Synergy of Ionic Liquids in Complex Catalysts for Acetylene Hydrochlorination

Author:Yu Dai

Supervisor:li xiao nian zhao jia


Degree Year:2019





Acetylene hydrochlorination for the production of vinyl chloride plays an important role in the economy of China.Since the traditional catalyst HgCl2 in industry causes serious pollution of the environment,it is imperative and in urgent need to develop mercury-free catalysts.However,existing mercury-free catalysts suffer from their poor performance and high cost,therefore,it has been a main theme in both academic research and in industry to design and develop high performance catalysts.Based on comprehensive study of existing works on acetylene hydrochlorination worldwide,in this thesis,we begin with the catalytic process of the reaction,to find possible ways to enhance the catalytic efficiency.We study how to develop high performance catalysts,utilizing the extraordinary properties of ionic liquids(ILs),and with the help of assistive secondary metal compounds.First,using the exceptional properties of ionic liquids,especially the high solubility and low volatility,etc,we have studied the catalytic performance of various metal chlorides using ionic liquids as the reaction media,and selected the ionic liquid[Prmim]Cl and CuCl2 as the best combination.This Cu/IL catalyst has been evaluated in a liquid-gas reaction set-up of acetylene chlorination.Its catalytic activity was much better than that of[Prmim]Cl alone and of CuCl2 supported on activated carbon(AC),under the same reaction conditions,revealing a synergy between IL and CuCl2.To understand the reaction mechanism,we have studied theoretically the main reaction paths of acetylene hydrochlorination via the CuCl2/[Prmim]Cl catalyst,using computational chemistry softwares Gaussian 16 and DMol3.Our calculations show that the polar[Prmim]Cl of high solubility makes it easy to dissolve and adsorb the reactants HCl and C2H2,and adsorb the CuCl2 complexes to the neighborhood of the N atoms of its imidazolium ring.Then C2H2 reacts with HCl to realize hydrochlorination via the assistance of CuCl2 in the intermediate states.Thus,via mutual corroboration from both experiment and theory,we have revealed the mechanism for the enhancement effect of IL on the catalytic performance.Next,we applied the IL to the Au/AC and prepared a supported IL phase(SILP)Au(III)catalytical system,Au-IL/AC.We focused our study on the effect of a varying content of the catalytically active Au species,and compare the performance of these catalysts with that of the usual Au/AC catalyst without the IL.Our results demonstrate that,under the synergy of IL,the optimal loading of Au is about 0.1 wt%,which is much lower than that for the conventional Au/AC catalyst.After 40 h reaction on stream,all our Au-IL/AC catalysts exhibited a much higher activity and stability than the conventional Au/AC under the same reaction condition.The acetylene conversion rate dropped only by 1-2%.Analyses by XRD and TEM/EDX show that,under the assistance of IL,the active species of the catalysts achieved a very high degree of dispersion,and remained highly stable throughout the reaction,without obvious clustering and substantial reduction of Au3+.Our XPS analsys shows that only a small fraction of Au3+was reduced.After 40 h’s reaction,the fraction of Au3+still remained very high,up to 53.8-79.8%,indicating the high stability of Au3+under the synergy of IL.Having established the enhancement effect of IL,we prepared AC supported ionic liquid phase Au(III)catalyst,Au-Cu-IL/AC,which was also modified by the additive CuCl2.Under very low loading of Au(0.1 wt%),the activity of this catalyst was much higher than other Au based catalysts that were not modified by CuCl2,under the same reaction conditions.Its initial activity is comparable to that of the Au/AC catalyst with the optimized Au loading(1.0 wt%),but exhibits a much higher long-term stability.As an important index for catalytic performance in acetylene hydrochlorination,the turnover frequency(TOF)of this catalyst is much higher than all other catalysts reported in the literature.We have proposed a model of redox cycles involving both Au and Cu species,in which the Cu2+species serve to re-oxidize the reduced Au0 and thus prevent Au3+from being reduced.This has also been confirmed by our catalytic gas-liquid phase reaction using the Au-Cu-IL catalyst without an AC support.CuCl2 plays a critical role for stabilizing the active Au3+species in this model.We believe that,with the help of both IL and CuCl2,the Au-Cu-IL/AC catalytic system has the potential to be the solution for preparing high performance,economy and stable,industry-level mercury-free catalyst,to achieve the ultimate goal of large-scale acetylene hydrochlorination.