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1.
Nat Commun ; 11(1): 653, 2020 Jan 31.
Artigo em Inglês | MEDLINE | ID: mdl-32005833

RESUMO

The industrial synthesis of ammonia (NH3) using iron-based Haber-Bosch catalyst requires harsh reaction conditions. Developing advanced catalysts that perform well at mild conditions (<400 °C, <2 MPa) for industrial application is a long-term goal. Here we report a Co-N-C catalyst with high NH3 synthesis rate that simultaneously exhibits dynamic and steady-state active sites. Our studies demonstrate that the atomically dispersed cobalt weakly coordinated with pyridine N reacts with surface H2 to produce NH3 via a chemical looping pathway. Pyrrolic N serves as an anchor to stabilize the single cobalt atom in the form of Co1-N3.5 that facilitates N2 adsorption and step-by-step hydrogenation of N2 to *HNNH, *NH-NH3 and *NH2-NH4. Finally, NH3 is facilely generated via the breaking of the *NH2-NH4 bond. With the co-existence of dynamic and steady-state single atom active sites, the Co-N-C catalyst circumvents the bottleneck of N2 dissociation, making the synthesis of NH3 at mild conditions possible.

2.
Nat Commun ; 11(1): 13, 2020 Jan 07.
Artigo em Inglês | MEDLINE | ID: mdl-31911625

RESUMO

A membrane with both high ion conductivity and selectivity is critical to high power density and low-cost flow batteries, which are of great importance for the wide application of renewable energies. The trade-off between ion selectivity and conductivity is a bottleneck of ion conductive membranes. In this paper, a thin-film composite membrane with ultrathin polyamide selective layer is found to break the trade-off between ion selectivity and conductivity, and dramatically improve the power density of a flow battery. As a result, a vanadium flow battery with a thin-film composite membrane achieves energy efficiency higher than 80% at a current density of 260 mA cm-2, which is the highest ever reported to the best of our knowledge. Combining experiments and theoretical calculation, we propose that the high performance is attributed to the proton transfer via Grotthuss mechanism and Vehicle mechanism in sub-1 nm pores of the ultrathin polyamide selective layer.

3.
Angew Chem Int Ed Engl ; 59(9): 3624-3629, 2020 Feb 24.
Artigo em Inglês | MEDLINE | ID: mdl-31773844

RESUMO

The construction of 2D and 3D covalent organic frameworks (COFs) from functional moieties for desired properties has gained much attention. However, the influence of COFs dimensionality on their functionalities, which can further assist in COF design, has never been explored. Now, by selecting designed precursors and topology diagrams, 2D and 3D porphyrinic COFs (2D-PdPor-COF and 3D-PdPor-COF) are synthesized. By model building and Rietveld refinement of powder X-ray diffraction, 2D-PdPor-COF crystallizes as 2D sheets while 3D-PdPor-COF adopts a five-fold interpenetrated pts topology. Interestingly, compared with 2D-PdPor-COF, 3D-PdPor-COF showed interesting properties, including 1) higher CO2 adsorption capacity; 2) better photocatalytic performance; and 3) size-selective photocatalysis. Based on this study, we believe that with the incorporation of functional moieties, the dimensionality of COFs can definitely influence their functionalities.

4.
Magn Reson Chem ; 2019 Oct 28.
Artigo em Inglês | MEDLINE | ID: mdl-31659777

RESUMO

Metal-organic frameworks (MOFs) are a class of important porous materials with many current and potential applications. Their applications almost always involve the interaction between host framework and guest species. Therefore, understanding of host-guest interaction in MOF systems is fundamentally important. Solid-state NMR spectroscopy is an excellent technique for investigating host-guest interaction as it provides information complementary to that obtained from X-ray diffraction. In this work, using MOF α-Mg3 (HCOO)6 as an example, we demonstrated that 13 C chemical shift tensor of organic linker can be utilized to probe the host-guest interaction in MOFs. Obtaining 13 C chemical shift tensor components (δ11 , δ22 , and δ33 , where δ11 ≥ δ22 ≥ δ33 ) in this MOF is particularly challenging as there are six coordinatively equivalent but crystallographically non-equivalent carbons in the unit cell with very similar local coordination environment. Two-dimensional magic-angle-turning experiments were employed to measure the 13 C chemical shift tensors of each individual crystallographically non-equivalent carbon in three microporous α-Mg3 (HCOO)6 samples with different guest species. The results indicate that the δ22 component (with its direction approximately being co-planar with the formate anion and perpendicular to the C-H bond) is more sensitive to the adsorbate molecules inside the MOF channel due to the weak C-H···O hydrogen bonding or the ring current effect of benzene. The 13 C isotropic chemical shift, on the other hand, seems much less sensitive to the subtle changes in the local environment around formate linker induced by adsorption. The approach described in this study may be used in future studies on host-guest interaction within MOFs.

