Three-Dimensionally Ordered Macroporous Iron Oxide for Removal of

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Three-Dimensionally Ordered Macroporous Iron Oxide for Removal of H2S at Medium Temperatures Hui-Ling Fan, Ting Sun, Yan-Peng Zhao, Ju Shangguan, and Jian-Ying Lin* State Key Laboratory of Coal Science and Technology, Co-founded by Shanxi Province and the Ministry of Science and Technology, Institute for Chemical Engineering of Coal, Taiyuan University of Technology, West Yingze Street Number 79, Taiyuan 030024, People’s Republic of China S Supporting Information *

ABSTRACT: A series of iron oxide sorbents with novel structures of threedimensionally ordered macropores (3DOM), ranging in size from 60 to 550 nm, were fabricated and creatively used as sorbents for the removal of H2S at medium temperatures of 300−350 °C. Evaluation tests using thermogravimetric analysis (TGA) and a fixed-bed reactor showed that, in comparison to the iron oxide sorbent prepared by a conventional mixing method, the fabricated iron oxide sorbent with a 3DOM structure exhibited much higher reactivity and efficiency, as well as high sorbent utilization with low regeneration temperature. The excellent performance of 3DOM iron oxide as a sulfur sorbent is attributed to its special texture, i.e., the open and interconnected macroporous, large surface area, and nanoparticles of iron oxide, which are revealed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and nitrogen adsorption techniques. The investigation results of the pore effect on the performance of the sorbent show that sorbents with pores size around 150 nm in diameter revealed the best performance. The reason is that pores of this size are large enough to allow gas to pass through even if the channel is partially blocked during the reaction process while remaining a large surface area that can provide more active sites for the reaction.

1. INTRODUCTION

Several researchers have reported on the important role of texture on the performance of hot-gas desulfurizerization,8−11 especially Palacios and co-workers, who mentioned the role of large pores in sulfidation.8 Silaban and Harrison also considered that pellet reactivity was more strongly dependent upon porosity than the surface for diffusion-controlled reactions.5 Jung et al. found that sorbents with different physical properties, such as pore volume and surface area, make a significant difference in desulfurization performance, and they suggested that both pore volume and surface area contribute to the capacity of the sorbent.12 However, few works have shown what the optimum texture for sulfidation is or what kind of texture should be applied for the sorbents. Investigation of the influence of textural parameters, especially the pore size, on the performance of iron oxide sorbents is thus quite important and very necessary, and it has been a focus of our work in recent years.13 To prepare a series of sorbents with different textural characteristics becomes the key, and two approaches were applied for this purpose. One is the conventional mechanical mixing method employing different sizes of polystyrene beads as the pore-making material, instead of traditional materials, such as starch. The other is to use a colloidal crystal templating

Integrated gasification combined cycle (IGCC) and fuel cells, especially coal polygeneration with IGCC, are a very promising and advanced clean coal technology because of the potential of a significant reduction in pollutant emissions and improvements in electrical generation efficiency.1−4 These clean coal technologies are of great significance for countries, such as China, whose main energy source is coal, and hot-gas desulfurization is a crucial process in these technologies. The desulfurization relies on regenerable metal oxides or mixed metal oxide sorbents, and it consists of two processes, sulfidation and regeneration. Sulfidation is the reaction between the metal oxide and the sulfur compound. During this process, metal oxides are converted to metal sulfides to catch the sulfur in coal gas. Because of the volume difference of oxygen and sulfur, the solid product occupies more space than the reactant, and thus, the porosity diminishes in the course of the reaction and even causes pore closure in some cases. This prevents the diffusion of reactive gases to the interior and may cause the reaction to cease before the solid reactant is completely used.1 Kinetic studies on the reaction between H2S and metal oxides indicate that the sulfidation reaction occurs under strong diffusional limitations.5−7 These results all point to the conclusion that the physical structure and/or texture of the sorbent determine the effect of the diffusion and, therefore, play a very significant role in the sulfidation reaction. © 2013 American Chemical Society

Received: Revised: Accepted: Published: 4859

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microscopy, respectively. X-ray photoelectron spectroscopy (XPS) measurements were made on a V.G. Scientific ESCALAB250 spectrometer.

