Review

Function-led design of new porous materials

+ See all authors and affiliations

Science  29 May 2015:
Vol. 348, Issue 6238, aaa8075
DOI: 10.1126/science.aaa8075

You are currently viewing the abstract.

View Full Text

It's all about the holes

From kitchen sieves and strainers to coffee filters, porous materials have a wide range of uses. On an industrial scale, they are used as sorbents, filters, membranes, and catalysts. Slater and Cooper review how each application will limit the materials that can be used, and also the size and connectivity of the pores required. They go on to compare and contrast a growing range of porous materials that are finding increasing use in academic and industrial applications.

Science, this issue 10.1126/science.aaa8075

Structured Abstract

BACKGROUND

Porous materials are important in established processes such as catalysis and molecular separations and in emerging technologies for energy and health. Porous zeolites have made the largest contribution to society so far, and that field is still developing rapidly. Other porous solids have also entered the scene in the past two decades, such as metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and porous organic polymers. No single class of porous material is ideal for all purposes. For example, crystallinity and long-range order might enhance selectivity for a molecular separation while also reducing mechanical stability or processability with respect to less ordered structures. To have an impact on real applications, porous materials must be scalable and must satisfy multiple functional criteria such as long-term stability, selectivity, adsorption kinetics, and processability, all within a viable cost envelope. This presents a broad design challenge, and it requires us to be able to control structure and to understand multiple structure-property relationships at a detailed level.

ADVANCES

In addition to MOFs, COFs, and porous polymer networks, other classes of molecular porous solids have emerged in the past 10 years, such as polymers of intrinsic microporosity and porous organic cages. The range of possible functions for porous solids is thus much broader than before. For example, conjugated microporous polymers and some COFs have extended, conjugated structures that are not present in zeolites or MOFs and have led to porous organic photocatalysts and electronic materials. The crystal engineering approaches developed for zeolites, MOFs, and COFs cannot be applied directly to amorphous solids such as porous polymers, but analogous modular strategies have allowed functions such as porosity and electronic band gap to be controlled by choosing the appropriate molecular building blocks. Rapid advances in the computational prediction of structure and function offer a strategy for identifying the best porous materials for specific applications, for example, via large-scale screening of gas adsorption in hypothetical MOFs.

OUTLOOK

Advances in synthesis have produced new classes of functional porous solids as well as fundamental breakthroughs in areas such as selective carbon dioxide capture, molecular separations, and catalysis. As yet, these rapid developments in basic understanding are unmatched by large-scale commercial implementation, but enhanced functions (such as enzyme-like CO2 selectivity) and new processing options (such as soluble porous solids) present exciting opportunities. A general challenge will be to reengineer porous materials where scale-up is prohibited by cost, retaining the advanced function but using cheaper and more sustainable building blocks. It is therefore important to develop structure-property relationships to understand how promising materials work. Not all future opportunities for porous solids involve improving on existing materials or the development of more scalable preparation routes. For example, porous photocatalysts that can perform direct solar water splitting might provide a completely new platform for energy production. As we seek increasingly complex functions for porous materials, the use of in silico computational design to guide experiment will become more important.

Porous materials can be defined by type or by function, but it is function that will determine the scope for practical applications.

Our ability to design functions in porous solids has advanced markedly in the past two decades as a result of developments in modular synthesis, materials characterization, and (more recently) computational structure-property predictions. This figure is based on the pore channels, shown in yellow, for an organic cage molecule, a new type of solution-processable porous solid developed over the past 6 years.

Abstract

Porous solids are important as membranes, adsorbents, catalysts, and in other chemical applications. But for these materials to find greater use at an industrial scale, it is necessary to optimize multiple functions in addition to pore structure and surface area, such as stability, sorption kinetics, processability, mechanical properties, and thermal properties. Several different classes of porous solids exist, and there is no one-size-fits-all solution; it can therefore be challenging to choose the right type of porous material for a given job. Computational prediction of structure and properties has growing potential to complement experiment to identify the best porous materials for specific applications.

View Full Text