CoFTS pkkJunBae 2009

8/13/2019 CoFTS pkkJunBae 2009

1/7


2/7

strong metal-support interaction, low surface area results in

formation of large cobalt particles. Due to better mechan-

ical properties of SiO2 and TiO2, these are widely used as

supports in a fixed-bed FTS reaction. Also, it has been

reported [6] that metal-support interaction affects the

reducibility of cobalt oxide species in the following order;

Al2O3[TiO2[SiO2 [6]. Oukaci et al. [10] reported that

the FTS activity of the cobalt-supported catalysts varied inthe order of Al2O3[ SiO2[TiO2 for a slurry bubble

column reactor (SBCR). However, a different trend

reported by Reuel and Bartholomew [4] on the catalytic

activity of cobalt-based catalyst as a function of support, is

also noteworthy. They described that the activity declines

in the following order; Co/TiO2[Co/Al2O3[Co/SiO2with respect to reducibility and cobalt particle size. In

addition, support having a large pore size is generally

responsible for the formation of large particle size with

facile reducibility [11]. Although there are a large number

of reports focusing on the influence of support with dif-

ferent textural properties, systematic and comparativestudy on the support such as c-Al2O3, SiO2 and TiO2 with

respect to intrinsic activity has not been established taking

into consideration of the pore size of the support, aggre-

gation phenomenon of the catalyst and particle size of

cobalt species in a slurry-phase FTS reaction.

In the present investigation, the cobalt-based FTS cata-

lysts with three different supports were prepared by

impregnation method and their catalytic activities were

compared with respect to intrinsic activity (TOF), nature of

catalyst aggregation and pore size of support in a slurry-

phase continuous stirred tank reactor (CSTR). The different

FTS activity onc-Al2O3, TiO2 and SiO2 support is further

substantiated by characterization techniques such as BET

surface area measurement, powder X-ray diffraction (XRD)

analysis, temperature-programmed reduction (TPR), H2chemisorption and O2titration.

2 Experimental

2.1 Catalyst Preparation

The cobalt based catalysts were prepared by impregnation

method in the slurry of supports such asc-Al2O3, SiO2and

TiO2with ethanol as a solvent. The i-Al2O3(Catapal B pre-

calcined at 623 K for 5 h, surface area (Sg)of 231 m2/g and

average pore diameter of 7.2 nm), TiO2(Degussa P25 with

size range of 74 * 117 lm which is previously calcined at

723 K for 5 h, Sg of 49 m2/g and average pore diameter of

48.7 nm) and SiO2 (Davisil 645 with size range of

173 * 221 lm used without calcination, Sg of 324 m2/g

and average pore diameter of 10.9 nm) were adopted as

supporting materials for the preparation of catalysts. Cobalt

nitrate (Co(NO3)2 6H2O) was used as a metal precursorand dissolved in ethanol so as to obtain 20 wt% cobalt on

support. The prepared catalyst was dried at 393 K for 12 h

and subsequently calcined at 773 K for 5 h. The final

catalysts i.e., Co/c-Al2O3, Co/SiO2 and Co/TiO2 are des-

ignated as CoA, CoS and CoT, respectively for ease of

presentation.

2.2 Catalytic Activity Test

The schematic reaction apparatus is shown in Fig. 1 that

includes a reactor with capacity of 600 mL equipped with a

magnetic stirrer and two traps (separation units such as hot

and cold trap) operating at 473 and 323 K, respectively to

remove the heavy hydrocarbons and water formed during

FTS reaction. Prior to activity test, the FTS catalyst was

activated at 673 K in a fixed-bed reactor (I.D. = 50 mm)

for 12 h with 5% H2/He. The activated (reduced) catalyst

was transferred to the reactor without the exposure to air.

