The Determination of Brønsted 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. 27Al NMR data show that the relative proportions of tetrahedral and octahedral Al in the resulting material are 70 and 30%, respectively. Three Brønsted 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) Å, b = 12.3676(1) Å, and c = 22.8336(2) Å, the combined Rwp factor was 6.3%, and χ2(reduced) was 1.48. Acid deuterons were located approximately 1 Å 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°), consistent with ab initio quantum mechanical calculations. This tilt clearly indicates an Al preference for site T4 over T6.