SiO2 modified Al2O3@Al supported cobalt for Fischer-Tropsch synthesis

Sep 3, 2018 - SiO2 modified high thermal conductive Al2O3@Al composites were synthesized and supported cobalt for Fischer-Tropsch synthesis to improve...
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Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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SiO2‑Modified Al2O3@Al-Supported Cobalt for Fischer−Tropsch Synthesis: Improved Catalytic Performance and Intensified Heat Transfer Da Wang,†,‡,§ Zhong Wang,†,§ Guangci Li,† Xuebing Li,*,† and Bo Hou*,‡

Ind. Eng. Chem. Res. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 09/14/18. For personal use only.

†

Key Laboratory of Biofuels, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, People’s Republic of China ‡ State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, People’s Republic of China S Supporting Information *

ABSTRACT: SiO 2-modified high thermal conductivity Al2O3@Al composites were synthesized and supported cobalt for Fischer−Tropsch synthesis to improve the catalytic performance and intensify the heat transfer within a fixedbed reactor. FTS results showed that the CO conversion and C5+ selectivity increased whereas CH4 selectivity decreased, which could be attributed to the reduced metal−support interaction via modification of SiO2 on the surface of Al2O3@ Al. The heat transfer coefficients of the Co/SAl2O3@Al−x were more than 30 times those of conventional Co/Al2O3 according to the laser flash measurement. The thermal conductive study showed that a homogeneous radial temperature gradient was maintained within the catalyst bed for Co/SAl2O3@Al-x even without using a heat diluter, indicating good thermal conductivity within the reactor. In addition, the temperature of the catalyst bed for Co/SAl2O3@Al-x was easier to reach a “steady state” compared to the reactor packed with traditional low thermal conductive Co/Al2O3, which could improve the operating flexibility of FTS.

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INTRODUCTION Fischer−Tropsch synthesis (FTS) is one of the preferred routes for the production of valuable chemicals and liquid fuels from fossil or biomass.1−3 The fixed-bed reactor packed with cobalt-based catalyst pellets has great potential in industrialization on account of the high activity, being more selective to heavy hydrocarbons, and having easy products separation.4−6 However, the heat transfer limitation within the fixed-bed reactor is the constraint for commercial utilization.7 FTS is a high exothermic reaction (ΔH = −165 kJ/molCO), which could cause an adiabatic temperature rising up to 1750 K.8 A high temperature gradient, even “thermal runway”, might occur within the catalyst bed, which could lead to catalyst deactivation and increased methane selectivity and threaten the safety of the plant provided that the generated heat could not be removed effectively.9,10 Utilizing high thermal conductivity FTS catalysts is an important way to intensify heat transfer in a fixed-bed reactor. Recently, high thermal conductivity structured catalysts, which use metallic monoliths or metallic foams as support and coating cobalt active phase, have attracted much interest.11−13 Both the heat transfer efficiency and the C5+ selectivity have been improved for these kinds of structured catalysts. Merino et al. prepared a metallic aluminum monolith and deposited © XXXX American Chemical Society

Co−Re/Al2O3 on the surface for FTS; they found that the good thermal conductivity of aluminum monolith permitted one to enhance the production of C5+ products at higher temperatures without significantly increasing methane selectivity.14 Usually a coating layer is needed on the metal substrate to “anchor” and disperse active components, but it is easy to damage during reaction, causing loss of the active phase. Another disadvantage is the complicated manipulation for loading the structured catalysts into the fixed-bed reactor. Carbon materials such as carbon nanotubes (CNTs), carbon nanofibers (CNFs), and carbon spheres (CSs) as well as mesoporous carbon (MC) have attracted much attention as supports for FTS because of the high thermal conductivity and relatively weak metal−support interaction.15,16 In addition, SiC was also employed as support to prepare Co/SiC for FTS because of its intrinsic high thermal conductivity.17,18 For instance, Lacroix et al. prepared Co/SiC catalyst for FTS, and they found that the C5+ selectivity (80%) was higher than conventional low thermal conductivity Co/Al2O3 (56%), Received: Revised: Accepted: Published: A

June 25, 2018 August 30, 2018 September 3, 2018 September 3, 2018 DOI: 10.1021/acs.iecr.8b02842 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

