Classifications

• • C—CHEMISTRY; METALLURGY
  • C07—ORGANIC CHEMISTRY
  • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
  • C07C51/00—Preparation of carboxylic acids or their salts, halides or anhydrides
  • C07C51/16—Preparation of carboxylic acids or their salts, halides or anhydrides by oxidation
  • C07C51/31—Preparation of carboxylic acids or their salts, halides or anhydrides by oxidation of cyclic compounds with ring-splitting
  • C07C51/313—Preparation of carboxylic acids or their salts, halides or anhydrides by oxidation of cyclic compounds with ring-splitting with molecular oxygen

Definitions

• the present invention relates to a process for the preparation of C-.-Cg aliphatic dibasic acids by oxidation of the corresponding saturated cycloaliphatic hydrocarbons in the presence of an organic acid solvent and a metal catalyst and subsequent recycling of the partly oxidized intermediates into the oxidation mixture for further oxidation.
• Adipic acid is a major article of commerce and its preparation has, therefore, attracted much attention. Consequently, many processes for the production of adipic acid have been proposed. For example, one process involves nitric acid oxidation of cyclohexanol, cyclohexanone, or mixtures thereof, which can in turn be obtained by air oxidation of cyclohexane or hydrogenation of phenol.
• Processes that have been proposed for preparing dibasic acids without the use of nitric acid include air oxidation of saturated cyclic hydrocarbons and/or corresponding cyclic ketones and/or alcohols.
• U.S. Patent 3,390,174 and British Patent 1,304,855 disclose processes requiring mixtures of two or more of these components.
• Catalytic air oxidation processes are believed to involve free radical oxidation.
• Such oxidations are complex systems in which many types of reactions other than oxidation can occur. Free radicals will attack any C-H bonds, regardless 5 of form, to an extent determined by bond strength and relative concentration of the specific C-H bond.
• various oxygenated compounds form, such as alcohols, aldehydes, ketones, and acids (including difunctional compounds having these functionalities), as well as other low molecular weight
• the aromatic acid products are extremely stable to further oxidation and can be subjected to extreme conditions under which a substantial amount of even seemingly
• inert acetic acid would be oxidized to C0 2 and water. Consequently, these aromatic acid products can be produced substantially free of oxidation bottoms, intermediates, derivatives, and the like at very high conversions of 95% or higher.
• Aliphatic diacids such as adipic acid, on the other hand, are subject to further oxidation because the C-H bonds of the methylene groups in such acids can more readily undergo free radical attack and oxidation. If subjected to forcing oxidation conditions at higher conversion, the various bottoms, intermediates, and derivatives will oxidize (as do similar aromatic compounds).
• This oxidation was carried out using a comparatively high concentration of cyclohexane (about 61 to 63 wt.%) in acetic acid solvent in the presence of air and various catalysts at temperatures of from 95 ⁇ C to 120 ⁇ C until a conversion level of about 23 to 24% was achieved.
• U.S. Patent 2,589,648 discloses a single-step oxidation process in which acetone is used instead of acetic acid as solvent.
• U.S. Patent 3,231,608 discloses another single-step direct oxidation process for the production of dibasic aliphatic acids.
• the reference teaches that certain critical ratios of solvent and catalyst to the saturated cyclic hydrocarbon can yield dibasic aliphatic acids under mild reaction conditions, usually at production rates of adipic acid of 3.5 to 4.0 wt.% per hour and at efficiencies generally around 73 to 76 wt.%.
• U.S. Patent 4,158,739 discloses a similar preparation of glutaric acid from cyclopentane in which the molar ratio of solvent to cyclopentane must be at least 1.5:1 and the amount of catalyst is relatively higher than for the process disclosed in U.S. Patent 3,231,608.
• British Patent 1,304,855 discloses the 0 direct oxidation of cyclohexanol, cyclohexanone, and cyclohexane, which is in a sense somewhat similar to oxidizing cyclohexane in the presence of recycled cyclohexanol and cyclohexanone.
• the references disclose a selectivity to adipic acid of only 54 mole% based on all oxidized cyclic 5 six carbon compounds.
• U.S. Patent 3,390,174 discloses the oxidation of saturated C 5 -C Intel cyclic hydrocarbons in the presence of the equil brium concentrations of the corresponding cyclic alcohols and ketones.
• Q the reference indicates that oxidation equilibrium levels are about 14 to 24% cyclohexanol and about 30 to 40% cyclohexanone, based on the amount of cyclohexane.
