Tuning the porosity of triangular supramolecular adsorbents for superior haloalkane isomer separations

Separation of cyclohexanol from cyclohexanol/cyclohexene mixtures by crystals of pillar[6]arene containing three benzoquinone units

Here, we develop a new absorbent for efficient separation of cyclohexanol (CHA-ol) and cyclohexene (CHA-ene) by using crystals of pillar[6]arene with three benzoquinone units (P3QA). P3QA crystals are found to show remarkable selectivity for CHA-ol in 50 : 50 (v/v) CHA-ol : CHA-ene mixtures with a purity of 95.2%, along with vapochromic behavior.

Cis-Trans and Length-Selective Molecular Discrimination of Halogenated Organic Compounds by a Crystalline Hybrid Macrocyclic Arene

The development of adsorbents with robust molecular discrimination capabilities for halogenated organic compounds (HOCs) holds significant importance due to their potential in adsorptive separation and mitigation of associated health risks. In this study, we report a molecular discrimination behavior based on crystalline hybrid macrocyclic arene H, offering precise capture of cis-trans isomers and length-selective separation of HOCs. The activated H crystals (Hα) demonstrate exceptional discrimination and separation performance by selectively capturing trans-1,2-dichloroethylene (trans-DCE) from cis/trans-isomer mixtures with a high selectivity of 98.8%. Evidenced by single-crystal X-ray diffraction and density functional theory (DFT) calculations, this high adsorption selectivity arises from the formation of more stable complex crystals between H and the preferred guest trans-DCE. Moreover, Hα exhibits the ability to selectively trap size-matched 1,2-dibromoethane (DBE) from mixtures of alkylene dibromides with varying alkane-chain lengths, although their capture and separation are recognized to be difficult as a consequence of low-polarity bonds. The solid-state transformations between guest-free and guest-containing Hα crystals indicate their recyclability, showcasing promising prospects for potential applications.

Selective Water Pore Recognition and Transport through Self-Assembled Alkyl-Ureido-Trianglamine Artificial Water Channels

In nature, aquaporins (AQPs) are proteins known for fast water transport through the membrane of living cells. Artificial water channels (AWCs) synthetic counterparts with intrinsic water permeability have been developed with the hope of mimicking the performances and the natural functions of AQPs. Highly selective AWCs are needed, and the design of selectivity filters for water is of tremendous importance. Herein, we report the use of self-assembled trianglamine macrocycles acting as AWCs in lipid bilayer membranes that are able to transport water with steric restriction along biomimetic H-bonding-decorated pores conferring selective binding filters for water. Trianglamine [(±)Δ, (mixture of diastereoisomers) and (R,R)3Δ and (S,S)3Δ], trianglamine hydrochloride (Δ.HCl), and alkyl-ureido trianglamines (n = 4, 6, 8, and 12) [(±)ΔC4, (±)ΔC8, (±)ΔC6, and (±)ΔC12] were synthesized for the studies presented here. The single-crystal X-ray structures confirmed that trianglamines form a tubular superstructure in the solid state. The water translocation is controlled via successive selective H-bonding pores (a diameter of 3 Å) and highly permeable hydrophobic vestibules (a diameter of 5 Å). The self-assembled alkyl-ureido-trianglamines achieve a single-channel permeability of 108 water molecules/second/channel, which is within 1 order of magnitude lower than AQPs with good ability to sterically reject ions and preventing the proton transport. Trianglamines present potential for engineering membranes for water purification and separation technologies.

Industrial Separation Challenges: How Does Supramolecular Chemistry Help?

The chemical industry and the chemical processes underscoring it are under intense scrutiny as the demands for the transition to more sustainable and environmentally friendly practices are increasing. Traditional industrial separation systems, such as thermally driven distillation for hydrocarbon purification, are energy intensive. The development of more energy efficient separation technologies is thus emerging as a critical need, as is the creation of new materials that may permit a transition away from classic distillation-based separations. In this Perspective, we focus on porous organic cages and macrocycles that can adsorb guest molecules selectively through various host-guest interactions and permit molecular sieving behavior at the molecular level. Specifically, we summarize the recent advances where receptor-based adsorbent materials have been shown to be effective for industrially relevant hydrocarbon separations, highlighting the underlying host-guest interactions that impart selectivity and permit the observed separations. This approach to sustainable separations is currently in its infancy. Nevertheless, several receptor-based adsorbent materials with extrinsic/intrinsic voids or special functional groups have been reported in recent years that can selectively capture various targeted guest molecules. We believe that the understanding of the interactions that drive selectivity at a molecular level accruing from these initial systems will permit an ever-more-effective "bottom-up" design of tailored molecular sieves that, in due course, will allow adsorbent material-based approaches to separations to transition from the laboratory into an industrial setting.

