Properties of a manganese oxide octahedral ...

Fuel Processing Technology 131 (2015) 238–246

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Properties of a manganese oxide octahedral molecular sieve (OMS-2) for adsorptive desulfurization of fuel gas for fuel cell applications Phuoc Hoang Ho a, Seong Chan Lee a, Jieun Kim b, Doohwan Lee b,⁎⁎, Hee Chul Woo a,⁎ a b

Department of Chemical Engineering, Pukyong National University, San 100 Yongdang-dong, Nam-gu, Busan 608-739, Republic of Korea Department of Chemical Engineering, University of Seoul, 13 Siripdae-gil, Dongdaemun-gu, Seoul 130-743, Republic of Korea

a r t i c l e

i n f o

Article history: Received 18 August 2014 Received in revised form 10 November 2014 Accepted 11 November 2014 Available online 6 December 2014 Keywords: Desulfurization Manganese oxide octahedral molecular sieve Fuel processing tert-Butylmercaptan Fuel cell

a b s t r a c t Properties and characteristics of cryptomelane manganese oxide octahedral molecular sieve (OMS-2)-based adsorbents for the adsorptive desulfurization of dimethyl sulfide (DMS), tert-butylmercaptan (TBM), and tetrahydrothiophene (THT) were investigated at ambient temperature and atmospheric pressure. OMS-2 adsorbents exhibited above 90% adsorption selectivity for TBM in a ternary mixture of DMS, TBM, and THT in a methane fuel stream, which is unique and unprecedented for zeolite-, metal oxide-, and activated carbonbased adsorbents. Hetero-metal doped M-OMS-2 (M = Ag, Ce, Co, Cu, Fe, La, Ni, Zn) adsorbents were prepared, and the effects of doped metal entities for TBM adsorption were studied. In particular, Cu-OMS-2 exhibited substantially enhanced TBM adsorption uptakes that were greater than 2.6 times that of the pristine OMS-2. A high TBM adsorption of 4.44 mmol S g−1 was achieved on Cu-OMS-2 (2.5 wt.% Cu doping), 2–7 times greater than the values reported for zeolite- and activated carbon-based adsorbents at similar experimental conditions. Structure and properties of OMS-2-based adsorbents were studied at various synthesis conditions and characterized with SEM, TEM, XRD, TGA, elemental analysis, N2 adsorption, and temperature programmed desorption (TPD) methods. The selective adsorption and thermal regeneration characteristics of OMS-2 were rationalized with a proposed mechanism. Thermal regeneration led to deactivation of OMS-2 due to a poisoning of adsorption sites by residual sulfur species and an alteration of the crystalline structure by reduction, in the case of thermal regeneration in an inert atmosphere, accompanying some collapse of the accessible pore structures. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Cryptomelane-type manganese oxide octahedral molecular sieve (OMS-2) has a one-dimensional pore structure consisting of a primary MnO6 octahedra building unit with linked edges and vertexes to form 2 × 2 square tunnels with an approximate pore size of 0.46 × 0.46 nm [1]. OMS-2 has a chemical composition of KMn8O16 · nH2O with potassium cations contained within the tunnels for charge balancing and that can be partly ion-exchanged with other cations [2,3]. A framework of manganese with mixed valances of +2, +3, and +4 yields an average oxidation state of ~ 3.8 [1,4]. The unique pore structure, ionexchange ability, and mixed valences of manganese of OMS-2 have attracted interest for use as oxidation catalysts [1,5–8], electrodes [9], battery materials [10], and adsorbents [11]. Selective adsorption of organosulfur species from fuel gases is considered as the most viable deep desulfurization method for fuel processing for polymer electrolyte membrane fuel cells (PEMFCs). An

⁎ Corresponding author. Tel.: +82 51 629 6436. ⁎⁎ Corresponding author. Tel.: +82 2 6490 2371. E-mail addresses: [email protected] (D. Lee), [email protected] (H.C. Woo).

http://dx.doi.org/10.1016/j.fuproc.2014.11.019 0378-3820/© 2014 Elsevier B.V. All rights reserved.

infinitesimal (~ ppm level) amount of sulfur species can irreversibly poison the reforming and electrode catalysts in the fuel cells [12], and conventional desulfurization methods that are performed at high temperatures and pressures, such as hydrodesulfurization (HDS), are not adequate for distributed or potable fuel cell systems [13]. In reality, a few organosulfur compounds, such as thiophenes, sulfides, and mercaptans, are usually added as odorants (several ppm level) in gaseous fuels to ensure detection of a gas leak: tetrahydrothiophene (THT), tert-butylmercaptan (TBM), and dimethyl sulfide (DMS) are the most utilized organosulfur species for natural and petroleum gas fuels [14–17]. Many studies have reported various adsorbents for desulfurization of fuels, including zeolites [14–20], metal impregnated oxides [21], and activated carbons [22,23]. These studies indicated that adsorption of TBM to these adsorbents was much weaker than that of THT or DMS, which led to limited or no selectivity for TBM removal from fuel gas when thiophenes or sulfides were present in the fuel stream. In practice, a combination of multiple adsorbents of different selectivity for specific sulfur species can be applied [24], but the overall desulfurization performance is governed primarily by its TBM capture properties [15–17,25]. In this work, we first report the studies on OMS-2-based materials for adsorptive desulfurization of fuel gas. OMS-2-based adsorbents

P.H. Ho et al. / Fuel Processing Technology 131 (2015) 238–246

exhibited markedly high adsorption selectivity for TBM (above 90%) in the presence of THT and DMS in fuel gases. OMS-2 adsorbents were prepared at various synthesis conditions, and their adsorptive desulfurization properties were investigated. Furthermore, the effects of heterometal dopants in M-OMS-2 (M = Cu, Co, Ni, Fe, Ag, La, Ce, and Zn) for TBM adsorption were studied. The TBM adsorption capacity was markedly enhanced on Cu-OMS-2, to approximately 2–3 fold greater than that of pristine OMS-2 and at least two-fold greater than literature values reported for zeolite-, metal oxide-, and activated carbon-based adsorbents. The structures of OMS-2-based adsorbents were characterized by various methods, and their desulfurization properties were rationalized with a proposed mechanism.

