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Journal of Hazardous Materials 248– 249 (2013) 81– 88
Contents lists available at SciVerse ScienceDirect
Journal of Hazardous Materials
jou rn al h om epage: www.elsev ier .com/ loc ate / jhazmat
ormation of (FexMn2−x)O3 solid solution and high sulfur capacity properties ofn-based/M41 sorbents for hot coal gas desulfurization
. Zhang, B.S. Liu ∗, F.M. Zhang, Z.F. Zhangepartment of Chemistry, Tianjin University, Tianjin 300072, PR China
i g h l i g h t s
High quality of MCM-41 was pre-pared from cheap and availableindustrial water glass.Formation of (FexMn2−x)O3 solidsolution enhanced greatly sulfurcapacity of sorbent.Mesoporous structure ofFexMnyOz/M41 improved thediffusion rate of H2S molecules.Result of XRD, XPS and LRS veri-fied that (FexMn2−x)O3 species wasmainly active phase.
g r a p h i c a l a b s t r a c t
r t i c l e i n f o
rticle history:eceived 31 October 2012eceived in revised form7 December 2012ccepted 28 December 2012vailable online 5 January 2013
a b s t r a c t
Several MCM-41 materials were synthesized at different conditions by hydrothermal procedure usingcheap and easily available industrial water glass as silica source. Fe doped manganese-based oxide/MCM-41 sorbents were prepared by a sol–gel method. The effects of loadings of metal oxide, Fe/Mn molar ratiosover MCM-41 and reaction temperature on the performance of sorbent for hot coal gas desulfurizationwere investigated. Various techniques such as BET, XRD, XPS, LRS and HRTEM were used to characterizethe sorbents. The result indicated Fe3+ ions could occupy a position of Mn3+ in cubic lattice of Mn2O3 and
eywords:ot coal gas desulfurizationolid solutionesoporous sorbentigh sulfur capacity
the (FexMn2−x)O3 solid solution is mainly active phase of sorbent. Moreover, the result of nine succes-sive sulfurization–regeneration cycles of sorbent showed high sulfur adsorption capacity and endurablestability of FeMn4Ox/MCM-41 for H2S removal.
© 2013 Elsevier B.V. All rights reserved.
heap material
. Introduction
Coal is the major fuel of electric power plants due to low costnd plentiful resource [1]. The conventional coal-fired systems suchs pressurized fluidized bed combustion (PFBC) and low emissionoiler system (LEBS) indicate that more than 60% of the energy
f coal is wasted during the operational process [2]. In order tomprove the energy efficiency, the integrated gasification combinedycle (IGCC) has attracted considerable attention [3]. However,∗ Corresponding author. Tel.: +86 2227892471; fax: +86 2287892946.E-mail address: [email protected] (B.S. Liu).
304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jhazmat.2012.12.053
some contaminants, composed of mainly H2S with small amountsof COS and CS2, are generated in the coal gasification process. All ofthese contaminants can pollute environment, corrode equipmentsand extremely endanger the human health [3,4]. Therefore, theremoval of H2S from hot coal gas is crucial for the protection ofequipment from corrosion and to meet the environmental criteriaand policy for sulfur emissions.
The hot coal gas desulfurization (HGD) can effectively utilizeoverall thermal efficiency compared to the removal of H2S at room
temperature (RT). Based on a free energy minimization princi-ple, Westmoreland and Harrison [5] have implemented extensivethermodynamic screening tests of desulfurization potentials for28 metal oxides at high temperature. They pointed out those 11
8 us Ma
edocnrcmoZrpmkmoippblpos
seira[frsfcbicaamdbsfMcaptsoe
2
2
FoNib
2 Y. Zhang et al. / Journal of Hazardo
lements, Fe, Zn, Mo, Mn, V, Ca, Sr, Ba, Co, Cu and W, have thermo-ynamic feasibility. Among these metal oxides, manganese-basedxide sorbents show high stability, high utilization, large sulfurapacity [5,6] and the ability of removing H2S and COS simulta-eously [7]. As reported in literature [8–10], the reactivity andegenerability of mixed metal oxides are significantly improvedompared with the single element sorbents. Therefore, dopedanganese-based sorbent has attracted the considerable attention
f many researchers, especially in the past decade. For example,hang et al. [11] prepared Mn–Fe–Zn mixed oxides to study theegeneration condition of sorbent. Cu doped manganese oxide wasroposed as regenerable sorbent [12–14] to further prevent for-ation of manganese sulfate during regeneration. However, to our
nowledge, there was not any description or comparison with pureanganese oxide properties concerning that the introduction of
ther metal oxides will enhance the breakthrough sulfur capac-ty of Mn-based sorbent. Indeed, breakthrough sulfur capacity isrecisely the core of commercialization of sorbent. Therefore, werepared Mn-based sorbent with high breakthrough sulfur capacityy doping proper iron species because ferric oxide also presented
arge sulfur capacity, high reaction rate and similar reaction tem-erature with manganese oxide [15–18]. Moreover, the formationf ferrimanganese oxide also improved the stability in structure oforbent during sulfurization or regeneration reaction [19].
