Title of Invention	"A PROCESS FOR THE PREPARATION OF A MIXTURE OF METHANOL AND FORMALDEHYDE THROUGH THE OXIDATION OF METHANE"
Abstract	This invention relates to a process for the preparation of a mixture of methanol and formaldehyde through the oxidation of methane. In the process the oxidation of methane is carried out using an oxidant and a solid salen transition metal complex as a catalyst. The source of methane may be either pure methane or natural gas. The organo transition metal complexes used as catalysts are solids insoluble in methane or the reaction products arising from oxidation wherein such changes are known to lead to catalyst deactivation problems. The oxidative stability as well as the catalytic activity of the metal salens in the oxidation of methane are enhanced by replacing the hydrogen from the salen by election withdrawing groups like the halogens or nitro groups thereby rendering the metal ions easier to reduce leading to an improved oxidation activity and stability of the catalysts during the reaction.

Title of Invention

"A PROCESS FOR THE PREPARATION OF A MIXTURE OF METHANOL AND FORMALDEHYDE THROUGH THE OXIDATION OF METHANE"

Abstract

This invention relates to a process for the preparation of a mixture of methanol and formaldehyde through the oxidation of methane. In the process the oxidation of methane is carried out using an oxidant and a solid salen transition metal complex as a catalyst. The source of methane may be either pure methane or natural gas. The organo transition metal complexes used as catalysts are solids insoluble in methane or the reaction products arising from oxidation wherein such changes are known to lead to catalyst deactivation problems. The oxidative stability as well as the catalytic activity of the metal salens in the oxidation of methane are enhanced by replacing the hydrogen from the salen by election withdrawing groups like the halogens or nitro groups thereby rendering the metal ions easier to reduce leading to an improved oxidation activity and stability of the catalysts during the reaction.

