Î³-Fe2O3-M41S Sorbents for H2S Removal: Effect ... - ACS Publications

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

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#-FeO-M41S Sorbents for HS Removal: Effect of Different Porous Structures and Silica Wall Thickness Claudio Cara, Elisabetta Rombi, Valentina Mameli, Andrea Ardu, Marco Sanna Angotzi, Daniel Niznansky, Anna Musinu, and Carla Cannas J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01487 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 20, 2018

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The Journal of Physical Chemistry

γ-Fe2O3-M41S Sorbents for H2S Removal: Effect of Different Porous Structures and Silica Wall Thickness

Claudio Cara,1,2,3 Elisabetta Rombi,1 Valentina Mameli,1,3 Andrea Ardu,1,2,3 Marco Sanna Angotzi, 1,3 Daniel Niznansky,4,5 Anna Musinu,1,2 and Carla Cannas1,2,3,* 1

Department of Chemical and Geological Sciences, University of Cagliari, S.S. 554 bivio per Sestu,

09042, Monserrato (CA), Italy 2

Consorzio AUSI, Palazzo Bellavista Monteponi, 09016 Iglesias, (CI), Italy

3

INSTM, Cagliari Unit, Monserrato (CA), Italy

4

Department of Inorganic Chemistry, Charles University of Prague, Prague 116 36, Czech Republic

5

Institute of Inorganic Chemistry of the AS CR, v.v.i., 250 68 Husinec-Řež 1001, Czech Republic

Abstract In this work, the effect of the M41S support pore structure (hexagonal or cubic) and of the wall thickness of the silica mesochannels have been evaluated aimed to achieve more and more efficient and regenerable iron oxide-based sorbents for H2S removal at mid-temperature. With this purpose, we set up a simple Pluronic-free synthetic strategy capable to produce silica supports with hexagonal (MCM-41) or cubic (MCM-48) pore structure with different wall thickness that have been used to fabricate the corresponding sorbents made up of iron oxide nanoparticles homogeneously dispersed into the mesochannels. The combined use of

57

Fe-Mӧssbauer

Spectroscopy and DC magnetometry has allowed to ascertain the presence of maghemite in form of ultrasmall nanoparticles in both composites and to give new insights on the influence of the ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

different silica matrices on the active phase features. The performances of the sorbents have been evaluated at mid-temperature (300 °C) through three repeated sulphidation and regeneration cycles and then correlated to their microstructure, and textural proprieties. Introduction Gasification of coal and biomass for syngas (CO + H2) production, under high temperature and low oxygen concentrations, results in a conversion of sulphur content of these resources mainly into H2S.1 Desulphidation cleaning processes (sweetening)1 able of reducing the H2S concentration to less than 100 ppm are required in order to render the resulting syngas usable as a source for integrated gasification combined cycle (IGCC) plants.2 Conventional techniques, based on H2S removal at low-temperature (40-50 °C) by using amine scrubbers or porous materials as zeolites3 or MOFs,4,5 feature tar condensation as the main drawback, that can cause plugging and fouling of the condenser and the downstream process piping.6 In addition, the scrubbed gas must then be reheated for downstream fuel synthesis, which occurs in the range 250-400 °C.6 These issues can be overcome by the use of mid- (300-600 °C) and high-temperature (600-850 °C) syngas sweetening processes. Since high-temperatures in strong reducing atmosphere induce thermal instability, sintering phenomena7 and/or volatilization of the metal (as the case of the ZnO)8, midtemperature range seems to be the most appropriate. Since the 1960s, metal oxides (Me = Zn, Cu, Fe, Ca, Mn, etc.)6,9,10 have been proposed and extensively investigated both for high and midtemperature desulphidation.1,10 However, conventional micrometric metal oxides showing low surface area and unreacted core phenomena are affected by the main disadvantage of low sulphur retention capacity and poor regenerability. The use of pure nanostructured metal oxide leads to higher performance and hamper unreacted core phenomena, but the typical issue of sintering related to pure phases cannot be avoided. In this context, the creation of a sorbent in which the metal oxide is in the form of a stable mesoporous structure should hamper the sintering and

