Investigating the impact of heating rates on hydrothermal conversion of heat-treated kaolin into Linde-type LTA zeolite for water vapor sorption

Abstract

This study focuses on the impact of heating rates on the hydrothermal conversion of heat-treated kaolin into zeolite LTA. Kaolin was heat-treated at three different heating rates: 5, 20, and 40 K min⁻¹. The synthesized zeolites were characterized using X-ray diffraction (XRD), Raman spectroscopy, ²⁷Al and ²⁹Si solid-state magic angle spinning (MAS)-nuclear magnetic resonance (NMR), field emission scanning electron microscopy (FE-SEM)/energy dispersive X-ray spectroscopy (EDX), N₂ sorption, Micromeritics AutoPore IV mercury porosimeter, and Fourier transform infrared spectroscopy (FTIR) techniques. XRD patterns showed relative crystallinities of 85.5%, 83.2%, and 81.0% for zeolite A synthesized from metakaolin samples MK-(5), MK-(20), and MK-(40), respectively. Samples synthesized from metakaolin at higher heating rates (20 and 40 K min⁻¹) also contained sodalite and quartz. SEM images revealed cubic particles and spine shaped crystals in all zeolite samples. The BET surface areas were 23, 29, and 31 m² g⁻¹ for the zeolite samples obtained using MK-(5), MK-(20), and MK-(40), respectively. Metakaolin was suitable for preparing zeolite A, and the water vapor adsorption capacity of the synthetic zeolite followed this order: Zeo-4A-MK-40 > Zeo-4A-MK-20 > Zeo-4A-MK-5, with maximum adsorption capacities were 15.3, 14.6, and 9.1 mg g⁻¹, respectively. GAB isotherm models were found to be more suitable for adjusting the experimental data, with R² values above 0.9.