Nonporous Adaptive Crystals Research Articles

Cover Picture: Tiara[5]arenes: Synthesis, Solid‐State Conformational Studies, Host–Guest Properties, and Application as Nonporous Adaptive Crystals (Angew. Chem. Int. Ed. 10/2020)

Tiara[5]arenes, five-fold-symmetric macrocycles consisting of phenolic units linked by para-substituted methylene bridges, were synthesized by H. Zuilhof, A. C.-H. Sue, and co-workers. In their Research Article on page 3994, the construction and derivatization are demonstrated, along with solid-state structures, host–guest properties in solution, and application as nonporous adaptive crystals for the separation of benzene and cyclohexane.

Separation of Benzene and Cyclohexane by Nonporous Adaptive Crystals of a Hybrid[3]arene.

The separation of benzene and cyclohexane is one of the most challenging tasks in the petrochemical field. However, conventional separation methods suffer from cumbersome operation, huge energy expenditure, or use of entrainers. Herein, we develop an environmentally friendly and energy saving adsorptive separation strategy using nonporous adaptive crystals of a hybrid[3]arene (1). Adaptive 1 crystals separate benzene from an equimolar benzene/cyclohexane mixture with a purity of 97.5%. The selectivity comes from the stability and variability of the new crystal structure upon capture of the preferred guest, benzene. Moreover, reversible transformations between the nonporous guest-free structure and the guest-containing structure make 1 highly recyclable.

Tiara[5]arenes: Synthesis, Solid-State Conformational Studies, Host-Guest Properties, and Application as Nonporous Adaptive Crystals.

Tiara[5]arenes (T[5]s), a new class of five‐fold symmetric oligophenolic macrocycles that are not accessible from the addition of formaldehyde to phenol, were synthesized for the first time. These pillar[5]arene‐derived structures display both unique conformational freedom, differing from that of pillararenes, with a rich blend of solid‐state conformations and excellent host–guest interactions in solution. Finally we show how this novel macrocyclic scaffold can be functionalized in a variety of ways and used as functional crystalline materials to distinguish uniquely between benzene and cyclohexane.

Tiara[5]arenes: Synthesis, Solid‐State Conformational Studies, Host–Guest Properties, and Application as Nonporous Adaptive Crystals

AbstractTiara[5]arenes (T[5]s), a new class of five‐fold symmetric oligophenolic macrocycles that are not accessible from the addition of formaldehyde to phenol, were synthesized for the first time. These pillar[5]arene‐derived structures display both unique conformational freedom, differing from that of pillararenes, with a rich blend of solid‐state conformations and excellent host–guest interactions in solution. Finally we show how this novel macrocyclic scaffold can be functionalized in a variety of ways and used as functional crystalline materials to distinguish uniquely between benzene and cyclohexane.

Highly selective removal of heterocyclic impurities from toluene by nonporous adaptive crystals of perethylated pillar[6]arene

We use nonporous adaptive crystals of perethylated pillar[6]arene to adsorb heterocyclic compounds from a mixture of toluene and heterocyclic compounds with high selectivity, ultimately improving the purity of toluene from 96.78% to 99.00%.

Separation of Bromoalkanes Isomers by Nonporous Adaptive Crystals of Leaning Pillar[6]arene

AbstractHaloalkanes are important chemicals in synthetic chemistry and petrochemical industry, but the separation of their isomers is a big hurdle. Herein, we report a facile energy‐efficient adsorptive separation strategy using a new class of nonporous adaptive crystals based on leaning pillar[6]arene. Desolvated perethylated leaning pillar[6]arene crystals (EtLP6) with interesting nonporous character show a preference for 1‐bromoalkane isomers over 2‐bromoalkane isomers. EtLP6 is capable of separating 1‐bromopropane, 1‐bromobutane, and 1‐bromopentane from the corresponding 1:1 (v/v) mixtures of 1/2‐positional isomers with purities from 89.6 % to 96.3 % in only one adsorption cycle. The selectivity is endowed by the different host–guest binding modes and different stabilities of EtLP6 crystalloids loaded with 1‐ and 2‐positional isomers. Significantly, the guest–adsorbed assemblies are highly stable at room temperature and EtLP6 can be reused many times without any decrease in performance.

Separation of Bromoalkanes Isomers by Nonporous Adaptive Crystals of Leaning Pillar[6]arene.

