Synthesis,structure, and fluorescence properties of a calcium-based metal–organic framework

The solvothermal reaction of a mixture of calcium acetylacetonate and 1,4-naphthalenedicarboxylic acid (H₂NDC) in a solution containing ethanol and distilled water gave rise to a metal–organic framework (MOF), {(H₃O⁺)₂[Ca(NDC)(C₂H₅O)(OH)]}₄·1.1H₂O. This MOF possesses a new structure composed of calcium clusters and H₂NDC linker anions and shows a unique fluorescence property; it exhibits a fluorescence peak at 395 nm (λ_ex = 350 nm) at room temperature, which is blue-shifted compared with that exhibited by the free H₂NDC ligand. One of the possible mechanisms for this fluorescence is likely attributable to a ligand-to-metal charge transfer (LMCT) transition and is the first example of a calcium-based MOF exhibiting blue-shifted fluorescence due to LMCT.

The solvothermal reaction of a mixture of calcium acetylacetonate and 1,4-naphthalenedicarboxylic acid (H₂NDC) in a solution containing ethanol and distilled water gave rise to a metal–organic framework (MOF), {(H₃O⁺)₂[Ca(NDC)(C₂H₅O)(OH)]}₄·1.1H₂O.     

CO2 and water vapor adsorption properties of framework hybrid W-ZSM-5/silicalite-1 prepared from RHA

Framework hybrid W-ZSM-5 and W-silicalite-1 zeolites were synthesized by hydrothermal methods using rice husk ash (RHA) as a silicon raw material. RHA is a low-cost precursor material, and its use can also alleviate the environmental and human health related problems that may occur when it is stacked in open fields. A series of comparative samples were characterized by XRD, FTIR, ICP-OES, SEM, N₂ adsorption–desorption and pore size analysis in order to examine their crystal structure, hybrid state, morphology and textural properties. The maximum CO₂ adsorption capacities of W-ZSM-5 and W-silicalite-1 are 81.69 and 69.96 cm³ g⁻¹, respectively, measured at 15 bar. The isotherms of CO₂, N₂ and O₂ are perfectly fitted by the Toth model, and it is noted that the presence of Al atoms increases the heterogeneity. It can be seen that the greater the heterogeneity of the adsorbent, the larger the CO₂ adsorption capacity achieved. The incorporation of tungsten into the framework does not affect the crystallization of the zeolite, but it prevents the formation of silanol and O–H groups at the adsorption sites. Therefore, the CO₂/H₂O selectivity of W-ZSM-5 is slightly higher than that of ZSM-5, and that of W-silicalite-1 is three times that of silicalite-1. W-ZSM-5/silicalite-1 are promising adsorbents for separating CO₂ under humid industrial conditions.

Framework hybrid W-ZSM-5 and W-silicalite-1 zeolites were synthesized by hydrothermal methods using rice husk ash (RHA) as a silicon raw material.     

Simultaneous adsorption of SO2 and CO2 in an Ni(bdc)(ted)0.5 metal–organic framework

The metal–organic framework Ni(bdc)(ted)_0.5 is a promising material for simultaneous capture of harmful gases such as SO₂ and CO₂. We found that SO₂ performs much better than CO₂ during adsorption, and the lack of physical insight was clarified through detailed analyses of the electronic structures obtained from density functional theory calculations. Our results showed that strong interactions of the d band of Ni atoms with the valence states (2n, 3n, and 4n) of SO₂ but almost not with those of CO₂ are the main reasons. Our finding is useful for the rational design of new metal–organic frameworks with suitable interactions for the simultaneous capture of not only SO₂ and CO₂ but also other gases.

SO₂ performs much better than CO₂ during co-adsorption due to the d-band of Ni atoms.     

Synthesis of bimetallic–organic framework Cu/Co-BTC and the improved performance of thiophene adsorption

A bimetallic–organic porous material (Cu/Co-BTC) with a paddle-wheel structure has been successfully synthesized by a solvothermal approach. The as-synthesized materials were characterized by XRD, SEM, ICP-AES, UV-Vis, TGA and N₂ adsorption at 77 K. The prepared Cu/Co-BTC samples were investigated in thiophene (TP) adsorption from model gasolines by the fixed bed adsorption method at 298 K. The results showed that only a small amount of Co could be successfully introduced into the framework of HKUST-1, and the introduction of Co had little effect on the crystalline structure, morphology, porosity, and thermal stability. The bimetallic Cu/Co-BTC with a Cu/Co ratio of 174 displayed significantly improved adsorption desulfurization performance, showing an increase in breakthrough volume by 30% compared with HKUST-1, implying that the central metal in the MOF plays an important role in adsorption desulfurization. The addition of toluene or cyclohexene (3.20–3.30 vol%) as a competitor in the model gasoline led to a decline in desulfurization performance, especially when cyclohexene was added. The bimetallic Cu/Co-BTC showed a slight loss in breakthrough volume by only 5% after regenerating 7 times, displaying an excellent regeneration property.

The adsorbent Cu/Co-BTC-174 exhibited the largest breakthrough sulfur capacity, with an improvement by 30% in comparison to HKUST-1.     

Synthesis of palm sheath derived-porous carbon for selective CO2 adsorption

Biomass-derived porous carbons are regarded as the most preferential adsorbents for CO₂ capture due to their well-developed textural properties, tunable porosity and low cost. Herein, novel porous carbons were facilely prepared by activation of palm sheath for the highly selective separation of CO₂ from gas mixtures. The textural features of carbon materials were characterized by the analysis of surface morphology and N₂ isotherms for textural characterization. The as-prepared carbon adsorbents possess an excellent CO₂ adsorption capacity of 3.48 mmol g⁻¹ (298 K) and 5.28 mmol g⁻¹ (273 K) at 1 bar, and outstanding IAST selectivities of CO₂/N₂, CO₂/CH₄, and CH₄/N₂ up to 32.7, 7.1 and 4.6 at 298 K and 1 bar, respectively. Also, the adsorption evaluation criteria of the vacuum swing adsorption (VSA) process, the breakthrough experiments, and the cyclic experiments have comprehensively demonstrated the palm sheath derived porous carbons as efficient adsorbents for practical applications.

