Synthesis of FeAPO-5 molecular sieves with high iron contents via improved ionothermal method and their catalytic performances in phenol hydroxylation

To synthesize zeolites in a safe and cost-saving manner is the first and the most important task for zeolite application in industry. 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 specific 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 94.1%.