PROCESS FOR PRODUCING CARBOXYLIC ACIDS

Abstract: A process for converting alcohols to carboxylic acids which process comprises contacting in the vapour phase one or more alcohols, water and added hydrogen at a temperature ranging from 200°C to 500°C with a catalyst comprising catalytically effective amounts of copper, zinc, chromium and a promoter selected from the group consisting of alkali metals, alkaline earth metals, rare earth metals and mixtures thereof supported on a porous refractory support.

Description: The present invention relates to an improved process for converting alcohols and water into carboxylic acids using a catalyst which comprises copper, zinc, chromium, alkali or alkaline earth metal or rare earth metal supported on an alumina support. Carboxylic acids find many uses in industry. The lower molecular weight acids are frequently used as intermediates; such as acetic acid being used in the preparation of cellulose plastics and esters. Intermediate molecular weight acids are used in the preparation of lubricants and lubricant additives. The higher molecular weight acids find use in the preparation of detergent products. In U.S. patent specification No. 4,220,803, reference is made to copper oxide and copper oxide-chromium oxide catalysts which are useful for the conversion of ethanol to acetic acid. Molecular oxygen was used as the oxidizing agent and the selectivity to acetic acid was lower than the selectivity to acetaldehyde. In Japanese patent specification No. 57/102835 reference is made to copper oxide-zinc oxide, copper oxide-chromium oxide or copper oxide-chromium oxide-manganese oxide catalysts which are useful for converting mixtures of ethanol or acetaldehyde and water to acetic acid. The present invention relates to a process for converting alcohols to carboxylic acids which process comprises contacting, in the vapour phase, one or more primary alcohols of the general formula RCH₂OH where R is hydrogen, aryl having up to 30 carbon atoms or alkyl having from 1 to 30 carbon atoms, water and at least one mole of added hydrogen per mole of alcohol at a temperature ranging from 200°C to 500°C with a catalyst comprising catalytically effective amounts of copper, zinc, chromium and a promoter selected from the group consisting of alkali metals, alkaline earth metals, rare earth metals and mixtures thereof supported on an inert porous refractory support. The use of catalysts of the process according to the present invention lower the aldehyde and ester yield while increasing the acid yield. The catalysts are suitably prepared by impregnating a porous alumina support with one or more solution(s) of copper, zinc, chromium, and/or promoter salts, drying the impregnated support, calcining the support in an oxidizing atmosphere at a temperature ranging from 100°C to 900°C and then activating the calcined material in a reducing environment. A key aspect of the invention is the addition of hydrogen to the feed in order to maintain the selectivity of the catalyst for the appropriate carboxylic acid, which is surprising in view of the fact that two moles of hydrogen are produced for every mole of carboxylic acid produced Optimum catalyst life is obtained by carrying out the reaction at pressures ranging between 2 to 5 atmospheres. The alcohols to be converted by the process according to the present invention are primary alcohols of the general formula RCH₂OH where R is hydrogen, aryl of up to C₃₀ or C₁ to C₃₀ alkyl. R may be substituted with inert substituents such as chloro, fluoro, alkyl, etc. These alcohols are converted to acids of the general formula RCO₂H wherein R is as defined above. Basically, catalyst preparation comprises (1) impregnating a porous inert support with one or more solutions of the requisite salts, (2) drying and calcining at a temperature ranging from 100°C to 900 °C, preferably from 200°C to 700°C and most preferably from 300°C to 600°C and (3) activating the calcined material in a reducing environment. The support or carrier employed in these catalysts in its broadest aspect is selected from the large number of conventional, porous refractory catalyst carriers or support materials which are considered relatively inert in the presence of the feeds, products and reaction conditions of the process according to the present invention. Such conventional materials are known to persons skilled in the art and may be of natural or synthetic origin and preferably have B.E.T. surface areas greater than 100 m²/g. Very suitable supports comprise those of aluminous or siliceous composition. Examples of supports that have been used as supports for different catalysts and which can be considered for use as supports for the present catalysts are the aluminum oxides (including the materials sold under the trade name "Alundum"), charcoal, pumice, magnesia, zirconia, keiselguhr, fullers' earth, silicon carbide, porous agglomerates comprising silicon and/or silicon carbide, silica, magnesia, selected clays, artificial and natural zeolites and ceramics. The catalysts to be used in the process according to the present invention comprise a porous gamma alumina having a B.E.T. surface area greater than 100m²/g. The B.E.T. method for determining surface area is described in Brunauer, S., Emmet, P.H. and Teller, E., J. Am. Chem. Soc., 60, 309-16 (1938). Regarding purity of the alumina, it may be stated that small amounts of impurities are not generally detrimental. The most suitable aluminas for use in the present invention are found to be those having a high surface area, for instance, aluminas having a surface area of at least 100 m²/g. The alumina may contain minor amounts of other compounds such as silica. Aluminas which are readily available commercially can be used in the process according to the present invention. The following table lists several commercial aluminas and their properties which are found suitable. The Kaiser KA-201 alumina is a preferred support. The first step in the preparation of the catalyst is to impregnate the support with solubilized salts of copper, zinc, chromium and/or promoter metal. The salts must be soluble in a suitable solubilizing medium, either organic or inorganic. Water is a preferred solubilizing medium. Lower alkanols also provide examples of suitable organic solvents. Suitable metal salts are, for example, chlorides, bromides, nitrates, acetates, lactates, carbonates, bicarbonates and the like. The impregnation of the support may be carried out in one step utilizing all the metal salts dissolved in a single solution, or it may be carried out in a multistep process, using one or more of the metal salts dissolved in individual impregnating solutions. A preferred impregnating process is the so-called "dry impregnation" where just a sufficient amount of impregnating solution containing all the requisite metal salts is used such that all the pore volume in the carrier is filled and no excess solution containing all the requisite metal salts is used such that all the pore volume in the carrier is filled and no excess solution is left after impregnation. The next step is to dry and calcine the impregnated material. The drying and calcining can be carried out in individual steps. For example, drying can be carried out at temperatures up to 150°C followed by a calcining step at temperatures ranging from 100°C to 900°C preferably from 300°C to 800°C. Preferably, the drying and calcining are carried out in one continuous step, heating the material slowly through the low temperature ranges to dry the material and then raising the temperature to the calcining conditions. The purpose of the calcining is to convert the soluble metal salts to oxides or oxygen-containing compounds upon the support material. Calcining is carried out in a neutral or oxidizing atmosphere, air being the preferred atmosphere. Nitrogen is also a suitable alternative atmosphere. The drying step is preferably carried out in the initial stages of the calcining step. Drying and calcining times are not critical and depend on temperatures. These are readily determined by simple experimentation. Five minutes to ten hours are usually sufficient, although longer times are acceptable. In the most preferred process, a solution containing the promoter metal salt is used to impregnate the support which is then dried and calcined. The calcined material is then impregnated with a solution having copper, zinc and chromium salts dissolved therein. This material is then dried, calcined and then activated prior to use. The amounts of metals deposited upon the support are not critical and may be varied through a wide range so long as they are present in sufficient amounts to be catalytically effective, a condition which is readily determined by experiment. Preferably the support will contain from 0.1 to 20% wt of copper, measured as metal per total weight of catalyst, from 0.1 to 10% wt of zinc, measured as metal per total weight of catalyst from 0.01 to 10% wt, more preferably from 0.1 to 5%wt of chromium, measured as metal per total weight of catalyst and from 0.001 to 10% wt, more preferably from 0.1 to 5% wt of promoter metal measured as metal per total weight of catalyst. The promoter metal is selected from alkali metals, alkaline earth metals, rare earth metals and mixtures thereof. After calcining, the catalyst is activated in a reducing environment. The reducing environment may be either a gaseous atmosphere or a suitable liquid solution. Suitable examples of a gaseous reducing atmosphere comprise hydrogen, ammonia, carbon monoxide, and the like. The preferred reducing atmosphere is hydrogen. Activation temperatures when utilizing a gaseous atmosphere range from 175°C to 550°C. The time needed for activation in a gaseous atmosphere will depend on the temperature; the higher the temperature, the shorter the time and vice versa, and typically, useful times have been found to range from 0.01 hour to 24 hours, although times outside these limits are also useful, economic considerations, however, tending to dictate against their use. Reducing solutions are those typically used in the art, such as, for example, aqueous or ammoniacial solutions of hydrazine, sodium borohydride or formaldehyde or solutions of, for example, triethyl aluminium or di-isobutyl aluminium hydride in an organic solvent such as heptane. Temperatures utilized with reducing solutions range from room temperature to 100°C or higher with times ranging from 0.01 to 10 hours or longer. Time and temperatures are not critical and will depend on the solution being utilized. They are readily determined by routine experimentation. Without wishing to be bound to