• the reported . selectivities of cyclohexane to adipic acid are again only about 45 to 52 mole%.
• Cyclohexanol and cyclohexanone are more readily oxidized to produce lower oxidation equilibrium levels than are cyclohexanol esters, which are among the primary constituents of the bottoms.
• an alcohol is oxidized four to five times faster on a relative molar rate basis than an ester of the same alcohol, resulting in a molar concentration of esters at oxidation equilibrium that is four to five times higher than the equilibrium level of the corresponding alcohol.
• the buildup of esters in the bottoms would be expected to adversely affect the efficiency of adipic acid production.
• the present invention relates to a process for the oxidative preparation of g-Cg aliphatic dibasic acids comprising (1) reacting
• reaction step (1) at least about 0.002 mole (preferably 0.015 to about 0.3 mole) per 1000 grams of of reaction mixture of a polyvalent heavy metal catalyst (preferably a cobalt salt of an organic acid); (2) removing the aliphatic dibasic acid (preferably as a precipitate collected by filtration or centrifugation); and (3) recycling intermediates, post-oxidation components, and derivatives thereof remaining after removal of the aliphatic dibasic acid (optionally in the presence of residual solvent, optionally after hydrolysis and isolation of additional aliphatic dibasic acid, and optionally after addition of further quantities of saturated cycloaliphatic hydrocarbon (a)) into reaction step (1) for reaction with oxygen gas or an oxygen- containing gas mixture in the presence of solvent (c) and catalyst (d) and subsequent isolation of aliphatic dibasic acid and, optionally, further recycling.
• a polyvalent heavy metal catalyst preferably a cobalt salt of an organic acid
• the process of this invention involves a single-stage oxidation that subjects a saturated cycloalkane and recycled bottoms to mild oxidation conditions, one of the import features being the intentional limitation of the extent of conversion of the cycloalkane.
• the primary product is adipic acid.
• Suitable cyclic hydrocarbons for use in the process of the invention include saturated cyclic hydrocarbons having from 5 to 8 ring carbon atoms and containing only primary and secondary hydrogen atoms.
• suitable saturated cyclic hydrocarbons include cyclopentane, cyclohexane, cyclo- heptane, cyclooctane, or hydrocarbon analogs or homologs thereof that contain only primary and secondary hydrogen atoms.
• a particularly preferred cyclic hydrocarbon is cyclohexane, which is a readily available commercial product.
• the cyclic hydrocarbon component may also contain any substantially inert diluent, such as another hydrocarbon containing only primary and secondary hydrogens. In general, however, such diluents are preferably removed, especially if present in larger amounts, because they necessarily occupy reactor space.
• a particularly beneficial inert diluent is benzene, the ultimate source of commercially produced cyclohexane. The presence of benzene can be economically advantageous for the production of adipic acid by enhancing the rate of oxidation and by allowing the use of lower grade cyclohexane from which benzene need not be removed.
• the cyclohexane when preparing adipic acid from cyclohexane according to the invention, can contain substantial amounts of benzene without detrimental effect on the oxidation process.
• benzene replaces a portion of solvent (c) (e.g., acetic acid)
• solvent (c) e.g., acetic acid
• the rate of oxidation may even be improved.
• the presence of up to about 60% by weight of benzene relative to cyclohexane can increase the oxidation rate.
• cyclohexane is produced commercially by hydrogenation of benzene.
• the conditions for hydrogenation are generally harsh, so as to optimize conversion and to minimize the cost associated with the difficult separation of benzene from the cyclohexane product.
• the use of cyclohexane containing 0.1 wt.% (or less) up to 5 or 10 wt.% of benzene can reduce raw material costs. Consequently, the more favorable oxidation kinetics and less demanding distillation requirements would permit substantially greater production of cyclohexane from an existing cyclohexane facility.
• the oxidant used in the process of the invention can be essentially any gas containing free molecular oxygen and optional substantially inert gaseous diluents.
• suitable oxygen-containing gases are air and oxygen-enriched air (tha is, air having an augmented oxygen content of, for example, 85 mole percent or more), as well as pure gaseous oxygen.
• tha is, air having an augmented oxygen content of, for example, 85 mole percent or more
• pure gaseous oxygen A mixture of 50 wt.% oxygen and 50 wt.% nitrogen, for example, provides atisfactory results. Even oxygen-depleted air can be used but is less preferred.
