Propene adsorption sites in zeolite ITQ-12: A combined synchrotron X-ray and neutron diffraction study

The adsorption site of propene in the small-pore, pure silica zeolite [Si ₂₄O ₄₈]-ITW-ITQ-12 has been characterized via Rietveld refinement of the crystal structure of propene-loaded ITQ-12 on the basis of synchrotron X-ray and neutron diffraction data taken at 298 K. The structure can be described with a monoclinic unit cell having Cm symmetry and unit cell parameters a = 10.436 Å, b = 15.018 Å, c = 8.855 Å, β = 105.74°, and volume = 1335.9 Å ³. Four-fold disordered adsorption sites that are nearly equivalent relative to the cage's 2/m pseudosymmetry are located near the center of each ellipsoidally shaped [4 ⁴5 ⁴6 ⁴8 ⁴] cage. At this site, the adsorbed propene molecule lies on a plane close and approximately parallel to the equatorial plane of the cage and is aligned with its methylene group pointing toward the pore's eight-ring window. The refined propene concentration, 1.8 per unit cell content, is close to one propene molecule per [4 ⁴5 ⁴6 ⁴8 ⁴] cage and the amount observed in adsorption experiments at 298 K and 1 atm propene partial pressure.