Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, 200433 Shanghai, PR China.
Chemistry. 2012 Jan 16;18(3):931-9. doi: 10.1002/chem.201102505. Epub 2011 Dec 14.
A series of core-shell-structured composite molecular sieves comprising zeolite single crystals (i.e., ZSM-5) as a core and ordered mesoporous silica as a shell were synthesized by means of a surfactant-directed sol-gel process in basic medium by using cetyltrimethylammonium bromide (CTAB) as a template and tetraethylorthosilicate (TEOS) as silica precursor. Through this coating method, uniform mesoporous silica shells closely grow around the anisotropic zeolite single crystals, the shell thickness of which can easily be tuned in the range of 15-100 nm by changing the ratio of TEOS/zeolite. The obtained composite molecular sieves have compact meso-/micropore junctions that form a hierarchical pore structure from ordered mesopore channels (2.4-3.0 nm in diameter) to zeolite micropores (≈0.51 nm). The short-time kinetic diffusion efficiency of benzene molecules within pristine ZSM-5 (≈7.88×10(-19) m(2) s(-1)) is almost retainable after covering with 75 nm-thick mesoporous silica shells (≈7.25×10(-19) m(2) s(-1)), which reflects the greatly opened junctions between closely connected mesopores (shell) and micropores (core). The core-shell composite shows greatly enhanced adsorption capacity (≈1.35 mmol g(-1)) for large molecules such as 1,3,5-triisopropylbenzene relative to that of pristine ZSM-5 (≈0.4 mmol g(-1)) owing to the mesoporous silica shells. When Al species are introduced during the coating process, the core-shell composite molecular sieves demonstrate a graded acidity distribution from weak acidity of mesopores (predominant Lewis acid sites) to accessible strong acidity of zeolite cores (Lewis and Brønsted acid sites). The probe catalytic cracking reaction of n-dodecane shows the superiority of the unique core-shell structure over pristine ZSM-5. Insight into the core-shell composite structure with hierarchical pore and graded acidity distribution show great potential for petroleum catalytic processes.
一系列核壳结构复合分子筛由沸石单晶（即 ZSM-5）作为核和有序介孔硅作为壳组成，通过在碱性介质中使用十六烷基三甲基溴化铵（CTAB）作为模板和正硅酸乙酯（TEOS）作为硅源的表面活性剂导向溶胶-凝胶法合成。通过这种涂层方法，均匀的介孔硅壳紧密地生长在各向异性沸石单晶周围，壳厚可通过改变 TEOS/沸石的比例轻松调节在 15-100nm 范围内。所得复合分子筛具有紧密的介孔/微孔交界处，形成了从有序介孔通道（直径 2.4-3.0nm）到沸石微孔（≈0.51nm）的分级孔结构。在覆盖 75nm 厚的介孔硅壳后，苯分子在原始 ZSM-5 中的短时间动力学扩散效率（≈7.88×10(-19)m(2)s(-1))几乎保持不变（≈7.25×10(-19)m(2)s(-1)），这反映了紧密相连的介孔（壳）和微孔（核）之间大大开放的交界处。核壳复合显示出对大分子（如 1,3,5-三异丙基苯）的吸附能力大大增强（≈1.35mmol g(-1))相对于原始 ZSM-5（≈0.4mmol g(-1))，这是由于介孔硅壳的存在。在涂层过程中引入 Al 物种时，核壳复合分子筛表现出从介孔的弱酸性（主要为路易斯酸位）到沸石核的可及强酸性（路易斯酸和布朗斯台德酸位）的梯度酸度分布。正十二烷的探针催化裂化反应表明，独特的核壳结构优于原始 ZSM-5。对具有分级孔和梯度酸度分布的核壳复合结构的深入了解显示出在石油催化过程中具有巨大的潜力。
