WO1992003225A1

WO1992003225A1 - Catalyst for and process of methane oxydehydrogenation

Info

Publication number: WO1992003225A1
Application number: PCT/US1991/005779
Authority: WO
Inventors: Heinz Heinemann; Pedro R. Pereira; Gabor A. Somorjai
Original assignee: The Regents Of The University Of California
Priority date: 1990-08-21
Filing date: 1991-08-21
Publication date: 1992-03-05
Also published as: ZA916611B; MX9100757A; MA22262A1; CN1061164A; CS258691A3; AU8529091A

Abstract

A mixed metal oxide catalyst (11) is described which permits the conversion of methane (18) to ethane, ethylene, and higher hydrocarbons at temperatures below 600 °C in the presence of oxygen (19) and steam (21) with selectivities of 80-100 % at methane conversions exceeding 10 %. The catalyst comprises an alkaline earth oxide, an oxide of a transition metal of groups VIB, VIIB and VIII of the periodic system and a small amount of an oxide of alkali. The catalyst is activated in oxygen and steam and remains active only in the presence of steam. This type of catalyst in the absence of steam and at temperatures above 600° converts methane with oxygen to higher hydrocarbons and carbon dioxide with selectivities to hydrocarbons below 70 %. High selectivity approaching 100 % can be achieved with a preferred catalyst containing an atomic ratio of Ca:Ni:K of 2-4:1:0.05-0.5 after pretreatment of the catalyst in oxygen.

Description

"CATALYST FOR AND PROCESS OF METHANE OXYDEHYDROGENATION."

BACKGROUND OF THE INVENTION

This invention relates to a method and catalyst compositions for oxidative coupling of hydrocarbons, particularlymethane, and more particularly to oxidative coupling processes over novel catalysts in the presence of steam for selectively converting Cj to C₂ and higher hydrocarbons, with selectivities approaching 100% and at least 8% conversion and yield.

This invention was made in the course of, or under Contract DE-AC03-76SF00098 between the United States Department of Energy and The University of California for the operation of Lawrence Berkeley Laboratory, according to which the U.S. Government has certain rights to this invention.

It has long been recognized bythe chemical industry that the production of higher chain length hydrocarbons from so called c, feedstocks, most notably methane, would be extremely valuable, principally because it would permit the conversion of abundant gaseous methane from natural gas into higher gaseous or liquid hydrocarbons, which could then be used to provide synthetic liquid fuels and other reagents for the chemical industry in general.

This goal has been pursued vigorously by many researchers. The literature onthe subject is voluminous and provides a large number of processes including catalyzed conversions using a wide variety of catalysts. It is indeed possible to convert C, to higher chain length hydrocarbons today, however, processes so far identified have considerable disadvantages, including unfavorable economics, excessive production of pollutants, and undesirable process parameters.

The major considerations in oxidative Cj coupling are the parameters of selectivity, conversion, and yield. The formation of CO and C0₂ is a major reaction which competes with the formation of C₂ hydrocarbons. The selectivity term expresses the desirable C₂, C₃ or C₄ product C ₊₎ formed as a percentage of the total Cj material converted. Lower selectivities thus implythat large quantities of CO_x are generated in the process. CO_x represents a loss of both C, reactant and oxygen but equally important, CO_x is an undesirable pollutant requiring removal from recycled unconverted methane by scrubbing with alkaline solution.

The term conversion is used to express the percentage of the C, feedstocks which has reacted in a single pass through the process reactor, i.e. conversion is the difference between the quantity of unreacted C_j in the product stream and.the C, quantity present in the initial feed stream. The reaction products in oxidative coupling processes are C₂ (and higher) hydrocarbons, water and CO-. The magnitude of the conversion is normally a function of the characteristics of the catalyst and of the reactor as well as the process variables and thus varies somewhat with the flow rates, catalyst quantities, process temperature, and the like. The conversion values for known processes and practical reactors are typically less than about 50%. Hence recirculation of product through the reactor is invariablyrequired in a practical plant. Since CO- build-up interferes with the reaction, a CO_x removal step is thus required for all coupling processes having low selectivity parameters, which includes all oxidative coupling processes known thus far.

Yield is defined as the product of selectivity and conversion. This parameter expresses, as a percentage, the quantity of the desired C₂ or higher hydrocarbon product formed from a quantity of C_ starting material for a given reactor under given operating conditions.

The important parameter for evaluating conventional processes is, of course, the comparative yield of C₂, and/or higher hydrocarbons. However, it is important to note that a process achieving 100% selectivity can be superior, from an overall cost point of view, to processes of lower selectivities even if these processes achieve higher yields and conversions.