2. Experimental 2.1. Preparation of adsorbents OMS-2 was hydrothermally prepared with a modification of the method reported in the literature [1]. Typically, an aqueous manganese sulfate solution was prepared by dissolving 26 mmol MnSO4∙ H2O (≥ 99%, Aldrich) in a mixture of 20 ml deionized H2O and 1.5 ml concentrated HNO3. An aqueous permanganate solution was separately prepared by dissolving 18.4 mmol KMnO4 (≥ 99%, Aldrich) in 40 ml deionized H2O, which was added drop-wise to the aqueous MnSO4 solution with vigorous stirring. The mixture was transferred to a Teflon-lined stainless steel autoclave and placed in an oven at 373 K for 24 h. The resulting precipitates were centrifuged and washed three times with deionized H2O, dried at 383 K overnight, and calcined at 723 K for 2 h in air. OMS-2 samples were also prepared at various hydrothermal temperatures (333, 373, 413, and 453 K) and reaction times (2, 6, 12, 24, 48 h) at 373 K. Hetero-metaldoped M-OMS-2 (M = Cu, Co, Ni, Fe, Zn, Ag, Ce, La) samples were prepared by adding hetero-metal nitrate salts (M/Mn = 10 wt.%) to the aqueous MnSO4 solution prior to the addition of KMnO4 solution. The subsequent procedures were the same as for OMS-2. In particular, Cu-OMS-2 samples were prepared with various Cu doping levels by varying the Cu concentration in the precursor solution from 0.18 to 0.55 M.

2.2. Characterizations Powder X-ray diffraction (XRD) patterns of OMS-2 and M-OMS-2 samples were obtained with graphite-monochromatized Cu Kα radiation operated at 40 kV and 30 mA (X'pert-MPD, Philips). The spectra were acquired in a 2θ range of 5 to 80° with a scanning rate of 4° min−1. The structure and morphology of the samples were characterized by field emission scanning electron microscopy (FE-SEM, JSM7500F, JEOL) and transmission electron microscopy (TEM, JEM-2100, JEOL), respectively. The chemical composition of the samples was characterized by inductively coupled plasma–atomic emission spectroscopy (ICP-AES, ICPE-9000, Shimadzu) in conjunction with a CCD detector. Sulfur content of the samples was quantified by X-ray fluorescence spectroscopy (XRF, XRF-1700, Shimadzu-Japan). Thermogravimetric analysis (TGA, TGA7-Pyris-1, Perkin-Elmer) on the samples was conducted at a ramping rate of 10 K min−1 from 323 to 1173 K in an N2 atmosphere. BET surface area and porosity of the samples were characterized from N2 adsorption–desorption isotherms obtained in a volumetric unit (Belsorp-Max, BEL Japan). The molecular size of THT and TBM was obtained from literature, while that of DMS was estimated by structure optimization using density functional theory (DFT) calculation with Becke's three-parameter exchange functional together with the correlation functionals of Lee–Yang–Parr (B3LYP) at 6-311+G(2d,p) basis sets. The calculation was conducted with Gaussian 09 software package (Gaussian).

239

2.3. Adsorption measurement and sample regeneration The adsorption uptake and selectivity of TBM, THT, and DMS on the adsorbents were obtained at 303 K and atmospheric pressure using a fixed-bed glass reactor (8 mm I.D.) packed with 100 mg of adsorbent (particle size: 160–225 μm). The uptake measurement and subsequent sample regeneration proceeded in three sequential steps: pretreatment, adsorption, and desorption. In the pretreatment step, the adsorbent was treated in He flow (99.999%) at 723 K for 2 h. In the adsorption uptake measurement step, a certified mixture of DMS (33.4 ppm), TBM (33.6 ppm), and THT (33.9 ppm) balanced in methane was fed at a flow rate of 50 ml min−1 at 303 K. The effluent concentration of these species was measured online as a function of time using a gas chromatograph (HP 5890 II) equipped with a capillary column (HP-1, 30 m–0.32 mm–0.25 μm) and a flame ionization detector (FID). The breakthrough adsorption capacity of an adsorbent was defined as the amount of a specific sulfur species adsorbed on the sample before its detection in an effluent stream (detection limit: ~0.1 ppm). The value was presented as the adsorption amount of a sulfur compound per unit mass of adsorbent (mmol S g−1), which was calculated from the inlet concentration of the sulfur compound (Cio), flow rate (q), breakthrough time (t), and mass of the adsorbent (mads) using the following equation: q −1 S mmol g ¼

