3D-Printed Eosin Y-Based Heterogeneous Photocatalyst for Organic Reactions

Heterogenization of Eosin Y by 3D-printing and its application in photocatalysis are reported. The approach allows a fine tuning of the photocatalyst morphology and its rapid preparation. Photocatalytic activity was evaluated through model organic reactions involving oxidation, reduction, and photosensitization pathways. The efficiency, recyclability and stability of 3D printed EY is remarkable paving the way to new generation of heterogeneous photocatalysts with a perfect control of their shape and adaptable to any photoreactors.

Enhancing Photoredox Catalysis in Aqueous Environments: Ruthenium Aqua Complex Derivatization of Graphene Oxide and Graphite Rods for Efficient Visible-Light-Driven Hybrid Catalysts

A ruthenium aqua photoredox catalyst has been successfully heterogeneneized on graphene oxide (GO@trans-fac-3) and graphite rods (GR@trans-fac-3) for the first time and have proven to be sustainable and easily reusable systems for the photooxidation of alcohols in water, in mild and green conditions. We report here the synthesis and total characterization of two Ru(II)-polypyridyl complexes, the chlorido trans-fac-[RuCl(bpea-pyrene)(bpy)](PF6) (trans-fac-2) and the aqua trans-fac-[Ru(bpea-pyrene)(bpy)OH2](PF6)2 (trans-fac-3), both containing the N-tridentate, 1-[bis(pyridine-2-ylmethyl)amino]methylpyrene (bpea-pyrene), and 2,2'-bipyridine (bpy) ligands. In both complexes, only a single isomer, the trans-fac, has been detected in solution and in the solid state. The aqua complex trans-fac-3 displays bielectronic redox processes in water, assigned to the Ru(IV/II) couple. The trans-fac-3 complex has been heterogenized on different types of supports, (i) on graphene oxide (GO) through π-stacking interactions between the pyrene group of the bpea-pyrene ligand and the GO and (ii) both on glassy carbon electrodes (GC) and on graphite rods (GR) through oxidative electropolymerization of the pyrene group, which yield stable heterogeneous photoredox catalysts. GO@trans-fac-3- and GR/poly trans-fac-3-modified electrodes were fully characterized by spectroscopic and electrochemical methods. Trans-fac-3 and GO@trans-fac-3 photocatalysts (without a photosensitizer) showed good catalytic efficiency in the photooxidation of alcohols in water under mild conditions and using visible light. Both photocatalysts display high selectivity values (>99%) even for primary alcohols in accordance with the presence of two-electron transfer processes (2e-/2H+). GO@trans-fac-3 keeps intact its homogeneous catalytic properties but shows an enhancement in yields. GO@trans-fac-3 can be easily recycled by filtration and reused for up to five runs without any significant loss of catalytic activity. Graphite rods (GR@trans-fac-3) were also evaluated as heterogeneous photoredox catalysts showing high turnover numbers (TON) and selectivity values.

Linker-Assisted CdS-TiO2 Nanohybrids as Reusable Visible Light Photocatalysts for the Oxidative Hydroxylation of Arylboronic Acids

A variety of phenols have been obtained in aqueous media with moderate to excellent chemical yields (≤100%) by using arylboronic acids and esters as substrates, a robust CdS-TiO₂ nanohybrid as a heterogeneous photocatalyst, visible light irradiation (467 nm), and an O₂-saturated atmosphere. The nanohybrid was prepared through a linker-assisted methodology that uses mercapto alkanoic acids as the organic linkers. The nanohybrid showed improved photocatalytic activity in the hydroxylation of substituted arylboronic acids and phenyl boronic esters compared with that of pristine CdS quantum dots. The nanohybrid can be reused in up to five photocatalytic cycles with ∼90% of its outstanding activity preserved.

Adducing Knowledge Capabilities of Instrumental Techniques Through the Exploration of Heterostructures' Modification Methods

The ongoing evolution of technology has facilitated the global research community to rapidly escalate the constant development of novel advancements in science. At the forefront of such achievements in the field of photocatalysis is the utilisation, and in oftentimes, the adaptation of modern instrumentation to understand photo-physical properties of complex heterostructures. For example, coupling in-situ X-ray Raman scattering spectroscopy for real-time degradation of catalytic materials.

