Nanofibrous and nanotubular supports for the immobilization of metalloporphyrins as oxidation catalysts

Nanofibrous and nanotubular materials, natural and synthetic, are important alternative matrices for the immobilization of metallocomplexes, especially metalloporphyrins, as oxidation catalysts. The process permits a regular and controllable distribution of the active phase at the outer and/or inner surfaces of the tubes, promoting a special environment for the approximation of a substrate to the catalytic active species. The immobilization also prevents the molecular aggregation and bimolecular self-destruction reactions, facilitates the recovery and reuse of the catalyst, reduce de cost of material preparation and environmental concerns. A variety of nanofibrous and nanotubular structures are presented and specific examples of immobilization of iron porphyrins in different supports and their oxidation catalytic activities are presented and discussed.     

Phosphinoyl-aziridines as a new class of chiral catalysts for enantioselective Michael addition

A series of new optically pure phosphine oxides containing chiral aziridine subunit were synthesized in good yields and applied as organocatalysts in asymmetric Michael reaction of various aliphatic aldehydes with β-nitrostyrene. The corresponding organocatalysts were synthesized starting from optically pure aziridines in a few simple efficient steps. The appropriate Michael adducts were obtained in most cases in very high chemical yield (up to 97%), excellent enantioselectivity (up to 98% of ee) and diastereoselectivity (up to 95:5 of dr).     

Formation of porous ceramic supports for catalysts by microarc oxidation

Microarc oxidation of aluminum in borate and silicate-alkaline electrolytes in order to form porous ceramic coatings that can be used as catalyst supports was subjected to an integrated study. The advantages of the technological modes of microarc oxidation of aluminum were analyzed. A catalytically active nickel oxide was deposited into pores of a ceramic coating.     

Zinc aluminates as supports for HDS catalysts

Surface behavior of zinc aluminates prepared according to three different methods has been investigated by TPD NH₃ and IR spectroscopy and compared with properties of alumina. It was stated that zinc affects the strength and distribution of acidic sites as well as HDS activity of Mo (CoMo) catalysts supported on Zn doped Al₂O₃.     

Iron oxide on carbon‐based supports as efficient catalysts for organic compounds oxidation

For the first time, iron oxide on carbon aerogel, amine functionalized carbon nanotube, black carbon and carboxylic acid functionalized carbon nanotube in the presence of H₂O₂ was reported as an efficient and stable catalyst for the selective oxidation of sulfides and alcohols. The catalysts were characterized by scanning electron microscopy, energy‐dispersive spectroscopy, transmission electron microscopy, X‐ray photoelectron spectroscopy, X‐ray diffraction, Fourier transform infrared spectroscopy and atomic absorption spectroscopy. In the next step, catalytic reactivity toward sulfide to sulfoxide and alcohol to aldehyde/ketone oxidation in the presence of H₂O₂ was studied and discussed.     

α-Zirconium phosphonates as new supports for metallocene catalysts

Homoleptic and mixed -zirconium phosphonates (ZrPs) -Zr(O₃PR)₂ (R = Me, Buⁿ, Buⁱ, Hex, Oct and Ph) and -Zr(O₃PR¹)_2−x(O₃PR²)_x were readily prepared in high yields from zirconyl choride and the corresponding phosphonic acids in suitable solvent mixtures under hydrothermal conditions at low fluoride concentrations. They form crystalline aggregates consisting of platelets from ca. 10–20 monolayers thickness, with well-defined surface structures. Impregnation with Cp₂ZrCl₂ by sublimation or slurry methods provided the first examples of ZrP-supported alkene polymerization catalysts. Crystal morphology and interlayer spacing are unaffected by the impregnation process. Solid-state NMR spectroscopy provides evidence for the integrity of the adsorbed metallocene structure. Covalent attachment of Cp^*ZrCl₃ to functionalized ZrPs of the type -Zr(O₃PR¹)_1.8(O₃PC_nH_2nOH)_0.2 is similarly possible. The new catalysts polymerize ethene with good to excellent activities under mild conditions, even at remarkably low methylalumoxane/zirconocene ratios of 10:1. The polymer is obtained as free-flowing particles, which reflect the morphology of the catalyst supports.     

