Positively Photo-Responsive Adsorption Over Binary Copper Porphyrin Framework and Graphene Film Sorbents.

Photo-responsive adsorption has emerged as a vibrant area because it provides a promising route to reduce the energy consumption of the traditional adsorption separation. However, the current methodology to fabricate photo-responsive sorbents is still subject to the photo-deforming molecular units. In this study, a new initiative of photo-dissociated electron-hole pairs is proposed to generate amazing adsorption activity, and prove its feasibility. Employing CuPP [PP = 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin] framework nanosheets compounded with graphene, binary film (BF) sorbents are successfully fabricated. The paradigmatic BF nanostructure brings about efficiently photo-excited electron-hole pairs with durable enough lifetime to meet the needs of microscopic adsorption equilibrium, which ultimately alters the electron density distribution of adsorption surface, and thus markedly modulates the adsorption activity. Therefore, an amazing photo-enhanced adsorption capability for the index gas CO can be gotten. Once exposed to the visible-light at 420 nm, the CO adsorption capacity (0 °C, 1 bar) is risen from 0.23 mmol g-1 in the darkness to 1.66 mmol g-1, changed by + 622%. This is essentially different from majority of current photo-responsive sorbents based on photo-deforming molecular units, of which adsorption capability is only decreased with photo-induction, and the maximum rate of change reported is just -54%.

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.

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.

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.

Metal-Organic Frameworks with Target-Specific Active Sites Switched by Photoresponsive Motifs: Efficient Adsorbents for Tailorable CO2 Capture.

Photoresponsive metal-organic frameworks (PMOFs) are of interest for tailorable CO2 adsorption. However, modulation of CO2 adsorption on PMOFs is based on steric hindrance or structural change owing to weak interactions between CO2 and active sites. It is challenging to fabricate PMOFs with strong but tailorable sites for CO2 adsorption. Now, the construction of PMOFs with target-specific (strong) active sites is achieved by introducing tetraethylenepentamine into azobenzene-functionalized MOFs for tailorable CO2 adsorption. Amines are specific active sites for CO2 , contributing to capture CO2 selectively. Cis/trans isomerization of azobenzene motifs trigged by UV/Vis light adjusts the electrostatic potential of amines significantly, leading to exposure/shelter of amines and modulation of CO2 adsorption on strong active sites. This system enables us to design adsorption processes for CO2 capture from mixtures, which is impossible to realize by traditional PMOFs.

Generation of Hierarchical Porosity in Metal-Organic Frameworks by the Modulation of Cation Valence.

Hierarchically porous metal-organic frameworks (HP-MOFs) have attracted great attention owing to their advantages over microporous MOFs in some applications. Despite many attempts, the development of a facile approach to generate HP-MOFs remains a challenge. Herein we develop a new strategy, namely the modulation of cation valence, to create hierarchical porosity in MOFs. Some of the CuII metal nodes in MOFs can be transformed into CuI via reducing vapor treatment (RVT), which partially changes the coordination mode and thus breaks coordination bonds, resulting in the formation of HP-MOF based on the original microporous MOF. Both the experimental results and the first-principles calculation show that it is easy to tailor the amount of CuI and subsequent hierarchical porosity by tuning the RVT duration. It is found that the resultant HP-MOFs perform much better in the capture of aromatic sulfides than the original microporous MOF.

Theoretical Study on Hydrogenolytic Cleavage of Intermonomer Linkages in Lignin.

