Non-linear ASF product distribution over alkaline-earth promoted molybdenum carbide catalysts for hydrocarbon synthesis

Please cite this article in press as: D.-V.N. Vo, et al., Non-linear ASF product distribution over alkaline-earth promoted molybdenum carbide

catalysts for hydrocarbon synthesis, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.02.002

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CATTOD-8336; No. of Pages 8

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Non-linear ASF product distribution over alkaline-earth promoted

molybdenum carbide catalysts for hydrocarbon synthesis

Dai-Viet N. Vo a,c, Viswanathan Arcotumapathy a, Bawadi Abdullahb, Adesoji A. Adesina a,∗

a Reactor Engineering & Technology Group, School of Chemical Engineering, The University of New South Wales, Sydney 2052, Australiab Chemical Engineering Department, Universiti Teknologi PETRONAS, 31750 Tronoh, Perak, Malaysiac Chemical Engineering Program, Texas A&M University at Qatar, P.O. Box 23874, Doha, Qatar

a r t i c l e i n f o

Article history:

Received 2 April 2012

Received in revised form 14 January 2013

Accepted 5 February 2013

Available online xxx

Keywords:

Fischer–Tropsch synthesis

Promoted molybdenum carbide

Temperature-programmed carburization

a b s t r a c t

Alkaline earth (Ba, Ca and Mg) promoted Mo carbide catalysts have been synthesized using temperature-

programmed carburization for Fischer–Tropsch synthesis. MoO3 precursor was converted completely to

final carbide form including �- and �-MoC1−x phases during carburization runs. Carbide production rate

increased with promoter addition whilst a reduction in activation energy of Mo carbide phase formation

was observed for promoted catalysts. CO2- and NH3-temperature-programmed desorption runs indi-

cated that Mo carbide catalysts possessed both acid and basic sites with each possessing both weak and

strong types. Promoter addition enhanced both CO2 and CO uptake but decreased H2 chemisorption. CO

adsorption appeared to be greater than H2 chemisorption with superior uptake and heat of desorption.

CO consumption rate improved with promoters in the order; Ca > Ba > Mg > undoped catalysts parallel to

the trend for strong basic site concentration and CO uptake. Optimal FT activity was observed at H2 mole

fraction of 0.75 for all catalysts. Alkaline-earth promoters increased chain growth probability by up to

63% whilst two different chain growth factors were detected for Ca- and Ba-promoted catalysts and may

be attributed to the existence of new FT sites in the CaMoO4 and BaMoO4 phases formed in these two

catalysts as confirmed by XRD measurements.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Molybdenum carbide has become an attractive and promising

catalyst for replacement of traditional Fischer–Tropsch (FT) cata-

lysts (such as Co- and Fe-based catalysts [1]) for higher hydrocarbon

production due to its resistance to sulphur poisoning [2] and carbon

deposition [3] since it was originally discovered to have Pt-like cat-

alytic properties by Levy and Boudart [4]. Although molybdenum

carbide may be produced by several methods, namely; pyroly-

sis of metal precursors, solution reactions, gas-phase reaction of

volatile compounds, alkalide reduction and sonochemical synthesis

[5], the most common technique for Mo carbide catalyst synthesis

is temperature-programmed carburization between MoO3 precur-

sor and a mixture of H2/hydrocarbon as carburizing agent [4,6].

The degree of carburization may be influenced by reaction tem-

perature, feed gas (carbon source) composition and carburization

time [7]. Xiao et al. found that the employment of higher hydro-

carbon during temperature-programmed carburization facilitated

∗ Corresponding author. Tel.: +61 2 9385 5268; fax: +61 2 9385 5966.

