Current Organic Chemistry - Volume 17, Issue 21, 2013

The “green” character of a series of reaction carried out under visible light photocatalysis has been evaluated by calculating the Process Mass Intensity (PMI) and by means of the EATOS software. The indexes obtained were likewise calculated for the thermal version of the same reactions for the sake of comparison. The role of solvents, of the photocatalyst used and of the irradiation apparatus are discussed.

This paper deals with the photocatalytic oxidation of benzyl amine by O2 on TiO2 under visible light irradiation, and it was found that the benzyl amine is selectively converted into N-benzylidenebenzylamine with high conversion (>80 %). The origin of visible light response is not due to the band-gap transition of TiO2, but to the excitation of the charge transfer surface complex constructed by the interaction of benzyl amine with the TiO2 surface, as characterized by spectroscopic analysis such as UV-Vis, FT-IR and DFT calculations. Moreover, investigations of the role of the surface OH groups on TiO2, the electron-transfer from the surface complex as well as kinetic studies provided insight into the reaction mechanisms.

Photocatalysis is becoming particularly important in the realization of partial and selective oxidations of organic substrates, including alcohols and polyols, and at the same time is an example of green catalysis since it moves towards a “sustainable chemistry” with a minimal environmental impact. Heterogeneous systems with well-defined textural characteristics represent a suitable means to tailor the selectivity of photocatalytic processes. Here, we describe the most significant features of photocatalytic systems and the achievements in the partial oxidation of various classes of alcohols, with a focus on titanium dioxide, other semiconducting oxides and polyoxotungstates, soluble models of semiconducting oxides. These compounds share similar primary photoreactions: light absorption causes a charge separation with formation of positive vacancies able to initiate the oxidation of many organic substrates. Works here discussed concern photooxidation reactions carried out in the presence of O2, whose role in not only that of scavenging the photogenerated electrons but also that of producing active oxygen species that take part to the overall oxidation process. We emphasize that the combination of the advances in the preparation of nanostructured materials with mechanistic knowledge, derived from surface science and molecular level investigations, is actually the goal in order to achieve optimal tailoring of photoactive materials for obtaining high selectivity.

Eco-friendly standards require organic synthetic protocols able to combine a low environmental impact with an economical convenience. The aerobic selective oxidation of organic compounds promoted by visible light radiation (including ambient sunlight) is candidate to become one of the most important, cheapest and greenest tools to transform raw materials into more complex molecules. Most of organic photochemical processes reported in the literature in the last century are promoted by high-energy ultraviolet radiation, this limiting the potential benefits of the proposed approaches. Nevertheless, in the last decade several inorganic, organometallic and organic photocatalysts (doped semiconductors, transition metal complexes, organic molecules and ions) have been developed, which are able to adsorb in the visible spectrum. Herein we provide a concise overview of the most significant and recent examples in which these photocatalysts are able to promote the functionalization of organic compounds via selective photooxidation in a molecular oxygen atmosphere.

This review summarizes the main results reported in papers concerning gas-solid photocatalytic partial oxidation of organic compounds with the aim to up-date the state of the art in the study of heterogeneous photocatalysis as an alternative green method to obtain high valuable chemical products in mild conditions. The literature works have focused mainly on the studying both new formulations of photocatalysts, as well as the functionalization of the catalyst surface and paying attention to the optimization of the operating conditions of photocatalytic process. However, the selectivity of the photoreactions depends not only by the catalysts formulation but evidenced strongly variations in dependence from the different design of the photoreactor. This review shows also the application of chemical engineering aspects devoted to the improvement of the performances of gas-solid photoreactors for oxidation reactions. In particular some process aspects have been developed, exploring the potential of gas-solid photocatalytic selective oxidations applications for industrial processes.

