Synthesis, Characterization, and Thiophene Hydrodesulfurization

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Synthesis, Characterization, and Thiophene Hydrodesulfurization Activity of Novel Macroporous and Mesomacroporous Carbon Murid Hussain,*,†,‡ Ji Sun Yun,† Son-Ki Ihm,† Nunzio Russo,‡ and Francesco Geobaldo‡ †

National Research Laboratory for Environmental Catalysis, Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), 373-1 Gusung-dong, Yusung-gu, Daejeon 305-701, South Korea ‡ Department of Materials Science and Chemical Engineering, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy ABSTRACT: Two different types of macroporous and mesomacroporous carbons have been synthesized using the templating method with polystyrene (PS) and SBA-15/PS (1:4) as templates. These synthesized carbon materials were exact replicas of the templates and showed a small window in the large pores of macroporous carbon in addition to mesophase in mesomacroporous carbon. Materials were characterized using X-ray diffraction (XRD), nitrogen adsorption-desorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), Fourier transform infrared (FT-IR), and CO chemisorption. Mo, Co, and CoMo catalysts were prepared by supporting the aforementioned metals on optimized macroporous and mesomacroporous carbon, while commercial activated carbon (Darco G-60) and alumina (γ-Al2O3) supported catalysts were used for comparison purposes. Thiophene hydrodesulfurization of these catalysts showed that mesomacroporous carbon supported catalysts had superior activity and selectivity compared to the others, which might be due to its better metal dispersion and bimodal porosity which made transport easier and minimized channel blocking.

1. INTRODUCTION One of the most urgent aims in the petroleum industry is the development of highly active hydrodesulfurization (HDS) catalysts, not only to protect the environment but also to efficiently utilize limited natural resources.1,2 The sulfides of transition metals (Mo, W, Co, or Ni) are currently of great industrial interest as catalysts in petroleum refining for hydroprocessing applications such as HDS.3 These metals have been mostly used for HDS catalysis, and no alternative to them has been reported to date. The catalytic activities of Mo, Co, and Co-Mo sulfide catalysts for HDS are influenced to a great extent by the support employed. Alumina is the most widely used support for hydrotreating catalysts. However, a strong interaction between metal and alumina is undesirable, as it leads to a negative effect on the HDS activity. The quest for a superior support system that avoids the main disadvantages of alumina has led researchers to explore alternative support materials.4 Carbon and silica are the most suitable alternatives to the alumina support. Carbon in particular seems to be very promising as a catalytic support material and has received a great deal of attention as a support for HDS catalysts since high HDS activities have been reported that may originate from a more favorable support/catalytic species interaction.2,5-7 Porous materials have long been studied and have been applied to commercial processes in various fields, such as in catalysis, adsorption, absorption, and separation because of their attractive material properties.8-12 Mesoporous carbon as well as mesoporous silica have already been studied in depth by researchers4,13-16 for HDS applications. These materials show better performances than commercial alumina or activated carbon. The surface area, porosity, and surface chemistry play key roles in this context. However, in this investigation, we have focused on the study of the novel macroporous carbon (MC) with small open windows and mesomacroporous carbon (MMC) for HDS application. These replica carbons were obtained by templating polystyrene r 2010 American Chemical Society

(PS) and SBA-15/PS, respectively. The HDS activities of macroporous and mesomacroporous carbon based Mo, Co, and CoMo catalysts have been compared with commercial activated carbon (AC) and γ-Al2O3 based catalysts to investigate the effect of the characteristics of the supports on the catalytic activity and on product selectivity.

2. EXPERIMENTAL SECTION 2.1. Synthesis of the Support Materials. MC and MMC were synthesized by the templating method14 using sucrose as a carbon precursor. PS and SBA-15/PS were used as the templates for the synthesis of the MC and MMC, respectively. SBA-15 mesophase, PS beads, and SBA-15/PS were synthesized according to the procedure reported in Ihm et al.,17 but a different SBA-15/PS1 ratio of 1:4 was adopted. Carbon precursor solution impregnation was used 1 and 2 times to make MC1,2 and MMC1,2. After carbonization in a furnace at 900 °C under vacuum, the silica template source SBA-15 was removed by dissolving with 5 wt % hydrofluoric acid (HF: 48-51%, J.T. Baker). PS was evaporated at a high carbonization temperature. Finally, the MC and MMC were dried at 100 °C. AC (Darco G-60) and γ-alumina were purchased from Norit and Strem Chemicals, respectively. 2.2. Catalyst Preparations. Equal numbers of metal atoms of Mo (6.8 wt %) and Co (4.2 wt %) were loaded on MC2, MMC2, AC, and alumina with ammonium heptamolybdate tetrahydrate (Aldrich) and cobalt nitrate tetrahydrate (Aldrich) by incipient wetness. In the CoMo catalysts, a Co/Mo ratio of 5:5 was fixed Special Issue: IMCCRE 2010 Received: March 10, 2010 Accepted: June 2, 2010 Revised: June 1, 2010 Published: June 15, 2010 2530

dx.doi.org/10.1021/ie100574w | Ind. Eng. Chem. Res. 2011, 50, 2530–2535

Industrial & Engineering Chemistry Research

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Figure 1. Thiophene hydrodesulfurization reaction system.

