WO2008136857A1

WO2008136857A1 - Process for the production of non-aromatic carboxylic acids

Info

Publication number: WO2008136857A1
Application number: PCT/US2007/083925
Authority: WO
Inventors: William H. Gong; Wayne P. Schammel; Victor A. Adamian; Chengxiang Zhou
Original assignee: Bp Corporation North America Inc.
Priority date: 2007-05-04
Filing date: 2007-11-07
Publication date: 2008-11-13

Abstract

A process is disclosed for producing at least one non-aromatic carboxylic acid by contacting a fluid reaction mixture comprising a non-alkylaromatic feedstock with oxygen in the presence of a bromine-free metal catalyst composition comprising at least three metal or metalloid components, wherein the components comprise palladium; antimony, bismuth or a combination thereof; and molybdenum, gold, niobium, vanadium, gallium, or a combination thereof. This process effectively produces non-aromatic carboxylic acids in one step, eliminates undesirable byproducts produced in commercial practice, and provides enhanced catalyst and product separation.

Description

PROCESS FOR THE PRODUCTION OF NON-AROMATIC CARBOXYLIC

ACIDS

FIELD OF THE INVENTION This invention relates generally to the production of non-aromatic carboxylic acids and, more particularly, to a process for the production of at least one non-aromatic carboxylic acid by the bromine-free catalytic oxidation of non-alkylaromatic feedstock materials.

BACKGROUND OF THE INVENTION Adipic acid, acrylic acid and other non-aromatic carboxylic acids are commodity chemicals that are widely used in the manufacture of polymers. Adipic acid is used in manufacturing plasticizer and lubricant components. Adipic acid is also applied in making polyester polyols for polyurethane systems and it is used in the synthesis of nylon-6,6 fibers. Acrylic acid is used in the manufacture of plastics, latex applications, floor polish, polymer solutions for coatings applications, emulsion polymers, paint formulations, leather finishings, and paper coatings. Acrylic acid is also used as a chemical intermediate. The global demand for adipic acid and acrylic acid continues to grow at a rapid pace. Adipic acid is currently made in a two-step process starting from cyclohexane. In the first step, cyclohexane is oxidized by contacting a reaction mixture containing a soluble, cobalt-salt catalyst with oxygen to produce cyclohexanone (ketone) and cyclohexanol (alcohol), known as "ketone and alcohol oil" or "KA oil," at a low conversion. Care is taken to avoid high conversion of cyclohexane since a significant oxidative degradation pathway for cyclohexane could occur and lead to higher manufacturing costs as a result of the cyclohexane being burned to carbon oxides, rather than converted to adipic acid. The KA oil is separated from the first step mixture, and then it is oxidized in a second step by nitric acid to produce adipic acid and nitrous oxide as a byproduct. Nitrous oxide poses a serious threat to the environment. It is classified as an ozone-depleting agent and it is a known greenhouse gas. Therefore, part of the process effort is to also capture and treat the nitrous oxide emission before it reaches the atmosphere.

The use of soluble catalyst metal ions, such as cobalt (II), poses significant challenges to the current adipic acid process because the separation of these catalysts from the reaction mixture containing dissolved

KA oil is very difficult. An ideal catalyst would be one that is insoluble in a highly soluble reaction mixture of both feedstock and final product. The insoluble catalyst could be removed by simply filtering the reaction mixture and then the filtered catalyst could be recycled for use again. A direct, one- step process for the production of adipic acid from cyclohexane utilizing an insoluble catalyst would not only eliminate the need to isolate the KA oil, but also the need to use nitric acid and capture the ensuing nitrous oxide.

Another difficulty in the manufacture of adipic acid has been from the use of bromine-promoted oxidation catalysts. Bromine sources used with the catalyst and reaction products thereof formed during oxidation are corrosive. To limit corrosion, process equipment including major equipment items, such as oxidation reactors, are normally constructed using titanium or other expensive, corrosion-resistant metals or alloys.

The current process for acrylic acid is a high yielding, two-step process that starts with propylene as the feedstock. Propylene is oxidized to acrolein at 320 - 330 ⁰C in an exothermic reaction. After the acrolein is isolated, it is subjected to another stage of oxidation with oxygen to produce acrylic acid at

210 - 255 ⁰C₁ which is also an exothermic reaction. The catalyst useful for the oxidation of propylene to acrolein is a mixed oxide containing molybdenum, antimony, or copper.

