REDUCIBILITY, LOCATION, AND ADSORBATE INTERACTIONS OF NI(I) IONS IN NI(II)-EXCHANGED SILICOALUMINOPHOSPHATE TYPE-41 STUDIED BY ELECTRON-SPIN-RESONANCE AND ELECTRON-SPIN ECHO MODULATION SPECTROSCOPIES

Medium-pore silicoaluminophosphate type 41 (SAPO-41) molecular sieve h as been synthesized by hydrothermal methods. The reducibility of Ni(II ) ions ion-exchanged into SAPO-41 (NiH-SAPO-41) by various reduction m ethods was studied by electron spin resonance spectroscopy. Dehydratio n of NiH-SAPO-41 at temperatures above 573 K produces an isolated Ni(I ) species with g(parallel to) = 2.498 and g(perpendicular to) = 2.115. When dehydrated and oxidized NiH-SAPO-41 is treated with hydrogen, mo re than one isolated Ni(I) species are observed depending on the reduc tion temperature. When NiH-SAPO-41 is gamma-irradiated at 77 K, Ni(I) is also formed. Adsorption of D2O into dehydrated NiH-SAPO-41 forms Ni (I)-(O-2)(n), indicating water decomposition by Ni(I). Adsorption of a mmonia on reduced NiH-SAPO-41 leads to Ni(I)-(ND3)(n), with similar ES R characteristics as that of the Ni(I)-(O-2)(n) complex produced by wa ter adsorption. The similar ESR spectra of these two species suggest t hat both species are localized at the same site in the molecular sieve structure and have similar symmetry with two or three molecular ligan ds. Methanol adsorption on NiH-SAPO-41 forms Ni(I)-(CH3OD)(I) with axi ally symmetric g values. Adsorption of ethylene on NiH-SAPO-41 reduces most Ni(I) ions formed during hydrogen reduction to Ni(0). The locati on of Ni(I) in NiH-SAPO-41 was determined by P-31 electron spin echo m odulation and is located in the main 10-ring channel near a six-ring w indow.