Figure 3. SEM images of (a, b) mesoporous silica (1:4) polystyrene as the template, (c, d) mesomacroporous carbon using 1 time carbon precursor solution impregnation (MMC1), and (e, f) mesomacroporous carbon using 2 times carbon precursor solution impregnation (MMC2).

Figure 2. SEM images of (a, b) polystyrene template, (c, d) macroporous carbon using 1 time carbon precursor solution impregnation (MC1), and (e, f) macroporous carbon using 2 times carbon precursor solution impregnation (MC2).

for the carbons and 3:7 for the alumina. All the catalysts were dried at 100 °C. 2.3. Characterization. The powder X-ray diffraction (XRD) patterns were recorded on a Rigaku Miniflex diffractometer with Cu KR radiation at 40 kV and 45 mA. The surface area, pore volume, and pore size were measured by the nitrogen adsorption-desorption method (ASAP 2000, Micromeritics). Information about the nature of the surface oxygen functional groups was obtained using Fourier transform infrared (FT-IR) spectroscopy. The spectra were

recorded on an FT-IR spectrometer (Nexus, Nicolet) equipped with a mercury cadmium telluride (MCT) detector with a resolution of 4 cm-1 and 200 scans per spectrum. A Philips CM200 instrument with 200 kV of acceleration voltage was used for the transmission electron microscopy (TEM) test. CO chemisorption was performed by a dynamic method in a once-through flow apparatus (Pulse Chemisorb 2705, Micromeritics) equipped with a thermal conductivity detector (TCD). A pulse of CO gas was introduced at 30 °C from a 6 port valve at an interval of 1 min. When the peaks attained a nearly constant area, the adsorption was assumed to reach saturation, and dispersion was calculated. 2.4. Thiophene Hydrodesulfurization Reaction. The thiophene HDS reaction was carried out at 400 °C in a stainless steel micro flow reactor (Figure 1), operated at 20 atm pressure. Before starting the reaction, each catalyst was calcined and then sulfided in situ at 300 °C for 2 h with a flow of H2S (10 vol %)/H2 mixture at 30 mL min-1. The HDS reaction was measured in the reactor with 0.1 g of catalyst and W/F of 2.22 g cat min/cc thiophene for 7 h. The reaction products were analyzed using a gas chromatograph (Hewlett-Packard 5710A) equipped with a TCD and packed column.

3. RESULTS AND DISCUSSION 3.1. Characterization of the Synthesized Carbons. Wellordered, 250 nm PS beads were obtained, as shown by the scanning electron microscopy (SEM) images in Figure 2a,b. These beads were then used as a template material for the synthesis of MC1,2 using 1 and 2 times carbon precursor solution impregnation in the voids. It can be seen in Figure 2c,d that 1 time sucrose solution impregnation was not enough to obtain the exact replica of PS and some void spaces were observed. Moreover, the resultant size of the 2531

dx.doi.org/10.1021/ie100574w |Ind. Eng. Chem. Res. 2011, 50, 2530–2535


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Figure 4. EDS analysis of the carbons obtained: (a) MC1, (b) MC2, (c) MMC1, and (d) MMC2.

Figure 6. TEM images of the mesomacroporous carbon (MMC2).

Figure 5. XRD patterns of (a) the mesoporous silica/PS template and (b) the mesomacroporous carbons.

