Catalysts for the oxidative dehydrogenation of hydrocarbons

Details Catalysts for the oxidative dehydrogenation of hydrocarbons Catalysts for the oxidative dehydrogenation of hydrocarbons

2002-04-16

2003-06-10

Wood, Elizabeth (Department: 1755)

Chemistry of hydrocarbon compounds

Unsaturated compound synthesis

By dehydrogenation

C585S661000, C546S352000, C558S383000

Reexamination Certificate

active

06576803

ABSTRACT:

FIELD OF THE INVENTION
This invention relates, in general, to the oxidative dehydrogenation of hydrocarbons. More particularly, the present invention relates to rare earth catalysts that provide unusually high selectivity to higher hydrocarbons and/or lower olefins when used for the oxidative dehydrogenation of a lower hydrocarbon at elevated pressure. Accordingly, the rare earth catalysts of the invention are particularly useful for coupling methane by oxidative dehydration to form ethane, ethylene and higher hydrocarbons, and for the oxidative dehydrogenation of ethane to form ethylene.
BACKGROUND OF THE INVENTION
Methane is an attractive raw material because it is widely available and inexpensive, but it is used mainly as a fuel. Natural gas liquids (ethane, propane, butane and higher hydrocarbons) are the major raw material for ethylene and propylene, from which many petrochemicals are produced. But the supply of natural gas liquids has not kept pace with increasing demand for olefins, so more costly cracking processes that use naphtha from petroleum are being commercialized. Therefore, the development of economical processes for manufacturing olefins and other hydrocarbons from methane is highly desirable.
Methane has low chemical reactivity, so severe conditions are required to convert it to higher hydrocarbons. Oxidative dehydrogenation is favored because conversion is not thermodynamically limited and reactions are exothermic. But selectively producing ethylene, ethane, and higher hydrocarbons by partial oxidation while avoiding complete oxidation to carbon oxides is difficult to achieve. Accordingly, those skilled in the art have expended much effort in attempts to develop selective catalysts for methane coupling. Rare earth oxycarbonate and oxide catalysts have been of particular interest.
U.S. Pat. No. 4,929,787 discloses a catalyst for oxidative coupling that contains at least one rare earth metal carbonate, which is defined to include simple carbonates and oxycarbonates and which comply approximately with the stoichiometric formulas M
2
(CO
3
)
3
, M
2
O
2
CO
3
, M
2
O (CO
3
)
2
, or M(OH)(CO
3
), which may be characterized by elementary analysis, where M is at least one rare earth metal. The rare earth oxycarbonates, M
2
O
2
CO
3
, are preferred, with lanthanum oxycarbonate, La
2
O
2
CO
3
, being most preferred. Only lanthanum, neodymium, and samarium are used in the examples. The catalysts may be prepared in several ways by thermal decomposition of a rare earth metal compound: carbonates may be directly decomposed; hydroxides, nitrates, carbonates, or carboxylates may be added to a solution of polycarboxylic acid (citric), dried, and roasted under vacuum or in air; carbonates, hydroxides, or oxides may be added to an acid (acetic), dried, and decomposed in air; carbonates or carboxylates (acetates) may be dissolved into aqueous carboxylic acid (formic or acetic), impregnated onto a carrier, and heated in air; or oxides may be contacted with carbon dioxide. These methods all specify decomposing the precursors at a temperature of 300° to 700° C., but the examples all use 525° to 600° C. The decomposition may be done outside or inside the reactor before passing the reacting gas mixture over the catalyst. In one example, the La
2
O
2
CO
3
catalyst was prepared by heating at 120° C. an acetic acid solution containing lanthanum acetate, reducing the volume of the solution by aspiration, drying the material at 150° C. under high vacuum, crushing the resultant foam to fine powder, and roasting the powder in air at 600° C. for two hours. In another example, the reactor was charged with anhydrous lanthanum acetate and treated with helium at 525° C. for one hour to form the La
2
O
2
CO
3
