MSU-H sorbents for hot coal gas desulfurization

Accepted Manuscript Title: High H2 O-resistance CaO-MnOx /MSU-H sorbents for hot coal gas desulfurization Author: Hong Xia Bingsi Liu PII: DOI: Reference:

S0304-3894(16)30983-9 http://dx.doi.org/doi:10.1016/j.jhazmat.2016.10.058 HAZMAT 18139

To appear in:

Journal of Hazardous Materials

Received date: Revised date: Accepted date:

14-7-2016 19-9-2016 24-10-2016

Please cite this article as: Hong Xia, Bingsi Liu, High H2O-resistance CaOMnOx/MSU-H sorbents for hot coal gas desulfurization, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2016.10.058 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

High H2O-resistance CaO-MnOx/MSU-H sorbents for hot coal gas desulfurization Hong Xia and Bingsi Liu*

Department of Chemistry, School of Science, Tianjin University and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China.

*

Corresponding author, Tel. 86-22-27892471, Fax 86-22-27403475, E-mail: [email protected] (B.S. Liu)

1

Graphical abstract

Highlights: 

CaO-MnOx/MSU-H exhibited high capacity and endurable regeneration ability.



Utilization of MSU-H improved dispersion of particles and diffusion rate of gas.



Ca species played dual roles for high dispersion of Mn2O3 and effective sorption of H2S/H2O.



90Mn10Ca/MSU-H sorbent presented outstanding H2O-resistance ability.

Abstract: A series of xMnyCa/MSU-H sorbents with various Mn/Ca molar ratio were first designed and synthesized with a sol-gel method. The desulfurization performance of the new sorbent was investigated at 600-800 oC in hot coal gas. 90Mn10Ca/MSU-H exhibited better desulfurization performance at 750 oC with a breakthrough sulfur capacity (BSC) of 18.69 g S/100 g sorbent compared to other supported Mn-based sorbents (13.2 g S/100 g sorbent) in similar desulfurization condition, and strong durability in multiple sulfidation-regeneration cycles using oxidation/reduction regeneration method which resolved the scientific issue of that CaSO4 is hardly 2

decomposed to CaO. The introduction of Ca species effectively promoted the dispersion of active constituents, which improved the desulfurization activity. More importantly, 90Mn10Ca/MSU-H showed excellent H2O-resistance ability due to the fact that CaO enhanced the sorption of H2O. Moreover, the utilization of MSU-H with large pore size and excellent thermal stability effectively assured fast mass-transfer and confined the migration of active particles, which led to long lifetime stability of sorbents.

Keywords: Desulfurization, Ca modified Mn-based sorbents, stable MSU-H, H2O-resistance, oxidation/reduction regeneration

1. Introduction Hydrogen sulfide (H2S) is one of the most noxious gases, which occurs naturally in water, crude oil as well as various industrial gases with its characteristic strong corrosiveness and odor[1]. It not only poses a threat to human health but also causes downstream catalyst poisoning and equipment corrosion in industrial processes[2]. Therefore, the removal of H2S is of great importance from the point of industrial catalytic and environment protection. Coal, as one of the primary energy sources, contains large amounts of sulfur, mostly in the form of H2S. Integrated gasification combined cycle (IGCC) is regarded as one of the most promising clean coal technologies and desulfurization is a critical process in this technology[3, 4]. To accomplish this task, many methods have been successfully employed for decades for H2S elimination. In view of thermal efficiency and capital costs, hot coal gas desulfurization is the optimal choice compared to other conventional desulfurization techniques. Metal oxides are the mainly active phases for hot coal gas desulfurization, such as zinc [5, 6], ferric [7-9], copper [10, 11] and molybdenum [12, 13]. These sorbents show high desulfurization efficiency and activity at lower than 600 °C. However, the metal oxides will be reduced into metallic states or form corresponding metal carbides 3

at higher than 700 °C, which is unfavourable for the removal of H2S. As reported by Westmoreland[14], manganese-based sorbents possess promising potential for desulfurization at 400-1000 °C, which have been widely researched because of high sulfur capacity and endurable regeneration ability for the last few decades[15-17]. In order to further improve desulfurization performance of manganese oxide, various metal oxides were doped in literatures [10, 12, 15, 18, 19]. But few can withstand or is effective for high temperature (700-850 °C) applications. Calcium oxide is stable at 800-1200 °C, which is generally used to adsorb SO2 [20]. Compared with other sorbents, calcium-based sorbents offer several advantages including great sulfur capacity, fast sulfidation rate, low cost and widespread availability such as limestone or dolomite. However, calcium oxide by itself suffers from the limitations for poor mechanical strength and regenerability. As a result, it has been widely regarded as a sorbent to be used once. To overcome these drawbacks, Jagtap[21] proposed regenerating calcium-based sorbents by alternately oxidizing and reducing the active constituent. Akiti et al. [22, 23] coated the calcium-based sorbent pellets with hydraulic cement to improve the mechanical properties of sorbents. Unfortunately, the coating layer tended to break apart when it was heated and reacted with H2S. Significantly, the introduction of calcium oxide into manganese-based sorbents presents synergetic effects, resulting in improving mechanical strength of the former and sulfur capacity of the latter, which is not reported in the literature so far. Moreover, this sorbent can be used at higher than 700 °C. Although the differences exist in composition, further improvement of desulfurization performance of pure metal oxides becomes difficult due to sintering, low dispersion and high gas diffusion resistance at high temperature [3]. The design and development of supported metal oxide sorbents is a powerful strategy. Mesoporous supports with high surface area and large pore volume have gained rapidly researchers’ attentions [2, 4]. Their unique pore architecture makes them very attractive for the high dispersion and long-term stabilization of nanometer-sized metal oxide particles. In general, large pore size and good pore interconnection can improve mass transfer and reduce pore blockage. 4

The steam resistance ability of sorbents should be taken into account due to significantly reduced desulfurization performance of sorbent by some water vapor in hot coal gas. However, the development of desulfurizer with high steam-resistance ability is still a challenging task. On account of the points mentioned above, a family of high-efficiency Mn-Ca sorbents based on cost-effective mesoporous silica (MSU-H) with large framework pore was originally designed. The sorbent exhibited the best desulfurization performance at 750 oC that was very close to the outlet temperature of coal gasification in IGCC. The addition of Ca to Mn-based sorbents played dual roles in improving effectively the dispersion of active components and increasing the surface basicity, which resulted in an improvement in desulfurization performance. In particular, the sorbent exhibited outstanding H2O-resistance ability due to the fact that Ca species enhanced the chemisorption of H2O. Besides, the regeneration ability of the sorbents was investigated using two types of regeneration methods. The properties of sorbents were characterized using BET, XRD, H2-TPR, CO2-TPD, HRTEM and ESR techniques. In view of experimental results, Mn-Ca/MSU-H sorbents were considered as the promising sorbents for the removal of H2S at high temperature.

