework reported here not only offers a combination of two opposite active sites in the same framework of materials but also increases mass transfer of the substrate, thus maximizing the efficiency and substrate selectivity of the bifunctional catalysts. The CPM200s showed the highest turnover frequency (TOF), outperforming that of the reported MOFs in tandem with the deacetalization-Knoevenagel reaction. A strong correlation between the TOF and charge-to-radius ratio (z/r) of metal ions in the CPM200s was observed for the first time. The bifunctional CPM200s catalysts can be reused five times without significant loss of activity.

Highlights

With the advantages of higher efficiency, less by-product, lower operating cost, and fewer purification steps, one-pot, multi-step cascading reactions are currently receiving considerable attention [1,2,3,4,5]
Among the different types of multifunctional catalysts, effective multifunctional heterogeneous catalysts have not been widely used in the multi-step cascading reactions, due to the challenge associated with the deactivation of different active sites, leaching of metal ions, and poor recyclability [7,8]
-In/Mg, -In/Co, and -In/Mn was proved by powder X-ray diffraction (PXRD) (Figure 1), which match with the simulated patterns, suggesting that the CPM200s were successfully prepared

Summary

Introduction

With the advantages of higher efficiency, less by-product, lower operating cost, and fewer purification steps, one-pot, multi-step cascading reactions are currently receiving considerable attention [1,2,3,4,5]. The applications of multi-functional catalysts in the one-pot reaction process simplify the operational procedure by omitting steps of the purification of the intermediate, and provides a useful strategy for the development of new reactions that cannot be realized with other methods [6]. Among the different types of multifunctional catalysts, effective multifunctional heterogeneous catalysts have not been widely used in the multi-step cascading reactions, due to the challenge associated with the deactivation of different active sites, leaching of metal ions, and poor recyclability [7,8]. The introduction of multi-functional catalytic active sites, both basic and acidic sites, on the same support has been considered a common strategy for developing heterogeneous catalysts for cascading reactions [9,10]. A lot of acid-base multi-functional catalysts have been prepared [11,12], including mesoporous organosilicas [13,14,15], zeolite [16], cross-linked micelles [17], Pickering emulsions [18], and hyper-cross-linked nanospheres [19].

Methods

Results

Conclusion