• Any gases that are substantially inert under the reaction conditions are suitable gaseous diluents.
• gaseous diluents examples include nitrogen, carbon dioxide, helium, neon and argon, as well as the normally gaseous paraffin hydrocarbons (such as methane, ethane, and propane). Mixtures of diluents can, of course, also be used.
• the molar quantity of oxygen should be at least sufficient to provide complete oxidation of the cycloaliphatic hydrocarbon to the corresponding dibasic acid (that is, at least 2.5 moles of 0 2 for each mole of cycloaliphatic hydrocarbon).
• the partial pressure of oxygen over the reaction mixture should be at least 0.10 atmosphere absolute and can be as high as 100 atmosphere absolute, or even higher.
• the preferred partial pressure of oxygen over the reaction mixture should be from at least about 0.10 to about 0.30 atmospheres absolute.
• the total pressure should be at least sufficient to keep the reactants in the liquid phase.
• the total reaction pressure that is employed will depend to a large extent on the particular oxygen-containing gas which is used, the composition of the reacting mixture, and temperature, which together determine the vapor pressure of the liquid reacting mixture.
• Suitable solvents for use as component (c) of the invention can be essentially any weak organic acids that contain only primary and/or secondary hydrogen atoms (that is, organic acids in which hydrogen atoms other than those of C00H groups are attached only to primary and/or secondary carbon atoms).
• Preferred solvents include lower aliphatic mono- carboxylic acids having 2 to about 6 carbon atoms, more preferably acetic acid.
• Mixed solvents including mixtures of solvents with inert diluents such as benzene) can, of course also be used.
• the amount of solvent used is selected so that from about 0.15 to about 15 moles (preferably about 0.25 to about 3 moles and more preferably 0.25 to 1.5 moles) of solvent are present in the oxidation zone for each mole of the cyclo ⁇ aliphatic hydrocarbon.
• this molar ratio range corresponds to a concentration of about 8.5 wt.% up to about 91 wt.% cyclohexane and about 91.5 wt.% down to about 9 wt.% acetic acid, with the preferred relative quantities corresponding approximately to 30-85 wt.% cyclohexane and 15-70 wt.% acetic acid and more preferably 50-85 wt.% cyclohexane and 15-50 wt.% acetic acid.
• Suitable oxidation catalysts are conventional in nature and include polyvalent heavy metal catalysts, especially those having atomic numbers from 23 to 29, as well as cerium. Particularly preferred catalysts are those containing cobalt, manganese, vanadium, and cerium and combinations thereof.
• These heavy metal catalysts are supplied to the oxidation zones in the form of compounds that are soluble or will become at least partially solubilized under the conditions of the oxidation reaction.
• Suitable such compounds include the oxides, hydroxides, and, preferably, the inorganic and organic salts of the metals. It is particularly preferred to use the catalyst metals as their acetates, naphthenates, and toluates, as well as various fatty acid salts, such as stearates, oleates, and the like.
• the preferred catalysts include essentially any cobalt salt of an organic acid. Examples of suitable such catalysts include cobalt acetate, cobalt propionate, and cobalt naphthenate. Materials which form such cobalt salts in situ can also be employed.
• cobalt oxide and acetic acid are suitable because they will form cobalt acetate in situ. It is particularly preferred for the cobalt salt to correspond to the salt of the acid which is used as the reaction solvent. Because acetic acid is the preferred solvent, cobalt(II) acetate is the most preferred catalyst.
• concentration of catalyst required within the oxidation zone is somewhat dependent on the desired rate of oxidation. Accordingly, preferred amounts of catalyst vary from about 0.005 to about 0.6 mole per 1000 grams of reaction mixture (more preferably between about 0.015 and about 0.3 mole per 1000 grams and most preferably between about 0.03 and about 0.20 mole per 1000 grams).
• substantially greater amounts of catalyst for example, up to 1 mole per-1000 grams or even more
• the use of such large quantities provides little advantage and, in fact, the use of such large quantities can cause problems in recovery of catalyst for recycle and re-use.
• extraneous promoters such as, for example, acetaldehyde, methyl ethyl ketone, cyclohexanol, cyclohexanone, and the like.
• conversion refers to the ratio (usually expressed as a percentage) of the quantity of cycloalkane that reacts in any manner to the quantity of cycloalkane in the feedstock.
