Separation is an important industrial step with critical roles in the chemical, petrochemical, pharmaceutical, and nuclear industries, as well as in many other fields. Although much progress has been made, the development of better separation technologies, especially through the discovery of high-performance separation materials, continues to attract increasing interest due to concerns over factors such as efficiency, health and environmental impacts, and the cost of existing methods. Metal-organic frameworks (MOFs), a rapidly expanding family of crystalline porous materials, have shown great promise to address various separation challenges due to their well-defined pore size and unprecedented tunability in both composition and pore geometry. In the past decade, extensive research is performed on applications of MOF materials, including separation and capture of many gases and vapors, and liquid-phase separation involving both liquid mixtures and solutions. MOFs also bring new opportunities in enantioselective separation and are amenable to morphological control such as fabrication of membranes for enhanced separation outcomes. Here, some of the latest progress in the applications of MOFs for several key separation issues, with emphasis on newly synthesized MOF materials and the impact of their compositional and structural features on separation properties, are reviewed and highlighted.

No abstract

A new redox-responsive hybrid nanogated ensemble has been developed by introducing the disulfide-linked polymeric network at the outlet of mesoporous silica. The cross-linked polymer works as gatekeeper to control molecule release from mesoporous silica. The presence of disulfide reducing agent can effectively open the polymeric network and release the loading in a tunable manner.

Metal-organic framework (MOF) materials have emerged as one of the favorite crystalline porous materials (CPM) because of their compositional and geometric tunability and many possible applications. In efforts to develop better MOFs for gas storage and separation, a number of strategies including creation of open metal sites and implantation of Lewis base sites have been used to tune host-guest interactions. In addition to these chemical factors, the geometric features such as pore size and shape, surface area, and pore volume also play important roles in sorption energetics and uptake capacity. For efficient capture of small gas molecules such as carbon dioxide under ambient conditions, large surface area or high pore volume are often not needed. Instead, maximizing host-guest interactions or the density of binding sites by encaging gas molecules in snug pockets of pore space can be a fruitful approach. To put this concept into practice, the pore space partition (PSP) concept has been proposed and has achieved a great experimental success. In this account, we will highlight many efforts to implement PSP in MOFs and impact of PSP on gas uptake performance. In the synthetic design of PSP, it is helpful to distinguish between factors that contribute to the framework formation and factors that serve the purpose of PSP. Because of the need for complementary structural roles, the synthesis of MOFs with PSP often involves multicomponent systems including mixed ligands, mixed inorganic nodes, or both. It is possible to accomplish both framework formation and PSP with a single type of polyfunctional ligands that use some functional groups (called framework-forming group) for framework formation and the remaining functional groups (called pore-partition group) for PSP. Alternatively, framework formation and PSP can be shouldered by different chemical species. For example, in a mixed-ligand system, one ligand (called framework-forming agent) can play the role of the framework formation while the other type of ligand (called pore-partition agent) can assume the role of PSP. PSP is sensitive to the types of inorganic secondary building units (SBUs). The coexistence of SBUs complementary in charge, connectivity, and so on can promote PSP. The use of heterometallic systems can promote the diversity of SBUs coexistent under a given condition. Heterometallic system with metal ions of different oxidation states also provides the charge tunability of SBUs and the overall framework, providing an additional level of control in self-assembly and ultimately in the materials' properties. Of particular interest is the PSP in MIL-88 type (acs-type topology) structure, which has led to a huge family of CPMs (called pacs CPMs, pacs = partitioned acs) exhibiting low isosteric heat of adsorption and yet superior CO uptake capacity.

A new pH-responsive hybrid nanogated ensemble has been developed by using acetal group linked gold nanoparticle capped mesoporous silica. The hydrolysis of acetal linker at acidic environment makes the gold nanoparticles work as a gatekeeper to control the release of guest molecules from mesoporous silica under different pH's.

Metal-organic frameworks (MOFs) with the highest CO(2) uptake capacity are usually those equipped with open metal sites. Here we seek alternative strategies and mechanisms for developing high-performance CO(2) adsorbents. We demonstrate that through a ligand insertion pore space partition strategy, we can create crystalline porous materials (CPMs) with superior CO(2) uptake capacity. Specifically, a new material, CPM-33b-Ni without any open metal sites, exhibits the CO(2) uptake capacity comparable to MOF-74 with the same metal (Ni) at 298 K and 1 bar.

Crystalline porous materials, especially inorganic porous solids such as zeolites, usually have negative frameworks with extra-framework mobile cations and are widely used for cation exchange. It is highly desirable to develop new materials with positive frameworks for selective anion exchange and separation or storage and delivery. Recent advances in metalorganic framework synthesis have created new opportunities in this direction. Here we report the synthesis of a series of positive indium metal-organic frameworks and their utilization as a platform for the anion exchange-based separation process. This process is capable of size-or charge-selective ion-exchange of organic dyes and may form the basis for size-selective ion chromatography. Ion-exchange dynamics of a series of organic dyes and their selective encapsulation and release are also studied, highlighting the advantages of metal-organic framework compositions for designing host materials tailored for applications in anion separation and purification.

Despite their having much greater potential for compositional and structural diversity, heterometallic metal-organic frameworks (MOFs) reported so far have lagged far behind their homometallic counterparts in terms of CO2 uptake performance. Now the power of heterometallic MOFs is in full display, as shown by a series of new materials (denoted CPM-200s) with superior CO2 uptake capacity (up to 207.6 cm(3)/g at 273 K and 1 bar), close to the all-time record set by MOF-74-Mg. The isosteric heat of adsorption can also be tuned from -16.4 kJ/mol for CPM-200-Sc/Mg to -79.6 kJ/mol for CPM-200-V/Mg. The latter value is the highest reported for MOFs with Lewis acid sites. Some members of the CPM-200s family consist of combinations of metal ions (e.g., Mg/Ga, Mg/Fe, Mg/V, Mg/Sc) that have never been shown to coexist in any known crystalline porous materials. Such previously unseen combinations become reality through a cooperative crystallization process, which leads to the most intimate form of integration between even highly dissimilar metals, such as Mg(2+) and V(3+). The synergistic effects of heterometals bestow CPM-200s with the highest CO2 uptake capacity among known heterometallic MOFs and place them in striking distance of the all-time CO2 uptake record.