A Novel Aluminum-Based Metal-Organic Framework with Uniform Micropores for Trace BTEX Adsorption

A Novel kgd-Topological Covalent Organic Framework with Optimum Pore Size for Efficient Benzene/Cyclohexane Separation

Separation of benzene and cyclohexane is essential for obtaining high-purity cyclohexane in the chemical industry and for resource recovery from exhaust gas, but is one of the most challenging separation processes due to their highly similar boiling points and kinetic diameters. Herein, based on the isoreticular contraction strategy, a novel covalent organic framework (i.e., HFPB-TAB-COF) with kgd topological structure and average pore size of 5.70 Å, between the kinetic diameters of benzene (5.60 Å) and cyclohexane (6.10 Å), is synthesized for benzene/cyclohexane separation by pore confinement effect. HFPB-TAB-COF has the highest ideal adsorbed solution theory (IAST) selectivity of 36.0 for benzene/cyclohexane separation, and can produce 0.48 mmol g-1 cyclohexane with purity of +99% from 50:50 (v:v) benzene/cyclohexane mixture under dynamic condition, higher than reported separation materials. Optimizing the pore size of COFs by isoreticular contraction strategy can trigger the pore confinement effect for better meeting the separation challenge in industry.

Trace benzene capture by decoration of structural defects in metal-organic framework materials

Capture of trace benzene is an important and challenging task. Metal-organic framework materials are promising sorbents for a variety of gases, but their limited capacity towards benzene at low concentration remains unresolved. Here we report the adsorption of trace benzene by decorating a structural defect in MIL-125-defect with single-atom metal centres to afford MIL-125-X (X = Mn, Fe, Co, Ni, Cu, Zn; MIL-125, Ti8O8(OH)4(BDC)6 where H2BDC is 1,4-benzenedicarboxylic acid). At 298 K, MIL-125-Zn exhibits a benzene uptake of 7.63 mmol g-1 at 1.2 mbar and 5.33 mmol g-1 at 0.12 mbar, and breakthrough experiments confirm the removal of trace benzene (from 5 to <0.5 ppm) from air (up to 111,000 min g-1 of metal-organic framework), even after exposure to moisture. The binding of benzene to the defect and open Zn(II) sites at low pressure has been visualized by diffraction, scattering and spectroscopy. This work highlights the importance of fine-tuning pore chemistry for designing adsorbents for the removal of air pollutants.

Enhanced Benzene Adsorption in Chloro-Functionalized Metal-Organic Frameworks

The functionalization of metal-organic frameworks (MOFs) to enhance the adsorption of benzene at trace levels remains a significant challenge. Here, we report the exceptional adsorption of trace benzene in a series of zirconium-based MOFs functionalized with chloro groups. Notably, MFM-68-Cl2, constructed from an anthracene linker incorporating chloro groups, exhibits a remarkable benzene uptake of 4.62 mmol g-1 at 298 K and 0.12 mbar, superior to benchmark materials. In situ synchrotron X-ray diffraction, Fourier transform infrared microspectroscopy, and inelastic neutron scattering, coupled with density functional theory modeling, reveal the mechanism of binding of benzene in these materials. Overall, the excellent adsorption performance is promoted by an unprecedented cooperation between chloro-groups, the optimized pore size, aromatic functionality, and the flexibility of the linkers in response to benzene uptake in MFM-68-Cl2. This study represents the first example of enhanced adsorption of trace benzene promoted by -CH···Cl and Cl···π interactions in porous materials.

Cross-Polarization Solid-State NMR Quantification of Species within Pores of Metal-Organic Frameworks: A Case Study of α-Mg3(HCOO)6

The quantitative measurement of adsorbed guest species within metal-organic framework (MOF) pores is of fundamental importance for evaluating the adsorption performance of MOFs. However, routine analytic techniques such as thermogravimetric analysis cannot distinguish the contribution from species adsorbed within pores, species adsorbed on the surface, and gas phase or liquid phase encapsulated in the inter-crystalline space. Herein, we developed a new quantification method based on the cross-polarization (CP) solid-state nuclear magnetic resonance (ssNMR) technique, in which only the species within MOF pores are selectively probed due to the dramatically reduced mobility. Using the commercialized MOF α-Mg3(HCOO)6 as an example, a good linear correlation between Areaguest/Areaframework (i. e., the integrated area of guest and framework 13C NMR signals) and guest loading can be observed for several representative molecules such as benzene, tetrahydrofuran (THF), and 1,4-dioxane, clearly revealing the feasibility of CP quantification approach. The effects of guest molecule and corresponding residual mobility on the CP quantification are further discussed by varying the geometry and size of guest molecules. This methodology thus provides an effective and irreplaceable route to evaluate the adsorption performance of porous materials in-depth, especially for liquid-phase adsorption and gas-phase adsorption in which the capillary condensation is not negligible.

