Preparation Of Hierarchical MEL Zeolite And Study On The Performance Of Alkylation Reaction

Hierarchical zeolite is a kind of porous catalytic material,which is widely used in adsorption,separation and chemical process due to its high specific surface area,strong acid center and good hydrothermal stability.In this paper,based on the previous work,some metal supported MEL zeolite with different Si/Al ratios were synthesized by hydrothermal method and post desilication-recrystallization,and their catalytic performance was evaluated in the liquid-phase Friedel crafts alkylation of mesitylene with benzyl alcohol.Firstly,the hydrothermal method and post desilication-recrystallization method were used to synthesize the hierarchical MEL zeolite with different Si/Al ratio.After a series of characterization,the pore structure parameters,morphology,acid properties and chemical state of iron species were explored.The catalytic effect of alkylation of mesitylene with benzol alcohol was evaluated,and the synergistic effect of acid and iron species on the alkylation process was investigated.After the process of post desilication-recrystallization,the crystal form has not been damaged,the mesoporous pore diameter was relatively uniform,the iron species was relatively uniform and mainly Fe³⁺,while the position of aluminum was relatively evenly distributed in the straight channel and cross channel.According to the activity test of liquid-phase alkylation of mesitylene with benzyl alcohol,the P-MEL has no activity,the conversion of benzyl alcohol of H-MEL was 28.24%,and the conversion of H-MEL@Fe-x benzyl alcohol was more than 70%.Therefore,the introduction of iron species greatly improves the activity of alkylation reaction.In addition,the introduction of iron species was not conducive to the alkylation process.Because of the existence of iron,oxidation-reduction properties were dominant and hinder the hydrolysis of dibenzyl ether.In general,the introduction of iron species improves the liquid-phase alkylation activity of mesitylene with benzyl alcohol,but was not conducive to the formation of BTMB.Secondly,a series of hierarchical MEL zeolite with different Si/Al ratios were synthesized by hydrothermal method and post desilication-recrystallization process,and the activity of the catalysts was evaluated in the liquid-phase alkylation of mesitylene with benzyl alcohol.After desilication-recrystallization process,aluminum was introduced to make the zeolite have Br?nsted acid,which is beneficial to the liquid-phase alkylation of mesitylene with benzyl alcohol.Hierarchical MEL zeolite mainly exist in the form of Cu²⁺,and were evenly dispersed in the skeleton.The alkylation process has a high selectivitydue to introduction of copper species promotes the activation of mesitylene,which makes it react with benzylcarbon cation to form BTMB,so the alkylation process has a high selectivity.Therefore,Br?nsted acid dominates the activity of alkylation,while copper species was related to selectivity.Thirdly,hierarchical MEL zeolite loaded with iron and copper were prepared by hydrothermal synthesis and post desilication-recrystallization.The activity of the catalysts was evaluated by the characterization of morphology,pore parameters,acidity and diffusion properties,and the liquid-phase alkylation of mesitylene with benzyl alcohol.With the increase of iron loading,the surface area and mesoporous pore volume were increased,and more Br?nsted acid was exposed.Moreover,trace metal was more evenly dispersed in the zeolite and the main forms of copper and iron species were Cu²⁺and Fe³⁺.With the increase of iron loading,the oxidation capacity increases,because the synergistic effect of bimetallic changes the oxidation-reduction capacity.After the Friedel-crafts alkylation of mesitylene with benzyl alcohol,the catalytic activity of hierarchical MEL zeolite supported only copper was low,but its selectivity was high.After loading bimetallic,the activity increased with the increase of iron loading,while the selectivity decreased sharply with the increase of iron loading.Compared with Fe³⁺,Cu²⁺has a weaker ability to form benzylcarbon positive ion due to its lower oxidation,but it activates trimethylene and reacts with benzyl carbocation,resulting in a higher BTMB selectivity,while Fe³⁺has a stronger ability to activate benzyl carbocation,which promotes the formation of dibenzyl ether by intermolecular dehydration.Dibenzyl ether can not be hydrolyzed into alkylation reagent in Fe³⁺to further react with trimethylene to form alkylation product,which results in the decrease of BTMB selectivity.In general,the activity and selectivity of alkylation can be regulated by changing the loading between copper and iron species.