Description:FIELD OF THE INVENTION
[0001] The present disclosure provides a method of direct hydrogenation of carbon dioxide to formic acid. The present disclosure also provides an iridium catalyst for direct hydrogenation of carbon dioxide to formic acid.

BACKGROUND OF THE INVENTION
[0002] Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
[0003] Formic acid is an important commodity chemical having wide industrial applications. Formic acid is used an intermediate in manufacturing of basic drugs, plant protection agents, pesticides, vulcanisation accelerators, antioxidants, cleaning agents and many other applications. The commercial production of formic acid is based on two-step process; in the first step, methanol is carbonylated with carbon monoxide to form methyl formate, and in the second step, methyl formate is hydrolysed in the presence of a base to obtain formic acid (Scheme-1). The other methods include, oxidation of methanol, oxidation of methane, hydrolysis of formamide and preparation from formats. Although, at present 80% of formic acid produced worldwide is based on the hydrolysis of methyl formate process, tremendous efforts have been directed over the last decades to develop a suitable formic acid production process using CO2 as a feedstock as the current formic acid production is highly carbon intensive. The current formic acid production process uses natural gas as a primary feedstock for syngas production which generates significant amount of CO2 as a by-product.
Step 1 : Methanol carbonylation : CH3OH + CO → HCOOCH
Step 2 : Methyl formate hydrolysis : HCOOCH3 + H2O → CH3OH + HCOOH
Scheme-1: The conventional process of formic acid production based on methanol carbonylation.
[0004] In view of the above, formic acid production using CO2 as feedstock would offers a carbon-neutral solution. However, to drive the activation of thermodynamically stable CO2 molecule, there is a need to develop highly active and selective catalyst system for hydrogenation of CO2 to formic acid.

OBJECTS OF THE INVENTION
[0005] An objective of the present invention is to provide a method of direct hydrogenation of carbon dioxide to formic acid.
[0006] Another objective of the present invention is to provide an iridium catalyst of formula (GS-1) for direct hydrogenation of carbon dioxide to formic acid.
[0007] Another objective of the present invention is to provide an iridium catalyst of formula (GS-2) for direct hydrogenation of carbon dioxide to formic acid.
[0008] Another objective of the present invention is to provide an iridium catalyst of formula (GS-3) for direct hydrogenation of carbon dioxide to formic acid.
[0009] Another objective of the present invention is to provide an iridium catalyst of formula (GS-4) for direct hydrogenation of carbon dioxide to formic acid.
[0010] Another objective of the present invention is to provide an iridium catalyst of formula (GS-5) for direct hydrogenation of carbon dioxide to formic acid.
[0011] Yet another objective of the present invention is to provide an iridium catalyst of formula (GS-6) for direct hydrogenation of carbon dioxide to formic acid.

SUMMARY OF THE INVENTION
[0012] This summary is provided to introduce a selection of concepts in a simplified form that are further described below in Detailed Description section. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.
[0013] The present disclosure provides a method of direct hydrogenation of carbon dioxide to formic acid comprising: contacting carbon dioxide (1-40 bar) with hydrogen (1-60 bar) in presence of a 2M base and 5-25 micro molar of an iridium catalyst to form a first reaction mixture; heating the first reaction mixture under condition with stirring to obtain a second reaction mixture; and cooling the second reaction mixture followed by acidification with an acid to obtain formic acid.
[0014] An aspect of the present disclosure relates to an iridium catalyst of formula (GS-1) for direct hydrogenation of carbon dioxide to formic acid comprising:

(GS-1)
wherein,
M= Ir;
L’= 1,2,3,4,5-Pentamethylcyclopentadiene (Cp*);
L= Cl, OH2; and
R= Me, Et, OMe, OH, NMe2, NH2 and NHMe.
[0015] Another aspect of the present disclosure relates to an iridium catalyst of formula (GS-2) for direct hydrogenation of carbon dioxide to formic acid comprising:

(GS-2)
wherein,
M= Ir;
L’= 1,2,3,4,5-Pentamethylcyclopentadiene (Cp*);
L= Cl, OH2; and
R= Me, Et, OMe, OH, NMe2, NH2 and NHMe.
[0016] Another aspect of the present disclosure relates to an iridium catalyst of formula (GS-3) for direct hydrogenation of carbon dioxide to formic acid comprising:

