Extensive research over the past few years has been focused on the synthesis and characterization of microporous materials with high internal surface areas. Metal-Organic Frameworks (MOFs), a crystalline subset of these materials, have shown promise in a wide range of applications from gas storage, chemical separations, chemical sensing, and catalysis, to ion exchange, light harvesting, and drug delivery. High internal surface area is one of the foremost attributes of MOFs and has been shown to be highly desirable in many potential applications involving catalysis or storage.

—Farha et al.

MOFs are composed of of organic linkers held together by metal atoms, creating a molecular cage-like structure. Currently, two MOFs with experimentally accessible BET surface areas slightly above 6,000 m²/g have been reported: MOF-210 and NU-100.

Many researchers believe that the surface areas for NU-100 and MOF-210 are close to the ultimate experimental limit for solid materials, the authors noted in their paper.

This belief no doubt stems from: a) simulations showing that the upper theoretical limit for MOF surface areas is about 10,500 cm²/g when linkers are constructed from repeating phenyl groups,25,34 and b) anticipated practical problems, such as poor solubility, low synthetic yields, and cumbersome purification protocols, for candidate linkers featuring very large numbers of phenyl repeat units. We reasoned, however, that both the experimental maximum and the perceived theoretical ceiling could be substantially increased by moving beyond phenyl-only struts to more “area-efficient” building blocks for MOF linkers.

—Farha et al.

The Northwestern team focused on paddlewheel connected MOF networks (rht-topology). A key feature of this is that catenation (interpenetration or interweaving of multiple frameworks) is mathematically precluded. Capitalizing on this topology, they synthesized two new materials with ultra-high surface areas, NU-109 and NU-110, from two new hexa-carboxylated linkers.

The computational modeling results clearly show that the strategy of using progressively more acetylenes in the organic linkers of MOFs, whether alone or with other molecular sub-units, has the potential of creating ordered structures with surface areas substantially higher than any previously envisioned for metal-organic framework materials. Importantly, it seems reasonable to conclude that even the record-high surface area of 7,140 m²/g for NU-110 does not define the practical experimental upper limit for surface areas of porous materials, as it corresponds to only about 49% of the theoretical upper limit for MOFs featuring acetylene-rich linkers. Indeed, it is conceivable that other linker motifs–for example, ones based on extended polyenes, or on connecting-atoms that are lighter than carbon–could yield even higher computational ceilings for surface areas.

—Farha et al.

Resources

• Omar K. Farha, Ibrahim Eryazici, Nak Cheon Jeong, Brad G Hauser, Christopher E Wilmer, Amy A. Sarjeant, Randall Q. Snurr, SonBinh T. Nguyen, Ahmet Özgür Yazaydin, and Joseph T. Hupp (2012) Metal-organic Framework Materials with Ultrahigh Surface Areas: Is the Sky the Limit? Journal of the American Chemical Society doi: 10.1021/ja3055639