In general, the non-supported metal oxides exhibited higherulfur sorption capacity than supported sorbent [20,21]. How-ver, the successive regeneration ability of these sorbents is poorn hot coal gas desulfurization due to the problems in evapo-ation, sintering and mechanical disintegrations. Supports, suchs Al2O3, SiO2, TiO2 and ZrO2 are applied as structure stabilizer2,22]. However, these conventional supports have low specific sur-ace area, which can only load a small amount of active species,esulting in a relatively low sulfur capacity (about 5 g S/100 g oforbent) [2,11,22,23]. Recently, MCM-41 and SBA-15 with high sur-ace area, large pore size and narrow pore size distribution hasaused the extensive interest of researchers as supports of sor-ents [24–27]. From the economic view-points, the utilization of
ndustrial water glass as silicon source can decrease greatly theost of preparing sorbent. Hence, high quality of MCM-41 (denoteds “M41” hereinafter) was synthesized from cheap and easily avail-ble industrial water glass and presented excellent characteristic ofesoporous structure. Next, a series of Fe–Mn/M41 sorbents with
ifferent Fe/Mn molar ratios and metal loadings were preparedy a sol–gel method, and the performance of high breakthroughulfur capacity for FeMn4/M41 sorbent could contribute to theormation of (FexMn2−x)O3 solid solution via Fe lattice replace in
n2O3. For comparison, 45%wt. Mn/M41 sorbent was also fabri-ated by aforementioned procedure. The desulfurization activitiesnd regeneration ability of these sorbents were investigated. Theroperties of sorbents were characterized using X-ray diffrac-ion (XRD), X-ray photoelectron spectroscopy (XPS), laser Ramanpectroscopy (LRS), Brunauer–Emmertt–Teller (BET) and high res-lution transmission electron micrograph (HRTEM) techniques toxplain the different reaction activities.
. Experimental
.1. Synthesis of M41
The M41 was synthesized at various conditions (Table 1).irstly, to improve the thermal stability of materials [28], 8.183 g
f industrial water glass (molecular formula determined wasa2O·2.84SiO2) and 50 mL deionized water (DW) were transferrednto a Teflon-lined autoclave and remained at 110 ◦C for 12 h. Then,ased on CTAB/SiO2 ratios, an appropriate amount of cetyltrimethyl
terials 248– 249 (2013) 81– 88
ammonium bromide (CTAB) was dissolved with 100 mL of DW(Table 1). After stirred at 60 ◦C for 0.5 h, the treated water glass wasadded dropwise to the aforementioned solution. The molar com-position of the synthesis solution was SiO2/xCTAB/120H2O. In themeantime, 1 mol/L of H2SO4 solution was used to adjust pH withstirring continually for another 2 h. Finally, the obtained sol wastransferred into a Teflon-lined autoclave and hydrothermal syn-thesis was conducted at 110 ◦C for 48 h. The products were filtered,washed to neutral and calcined at 550 ◦C for 6 h.
2.2. Preparation of Fe doped manganese-based sorbents
A series of sorbents were synthesized by means of a sol–gelmethod [29]. Taking the preparation of 45 wt.% Fe dopedmanganese-based sorbent (Fe/Mn molar ratio = 1:4) as an exam-ple. Approximately 11.84 g of 50% Mn(NO3)2 and 3.34 g of Fe(NO3)3were first dissolved in 50 mL HNO3 solution (0.01 mol/L) to pre-vent hydrolysis. After the addition of citric acid with molar amount1.5 times that of total metal ions, the as-prepared M41 was added.Next, the sol was continuously stirred at 60 ◦C until the formationof yellow gel. The product was aged and dried at room tempera-ture (RT) for a week, calcined at 550 ◦C for 6 h in air (100 mL/min).The obtained 45 wt.% FeMn4Oz/M41 sorbent was denoted as “45%FM4/M41” (FM4 denotes Fe/Mn molar ratio = 1:4 hereinafter). Sim-ilarly, 25% FM2/M41, 35% FM2/M41, 45% FM2/M41, 55% FM2/M41,45% FM6/M41 and 45% Mn/M41 were also prepared. All sorbentswere pressed and sieved through 20–40 meshes.
2.3. Characterization of synthesized materials
The nitrogen adsorption isotherms of M41 and sorbents weremeasured on a domestic N2 adsorption apparatus at 77 K [30]. Priorto analysis, the samples were treated in vacuum 200 ◦C for 2 h. TheBET surface area, pore volume and pore size of the samples werecalculated by Barrett–Joyner–Halenda (BJH) method via correlativeisotherms. Small- and wide-angle powder XRD investigation wereconducted over Rigaku D/max 2500 v/pc Automatic diffractometerand PANalytical X’Pert PRO diffractometer with Cu K� radiation(� = 0.15406 nm) and Ni filter at settings of “20 kV, 30 mA” and“40 kV, 50 mA”, respectively. XPS experiments were carried out ona RBD upgraded PHI-5000C ESCA system (Perkin Elmer) with MgK� radiation (h� = 1253.6 eV). The X-ray anode was run at 250 Wand high voltage was kept at 14.0 kV with a detection angle at 54◦.The whole spectra (0–1100 (1200) eV) and the narrow spectra of allthe elements with much high resolution were recorded using RBD147 interface (RBD Enterprises, USA) through the Auger Scan 3.21software. Binding energies were calibrated using Si 2p3/2 (103.3 eV)in support SiO2.
The LRS study was performed by single spectrum using a ThermoFischer DXR Raman Microscope. We select the 50× objective of theconfocal microscope together with a laser source 532 nm at 10 mWin mode laser power at 100%. Ten spectrum signals of 10 s expo-sures were averaged to improve the signal to noise ratio. Spectrawere analyzed using the OMNIC for Dispersive Raman Software.The HRTEM images and energy-dispersive X-ray (EDX) elementanalysis were performed on a Tecnai G2 F20 electron microscopyoperated at 200 kV.
2.4. Desulfurization and regeneration properties of sorbents
Desulfurization performance of sorbents was evaluated in afixed-bed reactor reported previously [25]. A thermocouple placed
in the center of the reactor was used to measure the reaction tem-perature. 0.5 g of sorbent was charged into the reactor and heated(10 ◦C/min) to required temperature in a N2 atmosphere. Then,simulative hot coal gas with composition of 0.33% H2S, 10.5% H2,
Y. Zhang et al. / Journal of Hazardous Materials 248– 249 (2013) 81– 88 83
Table 1Specific surface area (SBET), total pore volume (VT), micropore volume (Vmic), mesopore volume (Vmeso) and average pore size (Da) of M41 prepared with different conditions.
