SiO2 catalysts via Fischer-Tropsch synthesis

G Model

ARTICLE IN PRESS

CATTOD-10069; No. of Pages 8

Catalysis Today xxx (2016) xxx–xxx

Contents lists available at ScienceDirect

Catalysis Today journal homepage: www.elsevier.com/locate/cattod

The impact of Al2 O3 promoter on an efficiency of C5+ hydrocarbons formation over Co/SiO2 catalysts via Fischer-Tropsch synthesis Alexander P. Savost’yanov a , Roman E. Yakovenko a , Sergey I. Sulima a , Vera G. Bakun a , Grigoriy B. Narochnyi a , Victor M. Chernyshev a,∗ , Sergey A. Mitchenko a,b,∗∗ a b

Platov South-Russian State Polytechnic University (NPI), Prosveschenya 132, Novocherkassk 346428, Russia L.M. Litvinenko Institute of Physical Organic & Coal Chemistry, 70 R. Luxemburg Str., Donetsk 83114, Ukraine

a r t i c l e

i n f o

Article history: Received 27 December 2015 Received in revised form 23 February 2016 Accepted 25 February 2016 Available online xxx Keywords: Fischer-Tropsch synthesis Co/SiO2 catalyst Al2 O3 promoter Particle size C5+ selectivity

a b s t r a c t The influence of doping of Co/SiO2 catalysts with alumina on a performance of Fischer-Tropsch synthesis (FTS) was studied by extended FTS trials in a fixed-bed tubular pilot-scaled reactor. The addition of small amount of Al2 O3 causes apparent promotional effect on the catalysts activity and C5+ hydrocarbons selectivity. The largest promotion effect was observed for the catalysts with 1 wt.% of alumina loading. The modification of the catalyst with alumina (1 wt.%) changes molecular weight distribution of the resultant C5+ paraffins with increasing the fraction of C8 –C25 and decreasing the fraction of longer chain hydrocarbons. The addition of a proper amount of alumina into Co/SiO2 catalyst alters Co◦ particle size distribution making it narrower with the maximum at 8 nm and the same mean value for Co◦ particle size. A volcano-like dependence of CO chemisorption on alumina loadings with a maximum at 1 wt.% was observed. Relatively high CO chemisorption at the proper amount of alumina decreases the ratio of surface hydrogen to carbon monoxide and in such a way promotes formation of C5+ hydrocarbons. © 2016 Elsevier B.V. All rights reserved.

1. Introduction The Fischer-Tropsch synthesis (FTS) of hydrocarbons from syngas (a mixture of H2 and CO gases) is highly attractive way to convert carbon containing raw stocks (coal, natural or associated petroleum gas, biomass, etc.) into liquid hydrocarbons [1–4]. nCO + (2n + 1)H2 → Cn H2n+2 + NH2 O

(1)

The FTS is the central part of the gas-to-liquids (GTL) processes producing high quality liquid fuels, which are almost free from sulfur, nitrogen and aromatic compounds. The FTS process has been operated at a commercial scale since the 1930s and is still attracting much attention as a way of producing motor fuels owing to variety of raw materials that can be used for the syngas production [5,6]. Last years large investments have been made in the field of GTL. New powerful plants of unprecedented unit capacities are currently being designed and started worldwide, in particular, the Oryx (the output is now at about 34,000 barrels of diesel, naphtha and lique-

fied petroleum gas a day) and Pearl plants in Qatar [5,6]. In addition, biomass-to-liquids processes which produce renewable fuels from cellulosic biomass-derived syngas have reached pre-commercial scale [6]. It makes the FTS as a promising technology permissive achievement of the challenging aim of developing of economical and energy-efficient processes for sustainable production in commercial scale of fuels and chemicals as an alternative to the ones derived from crude petroleum. In this connection, the essential objective of FTS is to produce liquid paraffins and olefins of different chain length and to limit the formation of methane and carbon dioxide. Commercially scaled FTS processes usually require catalysts based on iron and cobalt [2,5,7–10]. Iron-based catalysts are often preferred for converting syngas with molar H2 /CO ratio less than 2 (the stoichiometric for FTS reaction), which is typical for syngas produced from coal or biomass [7,11,12]. Iron-based catalysts are active in water shift reaction (WGS, Eq. (2)) producing hydrogen and increasing in this way the H2 /CO ratio [7,12]. CO + H2 O  CO2 + H2

∗ Corresponding author. ∗∗ Corresponding author at: L.M. Litvinenko Institute of Physical Organic & Coal Chemistry, 70 R. Luxemburg Str., Donetsk 83114, Ukraine. E-mail addresses: [email protected], [email protected] (V.M. Chernyshev), samit [email protected] (S.A. Mitchenko).

(2)

Cobalt-based catalysts for FTS are in focus of current interest because of both their low WGS activity and higher productivity in synthesis of long-chain hydrocarbons C5+ (mostly waxes) as precursors for ultra-clean synthetic diesel and special lubricants [3,7,8].

http://dx.doi.org/10.1016/j.cattod.2016.02.037 0920-5861/© 2016 Elsevier B.V. All rights reserved.