The generation of paramagnetic intermediates upon photoinduced reduction of substituted nitroquinolones 1–6 in dimethylsulfoxide/methanol titania suspensions was investigated by in situ EPR spectroscopy. The assignment of the primary photogenerated paramagnetic signals was based on the results of cyclic voltammetry, amperostatic in situ spectroelectrochemistry and in situ EPR/UV-Vis spectroelectrochemistry in aprotic dimethylsulfoxide and dimethylsulfoxide/methanol mixed solvent. The primary reduction step in the cathodically- or in the photocatalytically-induced electron transfer process represents the formation of radical monoanion, the stability of which is crucially influenced by the 1-ethyl substitution at the nitrogen of the 4-pyridone ring of quinolone. 1-Ethyl 6-nitroquinolones typically form stable radical anions with well-resolved EPR spectra, with detailed interpretation of hyperfine coupling constants (hfcc) supported by theoretical calculations. On the other hand, the radical anions of nitroquinolones with amino hydrogen at nitrogen of the enaminone system (N–C=C–C=O) convert rapidly to diamagnetic s-dimer dianions, reduced in the second reversible reduction step to paramagnetic s-dimer radical trianions. The EPR spectra obtained upon prolonged irradiation of 1-ethyl nitroquinolones in titania suspensions were assigned to the R-NO•H intermediates produced via nitro group reduction. Experiments with deuterated methanol unambiguously confirmed the photoinduced reduction of the nitro group, including the interaction with hydrogen from the hydroxyl group of methanol. The generation of reactive radicals formed via methanol and dimethylsulfoxide oxidation in irradiated titania suspensions was investigated by EPR spin trapping technique. TiO2.

Both commercial and home prepared (HP) TiO2 samples have been tested for the photocatalytic reduction of CO2. (HP) TiO2 powders were prepared by using TiCl4 or Ti(OC4H9)4 as the precursors to obtain HP1 and HP2 samples, respectively. Also HP Cu-loaded and SiO2 supported TiO2 powders were prepared. The HP samples were more active than the commercial ones for the photoreduction of CO2 with and without water vapour. HP1 produced mainly formaldehyde, HP2 principally methane. Acetaldehyde was found to be the primary product obtained when HP1 was supported on SiO2. The addition of Cu increased the photocatalytic reactivity either of bulk and SiO2-supported HP1. In particular, 1 wt % of Cu improved the formaldehyde yield obtained with the bare HP1 by one order of magnitude. Differently, the presence of Cu or SiO2 in the HP2 samples markedly reduced the production of methane.

Photocatalytic reduction of carbon dioxide (CO2) was carried out using titanium(IV) oxide (TiO2). Methanol (CH3OH) was detected as the main product, and trace amounts of formic acid, carbon monoxide, methane, and hydrogen were also detected. The prepared decahedral-shaped anatase TiO2 with larger {011} and smaller {001} exposed crystal faces showed larger CH3OH generation than that of commercial anatase TiO2 powder, ST-01 (Ishihara Sangyo Co.). Photodeposition of silver and gold nanoparticles on the decahedral-shaped anatase TiO2 induced an increase in CH3OH production because the deposited metal particles work as reductive sites for multi-electron reduction of CO2.

Forming the heterojunction between two semiconductor photocatalysts is an effective method for achieving high photocatalytic activity in photocatalytic reaction. Here we have constructed the ZnGa2O4/Ga2O3 heterojunction by heating the mixture of GaOOH nanoplates and ZnO. Growing the ZnGa2O4 nanoparticles on the surface of Ga2O3 nanoplates not only improves the photocatalytic activity in CO2 photoreduction, but also increases the selectivity of CO2 photoreduction to CH4.

This tutorial review of the state of the art of organic reactions taking place with the help of photocatalysis provides definition of photocatalysis, considers major types of photocatalysts and reactors used for organic photocatalytic reactions. Important classes of photocatalytic organic reactions are exemplified: oxidation, reduction and hydrogenation, isomerization, cyclization, cycloaddition, substitution, formation of C-C bond, carbonylation, addition to double bond, dehydrogenation, decarboxylation, reduction of CO2 and photosynthesis mimicking. Some examples of industrial-scale reactions are adduced. It is concluded that modern acceleration of the research on photocatalytic organic reactions would undoubtedly lead to wider industrial applications of photocatalysis, especially solar photocatalysis, in the next ten years.

