Rhodium catalyst regeneration by thermal treatment
This invention is concerned with a process for improving the production of alkane polyols, as well as a variety of other chemicals, in particular, methanol, from the reaction of synthesis gas in a homogeneous liquid phase mixture containing a rhodium carbonyl complex catalyst. 
     There are described in U.S. Pat. No. 3,833,634 issued Sept. 3, 1974, and U.S. Pat. No. 3,957,857 issued May 18, 1976, processes for reacting hydrogen and oxides of carbon in the presence of rhodium carbonyl complex catalysts. The conditions employed in those processes involve reacting a mixture of an oxide of carbon and hydrogen with a catalytic amount of rhodium in complex combination with carbon monoxide, at a temperature of between about 100° C. to about 375° C. and a pressure of between about 500 psia to about 50,000 psia. 
     In addition to the aforementioned U.S. Patents, the following U.S. Patents amplify the development of the processes for making alkane polyols from gaseous mixtures containing hydrogen and carbon monoxide in the presence of a rhodium carbonyl complex catalyst. 
     
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U.S. Pat. No. 3,878,292                                                   
                  Patented April 15, 1975                                 
U.S. Pat. No. 3,878,290                                                   
                  Patented April 15, 1975                                 
U.S. Pat. No. 3,878,214                                                   
                  Patented April 15, 1975                                 
U.S. Pat. No. 3,886,364                                                   
                  Patented May 27, 1975                                   
U.S. Pat. No. 3,940,432                                                   
                  Patented February 24, 1976                              
U.S. Pat. No. 3,929,969                                                   
                  Patented December 30, 1975                              
U.S. Pat. No. 3,952,039                                                   
                  Patented April 20, 1976                                 
U.S. Pat. No. 3,948,965                                                   
                  Patented April 6, 1976                                  
U.S. Pat. No. 3,944,588                                                   
                  Patented March 16, 1976                                 
U.S. Pat. No. 3,957,857                                                   
                  Patented May 18, 1976                                   
U.S. Pat. No. 3,968,136                                                   
                  Patented July 6, 1976                                   
U.S. Pat. No. 3,974,259                                                   
                  Patented August 10, 1976                                
(formerly U.S. Ser. No. 455,380,                                          
filed March 27, 1974)                                                     
U.S. Pat. No. 3,989,799                                                   
                  Patented November 2, 1976                               
(formerly U.S. Ser. No. 455,379,                                          
filed March 27, 1974)                                                     
U.S. Pat. No. 4,013,700                                                   
                  Patented March 22, 1977                                 
(formerly U.S. Ser. No. 526,942,                                          
filed November 25, 1974)                                                  
U.S. Ser. No. 488,139                                                     
                  Filed July 12, 1974                                     
(now abandoned)                                                           
U.S. Ser. No. 506,862                                                     
                  Filed September 17, 1974                                
(now abandoned)                                                           
U.S. Pat. No. 4,001,289                                                   
                  Patented January 4, 1977                                
(formerly U.S. Ser. No. 506,864,                                          
filed September 17, 1974)                                                 
U.S. Ser. No. 506,865                                                     
                  Filed September 17, 1974                                
(now abandoned)                                                           
U.S. Ser. No. 615,093                                                     
                  Filed September 19, 1975                                
(now abandoned)                                                           
U.S. Ser. No. 537,885                                                     
                  Filed January 2, 1975                                   
(now abandoned)                                                           
U.S. Ser. No. 618,023                                                     
                  Filed September 30, 1975                                
(now abandoned)                                                           
U.S. Ser. No. 618,061                                                     
                  Filed September 30, 1975                                
(now abandoned)                                                           
U.S. Ser. No. 618,021                                                     
                  Filed September 30, 1975                                
U.S. Ser. No. 727,646                                                     
                  Filed September 29, 1976                                
(now abandoned)                                                           
U.S. Ser. No. 782,986                                                     
                  Filed March 30, 1977                                    
(now U.S. Pat. No. 4,111,975,                                             
patented September 5, 1978).                                              
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     The processes described in the foregoing patents are conducted in a homogeneous liquid phase, which means that the catalyst, the reaction products and the promoter, if present, are in a solution. The solution typically requires the presence of a solvent, primarily to keep the rhodium catalyst in solution. However, rhodium carbonyl complexes vary in structure depending upon many variables, such as, temperature, solvent, promoter, and the carbon monoxide and hydrogen pressure employed, such that a complex which may be extremely stable in solution at one set of operating conditions could precipitate out of solution, for example, at a different pressure, or temperature, or in the presence of a different promoter or solvent. In addition, it has been observed that even at seemingly constant operating conditions, rhodium catalyst is continuously lost from solution during periods of extended operation resulting in reduced productivity of reaction product. Consequently, it was heretofore necessary to periodically add soluble rhodium compounds to form new catalyst in order to maintain the desired level of reaction productivity. 
