The Catalytic Performance Of P-modified HZSM-5 Molecular Sieves In The Reaction Of Ethanol Dehydration To Ethylene

The reaction of ethanol dehydration to ethylene was studied over P-modified HZSM-5 zeolite catalyst in this thesis. Fractional factorial design of experiments was applied to synthesis of ZSM-5 zeolite by hydrothermal method, and P-modified HZSM-5 catalysts were prepared by impregnating. The activity and stability of the catalysts were tested in a continuous flow fixed-bed reactor. The parent and modified catalysts were characterized by XRD,SEM,XRF,NMR,MIP,BET,NH3-TPD,TG,XPS and IR. The possible molecular structure of the P-modified catalysts was investigated by computational chemistry.Using fractional factorial design, a mathematical model for ZSM-5 synthesis with ethanol as template was obtained from the experimental data. The model is: Y = 50.50–6.08A+14.77B+7.39C+7.66D+16.72AB–12.49AC–13.67AD Where Y is relative crystallinity of ZSM-5 samples, A, B, C, D are the molar ratios of H₂O/SiO₂, SiO₂/Al₂O₃, OH-/SiO₂, and C₂H₅OH/SiO₂ respectively in initial gel composition. Response optimization predicted by the model showed that, the response Y presented the maximum value at an optimal parameters of A (-1), B (-1), C (+1), D (+1), i.e., pure ZSM-5 could be prepared at a gel composition: H₂O/SiO₂ = 40, SiO₂/Al₂O₃ = 60, OH^-/SiO₂ = 0.15, C₂H₅OH/SiO₂ = 1.5.It was found that the main product is ethylene in a broader range of temperatures (573 713 K) over the P-modified HZSM-5 with a suitable P content. For the P-modified HZSM-5 catalysts with SiO₂/Al₂O₃ molar ratios of 25, 38, 50, the suitable P contents were 0.95, 1.1 and 1.0 (P/Al atomic ratio), respectively. On PZ-25-3.4 modified catalyst (P/Al = 0.95, SiO₂/Al₂O₃=25), catalytic activities indicated that the higher the reaction temperature, the higher the weight hourly space velocity. The effect of water content in feed on catalytic performance decreased with the increment of reaction temperature. The life-time testing for PZ-25-3.4 catalyst showed that ethanol conversion and ethylene selectivity were still above 99 % after 930 hours run at the reaction temperature from 553 K to 653 K. When the deactivated PZ-25-3.4 catalyst was regenerated, ethanol conversion and ethylene selectivity were 98.7 %, 99.9 %, respectively. The initial reaction temperature of the regenerated PZ-25-3.4 catalyst was the same as that of the fresh one.NMR, NH3-TPD characterizations indicated that phosphorus interacted with the oxygen of the bridging hydroxyl groups over framework aluminum atoms in tetrahedral coordination, leading to a decrease of the strong acid over P-modified catalysts. Furthermore, XPS, BET and IR characterizations showed that P valence was +5 oxidation state, and there may exists H 2 PO-4 structure over the modified catalysts. Combining this with the result of computational chemistry, the possible structure of P-modified catalyst was proposed in this study.