Mesoporous Silica for FischerâTropsch Synthesis - American

Article pubs.acs.org/JPCC

Co3O4@Mesoporous Silica for Fischer−Tropsch Synthesis: Core−Shell Catalysts with Multiple Core Assembly and Different Pore Diameters of Shell Prashant R. Karandikar, Yun-Jo Lee, Geunjae Kwak, Min Hee Woo, Seon-Ju Park, Hae-Gu Park, Kyoung-Su Ha, and Ki-Won Jun* †

Research Center for Green Catalysis, Korea Research Institute of Chemical Technology (KRICT), P.O. Box 107, Sinseongno 19, Yuseong, Daejeon 305-600, Republic of Korea ABSTRACT: A series of core−shell cobalt oxide catalysts were prepared by tuning the pore diameter of mesoporous silica shell and the influence of pore dimensions on Fischer− Tropsch synthesis was studied. TEM images of core−shell 20Co3O4@MSN-x catalysts showed an increase in the silica shell porosity with increasing molar ratio of the swelling agent to tetraethyl orthosilicate used during catalyst preparation. As a result of this porosity increase, the dispersion of congregated core Co oxide particles within the shell increased, resulting in multiple core centers. Reference catalysts were synthesized using conventional supports, and the effects of pore dimensions on the activity and hydrocarbon selectivity of the reference and the prepared core−shell catalysts were compared. In the experiments performed, 20Co3O4@MSN-x catalysts showed an increase in their CO conversion and C5+ selectivity with increasing silica shell porosity. Moreover, the prepared core−shell catalysts demonstrated higher selectivity toward C5−C18 hydrocarbon fractions than SBA−15- and SiO2-supported Co oxide catalysts. This could be due to the comparatively lower diffusion limitations and less probability of readsorption of intermediates in the confined space of the mesoporous silica shell. Further, the core−shell catalysts displayed higher stability (100 h on stream) than the reference catalysts, and this was attributed to their small diffusion length and less probability of metal sintering due to a protective silica shell around Co oxide particles.

1. INTRODUCTION Fischer−Tropsch synthesis (FTS) is a key industrial process to overcome the present crude-oil-dependent supply of liquid fuels, as well as provide high quality transportation fuels that fulfill stringent future environmental requirements.1 Cobalt oxide based catalysts are widely utilized for FTS because of their high activity, low water−gas shift activity, high selectivity toward long-chain hydrocarbons, and low operating temperature.2−6 Most studies on Co FTS catalysts have been carried out using metals supported on silica, alumina, and titania supports.7−9 Ordered mesoporous materials (OMMs), typically MCM-41 and SBA-15, show excellent structural features such as a large surface area and a uniform pore diameter owing to which they have emerged as promising heterogeneous catalysts over the last two decades.8−10 However, one of the main drawbacks associated with the catalytic applications of OMMs is their structural order. The most widely employed OMMs usually consist of large particles having densely packed, strictly parallel, and lack of interconnected mesopores, which result in a large diffusion length. Recently, mesoporous silica nanoparticles (MSNs) have been widely used for catalytic chemical transformations because of their small particle sizes, shorter channels and large surface areas.11,12 The consistent spherical shape of nanoparticles provides MSNs with more well-defined and readily accessible © 2014 American Chemical Society

catalytic sites than those on amorphous supports. Moreover, MSNs have shorter channels than bulky mesoporous silica materials such as MCM-41 or SBA-15; these channels could facilitate the easy diffusion of reactants and products. Prieto et al.13 previously reported that Co FTS catalysts supported on short-pore SBA-15 showed higher activity and C5+ selectivity than those supported on long-pore SBA-15. The deactivation of catalysts due to cobalt oxide sintering during the reaction has been reported by several authors.14,15 However, in the case of core−shell catalysts, each functional core is isolated by a permeable shell with a relatively homogeneous surrounding environment and thus, sintering between the functional cores is effectively prevented. There are many reports on metal catalysts stabilized by MSNs (impregnated or in situ-generated composite type) for successful chemical conversion. MSNstabilized Rh nanoparticles have been reportedly used to synthesize ethanol and acetaldehyde from syngas with improved selectivity.16 In another study, a Au@SiO2 framework of core−shell nanoparticles was used for the catalytic reduction of p-nitrophenol.17 Xie et al.18 reported a solvothermally prepared Co3O4@m-SiO2 core−shell catalyst for FT synthesis. Received: May 20, 2014 Revised: August 27, 2014 Published: August 27, 2014 21975