Synthesis conditions Physical properties of materials
x (CTAB/SiO2) pH SBET (m2/g) VT (mm3/g) Vmic (mm3/g) Vmeso (mm3/g) Da (nm)
0.1 9.5 596 700 123 577 4.70.2 9.5 807 830 124 706 4.10.3 9.5 721 780 185 595 4.90.2 10 1039 934 300 634 3.60.2 10.5 1114 1140 369 771 4.10.2 11 971 730 310 420 3.0
1cEootds
S
Wm1e8bia
3
3
taos(aisMrrraTavAsdg
3
b
High quality M41 [32] 1110 1060
8% CO, and 71.7% N2 was introduced and the flowing rates wereontrolled by mass flowmeters (D07-7B/ZM, Beijing Seven-starlectronics Co. Ltd., China). The concentration of H2S in inlet andutlet (denoted as Cin and Cout mg m−3) were analyzed by meansf iodimetry. The time at desulfurization breakthrough onset (i.e.he H2S concentration in outlet gas is less than 100 mg/m3) wasefined as the breakthrough time (t h). The sulfur capacity (SC) oforbent was calculated by following equation:
C(
g of sulfur100g of sorbent
)= WHSV × MS
Vm×
[∫ t
0(Cin − Cout) dt
]
×10−4 (1)
HSV is weight hourly space velocity (L h−1 g−1); MS stands forolar weight of S (32.06 g mol−1); Vm is molar volume of H2S at
atm. and 25 ◦C (24.5 L mol−1). The used sorbents were regen-rated at 700 ◦C in 5.0% (vol.) O2/N2 mixture with WHSV of
× 103 mL h−1 g−1 (2.4 × 103 h−1) until SO2 in the effluent cannote detected, finally, remained at 700 ◦C for 1 h in N2 atmosphere
n order to decompose sulfate formed at the regeneration processccording to report of Karayilan et al. [31].
. Results and discussion
.1. BET characterization of M41
As listed in Table 1, a high quality of M41 was synthesized byhe optimization of CTAB/SiO2 molar ratios (x) and pH. The N2dsorption isotherms and the corresponding pore size distributionsf M41 were shown in Fig. 1. The isotherms of samples synthe-ized at different condition could be classified as type-IV isothermFig. 1A). According to the report of Cai et al. [31], these materi-ls exhibited steep curves within a narrow p/p0 range (0.4–0.6),ndicating the formation of mesopores in the range of narrow poreize distribution. The maximum of the pore size distribution for41 (Fig. 1B) slightly increased with increasing CTAB/SiO2 molar
atios and declined in the range of pH = 10–11. Table 1 summa-ized physical properties of M41 prepared at different conditions. Itevealed that M41 prepared from industrial water glass at pH = 10.5nd x = 0.2 was comparable to high quality of M41 synthesized fromEOS [32] whereas the cost of industrial water glass was remark-bly low. In addition, the specific surface area (SBET) and total poreolume (VT) of M41 (Table 1) correlated closely with pH in solution.ccording to the report of Chen et al. [33], the acidity and basicity inynthesized solution can effectively control the rate of Si(OH)4 con-ensation. We may envisage that pH = 10.5 are favorable for waterlass condensation to form stable M41 structure.
.2. XRD, XPS and LRS analysis of sorbent
The small-angle XRD patterns of M41 and 45% FM4/M41 sor-ent were shown in Fig. 2A. There were a strong diffraction
– – 2.9
peak around 2� = 1.78◦ and three weak peaks at 2� = 3.1◦, 3.5◦
and 4.6◦ (Fig. 2A(a)), which could be indexed to (1 0 0), (1 1 0),(2 0 0) and (2 1 0) crystal plane belonged to P6mm space groupof two-dimensional-hexagonal symmetry. According to the reportof Renzo et al. [34], the thermal stability of mesoporous M41depended strongly on wall thickness. Therefore, we calculated wallthickness (1.6 nm) of as-prepared M41 via the difference of unitcell parameter (a0 = 2d1 0 0/31/2) with the maximum pore size (Dp),which is higher than that (0.6–1.5 nm) reported by Renzo et al.[35]. After Fe–Mn oxide is loaded, the diffraction peak of (1 0 0)crystal plane (Fig. 2A(c)) was shifted to 2� = 2.3◦ for 45% FM4/M41sorbent and corresponding interplanar spacing (d) became small(n� = 2d sin �). The estimated wall thickness was 1.9 nm.
The structure of FexMnyOz/M41 (Fe/Mn molar ratios = 1/2, 1/4,1/6 and 0) sorbents were characterized by means of wide-angleXRD technique. In 45% Mn/M41 sorbent (Fig. 2B(e)), the Mn2O3(2� = 23.1, 32.9, 38.2 and 55.1◦) with cubic structure was maincrystal phase [PDF#24-0508] companied with the formation oftetragonal structural Mn3O4 (2� = 29.0 and 36.1◦) [PDF#24-0734].After the 45% Mn/M41 and 45% FMy/M41 (y = 2, 4, 6) sorbents werecalcined in air for 12 h, there were still the diffraction peaks ofMn3O4 (Fig. 2B(f, g), not show the XRD patterns of 45% FMy/M41(y = 2, 6)), it meant that formation of Mn3O4 species was inevitableeven if adequate oxidation. Interestingly, consistent with the XRDpattern of 45% Mn/M41, all 45% FMy/M41 (y = 2, 4, 6) sorbents pre-sented the diffraction peaks of (Mn2−xFex)O3 and Mn3O4 crystalphase due to lattice replace between Fe3+ and Mn3+ (Fig. 2B(b–d)).As for used 45% FM4/M41 sorbent (Fig. 2B(h)), the diffraction peaksat 2� = 29.6◦, 34.3◦, 49.4◦ and 61.5◦ can be assigned to MnS withcubic structural [PDF#65-0891].
