High-Temperature Desulfurization of Coal Gas over Sm Doped Mn

Aug 13, 2015 - Hexagonally ordered mesoporous MSU-S was assembled from nanoclustered zeolite seeds. The XRD and BET results verified that after ...
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High-Temperature Desulfurization of Coal Gas over Sm Doped Mnbased/MSU‑S Sorbents F. Wang, B. S. Liu,* Z. F. Zhang, and S. Zheng Department of Chemistry, Tianjin University, and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, People’s Republic of China ABSTRACT: Hexagonally ordered mesoporous MSU-S was assembled from nanoclustered zeolite seeds. The XRD and BET results verified that after treated in 10%H2O/N2 atmosphere at 800 °C for 2 h, MSU-S still remained a well-ordered hexagonal structure due to rich acidic sites and excellent hydrothermal stability. A series of SmxMnyOz/MSU-S sorbents were prepared by a sol−gel method. The desulfurization performances of the 55%Sm5Mn95/MSU-S sorbent were improved significantly due to high hydrothermal stability of MSU-S with high surface area as well as the synergistic effect between Mn and Sm2O3. The result of eight successive sulfurization/regeneration cycles of sorbent illustrated that high breakthrough sulfur capacity and endurable stability of 55%Sm5Mn95/MSU-S correlated closely with the existence of the framework 4-coordination aluminum (NH3-TPD, 27 Al MAS NMR) and high dispersion of Sm/Mn species on MSU-S (XRD, TPR) for H2S removal. HRTEM images and SAED patterns confirmed that fresh 55%Sm5Mn95/MSU-S existed in high ordered mesoporous structure and the nanosized Sm2O3 and Mn3O4 particles occurred in highly dispersive polycrystallites. The valence state of Mn species and the regeneration process of used sorbent were characterized by X-ray photoelectron spectroscopy (XPS) and TG/DSC. In addition, the effects of reaction temperature, feed composition and Sm/Mn atomic ratio in sorbents on desulfurization performance were investigated. sulfidation and the manganese sulfate is formed below 800 °C in the regeneration process. Therefore, sulfidation and regeneration must be carried out at very high temperatures.9 However, it is well-known that the sorbents deteriorated by the sintering at high operating temperatures10 during the repeated cycles of desulfurization and regeneration, especially the temperature in bed enhanced rapidly due to exothermic sulfur oxidation reaction in air. Therefore, the control of operating temperatures and the optimization of regenerating conditions are very important in the desulfurization process. The samarium oxide, as one of the rare earth oxides (REOs), has received considerable attention because of high application potentials in microelectronics11 and heterogeneous catalysis.12 It is possible to combine the transition metal oxides with REOs in order to improve the stability and the total capacity of the sorbents. Samarium oxides can increase to a certain extent the number of surface oxygen vacancies and improve the oxygen mobility, leading to more surface active sites13 and more defects in the oxide lattice position often have better adsorption performance. In the meantime, not only can the presence of samarium oxide promote the high dispersion of Mn3O4 particles but the catalytic action of samarium oxysulfide (Sm2O2S) can result in the formation of elemental sulfur.14 It was reported that samarium oxides can affect the structure and desulfurization properties of sorbents. In our earlier works, the mixed metal oxides supported on mesoporous silica, such as SmxMeyOz/MCM-41,14 LaxMeyOz/SBA-1515 and Cu−Mn/ SBA-1516 presented good desulfurization performance due to

1. INTRODUCTION Recently, many countries have been developing the techniques on cleaning utilization of coal, such as the integrated gasification combined cycle (IGCC), solid oxide fuel cells (SOFC) and methanol production. Gas cleaning is a key process component for these clean coal technologies. In all cases, the development of these techniques depends on the ability to remove sulfur compounds (mainly H2S and COS) from the feed gases. Hydrogen sulfide is a toxic gas that can result in serious environmental problems.1−4 The desulfurization and regeneration reactions will carry out as shown in eqs 1−2 companied with the occurrence of an additional side reaction (eq 3) due to the formation of sulfate, which will lead to the decline of desulfurization activities.5 MexOy (s) + x H 2S(g) + (y − x)H 2(g) = x MeS(s) + y H2 O(g)

(1)

x MeS(s) + (x + y/2)O2 (g) = MexOy (s) + xSO2 (g) (2)

MeS(s) + 2O2 (g) = MeSO4 (s)

(3)

According to the report of Slimane and Hepworth,6 manganese-based sorbents can be safely utilized at more than 700 °C in H2S removal. Manganese oxide exhibits high sulfidation reactivity in thermobalance tests and the reactivity increased substantially by the addition of a mesoporous alumina or bentonite.7,8 The earlier kinetic studies by Westmoreland et al.2 showed that the reactivity of MnO is higher than those of CaO, ZnO and V2O3 in H2S removal and MnO is stable at 400−1000 °C, thereby allowing greater flexibility in sulfidation and regeneration temperatures without loss of sorbent. In the case of manganese-based sorbents, MnO is the stable phase in © XXXX American Chemical Society