ml min−1 t ð minÞ C i;o ðppmÞ 10−6 : 22:4 ml mmol−1 mads ðgÞ

After completion of each adsorption measurement, desorption characteristics of the adsorbed sulfur species were investigated by a temperature programmed desorption (TPD) method. The spent, sulfursaturated, sample was purged in flowing He for 1 h at 303 K, and the temperature was increased to 773 K (ramp rate = 10 K min−1) in a He flow (50 ml min−1). The desorption species were monitored online using a quadrupole mass spectrometer (HPR 20, Hiden Analytical) recording ionic currents of the main fragments of TBM, THT, DMS, and hydrocarbons. Desorption mechanism of the sulfur species and regenerability of OMS-2 and Cu-OMS-2 were studied by TPD, XRD, and elemental analysis methods. Thermal regeneration of OMS-2 and Cu-OMS-2 was also conducted in air (99.999%) at the same condition in order to investigate the effects of the regeneration atmosphere. 3. Results and discussion 3.1. Adsorption selectivity of the OMS-2 adsorbent Fig. 1 shows the time on stream of TBM, DMS, and THT adsorption on OMS-2 at ambient temperature and atmospheric pressure in an equimolar mixture flow of these compounds (~33 ppm for each species with CH4 balance). The results demonstrated above 90% adsorption selectivity of OMS-2 in favor of TBM, which is unprecedented and markedly different from previously reported adsorbents. An almost immediate breakthrough of DMS and THT occurred, indicating practically insignificant adsorption of these species on OMS-2, whereas TBM adsorption was extended to ~4.8 h on the sample after introduction of the feed stream (GHSV = 800 h−1). The adsorption properties of OMS-2 for these organosulfur compounds are unique and vastly different from those of zeolite [15,16] and activated carbon [25] based adsorbents. Studies on these adsorbents have reported a preferential removal of THT over TBM, in which strongly adsorbing THT prevented co-adsorption of TBM that exhibited relatively weak adsorptive interactions on these adsorbents. The marked TBM adsorption selectivity on OMS-2 in this work was not due to size discrimination, because small sizes of TBM (0.52 × 0.52 nm [16]), THT (0.53 × 0.35 nm [16]), and DMS (0.43 × 0.24 nm) molecules can be accommodated into the

240


1.2

DMS

1.0

C/C o

0.8 THT

0.6 0.4

TBM

0.2 0.0 0

4

8

12

16

Time (h) Fig. 1. Normalized concentration (C/C0) of DMS, TBM, and THT in the effluent from the OMS-2 adsorbent packed bed: inlet concentration (C0) = a certified mixture of TBM (33.6 ppm), THT (33.9 ppm), and DMS (33.4 ppm) balanced with CH4, GHSV = 800 h−1.

OMS-2 pore structure through the diagonal (0.65 nm) of the 2 × 2 square channel (0.46 × 0.46 nm). 3.2. Effects of hydrothermal synthesis temperature

Intensity (a.u.)

The hydrothermal synthesis temperature had substantial effects on the structure, morphology, adsorption capacity, and selectivity of OMS-2. Fig. 2 displays the XRD results for the samples obtained at various hydrothermal temperatures between 333 and 453 K for 24 h. The results clearly showed development of the characteristic diffraction pattern of cryptomelane OMS-2 with an increase in the hydrothermal temperature. Additional diffraction peaks appeared on the sample prepared at 453 K due to substantial formation of pyrolusite (MnO2, JCPDS-24-735). Fig. 3 shows FE-SEM (upper row, Fig. 3a–d) and TEM micrographs (bottom row, Fig. 3a1–d1) on OMS-2 prepared at different hydrothermal temperatures between 333 and 453 K. These micrographs clearly revealed the formation of typical OMS-2 structure with needle-shaped morphology on all samples. The structure was much less developed at a low temperature of 333 K, whereas needle-shaped OMS-2 crystallites clustered into bundles were clearly observed on the samples prepared at higher temperatures. The results demonstrated that length and thickness of these OMS-2 crystallites increased as