Hydrogenolysis is an important approach for depolymerization of lignin, which provides attractive new sustainable platforms of fuels, chemicals, and materials. The theory of lignin hydrogenolysis is, however, still unsound, which limits the development of this approach and causes inconsistencies among experimental studies. In this paper, density functional theory is employed to investigate the initial hydrogenolytic cleavages of recognized five different types of interaromatic unit linkages of lignin, assuming the presence of hydrogen free radicals. The relative free energies of reactant complexes, reaction free energy changes, and rate constants for candidate reactions are calculated comprehensively at 298-538 K. On the basis of the results of calculation and a rapid equilibrium hypothesis, the major reaction channel is decided for each linkage, and its kinetics is assessed. It is concluded that the hydrogenolysis occurs at ß-O-4 ether, diphenyl ether 4-O-5', and ß-1' diphenylmethane linkages instantaneously if these are accessible to hydrogen free radicals, while ß-5 phenylcoumaran and ß-ß' pinoresinol linkages are virtually inert to hydrogenolysis.

Excitation generated preferential binding sites for ethane on porous carbon-copper porphyrin sorbents: ethane/ethylene adsorptive separation improved by light.

Energy-efficient separation of C2H6/C2H4 is a great challenge, for which adsorptive separation is very promising. C2H6-selective adsorption has big implications, while the design of C2H6-sorbents with ideal adsorption capability, particularly with the C2H6/C2H4-selectivity exceeded 2.0, is still challenging. Instead of the current strategies such as chemical modification or pore space modulation, we propose a new methodology for the design of C2H6-sorbents. With a Cu-TCPP [TCPP = 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin] framework dispersed onto a microporous carbon and a hierarchical-pore carbon, two composite sorbents are fabricated. The composite sorbents exhibit enhanced C2H6-selective adsorption capabilities with visible light, particularly the composite sorbent based on the hierarchical-pore carbon, whose C2H6 and C2H4 adsorption capacities (0 °C, 1 bar) are targetedly increased by 27% and only 1.8% with visible light, and therefore, an C2H6-selectivity (C2H6/C2H4 = 10/90, v/v) of 4.8 can be realized. With visible light, the adsorption force of the C2H6 molecule can be asymmetrically enhanced by the excitation enriched electron density over the adsorption sites formed via the close interaction between the Cu-TCPP and the carbon layer, whereas that of the C2H4 molecule is symmetrically altered and the forces cancelled each other out. This strategy may open up a new route for energy-efficient adsorptive separation of C2H6/C2H4 with light.

Coumarin-functionalized metal-organic frameworks: adsorbents with photo-responsive active sites for adsorptive desulfurization.

Photo-responsive metal-organic frameworks (PMOFs) have great potential in on-demand controllable adsorption processes under distinct light conditions, which is challenging to realize by conventional adsorbents with static frameworks and properties. Here, we report a new type of adsorbent with photo-responsive active sites for adsorptive desulfurization. Coumarin monomers were incorporated in MIL-101(Cr) to construct the photo-responsive adsorbents by coordinating with unsaturated chromium sites. Trigged by UV light (>310 nm), coumarin monomers undergo dimerization to a cyclobutane structure, which forms a conjugation effect with π electrons on the aromatic ring of the sulfur compound, leading to remarkably increased uptake of 4,6-dimethyldibenzothiophene (4,6-DMDBT). Coumarin dimers can transform back to monomers under irradiation of 254 nm UV light, corresponding to decreased adsorption of 4,6-DMDBT. The change in adsorption amount between PMOF and the coumarin monomers and dimers is up to 62.8%. The present study might open up an avenue for the fabrication of adsorbents with on-demand active sites for various applications.

Transition Metal Single Atoms Constructed by Using Inherent Confined Space.

Single-atom catalysts (SACs) show expressively enhanced activity toward diverse reactions due to maximized atomic utilization of metal sites, while their facile, universal, and massive preparation remains a pronounced challenge. Here we report a facile strategy for the preparation of SACs by use of the inherent confined space between the template and silica walls in template-occupied mesoporous silica SBA-15 (TOS). Different transition metal precursors can be introduced into the confined space readily by grinding, and during succeeding calcination single atoms are constructed in the form of M-O-Si (M = Cu, Co, Ni, and Zn). In addition to the generality, the present strategy is easy to scale up and can allow the synthesis of 10 g of SACs in one pot through ball milling. The Cu SAC has been applied for CO2 cycloaddition of epichlorohydrin, and the activity is obviously higher than the counterpart prepared without confined space and various reported Cu-containing catalysts.