     In large-scale industrial processes, catalyst losses are generally unacceptable, particularly when the catalyst contains rhodium metal which is currently priced at about $500 per troy ounce. Thus, for example, in the processes described in the aforementioned patents, it is estimated that rhodium losses on the order of about 0.1% by weight on a per pass basis would be sufficient to make the process uneconomical. Accordingly, the commercialization of these processes necessitates that the loss of rhodium metal be kept well below 0.1%, by weight, if the cost of the reaction product is not to exceed the cost of such products when manufactured by other competitive processes. 
     There is described herein an improved process for continuously forming alkane polyols at high levels of productivity from the reaction of hydrogen and oxides of carbon in a homogeneous liquid mixture containing a rhodium carbonyl complex catalyst wherein the temperature of said liquid phase mixture is periodically lowered and thereafter raised to about its former value. 
     It has been found that the reaction productivity which generally declines over extended periods of time during normal operation can be restored to about its original level by lowering the normal reaction temperature, generally in the range of about 210° C. to about 300° C., for a period of time sufficient to at least partially convert the rhodium to a more active catalytic state and thereafter raising the temperature to about its former value. The loss of reaction productivity which occurs during extended periods of operation is believed to be due to conversion of rhodium to an inactive form, for example, precipitation of rhodium from the catalyst solution. Experimental evidence indicates that rhodium which is lost or precipitated from solution is resolubilized during such period of time that the reaction temperature is temporarily lowered thereby resulting in an increased level of productivity when the reaction is conducted, once again, at the normal reaction temperature. 
     To effect resolubilization of rhodium, the reaction temperature is lowered in accordance with the invention for a period of time sufficient to at least partially resolubilize rhodium precipitated from solution. The reaction temperature may be lowered by about 10° C. to 150° C., preferably about 10° C. to 30° C., relative to its normal operating temperature, although at a minimum, lowering the temperature even less than 10° C. will also effect a partial recovery of rhodium precipitated from solution. As a general rule, it is preferred to lower the reaction temperature as little as possible in order to minimize the energy requirement associated with cooling and reheating the reactor in accordance with the practice of the invention. 
     The amount of time required to convert the rhodium to a more active catalytic form at the lower reaction temperature will generally vary from the order of about several minutes to about 40 hours depending upon the extent to which the reactor is cooled and other reaction variables, such as, pressure and the H 2  /CO mole ratio. As a general rule, the more the reactor is cooled relative to its normal operating temperature, the shorter the time period necessary to effect recovery of rhodium precipitated from the reaction solution. Likewise, the higher the reaction pressure, the shorter the required time period to effect such resolubilization of rhodium. 
     Economic consideration will generally dictate the most convenient mode of operating the process of the invention. As described above, one embodiment of the process is characterized by a periodic interruption of the reaction whereby the reaction temperature is periodically lowered in order to effect at least a partial resolubilization of rhodium precipitated from solution and thereafter raised to about its former value, the reaction productivity being thus increased from its reduced level. The resolubilization of rhodium and/or its conversion to a more active catalytic form in such periodic manner may be effected whenever the reaction productivity declines to below a predetermined value, or at fixed intervals of time, whichever is more practical from an operational standpoint. Alternatively, the process may be operated continuously, i.e., without interruption of the reaction, at near constant productivity. In accordance with this embodiment of the invention, a portion of the liquid phase mixture has its temperature lowered outside of the reactor (or reaction zone) on a continuous or intermittent basis, depending upon the rate at which the rhodium in solution would otherwise tend to precipitate or become unstable, and then reintroduced into the reactor which is maintained at constant temperature. A portion of the catalyst solution is thus withdrawn from the reactor, preferably continuously, and cooled to the desired temperature, and then reintroduced into the reactor (or reaction zone). Such thermal treatment of the liquid phase mixture external to the reactor is intended to prevent rhodium in solution from reaching a condition where it will tend to precipitate from the catalyst solution or otherwise become converted to a less active form thereby enabling long-term uninterrupted operation of the process at near constant reaction productivity. 
     The rhodium carbonyl complex catalysts suitable for use herein may be in the form of rhodium carbonyl clusters. P. Chini, in a review article entitled &#34;The Closed Metal Carbonyl Clusters&#34; published in Review (1968), Inorganica Chimica Acta, pages 30-50, states that a metal cluster compound is &#34;a finite group of metal atoms which are held together entirely, mainly, or at least to a significant extent, by bonds directly between the metal atoms even though some non-metal atoms may be associated intimately with the cluster&#34;. The rhodium carbonyl cluster compounds contain rhodium bonded to rhodium or rhodium bonded to another metal, such as cobalt, and/or iridium. 
