the sol-gel derived co-mn/tio2 catalysts for light olefins production

JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY

Volume 42, Issue 2, Feb 2014 Online English edition of the Chinese language journal

Received: 11-Oct-2013; Revised: 04-Jan-2014 * Corresponding author. E-mail: [email protected] Copyright 2014, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

RESEARCH PAPERCite this article as: J Fuel Chem Technol, 2014, 42(2), 212218

The sol-gel derived Co-Mn/TiO2 catalysts for light olefins production Mohammad Mehdi Khodaei, Mostafa Feyzi*, Jahangir Shahmoradi, Mohammad Joshaghani Faculty of Chemistry, Razi University, Kermanshah 0098831, Iran

Abstract: In this research work, two 30%(Co-Mn)/TiO2 catalysts were prepared using sol-gel (catalyst A) and co-precipitation

(catalyst B) methods. The activity and selectivity to C2–4 light olefins in Fischer-Tropsch synthesis (FTS) has been studied in a

fixed-bed reactor under different operational conditions. These operational conditions were: temperature 220–280°C, and total pressure

from 0.1–0.6 MPa. The optimum operating conditions were investigated after steady state. As the results shown, the catalyst A was

more selective to C2–4 olefins (58.7% in 260°C) and catalyst B was more selective to C5+ hydrocarbons. Characterization of both

catalysts was carried out by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and N2 adsorption-desorption

measurements methods.

Keywords: Fischer-Tropsch synthesis; light olefins; characterization

Introduction of cheap, efficient and selective catalysts for production of fuels and heavier hydrocarbons from lighter ones is of great challenge in refineries and petrochemical industries. Cobalt-based catalysts exhibit characteristics which are superior to those of iron-based catalysts for the conversion of synthesis gas to hydrocarbons[1]. The Fischer-Tropsch synthesis (FTS) with supported cobalt catalysts has been studied by many investigators[2–4]. Most of these investigators showed that supported cobalt catalysts have a very low (or practically none at all) water-gas shift (WGS) reaction activity under conditions favoring the FTS, which is considered a disadvantage when a lean hydrogen feed is used. Modification of the traditional FTS catalysts (Mn, Ni, Co, Ru) by promoters and supports has provided one means of manipulating the FTS products spectrum[5]. Due to the thermodynamic and kinetic limitations of the reaction, few catalysts are able to amplify the C2–4 hydrocarbons fraction. However some examples are reported in the literature and these are Mn and Co based catalysts on partially reducible oxide supports such as MnO2, V2O5 and TiO2 instead of the conventional inert supports like SiO2 and Al2O3 were used in FTS[6–8]. There has been considerable interest for the modification of cobalt with manganese oxide and it has shown that a Co/MnO catalyst with Co/Mn molar ratio of unity can give decreased methane yields together with enhanced propylene formation[9–11], at atmospheric pressure and at low conversion, Reuel et al[12] reported, with 10% Co supported catalysts, an increase in

specific activity (p=0.1 MPa, t=225°C, H2/CO(volume ratio)=2) depending on the nature of the support in the following order: Co/MgO<Co/C<Co/SiO2<Co/Al2O3< Co/TiO2. However, the works of Iglesia et al[13] at higher pressure (p>0.5 MPa) and at high conversion indicate that the influence of the support on the specific activity of the methane and C5+ hydrocarbons selectivity can be neglected.

In this research work, we used the sol-gel method to investigate the effect of preparation condition on the structure of cobalt-manganese catalyst in FTS. Our work showed that the optimum catalyst is 30%(Co-Mn)/TiO2. However, not much systematic work about the other preparation methods of Co-Mn oxide catalysts and their catalytic properties has been reported in the literature. Thus, we planned to investigate in outline, the co-precipitation method and characterization of Co-Mn catalysts. We attempt to extensively report the influence of the effect of operation conditions on the catalytic performance of catalysts of 30%(Co-Mn)/TiO2. Characterization of precursor and catalysts were carried out by using X-ray diffraction, scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) specific surface area measurement. 1 Experimental 1.1 Catalyst preparation

Mohammad Mehdi Khodaei et al. / Journal of Fuel Chemistry and Technology, 2014, 42(2): 212218