2. Experimental section 2.1 Synthesis of supports MSU-H was prepared based on the procedure reported by Pinnavaia et al [24]. For the sake of synthesis cost, the synthesis was upgraded by using ordinary sodium silicate instead of expensive sodium silicate solution (e.g. 27% SiO2, 14% NaOH). 3.96 g of Pluronic P123 (Sigma-Aldrich) was dissolved in 66 mL of an aqueous solution of acetic acid (1.5 mol L−1). A solution of sodium silicate (9.20 g) in water (98 mL) poured into the surfactant solution under vigorous stirring. The mixture was heated at 60 oC for 20 h with stirring. The product was collected by filtration and calcined at 600 oC for 4 h. In order to investigate the hydrothermal stability of MSU-H, 0.5 g MSU-H was treated at 800 °C in 15% H2O/N2 mixture for 2 h. The treated support was characterized by nitrogen adsorption and Fourier transform infrared (FT-IR) 5

absorption spectroscopy. In contrast, FSM-16 was synthesized using cetyltrimethyl-ammonium bromide (CTAB) as a surfactant template and kanemite (δ-Na2Si2O5) as the silica source based on the method reported by Inagaki et al[25]. 2.2 Preparation of sorbents A series of fresh sorbents were prepared with a sol-gel method. This method involved the initial solvation of Mn(NO3)2 and Ca(NO3)2.6H2O to form a mixed salt solution, then citric acid with molar amount 1.5 times that of total metal ions was added to the solution to form metal complex. Next, a desired amount of MSU-H was introduced into the aforementioned solution. The mixture was stirred for 3 h to sure that the active particles entered fully the channels (with diameter of 9.8 nm) of MSU-H with the aid of capillary pressure. The sol was heated at 60 oC until it became a viscous yellow gel and then aged at room temperature (RT) for 3 days before dried at 60 °C. Finally, as-prepared sample was calcined in air at 550 oC for 6 h (Scheme 1). Similarly,

Mn/FSM-16,

Mn/MSU-H,

95Mn5Ca/MSU-H,

90Mn10Ca/MSU-H,

80Mn20Ca/MSU-H and 50Mn50Ca/MSU-H were prepared (the number in sorbents represented the molar ratio in metal oxides). The loading of active metal oxides for all samples was 50 wt%. 2.3 Characterization The Low-angle (2θ = 0.5-10o) X-ray diffraction (XRD) patterns were recorded with a Rigaku automatic diffractometer equipped with a Ni filtered Cu Kα radiation (λ = 0.15406 nm) source at settings of 20 kV and 30 mA. The wide-angle (2θ = 10-80o) XRD patterns were conducted on a PANalytical automatic diffractometer using Ni filtered Cu Kα radiation source (40 kV, 50 mA). The crystal sizes of active constituent were calculated with the width of diffraction profiles, referring to the full width of half-maximum (fwhm) of crystalline phase at (222) lattice plane of Mn2O3 and (211) lattice plane of Mn3O4, using the Debye-Scherrer formula: D

0.89  cos()

(1)

Where D is the crystal size, λ the wavelength of X-rays, Δ the fwhm of the diffraction 6

peak, and θ the diffraction angle corresponding to the peak. Nitrogen adsorption isotherms were carried out on a domestic adsorption system at 77 K [26]. The specific surface area, pore volume and average pore diameter were assessed from isotherm desorption branch by applying the Barrett-Joyner-Halenda (BJH) model. The morphology of supports were evaluated using a Tecnai G2 F20 transmission electron microscope (HRTEM) operating at 200 kV accelerating voltage. FT-IR spectra were collected using a BIO-RAD FTS 3000 spectrophotometer in the region of 400-4000 cm-1. H2-TPR for fresh sorbents and O2-TPO for used sorbents were conducted in a domestic system. More details regarding the operation have been described elsewhere[3]. Electron spin resonance (ESR) measurements were carried out at X-band (～9 GHz) on a Bruker-A300 spectrometer at room temperature. 2.4 H2S removal activity evaluation The desulfurization activity were evaluated at atmospheric pressure in a fixed-bed reactor (10 mm i.d.) in the range of 600-800 oC using simulated coal gas with or without steam (0.33% H2S, 10.5% H2, 18% CO and N2 balance). The gas flow rate was adjusted by a mass flow controller at a steady WHSV of 9 × 103 mL h−1 g−1. 0.5 g sorbent was packed into the reactor and the temperature was controlled by a K-type thermocouple. The concentration of H2S in inlet/outlet gas was analyzed using iodometric titration. The breakthrough curve was expressed as a plot of the outlet concentration of H2S versus time on stream. In addition, the breakthrough sulfur capacity (BSC), i.e. the amount of sulfur retained per unit mass of sorbent, was determined when the outlet concentration of H2S lowered than 50 mg·m-3 by the following formula:  g of sulfur  M S 22.4  t SC    Cin  Cout  dt  104   WHSV    0  100 g of sorbent V M   m H2S

(2)

Where SC represents the effective sulfur capacity of sorbent (g sulfur/100 g sorbent), WHSV is the weight hourly space velocity (L·h−1·g−1), Ms and MH2S are the molar weight of S (32.06 g·mol−1) and H2S (34.06 g·mol−1), respectively; Vm is the molar volume of H2S at 1 atm and 25 °C (24.5 L·mol−1), t is the reaction time for desulfurization (h), and Cin and Cout are the inlet and outlet concentration of H2S 7

(mg·m-3). 2.5 Regeneration tests The regeneration of used sorbents was performed in situ i.e., in the same experimental setup as the H2S removal tests (using two types of regeneration methods). They were diluted air regeneration (5% O2/N2) and the oxidation/reduction regeneration with O2/N2 and H2/N2. The sorbent was heated from 20 to 700 °C at 10 °C·min-1 with a N2 flow of 150 mL·min-1 which corresponded to a WHSV value of 9000 mL h−1 g−1. In diluted air regeneration process, 5 vol % O2/N2 mixtures were switched to regenerate the sulfided sorbent at 700 °C. In the oxidation/reduction regeneration process, the sulfided sorbent was first oxidized with 5 vol % O2/N2 and then reduced with 10 vol % H2/N2. When no sulfur element was formed or SO2 in outlet could not be detected using a KMnO4 solution, the regeneration was stopped. The regenerated sample was directly used for sulfidation test in the next cycle.

3. Results and discussion 3.1 Characters of support The N2 adsorption isotherms and small-angel XRD of MSU-H were shown in Fig. 1. Using the IUPAC classification, a type-IV sorption curve was characteristic of mesoporous materials (Fig. 1A). The step-like uptake of N2 in the range of p/p0 = 0.75-0.9 corresponded to capillary condensation within framework pores with a BJH diameter (Dp) at approximately 9.8 nm calculated from the adsorption branch of the N2 isotherms (Fig.1C), an indication of the uniform large channels existing within MSU-H. The corresponding textural properties, such as surface area, pore volume and average pore diameter were listed in Table 1. It indicated that MSU-H exhibited large surface area (831 m2 g-1) and pore volume (1.0 cm3 g-1). The powder X-ray diffractions pattern of MSU-H showed three typical diffraction peaks at 2θ = 0.5-2o, assigned to (100), (110) and (200) reflections of a two-dimensional (2-D) regular hexagonal arrangement of the channels (Fig. 1B) (space group P6mm)[24]. The pore wall thickness (h = 2.9 nm) of MSU-H was calculated by subtracting the maximum pore size (Dp= 9.80 nm) from the unit cell parameter (a0 = 2d100/30.5, based on the 8