hydrothermal temperature increased. Thermogravimetric analysis results indicated that OMS-2 prepared at 333 K had significantly lower thermal stability due to its low crystallinity (Supplementary data, Fig. S1). Table 1 displays the elemental composition, crystal size, and BET surface area of OMS-2 samples prepared at various hydrothermal temperatures and reaction times. The surface area of these samples was between 74 and 196 m2 g− 1, corroborating literature values for OMS-2 prepared at similar hydrothermal conditions [3]. The sample prepared at 453 K exhibited a particularly small surface area (32 m2 g− 1) primarily due to substantial formation of the pyrolusite MnO2 phase, as confirmed from the XRD results. TBM breakthrough uptake amounts of OMS-2 samples prepared at various hydrothermal temperatures are compared in Fig. 4, in which the uptakes were obtained with an inlet feed of 100 ppm TBM with CH4 balance (50 ml min−1). The highest TBM breakthrough uptake was attained on the sample prepared at 373 K with a value of 1.34 mmol S g−1, which decreased as hydrothermal synthesis temperature increased. The results demonstrated that the TBM adsorption capacity on OMS-2 was not proportional to the surface area, displaying a low TBM uptake on the sample prepared at 333 K which had the largest surface area. The results suggest that the crystallinity and crystal phase of the samples, rather than the surface area, likely played dominant roles on the TBM adsorption properties. The XRD (Fig. 2), FE-SEM, and TEM results (Fig. 3) consistently indicated that OMS-2 structure was not fully developed at the low hydrothermal temperature of 333 K, while formation of the pyrolusite MnO2 significantly increased at excessive high temperatures of 413 and 453 K accompanying scarification of the OMS-2 crystal phase. Qui et al. [1] reported similar results that crystalline OMS-2 was developed at 393 K, while an increasing formation of pyrolusite MnO2 phase was observed when the hydrothermal synthesis was conducted at higher temperatures. The decrease in TBM adsorption on the samples prepared at 413 and 453 K was reasonable considering that pyrolusite MnO2 possess a small pore channel with a size of 0.23 × 0.23 nm [26] that cannot accommodate larger TBM molecules. 3.3. Effects of hydrothermal synthesis time Fig. 5 displays TEM micrographs of OMS-2 prepared at 373 K for different hydrothermal synthesis times of 2 to 48 h. The results exhibited growth of crystalline OMS-2 fibrils and their aggregation into bundles with an increase in hydrothermal reaction time. Formation of other crystalline phases was not observed, which is consistent with the XRD results (Fig. 2). Fig. 6 shows TBM breakthrough adsorption uptake on these OMS-2 samples, which demonstrated a gradual decrease in TBM adsorption amount with an increase in hydrothermal reaction time for OMS-2. The moderate decrease in TBM breakthrough uptake with an increase in OMS-2 synthesis time was likely due to a gradual increase in the OMS-2 crystallite size (Fig. 5), which suggested diffusion controlled TBM adsorption on the large OMS-2 crystallites to some extent. Nonetheless, these OMS-2 samples presented significantly high TBM breakthrough adsorption capacities and selectivities compared to previously reported adsorbent materials. For example, TBM breakthrough adsorption uptake on OMS-2 prepared at 373 K for 2 h was 1.59 mmol S g− 1, which was approximately 1.7 times greater than those of NaY (0.95 mmol S g−1) and H-β (0.94 mmol S g−1) [15], 5.3 times greater than that of ETS-10 (0.3 mmol S g−1) [17], and 7 times greater than that of activated carbon (0.21 mmol S g−1) [25], previously reported to have relatively high TBM adsorption capacities. 3.4. Effects of hetero-metal dopant in M-OMS-2

2θ (Degree) Fig. 2. Powder XRD patterns of OMS-2 samples prepared at different temperatures from 333 to 453 K for 24 h (the reference diffraction patterns of OMS-2 and pyrolusite MnO2 are shown with their JCPDS file numbers).

The effects of various hetero-metal dopants in M-OMS-2 (M = Cu, Co, Ce, Ni, Fe, Ag, Zn, La) were investigated for TBM adsorption. Table 2 shows the elemental composition, crystal size, and BET surface area of M-OMS-2 adsorbents. XRD characterization results on these


(a) 333K

(c) 413K

(b) 373K

1༁ ༁

(d) 453K

1༁

(b1) 373K

(a1) 333K

500ༀ

241

1༁

(d1) 453K

(c1) 413K

500ༀ

1༁

500ༀ

500ༀ

Fig. 3. FE-SEM (a–d) and TEM (a1–d1) micrographs of OMS-2 prepared at (a) 333 K, (b) 373 K, (c) 413 K, and (d) 453 K for hydrothermal synthesis of 24 h.

Synthesis condition

333 K (24 h) 373 K (24 h) 413 K (24 h) 453 K (24 h) 2 h (373 K) 6 h (373 K) 12 h (373 K) 24 h (373 K) 48 h (373 K) a

Element compositiona K (mol)

Mn (mol)

0.096 0.120 0.128 0.087 0.101 0.118 0.116 0.120 0.125

1.144 1.129 1.151 1.170 1.123 1.123 1.134 1.129 1.100

Based on 100 g of the sample.

Crystal size (nm)

BET surface area (m2 g−1)

21.6 19.5 22.8 23.9 18.6 18.8 19.0 19.5 24.4

196 104 153 32 112 80 79 104 74

3.5. Effects of Cu doping amount in Cu-OMS-2 for TBM adsorption In order to investigate the effects of Cu doping in Cu-OMS-2, four different samples with various Cu contents were prepared: Cu-OMS2-I (23), Cu-OMS-2-II (30), Cu-OMS-2-III (36), and Cu-OMS-2-IV (39), where the number in parenthesis is the amount of Cu (mmol) in 100 g of each sample. The formation of hetero-crystalline phases on these Cu-OMS-2 samples was not observed, as confirmed from the XRD analysis (Supplementary data, Fig. S3). Table 3 summarizes the composition, crystal size, and BET surface area of these Cu-OMS-2 samples. The results showed that surface area and crystal size of Cu-OMS-2 were slightly dependent on the amount of introduced Cu. The increasing (M + K)/Mn molar ratio with an increase in Cu doping amount suggests that some of the Cu was introduced into the framework structure. Chen et al. also reported that Cu2+ could be more easily doped into the cryptomelane OMS-2 than other metal cations (Mg2+, Ni2 +, Al3 +, Zn2 +) substituting both K+ in the tunnel and Mn in the framework [3]. Consistent results were also reported in electron paramagnetic resonance [28] and X-ray absorption spectroscopy studies [29]. Fig. 9 displays breakthrough TBM adsorption uptakes on Cu-OMS-2 adsorbents as a function of Cu doping amount. The results indicated 2.0

200 Breakthrough TBM uptake Surface area

1.6

160

1.2

120

0.8

80

0.4

40

0.0

333 K

373 K 413 K Synthesis temperature (K)

453 K

Surface area (m2 g-1)

Table 1 Properties of OMS-2 adsorbents prepared at different hydrothermal synthesis conditions.

uptake of 3.55 mmol S g− 1, which was 2.7 times greater than that on pristine OMS-2. Conversely, TBM adsorption on Ce-OMS-2 was negligible indicating that the introduction of Ce adversely affected TBM adsorption.