E-mail addresses: [email protected],

[email protected] (A.A. Adesina).

the transformation of Mo oxide precursor to carbide phase with

lower carbide formation temperature and hence, higher BET sur-

face area of MoC1−x (0 ≤ x < 1) catalyst [6]. Vo and Adesina found

that 5H2:1C3H8 mixture was the optimal feed composition for

Mo carbide formation rate [8]. Thus, in this study, MoC1−x cata-

lyst preparation was carried out using H2/C3H8 = 5:1. Ca-promoted

transition metal FT catalysts have been shown to suppress methane

formation and improve C5+ selectivity as well as FT activity [9]. Mo

carbide catalyst system may have similar benefits from basic metal

oxide additives. Therefore, the objective of this paper was to inves-

tigate the role of alkaline-earth promoters on the physicochemical

properties of alumina-supported MoC1−x catalysts and FT reaction

metrics.

2. Experimental

Mo carbide catalyst system was prepared using co-

impregnation and temperature-programmed carburization

method with detailed description in previous studies [10,11]. An

accurately measured amount of (NH4)6Mo7O24·(4H2O (3.68 g)

and the corresponding promoter precursors including Ca(NO3)2

(2.46 g), Ba(NO3)2 (1.14 g) or Mg(NO3)2 (3.66 g) were dissolved

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2 D.-V.N. Vo et al. / Catalysis Today xxx (2013) xxx– xxx

in 40 ml purified H2O. This solution was used for impreg-

nation with 17.4 g of thermally pre-treated �-Al2O3 support

(heated up to 973 K in purified air for 6 h with a heating rate

of 5 K min−1 to ensure thermal stability). The slurry was stirred

for 3 h and subsequently dried in an oven for 16 h at 393 K

(named as uncalcined solid sample, UC-CAT). Calcination in air

was carried out for the resulting solid samples at 773 for 5 h

to produce 3 wt%X–10%MoO3/Al2O3 (X: Ca, Ba or Mg) catalysts.

Temperature-programmed carburization runs were conducted

in a computer-controlled fixed-bed reactor between calcined

solid samples and a mixture of 50 ml min−1 5H2:1C3H8 at 973 K

for 2 h for the formation of 3 wt%X–10%MoC1−x/Al2O3 catalysts.

The fixed-bed reactor was subsequently flushed with N2 flow to

cool down to FT reaction temperature at the end of carburization

runs. FTS evaluation was carried out in situ in the same fixed-bed

reactor with varying H2:CO = 1:5–5:1 at 473 K and atmospheric

pressure. Fibre glass insulation was placed at the top and bottom

ends of the fixed-bed reactor to minimize heat loss and pre-

vent product condensation. Additionally, heating tape wrapped

around reactor exit line kept it at 473 K to ensure that long chain

hydrocarbons remained in gas phase for online GC composition

measurement. Gas hourly space velocity (GHSV) was kept constant

at 10 L gcat−1 h−1 for all runs using 0.15 gcat per run and average

catalyst particle size, dp = 100 �m to ensure negligible transport

intrusions.

BET surface area and pore volume were determined at 77 K in

a Quantachrome Autosorb-1 unit using N2 physisorption. X-ray

diffraction measurements for both promoted and unpromoted Mo

carbide catalysts were carried out on a Philips X’pert Pro MPD

system with Ni-filtered Cu K� (� = 1.542 A) at 45 kV and 40 mA.

Temperature-programmed desorption (TPD) including NH3-, CO2-,

CO- and H2-TPD was measured on a Micromeritics 2910 AutoChem

unit at different ramping rates (5–30 K min−1) to estimate site

concentration (NH3, CO2, CO and H2 uptake respectively) and

site strength (heat of desorption, Ed). Approximately, 0.15 g of

MoC1−x catalyst placed in a quartz U-tube was exposed to 10%NH3,

10%CO2, 10%CO or 10%H2 diluted in inert gas for 1 h to conduct

corresponding NH3-, CO2-, CO- or H2-TPD measurements. Cata-

lyst samples were subsequently heated up to 973 K and desorbed

gases were detected by an online TCD detector. Thermogravi-

metric studies, namely; temperature-programmed calcination and

carburization runs were performed in a ThermoCahn TGA 2121

unit. In temperature-programmed calcination, about 65 mg of solid

sample, UC-CAT, was placed in a quartz sample boat and heated

up to 393 K in Ar flow. Catalyst was held isothermally at this

temperature for 30 min to remove moisture and any volatile com-

pounds followed by ramping up to 973 K in purified air and keeping

constant for 60 min before cooling down to 303 K in inert (Ar)

gas. Temperature-programmed carburization runs were performed

using 5H2:1C3H8 (50 ml min−1) from 303 to 973 K with different

heating rates in the range 10–20 K min−1.

