Zeolites have broad application prospects in adsorption, separation, catalysis, etc.. Improving their synthesis efficiency and reducing their production cost become a challenge. The synthesis rate of zeolite is low and the production period is long due to the prolonged nucleation and growth of crystals during the crystallization process. Furthermore, commercial high-quality zeolites are mainly synthesized with chemicals as raw materials, resulting in high costs. Therefore, it is significant to develop efficient and low-cost zeolite synthesis technologies. Some previous studies indicate that the use of free radicals could achieve a rapid synthesis of zeolites. Fly ash contains a large amount of silicon and aluminum elements. As a result, the synthesis of zeolite from fly ash reduces costs, and provides a feasible way for its high-value utilization. However, there is little work on the synthesis of zeolite from fly ash assisted by free radicals. It is thus necessary to explore the mechanism of free radical assisted synthesis of zeolite from fly ash, as well as the generation, types and properties of free radicals. Meanwhile, it is revealed that free radicals solve the problem of difficult activation of inert silicon and aluminum in fly ash. The theoretical basis is provided for the development of efficient and low-cost synthesis technology for the preparation of zeolite from fly ash assisted by free radicals, and a reference is provided for other coal based solid waste.

The existence of ·OH in zeolite synthesis systems was firstly discovered in 2016. ·OH was introduced into the synthesis system of NaA, Y, and ZSM-5 zeolites, thus greatly accelerating their crystallization process. This is a high-efficient method for zeolite synthesis besides a microwave method. Meanwhile, electron paramagnetic resonance (EPR) was used to determine hydroxyl radicals, and density functional theory (DFT) was used for the theoretical calculations to investigate the mechanism of hydroxyl radicals accelerating the synthesis rate of zeolite. The catalytic activity by ·OH is greater than that by OH^- for the depolymerization and polymerization of Si—O—Si and Si—O—Al bonds during the nucleation stage, especially in terms of depolymerization. With the continuous deepening of research, the promoting effects of other oxidizing active substances (i.e., reactive oxygen species, superoxide radicals, singlet oxygen, etc.) in zeolite synthesis were also discovered. Moreover, the introduction of free radical methods also opened up a new path for the synthesis of high-quality zeolites from some low-cost silicon aluminum sources with a lower activity. The problem of difficult activation of silicon and aluminum during the synthesis of zeolites from fly ash was discovered, and the positive effect of free radicals on the activation of inert silicon species was demonstrated through the simulation calculations. It is thus speculated that free radicals can improve the problems faced by coal-based solid waste synthesis of zeolite, represented by fly ash. However, little work on the relevant experimental research are made yet, and the further verification is needed. Therefore, free radical introduction methods suitable for fly ash synthesis of zeolite system were discovered, i.e., physical and chemical methods. Among them, adding initiators to produce free radicals in zeolite synthesis using chemical methods is a more feasible method. However, there exist still problems, i.e., some initiators unavailable for the alkaline environment of zeolite synthesis. The research on free radical generation methods suitable for zeolite synthesis environment will provide possibilities for constructing feasible methods for preparing zeolite from low-grade silicon aluminum sources, reducing the synthesis costs, and achieving the green development. Furthermore, some impurities existing in zeolization of fly ash are effectively removed by calcination and acid washing, thereby increasing its silicon and aluminum content, which is conducive to the synthesis of zeolites.