Reply to the 'Comment on "CO2 as oxidant: an unusual light-assisted catalyst free oxidation of aldehydes to acids under mild conditions"' by S. R. Khan, S. Saini, K. Naresh, A. Kumari, V. Aniya, P. K. Khatri, A. Ray and S. L. Jain, Chem. Commun., 2022, 58, 2208.

Our recent Communication (S. R. Khan et al., Chem. Commun., 2022, 58, 2208) suggested that CO2 can be used as a potential oxidant under light irradiation without using any catalyst for the oxidation of aldehydes to acids at room temperature. The Comment based on the published literature on the catalytic oxidation of aromatic aldehydes by CO2 and thermodynamical data argued on the realism of the experimental data.

Visible-light driven reaction of CO2 with alcohols using a Ag/CeO2 nanocomposite: first photochemical synthesis of linear carbonates under mild conditions.

The first photochemical synthesis of linear carbonates from the reaction of CO2 with alcohols using a silver-doped ceria nanocomposite at room temperature under visible light irradiation is described. DFT calculations suggested the electron transfer from Ag 4d states to Ce 4f states in the composite for the photoreaction.

Visible Light-Active Ternary Heterojunction Photocatalyst for Efficient CO2 Reduction with Simultaneous Amine Oxidation and Sustainable H2O2 Production.

The present work described a unique approach for CO2 reduction to methanol along with the oxidation of various amines to the corresponding imines and photocatalytic H2O2 production from H2O and molecular O2 using a heterojunction photocatalyst made up of ZnIn2S4/Ni12P5/g-C3N4(NCZ) under visible light irradiation. The photocatalysts were synthesized via a high-temperature treatment of nickel and phosphorous precursors with g-C3N4 followed by decoration of ZnIn2S4. The synthesized photocatalysts were characterized using various spectroscopic and microscopic techniques. The density functional theory (DFT) studies suggested the participation of the valence band maximum (VBM) from Ni12P5 and the conduction band maximum (CBM) from ZnIn2S4 in the ternary NCZ heterojunction. The ternary composite exhibited superior photocatalytic activity compared to that of its individual components due to the formation of a heterojunction, thereby enhancing the transfer efficiency of electrons from the conduction band of g-C3N4 to that of ZnIn2S4 using Ni12P5 as an electron bridge. Moreover, the reduced band gap of the ternary heterojunction played a key role in its higher efficiency.

Visible Light-Driven Metal-Organic Framework-Mediated Activation and Utilization of CO2 for the Thiocarboxylation of Olefins.

Visible light-mediated photoredox catalysis has emerged to be a fascinating approach for the activation of CO2 and its subsequent fixation into valuable chemicals utilizing renewable and inexhaustible solar energy. Although great progress has been made in CO2 photoreduction, visible light-assisted organic synthesis using CO2 as a reactive substrate is rarely explored. Herein, we report an efficient, facile, and economically viable photoredox-mediated approach for the synthesis of important ß-thioacids via carboxylation of olefins with CO2 and thiols over a porous functionalized metal-organic framework (MOF), Fe-MIL-101-NH2, as a photocatalyst under ambient conditions. This multicomponent reaction offers wide substrate scope, mild reaction conditions, easy work-up, cost-effective and reusable photocatalysts, and higher product selectivity. Computational studies suggested that CO2 interacts with the thiophenol-styrene adduct to facilitate the synthesis of ß-thioacids in almost quantitative yields.

CO2 as oxidant: an unusual light-assisted catalyst free oxidation of aldehydes to acids under mild conditions.

A novel visible light-driven catalyst-free oxidation of aldehydes using CO2 both in batch and flow photoreactors to get corresponding acids along with the formation of CO in the effluent gas is described.

Light-induced synthesis of unsymmetrical organic carbonates from alcohols, methanol and CO2 under ambient conditions.

The present work describes the first visible light-assisted, metal-free and organic base 1,1,3,3-tetramethyl guanidine (TMG) mediated synthesis of unsymmetrical methyl aryl/alkyl carbonates from the reaction of alcohols, methanol, and CO2 in high to excellent yields under atmospheric pressure and ambient temperature conditions.

Dinuclear uranium(VI) salen coordination compound: an efficient visible-light-active catalyst for selective reduction of CO2 to methanol.

