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Abstract Hierarchically structured zeolites can mitigate problems arising from the slow transport in the micropores, such as diffusion limitations. Hence, a manifold of different preparation methods for hierarchical zeolites have ...
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Abstract Hierarchically structured zeolites can mitigate problems arising from the slow transport in the micropores, such as diffusion limitations. Hence, a manifold of different preparation methods for hierarchical zeolites have been developed over the years and the corresponding zeolites showed superior performance due to enhanced mass transport caused by higher diffusivities in the additional pore system and reduced diffusion path length in the micropores. The dimensions of the additional secondary “transport” pores in these materials is a major factor affecting their diffusion properties and consequently their catalytic performance. Interestingly almost all reports focus on the generation of additional mesopores and study of catalytic performance of mesoporous zeolites. Larger macropores with pore diameters of more than 50?nm, however, have some intrinsic advantages over mesopores that make them very attractive for catalytic applications. In the last years, methods for the preparation of zeolites and zeolite composites with additional macropores have been developed. These developments have brought about a novel and promising class of hierarchically structured materials; zeolites with macropores. This review gives an overview about different preparation routes for zeolites with inter or intra crystalline macropores as well as zeolite composites with macropores and summarizes the first studies regarding catalytic their performance.
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Hierarchical (or mesoporous) zeolites have attracted significant attention during the first decade of the 21st century, and so far this interest continues to increase. There have already been several reviews giving detailed accoun...
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Hierarchical (or mesoporous) zeolites have attracted significant attention during the first decade of the 21st century, and so far this interest continues to increase. There have already been several reviews giving detailed accounts of the developments emphasizing different aspects of this research topic. Until now, the main reason for developing hierarchical zeolites has been to achieve heterogeneous catalysts with improved performance but this particular facet has not yet been reviewed in detail. Thus, the present paper summaries and categorizes the catalytic studies utilizing hierarchical zeolites that have been reported hitherto. Prototypical examples from some of the different categories of catalytic reactions that have been studied using hierarchical zeolite catalysts are highlighted. This clearly illustrates the different ways that improved performance can be achieved with this family of zeolite catalysts. Finally, future opportunities for hierarchical zeolite catalysts are discussed, and the virtues of various preparation methods are outlined, including a discussion of possible pitfalls in the evaluation of new, potential hierarchical zeolite catalysts.
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There has been growing interest in the research and development of hierarchical zeolites possessing at least two levels of porosity in order to reduce diffusion limitations in reactions catalyzed by microporous zeolites. Engineeri...
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There has been growing interest in the research and development of hierarchical zeolites possessing at least two levels of porosity in order to reduce diffusion limitations in reactions catalyzed by microporous zeolites. Engineering and manipulation of the hierarchy of pore structures are required to facilitate transport of large reactants and to improve the accessibility of active sites in zeolite catalysts. Various strategies and approaches have been proposed and developed in order to introduce secondary porosity, which is usually in the mesopore range, for conventional zeolites. From controllable post-synthetic modifications to designed direct synthesis with or without mesopore templating agents, significant progress has been made over the past several years in making hierarchical zeolites. In this paper, recent innovations in the synthesis of hierarchical zeolites by different methods will be reviewed, including post-synthetic demetallation, hard-and soft-templating approaches, and template-free routes. We will also discuss separately the progress of hierarchical structures derived from two-dimensional (2D) zeolite nanosheets.A summary and outlook will also be discussed with the focus on providing industrial perspectives of this field. (C) 2015 Elsevier B.V. All rights reserved.
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摘要 :
There has been growing interest in the research and development of hierarchical zeolites possessing at least two levels of porosity in order to reduce diffusion limitations in reactions catalyzed by microporous zeolites. Engineeri...
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There has been growing interest in the research and development of hierarchical zeolites possessing at least two levels of porosity in order to reduce diffusion limitations in reactions catalyzed by microporous zeolites. Engineering and manipulation of the hierarchy of pore structures are required to facilitate transport of large reactants and to improve the accessibility of active sites in zeolite catalysts. Various strategies and approaches have been proposed and developed in order to introduce secondary porosity, which is usually in the mesopore range, for conventional zeolites. From controllable post-synthetic modifications to designed direct synthesis with or without mesopore templating agents, significant progress has been made over the past several years in making hierarchical zeolites. In this paper, recent innovations in the synthesis of hierarchical zeolites by different methods will be reviewed, including post-synthetic demetallation, hard-and soft-templating approaches, and template-free routes. We will also discuss separately the progress of hierarchical structures derived from two-dimensional (2D) zeolite nanosheets.A summary and outlook will also be discussed with the focus on providing industrial perspectives of this field. (C) 2015 Elsevier B.V. All rights reserved.