Breakthrough TBM uptake (mmol-S g-1)

samples exhibited a typical OMS-2 diffraction pattern without the formation of hetero-crystalline phases (Supplementary data, Fig. S2). Fig. 7 displays FE-SEM micrographs on M-OMS-2 samples, in which a number of bundles of uniformly grown crystalline M-OMS-2 fibrils can be observed; the morphologies of Ce- and Fe-OMS-2 were somewhat different from other M-OMS-2 samples showing agglomeration of short nanofibers. The hetero-metal to manganese molar ratios (M/Mn) in these M-OMS-2 samples were largely different, although the M/Mn precursor ratio was kept the same (10 wt.%) for the syntheses of these samples. It has been reported that metal cations can be substituted into the framework or the external charge-balancing sites of OMS-2 depending on their physicochemical properties [3]. The charge-balancing metal (K + M) to framework Mn molar ratios, (K + M)/Mn, on OMS-2 and M-OMS-2 (M = Cu, Ni, Ag, Zn, La, Ce) samples were similar in a range of 0.11 to 0.15, suggesting that most of the hetero-metal dopants in these M-OMS-2 samples were substituted to the external charge-balancing sites. Differently, Fe-OMS-2 exhibited relatively larger (K + M)/Mn molar ratios of 0.19, which seemed to suggest an incorporation of Fe dopants into the framework. Previous studies reported that it occurs as the sizes and oxidation states of Fe cations are the same as those of Mn cations [27]. Surface area of M-OMS-2 samples was affected only slightly by the introduction of hetero-metals with values ranging from 73 to 98 m2 g−1, indicating that the structure and porosity of the samples remained intact. Fig. 8 shows the breakthrough TBM adsorption uptake amount on OMS-2 and hetero-metal-doped M-OMS-2 samples. The amount of hetero-metal introduced into M-OMS-2 is indicated in the parentheses: for example Zn (12) indicates that 12 mmol of Zn was present in 100 gram of Zn-OMS-2. The results revealed that most of these heterometal dopants do not constructively interplay with TBM adsorption, except Cu that promotes a substantial enhancement in TBM adsorption. It was notable that Cu-OMS-2 demonstrated a TBM breakthrough

0

Fig. 4. Breakthrough TBM adsorption capacity and surface are of OMS-2 adsorbents prepared at various synthesis temperatures for 24 h.

242


(a) 2h

(b) 12h

1༁

1༁ (c) 24h

(d) 48h

1༁ ༁

1༁

Fig. 5. TEM micrographs of OMS-2 adsorbents prepared at 373 K for different synthesis times: (a) 2 h, (b) 12 h, (c) 24 h, and (d) 48 h.

that the breakthrough TBM uptake on Cu-OMS-2 increased nearly proportionally to an increase in the Cu doping amount. The TBM adsorption amount on Cu-OMS-2-IV was 4.44 mmol S g−1, which was approximately two-fold greater than those of zeolite- and ETS-10based adsorbents that are known for their relatively high TBM adsorption capacities at similar adsorption condition; AgNaY: 2.15 mmol S g−1 [16], Cu-ETS-10: 2.5 mmol S g−1 [17]. The more than three-fold higher TBM adsorption capacity on Cu-OMS-2-IV (4.44 mmol S g−1) than pristine OMS-2 (1.34 mmol S g−1) suggests that the Cu dopants played a significant role in the adsorption of TBM, a result not achieved with other transition (Zn, Fe, Co, Ni, Ag) or lanthanide (La, Ce) dopant metal cations. Particularly strong attractive interactions between Cu and organosulfur compounds, such as methylmercaptan and THT, were reported to occur via strong π-complexation [19,30], which likely accounts for the enhancement of TBM adsorption on Cu-OMS-2 observed in this work. 200

Breakthrough TBM uptake Surface area 1.6

160

1.2

120

0.8

80

0.4

40

0.0

Surface area (m2 g-1)


2.0

0

2h

6h 12h 24h Synthesis time (h)

48h

Fig. 6. Effect of hydrothermal synthesis time on TBM adsorption capacity of OMS-2 adsorbents prepared at 373 K.

3.6. Regeneration of OMS-2 adsorbents After TBM adsorption, OMS-2 and Cu-OMS-2 were thermally regenerated in He or air flow by steadily increasing the temperature from 303 to 723 K. Fig. 10 displays the TBM breakthrough adsorption amount on the fresh and thermally regenerated OMS-2 and Cu-OMS-2. The results indicated that the TBM adsorption capacity of OMS-2 decreased by approximately 85% after the first thermal regeneration in both helium and air, while that of Cu-OMS-2 decreased, respectively, by 90% and 75%. These results suggest that detrimental alterations of the structures and properties of OMS-2 and Cu-OMS-2 occurred after the thermal regeneration that was not affected by the regeneration atmosphere. The mechanism of thermal regeneration of OMS-2 was further investigated using the temperature programmed desorption (TPD) method. Fig. 11 displays the TPD results on OMS-2 in the first thermal regeneration cycle in He flow. The results exhibited the strong appearance of hydrocarbon fragments of C3H5 (m/e = 41), (CH3)3C (m/e = 57), and isobutene (m/e = 56) at approximately 480 and 600 K, whereas desorption of typical sulfur-containing fragments, such as H2S Table 2 Properties of M-OMS-2 adsorbents doped with various hetero-metal components (M = Cu, Co, Ni, Ag, Fe, Zn, La, Ce). The samples were hydrothermally prepared by at 373 K for 24 h. Adsorbent