5.
J Am Chem Soc ; 141(45): 18318-18324, 2019 Nov 13.
Artigo em Inglês | MEDLINE | ID: mdl-31644275

RESUMO

A large amount of zeolite structures are still not synthetically available or not available in the form of aluminosilicate currently. Despite significant progress in the development of predictive concepts for zeolite synthesis, accessing some of these new materials is still challenging. One example is the IWR structure as well. Despite successful synthesis of Ge-based IWR zeolites, direct synthesis of aluminosilicate IWR zeolite is still not successful. In this report we show how a suitable organic structure directing agent (OSDA), through modeling of an OSDA/zeolite cage interaction, could access directly the aluminum-containing IWR structure (denoted as COE-6), which might allow access to new classes of materials and thus open opportunities in valuable chemical applications. The experimental results reveal that the COE-6 zeolites with a SiO2/Al2O3 ratio as low as 30 could be obtained. Very interestingly, the COE-6 zeolite has much higher hydrothermal and thermal stabilities than those of the conventional Ge-Al-IWR zeolite. In methanol-to-propylene (MTP) reaction, the COE-6 zeolite exhibits excellent selectivity for propylene, offering a potential catalyst for MTP reaction in the future.

6.
Chem Commun (Camb) ; 55(72): 10693-10696, 2019 Sep 16.
Artigo em Inglês | MEDLINE | ID: mdl-31414676

RESUMO

The inhomogeneous phenomenon with a gradient distribution in adsorption and diffusion behaviors of xenon within a large SAPO-34 crystal was revealed by 129Xe NMR, 2D EXSY NMR and 129Xe PFG NMR techniques at the micro-scale. A multi-layer adsorption and diffusion model for xenon in a single crystal was proposed.

7.
Chem Sci ; 10(23): 5875-5883, 2019 Jun 21.
Artigo em Inglês | MEDLINE | ID: mdl-31360391

RESUMO

Solid acids have been widely used as heterogeneous catalysts in developing green and sustainable chemistry. However, it remains a challenge to improve the mass transport properties and acid strength of solid acids simultaneously. Herein, we report a class of two dimensional (2D) layered hybrid solid acids with outstanding mass transfer and extremely high acid strength by incorporating sulfonated polymers in-between montmorillonite layers. The 2D layered structure and broad distribution of pore sizes allow for highly efficient mass transport of substrate molecules into and out of the solid acids. The acid strength of these solid acids was found to be stronger than that of 100% H2SO4, H3PW12O40 and any other reported solid acids to date, as determined by 1H and 31P solid-state NMR. These 2D solid acids show extraordinary catalytic performance in biomass conversion to fuels, superior to that of H3PW12O40, HCl and H2SO4. Theoretical calculations and control experiments reveal that H-bond based interactions between the polymer and montmorillonite facilitate the unusually high acid strengths found in these samples.

8.
ACS Appl Mater Interfaces ; 11(33): 29950-29959, 2019 Aug 21.
Artigo em Inglês | MEDLINE | ID: mdl-31352779

RESUMO

Hydrogen sulfide (H2S) is malodorous and highly toxic, and its selective removal from industrial feedstock is highly recommended for safety and environment protection. We report here a class of nitrogen-functionalized, hierarchical porous polymers (N-HPPs) synthesized from one-step alkylation-induced cross-linking without any involvement of templates. The as-engineered N-HPPs are large in BET surface area (792-1397 m2/g) and endowed with hierarchical porosity. The incorporated nitrogen species of N-HPPs act as structural base sites with properties that can be precisely controlled. By molecular simulation, the enhanced interactions between N-HPPs and H2S were verified. The synthesized N-HPPs show superb capacities for H2S adsorption (9.2 mmol/g at 0 °C, 1.0 bar) and display satisfactory IAST H2S/N2 and H2S/CH4 selectivity (88.3 and 119.6, respectively, at 0 °C). Catalyzed by the structural base sites located in the N-HPPs, the COS together with its derived H2S can be effectively eliminated under mild conditions.

