TRANSFORMING CO2 INTO FORMIC ACID BY INTEGRATED SOLAR-DRIVEN CATALYST-ENZYME COUPLED ARTIFICIAL PHOTOSYNTHETIC SYSTEM

Abstract

Photo-biocatalyst coupled systems offer a promising approach for converting solar energy into valuable fuels. The bio-integrated photocatalytic system sets a research benchmark by utilizing green energy for formic acid production, reducing CO2 emissions, and enhancing selectivity through bio-enzyme incorporation. This bio-photocatalytic are promising solutions for environmental remediation and energy production. This research reports the synthesis and application of a novel metal-free, nitrogen-enriched graphene composite photocatalyst (NenGCTPP) for artificial photosynthesis. NenGCTPP was synthesized by covalently coupling tetraphenyl porphyrin tetracarboxylic acid (TPP) with N-doped graphene via a polycondensation pathway. The photogenerated charge separation then facilitates the regeneration of enzymatically active coenzymes (NADH) for formic acid production catalyzed by formate dehydrogenase. The photocatalyst exhibited remarkable performance in photocatalytic regeneration of the coenzyme NADH from NAD+ with a high yield of 41.80%, as well as photocatalytic production of formic acid (HCO2H) as a solar fuel from CO2 with a yield of 99.12 µM. This innovative artificial photosynthetic system demonstrates an affordable, highly efficient, and selective approach for converting carbon dioxide into valuable solar fuels and regenerating NADH, addressing environmental concerns and contributing to sustainable energy solutions.

Specification

DESC:FIELD OF THE INVENTION
The present invention relates to Transforming CO2 into Formic Acid by Integrated Solar-Driven Catalyst-enzyme Coupled Artificial Photosynthetic System.
BACKGROUND OF THE INVENTION
References which are cited in the present disclosure are not necessarily prior art and therefore their citation does not constitute an admission that such references are prior art in any jurisdiction. All publications, patents and patent applications herein are incorporated by reference to the same extent as if each individual or patent application was specifically and individually indicated to be incorporated by reference.
The relentless rise in atmospheric carbon dioxide (CO2) levels has precipitated severe environmental consequences, including global warming, climate change, and ozone depletion. These adverse effects stem primarily from industrial activities and human actions that release vast quantities of greenhouse gases into the atmosphere.(1, 2) The rapid escalation of CO2 concentrations has given rise to numerous environmental issues, necessitating urgent attention and action.(3, 4) Nature, however, offers a remarkable solution through the process of photosynthesis, wherein carbon dioxide present in the Earth's atmosphere is effectively synthesized into organic compounds by plants, algae, and certain bacteria.(5, 6) This natural phenomenon inspires the exploration of utilizing carbon dioxide as a raw material and carbon source for chemical synthesis.(7) Unlike conventional carbon sources like carbon monoxide (CO) and phosgene (COCl2), carbon dioxide is cost-effective and abundantly available as a by-product of various chemical and energy processes. Indeed, carbon dioxide has already been employed as a chemical feedstock in industries for the conversion of certain organic compounds, such as urea, cyclic carbonates, and salicylic acid.(8, 9) Nevertheless, the utilization of carbon dioxide in this manner accounts for merely 1% of the total carbon dioxide emissions. In light of the pressing issue of climate change, it is advisable to transition towards renewable energy sources rather than relying on fossil fuel-based energy. This shift is crucial not only because the formation of fossil fuels takes billions of years but also because they have a significantly lesser negative impact on the environment compared to their non-renewable counterparts.(1) Consequently, the quest for sustainable and alternative energy sources has become an imperative endeavor.(10)
Photosynthesis, a biological process occurring in plants, algae, and certain bacteria, offers a promising avenue for harnessing light energy and converting it into chemical energy in the form of glucose.(10, 11) This intricate process involves two distinct photosystems: Photosystem I (PSI) and Photosystem II (PSII). PSI, a fundamental membrane complex, facilitates the photo-induced transport of electrons to PSII, ferredoxin (Fd), cytochrome complexes, and plastocyanin (PC). Ultimately, the light energy is stored in the biologically relevant form of NADH when the photo-induced electron reduces the oxidized nicotinamide adenine dinucleotide (NAD+) after passing through numerous electron transfer chains. (2, 12, 13) Inspired by the remarkable efficiency of natural photosynthesis, the development of artificial photosynthetic systems holds immense potential for addressing the global challenges of energy production and environmental sustainability. By mimicking the intricate mechanisms of photosynthesis, researchers aim to harness solar energy and convert carbon dioxide into valuable chemical compounds and fuels to power green operations.(14) This innovative approach not only promises to alleviate the burden on finite fossil fuel reserves but also offers a means to mitigate the detrimental effects of excess atmospheric carbon dioxide. The overarching objective of this research endeavour is to develop a highly efficient photocatalyst capable of converting atmospheric carbon dioxide (CO2) into solar fuels. This innovative approach directly addresses environmental concerns while contributing to the long-term sustainability of energy resources. Towards this end, we have synthesized a photoactive molecule termed a "light-harvesting photocatalyst," which possesses the remarkable ability to convert and store solar radiation into valuable solar fuels. Carbon dioxide (CO2) is a linear molecule composed of a single carbon atom covalently bonded to two oxygen atoms.(2) Due to its inert, stable, and non-toxic nature under standard temperature and pressure conditions, the conversion of CO2 requires a substantial amount of energy and pressure.(1) According to molecular orbital theory, CO2 possesses a relatively low-lying lowest unoccupied molecular orbital (LUMO) level, facilitating electron transfer from CO2 to form the corresponding radical anion (CO2-).(15-17) This initial conversion is a crucial step in the reduction of CO2 to various products, including formic acid (HCOOH), carbon monoxide (CO), methanol (CH3OH), formaldehyde (HCHO), and methane (CH4). Formic acid (HCOOH) stands out as a promising alternative fuel due to its efficiency, non-toxic nature, and excellent energy storage capacity.(18) Its applications span diverse industries, including leather tanning, dyeing, textiles, paper manufacturing, and as a preservative.(19) Consequently, converting CO2 into HCOOH simultaneously achieves two objectives: reducing atmospheric CO2 levels to maintain environmental balance and producing a valuable fuel source. While previous studies have explored electrocatalytic methods for CO2 reduction, exhibiting high selectivity,(20) these processes often require significant electrical input and involve fossil fuel consumption, rendering them environmentally hazardous. Furthermore, photocatalysts synthesized using graphene or metal oxides like TiO2 combined with graphene have faced limitations due to the energy level mismatch between the conduction band and metal edges, hindering efficient electron transfer from the rhodium complex to the metal oxides. Consequently, the low reduction efficiency has led to insufficient NADH regeneration.(21) To overcome these challenges, we have designed an ecologically benign and non-toxic metal-free photocatalyst, denoted as NenGCTPP, for the selective reduction of carbon dioxide (CO2).
