Synthesis of gasoline-range hydrocarbons over Mo/HZSM-5 catalysts§

Shetian Liu *, Amit C. Gujar, Peter Thomas, Hossein Toghiani, Mark G. White

Dave C. Swalm School of Chemical Engineering, Mississippi State University, Mississippi State, MS 39762, United States

Applied Catalysis A: General 357 (2009) 18–25

A R T I C L E I N F O

Article history:

Received 7 November 2008

Received in revised form 21 December 2008

Accepted 24 December 2008

Available online 4 January 2009

Keywords:

Fischer–Tropsch synthesis

Branched alkanes

Aromatics

Mo/HZSM-5

Zeolite Y

Syngas

Biomass gasification

A B S T R A C T

Mo/HZSM-5 has been found active in Fischer–Tropsch synthesis (FTS). The catalysts were evaluated under

various reaction conditions with low H2/CO (molar ratio �1.0) syngas typical of a synthesis gas derived

from biomass gasification. Liquid hydrocarbons formed on the Mo/HZSM-5 were composed mainly of

alkyl-substituted aromatics and lower branched and cyclized alkanes. Aliphatic hydrocarbons were

detected only in trace amounts. Lower hydrocarbons produced included mainly methane, ethane, propane

and iso-butane. Higher alcohols and carboxylic acids (C1–C6) were detected in the water phase liquids from

FTS. It is supposed that the formation of hydrocarbons on Mo/zeolite is through bifunctional-zeolite acidity

and molybdenum metal catalysis via mixed alcohols as the intermediates. Investigation on the effects of

zeolite structure indicated that zeolite Y is also an excellent support for Mo in the FTS.

� 2009 Elsevier B.V. All rights reserved.

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Applied Catalysis A: General

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1. Introduction

Fischer–Tropsch synthesis (FTS) is the core technology in theproduction of liquid transportation fuels from natural gas or othersolid carbon sources such as coal and plant biomass [1–4]. Fe-, Co-and Ru-based catalysts are the most active for FTS and have beenextensively studied in literature. Linear chain hydrocarbonsfollowing the Anderson–Schultz–Flory (ASF) distribution usuallydominate the FTS products, which need post-cracking or isomer-ization to obtain qualified transportation fuels. It is a greatchallenge in the catalyst development for the direct production ofhigh octane number gasoline through FTS reaction.

Literature efforts in circumventing the ASF distribution towardshigh quality gasoline production were mainly concerned onpromotion of secondary reactions such as isomerization, oligo-merization and alkylation of the FTS products over modified Fe- or

§ This report was prepared as an account of work sponsored by an agency of the

United States Government. Neither the United States Government nor any agency

thereof, nor any of their employees, makes any warranty, express or implied, or

assumes any legal liability or responsibility for the accuracy, completeness, or

usefulness of any information, apparatus, product, or process disclosed, or

represents that its use would not infringe privately owned rights. Reference herein

to any specific commercial product, process, or service by trade name, trademark,

manufacturer, or otherwise does not necessarily constitute or imply its endorse-

ment, recommendation, or favoring by the United States Government or any agency

thereof. The views and opinions of authors expressed herein do not necessarily state

or reflect those of the United States Government or any agency thereof.* Corresponding author. Tel.: +1 918 661 5765; fax: +1 918 662 1097.

E-mail address: shetian.liu@conocophillips.com (S. Liu).

0926-860X/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2008.12.033

Co-based catalysts, including the addition of a zeolite into the FTScatalysts [5–9], utilization of zeolite as the supports [10–13],modification of catalyst surface acidity [14], fabrication of zeolite-encapsulated or egg-shell catalysts [15–17] and the application ofa two-stage reaction [18]. Though the product selectivity could begreatly modified through the various catalyst designs, the activityand stability of the catalysts were often affected in a negative waycompared to the conventional Fe and Co catalysts, mainly owing toenhanced coke deposition inside the acidic zeolite cages/channelsand metal-support interactions which lowered the reducibility ofFe or Co, retarding the FTS reaction rates and increasing methaneformation. Zeolite-entrapped catalysts [19–22] were also appliedto FTS for shape-selective conversions of synthesis gas. The conceptof fabricating active sites for oligomerization, alkylation andisomerization inside zeolite cages and confining the chain-growingreaction in a nanospace seems to be promising in the developmentof FTS catalysts for high octane number gasoline production.

