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Modified temperature programmed desorption evaluation of hydrocarbon trapping by CsMOR zeolite under cold start conditions
Microporous and Mesoporous Materials 125 (2009) 35–38
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Modified temperature programmed desorption evaluation of hydrocarbon trapping by CsMOR zeolite under cold start conditions Paul J. Wesson, Randall Q. Snurr * Department of Chemical and Biological Engineering, Northwestern University, Evanston, IL 60208, USA
a r t i c l e
i n f o
Article history: Received 17 December 2008 Received in revised form 12 March 2009 Accepted 18 March 2009 Available online 25 March 2009 Keywords: Temperature programmed desorption Cold start Single-file diffusion Mordenite Hydrocarbon trap
a b s t r a c t A cesium-exchanged mordenite zeolite was tested as a hydrocarbon trap for cold-start automotive applications using a modified temperature programmed desorption (TPD) procedure. In the modified TPD procedure, a step increase in feed concentration was performed simultaneously with the start of the temperature ramp to more closely mimic cold start conditions. The feed stream contained mixtures of toluene, propane, and water. The cesium-exchanged mordenite trapped propane under these dynamic conditions, demonstrated improved propane trapping when toluene was coadsorbed, and retained its trapping capacity after nearly 40 h of hydrothermal treatment. Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction For automobiles equipped with modern three-way catalytic converters, the majority of hydrocarbon emissions occur during the so-called ‘‘cold-start” period, that is, during the first few minutes after engine ignition before the catalyst reaches its light-off temperature (typically around 250–300 °C). During the ‘‘cold-start” period, unburned hydrocarbons freely pass through the catalytic converter and account for up to 80% of non-methane hydrocarbon emissions during the US FTP75 and the European MEG tests [1–3]. Tighter emissions standards require that these emissions be reduced. One possible solution to this problem is to add an inline adsorbent upstream of the catalytic converter, which, ideally, would adsorb (or ‘‘trap”) hydrocarbons at low temperature and release them to the exhaust stream after the catalytic converter has reached light-off. By using this type of hydrocarbon trap, the cold start exhaust emissions can then be converted to carbon dioxide and water. A variety of zeolites have been investigated for this application [2–16]. In general, aromatic and other heavier hydrocarbons from the exhaust remain adsorbed until after catalyst light-off, while lighter hydrocarbons desorb before catalyst lightoff. Some of the recent work in this field includes using pairs of zeolites with different hydrophilicities to adsorb both water and hydrocarbons [13] and using zeolite beta to simultaneously trap and reform hydrocarbons [14].
* Corresponding author. Tel.: +1 847 467 2977; fax: +1 847 467 1018. E-mail address: [email protected] (R.Q. Snurr). 1387-1811/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2009.03.026
Czaplewski et al. [4] showed that, in one-dimensional zeolites, larger, more-strongly adsorbing molecules can ‘‘trap” smaller, less-strongly adsorbing molecules by blocking their diffusive motion (Fig. 1), potentially allowing them to trap even the lightest hydrocarbons in automobile exhaust. The concept exploits the phenomenon of ‘‘single-file” diffusion, where, in sufficiently narrow one-dimensional channels, molecules are unable to pass one another. Due to the lack of passing events, molecular motion becomes highly correlated, and the mean-squared displacement of molecules in single-file diffusion is proportional to the square root 2 of time, hrðtÞ i / t 1=2 . This breakdown of the usual Einstein diffusion behavior, where hrðtÞ2 i / t, has led to both theoretical and experimental interest in such systems [17–26]. In particular, Prof. Jörg Kärger and his colleagues have greatly increased our understanding of single-file diffusion in zeolites [17–22]. Using temperature programmed desorption (TPD) measurements, Czaplewski et al. [4] showed that toluene can trap propane in the one-dimensional channels of a mordenite (MOR) zeolite such that the desorption temperature of propane increases by 100 °C over its desorption temperature in the single-component case. Similarly, Iliyas et al. [15] found that ethylene could be trapped by toluene in the one-dimensional pores of ZSM-12 and SAPO-5. Both groups demonstrated that the trapping only occurs in materials with one-dimensional channels but not in other materials such as zeolites ZSM-5 or Y. Standard TPD measurements are performed by first equilibrating the adsorbent with constant concentrations of adsorbing species, then heating the loaded adsorbent under a constant flow of an inert carrier gas. This provides a well-defined starting point
36
P.J. Wesson, R.Q. Snurr / Microporous and Mesoporous Materials 125 (2009) 35–38
Fig. 1. Small molecules may be trapped in one-dimensional zeolite pores by larger, ‘‘blocking” molecules. The larger molecules cannot pass one another, resulting in ‘‘single-file” diffusion.
