Hexagonal Mesoporous

Nov 12, 2013 - A series of xMnyCe/hexagonal mesoporous silica (HMS) sorbents with wormhole-like structure was prepared by a sol–gel method, and thei...
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Fabrication and Performance of xMnyCe/Hexagonal Mesoporous Silica Sorbents with Wormhole-Like Framework for Hot Coal Gas Desulfurization Z. F. Zhang, B. S. Liu,* F. Wang, and J. F. Li Department of Chemistry, School of Science, Tianjin University, Tianjin 300072, People’s Republic of China ABSTRACT: A series of xMnyCe/hexagonal mesoporous silica (HMS) sorbents with wormhole-like structure was prepared by a sol−gel method, and their performance for hot coal gas desulfurization was investigated at 600 °C. All xMnyCe/HMS sorbents exhibited high breakthrough sulfur capacity, and the utilization of these sorbents was much higher than that of 10Mn/HMS. The breakthrough sulfur capacity over 4Mn1Ce/HMS sorbents was 121.7 mg of S/g of sorbent with the utilization of 82.4%. Such a behavior was maintained in eight consecutive desulfurization−regeneration cycles. The effects of the desulfurization temperature, H2 concentration, and 7% steam on the performance of 4Mn1Ce/HMS were examined. The fresh, used, and regenerated samples were characterized by means of X-ray diffraction, N2 adsorption, Fourier transform infrared absorption spectroscopy, and high-resolution transmission electron microscopy techniques. The results confirmed that the manganese oxide was dispersed highly on the HMS support because of the synergetic effects of manganese oxide and ceria oxide, and the wormhole-like structure in sorbents promoted the diffusion of H2S molecules. The Brunauer−Emmett−Teller results revealed that the wormhole-like mesoporous structure in sorbents remained intact even after the eighth successive desulfurization/regeneration cycle.

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et al.12 reported the pure CeO2 and Ce−Mn mixed oxide sorbents and found that the intimate combination of the two oxides improved the sulfur capacity of sorbents and desulfurization precision remarkably via the decline of oxygen vacancies in the CeO2 lattice but the utilization of γ-Al2O3 reduced the sulfur capacity because of Mn dissolution into the alumina support. It is reported that dispersing metal oxide onto a suitable highsurface-area support can ameliorate these drawbacks; for example, the reactivity of sorbents correlated directly to their surface area and pore volume.1 The high dispersion of active components as a result of the support with a high surface area and regular pore size distribution can promote the reactivity of H2S molecules. In comparison to zeolite, γ-Al2O3, SiO2, and active carbon, the mesoporous silica, such as MCM-41 and SBA-15, have received more attention for high surface area, large pore volume, and uniformly pore distribution.11−19 In recent years, the sorbents with single or mixed transition-metal oxides were supported on mesoporous silica, such as ZnO/ AlSBA-15, CuO/MCM-41, CuO/SBA-15, Cu−Mn/SBA-15, and Fe−Mn/MCM-41.20−25 All of them showed better desulfurization behavior than the pure metal oxide in the mid (300−600 °C) and high (>600 °C) temperature ranges. Besides, the sorbents with rare-earth/transition-metal oxides, such as LaxMeyOz/SBA-15, LaMeOx/MCM-41, and LaxFeyOz/ MCM-41 presented good desulfurization performance because of the synergetic effects of the mixed metal oxides.26−28 However, the structure of these sorbents was somewhat destroyed during the initial regeneration stage, and the sulfur