dx.doi.org/10.1021/jp504936k | J. Phys. Chem. C 2014, 118, 21975−21985

The Journal of Physical Chemistry C

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out during the formation of the mesoporous silica nanoparticles as shell. Additionally, the pore diameter of the silica shell was controlled by the addition of an appropriate amount of a swelling agent, SA (trimethylbenzene, TMB). The synthesis method is as follows. Mixture A was prepared by mixing 120 g of water, 14 g of cetyltrimethylammonium chloride as surfactant (CTACl, Sigma-Aldrich, 25% solution in water) and a certain amount of 1,2,4−trimethylbenzene (TMB, TCI) as a swelling agent to obtain the final mixture with the SA/ tetraethylorthosilicate (TEOS) mole ratios of 0, 0.6, 1.0, and 1.5 (However, the addition of a higher amount of the swelling agent resulted in structural deformation as well as increased aggregation of silica nanoparticles.). The mixture was heated up to 60 °C under stirring for 30 min. The black suspension of PVP-protected Co3O4 nanoparticles (equivalent to 20 wt % Co loading in the catalyst) prepared in the first part was added dropwise to the above surfactant solution and the whole mixture (mixture A) was stirred for another 3 h. In another autoclave, mixture B was prepared by mixing 10 g of tetraethylorthosilicate (TEOS, 98% purity, Samchun Chemicals) and 75 g of triethanolamine (TEA, 98% purity, Sigma-Aldrich). This mixture was placed in a closed Teflon vessel and heated in an oven at 90 °C for 20 min. This mixture B was added to mixture A above at room temperature and the resulting mixture was magnetically stirred for 48 h at 600 rpm. The product was then collected by filtration, washed thoroughly with distilled water and ethanol, and then dried at 60 °C. The dried product was then calcined at 500 °C for 6 h and denoted as 20Co3O4@MSN-x (where x = TMB/TEOS molar ratio; varied from 0 to 1.5). For a comparative study, a conventional mesoporous silica material, namely, SBA-15, was used to prepare cobalt oxide catalysts. SBA-15 was prepared by a procedure reported in the literature.22 Similarly, a cobalt oxide catalyst supported on commercial silica (SiO2, Davisil, grade 645, pore size: 150 Å, surface area: 300 m2/g) was also used for the comparative study. Supported cobalt oxide catalysts were prepared by impregnation of Co nitrate on SBA-15 and SiO2 materials. Briefly, a Co nitrate solution in ethanol was mixed with supports to obtain final catalysts with 20 wt % Co loading. The slurry was then stirred for 1 h, and this was followed by slow evaporation of the solvent in a rotary evaporator. Subsequently, the remaining slurry was dried under vacuum. After impregnation, the catalysts were dried at 100 °C for 10 h and calcined at 400 °C for 2 h under air. The catalysts were denoted as 20Co3O4−SBA-15 and 20 Co3O4/SiO2, respectively. 2.3. Catalyst Characterization. Powder X-ray diffraction (XRD) patterns of samples were obtained on a Rigaku D/ MAX-2200 V diffractometer using Cu/Kα radiation (λ = 0.154 056 nm) operating at 40 kV and 40 mA with a scanning rate of 5°/min from 5° to 70° to identify the crystalline phases of the prepared 20Co3O4@MSN-x catalysts. The surface morphology and compositions of the Co catalysts were measured using transmission electron microscopy (TEM, TECNAI G20 instrument), with the microscope operating at 200 kV. The morphologies of the core−shell catalysts and reference catalysts were characterized using scanning electron microscopy (SEM; JEOL, JSM6700F) for determining the dimensions of catalyst particles. The specimens of catalysts were coated with the thin layer of platinum before analysis. The Brunauer−Emmett− Teller (BET) surface area, pore volume, and pore size distribution of the samples were estimated from nitrogen

However, in the above study multiple-core cobalt oxide particles were found to congregate at the center of this catalyst; which could limit its catalytic activity to a certain extent. Moreover, the activity of core−shell catalysts with respect to the pore diameter of the shell has not yet been studied in detail. Many reports have been published on the influence of support morphology on FTS activity and hydrocarbon selectivity. Khodakov et al.19 studied the behavior of MCM41- and SBA-15-supported cobalt oxide catalysts, and showed that the CO conversion and C5+ selectivity increase with increasing pore diameter of the support. Another research article13 reported that the activity and C5+ selectivity of SBA-15supported cobalt oxide catalysts increased with decreasing pore length of SBA-15. Intrinsically, a wide range of hydrocarbons from C1 to >C100 is formed via FTS owing to its polymerization behavior. However, selectivity control of the desired products is receiving increasing attention by manipulating the pore structure and acidity of FTS catalyst. For example, Lira et al.20 reported that the Co/hexagonal mesoporous silica (HMS) catalyst resulted in a higher middle distillate fraction than that in the case of the Co3O4/SiO2 catalyst owing to the unique structure and dimension of the former catalyst. Likewise, there is a huge scope for the development of catalyst by controlling the pore dimensions of support material (pore length and pore diameter) to obtain the specific pattern of hydrocarbon products. In this paper, we focus on the structural effects of the core− shell morphology on the FTS performance. Mesoporous-silicastabilized multiple core cobalt oxide catalysts (20 wt % Co) were prepared by the in situ stabilization of Co in a mesoporous silica shell having different pore diameters. Further, a comparison between the prepared core−shell catalysts with unique pore characteristics and conventional silica-supported catalysts is presented in order to understand the effect of pore length and diameter on hydrocarbon selectivity. So far, to the best of our knowledge, there has been no report on FTS carried over a core−shell Co catalyst with a multiple-core assembly, as well as on the effect of different pore dimensions of the silica shell on the activity and selectivity of the catalyst.

