SBA-15 sorbents for desulfurization of hot coal gas

Journal of Hazardous Materials 233–234 (2012) 219–227

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Highly stable and regenerable Mn-based/SBA-15 sorbents for desulfurization of hot coal gas F.M. Zhang, B.S. Liu ∗ , Y. Zhang, Y.H. Guo, Z.Y. Wan, Fazle Subhan Department of Chemistry, Tianjin University, Tianjin 300072, PR China

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

 A series of mesoporous Cux Mny Oz /SBA-15 sorbents were fabricated for hot coal gas desulfurization.  1Cu9Mn/SBA-15 sorbent with high breakthrough sulfur capacity is high stable and regenerable.  Utilization of SBA-15 constrained the sintering and pulverization of sorbents.

a r t i c l e

i n f o

Article history: Received 11 April 2012 Received in revised form 3 July 2012 Accepted 5 July 2012 Available online 14 July 2012 Keywords: Hot coal gas Desulfurization Manganese oxide Thermal stability Mesoporous SBA-15

a b s t r a c t A series of mesoporous xCuyMn/SBA-15 sorbents with different Cu/Mn atomic ratios were prepared by wet impregnation method and their desulfurization performance in hot coal gas was investigated in a fixed-bed quartz reactor in the range of 700–850 ◦ C. The successive nine desulfurization–regeneration cycles at 800 ◦ C revealed that 1Cu9Mn/SBA-15 presented high performance with durable regeneration ability due to the high dispersion of Mn2 O3 particles incorporated with a certain amount of copper oxides. The breakthrough sulfur capacity of 1Cu9Mn/SBA-15 observed 800 ◦ C is 13.8 g S/100 g sorbents, which is remarkably higher than these of 40 wt%LaFeO3 /SBA-15 (4.8 g S/100 g sorbents) and 50 wt%LaFe2 Ox /MCM41 (5.58 g S/100 g sorbents) used only at 500–550 ◦ C. This suggested that the loading of Mn2 O3 active species with high thermal stability to SBA-15 support significantly increased sulfur capacity at relatively higher sulfidation temperature. The fresh and used xCuyMn/SBA-15 sorbents were characterized by means of BET, XRD, XPS, XAES, TG/DSC and HRTEM techniques, confirmed that the structure of the sorbents remained intact before and after hot coal gas desulfurization. © 2012 Elsevier B.V. All rights reserved.

1. Introduction In recent decades, integrated gasification combined cycle (IGGC) [1] technology and solid oxide fuel cell (SOFC) unit [2] were developed for high effective utilization of coal. Coal-derived gas, containing a certain concentration of sulfur compounds, nitrogen oxides, etc., cannot be applied in generating electricity directly. Sulfur compounds in hot coal gas not only result in the corrosion of pipe line and turbine, but also induce air pollution. So it is required to remove the sulfur compound from coal gas. Recently,

∗ Corresponding author. Tel.: +86 22 27892471; fax: +86 22 87892946. E-mail address: [email protected] (B.S. Liu). 0304-3894/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2012.07.023

conventional methods, such as wet limestone [3] and ammonia scrubber desulfurization [4] are unfavorable for reducing operation capital from the economic point of view. Therefore, the oxides of Fe, Zn, Ca, Mg, and Co as regenerative sorbents for hot coal gas desulfurization are studied extensively in recent years [5–13]. However, sintering, mechanical pulverization and reductive properties at temperature higher than 700 ◦ C limited the application of aforementioned metal oxides. From the economic viewpoints, it is required to increase the desulfurization temperature since the generation and combustion of coal-derived gas is in the range of 750–1000 ◦ C. Therefore, the studies on metal oxide sorbents which can be used at high temperature (>600 ◦ C) have attracted considerable attention of scientific researchers in the world.

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According to the report of Westmoreland and Harrison [5], manganese oxides are stable in the form of MnO at reductive atmosphere between 400 and 1000 ◦ C and have a superior initial desulfurization rate compared to oxides of V, Ca, and Zn [6]. Similarly, Slimane and Hepworth [14,15] also found that manganese-based sorbents presented high initial sorption rate, high sulfur capacity and good regenerative ability during hot coal gas desulfurization at 700–1000 ◦ C but with high H2 S equilibrium concentration (around 150 ppmv). Therefore, a variety of oxides, such as zinc oxide [16–19], iron oxides [18,20–22], copper oxides [19,21,23,24], and vanadium oxides [25,26] were incorporated into manganese oxides in order to increase the H2 S removal efficiency. However, the application of zinc and ferric oxides is limited to a maximum temperature of 600 ◦ C due to zinc evaporation and the reduction of ferric oxides at reductive atmosphere. The incorporation of copper oxides can enhance the dispersion of manganese oxides and improve the desulfurization performance of Mn-based adsorbents with H2 S pre-breakthrough concentration of 50 ppmv [19,21,23,24]. Furthermore, copper oxides can also maintain its desulfurization ability at above 800 ◦ C [5,27]. Therefore, the development of sorbents is focusing on the Cu and Mn mixed oxides with high sulfur capacity in the range of higher than 600 ◦ C. In addition, in order to resolve the scientific problems, such as sintering, low utilization of active components and mechanical attrition of sorbents, the metal oxides supported on zeolite [28,29], TiO2 [30] and ␥-Al2 O3 [18] indeed increased the stability of sorbents but decreased sulfur capacity. The decline of the surface area and porosity are unavoidable during successive desulfurization–regeneration cycles. Considering that SBA-15 and MCM-41 presented good thermal stability with high surface area and large pore volumes and were widely used in many domains [31–34], we reported Lax Mey Oz (Me = Fe, Co, Zn)/MCM-41 or SBA-15 desulfurization of hot coal gas in middle temperature (550–600 ◦ C) [7,8], which showed good regeneration stability and diffusion rate of gas. Furthermore, the CuO/SBA-15 or MCM-41 [35–37] also exhibited excellent performance for hot coal gas desulfurization. Therefore, in order to fabricate sorbents with high sulfur capacity and renewable ability at high temperature, a series of xCuyMn/SBA-15 were prepared due to the relative high hydrothermal stability of SBA-15 and their desulfurization activities were investigated. The properties of fresh and used xCuyMn/SBA-15 sorbents were characterized by means of nitrogen adsorption (BET), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM) and thermogravimetry/differential scanning calorimetry (TG/DSC).