Full Text	This invention relates to a process for the preparation of a mixture of methanol and formaldehyde through the oxidation of methane. More particularly the present invention relates to an improved process for the preparation of methanol and formaldehyde by the oxidation of methane, using an oxidant and a solid salen transition metal complex as a catalyst. The source of methane may be either pure methane or natural gas. Methane conversions are of particular interest due to large reserves of natural gas located in remote areas, far from their main centers of consumption. It is expensive to transport the gas to the latter location. It is preferable to convert it into liquids like methanol before transport. Large quantities of methane are also available from the decomposition of biomass, municipal waste, rice fields etc. Methane is one of the major greenhouse gases considered to be the primary causes of global warming. Hence technologies for the effective conversion of methane value added fuels or chemicals like methanol are desirable. The direct selective oxidation of methane to methanol is difficult to achieve. Considerable research is underway in an attempt to convert methane to methanol directly and bypass the costly reforming step. Present technology for methane conversion to to methanol is based on steam reforming or partial oxidation to synthesis gas (CO+H2) followed by methanol synthesis. Although well integrated processes have been developed for conventional technology through synthesis gas, there is no escaping from the fact that it is first necessary to conduct an energy intensive endothermic steam reforming step followed by a subsequent catalytic conversion step which has equilibrated limitations. If one could directly convert methane in a single exothermic oxidation step in high yield, this would be more attractive than current trends. One would in effect supplement a two step route having a costly endothermic first step with a direct one-step route which is highly exothermic and cogenerate energy. Methane is oxidised to methanol at ambient conditions by certain enzymatic catalyst systems. Certain bacteria have been found which rely exclusively on methane as their source of life sustaining carbon compounds and energy. The first and most difficult step in the processing of methane by these methanotrophic bacteria is its conversion into methyl alcohol. This conversion is catalyzed by a family of enzymes, the methane monooxygenases. These enzymes use C>2 as their oxygen source. Moreover, although methane is the only hydrocarbon known to sustain growth of the bacteria, methane monooxygenases are able to catalyze the oxidation of other hydrocarbons like ethane, propane and butane. Oxidation is accomplished by forming an activated oxygen : enzymes: substrate complex charged with two electrons acquired from a suitable donor such as NADH. The enzymatic systems, however, have practical limitations for industrial use because of the need for low concentrations of substrate, stiochiometric coreductants, narrow windows of temperature and pH etc. Synthetic bio-mimetics catalysts, dubbed enzymes can mimic enzymatic catalysis but suffer from similar drawbacks. Recent studies by Ellis et al ( J. Chem. Soc; Chem. Comm., 1989,1315) with higher hydrocarbons indicate that it may be possible to mimic some of the characteristics of the biological systems while catalyzing alkane oxidation using only oxygen and no coreductant with catalysts which are robust enough to survive industrial process conditions. U.S patent 5,345,011 claims the use of alummophosphates containing manganese in the structural framework as catalysts for the oxidation of methane to methanol in the vapour phase. However, methane conversion was low, being below 5 %. The selectivity for methanol was in the range 30-50 % mole. The use of ruthenium metal complex catalyst containing an end or bridged oxo group with a ligand L and a carboxylato group for the oxidation of methane to methanol was also claimed in U.S Patent 5,347,057. The catalyst turnover rate for the oxidation of reaction, however, was relatively low. In this system, the ligand and the ruthenium metal component are costly components and the recyclability of the catalysts remains questionable. The gas phase oxidation of natural gas to methanol by molecular oxygen has also been disclosed in U.S. Patent 4,618,732. A 13 % conversion of natural gas ( composition not mentioned) was claimed at 300-500°C and 10-100 atmospheres. U.S. Patents 4,918,249 and 5,132,472 used silicometallates as catalysts for the gas phase oxidation of methane to methanol and CO2 was reported. In all the gas phase oxidation of methane at temperatures above 100°C, referred to in the above patents, significant amounts of CO2 in excess of 10 % are produced. It is thus evident that there is a need for the development of a process for the low temperature oxidation of methane using solid, recyclable catalysts and operating at a low temperature (below 100°C, for example) to avoid the production of undesirable byproducts like carbon dioxide. It is, therefore, an object of the present invention to provide a process for the low temperature oxidation of methane to methanol and formaldehyde using a catalyst which would remain in the solid state at the end of the oxidation reaction thereby facilitating the easy separation, recovery and recycle of the catalyst from the reaction products without having any adverse impact on the environment. Another object of the present invention is to provide an improved process for the oxidation of methane at a temperature below that wherein the production of CO2 is significant. Salens are planar tetradentate ligand systems where metallic cations can be easily accommodated at the center of these ligands with the two oxygen and two nitrogen as the ligating atoms. Salen complexes are known to be useful as chemical reagents of a catalytic nature, more particularly in directing certain oxidative processes. Many known salens have been judged to suffer certain drawbacks by being deficient in the combination of properties desired for many candidate use, such as in the oxidation of alkanes and more particularly in the oxidation of methane. One major draw back of homogeneous salen catalysts in industrial oxidation processes is the formation of aggregates in solution which significantly deactivates these catalysts. Due to our continued research in this area the inventor of the present invention have observed that the organotransition metal complexes used as catalysts are solids insoluble in methane or the reaction products arising from oxidation of methane. Hence they do not undergo aggregation or change of phase during the oxidation wherein such changes are known to lead to catalyst deactivation problems. We have found that the oxidative stability as well as the catalytic activity of the metal salens in the oxidation of methane are enhanced by replacing the hydrogens from the salen by electron withdrawing groups like the halogens or nitro groups thereby rendering the metal ions easier to reduce leading to an improved oxidation activity and stability of the catalysts during the reaction. There are total eight hydrogen atom positions on such salen molecules which can in principle, be substituted by other substituents. We have observed that when some of the hydrogen atoms of the said salen are substituted by one or more electron withdrawing groups such as halogens or nitro groups or mixture of such groups there is substantial improvement in conversion. Accordingly, the present invention provides an improved process for preparation of a mixture of methanol and formaldehyde through the oxidation of methane which comprises reacting methane with an oxidant such as here in described in the presence of a solid organo transition metal complex as catalyst, the said organo transition metal complex is encapsulated in a solid matrix is a salen or substituted salen wherein some of the hydrogen atoms of the said organ transition metal complex have been substituted by one or more electron withdrawing groups selected from the halogens, the nitro group or mixture thereof, at a temperature in the range of 0 to 100 C, at a pressure in the range of 10 to 1000 psi pressure optionally in the presence of solvents, and a promoter such as herein described and isolating mixture of methanol and formaldehyde by conventional methods In an embodiment of the present invention the organ transition metal complex is selected from salen or substituted salens, wherein some or all of the hydrogen atoms have been substituted by one or more electron withdrawing groups such as halogen or nitro groups. In another embodiment of the present invention, the transition metal may be manganese and copper. Some no limiting examples of such oregano transition metal complexes used as catalysts in the oxidation