2. EXPERIMENTAL SECTION 2.1. Solvothermal Preparation of Co3O4 Nanoparticles. Co3O4@MSN core−shell catalysts were synthesized by in situ encapsulation and stabilization of Co oxide in a mesoporous silica framework during the synthesis of mesoporous silica nanoparticles (MSN). The catalysts synthesis was carried out in two steps. The Co oxide nanoparticles were prepared by solvothermal method similar to the procedure described reviously.18 According to this procedure, 4.5 g of polyvinylpyrrolidone (PVP) (average MW = 40 000, SigmaAldrich) and 2 g of Co(NO3)2·6H2O (97% purity, Samchun Chemicals) were dissolved in 300 mL ethanol under magnetic stirring. The solution was transferred to a Teflon-lined stainless autoclave and sealed; the autoclave was then heated at 180 °C for 4 h, and a black suspension was obtained. 2.2. Synthesis of Co3O4@Mesoporous Silica Catalysts. Mesoporous silica shell stabilization around the Co3 O4 nanoparticle core was carried out according to a procedure reported for colloidal mesoporous silica nanoparticles synthesis21 with process modifications at various stages. The in situ formation of a monodisperse Co oxide particle core was carried 21976



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desorption isotherms obtained at −196 °C using a constantvolume adsorption apparatus (Micromeritics, ASAP-2400). The pore volume of the samples was determined at a relative pressure (P/P0) of 0.99. The calcined samples were degassed at 300 °C under a He flow for 4 h before the measurements. The pore size distribution of the samples was calculated using the BJH (Barett−Joyner−Halenda) model from data corresponding to the desorption branch of the nitrogen isotherms. The prepared catalysts were subjected to temperature-programmed reduction (TPR) studies, and their reduction profiles were obtained using a Micromeritics ASAP 2920 instrument equipped with a thermal conductivity detector (TCD). Prior to the TPR experiments, the samples were pretreated under a He flow up to 350 °C and maintained at this temperature for 2 h to remove the adsorbed water and other contaminants; subsequently, they were cooled to 50 °C. A reducing gas (5 vol % H2/He) was passed over the samples at a flow rate of 30 mL/ min with a heating rate of 10 °C/min up to 750 °C; this temperature was maintained for 0.5 h. The dispersion and crystal size of reduced Co were measured by hydrogen chemisorption at 100 °C by using a Micromeritics ASAP 2020C instrument. The samples were reduced at 350 °C for 10 h, then cooled to 100 °C, and finally chemisorbed at the same temperature. Hydrogen chemisorption was performed to measure the Co metal size by assuming a stoichiometry of H/Co = 1. After H2 chemisorption measurements, the samples were reoxidized at 350 °C by 10% O2 in helium to determine the extent of reduction by O2 titration. 2.4. Catalytic Experiments. The catalysts (0.3 g) were separately mixed with 2.5 g α-alumina as an inert diluent and reduced in situ under a 5% H2/Ar (200 cc/min) flow at 400 °C for 5h. The FTS performance of all the cobalt oxide catalysts was tested using a fixed-bed reactor. Activity tests were conducted under the following reaction conditions: reaction T = 230 °C; Pg = 20 bar; SV (L/kgcat/h) = 2000; feed compositions (H2/CO/Ar; mole%) = 63.0/31.5/5.5, and reactor diameter =1/4 in. The effluent gas from the reactor was analyzed by an online gas chromatograph (YoungLin Acme 6000 GC) employing a GS GASPRO capillary column connected with a flame ionization detector (FID) for the analysis of hydrocarbons and Porapak Q/molecular sieve (5A)packed column connected with a thermal conductive detector (TCD) for the analysis of carbon oxides and hydrogen with an internal standard gas of Ar. Analysis of liquid products and wax in the trap collected under similar conditions was conducted offline on YoungLin ACME 6000 GC, equipped with a 30-m HP-1 capillary column.

Figure 1. Low-angle XRD patterns of core−shell catalysts.