(0.83, 1.39, and 2.78 g corresponding to SiO2 contents of 3, 5, and 10 wt %) was added to 20 mL of ethanol (Sinopharm Chemical Reagent Co. Ltd.) to mix evenly. Then 5 mL of H2O was added to the solution. Next, 8.00 g of the prepared Al2O3@Al composite was added to the above mixture and NH3.H2O (Sinopharm Chemical Reagent Co. Ltd.) was dropped in to adjust the pH to 10. Then the mixed solutions were stirred to dry at 353 K. Finally, the composite was dried at 373 K for 6 h and calcined at 823 K for 10 h. The obtained samples were labeled as SAl2O3@Al-x (x = 0, 3, 5, 10 wt %, where x represents the modified SiO2 content). The catalyst preparation was performed by the excessive wetness impregnation method. First, the prepared SAl2O3@Alx support (5.00 g) was added into 20 mL of an ethanol solution, which contained 4.65 g of Co(NO3)2·6H2O (Aladdin) (the cobalt loading was 15 wt %). Next, the mixed solutions were stirred to dry at RT. Last, the samples were dried at 373 K for 6 h and calcined at 673 K for 6 h in air at a heating rate of 1 K/min. The obtained samples were labeled as Co/SAl2O3@Al-x (x = 0, 3, 5, 10 wt %, where x represents the SiO2 content). Characterization of Supports and Catalysts. X-ray diffraction spectra of the supports and catalysts were recorded with a DX-2700 diffractometer using monochromatized Cu Kα radiation. The spectra were scanned at a rate of 2° min−1 in the range 2θ = 5−90°. N2 adsorption−desorption experiments were carried out with a Micromeritics ASAP 2020. The BET surface areas were obtained with P/P0 from 0.05 to 0.30. The total pore volumes were detected from the amount of adsorbed N2 at a relative pressure of 0.99. The pore size distributions were obtained using the Barrett−Joyner−Halenda (BJH) method. The macroporosity of the samples was obtained by mercury intrusion porosimetry in a Micromeritics Autopore IV 9500 instrument. The contact angle of 130° and cylindrical pore model were chosen for the calculations. The morphologies of the supports and the prepared catalysts were examined by a scanning electron microscope on a JSM-7100F. Prior to analysis the sample was covered by a thin layer of gold in order to avoid the problem of the charge effect. The thermal conductive properties of the catalysts were measured by the laser flash method over a LFA447 NanoFlash instrument. The electronic states of cobalt on the catalyst surface were characterized by X-ray photoelectron spectroscopy (AXISULTRA DLD) using Al Kα radiation. The fresh catalysts were pressed into thin disks and evacuated in the prechamber of the spectrometer at 10−9 mbar. The C 1s peak at 284.6 eV was used to correct for charging effects. The reduction behavior of supported oxidized cobalt phases was studied by hydrogen temperature-programmed reduction in a TP-5080 multipurpose automatic adsorption instrument. First, the catalyst (30 mg) was flushed with N2 for 1 h. Then a mixture of 10 vol % of H2 in N2 was passed through the catalyst and the temperature increased to 1173 K at a heating rate of 10 K/min. The thermal diffusivities (α) and specific heat capacity (Cp) could be directly obtained from the NanoFlash instrument. The thermal conductivity coefficient (λ) could be calculated as follows: λ = ραCp, where ρ is the density of the samples. Catalytic Testing. The Fischer−Tropsch synthesis reaction was performed in a tubular fixed-bed stainless steel reactor (i.d. = 10 mm). In a typical experiment, 1.60 g of catalyst (pellet size 0.18−0.25 mm) was mixed with 2.00 g of SiC and loaded into the reactor. The catalyst was reduced under 0.5 MPa of H2 at 673 K for 6 h. Then the temperature was cooled

which was attributed to the high heat transfer efficiency of SiC.19 However, the relatively complex synthesis method and expensive price hindered the wide application in industrialization for the above-mentioned high thermal conductivity supports. In addition, it should be mentioned that previous research of those high thermal conductivity catalysts mainly focused on investigating the intrinsic reaction behavior, lacking a deeper study on the heat transfer process within the reactor. The high thermal conductivity Al2O3@Al core−shell composite could be utilized as support for a high endothermic reaction.20,21 For the Al2O3@Al composite, the Al2O3 species could anchor and disperse the active phase whereas the internal Al phase could remove the generated heat during the reaction. In our previous work the Al2O3@Al core−shell composite was prepared by etching metallic aluminum powders with the NaOH method, and the composite was supported cobalt for FTS.22 However, the prepared Co/ Al2O3@Al catalyst exhibited a relatively higher CH4 selectivity and lower C5+ selectivity compared with those high thermal conductivity Co/SiC and Co/C FTS catalysts. This allowed us to explore the possible causes. It should be mentioned that the surface of SiC and carbon materials is inert; thus, the metal− support interaction was relatively weak for the prepared catalysts.15,17 However, a much stronger interaction existed between the cobalt phase and Al2O3 for Co/[email protected] It was believed that the metallic Co0 was the active phase for Fischer−Tropsch synthesis.24 For Co/Al2O3@Al, the stronger metal−support interaction altered the electronic state of the Co0, which could further decrease the CO dissociation. This might be the reason for the relatively high CH4 and low C5+ selectivity of Co/Al2O3@Al compared with Co/C and Co/ SiC.25,26 Therefore, reducing the interaction between cobalt phase and support might be an effective approach to improve the catalytic performance for Co/Al2O3@Al. Among the commonly used supports in FTS, the metal−support interaction is relatively weak for SiO2-supported catalysts, which usually favors high cobalt reducibility.24,27 Therefore, modifying Al2O3@Al with SiO2 might be an effective method to reduce the metal−support interaction and improve the catalytic performance. Herein, a series of SiO2-modified high thermal conductivity Al2O3@Al supports were synthesized and supported cobalt for Fischer−Tropsch synthesis with an attempt to improve the catalytic performance and intensify heat transfer within a ixedbed reactor. Characterization of the catalysts and their catalytic performance have been investigated in detail. The heat transfer properties of the fixed-bed reactor packed with these high thermal conductivity catalysts were also carefully studied.