INTRODUCTION Hot coal gas is the main energy resource for solid oxide fuel cell (SOFC) and integrated gasification combined cycle (IGCC) systems. Sulfur is converted to hydrogen sulfide during the coal gasification process, which needs to be removed to protect the equipment and environment.1,2 The conventional sulfur removal technologies, such as wet absorption or zinc oxide sorption, are operated at ambient temperature.2 However, hot coal gas must be cooled and then preheated to a high temperature before it is fed into a gas turbine or fuel cell, which results in significant loss in the thermal efficiency of the system.3 To satisfy the upcoming environmental regulation and decrease energy consumption, the hot coal gas desulfurization (HGD) technique has received increasing attention.4 One of the essential works in the development of the HGD technique for SOFC and IGCC systems is the need for a highsulfur-capacity, low-cost, eco-friendly, high-utilization, and regenerable sorbent. Several transition-metal oxides had been studied in the HGD technique through the thermodynamic method in 1976.5,6 Recently, Stephanopoulous et al.7,8 Dolley et al.,9 and Cheah et al.10 investigated the potential of rare-earth oxide sorbents using CeO2, La2O3, and Pr2O3. However, all of the pure metal oxide sorbents are confronted with the same problems, such as low-utilization, sintering, mechanical pulverization, and reductive effects during the desulfurization−regeneration process at a high temperature. It is wellknown that manganese is stable in the form of MnO in a reductive atmosphere and has a superior initial desulfurization rate.5 Bakker et al.11 reported the Mn/γ-Al2O3 sorbent to increase the utilization of manganese and verified that the desulfurization performance of manganese appeared to be stable during successive sulfidation−regeneration cycles. However, it is difficult to sulfidize fully because MnAl2O4 formed at reductive atmosphere until 800 °C. In addition, Li © XXXX American Chemical Society

Received: July 15, 2013 Revised: November 12, 2013

A

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Figure 1. Small- and wide-angle XRD patterns of (a) HMS, (b and f) fresh 4Mn1Ce/HMS, (c and h) used 4Mn1Ce/HMS, (d and i) eighth regenerated 4Mn1Ce/HMS, (e) 10Mn/HMS, and (g) 5Mn5Ce/HMS. 10Mn/HMS, 9Mn1Ce/HMS, 6Mn1Ce/HMS, 5Mn1Ce/HMS, 4Mn1Ce/HMS, 3Mn1Ce/HMS, and 5Mn5Ce/HMS, respectively; the numbers before Mn and Ce represented the molar number in mixed metal oxides. Characterization. The small-angle XRD patterns were recorded with a Rigaku D/MAX 2500 V/pc automatic diffractometer equipped with Ni-filtered Cu Kα radiation (20 kV and 30 mA). The wide-angle XRD patterns were recorded with a PANalytical automatic diffractometer using Ni-filtered Cu Kα radiation (λ = 0.154 06 nm) at settings of 40 kV and 50 mA. The average crystallite sizes of Mn3O4 (211) and CeO2 (011) were calculated by the Scherrer formula [D(hkl) = 0.89λ/β cos θ]. The N2 adsorption isotherms of HMS and sorbents were collected on a homemade system by the nitrogen adsorption at 77 K. Prior to analysis, the sample was treated in vacuum at 200 °C for 2 h. The Brunauer−Emmett−Teller (BET) specific surface area and pore volume were assessed from the adsorption data. The pore size distribution was determined by the Barrett−Joyner−Halenda (BJH) method. FTIR absorption spectroscopy was performed with a BIO-RAD FTS 3000 spectrophotometer at room temperature with the sample on a KBr wafer. The structure of HMS, fresh, and used sorbents was investigated by HRTEM on Tecnai G2 F20 electron microscopy operated at 200 kV. Desulfurization/Regeneration Performance. Desulfurization tests were performed at atmospheric pressure in a downflow fixedbed quartz-tube microreactor (10 mm inner diameter, loaded with 0.5 g of sorbent) using a simulated hot coal gas with a composition of 0.33% H2S, 10.6% H2, 28% CO, and N2 as balance. The flow rates of feed gas were controlled by a mass flowmeter (D07-7B/ZM, Beijing Sevenstar Electronics Co., Ltd., China). A thermocouple placed in the center of the sorbent bed was used to measure the reaction temperature. The sorbent bed was heated to the reaction temperature at the rate of 10 °C/min in a N2 atmosphere to remove the impurities adsorbed on the surface of the sorbent. After the desulfurization test, pure N2 was introduced to purge the system until the sorbent bed was cooled to ambient temperature. The concentration of H2S in the inlet/ outlet gas was analyzed by the method of iodometric titration with absolute deviation of 50 mg/m3. The breakthrough time was defined as the time when a concentration of H2S of 100 mg/m3 was detected in the outlet gas. Herein, the sulfur capacity (SC) and sorbent utilization (SU) were calculated by the formulas

