EPR and electron spin echo modulation spectroscopy of Cr-III and Cr-V in CrAPSO-5 molecular sieve: Evidence for framework substitution

CrAPSO-5 molecular sieve was synthesized to incorporate Cr-III into the mol ecular sieve framework using tripropylamine as a structure-directing agent. CrAPSO-5 was characterized by XRD, thermogravimetric analysis and IR spect roscopy. Treatment of as-synthesized CrAPSO-5 with a solution of CH3OH-HCl is effective in removing the tripropylamine. EPR shows Cr-III with g(1) = 5 .20, g(2) = 2.00 and g(3) = 0.98, which is assigned to Cr-III in a distorte d octahedral coordination. Spectral simulation indicates a zero-field split ting parameter of 0.50 cm(-1). Cr-III in ion-exchanged (L)Cr-SAPO-5 also sh ows octahedral coordination and is located in the main channel. After dehyd ration, Cr-III shows sharper EPR lines with g(perpendicular to) = 4.00 and g(parallel to) = 2.00 which are assigned to Cr-III in tetrahedral coordinat ion. After calcination and dehydration, the EPR of CrAPSO-5 shows Cr-V in t etrahedral coordination, characterized by g(perpendicular to) = 1.97 and g( parallel to) = 1.89, UV-VIS spectra support these coordination assignments. Electron spin echo modulation (ESEM) of P-31 indicates that Cr-V substitut es for phosphorus in a framework site in calcined CrAPSO-5 and that Cr-V is in a six-ring window in dehydrated, oxidized (L)Cr-SAPO-5. Deuterium ESEM indicates, that, in CrAPSO-5, Cr-V interacts with one molecule of ethylene via a weak sigma-bond, while in dehydrated, oxidized (L)Cr-SAPO-5, Cr-V int eracts with one molecule of ethylene via a pi-bond. These results seem cons istent with framework substitution in a phosphorus site in CrAPSO-5.