Modified silicas as supports for single-site zirconocene catalysts

The reactivity of Cp₂ZrCl₂ towards partially dehydroxylated silica was evaluated and the effects of chemical modification of this silica were studied. Different modified silicas were prepared by reaction of the original partially dehydroxylated silica with silicon ethers, EtOSiMe₃ and (Me₃Si)₂O, or a silazane, (Me₃Si)₂NH. The resulting materials were activated with MAO and the catalytic systems were evaluated in ethylene polymerization. The different reactions were monitored by FT-IR spectroscopy. The catalysts were characterized by elemental analysis as well as by infrared and UV–vis spectroscopy. Grafting of organosilanes occurs by reaction with reactive siloxane bridges. The new SiR₃ groups formed on the surface react with Cp₂ZrCl₂ to form volatile ClSiMe₃ and oxo zirconium species. These latter species are active, after the addition of MAO, in ethylene polymerization. The effects caused by changing the nature of the modifier in the grafting reaction with the metallocene, as well as the catalytic activities of the resulting materials, are presented and discussed.     

Supercritical extraction as a method for modifying the structure of supports and catalysts

Supercritical solvent extraction has been used for drying of wet gels in the production of SiO₂ monoliths, with methanol and liquid CO₂ used as solvents. The influence of various process parameters and nature of the solvent on the structural and mechanical properties of the monoliths obtained has been studied. Paper presented at the International Seminar on Monolith honeycomb supports and catalysts, St. Petersburg, September 19–22, 1995.     

Vanadium complexes immobilized on solid supports and their use as catalysts for oxidation and functionalization of alkanes and alkenes

This review mainly discusses the immobilization strategies that have been used for vanadium complexes, typically mesoporous material, zeolites and polymers, the characterization procedures for the obtained materials, and their catalytic applications. The retention of the active metal compound within the catalyst may be based on (i) adsorption, (ii) the formation of covalent bonds between metal ligand and support, (iii) ion exchange, (iv) encapsulation, or (v) entrapment. The heterogenized complexes are used as catalysts for oxidations and functionalization of alkanes, alkenes and other substrates, and an account of the various applications reported is given.     

Diruthenium(II,III) tetramidates as a new class of oxygenation catalysts

Two new diruthenium(II,III) tetramidate compounds, Ru(2)(NHOCC(CH(3))(2))(4)Cl (1) and Ru(2)(NHOCCH(2)CH(3))(4)Cl (2) have been prepared and structurally characterized by X-ray crystallography. The activity of promoting sulfide oxygenation using simple oxidants such as hydrogen peroxide (H(2)O(2)) and tert-butyl hydroperoxide (TBHP) was studied. A UV-kinetics study indicated that the initial rates of 1 and 2 are comparable to the previously studied diruthenium tetracarboxylates in promoting TBHP oxygenation of methyl phenyl sulfide (MPS). Using excess oxidant and CH(3)CN as the solvent, organic sulfides MPS and diphenyl sulfide (PPS) were oxidized using 1 mol% of the catalytic species. Compound 1 is more effective than 2 in converting sulfides to sulfoxide under the same conditions. Fast conversion was achieved when the reactions were carried out in the solvent-free conditions, and the major oxidation product was the sulfoxide. The electronic structure of the title compounds was studied with DFT calculations to gain an understanding of the activation of peroxy reagents.     

Synthesis of supports for reforming catalysts

The formation of surface defects in γ-Al₂O₃ obtained by pseudoboehmite calcination and plasticized with monobasic, dibasic, and tribasic organic acids is reported. The efficiency of catalysts in reforming reactions depends on the defectivity of the support.     

Pillared clays as efficient catalysts for cyclohexanol oxidation

Summary Iron, aluminium and mixed iron aluminium pillared clays were prepared by partial hydrolysis method and doped with 10% V and Mo. The samples were characterised by XRD, FTIR and surface area and pore volume measurements. Oxidation of cyclohexanol with hydrogen peroxide was done as probe reaction to test catalytic activity. Iron pillared systems exhibited maximum activity. The effects of various reaction variables on the reaction were studied.     

Polyferroorganosiloxanes as catalysts of oxidation processes

Polyferroorganosiloxanes were studied as catalysts for homogeneous oxidation of alkanes by hydrogen peroxide tinder mild conditions. In the oxidation of cyclohexane the catalysts are characterized by high efficiency (conversion of hydrogen peroxide is 25%) and stability Kip to 80 cycles per gat NJ The min product of the oxidation W do presence a 2,4,6-tri-tert-btitylphenol is cyclohexanol (tip to 35% per H₂O₂).     

Heteropolyanions as oxidation catalysts in a 2-phases system

Described is a system using heteropoly-11-tungstates as homogeneous catalysts for the oxidation of cyclohexene with hydrogen peroxide. The catalyst is dissolved in an organic solvent while the oxidant resides in the aqueous phase. The efficiency is high.     

