连续流微反应器中微纤结构化的Nafion/SiO2固体酸催化苯硝化反应

采用溶胶-凝胶组装方法制备了Nafion/SiO2胶体, 并涂附于烧结的316L不锈钢(SS-316L)金属微纤网络, 形成了空隙率为60%-75%(φ, 体积分数)的整体式微纤结构化的Nafion/SiO2固体酸催化剂. 使用傅里叶变换红外光谱(FT-IR), 热重分析(TGA), 扫描电子显微镜(SEM)及NH3吸附法对微纤结构化的Nafion/SiO2催化剂进行了表征. 结果显示, 溶胶-凝胶组装方法可以明显促进Nafion的分散, 导致酸性中心的暴露量显著增加; 结构化的Nafion/SiO2催化剂以200-400 nm颗粒堆积而成的多孔涂层形式存在. 在一种集换热、混合、催化(反应)功能于一体的微反应器中, 考察了整体式微纤结构化的Nafion/SiO2固体酸催化剂催化苯硝化的反应性能. Nafion在SiO2中的组装量为20%(w, 质量分数), 金属微纤网络上Nafion/SiO2担载量为36.3%(w)的优化催化剂上, 75 ℃时苯转化率为44.7%, 硝基苯选择性可达99.9%. 相近转化率下, Nafion/SiO2固体酸单位酸中心的催化效能约为硫酸的600倍.

Nitration of Benzene in a Continuous-Flow Microreactor Integrated with Microfiber-Structured Nafion/SiO2 Solid Acid Catalyst

We prepared novel monolithic microfiber-structured Nafion/SiO2 solid acid catalysts with 60%-75% (φ, volume fraction) void volume by coating Nafion/SiO2 sol-gel onto a sinter-locked stainless steel (SS-316L) microfibrous network. We characterized the microstructured Nafion/SiO2 catalysts using Fourier transform-infrared spectrometry (FT-IR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and NH3-chemisorption. Results show that the sol-gel technology results in highly dispersed Nafion particles which significantly increase the number of accessible acid sites. The Nafion/SiO2 catalyst coating layer is made up of 200-400 nmparticles and looks porous. The catalytic performance of the microstructured Nafion/SiO2 catalysts was tested using the nitration of benzene in a continuous-flow microreactor that integrated heat-exchange, mixture, and catalytic reactions. The best catalyst had a loading capacity of 36.3% (w, mass fraction) for the Nafion/SiO2 composite in which the Nafion content was 20% (w). At a reaction temperature of 75 ℃, this optimized microstructured solid acid catalyst resulted in a benzene conversion of 44.7% with a nitrobenzene selectivity of 99.9%. At an equivalent conversion level, the activity per acid site of the microstructured Nafion/SiO2 catalyst is nearly 600 times as that of liquid sulfuric acid.