Synthesis, electron paramagnetic resonance and electron spin echo modulation studies on synthesized NiAPSO-41 molecular sieve and comparison with ion-exchanged NiH-SAPO-41 molecular sieve

Abstract

Nickel-substituted silicoaluminophosphate type 41 (NiAPSO-41) molecular sieve has been synthesized hydrothermally using di-n-propylamine as the structure-directing agent. The reducibility of Ni ^II in NiAPSO-41 by various reduction methods was compared with that of NiH-SAPO-41, where Ni ^II was introduced into extraframework sites of SAPO-41 by partial ion exchange of H ^I by Ni ^II . Dehydration at elevated temperatures or hydrogen treatment at moderate temperatures produces Ni ^I species in NiH-SAPO-41. Two distinct Ni ^I species, assigned as isolated Ni ^I and as a Ni ^I –(H ₂ ) _n complex, are observed by electron paramagnetic resonance (EPR) in NiH-SAPO-41 after reduction in hydrogen at 573 K. By contrast, in NiAPSO-41 neither dehydration at high temperatures nor hydrogen reduction was effective in producing Ni ^I species. Isolated Ni ^I species could, however, be obtained in NiAPSO-41 by γ-irradiation of the materials at 77 K indicating the more stable nature of the Ni ^II sites in NiAPSO-41. Both NiAPSO-41 and NiH-SAPO-41 show differences in their EPR characteristics after reduction and adsorption of various adsorbates suggesting that Ni ^I in these two materials is in different sites. Electron spin echo modulation studies of ³¹ P modulation of Ni ^I in NiAPSO-41 and NiH-SAPO-41 also showed significant differences in their modulation patterns. Simulation of the spectrum observed for NiH-SAPO-41 shows two nearest-neighbour phosphorus atoms at a distance of 0.39 nm from Ni ^I suggesting that the Ni ^I ions are in the 10-membered ring channel near a 6-ring window. Simulation of the spectrum for NiAPSO-41 gives 12 nearest-neighbour phosphorus atoms at a distance of 0.54 nm and can be rationalized in terms of Ni ^I ions substituting into a framework phosphorus site.