Ind. Eng. Chem. Res. 1997, 36, 4595-4599
4595
Performance of Zeolite-Supported Catalysts for Selective Catalytic Reduction of Nitric Oxide and Oxidation of Methane Kylie A. Headon* and Dong-ke Zhang† Department of Chemical Engineering, The University of Adelaide, North Terrace, Adelaide, South Australia 5005, Australia
The selective catalytic reduction of nitric oxide by methane over copper ion-exchanged natural zeolites was investigated in a packed-bed tubular reactor. The catalytic activity of Cu-N and Cu-H-N catalysts was confirmed as NO, CH4, and O2 displayed very little reaction in the absence of any catalyst and zeolite without ion exchange was totally inactive. A maximum NO conversion of 33% at 650 °C for Cu-N-66 was achieved with 2% NO and 1% CH4 and a contact time of 0.9 g s cm-3, but the introduction of 2% O2 reduced the NO conversion to only 12%. Ion exchange for the H-form prior to copper ion exchange was essential for oxygen to promote catalytic activity over the temperature range 250-650 °C, with a maximum conversion of 30% at 450 °C with O2 present. The direct reduction of NO by methane was ruled out as a possible reaction pathway. Introduction The ever-increasing pollution problem posed by NOx emissions from anthropological sources has prompted many studies in recent years into new NOx removal processes. While selective catalytic reduction (SCR) technologies utilizing ammonia work well for stationary sources of NOx, mobile sources such as automobiles require a different solution. Current automotive catalysts remove between 80 and 90% (Harrison et al., 1981) of NOx from the exhaust stream when the engine operates with a stoichiometric air to fuel (A/F ) 14.7 (Shelef, 1995)) ratio. The current push is toward utilizing newly developed, more efficient lean-burn engines which operate with a higher A/F ()21-23 (Shelef, 1995)) ratio. However, existing automotive catalysts are unable to meet NOx emission standards under the oxygen-rich conditions which lean-burn engines impose. In recent years investigation into the SCR of NO over copper-loaded ZSM-5 zeolites using hydrocarbon reductants has been conducted (Shelef, 1995; Iwamoto et al., 1991; Yogo et al., 1992; Montreuil and Shelef, 1991; Bennett et al., 1992). This method holds particular interest as the reductants will already be present in the exhaust of the combustion process. One of the general features of the SCR reactions is that oxygen is necessary for a significant conversion of NO into nitrogen to occur (Shelef, 1995; Iwamoto et al., 1991; Montreuil and Shelef, 1991; Jen and Ghandi, 1994; Iwamoto and Hamada, 1991; Hamada et al., 1991). A maximum activity is observed at 450-550 °C, depending on the reductant used (Jen and Ghandi, 1994; Amiridis et al., 1996). Conversion of the hydrocarbons is high, reaching over 90% and almost always surpassing NO conversion (Iwamoto et al., 1991; Montreuil and Shelef, 1991; Jen and Ghandi, 1994). Cu-ZSM-5 is reported to not be deactivated by the presence of water vapor and SO2, although activity is inhibited (Iwamoto et al., 1991; Montreuil and Shelef, 1991; Iwamoto and Hamada, 1991). The main cause for concern with these catalysts is the hydrothermal stability (particularly if they are to be applied to diesel-fueled passenger cars (Amiridis et al., 1996)). * Author to whom correspondence should be addressed. E-mail:
[email protected]. † E-mail:
[email protected]. S0888-5885(97)00077-8 CCC: $14.00
Little has been done on the use of natural zeolite as a less expensive alternative to the use of ZSM-5. In this work the use of a copper ion-exchanged natural zeolite containing codominant quartz and zeolite (clinoptilolite and/or heulandite) is reported for the SCR of NO using methane as the reductant. Experimental Section Catalyst Preparation. The natural zeolite utilized contained codominant quartz and zeolite (clinoptilolite and/or heulandite) in the range 30-60%. The sample had a mean particle size of 100 µm and a Si/Al molar ratio of 4.8. Copper was ion-exchanged into the zeolite using an aqueous solution of 0.01 M copper acetate. Ionexchange time and temperature were varied to give the samples different copper loadings. The copper acetate solution was replaced after 30 and 58 h and the catalyst filtered and washed three times for 2 h each in deionized water between exchanges. For catalysts which underwent hydrogen ion exchange a 0.1 M solution of ammonium nitrate and exchange procedures as described for copper ion exchange were utilized. Ammonium ion exchange was conducted three times for 2 h each time at 50 °C with the ion-exchange solution replaced and the catalyst washed between each. Calcination of the catalyst in static air at 500 °C for 4 h after ammonium ion exchange yielded the H-form of the zeolite. Copper ion exchange was then performed on the H-form of the zeolite. The samples were not subjected to any pretreatment or calcination procedures prior to examination of the