Surface-engineered porous aromatic framework enables a monolithic aerogel for high-efficiency benzene adsorption

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.

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.

Selective extraction of uranium from water has attracted worldwide attention because the largest source of uranium is seawater with various interferenceions (Na+ , K+ , Mg2+ , Ca2+ , etc.). However, traditional adsorbents encapsulate most of their functional sites in their dense structure, leading to problems with low selectivity and adsorption capacities. In this work, the tailor-made binding sites are first decorated into porous skeletons, and a series of molecularly imprinted porous aromatic frameworks are prepared for uranium extraction. Because the porous architecture provides numerous accessible sites, the resultant material has a fourfold increased ion capacity compared with traditional molecularly imprinted polymers and presents the highest selectivity among all reported uranium adsorbents. Moreover, the porous framework can be dispersed into commercial polymers to form composite components for the practical extraction of uranium ions from simulated seawater.

Several new porous aromatic frameworks(PAFs) were designed by Li doping or B substitution based on the PAF-301 molecular model. The hydrogen storage capacities of these materials were investigated using quantum mechanics and molecular mechanics methods. First, the binding energies between H2 and the different molecular fragments were calculated using quantum mechanics, and the atomic charge distributions of the molecular fragments were calculated by the density-derived electrostatic and chemical charge(DDEC)method. Then, the adsorption equilibrium properties of H2 on the different PAFs were calculated at 77 and 298 K using grand canonical Monte Carlo(GCMC) simulations. The results indicate that the binding energy between H2 and benzene without Li doping is poor, while the binding energies between H2 and Li-doped six-member rings are improved. Li atoms doped into the benzene ring result in higher positive charges, and the electronegativity of the original carbon atoms in the benzene ring increase after its two carbon atoms are replaced with two boron atoms. Among these new materials, PAF-301 Li has the highest hydrogen storage capacity at 77 K, while PAFC4B2H4-Li2-Si and PAF-C4B2H4-Li2-Ge have better hydrogen storage capacities at room temperature than at 77 K. However, the hydrogen storage capacities of these various materials at room temperature are far below the capacities at cryogenic temperature. The preferential adsorption sites for H2 on the PAFs at 77 K were analyzed through the potential energy surfaces and mass center density distribution of the adsorption equilibrium. It was found that there are four obvious high-density adsorption regions in the frameworks of PAF-301 and PAF-301 Li because of their wide low-energy regions in the crystal center, while there are only two distinct high-density adsorption regions in the other three PAFs because of their narrow low-energy regions in the unit cell center.

The first example of an application of porous aromatic frameworks (PAFs) in synthesis of catalysts for oxidative desulfurization was described. The mesoporous aromatic framework with a diamondlike structure was modified with tungsten and molybdenum carbonyls, and synthesized catalysts were successfully used for low-temperature oxidation of organic sulfides. The obtained materials were characterized by low-temperature nitrogen adsorption/desorption, transmission electron microscopy, atomic emission spectroscopy, and X-ray photoelectron spectroscopy techniques. It was shown that the polymer did not undergo oxidation and retained its structure in the presence of hydrogen peroxide and tert-butyl hydroperoxide. The catalysts possessed high activity because of the hydrophobic nature of the carrier and their high surface area.

The Chan-Lam reaction was applied to the preparation of a porous aromatic framework (PAF) at room temperature in air without employing expensive noble metal catalysts. The PAF is shown to be a viable sorbent for use in solid-phase microextraction of antioxidants and preservatives as contained in cosmetics. The material was characterized by FT-IR, thermogravimetric analysis, scanning electron microscopy and transmission electron microscopy. Effects of various experimental parameters on the extraction efficiency were investigated. The analytes were extracted by using a fiber coated with the PAF. Following extraction, the fiber was directly inserted into a GC analyzer with flame ionization detection. Specifically, the following antioxidants and preservatives were extracted and detected: 2-tert-butyl-4-methoxyphenol (BHA), 2,6-bis(1,1-dimethylethyl)-4-methylphenol (BHT), methylparaben (MP), and propylparaben (PP). Under the optimum experimental conditions, the method has a low limit of detection (0.12–0.22 μg L−1 for antioxidants and 0.19–0.30 μg L−1 for preservatives) and a linear response in the 0.02 to 500 mg L−1 for concentration range. Relatives standard deviations (RSDs) for one fiber ranged from 1.2% to 7.2% (n = 6) while those for three different fibers (n = 3) were in the range between 4.3% and 11.0%. The fiber was successfully employed to the determination of antioxidants and preservatives in cosmetic samples with recoveries of 81.5%–110.2%.

