A metal catalyst and a process for catalytic conversion of ethanol to ethyl acetate and hydrogen

Abstract

The present invention provides a metal catalyst and a process for catalytic conversion of ethanol. The present invention provides the synthesis of highly valuable ethyl acetate from ethanol, employing a plethora of group (VIII) metal complexes. The present invention provides generation of green hydrogen gas as a source of clean energy and ethyl acetate as an important industrial substrate from ethanol, catalyzed by a range of group (VIII) metal complexes based on a variety of ligands. Herein, the low-cost ethanol is converted to industrially valuable ethyl acetate having numerous applications in paints and pharmaceutical industry.

Description

[0001] “A METAL CATALYST AND A PROCESS FOR CATALYTIC CONVERSION OF ETHANOL TO ETHYL ACETATE AND HYDROGEN” FIELD OF THE INVENTION The present invention relates to the field of catalysts and organic conversions. More particularly, the present invention relates to a metal catalyst and a process for catalytic conversion of ethanol. BACKGROUND OF THE INVENTION Ethyl acetate (EtOAc) is widely employed in the manufacture of bio-fuel, adhesives, paints and herbicides. This solvent is industrially manufactured from addition of ethanol to acetic acid by Fischer esterification, ethylene addition to acetic acid, or coupling of acetaldehyde. These processes require raw materials, such as ethylene and acetaldehyde that are obtained from fossil-based toxic feedstocks and acetic acid that causes corrosion. Hence, there is an intense need to improve the catalytic synthesis of ethyl acetate with minimum loss of raw materials and the formation of byproducts like primary alcohols, secondary alcohols, and methyl ethyl ketone. CN102671682B discloses about a catalyst for synthesizing ethyl acetate and preparation method and application of catalyst. The catalyst consists of titanium-dioxide-modified transition metal phosphides and a carrier, wherein the titanium dioxide accounts for 5.0 to 10.0 weight percent of the catalyst, the transition metal phosphides account for 5.7 to 22.4 weight percent of the catalyst and the carrier accounts for 70.6 to 88.3 weight percent of the catalyst. However, the disclosed method in the citation is complex and tedious that is time consuming and requires a very high temperature optimum condition. WO2011104738 discloses about a process for the production of ethyl acetate from ethanol. The catalyst comprises of a combination of copper chromite / metallic copper / alumina / barium chromate for producing ethyl acetate and CO-free hydrogen from ethanol. Further, the process for producing ethyl acetate comprising the step of contacting ethanol with the catalyst wherein ethanol is fed together with hydrogen and possibly an inert gas at a partial pressure of the hydrogen comprised between 0.1 and 4 bar. However, the process is carried out at a high range of temperature and pressure and, hence the method is not user-friendly. Nielsen et al., in Journal of the German Chemical Society 2011, 50, 9593-9597 discloses about efficient hydrogen production from alcohols under mild reaction conditions. The citation discloses the synthesis of ethyl acetate from ethanol and employed the complex RuH2(PPh3)3CO and an equimolar amount of bis(2-diisopropylphosphinoethyl)amine for the synthesis of acetaldehyde and ethyl acetate (formed as side product) at 90°C. However, the turnover frequency was obtained upto 1483 h-1. Waser and Zimmermann in Organic Process Research & Development 2018, 22, 862- 870, discloses about the SNS-ligands for Ru-catalyzed homogeneous hydrogenation and dehydrogenation reactions of various substrates. However, in case of ethanol to ethyl acetate transformation, a low yield of the product was obtained. The aforementioned state of the art discloses about catalysts and methods that are complex, inefficient and time consuming. None of the prior art discloses ethanol conversions at ambient conditions. Therefore, there is a need of an efficient, and easy to use metal catalyst and a cost effective process for the catalytic conversion of low cost ethanol into a value added product that is having high market value. OBJECT OF THE INVENTION The main object of the present invention is to provide a metal catalyst for conversion of ethanol. Another object of the present invention is to provide a process for catalytic conversion of ethanol. Yet another object of the present invention is to provide a process for catalytic conversion of ethanol into ethyl acetate and hydrogen. Yet another object of the present invention is to provide an improved catalytic synthesis of ethyl acetate with minimum loss of raw materials. Still another object of the present invention is to provide a mechanism for a metal catalysed conversion of low-cost ethanol into a value added product such as ethyl acetate and hydrogen. SUMMARY OF THE INVENTION The present invention relates to a metal catalyst and a process for synthesis of ethyl acetate and hydrogen by a metal catalysed acceptorless dehydrogenative coupling of ethanol. In an embodiment, the present invention provides a process for catalytic conversion of ethanol, comprising the steps of: taking a closed vessel of capacity of 5 mL inside a glove box and adding a predefined amount of a base and a metal catalyst into the glove box followed by adding a predefined volume of dry and degassed ethanol to obtain a mixture, tightly capping the vessel followed by bringing out the vessel from the glove box and heating the mixture in a preheated oil bath at a predefined temperature for a predefined time period and cooling down to room temperature to obtain a product. The present invention relates to a process for catalytic conversion of ethanol into ethyl acetate and hydrogen by employing a group (VIII) metal catalyst. The above objects and advantages of the present invention will become apparent from the hereinafter set forth brief description of the drawings, detailed description of the invention, and claims appended herewith. BRIEF DESCRIPTION OF THE DRAWINGS An understanding of the metal catalyst and the process for catalytic conversion of ethanol, of the present invention may be obtained by reference to the following figure: Figure 1 is a schematic representation of a mechanistic pathway for the acceptorless dehydrogenation coupling of ethanol illustrating, β-hydride elimination cycle in part (a); and Tischenko cycle in part (b); according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION The present invention will now be described hereinafter with reference to the accompanying drawings in which a preferred embodiment of the invention is shown. This invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiment set forth herein. Rather, the embodiment is provided so that this disclosure will be thorough, and will fully convey the scope of the invention to those skilled in the art. The present invention now will be described hereinafter with reference to the detailed description, in which some, but not all embodiments of the invention are indicated. Indeed, the invention may be embodied in many different forms and shouldnot be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout. The present invention is described fully herein with non- limiting embodiments and exemplary experimentation. The present invention provides a metal catalyst and a process for synthesis of ethyl acetate and hydrogen by a metal catalysed acceptorless dehydrogenative coupling of ethanol. In a preferred embodiment, the present invention provides a process for catalytic conversion of ethanol, comprising the steps of: (a) taking a closed vessel of capacity of 5 mL inside a glove box and adding a predefined amount of a base and a metal catalyst into the glove box followed by adding a predefined volume of dry and degassed ethanol to obtain a mixture; (b) tightly capping the vessel of step (a) followed by bringing out the vessel from the glove box and heating the mixture obtained in step (a) in a preheated oil bath at a predefined temperature for a predefined time period and cooling down to room temperature to obtain a product. Here, said predefined amount of the base is in a range of 0.0035-0.0045 g; said metal catalyst in step (a) is a group (VIII) pincer-complex in an amount of 0.0146-0.044 g; and said group (VIII) pincer-complex is selected from bis(imino)pyridine and 2, 6- bis(benzimidazole-2-yl) pyridine type NNN complexes having formula A1-A6, B1-B8 or C1-C4 with and without substituent A group. Preferably, said group (VIII) pincer-complex is tert-butoxide derivatives of complexes having formula B1-B8 or C1-C4.