method to produce three-dimensionally ordered macroporous (3DOM) sorbents with a very ordered porous structure that can easily be controlled, which makes the investigation easier and the results clearer. This study showed unexpected but encouraging results, indicating that iron oxide with a 3DOM structure is a very promising sorbent for desulfurization, especially for the ultrahigh levels of gas cleaning required by the process for high-value-added products. 3DOM materials with pore sizes in the sub-micrometer range have been extensively studied and have received much attention in recent years in the field of photonic crystals, electrodes, separation, and catalyst support.14−17 This ordered structure consists of a skeleton surrounding uniform close-packed macropores interconnected through windows. Furthermore, the 3DOM materials have high porosity, and the pore size can be adjusted in the range of several nanometers to several hundred nanometers, which makes it very attractive as a catalyst, especially for those processes in which the diffusion limitation is the controlling step. However, few reports of this special material used as a desulfurization sorbent have been found. This work will present the special features of the prepared 3DOM iron oxide in desulfurization and explore the possibility of this kind of structure being applied as a sulfur sorbent. In addition, the investigation results of the textural effects on the performance of the sorbent will also be presented.

3. RESULTS AND DISCUSSION 3.1. Preparation and Structural Features. Figure 1a shows typical SEM images of the polystyrene (PS) colloidal

Figure 1. SEM images of (a) PS colloidal crystal template with a diameter of 200 nm and (b) prepared sorbent with a 3DOM structure (3DOM-FS500) (inset is an enlarged magnification).

crystal templates. It can be seen that the PS beads are highly uniform and ordered into close-packed domains. Figure 1b shows the prepared sorbents with the 3DOM structure. As seen in the figure, all of the samples consisted of a uniform structure with well-ordered macropores that are interconnected through the windows where the original spheres touched. Normally, it was found that, during the preparation of the 3DOM material by the colloidal crystal template method, the resulting pore diameters were considerably smaller than the original PS sphere. The extent of this shrinkage, because of the melting of polymer templates and the sintering of the metal oxide during calcination, varied for the different materials. In this experiment, as shown by comparing the SEM images of the 3DOM sample and their corresponding templates (shown in Figure 2), the shrinkage ranged from 45 to 59% for the 3DOM-

2. MATERIALS AND METHODS 2.1. Sorbent Preparation. All chemicals used in this study were analytically pure. 3DOM sorbents were prepared by the colloidal crystal templating method. The detail procedure information is shown in the Supporting Information and can also reference the paper.18,19 The prepared 3DOM iron oxide (3DOM-F) and iron−silicon oxide (3DOM-FS) sorbents are list in Table S1 of the Supporting Information. 2.2. Performance Tests. The reactivity of the sorbent was tested using thermogravimetric analysis (TGA). Evaluations of sorbents for sulfidation were conducted in a 6 mm inner diameter upflow fixed-bed quartz reactor at atmosphere pressure, 350 °C, and space velocity of 34 000 h−1, using a mixture of 300 ppm H2S, 5% H2, and balance gas of nitrogen. The sorbents are in the form of small particles of 60−80 mesh. The sulfur compound in the gas exiting the reactor was measured continuously using a gas chromatograph equipped with a flame photometric detector (FPD). Sulfidation of the sorbent was continued until the breakthrough point. The breakthrough point was defined as the sulfur content exceeding 10% of the inlet gas sulfur content. Sulfur capacity is defined as grams of sulfur per 100 g of iron oxide in the sorbent at the breakthrough point. A temperature-programmed regeneration test was also performed in the reactor using 2 vol % O2 in N2, at atmosphere pressure and space velocity of 34 000 h−1. 2.3. Characterization Techniques. Nitrogen adsorption and desorption isotherms were determined with a micromeritics apparatus model 3H-2000PS2 system. The specific surface areas were calculated using the Brunauer−Emmet− Teller (BET) method, and the pore size distributions were calculated from the desorption branches of the isotherms. Powder X-ray diffraction (XRD) patterns were recorded with a Rigaku D/max-2500 diffractometer using Cu Kα radiation. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained with Nanosem430 and Tecnai G2 F20 electron

Figure 2. SEM images of (a) 3DOM-F550 (inset is the corresponding PS with a diameter of 1000 nm) and (b) 3DOM-FS200 (inset is the corresponding PS with a diameter of 320 nm).

F sorbents and from 37 to 45% for the 3DOM-FS sorbents. Table S1 of the Supporting Information shows the diameters of the spheres and the macropore sizes of the 3DOM sorbents. Figure 3 displays the TEM images of the microstructure of the inorganic skeleton of the prepared 3DOM sorbents. It shows that the walls are formed by the agglomeration of the fused nanocrystallized particles [see inset selected area electron diffraction (SAED) patterns in Figure 3], leading to significant textural mesoporosity within the walls of the structure. A noticeable result is that 3DOM-FS possesses somewhat smaller nanoparticles (5 nm) than 3DOM-F (25 nm), which is thought 4860

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3.2.2. Nitrogen Adsorption/Desorption. Figure 5 presents the N2 adsorption/desorption isotherms of 3DOM-F samples.

