Modeling the Adsorption of Carbon Monoxide on Zeolites. Eric Feise

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1 Modeling the Adsorption of Carbon Monoxide on Zeolites Eric Feise

2 Background The research on this topic involves two fundamental pieces: 1)The chemistry part: the physical realities that we are trying to analyze. Included in this are concepts of zeolites as filters, and the process of gas adsorption. 2)The simulation part: modeling the physical processes with computers. This involves not only programming, but figuring out how to accurately represent the chemistry that we are modeling.

The Simulation Part: The Basic Project Using our knowledge of zeolites and of gas adsorption we model the chemistry using computer simulation. There are a number of reasons to do this.

3 The Simulation Part: The Basic Project Using our knowledge of zeolites and of gas adsorption we model the chemistry using computer simulation. There are a number of reasons to do this. The most important reason is that it is difficult to collect the data that we want to collect in the laboratory. Also, since zeolites are filters with very specific structures, doing our work in a lab would require synthesis of our zeolite. We would also have to clean our zeolite filter after every data set, another difficult task.

The Chemistry Part: Zeolites Zeolites are porous crystal structures with channels. They are made up of silicon and oxygen but often have aluminum atoms as well.

4 The Chemistry Part: Zeolites Zeolites are porous crystal structures with channels. They are made up of silicon and oxygen but often have aluminum atoms as well. Straight Channel Ringed by 10 Al or Si atoms Sinusoidal Channel Ringed by 10 Al or Si atoms The straight and sinusoidal channels inside a zeolite tend to be at right angles with one another, but many zeolite structures are possible.

5 Zeolites can have a variety of crystal structures given a single chemical formula: ITQ-3 Si 96 O 192 ITQ-7 Si 96 O 192 Silicalite Si 96 O 192 By inserting various amounts of aluminum, one can produce many more zeolite crystals! NaY Na 56 [Al 56 Si 136 O 384 ] NaA Na 12 [Al 12 Si 12 O 48 ]

6 The Chemistry Part: Adsorption Adsorption is the attachment of particles to a surface. -An example of adsorption that you might be familiar with is the Removal of dissolved gases from tap water using charcoal. Zeolites also can function as filters. Our research focuses on gas adsorption within zeolites. Most gas molecules will adsorb within the large void spaces of the channels within a zeolite.

7 We want to find zeolites that are able to adsorb a lot of the gas that we are trying to collect, but we want it to do so selectively. That is, we want to be able to actually filter a gas mixture using our zeolite, so we need our zeolite to adsorb the gas of interest (adsorbate) strongly relative to other gases. C This project uses carbon monoxide as the adsorbate molecule. So far, all of the research on this project has been done using silicalite as the zeolite filter. O d + d - Silicalite

8 The Research Project Our Project: The Past Daniela Kohen s work prior to the research from the summer includes studies of the adsorption of CO 2 and N 2 as pure gases and as mixtures in the purely siliceous zeolites Silicalite, ITQ-3, and ITQ-7. Her research demonstrates the types of results we get from simulation. The most relevant of these to the current work with CO is the single-component isotherm, a chart that displays adsorption in (mol of a single adsorbate molecule) per (Kg of zeolite) at a variety of external pressures. Her previous work is very important to the present work with CO since the data we want for CO is analogous to the data collected previously for CO 2 and N 2.

9 amount adsorbed (mol/kg) CO 2 Simulation CO 2 Experiment CH 4 Simulation CH 4 Experiment N 2 Simulation N 2 Experiment P (bar) Experimental data from M. S. Sun et al, J. Phys. Chem. B 102 (1998) 1466 [CO2]; E. Buss et al., J. Chem. Soc. Faraday Trans. 93 (1997) , [CH4] ; K. Watanabe et al., Mol. Sim. 15 (1995) 197 [N2]. CH4 simulation data from A. I. Skoulidas et al, J. Phys. Chem. B, 105 (2001) 3151 The other piece of Daniela's work that is particularly important to the current research with CO is the validation of her simulation (above). The CO simulation data will need to be validated in the same fashion.

10 Single-component isotherms for N 2 and CO 2 amount adsorbed (mol/kg) 6 5 pure CO 2 on ITQ-3 pure CO 2 on ITQ pure CO 2 on silicalite pure N 2 on ITQ-3 pure N 2 on ITQ-7 pure N 2 on silicalite P (bar) Note that the above data come from six different simulations. Each represents the adsorption of a pure gas within one of the zeolites.

