Preparation And Performance On Doped Modification Of Pd-based/HY Mercury-free Catalysts For Acetylene Hydrochlorination

Polyvinyl chloride(PVC) is the main pillar for sustainable development of chlor-alkali industry, which is widely used in a ll walks of our life. Vinyl chloride monomer(VCM) production is one of the key technology for PVC manufacture, and ethane method、ethylene method and acetylene method(acetylene hydrochlorination method) are mainly three synthesis routes. Acetylene method is a coal-based chemical production process. Because of large coal reserves and less oil or gas, about 75% of VCM in China is derived from acetylene method. However, the traditional HgC l2/C catalysts which were used for acetylene hydrochlorination are highly toxic and harmful to human health and the environment. Therefore, it is imperative to explore non- mercury catalysts for acetylene hydrochlorination to promote the sustainable development of PVC industry.After years of efforts to research, many progress have been made for non- mercury catalysts systems and they have been proven to be potential, especially some metal chlorides, which are based on the labs at present. The reported non- mercury catalysts have good activities and VCM selectivities, but unsatisfactory in stabilities, so as the Pd/zeolite-based catalysts in our previous work. In this paper, aiming to improve the stability of the catalyst, it is successful to develop a new series of noble metal/zeolite(Pd/HY) catalysts for acetylene hydrochlorination. Combining with the catalytic tests and various characterization methods, the relationship between the structure, surface properties and catalytic performances in catalysts is studied and the interaction between supports and active components is explored. This study can promote the development and application of friendly non-mercuric catalysts with excellent performances. It has positive practical significant to achieve the sustainable development in the whole chlor-alkali industry. The main work and conclusions obtained are as follows:(1) The nit ro ge n- mo d ifie d Pd / HY ca ta lys ts w er e p repa re d us in g b y ultrasonic-assisted impregnation method. Their catalytic performances were tested at 160 °C, feed volume ratio VHCl/VC2H2 = 1.25, a C2H2 GHSV of 110 h-1. The structure and surface properties of nitrogen- modified supports and catalysts were characterized. The results showed that appropriate nitrogen-contained reagents could improve the stability of the Pd/HY catalyst. The Pd/4PANI-HY catalyst displayed the optimal catalytic performance for acetylene hydrochlorination among these catalysts, with the catalytic activity above 95% in 300 h, indicating the improvement catalytic stability of catalysts. The excellent stability of the Pd/PANI-HY catalyst was attributed to the presence of the PANI that changed the structural and surface properties of zeolites, partially weakened the occurrence of carbon deposition and the reduction of the Pd2+ active component on the catalyst surface. In addition, PEG modification or high-heat pretreatment were not valid for improvement the stability of the Pd/HY catalyst.(2) The HF、NH4F and NH4F-Urea modified Pd/HY catalysts were prepared and their catalytic performances were tested at 160 °C, feed volume ratio VHCl/VC2H2 = 1.25, a C2H2 GHSV of 110 h-1. The structure and surface properties of modified supports and catalysts were characterized. The results showed that the orders of fluoride-nitrogen modified catalysts’ stabilities were: Pd/NH4F-Urea-HY > Pd/NH4F-HY > Pd/HF-HY > Pd/HY. The acetylene conversion and VCM selectivity of the Pd/0.5HF-HY catalyst were 96% and 97% after 190 min, respectively; When the modified temperature was at 25 °C, the acetylene conversion and VCM selectivity of the Pd/10NH4F-HY catalyst both reached to 99% after 300 min. Under the same reaction conditions, the Pd/HY catalyst modified by NH4F-Urea had the optimal catalytic performance for acetylene hydrochlorination and the stability of the catalyst enhanced to about 800 min with 99% catalytic activity, which could improve the stability of the Pd/HY catalyst. The results of characterization revealed that the presence of F-contained species to Pd/HY catalysts could partially enhance the surface acidity of catalysts, weaken the occurrence of carbon deposition and the Pd loss, increase the Pd species dispersity, and inhibit the active component reduction, thus improving the catalytic performance of catalysts. In addition, F element from F-contained reagents could make oxo-bridged oxygen bond(-O-Al-) breakage and combine with H in the environment to generate the skeleton Al-OH and Al-F compounds, and the electronic absorption effect of F ions could make the skeleton Al-OH ionized, which affected the surface acidity and catalytic performance of catalysts.(3) The boron(B)、phosphorus(P) modified Pd/HY catalysts were prepared and their catalytic performances were tested at 160 °C, feed volume ratio VHCl/VC2H2 = 1.25, a C2H2 GHSV of 110 h-1. The structure and surface properties of modified supports and catalysts were characterized. The results showed that boron and phosphorus modified catalysts could improve the stability of Pd/HY catalysts and appropriate B and P modification could effectively improve the activity and stability of Pd/HY catalyst. At the same conditions, the Pd/B2-HY showed that the acetylene conversion could reach over 96% in 12 h and the Pd/P2-HY catalyst displayed that the acetylene conversion could reach over 99% within 8 h. The results showed that there were some by-products on the surface of the used Pd/B-HY catalysts, such as benzene, chlorobenzene, 1, 1-dichloroethane, 1-chlorobutylene and other unsaturated hydrocarbons, which increased the catalyst deactivation. After characterizations, the B and P adding could decrease the Pd loss and carbon deposition, inhibit the Pd reduction and change the acidity of catalysts, enhancing the catalytic performance of the Pd-based catalyst.(4) The KC l modified Pd/NFY catalysts were prepared and their catalytic performances were tested at 160 °C, feed volume ratio VHCl/VC2H2 = 1.25, a C2H2 GHSV of 110 h-1. The structure and surface properties of modified supports and catalysts were characterized. The results showed that KC l modification could effectively improve the stability of Pd/NFY catalysts and the Pd-7K/NFY catalyst displayed the optimal performance(the catalytic performance could reach over 99% within 50 h). Characterizations results showed that the K modifier could stabilize Pd2+ and weaken the occurrence of carbon deposition on the catalyst surface, thus improving the stability of the Pd/NFY catalysts. In addition, the modification orders of KC l could affect catalytic performances of catalysts and the catalytic performance of the Pd-7K/NFY catalyst was better than the Pd@7K/NFY and Pd7K/NFY catalysts.(5) The KF modified Pd/HY catalysts were prepared and their catalytic performances were tested at 160 °C, feed volume ratio VHCl/VC2H2 = 1.25, a C2H2 GHSV of 110 h-1. The structure and surface properties of modified supports and catalysts were characterized. The results showed that KF modification could effectively improve the stability of Pd/HY catalysts, and the methods of KF modification would effect on the stability of the catalyst. At the same time, the Pd-KF/HY catalyst displayed the optimal performance(the catalytic performance could reach over 99% within 18 h), and the less carbon deposition and Pd loss were the main reasons for the much better stability of the Pd-KF/HY catalyst.