One-Electron Oxidation Pathways during β-Cyclodextrin-Modified TiO2 Photocatalytic Reactions

Practical photocatalytic and sonophotocatalytic reduction of nitroarenes in water under blue LED irradiation using β-CD modified TiO2 as a green nest photocatalyst

Photocatalysis using natural photosynthesis is a green technology that is gaining popularity in a number of industries due to its potential for environmental applications and the use of solar energy. Focus is being placed on using inexpensive materials and light-emitting diodes (LEDs) of various wavelengths in photocatalytic reactions in order to improve the performance of solar-driven photocatalysts at a lower cost. In this study, a scalable, highly efficient photocatalytic and sonophotocatalytic method was investigated for the reduction of nitro-compounds by a water/titania/β-cyclodextrin system under sunlight and blue LED irradiation, using sodium sulfide as a sacrificial electron donor. β-Cyclodextrin, chemically bound to TiO2 nanoparticles as an encapsulating agent, hosted nitro compounds in aqueous media and formed an inclusion complex. In addition, this method was used to successfully carry out one-pot reduction-amidation of nitroarene compounds in the presence of acetic anhydride. Interestingly, it was found that ultrasound has a synergistic effect on photocatalytic reduction and considerably reduces the duration time. In this regard, a fast, practical sonophotocatalytic reduction of nitroarenes was carried out in an ultrasound bath.

Accelerated photoelectron transmission by carboxymethyl β-cyclodextrin for organic contaminants removal: An alternative to noble metal catalyst

Applications of noble metal decorated photocatalytic nanomaterials are restricted by its high cost. In this study, carboxymethyl β-cyclodextrin (CM-β-CD), as an alternative to gold nanoparticle, was used to modified titanium dioxide (CM-β-CD-P25) to accelerate photoelectron transmission and enhance the organic contaminants removal from water. Several of emerging organic contaminants, such as bisphenol A (BPA), phenol and sulphanilamide (SA), were used to evaluate their photocatalytic activities. Carboxymethyl-β-cyclodextrin not only provide hydrophobic sites to entrap organic contaminants but also provide a "bridge" for accelerated transmission of photogenerated charges without introducing the recombination interface. Consequently, 91.6 % of BPA, 71.9 % of phenol and 97.1 % of SA could be removed by CM-β-CD-P25(2:1) under 1 h UV light irradiation. The photooxidation rate constant of BPA, phenol and SA by CM-β-CD-P25(2:1) were 0.039 min^-1, 0.021 min^-1 and 0.062 min^-1, respectively, which are much higher than that of pristine P25 and Au-P25. Moreover, the photocatalytic activity of CM-β-CD-P25(2:1) remains almost unchanged in repeated cycle test owing to its high stability. The reasonable mechanism of CM-β-CD-P25 were investigated. CM-β-CD-P25 hybrid nanoparticles completely surpasses Au-P25 in organic contaminants removal, and shows great potential to replace noble metal as mediator.

Photocatalysis of Volatile Organic Compounds in water: Towards a deeper understanding of the role of cyclodextrins in the photodegradation of toluene over titanium dioxide

HYPOTHESIS

Cyclodextrin-assisted photodegradation of toluene was investigated in water in the presence of a photo-irradiated commercial titanium dioxide photocatalyst. It was expected that cyclodextrins could form water-soluble supramolecular host/guest complexes with the toluene and thus promote the approach of the pollutant on the TiO2 surface and enhance the phototocatalytic oxidation efficiency.

EXPERIMENTS

Photodegradation kinetics of toluene were investigated under UV-C and near-visible light radiation in aqueous suspensions of TiO2. Impact of cyclodextrin (CD) on the photocatalytic efficiency of TiO2 was evaluated with different cyclodextrins: α-CD, β-CD, γ-CD and RAME-β-CD. Host-guest association constants were determined by static headspace gas chromatography and affinity of cyclodextrins for the TiO2 surface by isothermal adsorption studies. Issue of the cyclodextrin stability during the degradation process was examined using Total Organic Carbon, NMR and MALDI-TOF analyses.