Concerning an innovative FTS catalyst development for highoctane number gasoline production, little attention has beenfocused on Mo/HZSM-5, which is an active catalyst for the dehy-droaromatization of methane [23–25]. Molybdenum carbide oroxycarbide formed during the reaction by methane reduction ofmolybdenum oxides anchored on the Bronsted acid sites of HZSM-5was generally accepted as the active species. Dimerization of surfaceCHx or methylene (CH2 = Mo) species was supposed to be a crucialstep in the oligomerization–aromatization process of methane,which is very similar to the supposed chain-growing carbidemechanism for Fischer–Tropsch reaction [26]. On the other hand,molybdenum carbides and sulfides were reported to be active for

S. Liu et al. / Applied Catalysis A: General 357 (2009) 18–25 19

mixed alcohol synthesis [27–33], and they were also reported to beactive catalysts for alkane isomerization [34–37]. Furthermore, Mo/HZSM-5 has been applied in the aromatization reactions ofmethanol and ethanol [38,39]. Dehydration of the alcohols onMo/HZSM-5 was supposed to happen during the aromatizationprocess, which is analogous to the CO insertion mechanism for F–Tsynthesis. The significant stabilization effect of CO on Mo/HZSM-5and its active incorporation into methane dehydroaromatizationindicated its high reactivity in the oligomerization of surfacehydrocarbon fragments (CHx and CxHy) [40,41]. These literatureresults proposed the potential of applying Mo/HZSM-5 as aninnovative catalyst for the FTS to produce high quality gasoline. Thepresent paper will show the unique characteristics of Mo/HZSM-5 inFTS reaction, most probably through a potential route that directlytransforms mixed alcohols to branched alkanes and aromatics.

2. Experimental

2.1. Catalyst preparation

Mo/Zeolite was prepared by incipient wetness impregnation of(NH4)6Mo7O24�4H2O (Fisher Scientific) aqueous solution with theammonium form of ZSM-5 (SiO2/Al2O3 = 23, 50, 80, and 280),Zeolite Y (SiO2/Al2O3 = 80) and Zeolite b (SiO2/Al2O3 = 25) obtainedfrom Zeolyst International. For comparison, Mo/SiO2 with 5%Moloading was also prepared using incipient wetness impregnation ofSiO2 (Cab-O-SilTM). The designated Mo loading amount was 5 wt.%or 10 wt.%. The samples were finally calcined in air at 773 K for 3 hand pelletized into 0.25–0.5 mm particles for activity test.

2.2. Catalytic reaction

The FTS reaction was performed using a continuous flow fixed-bed BTRS-Jr Laboratory Reactor Systems from Autoclave Engineers.Before the reaction, the catalyst (1.0 g) was pretreated in syngas(H2/CO = 1.0) flow at 673 K for 1 h or in methane gas flow at 923 Kfor 2 h. The gas hourly space velocity (GHSV) was 3000 h�1. Liquidproducts were collected using a condenser kept at 271 K and thepressure was 500 psig or 1000 psig, and the effluent gas from thecondenser was analyzed with an on-line gas chromatograph (GC,HP 6980) equipped with thermal conductive detector (TCD) andflame ionization detector (FID). A packed Molecular Seive 5Acolumn and a HP-1 capillary column were employed for separationof inorganic gases and light hydrocarbons. Liquid products,collected from the condenser, were separated into oil phase andwater phase, and analyzed with GC–mass spectrometer (Agilent)equipped with DB-Wax capillary column for oxygenated com-pounds and HP-5ms capillary column for hydrocarbons.

6%N2 was added into the syngas as internal standard for COconversion calculation. Selectivity of lower hydrocarbons wasestimated on carbon basis based on FID signal. The catalyst activityand selectivity were calculated according to Eqs. (1) and (2),respectively, where F0 and F are the flow rates of the syngas andeffluent gas after the reaction, respectively, C0

i and Ci are theconcentrations of component i in the syngas and effluent gas, and n

is the carbon number in a product i molecular:

conversion of CO ð%Þ ¼ F0C0CO � FCCO

F0C0CO

¼C0

CO � C0N2

CCO=CN2

C0CO

� 100

(1)

selectivity of producti ð%Þ ¼ FCin

F0C0CO� FCCO

¼C0

N2Cin

CN2C0

CO�C0N2

CCO�100

(2)

According to the stoichiometry of FTS reaction, the selectivity ofCO2 formation should be less than 50% (carbon basis). Actually,water and oxygenated products were always collected from theperformed FTS reactions. However, some data calculated using theinternal standard method showed CO2 selectivity higher than 50%,which is mainly due to experimental error such as error from GCanalysis and fluctuation of sampling pressure and flow rate control.