Novel porous carbons were facilely prepared by activation of palm sheath for the highly selective separation of CO₂ from gas mixtures.     

Boosting CO2 adsorption and selectivity in metal–organic frameworks of MIL-96(Al) via second metal Ca coordination

Aluminum trimesate-based MOF (MIL-96-(Al)) has attracted intense attention due to its high chemical stability and strong CO₂ adsorption capacity. In this study, CO₂ capture and selectivity of MIL-96-Al was further improved by the coordination of the second metal Ca. To this end, a series of MIL-96(Al)–Ca were hydrothermally synthesised by a one-pot method, varying the molar ratio of Ca²⁺/Al³⁺. It is shown that the variation of Ca²⁺/Al³⁺ ratio results in significant changes in crystal shape and size. The shape varies from the hexagonal rods capped in the ends by a hexagonal pyramid in MIL-96(Al) without Ca to the thin hexagonal disks in MIL-96(Al)–Ca4 (the highest Ca content). Adsorption studies reveal that the CO₂ adsorption on MIL-96(Al)–Ca1 and MIL-96(Al)–Ca2 at pressures up to 950 kPa is vastly improved due to the enhanced pore volumes compared to MIL-96(Al). The CO₂ uptake on these materials measured in the above sequence is 10.22, 9.38 and 8.09 mmol g⁻¹, respectively. However, the CO₂ uptake reduces to 5.26 mmol g⁻¹ on MIL-96(Al)–Ca4. Compared with MIL-96(Al)–Ca1, the N₂ adsorption in MIL-96(Al)–Ca4 is significantly reduced by 90% at similar operational conditions. At 100 and 28.8 kPa, the selectivity of MIL-96(Al)–Ca4 to CO₂/N₂ reaches up to 67 and 841.42, respectively, which is equivalent to 5 and 26 times the selectivity of MIL-96(Al). The present findings highlight that MIL-96(Al) with second metal Ca coordination is a potential candidate as an alternative CO₂ adsorbent for practical applications.

MIL-96(Al)–Ca1 shows the highest CO₂ adsorption capacity; while MIL-96(Al)–Ca4 displays a distinguished morphology with the highest selectivity of CO₂/N₂.     

Covalent organic framework with sulfonic acid functional groups for visible light-driven CO2 reduction

In this study, a covalent organic framework (TpPa–SO₃H) photocatalyst with sulfonic acid function groups was synthesized using a solvothermal method. The morphologies and structural properties of the as-prepared composites were characterized by X-ray diffraction, infrared spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, N₂ adsorption–desorption measurements, and field emission scanning electron microscopy. An electrochemical workstation was used to test the photoelectric performance of the materials. The results show that TpPa–SO₃H has –SO₃H functional groups and high photocatalytic performance for CO₂ reduction. After 4 h of visible-light irradiation, the amount of CO produced is 416.61 μmol g⁻¹. In addition, the TpPa–SO₃H photocatalyst exhibited chemical stability and reusability. After two testing cycles under visible light irradiation, the amount of CO produced decreased slightly to 415.23 and 409.15 μmol g⁻¹. The XRD spectra of TpPa–SO₃H were consistent before and after the cycles. Therefore, TpPa–SO₃H exhibited good photocatalytic activity. This is because the introduction of –SO₃H narrows the bandgap of TpPa–SO₃H, which enhances the visible light response range and greatly promotes the separation of photogenerated electrons.

In this study, covalent organic framework (TpPa–SO₃H) photocatalyst with sulfonic acid function groups was synthesized using a solvothermal method.     

Synthesis of ordered Ca- and Li-doped mesoporous silicas for H2 and CO2 adsorption at ambient temperature and pressure

Ca- and Li-doped mesoporous silicas have been prepared successfully using cetyltrimethylammonium bromide (CTAB) surfactant in basic media. Sol–gel synthesis and hydrothermal treatment produced highly ordered mesoporous Ca and Li loaded silica particles. The MCM-41 type mesostructures, the porosity, the pore sizes as well as the surface area of Ca- and Li-silicas have been thoroughly investigated using small angle X-ray scattering (SAXS), transmission electron microscopy (TEM), and N₂ sorption analysis. Samples prepared with varying amounts of Li and Ca loading have been further analyzed using inductive coupled plasma-atomic emission spectroscopy (ICP-AES) and field-emission scanning electron microscopy (FESEM) with an energy dispersive spectral attachment (EDS), which confirmed quite a large amount of Ca while the amount of Li was not enough. Additionally, H₂ and CO₂ gas uptake studies of these metal-loaded silicas have been carried out using a thermogravimetric analyzer (TGA) at normal temperature (25 °C) and pressure (1 atm). H₂ uptake of up to 10 mmol g⁻¹ by Ca-doped silica was recorded. CO₂ and H₂ selectivity were tested with both pure metal-MCM-41 and amine loaded silica using pure N₂ gas and a mixed flow of CO₂/N₂ and H₂/N₂. The effect of temperature on CO₂ uptake was also studied using Ca-MCM-41 materials.

Mesoporous Ca- and Li-doped silica materials have been synthesized in a surfactant mediated sol–gel method and the materials showed significant H₂ uptake capabilities at ambient temperature and pressure.     

Preparation and dye adsorption properties of an oxygen-rich porous organic polymer

Porous organic polymers (POPs), allowing fine synthetic control over their chemical structures, have shown great promise for addressing environmental issues. The high specific surface area and abundant porous structures of POPs can provide large storage space to adsorb dye molecules. Meanwhile, the introduction of polar groups, such as oxygen-containing functional groups in POPs, can not only improve the hydrophilicity, but also provide a strong interaction with dye molecules, thereby improving their adsorption performance. In this paper, an oxygen-rich porous polymer, POP-O, containing polar carbonyl and hydroxyl groups, was prepared by Sonogashira–Hagihara cross-coupling polycondensation. The characteristic results show that POP-O exhibits a hierarchical pore structure with a high specific surface area of 619 m² g⁻¹. The combination of abundant polar functional groups and high porosity endows POP-O with decent dye adsorption performance, and its theoretical maximum adsorption capacity for Rhodamine B (Rh B) is calculated to be 1012 mg g⁻¹.