Green Chemistry, Biocatalysis, and the Chemical Industry of the Future

In the movement to decarbonize our economy and move away from fossil fuels we will need to harness the waste products of our activities, such as waste lignocellulose, methane, and carbon dioxide. Our wastes need to be integrated into a circular economy where used products are recycled into a manufacturing carbon cycle. Key to this will be the recycling of plastics at the resin and monomer levels. Biotechnology is well suited to a future chemical industry that must adapt to widely distributed and diverse biological chemical feedstocks. Our increasing mastery of biotechnology is allowing us to develop enzymes and organisms that can synthesize a widening selection of desirable bulk chemicals, including plastics, at commercially viable productivities. Integration of bioreactors with electrochemical systems will permit new production opportunities with enhanced productivities and the advantage of using a low-carbon electricity from renewable and sustainable sources.

Exploring metallic and plastic 3D printed photochemical reactors for customizing chemical synthesis

Visible light photocatalysis is a rapidly developing branch of chemical synthesis with outstanding sustainable potential and improved reaction design. However, the challenge is that many particular chemical reactions may require dedicated tuned photoreactors to achieve maximal efficiency. This is a critical stumbling block unless the possibility for reactor design becomes available directly in the laboratories. In this work, customized laboratory photoreactors were developed with temperature stabilization and the ability to adapt different LED light sources of various wavelengths. We explore two important concepts for the design of photoreactors: reactors for performing multiple parallel experiments and reactors suitable for scale-up synthesis, allowing a rapid increase in the product amount. Reactors of the first type were efficiently made of metal using metal laser sintering, and reactors of the second type were successfully manufactured from plastic using fused filament fabrication. Practical evaluation has shown good accuracy of the temperature stabilization in the range typically required for organic synthesis for both types of reactors. Synthetic application of 3D printed reactors has shown good utility in test reactions-furan C-H arylation and thiol-yne coupling. The critical effect of temperature stabilization was established for the furan arylation reaction: heating of the reaction mixture may lead to the total vanishing of photochemical effect.

Organic dyes supported on silicon-based materials: synthesis and applications as photocatalysts

The most important advance in photocatalysis in the last decade has been the synthesis and application of organic compounds to promote this process.

Intensification of Heterogeneous Photocatalytic Reactions Without Efficiency Losses: The Importance of Surface Catalysis

Advances in LED and photoreactor technology have brought semiconductor photocatalysis to the verge of feasibility of industrial application for the synthesis of value-added chemicals. However, the often observed efficiency losses under intensified illumination conditions still present a great challenge. This perspective discusses the origin of these efficiency losses and what needs to be done to prevent or counteract it and pave the way for efficient, intensified heterogeneous photocatalytic processes. The role of surface catalysis is particularly highlighted as one of the rate-limiting steps.

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Nanosized CdS as a Reusable Photocatalyst: The Study of Different Reaction Pathways between Tertiary Amines and Aryl Sulfonyl Chlorides through Visible-Light-Induced N-Dealkylation and C-H Activation Processes

It has been found that the final products of the reaction of sulfonyl chlorides and tertiary amines in the presence of cadmium sulfide nanoparticles under visible light irradiation are highly dependent on the applied reaction conditions. Interestingly, with the change of a reaction condition, different pathways were conducted (visible-light-induced N-dealkylation or sp³ and sp² C-H activation) that lead to different products such as secondary amines and various sulfonyl compounds. Remarkably, all of these reactions were performed under visible light irradiation and an air atmosphere without any additive or oxidant in benign solvents or under solvent-free conditions. During this study, the CdS nanoparticles as affordable, heterogeneous, and recyclable photocatalysts were designed, successfully synthesized, and fully characterized and applied for these protocols. During these studies, intermediates resulting from the oxidation of tertiary amines are trapped during the photoinduced electron transfer (PET) process. The reaction was carried out efficiently with a variety of substrates to give the corresponding products at relatively short times in good to excellent yields in parallel with the use of the visible light irradiation as a renewable energy source. Most of these processes are novel or are superior in terms of cost-effectiveness, safety, and simplicity to published reports.