     The preferred rhodium carbonyl cluster compounds are those which contain rhodium-rhodium bonds. These compounds desirably contain carbon and oxygen in the form of carbonyl (--CO), in which the carbonyl may be &#34;terminal&#34;, &#34;edge-bridging&#34;, and/or &#34;face-bridging&#34;. They may also contain hydrogen and carbon in forms other than carbonyl. 
     The structures of the rhodium carbonyl clusters may be ascertained by X-ray crystal diffraction, nuclear magnetic resonance (NMR) spectra, or infrared spectra as disclosed in the article entitled &#34;Synthesis and Properties of the Derivatives of the [Rh 12  (CO) 30  ] 2-  Anion&#34; by P. Chini and S. Martinengo; appearing in Inorganica Chimica Acta, 3:2 pp 299-302, June (1969). Of particular analytical utility in the present invention is the use of infrared spectroscopy which allows for characterization of the particular rhodium carbonyl complex present during the operation of the process of the present invention. 
     The precise role of the rhodium carbonyl complexes, such as the rhodium carbonyl clusters, in the reaction of hydrogen with oxides of carbon to produce polyhydric alcohols is not fully appreciated at present. Under the reaction conditions of the present process the carbonyl complexes are believed to be anionic in their active forms. 
     Infrared spectra under reaction conditions of the present process have shown Rh(CO) 4   - , Rh 13  (CO) 24  H 3   -2 , Rh 6  (CO) 15  H - , Rh 13  (CO) 24  H 2   -3 , and [Rh 12  (CO) 34-36  ] 2-   anions, and other rhodium clusters to be present at various concentrations at different times of the reaction. These may represent the active rhodium carbonyl species responsible for polyhydric alcohol formation or may be merely symptomatic of some further intermediate transitory rhodium carbonyl structure which serves to convert the carbon monoxide and hydrogen to the polyhydric alcohol. 
     A number of nitrogen and/or oxygen-containing bases may also be used in the process of the present invention. For the purposes of this invention, the bases can be considered to promote the activity of the rhodium catalyst. 
     Nitrogen Lewis bases used as promoters generally contain hydrogen and nitrogen atoms. They may also contain carbon and/or oxygen atoms. They may be organic or inorganic compounds. With respect to the organic compounds, the carbon atoms can be part of an acyclic and/or cyclic radical such as aliphatic, cycloaliphatic, aromatic (including fused and bridged) carbon radicals, and the like. Preferably, the organic Lewis bases contain from 2 to 60,  most preferably 2 to 40 carbon atoms. The nitrogen atoms can be in the form of imino (--N═), amino (--N--), nitrilo (N═), etc. Desirably, the Lewis base nitrogen atoms are in the form of imino nitorgen and/or amino nitrogen. The oxygen atoms can be in the form of groups such as hydroxyl (aliphatic or phenolic), carboxyl ##STR1## carbonyloxy ##STR2## oxy (--O--), carbonyl ##STR3## etc., all of said groups containing Lewis base oxygen atoms. In this respect, it is the &#34;hydroxyl&#34; oxygen in the ##STR4## group and the &#34;oxy&#34; oxygen in the ##STR5## group that are acting as Lewis base atoms. The organic Lewis bases may also contain other atoms and/or groups as substituents of the aforementioned radicals, such as alkyl, cycloalkyl, aryl, chloro, trialkylsilyl substituents. 
     Illustrative of organic aza-oxa Lewis bases are, for example, the alkanolamines, such as, ethanolamine, diethanolamine, isopropanolamine, di-n-propanolamine, and the like; N,N-dimethylglycine, N,N-diethylglycine; iminodiacet acid, N-methyliminodiacetic acid; N-methyldiethanolamine; 2-hydroxypyridine, 2,4-dihydroxypyridine, 2-methoxypyridine, 2,6-dimethoxypyridine, 2-ethoxypyridine; lower alkyl substituted hydroxypyridines, such as 4-methyl-2-hydroxypyridine, 4-methyl-2,6-dihydroxypyridine, and the like; morpholine, substituted morpholines, such as 4-methylmorpholine, 4-phenylmorpholine; picolinic acid, methyl-substituted picolinic acid; nitrilotriacetic acid, 2,5-dicarboxypiperazine N-(2-hydroxyethyl) iminodiacetic acid, ethylenediamine-tetraacetic acid; 2,6-dicarboxypyridine; 8-hydroxyquinoline, 2-carboxyquinoline, cyclohexane-1,2-diamine-N,N,N&#39;,N&#39;-tetraacetic acid, the tetramethyl ester of ethylenediamine-tetraacetic acid, and the like. 
     Other Lewis base nitrogen containing compounds include organic and inorganic amines. 
