Zeolite Synthesis under Nonconventional Conditions: Reagents, Reactors, and Modi Operandi

A myriad of tetrahedral molecular sieve frameworks, often siliceous, can be calculated in silico. Only a tiny fraction (<0.1%) of these can be synthesized on purpose. Only a small fraction of these available frameworks, mostly those composed of only Si and Al as T atoms, i.e., true zeolites, are used commercially. A gap thus exists between what should be possible (thermodynamically) and what can be produced (kinetically) and used in real life. Even if a synthesis is successful (in industry or academia), flexibility with regard to synthesis parameters-in terms of time, amount of unit operations, OSDA efficiency, etc.-as well as the obtained material properties-in terms of Si/Al ratio, Al distribution, T atom variety, crystal size, etc.-remains limited. These limitations are not surprising, since conventional zeolite syntheses, i.e., hydrothermal synthesis in batch from amorphous or soluble Si and Al sources, have limited degrees of freedom (DOFs). Typically, the type of ingredients, their ratios, a constant temperature, the synthesis time, and the absence or presence of agitation are varied. In order to take new steps toward more cost-competitive syntheses, and more importantly zeolites with a greater flexibility in terms of structural properties, this Review highlights all DOFs that can be introduced in addition to or on top of the conventional way of synthesis. By doing this, a distinction is made between nonconventional DOFs that influence the chemistry of the system (e.g., interzeolite conversion, charge density mismatch, ionothermal or free-radical assisted synthesis) and nonconventional DOFs that influence the physical environment (e.g., ultrasounds, alternative energy via microwaves, or continuous setups). The Review concludes with learnings, practical insights, and future opportunities. In other words, which zeolite synthesis strategies really make a difference and which ones are just tweaking around the edges?