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EXPERIMENTAL SECTION Support Preparation and Catalyst Synthesis. The Al2O3@Al support was prepared by etching metallic Al powders with NaOH according to a previous report.22 First, 10.00 g of metallic Al powder (Aladdin, particle size 1−2 μm) was added into 1000 mL of NaOH (Sinopharm Chemical Reagent Co. Ltd.) solution with a concentration of 0.04 M. Then the solution was stirred for 12 h at RT, filtrated, and washed with deionized water until the pH of the filtrate was 7. Last, the filter cake was dried in air at 373 K for 10 h and calcined at 823 K for 6 h at a heating rate of 1 K/min. The SiO2-modified Al2O3@Al supports were prepared by depositing SiO2 on Al2O3@Al composites with tetraethoxysilane (Aladdin). First, a certain quality of tetraethoxysilane B

DOI: 10.1021/acs.iecr.8b02842 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 1. XRD patterns of (A) (a) SAl2O3@Al-0, (b) SAl2O3@Al-3, (c) SAl2O3@Al-5, and (d) SAl2O3@Al-10 and (B (a) Co/SAl2O3@Al-0, (b) Co/SAl2O3@Al-3, (c) Co/SAl2O3@Al-5, and (d) Co/SAl2O3@Al-10.

Figure 2. N2 physisorption curves of (A) (a) SAl2O3@Al-0, (b) SAl2O3@Al-3, (c) SAl2O3@Al-5, and (d) SAl2O3@Al-10 and (B (a) Co/SAl2O3@ Al-0, (b) Co/SAl2O3@Al-3, (c) Co/SAl2O3@Al-5, and (d) Co/SAl2O3@Al-10.

Figure 1A. As can be seen, the characteristic peaks located at 2θ = 38.6°, 45°, 65°, 78°, and 82.5° could be detected, which is corresponding to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) crystal planes of metallic Al. Moreover, diffraction peaks at 45.8° and 67° were also observed, demonstrating the existence of γ-Al2O3 phase. However, no diffraction peaks related to SiO2 phase were observed. This might be because the SiO2 particles were too small to be detected. The XRD patterns of Co/ SAl2O3@Al-x (x = 0, 3, 5, 10) catalysts are displayed in Figure 1B. Different diffraction peaks located at 2θ = 31.4°, 36.8°, and 59.4° were observed, which could be attributed to the Co3O4 crystalline phase.28 The average Co3O4 as well as Co0 crystallite sizes calculated by the Scherrer formula are given in Table S1. As can be seen, the Co0 crystallite size of Co/ SAl2O3@Al-0 was 9.3 nm, and it increased to 14.3 nm with the increase of SiO2 content to 10%. This might be because the modified SiO2 on Al2O3@Al reduced the interaction between cobalt phase and support, thus leading to formation of larger Co0 particles. The cobalt dispersion calculated by 96/d(Co0) is also shown in Table S1. A reduced cobalt dispersion tendency could be observed with the increase of SiO2 content. This could also be related to the weakened metal−support interaction with modification of SiO2 species. The N2 physorption isotherms of the SiO2-modified supports and the prepared catalysts are displayed in Figure 2. All samples exhibited nonreversible adsorption−desorption

down to room temperature and switched the synthesis gas (H2/CO/N2 in a volume ratio of 64/32/4, N2 was the internal standard) to pass through the reactor. Last, the pressure was adjusted to 2.0 MPa, and the temperature was increased to the desired value at a heating rate of 1 K/min. A constant gas hourly space velocity (GHSV) of 1.50 Lsyngas/(gcat·h) was applied for each reaction. The liquid products and waxes were collected in a cold trap at ca. 278 K and hot trap at ca. 393 K, respectively. The liquid hydrocarbons as well as waxes were weighted, dissolved in CS2, and analyzed offline on a GC-2010 chromatograph which was equipped with a 35 m OV-101 capillary column. The sweep gas was analyzed online on a GC920 chromatographs equipped with thermal conductivity detector (TCD) and flame ionization detector (FID). Nitrogen balance, oxygen balance, carbon balance, and total mass balance were satisfactory (100 ± 5%) to ensure reliable results. For the purpose of investigating the heat transfer properties within the fixed-bed reactor, a reactor (i.d. = 15 mm) that could measure the centerline and wall temperature was used. The catalysts reduction, evaluation, and products analysis remain the same with the above procedure.

