Carbon Dioxide Capture in Porous Aromatic Frameworks

Porous aromatic frameworks (PAFs), an important class of porous materials, have gained worldwide attentions and been greatly developed since the first PAF material PAF-1 was successfully designed and synthesized in 2009. More than 100 PAF materials with specific functionalities were synthesized and reported in the last decade. The high surface areas, light skeleton densities and high thermal and chemical stabilities make PAF materials good candidates in various applications, such as gas sorption and separation, catalysis, sensor and host-guest chemistry. The design of PAF-1 came from the structure of diamond, the most stable substance exiting in the nature at present. The carbon atoms in diamond are covalently bonded with four adjacent carbon atoms by sp3 hybridization to form tetrahedral units. This special structure makes diamond the hardest material. However, the short length of C–C bond results in a compact structure of diamond. Conceptually, if the C–C bonds were replaced with rigid linear units, the resulting material should not only retain the diamond structure but also present increasing internal surface area. By employing Ullmann coupling reaction of tetrakis(4-bromophenyl)methane, PAF-1 with ultrahigh surface area as well as high thermal and chemical stability was synthesized. The structural information of PAF-1 was obtained by various characterizations, which is consistent with P2 model structure simulated from diamond topology. Thus, tetrahedral building units are good candidates to construct PAFs with high surface area and high stabilities. Reticular chemistry was introduced into the design of porous materials in the end of 20th century, and has been well developed and employed with the concept of topology. Via the bottom-up and top-down ways, PAF materials derived from tetrahedral units may possess diamond-like structure thus they would exhibit high thermal stabilities. By adjusting the length of linkages between tetrahedral units, PAFs with tunable pore sizes can be targeted synthesized, which might even extend to mesopore range. In-situ synthesis and post-modification methods bring more possibilities for the functionalization of PAF materials. Various functional groups such as –NH2, –COOH, –OH can be embedded in PAFs skeletons. Metal cations can be introduced in the pores of PAF materials by ion-exchange method. Besides, metal nanoparticles would also be encapsulated in PAF materials with large pore size and specific interaction sites. In this review, we summarized the targeted synthesis of PAF materials derived from tetrahedral units with the guide of reticular chemistry and demonstrated the functionalization strategy. Meanwhile, the applications of PAF materials, including CO2 sorption and separation, small liquid molecule adsorption, iodine capture, catalysis and sensor were presented and the effect factors were discussed in detail. In addition, cost-effective PAF materials were synthesized with cheap catalysts such FeCl3 and AlCl3 to expand their applications in practice. The above achievements opened up new avenues for the targeted synthesis of porous materials with high surface areas and high stabilities and provided insights for the materials design with specific functionalities.

The creation of ultrahigh surface area materials are of great interest in academia and industry. In recent years, porous aromatic frameworks (PAF) were discovered and their porosity and properties were also explored. They are characterized by a rigid aromatic open-framework structure constructed by covalent bonds. The building block design, network formation method and relationship between functions and secondary building units were compiled in this highlight. In addition, advantages and challenges of predicted PAF derivatives were also discussed.

A series of porous aromatic frameworks (PAFs) were synthesized via a Yamamoto-type Ullmann reaction containing quadricovalent Si (PAF-3) and Ge (PAF-4). These PAFs are thermally stable up to 465 °C for PAF-3 and 443 °C for PAF-4, corresponding to a 5% weight loss according to the TG pattern. As PAF-1, they exhibit high surface areas (up to 2932 m2 g−1) and excellent adsorption ability to hydrogen, methane and carbon dioxide. Low pressure gas uptake experiments on PAFs show PAF-3 has the highest heat of adsorption (Qst) of hydrogen (6.6 kJ mol−1) and carbon dioxide (19.2 kJ mol−1), while PAF-4 has the highest Qst for methane adsorption (23.2 kJ mol−1) among PAFs. Gas molecule recognition at 273 K was performed and results show only greenhouse gases such as carbon dioxide and methane could be adsorbed onto PAFs.

Porous aromatic frameworks (PAFs) were first reported in 2009 and have quickly attracted much attention because of their exceptionally ultrahigh specific surface area (5800 m2·g-1). Uniquely, PAFs are constructed from carbon-carbon-bond-linked aromatic-based building units, which render PAFs extremely stable in various environments. At present, PAFs have been applied in many fields, such as adsorption, catalysis, ion exchange, electrochemistry, and so on. However, for such a unique material, its application in the biological fields is still rarely explored. Therefore, this Perspective introduces the reported application of PAFs in biological fields, for instance, diagnosis and treatment of diseases, artificial enzymes, drug delivery, and extraction of bioactive substances. Major challenges and opportunities for future research on PAFs in biology and biomedicine are identified in diagnostic platforms, novel drug carriers/antidotes, and novel artificial enzymes.

