The olefin to paraffin ratio as a function of catalyst particle size in Fischer–Tropsch synthesis by iron catalyst

Journal of Natural Gas Science and Engineering 14 (2013) 204e210

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The olefin to paraffin ratio as a function of catalyst particle size in FischereTropsch synthesis by iron catalyst Ali Nakhaei Pour*, Mohammad Reza Housaindokht Department of Chemistry, Ferdowsi University of Mashhad, P.O. Box 9177948974, Mashhad, Iran

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 May 2013 Received in revised form 10 June 2013 Accepted 20 June 2013 Available online

The dependence of olefins to paraffins ratio on catalyst particle size in FischereTropsch synthesis by iron catalyst is studied. A series of catalyst with different particle size is prepared by microemulsion method. The experimental results showed the FischereTropsch reaction rates passed from a maximum by decreasing the catalyst particle size. Studies on secondary reactions can also be observed from the dependency of the O/P ratio on chain length. The ratio of olefins to paraffins depends on catalyst type, structure and the reaction conditions. The results show that the olefin/paraffin ratio decreased with decreasing the catalyst particle size. Using the experimental results we concluded that the O/P ratio depended on solubility impact of produced hydrocarbons (vaporeliquid equilibrium) and surface phenomena of the catalysts. Ó 2013 Elsevier B.V. All rights reserved.

Keywords: Olefins to paraffins ratio FischereTropsch synthesis Iron catalyst Particle size

1. Introduction Liquid transportation hydrocarbon fuels can be produced from syngas via a well-known catalytic chemical reaction called Fischere Tropsch (FT) synthesis (Davis, 2005; de Deugd et al., 2003; Dry, 2002). A variety of catalysts can be used for FischereTropsch synthesis, but the most common are transition metals of iron, cobalt, nickel and ruthenium. FT catalyst development has largely been focused on the preference for high molecular weight linear alkanes and diesel fuels production. It has been stated that the FT reaction is a surface phenomenon, therefore for optimum catalyst performance; maximum metal usage must be achieved (Wang et al., 2001a; Bell, 2003; Bernhardt et al., 2007; Sun, 2007; Dantas Ramos et al., 2004). The products from the FischereTropsch synthesis form a complex multicomponent mixture with substantial variation in carbon number and product type. Main products are linear paraffins and aolefins (Dry, 1990; Dry et al., 2004; Steynberg et al., 2004). In 1946, Herington first treated the molar distribution of hydrocarbons from FTS in terms of a polymerization mechanism. The same formulation was rediscovered by Anderson et al. in 1951 and named the AndersoneSchulzeFlory (ASF) distribution (Dry, 1990; Dry et al.,

* Corresponding author. Tel./fax: þ98 5118795457. E-mail addresses: [email protected], [email protected] (A. Nakhaei Pour). 1875-5100/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jngse.2013.06.007

2004; Wang et al., 2003; Van Der Laan and Beenackers, 1999). In the ASF model, the formation of hydrocarbon chains was assumed as a stepwise polymerization procedure and the chain growth probability was assumed to be independent of the carbon number. However, significant deviations from the ideal ASF distribution have been observed in many studies (Gaube et al., 1986; König and Gaube, 1983; Patzlaff et al., 1999). The usual deviations of the distribution of the linear hydrocarbons are a relatively higher selectivity to methane, a relatively lower selectivity to ethene, and an increase in the chain growth probability with increasing molecular size in comparison to the ideal ASF distribution (König and Gaube, 1983; Huff and Satterfield, 1984; Satterfield et al., 1983; Donnelly et al., 1988). Kuipers et al. (1995) stated that the occurrence of secondary reactions (hydrogenation, reinsertion, hydrogenolysis, isomerization) gives the most reasonable explanation for these deviations of the ASF distribution. If a product is terminated by a reaction on an FT growth site to a paraffin or olefin, it is called a primary product. Readsorption of olefins on growth sites may also lead to primary products, whereas adsorption on other sites will produce secondary products due to hydrogenation or isomerization reactions. It is generally accepted that secondary reactions of olefins depend on the chain length, resulting in a decrease of the O/P ratio. The extent of secondary reactions can also be observed from the dependency of the O/P ratio or olefin content on chain length. On FT based catalysts, an exponential decrease with chain length is observed (Kuipers et al., 1995):

