Synthesis and Characterization of Gallosilicate Molecular Sieve with the MCM-22 Framework Topology

Ga-MCM-22 with high gallium content has been synthesized with hydrothermal method and characterized by means of X-ray diffraction, SEM, TGA, 71Ga and 29Si MAS NMR, BET surface area, and NH3-TPD. Ga-MCM-22 was obtained as very homogeneous particles of about 5-7 μm diameters and showed very similar morphologies to its Al analogue. The Na/Ga ratio of Ga-MCM-22 was much lower than 1.0, indicating that most of the negative framework charges should be compensated by protonated template, HMI. The results from 71Ga and 29Si MAS NMR demonstrated that gallium was incorporated into the tetrahedral framework of Ga-MCM-22. Also, the 29Si MAS NMR spectra showed that the as-synthesized Ga-MCM-22 has a different distribution of tetrahedral Si sites from its Al analogue. After calcination, however, the distribution of tetrahedral Si sites significantly changed, due to a large degallation from the framework of Ga-MCM-22. The NH3-TPD showed that Ga-MCM-22 possesses Brönsted and Lewis acid sites.