Cross-Reference to Related Application

This application claims the benefit of U.S. Provisional Application No. 62/442011, filed on 4 January 2017, which is incorporated herein by reference.

Field of the Invention

The invention relates to a process for the preparation of adipic acid.

Background of the Invention

In recent years there has been an increasing focus on how to decompose nitrous oxide (N ₂ 0), as it is an atmospheric ozone depletion gas (greenhouse gas) approximately 300 times more potent than CO2 on a per-mass basis. Nitrous oxide will be formed during the catalytic oxidation of ammonia in connection with nitric acid production and during oxidation of alcohols and ketones, for instance, in connection with adipic acid production.

Nitrous oxide is a powerful oxidizing agent that is kinetically stable under atmospheric conditions. Being 36% active oxygen by mass, nitrous oxide is one of the most atom economical and inexpensive oxidants, comparing favorably even to hydrogen peroxide. Moreover, the transfer of the active oxygen atom to substrate gives harmless nitrogen gas as the sole by-product. In contrast to oxygen gas, N2O oxidation pathways are non-radical, which can lead to higher product selectivity, fewer side products, and reduced risk of thermal runaway. Nitrous oxide is also an industrial waste product; e.g., in adipic acid manufacture, one mole of N2O by-product is produced for every mole of desired adipic acid. To prevent atmosphere emission, thermal or catalytic destruction of this potentially useful material is often carried out. Constructive utilization of N2O waste streams has potential for positive environmental and economic impact.

US 2844626 describes a process for the manufacture of adipic acid by the oxidation of cyclohexanol, cyclohexanone or a mixture of the two with nitric acid. The reaction is an exothermic reaction that produces adipic acid, N2O and small amounts of other nitric gases, mainly NO and NO2. These gaseous NOx species are converted in a subsequent step to nitric acid; however, the N2O is lost as by-product waste. The reaction mixture is cooled and the adipic acid crystallizes out.

US 6900358 describes a process for hydroxylating benzene under catalytic distillation conditions to produce hydroxylated products such as phenol. The process includes direct hydroxylation of liquid phase benzene with an oxidant and a zeolite catalyst under conditions effective to prevent coke formation on the catalyst.

Summary of the Invention

The invention provides a process comprising: a) contacting an aromatic compound comprising an aromatic ring and a substitutable hydrogen atom on said aromatic ring with an oxidation catalyst and an oxidant under conditions effective to hydroxylate the aromatic compound to produce a hydroxylated product and an unreacted aromatic compound, while maintaining at least a portion of the aromatic compound in a liquid phase; b) separating the hydroxylated product from the unreacted aromatic compound; c) hydrogenating the hydroxylated product to produce either cyclohexanol, cyclohexanone or a mixture thereof; and d) contacting at least a portion of the cyclohexanol, cyclohexanone or mixture thereof with nitric acid and a conversion catalyst to produce adipic acid and nitrous oxide.

Detailed Description of the Invention

The invention provides an improved process for producing adipic acid by recycling the N2O by-product waste stream to the front end of the adipic acid production process where it is used as a selective oxidant. Recycle of the N2O by-product waste stream as a process reactant improves adipic acid process economics and reduces the environmental impact of the process as nitrogen, N ₂ , is the only reaction by-product.

The first step of the process comprises contacting an aromatic compound with an oxidation catalyst and an oxidant to hydroxylate the aromatic compound to produce a hydroxylated product. The aromatic compound may be any aromatic compound with a substitutable hydrogen atom. For example, the aromatic compound may comprise benzene, fluorobenzene, chlorobenzene, toluene, ethylbenzene, and similar compounds. The aromatic compound is preferably benzene as the hydroxylation product of benzene is phenol.

Any suitable oxidant may be used, including nitrous oxide, oxygen, air and mixtures thereof. Nitrous oxide is a preferred oxidant. The molar ratio of the oxidant to the aromatic compound is at least about 1:100; preferably 1:10, more preferably 1:3 and most preferably 1:1. In a preferred embodiment, the oxidant to aromatic compound ratio is the stoichiometric ratio that will yield the desired product.