d-spacing of (100) crystal plane), which was remarkably higher than that (1.6 nm) of MCM-41 and that (2.3 nm) of MSU-S[16, 27]. The result indicated that thermal stability of MSU-H was higher than that of above-mentioned mesoporous materials due to the fact that thermal stability depended strongly on the wall thickness[27]. In an attempt to evaluate its hydrothermal stability, N2 adsorption isotherms, the pore size distribution plots and FT-IR spectra over MSU-H treated in 15% H2O/N2 mixture (denoted as MSU-H-T) were also displayed in Fig. 1. The mesoporous character of MSU-H was preserved after steam treatment, as revealed by the type-IV isotherm. The amount of adsorbed N2 decreased slightly, and the pore size distribution shifted to a lower region, reflecting the shrinkage of mesoporous structure to a certain extent. Despite of the fact that there was a decrease of SBET and Vt relative to MSU-H without steam-treatment, the MSU-H-T still remained with high SBET, Vt and uniform pore size distribution (Table 1). The FT-IR spectra of MSU-H and MSU-H-T were shown in Fig.1D. The band at 3424 cm-1 was belong to the O-H asymmetrical stretching vibration of structural water, and band at 1624 cm-1 was associated with the H-OH bending vibration of H2O adsorbed in capillary pores and on the sorbent surface[15]. The bands at 1225 and 825 cm-1 were attributed to Si-O-Si symmetric stretching vibrations; the peaks at 1077 and 468 cm-1 were ascribed to the Si-O-Si asymmetric stretching vibrations; the weak band at 961 cm-1 corresponded to defect sites (=Si-OH)[18, 28, 29]. It was worth noting that, the absorption peaks in IR spectra of MSU-H-T were almost the same as that of MSU-H, which demonstrated that the types of functional groups remained unchanged during steam-treatment. In other words, the hexagonal pore structure of MSU-H-T was well maintained, it was similar to the aforementioned results in N2 adsorption isotherms. Therefore, it was quite apparent that hydrothermal stability of MSU-H could meet the requirements of hot coal gas desulfurization. For comparison, FSM-16 was also prepared and the characterization results were shown in Fig. S1. The N2 adsorption isotherm and XRD indicated that an ordered mesoporous structure of FSM-16 was formed. The SBET of FSM-16 was higher than that of MSU-H whereas the Dp (9.8 nm) of MSU-H support was much

9

larger than that (2.1 nm) of FSM-16. As shown in Fig. 2c and 2d, the HRTEM images of MSU-H revealed a clear lattice fringe. When the electron beam was parallel to pore direction, well-ordered hexagonal array of mesopores could be seen, indicating a highly ordered 2D pore structure. The distance between two dark strips was estimated to be about 8.6 nm, which was very close to the most probable pore diameter (Fig.1c). In addition, it could be clearly seen that the structure of MSU-H was similar to that of FSM-16 (Fig. 2), whereas MSU-H possessed larger pore diameter compared to FSM-16. 3.2 Structural characterization of fresh and sulfided sorbents The wide-angle XRD of fresh and used sorbents were displayed in Fig. 3. It could be seen that no characteristic diffraction peaks of silicate were observed over all sorbents, which indicated that the interaction between metal oxides and MSU-H was very weak. For fresh 100Mn/MSU-H or 100Mn/FSM-16, the diffraction peaks at 23.1o, 32.9 o, 38.2o, 45.1o, 49.2o, 55.1o and 65.6o were assigned to the reflection of cubic Mn2O3 phase [PDF#78-0390] accompanied with the formation of small quantities of tetragonal structural Mn3O4 (2θ = 64.6o) that may stem from the decomposition of MnO2 during the calcination at high temperature on the basis of the equilibrium diagram for the Mn-O system. However, the diffraction signals of Mn2O3 gradually decreased with the incremental Ca doping ratio. No reflection signals corresponding to CaO were observed in the XRD pattern of fresh 80Mn20Ca/MSU-H, indicating that Ca species were well dispersed as an amorphous phase or as nano-crystallites. As shown in Fig. 3d, the characteristic peaks of Mn2O3 almost disappeared and the signals at 2θ = 29.6 and 32.9o became obvious when Ca doping was up to 50 at %, suggesting that Mn2O3 might be dissolved out and CaMn2O4 [PDF#74-2293] was formed. Marokite CaMn2O4 is generally referred to as the post-spinel phase that crystallizes in an orthorhombic structure, which is stable in air at high temperature in the Ca-Mn-O system [30]. The average crystallite sizes of 100Mn/FSM-16, 100Mn/MSU-H, 90Mn10Ca/MSU-H and 80Mn20Ca/MSU-H were listed in Table 1. It was worth noting that the particle size (27.4 nm) over 100Mn/FSM-16 was the largest. For Ca doped sorbents, the crystalline size decreased 10

with the incremental Ca content. 90Mn10Ca/MSU-H exhibited the minimum particle size, which indicated that Ca species could effectively improve the dispersion of manganese oxides due to the segregation of doped Ca2+ ions at the grain boundary. After sulfidation, the diffraction peaks of metal oxides disappeared (Fig. 3B) in all used samples, which illustrated that the active phases completely reacted with H2S and were transformed into metal sulfides. The reflection signals at 2θ = 29.6, 34.3, 49.3 and 61.5o were assigned to the diffraction of MnS [PDF#72-1534]. Only weak CaS [PDF#75-0261] peaks at 2θ = 31.5, 45.1 and 56.0o could be detected over the used 50Mn50Ca/MSU-H (Fig. 3d1). The physical textural properties of fresh, used and regenerated sorbents were characterized by means of nitrogen physisorption and pore size distribution. As shown in Fig. 4A, the nitrogen adsorption isotherms over different fresh sorbents revealed that the incorporation of active phase did not destroy remarkably the mesoporous structure of support. In comparison to bare support, there was a decline in the amount of N2 adsorption. The calculated surface area (SBET) and pore volume (Vt) (Table 1) of fresh sorbents were much lower than those of bare support due to partial filling of the mesopores by the nanosized metal oxides. Even after 50 wt% metal oxides loading, 100Mn/MSU-H still held ca. 30% of the initial pore volume, while there was only 24% of initial Vt in 100Mn/FSM-16. The SBET and Vt of 90Mn10Ca/MSU-H were higher than those over 100Mn/MSU-H due to lower O2- (0.14 nm)/Ca2+ ion radio in CaO compared to Mn2O3 despite the difference of ionic radius between Ca2+ (0.1 nm) and Mn3+ (0.066 nm) ions [3]. The pore size distribution of 100Mn/MSU-H (Fig. 4C) showed a narrow distribution centred at 5.9 nm, which was much lower than that (9.8 nm) of MSU-H (Fig. 1d), meaning that Mn2+ or its complex entered successfully the channels of MSU-H to form Mn2O3 and Mn3O4 nanoparticles (Fig.3A). While over 90Mn10Ca/MSU-H, Dp (7.6 nm) was slightly lower than that of bare support, which suggested that Mn-Ca composite oxides were dispersed in very small nanoparticles[2]. When Mn/Ca ratio was at or above 20 at %, there was a drastic decline in both SBET and Vt due to the gradual formation of CaMn2O4 with large molecular size that occupied the partial channel. These results implied that when a small quantity of Ca 11

species was doped, the dispersion of active constituent was improved and the numbers of active sites were increased, which was favorable for the H2S removal. After sulfidation at 750 oC, metal oxides were transformed into metal sulfides, an expansion in the volume of active phase would be expected due to the different molar volume values of metal oxides and corresponding sulfides. Such expansion led to a significant decrease in SBET and Vt for used 90Mn10Ca/MSU-H, which was actually supported by the results in Table 1. Further evidence for an increase on the active phase volume was provided by the Dp (6.0 nm) of the sulfided sorbent (Fig. 4D). Moreover, both SBET and Vt further declined with the increase of reaction temperature. As reported by Zhang et al[31], high-temperature could give rise to serious shrinkage in the mesoporous framework, leading to smaller pore diameter and lower surface area. The partial agglomeration of active nanoparticles at high temperature is also an important reason. For the 90Mn10Ca/MSU-H regenerated at 700 oC after seven cycles, approximately 32% of the SBET was still maintained, suggesting the stability of sorbent, which was advantageous to the diffusion of gas and heat transfer in large nanochannels of the sorbent. As for the ability of resistance-reduction of the sorbents, as shown in Fig. 5, H2-TPR profiles revealed that there were two obvious reduction peaks in the range of 300-500 °C over different pure manganese supported sorbents, which were attributed to the two-step conversion procedure of Mn2O3 [32]. The following equations described the reduction reaction of the Mn oxides:

3Mn 2O3  H2  2Mn3O4  H2O Mn3O4  H2  3MnO  H2O

(3) (4)

According to the report of Arena et al[33], MnO could not be reduced to elemental manganese at a temperature lower than 1200 °C under reduction atmosphere. In comparison to that (369 and 443 °C) of 100Mn/FSM-16 with narrow nanochannels (Fig. 2b), the reduction temperatures of 100Mn/MSU-H were lower (Fig. 3b). It was well known that highly dispersed active particles were reduced more easily due to the fact that the number of active sites to react with H2 was enhanced. Therefore, the 12

dispersion of active particles in MSU-H with large mesopores was higher than that in FSM-16, which was verified by the particle sizes calculated via Scherrer equation (Table 1) despite of aggregation on outer surface. In addition, the reduction peaks of 90Mn10Ca/MSU-H slightly shifted toward lower temperatures compared with 100Mn/MSU-H, indicating that small amounts of Ca could further decrease the particle size of manganese oxides owe to the segregation action of CaO. However, the reduction signals shifted to high temperature when Ca doping ratio exceeded 20 at %, which suggested that excessive Ca species would repress the reduction of Mn2O3. The formation of Mn-Ca complex was considered to be the important reason. Compared with manganese oxide, the CaMn2O4 (Fig. 3A) was more difficult to be reduced. For 7th regenerated 90Mn10Ca/MSU-H, only a big reduction peak was observed (Fig. 5e), corresponding to the reduction of Mn3O4 to MnO (448 oC). This indicated that MnS was converted into Mn3O4 instead of Mn2O3 in the case of low oxygen content, which was further confirmed by the XRD results hereinafter. ESR spectroscopy was employed to survey the oxidation states of Mn species, which was more sensitive than XPS that was difficult to identify valence state of Mn before and after desulfurization due to the fact that there was very little distinction on binding energies between Mn3+ and Mn2+ ions as reported in our previous researches[3, 12]. Fig. 6 showed X-band ESR spectra of fresh, used and regenerated 90Mn10Ca/MSU-H. For fresh sorbent, no ESR signals were observed (Fig. 6a). The absence of any ESR signal for a Mn-containing sorbent was attributed to the presence of Mn3+ species, since Mn3+ was usually ESR silent owing to its large zero-field splitting. As reported in literature[34], both Mn2+ and Mn4+ species could give similar ESR spectra, in which Mn4+ showed g values less than 2.0, whereas Mn2+ had g value above 2. Mn species found in used and regenerated 90Mn10Ca/MSU-H sorbents had g values above 2, which were consistent with Mn(II) ion. As expected, Mn2O3 was transformed into MnS after desulfurization, which was also confirmed by XRD (Fig. 3B(b1)). However, the ESR signal of Mn2+ species in used sorbent had a Lorentzian lineshape, which was no longer resolved and formed a single broad line at g = 2.004 with a peak-to-peak line-width of about 387 G (Fig. 6b). It was different from that of 13

typical Mn(II) ESR spectrum that showed six hyperfine lines[35, 36]. This variation may be explained by either magnetic exchange interactions or an unresolved sextet with a small hyperfine coupling constant[34]. Regeneration did not change the signal shape, but the peak-to-peak line-width of the signal increased slightly to ca. 555 G. However, the line height decreased seriously compared with that of Mn2+ in used 90Mn10Ca/MSU-H, indicating the coexistence of Mn2+ and Mn3+ ions. Therefore, after regeneration, MnS was transformed into Mn3O4, which was also supported by XRD analysis (hereinafter Fig. 12). 3.3 Investigation on desulfurization activity Fig. 7a-b showed the H2S breakthrough curves over 100Mn/FSM-16 and 100Mn/MSU-H. The breakthrough time and utilization of active ingredients of 100Mn/MSU-H were larger than those of 100Mn/FSM-16. 100Mn/MSU-H exhibited higher BSC compared with 100Mn/FSM-16 (inset in Fig. 7). Even though FSM-16 owned larger surface area, MSU-H with large pore size was more advantageous to the dispersion of active particles inside the channels, which was confirmed by H2-TPR and XRD. It is well known that sulfuration reaction is a typical gas-solid non-catalytic reaction in which gas diffusion is a key factor to control the reaction. For 100Mn/FSM-16, Mn2O3 particles were more likely to block the small channels of FSM-16, which further hindered the mass transfer. Besides, the conversion of Mn2O3 to MnS was the larger S2- ions instead of O2- ions, leading to product layer thickening and diffusion resistance increased. Thus, large pore diameter was very favorable for improving dispersion of active phase and decreasing the diffusion resistance. Another possible cause of low BSC for Mn/FSM-16 was the deterioration of mesoporous structure of FSM-16 at 750 °C, which would also block the diffusion of H2S, leading to a decrease in desulfurization efficiency. As a result, high BSC was dependent not only on the metal oxides but also on superior support. MSU-H with large pore size and high hydrothermal stability was an excellent candidate. To assess the influence of calcium additives on the desulfurization performance, the breakthrough curves over sorbents with different Mn/Ca atomic ratios were shown in Fig. 7c-f. It could be seen that calcium oxides affected seriously the desulfurization 14

ability of Mn-based sorbent, and the curves after the breakthrough point became steeper than that over pure Mn/MSU-H. Interestingly, the BSC of Ca doped Mn/MSU-H increased initially and then declined as the Ca content increased beyond 10 at %. 90Mn10Ca/MSU-H exhibited the highest BSC (18.69 g sulfur/100 g sorbent) with a utilization of about 87%, which was significantly higher than those of rare earth oxides doped manganese oxide sorbents[3, 27, 37]. High dispersion of particles and incremental surface basicity of sorbents contributed to the enhancement in desulfurization performance. However, the BSC over 50Mn50Ca/MSU-H was very low (Fig. 7f) due to two reasons, on the one hand, the formation of the marokite-type oxide CaMn2O4 (Fig. 3A(d)), which is one of the most dense forms of the AB2O4 structure, on the surface of sorbent limited the diffusion of gas molecules; on the other hand, the more declines of SBET and Vt over 50Mn50Ca/MSU-H as well as the aggregation of surfacial particles (Table 1) also suppressed the sulfidation reaction efficiency between active sites within nanochannel and H2S. These decreased further the utilization of 50Mn50Ca/MSU-H sorbents. Based on the kinetic studies, sulfidation reaction is controlled by the reaction rate of H2S with active components (S/O exchange) and gas diffusion efficiency (gaseous reactant H2S and gaseous product H2O), while these factors depend critically on temperature[38, 39]. Hence, the effect of reaction temperature on desulfurization performance over 90Mn10Ca/MSU-H was studied. A set of H2S breakthrough curves and the corresponding BSC in the range of 600-800 °C was shown in Fig. 8. One could see that the breakthrough time was prolonged from 112 to 142 min as temperature increased from 600 to 750 °C. There was no doubt that high temperature was beneficial for S/O exchange and gas diffusion, which resulted in enhancement in desulfurization performance. However, when the sulfidation temperature reached 800 °C, the breakthrough time descended to 138 min. As listed in Table 1, with an increase in temperature, the SBET and Vt of used 90Mn10Ca/MSU-H decreased, implying slight sintering of sorbent. The sintering effect hindered severely gas diffuse into the inner part of the sorbent, leading to a decline in BSC. Thus, 750 °C was determined as the most feasible sulfidation temperature, which exhibited the best 15