of methane to methanol and formaldehyde are iron manganese halosalens, coper halosalens, manganese nitrosalens and copper nitrosalens. In yet another embodiment of present invention the electron withdrawing groups attached to the organotransition metal complex is selected from the halogen like chlorine or bromine or nitro groups. In a preferred embodiment of the present invention, the oxidation of methane by molecular oxygen is catalyzed by the halogen or nitro salens of the manganese or copper. In yet another embodiment of the present invention, the oxidant used may be molecular oxygen, air or a mixture of oxygen and an inert gas diluent like nitrogen. Yet another embodiment of the present invention, the above mentioned reaction can be carried out in the presence or absence of solvents. It may be advantageous option to carry out the said oxidation reaction in the presence of a suitable solvent which would maintain the oxidation products like methanol in the dissolved state during the course of the reaction, thereby facilitating the separation of the methanol from the solid catalysts. Suitable solvents for such use include acetonitrile, benzonitrile and pyridine. Examples of such solvents which can be used in the process of the present invention include acetonitrile, acetone, benzene or any other organic solvent which is inert under the oxidation reaction conditions. In one advantageous embodiment of the present invention, the rates of the oxidation of methane to methanol may be significantly enhanced by addition of very small catalytic quantities of promoter. Examples of such promoters include alkyl hydroperoxide, dialkylperoxides and such compounds. Cyclohexyl hydroperoxide, cumyl peroxide, tertiary butyl hydroperoxide are some of the examples of such promoters which may be present in concentrations not exceeding 1 % by weight of methane and more preferably 0.1 % by weight of methane. In yet another advantageous embodiment of the present invention, the organotransition metal complex may be encapsulated in a solid matrix. Due to the greater dispersion of the organotransition metal complex catalyst in solid matrices and the consequent enhanced stability of the structural integrity of the catalyst significant process advantages like greater activity, stability and easy recovery and recyclability of the catalyst are observed. Examples of such solid matrices include inorganic oxide like silica, alumina, molecular sieves, zeolites and the like as well as organic polymeric materials. It is advantageous feature of the present invention that due to the high activity the catalysts used herein, the oxidation reaction can be carried out at temperatures much below those used in prior art and preferably below 100°C, thereby leading to much lower yields of undesired side products like CC^. The details of the present invention is described in the examples given below which are provided by way of illustration only and therefore should not be construed to limit the scope of the invention. Example 1 In an autoclave, 0.4 g of solid salen manganese(III), 30 g of acetonitrile, 120 psi methane, 120 psi air and 0.8 g of tertiary butyl hydroperoxide were stirred at 0°C for 8 hrs. At the end of the reaction, the gas was collected and analyzed for unreacted methane, carbon dioxide and formaldehyde using a Shimadzu GC-14B gas chromatograph equipped with a TCD detector and a Porapak N packed column. The solid catalyst were separated from the reaction products by filtration using G4 filtration assembly and analyzed by Perkin Elmer Auto System XL gas chromatograph equipped with a FID detector and a capillary column. The identity of the products was further confirmed by GC mass spectroscopy (Shimadzu GCMS-QP 5000) and also using standard compounds. Quantification of the products formed was carried out using gas chromatography techniques. The results are tabulated in table 1. Example 2 In an autoclave, 0.4 g of solid 3,3'-dichlorosalen manganese(III), 30 g of acetonitrile, 120 psi methane, 120 psi air and 0.8 g of tertiary butyl hydroperoxide were stirred at 0°C for 8 hrs. At the end of the reaction, the gas was collected and analyzed for unreacted methane, carbon dioxide and formaldehyde using a Shimadzu GC-14B gas chromatograph equipped with a TCD detector and a Porapak N packed column. The solid catalyst were separated from the reaction products by filtration using G4 filtration assembly and analyzed by Perkin Elmer Auto System XL gas chromatograph equipped with a FID detector and a capillary column. The identity of the products was further confirmed by GC mass spectroscopy (Shimadzu GCMS-QP 5000) and also using standard compounds. Quantification of the products formed was carried out using gas chromatography techniques. The results are tabulated in table 1. Example 3 In an autoclave, 0.4 g of solid 3,3'-dibromosalen manganese(III), 30 g of acetonitrile, 120 psi methane 120 psi air and 0.8 g of tertiary butyl hydroperoxide were stirred at 0°C for 8 hrs. At the end of the reaction, the gas was collected and analyzed for unreacted methane, carbon dioxide and formaldehyde using a Shimadzu GC-14B gas chromatograph equipped with a TCD detector and a Porapak N packed column. The solid catalyst were separated from the reaction products by filtration using G4 filtration assembly and analyzed by Perkin Elmer Auto System XL gas chromatograph equipped with a FID detector and a capillary column. The identity of the products was further confirmed by GC mass spectroscopy (Shimadzu GCMS-QP 5000) and also using standard compounds. Quantification of the products formed was carried out using gas chromatography techniques. The results are tabulated in table 1. Example 4 In an autoclave, 0.4 g of solid 3,3'-dinitrosalen manganese(III), 30 g of acetonitrile, 120 psi methane, 120 psi air and 0.8 g of tertiary butyl hydroperoxide were stirred at 0°C for 8 hrs. At the end of the reaction, the gas was collected and analyzed for unreacted methane, carbon dioxide and formaldehyde using a Shimadzu GC-14B gas chromatograph equipped with a TCD detector and a Porapak N packed column. The solid catalyst were separated from the reaction products by filtration using G4 filtration assembly and analyzed by Perkin Elmer Auto System XL gas chromatograph equipped with a FID detector and a capillary column. The identity of the products was further confirmed by GC mass spectroscopy ( Shimadzu GCMS-QP 5000) and also using standard compounds. Quantification of the products formed was carried out using gas chromatography techniques. The results are tabulated in table 1. (Table Removed) Example 5 In an autoclave, 0.4 g of solid 3,3',5,5'-tetrachlorosalen manganese(III), 30 g of acetonitrile, 120 psi methane, 120 psi air and 0.8 g of tertiary butyl hydroperoxide were stirred at 0°C for 8 hrs. At the end of the reaction, the gas was collected and analyzed for unreacted methane, carbon dioxide and formaldehyde using a Shimadzu GC-14B gas chromatograph equipped with a TCD detector and a Porapak N packed column. The solid catalyst were separated from the reaction products by filtration usingG4 filtration assembly and analyzed by Perkin Elmer Auto System XL gas chromatograph equipped with a FID detector and a capillary column. The identity of the products was further confirmed by GC mass spectroscopy ( Shimadzu GCMS-QP 5000) and also using standard compounds. Quantification of the products formed was carried out using gas chromatography techniques. The results are tabulated in table 1. Example 6 In an autoclave, 0.4 g of solid 3,3',5,5'-tetrabromosalen manganese(III), 30 g of acetonitrile, 120 psi methane, 120 psi air and 0.8 g of tertiary butyl hydroperoxide were stirred at 0°C for 8 hrs. At the end of the reaction, the gas was collected and analysed for unreacted methane, carbon dioxide and formaldehyde using a Shimadzu GC-14B gas chromatograph equipped with a TCD detector and a Porapak N packed column. The solid catalyst were separated from the reaction products by filtration using G4 filtration assembly and analyzed by Perkin Elmer Auto System XL gas chromatograph equipped with a FID detector and a capillary column. The identity of the products was further confirmed by GC mass spectroscopy ( Shimadzu GCMS-QP 5000) and also using standard compounds. Quantification of the products formed was carried out using gas chromatography techniques. The results are tabulated in table 1. In an autoclave, 0.4 g of solid salen complexes of copper, 30 g of acetonitrile, 120 psi methane, 120 psi air and 0.8 g of tertiary butyl hydroperoxide were stirred at 0°C for 8 hrs. At the end of the reaction, the gas was collected and analyzed for unreacted methane, carbon dioxide and formaldehyde using a Shimadzu GC-14B gas chromatograph equipped with a TCD detector and a poropak N packed column. The solid catalyst were separated from the reaction products