Zhangwen Wei

Zirconium-Metalloporphyrin PCN-222: Mesoporous Metal–Organic Frameworks with Ultrahigh Stability as Biomimetic Catalysts†

In nature, metalloporphyrins are well known for performing many biological functions in aqueous media, such as light harvesting, oxygen transportation, and catalysis. Heme, the iron–porphyrin derivative, is the cofactor for many enzyme/ protein families, including peroxidases, cytochromes, hemoglobins, and myoglobins. Using synthetic systems to mimic natural enzymes with high catalytic activity and substrate selectivity has been a sought-after goal in the last decade. Direct application of a heme as an oxidation catalyst in aqueous solution is usually challenging due to the formation of catalytically inactive dimers and catalyst self-destruction in the oxidizing reaction media. One promising approach is to load heme on supports, such as zeolites, clays, nanoparticles, hydrogels, or carbon materials, a practice which inevitably dilutes the density of active sites. An alternative approach is to protect the heme center by modifying the porphyrin to produce dendrimers or molecular crystals, which is a synthetically demanding method. Herein, we propose a unique strategy employing heme-like active centers as structural motifs for the assembly of highly stable porous materials, which should possess well-defined mesochannels and ultrahigh stability in aqueous solution. Metal-organic frameworks (MOFs) are a new class of crystalline porous materials with fascinating structures and intriguing properties, such as permanent porosity, high surface area, and uniform open cavities. The availability of various building blocks consisting of metals and organic linkers makes it possible to construct MOFs with unique properties for diverse applications. However, these desirable features of MOFs have rarely been applied to an enzymatic mimic, especially for catalysis in an aqueous medium, despite the fact that the assembly of ligands bearing high-density active sites into 3D frameworks may provide an ideal system to both enhance the catalytic activity and protect the cofactors. One of the main reasons is the lack of water-stable MOFs containing redox-active metal centers. Furthermore, most MOFs are microporous (pore size< 2 nm). Although they are suitable for gas storage, the small pore size slows down diffusion and limits the access of large substrate molecules to the active sites inside a MOF. Therefore, MOFs with mesopores, accessible redox sites, and ultrahigh stability, especially in aqueous media, are indispensible for any successful biomimetic attempt. Herein we have employed Fe-TCPP (TCPP= tetrakis(4carboxyphenyl)porphyrin) as a heme-like ligand and chosen highly stable Zr6 clusters as nodes for the assembly of stable Zr-MOFs. With carefully selected starting materials, we have successfully constructed a 3D heme-like MOF, designated as PCN-222(Fe) (Figure 1; PCN= porous coordination net-

Polyamine‐Tethered Porous Polymer Networks for Carbon Dioxide Capture from Flue Gas

One of the most pressing environmental concerns of our age is the escalating level of atmospheric CO2, which is largely correlated to the combustion of fossil fuels. For the foreseeable future, however, it seems that the ever-growing energy demand will most likely necessitate the consumption of these indispensable sources of energy. Carbon capture and sequestration (CCS), a process to separate CO2 from the flue gas of coal-fired power plants and then store it underground, has been proposed to reduce the anthropogenic CO2 emissions. Current CO2 capture processes employed in power plants worldwide are post-combustion “wet scrubbing” methods involving the chemical adsorption of CO2 by amine solutions such as monoethanolamine (MEA). The formation of carbamate from two MEA molecules and one CO2 molecule endows the scrubber with a high capacity and selectivity for CO2. However, this process suffers from a series of inherent problems, such as high regeneration costs that arise from heating the solution (ca. 30 % of the power produced by the plant), fouling of the equipment, and solvent boil-off. [1]

Construction of Ultrastable Porphyrin Zr Metal–Organic Frameworks through Linker Elimination

A series of highly stable MOFs with 3-D nanochannels, namely PCN-224 (no metal, Ni, Co, Fe), have been assembled with six-connected Zr6 cluster and metalloporphyrins by a linker-elimination strategy. The PCN-224 series not only exhibits the highest BET surface area (2600 m(2)/g) among all the reported porphyrinic MOFs but also remains intact in pH = 0 to pH = 11 aqueous solution. Remarkably, PCN-224(Co) exhibits high catalytic activity for the CO2/propylene oxide coupling reaction and can be used as a recoverable heterogeneous catalyst.

An Exceptionally Stable, Porphyrinic Zr Metal–Organic Framework Exhibiting pH-Dependent Fluorescence

A reaction between a Zr(IV) salt and a porphyrinic tetracarboxylic acid leads to a metal-organic framework (MOF) with two types of open channels, representing a MOF featuring a (4,8)-connected sqc net. The MOF remains intact in both boiling water and aqueous solutions with pH ranging from 1 to 11, a remarkably extensive pH range that a MOF can sustain. Given its exceptional stability and pH-dependent fluorescent intensity, the MOF can potentially be applied in fluorescent pH sensing.

Rigidifying fluorescent linkers by metal-organic framework formation for fluorescence blue shift and quantum yield enhancement.