2. Experimental 2.1. Preparation of SBA-15 and sorbents SBA-15 was prepared according to method reported by Zhao et al. [38]. First, 4 g of Pluronic P123 (Aldrich Co. MW 5800) was dissolved in 150 mL HCl (1.6 mol/L) with stirring at 40 ◦ C. 9 mL of TEOS was added dropwise with consistently stirring for another 20 h. The obtained mixture was transferred into the Teflon-lined stainless steel autoclave and treated at 100 ◦ C for 24 h. Then, the white precipitate was filtered, washed with deionized water (DW), dried at room temperature (RT) for two days and finally calcined at 550 ◦ C for 5 h in flowing air (500 mL/min). All sorbents were prepared by a sol–gel method. Taking the preparation of 1Cu9Mn/SBA-15 sorbent as an example, 10.50 g of 50 wt%Mn(NO3 )2 solution and 0.79 g of Cu(NO3 )2 ·3H2 O were dissolved in 25 mL DW followed by addition of 5 mL HNO3 (6 mol/L) in the solution. After the addition of citric acid with molar amount of 1.5 times that of total metal ions, 2.5 g of as-prepared SBA-15

was added to aforementioned solution. The mixture was kept at 60 ◦ C with constant stirring until it became a viscous gel and then dried at RT for two days. Finally, the samples were calcined in air at 550 ◦ C for 6 h. The obtained xCuyMn/SBA-15 sorbents with different Cu/Mn atomic ratios are denoted as 5Cu5Mn/SBA-15, 3Cu7Mn/SBA-15, 1Cu9Mn/SBA-15 and 10Mn/SBA-15, respectively (the numbers before Cu and Mn represent the molar number in mixed metal oxides). The amount supported CuO and Mn2 O3 in SBA-15 is 50 wt%. 2.2. Characterization of sorbents Nitrogen adsorption isotherms of SBA-15, fresh and used sorbents were investigated at 77 K in a homemade system [39]. Prior to analysis, the sorbents were treated in vacuum at 200 ◦ C for 2 h. BET surface area, pore volume and average pore diameter were calculated using adsorption isotherm while pore size distribution was estimated by Barrett, Joyner and Halenda (BJH) method [39]. The structures of fresh and used sorbents were investigated by HRTEM on a Tecnai G2 F20 electron microscopy operated at 200 kV. The small-angle XRD patterns were recorded with a Rigaku D/max 2500 v/pc Automatic Diffractometer equipped with Ni filtered Cu K␣ radiation (20 kV, 30 mA). Wide-angle XRD patterns (10–70◦ ) were measured with a PANalytical Automatic Diffractometer using Ni-filtered Cu K␣ radiation ( = 0.15406 nm) at settings of 40 kV and 50 mA. The XPS signals were obtained with a PHI-1600 ESCA spectrometer equipped with Mg K␣ X-ray source (1253.6 eV). The binding energies (BEs) of the samples were calibrated with the contaminant C 1s line (284.6 eV). The Cu 2p XPS and Mn LMM Auger spectra were fitted by 80% Gaussian–Lorentzian method using XPSPEAK software (version 4.1, Chinese University of Hong Kong) due to poor signal-to-noise ratios. TG/DSC analysis for 1Cu9Mn/SBA-15 precursor and 1Cu9Mn/SBA-15 used at 800 ◦ C was carried out in air. Approximately 10 mg of sample was heated from 30 to 1100 ◦ C at the rate of 10 ◦ C/min and the data was obtained on a STA 409 PC/PG model instruments. 2.3. Performance of sorbents for hot coal gas desulfurization The desulfurization performance of the sorbents for hot coal gas was tested in a fixed-bed micro-reactor. The experimental set up was described in details elsewhere [8]. The inlet stream was controlled by mass flow controllers (D07-7B/ZM, Beijing Sevenstar Electronics Co., Ltd., China) at 165 mL/min consisted of 72% N2 , 10.5% H2 , 17.1% CO and 0.33% H2 S. Approximately 0.5 g sorbent was packed into a quartz reactor (i.d. 10 mm) and the reaction temperature was controlled by a K-type thermal couple. The sorbents were first heated to the reaction temperature in N2 at the rate of 10 ◦ C/min and the simulated coal gas was then introduced to the reactor for desulfurization. The concentration of H2 S in inlet and outlet gas was analyzed by iodometry method to obtain H2 S breakthrough curve and breakthrough sulfur capacity (the outlet H2 S concentration of more than 100 ppmv is defined as the breakthrough point). The performance of sorbents was evaluated by the effective sulfur capacity (SC) according to Eq. (1):