Manganese triazacyclononane oxidation catalysts grafted under reaction conditions on solid cocatalytic supports

Manganese complexes of 1,4,7-trimethyl-1,4,7-triazacyclononane (tmtacn) are highly active and selective alkene oxidation catalysts with aqueous H(2)O(2). Here, carboxylic acid-functionalized SiO(2) simultaneously immobilizes and activates these complexes under oxidation reaction conditions. H(2)O(2) and the functionalized support are both necessary to transform the inactive [(tmtacn)Mn(IV)(μ-O)(3)Mn(IV)(tmtacn)](2+) into the active, dicarboxylate-bridged [(tmtacn)Mn(III)(μ-O)(μ-RCOO)(2)Mn(III)(tmtacn)](2+). This transformation is assigned on the basis of comparison of diffuse reflectance UV-visible spectra to known soluble models, assignment of oxidation state by Mn K-edge X-ray absorption near-edge spectroscopy, the dependence of rates on the acid/Mn ratios, and comparison of the surface structures derived from density functional theory with extended X-ray absorption fine structure. Productivity in cis-cyclooctene oxidation to epoxide and cis-diol with 2-10 equiv of solid cocatalytic supports is superior to that obtained with analogous soluble valeric acid cocatalysts, which require 1000-fold excess to reach similar levels at comparable times. Cyclooctene oxidation rates are near first order in H(2)O(2) and near zero order in all other species, including H(2)O. These observations are consistent with a mechanism of substrate oxidation following rate-limiting H(2)O(2) activation on the hydrated, supported complex. This general mechanism and the observed alkene oxidation activation energy of 38 ± 6 kJ/mol are comparable to H(2)O(2) activation by related soluble catalysts. Undesired decomposition of H(2)O(2) is not a limiting factor for these solid catalysts, and as such, productivity remains high up to 25 °C and initial H(2)O(2) concentration of 0.5 M, increasing reactor throughput. These results show that immobilized carboxylic acids can be utilized and understood like traditional carboxylic acids to activate non-heme oxidation catalysts while enabling higher throughput and providing the separation and handling benefits of a solid catalyst.     

Triazine-based polymers as nanostructured supports for the liquid-phase oxidation of alcohols

A covalent triazine framework (CTF) was used as support for palladium nanoparticles (NPs) and Pd/CTF was applied as the catalyst in the selective oxidation of benzyl alcohol. N groups in the CTF appeared more efficient than those created on carbon nanotubes (CNTs) by NH₃/high‐temperature treatment in stabilizing Pd NPs against growth during the immobilization step. This assured a high metal dispersion, which led to a highly active and stable catalyst in the alcohol oxidation reaction. Indeed, Pd on the CTF was more stable in recycling than Pd on activated carbon (AC) and on nitrogen‐doped CNTs, particularly avoiding leaching of Pd NPs. Moreover, Pd on the CTF was less sensitive than Pd on AC to the decrease of reactant concentration. This in turn led to a higher selectivity to benzaldehyde (98 %) with a considerable activity (turnover frequency 1453 h^?1).     

Silanediols: a new class of hydrogen bond donor catalysts

Silanediols are introduced as a new class of hydrogen bond donor catalysts for the activation of nitroalkenes toward nucleophilic attack. Excellent yields of product are obtained for the conjugate addition of indole to β-nitrostyrene catalyzed with a stable, storable dinaphthyl-derived silanediol. The preparation and structural characterization of a C(2)-symmetric chiral silanediol is also reported along with its ability to catalyze the conjugate addition reaction.     

Micelle-derived polymer supports for enantioselective catalysts

The BINOL moieties are successfully immobilized on the surface of a micelle-derived polymer. Ti-BINOLate complexes prepared from the polymer with Ti(O-i-Pr)₄ exhibited high catalytic activity for the asymmetric alkylation of benzaldehyde with Et₂Zn to afford the adduct in up to 96% yield with 84% ee.     

Structural-phase and morphological characteristics of silver catalysts on modified ceramic supports for selective oxidation of alcohols

New ceramic materials based on silicon nitride and obtained by self-propagating high-temperature (SPHT) synthesis were investigated. The Si₃N₄ materials modified with Zr and Al compounds were used as supports for silver-containing catalysts of selective oxidation of ethyleneglycol into glyoxal. The procedure for the synthesis of supported silver-containing catalysts affected the structural and morphological states of the active component before and after the catalytic reaction.     

Superhydrophobic materials as efficient catalysts for hydrocarbon selective oxidation

A new type of superhydrophobic material, FP-Co-SiO(2) was prepared with organic groups immobilized on the surface of the SiO(2)-based nanocomposite. This material showed much higher catalytic activity for selective oxidation of hydrocarbons than an equivalent hydrophilic catalyst.