A novel pillar[3]trianglimine macrocycle with a deep cavity used as a chiral selector to prepare a chiral stationary phase by thiol-ene click reaction for enantioseparation in high-performance liquid chromatography

A chiral pillar[3]trianglimine (C60 H72 N6 O6 ) with a deep cavity has been developed as a chiral selector and bonded to thiolated silica by thiol-ene click reaction to fabricate a novel chiral stationary phase for enantioseparation in high-performance liquid chromatography. The enantioseparation performance of the fabricated chiral stationary phase has been evaluated by separating various racemic compounds, including alcohols, esters, amines, ketones, amino acids, and epoxides, in both normal-phase and reversed-phase elution modes. In total, 14 and 17 racemates have been effectively separated in these two separation modes, respectively. In comparison with two widely used chiral columns (Chiralcel OD-H and Chiralpak AD-H), our novel chiral stationary phase offered good chiral separation complementarity, separating some of the tested racemates that could not be separated or were only partially separated on these two commercial columns. The influences of analyte mass, mobile phase composition, and column temperature on chiral separation have been investigated. Good repeatability, stability, and column-to-column reproducibility of the chiral stationary phase for enantioseparation have been observed. After the fabricated column had been eluted up to 400 times, the relative standard deviations (n = 5) of resolution (Rs) and retention time of the separated analytes were < 0.39% and < 0.20%, respectively. The relative standard deviations (n = 3) of Rs and retention time for column-to-column reproducibility were < 4.6% and < 5.2%, respectively. This study demonstrated that the new chiral stationary phase has great prospects for chiral separation in high-performance liquid chromatography.

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.

Water-Soluble Pd6L3 Molecular Bowl for Separation of Phenanthrene from a Mixture of Isomeric Aromatic Hydrocarbons

Phenanthrene is a high-value raw material in chemical industries. Separation of phenanthrene from isomeric anthracene continues to be a big challenge in the industry due to their very similar physical properties. Herein, we report the self-assembly of a water-soluble molecular bowl (TB) from a phenothiazine-based unsymmetrical terapyridyl ligand (L) and a cis-blocked 90° Pd(II) acceptor. TB featured an unusual bowl-like topology, with a wide rim diameter and a hydrophobic inner cavity fenced by the aromatic rings of the ligand. The above-mentioned features of TB allow it to bind polyaromatic hydrocarbons in its confined cavity. TB shows a higher affinity for phenanthrene over its isomer anthracene in water, which enables it to separate phenanthrene with ∼93% purity from an equimolar mixture of phenanthrene and anthracene. TB is also able to extract pyrene with around ∼90% purity from an equimolar mixture of coronene, perylene, and pyrene. Moreover, TB can be reused for several cycles without significant degradation in its performance as an extracting agent. This clean strategy of separation of phenanthrene and pyrene from a mixture of hydrophobic hydrocarbons by aqueous extraction is noteworthy.

Unravelling Structural Dynamics, Supramolecular Behavior, and Chiroptical Properties of Enantiomerically Pure Macrocyclic Tertiary Ureas and Thioureas

The introduction of urea or thiourea functionality to the macrocycle skeleton represents an alternative way to control conformational dynamics of chiral, polyamines of a figure-shaped periodical structure. Formally highly symmetrical, these macrocycles may adapt diverse conformations, depending on the nature of an amide linker and on a substitution pattern within the aromatic units. The type of heteroatom X in the N-C(═X)-N units present in each vertex of the macrocycle core constitutes the main factor determining the chiroptical properties. In contrast to the urea-containing derivatives, the electronic circular dichroism of thioureas is controlled by the chiral neighborhood closest to the chromophore. The dynamically induced exciton couplet is observed when the biphenyl chromophores are present in the macrocycle core. In the solid state, the seemingly disordered molecules may create ordered networks stabilized by intermolecular S···halogen, H···halogen, and S···H interactions. The presence of two bromine substituents in each aromatic unit in thiourea-derived trianglamine gives rise to a self-sorting phenomenon in the crystal. In solution, this particular macrocycle exists as a dynamic equimolar mixture of two conformational diastereoisomers, differing in the spatial (clockwise and counter clockwise) arrangement of the C-Br bonds. In the crystal lattice, macrocycles of a given handedness assemble into homohelical layers.