by filtration using G4 filtration assembly and analyzed by Perkin Elmer Auto System XL gas chromatograph equipped with a FID detector and a capillary column. The identity of the products was further confirmed by GC mass spectroscopy ( Shimadzu GCMS-QP 5000) and also using standard compounds. Quantification of the products formed was carried out using gas chromatography techniques. The results are tabulated in table 2. Example 8 In an autoclave, 0.4 g of solid 3,3'-dichlorosalen copper(II), 30 g of acetonitrile, 120 psi methane, 120 psi air and 0.8 g of tertiary butyl hydroperoxide were stirred at 0°C for 8 hrs. At the end of the reaction, the gas was collected and analyzed for unreacted methane, carbon dioxide and formaldehyde using a Shimadzu GC-14B gas chromatograph equipped with a TCD detector and a Porapak N packed column. The solid catalyst were separated from the reaction products by filtration using G4 filtration assembly and analyzed by Perkin Elmer Auto System XL gas chromatograph equipped with a FID detector and a capillary column. The identity of the products was further confirmed by GC mass spectroscopy (Shimadzu GCMS-QP 5000) and also using standard compounds. Quantification of the products formed was carried out using gas chromatography techniques. The results are tabulated in table 2. Example 9 n an autoclave, 0.4 g of solid 3,3'-dibromosalen copper(II), 30 g of acetonitrile, 120 psi methane, 120 psi air and 0.8 g of tertiary butyl hydroperoxide were stirred at 0°C for 8 hrs. At the end of the reaction, the gas was collected and analyzed for unreacted methane, carbon dioxide and formaldehyde using a Shimadzu GC-14B gas chromatograph equipped with a TCD detector and a Porapak N packed column. The solid catalyst were separated from the reaction products by filtration using G4 filtration assembly and analyzed by Perkin Elmer Auto System XL gas chromatograph equipped with a FID detector and a capillary column. The identity of the products was further confirmed by GC mass spectroscopy (Shimadzu GCMS-QP 5000) and also using standard compounds. Quantification of the products formed was carried out using gas chromatography techniques. The results are tabulated in table 2. Table 2. Results of methane oxidation using zeolite encapsulated copper salen complexes and molecular oxygen. Catalyst Conversion CH3OH HCHO HCOOH (mole%) (mole%) (mole %) (mole%) Cusalen-X 3.81 2.01 1.81 (Table Removed) Example 10 In an autoclave, 0.4 g of solid 3,3'-dinitrosalen copper(II), 30 g of acetonitrile, 120 psi methane, 120 psi air and 0.8 g of tertiary butyl hydroperoxide were stirred at 0°C for 8 hrs. At the end of the reaction, the gas was collected and analyzed for unreacted methane, carbon dioxide and formaldehyde using a Shimadzu GC-14B gas chromatograph equipped with a TCD detector and a Porapak N packed column. The solid catalyst were separated from the reaction products by filtration using G4 filtration assembly and analyzed by Perkin Elmer Auto System XL gas chromatograph equipped with a FID detector and a capillary column. The identity of the products was further confirmed by GC mass spectroscopy (Shimadzu GCMS-QP 5000) and also using standard compounds. Quantification of the products formed was carried out using gas chromatography techniques. The results are tabulated in table 2. Example 11 In an autoclave, 0.4 g of solid 3,3',5,5'-tetrachlorosalen copper(II), 30 g of acetonitrile, 120 psi methanol, 120 psi air and 0.8 g of tertiary butyl hydroperoxide were stirred at 0°C for 8 hrs. At the end of the reaction, the gas was collected and analysed for unreacted methane, carbon dioxide and formaldehyde using a Shimadzu GC-14B gas chromatograph equipped with a TCD detector and a poropak N packed column. The solid catalyst were separated from the reaction products by filtration using G4 filtration assembly and analyzed by Perkin Elmer Auto System XL gas chromatograph equipped with a FID detector and a capillary column. The identity of the products was further confirmed by GC mass spectroscopy ( Shimadzu GCMS-QP 5000) and also using standard compounds. Quantification of the products formed was carried out using gas chromatography techniques. The results are tabulated in table 2. Example 12 In an autoclave, 0.4 g of solid 3,3',5,5'-tetrabromosalen copper(II), 30 g of acetonitrile, 120 psi methane, 120 psi air and 0.8 g of tertiary butyl hydroperoxide were stirred at 0°C for 8 hrs. At the end of the reaction, the gas was collected and analysed for unreacted methane, carbon dioxide and formaldehyde using a Shimadzu GC-14B gas chromatograph equipped with a TCD detector and a Porapak N packed column. The solid catalyst were separated from the reaction products by filtration using G4 filtration assembly and analyzed by Perkin Elmer Auto System XL gas chromatograph equipped with a FID detector and a capillary column. The identity of the products was further confirmed by GC mass spectroscopy ( Shimadzu GCMS-QP 5000) and also using standard compounds. Quantification of the products formed was carried out using gas chromatography techniques. The results are tabulated in table 2. Example 13 In an autoclave, 0.4 g of copper salen complexes encapsulated in Y zeolites, 30 g of acetonitrile, 120 psi methane, 120 psi air and 0.8 g of tertiary butyl hydroperoxide were stirred at 0°C for 8 hrs. At the end of the reaction, the gas was collected and analyzed for unreacted methane, carbon dioxide and formaldehyde using a Shimadzu GC-14B gas chromatograph equipped with a TCD detector and a Porapak N packed column. The solid catalyst were separated from the reaction products by filtration using G4 filtration assembly and analyzed by Perkin Elmer Auto System XL gas chromatograph equipped with a FID detector and a capillary column. The identity of the products was further confirmed by GC mass spectroscopy ( Shimadzu GCMS-QP 5000) and also using standard compounds. Quantification of the products formed was carried out using gas chromatography techniques. The results are tabulated in table 2. Example 14 In an autoclave, 0.4 g of 3,3'-dinitrosalencopper(II) complexes encapsulated in Y zeolite , 30 g of acetonitrile, 120 psi methane, 120 psi air and 0.8 g of tertiary butyl hydroperoxide were stirred at 0°C for 8 hrs. At the end of the reaction, the gas was collected and analyzed for unreacted methane, carbon dioxide and formaldehyde using a Shimadzu GC-14B gas chromatograph equipped with a TCD detector and a Porapak N packed column. The solid catalyst were separated from the reaction products by filtration using G4 filtration assembly and analyzed by Perkin Elmer Auto System XL gas chromatograph equipped with a FID detector and a capillary column. The identity of the products was further confirmed by GC mass spectroscopy ( Shimadzu GCMS-QP 5000) and also using standard compounds. Quantification of the products formed was carried out using gas chromatography techniques. The results are tabulated in table 2. We Claim: 1 .An improved process for preparation of a mixture of methanol and formaldehyde through the oxidation of methane which comprises reacting methane with an oxidant such as here in described in the presence of a solid organo transition metal complex as catalyst, the said organo transition metal complex is encapsulated in a solid matrix is a salen or substituted salen wherein some of the hydrogen atoms of the said organ transition metal complex have been substituted by one or more electron withdrawing groups selected from the halogens, the nitro group or mixture thereof, at a temperature in the range of 0 to 100°C, at a pressure in the range of 10 to 1000 psi pressure optionally in the presence of solvents, and a promoter such as herein described and isolating mixture of methanol and formaldehyde by conventional methods. •4 2. An improved process as claimed in claim 1 wherein the transition metal is selected from manganese, copper or mixtures thereof. 3. An improved process as claimed in claim 1 wherein the oxidant used is molecular oxygen, air or a mixture of oxygen and an inert gas like nitrogen. 4. An improved process as claimed in claim 1 wherein the oxidation reaction is carried out optionally in the presence of solvents selected from acetonitrile, benzene and acetone. 5. An improved process as claimed in claim 1 wherein a promoter is selected from alkyl hydro peroxide, dialkyl peroxide or mixtures thereof in the reaction. 6. An improved process as claimed in claim 1 wherein the concentration of the promoter in the reaction mixture does not exceed 1% by weight of the methane. 7. An improved process as claimed in claim 1 wherein he solid matrix used is an inorganic oxide such as silica, alumina, aluminosilicates or molecular sieves. 8. An improved process for the preparation of a mixture of methanol and formaldehyde through the oxidation of methane with reference to the examples.

