Stable Mg Single-Atom Solid Base Catalysts Anchored on Metal-Organic Framework-Derived Nitrogen-Doped Carbon.

Solid base catalysts are widely used in the chemical industry owing to their advantages of environmental friendliness and easy separation. However, their application is limited by basic site aggregation and poor stability. In this study, we report the preparation of magnesium (Mg) single-atom catalysts with high activity and stability by a sublimation-trapping strategy. The Mg net was sublimated as Mg vapor at 620 °C, subsequently transported through argon, and finally trapped on the defects of nitrogen-doped carbon derived from metal-organic framework ZIF-8, producing Mg1/NC. Because of the atomically dispersed Mg sites, the obtained Mg1/NC exhibits high catalytic activity and stability for Knoevenagel condensation of benzaldehyde with malononitrile, which is a typical base-catalyzed reaction. The Mg1/NC catalyst achieves a high efficiency with a turnover frequency of 49.6 h-1, which is much better than that of the traditional counterpart MgO/NC (7.7 h-1). In particular, the activity of Mg1/NC shows no decrease after five catalytic cycles, while that of MgO/NC declines due to the instability of basic sites.

Construction of Nickel Single Atoms by Using the Inherent Confined Space in Template-Occupied Mesoporous Silica.

Due to their maximum atomic use of metal sites, single-atom catalysts (SACs) exhibit excellent catalytic activity in a variety of reactions. Although many techniques have been reported for the production of SACs, the construction of single atoms through a convenient strategy is still challenging. Here, we provide a facile method to prepare nickel SACs by utilizing the inherent confined space between the template and silica walls in template-occupied mesoporous silica KIT-6 (TOK). After the introduction of nickel-containing precursors into the inherent confined space of the TOK by solid-phase grinding, Ni SACs can be produced promptly during calcination. Single Ni atoms create a covalent Ni-O-Si structure in the TOK, as indicated by density functional theory (DFT) calculations and experimental data. This synthetic approach is easy to scale up, and 10 g of sample can be effortlessly synthesized using ball milling. The resultant Ni SACs were applied to the oxygen evolution reaction and exhibited higher catalytic activity and stability than the comparative sample synthesized in the absence of confined space.

Endowing Covalent Organic Frameworks with Photoresponsive Active Sites for Controllable Propylene Adsorption.

Photoresponsive covalent organic frameworks (PCOFs) have emerged as attractive candidates for adsorption, but it is challenging to construct PCOF adsorbents due to structural order loss of covalent organic frameworks (COFs) after introducing photoresponsive motifs and/or tedious steps of postmodification. Here, a facile strategy is developed, by dispersing photoresponsive metal-organic polyhedra (PMOP) into COFs, to endow COFs with photoresponsive adsorption sites. As a proof-of-concept study, a COF with pore size of 4.5 nm and PMOP with suitable molecular size (4.0 and 3.1 nm for trans and cis configuration, respectively) are selected to meet the requirements of proper accommodation space, good guest dispersion, and free isomerization. The structure of COF is well preserved after introducing PMOPs. Interestingly, the obtained photoresponsive host-guest composite (PHGC) adsorbents exhibit photomodulated adsorption capacity on propylene (C3 H6 ) and the change in adsorption capacity can reach up to 43.3% and is stable during multiple cycles. Density functional theory calculations reveal that visible-light irradiation drives the azobenzene motifs in PHGCs to the trans configuration and the adsorption sites are fully open and interact with C3 H6 . UV-light irradiation makes the azobenzene motifs transform to the cis configuration, leading to the shield of the adsorption sites and the consequent release of C3 H6 .

Photo-Induced Construction and Recovery of Cu+ Sites in Metal-Organic Frameworks.

