engleski

Kinetic Analysis of non-isothermal transformation of zeolite 4A into low-carnegieite

Kinetics of the non-isothermal amorphization of zeolite 4A and crystallization of low-carnegieite from the formed amorphous phase were investigated by powder X-ray diffraction method. Changes in the fractions fA of zeolite A, fa of amorphous aluminosilicate and fC of low-carnegieite during heating of zeolite A at three different heating rates (0.0833o/s, 0.1667o/s and 0.333o/s), showed that amorphization of zeolite 4A and crystallization of low-carnegieite take place simultaneously. Kinetic analyzes of the single processes (amorphization, crystallization) as well as of the entire transformation process (simultaneous transformation of zeolite A into amorphous aluminosilicate and its crystallization into low-carnegieite) have shown that the non-isothermal transformation takes place by the same mechanism as the isothermal transformation, i.e, amorphization of zeolite 4A takes place by a random, diffusion-limited agglomeration of the short-range-ordered aluminosilicate subunits formed by thermally induced breaking of Si-O-Si and Si-O-AL bonds between different building units of zeolite framework and crystallization of low-carnegieite occurs by homogeneous nucleation of low-carnegieite inside the matrix of amorphous aluminosilicate and diffusion-controlled, one-dimensional growth of the nuclei. On the other hand, kinetics of non-isotherlal processes are determined by by the changes of the rate constants during heating and thus on the apparent activation energies of amorphization and crystallization. Apparent activation energies of amorphization of zeolite A and crystallization of low-carnegieite are determined and kinetic equations for the mentioned processes are derived.

non-isothermal transformation; zeolite 4A; amorphous aluminosilicate; low carnegieite; kinetics; mechanism

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