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improve its regenerability. Among the mesoporous materials, the mesostructured ones represent ideal systems: the high surface area guarantees high performances, the dimension of the pores (250 nm) permits their interaction with a wide class of gaseous species, and the narrow pore size and wall thickness distributions assure the formation of a nanophase with homogeneous features and behaviors.11–13 To reach this goal two main routes can be followed: (i) the dispersion of the metal oxide active phase into a mesostructured inert support (nanocompositing),14 (ii) the creation of pure mesostructured metal oxides.15 ZnO-SBA15,16 Cu-ZnO-SBA1517, Fe2O3-SBA1512 and γ-Fe2O3MCM-4113 nanocomposites have been proposed in the literature as highly efficient and regenerable sorbents due to the effective dispersion of the active phase within the mesochannels of the hexagonal porous structure, while the rare attempts to use mesostructured pure metal oxides did not lead to satisfying performances in repeated cycles. Considering these results, with the aim of fabricating more and more efficient sorbents, we moved toward the design of different mesostructured silica supports (M41S), with different textural properties, for the dispersion of maghemite in the form of ultrasmall nanoparticles. In fact, the gas–solid reaction mechanism of H2S with metal oxides is influenced by product-layer diffusion, pore diffusion, and gas-film diffusion;16 therefore, the different porous structure or dimension of the pores can dramatically affect the H2S removal performance of the different sorbents. In this context, this work, being already proved that mesostructured γ-Fe2O3 sorbents show promising performances to H2S removal at mid-temperature,13 is focused on the design of new mesostructured supports able to improve the H2S removal capacity as well as the ability to be repeatedly used. Among the different mesostructured matrices, micrometric MCM-41 has been proved13 to be the most promising, due to the maximization of the surface area as a direct effect of the reduced pore size close to the mesoporous lower limit (2 nm). Starting from this point, two different strategies have been followed to facilitate the gas/solid interaction (H2S/active phase; regenerating gas/sulphided



deriving material) and to improve the lifetime of the sorbent. From one side, the pores interconnection, typical for a bi-continuous pore structure (cubic), should theoretically maximize the pore volume and the surface area allowing a better dispersion and a higher accessibility of the iron oxide. From the other one, the strengthening of the porous structure through the thickening of the silica pore walls should guarantee steady performances under a repeated number of sulphidation cycles. Methods Chemicals All chemicals were of analytical grade and used as received without further purification. Hexadecyltrimethylammonium bromide (CTAB, 98%), ethanol (EtOH, azeotropic 95.6%), ethyl acetate (EtOAc, 99.8%), ammonium hydroxide (NH4OH, 28% NH3 in H2O), tetraethyl orthosilicate (TEOS, 98%), iron (III) nitrate-nonahydrate (>99.5%), and n-Hexane (95%) were purchased from Sigma-Aldrich. Distilled water was used for all the experiments. Support preparation Hex and Hex_Hyd. Micrometric silica particles were synthesized as described elsewhere.13 Specifically, 1 g of CTAB was dissolved in 200 mL of distilled water and the solution kept under stirring at 300 RPM for 3 hours at room temperature. Then, 87 mL of EtOH and 21 mL of NH4OH were added; after 20 min, the stirring speed was increased to 600 RPM before adding 3.8 mL of TEOS. The starting transparent solution became white opalescent. After 1 hour under stirring, 100 mL of dispersion were poured into an autoclave (330 mL of total volume) and kept for 24 h at 100 °C (Hex_Hyd), while the remaining amount (Hex) was kept for 24 hours at RT under mild stirring (300 RPM).


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Finally, the resulting solids were recovered by centrifugation, washed three times with 20 mL of a 1:1 V/V mixture of ethanol and water, dried overnight at 80 °C and then calcined at 550 °C for 4 hours (heating rate, 5 °C min-1) to remove the organic template. The complete removal of the surfactant was verified by FTIR analysis (Figure S1).