Please cite this article in press as: A.P. Savost’yanov, et al., The impact of Al2 O3 promoter on an efficiency of C5+ hydrocarbons formation over Co/SiO2 catalysts via Fischer-Tropsch synthesis, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.037

G Model CATTOD-10069; No. of Pages 8 2

ARTICLE IN PRESS A.P. Savost’yanov et al. / Catalysis Today xxx (2016) xxx–xxx

Fig. 1. The general view (a) and principal scheme (b) of the pilot-scaled installation for FTS: 1–water jacket; 2–reactor; 3–reactor electrical furnace; 4–thermocouple; 5–steam chest; 6–liquid products condenser; 7–electromagnetic recirculating pump; 8–gasometer.

Fig. 3. X-Ray diffraction patterns for the Co-xAl2 O3 /SiO2 after air calination; x = 0 (a), 0.4 (b), 1 (c), 2 (d), 3 (e).

Fig. 2. Molecular weight distribution of C5+ paraffins produced over Co/SiO2 (a) and Co-1Al2 O3 /SiO2 (b) catalysts.

Moreover, Co-based catalysts possess longer life-time and higher CO conversion compared with iron-based catalysts [7,13,14]. Due to these features, cobalt is traditionally supported in FTS catalysts when it is possible to feed the reactor by syngas with H2 /CO ratio close to 2 [7,14,15]. Commercial Co-based catalysts are usually prepared by impregnation of a porous support (SiO2 , Al2 O3 , TiO2 , activated carbon, zeolite, etc.) with a solution of the metal precursor [7,10]. Cobalt nitrate is commonly used because of its high solubility in water, permitting single-step high metal loadings, and its easy decomposition into reducible Co oxides [7,10]. After impregnation and drying, the dried impregnate is calcined to form supported Co oxide nanocrystallites. Finally, the oxidic catalyst precursor is reduced in reactor under flow of H2 forming in situ the catalytically active Co◦ species. Promotion with noble metals [3,7,16–19] essentially increases the rate of cobalt reduction and greatly improves the catalysts performance. Despite of strong promotional effects of noble metals

for FTS catalysts, their commercial application is limited by their high cost [19,20]. Doping of the Co-based catalysts by metal oxides as promoters is more preferable way to improve the FTS catalyst owing to their low price and available supply [7,8,21]. Recently Zhang et al. [22] reported that incorporation of alumina into the Co(10 wt.%)/SiO2 catalysts significantly improved reaction activity and selectivity towards C5+ hydrocarbons at 240 ◦ C and P = 1 MPa, in a slurry-phase FTS reactor. Very close results were obtained by Pei et al. [23] in a fixed-bed tubular reactor over Co(15 wt.%)/AC (activated carbon) catalysts at 220 ◦ C and P = 3 MPa. In the both of these works the FTS experiments were performed in laboratory-scaled reactors and with relatively short time-on-stream (TOS) periods (up to 24 h [23]). In the present work a series of Al2 O3 promoted Co(∼20 wt.%)/SiO2 catalysts were studied by extended FTS trials (TOS > 150 h) and in fixed-bed tubular pilot-scaled reactor. This type of reactors is widely used in industry as well as commercial catalysts with cobalt loading of 20–25 wt.% are typically applied [24,25]. Results of the catalysts characterization by X-ray powder diffraction (XRD), H2 -temperature-programmed reduction, COtemperature-programmed desorption, SEM and TEM were discussed and correlated with their catalytic performances. 2. Experimental 2.1. Catalyst preparation To prepare catalysts, the silica support (Silica gel from Salavat Catalyst’s Plant Ltd., Public corporation Gazprom Salavat Neftekhim; the fraction of support particle size of 1–2 mm, BET

Please cite this article in press as: A.P. Savost’yanov, et al., The impact of Al2 O3 promoter on an efficiency of C5+ hydrocarbons formation over Co/SiO2 catalysts via Fischer-Tropsch synthesis, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.037

G Model

ARTICLE IN PRESS

CATTOD-10069; No. of Pages 8

A.P. Savost’yanov et al. / Catalysis Today xxx (2016) xxx–xxx

3

Table 1 Performance of FTS over Co-xAl2 O3 /SiO2 catalysts. Reaction conditions

x (wt.%)

T (◦ C)

CO conversion (%)

CH4

C2 –C4

C5+

P = 2.0 MPa, GHSV = 1000 h−1

0 0.4 1.0 2.0 3.0

213 213 214 219 217

49 51 54 46 23

15.4 10.8 10.7 17.6 23.5

6.6 9.3 8.4 7.7 13.7

P = 0.1 MPa, GHSV = 100 h−1

0 0.4 1.0 2.0

190 190 190 190

51 53 52 44

12.6 12.3 12.4 12.4

14.9 14.6 13.5 12.4

Hydrocarbons selectivity (%)

CO2 selectivity (%)

Catalyst productivity for C5+ hydrocarbons (kg/(m3 cat h))

