Zirconium Oxoclusters Research Articles

Ultrahigh surface area zirconium MOFs and insights into the applicability of the BET theory.

An isoreticular series of metal-organic frameworks (MOFs) with the ftw topology based on zirconium oxoclusters and tetracarboxylate linkers with a planar core (NU-1101 through NU-1104) has been synthesized employing a linker expansion approach. In this series, NU-1103 has a pore volume of 2.91 cc g(-1) and a geometrically calculated surface area of 5646 m(2) g(-1), which is the highest value reported to date for a zirconium-based MOF and among the largest that have been reported for any porous material. Successful activation of the MOFs was proven based on the agreement of pore volumes and BET areas obtained from simulated and experimental isotherms. Critical for practical applications, NU-1103 combines for the first time ultrahigh surface area and water stability, where this material retained complete structural integrity after soaking in water. Pressure range selection for the BET calculations on these materials was guided by the four so-called "consistency criteria". The experimental BET area of NU-1103 was 6550 m(2) g(-1). Insights obtained from molecular simulation suggest that, as a consequence of pore-filling contamination, the BET method overestimates the monolayer loading of NU-1103 by ∼16%.

Oxidative degradation of phenanthrene in the absence of light irradiatiion by hybrid ZrO2-acetylacetonate gel-derived catalyst

The intrinsic catalytic activity of a hybrid gel-derived ZrO2-acetylacetonate (HSGZ) material towards the oxidative degradation of phenanthrene (PHE), in aqueous solution and in the dark, was revealed for the first time.The HSGZ catalyst is a polymeric network of zirconium oxo-clusters on the surface of which part of Zr4+ ions are involved in strong complexation with acetylacetonate (acac) ligands. The HSGZ gave significant PHE degradation rates acting as radicals initiator without any light irradiation at 30°C.Free radicals were formed on the solid surface by the coexistence of Zr(IV)-acac and Zr(III)-acac• complexes in equilibrium at a given temperature, from which reactive oxygen species were produced in the presence of molecular O2. A direct evidence of radical's presence on HSGZ solid surface was obtained by EPR spectroscopy.The analysis of the degradation products confirmed that the reaction goes on through the formation of intermediate free radicals, leading to the first ring-opening and to the formation of phthalates as main intermediates. Subsequently, low molecular weight alkanes are produced. Finally, a deep oxidation of the intermediates occurs completing the mineralization process. The HSGZ catalyst showed a good stability under the reaction conditions, retaining its catalytic activity after repeated tests.

Hybrid organic–inorganic materials as coatings for protecting wood

A new coating based on organic–inorganic materials was prepared using concurrent sol–gel and polymerization techniques, and applied to wood using a dip coating method. Vinyl-functionalized zirconium oxoclusters were co-polymerized with vinyltrimethoxysilane on wood. The coating process was examined, also assessing the specific weight of hybrid polymer left on the wood after one or two coating steps. The efficacy of the process in consolidating and protecting the wood was investigated using high-temperature differential scanning calorimetry (DSC), environmental scanning electron microscopy (ESEM), infrared spectroscopy and solid state NMR spectroscopy. The coating did not affect the morphology and appearance of the wood. However, it did modify its behavior on exposure to fire and preliminary accelerated biological tests with the brown rot fungus Coniophora puteana showing an improved resistance to the fungal attack.

Use of a New Hybrid Sol–Gel Zirconia Matrix in the Removal of the Herbicide MCPA: A Sorption/Degradation Process

A class II hybrid sol-gel material was prepared starting from zirconium(IV) propoxide and 2,4-pentanedione and its catalytic activity in the removal of the herbicide 4-chloro-2-methylphenoxyacetic acid (MCPA) was revealed. The thermal and structural characterization, performed by thermogravimetry, differential thermal analysis, and diffuse reflectance Fourier transform infrared spectroscopy, demonstrated the hybrid nature of the material. The structure of the material can be described as a polymeric network of zirconium oxo clusters, on the surface of which large part of Zr(4+) ions are involved in strong complexation equilibria with acetylacetonate (acac) ligands. The incubation of MCPA in the presence of this material yielded an herbicide removal fraction up to 98%. A two-step mechanism was proposed for the MCPA removal, in which a reversible first-order adsorption of the herbicide is followed by its catalytic degradation. The nature of the products of the MCPA catalytic degradation as well as the reaction conditions adopted do not support typical oxidation pathways involving radicals, suggesting the existence of a different mechanism in which the Zr(4+):acac enol-type complex can act as Lewis acid catalyst.

