Metal organic frameworks based on Cu²⁺ and benzene-1,3,5-tricarboxylate as host for SO₂ trapping agents

Metal organic framework materials with Cu²⁺ as central cation and benzene-1,3,5-tricarboxylate (BTC) as linker were prepared via hydrothermal synthesis and impregnated with barium salts (chloride, nitrate, acetate) to explore the role of the Ba²⁺ counter ion on the SO₂ uptake. The impregnation of the metal organic framework materials with barium salts led to a decrease of pore volume through the (intra pore) formation of small Ba salt crystals. The structure of the Cu-BTC material was preserved after the impregnation with acetate and nitrate, but partially destroyed during impregnation with chloride. The complete loss of the BTC structure occurred through thermal decomposition at temperatures around 573 K. The sample impregnated with BaCl₂ showed a higher fraction of Cu²⁺ species compared to the other Ba/Cu-BTC samples. The SO₂ uptake capacity of the Ba/Cu-BTC(Cl^-) sample was the highest at temperatures below 673 K among all materials prepared and even higher compared to BaCO₃/Al₂O₃/ Pt based material. The comparison of the theoretical uptake (based on the stoichiometric formation of BaSO₄) with the maximum SO_x uptake achieved on the Ba/Cu-BTC samples clearly points out that a fraction of the SO_x is stored on the Cu species being part of the metal organic framework structure. With increasing temperature the framework is (partially) decomposed and highly dispersed Cu species are released, which act as additional SO_x storage sites in the high temperature region.