Electron spin resonance and electron spin echo modulation studies of adsorbate interactions with cupric ion on the aluminum content in Cu-AlMCM-41 materials

Employing electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopy, adsorbate interactions are studied in two types of ion-exchanged Cu-AlMCM-41 materials. One is structurally ordered Cu-AlMCM-41(60)-I, Cu-AlMCM-41(40)-I, Cu-AlMCM-41(20)-I, and the other is less structurally ordered Cu-AlMCM-41(15)-II. Four Cu(II) species are observed by ESR. Species A with ESR parameters g_∥^A = 2.406, A_∥^A = 0.0145 cm^-1, and g_⊥^A = 2.081 is assigned to octahedral Cu(II) and is observed in hydrated Cu-AlMCM-41. Species B with g_∥^B = 2.317, A_∥^B = 0.0176 cm^-1, and g_⊥^B = 2.076 is assigned to distorted octahedral Cu(II) and is observed in dehydrated samples. Species C with g_∥^C = 2.430, A_∥^C = 0.0125 cm^-1, and g_⊥^C = 2.091 is also assigned to octahedral Cu(II) and is observed after adsorption of CH₃OD. Species D contains square planar D1 Cu(II) with g_∥^D1 = 2.280, A_∥^D1 = 0.0168 cm^-1, and g_⊥^D = 2.058, and distorted octahedral D2 Cu(II) with g_∥^D2 = 2.249 and A_∥^D2 = 0.0190 cm^-1 after adsorption of ammonia. The accessibility to water, methanol, and ammonia adsorbates is similar for Cu(II) ions in Cu-AlMCM-41(60)-I, Cu-AlMCM-41(40)-I, and Cu-AlMCM-41(20)-I. This is confirmed by ESEM simulations which show that all Cu(II) in these samples coordinates to six D₂O or CH₃OD molecules. However, Cu(II) in Cu-AlMCM-41(15)-II is less accessible to adsorbates and interacts with two fewer D₂O or CH₃OD molecules than in Cu-AlMCM-41(n)-I samples. Therefore, the Cu(II) adsorption interactions do not depend on the Si/Al ratio but depend on structural ordering in different types of AlMCM-41 samples. This can be explained by different acidic sites in these types of AlMCM-41 materials.