11 Our Project: The Past Before any data could be obtained for CO, the code that was used to simulate CO 2 had to be modified appropriately. Single component isotherms for CO were then obtained, and compared to the gases studied previously. Simulation data was then compared with experimental data in an attempt to validate our model for CO.

How is CO different from CO 2? Predominant Resonance Structure Other Possible Resonance Structures Because CO has a triple bond, the C O bond length is only 1.128Å.

12 How is CO different from CO 2? Predominant Resonance Structure Other Possible Resonance Structures Because CO has a triple bond, the C O bond length is only 1.128Å. CO has a permanent dipole, but it is very small. The predominant form of CO has a positive charge on oxygen, the more electronegative atom. The other resonance structures do not provide carbon with a complete octet. One can also see this problem by looking at the molecular orbitals. As a result, the partial charges we use to model CO are very small (-.0975e for carbon and e for oxygen).

13 Preliminary Single-Component Isotherms (from Simulations) on Silicalite 4 Amount adsorbed (mol/kg) Pressure (bar) 308K 308K 308K

14 Comparing with Experimental Isotherms (Silicalite) 2.5 Amount adsorbed (mol/kg) Pressure (bar) CO 308K N2 308K CO experiment (305K) N2 experiment #1 (305K) N2 experiment #2 (298K) Data for Experiment #1: Golden, T. C.; S. Sircar (1994). J. Colloid Interface Sci. 1994, 162, 182. Data for Experiment #2: Watanabe, K.; Austin, N.; Stapleton, M. R. Mol. Sim. 1995, 15, 197.

15 Problems There is a substantial discrepancy between the two experimental isotherms for N 2. -Note that experiment #2 is the same one that was used to validate the CO 2 simulations. This cannot be accounted for by temperature difference alone. C O d - d + We have only one experimental data set for CO, and it does not correlate well with our simulation. However, the experimental data used for comparison (#1) may not be reliable. Since both experiments (#1 and #2) provide N 2 isotherms, and they do not agree with one another, it is not clear that we can rely on the data from experiment #1 at all, particularly the CO data that we want to use to validate our simulations.

16 Conclusions from the Past CO seems to adsorb almost as N 2 does, evident from the single-component isotherms. This makes sense because CO has almost no dipole, has the same mass, and almost the same size as N Å O C N N 1.5Å One (or both) of the experimental data sets is inaccurate. The agreement of one of the data sets with our simulation does not mean that it is more accurate than the other. Our simulation model has still not been validated.

17 Our Project: Recent Conclusions We searched for experimental results and potential parameters for CO in the literature. However, we have so far been unable to find additional data. We have been unable to compare the present results with additional experimental data. Because of the discrepancy between the two experimental isotherms, we could not evaluate the accuracy of the CO simulation data. It is not clear whether or not the present code acceptably simulates CO because we have been unable to compare our results to additional experimental data.

18 However, we tried to reproduce the results for CO 2 and using the code that was changed to accommodate CO. The isotherms we obtained for CO 2 using the CO code produced isotherms identical to those from earlier versions of the code. Although this does not assure that we are describing the CO interactions with the zeolite accurately, it does verify that the code itself is not faulty. Because it is clear that CO does not adsorb strongly (at least on silicalite) and because it is so similar to the previously studied N 2, we have decided to change the focus of our research for the future.

19 Our Project: The Future We recently have been gathering information so that we can simulate and study the adsorption of N 2 O. We currently have experimental isotherms at a variety of temperatures, and have recently obtained Lennard- Jones potential parameters for N and O of N 2 O, allowing us to begin our simulations. Although we must first compare of simulation data with the experimental isotherms, we will hopefully be able to analyze the adsorption of N 2 O on silicalite, using our simulations, in the near future. Unlike with CO, N 2 O has a significant dipole and very strong quadrupole. Therefore, we hope to see much more significant adsorption within our simulated zeolites.

20 Acknowledgements I would like to thank Meghan Thurlow and Greg Haman for their helpful discussions and insight into this project. I would also like to thank Daniela Kohen for her constant assistance with this project, and Chuck Carlin who was a great source of inspiration throughout the summer. I thank the Carleton College Chemistry department for supporting this project.

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