Canonical wisdom in zeolite science says that zeolites are, at best, thermodynamically meta-stable. Zeolites, as we have covered before, are materials that are made of little pyramids of silica (SiO2) and often alumina (AlO2) arranged in such a way that they form an ordered crystal with a complex, but ordered, network of pores running throughout it. New research suggests that certain zeolite frameworks may be thermodynamically stable.
For those who have not been fortunate enough to take a thermodynamics class, there are three regimes of stability as far a thermodynamics is concerned. First is a thermodynamically stable state, where something that left on its own will remain this way for all time—it will be stable in the thermodynamic limit. Often this is illustrated as a ball sitting in a valley: you can perturb it in any direction, but it will roll back to the bottom and sit there. On the complete opposite side is a thermodynamically unstable state—one where a small perturbation will completely disrupt the system. This can be viewed as a ball sitting atop a peak, where a small push in any direction will send it tumbling down hill. The final category, meta-stable states, can be thought of as somewhere between stable and unstable states. Using our hill/valley analogy, meta-stable states would be viewed as a little ditch on the hill side where our ball rests happily. If pushed in one direction it would roll back to its resting place, if pushed a little in the other direction it would again return to where it began. However, if it is pushed hard enough, it would bounce out of its little home and roll down to the bottom of the valley. This is how zeolites are viewed from a thermodynamic standpoint: they are trapped in some intermediate state, happy where they are, but given a kick or enough time they would find there way to a completely stable state.
New research* published on-line in advance of this week's edition of the Journal of Chemical Physics, used a computer model of silica to show that certain zeolite frameworks may actually represent thermodynamically stable states. Silica is notoriously difficult to model on a computer. Its silicon-oxygen bonds are neither purely ionic, nor purely covalent. In order to get quantitatively correct results, one needs to employ very complex equations describing these bonds. While other authors have done this, the computational cost needed to simply simulate the bonds limits what else one can then study. The authors of this work took a different approach; they developed a simplified model of silica that reduced all the complexity of each bond to a computationally very simple model. At first glance that may seem counterproductive, but prior work by the authors showed that the simple model was able to qualitatively capture a lot of the experimentally observed physical and thermodynamic properties of dense silica systems such as quartz, cristobalite, and coesite.
The new work reported here looked at not only dense forms of silica, but also included four common zeolite frameworks (there are currently over 170 recognized zeolite frameworks). Using a computing technique known as Monte Carlo simulations and some associated thermodynamic trickery; one can directly compute the free energy of a given model. A stable phase is defined as the one with the lowest Gibbs free energy at a given temperature and pressure. Armed with this, the researchers computed what phases would be stable at a range of temperatures and pressures. It was found that three of the four frameworks examined—SOD (sodalite); LTA (zeolite-A), and FAU (faujasite)—were not thermodynamically stable at any point of interest in the model, but it was found that silica, as defined by the model, arranged in a MFI (silicalite) framework (the model's rendition of this framework is pictured above) was thermodynamically stable at high temperatures and low pressures.
This is the first time such a result has been presented. Experimentally this has never been shown, but carefully carried out thermochemistry experiments have shown that at low pressures, the amount of zeolite "instability" decreases with increasing temperatures. Coupled with the results of this study, it could suggest that at even higher temperatures, certain types of real zeolites may be stable; to date, however, this has not been demonstrated experimentally. This work nicely illustrates, in my opinion, the use of computer models in modern science and engineering. The original model for silica was based on previous work on water that was adapted to silica. The authors then showed—in a previous paper—that the new model was capable of qualitatively reproducing well-known experimental results. This paper then extends that work by using the same model to attempt to make new predictions and gain new insights into complex systems. In my view, this is how computational science should work: models should be tested against known results, and only when they are shown to be capable of reproducing old work should new predictions be made with them.
* In the interest of full disclosure, I am the first author on the research being reported here.