Studies on zeolite-supported Mo and Re catalysts : catalytic performance in methane aromatization reaction and their structural characterization ゼオライト担持モリブデンおよびレニウム触媒の研究 : メタンの芳香族化反応の触媒特性と構造キャラクタリゼーション

博士論文

The main achievements in the present studies are summarized as 4 key points: 1) HZSM-5 supported Mo and Re catalysts are found to be quite active and selective for directly conyerting methane to benzene, naphthalene and C2 hydrocarbons. A great progress for methane aromatization has been made because of the discover of the two new catalysts. 2) Coke deposition on the catalyst for non-oxidative conversion of methane is solved by varying methane pressure combined with addition of CO2 in methane feed. Coke selectivity is about 0 at 3 atm methane and with 1-3% CO2 addition in methane feed. 3) Two reactions on Re/HZSM-5 catalyst are identifried: i) methane with CO at the nearly same consumption rate to produce benzene, naphthalene and C2 hydrocarbonss; ii) an improved reforming reaction of methane with CO2 to produce aromatics and synthesis gas. 4) MoO3/HZSM-5 and Re207/HZSM-5 are first endothermically converted to its reduced state before production of benzene. Mo exists in its carbide and is highly dispersed on the HZSM-5 support. The results are presented in details in 9 chapters. It was found in chapter 2 that Mo modified ZSM-5 catalysts exhibited excellent catalytic activity and selectivity to benzene for the conversion of methane at 973 K under nonoxidizing conditions. H2 and ethene may be the primary products in the conversion of methane to benzene. It was found in chapter 3 that the catalytic activity and stability of Mo/HZSM-5 can be enhanced by the addition of Zr or W and it is also very sensitive to pretreatment conditions. Pretreatment of catalysts under air atmosphere at a high temperature lead to an increase in pore size of the catalysts and a decrease in methane conversion and activity maintenance. However, the catalysts can be stabilized under the atmosphere of methane. A new Mo phase is formed under the methane atmosphere, which is a perquisite of the formation of arom atics and C2 hydrocarbons. In chapter 4, the TG/DTA/MS, XANES/EXAFS and SEM/TEM studies reveal that HZSM-5 supported Mo oxide is endothermally converted with methane around 955 K to molybdenum carbide (Mo2C) cluster (Mo-C, C.N.=1, R＝2.09 A; Mo-Mo, C.N.＝2.3-3.5; R＝2.98A). The formed Mo carbide exists on HZSM-5 support in the uniform particles. The highly dispersed metal carbide on HZSM-5 support acts as the active center to activate methane primarily to C2 hydrocarbons, which are secondarily dehydrocondensed towards benzene and naphthalene on HZSM-5 support. Methane aromatization with addition of C2 hydrocarbons such as ethane and ethene and acetylene on Mo/HZSM-5 catalyst is studied in chapter 5. Addition of all C2 hydrocarbons in methane feed lead to an increase of benzene formation rate and ethane addition is most effect to enhance the rate of benzene formation. A much high formation rate and selectivity of benzene is achieved by addition of about 4.3% ethane in methane feed. The formation rate and selectivity of naphthalene is most effectively enhanced by addition of 4.3% acetylene. The increase of benzene formation rate is due to high concentration of [C2Hy] intermediates for benzene formation by addition of C2. Addition of acetylene lead to enhancement of the secondary reaction of benzene to produce naphthalene, so the formation rate and selectivity of naphthalene is high. In chapter 6, catalytic dehydroaromatization of methane with CO and/or CO2 toward benzene and naphthalene on Mo/HZSM-5 and Fe/Co modified Mo/HZSM-5 in the prolonged time of 100 h is studied. The catalyst stability is enhanced and coke formation is suppressed by adding CO and/or CO2 to the methane feed and by varying methane pressure. Bifunctional catalysis of Mo/HZSM-5 is discussed by the mechanism that methane is activated on Mo sites to form CHx and C2-species, and then secondarily to form benzene and naphthalene. Dependence of activity and selectivity for methane aromatization on Mo/HZSM-5 on the reaction conditions such as pressure and specifric velocity of methane, CO2 addition at high SV and reaction temperature is investigated in chapter 7. A high and stable benzene formation rate of about 1700 nmole/s/g-cat is achieved for methane aromatization on Mo/HZSM-5 by increasing methane pressure from 1 to 3 atm, methane specifric velocity from 1440 to 10000 ml/h/g-cat and reaction temperature from 973 to 1073 K, and by adding about 2.1% CO2 to methane feed. The rate of benzene formatioh with time on stream is remarkably stabilized by varying methane pressure and addition of 2.1% CO2 in methane feed and greatly enhanced by increasing methane specific velocity from 1440 to 10000 ml/h/g-cat and reaction temperature from 973 to 1073 K. The selectivity of benzene and naphthalene and coke is also controlled by varying reaction conditions. High methane specific velocity lead to high benzene selectivity and low naphthalene selectivity. Catalytic features of Re/HZSM-5-based catalyst for aromatization reaction of methane are studied and compared with that of Mo/HZSM-5 in chapter 8. It is found in TPR of Re2O7/HZSM-5 in methane flow that methane activation process is initiated by reduction of Re2O7/HZSM-5, which is endothermic and company with production of hydrocarbon oxides and loss of catalyst weight. However, the reduction of catalyst in hydrogen stream is a exothermic process and takes place at a much lower temperature. We propose that rhenium carbide is formed by reductien of Re2O7/HZSM-5 in methane flow. The carbide is stable in the atmosphere of methane and catalyze the reaction of methane aromatization. The temperature for formation of rhenium carbide is about 100 K lower than that for formation of molybdenum carbide and Re/HZSM-5 shows much better low-temperature activity than Mo/HZSM-5-based catalyst. To compare with Mo/HZSM-5-based catalyst, Re/HZSM-5-based catalyst exhibits a better lower-temperature activity and a higher selectivity to C2 hydrocarbons. Catalytic conversion of methane with CO to aromatics on Re/HZSM-5-based catalyst are studied in chapter 9. The CO involves in the catalytic conversion with methane to produce aromatics and the catalytic activity for benzene formation, therefore, is greatly enhanced by adding CO to methane feed. Futhermore, the conversion of CO with methane also lead to formation of some intermediate products such as CO2 and H20, which eliminate coke on the catalyst. Therefore, the coke deposition is decreased and catalyst stability is enhanced by CO addition. Effect of CO2 addition on aromatization of methane on Re/HZ SM-5-based catalyst are studied in chapter 10. High and stable rate of benzene fomation is achieved at a high SV of methane on Re/HZSM-5 type catalyst by addition of about 1-3% CO2 to methane feed. A little amount of CO2 in methane feed is very effective to suppress deposition of coke on the catalyst and to enhance selectivity of hydrocarbon products. Seleetivity for coke deposition is about 0 at the presence of 1-3% CO2 in methane feed. The reaction of CO2 with coke to produce CO and the secondary reaction of produced CO with hydrogen to produce CHx species are responsible for the low coke selectivity and high hydrocarbon selectivity.