A. Nakhaei Pour, M.R. Housaindokht / Journal of Natural Gas Science and Engineering 14 (2013) 204e210

On ¼ eCn Pn

(1)

where On and Pn are the production rates or mole fractions of olefins and paraffins with carbon number n and C is a constant. The ratio of olefins to paraffins depends on catalyst type and structure and the reaction conditions. Iglesia et al. (Madon and Iglesia, 1993; Iglesia et al., 1997) studied the influence of chain length dependent diffusion coefficients on secondary reactions. They reported an empirical equation describing a strong influence of chain length on diffusivity for olefins and paraffins (Iglesia, 1997),

Dn fe0:3n

(2)

which was not verified by experimental data. Van der Laan and Beenackers concluded that one area of uncertainty is the impact of diffusion and/or solubility (vaporeliquid equilibrium) upon secondary reactions that alter the initial product distribution. Shi and Davis (2005) reported that diffusion limitations for the olefin products and their subsequent re-incorporation as chain initiators does not make a major impact on the product distribution. In this experiment the effect of catalyst particle size of iron catalysts on the secondary reactions and olefins to paraffins ratio in FTS studied. A series of iron catalysts with deferent particle and pore size are prepared. The olefins to paraffins ratio studied on a series of catalysts for evaluation of diffusion limitations. 2. Experimental 2.1. Catalyst preparation FeeCu nanoparticles were prepared by coprecipitation in a water-in-oil microemulsion (Pour et al., 2009; Pour et al., 2010; Pour et al., 2011, 2010a,b). The precipitation was performed in the single-phase microemulsion operating region. In order to achieve a series of catalysts displaying different hematite particle size the surfactant-to-oil (S/O) weight ratio was set to a value of 0.3 the water-to-surfactant molar ratio (W/S) was varied from 4 to 12.

205

The H2-TPD experiments were performed by means of the temperature-programmed desorption (TPD) of H2 on the catalyst (0.5 g), which was packed in a shallow-bed quartz reactor with a low dead volume. A thermal conductivity detector (TCD) was used to measure the H2 desorbed in the TPD quantitatively. The catalyst was first reduced with H2 at 673 K and 1 bar for 11 h. Then the sample was heated in argon from 323 to 673 K, held at 673 K until the baseline leveled off (ensuring complete removal of adsorbed species on the reduced catalyst surface has been achieved), and finally cooled to 323 K for TPD tests. In the subsequent steps H2 adsorption on the catalyst was performed at 323 K for 30 min. Then the sample was purged with argon for removed of weakly adsorbed species until the baseline leveled off. Following H2-TPD was being carried out while the temperature was increased to 1050 K. H2 chemisorptions uptakes were determined by integrating the area of H2-TPD curves as compared to the certain amounts of gas passed through the TCD. 2.3. Experimental apparatus and procedure Steady-state FTS reaction rates measured in a continuous spinning basket reactor. A detailed description of the experimental setup and procedures has been provided in our previous works (Pour et al., 2012). The fresh catalyst is crushed and sieved to particles with the diameter of 0.25e0.36 mm (40e60 ASTM mesh). The weight of the catalyst loaded was 2.5 g and diluted by 30 cm3 inert silica sand with the same mesh size range. The catalyst samples were activated by a 5% (v/v) H2/N2 gas mixture with space velocity equal to 15.1 nl/gcat./h at 1 bar and 1800 rpm. The reactor temperature increased to 673 K with a heating rate of 5 K/min, maintained for 1 h at this temperature, and then reduced to 543 K. The activation is followed by the synthesis gas stream with H2/CO ratio of 1 and space velocity equal to 3.07 nl/gcat./h for 24 h at 1 bar and 543 K. After catalyst activation, synthesis gas was fed to the reactor at conditions operated at 563 K, 17 bar, H2/CO ratio of 1 and a space velocity equal to 4.9 nl/gcat./h. After reaching steady state, the FTS reaction rate was measured.

