The field of metal–organic frameworks (MOFs) has been incorrectly believed to be purely crystalline. However, non-crystalline MOFs (amorphous MOFs, MOF liquids, and MOF glasses) are starting to emerge as alternative materials, beyond the dictatorial domain of crystalline MOFs. Non-crystalline MOFs present many opportunities, either as novel functional materials themselves, or as vehicles to create other materials. In this extensive Review, we describe the two approaches to preparing amorphous MOFs: (1) the amorphization of crystalline MOFs and (2) the direct synthesis. Special attention is paid to the relationship between preparation method, properties and applications of amorphous MOFs. We also explore the field of MOF liquids and their applications, centering our attention to the phenomenon of melting. Finally, MOF glasses are explained. We highlight the properties and applications of the MOF glasses that are not usually found in crystalline MOFs. New related glass materials such as MOF-blends, flux melted MOFs, MOF crystal-glass composites, MOF and inorganic glass composites, and MOF glass membranes are also reviewed. We conclude the fields of amorphous MOFs, MOF liquids, and MOF glasses by presenting our thoughts on the possible future research directions.

Metal–organic frameworks (MOFs) beyond crystallinity: amorphous MOFs, MOF liquids and MOF glasses

Metal–organic frameworks and covalent organic frameworks as disruptive membrane materials for energy-efficient gas separation

Conventional separation technologies to separate valuable commodities are energy intensive, consuming 15% of the worldwide energy. Mixed-matrix membranes, combining processable polymers and selective adsorbents, offer the potential to deploy adsorbent distinct separation properties into processable matrix. We report the rational design and construction of a highly efficient, mixed-matrix metal-organic framework membrane based on three interlocked criteria: (i) a fluorinated metal-organic framework, AlFFIVE-1-Ni, as a molecular sieve adsorbent that selectively enhances hydrogen sulfide and carbon dioxide diffusion while excluding methane; (ii) tailoring crystal morphology into nanosheets with maximally exposed (001) facets; and (iii) in-plane alignment of (001) nanosheets in polymer matrix and attainment of [001]-oriented membrane. The membrane demonstrated exceptionally high hydrogen sulfide and carbon dioxide separation from natural gas under practical working conditions. This approach offers great potential to translate other key adsorbents into processable matrix. Description Mixed-matrix membranes using nanosheets Selective adsorbents can show enhanced separation of components from mixed-gas streams, but these materials can often be difficult to fabricate into large-scale, robust membranes. Datta et al. report the synthesis and characterization of a mixed-matrix membrane. They first describe the synthesis of sheets of the metal organic framework (MOF) material AlFFIVE-1-Ni. By using nanosheets instead of the more commonly synthesized MOF nanoparticles, the authors were able to achieve much better alignment, loading fractions up to 60%, and better polymer-MOF compatibility when embedding the MOF nanosheets into a polymer matrix. These mixed-matrix membranes exhibited improved carbon dioxide and methane selectivity relative to many other comparable ones, as well as the ability to remove hydrogen sulfide. —MSL High loading of metal organic framework materials enables efficient upgrading of natural gas by removing carbon dioxide and hydrogen sulfide.

/pdf/rational-design-of-mixed-matrix-metal-organic-framework-i2kkurp2.pdf

Rational design of mixed-matrix metal-organic framework membranes for molecular separations

Propylene/propane separation is one of the most challenging separations, currently achieved by energy-intensive cryogenic distillation. Despite the great potential for energy-efficient membrane-based separations, no commercial membranes are currently available due to the limitations of current polymeric materials. Zeolitic imidazolate framework, ZIF-8, with the effective aperture size of ∼4.0 Å, has been shown to be very promising for propylene/propane separation. Despite the extensive research on ZIF-8 membranes, only a few reported ZIF-8 membranes have displayed good propylene/propane separation performances presumably due to the challenges of controlling the microstructures of polycrystalline membranes. Here we report the first well-intergrown membranes of ZIF-67 (Co-substituted ZIF-8) by heteroepitaxially growing ZIF-67 on ZIF-8 seed layers. The ZIF-67 membranes exhibited impressively high propylene/propane separation capabilities. Furthermore, when a tertiary growth of ZIF-8 layers was applied to heteroepitaxially grown ZIF-67 membranes, the membranes exhibited unprecedentedly high propylene/propane separation factors of ∼200 possibly due to enhanced grain boundary structure.

Heteroepitaxially-grown zeolitic imidazolate framework membranes with unprecedented propylene/propane separation performance

Membrane-based separations have garnered considerable attention owing to their high energy efficiency, low capital cost, small carbon footprint, and continuous operation mode. As a class of highly porous crystalline materials with well-defined pore systems and rich chemical functionalities, metal-organic frameworks (MOFs) have demonstrated great potential as promising membrane materials over the past few years. Different types of MOF-based membranes, including polycrystalline membranes, mixed matrix membranes (MMMs), and nanosheet-based membranes, have been developed for diversified applications with remarkable separation performances. In this comprehensive review, we first discuss the general classification of membranes and outline the historical development of MOF-based membranes. Subsequently, particular attention is devoted to design strategies for MOF-based membranes, along with detailed discussions on the latest advances on these membranes for various gas and liquid separation processes. Finally, challenges and future opportunities for the industrial implementation of these membranes are identified and outlined with the intent of providing insightful guidance on the design and fabrication of high-performance membranes in the future.

Advances in metal-organic framework-based membranes.

The combination of well-defined molecular cavities and chemical functionality makes crystalline porous solids attractive for a great number of technological applications, from catalysis to gas separation. However, in contrast to other widely applied synthetic solids such as polymers, the lack of processability of crystalline extended solids hampers their application. In this work, we demonstrate that metal–organic frameworks, a type of highly crystalline porous solid, can be made solution processable via outer surface functionalization using N-heterocyclic carbene ligands. Selective outer surface functionalization of relatively large nanoparticles (250 nm) of the well-known zeolitic imidazolate framework ZIF-67 allows for the stabilization of processable dispersions exhibiting permanent porosity. The resulting type III porous liquids can either be directly deployed as liquid adsorbents or be co-processed with state-of-the-art polymers to yield highly loaded mixed matrix membranes with excellent mechanical properties and an outstanding performance in the challenging separation of propylene from propane. We anticipate that this approach can be extended to other metal–organic frameworks and other applications. Solution processability is required for many industrial processes, but metal–organic frameworks are in general not dispersible, hindering their application. Here, a surface modification is reported that allows porous liquid formation and so synthesis of highly loaded and mechanically robust mixed matrix membranes.

/pdf/solution-processable-metal-organic-frameworks-for-mixed-3k2onks6af.pdf

Solution processable metal-organic frameworks for mixed matrix membranes using porous liquids.

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Solution processable metal-organic frameworks for mixed matrix membranes using porous liquids.