Electron spin resonance and electron spin echo modulation spectroscopic studies on the structure and reactivity of Pd(I) species in SAPO-11 molecular sleves

Dehydration, oxidation-reduction, and adsorbate (D₂O, CD₃OH, CO) interactions of PdSAPO-11 prepared by adding Pd(NH₃)₄Cl₂ during the synthesis of SAPO-11 have been studied and compared with (S)PdH-SAPO-11 and (i)PdH-SAPO-11 where (S) and (i) refer to solid-state ion exchange and impregnation, respectively. Diffuse reflectance spectra, electron spin resonance (ESR), and electon spin echo modulation (ESEM) spectroscopies are almost the same for the three differently prepared Pd-containing samples. ESR spectra of dehydrated samples at 600°C show g_∥ ≅ 2.9 and 2.6 and g_⊥ ≅ 2.1, which are assigned to two Pd(I) species. Adsorption of water on dehydrated samples induces decomposition of water and the formation of a Pd-oxygen radical complex. ESEM shows that, after adsorption of CD₃OH, the g_∥ = 2.9 component of Pd(I) disappears and the g_∥ = 2.6 Pd(I) coordinates to one methanol with a Pd-D distance of 0.39-0.40 nm. Pd(II) in SAPO-11 is reduced to Pd(I) by CO and forms at least two different Pd(I)-carbonyl complexes. Within the range of 0.5-1.0 mol % Pd in PdSAPO-11, it is most probable that the Pd ions are not in framework positions.