MC1 bead shrank to 231 nm compared to the 250 nm of PS. The 2 times sucrose solution impregnation to the PS beads produced a well-ordered MC2 and further shrunk the size to 193 nm, as shown in Figure 2e,f. It was also observed that the MC2 beads possessed small windows which might be interconnected and act as mesopores. A similar trend as in MC1,2 has also been observed for the synthesis of MMC1,2 with SBA-15(1:4)PS as the template; this trend is shown in Figure 3. However, a mesoporosity was obtained in MMC by the carbon replication of silica,14,18 together with an

open macroporous framework which was attained by PS.19 An energy dispersive spectroscopy (EDS) analysis (Figure 4) has confirmed the synthesis of the macroporous and mesomacroporous carbon supports, MC1,2 and MMC1,2, which were almost pure carbon materials, as they showed more than 90% carbon. Moreover, the silica template was almost completely dissolved by HF. Figure 5a shows the small-angle XRD pattern of the calcined SBA-15(1:4)PS template for the MMC. Large 2-D hexagonal mesoporous channels of SBA-15 are interconnected through microporous spacers.14 MMC1,2 show a well resolved XRD peak for the 2-D hexagonal space group (P6mm) (Figure 5b), which is similar to its silica template (Figure 5a). However, MMC2 showed better peak intensities compared to MMC1 and, hence, correlates with the SEM results (Figure 3). It has, therefore, been confirmed that MMC2 is the optimized exact replica of its parent template compared to MMC1. Pores in carbon materials play a major role because of their controlled applications.20 The TEM images shown in Figure 6 clearly show a biporous morphology, with both mesoscopic and macroscopic ranges. Perfectly spherical macropores can clearly be observed as imprints of the PS beads. The one-dimensional mesopore channel and hexagonal array that can be observed in SBA-15, and which remains on the walls of the macropores,17 has also been observed in the resultant replica MMC2. This TEM image is direct and conclusive evidence of an ordered mesomacroporous carbon with bimodal porosity. Table 1 shows the physical properties of silica and the resultant mesomacroporous carbon replica. The surface of 2532



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Table 1. Physical Properties of Silica and the Resultant Mesomacroporous Carbon Replica SBA-15 BET surface area (m2/g)

861

SBA-15 (1:4) PS1

meso/macroporous

meso/macroporous

carbon (1 HF)

carbon (2 HF)

741

339

640

pore volume (cm3/g)

1.0

1.0

0.5

0.8

pore size (nm)

5.1

5.6

4.7

5.2

the MMC2 after HF etching twice showed a higher surface area and pore volume than after 1 time HF etching of the silica template. The pore size was not influenced to any great extent by the replication. Although carbon is considered to be an inert material, in comparison to other supports, such as silica and alumina, its surface has a proportion of active sites as oxygen surface functional groups.6,15 These oxygen surface groups are by far the most important in influencing the surface characteristics and have attracted much attention by researchers. These groups could affect the carbon-metal interaction and catalytic activity of carbon-supported catalysts. The MC and MMC surface chemistry has been explored here by FT-IR reflectance spectra to find the qualitative oxygen surface functional groups (Figure 7). The FT-IR spectra of the carbons (Figure 7a,b) show characteristic bands at wave numbers ranging from 500 to 930 cm-1, which have been attributed to C-H vibrations.14 The bands appear at an intensity of 1030 to 1120 cm-1 and have been assigned to OH bendings, whereas the 1255-1320 cm-1 range covers the bands for C-O-C stretching, alcoholic, phenolic, and carboxylic groups. The band around 1590 cm-1 is due to the olefinic CdC bonds, and the band around 3500 cm-1 is due to the OH groups. MC2 has more oxygen surface functional groups than MC1. Similar behavior can be observed for the case of mesomacroporous carbon where MMC2 shows more surface oxygen groups than MMC1. This was also confirmed by the acid-base titration to see the acidic as well as basic surface functional groups quantitatively. 3.2. HDS Activity Test. The thiophene HDS activity, for the supported Mo, Co, and CoMo catalysts, was conducted at specific reaction conditions (presulfiding at: 300 °C, pressure: 20 atm, reaction temperature: 400 °C, reaction time: 7 h, H2/thiophene: 15). Figure 8 shows the conversion and selectivity results. For the Mo test (Figure 8a), the highest conversion was observed by a MMC2 supported Mo catalyst. The lowest conversion was obtained by alumina supported Mo catalysts whereas the MC2 and AC supported Mo catalysts showed intermediate results. Moreover, a slow deactivation trend was observed with the passage of time which might be due to pore blockage. A high surface area, well developed porosity, and surface functionality of the carbons are essential to achieve extensive metal dispersion.6 MMC2 showed good characteristics, regarding the above-mentioned properties, as confirmed by the characterizations performed. Moreover, the bimodal pore structure of MMC2, with higher oxygen surface functional groups, made it even better than the other supports used here and it acted as transport pores, which facilitated metal dispersion and transportation of the reactant and product molecules to reach the smaller pores situated in the interior of the carbon particles. The higher Mo dispersion on MMC2 was confirmed by the CO chemisorption (Table 2), and this can be considered direct proof of its activity. MC2 has low dispersion and lacks mesopores, which makes it less active than MMC2. AC also shows lower performance than MMC2. A very large proportion of its surface area is contained within micropores, which results in a low dispersion of the metal precursor and creates difficulty in the transportation of the reactants and products. Moreover, the alumina supported Mo

activated carbon 513 0.83 13.5

γ-alumina 220 0.35 6.5

Figure 7. FT-IR transmittance spectra for the surface functional groups: (a) the macroporous carbons and (b) the mesomacroporous carbons.