capacity is conspicuously declined in multiple cycles of regeneration. Tanev and Pinnavaia reported the synthesis of hexagonal mesoporous silica (HMS) in 1995 for the first time.29 The wormhole-like mesoporous silica received a lot of attention because of the small particle size, short channel, more exposed pore-opening, and textural mesopores properties, which are more favorable for mass transfer in the adsorption and catalysis processes than MCM-41 and SBA-15.30−32 The HMS was chosen as the support, and the active compound contains CeO2 and MnOx. The goal of this work is to prepare a series of 50% xMnyCe/HMS sorbents with high-sulfur-capacity, high-utilization, and complete-regeneration ability. Their desulfurization performances were investigated in a downflow fixed-bed reactor using a simulated hot coal gas. The structural features of the sorbent were assessed by X-ray diffraction (XRD) and highresolution transmission electron microscopy (HRTEM) technologies. Their textural properties were investigated by N2 adsorption and Fourier transform infrared (FTIR) absorption spectroscopy.

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EXPERIMENTAL SECTION

Sorbent Preparation. The wormhole-like HMS was prepared by exploiting N,N-dimethyldodecylamine as the template. First, 3.77 mL of surfactant was dissolved in 26.54 mL of ethanol by an ultrasonic method. Then, 26.64 mL of deionized water (DW) was added under vigorous stirring until the surfactant redissolved. Subsequently, 11.2 mL of tetraethyl orthosilicate (TEOS) was added dropwise during stirring until homogeneous dispersion. After aging at ambient temperature for 18 h, the wormhole-like HMS was obtained by filtration, washing, and drying at 80 °C overnight, with the final calcination in the air at 600 °C for 6 h. The 50% xMnyCe/HMS samples were prepared by a sol−gel method. When the preparation of the 4Mn1Ce/HMS sorbent was taken as an example, 7.36 g of 50 wt % Mn(NO3)2 solution and 2.23 g of Ce(NO3)3·6H2O were dissolved in 25 mL of DW. After the addition of citric acid with a molar amount of 1.5 times that of the total metal ions, 2.5 g of as-prepared HMS support was added to the aforementioned solution. The mixture was kept at 60 °C with constant stirring until a viscous gel was generated. Next, the gel was aged at ambient temperature for 3 days. The samples were heated to 600 °C at a rate of 5 °C min−1 and calcined at 600 °C for 6 h. The obtained xMnyCe/HMS sorbents with different compositions were denoted as B

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Figure 2. (A and B) Nitrogen adsorption isotherms and (C) pore size distributions of pure HMS, fresh, used, and eighth regenerated sorbents: (red ● and ○) fresh and used 10Mn/HMS, (blue ▲ and △) fresh and used 9Mn1Ce/HMS, (brown ◀ and ◁) fresh and used 6Mn1Ce/HMS, (maroon ▶ and ▷) fresh and used 5Mn1Ce/HMS, (pink ▼ and ▽) fresh and used 4Mn1Ce/HMS, (green ◆ and ◇) fresh and used 3Mn1Ce/ HMS, (orange ★ and ☆) fresh and used 5Mn5Ce/HMS, (purple □) eighth regenerated 4Mn1Ce/HMS, and (pink ◐) used 4Mn1Ce/HMS in the presence of 7% steam. The inset in panel A is the pore size distribution of HMS.

