Journal of Molecular Catalysis A: Chemical最新文献

Palladium substituted CeO₂, SnO₂ and their mixed oxides have been synthesized in quantitative yields out of non-toxic and inexpensive precursors using a simple and rapid single step solution combustion method. The resulting oxides, especially the mixed oxides Ce_0.79Sn_0.20Pd_0.01O_2-δ, Sn_0.79Ce_0.20Pd_0.01O_2-δ and Sn_0.99Pd_0.01O_2-δ proved to be highly active (TOF > 12,000 h⁻¹) for Suzuki-Miyaura cross-couplings of phenylboronic acid with various bromoarenes. The reactions were carried out in ambient air at moderate temperatures using environmentally friendly aqueous ethanol solutions as reaction solvents. Minimal amounts of palladium in the product solution ( < 0.14 mg/L), the reaction kinetics as well as catalyst poisoning tests support the thesis that the reaction proceeds via dissolved palladium species in a homogeneous reaction mechanism. Nevertheless, the synthesized catalysts could be reused for at least five times with only minor changes in activity and no changes in the crystal structure, indicating the high potential of the investigated catalysts as quasi-heterogeneous CC coupling catalysts.

Low temperature selective transformation of alkanes to useful products continues to be an important challenge in chemistry and industry. μ-Nitrido diiron phthalocyanines in combination with H₂O₂ have been recently identified as powerful oxidation catalysts for these challenging reactions due to the formation of ultra-high valent diiron oxo species PcFe(IV)μNFe(IV)O(Pc⁺). This very strong two-electron oxidizing species is generated from peroxo complex PcFe(IV)μNFe(III)OOR(Pc) (RH in the case of H₂O₂) via heterolytic OO bond cleavage. Therein we show that the evolution of the peroxo diiron complex depends on the peroxide structure. Using ^tBuOOH we have demonstrated the formation of an one-electron oxidizing PcFe(IV)μNFe(IV)O(Pc) and ^tBuO radical via homolytic OO cleavage of the peroxocomplex. The reactivity of the μ-nitrido diiron tetra-t-butylphthalocyanine − ^tBuOOH catalytic system was investigated in the oxidation of different CH bonds in alkanes, olefins, aromatic and alkylaromatic compounds. The main products of cyclohexane oxidation were cyclohexanone and cyclohexanol whereas bicyclohexyl was formed in minor amounts even in the presence of O₂ and ^tBuOOH. Under optimal conditions, the turnover numbers of almost 5300 have been achieved.

Complexes [M(P,SR)(diene)X] where (P,SR) = CpFe[1,2-C₅H₃(PPh₂)(CH₂SR)] (M = Ir, R = tBu or Bn diene = cod, X = Cl; M = Rh, diene = cod or nbd; X = BF₄ or Cl) were used as precatalysts for the redox isomerization of various allylic alcohols (7a–e) to the corresponding saturated ketones (8a–e) and or hydrogenation to the saturated alcohol (9a–e). In optimization studies using 1-phenyl-2-propen-1-ol (7a) in THF and in iPrOH/MeONa, the only observed product was the saturated alcohol 1-phenyl-1-propanol (9a) when working under a 30 bar H₂ pressure, but activation for only 1 min under H₂ pressure and then continuation under 1 bar of H₂ or Ar led to increasing amounts of the allylic isomerization product propiophenone (8a). Continued reaction under H₂ converted (8a) into (9a). The Rh precatalysts were more active than the Ir analogues. For the rhodium precatalysts (3) and (4), the redox isomerization reaction could be carried out after precatalyst activation in iPrOH/MeONa under Ar at 82 °C (without H₂) with complete conversion in 1 h (1% catalyst loading). However, longer reaction times resulted in slow transfer hydrogenation of (8a) leading to (9a) with low enantiomeric excess. Extension of the H₂-free activation of the Rh precatalysts in iPrOH to other allylic alcohol substrates (7b–d) yielded the corresponding ketones with good to excellent yields and excellent chemoselectivities under appropriate conditions.

