How microwaves dehydrate zeolites

Abstract

to predict stable hydrate complexes. Above and below the molecular plane the three predicted solvation sites are occupied. Overall, more water molecules are included in the first solvation shell according to the simulation results than is predicted from the solvation-site consideration. The hydration model developed by Hinton et al. by sequential energy minimization has 4 of the approximately 11 first-shell waters in place, and the quite reasonable results obtained for solvent effects on shielding constants may indicate that the most significant solvent molecules were included. Such a partial consideration of the hydration complex carries with it some risk of error. A complete first-shell model, with 11-12 water molecules, would be preferable to use as a model for the aqueous hydration complex of formamide. In this kind of study both the solvation site and optimized hydration models for predicting the structure of an aqueous hydration complex lack consideration of the basic space-filling nature of particles in condensed-phase systems and the spatial extent of energy minima in the solutewater pair interaction surface which may permit more than one water molecule to a single solvation site. Acknowledgment.The authors have experimentally studied the dehydration of 13X zeolite interacting with an electromagnetic field. They prove that the molecules are directly desorbed by the electromagnetic field, this process being independent of the temperature of the solid. The kinetic relationship is first order. The rate coefficient varies as the inverse of the square of the applied electric field intensity. The transport of water through the pores by convection is obtained by an overpressure, which can be evaluated from the model. The model presented also specifies the instantaneous dependence of the complex permittivity of the system during its changes.

Adsorbents regeneration under microwave irradiation for dehydration and volatile organic compounds gas treatment

“…The dielectric permittivity of a gas phase is always extremely low because molecules are no longer linked enough to each other to convert electromagnetic energy into heat by rotational polarization. As also concluded by Roussy et al [20] and Thiebaut et al [21], once they are desorbed from the surface, the circulating adsorbate molecules inside the pores of the solid do not contribute to microwave energy absorption any more. Electromagnetic energy is absorbed by the solid itself and the bound molecules at the surface.…”

VOC Recovery through Microwave Regeneration of Adsorbents: Process Design Studies

“…Singh 2 examined the feasibility of regenerating water from types 5A and 13x molecular sieves in a domestic-type microwave oven. Roussy et al, 3 in an experimental and theoretical study, concluded that water molecules are desorbed from 13X molecular sieves directly by the electromagnetic field; they found that the desorption kinetics were first order, varied with the square of the electric field intensity, and were independent of the temperature of the solid. Gibson et al 4 determined that microwave heating reduced the desorption times of ethylene oxide from polyvinyl chloride (PVC) by up to 400% over conventional heating for the same macroscopic temperature.…”

Microwave Catalysis in Organic Synthesis

“…A drastic reduction of the time of activation (from 9 h to 10 min) was observed in the activation of NaY zeolite catalyst by microwave dehydration in comparison with conventional thermal activation [30]. The very efficient activation and regeneration of zeolites by microwave heating can be explained by the direct desorption of water molecules from zeolite by the electromagnetic field; this process is independent of the temperature of the solid [31]. Interaction between the adsorbed molecules and the microwave field does not result simply in heating of the system.…”