Bok Tatiana O, Andriako Egor P, Knyazeva Elena E, Ivanova Irina I
A. V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences Laboratory No. 5, Leninskiy Prospect 29 119991 Moscow Russia
Department of Chemistry, Lomonosov Moscow State University Leninskie Gory 1, Bld.3 119991 Moscow Russia
RSC Adv. 2020 Oct 20;10(63):38505-38514. doi: 10.1039/d0ra07610d. eCollection 2020 Oct 15.
The mechanism of seeding of zeolite BEA steam assisted conversion has been studied using BEA seeds with different composition. The catalysts are characterized by X-ray diffraction, scanning and transmission electron microscopy, nitrogen adsorption-desorption, Hg-porosimetry, X-ray fluorescence and TPD of ammonia, and evaluated in benzene alkylation with propene. The results show that variation of the SiO/AlO ratio from 25 to 250 in BEA seeds changes the mechanism of seeding from "core-shell" to a "dissolution" mechanism, which can serve as a tool for engineering the morphological, textural and catalytic properties of BEA zeolites. Al-rich seeds (SiO/AlO = 25) do not dissolve during gel preparation and initiate dense oriented crystal growth on their surface resulting in the formation of large polycrystals (1-2 μm) with ordered densely intergrown nanocrystallites. In contrast, Si-rich seeds (SiO/AlO = 250) dissolve into tiny fragments, which serve as individual nuclei leading to formation of tiny isolated nanocrystallites aggregated into small hierarchical aggregates with high intercrystalline mesoporosity. The decrease of particle size and formation of intercrystalline mesoporosity in hierarchical aggregates improves the accessibility of acidic sites and facilitates the diffusion of reaction products, which leads to the significant improvement of catalytic activity and reduces the deactivation resulting in higher stability with time on stream in cumene synthesis from benzene and propylene.
利用不同组成的BEA晶种研究了BEA沸石蒸汽辅助转化的晶种形成机理。通过X射线衍射、扫描和透射电子显微镜、氮吸附-脱附、压汞法、X射线荧光和氨的程序升温脱附对催化剂进行了表征，并在苯与丙烯的烷基化反应中对其进行了评价。结果表明，BEA晶种中SiO/AlO比从25变化到250时，晶种形成机理从“核壳”转变为“溶解”机理，这可作为一种调控BEA沸石形态、织构和催化性能的工具。富铝晶种（SiO/AlO = 25）在凝胶制备过程中不溶解，并在其表面引发密集定向的晶体生长，从而形成具有有序密集共生纳米晶体的大尺寸多晶体（1-2μm）。相反，富硅晶种（SiO/AlO = 250）溶解成微小碎片，这些碎片作为单个晶核，导致形成微小孤立的纳米晶体，聚集成具有高晶间介孔率的小尺寸分级聚集体。分级聚集体中粒径的减小和晶间介孔的形成提高了酸性位点的可及性，并促进了反应产物的扩散，这导致催化活性显著提高，并减少失活，从而在由苯和丙烯合成异丙苯的过程中具有更高的随时间稳定性。