2.2. Catalyst characterization The surface area was calculated from the BrunauereEmmette Teller (BET) equation and pore volume, average pore diameter, and pore size distribution of the catalysts were determined by N2 physisorption using a Micromeritics ASAP 2010 automated system. A 0.5 g catalyst sample was degassed in the Micromeritics ASAP 2010 at 100  C for1 h and then at 573 K for 2 h prior to analysis. The analysis was done using N2 adsorption at 77 K. Both the pore volume and the average pore diameter were calculated by Barrete JoynereHallender (BJH) method from the adsorption isotherm. XRD was used to determine the phase composition of catalysts before and after pretreatments. The XRD spectrum of the catalysts were collected by an X-ray diffractometer, Philips PW1840 X-ray diffractometer, using monochromatized Cu/Ka radiation (40 kV, 40 mA). XRD peak identification was recognized by comparison to the JCPDS database software. The average crystallite size of samples, dXRD, can be estimated from XRD patterns by applying fullwidth half-maximum (FWHM) of characteristic peak (104) Fe2O3 located at 2q ¼ 33.3 peak to Scherrer equation:

dXRD ¼

0:9l FWHM cos q

(3)

where l is the X-ray wavelength (1.5406 A in this study) and q is the diffraction angle for the (104) plane.

Fig. 1. XRD pattern of the prepared catalysts after calcinations. (a:W/S ¼ 4, b:W/S ¼ 6, c:W/S ¼ 8, d:W/S ¼ 12).

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Table 1 H2 chemisorption, BET surface area and textural properties of prepared catalysts. W/Sc

Catalyst particle Sizea (nm)

BET surface area (m2/g)

Average pore size (nm)b

Pore volume (cm3/g)b

4

16

68.2

16.2

0.14

6 8 12

21 24 28

56.4 48.2 43.5

19.6 22.3 25.9

0.16 0.19 0.21

H2 uptake (  102 mmol H2/mmol Fe) 7.8

a b c

7.1 6.2 5.7

Calculated from XRD results. These values were calculated by BJH method from desorption isotherm. Water-to-surfactant molar ratio.

The out gas was analyzed by a gas chromatograph (Varian CP3800) equipped with TCD and FID detectors. The CO, CO2, N2, and O2 were analyzed through two packed column in series (Molecular sieve13  CP 81025 with 2 m length, and 3 mm OD, and Hayesep Q CP1069 with 4 m length, and 3 mm OD) connected to TCD detector. The C1eC5 hydrocarbons were analyzed via a capillary column (CP fused silica with 25 m  0.25 mm  0.2 mm film thickness) connected to FID detector. Hydrogen was analyzed through Shimatzu, GC PTF 4C, equipped with TCD detector and two column in series (Propack-Q with 2 m length, and 3 mm OD for CO2, C2H4 and C2H6 separation and molecular sieve-5A with 2 m length, and 3mmOD for CO, N2, CH4 and O2 separation), which were connected to each other via a three way valve. The collected liquid (Including hydrocarbons and oxygenates) were analyzed offline with Varian CP-3800 gas chromatograph equipped with capillary column (TM DH fused silica capillary column, PETRO COL 100 m  0.25 mm  0.5 mm film thickness) connected to FID detector. Total mass balances were performed with the carbon material balance closed between 97 and 103%. This criterion was adopted since compounds containing carbon and hydrogen might accumulate in the reactor, in the form of high molecular weight hydrocarbons.