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P-Methyacetophenone, the acylated product of toluene finds a wide range of applications in the flavors and fragrance industry. It is typically produced on an industrial scale by Friedel-Crafts acylation of toluene with acetic anhy...
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P-Methyacetophenone, the acylated product of toluene finds a wide range of applications in the flavors and fragrance industry. It is typically produced on an industrial scale by Friedel-Crafts acylation of toluene with acetic anhydride using homogeneous, corrosive and polluting acid catalysts such as aluminium chloride. The pollution problems related to this process such as the disposal of catalyst and treatment of acidic effluent needs to be replaced by a green process. The current work reports on the activity of hierarchical zeolite Beta in the liquid phase acylation of toluene with acetic anhydride. The liquid phase reactions were carried out in the temperature range of 60-140℃ in an autoclave. The effect of various reaction parameters such as time-on-stream (TOS), mole ratio of reactants, catalyst loading, and reaction temperature on the rates of reaction has been investigated. Under the optimum reaction conditions the performance of hierarchical zeolite Beta was compared with nanocrystalline zeolite Beta. It was found that hierarchical zeolite Beta catalyst exhibit higher activity, which is due to the hierarchical porosity and to the nano size of the Beta zeolite catalyst particles allows faster diffusion of the products out of the catalyst.
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The tailoring of hierarchical structures in beta zeolites of differing Al contents (Si/Al: 14-250) has been systematically investigated through a top-down base leaching process with various cationic tetraalkylammonium (TAA) salts ...
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The tailoring of hierarchical structures in beta zeolites of differing Al contents (Si/Al: 14-250) has been systematically investigated through a top-down base leaching process with various cationic tetraalkylammonium (TAA) salts and organic amines as pore-directing agents (PDAs) to examine their effectiveness in protecting beta zeolite framework while directing the formation of mesopores. The structure-property relationship was established between PDA structure and average mesopore sizes in resulting hierarchical zeolites, especially for high Si beta zeolites when the presence of PDA is necessary to avoid framework amorphization during NaOH desilication. All PDAs studied are effective in directing the mesopore formation in alkaline solutions without experiencing significant decrease in microporosity and crystallinity of parent beta zeolites. For high Si beta zeolites, hierarchical structures with non-ionic amines possess larger mesopores (average mesopore sizes, d(alpha): 5-8 nm) than those treated with TAA cationic PDAs (d(alpha): 2-6 nm), which should be due to better accommodation of TAA-type cationic PDAs within the inner channel of beta framework and more favorable interaction and protection of parent framework as compared to organic amines. For similarly structured TAA cations, the resulting mesopore sizes and surface areas are correlated with the molecular weights of different cations, i.e. higher molecular weight cations result in smaller da and correspondingly larger surface areas. Therefore, it is feasible to tailor the mesopore sizes by a top-down base leaching with an appropriate selection of PDAs for beta zeolites. The successful introduction of mesopores in beta zeolites also led to significantly improved catalytic conversion for the acid-catalyzed alpha-pinene isomerization, as compared to the parent beta catalysts.
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The development of biofuel production from biomass has become a promising breakthrough and could tremendously enhance the potential of industrial technology. In this case, zeolites have emerged as suitable materials for catalyzing...