Cub. In order to obtain micrometric silica with a cubic pore structure, the synthetic strategy used for preparing the micrometric hexagonal silica was varied using 3.79 mL of EtOAc soon after the addiction of 87 mL of EtOH and keeping all the other parameters unchanged. The particles were dried overnight at 80 °C and calcined at 550 °C for 4 hours (heating rate 5°C min-1) to decompose the organic template. Also in this case, the complete removal of the surfactant and of EtOAc was verified by FTIR analysis (Figure S1). Fe_Ref. A reference sorbent consisting in a nanostructured iron-oxide sample has been obtained by a chemical etching.18,19 Specifically, 0.8 g of Fe_Hex_Hyd was submitted twice to a etching step with 50 mL of NaOH 2M aqueous solution for 1 hour under stirring. Then, the recovered powder was washed with distilled water up to pH 7 is reached, and dried for 48 hours at 50 °C. Sorbents preparation Mesostructured silica-based iron oxide composites were prepared via a two-solvents incipient wetness impregnation route as described elsewhere.12 Typically, 0.1 g of dried silica samples (120 °C for 48 hours in air), were suspended in 12 mL of n-hexane and kept under stirring at 300 RPM for 2 hours at room temperature, then a suitable amount of the metal precursor aqueous solution (Fe(NO3)3·9H2O) was added drop-wise. The necessary volume of solution was calculated according to the pore volume determined by N2-physisorption analysis (Table S1), as requested for the incipient wetness impregnation methods.16 After 2 hours, the dispersion was heated at 80 °C in a ACS Paragon Plus Environment


hot-plate until complete evaporation of the hexane, and then kept in an oven at 80 °C overnight. Finally, the product was calcined at 500 °C (heating 2 °C min-1) for two hours in order to decompose the iron nitrate. The composites are labeled as Fe_support (Fe_Hex, Fe_Hex_Hyd, Fe_Cub). The theoretical amount of the active phase corresponds to the actual amount due to the fact that the method does not involve any liquid/solid separation and washing steps. Characterization Low-angle (2θ = 0.9°–6°) and wide-angle (2θ = 10°–70°) X-ray diffraction patterns were recorded on a Seifert instrument with a θ−θ geometry and a Cu Kα anode. The lattice parameter (a0) for hexagonal and cubic porous structure were respectively calculated according to the followed equations: =

√

= √6 ·

Hexagonal porous structure

Cubic porous structure

Where d100 and d221 are the d-spacing of the hexagonal and cubic structure. Textural analyses were carried out on a Micromeritics 2020 system by determining the nitrogen adsorption–desorption isotherms at -196 °C. Prior to analyses, the samples were heated for 24 hours under vacuum to 250 °C (heating rate, 1 °C min-1). The Brunauer–Emmett–Teller (BET) specific surface area was calculated from the adsorption data in the P/P0 range 0.05-0.17. The total pore volume was calculated at the point P/P0 = 0.99. The mean pore diameter was determined by applying the Barrett–Joyner–Halenda (BJH) model to the isotherm desorption branch. The wall thickness was calculated as the difference between the lattice parameter (a0) and the pore diameter (Dpore) for the hexagonal pore structure, while the following equation was used in the case of the cubic structure:


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=

− 3.092 2

Transmission electron microscopy (TEM) images were obtained by means of a JEOL JEM 2010 UHR microscope equipped with a Gatan Imaging Filter (GIF). Finely ground samples were dispersed in ethanol and sonicated, and the obtained suspensions were then dropped on carbon-coated copper grids. FTIR spectra were collected in the range 400-4000 cm-1 using a Bruker Equinox 55 spectrophotometer. The samples were analyzed after dispersing the powders in KBr pellets. 57

Fe Mössbauer spectra were measured for the Fe_Hex_Hyd and Fe_Cub samples in the

transmission mode with

57

Co diffused into a Rh matrix as the source, moving with constant

acceleration. The spectrometer (Wissel) was calibrated by means of a standard α-Fe foil and the isomer shift was expressed with respect to the standard at room temperature. The fitting of the spectra was performed with the help of the NORMOS program using Lorentzian profiles. DC magnetic properties have been studied by means of a Quantum Design PPMS Dynacool (Hmax = 90 kOe) by using the VSM module. Different kinds of magnetic measurements have been carried out. The field dependence of the magnetization has been studied at 5 K and 300 K between -90 kOe and +90 kOe. The temperature dependence of the magnetization has been studied by using Zero-Field-Cooled (ZFC), Field-Cooled (FC) protocols: the sample has been cooled down from 300 K to 2 K in zero magnetic field; then, the curves have been recorded under a static magnetic field of 250 Oe. MZFC has been measured during the warm-up from 2 K to 300 K, whereas MFC was recorded both during cooling and warm-up. From these data, the characteristics temperatures Tmax and Tirr have been