Optical filters and resonant cavities based on di-ureasil organic–inorganic hybrids

Sol–gel derived poly(oxyethylene)/siloxane organic–inorganic di-ureasil hybrids containing different amounts (20–60% mol) of methacrylic acid (McOH) modified zirconium oxo-clusters (Zr-OMc) were processed as thin films and transparent and shape controlled monoliths. Laser direct writing was used to create channel waveguides, Bragg gratings, Fabry–Perot cavities and optical filters. The resulting Fabry–Perot optical cavity displays a free spectral range of 16.55 GHz and a fringe intensity contrast of 5.35 dB. Optical rejection values between 6.7 and 10.4 dB were obtained by varying the amount of the Zr-OMc oxo-clusters.

Hybrid organic–inorganic materials using zirconium based NBBs and vinyl trimethoxysilane: Effect of pre-hydrolysis of silane

Various Nano Building Blocks (NBBs) such as polyhedral silsesquioxanes (POSS), functionalized metal oxide particles and transition metal oxoclusters have been already developed and used to improve thermal and mechanical properties of organic polymers. The NBBs ideal for the preparation of hybrid materials and nanocomposites are monodispersed, well-defined objects capped with polymerisable functions suitable for copolymerisation with organic monomers.In this study zirconium oxoclusters (ZrNBB) were obtained as a crystalline precipitate by reaction between zirconium propoxide and vinylacetic acid. They were post-functionalised by copolymerisation with vinyl trimethoxysilane in different molar ratios. The hybrid samples were prepared both with the organosilane pre-hydrolysis step and without adding water. Hybrid materials were obtained through the radical polymerization process by adding benzoyl peroxide (BPO). Silane pre-hydrolysis prevents bulk samples from being obtained.The polymerization process was studied using differential scanning calorimetry (DSC), and the shear storage modulus (G′) and loss modulus (G″) of hybrid polymers were investigated by dynamic mechanical spectroscopy (DMS). Multinuclear liquid- and solid-state NMR analyses and Fourier transform infrared (FTIR) spectroscopy were used to characterize the reagents and hybrid materials, and to study the influence of the synthesis conditions on condensation and polymerization.

Ligand dynamics on the surface of zirconium oxo clusters

The dynamics of carboxylate ligands on the surface of zirconium oxo clusters was investigated in two case studies. Zr4O2(methacrylate)12 was investigated by one- and two dimensional NMR spectra both in the solid state and in solution. In solution, the cluster is C2h symmetric; stepwise intramolecular exchange of the four non-equivalent ligands was observed when the temperature was raised from -80 degrees C to -50 degrees C. The individual exchange processes were assigned to different ligand positions. Ab initio molecular dynamics simulations were performed for Zr6(OH)4O4(formate)12 to study the trajectory for the rearrangement of three chelating ligands into bridging positions, i.e. the conversion of the C3-symmetric into an Oh-symmetric cluster. The observation of a dip in the energy barrier along the reaction coordinate was related to the intermediate formation of hydrogen bonds between the moving oxygen atom of the rearranging ligand and a micro3-OH group of the cluster. Thus, the motion of the ligand requires a concerted motion in three dimensions.