In the direct oxidation of propylene mainly to acrylic acid, the catalytic systems are generally based on cobalt and nickel molybdates and vanadium phosphate mixed oxides. In both cases, tellurium is the modifier used to enhance the reaction. The competitive product is acrolein, but the level of co- production of acrolein is dependent upon the compositional makeup of the catalyst. Propane would be a more preferable feedstock for the production of acrylic acid because propane is available in much higher volumes and lower costs than propylene. However, there are no current commercial processes for the production of acrylic acid from propane since extremely high temperatures are required to activate propane. The challenges for propane oxidation include the fact that the activation of the C-H bond of the methyl groups requires substantial energy, and that the oxygenated propane oxidation product is more active towards the oxidation of propane itself. The over-oxidation of oxygenated propane leads to carbon oxides as the final products. Three catalyst systems, namely vanadium phosphate catalysts, heteropoly acids and mixed metal oxides of transition metals, have been extensively examined for use in the oxidation of propane to acrylic acid, but have not been successful. Although there have been some instances of propane activation by mixed metal oxides (i.e., metals of high oxidation state and comprised mainly of molybdenum salts, although some also contain vanadium), the main products are carbon oxides, acetic acid and acetaldehyde.

U.S. Patent No. 5,864,051 discloses a process for the selective oxidation of alkanes and alkenes using a catalyst which comprises a noble metal component, an antimony oxide component and optionally a modifier.

The selective oxidation occurs at a temperature in the range of about 300 ⁰C about 600 ⁰C and the resultant products are aldehydes.

Accordingly, it would be desirable to provide a process for the production of non-aromatic carboxylic acids via the oxidation of non- alkylaromatic feedstock materials with a bromine-free metal catalyst composition, wherein the process does not yield any byproducts which are harmful to the environment. It would be also be desirable if the metal catalyst composition was insoluble in both the feedstock and final product to provide enhanced catalyst and product separation, as well as process simplification. SUMMARY OF THE INVENTION

The process of the invention, in its embodiments and features, is directed to the production of at least one non-aromatic carboxylic acid by contacting a fluid reaction mixture comprising a non-alkylaromatic feedstock with oxygen in the presence of a bromine-free metal catalyst composition containing at least three metal or metalloid components, wherein the components comprise (a) palladium, (b) antimony, bismuth or a combination thereof, and (c) molybdenum, gold, niobium, vanadium, gallium, or a combination thereof.

As used herein, "bromine-free" means the absence of reactive bromine in both the metal catalyst composition and the fluid reaction mixture. Unlike some commercial processes for making non-aromatic carboxylic acids, such as adipic acid from cyclohexane, the inventive process is "bromine-free" in that it is effective in the substantial or complete absence of bromine sources. While the process, in some of its embodiments, is tolerant of bromine in minor amounts, the presence of bromine in proportions commonly used in conventional commercial processes poisons the catalysts in the inventive process, either deactivating them or shifting selectivity away from non- aromatic carboxylic acid products toward non-aromatic species with less fully- oxidized substituent groups.

In one embodiment of the invention, the metal catalyst composition further comprises (d) molybdenum, gold, niobium, vanadium, gallium, or a combination thereof, provided component (d) is not the same as component

(C).

In another aspect, the invention provides a process for the production of each of acrylic acid and acetic acid by contacting a gas reaction mixture comprising a non-alkylaromatic feedstock selected from the group consisting of propane, propylene and mixtures thereof with oxygen in the presence of a bromine-free metal catalyst composition containing at least three metal or metalloid components on an inert support selected from the group consisting of titanium dioxide, silicon dioxide, aluminum oxide and mixtures thereof, wherein the components comprise (a) palladium, (b) antimony, bismuth or a combination thereof, and (c) molybdenum, gold, niobium, vanadium, gallium, or a combination thereof.

Another aspect of the invention is a process for the production of adipic acid comprising contacting a liquid reaction mixture comprising cyclohexane with oxygen in the presence of a bromine-free metal catalyst composition containing at least three metal or metalloid components on an inert support selected from the group consisting of titanium dioxide, silicon dioxide, aluminum oxide and mixtures thereof, wherein the components comprise (a) palladium, (b) antimony, bismuth or a combination thereof, and (c) molybdenum, gold, niobium, vanadium, gallium, or a combination thereof.

The present invention efficiently and effectively produces non-aromatic carboxylic acids in a direct, one-step process whereby the use of an insoluble, bromine-free metal catalyst composition eliminates undesirable byproducts produced in commercial practice which are harmful to the environment, and provides enhanced catalyst and product separation, as well as process simplification.

DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to a process for producing at least one non-aromatic carboxylic acid. In accordance with this invention, a fluid reaction mixture comprising a non-alkylaromatic feedstock is contacted with oxygen in the presence of a bromine-free metal catalyst composition comprising at least three metal or metalloid components, wherein the components comprise (a) palladium, (b) antimony, bismuth or a combination thereof, and (c) molybdenum, gold, niobium, vanadium, gallium, or a combination thereof.

