Title of Invention

AN IMPROVED PROCESS FOR THE CARBONYLATION OF METHANOL USING NOVEL RHODIUM COMPLEXES AS CATALYST

Abstract

'The present invention particularly provides an improved process for carbonylation of methanol to acetic acid and / or its derivatives especially methyl acetate using rhodium(I) complexes of the type [Rh(CO)2ClL] as catalyst precursors, where L is a pyridine based nitrogen donor ligands or nitrone N-Oxide donor ligands. The carbonylation reaction was carried out under CO pressure (1-20 bar) in the temperature range 120 - 160 °C for a period 60 - 90 minutes,

Full Text

The present invention relates to an improved process for carbonylation of methanol using novel rhodium metal complexes as catalyst. The present invention particularly provides an improved process for carbonylation of methanol to acetic acid and / or its derivatives especially methyl acetate using rhodium(I) complexes of the type [Rh(CO)2ClL] as catalyst precursors, where L is a pyridine based nitrogen donor ligands or nitrone N-Oxide donor ligands. The carbonylation reaction was carried out under CO pressure (1-20 bar) in the temperature range 120 - 160 °C for a period 60 - 90 minutes. In our co-pending patent Pat. Appl. No. NF ' „ 216 / 200$ we have disclosed & be. claimed a process for the preparation of novel rhodium complexes which can^used for the carbonylation of methanol for producing carboxylic acid and its ester. The commercial use of homogeneous catalyst species [Rh(CO)2l2]" for production of acetic acid and its ester by carbonylation reaction of methanol was first initiated by Monsato Company (F.E.Paulik and J.F.Roth, J.Chem.Soc. Chem.Comm., 1968, 1578). The process required a temperature - 150-200 °C, pressure - 25-45 atm. and the presence of I". Attempt to develop new catalysis species has been hampered by the relatively harsh condition under which the reaction is conducted commercially (because under such conditions, virtually any source of rhodium will be converted to [Rh(CO)2I2 (D. Forster. J. Am. Chem. Soc., 1976, 98, 846). In another patent U.S.Pat No. 5973197 wherein a method for preparing carboxylic acids by carbonylation of an alcohol in presence of rhodium complex catalyst. The carbonylation reaction was conducted at a temperature in the range 150 - 250°C and 1 - 100 b ressure. Again, the draw back is that the above process involves high temperature and pressure. In an U.S. Pat. No. 3769327 wherein methanol is carbonylated with carbon monoxide gas at 175°C and 1000 psig pressure to acetic acid using the catalyst [Rh(CO)2l2]'. The draw back of the process is that it involves sufficient high pressure and temperature for maintaining high absolute reaction rate, Dilworth et.al. (J.Chem.Soc.ChemComm., 1995, 1579) claimed that use of rhodium(I) complexes containing phosphino-thiolate and thioether ligands resulted about 4 times higher rate in carbonylation of methanol to ethanoic acid than that of [Rh(CO)2l2]". The reaction was carried out at 185°C and at 70 bar pressure. The draw back of the process is that it involves high temperature and pressure. Wegman et.al. disclosed (U.S.Pat. 4,990,654) the production of acetate ester from alcohol using rhodium complex catalysts. The process comprises catalytic reaction of a mixture of methanol and ethanol and carbon monoxide in contact with a homogeneous catalyst of rhodium complex containing the ligands like Ph2P(CH2)nP(0)Ph2 (Ph = phenyl, n = 1-4) or Ph2P(CH2)nCOOR (R = alkyl/aryl). The reaction was carried out at a temperature up to 130°C and reaction pressure up to about 250 psig. The main draw back of the process is that the conversion of methanol was 70%. Baker et.al. (J. Chem. Soc. Chem. Soc., 1995, 197) also disclosed that a catalyst cis- I RhI(CO)Ph2PCH2P(S)Ph2 is about 8 times more active than the classic Monsanto catalyst [Rh(CO)2l2]" for carbonylation of methanol at 185°C and at 70 bar pressure. Again, the main draw back of the above process is that it involves high temperature and pressure. In another U.S.Pat. 5488153 Baker et.al. disclosed a process for the liquid phase