Synthesis and Activity Comparison of Copper-Incorporated MCM-41

In the present study, novel copper-incorporated MCM-41-type high-surface-area mesoporous materials were synthesized following a direct hydrothermal ...
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Ind. Eng. Chem. Res. 2008, 47, 1035-1042

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Synthesis and Activity Comparison of Copper-Incorporated MCM-41-Type Sorbents Prepared by One-Pot and Impregnation Procedures for H2S Removal Zeynep Ozaydin, Sena Yasyerli,* and Gulsen Dogu Department of Chemical Engineering, Gazi UniVersity, Ankara, Turkey

In the present study, novel copper-incorporated MCM-41-type high-surface-area mesoporous materials were synthesized following a direct hydrothermal synthesis method and also by the impregnation of copper into the mesopores of the pre-synthesized MCM-41 structure. XRD results indicated the formation of a CuO phase within the MCM-41 structure. Sharp H2S breakthrough curves, quite high sorption rate parameters, and high sulfur retention capacities were obtained in high-temperature removal of H2S using these copperincorporated mesoporous materials. XRD patterns of the sulfided products indicated the formation of a Cu1.81S phase within the MCM-41 structure. A deactivation model gave good predictions of the experimental H2S breakthrough data. 1. Introduction Removal of H2S from process gases and from fuel gas produced by gasification of fossil fuels is a major environmental concern. In integrated gasification-power generation systems and in number of other applications, H2S removal from process gases has to be carried out at high temperatures using metal oxide sorbents. Development of novel regenerable sorbents for hightemperature H2S removal and novel catalysts for the selective oxidation of H2S to elemental sulfur attracted the attention of a number of researchers, and it was also one of the focused areas of research in our recent studies (Yasyerli et al., 2001, 2003, 2004, 2006; Karayilan et al., 2005). The mixed-oxide sorbents synthesized in these studies by the complexation technique have relatively low surface-area values. Sorbents with high surface area are needed to increase the sorption activity and to decrease the pre-breakthrough H2S concentration levels in fixed-bed adsorbers. Copper-based oxides and mixed oxides have attractive high-temperature sulfur capture capacities (Westmoreland and Harrison, 1976; Abbasian and Slimane, 1998; Wang and Lin, 1998; Garcia et al., 2000; Slimane and Abbasian, 2000; Ko et al., 2005; Jung et al., 2006). In the presence of hydrogen gas, reduction of CuO to Cu2O or Cu was expected prior to reaction with H2S (Patrick et al., 1989; Yasyerli et al., 2001). Metallic copper is not thermodynamically as favorable as oxides of copper for H2S removal. Consequently, complete reduction of CuO to metallic copper is not desired during the sulfidation process. Synthesis of high-surface-area mesoporous materials containing copper attracted the attention of number of researchers in recent years (Guo et al., 2004; Hadjiivanov et al., 2003; Tsoncheva et al., 2004). Copper-based materials are also considered as attractive catalysts for a number of catalytic reactions, including steam reforming of alcohols (Matter and Ozkan, 2005; Oguchi, et al., 2005). Discovery of the MCMtype silicate-structured mesoporous materials with a regular array of uniform pores and high surface areas (Kresge et al., 1992) initiated a significant amount of research related to chemical reaction engineering applications of such materials (Grun and Unger, 1999; Ciesla and Schu¨th, 1999). To improve the activity of these mesoporous materials, metals or metal * To whom correspondence should be addressed. E-mail: syasyerli@ gazi.edu.tr.

Figure 1. Nitrogen adsorption isotherms of MCM-41, Cu-MCM-41(I), and Cu@MCM-41(II).