The NenGCTPP photocatalyst is synthesized by combining N-doped graphene with tetraphenyl porphyrin tetracarboxylic acid (TPP), forming a selectively coupled product between the amino group of N-doped graphene and the carboxylic group of TPP, facilitated by the selective coupling agent HATU. Graphene possesses strong photocatalytic properties due to its high resistance, confined band gap, and excellent functionalization control. When graphene and porphyrin are combined, the photo-induced electron transfer process is significantly enhanced. (22-24) The proposed NenGCTPP photocatalyst exhibits substantial photo-induced electron transfer due to the remarkable physical and chemical characteristics imparted by TPP and N-doped graphene. This unique feature has led to an increased rate of CO2 reduction. In the realm of carbon dioxide (CO2) reduction, the artificial photocatalyst NenGCTPP exhibits remarkable potential due to the synergistic interaction of its inherent properties. Nicotinamide adenine dinucleotide (NADH), a vital coenzyme, is regenerated through the photo-induced electron transfer process facilitated by NenGCTPP. This regenerated NADH is subsequently channeled toward the selective reduction of CO2, preferentially yielding formic acid (HCO2H) via the catalytic activity of the enzyme formate dehydrogenase, which is intricately linked to the photocatalyst. (8, 25)
The regeneration of NADH plays a pivotal role in various biochemical processes, as it serves as a crucial cofactor for numerous enzymes involved in oxidation-reduction reactions. By harnessing the photo-induced electron transfer capabilities of NenGCTPP, this research aims to establish an efficient and sustainable method for NADH regeneration, circumventing the need for conventional chemical or electrochemical routes that often involve hazardous reagents or energy-intensive processes. Moreover, the selective production of formic acid from CO2 reduction represents a remarkable achievement, as formic acid is a valuable commodity with diverse applications, ranging from fuel cells to industrial processes.The bio-photocatalytic system (NenGCTPP + FDH enzyme) offer potential environmental benefits, including pollution reduction, CO2 utilization, renewable energy generation, sustainable chemical production, lower energy requirements, contributing to enhanced sustainability.
Several patents issued for photocatalysts but none of these are related to the present invention. Patent US7541509B2 discloses a photocatalyst nanocomposite which can be used to destroying biological agents includes a carbon nanotube core, and a photocatalyst coating layer covalently or ionically bound to a surface of the nanotube core. The coating layer has a nanoscale thickness. A method of forming photocatalytic nanocomposites includes the steps of providing a plurality of dispersed carbon nanotubes, chemically oxidizing the nanotubes under conditions to produce surface functionalized nanotubes to provide C and O including groups thereon which form ionic or covalent bonds to metal oxides, and processing a metal oxide photocatalyst sol-gel precursor in the presence of the nanotubes, wherein a nanoscale metal oxide photocatalyst layer becomes covalently or ionically bound to the nanotubes.
Another patent US9205420B2 relates to nanostructures and compositions comprising nanostructures, methods of making and using the nanostructures, and related systems. In some embodiments, a nanostructure comprises a first region and a second region, wherein a first photocatalytic reaction (e.g., an oxidation reaction) can be carried out at the first region and a second photocatalytic reaction (e.g., a reduction reaction) can be carried out at the second region. In some cases, the first photocatalytic reaction is the formation of oxygen gas from water and the second photocatalytic reaction is the formation of hydrogen gas from water. In some embodiments, a nanostructure comprises at least one semiconductor material, and, in some cases, at least one catalytic material and/or at least one photosensitizing agent.
Another patent US10449530B2 relates to photocatalysts for reduction of carbon dioxide and water are provided that can be tuned to produce certain reaction products, including hydrogen, alcohol, aldehyde, and/or hydrocarbon products. These photocatalysts can form artificial photosystems and can be incorporated into devices that reduce carbon dioxide and water for production of various fuels. Doped wide-bandgap semiconductor nanotubes are provided along with synthesis methods. A variety of optical, electronic and magnetic dopants (substitutional and interstitial, energetically shallow and deep) are incorporated into hollow nanotubes, ranging from a few dopants to heavily-doped semiconductors. The resulting wide-bandgap nanotubes, with desired electronic (p- or n-doped), optical (ultraviolet bandgap to infrared absorption in co-doped nanotubes), and magnetic (from paramagnetic to ferromagnetic) properties, can be used in photovoltaics, display technologies, photocatalysis, and spintronic applications.
Another patent CN107349937B discloses a preparation method of a graphene-based bimetallic sulfide nano composite photocatalyst, which is characterized in that a series of graphene-based bimetallic sulfide composite photocatalysts are synthesized by a simple and mild one-step hydrothermal method, and the morphology size and the oriented growth on the surface of graphene of bimetallic sulfide are regulated and controlled by controlling the hydrothermal reaction temperature, the reaction time, the addition amount of graphene and the content of a metal salt compound in a composite system; in addition, the preparation method provided by the invention is simple in process operation, low in raw material price and suitable for large-scale industrial production.
Another patent CN102921416B relates to a silver doped grapheme-zinc oxide nano composite photocatalytic material and a method for preparing the same and belongs to the technical field of nano composite materials and photocatalysis. Graphite oxide is subjected to ultrasonic dispersion to obtain a dispersion solution of graphene oxide; precursors of silver ions and zinc ions are added in the dispersion solution of the graphene oxide, and the mixture is placed into a reaction kettle to be subjected to hydro-thermal treatment to be prepared into the silver doped grapheme-zinc oxide nano composite photocatalytic material in situ after the pH is regulated to an alkaline condition. Photocatalytic degradation experiments show that the silver doped grapheme-zinc oxide nano composite photocatalytic material prepared through the method has good adsorption and visible light photocatalytic degradation effects on rhodamine B and is an ideal nano composite photocatalytic material.
