Process-Oriented Smart Adsorbents: Tailoring the Properties Dynamically as Demanded by Adsorption/Desorption

Synergistic adsorption and photocatalytic degradation of perfluorooctanoic acid in aqueous solution by a regenerable biochar-titania nanotube composite

Perfluorooctanoic acid (PFOA), a recalcitrant perfluoroalkyl substance, presents escalating challenges for aquatic decontamination due to its extreme persistence and bioaccumulation. A biochar-titania nanotube (TNTs@biochar) combining the advantages of biochar and TNTs was synthesized for the first time via an alkaline hydrothermal approach and explored for the adsorption and photodegradation of PFOA in aqueous solution. Titania nanotubes interacted with biochar to form TNTs@biochar. The optimal composite was obtained at a biochar : TiO2 mass ratio of 1 : 1 and a calcination temperature of 550 °C. The composite efficiently adsorbed ∼99% of PFOA through hydrophobic and anion-π interactions and hydrogen bonding, concentrating PFOA on photoactive sites. The incorporation of biochar with TNTs enhanced light absorption in the 200-700 nm range, lowered the band gap energy to 3.10 eV, improved the formation rate and separation efficiency of e--h+ pairs, and enhanced interfacial charge transfer, resulting in promoted photocatalytic activity. The degradation of pre-concentrated PFOA on TNTs@biochar reached up to 99%. The photodegradation also regenerated the composite, allowing for four successive adsorption-photodegradation cycles. Hydroxyl radical and h+-driven oxidation played a paramount part, leading to decarboxylation and C-F bond cleavage. The byproducts of the photodegradation demonstrated lower acute and chronic toxicity compared with PFOA. The composite exhibits synergistic adsorption and photocatalytic activity as well as offers efficiently and economically scalable solutions for PFOA-laden water remediation.

TiO2-ZnO functionalized low-cost ceramic membranes from coal fly ash for the removal of tetracycline from water under visible light

Hybrid wastewater treatment systems offer viable solutions to enhance the removal of complicated contaminants from aqueous system. This innovation has opened new avenues for advanced wastewater treatment processes. Herein, a novel TiO2-ZnO functionalized coal fly ash-based ceramic membrane was fabricated by utilizing a combined pressing and sintering method. The intrinsic properties of the functionalized membranes were characterized and their chemical and physical properties such as chemical stability, mechanical stability, water absorption, and porosity were established. The shape, crystallinity, thermal characteristics, and functional groups present were also determined using SEM, XRD, TGA, and FTIR studies, respectively. The results showed that the ceramic membrane functionalized with 0.5 g of TiO2-ZnO and sintered at 850 °C exhibited the best thermal, and chemical stability, and possessed the required porosity for ultrafiltration applications. Photocatalytic degradation of tetracycline (TC) as a model pollutant was examined and the optimum efficiency of 77% was achieved within 100 min of visible irradiation using the functionalized membrane. Moreso, the functionalized membrane was found to be stable with 73% degradation efficiency after 5 consecutive cycles of reusability study, showing negligible loss of efficiency. The scale-up of photocatalytic ceramic membranes and their utilization in real industrial applications will confirm their robustness.

Controlling Guest Diffusion by Local Dynamic Motion in Soft Porous Crystals to Separate Water Isotopologues and Similar Gases

