Tuning Hydrogen Sorption Properties of Metal–Organic Frameworks by Postsynthetic Covalent Modification

Postsynthetic modification is presented as a means to tune the hydrogen adsorption properties of a series of metal–organic frameworks (MOFs). IRMOF‐3 (isoreticular metal–organic framework), UMCM‐1‐NH₂ (University of Michigan crystalline material), and DMOF‐1‐NH₂ (DABCO metal–organic framework) have been covalently modified with a series of anhydrides or isocyanates and the hydrogen sorption properties have been studied. Both the storage capacities and isosteric heats of adsorption clearly show that covalent postsynthetic modification can significantly enhance the sorption affinity of MOFs with hydrogen and in some cases increase both gravimetric and volumetric uptake of the gas as much as 40 %. The significance of the present study is illustrated by: 1) the nature of the substituents introduced by postsynthetic modification result in different effects on the binding of hydrogen; 2) the covalent postsynthetic modification approach allows for systematic modulation of hydrogen sorption properties; and 3) the ease of postsynthetic modification of MOFs allows a direct evaluation of the interplay between MOF structure, hydrogen uptake, and heat of adsorption. The findings presented herein show that postsynthetic modification is a powerful method to manipulate and better understand the gas sorption properties of MOFs.     

Ionic absorption of polypropylene functionalized by surface grafting and reactions

Plasma-induced surface graft copolymerization of acrylic acid on polypropylene fibers and the subsequent reactions of the grafted carboxylic groups are reported. The extents of grafting was controlled by the plasma conditions. Reactions of the carboxylic acid with selected amines resulted in ion-exchanging and chelating functionalities. In general, ion adsorption is enhanced by higher levels of grafting and by raising temperature during adsorption. The adsorption level and preferences among ions of these functionalized fibers depend on the structure of the functional groups, i.e., the structure of the spacer and terminal groups. The carboxylic acid groups of the PP-g-AA fibers which behave like weakly acidic ion-exchangers are attributed to the low metal ion adsorption and the lack of ion preference. The F1 fibers with flexible  CH₂CH₂ spacer and small terminal  OH in the functional group exhibits highest ion adsorption among all functionalized fibers studied here. With benzene spacers, metal adsorption can be enchanced by the electron-donating nature of the terminal group. With the same ester end group in the functional structure, F3 fibers which contain benzene ring spacers show higher ion adsorption than F4 and F5 which have CH₂ and NH spacers, respectively. The ion preference and adsorption ability of the functionalized fibers, i.e., equilibrium binding constants (K_b) and saturation constants (K_s) derived from adsorption isotherms, also depend on the functional group structures. K_b increases with increasing grafting yield, increasing the electron donor atom in either terminal bonds or spacer, and reducing the steric hindrance of spacer. The K_s values are affected by the accessibility of functional groups, the size of spacer, and the terminal group structure. © 1997 John Wiley & Sons, Inc.     

Sterically‐Limited Self‐Assembly of Pt4 Macrocycles into Discrete Non‐covalent Nanotubes: Porous Supramolecular Tetramers and Hexamers

We report a template‐free strategy based on steric repulsion for the isolation of discrete columnar aggregates of macrocycles. Specifically, introduction of sterically‐demanding trityl‐derived substituents at the periphery of Pt₄ Schiff base macrocycles limits the otherwise infinite one‐dimensional columnar aggregation to discrete tetrameric and hexameric assemblies. Single crystal X‐ray diffraction studies of these compounds reveal discrete nanotubes of finite length that pack inefficiently resulting in three‐dimensional networks of interconnected void space. The discrete assemblies were studied by N₂ adsorption and show enhanced surface area when stacked. In the absence of bulky substituents the macrocycles are nonporous. This strategy for engineering discrete supramolecular macrocyclic aggregates may be generalized to other columnar assembling systems.     