The elemental valence states of fresh and used 45% FM4/M41can be obtained accurately from the XPS analysis. For fresh 45%FM4/M41, the XPS spectra at 642.1 and 653.7 eV (Fig. 3A) can beassigned to Mn 2p3/2 and Mn 2p1/2 [36,37]. And the variation ofbinding energies for Mn 2p3/2 before and after sulfidation is toosmall to precisely evaluate the valence state of manganese in MnOx.Therefore, the more sensitive Auger spectra of Mn LMM are also col-lected (Fig. 3B). It was reported that the kinetic energy for Mn0 was586.6 eV [38]. The kinetic energy of the Auger peak observed overfresh FM4/M41 is 582.5 eV whereas that over used one is 583.6 eV,approximately shifting of 1.1 eV toward high kinetic energy, indi-cating the transformation of Mn3+ to Mn2+ after desulfurization,in accordance with the analysis of XRD (Fig. 2B). The S 2p XPSspectra of fresh and used 45% FM4/M41 are shown in Fig. 4A.The presence of peak at 161.1 eV over used sample suggested theformation of metal sulfide [38] except for the peaks (154.3 eV)of silicon 2 s in M41. As shown in Fig. 4B, the peak at 711.1 eVcould be described to Fe 2p3/2 XPS spectrum in (FexMn2−x)O3
[39]. After sulfidation, the peak was shifted to 710.3 eV due tothe formation of FeS [38]. According to the analyses of XRD, therewas only the crystal phase of manganese oxide in fresh sam-ple due to the fact that Fe3+ ions (rFe3+ = 0.55 Å ) occupied a
84 Y. Zhang et al. / Journal of Hazardous Materials 248– 249 (2013) 81– 88
0.0 0.2 0.4 0.6 0.8 1.00
100
200
300
400
500
600
700
800
900
1000
20 40 60 80100
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7a
b
c
d
e
f
Volu
me(c
m3/g
)
Relative pressure (p/p0)
A
Pore diameter (angstrom)
B
Rela
tive p
ore
volu
me(m
3Å
-1g
-1)
a
b
c
d
e
f
F paredp
pcso
vuFwnbboBr
Fa
ig. 1. (A) N2 adsorption isotherms and (B) pore diameter distribution of M41 preH = 10.5, (f) pH = 11 in x = 0.2.
osition of Mn3+ (rMn3+ = 0.58 Å ) in cubic lattice of Mn2O3 viarystalline substitution. The (FexMn2−x)O3 solid solution has theame crystalline phase with Mn2O3 and so does the solid solutionf sulfide.
45% Mn/M41 and 45% FM4/M41 sorbents were characterizedia RT laser Raman spectroscopy (LRS) (Fig. 5b and c) in order tonderstand deeply the crystalline phase and action of Fe dopant.or comparison, the Raman spectroscopy of pure manganese oxideas also shown in Fig. 5a. According to Raman spectroscopy sig-als of bulk Mn2O3 and Mn3O4 reported by Han et al. [36], theands at 662 and 318 cm−1 detected for manganese oxide can
e ascribed to the symmetric stretch of MnOx groups and outf plane bending modes of Mn2O3/Mn3O4. Han et al. [36] anduciuman et al. [40] claimed that the peak at 365–370 cm−1 cor-elated closely with Mn3O4 species and the peak in intensity was10 2
Mn
Mn
(Mn2-
Mn
2 4 6 8
2 Theta (de gree)
c
a(100 ) (210 )
(200 )
(110 )
(100 )
Inte
nsity (
Arb
itra
ry u
nits)
A
ig. 2. (A) Small- and (B) wide-angle XRD patterns of (a) M41, (b, c, d, e) 45% FM2/M41,
nd 45% FM4/M41 calcined for 12 h and (h) used 45% FM4/M41.
under the conditions of (a) x = 0.1, (b) x = 0.2, (c) x = 0.3 in pH = 9.5, (d) pH = 10, (e)
remarkably higher than that at 318 cm−1, which is just oppositefrom our observation (Fig. 5). This suggested that the manganeseoxide used by us was the mixture of Mn2O3 and Mn3O4. When man-ganese oxide was loaded on M41 by a sol–gel method, as shownin Fig. 5b, the LRS band at 662 cm−1 was shifted to 652 cm−1 andbroadened significantly due to high dispersion of Mn2O3/Mn3O4particles over M41, this phenomenon was also observed by Zuoet al. [41,42]. Based on LRS band of Mn2O3/SBA-15 and pure bulkMn2O3 reported by Han et al. [36], the band at 504 cm−1 wasrelated to the asymmetric stretch of Mn O Mn bond of sup-ported Mn2O3 species. There was a new band at 610 cm−1 for
45% Mn/M41 sorbent, which can be contributed to the vibrationof Mn O Si bonding [40,43]. According to the report of Buciuman[40], the bands at 610–615 cm−1 was observed only for supportedsamples and the intensity of peak increased with the rise of Mn0 30 40 50 60 70
3O4
g
S
2 Theta (de gree)
b
c
d
e
f
h
xFe
x)O
3
2O3 orB
45% FM4/M41, 45% FM6/M41 and 45% Mn/M41 calcined for 6 h, (f, g) 45% Mn/M41
Y. Zhang et al. / Journal of Hazardous Ma
664 65 6 64 8 64 0
653.5
653.7
64
1.7
64
2.1
Mn 2p
Binding energ y (eV)
A
a
b
595 59 0 58 5 58 0
583.6
58
2.5
B
Mn LMM
b
a
Kinetic energ y (eV)
Fig. 3. (A) Mn 2p XPS and (B) Mn LMM Auger spectra of (a) fresh and (b) usedFM4/M41 after desulfurization at 600 ◦C.
