BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a process for preparing hydrocarbons, in particular, to a process for preparing hydrocarbons by hydrogenation of carbon dioxide over Fe-K/Al ₂ 0 ₃ catalyst, which is reduced under hydrogen atmosphere after activated in mixture of carbon dioxide and hydrogen.

Description of Related Art

Carbon dioxide is the so called 'greenhouse gas'. Accumulation of carbon dioxide in the atmosphere caused by a huge amount of the fossil fuel consumption is now regarded as one of the major reasons of the Global Warming.

To conserve the environment, it has to be restricted to use fossil fuel. However, it is expected that it will be necessary to use fuel and materials containing carbon atom continuously. A possible contribution to reducing carbon dioxide accumulation would be the chemical transformation of carbon dioxide into valuable compounds or fuel using suitable catalysts. It would provide the way of recycling carbon dioxide exhaustd from combustion of fuel.

The material manufactured from the reduction of carbon dioxide should be consumed in a wide scope because carbon dioxide is exhauted in enormous volume. Therefore, the effective conversion of carbon dioxide to hydrocarbons would be regarded as one of the mostly promising route for the carbon dioxide fixation by catalytic process.

Processes for preparing hydrocarbons by hydrogenation of carbon

monoxide has been studied widely and applied in commercial quantity. Catalysts and technology used for the hydrogenation of carbon monoxide also can be applied to hydrogenation of carbon dioxide. But chemical properties of carbon monoxide and carbon dioxide are quite 5 different from each other, so the yield of hydrocarbons, especially hydrocarbons having 2 or more carbon atoms (hereinafter called " C ₂₊ hydrocarbons" ) is extremely low.

It requires emergently new catalyst for preparing hydrocarbons in high yield without forrning of by-product such as carbon monoxide. i o C0 ₂ → Hydrocarbons

There are two general methods for preparing C ₂₊ hydrocarbons using carbon dioxide. The first method is comprised of two- step reactions, that is, conversion of carbon dioxide to methanol and continuous conversion of the methanol to hydrocarbons. The second 15 method is reacting carbon dioxide with hydrogen to obtain hydrocarbons directly.

Conventional methods for preparing C ₂₊ hydrocarbons using carbon dioxide according to the above first method are as follows.

Japanese patent non-examination No. 89 - 190,638 discloses a 0 method which carbon dioxide is reduced at 300 ) , 10 atm over catalysts in a fixed bed reactor . The catalysts used in the above reduction are composed of CuO - ZnO - A1 ₂ 0 ₃ and dealuminated H-Y Zeolite of Si0 ₂ /Al ₂ 0 ₃ molar ratio ≤ 10. As the result of reduction, the yield of hydrocarbon was 9.7% and the conversion of carbon 5 dioxide was 20.3 %.

Japanese patent non-examination No. 92 - 120,191 discloses a method which preparing of methanol from carbon dioxide, and C ₂₊ hydrocarbons from methanol are performed in two reactors. Carbon

dioxide is converted to methanol in first reactor, and mixture containing methanol is converted to C ₂₊ hydrocarbons in second reactor.

C0 ₂ + 3H ₂ → CH ₃ OH + H _j O CH ₃ OH → Hydrocarbons

The above two reactions are performed under different reaction conditions such as catalyst, temperature, pressure, etc. due to the difference of their optimum conditions at each reaction. The first reaction is performed appropriately at 250 X. , 80 atm, 4700 h ^"1 of the space velocity over CuO - ZnO - Cr ₂ 0 ₃ - A1 ₂ 0 ₃ catalyst. On the other hand the second reaction is performed appropriately at 300 13 , 1 atm, 1680 h ^"1 of the space velocity. As the result of the first reaction, the conversion of hydrocarbon was 32.1 %, the selectivities to methanol and carbon monoxide were 24.9 % and 7.2 % respectively. And mixture passed through the first reactor was introduced into the second reactor.

In the second reactor the conversion of carbon dioxide to hydrocarbon was not generated, but 32.1% of methanol formed in the first reactor was converted to hydrocarbons.

