The band gap energy (absorption edge energies) of the pure H₃[PMo₁₂O₄₀]·13H₂O and H₄[PVMo₁₁O₄₀]·13H₂O, respectively, supported on SiO₂ and SiC and some of its NH₄⁺ and Cs⁺ salts were determined by different methods. The influence of the counter-cations and the temperature on band gap energy was studied. In this purpose, the diffuse reflectance spectra of above mentioned compounds were registered at different temperatures, and it were transposed in the curves of the Kubelka-Munk function vs. wavelength. The band gap energies were determined by processing of low field energy of the ligand-metal charge transfer band (O²⁻ → Mo⁶⁺ and O²⁻ → V⁵⁺) usually observed between 200 and 400 nm on these curves. In this aim, the Tauc's relation was adapted for Kubelka-Munk function use and it was plotted for n = 1/2 (direct transition) and 2 (indirect transition) vs. wave energy (photon energy). The intersection of the curves’ tangent drawn to their point of inflection with horizontal axis gives the band gap energy. The other method for calculation of band gap energy was the differential calculus on the Kubelka-Munk function vs. wave energy curve where the x value corresponding to curves’ maximum is the found value. The comparison between experimental band gap values and literature data showed their partial fit. The higher temperature produces the band gap energy diminution as a result of a stronger interaction between Keggin Units, which occurs especially by the crystallization water loss. The Keggin Units isolation one from another by voluminous counter-ion or their spreading on a support leads to a weaker interaction between them and as a consequence, the increasing of absorption edge energy. A linear correlation of the crystallites size with band gap energy was observed.

The ordered mesoporous silica MCM-48 with cubic Ia3d structure was synthesized using the cationic surfactant hexadecyltrimethylammonium bromide (CTAB) as a template agent and tetraethylorthosilicate (TEOS) as a silica source. The obtained material was first functionalized with (3-glycidyloxypropyl)trimethoxysilane (KH560); further, two types of amination reagents were used: ethylene diamine (N2) and diethylene triamine (N3). The modified amino-functionalized materials were characterized by powder X-ray diffraction (XRD) at low angles, infrared spectroscopy (FT-IR) and nitrogen adsorption–desorption experiments at 77 K. Characterization from a structural point of view reveals that the ordered MCM-48 mesoporous silica has a highly ordered structure and a large surface area (1466.059 m²/g) and pore volume (0.802 cm³/g). The amino-functionalized MCM-48 molecular sieves were tested for CO₂ adsorption–desorption properties at different temperatures using thermal program desorption (TPD). Promising results for CO₂ adsorption capacities were achieved for MCM-48 sil KH560-N3 at 30 °C. At 30 °C, the MCM-48 sil KH560-N3 sample has an adsorption capacity of 3.17 mmol CO₂/g SiO₂, and an efficiency of amino groups of 0.58 mmol CO₂/mmolNH₂. After nine adsorption–desorption cycles, the results suggest that the performance of the MCM-48 sil KH N2 and MCM-48 sil KH N3 adsorbents is relatively stable, presenting a low decrease in the adsorption capacity. The results reported in this paper for the investigated amino-functionalized molecular sieves as absorbents for CO₂ can be considered as promising.