2.3. Catalyst characterization

Temperature programmed reduction/reaction (TPR) of 10%Mo/HZSM-5 was conducted with a Micromeritics Autochem 2910apparatus combined with a Dycor Dymaxion Quadrapole MassSpectrometer from Ametek Process Instruments. Concentrationchanges of both reactants (10%H2 in Ar, or 50%CO + 50%H2) andproducts were monitored. 300 mg catalyst particles of 40–60meshes was usually charged into the reactor. The reactant gas flowrate was 10 mL/min. The temperature ramping rate was 5 K/min or10 K/min. Before the TPR, sample was calcined in 21%O2 in He gasflow at 773 K for 1 h and then cooled down to room temperature tostart the TPR.

3. Results and discussion

3.1. Reaction of syngas on Mo/HZSM-5

The FTS reaction was first performed on 5%Mo/HZSM-5 (SiO2/Al2O3 = 50) at 500 psig and 623 K after ex situ pretreatment inmethane gas flow at 923 K for 2 h. Significant amounts of benzeneand naphthalene were produced during the pretreatment.

The catalyst was activated under methane atmosphere becausethis methane-treated Mo/HZSM-5 is active for the dehydroar-omatization of lower alkanes and for the deoxy-aromatization oflower alcohols [23,24,38,39]. Molybdenum carbide (MoCx) oroxycarbide (MoCxOy), which has been confirmed to be active inmixed alcohol synthesis [33,42], can be formed under methanearomatization reaction conditions. Analogously, studies of FTSwith the typical Fe- and Co-based catalysts also proposed thecorresponding metal carbides as the active phases. It was thusexpected that the reduced and carburized Mo/HZSM-5 maybe alsoactive in the direct FTS of aromatics rather than the aliphatichydrocarbons via alcohols as the intermediate compounds.Actually, a CO conversion about 15% (Fig. 1a) was obtained at623 K and 500 psig on the methane-activated Mo/HZSM-5.Methane, ethane and propane were the most abundant hydro-carbons in gas phase products. n-Butane, iso-butane, 2-methyl-butane, pentane and 2-methyl-pentane as well as trace amounts ofaromatics were also detected in the gas product using the on-lineGC. A liquid product, which composed of water and oil phases, wasalso successfully collected from the condenser kept at 271 K.

GC–mass spectrometer analysis of the oil-phase product(Fig. 2a) shows over 100 hydrocarbon compounds that can begrouped into three categories: (i) branched and cyclized C4–C8

alkanes such as methyl-butane, methyl-pentane, methyl-cyclo-hexane, etc.; (ii) aromatics including almost all the isomers ofalkyl-substituted benzenes, naphthalenes and indenes, withxylene, trimethyl-benzene and tetramethyl-benzene as the mostabundant components; (iii) C4–C20 aliphatic hydrocarbons, mostlyin minor amounts. These results clearly indicated that Mo/HZSM-5is active and selective for FT synthesis directly to aromaticcompounds, branched and cyclized alkanes of gasoline range,which is distinctly different from the yields obtained overconventional Fe- and Co-based catalysts producing aliphatichydrocarbons as the dominant products. GC–mass spectrometeranalysis of the water phase (Fig. 2b) indicated the presence ofoxygenates mainly composed of C1–C6 alcohols (methanol,

Fig. 1. CO conversion (a) and product selectivity (b) of FTS over 5%Mo/HZSM-5

(SiO2/Al2O3 = 50) at 623 K and 500 psig. (a) (&) Catalyst pretreated in methane at

923 K; (&) catalyst pretreated in syngas at 673 K and (b) (&) Total liquid

hydrocarbons; (&) CO2; (~) lower hydrocarbons (C1–3).

Table 1Contents of oxygenates (mg/mL) in water phase products from FTS on different

catalysts at 573 K and 1000 psig (H2/CO = 1.0).

Product Mo/HZSM-5 Mo/H-Y Mo/H-b Mo/SiO2

Methanol 11.53 7.10 19.21 51.44

High alcohols 13.81 11.42 14.46 15.49

Carboxylic acids 4.19 3.50 1.10 0.42

Acetone 0.24 0.39 0.18 0.03

Aldehydes 0.17 0.27 0.18 0.10

Total 29.94 22.69 35.13 67.48

S. Liu et al. / Applied Catalysis A: General 357 (2009) 18–2520

ethanol, propanol, n-butanol and 2-methyl-butanol) and car-boxylic acids (C1–C5). Acetone and aldehydes were also detected inthe products. The total concentration of these oxygenates in waterwas about 3 wt.% (Table 1). The presences of the variousoxygenates of appreciable amounts in the products and the high

Fig. 2. GC-mass identification of oil (a) and aqueous (b) phase product

formation of multi-alkyl-aromatics are indications that the FTSover Mo/HZSM-5 proceeds possibly via syngas to mixed alcohols tohydrocarbons transformation route.