computed as follows. Tmax is the temperature of the maximum in the Zero Field Cooled curve. Tirr has been calculated by considering a 3% difference between the MFC and the MZFC. Desulphidation activity and regenerability of the sorbents To determine the desulphidation and regeneration capacities, 50 mg of composite were placed on a quartz wool bed (50 mg) in a vertical quartz tubular reactor, coaxially located in an electrical furnace. Before desulphidation, a pre-treatment at 300 °C for 30 min under helium flow was performed to remove air and water from the sorbent and the reactor. Then, a reactant gas containing 15200 ppm of H2S in Helium (inlet flow, 20 cm3 min-1) was fed to the reactor and the H2S content in the outlet flow during the adsorption test was monitored by a quadrupole mass spectrometer (Thermo Electron Corporation), with a detection limit for H2S of about 50 ppm. At the same time, H2O and SO2 signals were also monitored. When the outlet concentration of H2S reached 15200 ppm, the measure was stopped and the system was purged in flowing helium (20 cm3 min-1) for 1 hour. The amount of sulphur retained per unit mass of sorbent was determined when the outlet H2S concentration attained 100 ppm by the formula:

sulphur retention capacity *SRC. =

*F0 ∗ B3 . W

where Fs is the mass flow rate of sulphur (mg of S s-1), Bt is the breakthrough time (s) and W is the sorbent weight (g), referring to the composite. The sulphur retention capacity of the sorbents was obtained as the difference between the Bt value of the composite and the Bt value of the bare support. The error on the SRC values was estimated at 2 mgS g-1sorbent by carrying out several sulphidation cycles on fresh portions of the commercial sorbent KatalkoJM 32-5. The regeneration process was performed on a Thermoquest 1100 TPD/R/O apparatus equipped with a thermal conductivity detector (TCD) and a quadrupole mass spectrometer (QMS) for monitoring SO2 and O2 signals. The composite was heated under air flow (20 cm3 min-1) up to 500 °C (heating rate, 10 ACS Paragon Plus Environment

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°C min-1) and the temperature was kept constant for 3 hours. To identify the samples after different cycles of sulphidation or regeneration processes, a letter (S o R, respectively) and a number (denoting successive cycles) were added in the sample name. Results Supports characterization. Effect of the hydrothermal treatment and of the addition of ethyl acetate on the support texture Figure 1a reports the low-angle XRD of the bare silica samples (Hex, Hex_Hyd, Cub). For the Hex and Hex_Hyd patterns, it is possible to note the presence of three clear signals ascribable to a hexagonal pore structure (space group p6mm). d-spacing (d100) and cell parameter (a0) have been calculated and reported in Table 1. The hydrothermal treatment causes an increase of both dspacing (from 3.5 to 3.8 nm) and cell parameter (from 4.0 to 4.4 nm), as well as an improvement of the pore order degree, as highlighted considering the relative intensities and the width of the low-intensity reflections [110] and [220]. The use of a proper amount of ethyl acetate in the support preparation causes the formation of a cubic pore structure (Ia3d) instead of the hexagonal one (P6mm), as evinced from the presence of two signals [211] [210], corresponding to a bicontinuous pore structure Ia3d (Figure 1a).


[210]

Hex

Cub 2

a)

3

4

2θ

5

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600

400

200

Hex_Hyd Hex Cub

0 0.0

6

b)

0.2

0.4

0.6

0.8

dV/dr (cm3nm-1g-1)

[211]

Hex_Hyd

Volume adsorbed (cm3g-1 STP)

[110]

[200]

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

[100]


1.0

Relative Pressure (P/P0)

0.1

0.0

Hex_Hyd Hex Cub 1.5

c)

2.0

2.5

3.0

3.5

4.0

Pore Diameter (nm)

Figure 1 Low-angle XRD patterns (a) N2 adsorption-desorption isotherms (b), pore size distributions (c) of Hex, Hex_Hyd and Cub silica.

Figure 1b reports N2-physisorption curves of the Hex, Hex_Hyd and Cub. The textural parameters of the different samples are listed in Table 1. According to the IUPAC classification, both hexagonal (Hex, Hex_Hyd) and cubic supports (Cub) show curves that can be classified as IVB type:20 besides a possible microporous contribution, a well-defined capillary condensation is observed in the range 0.2-0.4 P/P0, suggesting the presence of mesopores. The hydrothermal treatment induces a 17% reduction in the surface area (from 1063 to 877 m2g-1) and a corresponding reduction in the pore volume, while a 25% increase in the surface area (from 1063 to 1336 m2g-1) and a higher pore volume is observed when a cubic pore structure is formed. Pore Size Distribution (PSD) of the three samples is reported in Figure 1c. Compared to the Hex support, Hex_Hyd shows a slight increase in the average pore size (from 2.2 to 2.4 nm) and a narrower distribution, whereas the PSD of the Cub support results to be centered at a lower value (1.8 nm). Wall thickness values calculated by the correlation of the cell parameter and the average pore diameter have been reported in Table 1. The hydrothermal treatment, as wanted, causes an effective thickening of the walls, from 1.7 to 2.0 nm, while the use of a suitable amount of ethyl acetate leads to a cubic structure characterized by walls 0.3 nm thinner (1.4 nm) than the hexagonal one.