2q ¼ 33.3 corresponds to the hematite 104 plane was used to calculate the average metal particle size by the Scherrer equation. The calculated dXRD for the samples listed in Table 1. The H2-TPD results, BET surface area and textural properties and pore size of the fresh iron-based catalysts are shown in Table 1. As shown in Table 1, the average particle sizes of hematite are linearly correlated with the water-to-surfactant ratio used during the microemulsion catalyst preparation route. In fact, nanoparticles are formed in the internal structure of the microemulsion, which is determined by the ratio of water-to surfactant. At high oil concentration, the bicontinuous phase is transformed into a structure of small water droplets within a continuous oil phase (reverse micelles) when surfactant is added. Thus, the results show that the size of different droplets determines the iron particle size, depending on the amount of surfactant (Trépanier et al., 2010; Wang et al., 2001b). As shown in this table, surface area and particle size of catalysts are changed with W/S ratio in microemulsion system. When the total surfactants-to-oil phase weight ratio was constant, the particle size decreased and surface area increased slightly with the increase in the water content. Also Table 1 shows the H2 adsorbed on surface of catalysts increased by decreasing of catalyst particle size. 3.2. Catalyst activity and products distribution

3. Results and discussion 3.1. Catalyst characterization Nanostructure iron catalysts characterized by X-ray diffraction (XRD) measurement after calcination. Fig. 1 shows the XRD patterns of iron catalysts, the characteristic peaks corresponding to (012), (104), (110), (113), (024), (116), (018), (214), (300) planes are located at 2q ¼ 24.3, 33.3, 35.8, 40.8, 49.6, 54.1, 57.6, 64.1 and 65.6 , respectively. They show very close to the ones with cubic hematite structured Fe2O3 crystal in JCPDS database. It shows that the hematite structure once formed remains stable during subsequent aqueous impregnation and thermal treatment. This strongly infers that microemulsion system modules only physical properties of reaction medium without changing the reaction paths and arrangements of crystal structure. The characteristic peak at

The FT reaction yields organic compounds and the water and/or carbon dioxide as by-products. Thus, carbon monoxide can be consumed for the formation of organic compounds or carbon dioxide. Therefore, the rate of FischereTropsch Synthesis (FTS) reaction must be written as:

rFTS ¼ rCO  rCO2

(4)

where rCO2 is the rate of CO2 formation (water-gas shift reaction) and rCO is the rate of CO consumption. The FT rate equals the rate of formation of organic compounds on carbon basis. The experimental results of the effects of catalyst particle size on FTS reaction rates were listed in Table 2. Table 2 listed the change of syngas conversions, FTS reaction rates and hydrocarbons production rates (gHC(gcat h)1), with catalyst particle size. As shown in this table, the FTS reaction rates passed from a maximum by decreasing the

Table 2 Feed conversion, RFTS and hydrocarbons production rates of catalysts. Catalyst particle size (nm)

28 24 21 16

Conversion (%) CO

H2

70.1 75.2 79.7 73.1

68.0 74.1 78.0 71.7

RFTS (mol/gcat/h)

Hydrocarbons Production rates (gHC/gcat h) CH4

C2eC10

C11eC20

C21þ

0.050 0.055 0.059 0.055

0.112 0.134 0.158 0.155

0.503 0.580 0.628 0.602

0.063 0.046 0.032 0.018

0.020 0.010 0.004 0.001

Reaction conditions: 563 K, H2/CO ¼ 1, 17 bar, Space velocity ¼ 4.9 nl/gcat./h, time-on-stream 40 h.

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Table 3 The variation of olefins to paraffins ratio and empirical parameters (a and b in O/ P ¼ a exp (bn) equation) as a function of the catalyst particle size. Catalyst particle size (nm)

28 24 21 16

a

17.47 16.04 13.63 10.83

b

0.334 0.331 0.324 0.316

R2

0.8318 0.8344 0.8377 0.8657

O/P (mol %) C2eC4

C5eC10

C11eC17

1.555 1.470 1.339 1.228

0.646 0.640 0.613 0.581

0.415 0.412 0.398 0.333

Reaction conditions: 563 K, H2/CO ¼ 1, 17 bar, Space velocity ¼ 4.9 nl/gcat./h, timeon-stream 40 h.