3. Results and discussion

3.1. X-ray diffraction analysis

XRD patterns of both promoted and unpromoted Mo carbide

catalysts shown in Fig. 1 were analyzed basing on the Joint

Committee on Powder Diffraction Standards (JCPDS) database

[12]. These high intensity peaks detected at about 2� = 45.7◦ and

66.7◦ for all catalysts belonged to �-Al2O3 support. Interestingly,

doped and undoped MoC1−x catalysts possessed both hexagonal

closed packed (HCP) �-MoC1−x with characteristic peak located at

2� = 39.5◦ and face-centred cubic (FCC) �-MoC1−x with 2� = 36.6◦

and 61.3◦ for [1 1 1] and [2 2 0] phases respectively in agreement

Fig. 1. X-ray diffractograms for the Mo carbide catalyst system. (a)

10%MoC1−x/Al2O3 , (b) 3%Mg–10%MoC1−x/Al2O3 , (c) 3%Ca–10%MoC1−x/Al2O3 ,

(d) 3%Ba–10%MoC1−x/Al2O3. .

with other studies [13]. The typical peaks for MoO3 (2� = 23.40◦,

25.50◦ and 26.75◦) were not observed for all patterns indicating a

complete conversion of Mo oxide to final carbide phase at 973 K

and 5H2:1C3H8.

Fig. 1b shows the XRD pattern for 3%Mg–10%MoC1−x/Al2O3. The

peaks detected at 2� = 19.54◦ and 31.59◦ were assigned to the for-

mation of MgAl2O4 which may be produced during calcination via;

Mg(NO3)2 → MgO + 2NO2 +1

2O2 (1)

and

MgO + Al2O3 → MgAl2O4 (2)

As seen in Fig. 1c, CaMoO4 phase was detected on Ca-promoted

catalyst surface with characteristic peaks located at 2� of 18.65◦,

28.98◦, 31.28◦, 34.29◦, 47.09◦, 49.28◦, 53.14◦, 56.24◦, 58.19◦ and

59.24◦. The formation of CaMoO4 phase was most likely due to

thermal decomposition of Ca(NO3)2 precursor given as;

Ca(NO3)2 → CaO + 2NO2 +1

2O2 (3)

and

CaO + MoO3 → CaMoO4 (4)

The interaction between BaO promoter and MoO3 precursor

was also evidenced for 3%Ba–10%MoC1−x/Al2O3 catalyst as seen in

Fig. 1d. The BaMoO4 phase (with 2� = 32.06◦) was probably formed

from;

Ba(NO3)2 → BaO + 2NO2 +1

2O2 (5)

and

BaO + MoO3 → BaMoO4 (6)

The average crystallite size for Mo carbide catalysts (cf. Table 1)

was estimated from Scherrer equation [14] given as;

dp =0.9�

B cos �(7)

with dp, �, B and � being crystallite dimension, wavelength, peak

width and Bragg angle respectively. The small average crystallite

size (dp < 10 nm) and high BET surface area of 152–194 m2 gcat−1

close to that of pure Al2O3 support for both doped and undoped

MoC1−x/Al2O3 catalysts suggest that MoC1−x particles were finely

dispersed on support surface. Generally, MoC1−x crystallite size






D.-V.N. Vo et al. / Catalysis Today xxx (2013) xxx– xxx 3

Table 1

Physical properties of both promoted and unpromoted Mo carbide catalysts.