ConspectusThe precise and effective separation of similar mixtures is one of the fundamental issues and essential tasks in chemical research. In the field of gas/vapor separation, the size difference among the molecular pairs/isomers of light hydrocarbons and aromatic compounds is generally 0.3-0.5 Å, and the boiling-point difference is generally 6-15 K. These are necessary industrial raw materials and have great separation demands. Still, their separation mainly relies on energy-intensive distillation technology. On the other hand, remarkably similar substances such as oxygen/argon and isotopologues usually exhibit size differences of only 0-0.07 Å and boiling-point differences of only 1-3 K. Although their industrial separation can be realized, their efficiency is considerably low. Therefore, effectively separating remarkably similar mixtures is crucial in fundamental chemistry and industry, but it remains a significant challenge. Porous coordination polymers (PCPs) or metal-organic frameworks (MOFs) are emerging materials platforms for designing adsorbents for separating similar mixtures. However, the reported PCPs did not work well for separating remarkably similar substances. The framework structures of the mainstream PCPs remain unchanged (rigid) or significantly change (globally flexible) upon adsorption. However, rigid and globally flexible PCPs find controlling the pore aperture in subangstrom precision challenging, a prerequisite for distinguishing remarkably similar substances. Thus, novel mechanisms and materials design principles are urgently needed to realize PCPs-based adsorptive separation of remarkably similar mixtures.To confront the obstacles in separating remarkably similar mixtures, our group started contributing to this field in 2017. We employed locally flexible PCPs as the materials designing platform, whose local motions of the side substituent groups potentially regulate the pore apertures to design and control the gas/vapor diffusion in PCPs. Specifically, we encoded dynamic flipping molecular motions into the diffusion-regulatory gate functionality. The ligands were designed by integrating carboxylic coordination groups with nonplanar fused-ring moieties, with the latter moieties exhibiting flipping motion around their equilibrium positions with small energy increases. Such local motions of ligands lead to the dynamic opening and blocking of PCP channels, thus termed flipping dynamic crystals (FDCs). FDCs feature distinctive temperature-responsive adsorption behaviors due to the competition of thermodynamics and kinetics under diffusion regulation, enabling differentiation of remarkably similar mixtures by each gate-admission temperature much higher than the boiling-point temperature of each component. Even when the molecular sizes are the same in the water isotopologue mixtures, FDCs can separate each isotopologue by amplifying their diffusion-rate differences. Finally, by combining the thermodynamic and kinetic factors, FDCs achieve temperature-switched recognition of CO2/C2H2 and diffusion-rate sieving of C3H6/C3H8. Therefore, our work provides a platform for designing locally flexible PCPs by introducing subangstrom precision in flexibility. This opens up the feasibility of separating remarkably similar mixtures on scientific principles. In this Account, we summarize our above ongoing research contributions, including (i) the design of flipping ligands and FDCs, (ii) the characterization of flipping motions, (iii) the gas/isotopologue sorption behaviors, and (iv) the separation of gases and isotopologues. Overall, our studies offer a new aspect of soft porous crystals and provide future opportunities for relevant researchers in this field.

Spiropyran-Incorporated Y Zeolite: A Visible-Light-Responsive System for Controllable CO adsorption

The development of high-performance adsorbents is vital for adsorptive separation. Conventional adsorbents have limitations in combining selective adsorption and efficient desorption due to their fixed surface properties. In this work, we have constructed spiropyran (SP)-based visible-light-responsive adsorbents for controllable CO adsorption by synthesizing SP in situ in Y zeolite via the ship-in-the-bottle method. This avoids the drawbacks associated with the vast majority of systems that modulate adsorption capacity by UV light. SP molecules can undergo reversible isomerization within the Y zeolite, which exhibit the merocyanine (MC) state in the dark and revert to the SP form upon visible light stimulation. The results show that the isomerization of MC to SP leads to a tunable CO adsorption capacity of up to 34%. Simulations performed by density functional theory reveal that MC is more likely to trap CO molecules than SP due to its higher binding energy with CO. We further demonstrate that the isomerization-induced tunable adsorption capacity can be maintained during cycles without degradation.

Efficient removal of per- and polyfluoroalkyl substances from biochar composites: Cyclic adsorption and spent regenerant degradation

