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1.
Chemphyschem ; 2020 Feb 11.
Artigo em Inglês | MEDLINE | ID: mdl-32045082

RESUMO

The proton conduction properties of a phosphonato-sulfonate-based coordination polymer are studied by impedance spectroscopy using a single crystal specimen. Two distinct conduction mechanisms are identified. Water-mediated conductance along the crystal surface occurs by mass transport, as evidenced by a high activation energy (0.54 eV). In addition, intrinsic conduction by proton 'hopping' through the interior of the crystal with a low activation energy (0.31 eV) is observed. This latter conduction is anisotropic with respect to the crystal structure and seems to occur through a channel along the c axis of the orthorhombic crystal. Proton conduction is assumed to be mediated by sulfonate groups and non-coordinating water molecules that are part of the crystal structure.

2.
Dalton Trans ; 2020 Feb 13.
Artigo em Inglês | MEDLINE | ID: mdl-32052807

RESUMO

The new linker molecule (H2O3PCH2)2N-CH2C6H4SO3H, (4-{[bis(phosphonomethyl)amino]methyl}benzene-sulfonic acid, H5L), bearing both phosphonic and sulfonic acid groups, was employed for the synthesis of new coordination polymers (CPs). Four new CPs of composition [Mg(H3L)(H2O)2]·H2O (1), [Mg2(HL)(H2O)6]·2H2O (2), [Ba(H3L)(H2O)]·H2O (3) and [Pb2(HL)]·H2O (4), were discovered using high-throughput methods and all structures were determined by single-crystal X-ray diffraction (SCXRD). With increasing ionic radius of the metal ion, an increase in coordination number from CN = 6 (Mg2+) to CN = 9 (Ba2+) and an increase in the dimensionality of the network from 1D to 3D is observed. This is reflected in the composition of the IBU and the number of metal ions that are connected by each linker molecule, i.e. from three in 1 to ten in 4. The connection of the IBUs leads to 1D and 2D structures in 1 and 2 with non-coordinating sulfonate groups, while 3 and 4 crystallise in MOF-type structures and coordination of the sulfonate groups is observed. The compounds exhibit thermal stabilities between 200 (2) and 345 °C (4) as proven by variable temperature powder X-ray diffraction (VT-PXRD) measurements. Title compound 4 contains micropores of 4 × 2 Å and reversible H2O uptake of 50 mg g-1 was demonstrated by vapour sorption measurements, making it the first porous metal phosphonatosulfonate. Detailed characterisation, i.e. CHNS and TG analysis as well as NMR and IR spectroscopy measurements confirm the phase purity of the title compounds.

3.
ACS Appl Mater Interfaces ; 12(5): 5633-5641, 2020 Feb 05.
Artigo em Inglês | MEDLINE | ID: mdl-31940165

RESUMO

The synthetic flexibility of metal-organic frameworks (MOFs) with high loading capacities and biocompatibility makes them ideal candidates as drug delivery systems (DDSs). Here, we report the use of CAU-7, a biocompatible bismuth-based MOF, for the delivery of two cancer drugs, sodium dichloroacetate (DCA) and α-cyano-4-hydroxycinnamic acid (α-CHC). We achieved loadings of 33 and 9 wt % for DCA and α-CHC, respectively. Interestingly, CAU-7 showed a gradual release of the drugs, achieving a release time of up to 17 days for DCA and 31 days for α-CHC. We then performed mechanical and thermal amorphization processes to attempt to delay the delivery of guest molecules even more. With the thermal treatment, we were able to achieve an outstanding 32% slower release of α-CHC from the thermally treated CAU-7. Using in vitro studies and endocytosis inhibitors, confocal microscopy, and fluorescence-activated cell sorting, we also demonstrated that CAU-7 was successfully internalized by cancer cells, partially avoiding lysosome degradation. Finally, we showed that CAU-7 loaded either with DCA or α-CHC had a higher therapeutic efficiency compared with the free drug approach, making CAU-7 a great option for biomedical application.