9.
ACS Appl Mater Interfaces ; 11(26): 23112-23117, 2019 Jul 03.
Artigo em Inglês | MEDLINE | ID: mdl-31252486

RESUMO

For the first time, SSZ-39 zeolite has been directly prepared using conventional colloidal silica and sodium aluminate instead of using FAU zeolite as the raw material in the alkaline media. The adjustment of the Si/Al ratios in the starting materials to the suitable values is a key factor to prepare the aluminosilicate SSZ-39 zeolite. Various characterizations (for instance, X-ray diffraction, scanning electron microscopy, nitrogen sorption, solid 27Al NMR, and NH3-temperature-programmed desorption) display that the aluminosilicate SSZ-39 zeolite owns high crystallinity, uniform cuboid morphology, large surface area, four-coordinated aluminum species, and strong acidic sites. Inductively coupled plasma analysis shows that the SiO2/Al2O3 ratios of the SSZ-39 products are ranged from 12.8 to 16.8. Considering the special framework of the SSZ-39 zeolite, the yield of this synthesis is not higher than 21.3%. Moreover, the catalytic performance of Cu-SSZ-39 catalyst synthesized from this route is excellent in the selective catalytic reduction of NO x with NH3 (NH3-SCR).

10.
Phys Chem Chem Phys ; 21(6): 3287-3293, 2019 Feb 06.
Artigo em Inglês | MEDLINE | ID: mdl-30681681

RESUMO

A comparative study of the adsorption and desorption processes of methanol in two kinds of nanochannels (i.e. MCM-41 and SWNTs) is performed by in situ continuous-flow laser-hyperpolarized 129Xe NMR. The high sensitivity and short acquisition time of hyperpolarized 129Xe allow for probing the molecular dynamics in a confined geometry under real working conditions. Hyperpolarized 129Xe NMR spectra indicate that the methanol adsorption behavior in nanochannels is determined by the characters of adsorption sites and that the methanol adsorption rate in the nanochannels of SWNTs is faster than in MCM-41. The experimental data shown in this work also indicate that there is a change in gas phase 129Xe NMR signal intensity during the adsorption and desorption of methanol in SWNTs. This may be because there is a strong depolarization of hyperpolarized 129Xe in SWNTs.

11.
Chem Commun (Camb) ; 54(95): 13435-13438, 2018 Nov 27.
Artigo em Inglês | MEDLINE | ID: mdl-30430159

RESUMO

The acidity enhancement induced by the nonpolar solvent effect of naphthalene inside zeolites was unambiguously identified. The mechanism of such an effect due to the CH/π interaction in the nonpolar environment has been revealed based on the advanced 2D 1H-13C correlation NMR technique and DFT calculations for the first time.

12.
J Phys Chem Lett ; 9(24): 7137-7145, 2018 Dec 20.
Artigo em Inglês | MEDLINE | ID: mdl-30395479

RESUMO

Pentazole (HN5) and its anion ( cyclo-N5-) have been elusive for nearly a century because of the unstable N5 ring. Recently, Zhang et al. reported the first synthesis and characterization of the pentazolate anion cyclo-N5- in (N5)6(H3O)3(NH4)4Cl salt at ambient conditions ( Science 2017, 355, 374 ). However, whether the cyclo-N5- in (N5)6(H3O)3(NH4)4Cl salt is protonated or not has been debated ( Huang and Xu, Science, 2018, 359, eaao3672 ; Jiang et al. Science, 2018, 359, aas8953 ). Herein, we employed ab initio molecular dynamics (AIMD) simulations, which can well present the dynamic behavior at realistic experimental conditions, to examine the potential protonated state of cyclo-N5- in both crystal and dimethyl sulfoxide (DMSO) solvent. Our simulations revealed that the protonation reaction of (N5)6(H3O)3(NH4)4Cl → (N5)5(N5H)(H2O)(H3O)2(NH4)4Cl is thermodynamically spontaneous according to Δ G < 0, and the small energy barrier of 12.6 kJ/mol is not enough to prevent the partial protonation of cyclo-N5- due to the temperature effect; consequently, both deprotonated and protonated cyclo-N5- exist in the crystal. In comparison, the DMSO solvent effect can remarkably reduce the difference of proton affinities among cyclo-N5-, H2O, and NH3, and the temperature effect can finally break these hydrogen bonds and lead to the deprotonated cyclo-N5- in DMSO solvent. Our AIMD simulations reconcile the recent controversy.