Figure 5. Nitrogen adsorption and desorption isotherms of different pore size of 3DOM-F sorbents (inset is the corresponding pore size distribution of 3DOM-F158).

Figure 3. TEM images of prepared 3DOM materials with different magnifications: (a) 3DOM-F158 and (b) 3DOM-FS130 (inset is the corresponding SAED pattern).

It is noted that the isotherms of all of the 3DOM samples exhibit similar curves to a type II characteristic of a type H3 hysteresis loop in the relative pressure (p/p0) range of 0.8− 1.0.20,21 3DOM-FS samples show similar curves (see Figure S1 of the Supporting Information). The average pore size, calculated by the Barrett−Joyner−Halenda (BJH) method, is 20−33 and 6−17 nm for the 3DOM-F and 3DOM-FS samples, respectively (shown in Table S1 of the Supporting Information). This is attributed to the mesopore-sized void spaces between the crystallites, which are also observed in the TEM images. Furthermore, the results demonstrate that iron− silicon oxide samples have higher BET surface areas than the iron oxide samples, and the samples were observed to increase in surface area with a decreasing pore size, as shown in Table S1 of the Supporting Information. 3.2.3. XPS Studies. XPS analysis of 3DOM-F and 3DOM-FS sorbents (XPS spectra are presented in the Supporting Information) show that besides Fe2O3 (Fe 2p, 710.7 and 723.9 eV; O 1s, 530.4 eV),22−25 FeOOH (Fe 2p, 712.7 and 725.9 eV; O 1s, 531.4 eV)22,23 and some molecular water26 also occurred on the surface of the sorbent. The only one Si 2p peak at 102.4 eV has been assigned to SiO2,27 which is in a highly dispersed or amorphous form combined with the XRD result. The absence of any other features supports that silica is not forming any compound with iron oxide under the experimental conditions employed in the present study, in agreement with the XRD measurements. 3.3. 3DOM Iron Oxide or Iron−Silicon Oxide as a Sorbent for H2S Removal. 3.3.1. Sulfidation Test. The reactivity of the prepared 3DOM iron oxide as a sorbent for desulfurization was first examined in a thermogravimetric analyzer with a simple reactive gas containing 300 ppm H2S in nitrogen, at a temperature of 300 °C. Sorbent CM-F, prepared from an iron oxide by a conventional mechanical mixing method and with a BET surface area of 7.8 m2/g, was also tested for comparison. Before the experiments, the sorbents were crushed and sieved to small particles of 80−100 mesh. Figure 6 gives the resulting thermogravimetry (TG) curves of the two types of sorbents. As seen, the sorbents exhibited quite

to indicate that silicon can keep the iron grains separate and prevent them from growing larger during calcination. 3.2. Characterization of Sorbents. 3.2.1. XRD Studies. Figure 4 shows the XRD patterns of fresh 3DOM sorbents as

Figure 4. XRD patterns of fresh and sulfided 3DOM iron oxide and iron−silicon oxide sorbents.

prepared and after being used in H2S adsorption in an atmosphere of H2S/H2/N2. As noted from this figure, only one phase was found for each sample. For the fresh sorbent of 3DOM-F, the characteristic diffraction peaks correspond to αFe2O3, whereas for 3DOM-FS, the crystalline phase detected in the fresh sorbent was γ-Fe2O3, possibly because of retardation of the transformation of maghemite to hemite by silicon. No crystalline silicon features are apparent in 3DOM-FS. Furthermore, the broader diffraction peaks of γ-Fe2O3 indicate a smaller particle size compared to α-Fe2O3. The crystallite sizes of α-Fe2O3 and γ-Fe2O3 calculated on the basis of Scherer’s equation from line broadening of the highest intensity XRD peaks (104) and (311) are 26.04 and 5.47 nm, respectively. The values are in good agreement with the results provided by TEM. In the used samples, diffraction peaks assigned to FeS were observed for both types of sorbents. 4861