carbonylamethanol in presence of CO, a halogen promoter ( eg. CH3I), water, rhodium complexes of Ph2PCH2P(S)Ph2 or 2-(diphenylphosphino)-thiophenol. The reaction was performed in the temperature range of 25 - 250°C and at a pressure in the range 10 to 200 bar. The carbonylation rate was found to be considerably higher i.e. about 6 times higher than that in absence of the ligand in the system.The main draw back of the process is that it involves higher temperature and pressure for the reaction. Reference may be made to Wegman et.al. (J. Chem. Soc. Chem. Comm., 1987, 1891) wherein carbonylation in presence of catalyst [Rh(CO)2Cl(Ph2P(CH2)2P(O)Ph2)] was carried out at 80°C and 50 psig CO. The turnover frequency was 400 h"'. The draw back is the low production rate. The main object of the present invention is to provide an improved process for carbonylation of methanol using novel rhodium metal complexes as catalyst which obviates the drawbacks as mentioned above. Another object of the present invention is to provide a process that involves synthesis of novel rhodium metal complexes containing a pyridine based nitrogen donor ligand or a nitrone N-Oxide donor ligand. Still another object of the present invention is to provide a process for carbonylation of methanol at comparatively lower temperature and pressure. Accordingly, the present invention provides an improved process for carbonylation of methanol using novel rhodium metal complexes as catalyst, which comprises contacting carbon monoxide with liquid reaction composition comprising methanol, a halogen promoter preferably methyl iodide, water at a temperature in the range 120 to 160°C and at a pressure of 1 to 20 bar for a period of 60- 90 min. in presence of a-novel ^iex catalyst containing a pyridine based nitrogen donor ligand or a nilrone N-Oxide donor ligand synthesized by stirring a solution of dimeric complex [Rh(CO)2Cl]2 in an organic solvent with two molar equivalent of the respective ligand under nitrogen atmosphere at room temperature for n period of 10 - 60 min, recovering n mixture of acetic acid and its ester by conventional method. In an embodiment of the present invention the carbonylation reaction is exemplified by the reaction of methanol with carbon monoxide to form a mixture of acetic acid and methyl acetate. Yet another embodiment of the present invention provides the preparation of novel rhodium complexes [Rh(CO)2ClI 1, where L is a pyridine based nitrogen donor ligand like pyridine or methyl substituted pyridine or mtrone N-oxide donors ligands like N-phenyl nitrone or a-substituted N-phenyl nitrone, by stirring a solution of chlorobridged dimeric complex [Rh(CO)2Cl]2 in an organic solvent mostly halogenated hydrocarbon with two molar equivalent of pyridine or methyl substituted pyridine or N-phenyl nitrone or a-substituted N-phenyl nitrone under nitrogen atmosphere at room temperature for 10 - 60 min. Still in another embodiment of the present invention, the metal complex catalyst may be added directly to the catalytic system or may be generated in-situ by reacting [Rh(CO)2Cl]2 and the appropriate ligand L. The catalytic reaction was carried out in a 150 ml capacity teflon coated pressure reactor (Make Berghof, Model Heizug 75-150, Germany). The reactants such as methanol (3-4 ml), methyl iodide (1 ml), water (1 ml), ligand L ( 0.050 mmol) and [Rh(CO)2Cl]2 ( 0.025 mmol) were taken into the reaction vessel. Optionally, the metal as synthesized was added to the catalytic system. The reaction vessel was purged with CO gas for about 5 minutes and then pressured with CO gas up to 20 bar. The temperature of the reactor was raised to 120 - 160°C and the corresponding pressure were in the range 20 to 40 bar. The reaction was allowed for 30 to 90 minutes. After the catalytic reaction, the reaction products were collected and analyzed by Gas Liquid Chromatography. The IR spectra of the complexes [Rh(CO)2ClL], show two equally intense terminal carbonyl absorption band in the range 2000 - 2100 cm"1 attributable to the cis-disposition of the two carbonyl groups. The *H NMR spectra of