Selective and Generic Photocatalytic Oxidation of Alcohol with Pd-TiO2 Thin Films: Butanols to Butanal/Butanone with Different Morphologies of Pd and 0.5θPt -Pd Counterparts

The present study reports on the photocatalytic oxidation of butanols to butanal/butanone using thin film form of facet-dependent nano-Pd supported on commercial TiO₂ under one-sun condition and demonstrates the generic nature. Pd-nanocube (Pd_NC (100)), Pd-truncated octahedron (Pd_TO (100) and (111)), polycrystalline (Pd_PC ), and their counterparts with half-a-monolayer Pt-coated on Pd (0.5θ_Pt -Pd)) have been used as co-catalyst. A potentially scalable thin film form of Pd/TiO₂ photocatalyst, prepared by drop-casting method, has been employed to study oxidation of n-butanol, 2-butanol, and iso-butanol to corresponding aldehyde/ketone. 100% selectivity is demonstrated to respective aldehyde/ketone with any catalyst used in the present study with varying degree of butanols conversion by NMR. 0.5θ_Pt -Pd_TO /TiO₂ shows the highest conversion of 2-butanol to butanone (13.6% in 4 h). Continuous 10 h of reaction with the most active 0.5θ_Pt -Pd_TO /P25 catalyst demonstrates 31% conversion of 2-butanol to butanone, and catalyst recyclability has been demonstrated. The present protocol can be scalable to large scales to maximize the conversion in direct sunlight. Due to its generic nature, the current method can also be applied to many other alcohols and substrate molecules.

Titania/chitosan–lignin nanocomposite as an efficient photocatalyst for the selective oxidation of benzyl alcohol under UV and visible light

Developing functional materials from biomass is a significant research subject due to its unique structure, abundant availability, biodegradability and low cost. A series of chitosan–lignin (CL) composites were prepared through a hydrothermal method by varying the weight ratio of chitosan and lignin. Subsequently, these CL composites were combined with titania (T) to form a nanocomposite (T/CL) using sol–gel and hydrothermal based methods. T/CL nanocomposites exhibited improved photocatalytic performance in comparison with sol–gel and hydrothermally prepared pristine titania (SGH-TiO₂), towards the selective oxidation of benzyl alcohol (BnOH) to benzaldehyde (Bnald) under UV (375 nm) and visible light (515 nm). More specifically, the 75T/CL(25 : 75) nanocomposite (a representative photocatalyst from the 75T/CL nanocomposite series) showed very high selectivity (94%) towards Bnald at 55% BnOH conversion under UV light. Whereas, SGH-TiO₂ titania exhibited much lower (68%) selectivity for Bnald at similar BnOH conversion. Moreover, the 75T/CL(25 : 75) nanocomposite also showed excellent Bnald selectivity (100%) at moderate BnOH conversion (19%) under visible light. Whereas, SGH-TiO₂ did not show any activity for BnOH oxidation under visible light. XPS studies suggest that the visible light activity of the 75T/CL(25 : 75) nanocomposite is possibly related to the doping of nitrogen into titania from chitosan. However, according to UV-visible-DRS results, no direct evidence pertaining to the decrease in band-gap energy of titania was found upon coupling with the CL composite and the visible light activity was attributed to N-doping of titania. Overall, it was found that T/CL nanocomposites enhanced the photocatalytic performance of titania via improved light harvesting and higher selectivity through mediation of active radical species.

Combining titania with chitosan–lignin composites results in an active and selective photocatalyst for the oxidation of benzyl alcohol to benzaldehyde under green light (515 nm).

Steric Effects of Mesoporous Silica Supported Bimetallic Au-Pt Catalysts on the Selective Aerobic Oxidation of Aromatic Alcohols

Three series of catalysts consisting of gold (Au), platinum (Pt), or gold-platinum bimetallic nanoparticles (NPs) with controlled sizes (Au NPs 10 ± 2 nm, Pt NPs 6 ± 2 nm) anchored on hierarchical micro-/meso-/macroporous silica were successfully developed and systematically evaluated for the selective oxidation of aromatic alcohols to their corresponding aldehydes. The catalysts were prepared by the sol-immobilization method using as-made Au NPs and/or Pt NPs colloids; the silica supports were prepared with controlled pore structures and the hierarchical porous structures of catalysts were created by controllable desilication via the alkaline solution of the metal colloids. The catalysts were characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS), and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS), and these results showed no synergistic effect between Au and Pt on boosting the catalytic performance, whereas they demonstrated a clear dependence of catalytic conversions and reaction rates on the structural porosity of Au-Pt bimetallic catalysts. Our findings could potentially inspire peer researchers and scientists to develop designer porous catalysts and processes in the selective organic conversions.

Photocatalytic production of dihydroxyacetone from glycerol on TiO2 in acetonitrile

In this paper, photocatalytic production of dihydroxyacetone (DHA) from glycerol in acetonitrile on TiO₂ was investigated. HPLC-MS analysis showed that glycerol was converted to DHA, glyceraldehyde (GAD), glyceric acid and several other chemicals. Using acetonitrile as the reaction medium instead of water not only provided a more selective process for production of DHA but also increased the glycerol conversion. After 300 min, with 1 g L⁻¹ catalyst loading and 4 mM initial glycerol concentration, glycerol conversion and DHA selectivity were 96.8% and 17.8% in acetonitrile compared to 36.1% and 14.7% in water, respectively. The half-life of glycerol decreased by a factor of 6.2, from 467 min to 75 min, by changing the solvent from water to acetonitrile. Experiments using biodiesel-derived crude glycerol verified the effectiveness of the proposed process for the photocatalytic production of DHA from crude glycerol. A mechanism was proposed to explain the higher selectivity towards DHA over GAD in this process.

