Reversible Capture Mechanism of CO2 as a Zn(II)-Methylcarbonate

In this study, we elucidate the reaction mechanism for capturing CO2 with the ZnL1(MeOH) complex (L1=diacetyl-2-(4-methyl-3-thiosemicarbazone)-3-(2-hydrazinatopyridine)) in a methanol solution, using density functional theory calculations. One pathway involves the protonation of ZnL1(MeOH) by methylcarbonic acid, followed by ligand exchange of MeOH with MeOCO2 -. An alternative mechanism suggests a tautomerization between ZnL1(MeOH) and Zn(HL1)(OMe), followed by CO2 insertion. The latter pathway is energetically more favorable than the former and more complex than initially proposed. In fact, we unveiled that the solvent catalyzes tautomerization, as one explicit methanol molecule acts as a proton transfer agent. Then, Zn(HL1)(OMe) captures CO2, yielding a methylcarbonate bound to the metal center. The final step involves a rearrangement that leads to the cleavage of the Zn-O(Me)(COO) bond and the formation of a new Zn-O(COOMe) bond, along with the rotation of the methylcarbonate group. We consider an additional mechanism that combines tautomerization and ligand exchange but is endergonic and requires a high activation barrier for the ligand exchange. Furthermore, we evaluate the ligand basicity through the pKa calculated values of the Zn(II) complexes, the effects of varying the ligand from 4-methyl-thiosemicarbazone to 4-ethyl (L2), 4-phenethyl (L3), and 4-benzyl (L4) derivatives, and reversibility of the reaction in an argon environment.

Metal-Ligand Cooperativity Promotes Reversible Capture of Dilute CO2 as a Zn(II)-Methylcarbonate

In this study, a series of thiosemicarbazonato-hydrazinatopyridine metal complexes were evaluated as CO₂ capture agents. The complexes incorporate a non-coordinating, basic hydrazinatopyridine nitrogen in close proximity to a Lewis acidic metal ion allowing for metal-ligand cooperativity. The coordination of various metal ions with (diacetyl-2-(4-methyl-thiosemicarbazone)-3-(2-hydrazinopyridine) (H₂L¹) yielded ML¹ (M = Ni(II), Pd(II)), ML¹(CH₃OH) (M = Cu(II), Zn(II)), and [ML¹(PPh₃)₂]BF₄ (M = Co(III)) complexes. The ML¹(CH₃OH) complexes reversibly capture CO₂ with equilibrium constants of 88 ± 9 and 6900 ± 180 for Cu(II) and Zn(II), respectively. Ligand effects were evaluated with Zn(II) through variation of the 4-methyl-thiosemicarbazone with 4-ethyl (H₂L²), 4-phenethyl (H₂L³), and 4-benzyl (H₂L⁴) derivatives. The equilibrium constant for CO₂ capture increased to 11,700 ± 300, 15,000 ± 400, and 35,000 ± 200 for ZnL²(MeOH), ZnL³(MeOH), and ZnL⁴(MeOH), respectively. Quantification of ligand basicity and metal ion Lewis acidity shows that changes in CO₂ capture affinity are largely associated with ligand basicity upon substitution of Cu(II) with Zn(II), while variation of the thiosemicarbazone ligand enhances CO₂ affinity by tuning the metal ion Lewis acidity. Overall, the Zn(II) complexes effectively capture CO₂ from dilute sources with up to 90%, 86%, and 65% CO₂ capture efficiency from 400, 1000, and 2500 ppm CO₂ streams.

Gating effect for gas adsorption in microporous materials-mechanisms and applications

