Crystal Structure of Partially Pd~(2+)-Exchanged Zeolite X Dehydrated in Oxygen at 400 ℃ Formation of Linear Pd_2O_3 Clusters Proposed To Be HO-Pd~(IV)-O-Pd~(IV)-OH in (Pd~(2+))_(14)(HOPdOPdOH~(4+))_8(Na~+)_(32)-Si_(100)Al_(92)O_(384)

摘要

The crystal structure of partially Pd~(2+)-exchanged zeolite X, dehydrated at 400 ℃ in a flowing O_2 stream (a = 24.982(4) A), has been determined by single-crystal X-ray diffraction techniques in the cubic space group Fd3 at 21(1) ℃. The crystal was first Pd~(2+)-exchanged in a flowing stream of 0.05 M aqueous Pd(NH_3)_4Cl_2 for 3 days. After dehydration at 400 ℃ in flowing oxygen, the crystal was evacuated at 21(1) ℃ and 2 * 10~(-6) Torr for 2 h. The structure was refined to the final error indices R_1 = 0.070 and R_2 = 0.051 for the 196 reflections for which I > 2σ(I). In this structure, Pd~(2+) ions are found at four crystallographic sites: Na~+ ions fill just one, and nonframework oxygens are found at two. Eight Pd~(2+) ions eight O~(2-) 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 Pd~(2+) ions is octahedrally coordinated by framework oxygens. Sixteen Pd~(4+) ions at site I'(Pd-O = 2.103(13) a) 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-Pd~(IV)-O-Pd~(iv)-OH]~(4+) clusters with O-Pd-O-Pd-O linear. Thirty-two Na~+ ions fill site II and are recessed 1.03(1) A into the supercage from the single six-ring plane (Na-O = 2.258(11) A). About two Pd~(2+) ions at another site I' (Pd-O = 2.371(11) A) are displaced 1.11 A from six-ring plane into sodalite cages. About four Pd~(2+) ions lie at site III' in the supercage (Pd-O = 2.16(5) A).