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SEPARATIONS High-Capacity Sulfur Dioxide Absorbents for Diesel Emissions Control Liyu Li and David L. King* Materials Division, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99354
High-capacity sulfur dioxide absorbents based on manganese oxide octahedral molecular sieves (OMS) have been identified. These materials are based on MnO6 octahedra sharing faces and edges to form various tunnel structures (2 × 2, 2 × 3, 2 × 4, 3 × 3) differentiated by the number of octahedra on a side. The SO2 capacities of these materials, measured at 325 °C with a feed containing 250 ppmv SO2 in air, are as high as 70 wt % (w/w), remarkably higher than conventional metal oxide based SO2 absorbents. Among the OMS materials, the 2 × 2 member, cryptomelane, exhibits the highest capacity and absorption rate. Its SO2 absorption behavior has been further characterized as a function of temperature, space velocity, and feed composition. The dominant pathway for SO2 absorption is through the oxidation of SO2 to SO3 by Mn4+ followed by SO3 reaction with Mn2+ to form MnSO4. Absorption can occur in the absence of gas-phase oxygen, with a moderate loss in overall capacity. The inclusion of gases NO and CO in the feed does not reduce SO2 capacity. The absorption capacity decreases at high space velocity and low absorption temperature. A color change of cryptomelane from black to yellow-brown after SO2 absorption can be used as an indicator of absorption progress. Cryptomelane can be synthesized using MnSO4 as a reagent. Therefore, after full SO2 absorption, the product MnSO4 can be reused as raw material for a subsequent cryptomelane synthesis. Cryptomelane has a similarly high capacity toward SO3; therefore, it can be used for removal of all SOx species generated from a variety of combustion sources. Cryptomelane may find application as a replaceable absorbent for the removal of SOx from diesel truck exhaust, protecting downstream emissions control devices such as particulate filters and NOx traps. Introduction The emission of particulates and NOx from on-road diesel trucks is an important environmental problem. As a result, the EPA has mandated a 95% decrease in their production by 2010 relative to current standards.1 Major efforts are underway to reduce these emissions through the implementation of particulate filters and NOx conversion devices such as regenerable NOx traps, which store NOx as surface nitrates.2 Sulfur oxides (primarily SO2) that are present in the diesel exhaust will gradually decrease the effectiveness of NOx traps.3,4 SO2 is oxidized to SO3 over the NOx trap catalyst, and SO3 reacts to form sulfates that block NOx absorption sites. The sulfates are not removed during the rich gas regeneration period that converts absorbed nitrates to N2, leading to the need for a high-temperature desulfation step. This results in a gradual degradation of the NOx trap over the course of many cycles. One possible approach to improving NOx trap longevity is to develop a high-capacity sulfur oxide-specific trap that is located upstream and can be replaced at regular intervals during engine maintenance. Especially valuable would be a trap having high capacity toward both SO2 as well as SO3. Certain materials have been * To whom correspondence should be addressed. Tel.: (509) 375-3908. Fax: (509) 375-2186. E-mail:
[email protected].
identified as SO2 sorbers (alkalized alumina; alkaline earth oxides such as CaO and MgO; Ce/Al2O3).5,6 However, capacities of these materials are insufficient for an onboard replaceable absorbent of acceptable volume and weight, where SO2 uptake capacities as high as 40% (w/w) may be required to allow replacement at 30 000 mile service intervals. [This calculation is based on the following estimation: ∼240 g of sulfur is emitted from a typical diesel engine in 30 000 miles (fuel efficiency: 6 miles/gallon fuel, 15 ppm sulfur in fuel). A total of 1200 g of absorbent with 40 wt % SO2 capacity is needed to absorb this amount of sulfur, and this amount of absorbent can be coated on a reasonable size honeycomb monolith.] As compared to SO2, SO3 is generally more readily absorbed, typically forming stable surface or bulk sulfates. For this reason, SOx traps generally include an oxidation catalyst that converts the SO2 to SO3, facilitating absorption. There are problems with this approach, including the cost and the recovery of the oxidation catalyst (frequently a precious metal) during trap replacement, and the incomplete conversion of SO2 to SO3 at temperatures below 300 °C. Such lower temperatures may be encountered under actual engine operation. In the course of searching for high-capacity SO2 sorbers, we identified a promising class of absorbents based on manganese oxide octahedral molecular sieves (OMS). These materials comprise MnO6 octahedra that
10.1021/ie049111n CCC: $30.25 © 2005 American Chemical Society Published on Web 12/08/2004
Ind. Eng. Chem. Res., Vol. 44, No. 1, 2005 169 Table 1. Manganese Oxide OMS Materials Tested in This Work OMS structure
preparation
1 × 1 pyrolusite, MnO2 purchased from Strem, Inc. 2 × 2 cryptomelane Aa reflux of KMnO4 and MnSO4 mixture KxMn8O16 2 × 2 cryptomelane B mixing of O2 + MnSO4 + KxMn8O16 KOH and then calcination at 600 °C in air 2 × 3 romanechite calcination of birnessite at 500 °C Na2Mn5O10 2 × 4 sodium hydrothermal treatment manganese oxide, of birnessite Na2Mn6O12‚∼4H2O and NaCl at 210 °C 3 × 3 todorokite A, hydrothermal treatment of MgMgMn6O12‚∼4H2O exchanged birnessite at 150 °C 3 × 3 todorokite B, acquired from Engelhard MgMn6O12‚∼4H2O
surface area, m2/g 3.2 74 32 7.2 55 42 44
a For absorption studies, cryptomelane A was used unless otherwise specified.