This primary result encouraged us to investigate further intothe syngas catalysis over Mo/HZSM-5. In situ pretreatment of thecatalyst under syngas atmosphere was performed at 673 K. The COconversion reached 32% (Fig. 1a) at 500 psig and 623 K. Thepretreatment under syngas atmosphere at lower temperatureproduced higher activity of Mo/HZSM-5. The lower activity ofmethane pretreated Mo/HZSM-5 is probably related to the highpretreatment temperature, which might produce more coke on thecatalyst. Another possible reason leading to the lower activity ofthe methane pretreated catalyst may be related to the possible lossof Mo due to the higher pretreatment temperature and sublima-tion characteristic of MoO3. The product selectivity with reactiontime on stream is shown in Fig. 1b. It can be seen that the totalliquid product was about 13.1% with high formation of CO2

(�50.0%) and lower hydrocarbons (35.2%). To increase the liquidproducts formation, high pressure (1000 psig) and lower tem-perature (573 K) reaction was performed with the syngas-pretreated sample. The CO conversion was about 15%, but theliquid product selectivity increased to 25.8%. The catalyst was

s from FTS on 5%Mo/HZSM-5 (SiO2/Al2O3 = 50) at 623 K, 500 psig.

Fig. 3. CO conversion (a) and product selectivity (b) of FTS on 5%Mo/HZSM-5 (SiO2/

Al2O3 = 50) at 1000 psig and different temperatures (CO/H2 = 1.0). (&) CO2; (~)

lower alkanes; (&) CO conversion or liquid hydrocarbons.

Fig. 4. Distribution of gas phase hydrocarbons of FTS on 5%Mo/HZSM-5 (SiO2/

Al2O3 = 23) and 5%Mo/SiO2 at 573 K and 1000 psig.

S. Liu et al. / Applied Catalysis A: General 357 (2009) 18–25 21

stable and no catalyst deactivation was observed during thereaction.

The temperature dependence of FTS on 5%Mo/HZSM-5 (SiO2/Al2O3 = 50) was examined at 1000 psig. The CO conversion withreaction time on stream at different temperatures was shown inFig. 3a. The conversion of CO at 523 K was negligible. It reached 3–5% at 548 K. Liquid products were obtained, but the calculation ofliquid products selectivity was not reliable at this conversion level.The conversion of CO was 14% and 54% at 573 K and 623 K,respectively. It kept constant within 20 h reaction. The COconversion reached a maximum at 68% when the reactiontemperature reached 653 K, but it decreased to 62% within 20 hreaction time on stream, which is most probably due to catalystdeactivation at higher temperatures.

The product selectivity with reaction time on stream atdifferent temperatures is shown in Fig. 3b. Liquid productselectivity was around 20% and 15% at 573 K and 623 K,respectively. Production of lower alkanes was increased withthe increase of reaction temperature. CO2 formation remained ataround 50% at different temperatures. The decrease in liquidhydrocarbons formation at higher temperatures was accompaniedby the increase in lower alkanes production at similar level.

The liquid products obtained at different reaction temperatureson the 5%Mo/HZSM-5 were analyzed by GC–MS. The results were

Table 2Composition of liquid hydrocarbons from FTS on 5%Mo/HZSM-5 at different reactions

T (K) CO conversion (%) Effluent H2/CO SLiq. (

523 �1.0 1.00 –

548 3.8 1.01 –

573 14.0 1.05 18.4

623 54.0 1.29 14.6

653 64.4 1.32 9.9

summarized in Table 2. It can be seen that the formation ofaromatics was dominant in the products at all examinedtemperatures. Aromatics and iso-alkanes (branched + cyclizedalkanes) consist of more than 98% in the oil-phase products.Again, xylene, trimethyl-benzene and tetramethyl-benzene werethe most abundant components. The formation of linear alkaneswas only in minor amounts even at low temperatures (<573 K).This is interesting because the product distribution observed forour catalyst is different from the conventional Fe- or Co-basedcatalysts for which linear alkanes/olefins are always the majorproducts. It is an indication that FTS on Mo/HZSM-5 is mostprobably through a mechanism involving mixed alcohols as theintermediates for hydrocarbons formation. Zeolite acidity andisomerization activity of Mo-species greatly contribute to theisomerization and cyclization reactions. On the other hand, it isalso an indication that the active sites are located inside zeolitechannels. To confirm this, the FTS was performed on 5%Mo/SiO2