Haloalkanes are important chemicals in synthetic chemistry and petrochemical industry, but the separation of their isomers is a big hurdle. Herein, we report a facile energy-efficient adsorptive separation strategy using a new class of nonporous adaptive crystals based on leaning pillar[6]arene. Desolvated perethylated leaning pillar[6]arene crystals (EtLP6) with interesting nonporous character show a preference for 1-bromoalkane isomers over 2-bromoalkane isomers. EtLP6 is capable of separating 1-bromopropane, 1-bromobutane, and 1-bromopentane from the corresponding 1:1 (v/v) mixtures of 1/2-positional isomers with purities from 89.6 % to 96.3 % in only one adsorption cycle. The selectivity is endowed by the different host-guest binding modes and different stabilities of EtLP6 crystalloids loaded with 1- and 2-positional isomers. Significantly, the guest-adsorbed assemblies are highly stable at room temperature and EtLP6 can be reused many times without any decrease in performance.

Separation of Monochlorotoluene Isomers by Nonporous Adaptive Crystals of Perethylated Pillar[5]arene and Pillar[6]arene.

Separation of monochlorotoluene isomers is a vital process to obtain highly pure p-chlorotoluene, which is irreplaceable in the production of medicines and pesticides. However, traditional separation methods suffer from great energy consumption, cumbersome operation or use of organic desorbents. Herein, an energy-efficient and environmentally friendly method is developed through an absorptive separation strategy based on nonporous adaptive crystals of perethylated pillar[5]arene (EtP5) and pillar[6]arene (EtP6). EtP5 and EtP6 crystals separate p-chlorotoluene from a p-chlorotoluene/o-chlorotoluene equimolar mixture with purities of 99.1% and 96.1%, respectively and show no decrease in selectivity upon cycling. The selectivity is attributed to both the stability of the final crystal structure upon guest capture and suitable host cavity size/shape. Besides, we discovered the gate-opening behavior changes of EtP5 crystals at different temperatures after absorption of p-chlorotoluene/o-chlorotoluene mixtures with various p-chlorotoluene fractions, which is helpful to understand the thermodynamics of the absorption process.

Alkyl Chain Length-Selective Vapor-Induced Fluorochromism of Pillar[5]arene-Based Nonporous Adaptive Crystals.

Developing new vapochromic fluorescent sensing materials with selectivity is in great demand due to serious air pollution caused by volatile organic compounds. Here we report the first nonporous adaptive crystals with vapor-induced fluorochromism. The emission of anthracene conjugated pillar[5]arene P5en crystals blue shifts from bright yellow to green upon exposure to vapors of linear alkyl ketones with alkyl chain length selectivity. Powder and single crystal X-ray diffraction analyses reveal the relationship between vapor-induced crystal structural transformation and fluorochromism. After exposure to 2-butanone, 2-pentanone, and 2-hexanone, the anthracene groups in P5en exist as monomers in the solid state, while after exposure to heptanone and octanone, the anthracene groups instead aggregate as excimers in the solid state. By solid-state NMR and thermogravimetric analysis, we further find that the size fit between ketone molecules and interspaces in P5en crystals lead to the alkyl chain length selectivity. This work provides a new strategy to design vapochromic fluorescent crystalline materials and crystalline transformation systems.

Cis-Trans Selectivity of Haloalkene Isomers in Nonporous Adaptive Pillararene Crystals.

The separation of haloalkene cis-trans isomers is difficult to achieve, yet highly desired in the chemical industry. Here, we report an energy-efficient adsorptive separation of 1,4-dichloro-2-butene (DCB) cis-trans isomers using nonporous adaptive crystals of perethylated pillararenes. Adaptive perethylated pillar[6]arene (EtP6) crystals separate the trans-DCB isomer from its cis isomer with high selectivity while perethylated pillar[5]arene (EtP5) crystals adsorb cis-trans DCB isomers without selectivity. The selectivity of EtP6 derives from the difference in the thermodynamic stabilities of guest-loaded EtP6 crystal structures upon capture of cis-trans DCB isomers, while the structural similarity of guest-loaded EtP5 leads to the loss of selectivity. EtP6 is highly recyclable due to the reversible transformations between guest-free and guest-loaded structures.

Applications of pillararene NACs in adsorption and separation

This review describes a novel kind of materials for adsorption and separation, nonporous adaptive crystals (NACs), which are firstly mentioned and defined by us. Compared with traditional porous materials, NACs of pillararenes can be synthesized easily, have good chemical, humid and thermal stabilities, are soluble in many common organic solvents, and can be reused many times. Here, we focus on the discussion of pillararene-based NACs for adsorption and separation and the crystal structure transformations from the initial nonporous crystalline state to new guest-loaded structures during the adsorption and separation processes. Single-crystal X-ray diffraction, powder X-ray diffraction, gas chromatography and solution NMR are the main techniques to investigate the adsorption and separation processes and the structural transformations. It is expected that this kind of materials will show practical applications in the chemical industry.

