Heat induced transformation of pentasil gallosilicate

The changes in physico-chemical properties of gallosilicate molecular sieve s with MFI type structure were studied as a function of thermal treatment i n the temperature range 873-1393 K. The percentage relative crystallinity, specific surface area, sorption uptakes, and framework chemical composition of crystalline pentasil gallosilicate were found to be influenced by the t emperature and period of heat treatment. At least up to 2 h at 1213 K, no r emarkable changes were observed with respect to the stability of the MFI ph ase and a state of framework gallium. On the basis of severity of heat trea tment conditions, the progressive transformation of crystalline Na/GaZSM-5 to cristobalite phase was observed. The nature and concentration of framewo rk cations are also found to influence the rate of transformation of MFI st ructure into this phase. The correlation between temperature of the phase t ransformation and the mole ratio SiO2/Ga2O3 is systematically studied. The apparent activation energy of transformation of pentasil gallosilicate to c ristobalite is 279.6 kJ/mol. The variations in the physico-chemical propert ies were examined using characterization techniques such as XRD, IR, Si-29 MASNMR, SEM, AAS, sorption measurements and wet chemical analysis. (C) 1999 Elsevier Science B.V. All rights reserved.