An UV-Visible study of the stability of the ruthenium hexaammine cation in BEA zeolites—comparison with NaY

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Abstract
A series of Ru/zeolites with Ru content in the range of 1–7 wt.% was prepared by cationic exchange in an aqueous Ru(NH3)6Cl3 solution, at room temperature, of two H–BEA (different particle sizes but same Si/Al ratio of 13), a Cs–BEA (Si/Al=13) and a NaY (Si/Al=2.4), zeolitic supports. The quantitative UV-Visible analysis of the solutions during exchange showed that about 90% of the exchange process was over within the first hour of contact between the zeolite and the solution. This analysis also provided an accurate measurement of Ru content in the solids in real time and gave access to the Ru exchange capacities of the zeolitic supports. Diffuse reflectance UV-Visible spectroscopy showed that the [Ru(NH3)6]3+ cations were exchanged as such in all the BEA zeolites and were not modified after storage of the solids for several months in ambient conditions. On the contrary, the Ru hexaammine cations in NaY were gradually transformed into the Ru-red-wine [(NH3)5RuIIIO–RuIV(NH3)4–O–RuIII(NH3)5]6+ and Ru-brown [(NH3)5RuIV–O–RuIII(NH3)4–O–RuIV(NH3)5]7+ cationic trimers, in agreement with literature data. The transformation rate increased with the Ru loading. The reasons for the higher stability of the [Ru(NH3)6]3+ cations in the BEA series than in NaY are discussed.