SC = WHSV ×

M × Vm





(Cin − Cout ) dt × 10−4

(1)

where SC represents the effective sulfur capacity (g S/100 g sorbents); WHSV is weight hour space velocity (L/(h g)); M is molecular weight of sulfur (g/mol); Vm is the molar volume of H2 S at standard pressure and 25 ◦ C (24.5 L/mol); Cin and Cout are the inlet and outlet concentration (ppmv) of H2 S, respectively; t is the breakthrough time for desulfurization (h).

F.M. Zhang et al. / Journal of Hazardous Materials 233–234 (2012) 219–227

400

A

C

B

0.20 0.18

300 500

b 400 200

c d

300

a

e

200

100

e1

b c d

0.14

e

0.10

e1 e2

0.08

e3 e4 d11

d 11

0.12

0.06

0.02 0.00 -0.02

0 0.0 0.2 0.4 0.6 0.8 1.0

0.16

0.04

e2 e3 e4

100

a

3

600

Pore volume(cm /Ag)

3

Relative Volume adsorbed ( cm /g)

700

221

0.0 0.2 0.4 0.6 0.8 1.0 10

Relative pressure ( p/p o)

100

Pore Size (an gstrom)

Fig. 1. (A and B) N2 adsorption isotherms and (C) pore size distributions of (a) SBA-15, fresh (b) 5Cu5Mn/SBA-15, (c) 3Cu7Mn/SBA-15, (d) 10Mn/SBA-15, and (e) 1Cu9Mn/SBA15 (e1 –e4 ) 1Cu9Mn/SBA-15 used at 700, 750, 800 and 850 ◦ C, respectively, and (d11 ) 1Cu9Mn/SBA-15 regenerated after nine cycles.

The effective utilization of sorbent can be defined as the ratio of SC with the theoretical sulfur capacity (TSC). In these experiments, no formation of elemental sulfur was observed in exit cold line, suggesting that no additional decomposition of H2 S happened during the removal of H2 S. Using the aforementioned reactor, the used sorbent was first heated to 800 ◦ C in N2 (100 mL/min) at the rate of 10 ◦ C/min and was then regenerated in 6% O2 /N2 mixture (165 mL/min) at 800 ◦ C until the concentration of SO2 in exit gas cannot be detected by KMnO4 solution as a indicator. 3. Results and discussion 3.1. Characterization of sorbents To understand the change in structure of sorbents during the desulfurization, nitrogen adsorption isotherms and pore size distributions of SBA-15, fresh, used and regenerated sorbents are shown in Fig. 1. Pure SBA-15 shows type-IV isotherm characteristic of mesoporous materials defined by IUPAC classification [40]. A sharp inflection at the relative pressure (p/p0 ) of 0.6–0.85 is in accord with capillary condensation within uniform mesopores (Fig. 1C). In addition to the strongly mesoporous (9.0 nm) and microporous (1.7 nm) distributions for SBA-15, there are small mesopore with approximately 2.5–4 nm as the bridge-type channels between adjacent mesopores (9.0 nm) [41]. The specific surface area (SBET ), total pore volume (Vt ), mesopore volume (Vmeso ), micropore volume (Vm ) and average pore diameter (Da ) of SBA-15, fresh and used sorbents are listed in Table 1. The SBET of SBA-15 with a total pore volume of 0.91 cm3 /g is 867 m2 /g. Compared to SBA-15, both SBET and Vt of fresh sorbents decline to 180–243 m2 /g and 0.26–0.29 cm3 /g, respectively, which originate from the decrease of the mesopores (Fig. 1B(b–e)) and disappearance of micropores due to occupation of the active particles in channels of SBA-15 (Fig. 1C(b–e)). However, the BET surface areas (180–243 m2 /g) of fresh xCuyMn/SBA-15 sorbents are still larger than that (45 m2 /g) of Cu–Mn–O fabricated using a complexation method by Karayilan et al. [25] due to the utilization of SBA-15.

