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Hydrogenation of aromatic compounds

This application, pursuant to 35 U.S.C. § 120, claims benefit to U.S. patent application Ser. No. 11/389,955, filed Mar. 27, 2006. That application is incorporated by reference in its entirety.

1. Field of the Invention

The present invention relates to a process for hydrogenation of aromatic compounds such as the hydrogenation of benzene to cyclohexane and the supported nickel catalyst modified with up to 0.9 wt. % Cu therefore.

2. Background

Cyclohexane is the main precursor for the production of nylon products and as such, the demand remains strong. Cyclohexane was first obtained by the direct fractional distillation of suitable crude petroleum refinery streams. Now the major portion of cyclohexane is obtained from the direct hydrogenation of benzene. Conventionally the reaction is carried out in vapor or mixed phase using a fixed bed reaction. The reactor temperature is controlled to be between 350 to 500° F. Higher temperatures can lead to thermodynamic limitations on benzene conversion, thermal cracking and increased byproduct. In general, the amount of byproducts in the effluent stream from a hydrogenation reactor increases with hydrogenation temperature or conversion of benzene or both.

Peterson, in U.S. Pat. No. 2,373,501, discloses a liquid phase process for the hydrogenation of benzene to cyclohexane wherein a temperature differential is maintained between the top of the catalyst bed where benzene is fed and the outlet where substantially pure cyclohexane is withdrawn. The temperature differential is due to the change in the exothermic heat of reaction released as less and less benzene is converted as the concentration of benzene decreases. Specifically the top of the catalyst bed is at a higher temperature than the lower catalyst bed. Hydrogen is supplied countercurrent to the benzene/cyclohexane flow. Temperature control coils are disposed within the reactor to maintain the temperature differential if the exothermic heat of reaction is not sufficient or to cool the bed if too much heat is released. Peterson recognizes that although the bulk of his reaction takes place in the liquid phase a portion of the benzene and cyclohexane will be vaporized, especially near the top of the reactor where the benzene concentration is highest and conversion is highest. A reflux condenser is provided to condense the condensable material and return it to the reactor. Thus, a substantial portion of the heat of reaction is removed by condensation of the reactants vaporized throughout the reaction. Peterson maintains a liquid level above the topmost catalyst bed but allows room for vapors to escape to the condenser where the heat of reaction is removed.

Larkin et al., in U.S. Pat. No. 5,189,233, disclose another liquid phase process for the hydrogenation of benzene to cyclohexane. However, Larkin et al. utilize high pressure (2500 psig) to maintain the reactants in the liquid state. In addition Larkin, et al disclose the use of progressively more active catalyst as the concentration of benzene decreases to control the temperature and unwanted side reactions.

Hui et al, in U.S. Pat. No. 4,731,496, disclose a gas phase process for the hydrogenation of benzene to cyclohexane over a specific catalyst. The catalyst reported therein is nickel supported on a mixture of titanium dioxide and zirconium dioxide.

U.S. Pat. No. 6,750,374 discloses a process for the hydrogenation of benzene using hydrogen containing up to about 15 mole % impurities, such as carbon monoxide and light hydrocarbons with an alumina supported catalyst containing from about 15 to 35 wt. % Ni and from about 1 to 15 wt. % Cu. The catalyst may contain additional elements such as Mo, Zn, Co, Fe.

The present invention is a process and a catalyst used in the process for hydrogenation of aromatic compounds, such as benzene, aniline, naphthalene, phenol and benzene polycarboxylates, by hydrogenating the aromatic compound in the presence of a catalyst comprising from 4 to 14 wt. % Ni, preferably 9 to 10 wt. % Ni and up to about 0.9 wt. % Cu, preferably about 0.2 to 0.4 wt. % Cu deposited on a transition alumina support having a BET surface area from about 40 to 180 m2/g, and a pore volume from about 0.3 to about 0.8 cc/g. The hydrogenation of aromatic compounds is advantageously carried out in the presence of a high boiling solvent. The preferred solvent will have at least 10° F. higher boiling point than the aromatic compound to be hydrogenated and hydrogenated cyclic compound. The advantages of using a high boiling solvent are higher productivity of cyclohexane and keeping the temperature of catalytic reaction zone in a desired range. The high boiling solvent provides improvement in productivity of the reaction system whether or not the nickel catalyst is modified with copper.

