【作者】 李俊杰；

【导师】 宋春山； 郭新闻；

【作者基本信息】 大连理工大学 ， 工业催化， 2019， 博士

【摘要】 沸石是微孔结晶型材料,具有可调的酸性、优异的孔道择形性和高水热稳定性,在催化及分离等方面有广泛应用。ZSM-5类沸石是甲醇制丙烯(MTP)反应的催化材料,然而其微孔结构容易积炭,导致其在MTP反应中的稳定性较低。在沸石中引入介孔或大孔结构,制备多级孔沸石,可显著改善扩散性能。近些年,研究者们开发了许多制备多级孔沸石的方法,主要包括后处理法和模板法。本论文通过碱处理调变ZSM-5的孔道结构和酸性,探究孔道结构和酸性变化对MTP反应性能的影响,通过优化碱处理方法制备高性能MTP催化剂。主要研究结果如下:通过碱处理调变ZSM-5的孔结构,研究孔结构变化对MTP催化性能的影响。以四丙基氢氧化铵(TPAOH)为模板剂合成的ZSM-5外表面富铝,而内部富硅,且含有大量骨架缺陷。对其直接进行碱处理,晶体内部组分被选择性移除,而外表面组分的刻蚀程度较小,最终得到封闭的中空结构。在MTP反应中,中空ZSM-5的微孔外壳仍然不利于扩散,导致稳定性较碱处理前提高有限。通过NH4F改性调变铝配位状态,再经碱处理后可得到一系列具有不同多级孔结构的ZSM-5。在优化的处理条件下,成功引入均匀且贯通的介孔结构。在MTP反应中,具有均匀且贯通介孔结构的ZSM-5催化寿命是中空ZSM-5的3倍,同时丙烯选择性提高12%。通过碱处理调变ZSM-5的酸性,研究酸分布和酸量变化对MTP催化性能的影响。对于外表面富铝的ZSM-5,TPAOH处理选择性移除晶体内部富硅区域,被移除的硅在TPA+的导向作用下进行二次晶化,使硅重新分布,酸分布也发生改变。母体ZSM-5的外表面含有大量Bronsted酸,而中空ZSM-5的外表面几乎不含Bronsted酸。在MTP反应中,中空ZSM-5孔口处积炭反应减少,使催化寿命延长至母体的2倍;经NH4F改性后的ZSM-5,TPAOH处理同时脱硅和脱铝,被移除的硅和铝进行二次晶化,使硅铝重新分布,酸分布较母体更均匀。而且,NH4F-TPAOH联合处理得到介孔ZSM-5,扩散性能得到明显改善。在MTP反应中,具有均匀铝分布的介孔ZSM-5的催化寿命是母体的4倍。NaOH处理可调变ZSM-5的酸量,酸量变化规律与铝含量有关。经碱处理后,铝含量较高的样品的强酸量减少,而铝含量较低的样品的强酸量增加。铝含量不同的ZSM-5,经碱处理后,催化寿命均延长约2倍;而丙烯选择性与强酸量紧密相关,微孔和介孔ZSM-5的丙烯选择性均随强酸量增加而降低。优异的MTP催化剂兼具多级孔结构和适宜的酸量。然而适合碱处理造介孔的ZSM-5的Si/Al在25～50左右,处理得到的多级孔ZSM-5酸量较多,导致其丙烯选择性较低。高Si/Al ZSM-5经碱处理后的酸量适宜,但固体收率低于40%,同时碱处理产生大孔结构。采用碱处理硼硅MFI沸石(B-MFI)纳米团聚体时加入Al(NO3)3的方法,可得到由许多小空腔构成的肺泡状ZSM-5,总孔容达到1.28cm3/g,固体收率高于60%。与Silicalite-1(纯硅MFI沸石)相比,硼的加入有助于NaOH溶液中A13+插入沸石骨架,成为四配位骨架铝,使肺泡状ZSM-5具有适宜的酸量。在MTP反应中,其催化寿命是高Si/Al ZSM-5和多级孔ZSM-5的两倍多,同时丙烯选择性提高4%。在MTP反应中,高Si/A1(>300)ZSM-5的丙烯选择性非常高(>50%),但稳定性较差,少量活性位点被积炭覆盖后,即导致转化率和丙烯选择性大幅下降。反应过程中,在其转化率下降前引入新活性位点,可使其保持较高丙烯选择性的同时,提高催化稳定性。惰性Silicalite-1经AlCl3改性后产生优异的催化性能。通过NH3-TPD、27Al MAS NMR和OH-IR等证明,在反应过程中,不断有铝插入MFI沸石骨架,成为四配位骨架铝。通过调控反应过程中的原位铝迁移,制备了一系列高性能MTP催化剂。酸洗后的B-MFI沸石经AlCl3改性后,可保持100%转化率达360h,丙烯选择性稳定在50%以上,反应前期200h内,低碳烯烃选择性达到80%以上。Silicalite-1与Al2O3挤条成型并酸洗后,在MTP反应中可稳定运行437 h,丙烯选择性稳定在52%以上,丙烯选择性/乙烯选择性(P/E)为11。将原位铝迁移引入碱处理后的ZSM-5中,得到的催化剂在苛刻条件下可稳定运行968 h,丙烯选择性稳定在53%以上。更多还原

