WO1996025472A1

WO1996025472A1 - Process for the production of olefins using a molybdenum oxicarbide dehydrogenation catalyst

Info

Publication number: WO1996025472A1
Application number: PCT/FI1996/000094
Authority: WO
Inventors: Outi Krause; Marc Jacques Ledoux; Frédéric MEUNIER; Pascal Del Gallo; Cuong Pham-Huu; Vesa Niemi; Jyrki Hiltunen
Original assignee: Neste Oy
Priority date: 1995-02-17
Filing date: 1996-02-19
Publication date: 1996-08-22
Also published as: FI950751A0; EP0809684A1; AU4719996A; FI97546C; FI97546B

Abstract

A process for the production of light olefins, such as ethylene, propylene, butylene and/or amylene is described. The process comprises contacting a hydrocarbon feed comprising, for instance, a gasoline or naphta fraction containing alkanes having from 4 to 12 carbon atoms per molecule at an elevated temperature with a molybdenum oxicarbide catalyst. The process may be carried out in the presence of water vapour or free oxygen in order to stabilize the oxicarbide phase. The catalyst is preferably supported on silicon carbide optionally doped with at least one lanthanide, actinide, rare earth metal, or a mixture thereof.

Description

Process for the production of olefins using a molybdenum oxicarbide dehydrogenation catalyst .

Background of the Invention

Field of the Invention

The present invention relates to a process in accordance with the preamble of claim 1 for the production of light olefins by contacting a feed of dehydrogenable hydrocarbons at an elevated temperature with a catalyst in order to produce light olefins.

Description of Related Art

There are several ways of producing light olefins. These include steam cracking, fluidized bed catalytic cracking and dehydrogenation. In short these processes can be characterized as follows:

- In steam cracking the main product of steam cracking process is ethylene. Propylene and heavier olefins are the most important by-products and their yields cannot substantially be increased by a change of operating conditions. Other by-products are fuel gas, aromatic tar and coke, which are harmful for the process and have low or no value.

- Fluidized bed catalytic cracking (FCC) gives rise to a low yield of light olefins.

- Catalytic dehydrogenation of hydrocarbons takes place at relatively high temperatures. The dehydrogenation reaction is highly endothermic, requiring high, carefully controlled heat input to the reaction zone. This is the major challenge for process design.

The last group of processes is particularly interesting. It can be divided into two types depending on how the needed energy is used in the reaction zone:

1. Adiabatic processes: In adiabatic reactors both the catalyst bed temperature and the feed temperature are above the optimum process temperature at the beginning of the process cycle. 2. Isothermal processes: As far as the process technology is concerned, the reaction temperature should be as constant as possible. A temperature, which is too high, increases the coke formation and decreases the selectivity to olefins and a temperature, which is too low, decreases the conversion. In isothermal processes cracking cannot be prevented, because the temperature near the inner surface of the reactor tubes exceeds the safe temperature limit.

Commercial dehydrogenation catalysts are currently either chromium oxide or platinum on a support. These catalysts have several disadvantages: chromium has numerous oxidation states among which Cr(VI) is known to be carcinogenic and allergenic. The cost of platinum is very high. For these reasons the use of these catalysts is difficult in the processes where they can escape (e.g. in the fluidized bed processes where a part of the catalyst can vanish among the products). Other possible catalysts for dehydro¬ genation are based on vanadium oxide, molybdenum oxide, and metal carbides. These are presently in the developing stage.

As evident from what has been stated above there clearly exists a need for new dehydrogenation processes employing more effective catalysts.

Summary of the Invention

It is an object of the present invention to eliminate the problems of the prior art and to provide a novel process for producing olefins.

The present invention is based on the concept of replacing the commonly used catalysts of the conventional catalytic dehydrogenation processes with new materials recently developed. In particular, the new catalysts for the dehydrogenation process comprise molybdenum oxicarbide which optionally is supported on silicon carbide.

More specifically, the present invention is characterized by what is stated in the characterizing part of claim 1. Detailed Description of the Invention

As mentioned above, the present invention provides a process for the dehydrogenation of hydrocarbons at an elevated temperature with a catalyst comprising or consisting essentially of porous molybdenum oxicarbides in bulk form or, preferably, supported on silicon carbide, pure or doped. The process is carried out in the presence of water vapour or traces of free oxygen or oxygen containing compounds in order to stabilize the oxicarbide phase.