Catalyst BET surface

area

(m2 g−1)

Average

pore

volume

(cm3 g−1)

Average

crystallite

size (nm)

Pure Al2O3 179.3 0.68 –

10%MoO3/Al2O3 182.4 0.55 –

10%MoC1−x/Al2O3 194.0 0.73 9.8

3%Ca–10%MoC1−x/Al2O3 152.3 0.64 5.1

3%Ba–10%MoC1−x/Al2O3 169.8 0.71 6.1

3%Mg–10%MoC1−x/Al2O3 160.2 0.52 5.5

reduced with promoter addition from 9.8 to 5.1 nm. As seen in

Table 1, promoted catalysts possessed lower BET surface area and

average pore volume than those of undoped catalyst probably due

to pore blockage by alkaline-earth metal oxides.

3.2. Thermogravimetric studies

3.2.1. Temperature-programmed calcination

Fig. 2 shows the derivative weight profile of the temperature-

programmed calcination runs for UC-CAT samples. The low

temperature peak, P1 at 415–440 K observed in all runs may be

ascribed to Mo oxide production given as;

(NH4)6 + Mo7O24 → 6NH3 + 7MoO3 + 3H2O (8)

whilst the second peak P2 may belong to the formation of MgO

(516 K), CaO (510 K) and BaO (530 K) as seen in Eqs. (1), (3)

and (5) respectively. The high temperature peaks, P3 located at

705 K (Mg-doped), 625 K (Ca-doped) and 760 K (Ba-doped cat-

alyst) were probably due to the production of corresponding

MgAl2O4, CaMoO4, and BaMoO4 phases (cf. Eqs. (2), (4) and (6)

respectively). The formation of bimetallic oxides is consistent with

results from XRD measurements (cf. Fig. 1). As seen in Fig. 2,

there was no detectable peak beyond 773 K for all catalysts sug-

gesting that both promoted and unpromoted MoO3 precursors

were completely thermally decomposed to metal oxides at or

below 773 K.

3.2.2. Temperature-programmed carburization

The influence of alkaline-earth promoters on temperature-

programmed carburization of 10%MoO3/Al2O3 catalyst with

5H2/1 C3H8 is depicted in Fig. 3. The low temperature peak (P1)

and the second peak (P2) detected for both promoted and unpro-

moted catalysts were assigned to the formation of oxycarbide and

Fig. 2. Derivative weight profile for temperature-programmed calcination runs of

both promoted and unpromoted 10%MoO3/Al2O3 catalysts.

Fig. 3. Derivative weight profile for temperature-programmed carburization runs

of promoted 10%MoC1−x/Al2O3 catalysts.

carbide phases respectively. Carburization temperature seemed

to be higher for Ca- and Ba-promoted catalysts than undoped

catalyst whilst Mg promotion had a negligible effect on car-

bide formation temperature (cf. Fig. 3a). However, as seen in

Fig. 4, carbide formation rate estimated from derivative weight

profile during temperature-programmed carburization increased

with dopant addition in the order; Ca > Ba > Mg > unpromoted cat-

alyst.

Since peak temperature for both P1 and P2 increased linearly

with heating rate employed as seen in Fig. 3b, activation energy,

Fig. 4. Influence of promoter addition on carbide formation rate at 20 K min−1 .







Fig. 5. Activation energy, Ea for the formation of both oxycarbide and carbide phases

over Mo carbide catalysts.

Ea (cf. Fig. 5.) for the production of oxycarbide and carbide phases

may be estimated from Kissinger equation [15] given as;

ln

(

ˇ

T2P

)

= ln

(

AR

Ea

)

−Ea

RTP(9)

with ˇ, Tp and A being heating rate, peak temperature and pre-

exponential factor respectively whilst R is universal gas constant.

Interestingly, a reduction in activation energy for carbide phase

formation was observed for the three promoted catalysts and Ea

decreased in the order; unpromoted > Mg > Ba > Ca promoted cat-

alysts parallel to the trend for carbide formation rate. In general,

Ea of carbide phase seemed to be greater than that of oxycarbide

form for all catalysts in agreement with previous studies [10] indi-

cating that oxycarbide phase formation was more facile than the

production of final carbide phase.

