Modeling the Adsorption ofCarbon Monoxide on Zeolites

Eric Feise

The research on this topic involves two fundamentalpieces:

1)The chemistry part: the physical realities that we aretrying to analyze. Included in this are concepts ofzeolites as filters, and the process of gas adsorption.

2)The simulation part: modeling the physical processeswith computers. This involves not only programming,but figuring out how to accurately represent thechemistry that we are modeling.

Background

The Simulation Part: The Basic ProjectUsing our knowledge of zeolites and of gas adsorptionwe model the chemistry using computer simulation.There are a number of reasons to do this. The mostimportant reason is that it is difficult to collect the datathat we want to collect in the laboratory.

Also, since zeolites are filters with veryspecific structures, doing our work in alab would require synthesis of our zeolite. We would alsohave to “clean” our zeolite filter after every data set,another difficult task.

Zeolites are porous crystal structures with channels.They are made up of silicon and oxygen but often havealuminum atoms as well.

The Chemistry Part: Zeolites

The straight and sinusoidal channels inside a zeolitetend to be at right angles with one another, but manyzeolite structures are possible.

Sinusoidal Channel Ringed by10 Al or Si atoms

Straight Channel Ringed by10 Al or Si atoms

NaYNa56[Al56Si136O384]

NaANa12[Al12Si12O48]

ITQ-3 Si96O192

ITQ-7Si96O192

Zeolites can have a variety of crystal structures givena single chemical formula:

SilicaliteSi96O192

By inserting variousamounts of aluminum,one can produce manymore zeolite crystals!

The Chemistry Part: Adsorption

Zeolites also can function as filters. Our researchfocuses on gas adsorption within zeolites. Most gasmolecules will adsorb within the large void spaces of thechannels within a zeolite.

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 fromtap water using charcoal.

We want to find zeolites that are able to adsorb a lot ofthe gas that we are trying to collect, but we want it to doso selectively. That is, we want to be able to actuallyfilter a gas mixture using our zeolite, so we need ourzeolite to adsorb the gas of interest (adsorbate) stronglyrelative to other gases.

This project uses carbon monoxide as the adsorbatemolecule. So far, all of the researchon this project has been doneusing silicalite as the zeolite filter.

d+d-

CO

Silicalite

Our Project: The PastDaniela Kohen’s work prior to the research from thesummer includes studies of the adsorption of CO2 and N2as pure gases and as mixtures in the purely siliceouszeolites Silicalite, ITQ-3, and ITQ-7.Her research demonstrates the types of results we getfrom simulation. The most relevant of these to thecurrent work with CO is the single-component isotherm,a chart that displays adsorption in (mol of a singleadsorbate molecule) per (Kg of zeolite) at a variety ofexternal pressures.Her previous work is very important to the present workwith CO since the data we want for CO is analogous tothe data collected previously for CO2 and N2.

The Research Project

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amount adsorbed (mol/kg)

CH4 SimulationCH4 Experiment

CO2 SimulationCO2 Experiment

N2 Simulation

N2 Experiment

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) 1621-1628,[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 particularlyimportant to the current research with CO is thevalidation of her simulation (above). The CO simulationdata will need to be validated in the same fashion.

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amount adsorbed (mol/kg)

pure CO2 on ITQ-3

pure N2 on ITQ-3

pure CO2 on ITQ-7

pure N2 on ITQ-7pure N2 on silicalite

pure CO2 on silicalite

Single-component isotherms for N2 and CO2

Note that the above data come from six different simulations.Each represents the adsorption of a pure gas within one ofthe zeolites.

• Before any data could be obtained for CO, the codethat was used to simulate CO2 had to be modifiedappropriately.

• Single component isotherms for CO were thenobtained, and compared to the gases studiedpreviously.

• Simulation data was then compared with experimentaldata in an attempt to validate our model for CO.

Our Project: The Past

How is CO different from CO2?

• Because CO has a triple bond, the C—O bond length isonly 1.128Å.

• CO has a permanent dipole, but it is very small.

• The predominant form of CO has a positive charge onoxygen, the more electronegative atom.

• The other resonance structures do not provide carbonwith a complete octet. One can also see this problem bylooking at the molecular orbitals.

• As a result, the partial charges we use to model COare very small (-.0975e for carbon and +.0975e foroxygen).

Predominant ResonanceStructure

Other Possible Resonance Structures

Preliminary Single-Component Isotherms(from Simulations) on Silicalite

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CO2 @ 308K

N2 @ 308KCO @ 308KA

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Comparing with ExperimentalIsotherms (Silicalite)

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CO simulation @308K

N2 simulation @308K

CO experiment(305K)

N2 experiment #1(305K)N2 experiment #2(298K)

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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.

Problems…• There is a substantial discrepancy between the two

experimental isotherms for N2.-Note that experiment #2 is the same one that wasused to validate the CO2 simulations.

• This cannot be accounted for by temperaturedifference alone.

• We have only one experimental data set for CO, and itdoes not correlate well with our simulation.

• However, the experimental data used for comparison(#1) may not be reliable. Since both experiments (#1and #2) provide N2 isotherms, and they do not agreewith one another, it is not clear that we can rely on thedata from experiment #1 at all, particularly the COdata that we want to use to validate our simulations.

d+d-

CO

Conclusions from the Past• CO seems to adsorb almost as N2 does, evident

from the single-component isotherms. This makessense because CO has almost no dipole, has thesame mass, and almost the same size as N2.

NN

1.5ÅC

O

1.128Å

•One (or both) of the experimental data sets is inaccurate.•The agreement of one of the data sets with oursimulation does not mean that it is more accurate thanthe other.•Our simulation model has still not been validated.

Our Project: Recent Conclusions• We searched for experimental results and potential

parameters for CO in the literature. However, wehave so far been unable to find additional data.

• We have been unable to compare the present resultswith additional experimental data. – Because of the discrepancy between the two experimental isotherms, we could not evaluate the accuracy ofthe CO simulation data.

• It is not clear whether or not the present codeacceptably simulates CO because we have beenunable to compare our results to additionalexperimental data.

• However, we tried to reproduce the results for CO2

and using the code that was changed toaccommodate CO. The isotherms we obtained forCO2 using the CO code produced isotherms identicalto those from earlier versions of the code. Althoughthis does not assure that we are describing the COinteractions with the zeolite accurately, it does verifythat 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 thepreviously studied N2, we have decided to change thefocus of our research for the future.

Our Project: The Future• We recently have been gathering information so that

we can simulate and study the adsorption of N2O.• We currently have experimental isotherms at a variety

of temperatures, and have recently obtained Lennard-Jones potential parameters for N and O of N2O,allowing us to begin our simulations.

• Although we must first compare of simulation data withthe experimental isotherms, we will hopefully be ableto analyze the adsorption of N2O on silicalite, usingour simulations, in the near future.

• Unlike with CO, N2O has a significant dipole and verystrong quadrupole. Therefore, we hope to see muchmore significant adsorption within our simulatedzeolites.

Acknowledgements

I would like to thank Meghan Thurlow and Greg Haman fortheir helpful discussions and insight into this project. I wouldalso like to thank Daniela Kohen for her constant assistancewith this project, and Chuck Carlin who was a great source ofinspiration throughout the summer. I thank the CarletonCollege Chemistry department for supporting this project.