     Illustrative of such inorganic amines are, e.g., ammonia, hydroxylamine, and hydrazine. Primary, secondary, or tertiary organic amines are promoters. This includes the mono- and polyamines (such as di-, tri-, tetraamines, etc.) and those compounds in which the Lewis base nitrogen forms part of a ring structure as in pyridine, quinoline, pyrimidine morpholine, hexamethylenetetraamine, and the like. In addition any compounds capable of yielding an amino nitrogen under the reaction conditions of the present invention are promoters, as in the case of an amide, such as formamide, cyanamide, and urea, or an oxime. Further illustrative of Lewis base nitrogen compounds are aliphatic amines such as methylamine, ethylamine, n-propylamine, isopropylamine, octylamine, dodecylamine, dimethylamine, diethylamine, diisoamylamine, methylethylamine, diisobutylamine, trimethylamine, methyldiethylamine, triisobutylamine, tridecylamine, and the like; aliphatic and aromatic di- and polyamines such as 1,2-ethanediamine, 1,3-propanediamine, N,N,N&#39;,N&#39;-tetramethylenediamine, N,N,N&#39;,N&#39;-tetraethylethylenediamine, N,N,N&#39;,N&#39;-tetra-n-propylethylenediamine, N,N,N&#39;,N&#39;-tetrabutylethylenediamine, o-phenylenediamine, m-phenylenediamine, p-phenylenediamine, p-tolylenediamine, o-tolidene, N,N,N&#39;,N&#39;-tetramethyl-p-phenylenediamine, N,N,N&#39;,N&#39;-tetraethyl-4,4&#39;-biphenyldiamine, and the like; aromatic amines such as aniline, 1-naphthylamine, 2-naphthylamine, p-toluidine, o-3-xylidine, p-2-xylidine, benzylamine, diphenylamine, dimethylaniline, diethylaniline, N-phenyl-1-napthylamine, bis-(1,8)-dimethylaminonaphthalene, and the like; alicyclic amines such as cyclohexylamine, dicyclohexylamine, and the like; heterocyclic amines such as piperidine; substituted piperidines such as 2-methylpiperidine 3-methylpiperidine, 4-ethylpiperidine, and 3-phenylpiperidine; pyridine; substituted pyridines such as 2-methylpyridine, 2-phenylpyridine, 2-methyl-4-ethylpyridine, 2,4,6-trimethylpyridine, 2-dodecylpyridine, 2-chloropyridine, and 2-(dimethylamino) pyridine; quinoline; substituted quinolines, such as 2-(dimethylamino)-6-methoxyquinoline; 4,5-phenanthroline; 1,8-phenanthroline; 1,5-phenanthroline; piperazine; substitute piperazines such as N-methylpiperazine, N-ethylpiperazine, 2-methyl-N-methylpiperazine; 2,2&#39;-dipyridyl, methyl-substituted 2,2-dipyridyl; ethyl-substituted 2,2&#39;-dipyridyl; 4-triethylsilyl-2,2&#39;-dipyridyl; 1,4-diazabicyclo[2.2.2]octane, methyl substituted 1,4-diazabicyclo[2.2.2]octane, purine and the like. 
     Also included herein are the use of dimorpholine compounds characterized by the formula: ##STR6## wherein R is divalent alkylene of 1 to about 30 carbon atoms and 1,4-phenylene. 
     Under reaction conditions the promoter is preferably used in amounts from about 0.02 to about 40 equivalents of promoter, most preferably from about 0.1 to about 20 equivalents promoter, for every atom of rhodium in the reaction mixture. The number of equivalents of promoter is equal to the number of molecules of promoter times the number of nitrogen atoms in each molecule. 
     The process of this invention can also be carried out by providing salts in the homogeneous liquid phase reaction mixture. Suitable salts include any organic or inorganic salt which does not adversely affect the production of polyhydric alcohols. Experimental work suggest that many salts are beneficial as either a copromoter and/or in aiding in maintaining rhodium in solution during the reaction. Illustrative of useful salt promoters are the ammonium salts and the salts of the metals of Group I and Group II of the Periodic Table (Handbook of Chemistry and Physics--50th Edition) for instance the halide, hydroxide, alkoxide, phenoxide and carboxylate salts such as sodium fluoride, cesium fluoride, cesium pyridinolate, cesium formate, cesium acetate, cesium benzoate, cesium p-methylsulfonyl-benzoate (CH 3  SO 2  C 6  H 4  COO)Cs, rubidium acetate, magnesium acetate, strontium acetate, ammonium formate, ammonium benzoate and the like. The cesium, rubidium, potassium and ammonium salts are preferred. 
     Other organic salts useful in the practice of the present invention include the quaternized heterocyclic amine salts such as the pyridinium, piperidinium, morpholinium, quinolinium salts and the like, e.g., N-ethylpyridinium fluoride, N-methylmorpholinium benzoate, N-phenylpiperidinium hydroxide, N,N&#39;-dimethyl-2,2-bipyridinium acetate, and the like. 
