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 se veral adsorbates are compared to Ni(I) species formed in NiH-SAPO-34, where Ni(II) is incorporated by solid-state ion exchange into known extraframewo rk sites using electron spin resonance (ESR) and electron spin-echo modulat ion (ESEM) spectroscopies. Dehydration at temperatures above 573 K and hydr ogen treatment at 573 K as well as gamma-irradiation at 77 K produce one ni ckel species assigned by ESR as isolated Ni(I) in the two samples. Even tho ugh the ESR parameters of isolated Ni(I) species are similar after reductio n, NiAPSO-34 and NiH-SAPO-34 show noticeable differences in their ESR chara cteristics after adsorption of various adsorbates, suggesting that Ni(I) in these two materials is in different sites. As a supplement to this, ESEM s tudies of P-31 and Al-27, used to ascertain the location of the incorporate d paramagnetic transition metal ion, also show significant differences in t he modulation patterns. Simulation of the P-31 modulation observed for NiH- SAPO-34 shows two nearest-neighbor phosphorus atoms at 3.9 Angstrom and thr ee 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 red uction. The P-31 simulation for NiAPSO-34 shows three nearest-neighbor phos phorus atoms at a distance of 4 Angstrom and two next-nearest-neighbor phos phorus atoms at 5.3 Angstrom. This is consistent with Ni(I) ions substituti ng into a framework phosphorus site. In as-synthesized NiAPSO-34, the nicke l ion is possibly also coordinated to additional waters to give distorted o ctahedral coordination. On dehydration, tetrahedrally coordinated nickel in a framework site of SAPO-34 is formed.