Synthesis of zeolite A coatings

Abstract

Zeolites are microporous crystalline hydrated aluminosilicates with pores of molecular sizes in their framework. Zeolite A is a hydrophilic synthetic zeolite with the lowest Si/Al ratio possible for zeolites, which is 1 and has a 3-dimensional pore structure with effective pore diameter about 4 Å. During zeolite synthesis, an amorphous reactant mixture is converted into microporous crystalline product. In addition to the conventional powder and pellet forms, zeolites can be synthesized on various surfaces as coatings via different synthesis methods. These coatings may be used in separation and heating-cooling applications. Zeolite coatings prepared for use in separation applications, should be continuous thin coatings without defects. Two new methods developed recently in our laboratory for the preparation of zeolite coatings are conduction heating and induction heating methods. The main idea of these methods is that the substrate is heated by conduction or induction and the synthesis solution is kept at a lower temperature than the surface temperature of the substrate. In this way, crystallization on the substrate is favored with respect to that in the synthesis solution. The induction heating method was observed to be suitable for use in large-scale applications. The purpose of this study is to test the applicability of the induction heating method to the preparation of thin zeolite A coatings on cylindrical metal substrates that may be suitable to be used in separation applications. The effects of different substrate types and system/synthesis parameters on the properties and performance of the synthesized zeolite A coatings are examined. Different cylindrical metal substrates to be used in the coating experiments were designed and constructed with various types of 316L stainless steel wire mesh sheets. Zeolite A coatings were prepared by heating the metal substrate with induction power in a circulating synthesis solution system. The prepared coatings were weighed and their mass equivalent thickness values were calculated. The samples taken from the coatings were analyzed by thermogravimetric analysis (TGA) and X-ray diffraction (XRD) techniques. Nitrogen permeabilities of the coatings were also measured. Before the synthesis experiments, the dependence of reactor inlet and outlet temperatures and thus the temperature difference obtained across the reactor on other system parameters are determined with the help of calibration experiments carried out using water instead of synthesis solution. Zeolite A coating synthesis experiments were started by testing different substrate types, 24x110/500 (single outer layer), 24x100/500/500 (double outer layers) and 24x110/200x1400. An additional zeolite phase besides LTA, the LTN phase, was also seen to be present in smaller amounts, in the coatings obtained in these preliminary experiments. 24x110/200x1400 was observed to be the substrate type on which the most preferable coatings with lower amounts of LTN phase, and lower permeance and thicknesses values were obtained. For this reason, zeolite A coating experiments were continued using 24x110/200x1400 substrates. When experiments at different flow rates were conducted, it was observed that thinner and more closed coatings could be obtained using lower flow rates. For the investigation of the effects of inlet-outlet temperatures on the coating properties, experiments at different reactor inlet-outlet temperatures in the range of 30-35 to 55-60 °C, for two different synthesis times of 3 and 5 hours, were carried out. The coating thickness increased with the increase in reactor inlet and outlet temperatures, for both synthesis times. The gas permeance, however, seemed to decrease with the increase in coating thickness at low thickness values, while it started to increase again above coating thickness values of about 100 µm, for both synthesis times. In order to optimize the effects of reactor inlet-outlet temperatures to prepare thinner coatings with lower permeabilities, running the experiment in stages with different reactor inlet-outlet temperatures was attempted and successfully achieved. The smallest equivalent thickness obtained was 210 µm for the only coating synthesized with a gas permeance value lower than 10-9 mol/m2.s.Pa, in these initial two staged experiments. This coating was obtained with 3 hours of first stage at reactor inlet-outlet temperatures of 45-50°C, followed by 2 hours of second stage at reactor inlet-outlet temperatures of 55-60 °C. Two staged synthesis approach was further investigated by keeping one of the first or second stages same and changing the reactor inlet-outlet temperatures of the other stage. The change in reactor inlet-outlet temperatures in the second stage affected the coating thickness more significantly, while it affected the gas permeance less. The effect of the temperature change of the first stage against the temperature change of the second stage was more striking. As the temperature of the first stage increased in the range tested in these experiments, both the coating thickness and the gas permeance values decreased. The coating obtained from the experiment with the reactor inlet-outlet temperatures of 45-50 and 50-55 °C in the first and second stages, carried out for 3 and 2 hours, respectively, was observed to have an equivalent thickness value of 112 µm, almost half the value of 210 µm for the previous coating obtained at 45-50 and 55-60 °C. The actual effective thickness value of the coating across which gas permeation takes place is expected to be lower than the equivalent thickness value calculated, for the substrates used in this study. Both of these coatings had sufficiently low gas permeance values lower than 10-9 mol/m2.s.Pa. Finally, the results of XRD and TGA analyses revealed that both of these coatings were highly crystalline zeolite A coatings.