| Methanol to hydrocarbons?MTH?reaction,using methanol as raw material,is a process of directional synthesis of hydrocarbons like gasoline,olefins and aromatics over acid zeolite catalyst.ZSM-5 zeolite possesses ordered channel structure and strong acidity,showing excellent reactivity and product shape-selection in the MTH reaction.However,the pore channels of ZSM-5 are mainly micropores with small pore size?0.55 nm?,resulting in the difficulty for coke precursor to diffuse outside.Thus formed coke would cover the active sites and even block the channels,leading to the deactivation of ZSM-5 catalyst.Introducing mesopores into nano-ZSM-5 could further shorten the diffusion path length and improve the mass transfer performance of catalyst.Alkaline is commonly used to create mesopores as a simple and efficient method.But the conventional alkaline is seriously affected by the existence of Al distribution gradient.The formed mesopores are usually uneven.Furthermore,the alkaline would destroy the framework structure.It is the focus of finding a new mesoporous construction method in the study of hierarchical ZSM-5.In this work,a series of hierarchical ZSM-5 were synthesized by desilication-recrystallization,during which the tetrapropylammonium hydroxide?TPAOH?solution with Al source was added to the silicalite-1 zeolite.Thus prepared ZSM-5 possesses high catalytic stability.The microstructures of ZSM-5 were investigated by characterization analysis and their structure-activity relationship of MTH catalytic performance was defined by reaction evaluation.Main conclusions are as follows:?1?By comparing the ZSM-5 prepared by desilication-recrystallization of two silicalite-1 with different crystal sizes,it was found that the short diffusion path of the silicalite-1 with the crystal size of 200 nm made the desilicalition-recrystallization process easier,finally formed the hollow nano-ZSM-5 with mesopores in its shell.Whereas,the recrystallization of silicalite-1 with the crystal size of 900 nm was more difficult because of the long diffusion path,which need more liquid-to-solid ratio of TPAOH/silicalite-1 to obtain enough acidity for the micro-ZSM-5 with uniform mesopores in the bulk phase.The 29Si MAS NMR analysis revealed that the Si?4Si,0Al?and SiOH sites were reduce and the Si?3Si,1Al?was newly formed during the desilication-recrystallization.This indicated that not only the mesopores were directly constructed by this method,but also the acid sites of ZSM-5 could be directly formed.The MTH catalytic performance results showed that for highly mesoporous micro-ZSM-5 with large external surface area of 171 m2·g-1,its catalytic lifetime reached 192 h under the condition of WHSV of 4.7 h-1,four times greater than that of the micro-ZSM-5synthesized via the traditional hydrothermal synthesis.?2?Through adjusting the additive amount of NaAlO2 during the desilication-recrystallization of silicalite-1 with the crystal size of 200 nm,the SiO2/Al2O3 ratio of zeolites was changed.It was found that with the SiO2/Al2O3feed ratio decreased from 400 to 25,the acid amount of ZSM-5 gradually increased from 0.13 to 0.91 mmol·g-1 and the acid strength also enhanced.The mesoporous structure of ZSM-5 were determined by the change of Na+content in the solution caused by the variation of NaAlO2 amount.When the SiO2/Al2O3ratio?100,ZSM-5 exhibited simple hollow structure because of the weak adsorption of Na+.While the mesopores were created in the shell of ZSM-5 with decreasing the SiO2/Al2O3 ratio?SiO2/Al2O3?50?because of the enhanced competitive adsorption of Na+on surface with TPA+.Specially,the sample with SiO2/Al2O3 feed ratio of 50 exhibited largest external surface area of 133 m2·g-1,in addition to the proper acidity of 0.66 mmol·g-1,endowing it a longest lifetime of 132 h in MTH reaction under a WHSV of 8 h-1.Whereas,for the sample with SiO2/Al2O3 ratio of 25,the too strong acidity(0.91 mmol·g-1)would aggravate the reaction,resulting in its rapid deactivation and its lifetime was only 29 h.The Py-IR results demonstated that the strong Br?nsted acid of ZSM-5 enhanced with the decrease of the SiO2/Al2O3 ratio.This was good for the hydrogen transfer reaction,resulting that the aromatics selectivity gradually increased from 38.93 to 56.45%.?3?Two hierarchical ZSM-5 synthesized by conventional alkali treatment and desilication-recrystallization method were compared under the condition of similar Si/Al and particle size of zeolites.It was found that after the alkali treatment,the external surface area of ZSM-5 increased from 75 to 125 m2·g-1.However,the desilication mainly happened inside the bulk because of the existence of Al distribution gradient,thus the created mesopores were mainly distributed inside the crystal.Besides,alkali treatment would destroy the skeleton structure,resulting that the acid amount of ZSM-5 decreased from 0.63 to 0.57mmol·g-1.In comparison,for the ZSM-5 prepared by desilication-recrystallization,the introduced mesopores were uniformly distributed in the bulk phase of crystal and the external surface area reached 179 m2·g-1.More acid sites of ZSM-5 could be directly formed during the process of mesopores construction and the acid amount reached 0.67 mmol·g-1.Although the high acidity of ZSM-5 would aggravate the reaction,the larger external surface area,especially the more uniform mesopore distribution were more benefit for the diffusion.This made the ZSM-5 obtained by desilication-recrystallization exhibited a longer lifetime of 94h than the 70 h of ZSM-5 prepared by alkali treatment.After the deactivation and regeneration of the ZSM-5 prepared by desilication-recrystallization,its mesoporous properties was relatively easy to be recovered.Its lifetime reached149 h,1.6 times greater than that of the fresh ZSM-5. |
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