As used herein, "non-alkylaromatic" means non-benzylic. It is recognized that in the oxidation of an alkylaromatic hydrocarbon, it is the benzylic carbon that undergoes the oxidative transformation. The benzylic carbon is any carbon atom attached to an aromatic ring. Electronic influences of the aromatic ring upon this carbon results in selective oxidation of the benzylic carbon. These influences do not occur on any carbon beyond the immediate benzylic carbon. For an alkylaromatic hydrocarbon, the benzylic carbon is sp³ hybridized and, in addition to its attachment to the aromatic ring, it is also attached to three other hydrogen atoms. In the present invention, it is the other sp³ hybridized carbon atoms that are not benzylic which undergo oxidation. These non-benzylic carbons reside among alkanes, are sp³ hybridized, and are attached to at least two hydrogen atoms.

In accordance with one embodiment of this invention, the non- alkylaromatic feedstocks which may be used include acyclic alkanes, cyclic alkanes, acyclic alkenes, cyclic alkenes, and mixtures thereof. Preferable non- alkylaromatic feedstocks include, but are not limited to, methane, ethane, propane, butane, cyclohexane, propylene and mixtures thereof. Depending upon which non-alkylaromatic feedstock is utilized, the current invention may be used to produce a variety of different non-aromatic carboxylic acids. In one particular aspect of this invention, the non-aromatic carboxylic acids which are produced include acrylic acid, adipic acid and mixtures thereof. When the current invention is used to produce acrylic acid, the non-alkylaromatic feedstock is propane, propylene or a mixture thereof. When the present invention is used to produce adipic acid, the non-alkylaromatic feedstock is cyclohexane.

The oxidation of an alkane feedstock, such as propane, can also produce desirable oxygenate byproducts, such as alcohols, ketones, aldehydes and carboxylic acids. These oxygenates are highly desirable in a "gas to liquids" process for light alkanes since liquid oxygenates are valuable as chemical feedstocks and solvents, and as fuel and/or additives. In addition, the oxygenates can be easily and economically transported as liquids. The transportation of light alkanes as a compressed gas is much more expensive. The oxygenates which may be produced by the oxidation of propane include, but are not limited to propionaldehyde (a.k.a. propanal), acetic acid, acetone, and propionic acid (a.k.a. propanoic acid).

In accordance with the invention, the fluid reaction mixture comprises a non-alkylaromatic feedstock. By definition, a "fluid" can be a liquid or a gas. When the non-alkylaromatic feedstock is propane, propylene or a mixture thereof, the fluid reaction mixture is in the gas phase. If cyclohexane is utilized as the feedstock, then the fluid reaction mixture can be in either the gas or the liquid phase. No solvent is required when the fluid reaction mixture is in the gas phase. The cyclohexane can be oxidized in the gas phase by simply vaporizing it. Preferred solvents which may be used when the fluid reaction mixture is in the liquid phase include water and low molecular weight carboxylic acids, such as acetic acid. Other suitable solvents include fluorinated hydrocarbons, ionic liquids such as imidazolium salts, supercritical carbon dioxide, supercritical water and mixtures thereof. The bromine-free metal catalyst compositions which may be used according to aspects and embodiments of the invention are insoluble, heterogeneous catalysts. The metal catalyst comprises at least three metal or metalloid components. In a preferred embodiment, the metal catalyst composition comprises palladium, antimony and molybdenum. The palladium which is employed is a metal in the reduced, i.e., zero, oxidation state.

The expression "metal or metalloid" is used herein to refer collectively to metallic elements as well as semi-metallic and other elements not considered metals in a strict sense but having metal-like properties. In referring to metals and metalloids as components of the catalyst composition, it will be understood that the terms are used in a broad sense to include the metals and metalloids as such, as well as their compounds, complexes, alloys and combinations in other forms. Significance of distinctions between metals and metalloids is not readily apparent for purposes of the invention.

The metal catalyst composition preferably comprises four metal or metalloid components. Thus, in addition to (a) palladium, (b) antimony, bismuth or a combination thereof, and (c) molybdenum, gold, niobium, vanadium, gallium, or a combination thereof, the metal catalyst composition further comprises (d) molybdenum, gold, niobium, vanadium, gallium, or a combination thereof, provided component (d) is not the same as component (c). In a preferred embodiment of the invention, the metal catalyst composition comprises palladium, antimony, molybdenum and vanadium.