Fig. 1 Schematic representation for the catalyst test system and used reactor

Table 1 Catalytic performance of w(Co-Mn)/TiO2 (w=5%, 10%, 15%, 20%, 25%, 30%, 35% and 40%) catalysts prepared with sol-gel

method

40 35 30 25 20 15 10 5 w/%

32.7 34.7 35.1 32.6 29.8 25.4 21.5 18.3 CO conversion x/%

15.6 13.8 12.3 13.5 15.4 17.1 18.3 22.3 CH4

Product selectivity s/%

27.3 35.1 39.3 35.6 32.3 29.6 28.9 24.8 C2–4 olefins

23.5 23.3 23.0 24.3 24.9 25.1 23.4 22.5 C2–4 alkanes

11.1 10.4 9.5 9.3 8.2 9.0 9.4 10.3 CO2

22.5 17.1 15.9 17.3 19.2 19.2 20.0 20.1 C5+

1.17 1.50 1.70 1.46 1.29 1.17 1.23 1.10 olefins/alkanes

reaction conditions: GHSV=1200 h–1, H2/CO(volume ratio)=1/1, p=0.1 MPa and 260C

In this study sol-gel and co-precipitation methods were employed for catalyst preparation. At first for preparation of catalysts, the appropriate amount of materials cobalt nitrate (Co(NO3)2·6H2O) and manganese nitrate (Mn(NO3)2·4H2O) were dissolved in ethanol at 60°C, separately (I). The required amounts of tetra butoxy titane (Ti(OC4H9)4) as TiO2 source was dissolved in ethanol at 60°C and then gradually added to the solution containing cobalt-manganese (I) to produce: 5%, 10%, 15%, 20%, 25%, 30%, 35% and 40% (based on weight of TiO2) of Co/Mn=1/1 mixture solutions, respectively (II). An ethanol solution of oxalic acid (C2H2O4·2H2O, appropriate amount for hydrolysis of Ti(OC4H9)4 was added to a mixed solution (II) under constant stirring 500 r/min to obtain a gel form. The gel was dried in an oven (100°C, 12 h) to give a

material denoted as the catalyst precursor. Finally, the catalyst precursor was calcined in a furnace (600°C for 6 h) to give the final catalyst. Finally for investigation the effect of preparation method the catalyst containing 30% (Co-Mn)/TiO2 (catalyst B), was prepared by co-precipitation method. The aqueous solutions of Co(NO3)2·6H2O, Mn(NO3)2·4H2O and TiO2 were pre-mixed, and the resulting solution was heated to 70°C in a reflux flask equipped with a condenser. Aqueous Na2CO3 (0.25 mol/L) was added drop wise to the mixed nitrate solution with stirring, while the temperature was maintained at 70°C until pH = 8.0±0.1 was achieved. The precipitate material was then filtered and washed several times with warm distilled water until the Na+ <60 106 was observed in the filtrated water (tested by flame


atomic absorption). The precipitate was dried at 120°C for 16 h and subsequently calcined at 600°C for 6 h to give the final catalyst. 1.2 Catalyst characterization

XRD measurements were conducted with a Bruker axs D8

Advance diffractometer. Scans were taken with a 2 step size of 0.02 from 4° to 70° and a counting time of 1.0 s using Cu K radiation source generated at 40 kV and 30 mA. N2 physisorption were performed by using a NOVA 2000 instrument. Prior to the measurements, all catalyst samples were slowly degassed at 150°C for 4 h under inert N2 atmosphere. Then the samples were transferred to the adsorption unit for determine the textural properties. The morphology of catalyst and precursor was observed by means of an S-360 Oxford Eng scanning electron microscopy. 1.3 Fischer-Tropsch synthesis

Catalytic activity test were carried out in a fixed bed stainless steel reactor at different operation conditions, and the catalyst test system is shown in Figure 1.

Fig. 2 XRD patterns of w(Co-Mn)/TiO2 (w= 5%, 10%, 15%, 20%,

25%, 30%, 35% and 40% based on the TiO2 weight) catalysts

Fig. 3 XRD pattern for used 30%(Co-Mn)/TiO2 catalyst

: CoO(cubic); : Co2C(orthorhombic);

: CoTiO3(cubic); : MnO2(cubic); : MnO(cubic); : Co(cubic)

Table 2 Particle size of w(Co-Mn)/TiO2 catalysts

Size d/nm w/%

67 5

58 10

51 15

46 20

42 25

38 30

41 35

48 40

All catalysts were activated (reduced) for 16 h period on

line in pure hydrogen (0.1 MPa) at a temperature of 400°C at flow rate of 30 mLmin–1. Meshed catalyst (1.0 g) was held in the middle of the reactor (30 cm length and internal diameter is 7 mm). Reactant and stream products were analyzed on-line using a Varian gas chromatograph (Star 3600CX) equipped with a thermal conductivity detector (TCD) and a Chromosorb column. The heavy hydrocarbon products were off-line analyzed using a Varian CP 3800 with a Petrocol Tm DH100 fused silica capillary column and a flame ionization detector (FID). The conversion percentage of CO based on the fraction of CO forming carbon-containing products according to below equation:

