A light assisted, catalyst free oxidation of aldehydes to carboxylic acids using carbon dioxide

Abstract

The present invention relates to a novel method for visible light driven oxidation of aldehydes to carboxylic acid using CO2 as oxidant in the absence of any catalyst. In the disclosed process, aldehydes, when reacted with carbon dioxide (CO2) in an organic solvent both in batch as well as in continuous flow reactor under conditions of ambient temperature and pressure using a readily available household LED lamp, yield corresponding carboxylic acids along with the formation of CO in the effluent gas.

Description

FIELD OF THE INVENTIONThe present invention relates to a method for visible light driven catalyst free oxidation of aldehydes to carboxylic acid using CO2 as oxidant at ambient pressure condition.BACKGROUND OF THE INVENTIONCarbon dioxide (CO2) concentration is continuously increasing in the earth's atmosphere, attributed mainly due to industrialization and excessive consumption of fossil fuels. Such atmospheric CO2 increase is a major cause of climate change and is accompanied by global warming.Among the various options known for carbon dioxide mitigation, chemical fixation of CO2 to produce valuable chemicals has found considerable interest in recent years (Anwar et al. J. Environ. Management, 2020, 260, 110059; Yang and Lee, Chem. Sci. 2019,10,3905). However, due to the higher thermodynamic and kinetic stability, a significant amount of energy is required for the activation and conversion of CO2 into value-added products. In thermal catalysis, the requirements of significantly elevated temperatures for CO2 activation make these processes highly energy intensive and lead to catalyst deactivation as a result of coke formation typical of high temperature processes.In contrast to the thermal catalysis, single electron reduction of CO2 to radical anion assisted by light, which is cheap, clean, and inexhaustible, is desirable because it is simple, sustainable and uses mild operating conditions. So far, an extensive amount of work has been done for CO2 photoreduction using different photocatalysts including semiconductors, molecular complexes and composite materials for the production of chemicals such as CO, CH4, CH3OH etc.On the other hand, the use of CO2 as an oxidant is rarely reported in the literature. In open literature (Zhang et al J. Am. Chem. Soc. 2010, 132, 3, 914-915; Menon et al Org. Lett. 2010, 12, 2653-2655), there are reports of the oxidation of aldehydes to corresponding acids using CO2 as an oxidant in the presence of DBU as base and N-heterocyclic carbenes (NHCs) as catalysts at room temperature. The use of homogeneous additional base and organocatalyst limit the practical applicability of this developed process in the prior art. To the best of our knowledge, there is no prior art on the use of CO2 at ambient temperature and pressure as an oxidant under light irradiation without using any catalyst.Oxidation of aldehydes to carboxylic acids is one of the important transformations as carboxylic acids and their derivatives are important synthetic intermediates for the fine chemicals, pharmaceuticals, and functional materials. A number of methods using stoichiometric inorganic or organic oxidants have been developed for oxidizing aldehydes to carboxylic acids, however, generation of copious amounts of hazardous waste limited their practical applicability. Subsequently, a number of catalytic methods using environment-friendly oxidant, molecular oxygen have been developed (Choudhary et al (2011) Catal. Commun. (13), 82-86.; Vanoye et al. (2013) Org. Lett. (15), 5978-5981.; Shinji et al. (2016) Chem. Lett. (45), 188-190. ; Peixoto et al. (2017) J. Org. Chem. (82) 6232-6241). Recently, the base-promoted oxidation of aldehydes using transition metals such as (Rh, Ag, Cu, and Fe) with molecular oxygen as oxidant have been reported (Wang et al (2016) Green Chem.(18) 4605-4610. Although these strategies represent a considerable advancement, the expensive nature and multi step synthesis of metal catalysts along with the use of base and oxygen as oxidant leads to economical and environmental issues. Therefore, development of simple, environmentally benign, catalyst free synthesis of carboxylic acids using abundant, inexpensive CO2 as oxidant is highly desired.OBJECTIVE OF THE INVENTIONThe main object of the present invention is to provide a catalyst-free process for the oxidation of aldehydes to carboxylic acids using CO2 as oxidant under mild reaction conditions.Another object of the present invention is to provide a method for the catalyst free photochemical activation of CO2 to provide necessary oxygen for the oxidation along with the production of carbon monoxide as a by-product.Another objective of the present invention is to provide a novel catalyst free photochemical route for the preparation of carboxylic acids using CO2 as oxidant.Yet another objective of the present invention is to provide to a novel catalyst free photochemical route for the oxidation of aldehydes using CO2 as oxidant with the conversion in the range of 30-99 %.Yet another objective of the present invention is to provide a novel catalyst free photochemical route for the oxidation of aldehydes with CO2 at atmospheric pressure