1. Could you explain the significance of your article to the non-specialist?

Zeolites and related porous solids have the ability to retain (adsorb) gases. When the interaction forces involved are small, adsorption is reversible, i.e., the adsorbed gas is liberated by changing temperature (or pressure). Reversible gas adsorption can thus be used for such processes as gas separation, atmosphere pollution control, and gas storage and transport (a topical issue in the energy sector). 

Improvements on these processes can be expected from a better understanding of gas adsorption; and that is what our research is all about. By combining advanced methods of infrared spectroscopy and theoretical calculations, we hope to unveil hitherto unknown details about solid-gas interactions. 

 

2. What has motivated you to conduct this work?

Chemistry at interfaces is a fascinating subject, on which many relevant details are still poorly understood. Scientific curiosity is the main drive behind our research. However, one should also be aware that a better understanding of surface chemistry, and physics, should lead to technological advances which are much needed for sustainable development. The expectation that an in-depth understanding of solid-gas adsorption processes is in the horizon provides further motivation to explore the interplay between theoretical (and computational) chemistry and precise experimental measurements. That is why we are combining quantitative IR spectroscopy with calculations at the periodic DFT level. On account of their relative simplicity, alkali-metal ferrierites were chosen as a model system to explore the power of the method, but extension to more complex systems should be possible. 

 

3. Where do you see this work developing in the future?

Detailed understanding of reversible gas adsorption in porous solids should facilitate improvements on several technologically relevant processes. Among them, pressure-swing adsorption to separate gas mixtures (e.g. krypton from xenon), gas purification and (cost effective) storage and transport of flammable gases. In particular, the proposed use of hydrogen as an energy source (e.g. for vehicles and portable electronics) requires a means for safe and efficient storage. Trapping in microporous adsorbents is one possibility. For that, zeolites have the advantage of low cost and high stability. Their maximum loading capacity seems to be about 3% by weight (Zecchina et al., Phys. Chem. Chem. Phys., 2005, 7, 3948) and this is a disadvantage for mobile applications; but not so much for stationary usage. Other adsorbents, such as metal-organic frameworks (MOFs) show some advantages. Hydrogen uptake in some MOFs was recently reported to be as high as 7.5 wt % (A. G. Wong-Foy et al., J. Am. Chem. Soc., 2006, 128, 3494), and these materials offer good chances for tailored design and fine tuning of pore structures (for a recent review, see U. Mueller et al., J. Mater. Chem., 2006, 16, 626). Among other (related) materials, worth considering are some intrinsically microporous organic polymers. 

Aside from hydrogen storage, we also plan to apply combined theoretical and experimental procedures for detailed studies on adsorption of other gases, such as CO2 and carbon monoxide, which are ubiquitous environment pollutants (mainly) in industrial areas and large cities. And, no doubt, many other gas-solid systems merit consideration for different reasons. There seems to be ample room for creative thinking and fruitful research in this field. 

 

4. Are there any particular challenges facing future research in this area? 

There are challenges at both, the experimental and the computational level. Take, for instance, the case of hydrogen adsorption on zeolites. Alkali-metal-exchanged zeolites are known to show an adsorption enthalpy of about 5 to 10 kJ/mol. This is considerably larger than the gas liquefaction enthalpy (0.9 kJ/mol), hence favouring reversible hydrogen storage in cryogenically cooled vessels. However, a higher adsorption enthalpy would bring the operational temperature closer to ambient, with consequent advantages. Increasing intra-zeolite electric fields (using highly charged cations) should raise the gas-solid interaction energy. A further advantage can be expected from zeolites having a high concentration of extra-framework cations. However, because of the onset of multiple interactions involving adsorbed molecules, specific adsorption centres and the zeolite framework (which would develop an increased ionic character), experimental measurements become more demanding; particularly when multilayer gas adsorption is also under consideration. On the computational side, detailed understanding of complex gas-solid systems calls for methods that can duly account for dispersive forces, as well as for dynamic effects. Combined molecular dynamics and density functional theory would be desirable. On the other hand, grand canonical Monte Carlo simulations could help to understand pressure dependence of gas adsorption; and to predict relevant aspects of the expected adsorption behaviour. It should be clear, however, that this makes the theoretical approach highly demanding on both, complexity and computer time.