A Study On Catalytic Conversion Of Methanol To Olefins Over Rare Earth Metals (La, Y) Modified SAPO-34

Catalytic conversion of methanol to olefins (called MTO reaction) has been the subject that received a global particular concern, as it may resolve the problem of the deficiency of light olefins caused by the shortage of petroleum. The key issue for the MTO reaction is to develop a new catalyst with excellent catalytic performance. SAPO-34 molecular sieve has turned out to be a superior catalyst for the MTO reaction, due to its high activity for the reaction and high selectivity to ethylene and propylene. Nevertheless, as a micropore zeolite material, SAPO-34 is known to suffer rapid deactivation due to coke deposition during the MTO reaction, which leads to the serious decrease in activity and selectivity of the zeolite towards the aimed reaction. Consequently, how to reduce the coke formation over the zeolite so as to reduce the regeneration frequency of the catalyst is a subject that is urgently required to be solved for the practical application of MTO reaction. Thus, the following research work in our investigations was carried out and the obtained results were described below:(1) SAPO-34 was successfully modified by rare earth metals (La or Y) in solid state ion exchange or liquid-phase ion exchange methods. The MTO reaction results indicated that the Re-SAPO-34-L catalysts, which were prepared by the modification of SAPO-34 with the rare earth metals in liquid-phase ion exchange method, behaved no different catalytic performance from that of SAPO-34. However, the Re-SAPO-34-S catalysts, which were prepared by the modification of SAPO-34 with the rare earth metals in solid state ion exchange method, gave substantially improved catalytic performance compared with that of the parent SAPO-34. Under the reaction conditions, the lifetime of the Re-SAPO-34-S catalysts was significantly prolonged and the selectivity to the aimed products was largely increased. For example, the lifetime of the 3%Y-SAPO-34-S catalyst reached 185 minutes that was ca. 30 minutes longer than that of the parent SAPO-34, and the selectivity to C₂⁼-C₄⁼ over the catalyst increased to 95 % that was absolutely ca. 5% higher than that over SAPO-34. The improved catalytic performance over the 3%Y-SAPO-34-S catalyst can be correlated well with the distinct suppression of methane production in the reaction.(2) The structures of modified SAPO-34 catalysts were studied by means of XRD, FT-IR, BET, UV-vis-DRS, XRF and ICP. The results confirmed that rare earth metal ions in the solid-state ion exchanged samples had incorporated into the zeolite framework (substituted Al in the AlO₄ tetrahedron), whereas they rarely existed there for the liquid-phase ion exchanged samples. Therefore, the modified catalytic performance for the Re-SAPO-34-S catalysts can be reasonably attributed to the incorporation of rare earth metal ions into the zeolite framework.(3) The lifetime of the Re-SAPO-34-S catalysts, which was influenced by the reaction temperature, methanol weight hourly space velocity, molar ratio of methanol to water in the feed as well as the amount of rare earth metal in the catalyst, was investigated in the continuous flowing fixed bed reaction system. In terms of the lifetime of the catalyst, the optimum reaction conditions proposed in this paper are as follows: the reaction temperature is 450℃,the methanol weight hourly space velocity is 2 h^-1 and the molar ratio of methanol to water is 1:5.(4) The reproducibility of the 3%Y-SAPO-34-S catalyst for catalyzing the MTO reaction was also studied. It showed that the catalytic performance of the regenerated catalyst after 3 times changed little compared with the fresh one in terms of the lifetime and the conversion of methanol obtained over the catalyst.