Direct Synthesis And Properties Of Binder-free Hierarchical ZSM-5 Zeolite

ZSM-5 zeolite has a very broad application prospect in the industrial field,such as ion exchangers,adsorbents,catalysts,etc,attributing to its high thermal and hydrothermal stability,good acid resistance and selectivity.However,for industrial application the powder ZSM-5 zeolite needs to be shaped with binder(silica sol,meta-kaolin,aluminum glue,etc.).The introduction of binder will reduce the effective active components of the catalyst,block the pores of the zeolite,bring side reactions and other side effects.Generally,the catalytic performance of the shaped zeolite is only about 60% of the powder zeolite.Traditional ZSM-5 is microporous zeolite,the introduction of hierarchical pores can reduce the diffusion resistance of large molecules,increase the intra-crystalline diffusion rate and carbon deposition capacity,resulting to significant improvement of the catalytic performance and adsorption capacity of the zeolite.In view of this,this paper mainly explores how to directly synthesize binder-free hierarchical ZSM-5 zeolite and its catalytic performance in the methanol to propylene(MTP)reaction.The vapor phase transport(VPT)method was used to directly convert the shaped precursor into binder-free hierarchical ZSM-5 zeolite.Firstly,the effect of the crystallization time on the synthesis of binder-free hierarchical ZSM-5 zeolite and its catalytic performance in the methanol to propylene(MTP)reaction was investigated at identical temperature of 160 ℃.It was concluded that the typical MFI structure is obtained for 2 d crystallization,during the crystallization process of the VPT method which is a static crystallization process,the space vacated when the amorphous material was converted into crystals,forming a small amount of mesopore inside the crystals;the mesopore volume increased with the crystallization time owing to etching of alkali steam on zeolite crystal after 2 d and reached the maximum value for5 d;the amount of strong acid slowly decreased,while the amount of weak acid increased,with the increase of the crystallization time;the sample obtained for 5 d showed optimal catalytic performance in the MTP reaction.Then the effect of the crystallization temperature on the synthesis of binder-free hierarchical ZSM-5 zeolite and its catalytic performance in the MTP reaction was investigated for identical crystallization time of 5 d.It was concluded that shaped precursor could not form MFI structure at 120 ℃,only converted into incomplete MFI structure with defects on certain crystal face and transformed into full-crystalline ZSM-5 zeolite at 160 ℃ or above;the crystal size gradually increased,the aggregates were consisted of fewer large crystals and the surface of the aggregate gradually became smooth with the increase of the crystallization temperature at above 160 ℃,leading to reduction of contacting area with alkali in steam;it may result in the decrease of mesoporous volume obtained by etching;the amounts of strong and weak acids both increased slowly with the increase of the crystallization temperature;the sample obtained at 160 ℃ showed optimal catalytic performance in the methanol to propylene(MTP)reaction.XRD pattern of the deactivated samples showed that the ZSM-5 zeolite still holds a perfect complete MFI structure after deactivation,indicating its high structural stability and being able reused through regeneration.