OMS-2 Cu-OMS-2 Co-OMS-2 Ni-OMS-2 Ag-OMS-2 Fe-OMS-2 Zn-OMS-2 La-OMS-2 Ce-OMS-2 a

Element compositiona M (mmol)

K (mmol)

Mn (mmol)

(M + K)/Mn

30 46 10 34 92 12 9 25

120 110 119 120 89 110 118 116 105

1129 1117 1087 1126 1094 1047 1113 1121 1079

0.11 0.13 0.15 0.12 0.11 0.19 0.12 0.11 0.12


Crystal size (nm)


19.5 19.0 18.6 18.4 13.6 25.5 21.3 16.2 31.1

104 76 76 74 88 98 73 80 78


(a) Cu

243

(b) Co

2༁ ༁ (c) Ce

2༁ (d) Ni

2༁

2༁ (e) Fe

(f) Ag

2༁

2༁ (g) Zn

(h) La

2༁

2༁

Fig. 7. FE-SEM micrographs of M-OMS-2 adsorbents doped with hetero-metal cations (M). The samples were hydrothermally prepared by at 373 K for 24 h.

(m/e = 34), (CH3)2CSH (m/e = 75), and (CH3)3CSH (m/e = 90), were negligible. These results are significantly different from the TPD characteristics of TBM on Y zeolite- [15] and ETS-10-based adsorbents [17], where thermal desorption of both hydrocarbon and organosulfur species, such as H2S, C3H5, (CH3)3C, (CH3)2CSH, and (CH3)3CSH fragments, was reported at lower temperatures. Furthermore, the appearance of strong desorption peaks of CO2 (m/e = 44), SO2 (m/e = 64 and 48), and H2O (m/e = 18) was also observed above 600 K, indicating oxidative desorption of the adsorbed species from the samples even in an inert He environment. Based on these results, rational mechanisms for TBM desorption and interactions on OMS-2 during thermal regeneration could be proposed. (i) The majority of TBM adsorbed on OMS-2 was decomposed on the surface during the

thermal regeneration, and fragmented hydrocarbon species, such as (CH3)3C, isobutene, C3H5, and CH4, were desorbed without accompanying organosulfur fragments. (ii) Some residual sulfur and hydrocarbon species strongly adsorbed on OMS-2 were desorbed as oxidized SO2, CO2, and H2O at high temperatures above 600 K. It was likely that the lattice oxygen on OMS-2 was utilized for the oxidative desorption which resulted in deterioration of the structure and properties of OMS-2. The involvement of lattice oxygen was reported for oxidative dehydrogenation of 1-butene [31] and total oxidation of volatile organic compounds (VOCs) [5] on OMS-2, where the reactions were described to proceed via Mars–van Krevelen mechanism. (iii) Substantial amount of the sulfur moieties were unable to remove via thermal regeneration likely due to irreversible sulfurization of the substrate.



4.0 3.2 2.4 1.6 0.8 0.0

) ) ) ) S-2 (12 (92) (46) (10 (34 (30) La (9 (25) OM Zn Fe Co Ni Ag Cu Ce M-OMS-2

Fig. 8. Breakthrough TBM adsorption amount on OMS-2 and hetero-metal doped M-OMS-2 adsorbents (M = Zn, Fe, Co, Ni, Ag, Cu, La, Ce). The amount of hetero-metal introduced in M-OMS-2 is indicated in the parentheses (mmol).

Fig. 12 shows the XRD patterns on the fresh and the spent OMS-2 and Cu-OMS-2 after the first thermal regeneration treatment. The results revealed that OMS-2 and Cu-OMS-2 were partially converted from the KMn8O16 cryptomelane phase (JCPDS-29-1020) to Mn3O4 (JCPDS-24-734) after regeneration at 723 K in an inert He atmosphere, which was evidenced by the diffraction peaks of hausmannite Mn3O4 at 32.3, 36.1, and 58.5° (Fig. 12d and g). The intensity of these Mn3O4 peaks increased after the second cycle of TBM adsorption and thermal regeneration treatment. This was different for samples that were regenerated in air, in which the phase transformation did not clearly occur (Fig. 12e and h). The considerable phase transformation of the OMS-2 and Cu-OMS-2 structures by thermal regeneration in an inert He atmosphere was also supported by a significant and concomitant decrease in specific surface area (18–34%) after the regeneration treatment, as shown in Table 4. The decrease in surface area on these samples was approximately 2–5 times greater than that on the samples regenerated in air at the same temperature. However, the decrease in TBM adsorption capacity was not proportional to the extent of crystal phase change or destruction of the structure. For example, after the first regeneration cycle, the surface area of OMS-2 decreased by 23% (Table 4), but its TBM uptake decreased significantly by 80% (Fig. 10). Instead, the absence of organosulfur desorption fragments and a relatively weak intensity of SO2 desorption above 600 K in the thermal regeneration (Fig. 11) strongly suggest that significant portions of the adsorbed sulfur species persisted on the adsorbents, irreversibly blocking or altering the pristine adsorption sites, independent of regeneration atmosphere and temperature. Table 4 displays the elemental composition on the fresh and thermally regenerated OMS-2 and Cu-OMS-2 in an He or air atmosphere at various temperatures: the fresh samples contained some amount of sulfur (less than 0.5 wt.%) due to utilization of the MnSO4 precursor for the synthesis. The net increase in sulfur content on the samples (wt.%-S g−1) after the first adsorption–regeneration cycle was significant in the range of 1.4–1.7 Table 3 Properties of Cu-OMS-2 adsorbents doped with various amount of Cu. The samples were hydrothermally prepared at 373 K for 24 h. Adsorbent