Fig. 2. The variation of olefins to paraffins ratio as a function of the catalyst particle size. Reaction conditions: 563 K, H2/CO ¼ 1, 17 bar, Space velocity ¼ 4.9 nl/gcat h, timeon-stream 40 h.

catalyst particle size. Table 2 shows that the CO and H2 conversion increase by decreasing the catalyst particle size and passed from maximum. It is found that the hydrocarbon produced by the catalysts becomes lighter with decreasing the catalyst particle size. Many of previous results reported same size dependence of FTS activity (Trépanier et al., 2010; Murzin, 2009). The most accepted mechanism is CH2 insertion, which leads to conclude the carbide theory of FischereTropsch (Van Der Laan and

Beenackers, 1999). In this mechanism, chain termination occurs mainly with hydrogen b-elimination and 1-alkenes being desorbed as primary products. In the following reactions, readsorbed 1alkenes are hydrogenated to form intermediates for surface intermediates chain growth with C1 surface species. These C1 species have various hydrogenation degrees are as following: CO, HCO, HCOH, CH, and CH2. If chain termination occurs with C1 monomers that exhibited higher degree of hydrogenation (like CH2 species), the produced hydrocarbons becomes lighter than other C1 species. As shown in Table 1, the H2 concentration on catalyst surface decreased by increasing the catalyst particle size. Thus the concentration of monomers that exhibited higher degree of hydrogenation (like CH2 species) on the surface of catalyst increased with decreasing the catalyst particle size. Thus, the selectivity of lighter hydrocarbons increased with decreasing the catalyst particle size (as shown in Table 2). Fig. 2 illustrates the variation of olefins to paraffins ratio and Fig. 3 illustrates variation of olefins and paraffins production rates

Fig. 3. The variation of olefins and paraffins production rates [mmole(gcat h)1] as a function of the catalyst particle size. Reaction conditions: 563 K, H2/CO ¼ 1, 17 bar, Space velocity ¼ 4.9 nl/gcat/h, time-on-stream 40 h.

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Fig. 4. The variation of olefins to paraffins ratio as a function of the catalyst particle size for evaluation empirical equation Ol/P ¼ a exp (bn). Reaction conditions: 563 K, H2/CO ¼ 1, 17 bar, Space velocity ¼ 4.9 nl/gcat/h, time-on-stream 40 h.

[mmole (gcat h)1] as a function of the catalyst particle size. Also Table 3 listed the olefins to paraffins ratio as function of carbon number for catalysts with deferent particle size. The results presented in Figs. 2 and 3, as same as Table 3 show that the olefin/ paraffin ratios are decreased with decreasing the catalyst particle size. Also, these results predicted that the olefins to paraffins ratio decreased with carbon number for all the catalysts. It is postulated that 1-alkenes are primary products of the FTS reaction over iron-based catalyst and may undergo to paraffin by hydrogenation as a secondary reaction (Van Der Laan and Beenackers, 1999; Schulz, 2003; Schulz et al., 1988; Li et al., 2002;

Bartholomew et al., 1999). Secondary reactions occur when primary products desorbed from a site and then interact with another catalytic site before leaving the catalyst particles. Schulz (2003) and Schulz et al. (1988, 2001, 1994) mentioned secondary hydrogenation as the most important process for the selectivity of the FT products on iron catalysts. They concluded that secondary hydrogenation increases with carbon number due to increased adsorption strength. Ethene appeared very reactive for hydrogenation relative to heavier olefins. The extent of secondary reactions can also be observed from the dependency of the O/P ratio or olefin content on chain length. The ratio of olefins to paraffins depends on

Fig. 5. The variation of empirical parameters in equation Ol/P ¼ a exp (bn) with catalyst particle size.