In the past two decades, various microporous materials have been developed as useful adsorbents for gas adsorption for a wide range of industries. Considerable efforts have been made to regulate the pore accessibility in microporous materials for the manipulation of guest molecules' admission and release. It has long been known that some microporous adsorbents suddenly become highly accessible to guest molecules at specific conditions, e.g., above a threshold pressure or temperature. This anomalous adsorption behavior results from a gating effect, where a structural variation of the adsorbent leads to an abrupt change in the gas admission. This review summarizes the mechanisms of the gating effect, which can be a result of the deformation of the framework (e.g., expansion, contraction, reorientation, and sliding of the unit cells), the vibration of the pore-keeping groups (e.g., rotation, swing, and collapse of organic linkers), and the oscillation of the pore-keeping ions (e.g. cesium, potassium, etc.). These structural variations are induced either by the host-guest interaction or by an external stimulus, such as temperature or light, and account for the gating effect at a threshold value of the stimulus. Emphasis is given to the temperature-regulated gating effect, where the critical admission temperature is dictated by the combined effect of the gate opening and thermodynamic factors and plays a key role in regulating guest admission. Molecular simulations can improve our understanding of the gate opening/closing transitions at the atomic scale and enable the construction of quantitative models to describe the gated adsorption behaviour at the macroscale level. The gating effect in porous materials has been widely applied in highly selective gas separation and offers great potential for gas storage and sensing.

Efficient Gas and VOC Separation and Pesticide Detection in a Highly Stable Interpenetrated Indium-Organic Framework

The new indium-based organic framework {(Me₂NH₂)[In(BDPO)]·DMF·2H₂O}_n (1) was successfully constructed by using the oxalamide group modified ligand N,N'-bis(isophthalic acid)oxalamide (H₄BDPO). This framework presents a 2-fold interpenetrating structural characteristic, and the unique polar pore environment leads to a high capture ability for CO₂, C₂H_n and CH₃OH and good separation ability for CO₂ and C₂H_n over CH₄ as well as for CH₃OH over C₂H₅OH, which was further verified by an ideal adsorbed solution theory (IAST) calculation. Theoretical simulations pointed out the possible adsorption sites of different adsorbed gases in 1. In addition, the excellent chemical stability and strong luminescence of 1 give it an effective selective detection ability for 2,6-dichloro-4-nitroaniline (DCN) in water with a low detection limit of 3.85 ppm, and the detection mechanism is discussed in detail.

Paddlewheel SBU based Zn MOFs: Syntheses, Structural Diversity, and CO₂ Adsorption Properties

Four Zn metal⁻organic frameworks (MOFs), {[Zn₂(2,6-ndc)₂(2-Pn)]·DMF}n (1), {[Zn₂(cca)₂(2-Pn)]·DMF}n (2), {[Zn₂(thdc)₂(2-Pn)]·3DMF}n (3), and {[Zn₂(1,4-ndc)₂(2-Pn)]·1.5DMF}n (4), were synthesized from zinc nitrate and N,N'-bis(pyridin-2-yl)benzene-1,4-diamine (2-Pn) with naphthalene-2,6-dicarboxylic acid (2,6-H₂ndc), 4-carboxycinnamic acid (H₂cca), 2,5-thiophenedicarboxylic acid (H₂thdc), and naphthalene-1,4-dicarboxylic acid (1,4-H₂ndc), respectively. MOFs 1⁻4 were all constructed from similar dinuclear paddlewheel {Zn₂(COO)₄} clusters and resulted in the formation of three kinds of uninodal 6-connected non-interpenetrated frameworks. MOFs 1 and 2 suit a topologic 4⁸·6⁷-net with 17.6% and 16.8% extra-framework voids, respectively, 3 adopts a pillared-layer open framework of 4⁸·6⁶·8-topology with sufficient free voids of 39.9%, and 4 features a pcu-type pillared-layer framework of 412·6³-topology with sufficient free voids of 30.9%. CO₂ sorption studies exhibited typical reversible type I isotherms with CO₂ uptakes of 55.1, 84.6, and 64.3 cm³ g-1 at 195 K and P/P₀ =1 for the activated materials 1', 2', and 4', respectively. The coverage-dependent isosteric heat of CO₂ adsorption (Qst) gave commonly decreased Qst traces with increasing CO₂ uptake for all the three materials and showed an adsorption enthalpy of 32.5 kJ mol-1 for 1', 38.3 kJ mol-1 for 2', and 23.5 kJ mol-1 for 4' at zero coverage.

Gate-opening upon CO2 adsorption on a metal–organic framework that mimics a natural stimuli-response system

A dynamic d-champhorate-based protuberant-grid-type framework, undergoes gate opening and closing processes that were triggered by the stimuli of the adsorption or desorption of CO₂. It is able to specifically recognize CO₂ over than N₂ and H₂ and shows a high CO₂ uptake of 90 mg g⁻¹ under 35 bar at 298 K.