77.4 79.5 80.1 74.2 62.0

0.6 0.4 0.7 0.6 0.8

90 95 102 80 33

71.5 71.9 72.8 74.4

1.0 1.2 1.3 0.7

7.8 7.9 7.8 6.8

specific surface area of 365 m2 /g, pore diameter of 12 nm, total pore volume of 0.855 cm3 /g) was previously dried at 100 ◦ C overnight. The dried support was then impregnated at 75 ◦ C to incipient wetness with 55% aqueous solution of Co(NO3 )2 ·6H2 O (Aldrich) containing the required amount of aluminum nitrate. After impregnation, the samples were dried at 100–120 ◦ C for 4 h and then calcined at 300 ◦ C for 4 h. The loading of cobalt was 20 wt.% for all catalysts, the loading amounts of Al2 O3 were 0.4, 1.0, 2.0 and 3.0 wt.%. The unpromoted and alumina-promoted catalysts were denoted as Co/SiO2 and Co-xAl2 O3 /SiO2 , respectively, where x indicates the weight percentage of alumina relative to the initial weight of the dried support (x = 0.4, 1.0, 2.0 and 3.0). 2.2. FTS catalytic tests The FTS catalytic tests were carried out at syngas pressure of 0.1 or 2.0 MPa and GHSV of 100 or 1000 h−1 respectively in a downflow fixed-bed stainless steel reactor (internal diameter of 27 mm and 500 mm length) of the pilot-scaled installation (Fig. 1). The reactor was charged with catalyst in the calcined form (15 ml) diluted with quartz sand (15 ml) of the same fraction (particle size 1–2 mm). The catalyst then was reduced in situ in flowing H2 at GHSV = 1000 h−1 and 400 ◦ C for 1–4 h to achieve the degree of cobalt reduction of 50–55% (conditions for reduction of calcined catalysts were determined in preliminary experiments). After reduction, the temperature was lowered to 150 ◦ C under the flow of H2 , the gas flow was switched to syngas (H2 /CO = 2; GHSV = 100 or 1000 h−1 at pressures of 0.1 or 2.0 MPa, respectively) and the temperature raised at a rate of 2.5 ◦ C until the CO conversion of 44–54% has been achieved. The constancy of the conversion of CO was kept by varying temperature, which was measured by thermocouple placed in the midpoint of the reactor (Fig. 1). The axial gradient of temperature in the catalyst layer did not exceed 3 ◦ C. Therefore, the variation of FTS temperature was limited to ±3 ◦ C to minimize an influence of different temperatures on the product distribution in the compared catalytic runs. The reaction effluent passed through a condenser to collect the liquid products and separate them from gases. The moment when the reaction reached the designed CO conversion was taken as the starting time. After achieving a steady state of operation, activity (degree of CO conversion) and selectivity of the catalyst remained constant within experimental errors for the TOS period over 150 h. Feed gas and off-gas compositions were analyzed chromatographically with a Crystal-5000 GC system that was equipped with two thermal conductivity detectors and two chromatographic columns. A HayeSep R packed column was used to determine the C1 –C5 hydrocarbons (Ar as carrier gas at a flow rate of 15.0 ml/min), and the NaX column (He as carrier gas at a flow rate of 15.0 ml/min) was used to determine the CO, CO2 , H2 , and N2 . The total amounts

Fig. 4. Representative SEM micrographs of the Co/SiO2 (a) and Co-1Al2 O3 /SiO2 (b) catalysts after air calination.

of hydrocarbons and water condensed in the condenser (6) were measured by weighing after separation. The compositions of the condensed hydrocarbons were determined by GC–MS with an Agilent 7890A GC system, equipped with an Agilent 5975C massselective detector (electron impact, 70 eV) and a HP-5-MS column (30 m × 0.25 mm × 0.25 ␮m film), using He as carrier gas at a flow rate of 1.0 ml/min. The carbon monoxide and hydrogen conversions

Please cite this article in press as: A.P. Savost’yanov, et al., The impact of Al2 O3 promoter on an efficiency of C5+ hydrocarbons formation over Co/SiO2 catalysts via Fischer-Tropsch synthesis, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.037

G Model CATTOD-10069; No. of Pages 8

ARTICLE IN PRESS A.P. Savost’yanov et al. / Catalysis Today xxx (2016) xxx–xxx

4

Fig. 5. TPR profiles for (a) Co/SiO2 , (b) Co-0.4Al2 O3 /SiO2 , (c) Co-1Al2 O3 /SiO2 , (d) Co-2Al2 O3 /SiO2 and (e) Co-3Al2 O3 /SiO2 air-calcined catalysts.

were calculated based on the GC analysis of the inlet. The effluent gas and gas flow rates were measured at the inlet and outlet of the reactor. Hydrocarbon yields were calculated by using the amounts of hydrocarbons in the effluent gas. These yields were calculated based on GC analyses and the outlet gas flow rates, as well as the amounts of condensed hydrocarbons (calculated based on the total weights of the hydrocarbons condensed in the separator during the experiments and GC–MS analyses). The inaccuracies in the total mass balance for each accepted experiment could not exceed 2.5%; otherwise, the run was rejected. 2.3. Catalysts characterization Elemental composition analysis of the samples was performed by energy-dispersive X-ray spectroscopy in a Thermo Scientific ARL QUANT’X EDXRF Spectrometer. The BET surface areas of SiO2 and catalysts after calcinations were determined by argon desorption using Micromeritics ChemiSorb 2750 instrument. The X-ray diffraction (XRD) patterns of the calcined catalysts were recorded on a Thermo Scientific ARL X’TRA Powder Diffractometer using monochromatized Cu K␣ radiation in the region of 2␪ from 15◦ to 70◦ . The average Co3 O4 crystallite size was estimated by Sherrer’s equation [26] assuming a shape factor k = 0.9. The reduction behavior of the oxidic cobalt catalysts was studied by hydrogen temperature-programmed reduction (H2 -TPR) using a Micromeritics ChemiSorb 2750. The sample (100 mg) was heated in helium flow (20 ml/min) for 2 h at 200 ◦ C and after that was treated by a 10% H2 /N2 gas mixture in a flow rate of 20 ml/min at increasing temperature (a heating rate of 5 ◦ C/min) from ambient temperature to 800 ◦ C. The temperature-programmed desorption of CO (CO-TPD) was studied using the same instrument as H2 -TPR experiment. An oxidic cobalt catalyst sample weighing of about 0.1 g was placed in a U-shaped reactor and heated in flowing helium (20 ml/min) for 2 h at 200 ◦ C. After that it was reduced by a 10% H2 /He gas mixture in a flow rate of 20 ml/min at 400 ◦ C for 2 h, purged with helium for 30 min to remove adsorbed species from the surface and then cooled down to an ambient temperature. The sample surface was saturated by CO by pulsed adsorption of CO in a flow of helium