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The development of biofuel production from biomass has become a promising breakthrough and could tremendously enhance the potential of industrial technology. In this case, zeolites have emerged as suitable materials for catalyzing biomass conversion due to their outstanding catalytic properties, including the presence of the intrinsic acid sites, shape-selectivity properties, and high thermal stability. However, the sole micropores in zeolite cause diffusional limitation issues, especially for the bulky molecules involved in biomass feedstocks. For instance, many oxygenate molecules are too large to enter the micropores, so they cannot be converted into the product. In this case, hierarchical porosity could facilitate these molecules to access the acid site within the zeolite crystals. Research on synthetic strategies, modifications, and evaluations of their catalytic properties has consistently grown every year. This article reviews the recent development of hierarchical zeolite catalysts for biomass conversion to biofuel. Numerous strategies of hierarchical zeolite fabrication (bottom-up, top-down methods, and green synthesis approaches) and its modification (metals and functionalization with organic materials) and their characteristics are comprehensively reviewed. The key point in the fabrication of hierarchical zeolites is the development of notable mesopores while preserving the intrinsic micropores. Moreover, the role of hierarchical zeolites in various biofuel and bio-based chemicals for biofuel and biofuel additives production reactions, i.e., pyrolysis, hydrolysis, esterification and transesterification, isomerization, condensation, upgrading of bio-oil, and catalytic cracking reactions, are discussed in detail. Finally, the remaining challenges and insight that can be considered for further improvement are provided.
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Two series of BEA zeolites (Beta and Beta/mesa) have been prepared. A first series of the samples was obtained by a conventional aging of parent zeolite gel, while the second series (Beta/meso) was prepared by mesotemplate-free me...
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Two series of BEA zeolites (Beta and Beta/mesa) have been prepared. A first series of the samples was obtained by a conventional aging of parent zeolite gel, while the second series (Beta/meso) was prepared by mesotemplate-free method. In this method Beta nanoparticles are aggregated under acidic conditions with the formation of micro-mesoporous material. Both series (Beta and Beta/mesa) were doped with Fe, Cu and Co by ion-exchange method and tested as catalysts of N2O decomposition. The Cu-Beta catalyst was found to be the most active in the process of N2O decomposition conducted in inert gas atmosphere. However, in the process performed under conditions similar to those prevailing in waste gases emitted from nitric acid plants (one of the main sources of N2O emission) higher reaction rate was found for the Cu-Beta/meso catalyst. (C) 2014 Elsevier Inc. All rights reserved.
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Catalytic performance of Pt impregnated parent and desilicated nano-crystalline zeolites, ZSM-5 and Beta for n-hexane isomerization was studied. Difference in channel systems of the zeolites and absence/presence of mesopores there...
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Catalytic performance of Pt impregnated parent and desilicated nano-crystalline zeolites, ZSM-5 and Beta for n-hexane isomerization was studied. Difference in channel systems of the zeolites and absence/presence of mesopores therein were found to be reflected in product distributions. ZSM-5 was desilicated by NaOH and zeolite Beta with tetramethylammonium hydroxide (TMAOH.) Desilication was found to afford comparable catalytic performance to that of the parent counterpart at reaction temperature lower by 25 °C. Observed product distributions could be substantiated with characterizations such as ammonia TPD, surface area determination and SEM. Desilicated zeolite Beta was seen to be less prone to coking as deduced from the TGA study. Location of Pt with reference to proton sites within the channels and that inside the pores viz a viz external surface also have been discussed briefly.
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The activity,selectivity,and stability of microporous highly dispersed zeolites(H-Y,H-ZSM-5)and granulated hierarchical zeolites(H-Yh and H-ZSM-5h)were studied in the synthesis of quinolines by the Skraup reaction.Texture,porous s...
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The activity,selectivity,and stability of microporous highly dispersed zeolites(H-Y,H-ZSM-5)and granulated hierarchical zeolites(H-Yh and H-ZSM-5h)were studied in the synthesis of quinolines by the Skraup reaction.Texture,porous structure,morphology of crystals,acidic properties were characterized by XRD,XRF,N2 adsorption-desorption,SEM,NH3-TPD methods.It was found that quinoline,2-and 4-methylquinolines are the main products in the reaction of aniline with glycerol in the presence of the studied catalysts.H-ZSM-5h and H-Yh catalysts are more efficient than catalysts H-ZSM-5 and HY and provide a yield of quinolines to 72.4(H-ZSM-5h)and 60.2%(H-Yh)at 450℃,0.2 h~(-1),the molar ratio of aniline:glycerol=1:3.The high activity and selectivity of hierarchical zeolites are due to the presence of meso-and macropores,as well as nanocrystals of 15-100 nm size.It was reported that hierarchical catalysts exhibit higher stability in the reaction compared with microporous zeolites.
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