Preparation Of Salt-stabilized Recycled Nano-precious Metal Solution And Exploratory Research Of Its Catalytic Performance

Metal nanoparticles have small size,large specific surface area,abundant active sites,high catalytic activity and selectivity,and great catalytic value.As a reaction solvent,water is non-toxic,easy to obtain,can improve selectivity and accelerate the reaction,and is generally considered as a green and environmentally friendly reaction medium.Generally used to catalyze the reaction of organic substances in the aqueous phase are mainly metal colloids,supported,Pickering emulsions and other catalytic systems.At present,most of the metal colloids use polymers and ionic liquids as stabilizers,and there are few studies on the use of salt-stabilized colloids for aqueous phase catalysis.Salt is used to replace polymers and ionic liquids to stabilize nano-precious metals and to catalyze reactions in water systems,which is more economical and environmentally friendly.Firstly,a salt-stable recovered nano-precious metal solution was prepared by distillation reduction crystallization method and characterized.The waste acid of automobile exhaust catalyst is removed by vacuum distillation to remove excess hydrochloric acid,and the p H value of the remaining liquid can be controlled by controlling the amount of distilled acid,and hydrochloric acid with the highest concentration of 3-4 mol/L can be recovered.Then add an excessive amount of reducing agent and an appropriate amount of distilled water to stir to fully react,place to crystallize,separate the crystals,and obtain a series of salt-stable recovered nano-precious metal solutions.The single crystal X-ray diffraction of the obtained crystal found that the crystal Al H27S2O21 belongs to the cubic crystal system,which is a new type of crystal,and its structure was analyzed.The results of electron microscopy and ICP-MS show that the particle size of precious metals is about 10-20 nm,and the total content of precious metals(Pt,Rh,Pd)in each different catalyst in the dispersed salt solution is about1 mg.The prepared nano precious metal solution is used for hydrogenation and non-hydrogenation of nitrate ion and p-chloronitrobenzene.Nitrate ions will react under both non-hydrogenation and hydrogenation conditions.One part generates ammonium ions,and the other part may generate nitrogen.On the other hand,p-chloronitrobenzene does not react without hydrogenation,and a reduction reaction occurs under hydrogenation conditions.However,the reaction rate of p-chloronitrobenzene under hydrogenation conditions is very slow,and the product can only be detected by liquid-mass spectrometry.Therefore,neither nitrate nor p-chloronitrobenzene are suitable as reaction substrates for this catalyst.The prepared nano-precious metal solution is used for hydrogenation and non-hydrogenation research of nitrobenzene.Nitrobenzene does not react without hydrogenation,but will undergo a reduction reaction to form aniline under hydrogenation conditions.Under hydrogenation conditions,the yield of aniline per hour can reach 47.53%,but the yield of aniline will not increase with time.It can be seen in the mass spectrum of the product that in addition to the characteristic peaks of aniline,there are also characteristic peaks of azobenzene and azobenzene oxide and other impurity peaks.According to the mass spectrometry results,a possible reaction mechanism may be inferred: Nitrobenzene is first reduced to nitrosobenzene,and then may be reduced to aniline through a direct route,or azobenzene oxide may be generated through an indirect route,and then further reduced to aniline.The catalysts prepared by different methods are used to study the catalytic hydrogenation of nitrobenzene.The catalyst prepared by iron powder reduction has the best catalytic hydrogenation ability,and the yield of aniline can reach 47.53% per hour.Next,the effects of solvent,temperature,p H and the amount of catalyst on the catalytic hydrogenation of nitrobenzene were explored with the best catalyst.As the temperature increases,the rate of aniline formation gradually increases,and the yield can reach up to 98%.However,when the reaction progresses to a certain extent,the amount of aniline will not continue to increase,which may be due to the formation of other products such as azobenzene or azobenzene oxide during the reaction.The reaction rate in pure water solvent is significantly faster than that in the water/ethanol system.This is because nitrobenzene forms an oil-in-water emulsion due to the presence of nano-precious metal particles and salts in the water system,and the catalytic reaction occurs in water and oil.At the interface,the reaction rate is significantly faster than that of a homogeneous system.Under the condition of low p H(p H 0.00/0.15),because the product aniline is more likely to form ammonium salt and dissolve in the water phase,the reaction rate is faster.The more the total content of precious metals in the reaction system,the faster the catalytic hydrogenation reaction rate.When the content is 0.5 mg,the yield of aniline can reach 37.14%.