Metal-organic frameworks (MOFs) are porous crystalline materials with a high tunability. To improve the functionality of the original frameworks, several strategies, such as the use of different metal cations and organic ligands and post-synthetic modification, have been developed, enabling the use of MOFs in numerous practical applications in various fields. Recently, another approach, i.e., MOF-on-MOF architecturing, has been actively studied by combining two or more MOFs into a composite. MOF-on-MOF materials not only possess the intrinsic properties of each MOF but also exhibit unprecedented synergism within a single system, resulting in a considerable potential for various applications. This review summarizes the interesting areas of application of MOF-on-MOF architectures into three categories: separation, catalysis, and sensing. In particular, the synergism occurring within such MOF-on-MOF architectures is discussed.

Composite metal-organic frameworks (MOFs) tend to possess complex interfaces that prevent facile and rational design. Here we present a joint computational/experimental workflow that screens thousands of MOFs and identifies the optimal MOF pairs that can seamlessly connect to one another by taking advantage of the fact that the metal nodes of one MOF can form coordination bonds with the linkers of the second MOF. Six MOF pairs (HKUST-1@MOF-5, HKUST-1@IRMOF-18, UiO-67@HKUST-1, PCN-68@MOF-5, UiO-66@MIL-88B(Fe) and UiO-67@MIL-88C(Fe)) yielded from our theoretical predictions were successfully synthesized, leading to clean single crystalline MOF@MOF, demonstrating the power of our joint workflow. Our work can serve as a starting point to accelerate the discovery of novel MOF composites that can potentially be used for many different applications.

An isotope-selective responsive system based on molecular recognition in porous materials has potential for the storage and purification of isotopic mixtures but is considered unachievable because of the almost identical physicochemical properties of the isotopes. Herein, a unique isotope-responsive breathing transition of the flexible metal−organic framework (MOF), MIL-53(Al), which can selectively recognize and respond to only D 2 molecules through a secondary breathing transition, is reported. This novel phenomenon is examined using in situ neutron diffraction experiments under the same conditions for H 2 and D 2 sorption experiments. This work can guide the development of a novel isotope-selective recognition system and provide opportunities to fabricate flexible MOF systems for energy-efficient purification of the isotopic mixture.

Functional materials based on metal-organic frameworks (MOFs), especially MOF-on-MOF architectures, have attracted great interest for their potential application in a range of fields. The well-defined crystalline features of MOFs encourage...