Environ. Sci. Technol. 2005, 39, 9715-9720
SOx Removal by Calcined MgAlFe Hydrotalcite-like Materials: Effect of the Chemical Composition and the Cerium Incorporation Method MANUEL CANTU Ä , ESTEBAN LO Ä PEZ-SALINAS, AND JAIME S. VALENTE* Instituto Mexicano del Petro´leo, Eje Central La´zaro Ca´rdenas #152, Col. Sn. Bartolo Atepehuacan, C.P. 07730, G. A. Madero, Me´xico, D.F. RAMO Ä N MONTIEL Gerencia de Produccio´n PEMEX-Petroquı´mica, I y D Tecnolo´gico, Jacarandas 100 Frac. Rancho Alegre C.P. 52500, Coatz. Ver., Me´xico
Sulfur oxides are one of the most hazardous atmospheric pollutants since they contribute directly to acid rain formation. Consequently, stringent environmental regulations limit atmospheric SOx emissions, motivating research on efficient ways to reduce them. To supply an alternative to reduce these emissions in fluid catalytic cracking units, this study discloses efficient SOx reducing materials based on calcined MgAlFe hydrotalcite-like compounds (HT’s). Thus, HT materials were synthesized by several methods including cerium addition. The adsorption of SO2 was carried out by contacting the calcined solid with a mixture of SO2 (1%) in air at 650 °C. It was demonstrated that the isomorphic incorporation of iron increased its reduction capability which was reflected in higher reduction rates and metal sulfate reduction grade at 550 °C. Moreover, when cerium was present in the iron-containing materials the saturation rate was improved, because cerium oxide promotes the oxidation of SO2 to SO3. The way cerium is incorporated influences the SO2 adsorption capacity.
Introduction In the last few years great attention has been focused to control pollutant emissions. Among the major contributors to these emissions are energy power plants, which contribute with ∼65% of sulfur oxides, and petroleum refinery processes, more specifically the fluid catalytic cracking (FCC) process, with ∼7% (1). Sulfur oxides, a mixture of SO2 + SO3, commonly referred as SOx, are one of the most dangerous atmospheric pollutants since they contribute directly to acid rain formation and the destruction of the ozone layer. Thus, stringent environmental regulations limiting atmospheric SOx emissions encourage the research of more efficient ways to reduce them. Hence, due to the stronger acid character of SOx, basic oxides have been proposed to trap these molecules (2). SOx removal is closely related to the emission source, i.e., in power plants fuels and/or coal are burned to produce energy, where * Corresponding author phone: +52 (55) 9175-8444; e-mail:
[email protected]. 10.1021/es051305m CCC: $30.25 Published on Web 11/10/2005
2005 American Chemical Society
sulfur compounds are oxidized producing SOx emissions which are released to the atmosphere. Therefore, many strategies have been proposed, i.e., by changing the operating conditions and using fuels with lower sulfur contents or natural gas; however, the most popular and inexpensive method for SOx removal is the addition of selective sorbents with the fuel (3-6). In the FCC process (7, 8) the SOx production and removal mechanisms are different from those of energy power plants; for this purpose the characteristics of the sorbent should be different. After cracking reactions, the catalyst is deactivated and the coke deposited on it needs to be burned off to regenerate the catalyst activity; thus, sulfur compounds present in coke are oxidized to produce SOx emissions in the regeneration zone. Then in a first step, the sorbent, currently called an additive since it is added in small amounts to the total catalyst inventory, must have the property to oxidize SO2 to SO3 and produce a metal sulfate. In a second step, the additive and the cracking catalyst travel to the riser. In the riser the metal sulfate is reduced by hydrogen and other reducing gases to regenerate the metal oxide and/or produce a metal sulfide. Finally, the metal sulfide can be hydrolyzed by steam in the stripper to form the original metal oxide. Therefore, potential SOx additives must show the following characteristics: (i) be able to oxidize SO2 to SO3, (ii) be able to form a stable metal sulfate at regenerator conditions, (iii) be capable of releasing easily the sulfate species to regenerate the