OBJECTS OF THE INVENTION
Main object of the present invention is a transforming CO2 into formic acid by integrated solar-driven catalyst-enzyme coupled artificial photosynthetic system.
Another object of the present invention is to provide a system that efficiently captures and converts CO2 into formic acid using solar energy as the primary energy source, thereby offering a sustainable approach for carbon capture and utilization.
Another object of the present invention is to develop a catalyst-enzyme coupling mechanism that enhances the conversion of CO2 to formic acid by integrating an artificial photosynthetic catalyst with a formate dehydrogenase enzyme.
Another object of the present invention is to create a modular system design that allows for scalable application, enabling it to be adapted for various industrial settings.
Another object of the present invention is to develop a process that is environmentally benign, producing minimal by-products and eliminating the need for harmful chemicals.
Another object of the present invention is to design the system such that it can be easily integrated with existing CO2 capture technologies, allowing captured CO2 to be directly fed into the conversion system.
SUMMARY OF THE INVENTION
This summary is provided to introduce a selection of concepts, in a simplified format, that are further described in the detailed description of the invention.
This summary is neither intended to identify key or essential inventive concepts of the invention and nor is it intended for determining the scope of the invention.
To further clarify advantages and features of the present invention, a more particular description of the invention will be rendered by reference to specific embodiments thereof, which is illustrated in the appended drawings.
This invention presents a multi-step synthesis process for creating a NenGCTPP composite, a light-harvesting photocatalyst made from N-doped graphene and tetraphenyl porphyrin tetracarboxylic acid (TPP). The process begins with synthesizing N-doped graphene, where glucose acts as a carbon source while melamine and ammonium chloride provide nitrogen atoms. After dissolving and recrystallizing this mixture to remove impurities, the product undergoes high-temperature treatment at 1100°C to produce N-doped graphene with nitrogen atoms embedded within its hexagonal lattice.
The next step is synthesizing TPP through a condensation reaction of pyrrole and formyl benzoic acid in a water-methanol solvent, catalyzed by hydrochloric acid. Following purification through filtration and recrystallization, the intermediate is dissolved in dimethylformamide (DMF) and heated, producing TPP with a distinctive purple color.
Finally, the synthesis of the NenGCTPP composite involves dispersing N-doped graphene in DMF and combining it with TPP, hydroxybenzotriazole (HOBT), and triethylamine (TEA) in a reaction that creates amide bonds between the amino groups of N-doped graphene and the carboxyl groups of TPP. N,N'-dicyclohexylcarbodiimide (DCC) and hexafluorophosphate azabenzotriazole tetramethyl uronium (HATU) act as coupling agents, facilitating selective bond formation at an elevated temperature of 80°C. The resulting product, NenGCTPP, is isolated as a stable dark brown composite with strong photocatalytic properties. This composite leverages the structural stability of N-doped graphene and the light-absorbing capabilities of TPP, making it a promising material for applications in solar energy harvesting and artificial photosynthesis.
Herein enclosed an integrated solar-driven catalyst-enzyme coupled artificial photosynthetic system comprising of
ascorbic acid (AsA), acting as a sacrificial agent; triethylamine (TEA), a tertiary amine; Hydroxybenzotriazole (HOBT), a coupling reagent; dimethyl formamide (DMF), a polar aprotic solvent; ß-Nicotinamide adenine dinucleotide (ß-NAD+), a crucial coenzyme; a phosphate buffer solution for maintaining a controlled pH environment; melamine, a heterocyclic compound; ammonium chloride (NH4Cl) crystals; benzoic acid, an aromatic carboxylic acid; pyrrole, a heterocyclic organic compound; and Hexafluorophosphate Azabenzotriazole tetramethyl Uronium (HATU), a coupling agent facilitating amide bond formation.
A method for integrated solar-driven catalyst-enzyme coupled artificial photosynthetic system comprising the steps of
dissolving 2 grams of glucose in a solution containing 0.8 grams of melamine and ammonium chloride (NH4Cl) crystals;
doing dissolution of this mixture, followed by recrystallization to obtain a crude product;
subjecting the crude product to heat treatment at an elevated temperature of 1100°C, resulting in the formation of a black-colored material, in the desired N-doped graphene;
adding 2 millimoles of pyrrole and 2 millimoles of formyl benzoic acid to a reaction mixture comprising water and methanol as solvents;
introducing a catalytic amount of hydrochloric acid (HCl) into the reaction medium, allowed to stir for a period of 2 hours to facilitate the desired reaction;
filtering the reaction mixture to remove any insoluble impurities, and the filtrate was subjected to recrystallization after the completion of the reaction time;
dissolving the resulting crude product in dimethylformamide (DMF) and heated to a temperature of 60°C, leading to the formation of a purple-colored product tetraphenyl porphyrin tetracarboxylic acid (TPP) upon recrystallization;
dispersing 600 milligrams of N-doped graphene in dimethylformamide (DMF) and subjected to sonication, followed by centrifugation to obtain a stable suspension;
mixing this suspension with 30 milligrams of Hydroxybenzotriazole (HOBT) and triethylamine (TEA), and the resulting mixture was stirred at room temperature;
adding 100 milligrams of N,N'-Dicyclohexylcarbodiimide (DCC) and 200 milligrams of TPP to the reaction medium in the subsequent step; and
stirring the reaction mixture at an elevated temperature of 80°C, after the completion of the reaction, a workup procedure was performed using a DMF solution in a separating funnel, resulting in the isolation of a dark brown crude product NenGCTPP Composite (Light Harvesting Photocatalyst).
The Glucose serves as the carbon source for the formation of graphene, while melamine and ammonium chloride act as nitrogen precursors, facilitating the doping of nitrogen atoms into the graphene structure.
The recrystallization step helps purify the crude product by removing impurities and unwanted byproducts.
The high-temperature heat treatment at 1100°C is crucial for the graphitization process, where the carbon atoms arrange themselves into the characteristic hexagonal lattice structure of graphene, while the nitrogen atoms are incorporated into the lattice, forming the N-doped graphene material.
The Pyrrole and formyl benzoic acid serve as the precursors for the synthesis of TPP, undergoing a condensation reaction facilitated by the presence of the acid catalyst, hydrochloric acid (HCl).
