(7b) Effect of Reaction Conditions and SO2 Exposure on Cu Speciation in SSZ-13 Zeolites | AIChE

(7b) Effect of Reaction Conditions and SO2 Exposure on Cu Speciation in SSZ-13 Zeolites


Mandal, K. - Presenter, University of Virginia
Chen, Y. R., University of Virginia
Daya, R., Cummins Inc.
Epling, W., University of Virginia
A molecular level understanding of the active sites in metal-exchanged zeolites, and their reaction condition-dependent speciation is critical for fine-tuning operating conditions and synthesis techniques for prolonging catalyst longevity and regenerability. This is especially significant for Cu-exchanged SSZ-13 zeolites (Cu/SSZ-13) used for selective catalytic reduction (SCR) of NO in diesel engines where SO2 and SO3 readily poison these materials, deteriorating their catalytic performance. While the effect of SO2 on the predominant monatomic active sites in Cu/SSZ13, Cu2+ and [CuOH]+ has been explored, SO2 poisoning on Cu dimers ([Cu2O]2+ and [Cu2O2]2+) is unknown. While synthesis protocols dictate the extent of Cu dimers initially present in the zeolite, solvated [CuOH]+ at proximal Al exchange sites form Cu dimers and their protonated forms ([Cu2OxHy]2+, x,y=1-2) on dehydration. Here, we employed computational tools (AIMD simulations, density functional theory calculations) to develop thermodynamic models for the reaction of SOx with dimeric Cu motifs as a function of Al-Al proximity in SSZ13, and the reaction environment. Our results demonstrate that unlike Cu2+ and [CuOH]+, dimeric Cu species at the identified Al-Al configurations exhibit strong binding to SOx at a wide range of conditions. We observed desulfation at high temperatures (> 900 K), with variations seen across the Al-Al configurations. The most stable sulfated species for Cu dimers exchanged at Al pairs in the same eight-membered ring was found to be [Cu2SO4]+ while formation of [Cu2HSO4]+ was favorable at the oxygen-bridged sites. Moreover, interconversion of solvated ZCuOH to mono- and multinuclear Cu species was also strongly dependent on the proximity of Al-Al pairs and reaction conditions. Our thermodynamic phase diagrams indicate that at low temperatures, the presence of predominantly monatomic Cu inhibits SOx poisoning; with an increase in temperature, formation of Cu dimers becomes thermodynamically favorable rendering SSZ-13 zeolites more susceptible to SOx poisoning.