One of the main targets of chemical (as well as of other) research, nowadays, is to explore energy efficient and environmentally friendly synthetic routes to address the increasing global demand for energy and to reduce the use and the production of harmful chemicals. Photocatalytic synthesis routes are assumed to be alternative and promising eco-friendly methods for the production of organic compounds utilizing solar energy. The photoinduced charge separation occurring on the semiconductor surface simultaneously creates reduction and oxidation centers. This unique feature allows multisteps reactions to be carried out on the surface of a single photocatalyst particle. The fact that complex nitrogen containing organic compounds can be easily synthesized by this photocatalytic method starting from simple substrates is of great importance. However, numerous efforts are still required to enhance the efficiency of these photocatalytic reactions as well as to explore new aspects in this field. Previous reports demonstrate that different valuable nitrogen containing organic compounds can be photocatalytically synthesized starting even from simple molecules such as dinitrogen and hydrocarbons. The main results of these reports are being highlighted herein.

In this minireview, advances in the applications of graphene (GR)-based composite photocatalysts for selective organic synthesis have been summed up systematically. GR, with unique planar structure, excellent transparency, superior electron conductivity, high specific surface area and high chemical stability, is proven to be a promising component to construct effective GR-based composite photocatalysts for organic transformations, including selective oxidation of alcohols, epoxidation of alkenes, hydroxylation of phenol, and reduction of carbon dioxide (CO2). In addition to the well-known role of GR as a photoelectron reservoir, the novel functions of GR as an organic dye-like macromolecular photosensitizer and graphene oxide (GO) itself as a photocatalyst for selective synthesis are also presented.

Traditional processes for making chemicals are unsustainable in terms of resources and environmental impact. The present paper is a review of the most recent advances in the application of selective photocatalytic reactions to organic synthesis, which, in the last years, has attracted the interest of the scientific community addressing on development of environmentally benign synthetic processes. Indeed, selective photocatalysis is operated at ambient temperature and pressure, needs no complex equipments, does not use dangerous chemical reagents and solvents, does not release harmful wastes into the environment and can utilize solar light. Reactions discussed in this paper, as a green approach for valuable fine chemical synthesis, are: i) photocatalytic hydrogenation and/or transfer hydrogenation of ketones and nitrocompounds by using both UV and visible light; ii) photocatalytic hydrogenation and selective partial hydrogenation of unsaturated compounds; iii) selective partial oxidation of alkanes, alkenes, alcohols, aliphatic acids, benzene and other aromatic compounds by using both UV and visible light. Use of Photocatalytic Membrane Reactors (PMRs) in reductions (e.g. CO2 to fuels) and in selective partial oxidations (e.g. benzene to phenol) is reported, evidencing that opening up of a new avenue is expected in organic syntheses, thanks to the synergy of heterogeneous photocatalysis and membrane processes. Nevertheless, these studies are still at nascent stady and much work is needed before taking advantage of PMRs potentiality at industrial level in this interdisciplinary area.

The generation of renewable energy through photocatalysis is an attractive option to utilize the abundantly available solar radiation for a sustainable future. Photocatalysis refers to charge-carrier, i.e. electron and hole, mediated reactions occurring on a semiconductor surface in presence of ultraviolet or visible light radiation. Photocatalysis is a well established advanced oxidation technique for the decontamination of toxic organic pollutants to CO2 and H2O. However, the generation of energy in the form of hydrogen, hydrocarbon fuels and electricity via photocatalysis is an upcoming field with great many technical challenges towards practical implementation. This review will describe the fundamental reaction mechanism of (i) photocatalytic water splitting, (ii) photocatalytic H2 generation in presence of different sacrificial agents, (iii) H2 and electricity generation in a photofuel cell, (iv) photocatalytic reduction of CO2 to hydrocarbons and useful chemicals, and (v) photocatalytic water-gas shift reaction. A historic and recent perspective of the above conversion techniques, especially with regard to the development of TiO2-based and non-TiO2 materials is provided. The activity of different materials for the above reactions based on quantifiers like reaction rate, quantum yield and incident-photon-to-current efficiency is compared, and key design considerations of the “best” photocatalyst or photoelectrode is outlined. An overall assessment of the research area indicates that the presently achieved quantum efficiencies for the above reactions are rather moderate in the visible region, and the goal is to develop a catalyst that absorbs visible radiation, provides good charge-carrier separation, and exhibits high stability for long periods of usage.