FINDINGS

Toluene could be fully mineralized by TiO2 in water within hours, even if the presence of cyclodextrin caused a delay in the photodegradation process. The chemical nature of cyclodextrins was found to exert a significant influence on the extent of inhibitory effect, which was discussed in terms of balance between solubilization efficiency, substrate protection and coverage of active sites of TiO2 by competitive adsorption. The cyclodextrin degradation was also studied and discussed.

β-Cyclodextrin–TiO2: Green Nest for reduction of nitroaromatic compounds

A highly efficient, eco-friendly and selective photocatalytic reduction of the nitro group into amines and one-pot N-acylation and N-formylation are reported through the ‘Green Nest’ system.

Cyclodextrin-functionalized Fe3O4@TiO2: reusable, magnetic nanoparticles for photocatalytic degradation of endocrine-disrupting chemicals in water supplies

Water-dispersible, photocatalytic Fe3O4@TiO2 core-shell magnetic nanoparticles have been prepared by anchoring cyclodextrin cavities to the TiO2 shell, and their ability to capture and photocatalytically destroy endocrine-disrupting chemicals, bisphenol A and dibutyl phthalate, present in water, has been demonstrated. The functionalized nanoparticles can be magnetically separated from the dispersion after photocatalysis and hence reused. Each component of the cyclodextrin-functionalized Fe3O4@TiO2 core-shell nanoparticle has a crucial role in its functioning. The tethered cyclodextrins are responsible for the aqueous dispersibility of the nanoparticles and their hydrophobic cavities for the capture of the organic pollutants that may be present in water samples. The amorphous TiO2 shell is the photocatalyst for the degradation and mineralization of the organics, bisphenol A and dibutyl phthalate, under UV illumination, and the magnetism associated with the 9 nm crystalline Fe3O4 core allows for the magnetic separation from the dispersion once photocatalytic degradation is complete. An attractive feature of these "capture and destroy" nanomaterials is that they may be completely removed from the dispersion and reused with little or no loss of catalytic activity.

Mechanistic aspects of photooxidation of polyhydroxylated molecules on metal oxides

Polyhydroxylated molecules, including natural carbohydrates, are known to undergo photooxidation on wide-gap transition metal oxides irradiated by ultraviolet light. In this study, we examine mechanistic aspects of this photoreaction on aqueous TiO(2), α-FeOOH, and α-Fe(2)O(3) particles using electron paramagnetic resonance (EPR) spectroscopy and site-selective deuteration. We demonstrate that the carbohydrates are oxidized at sites involved in the formation of oxo-bridges between the chemisorbed carbohydrate molecule and metal ions at the oxide surface. This bridging inhibits the loss of water (which is the typical reaction of the analogous free radicals in bulk solvent) promoting instead a rearrangement that leads to elimination of the formyl radical. For natural carbohydrates, the latter reaction mainly involves carbon-1, whereas the main radical products of the oxidation are radical arising from H atom loss centered on carbon-1, -2, and -3 sites. Photoexcited TiO(2) oxidizes all of the carbohydrates and polyols, whereas α-FeOOH oxidizes some of the carbohydrates, and α-Fe(2)O(3) is unreactive. These results serve as a stepping stone for understanding the photochemistry on mineral surfaces of more complex biomolecules such as nucleic acids.

Efficient photodegradation of dyes using light-induced self assembly TiO(2)/β-cyclodextrin hybrid nanoparticles under visible light irradiation

A novel β-cyclodextrin (β-CD) grafted titanium dioxide (TiO(2)/β-CD) was synthesized through photo-induced self assembly methods, and its structure was characterized. The TiO(2)/β-CD hybrid nanomaterial showed high photoactivity under visible light irradiation (λ ≥ 400 nm and/or λ ≥ 420 nm) and simulated solar irradiation (λ ≥ 365 nm). Photodegradation of Orange II followed the Langmuir-Hinshelwood kinetics model. The initial rate R(0) of Orange II degradation increased by 6.9, 2.6 and 1.9 times in the irradiation conditions of λ ≥ 420nm, λ ≥ 400nm and λ ≥ 365 nm, respectively. β-CD increased the lifetimes of the excited states of the unreactive guests and facilitated the electron transfer from the excited dye to the TiO(2) conduction band, which enhanced the dye pollutant degradation. Superoxide radicals were shown to be the main reactive species that caused the degradation of Orange II under visible irradiation.