The activity test was carried out in a slurry-phase contin-uous stirred tank reactor (CSTR, I.D. = 80 mm) under the

following reaction conditions; liquid medium (squa-

lane) = 300 g; catalyst = 5.0 g; T= 493 and 513 K;

P = 2.0 MPa; space velocity (SV; L/kgcat/h) = 2,000 and

1,000; feed composition of H2/CO/CO2/Ar = 57.3/28.4/

9.3/5.0 mol%, respectively. Effluent gas from the CSTR

reactor was analyzed by an online gas chromatograph

(YoungLin Acme 6000 GC) employing GS-GASPRO

capillary column connected with flame ionization detector

(FID) for the analysis of hydrocarbons and Porapak Q/

molecular sieve (5A) packed column connected with

thermal conductivity detector (TCD) for the analysis of

carbon oxides and internal standard gas i.e., Ar.

Fig. 1 Schematic diagram of reaction apparatus for slurry-phase FTS

reaction

J. Oh et al.

1 3


3/7

2.3 Catalyst Characterization

The BET surface area, pore volume and pore size distri-

bution was estimated from nitrogen adsorption and

desorption isotherm data at 77 K using a constant-volume

adsorption apparatus (Micromeritics, ASAP-2400). The

pore volume of the samples was determined at a relative

pressure (P/Po) of 0.99. The calcined sample was degassedat 573 K with a He flow for 4 h before measurement. The

pore size distribution of the samples was determined by the

BJH (Barett-Joyner-Halenda) model from the data of

desorption branch of the nitrogen isotherms.

The powder X-ray diffraction (XRD) patterns of the

samples were obtained with a Rigaku diffractometer using

Cu-Ka

radiation. The reduced samples at 673 K for 12 h

with 5% H2/N2 flow, followed by passivation with 0.1%

O2/He for 0.5 h at room temperature (RT), were used to

identify the crystalline phases of Co3O4, CoO and Co metal

species. The cobalt content (wt%) was further determined

by using the X-ray fluorescence (XRF) analysis withSEA5120 equipment.

The temperature programmed reduction (TPR) experi-

ments were performed to determine the reducibility of

Co3O4. Prior to the TPR experiment, the sample was pre-

treated in a He flow up to 623 K and kept for 2 h to remove

the adsorbed water and other contaminants followed by

cooling to 323 K. The reducing gas containing 5% H2/Ar

mixture was passed over the sample at a flow rate of

30 mL/min with the heating rate of 10 K/min up to

1,000 K. The effluent gas was passed over a molecular

sieve trap to remove the generated water and analyzed by a

GC equipped with TCD.

H2 chemisorption measurement was carried out using a

Micromeritics ASAP 2020C. Before measurement, the

sample was dried in vacuum for 40 min at 673 K and

subsequently reduced at 673 K in flowing H2 for 12 h.

After reduction, the sample was evacuated for 2 h at the

same temperature and the H2 adsorption isotherm was

obtained at 373 K. The H/Co ratio at zero pressure was

found by extrapolation of the linear part of the isotherm.

Particle size estimation was first made based on hemi-

spherical geometry, assuming complete reduction and an

H/Co adsorption stoichiometry of 1 [12, 13] and recon-

sidered with the degree of reduction. The extent of cobalt

reduction was determined by O2 titration of reduced

sample at 673 K, using the same instrument mentioned

above assuming that all the metallic Co was converted to

Co3O4. The crystallite size of cobalt was recalculated by

taking the extent of reduction into consideration.

The calculation of metallic cobalt particle size (d(Co0);

nm) was carried out by using the following equation;

d Co0; nm

96=dispersion % degree of reduction [12, 13].

3 Results and Discussion

3.1 Physicochemical Properties of Catalysts

The summarized results of surface area and pore volume

for CoA, CoT and CoS catalysts are shown in Table 1 and

BJH pore size distribution is shown in Fig. 2. The surface

area increased in the order of CoS[CoA[CoT, how-ever, the average pore size is found to be smallest on CoA

catalyst. Upon impregnation of cobalt on the support, the

surface area decreased, especially on CoA catalyst. The

mono-modal pore size distribution of all catalysts revealed

a uniform distribution of cobalt species on each support.

