(50i) Molecular Simulation-Guided Design of Hydrophilic-Hydrophobic Interfaces for Hybrid MOF Microtanks for Methane Storage

Given the abundance of natural gas in the U.S., the widespread use of methane-powered vehicles seems an enticing prospect. The incumbent method to store methane in on-board tanks is by densification to 265 cc(STP)/cc through compression to 250 bar. However, the high storage pressure creates safety concerns that require the use of cumbersome tanks made of expensive materials to be addressed. Adsorption-based methane storage, which exploit gas-solid interactions to densify methane within a porous adsorbent has been considered one of the most promising ways to reduce the storage pressure. However, a series of high throughput screening studies have indicated that there may be material design constraint that fundamentally prevent reaching desirable methane adsorption levels in “stand alone” adsorbents such as metal-organic frameworks (MOFs).

A “hybrid” material design with potential to overcome those limits was introduced in experiments by Carreon and coworkers [1]. In the hybrid design, a millimeter-sized pellet of a porous material is coated by a thin membrane of the second porous material. The pellet can be filled with methane at high-pressure and held in place by reversibly adsorbing “sealant” molecules onto the thin membrane, blocking the stored methane from going out and keeping the external pressure low. While the operating principle was demonstrated experimentally, the described hybrid design is far from being optimized. Here, we first present grand canonical Monte Carlo simulations in model pores (graphene and graphene oxide) to demonstrate that a design principle that should be pursued in these hybrid systems is to make the pellet material hydrophobic and the membrane material hydrophilic, which creates a desirable distribution of methane and a hydrophilic sealant between the pellet and the membrane to prevent methane escape, as well as sealant invasion of the pellet. Second, we present a computational high throughput screening of a 2,000+ database of computational MOF prototypes constructed using the Topologically-Based Crystal Constructor (ToBaCCo) code to elucidate how to tune the chemistry and structure of MOFs to attain the desirable hydrophobic and hydrophilic character of the pellet and membrane materials for this methane storage application.

[1] Song et al. Nano Lett., 2016, 16, pp 3309–3313