Abstract:
Alcohols and/or ethers are synthesized from alkanes by mixing an alkane and bromine in a reactor to form alkyl bromide and hydrogen bromide. The alkyl bromide is directed into contact with metal oxide to form alcohol and/or ether and a metal bromide. The metal bromide is oxidized to metal oxide and bromine, both of which are recycled.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation-in-part application under 37 C.F.R. §1.53 of Application Ser. No. 09/886,078 filed Jun. 20, 2001, currently pending. 
    
    
     TECHNICAL FIELD 
     This invention relates generally to the synthesis of alcohols and ethers from alkanes, and more particularly to a method of and apparatus for manufacturing methanol and dimethyl ether from methane. 
     BACKGROUND OF THE INVENTION 
     Methane has previously been converted to methanol by the halogenation of methane followed by hydrolysis of the methyl halide to form methanol. For. example, gaseous chlorine has been used to chlorinate methane to form chlorinated methane, principally methyl chloride, together with other chlorides, i.e., dichloromethane, trichloromethane and carbon tetrachloride. Alternatively, methane has been subjected to oxychlorination with oxygen and hydrochloric acid to form the foregoing compounds. The chlorinated methanes produced are hydrolyzed in the vapor phase to produce methanol, formaldehyde, formic acid and by-products, including carbon dioxide and hydrochloric acid, depending on the chlorination selectivity. Hydrochloric acid is produced or used in the halogenation of methane by either method and must be recovered, dehydrated by azeotropic distillation and recycled. Corrosion and other problems involved with the handling of chlorine and hydrochloric acid are substantial. 
     U.S. Pat. No. 3,172,915 granted to Borkowski, et al. is directed to a process for converting methane to methanol. Borkowski discloses the chlorination of methane using ferric chloride at high temperatures to produce chloromethanes and hydrogen chloride. The process requires temperatures in the range of 220-8002° C., more preferably 250-450° C., and long residence times, e.g., more than one hour. Further, the process is hindered by the production of a mixture of chlorination products, e.g., chloromethane, dichloromethane, trichloromethane and carbon tetrachloride, which must be separated before hydrolysis to methanol. Other disadvantages result from the energy required to dry the ferric chloride and from the corrosion and handling problems inherent with hydrochloric acid. 
     U.S. Pat. No. 5,243,098 granted to Miller discloses another method for converting methane to methanol. In the Miller process the reaction of methane with cupric chloride produces chloromethane and hydrochloric acid. These intermediates are then reacted with steam and a catalyst containing magnesium oxide to produce methanol and magnesium chloride. Magnesium oxide is regenerated by treatment of the magnesium chloride by-product with air or oxygen. Cupric chloride is regenerated by treatment of the cuprous chloride by-product with air and hydrochloric acid. While these reactions proceed at favorable rates, attrition of the solid reactants, i.e., cupric and magnesium oxide, is significant. Special filters and processes are required to recover and regenerate the reactants in the required particle size. Miller also suggests cupric bromide and magnesium zeolite as alternative reactants. Because of the attrition of the reactants, difficulties associated with the handling of solids, and the special filters and processes required to regenerate the reactants, the Miller process has proven unsatisfactory. U.S. Pat. No. 5,334,777, also granted to Miller, discloses a nearly identical process for converting ethane to ethylene glycol. 
     U.S. Pat. No. 5,998,679 granted to Jorge Miller, discloses a process for converting alkanes and alkenes to the corresponding lower alkanols and diols. In the method of the invention, a gaseous halogen (bromine) is produced by decomposing a metal halide in a liquid having a melting point below and a boiling point above the decomposition temperature of the metal halide. The preferred liquid is molten hydrated ferric chloride maintained at a temperature between about 37-280° C. The lower alkane or alkene is halogenated in a gas phase reaction with the halogen. The resulting alkyl halide or alkyl dihalide is contacted with a metal hydroxide, preferably an aqueous solution of ferric hydroxide, to regenerate the metal halide and produce the corresponding lower alkanol or diol. Problems with this process include low monohalogenation selectivity, and corrosiveness of the hydrated ferric halides, which may present a containment problem if the process is run at 280° C., where high pressures of steam are required to maintain ferric halide hydration. Finally, the process produces a great deal of water and HCl or HBr, all of which are difficult to separate on a large scale from the desired product methanol. 