Figure 2. Pore size distribution of MCM-41, Cu@MCM-41(II), and CuMCM-41(I).

oxides were incorporated into their structure either by direct synthesis or postsynthesis methods such as impregnation, ion exchange, chemical vapor deposition, and so forth (Gucbilmez et al., 2005; Guo et al., 2004, Nalbant et al., 2007). In the present study, novel copper-incorporated MCM-41type high-surface-area mesoporous nanocomposite materials were synthesized, and the high-temperature H2S removal activity of these materials were tested. 2. Experimental Section Copper-incorporated MCM-41-type high-surface-area mesoporous materials were synthesized following a direct hydrothermal synthesis method (denoted as Cu-MCM-41) and also by the impregnation of copper into the mesopores of the presynthesized MCM-41 structure (denoted as Cu@MCM-41). In the synthesis of these materials, modified versions of the

10.1021/ie071039g CCC: $40.75 © 2008 American Chemical Society Published on Web 01/16/2008

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Table 1. Physical and Chemical Properties of Copper-Incorporated MCM-41 Sorbents One-pot Hydrothermal Route and Impregnation Procedure

sample

Cu/Si atomic ratio (EDS)

Cu/Si atomic ratio (XPS)

MCM-41 Cu@MCM-41(I) Cu@MCM-41(II) Cu@MCM-41(III) Cu-MCM-41(I) Cu-MCM-41(II)

0.09 0.56 0.70 0.07 0.12

0.10

a

0.024

BET area, m2/g

d(100) nm

lattice parameter a, nm

1040 1006 625 461 904 806

3.52 3.77 3.74 3.80 3.65 3.50

4.06 4.35 4.32 4.39 4.21 4.04

average pore diameter nm (BJH desorption)

pore wall thickness, nm

pore volume, cm3/g

2.43 2.80 2.26

1.63 1.55 2.06

1.03a 0.92 0.52

2.27

1.94

1.16

Sener et al., 2006.

procedures described in our previous studies for pure MCM41 and for the palladium-, vanadium-, and nickel-impregnated materials (Gucbilmez et al., 2005; Sener et al., 2006; Nalbant et al., 2007) were followed. In the synthesis of MCM-41 and Cu-MCM-41, an aqueous solution of sodium silicate containing 27 wt % SiO2 (Aldrich) was used as the silica source. Copper nitrate trihydrate (Merck) and hexadecyltrimethylammonium bromide (CTMABr, Sigma 99% pure) were used as the copper source and the surfactant, respectively. For the synthesis of MCM-41, sodium silicate solution was added to the surfactant solution dropwise with continuous stirring. The pH of the mixture was adjusted to about 11. The resulting gel was stirred for 1 h and then transferred into a Teflon-lined autoclave for hydrothermal synthesis. Hydrothermal synthesis was carried out at 120 °C for about 96 h. The solid product was washed thoroughly with deionized water and dried at 40 °C for 24 h in vacuum and then calcined in a tubular furnace in a dry-air flow. Calcination was achieved by heating the sample from ambient temperature to 550 °C at a heating rate of 1 °C/min and by keeping at this temperature for 6 h. For pure MCM-41 synthesis, the SiO2/CTMABr/H2O molar ratio of the reaction mixture was 0.0706/0.0363/4.83 and the volume of a typical reaction mixture was 120 mL. In the case of the one-pot synthesis of Cu-MCM41, a solution of copper nitrate trihydrate was added to the mixture of surfactant and sodium silicate before the hydrothermal synthesis step. The molar ratios of SiO2/CTMABr/Cu/H2O in the synthesis mixture prepared by the one-pot method adjusted to 0.0706/0.0363/0.0035/4.83 or to 0.0706/0.0363/0.0056/4.83. However, in the preparation of Cu@MCM-41, copper nitrate trihydrate solution was impregnated into the uncalcined MCM41, which was synthesized as described above. The mixture was stirred at 40 °C for 10 h and then dried in vacuum overnight. The product obtained was then calcined following a procedure as described above for Cu-MCM-41. The XRD patterns of the synthesized materials were determined by a Philips PW3040 employing a Cu KR diffractometer. Cu/Si mole ratios of the synthesized materials were determined by energy dispersive X-ray spectroscopy (EDS, JEOL JSM-6400), BET surface area, and BJH pore-size distributions of the synthesized materials were determined using a Quantachrome-AutoSorb-1 nitrogen adsorption system. XPS (SPECS) technique was used to determine the oxidation states and the external surface compositions of the synthesized materials. SEM (JEOL JSM-6335) photographs were used in the estimation of the particle size of the synthesized materials. These materials were also characterized by the temperature programmed reduction (TPR, Quantachrome Chembet 3000) technique using a gas mixture of 5% H2 in N2 and a heating rate of 10 °C/min. Dynamic H2S sorption experiments were carried out at 500 °C in a fixed-bed flow reactor using a gas stream containing 1% H2S, 10% H2 in helium at a flow rate of 100 cm3/min (measured at 25 °C). In these experiments, 0.2 g of sorbent was packed