catalyst. The catalyst may also contain one or more alkaline earth metal (Be, Mg, Ca, Sr, Ba) compounds to improve selectivity and a Group IVA metal (Ti, Zr, Hf) to increase activity. The reaction temperature specified is 300° to 950° C., preferably 550° to 900° C.; the examples are mainly at 600° to 750° C., but the catalysts are selective at temperatures exceeding 900° C. as well. The reaction pressure specified is 1 to 100 bars, particularly 1 to 20 bars, but the examples are all at atmospheric pressure. Carbon dioxide may be beneficially added (up to 20%) to the reaction gases as a diluent to increase yield by moderating the bed temperature and as a constituent to maintain a high activity of the carbonate catalyst. These catalysts are utilized in the related processes disclosed in U.S. Pat. Nos. 5,025,108 and 5,113,032.
The effect of reaction pressure on a catalyst disclosed in U.S. Pat. No. 4,929,787 was studied in M. Pinabiau-Carlier, et al., “The Effect of Total Pressure on the Oxidative Coupling of Methane Reaction Under Cofeed Conditions”, in A. Holmen, et al.,
Studies in Surface Science and Catalysis,
61,
Natural Gas Conversion,
Elsevier Science Publishers (1991). The catalyst (A) was a mechanical mixture of lanthanum oxycarbonate and strontium carbonate that was calcined in air at 600° C. for two hours. Increasing the pressure substantially decreased the selectivity to C
2
+ hydrocarbons (reaction temperature of 860° C.) from 72% at 1 bar to 39% (constant flow rate) or 35% (increased flow rate for constant conversion) at 7.5 bar (94 psig). Another catalyst (B) was a magnesia support impregnated with aqueous lanthanum and strontium nitrates and then calcined at 800° C. for two hours. This calcination temperature is above the maximum specified calcination temperature of 700° C. disclosed in U.S. Pat. No. 4,929,787 for producing oxycarbonate, and is a temperature at which predominantly lanthanum oxide, La
2
O
3
, is expected to form. The preparation furthermore did not include a carbon source from which oxycarbonate could be formed from the nitrate. Increasing the pressure significantly decreased the C
2
+ selectivity (900° C.) from 79% at 1.3 bar to 65% at 6 bar (72 psig) with constant flow rate. The study concluded that the reaction should be operated at pressures below 3 bar (29 psig).
A catalyst disclosed in U.S. Pat. No. 4,929,787 was used to study the effect of adding 10% ethane to oxidative coupling and pyrolysis reactors in series in H. Mimoun, et al., “Oxidative Coupling of Methane Followed by Ethane Pyrolysis”,
Chemistry Letters
1989: 2185. The catalyst was a mechanical mixture of lanthanum oxycarbonate and strontium carbonate. Ethane added to the coupling reactor (880° C. and one atmosphere) decreased methane conversion and increased ethylene and carbon monoxide production. The study concluded that oxygen preferentially dehydrogenates ethane instead of coupling methane; ethane is best separated from the natural gas feed and supplied to just the pyrolysis reactor, where it is cracked with high selectivity to olefins, as disclosed in U.S. Pat. No. 5,025,108.
U.S. Pat. No. 5,061,670 discloses a method for preparing a cocatalyst of lanthanide and alkaline-earth metal carbonates and/or oxycarbonates, which comprises forming an aqueous solution of lanthanide and alkaline-earth metal chlorides; adding alkali metal carbonate and optionally hydroxide to coprecipitate carbonates and/or hydroxycarbonates at a basic pH above 8; separating the coprecipitate from the reaction medium; washing away the alkali metal chlorides formed; and drying and calcining the coprecipitate at 400° to 1000° C. in air or an inert atmosphere. Scandium, yttrium, and lithium may be added as promoters. The examples form cocatalysts of barium with lanthanum or samarium.
Cocatalysts of BaCO
3
and La
2
O
2
CO
3
were studied in U. Olsbye, et al., “A Comparative Study of Coprecipitated BaCO
3
/La
2
O
n
(CO
3
)
m
Catalysts for the Oxidative Coupling of Methane”,
Catalysis Today
13: 603 (1992). They were prepared by mixing aqueous BaCl
2
and LaCl
3
with NaOH and Na
2
CO
3
at a pH above 8, washing and drying the precipitate, and calcining it at 500° C. in air. The reaction was done at 750° to 850° C. at atmospheric pressure. The catalysts were small crystals (300-500 Å) of BaCO
3
a

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