Fig. 6. Representative SEM micrographs of the reduced Co/SiO2 (a) and Co1Al2 O3 /SiO2 (b) catalysts.

(20 ml/min). CO desorption was carried out in a stream of helium by increasing temperature from ambient to 800 ◦ C at heating rate of 20 ◦ C/min. Signals of desorbed CO were detected by TCD. Transmission electron microscopy (TEM) characterization of selected samples was performed in a Tecnai G2 Spirit BioTWIN microscope operated at 120 kV. A few droplets of a suspension of the reduced and then passivated catalysts in ethanol were put on a copper TEM grid and allowed to dry at ambient temperature for TEM observations. Metal particles size histograms were generated upon measurement of 150–200 particles at several positions on the TEM grid. Surface-averaged particle size and standard deviation were calculated after correction for the passivation outlayer [27,28]. The surface morphology of selected samples was observed by scanning electron microscopy (SEM) in a Hitachi SU8000 field emission gun microscope (FE-SEM). An image capture was conducted in secondary electron imaging mode at an accelerating voltage of 2 kV and a working distance of 5–10 mm. Before recording, the samples were fixed on the surface of specimen stub using a conductive adhesive and coated with thin (20 nm) conductive carbon layer, which possesses a less defined structure compared to a metal coating [29]. Measurements were carried out as detailed elsewhere [30].

Please cite this article in press as: A.P. Savost’yanov, et al., The impact of Al2 O3 promoter on an efficiency of C5+ hydrocarbons formation over Co/SiO2 catalysts via Fischer-Tropsch synthesis, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.037

G Model

ARTICLE IN PRESS

CATTOD-10069; No. of Pages 8

A.P. Savost’yanov et al. / Catalysis Today xxx (2016) xxx–xxx Table 2 C5+ products distribution in the FTS over Co-xAl2 O3 /SiO2 at P = 2.0 MPa, GHSV = 1000 h−1 . x (wt.%)

Products distribution (wt.%) paraffins

0 1.0 3.0

olefins

alcohols

C5 –C18

C19 –C35

C35+

C5 –C18

C5 –C18

49.6 54.9 48.1

47.6 42.1 37.5

2.4 2.3 2.2

0.4 0.7 10.1

0.04 0.07 1.2

3. Results and discussion 3.1. Impact of alumina doping on the activity and selectivity of Co/SiO2 catalysts The impact of alumina dopant loading on the activity and selectivity of Co/SiO2 catalysts in the FTS was determined in the two sets of kinetic experiments at high and atmospheric pressure (Table 1). No apparent promoting effects of Al2 O3 additives to Co/SiO2 catalysts were observed at atmospheric pressure. At high pressure (P = 2 MPa) the CO conversion increased from 49 to 54% when Al2 O3 loading rose from 0 to 1%. Correspondingly, the selectivity towards methane dropped from 15.4 to 10.7%, while the selectivity towards C5+ hydrocarbons increased from 77.4 to 80.1% and the overall performance of the catalysts for C5+ hydrocarbons production increased from 90 to 102 kg/(m3 cat h). However, further increase in Al2 O3 loading over 1 wt.% produces the opposite effect: conversion of CO as well as selectivity and the overall performance of the catalysts for C5+ hydrocarbons production were decreased (Table 1). In the case of 3 wt.% Al2 O3 loading we were unable to enhance CO conversion by rising temperature within an interval of 6 ◦ C (see Section 2.2). These results are in line with recently reported promoting effect dependence on alumina loading into Co/AC catalysts with maximum at 1.9 wt.% providing 30% increase in CO conversion [23]. It is noteworthy that in the case of lower cobalt loading (10 wt.%) on SiO2 support the promoting effect of Al2 O3 is yet more: 47% increase in CO conversion at 10 wt.% of alumina loading [22]. The impact of alumina loading to Co/SiO2 catalysts on C5+ products distribution is summarized in Table 2. Paraffins are the main organic compounds produced over all the catalysts. Olefins and alcohols are also formed in trace amounts over the catalysts undoped with alumina. Output of alcohols and olefins is almost twice as much but remains still low at the optimal alumina loading of 1 wt.% and rises greatly when Al2 O3 loading is further increased. Histograms for molecular weight distribution of C5+ paraffins are shown in Fig. 2. Doping of the catalysts with alumina (1 wt.%) changes the molecular weight distribution making it narrower owing to increasing the fraction of C8 –C25 and decreasing the fraction of longer hydrocarbons. Thus, doping of Co/SiO2 with Al2 O3 causes a promotional effect on the catalysts activity and selectivity. Under the used reaction conditions the largest promotional effect is observed for the catalyst with alumina loading of 1 wt.%. Further increase in alumina loading causes the opposite effect. 3.2. Texture properties, XRD analysis and SEM of the calcined Co-xAl2 O3 /SiO2 catalysts The texture properties of the catalysts with or without alumina addition are compared in Table 3. There are no essential changes in average pore diameter for the support and the catalysts. Conversely, the BET surface and pore volume of the Co/SiO2 catalyst are significantly less than those of the support. Similar effects were observed recently for Co/AC catalysts [23] and were explained by

