Effect Of Acid-Base Properties Of SiO₂-supported Catalysts On Dehydrochlorination Of 1,1,2-trichloroethane To Dichloroethylene

Over the last few decades,it has been recognized that the massive emission of chlorinated hydrocarbons(such as 1,1,2-trichloroethane)is detrimental to the environment and human health.Considering that such chlorinated hydrocarbons are produced in large quantity during various industrial processes,the treatment of such compounds becomes an important environmental and social problem.Meanwhile,current industrial processes utilize the alkaline hydrolysis process for dehydrochlorination of 1,1,2-trichloroethane(TCE)into 1,1-dichloroethylene(VDC)in the liquid phase.However,the method led to a large amount of high-concentration salty wastewater and faces great environmental pressure.From the environmental and industrial point of view,gas phase catalytic dehydrochlorination of chlorinated hydrocarbons to valuable chemicals is an environmentally-friendly approach,which is of great practical potential.In this thesis,we have studied the dehydrochlorination of 1,1,2-trichloroethane over SiO2-supported alkali,transition metal catalyst and ionic liquid catalysts.The effects of surface basicity-acidity,active species content and reaction temperature on the catalytic behaviors(conversion and selectivity)were investigated,and exploring the reaction mechanism.Detailed contents of this thesis are summarized as follows:1.Dehydrochlorination of 1,1,2-trichloroethane over SiO2-supported alkali and transition metal catalysts:Tunable selectivity controlled by the acid-base properties of the catalysts and mechanism.The dehydrochlorination of TCE results in three main products:1,1-dichloroethene(VDC)and 1,2-dichloroethene(1,2-DCE)which includes cis-1,2-dicloroethene(cis-DCE)and trans-1,2-dichloroethene(trans-DCE).In this work,dehydrochlorination of 1,1,2-trichloroethane was performed on SiO2 supported alkali metal(Na.K,Cs)and transition metal(Mn,Ni,Zn)catalysts.The surface basicity or acidity of the catalyst exerted great influence on catalytic behaviors.The catalysts with higher amount of base or acid sites resulted in higher activity due to more adsorption sites for reactant.The basic catalysts(Cs/SiO2 and K/SiO2)tended to form vinylidene chloride with a selectivity up to 80%;the neutral catalysts(Na/SiO2 and SiO2)tended to form 1,2-dichloroethene with a selectivity up to about 75%;the acidic catalysts(Zn/SiO2,Mn/SiO2 and Ni/SiO2)tended to form 1,2-dichloroethene with a selectivity up to 96%.Characterization result forther porved to be the basicity of catalyst is a key factor for the generation of VDC and stronger base leads to higher VDC selectivity.The surface medium Lewis acid sites are advantageous for the generation of cis-DCE.Proposed reaction mechanisms included an Elcb mechanism with a carbanion intermediate on the basic catalysts,an E2 concerted mechanism on the neutral catalysts,and an E1 mechanism with a carbonium intermediate on the acidic catalysts.2.Dehydrochlorination of TCE into VDC was performed on SiO2 supported phosphonium salts ionic liquid(ILs)catalysts.Various characterizations revealed the interaction between ionic liquid and SiO2 surface,and the basicity of the catalyst declined with increasing loading of ionic liquid.The reaction results indicated that the catalyst with low ionic liquid loadings were beneficial to the formation of VDC(with a selectivity up to 91%),and the selectivity decreased with increasing ionic liquid content in the catalyst.It was also found that the mechanism of this reaction might follow the Elcb route under the roles of strong basicity of the catalyst.In addition,the catalysts showed good stability in long-term reaction,which showed good potential in practical application.