Tuning Gas Adsorption Selectivity and Diffusion Rates in Zeolites with Phosphonic Acid Monolayers

The distillation of light gases requires significant energy inputs. Sorbents could alleviate this issue, but they must be able to separate molecules of similar size and chemical functionality. Here, we report the enhanced separations of gases such as propane and propylene using zeolite 5A modified with phosphonic acids (PAs) of varying alkyl tail length. All PAs slow the diffusion of n-C4H10 but generally have a smaller effect on the diffusion rate and loading of smaller molecules such as CO2 and CH4. However, methylphosphonic acid (C1PA) decreases by >90% both the diffusion rates and apparent equilibrium uptakes of n-C4H10 and C3H6. The achievable loadings of C3H6, C2H6, CH4, and CO2 are affected much less, resulting in a C3H6-C3H8 ideal selectivity of 59 ± 14. The unique effects of C1PA are correlated to a much higher extent of deposition near the zeolite surface, while longer-chain PAs are confined to a monolayer on the external surface. [Display omitted] Zeolite 5A molecular sieves are modified with phosphonic acid ligandsLong-chain alkyl ligands localize at the outer surface and slow gas diffusionMethyl ligands penetrate the zeolite pores and substantially alter structureMethyl ligands dramatically improve propane-propylene selectivity Ellis et al. functionalize zeolite molecular sieves with organic ligands of controlled size as a strategy to control the selectivity of gas adsorption. Through the deposition of small methyl ligands that penetrate and modify the zeolite pores, propylene adsorption is strongly favored compared to propane, providing a basis for efficient alkane-alkene separation.