     Published international patent application WO 00/07718, naming Giuseppe Bellussi, Carlo Perego, and Laura Zanibelli as inventors, discloses a method for directly converting methane and oxygen to methanol over a metal halide/metal oxide catalyst. This is not a catalyst in the true sense of the word, however, because the reaction involves transfer of halide from a metal halide via reaction with methane to a different metal oxide producing the metal halide and methanol downstream. Eventually the halide is leached and the catalyst loses activity. 
     Olah et al. (George A. Olah, et al. J. Am. Chem. 
     Soc. 1985, 107, 7097-7105) discloses a method for converting methane to methanol via methyl halides (CH 3 Br and CH 3 Cl), which are then hydrolyzed to prepare methanol. In the process, CH 3 Br and CH 3 Cl are hydrolyzed over catalysts with excess steam generating a methanol, water, and HCl or HBr mixture. The separation of methanol (about 2% by mole) from HCl or HBr and water on an industry scale (2000 tons per day) requires an enormous amount of energy and generates a great deal of aqueous HCl or HBr waste. Aqueous HCl and HBr are very corrosive as well. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a process wherein bromine or a bromine containing compound is used as an intermediate to convert alkanes to alcohols, ethers, or olefins by reaction with oxygen or air. While the process can be used to convert a variety of alkanes, including methane, ethane, propane, butane, isobutane, pentanes, hexanes, cyclohexane, etc. to their respective alcohols, ethers, or olefins, the conversion of methane to methanol and dimethyl ether is illustrative. 
     Methane reacts with bromine over a catalyst to form CH 3 Br and HBr. CH 3 Br and HBr react with a metal oxide to form a variable mixture of dimethyl ether (DME), water and methanol, and the metal bromide. The metal oxide and molecular bromine are regenerated by reaction of the metal bromide with air and/or oxygen. The regenerated bromine is recycled to react with methane while the regenerated metal oxide is used to convert more methyl bromide to methanol and DME, completing the reaction cycle. 
     The process can be easily carried out in a riser reactor. Compared to the current industrial two step process, in which methane and steam are first converted to CO and H 2  at 800° C. followed by conversion to methanol over a Zn—Cu—Al—O catalyst at approximately 70-150 atmospheres, the process of the present invention operates at roughly atmospheric pressure and relatively low temperatures, thereby providing a safe and efficient process for methanol production. 
     The present invention operates with solid/gas mixtures at atmospheric pressure. In the process, the hydrogen halide is gaseous, and therefore not as corrosive as when aqueous at high temperatures. The reaction of Br 2  with an alkane can reach more than 90% selectivity and high conversion to alkane-monobromide. The main side products, alkane dibromides such as CH 2 Br 2  can be converted back to the monobromides by reaction with an alkane over another catalyst. Very few by-products are produced. 
     In the process most of the Br atoms are trapped in the solid state, making the system less corrosive. 
     Another advantage is that DME and alcohol (CH 3 OH) are not produced as a mixture with excess water. By controlling reaction conditions, almost pure DME and/or methanol is obtained directly so that it is not necessary to separate CH 3 OH from water. Finally, in the present process, methane and oxygen do not come into direct contact, resulting in improved safety. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the present invention may be had by reference to the following Detailed Description when taken in connection with the accompanying Drawings, wherein: 
     FIG. 1 is a schematic illustration of a method of and apparatus for synthesizing alcohols and/or ethers from alkanes comprising a first embodiment of the invention; 
     FIG. 2 is a schematic illustration of a method of and apparatus for synthesizing alcohols and/or ethers from alkanes comprising a second embodiment of the invention; 
     FIG. 3 is a schematic illustration of a method of and apparatus for synthesizing ethers and/or alcohols from alkanes comprising a third embodiment of the invention; and 
     FIG. 4 is a schematic illustration of a method of and apparatus for synthesizing ethers and/or alcohols from alkanes comprising a fourth embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     Alkanes (methane, ethane, propane, butane, isobutane, pentanes, hexanes, cyclohexane, etc.) react with molecular bromine over a catalyst composed of all possible metal compounds and mixtures thereof to form alkylbromides. For CH 4  (although the process may be applied to higher alkanes as well), the process of the present invention can convert more than 50% CH 4  to CH 3 Br and HBr, with selectivity higher than 90%. Most of the by-product is CH 2 Br 2  (with trace amounts of CHBr 3 , and CBr 4 ) (+2 HBr), which can be catalytically reconverted to CH 3 Br by reacting CH 2 Br 2  with CH 4 . 