for the analysis of TPD results. However, in automotive cold-start hydrocarbon trapping, molecules first diffuse into an empty sample at lower temperatures. Then, as the temperature of the adsorbent increases, the equilibrium adsorbed loading decreases and the driving force for diffusion changes direction, causing molecules to diffuse back out of the sample and into the gas stream. In this work, we extend the work of Czaplewski et al. [4] by introducing a modified TPD procedure in which a step increase in the feed concentration is applied simultaneously with the start of the temperature ramp. This allows investigation of competitive adsorption and desorption from an initially empty zeolite, which is closer to the conditions that would be experienced by an inline hydrocarbon trap in an automobile. We also consider the effect of water on the adsorptive capacity of the zeolite and the effect of extended hydrothermal treatment. In an exhaust stream, there is a significant amount of water, which may cause hydrothermal degradation of an adsorbent. Some zeolites are susceptible to hydrothermal degradation, but recent advances have improved the hydrothermal stability of these materials [7,27,28]. 2. Experimental Toluene (Sigma–Aldrich, St. Louis, MO) was used as received. Propane was purchased at 2.01% in helium (Airgas, Radnor, PA). MOR zeolite was purchased in the sodium form from Zeolyst (CP 500-11; Zeolyst International, Valley Forge, PA). The zeolite sample was ion exchanged to a cesium form by stirring 1 g of the as-purchased sodium mordenite in 100 mL of a 0.1 M cesium chloride (Sigma–Aldrich, St. Louis, MO) solution for 24 h. The resulting sample was rinsed with deionized water and dried at 110 °C for 8 h. This procedure was repeated three times. Elemental analysis of the zeolite sample was performed in house using energy dispersive X-ray analysis, which confirmed the Si:Al ratio at 5.4:1 and greater than 99% exchange from sodium to cesium. Particle size was 5 lm [4]. For the modified TPD measurements, a 500 mg sample of the cesium-exchanged MOR (CsMOR) was placed between acid washed glass wool in a thin wall 6.35 mm (0.25 in) outside diameter steel tube. The sample was calcined at 450 °C for 6 h under 120 cm3 min1 flow of pure helium (Airgas, Radnor, PA), then cooled under He flow to room temperature prior to testing. Concentrations in the outlet stream were determined with mass spectroscopy, using mass numbers 27 and 29 for propane, 91 and 92 for toluene, and 17 for water. A typical experiment consisted of concurrently initiating a temperature ramp and a step change in hydrocarbon and/or water concentrations. The temperature was ramped at 10 °C min1 from room temperature to 450 °C. Toluene and water were introduced by flushing He through separate parallel saturators at flow rates of 20 cm3 min1 and 10 cm3 min1, respectively. Propane, at 2.01% in He, was introduced at 30 cm3 min1 for a 0.5% final concentration. In all cases, pure He was used to adjust the total flow rate to 120 cm3 min1. Assuming that the He streams were saturated with toluene or water vapors at
40 °C, the feed concentrations were ca. 0.6% toluene and 0.6% water by mol. Hydrothermal treatments were performed by initiating a 10 °C min1 temperature ramp to 450 °C with all three components of the simulated exhaust, then holding the system for a predetermined amount of time under continuous flow of propane, toluene, and water. Then, the propane, toluene, and water concentrations were reduced to zero, and the system was flushed with 120 cm3 min1 of He for 6 h at 450 °C before allowing it to cool to room temperature and performing a modified TPD measurement. At the end of the modified TPD run, an additional period of hydrothermal treatment could be initiated, followed by another measurement. By repeating this cycle, the adsorption capabilities of a single sample could be measured after varying amounts of hydrothermal treatment. 3. Results and discussion Results of the modified TPD experiment for single-component propane are shown in Fig. 2 and Table 1. Although propane is introduced at t = 0, no propane appears in the exit stream until 550 s (in the absence of zeolite, a step change in feed concentration may be observed after 20 s). This delay is due to trapping of propane by the CsMOR zeolite. The maximum desorption occurs at 145 °C, which is significantly higher than the desorption temperature of 50 °C reported from a standard TPD experiment [4]. After the sharp peak, the exit concentration decays back to the feed concentration. The single-component results for toluene are shown in Fig. 3 and Table 1. There is again a long delay in the appearance of the hydrocarbon in the exit stream. The maximum desorption of toluene occurs at 355 °C, a significant increase from 260 °C observed in standard TPD [4]. While the profiles in Figs. 2 and 3 are qualitatively similar, we note that toluene desorbs at a much higher temperature and with a broader peak than propane. The increased desorption temperature and corresponding longer delay time are expected due to the more energetically favorable adsorption of toluene versus propane in MOR zeolites. One might expect that the increased temperature and delay time would also correspond to a larger loading; however, the CsMOR traps only 4.5 mg/g of toluene while it traps 11 mg/g of propane. For both systems, the adsorbing molecules initially diffuse into an empty zeolite, but, due to its smaller size and
Fig. 2. Propane desorption profile as a function of time. The solid black line is the propane signal at m/z = 29 divided by the pressure within the mass spectrometer and given in arbitrary units. The dashed line is the temperature of the zeolite bed.