Dihalobenzene Shape Sorting by Nonporous Adaptive Crystals of Perbromoethylated Pillararenes.

The separation of dihalobenzene isomers, such as dichlorobenzene isomers and difluorobenzene isomers, has a high practical value in both synthetic chemistry and industrial production. Herein we provide a simple to operate and energy-efficient adsorptive separation method using nonporous adaptive crystals of perbromoethylated pillar[5]arene (BrP5) and pillar[6]arene (BrP6). BrP6 crystals show a preference towards the ortho isomer of dichlorobenzene in isomer mixtures, but cannot discriminate difluorobenzene isomers. Single-crystal structures reveal that this selectivity is derived from the stability of the new host-guest crystal structure of BrP6 after uptake of the preferred guest and the binding strength of the host-guest interactions. Furthermore, because of the reversible transition between guest-free and guest-loaded structures, BrP6 crystals are recyclable.

Dihalobenzene Shape Sorting by Nonporous Adaptive Crystals of Perbromoethylated Pillararenes

AbstractThe separation of dihalobenzene isomers, such as dichlorobenzene isomers and difluorobenzene isomers, has a high practical value in both synthetic chemistry and industrial production. Herein we provide a simple to operate and energy‐efficient adsorptive separation method using nonporous adaptive crystals of perbromoethylated pillar[5]arene (BrP5) and pillar[6]arene (BrP6). BrP6 crystals show a preference towards the ortho isomer of dichlorobenzene in isomer mixtures, but cannot discriminate difluorobenzene isomers. Single‐crystal structures reveal that this selectivity is derived from the stability of the new host–guest crystal structure of BrP6 after uptake of the preferred guest and the binding strength of the host–guest interactions. Furthermore, because of the reversible transition between guest‐free and guest‐loaded structures, BrP6 crystals are recyclable.

Cyclic Ether Contaminant Removal from Water Using Nonporous Adaptive Pillararene Crystals via Host-Guest Complexation at the Solid-Solution Interface

The removal of soluble cyclic ether contaminants, such as dioxane and THF, produced in industrial chemical processes from water is of great importance for environmental protection and human health. Here we report that nonporous adaptive crystals of perethylated pillar[5]arene (EtP5) and pillar[6]arene (EtP6) work as adsorbents for cyclic ether contaminant removal via host-guest complexation at the solid-solution interface. Nonporous EtP6 crystals have the ability to adsorb dioxane from water with the formation of 1:2 host-guest complex crystals, while EtP5 crystals cannot. However, both guest-free EtP5 and EtP6 crystals remove THF from water with EtP5 having a better capacity. This is because EtP5 forms a 1:2 host-guest complex with THF via host-guest complexation at the solid-solution interface while EtP6 forms a 1:1 host-guest complex with THF. EtP6 also shows the ability to selectively remove dioxane from water even in the presence of THF. Moreover, the reversible transitions between nonporous guest-free EtP5 and EtP6 structures and guest-loaded structures make them highly recyclable.

Post-Synthetic Modification of Nonporous Adaptive Crystals of Pillar[4]arene[1]quinone by Capturing Vaporized Amines.

Postsynthetic modification in crystalline solids without disruption of crystallinity is very important for exerting control that is unattainable over chemical transformation in solution. This has been achieved in porous crystalline frameworks via solid-solution reactions to endow them with multiple functions. However, this is rather rare in nonporous molecular crystals, especially via solid-vapor reactions. Herein, we report unique solid-vapor postsynthetic modification of nonporous adaptive crystals (NACs) of a pillar[4]arene[1]quinone (EtP4Q1) containing four inert 1,4-diethoxybenzene units and one active benzoquinone unit. Amine vapors that can be physically adsorbed by EtP4Q1 NACs react with the EtP4Q1 backbone via Michael addition with in situ formation of new crystal structures. First, amines are physically adsorbed into cavities of EtP4Q1 molecules and slowly react due to their juxtapsition with the benzoquinone units. Amines that are too bulky to enter EtP4Q1 NACs do not react. Moreover, the process displays both reactant-size and -shape selectivities because of the rigid cavity of EtP4Q1 and the different binding strengths of various amines with EtP4Q1.