Full Text

This invention relates to a process for the preparation of a mixture of methanol and formaldehyde through the oxidation of methane. More particularly the present invention relates to an improved process for the preparation of methanol and formaldehyde by the oxidation of methane, using an oxidant and a solid salen transition metal complex as a catalyst. The source of methane may be either pure methane or natural gas. Methane conversions are of particular interest due to large reserves of natural gas located in remote areas, far from their main centers of consumption. It is expensive to transport the gas to the latter location. It is preferable to convert it into liquids like methanol before transport. Large quantities of methane are also available from the decomposition of biomass, municipal waste, rice fields etc. Methane is one of the major greenhouse gases considered to be the primary causes of global warming. Hence technologies for the effective conversion of methane value added fuels or chemicals like methanol are desirable. The direct selective oxidation of methane to methanol is difficult to achieve. Considerable research is underway in an attempt to convert methane to methanol directly and bypass the costly reforming step. Present technology for methane conversion to to methanol is based on steam reforming or partial oxidation to synthesis gas (CO+H2) followed by methanol synthesis. Although well integrated processes have been developed for conventional technology through synthesis gas, there is no escaping from the fact that it is first necessary to conduct an energy intensive endothermic steam reforming step followed by a subsequent catalytic conversion step which has equilibrated limitations. If one could directly convert methane in a single exothermic oxidation step in high yield,
this would be more attractive than current trends. One would in effect supplement a two step route having a costly endothermic first step with a direct one-step route which is highly exothermic and cogenerate energy. Methane is oxidised to methanol at ambient conditions by certain enzymatic catalyst systems. Certain bacteria have been found which rely exclusively on methane as their source of life sustaining carbon compounds and energy. The first and most difficult step in the processing of methane by these methanotrophic bacteria is its conversion into methyl alcohol. This conversion is catalyzed by a family of enzymes, the methane monooxygenases. These enzymes use C>2 as their oxygen source. Moreover, although methane is the only hydrocarbon known to sustain growth of the bacteria, methane monooxygenases are able to catalyze the oxidation of other hydrocarbons like ethane, propane and butane. Oxidation is accomplished by forming an activated oxygen : enzymes: substrate complex charged with two electrons acquired from a suitable donor such as NADH. The enzymatic systems, however, have practical limitations for industrial use because of the need for low concentrations of substrate, stiochiometric coreductants, narrow windows of temperature and pH etc. Synthetic bio-mimetics catalysts, dubbed enzymes can mimic enzymatic catalysis but suffer from similar drawbacks. Recent studies by Ellis et al ( J. Chem. Soc; Chem. Comm., 1989,1315) with higher hydrocarbons indicate that it may be possible to mimic some of the characteristics of the biological systems while catalyzing alkane oxidation using only oxygen and no coreductant with catalysts which are robust enough to survive industrial process conditions.
U.S patent 5,345,011 claims the use of alummophosphates containing manganese in the structural framework as catalysts for the oxidation of methane to methanol in the vapour phase. However, methane conversion was low, being below 5 %. The selectivity for methanol was in the range 30-50 % mole. The use of ruthenium metal complex catalyst containing an end or bridged oxo group with a ligand L and a carboxylato group for the oxidation of methane to methanol was also claimed in U.S Patent 5,347,057. The catalyst turnover rate for the oxidation of reaction, however, was relatively low. In this system, the ligand and the ruthenium metal component are costly components and the recyclability of the catalysts remains questionable. The gas phase oxidation of natural gas to methanol by molecular oxygen has also been disclosed in U.S. Patent 4,618,732. A 13 % conversion of natural gas ( composition not mentioned) was claimed at 300-500°C and 10-100 atmospheres. U.S. Patents 4,918,249 and 5,132,472 used silicometallates as catalysts for the gas phase oxidation of methane to methanol and CO2 was reported. In all the gas phase oxidation of methane at temperatures above 100°C, referred to in the above patents, significant amounts of CO2 in excess of 10 % are produced. It is thus evident that there is a need for the development of a process for the low temperature oxidation of methane using solid, recyclable catalysts and operating at a low temperature (below 100°C, for example) to avoid the production of undesirable byproducts like carbon dioxide.
It is, therefore, an object of the present invention to provide a process for the low temperature oxidation of methane to methanol and formaldehyde using a catalyst which would remain in the solid state at the end of the oxidation reaction thereby facilitating the
easy separation, recovery and recycle of the catalyst from the reaction products without
having any adverse impact on the environment.
Another object of the present invention is to provide an improved process for the
oxidation of methane at a temperature below that wherein the production of CO2 is
significant.
Salens are planar tetradentate ligand systems where metallic cations can be easily
accommodated at the center of these ligands with the two oxygen and two nitrogen as the ligating atoms. Salen complexes are known to be useful as chemical reagents of a catalytic nature, more particularly in directing certain oxidative processes. Many known salens have been judged to suffer certain drawbacks by being deficient in the combination of properties desired for many candidate use, such as in the oxidation of alkanes and more particularly in the oxidation of methane. One major draw back of homogeneous salen catalysts in industrial oxidation processes is the formation of aggregates in solution which significantly deactivates these catalysts. Due to our continued research in this area the inventor of the present invention have observed that the organotransition metal complexes used as catalysts are solids insoluble in methane or the reaction products arising from oxidation of methane. Hence they do not undergo aggregation or change of phase during the oxidation wherein such changes are known to lead to catalyst deactivation problems. We have found that the oxidative stability as well as the catalytic activity of the metal salens in the oxidation of methane are enhanced by replacing the hydrogens from the salen by electron withdrawing groups like the halogens or nitro groups thereby rendering the metal ions easier to reduce leading to an improved oxidation activity and stability of the catalysts during the reaction.
There are total eight hydrogen atom positions on such salen molecules which can in principle, be substituted by other substituents. We have observed that when some of the hydrogen atoms of the said salen are substituted by one or more electron withdrawing groups such as halogens or nitro groups or mixture of such groups there is substantial improvement in conversion.
Accordingly, the present invention provides an improved process for preparation of a mixture of methanol and formaldehyde through the oxidation of methane which comprises reacting methane with an oxidant such as here in described in the presence of a solid organo transition metal complex as catalyst, the said organo transition metal complex is encapsulated in a solid matrix is a salen or substituted salen wherein some of the hydrogen atoms of the said organ transition metal complex have been substituted by one or more electron withdrawing groups selected from the halogens, the nitro group or mixture thereof, at a temperature in the range of 0 to 100 C, at a pressure in the range of 10 to 1000 psi pressure optionally in the presence of solvents, and a promoter such as herein described and isolating mixture of methanol and formaldehyde by conventional methods
In an embodiment of the present invention the organ transition metal complex is selected from salen or substituted salens, wherein some or all of the hydrogen atoms have been substituted by one or more electron withdrawing groups such as halogen or nitro groups.
In another embodiment of the present invention, the transition metal may be manganese and copper.
Some no limiting examples of such oregano transition metal complexes used as catalysts in the oxidation of methane to methanol and formaldehyde are iron manganese halosalens, coper halosalens, manganese nitrosalens and copper nitrosalens.
In yet another embodiment of present invention the electron withdrawing groups attached to the organotransition metal complex is selected from the halogen like chlorine or bromine or nitro groups.
In a preferred embodiment of the present invention, the oxidation of methane by molecular oxygen is catalyzed by the halogen or nitro salens of the manganese or copper. In yet another embodiment of the present invention, the oxidant used may be molecular oxygen, air or a mixture of oxygen and an inert gas diluent like nitrogen. Yet another embodiment of the present invention, the above mentioned reaction can be carried out in the presence or absence of solvents. It may be advantageous option to carry out the said oxidation reaction in the presence of a suitable solvent which would maintain the oxidation products like methanol in the dissolved state during the course of the reaction, thereby facilitating the separation of the methanol from the solid catalysts. Suitable solvents for such use include acetonitrile, benzonitrile and pyridine. Examples of such solvents which can be used in the process of the present invention include acetonitrile, acetone, benzene or any other organic solvent which is inert under the oxidation reaction conditions.