the complexes containing pyridine based ligand show multiplate resonances in the range 8 9.00 - 6.00 ppm for ccH, PH and yH of the pyridine ring and singlet resonances in the range 8 1.00 - 3.00 ppm for substituted methyl group. On the other hand, the !H NMR spectra of the nitrone based complexes show a multiplate resonances in the range 8 6.00 - 7.50 ppm for aromatic proton and a singlet in the range 8 8.00 - 9.00 ppm for ot-H of the nitrone ligand. More than 30 years after the discovery, the Monsanto catalyst [Rh(CO)2l2]~ 's st'U me preferred commercial catalyst for carbonylation of methanol to acetic acid. It is known that the complexes which are more electron rich than [Rh(CO)2b]" will contribute higher carbonylation rate. It is anticipated that any 'Hard' donors like nitrogen or oxygen atoms which normally do not have or very low n-accepting capacity would increase the electron density on the central rhodium atom causing the complexes to behave as an efficient catalyst. In the present invention , the novelty of the catalyst lRh(CO)2ClL] lies on the use of ligands like pyridine based nitrogen donor ligand or nitrone N-Oxide donor ligand which contribute higher electron density on the central metal atom leading to higher «A>h' ciirhonylulioii reaction. Moreover, the possible chcluie fumuition of the nitrone ligands may lead to higher stability of the metal complexes during the catalytic process, which will increase the Turn over number. The following examples are given by way of illustration of the present invention and should not be construed to limit the scope of the present invention. EXAMPLE-1 Methanol (4 ml), methyl iodide (1 ml), water (1 ml) and [Rh(CO)2Cl]2 (10 mg, 0.026 mmol) were charged into a 150 ml capacity high pressure autoclave (Berghof, Germany) which was equipped with magnet-drive stirrer and heating arrangement. The reaction vessel (autoclave) was purged with carbon monoxide gas for about 5 minutes. The autoclave was then pressurized with carbon monoxide gas up to 20 bar (0.080 mol at about 30°C). The temperature was raised to about 130 °C and the corresponding pressure was about 35 atm. The reaction was allowed for about 60 minutes. After the catalytic reaction, the product was collected and analyzed by Gas chromatography. The product was a mixture of acetic acid (4 % wt/.wt) and methyl acetate (37 % wt./wt.) and the Turn Over Number (TON) was 760 per 60 min. (calculated on the basis of conversion of methanol). EXAMPLE - 2 Methanol (4 ml), methyl iodide (1 ml), water (1 ml), [Rh(CO)2Cl]2 (10 mg, 0.026 mmol) and the ligand Pyridine (Py) (4.07 mg, 0.051 mrncl) were charged into a 150 ml capacity high pressure autoclave (Berghof, Germany) which was equipped with magnet- drive ^^,^«*«snd heating arrangement. The reaction vessel (autoclave) was purged with carbon monoxide gas for about 5 minutes. The autoclave was then pressurized with carbon monoxide gas up to 20 bar (0.080 mol at about 30°C). The temperature was raised to about 130 ° C and the corresponding pressure was about 35 atm. The reaction was allowed for about 60 minutes. After the catalytic reaction, the product was collected and analyzed by Gas chromatography. The product was a mixture of acetic acid (6 % wt/.wt) and methyl acetate (44 % wt./wt.) and the Turn Over Number (TON) was 911 per 60 min. (calculated on the basis of conversion of methanol). EXAMPLE -3 Methanol (4 ml), methyl iodide (1 ml), water (1 ml) and [Rh(CO)2Cl(Py)] (14.07 mg, 0.051 mmol) were charged into a 150 nil capacity high pressure autoclave (Ucrghof, Germany) which was equipped with magnet-drive stirrer and heating arrangement. The reaction vessel (autoclave) was purged with carbon monoxide gas for about 5 minutes. The autoclave was then pressurized with carbon monoxide gas up to 20 bar (0.080 mol at about 30°C). The temperature was raised to about 130 °C and the corresponding pressure was about 35 atm. The reaction was allowed for about 60 minutes. After the catalytic reaction, the product was collected and analyzed by Gas chromatography. The product was a mixture of acetic acid (5 % wl/.wt) and methyl acetate (43 % wt./wt.) and the Turn Over Number (TON) was 875 per 60 min. (calculated on the basis of conversion of methanol). Methanol (4 ml), methyl iodide (1 ml), water (1 ml), [Rh(CO)2Cl]2 (10 mg, 0.026 mmol) and the ligand 2-methyl Pyridine (2-MePy) (4.79 mg, 0.051 mmol) were charged into a 150 ml capacity high pressure autoclave (Berghof, Germany) which was equipped with magnet-drive stirrer and heating arrangement. The reaction vessel (autoclave) was purged with carbon monoxide gas for about 5 minutes. The autoclave was then pressurized with carbon monoxide gas up to 20 bar (0.080 mol at about 30°C). The temperature was raised to about 130 ° C and the corresponding pressure was about 35 atm. The reaction was allowed for about 60 minutes. After the catalytic reaction, the product was collected and analyzed by Gas chromatography. The product was a mixture of acetic acid (7 % wt/.wt) and methyl acetate (41 % wt./wt.) and the Turn Over Number (TON) was 884 per 60 min. (calculated on the basis of conversion of methanol). EXAMPLE -5 Methanol (4 ml), methyl iodide (1 ml), water (1 ml) and [Rh(CO)2Cl(2-MePy)] (14.79 mg, 0.051 mmol) were charged into a 150 ml capacity high pressure autoclave (Berghof, Germany) which was equipped with magnet-drive stirrer and heating arrangement. The reaction vessel (autoclave) was purged with carbon monoxide gas for about 5 minutes. The autoclave was then pressurized with carbon monoxide gas up to 20 bar (0.080 mol at about 30°C). The temperature was raised to about 130 ° C and the corresponding pressure was about 35 atm. The reaction was allowed for about 60 minutes. After the catalytic reaction, the product was collected and analyzed by Gas chromat ^hy. the product was a mixture of acetic acid (8 % wt/.wt) and methyl acetate (39 % wt./wt.) and the Turn Over Number (TON) was 856 per 60 min. (calculated on the basis of conversion of methanol). EXAMPLE - 6 Methanol (4 ml), methyl iodide (1 ml), water (1 ml), [Rh(CO)2Cl]2 (10 mg, 0.026 mmol) and the ligand 3-methyl Pyridine (3-MePy) (4.79 mg, 0.051 mmol) were charged into a 150 ml capacity high pressure autoclave (Berghof, Germany) which was equipped with magnet-drive stirrer and heating arrangement. The reaction vessel (autoclave) was purged with carbon monoxide gas for about 5 minutes. The autoclave was then pressurized with carbon monoxide gas up to 20 bar (0.080 mol at about 30°C). The temperature was raised to about 130 ° C and the corresponding pressure was about 35 atm. The reaction was allowed for about 60 minutes. After the catalytic reaction, the product was collected and analyzed by Gas chromatography. The product was a mixture of acetic acid (12 % wt/.wt) and methyl acetate (54 % wt./wt.) and the Turn Over Number (TON) was 1206 per 60 min. (calculated on the basis of conversion of methanol). EXAMPLE -7 Methanol (4 ml), methyl iodide (1 ml), water (1 ml) and [Rh(CO)2Cl(3-MePy)] (14.79 mg, 0.051 mmol) were charged into a 150 ml capacity high pressure autoclave (Berghof, Germany) which was equipped with magnet-drive stirrer and heating arrangement. The reaction vessel (autoclave) was purged with carbon monoxide gas for about *-, •-"*•"' The autoclave was then pressurized with carbon monoxide gas up to 20 bar (0.080 mol at about 30°C). The temperature was raised to about 130 ° C and the corresponding pressure was about 35 atm. The reaction was allowed for about 60 minutes. After the catalytic reaction, the product was collected and analyzed by Gas chromatography. The product was a mixture of acetic acid (10 % wt/.wt) and methyl acetate (55 % wt./wt.) and the Turn Over Number (TON) was 1184 per 60 min. (calculated on the basis of conversion of methanol). EXAMPLE - 8 Methanol (4 ml), methyl iodide (1 ml), water (1 ml), [Rh(CO)2Cl]2 (10 mg, 0.026 mmol) and the ligand 4-methyl Pyridine (4-MePy) (4.79 mg, 0.051 mmol) were charged into a 150 ml capacity high pressure autoclave (Berghof, Germany) which was equipped with magnet-drive stirrer and heating arrangement. The reaction vessel (autoclave) was purged with carbon monoxide gas for about 