By the result of conventional studies, when C ₂₊ hydrocarbons was prepared from carbon dioxide, the conversion of carbon dioxide was less than 35% and the yield of hydrocarbon was less than 30 %.

G.A. Somorjai et al. reported that carbon dioxide was reduced by hydrogen at 300 "C , 6 atm over iron based catalyst to form hydrocarbons as desired products. As the result, 97 % of hydrocarbons exists as methane and C ₂₊ hydrocarbons formation is very low [J. Catal., 52, 291(1978)].

Also C. H. Bartholomew et al. reported that carbon dioxide was reduced at 450 ~ 630 K, 1 ~ 11 atm in the molar feed ratio of H ₂ /CQ ₂ = 4

over catalysts. The catalysts were prepared by impregnation of silica with transition metal such as Co, Fe, Ru, etc. in 15% to weight of carrier used. As the result, methane was formed as major, C ₂₊ hydrocarbons were less than 10 % of hydrocarbons and the conversion of carbon 5 dioxide was less than 15% [J. Catal. 87, 352(1984) ^" ].

Also M. D. Lee et al. reported that carbon dioxide was reduced at 320 XZ , 10 atm, 600 ml /g/h of the space velocity and H ₂ /C0 ₂ = 4 over K-Fe(K = 3 atom%) catalyst. As the result, the conversion of carbon dioxide was 35 % and the yields of hydrocarbons and C ₂₊ hydrocarbons i o s were 28 % and 23 %, respectively[Bull. Chem. Soc. Jpn, 62,

2756(1989)].

The inventors of this invention have investigated a new method for preparing hydrocarbons, especially C _{2 +} hydrocarbons by hydrogenation of carbon dioxide. As the result, this invention is i s completed by following method ; Fe-K/Al ₂ 0 ₃ catalyst was reduced with hydrogen and activated with the mixture of carbon dioxide and hydrogen, and then hydrogen and carbon dioxide (H ₂ /C0 ₂ = 1.0 ~ 5.0) were introduced to contact with the pretreated catalyst at 200 ~ 500 t , 1 - 100 atm, 500 ~ 20,000 h ^"1 of the space velocity. 0 In the process according to this invention, the conversion of carbon dioxide was raised highly more than 50 %, the yields of hydrocarbons and C ₂₊ hydrocarbons are more than 50 % and 45 % respectively, and carbon monoxide is generated in minimum.

5 SUMMARY OF THE INVENTION

The object of this invention is to provide a new process for preparing hydrocarbons which are valuable in chemical industry.

The other object of this invention is to provide a new process for

reducing the emission of carbon dioxide into the atmosphere.

This invention relates to a process for preparing hydrocarbon by hydrogenation of carbon dioxide over catalyst, wherein said catalyst is pretreated Fe-K/Al ₂ 0 ₃ with reduction and activation.

5

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to a method for preparing hydrocarbons by reducing and activating of Fe-K/Al ₂ 0 ₃ catalyst, and reacting carbon dioxide with hydrogen under said catalyst. i o Catalyst used in this invention is iron and potassium supported on alumina carrier, and a process for preparing the catalyst is as follows ; Solution of iron-containg salts such as iron chloride, iron nitrate, iron sulfate, iron acetate, iron oxalate etc. or potassium-containg salts such as potassium chloride, potassium carbonate, potassium nitrate,

15 potassium acetate etc. is impregnated in to alumina and mixed. Then it is dried at 80 - 200 XZ for 5 - 48 hours, and calcinated at 400 - 700 XZ for 5 - 48 hours. A catalyst obtained by the above becomes mixed state with iron oxide, potassium oxide and aluminum oxide. Preferably 5 - 50 wt% of iron to a whole catalyst is supported, 0.1 - 1.5 atomic ratio of 0 potassium to iron is mixed and γ -A1 ₂ 0 ₃ is used.

In previous method, potassium has been used as mixing with iron-based catalyst, but the atomic ratio of potassium to iron wasn't more than 0.05. According to this invention if atomic ratio of potassium to iron is less than 0.1, it isn't proper to activate acidic carbon

25 dioxide because basicity of a catalyst doesn't sufficiently increase. If the atomic ratio of potassium to iron is more than 1.5, the activity of catalyst is poor because total contents of iron as activating component is decreased greatly.

