Electron Spin Resonance and Electron Spin−Echo Modulation Studies of Synthesized NiAPSO-34 Molecular Sieve and Comparison with Ion-Exchanged NiH−SAPO-34 Molecular Sieve

Abstract

The formation of monovalent nickel in NiAPSO-34 where nickel is believed to be incorporated into the framework of SAPO-34, and its interaction with several adsorbates are compared to Ni(I) species formed in NiH−SAPO-34, where Ni(II) is incorporated by solid-state ion exchange into known extraframework sites using electron spin resonance (ESR) and electron spin−echo modulation (ESEM) spectroscopies. Dehydration at temperatures above 573 K and hydrogen treatment at 573 K as well as γ-irradiation at 77 K produce one nickel species assigned by ESR as isolated Ni(I) in the two samples. Even though the ESR parameters of isolated Ni(I) species are similar after reduction, NiAPSO-34 and NiH−SAPO-34 show noticeable differences in their ESR characteristics after adsorption of various adsorbates, suggesting that Ni(I) in these two materials is in different sites. As a supplement to this, ESEM studies of 31P and 27Al, used to ascertain the location of the incorporated paramagnetic transition metal ion, also show significant differences in the modulation patterns. Simulation of the 31P modulation observed for NiH−SAPO-34 shows two nearest-neighbor phosphorus atoms at 3.9 Å and three next-nearest-neighbor phosphorus atoms at 6.5 Å, indicating that Ni(I) is at site II‘ in the chabazite cage near a six ring window after reduction. The 31P simulation for NiAPSO-34 shows three nearest-neighbor phosphorus atoms at a distance of 4 Å and two next-nearest-neighbor phosphorus atoms at 5.3 Å. This is consistent with Ni(I) ions substituting into a framework phosphorus site. In as-synthesized NiAPSO-34, the nickel ion is possibly also coordinated to additional waters to give distorted octahedral coordination. On dehydration, tetrahedrally coordinated nickel in a framework site of SAPO-34 is formed.