Role Of Seed In Crystallization Process Of Sn-Beta Molecular Sieve

Seed plays an important role in the synthesis process of artificial zeolite.The addition of seed crystal in the synthesis of zeolite molecular sieve can shorten the crystallization time,reduce the formation of hybrid crystals,reduce the crystal size of zeolite,realize the synthesis of zeolite without template and induce the crystal transformation between different zeolite molecular sieves.As a solid acid catalyst,Sn-Beta molecular sieve can catalyze MeerweinPonndorf-Verley reaction and Baeyer-Villiger oxidation reaction via activation the carbonyl group of substrate molecules;Its Lewis acidity can also catalyze the reactions of biomass derived substrates,such as sugar isomerization,retro-aldol reaction,etc.SnBeta molecular sieves,which was synthesized by hydrothermal method in fluorinecontaining system,was considered to be the best catalyst for the conversion of sugar to lactate.Seed-assisted hydrothermal method has the advantages of shortening the crystallization time and reducing the formation of heterocrystals.However,the role of seed in the synthesis of molecular sieves is not clear,which needs to be further studied.This thesis studies the role of seed in the synthesis of Sn-Beta molecular sieve.Using pure Si-Beta molecular sieve as seed,Sn-Beta molecular sieve was prepared by hydrothermal method in fluorine medium.XRD and FT-IR were used to detect the changes of seed in the synthesis process of molecular sieve.The results showed that Si-Beta would dissolve and form fragments in the initial stage of crystallization.The fragments acted as crystal nucleus to accelerate the crystal growth process.Sn-Beta molecular sieves were synthesized by using Si-Beta containing template agent and without template agent as crystal seeds.It is found that the promoting effect of Si-Beta without template as seed on the crystallization process is better than that with template,because Si-Beta without template is easier to form seed fragments.SEM and deuterated acetonitrile adsorption infrared characterization results show that the acidity of Sn-Beta molecular sieve synthesized by seed-assisted method is similar to that synthesized by none seed method.The crystal size of Sn-Beta molecular sieve synthesized by seedassisted method is much smaller than that synthesized by none seed method.The crystal size of the synthesized Sn-Beta molecular sieve is related to the amount of crystal nuclei in the gel.Therefore,the synthesis of nano-sized Sn-Beta molecular sieves was studied by adjusting the amount of crystal nuclei.Si-Beta which was treated by hydrothermal method with different concentrations of tetraethylammonium hydroxide was used as seed.Using pure Si-Beta without template treated by 0.5 M tetraethylammonium hydroxide solution as crystal seed,Sn-Beta molecular sieve with minimum crystal size(about 500 nm)can be obtained.The supernatant and solid residue of the above seed mixture was used as seed to synthesize Sn-Beta,respectively.It was found that the crystal size distribution of the sample synthesized by supernatant as seed was narrower than that synthesized by solid residue as seed.