Two types of heterogeneous catalytic materials, Mn^II-L₃imid@Cox and Mn^II-L₃imid@PCox, have been synthesized and compared by covalent grafting of a catalytically active [Mn^II-L₃imid] complex on the surface of an oxidized activated carbon (Cox) and an oxidized pyrolytic carbon from recycled-tire char (PCox). Both hybrids are non-porous bearing graphitic layers intermixed with disordered sp²/sp³ carbon units. Raman spectra show that (I_D/I_G)_{activatedcarbon} > (I_D/I_G)_{pyrolyticcarbon} revealing that oxidized activated carbon(Cox) is less graphitized than oxidized pyrolytic carbon (PCox). The Mn^II-L₃imid@Cox and Mn^II-L₃imid@PCox catalysts were evaluated for alkene oxidation with H₂O₂ in the presence of CH₃COONH₄. Both showed high selectivity towards epoxides and comparing the achieved yields and TONs, they appear equivalent. However, Mn^II-L₃imid@PCox catalyst is kinetically faster than the Mn^II-L₃imid@Cox (accomplishing the catalytic runs in 1.5 h vs. 5 h). Thus, despite the similarity in TONs Mn^II-L₃imid@PCox achieved extremely higher TOFs vs. Mn^II-L₃imid@Cox. Intriguingly, in terms of recyclability, Mn^II-L₃imid@Cox could be reused for a 2th run showing a ∼20% loss of its catalytic activity, while Mn^II-L₃imid@PCox practically no recyclable. This phenomenon is discussed in a mechanistic context; interlinking oxidative destruction of the Mn-complex with high TOFs for Mn^II-L₃imid@PCox, while the low-TOFs of Mn^II-L₃imid@Cox are preventive for the oxidative destruction of the Mn-complex.

Zirconia modified with boric acid and/or tungstophosphoric acid calcined at 320 °C were prepared, characterized and used as catalysts in the production of acetates from diverse alcohols and phenols. Polyethylene glycol (PEG) of different molecular weight (400, 2000, 6000 Da) were added as low cost pore-forming agents during zirconia synthesis using zirconyl chloride as precursor and ammonium hydroxide as precipitating agent. The zirconias were impregnated with aqueous solutions of boric acid and/or tungstophosphoric acid (TPA). The borated zirconias, zirconias modified with TPA and zirconias doped with both boron and TPA were amorphous mesoporous materials with very strong acid sites, and specific surface areas S_BET of around 200, 100, and 150 m²/g, respectively. The FT-IR spectra of borated zirconias exhibited the bands of boron species, while the zirconias modified with TPA presented the characteristic bands of tungstophosphate anion, and the zirconias doped with both boron and TPA showed a degradation of the TPA anion, confirmed by ³¹P MAS-NMR. The borated zirconias and the zirconias modified with TPA gave excellent selectivity and yield in the 2-phenylethanol esterification with acetic acid. The use of these materials allows obtaining higher or similar results than those reported in the literature. Zirconias doped with both boron and TPA gave lower values, due to the transformation of the [PW₁₂O₄₀]³⁻ Keggin anion in to the [P₂W₂₁O₇₁]⁶⁻ and [PW₁₁O₃₉]⁷⁻ species. The reactivity towards acetylation with acetic acid of different alcohols and phenols using the best catalyst was ordered according to: primary alcohols > secondary alcohols > phenols. The reactivity difference of the alcohols and phenols was correlated with the electronic density on the oxygen atom and steric effects.

Gadolinium (Gd)-doped silver orthophosphate (Ag₃PO₄) nanoparticles, with spherical morphology, were synthesized by a co-precipitation method and their photocatalytic activities were evaluated in degradation of Reactive Blue 19 (RB19), as a model organic pollutant. The synthesized Ag₃PO₄ and Gd-doped Ag₃PO₄ were characterized using X-ray diffraction pattern analysis, scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, diffuse reflectance UV–vis spectroscopy and photoluminescence spectroscopy. In comparison with Ag₃PO₄, the experimental results revealed a considerable enhancement in photocatalytic activity of Gd-doped Ag₃PO₄. The optimum mole fraction of Gd loading in Ag₃PO₄ was 3%. Lower photocatalytic degradation efficiency of RB19 in the presence of oxalate anion and p-benzoquinone indicated the crucial role of photogenerated holes and superoxide anion-radicals in photocatalysis on Gd-doped Ag₃PO₄. The enhancement of photocatalytic activity was attributed to scavenge the photoexcited electrons, and to suppress their recombination with holes and generation of active oxygen species. A nonlinear empirical kinetic model was introduced to predict the apparent first order rate constant (k_app) as a function of operational parameters, including initial RB19 concentration, Gd-doped Ag₃PO₄ dosage and light intensity. Degradation byproducts were identified using gas chromatography-mass spectrometry and a probable pathway was proposed. Moreover, Gd-doped Ag₃PO₄ samples displayed adequate reusability after three successive experiments.

