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Cellulose-Nanofiber-Mediated Sorption-Benefitting Holed Silicalite-1 Crystals

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      Abstract

      Silicalite-1-type zeolites with unique intracrystal holes or cracks were successfully prepared using a cellulose nanofiber (CNF) as an additional mediating material, and their vapor phase adsorption properties toward methyl tert-butyl ether (MTBE) and n-nitrosodimethylamine (NDMA) were examined. It was found that the mixing protocol of CNF and structure-directing agents (SDAs), the addition amount of CNF, and the CNF/SDAs amount ratio play important roles in forming the holed silicalite-1. The synthesis route that preliminarily mixes CNF with SDAs in a series of controlled conditions is particularly beneficial for the formation of the holed silicalite-1 with mesoporosity and larger pores because the CNF-SDAs composite structure benefits the zeolite growth closely encompassing CNF inside the crystal structure. It also promotes the preferential formation of the orthorhombic phase vicinal to the CNF surface, namely, the surface of the formed internal holes or cracks, with the twin-type crystal size reduced as compared to the non-CNF-templated sample. On the contrary, the synthesis route that mixes CNF with SDAs–silicate composite ions tends to modify the twin-type crystal shape at the same time to form small but uniform well-crystallized particles with less holes or cracks and a dominative monoclinic phase. It was considered that both the inter-subunit structural defect and silanol defect whose content is increased with CNF addition influence the adsorptivity of MTBE and NDMA. Owing to the small twin-type crystal size, the smaller crystal subunits, and the favored short path from the surface of internal holes or cracks, the holed silicalite-1 derived from the CNF and SDA premixture assures the easiest access of adsorbate molecules to the most energetically favored sites and is most appropriate for the adsorption of both MTBE and NDMA among the examined zeolites.

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      Hierarchical zeolites: enhanced utilisation of microporous crystals in catalysis by advances in materials design.

      The introduction of synthetic zeolites has led to a paradigm shift in catalysis, separations, and adsorption processes, due to their unique properties such as crystallinity, high-surface area, acidity, ion-exchange capacity, and shape-selective character. However, the sole presence of micropores in these materials often imposes intracrystalline diffusion limitations, rendering low utilisation of the zeolite active volume in catalysed reactions. This critical review examines recent advances in the rapidly evolving area of zeolites with improved accessibility and molecular transport. Strategies to enhance catalyst effectiveness essentially comprise the synthesis of zeolites with wide pores and/or with short diffusion length. Available approaches are reviewed according to the principle, versatility, effectiveness, and degree of reality for practical implementation, establishing a firm link between the properties of the resulting materials and the catalytic function. We particularly dwell on the exciting field of hierarchical zeolites, which couple in a single material the catalytic power of micropores and the facilitated access and improved transport consequence of a complementary mesopore network. The carbon templating and desilication routes as examples of bottom-up and top-down methods, respectively, are reviewed in more detail to illustrate the benefits of hierarchical zeolites. Despite encircling the zeolite field, this review stimulates intuition into the design of related porous solids (116 references).
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        Amphiphilic organosilane-directed synthesis of crystalline zeolite with tunable mesoporosity.

        Zeolites are a family of crystalline aluminosilicate materials widely used as shape-selective catalysts, ion exchange materials, and adsorbents for organic compounds. In the present work, zeolites were synthesized by adding a rationally designed amphiphilic organosilane surfactant to conventional alkaline zeolite synthesis mixtures. The zeolite products were characterized by a complementary combination of X-ray diffraction (XRD), nitrogen sorption, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The analyses show that the present method is suitable as a direct synthesis route to highly mesoporous zeolites. The mesopore diameters could be uniformly tailored, similar to ordered mesoporous silica with amorphous frameworks. The mesoporous zeolite exhibited a narrow, small-angle XRD peak, which is characteristic of the short-range correlation between mesopores, similar to disordered wormhole-like mesoporous materials. The XRD patterns and electron micrographs of the samples taken during crystallization clearly showed the evolution of the mesoporous structure concomitantly to the crystallization of zeolite frameworks. The synthesis of the crystalline aluminosilicate materials with tunable mesoporosity and strong acidity has potentially important technological implications for catalytic reactions of large molecules, whereas conventional mesoporous materials lack hydrothermal stability and acidity.
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          Mesoporous Zeolite Single Crystals

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            Author and article information

            Affiliations
            []Environmental Management Research Institute, National Institute of Advanced Industrial Science and Technology , 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan
            []College of Architecture and Environment, Sichuan University , Chengdu 610065, Sichuan, China
            [§ ]Cellulose Materials Research Group, Research Institute for Sustainable Chemistry, National Institute of Advanced Industrial Science and Technology , Hiroshima 739-0046, Japan
            []College of Resources and Environment, Chengdu University of Information Technology , Chengdu 610225, China
            []China National Environmental Monitoring Centre , Beijing 100029, China
            Author notes
            Journal
            ACS Omega
            ACS Omega
            ao
            acsodf
            ACS Omega
            American Chemical Society
            2470-1343
            08 August 2019
            20 August 2019
            : 4
            : 8
            : 12995-13004
            6705092 10.1021/acsomega.9b00264
            Copyright © 2019 American Chemical Society

            This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.

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            ao-2019-00264s

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