Applications of Nanomaterials in Asymmetric Photocatalysis: Recent Progress, Challenges, and Opportunities

Asymmetric catalysis is one of the most attractive strategies to obtain important enantiomerically pure chemicals with high quality and production. In addition, thanks to the abundant and sustainable advantages of solar energy, photocatalysis possesses great potential in environmentally benign reactions. Undoubtedly, asymmetric photocatalysis meets the strict demand of modern chemistry: environmentally friendly and energy-sustainable alternatives. Compared with homogeneous asymmetric photocatalysis, heterogeneous catalysis has features of easy separation, recovery, and reuse merits, thus being cost- and time-effective. Herein, the state-of-the-art progress in asymmetric photocatalysis by heterogeneous nanomaterials is addressed. The discussion comprises two sections based on the type of nanomaterials: typical inorganic semiconductors like TiO₂ and quantum dots and emerging porous materials including metal-organic frameworks, porous organic polymers, and organic cages. Finally, the challenges and future developments of heterogeneous asymmetric photocatalysis are proposed.

Photocatalytic Properties of Bi2-xTi2O7-1.5x (x = 0, 0.5) Pyrochlores: Hybrid DFT Calculations and Experimental Study

The photocatalytic properties of Bi_2-xTi₂O_7-1.5x (x = 0, 0.5) pyrochlores are examined via ab initio calculations and experiments. A coprecipitation method is applied for the synthesis of nanopowder pyrochlores. The pyrochlore phase formation starts at 500 °C (Bi₂Ti₂O₇) and 550 °C (Bi_1.5Ti₂O_6.25). Nanopowders are found to be a metastable character of pyrochlore phases. The presence of bismuth and oxygen vacancies enhances the thermal stability of the Bi_1.5Ti₂O_6.25 phase in comparison with the Bi₂Ti₂O₇ phase. The estimated crystallite size is 30-40 nm with noticeable agglomerates of about 100-300 nm according to scanning electron microscopy (SEM) and with the formation of particles (510-580 nm) in the aqueous medium. The isoelectric points of the nanopowders seem to be shifted to the strongly acidic region, resulting in the formation of negative surface particle charges of -33 mV (Bi₂Ti₂O₇) and -27 mV (Bi_1.5Ti₂O_6.25) at pH 5.88 in distilled water. The specific surface area is 11.5 m²/g (Bi₂Ti₂O₇) and 12.00 m²/g (Bi_1.5Ti₂O_6.25). The use of the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional allows achieving an excellent agreement between theoretical and experimental structural parameters. The screened Coulomb hybrid HSE03 functional is the most appropriate for describing the optoelectronic properties. Bismuth titanate pyrochlores are wide-gap semiconductors with strong abilities to be active photocatalysts under visible irradiation. The optical E_g values for direct/indirect transition according to the experiment, 3.19/2.94 eV (x = 0) and 3.24/3.03 eV (x = 0.5), and the DFT/HSE03 calculations, 2.92/2.87 (x = 0) and 3.42/- eV (x = 0.5), are in the visible light region and are close. The calculated low effective masses of the charge carriers and suitable band edge positions confirm the ability of the pyrochlores to act as photocatalysts. The photocatalytic activity has been evaluated through the decomposition of rhodamine B under visible irradiation. Bi₂Ti₂O₇ shows the highest activity in comparison with Bi_1.5Ti₂O_6.25, which is in good agreement with theoretically predicted and experimentally revealed characteristics.