• conversion refers to the total moles of all dibasic acids produced (for example, the sum of adipic acid, glutaric acid, and succinic acid prepared when oxidizing cyclohexane) and the various by-products to the moles of the cycloalkane in the feedstock.
• the non-polar upper phase can be directly recycled for oxidation without costly processing (although it is, of course, possible to concentrate or otherwise process the non-polar phase before recycling for oxidation).
• the polar lower phase can be extremely rich in adipic acid that can be recovered in high yield by filtration or centrifugation, with the filtrate or supernatant, respec ⁇ tively, being to a large degree directly returned to oxidation without costly reprocessing.
• the bottoms contained in the small portion of filtrate that is further processed for recovery of succinic and glutaric acid can be subjected to hydrolysis (preferably thermal hydrolysis) to achieve further removal of dibasic acid and provide a recycle stream for oxidation that is richer in the more easily oxidizable alcohols.
• the reaction conversion will increase with reaction time to yield a higher concentration of dibasic acids.
• the selectivity to adipic acid diminishes as the conversion of cyclohexane is increased.
• the dibasic acid products are subject to further oxidation and thus a loss of selectivity. Therefore, it is preferable for the average concentration of the dibasic acids to be kept at the lowest practical level consistent with recovery costs.
• oxidizing cyclohexane for example, it is preferable to limit the conversion per pass of cyclohexane to no more than about 75%. Although it is possible to achieve higher conversions of cyclohexane, adipic acid selectivities typically deteriorate. In general, optimal production of adipic acid is
• reaction temperatures for the oxidation can vary • from about 60° to about 175"C, with preferred temperatures
• total pressure for the oxidation is generally not critical so long as it is sufficient to maintain a liquid
• the oxidation equipment normally dictate the use of pressures of from about 1 to about 100 atmospheres absolute (preferably from 1 to 70 atmospheres absolute and more preferably from 5 to 40 atmospheres absolute).
• the partial pressure of oxygen (measured in the vent gases from the oxidation zone) should be at least about 0.10 atmospheres absolute and preferably at least 0.3 atmospheres absolute. Maximum oxygen partial pressure is dictated almost entirely by flammability considerations.
• the oxidation mixture is preferably well agitated to insure better contact of the reactants. Agitation can be provided by mechanical stirring devices optionally aided by the ebullition caused by the introduction of the oxygen-containing gas into the liquid reaction mixture.
• the reaction time for the oxidation can vary from about ten minutes to about six hours (or even more), with preferred reaction times being about 0.25 to two hours.
• a key feature of the present invention is the unexpectedly advantageous ability to recycle the intermediates and derivatives back into the oxidation reaction, despite the multitude of bottoms components.
• the process complexities connected with the presence of bottoms are related to such factors as the rate of bottoms formation during oxidation and post-oxidation, the equilibrium rate under the specific oxidation conditions, and the existence of many hundreds of compounds.
• the difficulties of dealing with the bottoms can be further complicated by a low rate of dibasic acid formation and by catalyst considerations.
• the second group of compounds which includes the dibasic acid products and their derivatives, are, however, subject to post-oxidative attack that leads to loss of the desired product.
• the bottoms may contain a myriad of components that serve as intermediates because they can be further transformed to adipic acid. Some components of the bottoms can be considered both intermediates and products.
• Another, generally less significant group of bottoms are post-oxidation components and derivatives that are formed by oxidation of the dibasic acids. This group of post-oxidation components can be transformed to less desirable (but still valuable) glutaric and succinic acids. In general, it is highly desirable to recycle intermediate components and even post-oxidation components but undesirable to recycle derivatized product.
• Esters are typically the major component of the bottoms.
• the multitude of esters includes virtually all possible alcohol and acid functions (including difunctional and trifunctional compounds) derived from cyclohexanols, cyclic diols, cyclic hydroxy ketones, hydroxy acids, hydroxy diacids, acetic acid, and various diacids, as well as other linear multifunctional compounds.
• the esters of greatest prominence will be those in which the alcohol and acid moieties are present in the reaction environment in the greatest concentrations, whereas the esters of least prominence will be those in which the alcohol and acid moieties are present in the reaction environments in lowest concentrations.
• the amount of esters in the bottoms is a function not only of factors directly related to the oxidation procedure but also of factors related to the workup procedure. For example, when time, temperature, and concentration effects are greater after the oxidation than during oxidation, a greater amount of ester can be formed during workup.
