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Alternative TitleSynthesis of Nano-micro Sized ZSM-5 from Coal-series Kaolin and Investigation of the Catalytic Perfo
Thesis Advisor卢旭晨
Degree Grantor中国科学院研究生院
Degree Discipline化学工程
Keyword煤系高岭土   纳微尺度zsm-5   成核壁垒   液相机理   mto
AbstractZSM-5分子筛具有独特的孔道结构和易调变的骨架组成,在多相酸催化和氧化反应中表现出优异的择形催化能力,广泛应用于石油炼制与精细化工等领域,尤其是具有较小晶体尺寸的ZSM-5分子筛用作催化剂时表现出良好的催化性能。具有小晶粒ZSM-5分子筛通常以化工硅铝源为原料,采用水热法晶化合成。然而,以化工原料合成ZSM-5沸石存在成本高、效率低等问题。高岭土资源储量丰富,价廉易得,已成功制备出几十种硅铝酸盐分子筛。目前,以高岭土为原料制备ZSM-5分子筛存在成核壁垒,导致效率低、产品结晶度低和晶体较大。因此,本论文以煤系高岭土作为复合硅铝源,探索纳微尺度ZSM-5分子筛的高效绿色制备方法,克服其晶化过程中的成核壁垒,优化合成条件,分析晶化机理,并考察其催化性能。取得的创新性结果如下: (1)以煤系高岭土为原料,在无胺体系中成功合成了微米ZSM-5,将其作为晶种加入体系合成了高结晶度的亚微米ZSM-5。晶种的加入可为体系提供晶核,降低晶化所需的活化能,提高晶化速率,有效地缩短晶化时间至6h,减小晶体尺寸至亚微米级。所得到的亚微米ZSM-5呈六方片状(axis a × b × c: 800 ~1000nm × 400~500 nm × 50~100 nm),比表面积为341.7 m2?g-1。此外,晶种的加入扩大了制备高结晶度亚微米ZSM-5的“工作窗口”。在无胺体系中,Na+取代有机胺作为模板剂,诱导硅铝物种围绕其水合离子形成晶核并生长,并参与平衡骨架电荷。将煤系高岭土合成的ZSM-5分子筛用于甲醇转化制低碳烯烃(MTO)反应,在相同硅铝比和反应条件下,基于高岭土的亚微米ZSM-5的催化性能优于化工原料制备的ZSM-5。 (2)以煤系高岭土为原料,采用固相热反应的方法快速地合成纳米ZSM-5团聚体。固相热反应体系中的水仅来自于TPAOH溶液,含量极低,硅铝物种易达到过饱和状态,并围绕TPA+离子成核和生长。此外,TPA+离子起着晶核间的限域作用,使晶化产物为纳米ZSM-5颗粒的团聚体。与传统水热法相比,固相热合成具有无废液排放,晶化时间短及效率高等特点,在190℃晶化2h便可得到由20-40nm纳米晶体组成的高结晶度纳米ZSM-5团聚体,其团聚体大小约为200-600nm;比表面积高达446.4 m2?g-1。考察了晶化条件如模板剂用量、碱用量等对结晶度及形貌的影响,并对其晶化过程物相变化及硅铝物种配位关系进行了详细表征。将纳米ZSM-5团聚体用于MTO催化反应,与传统水热合成的ZSM-5分子筛相比,固相热合成的纳米ZSM-5团聚体具有更好的催化活性,相同条件下连续运行26h,甲醇的转化率仍可达到95%,C2-C4烯烃的最高选择性可高出水热法制备的ZSM-5分子筛19%。 (3)以煤系高岭土为原料,无钠体系蒸汽协助转化法直接合成出HZSM-5分子筛。以无机氨水作为蒸汽源,替代NaOH为体系提供碱度,并提供与Na+半径接近的NH4+进入骨架平衡电荷。系统地考察了氨水的作用及用量对HZSM-5晶化速率和晶体形貌的影响。结果表明,提高氨水用量可提高晶化速率,190℃晶化2h便可得到高结晶度的亚微米HZSM-5分子筛,但对晶体形貌无显著影响。该方法避免了进行铵交换和焙烧两道工序,不但可以降低生产成本,而且能提高生产效率。 (4)煤系高岭土合成纳微尺度ZSM-5分子筛遵循溶解结晶机理。综合分析了无胺水热体系、固相热合成和蒸汽协助转化合成ZSM-5分子筛晶化过程物相及硅铝物种配位关系的变化,分别讨论了体系中原料结构、碱度及水的作用。结果表明,高岭土原料经过焙烧和脱铝预处理后,具有很高的反应活性,在热碱性环境中发生“溶解”,围绕模板离子(Na+或TPA+)成核,生长成ZSM-5沸石分子筛。 (5)考察了分子筛的性能(晶体尺寸和硅铝比)和反应条件(反应温度和甲醇浓度)对MTO脱水过程的影响。结果表明,MTO过程甲醇的转化率和产物选择性与ZSM-5分子筛晶体尺寸、酸性有密切关系;较小的ZSM-5分子筛晶体有利于提高对烯烃的选择性;反应温度及甲醇浓度显著地影响产物的分布及选择性。
Other AbstractDue to its unique porous structure, good hydrothermal stability and selectivity, ZSM-5 has been widely used as catalytic materials and adsorption materials. Especially, ZSM-5 catalysts with small crystal show excellent catalytic performance. Generally, ZSM-5 is synthesized from chemical silicon and aluminum sources by hydrothermal method. However, some problems need to be solved, such as high costs of raw materials, low yield of products. Natural kaolin resource, which is abundant and relatively cheap, has been successfully applied in the synthesis of dozens of aluminosilicate molecular sieves. However, due to the existence of nucleation barrers, some problems also need to be solved, such as low crystallinity, large crystalline and low efficiency. In this research paper, we used coal-series kaolin as combinational Si and Al sources to synthesize nano-micro sized ZSM-5 zeolite efficiently, overcame the nuclear barriers during the crystallization process, investigated the optimized synthesis conditions, analyzed the crystallization mechanism, and investigated the catalytic