Modifying Adsorption Kinetics of Zeolites Using Self-Assembled Monolayers

Zeolites, due to their defined pore size and structure, are used to separate molecules based on their kinetic diameter in a process known as molecular sieving. Compared to their traditional polymer membrane counterparts, zeolite membranes show improved thermal and chemical stability in separation processes. This stability allows for the effective usage of zeolites in natural gas separations, such as the separation of CH₄ and CO₂. However, longer alkanes commonly found in natural gas feed streams such as propane and butane tend to adsorb in zeolite pores, blocking the pores and reducing separation selectivity. Thus, a method to selectively adsorb CO₂ during natural gas separations while reducing alkane adsorption is desirable.

The potential for self-assembled monolayers (SAMs) to control zeolite selectivity was evaluated in this study. SAMs have been used as surface modifications for catalytic applications. Specifically, self-assembled monolayers can tune the surface properties of catalysts and alter their selectivity. In principle, this same technique can be applied to tune zeolite surfaces to selectively control alkane fouling and ultimately reduce adsorption of these contaminants.

Zeolite surface modifications were applied on two commercially available zeolites, 5A and ferrierite, using alkylphosphonic acid self-assembled monolayers. The SAM monomers consisted of phosphonic acid head groups covalently bonded to the zeolite surface and n-alkane tail groups organized through Van der Waals attractions. The presence of well-ordered SAMs on the zeolite surfaces was confirmed using FT-IR analysis.

Alkane adsorption tests using butane and propane were also performed. The initial rates of alkane adsorption decreased on zeolites treated with the SAMs. This decrease in adsorption rate can be attributed to nonpolar interactions between the carbons in the alkanes and the carbons in the self-assembled monolayer. Additionally, the total adsorption and rate of adsorption of CO₂ were relatively unchanged, suggesting that these modified zeolites selectively adsorb CO₂ and decrease the rate of adsorption of contaminants such as propane and butane.