An oxygen-rich porous polymer containing polar carbonyl and hydroxyl groups, POP-O, was prepared, and the combination of abundant polar functional groups and high porosity endows POP-O with decent dye adsorption performance.     

Alkali modified P25 with enhanced CO2 adsorption for CO2 photoreduction

To improve the CO₂ adsorption on the photocatalyst, which is an essential step for CO₂ photoreduction, solid solutions were fabricated using a facile calcination treatment at 900 °C. Using various alkalis, namely NaOH, Na₂CO₃, KOH, K₂CO₃, the resulted samples presented a much higher CO₂ adsorption capacity, which was measured with the pulse injection of CO₂ on the temperature programmed desorption workstation, compared to the pristine Evonik P25. As a result, all of the fabricated solid solutions produced higer yield of CO under UV light irradiation due to the increased basicity of the solid solutions even though they possessed only the rutile polymorph of TiO₂. The highest CO₂ adsorption capacity under UV irradiation was observed in the sample treated with NaOH, which contained the highest amount of isolated hydroxyls, as shown in the FTIR studies.

Enhanced CO₂ adsorption capability and photocurrent with alkali modified P25 for CO₂ photoreduction.     

Synthesis,characterization, and properties of a novel aromatic ester-based polybenzoxazine

Polybenzoxazines with molecular design flexibility have excellent properties by using suitable raw materials. A new benzoxazine monomer terephthalic acid bis-[2-(6-methyl-4H-benzo[e][1,3]oxazin-3-yl)]ethyl ester (TMBE) with bis-ester groups has been synthesized from the simple esterification reaction of terephthaloyl chloride and 2-(6-methyl-4H-benzo[e][1,3]oxazin-3-yl)-ethanol (MB-OH). The chemical structure of TMBE was characterized by Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance spectroscopy (¹H-NMR, ¹³C-NMR). Polymerization behavior of TMBE was studied by differential scanning calorimetry (DSC) and FT-IR after each cure stage. The cross-linked polybenzoxazine (PTMBE) gave a transparent film through the thermal casting method. The dynamic mechanical analysis of PTMBE showed that the T_g was 110 °C. Thermogravimetric analysis reveals better thermal stability as evidenced by the 5% and 10% weight-loss temperatures (T_d5 and T_d10) of PTMBE, which were 263 and 289 °C, respectively, with a char yield of 27% at 800 °C. The tensile test of the film revealed that the elongation at break was up to 14.2%.

A novel benzoxazine monomer contain ester group was obtained by an indirect molecular design method. Its polymer showed excellent flexibility.     

Synthesis of a metal–organic framework by plasma in liquid to increase reduced metal ions and enhance water stability

Synthesis of a metal–organic framework by plasma in liquid was demonstrated with HKUST-1 as an example. HKUST-1 synthesized by this method contains a higher amount of monovalent copper ions than that synthesized by other conventional methods. The enhanced water stability was also confirmed.

Plasma in liquid provides a method for the synthesis of HKUST-1 with increased reduced metal ions and high water stability.

