Location of cupric ion and its adsorbate interactions in Cu(II)-exchanged mesoporous AlMCM-41 determined by electron spin resonance and electron spin echo modulation

Abstract

The location of Cu(II) exchanged into mesoporous AlMCM-41 material and its interaction with various adsorbate molecules were reinvestigated by electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopies. In contrast to previous work, it seems that Cu(II) is fully cordinated to adsorbates and interacts only with oxygen in an AlMCM-41 channel only in the absence of adsorbates. Cu(II) is octahedrally coordinated to six water molecules, as evidenced by an isotropic room temperature ESR signal. This species is located in an AlMCM-41 channel and rotates rapidly at room temperature. Evacuation at room temperature removes some of these water molecules, leaving the Cu(II) coordinated to fewer water molecules and anchored to oxygens in an AlMCM-41 channel wall. Adsorbate molecules such as water, methanol, ammonia, pyridine, aniline, acetonitrile, benzene and ethylene on dehydrated Cu-AlMCM-41 cause changes in the ESR spectrum of Cu(II), indicating complex formation with these adsorbates. Cu(II) forms a complex with six molecules of methanol, as evidenced by anisotropic room temperature ESR signal and ESEM data. Cu(II) also forms a square planar complex with four molecules of ammonia, pyridine and aniline, based on resolved nitrogen superhyperfine interaction and their ESR parameters. However, Cu(II) forms a complex with six molecules of acetonitrile, based on the ESR parameters. Only one molecule of benzene is directly coordinated to Cu(II). Only part of the Cu(II) coordinates weakly to one molecule of ethylene.