Received: March 30, 2015 Revised: July 31, 2015 Accepted: August 13, 2015

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DOI: 10.1021/acs.iecr.5b01145 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Sm5Mn95/MSU-S and 60%Sm5Mn95/MSU-S (the number after Sm and Mn represented the molar percentage in metal oxides). 2.3. Characterization. The nitrogen adsorption isotherms of MSU-S and as-prepared sorbents were obtained on a domestic N2 adsorption apparatus at 77 K.15 The samples were pretreated in a vacuum at 200 °C for 2 h before testing. The specific surface areas were calculated using the Brunauer− Emmett−Teller (BET) method and the pore size distributions were obtained from the adsorption branch of nitrogen isotherms by the Barrett−Joyner−Halenda (BJH) method. The small-angle XRD patterns were recorded on a Rigaku D/MAX 2500 V/pc automatic diffractometer equipped with Ni-filtered Cu Kα radiation (20 kV and 30 mA). The wideangle XRD patterns were done with a PANalytical automatic diffractometer using Ni-filtered Cu Kα radiation (λ = 0.154 06 nm) at settings of 40 kV and 50 mA. 27 Al Magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra were recorded using a Bruker MSL-300 spectrometer with a resonance frequency of 78.13 MHz, a pulse delay 3.0 s, a spectral width of 40.00 kHz and a spin rate of 8 kHz. NH3-TPD was conducted on an Auto Chem BET TPR/TPD (Quantachrome, USA). Approximately 100 mg of sorbent was charged in a U-shaped quartz tube and pretreated at 200 °C in argon (20 mL/min) for 30 min. When the sorbent was cooled to 100 °C, the ammonia was adsorbed up to saturation. After physiosorbed ammonia was purged with argon, TPD was carried out from 100 to 800 °C at heating rate of 10 °C/min. H2-TPR of sorbents was conducted in a domestic system.24 Approximately 60 mg of sorbent was charged in the U-shaped quartz tube and pretreated in N2 (60 mL/min) at 150 °C for 30 min. After cooling to RT, the sample was heated from RT to 850 °C at a rate of 10 °C/min. The amount of H2 uptake during the reduction was measured by a thermal conductivity detector (TCD). The structures and morphologies of MSU-S, fresh and used sorbents were investigated using a JP-S-4800 thermal field emission environmental scanning electron microscope and a Tecnai G2 F20 electron microscopy at 200 kV, respectively. The HRTEM images and energy-dispersive X-ray (EDX) element analysis were recorded. TG/DSC analysis for 55% Sm5Mn95/MSU-S used at 800 °C was carried out in air on a STA 409 PC/PG model instruments. Approximately 10 mg of sample was heated from 30 to 1000 °C at the rate of 10 °C/ min. The X-ray photoelectron spectroscopy (XPS) signals were obtained with a PHI-1600 ESCA spectrometer equipped with Al Kα X-ray source (1486.6 eV). The binding energies (BEs) of the sample were calibrated with the contaminant C 1s line (284.6 eV). 2.4. Desulfurization/Regeneration Performance. The performance of sorbents for hot coal gas desulfurization was evaluated in a fixed-bed microreactor at atmospheric pressure using simulated hot coal gas with composition (vol %) of 0.33% H2S, 10.5%H2, 17.1%CO2 and 72.1%N2. Approximately 0.5 g sorbent was packed into a quartz reactor (i.d. 0.9 cm and height 30 cm) and a thermocouple was placed at the center of sorbent bed to measure the reaction temperature. The sorbent was first heated to the desired temperature in N2 at rate of 10 °C/min and then simulated coal gas was introduced to the reactor with the weight hourly space velocity (WHSV) of 9 × 103 mL h−1 g−1 for desulfurization. In the meantime, the concentration of H2S in inlet and outlet gas was analyzed by iodometric titration to obtain H2S breakthrough curve (general standard of China,

the synergistic effect between REOs and the transition metal oxides. According to the report of Zhang et al.,17 the reactivity of carbonyl sulfide hydrolysis is improved when rare-earth oxide was added to single or mixed oxides. Recently, the mesoporous materials, such as MCM-41 and SBA-15 etc., have attracted much attention due to the uniform hexagonal structure, adjustable channel size, high specific surface area as well as highly dispersive ability for metal oxide particles18 and its potential applications in catalysis, separation and ionexchange.19−21 However, these materials are not as stable as zeolites with good framework stability owing to their amorphous feature, which induced by the amorphous pore wall structure limit their potential applications. To overcome these problems, Liu et al.22 synthesized the mesoporous molecular sieves Al-MSU-S derived from faujasitic zeolite type-Y seeds and found that these molecular sieves had high hydrothermal stability, strong acidity and high ion exchange ability. Similarly, Kim et al.23 also prepared ultrastable MSU-G molecular sieve catalysts and verified that MSU-G presented higher high-temperature resistance ability than MCM-41 and KIT-1. We expected that such a novel material with characters of zeolite and ordered mesoporous structure will be an ideal support for desulfurization sorbent. To ensure the hydrothermal stability of sorbents at high temperature, novel mesoporous molecular sieves MSU-S was synthesized by direct assembly of nanozeolitic precursors. A series of SmxMny/MSUS sorbents were prepared for the desulfurization of hot coal gas at high temperature.

2. EXPERIMENTAL SECTION 2.1. Sorbents Preparation. The parent MSU-S material was synthesized hydrothermally by direct assembly of nanocluster according to the procedure described by Liu et al.,22 i.e., 0.507 g of NaOH and 0.352 g of NaAlO2 were dissolved in 10 mL of deionized water (DW), then 25.5780 g of sodium silicates was added slowly into the aforementioned solution under vigorous stirring at 35 °C for 4 h. The obtained clear solution was transferred into an autoclave and statically synthesized at 110 °C for 12 h to form nanoaluminosilicate units. Subsequently, as-synthesized nanozeolite precursors were added dropwise to the solution of cetyltrimethylammonium bromide (CTAB) under moderate stirring at room temperature (RT). After the pH was adjusted to 9−10 using H2SO4 solution (6 mol/L), the resultant gel was crystallized in an autoclave at 140 °C for 48 h under static conditions. Finally, the precipitate was filtered, washed by DW, dried and calcined in air at 550 °C for 6 h to obtain the parent MSU-S (Si/Al = 48). 2.2. Preparation of Sm2O3 Doping Mn-based Sorbents. A series of sorbents were synthesized by means of a sol− gel method. Taking the preparation of 55 wt % Sm5Mn95/ MSU-S (Sm/Mn molar ratio = 5:95) as an example, 0.3182 g of Sm2O3 was dissolved in HNO3 solution (0.01 mol/L) and mixed with 12.4081 g of 50 wt % Mn(NO3)2 solution. After the addition of citric acid with a molar amount 1.5 times that of total metal ions, the as-prepared MSU-S was added to the aforementioned solution. The sol was continuously stirred at 60 °C until the formation of yellow gel. After the product was aged, dried at RT for 1 week and calcined at 550 °C for 6 h, the obtained 45%−60 wt % SmxMnyOz/MSU-S sorbents were sieved to 20−40 mesh and denoted as 45%Sm5Mn95/MSU-S, 50%Mn100/MSU-S, 50%Sm5Mn95/MSU-S, 50%Sm10Mn90/ MSU-S, 50%Sm20Mn80/MSU-S, 50%Sm50Mn50/MSU-S, 55% B