⎛ mg of sulfur ⎞ M 22.4 SC ⎜ [ ⎟ = GHSV S Vm M H2S ⎝ g of sorbent ⎠

∫0

× 10−4

SU = BSC/TSC × 100%

(Figure 1d), indicating the existence of the wormhole-like framework structure. As for the wide-angle XRD patterns of fresh, used, and regenerated sorbents with different Mn/Ce atomic ratios, as shown in Figure 1, the diffraction peaks at 23.1°, 32.9°, 38.2°, 49.3°, and 55.2° could be ascribed to the Mn2O3 phase [powder diffraction file (PDF) 41-1442] for the 10Mn/HMS (Figure 1e). The well-resolved diffraction peaks suggested the formation of large Mn2O3 particles. However, the broad diffraction peaks of Mn3O4 (PDF 18-0803) and CeO2 (PDF 34-0394) phases were detected, and the average crystallite sizes of Mn3O4 and CeO2 estimated from the Debye−Scherrer formula were about 6.8 and 8.2 nm, respectively, in 4Mn1Ce/ HMS (Figure 1f), indicating a high dispersion of Mn3O4 particles with the dope of CeO2. The main crystalline phase of manganese oxides in 4Mn1Ce/HMS was transformed from Mn2O3 to Mn3O4. With regard to CeO2 crystallites, the coordination numbers of cations and anions are 8 and 4 in the cubic fluorite structure, respectively. According to the reports of Murugan et al.33 and Li et al.,12 Mn ions entered initially the defect sites of CeO2 and substituted for Ce4+ in the CeO2 crystallites to form the Ce1−xMnxO2−δ solid solution. The particles of Mn3O4 and CeO2 reduced remarkably and were dispersed highly on the support as a result of the synergetic effects of MnOx and CeO2. Therefore, the diffraction peaks of Mn3O4 in the XRD pattern weaken. With the increasing amount of CeO2 in the sorbent, the peaks of CeO2 intensified in the XRD patterns of 5Mn5Ce/HMS (Figure 1g). In the XRD patterns of used 4Mn1Ce/HMS (Figure 1h), the diffraction peaks at 2θ = 29.5°, 34.3°, 49.3°, and 61.5° of MnS (PDF 65-0891) and weak reflections of Ce2O2S (PDF 261085) at 2θ = 29° and 45.6° were observed because of the low CeO2 content. The absence of Mn3O4 and CeO2 peaks illustrated the conversion of metal oxide to metal sulfide. After eight successive desulfurization−regeneration cycles, the diffraction peaks of Mn3O4 and CeO2 (Figure 1i) increased slightly because of the aggregation of Mn3O4 (from 6.8 to 9.2

t

(C in − Cout)dt ] (1) (2)

where GHSV is the gas hourly space velocity (L h−1 g−1), MS and MH2S are molar weights of S (32.06 g mol−1) and H2S (34.06 g mol−1), respectively; Vm is molar volume of H2S at 1 atm and 25 °C (24.5 L mol−1), Cin and Cout are the inlet and outlet concentrations of H2S (mg/m3), t is the duration time of desulfurization (h), and BSC and TSC are the breakthrough and theoretical sulfur capacities, respectively. The used sorbent was regenerated in a 5% O2/N2 gas mixture (165 mL/min) at 700 °C until no sulfur element formation, and the concentration of SO2 in the outlet gas cannot be detected by KMnO4 solution.

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RESULTS AND DISCUSSION Characterization of Sorbent. Figure 1 showed the XRD patterns of the HMS, fresh, used, and regenerated 4Mn1Ce/ HMS samples. The HMS (Figure 1a) exhibited a single and intense reflection peak at 2.06°, which was the typical reflection peak of the wormhole-like mesoporous silica. According to the report of Tanev et al.,26 the synthesis route of HMS was based on hydrogen bonding and self-assembly between neutral amine micelles (S0) and inorganic precursors (I0). The single and broad diffraction peak suggested a short-range structure with ordered pore distribution. For the fresh 4Mn1Ce/HMS sorbent (Figure 1b), only a weak peak was observed at 2θ = 1.35°, which indicated that the wormhole-like structure of HMS was still retained after the introduction of metal oxide to the channel of the support. It could be seen that the diffraction peak of used 4Mn1Ce/HMS (Figure 1c) further weaken because of the occupation of large metal sulfide in the channel of the used sorbent. It was worth noting that a weaker reflection peak was still observed for the eighth regenerated sample C

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the regeneration process. The XRD results also verified the formation of CeO2 and Mn3O4 (Figure 1i). However, the pore size distribution revealed that the mesoporous framework was slightly destroyed and the wormhole-like channel structure was retained partially in the regenerated sorbent, which was confirmed by the result of SAXRD. The HRTEM images of the fresh 4Mn1Ce/HMS sorbent were shown in Figure 3a. It revealed the active particle high