CO conversion x(%)=∑ niMi

MCO×100% (1)

where ni is the number of carbon atoms in product i, Mi is the mole number of product i and MCO is the mole number of CO in the syngas feed. The selectivity (s) of product i, is based on the total number of carbon atoms in the product and therefore is defined as:

si(%)=niMi

∑ niMi×100% (2)

2 Results and discussion 2.1 Effect of loading cobalt-manganese

To understand the influence of loading of cobalt-manganese

on the catalytic performance, a series of w(Co-Mn)/TiO2 catalysts (w= 5%, 10%, 15%, 20%, 25%, 30%, 35% and 40% based on the TiO2 weight) were prepared by using sol-gel method and tested under same reaction conditions (H2/CO(volume ratio)=1/1, GHSV=1200 h–1, p=0.1 MPa at 260°C). The results are shown in Table 1. According to the obtained results, the catalysts loaded with 30% of Co-Mn showed the best catalytic performance for light olefins especially propylene. The catalyst 30%(Co-Mn)/TiO2 was chosen as the optimal catalyst. As shown in Table 1, CO conversion increased with increasing weight percent of Co-Mn then decreased by excess of 30% of Co-Mn.


Fig. 4 SEM image of catalyst 30%(Co-Mn)/TiO2

(a): precursor; (b): calcined

Table 3 N2 adsorption-desorption measurements of w(Co-Mn)/TiO2 catalysts

w/% A/(m2·g–1) d/nm v/(cm3·g–1)

a b c a b c a b c

5 139.3 143.4 139.4 2.56 2.93 2.82 0.187 0.219 0.204

10 144.6 152.6 151.5 2.83 3.47 3.16 0.196 0.234 0.213

15 153.4 159.7 153.6 3.01 3.82 3.82 0.211 0.241 0.218

20 164.8 166.8 162.3 3.25 4.67 4.67 0.223 0.259 0.219

25 169.5 168. 1 157.2 3.58 4.38 4.95 0. 238 0.264 0.235

30 171.3 178.7 176.7 3.94 4.95 4.38 0.255 0.267 0.221

35 135.2 141.6 135.6 2.96 4.02 4.02 0.218 0.252 0.184

40 114.9 132.4 126.2 2.71 3.59 3.59 0.204 0.236 0.166

a: precursor; b: catalyst (before reaction); c: catalyst (after reaction)

Table 4 Effect of temperature on the catalytic performance of A catalyst

280 270 260 250 240 230 220 Temperature t/C

38.7 35.2 33.7 34.5 24.2 19.1 11.6 CO conversion x/%

11.6 9.1 7.4 6.7 5.0 4.8 4.1 CH4


44.9 49.7 58.7 52.6 54.6 53.6 52.4 C2–4 olefins

28.0 28.4 27.9 28.2 30.1 29.4 27.7 C2–4 alkanes

6.0 3.9 2.1 2.6 2.0 1.8 1.8 CO2

9.5 8.9 6.6 9.0 8.3 10.4 14.0 C5+

1.6 1.8 2.1 1.9 1.8 1.8 1.9 olefins/alkanes

reaction conditions: GHSV=1200 h–1, H2/CO(volume ratio)=1/1, p=0.1 MPa

The CO2 and CH4 selectivity decreased with increasing

weight percent of Co-Mn then increased by excess of 30% of Co-Mn. All prepared catalysts were characterized by XRD and shown in Figure 2. The actual phases were identified for these catalysts under the specified preparation conditions as CoMnO3 (rhombohedral), Co3O4 (cubic), (Co,Mn)(Co,Mn)2O4 (tetragonal) and TiO2 (tetragonal), although the relative diffracted intensities of these phases for the various catalysts were different. The catalyst containing 30%Co-Mn supported on TiO2 has shown the best catalytic performance than the other prepared catalysts. To identify the phase changes of 30%(Co-Mn)/TiO2 catalyst during the reactions, the catalyst

after reaction was characterized by XRD technique and shown in Figure 3. The actual phases identified in the XRD pattern of the catalyst (after reaction) were CoO (cubic), MnO2 (cubic), CoTiO3 (cubic), Co (cubic), MnO (cubic) and Co2C (orthorhombic). In Fe-based catalyst oxidic phases are highly selective for the preparation of olefins, and carbide phases are active in the hydrogenation of CO[14–18]. The XRD pattern of 30%(Co-Mn)/TiO2 catalyst after CO hydrogenation also shows the Co0 phase. Some reports claim that metallic Co is active for FTS. XRD technique was commonly used to evaluate the particle sizes using the Scherrer equation[19].


