Catalytic oxidation of hydrocarbons

Details Catalytic oxidation of hydrocarbons Catalytic oxidation of hydrocarbons

2000-08-18

2001-10-23

Aulakh, Charanjit S. (Department: 1625)

Organic compounds -- part of the class 532-570 series

Organic compounds

Carboxylic acids and salts thereof

C502S155000

Reexamination Certificate

active

06307100

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to production of chemical products via the catalytic oxidation of a hydrocarbon.
BACKGROUND OF THE INVENTION
Adipic acid (AA) is a raw material used in the production of many different materials including organic polymers (most notably Nylon-6,6), fibers, plasticizers, and food additives. The manufacture of these materials requires millions of tons of highly pure adipic acid each year.
Several methods of producing adipic acid are known. The currently predominant industrial process is performed in two separate steps. The first step is oxidizing cyclohexane to a mixture of cyclohexanone and cyclohexanol (KA mixture), and the second step is converting the KA mixture to adipic acid. The second step is typically performed using concentrated nitric acid (~55 wt % in the reaction) as an oxidant. Unfortunately, the nitric acid oxidation step results in the production of NO
x
(especially N
2
O) byproducts that can pollute the atmosphere and are not readily recyclable.
One step processes for preparing adipic acid by air or peroxide oxidation of cyclohexane have been reported. See, e.g., U.S. Pat. Nos. 5,221,800; 5,929,277; and Catalysis Today, 9: 237, 1991. These processes are typically performed using a Co(III) catalyst at high oxygen pressure (e.g., 20-30 atn) or N-hiydroxy- phthalimide/Co/Mn catalysts at low O
2
pressure (Iwahama, T.; Syojyo, K.; Sakaguchi, S.; Ishii, Y. Organic Proc. Res. Devel. 1998, 2, 255-260). Despite the potential efficiencies and cost savings associated with such one step processes, the two step process continues to be preferred in the industry because conventional one step processes have not been optimized for large scale syntheses. For example, conventional one step processes using a Co-based catalyst require that a very high concentration (e.g., about 0.01 M) of the catalyst be included in the reaction mixture. As this catalyst is relatively expensive, cost considerations mandate that it be recycled using an extraction procedure prior to reuse in additional runs. Conventional one step processes also offer relatively low selectivity, and result in an adipic acid product of relatively low purity (e.g., less than about 70% pure). For these reasons. cobalt-catalyzed one step oxidation processes generally involve costly purification/recycling steps for purifying the adipic acid from by-products of the reaction, and for recycling the catalyst.
SUMMARY
A chemical process has been developed for the catalytic, single step conversion of cyclohexane to solid adipic acid by air oxidation. A composition that catalyzes this process includes an transition metal such as iron or ruthenium in complex with a pyridyl ligand such as pyridine; 4,4′-diphenyl-2,2′-bipyridine (dpbp); 1,10 phenanthroliine (phen); 4,7-diphenyl-1,10-phenantlhroline (dpphen); or 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (dmdpphen). This composition can be readily dissolved in numerous cyclohexane/ cosolvent mixtures, permitting a reaction mixture comprising the catalytic composition, cyclohexane, and cosolvent to be in a homogeneous state (i.e., all components dissolved together in a liquid phase) at the initiation of the reaction. The reaction can be run under moderate temperatures and moderate pressures of air or other oxygen source to yield solid adipic acid precipitates from the reaction mixture solution upon cooling. Even unrecrystallized, the solid adipic acid precipitates can be greater than 95% pure (i.e., contaminated with less than 5% other substances). The solid adipic acid can then be filtered out of the reaction mixture, leaving behind a filtrate that can then be reused to produce additional adipic acid. Thus, the process is amenable to a continuous synthetic process whereby, for example, formed solid adipic acid can be removed from the reaction mixture as it is produced by repetitively cooling and then filtering different aliquots of the reaction mixture, and then returning the liquid filtrate to the reaction vessel where it is further reacted to produce additional adipic acid.
The invention thus offers several advantages over conventional two step and reported one step techniques. For example, compared to the conventional two step process, the invention allows relatively easier and less expensive isolation and purification of the adipic acid as the one step process can be performed at moderate temperatures and pressures to yield solid precipitates containing greater than about 95% pure adipic acid . The invention also avoids the use of the highly corrosive acidic solvents (e.g., nitric acid) used in conventional two step adipic acid syntheses, and therefore does not generate polluting nitrogen oxides. These advantages should decrease the costs associated with adipic acid production by reducing the complexity of plant design and maintenance (e.g., single step process vs. two step process), lowering energy costs due to ease of purification (e.g., higher purity solid recovered directly), reducing pollution (e.g., no NO
x
byproducts to recover), lowering raw material costs (e.g., air vs. nitric acid), and lowering new plant construction costs (e.g., noncorrosive conditions).
The invention also offers a marked improvement over conventional one step processes in that fewer or simpler purification/recycling steps are required to produce highly pure adipic acid in a cost efficient manner. For example, unlike conventional one step processes, the invention does not require a complex catalyst recovery procedure. Rather, recycling of the catalyst in the present invention presents much less of a problem because the catalysts within the invention are relatively inexpensive, are not required in high concentrations, and can be reused without being repurified.
Accordingly the invention features a catalyst for catalyzing the synthesis of a chemical product from a hydrocarbon and oxygen. In a preferred variation, the chemical product is adipic acid and the hydrocarbon is cyclohexane. The catalyst is made up of a transition element such as iron or ruthenium complexed with a polypyridyl ligand such as pyridine, dpbp, phen, dpphen, or dmdpphen. The transition element can be complexed with a counter ion such as ClO
4
−, Cl
−
, (CH
3
)
3
CCO
2
−
, or CF
3
SO
3
−
. Preferred catalysts of have between one and twelve mole equivalents of the pyridyl ligand per mole equivalent of the transition element.
Various catalysts within the invention can catalyze the production of adipic acid from cyclohexane in a single step process at a low concentration such as about 0.00002 to about 0.002 moles of the catalyst per mole of cyclohexane. In many cases, the catalysis occurs without nitrogen oxide production. Preferred catalysts within the invention have the ability to catalyze the production of a solid product from cyclohexane and oxygen in a single reaction vessel. The solid product being greater than about 70% or in some case greater than about 95% pure adipic acid.
In another aspect, the invention features a method of making a catalyst for the synthesis a chemical product from a hydrocarbon and oxygen. This method includes the steps of: (A) providing a transition element, a pyridyl compound, and a reaction mixture including the hydrocarbon; (B) adding the transition element and the pyridyl compound to the reaction mixture; and (C) placing the reaction mixture under conditions which cause the transition element and pyridyl compound to be able to function together as a catalyst for the synthesis of the chemical product from a hydrocarbon and oxygen. In variations of this method, the chemical product is adipic acid and the hydrocarbon is cyclohexane.
Other methods of making a catalyst for catalyzing the synthesis of adipic acid from cyclohexane and oxygen are also included in the invention. For example, preferred versions of such methods include the steps of: (A) mixing together a composition including a pyridyl compound and a composition including a transition element to form a reaction mixture; (B) allowing the reaction mixture to rea

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