and the results were compared with that obtained on Mo/HZSM-5.A CO conversion of 17% was reached at 573 K, 1000 psig, but thecollected liquid consisted only aqueous phase. Oxygenatesincluding alcohols and carboxylic acids were detected (Table 1).Methanol was the dominant compound in the oxygenatesproduced on Mo/SiO2. Its content in water was significantlyhigher than that produced with zeolite catalysts. The contents ofhigher alcohols (C2–C6 alc.) in water were all in the same level, butthe contents of other oxygenates, i.e. carboxylic acids, acetone andaldehydes, formed on Mo/SiO2 were much lower than that formedon other zeolite-supported catalysts. The formation of liquidhydrocarbons should be very low on this catalyst.

However, the comparison of gas phase hydrocarbons asillustrated in Fig. 4 allows us to draw some valuable conclusions.The formation of methane and ethane on Mo/SiO2 was muchhigher than on Mo/HZSM-5. It also produced more butane andpentane on Mo/SiO2. More significantly, Mo/SiO2 produced onlytrace amount of iso-butane and iso-pentane, while Mo/HZSM-5produced much more iso-alkanes. This result clearly indicated thatthe HZSM-5 as the support for Mo is superior to SiO2 in the FTSsynthesis for the production of iso-alkanes.

temperatures and 1000 psig (H2/CO = 1.0).

%) Selectivity of products (%, carbon basis)

Aromatics iso-Alkanes Linear alkanes

91.8 9.7 0

90.2 9.7 0.1

87.5 12.1 0.4

88.7 9.6 1.7

97.6 2.1 0.4

Fig. 5. TPR profile of 10%Mo/HZSM-5 (SiO2/Al2O3 = 23) in 10%H2 + N2.

S. Liu et al. / Applied Catalysis A: General 357 (2009) 18–2522

3.2. TPR of syngas on Mo/HZSM-5

To further understand the pretreatment effects, TPR analysiswere performed with Mo/HZSM-5 (SiO2/Al2O3 = 23) with 10%Moloading. Reduction of Mo species on Mo/HZSM-5 by hydrogenproceeded over the temperature range from 573 K to 973 K (Fig. 5).Two hydrogen consumption peaks can be observed. The lowertemperature peak appeared as a shoulder at around 723 K and can be

Fig. 6. TPR profile of 10%Mo/HZSM-5 (SiO2/Al2O3 = 23) in 50%CO

attributed to the reduction of crystallized MoO3 to MoO2, whereasthe main peak around 893 K is normally attributed to the reductionof well-dispersed MoO3 to MoO2 [43,44]. Further reduction of MoO2

at temperatures above 1023 K will produce Mo0. It seems to bedifficult to reduce MoO3 into metallic form (Mo0) at temperaturesbelow 773 K. Fig. 6a shows the TPR profile of a freshly calcined Mo/HZSM-5 in 50%H2 + 50%CO atmosphere. Decreases in H2 and COconcentrations were observed at temperatures as low as 473 K.However, no CO2 formation was observed until the temperaturereached 573 K. The weak water peak at around 473 K was mostprobably owing to the adsorbed water. It can be seen that thereduction of MoO3 by either H2 or CO started at around 623 K andsimultaneously produced water and CO2. Two peaks of both waterand CO2 formation appeared at 783 K and 953 K, respectively. On theother hand, methane and ethane started to form at around 673 K,slightly higher than the formation of water and CO2, suggesting thatthe reduced Mo species are active for the FTS. Methane formationcontinued up to 1023 K, but ethane formation stopped at around873 K. This implies that the oligomerization reaction of theintermediates from CO hydrogenation stopped at this temperature.Signals from methanol, benzene and toluene were also monitoredbut no significant change above the baseline was observed.

+ 50%H2. (a) Fresh sample and (b) after the first TPR run.

Fig. 6. (Continued ).

