Synthesis And Properties Of Zeolite / Mesoporous Silica And Carbon Nanotube / Mesoporous Carbon Core - Shell Composite

Porous materials have been widely applied in catalysis, adsorption and separation for their opened frameworks, large specific surface areas and ordered pore structures. Moreover, porous materials have brought huge economic benefits for human beings. Since the preparation of Y zeolite by the UOP Company in 1960 and ZSM-5 later by Mobil, zeolite molecular sieves have been used as catalysts in the catalytic cracking of petroleum, which have been changing the energy structure and chemical composition of the present society. However, traditional zeolite molecular sieves are largely restricted by their limited pore size (less than 1.3 nm) in many applications, which can not satisfy in the large-molecule involved applications, such as the catalytic conversion of heavy oil molecules. In 1992, the report of MCM-41 series mesoporous silica materials with large surface areas,2-D or 3-D ordered mesostructures by Mobil aroused widespread concerns. Afterwards, the successful synthesis of SBA series mesoporous silica materials with larger pore size and higher stability built a good foundation for their applications in large-molecule catalysis, fine chemicals, adsorption and separation, sensors and drug delivery etc. Nevertheless, the amorphous properties of these mesoporous materials resulted in the poor hydrothermal and mechanical stability, weak acidity. Therefore, these materials were failed to achieve the requirements of industrial applications.Functional design of nanomaterials is a strong driving force to promote the development of nanomaterials science. Core-shell composite materials are a new type of functional materials in terms of distributing different compositions with various functionalities and pore structures spatially on nanoscale. Based on the international trend in porous material synthesis and heavy oil conversion, this thesis presents a systematic study of the core-shell structured composite molecular sieves regarding structure construction and exploration, growth mechanism and application in catalysis etc. MWNT@mesoporous carbon core-shell composite were also constructed and the structure exploration and application in electrochemical energy storage were also taken into account.In chapter 2, the structure and acidity properties of hierarchically core-shell structured composite molecular sieves were explored. A series of core-shell structured composite molecule sieves comprising zeolite single-crystal (i.e., representative ZSM-5) as a core and ordered mesoporous silica as a shell were synthesized via a surfactant-directed sol-gel process in basic medium by using cetyltrimethyl-ammonium bromide (CTAB) as a template and tetraethylorthosilicate (TEOS) as a silica precursor. Through this coating method, uniform mesoporous silica shells closely grow around the anisotropic zeolite single-crystals, and the shell-thickness of which can easily be tuned in the range of 15-100 nm by changing the mass ratio of TEOS/zeolite. The obtained composite molecular sieves have compact meso-/micro-pore junctions to 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 m2/s) is almost retainable after covering with 75-nm-thick mesoporous silica shells (～7.25×10-19 m2/s), reflecting the highly opened junctions between closely connected mesopores (shell) and micropores (core). The core-shell composite shows greatly enhanced adsorption capacity (～1.35 mmol/g) for large molecule 1,3,5-triisopropylbenzene relative to that of pristine ZSM-5 (～0.4 mmol/g) 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 Br(?)nsted 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 potentials in petroleum catalytic processes.In chapter 3, we report an ultra-dilute liquid-phase coating strategy in an acidic medium for controllable synthesis of uniform micro/mesoporous core-shell composites zeolite@SBA-15 comprising zeolite cores and mesoporous silica SBA-15 shells using triblock compolymer Plunoric P123 as a template. Structural characterizations show that the core-shell composites possess tuneable specific surface areas (115-228 m2/g), large pores (～7.0 nm in diameter) with plenty of mesotunnels (～3.0 nm) from silica shells, original crystalline zeolite frameworks and opened junctions between micropores and mesopores. The silica shells have ordered 2-D hexagonal mesopore channels, most of which are annularly parallel (fingerprint-like arrangement) to the anisotropic zeolite faces. The shell-thickness is crystal face-dependent. Moreover, the synthesis parameters such as MgSO4 additive, stirring rate, acidity, temperature and reaction time show great influences on the formation of uniform core-shell composites. Post-hydrothermal treatment at 100℃has been for the first time adopted to improve mesostructural regularity of the core-shell composites. A scheme regarding surface-induced micellization and hydrothermal rearrangement of mesostructured silica shells in the coating process is proposed to illustrate the formation of core-shell composites. The core-shell composite HZSM-5@SBA-15 (HZ@S15) was employed as a catalyst for methanol to propylene (MTP) conversion, and shows excellent catalytic performance with high methanol conversion (～98%) and propylene to ethylene (P/E) ratio (～10.7) as well as propylene selectivity (～39%).In chapter 4, we report a liquid-phase coating strategy in an acidic medium for controllable synthesis of uniform meso-/micro-porous core-shell composites comprising zeolite cores and cage-like mesoporous silica shells using triblock compolymer Plunoric F108 as a template, TEOS as a silica source and ZSM-5 as core materials. The mesoporous silica shells are composed of cage-like pores with different pore sizes and the shell-thickness is～70 nm. The core-shell composites possess high specific surface areas, large pores (2.9-8.4 nm in diameter) from silica shells, original crystalline zeolite frameworks and opened junctions between micropores and mesopores. Pt nanoparticles with particle size of～3.2 nm were well-dispersed in the mesoporous silica shells. Toluene oxidation reaction of Pt supported catalysts shows that the zeolite cores with strong acidity facilitate the oxidation reaction and gives rise to high activity, but it may enhance coke formation and decrease the catalyst stability. Pt supported core-shell composites have a higher catalytic stability than the mixture catalysts.In chapter 5, based on the desired electrical conductivity and high specific-surface-area for carbon-based electrodes, we have designed and synthesized uniform multiwall carbon nanotube@mesoporous carbon (MWNT@mesoC) composites with core-shell configuration by combining sol-gel methods and nanocasting. Pristine MWNTs after acid treatment were first coated with uniform mesostructured silica shells to obtain the MWNT@mesoporous silica (MWNT@mesoS) composite using cationic surfactant cetyltrimethyl ammonium bromide (CTAB) as a template. Then, furfural alcohol (carbon source) and oxalic acid (catalyst) were impregnated into the template-free MWNT@mesoS composite and followed by carbonization. The removal of silica led to the replacement of the mesoC shells decorated on the surface of MWNTs. The obtained composite materials retain the one-dimension (1-D) tubular structure and three-dimension (3-D) entangled framework as the original MWNTs. Micro/nanostructure exploration demonstrates that each MWNT is uniformly coated by the mesoC shell with short-pore-length (～15 nm), which contributes above 300 m2/g to specific surface areas purely from bimodal-mesopores (3.9/8.9 nm in diameter). The MWNT@mesoC composite shows greatly increased specific capacitance from 9.0 to 48.4 F/g and 6.8 to 60.2 F/g in 1.0M (C2H5)4NBF4 and 6.0MKOH, good rate performance with～60% maintenance of the initial capacitance at the current density of 20 A/g and high cyclability (94% after 1000 cycles).