Mesostructured silica nanoparticles (MSN) and Ni loaded onto MSN (Ni/MSN) for the methanation of CO2 were prepared by the sol-gel and impregnation methods. Catalytic testing was conducted in the temperature range of 423-723 K under atmospheric pressure in the presence of H2. Ni supported on MSN was compared with others types of support such as MCM-41 (Mobile Crystalline Material), HY (protonated Y zeolite), SiO2 and γ-Al2O3. The activity of CO2 methanation followed the order: Ni/MSN > Ni/MCM-41 > Ni/HY > Ni/SiO2 > Ni/γ-Al2O3. The nitrogen physisorption and pyrrole adsorbed IR spectroscopy results indicated that the high activity of Ni/MSN is due to the presence of both intra-and interparticle porosity which led to the high concentration of basic sites. In addition, the correlation between N-H band intensity and the turnover frequency revealed that the methanation activity increased with increasing of the concentration of basic sites. The presence of defect sites or oxygen vacancies in MSN was responsible for the formation of surface carbon species, while Ni sites dissociated hydrogen to form atomic hydrogen. The surface carbon species then interacted with atomic hydrogen to form methane. The Ni/MSN catalyst performed with good stability and no deactivation up to 200 h.
Hierarchically organized porous materials can provide multidimensional spatial networks on different length scales with improved characteristics relevant to molecular diffusion. [1] Zeolites that are microporous crystalline materials having pores and channels at molecular dimensions are of great importance for industrial applications. [2] However, the presence of only micropores in zeolite frameworks often limits the molecular diffusion and therefore, restricts the transport of bulky molecules. This problem can be resolved by shortening the effective diffusion path lengths, which has been achieved by miniaturizing zeolite crystals, [3] delaminating or exfoliating layered zeolites, [4] synthesizing zeolite nanosheets, [5] and introducing mesopores into zeolite particles. [6][7][8][9][10][11][12] Among these solutions, the fabrication of hierarchical zeolites with micro-and mesoporosity is of interest because it combines intrinsic micropores with bypass-interconnected mesopores, and therefore, enhances both the micropore accessibility and molecular traffic within zeolite particles. [1, 7b, 12] Hierarchical zeolites have been produced using several techniques, including top-down desilication by alkali postsynthetic treatment [6] and bottom-up directed assembly by hard [7] or soft [8][9][10][11] templates. The hard-template method requires multistep procedures and is therefore unfavorable for large-scale production. Alternatively, the direct introduction of organic mesopore-generating agents (mesoporogens) during zeolite synthesis can create uniform mesopores. The use of such mesoporogens is currently one of the most promising methods for the single-step construction of hierarchical zeolites. Progress has been made using well-designed mesoporogens composed of long hydrophobic chains and hydrophilic zeolitic structure-directing groups to generate zeolites with tunable mesoporosity [9a, 10] and to direct the hierarchical assembly of zeolite nanosheets, yielding mesoporous zeolites with house-of-cards-like structures. [5a, 9c, 11] These hierarchical nanosheets showed excellent catalytic performance in several important reactions because the presence of thin layers with specific crystalline faces facilitates catalysis at the exteriors or pore mouths. [13] Such mesoporogens are probably necessary for the direct, singlestep synthesis of hierarchical zeolites.Herein we report an alternative, mesoporogen-free approach for the construction of hierarchically organized MFI zeolites by sequential intergrowth using a simple organic structure-directing agent (OSDA). The selection of an appropriate OSDA and optimized synthesis conditions that can form plate-like zeolites with enhanced 908 rotational intergrowths seems to be a key to achieving a hierarchical structure with three classes of porosity in one structure: the intrinsic microporosity of the zeolite framework together with mesoporosity existing within the zeolite plates and macroporosity stemming from the complex intergrown structure.Epitaxial and rotational int...
The unique three-dimensional pore structure of KCC-1 has attracted significant attention and has proven to be different compared to other conventional mesoporous silica such as the MCM-41 family, SBA-15, or even MSN nanoparticles. In this research, we carefully examine the morphology of KCC-1 to define more appropriate nomenclature. We also propose a formation mechanism of KCC-1 based on our experimental evidence. Herein, the KCC-1 morphology was interpreted mainly on the basis of compiling all observation and information taken from SEM and TEM images. Further analysis on TEM images was carried out. The gray value intensity profile was derived from TEM images in order to determine the specific pattern of this unique morphology that is found to be clearly different from that of other types of porous spherical-like morphologies. On the basis of these results, the KCC-1 morphology would be more appropriately reclassified as bicontinuous concentric lamellar morphology. Some physical characteristics such as the origin of emulsion, electrical conductivity, and the local structure of water molecules in the KCC-1 emulsion were disclosed to reveal the formation mechanism of KCC-1. The origin of the KCC-1 emulsion was characterized by the observation of the Tyndall effect, conductometry to determine the critical micelle concentration, and Raman spectroscopy. In addition, the morphological evolution study during KCC-1 synthesis completes the portrait of the formation of mesoporous silica KCC-1.
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