New high specific surface area silicon carbide or oxicarbide or carbides or oxicarbides of other elements such as Mo, W, V etc. have been recently developed. They are used in petrochemical catalyzed reactions or for the conversion of exhaust gas of IC engines (EP 0 313 480, EP 0 396 475, EP 0 474 570, EP 0 534 867 and EP 0 624 560). Said catalysts may be used as such and/or activated and/or supported and/or doped by elements such as rare earth metals. In particular molybdenum carbide and oxicarbide are advantageous in cracking, reforming and dehydrogenation reactions (EP 0 396 475).

The active phase of the catalysts used in the present process, the molybdenum oxicarbide, has a large specific surface area (1 to 200 m²/g). Its general formula is MoO_xC_y, wherein x is 0.01 to 5 and y is 0.01 to 10.

The molybdenum oxicarbides used in this invention can be made by any suitable method, preferably by the methods disclosed in the patent publications mentioned above.

By way of an example, it may be mentioned that the molybdenum oxicarbide can be prepared by reduction and carburation of the corresponding oxide (for instance MoO₃). The preparation process preferably consists of initially oxidizing the base metal in a current of air containing oxygen at a temperature between 300 and 450 °C for 3-15 hrs. This is preferably preceded by treatment in a flux of hydrogen at 600 to 800 °C for 1 to 2 hrs. The oxide formed, for instance MoO₃) is then subjected to a gaseous mixture of

H₂ and and an alkane, such as n-hexane, the transformation reaction being the isomerisation of n hexane. Carburation of the oxide catalyst is carried out in situ to produce the carbide/oxycarbide product.

In the catalyst, the molybdenum oxicarbide can be in mixture with oxicarbides of other metals, in particular tungstene (W) and/or chromium (Cr). The atomic ratio betwen Mo and the other metal(s) ranges from about 0.01 to 100, preferably about 0.05 to 50. In particular, for molybdenum tungstene mixed oxides, the preferred atomic ratio Mo/W is 1 to 30. For Mo/Cr mixed oxides the atomic ratio Mo/Cr is 0.1 to 10.

The active phase can be supported on a high specific surface area silicon carbide. Suitable silicon carbide materials are, for instance, disclosed in more detail in the following prior art publications: EP 2 621 904, EP 0 313 480 Bl, EP 0 440 569 A2, EP 0 511 919 Al, EP 0 543 752 Al, EP 0 543 751 Al, US 5 217 930, US 4 914 070. To mention just one exemplifying preparation process, finely divided SiC (submicron particles of surface area at least 200 πrg^"1) can be obtained by: generating SiO gas in a first reaction zone, heating SiO₂+Si at a temperature between 1100 and 1400 °C under a pressure of 0.1 to 1.5 mmPa, and, in a second reaction zone, reacting SiO gas with finely divided carbon of surface area at least 200 nrg^"1 at a temp of 1100 to 1400 °C. It is particularly preferred to form SiO at a temperature of 1200 to 1300 °C or 1100 to 1200 °C. The preferred temperature for the reaction of SiO with C is between 1100 and 1200 °C. The SiC can be doped as decribed below and then the product can be subjected to a postcalcination in air at a temperature between 600 and 800 °C for about 0.5 to 2 hrs. It is preferred to carry out the reaction in an inert atmosphere (argon or helium).

The silicon carbide support can be pure or doped with rare earth elements, lanthanides or actinides or mixtures thereof. As examples of preferred doping elements the following may be mentioned: Ce, U, Ti, Zr, and Hf. The doping can be carried out by impregnation by means of a solution, aqueous or other of a soluble compound (such as acetyl acetonate, nitrate etc.) decomposed by a heat treatment prior to the method of obtaining the heavy metal carbide. The specific surface area of the carbon is somewhat reduced during doping, but it generally remains higher than 200 m²/g. The amount of the doping element, such as Ce, is 0.5 to 20 % by weight, typically about 1 to 10 % by weight. The doping may be used to modify the catalytic properties of the catalysts. This feature will be apparent from a comparison of the results obtained in Example 3 and 5, respectively, given below; doping with Ce of the support of a SiC-supported molybdenum oxicarbide catalyst greatly increases the C₄-selectivity of the catalyst.