Thermodynamically and Kinetically Enhanced Benzene Vapor Sensor Based on the Cu-TCPP-Cu MOF with Extremely Low Limit of Detection

As a carcinogenic and highly neurotoxic hazardous gas, benzene vapor is particularly difficult to be distinguished in BTEX (benzene, toluene, ethylbenzene, xylene) atmosphere and be detected in low concentrations due to its chemical inertness. Herein, we develop a depth-related pore structure in Cu-TCPP-Cu to thermodynamically and kinetically enhance the adsorption of benzene vapor and realize the detection of ultralow-temperature benzene gas. We find that the in-plane π electronic nature and proper pore sizes in Cu-TCPP-Cu can selectively induce the adsorption and diffusion of BTEX. Interestingly, the theoretical calculations (including density functional theory (DFT) and grand canonical Monte Carlo (GCMC) simulations) exhibit that benzene molecules are preferred to adsorb and array as a consecutive arrangement mode in the Cu-TCPP-Cu pore, while the TEX (toluene, ethylbenzene, xylene) dominate the jumping arrangement model. The differences in distribution behaviors can allow adsorption and diffusion of more benzene molecules within limited room. Furthermore, the optimal pore-depth range (60-65 nm) of Cu-TCPP-Cu allows more exposure of active sites and hinders the gas-blocking process. The optimized sensor exhibits ultrahigh sensitivity to benzene vapor (155 Hz/μg@1 ppm), fast response time (less than 10 s), extremely low limit of detection (65 ppb), and excellent selectivity (83%). Our research thus provides a fundamental understanding to design and optimize two-dimensional metal-organic framework (MOF)-based gas sensors.

Double-walled Al-based MOF with large microporous specific surface area for trace benzene adsorption

Double-walled metal-organic frameworks (MOFs), synthesized using Zn and Co, are potential porous materials for trace benzene adsorption. Aluminum is with low-toxicity and abundance in nature, in comparison with Zn and Co. Therefore, a double-walled Al-based MOF, named as ZJU-520(Al), with large microporous specific surface area of 2235 m2 g-1, pore size distribution in the range of 9.26-12.99 Å and excellent chemical stability, was synthesized. ZJU-520(Al) is consisted by helical chain of AlO6 clusters and 4,6-Di(4-carboxyphenyl)pyrimidine ligands. Trace benzene adsorption of ZJU-520(Al) is up to 5.98 mmol g-1 at 298 K and P/P0 = 0.01. Adsorbed benzene molecules are trapped on two types of sites. One (site I) is near the AlO6 clusters, another (site II) is near the N atom of ligands, using Grand Canonical Monte Carlo simulations. ZJU-520(Al) can effectively separate trace benzene from mixed vapor flow of benzene and cyclohexane, due to the adsorption affinity of benzene higher than that of cyclohexane. Therefore, ZJU-520(Al) is a potential adsorbent for trace benzene adsorption and benzene/cyclohexane separation.

Spirobifluorene-Based Porous Organic Salts: Their Porous Network Diversification and Construction of Chiral Helical Luminescent Structures

Porous organic salts (POSs) are organic porous materials assembled via charge-assisted hydrogen bonds between strong acids and bases such as sulfonic acids and amines. To diversify the network topology of POSs and extend its functions, this study focused on using 4,4',4'',4'''-(9,9'-spirobi[fluorene]-2,2',7,7'-tetrayl)tetrabenzenesulfonic acid (spiroBPS), which is a tetrasulfonic acid comprising a square planar skeleton. The POS consisting of spiroBPS and triphenylmethylamine (TPMA) (spiroBPS/TPMA) was constructed from the two-fold interpenetration of an orthogonal network with pts topology, which has not been reported in conventional POSs, owing to the shape of the spirobifluorene backbone. Furthermore, combining tris(4-chlorophenyl)methylamine (TPMA-Cl) and tris(4-bromophenyl)methylamine (TPMA-Br), which are bulkier than TPMA owing to the introduction of halogens at the p-position of the phenyl groups with spiroBPS allows us to construct novel POSs (spiroBPS/TPMA-Cl and spiroBPS/TPMA-Br). These POSs were constructed from a chiral helical network with pth topology, which was induced by the steric hindrance between the halogens and the curved fluorene skeleton. Moreover, spiroBPS/TPMA-Cl with pth topology exhibited circularly polarized luminescence (CPL) in the solid state, which has not been reported in hydrogen-bonded organic frameworks (HOFs).