5

the pore blockage when cobalt was loaded on the support. Addition of alumina has very slight influence on the BET surface area and pore volume of the Co-xAl2 O3 /SiO2 catalysts. These results, in parallel to the previous observations [23], are suggestive that the improved performance of the modified catalysts should be attributed to the chemical effects of the added alumina. X-Ray diffraction patterns of the catalysts with different Al2 O3 loading after calcinations is depicted in Fig. 3. It was found that the cobalt species are mainly in Co3 O4 phase. From comparison of the X-ray diffraction patterns it follows that Al2 O3 loading does not influence the sample texture. The estimation from the broadening of diffraction peaks according to Scherrer’s equation gives crystallites size of ca. 20 nm regardless of Al2 O3 loading. The representative SEM micrographs for unmodified and modified with 1 wt.% of Al2 O3 catalysts after air calcination are depicted in Fig. 4. Microcrystallites of cobalt oxides on the surface of unmodified Co/SiO2 catalyst (Fig. 4a) are aggregated in agglomerates of the sizes of 50–300 nm and large (400–800 nm) clusters of agglomerates and granules; pores between agglomerates vary in the range of 50–200 nm. Granules and agglomerates of cobalt oxides on the catalyst’s surface with an optimum content of promoter, Co1Al2 O3 /SiO2 , are distributed more uniformly (Fig. 4b). The size of the agglomerates varies between 30 and 100 nm, pore size of 20–50 nm. Very few of large (300–400 nm) clusters of agglomerates are observed. 3.3. Characterization of the reduced catalysts 3.3.1. Temperature-programmed reduction of the oxidic Co-xAl2 O3 /SiO2 catalysts The H2 -TPR profiles for the air-calcined Co/SiO2 samples are plotted in Fig. 5. A very weak reduction peak is perceived around 150 ◦ C, which can be assigned to the decomposition of residual Co nitrate remaining after air calcinations [23,31]. Alongside this lowintensity peak, the catalysts shows two intense reduction features in the range of ∼220–500 ◦ C (peaking at 285–297 and 369–511 ◦ C), hereafter denoted as signal 1 and signal 2, respectively. These H2 consumptions are attributed to the stepwise reduction of Co3 O4 according to Eqs. (3) and (4): Co3 O4 + H2 → 3CoO + H2 O

(3)

CoO + H2 → Co◦ + H2 O

(4)

The areas ratio of (signal 2)/(signal 1) is of about 3 (Table 4), in fair agreement with the theoretical value expected from the stoichiometry of cobalt oxides reduction (Eqs. (3) and (4)). Finally, an additional weak signal at 652–688 ◦ C (signal 3), which is equivalently perceived for all the catalysts, is usually attributed [27,31] to surface cobalt hydroxy silicates which develop upon reaction of the metal species and the surface silanol groups of the silica support. The minor contribution of this cobalt silicate species to the H2 -TPR profiles is in line with previous observations [23,31] and indicates a negligible metal-support interaction [31]. Significant changes in the TPR profile in the range of the signal 2 (369–511 ◦ C) are observed with an increase in alumina loading up to 2 and 3 wt.%. A progressive broadening of the signal 2 is accompanied by the shift of the peaking towards high temperatures (Fig. 5d and e) in agreement with previously reported data [22]. Such changes are indicative of the strong interaction between supported cobalt oxide nanoparticles and added Al2 O3 species inhibiting the reduction of cobalt oxides. It was shown [22] earlier on the basis of XRD and H2 -chemisorption experiments that alumina-promoted catalysts form supported-cobalt particles with a higher dispersion. We have assured in this by means of scanning and transmission electron microscopy.

Please cite this article in press as: A.P. Savost’yanov, et al., The impact of Al2 O3 promoter on an efficiency of C5+ hydrocarbons formation over Co/SiO2 catalysts via Fischer-Tropsch synthesis, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.037

G Model

ARTICLE IN PRESS

CATTOD-10069; No. of Pages 8

A.P. Savost’yanov et al. / Catalysis Today xxx (2016) xxx–xxx

6

Fig. 7. Representative TEM micrographs and the corresponding Co particle size histograms for the reduced (a) Co/SiO2 , (b) Co-0.4Al2 O3 /SiO2 , (c) Co-1Al2 O3 /SiO2 and (d) Co-2Al2 O3 /SiO2 catalysts.