The hydrocarbon feed to be treated in the process of this invention, which in the following also will be called "the dehydrogenable hydrocarbons", can contain at least alkanes having from 2 to 12 carbon atoms per molecule. Non-limiting examples of suitable alkanes are: ethane, propane, n-butane, isobutane, 2-methylbutane, n-pentane, 2-methylpentane and n-hexane and the like. Preferred alkanes are those containing 2-5 carbon atoms per molecule.

The hydrocarbon feed can also contain at least one cycloalkane having from 5 to 10 carbon atoms per molecule. Non-limiting examples of suitable cycloalkanes are cyclopentane, cyclohexane, methylcyclohexane, 1,3-dimethylcyclohexane and ethylcyclohexane and the like.

The hydrocarbon feed can also be a fraction of petroleum crude oil, or a fraction of a catalytic cracker effluent, or a fraction of shale oil, or a fraction of a product produced by extraction or liquefaction of coal, or a similar hydrocarbon feedstock. Preferably, a petroleum fraction having a boiling point at atmospheric pressure in the range of about 0 °C to 220 °C, such as gasoline or naphtha fraction, is used as a feed. These fractions generally contain alkanes having from 4 to 12 carbon atoms per molecule as a major component.

The presence of water vapour in the reaction is preferred. Water vapour inhibits the formation of carbide phase which catalyses hydrogenolysis and increases methane content in the cracked product. The molar fraction of steam is generally 0.01 to 25 %, preferably from about 0.1 to 5 %. Water can be replaced by oxygen-containing compound, such as alcohols or ethers, in a concentration of 0.01 to 25 mol-%, or by a small amount, below 1 wt-%, of free oxygen in the gas phase. Any apparatus, which will afford an intimate contact of the hydrocarbon feed stream with the catalyst of this invention at an elevated temperature, can be employed. The process is in no way limited to a particular apparatus. The process can be carried out in a batch process, e.g. in an autoclave which can be heated and pressurized, and preferably containing internal agitation or circulation devices. The catalyst can be dispersed in the feed, or it can be used as a fixed bed. The process can also be carried out as a continuous process, e.g. in a tubular reactor containing the catalyst as a fixed bed, or in a fluidized bed reactor, where the flowing catalyst is separated after the reaction and regenerated preferably in another reactor. The term "hydrocarbon feed stream" is used herein to both batch and continuous process.

Any suitable temperature can be utilized in the dehydrogenation process of this invention. Generally the reaction temperature ranges from about 400 °C to about 700 °C, preferably from 500 °C to 600 °C due to thermodynamics.

Any suitable reaction pressure can be utilized in the dehydrogenation process of this invention. Generally the reaction pressure can be in the range from nearly vacuum (0.1 bar (abs)) to as high as 30 bar (abs).

Any suitable reaction time, i.e. the time of intimate contact of the hydrocarbon feed stream with the catalyst can be used in the process of this invention. The actual reaction time will greatly depend on such features as the effective reaction temperature, the type of feed used, the type of catalyst employed and its particle size. Generally, the reaction time ranges from about 0.5 to 100 seconds, preferably from about 1 to 50 seconds. In a continuous process, the reaction time is generally expressed in terms of the weight hourly space velocity (WHSV), which can be range from about 0.1 to about 12 t feed t catalyst/hour, preferably from 0.2 to 4 t feed /t catalyst /hour.

The dehydrogenated products formed in the process of this invention are preferably separated from the reaction mixture by any separation means, e.g. by fractional distillation. Unconverted feed hydrocarbons are preferably recycled to the reaction zone and hydrogen gas, which is formed during the process can be used as fuel or as reactant for chemical syntheses (hydrogenation).

If the catalyst of this invention loses its catalytic activity due to coking, the substantially deactivated catalyst can be regenerated in the same reactor by interrupting the flow of hydrocarbon feed and contacting the catalyst with a free oxygen containing gas, preferably air, at such regeneration conditions that will result in coke removal. This regeneration process can be carried out in a separate reactor as well. "Substantially deactivated catalyst" as used herein means a catalyst that has lost a sufficiently high portion of its initial activity and no longer converts the feed hydrocarbon to the desired products at commercially acceptable yields. Typical regeneration temperature is 400 to 800 °C using air or diluted air as a regenerating agent.

