Study On Nitration Of Benzene In A Microreactor Based On Metallic Microfibrous Media With Open Porous Structure

Highly exothermic reactions such as aromatic nitration can be carded out in a highly efficient manner in microreactors due to the high heat and mass transfer rates in such reactors.We have developed a technique of preparing the metallic microfibrous composite with three-dimensional network structure which facilitates mass and heat transfer,because of its beneficial properties in terms of large void volume,three-dimensional micron-grade pore structure,and large surface area to volume ratio.Note that the three-dimensional network structure acts as micromixer thereby leading to micronic segmentation and fast mixing of fluids.On the basis of this new material,a microreactor which integrated heat-exchange,mixture and catalytic reaction has been designed.This microreactor was evaluated by benzene nitration to demonstrate its potential applications for fast and exothermic liquid-liquid reactions.The results showed that nitration of benzene could proceed efficiently in this microreactor with high selectivity at a short residence time.When the voidage of reaction bed was 97.6%,residence time was 31 s,reaction temperature was 75℃, molar ratio of nitric acid to sulfuric acid was 0.95 and molar ratio of nitric acid to benzene was 2.24,the benzene conversion of 99.2%and the nitrobenzene selectivity of 99.6%could be achieved.In addition,this novel microfibrous media can also allow the entrapping of solid acid in its three-dimensional network to replace sulfuric acid as catalyst.Therefore, we have prepared microfibrous catalytic materials entrapped solid super acid,further developed the green,clean,and non-polluting aromatic nitration process in microreactor.The novel monolithic microfiber-structured H₃PW₁₂O₄₀/SiO₂ solid acid catalysts, with 60～75 vol%void volume,were prepared by coating H₃PW₁₂O₄₀/SiO₂ sol-gel onto the sinter-locked SS-316L microfibrous network.The catalytic performance of the microfiber-structured H₃PW₁₂O₄₀/SiO₂ catalyst was tested with nitration of benzene in the microractor.The benzene conversion of 38.2%and the nitrobenzene selectivity of 100%could be attained under the following conditions:H₃PW₁₂O₄₀ content in H₃PW₁₂O₄₀/SiO₂ of 15%,H₃PW₁₂O₄₀/SiO₂ loading in SS-316L fibers of 35%,nitric acid to benzene molar ratio of 3.1,residence time of 72 s,reaction temperature of 75℃.However,H₃PW₁₂O₄₀ will run off seriously as the reaction proceeds because it can be dissolved in water,thereby decreasing the stability of H₃PW₁₂O₄₀/SiO₂ catalyst obviously with the increase of reaction time.Accordingly,we used Nafion which has better stability and higher catalytic activity instead of H₃PW₁₂O₄₀ to prepare the monolithic microfiber-structured Nafion/SiO₂ solid acid catalyst,and the catalytic performance of the catalyst was evaluated in the microreactor.The microfiber-structured Nafion/SiO₂ catalyst was characterized by FT-IR,TG,SEM and NH₃-TG.The results showed that the sol-gel technology can promote a highly dispersion of Nafion particles,which significantly promoted the number of the accessible acid sites.The Nafion/SiO₂ catalyst coating layer looked porous and comprised 200-400 nm particles.When Nafion content in Nafion/SiO₂ was 20%,Nafion/SiO₂ loading in SS-316L fibers was 36.3%and reaction temperature was 75℃,the benzene conversion of 44.7%and the nitrobenzene selectivity of 99.9%could be attained.At equivalent conversion level, the activity per acid site of such microfiber-structured Nafion/SiO₂ catalyst was nearly 600 times than the liquid sulfuric acid as catalyst.