and ambient temperature ranges of 15-40 °C, preferably at about 25 °C.Yet another objective is to use a polar protic or aprotic solvent for the invention, selected from water, acetonitrile or dimethylformamide or dimethylsulfoxide or tetrahydrofuran, or mixtures thereof as may be appropriate, in particular pure acetonitrile.Yet another objective of the present invention is to provide a novel catalyst free photochemical route for the oxidation of aldehydes to carboxylic acids in the time ranging from 1 to 24 h.Yet another objective of the present invention is to use a visible light source having wavelength (λ) greater than 420 nm or a broad spectrum of solar energy containing such wavelengths for the activation of the carbon dioxide under mild conditions.Yet another objective of the present invention is to provide to a novel catalyst free photochemical route for the oxidation of aldehydes using CO2 as oxidant in a continuous flow reactor with the yield of the acid in the range of 20-80%.Yet another objective of the present invention is to provide a novel catalyst free photochemical reduction of CO2 to CO along with the simultaneous oxidation of aldehydes.SUMMARY OF THE INVENTIONAccordingly, present invention provides a process for a light assisted, catalyst free photochemical process for the oxidation of aldehydes of formula Iusing carbon dioxide comprising the step of:i. oxidizing the aldehyde with CO2 dissolved in a solvent under the light irradiation both in batch or continuous flow photoreactor at the temperature in the range of 20 to 40°C and pressure in the range of 1-5 bar for a period in the range of 1-24 h irradiation time in a batch and continuous flow photo-reactor followed by purification to give corresponding acid with the formation of carbon monoxide as a co-product in the gaseous phase.In an embodiment of the present invention, aldehyde is selected from aromatic aldehydes substituted by electron donating or electron deficient group or aliphatic aldehydes.In another embodiment of the present invention, aldehyde is preferably aryl aldehydes of formula IFormula Iwherein R is selected from the group consisting of hydrogen, halogen, methyl, ethyl, propyl, butyl, amyl, isopropyl, isobutyl, phenyl, tolyl, biphenyl, benzyl, or naphthyl.In yet another embodiment of the present invention, the light irradiation is done by using light source having wavelength λ greater than 420 nm, preferably house hold LED light of 10 to 50 W.In another embodiment of the present invention, solvent is selected from the group consisting of water, acetonitrile (ACN) or dimethylformamide (DMF) or dimethylsulfoxide (DMSO) or tetrahydrofuran (THF), or mixtures thereof as may be appropriate, in particular pure acetonitrile.In yet another embodiment of the present invention, the reaction time is preferably ranging between 1 to 15 h.In yet another embodiment of the present invention, the reaction is effective at ambient temperature (25°C) and at 1 atmospheric pressure of CO2.In yet another embodiment of the present invention, purification is carried out by column chromatography using silica gel bed.In yet another embodiment of the present invention, the yield of the product is ranging from 30-98% determined on the basis of isolated carboxylic acid or based on the recovered aldehyde substrate.Yet another embodiment of the present invention is to provide the first catalyst free photochemical activation of CO2 and its use as an oxygen transfer agent along with the production of CO in the effluent gas.Yet another embodiment of the present invention is to provide an innovative, facile, catalyst free, economically viable and environmentally friendly synthesis of carboxylic acids from oxidation of aldehydes using CO2 as an oxidant.Yet another embodiment of the present invention is to provide a catalyst free photochemical route for the oxidation of aldehydes with CO2 at atmospheric pressure of CO2 in typically ambient temperature ranges of 15-40°C, preferably at about 25°C.Yet another embodiment of the present invention is to provide to a catalyst free photochemical route for the oxidation of aldehydes using CO2 as oxidant in a continuous flow reactor with the yield of carboxylic acid in the range of 20-80%.Yet another embodiment is to provide a novel catalyst free photochemical reduction of CO2 to CO along with the simultaneous oxidation of aldehydes.In another embodiment of the present invention, the formation of CO in the gaseous effluent is determined by refinery gas analyzer (RGA) technique.In another embodiment of the present invention, the oxidation of aldehydes by CO2 to produce carboxylic acids is shown to be amenable to a continuous process.BRIEF DESCRIPTION OF THE DRAWINGSFigure 1 represents reactor set-up for the continuous flow process.DETAILED DESCRIPTION OF THE INVENTION The process comprises reacting an aldehyde having -CHO group in a polar organic solvent with carbon dioxide at temperature ranging from 20 to 40 °C and at atmospheric pressure under the visible light irradiation to prepare the corresponding carboxylic acid along with the production of CO as a co-product in gaseous effluent. As utilized herein including in the claims "oxidation of aldehyde" means insertion of an oxygen derived from CO2 in to the -CHO group of the compound