The Arrhenius parameters for both the oxycarbide and carbide

phase formation satisfied the relation;

ln Aj = aEaj+ b (10)

where a and b are constants (cf. Table 2) with correlation coeffi-

cient, R2 > 0.99 suggesting the presence of a ‘compensation effect’.

As seen in Fig. 6, activation energy and pre-exponential factor for

both carbide phases exhibited a good fit the compensation effect

model. Compensation effect and isokinetic relationship have been

employed to justify the similarity in reaction mechanism for a series

of reactions over a specified catalyst [16] or a single reaction over a

group of catalysts [17,18]. However, the occurrence of a compensa-

tion effect does not necessarily guarantee the existence of isokinetic

phenomenon. Based on the criteria recommended by Liu and Guo

[16], the requirement for the presence of isokinetic relationship

may be given as [19];

�G /=

j=

(

1

a− RT

)

ln

(

Ajh

kBT

)

+ b (11)

with h and kB being Planck and Boltzmann constants, respectively.

�Gj/= is the Gibbs free energy for the associated transition-state

complex during carburization. If the plots of �Gj/= against T exhibit

a common intersection point, the existence of isokinetic rela-

tionship may be postulated. As seen in Fig. 7, the unambiguous

Table 2

Estimates of associated model parameters for the compensation effect model.

Model parameters Oxycarbide

phase (P1)

Carbide

phase (P2)

a 1.87 × 10−4 1.61 × 10−4

b −1.91 −2.88

Fig. 6. Evidence for the existence of a compensation effect for both oxycarbide and

carbide phases of promoted and unpromoted Mo carbide catalysts.

inference of the isokinetic phenomenon may be deduced for oxy-

carbide and carbide phases with the isokinetic temperature, Tiso

of 645 and 746 K respectively. The presence of compensation effect

and isokinetic relationship suggest that the formation of oxycarbide

from the MoO3 precursor and the transformation of oxycarbide to

the final carbide phase for all catalysts were governed by a simi-

lar topotactic mechanism, in which oxygen atoms in the lattice of

Fig. 7. Proof for the presence of the isokinetic relationship for both oxycarbide and

carbide phases of the Mo carbide catalyst system.







Fig. 8. Effect of promoter on H2 and CO uptake for Mo carbide catalysts.

Mo oxide were replaced by carbon atoms with negligible structural

disruption.

3.3. Physicochemical properties

Table 3 summarizes the physicochemical properties of alkaline-

earth promoted Mo carbide catalysts. NH3-TPD and CO2-TPD

measurements showed that MoC1−x catalysts possessed both weak

(W) and strong (S) acid as well as basic sites consistent with other

studies [10,20]. Nevertheless, weak acid site was also identified for

pure Al2O3 support suggesting that the strong acid site may be

located in the MoC1−x phase or the Mo-support interface. Heat of

desorption, Ed for strong acid and basic sites was always greater

than that of corresponding weak sites. Acidic-to-basic site con-

centration ratio appeared to be higher than unity for both weak

and strong sites of the Mo carbide catalyst system indicating that

MoC1−x surface is comparatively acidic. Both basic site concentra-

tion (CO2 uptake) and specific basic site strength (product of CO2

heat of desorption and the CO2 site density) of the S-basic site were

enhanced with dopant addition. A companion reduction in NH3

uptake and specific site strength for S-acid site was also observed.

The H2- and CO-TPD properties on MoC1−x catalyst surface

are depicted in Figs. 8 and 9. Although H2 chemisorbed on both

promoted and unpromoted Mo carbide catalysts, H2 uptake and

heat of desorption were lower than the corresponding estimates

for CO suggesting that H2 may exist in gas phase and react with

molecularly adsorbed CO for chain growth [21]. Although there is

evidence for dissociative adsorption of CO on Mo carbide [22,23],

St. Clair et al. [24] reported molecular chemisorption on Mo

Fig. 9. Influence of promoter on H2 and CO heat of desorption for Mo carbide cata-

lysts.