nm) and CeO2 (from 8.2 to 9.6 nm) particles during the consecutive desulfurization−regeneration cycles. The N2 adsorption isotherms and pore size distribution for HMS, fresh, and used sorbents were shown in Figure 2. The HMS sample exhibited a type-IV isotherm with the uniform pore distribution (Figure 2A). On the basis of the maximum of the pore size distribution with 2.1 nm (inset of Figure 2A) and the unit cell constant a0 of 4.9 nm calculated by the small-angle X-ray diffraction (SAXRD) pattern, a0 = 2d/31/2, the wall thickness estimated was approximately 2.8 nm. Additionally, a significant uptake of N2 at relative pressures (p/p0 > 0.90) suggested a high degree of textural porosity for the HMS. To use the feature of high textural porosity, the metal oxides were not only anchored at the channel wall and exterior surface but also deposited on the surface of voids, as reported by Tanev and Pinnavaia29 and Pauly and Pinnavaia.32 For the xMnyCe/ HMS sorbent, SBET and VT (Table 1) became smaller and the Table 1. Specific Surface Area (SBET), Total Pore Volume (VT), Mesopore Volume (Vmeso), Micopore Volume (Vmic), Pore Diameter (Da) of HMS, Fresh, Used, and Regenerated xMnyCe/HMS Sorbents with Different Mn/Ce Atomic Ratios sample HMS 10Mn/HMS 9Mn1Ce/HMS 6Mn1Ce/HMS 5Mn1Ce/HMS 4Mn1Ce/HMS 3Mn1Ce/HMS 5Mn5Ce/HMS S-600-10Mn/HMS S-600-9Mn1Ce/HMS S-600-6Mn1Ce/HMS S-600-5Mn1Ce/HMS S-600-4Mn1Ce/HMS S-600-4Mn1Ce/HMS (7% steam) S-600-3Mn1Ce/HMS S-600-5Mn5Ce/HMS S-500-4Mn1Ce/HMS S-700-4Mn1Ce/HMS S-800-4Mn1Ce/HMS R-8-4Mn1Ce/HMSa

SBET (m2/g)

VT (mm3/g)

Vmeso (mm3/g)

Vmic (mm3/g)

Da (nm)

1248 286 348 311 301 275 293 245 256 313 284 237 230 116

1200 335 352 339 328 260 322 258 308 302 351 330 230 197

880 237 233 237 227 170 225 172 235 190 288 236 160 139

320 98 119 102 101 90 97 86 73 112 63 94 70 41

2.10 1.88 1.72 1.96 1.97 1.86 1.68 1.70 1.80 1.70 1.91 1.70 1.70 1.70

194 129 252 107 41 102

227 200 256 129 75 141

160 150 174 98 65 104

67 50 82 31 10 37

1.60 1.60 2.07 1.54 1.96 2.20

Figure 3. (a and b) HRTEM images and (c) EDX analysis of the fresh 4Mn1Ce/HMS sorbent.

dispersion over the HMS support, and no significant aggregation of active species was observed, which coincided with the XRD analysis (Figure 1f). Although the metal oxide content was 50 wt %, the wormhole-like channel can be seen clearly in the 4Mn1Ce/HMS sorbent, as shown in Figure 3b. The result of energy-dispersive X-ray (EDX) analysis (Figure 3c) indicated that the molar ratio of Mn/Ce was 3.7:1, close to the stoichiometric value (4:1). The HRTEM image of the used 4Mn1Ce/HMS sample (Figure 4a) revealed that the wormhole-like mesoporous structure still remained intact during the sulfurization at a high temperature. However, it is interesting to note that, except for these disorder network structures, some nanofibers were formed on the surface of the 4Mn1Ce/HMS sorbent (Figure 4b). To make the origination and composition of these nanofibers certain, the selected area election diffraction (SAED) and EDX analyses were used. The result of EDX analysis indicated that these are large amounts of oxygen, sulfur, and cerium species, expect for small amounts of Si and Mn (Figure 4c). This suggested that the formation of the nanofiber correlated closely with CeO2 species on the surface of sorbents. According to the report in the literature,33 more lattice defect sites and active oxygen species in the CeO2 fluorite structure because of the formation of Ce1−xMnxO2−δ solid solutions favor the adsorption sulfur compounds and the occurrence of the exchange reaction between oxygen and sulfur. Therefore, the formation of the nanofiber was related to isolated CeO2 on the outer surface of HMS, which would cause high sulfur capacity. The SAED pattern (Figure 4d) of the used sample presented weak rings and no regular bright spot, indicating that Ce2O2Sx species existed in amorphous or multi-crystal particles.

a

R-8-4Mn1Ce/HMS is denoted as 4Mn1Ce/HMS regenerated after eight cycles.

