Carbonic Anhydrase-Mimicking Supramolecular Nanoassemblies for Developing Carbon Capture Membranes.

As a ubiquitous family of enzymes with high performance in converting carbon dioxide (CO2) into bicarbonate, carbonic anhydrases (CAs) sparked enormous attention for carbon capture. Nevertheless, the high cost and operational instability of CAs hamper their practical relevance, and the utility of CAs is mainly limited to aqueous applications where CO2-to-bicarbonate conversion is possible. Taking advantage of the chemical motif that endows CA-like active sites (metal-coordinated histidine), here we introduce a new line of high-performance gas separation membranes with CO2-philic behavior. We first self-assembled a histidine-based bolaamphiphile (His-Bola) molecule in the aqueous phase and coordinated the resulting entities with divalent zinc. Optimizing the supramolecular synthesis conditions ensured that the resultant nanoparticles (His-NPs) exhibit high CO2 affinity and catalytic activity. We then exploited the His-NPs as nanofillers to enhance the separation performance of Pebax MH 1657. The hydrogen-bonding interactions allowed the dispersion of His-NPs within the polymer matrix uniformly, as confirmed by microscopic, spectroscopic, and thermal analyses. The imidazole and amine functionalities of His-NPs enhanced the solubility of CO2 molecules in the polymer matrix. The CA-mimic active sites of His-NPs nanozymes, on the other hand, catalyzed the reversible hydration of CO2 molecules in humid conditions, facilitating their transport across the membranes. The resulting nanocomposite membranes displayed excellent CO2 separation performance, with a high level of stability. At a filling ratio as low as 3 wt %, we achieved a CO2 permeability of >145 Barrer and a CO2/N2 selectivity of >95 with retained performance under humid continuous gas feeds. The bio-inspired approach presented in this work offers a promising platform for designing durable and highly selective CO2 capture membranes.

Efficient mercury removal from aqueous solutions using carboxylated Ti3C2Tx MXene.

Water supplies contaminated with heavy metals are a worldwide concern. MXenes have properties that make them attractive for the removal of metal ions from water. This work presents a simple one-step method of Ti3C2Tx carboxylation that involves the use of a chelating agent with a linear structure, providing strong carboxylic acid groups with high mobility. The carboxylation decreases the zeta-potential of Ti3C2Tx by ~16 to ~18 mV over a pH range of 2.0-8.5 and improves Ti3C2Tx stability in the presence of molecular oxygen. pH in the range of 2-6 has a negligible effect on the adsorption capacity of Ti3C2Tx and COOH-Ti3C2Tx. Compared to Ti3C2Tx, COOH-Ti3C2Tx has a slightly higher and much faster mercury uptake, and the concentration of mercury ions leached out from COOH-Ti3C2Tx is lower. For both Ti3C2Tx and COOH-Ti3C2Tx, the leached mercury ion concentration is far below the U.S.-EPA maximum level. At an initial Hg2+ concentration of 50 ppm and pH of 6, COOH-Ti3C2Tx has the equilibrium adsorption capacity of 499.7 mg/g and removes 95% of Hg2+ in less than 1 min. Moreover, it has an equilibrium time of 5 min, which is significantly shorter than that of Ti3C2Tx (~ 60 min). Finally, its mercury-ion uptake capacity is higher than commercially available adsorbents reported in the literature. Its mercury removal is mainly via chemisorption and monolayer adsorption.

Alginate beads impregnated with sulfonate containing calix[4]arene-intercalated layered double hydroxides: In situ preparation, characterization and methylene blue adsorption studies.

The present research describes the in situ preparation of novel alginate nanocomposite beads impregnated with unmodified MgAl-NO3 layered double hydroxide (LDH) and a sulfonate containing calix[4]arene-intercalated LDH (SuLDH). The effect of intercalation of LDH on the methylene blue adsorption efficiency of Alg/SuLDH beads was investigated and compared with Alg/LDH and pure alginate beads (Alg). The prepared beads were characterized by FTIR, XRD, TGA, SEM, BET, and swelling ratio analysis. Among samples, the Alg/SuLDH beads showed the highest thermal stability, crystallinity, water swelling ratio, and specific surface area. In addition, based on the batch adsorption studies, Alg/SuLDH beads exhibited higher adsorption efficiency compared to Alg/LDH and Alg beads. The obtained equilibrium adsorption data were modeled by the Langmuir and Freundlich adsorption isotherm models. Regarding the Langmuir model, the maximum adsorption capacity of Alg beads, Alg/LDH, and Alg/SuLDH nanocomposite beads was obtained 540.54, 598.80, and 653.59.36 mg/g, respectively. The kinetic adsorption studies also demonstrated that the adsorption of MB by all samples well obey the pseudo-second-order kinetic model. This study proposes the Alg/SuLDH beads as a highly developed adsorbent for removal of MB from wastewaters. The reusability experiments showed that the Alg/SuLDH beads retained 72.55% of initial activity after 5 cycles.