In one advantageous embodiment of the present invention, the rates of the oxidation of methane to methanol may be significantly enhanced by addition of very small catalytic quantities of promoter. Examples of such promoters include alkyl hydroperoxide, dialkylperoxides and such compounds. Cyclohexyl hydroperoxide, cumyl peroxide, tertiary butyl hydroperoxide are some of the examples of such promoters which may be present in concentrations not exceeding 1 % by weight of methane and more preferably 0.1 % by weight of methane.
In yet another advantageous embodiment of the present invention, the organotransition metal complex may be encapsulated in a solid matrix. Due to the greater dispersion of the organotransition metal complex catalyst in solid matrices and the consequent enhanced stability of the structural integrity of the catalyst significant process advantages like greater activity, stability and easy recovery and recyclability of the catalyst are observed. Examples of such solid matrices include inorganic oxide like silica, alumina, molecular sieves, zeolites and the like as well as organic polymeric materials. It is advantageous feature of the present invention that due to the high activity the catalysts used herein, the oxidation reaction can be carried out at temperatures much below those used in prior art and preferably below 100°C, thereby leading to much lower yields of undesired side products like CC^.
The details of the present invention is described in the examples given below which are provided by way of illustration only and therefore should not be construed to limit the scope of the invention.
Example 1
In an autoclave, 0.4 g of solid salen manganese(III), 30 g of acetonitrile, 120 psi methane, 120 psi air and 0.8 g of tertiary butyl hydroperoxide were stirred at 0°C for 8 hrs. At the end of the reaction, the gas was collected and analyzed for unreacted methane, carbon dioxide and formaldehyde using a Shimadzu GC-14B gas chromatograph equipped with a TCD detector and a Porapak N packed column. The solid catalyst were separated from the reaction products by filtration using G4 filtration assembly and analyzed by Perkin
Elmer Auto System XL gas chromatograph equipped with a FID detector and a capillary column. The identity of the products was further confirmed by GC mass spectroscopy (Shimadzu GCMS-QP 5000) and also using standard compounds. Quantification of the products formed was carried out using gas chromatography techniques. The results are tabulated in table 1.
Example 2
In an autoclave, 0.4 g of solid 3,3'-dichlorosalen manganese(III), 30 g of acetonitrile, 120 psi methane, 120 psi air and 0.8 g of tertiary butyl hydroperoxide were stirred at 0°C for 8 hrs. At the end of the reaction, the gas was collected and analyzed for unreacted methane, carbon dioxide and formaldehyde using a Shimadzu GC-14B gas chromatograph equipped with a TCD detector and a Porapak N packed column. The solid catalyst were separated from the reaction products by filtration using G4 filtration assembly and analyzed by Perkin Elmer Auto System XL gas chromatograph equipped with a FID detector and a capillary column. The identity of the products was further confirmed by GC mass spectroscopy (Shimadzu GCMS-QP 5000) and also using standard compounds. Quantification of the products formed was carried out using gas chromatography techniques. The results are tabulated in table 1.
Example 3
In an autoclave, 0.4 g of solid 3,3'-dibromosalen manganese(III), 30 g of acetonitrile, 120 psi methane 120 psi air and 0.8 g of tertiary butyl hydroperoxide were stirred at 0°C for
8 hrs. At the end of the reaction, the gas was collected and analyzed for unreacted methane, carbon dioxide and formaldehyde using a Shimadzu GC-14B gas chromatograph equipped with a TCD detector and a Porapak N packed column. The solid catalyst were separated from the reaction products by filtration using G4 filtration assembly and analyzed by Perkin Elmer Auto System XL gas chromatograph equipped with a FID detector and a capillary column. The identity of the products was further confirmed by GC mass spectroscopy (Shimadzu GCMS-QP 5000) and also using standard compounds. Quantification of the products formed was carried out using gas chromatography techniques. The results are tabulated in table 1.
Example 4
In an autoclave, 0.4 g of solid 3,3'-dinitrosalen manganese(III), 30 g of acetonitrile, 120 psi methane, 120 psi air and 0.8 g of tertiary butyl hydroperoxide were stirred at 0°C for 8
hrs. At the end of the reaction, the gas was collected and analyzed for unreacted methane, carbon dioxide and formaldehyde using a Shimadzu GC-14B gas chromatograph equipped with a TCD detector and a Porapak N packed column. The solid catalyst were separated from the reaction products by filtration using G4 filtration assembly and analyzed by Perkin Elmer Auto System XL gas chromatograph equipped with a FID detector and a capillary column. The identity of the products was further confirmed by GC mass spectroscopy ( Shimadzu GCMS-QP 5000) and also using standard compounds. Quantification of the products formed was carried out using gas chromatography techniques. The results are tabulated in table 1.
(Table Removed)
Example 5
In an autoclave, 0.4 g of solid 3,3',5,5'-tetrachlorosalen manganese(III), 30 g of acetonitrile, 120 psi methane, 120 psi air and 0.8 g of tertiary butyl hydroperoxide were stirred at 0°C for 8 hrs. At the end of the reaction, the gas was collected and analyzed for unreacted methane, carbon dioxide and formaldehyde using a Shimadzu GC-14B gas
chromatograph equipped with a TCD detector and a Porapak N packed column. The solid catalyst were separated from the reaction products by filtration usingG4 filtration assembly and analyzed by Perkin Elmer Auto System XL gas chromatograph equipped with a FID detector and a capillary column. The identity of the products was further confirmed by GC mass spectroscopy ( Shimadzu GCMS-QP 5000) and also using standard compounds. Quantification of the products formed was carried out using gas chromatography techniques. The results are tabulated in table 1.
Example 6
In an autoclave, 0.4 g of solid 3,3',5,5'-tetrabromosalen manganese(III), 30 g of acetonitrile, 120 psi methane, 120 psi air and 0.8 g of tertiary butyl hydroperoxide were stirred at 0°C for 8 hrs. At the end of the reaction, the gas was collected and analysed for unreacted methane, carbon dioxide and formaldehyde using a Shimadzu GC-14B gas chromatograph equipped with a TCD detector and a Porapak N packed column. The solid catalyst were separated from the reaction products by filtration using G4 filtration assembly and analyzed by Perkin Elmer Auto System XL gas chromatograph equipped with a FID detector and a capillary column. The identity of the products was further confirmed by GC mass spectroscopy ( Shimadzu GCMS-QP 5000) and also using standard compounds. Quantification of the products formed was carried out using gas chromatography techniques. The results are tabulated in table 1.
In an autoclave, 0.4 g of solid salen complexes of copper, 30 g of acetonitrile, 120 psi methane, 120 psi air and 0.8 g of tertiary butyl hydroperoxide were stirred at 0°C for 8 hrs. At the end of the reaction, the gas was collected and analyzed for unreacted methane, carbon dioxide and formaldehyde using a Shimadzu GC-14B gas chromatograph equipped with a TCD detector and a poropak N packed column. The solid catalyst were separated from the reaction products by filtration using G4 filtration assembly and analyzed by Perkin Elmer Auto System XL gas chromatograph equipped with a FID detector and a capillary column. The identity of the products was further confirmed by GC mass spectroscopy ( Shimadzu GCMS-QP 5000) and also using standard compounds. Quantification of the products formed was carried out using gas chromatography techniques. The results are tabulated in table 2.
Example 8
In an autoclave, 0.4 g of solid 3,3'-dichlorosalen copper(II), 30 g of acetonitrile, 120 psi methane, 120 psi air and 0.8 g of tertiary butyl hydroperoxide were stirred at 0°C for 8 hrs. At the end of the reaction, the gas was collected and analyzed for unreacted methane, carbon dioxide and formaldehyde using a Shimadzu GC-14B gas chromatograph equipped with a TCD detector and a Porapak N packed column. The solid catalyst were separated from the reaction products by filtration using G4 filtration assembly and analyzed by Perkin Elmer Auto System XL gas chromatograph equipped with a FID detector and a capillary column. The identity of the products was further confirmed by GC mass spectroscopy (Shimadzu GCMS-QP 5000) and also using standard
compounds. Quantification of the products formed was carried out using gas chromatography techniques. The results are tabulated in table 2.
Example 9
n an autoclave, 0.4 g of solid 3,3'-dibromosalen copper(II), 30 g of acetonitrile, 120 psi methane, 120 psi air and 0.8 g of tertiary butyl hydroperoxide were stirred at 0°C for 8 hrs. At the end of the reaction, the gas was collected and analyzed for unreacted methane, carbon dioxide and formaldehyde using a Shimadzu GC-14B gas chromatograph equipped with a TCD detector and a Porapak N packed column. The solid catalyst were separated from the reaction products by filtration using G4 filtration assembly and analyzed by Perkin Elmer Auto System XL gas chromatograph equipped with a FID detector and a capillary
column. The identity of the products was further confirmed by GC mass spectroscopy (Shimadzu GCMS-QP 5000) and also using standard compounds. Quantification of the products formed was carried out using gas chromatography techniques. The results are tabulated in table 2.
Table 2. Results of methane oxidation using zeolite encapsulated copper salen complexes and molecular oxygen.
Catalyst Conversion CH3OH HCHO HCOOH
(mole%) (mole%) (mole %) (mole%)
Cusalen-X 3.81 2.01 1.81
(Table Removed)
Example 10
In an autoclave, 0.4 g of solid 3,3'-dinitrosalen copper(II), 30 g of acetonitrile, 120 psi methane, 120 psi air and 0.8 g of tertiary butyl hydroperoxide were stirred at 0°C for 8 hrs. At the end of the reaction, the gas was collected and analyzed for unreacted methane, carbon dioxide and formaldehyde using a Shimadzu GC-14B gas chromatograph equipped with a TCD detector and a Porapak N packed column. The solid catalyst were separated from the reaction products by filtration using G4 filtration assembly and