Fig. 10. Effect of feed composition on (−rCO) over Mo carbide catalysts at 473 K.

carbide catalyst surface under conditions similar to those used in

this study. Furthermore, the synthesis of alcohols over Mo carbide

implicates the hydrogenation of molecular surface CO [21,25,26]

within the FTS temperature range. Promoter addition improved

CO uptake in the order; Ca > Ba > Mg > unpromoted Mo carbide cat-

alysts since increasing electron density on catalyst surface donated

by basic promoters may facilitate CO chemisorption [27,28].

However, alkaline-earth promoters appeared to be detrimental to

H2 adsorption (cf. Fig. 8).

3.4. Fischer–Tropsch synthesis evaluation

3.4.1. Effect of feed composition

The influence of feed composition on CO consumption rate,

(−rCO) over both unpromoted and promoted MoC1−x catalysts is

displayed in Fig. 10. It is evident that consumption rate increased

with H2 mole fraction, yH2and peaked at yH2

= 0.72 for all cat-

alysts. Generally, FT activity improved with promoter addition in

the order; Ca > Ba > Mg > unpromoted 10%MoC1−x/Al2O3 catalysts.

Interestingly, the trend for (−rCO) was parallel to that for CO uptake

suggesting that CO chemisorption site may be the active site for

Fischer–Tropsch synthesis over Mo carbide catalyst. As seen in

Fig. 11, the nonlinear (power-law) increase in CO consumption rate

with increasing CO uptake regardless of feed composition further

supports this proposition.

Fig. 11. Effect of CO uptake on (−rCO) over Mo carbide catalysts at 473 K.







Table 3

Physicochemical properties of both doped and undoped Mo carbide catalysts.

Catalyst MoC1−x/Al2O3 Ca–MoC1−x/Al2O3 Ba–MoC1−x/Al2O3 Mg–MoC1−x/Al2O3 Al2O3 support

NH3 uptake (mol NH3 m−2 × 107) W 6.65 7.09 4.22 6.49 10.50

S 11.40 5.31 6.42 7.80 –

Heat of desorption for NH3 (kJ mol−1) W 37.91 37.64 47.94 46.13 50.00

S 320.12 53.02 70.82 79.17 –

Specific acid site strength (kJ m−2 × 105) W 2.52 2.67 2.02 2.99 5.24

S 36.60 2.81 4.55 6.18 –

CO2 uptake (mol CO2 m−2 × 107) W 1.40 0.37 0.44 0.14 1.47

S 0.32 4.99 2.99 2.71 0.91

Heat of desorption for CO2 (kJ mol−1) W 34.91 35.90 25.80 57.72 50.00

S 137.07 167.41 83.66 69.91 57.42

Specific basic site strength (kJ m−2 × 105) W 0.49 0.13 0.11 0.08 0.74

S 0.44 8.35 2.50 1.89 0.52

Acid:basic site ratio W 4.74 18.95 9.69 45.22 7.12

S 35.81 1.06 2.15 2.88 –

Table 4

Estimates of associated ROP model parameters for alkaline-earth promoted Mo carbide catalysts at 473 K and H2/CO = 2:1.

Catalyst 10%MoC1−x/Al2O3 3%Ca–10%MoC1−x/Al2O3 3%Ba–10%MoC1−x/Al2O3 3%Mg–10%MoC1−x/Al2O3

ROPmax 2.97 ± 0.08 0.72 ± 0.02 1.87 ± 0.01 0.70 ± 0.01

nmax 3.09 ± 0.08 3.32 ± 0.03 3.28 ± 0.01 3.62 ± 0.01

bROP 1.19 ± 0.03 1.09 ± 0.08 1.04 ± 0.01 0.98 ± 0.02

3.4.2. Fischer–Tropsch product distribution

Olefin-to-paraffin ratio (ROP) for C2+ hydrocarbons may be esti-

mated from;

ROPCn =rOlefinCn

rParaffinCn

(12)

with rOlefinCn

and rParaffinCn

being olefin and paraffin formation rates

with carbon number n respectively.

