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Selective F-T Synthesis for the Production of Middle Distillates

Yuhan Sun, Yao Xu, and Jiangang Chen. State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taoyuan SouthRoad 27, Taiyuan, 030001, China

Introduction

Two categories of Co-based FTS catalysts are being developed for the production of heavy hydrocarbons and middle distillates, respectively. Several methods including sol-gel method, confinement by mesopores, or localized by polymer were adopted to prepare different Co catalysts. By tuning both pore structure and support surface, the Co-catalysts with adjustable conversion of CO and controllable distribution of products were developed with lower CH4 selectivity than 5%. Besides, the correlation of the catalyst structure with the FT performance is being well established.   Research Development of Co-based F-T Catalysts

Co Catalyst Preparation. Co catalysts are usually prepared by the impregnation methods while the co-precipitation and melting methods are preferred for Fe catalyst. Although these techniques have great practical simplicity, their drawbacks are the low maximum loadings and the sometimes-unsatisfactory distribution and low dispersion of the active phase in the ultimate catalysts. The homogeneous deposition-precipitation (HDP) has been developed for the preparation of highly loaded and highly dispersion oxide-supported metal catalyst. In this case, a solvated metal precursor is deposited exclusively onto the surface of a suspended support by the slow and homogeneous introduction of a precipitating agent, which then avoid nucleation of solid precursor compound in the bulk solution.

l        Surface organic modified Co-catalysts. CH3-modified SiO2 (CH3-SiO2) (CH3)2-modified SiO2 ((CH3)2-SiO2), (CH3)3-modified SiO2 ((CH3)3-SiO2), NH2-modified SiO2 (NH2-SiO2) and NH2(CH2)2NH-modified SiO2 (en-SiO2) were prepared through the surface reaction between organotrialkoxysilane and Si-OH groups on SiO2 aerogel.

l        Polymer localized Co-catalysts. The Co/SiO2 catalysts were synthesized by a sol-gel process from 3-aminopropyltriethoxysilane (APTS) as aminopropyl introducer for choice addition on certain condition, polymethylhydrosiloxane (PMHS) as methyl introducer and TEOS as main silica source. The catalysts were named as SipA-RNHB, in which A meant the molar ratio of Si from PMHS to Si from other silica source; B meant the molar ratio of aminopropyl groups in APTS to Co2+ ions.

l        Mesopore confined Co-catalysts. Hollow mesoporous silica sphere (HMSS) was prepared according to literature 1.The supported cobalt catalyst containing 30 wt% Co was prepared by the "two-solvent" technique which reported by this literature 2. The catalyst was eroded to confirm the particle size of Co3O4.   Kinetics

The active sites of Co/SiO2 catalysts for FT synthesis are discriminated kinetically. Several Co/SiO2 catalysts with well-defined structure were prepared. By sol-gel route, silica of various porosity, but of similar chemical property were realized. In TPR profiles, there were four peaks after deconvolution. The relative intensity of those peaks changed as the silica pore size increased, with the low-temperature peaks (547, 588K) enlarging at expense of high temperature peaks. Quantitative analysis suggests that the extent of reduction increased gradually. Those catalysts were evaluated in FT synthesis under conditions of 2.0MPa, 1500GHSV and H2/CO =2. The CH4 production was insensitive to the reducibility whereas the C2+ yield responded to reducibility regularly. This meant that the CH4 formation depended on the total cobalt, no matter how it was reduced. In contrast, the C2+ products (featured with carbon chain growth) only formed on reduced Co. On light of the above results, the F-T synthesis over cobalt was simplified as two reactions with the same feedstock. The product of reaction 1 was CH4 and that of 2 was C2+. These two reactions were independent and was first order to H2 partial pressure. The pre-exponential factors and active energies were derived according to the parallel reactions mechanism. The difference of kinetic constants between two reactions was remarkable. The coincidence of C2+ reaction parameter might confirmed the uniform of active sites metallic Co.   Catalytic performance