Figure 2. Wide-angle XRD patterns of core−shell catalysts.

which coincide well with the values for the Co3O4 spinel phase.25 The size of Co3O4 particles calculated using the Scherrer equation20 varies slightly depending on the different catalysts even though the particles are presyntheszed (Table 1). The cobalt oxide particles size mentioned here is the average size determined by XRD and consequently the difference seemed within the margin of error. Another probability of insignificantly increased cobalt oxide particle size might come from the calcination process at 500 °C. The effect is more pronounced for the dispersed cobalt oxide particles in the shell as compared to the congregated cobalt oxide particles at the center. The Co3O4 particle size for conventional catalysts is also shown in Table 1. TEM images of core−shell and conventional catalysts are shown in Figure 3. It can be observed that in the case of 20Co3O4@MSN-x catalysts (Figure 3a−d), a nearly perfect core−shell morphology was obtained in which the cobalt oxide core particles found to encapsulate in the spherical silica shell with an average catalyst pore length of ∼100 nm (Table 1). Careful observation of the TEM images revealed that cobalt oxide particles are congregated like a single core at the center of the mesoporous silica sphere in the case of the 20Co3O4@ MSN-0 catalyst. Similar type of morphology was observed in the TEM images of previous report18 and core cobalt oxide particles found to congregate at the center which could restrict the number of active sites and ultimately FTS activity to certain extent. However, in the present case, with increasing SA/TEOS

3. RESULTS AND DISCUSSION 3.1. Characterization of Catalysts. The XRD profiles of 20Co3O4@MSN-x catalysts with low- and wide-angle diffraction patterns are shown in Figures 1 and 2, respectively. All the samples showed broad single diffraction patterns at low angles (Figure 1) owing to the wormhole structure formation with short-range mesoporosity,23,24 which is evidenced by the TEM images in Figure 3. The XRD patterns showed a shift in the position of the diffraction maximum to a lower 2θ value with increasing in the SA/TEOS molar ratio (x) from 0 to 1.5. This is due to the increase in the pore−pore correlation distance with the pore diameter which increased continuously from [email protected] to [email protected] catalysts. The wide-angle XRD patterns of all the catalysts showed 2θ values of 31.2°, 36.8°, 38.4°, 44.7°, 55.6°, 59.3°, and 65.2° (Figure 2), 21977



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Figure 3. TEM images of (a) 20Co3O4@MSN-0, (b) [email protected], (c) 20Co3O4@MSN-1, (d) [email protected], (e) 20Co3O4−SBA-15, (f) 20Co3O4/SiO2, and (g) representative spent 20Co3O4@MSN-1 catalyst sample.

propose that the addition of increasing amount of swelling agent (SA/TEOS > 0) not only increased the porosity of silica shell but also was responsible for the dispersion of polymer coated cobalt oxide nanoparticles and the formation of multiple core centers within the shell. In other words, the dispersion of cobalt oxide particles increased with increasing porosity of the

ratio (x = 0.6−1.5), cobalt oxide particles with an almost uniform particle size are observed to be dispersed within the mesoporous silica shell (multiple core centers). It is previously reported that the metal nanoparticles showed higher dispersion in the presence of weakly polar or nonpolar aromatic hydrocarbon moieties like trimethylbenzene.26 Here we 21978



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Table 1. Physicochemical Properties of Catalysts According to Co Particle Sizes, As Determined by XRD Results sample

surface area (m2/g)

pore volume (cm3/g)

pore size (nm)a

average pore lengthb (nm)

XRD, Co3O4c,d size (nm)

20Co3O4@MSN-0 (20% cobalt) [email protected] (20% cobalt) 20Co3O4@MSN-1 (20% cobalt) [email protected] (20% cobalt) 20Co3O4−SBA-15 (20% cobalt) 20Co3O4/SiO2 (20% cobalt)

734 640 606 565 615 236

0.20 0.30 0.37 0.43 0.90 0.80

1.9 2.4 3.3 4.4 3.8 11.4

∼100 ∼100 ∼100 ∼100 >1000 >1000

14.6 15.0 15.6 15.4 14.4 20.0

a Maximum of pore size distribution. bDetermined by SEM measurements. cParticle sizes of Co3O4 were calculated from the most intense XRD peaks located at 2θ = 36.8°. dCo amount in core−shell catalysts was determined via XRF analysis performed on a standard sample.

Figure 4. SEM images of (a) 20Co3O4@MSN-0, (b) 20Co3O4@MSN−0.6, (c) 20Co3O4@MSN-1, (d) 20 [email protected], (e) 20Co3O4−SBA15, and (f) 20Co3O4/SiO2.