The water-methanol solvent system provides a suitable reaction medium for the condensation process.
The filtration and recrystallization steps are employed to purify the crude product, removing any unreacted starting materials or byproducts.
The subsequent dissolution in DMF and heating at 60°C are crucial steps for promoting the cyclization and aromatization of the intermediate porphyrinogen, leading to the formation of the desired TPP product with its characteristic purple color.
The catalytic amount of Hexafluorophosphate Azabenzotriazole tetramethyl Uronium (HATU) was introduced to facilitate selective coupling between the amino groups of N-doped graphene and the carboxylic groups of TPP.
The HOBT and DCC act as coupling agents, facilitating the formation of the amide linkages, while HATU serves as a selective coupling agent, promoting the desired coupling between the specific functional groups.
The Triethylamine acts as a base, neutralizing the acidic byproducts formed during the reaction, the elevated temperature of 80°C provides the necessary thermal energy to overcome the activation barrier and drive the reaction towards completion, wherein the workup procedure involving DMF, and a separating funnel allows for the isolation of the desired NenGCTPP composite from the reaction mixture.
BRIEF DESCRIPTION OF THE DRAWINGS
The illustrated embodiments of the subject matter will be understood by reference to the drawings, wherein like parts are designated by like numerals throughout. The following description is intended only by way of example, and simply illustrates certain selected embodiments of devices, systems, and methods that are consistent with the subject matter as claimed herein, wherein:
Figure 1. TPP and N-doped graphene are coupled via a polycondensation process resulting in the light-active NenGCTPP-Photocatalyst.
Figure 2. (a) Diagrammatic representation of DRS of NenGCTPP Photocatalyst (blue) and TPP (green) along with tauc plot (inset);(b) FT-IR spectrum of NenGCTPP Photocatalyst (blue), TPP (green) and N-doped graphene (red).
Figure 3. Diffraction pattern of NenGCTPP (blue), TPP (green), and N-doped graphene (red) photocatalyst.
Figure 4. (a) Cyclic voltammetry(CV) of NenGCTPP (blue), TPP (green); (b) EIS of NenGCTPP (blue), TPP (green); (c) Tafel NenGCTPP (blue), TPP (green);(d) Chronopotentiometry of NenGCTPP (blue), TPP (green).
Figure 5. (a) Energy-dispersive X-ray spectroscopy (EDX) of NenGCTPP photocatalyst; (b) Scanning Electron Microscopy (SEM) spectrum of NenGCTPP photocatalyst.
Figure 6. (a) Photo-regeneration of NADH, and (b) Formic acid production.
DETAILED DESCRIPTION OF THE INVENTION
In some embodiments of the present invention, the synthesis of N-doped graphene was achieved through a multi-step process. Initially, 2 grams of glucose were dissolved in a solution containing 0.8 grams of melamine and ammonium chloride (NH4Cl) crystals.
In some embodiments of the present invention, this mixture underwent dissolution, followed by recrystallization to obtain a crude product. Subsequently, the crude product was subjected to heat treatment at an elevated temperature of 1100°C, resulting in the formation of a black-colored material, which is the desired N-doped graphene.(22, 26, 27) Glucose serves as the carbon source for the formation of graphene, while melamine and ammonium chloride act as nitrogen precursors, facilitating the doping of nitrogen atoms into the graphene structure.
In some embodiments of the present invention, the recrystallization step helps purify the crude product by removing impurities and unwanted byproducts. The high-temperature heat treatment at 1100°C is crucial for the graphitization process, where the carbon atoms arrange themselves into the characteristic hexagonal lattice structure of graphene, while the nitrogen atoms are incorporated into the lattice, forming the N-doped graphene material.
In some embodiments of the present invention, the synthesis of tetraphenyl porphyrin tetracarboxylic acid (TPP) was carried out through a multi-step procedure. In the initial step, 2 millimoles of pyrrole and 2 millimoles of formyl benzoic acid were added to a reaction mixture comprising water and methanol as solvents.
In some embodiments of the present invention, a catalytic amount of hydrochloric acid (HCl) was introduced into the reaction medium, which was then allowed to stir for a period of 2 hours to facilitate the desired reaction. After the completion of the reaction time, the reaction mixture was filtered to remove any insoluble impurities, and the filtrate was subjected to recrystallization.
In some embodiments of the present invention, the resulting crude product was then dissolved in dimethylformamide (DMF) and heated to a temperature of 60°C, leading to the formation of a purple-colored product upon recrystallization.[28] Pyrrole and formyl benzoic acid serve as the precursors for the synthesis of TPP, undergoing a condensation reaction facilitated by the presence of the acid catalyst, hydrochloric acid (HCl).
In some embodiments of the present invention, the water-methanol solvent system provides a suitable reaction medium for the condensation process. Filtration and recrystallization steps are employed to purify the crude product, removing any unreacted starting materials or byproducts. The subsequent dissolution in DMF and heating at 60°C are crucial steps for promoting the cyclization and aromatization of the intermediate porphyrinogen, leading to the formation of the desired TPP product with its characteristic purple color.
Herein enclosed an integrated solar-driven catalyst-enzyme coupled artificial photosynthetic system comprising of
ascorbic acid (AsA), acting as a sacrificial agent; triethylamine (TEA), a tertiary amine; Hydroxybenzotriazole (HOBT), a coupling reagent; dimethyl formamide (DMF), a polar aprotic solvent; ß-Nicotinamide adenine dinucleotide (ß-NAD+), a crucial coenzyme; a phosphate buffer solution for maintaining a controlled pH environment; melamine, a heterocyclic compound; ammonium chloride (NH4Cl) crystals; benzoic acid, an aromatic carboxylic acid; pyrrole, a heterocyclic organic compound; and Hexafluorophosphate Azabenzotriazole tetramethyl Uronium (HATU), a coupling agent facilitating amide bond formation.