analyzed by Perkin Elmer Auto System XL gas chromatograph equipped with a FID detector and a capillary column. The identity of the products was further confirmed by GC mass spectroscopy (Shimadzu GCMS-QP 5000) and also using standard compounds. Quantification of the products formed was carried out using gas chromatography techniques. The results are tabulated in table 2.
Example 11
In an autoclave, 0.4 g of solid 3,3',5,5'-tetrachlorosalen copper(II), 30 g of acetonitrile, 120 psi methanol, 120 psi air and 0.8 g of tertiary butyl hydroperoxide were stirred at 0°C for 8 hrs. At the end of the reaction, the gas was collected and analysed for unreacted methane, carbon dioxide and formaldehyde using a Shimadzu GC-14B gas chromatograph equipped with a TCD detector and a poropak N packed column. The solid catalyst were separated from the reaction products by filtration using G4 filtration assembly and analyzed by Perkin Elmer Auto System XL gas chromatograph equipped with a FID detector and a capillary column. The identity of the products was further confirmed by GC mass spectroscopy ( Shimadzu GCMS-QP 5000) and also using standard compounds. Quantification of the products formed was carried out using gas chromatography techniques. The results are tabulated in table 2.
Example 12
In an autoclave, 0.4 g of solid 3,3',5,5'-tetrabromosalen copper(II), 30 g of acetonitrile, 120 psi methane, 120 psi air and 0.8 g of tertiary butyl hydroperoxide were stirred at 0°C for 8 hrs. At the end of the reaction, the gas was collected and analysed for unreacted
methane, carbon dioxide and formaldehyde using a Shimadzu GC-14B gas chromatograph equipped with a TCD detector and a Porapak N packed column. The solid catalyst were separated from the reaction products by filtration using G4 filtration assembly and analyzed by Perkin Elmer Auto System XL gas chromatograph equipped with a FID detector and a capillary column. The identity of the products was further confirmed by GC mass spectroscopy ( Shimadzu GCMS-QP 5000) and also using standard compounds. Quantification of the products formed was carried out using gas chromatography techniques. The results are tabulated in table 2.
Example 13
In an autoclave, 0.4 g of copper salen complexes encapsulated in Y zeolites, 30 g of acetonitrile, 120 psi methane, 120 psi air and 0.8 g of tertiary butyl hydroperoxide were stirred at 0°C for 8 hrs. At the end of the reaction, the gas was collected and analyzed for unreacted methane, carbon dioxide and formaldehyde using a Shimadzu GC-14B gas chromatograph equipped with a TCD detector and a Porapak N packed column. The solid catalyst were separated from the reaction products by filtration using G4 filtration assembly and analyzed by Perkin Elmer Auto System XL gas chromatograph equipped with a FID detector and a capillary column. The identity of the products was further confirmed by GC mass spectroscopy ( Shimadzu GCMS-QP 5000) and also using standard compounds. Quantification of the products formed was carried out using gas chromatography techniques. The results are tabulated in table 2.
Example 14
In an autoclave, 0.4 g of 3,3'-dinitrosalencopper(II) complexes encapsulated in Y zeolite , 30 g of acetonitrile, 120 psi methane, 120 psi air and 0.8 g of tertiary butyl hydroperoxide were stirred at 0°C for 8 hrs. At the end of the reaction, the gas was collected and analyzed for unreacted methane, carbon dioxide and formaldehyde using a Shimadzu GC-14B gas chromatograph equipped with a TCD detector and a Porapak N packed column. The solid catalyst were separated from the reaction products by filtration using G4 filtration assembly and analyzed by Perkin Elmer Auto System XL gas chromatograph equipped with a FID detector and a capillary column. The identity of the products was further confirmed by GC mass spectroscopy ( Shimadzu GCMS-QP 5000) and also using standard compounds. Quantification of the products formed was carried out using gas chromatography techniques. The results are tabulated in table 2.