13.
Phys Chem Chem Phys ; 20(41): 26522-26531, 2018 Nov 07.
Artigo em Inglês | MEDLINE | ID: mdl-30306980

RESUMO

The direct conversion of methane to methanol on [Cu(µ-O)M]2+ (M = Cu, Ag, Zn, Au) bimetal centers in ZSM-5 zeolite is investigated using periodic density functional theory for the first time. Some conclusions are drawn: (1) methane activation on [Cu(µ-O)M]2+ (M = Cu, Ag, Zn, Au) in the ZSM-5 zeolite proceeds through radical-like transition states, and the ability for CH4 activation decreases in the sequence [Cu(µ-O)Ag]2+ > [Cu(µ-O)Au]2+ > [Cu(µ-O)Cu]2+ > [Cu(µ-O)Zn]2+. (2) There are two factors that can dramatically enhance C-H bond activation: a greater spin density and a less negative charge of the µ-O atom. (3) The angles ∠CuOM play a minor role in the reactivity difference among [CuOM]2+-ZSM-5 (M = Cu, Ag, Zn, Au). Our findings will provide insight into methane activation for designing highly effective catalysts applied in industrial processes.

14.
Chem Sci ; 9(31): 6470-6479, 2018 Aug 21.
Artigo em Inglês | MEDLINE | ID: mdl-30310577

RESUMO

The methanol-to-olefin (MTO) reaction is an active field of research due to conflicting mechanistic proposals for the initial carbon-carbon (C-C) bond formation. Herein, a new methane-formaldehyde pathway, a Lewis acid site combined with a Brønsted acid site in zeolite catalysts can readily activate dimethyl ether (DME) to form ethene, is identified theoretically. The mechanism involves a hydride transfer from Al-OCH3 on the Lewis acid site to the methyl group of the protonated methanol molecule on the adjacent Brønsted acid site leading to synchronous formation of methane and Al-COH2 + (which can be considered as formaldehyde (HCHO) adsorbed on the Al3+ Lewis acid sites). The strong electrophilic character of the Al-COH2 + intermediate can strongly accelerate the C-C bond formation with CH4, as indicated by the significant decrease of activation barriers in the rate-determining-step of the catalytic processes. These results highlight a synergy of extra-framework aluminum (EFAl) Lewis and Brønsted sites in zeolite catalysts that facilitates initial C-C bond formation in the initiation step of the MTO reaction via the Al-COH2 + intermediate.

15.
ACS Appl Mater Interfaces ; 10(39): 33214-33220, 2018 Oct 03.
Artigo em Inglês | MEDLINE | ID: mdl-30203642

RESUMO

Modern methodologies for synthesizing zeolites typically involve the employment of costly organic structure-directing agents. Herein, we report the design synthesis of aluminosilicate zeolite with ITE structure using an inexpensive nickel-amine complex (nickel-pentaethylenexamine) as a novel structure-directing agent. Characterizations including X-ray diffraction, scanning electron microscopy, N2 sorption isotherms, and 27Al magic-angle spinning NMR techniques show that the ITE zeolite has high crystallinity, perfect crystals, large surface area, and abundant aluminum species in the framework. More importantly, catalytic tests on the hydrogenation of CO2 into methane show that the Ni-ITE zeolite exhibits better catalytic performance than aluminosilicate-supported and silica-supported nickel catalysts. Obviously, the use of nickel-amine complex offers an alternative and facile way to synthesize aluminosilicate zeolites.

16.
J Am Chem Soc ; 140(34): 10764-10774, 2018 08 29.
Artigo em Inglês | MEDLINE | ID: mdl-30070481

RESUMO

Post-synthetic dealumination treatment is a common tactic adopted to improve the catalytic performance of industrialized zeolitic catalysts through enhancements in acidity and stability. However, among the possible extra-framework aluminum (EFAL) species in dealuminated zeolites such as Al3+, Al(OH)2+, Al(OH)2+, AlO+, AlOOH, and Al(OH)3, the presence of tri-coordinated EFAL-Al3+ species, which exhibit large quadrupolar effect due to the lack of hydrogen-bonding species, was normally undetectable by conventional one- and two-dimensional 1H and/or 27Al solid-state nuclear magnetic resonance (SSNMR) techniques. By combining density functional theory (DFT) calculations with experimental 31P SSNMR using trimethylphosphine (TMP) as the probe molecule, we report herein a comprehensive study to certify the origin, fine structure, and possible location of tri-coordinated EFAL-Al3+ species in dealuminated HY zeolite. The spatial proximities and synergies between the Brønsted and various Lewis acid sites were clearly identified, and the origin for the observed EFAL-Al3+ species with ultra-strong Lewis acidity was deduced to be at the expense of adjacent Brønsted acid sites. The excellent performance of such tri-coordinated EFAL species was furthermore confirmed by glucose isomerization reactions.