UV-photopolymerisation of poly(methyl methacrylate)-based inorganic–organic hybrid coatings and bulk samples reinforced with methacrylate-modified zirconium oxocluster

In this work, we developed and optimised a synthetic route which enables to produce by spray-coating hard, transparent and stable inorganic–organic hybrid coatings for a wide variety of different substrates (e.g. stone, stainless steel, polymethylmethacrylate, polyethylene, wood, anodized aluminum). Chemically and thermally stable acrylate-based hybrid materials embedding the zirconium oxocluster Zr4O2(OMc)12, OMc(CH2C(CH3)C(O)O) were prepared and UV-cured.The coatings of different compositions on different substrates were characterized by numerous analytical and spectroscopic methods such as FT-IR spectroscopy, Angle Resolved-XPS (AR-XPS), Secondary Ion Mass Spectrometry (SIMS), X-ray Absorption Spectroscopy (XAS), Environmental Scanning Electron Microscopy (ESEM) and Energy Dispersive X-ray analysis (EDX). Bulk samples were also prepared for additional characterizations. The bulk samples were analysed by FT-IR, whereas the cross-linking degree was qualitatively evaluated by swelling experiments. As far as the mechanical properties are concerned, the shear storage modulus (G′) and loss modulus (G″) were measured by Dynamical Mechanical Thermal Analysis (DMTA) technique. Moreover, the best conditions for the curing and cross-linking processes of the hybrid materials were studied up to 200°C by using Differential Scanning Calorimetry (DSC). The thermal stability of the hybrid samples was evaluated by Thermogravimetric Analysis (TGA).

Waveguides and gratings fabrication in zirconium-based organic/inorganic hybrids

Sol–gel derived poly(oxyethylene)/siloxane organic/inorganic di-ureasil hybrids containing different amounts of methacrylic acid (McOH, CH2=C(CH3)COOH)) modified zirconium oxo-clusters (Zr-OMc) were processed as thin films deposited in glassy substrates via spin coating and as transparent and shape controlled monoliths. Channel monomode waveguides and diffraction gratings were UV patterned using the Talbot interferometer and the Lloyd mirror interferometer experimental setups. The time dependence of the diffraction gratings efficiency was studied for hybrids containing different amounts of Zr-OMc. Finally, the number of propagating modes and the refractive index gradient within the waveguide region, determined as a Gaussian section located below the patterned channel, was evaluated and modeled, a maximum index contrast of 2.43 × 10−5 being estimated.

Si and Zr based NBBS for hybrid O/I macromolecular materials starting by preformed zirconium oxo-clusters

In this study surface-functionalized zirconium oxoclusters (ZrNBB), formed by reaction between vinylacetic acid and zirconium propoxide, were chemically modified through condensation with siloxane groups, so that the core integrity of ZrNBB was preserved. The direct coupling between zirconium oxo-clusters and siloxane moiety was performed by co-polymerisation with vinyl trimethoxysilane (VTMS) with Zr:Si molar ratio 1:2 in air at room temperature and benzoyl peroxide (BPO) was used as an initiator to start radical polymerization. The polymerisation was studied by differential scanning calorimetry (DSC) and Fourier Transform Infra Red (FTIR) spectroscopy. 29Si and 13C liquid and solid state NMR analyses were performed in order to study the microstructure of the hybrid material and the role of the clusters in polymerization and condensation. Beyond this, shear storage modulus (G′) and loss modulus (G′′), were investigated by dynamical mechanical spectroscopy (DMS). The results indicated a great stability at high temperature without any viscous flow during glass transition.

Inorganic–organic hybrid materials with zirconium oxoclusters as protective coatings on aluminium alloys

