Controlled Design And Performance Study Of Structures In Acetylene Hydrochlorinated Carbon And Nitrogen Materials

Polyvinyl chloride（PVC）is a plastic material used frequently in construction,package,furniture,and other industries,it is synthesized from vinyl chloride（VCM）monomer.The main production processes of VCM include the methods of ethane,ethylene and acetylene.The acetylene method uses coal as raw material and mercury as catalyst,which possesses the advantages of low cost,high product selectivity and compliance with our energy structure,and accounts for 75%of the VCM production process in China.However,mercury is unstable at high temperature,which easily causes environmental pollution and endangers human health.Currently,the research on non-mercury catalysts is mainly at the stage of laboratory research,and there is no large-scale industrial production technology yet.Therefore,it is extremely urgent to explore green and efficient non-mercury catalysts.Carbon-based catalysts have attracted much attention in acetylene hydrochlorination reactions because of their excellent physicochemical properties.Compared with the industrial catalyst,the carbon-nitrogen catalysts catalytic activity and stability were not yet satisfactory.Therefore,it is of great significance to investigate the excellent performance of acetylene hydrochlorination carbon-nitrogen material catalysts.In this thesis,aiming to improve the catalytic activity and stability of the catalyst,it is successful to develop a new series of carbon-nitrogen catalysts for acetylene hydrochlorination.We enhance the catalytic activity and stability through the precise and controllable design of large pore and defective structures with catalysts and novel regeneration methods,and initially explore the structural modulation of high-density pyridinic-Fe N₄ based on carbon-nitrogen materials.The main results are summarized as the following:（1）To investigate the effect of the macroporous structure on the catalytic performances.The macroporous carbon catalysts with high N content were prepared by introducing melamine as an in situ doped N-precursor,and effect of pore structure on the catalytic activity of acetylene hydrochlorination was evaluated.The results showed that the pore size and surface area gradually increased with the increase of N content,the catalytic activity of catalysts increased.The macroporous carbon catalyst CN-2 showed excellent catalytic performance（98%acetylene conversion and 99%VCM selectivity）.According to the results of X-ray photoelectron spectroscopy（XPS）and other characterization analysis,CN-2 has a high nitrogen content of 9.74%,a pore size of 300 nm.temperature programmed desorption（TPD）experiments verified the strong adsorption of C₂H₂ and HCl.The high activity and stability of CN-2 indicated its great potential as a carbon catalyst for acetylene hydrochlorination reaction.The catalyst was deactivated due to carbon deposition and active site reduction,and the NH₃ treatment increased the active site pyridinic N content of the catalyst and restored the initial catalytic performance,indicating that NH₃ treatment is one of the methods for catalyst regeneration.（2）To investigate the effect of defect structure on catalytic performance,one simple strategy was developed to synthesize carbon materials with high nitrogen and rich carbon defects.Synthesis of precursors by polymerization strategy using 2,6-diaminopyridine and ammonium persulfate,carbon-based catalysts with different defect and nitrogen content were prepared at different pyrolysis temperatures.As the defect number increased,the specific surface area and the content of active pyridinic N sites increased,which results in promotion of catalytic activity The NC-800 catalyst showed outstanding catalytic activity and stability in acetylene hydrochlorination reaction（98%acetylene conversion and its stability up to 500h）.The excellent catalytic activity and stability of NC-800 were attributed to the high nitrogen content of13.46%,the maximum specific surface area and high number of defects,and the strong adsorption of C₂H₂and HCl.The excellent catalytic activity and stability of NC-800 were attributed to the high nitrogen content of 13.46%,the maximum specific surface area and high number of defects,and the strong adsorption of C₂H₂ and HCl.The deactivation was manifested by the carbon deposition on the catalyst surface and the reduction of active sites.（3）The low stability of nitrogen-doped carbon materials limited their development in acetylene hydrochlorination,and study on the deactivation reasons and regeneration methods of carbon catalysts has become the focus of research.In the previous chapters,carbon-nitrogen materials were deactivated due to carbon deposition and active site reduction,and NH₃ regeneration methods can restore the number of active sites of catalysts,but not solved the problem of carbon deposition and pore blockage by designing of macroporous structure catalyst.The deactivation causes of nitrogen-doped carbon materials in acetylene hydrochlorination and a novel regeneration method were successfully proposed.Using D（+）-Glucosamine hydrochloride biomass material with high nitrogen content,the acetylene conversion of D-GH-800 catalyst was up to 99%by one-step pyrolysis,but the catalytic activity decreased by 30%after 60 h.Thermogravimetric analysis showed carbon accumulation of 5.87%.TPD verified that the deactivation was due to the D-GH-800 catalyst’s strong adsorption and difficult desorption of acetylene,resulting in the accumulation of acetylene on the catalyst surface to form macromolecular carbon polymers and leading to the pore blockage.Based on the catalyst deactivation caused by carbon deposition,the new idea of regeneration by Zn Cl₂ activation was used to eliminate the carbon deposition in the pores of the deactivated catalyst.The activity of D-GH-800 catalyst was restored,and its lifetime was extended which improved the utilization rate of D-GH-800 catalyst.This study elucidates the mechanism of carbon deposition in the cause of catalyst deactivation by carbon-nitrogen materials,which plays an important role in improving catalytic stability and provides a promising regeneration method.（4）Based on the previous three chapters,it was found that the catalytic efficiency of the carbon-nitrogen materials itself is low and its stability still does not reach the industrial requirements.Single-atom catalysts with dense Fe-N₄ active sites were prepared based on carbon-nitrogen materials to investigate their catalytic performance.the catalytic performance of Fe-N_x catalysts with a high density of active sites prepared based on carbon-nitrogen materials was explored.Cs-Fe-600 with Fe-content up to 1.92 wt.%were synthesized by using the abundant pyridinic N anchored Fe in the chitosan.Cs-Fe-600 with dense pyridinic-Fe N₄ sites showed excellent performance.Spherical aberration-corrected high-angle annular dark field scanning transmission microscope（AC-HAADF-STEM）and X-ray absorption spectra（XAS）characterization analysis showed that the dense Fe single-atom was uniformly distributed on the carrier as stable pyridinic-Fe N₄ active sites.TPD analysis and density flooding theory calculations confirmed the strong adsorption capacity of pyridinic-Fe N₄on acetylene,which was the main active site for the reaction.The Cs-Fe-600 catalyst was deactivated by Fe metal agglomeration and pyridinic-Fe N₄ active site loss.However,NH₃ regeneration restored pyridinic-Fe N₄ active sites and their initial catalytic activity.This study has promising applications by controlling the specific N-ligand Fe environment and having a recoverable catalyst.