4.
Chemistry ; 2020 Jan 28.
Artigo em Inglês | MEDLINE | ID: mdl-31991507

RESUMO

We report the utilization of acetonitrile as a new solvent for the synthesis of the three significantly different benchmark metal-organic frameworks CAU-10, Ce-UiO-66 and Al-MIL-53 of idealized composition [Al(OH)(ISO)], [Ce6O4(OH)4(BDC)6] and [Al(OH)(BDC)], respectively (ISO2-: isophthalate, BDC2-: terephthalate). Its use allowed the synthesis scale-up of Ce-UiO-66 to the gram-scale. Its use allowed the synthesis scale-up of Ce-UiO-66 to the gram-scale.While CAU-10 and Ce-UiO-66 exhibit properties similar to those reported elsewhere for these two materials, the obtained Al-MIL-53 shows no structural flexibility upon adsorption of hydrophilic or hydrophobic guest molecules like water and xenon and is stabilized in its large pore form over a broad temperature range (130 - 450 K). The stabilization of the large pore form of Al-MIL-53 was attributed to an unusually high percentage of non-coordinating -COOH groups as determined by solid-state NMR spectroscopy. The defective material shows an unusually high water uptake of 310 mg/g within the range of 0.45 to 0.65 p/p°. Despite of showing no breathing effect upon water adsorption it exhibits distinct mechanical properties. Thus, mercury intrusion porosimetry studies revealed that the solid can be reversibly forced to breath by applying moderate pressures (~60 MPa).

5.
Sci Rep ; 9(1): 19775, 2019 Dec 24.
Artigo em Inglês | MEDLINE | ID: mdl-31875003

RESUMO

This work presents the suppression of abnormal grain growth in bulk ceramic K0.5Na0.5NbO3 (KNN). The suppression is enabled by precise control of the starting powder morphology through match of milling and calcination duration. A comparative temperature-dependent analysis of the resulting sample morphology, phase transitions and related electronic material properties reveals that abnormal grain growth is indeed a major influence in material property deterioration, as has theoretically been suggested in other works. However, it is shown that this abnormal grain growth originates from the calcined powder and not from sintering and that all subsequent steps mirror the initial powder morphology. In specific, the results are discussed with respect to the predictions of the compatibility theory and microstructure. Despite the material's multi-scale heterogeneity, the suppression of abnormal grain growth allows for the achievement of significantly improved functional properties and it is reported that this development is correctly predicted by the compatibility theory within the borders of microstructural integrity. It could be demonstrated that functional fatigue is strongly minimised, while thermal and electronic properties are improved when abnormal grain growth is suppressed by powder morphology control.

6.
Dalton Trans ; 48(44): 16737-16743, 2019 Nov 12.
Artigo em Inglês | MEDLINE | ID: mdl-31670727

RESUMO

A metallocene based linker 1,1'-ferrocenedicarboxylic acid (H2FcDC) was used to synthesise the first permanently porous ferrocenedicarboxylate, exhibiting a MIL-53 architecture. This compound Al-MIL-53-FcDC [Al(OH)(FcDC)] is obtained in glass vials under mild synthesis conditions at ≤100 °C and after a short reaction time of 90 min. The crystal structure was determined from powder X-ray diffraction data and the compound shows porosity towards N2 and H2O, exhibiting a BET surface area of 340 m2 g-1. Furthermore, the MOF was characterised via EPR and Mössbauer spectroscopy. The Mössbauer spectrum of Al-MIL-53-FcDC shows a characteristic doublet with an isomeric shift of 0.34 mm s-1 and a quadrupole splitting of 2.39 mm s-1, proving the persistence of the ferrocene moiety. A negligibly small amount of impurities of ferrocenium ions could be detected by EPR spectroscopy as a complementary technique. Cyclic voltammetric experiments demonstrated the accessible redox activity of the linker molecule FcDC2- in Al-MIL-53-FcDC. A reversible oxidation and reduction signal (0.75 V and 0.64 V, respectively, vs. Ag) of FcDC2- was observed and maintained during forty CV cycles, while the crystallinity of the MOF remained unchanged after the experiment.