730 72 0 71 0 70 0
B
Binding energy (eV)
a
b
Fe 2p3/2
71
0.3
71
1.1
175 17 0 16 5 16 0 15 5
A
Binding energy (eV)
a
b
S 2p
161.1
154.3
Fig. 4. (A) S 2p and (B) Fe 2p3/2 XPS spectra of (a) fresh and (b) used FM4/M41 afterdesulfurization at 600 ◦C.
200 30 0 40 0 50 0 60 0 70 0 80 0
Inte
nsity (
Arb
itra
ry u
nits)
Raman shift ( cm-1 )
662
369
318
a
200 30 0 40 0 50 0 60 0 70 0 80 0
Raman shift ( cm-1 )
652610
504
367318
610
487
367
b
C
Fig. 5. Laser Raman spectroscopy (LRS) of (a) non-supported manganese oxide, (b)45% Mn/M41 and (c) 45% FM4/M41.
terials 248– 249 (2013) 81– 88 85
loadings. Therefore, the band at 610 cm−1 can be described to man-ganese species on the surface of M41. As for FM4/M41 sorbent(Fig. 5c), after Fe was doped in Mn2O3, the Raman bands of sam-ple were down-shifting significantly, the band at 652 cm−1 shiftedto 610 cm−1, which can be explained by the enlarged Mn O bondlengths resulted from lattice distortions of (FexMn2−x)O3 (rFe3+ =0.55 Å , rMn3+ = 0.58 Å ). In the meantime, the peak at 367 cm−1,which was considered to be characteristic band of Mn3O4 species[36], weakened in intensity. The decline of Mn3O4 species is favor-able for desulfurization activity of 45% FM4/M41 sorbent.
3.3. BET and TEM characterization of sorbents
After the loading of active species, the N2 adsorption isothermsand pore diameter distributions (Fig. 6) of fresh and used sorbentsstill presented typical feature of mesoporous martial and theirphysical properties were listed in Table 2. Compared to pure M41(Fig. 1e), the SEBT and VT of as-prepared sorbents decreased gradu-ally with incremental metal oxide loadings due to the coverage ofinner surface of channels. As far as the same loadings and differentFe/Mn ratios is concerned, the variation in the SEBT, VT and Da werevery small due to similar ion radius between Fe3+ and Mn3+ ions.For used 45% FM4/M41 sorbent, the SEBT and VT became smallerafter sulfidation because O2− (ionic radius 0.14 nm) in sorbent wassubstituted by S2− (0.184 nm).
In order to further investigate the stability of mesoporous struc-ture, HRTEM images of 45% FM4/M41 sorbents before and aftersulfidation were shown in Fig. 7. HRTEM images of fresh 45%FM4/M41 sorbent revealed the existence of a highly ordered meso-porous structure similar to the HRTEM images of LaFeO3/SBA-15synthesized from TEOS as siliceous source [25]. After sulfidation,HRTEM image of used 45% FM4/M41 sorbent showed that themesoporous structure of sample remained intact (Fig. 7C). The EDXanalysis of used sorbent (Fig. 7D) revealed that Fe–Mn oxides wereentirely converted to sulfide because atomic ratio of oxygen andsilicon was 2:1.
3.4. Desulfurization activity evaluation of sorbents
3.4.1. Effect of metal oxide loadings over M41 and temperatureson sulfidation performance
The H2S breakthrough curves over the sorbents with differentmetal oxide loadings were shown in Fig. 8A. The breakthroughsulfur capacity of different sorbents increased with incrementalmetal oxide loadings and 55% FM2/M41 sorbent exhibited the high-est breakthrough sulfur capacity. However, it can be seen thatafter breakthrough point, the H2S breakthrough curve over 55%FM2/M41 sorbent became smooth (not steep) due to the fact thatdiffusion resistance of H2S through thick sulfide layer increasedso that the reaction rate with inner oxide reduced. Therefore,45% FM2/M41 is considered to be more available. As shown ininset of Fig. 8B, the H2S breakthrough sulfur capacity changedslightly with the variation in temperature. The breakthrough sulfurcapacity at 600 ◦C was the highest and there were steep deactiva-tion rate curves (Fig. 8B), which originated from the competitionbetween sulfidation and reduction of metal oxide in reductive hotcoal gas.
3.4.2. Effect of Fe/Mn molar ratios on sulfurization performanceAs shown in Fig. 9, the breakthrough sulfur capacity of sorbents
depended strongly on Fe/Mn molar ratios. The desulfurization per-
formance of sorbent enhanced with the increase of Mn contentsin Fe/Mn mixed oxides due to high sulfur capacity properties ofMn2O3. However, 45% FM6/M41 and 45% Mn/M41 sorbents exhib-ited relatively lower desulfurization efficiency. Especially for 45%
86 Y. Zhang et al. / Journal of Hazardous Materials 248– 249 (2013) 81– 88
0.0 0.2 0.4 0.6 0.8 1.0
50
100
150
200
250
20 40 60 8010 0
0.00
0.05
0.10
0.15
0.20
0.25
Vo
lum
e (
cm3/g
)
Relative p ressure (p/p0)
a
b
c
d
e
f
g
h
A B
Re
lative
po
re v
olu
me
(cm
3Å
-1g
-1)
Pore diame ter (angstrom)
a
b
c
d
e
f
g
h
F FM2/M4
MbtXoTsw
FF
ig. 6. (A) N2 adsorption isotherms and (B) pore diameter distributions of (a) 25%
5% FM6/M41, (g) 45% Mn/M41 and (h) used 45% FM4/M41.
n/M41, not only is the breakthrough sulfur capacity over it low,ut also the deactivation rate declined remarkably after break-hrough point due to the formation of perfect Mn2O3 crystallites.RD results (Fig. 2B(d–e)) verified that the particle size of Mn2O3r (FexMn2−x)O3 enlarged and the contents of Mn3O4 increased.
he 45% FM4/M41 sorbent, showing the highest breakthroughulfur capacity, presented the largest amount of (FexMn2−x)O3ithout Mn3O4 diffraction peaks in XRD pattern (Fig. 2B(c)). Weig. 7. HRTEM images of fresh FM4/M41 sorbent taken with the electron beam (A) perpenM4/M41 sorbent.