DOI: 10.1021/acs.iecr.5b01145 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research GB/T 11060.1-1998). The breakthrough time was defined as the starting time when H2S concentration in outlet reached 100 mg m−3 (limit concentration of H2S detected by iodometric titration is 50 mg m−3). The amount of sulfur captured by sorbent before breakthrough time was denoted as the breakthrough sulfur capacity (BSC), which can be calculated by the following equation:25 SC = WHSV ×

Ms 22.4 × ×[ Vm M H 2S

× 10−3

∫0

Obviously, the adsorption isotherm of MSU-S exhibited a steep inflection point at p/po = 0.30−0.50, a typical of capillary condensation process within uniform mesopores,27,28 which was verified by the narrow pore size distribution calculated by the BJH method for cylindrical pores (Figure 1B). The specific surface area (SBET), total pore volume (Vt), mesoporous volume (Vmeso), microporous volume (Vm) and average pore diameter (Da) of MSU-S are listed in Table 1. It can be seen that the MSU-S treated at 800 °C in 10% H2O/N2 mixture still remains high SBET, VT and uniform pore size distribution despite that there was slight decrease of SBET and VT relative to as-prepared MSU-S. In contrast, the SBET and VT of MCM-41 (Si/Al = 48:1) assembled from aluminosilicate anions declined remarkably under equivalent steam-treatment condition (Table 1). Therefore, the stability of hexagonal aluminosilicate mesostructure in steam can be improved substantially by the assembly of nanocluster precursors that normally nucleated the crystallization of microporous zeolites.22 In addition, 27Al MAS NMR results (Figure 2A) indicated that the chemical shift of framework 4-coordination aluminum is 55 ppm in MSU-S and there was no extra-framework 6-coordination aluminum at 0 ppm, which illustrated that the hydrothermal stability of mesoporous MSU-S originated mainly from small domains of zeolites with the FAU structure and all aluminum was incorporated into the framework of MSU-S mesoporous structure.21 NH3-TPD is widely used to determine the acidic sites of solid MSU-S. As shown in Figure 2B(a), a NH3 desorption peak at approximately 271 °C can be assigned to moderately ammonia adsorption sites and the peak at 558 °C was ascribed to ammonia adsorption sites correlated with strong Lewis acidity. According to report in literature,33 the hydrothermal stability of the support increases with increasing acid sites. The amount of ammonia desorbed at a given temperature indicated the numbers of acid sites, and there are more Lewis acid sites on the surface of MSU-S.29 In addition, the Bronsted and Lewis acid sites on MSU-S reduced slightly with the loadings of Sm/ Mn oxides due to the interaction of metal species with −OH groups (Figure 2B(b)), which will result in the enhancement in pore wall thickness and thermal stability. 3.2. XRD and BET Characterization of MSU-S and Sorbents. The small-angle XRD pattern of MSU-S (Figure 3A) illustrated the presence of an ordered hexagonal array of mesoporous structure.27 The diffraction peaks at 2θ = 1.98°, 3.4° and 3.9° can be indexed to (100), (110) and (200) crystal plane of sample belonged to the P6mm space group of hexagonal symmetry.30 According to the report of Renzo et al.,31 the thermal stability of mesoporous MSU-S depended strongly on the wall thickness. In other words, the wall thickness (2.3 nm) of as-prepared MSU-S calculated via the difference of unit cell parameter (a0 = 2d100/√3) with the maximum pore size (Dp) was remarkably higher than that (1.6 nm) of MCM-41 reported by Zhang et al.32 The XRD patterns (Figure 3A(b)) of MSU-S treated at 800 °C in 10% (v/v) H 2 O/N 2 mixture for 2.0 h verified that the MSU-S mesostructure still remained a well-ordered hexagonal array30 with similar pore wall thickness, similar to the aforementioned observation in N2 adsorption isotherms (Figure 1A). Similarly, small-angle XRD patterns of sorbents illustrated that the introduction of Sm/Mn metal oxides on MSU-S did not destroy significantly the hexagonal mesoporous structure of support (Figure 3A). The diffraction angle of the (100) crystal plane was shifted to 2θ = 2.1° and corresponding interplanar

t

(C in − Cout)dt ] (4)

where SC is the effective sulfur capacity of sorbent (mg-sulfur/ g-sorbent); WHSV is weight hourly space velocity (L h−1 g−1); Ms and MH2S are the molar weight of S (32.06 g/mol) and H2S (34.06 g/mol), respectively; Vm is molar volume of H2S at 1 atm and 25 °C (24.5 L mol−1); t is the time of total desulfurization reaction and the reaction time when Cout = 100 mg m−3 is the breakthrough time of sorbent (h); Cin and Cout are the inlet and outlet concentrations of H2S (mg m−3) respectively. The used sorbent was regenerated at 700 °C in a 5% O2/N2 mixture at WHSV of 9 × 103 mL h−1g −1 until there is no detection of SO2 in the effluent (using a KMnO4 solution for indication). The utilization (SU) of 50%Sm5Mn95/MSU-S sorbent is defined as a ratio of breakthrough sulfur capacity (BSC) and theoretical sulfur capacity (TSC) (TSC is estimated approximately only based on the content of Mn3O4 based on XRD according to the chemical equation Sm2O3 + Mn3O4 + H2 + 4H2S = Sm2O2S + 3MnS + 5H2O) as follows SU(%) =