catalysts showed bigger particle dimensions of ∼200 μm (Figure 4f)). The textural properties of core−shell and conventional catalysts are listed in Table 1. The effect of a swelling agent on the pore size distribution of catalyst (silica shell) is shown in Figure 5. The 20Co3O4@MSN-0 catalyst sample without the addition of a swelling agent (x = 0) showed a surface area of 734 m2/g, pore volume of 0.2 cm3/g, and pore diameter of 1.9 nm. However, with increasing SA/TEOS molar ratio (x = 0.6− 1.5), pore diameters (Figure 5) and pore volumes increased consistently at the expense of decreasing surface area of the silica shell, as shown in Table 1. The surface properties of cobalt oxide catalysts supported on SBA-15 and SiO2 are also listed in Table 1. Twenty Co3O4−SBA-15 catalyst showed the surface area and pore diameter within the spectrum of respective surface properties of core−shell catalysts. TPR experiments were carried out to evaluate the reduction behavior of cobalt oxide catalysts, and the profiles are shown in Figure 6. The 20Co3O4@MSN-0 catalyst (Figure 6a) showed a single peak at a low reduction temperature (270 °C) for the

silica shell. TEM images of 20Co3O4−SBA-15 and 20 Co3O4/ SiO2 catalysts are shown in Figure 3e,f). It can be observed that the Co oxide particles of different sizes are nonuniformly dispersed over both the conventional supports. To examine the long-term reaction stability (∼100 h) the representative TEM image of a spent 20Co3O4@MSN-1 catalyst is presented in Figure 3 g). The images showed some amount of aggregation of catalyst particles with the negligible amount of sintering of Co oxide. SEM images of all the catalysts are shown in Figure 4. It can be observed that the core−shell catalysts are composed of relatively uniform, spherical particles with a diameter of ∼100 nm (Figure 4a−d). In some cases, the catalyst particles are connected to each other by means of thin silica links owing to Ostwald ripening. Several studies suggested that Ostwald ripening has been involved in the silica nanoparticles growth.27,28 In the case of the SBA-15-supported catalyst, rod-like particles with a size of more than 1 μm are observed (Figure 4e). These rods are straight, although some particles showed a limited degree of curvature. Twenty Co3O4/SiO2 21979



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(particularly for 20Co3O4@MSN-0 and [email protected] catalysts) and possibly functioned as bulk Co oxide particles during the TPR measurements. Similarly, other 20Co3O4@ MSN-x catalysts (x = 1 and 1.5) showed intense lowtemperature peak for the reduction of bulk like Co3O4 to CoO and some amount of CoO to metallic Co (Figure 6c,d). Moreover, a couple of small peaks are also observed along with these highly intense low temperature reduction peak, assigned to the multiple-step reduction of dispersed Co oxide particles. For the 20Co3O4@MSN-0 catalyst, Co oxide particles are assembled at the center of the shell. It is relatively difficult for hydrogen to diffuse along the mesoporous channels and reduce the congregated cobalt oxide particles. Hence the catalyst showed reduction peak with low intensity. It is previously reported that the longer diffusion time of hydrogen within the thicker silica shell delayed the reduction process and the reduction peak shifted to higher temperature.31,32 In the case of other core−shell catalysts (20Co3O4@MSN-x; x > 0) the first reduction peak observed to shift continuously toward low reduction temperature with increasing catalyst shell porosity. Careful observation of TEM images of these catalysts reveals that the cobalt oxide particles are dispersed within the porous shell due to the presence of swelling agent. Hence the improvement in the reducibility of 20Co3O4@MSN-x catalysts (x = 0.6−1.5) is due to the comparatively easy diffusion of reducing gas and increase in the number of active sites,33−35 within the shell. [email protected] catalyst with higher pore diameter and metal dispersion showed higher reducibility compared to other catalysts. The 20Co-SBA-15 catalyst displayed first reduction peak similar to the core−shell catalysts showing a broad reduction peak for the stepwise reduction of Co3O4 to CoO and multiple reduction peaks at 394 and 498 °C for the reduction of CoO to Co metal species.33,36 The hightemperature reduction peak at 843 °C is attributed to the Co silicate species5,30 which are difficult to reduce. SiO2 supported catalyst showed two well-defined doublets in the range of 250 to 350 °C and small peak at higher reduction temperature of around 569 °C. The low temperature doublet is assigned to the two stage reduction of Co3O4. H2 chemisorption and O2 titration results provided useful information about cobalt dispersion, average particle size, and percentage reduction for all the catalysts, and the results are listed in Table 2. Co3O4 nanoparticles were solvothermally prepared in the presence of PVP as the capping agent. H2

Figure 5. Pore size distribution of core−shell catalyst samples.

Figure 6. TPR profiles of (a) 20Co3O4@MSN-0, (b) 20Co3O4@ MSN-0.6, (c) 20Co3O4@MSN-1, (d) [email protected], (e) 20Co3O4−SBA-15, and (f) 20Co3O4/SiO2.

reduction of bulk-like Co3O429 and a broad peak at a high temperature of 826 °C for the reduction of less reducible Co oxide species5 or surface Co−support compounds.30 From the TEM images (Figure 3), it can be observed that many cobalt oxide particles are assembled at the center of the catalyst shell

Table 2. H2 Chemisorption and O2 Titration Results for Core−Shell and Conventional Catalysts sample 20Co3O4@ MSN-0 20Co3O4@ MSN-0.6 20Co3O4@ MSN-1 20Co3O4@ MSN-1.5 20Co3O4−SBA15 20Co3O4/SiO2