41, (b) 35% FM2/M41, (c) 45% FM2/M41, (d) 55% FM2/M41, (e) 45% FM4/M41, (f)
may envisage that the formation of (FexMn2−x)O3 solid solutionvia Fe3+ lattice replace was crucial factor. The introduction ofFe3+ ions effectively improved the dispersion of particles andlimited the reduction of Mn3+ in Mn2O3. The XPS analysis of 45%FM4/M41 sorbent also confirmed the existence of Fe3+ and Mn3+
(Figs. 3 and 4). The H2S breakthrough sulfur capacity over 45%FM4/M41 sorbent was 15.5 g S/100 g sorbent, significantly higherthan those over 50% LaxFeyOz/M41 (La:Fe = 1:2) (5.37 g) and 40%
dicular and (B) parallel to pore axis, (C) HRTEM image and (D) EDX analysis of used
Y. Zhang et al. / Journal of Hazardous Materials 248– 249 (2013) 81– 88 87
Table 2Specific surface area (SBET), total pore volume (VT), micropore volume (Vmic), mesopore volume (Vmeso) and average pore size (Da) of fresh and used sorbents.
Samples SBET (m2/g) VT (mm3/g) Vmic (mm3/g) Vmeso (mm3/g) Da (nm)
25% FM2/M41 585 347 174 173 2.3735% FM2/M41 428 242 136 106 2.2645% FM2/M41 362 226 112 114 2.555% FM2/M41 244 149 79 70 2.4445% FM4/M41 373 241 114 127 2.5845% FM6/M41 332 199 99 100 2.445% Mn/M41 392 308 129 179 3.14Used 45% FM4/M41 215 129 63 65 2.4
0 50 10 0 15 0 20 0 25 0
0
500
1000
1500
2000
2500
3000
0 50 10 0 15 0 20 0
0
500
1000
1500
2000
2500
3000
3500
Exi
t H2S
con
cent
rati
on (
mg/
m3 )
Duration time (min)
Exi
t H2S
con
cent
rati
on (
mg/
m3 )
Duration time (min)
25% FM2/M41
35% FM2/M41
45% FM2/M41
55% FM2/M41(A)
Temperture ( ) C
50 0oC
55 0oC
60 0oC
65 0oC(B)
500 550 600 6500
2
4
6
8
10
12
Bre
ak
thro
ug
h s
ulf
ur
ca
pa
cit
y
(g S
/10
0 g
so
rbe
nt)
F gs at
◦
( 7% N2
Lt
3
bTtd
Frc
76.5% of breakthrough sulfur capacity of fresh one. Combined withthe relatively low price of raw material, the potential of industrialapplication was promising in the future.
ig. 8. H2S breakthrough curves of (A) FM2/M41 sorbents for different loadinWHSV = 2 × 104 mL h−1 g−1, feed composition: 0.33% H2S, 10.5% H2, 18% CO and 71.
aFeO3/SBA-15 (4.85 g) reported by Liu et al. [24,25] in similar reac-ion conditions.
.4.3. Successive sulfurization–regeneration of FM4/M41 sorbentFig. 10 showed the breakthrough curves of 45% FM4/M41 sor-
ent at 600 ◦C for nine successive sulfidation–regeneration cycles.here was significant decrease in breakthrough sulfur capacity afterhe first cycle of sorbent, plausibly due to formation of sulfate, SBETecline or aggregation of active species, and after that it almost
0 50 100 150 200 250
0
500
1000
1500
2000
2500
3000
3500
Exi
t H
2S c
once
ntra
tion
(m
g/m
3 )
Duration time (min)
45% F M2/M41
45% F M4/M41
45% F M6/M41
45% Mn/M41
1:2 1:4 1:6 pu re Mn0
2
4
6
8
10
12
14
16
Bre
ak
thro
ug
h s
ulf
ur
ca
pa
cit
y
(g S
/10
0 g
so
rbe
nt)
Fe/Mn ratio
ig. 9. H2S breakthrough time curves over sorbents with different Fe/Mn molaratios (reaction temperature: 600 ◦C, loadings: 45%, WHSV = 2 × 104 mL h−1 g−1, feedomposition: 0.33% H2S, 10.5% H2, 18% CO and 71.7% N2).
550 C and (B) 45 wt.% FM2/M41 sorbents for different reaction temperatures).
kept a constant. The regeneration sorbent remained approximately
0 50 100 150 200
0
500
1000
1500
2000
2500
3000
3500
Exi
t H
2S c
once
ntra
tion
(m
g/m
3 )
Duration time (min)
1
2
3
4
5
6
7
8
9
1 2 3 4 5 6 7 8 90
2
4
6
8
10
12
14
16
Bre
ak
thro
ug
h s
ulf
ur
ca
pa
cit
y
(g S
/10
0 g
so
rbe
nt)
Cycle number
Fig. 10. H2S breakthrough time curves of FM4/M41 sorbents for nine successivesulfidation and regeneration cycles (reaction temperature: 600 ◦C, loadings: 45%,WHSV = 2 × 104 mL h−1 g−1, feed composition: 0.33% H2S, 10.5% H2, 18% CO and 71.7%N2).