Interior and Exterior Surface Modification of Zr-Based Metal-Organic Frameworks for Trace Benzene Removal

The emission of volatile organic compounds (VOCs) significantly contributes to air pollution and poses a serious threat to human health. Benzene, one of the most toxic VOCs, is difficult for the human body to metabolize and is classified as a Group 1 carcinogen. The development of efficient adsorbents for removing trace amounts of benzene from ambient air is thus of great importance. In this work, we studied the benzene adsorption properties of four Zr-based metal-organic frameworks (Zr-MOFs) through static volumetric and dynamic breakthrough experiments. Two previously reported Zr-MOFs, BUT-12 and STA-26, were prepared with a tritopic carboxylic acid ligand (H3L1) functionalized with three methyl groups, and STA-26 is a 2-fold interpenetrated network of BUT-12. Two new isoreticular Zr-MOFs, BUT-12-Et and STA-26-Et, were synthesized using a similar ligand, H3L2, where the methyl groups are replaced with ethyl groups. There are mesopores in BUT-12 and BUT-12-Et and micropores in STA-26 and STA-26-Et. The four Zr-MOFs all showed high stability in liquid water and acidic aqueous solutions. The microporous STA-26 and STA-26-Et showed much higher benzene uptakes than mesoporous BUT-12 and BUT-12-Et at room temperature under low pressures. Particularly, the benzene adsorption capacity of STA-26-Et was high up to 2.21 mmol/g at P/P0 = 0.001 (P0 = 12.78 kPa), higher than those of the other three Zr-MOFs and most reported solid adsorbents. Breakthrough experiments confirmed that STA-26-Et could effectively capture trace benzene (10 ppm) from dry air; however, its benzene capture capacity was reduced by 90% under humid conditions (RH = 50%). Coating of the crystals of STA-26-Et with polydimethylsiloxane (PDMS) increased the hydrophobicity of the exterior MOF surfaces, leading to a more than 2-fold improvement in its benzene capture capacity in the breakthrough experiment under humid condition. PDMS coating of STA-26-Et likely slowed down the water adsorption process, and thus, the adsorbent afforded more efficient capture of benzene. This work demonstrates that modifying both the interior and exterior surfaces of MOFs can effectively enhance their performance in capturing trace benzene from ambient air, even under humid conditions. This finding is meaningful for the development of new adsorbents for effective air purification applications.

Methyl-Functionalized Al-Based MOF ZJU-620(Al): A Potential Physisorbent for Carbon Dioxide Capture

Developing Al-based metal-organic frameworks (MOFs) with moisture-resistance ability is a challenge for carbon dioxide (CO2) capture. Methyl-functionalized Al-based MOF ZJU-620(Al), with excellent chemical-thermal stability and specific surface area of 1347 m2/g, observed here, is a potential adsorbent for CO2 capture with good recyclability and large capacity up to 4.25 mmol/g at 298 K and 1 atm. CO2 molecules are largely trapped on two types of sites. One (site I) is near the AlO6 clusters, and another (site II) is between two parallel benzene rings with a distance of 6.64 Å. ZJU-620(Al) can be used for CO2/N2 (15/85) separation with the excellent selectivity up to 107.20 at 273 K and 31.93 at 298 K, and the separation factor of 13.68. It is also with excellent moisture-resistance ability due to 5% breakthrough time (outlet concentration reached the 5% of inlet concentration) without reduction at 80% relative humidity than under dry conditions. Water molecules occupy a small amount of CO2 adsorption site I, but they almost do not occupy the CO2 adsorption site II due to hydrophobic methyl-functional ligands. Moreover, CO2 can be adsorbed on the ZJU-620(Al) surface through C═O···H binding of water molecules with high affinity. Thus, ZJU-620(Al) is a candidate adsorbent for CO2 capture and separation especially under humidity conditions.