[0002] . Additionally, said predefined volume of ethanol in step (a) is in a range of 0.1-0.2 mL; said predefined temperature and said predefined time period in step (b) is in a range of 130- 150°C and 20-30 hours; and said product obtained in step (b) is ethyl acetate and hydrogen. Further, said base is selected from potassium tertiary butoxide (KOtBu), sodium tertiary butoxide (NaOtBu), potassium hydroxide (KOH), sodium hydroxide (NaOH), sodium bicarbonate (NaHCO3), sodium carbonate (Na2CO3), potassium carbonate (K2CO3), cesium carbonate (Cs2CO3), sodium (Na) or sodium ethoxide (NaOEt). Preferably, said base is NaOEt. Moreover, said process yields ethyl acetate in a range of 22-97% and hydrogen in a range of 22-97%. Preferably, the predefined temperature is 140°C and the predefined time period is 24 hours. Referring to Figure 1, a schematic representation of a mechanistic pathway for the acceptorless dehydrogenation coupling of ethanol illustrating, β-hydride elimination cycle in part (a) and Tischenko cycle in part (b), is depicted. EXAMPLE 1 For Experimentation Details Materials and Methods Materials All manipulations were carried out under purified argon (Ar) by taking a standard double manifold or a glove box. The solvents such as tetrahydrofuran (THF), hexane and toluene were dried by double distillation over sodium (Na) / Benzophenone prior to experiment. Ethanol was dried and distilled under argon atmosphere. All other chemicals such as dimethyl sulfoxide (DMSO) and, deuterated chloroform (CDCl3) were purchased from MERCK or Sigma-Aldrich and were employed as such. All catalytic reactions were set up out under an argon atmosphere with the help of dried glassware. Group (VIII) metal catalysts for catalytic conversion of ethanol into ethyl acetate and hydrogen The present invention synthesized group (VIII) metal catalyst for conversion of ethanol into ethyl acetate and hydrogen. The metal catalyst is a pincer-metal complex having formula I: Here, M=Group VIII metal selected from iron (Fe), ruthenium (Ru) or osmium (Os); X= C, N, O; E=N, O, P, As; Z=alkyl, aryl; and Y=CH2, O, NH, S. Preferably M is Ru. Further, the formula I is selected from a group of bis(imino)pyridine and 2, 6- bis(benzimidazole-2-yl) pyridine type NNN complexes having formula A1-A6, B1-B8 or C1-C4 with and without substituted A group that is A is either H group or A is selected from group of OH, OMe, OK or NMe2. Preferably, the group (VIII) pincer-complex is tert- butoxide derivatives of complexes having formula B1-B8 or C1-C4.