In the production cyclohexane, benzene is present in the reaction stream in an amount of 1 to 60 wt. %, preferentially of 3 to 40 wt. %. The hydrogen stream may be pure hydrogen or may contain up to 5 mole % impurities including carbon monoxide. The remaining components in the reaction stream can be cyclohexane, a high boiling solvent or a mixture of cyclohexane and a high boiling solvent. If a high boiling solvent is used, the solvent may comprise 10 to 90 wt. %, preferably 20 to 80 wt % of the reaction stream. The high boiling solvent may be recovered from the effluent recycle stream and recycled to hydrogenation reactor. The content of cyclohexane in the recycle solvent stream can be 0.0 to 80 wt. %, preferably from 0.5 to 30 wt %.

The present invention also includes a copper modified nickel catalyst used in the hydrogenation of aromatic compound to produce a hydrogenated cyclic compound comprising 4 to 14 wt. % Ni and about 0.2 to 0.4 wt. % Cu deposited on a transition alumina support having a BET surface area from about 40 to 180 m2/g and a pore volume from about 0.3 to about 0.8 cc/g.

Other aspects and advantages will be apparent from the following description and the appended claims.

This invention pertains to a catalytic hydrogenation process of aromatic compounds such as benzene, aniline, naphthalene and phenol in the presence of improved copper modified nickel-based catalyst supported on a porous support. Hydrogenation of benzene yields cyclohexane. But the hydrogenation product stream from the catalytic reactor contains other undesired by-products such as pentane, cyclopentane, methyl cyclopentane, n-hexane and methyl cyclohexane. The product stream usually contains a trace amount of benzene, up to about 200 ppm by weight. Less than 10 ppm benzene is highly desirable for the production of high purity cyclohexane. In general, the amount of by-products in the effluent stream from a hydrogenation reactor increases with hydrogenation temperature or conversion of benzene or both. Especially the amount of the by-products rapidly increases with hydrogenation temperature higher than about 340° F. The hydrogenation of aniline yields cyclohexylamine. But the undesired side reactions are deamination, formation of di and triphenylamine and various heavier products. An advantage of the present invention is reduced by-products so that simple distillation of hydrogenation product stream produces high purity cyclohexane product. Preferably the cyclohexane product contains no more than about 50 ppm, preferably no more 30 ppm by weight impurities including unconverted benzene excluding impurities came in with the feed benzene. The hydrogenation reactor effluent contains small amounts of cyclohexene. Since cyclohexene can easily be hydrogenated to cyclohexane by recycling the reactor effluent or using a small separate reactor without producing significant amounts of by-product, it is not considered as an undesired by-product. The present invention provides a significant improvement of the productivity of cyclohexane as well as reducing total impurities in the product cyclohexane stream from the catalytic reaction zone to less than 10 ppm, if the benzene hydrogenation is carried out in the presence of a heavy solvent such as decalin and decane. An additional advantage of using heavy solvent is substantially less recycle of hydrogen.

The hydrogenation reaction can be carried out in any physical device such as catalytic distillation column, fixed bed reactor, boiling point reactor, stirred tank reactor, trickle bed reactors or any combination of these. Since the benzene hydrogenation reaction is exothermic reaction, the hydrogenation reaction for the traditional fixed bed operation is preferably carried out by recycling the reactor effluent stream to dilute the fresh benzene feed, which dilutes the heat of reaction. Although recycling the reactor effluent is not necessary for the catalytic distillation reactor, one may choose to do so.