8 us Ma
4
tptuiafptFsHtssc
A
NC
R
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
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[
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8 Y. Zhang et al. / Journal of Hazardo
. Conclusions
The M41 was synthesized from cheap and easily available indus-rial water glass. A series of (FexMn2−x)O3/M41 sorbents wererepared by a sol–gel method. The BET, XRD, XPS, LRS and HRTEMechniques were employed to characterize the support, fresh andsed sorbents. The results showed that the M41 prepared from
ndustrial water glass presented excellent structural propertiesnd even was comparable to high quality of M41 synthesizedrom TEOS. XRD and XPS results revealed that Fe3+ ions occu-ied a position of Mn3+ in solid solution of Mn2O3. XRD indicatedhat the amount of (FexMn2−x)O3 species could be regulated bye/Mn molar ratios, which is mainly active phase of sorbent. LRShowed the structural change when Fe was doped in sorbent.RTEM images of sample verified the stability of mesoporous struc-
ure for sorbents before and after sulfidation. The 45% FM4/M41orbent presented high sulfur adsorption capacity and endurabletability for the nine successive sulfidation and regenerationycles.
cknowledgments
We gratefully acknowledge the joint financial support ofational Natural Science Foundation of China and BAOSTEEL Grouporporation (grant no. 50876122).
eferences
[1] K.C. Kwon, Y.K. Park, S.K. Gangwal, K. Das, Reactivity of sorbent with hot hydro-gen sulfide in the presence of moisture and hydrogen, Sep. Sci. Technol. 12(2003) 3289–3311.
[2] 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.
[3] J.H. Swisher, K. Schwerdtfeger, Review of metals and binary oxides as sorbentsfor removing sulfur from coal-derived gases, J. Mater. Eng. Perform. 1 (1992)399–408.
[4] K.A. Brenneman, R.A. James, E.A. Gross, D.C. Dorman, Olfactory neuron lossin adult male CD rats following subchronic inhalation exposure to hydrogensulfide, Toxicol. Pathol. 28 (2000) 326–333.
[5] 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.
[6] W.J.W. Bakker, F. Kapteijn, J.A. Moulijn, A high capacity manganese-basedsorbent for regenerative high temperature desulfurization with direct sulfurproduction conceptual process application to coal gas cleaning, Chem. Eng. J.96 (1/3) (2003) 223–235.
[7] J.P. Wakker, A.W. Gerritsen, J.A. Moulijin, High temperature H2S and COSremoval with MnO and FeO on �-Al2O3 acceptors, Ind. Eng. Chem. Res. 32 (1)(1993) 139–149.
[8] M.C. Woods, S.K. Gangwal, D.P. Harrison, K. Jothimurugesan, Kinetics of thereactions of a zinc ferrite sorbent in high-temperature coal gas desulfurization,Ind. Eng. Res. 30 (1991) 100.
[9] J.F. Akyurtlu, A. Akyurtlu, Hot gas desulfurization with vanadium-promotedzinc ferrite sorbents, Gas Sep. Purif. 9 (1995) 17.
10] T.J. Lee, W.T. Kwon, C.C. Won, A study of regeneration of zinc titanate sorbentsfor H2S removal, Korean J. Chem. Eng. 14 (1997) 513.
11] J.C. Zhang, Y.H. Wang, D.Y. Wu, Effect investigation of ZnO additive on Mn–Fe/�-Al2O3 sorbents for hot gas desulfurization, Energ. Convers. Manage. 44 (2003)357–367.
12] D. Karayilan, T. Dogu, S. Yasyerli, G. Dogu, Mn–Cu and Mn–Cu–V mixed-oxideregenerable sorbents for hot gas desulfurization, Ind. Eng. Chem. Res. 44 (2005)5221–5226.
13] E. García, J.M. Palacios, L. Alonso, R. Moliner, Performance of Mn and Cu mixedoxides as regenerable sorbents for hot coal gas desulfurization, Energy Fuels14 (2000) 1296–1303.
14] F.M. Zhang, B.S. Liu, Y. Zhang, Y.H. Guo, Z.Y. Wan, F. Subhan, Highly stable andregenerable Mn-based/SBA-15 sorbents for desulfurization of hot coal gas, J.Hazard. Mater. 233/234 (2012) 219–227.
15] Y.G. Pan, J.F. Perales, E. Velo, L. Puigjaner, Kinetic behaviour of iron oxide sorbentin hot gas desulfurization, Fuel 84 (9) (2005) 1105–1109.
[
terials 248– 249 (2013) 81– 88
16] J.D. White, F.R. Groves Jr., D.P. Harrison, Elemental sulfur production duringthe regeneration of iron oxide high-temperature desulfurization sorbent, Catal.Today 40 (1) (1998) 47–57.
17] R.B. Slimane, J. Abbasian, Utilization of metal oxide-containing waste materialsfor hot coal gas desulfurization, Fuel Process. Technol. 70 (2001) 97–113.
18] S.S. Tamhankar, M. Hasatani, C.Y. Wen, Kinetic studies on the reactions involvedin the hot gas desulfurization using a regenerable iron oxide sorbent, Chem.Eng. Sci. 36 (1981) 1181–1191.
19] R.E. Ayala, D.W. Marshi, Characterization and long-range reactivity of zinc fer-rite in high temperature desulfurization processes, Ind. Eng. Chem. Res. 30 (1)(1991) 55–60.
20] J.B. Chung, J.S. Chung, Desulfurization of H2S using cobalt-containing sorbentsat low temperatures, Chem. Eng. Sci. 60 (2005) 1515–1523.
21] J.K. Thomas, K. Gunda, P. Rehbein, F.T.T. Ng, Flow calorimetry and adsorp-tion study of dibenzothiophene, quinoline and naphthalene over modified Yzeolites, Appl. Catal. B: Environ. 94 (2010) 225–233.