     Referring to the Drawings, and particularly to FIG. 1, a method and apparatus  10  for synthesizing alcohols and ethers from alkanes using bromine comprising a first embodiment of the invention is schematically illustrated. In the operation of the method and apparatus  10 , bromine is received from a suitable source through a line  12  and is directed to a bromine storage container  14 . For example, bromine is easily manufactured from bromide, which is readily available from sea water. 
     As is well known, bromine is a liquid at room temperature. Liquid bromine from the storage container  14  is directed through a line  16  to a bromine vaporizer  18  wherein the bromine is converted from the liquid phase to the gas phase. From the vaporizer  18  the gaseous bromine is directed through a line  20  to a reactor  22 . 
     Methane from a suitable source is directed to the reactor  22 , which contains a bromination catalyst, through a line  24 . Within the reactor  22  the methane and the gaseous bromine are mixed together and the temperature of the mixture is raised to between about 20° C. and about 600° C., thereby converting the methane and the bromine to methyl bromide (CH 3 Br) and hydrogen bromide (HBr). 
     From the reactor  22 , the CH 3 Br, the HBr, any unreacted methane and by products CH 2 Br 2 , CHBr 3 , and CBr 4  are directed to a condenser  34 . The by products CH 2 Br 2 , CHBr 31  and CBr 4  now in the liquid states are sent to a converter  28  with methane from the line  24 . In converter  28 , methane reacts with the by products CH 2 Br 2 , CHBr 3 , and CBr 4  to form CH 3 Br. The newly formed CH 3 Br and any unreacted CH 2 Br 21 , CHBr 3 , CBr 4  and/or methane are sent to the condenser  34 . From the condenser  34 , the gas phase methane, HBr, and CH 3 Br are sent to a converter  52 . In the converter  52  HBr and CH 3 Br react with metal oxide to form CH 3 OCH 31 , CH 3 OH, and H 2 O, which are sent to a separator  44  along with unreacted methane and CH 3 Br. In the separator, methanol and dimethyl ether are separated as products. CH 3 Br is sent back to the converter  52 . Methane is sent back to the bromination reactor  22 . In the converter  52 , the original metal oxide has been converted to metal bromide after reaction. The metal bromide is sent to a converter  58  to react with oxygen (from source  74 ) to regenerate bromine and metal oxide. The regenerated metal oxide is sent back to the converter  52 , while the bromine and unreacted oxygen are sent to a condenser  62 , then to a separator  68 . The liquid bromine is sent to the storage container  14 , while oxygen is sent to the converter  58 . 
     Referring to FIG. 2, there is shown a method of and apparatus  100  for synthesizing alcohols and ethers from alkanes comprising a second embodiment of the invention. Methane and bromine are directed to a heated vaporizer  102  where they are heated to form a gas mixture. The gas mixture is passed into a bromination reactor  104 , containing a bromination catalyst, if desired. After the reaction, the mixture is directed to a condenser  106 . The liquid phase contains by products CH 2 Br 2 , CHBr 3 , and CBr 4 , while the gas phase contains bromomethane, HBr, and unreacted methane. 
     The liquid by products CH 2 Br 2 , CHBr 3 , and CBr 4  are sent to a converter  108  where they react with methane to form bromomethane. After reaction the mixture is sent to the condenser  106 , where any remaining byproducts can once again be cycled to the converter  108 . 
     The gas phase mixture from condenser  106  is passed through a converter  110 , where HBr reacts with metal oxide to form metal bromide and water. The metal bromide is sent to a regenerator  120  to regenerate metal oxide. From the converter  110 , the water, bromomethane, and methane are separated. Methane is recycled to the converter  108  and the vaporizer  102 . Bromomethane is sent to a reactor  114 . Water is sent to a reactor  118 . 
     In the reactor  114 , bromomethane reacts with metal oxide to generate dimethyl ether (DME) and metal bromide. Metal bromide is sent to the regenerator  120 . 
     The mixture of bromomethane and DME from the reactor  114  is sent to a separator  116 . Bromomethane from the separator  116  is recycled to the reactor  114 , while DME is obtained as a product or directed to the reactor  118 . 
     In the reactor  118 , DME reacts with water, e.g., from the separator  112 , over a suitable catalyst to form methanol. 
     In the regenerator  120 , metal bromide from the converter  110  and the reactor  114  reacts with air or oxygen to regenerate metal oxide and bromine. After regeneration, metal oxide is sent to the converter  110  and the reactor  114 , while bromine is sent to the vaporizer  102 . If air is used as the source of oxygen, nitrogen can be continuously purged from the system through a separator  122 . 