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RESULTS AND DISCUSSION Phase, Texture, and Morphologies. The XRD patterns of the SAl2O3@Al-x (x = 0, 3, 5, 10) supports are shown in C

DOI: 10.1021/acs.iecr.8b02842 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 3. SEM images of (a) Al powders, (b) SAl2O3@Al-0 support, (c) Co/SAl2O3@Al-0 catalyst. SEM mappings of (d) Co/SAl2O3@Al-0, (e) Co/SAl2O3@Al-3, (f) Co/SAl2O3@Al-5, and (g) Co/SAl2O3@Al-10.

macropores. Actually, the macro−mesoporous structure of the Co/SAl2O3@Al-x (x = 0, 3, 5, 10) catalysts could favor the formation of high carbon number products because of the intensified mass transfer process.22 The SEM images of the SiO2-modified supports and the prepared catalysts are shown in Figure 3. As can be seen, the surface of the raw metallic aluminum powder was smooth (Figure 3a). However, a “plate-like” structure was formed after etching with NaOH solution (Figure 3b). The plate-like structure further confirmed the N2 adsorption−desorption results, which the pores of the supports were slit-shaped pores formed by the aggregating of plate-like particles. For the prepared catalyst (Figure 3c), the plate-like structure could not be clearly observed, which might be attributed to pore blockage by cobalt species. However, this did not mean the

isotherms with a hysteresis loop, which indicated the mesoporous structure of the samples. In addition, the isotherms continuously increased until saturation (P/P0 = 1) and did not reach a plateau, which were the standard characteristics of H3 and H4 hysteresis loops according to the IUPAC recommendations for gas physorption analysis.29 The N2 physorption results suggested that the pores of the samples might be “slit-shaped” pores generated from aggregating of plate-like particles and macropores might exist. Mercury intrusion porosimetry was also carried out in order to confirm the existence of macropore structure. As can be observed in Table S1, the volumes of macropores in SAl2O3@Al-x (x = 0, 3, 5, 10) supports and the prepared catalysts were more than 10% of the total pore volume measured by N2 physisorption, indicating the existence of D

DOI: 10.1021/acs.iecr.8b02842 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 4. (A) XPS wide spectra of (a) Co/SAl2O3@Al-0, (b) Co/SAl2O3@Al-3, (c) Co/SAl2O3@Al-5, (d) Co/SAl2O3@Al-10. (B) Co 2p XPS spectra of (a) SAl2O3@Al-0, (b) SAl2O3@Al-3, (c) SAl2O3@Al-5, and (d) SAl2O3@Al-10.

changed slightly for further increasing of SiO2 content to 10 wt %, which could also be attributed to the slight variation of SiO2 coverage degree between Co/SAl2O3@Al-5 and Co/SAl2O3@ Al-10. Catalysts Reduction Behavior. Figure 5 shows the H2TPR profiles of the prepared Co/SAl2O3@Al-x catalysts. As

disappearance of macropores for the catalysts, since the macropores still be examined by mercury intrusion porosimetry results. SEM mapping was also applied to characterize the structure and composition of the prepared catalyst (Figure 3d−g). As can be seen, only Co and Al signals could be observed for Co/SAl2O3@Al-0, whereas Co, Al, and Si signals could be detected for Co/SAl2O3@Al-x (x = 3, 5, 10) catalysts. This confirmed that SiO2 was successfully introduced on the Al2O3@Al support. In addition, the cobalt element could be observed to be uniformly distributed on the surface of SAl2O3@Al supports, which demonstrated a cobalt dispersion of the Co/SAl2O3@Al-x (x = 3, 5, 10) catalyst. Surface Spectroscopic Analysis. XPS measurements were carried out to investigate the surface chemical nature of the catalysts. From the XPS wide spectra (Figure 4A), the Si 2p signal was detected on Co/SAl2O3@Al-x (x = 3, 5, 10), indicating that the SiO2 phase successfully deposited on the surface of Al2O3@Al. For the Co 2p XPS spectra (Figure 4B), two peaks located at about 795 and 780 eV were observed, which could be attributed to the Co 2p1/2 and Co 2p3/2 peaks of Co3O4.30 The higher binding energy of Co 2p3/2 on Co/ SAl2O3@Al-0 could be due to the stronger interaction between cobalt phase and Al2O3. For all of the SiO2-modified cobalt catalysts it can be observed that the binding energy of Co 2p3/2 decreased with the increase of SiO2 content. The result confirmed that the modified SiO2 species might locate on the surface of Al2O3@Al and part of cobalt phase directly contacted with the SiO2 species; thus, the interaction between the cobalt phase and the support became weaker. The molar ratio of Si/Al, Co/Al, as well as Co/Si is shown in Table S2. As can be seen, with the increase of SiO2 content from 3 to 5 wt %, the Si/Al atomic ratio increased to 0.33. No significant Si/Al ratio change was observed for further increasing the modified SiO2 content to 10 wt %. Actually, the modified SiO2 species mainly deposited on the surface of Al2O3@Al. The SiO2 coverage on Al2O3@Al could not increase continuously with the increase of SiO2 content because of the aggregation of SiO2 particles. Therefore, the slight variation of Si/Al ratio between Co/SAl2O3@Al-5 and Co/SAl2O3@Al-10 could be attributed to the similar SiO2 coverage on Al2O3@Al. Similarly, the molar ratio of Co/Al increased from 0.11 to 0.37 with the increase of SiO2 content from 3 to 5 wt %. This suggested that more Al cobalt atoms were covered with SiO2 species. However, the Co/Al ratio