BSC × 100% TSC

(5)

3. RESULTS AND DISCUSSION 3.1. BET, 27Al MAS NMR and NH3-TPD Characters of MSU-S Support. The nitrogen adsorption isotherms and the pore size distributions of MSU-S are shown in Figure 1A,B,C. It can be clearly observed that the N2 adsorption isotherm of MSU-S contributed to type-IV (Figure 1A), a characteristic of mesoporous materials according to the IUPAC classification.26

Figure 1. (A) N2 adsorption isotherms and (B, C) pore diameter distributions of MSU-S (circle) and MCM-41 (square) with (○, □) and without (●, ■) treatment in 10% steam at 800 °C for 2 h. C

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Table 1. Specific Surface Area (SBET), Total Pore Volume (Vt), Micropore Volume (Vmic)/Vt (%), Mesopore Volume (Vmeso)/Vt (%) and Average Pore Size (Da) of MSU-S, Fresh and Used SmxMny/MSU-S Sorbents with Different Sm/Mn Atomic Ratiosa sample

SBET (m2/g)

Vt (cm3/g)

Vmic/Vt (%)

Vmeso/Vt (%)

Da (nm)

MSU-S MCM-41 MSU-S treated in10% (v/v) H2O/N2 mixture MCM-41 treated in 10% (v/v) H2O/N2 mixture 50%Sm5Mn95/MSU-S 50%Sm10Mn90/MSU-S 50%Sm20Mn80/MSU-S 50%Sm50Mn50/MSU-S 55%Sm5Mn95/MSU-S S-800-50%Sm5Mn95/MSU-S S-800-50%Sm10Mn90/MSU-S S-800-50%Sm20Mn80/MSU-S S-800-55%Sm5Mn95/MSU-S, (3% steam) S-800-55%Sm5Mn95/MSU-S S-700-55%Sm5Mn95/MSU-S S-750-55%Sm5Mn95/MSU-S R-8-55%Sm5Mn95/MSU-S

880 945 651 486 303 227 239 229 250 51 50 43 22 36 84 82 26

0.706 0.741 0.561 0.497 0.219 0.334 0.348 0.367 0.345 0.076 0.087 0.081 0.071 0.051 0.130 0.141 0.037

37.5 40.8 39.4 33.0 37.0 25.8 21.3 19.6 25.2 21.1 26.4 18.5 9.9 11.8 17.7 16.3 21.6

62.5 59.2 60.6 67.0 63.0 74.3 78.7 80.4 74.8 9.0 73.6 81.5 90.1 88.2 82.3 83.7 73.4

2.9 3.1 2.9 4.1 5.5 5.9 5.8 6.4 5.5 5.9 7.0 11.7 13.2 5.7 6.2 6.9 5.6

a S-800-50% Sm5Mn95/MSU-S denoted as 50%Sm5Mn95/MSU-S sulfurized at 800 °C. R-8-55% Sm5Mn95/MSU-S denoted as 55%Sm5Mn95/MSU-S regenerated after eight cycles.

Figure 2. (A) 27Al Magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectrum and (B) NH3-TPD curves of MSU-S (a) and 55%Sm5Mn95/MSU-S (b).

Figure 3. Small- and wide-angle XRD patterns of before (a) and after (b) exposure to 10% v/v H2O/N2 mixture at 800 °C for 2.0 h (c) fresh 55%Sm5Mn95/MSU-S (d) used 55%Sm5Mn95/MSU-S, (e) fresh 100% Sm5Mn95, (f) fresh 50%Mn100/MSU-S, (g) eight regenerated 55% Sm5Mn95/MSU-S, (h) fresh 50%Sm50Mn50/MSU-S.

spacing (d) became small (nλ = 2dsin θ, λ = 0.154 06 nm), whereas the diffraction peak in intensity decreased largely, meaning that the shrinkage of the mesoporous structure and the well-ordered array of 55%Sm5Mn95/MSU-S sorbent deteriorated to a certain extent. After desulfurization at 800 °C, the 55%Sm5Mn95/MSU-S sorbent still exhibited three diffraction peaks at 2θ = 2.2°, 3.97° and 4.6°, which means the existence of a well-ordered mesoporous framework. As shown in wide-angle XRD patterns of sorbents, after the loadings of active species (Figure 3B(c,e,f,h)), Mn 2 O 3 [PDF#41-1442] and Mn3O4 [PDF#18-0803] were the main crystalline phase. No diffraction peak of silicates meant that no chemical reaction occurred between metal oxides and support. For 50%Mn100/MSU-S sorbent (Figure 3B(f)), the Mn2O3 (2θ = 23.23°, 33.05°, 38.36°, 45.35°, 55.27° and 66.02°) [PDF#411442] with cubic structure was the main crystal phase and there was no Mn3O4 (2θ = 36.15°) [PDF#18-0803] with a tetragonal structure. It can be seen that the doping of a small amount of Sm2O3 results in a decrease of Mn2O3 crystallites (Figure 3B(c)), and the average sizes of Mn3O4 and Mn2O3 estimated

by the Debye−Scherrer formula were about 7.9 and 8.2 nm, respectively, which indicated that the synergistic action between Sm2O3 and Mn2O3 nanoparticles was favorable for the dispersion of active species. After the desulfurization in hot coal gas at 800 °C, the used 55%Sm5Mn95/MSU-S sorbent (Figure 3B(d)) presented the diffraction peaks of Sm2O2S (2θ = 26.64° and 29.74°) [PDF#65-3451] and α-MnS (2θ = 34.39°, 49.27° and 61.47°) [PDF#65-0891] due to the occurrence of following reaction: Sm2O3 + Mn2O3 + H2 + 3H2S = Sm2O2S + 2MnS + 4H2O. After used 55%Sm5Mn95/MSU-S was regenerated, there was not a significant variation in structure compared to fresh one except that corresponding diffraction peaks intensified owing to the aggregation of partially active particles (Figure 3B(g)) during desulfurization and regeneration cycles. D