As listed in Table 1, for 1Cu9Mn/SBA-15 after desulfurization at 700–850 ◦ C, the SBET and Vt decrease remarkably due to the fact that metal sulfides with larger molecular sizes replaced metal oxides in sorbents, meaning the blockage of partial microporous structure. In addition, the properties of xCuyMn/SBA-15 sorbents to certain extent depended on the Cu/Mn atomic ratios. It can be seen that the SBET and Vt of sorbents decline with the increment of Cu content due to the fact that the radius of Cu+ ions (96 pm) is larger than that of Mn2+ (80 pm). However, the SBET and Vt of 10Mn/SBA15 used at 800 ◦ C are slightly lower than those of 3Cu7Mn/SBA-15 and 1Cu9Mn/SBA-15 plausible due to the enhancement of sulfide molecules formed and high dispersion of Mn particles. However, the N2 adsorption isotherms of samples (Fig. 1B) indicate that the hexagonal mesoporous structure of SBA-15 still preserves after desulfurization. HRTEM images were taken to investigate the fine structures of fresh and used 1Cu9Mn/SBA-15. As shown in Fig. 2a, fresh 1Cu9Mn/SBA-15 exhibited ordered hexagonal arrays of 2-D mesoporous channels [38]. Due to high loadings of active species, there is slight aggregation of active particles outside channels. After desulfurization, the framework of 1Cu9Mn/SBA-15 still remained intact (Fig. 2c), indicating that this sorbent is of remarkable properties of resistance to high temperature (750–850 ◦ C) compared to LaMeOx based sorbents reported in literature [7,8]. The EDX analyses of fresh and used 1Cu9Mn/SBA-15 (Fig. 2b and d) revealed that the decline of oxygen contents and the occurrence of sulfur element in sorbent before and after desulfurization, meant the formation of metal sulfides. In addition, the aggregation of active particles seems more obvious after desulfurization. As shown in Fig. 3a, the small-angle XRD patterns of SBA-15 show three well resolved peaks at around 2 = 0.96◦ , 1.60◦ and 1.86◦ , which can be indexed as (1 0 0), (1 1 0) and (2 0 0) diffraction crystal planes of two-dimensional (2-D) hexagonal structure [38]. The introduction of metal oxides on SBA-15 does not destroy the hexagonal mesoporous structure of support (Fig. 3d). However, the drastic decrease in intensity of peaks at about 2 = 0.94◦ , 1.64◦ and 1.88◦ (Fig. 3d) verified the reduction of regularity degree in framework. Based on the d1 0 0 -spacing of fresh 1Cu9Mn/SBA15 crystal plane (Fig. 3d), the pore wall thickness (3.34 nm) of

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Table 1 Total pore volume (Vt ), mesopore volume (Vmeso ), micropore volume (Vm ), average pore diameter (Da ) and BET surface area (SBET ) of SBA-15, fresh and used xCuyMn/SBA-15. Sample

Vt (mm3 /g)

Vmeso (mm3 /g)

Vm (mm3 /g)

Da (nm)

SBET (m2 /g)

SBA-15 5Cu5Mn/SBA-15 3Cu7Mn/SBA-15 1Cu9Mn/SBA-15 10Mn/SBA-15 S-700-1Cu9Mn/SBA-15 S-750-1Cu9Mn/SBA-15 S-800-1Cu9Mn/SBA-15 S-850-1Cu9Mn/SBA-15 S-800-3Cu7Mn/SBA-15 S-800-5Cu5Mn/SBA-15 S-800-10Mn/SBA-15 R-9-1Cu9Mn/SBA-15

910 260 290 260 270 240 170 170 130 160 64 95 25

640 200 220 190 210 200 130 150 130 131 56 81 18.5

270 60 70 70 60 40 40 20 0.14 29 8 14 6.5

2.1 2.5 2.5 2.1 3.0 2.7 2.7 4.8 4.0 4.6 3.1 3.2 3.9

867 206 239 243 181 178 126 70 65 69 41 60 13

Notes: 1Cu9Mn/SBA-15 used at 700 ◦ C is denoted as S-700-1Cu9Mn/SBA-15. R-9-1Cu9Mn/SBA-15 is denoted as 1Cu9Mn/SBA-15 regenerated after nine cycles.

1Cu9Mn/SBA-15 can be estimated from the difference between √ unit cell constant (a = 2d1 0 0 / 3) and the pore diameter (7.5 nm) (Fig. 1c), which is higher than the results (1.8 nm) of SBA-15, indicating that the introduction of metal oxides enhanced the wall thickness of SBA-15 and improved thermal stability of sorbents. The similar results were observed in literature [42,43]. As shown in Fig. 3d1 , 1Cu9Mn/SBA-15 sorbent after desulfurization at 800 ◦ C

still exhibited three peaks between 0.9◦ and 2◦ but the intensity for each peak became weak. It means that the mesoporous framework of SBA-15 deteriorates during the desulfurization. The wide-angle XRD patterns of fresh and used 5Cu5Mn/SBA-15, 3Cu7Mn/SBA-15, 1Cu9Mn/SBA-15 and 10Mn/SBA-15 are shown in Fig. 3. The well resolved diffraction peaks were assigned to metal oxides due to the high metal oxide loading in fresh

Fig. 2. HRTEM images and EDX analysis for (a and b) fresh and (c and d) used 1Cu9Mn/SBA-15 after desulfurization at 800 ◦ C.