to convert it into -COOH. As utilized herein including in the claims "reactants" collectively references both aldehyde and CO2 (oxidant) and the "solvents" including both aqueous and organic reaction media. Within this disclosure, "visible light" means light having a wavelength (λ) greater than 420 nm. In a preferred embodiment of this invention, any compound having -CHO group can be employed in the process described by this invention. Aromatic aldehydes, - whether substituted by electron donating or electron deficient group - are preferred, but aliphatic aldehydes - both - unsaturated and linear can also be employed. Most of the aforementioned aldehydes are available commercially and used as received. Constituents ReactantsThe present invention is related to the oxidation of the aldehydes with CO2 without using any catalyst under light irradiation. The aldehyde used in the present invention can be an organic compound containing -CHO group. Aromatic aldehydes, which are suitable for the process of this invention include benzaldehyde and its substituted derivatives, heterocyclic aldehydes and analogously, the cyclic, or branched or chain isomers of aliphatic aldehydes both - saturated or unsaturated - containing C4 to C10 carbon and their substituted variants. Oxidant (Carbon dioxide) In the present invention carbon dioxide is used as oxidant in place of conventional oxidants like O2 or H2O2 under visible illumination. Carbon dioxide, among the two oxygen atoms utilized one oxygen atom for the oxidation of -CHO to -COOH and converted to carbon monoxide during the process. The reaction mixture containing substrates and solvent was either saturated with CO2 or purged continuously with CO2 flow for effective oxidation. Organic solvents Substrates (aldehyde and CO2) used in the present invention are preferably dissolved in organic solvents. Specifically polar organic solvents were used mainly due to the higher solubility of CO2 in polar solvents. Suitable organic solvents include specifically, but not limited to dimethylformamide (DMF), dimethylacetamide (DMA), acetonitrile (ACN), dimethylsulfoxide (DMSO) and N-methyl pyrollidone (NMP) or mixtures thereof.Standard protocol (Batch and Continuous process)According to the present invention, aldehyde substrate in a polar organic solvent, preferably acetonitrile in (1:2 to 1:10 weight ratio with respect to the substrate) was taken in to a 60 ml vessel for a batch process. The reaction mixture was saturated with CO2 by purging. The reaction vessel was sealed and irradiated with 20 W LED light for 1.0-15 h. The intensity of the LED light at the reaction flask was measured to be 80-100 W / m2 by intensity meter. The progress of the reaction was monitored by thin layer chromatography using silica gel. After completion of the reaction, the solvent was evaporated under reduced pressure and the concentrated residue was subjected to column chromatography on a silica gel (100-200 mesh) column using 9:1 hexane-ethyl acetate solvent mixture as eluent to afford pure carboxylic acid. The yield of the acid was obtained in the range of 30-98%. The formation of CO in the effluent gas was confirmed by Residual Gas Analysis (RGA).In case of continuous process, the experimental setup (Fig. 1) consists of a continuousflow reactor assembly which includes a syringe pump, a digital mass flow controller andnon-return valves that are connected to a T-micro mixer for efficient mixing of reactantbefore sending to the reaction flow line. The continuous flow reactor line is 1000 mm inlength with outer dia: 3 mm, inner dia:1.5 mm that is connected to the back-pressureregulator (BPR). The reactor was illuminated with visible light of intensity 80-100watt / m2 and the product mixture was collected at the outlet of the rector.While the invention is valuable for oxidizing aldehydes in general to obtain carboxylicacids, it is of particularly useful for obtaining aromatic carboxylic acids from thecorresponding aryl aldehydes such as benzaldehyde, tolualdehyde, anisaldehyde, chlorobenzaldehyde,and homologues thereof as well as heterocyclic aldehydes such as furfuraland analogues thereof.The preferred aldehydes used in the invention are aryl aldehydes corresponding to theformula:Thus, the R may be hydrogen, halogen, methyl, ethyl, propyl, butyl, amyl, isopropyl,isobutyl, phenyl, tolyl, biphenyl, benzyl, or naphthyl, for example. Preferably,benzaldehyde and its substituted derivatives are utilized.EXAMPLESFollowing examples are given by way of illustration and therefore should not beconstrued to limit the scope of the invention.Example 1: General procedure for the oxidation of benzaldehydeBenzaldehyde (1a) and polar organic solvent, preferably acetonitrile in (1:2 to 1:10weight ratio with respect to the substrate) was taken in to a 60 ml vessel. The reactionmixture was saturated with CO2 by purging. The reaction vessel was sealed and irradiatedwith 20 W LED light for 2h. The intensity of the LED light at the reaction flask wasmeasured to be 86 W / m2 by intensity meter. The progress of the reaction was monitoredby thin layer chromatography using silica gel. After completion of the reaction, the solvent was evaporated under reduced pressure and the resulting residue The concentrated residue was subjected to column