The Catalytic Decarboxylation Of Terephthalic Acid To Benzene

Oxidation residue discharged from the PTA oxidation unit contains abundant aromatic acids, trace colored impurities, metal catalysts, etc. Terephthalic acid is the main component in PTA residue. This thesis focuses on how to deal with PTA residue efficiently. We first propose the catalytic decarboxylation method to prepare aromatics from PTA residue. It’s easy to achieve effective utilization of organic resources and environmental protection by PTA decarboxylation process.Firstly, some metal oxides were investigated to figure out a catalyst with high activity, and to futher explore the decarboxylation mechanism. Then, based on the decarboxylation mechanism, we study the thermal decomposition behavior, products and thermal decomposition kinetics of terephthalates under nitrogen atmosphere. Finally, the influence of various factors was investigated on the stability and catalytic activity of composite oxides. The thesis focuses on the three parts as follows:(1) Several metal oxides and USY zeolite were prepared for catalytic decarboxylation of terephthalic acid. Catalytic activities and reaction products of terephthalic acid over different catalysts were compared to select zinc oxide as decarboxylation catalyst, which shows high catalytic activity and costs inexpensively. The decarboxylation ratio of terephthalic acid over ZnO was about100%and the content of benzene was over40%at550℃and weight hourly space velocity (WHSV) of0.48h1-1The results of FT-IR, XRD and Py-GC/MS characterization illustrated that zinc terephthalate intermediate was identified on ZnO after decarboxylation reaction. The decarboxylation of terephthalic acid over ZnO involved two steps:chemical adsorption and thermal decomposition of zinc terephthalate to produce benzene and CO2. The deep dehydrogenation of Zinc terephthalate resulted in carbon deposition on ZnO catalyst surface.(2) Four terephthalates were characterized by FT-IR, Py-GC/MS and TG. Zinc ion and carboxylate radicals of zinc terephthalate ligand are combined with each other mainly in bridge carboxylate coordination bonding mode. The thermal decomposition of terephthalates proceeds stepwise as follows:the loss of crystal water and solvent, the thermal decomposition of the anhydrous salts; the decomposition remnants of terephthalates are metal oxides or carbonates. The thermal decomposition of terephthalates under nitrogen atmosphere results in C-O and C-C bonds cleavage into benzoyl and phenyl radicals, which can be combined to obtain various aromatics. Zinc terephthalate decomposed mainly into benzene and benzophenone at lower temperature.(3) Supported zinc oxides were prepared by chemical coprecipitation method dispersed in the scale of nanometer and exhibit higher catalytic decarboxylation activities than pure zinc oxide. Terephthalic acid can be completely converted and benzene yield reach86%at500℃over supported zinc oxides without changing other conditions. Zn/Al2O3catalyst displays slow deactivation trend with the decreasing benzene selectivity, while Zn/SiCO2catalyst shows more stable decarboxylation performance. BET and XRD results show that carbonaceous deposition and solid solution formation both infulenced catalyst activity. Carbonaceous formation is the main factor lead to the deactivation of decarboxylation catalyst.