1. Performance of surface organic modified Co-catalysts

(CH3-SiO2), (CH3)2-SiO2 and (CH3)3-SiO2 reduced the surface silanol (Si-OH) concentration of SiO2 support, suppressed the interaction between cobalt and silica, enhanced the reducibility of the supported cobalt species, and thus increased the catalytic activity of Co catalysts for FT synthesis (see Table 1). However, coordination compounds wereformed between NH2-SiO2 and Co2+ cations, and thus the interaction between cobalt and silica was enhanced, the reducibility of Co catalysts for FT synthesis was decreased. Because chelated compounds were formed between en-SiO2 and Co2+ cations the supported cobalt catalyst showed the worst performance in FT synthesis (see Table 1).

Table 1. FTS performance of Surface organic modified catalysts

Sample

CO Conversion (%)

Hydrocarbon distribution (wt%)

 

C1

C2-C4

C5+

C5-C11

C12-C18

C19+

Co/SiO2

21.4

36.5

13.5

50.0

34.8

11.3

3.9

Co/en-SiO2

0

-

-

-

-

-

-

Co/NH2-SiO2

4.43

7.0

 

88.3

33.6

48.1

6.6

Co/(CH3)3-SiO2

34.3

19.8

12.0

68.2

29.5

30.2

8.5

Co/(CH3)2-SiO2

45.7

13.7

11.3

75.0

31.1

25.2

18.7

Co/CH3-SiO2

51.8

10.5

10.0

79.5

24.6

30.0

24.9

 

2. Performance of Polymer localized Co-catalysts

Catalyst Sip1.8-RNH0 performed obviously higher catalytic activity in FT synthesis than Sip1.8-RNH0.6 (see Table 2). And hydrocarbons obtained by Sip1.8-RNH0 catalysis were mainly low valuable C1-C4 gas, while high valuable heavy wax was main product from the FT reaction catalyzed by Sip1.8-RNH0.6. The large influence on selectivity might result from the variety of organic groups, because of small difference in other characterization.

Table 2. FTS performance of polymer localized Co-catalysts

Sample

T (C)

CO Conversion (%)

Hydrocarbon distribution (wt%)

C1

C2-C4

C5-C11

C12-C18

C19+

Sip1.8-RNH0

200.1

70.42

68.21

25.98

5.70

0.09

0.03

210.3

89.79

73.22

22.15

4.30

0.24

0.08

Sip1.8-RNH0.6

200.6

39.21

14.57

5.07

1.88

0.28

78.19

210.0

61.78

7.46

2.01

6.70

0.33

83.49

 

3. Performance of mesopore confined Co-catalysts

The Co3O4 particle clusters were 100200 nm, but XRD illuminated the particle size of Co3O4 was 1020 nm. Therefore the Co3O4 particles cluster was composed by smaller mono-dispersed Co3O4 nano-particles, these nano-particles were 1020 nm. Actually, these mono-dispersed Co3O4 nano-particles were loaded equably in the pore channels of the HMMS and were divided by the pore walls with each other. From Table 3 it can be seen that at reaction temperature 210 C the catalyst had good performance and hydrocarbon distribution which concentrated on C5C18 in FT synthesis, methane selectivity was only 4.8 wt%, C5+ selectivity reached up to 93.6 wt%.

Table 3. FTS performance of mesopore confined Co-catalysts

CO Conversion (%)

Hydrocarbon distribution (wt%)

C1

C2-C4

C5-C18

C19-C25

C26+

C5+

83.1

4.8

1.6

70.8

15.7

9.9

93.6

<>Outlook

In summary, the selective synthesis of hydrocarbon could be more controllably carried out via appreciate surface modification or confinement to supported Co-catalysts. This may provide a potential solution for adjusting the products distribution

 

Acknowledgment

This work was supported by the Natural Science Foundation of China (Contract Nos. 20590361 and 20303026) and State Key Foundation Program for Development and Research of China (Contract No. 2005CB221402).