Metal–organic frameworks (MOFs) are a class of hybrid materials composed of organic linkers and metal nodes. They have high surface areas with tunable pore size and functionality.^1,2 Because of these features, MOFs have potential applications in the fields of gas adsorption/storage,^3,4 catalysis,^5,6 drug delivery,^7,8 and fabrication of luminescent materials.^9,10 Many methods have been proposed for synthesizing MOFs; these include solvothermal methods,^11,12 microwave-assisted methods,^13,14 ultrasonic-assisted methods,^15,16 mechanochemical methods,¹⁷ and electrosynthesis methods.¹⁸ Different synthesis methods result in the different crystallinity and size. Further, the synthesis method can influence the physicochemical and semiconductor properties.¹⁹Recently, a method for synthesizing MOFs in liquid contacting gas-phase dielectric barrier discharges has been reported.^20,21 However, MOFs synthesized by plasma have not been sufficiently characterized yet, thereby demanding further research. Plasma is a unique reaction field and has the advantage of being able to reduce and functionalize species using electrons and radicals. These characteristics have been widely utilized even in liquid phase for the synthesis of various materials such as carbon materials^22,23 and metal nanoparticles^24,25 and for introducing functional groups in materials.²⁶ Using plasma, it may be possible to synthesize functional MOFs with reduced metal ions or additional functional groups. Plasma in liquid generally have higher density of electrons and radicals than plasma in the gas phase,²⁷ and the influence of reactive species can be observed more prominently.In this study, we focused on [Cu₃(BTC)₂]_n (BTC = 1,3,5-benzenetricarboxylate). This is known as HKUST-1 and is one of the most widely researched MOFs. This MOF is assembled from copper nodes and BTC, with each copper coordinated with four oxygen atoms. HKUST-1 is a promising candidate for applications such as H₂ and SO₂ storage.^28,29 However, the crystal structure of HKUST-1 is changed by water molecules, and its surface area decreases.^30,31 To ensure the practical applications of HKUST-1, it is important to increase its resistance to water. To improve its stability to water, strategies such as incorporation of other materials, including graphite oxide³² and carboxyl-functionalized attapulgite,³³ have been developed. In the post-processing using plasma, the adsorption of water molecules on HKUST-1 is inhibited by introducing hydrophobic groups with perfluorohexane.³⁴ In another method, the adsorption of water molecules on unsaturated metal sites was prevented by irradiating the HKUST-1 with oxygen plasma.³⁵ In addition, to improve the water tolerance, reduction of Cu(ii) ions in HKUST-1 was also proposed.³⁶ However, in the above-mentioned methods, mixing of other materials or post-treatment steps was essential.In this work, we synthesized HKUST-1 containing Cu(i) by plasma in liquid; the method involved in situ plasma treatment during the synthesis activated also by plasma. Its resistance to water was compared with HKUST-1 synthesized by conventional methods such as heating or addition of triethylamine at room temperature.HKUST-1 was synthesized using the plasma generated by applying a bipolar pulse voltage to the electrode in an ethanol–water solution containing copper nitrate trihydrate and BTC (Fig. S1 and S2^†). Hereafter in this paper, the thus-synthesized HKUST-1 will be referred to as PL-HKUST-1. For comparison, HKUST-1 was synthesized by the conventional heating method and by the addition of triethylamine at room temperature,¹⁹ and the HKUST-1 formed are referred to as CH-HKUST-1 and RT-HKUST-1, respectively. The details of the synthetic methods are described in the ESI.^†All the samples were characterized by X-ray diffraction (XRD) and thermogravimetric analysis. The XRD patterns of CH-HKUST-1, RT-HKUST-1, and PL-HKUST-1 are shown in Fig. 1. In all the cases, the characteristic diffraction peaks of HKUST-1 were identified, confirming the phase-pure formation of HKUST-1 samples irrespective of the synthetic method.^37,38 Thermogravimetric analysis of HKUST-1 revealed that the thermal stability of PL-HKUST-1 was similar to those of CH-HKUST-1 and RT-HKUST-1 (Fig. S3^†).Open in a separate windowFig. 1XRD patterns of CH-HKUST-1, RT-HKUST-1, and PL-HKUST-1 before and after immersion in water. Fig. 2 shows the fluorescence (FL) spectra of CH-HKUST-1, RT-HKUST-1, and PL-HKUST-1 upon excitation at 310 nm at room temperature. Note that a broad peak around 520 nm was observed only for PL-HKUST-1 and not for CH-HKUST-1 and RT-HKUST-1. Such a behavior has been previously reported for Cu(i) complexes and is attributed to the metal-to-ligand charge transfer (MLCT) from Cu(i) to an empty antibonding π* orbital of the ligand.^39–42 Thus, the FL spectrum indicates that a certain amount of Cu(i) is formed in PL-HKUST-1, which highly contrasted to CH-HKUST-1 and RT-HKUST-1.Open in a separate windowFig. 2FL spectra of CH-HKUST-1, RT-HKUST-1, and PL-HKUST-1 (λ_ex = 310 nm).The existence of Cu(i) in PL-HKUST-1 was also confirmed by X-ray photoelectron spectroscopy (XPS). The XPS spectrum of PL-HKUST-1 showed peaks at 932.2 and 934.6 eV, assignable to Cu(i) and Cu(ii), respectively (Fig. S4^†).^43,44 Although the reduction to Cu(i) inevitably proceeded upon X-ray irradiation,⁴⁵ the concentration of Cu(i) in PL-HKUST-1 was apparently higher than those in CH-HKUST-1 and RT-HKUST-1, which was consistent with the FL spectra. The plasma has ample free electrons and ions, which can reduce metal ions via electrochemical reactions.⁴⁶ The temperature of the plasma in liquid could be as high as 1000–7000 K,²⁷ and the reduction of Cu(ii) to Cu(i) by heat might occur.⁴⁷To examine the water stability of the MOFs, HKUST-1 was immersed in water at room temperature for 12 h. The XRD patterns of the samples after immersion in water were compared with those of the as-synthesized samples. The water treatment resulted in the appearance of new peaks at 2θ ≈ 9.4°–9.7°, 10.1°, 11.1°, 17.3°, and 19.6° for CH-HKUST-1 and RT-HKUST-1 (Fig. 1). Notably, the XRD patterns of PL-HKUST-1 before and after immersion in water were identical, with no detectable changes.The effect of water treatment on the morphology of HKUST-1 was also investigated using scanning electron microscopy (SEM). Particles with octahedral shape were obtained by the plasma in liquid method, similar to those obtained by the conventional heating method (Fig. 3). After immersion in water, the formation of particles with a thread-like morphology was observed for CH-HKUST-1 and RT-HKUST-1, suggesting the decomposition of HKUST-1.^30,48 In contrast, although PL-HKUST-1 contained a portion of the etched corners, the particle shape and size remained unchanged after water treatment.Open in a separate windowFig. 3SEM images of (a and b) CH-HKUST-1, (c and d) RT-HKUST-1, and (e and f) PL-HKUST-1 before and after immersion in water.Fourier transform infrared (FTIR) spectra of HKUST-1 before and after the water treatment are shown in Fig. 4. All as-synthesized samples showed the typical characteristic peaks of HKUST-1. The bands from 1300 to 1700 cm⁻¹ are associated with the carboxylate group of the BTC ligand.⁴⁹ The two peaks in the range 1371–1448 cm⁻¹ correspond to the symmetric stretching vibrations of the carboxylate group.⁵⁰ The bands at 730 and 758 cm⁻¹ correspond to the in-plane C–H bending mode.⁴⁹ Peaks observed below 600 cm⁻¹ correspond to the bonds involving copper ions.⁴⁹ After immersion in water, the FTIR spectrum of PL-HKUST-1 did not show any significant change whereas those of CH-HKUST-1 and RT-HKUST-1 showed shifts in the peaks and formation of new peaks. When exposed to water, peaks corresponding to COO vibrations were observed for CH-HKUST-1 and RT-HKUST-1 between 1200 and 1700 cm⁻¹ due to a change in the environments of the linker BTC.⁴⁸ The peak around 1640 cm⁻¹ shifted to longer wavelengths, and a new peak appeared around 1570 cm⁻¹, which is consistent with previous research.³⁸ The Raman spectra of CH-HKUST-1 and RT-HKUST-1 also showed the structural change in the coordination bonding upon the water treatment (Fig. S5^†).⁵¹Open in a separate windowFig. 4FTIR spectra of CH-HKUST-1, RT-HKUST-1, and PL-HKUST-1 before and after immersion in water. Fig. 5). The BET surface areas of CH-HKUST-1, RT-HKUST-1, and PL-HKUST-1 were 888, 574, and 739 m² g⁻¹, respectively. After immersion in water, the amount adsorbed by CH-HKUST-1 and RT-HKUST-1 was lowered compared to that before immersion, and their BET surface areas were 407 and 351 m² g⁻¹, respectively. By contrast, in the case of PL-HKUST-1, the adsorption amount remained high even after immersion in water, and its BET surface area was 811 m² g⁻¹.Brunauer–Emmett–Teller surface areas of CH-HKUST-1, RT-HKUST-1, and PL-HKUST-1 before and after immersion in water