In order to solve the problem of efficient desorption of per- and polyfluoroalkyl substances (PFAS) and regeneration of adsorbents, a novel biochar composite was prepared based on the quaternary ammonium groups and hydrophobicity of sulfobetaine polymer, which can be used for the efficient removal of various PFASs and has great regeneration ability. Through adsorption, regeneration and degradation experiment, the comprehensive effect of the novel biochar composite on the whole process of removal of PFAS was systematically investigated. The results showed that the maximum adsorption capacity of PFOS, PFOA, PFBS, and PFBA reached 634 mg/g, 536 mg/g, 301 mg/g and 264 mg/g, respectively. The adsorption process involved hydrophobicity, electrostatic, pore diffusion and complexation. The NaI + NaOH solution was used at 50 °C to achieve efficient regeneration of the adsorbent, which can be recycled more than 4 times. When the vacuum-ultraviolet (VUV)/sulfite reduction system was used for deep degradation of the regenerated solution, the effect of hydrated electrons on PFAS was enhanced due to the inclusion of NaI and NaOH in the regeneration reagent, resulting in an increase in the degradation efficiency (89.1%-99.9%) and defluorination efficiency (63.3%-84.1%). Based on the performance of BC-P(SB-co-AM) and the treatment efficiency of PFAS, the design idea of the whole process treatment technology of PFAS proposed in this work is expected to hold great promise in environmental applications. This work provides a novel idea and system for the efficient adsorption removal and desorption of PFAS, and subsequent deep degradation.

Potential of nonporous adaptive crystals for hydrocarbon separation

Hydrocarbon separation is an important process in the field of petrochemical industry, which provides a variety of raw materials for industrial production and a strong support for the development of national economy. However, traditional separation processes involve huge energy consumption. Adsorptive separation based on nonporous adaptive crystal (NAC) materials is considered as an attractive green alternative to traditional energy-intensive separation technologies due to its advantages of low energy consumption, high chemical and thermal stability, excellent selective adsorption and separation performance, and outstanding recyclability. Considering the exceptional potential of NAC materials for hydrocarbon separation, this review comprehensively summarizes recent advances in various supramolecular host-based NACs. Moreover, the current challenges and future directions are illustrated in detail. It is expected that this review will provide useful and timely references for researchers in this area. Based on a large number of state-of-the-art studies, the review will definitely advance the development of NAC materials for hydrocarbon separation and stimulate more interesting studies in related fields.

Selective Adsorption and Recovery of Silver from Acidic Solution Using Biomass-Derived Sulfur-Doped Porous Carbon

It is vital to recycle precious metals effectively such as silver from waste sources because of limited natural reserves. Herein, passion fruit (Passiflora edulis Sims) shell-derived S-doped porous carbons (SPCs) were newly synthesized by hydrothermal carbonization and following with activation by KOH/(NH4)2SO4, and the adsorption of Ag+ on SPC under acidic solutions was investigated. It was found that the activator of (NH4)2SO4 can not only introduce the doping of S elements but also increase the proportion of mesopores in the as-prepared SPC. As the active site, the increasing S doping can improve the adsorption of Ag+ on SPC. The kinetic data of Ag+ adsorption by SPC was consistent with the pseudo-second-order kinetic model. The Langmuir isothermal model was used to well fit the Ag+ adsorption isotherms of SPC, and the maximum adsorption capacity of the optimized SPC-3 for Ag+ is up to 115 mg/g in 0.5 mol/L HNO3 solution. SPC-3 showed good selectivity toward Ag+ over diverse competing cations, which is mainly attributed to the strong bonding between Ag+ ions and the sulfur-containing functional groups on the surface of SPC-3 resulting in the formation of Ag2S nanoparticles. The adsorbed Ag could be recovered as an elemental form by a simple calcination. This study provides a new insight into the design of an environmentally friendly and efficient adsorbent for the selective recovery of silver from acidic aqueous media.

Strengthening Intraframework Interaction within Flexible MOFs Demonstrates Simultaneous Sieving Acetylene from Ethylene and Carbon Dioxide