Figure 1. Polyhedral representations of the crystal structures of manganese oxide octahedral molecular sieves (OMS): pyrolusite (top left), 2 × 4 sodium manganese oxide (top right), cryptomelane (middle), romanechite (bottom left), and todorokite (bottom right). Reproduced with permission from ref 7. Copyright 1999 National Academy of Sciences.
are assembled to share faces and edges, resulting in a family of porous absorbents differentiated by the number of octahedra on a side, as shown in Figure 1.7 The SO2 and SO3 absorption properties of these materials are the focus of this paper, especially the 2 × 2 structure cryptomelane, which was measured to have the highest capacity and fastest absorption kinetics toward SO2. The efficacy of these materials is a result of the high oxidation state of manganese (4+) present in the framework. According to a simplified reaction scheme, Mn4+ oxidizes SO2 to SO3, with simultaneous reduction of the manganese cation to Mn2+ (formally MnO). The SO3 produced then reacts with MnO to form MnSO4. Not unexpectedly, these materials also react directly with SO3. In this paper, we will describe the absorption behavior and properties of the OMS materials toward SOx, with a major focus on cryptomelane absorption of SO2. Experimental Section Preparation of OMS Manganese Oxide Materials. 2 × 2, Cryptomelane, OMS-2. The octahedral manganese oxide molecular sieve (tunnel structure cryptomelane) was prepared using the methods developed by DeGuzman et al.8 A typical synthesis (reflux method) was carried out as follows: 11.78 g (74 mmol) of KMnO4 in 200 mL of water was added to a solution of 23.2 g of MnSO4‚4H2O (104 mmol) in 60 mL of water and 6 mL of concentrated HNO3. The solution was refluxed at 100 °C for 24 h, and the product was washed and dried at 120 °C. The yield was 18.3 g. An alternative synthesis (oxidation method) for cryptomelane involved bubbling O2 through a solution of MnSO4 and KOH, followed by calcining the recovered solid at 600 °C.8 In a typical preparation, a solution of 15.7 g of KOH (280 mmol) in 100 mL of water was added to a solution of 14.9 g of MnSO4‚H2O (88 mmol) in 100 mL of water. Oxygen gas was bubbled (about 10 L/min) through the solution for 4 h. The product was washed with water and calcined in air for 20 h. The yield was 4.9 g.
2 × 3, 3 × 3, and 2 × 4 OMS Materials. Birnessite was synthesized and used as a precursor for the synthesis of the 2 × 3, 3 × 3, and 2 × 4 OMS materials. Birnessite (a layered manganese oxide, Na0.55Mn2O4‚ 1.5H2O) was prepared using a method described by Golden et al.9,10 A typical synthesis was carried out as follows: 250 mL of a 6.4 M NaOH solution was mixed with 200 mL of 0.5 M MnSO4 at room temperature. Oxygen was immediately bubbled through a glass frit at a rate of 4 L/min. After 4.5 h, the oxygenation was stopped, and the precipitate was filtered, washed with deionized water, and dried in air at 100 °C. About 13 g of birnessite product was obtained. Romanechite (sodium manganese oxide with 2 × 3 channels of MnO6 units) was prepared by calcination of birnessite in air for 12 h at 500 °C.11 Todorokite (magnesium manganese oxide with 3 × 3 channels of MnO6 units, OMS-1) was prepared according to literature recipes.9,12 Approximately 3 g birnessite was added to 100 mL of a 1 M MgCl2 solution, and the slurry was stirred overnight at room temperature to ion exchange Mg2+ for Na+. The product was washed with deionized water, then added to an autoclave with 25 mL of H2O and heated at 150 °C for 48 h. The product was washed and dried in air at 100 °C. About 2 g of the todorokite product was obtained. An experimental todorokite sample was also obtained from Engelhard Corp. Sodium manganese oxide with 2 × 4 channels of MnO6 units was prepared using a method developed by Xia et al.13 About 5 g of birnessite, together with 25 mL of a 2.5 M NaCl solution, was autoclaved at 210 °C for 48 h. The product was washed and dried in air at 100 °C. About 4.4 g of pure product was obtained. A summary of the manganese oxide OMS materials evaluated in this study, along with their properties, is provided in Table 1. SO2 and SO3 Uptake Measurements. The test setup employed a small fixed bed reactor (quartz tube, 3.9 mm i.d.), which was heated by a small clam-shell furnace. Reactant gases were metered using mass flow controllers. The analytical system comprised a HP6890 gas chromatograph equipped with a sulfur chemiluminescent detector (SCD). The analytical system, which is capable of detecting both SO2 and SO3, has been described previously.14 During the experimental run, the analytical system operated continuously, sampling the effluent every 2 min. The accuracy of the system at maximum sensitivity of the system to SO2 (with SO2