Inorganic–organic hybrid materials are attracting a strong scientific interest mainly for their outstanding inherent mechanical and thermal properties, which can be traced back to the intimate coupling of both inorganic and organic components. By carefully choosing the experimental parameters used for their synthesis, chemically and thermally stable acrylate-based hybrid material embedding the zirconium oxocluster Zr 4O 4(OMc) 12, where OMc CH 2 C(CH 3)C(O)O, can be deposited as UV-cured films on aluminium alloys. In particular, the molar ratios between the oxocluster and the monomer, the polymerisation time, the amount of photo-initiator and the deposition conditions, by using an home-made spray-coating equipment, were optimised in order to obtain the best performing layers in terms of transparency and hardness to coat aluminium alloy (AA1050, AA6060 and AA2024) sheets. Furthermore, it was also evaluated whether the hybrid coatings behave as barrier to corrosion. Several coated samples were prepared and characterised. Environmental scanning electronic microscopy (ESEM) and scratch test were used to investigate the morphology of the films and to evaluate their scratch resistance, respectively. Electrochemical impedance spectroscopy (EIS) was performed in order to evaluate if the coatings actually protect the metallic substrate from corrosion. In order to measure shear storage modulus ( G′) and loss modulus ( G″) of the materials used for coatings, bulk samples were also obtained by UV-curing of the precursors solution. Dynamical mechanical thermal analysis (DMTA) was performed in shear mode on cured disks of both the hybrid materials and pristine polymer for comparison. The values of T g were read off as the temperatures of peak of loss modulus. The length and mass of all the samples were measured before and after the DMTA analysis, so that the shrinkage of the materials in that temperature range was exactly evaluated.

Thiol‐ene Hybrid Organic/Inorganic Nanostructured Coatings Based on Thiol‐Functionalized Zirconium Oxoclusters

AbstractThe thiol‐functionalized zirconium oxocluster Zr12(µ3‐O)8(µ3‐OH)8(MP)24 · 4MPA was used as inorganic nanosized building block in the thiol‐ene photopolymerization of APE and TH in a 1:1 molar mixture. Transparent and crack‐free coatings were obtained, and TEM analysis showed that the inorganic particles are well dispersed within the polymeric network with no significant macroscopic agglomeration. An increase of Tg values, storage modulus in the rubbery region, and thermal stability were evidenced by increasing the zirconium oxocluster content in the photocurable formulations. XPS analysis and SIMS depth profile were carried out on UV cured films and showed the presence of a homogeneously distributed zirconium oxocluster.magnified image

Suppression of Crazing in Polystyrene Crosslinked with a Multifunctional Zirconium Oxo Cluster Observed In Situ during Tensile Tests

AbstractThe structural development of polystyrene, which was crosslinked with varying amounts of the multifunctional cluster Zr6O4(OH)4(methacrylate)12, was followed in situ by small‐angle X‐ray scattering at a synchrotron radiation source. It was observed that crazing strongly diminishes with increasing cluster proportion and disappears nearly completely for the material with the highest cluster proportion of 0.87 mol‐%. Accordingly, the maximum stress before failure is strongly enhanced, whereas the maximum strain decreases.magnified image

Hybrid Organic‐Inorganic Nanostructured Acrylic Films Based on Methacylate Modified Zirconium Oxocluster

AbstractOrganic‐inorganic nanostructured acrylic films, based on methacrylate‐modified zirconium oxocluster, were prepared by UV‐induced polymerization, which yielded transparent and crack‐free layers. TEM analysis evidenced that all the inorganic particles are well dispersed with no significant macroscopic agglomerations. FT‐IR kinetic investigations showed that the presence of the zirconium oxocluster does not affect the photopolymerization rate or acrylic double bond conversion. An increase on Tg values of the cured films is evident at 15 wt.‐% load of the inorganic filler. A thermal stability increase was also achieved in the presence of the zirconium oxocluster. Surface studies performed by XPS analyses showed the presence of homogeneously distributed zirconium oxoclusters, which induces important improvements of the surface hardness evidenced on the basis of the pencil test.magnified image

Crosslinking vs. filler effect of carboxylate-substituted zirconium oxo clusters on the thermal stability of polystyrene

Inorganic–organic hybrid materials were prepared by step-wise free radical polymerization of styrene in the presence of zirconium oxo clusters of the general compositions Zr6O4(OH)4(carboxylate)12 (carboxylate=5-norbornene-2-carboxylate or isobutyrate/methacrylate) and Zr12O8(OH)8(carboxylate)24 (carboxylate=acetate/propionate, vinylacetate, or acetate/methacrylate). Clusters with non-polymerizable ligands resulted in cluster/polystyrene blends which were soluble in toluene, whereas clusters with polymerizable ligands gave cluster-crosslinked, swellable polymers for which solvent uptake correlated with the functionality of the corresponding cluster. The onset temperatures of thermal decomposition and the glass transition temperatures of all cluster-containing polymers were higher than that of neat polystyrene, independent of whether the clusters were crosslinking or blended into the polymer.

