|其他摘要||Light olefins, such as ethylene and propylene, are very important raw materials in modern chemical industry. As the foreseeable depleting of world oil reserves, MTO (methanol to olefins) technology draws more and more attention for the production of light olefins instead of the traditional production of light olefins from naphtha. In recent years, demand of propylene increases rapidly, ethylene is the main product in the traditional production of light olefins from naphtha, with the production of propylene as by-product. As the ethylene feedstock becomes more and more light and diverse, conversion of methanol with high propylene selectivity has more economic value and practical significant.
ZSM-5 zeolite and SAPO-34 molecular sieve are regarded as the most valuable MTO catalysts and applied in commercial MTP (methanol to propylene) and MTO technology, respectively. Although SAPO-34 molecular sieve gives high light olefins selectivity, it deactivates rapidly due to coke deposition and necessitates to be regenerated frequently with high consumption of energy and investment. ZSM-5 zeolite is more resistant to deactivation, but its medium channel dimension leads to a low propylene selectivity and forms a large amount of C5+ hydrocarbons which is necessary to be recycled and cracked to produce propylene. At present, the greatest challenge for MTO research is still the development of effective catalyst.
MCM-22 molecular sieve has a unique lamellar structure, its acidity is similar to ZSM-5 zeolite, both of the dimensions of 10MR channels are wider than that of SAPO-34 molecular sieve and slightly narrower than that of ZSM-5 zeolite, therefore, MCM-22 molecular sieve is more selective to produce propylene than ZSM-5 zeolite. Moreover, the sinusoidal channels of MCM-22 molecular sieve is more resistant to deactivation, thus, it is a potential catalyst for MTO reaction.
In this thesis, MCM-22 molecular sieve was used as catalyst in MTO reaction. Morphology and acidity of MCM-22 molecular sieve was adjusted to improve its catalytic performances. The main contents and conclusions in this study are as follow:
(1) First MCM-22 molecular sieve with different Si/Al (silicon/aluminum) ratios (ranging from 10 to 50) was hydrothermally synthesized, the results from characterization and catalytic tests showed that the acidity of MCM-22 molecular sieve and its catalytic performances in MTO reaction were strongly affected by its Si/Al ratio. The stability and light olefins selectivity of MCM-22 molecular sieve in MTO reaction increased with its Si/Al ratio. However, high Si/Al ratio (Si/Al=49.8) gave rise to low crystallinity of MCM-22 molecular sieve, which led to the decrease of its stability and light olefins selectivity in MTO reaction. MCM-22 molecular sieve with Si/Al=35.1 gave the best catalytic performance, its catalytic life, defined as the time in which methanol was completely converted, light olefins selectivity, propylene selectivity and P/E ratio in the reaction products were 48 h, 68.1%, 39.9% and 7.6, respectively.
(2) Based on the results above, in order to increase the Si/Al ratio of MCM-22 molecular sieve and improve its catalytic performances in MTO reaction, MCM-22 molecular sieve was dealuminated with nitic acid, oxalic acid and ammonium hexafluorosilicate (AHFS). The results from ICP AES and 27Al MAS NMR showed, comparing with nitric and oxalic acid, AHFS could effectively decrease the content of aluminum in MCM-22 molecular sieve. Nitric acid is selective to eliminate framework aluminum (AlF), while AHFS is selective to eliminate extra-framework aluminum (AlEF), and for oxalic acid, framework aluminum and extra-framework aluminum are eliminated proportionally. The results from MTO reaction proved the lifetime of MCM-22 molecular sieve was affected by total acid sites and AlF/AlEF ratio, propylene selectivity was related to the amount of aluminum eliminated, proper dealumination was benefit for the increase of propylene selectivity and excess dealumination led to the decrease of propylene selectivity. MCM-22 molecular sieve dealuminated with 0.01mol/L AHFS solution (Si/Al ratio=100.9) had the longest lifetime, the highest light olefins selectivity and propylene selectivity, which were 54 h, 74.6% and 46%. The P/E ratio in reaction products was 10.9.
(3) For the further improvement of catalytic performances of MCM-22 molecular sieve in the MTO reaction, hierarchical MCM-22 molecular sieve was synthesized with carbon black used as the hard template and effects of the content of carbon black in the synthetic gel on the properties of hierarchical MCM-22 molecular sieve and its catalytic performances were investigated. The results from characterization proved that hierarchical MCM-22 molecular sieve was hollow nest-like aggregates and possessed exotic meso/macropores of 3-150 nm in diameter. Increasing the content of carbon black in the synthetic gel was benefit for the formation of the hollow nest-like aggregates and increased its meso/macropores. Comparing with the conventional MCM-22 molecular sieve, hierarchical MCM-22 molecular sieve gave longer lifetime and higher light olefins selectivity and propylene selectivity in the MTO reaction, but decreased P/E ratio in the products. Hierarchical MCM-22 molecular sieve synthesized with m(SiO2):m(carbon black)=4:2 gave the longest lifetime and highest light olefins selectivity and propylene selectivity, which were 108h, 70.6% and 41.6%, respectively. P/E ratio was 5.6. Hierarchical MCM-22 molecular sieve could decrease the coking rate in MTO reaction and among them, MCM-22(2C) gave the lowest coking rate. After dealuminated with 0.02mol/L AHFS solution, the lifetime, light olefins selectivity and propylene selectivity of hierarchical MCM-22 molecular sieve (Si/Al=85.1) in the MTO reaction increased, which were 124h, 74.2% and 44.6%, respectively. P/E ratio was 8.5.|