Basis Study Of The Hydrogenation Of Polystyrene To Produce Novel Polymer Poly(Vinylcyclohexane)

The heterogeneous hydrogenation of PS to produce polycyclohexylethylene (PCHE) has been proved to be a potential polymer modification process because of the greatly improved heat, oxidation, and UV resistance of PCHE compared with PS. It has been well-known that the characteristics of PS hydrogenation are quite different from that of hydrogenation of small molecules, because the PS molecules have large size and the solution has high viscosity. The low catalytic reactivity and difficult scale-up associated with the traditional powder catalysts are impediments to the development of PS hydrogenation technology. The structure of catalyst plays a vital role in the diffusion of reaction species, especially for the reaction of high-molecular polymers. However, there are very few studies on the catalyst design and mechanism study of PS hydrogenation.In this dissertation, the effect of catalyst structure on PS hydrogenation activity was firstly investigated, and the interplay of chemical kinetics and mass transport in polymer hydrogenation was studied. Base on the above conclusions, high-performance Pd/CNT catalysts were introduced and investigated in PS hydrogenation system. Furthermore, monolithic macroporous ceramic foam catalysts which could offer minimal resistance to flowing solutions and possess good mechanical properties were developed.The main work and results in this dissertation are listed as follows:(1) A variety of barium sulfate (BaSO4) carriers with or without mesopore structure were synthesized and used as catalyst carriers for PS hydrogenation. The Pd/BaSO4catalysts were prepared and characterized by N2physisorption, transmission electron microscopy (TEM), X-ray diffraction (XRD) to investigate the effect of carrier structure on the dispersion and geometric location of active metal. The catalytic activities were evaluated and kinetics studies in PS hydrogenation were investigated. It was found that the pore structure of carrier played an important role in the dispersion and location of Pd grains. The Pd/BSC-6H without mesopores had Pd grains deposited on the external surface of the carrier, and exhibited better activity than the mesoporous catalysts, which could be attributed to the absence of pore diffusion of PS coils in the hydrogenation. The PS hydrogenation catalyzed by Pd/BaSO4catalysts can be described by a pseudo-first-order equation with respect to the concentration of aromatic rings of PS and zero-order equation with respect to H2concentration. (2) The carbon nanotube (CNT) supported Pd catalysts were synthesized by the impregnation method and applied in the hydrogenation of PS for the first time. The Pd/CNT catalysts displayed excellent hydrogenation activities compared with those of traditional catalysts, e.g., Pd/AC or Pd/BaSO4, and allowed the reaction to be carried out under milder reaction conditions. The physical and chemical properties of Pd/CNTs were characterized by inductively coupled plasma-atomic emission spectrometry (ICP-AES), N2physisorption, TEM, XRD, CO chemisorption, and kinetics analysis. The catalyst characterization results showed that the active metal deposited on the external surface of CNTs with good dispersion. Kinetics analysis showed that the reaction order with respect to the concentration of aromatic rings and H2was zero. The nanoscale tubular structure of CNTs could not only eliminate pore diffusion of PS coils but also interact with PS coils to allow more segments to adsorb on the CNTs, therefore, leading to the high performance of Pd/CNTs.(3) Monolithic TiO2ceramic foams (CFs) supported Pd nanoparticles, possessing three-dimensional structure with open accessible pores, were developed as effective catalysts for the hydrogenation of PS. The monolithic CFs were synthesized through uniform coating of TiO2on synthetic template and partial sintering. The relationships between the preparation conditions and the mechanical and structural properties of CFs were investigated. The optimized CFs possessed interconnected cell windows of400-600μm and macropores of200-300nm on the struts. The distribution and texture of Pd nanoparticles on the CFs and their hydrogenation performances were studied. Furthermore, the internal mass transfer analysis of PS coils during the hydrogenation was evaluated. The Pd nanoparticles were located on the surface of the TiO2struts. As the Pd loading increased, the dispersion of Pd on the Pd/CFs significantly decreased. For Pd/CFs catalysts, the Weisz modulus was calculated to be less than0.30, indicating that internal diffusion limitation inside the Pd/CFs catalysts could be neglected for unique macropores on the struts.(4) Macroporous ceramic foams had good potential in the high viscosity system, especially polymer hydrogenation. HF-etching and Sol-Gel coating was used to modify the structure of CF and increase the specific surface area. By optimizing the etching time, HF-etching could only remove the SiO2to generate pores (0.5-1.0μm) on the surface of CF while the SiO2inside the struts remained. By Sol-Gel coating, mesoporous SiO2coating with good adhesive strength were fabricated on the surface of CF base. The modified Pd/CFs catalyst showed increased specific surface area, better dispersion of Pd, and enhanced PS hydrogenation activity.(5) Carbon-based nanostructured composite was fabricated by growing carbon nanofibers (CNFs) directly on CFs via catalytic chemical vapor deposition and used as monolithic catalyst carrier for PS hydrogenation. The surface of the CFs was covered with a significant amount of CNFs with intertwined morphology. The amount of CNF could be controlled by varying the growth time. The Pd/CNF-CF catalyst showed high PS hydrogenation activity, which could be due to the increase of specific surface area and the affinity between Pd and CNF.