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Sorbent characterization Figure 2a reports the low-angle XRD of the Fe_Hex_Hyd and Fe_Cub composites and of the corresponding supports. The characterization of the Fe_Hex sample has already been reported in a previous paper.13 In both the Fe_Hex_Hyd and Fe_Cub composites, the hexagonal (space group

p6mm) and cubic pore structure (space group Ia3d), respectively, are well retained after the impregnation and calcination procedures. The corresponding d-spacing (d100 and d211 for hexagonal and cubic structures, respectively) and cell parameter (a0) have been calculated and reported in Table 1. Figure 2b reports wide-angle patterns of the bare MCM-41 (Hex_Hyd) and MCM-48 (Cub) supports and of the corresponding composites made up with 10% w/w of Fe2O3 active phase. For Fe_Hex_Hyd and Fe_Cub, the typical broad reflection of amorphous silica at about 22° is accompanied by two weak and broad additional reflections at about 35° and 62° that can be considered an indication of the high dispersion of the iron-bearing phase into the matrix in the form of small nanoparticles or clusters. Aware that the sulphidation performances are strictly related to the iron oxide crystalline phase,21,22

57

Fe Mӧssbauer Spectroscopy has been also proposed, being capable to distinguish

maghemite, from hematite and magnetite, even in nanostructured materials.23–25 To this end, room temperature 57Fe Mӧssbauer Spectroscopy has been performed (Figure 2c) on Fe_Hex_Hyd and Fe_Cub composites. For both samples, the spectra consist of a doublet showing an isomer shift of 0.34 ± 0.01 mm s-1, which confirms the formation of very small maghemite nanoparticles.13,26,27 The FWHM has been found equal to 0.54 ± 0.01 mm s-1 and 0.65 ± 0.01 mm s1

for Fe_Hex_Hyd and Fe_Cub, respectively. This difference suggests a different particles size or

particle size distribution in the two samples. Moreover, the Mӧssbauer data recorded on the Fe_Hex_Hyd sample are very close to the values found for the Fe_Hex composite (IS of 0.34 ± 0.01 ACS Paragon Plus Environment


mm s-1 and FWHM of 0.49 ± 0.01 mm s-1).13 In order to get further details on the active phase, the dependence of magnetization on temperature (M vs. T) and magnetic field (M vs. H) has been investigated for the Fe_Hex_Hyd and Fe_Cub composites, by taking Fe_Hex as the reference. The temperature dependence of the magnetization has been recorded by means of the zero field

Fe_Cub

[110]

[200]

[100]

Cub

* * Fe_Cub Cub

* *

Fe_Hex_Hyd

4

5

b)

2θ

20 30 40 50 60 70

d)

250 Oe

30

1.6 1.2 Fe_Hex Fe_Hex_Hyd Fe_Cub

0.8 0.4 0

10

20

30

40

T (K)

50

60

70

-10

c)

2θ

2.0

0.0

Fe_Cub

Hex_Hyd

6

M (emu/gact)

a)

3

Fe_Hex_Hyd

Fe_Hex_Hyd

Hex_Hyd 2

Intensity (a.u.)

Intensity (a.u.)

Intensity (a.u.)

[210]

[211]

cooled-field cooled (ZFC-FC) protocol (Figure 2d).

M (emu/gact)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-5

0

5

10

Velocity (mm/s)

5K

20 Fe_Hex 10 Fe_Hex_Hyd Fe_Cub 0

4

-10

2

-20

-2

-30

-4 -2

0

-90

-60

e)

-30

0

-1

30

0

60

1

2

90

H (kOe)

Figure 2. Low-angle (a) and wide-angle XRD (b) patterns of the bare supports and of the corresponding Fe_Hex_Hyd and Fe_Cub composites; 57Fe Mӧssbauer spectra of the Fe_Hex_Hyd and Fe_Cub composites (c); dependence of magnetization on temperature (d) and magnetic field (e) for the Fe_Hex, Fe_Hex_Hyd and Fe_Cub samples.