catalyst showed lower dispersion than the MMC2 supported Mo catalyst (Table 2), due to its less effective characteristics which led to the lower conversion. The alumina surface is considered to be less inert than carbon, which leads to a strong metal-support interaction and, hence, less activity.6 Another important characteristic of the carbon here is the better adsorption of the reactant during the reaction which makes it better than alumina. Figure 8b shows the conversion results for the Co supported catalysts. The main trend was similar to that observed in the Mo supported catalysts; the MC2 and AC supported Co catalysts, however, showed a similar conversion. This might be due to the higher dispersion of the Co metal on the carbon surface,14 which was confirmed by CO chemisorption (Table 2) and which was due to the pore blockage in the micropores of AC reduced the activity. The intensive chemical interaction of Co metal with alumina support nullifies the activity. An overall higher conversion of thiophene was observed for the CoMo supported catalysts, as shown in Figure 8c. However, the trend was the same as in the case of the Co supported catalysts, and only the synergistic effect between Co and Mo improved the overall activity. 2533



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Figure 8. Thiophene HDS conversion at specific conditions by (a) Mo, (b) Co, and (c) CoMo supported catalysts and (d) selectivity by CoMo supported catalysts.

Table 2. Metal Dispersion by CO Chemisorption at 30 °C sulfided catalysts

metal dispersion (%)

Mo(6.8)/MMC2

25

Mo(6.8)/MC2

21

Mo(6.8)/AC

18

Mo(6.8)/alumina Co(4.2)/MMC2

7 60

Co(4.2)/MC2

51

Co(4.2)/AC

50

Co(4.2)/alumina

4

CoMo(5:5)/MMC2

50

CoMo(5:5)/MC2

45

CoMo(5:5)/AC

45

CoMo(3:7)/alumina

10

The HDS of thiophene mainly takes place in three pathways: HDS via hydrogenation followed by desulfurization (hydrogenation pathway I), C-S bond scission (hydrogenolysis pathway II), or direct desulfurization (direct pathway III).14 The selectivity results of CoMo supported catalysts are shown in Figure 8d. The MC2, AC, and alumina supported CoMo catalysts showed more tetrahydrothiophene (pathway I) than MMC2. However, MMC2 supported CoMo catalysts showed more butene (pathways II or III) than the others. The great number of acidic surface functional groups (carboxylic, lactonic, phenolic measured by acid-base titration) on MMC2 might favor the direct desulfurization pathway.

4. CONCLUSIONS Macroporous and mesomacroporous carbons have been successfully synthesized and optimized through the templating method and have been shown to possess better characteristics than conventional AC or alumina. MMC2 supported Mo, Co,

and CoMo catalysts have shown a better performance, regarding conversion and selectivity of thiophene, compared to MC2, AC, and alumina supported catalysts. This has been attributed to the superior characteristics of MMC2, which include bimodal porosity, higher adsorption capacity, and higher surface functional groups which induced higher metal dispersion leading to higher conversion and selectivity. These results suggest that MMC2 used as a HDS catalyst support could be a better promising support in the future for combined double action (adsorption þ HDS) and for HDS of bulky molecules (dibenzothiophene, 4,6dimethyldibenzothiophene), compared to past conventional supports.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: þ39-011-0904720. Fax: þ39-011-5644699. E-mail: [email protected].

’ ACKNOWLEDGMENT Financial support through a grant from the Brain Korea 21 (BK21) Project and in part from the National Research Laboratory Project of Korea Ministry of Science and Technology is gratefully acknowledged. ’ REFERENCES (1) Topsoe, H.; Clausen, B. S.; Massoth, F. E. Hydrotreating Catalysis; Springer: Berlin, 1996. (2) Babich, I. V.; Moulijn, J. A. Science and Technology of Novel Processes for Deep Desulfurization of Oil Refinery Streams: A Review. Fuel 2003, 82, 607. (3) Farag, H.; Whitehurst, D. D.; Mochida, I. Synthesis of Active Hydrodesulfurization Carbon-Supported Co-Mo Catalysts. Relationships 2534



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Synthesis, Characterization, and Thiophene Hydrodesulfurization

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