Dual organic structure-directing agents in zeolite synthesis: cooperation or competition?

Organic structure-directing agents (OSDAs) play a vital role in the structural arrangement and compositional diversity of zeolites. The synthesis procedure and properties of zeolites can be improved through the "dual-OSDA" approach, which involves cooperation and/or competition of different OSDAs in the synthesis media. Two OSDAs achieving results that neither of the OSDAs can deliver on its own is referred to as the "cooperative OSDA" approach. In this manner, one can enhance zeolite properties by stabilizing different parts of the framework and altering the synthesis outcome, such as acidity and/or Al distribution. However, cooperation can easily be disrupted, and one of the challenges in dual-OSDA zeolite synthesis is determining the conditions under which OSDA molecules can function in harmony to affect zeolite properties and control phase selection. On the other hand, competition does not always result in negative outcomes (e.g., intergrowth materials). In this review, we discussed the importance of cooperative dual-OSDA synthesis in certain cases, explained the outcomes of this approach, and described the different behaviors key to cooperative systems.