7.
Nat Commun ; 10(1): 3025, 2019 Jul 09.
Artigo em Inglês | MEDLINE | ID: mdl-31289274

RESUMO

Efficient use of energy for cooling applications is a very important and challenging field in science. Ultra-low temperature actuated (Tdriving < 80 °C) adsorption-driven chillers (ADCs) with water as the cooling agent are one environmentally benign option. The nanoscale metal-organic framework [Al(OH)(C6H2O4S)] denoted CAU-23 was discovered that possess favorable properties, including water adsorption capacity of 0.37 gH2O/gsorbent around p/p0 = 0.3 and cycling stability of at least 5000 cycles. Most importantly the material has a driving temperature down to 60 °C, which allows for the exploitation of yet mostly unused temperature sources and a more efficient use of energy. These exceptional properties are due to its unique crystal structure, which was unequivocally elucidated by single crystal electron diffraction. Monte Carlo simulations were performed to reveal the water adsorption mechanism at the atomic level. With its green synthesis, CAU-23 is an ideal material to realize ultra-low temperature driven ADC devices.

8.
Dalton Trans ; 48(23): 8433-8441, 2019 Jun 21.
Artigo em Inglês | MEDLINE | ID: mdl-31116214

RESUMO

Six different chiral and achiral alkane dicarboxylic C4-acids, i.e. succinic acid (H2SUC), dl-2-methylsuccinic acid (H2MS), 2,3-dimethylsuccinic acid (H2DMS) and aspartic acid (d-, l- and dl-H2ASP), were used to obtain Ce(iv)-MOFs via microwave assisted reactions. In water-based syntheses, MOFs with three different topologies, denoted as UiO-66 (fcu), CAU-41 (bcu) and CAU-44 (bct), were obtained within 30 min under mild reaction conditions. The MOFs were fully characterized and their structures were refined from PXRD data. The chirality of the incorporated linker molecules was confirmed by circular dichroism spectroscopy. The optical activities were also investigated by second-harmonic generation (SHG) measurements. The use of H2MS, H2DMS and H2ASP leads to the formation of a UiO-66-type structure, and a hydrated form of the common hexanuclear cluster, i.e. [Ce6(µ3-O)4(µ3-OH)4(H2O)6]12+ was observed. Rietveld refinement of the PXRD data showed an ordered arrangement of the d- and l-enantiomers in Ce-UiO-66-dl-ASP. By employing H2SUC as the linker, the other two title compounds are obtained. CAU-41, [Ce6(µ3-O)4(µ3-OH)4(SUC)4(OH)4(H2O)4], exhibits the well-known eight-fold connectivity of the hexanuclear cluster, whereas in CAU-44, [Ce6(µ3-O)4(µ3-OH)4(SUC)4(NO3)2(OH)2(H2O)2], the connection of the clusters is also achieved by nitrate ions resulting in a framework with bct topology.

9.
Angew Chem Int Ed Engl ; 58(32): 10995-11000, 2019 Aug 05.
Artigo em Inglês | MEDLINE | ID: mdl-31087755

RESUMO

Two new zirconium-based metal-organic frameworks with the composition [Zr6 O4 (OH)4 (OAc)6 (BDC)3 ] (CAU-26) and [Zr5 O4 (OH)4 (OAc)4 (BDC)2 ] (CAU-27) are reported, which were synthesized from acetic acid, a rarely utilized but green and sustainable solvent (BDC2- : 1,4-benzenedicarboxylate). Structure determination aided by automated electron diffraction tomography revealed that CAU-26 is composed of layers of well-known {Zr6 O8 } clusters interconnected by terephthalate ions. In contrast CAU-27 exhibits a three-dimensional structure with a so far unknown type of one-dimensional inorganic building unit (IBU), which can be rationalized as condensed polyhedron-sharing chains of {Zr6 O8 } clusters. CAU-26 occurs as an intermediate of the CAU-27 synthesis and can be isolated easily, when reaction temperature and time are decreased. We were also able to synthesize two isoreticular derivatives of CAU-27 with extended linker molecules by implementing 4,4'-biphenyldicarboxylic acid (H2 BPDC) and 5,5'-dicarboxy-2,2'-bipyridine (H2 BIPY). All materials show high thermal and chemical stability as well as permanent microporosity. The excellent stability of CAU-27-BIPY was exploited to synthesize a performant iridium-supported heterogeneous MOF-based catalyst for the direct C-H borylation of arenes.

