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MULTIFUNCTIONAL CATALYSTS FOR PRODUCTION OF SYNTHESIS-GAS AND OXYGENATES BY DRY REFORMING OF METHANE

Abstract:

The Co-containing catalysts promoted by noble metals have been studied in the dry reforming of methane. It has been observed that the catalysts show the multifunctional properties. Under atmospheric pressure they carry out the reforming of methane by carbon dioxide with production of synthesis gas, and under elevated pressures they also promote the formation of oxygenates.

Keywords: Dry reforming, methane, synthesis-gas, oxygenates, Co-containing catalysts

1. Introduction

The main gas-to-liquids (GTL) interest now is in Fischer–Tropsch synthesis of hydrocarbons. While synthesis gas (syngas) for GTL can be produced from any carbon-based feedstock the lowest cost routes to syngas so far are based on natural gas [1]. There are three ways of natural gas conversion into synthesis gas: partial oxidation (eq. 1), steam reforming (eq. 2) and carbon dioxide reforming of methane (so-called “dry reforming”) (eq. 3):

CH4+1/2O2 ? CO + 2H2 (eq.1)

CH4 + H2O ? CO + 3H2 (eq.2)

CH4 + CO2 ? 2CO + 2H2 (eq.3)

The last one seems to be more attractive and challenging subject for the chemical utilization of natural gas and carbon dioxide, which are substances intimately related to the greenhouse effect and energy resources [2,3]. From an industrial viewpoint, the reaction is also potentially beneficial because it produces CO-rich synthesis gas (H2/CO), in comparison with that from either steam reforming or partial oxidation of methane [3]. A low H2/CO ratio is more suitable for the Fischer–Tropsch synthesis of higher hydrocarbons and for the oxo-synthesis or synthesis of oxygenates [4-6].

Most of the group VIII metals are more or less catalytically active toward the CH4 +CO2 reaction. The numerous supported metal catalysts such as Ni-based catalysts [7-13, et al.] as well as supported noble metal catalysts [2, 14-17 et al.] have been found to exhibit promising catalytic performance in terms of methane conversion and selectivity to synthesis gas. The Co-containing catalysts were less used because of their rapid deactivation due to coke formation and also due to oxidation of the metallic sites by CO2 as it was shown by authors [4]. The catalysts based on noble metals have been reported to be more active and less sensitive to coking [16], but their availability is limited due to high cost. For this reason it is more practical to develop Co- or Ni-based catalysts which are resistant to carbon deposition and exhibit high activity for the reaction.

F. Fischer and H. Tropsch were first observed the high activity of Co in both reactions of dry reforming of methane (1) and synthesis gas conversion (2) [18]. Considering this aspect in this work it has been supposed that syngas produced by the first reaction can be converted into valuable products at the same reactor over the same catalyst under certain conditions. Taking into account the suitable resistance to coking of noble metals, the supported bimetallic catalysts containing both Co and one of noble metals have been synthesized and studied in dry reforming of methane. This work is continuation and a more detailed part of the previous studies [19].

2. Experimental

The bimetallic Co-containing catalysts were prepared by impregnating alumina with a solution containing both the Co and M (M is VIII Group metal) compounds and following thermal treatment. The total metal content in catalysts was 5 wt.%. The ratio of Co:M was 1:1.

The physico-chemical properties of the catalysts at different stages of their preparation and operation were studied by using: TEM, TPR and IR-spectroscopy.

The process was carried out in a flow stainless steel reactor under pressure 0.1 - 2.0 MPa and varying experiment temperature from 200 to 600oC and a quartz flow micro-reactor within temperature region of 200-800oC under atmospheric pressure. Space velocity was varied from 500 to 2000 hr-1. Ratio CO2:CH4 = 1:1 was constant, content of each gas in the initial reacting mixture with Ar was 10 vol. %.

Reactants and products were on-line analysed by using GC’s and also by IR-spectroscopy.

3. Results and Discussion

3.1. Physico-chemical characteristics of the catalysts

The catalysts have been studied by TEM. It has been detected that the bimetallic catalysts have the uniform distribution of metal nano-particles with a size of 1-2 nm. No graphite formation has been found over the Co-M/Al2O3 catalysts used in the process of CO2+CH4.

By TPR it has been shown that introducing the second noble metal into the Co/Al2O3 catalyst leads to shifting temperature of Co reduction into lower values. This is a result of the effect of noble metals facilitated the Co reduction.

The bands of absorption at 2000-2100 cm-1, 2800-2900 cm-1 and 3200-3600 cm-1 observed in IR-spectra of the bimetallic catalysts after adsorption and reaction of CO2+CH4 mix are assigned to OH-, CHx- and CO- adsorbed groups respectively. These results indicate that dissociative adsorption of both CO2 and CH4 are occurred. The following reactions can take a place:

CO2 ? COads + Oads (eq.4)

CH4 ? CHx ads + Hads (eq.5)

Oads + Hads ? OHads (eq.6)

Also, the absorption band at 2310 cm-1 and bands in region of 1440-1600 cm-1 assigned to physically adsorbed CO2 and carbonate-carboxylate groups respectively have been detected.