Figure 5. TPR profiles of (a) Co/SAl2O3@Al-0, (b) Co/SAl2O3@Al3, (c) Co/SAl2O3@Al-5, and (d) Co/SAl2O3@Al-10.

can be seen, three reduction peaks could be detected. The first two peaks at the low-temperature range (500−700 K) could be assigned to the reduction of Co3O4 to CoO and CoO to Co0, whereas the third peak (above 850 K) could be attributed to the reduction of barely reducible cobalt aluminates.31,32 The highest T2max (CoO to Co0) value of Co/SAl2O3@Al-0 indicates the strongest cobalt−support interaction. For the SiO2-modified catalysts it can be observed that the T2max decreased with the increase of SiO2 to 5 wt % and then remained nearly unchanged, further increasing the SiO2 content to 10 wt %. This suggested that the introduction of SiO2 on the surface of Al2O3@Al reduced the interaction between cobalt phase and the support. Meanwhile, the decreased intensity of the third reduction peak could also be observed, which further indicated the reduction of barely reducible cobalt aluminates after modifying SiO2 species. The reduction degrees of Co estimated by H2-TPR experiments are shown in Table 1. Co/SAl2O3@Al-0 possessed the lowest Co reduction degree, and it increased to 66% for Co/SAl2O3@ E

DOI: 10.1021/acs.iecr.8b02842 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 1. Thermal Conductivity Coefficients of Supports and the Prepared Catalystsa samples

α (mm2s−1)

Cp (J k−1 g−1)

ρ (g cm−3)

λ (W m−1 K−1)

Al powders SAl2O3@Al-0 SAl2O3@Al-3 SAl2O3@Al-5 SAl2O3@Al-10 Co/SAl2O3@Al-0 Co/SAl2O3@Al-3 Co/SAl2O3@Al-5 Co/SAl2O3@Al-10 Co/Al2O3

19.65 8.62 8.58 8.83 9.11 9.04 9.20 8.95 9.30 0.25

0.94 0.80 0.83 0.74 0.76 0.83 0.78 0.81 0.82 0.86

2.32 1.83 1.79 1.81 1.72 1.80 1.82 1.79 1.75 1.68

42.85 12.62 12.75 11.12 11.91 13.50 13.06 12.98 13.35 0.36

λ = ραCp, λ is the thermal conductivity coefficient, α is the thermal diffusivities, Cp is the specific heat capacity, and ρ is the density of the sample.

a

Table 2. FTS Catalytic Performance of the Co/SAl2O3@Al-x (x = 0, 3, 5, 10) Catalystsa products selectivity (wt %) catalyst

conv. (%)

CH4

C2−C4

C5+

C5−C18

C19+

CO2

CoTYb

Co/SAl2O3@Al-0 Co/SAl2O3@Al-3 Co/SAl2O3@Al-5 Co/SAl2O3@Al-10

40.2 43.9 49.6 51.5

10.4 8.9 7.4 7.7

9.5 7.9 7.6 7.1

80.1 82.7 84.4 84.7

40.6 38.1 36.7 36.1

39.5 44.6 47.7 48.6

0.6 0.5 0.6 0.5

1.40 1.53 1.73 1.79

−1 Reaction conditions: n(H2)/n(CO) = 2, GHSV = 1.5 Lsyngasgcath−1, T = 483 K, P = 2.0 MPa, TOS = 48 h. bCobalt time yield (10−5 molCO g−1 Co s , molar CO conversion rate per gram of cobalt per second).

a

seen in Figure S1, the CoTY increased from 1.40 to 1.73 × −1 10−5 molCO g−1 with the increase of SiO2 content to 5 wt Co s %. No obvious change was observed, further increasing the SiO2 content to 10 wt %. It was accepted that a balanced interaction between support and the active phase was particularly important for FTS.33 Bartholomew et al. carefully investigated the cobalt-based Fischer−Tropsch synthesis on different supports, and they found that the CO conversion decreased in the order of Co/SiO2 > Co/Al2O3 > Co/C > Co/ MgO.34 Herein, according to the H2-TPR and XPS results, the cobalt reduction degree and the surface molar ratio of Co/Al increased with the increase of SiO2 content from 3 to 5 wt %. This should be responsible for the improved FTS activities. For a further increase of SiO2 content to 10 wt % the cobalt reduction as well as the surface molar ratio of Co/Al changed slightly, which could be the reason for no obvious change on CO conversion. The product selectivity of the Co/SAl2O3@Al-x (x = 0, 3, 5, 10) catalysts is displayed in Figure 6. As can be seen, Co/ SAl2O3@Al-0 exhibits the highest CH4 selectivity. The CH4 selectivity decreased for the catalysts supported on SiO2modified Al2O3@Al. It is believed that the metallic Co0 was the active phase and oxidized cobalt species favored CH4 formation in FTS.24,35 According to the XPS and H2-TPR analysis, the interaction between the cobalt phase and the support became weaker and the cobalt reduction degree increased with the increase of SiO2 content. In addition, the decreased intensity of the reduction peak at high temperature from H2-TPR analysis for Co/SAl2O3@Al-x (x = 0, 3, 5, 10) also confirmed the decrease of barely reducible cobalt aluminates after introduction of SiO2. Therefore, the reduced CH4 selectivity for the Co/SAl2O3@Al-x (x = 0, 3, 5, 10) could be attributed to the improved cobalt reduction degree. The improved C5+ selectivity with the increase of SiO2 content for Co/SAl2O3@Al-x (x = 0, 3, 5, 10) could be observed. Brog et al. found positive correlations between the C5+ selectivity and the cobalt particle size, and they pointed out that the C5+

