334139 Computational Screening of Microporous Organic Solids: Development of Design Principles and Analysis of BET Theory
Organic molecules and dendrimers of intrinsic microporous (OMIMs and DIMs) are amorphous, organic microporous materials, which are of interest for a variety of applications, including gas separation and storage. They incorporate rigid and awkward structures to promote inefficient packing, similar to polymers of intrinsic microporosity (PIMs). Molecular simulations have been utilized to screen an array of >30 OMIMs and DIMs with a wide variety of structures in order to guide future synthetic efforts. Analysis of the simulations has revealed the role of three structural design aspects for increasing porosity in these types of materials. First, molecular rigidity has been observed to have a profound effect on pore formation, and simulations of ideal rigid-body OMIMs have resulted in materials with surface areas larger by an order of magnitude. Second, efficient packing and molecular interpenetration is hindered by the inclusion of bulky terminal units, such as tert-butyl and adamantyl functional groups. Third, molecular structures that are more three-dimensional, instead of planar, introduce greater “internal molecular free volume” and help promote more inefficient packing to increase porosity.
In addition to providing insight into important design principles, our computational screening has enabled a closer look into the applicability of BET theory for deriving apparent surface areas from adsorption data in these types of microporous materials. Specifically, we have compared geometric surface areas measured from the simulations with BET surface areas calculated from simulated nitrogen adsorption isotherms at 77 K, as is done experimentally. Our results have shown a systematic over-prediction by the BET surface areas, especially for the ultramicroporous materials. We utilize the array of intrinsically microporous materials we have simulated, which have surface areas covering the range of 100 to >1500 m2 g-1, to improve understanding of this relationship.