3.2. Catalysts testing

Testing the catalysts in dry reforming of methane under atmospheric pressure showed that the process of complete methane conversion occurred within temperature region of 650-800oС depending on the catalyst nature (Table 1). No 100% conversion of CO2 was observed. But it needs to take into account that carbon dioxide can be a reaction product formed due to the secondary reactions, which take a place under the highest process temperatures. Due to this reason the real CO2 conversion can be higher than calculated on a base of difference of CO2 content in the initial and reacted gaseous mix.

Table 1. Comparative characteristics of the bimetallic Co-containing catalysts (Со-М =1:1) in the СО2+СН4 reaction: P=1 atm and S.V. = 1000 hr-1

 

Catalyst

T, °C

Degree of conversion, %

СО2

СН4

5%Co-M1/Al2O3

700

92.4

100

5%Co-M2 /Al2O3

650

97.2

100

5%Co-M3 /Al2O3

800

97.1

100

5%Co-M4 /Al2O3

800

94.0

100

Synthesis gas is a main reaction product over these catalysts under atmospheric pressure. In some cases also water is produced.

Under the elevated pressures the behaviour of catalysts is changed. In parallel with synthesis gas oxygenates and water are formed over all the catalysts synthesised (Table 2). Basic oxygenates are methanol and ethanol, but also a little amount of acids, aldehydes and higher alcohols are formed (T = 600°C, Р = 10 atm and space velocity - S.V. = 1000 hr-1). The formation of oxygenates can run due to the conversion of synthesis gas formed and/or direct interaction adsorbed intermediates between each other or with synthesis-gas. The variety of intermediates and products provides a wide number of final products formed.

Table 2. Comparative characteristics of Co-containing catalysts (Со-М =1:1) in the СО2+СН4 (1:1) reaction (T = 600°C, Р=10 atm, and S.V. = 1000 hr-1)

 

Catalyst

Degree of conversion, %

Selectivity of product formation

CO2

CH4

SCO, %

SH2, %

SMeOH, %

SEtOH,%

5% Co-M1/Al2O3

30.0

36.0

84.6

89.5

2.0

3.0

5%Co-M2 /Al2O3

37.7

33.3

83.7

86.4

2.9

2.4

5%Co-M3 /Al2O3

39.0

37.1

86.7

87.7

2.2

2.2

5%Co-M4 /Al2O3

29.7

24.4

82.5

91.7

8.0

0.6

Table 3. The effect of pressure on yield of oxygenates in CO2+CH4 reaction over Co-M4/Al2O3

(CO2:CH4=1:1, S.V. =1000 hr-1, T=600oC)

 

P, atm

Conversion, %

Selectivity, %

CO2

CH4

CO

H2

CH3OH

C2H5OH

Aldehydes +acids+high alcohols

1

51.7

55.6

97.8

97.7

-

-

-

2

48.3

48.1

96.1

98.7

-

-

-

5

34.4

38.8

96.2

91.6

0.4

traces

-

8

28.1

31.9

90.2

98.3

2.2

2.2.

traces

10

24.4

29.7

82.5

81.7

8.0

0.6

0.2

12

24.0

26.1

92.4

89.3

6.0

0.1

0.1

15

20.9

23.1

92.2

87.3

6.0

0.2

0.2

Increase in pressure influences on the process of dry reforming by the same manner over all the catalysts. The effect of pressure is shown for example of the Co-M4/Al2O3 catalyst (Table 3). Degree of conversion of both carbon dioxide and methane decreases from 51.7 and 55.6 to 20.9 and 23.1% respectively with increasing pressure from 1 to 15 atm (S.V. = 1000 hr-1, T = 600oC). It is not wonder taking into account that reaction (eq.2) runs with increasing volume. It is has been shown that the formation of oxygenates is started under pressure higher then 2 atm. Their yield is increased with growing pressures from 5 to 10 atm. The following increasing pressure to 15 atm does not significantly effect on oxygenate yield (Table 3).

Thus, it is possible to conclude that the Co-M/Al2O3 catalysts exhibit the multifunctional properties: under atmospheric pressure they carry out the dry reforming of methane with formation of syngas, and under the elevated pressures in addition they carry the formation of oxygenates. It needs to take into account, that the reaction of polymerisation is occurred for the formation of highest oxygenates.

4. Conclusions

The bimetallic catalysts synthesised have the high activity, stability and resistance to coke formation in the dry reforming of methane.

The Co-containing catalysts promoted by noble metals act as the multifunctional catalysts: - under the atmospheric pressure they decompose methane and carbon dioxide with formation of synthesis-gas; - under elevated pressures – they also provide the formation of oxygenates.

Acknowledgement

Author is grateful to the academician of NAS of RK, Prof. G.D. Zakumbaeva for her assistance and consultation. The special thanks to the laboratory of physico-chemical study of catalysts of IOCE for carrying out the catalysts investigation.

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