Suitable oxidation catalysts are any that will catalyze the hydroxylation of an aromatic compound in the presence of an oxidant. These catalysts include molecular sieves, including zeolites and non- zeolite materials. The zeolite catalyst may have any of the following structures: MFI, MEL, FER, FAU, BEA, MFS, NES, MOR, CHA, MTT, MWW, EUO, OFF, MTW, ITQ-1, ITQ-2, MCM-56, MCM-49, ZSM-48, SSZ-35, or SSZ- 39. Preferred zeolite catalysts are modified zeolites, preferably of the MFI structural type, most preferably ZSM-5, ZSM-11 or beta zeolite. The zeolite catalyst preferably comprises one or more metals selected from the group consisting of ruthenium, rhodium, iron, magnesium, cobalt, copper, titanium, rhenium and iridium. The metal may be present in an amount of from 0.01 to 1.5 wt%, preferably in an amount of from 0.1 to 0.5 wt%.

Preferred non- zeolite molecular sieves are aluminum phosphates (AlPO's) or silica aluminum phosphates (SAPO's). These non-zeolites preferably comprise metals, for example, cobalt, vanadium, manganese, magnesium or iron. The catalyst may consist of AlPO's or SAPO's with a fraction of the aluminum or phosphate ions being replaced during synthesis by a transition metal ion. In another embodiment, the transition metal may be incorporated into the framework of the catalyst after synthesis using known means including ion exchange, impregnation, co-mulling, and physical admixing. Another suitable type of non- zeolite catalyst includes vanadium-peroxide complexes formed by using hydroquinones to produce peroxide species which are transferred to the vanadium complexes.

The reaction may be carried out under catalytic distillation conditions. A portion of the aromatic compound is maintained in a liquid phase and the reaction is carried out to manage the heat generated by the exothermic hydroxylation reaction. The reflux of the unreacted aromatic compound makes the reaction substantially isothermal.

The catalytic distillation reactor preferably provides both catalytic zones and distillation zones. The catalytic zone is defined as the portion of the reactor containing the catalyst where the oxidant and aromatic compound react to form hydroxylated product. The distillation zone, also called the fractionation zone is defined as the portion of the reactor adapted to separate the hydroxylated product from the unreacted aromatic compound. The distillation zone is a conventional fractionation column design, preferably integral with and downstream of the reaction zone. In another embodiment, the distillation zone may be a separate column. The catalytic distillation reactor can be configured as an up flow reactor, a down flow reactor or a horizontal flow reactor.

In a preferred embodiment, the catalytic zone and the distillation zone are in a single column. The catalytic zone contains an amount of catalyst and the distillation zone contains a number of conventional separation trays. The aromatic compound preferably is delivered to the column above the catalyst and the oxidant is fed to the column below the catalyst. Any unreacted aromatic compound is either withdrawn from the column once it leaves the catalytic zone, preferably as a vapor, and supplied as makeup or allowed to reflux. The overhead is withdrawn from the column above the catalytic zone and typically will contain a mixture of oxidant and a small amount of aromatic compound. The oxidant is preferably separated from the aromatic compound by conventional means and recycled as makeup.

During the catalytic distillation, the hydroxylation reaction occurs simultaneously with the distillation, and the hydroxylated product is removed from the catalytic zone as it is formed. Removal of the hydroxylated product minimizes side reactions and

decomposition of the hydroxylated product. The temperature and pressure of the distillation zone of the reactor is controlled to keep any unreacted aromatic compound that travels from the catalytic zone to the distillation zone in the vapor phase, preferably at or above the boiling point of the aromatic compound at the given pressure. The catalytic zone is maintained at a temperature that is below the boiling point of the hydroxylated product. The unreacted aromatic compound eventually reaches a point in the reactor where it boils, and, as a result, the temperature of the reactor is controlled by the boiling point of the aromatic compound at the system pressure. The exothermic heat of the hydroxylation reaction will vaporize a portion of the unreacted liquid aromatic compound but will not increase the temperature in the reactor. The hydroxylation reaction has an increased driving force because the hydroxylated product is removed and cannot contribute to a reverse reaction.

In one embodiment, an aromatic compound is hydroxylated using catalytic distillation to form a hydroxylated product having a higher boiling point than the aromatic compound. The hydroxylation reaction is catalyzed by an oxidation catalyst in the presence of an oxidant in a catalytic distillation reactor at conditions that also result in fractional distillation.

The oxidant preferably remains in a gaseous state and unreacted oxidant is withdrawn as overhead. The unreacted aromatic compound may be allowed to reflux or it may be withdrawn from the distillation zone and added to the original aromatic compound feed as makeup.

