Abstract:
Acetaldehyde is prepared in good yield from methanol and synthesis gas under mild reaction conditions by contacting a mixture of methanol, carbon monoxide and hydrogen with an iodide or iodine free catalyst composition comprising (1) ruthenium powder, (2) a cobalt-containing compound (3) a rhodium-containing compound, and (4) an onium salt or base, and heating the resulting mixture under mild temperature and pressure for sufficient time to produce the desired acetaldehyde, and then recovering the same from the reaction mixture.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a new process for preparing acetaldehyde. More particularly, the invention relates to a new process for preparing acetaldehyde from methanol and synthesis gas using a novel catalyst composition. 
     Specifically, the invention provides a new and improved process for preparing acetaldehyde from methanol and syngas in good yield under mild reaction conditions, which process comprises contacting a mixture of methanol, carbon monoxide and hydrogen with an iodide or iodine free catalyst composition comprising (1) ruthenium powder, (2) a cobalt-containing compound, (3) a rhodium-containing compound, and (4) an onium salt or base, and heating the resulting mixture under mild temperature and pressure for sufficient time to produce the desired acetaldehyde, and then recovering the same from the reaction mixture. 
     2. Prior Art 
     Acetaldehyde is an important chemical of commerce used in a great variety of applications, such as, for example, in the preparation of acetic acid, vinyl acetate, chloral, cyanohydrins and polyhydric alcohol derivatives, such as the glycol monoalkyl ethers. Acetaldehyde has been produced heretofore by methods, such as the hydration of acetylene or the oxidation of ethylene. Such methods, however, have their limitations, particularly as to cost, and it would be desirable to find a more economical method for producing this compound. 
     U.S. Pat. No. 4,151,208 discloses a method for producing acetaldehyde from methanol and syngas using a catalyst comprising a special cobalt compound and an iodine promoter. This process is limited, however, because of the serious corrosion problems due to the presence of the iodine promoter. U.S. Pat. No. 4,201,868 discloses a process for preparing a mixture of products containing acetaldehyde using a catalyst comprising a cobalt carbonyl and an organic nitrogen-containing ligand. This process, however, is limited by its poor selectivity. 
     It is an object of the invention, therefore, to provide a new and improved process for preparing acetaldehyde. It is a further object to provide a process for preparing acetaldehyde from methanol and syngas using a new and improved catalyst system. It is a further object to provide a new process for preparing acetaldehyde from methanol and syngas which gives good selectivity and yield of the desired product. It is a further object to provide a process for preparing acetaldehyde from methanol and syngas which utilizes milder reaction conditions, particularly as to the use of lower pressures and temperatures, than used heretofore. It is a further object to provide a process for preparing acetaldehyde which utilizes a catalyst free of corrosive elements and is capable of being used on a large commercial scale. These and other objects of the invention will be apparent from the following detailed description thereof. 
     SUMMARY OF THE INVENTION 
     It has now been discovered that these and other objects may be accomplished by the process of the invention comprising contacting a mixture of methanol, carbon monoxide and hydrogen with an iodide or iodine free catalyst composition comprising (1) ruthenium powder, (2) a cobalt-containing compound, (3) a rhodium-containing compound, and (4) an onium salt or base, and heating the resulting mixture under mild temperature and pressure for sufficient time to produce the desired acetaldehyde, and then recovering the same from the reaction mixture. It was surprising to find that this new catalyst system was highly selective for the conversion of methanol to the desired acetaldehyde in view of discouraging results obtained with related catalyst systems. Further the new process avoids the use of catalysts containing corrosive elements, such as iodine promoters, and is capable of being used on a large commercial use. 
     Further surprising advantage is found in the fact that the process can be operated under mild temperatures and pressure. For example, by the use of the present process the desired results can be obtained by the use of lower pressures, say from about 1000 psi to about 5000 psi, where normally pressures would preferably range from 5000 psi to about 7500 psi. The improvement in this regard over the new process for preparing acetaldehyde described and claimed in our copending patent application Ser. No. 344,430 filed this same date, is shown in the working examples at the end of the specification. 
