Sustainability in Catalytic Cyclohexane Oxidation: The Contribution of Porous Support Materials

Synthesis and characterization of mesoporous silica supported metallosalphen-azobenzene complexes: efficient photochromic heterogeneous catalysts for the oxidation of cyclohexane to produce KA oil

The oxidation of cyclohexane to produce KA oil (cyclohexanone and cyclohexanol) is important industrially but faces challenges such as low cyclohexane conversion at high KA oil selectivity, and difficult catalyst recyclability. This work reports the synthesis and evaluation of new heterogeneous catalysts consisting of Co(ii), Mn(ii), Ni(ii) and Cu(ii) salphen-azobenzene complexes [ML1] immobilized on amino-functionalized mesoporous silica (SBA-15, MCM-41, MCM-48) through coordination bonding. In the first step, the salphen-azobenzene ligand was synthesized and complexed with Co, Mn, Ni and Cu metal ions. In the second step, aminopropyltriethoxysilane (APTES) was grafted onto the surface of different types of commercial mesoporous silica. The immobilization of [ML1] onto the mesoporous silica surface and the thermal stability of the obtained materials were confirmed using different characterization techniques such as FT-IR, powder XRD, SEM, TEM, BET, and TGA. The obtained results revealed high dispersion of [ML1] through the silica surface. The catalytic activity of the prepared materials Silica-N-ML1 was evaluated on the cyclohexane oxidation to produce KA oil using various oxidants. The cis-trans isomerization of the azobenzene upon UV irradiation was found to affect the catalytic performance of Silica-N-ML1. The cis isomer of SBA-15-N-CoL1 exhibited the highest cyclohexane conversion (93%) and KA selectivity (92%) under mild conditions (60 °C, 6 h) using m-CPBA as oxidant. Moreover, The SBA-15-N-CoL1 showed high stability during four successive cycles.

Outstanding photo-thermo synergy in aerobic oxidation of cyclohexane by bismuth tungstate-bismuth oxychloride high-low heterojunction

The difficulty of achieving both high conversion rate and high selectivity is a huge challenge in the catalytic aerobic oxidation of cyclohexane. In this paper, bismuth tungstate-bismuth oxychloride (Bi2WO6-BiOCl) nanoflower heterojunctions prepared via a one-step solvothermal process were applied in the photo-thermo synergetic catalytic oxidation of cyclohexane in the dried air. With the addition of little water at different reaction temperature, the ratio of bismuth to tungsten and the mass ratio of Bi2WO6 to BiOCl can be precisely tailored in the nanoflower sphere composites with thin nanosheets. Their microscopic morphology, elemental composition, crystal structure, and photoelectrochemical characteristics were explored by different characterization methods. The Bi2WO6-BiOCl composites possessed poor photocatalytic and thermal performances with the low conversion rates of 1.43% and 2.68%, respectively. However, through the photo-thermo catalytic oxidation process, an exceptional conversion rate of 13.32% was achieved with excellent selectivity of 99.22% for cyclohexanone and cyclohexanol (KA oil) using the same Bi2WO6-BiOCl composites. This superior performance outstrips Bi2WO6 flowers, BiOCl nanosheets and Bi2WO6-BiOCl composites with other compounding ratios. The creation of a high-low heterojunction in the Bi2WO6-BiOCl composite was confirmed by band energy analysis. The opto-electronic analysis, band energy analysis, sacrifice experiments, and active radical analysis were employed to elucidate the mechanism for the exceptional photo-thermo catalytic performance in detail. This work offers an exploratory solution to the challenges of high energy consumption and the difficulty in simultaneously achieving high selectivity and high conversion rates in cyclohexane oxidation, thus holding significant value.

Oxido- and Dioxido-Vanadium(V) Complexes Supported on Carbon Materials: Reusable Catalysts for the Oxidation of Cyclohexane

Oxidovanadium(V) and dioxidovanadium(V) compounds, [VO(OEt)L] (1) and [Et3NH][VO2L] (2), were synthesized using an aroylhydrazone Schiff base (5-bromo-2-hydroxybenzylidene)-2-hydroxybenzohydrazide (H2L). They were characterized by elemental analysis, Fourier-transform infrared spectroscopy (FT-IR), (1H and 51V) nuclear magnetic resonance (NMR), electrospray ionization mass spectrometry (ESI-MS) and single crystal X-ray diffraction analyses. Both complexes were immobilized on functionalized carbon nanotubes and activated carbon. The catalytic performances of 1 and 2, homogenous and anchored on the supports, were evaluated for the first time towards the MW-assisted peroxidative oxidation (with tert-butylhydroperoxide, TBHP) of cyclohexane under heterogeneous conditions. The immobilization of 1 and 2 on functionalized carbon materials improved the efficiency of catalytic oxidation and allowed the catalyst recyclability with a well-preserved catalytic activity.

Supported Palladium Nanocatalysts: Recent Findings in Hydrogenation Reactions

Catalysis has witnessed a dramatic increase on the use of metallic nanoparticles in the last decade, opening endless opportunities in a wide range of research areas. As one of the most investigated catalysts in organic synthesis, palladium finds numerous applications being of significant relevance in industrial hydrogenation reactions. The immobilization of Pd nanoparticles in porous solid supports offers great advantages in heterogeneous catalysis, allowing control of the major factors that influence activity and selectivity. The present review deals with recent developments in the preparation and applications of immobilized Pd nanoparticles on solid supports as catalysts for hydrogenation reactions, aiming to give an insight on the key factors that contribute to enhanced activity and selectivity. The application of mesoporous silicas, carbonaceous materials, zeolites, and metal organic frameworks (MOFs) as supports for palladium nanoparticles is addressed.