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Figure 2. SO2 absorption by cryptomelane KxMn8O16 at 325 °C. Table 2. SO2 Breakthrough Capacities of Some Candidate Materials at 325 °C, 8000 h-1 GHSV, 250 ppm SO2 in Air
material tested Al2O3 CaO Ca(OH)2 MgO Mn2O3 ZrO2 ZrO2-CeO2 (26, 74 wt %) ZrO2-CeO2-La2O3 (29, 62, 9 wt %)
source or method of preparation γ-Al2O3 (Catapal A) thermal decomposition of Ca(OH)2 at 650 °C, 12 h Aldrich thermal decomposition of Mg(OH)2 at 400 °C, 3 h Alfa Aesar, Inc. Daiichi Kigenso Kagaku Kogyo Co., Ltd. Daiichi Kigenso Kagaku Kogyo Co., Ltd.
packing density, g/cm3
SO2 breakthrough capacity, % (100 × g of SO2/ g of material)
150 10
0.65 0.86
1.1 3.6
16.4 143
0.94 0.71
3.2 2
1.84 95.7 53.5
1.9 0.81 1.06
0.2 1.6 2.0
69.1
1.01
3.2
surface area, m2/g
Daiichi Kigenso Kagaku Kogyo Co., Ltd.
feed levels 10 ppm) is approximately (100 ppb, and that to SO3 (with 250 ppm SO3 feed) is approximately (3 ppm. Typical measurements employed a 0.5 g sample, sieved to 40-80 mesh, which gave a less than 5 psig pressure drop under tested conditions. We note that under on-road conditions on a vehicle, this material would be provided in monolith form rather than powders or pellets, providing a low pressure drop. In addition to the manganese oxide materials listed in Table 1, the performance of several conventional metal oxide SOx absorbents was also tested. In this paper, we employ the term absorbents rather than adsorbents in those cases when uptake of SO2 into the bulk of the material appears to be occurring, or when new compounds (sulfates) are formed as a result of reaction at the surface. Each sample was pretreated in flowing air (100 sccm) at 500 °C for 2 h prior to measuring SO2 uptake. For SO3 uptake measurements, a catalyst comprising 5 wt % Pt supported on SiO2 was used upstream of the absorbents to convert SO2 to SO3. This oxidation catalyst was prepared by impregnation of silica (Cabosil) using an aqueous solution of H2PtCl6, followed by drying and calcination at 500 °C in air. To characterize the structural change before and after SO2 absorption,
powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and BET surface area data were collected on some of the tested materials. Because CO, hydrocarbons, and NO are also present in the diesel exhaust, separate tests were performed to study the effect of these components on the behavior of the SOx absorbent. Simulated lean exhaust containing 500 ppm CO, 380 ppm C3H6, and 500 ppm NO was used, and the tests were carried out at 30 000 h-1 GHSV between 25 and 400 °C. The concentrations of CO, C3H6, and CO2 were measured using an Agilent Quad Series Micro GC, and the NO, NO2, and total NOx were measured using a 600-HCLD Digital NOx meter (California Analytical Instruments, Inc.). Results Table 2 provides SO2 breakthrough capacities of several candidate metal oxide materials, measured at 325 °C with 250 ppm SO2 in air as the feed gas at a space velocity of 8000 h-1 (GHSV). The breakthrough capacity was defined as the point where the SO2 out exceeded 1% of the SO2 in, or 2.5 ppm. Under the tested conditions, the SO2 capacities of these materials are all less than 5 wt %.
Ind. Eng. Chem. Res., Vol. 44, No. 1, 2005 171 Table 3. SO2 Absorption Performance of Manganese Oxides OMS Materials at 325 °C
material 1 × 1 pyrolusite 2 × 2 cryptomelane A 2 × 2 cryptomelane B 2 × 3 romanechite 2 × 3 romanechite 2 × 3 romanechite 2 × 4 sodium manganese oxide 3 × 3 A torodokite 3 × 3 B torodokite EMD MnO2b
SO2 breakSO2 total packing through capacity capacity (100 density, (100 × g of SO2/ × g of SO2/g g of absorbent) of absorbent) GHSVa g/cm3 1.34 0.66