5 minutes. The autoclave was then pressurized with carbon monoxide gas up to 20 bar (0.080 mol at about 30°C). The temperature was raised to about 130 ° C and the corresponding pressure was about 35 atm. The reaction was allowed for about 60 minutes. After the catalytic reaction, the product was collected and analyzed by Gas chromatography. The product was a mixture of acetic acid (11 % wt/.wt) and methyl acetate (49 % wt./wt.) and the Turn Over Number (TON) was 1087 per 60 min. (calculated on the basis of conversion of mclhanol). EXAMPLE -9 Methanol (4 ml), methyl iodide (1 ml), water (1 ml) and [Rh(CO)2Cl(4-MePy)] (14.79 mg, 0.051 mmol) were charged into a 150 ml capacity high pressure (iniu'jiavc (Berghof, Germany) which was equipped with magnet-drive stirrer and heating arrangement. The reaction vessel (autoclave) was purged with carbon monoxide gas for about 5 minutes. The autoclave was then pressurized with carbon monoxide gas up to 20 bar (0.080 mol at about 30°C). The temperature was raised to about 130 ° C and the corresponding pressure was about 35 atm. The reaction was allowed for about 60. minutes. After the catalytic reaction, the product was collected and analyzed by Gas chromatography. The product was a mixture of acetic acid (10 % wt/.wt) and methyl acetate (50 % wt./wt) and the Turn Over Number (TON) was 1093 per 60 min. (calculated on the basis of conversion of methanol). EXAMPLE -10 Methanol (3.16 ml), methyl iodide (1 ml), water (1 ml) and [Rh(CO)2Cl]2 (10 mg, 0.026 mmol) were charged into a 150 ml capacity high pressure autoclave (Berghof, Germany) which was equipped with magnet-drive stirrer and heating arrangement. The reaction vessel (autoclave) was purged with carbon monoxide gas for about 5 minutes. The autoclave was then pressurized with carbon monoxide gas up to 6 bar (0.036 mol at about 30°C). The temperature was raised to about 150 ° C and the corresponding pressure was about 20 atm. The reaction was allowed for about 90 minutes. After the catalytic reaction, the product was collected and analyzed by Gas chromatography. The product WQS a i]^ Jg»of acetic acid (12 % wt/.wt) and methyl aectulc (52 % vvt./wt.) mid the Turn Over Number (TON) was 921 per 90 min. (calculated on the basis of conversion of methanol). EXAMPLE-11 Methanol (3.16 ml), methyl iodide (1 ml), water (1 ml), [Rh(CO)2Cl]2 (10 mg, 0.026 mmol) and cc.N- diphenyl nitrone (Ph-CH=N(0)Ph) (10.13 mg, 0.051 mmol) were charged into a 150 ml capacity high pressure autoclave (Berghof, Germany) which was equipped with magnet-drive stirrer and heating arrangement. The reaction vessel (autoclave) was purged with carbon monoxide gas for about 5 minutes. The autoclave was then pressurized with carbon monoxide gas up to 6 bar (0.036 mol at about 30°C). The temperature was raised to about 150° C and the corresponding pressure was about 20 atm. The reaction was allowed for about 90 minutes. After the catalytic reaction, the product was collected and analyzed by Gas chromatography. The product was a mixture of acetic acid (less than 43 % wt./wt) and methyl acetate (40 % wt./wt.) and the Turn Over Number (TON) was 1195 per 90 min (calculated on the basis of conversion of methanol). EXAMPLE-12 Methanol (3.16 ml), methyl iodide (1 ml), water (1 ml) and [Rh(CO)2Cl(Ph-CH=N(0)Ph)] (20.13 mg, 0.051 mmol) were charged into a 150 ml capacity high pressure autoclave (Berghof, Germany) which was equipped with magnet-drive stirrer and heating arrangement. The reaction vessel (autoclave) was purged with carbon monoxide gas for about 5 j^*£S- The autoclave was then pressurized with carbon monoxide gas up to 6 bar (0.036 mol at about 30°C). The temperature was raised to about 150 ° C and the corresponding pressure was about 20 atm. The reaction was allowed for about 90 minutes. After the catalytic reaction, the product was collected and analyzed by Gas chromatography. The product was a mixture of acetic acid (less than 41 % wt./wt) and methyl acetate (38 % wt./wt,) and the Turn Over Number (TON) was 1144 per 90 min (calculated on the basis of conversion of methanol). EXAMPLE -13 Methanol (3.16 ml), methyl iodide (1 ml), water (1 ml), [Rh(CO)2Cl]2 (10 mg, 0.026 mmol) and a-styryl, N-phenyl nitrone (Ph-CH=CH-CH=N(0)Ph) (11.47 mg, 0.051 mmol) were charged into a 150 ml capacity high pressure autoclave (Berghof, Germany) which was equipped with magnet-drive stirrer and heating arrangement. The reaction vessel (autoclave) was purged with carbon monoxide gas for about 5 minutes. The autoclave was then pressurized with carbon monoxide gas up to 6 bar (0.036 mol at about 30°C). The temperature was raised to about 150 ° C and the corresponding pressure was about 20 atm. The reaction was allowed for about about 90 minutes. Alter the catalytic reaction, the product was collected and analyzed by Gas chromatography. The product was a mixture of acetic acid (55 % wt.wt) and methyl acetate (36 % wt./wt.) and the Turn Over Number (TON) was 1310 per 90 min. (calculated on the basis of conversion of methanol). EXAMPLE-14 Methanol (3.16 ml), methyl iodide (1 ml), water (1 ml), lRh(CO)2Cl(I)h-Cll-CH-CH=N(O)Ph)] (21.47 mg, 0.051 mmol) were charged into a 150 ml capacity high pressure autoclave (Berghof, Germany) which was equipped with magnet-drive stirrer and heating arrangement. The reaction vessel (autoclave) was purged with carbon monoxide gas for about 5 minutes. The autoclave was then pressurized with carbon monoxide gas up to 6 bar (0.036 mol at about 30°C). The temperature was raised to about 150 ° C and the corresponding pressure was about 20 atm. The reaction was allowed for about about 90 minutes. After the catalytic reaction, the product was collected and analyzed by Gas chromatography. The product was a mixture of acetic acid (49 % wt.wt) and methyl acetate (40 % wt./wt.) and the Turn Over Number (TON) was 1272 per 90 min. (calculated on the basis of conversion of methanol). EXAMPLE-15 Methanol (3.16 ml), methyl iodide (1 ml), water (1 ml), [Rh(CO)2Cl]2 (10 mg, 0.026 mmol) and a-styryl, NX-diphenyldinitrone (Ph-N(0)=CH-CH=N(O)Ph) (12.35 mg, 0.051 mmol) were charged into a 150 ml capacity high pressure autoclave (Berghof, Germany) which was equipped with magnet-drive stirrer and heating arrangement. The reaction vessel (autoclave) was purged with carbon monoxide gas for about 5 minutes. The autoclave was then pressurized with carbon monoxide gas up to 6 bar (0.036 mol at about 30°C). The temperature was raised to about 150 °C and the corresponding pressure was about 20 atm. The reaction was allowed for about about 90 minutes. After the catalytic reaction, the product was collected and analyzed by Gas chromatography. The product mixture of ncclic acid (55 % wi.wt) and methyl ncclatc (41 % wl./wt.) and the Turn Over Number (TON) was 1382 per W min. (calculated on the basis of conversion of methanol). KXAMI'U'M6 Methanol (.1.16 ml), methyl Iodide (I ml), waler (1 ml). |Kh(n)hri(l'h-N(OH'll. CH=N(0)Ph)] (22.35 mg, 0.051 mmol) were charged into a 150 ml capacity high pressure autoclave (Berghof, Germany) which was equipped with magnet-drive stirrer and heating arrangement. The reaction vessel (autoclave) was purged with carbon monoxide gas for about 5 minutes. The autoclave was then pressurized with carbon monoxide gas up to 6 bar (0.036 mol at about 30"C). The temperature was raised to about 150 ° C and the corresponding pressure was about 20 aim. The reaction was allowed for about about 90 minutes. After the catalytic reaction, the product was collected and analyzed by Gas chromatography. The product was a mixture of acetic acid (51 % wt.wt) and methyl acetate (44 % wt./wt.) and the Turn Over Number (TON) was 1358 per 90 min. (calculated on the basis of conversion of methanol). EXAMPLE-17 Methanol (3.16 ml), methyl iodide (1 ml), water (1 ml), [Rh(CO)2Cl]2 (10 mg, 0.026 mmol) C-(2-furyl)-N-phenylnitrone (2-furyl-CH=N(0)Ph) (9.62 mg, 0.051 mmol) were charged into a 150 ml capacity high pressure autoclave (Berghof, Germany) which was equipped with magnet-drive stirrer and heating arrangement. The reaction vessel (autoclave) was purged with carbon monoxide gas for about 5 minutes. The autoclave was th^-°ssurized with carbon