SiC-10 catalyst. It also demonstrated that the modified SiO2 on the surface of Al2O3@Al decreased the interaction between cobalt phase and support, which favored the reduction of cobalt species. Thermal Conductive Properties of Supports and Catalysts. The thermal conductivity coefficients (λ) of the supports and catalysts measured by the laser flash technique are displayed in Table 1. The samples were pressed to get tablets with a thickness of about 3 mm for measuring. Although the measured λ value could not represent the real thermal conductive properties of the catalyst bed during reaction, it could directly reflect the intrinsic thermal conductivity of catalysts. The thermal conductivity coefficient (λ) of the original metallic Al powders was high, that is, 42.85 W m−1 k−1. The λ value sharply decreased to 12.62 W m−1 k−1 after being etched with NaOH solution. This could be attributed to the poor thermal conductivity of the formed Al2O3 species on SAl2O3@Al-0 support. Furthermore, no obvious λ value changed for SAl2O3@Al-x (x = 3, 5, 10) and Co/SAl2O3@Al-x (x = 3, 5, 10) compared to SAl2O3@Al-0, suggesting that the introduction of SiO2 and cobalt phase did not affect the thermal conductivity of SAl2O3@Al-0. It should be mentioned that the thermal conductivity coefficient of Co/ Al2O3 was only 0.36 W m−1 k−1. However, the lowest thermal conductivity coefficient of Co/SAl2O3@Al-x (x = 3, 5, 10) catalysts was 12.98 W m−1 k−1, which was more than 30 times that of Co/Al2O3. This suggested that the SiO2-modified Co/ SAl2O3@Al catalysts had a much better heat transfer ability than traditional Co/Al2O3. Catalytic Test Results. All of the catalytic data were collected after 48 h on stream and are listed in Table 2. As can be seen, the CO conversion of Co/SAl2O3@Al-0 was 40.2%. With the increase of the SiO2 content from 3 to 5 wt %, CO conversion increased from 43.9% to 49.6%. Further increasing the SiO2 content to 10 wt % resulted in slight variation on CO conversion. The cobalt time yield as a function of SiO2 content exhibited the same tendency with CO conversion. As can be F

DOI: 10.1021/acs.iecr.8b02842 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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conclusion. Compared to Co/SAl2O3-0, it could be clearly observed that the peaks intensity became weak and peaks E and F even disappeared for Co/SAl2O3-5. This suggests that the modified SiO2 species covered the Al2O3 phase on the surface of Al2O3@Al, leading to the decrease of acid sites. Figure 7 shows the calculated CO conversion, CH4 selectivity, and C5+ selectivity of Co/SAl2O3@Al-5 under different reaction temperatures. As can be seen, the CO conversion and CH4 selectivity increased whereas C 5+ selectivity decreased with the increase of reaction temperature. This could be attributed to the fact that a high reaction temperature favored the hydrogenation of primary products; thus, more light hydrocarbons were produced.38,39 In addition, the variation of CH4 selectivity was less obvious than C5+ selectivity. This might be attributed to the high thermal conductivity of catalyst body; thus, the temperature did not exhibit a large effect on CH4 selectivity.14 Thermal Conductive Properties. Usually the evaluated FTS catalysts always diluted with high thermal conductivity SiC particles for evaluation to avoid the overheating effect and improve the temperature distribution within a fixed-bed reactor.24,40 Herein, Co/SAl2O3@Al-0 and Co/SAl2O3@Al-5 were evaluated with and without using SiC heat diluter to investigate the thermal conductivity in a specific fixed-bed reactor that could measure the centerline and wall temperature (Figure S3). Temperature difference between the reactor wall and the centerline (ΔT), CO conversion, as well as products selectivity were carefully studied. Traditional low thermal conductivity Co/Al2O3 was tested under the same condition for comparison. About 70% of CO conversion was chosen for all of the diluted catalysts in this study in order to ensure a similar heat release. As can be seen in Table 3, the temperature differences between the centerline and the wall (ΔT) were almost the same for Co/SAl2O3@Al-0 evaluated with and without using a SiC heat diluter. This indicated that the reaction heat could be effectively removed from the catalyst bed even without using a SiC heat diluter. In addition, this also led one to the conclusion that a good thermal conductivity was achieved for the reactor packed with undiluted Co/SAl2O3@Al-0, which might be due to it intrinsic high thermal conductivity (13.50 W m−1 K−1 measured by the laser flash technique). Likewise, no obvious ΔT change was observed for diluted and undiluted Co/ SAl2O3@Al-5 catalysts. This suggested that the modification of SiO2 species on Al2O3@Al made a negligible influence on the thermal conductivity. For the traditional low thermal