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P.J. Wesson, R.Q. Snurr / Microporous and Mesoporous Materials 125 (2009) 35–38 Table 1 Loadings and desorption temperatures for single component studies. Adsorbate
Loading (mg/g)
Propane Toluene
11 4.5
Desorption temperature (°C) Current work
Ref. [4]
145 355
50 260
Fig. 4. Propane and toluene binary desorption profile. The propane spectrum at m/ z = 29 is in black, the toluene spectrum at m/z = 92 is in grey, and the dashed line is temperature. The initial desorption of propane starts at 115 °C, but there are shoulders in the spectrum at 240 and 350 °C, indicating significant trapping of propane by toluene.
Table 2 Loadings and desorption temperatures for multi-component studies. Fig. 3. Toluene desorption profile as a function of time. The solid grey line is the toluene signal at m/z = 92 divided by the pressure within the mass spectrometer and given in arbitrary units. The dashed line is the temperature of the zeolite bed.
Figures
Adsorbates
Loading (mg/g)
Desorption temperatures (°C)
Fig. 4
Propane Toluene Propane Toluene Propane Toluene Propane Toluene
18 3.5 22 3.1 23 2.2 9 43
140 365 150 375 150 370 165 70
Fig. 5
weaker adsorption, propane diffusion is expected to be significantly faster than toluene diffusion in CsMOR. Additionally, we speculate that propane is small enough that molecules can pass one another in the MOR channels, whereas toluene undergoes single-file diffusion. The single-file behavior for toluene will additionally hinder its ability to fill the entire pore, resulting in a lower trapping per gram of zeolite. For both molecules, as the temperature increases, the equilibrium loading decreases, causing a driving force for molecules to diffuse back out of the zeolite. Again, toluene diffusion will be much slower than propane diffusion, resulting in the broader peak. Fig. 4 and Table 2 present the results when propane and toluene are simultaneously introduced to the CsMOR bed. There are slight shifts in the maximum desorption temperatures to 140 and 365 °C, respectively. More significantly, two shoulders appear in the propane desorption profile at 240 and 350 °C, indicating that the presence of toluene traps some of the propane until much higher temperatures. In addition, less propane is desorbed in the first peak at 140 °C than in the peak for the tests without toluene (Fig. 2). The maximum desorption rate of propane in the propane/toluene mixture was 96% of that observed in the case of propane, while the amount of propane desorbed during the first 200 s after propane breakthrough decreased from 11.7 mg/g (pure propane) to 11.1 mg/g (mixture). Following the suggestion of Czaplewski et al. [4], we attribute the peak at 240 °C to the initiation of ‘‘passing” events. These passing events are activated processes, and, as the temperature increases, propane molecules are more likely to have enough kinetic energy to overcome the energetic barrier for passing toluene molecules and then diffusing out of the pores. As toluene desorbs and exits the MOR pores, the remaining trapped propane molecules can also diffuse out, resulting in the second propane shoulder occurring at 350 °C. This shoulder occurs slightly before the toluene peak, and may be due to the greater diffusivity of propane than toluene. Note that, in these non-equilibrium
Fig. 6 Ref. [4]
240
350
225
340
245
350
280
Multiple desorption temperatures specify individual shoulders in the desorption peaks.
Fig. 5. Desorption profiles in the presence of water. The propane spectrum at m/ z = 29 is in black, the toluene spectrum at m/z = 92 is in dark grey, the water spectrum at m/z = 17 is in light grey, and the dashed line is temperature.
