Investigation Of Direct Synthesis Of Acetic Acid From Ethylene Over Pd-based Catalysts

Acetic acid is a very important monoatomic fatty acid and has been widely used in chemical industry. As an important chemical product, the development of acetic acid often reflects the current situation and future of chemical industry. Among the many synthesis techniques of acetic acid, ethylene direct oxidation is a new route, which possesses the advantages of simple process, stable operation, less waste, flexible equipment scale and so on. In this thesis, ethylene direct oxidation was investigated using the catalyst system which Pd and heteropolyacids were contained. By researching influence of different carriers on the performance of the catalyst, the results reveal that the activity and selectivity toward acetic acid of Pd-Se-Ru-SiW₁₂/SiO₂ were prior to those of other carriers supported Pd-Se-Ru-SiW₁₂ catalysts. When activated carbon (AC) was used as carrier, most of C₂H₄ was adsorbed in capillary pore of activated carbon, and desorbed at 350 K, while MCM-22 was used as support, most of C₂H₄ were adsorbed on strong active sites, and desorbed at 675K and 850 K. Most of C₂H₄ was adsorbed on the moderate sites of Pd-Se-Ru-SiW₁₂/SiO₂ catalyst and desorbed at 423-453K, which favored acetic acid formation during ethylene oxidation reaction. By investigating influence of different heteropolyacids, it is found that loading silicotungstic acid (H₄SiW₁₂O₄₀) does good to the oxidation of ethylene to acetic acid and the favorable amount of H₄SiW₁₂O₄₀-loaded occupies 30% of the mass of carrier. With the promoters of alkali metals, alkali earth metals, noble metals, rare metals etc, further research by evaluation experiments and analysis experiments reveals that when Pd, Se, Na, Ru and H₄SiW₁₂O₄₀ are dopedon the carrier SiO2, the amount of the absorbed ethylene is the largest among them and the catalyst behaves best.Optimization of the reaction conditions in the synthesis of acetic acid was also conducted in this thesis. The influence of feed proportion, the reaction temperature and the space velocity on the space-time yield and selectivity of acetic acid was investigated. The results reveal that the optimal reaction conditions are: feed proportion C2H4:N2:O2:H2O = 55:14:9:22 (mole ratio), the reaction temperature T=150¹70°C, the reaction pressure P=1.4 MPa and the space velocity GHSV=3 700-4000 h"1. Under these conditions, the one-pass conversion of ethylene reaches 8%, the selectivity of acetic acid reaches 87.4% and the one-pass space-time yield of acetic acid reaches 372 g /Lh.