pore size distributions became less uniform, because of the fact that the molecules of CeO2 and MnOx were loaded in the inner surface of the HMS channel. However, because of pure manganese oxide aggregation and the blockage channel of the support during the preparation process, both SBET and VT of 10Mn/HMS were smaller than those of 9Mn1Ce/HMS. The N2 adsorption volumes and pore size of used sorbents decreased slightly compared to fresh samples because of the formation of the metal sulfides with a larger molecular size. After eighth regeneration, SBET and VT of the 4Mn1Ce/HMS sorbent were 102 m2/g and 141 mm3/g, respectively, which are remarkably lower than those of the fresh sample; this is because Mn−Ce−O particles partially aggregate or sinter when the metal sulfide was converted again into the metal oxide during D

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were assigned to the Si−O bending vibration, as reported by Selvaraj et al.34 As for the fresh and regenerated sorbent, the absorption peak at 605 cm−1 was attributed to the Mn−O vibrations of bivalent manganese ions in tetrahedral coordination in Mn3O4.35 There was no typical absorption peaks of MnSO4 (Figure 5a) at 625, 652, and 1013 cm−1 for the eighth regenerated 4Mn1Ce/HMS sorbent, indicating that no manganese sulfate formed during the eight consecutive desulfurization−regeneration cycles. Effect of the Composition of xMnyCe/HMS on the Desulfurization Performance. The effect of the composition of xMnyCe/HMS on the desulfurization performance and blank test over the HMS support was shown in Figure 6. It can

Figure 6. Desulfurization behavior over different xMnyCe/HMS sorbents and pure HMS support at 600 °C: (a) 10Mn/HMS, (b) 9Mn1Ce/HMS, (c) 6Mn1Ce/HMS, (d) 5Mn1Ce/HMS, (e) 4Mn1Ce/HMS, (f) 3Mn1Ce/HMS, (g) 5Mn5Ce/HMS, and (h) blank run over HMS (GHSV, 2 × 104 mL h−1 g−1; feed composition, 0.33% H2S, 10.6% H2, 18% CO, and N2 balance gas).

Figure 4. (a−c) HRTEM, (d) EDX analysis, and (e) SAED of the used 4Mn1Ce/HMS sorbent.

The formation of MnSO4 was the main reason for the reduction of the sulfur capacity on the Mn-based sorbent during the regeneration according to the reports in the literature.1,11,25 To investigate the effect of regeneration ways on the formation of MnSO4, the FTIR spectra for fresh and eighth regenerated 4Mn1Ce/HMS sorbents as well as MnSO4 were shown in Figure 5. For the support, fresh, and regenerated samples (spectra b−d of Figure 5), the peaks at 812 and 1080 cm−1 were attributed to the symmetric and asymmetric stretching vibrations of Si−O−Si and the bands at 475 cm−1

be seen that the removal of H2S over the HMS support is negligible (Figure 6h). The breakthrough sulfur capacity of 10Mn/HMS was 121.6 mg of S/g of sorbent with the utilization of only 60%. Ranade et al.36 reported that the gas−solid reaction between H2S and MeOx occurred first on the surface and then extended to the bulk phase. A low utilization of 10Mn/HMS was due to the large Mn2O3 particles and the slow gas film diffusion rate based on the reaction kinetics. XRD analysis (Figure 1e) also verified the formation of large Mn2O3 and Mn3O4 particles, and we observed that, after the breakthrough time point, the variation of the deactivation curve with the duration time became slow clearly (Figure 6a). For the 5Mn1Ce/HMS and 4Mn1Ce/HMS sorbents, the breakthrough curve was quite sharp after the breakthrough time point; this suggested that the high dispersion of active species enhanced the interaction rate of the sorbent with H2S molecules remarkably because of the formation of Ce1−xMnxO2−δ solid solution. Therefore, with the incorporation of CeO2, the utilizations of sorbents were much higher than those of 10Mn/HMS. For example, the utilization of the 4Mn1Ce/HMS sorbent was 82.4%, nearly twice as much as that over the mixed Ce1Mn3 sorbent reported by Yasyerli.37 In addition, the introduction of a small amount of CeO2 in the Mn-based sorbent could improve the dispersion of Mn species and increase the desulfurization efficiency. However, the sulfur retention capacity of CeO2 itself is low (90 mg of S/g of CeO2), and the reaction rate with H2S molecules was slow. Hence, the