experiments, the relative amounts of adsorbed propane and toluene are significantly different than in traditional experiments (Table 2 and Ref. [4]). Standard TPD experiments begin with an
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P.J. Wesson, R.Q. Snurr / Microporous and Mesoporous Materials 125 (2009) 35–38
low temperatures and out of the zeolite at elevated temperatures. The results show that toluene can trap propane in one-dimensional zeolite pores, causing retention of much of the propane to significantly higher temperatures than in the absence of toluene. Tests including water in the feed produced similar trapping of toluene and propane, and also demonstrated that CsMOR maintains its trapping ability after extended hydrothermal treatment. Future studies of cold-start hydrocarbon trapping using this method may focus on comparing the trapping efficiency of various forms of the MOR zeolite, or incorporating increased concentrations of water or additional gases (e.g. carbon dioxide) into the feed stream to more closely mimic automotive exhaust conditions. Acknowledgments The work was supported by the NSF through Grant CTS0302428. We thank Prof. Jörg Kärger for stimulating our thinking about single-file diffusion in zeolites and for many years of enjoyable collaborations and discussions. Fig. 6. Propane, toluene and water desorption profiles after almost 40 h of hydrothermal treatment. The propane spectrum at m/z = 29 is in black, the toluene spectrum at m/z = 92 is in dark grey, the water spectrum at m/z = 17 is in light grey, and the dashed line is temperature.
equilibrium adsorption state (where toluene is preferentially adsorbed), whereas the modified experiments preferentially trap the more rapidly diffusing propane molecules. Importantly, single-file diffusion allows a small amount of toluene to trap a significant amount of propane. To test the effect of water on the trapping capacity of CsMOR, we performed the modified TPD experiment with a simulated exhaust stream containing propane, toluene, and water. As shown in Fig. 5, the desorption profiles of propane and toluene are similar to those obtained without water (Fig. 4). There are only slight shifts in the temperatures; the propane maximum desorption occurs at 150 °C, with secondary peaks at 225 and 340 °C, whereas toluene desorbs at 375 °C. Water partially desorbs at 290 °C before attaining its steady-state concentration at 395 °C. Results after almost 40 h of hydrothermal treatment with simulated exhaust at 450 °C are shown in Fig. 6. The propane peaks shift to 150, 245, and 350 °C, the toluene peak to 370 °C, and the water peaks are present at 260 and 370 °C. The zeolite maintains its adsorption and trapping properties after the hydrothermal treatment, as evidenced by the high temperature peaks or shoulders for propane and the essentially unaffected toluene desorption profile. The increased intensity of the water and propane peaks after hydrothermal treatment suggests that the zeolite has been modified by the hydrothermal treatment, but, importantly, the zeolite continues to trap both propane and toluene. 4. Conclusions A modified TPD procedure was used to test one-dimensional CsMOR zeolites for cold-start hydrocarbon trapping. The modified procedure more closely mimics the automotive cold start cycle, as it includes diffusion of molecules both into the empty zeolite at
References [1] R.J. Farrauto, R.M. Heck, Catal. Today 51 (1999) 351. [2] D. Caputo, C. Colella, F. Iucolano, P. Corbo, F. Migliardini, R. Aiello, F. Crea, Stud. Surf. Sci. Catal. 154 (2004) 2034. [3] D.S. Lafyatis, G.P. Ansell, S.C. Bennett, J.C. Frost, P.J. Millington, R.R. Rajaram, A.P. Walker, T.H. Ballinger, Appl. Catal. B-Environ. 18 (1998) 123. [4] K.F. Czaplewski, T.L. Reitz, Y.J. Kim, R.Q. Snurr, Micropor. Mesopor. Mater. 56 (2002) 55. [5] F. Brandani, D.M. Ruthven, Ind. Eng. Chem. Res. 43 (2004) 8339. [6] P.L. Burk, J.K. Hochmuth, D.R. Anderson, S. Sung, A. Punke, U. Dahle, S.J. Tauster, C.O. Tolentino, J. Rogalo, G. Miles, M. Mignano, M. Niejako, Stud. Surf. Sci. Catal. 