chromatography on a silica gel (100- 200 mesh) column using 9:1 hexane-ethyl acetate solvent mixture as eluent to afford pure benzoic acid as white crystals in 96% isolated yield.Example 2: Oxidation of benzaldehyde in continuous flow reactorBenzaldehyde (1a) is mixed with acetonitrile solvent in 1:2 to 1:5 weight ratio and pumped continuously through a syringe pump. The reactant solution was pumped at a flow rate of 72μL / min and directed into the tubing of the reactor via T-micromixer. The CO2 is introduced into the system through MFC (mass flow controller) and was mixed with the solution in the T-micromixer to form a CO2-saturated reactant solution stream before sending it into a photochemical reactor. The combined flow rate was adjusted to the reactor volume of 7 ml with a residence time of 4 min at 20 °C. A terminal BPR was connected with a 60 psi to maintain the system pressure and prevent out-gassing. Benzoic acid product was obtained with 75 % yield.Example 3: Oxidation of benzaldehyde in darkBenzaldehyde (1a) and polar organic solvent, preferably acetonitrile in (1:2 to 1:10 weight ratio with respect to the substrate) was taken in to a 60 ml vessel. The reaction mixture was saturated with CO2 by purging. The reaction vessel was sealed and kept in the dark for 6h. After completion of the reaction, the solvent was evaporated under reduced pressure and the resulting residue was analyzed by GC-FID and 1H NMR. There was no reaction observed and original substrate could recover after the reaction. It confirmed that light irradiation is essentially required for the activation of CO2 to provide necessary oxygen for the oxidation reaction.Example 4: Oxidation of benzaldehyde without CO2Benzaldehyde (1a) and polar organic solvent, preferably acetonitrile in (1:2 to 1:10 weight ratio with respect to the substrate) was taken in to a 60 ml vessel. The reaction mixture was purged with N2 in place of CO2. The reaction vessel was sealed and irradiated with 20 W LED light for 6h. After completion of the reaction, the solvent was evaporated under reduced pressure and the resulting residue was analyzed by GC-FID and 1H NMR. There was no reaction observed and original substrate could recover afterthe reaction. It confirmed that the required oxygen for the oxidation of benzaldehyde was derived from carbon dioxide.Example 5: Oxidation of benzaldehyde using different solventsBenzaldehyde (1a) and an organic solvent (1:2 to 1:10 weight ratio with respect to the substrate) were taken in to a 60 ml vessel. The reaction mixture was saturated with CO2 by purging. The reaction vessel was sealed and irradiated with 20 W LED light for 2h. The intensity of the LED light at the reaction flask was measured to be 86 W / m2 by intensity meter. After completion of the reaction, the solvent was evaporated under reduced pressure and the resulting residue was subjected to column chromatography on a silica gel (100-200 mesh) column using 9:1 hexane-ethyl acetate solvent mixture as eluent to afford pure benzoic acid. The results of the experiments in different solvents are summarized in Table 1, entries 1, 5-9.Example 5: Oxidation of different aldehydes under optimized conditionsAldehyde (1b-1h) and polar organic solvent, preferably acetonitrile in (1:2 to 1:10 weight ratio with respect to the substrate) was taken in to a 60 ml vessel. The reaction mixture was saturated with CO2 by purging. The reaction vessel was sealed and irradiated with 20 W LED light for 2-10 h. The intensity of the LED light at the reaction flask was measured to be 86 W / m2 by intensity meter. The progress of the reaction was monitored by thin layer chromatography using silica gel. After completion of the reaction, the solvent was evaporated under reduced pressure and the resulting residue was subjected to column chromatography on a silica gel (100- 200 mesh) column using 9:1 hexane-ethyl acetate solvent mixture as eluent to afford pure acid. The isolated yield and the reaction time of these experiments are summarized in Table 1, entries 10-19.Table 1: Photochemical oxidation of aldehydes with CO2a Isolated yield; b Yield based on recovered aldehyde; c in a flow photoreactor; dunder dark condition; ewithout CO2 under N2.ADVANTAGES OF THE INVENTION- The present invention discloses the first catalyst free light assisted utilization of CO2 for chemical synthesis.- The present invention discloses first catalyst free photochemical synthesis of carboxylic acids from oxidation of aldehydes using CO2 as an oxidant under ambient temperature and pressure conditions. The use of CO2 as an oxidant offer several advantages as it is abundantly available, safe, and inexpensive; also it provided carbon monoxide, an important building block as a co-product during the oxidation process.- Using benzladehyde and its derivatives substituted with electron donating groups as substrates the conversion and yield of the corresponding acids remained higher than the substrates having electron withdrawing groups.- The best results were obtained in acetonitrile solvent; whereas, DMF and DMA being a CO2-philic solvent showed moderate reactivity.- The present invention provides a viable approach that can be served as a tool for chemical fixation of CO2 in a sustainable way under mild operating conditions.

