Synthesis And Catalytic Performance Of Plate-like And Hierarchical MFI Zeolite Single Crystals

Due to unique microporous structure,high thermal stability,hydrothermal stability and unique shape selectivity,zeolites with MFI structure are widely used as catalysts for many industrial reactions.For this kind of MFI zeolite,the primary transport ways of molecules inside their crystals are straight channels parallel to the b-axis of the crystal.The zeolites coming from traditional synthesis methodology generally exist as the crystals with spherical morphology and large size.When they were applied as catalyst for the reaction involving bulky molecules,a strong diffusion restriction occurs due to the molecules have to be transported along the long microporous channels inside the crystals,which often makes the mass transfer become rate-controlling step of the catalytic reaction.Hence,for the zeolite being used as catalyst,to overcome the limitation of molecular mass transfer in their long channels,plate-like zeolites with short channels along the b-axis or hierarchical zeolites are highly desired.However,the synthesis of plate-like and hierarchical zeolites reported in literature are either suffered from environmentally unfriendly,expensive templates has to be applicated,or poor hydrothermal stability of the product,which are not conducive to the industrial application.Hence,this thesis is devoted to study the synthesis the MFI zeolite with plate-like morphology or hierarchical structure through low-cost and environmentally friendly route.A method for synthesizing plate-like pure silica zeolite silicalite-1（S-1）,titanosilicate zeolite TS-1 and aluminosilicate zeolite ZSM-5 using guanidine compounds as crystal growth modifiers was studied and proposed in this thesis.This kind of crystal growth modifiers is cheap and non-toxic,and the water amount for preparing the gel could be significantly depressed when this kind of modifiers is applied in the zeolite synthesis.Hence,the synthesis process in this method has the advantages that waste water releasing is much less,the zeolite production is low-cost and environment-friendly.Three guanidine compounds,including tetramethylguanidine（TMG）,dodecylguanidine hydrochloride（DGH）and polyhexamethylene biguanidine hydrochloride（PHMB）,modifying the crystal morphology were studied in the S-1,TS-1 and ZSM-5 zeolite syntheses,and it was found that the all of them can effectively modify the crystals growth into plate-like morphology for three MFI zeolites.Plate-like S-1zeolite with a short length along the b-axis（L_b）of 75 nm and the length ratio along the c-axis and b-axis（L_c/L_b）of 12 was successfully synthesized from a gel with the TMG/Si mole ratio of0.3 that was stirred at 80 ^oC for 24 h and crystallized at 170 ^oC for 12 h.The crystallization time（12 h）is only half of that（24 h）reported in literature,in which the plate-like zeolite was synthesized most rapidly up to date.On the other hand,the water used in the synthesis（H₂O/Si mole ratio of 14）by using this method is even less than the half of that（H₂O/Si mole ratio of30～236）reported in literature for synthesizing plate-like zeolites.Why guanidine compounds led to plate-like crystals formation for the MFI zeolite syntheses?It can be ascribed to their imino groups and/or amino groups,with which the guanidine molecules predominantly adsorbed on the（010）plane of zeolite crystals when some crystal nuclei are generated,and thereby made the crystals mainly grow along the a-and c-axis.It was found that,the DGH has the strongest ability among the three guanidine compounds being studied to modify the crystal growth,which should be related to the stronger obstruction effect of the long dodecyl chain in its molecule on crystal growth in the b-axis direction.The plate-like S-1 zeolite with L_b of 105nm,L_c/L_b of 17 was obtained from a gel with the DGH/Si mole ratio of 0.1 in the synthesis.In the thesis,it is found that,some mesopores can be introduced into the ZSM-5 zeolite crystal when guanidine compounds are used as crystal growth modifiers for the synthesis of ZSM-5 zeolite.Singe-crystalline hierarchical plate-like ZSM-5 zeolite with a large number of mesopores of 2–6 nm,L_b of 85 nm and L_c/L_b of 10 was obtained from the gel with TMG/Si mole ratio of 0.3.The catalytic performance of the zeolite in methanol-to-olefin reaction is significantly superior to that of the conventional microporous spherical ZSM-5 zeolite.It was also found that,for ZSM-5 zeolite,the L_c/L_b in the range of 2–15 could be regularly controlled by adjusting the synthesis conditions including TMG/Si mole ratio,time used for crystallization and the seeds amount being added to the gel.As for the mesopores generated in the ZSM-5zeolite single crystal,it can be considered as the spaces left from non-tight attachment and epitaxial growth of colloidal nanoparticles,as the nanoparticles mainly close to the a-c plane of the microcrystal and epitaxially grow on the microcrystal due to the directional obstruction effect of guanidine compounds adsorbed on the microcrystal surface.