Modification Of FeAPO-5 Zeolite Catalyst And Its Catalytic Performance For Phenol Hydroxylation

The direct hydroxylation of phenol to dihydroxybenzene over iron-based zeolitic catalysts exhibits many advantages,such as cheap raw materials,mild reaction conditions,green and efficient catalytic process.This catalytic process has gradually been becoming a hot topic in recent years.However,there are still some controversies about the catalytic performance of different iron species in the iron-based molecular sieve catalysts,and the problems such as the low selectivity of hydroquinone and the easy loss of iron species in the reaction process have not been effectively solved.In this paper,FeAPO-5 molecular sieves with different iron contents were synthesized by traditional hydrothermal method.The distribution of iron species was investigated,and the performance of FeAPO-5 molecular sieves in phenol hydroxylation reaction was evaluated.Then,the synthesized molecular sieves were treated by acid washing to remove the non framework iron and determine the active components in phenol hydroxylation reaction.In order to solve the problem that the iron species of catalyst are easy to lose in the reaction process,the core-shell molecular sieve was designed and synthesized to reduce the loss rate of iron.The surface of molecular sieve was modified by hydrophilic and hydrophobic modification to improve the selectivity and yield of hydroquinone.The main results are as follows:1 A series of FeAPO-5 zeolite catalysts with different iron contents were synthesized.FeAPO-5 zeolites were characterized by XRD,SEM,ICP-AES,N₂physical adsorption and UV Vis spectroscopy.The results show that the catalyst has isolated Fe³⁺at the skeleton site and oligomeric iron species outside the skeleton.The conversion of phenol was 64.6%and the yield of hydroquinone was 37.5%.Furthermore,the results also showed that 60%of non framework iron was removed by acid pickling,and the performance of the catalyst was lower than that of the sample before treatment（the selectivity of benzenediol was 58.9%）.It can be concluded that for iron-based aluminophosphate molecular sieves,framework iron and non framework iron can catalyze the hydroxylation of phenol,and non framework iron has weak activity in the reaction.In addition,in the repeated test of the catalyst,due to the loss of more iron species,the catalytic results are quite different.2 The core-shell molecular sieves with higher specific surface area and microporous mesoporous volume were synthesized,which showed better catalytic performance in the hydroxylation of phenol.The selectivity of benzenediol was increased from 66.7%to 85.4%.The core shell molecular sieve can also be obtained by changing the feeding order of the initial gel,but it shows a lower selectivity than the nuclear phase FeAPO-5-0.4 sample.It is attributed to the single isolated Fe³⁺species.Therefore,it can be concluded that for the iron based aluminum phosphate molecular sieve,the hydroxylation of phenol is co-catalyzed by skeleton iron and non skeleton iron,and the activity of skeleton iron is much higher than that of non skeleton iron.In addition,the appearance of core-shell structure has a slow effect on the loss of iron species in catalytic reaction,and effectively protects iron species.In the reuse of catalyst,the core-shell molecular sieve still shows high conversion rate and selectivity.3 Three different modifiers,namely triaminopropyl triethoxysilane（KH-550）,cetyltrimethylammonium bromide（CTMAB）and sodium dodecylbenzene sulfonate（SDBS）,were used to modify the surface of the catalyst.The results showed that KH-550 modified zeolite exhibited hydrophobic state.The conversion of phenol was43.2%,and the yield of product was 44.9%;while CTMAB and SDBS modified zeolite made the catalytic performance weaker than before,with 48.5%and 47%selectivity of benzenediol.Therefore,hydrophobic modification on the surface of iron-based aluminophosphate zeolite is an effective method to improve the selectivity and yield of benzenediol.