The adjustment of the valence state of metal ions is crucial for various applications because peculiar activity originates from metal ions with specific valence. Cu+ can interact with molecules possessing unsaturated bonds like CO via π-complexation, while Cu2+ doesn't have such ability. Meanwhile, Cu+ sites are easily oxidized to Cu2+ , leading to the loss of activity. Despite great efforts, the development of a facile method to construct and recover Cu+ sites remains a pronounced challenge. Here, for the first time a facile photo-induced strategy is reported to fabricate Cu+ sites in metal-organic frameworks (MOFs) and recover Cu+ after oxidation. The Cu2+ precursor was loaded on NH2 -MIL-125, a typical visible-light responsive Ti-based MOF. Visible light irradiation triggers the formation of Ti3+ from Ti4+ in framework, which reduces the supported Cu2+ in the absence of any additional reducing agent, thus simplifying the process for Cu+ generation significantly. Due to π-complexation interaction, the presence of Cu+ results in remarkably enhanced CO capture capacity (1.16 mmol g-1 ) compared to NH2 -MIL-125 (0.49 mmol g-1 ). More importantly, Cu+ can be recovered conveniently via re-irradiation when it is oxidized to Cu2+ , and the oxidation-recovery process is reversible.

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

Adsorptive separation plays a critical role in chemical, food, pharmaceutical, and environmental industries, as well as in many other industrial areas. Adsorbents are most important for adsorptive separation and undergo adsorption and desorption processes to accomplish the specific tasks of separation. In the process of adsorption, adsorbates diffuse into the pore spaces of adsorbents through pore openings, adsorb on active sites via physical or chemical interactions, and subsequently are regenerated by temperature or pressure swings during desorption. In the process of adsorption and desorption, however, the requirements for pore structures and surface properties of adsorbents are different. In general, adsorbents with small pore openings can realize selective adsorption and do not favor desorption; on the other hand, adsorbents with large pore openings are efficient in desorption but at the expense of adsorption selectivity. Remarkably, active sites possessing strong interactions with adsorbates contribute to high selectivity for adsorption, while desorption becomes difficult. The trade-off between adsorption and desorption presents an enormous challenge to develop high-efficiency adsorbents. Restricted by their fixed structures and surface properties, conventional adsorbents are unable to meet the demands of adsorption and desorption processes simultaneously.To confront the obstacles, the development of advanced adsorbents to meet the demand of adsorptive separation are urgent. A key strategy to address such issues lies in dynamically adjusting the pore structures or the surface properties of adsorbents with controllability according to the demands of adsorption/desorption. For instance, pursuant to the requirements of varying pore structures during adsorption/desorption, the pore openings of adsorbents can be customized through dynamic structural change of the decorated stimuli-sensitive motifs by suitable external intervention. In addition, the active sites within the adsorbents can be exposed to enhance the adsorption selectivity or sheltered to accelerate the desorption through stimuli-triggered adsorbent-adsorbate interactions. Hence, we proposed a concept of process-oriented smart adsorbents (POSAs) on the basis of the requirements of the adsorption/desorption processes. The design and development of such POSAs are based on three aspects, namely, pore openings, pore spaces, and adsorption sites of adsorbents.In this Account, we present the progress in the development of POSAs according to the demands of adsorption/desorption processes. A series of POSAs with incorporated stimuli-sensitive motifs were successfully achieved. The versatility of incorporated motifs allows them to tune the pore structures and surface properties of adsorbents dynamically and further to enhance the adsorption and desorption efficiency simultaneously. Based on the concept of POSAs, we hope that this Account could contribute to the development of high-efficiency adsorbents and ultimately promote their applications in practical industrial separation. Moreover, we present an outlook on future trends and challenges on the road toward the development and applications of POSAs.

Photomodulation on Active Sites of Adsorbents: Controllable Adsorption Processes with Improved Efficiency.