Proportions of the metal or metalloid components of the catalyst compositions can vary. Preferably, the components are present in amounts such that the weight ratio of (a) palladium to (b) antimony, bismuth or a combination thereof to (c) molybdenum, gold, niobium, vanadium, gallium, or a combination thereof to (d) molybdenum, gold, niobium, vanadium, gallium, or a combination thereof, provided component (d) is not the same as component (c), is 1 :1 :1 :1 , 4:1 :1 :1 , 1 :1 :4:1 or 1 :1 :1 :4. The most preferred weight ratio of the four components calculated as elements is 1 :1 :1 :1 . Proportions of the metal and metalloid elements in various combinations can be determined and optimized for particular combinations and usages by persons skilled in catalytic oxidations for the manufacture of non-aromatic carboxylic acids guided by the description and examples appearing herein.

The heterogeneous catalyst also comprises an inert support capable of withstanding an acidic process environment without dissolving or significant loss of catalyst metals. Preferred support materials are solids that are stable in the sense of maintaining physical integrity and catalyst metal loadings suitable to process operation over prolonged exposures to process conditions and use. Carbons, high strength, acid stable silicon carbides and various metal oxides such as silicas, aluminas, titanias, including anatase and rutile phases thereof and mixed phase forms, and zirconia are examples of suitable support materials. Preferable supports include titanium dioxide ("titania"), silicon dioxide ("silica"), aluminum oxide ("alumina") and mixtures thereof.

In yet another aspect of this invention, the metal catalyst composition is also free of halogens other than bromine. The absence of all halogens from the inventive process is preferred since halogens are known to cause undesirable corrosion and adversely affect the activity or selectivity of the metal catalyst compositions. Maximum benefit is achieved by eliminating the use of bromine and not replacing it with one or more other halogens.

Proportions of the non-alkylaromatic feedstock, catalyst and oxygen in oxidation are not critical and vary not only with choice of feed materials and intended product, but also choice of process equipment and operating factors. Oxygen or air or other mixtures of oxygen and nitrogen may be further introduced to contact the non-alkylaromatic feedstock and metal catalyst composition. The rate of oxygen feed must be at least sufficient to fully oxidize all of the non-alkylaromatic feedstock to the non-aromatic carboxylic acid. The concentration of oxygen in any vapor phase of the process should be kept below about 12% by volume to prevent flammability of any non- alkylaromatic feedstock and oxygen mixtures. However, those skilled in the art will recognize that when the oxidation of cyclohexane is conducted in a batch process in the liquid phase, an oxidation concentration less than about 9% should be used so that flammability cannot be achieved. The metal catalyst compositions are suitably used in weights providing about 100 ppm to about 10,000 ppm catalyst metal based on the water plus feed weight. Although the present invention can be carried out as a batch, semi- continuous or continuous process, it is preferred that the process flow move in a continuous mode. Suitable oxidation reactors which may be used in the process include, but are not limited to slurry, CSTR, slurry bubble column, ebulating bed, fixed or packed bed, trickle bed and bubble column reactors. The oxidation reaction may be staged in multiple reactors, if desired, with either feedstock, oxygen or both being introduced in one or more reactors by one or more inlet streams. Depending on the choice of concentrations of reagents and products and other operating parameters, the intermediates and non-aromatic carboxylic acid products may be kept in solution or allowed to solidify in the process after formation. In either case, suitable means to separate products from catalyst and oxidation medium are employed to allow efficient recovery of non-aromatic carboxylic acid product.

The temperature, pressure and residence times are dependent upon the non-aromatic carboxylic acid being produced and can be determined and optimized by persons skilled in catalytic oxidations for the manufacture of non- aromatic carboxylic acids guided by the description and examples appearing herein. Generally, the temperature of the fluid reaction mixture during oxidation should be maintained in the range of about 100 ⁰C to about 320 ⁰C and, preferably, in the range of about 160 ⁰C to about 280 ⁰C. Given the physical properties of the system components, these temperatures will result in a system pressure of about 1 bar absolute (bara) to about 100 bara. The fluid reaction mixture is oxidized for about 0.01 hours to about 4 hours and, preferably, for about 0.5 hours to about 2 hours.

The process of the present invention effectively produces non-aromatic carboxylic acids, particularly adipic acid, acrylic acid and acetic acid, without undesirable byproducts, such as byproducts produced in current commercial practice which deplete the ozone layer and produce acid rain. By contacting a fluid reaction mixture comprising a non-alkylaromatic feedstock with oxygen in the presence of a bromine-free catalyst composition comprising at least three metal or metalloid components, wherein the components comprise palladium; antimony, bismuth or a combination thereof; and molybdenum, gold, niobium, vanadium, gallium, or a combination thereof, the inventors have surprisingly discovered that non-aromatic carboxylic acids can be effectively produced in one step. The use of an insoluble, heterogeneous catalyst further provides for enhanced catalyst and product separation. The current process is also economically appealing since savings are achieved through reductions in capital and operating costs. Without the use of reactive bromine or other halogens, the need for expensive corrosion-resistant equipment is eliminated.