• esters of the bottoms because of their differing component parts, will have different effects on the overall oxidation process, some positive and some negative. Those esters having component parts that can be oxidized to form the desired products can be considered intermediates, whereas, other esters, such as those derived from the desired final dibasic acid products, are subject to post-oxidative attack that can lead to loss of the desired product.
• the removal of the aliphatic dibasic acid from the reaction mixture in a manner consistent with recycling can be carried out by a variety of separation and isolation methods known in the art.
• the dibasic acid is removed by (i) cooling the reaction mixture to induce precipitation of the aliphatic dibasic acid and separation of the liquid portion of the reaction mixture into a polar liquid phase and a non-polar liquid phase and (ii) separating the resultant precipitated aliphatic dibasic acid, polar liquid phase, and non-polar liquid phase from one another (preferably by phase separation followed by filtration or centrifugation or by filtration or centrifugation followed by phase separation).
• Recycling is then carried out by (i) transferring the non-polar liquid phase (which may contain residual solvent, as well as certain other constituents having a degree of solubility or miscibility in the non-polar phase), optionally, after addition of further quantities of saturated cycloaliphatic hydrocarbon (a), into reaction step (1) for further oxidation and isolation of aliphatic dibasic acid and, optionally, further recycling and (ii) transferring at least a portion of the polar liquid phase, optionally after hydrolysis and isolation of additional aliphatic dibasic acid and, optionally, after addition of further quantities of saturated cycloaliphatic hydrocarbon (a), into reaction step (1) for further oxidation and isolation of aliphatic dibasic acid and, optionally, further recycling.
• the polar phase With respect to recycling the polar phase, it is in general suitable to transfer about 10 to about 98% by weight (preferably 50 to 95% by weight and more preferably 60 to 90% by weight) of the polar phase, optionally, after the inter ⁇ mediate treatments mentioned above, into reaction step (1).
• the portion of the polar lower phase mother liquor that is not recycled directly back to oxidation (that is, the portion from which glutaric and succinic acids must be removed) is subjected to hydrolysis and recovery of free diacid so that the recycled stream has a higher relative amount of the more easily oxidizable alcohol function and a lower relative amount of the diacid.
• This hydrolysis step can provide effective control of bottoms at equilibrium during oxidation and facilitate adipic acid purification.
• the water produced during the oxidation reaction, as well as any water added during he optional hydrolysis step, can be removed at any of several points in the overall process of the invention.
• water can be removed by methods 5 known in the art from the reaction mixture during the reaction or from the polar phase (preferably by distillation) before the polar phase is recycled.
• the recycling steps of the invention may, of course, be carried out batchwise (as illustrated in the examples) or 0 continuously using methods known in the art. Whatever the specific recycling methods employed, however, the molar ratio of solvent (c) to cycloalkane (a) should be maintained within the specified range.
• Reaction Rate The weight percent (based on the total liquid- phase reaction medium) of adipic acid produced per hour (based on an extrapolation of the actual amount produced in the time described in each example).
• Selectivity to Adipic Acid The ratio (expressed as a percentage) of the moles of adipic acid formed to the number of moles of cyclohexane reacted and which cannot be recovered or recycled. This term is indicative of ultimate yield when non-product can be recovered and recycled.
• Selectivity to Dibasic Acids The ratio (expressed as a percentage) of the moles of all di sic acid formed to the number of moles of cyclohexane reached and which cannot be recovered or recycled. This term is indicative of ultimate yield.
• the following gas-liquid chromatography (GLC) method was used for analytical determinations.
• the GLC analyses were carried out using columns having a diameter of 0.125 inch (ca. 3.2 mm) and varying lengths were packed with PEG 20M (that is, polyethylene glycol having a molecular weight of about 20,000) as the liquid substrate and an 80-100 mesh CHR0M0S0RB W support (available from Supelco Inc.; CHR0M0S0RB is a trademark of Johns-Manville Corporation) and fitted with a thermal conduc ⁇ tivity detector. Samples were prepared for GLC analysis by converting the carboxylic acids to methyl esters.
• the methyl esters were prepared by mixing the sample with a fixed amount of excess methanol and about 5-10 wt.% of predried AMBERLYST 15 sulfonic acid resin (available from Rohm and Haas Company) and then heating over a steam bath for about one hour.
• Analytical standards for adipic acid, glutaric acid, and succinic acid, acetic acid were prepared by the same technique using mixtures of known quantities of the pure compounds in ratios approxi ⁇ mating those of the test samples.
