(339e) Modification of Zeolites with Organic Phosphonic Acids for Adsorptive Separation of Gases

The adsorptive separation of propylene and propane gases using molecular sieves is a challenging process due to the similarity in molecular sizes for the two molecules. We deposited organic phosphonic acid (PA) monolayers on zeolites to tune the diffusion rates and adsorptive selectivity of various gases including propylene and propane. The PAs with different chemical functional groups, chain lengths, and steric structures were used to modify zeolites. The PA monolayers can hypothetically function as “gate-keepers” by adding a tunable diffusion barrier on the zeolite surface. We used pressure-decay adsorption measurements to demonstrate that 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. Moreover, we used pulse method for adsorption measurements of propylene and propane mixture followed by temperature-programmed desorption to determine the mixture selectivity.

For single gas adsorption, on zeolite 5A coated with n-octadecylphosphonic acid (ODPA), 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 propylene and propane molecules was limited by the PA monolayer at the external surface of zeolite. In contrast, for uncoated 5A, it was controlled by the pore channels of zeolite, so that the enhanced kinetic selectivity from ODPA-coated 5A 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 propylene and propane were both small. Modification of 5A with other PAs, including those with methyl, n-butyl, and tert-butyl groups, also increased the kinetic selectivity. However, modification with methylphosphonic acid, which partially penetrated the near-surface region of zeolite, severely lowered diffusion rates of propylene and propane. 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.

Moreover, our recent studies on propylene and propane mixture adsorption on zeolite 5A indicated that there was competitive adsorption between propylene and propane and the mixture selectivity of C₃H₆/C₃H₈ was higher than the ideal selectivity, which was ascribed to the stronger interaction forces of zeolites with propylene than propane.

The use of organic films may enable rational design of selective adsorbents based on providing gas-specific resistance at the pore entrance. Coatings with PA monolayers on zeolites has potential to provide another lever to tune the diffusion rates of gases by choosing suitable PAs and zeolites. Such an approach may be broadly applicable to diverse adsorbents, i.e., a kinetically selective coating can enhance performance for an underlying equilibrium-selective material.