Comparison of mesoporous solid acid catalysts in the production of DABCO by cyclization of ethanolamine I. Synthesis and characterization of mesoporous solid acid catalysts

Mesoporous silica molecular sieves MCM-41 containing zinc and aluminum ions (Zn-Al-MCM-41) with Si/(Zn + Al) ratio equal to 75, 151, 228, 304 and 380 and only aluminum ions (Al-MCM-41) with Si/Al ratio equal to 21, 42, 62, 83 and 104 respectively, were synthesized under hydrothermal conditions using cetyltrimethylammonium (CTMA⁺) surfactants as template in the absence of auxiliary organics. The mesoporous materials viz. Zn-Al-MCM-41 and Al-MCM-41 were characterized using several techniques, e.g. ICP-AES, XRD, FTIR, TG/DTA, Nitrogen adsorption, ²⁷Al-MAS-NMR, SEM, TEM, XPS and pyridine acidity (TPD and FTIR) measurements. The d-spacing value and unitcell parameter in the Zn-Al-MCM-41 and Al-MCM-41 decreases by increasing the metal ions content. FT-IR studies showed that zinc ion was incorporated into the hexagonal mesoporous Al-MCM-41 materials, however, the wavenumber of the anti-symmetric Si-O-Si vibration bands (1096 cm^-1) in Zn-Al-MCM-41 are higher than those in Al-MCM-41 (1083 cm^-1). These bands should be due to the increase of the mean Si-O distance in the wall caused by the substitution of the small ionic radii of silicon by the larger ionic radii of zinc. The surface area, pore diameter, pore volume of Zn-Al-MCM-41 is smaller than that of Al-MCM-41 while the hydrothermal stability and acid sites in the Zn-Al-MCM-41 are higher than that of Al-MCM-41 due to increasing protons with the aluminum ions non-framework by the introduction of zinc ions. Thus Bronsted and Lewis acid sites in Zn-Al-MCM-41 are same, but its acid sites are higher than Al-MCM-41.