The Tapered Element Oscillating Microbalance (TEOM) was demonstrated to be a valuable tool for elucidating the adsorption/desorption characteristics of alkylation reactants on microporous and mesoporous catalysts.. The catalyst is loaded onto the element, and changes in the oscillation frequency sensitively translate into small changes of the catalyst mass. This unique feature provides real-time changes in the mass of the catalyst during adsorption and desorption of reactant/product molecules from which a host of insights into the physicochemical processes occurring in the catalyst pores may be obtained.
Equilibrium adsorption isotherms were obtained on these catalysts using n-butane, isobutane, and propane as proxy reactant molecules (T = 303-398 K, adsorbate partial pressure 0-1.2 bar) and 2,2,4-trimethylpentane (2,2,4-TMP) as proxy product molecule (T = 298-473 K, P224-TMP = 0-0.3 bar). Analysis of the transient adsorption/desorption profiles of the reactant molecules demonstrate that diffusion in the secondary meso-/macroporous structure formed in either the catalyst packing or the pelletized cataysts control the overall sorption rates. The experimental adsorption/desorption profiles from the pelletized zeolites were regressed with dynamic models to obtain effective meso-/macropore diffusivities for reactant molecules. Neither USY nor beta zeolites appear to be durable catalysts for alkylation. The alkylate product (C8) is retained in the pores, and even reactant molecules (C4's) are slow to desorb. Mesoporous silica gels provide good pore accessibility for C8 alkylates as evidenced by complete desorption of 224-TMP even at mild temperatures. These results suggest that the design of solid acid catalysts for n-butane + isobutane alkylation must focus on mesoporous structures for better pore accessibility and extended durability.