96 (1995) 919. [7] S.P. Elangovan, M. Ogura, M.E. Davis, T. Okubo, J. Phys. Chem. B 108 (2004) 13059. [8] J. Jirat, M. Kubicek, M. Marek, Chem. Eng. Sci. 56 (2001) 1597. [9] D.J. Kim, J.M. Kim, J.E. Yie, S.G. Seo, S.C. Kim, J. Colloid Interf. Sci. 274 (2004) 538. [10] N. Noda, A. Takahashi, Y. Shibagaki, H. Mizuno, SAE 980423 (1998) 71. [11] K. Otto, C.N. Montreuil, O. Todor, R.W. McCabe, H.S. Gandhi, Ind. Eng. Chem. Res. 30 (1991) 2333. [12] R.G. Silver, D. Dou, C.W. Kirby, R.P. Richmond, J. Balland, S. Dunne, SAE 972843 (1997) 11. [13] R.W. Dorner, M. Deifallah, C.R.A. Catlow, F. Cora, S.P. Elangovan, T. Okubo, G.J. Sankar, Phys. Chem. C 112 (2008) 4187. [14] M. Ogura, T. Okubo, S.P. Elangovan, Catal. Lett. 118 (2007) 72. [15] A. Iliyas, M.H. Zahedi-Niaki, M. Eic, S. Kaliaguine, Micropor. Mesopor. Mater. 102 (2007) 171. [16] S.P. Elangovan, M. Ogura, Y. Zhang, N. Chino, T. Okubo, Appl. Catal. B-Environ. 57 (2005) 31. [17] J. Kärger, D.M. Ruthven, Diffusion in Zeolites and Other Microporous Solids, John Wiley, New York, 1994. [18] K. Hahn, J. Kärger, V. Kukla, Phys. Rev. Lett. 76 (1996) 2762. [19] H. Jobic, K. Hahn, J. Kärger, M. Bee, A. Tuel, M. Noack, I. Girnus, G.J. Kearley, J. Phys. Chem. B 101 (1997) 5834. [20] V. Kukla, J. Kornatowski, D. Demuth, I. Gimus, H. Pfeifer, L.V.C. Rees, S. Schunk, K.K. Unger, J. Kärger, Science 272 (1996) 702. [21] K. Hahn, J. Kärger, J. Phys. A-Math. Gen. 28 (1995) 3061. [22] C. Rodenbeck, J. Kärger, K. Hahn, J. Catal. 157 (1995) 656. [23] M.S. Okino, R.Q. Snurr, H.H. Kung, J.E. Ochs, M.L. Mavrovouniotis, J. Chem. Phys. 111 (1999) 2210. [24] C. Saravanan, S.M. Auerbach, J. Chem. Phys. 107 (1997) 8120. [25] D.S. Sholl, C.K. Lee, J. Chem. Phys. 112 (2000) 817. [26] G.D. Lei, B.T. Carvill, W.M.H. Sachtler, Appl. Catal. A-Gen. 142 (1996) 347. [27] S.P. Elangovan, M. Ogura, E.C. Stefan, M. Hartmann, S. Tontisirin, M.E. Davis, T. Okubo, Micropor. Mesopor. Mater. 96 (2006) 210. [28] M. Kanezashi, J. O’Brien, Y.S. Lin, Micropor. Mesopor. Mater. 103 (2007) 302.
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Modified temperature programmed desorption evaluation of hydrocarbon trapping by CsMOR zeolite under cold start conditions Paul J. Wesson, Randall Q. Snurr * Department of Chemical and Biological Engineering, Northwestern University, Evanston, IL 60208, USA
a r t i c l e
i n f o
Article history: Received 17 December 2008 Received in revised form 12 March 2009 Accepted 18 March 2009 Available online 25 March 2009 Keywords: Temperature programmed desorption Cold start Single-file diffusion Mordenite Hydrocarbon trap
a b s t r a c t A cesium-exchanged mordenite zeolite was tested as a hydrocarbon trap for cold-start automotive applications using a modified temperature programmed desorption (TPD) procedure. In the modified TPD procedure, a step increase in feed concentration was performed simultaneously with the start of the temperature ramp to more closely mimic cold start conditions. The feed stream contained mixtures of toluene, propane, and water. The cesium-exchanged mordenite trapped propane under these dynamic conditions, demonstrated improved propane trapping when toluene was coadsorbed, and retained its trapping capacity after nearly 40 h of hydrothermal treatment. Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction For automobiles equipped with modern three-way catalytic converters, the majority of hydrocarbon emissions occur during the so-called ‘‘cold-start” period, that is, during the first few minutes after engine ignition before the catalyst reaches its light-off temperature (typically around 250–300 °C). During the ‘‘cold-start” period, unburned hydrocarbons freely pass through the catalytic converter and account for up to 80% of non-methane hydrocarbon emissions during the US FTP75 and the European MEG tests [1–3]. Tighter emissions standards require that these emissions be reduced. One possible solution to this problem is to add an inline adsorbent upstream of the catalytic converter, which, ideally, would adsorb (or ‘‘trap”) hydrocarbons at low temperature and release them to the exhaust stream after the catalytic converter has reached light-off. By using this type of hydrocarbon trap, the cold start exhaust emissions can then be converted to carbon dioxide and water. A variety of zeolites have been investigated for this application [2–16]. In general, aromatic and other heavier hydrocarbons from the exhaust remain adsorbed until after catalyst light-off, while lighter hydrocarbons desorb before catalyst lightoff. Some of the recent work in this field includes using pairs of zeolites with different hydrophilicities to adsorb both water and hydrocarbons [13] and using zeolite beta to simultaneously trap and reform hydrocarbons [14].
