Synthesis And Catalytic Performance Of Nanosized Beta Molecular Sieves

As a by-product of biodiesel,the excess production of glycerol has become an urgent problem for the sustainable development of biodiesel industry.At present,Beta molecular sieves is considered as one of the most promising catalysts for glycerol dehydration.However,at present,the key technical problems restricting the industrial application of Beta zeolite catalysts are easy coking and poor stability.Because of its larger specific surface area,shorter pore length and more exposed catalytic active sites,nano-Beta molecular sieves can significantly reduce the rate of carbon deposition and improve the catalytic efficiency of molecular sieves.However,the nano-molecular sieves prepared by current methods usually have poor crystallinity and hydrothermal stability,low solid yield and high synthesis cost,which is not conducive to catalytic reaction and industrial production.As we all know,the crystal morphology and particle size of Beta molecular sieve have great influence on its performance.In this paper,based on the development of nanometer Beta molecular sieve catalyst and the study of the relevant factors affecting the catalyst activity,the corresponding nano-Beta molecular sieve catalyst was synthesized by changing the preparation conditions.Through performance evaluation and structure characterization,the effects of various synthesis conditions on the crystal morphology,size and structure of molecular sieves were systematically studied,with emphasis on the effects of pore size,pore structure,acid content,acid strength and reaction conditions on the dehydration performance.The main contents and results are as follows:Firstly,in the traditional hydrothermal synthesis system,using tetraethyl orthosilicate as silicon source and tetraethyl ammonium hydroxide as template,the morphology and particle size of molecular sieve were characterized by scanning electron microscope（SEM）and X-ray diffraction（XRD）.One-step method and two-step variable temperature method were used to explore the effects of crystallization time,crystallization temperature and crystallization steps on the crystallization process of Beta molecular sieve.To realize the regulation of molecular sieve morphology and particle size.It is found that,compared with the traditional one-step method,the crystal size of the two-step method is much smaller than that of the one-step method under the same conditions,which greatly shortens the crystallization time needed for the preparation of molecular sieves and saves energy.The process of glycerol dehydration was optimized with the catalyst prepared by crystallization at 80℃and 24h+140℃for 24 h.The glycerol dehydration reaction showed a high conversion rate of glycerol.The optimum conditions were as follows:reaction temperature 320℃,feed rate 0.10m L/min,glycerol content 10wt%.Then,on the basis of two-step temperature change,the effects of silicon source,Si-Al ratio,water-to-silicon ratio and template dosage on the synthesis of Beta molecular sieve were further investigated.By studying the effects of silicon source,Si/Al ratio,water/Si ratio and Si/Si ratio on the synthesis under two-step variable temperature synthesis,the optimum crystallization conditions are obtained as follows:the gel group is1.0Si O₂:0.5TEAOH:0.02Al₂O₃:0.08Na₂O:20H₂O,synthesized Beta molecular sieve has smaller crystal size,more acid content,faster diffusion efficiency and more active sites,which shows better catalytic activity in glycerol dehydration reaction.Finally,based on the optimal preparation conditions of two-step variable temperature method,nano-Beta zeolites were prepared by comprehensively using the adsorption of surfactants and two-step crystallization of Beta zeolites.Nano-Beta zeolites were prepared by hydrothermal synthesis using five typical surfactants combined with variable temperature crystallization,and applied to catalyze glycerol dehydration to propenal.The best Nonionic surfactant polyoxyethylene（15）oil ether molecular sieve and its dosage were selected.the molecular sieve has micro-mesoporous structure and nano-size beta molecular sieve and shows high catalytic activity and catalyst life in glycerol dehydration reaction.