F.M. Zhang et al. / Journal of Hazardous Materials 233–234 (2012) 219–227

(100)

223

(A)

(110) (200) a d d1

*5

1

2

3

4

5

6

7

8

9

10

2 Theta (degree)

(C)

SiO2

(B)

Cu2S

Mn2O3

MnS

Cu1.5Mn1.5O4 CuO

d11 b1

b c d e 10

c1 e1 d1 20

30

40

50

60

70 10

2 Theta(degree)

20

30

40

50

60

70

2 Theta (degree)

Fig. 3. (A) Small-angle XRD patterns of (a) SBA-15, (d) fresh and (d1 ) used 1Cu9Mn/SBA-15; (B and C) wide-angle XRD patterns of (b, b1 ) 5Cu5Mn/SBA-15, (c, c1 ) 3Cu7Mn/SBA15, (d, d1 ) 1Cu9Mn/SBA-15 (e, e1 ), 10Mn/SBA-15 and (d11 ) regenerated 1Cu9Mn/SBA-15 (subscription “1” denoted as used sorbents).

sorbents. No diffraction peak of silicates meant that no chemical reaction occurred between metal oxides and support. As for wide-angle XRD patterns of fresh sorbents (Fig. 3B), we observed the diffraction peaks of CuO (2 = 35.7◦ , 38.96◦ , 49.2◦ ) [PDF#011117] and Mn2 O3 (2 = 23.1◦ , 32.96◦ , 38.2◦ , 45.2◦ , 49.4◦ , 55.2◦ , 64.1◦ and 65.8◦ ) [PDF#65-7467]. The copper and manganese composite oxides existed in a spinel structure such as Cu1.5 Mn1.5 O4 (2 = 18.5◦ , 30.5◦ , 35.9◦ , 57.7◦ , 63.4◦ ) [PDF#70-0262]. After desulfurization, the CuO and Mn2 O3 were transformed into Cu2 S (2 = 27.8◦ , 32.1◦ , 46.1◦ ) [PDF#53-0522] and ˛-MnS (2 = 29.6◦ , 34.3◦ , 49.3◦ , 58.6◦ , 61.5◦ ) [PDF#65-0891], according to following equations: 4Cu1.5 Mn1.5 O4 + 9H2 S + 7H2 = 3Cu2 S + 6MnS + 16H2 O 2CuO + H2 S + H2 = Cu2 S + 2H2 O Mn2 O3 + 2H2 S + H2 = 2MnS + 3H2 O

to the aggregation of partial active particles during desulfurization and regeneration cycles. However, there is no remarkable diffraction peak of MnSO4 . The XPS and LMM spectra of Mn in fresh and used 1Cu9Mn/SBA15 are shown in Fig. 4. The XPS peaks at 641.6 and 653.4 eV (Fig. 4A) are assigned to Mn 2p3/2 and Mn 2p1/2 in Mn2 O3 , respectively, which is in agreement with the report of Han et al. [43,44]. According to the report of Polychronopoulou et al. [45], Mn 2p3/2 BEs (642.3–642.5 eV) in all fresh Fe–Mn–Zn–TiO solid corresponded to Mn4+ , which was in harmony with the presence of the ZnMnO3 phase. After sulfidation, the Mn4+ in ZnMnO3 was reduced into Mn3+ at 25–100 ◦ C based on following equation (reduction by H2 S):

(I) (II) (III)

After used 1Cu9Mn/SBA-15 was regenerated (Fig. 3B(d11 )), there is no significant variation in structure compared to fresh sorbent except that corresponding diffraction peaks intensified owing

2ZnMnO3 + 6H2 S = Mn2 S3 + S + 2ZnS + 6H2 O Therefore, Mn 2p3/2 BEs equal to 641.6 eV, different from our result abovementioned due to different calcined and sulfidation temperature. In the meantime, the ESR analysis of Dhage et al. [46] also revealed that reduction of Mn3+ to Mn2+ reflected the reaction with H2 S not with H2 at below 100 ◦ C. However, we find that there is no

F.M. Zhang et al. / Journal of Hazardous Materials 233–234 (2012) 219–227

b 653.4

b

8

(A)

0

DTG

80 60

293 275

Weight(%)

4 0 -4 -8 -12

100

(B)

40 140

a

275

TG

310 377

502

12

638

DSC

120 100

8 4

TG

0 0

582.4

846

(C)

Derivative weight (mg/min)

641.6

DSC

16

293

653.7

310

24

Heat Flow (mW/mg)

583.6

exo

Mn LMM

exo

641.6

Mn 2p

Weight(%) Heat Flow(mW/mg)

224

100 200 300 400 500 600 700 800 900 10001100 o

Temperature( C)

a

(A) 670

660

650

640

Fig. 6. TG/DTG and DSC curves for (A and B) 1Cu9Mn/SBA-15 precursors before calcinations and (C) used 1Cu9Mn/SBA-15 after desulfurization at 800 ◦ C.