A new dinuclear uranyl salen coordination compound, [(UO2)2(L)2]·2MeCN [L = 6,6'-((1E,1'E)-((2,2-dimethylpropane-1,3-diyl)bis(azaneylylidene))-bis(methaneylylidene))bis(2-methoxyphenol)], was synthesized using a multifunctional salen ligand to harvest visible light for the selective photocatalytic reduction of CO2 to MeOH. The assembling of the two U centers into one coordination moiety via a chelating-bridging doubly deprotonated tetradentate ligand allowed the formation of U centers with distorted pentagonal bipyramid geometry. Such construction of compounds leads to excellent activity for the photocatalytic reduction of CO2, permitting a production rate of 1.29 mmol g-1 h-1 of MeOH with an apparent quantum yield of 18%. Triethanolamine (TEOA) was used as a sacrificial electron donor to carry out the photocatalytic reduction of CO2. The selective methanol formation was purely a photocatalytic phenomenon and confirmed using isotopically labeled 13CO2 and product analysis by 13C-NMR spectroscopy. The spectroscopic studies also confirmed the interaction of CO2 with the molecule of the title complex. The results of these efforts made it possible to understand the reaction mechanism using ESI-mass spectrometry.

Highly efficient conversion of the nitroarenes to amines at the interface of a ternary hybrid containing silver nanoparticles doped reduced graphene oxide/ graphitic carbon nitride under visible light.

In this report, a ternary Ag-rGO/g-C3N4 hybrid was synthesized by a simple hydrothermal approach for the photocatalytic reduction of nitroarene compounds into their corresponding amines under visible light. Importantly, the present method did not require reducing agents, like hydrazine hydrate; instead methanol has been used as a source of electrons and protons for the photoreduction process. After grafting of Ag NPs, a significant enhancement in the efficiency of the rGO/g-C3N4 for the reduction of nitrobenzenes was observed. Under optimized experimental conditions, the conversion of nitrobenzene and yield of aniline were determined to be 99% and 98%, respectively under visible light illumination for 4 h. The nitrobenzene compounds bearing both electron donating and withdrawing groups were selectively converted into their corresponding aniline products without altering the functionality. The enhanced performance of the developed photocatalyst attributed to the effective separation of photoexcited electrons on the photocatalyst surface and their subsequent transfer for the reduction of nitrobenzene molecules.

Ternary hybrid TiO2-PANI-AuNPs for photocatalytic A3-coupling of aldehydes, amines and alkynes: First photochemical synthesis of propargyl amines.

The present paper describes the successful synthesis and application of a ternary hybrid consists of gold nanoparticles decorated on titania-polyaniline (TiO2-PANI-AuNPs) for A3-coupling between aldehydes, terminal alkynes and amines under visible irradiation. The synthesis of ternary hybrid involved the coating of PANI on TiO2 via an oxidative polymerization method followed by the deposition of AuNPs on TiO2-PANI using trisodium citrate as a reducing agent. The synthesized photocatalyst was characterized by various techniques like FT-IR, SEM, HR-TEM, XRD, UV-VIS, ICP-AES, TGA, XPS and PL spectroscopy. The synthesized photocatalyst exhibited higher efficiency which was presumed due to the suppressed electron-hole recombination and better electron mobility at the surface of the photocatalyst. In the hybrid, polyaniline was found to enhance the activity as well as stability of the photocatalyst owing to its chemical interaction with titania and gold. Furthermore, after the reaction, the photocatalyst could readily be recovered and recycled for several runs with consistent photoactivity.

Synthesis and evaluation of CoPc grafted bismuth oxyhalide (Bi24O31Br10): a visible light-active photocatalyst for CO2 reduction into methanol.

The present study describes the synthesis, characterization, and evaluation of cobalt(ii) phthalocyanine embedded bismuth oxyhalide (CoPc@Bi24O31Br10) for the photoreduction of CO2 into methanol selectively using triethylamine (TEA) as a sacrificial donor under visible light illumination. The highest yield of methanol was found to be 3485.3 µmol per g of the photocatalyst with a methanol formation rate of 145.2 µmol h-1 g-1. The developed photocatalyst was found to be quite stable and showed comparable results for five recycling runs without any significant loss in efficiency. Importantly, no significant leaching of the complex was observed even after the fifth run.

Highly improved photoreduction of carbon dioxide to methanol using cobalt phthalocyanine grafted to graphitic carbon nitride as photocatalyst under visible light irradiation.

A substantially improved methanol yield was achieved from the photoreduction of carbon dioxide under visible light by using a hybrid photocatalyst consisting of molecular cobalt phthalocyanine tetracarboxylic acid (CoPc-COOH) complex immobilized to the organic semiconductor graphitic carbon nitride (g-C3N4) and triethylamine as sacrificial electron donor. The structural and morphological features of the hybrid photocatalyst determined by various techniques like FTIR, UV-Vis, Raman, XPS, TGA, BET etc. After 24 h of light irradiation, the methanol yield by using g-C3N4/CoPc-COOH photocatalyst (50 mg) was found to be 646.5 µmol g-1cat or 12.9 mmol g-1cat with conversion rate 538.75 µmol h-1 g-1cat. However, the use of homogeneous CoPc-COOH (6.5 µmol Co, equivalent to g-C3N4/CoPc-COOH) and g-C3N4 (50 mg) provided 88.5 µmol (1770 µmol g-1cat) and 59.2 µmol (1184 µmol g-1cat) yield of methanol, respectively under identical conditions. The improved photocatalytic efficiency of the hybrid was attributed to the binding ability of CoPc-COOH to CO2 that provided the higher CO2 concentration on the support. Further, the semiconductor support provided better electron mobility and charge separation with the integrated benefit of facile recovery and recycling of the material at the end of the reduction process.

