Meso-macroporous

In China, most of the traditional RFCC catalysts (such as Orbit, DVR, and MFC mentioned above) are based on alnmina matrix, and the most widely used materials for alumina matrix preparation are alumina sol and modified active alumina [4]. Alumina matrix combines the virtnes of alumina-sol (better attrition resistance and coke selectivity) and active alnmina (higher cracking activity), thus improving the cracking activity and selectivity of the catalysts. However, the coke selectivity of the alumina matrix is nnsatisfactory when processing resid feed due to the insufficient amount of meso/macropores and higher concentration of acid sites. 

In an experiment carried out with 100 mg/1 methylene blue concentration the behaviour was the same as described before, but, there was a time in which SC-155 reached the saturation and the material stopped the adsorption the AC-ref instead continued the adsorption at longer times due to its higher carbon contents. Then, the great difference in adsorption kinetics observed between SC-155 and AC-ref is justified by the more expanded structure of carbon microdomains of SC-155 than the reference, and by the higher radius of meso-macropores observed for the SC-155 the last point provides an easy access of molecules to be adsorbed into the grains of the material, minimizing diffusional problems. 

The physical and chemical activation processes have been generally employed to prepare the porous carbons.18"35 However, the pore structures are not easily controlled by the activation processes and the size of the pores generated by the activation processes is limited to the micropore range only. Recently, much attention has been paid to the synthesis of meso/macroporous carbons with various pore structures and pore size distributions (PSD) by using various types of such inorganic templates as silica materials and zeolites.17,36 55... 

The meso/macroporous carbons have attracted much attention in their application as electrode materials in EDLCs, since the meso/macropores promote the formation of an effective doublelayer or the transfer of ions into the pores, resulting in the increases in the electrolyte wettability and the rate capability.67,68 In this regard, there has been considerable research targeted towards developing the synthetic methods of novel meso/macroporous carbons.17,36"55,69 72 Various types of such inorganic templates as silica materials and zeolites are widely used for the synthesis of the meso/macroporous carbons, since it was revealed17,36"55 that these inorganic templates contribute to the formation of the meso/macropores with various pore structures and broad PSD. 

Figure 1. Plots of differential pore volume against pore diameter calculated from the N2 gas adsorption isotherms obtained from meso/macroporous carbon specimens I (-0-), II (- -), and III (-A-) using Barrett-Joyner-Halenda (BJH) method. Reprinted with permission from G. -J. Lee and S. -I. Pyun, Carbon, 43 (2005) 1804. Copyright 2005, with permission from Elsevier.

In the case of the meso/macropores, the physical adsorption related to the capillary condensation is generally described with the aid of following Kelvin equation94103... 

Based upon the Kelvin equation, the PSD of the meso/macropores has been generally determined by Barrett, Joyner, and Halenda (BJH) method.104 Furthermore, the density functional theory94 which is based upon a molecular-based statistical thermodynamic theory was recently introduced in order to analyze... 

The present article first provided the brief overview of the synthetic methods of the porous carbons. In order to prepare the microporous carbons with high surface area, the physical/chemical activation methods have been widely used for a long time.18"35 Recently, the meso/macroporous carbons with various pore structures are prepared by templating methods by using various templates and changing sol-gel reaction conditions, e.g., pH, amount of template, and gelation temperature.17,36 55... 

Shiraishi, S., Kurihara, H., Shi, L., Nakayama, T., and Oya, A. Electric double-layer capacitance of meso/macroporous activated carbon fibers prepared by the blending method. J. Electrochem. Soc. 149, 2002 A855-A861. 

The strategy of this method is to utilize the inherent porosity of bulky substrates in the construction of hierarchical stractures by incorporating additional pore systems. Diatoms are unicellular algae whose walls are composed of silica with an internal pore diameter at submicron to micron scales. Zeolitization of diatoms, in which zeolite nanoparticles are dispersed on the surface of diatoms followed by a hydrothermal conversation of a portion of the diatom silicas into zeolites, resulted in the formation of a micro/mesoporous composite material. Similarly, wood has also been used as a substrate to prepare meso/macroporous composites and meso/macroporous zeolites. After the synthesis, wood is removed by calcination. ... 

Amorphous silica-alumina (Si02-AI2O3) has been also tested for the catalytic cracking of polyolefins [36, 37, 43]. This acid solid is featured by having a broad distribution of pore sizes, which is determined by the synthesis procedure. Moreover, the occurrence of a bimodal pore size distribntion (e.g. meso-macroporous) is usually present. The aluminium... 

They consist of a thin layer (<10 fxm) of a nanoporous (3-1OA) carbon film supported on a meso-macroporous inorganic solid (alumina) or on a carbonized polymeric structure [15]. They are produced by pyrolysis of polymeric films. The following two types of membranes are produced ... 

The magnitudes of gas diffusivities by different mechanisms follow the order meso-macropore gas diffusivity (Dp) > surface diffusivity (D ) in those pores > micropore diffusivity (Dm). For example. Dp... 

The diffusivity of a liquid adsorbate through the meso-macropores is much slower than that for a gas. For example, the diffusivity of bulk liquid phase water from ethanol into an alumina sample was... 

The nanoporous carbon membrane consists of a thin layer (<10pm) of a nanoporous (3-7 A) carbon film supported on a meso-macroporous solid such as alumina or a carbonized polymeric structure. They are produced by judicious pyrolysis of polymeric films. Two types of membranes can be produced. A molecular sieve carbon (MSC) membrane contains pores (3-5 A diameters), which permits the smaller molecules of a gas mixture to enter the pores at the high-pressure side. These molecules adsorb on the pore walls and then they diffuse to the low-pressure side of the membrane where they desorb to the gas phase. Thus, separation is primarily based on differences in the size of the feed gas molecules. Table 7 gives a few examples of separation performance of MSC membranes. ° Component 1 is the smaller component of the feed gas mixture. 

Complex templates combine soft and hard template techniques. This methodology is used for synthesizing hierarchically bimodal and trimodal meso-macroporous materials with interconnected pore channels combining a surfactant template with a colloidal crystal template (Yuan and Su, 2004). 

V ultra-micropore (mL/g) V super-micropore (mL/g) V meso-, macropore (mL/g) Ssuper-micro-, meso-, macropore ( l /s) Cbet... 

As a rule, ACF not only presents a higher adsorption capacity than conventional GAC, but the pore network is also different due to the fibril structure, which ensures a much higher adsorption kinetics. The reason is that in GAC, the adsorbate must diffuse throughout the macro and mesopores before reaching the micropore or adsorption sites, whereas the micropores are directly accessible ftom the external surface in the ACF (Fig. 23). Consequently, there is no resistance to the diffusion of adsorbates through to the adsorption pores because there is no meso/macropore network. 

---