铁脂质体/水分配系数测定及影响因素考察

基于PAF-301分子模型通过Li 掺杂或B取代等模式设计了几种新型多孔芳香骨架(PAFs)材料, 采用量子力学和分子力学方法对新材料的储氢性能进行研究. 由量子力学计算得到了不同分子片段与H₂之间的结合能, 并结合DDEC方法计算了各分子片段的原子电荷分布. 利用巨正则蒙特卡洛(GCMC)模拟方法计算了77和298 K下H₂在不同PAFs材料中的吸附平衡性质. 结果表明, H₂直接与苯环的结合能较低, 但掺杂Li 原子能够提高H₂与六元环的结合能, 同时Li 原子体现出较高的正电性质, B原子取代苯环中的两个C原子后, 使得原有C原子电负性增强; 77 K下PAF-301Li 具有最高的储氢性能, 而PAF-C₄B₂H₄-Li₂-Si 和PAF-C₄B₂H₄-Li₂-Ge体现出较好的常温储氢性能, 各种材料的常温储氢性能远低于其低温储氢性能. 通过77 K下H₂在PAFs材料中的等位能面分布和吸附平衡质心密度分布对H₂在PAFs 材料中的优先吸附位置进行分析, 发现在PAF-301 和PAF-301Li 骨架中, 由于中心能量较低的等位能区域范围较宽, H₂在其中存在四个明显的吸附高密度分布区域, 而其它三种PAFs晶胞中心能量较低的等位能区域范围较窄, 使得H₂在其中只存在两个明显的吸附高密度分布区域.     

Oxygen adsorption characteristics on hybrid carbon and boron‐nitride nanotubes

In this work, first‐principles density functional theory (DFT) is used to predict oxygen adsorption on two types of hybrid carbon and boron‐nitride nanotubes (CBNNTs), zigzag (8,0), and armchair (6,6). Although the chemisorption of O₂ on CBNNT(6,6) is calculated to be a thermodynamically unfavorable process, the binding of O₂ on CBNNT(8,0) is found to be an exothermic process and can form both chemisorbed and physisorbed complexes. The CBNNT(8,0) has very different O₂ adsorption properties compared with pristine carbon nanotubes (CNTs) and boron‐nitride nanotube (BNNTs). For example, O₂ chemisorption is significantly enhanced on CBNNTs, and O₂ physisorption complexes also show stronger binding, as compared to pristine CNTs or BNNTs. Furthermore, it is found that the O₂ adsorption is able to increase the conductivity of CBNNTs. Overall, these properties suggest that the CBNNT hybrid nanotubes may be useful as a gas sensor or as a catalyst for the oxygen reduction reaction. © 2014 Wiley Periodicals, Inc.     

A Hydrophobic Metal–Organic Framework Based on Cubane‐Type [Co4(μ3‐F)3(μ3‐SO4)]3+ Clusters for Gas Storage and Adsorption Selectivity of Benzene over Cyclohexane

Hydrophobic metal‐organic frameworks (MOFs) not only have high water stability, but also exhibit high adsorption capacity towards organic molecules, in particular hydrocarbons. Herein we report a rare metal fluoride organic framework MFOF‐1 with high hydrophobicity, which is constructed from unprecedented fluoride‐ and sulfate‐bridged cubane‐type tetranuclear cobalt clusters. MFOF‐1 consists of three types of polyhedral cages with face‐sharing configurations, and possesses a novel (3,9)‐connected 3D+3D→3D self‐interpenetrating array or the rare pyr topology. MFOF‐1 shows high thermal stability and high stability in water and even acid/base aqueous solutions, and exhibits rather high H₂ and CO₂ storage capacities at ambient pressure. Remarkably, MFOF‐1 shows little adsorption of water but considerably high uptakes of methanol, n‐hexane, cyclohexane, and benzene, and exhibits a certain degree of adsorption selectivity of benzene over cyclohexane.     

