Crystal Structure of Partially Pd²⁺-Exchanged Zeolite X Dehydrated in Oxygen at 400 °C. Formation of Linear Pd₂O₃ Clusters Proposed To Be HO−Pd^IV−O−Pd^IV−OH in (Pd²⁺)₁₄(HOPdOPdOH⁴⁺)₈(Na⁺)₃₂−Si₁₀₀Al₉₂O₃₈₄

Abstract

The crystal structure of partially Pd2+-exchanged zeolite X, dehydrated at 400 °C in a flowing O2 stream (a = 24.982(4) Å), has been determined by single-crystal X-ray diffraction techniques in the cubic space group Fd3̄ at 21(1) °C. The crystal was first Pd2+-exchanged in a flowing stream of 0.05 M aqueous Pd(NH3)4Cl2 for 3 days. After dehydration at 400 °C in flowing oxygen, the crystal was evacuated at 21(1) °C and 2 × 10-6 Torr for 2 h. The structure was refined to the final error indices R1 = 0.070 and R2 = 0.051 for the 196 reflections for which I > 2σ(I). In this structure, Pd2+ ions are found at four crystallographic sites: Na+ ions fill just one, and nonframework oxygens are found at two. Eight Pd2+ ions and eight O2- ions fill the 16 double six-oxygen ring (D6R) centers (site I) per unit cell; this interpretation of the electron density at site I behaves well in least-squares refinement. Each of these Pd2+ ions is octahedrally coordinated by framework oxygens. Sixteen Pd4+ ions at site I‘ (Pd−O = 2.103(13) Å) lie in six-ring planes. With the eight oxide ions at site I at central positions and 16 more teriminal, they form eight linear O−Pd−O−Pd−O clusters along 3-fold axes per unit cell. Each passes through the center of a D6R and extends into its two adjacent sodalite cavities. Considering bond lengths and charge balance, it is proposed that they are [HO−PdIV−O−PdIV−OH]4+ clusters with O−Pd−O−Pd−O linear. Thirty-two Na+ ions fill site II and are recessed 1.03(1) Å into the supercage from the single six-ring plane (Na−O = 2.258(11) Å). About two Pd2+ ions at another site I‘ (Pd−O = 2.371(11) Å) are displaced 1.11 Å from six-ring plane into sodalite cages. About four Pd2+ ions lie at site III‘ in the supercage (Pd−O = 2.16(5) Å).