Efficient separation of acetylene (C₂ H₂ )/ethylene (C₂ H₄ ) and acetylene/carbon dioxide (CO₂ ) by adsorption is an industrially promising process, but adsorbents capable of simultaneously capturing trace acetylene from ethylene and carbon dioxide are scarce. Herein, a gate-opening effect on three isomorphous flexible metal-organic frameworks (MOFs) named Co(4-DPDS)₂ MO₄ (M = Cr, Mo, W; 4-DPDS = 4,4-dipyridyldisulfide) is modulated by anion pillars substitution. The shortest CrO₄ ^2- strengthens intraframework hydrogen bonding and thus blocks structural transformation after activation, striking a good balance among working capacity, separation selectivity, and trace impurity removal of flexible MOFs out of nearly C₂ H₂ /C₂ H₄ and C₂ H₂ /CO₂ molecular sieving. The exceptional separation performance of Co(4-DPDS)₂ CrO₄ is confirmed by dynamic breakthrough experiments. It reveals the specific threshold pressures control in anion-pillared flexible materials enabled elimination of the impurity leakage to realize high purity products through precise control of the intraframework interaction. The adsorption mechanism and multimode structural transformation property are revealed by both calculations and crystallography studies. This work demonstrates the feasibility of modulating flexibility for controlling gate-opening effect, especially for some cases of significant aperture shrinkage after activation.

Facile Strategy for Fabricating an Organosilica-Modified Fe3O4 (OS/Fe3O4) Hetero-nanocore and OS/Fe3O4@SiO2 Core-Shell Structure for Wastewater Treatment with Promising Recyclable Efficiency

The development of a sustainable process for heavy metal ion remediation has become a point of interest in various fields of research, including wastewater treatment, industrial development, and health and environmental safety. In the present study, a promising sustainable adsorbent was fabricated through continuous controlled adsorption/desorption processes for heavy metal uptake. The fabrication strategy is based on a simple modification of Fe3O4 magnetic nanoparticles with organosilica in a one-pot solvothermal process, carried out in order to insert the organosilica moieties into the Fe3O4 nanocore during their formation. The developed organosilica-modified Fe3O4 hetero-nanocores had hydrophilic citrate moieties, together with hydrophobic organosilica ones, on their surfaces, which facilitated the further surface coating procedures. To prevent the formed nanoparticles from leaching into the acidic medium, a dense silica layer was coated on the fabricated organosilica/Fe3O4 (OS/Fe3O4). In addition, the prepared OS/Fe3O4@SiO2 was utilized for the adsorption of cobalt(II), lead(II), and manganese(II) from the solutions. The data for the adsorption processes of cobalt(II), lead(II), and manganese(II) on OS/(Fe3O4)@SiO2 were found to follow the pseudo-second-order kinetic model, indicating the fast uptake of heavy metals. The Freundlich isotherm was found to be more suitable for describing the uptake of heavy metals by OS/Fe3O4@SiO2 nanoparticles. The negative values of the ΔG° showed a spontaneous adsorption process of a physical nature. The super-regeneration and recycling capacities of the OS/Fe3O4@SiO2 were achieved, comparing the results to those of previous adsorbents, with a recyclable efficiency of 91% up to the seventh cycle, which is promising for environmental sustainability.

Adsorptive removal of aromatic diamines from water using metal-organic frameworks functionalized with a nitro group

Adsorptive removal of aromatic diamines such as methylenedianiline (MDA) and p-phenylenediamine (PPD) was firstly investigated with nitro-functionalized metal-organic frameworks (MOFs, MIL-101(Cr)-NO₂). The MIL-101(Cr)-NO₂ showed much better performances in the removal of MDA and PPD, in both adsorption capacity and kinetics, than any other adsorbents. For example, MIL-101(Cr)-NO₂ had a much higher maximum adsorption capacity for MDA (1111 mg·g^-1) than activated carbon (208 mg·g^-1) or a reported adsorbent (391 mg·g^-1). Based on experimental results, hydrogen bonding (especially, via the formation of a 6-membered ring (6-MR) between -NO₂ of the adsorbent and -NH₂ of the adsorbates) could be suggested as the main mechanism to interpret the noticeable adsorption of the diamines. Importantly, this is the first example to confirm that MOFs with nitro group can be a competitive adsorbent to remove organics composed of amino group, especially via making 6-MR through hydrogen bonding. Higher adsorption of MDA than that of PPD over MIL-101(Cr)-NO₂ might be explained with π-π interaction between aromatic rings (π-lean aromatics of MOF and π-rich aromatics of the adsorbates). Moreover, MIL-101(Cr)-NO₂ could be recycled after simple washing, suggesting the potential use of the MOF in adsorptive purification of contaminated water with organics with amino groups.