* Corresponding author. Tel.: +1 847 467 2977; fax: +1 847 467 1018. E-mail address: [email protected] (R.Q. Snurr). 1387-1811/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2009.03.026
Czaplewski et al. [4] showed that, in one-dimensional zeolites, larger, more-strongly adsorbing molecules can ‘‘trap” smaller, less-strongly adsorbing molecules by blocking their diffusive motion (Fig. 1), potentially allowing them to trap even the lightest hydrocarbons in automobile exhaust. The concept exploits the phenomenon of ‘‘single-file” diffusion, where, in sufficiently narrow one-dimensional channels, molecules are unable to pass one another. Due to the lack of passing events, molecular motion becomes highly correlated, and the mean-squared displacement of molecules in single-file diffusion is proportional to the square root 2 of time, hrðtÞ i / t 1=2 . This breakdown of the usual Einstein diffusion behavior, where hrðtÞ2 i / t, has led to both theoretical and experimental interest in such systems [17–26]. In particular, Prof. Jörg Kärger and his colleagues have greatly increased our understanding of single-file diffusion in zeolites [17–22]. Using temperature programmed desorption (TPD) measurements, Czaplewski et al. [4] showed that toluene can trap propane in the one-dimensional channels of a mordenite (MOR) zeolite such that the desorption temperature of propane increases by 100 °C over its desorption temperature in the single-component case. Similarly, Iliyas et al. [15] found that ethylene could be trapped by toluene in the one-dimensional pores of ZSM-12 and SAPO-5. Both groups demonstrated that the trapping only occurs in materials with one-dimensional channels but not in other materials such as zeolites ZSM-5 or Y. Standard TPD measurements are performed by first equilibrating the adsorbent with constant concentrations of adsorbing species, then heating the loaded adsorbent under a constant flow of an inert carrier gas. This provides a well-defined starting point
36
P.J. Wesson, R.Q. Snurr / Microporous and Mesoporous Materials 125 (2009) 35–38
Fig. 1. Small molecules may be trapped in one-dimensional zeolite pores by larger, ‘‘blocking” molecules. The larger molecules cannot pass one another, resulting in ‘‘single-file” diffusion.
for the analysis of TPD results. However, in automotive cold-start hydrocarbon trapping, molecules first diffuse into an empty sample at lower temperatures. Then, as the temperature of the adsorbent increases, the equilibrium adsorbed loading decreases and the driving force for diffusion changes direction, causing molecules to diffuse back out of the sample and into the gas stream. In this work, we extend the work of Czaplewski et al. [4] by introducing a modified TPD procedure in which a step increase in the feed concentration is applied simultaneously with the start of the temperature ramp. This allows investigation of competitive adsorption and desorption from an initially empty zeolite, which is closer to the conditions that would be experienced by an inline hydrocarbon trap in an automobile. We also consider the effect of water on the adsorptive capacity of the zeolite and the effect of extended hydrothermal treatment. In an exhaust stream, there is a significant amount of water, which may cause hydrothermal degradation of an adsorbent. Some zeolites are susceptible to hydrothermal degradation, but recent advances have improved the hydrothermal stability of these materials [7,27,28]. 2. Experimental Toluene (Sigma–Aldrich, St. Louis, MO) was used as received. Propane was purchased at 2.01% in helium (Airgas, Radnor, PA). MOR zeolite was purchased in the sodium form from Zeolyst (CP 500-11; Zeolyst International, Valley Forge, PA). The zeolite sample was ion exchanged to a cesium form by stirring 1 g of the as-purchased sodium mordenite in 100 mL of a 0.1 M cesium chloride (Sigma–Aldrich, St. Louis, MO) solution for 24 h. The resulting sample was rinsed with deionized water and dried at 110 °C for 8 h. This procedure was repeated three times. Elemental analysis of the zeolite sample was performed in house using energy dispersive X-ray analysis, which confirmed the Si:Al ratio at 5.4:1 and greater than 99% exchange from sodium to cesium. Particle size was 5 lm [4]. For the modified TPD measurements, a 500 mg sample of the cesium-exchanged MOR (CsMOR) was placed between acid washed glass wool in a thin wall 6.35 mm (0.25 in) outside diameter steel tube. The sample was