10.
Chem Sci ; 10(12): 3616-3622, 2019 Mar 28.
Artigo em Inglês | MEDLINE | ID: mdl-30996954

RESUMO

C-H activation reactions are generally associated with relatively low turnover numbers (TONs) and high catalyst concentrations due to a combination of low catalyst stability and activity, highlighting the need for recyclable heterogeneous catalysts with stable single-atom active sites. In this work, several palladium loaded metal-organic frameworks (MOFs) were tested as single-site catalysts for the oxidative coupling of arenes (e.g. o-xylene) via C-H/C-H activation. Isolation of the palladium active sites on the MOF supports reduced Pd(0) aggregate formation and thus catalyst deactivation, resulting in higher turnover numbers (TONs) compared to the homogeneous benchmark reaction. Notably, a threefold higher TON could be achieved for palladium loaded MOF-808 due to increased catalyst stability and the heterogeneous catalyst could efficiently be reused, resulting in a cumulative TON of 1218 after three runs. Additionally, the palladium single-atom active sites on MOF-808 were successfully identified by Fourier transform infrared (FTIR) and extended X-ray absorption fine structure (EXAFS) spectroscopy.

11.
ACS Appl Mater Interfaces ; 11(5): 5325-5335, 2019 Feb 06.
Artigo em Inglês | MEDLINE | ID: mdl-30600988

RESUMO

Carbon-based fibrous scaffolds are highly attractive for all biomaterial applications that require electrical conductivity. It is additionally advantageous if such materials resembled the structural and biochemical features of the natural extracellular environment. Here, we show a novel modular design strategy to engineer biomimetic carbon fiber-based scaffolds. Highly porous ceramic zinc oxide (ZnO) microstructures serve as three-dimensional (3D) sacrificial templates and are infiltrated with carbon nanotubes (CNTs) or graphene dispersions. Once the CNTs and graphene coat the ZnO template, the ZnO is either removed by hydrolysis or converted into carbon by chemical vapor deposition. The resulting 3D carbon scaffolds are both hierarchically ordered and free-standing. The properties of the microfibrous scaffolds were tailored with a high porosity (up to 93%), a high Young's modulus (ca. 0.027-22 MPa), and an electrical conductivity of ca. 0.1-330 S/m, as well as different surface compositions. Cell viability, fibroblast proliferation rate and protein adsorption rate assays have shown that the generated scaffolds are biocompatible and have a high protein adsorption capacity (up to 77.32 ± 6.95 mg/cm3) so that they are able to resemble the extracellular matrix not only structurally but also biochemically. The scaffolds also allow for the successful growth and adhesion of fibroblast cells, showing that we provide a novel, highly scalable modular design strategy to generate biocompatible carbon fiber systems that mimic the extracellular matrix with the additional feature of conductivity.

12.
Angew Chem Int Ed Engl ; 58(8): 2423-2427, 2019 Feb 18.
Artigo em Inglês | MEDLINE | ID: mdl-30548136

RESUMO

Metal-organic frameworks (MOFs) enable the design of host-guest systems with specific properties. In this work, we show how the confinement of anthracene in a well-chosen MOF host leads to reversible yellow-to-purple photoswitching of the fluorescence emission. This behavior has not been observed before for anthracene, either in pure form or adsorbed in other porous hosts. The photoresponse of the host-guest system is caused by the photodimerization of anthracene, which is greatly facilitated by the pore geometry, connectivity, and volume as well as the structural flexibility of the MOF host. The photoswitching behavior was used to fabricate photopatternable and erasable surfaces that, in combination with data encryption and decryption, hold promise in product authentication and secure communication applications.

