Study On Catalyst And Process For The Hydrogenation Of Polystyrene

Polystyrene（PS）is one of the most commonly used thermoplastics on the market.It has the advantages of good permeability,water and corrosion resistance,good thermal insulation,high coloration,good processing fluidity,and easy-to-shape features.It is widely used in electronic products,communication equipment,optical research equipment,transparent models,food packaging materials and daily necessities.However,PS is brittle,has poor environmental stress resistance,low tensile strength,and is easy to oxidize,which limits its application in high-end fields.PS can be hydrogenated to produce polycyclohexylethylene（PCHE）,which greatly improves its properties:increased glass transition temperature,enhanced mechanical properties,better heat resistance,and lower density and water absorption performance,etc.PCHE has a broader market prospect in high-end fields such as electronic communications,optical materials,aerospace,urban rail transit,and optical storage media.The key to realize the efficient hydrogenation of PS to PCHE is the catalyst.At present,there are relatively few studies on PS hydrogenation catalyst systems,which are necessary to further develop inexpensive and efficient catalysts.In this paper,carbon based catalysts（including activated carbon and carbon nanotubes）carried active component Pd,explored the effect of the catalyst on the hydrogenation performance of PS,and investigated the effect of Ni incorporation on the carbon based catalyst.The pore structure,metal valence state and hydrogen absorption and desorption capacity of the catalysts were characterized by BET,XRD,H₂-TPR,XPS,TEM and ICP.The structure-activity relationship of the catalysts was analyzed combined with the catalytic performance,and the process conditions of PS hydrogenation were optimized.The specific results are as follows:By comparing the performance of Pd catalysts supported by MCM-41,Al₂O₃,mesoporous carbon（MC）and activated carbon（C）in PS hydrogenation,the Pd/C catalyst with higher Pd dispersion showed excellent catalytic activity and stability,and avoided serious chain breaking phenomenon.With the increase of catalyst dosage,stirring speed,reaction temperature,pressure,time and other conditions,the conversion rate of PS showed an increasing trend.The increase of catalyst dosage is beneficial to maintain the high molecular weight of the product.High stirring speed,reaction temperature and too long reaction time will cause severe chain scission.The change of reaction pressure has little effect on the chain length.The stirring speed was adjusted to 400rpm at 160°C and 6 MPa,and 10%catalyst was added to react for 12 h.The conversion rate of PS hydrogenation was 99.3%.The chain length retention rate was 76%.The prepared PCHE has excellent properties:glass transition temperature of 130.7℃,density of 0.93 g/cm³,water absorption of 0.16%,and tensile strength greater than 60 MPa.In order to reduce the chain scission of PS molecules in the process of catalytic hydrogenation,CNTs with larger external specific surface area are selected as carriers to support the active component Pd.The results show that compared with Pd/C,PS on Pd/CNT can better maintain the chain length,even at a high temperature of 200°C,the chain length can maintain 68%of the original.According to the experimental results and characterization analysis,the optimal preparation conditions of the catalyst are obtained as follows:5%Pd loading,calcination at 400℃for4 h,and reduction at 350℃for 2 h.The optimal reaction conditions:180℃,6 MPa,stirring speed of 400 rpm,10%catalyst dosage,6 h.At this time,the conversion rate of obtaining PS hydrogenation was 99.6%.The catalyst has excellent stability,and the activity does not show a significant decrease after 10 times of use.In order to further improve the catalytic activity of the Pd/CNT catalyst,a second metal Ni is introduced into the Pd/CNT system as a cocatalyst.The addition of Ni greatly enhanced the catalytic activity,and at the same time the chain length retention rate also increased by 16%.Through investigation,it can be determined that the optimal preparation conditions of Ni-promoted Pd/CNT catalyst are:the loading of Pd was5%,the loading of Ni was 5%,using nickel nitrate as precursor,calcination at 400°C for 4 h,carbonization at 350°C for 2 h;the optimal reaction conditions determined was 160°C,6 MPa,stirring speed of 400rpm,10%catalyst,8 h.The conversion rate of PS hydrogenation was98.6%.The chain length retention rate was 89%.The improved activity of Ni-doped Pd/CNT catalysts shortens the reaction time and effectively reduces the scission of molecular chains.