Organic-coated zeolites for selective gas adsorption: Effect of functional group identity and coating density

Abstract

Adsorptive separation of propylene (C₃H₆) and propane (C₃H₈) is an alternative to energy-intensive distillation, but improving kinetic selectivity is challenging for molecules with similar sizes. Composite materials consisting of a barrier organic film on the external surface of a zeolite have shown higher selectivity; however, structure–function relationships for these materials are lacking. Here, gas adsorption rates on zeolite 5A were controlled by varying the terminal functional group (amine or carboxylic acid) and coating density of organic phosphonic acid (PA) modifiers. Single-gas, pressure-decay adsorption measurements showed that with a complete n-butylphosphonic acid (BPA) monolayer, the C₃H₆/C₃H₈ kinetic selectivity was > 5 initially, and it approached the equilibrium selectivity of ∼ 1.2 after 20 min, whereas a coating of 4-phosphonobutyric acid (COOHC₃PA) with a similar chain length as BPA yielded a selectivity of 15 at 60 min. Coating with 3-aminopropyl phosphonic acid (NH₂C₃PA) resulted in high resistance to gas diffusion. To investigate whether the slow adsorption was attributable to excessive NH₂C₃PA, the coating density was tuned by varying PA concentration for deposition. As the coating density decreased, the initial adsorption rates increased. With an ∼ 0.1 monolayer NH₂C₃PA coating, the C₃H₆/C₃H₈ kinetic selectivity was > 15 for 60 min. Temperature-programmed desorption of n-propylamine suggested that the improved selectivity of NH₂C₃PA coating may be associated with the affinity of the amine group for the zeolite surface. This study demonstrates that gas adsorption rates and selectivities in zeolites are highly sensitive to the composition and density of monolayer films on the external surface.