(B) 590

630

Binding energy (eV)

580

570

560

550

Kinetic energy(eV)

shifts toward lower BEs (932.3 eV), exhibiting the characteristic XPS spectrum of Cu+ in Cu2 S [50]. In addition, two bread-type peaks at 161.8 and 168 eV can be attributed to the signals of S 2p3/2 in sulfides and sulfates, respectively (magnification curves in Fig. 5B), similar to the report by Liu et al. [8]. The BEs of O 1s for fresh samples at 532.9 eV and 529.6 eV (Fig. 5C) are assigned to oxygen in SiO2 [51], CuO [52] and Mn2 O3 [48], respectively. After desulfurization, the diminution of the O 1s peak at 529.6 eV indicated the transformation of CuO and Mn2 O3 to Cu2 S and MnS in hot coal gas desulfurization [21]. TG/DSC analysis of precursor (before-calcinations) and used 1Cu9Mn/SBA-15 at 800 ◦ C is shown in Fig. 6. The weight loss at 275 ◦ C in DTG curve is due to the dehydration of gel [53], corresponding to the occurrence of endothermic peak at 275 ◦ C in DSC curve (Fig. 6A). There are the other two peaks of weight loss in the range of 286–350 ◦ C and two corresponding exothermic peaks

Fig. 4. (A) Mn 2p and (B) Mn LMM spectra of (a) fresh and (b) used 1Cu9Mn/SBA-15 after desulfurization at 800 ◦ C.

variation in BEs of Mn 2p3/2 after desulfurization [47]. Therefore, the more sensitive Auger lines of Mn LMM (Mg anode) over fresh and used sorbents are also collected to confirm the valence state of Mn. The reported kinetic energy (KE) value for Mn0 is 586.6 eV [48]. As shown in Fig. 4B, KE of the Auger peak observed over fresh 1Cu9Mn/SBA-15 is 582.4 eV whereas that over used one is 583.6 eV. A shift of 1.2 eV toward high KE region suggests the reduction of Mn3+ to Mn2+ during desulfurization. The XPS spectra of Cu 2p, S 2p and O 1s are shown in Fig. 5. The fresh 1Cu9Mn/SBA-15 presented Cu 2p3/2 peak at 933.7 eV (Fig. 5A(a)), which is characteristic of Cu2+ in CuO [49]. Over used sample, the main peak of Cu 2p3/2

932.3

b

Si 2s 154.4

S 2p

933.7

a*5

a

529.6

b b (A) 950

940

930

O 1s

161.8

b*5

960

532.9

168.0

Cu 2p

a (B)

a 170

165

160

155 540

(C) 535

530

525

520

Binding Energy (eV) Fig. 5. (A) Cu 2p, (B) S 2p, and (C) O 1s XPS spectra of (a) fresh and (b) used 1Cu9Mn/SBA-15 after desulfurization at 800 ◦ C.

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are also observed in DSC curves. The similar phenomenon of the auto-catalyzed oxidation of citrate happened in the presence of nitrate has been reported in literature [54–56]. There is no significant weight loss above 350 ◦ C, indicating that the calcination temperature at 400–900 ◦ C for 1Cu9Mn/SBA-15 precursor is almost reasonable. As shown in Fig. 6C, an increase of 24.1% in weight between 250 and 700 ◦ C can be divided into three stages. In the first stage, the minor weight gain contributed to the formation of manganese oxysulfides [Mn(SO4 )0.5 O0.6 ] [57], corresponding to the exothermic peak at 377 ◦ C in DSC curve. In the second stage, the increase in weight is due to the formation of CuSO4 [58] and the successive oxidation of manganese oxysulfides to Mn(SO4 )0.6 O0.6 species [57] while there is a broad exothermic peak at 502 ◦ C. Finally, incremental weight can be ascribed to the formation of CuSO4 [58] and MnSO4 [57], correlating with exothermic peak at 638 ◦ C. Next, the weight of sample decreased gradually (ca. 27%) with incremental temperature due to the decomposition of CuSO4 and MnSO4 . 3.2. Effect of Cu/Mn atomic ratios on performance of sorbents The effect of different Cu/Mn atomic ratios in manganesebased sorbents on desulfurization performance is shown in Fig. 7. As can be seen that 10Mn/SBA-15 sorbent exhibited appropriate H2 S breakthrough time but low deactivation rate (Fig. 7a). This indicated that Mn2 O3 particles aggregated in 10Mn/SBA-15 and suppressed the diffusion of H2 S molecules from surface to bulk. We also observed that the diffraction peaks of Mn2 O3 in wide-angle XRD patterns became weak in intensity and width (Fig. 3) with the increment of Cu contents. The breakthrough time observed over different sorbents increased with the decline of copper loadings. These results indicate that the introduction of small amount of copper in Mn-based sorbents can improve the dispersion of Mn2 O3 particles and enhances the desulfurization efficiency. Among all four sorbents, the performance of 1Cu9Mn/SBA-15 is the best with the breakthrough sulfur capacity of 13.8 g S/100 g sorbents and the utilization rate of 71.54%, which is significantly higher than