ω-Mercapto-functionalized hafnium- and zirconium-oxoclusters as nanosized building blocks for inorganic–organic hybrid materials: synthesis, characterization and photothiol-ene polymerization

Two isostructural mercapto-functionalized zirconium- and hafnium-oxoclusters [M12(μ3-O)8(μ3-OH)8(MP)24·n(MPA), MPA = HS–(CH2)2–C(O)OH; MP = HS–(CH2)2–C(O)O−; M= Zr, Hf; n = 4 for Zr, n = 5 for Hf] were prepared by reacting 3-mercaptopropionic acid with zirconium and hafnium butoxide, respectively, in an alkoxide : acid 1 : 7 molar ratio. The two oxoclusters Zr12 and Hf12, which are composed of two Zr6 and Hf6 sub-units bridged by four carboxylate ligands, were thoroughly characterized by single crystal X-ray diffraction, infrared spectroscopy (FT-IR), 1H and 13C nuclear magnetic resonance (NMR) spectroscopy, thermogravimetry (TGA), differential thermoanalysis (DTA) and extended X-ray absorption spectroscopy (EXAFS). The zirconium oxocluster was then embedded in a polymethacrylic matrix by a photothiol-ene polymerization reaction by using different cluster : monomer molar ratios. EXAFS measurements performed on the polymethacrylic-based hybrid materials showed that the structure of the oxocluster is retained even after being embedded in the polymer matrix. Solid-state 13C NMR spectroscopy as well as FT-IR spectroscopy demonstrated that the inorganic–organic hybrid materials are characterized by an almost complete polymerization of methacrylic acid, in which the oxoclusters are embedded in and covalently bound to the polymer matrix.

HfO2–ZrO2 Doped Silica Thin Films by XPS

Silica thin films embedding ZrO2 and HfO2 were prepared by spin-coating on silica glass via a modified sol-gel processing. The novel synthetic route is based on the co-polymerization of two organically modified oxozirconium and oxohafnium clusters (M4O2(OMc)12 with M = Zr, Hf and OMc = OC(O)–C(CH3)=CH2)) with (methacryloxypropyl)trimethoxysilane (MAPTMS). The crystalline clusters, which are the precursors for the corresponding metal oxides (MO2) were prepared via the sol-gel route by reacting zirconium or hafnium butoxide with methacrylic acid. The copolymerization of the cluster with previously prehydrolyzed methacrylate-functionalized siloxane, allows the anchoring of the oxoclusters to the forming silica network. Thin films were prepared starting from a THF (tetrahydrofurane) solution with molar ratios Hf4O2(OMc)12: Zr4O2(OMc)12:MAPTMS of 1:1:88. After deposition, the films were annealed 3 h at 800 °C in air to promote the decomposition of the hafnium and zirconium oxoclusters to give the corresponding HfO2 and ZrO2 oxides. The obtained HfO2–ZrO2–SiO2 films resulted transparent, homogeneous and displayed a very good adhesion to the substrate. The composition of the films was investigated by secondary ionization mass spectrometry (SIMS) and x-ray photoelectron spectroscopy (XPS). The depth profiles evidenced a very homogenous distribution of both zirconium or hafnium species within the whole silica films and sharp film-substrate interfaces. As far as XPS analyses are concerned, the main XPS core-levels were analyzed for the annealed sample and the formation of hafnium and zirconium oxides was evidenced.