Solvent-free Synthesis Of Small Grain FeZSM-5 Molecular Sieves And Their Phenol Hydroxylation Properties

Hydroquinone(catechol and hydroquinone)is widely used as an important chemical intermediate in various industries.The catalytic reaction of phenol hydroxylation by iron-based zeolite molecular sieves has become a hot research topic in the field of benzenediol synthesis due to its simple reaction process,mild reaction conditions and green reaction products.The main bottleneck of iron-based zeolite molecular sieves in phenol hydroxylation reactions is the low product selectivity.Rational design of the zeolite structure and properties,as well as the nature of the iron species in the active centre,is an effective way to solve the above problems.In this study,the mechanism of accelerated zeolite crystallisation by hydroxyl radical effect and the mechanism of limiting zeolite nucleus growth by crystal growth inhibitors were combined to prepare small crystal size FeZSM-5 molecular sieves by solvent-free synthesis and evaluate their catalytic performance in phenol hydroxylation reactions.The main studies and results are presented below:1.FeZSM-5 molecular sieves were prepared under solvent-free synthesis conditions using Fenton’s reagent as a promoter and the addition of crystal growth inhibitors.The system consisted of a two-step process of low temperature(80°C)pretreatment and high temperature(180°C)crystallisation,where the low temperature pretreatment was used to accelerate zeolite nucleation.The synthesis method was improved by varying the crystal growth inhibitor addition time,the grinding method and the addition of steam-assisted crystallisation.The analytical test results of the synthesized FeZSM-5 molecular sieve showed that the FeZSM-5 molecular sieve synthesized by this method effectively reduced the crystal size and exhibited good catalytic performance in the phenol hydroxylation reaction,especially the selectivity of 91.9% for benzenediol.2.FeZSM-5 molecular sieves with different iron contents were synthesised by a solvent-free method using Fenton’s reagent as promoter,with the highest iron content of 3.67%.Analytical tests on the synthesised molecular sieves showed the simultaneous presence of isolated skeletal iron species in the iron active site and exoskeletal iron species in the low aggregated state for the molecular sieves with different iron contents.In the phenol hydroxylation reaction,a selectivity of 94.3%for benzene and a conversion of 61.7% for phenol were obtained.The catalyst was treated with nitric acid pickling and the FeZSM-5-0.08-y after pickling was characterised.The results showed that after the pickling treatment,the outer iron species of the skeleton were significantly reduced,and the selectivity of benzene and the conversion of phenol in the phenol hydroxylation reaction were significantly lower,indicating that the catalytic performance of FeZSM-5 molecular sieve in the phenol hydroxylation was a combination of skeletal and non-skeletal iron,and the skeletal iron played a major role.In this paper,hydroxyl radical effect and crystal growth inhibitors are used to prepare small grain FeZSM-5 molecular sieves,which exhibit good benzene-phenol selectivity in the phenol hydroxylation reaction.The synthesis method provides a new idea for an innovative route to the synthesis of small grain zeolite molecular sieves.