Cr(VI) genesis and dynamics in Ferralsols developed from ultramafic rocks: The case of Niquelandia, Brazil

Despite high chromium concentrations in soils developed on ultramafic rocks, Cr availability is generally low, as Cr-bearing minerals are considered stable in supergene conditions. However, KH2PO4 extractions have shown high hexavalent chromium availability in Ferralsols developed on the ultramafic massif of Niquelandia (Brazil). A study combining mineralogical, geochemical, mu SXRF and XANES approaches was performed to assess the solid speciation of Cr and to understand its genesis. Chromium is mostly present in the trivalent form and is included in chromites and Fe-oxides. Nevertheless, a large amount of Cr(VI) is associated with the clay-sized fraction in the mineral horizons. Mn-oxides, the only natural Cr(III) oxidants, are also present in these horizons. Microscale investigations revealed the close association of Cr(VI) with Mn-oxides and strongly suggest that Mn-oxides can oxidize Cr(III) into Cr(VI). The XANES analyses show the occurrence of Cr(VI), which is removed by KH2PO4 extraction, demonstrating that the Cr(VI) is completely sorbed onto the soil matrix, i.e., Fe-oxides. Finally, the high mobility of Cr(VI), associated with the finest Fe-oxide particles mobilized during runoff following rainy events, may have harmful environmental consequences. This approach of combining direct and indirect observations allows us to characterize the total extractable Cr(VI) pool of these soils and to give key information on its mobility and localization

A Silica-Supported Double-Decker Silsesquioxane Provides a Second Skin for the Selective Generation of Bipodal Surface Organometallic Complexes

A well-defined silica-based material with a homogeneous nanolayer presenting identical pairs of vicinal silanols has been prepared by reaction of the surface organometallic species[ SiOZr(CH2CMe3)(3)], obtained on a silica dehydroxylated at 900 degrees C, with the double-decker-shaped silsesquioxane (OH)(2)DD(OH)(2). The surface structure has been established using extensive NMR characterization (H-1, C-13, Si-29, HETCOR, double-quantum, triple-quantum). Treatment with Zr(CH2CMe3)(4) leads to the first well-defined single-site bipodal grafted bis-neopentyl zirconium complex

A well-defined mesoporous amine silica surface via a selective treatment of SBA-15 with ammonia

2D double-quantum H-1-H-1 NMR unambiguously shows that the ``isolated'' Si-OH surface silanols of dehydroxylated SBA-15 are converted upon treatment with ammonia into single silylamine surface site Si-NH2. The ``gem'' di-silanols (= Si(OH)(2)) remain intact. Treatment using HMDS produces (= Si(OSiMe3)(2)) but leaves Si-NH2 untouched. The resulting surface is hydrophobic and stable

Solid-State NMR and DFT Studies on the Formation of Well-Defined Silica-Supported Tantallaaziridines: From Synthesis to Catalytic Application

Single-site, well-defined, silica-supported tantallaaziridine intermediates [Si-O-Ta((2)-NRCH2)(NMe2)(2)] [R=Me (2), Ph (3)] were prepared from silica-supported tetrakis(dimethylamido)tantalum [Si-O-Ta(NMe2)(4)] (1) and fully characterized by FTIR spectroscopy, elemental analysis, and H-1,C-13 HETCOR and DQ TQ solid-state (SS) NMR spectroscopy. The formation mechanism, by -H abstraction, was investigated by SS NMR spectroscopy and supported by DFT calculations. The C-H activation of the dimethylamide ligand is favored for R=Ph. The results from catalytic testing in the hydroaminoalkylation of alkenes were consistent with the N-alkyl aryl amine substrates being more efficient than N-dialkyl amines

Methane Reacts with Heteropolyacids Chemisorbed on Silica to Produce Acetic Acid under Soft Conditions

International audienceSelective functionalization of methane at moderate temperature is of crucial economic, environmental, and scientific importance. Here, we report that methane reacts with heteropolyacids (HPAs) chemisorbed on silica to produce acetic acid under soft conditions. Specially, when chemisorbed on silica, H4SiW12O40, H3PW12O40, H4SiMo12O40, and H3PMo12O40 activate the primary C-H bond of methane at room temperature and atmospheric pressure. With these systems, acetic acid is produced directly from methane, in a single step, in the absence of Pd and without adding CO. Extensive surface characterization by solid-state NMR spectroscopy, IR spectroscopy, cyclic voltammetry, and X-ray photoelectron spectroscopy suggests that C-H activation of methane is triggered by the protons in the HPA-silica interface with concerted reduction of the Keggin cage, leading to water formation and hydration of the interface. This is the simplest and mildest way reported to date to functionalize methane