OBJECTS OF THE INVENTION

Accordingly, it is a principal object of this invention to provide a process for oxidative coupling of methane which is characterized by higher selectivity for C-, and hi liex hydrocarbons,

Another object of the invention is to provide an oxidative coupling process in which the formation of CO_x is eliminated to as large an extent as possible and to thus obviate the need of CO_x removal entirely. Yet another object of this invention is to provide catalysts which promote oxidative coupling with attendant high selectivities and which can be readily and reproducibly made.

Still another object of the invention is to provide a process of improved economics in regard to reagent requirements, minimizing the use of oxygen and employing steam, thus permitting the process to be carried out safely below the flammability limit.

These and other objects and advantages of the present invention, including the ability to operate at lower temperatures than used in other proposed processes, will become apparent to those skilled in the art upon consideration of the following description and drawings.

SUMMARY OF THE INVENTION

This invention is a process for the oxidative coupling of hydrocarbons of lower chain length particularly methane, wherein methane is passed over a catalyst comprised of the oxides of an alkali, an alkaline earth and a transition metal together with steam and a small amount of oxygen at relatively low temperatures in the range of about 500 to about 650°C. The optimal mole ratio of the feed gas mixture is about 3:1:6:5 of methane:oxygen:water. The pressure may be between atmospheric and about 500 psig.

A significant aspect of the invention relates to the catalyst composition and its preparation. The selectivity of the present catalysts has been generally superior to those of the prior art, i.e. 80% or higher, and essentially 100% for the preferred catalysts of this invention. However, within this range, establishing optimum activities and selectivities depends remarkably upon variations in chemical composition and preparation of the catalyst. The preferred catalyst is a mixture of calcium, nickel, and potassium oxides in an atomic weight ratio of about 2:1:0.01.

The catalysts are prepared by drying, decomposing and oxidizing mixed aqueous solutions of calcium and nickel nitrates in air at elevated temperatures, and adding small amounts of potassium nitrate in the process.

Optimum catalysts have also been prepared in similar fashion from the corresponding hydroxides.

DETAILED DESCRIPTION OF THE INVENTION CATALYST COMPOSITION

The catalyst formulations discovered in the course of the present invention are generally superior over those known heretofore, particularly in terms of their selectivities. However, themost dramatic aspect of this invention relates to he discovery of catalyst formulations and preparation which allow achievement of selectivities of 599%+, since they permit oxidative coupling of methane to be carried out without evolution of CO_x and hence without the need of separating CO_x from the product stream prior to recirculation or recovery. These catalysts will be discussed below under the heading of preferred catalysts.

Work is underway to characterize the catalysts and the mechanism of the coupling reaction. While not complately understood at this point, it appears that the coupling reaction takes place at the surface of the catalyst and that gas phase reactions are not involved, in contrast with the majority of reports in the literature. Carbon balance studies comparing reactor inputs and outputs show that there is a loss of carbon within the reactor during start-up. This carbon is believed to be deposited on the catalyst and appears to be an important participant in the catalytic process.

THE PREFERRED CATALYSTS AND THEIR PREPARATION

The composition of the catalyst with which *=100% selectivity was obtained in steam oxidative methane coupling were mixtures of activated oxides of calcium, nickel, and potassium at a nominal atomic ratio of about 2:1:0.1.

Small amounts of CO_x may be tolerable, but become undesirable at even only a few percent, since with recirculation, the C0_X quantity builds up, requiring purification of the reactor effluent. The permissible variation of the catalyst composition is as follows: The Ca/Ni ratio may vary from about 2:1 to about 4:1, and the Ni/K ratio from about 1:0.05 to about 1:0.2.

As has been indicated above, the preparation of the catalyst may play an important role in obtaining catalysts of the desired characteristics. The following processes have been found to routinely yield 599%+ selectivity catalysts.

The catalyst may be prepared from aqueous solutions of calcium nitrate, nickel nitrate, and potassium nitrate. Alternatively the catalyst may be prepared from the hydroxides or carbonates also. It appears to be important that the anions of the solutions are identical. It is preferred to produce the catalyst starting from aqueous nitrate solutions. Production of catalyst from hydroxide or carbonate would follow the same procedure.

The aqueous solutions of the nitrates are prepared, in concentrations as high as their solubilities permit.

