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0 10 20 30 40 50

P(bar)

amount adsorbed (mol/kg)

pure CO 2 on ITQ-3pure CO 2 on ITQ-7

pure CO 2 on silicalite

SilicaliteITQ-3 ITQ-7

Amount adsorbed (mol/kg)

Pressure (bar)

Behavior within Zeolites III: Atomistic Simulations of CO2

Diffusion in Silica ZeolitesDisan Davis and Daniela Kohen

Carleton College

INTRODUCTION

SIMULATING MOTION IN SILICALITE

VALIDATION OF METHODS

SIMULATING MOTION INDIFFERENT MATERIALS

ACKNOWLEDGEMENTS

Zeolites are made of silicon, oxygen, and sometimes aluminum, assembled in a crystaline structure. This structure has pores inside which work well for trapping small molecules (such as CO2). When these molecules become trapped on the surfaces of the zeolite pores, it is called adsorption.

Dani’s group has been studying zeolites using computer simulation. We want to discover what makes some zeolites such great adsorbers of CO2 . Work started by comparing the adsorption of CO2 on three similar zeolites.

Molecular Dynamics (MD) is a computational method that simulates the motions of particles over time using Newton’s equation (F=ma). We use MD to calculate the trajectories of adsorbates through zeolite crystals in hopes of better understanding how particle motion affects adsorption. Using trajectories recorded from MD, we can calculate diffusion in a few key ways:

Self-diffusion (Ds) is the area that one particle explores in a given amount of time (t).

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Amount adsorbed (mol/kg)

P (bar)

pure CO

Greater interactions with the zeolite cause higher adsorption, and thus CO2’s slower diffusion correlates with higher adsorption. However, we can also see that the diffusion of CO2 is significant enough to suggest that the molecules are moving well within the pores, which gives us confidence in our equilibrium data.

Thank you to Dorissa Lahner, Jayme Dahlin, Meghan Thurlow, and Eric Feise for their guidance and assistance. Thank you to HHMI and Carleton College for funding and resources.

this graph shows the relative adsorbance of the three materials of interest, where ITQ-3 can hold more CO2 and silicalite can hold less CO2 than the other materials

Self-diffusion results for silicalite and ITQ-3 fit well with expectations based on adsorption trends as well as structural factors in the various materials. Higher isosteric heat (the heat of desorption) corresponds to lower self- diffusion because of the stronger adsorbate-adsorbant interactions.

more importantly than adsorption itself, the isosteric heat corresponds to the extent of self-diffusion found

in the particular system.ITQ-7 has the lowest isosteric heat,

corresponding to fewer interactions in the system and thus overall faster

diffusion

In addition to comparing the overall self-diffusion between materials, the trends in the x, y, and z directions of both materials show that diffusion is dependent upon the material structure, relating to channel sizes and types.

Further analysis of transport diffusion is not shown because there are not yet enough runs to converge to accurate results. I am also beginning to extend this research to look at the third material, ITQ-7, to further extend our understanding of the dynamics involved in CO2 adsorption in zeolites.

(above) the largest pores in silicalite are in the x and y-directions and are associated with the higher diffusion values. For ITQ-3, the

y-direction contains the only significantly large pores(below) this movement can be verified with “pictures” of

molecules within the various systems where silicalite shows much more motion in all directions, especiallly x and y, as opposed to

ITQ-3 with its 1-dimensional movement constrained to the y-axis

THE TRENDS

Results collected using our simulation are compared to other published results with similar MD methods and with experimental results.

(above) self-diffusion shows excellent agreement with Jobic et al

under very similar simulation conditions

(at left) transport diffusion shows very good agreement with all

simulation results, collected under very similar simulation conditions

From the graphs above, the general diffusion trends can be analyzed. Self-diffusion is shown to decrease with increasing pore loading. This is due to increased interactions between molecules, essentially making the environment more crowded. Transport diffusion, however, shows an increase over increased pore loading. The collective average component is roughly constant and independent of pore loading, and the thermodynamic correction factor increases with pore loading from increased overall adsorption, causing the increasing transport diffusion.

CO2 on silicalite

CO2 on ITQ-3

Image from “Atlas of Zeolite Structure Types”

Image from “Atlas of Zeolite Structure Types”

Silicalite ITQ-3

down z-axis

down x-axis down y-axis

Silicalite ITQ-3

SIMULATING MOTION IN ZEOLITES

Transport diffusion (Dt), is a property of the group of particles, and it is the proportionality constant between the flux (J) and the concentration gradient (c).

Transport diffusion can be calculated computationally using the average of many trajectories collectively times a thermodynamic correction factor based on the change in pressure (as fugacity, f ) over concentration (c).

−= ∑=∞→

2

101

61)( lim

N

lll

ts )(r(t)r

NtcD

over one trajectory

T

N

lll

tt c

frtrtN

cD

∂∂

⋅−= ∑=∞→ ln

ln)0()(161)(

2

1lim

over many trajectories

ccDJ t ∆⋅−= )(

silicalite

ITQ-3

My goal is to study the way that gas molecules move through the materials as a means of verifying that our equilibrium description is accurate and to learn more about the motion of gas molecules within the zeolite pores. After study of diffusion coefficients for the three materials, I will collect data on mixtures of CO2 and N2 within these same materials.

down y-axisdown x-axis

CO2 on ITQ-7 0

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Q st (kJ /mol)

amount ads orbed (mol/kg)

3

N2 on IT Q-7

on s ilicalite

0

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Q st (kJ /mol)

amount ads orbed (mol/kg)

C O 2 on IT Q-

N2 on IT Q-3

C O 2 on IT Q-7

N2 on IT Q-7

N2 on s ilicalite

C O2 on s ilicalite

Image from “Atlas of Zeolite Structure Types”

Image from “Atlas of Zeolite Structure Types”

Image from “Atlas of Zeolite Structure Types”

Diffusion values for silicalite (above left), ITQ-3 (above right), and ITQ-7

(right) vary depending on the direction, where the larger pores

allow for faster diffusion through the material. Different materials have different dimensions accessible to

molecules such as CO2, where silicalite and ITQ-7 are accessible in all three dimensions, but ITQ-3 only allows diffusion in the y-dimension.

The slight increase in diffusion at higher pore loadings is consistent

with one 1-dimensional flow model.

y-direction

x-direction

z-direction

y-direction

y-direction

x-direction

x-direction

z-direction

z-directionSIMULATIONS WITH N2

In order to study mixtures of CO2 and N2, I have been verifying that the code works for N2 as a single-component in comparison with published results.

(left) GCMC results for N2 at 308K (below) MD results for N2 at 200K

EXP PUBLISHEDDs 7.18 x 10 7.0 x 10 cm /secD0 9.0 x 10 9 x 10 cm /sec

Jobic et al 2004, 300K MDMakrodimitris et al Experimental 2001, 300KMakrodimitris et al 2001, 303K MD EMP2potMakrodimitris et al 2001, 303K MD EPpotMy Results, 300K

Jobic et al 2004, experimentalJobic et al 2004, simulationPapadapoulos et al 2004, experimentalPapadapoulos et al 2004, simulationMy Results, 300K

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