Influence of carbohydrate polymer shaping on organic dye adsorption by a metal–organic framework in water

A number of studies have been conducted to develop new metal–organic frameworks (MOFs) as adsorbents for the removal of contaminants from polluted water. However, few reports exist describing detailed and thorough examinations of the effects of shaping on the adsorption properties of MOFs. In this study, a thorough analysis and comparison was conducted of the Orange II and Rhodamine B dye adsorption properties of unshaped MIL-100(Fe) (MIL) particles and alginate polymer-shaped MIL beads (MIL-alg). The adsorption affinities of Orange II and Rhodamine B for unshaped MIL were observed to be higher than those for shaped MIL-alg because partial coating of the surface of MIL particles by alginate polymer weakens adsorption forces. Kinetic analysis using a two-compartment model indicates that the contribution of the slow step in the mechanistic pathway for adsorption is more pronounced in MIL-alg compared to MIL because slow dye diffusion takes place in the alginate polymer. We believe that these fundamental findings will have a beneficial impact on approaches to design shaped MOFs that display improved dye removal performance.

A thorough analysis and comparison was conducted of the Orange II and Rhodamine B dye adsorption properties of unshaped MIL-100(Fe) (MIL) particles and alginate polymer-shaped MIL beads (MIL-alg).     

Synthesis and characterization of a new ZIF-67@MgAl2O4 nanocomposite and its adsorption behaviour

Fabricating suitable adsorbents with low-cost and high efficiency extraction for measurement of very small amounts of agricultural pesticides in food and water is playing a vital key role in personal and environmental health. Here, a new composite of zeolitic imidazolate framework-67@magnesium aluminate spinel (ZIF-67@MgAl₂O₄) has been fabricated by a simple method at room temperature with different weight ratios. Several techniques such as FE-SEM, BET, XRD, and TGA have been used to confirm the structural characterization of the obtained materials. The obtained ZIF-67@MgAl₂O₄ was utilized as an adsorbent in the solid phase microextraction technique to extract and preconcentrate the herbicide molinate (as an analyte) in aqueous solution. Corona discharge ionization-ion mobility spectrometry (CD-IMS) was applied for quantification of the analyte molecules. Extraction temperature, extraction time, stirring rate, and sample pH as the main parameters that affected the extraction proficiency were chosen and considered. Under optimal conditions, the linear dynamic range (LDR) of the various concentrations of the molinate and correlation coefficient were 10.0–100.0 μg L⁻¹ and 0.9961, respectively. The limit of quantification (LOQ) and method detection limit (MDL) were 10.0 μg L⁻¹ and 3.0 μg L⁻¹, respectively. The relative standard deviation (RSD) of the ZIF-67@MgAl₂O₄ for extracting the molinate molecules (molinate concentration; 50 μg L⁻¹) was calculated to be 4% and the enrichment factor (EF) was ∼5.

New nanocomposite of zeolitic imidazolate framework-67@magnesium aluminate spinel (ZIF-67@MgAl₂O₄) has been fabricated by a simple method at room temperature with different weight ratios.     

Selective separation of Xe/Kr and adsorption of water in a microporous hydrogen-bonded organic framework

We have studied the adsorption properties of Xe and Kr in a highly microporous hydrogen-bonded organic framework based on 1,3,5-tris(4-carboxyphenyl)benzene, named HOF-BTB. HOF-BTB can reversibly adsorb both noble gases, and it shows a higher affinity for Xe than Kr. At 1 bar, the adsorption amounts of Xe were 3.37 mmol g⁻¹ and 2.01 mmol g⁻¹ at 273 K and 295 K, respectively. Ideal adsorbed solution theory (IAST) calculation predicts selective separation of Xe over Kr from an equimolar binary Xe/Kr mixture, and breakthrough experiments demonstrate the efficient separation of Xe from the Xe/Kr mixture under a dynamic flow condition. Consecutive breakthrough experiments with simple regeneration treatment at 298 K reveal that HOF-BTB would be an energy-saving adsorbent in an adsorptive separation process, which could be attributed to the relatively low isosteric heat (Q_st) of adsorption of Xe. The activated HOF-BTB is very stable in both water and aqueous acidic solutions for more than one month, and it also shows a well-preserved crystallinity and porosity upon water/acid treatment. Besides, HOF-BTB adsorbs about 30.5 wt%, the highest value for HOF materials, of water vapor during the adsorption–desorption cycles, with a 19% decrease in adsorption amounts of water vapor after five cycles.

A highly robust microporous hydrogen-bonded organic framework selectively separates Xe from Kr, as well as efficiently adsorbs water vapor.     

Synthesis of heterometallic metal–organic frameworks and their performance as electrocatalyst for CO2 reduction

Herein we report the solventless synthesis and doping of the benchmark HKUST-1(Cu) as a facile route to afford heterometallic metal–organic frameworks (MOFs) having proficient behavior as electrocatalytic materials in the reduction of carbon dioxide. Zn(ii), Ru(iii) and Pd(ii) were selected as doping metals (M_D) with the aim of partially replacing the Cu(ii) atoms of the pristine structure to afford HKUST-1(Cu,M_D) type materials. Apart from the high yield and good crystallinity of the obtained materials, the extremely high reagent concentration that the reaction conditions imply makes it feasible to control dopant loading in all cases. Prepared samples were processed as electrodes and assembled in a continuous flow filter-press electrochemical cell. Faraday efficiency to methanol and ethanol at Ru(iii)-based electrodes resulted in activity as high as 47.2%, although the activity of the material decayed with time. The interplay of the dopant metal and copper(ii), and the long-term performance are also discussed.