combination of S/O exchange and gas diffusion. 3.4 Effect of composition gas on the H2S removal In coal gasification, the raw gas usually contained a large amount of reducing gas (H2 and CO) and partial steam, which depended on the type of coal and gasification conditions. These gases could cause a deterioration of desulfurization performance of sorbents. Hence, the H2O-resistance and reduction-resistance abilities of sorbents were assessed to explore the impact of steam and reducing gas on H2S removal over 90Mn10Ca/MSU-H. As shown in Fig. 9a-c, the presence of 5% steam in hot coal gas significantly shortened the breakthrough time and reduced to approximately 76.83 % of the initial BSC obtained in the absence of steam. According to the following reaction equation: Mex Oy (s)  yH2S(g)  yMe(x / y)S(s)  yH2O(g)

(5)

H2O, as a product of sulfidation reaction, drives the equilibrium to the left, which leads to a decrease in BSC. However, as the steam content increased from 5% to 15%, the BSC of 90Mn10Ca/MSU-H only decreased slightly. We found that the BSC (12.80 g S/100 g sorbent) observed in 15 % steam was much higher than that (6.61 g S/100 g sorbent) over Sm5Mn95/MSU-S and that (5.51 g S/100 g sorbent) over 4Mn1Ce/HMS

in

the presence of 7% steam

[27,

37], indicating that

90Mn10Ca/MSU-H presented an excellent H2O-resistance ability. This could be attributed to such a fact that MSU-H possessed an excellent hydrothermal stability and CaO could readily absorb moisture and be transformed into Ca(OH)2, which consumed water vapor in time and suppressed aforementioned sulfidation reaction (Eq. 5) to occur inversely. The sulfidation reactions of calcium oxide under the existence of steam were as follows: CaO + H2O = Ca(OH)2 Ca(OH)2 + H2S = CaS + 2H2O CaO + H2S = CaS + H2O

(6) (7) (8)

In addition, when the amount of CO and H2 was increased to 32 % and 39.6 %, respectively, BSC almost remained a constant. Although Mn2O3 was readily reduced 16

to oxides with low valence that had relatively poor desulfurization ability, the reverse water-gas shift reaction ( CO(g)  H2O(g)  CO2 (g)  H2 (g) ) was favorable for the sulfidation reaction, which compensated the decrease in sulfur capacity that was caused by the reducing action to some extent. In a word, 90Mn10Ca/MSU-H exhibited excellent H2O-resistance and reducing atmosphere-resistance abilities in the desulfurization process. This is a significant superiority for industrial applications. 3.5 Sorbent regeneration Diluted air regeneration and alternate oxidation/reduction regeneration were performed to investigate the regeneration performance of 90Mn10Ca/MSU-H. H2S breakthrough curves for the regeneration with diluted air (5% O2/N2) were shown in Fig. 10a. The BSC of the second and third cycles were 14.62 and 14.42 g S/100 g sorbent, which remained about 78.22 and 77.15 % of the initial sulfur capacity, respectively. In diluted air regeneration, the following reactions would occur: 4MnS + 7O2 = 2Mn2O3 + 4SO2

(9)

3MnS + 5O2 = Mn3O4 + 3SO2

(10)

MnS + 2O2 = MnSO4 CaS + 2O2 = CaSO4 CaS + 3/2O2 = CaO+SO2 CaO + SO2 +1/2O2 = CaSO4

(11)

(12) (13) (14)

The O2-TPO experiments confirmed the oxidation of MnS and CaS (Fig.11). Equation (9), (10) and (13) were the desired regeneration reactions, which translated the metal sulfides into metal oxides. However, sulfates (MnSO4 and CaSO4) were undesired by-products, which were formed on the outside surface of the nanoparticles and may inhibit the diffusion of O2, thereby restraining the regeneration of the interior region of the sorbent[20]. To identify specific compositions, the regenerated sorbent was determined by XRD as shown in Fig. 12A(b). It could be seen that there were the characteristic diffraction peaks of CaSO4, except for the phase of Mn2O3 and tetragonal structural Mn3O4. In addition, in corresponding FT-IR spectra (Fig. 12B(b)), the band at 957 cm-1 attributed to the adsorption of SO42- was observed, which further 17

verified the results of XRD. The formation of Mn3O4 was plausibly due to the low oxygen atmosphere[3]. The absence of manganese sulfate in XRD suggested that it could thoroughly decompose under the regeneration temperature (700 oC) as reported previously [12]. However, thermodynamically, the decomposition of CaSO4 requires a temperature as high as 1400 oC, which is not practicable for regenerating the used sorbent. The existence of inert CaSO4 would lead to a decrease in sulfur capacity, because calcium sulfate played only a structural accessory ingredient role in late sulfidation/regeneration cycles. Hence, the sulfided sorbent was regenerated incompletely with diluted air. In order to completely regenerate the used sorbent, reducing gas like CO or H2, which was beneficial to the decomposition of CaSO4 to CaO, was introduced into the regeneration process. By combining the reductive decomposition with the oxidation process, CaS could be regenerated rapidly and completely. In the oxidation/reduction regeneration process, the sulfided sorbent was first oxidized with 5% O2-N2. When all of the sulfides were converted to oxides or sulfates, 5% O2-N2 was switched to a mixture of 10 % H2-N2 for decomposing CaSO4. Fig. 10b showed that the BSC of the 2nd cycles was 16.08 g S/100 g sorbent, which was higher than that of 14.62 g S/100 g sorbent regenerated only by 5% O2-N2. Fig. 12A(c) illustrated the composition of sorbent after regeneration using oxidation/reduction operation. CaSO4 crystal phase disappeared, indicating that CaSO4 was converted to CaO. The regeneration reduction reaction is given as following: CaSO4 + H2 = CaO + SO2 + H2O

(15)