Ethylene tetramerization with cationic chromium(I) complexes

Various [Cr(CO)(4)(P boolean AND P)](+) complexes have been synthesized and tested for the tetramerization of ethylene to give principally 1-octene. The effect of the anion has been investigated, and it has been found that for successful catalysis an extremely weakly coordinating anion is required. This study shows that when [Al(OC(CF3)(3))(4)](-) is employed as the anion, along with triethylaluminum and PNPiPr as the coordinated diphosphine, the system is active for the tetramerization of ethylene. This is the first example of Cr-I being used as a catalyst precursor for selective tetramerization and gives evidence toward the catalytic cycle acting through a Cr-I -&gt; Cr-III cationic mechanism.</p

Methane Reacts with Heteropolyacids Chemisorbed on Silica to Produce Acetic Acid under Soft Conditions

Selective functionalization of methane at moderate temperature is of crucial economic, environmental, and scientific importance. Here, we report that methane reacts with heteropolyacids (HPAs) chemisorbed on silica to produce acetic acid under soft conditions. Specially, when chemisorbed on silica, H4SiW12O40, H3PW12O40, H4SiMo12O40, and H3PMo12O40 activate the primary C-H bond of methane at room temperature and atmospheric pressure. With these systems, acetic acid is produced directly from methane, in a single step, in the absence of Pd and without adding CO. Extensive surface characterization by solid-state NMR spectroscopy, IR spectroscopy, cyclic voltammetry, and X-ray photoelectron spectroscopy suggests that C-H activation of methane is triggered by the protons in the HPA-silica interface with concerted reduction of the Keggin cage, leading to water formation and hydration of the interface. This is the simplest and mildest way reported to date to functionalize methane

Investigation of Surface Alkylation Strategy in SOMC: In Situ Generation of a Silica-Supported Tungsten Methyl Catalyst for Cyclooctane Metathesis

An efficient and potentially scalable method is described for the synthesis of the silica-supported complexes [(equivalent to Si-O)WMe5] and [(equivalent to:Si-O)WMe2(equivalent to CH)] obtained by in situ alkylation of the surface-grafted tungsten chloride [(equivalent to Si-O)WCl5] (1). [(equivalent to Si-O)WCl5] can be readily prepared by the reaction of commercially available and stable tungsten hexachloride WCI6 with partially dehydroxylated silica at 700 degrees C (SiO2-700). Further reaction with ZnMe2 at room temperature rapidly forms a mixture of surfacealkylated tungsten complexes. They were fully characterized by microanalysis, FTIR, mass balance, and solid-state NMR (H-1, C-13, H-1-C-13 HETCOR, H-1-H-1 double quantum and triple quantum) and identified as [(equivalent to Si-O)WMes] and another product, [(equivalent to Si-O)WMe2(equivalent to CH)]. The latter might have been generated by partial decomposition of the tungsten methyl chloride compound, which is formed during the stepwise alkylation of [(equivalent to Si-O)WCl3]. DFT calculations were carried out to check the relative stability of the tungsten methyl chloride intermediates and the feasibility of the reaction and corroborate the experimental results. This tungsten complex and its derivative were found to be active catalysts for the metathesis of cydooctane

Titanacalixarenes in Homogeneous Catalysis: Synthesis, Conformation and Catalytic Activity in Ethylene Polymerisation

International audienceVariously substituted titanacalixarenes were synthesised and tested for their performance as catalysts in ethylene polymerisation: p-tert-butylcalix[4]arene was derivatised to afford distal diether p-tert-butylcalix[4]arenes L1–L4, distal dihydroxy-depleted L5, proximal dihydroxy-depleted L6, proximal diether p-tert-butylcalix[4]arene L7, proximal intrabridged disiloxane L8 and proximal monoether L9. (Dichlorotitana)-p-tert-butylcalix[4]arenes 1–8 and (monochlorotitana)-p-tertbutylcalix[4]arene 9 have been successfully synthesised by reacting L1–L4 with TiCl4·2THF or L5–L9 with TiCl4. The structural conformation of complexes 1–9 in solution were determined by 1H NMR, 13C NMR, NOESY and COSY spectroscopic studies. Compounds 2, 3 and 4 have been structurally characterised by single-crystal X-ray diffraction. Upon activation with methylaluminoxane, compounds 1–8 revealed low to moderate activities for the production of high-density polyethylene (HDPE) to ultra-high-molecular-weight polyethylene (UHMWPE). 6/MAO gave the best activity at 770 kgPE (mol Ti)–1 h–1