3.3.2. SEM and TEM characterization of the reduced Co-xAl2 O3 /SiO2 catalysts Representative SEM micrographs for unmodified and modified by alumina with the optimum 1 wt.% of Al2 O3 loading catalysts after

reduction and subsequent passivation are depicted in Fig. 6. Similar to the oxidic form of the catalysts (Fig. 4) microcrystallites of cobalt on the surface of the reduced unmodified Co/SiO2 catalyst (Fig. 6a)

Table 3 Texture properties of the calcined SiO2 support and Co-xAl2 O3 /SiO2 catalysts. Sample

BET surface area (m2 /g)

Average pore diameter (nm)

Total pore volume (cm3 /g)

SiO2 Co/SiO2 Co-0.4Al2 O3 /SiO2 Co-1Al2 O3 /SiO2 Co-2Al2 O3 /SiO2 Co-3Al2 O3 /SiO2

365 236 232 239 227 227

12 11.2 10.9 11.0 11.2 10.4

0.85 0.66 0.63 0.66 0.64 0.59

Please cite this article in press as: A.P. Savost’yanov, et al., The impact of Al2 O3 promoter on an efficiency of C5+ hydrocarbons formation over Co/SiO2 catalysts via Fischer-Tropsch synthesis, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.037

G Model CATTOD-10069; No. of Pages 8

ARTICLE IN PRESS A.P. Savost’yanov et al. / Catalysis Today xxx (2016) xxx–xxx

7

Table 4 H2 -TPR features for the air-calcined catalysts. Catalyst

Co/SiO2 Co–0.4Al2 O3 /SiO2 Co–1Al2 O3 /SiO2 Co–2Al2 O3 /SiO2 Co–3Al2 O3 /SiO2

Signal 1

Signal 2

Signal 3

Peaking (◦ C)

Peak area (%)

Peaking (◦ C)

Peak area (%)

Peaking (◦ C)

Peak area (%)

297 285 295 292 292

23 25 25 24 24

369 353 371 380 511

71 69 72 73 74

678 652 670 655 688

6 6 3 3 2

Table 5 Mean crystallite size (nm) of Cо◦ nanoparticles in the Co-xAl2 O3 /SiO2 reduced catalysts. x (wt.%) 0 12.4 ± 2.4

0.4 8.0 ± 1.6

1.0 8.0 ± 1.5

2.0 10.7 ± 2.1

are aggregated in small agglomerates of the sizes of 50–300 nm and large (≥500 nm) clusters of agglomerates. Pores between agglomerates vary in the range of 50–100 nm. Granules and agglomerates of cobalt on the catalyst’s surface with an optimum content of promoter, Co-1Al2 O3 /SiO2 , are distributed more uniformly (Fig. 6b). The size of the agglomerates varies between 30 and 200 nm, pore size of 30–60 nm. Very few of agglomerates are larger than 400 nm in size. Representative TEM micrographs and the corresponding particle size distribution for selected reduced catalysts are shown in Fig. 7. It is observable that the increase in alumina content up to 1.0 wt.% causes clear decrease in cobalt particle size (increase in dispersion) and narrowing of the particle size distribution. However, further increase in alumina content to 2.0 wt.% leads to the opposite effects, i.e. to enlargement of Co nanoparticles and broadening of the size distribution. Mean Co◦ particle size values, which have been corrected by accounting for the volume contraction upon reduction of the 3 nm CoO passivation shell [27,28,31], are collected in Table 5. Thus, Al2 O3 loading up to 1 wt.% leads to a more narrow size distribution of Co◦ particles with the maximum at 8 nm and the same value for the mean particle size. Further increasing in alumina loading causes the opposite effects. Such a volcano-like dependence for the cobalt nanoparticles size on the loaded Al2 O3 is in line with the recently obtained results for Co/AC catalysts promoted by alumina [23]. It can be suggested that a proper amount of Al2 O3 dopant could inhibit the aggregation of metallic cobalt nanoparticles leading to the observable decrease in mean particle size. The higher dispersion of metallic cobalt increases the total amount of active cobalt atoms exposed to outer surface of cobalt nanoparticles thereby increasing the activity of the catalyst. It is noteworthy that analogous influence of cobalt particle size on the activity and C5+ selectivity with a maximum observed at 7–8 nm was reported recently for alumina supported cobalt catalysts [32]. The noticed effect of average size of cobalt nanoparticles on the selectivity of FTS over Co/SiO2 can arise from the activity of cobalt nanoparticles in hydrogenolysis of the resultant hydrocarbons. Recently it has been shown [33] that the highest activity in the selective hydogenolysis of hexadecane into lighter (C10 –C15 ) hydrocarbons is observed for catalysts with the average size of cobalt nanoparticles of just 8 nm. So, it can be suggested that more extensive subsequent hydrogenolysis over Co◦ nanoparticles of long-chain hydrocarbons gives reasons for the increase in the fraction of C8 –C25 at the expense of longer hydrocarbons that was observed for the products formed over Co-1Al2 O3 /SiO2 catalyst as compared to Co/SiO2 (Fig. 2).

Fig. 8. CO-TPD profiles for the reduced catalysts (a) Co/SiO2 ; (b) Co–0.4Al2 O3 /SiO2 (c) Co-1Al2 O3 /SiO2 , (d) Co–2Al2 O3 /SiO2 , (e) Co–3Al2 O3 /SiO2 .