     As noted, the process of the invention is particularly characterized by the good selectivity in the conversion of the methanol and syngas to acetaldehyde as represented by the following equation: 
     
         CH.sub.3 OH+CO+H.sub.2 →CH.sub.3 CHO+H.sub.2 O      (1) 
    
     Typical conversions of the methanol to the acetaldehyde range from about 58% to about 94% with selectivities to acetaldehyde of the order of about 50%. Valuable by-products of the reaction include ethanol, methyl acetate, ethyl acetate, and the like. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In the operation of the process of the invention, the acetaldehyde, along with the above-noted by-products, are produced concurrently from the methanol and syngas by a process comprising the following steps: 
     (a) contacting a mixture of methanol, carbon monoxide and hydrogen with a catalyst composition comprising (1) ruthenium metal powder, (2) a cobalt-containing compound, (3) a rhodium-containing compound, and (4) an onium base or salt, said reaction mixture can and sometimes preferably does contain a solvent, such as p-dioxane, 
     (b) maintaining the said mixture under mild reaction conditions, e.g. temperatures between 150° C. and 350° C. and a pressure between about 1000 psi and 5000 psi, with sufficient carbon monoxide and hydrogen to satisfy the stoichiometry of the desired acetaldehyde synthesis as noted above, until the formation of the desired acetaldehyde has been achieved, and, 
     (c) preferably isolating the said acetaldehyde and minor by products from the reaction mixture, as by distillation. 
     In order to present the inventive concept of the present invention in the greatest possible detail, the following supplementary disclosure is submitted. The process of the invention is practiced as follows: 
     As noted, the new catalyst system used in the process of the invention contains ruthenium metal powder, a cobalt-containing compound, a rhodium-containing compound and an onium salt or base. The ruthenium metal powder can be powdered ruthenium metal of any mesh size. 
     The cobalt-containing compound to be used in the catalyst composition may take many different forms. For instance, the cobalt may be added to the reaction mixture in the form of an oxide, salt, carbonyl derivative and the like. Examples of these include, among others, cobalt oxides Co 2  O 3 , Co 3  O 4 , Co 3  O 4 , CoO, cobalt (II) bromide, cobalt(II) thiocyanate, cobalt(II) hydroxide, cobalt(II) carbonate, cobalt(II) nitrate, cobalt(II) phosphate, cobalt acetate, cobalt naphthenate, cobalt benzoate, cobalt valerate, cobalt cyclocohexanoate, cobalt carbonyls, such as dicobalt octacarbonyl Co 2  (CO) 8 , tetracobalt dodecacarbonyl Co 4  (CO) 12  and hexacobalt hexadecacarbonyl Co 6  (CO) 16  and derivatives thereof by reaction with ligands and preferably group V donors, such as the phosphines, arsines and stilbines, such as (Co(CO) 3  L) 2  wherein L is PR 3 , AsR 3  and SbR 3  wherein R is a hydrocarbon radical, cobalt carbonyl hydrides, cobalt carbonyl bromide, cobalt nitrosyl carbonyls as CoNO(CO) 3 , Co(NO)(CO) 2  PPh 3 , cobalt nitrosyl bromides, organometallic compounds obtained by reacting cobalt carbonyls with olefins, allyl and acetylene compounds, such as bis(π-cyclopentadienyl) cobalt (πC 5  H 5 ) 2  Co, cyclopentadienyl cobalt dicarbonyl, bis(hexamethylene-benzene) cobalt. 