13.
J Am Chem Soc ; 140(50): 17379-17383, 2018 Dec 19.
Artigo em Inglês | MEDLINE | ID: mdl-30497258

RESUMO

Bimetallic Ce/Zr-UiO-66 metal-organic frameworks (MOFs) proved to be promising materials for various catalytic redox applications, representing, together with other bimetallic MOFs, a new generation of porous structures. However, no direct proof for the presence of both metals in a single cornerstone of UiO-type MOFs was reported so far. Employing element-selective X-ray absorption spectroscopy techniques herein, we demonstrate, for the first time, that our synthesis route allows obtaining Ce/Zr-UiO-66 MOFs with desired Ce content and bimetallic CeZr5 cornerstones. Performing multiple-edge extended X-ray absorption fine structure analysis, we determine the exact stoichiometry of the cornerstones, which explains the dependence of thermal and chemical stability of the materials on Ce content.

14.
Chemistry ; 24(66): 17429-17433, 2018 Nov 27.
Artigo em Inglês | MEDLINE | ID: mdl-30288800

RESUMO

Single-crystal electron diffraction has shown to be powerful for structure determination of nano- and submicron-sized crystals that are too small to be studied by single-crystal X-ray diffraction. However, it has been very challenging to obtain high quality electron diffraction data from beam sensitive crystals such as metal-organic frameworks (MOFs). It is even more difficult to locate guest species in the pores of MOF crystals. Here, we present synthesis of a novel porous cobalt MOF with 1D channels, [Co2 (Ni-H4 TPPP)]⋅2 DABCO⋅6 H2 O, (denoted Co-CAU-36; DABCO=1,4-diazabicyclo[2.2.2]octane), and its structure determination using continuous rotation electron diffraction (cRED) data. By combining a fast hybrid electron detector with low sample temperature (96 K), high resolution (0.83-1.00 Å) cRED data could be obtained from eight Co-CAU-36 crystals. Independent structure determinations were conducted using each of the eight cRED datasets. We show that all atoms in the MOF framework could be located. More importantly, we demonstrate for the first time that organic molecules in the pores, which were previously difficult to find, could be located using the cRED data. A comparison of eight independent structure determinations using different datasets shows that structural models differ only on average by 0.03(2) Šfor the framework atoms and 0.10(6) and 0.16(12) Šfor DABCO and water molecules, respectively.

15.
Inorg Chem ; 57(20): 12820-12826, 2018 Oct 15.
Artigo em Inglês | MEDLINE | ID: mdl-30256108

RESUMO

1- H-Pyrazole-3,5-dicarboxylic acid (H2PZDC), a small, strongly bent linker molecule with an angle of 147.4° between the carboxylate groups, was used in the synthesis of metal organic frameworks (MOFs) with fcu, bcu and reo topology. In systematic studies of the chemical system Ce4+/Zr4+/H2PZDC/HCOOH, their fields of formations were established. The decisive factors for the product formation and hence the transition between the framework topologies are the HCOOH/metal ratio and the molar ratio of Ce4+/Zr4+ employed in the synthesis. All title compounds crystallize with the well-known hexanuclear cluster {M6(µ3-O)4(µ3-OH)4(-CO2) n}, with n = 8 or 12 and M = Ce4+ and Zr4+, as the inorganic building unit (IBU). Connection through 12 or eight linker molecules leads to three framework topologies: fcu, bcu, and reo, respectively. The dominant phase observed in this system crystallizes with reo topology and is known as DUT-67. The pure Zr-MOF of composition [Zr6(µ3-O)4(µ3-OH)4(PZDC)4(OH)2(H2O)2] (Zr-DUT-67-PZDC) as well as the mixed-metal compounds Ce/Zr-DUT-67-PZDC are accessible and the molar ratio Ce4+/Zr4+ can be adjusted between 0 and 1. At low HCOOH/metal ratios, surprisingly, the UiO-66 type structure with fcu topology is formed despite the nonlinear geometry of the linker. Thus, using exclusively Zr4+ ions in the starting mixture the pure Zr-MOF with ideal composition [Zr6(µ3-O)4(µ3-OH)4(PZDC)6] (Zr-UiO-66-PZDC) was obtained. Variation of the Ce/Zr molar ratio leads to a continuous increase in linker defects with increasing Ce content in the MOF. At a Ce/Zr value of ∼ 1:1 a transition from the fcu to the reo framework topology takes place. Using high HCOOH/metal ratios, a transition from the reo to the bcu topology is observed when a molar ratio of Ce/Zr ≥ 1:5 is employed. Irrespective of the molar ratio used in the reaction mixture, the mixed-metal MOF of composition [CeZr5(µ3-O)4(µ3-OH)4(PZDC)4(OH)2(H2O)2] (Ce/Zr-CAU-38-PZDC) is always formed as confirmed by comprehensive EDX analyses. Rietveld refinement strongly indicates the presence of exclusively hexanuclear {CeZr5(µ3-O)4(µ3-OH)4} clusters and thus CAU-38 is the first Ce/Zr-MOF which solely occurs at a specific metal stoichiometry. In addition to the detailed synthetic study, the compounds were thoroughly characterized regarding their composition, lattice parameters, and porosity, as well as thermal and chemical stability.