In this invention, y -A1 ₂ 0 ₃ not only playes a role of carrier but also increases activity and selectivity of the catalyst through interaction with iron. In case of using silica which only playes a role of carrier, activity and selectivity of iron-based catalyst is not increased. If the

5 content of supported iron is less than 5 wt%, activity of catalyst is decreased and if it is more than 50 wt%, advantages due to interaction with aluminum oxide cannot be gained.

According to this invention to obtain effective catalyst, Fe-K/ A1 ₂ 0 ₃ catalyst must be pretreated by process which is consisted of reduction i o and activation of the catalyst. Reduction is performed by flowing hydrogen into the catalyst at 1 - 10 arm, 300 - 500 t , 20 - 100 ml/g- cat. /min of the flow rate, and iron oxide reduced changes to iron metal. Activation is performed by flowing mixture of hydrogen and carbon dioxide into Fe-K/Al ₂ 0 ₃ at 200 - 400 XZ , 10 - 40 atm, 2 - 200 ml

15 /g-cat. /min of the flow rate, and flowing carrier gas selected from the group which is consisted of nitrogen, argon and helium into Fe- K/A1 ₂ 0 ₃ at 100 - 400 t , 1 - 10 atm, 10 - 100 ml /g-cat. /min. Iron of the activated Fe-K/Al ₂ 0 ₃ catalyst exists as carburized state appropriately, and this effectively acts on activating of carbon dioxide. 0 The reactor for preparing hydrocarbons using carbon dioxide can be fixed bed reactor, fluid ized bed reactor or slurry type reactor of liquid phase. The hydrogenation of carbon dioxide is performed under mixture gas (H ₂ /CO ₂ =1.0 - 5.0 v/v) at 200 - 500 t , 1 - 100 atm, 500 ~ 20,000 h ^"1 of the space velocity. Stoichiometric ratio of the 5 mixture gas (H ₂ /C0 ₂ ) is 3 - 4 v/v. If the condition of hydrogenation is out of this range, the conversion of carbon dioxide goes down. If the ratio of mixture gas is less than 1.0 v/v or more than 5.0 v/v, the conversion of carbon dioxide become too low. If reaction temperature

is lower than 200 XZ , the conversion goes down, and if it is higher than 500 XZ , the selectivity to C ₂₊ hydrocarbons become low because the more reaction temperature is increased, the more formation of methane is increased. If reaction pressure is less than 1 atm, reaction rate is too 5 slow and if it is more than 100 atm, the control of reactions conditions controlling is difficult. If the space velocity is less than 500 h" ¹ , the selectivity is too low and if it is more than 20,000 h ^"1 , the conversion is low because the contact time between reactant and catalyst is too short.

According to the above description the process for preparing i o hydrocarbons is consisted of pretreatment of Fe-K/Al ₂ 0 ₃ catalyst through reduction by hydrogen and activation by carbon dioxide and hydrogen, and the hydrogenation of carbon dioxide over Fe-K/Al ₂ 0 ₃ catalyst pretreated.

Accordingly, this invention is useful to the preparation of C ₂₊ 15 hydrocarbons from carbon dioxide because the conversion of carbon dioxide is high and the selectivity to C ₂₊ hydrocarbons is very high.

This invention may be illustrated in more detail by the following examples but it is not limited by the examples. EXAMPLE 1 0 In 15g of Fe(N0 ₃ ) ₃ • 9H ₂ 0 and 1.3 g of K ₂ C0 ₃ in 100 g of water

10 g of -A1 ₂ 0 ₃ was added and vigorously stirred to evaporate water. After the evaporation of water, the reaction mixture was dried at 120 XZ for 24 hours and calcinated at 500 XZ for 12 hours.

