| Nitroaromatic compounds play important roles in organic and fine chemicals,which are extensively used in the manufacture of pharmaceuticals,perfumes,plastics,explosives,etc.Among different nitroaromatic compounds,dinitronaphthalenes,prepared by nitration of 1-nitronaphthalene or naphthalene,are the raw materials for the fabrication of polyurethane,dyes,and sensitizing agents.Especially,1,5-dinitronaphthalene is mainly used for the synthesis of 1,5-naphthalene diisocyanate（NDI）,which is a key intermediate for the preparation of high-performance polyurethane elastomer materials.In industry,the1,5-dinitronaphthalene is usually synthesized by the nitration of naphthalene with a“mixed-acid”method（concentrated nitric acid and fuming sulfuric acid）.However,this method leads to a lot of acidic wastewater,corrosion of equipment as well as poor target product selectivity（～35%）.Therefore,it is urgent to develop green nitration systems by using efficient catalysts to replace the traditional nitration process.In this work,a method for the highly selective preparation of 1,5-dinitronaphthalene from catalytic nitration of 1-nitronaphthalene or naphthalene under mild conditions employing nitrogen dioxide-dioxygen replacing nitric acid and novel solid catalysts replacing sulfuric acid was studied.Firstly,the uncatalyzed nitration of 1-nitronaphthalene with several typical nitration systems was investigated,and the distribution of the products was analyzed.The results showed that the NO2-O2-Ac2O system appeared to play a positive role in attaining a higher 1,5-:1,8-dinitronaphthalene ratio under mild conditions.As revealed by the density functional theory（DFT）calculation,the reason for the favorable selectivity to 1,5-dinitronaphthalene in NO2-O2-Ac2O compare to the HNO3 system.A series of solid acid catalysts were designed for the catalytic nitration of1-nitronaphthalene,and the solid superacid SO42-/Zr O2 catalyst with a tetragonal crystal phase exhibited high activity and selectivity.The results show that the synergic effect of SO42-/Zr O2 and acetyl nitrate（Ac ONO2）in NO2-O2-Ac2O system remarkably improved the selectivity to 1,5-dinitronaphthalene.Under the optimal conditions,91.9%of 1-nitronaphthalene conversion and 53.3%1,5-dinitronaphthalene selectivity were achieved.The characterization results displayed that the sulfate group was double-coordinated to the catalyst surface by covalent form.The introduction of the sulfate group changes the acid sites of the catalyst and provides an essential acidic site for the aromatic nitration reaction.Furthermore,the Fe and Mn-modified SO42-/Zr O2(SO42-/Fe1.5Mn0.5-Zr O2)showed higher catalytic activity compared with the unmodified ones,and 92.6%of1-nitronaphthalene conversion with 57.2%of selectivity to 1,5-dinitronaphthalene were obtained under the optimal conditions.Meanwhile,the catalyst showed good stability in the present reaction.The characterization results unveiled that S,Fe and Mn elements were highly dispersed in the entire Zr O2architecture,which was effective to prevent the loss of sulfate groups,decreasing the crystallite size and increasing the acid sites.Combined with the experimental and XPS characterization results,the formation of the intermediate Ac ONO2 in the NO2-O2-Ac2O system was proved.The DFT showed that the Ac ONO2was activated by the acid sites on the catalyst surface,and a possible catalytic nitration reaction mechanism was proposed.To further improve the selectivity of 1,5-dinitronaphthalene,a mesoporous submicron Fe-and Cu-modified SO42-/Zr O2catalyst(sub-SO42-/Fe1.5Cu0.5-Zr O2)was synthesized by a two-step hydrothermal impregnation.The sub-SO42-/Fe1.5Cu0.5-Zr O2catalyst showed excellent regioselectivity and stability in the nitration of1-nitronaphthalene with NO2-O2-Ac2O system.The conversion of 1-nitronaphthalene and selectivity to 1,5-dinitronaphthalene reached 94.9%and 62.1%,respectively,under the optimal conditions.Furthermore,the microscopic morphology and physicochemical properties of the catalyst were characterized,and the results showed that the sub-SO42-/Fe1.5Cu0.5-Zr O2 catalyst has a uniform and unagglomerated submicron particles and microcrystalline tetragonal phase zirconia.Meanwhile,the correlation between the physicochemical properties and catalytic performance of the catalysts prepared by different methods was further analyzed.The results suggest that the crystalline phase,strong acid center sites and Lewis acid sites of catalysts might be the factors affecting the catalytic activity and regioselectivity.Finally,an efficient approach for the selective preparation of1,5-dinitronaphthalene by one-step nitration of naphthalene over Fe-and Cu-modified persulfate solid superacid catalysts was investigated.It was found that the S2O82-/Fe2.0Cu0.5-Zr O2 catalyst exhibited better catalytic activity and stability for the nitration of naphthalene with NO2-O2-Ac2O system,99.5%of naphthalene conversion with 56.9%of selectivity to 1,5-dinitronaphthalene was achieved under the optimal conditions.Meanwhile,the nitration of 1-nitronaphthalene with NO2-O2-Ac2O system over the present catalyst was investigated,and the 1-nitronaphthalene conversion reached 94.2%with 62.3%of 1,5-dinitronaphthalene selectivity.The characterization results displayed that the physicochemical properties and activity of the catalyst was greatly affected by the modification amount of S,Fe and Cu.The presence of sulfur and metal dopants prevented the transformation of active tetragonal Zr O2 to the monoclinic phase.Moreover,the acid sites and surface properties of the catalysts with different sulfur sources were analyzed.It was found the S2O82-/Fe2.0Cu0.5-Zr O2 has more strong acid center sites and Lewis acid sites.A plausible reaction mechanism for catalytic nitration of naphthalene with NO2-O2-Ac2O system over S2O82-/Fe Cu-Zr O2catalyst was discussed.In this work,an green and efficient strategy for the synthesis of1,5-dinitronaphthalene was successfully developed.The combined use of nitrogen dioxide and solid superacid catalysts in place of the traditional concentrated nitric acid-sulfuric acid system appears to be attractive.Overall,the developed catalytic nitrification system is environmentally friendly,atom economy and demonstrates high selectivity to 1,5-dinitronaphthalene,which has important theoretical significance and shows potential in industrial application. |
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