We Claim:
1 .An improved process for preparation of a mixture of methanol and formaldehyde through the oxidation of methane which comprises reacting methane with an oxidant such as here in described in the presence of a solid organo transition metal complex as catalyst, the said organo transition metal complex is encapsulated in a solid matrix is a salen or substituted salen wherein some of the hydrogen atoms of the said organ transition metal complex have been substituted by one or more electron withdrawing groups selected from the halogens, the nitro group or mixture thereof, at a temperature in the range of 0 to 100°C, at a pressure in the range of 10 to 1000 psi pressure optionally in the presence of solvents, and a promoter such as herein described and isolating mixture of methanol and formaldehyde by conventional methods.
•4
2. An improved process as claimed in claim 1 wherein the transition metal is selected from
manganese, copper or mixtures thereof.
3. An improved process as claimed in claim 1 wherein the oxidant used is molecular oxygen, air
or a mixture of oxygen and an inert gas like nitrogen.
4. An improved process as claimed in claim 1 wherein the oxidation reaction is carried out
optionally in the presence of solvents selected from acetonitrile, benzene and acetone.
5. An improved process as claimed in claim 1 wherein a promoter is selected from alkyl hydro
peroxide, dialkyl peroxide or mixtures thereof in the reaction.
6. An improved process as claimed in claim 1 wherein the concentration of the promoter in the
reaction mixture does not exceed 1% by weight of the methane.
7. An improved process as claimed in claim 1 wherein he solid matrix used is an inorganic oxide
such as silica, alumina, aluminosilicates or molecular sieves.
8. An improved process for the preparation of a mixture of methanol and formaldehyde through
the oxidation of methane with reference to the examples.

Documents:

342-del-2001-abstract.pdf

342-del-2001-claims.pdf

342-del-2001-correspondence-others.pdf

342-del-2001-correspondence-po.pdf

342-del-2001-description (complete).pdf

342-del-2001-form-1.pdf

342-del-2001-form-18.pdf

342-del-2001-form-2.pdf

342-del-2001-form-3.pdf

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Patent Number

231586

Indian Patent Application Number

342/DEL/2001

PG Journal Number

13/2009

Publication Date

27-Mar-2009

Grant Date

06-Mar-2009

Date of Filing

23-Mar-2001

Name of Patentee

COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH

Applicant Address

RAFI MARG, NEW DELHI-110001, INDIA.

Inventors:

#	Inventor's Name	Inventor's Address
1	PUTHUSSERIL VARKEY	NATIONAL CHEMICAL LABORATORY, PUNE 411 008, MAHARASTRA, INDIA.
2	CHANDRA RATNASAMY	NATIONAL CHEMICAL LABORATORY, PUNE 411 008, MAHARASTRA, INDIA.
3	SARADA GOPINATHAN	NATIONAL CHEMICAL LABORATORY, PUNE 411 008, MAHARASTRA, INDIA.

PCT International Classification Number

C07C 47/00

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA