(335h) MOF Straws: Development of Design Rules for Metal-Organic Framework Hollow Fibers

A current S&T thrust for the U.S. Army is the minimization of the physiological and logistical burden incurred by the Warfighter by the modernization of our protective capabilities. Among this effort are attempts to develop novel form factors that incorporate solid adsorbates in functional entities that provide lightweight alternatives to current technology and are scalable for tactical deployment. In the past several years the DoD has spent a considerable amount of time and funding for the development of metal-organic frameworks (MOFs) for their ability to mitigate the risk of highly toxic compounds such as toxic industrial chemicals (TICs) and chemical warfare agents (CWAs).\However, native MOF materials in powder form must be incorporated into functional form factors, typically as MOF/polymer composites.

Recently, we have developed and patented a process to produce robust MOF hollow fibers. Simply, a MOF/polymer solution is injected into a silicone tubing mold and allowed to dry. As the solution dries, the polymer gel contracts on itself forming the fiber. This process has been demonstrated to produce meter long fibers. Several macroscopic and microscopic parameters for this synthesis can be tuned such as the mold diameter, mold length, solvent, MOF:polymer ratio, solution concentration, and drying temperature. All these parameters can impact the MOF hollow fiber morphology and thereby impact chemical performance. It is desirable to understand how processing factors impact the hollow fiber.

Due to the large number of factors that can be varied, traditional experimental methods of changing a single variable at a time would lead to an unwieldy amount of samples needed to develop design rules. A design of experiments (DOE) process was used to minimize the amount of samples and testing. In addition to physical properties of the hollow fibers, chemical capacity and mass transfer rates are measured for the hollow fibers. Based on the results, DOE modeling can be used to develop design rules for synthesizing optimal hollow fibers.

A proposed application space for these materials is to create a hollow fiber array, similar to a monolith structure. However, the mechanical properties of the polymer will allow the arrays to be highly flexible. This type of structure could be used in the design of new protective equipment, replacing the traditional packed bed canisters used in military masks. In addition to allowing novel mask design, a filter developed from a hollow fiber array could have better transport properties resulting in lower pressure drops and better heat transfer. To explore this possibility, finite elemental modeling is employed using the software COMSOL. With this modeling, various array designs can be developed using various hollow fiber morphologies. The arrays can be optimized for chemical performance to provide predictions for experiments. Based on designs from the array modeling and design rules for hollow fiber synthesis, a hollow fiber array will be produced as proof-of-concept in developing a flexible filter capable of replacing packed beds.

Acknowledgements:
Funding was provided by the U.S. Army via the Chemical Biological Advanced Manufacturing Program (PE 060112A Project VR9) at the Combat Capabilities Development Command Chemical Biological Center.