H2 uptake (μ mol/g)

uncorrected dispersion (%)a

uncorrected Co diameter (nm)b

O2 uptake (μ mol/g)

reduction degree (%)c

corrected dispersion (%)d

corrected Co diameter (nm)e

34

2.1

44.7

867

41.1

5.2

18.4

37

2.8

34.6

923

51.9

5.3

18.0

44

3.3

29.1

1031

58.0

5.7

16.9

50

3.7

25.6

1144

64.3

5.8

16.5

62

4.0

24.2

1360

65.2

6.1

15.8

60

4.5

21.3

1407

79.1

5.7

16.9

dispersion (D) = number of surface Co0 atoms/total number of Co atoms × 100, assumed stoichiometric adsorption ratio of H2/Co = 1/2. bCo particle size calculated from H2 chemisorption using d (Co) = 96/D. cCalculated from O2 uptake. dCorrected dispersion (D) = number of surface Co0 atoms/total number of reduced Co0 atoms × 100 = number of surface Co0 atoms/(total number of Co atoms × reduced fraction) × 100. e Corrected Co diameter = uncorrected Co diameter × reduced Co fraction. a

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Table 3. H2 Chemisorption and O2 Titration Results for Core−Shell and Conventional Catalystsa CO conversion (%)

TOF (S1−)d

hydrocarbon selectivity (%)

sample

To HC

to CO2

CH4

C2−C4

C5+

C19+/C5−C18c

20Co3O4@MSN-0 [email protected] 20Co3O4@MSN-1 [email protected] 20Co3O4−SBA-15 20Co3O4/SiO2

53.3 54.7 62.5 71.5 75.2 67.4

1.6 1.2 0.8 2.4 4.5 3.7

8.7 7.0 6.5 5.2 5.4 9.6

4.7 4.1 3.0 3.3 2.5 2.8

86.5 88.9 90.5 91.5 92.0 89.2

0.12 0.11 0.15 0.16 0.34 0.32

b

0.110 0.103 0.099 0.100 0.085 0.078

Reaction conditions: Catalyst: 0.3 g + 1.5 g α-alumina; T = 230 °C; Pg = 20 bar; SV (L/kgcat/h) = 2000; feed composition: H2/CO/Ar = 63.0/ 31.5/5.5 (mol %; TOS = 50 h, reactor dia.: 1/4 in. bHydrocarbons. cAnalysis of liquid products and wax in the trap collected under similar conditions were analyzed offline on Young Lin ACME 6000 GC equipped with a 30-m HP-1 capillary column. The results fall within the 5% error margin for all catalysts. dTOF is the moles of CO converted per total moles of Co active sites per second. a

mesoporous silica shell and the [email protected] catalyst showed the highest conversion up to 71.5%. Many researchers studied the influence of the support pore diameter on the activity and selectivity of cobalt oxide catalysts and reported that a high pore diameter resulted in a positive influence on catalyst activity.19,39,40 In the present study along with the pore diameter, the higher number of metal active sites (multiple core centers) also increased the reducibility and activity of the 20Co3O4@MSN-x catalysts similar to the previous reports.41−43 Hence the best performance of the 20Co3O4@ MSN-1.5 catalyst is also ascribed to the higher metal dispersion and reducibility values determined by chemisorption and O2 titration data (Table 2). The conventional 20Co3O4−SBA-15 and 20Co3O4/SiO2 catalysts showed comparable performance to that of the highly efficient [email protected] catalyst under similar reaction conditions. Even though H2 chemisorption and O2 titration data showed the higher reducibility for both the conventional catalysts as compared to the core− shell catalysts, the longer diffusion path and progressive filling of catalyst pores with the liquid waxes gave rise to the decreased diffusion rate which limits the catalytic activity. The C5+ selectivity for 20Co3O4@MSN-x catalysts increased slightly from 86.5 to 91.5% and methane selectivity decreased slightly from 8.7 to 5.2% with increasing silica shell porosity (x = 0 to 1.5). For the 20Co3O4@MSN-0 catalyst, Co oxide particles are assembled at the center of the silica shell with comparatively small pore size. Hence, it would be relatively difficult for CO to diffuse along the mesoporous channels to react with the active core center. This diffusion constrain restricts the activity of the catalyst at 53.3% up to 50 h on stream (Table 3). It is known that CO diffusion limitations increase the effective H2/CO ratio and encourage the formation of methane13,19 and 20Co3O4@MSN-0 catalyst showed comparatively higher methane formation as compared to the other core−shell catalysts. Previously, it was reported that FTS product distribution shifted to lighter hydrocarbons with the increase in the shell thickness due to the diffusion limitations.31,32 However, in the case of other 20Co3O4@ MSN-x catalysts with the increase in pore diameter (x > 0), we observed that CO conversion and C5+ selectivity increased and methane selectivity decreased (Table 3) and the results are similar to the previously reported literature.44,45 When the pore size of shell and dispersion of metal particles increased due to the addition of swelling agent, CO diffusion and access to the active center would be comparatively facile. This is reflected in the results of CO conversion and 20Co3O4@MSN-1,5 catalyst with nearly two times higher pore size and multiple core centers showed 71.5% conversion up to 50 h on stream. The