16.
Inorg Chem ; 57(16): 10352-10363, 2018 Aug 20.
Artigo em Inglês | MEDLINE | ID: mdl-30070474

RESUMO

With the employment of high-throughput methods, the system Bi3+/4,8-disulfonyl-2,6-naphthalenedicarboxylic acid (H4DSNDC)/H2O/additive (HNO3 or NaOH) was systematically investigated under hydrothermal reaction conditions. The influence of the molar ratio of the starting materials, pH, and reaction temperature and time was investigated in more than 500 reactions. The product formation is highly sensitive toward small changes of the synthesis parameters, but six new bismuth sulfonatocarboxylates were reproducibly obtained starting from clear solutions of the reactants. All compounds were structurally characterized from single-crystal X-ray diffraction data. Fully deprotonated linker ions are found in [Bi6O6(OH)2(H2O)4(DSNDC)] (1), [Bi2(OH)2(DSNDC)] (2), [Bi8O7(OH)2(H2O)2(DSNDC)2] (3), and [Bi7O5(OH)3(H2O)4(DSNDC)2]·4H2O (4), while the presence of larger amounts of acid or short reaction times leads to compounds with noncoordinating -COOH groups, [Bi2(OH)2(H2O)2(DSNDC)(H2DSNDC)] (5) and [Bi6O4(OH)4(H2O)12(H2DSNDC)3]· xH2O (6), respectively. The inorganic building units (IBUs) in all six structures differ substantially from each other; the IBUs found in 2 ({Bi2(OH)2}), 5 (BiO8 polyhedron), and 6 ({Bi6O4(OH)4} cluster) have been reported in the literature, while new IBUs are observed for 1 (chains of composition {Bi3O3(OH)}∞), 3 ({Bi16O14(OH)4} cluster), and 4 ({Bi7O5(OH)3} cluster). Systematic variation of the reaction temperature and time indicated their distinct influence on product formation. Hence, in situ powder X-ray diffraction measurements at Deutsches Elektronen-Synchrotron, Hamburg, Germany, employing synchrotron radiation were carried out. In all studied in situ reactions, compound 6 is first observed and subsequently transformed to 1, 2, 4, and 5, depending on the reaction time and temperature as well as concentration of the starting materials.