     A method and apparatus  121  for converting methane to dimethyl ether and/or methanol comprising a third embodiment of the invention is illustrated in FIG.  3 . Liquid bromine is stored in a bromine storage container  123  and is directed therefrom through a line  124  through a bromine vaporizer  126 . From the bromine vaporizer  126  the bromine vapor passes through a line  128  to a primary reactor  130  which also receives methane from a suitable source through a line  132 . 
     Within the primary reactor  130 , the bromine and methane react over a solid catalyst, if desired, to form CH 3 Br, CH 2 Br 2 , CHBr 3 , CBr 4  and HBr. The reaction products together with any unreacted methane are directed to a condenser separator  134  through a line  136 . The condenser separator  134  directs CH 3 Br, CH 2 Br 2 , CHBr 3 ; and CBr 4  to a separation (distillation) apparatus  138  through a line  140 . The separation (distillation) apparatus  138  directs CH 2 Br 2 , CHBr 3 , and CBr 4  to a secondary reactor  142  through a line  144 . The secondary reactor  142  also receives methane from a suitable source through a line  146 . 
     The secondary reactor  142  converts the CH 2 Br 2 , CHBr 3 , and CBr 4  received through the line  144  to CH 3 Br. CH 3 Br and excess methane are directed from the secondary reactor  142  to the line  136  through a line  148 . 
     The condenser separator  134  also directs CH 4  and HBr to an HBr converter  150  through a line  152 . The HBr converter  150  is filled with a metal oxide. Within the HBr converter  150  the metal oxide and the HBr react to form metal bromide and water. The water and the unreacted methane are directed from the HBr converter  150  to a condenser separator  154  through a line  156 . The water is recovered from the condenser separator  154  through an outlet  158 , while the unreacted methane is directed through a line  160  to a blower  162  and from the blower  162  through a line  164  to the line  132 . 
     Metal bromide formed during operation of the HBr converter  150  is directed to a bromide to oxide converter (regenerator)  166  through a line  168 . The bromide to oxide converter  166  receives air or oxygen through a line  170  and functions to convert the metal bromide back to metal oxide. The metal oxide is returned to the HBr converter  150  through a line  172 . 
     Operation of the bromide to oxide converter  166  also produces bromine. Bromine and excess air or oxygen are directed through a line  174  to a blower  176 , and from the blower  176  to a condenser separator  178 . Operation of the condenser separator  178  produces liquid bromine which is directed to the bromine storage container  123  through a line  180 . Excess air and/or oxygen is recovered from the condenser separator  178  through an outlet  182  and is directed therefrom through a blower  184  and a line  186  to the line  170 . 
     The separation (distillation) apparatus  138  directs CH 3 Br to an oxide to bromide converter  188  through a line  190 . The oxide to bromide converter  188  is filled with a metal oxide, which may be the same metal oxide that is utilized in the operation of the HBr converter  150 . Operation of the oxide to bromide converter  188  produces dimethyl ether, which together with unreacted bromomethane is directed through a line  192 , a compressor  194 , and a line  196  to a condenser separator  198 . From the condenser separator  198  dimethyl ether may be recovered as a final product at an outlet  200 . The bromomethane is sent back to the converter  188 . Alternatively, the dimethyl ether may be directed to a methanol reactor  202 , which receives water through an inlet  204 . The dimethyl ether and the water react in the methyl reactor  202  to form methanol, which is recovered at an outlet  206 . 
     Operation of the oxide to bromide converter  188  converts the metal oxide contained therein to metal bromide, which is directed to the bromide to oxide converter  166  through a line  208 . Operation of the bromide to oxide converter  166  converts the metal bromide to the original metal oxide, which is returned to the oxide to bromide converter  188  through a line  210 . 
     EXAMPLES 
     Reaction 1 
     Catalyst Preparation 
     Nb 2 O 5  (0.8000 g) was mixed with 0.500 ml 96(w)% H 2 SO 4 , then the mixture was heated at 110° C. for 4 hours. The temperature increased to 500° C. within 6 hours, and kept at 500° C. for 4 hours. Catalyst C 1  was obtained. 
     Testing 
     Reaction conditions: 
     The catalyst was tested at a methane flow of 1.5 ml/minute and Br 2  flow of 0.07 ml/hour. The reaction temperature was 400° C. The reaction was carried out in a microreactor system. After 6 hours on line reaction, the reaction effluent was analyzed by a GC/MS. A methane conversion of 24% (mol) with 95% CH 3 Br was obtained. Summarizing the overall process in Reaction 1: 
     
