Interactions of Cu(II) Ions with Framework Al in High Si:Al Zeolite Y as Determined from X- and W-Band Pulsed EPR/ENDOR Spectroscopies

Abstract

A combination of continuous wave (CW) electron paramagnetic resonance (EPR), pulsed EPR, and pulsed electron−nuclear double resonance (ENDOR) techniques were used to obtain structural information about the Cu2+ ions in hydrated, room-temperature evacuated, and dehydrated Cu−Y (Si:Al = 12 and 5) with a particular emphasis on framework Al interactions. W-band 1H ENDOR was used to probe the water ligands, whereas X-band hyperfine sublevel correlation (HYSCORE) spectroscopic measurements were employed to detect 27Al hyperfine couplings. The X-band CW EPR spectra show that a total of three Cu2+ species (A, B, and C) are present in the samples. ENDOR measurements of hydrated and evacuated Cu−Y-12 indicate that species A and B have a complete coordination sphere of water and, on the basis of the absence of 27Al signals in the HYSCORE spectrum, the Cu2+ is not directly bonded to the zeolite framework. Evacuation converted species B to species A. 1H ENDOR spectra combined with simulations show that upon freezing, the equatorial and axial water ligands of species A and B have a distribution of orientations with respect to the Cu−O bond. The CW EPR spectrum of dehydrated Cu−Y-12 shows a single species (species C) and the HYSCORE spectrum exhibits cross-peaks from 27Al with an isotropic coupling, aiso, of 1.5 MHz. Unlike Cu−Y-12, evacuated Cu−Y-5 consists of species A and C, and the HYSCORE spectrum clearly shows a doublet of 27Al cross-peaks with aiso = 3.0 MHz assigned to species C. Upon dehydration, the 27Al coupling decreases to 2.6 MHz. This indicates that in species C the Cu2+ is bound to framework oxygens which are bonded to an Al nucleus. For Si:Al = 5, the zeolite framework becomes negative enough that it can replace water ligands even after mild evacuation. Simulations of 27Al HYSCORE spectra indicate that species C is Cu2+ bound to the framework oxygens primarily near a single Al nucleus with a large quadrupole coupling constant.