稀释蒸汽中Na+及积炭对甲醇制丙烯催化剂性能影响

考察了稀释蒸汽中Na⁺及积炭对甲醇制丙烯(MTP)催化剂物理化学性质和催化性能影响, 及离子交换后催化性能. 采用X射线衍射(XRD)、扫描电镜(SEM)、X射线荧光(XRF)光谱、N₂吸附-脱附、程序升温氨脱附(NH3-TPD)和热重(TG)分析等方法对失活和再生催化剂进行了表征, 并在101325 Pa、470℃ 和甲醇空速(WHSV)为1.0-3.0 h^-1的反应条件下, 采用连续流动固定床微型反应器考察其催化甲醇制丙烯性能. 结果表明: MTP反应970 h后的催化剂晶体结构和形貌没有受到明显破坏, 但稀释蒸汽中Na⁺极易扩散至催化剂表面,部分取代H质子的位置, 从而使催化剂酸性逐渐下降而中毒失活; 另外, MTP催化剂表面的积炭导致分子筛微孔堵塞是造成其失活的主要原因, 可通过烧炭再生过程消除, 而水蒸汽脱铝对催化剂性能的影响缓慢但更严重. 用再生和离子交换处理后, Na⁺中毒催化剂MTP反应性能基本完全恢复. 在470 h反应过程中, 甲醇转化率保持在99%以上, 丙烯选择性大于46%, 且随着反应时间的延长, 丙烯选择性逐渐升高、乙烯选择性逐渐下降.

Effects of Na+ in Dilution Steam and Coke Deposition on Catalytic Performance of Methanol-to-Propylene Catalysts

The effects of Na⁺ in dilution steam and coke deposition on the physicochemical properties andcatalytic performance of ZSM-5 catalysts for the methanol-to-propylene (MTP) reaction were investigated.The deactivated and regenerated catalysts were characterized by means of X-ray diffraction (XRD),scanning electron microscopy (SEM), X-ray fluorescence (XRF) spectrum, nitrogen adsorption/desorption,temperature-programmed desorption of ammonia (NH₃-TPD), and thermogravimetry (TG). Their catalyticperformance for MTP reaction was tested in a continuous flow fixed-bed micro-reactor at 470℃, 101325Pa, and with methanol weight hourly space velocity (WHSV) in the range of 1.0-3.0 h^-1. The resultsindicated that the catalyst crystal structure and morphology was not significantly altered after 970 h onstream. In the MTP reaction, Na⁺ in the dilution steam can easily enter the pore channels of the catalyst,and partially replace H protons, thereby gradually decreasing the amount of acidity and acid strength of thecatalyst, which eventually causes deactivation. In addition, coke deposits on the catalyst surface blocking its micropores are the main reason for deactivation of the MTP catalyst. Coke deposits are mostlyeliminated through the burning charcoal regeneration process. The effect of framework dealumination fromthe catalyst by steam in the MTP process is slow but more serious. Through regeneration and ionexchange process, the catalytic activity of the deactivated catalyst can be fully restored. The conversion ofmethanol is consistently above 99%, and propylene selectivity is greater than 46% even after 470 h onstream. With increasing reaction time, the propylene selectivity gradually increases, while ethyleneselectivity gradually decreases.