Separation of Aromatics/Cyclic Aliphatics by Nonporous Adaptive Pillararene Crystals

The separation of cyclic aliphatics of high purity, which are produced from hydrogenation of the corresponding aromatics, is highly desired in the chemical industry. An energy-efficient and environmentally friendly adsorptive separation method using nonporous adaptive crystals of perethylated pillar[5]arene (EtP5) and pillar[6]arene (EtP6) is described. Adaptive EtP5 crystals separate toluene from methylcyclohexane with 98.8 % purity, while adaptive EtP6 crystals separate methylcyclohexane from toluene with 99.2 % purity. The selectivities come from the stability of new EtP5 and EtP6 crystal structures upon capture of toluene and methylcyclohexane, respectively. The reversible transformations between nonporous guest-free EtP5 or EtP6 structures and guest-loaded structures make them highly recyclable.

Separation of Aromatics/Cyclic Aliphatics by Nonporous Adaptive Pillararene Crystals

AbstractThe separation of cyclic aliphatics of high purity, which are produced from hydrogenation of the corresponding aromatics, is highly desired in the chemical industry. An energy‐efficient and environmentally friendly adsorptive separation method using nonporous adaptive crystals of perethylated pillar[5]arene (EtP5) and pillar[6]arene (EtP6) is described. Adaptive EtP5 crystals separate toluene from methylcyclohexane with 98.8 % purity, while adaptive EtP6 crystals separate methylcyclohexane from toluene with 99.2 % purity. The selectivities come from the stability of new EtP5 and EtP6 crystal structures upon capture of toluene and methylcyclohexane, respectively. The reversible transformations between nonporous guest‐free EtP5 or EtP6 structures and guest‐loaded structures make them highly recyclable.

Nonporous Adaptive Crystals of Pillararenes.

Porous materials with high surface areas have drawn more and more attention in recent years because of their wide applications in physical adsorption and energy-efficient adsorptive separation processes. Most of the reported porous materials are macromolecular porous materials, such as zeolites, metal-organic frameworks (MOFs), or porous coordination polymers (PCPs), and porous organic polymers (POPs) or covalent organic frameworks (COFs), in which the building blocks are linked together by covalent or coordinative bonds. These materials are barely soluble and thus are not solution-processable. Furthermore, the relatively low chemical, moisture, and thermal stability of most MOFs and COFs cannot be neglected. On the other hand, molecular porous materials such as porous organic cages (POCs), which have been developed very recently, also show promising applications in adsorption and separation processes. They can be soluble in organic solvents, making them solution-processable materials. However, they are usually sensitive to acid/base and humid environments since most of them are based on dynamic covalent bonding. These macromolecular and molecular porous materials usually have two similar features: high Brunauer-Emmett-Teller (BET) surface areas and rigid pore structures, which are stable during adsorption and separation processes. In this Account, we describe a novel class of solid materials for adsorption and separation, nonporous adaptive crystals (NACs), which function at the supramolecular level. They are nonporous in the initial crystalline state, but the intrinsic or extrinsic porosity of the crystals along with a crystal structure transformation is induced by preferable guest molecules. Unlike solvent-induced crystal polymorphism phenomena of common organic crystals that occur at the solid-liquid phase, NACs capture vaporized guests at the solid-gas phase. Upon removal of guest molecules, the crystal structure transforms back to the original nonporous structure. Here we focus on the discussion of pillararene-based NACs for adsorption and separation and the crystal structure transformations from the initial nonporous crystalline state to new guest-loaded structures during the adsorption and separation processes. Single-crystal X-ray diffraction, powder X-ray diffraction, gas chromatography, and solution NMR spectroscopy are the main techniques to verify the adsorption and separation processes and the structural transformations. Compared with traditional porous materials, NACs of pillararenes have several advantages. First, their preparation is simple and cheap, and they can be synthesized on a large scale to meet practical demands. Second, pillararenes have better chemical, moisture, and thermal stability than crystalline MOFs, COFs, and POCs, which are usually constructed on the basis of reversible chemical bonds. Third, pillararenes are soluble in many common organic solvents, which means that they can be easily processed in solution. Fourth, their regeneration is simple and they can be reused many times with no decrease in performance. It is expected that this class of materials will not only exert a significant influence on scientific research but also show practical applications in chemical industry.

Linear Positional Isomer Sorting in Nonporous Adaptive Crystals of a Pillar[5]arene.

Here we show a new adsorptive separation approach using nonporous adaptive crystals of a pillar[5]arene. Desolvated perethylated pillar[5]arene crystals (EtP5α) with a nonporous character selectively adsorb 1-pentene (1-Pe) over its positional isomer 2-pentene (2-Pe), leading to a structural change from EtP5α to 1-Pe loaded structure (1-Pe@EtP5). The purity of 1-Pe reaches 98.7% in just one cycle and EtP5α can be reused without losing separation performance.