Heterogeneous photocatalysis in flow chemical reactors

The synergy between photocatalysis and continuous flow chemical reactors has shifted the paradigms of photochemistry, opening new avenues of research with safer and scalable processes that can be readily implemented in academia and industry. Current state-of-the-art photocatalysts are homogeneous transition metal complexes that have favourable photophysical properties, wide electrochemical redox potentials, and photostability. However, these photocatalysts present serious drawbacks, such as toxicity, limited availability, and the overall cost of rare transition metal elements. This reduces their long-term viability, especially at an industrial scale. Heterogeneous photocatalysts (HPCats) are an attractive alternative, as the requirement for the separation and purification is largely removed, but typically at the cost of efficiency. Flow chemical reactors can, to a large extent, mitigate the loss in efficiency through reactor designs that enhance mass transport and irradiation. Herein, we review some important developments of heterogeneous photocatalytic materials and their application in flow reactors for sustainable organic synthesis. Further, the application of continuous flow heterogeneous photocatalysis in environmental remediation is briefly discussed to present some interesting reactor designs that could be implemented to enhance organic synthesis.

Synthetic Approaches for C-N Bonds by TiO2 Photocatalysis

Nitrogen-containing organic compounds possess the most important status in drug molecules and agricultural chemicals. More than 80% currently used drugs have at least a C-N bond. The green and mild methodology to prepare diverse C-N bonds to replace traditional harsh preparation protocols is always a hotspot in modern synthetic chemistry. TiO₂-based nanomaterials, considered as environmentally benign, stable, and powerful photocatalysts, have recently been applied in some certain challenging organic synthesis including construction of useful C-N compounds under mild conditions that are impossible to complete by conventional catalysis. This mini review would present state-of-the-art paragon examples of TiO₂ photocatalyzed C-N bond formations. The discussion would be divided into two main sections: (1) N-alkylation of amines and (2) C-N formation in heterocycle synthesis. Especially, the mechanism of TiO₂ photocatalytic C-N bond formation through activating alcohol into C=O by photo-induced hole followed by C=NH-R formation and finally hydrogenating C=NH-R into C-N bonds by combination of photo-induced electron/H⁺ assisted with loaded-Pt would be covered in detail. We believe that the mini-review will bring new insights into TiO₂ photocatalysis applied to construct challenging organic compounds through enabling photo-induced hole and electron in a concerted way on coupling two substrate molecules together with respect to their conventionally independent catalysis behavior.

Practical heterogeneous photoredox/nickel dual catalysis for C-N and C-O coupling reactions

Efficient C-N and C-O coupling reactions of aryl halides with amines and alcohols have been developed by using the strategy of heterogeneous visible light photoredox and nickel dual catalysis. Obviously, the joint use of inexpensive and bench-stable CdS and nickel salts, together with mild reaction conditions, makes these two transformations attractive for the synthetic community. This heterogeneous dual catalysis system also proved to be successful in the ligand-free catalytic hydroxylation of aryl bromide with water as a nucleophile. The practicality of this protocol is further emphasized by the scaled-up reaction and the reusability of heterogeneous photocatalysts.

TiO2 Photocatalyzed C–H Bond Transformation for C–C Coupling Reactions

Fulfilling the direct inert C–H bond functionalization of raw materials that are earth-abundant and commercially available for the synthesis of diverse targeted organic compounds is very desirable and its implementation would mean a great reduction of the synthetic steps required for substrate prefunctionalization such as halogenation, borylation, and metalation. Successful C–H bond functionalization mainly resorts to homogeneous transition-metal catalysis, albeit sometimes suffering from poor catalyst reusability, nontrivial separation, and severe biotoxicity. TiO2 photocatalysis displays multifaceted advantages, such as strong oxidizing ability, high chemical stability and photostability, excellent reusability, and low biotoxicity. The chemical reactions started and delivered by TiO2 photocatalysts are well known to be widely used in photocatalytic water-splitting, organic pollutant degradation, and dye-sensitized solar cells. Recently, TiO2 photocatalysis has been demonstrated to possess the unanticipated ability to trigger the transformation of inert C–H bonds for C–C, C–N, C–O, and C–X bond formation under ultraviolet light, sunlight, and even visible-light irradiation at room temperature. A few important organic products, traditionally synthesized in harsh reaction conditions and with specially functionalized group substrates, are continuously reported to be realized by TiO2 photocatalysis with simple starting materials under very mild conditions. This prominent advantage—the capability of utilizing cheap and readily available compounds for highly selective synthesis without prefunctionalized reactants such as organic halides, boronates, silanes, etc.—is attributed to the overwhelmingly powerful photo-induced hole reactivity of TiO2 photocatalysis, which does not require an elevated reaction temperature as in conventional transition-metal catalysis. Such a reaction mechanism, under typically mild conditions, is apparently different from traditional transition-metal catalysis and beyond our insights into the driving forces that transform the C–H bond for C–C bond coupling reactions. This review gives a summary of the recent progress of TiO2 photocatalytic C–H bond activation for C–C coupling reactions and discusses some model examples, especially under visible-light irradiation.