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Industrial & Engineering Chemistry Research As listed in Table 1, the SBET and VT values of supported sorbents decreased remarkably compared to as-prepared MSUS because the nanoparticles of Sm2O3 and MnOx occupied the inner surface of channels via the interaction of metal oxide with the Si−OH groups of MSU-S.32 In addition, the properties of SmxMny/MSU-S sorbents to a certain extent depended on the Sm/Mn molar ratios. It can be seen that the SBET and Vt values of sorbents decline with the incremental Sm contents due to the fact that the radius (95 pm) of Sm3+ ions is larger than that (80 pm) of Mn3+. In the meantime, a steep inflection point at p/po = 0.30−0.50 in nitrogen adsorption isotherms disappeared for fresh sorbents (Figure 4A) compared to MSU-S. After

Figure 5. H2-TPR profiles for fresh (a) 50%Mn100/MSU-S, (b) 50% Sm5Mn95/MSU-S, (c) 55%Sm5Mn95/MSU-S, (d) 50%Sm10Mn90/ MSU-S, (e) 50%Sm20Mn80/MSU-S, (f) 50%Sm100/MSU-S.

MSU-S and 55%Sm5Mn95/MSU-S (Figure 5b,c) had not varied remarkably due to the high SBET value (Table 1) of MSU-S. It may envisage that metal oxides are highly dispersed on the surface as well as in the channels of MSU-S due to the segregation action of Sm2O3 and there is not the reduction peaks of larger Mn3O4 particles at approached to 642 °C. 3.4. SEM and HRTEM Images of MSU-S and Sorbents. As shown in Figure 6A,B, MSU-S presented a regular strip structure with hexagonal channels of ca. 3.2 nm taken with the electron beam perpendicular to pore direction, very close to the average pore diameter (2.9 nm) listed in Table 1. An array of ordered bright spot characteristics in fast Fourier transform (FFT) pattern suggested the existence of single-channel amorphous structure (Figure 6C) consisted of SiO2 and Al2O3 components from energy dispersive X-ray spectroscopy (EDX) analysis (Figure 6D). The SEM images (Figure 6E) revealed that the MSU-S presented a snow flower-like structure. As shown in Figure 7A,D, HRTEM images of fresh 55% Sm5Mn95/MSU-S sorbent revealed the existence of a highly ordered mesoporous structure and the active components were highly dispersed in the channel of MSU-S. The occurrence of dispersive circles in the SAED pattern (Figure 7B) of fresh 55% Sm5Mn95/MSU-S indicated that the nanosize metal particles were small and occurred in highly dispersive polycrystallites, although the metal oxide content was up to 55 wt %. However, it can be seen that the narrowing in distance between two black strips illustrated a decrease of pore diameter, in agreement with the most probable pore diameter (Figure 4B) of the sorbent. The SEM images of fresh sorbent also suggested that the particles agglomerated slightly on the surface of MSU-S (Figure 7E), but better than Mn−Fe−Zn−O/γ-Al2O3.10 According to the results of the EDX (Figure 7C), the Cu and carbon signals in EDX analysis originated from the copper web of sample except for the detection of Si, Al, O, Sm and Mn elements. 3.5. Analysis of Mn Valence State in Sorbents by XPS. The XPS and LMM spectra of Mn over fresh 55%Sm5Mn95/ MSU-S, fresh and used Sm5Mn95 sorbents are shown in Figure 8. All C 1s peaks (284.6 eV) in Figure 8A originated from the contaminant carbon species and the enhancement of peak area after desulfurization (Figure 8A(c)) correlated closely with the CO disproportionation reaction. The XPS spectra at 641.6 and

Figure 4. (A) N2 adsorption isotherms and (B) pore diameter distributions of fresh, used sorbents; (orange ★, ☆) fresh and used 55%Sm5Mn95/MSU-S (3% steam), (olive ▼, ▽) fresh and used 50% Sm10Mn90/MSU-S, (red ●, ○) fresh and used 50%Sm5Mn95/MSU-S, (black ■, □) fresh and used 55%Sm5Mn95/MSU-S, (magenta ⧫, ◊) fresh and used 50%Sm20Mn80/MSU-S, (blue □) 700 °C 55% Sm5Mn95/MSU-S, (pink △) 750 °C 55%Sm5Mn95/MSU-S and (pink ★) 55%Sm5Mn95/MSU-S regenerated after eight cycles.

desulfurization at 700−800 °C, the SBET and Vt values over 55% Sm5Mn95/MSU-S (Table1) decreased remarkably due to the fact that metal oxides in sorbents were transformed into metal sulfides with large molecular size resulting in blockage of partial microporous channel. 3.3. H2-TPR Measurements. Hydrogen in hot coal gas can diminish the sulfur capacity of metal oxides, so it was necessary to understand deeply the reduction property of sorbents.29,34 As shown in Figure 5a, the TPR profiles over 50%Mn100/MSUS revealed that two peaks centered at 402 and 501 °C can be contributed to reduction of Mn2O3 to Mn3O4 and then to MnO, which cannot be reduced further to elemental manganese35 in reduction atmosphere. Thus, manganesebased sorbents offered the advantage of high thermal stability at 800 °C.36 In addition, the small peak at 642 °C was attributed to the reduction of the larger Mn3O4 particles existed in external surface of MSU-S.37 After the doping of Sm2O3 species, there is the existence of more Mn3O4 phase on the surface of sorbents (Figure 4B(b−e)), so the reduction process of partial Mn2O3 to Mn3O4, then to MnO will conduct successively based on the TPR profiles of sorbent because the 50%Sm100/MSU-S (Figure 5f) cannot be reduced in the range of 800 °C.38 In addition, the TPR profiles over 50%Sm5Mn95/ E

DOI: 10.1021/acs.iecr.5b01145 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 6. HRTEM images of (A and B) MSU-S perpendicular to pore direction; insets are (C) FFT pattern, (D) EDX analysis and (E) SEM image of MSU-S support.