Table 5 Effect of temperature on the catalytic performance of B catalyst

280 270 260 250 240 230 220 Temperature t/C

49.5 46.1 44.2 44.0 37.5 29.8 25.2 CO conversion x/%

13.6 9.9 7.9 7.2 7.1 6.9 6.9 CH4

Product selectivity

s/%

37.1 44.4 52.0 54.4 47.4 39.2 37.6 C2–4 olefins

23.7 22.0 17.3 17.1 18.8 19.8 17.8 C2–4 alkanes

8.4 7.0 6.2 6.0 6.1 6.1 6.0 CO2

17.2 16.7 16.6 15.3 20.6 28.0 31.7 C5+

1.6 2.0 3.0 3.2 2.5 2.0 2.1 olefins/alkanes

reaction conditions: GHSV= 1200 h–1, H2/CO(volume ratio)=1/1, p=0.1 MPa

Table 6 Effect of reaction pressure on the catalytic performance of catalyst A

0.6 0.5 0.4 0.3 0.2 0.1 Pressure p/MPa

45.8 43.6 41.3 39.4 37.9 33.7 CO conversion x/%

3.9 4.0 4.1 4.3 4.3 4.7 CH4

Product selectivity

s/%

43.2 47.4 50.5 54.7 59.3 58.7 C2–4 olefins

19.4 23.0 23.1 24.3 26.1 27.9 C2–4 alkanes

3.5 3.5 3.4 3.4 3.1 2.1 CO2

30.0 22.1 18.9 13.3 7.2 6.6 C5+

2.22 2.06 2.18 2.25 2.27 2.10 olefins/alkanes

reaction conditions: GHSV=1200 h–1, H2/CO(volume ratio)=1/1, t=260C

The particle sizes of cobalt phase in the calcined catalysts

with different weight percent of Co-Mn are shown in Table 2. As it is previously shown, in Table 1, CO conversion and products selectivity are dependent on weight percent of Co-Mn. A comparison of the results in Table 1 and Table 2 indicated that CO conversion is dependent on catalyst particle size. The results show that decreasing catalyst particle size leads to increasing CO conversion and hydrocarbon selectivity. The catalyst 30%(Co-Mn)/TiO2 has an average particle sizes about 38 nm. In view of this, a detailed SEM study of both precursor and calcined catalyst 30%(Co-Mn)/TiO2 was taken and the results are given in Figure 4. SEM observations have shown differences in morphology of precursor and calcined catalysts. The image obtained from catalyst precursor depicts several larger agglomerations of particles (Figure 4 (a)) and show that this material has a less dense and homogeneous morphology. After the calcination at 500°C, 6 h and heating rate of 2°C·min–1, the morphological features are different with the precursor sample and shows that the agglomerate size is greatly reduced compared to the precursor (Figure 4 (b)). It may be due to this reason the calcined catalyst surface is covered with small crystallite of manganese and cobalt oxides, in agreement with XRD results. The specific surface area, pore volume and pore diameter of the precursors and calcined catalysts (before and after reaction) are given in Table 3. According to the obtained results, the specific surface areas are dependent on Co-Mn loading. It can be clearly illustrated that for calcined catalysts, with increasing of weight percent of Co-Mn, at first, these entire

physical characteristic feature were increased then decreased by excess of 30% Co-Mn. The N2 absorption-desorption data calculated catalysts properties are shown in Table 3. The catalyst 30%(Co-Mn)/TiO2 has higher specific surface area, pore volume and pore diameter, than the other calcined catalysts. The catalyst 30%(Co-Mn)/TiO2 also had a larger average pore diameter 4.95 nm than the other prepared catalysts. It can be found from Table 3 pore volume for the optimal catalyst was significantly larger than the pore volume for the other prepared catalysts. This phenomenon is very interesting and this might be reasons why the 30%(Co-Mn)/TiO2 catalyst shows a better catalytic performance than the other tested catalysts. For improve this study the catalyst 30%(Co-Mn)/TiO2 was prepared by using co-precipitation method (catalyst B) and comparison its catalytic performance with catalyst A. 2.2 Effect of reaction temperature

The effect of reaction temperature on catalytic performance

of two catalysts (A and B) containing 30%(Co-Mn)/TiO2 studied at a range of temperatures between 220280°C under the same reaction conditions (p=0.1 MPa, H2/CO(volume ratio)=1/1 and GHSV=1200 h–1). The results are presented in Tables 4 and 5. The CO conversion strongly increases with increasing temperature for both catalysts. Selectivity to methane also increases with temperature, but it has a stronger effect on catalyst B.