Adsorption is a widely applied technique in producing high-purity chemicals with advantages of low energy consumption, high selectivity, and mild operating conditions. However, traditional adsorbents have inflexible properties and suffer from the trade-off between selective adsorption and efficient desorption. Recently, the emerging photoresponsive adsorbents have provided new avenues for adsorption techniques. Active sites of photoresponsive adsorbents can be regulated through steric hindrance or tunable adsorbent-adsorbate interactions. Therefore, variation in adsorptive capacity is able to readily achieve through photomodulation, and the corresponding adsorption/desorption cycles are energy-saving. This concept mainly summarizes recent efforts on the fabrication and application of photoresponsive adsorbents with tunable active sites. Also, the future opportunities and critical challenges of photoregulation on adsorptive sites are presented.

Azobenzene-Functionalized UiO-66-NH2: Solid Base Catalysts with Photocontrollable Activity.

Heterogeneous solid base catalysts are highly expected due to their high activity and environmentally friendly nature in a variety of reactions. However, the catalytic activity of traditional solid base catalysts is controlled by external factors (such as temperature and pressure), and regulation of the activity by in situ changing their own properties has never been reported. Herein, we report a smart solid base catalyst by chemically anchoring the photoresponsive azobenzene derivative p-phenylazobenzoyl chloride (PAC) onto the metal-organic framework UiO-66-NH2 (UN) for the first time, which can regulate the catalytic activity through remote control of external light. The prepared catalysts have a regular crystal structure and photoresponsive properties. It is fascinating that the configuration of PAC can be isomerized easily during UV- and visible-light irradiation and resulted in regulation of the catalytic activity. In the Knoevenagel condensation of 1-naphthaldehyde and ethyl cyanoacetate to ethyl 2-cyano-3-(1-naphthalenyl)acrylate, the optimal catalyst shows up to 56.2% of change after trans/cis isomerization, while the change of the yield over UN is negligible. The regulated catalytic behavior can be assigned to the steric hindrance change of the catalysts under external light irradiation. This work may shed light on the design and construction of smart solid base catalysts with tailorable properties for various reactions.

Transformation of Type III to Type II Porous Liquids by Tuning Surface Rigidity of Rhodium(II)-Based Metal-Organic Polyhedra for CO2 Cycloaddition.

Porous liquids (PLs), a summation of porous hosts and bulky solvents bestowing permanent cavities, are the prominent emerging materials. Despite great efforts, exploration of porous hosts and bulky solvents is still needed to develop new PL systems. Metal-organic polyhedra (MOPs) with discrete molecular architectures can be considered as porous hosts; however, many of them are insoluble entities. Here we report the transformation of type III PL to type II PLs by tuning the surface rigidity of insoluble MOP, Rh24 L24 , in a bulky ionic liquid (IL). Functionalization of N-donor molecules on Rh-Rh axial sites ensue their solubilization in bulky IL which confer type II PLs. Experimental and theoretical studies reveal the bulkiness of IL as per the cage apertures, and the cause of their dissolution as well. The obtained PLs, capturing more CO2 than neat solvent, have depicted higher catalytic activity for CO2 cycloaddition compared to individual MOPs and IL.

Detachable Porous Organic Polymers Responsive to Light and Heat.

Stimuli-responsive porous materials have captured much attention due to the on-demand tunable properties. Most reported stimuli-responsive porous materials are based on molecule isomerism or host-guest interaction, and it is highly desired to develop new types based on different responsive mechanism. Herein, inspired by natural cells which have the ability to fuse and divide induced by external stimulation, we report a new type of stimuli-responsive porous material based on detachment mechanism. A detachable porous organic polymer, namely DT-POP-1, is fabricated from the polymerization of anthracene-containing monomer (AnMon) when irradiated by 365 nm UV light. DT-POP-1 can detach into the monomer AnMon when irradiated with 275 nm UV light or heat. Such polymerization/detachment is reversible. The detachment results in a big difference in porosity and adsorption capacity, making the present detachable porous polymer highly promising in adsorptive separation and drug delivery.

Enhanced CO Adsorption by Modulating the Electron Density Distribution of Graphite-Copper Porphyrin Sorbents with Light.