3.6. TG/DSC Analysis of Used 55%Sm5Mn95/MSU-S Sorbent. TG/DSC profiles of used sorbents were shown in Figure 9. It can be seen that there are three incremental stages in weight and a weigh loss region in the DTG curves. The first weight increase at 246 °C in DTG curve is due to the formation of MnS0.4 (SO4)0.3O0.5 originated from α-MnS42 (Figure 3), corresponding to the occurrence of an exothermic peak at 254 °C. Moreover, the oxidation products of manganese sulfide can be represented by the following formula, MnSx (SO4)yOz, where x = 0 if the temperature is more than 400 °C.43 In the second stage, the weight gain contributed to the formation of manganese oxysulfides, Mn(SO4)0.5O0.8, corresponding to a big exothermic peak at 377 °C in the DSC curve. Next, the increase in weight is due to the successive oxidation of Mn(SO4)0.5O0.8 to Mn(SO4)0.6O0.6 species while there is a broad exothermic peak at 439 °C. Finally, the weight loss (ca. 21.5%) of sample with incremental temperature is due to the removal of SO2 and Mn3O4 formation (Figure 3B). 3.7. Investigation on Performance of Sorbents. 3.7.1. Relation of Sm/Mn Molar Ratios in 50%SmxMny/ MSU-S with Desulfurization Performance. The relationship of different Sm/Mn molar ratios with desulfurization performance of 50%SmxMny/MSU-S sorbent was investigated. As shown in Figure 10, the utilization (52%) of 50%Sm5Mn95/MSU-S sorbents is largest than other sorbents.44 Low utilization of 50% Mn100/MSU-S sorbent was due to the aggregation of Mn2O3 crystallites, which will reduce the gas film diffusion rate based

653.6 eV, respectively can be ascribed to Mn 2p3/2 and Mn 2p1/2 in Mn2O3 (Figure 8B). However, it is difficult to make certain of oxidation states for Mn because the Mn 2p peak has relatively small shifting as a function of oxidation state.40 Therefore, the more sensitive Auger lines (Figure 8C) of Mn LMM (Al anode) over fresh and used sorbents are collected in order to verify the valence state of Mn. According to the kinetic energy (KE, 586.6 eV) of Mn0 reported,39 the Auger peaks at 583.2 eV for fresh 55%Sm 5 Mn 95 /MSU-S and fresh 100%5Sm95Mn are attributed to Mn3+ whereas that at 583.9 eV for used 100%Sm5Mn95 is ascribed to Mn2+ (Figure 8C) in MnS. An approximate shift of 0.7 eV toward the high KE region suggested the reduction of Mn3+ to Mn2+ during desulfurization. As shown in Figure 8D,E, the Mn 3p spectra are strongly influenced by the variation of the outer valence electron configuration. Therefore, the broadening of the Mn 3p peak in Figure 8D verified the different valence states of Mn in fresh 55%Sm5Mn95/MSU-S and 100%Sm5Mn95 sorbents and after smoothing and background subtraction, there is approximately equal peak area of Mn3+/Mn2+ by a fitting of the Mn 3p spectrum, which means the coexistence of both Mn3+ (49.5 eV) and Mn2+ (48.2 eV)41 ions whereas there were more Mn3+ ions to appear in 100%Sm5Mn95. As can be expected, Mn2O3 or Mn3O4 was transformed into MnS after desulfurization of hot coal gas with the BE values of 48.2 eV (Figure 8E), in accordance with XRD analysis (Figure 3). F

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Figure 7. HRTEM images of (A, D) fresh 55%Sm5Mn95/MSU-S taken with the electron beam perpendicular and parallel to pore direction, respectively; insets are (B) SAED pattern (C) EDX analysis and (E) SEM image of fresh 55%Sm5Mn95/MSU-S.

of sorbent. For the nonsupported 100%Sm5Mn95 mixture, the H2S breakthrough curve became smoother after the breakthrough time point (Figure 11f) due to the existence of large Mn2O3 and Mn3O 4 particles (Figure 3e). Hence, the desulfurization efficiency of 100%Sm5Mn95 sorbents declines. However, the H2S breakthrough curve on 55%Sm5Mn95/MSUS sorbent is quite sharply after the breakthrough time point due to the high dispersion of active species (Figure 11c). TPR results verified that there were similar dispersion of particles over 50%Sm5Mn95/MSU-S and 55%Sm5Mn95/MSU-S because of high BET surface areas. The shapes of the breakthrough curves (Figure 11c,e) are basically the same, but the breakthrough time over 55%Sm5Mn95/MSU-S (H-type) sorbent is shorter than that over the same sorbent (Na-type) due to the interaction between H2S and alkaline support (Natype). Therefore, 55%Sm5Mn95/MSU-S (Na-type) was chosen as the best sorbent for the optimization of experimental parameters hereinafter. 3.7.3. Effect of Temperature on Desulfurization Performance of 55%Sm5Mn95/MSU-S. The effect of reaction temperature on desulfurization performance for the removal of H2S is shown in Figure 12. According to the kinetic studies, the desulfurization reaction rate between H2S and metal oxides depended strongly on the limitation of gas diffusion.46 Solid− gas diffusion is controlled by temperature to a large extent and is the dominant factor of the desulfurization process at 700− 850 °C.6 The 55%Sm5Mn95/MSU-S sorbent exhibited the