into a quartz tubular reactor and supported by quartz wool from both sides. An FTIR spectrometer (PerkinElmer) connected to the exit stream of the reactor allowed for the on-line chemical analysis of H2S, SO2, and H2O. Further details of the H2S sorption system were given elsewhere (Yasyerli et al., 2001, 2003, 2004). 3. Results and Discussion 3.1. Characterization of the Synthesized Materials. In this study, copper-incorporated MCM-41 sorbents were prepared using one-pot direct hydrothermal and impregnation techniques. Physical and chemical properties of these materials are given in Table 1. Pure MCM-41 had a surface area of 1040 m2/g. The surface area of the copper-impregnated MCM-41 materials decreased with an increase in the Cu/Si ratio in these synthesized materials. For instance, for the material containing a Cu/Si mole ratio of 0.7 (Cu@MCM-41(III), the surface area was decreased to 461 m2/g. However, such a surface area is quite high compared to the surface area of pure CuO (115 m2/g), which was synthesized by the complexation technique (Yasyerli et al., 2001). The increase of the Cu/Si ratio of the copper-impregnated MCM-41-type materials also caused a decrease in the pore volume of these materials (Table 1). For instance, pore volume decreased from 0.92 to 0.52 cm3/g with an increase in the Cu/ Si atomic ratio from 0.09 to 0.56. At high Cu/Si ratios, plugging of some of the smaller pores by copper clusters is expected. In the case of direct hydrothermal synthesis route, the surface area values were 904 and 806 m2/g for the materials having Cu/Si atomic ratios of 0.07 and 0.12, respectively (Table 1). These materials have relatively high pore volumes. For instance, a pore volume of Cu-MCM-41(I) (containing a Cu/Si molar ratio of 0.07) is 1.16 cm3/g, which is very close to the pore volume of pure MCM-41 (1.03 cm3/g, Sener et al. 2006). The average pore diameters estimated from the BJH isotherms were found to be around 2.5 nm for all of the materials (Table 1). The nitrogen adsorption isotherms of these materials are typical type 4 isotherms, indicating mesoporous structure (Figure 1). For the copper-incorporated MCM-41-like materials synthesized by the one-pot and impregnation procedures, the capillary condensation of nitrogen was observed in the relative pressure range of 0.300.35. These materials have narrow pore-size distributions between 2 and 4 nm. Typical pore-size distributions of MCM41 and copper-incorporated MCM-41 (synthesized by direct and impregnation procedures) are shown in Figure 2. As shown in this figure, some pores larger than 20 nm were also observed for Cu-MCM-41(I) (direct synthesis product). Typical SEM photographs of the synthesized materials are shown in Figure 3. Particles having dimensions between 0.2 and 0.8 µm were observed in the SEM photographs. The XRD patterns of the copper-incorporated MCM-41-like materials prepared by the impregnation procedure (Cu@MCM-

Ind. Eng. Chem. Res., Vol. 47, No. 4, 2008 1037

Figure 5. XRD patterns of MCM-41 and Cu-MCM-41 prepared by the direct synthesis route. (a) MCM-41, (b) Cu-MCM-41(I) (Cu/Si:0.07), and (c) Cu-MCM-41(II) (Cu/Si:0.12).

Figure 3. SEM photographs of (a) Cu@MCM-41(II) (Cu/Si:0.56) and (b) Cu-MCM-41(I) (Cu/Si:0.07).

Figure 6. Hydrogen TPR profiles of (a) Cu@MCM-41(II) and (b) CuMCM-41(I).