Benchmarking of DFT functionals for the kinetics and mechanisms of atmospheric addition reactions of OH radicals with phenyl and substituted phenyl‐based organic pollutants

^•OH addition reactions play a pivotal role in the atmospheric transformation of a number of phenyl and substituted phenyl‐based persistent and toxic organic pollutants. Here, we screened appropriate DFT functionals to predict reaction mechanisms and rate constants (k_OH) of the ^•OH additions by taking benzene and substituted benzenes (C₆H₅F, C₆H₅Cl, C₆H₅Br, C₆H₅CH₃, C₆H₅OH) as model compounds. By comparing the k_OH values calculated with DFT methods to experimental values, we found that the ωB97 functional is the best among the 18 functionals considered (using the basis sets 6‐31 + G(d,p) for optimizations and 6‐311++G(3df,2pd) for single point energy calculations) in the temperature range of 230‐330 K. In addition, we found that some other functionals performed well in specific conditions, e.g., BMKD3 is good for benzene, halogenated benzenes and C₆H₅CH₃, and CAM‐B3LYP is good for the reaction of C₆H₅OH at room temperature. Based on the diversity of the electronic structures of the selected model compounds and the frequent occurrence of certain substituents ( CH₃,  OH,  F,  Cl, and  Br) in the target compounds, the functionals recommended here can be used for future study of the reaction mechanisms and k_OH values for ^•OH addition to phenyl and substituted phenyl‐based persistent and toxic organic pollutants.     

Benzene and toluene adsorption at low concentration on activated carbon fibres

The present study analyses the preparation of activated carbon fibres (ACFs) by the so-called “physical” activation method with steam or carbon dioxide and their application for benzene and toluene adsorption at low concentration (200 ppmv). ACFs have been scarcely studied for the adsorption of these pollutants at low concentration in gaseous phase, despite their interesting features regarding adsorption kinetics, bed pressure drop, possibility of conformation and others. Our results have shown that the preparation method used is suitable to produce ACFs with high adsorption capacities for benzene and toluene at the low concentration used. The fibre morphology of the ACFs does not enhance their performance, which results to be similar to other non-fibrous activated carbons such as granular, pellets and powders. Such good performance of the ACFs, leading to benzene and toluene adsorption capacities as large as 31 g benzene/100 g ACF or 53 g toluene/100 g ACF, can be explained due to their large volume of narrow micropores (<0.7 nm) developed upon activation and their low content in surface oxygen groups. Our results have also shown very good agreement between the adsorption results derived from dynamic adsorption experiments and from adsorption isotherms. As the relative pressure of the organic compound increases the corresponding fraction of narrow micropore volumes filled by benzene and toluene increases. For a given low and comparable relative pressure, toluene always occupies a larger fraction of narrow micropores than benzene.     

C?H Activation of Benzene by a Photoactivated NiII(azide): Formation of a Transient Nickel Nitrido Complex

Photochemical activation of nickel‐azido complex 2 [Ni(N₃)(PNP)] (PN^HP=2,2′‐di(isopropylphosphino)‐4,4′‐ditolylamine) in neat benzene produces diamagnetic complex 3 [Ni(Ph)(PN^PN^H)], which is crystallographically characterized. DFT calculations support photoinitiated N₂‐loss of the azido complex to generate a rare, transient Ni^IV nitrido species, which bears significant nitridyl radical character. Subsequent trapping of this nitrido through insertion into the Ni P bond generates a coordinatively unsaturated Ni^II imidophosphorane PN donor. This species shows unprecedented reactivity toward 1,2‐addition of a C H bond of benzene to form 3 . The structurally characterized chlorido complex 4 [Ni(Cl)(PN^PN^H)] is generated by reaction of 3 with HCl or by direct photolysis of 2 in chlorobenzene. This is the first report of aromatic C H bond activation by a trapped transient nitrido species of a late transition metal.     

Synthesis,Structure, and Reactivity of Hydridoiridium Complexes Bearing a Pincer‐Type PSiP Ligand

