Tetrabutylphosphonium Cation as a Template in the Kinetics of the Crystallization of B-ZSM-5

Abstract Pentasil zeolite, B-ZSM-5 and Al-ZSM-5, have been synthesized hydrothermally using orthoboric acid and sodium aluminate, respectively, with ethyl silicate-40 so as to obtain a gel mix with SiO2/M2O3 = 22 (where M = B or Al) employing a tetrabutylphosphonium bromide (TBPBr) template at temperatures of 423, 448, and 473 K. The crystallization kinetics followed by XRD, FTIR, and SEM studies clearly demonstrate the influence of the size of the hetero atoms on the rate of nucleation and crystallization. The values of the apparent activation energies for nucleation and crystal growth indicate that both nucleation and crystal growth are faster for the Al-ZSM-5 system than for the B-ZSM-5 system. Avrami-Erofeev parameters for both systems have been evaluated. Correlations were found between the kinetics of crystallization and the characteristics of the formed crystal. SEM pictures show similar, but larger sized, crystals for the Al-ZSM-5 system than those of the B-ZSM-5 system. XRD, FTIR, and chemical analysis have revealed that the mechanism operating during crystallization is the same, and that hetero atoms become incorporated with the framework during the nucleation step, itself, with boron at a slower rate.