EXAMPLES

The following examples are intended to be illustrative of the present invention and to teach one of ordinary skill how to make and use the invention. These examples are not intended to limit the invention or its protection in any way.

Example 1

The performance of 160 catalyst compositions was investigated using the following metal or metalloid components and supports in various combinations and ratios: o Pd o X = Sb₁ Bi o Y = Mo₁ Au₁ Nb₁ V₁ Ga o Z = Mo₁ Au₁ Nb₁ V₁ Ga o Y ≠ Z o Support = titania, silica o Pd/X/Y/Z ratio = 1 :1 :1 : 1 , 4:1 :1 :1 , 1 :1 :4:1 and 1 :1 : 1 :4

The catalysts were prepared from the metal precursors shown below in

Table 1. The precursor solutions for Mo₁ Sb₁ and V were prepared by dissolving the metal salts in a warm (60 ⁰C) solution of citric acid and water, where the ratio of metal salt to citric acid was 1 :2 on a mass basis. The starting Pd nitrate solution (35.34% Pd nitrate) was diluted with water to obtain a solution of 0.25 g Pd-nitrate/mL solution.

Table 1

Two gram samples were prepared by mixing the supports with the necessary amount of the precursor solutions to obtain catalyst compositions having the desired weight ratio of components. Since sufficient solution volume is required to obtain homogeneous distribution of the precursor solutions upon mixing with the support, extra water was added to obtain a homogeneous slurry.

All of the samples were dried for more than 48 hours at 50 ⁰C, followed by 2 hours at 120 ⁰C (2 °C/min, heating rate). After drying, the samples appeared homogeneous by visual observation. The catalysts were next calcined in air by heating at a rate of 1 °C/min from room temperature to 400 °C, and then maintaining the temperature at 400 °C for 2 hours.

Prior to the final, reduction step, the samples were ground to a powder in a mortar. The ground samples were transferred to crucibles for reduction under a flow of 100 mL 7% H₂ and 93% N₂. The temperature was first kept at room temperature for one hour, and then heated to 250 ⁰C at a rate of 1 °C/min. The samples were kept at 250 ⁰C for 5 hours. The catalysts for the propane and propylene oxidation studies were further processed for gas- phase primary screening. The reduced catalysts were then shaped, crushed and sieved to a particle size ranging from 200 to 400 microns. Example 2

The 160 catalyst compositions were examined at 140 ⁰C and 180 ⁰C for the liquid phase oxidation of cyclohexane. This resulted in a total of 320 experiments. Additionally, 16 experiments were "blanks" and replicates, bringing the total to 336.

High temperature-high pressure batch reactors were used to conduct the experiments. The reactors each had a volume of 8 ml_, of which the ideal working volume was 1 -1 .5 mL, and were stirred magnetically. The reaction mixture was heated in the heating zone to provide a maximum operating temperature of approximately 220 ⁰C.

The heterogeneous catalyst was weighed into a disposable Teflon insert containing a magnetic stir bar. The starting cyclohexane feedstock (800 μ\) was dispensed and weighed before the reactors were closed and pressurized to 60 bar with 40% O₂ and 60% N₂ at room temperature. The upper part of the reactor block was set at a temperature that was 15 ⁰C lower than the desired reaction temperature, with 160 ⁰C as the upper limit. The temperature of the heating block was set to reach a temperature of 140 ⁰C or 180 ⁰C in the reaction mixture. The reactors were put into the pre-heated reactor block at time = 0 min. Following the reaction (time = 180 min), the reactors were removed from the heating block and cooled rapidly in an ice bath. After the addition of an analytical standard and a diluent, the reaction mixtures were stirred to dissolve all of the solid products, and then the samples were diluted further for analysis by gas chromatography.

The oxidation experiments conducted at 140 ⁰C using no catalysts or "blanks" established baseline results for comparison to the oxidations using the catalyst compositions of the present invention. Table 2 lists the results of these three blanks that were conducted under identical conditions (Comparative Example Nos. l-lll) and an average of these results to be used for comparisons (Comparative Average). As shown below in Table 2, no carboxylic acids were observed in any of the product analyses. Table 2

^*(CH = Cyclohexane, A = Cyclohexanol, K = Cyclohexanone)