     Preferred cobalt-containing compounds to be used in the catalyst system comprise those having at least one cobalt atom attached to carbon, such as the cobalt carbonyls and their derivatives as, for example, dicobalt octacarbonyl, tetracobalt dodecacarbonyl, (Co(CO) 3  P(CH 3 ) 3 ) 2 , organometallic compounds obtained by reacting the cobalt carbonyls with olefins, cycloolefins, allyl and acetylene compounds such as cyclopentadienyl cobalt dicarbonyl, cobalt carbonyl bromides, cobalt carbonyl hydrides, cobalt nitrosyl carbonyls, and the like, and mixtures thereof. 
     Particularly preferred cobalt-containing compounds to be used in the catalyst comprise those having at least one cobalt atom attached to at least three separate carbon atoms, such as for example, the dicobalt octacarbonyls and their derivatives. 
     The quaternary onium salt or base to be used in the catalyst composition may be any onium salt or base, but are preferably those containing phosphorous or nitrogen, such as those of the formula ##STR1## ps wherein Y is phosphorous or nitrogen, R 1 , R 2 , R 3  and R 4  are organic radicals preferably alkyl, aryl or alkaryl radicals, and X is an anionic species. The organic radicals useful in this instance include those alkyl radicals having from 1 to 20 carbon atoms in a branched or linear alkyl chain, such as methyl, ethyl, n-butyl, isobutyl, octyl, 2-ethylhexyl and dodecyl radicals. Tetraethylphosphonium bromide and tetrabutylphosphonium bromide are typical examples presently in commercial production. The corresponding quaternary phosphonium or ammonium acetates, hydroxides, nitrates, chromates, tetrafluoroborates and the corresponding chlorides are also satisfactory. 
     Equally useful are the phosphonium and ammonium salts containing phosphorous or nitrogen bonded to a mixture of alkyl, aryl and alkaryl radicals, which radicals preferably contain from 6 to 20 carbon atoms. The aryl radical is most commonly phenyl. The alkaryl group may comprise phenyl substituted with one or more C 1  to C 10  alkyl substituents, bonded to phosphorous or nitrogen through the aryl function. 
     Illustrative examples of suitable quaternary onium salts or bases include tetrabutylphosphonium bromide, n-heptyltriphenylphosphonium bromide, tetrabutylammonium chloride, tetrabutylphosphonium nitrate, tetrabutylphosphonium hydroxide, tetrabutylphosphonium chromate, tetraoctylphosphonium tetrafluoroborate, tetrahexyl phosphonium acetate and tetraoctylammonium bromide. 
     The preferred quaternary onium salts and bases to be used in the process comprise the tetralkylphosphonium salts containing alkyl groups having 1 to 6 carbon atoms, such as methyl, ethyl, butyl, hexyl, heptyl and isobutyl. Tetralkylphosphonium salts, such as the bromides and chlorides acetate and chromate salts and hydroxide base, are the most preferred. 
     The rhodium-containing compound to be included in the catalyst can take many different forms. For instance the rhodium may be added to the reaction mixture as an oxide, as in the case of, for example, rhodium(III) oxide hydrate, rhodium(IV) dioxide, and rhodium sesquioxide (Rh 2  O 3 ). Alternatively, it may be added as the salt of a mineral acid, as in the case of rhodium(II) chloride hydrate, rhodium(III) bromide, chlorodicarbonyl rhodium(I) dimer, anhydrous rhodium(III) chloride and rhodium nitrate, or as the salt of a suitable organic carboxylic acid, for example, rhodium(II) formate, rhodium(II) acetate, rhodium(II) propionate, rhodium(II) butyrate, rhodium(II) valerate, rhodium(III) naphthenate, rhodium(III) acetylacetonate, etc. The rhodium may also be added as a carbonyl or hydrocarbonyl derivative. Here, suitable examples include tetrarhodium dodecacarbonyl, dirhodium octacarbonyl, hexarhodium hexadecacarbonyl, rhodium tetracarbonyl salts, and substituted carbonyl species such as rhodium dicarbonyl acetylacetonate. 