The solventless synthesis of heterometallic metal–organic frameworks and their proficient behavior as electrocatalysts in the CO₂ reduction to alcohols is presented.     

Regulation and photocatalytic degradation mechanism of a hydroxyl modified UiO-66 type metal organic framework

Photocatalytic performance can be effectively improved by modifying the functional groups on the organic ligands of metal organic frameworks (MOFs). Herein, the hydroxyl-modified UiO-66 type MOF: UIO-66-2OH(2,3), was successfully synthesized by the method of ligand exchange by the 2,3-dihydroxyterephthalic acid and UIO-66 as raw materials. The mechanism of photocatalytic degradation of methylene blue (MB) by UIO-66-2OH(2,3) shows that the hydroxyl functional group on the organic ligand regulates its electronegativity and expands its light absorption range. The decomposition of MB is carried out in multiple steps under the oxidation of the hydroxyl radical (˙OH). This research result shows the direction for guiding the synthesis of efficient photocatalysts and clarifying the light absorption of MOFs regulated by hydroxyl functional groups.

The regulation and mechanism of UIO-66-2OH(2,3) photocatalytic degradation of methylene blue (MB) shows that the hydroxyl functional group on the organic ligand regulates its electronegativity and expands its light absorption range.     

Synthesis and adsorption performance of La@ZIF-8 composite metal–organic frameworks

In this study, ZIF-8 with a rhombic dodecahedron structure was prepared by a hydrothermal method. Then La(OH)₃, was successfully loaded onto the ZIF-8 by an immersion deposition method, to form a lanthanide-based metal–organic framework (La@ZIF-8) composites. The structure and properties of La@ZIF-8 were verified by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and zeta potential measurements. The optimum process conditions are discussed within the materials and methods. The effects of initial phosphorus concentration, dosage, pH and contact reaction time on the phosphorus removal performance of the nanomaterial were investigated. The results indicated that La@ZIF-8 exhibited an excellent adsorption capacity (147.63 mg g⁻¹) and its phosphorus removal efficiency could reach as high as 99.7%. Experimental data were interpreted using different adsorption kinetic and isotherm models. The kinetic behavior conformed to the pseudo-second-order kinetic model, which indicated the chemisorption of phosphorus by La@ZIF-8. The adsorption behavior of phosphorus by La@ZIF-8 fitted well to the Langmuir isotherm model, suggesting a monolayer chemical adsorption process. The majority of the adsorbed phosphate could be desorbed by NaOH (2 mol L⁻¹), and the removal efficiency of the recycled La@ZIF-8 reached 90%, even after the fifth cycle. The obtained results demonstrate the great application potential of the prepared La@ZIF-8 as a fascinating adsorbent for the removal of phosphate.

In this study, La(OH)₃ was successfully loaded on ZIF-8 by immersion deposition method, to form lanthanide-based metal–organic frameworks (La@ZIF-8) composites.     

A multifunctional microporous metal–organic framework: efficient adsorption of iodine and column-chromatographic dye separation

In this work, a multifunctional microporous metal–organic framework (MOF), [Cd(ABTC)(H₂O)₂(DMA)]·4DMA (JLNU-4; JLNU = Jilin Normal University; H₄ABTC = 3,3′,5,5′-azobenzenetetracarboxylic acid), has been synthesized based on the ligand H₄ABTC under solvothermal conditions. JLNU-4 shows excellent uptake of iodine both in solution and in the vapor phase, owing to the existence of a microporous structure in JLNU-4. The adsorption kinetics during the process of iodine adsorption were analyzed via a series of qualitative and quantitative analyses, such as the Langmuir and Freündlich adsorption isotherms. In addition, according to UV/vis spectroscopy analysis and the colour variance of JLNU-4, the relatively small sized dye methylene blue (MB) could be efficiently adsorbed by JLNU-4, through size-exclusion effects. Particularly, JLNU-4 can serve as a column-chromatographic filler for the separation of dye molecules. Therefore, JLNU-4 is a multifunctional microporous MOF for iodine adsorption and column-chromatographic dye separation.

JLNU-4 shows excellent uptake of iodine and could selectively adsorb dyes; therefore it can be used for column-chromatographic dye separation.     

Room-temperature preparation of a chiral covalent organic framework for the selective adsorption of amino acid enantiomers

Herein, we have reported the facile room-temperature synthesis of a chiral covalent organic framework (CCOF) for the enantioselective adsorption of amino acids. The prepared CCOF provides various stereoscopic interactions with amino acids for highly selective adsorption of their enantiomers.

A chiral COF CTzDa was synthesized at room temperature for the selective enantioselective adsorption of amino acids.