on the reaction kinetics (Figure 10a). The XRD and TPR results verified that the addition of small amount of Sm in Mnbased sorbents can improve remarkably the dispersion of Mn2O3 particles and the reduction temperature of Mn3+ decreased with the dispersion of Mn2O3 particles (Figure 5). Thereby, the desulfurization efficiency enhanced. The desulfurization performance of 50%Sm5Mn95/MSU-S performed the best with the BSC of 99.1 mg-S/g sorbent and the utilization of 52%. However, it is interesting to note that the desulfurization performance of 50%Sm50Mn50/MSU-S is very poor (Figure 10e) with the BSC of 45.2 mg-S/g sorbent, plausible due to low sulfur capacity character of Sm2O3 itself, indicated that too many Sm2O3 loadings is disadvantage for the enhancement of sulfur capacity. 3.7.2. Effect of Amount of Metal Oxide Loadings and MSU-S Type on H2S Removal. The H2S breakthrough curves over sorbents with different metal oxide loadings are shown in Figure 11. It can be seen that the breakthrough sulfur capacity increases with the increasing metal oxides (from 45% to 60%, Figure 11). It is well-known that a large amount of metal oxide loading can result in the formation of more metal sulfides, but more loadings can promote the aggregation or sintering of sulfide particles during prolonged desulfurization of hot coal gas.45 According to the kinetic model of shrinking core reported by Slimane and Hepworth,6 the existence of dense metal sulfide layers increased remarkably the mass transfer resistance of H2S molecules in hot coal gas into the inner core G

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Figure 8. (A) C 1s (B) Mn 2p (C) Mn LMM (D, E) Mn 3p spectra of (a) fresh 55%Sm5Mn95/MSU-S (b) fresh 100%Sm5Mn95 (c) used 100% Sm5Mn95.

increased from 700 to 800 °C. The initial desulfurization rate and sulfur uptake capacity increased because the activated process of solid−gas state diffusion became fast with incremental temperature despite the destruction of the channel and aggregation of 55%Sm5Mn95/MSU-S sorbent. Sintering was strongly temperature dependent according to the report of Moulijn et al.47 The diffusion in the solid phase compensates the decrease in sulfur capacity that is caused by the loss of surface area in the range of 700−800 °C to some extent. The sulfur removal efficiency over 55%Sm5Mn95/MSU-S decreased slightly at 850 °C and its desulfurization curve was quite smooth after the breakthrough point, plausible due to serious collapse of the mesoporous framework or the aggregation of Mn particles in the sorbent, which hinder severely the gas− solid spread into the inside of the sorbent and made the chemical reaction between the sorbent and the gas not sufficient. Thus, the effective of desulfurization at 800 °C was better than that at 850 °C. 3.7.4. Influence of H2S Concentration on Performance of 55%Sm5Mn95/MSU-S. As shown in Figure 13, the variation of H2S concentration (from 0.23 to 0.53 vol %) in hot coal gas affected markedly the desulfurization feature of 55%Sm5Mn95/

Figure 9. TG/DTG and DSC curves for 55%Sm5Mn95/MSU-S after desulfurization at 800 °C.

highest sulfur capacity (152 mg-S/g sorbent) at 800 °C. It can be noticed that an effectiveness of the desulfurization process H

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Figure 10. Desulfurization behavior over different SmxMny/MSU-S sorbents at 800 °C: (a) 50%Mn100/MSU-S (b) 50%Sm5Mn95/MSU-S, (c) 50%Sm10Mn90/MSU-S, (d) 50%Sm20Mn80/MSU-S, (e) 50% Sm50Mn50/MSU-S, (WHSV = 9 × 103 mL h−1 g−1; feed composition, 0.33 vol %-H2S, 10.5 vol %-H2, 18 vol %-CO and N2 balance gas). The data in and out of the parentheses are the breakthrough sulfur capacities (mg-S/g sorbents) and utilization (%), respectively.

Figure 12. H2S breakthrough curves of 55%Sm5Mn95/MSU-S sorbent for different reaction temperatures; (a) 700, (b) 750, (c) 800, (d) 850 °C (WHSV = 9 × 103 mL h−1 g−1, feed composition: 0.33 vol%-H2S, 10.5 vol %-H2, 18 vol %-CO and N2 balance gas). The data in the parentheses are the breakthrough sulfur capacities (mg-S/g sorbents).

Figure 11. H2S breakthrough curves of sorbents for different loadings at 800 °C; (a) 45%Sm5Mn95/MSU-S, (b) 50%Sm5Mn95/MSU-S, (c) 55%Sm5Mn95/MSU-S, (d) 60%Sm5Mn95/MSU-S, (e) 55%Sm5Mn95/ MSU-S (H type), (f) 100%Sm5Mn95, (WHSV = 9 × 103 mL h−1 g−1, feed composition: 0.33 vol%-H2S, 10.5 vol %-H2, 18 vol %-CO and N2 balance gas). The data in the parentheses are the breakthrough sulfur capacities (mg-S/g sorbents).

Figure 13. H2S breakthrough time curves of 55%Sm5Mn95/MSU-S sorbent at different inlet H2S concentrations (mg/m3); (a) 2300 (b) 3300 (c) 4300 (d) 5300 (800 °C ; WHSV = 9 × 103mL h−1 g−1; 10.5 vol %-H2, 18 vol %-CO and N2 balance gas). The data in parentheses are the breakthrough sulfur capacities (g-S/100 g sorbent).

