Effects Of Metal Additives On Catalystic Performance Of Pd-based Catalysts For Acetylene Hydrochlorination

Polyvinyl chloride(PVC)is an important product for the chlor-alkali industry,and widely used in many fields.During the synthesis process of PVC,the raw material vinyl chloride(VCM)is irreplaceable.Based on the “meager oil,rich coal” energy situation in C hina,VCM was mainly produced from acetylene hydrochlorination process,and catalyst is critical for the technique.However,for the acetylene hydrochlorination reaction,HgC l2/C was used as catalyst for the production of VCM,which would jeopardize the human health and environment,and it also confront shortage problem of mercury resource.Therefore,developing an environmental friendly mercury-free acetylene hydrochlorination catalyst is extremely imperative for the PVC industry.At present,the research about acetylene hydrochlorination catalyst has been performed by scientists around the world,especially noble metals mercury-free catalyst.It is found that the catalytic performance(activity and selectivity)of noble metal Pd-based catalyst is up to the level of the industrial high mercury catalyst under certain condition,however,due to the poor lifetime and stability of Pd-based catalyst,it is still need to do much efforts to replace the mercury catalyst.Consequently,in this paper,in order to increase the life time and stability of catalyst,we adopted the method of using metal additives to modified Pd-based catalyst,also investigated the influences of preparation methods on catalytic performance of catalyst,and combined with the corresponding characterization methods to analyze the reason for deactivation of Pd-based catalysts modified by metal additives and its modification to effects on its catalytic porfomance.Their catalytic performances were tested at T = 160 oC,GHSV=120 h-1,V(HCl):V(C2H2)=1.15.The main work and research achievements of this paper are as follows:(1)The effects of the impregnation method of metal additives(alkali metal,alkaline-earth metal,rare earth metal and transition metal)and Pd on catalytic performance of the catalyst was investigated.Through catalytic evaluation results,these demonstrated that the most suitable impregnation method of Pd-based catalyst modified with alkali metal(Li,Na,K),alkaline-earth metal(Ca,Mg,Ba),rare earth metal(La,Ce)and transition metal(Fe,C u,Zn),respectively,was Pd with Li co-impregnation,Pd-Ca,La-Pd and Pd-Zn successive impregnation method.(2)Studied the effects of the content of metal additives(Li,Ca,La and Zn)on the catalytic performance of Pd-based catalyst and the reason for deactivation of the catalysts.It is found that: the optimum content that Pd-based catalysts modified by prmoter(Li,Ca,La and Zn)is 0.5%,3.0%,3.0%,1.0%.Their modification can increase the dispersion of Pd active species,decrease the formation of coke deposition,thus promote the catalytic performance of catalyst;The effect of the modified Pd-based catalysts is in the order of Li > Zn > Ca > La,besides,reduction of Pd species and carbon deposition during the catalytic reaction are the main reason for the deactivation of Pd-based catalyst,and the modification of Li can inhibit this process.(3)Explored the catalytic performance of Pd-based catalyst modified by dual metal.The experimental results showed that: in catalysts(La3.0-Pd0.5-Zn1.0/C,Ca3.0-Pd0.5-Zn1.0/C),La and Zn,Ca and Zn played an antagonism role during the catalytic reaction,which may due to the decrease of dispersio n of active components,so as to reduced catalytic performance of the catalyst;La and Ca,La and Li,Li and Zn,Ca and Zn all displayed a synergistic effects,La3.0Ca3.0-Pd0.5/C,La3.0-Pd0.5Li0.5/C,Ca3.0-Pd0.5Li0.5/C and Pd0.5Li0.5-Zn1.0/C showed elevated catalytic performance.Which may due to the enhancement of the interaction between the active component Pd and carbon support and boost the dispersion of active components,consequently,increased catalyst stability;The effect of six groups of bimetallic modified Pd-based catalysts is in the order of Li and Zn > Li and La > Li and Ca > Ca and Ca > Ca and Zn > La and Zn.(4)Explored the reason for which the effect on catalytic performance of Pd-baesd catalysis modified by bimetallic modification.It is showed that the modification of La,Ca and Zn mainly improved the initial activity of the catalysts,and the addition of Li promoted the stability and lifetime of the catalysts.In addition,active Pd species of the catalysts(La3.0-Pd0.5Li0.5/C,Ca3.0-Pd0.5Li0.5/C and Pd0.5Li0.5-Zn1.0/C)modified by Li and other metal additives(Ca,La,Zn),is mainly PdO,which its dispersion is gooder,increase the active site Pd2+,and inhibit the reduction of Pd species to some extent during the preparation of catalyst,thus enhancing the lifetime and stability of Pd-based catalyst.