calcined at 450 °C for 6 h under 120 cm3 min1 flow of pure helium (Airgas, Radnor, PA), then cooled under He flow to room temperature prior to testing. Concentrations in the outlet stream were determined with mass spectroscopy, using mass numbers 27 and 29 for propane, 91 and 92 for toluene, and 17 for water. A typical experiment consisted of concurrently initiating a temperature ramp and a step change in hydrocarbon and/or water concentrations. The temperature was ramped at 10 °C min1 from room temperature to 450 °C. Toluene and water were introduced by flushing He through separate parallel saturators at flow rates of 20 cm3 min1 and 10 cm3 min1, respectively. Propane, at 2.01% in He, was introduced at 30 cm3 min1 for a 0.5% final concentration. In all cases, pure He was used to adjust the total flow rate to 120 cm3 min1. Assuming that the He streams were saturated with toluene or water vapors at
40 °C, the feed concentrations were ca. 0.6% toluene and 0.6% water by mol. Hydrothermal treatments were performed by initiating a 10 °C min1 temperature ramp to 450 °C with all three components of the simulated exhaust, then holding the system for a predetermined amount of time under continuous flow of propane, toluene, and water. Then, the propane, toluene, and water concentrations were reduced to zero, and the system was flushed with 120 cm3 min1 of He for 6 h at 450 °C before allowing it to cool to room temperature and performing a modified TPD measurement. At the end of the modified TPD run, an additional period of hydrothermal treatment could be initiated, followed by another measurement. By repeating this cycle, the adsorption capabilities of a single sample could be measured after varying amounts of hydrothermal treatment. 3. Results and discussion Results of the modified TPD experiment for single-component propane are shown in Fig. 2 and Table 1. Although propane is introduced at t = 0, no propane appears in the exit stream until 550 s (in the absence of zeolite, a step change in feed concentration may be observed after 20 s). This delay is due to trapping of propane by the CsMOR zeolite. The maximum desorption occurs at 145 °C, which is significantly higher than the desorption temperature of 50 °C reported from a standard TPD experiment [4]. After the sharp peak, the exit concentration decays back to the feed concentration. The single-component results for toluene are shown in Fig. 3 and Table 1. There is again a long delay in the appearance of the hydrocarbon in the exit stream. The maximum desorption of toluene occurs at 355 °C, a significant increase from 260 °C observed in standard TPD [4]. While the profiles in Figs. 2 and 3 are qualitatively similar, we note that toluene desorbs at a much higher temperature and with a broader peak than propane. The increased desorption temperature and corresponding longer delay time are expected due to the more energetically favorable adsorption of toluene versus propane in MOR zeolites. One might expect that the increased temperature and delay time would also correspond to a larger loading; however, the CsMOR traps only 4.5 mg/g of toluene while it traps 11 mg/g of propane. For both systems, the adsorbing molecules initially diffuse into an empty zeolite, but, due to its smaller size and
Fig. 2. Propane desorption profile as a function of time. The solid black line is the propane signal at m/z = 29 divided by the pressure within the mass spectrometer and given in arbitrary units. The dashed line is the temperature of the zeolite bed.
37
P.J. Wesson, R.Q. Snurr / Microporous and Mesoporous Materials 125 (2009) 35–38 Table 1 Loadings and desorption temperatures for single component studies. Adsorbate
Loading (mg/g)
Propane Toluene
11 4.5
Desorption temperature (°C) Current work
Ref. [4]
145 355
50 260
Fig. 4. Propane and toluene binary desorption profile. The propane spectrum at m/ z = 29 is in black, the toluene spectrum at m/z = 92 is in grey, and the dashed line is temperature. The initial desorption of propane starts at 115 °C, but there are shoulders in the spectrum at 240 and 350 °C, indicating significant trapping of propane by toluene.
Table 2 Loadings and desorption temperatures for multi-component studies. Fig. 3. Toluene desorption profile as a function of time. The solid grey line is the toluene signal at m/z = 92 divided by the pressure within the mass spectrometer and given in arbitrary units. The dashed line is the temperature of the zeolite bed.