that (ca. 5 g S/100 g sorbents) over 40 wt%LaFeO3 /SBA-15 reported previously [8]. However, the breakthrough sulfur capacity over xCuyMn/SBA-15 sorbents decline with incremental Cu loadings. This is due to the fact that copper oxide itself has the properties of low sulfur capacity. Karvan and Atakül [35] and Ozaydin et al. [36] reported that both CuO/SBA-15 and CuO/MCM-41 sorbents exhibited low sulfur capacity (1.72 and 3.27 g S/100 g sorbents) and utilization rate of active species (16.9% and 31.3%), respectively. Furthermore, the operational temperatures for these sorbents also are low (500–515 ◦ C). 3.3. Effect of desulfurization temperature on the performance of 1Cu9Mn/SBA-15 To investigate the effect of reaction temperature on the performance of sorbents, the H2 S breakthrough curves over 1Cu9Mn/SBA-15 at different temperatures are shown in Fig. 8. The breakthrough sulfur capacity over 1Cu9Mn/SBA-15 increases with the increase of desulfurization temperature in the range of 700–800 ◦ C (inset in Fig. 8). This is because the initial desulfurization rate or sulfur uptake capacity increased with incremental temperature [6] despite the fact that from the thermodynamic viewpoints, the equilibrium constant (activity) of manganese oxides and H2 S reaction decreased when the temperature increased from 400 to 1127 ◦ C [59]. According to reports in literature [7,8,18,20], the manganese-based sorbents exhibited high efficiency of H2 S removal even at temperature higher than 800 ◦ C. We observed that the sulfur removal efficiency over 1Cu9Mn/SBA-15 decreased slightly at 850 ◦ C, plausible due to the partial collapse of mesoporous framework or the aggregation of Mn particles in sorbent. The SBET and Vt of 1Cu9Mn/SBA-15 sorbent before and after sulfidation at 850 ◦ C also declined remarkably (Table 1). Furthermore, there was a gradual decline in SBET and Vt of 1Cu9Mn/SBA-15 and the increase of average pore diameters with incremental desulfurization temperature (700–850 ◦ C), this meant that the walls of partial mesoporous channels collapsed or pulverized at 850 ◦ C. However, HRTEM image of sample used at 800 ◦ C revealed that

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Fig. 8. H2 S breakthrough curves for 1Cu9Mn/SBA-15 sorbents at different temperatures. WHSV = 19,800 mL/(h g), feed composition: 72% N2 , 10.5% H2 , 17.1% CO, and 0.33% H2 S.

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mesoporous structure of sorbents remain intact (Fig. 3). In a word, 800 ◦ C is considered as the more appropriate temperature during hot coal gas desulfurization.

3.4. Investigation on desulfurization and regeneration of sorbents In order to understand deeply the regeneration stability, mechanical strength and sintering-resistant performance of 1Cu9Mn/SBA-15 sorbent, successive nine desulfurization regeneration studies were carried out at 800 ◦ C. As shown in Fig. 9, at initial desulfurization–regeneration stage, the breakthrough sulfur capacity observed over 1Cu9Mn/SBA-15 is 13.8 g S/100 g sorbents, which is remarkably higher than these over 40 wt%LaFeO3 /SBA-15 [8] (4.8 g S/100 g sorbents) and 50 wt%LaFe2 Ox /M41 (5.58 g S/100 g sorbents) [7], and the latter is used only at 500–550 ◦ C. This indicated that the addition of Mn active species in mesoporous SBA-15 support increased significantly sulfur capacity of sorbent and improved the sulfidation temperature. The incorporation of small amount of Cu species into Mn/SBA-15 also increased the breakthrough sulfur capacity of sorbent. According to the report of Karayilan et al. [25], the breakthrough capacity over equimolecular Mn–Cu oxide sorbent with the SBET of 45 m2 /g is 10.5 g S/100 g sorbent, which is remarkably lower than the result (27.5 g S/100 g active species) estimated based on pure active species. This suggested that the employment of SBA-15 with high SBET increased greatly the utilization rate of Mn–Cu components. However, as shown in Fig. 9, the breakthrough sulfur capacity over 1Cu9Mn/SBA-15 sorbent declined gradually with incremental number of regeneration. This is attributed to the decline in SBET of sorbents (Fig. 1B(d11 )) and the aggregation or sintering of partial Mn–Cu particles supported on SBA-15. The XRD results confirmed that the diffraction peaks of Mn2 O3 in intensity enhanced after the regeneration of used 1Cu9Mn/SBA-15 (Fig. 3d11 ). After successive nine desulfurization and regeneration cycles, the breakthrough sulfur capacity over 1Cu9Mn/SBA-15 still is 10.2 g S/100 g sorbents, meaning that approximately 74% initial activity of sorbents can be retained. These results demonstrate that 1Cu9Mn/SBA-15 is a good regenerable sorbent with large sulfur capacity.

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Duration time (min) Fig. 9. Successive nine desulfurization–regeneration cycles over 1Cu9Mn/SBA-15 sorbent. Desulfurization: 800 ◦ C; WHSV = 19,800 mL/(h g), feed composition: 72% N2 , 10.5% H2 , 17.1% CO, 0.33% H2 S. Regeneration: 800 ◦ C; WHSV = 19,800 mL/(h g); feed composition: 6% O2 /N2 . Inset is breakthrough sulfur capacity.