(GS-3)
wherein,
M= Ir;
L’= 1,2,3,4,5-Pentamethylcyclopentadiene (Cp*);
L= Cl, OH2; and
R= Me, Et, OMe, OH, NMe2, NH2 and NHMe.
[0017] Another aspect of the present disclosure relates to an iridium catalyst of formula (GS-4) for direct hydrogenation of carbon dioxide to formic acid comprising:

(GS-4)
wherein,
M= Ir;
L’= 1,2,3,4,5-Pentamethylcyclopentadiene (Cp*);
L= Cl, OH2; and
R= Me, Et, OMe, OH, NMe2, NH2 and NHMe
[0018] Another aspect of the present disclosure relates to an iridium catalyst of formula (GS-5) for direct hydrogenation of carbon dioxide to formic acid comprising:

(GS-5)
wherein,
M= Ir;
L’= 1,2,3,4,5-Pentamethylcyclopentadiene (Cp*);
L= Cl, OH2; and
R= Me, Et, OMe, OH, NMe2, NH2 and NHMe.
[0019] Further aspect of the present disclosure relates to an iridium catalyst of formula (GS-6) for direct hydrogenation of carbon dioxide to formic acid comprising:

(GS-6)
wherein,
M= Ir;
L’= 1,2,3,4,5-Pentamethylcyclopentadiene (Cp*);
L= Cl, OH2; and
R= alkyl, aryl, heterocyclic and carbocyclic
[0020] Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS
[0021] The accompanying drawings are included to provide a further understanding of the present disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure.
[0022] Figure 1 showed formic acid formation with increase in pressure.

DETAILED DESCRIPTION OF THE INVENTION
[0023] The following is a detailed description of embodiments of the disclosure. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
[0024] Unless the context requires otherwise, throughout the specification which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense that is as “including, but not limited to.”
[0025] As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. It should also be noted that the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise.
[0026] In some embodiments, the numbers expressing quantities of ingredients, properties such as concentration, reaction conditions, and so forth, used to describe and claim certain embodiments of the invention are to be understood as being modified in some instances by the term “about.” Accordingly, in some embodiments, the numerical parameters set forth in the written description are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable.
[0027] The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it is individually recited herein.
[0028] All processes described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.
[0029] The headings and abstract of the invention provided herein are for convenience only and do not interpret the scope or meaning of the embodiments.
[0030] The following discussion provides many example embodiments of the inventive subject matter. Although each embodiment represents a single combination of inventive elements, the inventive subject matter is considered to include all possible combinations of the disclosed elements. Thus if one embodiment comprises elements A, B, and C, and a second embodiment comprises elements B and D, then the inventive subject matter is also considered to include other remaining combinations of A, B, C, or D, even if not explicitly disclosed.
[0031] All publications herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
[0032] Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member can be referred to and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group can be included in, or deleted from, a group for reasons of convenience and/or patentability. When any such inclusion or deletion occurs, the specification is herein deemed to contain the group as modified thus fulfilling the written description that follows, and the embodiments described herein, is provided by way of illustration of an example, or examples, of particular embodiments of the principles and aspects of the present disclosure. These examples are provided for the purposes of explanation, and not of limitation, of those principles and of the disclosure.
[0033] It should also be appreciated that the present invention can be implemented in numerous ways, including as a system, a method or a device. In this specification, these implementations, or any other form that the invention may take, may be referred to as processes. In general, the order of the steps of the disclosed processes may be altered within the scope of the invention.
[0034] Various terms as used herein are shown below. To the extent a term used in a claim is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in printed publications and issued patents at the time of filing.
[0035] With the objective of developing active and selective catalyst system for direct hydrogenation of CO2 to formic acid, various classes of N-heterocyclic compounds-based ligands to make novel Iridium complex for generating a series of iridium based efficient homogeneous catalysts (Scheme-2).

Scheme 2: Direct hydrogenation of CO2 to formic acid using Iridium catalysts.