Here, we report the fabrication of palladium nanoparticles on porous aromatic frameworks (Pd/PAF-6) using a facile chemical approach, which was characterized by various spectro- and electrochemical techniques. The differential pulse voltammetry (DPV) response of Pd/PAF-6 toward the vanillin (VA) sensor shows a linear relationship over concentrations (10-820 pM) and a low detection limit (2 pM). Pd/PAF-6 also exhibited good anti-interference performance toward 2-fold excess of ascorbic acid, nitrophenol, glutathione, glucose, uric acid, dopamine, ascorbic acid, 4-nitrophenol, glutathione, glucose, uric acid, dopamine, and 100-fold excess of Na(+), Mg(2+), and K(+) during the detection of VA. The developed electrochemical sensor based on Pd/PAF-6 had good reproducibility, as well as high selectivity and stability. The established sensor revealed that Pd/PAF-6 could be used to detect VA in biscuit and ice cream samples with satisfactory results.

Rhenium is a rare and significant metal, making its recovery crucial for resource management. Herein, to address these issues, three porous aromatic frameworks with high-density ionic sites were successfully synthesized using a multivariate strategy. These MTV-iPAFs are named PAF-271, PAF-272 and PAF-273, respectively, and were employed for the ultrafast and selective removal of ReO4 -. These materials exhibit varying high specific surface areas, porosities, and imidazolium densities. The results demonstrated that compared to PAF-272 with low adsorption site density and PAF-273 with high specific surface area, PAF-271 with higher imidazolium density exhibited superior performance for ReO4 - adsorption. Specifically, PAF-271 could remove 98 % of ReO4 - within 2 min, achieving a maximum adsorption capacity was 707 mg g-1. Moreover, PAF-271 also had excellent stability, making it highly effective for ReO4 - adsorption under strong acid or alkali, radiation, high anion concentration and even in Hanford simulated waste liquid, it is expected to be applied to adsorb TcO4 -. Embedding high-density ionic sites within PAFs by multivariate strategy result in highly stable and effective ReO4 - disposal approach.

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.

Porous aromatic frameworks (PAFs) with rich metal coordination sites are highly effective support materials for gold nanoparticles (AuNPs), which would not only prevent AuNPs agglomeration but also facilitate mass transfer during the catalytic process. In this work, PAF-160, -161, and -162 bearing diphosphine units are synthesized via the Friedel-Crafts alkylation reaction to act as efficient platforms for AuNPs immobilization. These PAFs possess high surface areas (up to 655 m2 g-1) together with excellent stabilities, and the different linkage lengths between P centers allow more scattered and accessible sites for gold coordination. In the resultant Au-PAFs, AuNPs with uniform sizes are stabilized dispersedly. The catalytic performances of these Au-PAFs are monitored by the reduction of 4-nitrophenol (4-NP), and all materials exhibit excellent catalytic activities on the reduction of 4-NP, especially Au-PAF-162 with the apparent rate constant (kapp) up to 0.019 s-1. Additionally, the reductions of various nitroarenes with different functional groups are explored and all Au-PAFs can convert most nitroaromatic derivatives to the corresponding arylamines with high conversions of 99%, in which the reaction mechanism is also proposed. Furthermore, a continuous catalytic device with Au-PAF-160 catalyst is explored, and Au-PAF-160 can convert 1-chloro-4-nitrobenzene, 2,6-dichoronitrobenzene and 1-chloro-2,4-dinitrobenzene into the corresponding amines in sequence in the continuous flow catalytic experiments. This work has enriched the variety of porous materials for noble metal immobilization and promotes their applications in heterogeneous catalysis.