activity of the sorbent, and (iv) not modify the conversion and selectivity of FCC products. Worldwide research groups have studied several materials for this end, i.e., MgO, Al2O3, and MgAl spinels were evaluated as possible additives. However, their performance was limited, since MgO forms very stable MgSO4 compounds, restricting the additive regeneration; besides it has a low density and attrition index. Al2O3 showed a low SOx removal capacity because the Al2(SO4)3 formed is very unstable at the regenerator temperature so it releases the sulfate species as produced in the regenerator. Eventually, MgAl2O4 spinels were also used, but they had low SOx removal capacity and sulfate reduction which causes the solid’s deactivation (913). Cerium oxide has been reported as an oxidation catalyst, more specifically as a promoter for the oxidation of SO2 in the De-SOx process from FCC units (14-18). Indeed, cerium oxide has proved to be an excellent oxidation catalyst, since CeO2 promotes SO2 to SO3 oxidation and its basic character allows the adsorption of SOx species (19-21). However, cerium oxide is not used as a single phase mainly for its high cost. For this reason, instead of cerium the use of iron spinels was proposed; the iron has the ability to play a dual role, as an oxidizing and a reducing catalyst. Notwithstanding, studies showed that iron could be an undesirable constituent of a SOx removal additive, in particular for coke formation, due to the important role of coke in the overall thermal balance of the FCC process. Nonetheless, it has been mentioned that this effect can be minimized or even eliminated by controlling the iron content (22). In recent years, basic mixed oxides obtained from hydrotalcite-like (HT) compounds have shown good SOx and NOx activities to reduce these emissions from several sources, including FCC (3, 4, 23-25). An HT structure is created by replacing some of the M2+ cations by M3+ trivalent cations turning the layered array positively charged. These positively charged layers are electrically compensated for by anions which are located in the interlayer region; hence, a wide variety of synthetic HT materials can be prepared. Furthermore, it was previously VOL. 39, NO. 24, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. XRD patterns of the MgAl, MgFe, MgAlFe, and the Ce-containing hydrotalcite-like compounds. reported that physicochemical properties of HT compounds and the solid solutions produced after their calcination can be easily tailored by changing the nature and amount of metal cations and anions (26, 27). Therefore, to elucidate the role of the chemical composition and the cerium incorporation method on the catalytic SO2 removal performance, a series of HT materials containing iron and cerium were prepared.
Experimental Section Sample Synthesis. MgAl and MgFe HT materials were prepared by coprecipitation following a procedure described elsewhere (26-29). For detailed procedure see the Supporting Information. Characterization of Solids. The solids were characterized by inductively coupled plasma atomic emission spectrometry (ICP-AES), X-ray diffraction (XRD), thermal analyses, N2 adsorption-desorption, scanning electron microscopy, (SEM) and energy-dispersive X-ray emission (EDX) techniques. For a full description see the Supporting Information. SO2 Adsorption-Reduction Test. An amount of 30-50 g of hydrotalcite-like compound was annealed at 700 °C in air for 4 h. Then, the solid was placed in the platinum holder of the Perkin-Elmer TGA-7 equipment and reactivated at 650 °C. SO2 adsorption was carried out by contacting the solid with a mixture of SO2 (1%)/air (20 cm3/min). The saturation time and adsorption capacity depends on the inherent properties of each sample. After reaching saturation, the system is flushed with a N2 flow (20 cm3/min) and then reduced with a H2 flow (20 cm3/min) at 550 and/or 650 °C, depending on the sample’s requirements. To disclose the solid reusability a new adsorption-reduction cycle was done. The performance of our materials was compared with that of a commercial SOx reducing additive (COM). The additive is made up mainly of 61% MgO, 18% Al2O3, 16% CeO2, and 4.5% V2O5, showing XRD patterns corresponding to hydrotalcite and CeO2.
