Behavior of CO2 within Zeolites I: Aluminum Substituted Zeolites

Dorissa Lahner and Daniela KohenChemistry Department, Carleton College, Northfield, MN, 55057

INTRODUCTION

BACKGROUND RESEARCH

ALUMINUM SUBSTITUTED ZEOLITES CONTINUED...

Due to the connection between increasing atmospheric levels of the greenhouse gas CO2 and global warming, it is of the utmost importance that new technologies for sequestration of CO2 are developed. The separa-tion of CO2 from a mixture of gaseous species requires a filter that strongly selects CO2 relative to other non-hazardous atmospheric gases, such as N2. Zeolite crystals act as molecular sponges that soak up CO2 in a network of “pores” during a process called adsorption. Zeolites are crystals that are composed of aluminosilicates in a wide variety of crystaline structures (Figure 1). In order to quantitatively describe the adsorption process, ad-sorption isotherms are used (Figure 2).

ACKNOWLEDGEMENTSWe gratefully acknowledge assistance from Disan Davis, Jayme Dahlin, and Meghan Thurlow. In addition, funding from the Dreyfus Foundation and sup-port from the Carleton Chemistry Department community have been essen-tial to our research.

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ITQ-3 ITQ-7 SilicaliteDensity: 1.79 g/cmVoid Space: 0.08 g/cmPore Diameter: ~5 A

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Density: 1.63 g/cmVoid Space: 0.11 g/cmPore Diameter: ~4 A

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Density: 1.54 g/cmVoid Space: 0.15 g/cmPore Diameter: ~6 A

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Figure 1. Zeolite structure. (a) The structure of a generic aluminosilicate zeolite. (b) ITQ-3, ITQ-7, and Silicalite are all-silica zeolites used in the Kohen group. The straight channels of silicalite are emphasized here (blue circle). These zeolite struc-tures are just a few examples of many diverse zeolite crystal structures. Images from “Atlas of Zeolite Structure Types”.

Figure 2. Adsorption Isotherms. (a) An overlay of CO2 and N2 isotherms for ITQ-3, ITQ-7 and Silicalite show ITQ-3 is the best adsorbent (2). (b) An overlay of several adsorption isotherms for both CO2 and N2 on Silicalite show preferential adsorption of CO2 relative to N2 from an equimolar mixture of both gases.

REFERENCES(1) Beerdsen, E.; Smit, B.; Calero, S. J. Phys. Chem. B 2002, 106, 10659.(2) Goj et al. J. Phys. Chem. B 2002,106.

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pure N2

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pure CO2

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pure CO2 on ITQ-7

pure C02 on ITQ-3

pure CO2 on Silicalite

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pure N2 on:

ALUMINUM SUBSTITUTED ZEOLITES

FUTURE RESEARCH

Changing the Al/Si ratio within a zeolite crystal has been shown to alter adsorp-tion characteristics of the zeolite (1). We are investigating the impact of Al sub-stitution in the three zeolites shown in Figure 1. Computational techniques allow for observation of adsorption processes on an atomic scale, which may be highly useful to engineers seeking to synthesize zeolites for CO2 sequestration. Adsorption characteristics shown in Figure 2 were obtained using Grand Co-nonical Monte Carlo (GCMC) simulations. GCMC simulates equilibrium and makes use of random number generation and the Boltzmann distribution to test the most probable re-locations of a CO2 molecule in the zeolite framework. Re-sults show that ITQ-3 is best at adsorbing CO2. However, results from “fake” GCMC simulations, where all coulombic interactions are absent, indicate that the adsorption ability of ITQ-3 relies on coulombic interactions (Figure 3).

Pressure (bar)

pure C02 on ITQ-3pure CO2 on ITQ-7

pure CO2 on Silicalite

Amount adsorbed (mol/kg)

pure N2 on ITQ-3pure N2 on ITQ-7pure N2 on Silicalite

Figure 3. No Coulombic Interactions. An overlay of CO2 and N2 isotherms for ITQ-3, ITQ-7 and Silicalite show ITQ-3 is no longer the best adsorbent when coulombic interactions are neglected (2). This finding suggests that the coulombic environment of a zeolite plays an important role in its adsorption characteristics.

The dependence of adsorption properties on coulombic interactions is the moti-vation for our research of Al substitution in zeolites. By altering the Al/Si ratio within ITQ-3, ITQ-7, and Silicalite, we will increase the coulombic interactions in the zeolite crystal and evaluate the effects of this change on adsorption charac-teristics.

The substitution of Si with Al results in the generation of a negative charge--this charge discrepancy is resolved by the association of a cation, in this case sodium (Figure 4).

Na

Figure 4. Al Substitution in Zeolites. Substitution of Si with Al results in the association of a cation and an alteration of the electric field within the zeolite.

The current method of potential energy (PE) calculation in our code uses a grid of 3-D coordinates within the zeolite with each point in the grid corresponding to a stored PE (Figure 5). As the zeolite crys-tal changes by means of Al substitution, these stored PE values are no longer valid. Therefore, it is necessary to alter the code to bypass interpolation and cal-culate PE directly based on the location of the CO2 molecule relative to the zeo-lite.

Now that we have produced a modified code that bypasses the PE interpola-tion scheme, we will proceed with the following research goals:

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Lennard-Jones Potential

U(r)Energy

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Figure 5. PE calculation. Potential energy in the unmodified code is inter-polated based on the position of CO2 molecules relative to stored values (En-

int) corresponding to grid locations (numbered green circles). The modified code bypasses use of the grids and cal-culates potential energy directly (Edirect) using the Lennard-Jones (LJ) Potential.

Validate interpolation bypass in forces

Create modified xyz input files

Choose Al positions for Silicalite and create a separate code that will replace some Si by Al

Figure 6. Validation of Modified Code. An overlay of potential energy (PE) data from the unmodified code (purple triangle) and the modified code (blue triangle) that by-passes interpolation. Results indicate that the modified code is correctly calculating PE in the absence of the grid.

Figure 7. Preliminary Results for Force Calculation. An overlay of force data from the unmodified code (purple tri-angle) and the modified code (blue tri-angle) that bypasses interpolation. This plot indicates an encouraging relation-ship between the data that awaits fur-ther evaluation.

Position in grid

Energy (arbitrary units) Force (arbitrary units)

Position in grid

We wrote a new function in the unmodified code that calculates PE directly based on the LJ potential and the position of a CO2 molecule relative to the zeolite crystal (Figures 5 and 6). It is also necessary to modify the code to remove interpolation of forces. Forces are used in Molecular Dynamics (MD) simulations and are calculated using classical mechanics. Similar to poten-tial energy, pre-calculated force values are stored with corresponding grid locations. Preliminary results from modified code that calculates forces di-rectly based on the position of a CO2 molecule relative to the zeolite crystal are shown in Figure 7. The relationship between the interpolated and di-rectly calculated forces is encouraging; however, these data await rigorous evaluation to confirm their validity.