Figure 4. XRD patterns of MCM-41 and Cu@MCM-41 prepared by the impregnation procedure. (a) MCM-41, (b) Cu@MCM-41(I) (Cu/Si:0.09), (c) Cu@MCM-41(II) (Cu/Si:0.56), and (d) Cu@MCM-41(III) (Cu/Si:0.70).

41(I), Cu@MCM-41(II), and Cu@MCM-41(III)) and by the direct hydrothermal synthesis route (Cu-MCM-41(II) with Cu/ Si ) 0.07) showed that the charasteristic MCM-41 structure was not distorted by the incorporation of copper in these materials (Figures 4and 5). The characteristic Bragg peaks of MCM-41 structure corresponding to d(100) and the three reflections were observed in the XRD patterns of all of these materials. The characteristic peaks corresponding to d(100) were observed at 2θ values between 2.3 and 2.5. A list of 2θ values of the peaks corresponding to d(100) and the three reflections are listed in Table 2 for all of the synthesized materials. An increase in the Cu/Si atomic ratio caused a slight increase in the d(100) values of the impregnated materials (Table 1). The characteristic peaks observed at 2θ values of 38.68 and 35.52 for the impregnated materials and at 38.72 and 35.54 for the materials synthesized

by direct synthesis route indicated the formation of a CuO phase within the MCM-41 structure both for impregnated and for onepot hydrothermally synthesized materials. The characteristic lattice parameter values of these materials are evaluated from a ) (2/x3)d(100) (Sener et al., 2006), and the results are reported in Table 1. The pore wall thickness values were then estimated from the difference of the lattice parameter and the average pore diameter. The pore wall thickness values of pure MCM-41 and Cu@MCM-41(I) (containing a Cu/Si ratio of 0.09) were estimated to be about 1.6 nm (Table 1). However, for the materials containing higher amounts of copper impregnated into the MCM-41, an increase in pore wall thickness was observed. For instance, the pore wall thickness of Cu@MCM-41(II) (containing a Cu/Si atomic ratio of 0.56) is about 2.1 nm. These results indicated that, by the deposition of copper on the pore walls, pore wall thickness was increased. The Cu/Si atomic ratios of the direct synthesis materials evaluated by the EDS technique were found to be higher than the Cu/Si ratios in the synthesis solution. For instance, for CuMCM-41(I) and Cu-MCM-41(II), the synthesis solutions contained Cu/Si ratios of 0.05 and 0.08, respectively. However, EDS results gave Cu/Si ratios of 0.07 and 0.12 in these materials,

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Figure 7. XPS of (a) Cu-MCM-41(I) and (b) Cu@MCM-41(II). Table 2. 2θ and d-spacing Values between 0 and 4 Degrees for Copper-Incorporated MCM-41 Prepared by the One-Pot Route and Impregnation Procedures 2θ and d-spacing (nm) Values MCM-41