Before and after the regeneration the catalyst is stripped by passing nitrogen or other inert gas through the catalyst to prevent the catalyst for overheating, which could induce safety consequences.

Depending on the hydrocarbon feed, the catalyst and the process conditions, light olefins, in particular ethylene, propylene, butylenes and/or amylenes are produced by the present process, the butylenes generally being the main component of the reaction product mixture.

The following examples are presented to further illustrate this invention without limiting the scope of this invention.

Example 1

This example illustrates the dehydrogenation of n-butane over unsupported Mo oxicarbide catalyst. The catalyst can be prepared as described in EP 0 396 475).

Test conditions. The reactant feed was a mixture of n-butane (WHSV 2 h^'1) and hydrogen. The molar gas ratio was H₂/n-C₄ = 9 and 500 mg of catalyst was used. The temperature was increased from 350 °C to 550 °C, the temperature of the reaction, at a rate of 50 °C/min. The reaction pressure was atmospheric pressure. No water vapour or oxygen was used during the reaction.

Cycle of regeneration. Oxidative regeneration was performed in flowing air (20 cmVmin) at the same reaction temperature for 10 min after flushing the apparatus with He (50 cmVmin).

The results of these tests are shown in Table 1.

Example 2

This example illustrates the dehydrogenation of n-butane over a catalyst containing a physical mixture of Mo oxicarbide (500 mg) and silicon carbide (400 mg) (The SiC can be prepared as described in one of the references mentioned in the general part of the description).

7e5t conditions. Reaction conditions were the same as in Example 1.

Cycle of regeneration. The catalyst was regenerated 2 hrs at 350 °C under a stream of flowing air (20 cm³/min).

The results of these tests are shown in Table 2.

Example 3

This example illustrates the dehydrogenation of n-butane over a supported molybdenum oxicarbide. The catalyst is SiC-supported molybdenum oxicarbide (10 - 20 wt-%). 14 Torr of water vapour in the gas stream is used during the processes.

Test conditions . Reaction conditions were the same as in Example 1 except that the total mass of the catalyst sample was 1500 mg (WHSV = 0.67h^"'). Cycle of regeneration. The regeneration conditions were the same as in Example 1, the only difference was the presence of water vapour during this operation.

The results of these tests are shown in Table 3.

Example 4

This example illustrates the dehydrogenation of n-butane over a SiC-supported Mo oxicarbide (10 - 20 wt-%) in the presence of 14 Torr water vapour in the gas stream.

Test conditions. The total mass of the catalyst was 5000 mg (WHSV 0.2 h^"1). After the second regeneration the temperature of the dehydrogenation reaction was 570 °C.

Cycle of regeneration. The regeneration conditions were equal as in Example 3.

The results of these tests are shown in Table 4.

Example 5

This example illustrates the dehydrogenation of n-butane over molybdenum oxicarbide

(10 - 20 wt-%) supported on a cerium doped SiC .

Test conditions. The reaction conditions were the same as in Example 3.

Cycle of regeneration. The regeneration conditions were equal to those of Example 3.

The results of this test are shown in Table 5.

As will be apparent from the results given in Tables 1 to 5, dilution of the bulk molybdenum oxicarbide catalyst and furtheron arranging the active phase on a SiC- support enhance the selectivity of the catalyst. The presence of water vapour in the feed stabilizes the catalytically active oxicarbide phase and further increases remarkably the effectiveness of the catalyst. In addition, the isomerization activity of the catalyst is increased by the usage of water vapour in the feed. The selectivity of the catalyst can still be increased by doping the SiC-support.

Example 6

Two types of mixed oxide based catalysts were synthesized. The first type included molybdenum/tungstene unsupported mixed oxides with an atomic ratio of Mo-to-W in the range of 30 to 1. The second type comprised molybdenum/chromium SiC-supported and unsupported mixed oxides with an atomic ratio of Mo-to-Cr in the range of 0.1 to 10. For the supported oxide the weight ratio of Cr was 3.9 % and 9.88 % for molybdenum.