Appropriate quantities of the solutions are mixed to provide the desired Ca/Ni ratio. An appropriate quantity of potassium nitrate may be mixed in at this point or later as discussed below. The mixed solution is then dried and decomposed in air at about 700°C to form the corresponding oxides.

The potassium nitrate solution may alternatively be added to be dried and decomposed mixture of calcium and nickel oxides, followed by positive drying and decomposition in air at 700°C until a powdery mixture is formed. The catalyst may be used in this form or may be pelletized by pressing or extruding.

Unsupported catalyst has been found satisfactory for loading into and use in the reactor. However, it may be desired to use an inert support, such as a porous ceramic of AL^ or Si0₂ to increase the surface area and permit transport of feedstock over the catalyst at higher space velocities or higher conversions. To produce supported catalyst, the support material is added to the solution mixture prior to drying followed by processing as described.

Prior to use, the catalyst must be activated. Activation may be accomplished prior to loading the catalyst into the reactor, or preferably by treatment in the reactor after loading, but prior to exposure to feedstock. To activate the catalyst, it is exposed to flowing oxygen or air, e.g. at a rate of about 2cc 0₂ per gram of catalyst, εt a mperatare ^cit 680°C

The time period for which the catalyst is activated is important in that activation for longer periods increases conversion and also increases specific selectivity for hydrocarbons of greater chain length, and also affects the olefin/paraffin ratio, as shown in Table 1 below. The present catalysts thus provide an important ability to convert methane to C₃ and C₄ hydrocarbons, and the activation step provides the means for optimizing production of the desired hydrocarbon species.

TABLE 1

Effect of the in-situ activation of the catalyst. Activation was with oxygen prior to reaction.

Selectivity was assumed 100% because C0₂ production was always below the blank run values and approximately constant.

THE GENERAL CLASS OF CATALYSTS

Mechanistic studies and experimentation have shown, that while the above specific catalyst formulation provide nearly 100% selectivity, dramatically improved selectivities are obtained from the general class of catalysts comprising composites of the oxides of an alkaline earth and a transition metal mixed with small amounts of an oxide of an alkali. By small amounts we mean less than 1/20, or from about 1/20 to about 1/100 by weight of the total amount. More particularly, the preferred alkaline earth group of interest for the present catalyst comprises magnesium, calcium, and barium; the preferred transition metals are iron, cobalt and nickel; and the preferred alkalis are lithium, sodium, and potassium. The preferred molar ranges of the alkaline earth/transition metal varies from about 2:1 to about 4:1, and the preferred molar ranges of transition metal/alkali is from about 1:0.05 to about 1:0.2.

The preparation of these catalysts and their activation is carried out in the same manner as described for the preferred catalysts, i.e. initial mixture of soluble salts, of the metal and identical anionε, preferably nitrates with or without support materials, drying in air at 650 to 750°C to convert the nitrates to oxides, followed by oxygen activation from 1 to about 48 hours at about 700°C.

On the basis of our investigations, we believe that aside from the preparation of the catalyst as described thus far, the following factors play an important role in the action of the catalyst: Material balance measurements show that during startup of the oxidative coupling reaction a carbon deficit occurs which we believe results in the incorporation of carbon in the catalyst surface. As shown in Table 2 the material balance does not reach 100% until after about 3 hours of operation.

TABLE 2

Secondly, the effect of carrying out the oxidative coupling reaction with water is striking, in that the selectivity is seriously compromised if water is replaced by an inert gas in the process. Table 3 provides data for a run in which steam is replaced with helium, demonstrating the dramatic drop in both hydrocarbon selectivity and conversion, followed by the restoration of the original values when helium is replaced by steam again.

TABLE3

X-ray photoemission studies of active and inactive catalysts show signals associatedwith Ni(OH)₂ and higher oxidation states of Ni for the active catalyst, compared with NiO for less active catalyst.

THE OXIDATION COUPLING PROCESS

The use of the above described catalysts in a process for coupling hydrocarbons will now be discussed with reference to Figure 1, which schematically represents the experimental set up used to investigate the present invention. A predominant aspect of the invention is the conversion of methane to ethane and ethylene, however, as discussed above, higher hydrocarbons are formed also, and depending, upon the catalyst pretreatment , may be formed at higher conversion rates. The process may also have utility to couple hydrocarbons including methane and higher chain length olefinic or paraffinic species to form still higher chain length hydrocarbons, including, for example C₃-C,₀.