Cu@MCM-41(I) Cu/Si:0.09

Cu@MCM-41(II) Cu/Si:0.56

Cu@MCM-41(III) Cu/Si:0.70

Cu-MCM-41(I) Cu/Si: 0.07

Cu-MCM-41(II) Cu/Si: 0.12

2θ

d

2θ

d

2θ

d

2θ

d

2θ

d

2θ

d

2.51 4.22 4.85 6.25

3.52 2.09 1.82 1.41

2.34 3.96 4.50 6.08

3.77 2.22 1.96 1.45

2.36 4.04 4.62 6.12

3.74 2.18 1.91 1.44

2.32 4.00 4.72 6.10

3.80 2.20 1.87 1.45

2.42 4.10 4.74 6.22

3.65 2.15 1.86 1.42

2.52 4.24 4.72

3.50 2.08 1.87

respectively. Considering that the particle size range of the synthesized materials was between 0.2 and 0.8 µm, EDS technique gives essentially the bulk composition of the material. By this technique (EDS), chemical analysis of the material up to a depth of 1.0 µm is obtained. These results indicated that copper was successfully incorporated into the MCM-41 structure during synthesis, whereas some of the silicon was washed away. Similar results were reported for V-MCM-41 and Pd-MCM41 in the earlier studies (Gucbilmez et al., 2005; Sener et al., 2006). In those studies, EDS results were also compared with XRF, and atomic adsorption analysis of the materials and the results were found to be quite consistent. Formation of palladium nanoballs within the lattice of MCM-41 was shown by Sener et al. (2006). The temperature programmed reduction (TPR) profile of the direct synthesis material Cu-MCM-41(I) indicated three peaks in the temperature range between 188 and 547 °C (part b of Figure 6). The first two peaks have maximums at 249 and 393 °C. The third wide peak that appears between 400 and 547 °C probably corresponds to the reduction of copper, deep within the lattice of the MCM-41 structure. It was expected that the reduction of copper occurred in two steps, Cu2+ f Cu1+ f Cu0 (Tsoncheva et al., 2004). For pure CuO, this reduction process is expected to be completed below 390 °C. These results indicated that the reduction of CuO within MCM-41 was more difficult than the reduction of pure CuO. This is a desired property of a copper-based sorbent for H2S removal. In the case of the removal of H2S in the presence of hydrogen gas, the reduction of sorbent and sulfide formation reactions take place in parallel. Cu2+ was reported to be more reactive than its reduced forms in H2S removal. Reduction of the copper to Cuo is expected to cause a decrease in the H2S sorption rate. Similar results were obtained for the copper-impregnated material. However, for this case the third peak that was observed at high temperatures was quite small (part a of Figure 6). TypicalXPSanalysisresultsofcopper-impregnated(Cu@MCM41(II)) and direct synthesis (Cu-MCM-41(I)) materials are shown in Figure 7. The characteristic Si2p band corresponding to SiO2 was observed at 102.9 and 103.5 eV for Cu@MCM41(II) and Cu-MCM-41(I), respectively. For both samples, 2p3/2 corresponding to copper gave rather wide bands, giving a

maximum at about 932.7 eV. However, this band extended over 935 eV, making a shoulder. For CuO (Cu2+ oxidation state), the copper 2p3/2 XPS band is expected at 933.2 eV, whereas for Cu1+ and Cuo oxidation states of 2p3/2, XPS bands were expected at 932.4 and 932.67 eV, respectively (Wagner, 1991). These results indicated that copper on the surface of the synthesized materials was partly in +2 and partly in +1 oxidation states. However, the XRD patterns had indicated the presence of a CuO phase within these materials. The indication of some Cu1+ on the surface by the XPS analysis might also be caused by the partial reduction of the surface copper atoms during argon bombardment of the surface before the XPS analysis. Cu/Si atomic ratios evaluated from the XPS analysis are also reported in Table 1. For the copper-impregnated samples, Cu/Si ratios evaluated from the XPS (external surface composition of particles) and from EDS (bulk analysis) are quite close to each other. For instance, for Cu@MCM-41(I), XPS and EDS results for the Cu/Si ratio are 0.100 and 0.09, respectively. This result supported the conclusion that copper had entered into the pores of MCM-41 and formed CuO nanoballs and nanowires within the pores and/or a CuO layer on the pore surfaces. If most of the copper had been deposited at the external surface and at the pore mouths of the mesoporous MCM-41, XPS results would give higher Cu/Si ratios than EDS analysis. As far as the Cu-MCM-41-type materials synthesized by the one-pot direct synthesis procedure are concerned, XPS analysis gave lower a Cu/Si ratio than the EDS analysis (Table 1). This result indicated that most of the copper was deep within the lattice of the synthesized material, rather than being on the external surface. 3.2. H2S Sorption Results. Analysis of the reactor effluent stream by the FTIR spectrometer connected on-line to the reactor exit stream gave H2S breakthrough curves as well as H2O concentration profiles. No SO2 formation was observed during these H2S sorption experiments. However, the formation of some SO2 during the initial stages of sorption was reported in the literature (Kyotoni et al., 1989; Yasyerli, 2006) for some other sorbents. A typical result showing the variation of H2S and H2O concentrations at the reactor outlet stream is given in Figure 8. As shown in this figure, a sharp increase in H2O concentration was observed at the initial times of the sorption reaction. H2O concentration gives a maximum at about the breakthrough point

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Figure 8. Concentration variations of H2S and H2O as a function of time outlet using Cu@MCM-41(II) as the sorbent (reaction temperature, 500 °C; sorbent amount, 0.2 g). Figure 10. H2S breakthrough curves for Cu-MCM-41 sorbents preapared by the one-pot hydrothermal synthesis route. (a) MCM-41, (b) Cu-MCM41(I) (Cu/Si, 0.07), and (c) Cu-MCM-41(II) (Cu/Si, 0.12). (Reaction temperature, 500 °C; feed composition, 1%H2S, 10%H2 in helium).