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catalyst type, structure and the reaction conditions. Slow removal of reactive products (e.g., a-olefins) due to a decrease of diffusion coefficients with increasing chain length can influence the FTS reaction rate and selectivity. Van Der Laan and Beenackers (1999) emphasized the solubility impact (vaporeliquid equilibrium) over diffusion as the main contributor on secondary reactions. As demonstrated in Table 3, the secondary reactions increase (O/P decreased) in lower iron particle size because of some difficulties in olefin diffusion in nanoparticles with narrow pores. On the other hand, the steric hindrance can be limited the competitive adsorption of bigger molecules on smaller crystal sizes. Higher hydrogen concentrations on smaller particles (Table 1) enhance the hydrogenation of olefin and hydrogenation of 1-alkenes to paraffin. Fig. 4 shows the variation of olefins to paraffins ratio as a function of the catalyst particle size. Because of a relatively lower selectivity to ethane, the O/P ration for C2 hydrocarbons is eliminated in this Figure Secondary reactions are often reported as the most possible reason for the anomalies of C2 products (Van Der Laan and Beenackers, 1999). As shows in this Figure, the olefin/ paraffin (O/P) ratio can explain with following equation:

On ¼ aebn Pn

(5)

Equation (5) is same to Equation (1), but the pre exponential (a) added for better fitting of experimental data. The empirical parameters a and b (Equation (5)) evaluated in Fig. 4 and listed in Table 3. In Table 3, R2 is a parameter for discrimination of models and it compares a calculated and experimental O/P result which is defined as:

R2 ¼ 1 

Residual sum of squares Corrected sum of squars

(6)

As shown in Table 3, a decreased with decreasing of catalyst particle size and changed from 17.47 to 10.83. Also the empirical parameter b decrease from 0.334 to 0.316 with decreasing of catalyst particle size (from 28 to 16 nm). Fig. 5 shows the variation of empirical parameters a and b with catalyst particle size. As shown in this Figure, both parameters increased linearly with catalyst particle size and intercept of b parameter is 0.3. Iglesia (1997) reported an empirical equation describing a strong influence of chain length on diffusivity for olefins and paraffins as Equation (2). As shown in this Equation, the exponential parameter equal to 0.3. Using the experimental results we concluded that the O/P ratio or the extent of secondary reactions depended on the solubility impact (vaporeliquid equilibrium) over diffusion and surface phenomena of the catalysts. The a parameter in Equation (5) related on surface characters of catalyst. Exponential part of Equation (5) related to diffusion coefficients of hydrocarbons products. For better fitting of experimental results the Equation (2) can be modify as:



DO DP



fe0:3n

(7)

n

DO is diffusion coefficient of olefins and DP is diffusion coefficient of Paraffins.

4. Conclusion The experimental results showed the FTS reaction rates passed from a maximum by decreasing the catalyst particle size. It is found that the hydrocarbon produced by the catalysts becomes lighter with decreasing the catalyst particle size. Studies on secondary