• the standard was prepared using a mixture of acetic acid, water, adipic acid, glutaric acid, and succinic acid in a weight ratio of 5:1.25:1:0.1:0.05 and six parts of methanol for each part of the mixture.
• impurities for example, the amounts of glutaric and succinic acids in adipic acid isolates
• concentrated bottoms the samples were prepared using 12 parts of methanol for each part sample.
• the standard for concentrated bottoms (which contains little if any cyclohexane, water, and acetic acid) can be prepared, for example, using a mixture of adipic acid, glutaric acid, and succinic acid in a weight ratio of 1:1:0.5 and 12 parts of methanol for each part of the mixture.
• adipic acid glutaric acid
• succinic acid in a weight ratio of 1:1:0.5 and 12 parts of methanol for each part of the mixture.
• Oxidations are characterized by a typical "S curve" in graphs having oxidation rate plotted along the ordinate (i.e., the y axis) and time plotted along the abscissa (i.e., the x axis).
• the lower portion of the S curve is a combination of chemical and physical characteristics, being both a combination of a chemical induction period and heatup to reaction temperature.
• the sloping portion of the S curve is usually nearly constant for the major portion of the oxidation and serves as a good indicator of the rate of oxidation.
• the slope of the curve bends downward to become the top of the S curve only with the occurrence of a substantial reduction in the reactants and/or other oxidation limiting factors (such as phase separation), with an attendant drop in oxidation rate.
• the starting time in the oxidations described in the examples is taken as the intercept of the slope of the S curve with the x-time axis. Furthermore, it is preferred to quench the oxidation before the top of the S curve is reached. In the zone where phase separation is beginning to occur, a disproportionate amount of oxidation of the catalyst-rich polar phase is believed to occur. This phase is rich in adipic acid and low in cyclohexane, conditions conducive to post-oxidation of adipic acid and deterioration in selectivity. Furthermore, this phase of the oxidation is a controllable feature of the reaction through removal of some water of reaction.
• reaction mixture was formed by dissolving the catalyst in the solvent, adding the charge stock (which was cyclohexane in all of the examples), and an initiator to a 500-ml 316 stainless steel reactor equipped with a rotating magnetic agitator. The reactor was then sealed and pressurized to 14 atmospheres absolute with a mixture of 50% oxygen gas and 50% nitrogen gas. The reaction mixtures were heated to the reaction temperature, and the progress of oxidation was measured by the pressure drop due to oxygen consumption. When approximately 70-80% of the oxygen was consumed, the system was repressurized with oxygen to the starting pressure. At the desired conversion level, the reaction was quickly quenched by cooling by about 20-30'C and then worked up.
• reaction effluent was cooled to induce crystallization
• the resultant three-phase effluent was filtered to recover adipic acid.
• the crystalline adipic acid was washed with cold acetic acid-cyclohexane and dried.
• the two-phase mother liquor and washes were handled in a manner designed to simulate a continuous operation, with all of the non-polar upper phase and most of the polar lower phase being directly recycled and not subjected to conditions that would form additional bottoms (i.e., reaction residue).
• the simulation was achieved by removing the water of reaction, as well as additional adipic acid, using conditions under which minimal ester formation occurs, that is, by removing cyclohexane, water, and most of the acetic acid at temperatures below 60 ⁇ C (usually 50 C C) under vacuum.
• additional adipic acid was recovered from the resultant concentrate and washed with cold cyclohexane-acetic acid.
• the mother liquor which contained bottoms, unrecovered adipic acid, and glutaric and succinic acids, was intensely dark green, a color indicative of cobalt(III) ion, the active free-radical-generating form of the catalyst.
• the mother liquor was analyzed for bottoms content and then reconstituted with additional cyclohexane, acetic acid, and a small amount of catalyst.
• the cyclohexane content was brought to the initial concentration; the additional acetic acid equaled the original starting quantity less the amount of recycled bottoms and unrecovered dibasic acids; and the additional catalyst replaced the amount removed during sampling.
• the reconstituted reaction mixture containing recycle bottoms was oxidized under * same conditions as used in the initial reaction but was usually stopped after about 80% oxygen uptake. After quenching the oxidation by cooling the reaction mixture by about 20-30 * C, the reaction mixture was manipulated in the same way as the first sequence described above for recovery of adipic acid and preparation for additional recycle.
• a second experimental setup used a one-liter glass reactor equipped with a turbine stirrer.