     In addition, the anion of the above salt may be any of the rhodium carbonyl anions. Suitable rhodium carbonyl anions include [Rh 6  (CO) 15  ] 2-  ; [Rh 6  (CO) 15  Y] -   wherein Y may be halogen, such as chlorine, bromine, or iodine, [Rh 6  (CO) 15  (COOR&#34;] -  wherein R&#34; is lower alkyl or aryl such as methyl, ethyl, or phenyl; [Rh 6  (CO) 14  ] 2-  ; [Rh 7  (CO) 16  ] 3-  ; [Rh 12  (CO) 30  ] 2-  ; Rh 13  (CO) 24  H 3   -2  ; and Rh 13  (CO) 24  H 2   -3 . 
     Under reaction conditions where a salt is employed the salt is desirably added with the initial charge of reactants in amounts of from about 0.5 to about 2.0 gram moles, preferably from about 0.8 to about 1.6 moles, and most preferably from about 0.9 to 1.4 moles of salt for every five gram atoms of rhodium present in the reaction mixture. 
     Illustrative solvents which are generally suitable in making the homogeneous mixture include, for example, ethers such as tetrahydrofuran, tetrahydropyran, diethyl ether, 1,2-dimethoxybenzene, 1,2-diethoxybenzene, the mono- and dialkyl ethers of ethylene glycol, of propylene glycol, of butylene glycol, of diethylene glycol, of dipropylene glycol, of triethylene glycol, of tetraethylene glycol, of dibutylene glycol, of oxyethylenepropylene glycol, etc; alkanols such as methanol, ethanol, propanol, isobutanol, 2-ethylhexanol, etc.; ketones such as acetone, methyl ethyl ketone, cyclohexanone, cyclopentanone, etc.; esters such as methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl butyrate, methyl laurate, etc.; water; gamma-butyrolactone, deltavalerolactone; substituted and unsubstituted tetrahydrothiophene-1,1-dioxides (sulfolanes) as disclosed in U.S. application Ser. No. 537,885, filed on Jan. 2, 1975, the disclosure at pages 6 and 7 of the specification of which is incorporated herein by reference. 
     Also, the crown ethers are suitable herein, particularly those as described in U.S. Patent Application Ser. No. 832,384 filed Sept. 13, 1977, which application is incorporated herein by reference and is now U.S. Pat. No. 4,162,261 issued July 24, 1979. The crown ethers described therein contain at least four oxygen heteroatoms and include [18]-crown-6 and [15]-crown-5. 
     Particularly desirable solvents are tetraglyme, sulfolane, gamma-butyrolactone and the crown ethers. Other very desirable solvents include mixtures of tetraglyme and sulfolane, mixtures of sulfolane and butyrolactone, mixtures of crown ethers and sulfolane, mixtures of crown ethers and tetraglyme, mixtures of crown ethers and butyrolactone, mixtures of tetraglyme and butyrolactone. 
     In practicing the method of the present invention, the synthesis of the desired alkane diols and derivatives thereof, by the reaction of hydrogen with an oxide of carbon is suitably conducted under operative conditions, as heretofore described, which give reasonable reaction rates and/or conversions. 
     The process is suitably effected over a wide superatmospheric pressure range of from about 500 psia to about 50,000 psia. Pressures as high as 50,000 psia, and higher can be employed but with no apparent advantages attendant thereto which offset the unattractive plant investment outlay required for such high pressure equipment. Therefore, the upper pressure limitation is desirably approximately 16,000 psia. Effecting the present process below about 16,000 psia, especially below about 13,000 psia, and preferably at pressures below about 8000 psia, results in cost advantages which are associated with low pressure equipment requirements. In attempting to foresee a commercial operation of this process, pressures between about 4,000 psia and 16,000 psia appear to represent most realistic values. 
     In a preferred embodiment of the present invention the pressures referred to above represent the total pressures of hydrogen and oxides of carbon in the reactor. 
     The temperature which may be employed can vary over a wide range of elevated temperatures. In general, the process can be conducted at a temperature in the range of from about 100° C. and upwards to approximately 375° C., and higher. Temperatures outside this stated range are not excluded from the scope of the invention. At the lower end of the temperature range, and lower, the rate of reaction to desired product becomes markedly slow. At the upper temperature range, and beyond, signs of some catalyst instability are noted. Notwithstanding this factor, reaction continues and alkane polyols and/or their derivatives are produced. Additionally, one should take notice of the equilibrium reaction for forming ethylene glycol: 
     
         2CO+3H.sub.2 ⃡HOCH.sub.2 CH.sub.2 OH 
    
     At relatively high temperatures the equilibrium increasingly favors the left hand side of the equation. To drive the reaction to the formation of increased quantities of ethylene glycol, higher partial pressures of carbon monoxide and hydrogen are required. Processes based on correspondingly higher pressures, however, do not represent preferred embodiments of the invention in view of the high investment costs associated with erecting chemical plants which utilize high pressure utilities and the necessity of fabricating equipment capable of withstanding such enormous pressures. Suitable temperatures are between about 150° C. to about 320° C., and desirably from about 210° C. to about 300° C. 
