Synthesis Of Hierarchical Metal-containing Zeolite And Its Application In Phenol Hydroxylation

Dihydroxybenzenes are important chemical intermediates,which can be widely used in medicine,dyes,pesticides,etc.Phenol hydroxylation,as an environmental-friendly and efficient technology to produce dihydroxybenzenes,is recognized by many researchers.Metal-containing MFI-type zeolites,mainly including TS-1,Fe-ZSM-5,etc.,show good catalytic performance in the reaction.In this work,different metal-containing MFI-type zeolites were synthesized,and the catalytic activity of these samples in phenol hydroxylation was studied.In order to improve the catalytic activity of the samples,desilication method was used to introduce mesopores into the zeolites.The main works of this article are as follow:（1）S-1,ZSM-5,TS-1,Fe-S-1 and Fe-TS-1were synthesized through a one-step hydrothermal method.The physicochemical properties of the prepared samples were investigated by X-ray diffraction,N₂ adsorption-desorption,Field-emission scanning electron microscopy,Fourier transform infrared spectroscopy,Ultraviolet visible spectroscopy,etc.The catalytic performance of these samples was evaluated by conducting phenol hydroxylation experiments.It is found that S-1 and ZSM-5 showed no catalytic activity,while framework Fe and framework Ti were considered the active sites for phenol hydroxylation.Compared to TS-1,Fe-S-1 and their mixture,Fe-TS-1 showed higher conversion and selectivity,indicating there might be a synergistic effect between framework Ti and framework Fe in phenol hydroxylation.After the reaction was conducted five times,the catalytic performance of Fe-TS-1 was similar to that in the first reaction,which illustrated the sample owned good stability.（2）Fe-TS-1 samples with different iron content were prepared,and were desilicated by Na OH as the alkali media,which introduced mesopores into the samples.It is found that the manipulation of mesopores can be achieved by changing the amount of Fe.The more amount of iron species,the smaller volume and diameter of mesopores would be,indicating that the iron species could protect the structure of zeolite during Na OH desilication.By comparing the physicochemical properties of the acid-treated samples and the untreated samples after desilication process,the iron oxides,which could be removed by acid treatment,were found to show no protection to the skeleton structure of the zeolite.The protective effect of iron species on the structure was mainly caused by the framework Fe of the sample.（3）A series of hierarchical MFI-type zeolites were prepared through desilication and used as catalysts in phenol hydroxylation.The reaction results showed that the desilication process could effectively improve the catalytic performance by introducing abundant mesopores.Among all the samples,Fe-TS-1（50）-D showed the best catalytic performance with a phenol conversion of 45.2%and a dihydroxybenzenes selectivity of 95.2%,when the phenol/H₂O₂ molar ratio was 1:1.Fe-TS-1（50）-D still showed an excellent catalytic performance and basically unchanged metal distribution after the reaction was conducted five times,which confirmed the good stability and reusability of the sample.