Mechanism of microwave heating of zeolite A

The mechanism of microwave heating of A zeolite was studied by comparing the heating properties, cation distributions and dielectric properties of 3A, 4A and 5A zeolites. It was easy to heat hydrated 4A zeolite to a glowing (melting) temperature by microwave (2.45 GHz) radiation from room temperature but was difficult to heat the same zeolite with little hydration. When 4A zeolite was preheated to ≥ 120°C, it could be easily heated by microwave radiation. However, 3A zeolite was not heated by microwaves under the above condition used for 4A zeolite. 3A zeolite with little hydration at room temperature could not be heated by microwaves but could be heated after hydrating it to ≥5 H₂O per unit cell (puc) or preheating 3A to ≥254°C. 5A zeolite could not be heated at all. The easiness of microwave heating was in the order of 4A > 3A ≫ 5A. There are differences in the cation distribution among these zeolites, i.e., 5A zeolite has no cations on the 4- and 8-membered oxygen ring sites but the other two have. It was expected from dielectric properties reported so far that the cation on 4-ring site can absorb microwave at >450°C with a higher efficiency but cations in other locations are ineffective for absorption. The following mechanism for the microwave heating was proposed: In the initial period, the hydrated zeolite absorbs the microwaves through its adsorbed water and its temperature rises. The adsorbed water completes desorption by ca. 400-470°C. In this temperature range the zeolite begins to absorb microwaves and the absorption efficiency becomes high with increasing temperature. When zeolite reaches 450-500°C thermal runaway starts. 5A zeolite cannot reach thermal runaway conditions because of absence of cation on 4-ring site. The adsorbed water plays the role of preheating agent in the initial period of microwave heating. Its role, however, can be substituted by other ways, e.g., preheating by conventional heating.