any particular theory it may well be that the activation of the catalyst in a reducing environment serves to at least partially reduce the copper from the +2 valence state to the +1 and/or 0 valence state, which is believed to contribute to the catalytic activity of the catalyst. Reducing conditions, however, should not be so severe as to reduce ZnO and Cr₂O₃. The promoter metals, i.e., the alkali metal, alkali earth metals and rare earth metals will not, even after reduction, be present on the catalyst in the form of the active free metal but will be present in oxidized form. They are believed to be present in the form of oxides or oxygen-containing compounds and may be combined with the support and/or other catalyst components. The appropriate activation conditions can readily be determined by experimentation. For example, times and temperatures can be varied and the resulting catalytic material can be examined by x-ray photoelectron spectroscopy in order to determine the activation state of copper. Alternatively, the catalyst's catalytic activity can be determined and utilized to determine optimum activating conditions. In practice, the alcohol to be converted is contacted along with water and hydrogen, all in the vapour phase with the catalyst to be used in the process according to the present invention. At least 1 mole of water per mole of alcohol is utilized. Preferably, excess water is used, e.g. a water to alcohol mole ratio of 3:1, preferably 5:1, and more preferably 10:1. Although hydrogen is generated in the reaction, the presence of an external supply of hydrogen is needed apparently to prevent the catalyst from rapidly losing activity as a function of time. At least 1 mole of hydrogen per mole of alcohol is used. Suitable limits range between 1:1 to 1:10, more preferably between 2:1 to 8:1 moles of hydrogen per mole of alcohol. The catalyst to be used in the process according to the invention can be utilized in fluidized beds or packed columns, preferably the latter. The reaction is suitably carried out at a temperature ranging from 200°C to 500°C, preferably from 250°C to 400°C. The reaction is carried out at relatively low pressures, e.g. from 0.1 to 10 bar, in particular from 0.1 to 4 bar. While the thermodynamics of the alcohol to acid reaction favours low pressures, it has been found that maximum catalyst life can be obtained when operating at pressures ranging from 2 to 5 bar. The alcohol is fed to the reactor at liquid hourly space velocities ranging from 1 to 100, preferably from 2 to 15. The reaction may be conducted batchwise or in a continuous process. The batchwise process is suitably carried out in a high pressure autoclave which is charged with alcohol and water and pressurized with hydrogen and heated to reaction temperature. After the reaction is allowed to proceed for the desired length of time, the autoclave is cooled, the excess hydrogen vented, and the products worked up by conventional methods. The continuous process is suitably carried out in a vertical, high-pressure column which is charged with catalyst; and alcohol and water are supplied at one end of the column, e.g. the top end. At the same time hydrogen is metered into the column in cocurrent flow. During the reaction, appropriate conditions of temperature and pressure are maintained. The reaction product is removed from the bottom of the column, freed from hydrogen and worked up by conventional methods. Hydrogen is advantageously recycled to the reactor. As used herein, the term "catalytically effective amount" of a catalyst component means an amount of a particular component that provides catalytic activity or a change in the catalytic properties of a catalyst. Examples of catalytic properties include, inter alia , operability (resistance to runaway), selectivity, activity, conversion yield and stability. The invention will now be illustrated by the following Examples. EXAMPLE I A catalyst (1) to be used in the process according to the present invention was prepared as follows. 0.21 Grams of lithium hydroxide promoter were dissolved in 12 ml of water and added to 20g of 14-30 mesh (corresponding particle diameters of between 0.55-1.19 mm) Kaiser KA-201 alumina. This material was heated in air in a muffle furnace for 1 hr each at 100, 200, and 300 °C. 9.5 Millilitres from a 250 ml stock solution of 0.69M cupric nitrate, 0.40M zinc nitrate, and 0.11M chromium nitrate were diluted with water to 12 ml and used to impregnate the promoter treated support. The mixture was mulled during a 30 min period to allow even penetration and then transferred to a Vycor tube. A 200 ml air stream was passed over the catalyst while the temperature was raised to 500 °C over a 4 hr period. Analysis of the dry catalyst showed the following weight %; Cu= 6.6; Zn 4.6; Cr 0.81; Li 0.17 (as metals). EXAMPLE II A process according to the present invention was carried out using catalyst (1). 