Topotactic route to novel layered titanosilicates via mild solution treatment of AM-4 at ambient temperature

Layered hydrated titanosilicates capable of topotactic transformations are known in nature. However, their preparation in laboratory conditions is challenging. This work demonstrated the synthesis and characterization of two novel layered titanosilcates, MSA-1, Na 2 [Ti 2 Si 4 O 12 (OH) 2 ]∙1.6H 2 O ( C 2 /c , a = 27.6518(12), b = 8.68987(37), c = 5.26316(21) Å, β = 90.8916(33) o , V = 1264.501(91) Å 3 ) and MSA-2, HTi 2 Si 4 O 11 (OH)(OH) 2 ∙ n H 2 O ( n is close to 1) ( C 2 /c , a = 26.4573(21), b = 8.75932(72), c = 5.21790(39) Å, β = 92.2808(63) o , V = 1208.28(17) Å 3 ), structurally similar to the minerals eliseevite and punkaruaivite, respectively. These materials were synthesized at room temperature by a 2D-2D transformation of AM-4 [Na 3 (Na,H)Ti 2 O 2 [Si 2 O 6 ] 2 ∙2H 2 O] titanosilicate, a synthetic sodium counterpart of the mineral lintisite. The transformations involved a gradual lattice contraction due to the removal of Na + cations from the interlayer space. This loss of positive charge was compensated by the protonation of underbonded framework oxygen atoms, resulting in the formation of hydroxyl groups (OH − ). These OH groups were structurally integrated into the titanosilicate framework, which helped to preserve charge neutrality. Other transformations were also found by heating MSA-2 to 200 °C which led to a lattice contraction, decreased symmetry ( P 2 1 /c , a = 11.9484(6), b = 8.7067(4), c = 5.2227(2) Å, β = 101.043(4) o , V = 533.26(4) Å 3 ) and resulted in the formation of the layered titanosilicate L3 [Ti(Si 2 O 5 )(OH)(OH)]. Further heating of MSA-2 to 300 °C led to the formation of a phase similar to L3 but with a substantially smaller lattice ( P 2 1 /c , a = 11.6257(8), b = 8.6809(5), c = 5.2235 Å, β = 100.906(4) o , V = 517.65(5) Å 3 ). The MSA-2 showed high CO 2 adsorption capacity. • Room-temperature topotactic transformation reveals new layered titanosilicates. • Immersing AM-4 in diluted H 2 O 2 results in the titanosilicate MSA-1, analogue of the mineral eliseevite. • Immersing AM-4 in diluted H 2 SO 4 results in the titanosilicate MSA-2, analogue of the mineral punkaruaivite. • MSA-2 titanosilicate reveals CO 2 adsorption properties.