To reduce the catalyst hydrogen was flowed into 0.5 g of

25 calcination-completed Fe-K/Al ₂ 0 ₃ catalyst at 450 XZ , 60 ml /g-cat. / min for 24 hours in a flow-type reactor. To activate the catalyst carbon dioxide and hydrogen(H ₂ /C0 ₂ = 3 v/v) were flowed into Fe-

K/A1 ₂ 0 ₃ catalyst at 10 atm, 300 XZ , 32 ml /g-cat. /min for 16 hours,

and then nitrogen was flowed into Fe-K/ A1 ₂ 0 ₃ catalyst at 200 XZ , 1 atm, 20 ml /g-cat. /min for 1 hour .

To prepare hydrocarbons mixture gas of carbon dioxide and hydrogen(H ₂ /C0 ₂ = 3 v/v) was passed through the pretreated Fe- 5 K/A1 ₂ 0 ₃ catalyst at 300 XZ , 10 atm, 1330 h ^"1 of the space velocity.

The results are given in the following Table 1. EXAMPLE 2

Mixture gas of carbon dioxide and hydrogen(H ₂ /C0 ₂ = 3 v/v) was passed through the Fe-K/Al ₂ 0 ₃ catalyst pretreated by the above i o Example 1 at 300 XZ , 20 atm, 1330 h ^"1 of the space velocity to prepare hydrocarbons.

The results are given in the following Table 1. EXAMPLE 3

Mixture gas of carbon dioxide and hydrogen(H ₂ /C0 ₂ = 3 v/v) is was passed through the Fe-K/Al ₂ 0 ₃ catalyst pretreated by the above

Example 1 at 400 XZ , 20 atm, 1330 h ^"1 of the space velocity to prepare hydrocarbons.

The results are given in the following Table 1. EXAMPLE 4 0 In 15g of Fe(NC- ₃ ) ₃ • 9H ₂ 0 and 0.51g of K ₂ C0 ₃ in 100 g of water

10 g of y -A1 ₂ 0 ₃ was added and vigorously stirred to evaporate water. After the evaporation of water, the reaction mixture was dried at 120 XZ for 24 hours and calcinated at 500 XZ for 12 hours.

To reduce the catalyst hydrogen was flowed into 0.5 g of

25 calcination-completed Fe-K/Al ₂ 0 ₃ catalyst at 400 : , 60 ml /g-cat. / min for 24 hours in a flow-type reactor. To activate the of catalyst carbon dioxide and hydrogen(H ₂ /C0 ₂ = 3 v/v) were flowed into Fe-

K/A1 ₂ 0 ₃ catalyst at 20 atm, 300 XZ , 32 ml /g-cat. /min for 16 hours,

and then nitrogen was flowed into Fe-K/ A1 ₂ 0 ₃ catalyst at 300 XZ , 1 atm, 20 ml /g-cat. /min for 1 hour .

To prepare hydrocarbons mixture gas of carbon dioxide and hydrogen(H ₂ /C0 ₂ = 3 v/v) was passed through the pretreated Fe- 5 K/A1 ₂ 0 ₃ catalyst at 400 XZ , 20 atm, 1330 h ^"1 of the space velocity.

The results are given in the following Table 1. EXAMPLE 5

In 15g of Fe(N0 ₃ ) ₃ ^• 9H ₂ 0 and 2.6 g of K ₂ C0 ₃ in 100 g of water lO g ofy -A1 ₂ 0 ₃ was added and vigorously stirred to evaporate water. ι o After the evaporation of water, the reaction mixture was dried at 120 XZ for 24 hours and calcinated at 500 XZ for 12 hours.

To reduce the catalyst, hydrogen was flowed into 0.5 g of calcination-completed Fe-K/Al ₂ 0 ₃ catalyst at 450 XZ , 60 ml /g-cat. / min for 24 hours in a flow-type reactor. To activate the catalyst 15 carbon dioxide and hydrogen(H ₂ /C0 ₂ = 3 v/v) were flowed into Fe-

K/A1 ₂ 0 ₃ catalyst at 20 atm, 350 XZ , 32 ml /g-cat. /min for 16 hours, and then nitrogen was flowed into Fe-K/ A1 ₂ 0 ₃ catalyst at 300 XZ , 1 atm, 20 ml /g-cat. /min for 1 hour .