The larger pore size of 25.1 nm and small surface area of

40 m2/g was observed on CoT catalyst. Support having a

large pore size such as TiO2 and SiO2 is generally

responsible for the formation of large cobalt particles

facilitating the reduction [11]. The small pore size of

6.9 nm and small pore volume on CoA is not beneficial for

diffusion of FTS products generated. Due to uniform poresize distribution of cobalt impregnated on Al2O3 support,

the catalyst shows reduced pore volume and average pore

Table 1 Physical properties of FTS catalysts

Notation Surface

area

(m2/g)

Pore

volume

(cm3/g)

Average

pore

diameter

(nm)

Support

Surface

area

Pore

volume

Pore

diameter

CoA 139 0.326 6.9 231 0.474 7.2

CoT 40 0.263 25.1 49 0.204 16.1

CoS 247 0.992 12.9 336 1.113 11.0

CoA = 20 wt% Co/c-Al2O3; CoT = 20 wt% Co/TiO2; CoS =

20 wt% Co/SiO2

Pore diameter (nm)

001011

dV/dlog

(D)porevo

lume

(cm

3/g)

0

1

2

3

4

5

6

CoA

CoT

CoS

Al2O

3

TiO2

SiO2

Fig. 2 Pore size distribution of CoA, CoT and CoS catalysts with

each support of Al2O3, TiO2 and SiO2

Slurry-Phase FischerTropsch Synthesis

1 3


4/7

size which means efficient deposition of cobalt species in

the inner pore of Al2O3 support. Although the surface area

after cobalt impregnation decreased, the average pore size

of CoT and CoS catalysts slightly increased. This may be

considered mainly due to deposition of large cobalt parti-

cles on the outer surface of SiO2and TiO2. Thus, the inter-

particular large pores induced from deposited cobalt oxidescontribute to the increased average pore size on CoT and

CoS catalysts. The initial average pore size of SiO2 and

TiO2 support is around 11.0 and 16.1 nm, respectively

which increases to 12.9 and 25.1 nm catalyst after cobalt

loading.

Normally, cobalt-support interaction depends on the

type of support and cobalt particle size, and its interaction

is generally stronger on TiO2and weaker on SiO2[46,9].

In the case of CoS catalyst, the interaction of the metal salt

with support is small thus leading to high degree of

reduction of cobalt species. But, since CoA and CoT cat-

alysts show a strong metal-support interaction and are

difficult to reduce completely, the estimated particle size

assuming complete reduction may not always be accurate.

The summary of cobalt particle size, surface area of

metallic cobalt and its dispersion which is recalculated by

considering the reduction degree by O2 titration and cobalt

content by XRF analysis is shown in Table 2. It is observed

that the Co particle size in CoA (15.3 nm) is smaller and

therefore it gives high surface area and larger Co dispersion

in comparison to CoS and CoT catalysts. Due to the weak

metal-support interaction and medium pore size of CoS

catalyst, reducibility of cobalt species is slightly easier thus

yielding medium cobalt particle size.

From XRD analysis, the crystalline phases of cobalt

species in the reduced and successively passivated FTS

catalysts are shown in Fig.3. It is found that the presence

of Co3O4 in CoA catalyst is more prominent than in CoT

and CoS catalysts suggesting relatively homogeneous dis-

tribution of cobalt particles on Al2O3support. To verify the

above result, the particle size of Co3O4is further estimated

from X-ray line broadening using the Scherrers equation.

The particle size of Co3O4at 2h = 36.8and CoO at 62.0

have similar dimensions and its size is around 9.6, 26.8 and

13.9 nm on CoA, CoT and CoS, respectively. Thus, the

particle size of cobalt was bigger in case of CoT and

smalller in CoA catalyst.

Temperature-programmed reduction profiles of FTS

catalyst prepared with different supports are shown in

Fig.4. The TPR data reveal that the different supports

influence the reduction behavior of the catalysts in a dif-

ferent way. The reducibility reflects the extent of metal-

support interaction. As shown in Fig.4, CoA and CoT

show two major peaks with different relative intensity and

CoS had an additional small third peak at around 970 K.

At *620 K, CoT and CoS show the first reduction peak

which is attributed to the reduction of Co3O4 to CoO.