S. Liu et al. / Applied Catalysis A: General 357 (2009) 18–25 23

After the TPR run with syngas up to 1173 K, the samples wascooled down naturally in the syngas flow to room temperature, andthe TPR was performed again. The profiles are shown in Fig. 6b. Anincrease in H2 concentration was observed, which is most probablydue to hydrogen adsorption on the catalyst. All the products, H2O,CO2, CH4 and C2H6 showed single peak profiles. The startingtemperatures of all these products are much lower than that in theprevious TPR run. As the catalyst was already reduced, theformation of H2O, CO2, CH4 and C2H6 are mainly from the synthesisreaction. Ethane was formed in a temperature window from 473 Kto 873 K, which may be the possible temperature region of FTS onthe Mo/HZSM-5 catalysts. However, no methanol and no tolueneformation was observed under the present TPR condition.

3.3. Effects of zeolite acidity on FTS

Acidity of HZSM-5 is a crucial factor for its catalyticperformance in aromatization, isomerization as well as alkylationreactions. As a part of catalyst screening of the Mo-based zeolitecatalysts, Mo/HZSM-5 with 5 wt.% Mo loading and different SiO2/Al2O3 ratio was investigated to understand the effect of acidity onthe catalytic performances in FTS reaction.

Table 3 summarizes the reaction results of 5%Mo/HZSM-5 withdifferent SiO2/Al2O3 ratios. The product distribution in Table 3 wasexpressed in moles of product formed per mole of CO2 producedduring the reaction to provide a comparison of product formationin relation to CO2 production. Samples with SiO2/Al2O3 of 50 and 80showed higher production of liquid products. The formation ofliquid hydrocarbons was 0.52 mol/mol-CO2 at 537 K, 1000 psigover 5%Mo/HZSM-5 with SiO2/Al2O3 = 50. The other two sampleshaving lower (23) and higher (280) SiO2/Al2O3 ratios producedmore lower alkanes (C1–C3) than liquid hydrocarbons. As thezeolite acidity decreases with the increase of SiO2/Al2O3 ratio, theoptimum feature of liquid hydrocarbons production at SiO2/Al2O3

around 50–80 seems to be related to an optimum distribution ofthe acid sites and active molybdenum species. Molybdenum oxidesloaded on the HZSM-5 prefer to interact with acid sites. Theframework aluminum in the zeolite can be easily extracted by theinteraction with Mo species leading to the formation of non-framework Al and Al2(MoO4)3 phase, which is most probably notactive for the FTS reaction [45–47]. At lower SiO2/Al2O3 ratio therewill be more Al2(MoO4)3 formation leaving fewer active molyb-denum species, while at higher SiO2/Al2O3 ratio there will be feweracid sites available for further conversion of the intermediate

Table 3Catalytic performance of Mo/zeolite catalysts for FT synthesis at 623 K, 500 psig and 573 K, 1000 psig (H2/CO = 1.0).

Catalyst SiO2/Al2O3 T (K) P (psig) CO conversion (%) Products distributiona

C1–3 Liquid products

5%Mo/HZSM-5 23 573 1000 10.5 0.26 0.05

623 500 47.5 0.87 –

50 573 1000 15.2 0.49 0.52

623 500 31.8 0.68 0.25

80 573 1000 32.4 0.43 0.18

623 500 51.2 0.73 0.01

280 573 1000 20.6 0.38 0.16

623 500 44.6 0.59 0.03

10%Mo/HZSM-5 23 573 500 15.8 0.53 0.27

5%Mo/zeolite-Y 80 573 1000 13.9 0.79 0.80

5%Mo/zeolite-b 25 573 1000 13.5 0.64 –

623 500 39.1 0.63 –

a Product distribution was based on carbon basis and 1 mol CO2 formation.

S. Liu et al. / Applied Catalysis A: General 357 (2009) 18–2524

mixed alcohols formed on molybdenum sites. This is furtherevidenced by increasing the Mo loading on HZSM-5 (SiO2/Al2O3 = 23). It can be seen from Table 3 that the CO conversionon 10%Mo/HZSM-5 was very close to that over 5%Mo/HZSM-5. Theformation of liquid hydrocarbons was greatly increased. It is thusproposed that the FTS on Mo/HZSM-5 catalysts is a synergisticbifunctional-molybdenum metal function with zeolite acid func-tion catalytic process.

3.4. Effects of zeolite structure

Table 3 also summarizes the reaction results of using differentzeolite-supported Mo catalysts. The best result was obtained withzeolite Y having a SiO2/Al2O3 = 80, on which the formation of liquidproducts reached 0.8 mol/mol-CO2 at a CO conversion of 14% at573 K, 1000 psig. ZSM-5 with SiO2/Al2O3 = 50, 80 also gave goodresults, but the zeolite b (SiO2/Al2O3 = 25) as the support for Moproduced much lower amount of liquid hydrocarbons.