Figure 9. XRD patterns of fresh and sulfided samples (a) Cu-MCM-41(I) (Cu/Si:0.07), fresh; (b) Cu-MCM-41(I), after sulfidation; (c) Cu@MCM41(II) (Cu/Si:0.56), fresh; and (d) Cu@MCM-41(II), after sulfidation.

of the H2S curve. Water in the exit stream approached zero at about 12 min, whereas the concentration of H2S reached about 90% of the initial H2S concentration during the same time period. After 12 min, H2S concentration slowly increased, reaching the inlet H2S concentration at about 20 min. These results indicated that a small fraction of CuO (less than 10%) was reduced to metallic copper during the initial stages, and this metallic copper reacted with H2S slowly after the fast initial sorption of H2S over oxides of copper. The following reaction sequence is also consistent with our earlier work (Yasyerli et al., 2001).

2CuO + H2 f Cu2O + H2O Cu2O + H2S f Cu2S + H2O Cu2O + H2 f 2Cuo + H2O 2Cuo + H2S f Cu2S + H2 (at later stages of sorption) To clarify the structure of the sulfided product after the H2S sorption experiments, XRD patterns of the materials after the sulfidation experiments were also determined. Results indicated that XRD peaks corresponding to CuO were completely disappeared after the sulfidation, and some new peaks were observed (Figure 9). These peaks observed in the XRD patterns of the sulfided samples are consistent with Cu1.81S structure. This result was supported by the EDS analysis of the sulfided samples, which gave a Cu/S atomic ratio of 1.83. Some reduction of the surface areas of the samples was observed after the sulfidation experiment (Table 3). For instance, the surface area of Cu@MCM-41(I) (having a Cu/Si ratio of 0.09) decreased from 1006 to 881 m2/g after sulfidation. This decrease is more for materials containing higher amounts of copper. For instance,

Figure 11. H2S breakthrough curves for Cu@MCM-41 sorbents prepared by the impregnation procedure (a) MCM-41, (b) Cu@MCM-41(I) (Cu/Si, 0.09), (c) Cu@MCM-41(II) (Cu/Si, 0.56), and (d) Cu@MCM-41(III) (Cu/ Si, 0.70). (Reaction temperature, 500 °C; feed composition, 1%H2S, 10%H2 in helium). Table 3. Surface Areas and Sulfur Retention Capacities for Copper-Incorporated MCM-41 Sorbents after Sulfidation at 500 °C

sample Cu@MCM41(I) Cu@MCM41(II) Cu@MCM41(III) Cu-MCM41(I) Cu-MCM41(II)

Cu/Si sulfur retention sulfur retention BET area atomic ratio capacities capacities m2/g (EDS) (g S/ g Sorbent) (g S/g CuO) (single point) 0.09

0.0244

0.23

881

0.56

0.0435

0.11

312

0.70

0.0787

0.15

222

0.07

0.0145

0.19

696

0.12

0.0174

0.13

667

for the material containing a Cu/Si ratio of 0.70 (Cu@MCM41(III)) about a 50% reduction was observed in the surface area. These results indicated closure of some of the smaller pores by the product layer as a result of the sulfidation experiments. H2S breakthrough curves obtained at 500 °C with the CuMCM-41- and Cu@MCM-41-type materials containing different Cu/Si ratios are shown in Figure 10 and 11, respectively. These materials showed quite high activity for H2S removal, and the breakthrough curves are quite sharp. In the case of copperimpregnated materials containing very high Cu/Si ratios, breakthrough curves became less sharp (Figure 11), indicating the increased significance of pore diffusion resistance caused by partial closure of some of the pores by the sulfided product. Sulfur retention capacities of the synthesized materials were evaluated by the numerical integration of the H2S breakthrough curves, and the results are listed in Table 3. The sulfur retention