[0003] Optimization of reaction conditions For optimizing the reaction conditions, the influence of base on the product yield was first evaluated and the results are summarized in Table 1. In the initial optimization, the reactions were started with neat EtOH in presence of 3 mol% potassium tertiary butoxide (KOtBu) and 1 mol% of (R2NNN)MCl2(PPh3) as catalyst (S. No. 1, Table 1). The major product of the reaction was ethyl acetate with 1-butanol being formed as side product. Lower yields were obtained with sodium tertiary butoxide (NaOtBu) (S. No.2, Table 1) and higher yields were obtained with KOH (S. No.3, Table 1). The bases such as sodium hydroxide (NaOH), sodium carbonate (Na2CO3), potassium carbonate (K2CO3), cesium carbonate (Cs2CO3), sodium bicarbonate (NaHCO3) and sodium (Na) metal (S. No.4-9, Table 1) gave lower yields of the desired product. Taking NaOEt as base gave the highest yield of ethyl acetate among all the bases screened (S. No.10, Table 1). Taking toluene as a solvent further increased the yield (S. No.11, Table 1). The yields got further reduced on further decreasing the catalyst loading (S. No.12 and 13, Table 1) and changing the solvent to mesitylene (S. No.14, Table 1). There was no reactivity observed in absence of base or catalyst, thereby pointing towards the importance of both in the catalytic reaction (S. No.15 and 16, Table 1). The periodic venting of the hydrogen gas pushed reaction in forward direction, thereby giving good yields (95-97%), as depicted in S. No.17 in Table 1. Table 1: Optimization studies towards the transformation of ethanol to ethyl acetate with (R2NNN)MCl2(PPh3)a aReaction condition: Neat Ethanol (0.117 ml, 2 mmol), base (3 mol%), [M] (1 mol%) at 140ºC where M is group VIII metal.bThe yield was calculated by1H NMR spectroscopy by taking toluene as an internal standard.cToluene (0.6 mL) was taken as solvent and dimethyl sulfoxide was taken as internal standard.d0.5 mol% [M] was taken.e0.2 mol% [M] was employed.fmesitylene (0.6 mL) was taken as solvent.gNo base was added to the reaction.hNo catalyst was added to the reaction.iThe flask was opened at regular intervals to vent the hydrogen. The mechanistic pathway for the acceptorless dehydrogenative coupling of ethanol is shown in Figure 1. The first step is the dissociation of -PPh3that results into a 16-electrons pentacoordinate species that undergoes salt metathesis in the presence of ethanol and base to give M-ethoxide species. The β-hydride elimination of M-ethoxide species forms the M- H species that further releases hydrogen in the presence of another molecule of ethanol thereby completing the cycle. The acetaldehyde formed in the cycle inserts into M- ethoxide species followed by β-hydride elimination to yield ethyl acetate and M-H species. The M-H species undergoes σ-bond metathesis in the presence of ethanol, to generate another molecule of hydrogen and M-ethoxide intermediate. General procedure for the acceptorless dehydrogenative coupling of ethanol In a closed vessel of capacity of 5 mL inside glove box, 0.0041 g (0.06 mmol) of NaOEt and 0.0146 g (1 mol%, 0.02 mmol) of (R2NNN)MCl2(PPh3) complex were added inside the glove box followed by addition of 0.117 ml (2 mmol) of dry and degassed ethanol. The vessel was tightly capped and brought out of glove box. The mixture was heated in a pre- heated oil bath at a temperature of 140°C and the reaction was run for 24 hours and cooled down to room temperature. An aliquot of 10 mg was withdrawn from reaction mixture and the NMR yield of the ethyl acetate was determined by1H NMR by taking CDCl3as solvent and toluene as internal standard (known amount added in the flask). In case, when toluene was taken as solvent, dimethyl sulfoxide was taken as an internal standard. EXAMPLE 2 Characterization Physical Measurements1H,2H,13C,31P NMR were recorded on a Bruker ASCEND 600 operating at 600 MHz for1H and 150 MHz for13C or Bruker AVANCE 400 operating at 400 MHz for1H, 100 MHz for13C or on a Bruker AVANCE 500 operating at 500 MHz for1H and 125 MHz for13C. Chemical shifts (δ) were reported in ppm, spin−spin coupling constant (J) were expressed in Hz, and other data were reported as follows: s=singlet, d=doublet, t=triplet, m=multiplet, q=quartet, and brs=broad singlet. GC analyses were performed on a Agilent 7820-GC instrument fitted with Agilent Front SS7 inlet N2 HP-PLOT Q column (30 m length × 530 μm × 40 μm) by the following method: Agilent7820-GC back detector; TCD starting temperature: 40°C; time at starting temp: 0 minutes; ramp: 40°C / minute up to 250°C having a hold time of 10 minutes; flow rate (carrier): 25 mL / minutes (N2); split ratio: 195; inlet temperature: 40°C; detector temperature: TCD: 250°C, and FID: 250°C. Therefore, the present invention provides group (VIII) pincer-metal complex as catalysts for synthesis of ethyl acetate and hydrogen from ethanol; and a mechanistic pathway for the acceptorless dehydrogenation coupling of ethanol. Many modifications and other embodiments of the invention set forth herein will readily occur to one skilled in the art to which the invention pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

