| ZSM-5 molecular sieves have been widely used in the chemical industry because of their high catalytic activity,good hydrothermal stability and unique three-dimensional pore structures which can provide abundant channels for catalytic reaction and facilitate the timely diffusion of reactants and products.At present,most of the ZSM-5 molecular sieves are synthesized through using organic amines as the template,but such methods can cause serious environmental pollution and energy consumption and increase production costs at the same time.Therefore,the template-free synthesis of ZSM-5 molecular sieves has received extensive attention in recent years.In this article,ZSM-5 molecular sieves were prepared by three methods in the template-free system.Firstly,one-step method was used to explore the suitable ratio for synthesizing ZSM-5 molecular sieves.The effect of SiO2/Al2O3ratio on the morphology of ZSM-5 was investigated and the nano-ZSM-5 was successfully prepared.Then ZSM-5 molecular sieves were synthesized by two-step method and seed-induced method respectively.Besides,XRD,SEM,TEM and Py-IR were utilized to characterize the structures,morphologys and acidic properties of molecular sieves,and the effects of different crystallization processes on the morphology,crystal structure and acidity of ZSM-5 were investigated.The conclusions are as follows:1)In the template-free system,a series of ZSM-5 molecular sieves were successfully prepared by one-step method through adjusting the alkalinity,water content and SiO2/Al2O3 ratio in the synthesis solution when the mole ratio of ZSM-5 sol was 12Na2O:（70140）SiO2:2Al2O3:2500H2O which processed by aging at room temperature for 24 hours and then crystallizing at 180℃for24h48h,and the effects of SiO2/Al2O3 ratio on the structure,morphology and acidity of ZSM-5 were investigated.The results show that when NaAlO2 is used as the aluminum source,ZSM-5 nanocrystals of about 100nm can be obtained at the condition of SiO2/Al2O3=60 and 70;when SiO2/Al2O3=40 and 50,uniform-sized and about 20μm?10μm ZSM-5 braided large single crystal with a layered roughness on the surface can be obtained.Compared with the commercial ZSM-5 molecular sieve of SiO2/Al2O3=38（hexagonal columnar crystals about 2.5μm?1.5μm）,the B,L acid increase,B/L ratio decrease,which indicates that in the case of lower SiO2/Al2O3 ratio and no organic template involved in crystallization,it is easier to synthesize big single crystal which contains large amount of acid and low B/L ratio.One the other hand,when pseudo-boehmite is used as the aluminum source,the ZSM-5 molecular sieve can be obtained at SiO2/Al2O3=35,40 and 50 which is incompletely crystallized,including hexagonal columnar large single crystals of about 10μm?8μm and incompletely crystallized cauliflower-like structure;when SiO2/Al2O3=60,uniform ZSM-5 spherical crystals of about 50nm can be obtained.Compared with ZSM-5 synthesized by NaAlO2 as the aluminum source under the same SiO2/Al2O3 ratio,the amount of B acid increases,the amount of L acid decreases,and the ratio of B/L increases.Thus,this indicates that the pseudo-boehmite has higher activity as the aluminum source and it is difficult to control crystal growth under low SiO2/Al2O3 ratio,but it is easier to obtain nanocrystals which have a smaller size and a higher B/L ratio when the SiO2/Al2O3 ratio is higher.2)In the system without organic template,ZSM-5 molecular sieves were synthesized by the two-step method and the mole ratio of ZSM-5 sol was12Na2O:（80、140）SiO2:2Al2O3:2500H2O.When NaAlO2 is used as the aluminum source,ZSM-5 can not be obtained with SiO2/Al2O3=100;the optimal crystallization process corresponding to SiO2/Al2O3=70 is pre-crystallization at220°C for 5h and then re-crystallization at 180°C for 48h.And ZSM-5 single crystal has a hexagonal columnar structure with a grain size of about 10μm?5μm,but part of it is not crystallized completely;the optimal crystallization process corresponding to SiO2/Al2O3=40 is pre-crystallization at 220°C for 4h,then re-crystallization at 180°C for 40h.And the ZSM-5 single crystal has a braid-like structure with a grain size of about 15μm?8μm.Compared with one-step method,the particle size is similar and the crystallization is more complete.While compared with the commercial ZSM-5 with SiO2/Al2O3=38,the total acid amount increases and the B/L ratio decreases,which is the same as the result of one-step method.When pseudo-boehmite is used as the aluminum source,ZSM-5 can not be obtained with SiO2/Al2O3=70 and 100;the optimal crystallization process corresponding to SiO2/Al2O3=40 is pre-crystallization at200°C for 4h and then re-crystallization at 180°C for 24h.The ZSM-5 single crystal has a flat hexagonal prism shape with a size of about 5μm?3μm.Compared with one-step method,the grain size is reduced but the crystallization completeness is not improved.At the same time,the two-step method shortens the crystallization period of ZSM-5 molecular sieve in the template-free system.3)In the template-free system,the seed-induced method was applied to synthesize ZSM-5 successfully when NaAlO2 is the aluminum source,mole ratio is 12Na2O:（80、140、200）SiO2:2Al2O3:2500H2O and crystallizing at180℃,and the effects of SiO2/Al2O3 ratio on the structure,morphology and acidity of ZSM-5 were investigated.When SiO2/Al2O3=100,the crystallinity of ZSM-5 is low and most of the crystal grains are not crystallized;when SiO2/Al2O3=70,hexagonal columnar ZSM-5 grains with a size of about8μm?3μm are synthesized and long strip of mordenite crystal attached to the surface;when SiO2/Al2O3=40,ZSM-5 particles have a uniform braid-like structure with a size of about 5μm?3μm.Compared with commercial ZSM-5molecular sieve with SiO2/Al2O3=38,the amout of B and L acid is greatly increased,the B/L ratio is greatly reduced and the percentage of strong acid is basically similar.In summary,the seed-induced method greatly shortens the synthesis cycle of ZSM-5 in the template-free system,improves the crystallinity of products and provides a possibility for synthesizing ZSM-5 with high SiO2/Al2O3 ratio in the template-free system. |
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