Study On Catalytic Behavior Of Au-Based Complex Catalyst For Acetylene Hydrochlorination

Vinyl chloride monomer?VCM?is the main chemical intermediate in the manufacturing process of polyvinyl chloride?PVC?.PVC has high resistance to light and chemicals,and is widely and significantly used in the construction industry,packaging,electrical and garment industries as the second-largest general-purpose resin.Two synthetic methods are used to produce PVC:?i?ethylene oxychlorination and?ii?acetylene hydrochlorination.The route of producing VCM by ethylene oxychlorination is more dependent on petroleum resources,which is mainly adopted by western countries with developed oil and gas resources.Acetylene hydrochlorination is mainly used in regions rich in coal resources.For example,more than 70%of the VCM produced in China is synthesized by acetylene hydrochlorination.Currently,the most widely used catalyst in this synthesis route is activated-carbon-supported mercury?II?chloride?HgCl₂?.However,the active components are easily sublimated and lost at the reaction temperature,which seriously affects the service life of the catalyst.The loss of Hg not only causes catalyst deactivation but also causes severe environmental pollution and human health risks.Therefore,a solution to the pollution problem caused by the Hg catalyst is needed,and the research and development of alternative non-Hg-based catalysts is underway.Over the years of research,it has been found that the catalyst with the most industrial potential to replace the mercury-based catalyst is the gold-based catalyst,but the activity and stability of the gold-based catalyst still need to be further improved.Therefore,this paper aimed at this problem and carried out the following work:?1?A series of Au catalysts were prepared with several representative solvents and evaluated for acetylene hydrochlorination.Solvent was selected according to the change of polarity and boiling point.The seven solvents were water,aqua regia,methanol,ethanol,isopropanol,n-propanol and isobutanol.The catalytic performance of the catalysts increased with decreasing polarity of the solvents,and superior performance was achieved over the Au-isopropanol/AC catalyst,with a 84%stable conversion under reaction conditions of 180 ^oC and a gas hourly space velocity?GHSV?of 1200 h^-1;relative increases of 700.0%and 483.3%,respectively,in acetylene conversion were achieved compared with that achieved with traditional catalysts prepared with water and aqua regia.Characterization and analysis of a series of catalysts showed that the substitution of the highly polar water and aqua regia with weakly polar and volatile alcohols altered the crystallisation process of Au nanoparticles?NPs?during their formation.The various edges or defects in the formed multiple-twinned or polycrystalline particles provided new active sites for reactants.The altered solvents may enhance the interaction between the support and the Au species,highly dispersing and anchoring the active species and inhibiting their agglomeration and loss during the reaction.Moreover,the interaction also strengthens the adsorption capacity for reactants of the catalysts,enhancing their catalytic performance.?2?As we know,Au ions have similar peripheral s,p,d equivalent orbits,so it is easier to form hybrid orbits with powerful bonding ability to accept the lone pair electrons provided by the hetero-atomic ligands.Etidronic acid?HEDP?and Nitrilotri?methylphosphonic acid??ATMP?were selected as heteroatoms-containing ligands to modify the Au catalysts using the selected isopropanol as the dispersing solvent for acetylene hydrochlorination.The results uncovered that the optimal mole ratio of Au/ligand?L?was 1:1.The results showed that the highest acetylene conversion rate was 97.4%and the selectivity of vinyl chloride was higher than 99%under reaction conditions of 180 ^oC and a gas hourly space velocity?GHSV?of 1200h^-1;Characterization and analysis of a series of catalysts showed that:The coordination between Au and heteroatoms,especially the collocative presence of N,P,and O,promoted the electron transfer from heteroatoms to Auⁿ⁺and increased the density of electron cloud around active centers.This stabilized Auⁿ⁺and increased the difficulty in the reduction of Au species,and even re-oxidized the Au⁰ to active Auⁿ⁺species.Moreover,the coordination also inhibited the migration of active components;the increased density of electron cloud around active centers can also weaken the adsorption capacity for C₂H₂ and strengthen the adsorption ability for HCl of the catalysts,which is conductive to constructing the micro-environment favorable to the reaction.DFT calculations indicated the stabilization effect of the heteroatoms-contained ligands,and the energy sequence of the transition states was also in accord with the experimental results.?3?According to the previous work,the activity and stability of catalysts were significantly improved due to the presence of N,P or O elements in organic ligands.Therefore,the effects induced by ligands existing alone with heteroatoms were studied.Three kinds of ligands containing N,P and O heteroatoms were selected and studied respectively.Ligands were selected according to the variety and number of substituents around heteroatoms.A total of 17 kinds of catalysts of 3 types were prepared for acetylene hydrochlorination.The results showed that the catalysts of Au-N₄/AC,Au-P₁/AC and Au-O₁/AC had the best catalytic performance,and their acetylene conversion rates were up to 91.3%,94.5%and 95.5%,respectively.Their vinyl chloride selectivity was higher than 99%under reaction conditions of 180 ^oC and a gas hourly space velocity?GHSV?of 1200 h^-1.The characterization and analysis of the catalyst showed that the introduction of coordination heteroatoms increased the electron cloud density around the active center Auⁿ⁺,inhibited the reduction of Auⁿ⁺species to Au⁰species,and thus improved the stability of Auⁿ⁺.In addition,the interaction between Au and heteroatoms in the ligand can improve the adsorption ability of the catalyst to the reactant hydrogen chloride and reduce its adsorption ability to the reactant acetylene,thus reducing the possibility of forming oligomers on the surface of the catalyst,inhibiting the generation of carbon deposition,and improving the catalytic activity and stability.