4. Conclusions A series of xCuyMn/SBA-15 sorbents with high SBET were prepared by sol–gel method. By the optimization of the Cu/Mn atomic ratios and the reaction temperature, 1Cu9Mn/SBA-15 performed the best at 800 ◦ C with effective sulfur capacity of 13.8 g S/100 g sorbents and high utilization (71.54%) of active species. The BET, HRTEM and small-angle XRD characterizations verified that the mesoporous structure of sorbents remained intact with minor decline of SBET after sulfidation and regeneration process. The wide-angle XRD patterns of xCuyMn/SBA-15 revealed that the incorporation of a small amount of copper to Mn/SBA-15 suppressed the aggregation of Mn2 O3 particles. The results of successive nine desulfurization and regeneration cycles at 800 ◦ C showed that 1Cu9Mn/SBA-15 sorbent still kept 74% of its initial sulfur capacity. Acknowledgments We thank the financial support of National Natural Science Foundation of China and BAOSTEEL Group Corporation (Grant No. 50876122). We are grateful to the Analysis Center of Tianjin University for XRD, XPS, TG/DSC and HRTEM characterizations of samples. References [1] Y. Huang, S. Rezvani, D. Mcllveen-Wright, A. Minchener, N. Hewitt, Technoeconomic study of CO2 capture and storage in coal fired oxygen fed entrained flow IGCC power plants, Fuel Process. Technol. 89 (2008) 916–925. [2] R.A. George, Status of tubular SOFC field unit demonstrations, J. Power Sources 86 (2000) 134–139. [3] R. Álvarez-Rodríguez, C. Clemente-Jul, Hot gas desulphurization with dolomite sorbent in coal gasification, Fuel 87 (2008) 3513–3521. [4] J. Yan, L. Yang, J. Bao, et al., Impact property on fine particles from coal combustion in ammonia flue gas desulfurization process, Proc. CSEE 29 (2009) 21–26. [5] P.R. Westmoreland, D.P. Harrison, Evaluation of candidate solids for hightemperature desulfurization of low-btu gases, Environ. Sci. Technol. 10 (1976) 659–661. [6] P.R. Westmoreland, J.B. Glbson, D.P. Harrison, Comparative kinetics of high temperature reaction between hydrogen sulfide and selected metal oxides, Environ. Sci. Technol. 11 (1977) 488–491. [7] Z.Y. Wan, B.S. Liu, F.M. Zhang, X.H. Zhao, Characterization and performance of Lax Fey Oz /MCM-41 sorbents during hot coal gas desulfurization, Chem. Eng. J. 171 (2011) 594–602. [8] B.S. Liu, X.N. Wei, Y.P. Zhan, R.Z. Chang, F. Subhan, C.T. Au, Preparation and desulfurization performance of LaMeOx /SBA-15 for hot coal gas, Appl. Catal. B: Environ. 102 (2011) 27–36. [9] D. Wang, J. Yu, L. Chang, D. Wang, Effects of addition of Mo on the sulfidation properties of Fe-based sorbents supported on fly ash during hot coal gas desulfurization, Chem. Eng. J. 166 (2011) 362–367. [10] X. Bu, Y. Ying, C. Zhang, W. Peng, Research improvement in Zn-based sorbent for hot gas desulfurization, Powder Technol. 180 (2008) 253–258. [11] W. Bao, Z. Zhang, X. Ren, F. Li, L. Chang, Desulfurization behavior of iron-based sorbent with MgO and TiO2 additive in hot coal gas, Energy Fuels 23 (2009) 3600–3604. [12] H. Fan, K. Xie, J. Shangguan, F. Shen, C. Li, Effect of calcium oxide additive on the performance of iron oxide sorbent for high-temperature coal gas desulfurization, J. Nat. Gas Chem. 16 (2007) 404–408. [13] H.K. Jun, J.H. Koo, T.J. Lee, S.O. Ryu, C.K. Yi, C.K. Ryu, J.C. Kim, A study of Zn–Tibased H2 S removal sorbents promoted with cobalt and nickel oxides, Energy Fuels 18 (2004) 41–48. [14] R.B. Slimane, M.T. Hepworth, Desulfurization of hot coal-derived fuel gases with manganese-based regenerable sorbents. 1. Loading (sulfidation) tests, Energy Fuels 8 (1994) 1175–1183. [15] R.B. Slimane, M.T. Hepworth, Desulfurization of hot coal-derived fuel gases with manganese-based regenerable sorbents. 3. Fixed-bed testing, Energy Fuels 9 (1995) 372–378. [16] L. Alonso, J.M. Palacios, Performance and recovering of a Zn-doped manganese oxides as a regenerable sorbent for hot coal gas desulfurization, Energy Fuels 16 (2002) 1550–1556. [17] T.H. Ko, H. Chu, Y.J. Liou, A study of Zn–Mn based sorbents for the hightemperature removal of H2 S from coal-derived gas, J. Hazard. Mater. 147 (2007) 334–341. [18] J. Zhang, Y. Wang, R. Ma, D. Wu, A study on regeneration of Mn–Fe–Zn–O supported upon ␥-Al2 O3 sorbents for hot gas desulfurization, Fuel Process. Technol. 84 (2003) 217–227.

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