[0036] The performance of the synthesized catalysts was evaluated in a batch reactor and CO2 conversion and formic acid selectivity were determined experimentally.
[0037] An aspect of the present disclosure provides a method of direct hydrogenation of carbon dioxide to formic acid comprising: contacting carbon dioxide (1-40 bar) with hydrogen (1-60 bar) in presence of a 2M base and 5-25 micro molar of an iridium catalyst to form a first reaction mixture; heating the first reaction mixture under condition with stirring to obtain a second reaction mixture; and cooling the second reaction mixture followed by acidification with an acid to obtain formic acid.
[0038] In an embodiment, the base is selected from a group consisting of potassium bicarbonate, sodium hydroxide, potassium hydroxide, sodium bicarbonate, cesium hydroxide, ammonium hydroxide and combination thereof.
[0039] In an embodiment, the first reaction mixture is heated at a temperature in the range of 50-100 °C for a period in the range of 1-24 hrs, preferably at a temperature in the range of 50-80 °C for a period in the range of 6-12 hrs. The stirring is carried out at a speed in the range of 400-600 rpm, preferably at a speed in the range of 450-550 rpm.
[0040] In an embodiment, the acidification is carried out by an acid is selected from a group consisting of hydrochloric acid, nitric acid, sulphuric acid and combination thereof, preferably the acid is hydrochloric acid. The acid is added to obtain a pH in the range of 2 to 4, preferably pH is 3.
[0041] An embodiment of the present disclosure provides an iridium catalyst of formula (GS-1) for direct hydrogenation of carbon dioxide to formic acid comprising:

(GS-1)
wherein,
M= Ir;
L’= 1,2,3,4,5-Pentamethylcyclopentadiene (Cp*);
L= Cl, OH2; and
R= Me, Et, OMe, OH, NMe2, NH2 and NHMe.
[0042] GS-1 is an aromatic amine based 2-imidazoline derivatised amido complex. In an embodiment, the catalyst of GS-1 is selected from:
, and
(1) (2).
[0043] An embodiment of the present disclosure provides an iridium catalyst of formula (GS-2) for direct hydrogenation of carbon dioxide to formic acid comprising:

(GS-2)
wherein,
M= Ir;
L’= 1,2,3,4,5-Pentamethylcyclopentadiene (Cp*);
L= Cl, OH2; and
R= Me, Et, OMe, OH, NMe2, NH2 and NHMe.
[0044] GS-2 is an aromatic amine based 2-benzomidazole derivatised amido complex In an embodiment, the catalyst of GS-2 is selected from:

(3).

[0045] An embodiment of the present disclosure provides an iridium catalyst of formula (GS-3) for direct hydrogenation of carbon dioxide to formic acid comprising:

(GS-3)
wherein,
M= Ir;
L’= 1,2,3,4,5-Pentamethylcyclopentadiene (Cp*);
L= Cl, OH2; and
R= Me, Et, OMe, OH, NMe2, NH2 and NHMe.
[0046] GS-3 is an aromatic amine based 2-pyrimidyl derivatised amido complex. In an embodiment, the catalyst of GS-3 is:

(4).

[0047] An embodiment of the present disclosure provides an iridium catalyst of formula (GS-4) for direct hydrogenation of carbon dioxide to formic acid comprising:

(GS-4)
wherein,
M= Ir;
L’= 1,2,3,4,5-Pentamethylcyclopentadiene (Cp*);
L= Cl, OH2; and
R= Me, Et, OMe, OH, NMe2, NH2 and NHMe.
[0048] GS-4 is N,N chelated complex with the derivative of 2-pyrimidyl -2-imidazoline compounds, In an embodiment, the catalyst of GS-4 is:

(5).
[0049] An embodiment of the present disclosure provides an iridium catalyst of formula (GS-5) for direct hydrogenation of carbon dioxide to formic acid comprising:

(GS-5)
wherein,
M= Ir;
L’= 1,2,3,4,5-Pentamethylcyclopentadiene (Cp*);
L= Cl, OH2; and
R= Me, Et, OMe, OH, NMe2, NH2 and NHMe.
[0050] GS-5 is N,N chelated complex with S-triazene substituted 2-amino pyridine derivatives. In an embodiment, the catalyst of GS-5 is:

(6).
[0051] An embodiment of the present disclosure provides an iridium catalyst of formula (GS-6) for direct hydrogenation of carbon dioxide to formic acid comprising:

(GS-6)
wherein,
M= Ir;
L’= 1,2,3,4,5-Pentamethylcyclopentadiene (Cp*);
L= Cl, OH2; and
R= alkyl, aryl, heterocyclic and carbocyclic.

