We discuss methods to manipulate the synthesis and crystallization of chabazite (CHA, SSZ-13) zeolites to obtain different arrangements of isolated and paired framework Al atoms at fixed elemental composition (Si/Al). Six-membered rings (6-MR) in CHA containing two Al atoms (paired Al) can be titrated and quantified by Co2+ cations, which give rise to UV-Visible absorption bands for Co2+ d-d transitions (19,000 cm-1) that increase linearly in area with Co content until saturation. Each exchanged Co2+ cation replaces two H+ sites, as determined by NH3 temperature-programmed desorption (TPD) methods that quantify residual H+ sites in metal-exchanged zeolites after their saturation with gaseous NH3 (433 K) and purging in wet helium (&lt;3% H2O, 433 K) to selectively retain NH4+ species. On Cu-SSZ-13 zeolites (Si/Al = 5-25), NH3 titration and TPD methods show that Cu2+ and [CuOH]+ species exchange for two and one H+ site, respectively. Cu2+ cations preferentially exchange at paired Al sites until saturation, beyond which [CuOH]+ species exchange at isolated Al atoms. We discuss the implications of extraframework Cu2+ and [CuOH]+ speciation in Cu-SSZ-13, which is determined by the underlying framework Al distribution, for the selective catalytic reduction (SCR) of NOx with ammonia. Synthetic methods to control the distribution and arrangement of isolated and paired framework Al atoms broaden the catalytic diversity of zeolites, especially those of fixed bulk elemental composition.

Synthetic methods to control framework aluminum distribution in chabazite zeolites and consequences for NOx selective catalytic reduction with ammonia

251st National Meeting (2016)

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