Table 7 Effect of reaction pressure on the catalytic performance of catalyst B

0.6 0.5 0.4 0.3 0.2 0.1 Pressure p/MPa

64.3 61.3 59.2 56.3 47.4 44.2 CO conversion x/%

9.4 9.4 9.2 8.1 7.9 7.9 CH4


30.1 32.8 34.2 41.7 47.6 52.0 C2–4 olefins

24.1 27.7 28.4 21.8 19.1 17.3 C2–4 alkanes

5.1 5.2 5.4 6.1 6.2 6.2 CO2

31.3 24.9 22.8 20.3 19.2 16.6 C5+

1.07 1.18 1.20 1.91 2.49 3.0 olefins/alkanes

reaction conditions: GHSV=1200 h–1, H2/CO(volume ratio)=1/1, t=250C

In addition, selectivity toward light olefin for catalyst A

increases until 240°C and then decreases with temperature, while for catalyst A the maximum selectivity to light olefins occurs at 240°C. Reaction temperature has a modest effect on chain growth probability (C5+ selectivity) for catalyst A while there was a significant effect for catalyst B. Generally the increase in the reaction temperature leads to the increase in the catalytic performance. Furthermore, it was shown that the reaction temperature should not be too low[20]. At low reaction temperatures, the conversion percentage of CO was low, so it caused a low catalytic performance. The methane selectivity increased with increasing temperature. From Tables 4 and 5, it can also be seen that the selectivity to light olefins is relatively high, and increased with increasing temperature except for ethylene, which decreased from 16.5% to 10.7%. From the results, it can also be concluded that higher temperature is preferential for chain termination to produce light hydrocarbons, while lower temperature is preferential for chain growth and the production of heavy hydrocarbons[21]. Because of high CO conversion, total selectivity of light olefin products, low CH4 and CO2 selectivity, the temperature 250°C was considered to be the optimum operating temperature for catalyst (B). Comparison of the two catalysts performance shows that the sol-gel catalyst (A) has higher activity, lower selectivity to CH4 and CO2 selectivity, and higher selectivity to light olefin. 2.3 Effects of reaction pressure

In commercial process, the FTS reaction usually operates

under high pressure. The increase in total pressure would generally result in condensation of hydrocarbons, which are normally in the gaseous state at atmospheric pressure. Higher pressures and higher carbon monoxide conversions would probably lead to saturation of catalyst pores by liquid reaction products[22] and it can influences on the catalytic performance. The effect of pressure on the FTS for catalyst A at 260°C and for catalyst B at 250°C are shown in Tables 6 and 7. The research conditions are at different pressure levels (0.1–0.6

MPa) and H2/CO(volume ratio) = 1/1. As shown in these Tables, for catalyst A the increase of total pressure slightly increases the CO conversion from 33.7% at 0.1 MPa to 45.8% at 0.6 MPa and for catalyst B, the CO conversion increases from 44.2% to 64.3%. In contrast, the selectivity to heavier hydrocarbons (C5+) strongly increases with pressure, while the selectivity to methane and olefins declines. The effects of pressure on process selectivity can be interpreted by considering the olefins reactivity during low-temperature FTS.

As a result, before the reactants reach the catalyst surface they have to diffuse inside this layer, while reaction products have to do the same in the opposite direction before being desorbed. It is well known that olefins, in contrast to paraffins, can be readsorbed on the active sites, reinserted in the chain growth process, or can be hydrogenated to the corresponding paraffins[23–25]. The results indicate that catalyst B has higher selectivity toward methane and C5+ products than catalyst A. In addition, at 0.1 MPa, CO2 selectivity for catalyst B is higher than that for catalyst A. As it can be seen in Tables 6 and 7, at the ranges of 0.1–0.6 MPa total pressure, the light olefins selectivity were changed and the results indicate that at the total pressure of 0.2 MPa, the catalyst A showed the highest total selectivity of 59.3% with respect to C2–4 light olefins and for catalyst B at total pressure of 0.1 MPa is higher (52.0%). 3 Conclusions

In this study, two methods were employed to the preparation of 30%(Co-Mn)/TiO2 catalyst, namely the sol-gel (catalyst A) and co-precipitate (catalyst B) procedures and the effect of reaction temperature and pressure on the activity and hydrocarbon selectivity to especially light olefinic products has been studied. The optimum conditions were found to be 260°C, H2/CO = 1/1(volume ratio), and 0.2 MPa for catalyst A, whereas 250°C, H2/CO=1/1 and 0.1 MPa were the optimum operating conditions for catalyst B. However, catalyst A had selectivity to higher light olefins and lower selectivity to methane. The results are shown that the catalyst


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