209

reactions can also be observed from the dependency of the O/P ratio or olefin content on chain length. The ratio of olefins to paraffins depends on catalyst type, structure and the reaction conditions. The results show that the olefin/paraffin ratios are decreased with decreasing the catalyst particle size. Also, these results predicted that the olefins to paraffins ratio decreased with carbon number for all the catalysts. Using the experimental results we concluded that the O/P ratio or the extent of secondary reactions depended on the solubility impact (vaporeliquid equilibrium) over diffusion and surface phenomena of the catalysts. Acknowledgments Financial support of the Ferdowsi University of Mashhad, Iran (2/26310-11/2/92) is gratefully acknowledged. References Bartholomew, C.H., Stoker, M.W., Mansker, L., Datye, A., 1999. Effects of pretreatment, reaction, and promoter on microphase structure and FischereTropsch activity of precipitated iron catalysts. In: Delmon, B., Froment, G.F. (Eds.), Studies in Surface Science and Catalysis. Elsevier, pp. 265e272. Bell, A.T., 2003. The impact of nanoscience on heterogeneous catalysis. Science 299, 1688e1691. Bernhardt, T., Heiz, U., Landman, U., 2007. Chemical and catalytic properties of sizeselected free and supported clusters. In: Heiz, U., Landman, U. (Eds.), Nanocatalysis. Springer, Berlin Heidelberg, pp. 1e191. Dantas Ramos, A.L., Alves, P.d.S., Aranda, D.A.G., Schmal, M., 2004. Characterization of carbon supported palladium catalysts: inference of electronic and particle size effects using reaction probes. Applied Catalysis A: General 277, 71e81. Davis, B., 2005. FischereTropsch synthesis: overview of reactor development and future potentialities. Topics in Catalysis 32, 143e168. de Deugd, R., Kapteijn, F., Moulijn, J., 2003. Trends in FischereTropsch reactor technologydopportunities for structured reactors. Topics in Catalysis 26, 29e39. Donnelly, T.J., Yates, I.C., Satterfield, C.N., 1988. Analysis and prediction of product distributions of the FischereTropsch synthesis. Energy & Fuels 2, 734e739. Dry, M., 1990. FischereTropsch synthesis over iron catalysts. Catalysis Letters 7, 241e251. Dry, M.E., 2002. The FischereTropsch process: 1950e2000. Catalysis Today 71, 227e241. Dry, M.E., 2004. Chapter 7-FT catalysts. In: André, S., Mark, D. (Eds.), Studies in Surface Science and Catalysis. Elsevier, pp. 533e600. Gaube, J., Herzog, K., König, L., Schliebs, B., 1986. Kinetische Untersuchungen der Fischer-Tropsch-Synthese zur Klärung der Wirkung des Alkali als Promotor in Eisen-Katalysatoren. Chemie Ingenieur Technik 58, 682e683. Huff Jr., G.A., Satterfield, C.N., 1984. Evidence for two chain growth probabilities on iron catalysts in the FischereTropsch synthesis. Journal of Catalysis 85, 370e379. Iglesia, E., 1997. Design, synthesis, and use of cobalt-based FischereTropsch synthesis catalysts. Applied Catalysis A: General 161, 59e78. Iglesia, E., 1997. FischereTropsch synthesis on cobalt catalysts: structural requirements and reaction pathways. In: de Pontes, R.L.E.C.P.N.J.H.S.M., Scurrell, M.S. (Eds.), Studies in Surface Science and Catalysis. Elsevier, pp. 153e162. König, L., Gaube, J., 1983. FischereTropsch-Synthese. Neuere Untersuchungen und Entwicklungen. Chemie Ingenieur Technik 55, 14e22. Kuipers, E.W., Vinkenburg, I.H., Oosterbeek, H., 1995. Chain length dependence of a-olefin readsorption in FischereTropsch synthesis. Journal of Catalysis 152, 137e146. Li, S., Krishnamoorthy, S., Li, A., Meitzner, G.D., Iglesia, E., 2002. Promoted ironbased catalysts for the FischereTropsch synthesis: design, synthesis, site densities, and catalytic properties. Journal of Catalysis 206, 202e217. Madon, R.J., Iglesia, E., 1993. The importance of olefin readsorption and H2/CO reactant ratio for hydrocarbon chain growth on ruthenium catalysts. Journal of Catalysis 139, 576e590. Murzin, D., 2009. Size dependent interface energy and catalytic kinetics on nonideal surfaces. Reaction Kinetics and Catalysis Letters 97, 165e171. Pour, A. Nakhaei, Housaindokht, M.R., Tayyari, S.F., Zarkesh, J., Alaei, M.R., 2010. Deactivation studies of FischereTropsch synthesis on nano-structured iron catalyst. Journal of Molecular Catalysis A: Chemical 330, 112e120. Patzlaff, J., Liu, Y., Graffmann, C., Gaube, J., 1999. Studies on product distributions of iron and cobalt catalyzed FischereTropsch synthesis. Applied Catalysis A: General 186, 109e119. Pour, A.N., Taghipoor, S., Shekarriz, M., Shahri, S.M.K., Zamani, Y., 2009. Fischere Tropsch synthesis with Fe/Cu/La/SiO2 nano-structured catalyst. Journal of Nanoscience and Nanotechnology 9, 4425e4429. Pour, A.N., Housaindokht, M.R., Tayyari, S.F., Zarkesh, J., 2010a. FischereTropsch synthesis by nano-structured iron catalyst. Journal of Natural Gas Chemistry 19, 284e292.

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