TEOM analysis of the equilibria and kinetics of n-hexane and n-heptane adsorption on FCC catalyst/silicalite

A tapered element oscillatory microbalance is used for the measurement of uptake rates of n-hexane and n-heptane on fluid catalytic cracking catalyst and silicalite adsorbent. Investigations are considered over the temperature range of 373 to 513 K, and adsorbate partial pressures up to 0.12 bar. Based on measured adsorption isotherms, and through mechanistic descriptions of the diffusion process, a mathematical model is developed to describe the transient response of the microbalance, and subsequently the kinetics of adsorption and desorption. For example, for the hexane-silicalite system at a temperature of 473 K, the diffusion coefficient at zero-surface coverage is estimated as 2.9 x 10(-11) m(2)/s, with an activation energy of 17 kJ/mol. This is consistent with published data based on other transient analysis methods. At relatively low temperatures of operation, i.e. less than 450 K, a single resistance model for mass transfer failed to accurately predict the desorption profiles for hexane on silicalite. As a means of quantifying the observed desorption rates, a dual-resistance model is introduced in which two different diffusion rates are assumed to take place above and below a threshold value of adsorbate concentration. Such a model may be used to account for silicalite phase transitions at high adsorbate loading. (C) 2003 Elsevier Science Ltd. All rights reserved.