We demonstrate that rigidifying the structure of fluorescent linkers by structurally constraining them in metal-organic frameworks (MOFs) to control their conformation effectively tunes the fluorescence energy and enhances the quantum yield. Thus, a new tetraphenylethylene-based zirconium MOF exhibits a deep-blue fluorescent emission at 470 nm with a unity quantum yield (99.9 ± 0.5%) under Ar, representing ca. 3600 cm(-1) blue shift and doubled radiative decay efficiency vs the linker precursor. An anomalous increase in the fluorescence lifetime and relative intensity takes place upon heating the solid MOF from cryogenic to ambient temperatures. The origin of these unusual photoluminescence properties is attributed to twisted linker conformation, intramolecular hindrance, and framework rigidity.

Metal–Organic Frameworks as Biomimetic Catalysts

In this Minireview, we have summarized the recent progress of biomimetic catalysis in the field of metal‐organic frameworks (MOFs) with a focus on the implantation of biomimetic active sites into a stable MOF. In addition, the potential of creating highly selective catalytic pockets and diffusion‐favored hierarchical structures in MOFs has also been explored. Furthermore, we have highlighted the achievements of MOF catalysts in the applications as mimics of peroxidase, cytochromes P450, hemoglobin, and photosynthetic systems.

Stable metal-organic frameworks containing single-molecule traps for enzyme encapsulation

Enzymatic catalytic processes possess great potential in chemical manufacturing, including pharmaceuticals, fuel production and food processing. However, the engineering of enzymes is severely hampered due to their low operational stability and difficulty of reuse. Here, we develop a series of stable metal-organic frameworks with rationally designed ultra-large mesoporous cages as single-molecule traps (SMTs) for enzyme encapsulation. With a high concentration of mesoporous cages as SMTs, PCN-333(Al) encapsulates three enzymes with record-high loadings and recyclability. Immobilized enzymes that most likely undergo single-enzyme encapsulation (SEE) show smaller Km than free enzymes while maintaining comparable catalytic efficiency. Under harsh conditions, the enzyme in SEE exhibits better performance than free enzyme, showing the effectiveness of SEE in preventing enzyme aggregation or denaturation. With extraordinarily large pore size and excellent chemical stability, PCN-333 may be of interest not only for enzyme encapsulation, but also for entrapment of other nanoscaled functional moieties.

Topology-Guided Design and Syntheses of Highly Stable Mesoporous Porphyrinic Zirconium Metal–Organic Frameworks with High Surface Area

Through a topology-guided strategy, a series of Zr6-containing isoreticular porphyrinic metal-organic frameworks (MOFs), PCN-228, PCN-229, and PCN-230, with ftw-a topology were synthesized using the extended porphyrinic linkers. The bulky porphyrin ring ligand effectively prevents the network interpenetration which often appears in MOFs with increased linker length. The pore apertures of the structures range from 2.5 to 3.8 nm, and PCN-229 demonstrates the highest porosity and BET surface area among the previously reported Zr-MOFs. Additionally, by changing the relative direction of the terminal phenyl rings, this series replaces a Zr8 cluster with a smaller Zr6 cluster in a topologically identical framework. The high connectivity of the Zr6 cluster yields frameworks with enhanced stability despite high porosity and ultralarge linker. As a representative example, PCN-230, constructed with the most extended porphyrinic linker, shows excellent stability in aqueous solutions with pH values ranging from 0 to 12 and demonstrates one of the highest pH tolerances among all porphyrinic MOFs. This work not only presents a successful example of rational design of MOFs with desired topology, but also provides a strategy for construction of stable mesoporous MOFs.

A Highly Stable Porphyrinic Zirconium Metal–Organic Framework with shp-a Topology

Through a kinetically controlled synthetic process, we synthesized PCN-223, a new porphyrinic Zr-MOF constructed from the newly reported hexagonal prismatic 12-connected Zr6 cluster through an unusual disordered arrangement, giving rise to the first example of the shp-a network in MOFs. With its extremely high connectivity, PCN-223 shows high stability in aqueous solutions with a wide range of pH. Cationic PCN-223(Fe) formed by postsynthetic treatment is an excellent recyclable heterogeneous catalyst for the hetero-Diels-Alder reaction.

Metal–Organic Frameworks Based on Previously Unknown Zr8/Hf8 Cubic Clusters

The ongoing study of zirconium- and hafnium-porphyrinic metal-organic frameworks (MOFs) led to the discovery of isostructural MOFs based on Zr8 and Hf8 clusters, which are unknown in both cluster and MOF chemistry. The Zr8O6 cluster features an idealized Zr8 cube, in which each Zr atom resides on one vertex and each face of the cube is capped by one μ4-oxygen atom. On each edge of the cube, a carboxylate from a porphyrinic ligand bridges two Zr atoms to afford a 3D MOF with a very rare (4,12)-connected ftw topology, in which two types of polyhedral cages with diameters of ∼1.1 and ∼2.0 nm and a cage opening of ∼0.8 nm are found. The isostructural Zr- and Hf-MOFs exhibit high surface areas, gas uptakes, and catalytic selectivity for cyclohexane oxidation.