Photo-Responsive Carbon Capture over Metalloporphyrin-C60 Metal-Organic Frameworks via Charge-Transfer.

Great efforts have been devoted to the study of photo-responsive adsorption, but its current methodology largely depends on the well-defined photochromic units and their photo-driven molecular deformation. Here, a methodology to fabricate nondeforming photo-responsive sorbents is successfully exploited. With C60-fullerene doping in metalloporphyrin metal-organic frameworks (PCN-M, M = Fe, Co, or Ni) and intensively interacting with the metalloporphyrin sites, effective charge-transfer can be achieved over the metalloporphyrin-C60 architectures once excited by the light at 350 to 780 nm. The electron density distribution and the resultant adsorption activity are thus changed by excited states, which are also stable enough to meet the timescale of microscopic adsorption equilibrium. The charge-transfer over Co(II)-porphyrin-C60 is proved to be more efficient than the Fe(II)- and Ni(II)-porphyrin-C60 sites, as well as than all the metalloporphyrin sites, so the CO2 adsorption capacity (CAC; at 0 °C and 1 bar) over the C60-doped PCN-Co can be largely improved from 2.05 mmol g-1 in the darkness to 2.69 mmol g-1 with light, increased by 31%, in contrast to photo-irresponsive CAC over all C60-undoped PCN-M sorbents and only the photo-loss CAC over C60.

Calcium oxide-modified mesoporous silica loaded onto ferriferrous oxide core: Magnetically responsive mesoporous solid strong base.

The design of new type of solid strong base with ideal activity, stability, and reusability is strongly urged by the growing demand of green chemistry and sustainable development. In this study, a new type of mesoporous solid strong base, denoted as CaO/mSiO2/Fe3O4, is successfully fabricated by successively coating SiO2 onto Fe3O4 magnetic nanoparticles and loading CaO into the mesoporous SiO2. Compared with a series of other typical solid bases, the CaO/mSiO2/Fe3O4 exhibits higher activity towards the synthesis of dimethyl carbonate by the transesterification of ethylene carbonate and methanol. The activity of the CaO/mSiO2/Fe3O4 is not observed to decrease obviously even after sextic catalyst recirculation, and in particular, the recovery of the catalyst without quality loss is very convenient due to the good magnetic responsiveness of the Fe3O4 cores.

Controlled Construction of Cu(I) Sites within Confined Spaces via Host-Guest Redox: Highly Efficient Adsorbents for Selective CO Adsorption.

Here, we designed a double-solvent/host-guest redox combined strategy to construct Cu+ sites in metal-organic frameworks (MOFs) for the first time. As a proof of concept, a representative MOF MIL-100(Fe) with tunable valence states of cations was employed as the host. The combined strategy realizes selective introduction of Cu2+ precursors to the interior pores of MIL-100(Fe), remarkably minimizing the aggregation of Cu2+ and subsequently formed Cu+ species. Owing to the proper reducibility of in situ formed Fe2+ in the frameworks, controlled conversation of Cu2+ to Cu+ with ∼100% yield is achieved in the absence of any additional reducing agents. These characteristics make the obtained materials Cu+-modified MIL-100(Fe) highly active in selective CO adsorption. The CO adsorption capacity is up to 3.75 mmol·g-1 at 298 K and 1 bar, which is superior to all other Cu+-containing adsorbents reported so far such as CuCl/activated carbon (2.5 mmol·g-1), CuCl/γ-Al2O3 (1.0 mmol·g-1), and CuCl/SBA-15 (0.50 mmol·g-1). The same adsorbent also exhibits quite high selectivity of CO over N2, and the ideal adsorption solution theory selectivity reaches 424. The outstanding CO adsorption performance make the present adsorbents great potential in separation of CO from various mixtures.