Targeted synthesis of novel porous aromatic frameworks with selective separation of CO2/CH4 and CO2/N2

Triclosan (TCS) has been regarded as an emerging contaminant in aquatic systems, making its efficient removal of great significance. In this study, NPVMo@iPAF-1, with a specific surface area of 665 m2/g, was prepared by incorporating (NH4)5H6PV8Mo4O40 into porous aromatic frameworks (PAF). The maximum adsorption capacity of TCS on NPVMo@iPAF-1 reached 917.1mg/g, as calculated from the Langmuir model. Fixed-bed columns packed with NPVMo@iPAF-1 were employed for TCS removal; the experiment data strongly correlated with the Thomas and Yoon-Nelson models under different operational conditions. Pore preservation, electrostatic effects, and the synergistic effect of π-π interactions contributed to the effective adsorption of TCS onto NPVMo@iPAF-1. The NPVMo@iPAF-1 fixed-bed column could be effectively regenerated through in-situ ozonation for more than 10 regeneration cycles. NPVMo@iPAF-1 turned out to be a promising adsorbent for removing TCS not only from pure water but also from reclaimed water and surface water samples.

Porous aromatic frameworks (PAFs) are a fundamental group of porous materials characterized by their distinct structural features and large surface areas. These materials are synthesized from aromatic building units linked by strong carbon-carbon bonds, which confer exceptional rigidity and long-term stability. PAFs functionalities may arise directly from the intrinsic chemistry of their building units or through the postmodification of aromatic motifs using well-defined chemical processes. Compared to other traditional porous materials such as zeolites and metallic-organic frameworks, PAFs demonstrate superior stability under severe chemical treatments due to their robust carbon-carbon bonding. Even in challenging environments, the chemical stability and ease of functionalization of PAFs demonstrate their flexibility and specificity. Research on PAFs has significantly expanded and accelerated over the past decade, necessitating a comprehensive overview of key advancements in this field. This review provides an in-depth analysis of the recent advances in the synthesis, functionalization, and dimensionality of PAFs, along with their distinctive properties and wide-ranging applications. This review explores the innovative methodologies in PAFs synthesis, the strategies for functionalizing their structures, and the manipulation of their dimensionality to tailor their properties for specific potential applications. Similarly, the key application areas, including batteries, absorption, sensors, CO2 capture, photo-/electrocatalytic usages, supercapacitors, separation, and biomedical are discussed in detail, highlighting the versatility and potential of PAFs in addressing modern scientific and industrial challenges.

The adsorption process is widely used for the treatment of wastewater containing organic pollutants. We fabricated highly branched pillar[5]arene-based porous aromatic frameworks (PAFs), PAF-P5, for the adsorption and removal of organic pollutants (short-chain alkyl derivatives 1-3 and pesticide molecules 4-6) from water with high removal efficiency (RE). However, PAF-P5 was incapable of adsorbing aromatic organic dyes 7-9. Adsorption kinetic studies indicated that the adsorption is mainly driven by strong host-guest interactions between 1-3 and the pillar[5]arene units in PAF-P5, while 4-6 only weakly interacted with the pillar[5]arene units in PAF-P5. Moreover, chemically breaking down the pillar[5]arene rings in PAF-P5 caused changes in the pore size, the microenvironment inside of the pores, and the frame morphology, and the resultant frameworks, PAF-DeP5, exhibited poor adsorption toward 1-6 but adsorbed 7-9 possibly through physical adsorption as implied by fitting the experimental data into the adsorption kinetic models.

The targeted synthesis of a series of novel charged porous aromatic frameworks (PAFs) is reported. The compounds PAF-23, PAF-24, and PAF-25 are built up by a tetrahedral building unit, lithium tetrakis(4-iodophenyl)borate (LTIPB), and different alkyne monomers as linkers by a Sonogashira-Hagihara coupling reaction. They possess excellent adsorption properties to organic molecules owing to their "breathing" dynamic frameworks. As these PAF materials assemble three effective sorption sites, namely the ion bond, phenyl ring, and triple bond together, they exhibit high affinity and capacity for iodine molecules. To the best of our knowledge, these PAF materials give the highest adsorption values among all porous materials (zeolites, metal-organic frameworks, and porous organic frameworks) reported to date.