EPR and electron spin echo modulation spectroscopy of CrIII and CrV in CrAPSO-5 molecular sieve: Evidence for framework substitution

Abstract

CrAPSO-5 molecular sieve was synthesized to incorporate Cr^III into the molecular sieve framework using tripropylamine as a structure-directing agent. CrAPSO-5 was characterized by XRD, thermogravimetric analysis and IR spectroscopy. Treatment of as-synthesized CrAPSO-5 with a solution of CH₃OH–HCl is effective in removing the tripropylamine. EPR shows Cr^III with g₁=5.20, g₂=2.00 and g₃=0.98, which is assigned to Cr^III in a distorted octahedral coordination. Spectral simulation indicates a zero-field splitting parameter of 0.50 cm^-1. Cr^III in ion-exchanged (L)Cr-SAPO-5 also shows octahedral coordination and is located in the main channel. After dehydration, Cr^III shows sharper EPR lines with g_⊥=4.00 and g_‖=2.00 which are assigned to Cr^III in tetrahedral coordination. After calcination and dehydration, the EPR of CrAPSO-5 shows Cr^V in tetrahedral coordination, characterized by g_⊥=1.97 and g_‖=1.89. UV–VIS spectra support these coordination assignments. Electron spin echo modulation (ESEM) of ³¹P indicates that Cr^V substitutes 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 σ-bond, while in dehydrated, oxidized (L)Cr-SAPO-5, Cr^V interacts with one molecule of ethylene via a π-bond. These results seem consistent with framework substitution in a phosphorus site in CrAPSO-5.