Cu-OMS-2-I Cu-OMS-2-II Cu-OMS-2-III Cu-OMS-2-IV a

Element compositiona Cu (mmol)

K (mmol)

Mn (mmol)

(M + K)/Mn

23 30 36 39

115 110 107 108

1100 1117 1044 1081

0.13 0.13 0.14 0.14


4.8 4.0 3.2 2.4 1.6 0.8 0.0

0.0

0.1

0.2

0.3

0.4

Cu-doping amount (mmol-Cu g-1)

Crystal size (nm)


15.6 19.0 16.6 17.0

90 76 91 70

Fig. 9. Breakthrough TBM adsorption uptakes on Cu-OMS-2 adsorbents as a function of Cu doping amounts. The samples were hydrothermally prepared at 373 K for 24 h.

for OMS-2 and Cu-OMS-2 treated in He and at approximately 2.2 for the samples treated in air, nearly independent of the Cu doping in OMS-2. These results suggested that the significant amount of sulfur species adsorbed irreversibly on the framework rather than on the external metal cation sites. The net quantities of residual sulfur on the samples after the first thermal regeneration in He (OMS-2 = 0.9, Cu-OMS-2 = 1.1 mmol S g− 1) were equivalent to ~ 67 and ~ 31% of the TBM adsorption amount on the fresh OMS-2 (1.34) and Cu-OMS-2 (3.55 mmol S g−1) samples, respectively. 3.7. Characteristics of TBM adsorption on OMS-2-based adsorbents OMS-2 and Cu-OMS-2 adsorbents in this work exhibited markedly high adsorption selectivity for TBM (above 90%) over DMS and THT in a CH4 fuel stream at ambient temperature and atmospheric pressure. The TBM adsorption capacity of Cu-OMS-2 (4.44 mmol S g−1) was particularly high with values at least two-fold greater than those of other activated carbon- and zeolite-based adsorbents reported in the literature. The extensive characterization results collectively suggest that this unusual, exclusive TBM selectivity on OMS-2-based adsorbents was primarily due to strong adsorptive interactions of TBM with the framework of OMS-2 rather than with external metal cations. In particular, doped Cu cations provided additional adsorption sites for TBM adsorption in Cu-OMS-2. The TBM adsorption on OMS-2 appears to be exceptionally strong, resulting in a considerable amount of



244

4 fresh spent 3

2

1

0

OMS-2 in He

OMS-2 in Air

Cu-OMS-2 in He

Cu-OMS-2 in Air

Fig. 10. TBM breakthrough adsorption amount on the fresh and the spent OMS-2 and Cu-OMS-2 adsorbents. The uptake measurements on the spent sample was conducted after a sample regeneration in different atmosphere either with He or air (723 K, ramping rate = 10 K min−1).


245

Fig. 11. Temperature programmed desorption (TPD) of TBM adsorbed on the fresh OMS-2 adsorbent. TPD was conducted in a flow of He (ramping rate = 10 K min−1).

irreversibly adsorbed sulfur species on the samples even after thermal regeneration at an elevated temperature of 723 K. The extent of adsorptive interactions of DMS and THT with the OMS-2 framework was much less and negligible compared to that of TBM. It is reasonable that the regeneration atmosphere had considerable effects on the crystalline structures of OMS-2 and Cu-OMS-2 considering that the materials are partially reducible. Thermal regeneration of the samples in an inert He atmosphere induced progressive phase transformation from cryptomelane KMn8O16 to hausmannite Mn3O4 accompanying a significant decrease in surface area and accessible open pores. The results suggest that organosulfur species adsorbed on the framework facilitated reduction of the samples during thermal regeneration, which induced the phase transformation and a considerable collapse of the pristine pore structure. Alternatively, thermal regeneration in air had a much weaker effect on the structure, preserving the pristine accessible pores for the guest molecules. Therefore, TBM adsorption uptake on the regenerated OMS-2 and Cu-OMS-2 was greater when the samples were regenerated in air rather than an inert atmosphere (Fig. 10). Although the OMS-2-based adsorbents in this work showed significant potential as new adsorbents for adsorptive desulfurization of fuel gas, their lack of regenerability hinders practical applicability. However, their unprecedented TBM adsorption selectivity with high adsorption capacity is an intriguing feature that has not been reported on porous adsorbents for adsorptive desulfurization of hydrocarbon fuels. We are currently investigating the nature of sulfur species irreversibly sustained on OMS-2 and effective physicochemical modification and regeneration methods of OMS-2 that can substantially alleviate the low regenerability. The exceptional adsorptive desulfurization properties of OMS-2-based adsorbents for organosulfur species containing fuel gases could provide significant advances in fuel processing for fuel cells, where the quality of hydrogen is crucial for consistent and efficient operation of the fuel cell systems.