In addition, despite the fact that no diffraction peak of elemental sulfur was detected in XRD pattern (Fig. 12A), elemental sulfur was observed at the exit of the sorbent bed under the two regeneration processes and was verified by time of flight mass spectroscopy[40]. In order to more obviously illustrate the difference of two regeneration methods, 80Mn20Ca/MSU-H with high Ca species was used as regeneration test (Fig. 13). The BSC of the second and third cycles using oxidation/reduction regeneration were much higher than that only using diluted air regeneration. Therefore, oxidation/reduction 18

regeneration was an effective method to restore the used Ca-containing sorbents. For evaluating the regeneration stability, seven successive sulfidation/regeneration cycles over 90Mn10Ca/MSU-H sorbent were performed (Fig. 14). It could be observed that the breakthrough curves of regenerated sorbent were very close to that of fresh sorbent. The BSC after the 2nd cycles was highly stable with ca. 81% of the initial sulfur capacity (18.69 g S/100 g sorbent). Fig. 12A (c) showed that Mn3O4 was the main phase after seven sulfidation/regeneration cycles, which was different from the fresh sorbent, suggesting that Mn3O4 presented the most stable spinel structure in the regenerated sorbent. Besides, corresponding FT-IR spectra (Fig. 12B(c)) revealed the formation of a little amount of sulfates. According to the results calculated using Scherrer equation, the average crystalline sizes of Mn3O4 after seventh regeneration cycles was 20.2 nm, which was almost the same as the fresh one (18.4 nm). Despite a decrease in SBET and Vt, it was remarkably superior to that of 1Cu9Mn/SBA-15 and 5Sm95Mn/MSU-S [10, 27]. These results demonstrated that stable MSU-H could effectively reduce sintering and agglomeration of active species during the harsh sulfidation-regeneration cycles. As a result, 90Mn10Ca/MSU-H was a good regenerable sorbent, which offered promising solutions for industrial applications in the near future.

4. Conclusion A series of Ca modified Mn/MSU-H with different Mn/Ca molar ratios were synthesized by a sol-gel method. The desulfurization performance of this new sorbents was investigated at 600-800 oC. 90Mn10Ca/MSU-H exhibited the highest BSC of 18.69 g S/100 g sorbent. The introduction of Ca acted as a key factor in promoting the dispersion of active constituents and enhancing the sorption of H2O. The sorbents showed outstanding reducing atmosphere-resistance and H2O-resistance abilities. After multiple sulfidation-regeneration cycles, 90Mn10Ca/MSU-H could still retain 81% of the initial sulfur capacity, which suggested that the sorbent possessed endurable regeneration ability. The utilization of MSU-H confined the migration of active particles, which ensured long lifetime stabilities of sorbents. 19

Therefore, the new xMnyCa/MSU-H could be considered as promising sorbents for high temperature H2S removal.

Acknowledgments This work was supported by National Natural Science Foundation of China and BAOSTEEL Group Corporation (Grant 50876122) and Tianjin (china) Training Programs of Innovation and Entrepreneurship for Undergraduates (201410056244).

References [1] Y. Cao, X. Hu, X. Lin, Y. Lin, R. Huang, L. Jiang, K. Wei, Low-temperature desulfurization on iron oxide hydroxides: Influence of precipitation pH on structure and performance, Ind. Eng. Chem. Res., 54 (2015) 2419-2424. [2] M. Mureddu, I. Ferino, A. Musinu, A. Ardu, E. Rombi, M.G. Cutrufello, P. Deiana, M. Fantauzzi, C. Cannas, MeOx/SBA-15 (Me = Zn, Fe): highly efficient nanosorbents for mid-temperature H2S removal, J. Mater. Chem. A, 2 (2014) 19396-19406. [3] H. Xia, F.M. Zhang, Z.F. Zhang, B.S. Liu, Synthesis of functional xLayMn/KIT-6 20

and features in hot coal gas desulphurization, Phys. Chem. Chem. Phys., 17 (2015) 20667-20676. [4] 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. [5] M. Mureddu, I. Ferino, E. Rombi, M.G. Cutrufello, P. Deiana, A. Ardu, A. Musinu, G. Piccaluga, C. Cannas, ZnO/SBA-15 composites for mid-temperature removal of H2S: Synthesis, performance and regeneration studies, Fuel, 102 (2012) 691-700. [6] S.Y. Jung, H.K. Jun, S.J. Lee, T.J. Lee, C.K. Ryu, J.C. Kim, Improvement of the desulfurization and regeneration properties through the control of pore structures of the Zn−Ti-based H2S removal sorbents, Environ. Sci. Technol., 39 (2005) 9324-9330. [7] H.L. Fan, T. Sun, Y.P. Zhao, J. Shangguan, J.Y. Lin, Three-dimensionally ordered macroporous iron oxide for removal of H2S at medium temperatures, Environ. Sci. Technol., 47 (2013) 4859-4865. [8] Y.H. Lin, Y.C. Chen, H. Chu, The mechanism of coal gas desulfurization by iron oxide sorbents, Chemosphere, 2015, 62-67. [9] B. Zeng, H. Yue, C. Liu, T. Huang, J. Li, B. Zhao, M. Zhang, B. Liang, Desulfurization behavior of Fe–Mn-based regenerable sorbents for high-temperature H2S removal, Energy Fuels, 29 (2015) 1860-1867. [10] F.M. Zhang, B.S. Liu, Y. Zhang, Y.H. Guo, Z.Y. Wan, F. Subhan, Highly stable and regenerable Mn-based/SBA-15 sorbents for desulfurization of hot coal gas, J. Hazard. Mater., 233–234 (2012) 219-227. [11] D. Liu, S. Chen, X. Fei, C. Huang, Y. Zhang, Regenerable CuO-based adsorbents for low temperature desulfurization application, Ind. Eng. Chem. Res., 54 (2015) 3556-3562. [12] Z.F. Zhang, B.S. Liu, F. Wang, S. Zheng, High-temperature desulfurization of hot coal gas on Mo modified Mn/KIT-1 sorbents, Chem. Eng. J., 272 (2015) 69-78. [13] 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. 21

[14] P.R. Westmoreland, D.P. Harrison, Evaluation of candidate solids for high-temperature desulfurization of low-Btu gases, Environ. Sci. Technol., 10 (1976) 659-661. [15] D. Liu, W. Zhou, J. Wu, CeO2–MnOx/ZSM-5 sorbents for H2S removal at high temperature, Chem. Eng. J., 284 (2016) 862-871. [16] Y. Zhang, B.S. Liu, F.M. Zhang, Z.F. Zhang, Formation of (FexMn2−x)O3 solid solution and high sulfur capacity properties of Mn-based/M41 sorbents for hot coal gas desulfurization, J. Hazard. Mater., 248–249 (2013) 81-88. [17] W.J. Bakker, F. Kapteijn, J.A. Moulijn, A high capacity manganese-based sorbent for regenerative high temperature desulfurization with direct sulfur production: Conceptual process application to coal gas cleaning, Chem. Eng. J., 96 (2003) 223-235. [18] Z.B. Huang, B.S. Liu, F. Wang, R. Amin, Performance of Zn–Fe–Mn/MCM-48 sorbents for high temperature H2S removal and analysis of regeneration process, Appl. Surf. Sci., 353 (2015) 1-10. [19] T.H. Ko, H. Chu, Y.J. Liou, A study of Zn–Mn based sorbent for the high-temperature removal of H2S from coal-derived gas, J. Hazard. Mater., 147 (2007) 334-341. [20] J. Wang, J. Guo, R. Parnas, B. Liang, Calcium-based regenerable sorbents for high temperature H2S removal, Fuel, 154 (2015) 17-23. [21] S.B. Jagtap, T.D. Wheelock, Regeneration of sulfided calcium-based sorbents by a cyclic process, Energy Fuels, 10 (1996) 821-827. [22] T.T. Akiti, K.P. Constant, L.K. Doraiswamy, T.D. Wheelock, A regenerable calcium-based core-in-shell sorbent for desulfurizing hot coal gas, Ind. Eng. Chem. Res., 41 (2002) 587-597. [23] T.T. Akiti Jr, K.P. Constant, L.K. Doraiswamy, T.D. Wheelock, Development of an advanced calcium-based sorbent for desulfurizing hot coal gas, Adv. Environ. Res., 5 (2001) 31-38. [24] S.S. Kim, T.R. Pauly, T.J. Pinnavaia, Non-ionic surfactant assembly of ordered, very large pore molecular sieve silicas from water soluble silicates, Chem. Commun., 22

