Preparation And Catalytic Hydrodechlorination Behaviours Of Confined Ni-based Catalysts

Volatile chlorinated hydrocarbons are commonly used in industrial processes.Because of their stable chemical properties,they will evoke tong-term environmental pollution upon discharge into the environment.Considering their strong mutagenic and carcinogenic nature,volatile chlorinated hydrocarbons are listed as priority pollutants by the U.S.Environmental Protection Agency(USEPA).A variety of methods have been explored for removal of those pollutants,including adsorption,catalytic oxidation?biological degradation and catalytic hydrodechlorination(HDC).Gas phase catalytic HDC is a cost-effective method for the detoxification of chlorinated hydrocarbons,which is capable of converting chlorinated hydrocarbons into useful chemicals and/or less toxic byproducts.1,2-dichloroethane are commercially important chemicals that can be used as raw material in the production of vinyl chloride,trichloroethane and tetrachloroethylene.Supported noble metals(e.g.,Pd,Pt and Rh)are commonly used catalysts in catalytic HDC processes.1,2-dichloroethane is mainly converted into ethane on the noble metal catalysts with high conversion.However,the scarcity and low ethylene selectivities of noble metals limit their large-scale applications.As alternative to noble metal catalysts,Ni-based catalysts have been widely studied in the gas phase catalytic HDC.In comparison with noble metal catalysts,nickel-based catalysts normally have high selectivity and stability but low activity in catalytic HDC processes.Besides,the Ni particles aggregation usually occurred at high temperature and further led to the low catalytic activity.The confined metal catalysts could enhance the metal-support interaction and suppress metal agglomeration,which further improve the catalytic activity.In the thesis,three methods were explored to prepare confined Ni-based catalysts,and the catalytic hydrodechlorination of 1,2-dichloroethane on the catalysts was investigated.The structure-catalytic property relationship of the catalysts was analyzed through a series of characterizations.Based on the results of the hydrodechlorination of 1,2-dichloroethane and characterization,the correlation mechanism between catalyst structure and catalytic efficiency was discussed.The main results are as fellows:1.N-doped porous carbon supported Ni catalysts(denoted as Ni@NC)were prepared using furfuryl alcohol(FA)loaded Ni-MOF-74 as the precursor followed by NH4OH treatment and carbonized under N2 atmosphere.For comparison purpose,Ni catalysts supported on porous carbon(denoted as Ni@C)were also prepared by direct carbonization of Ni-MOF-74.The gas phase catalytic hydrodechlorination of 1,2-dichloroethane was carried out to evaluate the catalytic performances of the catalysts.The TEM characterization results showed that for Ni catalysts prepared at the same carbonization temperature,Ni particle sizes in Ni@NC catalysts were significantly smaller than that of Ni@C.Increasing pyrolysis temperature could result in grown Ni particles.The H2-TPR characterization results suggested that the marked enhancement effects on H2 spillover in Ni@NC,which was likely due to the effective anchoring of Ni particles on N-doped carbon,resulted in enhanced metal-support interactions.All suported Ni catalysts are highly selective towards to ethylene with ethylene selectivities above 95%in the process of catalytic hydrodechlorination of 1,2-dichloroethane.Furthermore,supported Ni catalysts obtained at lower carbonization temperature have higher catalytic activities owning to their smaller Ni particles.For catalysts obtained at the same carbonization temperature,N-doping prominently enhances the activity of the catalysts due to effective suppression of Ni particle growth and the enhanced metal-suport interactions.2.Required amounts of Ce(NO3)3 and ZrOCl2 were introduced into the pores of Ni-MOF-74 through the impregnation method.After pyrolysis under N2 atmosphere,Ni@C-CeO2 and Nj@C-Ce0.5Zr0.5O2 were obtained.Besides,MOFs infiltrated with Ce(NO3)3 and ZrOCl2 was calcinated in air and the obtained catalyst was denoted as Ni@Ce0.5Zr0.5O2.The gas phase hydrodechlorination of 1,2-dichloroethane was used as the model reaction to e valuate the catalytic performances of the catalysts.The results showed that for Ni@C-CeO2and Ni@C-Ce0.5Zr0.5O2 with similar Ni loading amount,the catalytic activity of Ni@C-Ce0.5Zr0.5O2 was much higher than that of Ni@C-CeO2,suggesting the stronger metal-support interactions and smaller Ni particles in Ni@C-Ce0.5Zr0.5O2,which could be identified by the TEM characterization results.For the catalysts of Ni@C-Ce0.5Zr0.5O2 and Ni@Ce0.5Zr0.5O2-cal,the catalytic activity of the Ni@C-Ce0.5Zr0.5O2 was markedly higher than that of Ni@Ce0.5Zr0.5O2,likely due to the presence of carbon that enhanced the metal-support interaction,which further led to the strong H2 spillover effect.3.Different amounts of nanosized Ni-MOF-74 grew in situ inside the mesopores of SBA-15 at room temperature(denoted as Ni-MOF-74@SBA-15).A series of Ni@C-SBA-15 with different Ni loading amounts were obtained through pyrolysis of Ni-MOF-74@SBA-15 under N2 atomosphere.For comparison,the Ni@C catalyst was prepared through direct carbonization of Ni-MOF-74.Meanwhile,supported Ni catalysts on SBA-15(denoted as Ni/SBA-15)with similar Ni loading amounts synthesized by the conventional impregnation method were also included.Besides,in order to investigate the role of carbon in the catalysts,Ni@SBA-15 catalyst without carbon was obtained by calcining Ni@C-SBA-15 in air to remove C.The characterization results demonstrated that the catalyst 1Ni@C-SBA-15(1 represents the loading amount)has very small Ni particle sizes and high metal dispersions.The presence of carbon in the catalysts effectively enhanced H2 spillover effect,which was confirmed by the H2-TPD result Thus,the catalyst of 1Ni@C-SBA-15 displayed excellent catalytic performance toward the catalytic hydrodechlorination of 1,2-dichloroethane,which could be ascribed to the confinement effect offered by SBA-15 and C-doping,leading to strong metal-support interaction and enhanced hydrogen spillover effect.4.Supported Ni catalysts on three mesoporous SiO2 supports(i.e.,SBA-15,MCM-41 and HMS)were prepared using a solid-state reaction between Ni(NO3)2 and organic-occluded mesoporous SiO2,For comparison supported Ni catalysts on mesoporous SiO2 with similar Ni loading amounts synthesized by the conventional impregnation method were also included.The catalysts were systematically characterized.The results of UV-Vis showed that upon grinding Ni(NO3)2 with template-occluded mesoporous SiO2(HMS),strong coordination between Ni2+and dodecylamine was identified in the Ni(NO3)2-HMS system.Additionally,the results of temperature-programmed H2 reduction revealed that there is a strong interaction between NiO and mesoporous silica in NiO@HMS.For the catalytic hydrodechlorination of 1,2-dichloroethane,Ni@MCM-41 synthesized by the solid-state reaction method exhibited a catalytic activity similar to that prepared by the impregnation method(im-Ni/MCM-41),while higher catalytic activities were observed on Ni@HMS and Ni@SBA-15 than on their counterparts prepared by the impregnation method.Furthermore,a significantly higher conversion was identified on Ni@HMS than on Ni@SBA-15 and Ni@MCM-41.The present findings suggest that the different interaction between metal precursor and template could influence the dispersions of Ni in mesoporous silica.