However, in case of CoA, reduction of Co3O4 was much

difficult and needed higher temperature (*750 K). Simi-

larly, second peak is due to metallic cobalt arising out of

reduction of CoO. A third peak in case of CoS catalyst at

around 950 K is considered from the reduction of cobalt

silicate based on the literature [14]. From Fig. 4, one can

see that CoS catalyst is the easiest to reduce to Co metal

but CoA is relatively difficult. The O2titration method was

Table 2 Cobalt particle size and surface area measured by H2 chemisorption and reduction degree by O2 titration

Notation H2 chemisorption Reduction

degree (%)bParticle size

of Co3O4from XRD

Co content (wt%)

from XRFH2 uptakes

(mmol/g)

Co particle

size (nm)aCo surface area

(m2/g-metal)

Co dispersion

(%)a

CoA 0.06015 15.3 44.1 6.5 65.1 9.6 19.8

CoT 0.01678 61.4 11.0 1.6 73.0 26.8 19.9

CoS 0.04685 26.0 25.9 3.8 86.4 13.9 19.8

a Cobalt particle size and dispersion was corrected by considering the reduction degree and Co content which is verified by XRF analysisb The degree of reduction of cobalt oxide is measured by O2 titration by considering theoretical O2 uptake with the following equation;

3Co + 2O2 ! Co3O4

2 Theta (degree)

10 20 30 40 50 60 70 80

Intensity

(a.u.

)

CoA

CoT

CoS

Co3O4

Co

CoO

TiO

Fig. 3 X-ray diffraction (XRD) patterns of reduced and subsequently

passivated FTS catalysts

J. Oh et al.

1 3


5/7


6/7


7/7

deactivation of catalyst. The TOF values of*10 nm cobalt

species without the support are reported in the range of

1.6 9 10-3 * 3.0 9 10-3/s [3, 9, 22]. Therefore, we

expected almost similar TOF from our samples having

particle size about*15 nm. In practice, however, we found

that TOF value is significantly altered. It is therefore

appropriate to consider the difference in TOF values from

our samples due to the difference in pore size of the cata-lysts. The larger the pore size of support e.g., CoT, the higher

the TOF value at all reaction conditions, even though it may

have large cobalt particle size and lower reducibility. In

addition, the observed low TOF value at low space velocity

of 1,000 L/kgcat/h could be attributed to the low concen-

tration of reactants in reaction medium with high CO con-

version. In general, water produced could alter the catalytic

activity depending on its concentration, cobalt particle size

and average pore size of catalysts [20]. It is reported that

water shows a positive effect for SiO2 support (high CO

conversion and selectivity to C5?) at high CO conversion.

But it shows, a negative effect for Al2O3support and only alittle effect for TiO2 [20]. In addition, at a high partial

pressure regime of water, the water was found to be irre-

versibly affecting the catalytic activity. In the present studies

a higher CO conversion on CoS and CoA catalysts due to

narrow pore size distribution and small cobalt particle size

was observed but the abrupt catalyst deactivation could be

attributed to the irreversible effect of water by oxidation of

small cobalt particles. In summary, the present investigation

reveals that cobalt particle size and average pore size of

catalyst simultaneously affect the intrinsic activity in a

slurry-phase FTS reaction and the contribution of pore size

is more significant than particle size of cobalt species. Fur-

thermore, the catalyst aggregation during FTS reaction

could be suppressed by the extent of macro-emulsion for-

mation from water and trace amount of oxygenates includ-

ing higher alcohols in the case of catalysts having a large

pore size such as CoT and CoS.