For dehydrogenation of n-butane, two catalyst samples were used, viz. one catalyst of the molybdenum/tungstene unsupported type having a Mo/W ratio of 15, and one catalyst of the supported Mo/Cr type having a Mo/Cr ratio of 1.5. In both cases, the reaction temperature = 550 °C, WHSV = 2 h^'1, H,/n-C₄ = 9.1, water = 14 torr and m (of catalyst) = 500 mg.

The results are shown in Tables 6 and 7:

Table 6. Conversion of n-butane and selectivity in C₄ products over Mo/Cr mixed oxides

Time on Stream, min

0 10 20 50 100 200 450

Selectivity, % 85 83 86 88 90 93 95

Conversion, % 19.0 17.2 14.5 12.2 11.6 9.0 7.1

Table 7. Conversion of n-butane and selectivity in C₄ products over Mo/W mixed oxides

Time on Stream, min

0 10 20 50 100 200 400

Selectivity, % 68 82 83 85 84 82 75

Conversion, % 2.0 10.0 9.0 6.6 5.0 3.2 2.5

Claims

IN THE CLAIMS:

1. A process for the production of light olefins, comprising contacting a feed containing at least one dehydrogenable hydrocarbon compound at an elevated temperature with a molybdenum oxicarbide catalyst.

2. The process of claim 1 , wherein the feed further contains at least one oxygen- containing compound

3. The process of claim 1 or 2, wherein the specific surface of the catalyst is 1 to 1000 m²/g.

4. The process of any of claims 1 to 3, wherein the catalyst comprises mixed oxicarbides of molybdenum and tungstene and/or chromium.

5. The process of claim 4, wherein the atomic ratio of molybdenum to the other metal(s) of the catalyst is 0.01 to 100, in particular 0.05 to 50.

6. The process of any of claims 1 to 5, wherein the catalyst is supported on silicon carbide.

7. The process of claim 6, wherein the specific surface of the SiC support is 1 to 1000 m²/g.

8. The process of claim 6 or 7, wherein the support is doped with at least one lanthanide, actinide, rare earth metal, or a mixture thereof.

9. The process of claim 8, wherein the support contains 0.5 to 20 wt-% of at least one lanthanide, actinide, rare earth metal, or a mixture thereof.

10. The process of claim 8 or 9, wherein the support contains 0.5 to 20 wt-% of Ce optionally together with at least one other lanthanide and or actinide and/or rare earth metal.

1 1. The process of any of the previous claims, wherein the feed is contacted with the catalyst at a temperature of from 400 to 700 °C.

12. The process of any of the previous claims, wherein the feed is contacted with the catalyst during 0.5 to 100 seconds.

13. The process of any of the previous claims, wherein the feed mass flow contacted with the catalyst per catalyst mass (WHSV) is from 0.1 to 12 t feed/t catalyst/hour.

14. The process of any of claims 2 to 13, wherein the feed is contacted with the catalyst in the presence of 0.01 to 25 mol-% water vapour or oxygen containing compound.

15. The process of claim 14, wherein the oxygen containing compound comprises at least one alcohol or ether.

16. The process of any of claims 2 to 13, wherein the feed is contacted with the catalyst in the presence of 0,01 to 1 wt-% of free oxygen.

17 The process of any of claims 2 to 16, wherein water vapour, the at least one oxygen containing compound or the free oxygen is contained in the feed.

18. The process of any of the previous claims, wherein the hydrocarbon feed contains alkanes having from 2 to 20 carbon atoms and or cycloalkanes having from 5 to 20 carbon atoms.

19. The process of any of the previous claims, wherein the hydrocarbon feed comprises a fraction of petroleum crude oil, or a fraction of a catalytic cracker effluent, or a fraction of shale oil, or a fraction of a product produced by extraction or liquefaction of coal.

20. The process of claim 19, wherein the hydrocarbon feed comprises a petroleum fraction having a boiling point at atmospheric pressure in the range of about 0 to 220 °C.

21. The process of claim 20, wherein the hydrocarbon feed comprises a gasoline or naphtha fraction containing alkanes having from 4 to 12 carbon atoms per molecule.

22. The process of any of the previous claims, wherein ethylene, propylene, butylenes and/or amylenes are produced.