       
         CH 4 +Br 2 &gt;HBr+CH 3 Br+CH 2 Br 2 +CHBr 3 cat  (1 
       
     
     Reaction 2 
     Metal Oxide Preparation 
     Zr Solution Preparation 
     Zr(OCH 2 CH 2 CH 3 ) 4  (70(w)% in isopropanol, 112.6 ml) was dissolved into acetic acid (275 ml) under stirring. After stirring for 10 minutes, the solution was diluted by water to make a total volume of 500 ml. A solution with a Zr concentration of 0.5M was obtained. 
     Preparation of M1 
     Cu(NO 3 ) 2  (0.5M, 7.200 ml) solution was added into BaBr 2  (0.5M, 0.800 ml). A clear solution was obtained. To this solution, Zr solution (0.5M) as prepared above was added under stirring. After stirring a few seconds, a gel was obtained. The gel was dried at 110° C. for 4 hours, then heated to 500° C. within 6 hours, and kept at 500° C. for 4 hours. M1 was obtained. The metal oxide mixture was tested at a CH 3 Br flow of 1.0 ml/minute at 230° C. In the first half hour, the average CH 3 Br conversion is 65%, and the average dimethyl ether selectivity is 90.5%. 
     Catalyst Preparation 
     ZrO 2  (2.0000 g) was mixed with H 2 SO 4  (3.000 ml, 96(w)%), then the mixture was heated at 110° C. for 4 hours. The temperature increased to 500° C. within 6 hours, and kept at 500° C. for 4 hours. Catalyst C2 was obtained. 
     Preparation of M2 
     Cu(NO 3 ) 2  (0.5M, 40.000 ml) solution was added into Zr solution (0.5M, 30.000 ml as prepared above). After stirring a few seconds, a gel was obtained. The gel was dried at 110° C. for 4 hours, then heated to 500° C. within 6 hours, and calcined at 500° C. for 4 hours. M2 was obtained. 
     Testing 
     The catalyst C2 (2.0000 g) was loaded in the first reactor (R1). A trap was loaded 2.0000 g M2 in the oxide form. A second reactor (R2) was also loaded with M2 in the oxide form(0.8500 g). 
     Reactants methane and bromine were fed into the first reactor (methane flow of 1.5 ml/minute, Br 2  flow of 0.07 ml/hour). The reaction temperature was 390° C. After the reaction in R1 (stabilized by online reaction for more than 8 hours), the products generated in R1 were passed through the trap and HBr was removed. Following removal of HBr, a mixture of methane and CH 3 Br (containing 20% mol of CH 3 Br) was obtained. This gas mixture was directly fed into R2 at 220° C. In the first one hour, an average CH 3 Br conversion of 91% with an average dimethyl ether selectivity of 75% was obtained. 
     Summarizing the overall process in Reaction 2: 
     
       
         CH 3 Br+HBr+CuO&gt;CH 3 OH+CuBr 2   (2) 
       
     
     Possible variations of Reaction 2: 
     
       
         2HBr+CuO&gt;H 2 O+CuBr 2  (reaction occurring in the trap)  (2a) 
       
     
     
       
         2CH 3 Br+CuO&gt;CH 3 OCH 3 +CuBr 2   (2b) 
       
     
     Reaction 3 
     The solid CuBr 2 /ZrO 2  is transferred from Reactor 2 to Reactor 3 and treated with O 2  at 300° C. to yield Br 2  and CuO/ZrO 2  in 100% yield and conversion. This reaction may be run at space velocity 100 h −1 . Summarizing the overall process in Reaction 3: 
     
       
         CuBr 2 /ZrO 2 +½O 2 &gt;Br 2 +CuO/ZrO 2   (3) 
       
     
     Overall: 
     
       
         CH 4 +½O 2 &gt;CH 3 OH  (A) 
       
     
     Possible variation: 
     
       
         CH 4 +½O 2 &gt;½CH 3 OCH 3 +½H 2 O  (B) 
       
     
     It will therefore be understood that the method and apparatus of the present invention operates on a continuous or batch basis to convert alkanes to alcohols and ethers. The method and apparatus of the present invention operates at relatively low temperatures and at low pressures and is therefore economical for manufacture and use. The bromine, which is utilized in the method and apparatus of the present invention, is continuously recycled. The metal oxide catalyst, which is utilized in the process is continuously refreshed. 
     Although preferred embodiments of the invention have been illustrated in the accompanying drawings and described in the foregoing detailed description, it will be understood that the invention is not limited to the embodiments disclosed but is capable of numerous rearrangements, modifications, and substitutions of parts and elements without departing from the spirit of the invention.