chemisorption and O 2 titration results for core−shell 20Co3O4@MSN-x catalysts showed an increase in the dispersion of Co-oxide to some extent with increasing amount of swelling agent added during synthesis (x > 0), and these results corroborate the previous discussion in TEM section. The core−shell assembly formed by the polycrystallization of monodispersed Co3O4 particles with almost uniform sizes was followed by the formation of a mesoporous silica shell through a co-condensation reaction of alkoxysilane with the micelleforming surfactant or structure-directing agent in an alkaline medium. The Co oxide particles assembled or congregated like a single core and were encapsulated by the mesoporous silica shell. The addition of a swelling agent not only enlarged the pore size of shell but also scattered the core Co oxide particles within the shell to some extent (multiple core), which is observed in the TEM images. Hence, it is observed that for the similar amount of Co loading, the reducibility of catalysts increased with increasing number of active sites of Co,37 and that the 20Co3O4@MSN−1.5 catalyst showed highest dispersion (5.8%) and reducibility (64.3%) among the prepared core−shell catalysts. 20Co3O4−SBA-15 and 20Co3O4/SiO2 reference catalysts showed higher dispersion and reducibility than 20Co3O4@MSN-x catalysts did. For SBA-15-supported catalysts, even though the cobalt oxide particle size is comparatively small, some amount of large Co-oxide particles are present on the external surface (Figure 3 e), which are responsible for the higher reducibility. H2 chemisorption showed that the mean cobalt particle size of all the catalysts is bigger than the Co3O4 crystal sizes determined by XRD (Table 1). This is possibly due to some amount of sintering of cobalt particles at the catalyst reduction temperature.13,38 3.2. Fischer−Tropsch Synthesis. The activities of 20Co3O4@MSN-x and reference catalysts after reduction at 400 °C for 5 h were measured in a fixed-bed reactor under the following conditions: 230 °C, 20 bar, H2/CO = 2, and SV (L/ kgcat/h) = 2000. The results for catalyst activity and hydrocarbon selectivity after 50 h of reaction time are summarized in Table 3. In this work, along with obtaining the advantage of lower diffusion limitation through the core− shell morphology (small size support), we modified the procedure to obtained the catalyst with the tunable shell porosity and multiple core assembly and studied the influence on the FTS activity. In the case of core−shell 20Co3O4@MSNx catalysts, the addition of a swelling agent increased the pore size of the silica shell and [email protected] catalyst showed the pore size of almost twice that of the 20Co3O4@MSN-0 catalyst. Consequently syngas conversion over 20Co3O4@ MSN-x catalysts increased with increasing porosity of the 21981



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Figure 7. Carbon number distribution for C8+ hydrocarbons prepared using core−shell and conventional catalysts: (a) 20Co3O4@MSN-0, (b) [email protected], (c) 20Co3O4@MSN-1, (d) [email protected], (e) 20Co3O4−SBA-15, and (f) 20Co3O4/SiO2.

The C19+/C5−C18 hydrocarbon fraction ratio for the reference catalysts was found to be more than two times that for the core−shell catalysts. This is because in the case of the core− shell catalysts, the domain of the mesoporous silica shell is so small (∼100 nm, SEM images) for readsorption of intermediates such as olefin molecules that the readsorption took place with far less probability and they did not seem to grow and form the heavy hydrocarbons.18,46 Previously, it was reported that the FTS catalysts with the short pore length resulted in the higher middle distillate fractions than the amount obtained using the conventional catalysts.18,20,23,47 The 20Co3O4/SiO2 catalyst showed comparatively higher amounts of heavy hydrocarbon formation (>C19+) due to the readsorption of product intermediates and chain propagation. In the case of SBA-15-supported catalyst, even though the regular pore structure and narrow pore size restrict the formation of long-chain hydrocarbons,48 a part of large Co oxide particles

higher CO conversion produced the higher average water partial pressure which would inhibit the secondary hydrogenation and increased the chain growth probability by reducing the diffusion limitation and chain termination. Hence, we observed that with the increase in the pore diameter of silica shell and dispersion of core cobalt oxide particles, methane selectivity decreased with the increase in the C5+ selectivity as shown in Table 3. SiO2- and SBA-15-supported cobalt oxide catalysts showed comparable C5+ selectivity (89.2 and 92%, respectively) to those shown by the core−shell catalysts. Further, the liquid products and wax in trap collected under similar conditions were analyzed by an offline GC equipped with a 30-m HP-1 capillary column. It was observed that 20Co3O4@MSN-x catalysts produced higher C5−C18 fractions and showed lower selectivity toward both CH4 and C19+ products than those shown by 20Co3O4−SBA-15 and 20Co3O4/SiO2 catalysts under similar conditions (Table 3). 21982