A method for integrated solar-driven catalyst-enzyme coupled artificial photosynthetic system comprising the steps of
dissolving 2 grams of glucose in a solution containing 0.8 grams of melamine and ammonium chloride (NH4Cl) crystals;
doing dissolution of this mixture, followed by recrystallization to obtain a crude product;
subjecting the crude product to heat treatment at an elevated temperature of 1100°C, resulting in the formation of a black-colored material, in the desired N-doped graphene;
adding 2 millimoles of pyrrole and 2 millimoles of formyl benzoic acid to a reaction mixture comprising water and methanol as solvents;
introducing a catalytic amount of hydrochloric acid (HCl) into the reaction medium, allowed to stir for a period of 2 hours to facilitate the desired reaction;
filtering the reaction mixture to remove any insoluble impurities, and the filtrate was subjected to recrystallization after the completion of the reaction time;
dissolving the resulting crude product in dimethylformamide (DMF) and heated to a temperature of 60°C, leading to the formation of a purple-colored product tetraphenyl porphyrin tetracarboxylic acid (TPP) upon recrystallization;
dispersing 600 milligrams of N-doped graphene in dimethylformamide (DMF) and subjected to sonication, followed by centrifugation to obtain a stable suspension;
mixing this suspension with 30 milligrams of Hydroxybenzotriazole (HOBT) and triethylamine (TEA), and the resulting mixture was stirred at room temperature;
adding 100 milligrams of N,N'-Dicyclohexylcarbodiimide (DCC) and 200 milligrams of TPP to the reaction medium in the subsequent step; and
stirring the reaction mixture at an elevated temperature of 80°C, after the completion of the reaction, a workup procedure was performed using a DMF solution in a separating funnel, resulting in the isolation of a dark brown crude product NenGCTPP Composite (Light Harvesting Photocatalyst).
The Glucose serves as the carbon source for the formation of graphene, while melamine and ammonium chloride act as nitrogen precursors, facilitating the doping of nitrogen atoms into the graphene structure.
The recrystallization step helps purify the crude product by removing impurities and unwanted byproducts.
The high-temperature heat treatment at 1100°C is crucial for the graphitization process, where the carbon atoms arrange themselves into the characteristic hexagonal lattice structure of graphene, while the nitrogen atoms are incorporated into the lattice, forming the N-doped graphene material.
The Pyrrole and formyl benzoic acid serve as the precursors for the synthesis of TPP, undergoing a condensation reaction facilitated by the presence of the acid catalyst, hydrochloric acid (HCl).
The water-methanol solvent system provides a suitable reaction medium for the condensation process.
The filtration and recrystallization steps are employed to purify the crude product, removing any unreacted starting materials or byproducts.
The subsequent dissolution in DMF and heating at 60°C are crucial steps for promoting the cyclization and aromatization of the intermediate porphyrinogen, leading to the formation of the desired TPP product with its characteristic purple color.
The catalytic amount of Hexafluorophosphate Azabenzotriazole tetramethyl Uronium (HATU) was introduced to facilitate selective coupling between the amino groups of N-doped graphene and the carboxylic groups of TPP.
The HOBT and DCC act as coupling agents, facilitating the formation of the amide linkages, while HATU serves as a selective coupling agent, promoting the desired coupling between the specific functional groups.
The Triethylamine acts as a base, neutralizing the acidic byproducts formed during the reaction, the elevated temperature of 80°C provides the necessary thermal energy to overcome the activation barrier and drive the reaction towards completion, wherein the workup procedure involving DMF, and a separating funnel allows for the isolation of the desired NenGCTPP composite from the reaction mixture.
EXAMPLE 1
EXPERIMENTAL SECTION
Materials and Methods
The chemicals employed in this study were procured from reputable suppliers, Sigma Aldrich and TCI, and utilized without further purification. These included ascorbic acid (AsA), acting as a sacrificial agent; triethylamine (TEA), a tertiary amine; Hydroxybenzotriazole (HOBT), a coupling reagent; dimethyl formamide (DMF), a polar aprotic solvent; ß-Nicotinamide adenine dinucleotide (ß-NAD+), a crucial coenzyme; a phosphate buffer solution for maintaining a controlled pH environment; melamine, a heterocyclic compound; ammonium chloride (NH4Cl) crystals; benzoic acid, an aromatic carboxylic acid; pyrrole, a heterocyclic organic compound; and Hexafluorophosphate Azabenzotriazole tetramethyl Uronium (HATU), a coupling agent facilitating amide bond formation.
Synthesis of N-doped graphene
The synthesis of N-doped graphene was achieved through a multi-step process. Initially, 2 grams of glucose were dissolved in a solution containing 0.8 grams of melamine and ammonium chloride (NH4Cl) crystals. This mixture underwent dissolution, followed by recrystallization to obtain a crude product. Subsequently, the crude product was subjected to heat treatment at an elevated temperature of 1100°C, resulting in the formation of a black-colored material, which is the desired N-doped graphene.(22, 26, 27) Glucose serves as the carbon source for the formation of graphene, while melamine and ammonium chloride act as nitrogen precursors, facilitating the doping of nitrogen atoms into the graphene structure. The recrystallization step helps purify the crude product by removing impurities and unwanted byproducts. The high-temperature heat treatment at 1100°C is crucial for the graphitization process, where the carbon atoms arrange themselves into the characteristic hexagonal lattice structure of graphene, while the nitrogen atoms are incorporated into the lattice, forming the N-doped graphene material.
Synthesis of tetraphenyl porphyrin tetracarboxylic acid (TPP)
The synthesis of tetraphenyl porphyrin tetracarboxylic acid (TPP) was carried out through a multi-step procedure. In the initial step, 2 millimoles of pyrrole and 2 millimoles of formyl benzoic acid were added to a reaction mixture comprising water and methanol as solvents. Subsequently, a catalytic amount of hydrochloric acid (HCl) was introduced into the reaction medium, which was then allowed to stir for a period of 2 hours to facilitate the desired reaction. After the completion of the reaction time, the reaction mixture was filtered to remove any insoluble impurities, and the filtrate was subjected to recrystallization. The resulting crude product was then dissolved in dimethylformamide (DMF) and heated to a temperature of 60°C, leading to the formation of a purple-colored product upon recrystallization.[28] Pyrrole and formyl benzoic acid serve as the precursors for the synthesis of TPP, undergoing a condensation reaction facilitated by the presence of the acid catalyst, hydrochloric acid (HCl). The water-methanol solvent system provides a suitable reaction medium for the condensation process. Filtration and recrystallization steps are employed to purify the crude product, removing any unreacted starting materials or byproducts. The subsequent dissolution in DMF and heating at 60°C are crucial steps for promoting the cyclization and aromatization of the intermediate porphyrinogen, leading to the formation of the desired TPP product with its characteristic purple color.