In the catalytic distillation reactor, there is a liquid phase, or internal reflux, and a vapor phase. The liquid phase is denser than the gas phase and it allows for a more dense concentration of molecules for reaction over the catalyst. The fractionation or distillation separates the hydroxylated product from unreacted materials, providing the benefits of a combined liquid phase and vapor phase system while avoiding continual contact between the catalyst, the reactants and the products

In a preferred embodiment, the aromatic compound is benzene. The catalytic distillation is carried out in a catalytic distillation reactor at a temperature and pressure effective to hydroxylate the benzene while fractionating or removing the hydroxylated product, phenol, from the oxidant and unreacted benzene. The temperature in the distillation zone of the reactor is higher than the temperature in the catalytic zone of the reactor, creating a temperature gradient within the reactor of from about 50 °C to about 400 °C, preferably from about 80 °C to about 300 °C such that the lower boiling components are vaporized and migrate toward the upper portion of the reactor while the higher boiling components migrate toward the lower portion of the reactor. The temperature in the lower portion of the reactor preferably is higher than the boiling point of benzene but lower than the boiling point of the phenol product to achieve an effective separation of the phenol product from the benzene. The pressure in the reactor is from about 20 kPa to about 5.1 MPa, preferably from about 50 kPa to about 3 MPa.

The benzene may be added at any point in the reactor, for example, it may be added to the fixed bed catalyst or to the reflux as makeup. At least a portion of the benzene, preferably from about 10 wt% to about 100 wt%, is fed to the reactor in a liquid state. The oxidant is preferably a gas, and is fed to the reactor at a point below the catalyst bed allowing the oxidant to flow upward into the catalyst bed where the oxidant contacts and reacts with the benzene. Once in the reactor, the benzene contacts the catalyst and the oxidant, and the benzene is hydroxylated to form phenol. Phenol has a higher boiling point (182 °C) than benzene (80 °C) which allows for easy separation by fractional distillation.

The overhead taken from the distillation column preferably is partially condensed to separate the unreacted benzene from the unreacted oxidant. The partially condensed overheads are passed to an accumulator where benzene is collected and the gaseous oxidant is taken off. The benzene and the oxidant can be fed back to the distillation column. The heat generated by the hydroxylation reaction is removed from the reactor by the reflux of unreacted organic compounds, allowing for isothermal operation of the system. The hydroxylated product is subjected to catalytic hydrogenation to form the respective ketone and alcohol. In one embodiment, when the hydroxylated product is phenol, the respective ketone is cyclohexanone and the respective alcohol is cyclohexanol. This reaction may be carried out in the presence of a catalyst comprising platinum, palladium, ruthenium, rhodium, iridium, rubidium, osmium and mixtures thereof. In another embodiment, the phenol may be hydrogenated in the presence of a metallic nickel hydrogenation catalyst. The catalyst may be a supported catalyst.

The hydrogenation reaction may be carried out at a temperature in the range of from 100 to 230 °C. The hydrogenation reaction may be carried out in any reactor suitable for this reaction, including packed bed reactors, slurry reactors, and shell and tube heat exchange reactors.

The cyclohexanone and cyclohexanol mixture, hereinafter referred to as KA oil is converted to adipic acid by contacting the KA oil with nitric acid in the presence of a conversion catalyst. The reaction is exothermic and occurs with the evolution of nitric and nitrous oxides and carbon dioxide. The reaction products are cooled and the adipic acid which crystallizes out can be filtered or centrifuged. The adipic acid can then be purified as needed.

Suitable conversion catalysts comprise one or more of the following metals:

vanadium, manganese, copper, cobalt, molybdenum, nickel, lead, chromium, iron and mercury. The conversion catalyst may comprise an oxide, nitrate or acetate of one or more of the metals listed. In another embodiment, the catalyst may comprise vanadium pentoxide.

In one embodiment, the nitrous oxide produced in the KA oil conversion step can be recycled as an oxidant to the step of oxidizing an aromatic compound. The nitric oxide components may be removed from the nitrous oxide by contacting the nitrous and nitric oxides with ammonia and a Selective Catalytic Reduction (SCR) deNOx catalyst. In another embodiment, the nitric oxides may be converted to nitric acid which can be recycled to the KA oil conversion step.

In the embodiment where the nitrous oxide is recycled to the aromatic compound oxidation step, additional nitrous oxide may be added to the recycled nitrous oxide stream.