Synthesis and tribological behavior of fatty acid constituted tetramethylguanidinium (TMG) ionic liquids for a steel/steel contact.

In the present study a number of fatty acid constituted ionic liquids having tetramethylguanidinium ion as a cationic counterpart were synthesized by neutralization of 1,1,3,3­tetramethyguanidine (TMG) with fatty acids having varying degree of alkyl chain and olefinic bonds. The structure of the synthesized ionic liquids was thoroughly characterized using a number of analytical tools such as TGA, FT-IR, 1H and 13C NMR spectroscopy. The tribo-properties of the obtained ionic liquids as high performance anti-friction and wear reducing additives were studied in different dosage to mineral base oil under condition of mixed/boundary lubrication. It was found that the anti-wear and friction reduction properties of blends were improved with increasing the alkyl chain in constituted fatty acid ionic liquids.

Visible light assisted hydrogen generation from complete decomposition of hydrous hydrazine using rhodium modified TiO2 photocatalysts.

Hydrogen is considered to be an ideal energy carrier, which produces only water when combined with oxygen and thus has no detrimental effect on the environment. While the catalytic decomposition of hydrous hydrazine for the production of hydrogen is well explored, little is known about its photocatalytic decomposition. The present paper describes a highly efficient photochemical methodology for the production of hydrogen through the decomposition of aqueous hydrazine using titanium dioxide nanoparticles modified with a Rh(i) coordinated catechol phosphane ligand (TiO2-Rh) as a photocatalyst under visible light irradiation. After 12 h of visible light irradiation, the hydrogen yield was 413 µmol g-1 cat with a hydrogen evolution rate of 34.4 µmol g-1 cat h-1. Unmodified TiO2 nanoparticles offered a hydrogen yield of 83 µmol g-1 cat and a hydrogen evolution rate of only 6.9 µmol g-1 cat h-1. The developed photocatalyst was robust under the experimental conditions and could be efficiently reused for five subsequent runs without any significant change in its activity. The higher stability of the photocatalyst is attributed to the covalent attachment of the Rh complex, whereas the higher activity is believed to be due to the synergistic mechanism that resulted in better electron transfer from the Rh complex to the conduction band of TiO2.

A Prussian blue/carbon dot nanocomposite as an efficient visible light active photocatalyst for C-H activation of amines.

A Prussian blue/carbon dot (PB/CD) nanocomposite was synthesised and used as a visible-light active photocatalyst for the oxidative cyanation of tertiary amines to α-aminonitriles by using NaCN/acetic acid as a cyanide source and H2O2 as an oxidant. The developed photocatalyst afforded high yields of products after 8 h of visible light irradiation at room temperature. The catalyst was recycled and reused several times without any significant loss in its activity.

Nickel Decorated on Phosphorous-Doped Carbon Nitride as an Efficient Photocatalyst for Reduction of Nitrobenzenes.

Nickel nanoparticle-decorated phosphorous-doped graphitic carbon nitride (Ni@g-PC3N4) was synthesized and used as an efficient photoactive catalyst for the reduction of various nitrobenzenes under visible light irradiation. Hydrazine monohydrate was used as the source of protons and electrons for the intended reaction. The developed photocatalyst was found to be highly active and afforded excellent product yields under mild experimental conditions. In addition, the photocatalyst could easily be recovered and reused for several runs without any detectable leaching during the reaction.

A novel Ru/TiO2 hybrid nanocomposite catalyzed photoreduction of CO2 to methanol under visible light.

A novel in situ synthesized Ru(bpy)3/TiO2 hybrid nanocomposite is developed for the photoreduction of CO2 into methanol under visible light irradiation. The prepared composite was characterized by means of SEM, TEM, XRD, DT-TGA, XPS, UV-Vis and FT-IR techniques. The photocatalytic activity of the synthesized hybrid catalyst was tested for the photoreduction of CO2 under visible light using triethylamine as a sacrificial donor. The methanol yield for the Ru(bpy)3/TiO2 hybrid nanocomposite was found to be 1876 µmol g(-1) cat (ϕMeOH 0.024 mol Einstein(-1)) that was much higher in comparison with the in situ synthesized TiO2, 828 µmol g(-1) cat (ϕMeOH 0.010 mol Einstein(-1)) and the homogeneous Ru(bpy)3Cl2 complex, 385 µmol g(-1) cat (ϕMeOH 0.005 mol Einstein(-1)).

