(85g) Adsorption/desorption Studies Of CO2, Isobutane And 2,2,4-Trimethylpentane In Beta-Zeolite And Usy-Zeolite Using A Tapered Element Oscillating Microbalance (Teom)

In developing stable and durable solid acid catalysts for 1-butene+isobutane alkylation, reliable understanding of the adsorption and desorption rates of reactants, products and solvents in catalysts is essential to screen potential solid acid catalysts. Toward this end, we are currently investigating the adsorption/desorption phenomena of model compounds (olefins and paraffins) and potential solvents (i.e. CO₂ media have been demonstrated to enhance pore-accessibility and catalyst stability) on various solid acid catalysts and supports. Complementary theoretical models are implemented to reliably interpret the experimental data and obtain fundamental parameters. Equilibrium adsorption isotherms and adsorption/desorption rates of CO₂, isobutane and 2,2,4-trimethylpentane in β-zeolite and USY-zeolite have been experimentally investigated by using a tapered element oscillating microbalance (TEOM) which provides excellent sensitivity, allowing mass changes as little as 10 µg to be detected. It was found that CO₂ and isobutane adsorption and desorption rates on β-zeolite and USY-zeolite are fast (at 303-373 K, under CO₂ or isobutane partial pressure 0-1.4 bar). At all temperatures investigated, saturation adsorption is not achieved even up to 1.4 bar for CO₂. The adsorption uptakes of 224-TMP in β-zeolite or USY-zeolite change dramatically (the time to approach equilibria can differ by 2-3 orders of magnitude) depending on the operating conditions. The adsorption/desorption dynamics of 224-TMP in both zeolites are much slower at ambient temperatures preferred for alkylation. The significant buildup of 224-TMP can impose significant transport limitations inside the β-zeolite and USY-zeolite pores. The accumulation of 224-TMP and other heavy compounds inside the pores will result in catalyst deactivation. A GCMC (Grand Canonical Monte Carlo) simulation program has been employed to understand the relationship between molecular-level structure and observable macroscopic properties in this adsorption system. A meso-scale model has been developed to estimate the effective diffusivity and adsorption/desorption parameters.