On the other hand, doping of Co/SiO2 with alumina could create some acidity of the support [34], and acid catalysis of further hydrogenolysis of hydrocarbons produced via FTS cannot be excluded [33,35]. In particular, the observed drastic increase in CH4 and olefins selectivities over Co-3Al2 O3 /SiO2 as against Co1Al2 O3 /SiO2 catalyst (Table 1) can be a consequence of anticipated greater concentration of the acidic sites on the surface of the first catalyst and successive demethylation of hydrocarbons over them. 3.3.3. Temperature-programmed desorption of CO (CO-TPD) CO-TPD was used to study the effect of alumina loading on the CO adsorption behavior of the reduced cobalt catalysts. The COTPD profiles for the reduced Co-xAl2 O3 /SiO2 samples are plotted in Fig. 8. A very weak desorption peak is perceived around 70 ◦ C, which can be assigned to the physisorbed CO desorption. Besides of this low-intensity peak, the catalysts show two major desorption signals: a low-temperature signal in the range of 170–250 ◦ C (peaking at ca. 220 ◦ C), and a high-temperature (peaking at ca. 750 ◦ C) signal which can be attributed for desorption of CO2 formed from chemisorbed CO via the Boudouard reaction [36]. Addition of alumina does not affect the position of the low temperature peaks indicating that the ability of the catalysts for the activation of CO is not influenced by the presence of the Al2 O3 dopant. At the same time, with increase in alumina content until 1 wt.% the low-temperature signals show a monotonic increase in their area (Table 6). However, the further increase in alumina loadings over 1 wt.% causes the opposite effect: the low-temperature signals decrease in their area. Such a dependence of CO chemisorption on alumina loadings with maximum at 1 wt.% is obviously a consequence of the highest dispersion of the metallic cobalt which is acquired just at this amount of the dopant. It is noteworthy that

Please cite this article in press as: A.P. Savost’yanov, et al., The impact of Al2 O3 promoter on an efficiency of C5+ hydrocarbons formation over Co/SiO2 catalysts via Fischer-Tropsch synthesis, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.037

G Model

ARTICLE IN PRESS

CATTOD-10069; No. of Pages 8

A.P. Savost’yanov et al. / Catalysis Today xxx (2016) xxx–xxx

8 Table 6 CO-TPD features for the reduced catalysts. Catalyst

Co/SiO2 Co–0.4Al2 O3 /SiO2 Co–1Al2 O3 /SiO2 Co–2Al2 O3 /SiO2 Co–3Al2 O3 /SiO2

Low temperature peak Peaking (◦ C)

Relative peak areaa (%)

207 207 220 219 221

12 15 18 8 7

a Peak area normalized to the total area of all observed peaks in the CO-TPD profiles.

electronic effects can play definite role besides these structural factors. It is known that alumina possesses an electron-donating character [23]. The presence of an electron-donating species on the catalyst surface favors the CO adsorption, since carbon monoxide is an ␲-acceptor. This implies that alumina can be considered both as a structural promoter leading to increased cobalt dispersion and as an electronic promoter modifying chemisorption properties of the active metal. Both of these alumina functions favor for CO adsorption. On the other hand, it was shown recently [23] that Al2 O3 as promoter inhibits H2 adsorption. These should decrease H/C ratio on the surface of the promoted catalysts. It is noteworthy that the surface H/C ratio is associated with selectivity and product distribution in the FTS reaction [37–39]. As suggested [37], the higher hydrogen accessibility would increase CO hydrogenation facility leading to increase in methane selectivity, while higher surface carbon monoxide concentration would facilitate the carbon chain growth probability resulting in more long-chain products. Probably these effects are responsible for the improved selectivity towards C5+ hydrocarbons on alumina promoted Co/SiO2 catalysts.

The authors also thank the Shared Research Center “Nanotechnologies” of the Platov South-Russian State Polytechnic University and the Department of Structural Studies of Zelinsky Institute of Organic Chemistry for analytical services. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]

[21] [22]

4. Conclusions

[23]

The doping of Co/SiO2 FTS catalysts (∼20 wt.% of Co) with Al2 O3 causes promotional effect on their activity and selectivity to C5+ hydrocarbons with increasing amount of C8 –C25 fraction. At P = 2 MPa, GHSV = 1000 h−1 and 213 ◦ C the largest promotion effect is observed for the catalysts with 1 wt.% of Al2 O3 . Decrease of pressure, GHSV and temperature up to 0.1 MPa, 100 h−1 and 190 ◦ C, respectively, diminishes the promotional effect. The addition of alumina into Co/SiO2 catalyst alters Co◦ particle size distribution and provides a volcano-like dependence of CO chemisorption on alumina loadings. The lowest mean value for Co◦ particle size of 8 nm and maximum of CO chemisorption were observed for the catalyst with 1 wt.% of Al2 O3 loading. Increase in CO chemisorption at the proper amount of alumina decreases the ratio of surface hydrogen to carbon monoxide and in such a way promotes formation of C5+ hydrocarbons. The obtained results of extended trials of the alumina doped catalysts in a fixed-bed tubular pilot-scaled reactor demonstrate high efficiency of alumina as an inexpensive promoter for industrial Co/SiO2 catalysts of FTS in GTL processes.

[24]

[25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39]