Synthesis of NenGCTPP Composite (Light Harvesting Photocatalyst)
The synthesis of the NenGCTPP composite, a light-harvesting photocatalyst, was achieved through a stepwise poly-condensation method involving N-doped graphene and tetraphenyl porphyrin tetracarboxylic acid (TPP) (Figure 1). Initially, 600 milligrams of N-doped graphene were dispersed in dimethylformamide (DMF) and subjected to sonication, followed by centrifugation to obtain a stable suspension. This suspension was then mixed with 30 milligrams of Hydroxybenzotriazole (HOBT) and triethylamine (TEA), and the resulting mixture was stirred at room temperature. In the subsequent step, 100 milligrams of N,N'-Dicyclohexylcarbodiimide (DCC) and 200 milligrams of TPP were added to the reaction medium. A catalytic amount of Hexafluorophosphate Azabenzotriazole tetramethyl Uronium (HATU) was introduced to facilitate selective coupling between the amino groups of N-doped graphene and the carboxylic groups of TPP.
The reaction mixture was then stirred at an elevated temperature of 80°C. After the completion of the reaction, a workup procedure was performed using a DMF solution in a separating funnel, resulting in the isolation of a dark brown crude product.[23] The poly-condensation method employed in this synthesis involves the formation of amide bonds between the amino groups of N-doped graphene and the carboxylic acid groups of TPP. HOBT and DCC act as coupling agents, facilitating the formation of the amide linkages, while HATU serves as a selective coupling agent, promoting the desired coupling between the specific functional groups. Triethylamine acts as a base, neutralizing the acidic byproducts formed during the reaction. The elevated temperature of 80°C provides the necessary thermal energy to overcome the activation barrier and drive the reaction towards completion. The workup procedure involving DMF, and a separating funnel allows for the isolation of the desired NenGCTPP composite from the reaction mixture.
EXAMPLE 2
Result and discussion
Results and Discussion
UV-visible spectroscopy, involving light absorption measurement to determine a sample's unique absorption properties, is explained. The analysis of NenGCTPP's UV-visible spectrum revealed a strong resemblance to free TPP. However, significant changes in peak profiles were observed upon covalent attachment of TPP to N-doped graphene. The key result highlights a redshift (bathochromic shift) in the absorption peak of NenGCTPP compared to free TPP. The NenGCTPP material displayed an absorption peak at 520 nm, while free TPP exhibited its peak at 420 nm. This 100 nm increase in the absorption capability of NenGCTPP suggests stronger light absorption and potential for photocatalytic applications. The redshift also implies the formation of an amide bond between TPP and N-doped graphene. UV-visible diffuse reflectance spectroscopy (DRS) is a highly valuable technique for analyzing the electrical structure of materials. The band gap energy (2.20eV) is a crucial parameter that influences the photocatalyst's ability to absorb light and generate charge carriers for photocatalytic reactions (Figure 2a) enabling a comprehensive understanding of the photocatalytic behavior of NenGCTPP. These findings contribute to the broader knowledge of the material's electronic structure and its relevance in photocatalysis.(12, 28)
FT-IR spectroscopy was employed to identify the functional groups present in TPP, N-doped graphene, and the synthesized NenGCTPP photocatalyst. The analysis focused on characteristic peaks indicative of amide bond formation between TPP and N-doped graphene. The FT-IR spectra of TPP (green), N-doped graphene (red), and NenGCTPP (blue) are presented in Figure 2b. A new peak observed at 1507 cm?¹ in the NenGCTPP spectrum can be attributed to C-N stretching vibrations, suggesting a successful covalent coupling between TPP and the nitrogen moieties within N-doped graphene.(11) Furthermore, the absence of a peak at 1734 cm?¹, characteristic of the carbonyl group in carboxylic acids (present in TPP), and the emergence of a new peak at 1693 cm?¹ in NenGCTPP, provide compelling evidence for the formation of amide C=O bonds.(23, 29) These peak shifts support the successful covalent attachment of TPP to N-doped graphene and confirm the formation of the desired NenGCTPP photocatalyst.(30)
X-ray diffraction (XRD) was employed to investigate the crystalline properties and phase characteristics of TPP, N-doped graphene, and the synthesized NenGCTPP photocatalyst (Figure 3). The XRD pattern of TPP (green curve in Figure 3) displays a broad peak-less region, suggesting its semi-crystalline nature. In contrast, the XRD pattern of N-doped graphene (red curve) exhibits sharp peaks at around 27.42°, indicative of its crystalline structure. The XRD pattern of NenGCTPP (blue curve) is particularly noteworthy. It retains the characteristic peak from N-doped graphene at approximately 27.42° but with a slight shift to a higher angle (27.54°). This peak shift signifies the successful coupling of TPP with N-doped graphene.(11, 31)
CV measurements were performed using a klyte electrochemical workstation to assess the band gap of the materials (Figure 4a). The cyclic voltammograms of TPP (green curve) and NenGCTPP (blue curve) are presented in Figure 4a. The reduction and oxidation potential values of NenGCTPP were experimentally determined to be -1.14 V and +1.24 V, respectively. From the CV data, the energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of NenGCTPP were calculated. The band gap energy (Eg) of NenGCTPP was then determined to be 2.38 eV by calculating the difference between the HOMO and LUMO energy levels. This band gap energy plays a critical role in the photoexcitation of electrons during light absorption and subsequent photocatalytic reactions. (11, 32)
EIS is a powerful technique for investigating the resistance at the electrode interface during charge transfer processes. Nyquist plots, which graphically represent the impedance data in a complex plane format, are typically used to analyze EIS results (Figure 4b). The Nyquist plot of NenGCTPP (blue curve in Figure 4b) exhibits a significantly smaller arc radius compared to that of free TPP (green curve). The width of the semicircle in the Nyquist plot is directly correlated with the charge transfer resistance (Rct). A lower Rct signifies a more efficient and reversible process of electron transfer and other interfacial interactions. The observed decrease in the arc radius for NenGCTPP suggests a reduced barrier to charge transfer, indicating enhanced efficiency in electron transport at the electrode interface. This translates to favorable electrochemical characteristics for NenGCTPP in photocatalytic applications.(33)
Tafel analysis, a technique used to study the kinetics of electrode reactions, was performed to gain further insights into the electron transfer behavior of NenGCTPP and TPP (Figure 4c). A Tafel plot typically depicts the relationship between the electrode potential (x-axis) and the logarithm of current density (log(i) on the y-axis. By comparing the Tafel slopes of NenGCTPP and TPP in Figure 4c, a clear distinction can be observed. TPP exhibits a steeper slope compared to NenGCTPP. This difference in slopes signifies contrasting activation energies required for the electrode reactions at each catalyst. A lower Tafel slope, as observed for NenGCTPP, indicates a lower activation energy and faster electron transfer kinetics. This translates to a reduced rate of charge recombination in NenGCTPP, promoting more efficient charge transfer processes at the electrode interface. Consequently, the enhanced charge transfer efficiency likely contributes to the faster photocatalytic activity of NenGCTPP compared to free TPP (34).