The photo-responsive adsorption has emerged as a vibrant area, but its current methodology is limited by the well-defined photochromic units and their molecular deformation driven by photo-stimuli. Herein, a methodology of nondeforming photo-responsiveness is successfully exploited. With the exploiting agent of Cu-TCPP framework assembled on the graphite and strongly interacted with it, the sorbent generates two kinds of adsorption sites, over which the electron density distribution of the graphite layer can be modulated at the c-axis direction, which can further evolve due to photo-stimulated excited states. The excited states are stable enough to meet the timescale of microscopic adsorption equilibrium. Independent of the ultra-low specific surface area of the sorbent (20 m2 g-1 ), the CO adsorption capability can be improved from 0.50 mmol g-1 at the ground state to 1.24 mmol g-1 (0 °C, 1 bar) with the visible light radiation, rather than the photothermal desorption.

Solitary Medium of a Multifunctional Ionic Liquid for Crystallizing Hierarchically Porous Metal-Organic Frameworks.

Hierarchically porous metal-organic frameworks (HP-MOFs), dominating both the micro- and mesoporous regimes, show high potentials in various applications especially those involving bulky biomolecules. The templating method has been proven to be effective in the fabrication of HP-MOFs; however, complicated synthetic systems containing solvents, templates, and additives are frequently employed. Here we report the first example of designing a poly(ethylene glycol)-based alkylammonium and bromide multifunctional ionic liquid (IL) as a solitary medium to construct HP-MOFs, avoiding the involvement of any additional media. Besides the ready solubilization of MOF precursors in the multifunctional IL due to a poly(ethylene glycol) chain as the solubilizer, the ionic moiety facilitates electrostatic interaction to create a templating effect. Hence, UiO-66 with hierarchical porosity has been successfully fabricated, and such a methodology can also be applied to the construction of other HP-MOFs. The resultant HP-UiO-66 is efficient in the encapsulation of bulky biomolecule cytochrome c, and the adsorption capacity is obviously superior to that of the microporous counterpart.

Porous Liquids Responsive to Light.

A porous liquid is a unique liquid medium that combines the cavity of porous solids with the fluidity of liquids. This special characteristic offers potential in various applications. Here we report a type II photoresponsive porous ionic liquid (PPIL) from dissolving a photoresponsive metal-organic polyhedron (PMOP, constructed from dicopper and azobenzene-containing carboxylate) in a polyethylene-glycol-functionalized bulky ionic liquid (IL). Owing to favorable ion interactions, bulky IL molecules encircle outside PMOP, and the inter cavities are maintained. The azobenzene moieties can be isomerized freely in the PPILs to expose and shelter active sites upon visible and UV light irradiation. Hence, the adsorption capacity of PPILs is controllable by light irradiation, and the change in CO2 uptake is up to 30 % compared to neat IL. This study may inspire the development of new adsorption process regulated by light instead of pressure and temperature swing adsorption.

Reversible Light-Controlled CO Adsorption via Tuning π-Complexation of Cu+ Sites in Azobenzene-Decorated Metal-Organic Frameworks.

Light-responsive adsorbents capture significant attention due to their tailorable performance upon light irradiation. The modulation of such adsorbents is mainly based on weak (physical) interactions caused by steric hindrance while tuning strong interaction with target adsorbates is scarce. Here we report smart π-complexation adsorbents, which can adjust the π-complexation of active sites via light irradiation. A typical metal-organic framework, MIL-101-NH2 , was decorated with azobenzene motifs, and Cu+ as π-complexation active sites were introduced subsequently. The reversible light-induced isomerization of azobenzene regulates the surface electrostatic potentials around Cu+ from -0.038 to 0.008 eV, causing shielding and exposure effects. The alteration of CO uptake is achieved up to 54 % via changing light, while that on MIL-101-NH2 is negligible. This study provides a clue for designing target-specific smart materials to meet the practical stimuli-responsive adsorption demands.

Catalytically Stable Potassium Single-Atom Solid Superbases.