[0052] GS-6 is N,N-chelated complex with aliphatic amine substituted biguanide derivative. In an embodiment, the catalyst of GS-6 is selected from:
and
(7) (8).

[0053] In a typical procedure, a 250 ml autoclave was charged with 2M KHCO3 (60 ml) and 5-25 micro molar of an iridium complex. The autoclave was closed and pressurized with carbon dioxide (1-40 bar) and hydrogen (1-60 bar). The reaction was conducted at 50-100 0C for a duration of 1-24 hours. The autoclave was depressurized carefully and the liquid was acidified with 1N HCl. The formation of product was analysed by Agilent HPLC using C18 column.
[0054] While the foregoing describes various embodiments of the disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof. The invention is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.

EXAMPLES
[0055] The present invention is further explained in the form of following examples. However, it is to be understood that the following examples are merely illustrative and are not to be taken as limitations upon the scope of the invention.
[0056] Catalyst Preparation: To a solution of Cp*Ir(OH2)3 SO4 in water was added heterocyclic ligand and the mixture was stirred at room temperature for 16 hours. The solution was filtered and the filtrate was dried under reduced pressure to affort the Iridium catalyst as yellow-brown solid.
Example 1
[0057] A 250 mL autoclave was charged with 2M KHCO3 (60 mL) and Cat-1 (GS-1). The autoclave was sealed and pressurized with CO2 (20 bar) and H2 (20 bar). It was heated to 50 °C and stirred (500 rpm) for 6-12 h. Cooled to room temperature and the liquid was carefully acidified with HCl to pH~3. The concentration of formic acid was analysed by HPLC using C-18 column. Formic acid concentration: 0.8 M.
Examples 2
[0058] A 250 mL autoclave was charged with 2M KHCO3 (60 mL) and Cat-2 (GS-1). The autoclave was sealed and pressurized with CO2 (20 bar) and H2 (20 bar). It was heated to 50 °C and stirred (500 rpm) for 6-12 h. Cooled to room temperature and the liquid was carefully acidified with HCl to pH~3. The concentration of formic acid was analysed by HPLC using C-18 column. Formic acid concentration: 0.4 M.
Example 3
[0059] A 250 mL autoclave was charged with 2M KHCO3 (60 mL) and Cat-3 (GS-2). The autoclave was sealed and pressurized with CO2 (20 bar) and H2 (20 bar). It was heated to 50 °C and stirred (500 rpm) for 6-12 h. Cooled to room temperature and the liquid was carefully acidified with HCl to pH~3. The concentration of formic acid was analysed by HPLC using C-18 column. Formic acid concentration: 0.23 M.
Example 4
[0060] A 250 mL autoclave was charged with 2M KHCO3 (60 mL) and Cat-4 (GS-3). The autoclave was sealed and pressurized with CO2 (20 bar) and H2 (20 bar). It was heated to 50 °C and stirred (500 rpm) for 6-12 h. Cooled to room temperature and the liquid was carefully acidified with HCl to pH~3. The concentration of formic acid was analysed by HPLC using C-18 column. Formic acid concentration: 0.58 M.
Example 5
[0061] A 250 mL autoclave was charged with 2M KHCO3 (60 mL) and Cat-5 (GS-4). The autoclave was sealed and pressurized with CO2 (20 bar) and H2 (20 bar). It was heated to 50 °C and stirred (500 rpm) for 6-12 h. Cooled to room temperature and the liquid was carefully acidified with HCl to pH~3. The concentration of formic acid was analysed by HPLC using C-18 column. Formic acid concentration: 0.48 M
Example 6
[0062] A 250 mL autoclave was charged with 2M KHCO3 (60 mL) and Cat-6 (GS-5). The autoclave was sealed and pressurized with CO2 (20 bar) and H2 (20 bar). It was heated to 50 °C and stirred (500 rpm) for 6-12 h. Cooled to room temperature and the liquid was carefully acidified with HCl to pH~3. The concentration of formic acid was analysed by HPLC using C-18 column. Formic acid concentration: 0.60 M.
Example 7
[0063] A 250 mL autoclave was charged with 2M KHCO3 (60 mL) and Cat-7 (GS-6). The autoclave was sealed and pressurized with CO2 (20 bar) and H2 (20 bar). It was heated to 50 °C and stirred (500 rpm) for 6-12 h. Cooled to room temperature and the liquid was carefully acidified with HCl to pH~3. The concentration of formic acid was analysed by HPLC using C-18 column. Formic acid concentration: 0.62 M.