A series of iridium tetrahydride complexes [Ir(H)₄(PSiP‐R)] bearing a tridentate pincer‐type bis(phosphino)silyl ligand ([{2‐(R₂P)C₆H₄}₂MeSi]⁻, PSiP‐R, R=Cy, iPr, or tBu) were synthesized by the reduction of [IrCl(H)(PSiP‐R)] with Me₄N ⋅ BH₄ under argon. The same reaction under a nitrogen atmosphere afforded a rare example of thermally stable iridium(III)–dinitrogen complexes, [Ir(H)₂(N₂)(PSiP‐R)]. Two isomeric dinitrogen complexes were produced, in which the PSiP ligand coordinated to the iridium center in meridional and facial orientations, respectively. Attempted substitution of the dinitrogen ligand in [Ir(H)₂(N₂)(PSiP‐Cy)] with PMe₃ required heating at 150 °C to give the expected [Ir(H)₂(PMe₃)(PSiP‐Cy)] and a trigonal bipyramidal iridium(I)–dinitrogen complex, [Ir(N₂)(PMe₃)(PSiP‐Cy)]. The reaction of [Ir(H)₄(PSiP‐Cy)] with three equivalents of 2‐norbornene (nbe) in benzene afforded [Ir^I(nbe)(PSiP‐Cy)] in a high yield, while a similar reaction of [Ir(H)₄(PSiP‐R)] with an excess of 3,3‐dimethylbutene (tbe) in benzene gave the C H bond activation product, [Ir^III(H)(Ph)(PSiP‐R)], in high yield. The oxidative addition of benzene is reversible; heating [Ir^III(H)(Ph)(PSiP‐Cy)] in the presence of PPh₃ in benzene resulted in reductive elimination of benzene, coordination of PPh₃, and activation of the C H bond of one aromatic ring in PPh₃. [Ir^III(H)(Ph)(PSiP‐R)] catalyzed a direct borylation reaction of the benzene C H bond with bis(pinacolato)diboron. Molecular structures of most of the new complexes in this study were determined by a single‐crystal X‐ray analysis.     

Synthesis,structural analysis,and self-assembly of phenylene ethynylene oligomers and their ?F, ?CF3, and ?CH3 substituted derivatives

Oligomers consisting of aromatic building blocks separated by alkynyl units were synthesized by Sonogashira cross-coupling of aryl halides with terminal acetylenes. Strong electron acceptors such as  F and  CF₃ and weak electron donors like  CH₃ were placed as substituents on one of the benzene rings. Acetyl-protected sulfhydryl groups were attached to one end of these molecules to promote their self-organization on gold surfaces. The electron-transport properties of such self-assembled monolayers (SAMs) are highly sensitive to the local order of the molecules in the solid state. Single crystals were analyzed by X-ray diffraction experiments that revealed structural details that could lead to a better understanding of the electron-transport properties. The unsymmetrical substitution of the aromatic rings by electron-active groups in the ortho-, meta-, or para positions resulted in changes of such molecular parameters as bonding and torsion angles and planarity. These parameters, in turn, can affect the angle of the molecular attachment to a gold substrate and the density of the resulting SAMs. Patterned SAMs of some of these molecules and comparison alkane thiols were obtained on gold by microcontact printing or flooding. The SAM thickness was determined by spectroscopic ellipsometry. Surface potential differences between adjacent SAMs or between SAMs and the gold substrate were measured by scanning surface potential microscopy under ambient conditions. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 541–550, 2004     

Dynamic Molecular Tweezers Composed of Dibenzocyclooctatetraene Units: Synthesis,Properties, and Thermochromism in Host–Guest Complexes

Novel dynamic molecular tweezers (DMTs) 3 a , 3 b , 4 a , 4 b , and 5 b , composed of two tub‐shaped dibenzocyclooctatetraene (DBCOT) units, were designed and synthesized. The cyclooctatetraene (COT) rings of these DMTs readily invert in solution, and the molecular structure shows rigid syn and anti forms in an equilibrium mixture in solution. The syn and anti conformers can be observed by NMR. The isomerization barriers of 3 a , 3 b , 4 a , 4 b , and 5 b are in the range of 16.5–21.3 kcal mol^?1, depending on steric repulsion between substituents of the COT rings and protons of the central benzene ring. These DMTs form complexes with 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ) and 1,2,4,5‐tetracyano‐benzene (TCNB) in solution and in the solid state. The binding abilities of these DMTs increase with electron‐donating substituents on COT, which increase the electron densities of the cavity of the syn form, as supported by theoretical calculations. In addition, elongation of the terminal alkoxy chains of the DMTs was found to cause the enhancement of van der Waals contact with guest molecules. Therefore, 5 b , which has CH₂OMe groups on the COT rings and longer ethoxy groups on the terminal benzene rings, showed the highest electron density of the cavity and hence the highest binding ability with the electron‐deficient guest molecules. Interestingly, solutions of 3 b , 4 b , and 5 b show thermochromism in the presence of DDQ. A solution of 3 b or 4 b with DDQ in CHCl₃ is green due to charge‐transfer interaction at room temperature and the color changes from green to yellow upon heating to 60 °C and from green to blue upon cooling to ?40 °C, whereas the high complexation ability of 5 b with DDQ only shows a change in the shade of blue.     