Solid superbases can catalyze diverse reactions under mild conditions, while they suffer from aggregation of basic sites and poor stability during recycling. Here we report a new generation of solid superbases derived from K single atoms (SAs) prepared by a tandem redox strategy. The initial redox reaction takes place between base precursor KNO3 and graphene support, producing K2 O at 400 °C. Further increasing the temperature to 800 °C, the graphene reduces K2 O to K anchored by its vacancies, leading to the generation of K SAs (denoted as K1 /G). The source of basicity in the K1 /G is K SAs, and neighboring single atoms (NSAs) possess superbasicity, which is different from conventional basicity originated from oxygen and nitrogen atoms. Due to the superbasicity as well as high dispersion and anchoring of basic sites, the K1 /G shows excellent catalytic activity and stability in transesterification reaction, which is much superior to the reported catalysts.

Hybridization with Ti3C2Tx MXene: An Effective Approach to Boost the Hydrothermal Stability and Catalytic Performance of Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) have attracted increasing research enthusiasm owing to their tunable functionality, diverse structure characteristics, and large surface area. However, poor hydrothermal stability restricts the utilization of some MOFs in practical applications. Our work aims at improving the hydrothermal stability of a representative MOF, namely, HKUST-1, by incorporating a two-dimensional material Ti3C2Tx MXene for the first time. A new type of hybrid material is synthesized through the hybridization of HKUST-1 and Ti3C2Tx, and the obtained hybrids show improved hydrothermal stability as well as catalytic performance. The porosity of hybrids is enhanced when incorporating an appropriate amount of Ti3C2Tx, and the surface area can reach 1380 m2·g-1, while the pristine HKUST-1 is 1210 m2·g-1. After the hydrothermal treatment (hot water vapor, 70 °C), the structure of hybrid materials maintains well, while the framework of HKUST-1 is severely destroyed. When catalyzing the ring-opening reaction of styrene oxide, the conversion reaches 76.7% only for 20 min, which is much higher than that of pure HKUST-1 (23.1% for 20 min). More importantly, the catalytic activity could recover without loss even after six cycles. Our hybrid materials are promising in practical catalytic applications due to their excellent hydrothermal stability, catalytic activity, and reusability.

Controllable Microporous Framework Isomerism within Continuous Mesoporous Channels: Hierarchically Porous Structure for Capture of Bulky Molecules.

To date, some attempts have been made to synthesize hierarchically porous metal-organic frameworks (HPMOFs), and in most cases, mesopores are formed in microporous frameworks. However, mass transfer and diffusion are still limited in such HPMOFs since micropores connect mesopores and mesopores are noncontinuous. Here, we fabricate a new hierarchical structure through the formation of microporous MOFs within continuous mesoporous channels. Confined space in the as-prepared mesoporous silica-containing template was used to prepare well-dispersed metal precursor of ZnO. The strategy of ligand vapor-induced crystallization was then designed to construct MOFs inside mesoporous channels, in which vapored ligand at elevated temperature diffuses and reacts with metal precursor. Our results indicate that framework isomerism is controllable by adjusting the crystallization conditions. In comparison to their microporous and mesoporous counterparts, the hierarchically porous materials show obviously enhanced adsorption performance on a series of bulky molecules including dye, enzyme, and metal-organic polyhedron.

Fabrication of Microporous Metal-Organic Frameworks in Uninterrupted Mesoporous Tunnels: Hierarchical Structure for Efficient Trypsin Immobilization and Stabilization.

Hierarchically porous metal-organic frameworks (HP-MOFs) are promising in various applications. Most reported HP-MOFs are prepared based on the generation of mesopores in microporous frameworks, and the formed mesopores are connected by microporous channels, limiting the accessibility of mesopores for bulky molecules. A hierarchical structure is formed by constructing microporous MOFs in uninterrupted mesoporous tunnels. Using the confined space in as-prepared mesoporous silica, highly dispersed metal precursors for MOFs are coated on the internal surface of mesoporous tunnels. Ligand vapor-induced crystallization is employed to enable quantitative formation of MOFs in situ, in which sublimated ligands diffuse into mesoporous tunnels and react with metal precursors. The obtained hierarchically porous composites exhibit record-high adsorption capacity for the bulky molecule trypsin. The thermal and storage stability of trypsin is improved upon immobilization on the composites.