     Preferred rhodium-containing compounds include oxides of rhodium, rhodium salts of a mineral acid, rhodium salts of organic carboxylic acids and rhodium carbonyl or hydrocarbonyl derivatives. Among these, particularly preferred are rhodium(III) chloride, rhodium(III) acetylacetonate, rhodium sesquioxide, rhodium dicarbonyl acetylacetonate, rhodium(II) acetate, rhodium(II) propionate and hexarhodium hexadecacarbonyl. 
     The quantity of the ruthenium powder and the cobalt-containing compound to be used in the process of the invention may vary over a wide range. The process is conducted in the presence of a catalytically effective amount of the active ruthenium powder and the active cobalt-containing compound which gives the desired product in a reasonable yield. The reaction proceeds when employing as little as about 1×10 -6  weight percent, and even lesser amounts of the ruthenium powder, together with as little as about 1×10 -6  weight percent of the cobalt-containing compound, or even lesser amounts, based on the total weight of the reaction mixture. The upper concentration is dictated by a variety of factors including catalyst cost, partial pressures of carbon monoxide, operating temperature, etc. An amount of the ruthenium powder of from about 1×10 -5  to about 5 weight percent in conjunction with a cobalt-containing compound concentration of from about 1×10 -5  to about 5 percent, based on the total weight of the reaction mixture is generally desireable in the practice of this invention. The preferred ruthenium to cobalt atomic ratios are from about 10:1 to 1:10. 
     Generally, in the catalyst system used in the process of the invention, the molar ratio of the ruthenium powder to the quaternary onium salt or base will range from about 1:0.1 to about 1:100 or more, and preferably will be from about 1:1 to about 1:20. 
     The amount of the rhodium-containing compound to be used in the catalyst system of the present invention may vary over a considerably range. In general, the amount of the rhodium-containing compound to be used may vary from about 0.01 to 20 moles per mole of the ruthenium powder, and preferably from about 0.1 to 10 moles per mole of the ruthenium powder. 
     Particularly superior results are obtained when the above-noted four components of the catalyst system are combined in a molar basis as follows: ruthenium powder 0.1 to 4 moles, cobalt-containing compound 0.1 to 8 moles, the quaternary onium salt or base 0.4 to 60 moles, and rhodium-containing compound 0.01 to 20 moles. Still more preferably the components are combined in the following molar ratios; ruthenium powder 1 to 4 moles, cobalt-containing compound 2 to 4 moles, quaternary onium base or salt 10 to 50 moles, and rhodium-containing compound 0.1 to 20 moles. 
     Solvents may be and sometimes preferably are employed in the process of the invention. Suitable solvents for the process include the oxygenated hydrocarbons, e.g. compounds possessing only carbon, hydrogen and oxygen and one in which the oxygen atom present is in an ether, ester, ketone carbonyl or hydroxyl group or groups. Generally, the oxygenated hydrocarbon will contain from about 3 to 12 carbon atoms and preferably a maximum of three oxygen atoms. The solvent must be substantially inert under the reaction conditions, must be relatively non-polar and preferably must be one which has a normal boiling point of at least 65° C. at atmospheric pressure and still more preferably the solvent will have a boiling point greater than that of the ester and other products of the reaction so that recovery of the solvent by distillation is facilitated. 
     Preferred ester type solvents are the aliphatic, cycloaliphatic and aromatic carboxylic acid esters as exemplified by methyl benzoate, isopropyl benzoate, butyl cyclohexanoate, as well as dimethyl adipate. Useful alcohol-type solvents include the monohydric alcohols as cyclohexanol and 2-octanol, etc. Suitable ketone-type solvents include, for example, cyclic ketones, such as cyclohexanone, 2-methylcyclohexanone, as well as acyclic ketones, such as 2-pentanone, butanone, acetophenone, etc. Ethers which may be utilized as solvents include cyclic, acyclic, and heterocyclic materials. Preferred ethers are the heterocyclic ethers as illustrated by 1,4-dioxane and 1,3-dioxane. Other suitable ethers include isopropyl dibutyl ether, diethylene glycol dibutyl ether, diphenyl ether, dibutyl ether, heptyl phenyl ether, anisole, tetrahydrofurane, etc. The most useful solvents of all of the above groups include the ethers, as represented by the polycyclic, heterocyclic ethers such as diphenyl ether and 1,4-dioxane, etc. 
