Green Synthesis Of FeAPO-5 Molecular Sieves With High Crystallinity And Iron Contents And Their Catalytic Performance In Phenol Hydroxylation

Aluminosilicate and aluminophosphate zeolites are two of the most important crystalline microporous materials,among them,aluminophosphate molecular sieves?AlPO₄-based?as the third generation of molecular sieve family,which were firstly synthesized in 1982,recently attracted much attention from the researcher.Owing to their unique frameworks with pores and channels,they have been widely used in different areas of chemical industry such as adsorption and separation,ion exchange and catalysis.Generally,these fascinating materialscanbesynthesizedbyseveralsynthesismethods,including hydrothermal/solvothermal and ionothermal synthesis.To promote the application of these zeolitic materials,the scientists in the zeolite community need to solve a series of problems,of which to synthesize zeolites in a safe and cost-saving manner is the frst and the most important one.Recently,several novel synthetic methods,including ionothermal synthesis and solvent-free synthesis have been successfully developed,which can well address the safety and cost issues of zeolite synthesis.In this work,AFI-type iron-based aluminophosphate molecular sieve with high crystallinity and iron content was synthesized by modified ionothermal and solvent-free methods.The main contents are as follows:In the present work,FeAPO-5 aluminophosphate molecular sieve with high iron content was ionothermally synthesized by microwave irradiation at ambient pressure using low dosage of eutectic mixture as the solvent.The influences of seed crystals and mechanochemical pretreatment on the synthesis of FeAPO-5 were investigated in detail.The resultant relatively pure FeAPO-5 molecular sieves were characterized by XRD,ICP-AES,SEM,N₂ physisorption and UV–Vis diffuse reflectance spectrometer,and their catalytic performances were studied using phenol hydroxylation as a probe reaction.The XRD results revealed that pure FeAPO-5 can be readily obtained from mechanochemically pretreated reactants,regardless of whether or not the seed crystals were added.The ICP-AES measurements suggested that only by using specifc seed crystals can pure FeAPO-5 molecular sieve with high iron content be synthesized.The SEM analysis implied that seed crystals may play a role of morphology-directing agent in these syntheses.The catalytic results demonstrated that longer pronounced induction period were only observed over those FeAPO-5 catalysts with sole framework iron species.The different catalytic activity among five selected catalysts was largely determined by their different densities of framework iron species,while the selectivity towards dihydroxybenzene was mainly controlled by the diffusion path length.Under given reaction conditions,FeAPO-5 catalyst with up to 6.79 wt%of iron content gave 42.0%dihydroxybenzene yield with a selectivity of 95.1%.Hierarchical porous FeAPO-5 catalysts with high iron content and crystallinity were successfully synthesized by a modified solvent-free method.The effects of mechanochemical and low-temperature pretreatment and the addition of hydrogen peroxide and seed crystals on the synthesis of Fe APO-5 molecular sieves were studied in detail.The relatively pure resultant products were characterized by XRD,SEM,ICP-AES,CHN analysis,TG-DTA,UV-vis diffuse reflectance spectroscopy and N₂ physisorption.The results reveal that the obtained FeAPO-5 catalysts by the solvent-free route exhibit remarkably higher crystallinity and iron content than those of reference sample by conventional hydrothermal synthesis.Moreover,N₂ physisorption measurements illustrate that the former catalysts are hierarchical porous materials with both micropores and macropores.The low-temperature pretreatment towards H₂O₂-containing precursor mixtures can significantly improve the crystallinity of FeAPO-5 molecular sieve.On the other hand,no any beneficial effects were observed when mechanochemical pretreatment and seed crystals were used to modify the solvent-free synthesis.The catalytic performances of five selected FeAPO-5 catalysts were compared using the phenol hydroxylation as a probe reaction.Among these catalysts,the one with the optimum density of framework iron species exhibits the best catalytic performance,giving phenol conversion of about 40%and selectivity of dihydroxybenzene up to 80%at mild reaction conditions.