Porous aromatic frameworks (PAFs) with robust structure, high stability, and high surface area have attracted intense interest from scientists in diverse fields. However, there are still very few reports on the adsorption of organic dyes by PAFs. In this work, four new PAFs have been facilely synthesized by the polymerization of a tetrahedral-shaped (four-node) monomer with a series of three-node monomers through Suzuki-Miyaura coupling reactions. All the obtained materials possess hierarchical porous structures and show high thermal and chemical stability. The Brunauer-Emmett-Teller (BET) surface areas of these PAFs were determined to be 857 m2 g-1 for PAF-111, 526 m2 g-1 for PAF-112A, 725 m2 g-1 for PAF-112B, and 598 m2 g-1 for PAF-113. Rhodamine B was selected as a model organic dye to test the adsorption capacities of the obtained PAF materials. PAF-111 showed a maximum adsorption capacity of 1666 mg g-1 (167 wt %) for Rhodamine B, which is among the highest values reported to date for porous organic materials. It is noteworthy that PAF-111 could be reused in at least ten cycles under the adsorption conditions without any loss of adsorption capacity. Our study has revealed the great potential and advantages of PAFs as ultrastable adsorption materials for the removal of organic dyes.

Facile construction of highly porous carbon materials derived from porous aromatic frameworks for greenhouse gas adsorption and separation

Fully conjugated porous aromatic frameworks (PAFs) have been constructed through Gilch reaction. The obtained PAFs have rigid conjugated backbones, high specific surface area, and excellent stability. The prepared PAF-154 and PAF-155 have been successfully applied in the perovskite solar cells (PSCs) by doping into the perovskite layer. The champion PSC devices afford a power conversion efficiency of 22.8% and 22.4%. It is found that the PAFs can be used as an efficient nucleation template, thus regulating the perovskite crystallinity. Meanwhile, PAFs can also passivate defects and promote carriers transporting in the perovskite film. By the comparative study with their linear counterpart, we unravel that the efficacy of PAFs is highly related to their porous structure and rigid fully conjugated networks. The unencapsulated devices with PAFs doping exhibit outstanding long-term stability, retaining 80% of their initial efficiencies after half-year storage in ambient conditions.

Fully conjugated porous aromatic frameworks (PAFs) have been constructed through Gilch reaction. The obtained PAFs have rigid conjugated backbones, high specific surface area, and excellent stability. The prepared PAF‐154 and PAF‐155 have been successfully applied in the perovskite solar cells (PSCs) by doping into the perovskite layer. The champion PSC devices afford a power conversion efficiency of 22.8 % and 22.4 %. It is found that the PAFs can be used as an efficient nucleation template, thus regulating the perovskite crystallinity. Meanwhile, PAFs can also passivate defects and promote carriers transporting in the perovskite film. By the comparative study with their linear counterpart, we unravel that the efficacy of PAFs is highly related to their porous structure and rigid fully conjugated networks. The unencapsulated devices with PAFs doping exhibit outstanding long‐term stability, retaining 80 % of their initial efficiencies after half‐year storage in ambient conditions.

Biofouling is a major obstacle to the efficient extraction of uranium from seawater due to the numerous marine microorganisms in the ocean. Herein, we report a novel amidoxime (AO) crystalline cova...

Porous aromatic frameworks (PAFs) represent an important category of porous solids. PAFs possess rigid frameworks and exceptionally high surface areas, and, uniquely, they are constructed from carbon-carbon-bond-linked aromatic-based building units. Various functionalities can either originate from the intrinsic chemistry of their building units or are achieved by postmodification of the aromatic motifs using established reactions. Specially, the strong carbon-carbon bonding renders PAFs stable under harsh chemical treatments. Therefore, PAFs exhibit specificity in their chemistry and functionalities compared with conventional porous materials such as zeolites and metal organic frameworks. The unique features of PAFs render them being tolerant of severe environments and readily functionalized by harsh chemical treatments. The research field of PAFs has experienced rapid expansion over the past decade, and it is necessary to provide a comprehensive guide to the essential development of the field at this stage. Regarding research into PAFs, the synthesis, functionalization, and applications are the three most important topics. In this thematic review, the three topics are comprehensively explained and aptly exemplified to shed light on developments in the field. Current questions and a perspective outlook will be summarized.

Natural gas, a lower emission alternative than its fossil fuel counterparts, requires the removal of carbon dioxide, known as “sweetening”, prior to its use. In this study we computationally explore the separation of methane and carbon dioxide using a new adsorbent consisting of lithium-decorated fullerenes (Li6C60) impregnated within a series of porous aromatic frameworks (PAFs) of various pore sizes. The strong affinity of CO2 with the impregnated frameworks, confirmed by density functional theory, leads to selective adsorption over CH4. The impregnation can also double the CO2 adsorption capacity compared to the bare PAF and increase selectivity of CO2/CH4 up to 48 for an optimum amount of Li6C60, which is above the current industry benchmark. Overall, the study reveals physical insights and proposes impregnated PAFs to be promising candidates for CO2/CH4 separations for natural gas purification.