Chronopotentiometry (CP) was employed to evaluate the current-carrying capacity of TPP and the NenGCTPP photocatalyst (Figure 4d). The CP analysis revealed that NenGCTPP exhibits a higher current-carrying capacity compared to TPP. This characteristic aligns with its enhanced photocatalytic behavior in reactions like NADH regeneration and formic acid production. Furthermore, the NenGCTPP photocatalyst demonstrates a better recovery of current upon switching from dark conditions to light irradiation (Figure 4d). This observation suggests a superior ability of NenGCTPP to maintain its current-carrying capability compared to TPP. This improved performance can be attributed to the more favorable charge transfer properties and reduced recombination rates in NenGCTPP.(11)
Energy-dispersive X-ray spectroscopy (EDX) was employed to determine the elemental composition of the NenGCTPP photocatalyst (Figure 5a). The EDX spectrum confirms the presence of expected elements, including carbon (C), oxygen (O), and nitrogen (N). For comparison purposes, the EDX spectra of N-doped graphene (containing C and N) and TPP (containing C, O, and N) are referenced in the literature.(35, 36) The presence of C, N, and O in the EDX spectrum of NenGCTPP photocatalyst signifies the successful incorporation of elements from both TPP and N-doped graphene into the final hybrid material.
Scanning electron microscopy (SEM) was used to investigate the surface morphology of the synthesized materials. The SEM image of pristine N-doped graphene(36) typically exhibits a flat, crumpled structure. In contrast, the SEM micrograph of the NenGCTPP photocatalyst (Figure 5b) reveals a distinct morphology characterized by tubular or wrinkle-like features. This significant change in morphology from N-doped graphene to NenGCTPP suggests a successful coupling between TPP and the N-doped graphene support. The observed tubular or wrinkle-like structures could potentially influence the surface area and light absorption properties of the NenGCTPP photocatalyst, potentially impacting its overall photocatalytic performance.
Photocatalytic Regeneration of NADH (Reduced NAD+)
The photocatalytic regeneration of NADH and its subsequent utilization for formic acid production were investigated in a quartz reactor maintained at room temperature under an inert atmosphere. The reaction mixture contained ascorbic acid (sacrificial agent), a rhodium complex, NAD+ co-factor, the newly developed NenGCTPP photocatalyst, and a phosphate buffer solution (pH=7). To assess the role of light irradiation in NADH regeneration, a two-step experiment was conducted. In the first step, the reaction mixture was kept in the dark to allow for the establishment of adsorption-desorption equilibrium between the reactants and the catalyst. As expected, no NADH regeneration was observed under these dark conditions. Subsequently, the reaction mixture was exposed to light irradiation (450 W) for 90 minutes. The concentration of NADH was monitored via UV-Vis spectroscopy at 30-minute intervals, revealing a progressive increase in NADH formation (reaching a maximum of 41.80% after 90 minutes) (Figure 6a).(11, 37, 38)
Photocatalytic Production of Formic Acid (Fixation of CO2)
The photocatalytic production of formic acid, achieved through CO2 fixation, was investigated in a quartz reactor (Figure 6b). The reaction mixture contained 1.24 µmol of ascorbic acid (sacrificial agent), 0.62 µmol of a rhodium complex synthesized according to a previously reported procedure,(37) 1.24 µmol of NAD+ co-factor, 0.5 mg of the NenGCTPP photocatalyst, a phosphate buffer solution (pH 7), and 3 units of formate dehydrogenase enzyme. CO2 gas continuously flowed through the reaction mixture at a rate of 0.5 mL per minute. To isolate the effect of light irradiation on formic acid production, a two-step experiment was conducted. First, the reaction mixture was purged with CO2 for one hour in the dark. Under these dark conditions, no significant formic acid formation was observed. Subsequently, the reaction mixture was exposed to visible light irradiation, leading to the production of formic acid with a yield of 99.12 µM after one hour. The concentration of formic acid was quantified using methods described in previous reports.(8, 21)
Selectively of Enzymatically Active Form of NADH
The selectivity of the photocatalytic approach for generating enzymatically active NADH is crucial for its application in enzymatic reactions. Traditional electrochemical methods for NAD+ reduction often suffer from high overpotentials, leading to the production of unwanted radicals and non-selective protonation. These undesirable side reactions result in the formation of various inactive NADH isomers. The radical intermediates formed during electrochemical reduction can follow multiple pathways (routes 3 and 4). Unfortunately, only the 1,4-dihydro derivative (isomer 3a') possesses enzymatic activity. This study demonstrates the successful selective synthesis of the enzymatically active 1,4-dihydro NADH (isomer 3a') via an artificial photosynthetic pathway mediated by the NenGCTPP photocatalyst. This approach offers a significant advantage compared to conventional electrochemical methods by minimizing the formation of inactive isomers and radicals.(12, 39) The photogenerated charge separation within the NenGCTPP photocatalyst facilitates the targeted reduction of NAD+ to the enzymatically active 1,4-dihydro form (isomer 3a').