Figure 5. FTIR spectra: (a) MnSO4, (b) HMS, (c) fresh 4Mn1Ce/ HMS, and (d) R-8-4Mn1Ce/HMS. E

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BSC and the utilization of 3Mn1Ce/HMS and 5Mn5Ce/HMS declined with the increase of Ce contents. Effect of the Desulfurization Temperature on the Performance of 4Mn1Ce/HMS. Figure 7 showed that the

Figure 8. H2S breakthrough curves for the 4Mn1Ce/HMS sorbent at different inlet H2 concentrations (600 °C; GHSV, 2 × 104 mL h−1 g−1; feed composition, 0.33% H2S, 18% CO, and N2 balance gas; and the contents of hydrogen are 10.6% in the case of 7% H2O). The data in parentheses are the breakthrough sulfur capacities (mg of S/g of sorbent).

Figure 7. H2S breakthrough curves over the 4Mn1Ce/HMS sorbent at different temperatures (GHSV, 2 × 104 mL h−1 g−1; feed composition, 0.33% H2S, 10.6% H2, 18% CO, and N2 balance gas). The data in parentheses are the breakthrough sulfur capacities (mg of S/g of sorbent).

stable in the atmosphere of hydrogen.27 The sulfur sorption process in the sorbent could be explained by the following reactions:

H2S breakthrough curves and sulfur retention capacity over the 4Mn1Ce/HMS sorbent varied with the reaction temperature. It could be noted that the breakthrough sulfur capacity of sorbents first increased and then declined with incremental temperature. The highest BSC was obtained at 600 °C (121.7 mg of S/g of sorbent). According to the reports of Ben-Slimane and Hepworth,38−40 the Mn-based oxides exhibited a high desulfurization efficiency in the range of 400−850 °C. On the basis of the results of chemical kinetic calculations, the reaction rate of hot coal gas desulfurization enhanced with an incremental reaction temperature; i.e., the BSC at 500 °C is relatively low because only the metal oxides on the surface were sulfided. From the viewpoint of thermodynamics,41 the equilibrium constants (Keq = 2.569 × 103 at 527 °C and Keq = 1.072 × 103 at 627 °C) of MnO reacted with H2S reduced with an increasing reaction temperature (MnO + H2S = MnS + H2O).24 When the desulfurization temperature is higher than 600 °C, the BSC declined slightly with the rise of the temperature plausibly because of the fact that the sulfidation reaction between metal oxides and H2S reduced, owing to the limitation of H2S diffusion. The decrease of SBET and VT for used 4Mn1Ce/HMS at 700 and 800 °C (Table 1) also confirmed the partial collapse of the mesoporous framework or the agglomeration of the active phase. In a word, we selected 600 °C as the best desulfurization condition. Influence of H2 Contents and Steam on the Performance of 4Mn1Ce/HMS. The influence of the H2 concentration on the performance of the 4Mn1Ce/HMS sorbent was shown in Figure 8. The result indicated that the shape of the breakthrough curve was quite smooth and the BSC was low (113.3 mg of S/g of sorbent) when there was no H2 in the feed gas. The breakthrough curves were quite sharp when the H2 concentration exceeded 10.6%, and the breakthrough sulfur capacity of the 4Mn1Ce/HMS sorbent was not significantly affected by the increase of the H2 concentration because of the high thermal stability of MnO and Ce2O3 species,38−40 which was quite different from the LaxFeyOz/MCM-41 sorbent as a result of the fact that FeOx was reduced easily but MnO was

Mn3O4 + H 2 = 3MnO + H 2O

(3)

MnO + H 2S = MnS + H 2O

(4)

CeO2 + 0.5H 2 = 0.5Ce2O3 + 0.5H 2O

(5)

Ce2O3 + H 2S = Ce2O2 S + H 2O

(6)

