Investigation of NH3 Desorption Kinetics on the LTA and SOD Zeolite Membranes

Abstract

The acidity characteristics of zeolite are highly significant, and understanding the acidic properties is essential for developing new types of zeolite catalysts. Zeolite membranes were synthesized using metakaolin, sodium hydroxide, and alumina with a molar ratio of 6Al2Si2O7:12NaOH:2Al2O3 as the starting ingredients. X-ray diffraction, scanning electron microscopy, and infrared spectroscopy were used for this study. N2 adsorption measurements determined the surface areas of the SOD zeolite membrane (115 m2/g) and the LTA membrane (150 m2/g). The units of absorbed water vapor were 40 and 60 wt% for the SOD membrane and the LTA membrane, respectively. The strength and number of acid sites of the synthesized LTA and SOD zeolite membranes were determined by temperature-programmed desorption of ammonia. As a result, the value of the total acidity of the LTA zeolite membrane is in the range of 0.08 × 1019 units/m2 while that of the sodalite membrane is an order of magnitude lower and is 0.006 × 1019 units/m2. The apparent activation energy values for desorption of ammonia from LTA and SOD zeolite membranes were calculated using data on the kinetics of desorption of ammonia at different heating rates. It was found that at temperatures below 250 °C, the degree of conversion of the activation energy values is no more than 35 kJ/mol, which corresponds to the desorption of physically bound ammonia. An increase in the activation values up to 70 kJ/mol (for SOD) and up to 80 kJ/mol (for LTA) is associated with the desorption of chemically bound ammonia from the samples.