Visible light mediated metal-free thiol-yne click reaction

The carbon-sulfur bond formation reaction is of paramount importance for functionalized materials design, as well as for biochemical applications. The use of expensive metal-based catalysts and the consequent contamination with trace metal impurities are challenging drawbacks of the existing methodologies. Here, we describe the first environmentally friendly metal-free photoredox pathway to the thiol-yne click reaction. Using Eosin Y as a cheap and readily available catalyst, C-S coupling products were obtained in high yields (up to 91%) and excellent selectivity (up to 60 : 1). A 3D-printed photoreactor was developed to create arrays of parallel reactions with temperature stabilization to improve the performance of the catalytic system.

Reduced Graphene Oxide-Immobilized Tris(bipyridine)ruthenium(II) Complex for Efficient Visible-Light-Driven Reductive Dehalogenation Reaction

A sodium benzenesulfonate (PhSO3Na)-functionalized reduced graphene oxide was synthesized via a two-step aryl diazonium coupling and subsequent NaCl ion-exchange procedure, which was used as a support to immobilize tris(bipyridine)ruthenium(II) complex (Ru(bpy)3Cl2) by coordination reaction. This elaborated Ru(bpy)3-rGO catalyst exhibited excellent catalytic efficiency in visible-light-driven reductive dehalogenation reactions under mild conditions, even for ary chloride. Meanwhile, it showed the comparable reactivity with the corresponding homogeneous Ru(bpy)3Cl2 catalyst. This high catalytic performance could be attributed to the unique two-dimensional sheet-like structure of Ru(bpy)3-rGO, which efficiently diminished diffusion resistance of the reactants. Meanwhile, the nonconjugated PhSO3Na-linkage between Ru(II) complex and the support and the very low electrical conductivity of the catalyst inhibited energy/electron transfer from Ru(II) complex to rGO support, resulting in the decreased support-induced quenching effect. Furthermore, it could be easily recycled at least five times without significant loss of catalytic reactivity.

The application of heterogeneous visible light photocatalysts in organic synthesis

The recent advances of organic synthesis reactions based on heterogeneous visible-light photocatalysis are reviewed.

Photocatalysis with TiO2 Applied to Organic Synthesis

Titanium dioxide is a versatile heterogeneous catalyst. Absorption of light by a TiO2 particle leads to the formation of an electron–hole pair. Electron transfer from or to the particle induces redox reactions. Although mainly applied in the context of environmental chemistry, these processes are also used to selectively transform organic compounds. Oxidations and reductions have been carried out. Applications to the synthesis of heterocycles have been reported. Many C–C bond formation reactions have been performed. Owing to adsorption of the substrates or by different surface modifications, visible light can be used to excite the catalytic system, which generates mild reaction conditions.

Light-mediated cascade transformation of activated alkenes: BiOBr nanosheets as efficient photocatalysts for the synthesis of α-aryl-β-trifluoromethyl amides

A facile light induced, BiOBr nanosheet promoted one-pot consecutive trifluoromethylation/aryl migration/desulfonylation and N–H bond formation of activated alkenes is proposed.

Graphitic carbon nitride polymer as a recyclable photoredox catalyst for fluoroalkylation of arenes

Heterogeneous catalysis for trifluoromethylations and perfluoroalkylations has been performed. Through the usage of cheap, metal-free and recyclable mesoporous graphitic carbon nitride (mpg-CN) it was possible to fluoroalkylate various arenes by the reductive activation of sulfonyl chlorides with visible light. Thus, we were able to demonstrate the robustness and versatility of mpg-CN as a photoredox catalyst beyond water splitting and the activation of oxygen.