22] T.H. Ko, H. Chu, L.K. Chaung, The sorption of hydrogen sulfide from hot syngasby metal oxides over supports, Chemosphere 58 (2005) 467–474.
23] J.C. Wang, B. Qiu, L. Han, G. Feng, Y.F. Hu, L.P. Chang, W.R. Bao, Effect of precursorand preparation method on manganese based activated carbon sorbents forremoving H2S from hot coal gas, J. Hazard. Mater. 213-214 (2012) 184–192.
24] Z.Y. Wan, B.S. Liu, F.M. Zhang, X.H. Zhao, Characterization and performance ofLaxFeyOz/MCM-41 sorbents during hot coal gas desulfurization, Chem. Eng. J.171 (2011) 594–602.
25] B.S. Liu, X.N. Wei, Y.P. Zhan, R.Z. Chang, F. Subhan, C.T. Au, Preparation anddesulfurization performance of LaMeOx/SBA-15 for hot coal gas, Appl. Catal. B:Environ. 102 (2011) 27–36.
26] X.H. Wang, J.P. Jia, L. Zhao, T.H. Sun, Mesoporous SBA-15 supported iron oxide:a potent catalyst for hydrogen sulfide removal, Water Air Soil Poll. 193 (2008)247–257.
27] Y. Mathieu, M. Soulard, J. Patarin, M. Molière, Mesoporous materials for theremoval of SO2 from gas streams, Fuel Process. Technol. 99 (2012) 35–42.
28] J.Q. Xu, W. Chu, S.Z. Luo, Synthesis and characterization of mesoporous V-M41molecular sieves with good hydrothermal and thermal stability, J. Mol. Catal.A: Chem. 256 (1/2) (2006) 48–56.
29] B.S. Liu, C.T. Au, Carbon deposition and catalyst stability over La2NiO4/�-Al2O3
during CO2 reforming of methane to syngas, Appl. Catal. A: Gen. 244 (1) (2003)181–195.
30] B.S. Liu, Y. Zhang, J.F. Liu, M. Tian, F.M. Zhang, C.T. Au, A.S.-C. Che-ung, Characteristic and mechanism of methane dehydroaromatization overZn-based/HZSM-5 catalysts under conditions of atmospheric pressure andsupersonic jet expansion, J. Phys. Chem. C 115 (2011) 16954–16962.
31] Q. Cai, Z.S. Luo, W.Q. Pang, Dilute solution routes to various controllable mor-phologies of MCM-41 silica with a basic medium, Chem. Mater. 13 (2001)258.
32] P. Van Der Voort, M. Mathieu, F. Mees, E.F. Vansant, Synthesis of high-qualityMCM-48 and MCM-41 by means of the GEMINI surfactant method, J. Phys.Chem. B 102 (1998) 8847–8851.
33] Q.R. Chen, L. Han, C.B. Gao, S.A. Che, Synthesis of monodispersed mesoporoussilica spheres (MMSSs) with controlled particle size using gemini surfactant,Microporous Mesoporous Mater. 128 (2010) 203–212.
34] F.D. Renzo, N. Coustel, M. Mendiboure, H. Cambon, F. Fajula, Progress in Zeoliteand Microporous Materials, in: H. Chon, S.K. Him, Y.S. Uh (Eds.), Studies in Surf.Sci. Catal. Part A, vol. 105, Elsevier, Amsterdam, 1997, p. 69.
35] F.D. Renzo, F. Testa, J.D. Chen, H. Cambon, A. Galarneau, D. Plee, F. Fajula,Textural control of micelle-templated mesoporous silicates: the effects of co-surfactants and alkalinity, Microporous Mesoporous Mater. 28 (1999) 437–446.
36] Y.F. Han, F.X. Chen, Z.Y. Zhong, K. Ramesh, L.W. Chen, E. Widjaja, Controlledsynthesis, characterization, and catalytic properties of Mn2O3 and Mn3O4
nanoparticles supported on mesoporous silica SBA-15, J. Phys. Chem. B 110(2006) 24450–24456.
37] Y.F. Han, F.X. Chen, K. Ramesh, Z.Y. Zhong, E. Widjaja, L.W. Chen, Preparation ofnanosized Mn3O4/SBA-15 catalyst for complete oxidation of low concentrationEtOH in aqueous solution with H2O2, Appl. Catal. B: Environ. 76 (2007) 227–234.
38] C.D. Wagner, W.M., Riggs, L.E., Davis, J.F., Moulder, G.E. Mullenberg, Handbookof X-ray Photoelectron Spectroscopy, Physical Electron Division, Perkin–ElmerCorporation, Eden Prairie, MN, 1992.
39] A. Glisenti, The reactivity of a Fe–Ti–O mixed oxide under different atmo-spheres: study of the interaction with simple alcohol molecules, J. Mol. Catal.A: Chem. 153 (1/2) (2000) 169–190.
40] F. Buciuman, F. Patcas, R. Cracium, D.R.T. Zahn, Vibrational spectroscopy of bulkand supported manganese oxides, Phys. Chem. Chem. Phys. 1 (1999) 185.
41] J. Zuo, C. Xu, Y. Liu, Y. Qian, Crystallite size effects on the Raman spectra ofMn3O4, Nanostruct. Mater. 10 (1998) 1331.
42] M. Ludvigsson, J. Lindgren, J. Tegenfeldt, Incorporation and characterisation of
oxides of manganese, cobalt and lithium into Nafion 117 membranes, J. Mater.Chem. 11 (2001) 1269.43] Q. Zhang, Y. Wang, S. Itsuki, T. Shishido, K. Takehira, Manganese-containingMCM-41 for epoxidation of styrene and stilbene, J. Mol. Catal. A: Chem. 188(2002) 189–200.