Fig. 12 shows that ROP was a function of carbon number, n

and exhibited an optimal value at n = 3–4. However, a reduction in

olefin-to-paraffin ratio beyond n = 4 was observed for all catalysts

most likely due to lower diffusion coefficients [29] or increasing

adsorptivity [30,31] for �-olefins with chain length. The lower

ROP value for C2 than that of C3 hydrocarbon may be assigned to

the rapid readsorption of ethene for secondary reactions, namely;

incorporation of ethene for chain propagation [31,32] and ethene

hydrogenolysis [33]. As seen in Fig. 12, ROP reduced with alkaline-

earth promoters and Mg-doped catalyst gave the poorest ROP

value. This is somewhat unexpected but may be due to stronger

olefin adsorption compared to paraffin in the promoted catalysts

Fig. 12. Effect of promoter addition on ratio olefin-to-paraffin (ROP) for Mo carbide

catalysts at 473 K and H2:CO = 2:1.

although the total amount of olefins produced was still higher

on the latter catalyst class. The CO consumption rate-composition

envelopes (cf. Fig. 10) and ROP profiles retained essentially the

same shape suggesting that the FT mechanism was unaffected by

the lack or type of promoter although the relative rates of individual

elementary steps may change.

The ROP profile in Fig. 12 may be expressed by;

ROP =ROPmax

1 + ((n − nmax)/bROP)2

(13)

where ROPmax is the optimal ROP value for hydrocarbon with car-

bon number nmax whilst bROP is an associated model parameter. The

estimates for the ROP model parameters are summarized in Table 4.

The parity plot (cf. Fig. 13) shows a good fit of experimental data to

the ROP model.

Fig. 14 shows the effect of promoter addition on total olefin-to-

paraffin ratio (TOPR) defined as Eq. (14).

TOPR =

∑∞

n=1olefin

∑∞

n=1rparaffinCn

(14)

Fig. 13. Parity plot for ROP model over the Mo carbide catalyst system at 473 K and

H2:CO = 2:1.







Fig. 14. Total olefin-to-paraffin ratio over Mo carbide catalysts at 473 K and different

feed compositions.

TOPR value increased with decreasing H2 mole fraction and

attained the optimum at yH2= 0.17 to 0.25 for all catalysts. The

similarity in TOPR-composition profiles for both promoted and

unpromoted catalysts further confirmed that hydrocarbon pro-

duction over MoC1−x catalysts was governed by the same FT

mechanism. The reduction in TOPR with increasing yH2was due

to increased termination rate of olefinic species on catalyst surface

to paraffins [8]. Ba promoter appeared to be the optimal dopant in

terms of olefin selectivity and better than the unpromoted catalyst.

However, TOPR values for both Ca- and Mg-doped catalysts were

lower than in the undoped Mo carbide.

Fischer–Tropsch product distribution is generally governed

by a polymerization scheme which may be captured by the

Anderson–Schulz–Flory (ASF) model [34] given as;

rn = kASF (1 − ˛)2˛n−1 (15)

where being chain growth probability. rn is hydrocarbon

formation rate with carbon number, n whilst kASF is an

Anderson–Schulz–Flory constant. As seen in Fig. 15, experimental

data exhibited a good fit to ASF model for unpromoted and Mg-

doped catalysts. However, FT product distribution over both Ca-

and Ba-promoted MoC1−x catalysts showed a deviation from the

standard linear ASF plots with two chain growth factors (low ˛1

Fig. 15. ASF plots for Mo carbide catalysts at 473 K and H2/CO = 2:1.

Fig. 16. ASF plots for Ba-doped Mo carbide catalyst at different feed compositions

and 473 K.

value for C1–C6 and high ˛2 for C7+) indicating different polymer-

ization pathways. Two values were also observed for ASF plots

at different feed compositions (cf. Fig. 16). The nonlinear-ASF dis-

tribution for Ca- and Ba-doped catalysts was probably due to the

appearance of two different active sites for chain growth (MoC1−x

and CaO·MoO3 or BaO·MoO3 phases for Ca- or Ba-promoted cata-

lysts respectively as seen in XRD patterns (cf. Fig. 1)) [35,36]. Indeed,

Bian et al. reported that MoO3 was also an active site for CO hydro-

genation [37].