The ZFC-FC curves appear overlapped at high temperature, while at 25 K (Tirr), some differences has been observed below such temperature: the curves of Fe_Hex and Fe_Hex_Hyd clearly exhibit a maximum in the ZFC curves at about 10 K (5678 ), while the ZFC profile of Fe_Cub shows a narrower peak with a maximum at lower temperatures (6 K). Assuming the presence of negligible


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interparticle interactions, due to the low Fe2O3 loading, the

5678

value can be considered as

directly related to the average volume of all the particles, while 59 is related to the volume of the biggest ones.28,29 Therefore the

*59 − 5678 .

values can provide a qualitative measure of the

volume size distribution.29 Accordingly, it can be proposed that the Fe_Hex and Fe_Hex_Hyd samples are constituted by particles with similar volume and particles size distribution, which appear to be larger than those in the Fe_Cub composite. Moreover, the latter composite shows a narrower pore size distribution with respect to the former two samples. The M vs. H curves at 5 K for all samples show a S-shaped hysteretic behavior typical for ferro-ferrimagnetic phases, with calculated coercivity field values equal to ∼ 1 kOe for Fe_Hex and Fe_Hex_Hyd and ∼ 0.4 kOe for Fe_Cub. In agreement with the difference in Tmax, such values confirm the different particles size of the two hexagonal composites with respect to the cubic one, as already indicated by the Mӧssbauer data, which showed a higher FWHM value for the cubic Fe-Cub sample, suggesting that this parameter and the average particle size are inversely related. Therefore, both the Mӧssbauer and magnetic data indicate that, compared to the MCM41-based (Hex, Hex_Hyd) sorbents, slightly smaller maghemite particles are present in the Fe_Cub sample, for which the narrower peak in the ZFC curve also indicates a narrower particle size distribution. These results can be explained by taking into account the higher surface area of the cubic support, which should permit a higher dispersion of the active phase. Figures 3a,b and 3c,d report representative TEM images of the Fe_Hex_Hyd and Fe_Cub composites, respectively. Micrometric particles are observed for each composite, and, in agreement with the low-angle XRD results, the presence of a well ordered porous structure, with pores dimensions in the 2-3 nm range, is confirmed.



Figure 3. Bright field TEM images at different magnification of Hex _Hyd (a, b,) and Cub (c, d) at different magnifications.

The nitrogen adsorption-desorption isotherms and pore size distributions of the Hex_Hyd and Cub supports and relative composites are shown in Figures 4a,b and 4d,e, respectively, and the textural parameters are listed in Table 1. As it can be observed from the Figure, the sorbents exhibit the same IVB type isotherms of the bare supports.30 As expected, the impregnation process induces a reduction in the surface area values, as well as in the pore volume (mainly due to a significant decrease in the extent of mesopores adsorption), while the dimension of the pores is retained (Table 1). In order to estimate the micro and mesoporous volumes of the samples, the tplot method has been used. The determination of the average thickness t(P) of the film adsorbed on the flat surface of the reference as a function of the pressure P has been performed following the Gaterneau approach.31 In the case of micropores-containing materials, none of the linear regimes pass through the origin of the t-plot, and the intercept of the linear fit in the low-pressure range is classically taken as the microporous volume of the solid.31 Figures 4c and 4f report the tplot curves for the bare Hex_Hyd and Cub supports and for the corresponding Fe_Hex_Hyd and ACS Paragon Plus Environment

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Fe_Cub_Hyd composites. As it is possible to evince from the graphs, the intercepts of the linear fit of the t-plot in the low-pressure range (corresponding to the 0.2-0.3 nm range) are close to the origin, pointing out that a very little microporous contribute is present in all the samples, as reported in Table 1 (Vmicro t-plot column). Furthermore, considering the value corresponding to the first point of the linear regime after condensation within the mesopores, it is possible to calculate the total pore volume (Vtotal) from the t-plot curves, whose values are reported in Table 1 (Vtot t-plot column). The difference between the two pore volume values calculated at 0.99 P/P0 from the physisorption isotherm (Vp) and from the t-plot (Vtotal) can be justified considering the N2 condensation in the interparticles voids at around 0.99, that can cause an overestimation of the Vp value.


