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Beyond Lattice Models of Activated Transport in Zeolites: High-Temperature Molecular Dynamics of Self-Diffusion and Cooperative Diffusion of Benzene in NaX
 

Summary: Beyond Lattice Models of Activated Transport in Zeolites: High-Temperature Molecular
Dynamics of Self-Diffusion and Cooperative Diffusion of Benzene in NaX
Harikrishnan Ramanan,, Scott M. Auerbach,*,, and Michael Tsapatsis,
Departments of Chemical Engineering and of Chemistry, UniVersity of Massachusetts,
Amherst, Massachusetts 01003, and Department of Chemical Engineering and Materials Science,
UniVersity of Minnesota, Minneapolis, Minnesota 55455
ReceiVed: June 2, 2004; In Final Form: August 11, 2004
We employ high-temperature molecular dynamics to investigate self-transport and cooperative transport of
benzene in NaX (Si:Al ) 1.2). We have refined the benzene-NaX force field for use with our previously
developed framework force field for aluminosilicates, which explicitly distinguishes between Si and Al atoms
in the frame, and also between oxygen atoms in Si-O-Si and Si-O-Al environments. Energy minimizations
and molecular dynamics simulations performed to test the new force field give excellent agreement with
experimental data on benzene heats of adsorption, benzene-Na distances, and Na distributions for benzene
in NaY (Si:Al ) 2.4) and NaX (Si:Al ) 1.2). Molecular dynamics simulations are performed over a range
of temperatures (600-1500 K) and loadings (infinite dilution to four benzenes per supercage) to evaluate
simultaneously the self-diffusivities and cooperative (alternatively Maxwell-Stefan) diffusivities. The simulated
diffusivities agree well with pulsed field-gradient NMR and quasi-elastic neutron scattering data. Despite
this agreement, we show in the following companion paper that membrane fluxes calculated with our
diffusivities overestimate experiments by 1 order of magnitude when support resistance is accounted for in
the transport model, and by about 2 orders of magnitude when support resistance is neglected. This discrepancy

  

Source: Auerbach, Scott M. - Department of Chemistry, University of Massachusetts at Amherst

 

Collections: Chemistry