Figure 6. Products selectivity of the Co/SAl2O3@Al-x (x = 0, 3, 5, 10) catalysts. Reaction conditions: n(H2)/n(CO) = 2, GHSV = 1.5 Lsyngas gcat h−1, T = 483 K, P = 2.0 MPa, TOS = 48 h.

selectivity is controlled by the cobalt particles sizes.36 According to the mechanism study, the bridge-type adsorbed CO was more easily formed on large Co particles and dissociated to active carbon and oxygen species because of the weaker C−O bond. As a result, the CHx (1 ≤ x ≤ 3) species were easily formed on a large Co particle, which was beneficial to chain growth.37 For Co/SAl2O3@Al-x (x = 0, 3, 5, 10), the modification of SiO2 on the surface of Al2O3@Al reduced metal−support interaction; thus, larger cobalt particles were formed, which was also evidenced by the XRD technique. Furthermore, as can be seen, the CH4 as well as C5+ selectivity changed slightly for Co/SAl2O3@Al-5 and Co/SAl2O3@Al-10. According to the XPS result, the Si/Al atomic ratio between Co/SAl2O3@Al-5 and Co/SAl2O3@Al-10 changes slightly and the coverage degree of Al2O3@Al was similar for the supports. Therefore, the chemical character of cobalt species might be similar for Co/SAl2O3@Al-5 and Co/SAl2O3@Al-10. Also, this should be the reason for the comparable CH4 and C5+ selectivity for the two catalysts. In addition, the C5+ products mainly included middle distillate (C5−C18) and heavy components (C19+). As can be seen in Table 2, the C5−C18 decreased while C19+ increased with the increase of SiO2 content to 10 wt %. This could be related to the different acid properties of Co/SAl2O3@Al-x (x = 0, 3, 5, 10), since the acid site was in favor of the cracking reaction, resulting in the formation of light fraction products.33 The NH3-TPD result (Figure S2) is also in line with that

Figure 7. FTS activity (A) and products selectivity (B) of Co/SAl2O3@Al-5 at different reaction temperatures. G

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observed for diluted/undiluted Co/SAl2O3@Al-5. However, the CH4 selectivity increased whereas C5+ selectivity decreased for undiluted Co/Al2O3 compared to diluted Co/Al2O3. It was widely accepted that FTS was sensitive to the reaction temperature and high reaction temperature in favor of the formation of undesired light hydrocarbons.41,42 The accumulated reaction heat resulted in the increase of temperature in the catalyst bed because of the poor thermal conductivity for undiluted Co/Al2O3, which caused the increase of CH4 selectivity and decrease of C5+ selectivity. However, for the undiluted Co/SAl2O3@Al-5 the good thermal conductivity within the reactor was beneficial to removing the reaction heat; thus, similar product selectivity was achieved compared to undiluted Co/SAl2O3@Al-5. In brief, by contrasting the diluted/undiluted data of Co/ SAl2O3@Al and Co/Al2O3, we could conclude that a good thermal conductivity was achieved for the reactor packed with Co/SAl2O3@Al even without diluting SiC particles. The intrinsic high thermal conductivity of Co/SAl2O3@Al-x could be responsible for that. Even though the heat transfer limitation existed between catalyst particles because of relatively loose contact, the liquid products could enhance the effective thermal conductivity coefficient of the bed,8 that is, the high thermal conductivity of the catalyst body could improve the heat transfer ability within the fixed-bed reactor during the reaction. In addition, the heat diluter was not necessary for Co/SAl2O3@Al-x, which could increase the volume availability of the reactor. Because of the strong exothermic characteristic of FTS, the changes of operating condition may lead to temperature changes within catalyst bed and even “temperature runway” might occur especially in industrialization. Figure 8 shows the temperature changes of Twall and Tcenterline for the reactor packed with undiluted Co/SAl2O3@Al-5 and Co/Al2O3 while increasing the oven temperature (Toven) from 503 to 508 K. As can be seen, Twall and Tcenterline increased for the reactor packed with undiluted Co/SAl2O3@Al-5, and it took 13 min to reach a steady state. However, a longer time, that is, 17 min, was taken for the Co/Al2O3 bed to reach the steady state. This further demonstrated that the reactor packed with Co/SAl2O3@Al-5 had good thermal conductivity and the temperature of catalyst bed could be easily controlled compared to those reactors packed with a low thermal conductivity catalyst.