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The large difference of desulfidation behaviors in terms of efficiency, reactivity, and sorbent utilization between the two types of sorbents is believed to be due to the different physical properties of the pore structures and surface areas of the two sorbents. As mentioned in the Introduction, sulfidation is a non-catalytic gas−solid reaction that is accompanied by a volume increase because of the solid product occupying more space than the solid reactant. The pores, as the channels for transporting the reactant gas, may become narrow or even be closed in some cases during the process. Sulfidation thus may slow or even stop before the sorbent is completely used because the reactant gas has difficulty reaching or cannot reach fresh active components in the inner sorbent for further reaction. It is generally considered that the global reaction rate for desulfurization is controlled by transport resistance,5,6 and increasing porosity should increase the rate of diffusion and, consequently, increase reactivity, as well as increase the utilization of the sorbents. The 3DOM sorbent possesses very ordered macropores that are open and interconnected with each other. Such a structure can greatly improve the diffusion of a gaseous reactant to the inner part of the sorbent for continuing the reaction. A comparison TG test of the sulfidation of 3DOM iron oxide in the form of large and small particles (5 mm and 80−100 mesh, respectively) may illustrate that suggested. In Figure 6, the TG curves show that the reactivity of the sorbent for both cases is almost the same, indicating that intraparticle diffusion in the 3DOM largeparticle sample is almost as small as in the small-particle sample, and thus, the diffusion resistance of the sorbent may be significantly alleviated by making the 3DOM structure. In addition, in comparison to CM-F, the 3DOM sorbent has a large surface area of 34.26 m2/g because of the mesopores between the nanoparticles of the iron oxide, as revealed in the TEM results, which makes more of the active component available for the sulfidation reaction. In summary, the improved pore structure and large surface area of the 3DOM sorbent are believed to be responsible for its excellent desulfurization performance. It should be pointed out that the significance of this result is that the 3DOM structure may have special meaning for ultradeep gas desulfurization. 3.3.2. Test of Regeneration. A simple regeneration experiment was performed to test the regenerability of the sorbent with the 3DOM structure using a flow of 2 vol % O2 in N2. A sulfur-saturated 3DOM iron−silicon oxide was used as the sample. The regeneration test was carried out initially at 100 °C and held at that temperature until the SO2 concentration in the outlet was less than 3 ppm. The temperature was then increased to 200 °C at a rate of 1 °C/min and was maintained at that temperature until the SO2 concentration in the outlet was less than 3 ppm. The regeneration gas was switched off during the temperature increase process. The same procedure was also carried out at 300 and 400 °C. Figure 8 shows the SO2 concentration in the outlet of the reactor. It can be seen that regeneration does not occur at such a low temperature of 100 °C; however, increasing the temperature could help to release sulfur. The concentration of SO2 was high, and it lasted for only a few minutes at temperatures of 200 and 300 °C. After that, it sharply changed to steady, very low values. Sulfur element analysis by the coulometric method proved that only 1.03% sulfur in the sorbent remained in the regenerated sorbent, indicating that almost complete sorbent regeneration was obtained at temperatures below 400 °C, whereas most of the sulfur could be desorbed at a low temperature of 300 °C.

Figure 6. Comparison of TG curves of 3DOM-F and CM-F.

different behaviors. In the case of the 3DOM sorbent, the weight increased dramatically with a very sharp slope, achieving 22% weight gain in only 35 min. In the case of sorbent CM-F, however, the whole TG curve increases smoothly with a slowly changing slope, and only a 10% weight gain was obtained using the same sulfidation time of 35 min. Obviously, therefore, the reactivity of iron oxide with the 3DOM structure is much higher than that of the sorbent prepared by the conventional mixing method. A comparison evaluation for sulfidation of 3DOM-F and CM-F was also performed in a fixed-bed reactor with respect to the H2S breakthrough curves, as shown in Figure 7. As

Figure 7. Breakthrough curves of sorbents with different preparation methods.

expected, the sorbent CM-F prepared by a conventional mixing method exhibits quite poor in breakthrough behaviors; the H2S concentration at the outlet is high, ranging from 7 to 28 ppm, even in the beginning sulfidation time of 100 min. In contrast, the 3DOM sorbent shows obviously excellent desulfurization performance. The ultralow H2S level (below the FPD limit of 0.2 ppm) prior to breakthrough and the sharp breakthrough curve show a very high efficiency for sulfur removal, with almost complete removal of H2S. Furthermore, it was found that the breakthrough sulfur capacity of 3DOM-F158 and CMF was 37.18 and 15.3%, corresponding to the sorbent utilization of 93 and 38.2%, respectively. The results are indicative of very high reactivity of the 3DOM sorbent, which is in good agreement with the fast reaction rate observed in the TG experiments. 4862

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Figure 8. Regeneration curves of 3DOM-FS. Figure 10. H2S breakthrough curves of 3DOM-F with different pore diameters.