Results and Discussion Chemical Compositions. Even if from the synthesis step a nominal molar ratio M2+/M3+ ) 3 was fixed, the real molar ratios varied from 2.21 for MgFe to 3.25 for MgAlFe, which can be attributed to an incomplete incorporation of the cations inside the layers. Although cerium was not incorporated inside the layers (as confirmed by XRD), it was included in the chemical formulas considering that cerium is also part of the material (see the Supporting Information). 9716
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TABLE 1. Average Crystal Size and Cell Parameters for Different HT Compounds sample
L003 (Å)
L110 (Å)
a (Å)
L111 (Å) fresha
L111 (Å) calcineda
MgAl MgFe MgAlFe MgAlFe-Ce-I MgAlFe-Ce-II MgAlFe-Ce-III
164 154 191 122 114 176
266 339 237 240 321 301
3.054 3.111 3.067 3.089 3.085 3.092
137 118 104
152 134 130
a
Crystal size of cerium oxide.
XRD Powder Diffraction. Figure 1 shows the XRD powder patterns of the MgAl, MgFe, and MgAlFe dried at 100 °C; all show the characteristic reflections corresponding to hydrotalcite-like structure. There were no different crystalline phases detected, which is in good agreement with that reported in the literature (30). Unit cell parameters were calculated assuming a 3R stacking sequence; therefore, a ) 2d110 and c ) 3d003, where c is the interlayer distance regulated by the size and charge of the anion placed between the brucite-like layers and the cell parameter a is the average metal-metal distance inside the brucite-like layers (31). The results are reported in Table 1. A way to check the isomorphic substitution of Mg2+ by Al3+ and/or Fe3+ is by analyzing the variation of cell parameter a. In Table 1, the a value varied from 3.054 for MgAl to 3.111 for MgFe; this variation can be explained by cation size differences, since the ionic radius of Fe3+ is bigger than that of Al3+, 0.690 and 0.675 Å, respectively. All samples afford crystals that grew preferentially in the (110) direction rather than (003), see Table 1. The XRD powder patterns of the Ce-containing solids are shown in Figure 1. The diffraction pattern of CeO2 was identified along with that of the hydrotalcite phase. Important differences are evident between them, depending on the way cerium was incorporated, i.e., MgAlFe-Ce-III has the biggest crystals related to the HT structure and the smallest ones related to cerium oxide (see Figure 1 and Table 1), probably due to the fact that cerium was incorporated in one-pot synthesis so any possible redisolution does not took place, while in the case of MgAlFe-Ce-I and MgAlFe-Ce-II the HT structure was previously collapsed prior to cerium incorporation. XRD powder patterns of the calcined Ce-containing materials, where CeO2, MgO, and traces of maghemite phases
FIGURE 2. XRD patterns of the Ce-containing samples calcined at 700 °C for 4 h.
TABLE 2. Textural Properties of Calcined Solids at 700 °C/4 h sample MgAl MgFe MgAlFe MgAlFe-Ce-I MgAlFe-Ce-II MgAlFe-Ce-III COM
BET total pore (m2/g) volume (cc/g) 204 74 137 149 144 109 92
0.85 0.70 0.90 0.56 1.09 0.74 0.27
average pore diameter (Å) I II 117 30 33 55 31 121
435 380 353 456 592 575
are present, are shown in Figure 2. As reported earlier, when a hydrotalcite MgAl is calcined between 400 and 800 °C only the MgO phase is detected (32). It is worth remarking that after calcination the crystal sizes of CeO2 were uniform for MgAlFe-Ce-II and MgAlFe-Ce-III, being 134 and 130 Å, respectively, while in the case of MgAlFe-Ce-I the size increased to 152 Å (see Table 1). This result indicates that cerium oxide is better dispersed in MgAlFe-Ce-II and MgAlFe-Ce-III than in MgAlFe-Ce-I. Thermal Analysis. Decomposition of hydrotalcite-like compounds occurs at variable temperatures according to their chemical composition (26). DTA profiles for MgAl, MgFe, MgAlFe, and MgAlFe-Ce-II with different endothermic peaks can be appreciated (see the Supporting Information). The total weight loss oscillates from 36% to 46 wt %. In fact, MgAl presented the most significant weight loss, 46%, and at least five steps can be noticed in its thermogram. The peaks at lower temperature,