performance in MTO process. The creative results are as follows: (1) Submicron ZSM-5 has been synthesized successfully without organic template in the assistance of crystal seeds by using coal-series kaolin as raw materials. The crystal seeds could provide crystal nucleus, lower the activiation energy needed during the crystallization process, enhance the crystallization rate, shorten the crystallization time to 6 h effectively, and reduce the crystal size to submicron size. The submicron ZSM-5 showed hexagonal plates (axis a × b × c: 800 ~1000nm × 400~500 nm × 50~100 nm) with high specific surface area of 341.7 m2?g-1. Furthermore, the “working window” was enlarged as the crystal seeds were added into the reaction system. In the orgainic template free system, Na+ acted as template agent, induced the Si and Al species to form crystal nucleus and grow. Moreover, Na+ got into the skelecton and banlance the charge. When the submicron ZSM-5 was used as catalyst in methanol to olefins (MTO) reaction, the catalytic performance was better than that of ZSM-5 synthesized from chemical materials. (2) Fast route for the synthesis of nano-sized ZSM-5 aggregates by solid-state conversion. In the solid-state reaction system, Si and Al species could reach supersaturation easily because the content of water was quite low, and form crystal nucleus around the TPA+. In addition, TPA+ acted as comfined space agents, which prevented the nano-sized crystal to grow furtherly. Compared with the traditional hydrothermal method, solid-state conversion has the following obvious advantages: high yields of products; reducing the pollutant significantly; shortening the crystallization time greatly and enhancing the production efficiency. Highly crystalline nano ZSM-5 aggregates were obtained within 2h at 190℃, the crystal size was about 200-600 nm which consisted of 20-40 nm particles. The specific surface area was as high as 446.4 m2?g-1.The effect of synthesis conditions such as content of TPAOH, NaOH/SiO2 ratios, and the change of phases and the coordination relationship of silicon aluminum species during the crystallization process were researched systematically. Compared with the traditional hydrothermal method, the sample synthesized by solid-state conversion exhibited a longer lifetime to endure coke deposition in methanol dehydration reaction. (3) Direct synthesis of HZSM-5 zeolite by steam assisted conversion (SAC) method in the absence of Na+. During the synthesis process, we used NH3?H2O as alkali source to replace the NaOH. NH3?H2O not only provided the basicity, but also provided NH4+ to balance the charge in the framework. The effect of NH3 stream on the crystallization rate and crystal size was investigated systematically. The results demonstrated that the NH3?H2O increased influence the crystallization rate obviously, but had no significant effect on the the morp
Document Type学位论文
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GB/T 7714
潘锋. 基于煤系高岭土合成纳微尺度ZSM-5及其催化性能研究[D]. 中国科学院研究生院,2014.
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