Studies On The Structural Modulation And Catalytic Performance Of Catalyst For Methanol To Propylene

Methanol to propylene (MTP) technology is a relatively independent method for propylene production. The successful development and wide application in petrochemical industry of MTP mean that the nontraditional paths to increase propylene production by coal and natural gas (or shale gas) is becoming riper, which is very significant to guarantee our energy security and to establish the pattern of chemical engineering industry in the future. To obtain a high selectivity and a stabilized yield of propylene are the primary targets in the development of MTP. Meanwhile they are also the key signs of the technological competitiveness. The regulation manners to improve the selectivity of catalyst are based on two orientations. One is to improve the diffusion ability of the pores, the other is to build or modify the acid distribution of the catalyst. However, these means are generally "offline". They didn’t take the reaction factors in account, which had "timely" effect on the catalyst structure. Thus they can’t well satisfy the dual requirements of catalyst for high selectivity and stable yield of propylene.In this thesis, the researches were carried out on the modulation of HZSM-5 zeolite by reaction factors. At the beginning, the relationship between the pores atmosphere and the diffusion behavior of dissolvent in the pores was built. The suppressive effect of steam aging on the coke deposition and the guiding effect of pre-coking and regeneration on the secondary coke deposition were studied. Afterwards, the modulation mechanism of reaction atmosphere, coking and regeneration on the catalyst pores and acid sties were revealed and an alternative scheme by using coking modulation cooperated with steam modulation was proposed. Furthermore, the application of coking modulation cooperated with steam modulation technology on the two-step MTP process was studied and a management concept of "carbon cycle" in moving bed reactor of MTP was proposed. Based on the steam aging catalyst, a kinetic model of MTP was established which considered the coke deactivation of catalyst, thus providing a significant guidance for the development of moving bed reactor for MTP reaction. The main work and achievement of this thesis can be summarized as follows.The delay mechanism for coke deposition of steam aging was studied. With the gravimetric method, the diffusion behaviors of methanol and toluene molecules in various steam aging catalysts with different structural parameters were investigated. The relationship between pores atmosphere and diffusion behaviors was established, and the mesoporous form which could improve the diffusion properties of catalyst was proposed. By comparative investigation of the structure and reactivity of catalyst after hydrothermal treatment under different aging temperature, WHSV and amount of steam conditions, the method for the introduction of "export-oriented" mesopores was proposed which could improve the diffusion properties of catalyst.The distribution of products under multi-dilution ratio with H2O and N2 was investigated in order to study their different diluent effects on the methanol to propylene (MTP) reaction with HZSM-5 zeolite. The different diluent mechanisms of N2 and H2O were presented. The chemisorption effect of water on the acid sties varied with time was studied. The results showed that the adsorption effect of H2O on acid sites would cause the remove of extra-framework aluminum, which mainly happened during the initial period of reaction. By steam aging of catalyst for 24 h, the unstable framework aluminum was removed, the acid density and the adsorption sites were reduced, and the high selectivity and stabilized yield of propylene were achieved.The effects on the catalyst acidity of citric acid treatment and "online" supplement of phosphorous for the long period MTP reaction were studied. The influences of acid treatment temperature, acid concentration and the time on the distribution of acid sties were investigated. Based on the mechanism that the citric acid could translate some non-framework aluminum into framework aluminum, thus increase the acid sites of catalyst, a steam aging technology combined with citric acid treatment was proposed. Meanwhile the effects of phosphorus content, precursor concentration, temperature, and time on the phosphorus shape, aluminum shape and their distribution were investigated. It was found that the high temperature was the direct factor to induce the combination of phosphorus atoms with aluminum atoms. A further investigation was made by studying the stability of these modified catalysts. The results verified that the catalyst which proved that the phosphorus had real effect to stabilize the framework aluminum by online supplement phosphorus.The modulation means with pre-coking technology was studied and established. The coke forms changed by the temperature and the time were investigated. In the meantime, the evolution of low-temperature coke to high-temperature coke was revealed. The pre-coked catalysts under different temperatures, times and raw materials were prepared. Moreover, the modulation effect of pre-coke on the structure of catalyst and the guiding effect on the secondary coking were studied. The technology using coke modulation cooperated with steam modulation was proposed. A "carbon cycle" management route in the moving bed MTP process was proposed as the result of the investigation of coke behavior of high-temperature coke under low temperature, and low-temperature coke under high temperature.The growing law of coke and the deactivation law of catalyst in each reaction-regeneration cycles were studied. The coke distribution of deactivated catalyst in a high methanol WHSV and a low methanol WHSV were investigated. The results showed that different coke location would lead to different deactivation mechanism, the high methanol WHSV resulted in shell coke deactivation, while the lower resulted in whole crystal coke deactivation. By tracking the diffusion behavior of methanol, toluene and isopentane in the catalysts with different coke contents, the coke growing process "from inside to outside" was proposed. Moreover, the competitive effects of water in the first cycle and coke in the second cycle on the catalyst deactivation were revealed by comparing the coke behavior and catalytic performance of fresh catalyst with regenerated catalyst.By using the steam aging catalyst, the influence of water on the product distribution was eliminated. In this case, the rules of MTP reaction with methanol feeding and methanol-olefins co-feeding in a methanol WHSV about 3-40 (gmethanoi/ gcat h) and temperature about 420～500℃ were studied. In order to study the generation path of aromatics and propane, the propylene was added to the methanol as the accelerator to produce aromatics and propane in a methanol WHSV about 0.5-5 (gmethanoi/gcat h)-Considering the coke deactivation of catalyst, kinetic model contained coke content was established, which could precisely describe the generation rate and consumption rate of 9 kinds of components. Furthermore, this model can be applied to predict the products distribution of MTP and MTP with recycle hydrocarbon reaction at the initial stage and the stages with different coke content.