Based on the Comparative Average, two catalyst compositions had improved performances over that of the Comparative Average based solely on the amount of cyclohexanol produced. These results are shown below in Table 3. Again, under these conditions, no detectable levels of carboxylic acid products were observed. As demonstrated in Table 3, the nature of the support, the selection of metals, and the ratios of the metals on a given support affected the performance of the catalyst. For instance, Example No. 2C was supported on silica, and it produced a lower conversion of cyclohexane and lower concentrations of oxidation products than the Comparative Average. Also, when a higher concentration of palladium was used so that the Pd/Sb/V/Ga ratio was 4:1 :1 : 1 on the same silica support as Example No. 2C, the oxidation produced a conversion of cyclohexane, but no detectable levels of any cyclohexanol and cyclohexanone (see Example No. 2D). Similarly, individual increases in the level of V (Example No. 2E) or Ga (Example No. 2F) also resulted in lower concentrations of cyclohexanol than the Comparative

Average. The most effective catalyst composition at 140 ⁰C was Example No. 2B, where Pd/Sb/Au/V was used at 1 :1 :1 :1 ratio on silica. In this case, when the support was changed to titania (Example No. 2G), the resulting cyclohexane conversion, and concentrations of cyclohexanol and cyclohexanone were lower. Based on the data in this Example, it was determined that an effective and efficient catalyst composition contains palladium, antimony and vanadium. Table 3

^*Component is 4-times the concentration of any of the other metals in the catalyst.

Example 3 Catalyst compositions containing palladium, antimony or bismuth, and vanadium in a 1 :1 :1 :1 ratio were analyzed by gas chromatography to quantify cyclohexanol and cyclohexanone, as well as adipic acid. (ADA). The results are shown below in Table 4. These experiments were conducted twice at 180 0C, so the results for the cyclohexane conversion, cyclohexanol and cyclohexanone produced are averages of both experiments. The adipic acid concentration, however, was only measured for the duplicated experiment. A comparison of Example Nos. 3B and 3C indicates that antimony had a stronger influence on the production of adipic acid than bismuth. Antimony produced twice the concentration of adipic acid as bismuth. Also, the catalyst composition of Pd/Sb/Mo/V in Example No. 3E produced the highest concentration of adipic acid at 0.0793 wt%.

Table 4

Example 4

Another set of experiments was conducted to explore variables such as support, solvent, and feedstocks in the evaluation of catalyst compositions. Cobalt (1000 ppmw based on feedstock), which is a homogeneous catalyst that requires no support, was used as a reference catalyst. The form of cobalt used was dependent upon the conditions, i.e., a more hydrocarbon-soluble cobalt catalyst (cobalt (II) naphthenate) was used in the absence of a solvent, whereas cobalt(ll) acetate tetrahydrate was used when acetic acid (HOAc) or water was employed as the solvent. The metals were fixed at a 1 :1 :1 :1 ratio with palladium and all other metals at 1.5 wt% loading on the support. The reaction temperature was set at 180 ⁰C for three hours. The variables were as follows: o Support = γ-alumina or titania o Solvent = water, acetic acid or none o Feedstock = cyclohexane or cyclohexanol

The results are shown below in Table 5. Example No. 4A used cobalt as the reference catalyst in the oxidation of cyclohexane and water. Most of the catalysts used in this set of experiments produced no detectable levels of adipic acid. The only two catalysts that produced adipic acid were Example

Nos. 4D and 4F, namely Pd/Sb/Mo/V and Pd/Sb/Au/V on a titania support.

When water was replaced by acetic acid in the oxidation of cyclohexane, the cobalt reference catalyst produced detectable levels of adipic acid. Differences in the effectiveness of the support under these acetic acid-based conditions were noted. The titania and alumina support for the catalyst Pd/Sb/Nb/V generated equivalent adipic acid, so this catalyst was not sensitive to the support. But, for the Pd/Sb/Mo/V catalyst (Example No. 4K), it was again the titania support that was more effective than the alumina support (Example No. 4L). This same trend was seen in the Pd/Sb/Au/V catalyst where the titania support (Example No. 4M) produced a higher concentration of adipic acid than the alumina support (Example No. 4N). In the absence of any solvent for the oxidation of cyclohexane in Example Nos. 40 to 4T, titania was an effective support for the Pd/Sb/Nb/V and Pd/Sb/Mo/V catalysts.

When cyclohexanol was used as the feedstock (to evaluate the oxidation of a cyclohexane intermediate to adipic acid) and water as the solvent, the use of cobalt produced very little adipic acid at 0.004 wt% (Example No. 4U). Surprisingly, the alumina support performed more effectively to produce a higher adipic acid concentration for the Pd/Sb/Nb/V (Example Nos. 4V and 4W) and Pd/Sb/Mo/V (Example Nos. 4X and 4Y) catalysts. However, the Pd/Sb/Au/V catalyst was less sensitive as similar concentrations of adipic acid were produced (Example Nos. 4Z and 4AA). These results indicate that the catalyst composition of the present invention is not deactivated by water in the oxidation of cyclohexanol.

