| Dehydrogenation of ethylbenzene(EB)with CO2(CO2-ODEB)is a new technology for the styrene(ST)production and is characteristic of high efficiency,energy conservation and environmental friendliness.The commercialization of this technology depends on the development of highly efficient catalysts.Al2O3 is the most used catalyst support in the field of CO2-ODEB and its surface properties,such as acid-basic properties and metal-support interactions,have significant influences on the performance of the catalysts.Recently,it was found that using Al2O3 with advanced morphologies as the catalyst support could tune the physicochemical properties of the catalysts,and appeared to be a new strategy to develop highly efficient catalyst materials.Herein,using Al2O3 nanoflake materials as the catalyst supports,three types of supported catalysts based on iron oxides,CeO2-ZrO2 and carbon were prepared and used in CO2-ODEB reaction.The effects of the morphology,structure and surface properties of the Al2O3 nanoflake support on the physicochemical properties of the catalysts and the catalytic performances in CO2-ODEB were investigated systematically,in comparison with conventional Al2O3 supports.The reveal of structure-activity correlation provided some theoretical and experimental guidance to develop efficient catalysts for CO2-ODEB.The main research contents and results are as followings:(1)Supported iron oxides catalysts were prepared by the incipient wetness method using an Al2O3 nanoflake as the support,in comparison with a conventional Al2O3 support.The catalytic results indicated that the Al2O3 nanoflake-supported iron oxides were more active than the conventional Al2O3-supported counterparts.At 550℃,the former possessed an EB conversion of 51.4%,which was 1.6 times than the latter.By characterizing the morphology,structure and surface properties of both supports and catalysts,it was revealed that the Al2O3 nanoflake was rich in surface O2-basic sites(9.93 μmol/g),as a result of the removel of more-OH groups.These O2-basic sites improved the dispersion of iron oxides via interfacial chemistry during the impregnation process.As a result,highly dispersed oligomeric FexOy clusters mainly formed on the resulted catalyst,and these iron species were highly active.By contrast,the conventional Al2O3 had much less surface O2-basic sites with a concentration of 3.48 μmol/g.Consequently,large Fe2O3 particles with less activity were present on the surface of conventional Al2O3-supported catalysts.(2)Supported CeO2-ZrO2 catalysts were prepared by the deposition-precipitation method using an Al2O3 nanoflake as the support,in comparison with a conventional Al2O3 support.The catalytic results indicated that the Al2O3 nanoflake-supported CeO2-ZrO2 possessed both higher activity and stability,with an initial EB conversion of 58.6%and a decrease of 9.3%after 10 h at 600℃.Via the characterization towards the morphology,structure and surface properties of both supports and catalysts,it was revealed that the Al2O3 nanoflake had numerous slit-shape pores.This unique pore was crucial to the structure of the CeO2-ZrO2 precursor of precipitates during the deposition-precipitation process,thus improving the dispersion of CeO2-ZrO2 species and the concentration of surface oxygen vacancies of the catalyst.By comparison,only intraparticle pores were present in the case of the conventional Al2O3 support and the resulted catalyst possessed a poorer dispersion of Ce02-ZrO2 and a lower concentration of surface oxygen vacancies.(3)A series of carbon-covered Al2O3(CCA)materials were prepared by an infiltration-carbonization method using a conventional Al2O3 as the support.And a series of Al2O3 nanoflake@C(Al2O3-n@C)materials with a core-shell structure were also synthesized via an hydrothermal carbon coating method using a γ-AlOOH nanoflake as the support.The structural properties of the two types of C-Al2O3 composites,as well as their catalytic performances in CO2-ODEB,were investigated.It was found that the CCA materials possessed the highest catalytic activity at the optimum carbon loading(18.6 wt%)with which the pore structure derived from the support was preserved.When the pore-blockage occurred,the catalytic performances of CCA materials decreased drastically.Al2O3-n@C materials with a carbon loading of 9.2 wt%possessed the largest outer surface area of the carbon shell and therefore the highest catalytic activity was achieved.Further increase in the carbon loading had little influences on the performance of the Al2O3-n@C materials.Unlike to the CCA materials,the Al2O3-n@C materials were almost free of the problem of pore-blockage and thus had potential advantages in further application.In addition,for the both types of C-Al2O3 composites,a lower pyrolysis temperature led to a higher graphitization of the carbon species,which were beneficial to the formation of C=O groups,a type of active sites for dehydrogenation.Therefore,the composites synthesized at the lower temperature possessed better catalytic performances. |