Pushing Rubbery Polymer Membranes To Be Economic for CO2 Separation: Embedment with Ti3C2Tx MXene Nanosheets.

Sustainable and energy-efficient molecular separation requires membranes with high gas permeability and selectivity. This work reports excellent CO2 separation performance of self-standing and thin-film mixed matrix membranes (MMMs) fabricated by embedding 2D Ti3C2Tx MXene nanosheets in Pebax-1657. The CO2/N2 and CO2/H2 separation performances of the free-standing membranes are above Robeson's upper bounds, and the performances of the thin-film composite (TFC) membranes are in the target area for cost-efficient CO2 capture. Characterization and molecular dynamics simulation results suggest that the superior performances of the Pebax-Ti3C2Tx membranes are due to the formation of hydrogen bonds between Ti3C2Tx and Pebax chains, leading to the creation of the well-formed galleries of Ti3C2Tx nanosheets in the hard segments of the Pebax. The interfacial interactions and selective Ti3C2Tx nanochannels enable fast and selective CO2 transport. Enhancement of the transport properties of Pebax-2533 and polyurethane when embedded with Ti3C2Tx further supports these findings. The ease of fabrication and high separation performance of the new TFC membranes point to their great potential for energy-efficient CO2 separation with the low cost of $29/ton separated CO2.

Rational Tuning of Zirconium Metal-Organic Framework Membranes for Hydrogen Purification.

The effect of organic ligands on the separation performance of Zr based metal-organic framework (Zr-MOF) membranes was investigated. A series of Zr-MOF membranes with different ligand chemistry and functionality were synthesized by an in situ solvothermal method and a coordination modulation technique. The thin supported MOF layers (ca. 1 µm) showed the crystallographic orientation and pore structure of original MOF structures. The MOF membranes show excellent selectivity towards hydrogen owing to the molecular sieving effect when the bulkier linkers were used. The molecular simulation confirmed that the constricted pore apertures of the Zr-MOFs which were formed by the additional benzene rings lead to the decrease in the diffusivity of larger penetrants while hydrogen was not remarkably affected. The gas mixture separation factors of the MOF membranes reached to H2 /CO2 =26, H2 /N2 =13, H2 /CH4 =11.

Tuning the Gas Selectivity of Tröger's Base Polyimide Membranes by Using Carboxylic Acid and Tertiary Base Interactions.

Polyimide-based materials provide attractive chemistries for the development of gas-separation membranes. Modification of inter- and intra-chain interactions is a route to improve the separation performance. In this work, copolyimides with Tröger's base (TB) monomers are designed and synthesized. In particular, a series of copolyimides is synthesized with different contents of carboxylic acid groups (0-50 wt %) to alter the inter- and intra-chain interactions and enhance the basicity of the TB-polyimides. A detailed thermal and structural analysis is provided for the new copolyimides. Gas permeation data reveal a tunable trend in separation performance with increasing carboxylic acid group content. Importantly, this is one of the few examples of copolyimide membranes materials that show enhanced plasticization resistance to high-pressure gas feeds through physical cross-linking.

A facile synthesis of contorted spirobisindane-diamine and its microporous polyimides for gas separation.

Microporous polyimides (PIM-PIs, KAUST-PIs) and polymers containing Tröger's base (TB) derivatives with improved permeability and selectivity have great importance for separation of environmental gas pairs. Despite the tremendous progress in this field, facile synthesis of microporous polymers at the industrial scale via designing new monomers is still lacking. In this study, a new potential approach for large scale synthesis of spirobisindane diamine (DAS) (3) has been reported from commercially available 5,5',6,6'-tetrahydroxy-3,3,3',3'-tetramethyl-1,1'-spirobisindane (TTSBI) and 3,4-difluoronitrobenzene. A series of DAS diamine based microporous polyimides were also synthesized. The resulting polymer membranes showed high mechanical and thermal properties with tunable gas separation performance.