15 Millilitres of the catalyst from the catalyst preparation as described in Example I were placed in a 25 mm stainless steel reactor tube that was heated with an electric furnace. Reduction of the catalyst was carried out at 1 bar, using a mixture of 200 ml/min of nitrogen and 100 ml/min hydrogen. The temperature was programmed to 100 °C in 15 min, and then to 325 °C in 3 hrs. The temperature was set at 310°C for the start of the alcohol to acid reaction. The nitrogen flow was stopped and the hydrogen flow was increased to 3.10 bar. Nonyl alcohol and water were pumped into the reactor at 15 ml/hr each. After running for 2 hrs a sample was removed for gas chromatograph analysis. The analysis gave a 71.9% conversion of the alcohol. The weight % selectivities to the various products were as follows; hydrocarbon 0.17; nonanal = 42.3; nonanoic acid = 49.5 aldol condensates 2.09; C₁₇ ketone = 0.68; and nonyl nonoate = 5.27. EXAMPLE III A series of Cu-Zn-Cr on KA-201 alumina catalysts with varying amounts of Li (added as LiOH) were prepared and tested using nonyl alcohol as the feed. Samples were analyzed after 1.2 hours at reaction temperature and the results are shown in Table I. EXAMPLE IV A series of Cu-Zn-Cr on KA-201 alumina catalysts with varying amounts of Na (added as NaOH) were prepared and tested using nonyl alcohol as the feed. Samples were analyzed after 1-2 hours at reaction temperature and the results are shown in Table II. EXAMPLE V A series of Cu-Zn-Cr on KA-201 alumina catalysts with varying amounts of Ca (added as Ca(NO₃)₂) were prepared and tested using nonyl alcohol as the feed. Samples were analyzed after 1-2 hours at temperature and the results are shown in Table III. EXAMPLE VI A series of Cu-Zn-Cr on KA-201 alumina catalysts with different promoters were prepared and tested using nonyl alcohol as the feed. Samples were analyzed after 1-2 hours at temperature and the results are shown in Table IV. EXAMPLE VII Comparative life tests were made on two Cu-Zn-Cr-Na/Ra-1 alumina catalysts. The feed in both cases consisted of 15 ml/hr of nonyl alcohol and 15 ml/hr of H₂O. For Example VII-1, hydrogen (125 ml/min) was fed continuously through the run. For Example VII-2 hydrogen (125 ml/min) was fed only during the first two hours and then cut off. The ratio of carboxylic acid to aldehyde produced was determined and reported in Table V. As can be seen from the results, hydrogen must be added to the test to maintain the long term activity of the catalyst, although there may be a short term increase in acid production due to a more favourable thermodynamic equilibrium situation. EXAMPLE VIII A catalyst was prepared in a manner similar to that described above using as a support Davison Grade 57 SiO₂ (Surface area - 300 m²/g; pore vol. - 1.0 cm³/g; density - 0.4 g/cm³). The catalyst was tested using nonyl alcohol as fed. Samples were analyzed after 1-2 hours at temperature and the results are shown in Table VI.

Claims: A process for converting alcohols to carboxylic acids which process comprises contacting in the vapour phase one or more primary alcohols of the general formula RCH₂OH where R is hydrogen, aryl having up to 30 carbon atoms or alkyl having from 1 to 30 carbon atoms, water and at least one mole of added hydrogen per mole of alcohol at a temperature ranging from 200°C to 500°C with a catalyst comprising catalytically effective amounts of copper, zinc, chromium and a promoter selected from the group consisting of alkali metals, alkaline earth metals, rare earth metals and mixtures thereof supported on a porous refractory support. A process according to claim 1, characterized in that the refractory support is alumina. A process according to claim 2, characterized in that the alumina has a surface area greater than 100 m²/g. A process according to any one of claims 1-3, characterized in that the temperature ranges from 250°C to 400°C. A process according to any one of claims 1-4, characterized in that the pressure ranges from 0.1 to 10 bar. A process according to claim 5, characterized in that the pressure ranges from 0.1 to 4 bar. A process according to any one of claims 1-6, characterized in that copper, measured as metal, ranges from 0.1 to 20 percent by weight of total catalyst, zinc, measured as metal ranges from 0.1 to 10 percent by weight of total catalyst, chromium, measured as metal, ranges from 0.01 to 10 percent by weight of total catalyst and the promoter, measured as metal, ranges from 0.001 to 10 percent by weight of total catalyst. A process according to any one of claims 1-7, characterized in that chromium, measured as metal, ranges from 0.1 to 5 percent by weight of total catalyst and the promoter, measured as metal, ranges from 0.1 to 5 percent by weight of total catalyst. A process according to any one of claims 1-8, characterized in that the promoter is selected from sodium, potassium, lithium, calcium, magnesium, lanthanum and mixtures thereof. A catalyst comprising catalytically effective amounts of copper, zinc, chromium and a promoter selected from the group consisting of alkali metals, alkaline earth metals, rare earth metals and mixtures thereof supported on a porous refractory support, characterized in that copper measured as metal, ranges from 0.1 to 20 percent by weight of total catalyst, zinc, measured as metal, ranges from 0.1 to 10 percent by weight of total catalyst, chromium, measured as metal, ranges from 0.01 to 10 percent by weight of total catalyst and the promoter, measured as metal, ranges from 0.001 to 10 percent by weight of total catalyst.

Inventors: SLAUGH, LYNN HENRY
Applicants: SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1987
Main classification: C07C53
IPC codes: C07C51, B01J23, C07B61, C07C53, C07C67, C07C45