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Ind. Eng. Chem. Res., Vol. 47, No. 4, 2008 Table 4. Deactivation Rate Parameters for Copper-Incorporated MCM-41 Sorbents at 500 °C and Comparison with the Literature

sample

Figure 12. Comparison of sulfur retention capacities of different copperbased sorbents, +sulfur retention capacies of Cu-MCM-41 and Cu@MCM41-type materials are evaluates as g S/g CuO within the material (at 500 °C), *Yasyerli et al. (2001, 2003), **Karayilan et al. (2005), and ***Caglayan et al. (2006).

Figure 13. Typical H2S breakthrough curves and deactivation model predictions for Cu-MCM-41(I) and Cu@MCM-41(II).

Cu@MCM-41(I) Cu@MCM-41(II) Cu@MCM-41(III) Cu-MCM-41(I) Cu-MCM-41(II) CuOb Cu-Mnc Cu-Mod Cu-Vd Cu-V-Mod Cu-V-Mnc Cu-Fe-Mne

Cu/Si atomic ratio (EDS)

deactivation rate constant kd (min-1)

43.0a

0.09 0.59 0.70 0.07 0.12

1.05 1.06 0.39 1.78 1.27 0.15 0.30 0.16 0.12 0.17 0.19 0.72

23.3a 10.0a 62.1a 33.9a 7.7 8.8 5.7 5.5 5.1 3.4 13.0

R2 0.9801 0.9913 0.9830 0.9862 0.9299 0.9979 0.9961 0.9923 0.9962 0.988

a

Based on the CuO content of the sorbent. b Yasyerli et al., 2001 (600 °C). c Karayilan et al., 2005 (627 °C). d Yasyerli et al., 2003 (600 °C). e Caglayan et al., 2006 (600 °C).

synthesis procedure. This conclusion was also supported by the XPS results discussed in the previous section. One of the important parameters of such metal oxide sorbents is the sorption rate constant of H2S. To evaluate the sorption rate parameters and to predict the H2S breakthrough curves, model equations derived in our earlier studies using a deactivation model were used. The analysis of the experimental breakthrough data with the breakthrough equation predicted from the concentration-dependent deactivation model gave the initial H2S sorption rate and the deactivation rate constants. According to this model, the decrease of the solid reactant concentration with the reaction extent and the effects of the textural changes of the solid reactant on the observed rate of sorption were expressed in terms of activity term a introduced into the rate expression. For this model, the rate of change of the activity of the solid reactant with respect to time and the species conservation equation for the reactant gas (H2S) were written as,

capacities of these materials based on their copper oxide contents are quite high. A comparison of sulfur retention capacities of Cu-MCM-41- and Cu@MCM-41-type sorbents synthesized in this work (as g S/g CuO in the sorbent) with the sulfur retention capacities of pure CuO and other mixed-oxide sorbents reported in the literature is given in Figure 12. If all of the CuO had been converted to Cu1.81S, the sulfur retention capacity could reach to 0.22 g S/g CuO. Experimental sulfur retention capacities of synthesized materials containing Cu/Si ratios of 0.09 and 0.07 approached this theoretical value. Although some decrease in sulfur retention capacities were observed with an increase in the Cu/Si ratio, quite high sulfur retention capacities were observed for all of the samples. However, the decrease in sulfur retention capacity with an increase in the Cu/Si ratio is more significant for the materials prepared by the direct synthesis route. For instance, for the direct synthesis material containing a Cu/Si ratio of 0.12 (Cu-MCM-41(II)) sulfur retention capacity was evaluated as 0.13 g S/g CuO. However, for the impregnated material containing a much higher amount of copper (Cu@MCM41(III) with a Cu/Si ratio of 0.70), sulfur retention capacity was obtained as 0.15 g S/g CuO. This is mostly due to the presence of some copper atoms deep in the lattice of the MCM-41 structure in the materials synthesized by the one-pot direct

initial sorption rate constant ko (cm3/min‚g sorbent) × 10-3

da ) kdaCA dt

(1)

dCA - koCAa ) 0 dW

-Q

(2)

The iterative solution of these equations gave the following approximate expression for the breakthrough curves (Yasyerli et al., 2001, 2003; Karayilan et al., 2005).