Variation of the composition of liquid hydrocarbons obtainedon different zeolite-supported Mo catalysts was shown in Fig. 7.It can be seen that branched and cyclized alkanes and aro-matic compounds are dominated in the oil produced on Mo/HZSM-5 or Mo/H-b. Aromatic compounds reached 30–78%on these catalysts depending on the reaction conditions. In

Fig. 7. Product distributions in the oil phase from FTS over different catalysts. (&)

contrast, Mo/H-Y produced linear and isomerized alkanes as themajor products. Aromatic compounds were about 11% and 7% at623 K, 500 psig and 573 K, 1000 psig, respectively, much lessthan that of Mo/HZSM-5 or Mo/H-b. The zeolite structure seemsto affect the formation of aromatic compounds greatly. It is alsonoticed from Fig. 7 that Mo/H-b and Mo/HZSM-5 with a SiO2/Al2O3 ratio of 80 produced the highest amount of aromaticcompounds, but with little formation of the linear alkenes. Itsuggests that these two catalysts have the highest aromatizationactivity, and the linear alkenes are the intermediates foraromatic compounds formation. It is generally agreed that theacid sites are responsible for the aromatization and isomeriza-tion reactions. However, the present results showed a higherformation of aromatic compounds on Mo/HZSM-5 with higherSiO2/Al2O3 ratio.

Fig. 7 also shows the data at different reaction conditions. Lowertemperature and higher pressure decreased the formation ofaromatic compounds and increased the formation of linear,isomerized and cyclized alkanes. A product with less than 8% ofaromatic compounds and more than 50% isomerized and cyclizedalkanes was obtained at 573 K, 1000 psig over 5%Mo/H-Y catalyst.As has been shown in Table 3, this catalyst is one of the most activeand selective catalysts we have obtained. Oil produced with thiscatalyst is very promising for high quality gasoline products.

Linear alkanes; (&) linear alkenes; (&) aromatics; (&) branched and cyclized.

S. Liu et al. / Applied Catalysis A: General 357 (2009) 18–25 25

4. Conclusions

The following conclusions can be drawn from the above resultsand discussions:

Mo/HZSM-5 has been found active in FTS reaction. Aromaticsand branched/cyclized alkanes are major liquid-phase products ofFTS on Mo/HZSM-5. FTS on Mo/HZSM-5 proceeds via mixed alcoholformation as the first step for hydrocarbons formation, probablyinside the zeolite channels. Decreasing the formation of CO2 andlower hydrocarbons will be the major task in the furtherdevelopment of Mo/zeolite-based FTS catalysts.

Acknowledgments

This material is based upon work performed through theSustainable Energy Research Center at Mississippi State Universityand is supported by the Department of Energy under AwardNumber DE-FG3606GO86025. MGW acknowledges the generoussupport offered by the Earnest W. Deavenport, Jr. Endowed Chair.

References

[1] A.P. Steynberg, Stud. Surf. Sci. Catal. 152 (2004) 1.[2] A. Demirbas, Energy Sources, Part A 29 (2007) 1507.[3] X. Hao, G. Dong, Y. Yang, Y. Xu, Y. Li, Chem. Eng. Technol. 30 (2007) 1157.[4] R.R. Keshav, S. Basu, Fuel Process. Technol. 88 (2007) 493.[5] A.N. Pour, Y. Zamani, A. Tavasoli, S.M. Kamali Shahri, S.A. Taheri, Fuel 87 (2008)

2004.[6] A. Martinez, C. Lopez, Appl. Catal. A: Gen. 294 (2005) 251.[7] F.G. Botes, W. Bohringer, Appl. Catal. A: Gen. 267 (2004) 217.[8] C. Ngamcharussrivichai, A. Imyim, X. Li, K. Fujimoto, Ind. Eng. Chem. Res. 46

(2007) 6883.[9] Y. Li, T. Wang, C. Wu, Y. Lu, N. Tsubaki, Energy Fuels 22 (2008) 1897.

[10] S. Bessell, Appl. Catal. A: Gen. 96 (1993) 253.[11] A.S. Zola, A.M.F. Bidart, A. do C. Fraga, C.E. Hori, E.F. Sousa-Aguiar, P.A. Arroyo,

Stud. Surf. Sci. Catal. 167 (2007) 129.[12] R. Ravishankar, M.M. Li, A. Borgna, Catal. Today 106 (2005) 149.