The coupling reaction takes place in reactor 10, which comprises flow through catalyst chamber 11, surrounded by heater means 12, whereby the temperature of the reactor may be regulated and maintained at the desired value. The reactor is conventionally instrumented to permit measurement and control of the pressure, temperature and flow rate, using temperature control 13, temperature sensor 14, pressure gauges 15, and mass flow controller 16.

Reactor input port 17 receives the feedstream comprising methane, oxygen, and steam which pass over the catalyst in the catalyst chamber 11. In the experimental set-up shown, methane is supplied frommethane bottle 18, oxygen from oxygen reservoir 19, and water from syringe pump 20. Water is converted into steam in heated conduit 21. It should be readily appreciated that in a production plant the methane, oxygen and steam would be supplied by suitable industrial sources. Also, since only a fraction of the initial hydrocarbon feed is converted tohigherhydrocarbons, recirculation of a corresponding fraction of the reactor output would be required as indicated by dashed line 22 leading from reactor output port 23 to the reactor input.

The product issuing from output port 23 is put through expander 24. Water is separated in water recipient 25. In production equipment where removal of CO_χ is required due to use of catalysts having selectivities less than 100%, the recirculation loop would also have to include a conventional scrubber for this purpose. The product is analyzed" and monitored by attendant analytical instrumentation, shown here to comprise ad chromatography equipment 26 including reservoir 27 and valved lines 28 for supplying a known calibrating 29 mixture. In operation, the catalyst is loaded into the reactor and activated as discussed above. In a production reactor two catalyst chambers may be employed to permit continuous operation through alternating chambers, i.e. feedstock is processed in one chamber while catalyst is loaded and activated in the other chamber. Catalyst may, of course, be activated prior to loading as discussed above. After catalyst activation, the reactor temperature is raised to between about 550 and 650°C, steam is generated by heating conduit 21 to about 140°C, and methane, oxygen and steam are introduced into the reactor. The composition of the feedstream expressed in terms of the molar ratio of CH₄/0₂/H₂0 may vary from 2/1/4 to 5/1/6, with che optimum ratio being about 3/1/6.5. It should be noted that the process is carried out below the explosive limit for methane and oxygen. The use of steam as well as the reduced need for oxygen and operation at relatively low temperatures (processes described in the literature operate at 705-900*C) are considered major advantages of the present process generally and also facilitate safe operation. The preferred flow rate of the feedstream is about 40 moles of methane per gram of catalyst per hour. As indicated above, during the start-up phase of the reaction, the material balance of carbon shows that carbon is absorbed by the catalyst for about 3 hours. Figure 2 shows a graph of the material balance with time. Once a 100% carbon balance has been achieved, catalyst life is long and constanthydrocarbonyield prevails over at least two days of operation. Small but tolerable quantities of CO_x may be generated in the course of preheating. Blank runs have shown that the C0₂ results from gas phase reactions int he preheat zone and are not attributable to lower catalyst selectivities. Comparison of blank runs over inert ceramic granules and catalyst show that gas phase conversion of methane to C0₂ is in fact reduced by the catalyst.

Figure 3 shows the variation of selectivity, conversion, and yield for a Ca/Ni/K = 2:1:0.1 catalyst as the temperature is increased from 600°C to 750°C and then dropped to 600°C again. As can be seen, the selectivity dramatically deteriorates from early 100% to less than 10% and is restored to better than 90% upon return of the temperature to 600°C again.

Examples Example 1:

A catalyst comprising oxides of calcium and nickel in an atomic ratio of 2:1 was prepared from a solution of the nitrates by drying in air decomposing the nitrates to oxides in air at 700^βC and activating the catalyst in flowing oxygen at 680°C for 20 hours. The catalyst was charged to the reactor and a gaseous mixture of CH₄, 0₂ and water in a 1 mole ratio of 3:1:6.5 was passed over the catalyst at 600C and atmospheric pressure at a rate of 40 moles of CH₄ per gram of catalyst per hour. The selectivity to higher hydrocarbons in the product was about 80% at about 10% CH₄ conversion for a yield of 8% C₂ and C₃.

Example 2: A catalyst comprising oxides of calcium, nickel and potassium oxides in an atomic ratio of 2:1:0.1 was prepared from a solution of the nitrates by drying the Ca and Ni nitrates in air decomposing them to the oxides in air at 700°C, adding a solution of potassium nitrate, drying and decomposing again at 700°C and activating the catalyst in flowing oxygen at 600°C for 20 hours. When a mixture of CH₄, 0₂ and water was passed in a mole ratio of 3:1:6.5 over this catalyst at 600°C and atmospheric pressure at a rate of 40 moles of CH₄ per gram of catalyst per hour, a selectivity to C₂₊ hydrocarbons in the product of 100% was achieved at about 11% CH₄ conversion for a yield of 11% C₂₊.