17.
Angew Chem Int Ed Engl ; 57(31): 9833-9837, 2018 Jul 26.
Artigo em Inglês | MEDLINE | ID: mdl-29893503

RESUMO

Fast switching of spontaneous polarization (Ps ) is one of the most essential requirements for ferroelectrics used in the field of data storage. However, in contrast to inorganic counterparts, the low operating frequency (<500 Hz) for molecular ferroelectrics severely hinders their large-scale applications. Herein, for the first time, we achieved the room-temperature fastest switching of the Ps in a new molecular ferroelectric, N-methylmorpholinium trinitrophenolate (1), which displays notable ferroelectricity (Ps =3.2 µc cm-2 ). Strikingly, electric polarizations of 1 have been switched under a record-high frequency of 263 kHz, and this performance remains stable without any obvious fatigue after ca. 2×105 switching cycles. To our knowledge, 1 is the first organic ferroelectric to switch polarization at such a high operating frequency, exceeding the majority of organic ferroelectrics, which opens up new possibilities for its potential in the field of non-volatile memory.

18.
Angew Chem Int Ed Engl ; 57(42): 13800-13804, 2018 Oct 15.
Artigo em Inglês | MEDLINE | ID: mdl-29864237

RESUMO

Designing heterogeneous metal-free catalysts for hydrogenation is a long-standing challenge in catalysis. Nanodiamond-based carbon materials were prepared that are surface-doped with electron-rich nitrogen and electron-deficient boron. The two heteroatoms are directly bonded to each other to form unquenched Lewis pairs with infinite π-electron donation from the surrounding graphitic structure. Remarkably, these Lewis pairs can split H2 to form H+ /H- pairs, which subsequently serve as the active species for hydrogenation of different substrates. This unprecedented finding sheds light on the uptake of H2 across carbon-based materials and suggests that dual Lewis acidity-basicity on the carbon surface may be used to heterogeneously activate a variety of small molecules.

19.
J Am Chem Soc ; 140(25): 7885-7895, 2018 06 27.
Artigo em Inglês | MEDLINE | ID: mdl-29799200

RESUMO

Aquaporin Z (AqpZ) is an integral membrane protein that facilitates transport of water across Escherichia coli cells with a high rate. Previously, R189, a highly conserved residue of the selective filter of AqpZ, was proposed as a gate within the water channel on the basis of the observation of both open and closed conformations of its side chain in different monomers of an X-ray structure, and the observation of rapid switches between the two conformations in molecular dynamic simulations. However, the gating mechanism of the R189 side chain remains controversial since it is unclear whether the different conformations observed in the X-ray structure is due to different functional states or is a result of perturbation of non-native detergent environments. Herein, in native-like synthetic bilayers and native E. coli membranes, a number of solid-state NMR techniques are employed to examine gating mechanism of the R189 side chain of AqpZ. One R189 side-chain conformation is highly evident since only a set of peaks corresponding to the R189 side chain is observed in 2D 15N-13C spectra. The immobility of the R189 side chain is detected by 1H-15N dipolar lineshapes, excluding the possibility of the rapid switches between the two side-chain conformations. High-resolution monomeric structure of AqpZ, determined by CS-Rosetta calculations using experimentally measured distance restraints related to the R189 side chain, reveals that this side chain is in an open conformation, which is further verified by its water accessibility. All the solid-state NMR experimental results, combining with water permeability essay, suggest a permanently open conformation of the R189 side chain in the synthetic bilayer and native membranes. This study provides new structural insights into the gating mechanism of aquaporins and highlights the significance of lipid bilayer environments in elucidating the molecular mechanism of membrane proteins.

20.
Phys Chem Chem Phys ; 20(17): 11702-11712, 2018 May 03.
Artigo em Inglês | MEDLINE | ID: mdl-29683160

RESUMO

In this work, a density functional theory (DFT) study was carried out to address the fundamental description of the effective phosphorous species that could improve the framework stability and reduce the coke deposition formation on the P-ZSM-5 zeolite. On the basis of the dealumination barriers of ZSM-5 with all the possible phosphorous species bound on the zeolite framework, PO4H4 was ascertained to be the effective phosphorous species that could improve the ZSM-5 zeolite hydrothermal stability and reduce its acid strength. Apart from this, the olefin polymerization reaction is the main cause of coking deactivation for ZSM-5. Thus, the effect of the modification P-ZSM-5 on the reactivity of light olefins dimerization was also studied. Compared to the unmodified ZSM-5, the activation energy of the rate-limiting step of ethylene dimerization was increased from 20.3 kcal mol-1 to 34.6 kcal mol-1, thereby apparently inhibiting ethylene dimerization and improving the resistance to coke deposition for P-ZSM-5. Our calculation results should provide a beneficial theoretical guide for designing and improving a catalyst for the methanol-to-olefins process and bioethanol dehydration.

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