4 Conclusions

Various supports such asc-Al2O3, TiO2, and SiO2influence

the physico-chemical and catalytic properties of cobalt-

based catalysts in the slurry-phase FTS reaction. Co/c-Al2O3catalyst shows the smallest cobalt particle size and the

highest dispersion but a poor degree of reduction due to the

strong metal-support interaction. Co/SiO2 catalyst demon-

strated the best catalytic performance because of high degree

of reduction of cobalt species. Co/TiO2 catalyst has rela-

tively bigger pore size and it aids an easy diffusion of FTS

products. After the FTS reaction with Co/c-Al2O3catalyst, a

lump of catalyst aggregation was induced by heavy hydro-

carbons formed during FTS reaction. Low concentration of

oxygenates, including C7C12 alcohols, is responsible for

the difficult formation of macro-emulsion and eventually

increased the catalyst aggregation, especially on the catalyst

having small pore size like CoA, and fast catalyst deacti-

vation by the possible irreversible oxidation under the con-

ditions of high water concentration with high CO

conversion. Although the cobalt particle size above 15 nm

has trivial effects on enhancing the intrinsic activity, averagepore size of catalyst significantly affects the intrinsic activity

in a slurry-phase FTS reaction due to the facile diffusion of

heavy hydrocarbons formed inside of catalyst pore.

Acknowledgments The authors would like to acknowledge the

financial support of KEMCO and GTL Technology Development

Consortium (Korea National Oil Corp., Daelim Industrial Co., Ltd,

Doosan Mecatec Co., Ltd, Hyundai Engineering Co. Ltd and SK

Energy Co. Ltd) under Energy & Resources Technology Develop-

ment Programs of the Ministry of Knowledge Economy, Republic of

Korea. P. K. Khanna thanks KOSEF for a Brain Pool fellowship.

References

1. Zhang Y, Liu Y, Yang G, Sun S, Tsubaki N (2007) Appl Catal A

321:79

2. Storster S, Ttda B, Walmsley JC, Tanem BS (2005) J Catal

236:139

3. Iglesia E (1997) Appl Catal A 161:59

4. Reuel RC, Bartholomew CH (1984) J Catal 85:78

5. Riva R, Miessner H, Vitali R, Del Piero G (2000) Appl Catal A

196:111

6. Jacobs G, Das TK, Zhang Y, Li J, Racoillet G, Davis BH (2002)

Appl Catal A 233:263

7. Panpranot J, Goodwin J G Jr, Sayari A (2002) J Catal 211:530

8. Bae JW, Lee YJ, Park JY, Jun KW (2008) Energy Fuels 22:28859. Khodakov AY, Chu W, Fongarland P (2007) Chem Rev

107:1692

10. Oukaci R, Singleton AH, Goodwin J G Jr (1999) Appl Catal A

186:129

11. Khodakov AY, Girardon JS, Griboval-Constant A, Lermontov

AS, Chernavskii PA (2004) Stud Surf Sci Catal 147:295

12. Zhang Y, Wei D, Hammache S, Goodwin JG Jr (1999) J Catal

188:281

13. Xiong J, Borg O, Blekkan EA, Holmen A (2008) Catal Commun

9:2327

14. Moradi GR, Basir MM, Taeb A, Kiennemann A (2003) Catal

Comm 4:27

15. Chu W, Chernavskii PA, Gengembre L, Pankina GA, Fongarland

P, Khodakov AY (2007) J Catal 252:215

16. Park SJ, Bae JW, Oh JH, Chary KVR, Sai Prasad PS, Jun KW,Rhee YW (2009) J Mol Catal A 298:81

17. Schulz H, Nie Z, Ousmanov F (2002) Catal Today 71:351

18. Bae JW, Kim SM, Lee YJ, Lee MJ, Jun KW Catal Commun doi:

10.1016/j.catcom.2009.02.023

19. Okabe K, Li XH, Wei MD, Arakawa H (2004) Catal Today

89:431

20. Dalai AK, Davis BH (2008) Appl Catal A 348:1

21. Chakrabarty T Wittenbrink RJ Berlowitz PJ Ansell LL USP

6677388 B2

22. Bezemer GL, Bitter JH, Kuipers HPCE, Oosterbeek H, Holewijn

JE, Xu X, Kapteijn F, van Dillen AJ, de Jong KP (2006) J Am

Chem Soc 128:3956

Slurry-Phase FischerTropsch Synthesis

1 3
http://dx.doi.org/10.1016/j.catcom.2009.02.023http://dx.doi.org/10.1016/j.catcom.2009.02.023

Documents

CoFTS pkkJunBae 2009