(2000) 1661-1662. [25] S. Inagaki, A. Koiwai, N. Suzuki, Y. Fukushima, K. Kuroda, Syntheses of highly ordered mesoporous materials, FSM-16, derived from kanemite, Bull. Chem. Soc. Jpn, 69 (1996) 1449-1457. [26] B.S. Liu, Y. Zhang, J.F. Liu, M. Tian, F.M. Zhang, C.T. Au, A.S.C. Cheung, Characteristic

and

mechanism

of

methane

dehydroaromatization

over

Zn-based/HZSM-5 catalysts under conditions of atmospheric pressure and supersonic jet expansion, J. Phys. Chem. C, 115 (2011) 16954-16962. [27] F. Wang, B.S. Liu, Z.F. Zhang, S. Zheng, High-temperature desulfurization of coal gas over Sm doped Mn-based/MSU-S sorbents, Ind. Eng. Chem. Res., 54 (2015) 8405-8416. [28] A. Lapkin, B. Bozkaya, T. Mays, L. Borello, K. Edler, B. Crittenden, Preparation and characterisation of chemisorbents based on heteropolyacids supported on synthetic mesoporous carbons and silica, Catal. Today, 81 (2003) 611-621. [29] M.A. Camblor, A. Corma, J. Perez-Pariente, Infrared spectroscopic investigation of titanium in zeolites. A new assignment of the 960 cm-1 band, J. Chem. Soc., Chem. Commun., (1993) 557-559. [30] J. Du, Y. Pan, T. Zhang, X. Han, F. Cheng, J. Chen, Facile solvothermal synthesis of CaMn2O4 nanorods for electrochemical oxygen reduction, J. Mater. Chem., 22 (2012) 15812-15818. [31] F.Q. Zhang, Y. Yan, H.F. Yang, Y. Meng, C.Z. Yu, B. Tu, D.Y. Zhao, Understanding effect of wall structure on the hydrothermal stability of mesostructured silica SBA-15, J. Phys. Chem. B, 109 (2005) 8723-8732. [32] X. Tang, J. Li, L. Sun, J. Hao, Origination of N2O from NO reduction by NH3 over β-MnO2 and α-Mn2O3, Appl. Catal., B: Environ., 99 (2010) 156-162. [33] F. Arena, T. Torre, C. Raimondo, A. Parmaliana, Structure and redox properties of bulk and supported manganese oxide catalysts, Phys. Chem. Chem. Phys., 3 (2001) 1911-1917. [34] J. Xu, Z. Luan, T. Wasowicz, L. Kevan, ESR and ESEM studies of Mn-containing MCM-41 materials1, Micropor. Mesopor. Mater., 22 (1998) 179-191. 23

[35] D. Zhao, D. Goldfarb, Synthesis of mesoporous manganosilicates: Mn-MCM-41, Mn-MCM-48 and Mn-MCM-L, J. Chem. Soc., Chem. Commun., (1995) 875-876. [36] Z. Olender, D. Goldfarb, J. Batista, Magnetic resonance studies of SAPO-44 and MnAPSO-44, J. Am. Chem. Soc., 115 (1993) 1106-1114. [37] Z.F. Zhang, B.S. Liu, F. Wang, J.F. Li, Fabrication and performance of xMnyCe/hexagonal mesoporous silica sorbents with wormhole-like framework for hot coal gas desulfurization, Energy Fuels, 27 (2013) 7754-7761. [38] H.L. Fan, Y.X. Li, C.H. Li, H.X. Guo,. K.C. Xie, The apparent kinetics of H2S removal by zinc oxide in the presence of hydrogen, Fuel, 81 (2002) 91-96. [39] R. Ben-Slimane, M. Hepworth, Desulfurization of hot coal-derived fuel gases with manganese-based regenerable sorbents. 1. Loading (sulfidation) tests, Energy Fuels, 8 (1994) 1175-1183. [40] H. Xia, B.S. Liu, Q. Li, Z.B. Huang, A.S.C. Cheung, High capacity Mn-FeMo/FSM-16 sorbents in hot coal gas desulfurization and mechanism of elemental sulfur formation, Appl. Catal. B: Environ., 200 (2017) 552–565.

24

Fig. 1 (A) Nitrogen adsorption isotherms; (B) small-angle XRD; (C) pore size distributions and (D) FT-IR spectra of MSU-H with (□) and without (■) treatment in 15 % steam/N2 mixture. Fig. 2 HRTEM images of (a) FSM-16 and (c) MSU-H; (b) and (d) corresponding high magnification HRTEM images. Fig. 3 Wide-angle XRD of (A) fresh and (B) used sorbents of (x) 100Mn/FSM-16, (a) 100Mn/MSU-H, (b, b1) fresh and used 90Mn10Ca/MSU-H; (c, c1) fresh and used 80Mn20Ca/MSU-H; (d, d1) fresh and used 50Mn50Ca/MSU-H. Fig. 4 (A, B) N2 adsorption isotherms and (C, D) pore size distribution of fresh (■) 100Mn/MSU-H, (■) 100Mn/FSM-16, (■) 90Mn10Ca/MSU-H, (■) 80Mn20Ca/MSU-H, and (■) 50Mn50Ca/MSU-H; (□, □, □) 90Mn10Ca/MSU-H used at 750, 600 and 800 oC, respectively; (□, □) 90Mn10Ca/MSU-H regenerated at 700 oC after three cycles and seven cycles, respectively. Fig. 5 TPR profiles of fresh (a) 100Mn/FSM-16, (b) 100Mn/MSU-H, (c) 90Mn10Ca/MSU-H, (d) 80Mn20Ca/MSU-H and (e) regenerated 90Mn10Ca/MSU-H. Fig. 6 X-band ESR spectra of (a) fresh 90Mn10Ca/MSU-H, (b) used 90Mn10Ca/MSU-H and (c) regenerated 90Mn10Ca/MSU-H. Fig.