When water was replaced by acetic acid, the Pd/Sb/Nb/V catalyst was insensitive to the support with regard to the adipic acid produced (Example Nos. 4BB and 4CC). However, when Pd/Sb/Mo/V was examined, the alumina support produced the highest concentration of adipic acid (Example No. 4EE) in comparison to the titania support in Example No. 4DD. This trend was reversed when the Pd/Sb/Au/V catalyst was examined. In this case, the titania supported catalyst in Example No. 4FF outperformed the alumina supported catalyst in Example No. 4GG.

Examples Nos. 4HH through 4MM examined the catalyst compositions in the oxidation of cyclohexanol with no solvent. In all cases, the catalysts that performed well were those that were supported on alumina. Thus, these experiments illustrate that catalytic performance is dependent on what feedstock and solvent (if any) are used. The experiments also demonstrate that the oxidation catalyst is not deactivated by acetic acid or water, and that it can perform in the absence of any solvent. Table 5

Example 5

Catalyst screening experiments were performed in an experimental set-up known as a "continuous oxidation system" that consisted of four blocks, each containing 16 micro fixed bed reactors with a manifold that was fed by two feed lines. The manifold distributed a gaseous feed to each of the 16 reactors of each of the blocks from each of the feed lines going to the manifold. Because each block was independently heated, it was possible for four separate temperatures to be examined for the continuous, gas-phase oxidations of propane and propylene (IUPAC name propene). For the investigation of these catalyst compositions, one feed line contained either propane or propylene plus molecule oxygen, a diluent gas such as nitrogen, and helium, which was used as an internal standard. A second feed line delivered water. On the downstream end of these continuous reactors of these four blocks was a back pressure regulator device for each of the blocks to control gas exit in order to contain a set pressure for each of these blocks. Further downstream from the back pressure regulator devices were selection valves which were computer controlled to direct the exit gaseous stream to one of two gas chromatographs designed to quantify the anticipated propane or propylene oxygenation products, byproducts and carbon oxides, as well as helium. A catalyst volume of 50, 100 or 200 microliters was loaded into the reactor. A diluent guard bed (25 microliters of zirblast, inert ceramic beads) was added at the top of the catalyst bed. The catalyst compositions for the propane and propylene oxidations were then sieved and those having a particle size in the range of 200 to 400 microns were tested.

Using the fixed bed catalyst system described above at 280 ⁰C with a propane flow of 8.30 mL/min, an O₂ flow of 15.5 mL/min, and a gram-hour space velocity of 1202 hr^"\ the results shown below in Table 6 were obtained. As demonstrated in Table 6, the use of a Pd/Sb/MoA/ catalyst supported on silica was effective in producing acrylic acid without any acetic acid, acetaldehyde or acetone.

Table 6

Using the fixed bed catalyst system at 205 ⁰C with a propylene flow of 17.05 mL/min, an O₂ flow of 31 .01 mL/min, and a gram-hour space velocity of 2535 hr^"\ the results shown below in Table 7 were obtained. As demonstrated in Table 7, the Pd/Sb/Mo/V catalyst at 1 :1 :1 :1 on silica generated 2% conversion of propylene at a low temperature of 205 ⁰C, with acrylic acid produced in 15.65% selectivity. The degradation organic products generated, i.e., acetic acid and acetaldehyde, were below a total of 2%. Non-degradative oxidation products, namely acetone and 2-propanol (a.k.a. isopropyl alcohol), were also produced. Table 7

Using the fixed bed catalyst system at 235 ⁰C with a propylene flow of 16.34 mL/min, an O₂ flow of 31.0 ml_/min, and a gram-hour space velocity of 2535 hr^'1, the results shown below in Table 8 were obtained. As demonstrated in Table 8, the oxidation occurred at a slightly higher temperature of 235 ⁰C, which increased the propylene conversion to 10.27%. The acrylic acid product selectivity was 8.64%. The selectivity was lower than in Table 7, as a result of the higher conversion. The acetic acid and acetaldehyde were at about 4% selectivity, and the sum of selectivities of acetone and 2-propanal was below 3%.

Table 8

While the present invention is described above in connection with preferred or illustrative embodiments, these embodiments are not intended to be exhaustive or limiting of the invention. Rather, the invention is intended to cover all alternatives, modifications and equivalents included within its spirit and scope, as defined by the appended claims.

Claims

What is claimed is:

1. A process for the production of at least one non-aromatic carboxylic acid comprising contacting a fluid reaction mixture comprising a non-alkylaromatic feedstock with oxygen in the presence of a bromine-free metal catalyst composition comprising at least three metal or metalloid components, wherein the components comprise (a) palladium, (b) antimony, bismuth or a combination thereof, and (c) molybdenum, gold, niobium, vanadium, gallium, or a combination thereof.

