Hydrogen tetratitanate (H2Ti4O9) is synthesized from the precursor potassium tetratitanate (K2Ti4O9) by proton exchange in acidic solution and subsequently subjected to organic functionalization with n-propylamine (PA) pillars, resulting in interlayer expansion as illustrated in Fig. . This synthesis has been described previously by our group. Powder X-ray diffraction (XRD) pattern of HTO and HTO-PA are shown in Fig. . The shift of the signal corresponding to the (200) set of planes corresponds to an interlayer expansion from 0.86 nm in HTO to 1.58 nm in HTO-PA. This confirms the successful interlayer functionalization with n-propylamine, in agreement with our previous work. The shoulder of the (200) signal of HTO towards lower 2θ angles is a result of inhomogeneous interlayer hydration. Thermogravimetric analysis (TGA) is used to analyze the content of confined volatile species in the HTO interlayer (Fig. ). After heating the samples to 600 °C, full dehydration of the H2Ti4O9 lattice yields 4 TiO2. For pristine HTO, an overall mass loss of 14.3 % is measured, indicating an initial composition with a structural formula H2Ti4O9•1.96H2O, in good agreement with the refined structure of a previous study. The measured overall mass loss for HTO-PA is 19.6 %, indicating the initial composition H2Ti4O9•1.01PA.