Zeolite Synthesis under Nonconventional Conditions: Reagents, Reactors, andModi Operandi

Abstract

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 (DOF). Typically, the type of ingredients, their ratios, a constant temperature, synthesis time and the absence or presence of agitation are varied. In order to take new steps towards 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 non-conventional DOFs that influence the chemistry of the system (e.g. interzeolite conversion, charge density mismatch approach, ionothermal or free-radical assisted synthesis) and non-conventional DOFs that influence the physical environment (e.g. ultrasounds, alternative energy via microwaves or continuous set-ups). 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?

On the key role of aluminium and other heteroatoms during interzeolite conversion synthesis

“…1). [1][2][3] Typically this is done in a classic hydrothermal synthesis in batch (opposed to nonconventional modi operandi). 3 Oen the term 'interzeolite transformation' is also used instead of IZC.…”

“…[1][2][3] Typically this is done in a classic hydrothermal synthesis in batch (opposed to nonconventional modi operandi). 3 Oen the term 'interzeolite transformation' is also used instead of IZC. 4,5 However, certain polymorphic transformations can also be dened as 'interzeolite transformation', which may lead to confusion.…”

“…32,33 More framework discoveries are expected as the synthesis eld is taking into account more and more (unconventional) degrees of freedom (DOFs) impacting the (kinetic) route towards new framework materials and compositions. 3 Among these advanced zeolite synthesis methods, IZC is a promising one. The renewed interest in small pore zeolites, and the promising results of IZC for synthesis of the latter has invigorated the interest in IZC.…”

“…However, recent efforts have demonstrated that these simple criteria are inadequate for generalization. 3,5,40,41 A rst remark is given on the thermodynamics. Dense frameworks are indeed more stable than low density frameworks in the case of defect-free silicates, as backed by computational studies.…”

“…95 The temperatureinduced increase in kinetics are believed to be so large that reactor wall conductivity becomes a limiting factor. 3,65 The last physical DOF is mixing, which is believed to be a crucial parameter as it is a determinant for the local availability of Si and Al for further (re-)dissolution and in consecutive processes during the formation of (pre-) nuclei and crystallization. 3 The role of the last key ingredient (SDA) can be extrapolated from investigations on desilication procedures.…”