Zeolite-templated microporous carbons high-surface-area carbon

Zeolites are widely used as acid catalysts, especially in the petrochemical industry. Zeolites have several attractive properties such as high surface area, adjustable pore size, hydrophilicity, acidity, and high thermal and chemical stability. In order to fully benefit from the unique sorption and shape-selectivity effects in zeolite micropores in absence of diffusion limitation, the diffusion path length inside the zeolite particle should be very short, such as, e.g., in zeolite nanocrystals. An advantageous pore architecture for catalytic conversion consists of short micropores connected by meso- or macropore network [1]. Reported mesoporous materials obtained from zeolite precursor units as building blocks present a better thermal and hydrothermal stability but also a higher acidity when compared with amorphous mesoporous analogues [2-6]. Alternative approaches to introduce microporosity in walls of mesoporous materials are zeolitization of the walls under hydrothermal conditions and zeolite synthesis in the presence of carbon nanoparticles as templates to create mesopores inside the zeolite bodies [7,8]. 

As shown in Fig. 2.19, the nan(x asting pathway is also powerful to produce nano-porous carbons with remaikably high surface areas (up to 4,000 m g ) and precisely controlled microporous structures (0.5-1.5 mn) that are well suitable for CO2 capture [97]. The efforts to construct such molecular-sieve-type porous carbon using well-crystalline zeolite or MOFs-related materials (ZIFs, MOCPs) as sacrificial templates still continue. 

Because of the uniquely ordered structure of template-synthesized porous carbons, they themselves can be used as templates to replicate other materials with an ordered porous structure that is difficult to make using traditional methods [121,338-348], Thus, CMK-3 carbon was first demonstrated as a template to prepare mesoporous silica [121,338]. An OMC prepared from an MCM-48 template was used to prepare nanostructured silica [339]. Nanocasting of CMK-3 using ZSM-5 crystals yielded a mesoporous zeolite with both mesopores and micropores [340]. More recently, the preparation of a novel class of mesoporous aluminosilicate molecular sieves was described [341]. The preparation of mesoporous boron nitride (MBN) and mesoporous carbon nitride (MBCN) with very high surface area and pore volume was recently realized using a well-ordered hexagonal mesoporous carbon as a template and boron trioxide as a boron source [342]. Nonspherical silica nanocases with a hollow core and mesoporous shell were also produced [343]. 

A series of ZSM-5 zeolites synthesized using carbon black Black Pearls 2000 as secondary template was compared with ZSM-5 prepared by confined space synthesis and carbon-free ZSM-5. By secondary templating approach it is possible to prepare highly crystalline materials with mesopore diameter of 12 ran corresponding to the size of carbon black particles. Mesopore volume and mesopore surface area of obtained materials increase with increasing amount of carbon black in the reaction mixture. Observed decrease in micropore volume can be attributed to the increasing amount of Si04 tetrahedra on the mesopore surface. In the case of the sample prepared by eonfined space... 

In order to improve the structural ordering of zeolite-templated carbons, Ma et al. have investigated systematically the synthesis of microporous carbons using zeolite Y as hard template. They used a two-step method to prepare an ordered, microporous carbon with high surface area, which retained the structural regularity of zeolite Y by filling as much carbon precursor as possible into the zeolite pores so as to prevent any subsequent partial collapse of the resulting carbon framework. In the... 

Figure 4.2 XRD patterns of (a) zeolite Y and (b) zeolite Y-templated carbon liberated from the carbon/zeolite Y composite. Reprinted with permission from Z.X. Ma, T. Kyotani and A. Tomita, Chem. Comm, Preparation of a high surface area microporous carbon having the structural regularity of Y zeolite. Issue 23,2365-2366. Copyright (2000) RSC Publishing...

Figure 4.3 HRTEM image and corresponding diffraction pattern (inset) of zeolite Y-templated ordered microporous carbon.Reprinted with permission from Z.X. Ma, T. Kyotani, Z. Liu, O. Terasaki and A. Tomita, Chem. Mater., Very High Surface Area Microporous Carbon with a Three-Dimensional Nano-Array Structure Synthesis and its Molecular Structure. 13, 4413. Copyright (2001) American Chemical Society...

Apart from zeolites and clays, other materials, such as metal-organic frameworks (MOFs), have also been explored as template to produce porous carbons. Recently, Liu et al. have synthesised porous carbon by heating the carbon precursor furfuryl alcohol within the pores of MOF-5. The resultant carbon exhibits high specific surface area up to 2872 m g and high pore volume of 2 cm g but possesses both micropores and mesopores. This porous carbon material shows good hydrogen uptake of 2.6 wt% at 760 Torr and —196 °C, as well as excellent electrochemical properties as an electrode material for an electrochemical double-layered capacitor. 

Kyotani and coworkers [81] systematically demonstrated the preparation of micro-porous carbons using zeolite templates, whereas early studies [82-84] described the pyrolysis of carbon precursors to make carbon materials in the presence of zeolites. Subsequently, Mallouk and coworkers [85] employed zeolites Y, L, and P as templates to prepare microporous carbons with a specific surface area as high as 1580 m /g. It was reported that the zeolite template has a direct relationship with the structural and topological properties of the resultant carbon. Rodri-guez-Mirasol et al. [71] described the preparation of microporous carbons with a wide distribution of pore sizes, well-developed mesoporosity, and high adsorption capacity. Zeolite Y was used as template, and a chemical vapor infiltration method was employed to deposit carbon in the template pores. It was found that the apparent surface area of the resultant carbons increased with increasing deposition temperatures. Meyers et al. [72] synthesized porous carbon materials with a surface area of about 1000 m /g using zeolites Y, p, and ZSM-5 as templates and acrylonitrile, FA, pyrene, and vinyl acetate as carbon precursors. The template-encapsulated carbon precursors were pyrolyzed at 600 C, and the resultant materials were observed to be composed of disordered carbon arrays. 

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