     The amount of the solvent employed may vary over a wide range. In general, it is desirable to use sufficient solvent to fluidize the catalyst system. 
     The relative amounts of carbon monoxide and hydrogen which may be initially present in the syngas mixture can be varied widely. In general, the mole ratio of CO to H 2  is in the range from about 20:1 to 1:20, preferably from about 5:1 to 1:5. Particularly in continuous operations, but also in batch experiments, the carbon monoxide-hydrogen gaseous mixture may also be used in conjunction with up to 50% by volume of one or more other gases. These other gases may include one or more inert gases such as nitrogen, argon, neon and the like, or they may include gases that may or may not undergo reaction under CO hydrogenation conditions, such as methane, ethane, propane, and the like, ethers, such as dimethyl ether and diethyl ether. 
     The temperature used in the process of the invention may vary over a considerable range, but as noted above, a distinct advantage of the present process is that it can be operated at the more moderate temperatures such as, for example, those within the range of 100° C. to about 350° C. The exact temperature selected will depend upon experimental factors, such as the pressure, the concentration and choice of the particular catalyst and cocatalyst selected, etc. Preferred temperatures range from about 150° C. to about 250° C. 
     As noted above, a special advantage of the present process is the use of milder reaction conditions, and particularly lower pressures, than would preferably be used in the other processes. For example, preferred pressures are the more moderate temperatures e.g. those that range from about 1000 psi to about 5000 psi, although higher or lower pressures may be used as desired. The pressures referred to herein represent the total pressure generated by all the reactants, although they are substantially due to the carbon monoxide and hydrogen reactants. 
     The desired product of the reaction, acetaldehyde, will be formed in significant quanities generally varying up to about 46% selectivity. Also formed will be minor by-products including ethanol, methyl and ethyl acetate, and other lower oxygenated products. The acetaldehyde and the by-products can be recovered from the reaction mixture by conventional means, e.g. fractional distillation in vacuo. 
     The novel process of the invention can be conducted in a batch, semi-continuous or continuous manner. The catalyst can be initially introduced into the reaction zone batchwise, or it may be continuously or intermittently introduced into such a zone during the course of the synthesis reaction. Operating conditions can be adjusted to optimize the formation of the desired acetaldehyde product, and said material may be recovered by methods known to the art, such as distillation, fractionation, extraction and the like. A fraction rich in the catalyst components may then be recycled to the reaction zone, if desired, and additional product generated. 
     The products have been identified in this work by one or more of the following analytical procedures; viz, gas-liquid phase chromatography (glc), infrared (ir) mass spectrometry, nuclear magnetic resonance (nmr) and elemental analyses, or a combination of these techniques. Analyses have, for the most part, been by parts by weight; all temperatures are in degree centigrade and all pressures in pounds per square inch (psi). 
    
    
     To illustrate the process of the invention, the following examples are given. It is to be understood, however, that the examples are given in the way of illustration and are not to be regarded as limiting the invention in any way. 
     EXAMPLE I 
     This example illustrates the results obtained by using the catalyst composition of the present invention. 
     A glass liner was charged with 0.25 mmole of ruthenium metal powder, 0.5 mmole dicobalt octacarbonyl, 5 mmole of tetra-n-butylphosphonium bromide and 0.5 mmole of hydrated rhodium trichloride, 4.0 g of methanol and 15 g of p-dioxane. The glass liner was placed in a stainless steel reactor, the reactor was purged of air and pressured to 2000 psi with a mixture of carbon monoxide and hydrogen (1:2 molar) and then heated to 200° C. while it was agitated by rocking. The pressure was brought up to 4000 psi and the constant pressure maintained by repressuring with additional amounts of carbon monoxide-hydrogen from a surge tank. 