Different functional groups (i.e. NH₂, COOH, OH and nitrogen-doping) modified CNTs (denoted as AMCN, CMCN, HMCN and NMCN, respectively) supported ruthenium catalysts (Ru/AMCN, Ru/CMCN, Ru/HMCN and Ru/NMCN) were prepared by incipient wetness impregnation method. They were fully characterized by XRD, TG, Raman, XPS, TPD and TEM to elucidate the relationship between the physical property and their catalytic performance. TEM results shown that Ru particles were well dispersed on the surface for all the samples with the size of 1.48–1.99 nm. The effects of functional groups of carbon nanotubes (CNTs), nitrogen doping and base additive types on activity and selectivity of ethylene glycol (EG) and propylene glycol (1,2-PD) were investigated. In addition, the activity and final products distribution were much influenced by the properties of functional groups on CNTs and the type of metal cation of the base promoters, which probably participated in the reaction for accelerating a retro-aldol reaction for CC cleavage. Among the catalysts, Ru supported on AMCN exhibited the best catalytic activities and glycols selectivities than on MCN, CMCN, HMCN and NMCN.

Protonation of copper-triazolide is a distinctive and final part in CuAAC reaction which has kinetic importance such that it can even affect the product distribution. In the context of this study, the protonation mechanism of copper-triazolide was investigated with quantum mechanical calculations to have a deeper understanding of the mechanism. In aprotic conditions where the alkyne is considered as proton donor, the key finding of DFT calculations performed in this study is that the activation energy barrier for the protonation step is greater than the cycloaddition step. In the absence of a strong proton donor the final protonation step is coupled with the alkyne deprotonation step in the catalytic cycle, which slows down the reaction. A conceivable pathway for acetic acid promoted CuAAC reaction on the basis of experimental and computational studies was also sought. With acetate as ligand, cycloaddition is a facile reaction and the energetics shows that it speeds up the cycloaddition step. Acetic acid in CuAAC reaction provides proton for the final protonation step in the catalytic cycle, decouples the protonation/deprotonation step by acting as a strong proton donor in the last step and facilitates protonation. The energetics presented herein are in accordance with the experimental proposals on rate-determining step in aprotic conditions and decreased reaction times obtained in the experiments in the presence of acetic acid.

A simple and efficient CH activation catalyst was identified through a model structure-activity screening applied to a noncooperative, nonsymmetric bimetallic palladium(II)-N-heterocyclic carbene complex. Mechanistic studies based on kinetics and DOSY NMR spectroscopy provided the origin of the higher efficiency of the identified catalyst.

Scorpionate ligands provide the benefit of hemilability while minimizing complete dissociation of the ligand. Previous investigations into Tp′PtL_nX_m complexes [Tp′ = hydridotris(3,5-dimethylpyrazolyl)borate] revealed the importance of hemilability as the Tp′ ligand facilitates Pt(II/IV) interconversions. Here we discuss the synthesis and metalation of a series of asymmetric scorpionate ligands bearing two pyrazolyl rings and one triazolyl ring. In addition to utilizing triazole donor arms with differing substituents, we also compare octahedral structures of Pt(IV) complexes with P = O and CH and BH caps at the pole of the facial tridentate umbrella. Oxidation from Pt(II) to Pt(IV) with electrophilic reagents, simple acids and acid chlorides, leads to isomers in some cases, and the binding properties of the various donor arms dictate the stereochemistry of the products. Investigations into the reactivity of heteroscorpionate tridentate ligands bound to platinum(II) led to CCl activation reactions with methylene chloride and 1,2-dichloroethane. Isolation of a dinuclear platinum complex bridged by an ethylene unit produced an unusual proton NMR AA′XX′ pattern in the ¹H NMR spectrum due to chirality at each platinum center.