monoxide gas up to 6 bar (0.036 mol at about 30°C). The temperature was raised to about 150 °C and the corresponding pressure was about 20 atm. The reaction was allowed for about about 90 minutes. After the catalytic reaction, the product was collected and analyzed by Gas chromatography. The product was a mixture of acetic acid (37 % wt.wt) and methyl acetate (50 % wt./wt.) and the Turn Over Number (TON) was 1252 per 90 min. (calculated on the basis of conversion of methanol). EXAMPLE -18 Methanol (3.16 ml), methyl iodide (1 ml), water (1 ml), [Rh(CO)2Cl(2-furyl-CH=N(0)Ph)] (19.62 mg, 0.051 mmol) were charged into a 150 ml capacity high pressure autoclave (Berghof, Germany) which was equipped with magnet-drive stirrer and heating arrangement. The reaction vessel (autoclave) was purged with carbon monoxide gas for about 5 minutes. The autoclave was then pressurized with carbon monoxide gas up to 6 bar (0.036 mol at about 30°C). The temperature was raised to about 150 ° C and the corresponding pressure was about 20 atm. The reaction was allowed for about about 90 minutes. After the catalytic reaction, the product was collected and analyzed by Gas chromatography. The product was a mixture of acetic acid (41 % wt.wt) and methyl acetate (44 % wt./wt.) and the Turn Over Number (TON) was 1215 per 90 min. (calculated on the basis of conversion of methanol). In summary, the metal complexes [Rh(CO)2ClL] L is a pyridine based nitrogen donor ligand like Pyridine or methyl substituted pyridine or nitrone N-Oxide donor ligand like N-phenyl nitrone or a-substituted N-phenyl nitrone etc. which enhances the electron density central metal atom and subsequently enhances the activity for carbonylation reaction. Moreover, the stability of the metal complexes nrc high which may be responsible for enhancing the yield. The selectivity depends up on the temperature and pressure of the carbonylation reaction. The main advantages of the present invention are : (i) The carbonylation reaction can be carried at a temperature below 1 60°C and pressure below 20 bar. (ii) The metal complexes [Rh(CO)2CL] can be added directly to the catalytic system or can be generated in-situ by reacting [Rh(CO)2CJ2 or with appropriate ligand L and (iii) The metal complexes are very stable in air as well as in solution. we have described and claimed aIn our copending patent application no. n we nave of novel rhodium complexes. We claim 1. An improved process for carbonylation of methanol using novel rhodium metal complexes as catalyst which comprises contacting carbon monoxide with liquid reaction composition comprising methanol, a halogen promoter preferably methyl iodide, water at a temperature in the range 120 to 160°C and at a pressure of 1 to 20 bar for a period of 60- 90 min. in presence of a novel rhodium complex catalyst containing a pyridine based nitrogen donor ligand or a nitrone N-Oxide donor ligand or prepared in situ by stirring a solution of dimeric complex [Rh(CO)2Cl]2 in an organic solvent with two molar equivalent of the respective ligand under nitrogen atmosphere at room temperature for a period of 1 0 - 60 min, recovering a mixture of acetic acid and its ester. 2. A process as claimed in claim 1 wherein the rhodium metal complex is selected from [Rh(CO)2Cl(Pyridine)] [Rh(CO)2Cl{substituted-Pyridine)] o*«l[Rh(CO)2Cl(Nitrone N- Oxide)] 3. A process as claimed in claims 1 & 2 wherein the substituted pyridine /• 2-melhyl pyridine, 3-methyl pyridine and 4-methyl pyridine. 4. A process as claimed in claims 1 & 3 wherein Nitrone N-Oxides donors^ diphenyl nitrone, cc-styril-N-phenyl nitrone, N,N-diphenyldinitrone and C-(2-furyI)-N-phenyl nitrone. 5. A process /claimed in claims 1-4 wherein the metal complex catalyst [Rh(CO)2ClL] can be added directly to the catalytic system or the catalyst can be generated in situ by reacting [Rh(CO)2 I2 ]" with the ligand L. 6. An improved process for carbonylation of methanol using novel rhodium metal complexes as catalyst substantially as herein described with reference to the examples.

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