Example 8
[0064] A 250 mL autoclave was charged with 2M KHCO3 (60 mL) and Cat-8 (GS-6). The autoclave was sealed and pressurized with CO2 (20 bar) and H2 (20 bar). It was heated to 50 °C and stirred (500 rpm) for 6-12 h. Cooled to room temperature and the liquid was carefully acidified with HCl to pH~3. The concentration of formic acid was analysed by HPLC using C-18 column. Formic acid concentration: 0.60 M.
[0065] The activity of the catalyst is evaluated based on the concentration of formic acid formed from different catalysts under identical conditions as shown in Table 1. All the catalysts reported are found to be completely selective to formic acid and no formation of side products (methanol, formaldehyde, methylformate etc) have been observed.
Table 1: Formic acid concentration and selectivity by using iridium catalysts.
Example Catalyst HCOOH (M) Selectivity
1 Cat-1
0.80 >99%
2 Cat-2
0.40 >99%
3 Cat-3
0.35 >99%
4 Cat-4
0.58 >99%
5 Cat-5
0.48 >99%
6 Cat-6
0.60 >99%
7 Cat-7
0.62 >99%
8 Cat-8
0.60 >99%
10
0.35 RSC Adv., 2018, 8, 1346–1350

[0066] A series of water-soluble electron rich novel iridium complexes were synthesized for the direct hydrogenation of carbon dioxide to formic acid. The catalysts were prepared in a single step by reacting the ligand with the iridium precursor. The CO2 hydrogenation was carried out at low temperature (50 °C) and under moderate pressure (40 bar). Iridium catalysts reported here-in are found to be very active and completely selective towards CO2 hydrogenation to formic acid and no formation of side products were observed. Partial pressure of reactants (CO2 and hydrogen) directly linked with formic acid formation and higher concentration of formic acid is achieved with increasing pressure of reactants (Table 2 and Figure 1). The formation of formic acid is also dependent on the temperature and highest conversion to formic achieved at 60 °C. However, further increase in the temperature resulted in the diminished concentration for formic acid possible due to the favourable dehydrogenation at the elevated temperature.
Table 2: Change in formic acid concentration with increase in pressure for Cat-1.
Pressure (bar) FA Conc (Molar)
0 0
2 0.04
10 0.2
20 0.37
40 0.62
60 0.83
80 0.99