The influence of H2 mole fraction on chain growth factor of Mo

carbide catalysts is shown in Fig. 17. Generally, value exhibited a

maximum at yH2= 0.25–0.33 for both promoted and unpromoted

catalysts consistent with other studies [38]. However, the chain

growth probability decrease with increasing yH2beyond 0.33 may

be ascribed to an increase in the ratio of termination to propagation

rate. The second chain growth factor, ˛2 was always higher than ˛1

for all feed compositions over Ba- and Ca-promoted catalysts. The

optimal chain growth probability improved with promoter addi-

tion from 0.52 to 0.85 and generally, Ba-promoted catalyst gave

the highest chain growth factor followed by Ca and Mg promoters.

Fig. 17. Influence of feed composition on chain growth probability of Mo carbide

catalysts.







Table 5

CO conversion over alkaline-earth promoted 10%MoC1−x/Al2O3 catalysts at 473 K and H2/CO = 2:1.

Catalyst 10%MoC1−x/Al2O3 3%Ca–10%MoC1−x/Al2O3 3%Ba–10%MoC1−x/Al2O3 3%Mg–10%MoC1−x/Al2O3

CO conversion (%) 4.76 22.00 9.65 5.07

Fig. 18. Hydrocarbon selectivity over the Mo carbide catalyst system at 473 K and

H2/CO = 2:1.

The promotional effect of alkaline-earth metal oxides on FT

product selectivity (ratio of the formation rate for a given species

to that of the total hydrocarbon production) is shown in Fig. 18.

Methane selectivity was suppressed with promoter addition from

69.5% (undoped catalyst) to 58.5% (Ba-promoted) whilst C5+ selec-

tivity increased in the order; Ba > Ca > Mg > undoped catalyst in

agreement with the trend for chain growth probability (cf. Fig. 17).

As seen in Table 5, alkaline-earth promoted catalysts enhanced

CO conversion from 4.76% (unpromoted) to 22.00% (Ca-promoted

catalyst).

4. Conclusions

Alkaline earth-promoted MoC1−x/Al2O3 catalysts produced

by temperature-programmed carburization have been evalu-

ated for CO hydrogenation. Both �- and �-MoC1−x phases were

formed during temperature-programmed carburization runs at

973 K and 5H2/1 C3H8. Promoters improved carbide formation

rate but increased carburization temperature. Mo carbide cat-

alysts possessed both acid and basic centres. There was an

increase in strong basic site concentration and CO uptake with

promoter addition. Although H2 chemisorbed on MoC1−x cata-

lyst surface, H2 chemisorption seemed to be weaker than CO

adsorption with higher CO uptake and heat of desorption. CO

consumption rate increased with promoter addition in the order;

Ca > Ba > Mg > unpromoted MoC1−x/Al2O3 catalysts due to enhance-

ment of CO uptake and reached an optimum at yH2= 0.75 for

all catalysts. Chain growth probability was improved by up to

63% with promoter addition and value decreased in the order;

Ba > Ca > Mg > undoped catalysts. Alkaline-earth promoters sup-

pressed CH4 formation but improved C5+ selectivity. Ba-promoted

MoC1−x catalyst seemed to be the optimal catalyst for highest TOPR

value. The nonlinear-ASF product distribution was observed for

both Ca- and Ba-promoted catalysts most likely due to formation

of new FT site in the CaMoO4 or BaMoO4 phase in addition to that

in the MoC1−x phase present in all catalysts.

Acknowledgements

The authors acknowledge the financial support of the Australian

Research Council (ARC).

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Non-linear ASF product distribution over alkaline-earth promoted molybdenum carbide catalysts for hydrocarbon synthesis