Acknowledgment

M.E. Dry, Catal. Today 71 (2002) 227–241. C.H. Bartholomew, Catal. Lett. 7 (1990) 303–315. W. Chu, J. Xu, J. Hong, T. Lin, A. Khodakov, Catal. Today 256 (2015) 41–48. P. Guo, W. Saw, P. van Eyk, P. Ashman, G. Nathan, E. Stechel, Energy Procedia 69 (2015) 1770–1779. E. van Steen, M. Claeys, Chem. Eng. Technol. 31 (2008) 655–666. M. Shao, Y. Li, J. Chen, Y. Zhang, Adv. Chem. Eng. 47 (2015) 337–387. A.Y. Khodakov, W. Chu, P. Fongarland, Chem. Rev. 107 (2007) 1692–1744. H. Jahangiri, J. Bennett, P. Mahjoubi, K. Wilson, S. Gu, Catal. Sci. Technol. 4 (2014) 2210–2229. Q. Zhang, K. Cheng, J. Kang, W. Deng, Y. Wang, ChemSusChem 7 (2014) 1251–1264. M. Arsalanfar, A.A. Mirzaei, H.R. Bozorgzadeh, A. Samimi, Phys. Chem. Res. 2 (2014) 179–201. S.A. Gardezi, B. Joseph, F. Prado, A. Barbosa, Biomass Bioenergy 59 (2013) 168–186. K. Mai, T. Elder, L.H. Groom, J.J. Spivey, Catal. Commun. 65 (2015) 76–80. H.M. Torres Galvis, K.P. de Jong, ACS Catal. 3 (2013) 2130–2149. A. Di Fronzo, C. Pirola, A. Comazzi, F. Galli, C.L. Bianchi, A. Di Michele, R. Vivani, M. Nocchetti, M. Bastianini, D.C. Boffito, Fuel 119 (2014) 62–69. A.P. Savost’yanov, R.E. Yakovenko, G.B. Narochnyi, A.L. Lapidus, Solid Fuel Chem. 49 (2015) 356–359. N. Tsubaki, S. Sun, K. Fujimoto, J. Catal. 199 (2001) 236–246. W. Chu, P.A. Chernavskii, L. Gengembre, G.A. Pankina, P. Fongarland, A.Y. Khodakov, J. Catal. 252 (2007) 215–230. J.-S. Girardon, E. Quinet, A. Griboval-Constant, P.A. Chernavskii, L. Gengembre, A.Y. Khodakov, J. Catal. 248 (2007) 143–157. M. de Beer, A. Kunene, D. Nabaho, M. Claeys, E. van Steen, J. South Afr. Inst. Min. Metall. 114 (2014) 157–165. L. París, J. Lopez, F. Barrientos, M. Pardo, Catalytic conversion of biomass-derived synthesis gas to fuels, in: J.J. Spivey, Y.-F. Han, K.M. Dooley (Eds.), Catalysis, vol. 27, RSC, Cambridge, 2015, pp. 62–143 (Chapter 3). G.R. Johnson, A.T. Bell, ACS Catal. 6 (2016) 100–114. Y. Zhang, S. Nagamori, S. Hinchiranan, T. Vitidsant, N. Tsubaki, Energy Fuels 20 (2006) 417–421. Y. Pei, Y. Ding, H. Zhu, J. Zang, X. Song, W. Dong, T. Wang, Y. Lu, Catal. Lett. 144 (2014) 1433–1442. A. de Klerk, Y.-W. Li, R. Zennaro, Fischer-Tropsch technology, in: P.M. Maitlis, A. de Klerk (Eds.), Greener Fischer-Tropsch Processes for Fuels and Feedstocks, Wiley, Weinheim, 2013, pp. 53–79. R.E. Yakovenko, G.B. Narochnyi, A.P. Savost’yanov, V.A. Kirsanov, Chem. Pet. Eng. 51 (2015) 159–163. The Rietveld Method, in: R.A. Young (Ed.), University Press, Oxford, 1993. G. Prieto, A. Martínez, P. Concepción, R. Moreno-Tost, J. Catal. 266 (2009) 129–144. G.L. Bezemer, J.H. Bitter, H.P.C.E. Kuipers, H. Oosterbeek, J.E. Holewijn, X. Xu, F. Kapteijn, A.J. van Dillen, K.P. de Jong, J. Am. Chem. Soc. 128 (2006) 3956–3964. A.S. Kashin, V.P. Ananikov, Russ. Chem. Bull. 60 (2011) 2602–2607. V.V. Kachala, L.L. Khemchyan, A.S. Kashin, N.V. Orlov, A.A. Grachev, S.S. Zalesskiy, V.P. Ananikov, Russ. Chem. Rev. 82 (2013) 648–685. G. Prieto, P. Concepción, R. Murciano, A. Martínez, J. Catal. 302 (2013) 37–48. Ø. Borg, P.D.C. Dietzel, A.I. Spjelkavik, E.Z. Tveten, J.C. Walmsley, S. Diplas, S. Eri, A. Holmen, E. Rytter, J. Catal. 259 (2008) 161–164. X. Peng, K. Cheng, J. Kang, B. Gu, X. Yu, Q. Zhang, Y. Wang, Angew. Chem. 127 (2015) 4636–4639. U.A. Sedran, N.S. Figoli, Appl. Catal. 19 (1985) 317–325. N. Kruse, A.G. Machoke, W. Schwieger, R. Güttel, ChemCatChem 7 (2015) 1018–1022. N. Fischer, E. van Steen, M. Claeys, J. Catal. 299 (2013) 67–80. M. Claeys, E. van Steen, Stud. Surf. Sci. Catal. 152 (2004) 601–680. A. Dinse, M. Aigner, M. Ulbrich, G.R. Johnson, A.T. Bell, J. Catal. 288 (2012) 104–114. J. Li, C. Zhang, X. Cheng, M. Qing, J. Xu, B. Wu, Y. Yang, Y. Li, Appl. Catal. A 464–465 (2013) 10–19.

This work was financially supported by Russian Science Foundation through grant No. 14-23-00078.

Please cite this article in press as: A.P. Savost’yanov, et al., The impact of Al2 O3 promoter on an efficiency of C5+ hydrocarbons formation over Co/SiO2 catalysts via Fischer-Tropsch synthesis, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.037