(278g) Zeolites Coated with Organic Phosphonic Acids for Adsorptive Separation of Hydrocarbons

The separation of olefin/alkyne/paraffin mixtures to produce pure olefins for the manufacture of polymers is of great importance, and energy-efficient adsorption is a promising alternative separation method to the industrial cryogenic distillation. Zeolites are potential adsorbents for the adsorptive separation by their well-defined pores and high chemical, thermal, and mechanical stabilities. However, the adsorptive separation using zeolites for hydrocarbons with similar molecular sizes is challenging. We deposited organic phosphonic acid (PA) monolayers on zeolites to tune the diffusion rates and improve adsorptive selectivity of gases.

The pressure-decay measurements for single-gas adsorption showed that the PA monolayers on zeolites significantly enhanced the ideal kinetic selectivity of gas adsorption by changing the diffusion mechanism based on the properties of the alkyl tail. The alkyl chain lengths, steric structures, and chemical functional groups of PAs affected the assembly of coating layers on zeolite surfaces and therefore created different resistances for gas diffusion. With an n-octadecylphosphonic acid (ODPA) coating on the external surfaces of zeolite 5A, the kinetic selectivity of propylene/propane was initially >8 at 25 ^oC, whereas for uncoated 5A, it was limited to ~1.2. The coating with n-butylphosphonic acid (BPA) yielded lower kinetic selectivity than ODPA, ostensibly due to its shorter alkyl tail. The sterically bulkier PAs created less dense coating layers on zeolite surfaces and consequently they had less effect on gas diffusion than the linear chain PAs. Moreover, the PAs with amino or carboxyl group resulted in denser coating layers on zeolite surfaces and achieved higher kinetic selectivity than those without functional groups.

The pulse injection measurements for mixture adsorption of propylene/propane on uncoated zeolite 5A showed that the selectivity increased markedly with cumulative gas exposure, such that it reached 26 at high exposures. The mixture selectivity of propylene/propane was higher than the ideal selectivity because propylene, having a stronger affinity for the zeolite, hindered propane adsorption and displaced pre-adsorbed propane. When the zeolite 5A was coated with PAs, the initial mixture selectivity was higher than on the uncoated, and at the same propylene loadings, the mixture selectivity was higher on PA-coated 5A.

The tuning of gas diffusion rates and adsorption selectivity with organic coatings on zeolite surfaces may enable the rational design of selective adsorbents based on providing gas-specific resistance at the pore entrance by changing alkyl chain lengths, steric structures, and chemical functional groups of PAs. The insights on exposure-dependent competitive diffusion and surface displacement in mixture adsorption contribute to the optimization of separation processes of hydrocarbon mixtures.