Chirality is one of the most common properties of natural compounds including proteins, polysaccharides, nucleic acids and enzymes, and it plays an extremely important role in life activities.^1,2 However, the selective recognition and interaction of their enantiomers with organisms make a huge difference in activity, toxicity, adsorption, transfer, metabolism and elimination. Therefore, the exploration of efficient ways to obtain pure enantiomers becomes more and more urgent; however, this is highly challenging owing to the dramatic similarity of the physicochemical properties of two enantiomers.^3,4 To date, various chiral separation techniques have been proposed such as chromatography,^5,6 crystallization^7,8 and extraction.^9,10 Adsorption separation based on porous materials has shown advantages due to their strong chiral recognition ability, long-term stability, and less complexity.¹¹The exploration of chiral-functionalized porous materials as adsorbents for the highly efficient resolution of enantiomers has received extensive attention; these materials include metal–organic frameworks,^12,13 porous organic cages,^14,15 metal–organic cages^16,17 and composite porous materials.¹⁸ However, the type of adsorbents for enantioselective adsorption was far more enough due to its challengeable preparation. As a consequence, it is necessary to design and prepare more new adsorbents with excellent stability and rapid kinetics for the selective adsorption of enantiomers.Covalent organic frameworks (COFs)^19,20 are crystalline organic porous materials with broad applications in diverse fields including chromatography separation,^21,22 heterogeneous catalysis,^23,24 fluorescence sensing^25,26 and optoelectronic materials.^27,28 The large surface area, excellent stability and the number of duplicate ordered units of COFs allow numerous interactions between the host and guests, such as hydrogen bonding, π–π interactions, hydrophobic interactions and molecular sieving, indicating COFs as a convenient platform for enantioselective adsorption. Chiral covalent organic frameworks (CCOFs) have been explored as the stationary phase in chiral chromatography and as catalysts in asymmetric catalysis.^29–31 However, the application of CCOFs as adsorbents for selective adsorption has been rarely reported.Here, we have reported the design and room-temperature (RT) synthesis of a CCOF, CTzDa, via the post-modification of the COF TzDa for the selective adsorption of the enantiomers of amino acids (AAs). TzDa consisting of 4,4′,4''-(1,3,5-triazine-2,4,6-triyl)trianiline (Tz) and 1,4-dihydroxyterephthalaldehyde (Da) was chosen as the platform for the preparation of chiral COF due to its high stability, easy synthesis and abundant active groups (–OH).³²d-Camphoric acid was converted to its acid chloride to react with the hydroxyl group of TzDa for obtaining CTzDa. The application of CTzDa as the adsorbent for the chiral separation of AAs was further investigated via detailed experimental characterizations and computational modeling. This work shows high potential of chiral COFs as adsorbents in enantioselective adsorption.The COFs used as adsorbents should possess great stability, high crystallinity and large surface areas. Moreover, the introduction of a chiral environment into the COF structure via a post-modification strategy is a widely accessible way to prepare CCOFs. In this work, TzDa, which possessed a highly ordered and stable structure with abundant active groups (–OH) for further modification, was chosen as the COF platform for preparing CCOF. As shown in Fig. 1, we synthesized TzDa by condensing Tz and Da at RT instead of high temperature and pressure (Fig. S1, ESI^†).Open in a separate windowFig. 1Room-temperature synthesis: (a) TzDa; (b) CTzDa. d-Camphor acid chloride (d-cam-ClO) (Fig. S2 and S3, ESI^†) prepared from d-camphor acid was then applied to react with hydroxyl groups to introduce the chiral moiety into the channel of TzDa for preparing CTzDa.The Fourier transform infrared (FTIR) spectra of TzDa show the C N peak at 1665 cm⁻¹ along with the disappearance of the peaks for the C O and NH₂ bonds for the starting materials, indicating the successful condensation of Tz and Da (Fig. S4, ESI^†). Compared with TzDa, CTzDa exhibited additional peaks at 1805 cm⁻¹, 1741 cm⁻¹ and 1259 cm⁻¹ for the C O bond of the carboxyl group and C O and C–O bonds of the ester group, respectively, but no peaks for the C O bond of acid chloride (Fig. 2a and S5, ESI^†). The result reveals the successful grafting of the chiral d-camphoric acid moiety on TzDa. The modification ratio of d-camphoric acid on TzDA was calculated to be 41% using a toluidine blue O (TBO) dye assay (ESI^†).Open in a separate windowFig. 2(a) FTIR spectra of TaDa and CTzDa. (b) PXRD patterns of TaDa and CTzDa. (c) Zeta potential of TzDa and CTzDa. (d) PXRD patterns of CTzDa after immersing in various solvents.The powder X-ray diffraction (PXRD) pattern of TzDa prepared via the RT approach not only matched well with the simulated PXRD pattern, but also showed all the characteristic peaks of TzDa obtained with the solvothermal approach, indicating the formation of the reported ordered structure of TzDa (Fig. S6, ESI^†). All the PXRD peaks of TzDa remained after modification with d-cam-ClO, indicating no change in the crystal structure. The coupling reaction of camphoric acid with its hydroxyl group prevents the formation of intramolecular hydrogen bonds between the hydroxyl groups (Da) on formaldehyde (Tz), which results in a decrease in the crystallinity of the synthesized CTzDa (Fig. 2b and S7, ESI^†).The grafting of d-cam made the zeta potential of COF more negative from −8.2 mV (TzDa) to −47.3 mV (CTaDa) due to the introduction of the hydroxyl group of d-cam (Fig. 2c; Table S1, ESI^†). There was no variation in the PXRD patterns and FTIR spectra of CTzDa after immersing in various solvents including tetrahydrofuran (THF), acetonitrile (ACN), dimethyl formamide (DMF), water, 0.1 M HCl and 0.1 M NaOH for 1 day, demonstrating the high chemical stability of CTzDa (Fig. 2d and S8, ESI^†). The prepared CTzDa also had high thermal stability up to 200 °C (Fig. S9, ESI^†).The transmission electron microscopy (TEM) images show a layer-like structure for both TzDa and CTzDa and no obvious change in morphology after the grafting of d-camphor acid onto TzDa (Fig. S10, ESI^†). The scanning electron microscopy (SEM) images indicate that the surface of CTzDa is rougher than that of TzDa (Fig. S11, ESI^†). The Brunauer–Emmett–Teller (BET) surface area and the pore size of TzDa were calculated to be 1380 m² g⁻¹ and 3.2 nm, respectively, while those of CTzDa decreased to 403 m² g⁻¹ and 1.8 nm, respectively, due to the introduction of d-camphor acid (Fig. S12 and Table S2, ESI^†).The introduction of a chiral moiety caused various stereoscopic interactions in the COF, which could improve the enantioselective ability of CTzDa. Thus, we employed the synthesized porous material CTzDa for the selective adsorption of chiral AAs (tryptophan (Trp), histidine (His), aspartic acid (Asp) and serine (Ser)). The effect of the concentration of AAs on the adsorption capacity indicated the appropriate concentrations of AAs in adsorption (Fig. S13, ESI^†). The effect of pH on the AAs adsorption showed that the adsorption process was favorable near the isoelectric point (Fig. S14, ESI^†). In comparison with TzDa, CTzDa exhibited obviously higher enantioselectivity and adsorption capacity to l-AAs than d-AAs (Fig. 3 and S15, ESI^†).Open in a separate windowFig. 