The Preparation Of Cobalt(?)-containing Zeolite And Their Application In Epoxidation Of Styrene

Styrene oxide is an important organic intermediat in food,perfume and pharmaceuticals industry,and styrene oxide production via the epoxidation of styrene is an important process in chemical industry,which can traditionally be synthesized by using the routes of peroxy acid method and halohydrin method.While,for those two routes,they use peroxy acid or halohydrin as the oxidizing agent to oxidize the reactant.Howerver,here comes the problem of equipment corrosion and environmental pollution when using the traditional production methods due to the utilization of liquid acids.With the increasingly stringent laws and regulations of environment,developing a new green technology to produce styrene oxide will be a challenging work but meaningful for us to do.Therefore,the aim in this study is to develop cobalt-containing zeolite as catalyst and green reagent oxygen as oxidizing agent to replace the tradition processes.In this study,IM-Co/Y catalyst and IE-Co/Y catalyst were prepared by impregnation(IM)and ion exchange(IE)methods respectively.And then,the preparation method together with cobalt source and the process conditions were optimized.After that,alkaline and alkaline earth metal were added to modify the catalysts by repeatedly ion exchange and IE-MCo/Y catalysts were maded(M=Ca,Sr,K,Cs,Ba).The catalysts were examined and characterized by means of XRD,H2-TPR,XRF,UV-vis,NH3-TPD,XPS and so on.The main results are demonstrated as follows:IE-Co/Y catalyst prepared by ion exchange method showed the better catalytic performance than IM-Co/Y catalyst prepared by impregnation method.For IE-Co/Y catalyst,the conversion of styrene was 51.2%and the selectivity of styrene oxide was 48.5%.While,the conversion of styrene was only 28.3%and the selectivity of styrene oxide was 39.8%for IM-Co/Y catalyst.Using zeolite Y as support and choosing cobalt nitrate as cobalt source to prepare IE-Co/Y catalyst would lead to a better catalytic performance.Futher study showed that the preparation method of catalyst would affect existential state of cobalt.When using the impregnation method to prepare the catalysts,cobalt existed as Co3O4 crystalline phase and stayed outside the pore of zeolites.Nevertheless,when using the ion exchange method,cobalt existed as single-site Co(II)form inside the pore of zeolites.After ulteriorly optimizing the preparation method,it is found that the calcination temperature of the catalysts may affect the coordination form of cobalt.Therefore,IE-Co/Y catalyst calcinated at 300?,500? and 650? in the air were attempted.When the catalyst was calcinated at 500 ?,cobalt change from octahedral coordination form as[Co(H2O)6]2+ to tetrahedral coordination form compared with the sample only dried at 110? in the air.Besides,the reactivity of the catslyst calcinated at 500 ? was distinctly higher than the catalyst without calcination.Afterwards,the reaction condition of the styrene epoxidation was optimized.By setting reaction temperature as 80 ? and suing TBHP(t-butylhydroperoxide)together with O2 as oxidizing agent,the conversion of styrene was 100%and the selectivity of styrene oxide was nearly 70%after reacting for 5 hours.TBHP is a kind of free radical nitiator and it was reported in literature that the reaction mechanism of the epoxidation of styrene was free radical reaction mechanism which may explain the reactivity improved significantly by adding a small amount of TBHP.In addition,TBHP can oxidize the styrene solely without oxygen which meant that there was some synergistic effect between TBHP and oxygen.Eventually,the IE-Co/Y catalyst was modified by alkaline and alkaline earth metal.Alkaline and alkaline earth metals could adjust the acidity of catalyst and affect the reactivity of catalysts.IE-MCo/Y(M=Ca?Sr?K?Cs?Ba)catalysts were obtained by repeatedly ion exchange.As for the IE-CaCo/Y catalyst which had the best reactivity performance among the as-prepared catalysts,Ca2+ could not only adjust the acidity of catalyst but also promote the epoxidation of styrene.By adding Ca2+,the conversion of styrene was 67.5%and the selectivity of styrene oxide was nearly 47.8%.However,the conversion of styrene was 29.6%and the selectivity of styrene oxide was 38.6%for the control group catalyst(IE-Co/Y).And we suspected that Ca2+ could affect the location of Co2+ in the zeolite.Ca2+ preferred the site ?,sites ?'and ? where the Co2+ had the poor or no reactivity,which urged Co2+ to occupy the site ?'(thought to be the more active site)so that IE-CaCo/Y had the better reactivity performance.