Density functional theory study on the possibility of Si‐, Ge‐, and Sn‐doped carbon nanotubes as efficient support materials for platinum

PBEPBE‐D3 calculations were performed to investigate how platinum (Pt) interacts with the internal and external surfaces of single‐walled pristine, Si‐, Ge‐, and Sn‐doped (6,6) carbon nanotubes (CNTs). Our calculations showed that atomic Pt demonstrates stronger binding strength on the external surfaces than the internal surface adsorption for the same type of nanotube. In cases of external surface adsorptions, Si‐, Ge‐, and Sn‐doped CNTs show comparable binding energies for Pt, at least 1.40 eV larger than pristine CNT. This enhancement can be rationalized by the strong covalent interactions between Pt and X? C (X = Si, Ge, and Sn) pairs based on structural and projected density of states analysis. In terms of internal surface adsorptions, Ge and Sn doping could significantly enhance the binding of Pt. Pt atom shows much more delocalized and bonding states inside Ge‐ and Sn‐doped CNTs, indicating multiple‐site interaction pattern when atomic Pt is confined inside the nanotubes. However, the internal surface of Si‐doped CNT presents limited enhancement in Pt adsorption with respect to that of pristine CNT because of their similar binding geometries. © 2016 Wiley Periodicals, Inc.     

A Facile Synthesis of a Wide Variety of Cationic Ruthenium Hydrido‐Arene Complexes of binap (=1,1′‐Binaphthalene‐2,2′‐diylbis(diphenylphosphane)) and MeO?biphep (=6,6′‐Dimethoxybiphenyl‐2,2′‐diylbis(diphenylphosphane))

A straightforward high‐yield synthetic route to the cationic hydrido‐arene complexes [RuH(η⁶‐arene)(binap or MeO biphep)](CF₃SO₃), with a variety of arenes containing both donor and acceptor substituents, is described. ¹³C‐NMR Data for these complexes are reported. Several of these Ru‐complexes have been used as transfer‐hydrogenation catalysts in the reduction of acetophenone.     

Amine-functionalized low-cost industrial grade multi-walled carbon nanotubes for the capture of carbon dioxide

Industrial grade multi-walled carbon nanotubes (IG-MWCNTs) are a low-cost substitute for commercially purified multi-walled carbon nanotubes (P-MWCNTs). In this work, IG-MWCNTs were functionalized with tetraethylenepentamine (TEPA) for CO₂ capture. The TEPA impregnated IG-MWCNTs were characterized with various experimental methods including N₂ adsorption/desorption isotherms, elemental analysis, X-ray diffraction, Fourier transform infrared spectroscopy and thermogravimetric analysis. Both the adsorption isotherms of IG-MWCNTs-n and the isosteric heats of different adsorption capacities were obtained from experiments. TEPA impregnated IG-MWCNTs were also shown to have high CO₂ adsorption capacity comparable to that of TEPA impregnated P-MWCNTs. The adsorption capacity of IG-MWCNTs based adsorbents was in the range of 2.145 to 3.088 mmol/g, depending on adsorption temperatures. Having the advantages of low-cost and high adsorption capacity, TEPA impregnated IG-MWCNTs seem to be a promising adsorbent for CO₂ capture from flue gas.     

Hydrogen storage in porous structures of adamantane‐based nitrogen‐heterocyclic ring with diamond‐like structure

Using the density functional theory and molecular mechanics methods, we calculated the binding energy and parameters about the primitive cell designed by us with the adamantane and the nitrogen heterocyclic ring, the vibrational frequencies about the small complexes. Grand canonical Monte Carlo simulations were performed to predict the capacities for the hydrogen storage and adsorption isotherms. The results show the positive effects of bigger specific surface area and pore volume on hydrogen storage and isosteric heat. The gravimetric hydrogen uptake of adamantane‐based nitrogen‐heterocyclic ring of quaterpyridyl can reach 9.02 wt % at room temperature and 100 bar. But the volumetric H₂ capacities of the four materials are low at T = 298 K because of weak interaction between the materials and H₂ molecule. © 2014 Wiley Periodicals, Inc.     