• a F00 g liquid charge (a volume of about 600 to 620 ml) at room temperature was used.
• Oxygen was used as the oxidant at a partial pressure of 5 to 15 psia, using a minimal flow of excess oxygen.
• the workup and reconstitution of each subsequent oxidation charge was identical to the smaller 75 g scale recycle runs described above.
• Example 1 To the 500-ml 316 stainless steel reactor was added
• the reaction mixture was worked up as described above to recover adipic acid and to prepare the bottoms (which contained catalyst) for recycling.
• the accumulating bottoms and catalyst were recycled three times for a total of four runs with the following results.
• the average rate of adipic acid production for the four runs was 12.8 wt%/hr based on the amount of the reaction mixture.
• the average conversion of cyclohexane was 21.2%.
• the selectivity to adipic acid was 87.2 mole%, to glutaric acid was 6.7 mole%, and to succinic acid was 3.7 mole%.
• adipic acid About 69% of the total adipic acid was recovered by direct filtration of the cooled oxidation effluent. Based on equivalent processing of recycling all of the upper phase and 75% of the lower phase, the amount of bottoms at oxidation equilibrium was 4.3 wt% based on cyclohexane. The adipic acid that could be recycled was about 24.1% of the adipic acid made or about 7.7 wt% based on cyclohexane.
• Example 1 when compared with the previously known methods, provides a much higher oxidation rate, significantly higher adipic acid selectivity, and greater ease of adipic acid recovery.
• Exampl e 2 when compared with the previously known methods, provides a much higher oxidation rate, significantly higher adipic acid selectivity, and greater ease of adipic acid recovery.
• the rate of adipic acid formation dropped by about
• the average rate of adipic acid production for the four runs was 4.2 wt%/hr based on the amount of the reaction mixture.
• the average conversion of cyclohexane was 34.5%.
• the selectivity to adipic acid was 82.3 mole%.
• About 21% of the total adipic acid was recovered by direct filtration of the cooled oxidation effluent. Based on equivalent processing of recycling all of the upper phase and 75% of the lower phase, the amount of bottoms at oxidation equilibrium was 22.3 wt% based on cyclohexane.
• the adipic acid that could be recycled was about 59.3% of the adipic acid made or about 29.3 wt% based on cyclohexane. Over the course of the four runs, the rate of adipic acid formation dropped by about 33%.
• the reaction mixture was worked up as described above to recover adipic acid and to prepare the bottoms (which contained catalyst) for recycling.
• the accumulating bottoms and catalyst were recycled three times for a total of four runs with the following results.
• the average rate of adipic acid production for the four runs was 3.0 wt%/hr based on the amount of the reaction mixture.
• the average conversion of cyclohexane was 36.9%.
• the selectivity to adipic acid was 83.1 mole%, to glutaric acid was 9.1 mole%, and to succinic acid was 4.6 mole%.
• Examples 2 to 4 when compared with Example 1, show that a substantially lower cyclohexane concentration in the oxidation step, although giving much lower reaction rates (approximating those of the better published methods), generally leads to higher selectivity and better direct recovery of adipic acid.
• the low cyclohexane concentration used in Example 4 is not conducive to precipitation of adipic acid.
• the lower conversion conditions used in Examples 3 and 4 provide greater selectivity than the higher conversion conditions of Example 2 but with a reduced direct recovery of adipic acid.
• Example 5
• the reaction mixture was worked up as described above to recover adipic acid and to prepare the bottoms (which contained catalyst) for recycling.
• the accumulating bottoms and catalyst were recycled three times for a total of four runs with the following results.
• the average rate of adipic acid production for the four runs was 4.5 wt%/hr based on the amount of the reaction mixture.
• the average conversion of cyclohexane was 23.5%.
• the selectivity to adipic acid was 74.2 mole%. About 61% of the total adipic acid was recovered by direct filtration of the cooled oxidation effluent.
• the amount of bottoms at oxidation equilibrium was 29.0 wt% based on cyclohexane.
• the adipic acid that could be recycled was about 29.3% of the adipic acid made or about 8.8 wt% based on cyclohexane.
• the rate of adipic acid formation dropped by about 38%.
• Examples 5 and 6 provide improved adipic acid recovery and excellent selectivity and show the advantages of high cyclohexane concentration and higher temperatures on rate even when using lower catalyst concentrations.
• Example 7 shows that good results can be obtained even when using very low catalyst concentrations.

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