Dual catalysis with magnetic chitosan: direct synthesis of cyclic carbonates from olefins with carbon dioxide using isobutyraldehyde as the sacrificial reductant.

Chitosan coated magnetic nanoparticles were synthesized and used as a support for the immobilization of the cobalt(II) acetylacetonate complex [Co(acac)2] and quaternary triphenylphosphonium bromide [P(+)Ph3Br(-)] targeting -NH2 and -OH moieties located on the surface of chitosan. The synthesized material was used as a catalyst for one pot direct synthesis of cyclic carbonates from olefins via an oxidative carboxylation approach with carbon dioxide using isobutyraldehyde as the sacrificial reductant and molecular oxygen as the oxidant. After the reaction, the catalyst was recovered by applying an external magnet and reused for several runs without significant loss in catalytic activity and no leaching was observed during this course.

Photo-induced reduction of CO2 using a magnetically separable Ru-CoPc@TiO2@SiO2@Fe3O4 catalyst under visible light irradiation.

An efficient photo-induced reduction of CO2 using magnetically separable Ru-CoPc@TiO2@SiO2@Fe3O4 as a heterogeneous catalyst in which CoPc and Ru(bpy)2phene complexes were attached to a solid support via covalent attachment under visible light is described. The as-synthesized catalyst was characterized by a series of techniques including FTIR, UV-Vis, XRD, SEM, TEM, etc. and subsequently tested for the photocatalytic reduction of carbon dioxide using triethylamine as a sacrificial donor and water as a reaction medium. The developed photocatalyst exhibited a significantly higher catalytic activity to give a methanol yield of 2570.78 µmol per g cat after 48 h.

Graphene oxide supported molybdenum cluster: first heterogenized homogeneous catalyst for the synthesis of dimethylcarbonate from CO2 and methanol.

The octahedral molybdenum cluster-based compound, Cs2 Mo6 Br(i) 8 Br(a) 6 was immobilized on graphene oxide (GO) by using a facile approach. High resolution transmission electron microscopy results revealed that molybdenum clusters were uniformly distributed on the GO nanosheets. Cs2 Mo6 Br(i) 8 Br(a) 6 was attached to the GO support via chemical interaction between apical ligands of Mo6 Br(i) 8 Br(a) 6 cluster units and oxygen functionalities of GO, as revealed by XPS studies. The developed material was used for the synthesis of dimethyl carbonate by reduction of carbon dioxide. The synthesized catalyst, that is, GO-Cs2 Mo6 Br(i) 8 Br(a) x , exhibited higher catalytic efficiency than its homogeneous analogue without using dehydrating agent. The catalyst was found to be efficiently recyclable without significant loss of catalytic activity.

Cobalt phthalocyanine immobilized on graphene oxide: an efficient visible-active catalyst for the photoreduction of carbon dioxide.

New graphene oxide (GO)-tethered-Co(II) phthalocyanine complex [CoPc-GO] was synthesized by a stepwise procedure and demonstrated to be an efficient, cost-effective and recyclable photocatalyst for the reduction of carbon dioxide to produce methanol as the main product. The developed GO-immobilized CoPc was characterized by X-ray diffraction (XRD), FTIR, XPS, Raman, diffusion reflection UV/Vis spectroscopy, inductively coupled plasma atomic emission spectroscopy (ICP-AES), thermogravimetric analysis (TGA), Brunauer-Emmett-Teller (BET), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). FTIR, XPS, Raman, UV/Vis and ICP-AES along with elemental analysis data showed that Co(II) -Pc complex was successfully grafted on GO. The prepared catalyst was used for the photocatalytic reduction of carbon dioxide by using water as a solvent and triethylamine as the sacrificial donor. Methanol was obtained as the major reaction product along with the formation of minor amount of CO (0.82 %). It was found that GO-grafted CoPc exhibited higher photocatalytic activity than homogeneous CoPc, as well as GO, and showed good recoverability without significant leaching during the reaction. Quantitative determination of methanol was done by GC flame-ionization detector (FID), and verification of product was done by NMR spectroscopy. The yield of methanol after 48 h of reaction by using GO-CoPc catalyst in the presence of sacrificial donor triethylamine was found to be 3781.8881 µmol g(-1) cat., and the conversion rate was found to be 78.7893 µmol g(-1) cat. h(-1). After the photoreduction experiment, the catalyst was easily recovered by filtration and reused for the subsequent recycling experiment without significant change in the catalytic efficiency.