Table 3. Temperature and Activity Variations of the Catalysts with and without Using SiC Heat Dilutera catalyst

Twall (K)

Tcenterline (K)

ΔTd (K)

conv.e (%)

Co/SAl2O3@Al-0 useb Co/SAl2O3@Al-0 nonusec Co/SAl2O3@Al-5 use Co/SAl2O3@Al-5 nonuse Co/Al2O3 use Co/Al2O3 nonuse

503.1 503.0 497.9 497.6 507.2 507.4

507.2 507.4 502.2 502.1 512.7 515.3

4.1 4.4 4.5 4.3 5.5 7.9

68.3 69.0 71.1 70.6 70.7 80.8

Reaction conditions: n(H2)/n(CO) = 2, GHSV = 1.5 Lsyngas gcat−1 h−1, TOS = 48 h. bCatalyst evaluated using SiC heat diluter. cCatalyst evaluated without using heat diluter. dΔT = Tcenterline − Twall. eThe CO conversion rate. a

conductivity Co/Al2O3, the ΔT of the diluted catalyst was 5.5 K; however, it increased to 7.9 K for the undiluted catalyst. We believed that this could be related to the different thermal conductivities of diluted/undiluted Co/Al2O3 within the fixedbed reactor. The Co/Al2O3 using a SiC diluter had good heat transfer ability within the reactor, and a relatively homogeneous radial temperature gradient was maintained. However, the thermal conductivity in a fixed-bed reactor packed with undiluted Co/Al2O3 was poor, and the reaction heat was accumulated, resulting in a higher temperature difference (ΔT). It could be observed that the CO conversion for the diluted/undiluted catalysts also followed the same rule with ΔT. For the undiluted Co/SAl2O3@Al-0 and Co/SAl2O3@Al5, the CO conversion changed less compared to the diluted experiments. However, the CO conversion of the undiluted Co/Al2O3 was 10.1% higher than the diluted Co/Al2O3. We believe that this could also be attributed to the different thermal conductivities within the fixed-bed reactor. The reaction heat accumulated in the catalyst bed because of the poor thermal conductivity for the reactor packed with undiluted Al2O3, which further increased the FTS activity. For Co/SAl2O3@Al-0 and Co/SAl2O3@Al-5, the good thermal conductivity within the reactor facilitated the effective removal of reaction heat even without using a heat diluter; thus, heat accumulation did not occur in the catalyst bed, leading to the negligible CO conversion variation. Figure S4 shows the CH4 and C5+ selectivities of Co/ SAl2O3@Al-5 and Co/Al2O3 with and without using a SiC heat diluter. No obvious CH4 and C5+ selectivities changes were

Figure 8. Temperature of reactor wall and centerline changes with the variation of oven temperature for (A) Co/SAl2O3@Al-5 and (B) Co/Al2O3. H

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CONCLUSION A series of SiO2-modified high thermal conductivity Al2O3@Al composites was prepared and supported cobalt for the purpose of improving the catalytic performance and intensifying heat transfer of Fischer−Tropsch synthesis. The prepared Co/ SAl2O3@Al-x (x = 0, 3, 5, 10) had a macro−mesoporous structure. The introduction of SiO2 and cobalt species had less of an influence on the intrinsic thermal conductivity of Al2O3@ Al. According to the laser flash measurement, the thermal conductivity coefficient of Co/SAl2O3@Al-x (x = 0, 3, 5, 10) was more than 30 times that of conventional Co/Al2O3. FTS results showed that the CO conversion and C5+ selectivity increased whereas CH4 selectivity decreased with the increase of SiO2 content to 5 wt %. This could be attributed to the improved cobalt reduction degree with the modification of SiO2 species on the surface of Al2O3@Al, which reduced the metal−support interaction. Further increasing the SiO2 content to 10 wt % led to a negligible change of FTS performance, which might due to the constant SiO2 coverage on the surface of the support. The CO conversion, products selectivity, and radial temperature gradient within the catalyst bed was significantly different for a traditional low thermal conductivity Co/Al2O3 evaluated with and without using a SiC heat diluter. However, no obvious variation of FTS performance and radial temperature gradient within the catalyst bed were observed for diluted/undiluted Co/SAl2O3@Al-x, which suggested a good thermal conductivity within the reactor. In addition, the temperature of the catalyst bed for Co/SAl2O3@Al-x more easily to reached a “steady state” while changing the oven temperature compared with the reactor packed with low thermal conductivity Co/Al2O3, which could improve the operating flexibility of the Fischer−Tropsch synthesis.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b02842.

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Additional tables and figures (PDF)

AUTHOR INFORMATION

Corresponding Authors

*Tel: (+86) 0532-80662759; E-mail: [email protected]. *Tel: (+86) 0351-4121877; E-mail: [email protected]. ORCID

Da Wang: 0000-0002-8477-6636 Xuebing Li: 0000-0002-1876-2308 Author Contributions §

D.W. and Z.W.: These authors contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support from the National Natural Science Foundation of China (21808235). REFERENCES

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