[

(

{

koW [1 - exp(- kdt)] 1 - exp CA Q ) exp CA0 1 - exp(-kdt)

})

exp(-kdt)

]

(3)

The predicted curves of the deactivation model and the experimental breakthrough data gave good agreement for copper-incorporated MCM-41 mesoporus materials (Figure 13). The evaluated values of the sorption rate constant ko and the deactivation rate constant kd are given in Table 4 (based on the mass of CuO in the sorbent) at 500 °C. The corresponding values of ko and kd reported in the literature for pure CuO prepared by the complexation technique were 7.7 × 103 cm3/min‚g and 0.17 min-1 at 600 °C, respectively (Yasyerli et al., 2001). The sorption rate constants of copper-incorporated MCM-41 sorbents (based on CuO content) synthesized here are much higher than the corresponding value reported for pure CuO. The initial

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sorption rate-constant values reported in the literature for other mixed-oxide sorbents were all lower than the ko values obtained with copper-incorporated MCM-41-like materials synthesized in this work (Table 4). The initial sorption rate constants (based on mass of CuO) of both impregnated and direct synthesis materials having low Cu/Si ratios (Cu@MCM-41(II) and Cu-MCM-41(I)) are about an order of magnitude higher than the ko value reported in the literature for pure CuO synthesized by the complexation technique. This is mostly due to the very high surface-area values of the materials synthesized in this study. With an increase in the Cu/Si ratio of the impregnated materials, a decrease in the ko value was observed (Table 4). As shown in Table 1, an increase in the Cu/Si ratio also caused a decrease in surface area. These results also justified the increased significance of diffusion resistance in materials containing high Cu/Si ratios. All of these results showed that the materials synthesized by the direct synthesis route had higher initial sorption activities and initial sorption rate constants. However, if the sulfur retention capacities were compared, impregnated materials had higher sulfur sorption capacities than the materials synthesized by direct synthesis (having smaller Cu/Si ratios) route. Because of these significantly higher values of ko, quite sharp H2S breakthrough curves were obtained with Cu-MCM-41. Such a high sorption rate constant is a prerequisite to obtain low pre-breakthrough H2S concentrations. These results are in agreement with the implications of the hydrogen TPR results that showed that the reduction of Cu2+ in CuMCM-41(I) was more difficult than the reduction of Cu2+ in Cu@MCM-41 in the presence of hydrogen gas, and, consequently, the H2S removal reactivity of Cu-MCM-41(I) was higher. 4. Concluding Remarks Copper-incorporated MCM-41-like sorbents showed very high sorption-rate values for high-temperature removal of H2S in the presence of hydrogen gas. The deactivation model gave a good prediction of the experimental H2S breakthrough data. Copper-incorporated MCM-41-type materials synthesized in this work have very high surface-area values as compared to the pure CuO synthesized by the complexation technique. This is the major reason for having high sorption rate parameters of H2S on these materials. Sulfur retention capacities of the materials prepared by the impregnation procedure are higher than the corresponding values for the materials prepared by the direct synthesis route. This is mainly due to the presence of some of the copper atoms deep in the lattice of MCM-41 structure, in the case of direct synthesis route. The results also showed that the characteristic MCM-41 structure did not change much from the incorporation of copper into these materials. However, the surface area showed a decreasing trend with an increase in the Cu/Si ratio of these materials. Nomenclature a ) activity CA0 ) inlet concentration of H2S ko) initial sorption rate constant, cm3g-1min-1 kd ) deactivation rate constant, min-1 Q ) gas low rate, cm3min-1 W ) mass of the sorbent, g Acknowledgment The author would like to acknowledge Dr. Timur Dogu of Middle East Technical University for his help and valuable comments and Dr. Suna Balci of Gazi University

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