Claims

CLAIMS We claim:

1. A process for catalytic conversion of ethanol comprising the steps of: (a) taking a closed vessel of capacity of 5 mL inside a glove box and adding a predefined amount of a base and a metal catalyst into the glove box followed by adding a predefined volume of dry and degassed ethanol to obtain a mixture; and (b) tightly capping the vessel of step (a) followed by bringing out the vessel from the glove box and heating the mixture obtained in step (a) in a preheated oil bath at a predefined temperature for a predefined time period and cooling down to room temperature to obtain a product; wherein, said predefined amount of the base is in a range of 0.0035-0.0045 g; said metal catalyst in step (a) is a group (VIII) pincer-complex in an amount of 0.0146-0.044g; said predefined volume of ethanol in step (a) is in a range of 0.1-0.2 mL; said predefined temperature and said predefined time period in step (b) is in a range of 130-150°C and 20-30 hours; and said product obtained in step (b) is ethyl acetate and hydrogen.

2. The process as claimed in claim 1, wherein said base in step (a) is selected from potassium tertiary butoxide (KOtBu), sodium tertiary butoxide (NaOtBu), potassium hydroxide (KOH), sodium hydroxide (NaOH), sodium bicarbonate (NaHCO3), sodium carbonate (Na2CO3), potassium carbonate (K2CO3), cesium carbonate (Cs2CO3), sodium (Na) or sodium ethoxide (NaOEt).

3. The process as claimed in claim 1, wherein said base in step (a) is preferably NaOEt.

4. The process as claimed in claim 1, wherein said process yields ethyl acetate in a range of 22-97%.

5. The process as claimed in claim 1, wherein said group (VIII) pincer-complex is selected from bis(imino)pyridine and 2, 6-bis(benzimidazole-2-yl) pyridine type NNN complexes having formula A1-A6, B1-B8 or C1-C4:.

6. The process as claimed in claim 1, wherein said group (VIII) pincer-complex is preferably tert-butoxide derivatives of complexes having formula B1-B8 or C1-C4.

7. The process as claimed in claim 1, wherein said predefined temperature is preferably 140°C.

8. The process as claimed in claim 1, wherein said predefined time period in step (b) is preferably 24 hours.

9. The process as claimed in claim 1, wherein said process yields hydrogen in a range of 22-97%.

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