2. The process of claim 1 wherein the non-alkylaromatic feedstock is selected from the group consisting of acyclic alkanes, cyclic alkanes, acyclic alkenes, cyclic alkenes and mixtures thereof.

3. The process of claim 2 wherein the non-alkylaromatic feedstock is selected from the group consisting of methane, ethane, propane, butane, cyclohexane, propylene and mixtures thereof.

4. The process of claim 3 wherein the non-alkylaromatic feedstock is selected from the group consisting of propane, propylene and mixtures thereof, and the non-aromatic carboxylic acid is acrylic acid.

5. The process of claim 4 wherein the fluid reaction mixture is in the gas phase.

6. The process of claim 3 wherein the non-alkylaromatic feedstock is cyclohexane and the non-aromatic carboxylic acid is adipic acid.

7. The process of claim 6 wherein the fluid reaction mixture is in the gas phase.

8. The process of claim 6 wherein the fluid reaction mixture is in the liquid phase.

9. The process of claim 8 wherein the fluid reaction mixture further comprises a solvent selected from the group consisting of water and a low molecular weight carboxylic acid.

10. The process of claim 1 wherein the metal catalyst composition comprises palladium, antimony and molybdenum.

1 1 . The process of claim 1 wherein the metal catalyst composition further comprises (d) molybdenum, gold, niobium, vanadium, gallium, or a combination thereof, provided component (d) is not the same as component (c).

12. The process of claim 11 wherein the metal catalyst composition comprises palladium, antimony, molybdenum and vanadium.

13. The process of claim 1 1 wherein the weight ratio of the four metal or metalloid components calculated as elements is about 1 :1 :1 :1.

14. The process of claim 1 wherein the metal catalyst composition is on an inert support selected from the group consisting of titanium dioxide, silicon dioxide, aluminum oxide, and mixtures thereof.

15. The process of claim 1 wherein the metal catalyst composition is free of halogens other than bromine.

16. The process of claim 1 wherein the fluid reaction mixture is contacted at a temperature in the range of about to 100 ⁰C about 320 ⁰C.

17. The process of claim 1 wherein the fluid reaction mixture is contacted at a temperature in the range of about 160 ⁰C to about

280 ⁰C.

18. A non-aromatic carboxylic acid produced by the process of claim 1.

19. A process for the production of acrylic acid comprising contacting a gas reaction mixture comprising a non-alkylaromatic feedstock selected from the group consisting of propane, propylene and mixtures thereof with oxygen in the presence of a bromine-free metal catalyst composition comprising at least three metal or metalloid components on an inert support selected from the group consisting of titanium dioxide, silicon dioxide, aluminum oxide, and mixtures thereof, wherein the components comprise (a) palladium, (b) antimony, bismuth or a combination thereof, and (c) molybdenum, gold, niobium, vanadium, gallium, or a combination thereof.

20. The process of claim 19 wherein the metal catalyst composition further comprises (d) molybdenum, gold, niobium, vanadium, gallium, or a combination thereof, provided component (d) is not the same as component (c).

21 . A process for the production of acetic acid comprising contacting a gas reaction mixture comprising a non-alkylaromatic feedstock selected from the group consisting of propane, propylene and mixtures thereof with oxygen in the presence of a bromine-free metal catalyst composition comprising at least three metal or metalloid components on an inert support selected from the group consisting of titanium dioxide, silicon dioxide, aluminum oxide, and mixtures thereof, wherein the components comprise (a) palladium,

(b) antimony, bismuth or a combination thereof, and (c) molybdenum, gold, niobium, vanadium, gallium, or a combination thereof.

22. The process of claim 21 wherein the metal catalyst composition further comprises (d) molybdenum, gold, niobium, vanadium, gallium, or a combination thereof, provided component (d) is not the same as component (c).

23. A process for the production of adipic acid comprising contacting a liquid reaction mixture comprising cyclohexane with oxygen in the presence of a bromine-free metal catalyst composition comprising at least three metal or metalloid components on an inert support selected from the group consisting of titanium dioxide, silicon dioxide, aluminum oxide, and mixtures thereof, wherein the components comprise (a) palladium, (b) antimony, bismuth or a combination thereof, and (c) molybdenum, gold, niobium, vanadium, gallium, or a combination thereof.

24. The process of claim 23 wherein the metal catalyst composition further comprises (d) molybdenum, gold, niobium, vanadium, gallium, or a combination thereof, provided component (d) is not the same as component (c).

25. The process of claim 23 wherein the liquid reaction mixture further comprises a solvent selected from the group consisting of water and a low molecular weight carboxylic acid.