Photocatalytic conversion of glycerol and selectivity for dihydroxyacetone production was increased by using acetonitrile as the reaction medium.

Visible Light-Emitting Diode Light-Driven Cu0.9Fe0.1@RCAC-Catalyzed Highly Selective Aerobic Oxidation of Alcohols and Oxidative Azo-Coupling of Anilines: Tandem One Pot Oxidation-Condensation to Imidazoles and Imines

Here, we have demonstrated visible light-emitting diode light-driven selective and efficient aerobic oxidation of primary/secondary alcohols to aldehydes/ketones and oxidative azo-coupling of anilines using biomass rice husk-derived chemically activated carbon sheet-supported copper-iron bimetallic hybrid nanomaterials (Cu _x Fe_1-x @RCAC) under oxidant and additive-free conditions. The catalytic activity of the Cu _x Fe_1-x @RCAC materials has been investigated for the oxidation of alcohols and anilines, and Cu_0.9Fe_0.1@RCAC was established as the best catalyst. Moreover, a tandem one-pot protocol has been developed for the sequential oxidation of alcohols followed by condensation to functionalized imidazole and imine derivatives in high isolated yields. The hybrid materials were highly robust and stable under the reaction conditions and were recovered simply by filtration and recycled up to 12th run without considerable loss in catalytic activity.

Supported Gold Nanoparticles as Catalysts for the Oxidation of Alcohols and Alkanes

Supporting gold nanoparticles have shown to be extremely active for many industrially important reactions, including oxidations. Two representative examples are the oxidation of alcohols and alkanes, that are substrates of industrial interest, but whose oxidation is still challenging. This review deals with these reactions, giving an insight of the first studies performed by gold based catalysts in these reactions and the most recent developments in the field.

CO Preferential Photo-Oxidation in Excess of Hydrogen in Dark and Simulated Solar Light Irradiation over AuCu-Based Catalysts on SBA-15 Mesoporous Silica-Titania

In this work, SBA-15 silica and silica-titania have been used as supports for photocatalysts based on AuCu alloy (Au:Cu = 1) to be used in the preferential oxidation of CO (CO-PROX) in excess of hydrogen at room temperature and atmospheric pressure both in the dark and under simulated solar light irradiation. To study their textural, structural, chemical and optical properties, the samples were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), adsorption-desorption of N₂ at -196 °C, 13C and 29Si solid state nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS) and diffuse reflectance ultraviolet-visible (DRUV-vis) spectroscopy. Titanium was present mainly in the form of titania aggregates, but also as small particles interacting with the SBA support. In both catalysts, the metal alloy nanoparticles displayed an average size of 4 nm as demonstrated by TEM measurements. AuCu/Ti-SBA turned out to be photoactive and selective in the photo-CO-PROX reaction showing the highest activity, with conversion and selectivity towards CO₂ of 80%, due both to the presence of titania incorporated in SBA-15 and to the synergistic effect of Cu when alloyed with Au.

Au and AuCu Nanoparticles Supported on SBA-15 Ordered Mesoporous Titania-Silica as Catalysts for Methylene Blue Photodegradation

The photocatalytic degradation of methylene blue (MB) dye has been performed under UV irradiation in aqueous suspension, employing photocatalysts based on Au (1.5 wt %) and AuCu (Au/Cu = 1, 2.0 wt %), and supported on SBA-15-ordered mesoporous silica, with and without titania (Si/Ti = 3), in order to evaluate the versatility of this mesoporous support in this type of reaction of great impact from the environmental point of view. Samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), N₂ adsorption-desorption at -196 °C, and X-ray photoelectron spectroscopy (XPS), so as to study their structural, optical, and chemical properties. All the prepared catalysts were found to be active in the test reaction. The bimetallic AuCu-based catalysts attained very high MB degradation values, in particular AuCu/SBA-15 titania-silica sample reached 100% of dye oxidation after the monitored reaction period (120 min).

Size-Controlled Synthesis of Anatase in a Mesoporous Silica, SBA-15

The preparation of anatase in the cylindrical mesopore of SBA-15 (pore size of 8 nm) was done by the impregnation of tetraisopropyl orthotitanate and its subsequent crystallization. The impregnation was done without a solvent. Hydrolysis and condensation were promoted by the HCl vapor to encapsulate a larger amount of titanium oxo species in the mesopore and to suppress the desorption of the titanium oxo species during crystallization to anatase. After the reaction, the shape of the N₂ adsorption isotherm changed significantly, indicating the decrease of the Brunauer-Emmett-Teller surface area from 743 to 283 m²/g and of the pore volume from 1.27 to 0.26 cm³/g, respectively. After the crystallization to anatase, the TiO₂ content in the product was estimated to be 62 mass %, filling 30% of the pore volume of SBA-15. The homogeneous distribution of titanium in the SBA-15 sample was confirmed by elemental mapping based on scanning electron microscopy/energy-dispersive X-ray spectrometry. The crystal size of the anatase was determined to be ca. 8.1 nm, which is consistent with the pore size of the used SBA-15 (8.0 nm, derived from the Barrett-Joyner-Halenda analysis of the nitrogen adsorption isotherm). The zeta potential measurements showed the absence of anatase as isolated particles or on the surface of SBA-15 particles. All of these characterizations confirmed the successful size-controlled synthesis of anatase in the mesopore of SBA-15.

Preparation of 3,3,3-trifluoropropyl functionalized hydrophobic mesoporous silica and its outstanding adsorption properties for dibutyl phthalate

In this study, 3,3,3-trifluoropropyltrimethoxysilane was first used as a modification agent to synthesize a highly hydrophobic mesoporous silica, TFP-MCM-41, using a refluxing method.