【Abstract】 Zeolites are microporous crystalline materials with tunable acidity,shape-selective pores,and high hydrothermal stability.They have been widely used in industrial applications including catalysis and separation.While zeolite of ZSM-5 type is important as catalytic material for the methanol to propylene(MTP)process,it deactivates quickly due to the intrinsically dominant micropores which can be blocked by the coke.Introducing additional mesopores or macropores into zeolites significantly increases the catalytic stability.In recent years,researchers have developed many ways for preparing hierarchical zeolites,including demetallation and template methods.This doctoral thesis studies the influences of alkaline treatment on ZSM-5 pore structure and acidity.It addresses the effects of pore structure and acidity on the catalytic performance.Superior MTP catalysts were obtained by tailoring pore structure and acid properties simultaneously.A series of materials and treatments were examined and the results are summarized as follows.TPAOH templated ZSM-5 showed an aluminum-rich exterior and a silicon-rich defective interior.NaOH treatment selectively removed the inner portion,while the external surface was less corroded,resulting in a hollow ZSM-5.This material was ineffective for suppressing coke deposition,and only limited improvement of stability was observed.The NH4F treatment can modify A1 coordination in ZSM-5,thus distinct hierarchical structures were introduced into ZSM-5 by a subsequent NaOH treatment.Interonnetcted and uniform mesopores were introduced into Al-zoned ZSM-5 by optimized treatment methods,which resulted in a longer lifetime that was three times longer than hollow ZSM-5,while the propylene selectivity increased by 12 percent.The acid property,including acid amount and distribution,can be tailored by alkaline treatment.TPAOH treatment selectively removed the silicon-rich interior of Al-zoned ZSM-5,meanwhile the removed Si species recrystallized on the zeolite surface by TPA+ template,and the acid sites distribution was varied by the redistribution of Si sites.Parent ZSM-5 showed a large amount of external Bronsted acid sites,while there were few external Bronsted acid sites on the hollow ZSM-5,which inhibited external coke deposition during the MTP reaction.Therefore the lifetime of the hollow ZSM-5 with more silica-rich shell was doubled when compared with the parent ZSM-5.TPAOH treatment of NH4F modified ZSM-5 removed both Si and A1 sites,meanwhile Si and Al sites redistributed through the recrystallization,obtaining a more uniform A1 distribution.Besides,mesoporous ZSM-5 was obtained by sequential NH4F and TPAOH treatments.Mesoporous ZSM-5 with a more uniform acid sites distribution showed four times longer lifetime of parent ZSM-5.NaOH treatment could modulate acid amount,and the variation of the acid amount was directly related with the A1 content in ZSM-5.The amount of strong acid decreased significantly after NaOH treatment of samples with a higher Al content,while it increased for samples with a lower A1 content.Alkaline treated ZSM-5 showed three times longer lifetime than the parent ZSM-5.But the propylene selectivity showed close relationship with the strong acid amount,the propylene selectivity decreased with the increasing amount of strong acid over microporous and mesoporous ZSM-5.A superior MTP catalyst may have a hierarchical structure and a suitable amount of acid sites.A suitable Si/Al ratio for alkaline treatment was between 25 and 50,the corresponding mesoporous ZSM-5 obtained by alkaline treatment contains too many acid sites,which showed a low propylene selectivity.Catalysts with suitable amounts of acid sites were observed by alkaline treatment of ZSM-5 with a higher Si/Al ratio,but it showed low solid yield below 40%,and some macropores were generated.Hierarchical alveolus-like ZSM-5,composed of numerous hollow crystals,was obtained by NaOH treatment of B-MFI nano-aggregates with an addition of Al(NO3)3 solution,it showed a higher pore volume of 1.28 cm3/g and a higher solid yield above 60%.The addition of boron facilitated the insertion of Al3+ in the framework structure,as framework tetrahedral Al sites.Alveolus-like ZSM-5 with a suitable amount of acid sites showed two times longer lifetime than ZSM-5 with a high Si/Al ratio and hierarchical ZSM-5 during MTP reaction,and the propylene selectivity increased by four percent.ZSM-5 with a much higher Si/Al ratio(>300)showed higher propylene selectivity(>50%)during the MTP reaction,but a low stability.Once the active sites were covered by coke,the methanol conversion and propylene selectivity decreased sharply.If some new active sites could be introduced into ZSM-5 with a high Si/Al ratio,during the reaction,higher propylene selectivity and stability could be obtained.A superior catalytic performance was obtained with otherwise inactive Silicalite-1 after AICl3 modification.By using NH3-TPD,27Al MAS NMR,and OH-IR characterization,it is determined that a portion of Al atoms were inserted into the framework structure,serving as four-coordinate framework Al.A series of superior catalysts were tailor-made by regulating in-situ Al migration.Large amounts of defects were generated after acid washing of borosilicate MFI,producing a superior catalyst with a longer lifetime of 360 h,stable propylene selectivity higher than 50%,and a high selectivity of light olefin above 80%before 200 h TOS.Al2O3 extruded Silicalite-1 showed a life-span of 437 h,a stable high propylene selectivity above 52%,and a high propylene/ethylene(P/E)ratio of 11 after acid washing.A superior catalyst,with longer lifetime of 968 h and higher propylene selectivity of 53%,was designed by introducing in-situ Al migration into hierarchical ZSM-5,which combined the modulation of acidity and pore structure.更多还原