Mechanism of selectivity control for zeolites modified with organic monolayers

The adsorptive separation of C3H6 and C3H8 gases using molecular sieves is a challenging process due to the similarity in molecular sizes for the two molecules. In this work, we report that organic phosphonic acid (PA) monolayers on zeolites significantly enhanced the ideal selectivity of C3H6/C3H8 adsorption by changing the diffusion mechanism based on the properties of the alkyl tail. With an n-octadecylphosphonic acid (ODPA) coating on zeolite 5A, the kinetic selectivity of C3H6/C3H8 was initially >8 at 25 °C, whereas for uncoated 5A, it was limited to ∼1.2. Kinetic modeling showed that in ODPA-coated 5A, the diffusion of C3H6 and C3H8 was limited by the PA monolayer at the external surface of the zeolite. In contrast, for uncoated 5A, it was controlled by the pore channels, so that the enhanced kinetic selectivity from 5A-ODPA was related to a different limiting transport mechanism. The kinetic selectivity was not temperature sensitive in the range of 25–150 °C in 5A-ODPA as the diffusion activation energies of C3H6 and C3H8 were both small. Modification of 5A with other PAs also increased the kinetic selectivity. Coating with n-butylphosphonic acid yielded lower kinetic selectivity than ODPA, ostensibly due to its shorter alkyl tail. Coating with tert-butylphosphonic acid, a sterically bulky ligand, decreased kinetic selectivity still further. However, methylphosphonic acid, which partially penetrated the near-surface region of the zeolite, severely lowered the diffusion rates. The use of organic films may enable rational design of selective adsorbents based on providing gas-specific resistance at the pore entrance.