Construction of Photoresponsive 3D Structures Based on Triphenylethylene Photochromic Building Blocks

Photoresponsive materials have been widely used in sensing, bioimaging, molecular switches, information storage, and encryption nowadays. Although a large amount of photoresponsive materials have been reported, the construction of these smart materials into precisely prescribed complex 3D geometries is rarely studied. Here we designed a novel photoresponsive material methyl methacrylate containing triphenylethylene (TrPEF₂-MA) that can be directly used for digital light processing (DLP) 3D printing. Based on TrPEF₂-MA, a series of photoresponsive 3D structures with reversible color switching under ultraviolet/visible light irradiations were fabricated. These complex photoresponsive 3D structures show high resolutions (50 μm), excellent repeatability (25 cycles without fatigue), and tunable saturate color degrees. Multicomponent DLP 3D printing processes were also carried out to demonstrate their great properties in information hiding and information-carrying properties. This design strategy for constructing photoresponsive 3D structures is attractive in the area of adaptive camouflage, information hiding, information storage, and flexible electronics.

Functionalized metal-organic framework-derived carbon: Effective adsorbent to eliminate methylene blue, a small cationic dye from water

Elimination of organic dyes from wastewater is very important for our safe environment and sound health. In this work, adsorptive removal of cationic dyes, especially small ones, was investigated with carbonaceous materials to develop a competitive adsorption technology. To improve the performance of metal-organic framework (MOF)-derived carbons (MDCs) in dye adsorption, an MDC, derived from a MOF (MAF-6), was oxidatively functionalized with ammonium persulfate solutions (APSs). Although the porosity of pristine MDC decreased with functionalization via oxidation, functionalized MDCs (FMDCs), especially FMDC(1.0) that was obtained via treating MDC with APS (1.0 M), showed remarkable performances in the adsorption of small cationic dyes like methylene blue (MB) and azure B. For example, FMDC(1.0) had the maximum adsorption capacity (Q_o) of 625.0 mg/g (for MB) which is larger than any reported value with carbonaceous materials. Moreover, the obtained Q_o was around 4 and 2 times that of activated carbon with Q_o of 160 mg/g and MDC with Q_o of 298 mg/g, respectively. On the contrary, oxidative treatment of MDC was negative in adsorption of an anionic dye such as methyl orange. Moreover, the functionalized MDC was not very effective in the adsorption of cationic dyes with large sizes (like brilliant green, crystal violet, Janus green B, and rhodamine B) because of the limited pore size of the studied adsorbent FMDC(1.0). The remarkable adsorption of MB over FMDC(1.0) could be explained by electrostatic and π-π interactions. Finally, the facile recyclability of the FMDC(1.0) in MB adsorption was confirmed via successive adsorptions, FT-IR, and nitrogen adsorption; therefore, FMDC(1.0) can be suggested as a potential adsorbent to remove cationic dyes, especially with small molecular sizes.

Rational Design of Smart Metal-Organic Frameworks for Light-Modulated Gas Transport

Smart metal-organic frameworks (MOFs) are constructed by introducing stimuli-responsive functional groups into MOF platforms. Through membrane systems containing smart MOFs, external field-modulated gas transport can be achieved, which finds potential applications in chemical engineering. In this work, we design a series of Mg-MOF-74-III-based frameworks functionalized by arylazopyrazole groups. Methyleneamine chains with various lengths are attached to the photoresponsive azopyrazole moiety. Molecular dynamics simulations show that CO₂ diffusion can be remarkably changed by controlling the cis-to-trans isomerization of the functional unit due to the tunable adsorbate-adsorbent and adsorbate-adsorbate interactions of the two states. With the optimal length of the functional chain, the spatial hindrance and adsorbate-adsorbent interaction exhibit a synergetic effect to maximize the stimuli-responsive kinetic separation of N₂ over CO₂. This work provides a promising strategy for elevating smart MOFs' potential in gas separation.