     The novel process is effected for a period of time sufficient to produce the alkane polyols and/or derivatives thereof. In general, the residence time can vary from minutes to several hours, e.g., from a few minutes to approximately 24 hours, and longer. It is readily appreciated that the residence period will be influenced to a significant extent by the reaction temperature, the concentration and choice of the catalyst, the total gas pressure and the partial pressures exerted by its components, the concentration and choice of diluent, and other factors. The synthesis of the desired product(s) by the reaction of hydrogen with an oxide of carbon is suitably conducted under operative conditions which give reasonable reaction rates and/or conversions. 
     The relative amounts of carbon monoxide and hydrogen which are initially present in the reaction mixture can be varied over a wide range. In general, the mole ratio of CO:H 2  is in the range of from about 20:1 to about 1:20, suitably from about 10:1 to about 1:10, and preferably from about 5:1 to about 1:5. 
     It is to be understood, however, that molar ratios outside the aforestated broad range may be employed. Substances or reaction mixtures which give rise to the formation of carbon monoxide and hydrogen under the reaction conditions may be employed instead of mixtures comprising carbon monoxide and hydrogen which are used in preferred embodiments in the practice of the invention. For instance, polyhydric alcohols are obtained by using mixtures containing carbon dioxide and hydrogen. Mixtures of carbon dioxide, carbon monoxide and hydrogen can also be employed. If desired, the reaction mixture can comprise steam and carbon monoxide. 
     The active forms of the rhodium carbonyl clusters may be prepared by various techniques. They can be preformed and then introduced into the reaction zone. Alternatively, any of the host of rhodium-containing substances can be introduced into the reaction zone and, under the operative conditions of the process (which of course includes hydrogen and carbon monoxide), the active rhodium carbonyl cluster can be generated in situ. Illustrative of rhodium-containing substances which can be conveniently introduced or placed in the synthesis zone include, for example, rhodium oxide (Rh 2  O 3 ), tetrarhodium dodecacarbonyl, dirhodium octacarbonyl, hexarhodium hexadecacarbonyl (Rh 6  (CO) 16 ), rhodium(II) formate, rhodium(II) acetate, rhodium(II) propionate, rhodium(II) butyrate, rhodium(II) valerate, rhodium(III) naphthenate, rhodium dicarbonyl acetylacetonate, rhodium tri(acetylacetonate), rhodium trihydroxide, indenylrhodium dicarbonyl, rhodium dicarbonyl (1-phenylbutane-1,3-dione), tris(hexane-2,4-dionato)rhodium(III), tris(heptane-2,4-dionato)rhodium(III), tris(1-phenylbutane-1,3-dionato)rhodium(III), tris(3-methylpentane-2,4-dionato)rhodium(III), tris(1-cyclohexylbutane-1,3-dionato)rhodium(III), triacontacarbonyl rhodium salts and rhodium-containing compounds deposited on porous supports or carriers capable of providing rhodium carbonyls in solution, and others. 
     The preparation of the rhodium carbonyl complex compounds can be conveniently carried out in the solvent mixture. Tetrarhodium dodecacarbonyl, though of limited solubility, can be added to the solvent in a finely divided form. Any of several of the rhodium-containing compounds illustrated previously can be employed in lieu of tetrarhodium dodecacacarbonyl. The rhodium carbonyl complex or cluster forming reaction can be effected under a carbon monoxide pressure, with or without H 2 , of about 1 to 15 atmospheres, and higher, using a temperature of about 30° C. to about 100° C., for a period of time ranging from minutes to a few days, generally from about 30 minutes to about 24 l hours. The resulting rhodium carbonyl complex contained in the solvent mixture is catalytically active in this process. 
    
    
     In order to illustrate the invention reference is made to the drawing which depicts a schematic flowsheet describing a liquid recycle system for the continuous operation of the process of the invention. 
    
    
     Reactor 2 is a heat jacketed stainless steel autoclave capable of withstanding reaction pressures of up to 2,000 atmospheres. Provided in reactor 2 is the liquid phase mixture containing the rhodium carbonyl complex catalyst dissolved therein. Hydrogen and carbon monoxide are mixed in the desired molar ratio using a metering system (not shown) which allows the composition of the exit gas to vary from pure H 2  to pure CO. When reactor 2 is operated in an overflow (or liquid full) configuration, the resultant gas feed is passed through gas compressor 1 and fed to reactor 2 in quantities sufficient to saturate the liquid phase mixture in the reactor but less than an amount whereby a discrete gas phase will form in the reaction zone. Reactor 2 is provided with an agitator to uniformly distribute the H 2  /CO gas mixture in the liquid body thus providing intimate gas-liquid contact. 
