Synthesis and characterization of gallosilicate molecular sieve with the MCM-22 framework topology

Abstract

Ga-MCM-22 with high gallium content has been synthesized with hydrothermal method and characterized by means of X-ray diffraction, SEM, TGA, Ga-71 and Si-29 MAS NMR, BET surface area, and NH3-TPD. Ga-MCM-22 was obtained as very homogeneous particles of about 5-7 mum 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 Ga-71 and Si-29 MAS NMR demonstrated that gallium was incorporated into the tetrahedral framework of Ga-MCM-22. Also, the Si-29 MASNMR 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 Bronsted and Lewis acid sites.