Redox Properties of CeO2at Low Temperature: The Direct Synthesis of Imines from Alcohol and Amine

Towards highly active heterogeneous catalysts via a sequential noncovalent bonding strategy

The synthesis strategy of Cu/CeO2 and Pd–Cu/CeO2 nanocatalysts.

Enhanced Catalytic Performance of Quasi-HKUST-1 for the Tandem Imine Formation

Copper-based metal-organic framework (Cu₃ (BTC)₂ (H₂ O)₃ ]_n ⋅nH₂ OMeOH (HKUST-1) has been subjected to thermolysis under air atmosphere at different temperatures ranging from 100 to 300 °C. This treatment produces the partial removal of ligands, the generation of structural defects and additional porosity in a controlled way. The resulting defective materials denoted according to the literature as quasi-MOFs, were subsequently employed as heterogeneous catalysts in the one pot synthesis of N-benzylideneaniline from aniline and benzyl alcohol in open air as terminal oxidant at 70 °C under base- and dehydrating agent-free conditions. The Q-HKUST catalysts calcined at 240 °C (QH-240) was the most efficient in the series, promoting imine synthesis. Data from Knoevenagel condensation of malononitrile shows that in QH-240 the distances of Cu ions in HKUST-1 cavities are preserved, increasing the Knoevenagel activity, but a strong rearrangement takes place at 300 °C or above. The unsaturated copper active sites with simultaneous presence of micro- and mesopores in QH-240 are responsible for this excellent catalytic performance. The effective parameters on catalytic activity of QH-240 including deligandation temperature, the amount of catalyst, the ratio of reactants, and reaction temperature as well as the stability and recyclability of the catalyst were also investigated. The possible mechanism used by QH-240 follows alcohol aerobic oxidation and subsequent anaerobic condensation of aldehyde intermediate with aniline.

Aprotic Amine-modified Manganese Dioxide Catalysts for Selectivity-tunable Oxidation of Amines

Organic modifiers have shown promising potential for regulating the activity and selectivity of heterogeneous catalysts via tuning their surface properties. Despite the increasing application of organic modification technique in regulating the redox-acid catalysis of metal oxides, control of the acidity of metal oxide catalysts for enhanced reaction selectivity without sacrificing their redox activity remains a substantial challenge. Herein, we show the successful control of redox-acid catalysis of metal oxides with aprotic tertiary amine modifiers. Robust modification of manganese dioxide catalysts with N,N-dialkylcyclohexylamine selectively blocks the Lewis acid sites, with their redox activity mostly unaffected. This enables efficient synthesis of imines in high to excellent selectivity via aerobic oxidation of structurally diverse aryl amines.

Efficient Imine Formation by Oxidative Coupling at Low Temperature Catalyzed by High-Surface-Area Mesoporous CeO2 with Exceptional Redox Property

High-surface-area mesoporous CeO₂ (hsmCeO₂ ) was prepared by a facile organic-template-induced homogeneous precipitation process and showed excellent catalytic activity in imine synthesis in the absence of base from primary alcohols and amines in air atmosphere at low temperature. For comparison, ordinary CeO₂ and hsmCeO₂ after different thermal treatments were also investigated. XRD, N₂ physisorption, UV-Raman, H₂ temperature-programmed reduction, O₂ temperature-programmed desorption, EPR spectroscopy, and X-ray photoelectron spectroscopy were used to unravel the structural and redox properties. The hsmCeO₂ calcined at 400 °C shows the highest specific surface area (158 m²  g^-1 ), the highest fraction of surface coordinatively unsaturated Ce³⁺ ions (18.2 %), and the highest concentration of reactive oxygen vacancies (2.4×10¹⁵  spins g^-1 ). In the model reaction of oxidative coupling of benzyl alcohol and aniline, such an exceptional redox property of the hsmCeO₂ catalyst can boost benzylideneaniline formation (2.75 and 5.55 mmol  g ceria - 1  h^-1 based on >99 % yield at 60 and 80 °C, respectively) in air with no base additives. It can also work effectively at a temperature of 30 °C and in gram-scale synthesis. These are among the best results for all benchmark ceria catalysts in the literature. Moreover, the hsmCeO₂ catalyst shows a wide scope towards primary alcohols and amines with good to excellent yield of imines. The influence of reaction parameters, the reusability of the catalyst, and the reaction mechanism were investigated.

Insight into the optical, color, photoluminescence properties, and photocatalytic activity of the N–O and C–O functional groups decorating spinel type magnesium aluminate

MgAl₂O₄ with a spinel structure was successfully synthesized using a gamma-ray irradiation assisted polyacrylamide gel method.

Nanoceria-promoted low Pd–Ni catalyst for the synthesis of secondary amines from aliphatic alcohols and ammonia

Pd–Ni catalysts supported over nanoceria with very low Ni and Pd loading (<0.5 wt%) showed high activity and selectivity for the direct amination of 1-octanol with ammonia towards dioctylamine with 80% yield.

Crystal plane dependent dopant migration that boosts catalytic oxidation

CeO₂ rods with {110} facets and cubes with {100} facets were utilized as catalyst supports to probe the effect of crystallographic facets on the iron species and the structure-dependent catalytic performance.

A Reusable Co Catalyst for the Selective Hydrogenation of Functionalized Nitroarenes and the Direct Synthesis of Imines and Benzimidazoles from Nitroarenes and Aldehydes

The use of abundantly available transition metals in reactions that have been preferentially mediated by rare noble metals, for example, hydrogenations, is a desirable aim in catalysis and an attractive strategy for element conservation. The observation of novel selectivity patterns with such inexpensive metal catalysts is especially appealing. Herein, we report a novel, robust, and reusable cobalt catalyst that permits the selective hydrogenation of nitroarenes in the presence of highly hydrogenation-sensitive functional groups, as well as the direct synthesis of imines from nitroarenes and aldehydes or ketones in the presence of such substituents. Furthermore, we introduce the first base-metal-mediated direct synthesis of benzimidazoles from nitroarenes and aldehydes. Functional groups that are easy to hydrogenate are again well tolerated.