Preparation Of Borosilicate Molecular Sieves And Their Catalytic Performance For Oxidative Dehydrogenation Of Propane

Oxidative dehydrogenation of propane（ODHP）to propylene has attracted much attention due to its obvious advantages of no equilibrium limitations,no carbon deposition and low energy consumption.Metal oxides catalysts,such as vanadium-based catalysts,can effectively promote oxidative dehydrogenation of propane,but it still surfer from low yield of propylene caused by over-oxidation of propylene to CO_x.Therefore,the key of propane oxidative dehydrogenation is to develop a new catalytic system,which can inhibit over-oxidation and improve the selectivity of target products.Boron nitride,as a new type of non-metallic catalyst,shows higher conversion of propane and higher selectivity of propylene in oxidative dehydrogenation of propane reaction,and produces little by-product CO₂,but it is limited by high price and low specific area,there will be much room for improving the catalytic performance by optimum and design of the boron-containing catalyst.For example,high specific surface area and porous materials can be used to disperse boron active sites.Borosilicate zeolites not only have higher surface area and well-developed pores,but also their own active boron species can catalyze oxidative dehydrogenation.In this paper,borosilicate zeolites catalysts with different topologies were prepared,and their properties and influence factors in oxidative dehydrogenation of propane were investigated.MWW topological structure borosilicate zeolites were synthesized,and the effects of calcination conditions and boron content on the reaction performance were investigated.The results showed that the sample calcined at 500oC for 12 h with the gel molar ratio of Si/B=1（boron content 2.5 wt%）exhibited the best performance.When the reaction temperature is 540oC,³0%propane conversion and⁷0%propylene selectivity can be obtained,the yield rate of propylene is 2.8×10^-3 s^-1,and the activity of the catalyst has no obvious change after 20 h stability test.The changes of boron species contents in the samples of different treatment stages were characterized by ¹¹B MAS NMR spectra,and the data were correlated with the corresponding catalytic activities.It was suggested that defective trigonal boron species B（OSi）_3-x（HO）_x（x=1,2）are the possible active sites.Borosilicate molecular sieves with MFI topology were synthesized as catalysts for oxidative dehydrogenation of propane,the key factors affecting the performance were investigated by adjusting the proportion of raw materials and hydrothermal synthesis conditions.The results show that the samples with molar ratio of Si/B=3 and hydrothermal condition of150oC for 144 h（B-MFI-5）or 160oC for 120 h（B-MFI-6）both show better performance.For instance,the conversion of propane was 10%and selectivity of total olefin（ethylene and propylene）reached to⁸3%over B-MFI-5 catalyst at 485oC;When the reaction temperature was increased to 525oC,the propane conversion was about 26%with 81%selectivity of propylene,after 10 h stability test,the catalytic activity had no obvious change.The various boron species content during the activation of B-MFI-5 sample were tracked by ¹¹B MAS NMR spectra,and the changes were correlated with the corresponding catalytic activity data.It is suggested that defective trigonal boron species B（OSi）_3-x（HO）_x（x=1,2）are the possible active sites.