Lower temperatures for the preparation of thinner zeolite A coatings

The possibility of preparing thin zeolite 4A coatings on quartz and stainless steel substrates by direct synthesis from a clear aluminosilicate solution has been investigated. In this context, the effects that originate from the synthesis temperature on the thickness of the zeolite coatings have been determined. The nucleation curves and the crystal growth rates pertaining to the coatings prepared at different temperatures have been obtained by taking into account the variation of the numbers and sizes of the zeolite particles and the sizes of the largest crystals on the substrates, respectively, with respect to time. The apparent activation energies corresponding to nucleation and crystal growth have been estimated from these data. It has been determined that the thickness of closed zeolite 4A layers may be decreased at relatively lower synthesis temperatures in case relatively longer synthesis times are employed. The reduction obtained in the thickness of the zeolite coatings with decreasing temperature is due to the rate of nucleation being less temperature sensitive than the rate of crystal growth. The crystal growth activation energies corresponding to the synthesis of zeolite 4A coatings on stainless steel plates are noticeably higher than those obtained when quartz plates are employed while the apparent activation energies for nucleation on these two substrates are quite close. Zeolite 4A coatings of 1 and 0.7 mum thickness have been prepared on quartz and stainless steel substrates, respectively, at 45 degreesC. (C) 2001 Elsevier Science B.V. All rights reserved.