自由基辅助粉煤灰合成沸石分子筛的研究进展

Research Progress on Free Radical Assisted Synthesis of Zeolite from Fly Ash

沸石分子筛(简称沸石)在吸附、分离和催化等领域具有广阔应用前景,但沸石合成速率慢、工业化生产效率低;同时高品质沸石主要由化学原料合成,制备成本高,因此,利用自由基提高合成速率,以粉煤灰为硅铝源降低合成成本,对沸石的工业化生产和大宗固废的消纳具有重要的现实意义。本文对自由基促进沸石合成的作用机理进行调研,同时探究自由基促进粉煤灰硅铝活化的可能机制,挖掘自由基在沸石合成体系中的产生及存在,并针对粉煤灰来源广、成分杂问题,系统梳理了各类预处理方法,在此基础上,提出了利用自由基有望解决粉煤灰中硅铝活化难的问题,这将为粉煤灰大规模、高值化利用提供可能途径。

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.Summary and prospects Compared to OH~-,the breaking and formation of Si—O—Al and Si—O—Si bonds were achieved with lower energy barriers in the presence of free radicals,thus achieving the efficient synthesis of zeolites.Fly ash as a raw material could effectively reduce the cost of synthesizing zeolite.However,activating these inert silicon and aluminum was a challenge for synthesizing zeolite from fly ash.Free radicals played a catalytic role in the cleavage and formation of Si—O—Si bonds in inert silicon species.It could be inferred that free radicals could play a similar catalytic role in the synthesis of zeolite from fly ash.The excavation of free radical introduction methods (i.e.,using chelates to chelate Fe~(2+)to broaden the pH range of the Fenton reagent applications or using persulfate ions) could make up for the lack of experimental research on free radical assisted fly ash synthesis of zeolite.Besides,for fly ash with wide source and complex composition,using calcination and acid washing could effectively remove some impurities in the fly ash to improve the product quality.A reference for the resource utilization of other similar coal based solid waste could be provided via studying the free radical assisted synthesis route of zeolite from fly ash.In addition,widening the silicon aluminum ratio of fly ash and expanding the range of synthetic zeolite are also a challenge in the future.