A Cavity-Tailored Metal-Organic Tetrahedral Nanocage and Gas Adsorption Property

Porous organometallic nanomaterials are a new class of materials based on a three-dimensional structure. They have excellent applications in different fields, but their applications in gas storage and separation have not been fully developed. CO₂ adsorption storage and hydrocarbon separation has been a challenging industrial problem. Several typical molecular adsorbents have been used to study the separation, but the problems of long-term stability, high selectivity and synthetic complexity of these adsorbents remain to be solved. Here, we have designed and synthesized tetrahedral metal supramolecular nanocage with custom cavities based on the unique rigid structure of triptycene derivatives. Using the unique discrete porous structure of tetrahedral metal nanocages, the gas adsorption and separation performance of the metal supramolecular nanocage was investigated. By analyzing the adsorption and desorption isotherms and the multi-component competitive adsorption curves, we noticed that the tetrahedral supramolecular nanocages had good CO₂ storage capacity and good separation capacity for C₂H₂/CO₂ and C₂H₂/N₂. All these indicate that porous organic metal nanomaterials are expected to be a new energy saving separation material.

Efficient Purification of 2,6-Lutidine by Nonporous Adaptive Crystals of Pillararenes

2,6-Lutidine (2,6-LT) is a very important raw material in the chemical industry, but the impurities of 3-picoline (3-PC) and 4-picoline (4-PC) existing in 2,6-LT seriously affect its quality. Considering different molecular sizes of these three compounds, herein, we exploit nonporous adaptive crystals (NACs) of pillararenes as purifying agents for removal of 3-PC and 4-PC in 2,6-LT. We find that per-ethylated pillar[5]arene (EtP5) can selectively adsorb 3-PC and 4-PC, while negligible capture of 2,6-LT is observed, resulting in improvement of the purity of 2,6-LT up to 94.9%. Single-crystal structures indicate that the excellent selectivity originates from the size match and complexation stability differences among different host/guest pairs. After purification, NACs of EtP5 can be easily regenerated and used in the next run without a significant performance degradation.

Symmetry-driven diastereoselective functionalization of simple trianglamine

We have found that derivatization of the trianglamine macrocycle by aliphatic aldehydes leads selectively to one of the two possible diastereomeric aminal products. X-ray analysis, NMR measurements and DFT calculations pointed to the product possessing a higher symmetry.

Preparation of chiral stationary phase based on a [3+3] chiral polyimine macrocycle by thiol-ene click chemistry for enantioseparation in normal-phase and reversed-phase high performance liquid chromatography

Polyimine macrocycles are a new class of organic macrocycles with cyclic structures, well-defined molecular cavities, and multiple cooperative binding sites, which have recently aroused considerable research interest in molecular recognition and separation. Herein, we report the bonding of a [3+3] chiral polyimine macrocycle (H₃L, C₇₈H₇₈N₆O₃) on thiol-functionalized silica gel using thiol-ene click chemistry to prepare a chiral stationary phase (CSP) for high performance liquid chromatography (HPLC). The fabricated column exhibited excellent chiral separation capability under both normal-phase and reversed-phase conditions. Fourteen and 10 racemates were well resolved on the column in normal-phase mode (using n-hexane/isopropanol as the mobile phase) and reversed-phase mode (using methanol/water as the mobile phase), respectively, including alcohols, esters, ethers, ketones, aldehydes, epoxides and organic acids. Moreover, the column also shows good selectivity toward positional isomers. Six positional isomers (dinitrobenzene, chloroaniline, bromoaniline, iodoaniline, nitrobrobenzene and nitrochlorobenzene) were well separated on the column. In addition, the effects of the injection mass and mobile phase composition on the separation were investigated. The column shows good reproducibility and stability after multiple injections with the relative standard deviation (RSD) (n = 5) of the retention time and resolution being < 0.96 % and 0.65 %, respectively. This study indicates that this type of chiral polyimine macrocycles is a promising chiral selector for HPLC enantioseparation and will push forward the applications of more novel chiral macrocycles for chiral chromatographic separation.

Guest-induced amorphous-to-crystalline transformation enables sorting of haloalkane isomers with near-perfect selectivity

The separation of haloalkane isomers with distillation-free strategies is one of the most challenging research topics in fundamental research and also gave high guiding values to practical industrial applications. Here, this contribution provides a previously unidentified solid supramolecular adsorption material based on a leggero pillararene derivative BrP[5]L, which can separate 1-/2-bromoalkane isomers with near-ideal selectivity. Activated solids of BrP[5]L with interesting amorphous and nonporous features could adsorb 1-bromopropane and 1-bromobutane from the corresponding equal volume mixtures of 1-/2-positional isomers with purities of 98.1 and 99.0%, respectively. Single-crystal structures incorporating theoretical calculation reveal that the high selectivity originates from the higher thermostability of 1-bromoalkane-loaded structures compared to its corresponding isomer-loaded structures, which could be further attributed to the perfect size/shape match between BrP[5]L and 1-bromoalkanes. Moreover, control experiments using its counterpart macrocycle of traditional pillararene demonstrate that BrP[5]L has better adsorptive selectivity, benefiting from the intrinsic free-rotation phenylene subunit on its backbone.