With regard to CeO2, the removal of oxygen vacancies and reduction effect (Ce4+/Ce3+) could enhance the H2S sorption rate over the sorbent.36,42 The incorporation of CeO2 in the sorbent favorably eliminated the sensitivity of the variation of the H2 concentration in the feed gas. As shown in Figure 8, if 7.0% steam was introduced to simulate hot coal gas, the breakthrough sulfur capacity over the 4Mn1Ce/HMS sorbent declined significantly because of the fact that the presence of steam will greatly suppress the occurrence of the aforementioned reactions (eqs 3−6). In addition, SBET and VT (Table 1) of the used 4Mn1Ce/HMS sorbent in the presence of steam declined slightly compared to the case without the presence of steam, indicating that the hydrothermal stability of the HMS support and the breakthrough sulfur capacity (55.1 mg of S/g of sorbent) over 4Mn1Ce/HMS is slightly higher than the result (50.2 mg of S/g of sorbent) over Mn12Ce3La mixed oxides.12 4Mn1Ce/HMS Sorbent Desulfurization Regeneration Cycles. The eight consecutive desulfurization−regeneration cycles were performed on the 4Mn1Ce/HMS sorbent to investigate the practical ability of the sorbent in industry on a large scale. As shown in Figure 9, the shapes of the breakthrough curves were almost the same for every regeneration cycle and the breakthrough sulfur capacity only had a slight fluctuation (101.0−112.7 mg of S/g of sorbent). The decrease of the sulfur capacity was attributed to the crystallite growth or sintering in sorbents, and the XRD results also verified that the average crystalline sizes of Mn3O4 and CeO2 decreased from 6.8 to 9.2 nm and from 8.2 to 9.6 nm, respectively, after the eighth regeneration cycle. In the meantime, the breakthrough sulfur capacity (112.7 mg of S/g of sorbent) was slightly lower than that over the fresh one F

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the joint financial support of the National Natural Science Foundation of China and BAOSTEEL Group Corporation (Grant 50876122).

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Figure 9. H2S breakthrough curves over desulfurization−regeneration cycles over 4Mn1Ce/HMS Sulfurization: 600 °C; GHSV, 2 × 104 mL h−1 g−1; and feed composition, 0.33% H2S, 10.6% H2, 18% CO, and N2 as balance gas. Regeneration: 700 °C; GHSV, 1.65 × 104 mL h−1 g−1; and 5% O2/N2 mixture).

(121.7 mg of S/g of sorbent) and significantly different from Mn-based/SBA-15 and Mn-based/M41 sorbents because the sulfur capacity of the latter reduced remarkably after the first regeneration cycle.24,25 According to the analysis of the H2S concentration (∼100 mg/m3) before breakthrough time, the efficiency (∼96%) of H2S removal over the 4Mn1Ce/HMS sorbent at 600 °C approached theoretical equilibrium desulfurization efficiency (∼98.5%) of MnO, which meets the requirement that 95% equilibrium desulfurization will be the minimum acceptable in industry.5,13 On the basis of the analysis of SAXRD and N2 adsorption, the sorbent still retained the mesoporous channel structure (Figure 1d) and a large surface area (Table 1) after the eighth regeneration cycle. The result demonstrated that 4Mn1Ce/HMS was a good sorbent with complete regeneration ability and was potentially promising for application in the future.

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CONCLUSION xMnyCe/HMS sorbents with wormhole-like structure were synthesized for hot coal gas desulfurization. The fresh, used, and regenerated samples were characterized by means of XRD, N2 adsorption, FTIR absorption spectroscopy, and HRTEM techniques. High sulfur capacity (121.7 mg of S/g of sorbent) and utilization (82.4%) of the 4Mn1Ce/HMS sorbent at 600 °C were achieved as a result of the high stable structure of wormhole-like mesoporous HMS and synergetic effects of manganese and ceria oxide. The 4Mn1Ce/HMS sorbent exhibited complete regeneration ability, and no obvious deactivation was observed after eight consecutive desulfurization−regeneration cycles. The XRD patterns revealed that the active species of Mn3O4 and CeO2 were dispersed highly on the HMS support in the 4Mn1Ce/HMS sorbent and no MnSO4 phase formed during the desulfurization/regeneration cycles, which was also confirmed by the FTIR absorption spectroscopy technique. The results of SAXRD, N2 adsorption, and HRTEM images showed that the surface area of the sorbent declined slightly after the eighth regeneration cycle but the wormholelike structure still remained intact in the 4Mn1Ce/HMS sorbent after sulfidation.

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