MSU-S. The breakthrough time became short and the slope of curves became steep with the increase of initial concentration of H2S (Figure 13a−d). In the meantime, the breakthrough sulfur capacity of the sorbent (Figure 13) increased as the initial concentration of H2S increased (from 0.23% to 0.33%) because the chemical equilibrium shifted toward the positive direction when the H2S concentration increased according to Le Chatelier’s principle. However, when the initial concentration of H2S increased from 0.33% to 0.53%, the BSC decreased. This was because high H2S concentration resulted in fast formation of a metal sulfide (MnS and Sm2O2S) layer, according to the following reaction:

interaction between H2S and internal active species of sorbent.48 3.7.5. Influence of Steam Contents in Hot Coal Gas on Performance of 55%Sm5Mn95/MSU-S. The desulfurization performance of 55%Sm5Mn95/MSU-S sorbent was evaluated using hot coal gas with steam (0−9 vol %) due to the fact that there are different amount of steam during the coal gasification. As shown in Figure 14, the BSC value over 55%Sm5Mn95/ MSU-S declined significantly with incremental contents of steam. According to the reaction equation MnO(s) + H2S = MnS(s) + H2O, if the chemical reaction quotient, [H2O]/ [H2S], is thermodynamically larger than the equilibrium constant (Keqb), the aforementioned sulfidation reaction occurred inversely.45 The sorption process is inhibited due to the water−gas shift reaction. However, the breakthrough sulfur capacity (66.1 mg-S/g sorbent) observed over 55%Sm5Mn95/ MSU-S at 800 °C is higher than that (55.1 g-S/g sorbent) over

Sm2O3 + Mn2O3 + 3H 2S + H 2 = Sm2O2 S + 2MnS + 4H 2O

(6)

The formed dense metal sulfide layers increased the internal mass transfer resistance of H2S molecules and lowered the I

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curves over 55%Sm5Mn95/MSU-S sorbent at 800 °C for eight successive sulfidation−regeneration cycles. Although the BSC decreased slightly after an initial sulfurization and regeneration cycle (Figure 15), it almost maintained a constant and no degradation was observed in the later cycles. Therefore, the 55%Sm5Mn95/MSU-S sorbent with high performance was renewable and durable. However, it can be seen that the BSC value (124.8 mg-S/g sorbent) over regenerated 55%Sm5Mn95/ MSU-S was slightly lower than that (152 mg-S/g sorbent) over fresh sorbent due to the sinter and aggregation of active species47 or the collapse of partial mesoporous MSU-S structure with incremental regeneration cycles. This result is significantly different from that for Mn-based/MCM-41 sorbents because the BSC value of the latter reduced remarkably after the first regeneration cycle32 due to the fact that thermal stability or hydrothermal stability of MCM-41 deteriorated at 600 °C. The results of N2 adsorption and smallangle XRD for MCM-41 (Figures 1 and 3) treated at 800 °C in 10% H2O/N2 atmosphere also confirmed the aforementioned conclusion. In addition, the hot spots can be formed during the regeneration because of the exothermicity of the regeneration reaction (Figure 9). We had resolved the problem in hot spots by using a 5% O2/N2 mixture. High loading 55%Sm5Mn95/ MSU-S sorbent after eight successive sulfidation−regeneration cycles is remarkably superior to 1Cu9Mn/SBA-15 in regards to the SBET value (26 vs 13 m2/g).15 Thus, the high structural stability of MSU-S reduced effectively sintering of metal oxides and inhibited surface migration of the active phase. The results demonstrated that the 55%Sm5Mn95/MSU-S sorbent was a regenerable and highly stable with a large sulfur capacity.

Figure 14. H2S breakthrough time curves of 55%Sm5Mn95/MSU-S sorbent at different steam content; (a) 0% steam, (b) 3% steam, (c) 5% steam, (d) 7% steam, (e) 9% steam, at 800 °C ; WHSV = 9 × 103 mL h−1 g−1; 0.33 vol %-H2S, 10.5 vol %-H2, 18 vol %-CO and N2 balance gas). The data in parentheses are the breakthrough sulfur capacities (mg-S/g sorbents).

4Mn1Ce/HMS sorbent at 600 °C reported by Zhang et al.44 in the presence of 7% steam due to high stability of the sorbent. This indicated that MSU-S presented an excellent hydrothermal stability of high-temperature resistance compared to HMS support. The BET result (Table 1) of MSU-S treated at 800 °C in 10% H2O/N2 atmosphere also verified the hydrothermal stability of mesoporous MSU-S with FAU structure. According to the report of Liu et al.,22 the stability AlMSU-S correlated strongly with the composition of its pore wall with faujasitic Y zeolite seeds. The results of NH3-TPD and 27Al MAS NMR (Figure 2) revealed the existence of framework 4-coordination aluminum with strong Lewis acidic sites. 3.7.6. Successive Sulfurization-Regeneration Cycles of 55%Sm5Mn95/MSU-S. Figure 15 shows the breakthrough

4. CONCLUSION A series of SmxMnyOz/MSU-S sorbents were prepared by a sol−gel method. By the optimization of the Sm/Mn atomic ratios and the reaction temperature, 55%Sm5Mn95/MSU-S performed the best at 800 °C with an effective breakthrough sulfur capacity of 152 mg-S/g sorbents and high utilization (71.2%) of active species. The results of N2 adsorption, 27Al MAS NMR, NH3-TPD and HRTEM indicated that the high desulfurization performance of 55%Sm5Mn95/MSU-S sorbent depended strongly on excellent hydrothermal stability and/or structural properties of MSU-S, obviously superior to high quality of MCM-41. After incorporation with rare earth oxide Sm2O3, the active Mn2O3 species was highly dispersed on the pore wall of MSU-S. Therefore, the results of eight successive desulfurization−regeneration cycles verified that the highperformance 55%SmxMny/MSU-S sorbent was renewable and durable. The result of XPS verified the coexistence of both Mn3+ (49.5 eV) and Mn2+ (48.2 eV) in 55%Sm5Mn95/MSU-S and The TG/DSC of used 55%Sm5Mn95/MSU-S suggested that no MnSO4 phase formed during the desulfurization/ regeneration cycles.

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AUTHOR INFORMATION

Corresponding Author

*B. S. Liu. Tel.: 86-22-27892471. E-mail: [email protected].

Figure 15. Sulfurization−regeneration cycles over 55%Sm5Mn95/ MSU-S with WHSV = 9 × 103 mL h−1 g−1; sulfurization T = 800 °C (0.33 vol %-H2S, 10.5 vol %-H2, 18 vol %-CO and N2 balance gas); regeneration T = 700 °C. Inset shows the breakthrough sulfur capacity.

Notes

The authors declare no competing financial interest. J

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

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