Preparation,Optical Properties And Propane Dehydrogenation Over Doped Graphitic Carbon Nitride

C₃N₄is a semiconductor material with good stability,low price and low toxicity.It has become an important material in contemporary catalysis and can be widely used in multiphase catalysis,photocatalysis and organic degradation.However,the application of C₃N₄in catalysis is limited due to its small specific surface area,narrow response range of visible light and low charge transfer ability.In order to solve these problems,element doping of C₃N₄material can change its morphology,crystal size,widen its band gap,adjust the optical properties of C₃N₄material,and increase the catalytic active site.In addition,propylene is an important raw material in the petrochemical industry,and the current market is in short supply.Propane dehydrogenation to propylene is a strong competitor in the current propylene supply market due to its cheap raw material,simple process and easy operation.However,the existing propane dehydrogenation to propylene process package and catalyst,are monopolized by foreign companies,high patent costs;There is still a certain gap between Pt and Cr catalysts currently studied and those that have been industrialized.In order to develop a new catalyst for propane dehydrogenation,this paper prepared a unique C₃N₄catalyst by doping V and Cr in C₃N₄material to change the characteristics of the catalyst,and used in the catalytic reaction of propane dehydrogenation.The main work contents are as follows:（1）With urea as constant quantity,C₃N₄materials with low vanadium load and high vanadium load were prepared by controlling the addition amount of sodium vanadate and adjusting the ratio between sodium vanadate and urea,and the corresponding optical and physical properties were explored.Using XRD,SEM,FTIR,PL,UV-VIS characterization,in the infrared band,visible band,UV band.It is found that the C₃N₄materials doped with V exhibit different properties.The grain size of C₃N₄material with low vanadium content doping was smaller and dispersed with the increase of vanadium content,and V appeared in the form of VO_xspecies.When C₃N₄material with high vanadium content is doped,V mainly appears in the form of V₂O₅and VO₂mixture with the increase of vanadium content.With the increase of vanadium addition,the morphology changes from fine block to rod structure.By doping V in C₃N₄,it is proved that the band gap of C₃N₄can be adjusted by doping V.The work in this part also lays a foundation for further exploration of the optical properties of V-doped C₃N₄materials.（2）A series of V-doped g-C₃N₄catalysts were prepared by combining the thermal decomposition of urea and sodium metavanadate.The catalytic performance of these catalysts was evaluated in the propane dehydrogenation to propylene plant.The results of TEM,SEM,XRD,FTIR,Raman and XPS analysis showed that the VO_xspecies was successfully introduced into g-C₃N₄and maintained its layered structure during the process of V doping.Catalyst performance evaluation is as follows:When the reaction temperature is 600℃,the propene selectivity of V₁-g-C₃N₄nanosheet is 68.6%and the propene yield is 16.16%.The propene yield of g-C₃N₄nanosheet without doping V is5.42%,and the propene yield of V₁-g-C₃N₄nanosheet is three times that of g-C₃N₄nanosheet without doping V.The reaction path of V₁-g-C₃N₄and undoped V-g-C₃N₄was speculated.The study showed that the interaction between VO_xand g-C₃N₄nanosheet carrier enhanced the propane conversion rate,providing a new idea for exploring the preparation of cheap propane dehydrogenation catalyst with high propane conversion rate and high propylene selectivity.（3）g-C₃N₄material was prepared by melamine,and chromium acetate was added to g-C₃N₄material in different proportions.0.5-Cr-g-C₃N₄,5-Cr-g-C₃N₄and10-Cr-g-C₃N₄catalysts were prepared by two-step method,and then these catalysts were put into a fixed bed quartz reactor.The catalytic activity of PDH reaction was tested.Using TEM,SEM,XRD,FTIR,Raman and XPS,the results showed that Cr O_xspecies were successfully introduced into g-C₃N₄carrier during The Process of Cr doping,and were uniformly distributed on g-C₃N₄,presenting a thin two-dimensional nanosheet structure.The catalyst performance was evaluated as follows:the propane conversion rate and propylene yield increased with the increase of Cr content;When the reaction temperature was 600℃,the propene selectivity and propene yield of10-Cr-g-C₃N₄catalyst with better catalytic performance were 42.61%and 10.78%,indicating that the introduction of more Cr and Cr oxide clusters led to the improvement of propane conversion and improved the catalytic performance of propane conversion on a certain basis.This provides a new way to prepare cheap catalyst for propane dehydrogenation.