Gas storage in porous aromatic frameworks (PAFs)

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

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.

A porous aromatic framework (referred as PAF-39) based on tetrakis(4-(9H-carbazol-9-yl)phenyl)methane has been designed and synthesized by oxidative coupling polymerization. PAF-39 exhibits high surface area and high heat of adsorption (Qst ) of CO2 . Especially, the PAF material was applied for non-enzymatic H2 O2 sensing.

A new facile method for synthesis of porous azo-linked polymers (ALPs) is reported. The synthesis of ALPs was accomplished by homocoupling of aniline-like building units in the presence of copper(I) bromide and pyridine. The resulting ALPs exhibit high surface areas (SABET = 862–1235 m2 g–1), high physiochemical stability, and considerable gas storage capacity especially at high-pressure settings. Under low pressure conditions, ALPs have remarkable CO2 uptake (up to 5.37 mmol g–1 at 273 K and 1 bar), as well as moderate CO2/N2 (29–43) and CO2/CH4 (6–8) selectivity. Low pressure gas uptake experiments were used to calculate the binding affinities of small gas molecules and revealed that ALPs have high heats of adsorption for hydrogen (7.5–8 kJ mol–1), methane (18–21 kJ mol–1), and carbon dioxide (28–30 kJ mol–1). Under high pressure conditions, the best performing polymer, ALP-1, stores significant amounts of H2 (24 g L–1, 77 K/70 bar), CH4 (67 g L–1, 298 K/70 bar), and CO2 (304 g L–1, 298 K/40 bar).

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.

Porous aromatic frameworks (PAFs) are novel materials with diamond topology. With the aim of enhancing their CO(2) capture and storage capacity and investigating the effect of nitrogen and/or -COOH decorations on CO(2) adsorption in PAFs, a series of N-containing PAFs were designed based on ab initio results. The interaction energies (E(int)) between CO(2) and each six-membered ring were calculated at the B2PLYP-D2/def2-TZVPP level, then the six-membered rings with high CO(2) -binding affinity were selected and used in the PAFs. To explore the performance of the designed PAFs, the CO(2) uptake, selectivity of CO(2) over CH(4), H(2), and N(2), and the E(int) value of CO(2) in PAFs were investigated by using grand canonical Monte Carlo (GCMC) simulations and ab initio calculations. This work shows that pyridine with one nitrogen atom can provide a strong physisorption site for CO(2), whereas more nitrogen atoms in heterocycles will reduce the interaction, especially at relatively low pressure. PAFs with -COOH groups show high CO(2) capacity. Our work provides an efficient way to understand the adsorption mechanism and a supplemental approach to experimental work.

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.

The structural and dynamic factors governing porosity in porous aromatic frameworks (PAFs) are investigated using coarse-grained molecular dynamics simulations. PAFs form amorphous, porous networks with potential for gas storage and separation applications. We focus on a series of four PAFs—PAF-1, PPN-1, PPN-2, and PPN-3—which exhibit an unexpected trend in porosity as the structure of the PAF monomer is varied. The simulations suggest that nonbonding dispersion interactions that stabilize misbound monomer configurations play an essential role in the formation of porosity-reducing interpenetrated frameworks in PAFs comprising the larger PPN-1 and PPN-2 monomers; on the other hand, the simulations indicate that the steric bulk of a key reaction intermediate acts to limit interpenetration in PAFs made up of the smaller PAF-1 and PPN-3 monomers. The simulations also show that the rate of cluster growth, which depends largely on the monomer concentration used in the experimental synthesis, is significantly higher for PPN-1 and PPN-2, which would exacerbate the kinetic trapping of interpenetrated misbound configurations. This work provides design rules for synthesizing highly porous amorphous networks through the choice of monomer structure and reaction conditions that limit framework interpenetration.

The selective adsorption of target acid gas molecules from binary gas mixtures by porous aromatic frameworks (PAFs) with two identical functional groups per aromatic ring (PAF-R2) was computationally investigated using grand canonical Monte Carlo simulations. PAF-R2 adsorption was considered for three binary mixtures of small molecular concentrations of acid gas and abundant nitrogen gas (CO2/N2, SO2/N2, and H2S/N2). The results indicate that additional functional groups enhance acid gas loadings and selectivity, compared with pristine PAF and single-functionalized PAFs. Low pressures yield linearly increasing gas loadings and constant selectivity, while high pressures yield much higher adsorption and selectivity. In particular, SO2 loading and selectivity under high pressures are heavily influenced by the PAF's maximum adsorption limit, which can be linked back to the functional groups and their configuration. In summary, PAF-(3,5)-(COOH)2 (nomenclature of PAFs is provided in the Appendix in the Supporting Information) and many other PAF with the same two electron-withdrawing groups are predicted to have great acid gas adsorption and selectivity from gas mixtures, while PAF-(3,5)-(OH)2 (one of PAFs with two identical electron-donating groups) is predicted to have good adsorption and selectivity, especially under elevated pressures. The results of this work can provide insights into various types of PAFs with great selective adsorption ability and their corresponding conditions. The simulation procedures and results may inspire the exploration and screening of other types of PAFs or porous materials, for acid gas absorption.

This study explores the room-temperature synthesis of porous materials and the immobilization of CO2 without the use of metals. The porous aromatic frameworks synthesized at room temperature retain the important functional group structure, and the abundance of carbon-chlorine bonds creates an excellent environment for imidazole linkage. Consequently, a catalyst conducive to the cycloaddition of carbon dioxide is obtained. Hexachloro-p-xylene is explored as the precursor, and a catalyst conducive to carbon dioxide cycloaddition is obtained. The functionalized porous aromatic frameworks (PAF-280-I/B) possess a conversion of 99.6% with a selectivity of 98.9% toward styrene carbonate (SC). The findings of this study can help mitigate the impact of greenhouse gases and enable the production of organic compounds in the circular carbonate platform, turning waste into valuable resources.

Construction and sorption properties of pyrene-based porous aromatic frameworks

The adsorption isotherms of CO2 in several porous aromatic frameworks (PAFs) have been simulated with Grand Canonical Monte Carlo technique, to support the synthesis of new materials for efficient carbon dioxide capture and storage. The simulations covered the 0-60 bar pressure range and were repeated at 273, 298, and 323 K. The force field employed in the simulations was optimized to fit the correct behavior of the free gas and to reproduce the CO2-phenyl interactions computed at high quantum mechanical level. PAFs are based on the diamond structure, with polyaromatic chains inserted in C-C bonds. We examined four PAF-30n (n being the number of phenyl rings in the aromatic linkers), finding that PAF-302 is overall the best performing, although PAF-301 provides higher adsorbed densities at very low pressure. The CO2 adsorption then was simulated in a number of modified PAF-302, with different functional groups (aminomethane, toluene, pyridine, and imidazole) attached to the phenyl chains; different degrees of substitution (25%, 50%, and 100% derivatized rings) were considered. The effects of functionalization and the dependence on the substitution degree are carefully discussed, to determine the most promising materials at low, intermediate, and high pressures.

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.