The determination of Bronsted acid sites in zeolite ERS-7 by neutron and X-ray powder diffraction

The deuterated acid form of zeolite ERS-7 (Si/Al = 8.4) has been prepared by repeated cycles of D2O exposure and calcination. Al-27 NMR data show that the relative proportions of tetrahedral and octahedral Al in the resulting material are 70 and 30%. respectively. Three Bronsted sites were identified by a combined Rietveld refinement that simultaneously employed synchrotron X-ray powder diffraction data and neutron powder diffraction data. Final lattice parameters were a = 9.7843(1) Angstrom, b = 12.3675(1) Angstrom, and c = 22.8336(2) Angstrom, the combined R-wp factor was 6.3%. and x(2)(reduced) was 1.48. Acid deuterons were located approximately 1 A from oxygens O3, O5, and O7, with occupancy factors of 0.16(2), 0.16(3), and 0.18(2), respectively. These correpsond to 1.28(16), 0.64(12), and 1.44(16) deuterons per unit cell, respectively, for a total of 3.36(26) acid sites per unit cell, which is consistent with the tetrahedral Al content. Only deuteron D5 lies on a special position. T-O bond lengths suggest a site preference for Al at T4, T1, and possibly T2, each of which is bonded to at least one of the observed acid-site oxygens. A bridging framework oxygen site was identified in both the PND and PXD difference Fourier maps that appears to be disordered over at least two distinct sites; this corresponds to different local arrangements of Al at the neighboring T-sites. The out-of-plane framework hydroxyl tilt angles at each acid site were comparable to those of H+ SSZ-13 and D+ RHO. Acid site D7 demonstrated a significant in-plane hydroxyl tilt angle (7.5 degrees), consistent with ab initio quantum mechanical calculations. This tilt clearly indicates an Al preference for site T4 over T6.