To prepare hydrocarbons mixture gas of carbon dioxide and 0 hydrogen(H ₂ /C0 ₂ = 3 v/v) was passed through the pretreated Fe-

K/A1 ₂ 0 ₃ catalyst at 350 XZ , 20 atm, 1330 h ^"1 of the space velocity.

The results are given in the following Table 1. EXAMPLE 6

In 15g of Fe(N0 ₃ ) ₃ • 9H ₂ 0 and 2.6 g of K ₂ C 0 ₃ in 100 g of 5 water,10 g of y -A1 ₂ 0 ₃ was added and vigorously stirred to evaporate water. After the evaporation of water, the reaction mixture was dried at 120 XZ for 24 hours and calcinated at 500 XZ for 12 hours.

To reduce the catalyst, hydrogen was flowed into 0.5 g of

calcination-completed Fe-K/Al ₂ 0 ₃ catalyst at 450 XZ , 80 ml /g-cat. / min for 24 hours in a flow-type reactor. To activate catalyst, carbon dioxide and hydrogen(H ₂ /CO _z = 3 v/v) was flowed into Fe-K/Al ₂ 0 ₃ catalyst at 30 atm, 300 XZ , 32 ml /g-cat. /min for 16 hours, and then 5 nitrogen was flowed into Fe-K/Al ₂ 0 ₃ catalyst at 300 X , 1 atm, 40 ml/ g-cat. /min for 2 hours .

To prepare hydrocarbons mixture gas of carbon dioxide and hydrogen(H ₂ /C0 ₂ = 3 v/v) was passed through the pretreated Fe- K/ A1 ₂ 0 ₃ catalyst at 300 XZ , 30 atm, 1330 h ^'1 of the space velocity. ι o The results are given in the following Table 1.

COMPARATIVE EXAMPLE 1

Hydrocarbons were prepared by the same manner with the above Example 1 except Fe-K /silica catalyst was used instead of Fe- K/A1 ₂ 0 ₃ catalyst. 15 The results are given in the following Table 1.

COMPARATIVE EXAMPLE 2

Hydrocarbons were prepared by the same manner with the above Example 1 except Fe-K/silica catalyst was used instead of Fe- K/A1 ₂ 0 ₃ catalyst and K ₂ C0 ₃ wasn't added. 0 The results are given in the following Table 1.

COMPARATIVE EXAMPLE 3

Hydrocarbons were prepared by the same manner with the above Example 1 except activation of catalyst was not performed.

The results are given in the following Table 1. 5 COMPARATIVE EXAMPLE 4

Hydrocarbons were prepared by the same manner with the above Example 4 except for using 0.13g of K ₂ C0 ₃ .

The results are given in the following Table 1.

COMPARATIVE EXAMPLE 5

Hydrocarbons were prepared by the same manner with the above Example 1 except for using catalyst prepared as follows ;

Ammonium hydroxide was slowly added in 15g of Fe(N0 ₃ ) ₃ • 9H ₂ 0 in lOOg of water to be pH 8.5 and to form precipitates. Then the precipitates were dried at 120 XZ for 24 hours and calcinated at 500tl for 12 hours. This was mixed with 1.3 g of potassium carbonate solution in 100 g of water and vigorously stirred, and water was evaporated by heating. After the evaporation of water, it was dried at 120 tl for 12 hours and calcinated at 500 tl for 3 hours.

The results are given in the following Table 1. Table 1.

Conversion Yield (C %)

Example rate (%) hydrocarbons c,*

Example 1 57.7 50.4 3.6 46.8

Example 2 56.0 54.7 4.3 50.4

Example 3 69.6 66.9 10.7 56.2

Example 4 67.3 63.3 11.5 51.8

Example 5 70.4 68.6 7.8 60.8

Example 6 61.8 60.1 3.3 56.8

Comp. Example 1 14.7 5.3 0.7 4.6

Comp. Example 2 22.1 8.5 5.1 3.4

Comp. Example 3 31.8 25.6 3.5 22.1

Comp. Example 4 49.7 45.7 20.7 25.0

Comp. Example 5 27.7 21.0 2.3 18.7

C,* : methane

C ₂₊ ** : hydrocarbons having 2 or more carbon atoms