Fig. 12. XRD patterns on OMS-2 and Cu-OMS-2 taken before and after the thermal regeneration treatment: (*) indicates for the peak of Mn3O4 phase.

4. Conclusions The properties and characteristics of OMS-2-based adsorbents for adsorptive desulfurization of fuel gas were first studied at ambient temperature and atmospheric pressure for potential fuel cell applications. These adsorbents exhibited markedly high adsorption selectivity for TBM (above 90%) over THT and DMS in CH4 fuel stream, which was vastly different from the properties of zeolite-, activated carbon-, and metal oxide-based porous adsorbents previously reported in the literature. A hetero-metal doping with Cu in OMS-2 produced a considerable increase in TBM adsorption capacity with 4.44 mmol S g−1 on CuOMS-2 (2.5 wt.% of Cu), which was approximately 2–7 times greater than the values reported for activated carbon- and zeolite-based adsorbents at similar conditions. The thermal regeneration atmosphere had considerable effects on the structure of OMS-2. Thermal regeneration Table 4 Properties of OMS-2 and Cu-OMS-2 adsorbents regenerated in different atmosphere and temperature conditions. Regenerated samples

Fresh OMS-2 OMS-2 in Helium 723 K OMS-2 in Air 723 K Fresh Cu-OMS-2 Cu-OMS-2 in Helium 723 K Cu-OMS-2 in Air 723 K Cu-OMS-2 in Air 673 K Cu-OMS-2 in Air 573 K a

Analyzed by XRF.

Element composition (wt.%)a Cu

K

Mn

S

– – – 3.77 3.66 3.84 3.64 3.76

8.40 8.46 8.50 7.55 8.14 7.81 7.95 8.27

91.22 89.71 88.97 88.16 85.98 85.61 86.04 85.21

0.38 1.82 2.53 0.53 2.22 2.74 2.37 2.76

BET surface area (m2 g−1) 104 69 97 76 62 70 73 73

246


in an inert He atmosphere induced a substantial and progressive phase transformation of KMn8O16 to Mn3O4 with destruction of the accessible pore structure. The TBM adsorption on OMS-2 and Cu-OMS-2 was exceptionally strong, resulting in deactivation of the adsorbents by considerable amount of residual sulfur species on the structure that could not be removed even by elevated temperature thermal regeneration (723 K). This was not affected by the regeneration atmosphere with inert helium or air. Acknowledgments H. C. Woo acknowledges the support by Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education (C-D-2014-0807). D. Lee acknowledges the financial support from the Korea Research Foundation (Grant 2014-R1A1A2053895). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.fuproc.2014.11.019. References [1] G. Qiu, H. Huang, S. Dharmarathna, E. Benbow, L. Stafford, S.L. Suib, Hydrothermal synthesis of manganese oxide nanomaterials and their catalytic and electrochemical properties, Chemistry of Materials 23 (2011) 3892–3901. [2] S. Ching, J.L. Roark, N. Duan, S.L. Suib, Sol-gel route to the tunneled manganese oxide cryptomelane, Chemistry of Materials 9 (1997) 750–754. [3] C.K. King'ondu, N. Opembe, C.-h. Chen, K. Ngala, H. Huang, A. Iyer, H.F. Garcés, S.L. Suib, Manganese oxide octahedral molecular sieves (OMS-2) multiple framework substitutions: a new route to OMS-2 particle size and morphology control, Advanced Functional Materials 21 (2011) 312–323. [4] Q. Feng, H. Kanoh, Y. Miyai, K. Ooi, Alkali metal ions insertion/extraction reactions with hollandite-type manganese oxide in the aqueous phase, Chemistry of Materials 7 (1995) 148–153. [5] H.C. Genuino, S. Dharmarathna, E.C. Njagi, M.C. Mei, S.L. Suib, Gas phase total oxidation of benzene, toluene, ethylbenzene, and xylenes using shape-selective manganese oxide and copper manganese oxide catalysts, The Journal of Physical Chemistry C 116 (2012) 12066–12078. [6] W.Y. Hernández, M.A. Centeno, S. Ivanova, P. Eloy, E.M. Gaigneaux, J.A. Odriozola, Cu-modified cryptomelane oxide as active catalyst for CO oxidation reactions, Applied Catalysis B: Environmental 123–124 (2012) 27–35. [7] T. Oishi, K. Yamaguchi, N. Mizuno, Conceptual design of heterogeneous oxidation catalyst: copper hydroxide on manganese oxide-based octahedral molecular sieve for aerobic oxidative alkyne homocoupling, ACS Catalysis 1 (2011) 1351–1354. [8] H. Sun, S. Chen, P. Wang, X. Quan, Catalytic oxidation of toluene over manganese oxide octahedral molecular sieves (OMS-2) synthesized by different methods, Chemical Engineering Journal 178 (2011) 191–196. [9] O. Ghodbane, F. Ataherian, N.-L. Wu, F. Favier, In situ crystallographic investigations of charge storage mechanisms in MnO2-based electrochemical capacitors, Journal of Power Sources 206 (2012) 454–462. [10] C.H. Jiang, S.X. Dou, H.K. Liu, M. Ichihara, H.S. Zhou, Synthesis of spinel LiMn2O4 nanoparticles through one-step hydrothermal reaction, Journal of Power Sources 172 (2007) 410–415.

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