The nature of the N?O bond in amine oxides

Despite the ubiquitous presence of amine oxides in chemistry, there is no consensus about the nature of the N O bond in these compounds. In this work, we have used electron density analysis to investigate the nature of this bond in substituted amine oxides, R₃NO, and have compared it with the nature of the N O bond in hydroxylamines, R₂NOR, and model molecules that have well-established chemical bond character. The results showed that the N O bond length and relative stability are proportional to the inductive effect of the substituents. Quantum chemical topology, natural bond orbitals (NBO), and natural resonance theory (NRT) analyses indicated that the N O bond is polar covalent in all the studied amine oxides, but the ionic contribution is different. NBO and NRT analyses revealed that molecules with more electronegative substituents have strongly delocalized N O and N R bonds, whereas molecules with electropositive substituents have localized bonds.     

The σ‐donating and π‐accepting properties of ortho‐Si(CH3)3 phosphinine macrocycles

A theoretical investigation of both the ortho‐Si(CH₃)₃ phosphinine and some silacalix[n]phosphinines was performed. The optimized geometries agree well with those reported from X‐ray analysis and other structural studies. The silacalix[n]phosphinine macrocyle is very flexible because of the C Si C bridges. This, in turn, makes possible the formation of strained configurations in solid packed structures. In the silacalix[3]phosphinine, a P P bonding interaction that is presumably responsible for its structural and electronic features seems to exist. The molecular orbital calculations corroborate that both the π‐accepting properties and the σ‐donating capacities of the phosphinine unit may be enhanced by ortho‐Si(CH₃)₃ substitution. These features satisfy the proposal of the synthesizers as regards the production of macrocyclic phosphorus compounds, with good π‐accepting properties and strong σ‐donating capacities, which are sufficiently flexible as to encapsulate metals with coordination spheres of different geometries. © 2003 Wiley Periodicals, Inc. Heteroatom Chem 14:160–169, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.10118     

Density functional calculations on CO attached to PtnRu(10−n) (n = 6–10) clusters

B3LYP and SCF‐Xα calculations have been performed on Pt_nRu_(10−n)CO (n = 6–10) clusters. The work aims to simulate the adsorption of CO on the (111) surface of platinum metal and to examine the electronic effects that arise when some Pt atoms are replaced with Ru. Adsorption energies and Pt C and C O stretching frequencies have been calculated for each cluster. Ru does affect the electronic structure of the clusters, the calculated adsorption energies, and frequencies, the Pt C frequency more than the C O. The donation‐backbonding mechanism that accompanies the shift in CO stretching frequency that occurs when CO adsorbs on platinum does not explain the differences in frequency shift observed in CO on various Pt/Ru surfaces. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 589–598, 2000     

One‐Step Solvothermal Synthesis of Single‐Crystalline TiOF2 Nanotubes with High Lithium‐Ion Battery Performance

Single‐crystalline TiOF₂ nanotubes were prepared by a one‐step solvothermal method. The nanotubes are rectangular in shape with a length of 2–3 μm, width of 200–300 nm, and wall thickness of 40–60 nm. The formation of TiOF₂ nanotubes is directly driven by the interaction between TiF₄ and oleic acid in octadecane to form the 1D nanorods, and this is followed by a mass diffusion process to form the hollow structures. The synthesis approach can be extended to grow TiOF₂ nanoparticles and nanorods. Compared with TiO₂, which is the more commonly considered anode material in lithium‐ion batteries, TiOF₂ has the advantages of a lower Li‐intercalation voltage (e.g., to help increase the total voltage of the battery cell) and higher specific capacities. The TiOF₂ nanotubes showed good Li‐storage properties with high specific capacities, stable cyclabilities, and good rate capabilities.