17.
Chem Sci ; 9(24): 5467-5478, 2018 Jun 28.
Artigo em Inglês | MEDLINE | ID: mdl-30009015

RESUMO

The Ni-metallated porphyrin-based tetraphosphonic acid (Ni-tetra(4-phosphonophenyl)porphyrin, Ni-H8TPPP) was used for the synthesis of highly porous metal phosphonates containing the tetravalent cations Zr4+ and Hf4+. The compounds were thoroughly characterized regarding their sorption properties towards N2 and H2O as well as thermal and chemical stability. During the synthesis optimization the reaction time could be substantially decreased under stirring from 24 to 3 h in glass vials. M-CAU-30, [M2(Ni-H2TPPP)(OH/F)2]·H2O (M = Zr, Hf) shows exceptionally high specific surface areas for metal phosphonates of aBET = 1070 and 1030 m2 g-1 for Zr- and Hf-CAU-30, respectively, which are very close/correspond to the theoretical values of 1180 and 1030 m2 g-1. CAU-30 is always obtained as mixtures with one mol ZrO2/HfO2 per formula unit as proven by TEM, electron diffraction, TG and elemental analysis. Hence experimentally derived specific surface areas are 970 and 910 m2 g-1, respectively. M-CAU-30 is chemically stable in the pH range 0 to 12 in HCl/NaOH and thermally up to 420 °C in air as determined by variable-temperature powder X-ray diffraction (VT-PXRD). The crystal structure of M-CAU-30 was determined by combining electron diffraction tomography for structure solution and powder X-ray diffraction data for the structure refinement. The crystal structure consists of chains of corner sharing MO6 octahedra interconnected by the partly deprotonated linker molecules Ni-H2TPPP6-. Thus 1D channels with pore diameters of 1.3 × 2.0 nm are formed. The redox activity of Zr-CAU-30 was investigated by cyclic voltammetry resulting in a reversible redox process at a half-wave potential of E1/2 = -0.649 V.

18.
J Am Chem Soc ; 140(26): 8206-8217, 2018 07 05.
Artigo em Inglês | MEDLINE | ID: mdl-29890070

RESUMO

The mechanism of the Heck C-C coupling reaction catalyzed by Pd@MOFs has been investigated using operando X-ray absorption spectroscopy (XAS) and powder X-ray diffraction (PXRD) combined with transmission electron microscopy (TEM) analysis and nuclear magnetic resonance (1H NMR) kinetic studies. A custom-made reaction cell was used, allowing operando PXRD and XAS data collection using high-energy synchrotron radiation. By analyzing the XAS data in combination with ex situ studies, the evolution of the palladium species is followed from the as-synthesized to its deactivated form. An adaptive reaction mechanism is proposed. Mononuclear Pd(II) complexes are found to be the dominant active species at the beginning of the reaction, which then gradually transform into Pd nanoclusters with 13-20 Pd atoms on average in later catalytic turnovers. Consumption of available reagent and substrate leads to coordination of Cl- ions to their surfaces, which causes the poisoning of the active sites. By understanding the deactivation process, it was possible to tune the reaction conditions and prolong the lifetime of the catalyst.

19.
Chemphyschem ; 19(4): 373-378, 2018 02 19.
Artigo em Inglês | MEDLINE | ID: mdl-29027736

RESUMO

The introduction of Ce4+ as a structural cation has been shown to be a promising route to redox active metal-organic frameworks (MOFs). However, the mechanism by which these MOFs act as redox catalysts remains unclear. Herein, we present a detailed study of the active site in [Ce6 O4 (OH)4 ]-based MOFs such as Ce-UiO-66, involved in the aerobic oxidation of benzyl alcohol, chosen as a model redox reaction. X-ray absorption spectroscopy (XAS) data confirm the reduction of up to one Ce4+ ion per Ce6 cluster with a corresponding outwards radial shift due to the larger radius of the Ce3+ cation, while not compromising the structural integrity of the framework, as evidenced by powder X-ray diffraction. This unambiguously demonstrates the involvement of the metal node in the catalytic cycle and explains the need for 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) as a redox mediator to bridge the gap between the one-electron oxidation of the Ce4+ /Ce3+ couple and the two-electron alcohol oxidation. Finally, an improved catalytic system with Ce-MOF-808 and TEMPO was developed which outperformed all other tested Ce4+ -MOFs.

20.
Chem Commun (Camb) ; 54(8): 876-879, 2018 Jan 23.
Artigo em Inglês | MEDLINE | ID: mdl-29236104

RESUMO

A precursor method has been developed to synthesize Ce(iv) MOFs that could not be prepared directly from Ce(iv) salts. Starting from Ce6 clusters, two Ce-UiO-66 analogues and four tetracarboxylate-based Ce(iv) MOFs could be synthesized. The applied method facilitates framework formation by evading reactive individual Ce(iv)-ions thereby paving the way for further development of Ce-MOFs.

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