Claims

1. A process for a light assisted, catalyst free photochemical process for the oxidation of aldehydes of formula I using carbon dioxide comprising the step of: i. oxidizing the aldehyde with CO2 dissolved in a solvent under the light irradiation both in batch or continuous flow photoreactor at the temperature in the range of 20 to 40°C and pressure in the range of 1-5 bar for a period in the range of 1-24 h irradiation time in a batch and continuous flow photo-reactor followed by purification to give corresponding acid with the formation of carbon monoxide as a co-product in the gaseous phase.

2. The process as claimed in claim 1, wherein aldehyde is selected from aromatic aldehydes substituted by electron donating or electron deficient group or aliphatic aldehydes.

3. The process as claimed in claim 1, wherein aldehyde is preferably aryl aldehydes of formula I Formula I wherein R is selected from the group consisting of hydrogen, halogen, methyl, ethyl, propyl, butyl, amyl, isopropyl, isobutyl, phenyl, tolyl, biphenyl, benzyl, or naphthyl.

4. The process as claimed in claim 1, wherein the light irradiation is done by using light source having wavelength λ greater than 420 nm, preferably house hold LED light of 10 to 50 W.

5. The process as claimed in claim 1, wherein solvent is selected from the group consisting of water, acetonitrile (ACN) or dimethylformamide (DMF) or dimethylsulfoxide (DMSO) or tetrahydrofuran (THF), or mixtures thereof as may be appropriate, in particular pure acetonitrile.

6. The process of as claimed in claim 1, wherein the reaction time is preferably ranging between 1 to 15 h.

7. The process as claimed in claim 1, wherein the reaction is effective at ambient temperature (25°C) and at 1 atmospheric pressure of CO2.

8. The process as claimed in claim 1, wherein purification is carried out by column chromatography using silica gel bed.

9. The process as claimed in claim 1, wherein the yield of the product is ranging from 30-98% determined on the basis of isolated carboxylic acid or based on the recovered aldehyde substrate.