0.12

200

a)

0.2

0.6

0.8

0.06 0.04 0.02

Fe_Hex_Hyd Hex_Hyd

-0.02

1.0


2

b)

3

0.14

200

-1 -1

0.10

3

dV/dr (cm nm g )

0.12

400

0.06

0 0.2

0.4

0.6

0.8

0.08

0.04 0.02 0.00

Cub Fe_Cub

Cub Fe_Cub

-0.02

1.0

2


e)

Page 16 of 33

500

3

Vtotal 300

200

100

Fe_Hex_Hyd Hex_Hyd

0 0.0

0.2

0.4

0.6

t (nm)

c)

500

Vtotal

400

Vtotal

300

200

100

Cub Fe_Cub

0

4

Pore Diameter (nm)

Vtotal

400

4

Pore Diameter (nm)

600

0.0

d)

0.4

0.08

0.00

Fe_Hex_Hyd Hex_Hyd

0

0.10

Volume adsorbed (cm3 g-1STP)

dV/dr (cm3nm-1g-1)

400

Volume adsorbed (cm3 g-1STP)

0.14

0.0

Volume adsorbed (cm3g-1 STP)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Volume adsorbed (cm3g-1STP)


0.0

f)

0.2

0.4

0.6

t (nm)

Figure 4. N2 adsorption-desorption isotherms (a, d), pore size distributions (b, e) and t-plot (c, f) of the bare Hex_Hyd ,Cub and their corresponding iron oxide-based sorbents Fe_Hex_Hyd and Fe_Cub.

Sample

SBET

Vp

Vtot t-plot

Vmicro t-plot Dp

wt

d100 a0

(m2g-1) (cm3g-1) (cm3g-1) (cm3g-1)

(nm) (nm) d211 (nm)

Hex*

1063

0.76

0.68

0.01

2.2

1.7

3.5

4.0

Hex_Hyd

877

0.69

0.59

0.01

2.4

2.0

3.8

4.4

Fe_Hex_Hyd 744

0.60

0.48

0.01

2.4

2.0

3.8

4.4

Cub

1336

0.83

0.75

0.01

2.1

1.4

3.1

7.7

Fe_Cub

911

0.65

0.57

0.01

2.1

1.4

3.1

7.6

Table 1 Textural features obtained by N2-physisorption data for the supports and the corresponding composites. SBET=Surface area; Vp=Pore volume; Dp=pore diameter; Vtot t-plot: total volume calculated by t-plot graphs; Vmicro t-plot: microporous volume calculated by t-plot graphs; wt= walls thickness. Relative standard deviation: %RSD(SBET)=2.1%; %RSD(Vp)=1.1%; %RSD(Dp)=1.8%. d-spacing (d100 and d211 for hexagonal and cubic structure, respectively) and lattice parameter (a0) obtained from the X-Ray diffraction data. *Values reported in the reference13


Page 17 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Sulphidation and regeneration processes For each sorbent, three sulphidation runs have been carried out (breakthrough curves in Figure 5), and the corresponding Bt and SRC values are reported in Table 2. In order to study the effect of the different textural properties (wall thickness and porous structure) on the sulphidation performances, the hydrothermally treated sorbent with hexagonal pore structure (Fe_Hex_Hyd) and the one with cubic pore structure (Fe_Cub) have been compared with the Fe_Hex sorbent (reported as a reference)13 Furthermore, in order to compare the behavior of the composites with their counterpart made up of free nanostructured iron oxide, an ad-hoc sample (Fe_Ref) has been prepared by a proper etching of the composite and tested. A further comparison was also performed by testing a ZnO-based commercial sorbent (KatalkoJM 32-5) (Table 2). Sample

Sulphidation

Bt (s)

run

SRC

SRC

(mgS gsorbent-1)

(mgS g acitve phase-1)