     The reaction was terminated after 18 hours and the reactor was cooled to room temperature. Analysis of the resulting liquid product (21.0 g) was by glc and the calculations of product selectivity showed as follows: 
     46% by weight acetaldehyde 
     11% by weight ethanol 
     20% by weight methyl acetate 
     11% by weight ethyl acetate 
     Methanol conversion was 87%. 
     EXAMPLE I 
     Comparative Tests 
     The following experiment demonstrates the surprising nature of the above-noted results in relation to results obtained with catalysts which contain only three components and do not contain the rhodium compound. 
     The above procedure was repeated with the exception that the catalyst system comprised 0.25 mmole (0.025 g) of ruthenium powder, 0.5 mmole (0.17 g) dicobalt octacarbonyl, 5.0 mmoles (1.7 g) of tetra-n-butylphosphonium bromide, 4.0 g of methanol and 15.0 g of p-dioxane were added to the reaction mixture along with the above-noted catalyst. Synthesis gas was added in a 2:1 (H 2  :CO) molar mix, the operating temperature was 200° C. and pressure was 4000 psi for the reaction period of 18 hours. The liquid product (21.8 g) was analyzed by glc and showed the following: 
     6.5% by weight acetaldehyde 
     6.0% by weight methanol 
     1.2% by weight ethanol 
     2.2% by weight methyl acetate 
     2.8% by weight ethyl acetate 
     6.4% water 
     73.4% by weight p-dioxane. 
     The product selectivities (excluding p-dioxane and water) were calculated to be: 
     46% acetaldehyde 
     9% ethanol 
     15% methyl acetate 
     20% ethyl acetate 
     The methanol conversion was 70%. 
     Here is should be noted that the methanol conversion of 87% obtained by the above-noted process of the invention using the rhodium-containing compound is better than the 70% conversion obtained in this comparative synthesis without the rhodium compound, when both synthesis were conducted at the low operating pressure of 4000 psi. 
     EXAMPLES II TO IV 
     The preceding example was repeated with the exception that the amount of the rhodium chloride and the other components of the catalyst system were varied. The results are shown in Table I. 
     Here is may be noted that in Examples III and IV the synthesis of acetaldehyde from methanol is effected with the ruthenium powder, tetra-n-butylphosphonium bromide, dicobalt octacarbonyl, rhodium(III) chloride catalyst combinations at operating pressures of 3200 to 2000 psi and operating temperatures of 180° C. 
     
                                           TABLE No. I__________________________________________________________________________   Ru/n-Bu.sub.4 PBr/Co.sub.2 (CO).sub.8 /RhCl.sub.3                               product selectivities (wt %)           Methanol      Methanol                      Wt.   Catalyst           &amp;      Reaction                         Conversion                               H.sub.2 O %             gram   (mmole used)           Solvent                  Conditions                         (%)   (K.F.)                                   CH.sub.3 CHO                                         C.sub.2 H.sub.5 OH                                              CH.sub.3 OAc                                                   EtoAc                                                       (g)__________________________________________________________________________EXAMPLE II   (0.25:5:0.5:0.5)           CH.sub.3 OH 40 g                  6500 psi                         96    9.73                                   42    17   5    10  1.5           p-dioxane                  200° C.           15 g   18 hrs.EXAMPLE III   (0.5:10:1:1)           CH.sub.3 OH 8 g                  3200-2000 psi                         75    5.16                                   36    7    20   23  3.0           p-dioxane                  200° C.           25 g   18 hrs.EXAMPLE IV   (0.5:10:1:2)           CH.sub.3 OH 8 g                  3000 psi                         80    7.08                                   33    33   20   10  --           p-dioxane                  180° C.           25 g   18 hrs.__________________________________________________________________________