[13] H. Li, J. Li, H. Ni, D. Song, Catal. Lett. 110 (2006) 71.[14] Y. Bi, A.K. Dalai, Can. J. Chem. Eng. 81 (2003) 230.[15] G. Yang, J. He, Y. Zhang, Y. Yoneyama, Y. Tan, Y. Han, T. Vitidsant, N. Tsubaki,

Energy Fuels 22 (2008) 1463.[16] G. Yang, J. He, Y. Yoneyama, Y. Tan, Y. Han, N. Tsubaki, Appl. Catal. A: Gen. 329

(2007) 99.[17] E. Iglesia, S.L. Soled, J.E. Baumgartner, S.C. Reyes, J. Catal. 153 (1995) 108.[18] T.-S. Zhao, J. Chang, Y. Yoneyama, N. Tsubaki, Ind. Eng. Chem. Res. 44 (2005) 769.[19] D. Fraenkel, B.C. Gates, J. Am. Chem. Soc. 102 (1980) 2478.[20] T.J. Lee, B.C. Gates, J. Mol. Catal. 71 (1992) 335.[21] D.J. Koh, J.S. Chung, Y.G. Kim, Ind. Eng. Chem. Res. 34 (1995) 1969.[22] D. Ballivet-Tkatchenko, G. Coudurier, H. Mozzanega, I. Tkatchenko, A. Kienne-

mann, J. Mol. Catal. 6 (1979) 293.[23] Y. Xu, S. Liu, L. Wang, M. Xie, X. Guo, Catal. Lett. 30 (1995) 135.[24] S. Liu, L. Wang, R. Ohnishi, M. Ichikawa, J. Catal. 181 (1999) 175.[25] Y. Xu, X. Bao, L. Lin, J. Catal. 216 (2003) 386.[26] E.V. Slivinskii, P. Yu, Voitsekhovskii, Russ. Chem. Rev. 58 (1989) 57.[27] J. Bao, Y. Fu, G. Bian, Catal. Lett. 121 (2008) 151.[28] Z. Li, Y. Fu, J. Bao, M. Jiang, T. Hu, T. Liu, Y. Xie, Appl. Catal. A: Gen. 220 (2001) 21.[29] Z. Li, Y. Fu, M. Jiang, Appl. Catal. A: Gen. 187 (1999) 187.[30] G. Bian, L. Fan, Y. Fu, K. Fujimoto, Ind. Eng. Chem. Res. 37 (1998) 1736.[31] G. Bian, Y. Fu, M. Yamada, Appl. Catal. A: Gen. 144 (1996) 79.[32] H.C. Woo, K.Y. Park, Y.G. Kim, I.S. Nam, J.S. Chung, J.S. Lee, Appl. Catal. 75 (1991)

267.[33] M. Xiang, D. Li, H. Qi, W. Li, B. Zhong, Y. Sun, Fuel 86 (2007) 1298.[34] G. Boskovic, P. Putanov, K. Foettinger, H. Vinek, Appl. Catal. A: Gen. 317 (2007)

175.[35] Y. Ono, Catal. Today 81 (2003) 3.[36] C. Bouchy, C. Pham-Huu, B. Heinrich, C. Chaumont, M.J. Ledoux, J. Catal. 190

(2000) 92.[37] A.P.E. York, C. Pham-Huu, P. Del Gallo, M.J. Ledoux, Catal. Today 35 (1997) 51.[38] R. Barthos, T. Bansagi, T.S. Zakar, F. Solymosi, J. Catal. 247 (2007) 368.[39] R. Barthos, A. Szechenyi, F. Solymosi, J. Phys. Chem. B 110 (2006) 21816.[40] S. Liu, Q. Dong, R. Ohnishi, M. Ichikawa, Chem. Commun. (1998) 1217.[41] R. Ohnishi, S. Liu, Q. Dong, L. Wang, M. Ichikawa, J. Catal. 182 (1999) 92.[42] M. Xiang, D. Li, H. Qi, W. Li, B. Zhong, Y. Sun, Fuel 87 (2008) 599.[43] J.R. Regalbuto, J.W. Ha, Catal. Lett. 29 (1994) 189.[44] H. Jiang, L. Wang, W. Cui, Y. Xu, Catal. Lett. 57 (1999) 95.[45] S. Al-Kandari, H. Al-Kandari, F. Al-Kharafi, A. Katrib, Appl. Catal. A: Gen. 341

(2008) 160.[46] R.W. Borry, Y.H. Kim, A. Huffsmith, J.A. Reimer, E. Iglesia, J. Phys. Chem. B 103

(1999) 5787.[47] W. Liu, Y. Xu, L. Li, H. Hu, Wuli Huaxue Xuebao 13 (1997) 693.