Pillar[3]trianglamines: deeper cavity triangular macrocycles for selective hexene isomer separation

The separation of α-olefins and their corresponding isomers continues to be a big challenge for the chemical industry due to their overlapping physical properties and low relative volatility. Herein, pillar[3]trianglamine (P-TA) macrocycles were synthesized for the molecular-sieving-like separation of 1-hexene (1-He) selectively over its positional isomer trans-3-hexene (trans-3-He) in the vapor and liquid state. This allyl-functionalized macrocycle features a deeper cavity compared to the previously reported trianglamine host molecules. Solid-vapor sorption experiments verified the successful separation of 1-He from an equimolar mixture of 1-He and trans-3-He. Single-crystal structures and powder X-ray diffraction patterns suggest that this selective adsorption arises from the formation of a thermodynamically stable host-guest complex between 1-He and P-TA. A reversible transformation between the nonporous guest-free structure and the guest-containing structure shows that 1-He separation can be carried out over multiple cycles without any loss of performance. Significantly, P-TA can separate 1-He directly from a liquid isomeric mixture and thus P-TA modified silica sieves (SBA-15) showed the ability to selectively separate 1-He when utilized as a stationary phase in column chromatography. This capitalizes on the prospects of employing macrocyclic hosts as molecular recognition units in real-life separations for sustainable and energy-efficient industrial practices.

Surface environment complication makes Ag29 nanoclusters more robust and leads to their unique packing in the supracrystal lattice

Silver nanoclusters have received unprecedented attention in cluster science owing to their promising functionalities and intriguing physical/chemical properties. However, essential instability significantly impedes their extensive applications. We herein propose a strategy termed “surface environment complication” to endow Ag₂₉ nanoclusters with high robustness. The Ag₂₉(S-Adm)₁₈(PPh₃)₄ nanocluster with monodentate PPh₃ ligands was extremely unstable and uncrystallizable. By substituting PPh₃ with bidentate PPh₂py with dual coordination sites (i.e., P and N), the Ag₂₉ cluster framework was twisted because of the generation of N–Ag interactions, and three NO₃ ligands were further anchored onto the nanocluster surface, yielding a new Ag₂₉(S-Adm)₁₅(NO₃)₃(PPh₂py)₄ nanocluster with high stability. The metal-control or ligand-control effects on stabilizing the Ag₂₉ nanocluster were further evaluated. Besides, Ag₂₉(S-Adm)₁₅(NO₃)₃(PPh₂py)₄ followed a unique packing mode in the supracrystal lattice with several intercluster channels, which has yet been observed in other M₂₉ cluster crystals. Overall, this work presents a new approach (i.e., surface environment complication) for tailoring the surface environment and improving the stability of metal nanoclusters.

A strategy of “surface environment complication” has been exploited to endow Ag₂₉ nanoclusters with high robustness and a unique packing mode in the supracrystal lattice.

Selective separation of planar and non-planar hydrocarbons using an aqueous Pd6 interlocked cage

Polycyclic aromatic hydrocarbons (PAHs) find multiple applications ranging from fabric dyes to optoelectronic materials. Hydrogenation of PAHs is often employed for their purification or derivatization. However, separation of PAHs from their hydrogenated analogues is challenging because of their similar physical properties. An example of such is the separation of 9,10-dihydroanthracene from phenanthrene/anthracene which requires fractional distillation at high temperature (∼340 °C) to obtain pure anthracene/phenanthrene in coal industry. Herein we demonstrate a new approach for this separation at room temperature using a water-soluble interlocked cage (1) as extracting agent by host–guest chemistry. The cage was obtained by self-assembly of a triimidazole donor L·HNO₃ with cis-[(tmeda)Pd(NO₃)₂] (M) [tmeda = N,N,N′,N′-tetramethylethane-1,2-diamine]. 1 has a triply interlocked structure with an inner cavity capable of selectively binding planar aromatic guests.

We report here a triply interlocked cage with the ability to encapsulate planar guests in aqueous medium. This property was then employed to efficiently separate planar and non-planar aromatic hydrocarbons by aqueous extraction.

Highly selective molecular sieving of cis- over trans-1,2-dichloroethene isomers

An economic and energy-efficient adsorptive molecular sieving approach is demonstrated for the separation of the cis-DCE isomer with a purity of ∼96% using trianglimine macrocycles.