Titration of Brønsted Acid Sites in Cu-Zeolites and Their Mechanistic Roles in Standard SCR

Small-pore, bifunctional, Cu-exchanged zeolites with the chabazite (CHA) topology are used in commercial deNO_X applications using selective catalytic reduction (SCR) with NH₃. Here, we discuss methods that enable using NH₃ as a stoichiometric chemical titrant for H⁺ sites in ZSM-5 (MFI) and SSZ-13 (CHA) zeolites in their H- and partially-Cu-exchanged forms, and discuss how these methods provide new insight into the mechanistic roles of H⁺ sites in standard SCR.

In H-ZSM-5 and Cu-ZSM-5 samples with varying aluminum content (Si/Al = 17-90), H⁺ sites were titrated selectively and counted consistently using three different NH₃ titration procedures and one reactive n-propylamine titration method. The number of H⁺ sites on ZSM-5 zeolites measured by direct chemical titration was often less than the number of framework Al (Al_f) atoms determined from ²⁷Al MAS NMR spectra (H⁺:Al_f = 0.7–1.0), providing yet another demonstration that Al_f structures can be imprecise surrogates for active H⁺ sites on zeolites. In contrast with ZSM-5, the number of H⁺ sites accessible to NH₃ on H-SSZ-13 (Si:Al = 4.5) and Cu-SSZ-13 (Cu:Al = 0-0.20) was higher by at least a factor of four than the number of H⁺ sites accessible to n-propylamine under the saturation conditions studied. The number of H⁺ sites on these Cu-SSZ-13 samples decreased with increasing Cu loading with a 2:1 stoichiometry, consistent with the exchange of isolated Cu²⁺ ions for two H⁺ sites associated with proximal Al_f atoms. We conclude that NH₃ is more appropriately suited to titrate all accessible H⁺ sites in small-pore Cu-zeolites, because residual OH groups in SSZ-13 were not detected in infrared spectra after NH₃ exposure, and because NH₃ is the reductant in NO_X SCR reactions [1].

Standard SCR rates were measured under differential conditions and normalized by the number of isolated Cu²⁺ sites, quantified by combining elemental analysis with ambient UV-Visible and X-ray absorption spectral data [2]. SCR turnover rates (473 K, per Cu²⁺) show a zero-order dependence on the concentration of residual H⁺ sites that remain after Cu²⁺ exchange on Cu-SSZ-13 [1]. The primary function of these residual H⁺ sites appears to be for NH₃ storage under the conditions studied. We also show how NH₃ titrations performed under different gaseous environments can probe the nature of active H⁺ sites that participate in standard SCR turnovers.

References:

[1] Bates, S. A.; Delgass, W. N.; Ribeiro, F. H.; Miller, J. T.; Gounder, R. J. Catal. 2014, 312, 26.

[2] Bates, S. A.; Verma, A. A.; Paolucci, C. P.; Parekh, A. A.; Anggara, T.; Yezerets, A.; Schneider, W. F.; Miller, J. T.; Delgass, W. N.; Ribeiro, F. H. J. Catal. 2014, 312, 87.