     In operating reactor 2 in an overflow (or liquid full) configuration, the liquid phase mixture comprising the catalyst solution, reaction products and dissolved reactant gas is withdrawn from the top of reactor 2 and is passed through pressure reducing value 10 to a liquid-vapor separator 3 where a portion of the unconverted reactant gas dissolved in the liquid mixture comes out of solution at the reduced pressure of the separator and is vented through line 6. The liquid mixture leaving separator 3 is either returned directly to reactor 2 via line 7, pressure reducing valve 12 and liquid pump 4, or alternatively, sent through line 8, and pressure reducing valve 11 to CO stripper 5 for recovery of product, predominantly ethylene glycol with varying amounts of polyols such as glycerine, propylene glycol and methanol. Stripper 5 is a vertical packed column filled with a high surface area packing, the liquid feed entering near the top being stripped by a counter-current flow of carbon monoxide or optionally a CO/H 2  mixture which is then further processed for product recovery. The liquid mixture exiting at the bottom of stripper 5 is recycled to reactor 2 through line 9, valve 13 and pump 4. Valve 13 prevents the flow of liquid mixture leaving valve 12 from entering stripper 5; valve 13 being closed when the liquid mixture in line 7 bypasses stripper 5. 
     As an alternative to operating reactor 2 in an overflow configuration, as described above, reactor 2 may operate as a vapor-stripping reactor in a gas recycle system. In this mode of operation, the liquid level of catalyst solution in reactor 2 is controlled such that a liquid-vapor interface is maintained in the reactor, the vapor space above the liquid phase mixture being composed of excess reactant gas and reaction product. An excess amount of H 2  and CO reactant gas mixture is thus introduced into the bottom of reactor 2 to contact the catalyst solution therein and strip therefrom vaporous products of the reaction and solvent. The CO/H 2  gas mixture in the vapor space is thus saturated with vaporous reaction products and solvent vapors and passes from the top of reactor 2 into vapor-liquid separator 3. For a gas recycle system, the operating pressure of separator 3 is preferably maintained at the pressure of reactor 2. A heat exchanger may be provided wherein all but the CO/H 2  mixture is liquified and removed from line 7, the CO/H 2  mixture being withdrawn through line 6 and recycled to reactor 2 (gas recycle not shown in drawing). The consensed liquid mixture in line 7 is then sent to stripper 5, as previously described, for recovery of product, the remaining liquid in stripper 5 exiting through line 9 for recycle to reactor 2. 
     Resolubilization of rhodium precipitated from the catalyst solution may be effected by periodically reducing the temperature of reactor 2 as previously described. This is conveniently accomplished by allowing the reactor to cool to a predetermined lower temperature, maintaining the catalyst solution at the lower temperature for the requisite amount of time necessary to effect at least a partial resolubilization of precipitated rhodium and thereafter raising the reactor temperature to about its former value. Alternatively, the temperature of reactor 2 may be maintained constant. In accordance with this embodiment of the invention, a portion of the liquid phase mixture is withdrawn from reactor 2 on a periodic or continuous basis via line 17 and liquid pump 14, cooled in heat exchanger 15 and then maintained at such lower temperature for the desired residence time by adjusting the flow rate of solution in line 17, or by providing a holding tank (not shown) for retaining the cooled catalyst solution, the solution being thereafter reheated to the reaction temperature in heat exchanger 16 prior to being reintroduced into reactor 2. If desired, the catalyst solution cooled in heat exchanger 15 may be heated to the reaction temperature in reactor 2 thus eliminating the need for heat exchanger 16. 
     To further illustrate the invention, the following examples were operated in accordance with the liquid recycle system illustrated in FIG. 1. Reactor 2 was operated in a liquid-full configuration. In the specific practice of the examples, a 30 or 60 pound charge of solution, as the case may be, containing solvent, rhodium dicarbonyl acetylacetonate, and promoters was initially introduced into a holding tank (not shown in the drawing) under nitrogen pressure. A 1:1 mole mixture of H 2  /CO, at the desired gas feed rate, was introduced into a 1.5 or 6 liter capacity stainless steel reactor (depending upon the particular example) until the desired reactor pressure was established in the system. The catalyst solution was then charged from the holding tank into the reactor and the reactor then heated to the desired reaction temperature. 
     The dimensions of the 1.5 and 6 liter reactors used in the examples were as follows: 35/8&#34; I.D.×9&#34; and 51/2&#34; I.D.×18&#34;. The liquid-vapor separator was a 2 liter capacity stainless steel vessel, 3&#34; I.D., 18&#34; long. The CO stripper was a 2&#34; I.D. tube, 8 ft. long. 