3Time-dependent enantioselective adsorption of AAs on CTzDa at 293 K: (a) d-Trp and l-Trp (50 mg L⁻¹); (b) d-His and l-His (20 mg L⁻¹); (c) d-Asp and l-Asp (red, 20 mg L⁻¹); (d) d-Ser and l-Ser (20 mg L⁻¹).We further investigated the kinetics and adsorption isotherms of AAs on CTzDa. The time-dependent adsorption capacity (q_t) of AAs at three initial concentrations at 293 K showed that the adsorption equilibrium of AAs on CTzDa was achieved within 30 min, indicating the rapid adsorption of AAs on CTzDa (Fig. S16, ESI^†). The adsorption followed the pseudo-second-order kinetic model rather than the pseudo-first-order kinetic model (Fig. S17 and S18, ESI^†). The larger k₂ values of d-AAs than those of l-AAs indicate different interactions of CTzDa with d-AAs and l-AAs (Table S3, ESI^†).^33,34The adsorption isotherms were evaluated in an initial concentration range of 10–100 mg L⁻¹ at four different temperatures (20–50 °C) (Fig. S19, ESI^†). The adsorption isotherms of AAs could be better described by the Langmuir model than the Freundlich model (Table S4, ESI^†), indicating monolayer adsorption of AAs on CTzDa. The calculated maximum adsorption capacities (q_m) of l-AAs were higher than those of d-AAs, indicating the selective adsorption of chiral AAs on CTzDa. The adsorption enantioselectivity values of CTzDa were 4.20, 2.59, 2.60 and 1.62 for the enantiomers of Trp, His, Asp and Ser, respectively (Table S5, ESI^†). Compared with previous adsorbents, the developed CTzDa exhibited higher enantioselectivity (Table S6, ESI^†), showing the great potential of CTzDa as an adsorbent in the enantioselective adsorption of AAs.Efficient desorption and reusability are essential for adsorbents. Different types of eluents were used for the desorption of AAs from CTzDa at 60 °C under ultrasonication for 5 min (Fig. S20^†). The results showed that organic solvents were not favourable for AA desorption. The adsorbed AAs could be well desorbed from CTzDa with water (pH = 4 or 8) (Fig. S20, ESI^†) due to the increase in the hydrophilicity of AAs.³⁵ After five adsorption–desorption cycles, CTzDa exhibited no significant decrease in adsorption capacity, indicating the good reusability of CTzDa for the adsorption of AAs (Fig. S21, ESI^†). There was no obvious change in the PXRD pattern and FTIR spectra after five adsorption–desorption cycles, suggesting that CTzDa was stable during adsorption and desorption (Fig. S22, ESI^†).The adsorption thermodynamics was assessed by the change in Gibbs free energy (ΔG), enthalpy (ΔH) and entropy (ΔS) (Fig. S23, S24 and Table S7, ESI^†). The negative ΔG value indicated that the adsorption of AAs on CTzDa was thermodynamically spontaneous. The negative ΔH value suggested the presence of an exothermic process, which was related to the decrease in adsorption capacity at high temperatures. The negative ΔS value demonstrated the AAs lost freedom during the adsorption process.AutoDock Vina (ADVina) was used to perform docking calculations.^36,37 The calculated binding energy (BE, kcal mol⁻¹) represents the generated energy in adsorption (Table S8, ESI^†). The existing interaction modes between CTzDa and AAs are shown in Fig. 4. The binding interactions between AAs and the building unit mainly included π–π interactions, C–H⋯π interactions and H-bonds, but the strengths related to the stereoscopic interactions were different, which originally resulted in distinct adsorptions. For Trp, the carboxyl and amino groups of l-Trp could both form hydrogen bonds with CTzDa, while the different stereoscopic positions of d-Trp led to only carboxyl group forming aromatic H-bonds with CTzDa (Fig. 4a). The carboxyl group of l-His or l-Ser formed hydrogen bonds with CTzDa. On the contrary, the corresponding hydrogen bond of d-His or d-Ser between the carboxyl group and CTzDa was absent due to the large distance (Fig. 4b and d). The hydrogen bond length between l-Asp and CTzDa (1.95 Å) was shorter than that between d-Asp and CTzDa (2.61 Å) (Fig. 4c). The above-mentioned different stereoscopic interactions made the BE between the main framework and racemic AAs follow the order l-AAs > d-AAs, indicating the stronger adsorption of l-AAs than that of d-AAs on CTzDa (Table S8^†). The K_L/K_D ratios were 1.97, 1.66, 1.18 and 1.40 for Trp, His, Aps and Ser, respectively. K_L/K_D > 1 also indicated that CTzDa exhibited stronger adsorption of l-AAs than that of d-AAs.Open in a separate windowFig. 4Molecular docking modes between CTzDa and AAs: (a) Trp; (b) His; (c) Asp; (d) Ser. The receptor COF unit is displayed with thin stick style by marking C in yellow, O in red, N in blue and H in white. The AAs are displayed with thick stick style by marking C in green, O in red, N in blue and H in white. Blue, green and yellow dotted lines represent the π–π interaction, n–π interaction and hydrogen bond between CTzDa and AAs, respectively. Thin figure represents the distance of atoms.In summary, we have designed and synthesised the chiral COF CTzDa through introducing a chiral selector (d-cam) in the COF TzDa at room temperature in a facile manner. The prepared CTzDa showed good stability in various solvents, which was favourable for adsorption. CTzDa also exhibited rapid kinetics and high selectivity for the adsorption separation of the enantiomers of amino acids. Docking calculations showed that the difference in the stereoscopic hydrogen bonds between l-AAs and d-AAs is the key interaction for the enantioselective adsorption of AAs on CTzDa. This work provides a facile strategy for highly selective adsorption of AA enantiomers. Further research will focus on the potential of CTzDa in the chiral chromatographic separation of AAs.