Structures of dehydrated potassium zeolite L at 298 and 78K and at 78K containing sorbed perdeuteriobenzene

Abstract

The complete structures of dehydrated potassium zeolite L, K{sub 9}Al{sub 9}Si{sub 27}O{sub 72}, at 298 and 78 K and of the same zeolite at 78 K containing, on average, 1 molecule of perdeuteriobenzene/unit cell have been determined by powder neutron diffraction. Comparison with earlier literature data indicates that little change in structure accompanies dehydration, benzene sorption, or temperature change 298 K {ge} T {ge} 78 K. The use of powder neutron diffraction has enabled a direct measure of the aluminum partitioning between the two inequivalent T-sites in the structure that, consistent with the complementary measure from bond length arguments, demonstrates a distinct aluminum preference for the 12-ring, Sil (T4) sites. The nonframework K{sup +} configuration shows only subtle changes over the range of conditions studied. Perdeuteriobenzene is observed at 78 K in capping positions above the channel wall (type D) K{sup +} cations. Simple atom-atom potential modeling of the benzene site based on the structural results yields only a small activation barrier to benzene molecule reorientation.