Conclusion
This research successfully demonstrates the synthesis of a metal-free, light-activated photocatalyst, NenGCTPP, via a polycondensation pathway. NenGCTPP exhibits promising potential for developing an artificial photosynthetic system capable of mimicking natural photosynthesis due to its remarkable performance in NADH regeneration and formic acid production. The synthesized NenGCTPP photocatalyst exhibits remarkable activity in regenerating NADH and facilitating formic acid production. Its ability to mediate the ambient reduction of NAD+ to NADH in the presence of a Rh complex highlights its potential for efficient electron transfer and photocatalytic reactions. The regeneration of NADH with an optical band gap energy of 2.38 eV suggests high efficiency, potentially replicating the Z-scheme observed in natural photosynthesis. Comprehensive characterization techniques, including UV-visible diffuse reflectance spectroscopy, FT-IR, XRD, SEM, and EDX, provided strong evidence for the successful formation of the NenGCTPP photocatalyst. These findings support its great potential for applications in chemical synthesis. CV and EIS analyses revealed favorable properties for NenGCTPP, including a suitable band gap, reduced charge transfer resistance, and faster electron transfer kinetics for NenGCTPP compared to free TPP. The fabricated NenGCTPP photocatalyst represents an innovative approach to utilizing solar energy for the production of valuable solar fuels like formic acid and the regeneration of NADH, a crucial coenzyme in many biological processes. This work paves the way for further research and development towards creating affordable, highly efficient, and selective systems capable of converting carbon dioxide into valuable fuels using solar energy. The bio-photocatalytic systems provide significant environmental advantages, such as effective pollutant degradation, conversion of CO2 into useful products, harnessing solar energy for chemical processes, facilitating the sustainable synthesis of fuels and chemicals, reducing reliance on non-renewable energy, and fostering a healthier biodiversity. Future efforts could focus on optimizing the photocatalytic performance of NenGCTPP, exploring its activity in other reactions, and integrating it into functional devices for practical applications.
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,CLAIMS:1. An integrated solar-driven catalyst-enzyme coupled artificial photosynthetic system comprising of
ascorbic acid (AsA), acting as a sacrificial agent; triethylamine (TEA), a tertiary amine; Hydroxybenzotriazole (HOBT), a coupling reagent; dimethyl formamide (DMF), a polar aprotic solvent; ß-Nicotinamide adenine dinucleotide (ß-NAD+), a crucial coenzyme; a phosphate buffer solution for maintaining a controlled pH environment; melamine, a heterocyclic compound; ammonium chloride (NH4Cl) crystals; benzoic acid, an aromatic carboxylic acid; pyrrole, a heterocyclic organic compound; and Hexafluorophosphate Azabenzotriazole tetramethyl Uronium (HATU), a coupling agent facilitating amide bond formation.
2. A method for integrated solar-driven catalyst-enzyme coupled artificial photosynthetic system as claimed in claim 1, wherein the said method comprising the steps of
i. dissolving 2 grams of glucose in a solution containing 0.8 grams of melamine and ammonium chloride (NH4Cl) crystals;
ii. doing dissolution of this mixture, followed by recrystallization to obtain a crude product;
iii. subjecting the crude product to heat treatment at an elevated temperature of 1100°C, resulting in the formation of a black-colored material, in the desired N-doped graphene;
iv. adding 2 millimoles of pyrrole and 2 millimoles of formyl benzoic acid to a reaction mixture comprising water and methanol as solvents;
v. introducing a catalytic amount of hydrochloric acid (HCl) into the reaction medium, allowed to stir for a period of 2 hours to facilitate the desired reaction;
vi. filtering the reaction mixture to remove any insoluble impurities, and the filtrate was subjected to recrystallization after the completion of the reaction time;
vii. dissolving the resulting crude product in dimethylformamide (DMF) and heated to a temperature of 60°C, leading to the formation of a purple-colored product tetraphenyl porphyrin tetracarboxylic acid (TPP) upon recrystallization;
viii. dispersing 600 milligrams of N-doped graphene in dimethylformamide (DMF) and subjected to sonication, followed by centrifugation to obtain a stable suspension;
ix. mixing this suspension with 30 milligrams of Hydroxybenzotriazole (HOBT) and triethylamine (TEA), and the resulting mixture was stirred at room temperature;
x. adding 100 milligrams of N,N'-Dicyclohexylcarbodiimide (DCC) and 200 milligrams of TPP to the reaction medium in the subsequent step; and
xi. stirring the reaction mixture at an elevated temperature of 80°C, after the completion of the reaction, a workup procedure was performed using a DMF solution in a separating funnel, resulting in the isolation of a dark brown crude product NenGCTPP Composite (Light Harvesting Photocatalyst).
wherein the Glucose serves as the carbon source for the formation of graphene, while melamine and ammonium chloride act as nitrogen precursors, facilitating the doping of nitrogen atoms into the graphene structure; and the recrystallization step helps purify the crude product by removing impurities and unwanted byproducts.
3. The method as claimed in claim 2, wherein the high-temperature heat treatment at 1100°C is crucial for the graphitization process, where the carbon atoms arrange themselves into the characteristic hexagonal lattice structure of graphene, while the nitrogen atoms are incorporated into the lattice, forming the N-doped graphene material.
4. The method as claimed in claim 2, wherein the Pyrrole and formyl benzoic acid serve as the precursors for the synthesis of TPP, undergoing a condensation reaction facilitated by the presence of the acid catalyst, hydrochloric acid (HCl).
5. The method as claimed in claim 2, wherein the water-methanol solvent system provides a suitable reaction medium for the condensation process.
6. The method as claimed in claim 2, wherein filtration and recrystallization steps are employed to purify the crude product, removing any unreacted starting materials or byproducts.
7. The method as claimed in claim 2, wherein the subsequent dissolution in DMF and heating at 60°C are crucial steps for promoting the cyclization and aromatization of the intermediate porphyrinogen, leading to the formation of the desired TPP product with its characteristic purple color.
8. The method as claimed in claim 2, wherein a catalytic amount of Hexafluorophosphate Azabenzotriazole tetramethyl Uronium (HATU) was introduced to facilitate selective coupling between the amino groups of N-doped graphene and the carboxylic groups of TPP.
9. The method as claimed in claim 2, wherein HOBT and DCC act as coupling agents, facilitating the formation of the amide linkages, while HATU serves as a selective coupling agent, promoting the desired coupling between the specific functional groups.
10. The method as claimed in claim 2, wherein Triethylamine acts as a base, neutralizing the acidic byproducts formed during the reaction, the elevated temperature of 80°C provides the necessary thermal energy to overcome the activation barrier and drive the reaction towards completion, wherein the workup procedure involving DMF, and a separating funnel allows for the isolation of the desired NenGCTPP composite from the reaction mixture.

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