     The separator was operated at about 2,000 psig and the pressure of the CO stripper was about 15 psig. 
     EXAMPLE I 
     A 60 pound charge of catalyst solution containing about 250 ppm by weight of rhodium (added as rhodium dicarbonyl acetylacetonate) and having a 6 to 1 molar ratio of rhodium to cesium (50 ppm added as cesium fluoride) and a 4:1 molar ratio of 2-hydroxypridine to rhodium in tetraglyme solvent was introduced into a liquid recycle system, as described above, having a 6 liter stainless steel reactor. The feed gas was a 1:1 mole ratio of H 2  /CO, the reactor temperature was 210° C. and the reactor pressure was 10,000 psig. After 150 hours of operation, the rhodium concentration in solution had declined to about 150 ppm while the cesium concentration was about 30 ppm. The reactor temperature was then lowered to 100° C. without changing any of the other reaction variables. After 40 hours of operation the rhodium concentration had increased to about 500 ppm and the cesium concentration to about 60 ppm. The apparent resolubilization of rhodium in amounts greater than that which was lost or precipitated from solution is believed to be due to the resolubilization of rhodium which was precipitated from other catalyst charges during the course of prior experiments conducted in the same reactor. 
     EXAMPLE II 
     An experiment was carried out using a catalyst solution similar in volume, composition and mole ratios of rhodium to promoters to that described in Example 1 except that the rhodium concentration was about 320 ppm by weight and the cesium concentration was about 70 ppm. The reaction temperature, pressure, and H 2  /CO gas mixture were all as described in Example 1. The reactor volume was 6 liters. After 76 hours of reaction, the rhodium concentration had declined to about 170 ppm and cesium to about 55 ppm whereupon the reaction temperature was reduced to 100° C. and nitrogen was substituted for the H 2  /CO feed gas. After 12 hours at these conditions the rhodium concentration was still at approximately 170 ppm. The cesium concentration was not determined. The feed gas was then switched to a 1:1 mole ratio of H 2  /CO while maintaining all other reaction conditions constant. After 12 hours of operation, the rhodium concentration was about 330 ppm and the cesium concentration about 67 ppm. 
     The substitution of N 2  for the H 2  /CO feed, as described above, did not result in any measured increase in rhodium concentration and appears to indicate that the resolubilization of rhodium at a lowered reaction temperature in accordance with the invention does not occur when the catalyst solution is under an inert gas pressure rather than that of the H 2  /CO reactant gas. 
     EXAMPLE III 
     A catalyst solution was used similar in volume, composition and mole ratios of rhodium to promoters to that described in Example 1 except that the rhodium concentration was about 830 ppm. The reaction temperature was 220° C. while the reaction pressure and H 2  /CO feed gas were as set forth in Example 1. The reactor volume was 6 liters. After 168 hours of operation, the concentration of rhodium in solution had declined to about 100 ppm. The reactor temperature was then decreased to about 100° C. and the liquid circulation in the system was stopped while maintaining the H 2  /CO feed. After 24 hours at these conditions the catalyst solution in the reactor was sampled and found to contain in excess of 5600 ppm rhodium. The circulation of solution was then restarted and after becoming well mixed, once again, the solution was analyzed and the concentration of rhodium in solution determined to be about 830 ppm. 
     The measured concentration of rhodium above 5600 ppm, as described above, appears to indicate that the original loss of rhodium from solution and subsequent resolubilization occurred predominantly in the reactor and not elsewhere in the system. 
     EXAMPLE IV 
     A 30 lb. charge of catalyst solution containing about 112 grams of rhodium (added as rhodium dicarbonyl acetylacetonate), potassium (added as potassium bicarbonate) in a 5:1 molar ratio of RH/K and N-methylmorpholine at a concentration of about 1.5 weight % in a solvent consisting of the following: 18-Crown-6 (about 75 weight %) and triphenyl phosphine oxide (about 20 weight %), with the balance being reaction products, was introduced into a liquid recycle system having a 1.5 liter stainless steel reactor. The feed gas was a 0.5:1 mole ratio of hydrogen to carbon monoxide, the reaction temperature was 265° C. and the reaction pressure was about 15,000 psig. The initial productivity of ethylene glycol was about 4.5 gram-moles per hour. After about 100 hours of operation, the amount of rhodium in solution had decreased to about 78 grams, the productivity declining to about 3.5 gram-moles of glycol per hour. The reactor temperature was then reduced to 230° C. while maintaining all other reaction conditions constant. After 6 hours of operation the weight of rhodium in solution had increased about 110 grams. After an interval during which another experiment was conducted with the catalyst system which involved decreasing the molar ratio of Rh/K to about 2.5:1, the temperature was, once again, elevated to 265° C. The initial productivity was thereafter about 4.2 gram-moles per hour.