catalytic activity. Characterization of the zeolite was achieved by X-ray diffraction (XRD) to determine the mineral content and structural crystallinity. The metal content of the zeolite was determined using inductively coupled plasma (ICP) and whole rock fusion techniques. Table 1 details the analysis results. Copper ion-exchange levels were calculated on the basis of the assumption that one Cu2+ ion is associated with two Al3+ ions in the zeolite structure. The catalyst samples have been identified by the ion-exchange processes which they have undergone and the copper ion-exchange level. For example, Cu-H-N-90 is a natural zeolite that has first been H-exchanged, followed by copper ion exchange which resulted in a copper ionexchange level of 90%. © 1997 American Chemical Society
4596 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 Table 1. Average Metal Content of Natural Zeolites As Determined by ICP and Whole Rock Fusion Analysis (Only Major Components Are Listed) metal compound concentration (wt %) sample
Cu
Al2O3
SiO2
CaO
MgO
Na2O
no exchange Cu-N-24 Cu-N-23 Cu-N-43 Cu-N-66 Cu-N-32 Cu-N-64 Cu-N-115 Cu-H-N-50 Cu-H-N-22
400 °C also supports this theory. However, more experimental data are needed to explore further the actual reaction mechanism. In the absence of any catalyst the gas phase reactions between NO and CH4, CH4 and O2, and NO, CH4, and O2 were examined with the same concentrations as those used for catalytic activity measurements. When all three gases were present in the inlet stream, no reaction occurred with methane and no nitrogen or N2O was formed. The oxygen concentration decreased significantly and the formation of NO2 must have occurred although this was not monitored. As no nitrogen formation occurred under any conditions, the catalytic nature of the SCR of NO was confirmed. When (1) NO and CH4 and (2) CH4 and O2 were present in the feed stream, no reaction was observed at any of the temperatures investigated (250-650 °C). D’Itri and Sachtler (1991) suggested that methane reacts preferentially with oxygen rather than NO in the absence of any catalyst, but these results cannot confirm or deny this theory as no reaction occurs in either case. The natural zeolite alone, without any copper or hydrogen ion exchange conducted on it, was tested with 0.4% NO, 0.4% CH4, and 2% O2, with a contact time of 0.2 g s cm-3. The sample displayed no activity at all under these conditions, confirming that the H ion exchange and copper ion exchange were responsible for the catalytic activity of the natural zeolite. The catalytic activity for methane oxidation was also tested over Cu-N-66 in the absence of NO. The catalyst promoted the oxidation of methane at and above 550 °C, showing the natural zeolite behaves in a way similar to that of Cu-ZSM-5 in this respect, promoting the hydrocarbon oxidation at lower temperatures (Sasaki et al., 1992). The promotion of methane oxidation at temperatures >450 °C means that the SCR of NO and oxidation of the hydrocarbon are now competing reactions. This causes the decrease in conversion of NO to nitrogen at temperatures >450 °C, as illustrated in Figure 4.
Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4599
Conclusions The natural zeolite under investigation is active for the SCR of NO using methane when ion-exchange procedures have been conducted on it. The SCR of NO is a catalytic process, as confirmed by examination of the gas phase reactions and unexchanged catalytic activity. It has been established that ion exchange for the H-form prior to copper ion exchange is essential for catalytic activity to not be poisoned by oxygen over the entire temperature range investigated. While the natural zeolites show promising activity under the conditions tested, it remains to test the activity under simulated exhaust conditions. No reaction mechanism could be proposed from the experimental data obtained; however, the direct decomposition of NO with methane acting to remove oxygen from the active copper sites could be rejected. Acknowledgment The authors acknowledge the financial support received for this research from the State Energy Research Advisory Committee (SENRAC) of South Australia, Australian Research Council (ARC), Zeolite Australia Limited, and The University of Adelaide. K. H. also gratefully thanks the Energy Research and Development Corp. for the ERDC Postgraduate Award.
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Received for review January 27, 1997 Revised manuscript received May 20, 1997 Accepted June 27, 1997X
Literature Cited Amiridis, M. D.; Zhang, T.; Farrauto, R. J. Appl. Catal. B 1996, 10, 203-227. Bennett, C. J.; Bennett, P. S.; Golunski, S. E.; Hayes, J. W.; Walker, A. P. Appl. Catal. A 1992, 86, L1-L6. Brand, H.; Curtis, L.; Iton, L. J. Phys. Chem. 1993, 97 (49), 1277312782. Burch, R.; Millington, P. J. Appl. Catal. B 1993, 2, 101-116.
IE9700775
Abstract published in Advance ACS Abstracts, October 1, 1997. X