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prevented by the protective shell. The TEM image (Figure 3 g) of the representative spent 20Co3O4@MSN-1 catalyst after the reaction for up to 100 h shows that the multiple core centers are almost protected by the silica shell with negligible amount of Co oxide sintering. On the contrary, in the case of the conventional catalysts, longer diffusion length and higher pore residence time for water formed in the reaction possibly gives rise to pore plugging and significant metal sintering respectively which is responsible for the deactivation of the catalysts.13 Briefly, the development of multiple core centers and higher shell porosity were found to enhance the activity and selectivity of hydrocarbon products in our research proposition. We found that the small particle size of the silica shell along with the multiple core centers had a positive influence on the activity and selectivity of catalysts, which was not observed in the case of conventional supports.

located on the external surface led to the formation of heavy hydrocarbon products.49 Figure 7 shows the typical carbon number distribution charts for the C8+ hydrocarbon fraction produced by core−shell catalysts (a−d) and reference catalysts (e,f). It can be clearly observed that the conventional catalysts showed higher amounts of heavy hydrocarbons formation, whereas for core−shell catalysts, the hydrocarbon selectivity spectrum shifted toward the middle distillate region. Similarly, when the activity and selectivity of reported catalyst are compared, conversion and C5+ selectivity of [email protected] at 230 °C are higher by 10.3% and 11.2% respectively than the core−shell catalysis results reported by Xie et al.18 even though GHSV of our study was two times faster. Especially, our catalyst showed more advanced selectivity controlling capability and the C19+/C5−C18 value in our case was 0.16 with respect to theirs value of 0.32. In the present article, the core−shell catalyst with the establishment of higher porosity and multiple core assembly showed significantly higher activity and selectivity as compared to the reported core−shell catalyst18 for FT synthesis. The catalytic activity expressed in terms of turnover frequency (TOF S1−) is given in Table 3. There is no major difference in the TOF values of all the core−shell catalysts. Even though there is no considerable difference in the particles size of cobalt oxide as shown in Table 1, the difference in the activity of 20Co3O4@MSN-x catalysts (x > 0) is due to the increasing dispersion and reducibility values (Table 2). The long-term stability of core−shell and conventional catalysts was compared for up to 100 h on stream and the results obtained are shown in Figure 8. All the catalysts showed

4. CONCLUSIONS Core−shell catalysts with controlled pore dimensions were prepared by the in situ immobilization of Co oxide nanoparticles during the preparation of a mesoporous silica shell. Catalysts with a particle length of ∼100 nm showed an increase in dispersion and reducibility of Co oxide with increasing porosity. TEM images showed fairly dispersed and mostly uniform Co oxide particles in the mesoporous silica shell. Further, the catalyst activity increased with increasing silica shell porosity; however, C5+ selectivity increased slightly. The core−shell catalysts resulted in a higher fraction of middle distillates, formed owing to the confined effect of pore dimensions and lower diffusion limitations. In contrast, conventional catalysts resulted in a greater amount of heavy hydrocarbon products (C19+) owing to the presence of large Co oxide particles that facilitated the diffusion-enhanced readsorption of primary α-olefins. The core−shell catalysts showed higher reaction stability up to 100 h on stream owing to easy diffusion access as well as suppressed metal sintering. However, the longer diffusion path of conventional supports (pore length >1000 nm) promoted progressive filling of catalyst pores with liquid waxes that deactivates the catalyst. Briefly, the present article suggests that the use of core−shell catalysts with controlled pore dimensions is a more successful strategy for achieving improved catalytic activity and middle distillate selectivity than the use of conventional catalysts.

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Figure 8. Change in CO conversion with time on stream for core− shell and reference catalysts.

AUTHOR INFORMATION

Corresponding Author

*Phone: +82-42-860-7671; e-mail: [email protected]. Notes

significant increase in the activity up to 20 h on stream before the activity got stabilized. It is known that the FT activity and selectivity develop during the initial time of run in the process of reconstruction of cobalt crystalline planes. This reconstruction creates defect sites which drive the catalytic reaction.50 In the present case, similarly, the cobalt particles likely experience the self-organization and reconstruction process before reaching the steady state activity. It was found that core−shell catalysts show better stability than the conventional catalysts up to 100 h on stream. This is because in the case of the core− shell catalysts, a relatively shorter particle length likely makes diffusion access easy for the products. Moreover, in the case of the core−shell catalysts, since each functional core is isolated by a permeable shell with relatively homogeneous surroundings, sintering between the core Co oxide species can be effectively

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) under “Energy E fficiency & Resources Programs” (Project No . 2013201020178B) of the Ministry of Trade, Industry and Energy (MOTIE) of the Republic of Korea.

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REFERENCES

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