As a class of materials with large specific surface area and chemical stability, porous aromatic frameworks (PAFs) have attracted much attention in the fields of gas adsorption, separation, and catalysis. However, synthetic methods for PAFs have been limited to a few coupling reactions, and PAF powders were usually obtained with a diameter of micrometer size. Here, we demonstrate an efficient N-H insertion reaction of diazoesters in the synthesis of PAFs with a diameter <200 nm. The established polymerization can be performed at room temperature, and four PAFs with different skeletons and composition can be obtained in high yields. The prepared PAFs have appreciable thermal and chemical stabilities. Because of the presence of ester groups in the backbone, the prepared PAFs with α-phenylglycine fragments can be easily obtained through the successive hydrolysis of the ester groups. The synthesized PAFs bearing phenylglycine moieties exhibit good water dispersibility and low cytotoxicity. We further show the potential of these PAFs in drug loading and photodynamic therapy.

ConspectusPorous frameworks possess high porosity and adjustable functions. The two features conjointly create sufficient interfaces for matter exchange and energy transfer within the skeletons. For crystalline porous frameworks, including metal organic frameworks (MOFs) and covalent organic frameworks (COFs), their long-range ordered structures indeed play an important role in managing versatile physicochemical behaviors such as electron transfer or band gap engineering. It is now feasible to predict their functions based on the unveiled structures and structure-performance relationships. In contrast, porous organic frameworks (POFs) represent a member of the porous solid family with no long-range regularity. For the case of POFs, the randomly packed building units and their disordered connections hinder the electronic structural consistency throughout the entire networks. However, many investigations have demonstrated that the functions of POFs could also be designed and originated from their local motifs.In this Account, we will first provide an overview of the design and synthesis principles for porous aromatic frameworks (PAFs), which are a typical family of POFs with high porosity and exceptional stability. Specifically, the functions achieved by the specific design and synthesis of in-framework motifs will be demonstrated. This strategy is particularly intuitive to introduce desired functions to PAFs, owing to the exceptional tolerance of PAFs to harsh chemical treatments and synthetic conditions. The local structures can be either obtained by selecting suitable building units, sometimes with the aid of computational screening, or emerge as the product of coupling reactions during the synthetic process. Radical PAFs can be obtained by incorporating a persistent radical molecule as a building unit, and the rigid and porous framework may facilitate the formation of radical species by trapping spins in the organic network, which could avoid the delocalizing and recombining processes. Alternatively, radical motifs can also be formed during the formation of the framework linkages. The coupling reaction plays an important role in the construction of functional motifs like diacetylene. The highly porous, radical PAFs showed significant performance as anodes of lithium-ion batteries. To improve the charge transport within the framework, the building units and their connecting manner were cohesively considered, and the framework with a fully conjugated backbone was built up. In another case, the explicit product of the cross-coupling reaction ensured the precise assembly of two building units with electron donating and accepting abilities; therefore, the moving direction of photogenerated electrons was rationally controlled. Constructing a fully conjugated backbone or rationally designing a D-A system for charge transfer in porous frameworks introduced exciting properties for photovoltaic and photocatalysis, and their highly porous, stable frameworks improved their functional applications for perovskite solar cells and chemical productions. These investigations shed light on the designable combination of intrinsic functional motifs with highly porous organic frameworks for effective energy storage and conversion.

The adsorption of pure SO2, a sulfur-containing acid gas, by porous aromatic frameworks (PAFs) was investigated with a range of computational methods including first-principles density functional theory and grand canonical Monte Carlo calculations. We found that the presence of combinations of functional groups, including the electron-donating groups −CH3, −OH, and −NH2, and the electron-withdrawing groups −CN, −COOH, and −NO2, within the PAF structures was predicted to enhance SO2 uptake at low pressure. In particular, the simulations predicted that the functionalized PAFs, especially double-functionalized PAFs, PAF-(OH)2 and PAF-(COOH)2, as well as mixed-functionalized PAFs, PAF-2-CN-3-NO2 and PAF-3-OH-5-NH2, were able to capture high loadings of SO2 pure gas under a very low pressure at 298 K. The additional functional groups were able to strengthen the interactions between the PAF frameworks and the acid gas molecules. At the same time, introducing two functional groups to PAFs generally decreases the maximum adsorption limit, due to the smaller pore volume available to the gases. In this work, we created a library of various functionalized PAFs, as well as simulated their adsorption isotherms. The results of this work can be used as a guideline for other combinations of functionalized PAFs and their experimental synthesis for maximal acid gas adsorption.

Dynamic Covalent Synthesis of Crystalline Porous Graphitic Frameworks