7

Desulfurization

behavior

over

(a)

100Mn/FSM-16,

(b)100Mn/MSU-H

(c)

95Mn5Ca/MSU-H (d) 90Mn10Ca/MSU-H (e) 80Mn20Ca/MSU-H and (f) 50Mn50Ca/MSU-H at 750 °C. Fig. 8 H2S breakthrough curves for 50% 90Mn10Ca/MSU-H at different temperature. Fig. 9 Effect of gas composition on H2S breakthrough curves over 50% 90Mn10Ca/MSU-H: (a) 5% steam; (b) 10% steam; (c) 15% steam and (d) 39.6% H2, 32% CO, 15% steam (0.33% H2S and N2 balance gas). Fig. 10 H2S breakthrough curves of 90Mn10Ca/MSU-H in 3 cycles for (a) regeneration with diluted air and (b) regeneration with oxidation/reduction using 5 % O2/N2 and 10 % H2/N2 at 700 °C. The data in parentheses in upper left is the corresponding BSC. Fig. 11 TPO profiles of Mn100/MSU-H and 90Mn10Ca/MSU-H used at 750 oC. Fig. 12 (A) XRD and (B) FT-IR spectra of 90Mn10Ca/MSU-H after regeneration with (b) diluted air and (c) oxidation/reduction using 5% O2-N2 and 10% H2-N2. Fig. 13 H2S breakthrough curves of 80Mn20Ca/MSU-H in 3 cycles for (a) regeneration with oxidation/reduction method and (b) regeneration with diluted air at 700 °C. 25

Fig. 14 H2S breakthrough curves of seven successive sulfidation-regeneration cycles for 90Mn10Ca/MSU-H (regeneration with oxidation/reduction method); the data in parenthesis is the BSC. Scheme 1. Synthetic diagram of supported metal sorbent

26

450

3424(OH)

(D)

(A)

600

1225(Si-O-Si)

0.00

Pore diameter (angstom)

2

3

4

2 Theta(degree)

5 4000

961(Si-OH) 825(Si-O-Si) 468(O-Si-O)

-1 -1

0.02

100

(B) 1

(C)

200

d (nm) 11.0 6.42 5.13

0.04

3

100

hkl 100 110 200 110

Intensity(a.u.)

0 0.0 0.2 0.4 0.6 0.8 1.0 Relative pressure (p/p0)

1077(Si-O-Si)

150

1624(OH)

300

dv/dr(cm A g )

3

-1

Volume (cm g )

750

3000 2000 1000 -1 Wavenumber(cm )

Fig. 1

27

Fig. 2

28

c

(211)

(400)

b



30

40

50



a



x

60

(220)

(111) (200)









 MnS CaS



c1

(400)



d1

(220)



d (622)

 

(440)

(332)

(400) 

(132)

 Mn3O4



20



 CaMn2O4

(222)

B

(200)



(222) 

Intensity (a.u.)

 Mn2O3

(320)

(011)

A



b1

70 20 30 2 Theta(deg.)

40

50

60

70

Fig. 3

29

3

-1

Relative volume absorbed (cm g )

(A)

600

400

550

350

500

300

450

250

400

200

350

150

300

100

250

50

200

0

0.0 0.2 0.4 0.6 0.8 1.0

(B)

0.0 0.2 0.4 0.6 0.8 1.0

(C)

0.15

3

-1

-1

Relative pore volume(cm A g )

Relative pressure (p/p0)

0.15

(D)

0.12

0.12

0.09

0.09 0.06

0.06

0.03

0.03 0.00

0.00 100

100 Pore diameter (angstrom)

Fig. 4

30

448

e

Intensity (a.u.)

413

287

d

406

334

338 422

c 443

b

369

a 100

200

300

400

500

600

700

o

Temperature ( C)

Fig. 5

31

b used sorbent

regenerated sorbent

Intensity

c a fresh sorbent

g = 2.004 H = 387G

2500

3000

3500

4000

4500

Magnetic field (G)

Fig. 6

32

3.0 2.5 2.0 1.5

20

100

16

80

Utilization(%)

3.5

BSC (g S/100 g sorbent)

-3

Exit H2S concentration (g m )

4.0

12

60

8

40

4 0

a

b

c

d

e

f

a b c d e f

20

1.0 0.5 0.0 0

50

100 150 Duration time (min)

200

Fig. 7

33

90

18

85

16

3.0 2.5

14

80

12

75

10

2.0 1.5

70

8

Utilization (%)

3.5

BSC (g S/100g sorbent)

-3

Exit H2S concentration(g m )

4.0

65

6

600

700

800

1.0 0.5

o

600 C o 700 C o 750 C o 800 C

0.0 0

50

100 150 Duration time (min)

200

Fig. 8

34

a (14.36)

-3

Exit H2S concentration (g m )

3.5

b (14.27) c (12.80) d (12.73)

3.0 2.5 2.0 1.5 1.0 0.5 0.0

0

20

40 60 80 100 120 140 160 180 Duration time(min)

Fig. 9

35

-3

Exit H2S concentration(g m )

S-1 (18.69) S-2 (14.62) S-3 (14.42)

3.5 3.0 2.5

3.5 3.0 2.5

2.0

2.0

1.5

1.5

1.0

1.0

0.5

S-1 (18.69) S-2 (16.08) S-3 (14.83)

0.5

a

0.0 0

b 0.0 50 100 150 200 250 0 50 100 150 200 250 Duration time (min)

Fig. 10 Fig. 10

36

TCD signal(a.u.)

337

100Mn/MSU-H 378 90Mn10Ca/MSU-H

100

200

300 400 500 o Temperature( C)

600

700

800

Fig. 11

37

A

Mn3O4

Mn2O3

CaSO4





b 3rd c 7th

(224)

(211)

(200)

Intensity (a.u.)



B

a fresh

c 7th





957 



 



20

30

40

50

60

2 Theta(deg.)



b 3rd

70 1600

1200

800

400 -1

Wavenumber(cm )

Fig. 12

38

-3

Exit H2S concentration (g m )

4.0

4.0

S-1 (13.89) S-2 (11.89) S-3 (11.60)

3.5 3.0 2.5

3.5 3.0 2.5

2.0

2.0

1.5

1.5

1.0

1.0

0.5 0.0

a 0

50

S-1 (13.89) S-2 (9.78) S-3 (9.17)

0.5 0.0

b

100 150 200 0 50 100 150 200 Duration time (min)

Fig. 13

39

S-1 (18.69) S-2 (16.08) S-3 (14.83) S-4 (15.40) S-5 (14.72) S-6 (15.21) S-7 (14.78)

-3

Exit H2S concentration (g m )

3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0

50

100 150 Duration time (min)

200

250

Fig. 14

Scheme 1

40

Table 1 Properties of MSU-H, FSM-16, fresh, used and regenerated xMnyCa/MSU-H samples Sample

D(cs)a (nm)

SBET (m ·g )

(cm ·g )

(cm ·g )

(cm ·g )

Da (nm)

MSU-H

-

831

1.0

0.30

0.70

4.80

FSM-16

-

1079

1.0

0.34

0.66

4.02

MSU-H-T

-

507

0.57

0.16

0.41

4.50

100Mn/MSU-H

21.23

238

0.30

0.08

0.22

5.08

100Mn/FSM-16

27.40

301

0.24

0.10

0.14

3.19

90Mn10Ca/MSU-H

18.40

369

0.40

0.14

0.26

4.33

80Mn20Ca /MSU-H

19.71

219

0.28

0.05

0.23

5.10

50Mn50Ca/MSU-H

24.64

202

0.26

0.06

0.20

5.13

S-750-90Mn10Ca/MSU-H

-

187

0.27

0.05

0.22

5.73

S-600-90Mn10Ca/MSU-H

-

220

0.40

0.07

0.33

7.30

S-800-90Mn10Ca /MSU-H

-

157

0.26

0.05

0.21

6.60

R3-90Mn10Ca/MSU-H

36.31

168

0.25

0.05

0.20

5.95

R7-90Mn10Ca/MSU-H

20.20

118

0.18

0.02

0.16

6.10

2

Vt -1

Vmic 3

-1

3

Vmec -1

3

-1

a

Crystalline size D(cs) of Mn2O3 or Mn3O4 are calculated from (222) lattice plane of Mn2O3 and (211) lattice plane of Mn3O4, respectively.

42