Structure, Dynamics and Rotational Tunneling of Methane in Metal-Organic Frameworks

 

Wei Zhou and Taner Yildirim

 

Storing methane effectively is important for fuel-cell related applications. Metal-organic framework (MOF) compounds, which consist of metal-oxide clusters connected by organic linkers, are a relatively new class of nano-porous material that show promise for methane storage. Here we present a detailed study of methane adsorption in MOF5 using elastic and inelastic neutron scattering (INS) along with first-principles calculations. We developed a fully computer-controlled Sieverts apparatus to measure the adsorption capacity and the heat of adsorption over a large temperature and pressure range. We found that the widely used van der Waals equation of state (EOS) is not adequate and the NIST EOS should be used in the data analysis. The adsorption sites were directly determined from the difference-Fourier analysis of neutron powder diffraction data. Initial adsorption occurs at the MOF5 cup site with a well defined CH4 orientation. Using INS, we studied the methane dynamics, including the CH4 phonons and librons, and the rotational tunnel splitting. The later is a very sensitive probe to measure not only the rotational potential's second derivative at the minima but also the energy barriers between the local minima. The experimental data including absorption site positions, phonon-libron energies and the rotational tunnel-spectrum is then used to test widely used phenomenological force fields and potentials obtained from first-principles. All these results hold the key to tuning MOF structures further for better methane storage applications.

 

 

 Author Information

            Name:                         † Wei Zhou

            Mentorís name:            Taner Yildirim

            Division:                       NIST Center for Neutron Research

            Laboratory:                 † Materials Science and Engineering Laboratory

            Building:††††††††††††††††††††† †235

††††††††††† Room:††††††††††††††††††††††† †††E17

            Mail Stop:                     8562

            Telephone #:                 301-975-8169

            Fax #:                            301-921-9847

            Email:                            wzhou@nist.gov

            Sigma Xi:                      full member

            Category:                   †   Materials Science