Fe_ Hex*

1st

295

38*

380

Fe_ Hex_R1

2nd

169

22*

220

Fe_ Hex_R2

3rd

169

22*

220

Fe_ Hex_Hyd

1st

125

17

170

Fe_ Hex_Hyd_R1

2nd

161

21

210

Fe_ Hex_Hyd_R2

3rd

169

22

220

Fe_ Cub

1st

128

17

170

Fe_ Cub_R1

2nd

98

13

130

Fe_ Cub_R2

3rd

84

11

110

Fe_Ref

1st

1121

146

146



Fe_Ref_R1

2nd

525

69

69

Fe_Ref_R2

3rd

363

47

47

KatalkoJM 32-5*

1st

122

16

16

KatalkoJM 32-5_R1

2nd

10

1

1

KatalkoJM 32-5_R2

3rd

13

2

2

Table 2 Breakthrough time (Bt) and sulphur retention capacity (SRC) of fresh and regenerated iron oxide-based sorbents. R1 and R2 -1 * refer to the first and the second regeneration, respectively. The error in SRC value is estimated to be ± 2 mgS gsorbent . Values 13 reported in the reference 300

300

1S 2S 3S

200

1S 2S 3S

250

H2S (ppmv)

250

H2S (ppmv)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 33

150 100 50

200 150 100 50

0 0

a)

0

Fe_Hex_Hyd

-50

100

200

Time (s)

300

Fe_Cub 0

b)

100

200

300

Time (s)

Figure 5. H2S breakthrough curves for three sulphidation–regeneration cycles (1S = first sulphidation, 2S = second sulphidation; 3S = third sulphidation) for the sample Fe_Hex_Hyd (a) and Fe_Cub (b).

The Fe_Hex_Hyd and Fe_Cub composites show similar SRC values only in the first sulphidation (17 mgs gsorbent-1) whereas, in the successive runs, an opposite trend has been observed: if in the case of Fe_Cub a gradual worsening of the performance is observed during the successive two sulphidation steps (13 mgs gsorbent-1 and 11 mgs gsorbent-1), a SRC improvement up to 21 mgs gsorbent-1 has been reached in the case of Fe_Hex. Taking into account the last two SRC values for each sorbent, it turns out that the Fe_Hex_Hyd composite has a sulphur retention capacity nearly doubled with respect to that of Fe_Cub, and shows a very similar performance compared to Fe_Hex. As expected, for pure phases, a decrease of the H2S performance was found for both reference samples (Fe_Ref and KatalkoJM 32-5) over the repeated sulphidation cycles, probably due to intrinsic sintering phenomena during the sulphidation (at 300°C) and regeneration (at 500°C) steps. The comparable first sulphidation SRC of the nanocomposite (Fe_Hex_Hyd) with the ACS Paragon Plus Environment

Page 19 of 33

corresponding free nanostructured iron oxide (Fe_Ref) point out the similitude of the active phase in the two sorbents (see Figure S4) . The lower SRC value of the commercial sorbent can be

Fe_Hex_Hyd Fe_Cub 500

a)

1000

Time (s)

400

300

QMS (O2) 200 100

TCD (O2 +SO2)

QMS (SO2) 0

1500

500

2000

b)

4000

Time (s)

0

6000

Fe_Cub T (°C)

500

400

300

200

QMS (O2) 100

TCD (SO2+O2) QMS (SO2) 0

c)

2000

4000

Temperature (°C)

SO2

T (°C)

TCD and QMS signals (a.u.)

H2O

Fe_Hex_Hyd

Temperature (°C)

H2S

TCD and QMS signals (a.u.)

explained considering the differences in the active phase (ZnO) and crystallite size.

QMS signal (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

6000

Time (s)

Figure 6 (a) H2S, SO2 and H2O profiles of the Fe_Hex_Hyd (purple) and Fe_Cub (orange) during the first sulphidation run (b, c). Thermal Conductivity Detector (TCD) profile and SO2 and O2 Quadrupole Mass Spectrometer (QMS) signals of Fe_Hex_Hyd and Fe_Cub during the first regeneration run.

Figure 6a reports the H2S, SO2 and H2O profiles recorded during the first sulphidation run for the Fe_Hex_Hyd and Fe_Cub composites. In both cases, it appears that the H2S retention corresponds to the formation of a large amount of H2O, indicating that the substitution reaction governs the sulphidation processes, in accordance to the following reactions:13 ; 3? @ → ; 3? =

equation 3.1

Being Fe2S3 thermodynamically unstable phase, it easily converts to pyrite (FeS2)12 and Fe3S422,32 (equation 3.2) or pyrite, pyrrhotite (FeSx) and sulphur33 (equation 3.3, where α, β, γ are the relative molar amounts). 2 ;

Î³-Fe2O3-M41S Sorbents for H2S Removal: Effect ... - ACS Publications

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