Maximizing Photoresponsive Efficiency by Isolating Metal-Organic Polyhedra into Confined Nanoscaled Spaces.

Photoresponsive metal-organic polyhedra (PMOPs) have attracted expanding interests due to their modular nature with tunable functionality and variable responsive behaviors tailored conveniently by external-stimulus. However, their photoresponsive efficiency is often compromised after activation because of desorption-triggered aggregation into bulk PMOPs, which limits their utility in stimuli-responsive applications. Here, we report a case system that can overcome the aggregation problem and achieve maximized photoresponsive efficiency by polyhedral isolation in the nanoscaled spaces of mesoporous silica (MS). Through confinement, amount-controllable PMOPs are well dispersed in the nanoscaled spaces of MS, avoiding aggregation that commonly takes places in bulk PMOPs. Furthermore, reversible trans/ cis isomerization of photoresponsive groups can be realized freely during ultraviolet/visible light irradiation, maximizing control over photoresponsive guest adsorption behaviors. Remarkably, after trans/ cis isomerization, the confined PMOP-1 shows 48.2% of change in adsorption amount for propene with small molecular size and 43.9% for brilliant blue G (BBG) with large molecular size, which is significantly higher than that over bulk PMOP-1 with 11.2% for propene and 7.8% for BBG, respectively. Therefore, our work paves a way for the design and construction of multifunctional composite materials toward efficient stimuli-responsive needs.

Fabrication of Cu(I)-Functionalized MIL-101(Cr) for Adsorptive Desulfurization: Low-Temperature Controllable Conversion of Cu(II) via Vapor-Induced Reduction.

Because of their nontoxicity, economic applicability, and excellent performance on adsorptive desulfurization, the fabrication of Cu(I) sites onto porous supports has drawn much attention. However, high temperatures (usually ≥700 °C) are required for the formation of Cu(I) sites from Cu(II) species through the traditional autoreduction method, which is unworkable for thermolabile metal-organic frameworks (MOFs). Here, we report a strategy named vapor-induced reduction (VIR) to convert Cu(II) species to Cu(I) in MIL-101(Cr), in which ethanol is used as an environmentally benign reductant. The entire formation of Cu(I) from Cu(II) with more than 96% selectivity is allowed, at a relatively low temperature of 200 °C, and well-maintains the structure of the MOF. Moreover, the generated Cu(I) sites exhibit good performances in adsorption desulfurization with regard to both activity and reusability.

Significant Decrease in Activation Temperature for the Generation of Strong Basicity: A Strategy of Endowing Supports with Reducibility.

Mesoporous solid strong bases are quite attractive due to their good catalytic performance for applications as environmentally friendly catalysts in various reactions. However, pretty harsh conditions are usually compulsory for the fabrication of strong basicity by using traditional thermal activation (e.g., 700 °C for the activation of base precursor KNO3 supported on mesoporous Al2O3). This is energy intensive and harmful to the mesoporous structure. In this study, we report a strategy of endowing supports with reducibility (ESWR) by doping low-valence Cr3+ into mesoporous Al2O3, so that the activation temperature for basicity generation is decreased significantly. Fascinatingly, KNO3 on mesoporous Al2O3 can be motivated to basic sites completely at the temperature of 400 °C via the ESWR strategy, which is much lower than the conventional thermal activation (700 °C). We have demonstrated that the redox reciprocity between KNO3 and Cr3+ is responsible for the low-temperature conversion, and Cr6+ is formed quantitatively as the oxidation product. The obtained solid bases possessing ordered mesostructure and strong basicity provide promising candidates for base-catalyzed synthesis of dimethyl carbonate via transesterification. The catalytic activity is obviously higher than a typical solid base like MgO as well as a series of reported basic catalysts containing alkali metal and alkaline-earth metal oxides.