The isothermal regeneration test at 400 °C in an atmosphere of 2% O2−15% vapor-N2 also confirmed the above results. Thus, the regeneration temperature for sulfided 3DOM iron oxide sorbent is much lower than that reported in the literature.28,29 In addition, the breakthrough curves of regenerated 3DOM-FS sorbents are very close to that of the fresh sorbent (see Figure S3 of the Supporting Information). SEM images of the regenerated sorbents (Figure 9 and Figure S4 of the Supporting

performance were also observed for the sorbents of different pore sizes. When the pore diameter was smaller than 158 nm, the increase in the pore size led to an increase in efficiency and breakthrough sulfur capacity, whereas in the case in which the pore diameter was larger than 158 nm, an increase in the pore size resulted in a decrease of the sorbent performance. Among the samples, the 3DOM-F158 sorbent, with the macropore size of 158 nm, exhibited the longest breakthrough time. The test using 3DOM-FS sorbents also give similar results (see Figure S5 of the Supporting Information). 3DOM-FS156 exhibited the best performance, with the highest sulfur capture capacity of 38.92%, which is very close to the theoretical value (40%). This high utilization of iron oxide of nearly 97.3% indicates a much high reactivity. Obviously, there is an optimum pore size of around 150 nm for the performance of H2S sorbents. This result is similar to that reported in a previous paper, which shows that a pore size of 200 nm is good for sulfidation.30 The elimination of pore diffusion resistance above a certain size of pore has been cited as a reasonable reason for these results. As stated above, the reaction for the removal of H2S by a metal oxide is characterized by forming a product with a larger volume than reactant solids, with porosity diminishing, and pore surface area loss may occur because the pores are becoming plugged with solid product during the course of sulfidation. Sulfidation occurs under strong intraparticle diffusion limitations. Small pores tend to suffer this plugging more easily than macropores, which may result in a severe transport resistance. Macropores, however, can prevent the plugging and present enough space for gaseous reactant diffusion, even in some severe cases. As the pore size increases, diffusion resistance in macropores will decrease. Once the pore size is large enough (around 150 nm, as shown in this paper), sulfidation is no longer dominantly influenced by pore diffusion; instead, the BET surface area, which decreases with the increase of pore size, may then play a crucial role in the desulfurization process. In this case, a large surface area is favorable because of the availability of more of the active component for reaction. The effects are confirmed in Figure 11, in which the sulfur capacity is shown as a function of the pore size and surface area. Thus, it is suggested that the sorbent with macropores that are large enough and with a high surface area could exhibit good performance because the pore size enables the reactant gas to diffuse more easily through the pores and the high surface area provides more active sites for easy and rapid absorption.

Figure 9. SEM images of regenerated 3DOM-FS sorbent.

Information) illustrate that, despite some cracking occurring at the wall, the macroporosity and ordering of the structure is still intact and the overall 3DOM structure is retained after regeneration. Although further study about regeneration and improvement of sorbent preparation is necessary, one can expect that a perfect 3DOM structure can be prepared under optimized conditions. The good stability and low regeneration temperature prove again that the 3DOM material holds considerable promise as a sulfur-capturing sorbent material. 3.4. Effect of the Pore Size on the Performance of the 3DOM Sorbent for H2S Removal. To investigate the effect of the pore size on desulfurization performance, seven 3DOMF sorbents with different pore sizes were subjected to sulfidation in terms of breakthrough curves in the fixed-bed reactor. The macropore sizes of the sorbents were 65, 90, 158, 205, 367, and 550 nm. As shown in Figure 10 for all of the sorbents with the 3DOM structure, an obvious feature is the high efficiency for sulfur removal, with a nearly complete reduction of the H2S concentration prior to breakthrough. Prebreakthrough H2S levels in the gas stream leaving the reactor remained below the detection limit. All curves increased sharply close to the breakpoint, and all 3DOM-F samples behaved as very reactive sorbents. However, differences in the sulfidation 4863

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Figure 11. Plot of sulfur capacity and surface area versus pore diameter.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Sorbent preparation and figures on nitrogen adsorption and desorption isotherms of 3DOM-FS sorbents, XPS results, breakthrough curves of all 3DOM-FS sorbents in a fixed-bed reactor, and SEM images of regenerated 3DOM-FS. This material is available free of charge via the Internet at http:// pubs.acs.org. Corresponding Author

*Telephone: 0086-351-6018534. E-mail: [email protected]. cn and/or [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Nature Science Fundamental (20976114). Additional support was provided by the Nature Science Fundamental of Shanxi Province (2011011008-2) and the Research Project Supported by Shanxi Scholarship Council of China (2011-40). The authors are also grateful for the support from the key fostering disciplines of Shanxi Province.

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