ADVANTAGES OF THE PRESENT INVENTION
[0067] Formic acid production is achieved from CO2, a greenhouse gas. Converting CO2 into useful feed stock chemicals and fuels represents another important strategy that not only removes CO2 from the atmosphere, but also reduces dependence on petrochemicals.
[0068] Several novel electron rich water soluble Iridium catalyst have been developed in a single step from the iridium precursor.
[0069] Catalysts reported here are very active and completely selective to formic acid production and no by product was observed.
[0070] Iridium based catalyst developed herein are more active than other catalysts reported (Ruthenium, Rhodium, Iron, Copper and manganese based catalysts).
[0071] The procedure developed here in uses inorganic bases (unlike organic bases used for Ruthenium counterpart) and makes the isolation of formic acid easy and economically viable.
[0072] The method uses water (as opposed to organic solvent) as the solvent thus makes the process green.
[0073] A skilled artisan will appreciate that the quantity and type of each ingredient can be used in different combinations or singly. All such variations and combinations would be falling within the scope of present disclosure.
[0074] The foregoing examples are merely illustrative and are not to be taken as limitations upon the scope of the invention. Various changes and modifications to the disclosed embodiments will be apparent to those skilled in the art. Such changes and modifications may be made without departing from the scope of the invention.
, Claims:
1. A method of direct hydrogenation of carbon dioxide to formic acid comprising:
contacting carbon dioxide (1-40 bar) with hydrogen (1-60 bar) in presence of a 2M base and 5-25 micro molar of an iridium catalyst to form a first reaction mixture;
heating the first reaction mixture under condition with stirring to obtain a second reaction mixture; and
cooling the second reaction mixture followed by acidification with an acid to obtain formic acid.
2. The method as claimed in claim 1, wherein the base is selected from a group consisting of potassium bicarbonate, sodium hydroxide, potassium hydroxide, sodium bicarbonate, cesium hydroxide, ammonium hydroxide and combination thereof.
3. The method as claimed in claim 1, wherein the first reaction mixture is heated at a temperature in the range of 50-100 °C for a period in the range of 1-24 hrs.
4. The method as claimed in claim 1, wherein the stirring is carried out at a speed in the range of 400-600 rpm.
5. The method as claimed in claim 1, wherein the second reaction is cooled at a temperature in the range of 20-35 °C.
6. The method as claimed in claim 1, wherein the acid is selected from a group consisting of hydrochloric acid, nitric acid, sulphuric acid and combination thereof.
7. The method as claimed in claim 1, wherein the acid is added to obtain a pH in the range of 2 to 4.
8. An iridium catalyst of formula (GS-1) for direct hydrogenation of carbon dioxide to formic acid comprising:

(GS-1)
wherein,
M= Ir;
L’= 1,2,3,4,5-Pentamethylcyclopentadiene (Cp*);
L= Cl, OH2; and
R= Me, Et, OMe, OH, NMe2, NH2 and NHMe.
9. The catalyst as claimed in claim 8, wherein the catalyst is selected from:
, and
(1) (2).
10. An iridium catalyst of formula (GS-2) for direct hydrogenation of carbon dioxide to formic acid comprising:

(GS-2)
wherein,
M= Ir;
L’= 1,2,3,4,5-Pentamethylcyclopentadiene (Cp*);
L= Cl, OH2; and
R= Me, Et, OMe, OH, NMe2, NH2 and NHMe.
11. The catalyst as claimed in claim 10, wherein the catalyst is selected from:

(3).
12. An iridium catalyst of formula (GS-3) for direct hydrogenation of carbon dioxide to formic acid comprising:

(GS-3)
wherein,
M= Ir;
L’= 1,2,3,4,5-Pentamethylcyclopentadiene (Cp*);
L= Cl, OH2; and
R= Me, Et, OMe, OH, NMe2, NH2 and NHMe.
13. The catalyst as claimed in claim 12, wherein the catalyst is

(4).
14. An iridium catalyst of formula (GS-4) for direct hydrogenation of carbon dioxide to formic acid comprising:

(GS-4)
wherein,
M= Ir;
L’= 1,2,3,4,5-Pentamethylcyclopentadiene (Cp*);
L= Cl, OH2; and
R= Me, Et, OMe, OH, NMe2, NH2 and NHMe.
15. The catalyst as claimed in claim 14, wherein the catalyst is

(5).
16. An iridium catalyst of formula (GS-5) for direct hydrogenation of carbon dioxide to formic acid comprising:

(GS-5)
wherein,
M= Ir;
L’= 1,2,3,4,5-Pentamethylcyclopentadiene (Cp*);
L= Cl, OH2; and
R= Me, Et, OMe, OH, NMe2, NH2 and NHMe.
17. The catalyst as claimed in claim 16, wherein the catalyst is

(6).
18. An iridium catalyst of formula (GS-6) for direct hydrogenation of carbon dioxide to formic acid comprising:

(GS-6)
wherein,
M= Ir;
L’= 1,2,3,4,5-Pentamethylcyclopentadiene (Cp*);
L= Cl, OH2; and
R= alkyl, aryl, heterocyclic and carbocyclic.

19. The catalyst as claimed in claim 18, wherein the catalyst is selected from:

and
(7) (8).