Study On The Design Of Catalytic Au Active Phase And Reaction Pathway Regulation For Acetylene Hydrochlorination

Polyvinyl chloride?PVC?,as the second largest general synthetic resin,has excellent physic,chemical and mechanical properties and has been widely applied in the fields of industry,construction,agriculture,packaging and electric power,and so on.The increasing PVC demand promotes the domestic production of vinyl chloride.Vinyl chloride monomer is mainly synthesized by the ethylene-based process and the acetylene-based process.Due to the characteristics of energy structure of China,the shortage of oil and gas and the enrichment of coal resources,the acetylene-based process from the coal chemical route to produce vinyl chloride has the more distinguished advantages in energy and cost than the ethylene-based process from the petrochemical route.Mercuric chloride supported on activated carbon is the industrial catalyst for the synthesis of vinyl chloride by the acetylene-based process.However,the characteristic of easy volatilization of HgCl₂ leads to the mercury loss,resulting in the obvious decrease of the catalyst activity and the reduction of the product quality.The loss of mercuric chloride by volatilization may cause severe environmental problems.Mercuric chloride catalyst has been strictly limited to use in international society.According to Comprehensive Prevention Programs of Mercury Pollution in PVC Industry based on Calcium Carbide issued by Ministry of Industry and Information Technology of the People's Republic of China,the whole PVC industry will use the low-mercury catalyst at the year of 2015 and eliminate mercury-based catalyst entirely at 2020.Therefore,development of the non-mercury-based catalysis technologies and relative clean production routes by the acetylene-based process is the key to realizing the sustainable development of PVC industry.In the system of non-mercury catalysts for the synthesis of PVC by the hydrochlorination of acetylene,activated carbon supported AuCl₃ catalyst has high catalytic activity and selectivity and is considered to be the potential catalyst for substituting mercuric chloride catalyst.In this dissertation,on the basis of the systematical review of the catalysts for hydrochlorination of acetylene to vinyl chloride,gold was selected as the active component to investigate the catalytic performance of activated carbon supported gold catalyst for the hydrochlorination of acetylene.In order to solve the problem of the instability of AuCl₃ catalyst,modification and optimization to the catalyst were carried out from the two following aspects:?1?Modifying activated carbon support by doping N;?2?Constructing the Au-Cs inorganic complex as active phase.Some preliminary achievements on the above work have been made,providing some scientific basis and reference for further development of the industrial catalyst for hydrochlorination of acetylene to vinyl chloride.The main results obtained are as follows:Using urea as the nitrogen source,a small amount of nitrogen was introduced into activated carbon support by the post-treatment way of doping nitrogen.The catalytic performance of AuCl₃ supported on N-contained activated carbon was studied.The research shows that compared with the undoped activated carbon support,the active center Au-N constructed by doping nitrogen in activated carbon can promote the dispersion of Au nano-particles,enhance the strength of chemical bond between metal and support,and also modify the electronic structure of nano-particles on the catalyst,resulting in the improvement of the activity and stability of the catalyst.Under reaction conditions,active phase AuCl₃ can be reduced by the strong reducing property of acetylene or thermally decomposed into Au0,leading to the deactivation of gold catalyst.To solve the above problems,an inorganic complex CsAuCl₄ with definite microstructure and composition have been designed and synthesized.In the active phases of CsAuCl₄,the electrode potential of Au is decreased by the orbital hybridization,making the oxidation states of Au species difficult to be reduced.Furthermore,the large difference of ionic potential between Au³⁺ and Cs+ and the weak polarization of Cs+ to Cl-are beneficial to the maintenance of Au species in oxidation states under high reaction temperatures,making it difficult to thermolysis to Au⁰ and consequently loses its activity.The catalytic performance of Au-Cs/AC catalyst with the CsAuCl₄ as an active phase and the Au loading of 1 wt% was investigated.Acetylene conversion and the stability of the catalyst were improved obviously on Au-Cs/AC catalyst compared with 1Au/AC catalyst.For Au-Cs/AC catalyst,the main reason for the improvement of the catalytic activity is the great increase of Au species in oxidation state,which have high catalytic activity.The stability of the catalyst is enhanced by two reasons.On the one hand,Au species in oxidation state are more stable in the active phase of CsAuCl₄.On another hand,Au-Cs/AC catalyst can greatly inhibit the occurrence of coke deposition on the catalyst surface,thereby improving the activity and long-term stability of the Au-based catalysts.Since Au⁺ has better catalytic performance than Au³⁺ in acetylene hydrochlorination reaction,we designed a mixed-valence complexes of Au⁺/Au³⁺ in Cs₂Au₂Cl₆ as an active phase of gold catalyst on the basis of previous study of the active phase CsAuCl₄.An impregnation method was employed to prepare CsAu/AC catalyst,on which Cs₂Au₂Cl₆ is the active phase and Au loading is 0.25 wt%.The characterization results of XRD,STEM,H₂-TPR,C₂H₂-TPD and HCl-TPD revealed that the main reason for the improvement of the catalytic activity of CsAu/AC is the great increase of Au species in oxidation state compared with 0.25Au/AC catalyst,and the reasons for the improvement of the stability of CsAu/AC are the higher reduction temperature,the weaker adsorption capacity to C₂H₂ and the stronger adsorption capacity to HCl compared with 0.25Au/AC.The hydrochlorination of acetylene to vinyl chloride was studied over CsAu/AC catalyst.The result shows that CsAu/AC catalyst showed comparable reactivity to 1 wt % Au catalyst and significantly improved stability when compared with 1Au/AC with AuCl₃ as an active phase and 1Au₄Cs/AC with CsAuCl₄ as an active phase of gold catalyst.Under the reaction conditions of T = 180 °C,P = 0.1 MPa,GHSV = 50 h-1,n?HCl?/n?C₂H₂?= l.2,CsAu/AC catalyst had been evaluated for 1800 h,with only 10% decrease of acetylene conversion.The acetylene conversion of Cs Au/AC catalyst only decreased from 99.9% to 90.7% after 1800 h,indicating that the CsAu/AC presented good catalytic stability.Overall,CsAu/AC featured in low loading,high catalytic activity and good stability,making it a potentially mercury-free industrial catalyst for acetylene hydrochlorination.