Figures
Adsorbates
Loading (mg/g)
Desorption temperatures (°C)
Fig. 4
Propane Toluene Propane Toluene Propane Toluene Propane Toluene
18 3.5 22 3.1 23 2.2 9 43
140 365 150 375 150 370 165 70
Fig. 5
weaker adsorption, propane diffusion is expected to be significantly faster than toluene diffusion in CsMOR. Additionally, we speculate that propane is small enough that molecules can pass one another in the MOR channels, whereas toluene undergoes single-file diffusion. The single-file behavior for toluene will additionally hinder its ability to fill the entire pore, resulting in a lower trapping per gram of zeolite. For both molecules, as the temperature increases, the equilibrium loading decreases, causing a driving force for molecules to diffuse back out of the zeolite. Again, toluene diffusion will be much slower than propane diffusion, resulting in the broader peak. Fig. 4 and Table 2 present the results when propane and toluene are simultaneously introduced to the CsMOR bed. There are slight shifts in the maximum desorption temperatures to 140 and 365 °C, respectively. More significantly, two shoulders appear in the propane desorption profile at 240 and 350 °C, indicating that the presence of toluene traps some of the propane until much higher temperatures. In addition, less propane is desorbed in the first peak at 140 °C than in the peak for the tests without toluene (Fig. 2). The maximum desorption rate of propane in the propane/toluene mixture was 96% of that observed in the case of propane, while the amount of propane desorbed during the first 200 s after propane breakthrough decreased from 11.7 mg/g (pure propane) to 11.1 mg/g (mixture). Following the suggestion of Czaplewski et al. [4], we attribute the peak at 240 °C to the initiation of ‘‘passing” events. These passing events are activated processes, and, as the temperature increases, propane molecules are more likely to have enough kinetic energy to overcome the energetic barrier for passing toluene molecules and then diffusing out of the pores. As toluene desorbs and exits the MOR pores, the remaining trapped propane molecules can also diffuse out, resulting in the second propane shoulder occurring at 350 °C. This shoulder occurs slightly before the toluene peak, and may be due to the greater diffusivity of propane than toluene. Note that, in these non-equilibrium
Fig. 6 Ref. [4]
240
350
225
340
245
350
280
Multiple desorption temperatures specify individual shoulders in the desorption peaks.
Fig. 5. Desorption profiles in the presence of water. The propane spectrum at m/ z = 29 is in black, the toluene spectrum at m/z = 92 is in dark grey, the water spectrum at m/z = 17 is in light grey, and the dashed line is temperature.
experiments, the relative amounts of adsorbed propane and toluene are significantly different than in traditional experiments (Table 2 and Ref. [4]). Standard TPD experiments begin with an
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low temperatures and out of the zeolite at elevated temperatures. The results show that toluene can trap propane in one-dimensional zeolite pores, causing retention of much of the propane to significantly higher temperatures than in the absence of toluene. Tests including water in the feed produced similar trapping of toluene and propane, and also demonstrated that CsMOR maintains its trapping ability after extended hydrothermal treatment. Future studies of cold-start hydrocarbon trapping using this method may focus on comparing the trapping efficiency of various forms of the MOR zeolite, or incorporating increased concentrations of water or additional gases (e.g. carbon dioxide) into the feed stream to more closely mimic automotive exhaust conditions. Acknowledgments The work was supported by the NSF through Grant CTS0302428. We thank Prof. Jörg Kärger for stimulating our thinking about single-file diffusion in zeolites and for many years of enjoyable collaborations and discussions. Fig. 6. Propane, toluene and water desorption profiles after almost 40 h of hydrothermal treatment. The propane spectrum at m/z = 29 is in black, the toluene spectrum at m/z = 92 is in dark grey, the water spectrum at m/z = 17 is in light grey, and the dashed line is temperature.
equilibrium adsorption state (where toluene is preferentially adsorbed), whereas the modified experiments preferentially trap the more rapidly diffusing propane molecules. Importantly, single-file diffusion allows a small amount of toluene to trap a significant amount of propane. To test the effect of water on the trapping capacity of CsMOR, we performed the modified TPD experiment with a simulated exhaust stream containing propane, toluene, and water. As shown in Fig. 5, the desorption profiles of propane and toluene are similar to those obtained without water (Fig. 4). There are only slight shifts in the temperatures; the propane maximum desorption occurs at 150 °C, with secondary peaks at 225 and 340 °C, whereas toluene desorbs at 375 °C. Water partially desorbs at 290 °C before attaining its steady-state concentration at 395 °C. Results after almost 40 h of hydrothermal treatment with simulated exhaust at 450 °C are shown in Fig. 6. The propane peaks shift to 150, 245, and 350 °C, the toluene peak to 370 °C, and the water peaks are present at 260 and 370 °C. The zeolite maintains its adsorption and trapping properties after the hydrothermal treatment, as evidenced by the high temperature peaks or shoulders for propane and the essentially unaffected toluene desorption profile. The increased intensity of the water and propane peaks after hydrothermal treatment suggests that the zeolite has been modified by the hydrothermal treatment, but, importantly, the zeolite continues to trap both propane and toluene. 4. Conclusions A modified TPD procedure was used to test one-dimensional CsMOR zeolites for cold-start hydrocarbon trapping. The modified procedure more closely mimics the automotive cold start cycle, as it includes diffusion of molecules both into the empty zeolite at
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