Example 3: A catalyst comprising oxides of calcium, nickel and potassium in atomic ratio of 4:1:0.1 was prepared by the method described in example 2. This- catalyst gave the same conversion and selectivity as the catalyst of example 2.

Example 4:

A catalyst comprising oxides of Ca, Ni and K in an atomic ratio of 2:1:0.1 and prepared as in example 2 was activated in flowing oxygen at 700°C for 1 hour. On testing this catalyst for the oxidative coupling of CH₄ under the conditions described in example l, a selectivity of 100% to C₂₊ hydrocarbons was obtained at a CH₄ conversion of only 2%. Selectivity to C₂ hydrocarbons was 98% and to C₃ hydrocarbons was 2%. The olefin/paraffin ratio of the C₂₊ hydrocarbons was 0.2.

Example 5:

The catalyst prepared according to example 2 was activated for 40 hours in flowing 0₂ at 680°C. On testing this catalyst for the oxidative coupling of CH₄ under the conditions described in example 1, a selectivity of 100% to C₂₊ hydrocarbons was obtained at 9.5% conversion of CH₄. Selectivity to C₂ hydrocarbons was 86% to C₃ hydrocarbons 11% and to C₄ hydrocarbons 3%. The olefin/paraffin ratio was 0.8.

Having thus described the invention, it will be obvious to those skilled in the art that various modification may be made in without departing from the spirit and scope of the invention, which should therefore be only limited by the following claims.

Claims

We claim :

1. A catalyst for use in oxidative coupling of lower hydrocarbons to form hydrocarbons of higher chain lengths, which comprises an alkaline earth oxide, an oxide of a transition metal, and a small amount of an oxide of an alkali.

2. The catalyst of claim 1, wherein said alkaline earth is selected from the group consisting of magnesium, calcium, strontium and barium.

3. The catalyst of claim 1, wherein said transition metal is selected from the group consisting of iron, cobalt, and nickel.

4. The catalyst of claim 1, wherein said alkali is lithium, sodium, or potassium.

5. The catalyst of claim l, wherein the atomic ratio of alkaline earth to transition metal varies between about 2:1 and 4:1 and the atomic ratio of transition metal to alkali is between about 1:0.05 and 1:0.2.

6. The catalyst of claim l, which has been activated by exposure to oxygen at elevated temperatures for a period of one to forty-eight hours.

7. The catalyst of claim 6, wherein the temperature is 700°C.

8. The catalyst of claim 1, wherein the catalyst is formed by mixing aqueous solutions of salts comprised of said alkaline earth, transition metal and alkali and identical anions, heating said solutions in the presence of air to dry, decompose, and oxidize said alkaline earth, transition metal, and alkali.

9. The catalyst of claim 8, wherein said anion is nitrate or hydroxide.

10. The catalyst of claim 1, wherein said catalyst is disposed on the surface of a porous granular, inert substrate.

11. The catalyst of claim 1, wherein said catalyst is pelletized.

12. A catalyst for oxidative coupling of hydrocarbons, comprising a mixture of the oxides of calcium, nickel, and potassium.

13. The catalyst of claim 12, where in the mole ratio of calcium and nickel is between about 2:1 and 4:1 and the mole ratio of nickel and potassium is between about 1:0.05 and 1:0.2.

14. A process to convert methane to paraffins and olefins of the C₂, C₃, and C₄ hydrocarbons with selectivities about 80% by passing the methane in admixture with oxygen and steam at temperatures in the range of 500-600°C and pressures of atmospheric to 500 psig over a catalyst comprising a composite of an alkali, an earth alkali and a transition metal oxide.

15. A process in accordance with claim 14, in which the catalyst comprises a composite of calcium, nickel and potassium oxides.

16. A process in accordance with claim 14 in which the catalyst comprises a composite of calcium, nickel and potassium oxides in atomic proportion of about 2:1:0.1 and in which the catalyst has been oxidized in an oxygen stream at 700°C for period of 1 to 48 hours prior to the conversion reaction.

17. A process in accordance with claim 14 in which the catalysts comprises composites of oxides of magnesium, iron and sodium.

18. The process of claim 14, wherein the molar ratio of methane, oxygen and steam is between 2:1:4 and 5:1:8.

19. The process of claim 18, wherein the molar ratio of methane, oxygen and steam is about 3:1:6.5 and the temperature is about 600°C.