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

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 gamma-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 P-31 and Al-27, used to ascertain the location of the incorporated paramagnetic transition metal ion, also show significant differences in the modulation patterns. Simulation of the P-31 modulation observed for NiH-SAPO-34 shows two nearest-neighbor phosphorus atoms at 3.9 Angstrom and three next-nearest-neighbor phosphorus atoms at 6.5 Angstrom, indicating that Ni(I) is at site II' in the chabazite cage near a six ring window after reduction. The P-31 simulation for NiAPSO-34 shows three nearest-neighbor phosphorus atoms at a distance of 4 Angstrom and two next-nearest-neighbor phosphorus atoms at 5.3 Angstrom. 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.