Carbon mesoporous structures

As seen in the comparison of mesoporous silica and PMO in protein adsorption behavior, the nature of the framework of mesoporous materials has unavoidable influence on the protein adsorption. Therefore, adsorption of protein on mesoporous structures composed of hydrophobic materials such as carbon is worthy of detailed investigation. In this section, systematic research on protein adsorption on mesoporous carbon materials by Vinu and coworkers is mainly introduced. 

The aluminum is incorporated in a tetrahedral way into the mesoporous structure, given place to Bronsted acidic sites which are corroborated by FTIR using pyridine as probe molecule. The presence of aluminum reduces the quantity of amorphous carbon produced in the synthesis of carbon nanotubes which does not happen for mesoporous silica impregnated only with iron. It was observed a decrease in thermal stability of MWCNTs due to the presence of more metal particles which help to their earlier oxidation process. 

Nickel containing MCM-36 zeolite was used as new catalyst in the ethylene oligomerization reaction performed in slurry semi-batch mode. This catalyst, with micro-mesoporous structure, mild acidity and well balanced Ni2+/acid sites ratio, showed good activity (46 g of oligomers/gcataLh) and selectivity (100% olefins with even number of carbon atoms). The NiMCM-36 behaviour was compared to those obtained with NiMCM-22, NiY, NiMCM-41 and NiMCM-48 catalysts. 

FTIR spectra of the polymeric carbon formed in the channels of the MCM-41 by both the photochemical polymerization of C4I2 and the polymerization of Cmesoporous structure of the MCM-41 protects the reactive polyyne chains. Moreover, the confining of C6HI in the channels of the MCM-41 eliminates the explosive behaviour of the polymeric product. 

X-ray absorption spectroscopy has proved the presence of rhenium dioxide within this nanostructure [12]. Extraction of the surfactant with various solvents remained inefficient since either the surfactant persists within the composite or the nanostructure is lost. Calcination at mild temperatures as low as 300-350°C in nitrogen atmosphere leads to a mass loss under these pyrolytic conditions of about 50% with only little loss of the nanostructure. Similar results are obtained when the composite is oxidatively treated in an oxygen plasma for not more than ten minutes. Physisorption measurements on the calcined or plasma treated samples show only very small surface areas, which cannot be assigned to a mesoporous structure. Right now we believe that residual carbon may introduce some pore blocking effects within the nanostructure preventing good access of the inner pore surfaces. 

Therefore, MCM-41 is a possible adsorbent to substitute activated carbon for controlling VOCs [39,119,120], But, the adsorption equilibrium of VOCs on MCM-41 frequently shows very low adsorption capacity in the low-concentration region, owing to its mesoporous structure, which considerably limits the application of MCM-41 as an adsorbent for low-concentration VOC removal [39],... 

Particularly for adsorption studies, it is expected that the well-defined mesoporous structures would be useful as a reference pore system for the development of characterization methods and theoretical modeling for the adsorption in carbon pores. In this regard, we briefly review on the synthesis strategy, structure characterization, and their perspectives. 

CMK-1 carbon was the first carbon material reported to exhibit well-resolved XRD lines characteristic of ordered arrays of carbon mesopores [3]. The synthesis of the carbon was achieved by carbonization of sucrose inside the MCM-48 mesoporous silica. As shown in Fig. 2, the XRD pattern exhibits a new diffraction line around 1.4 compared with its MCM-48 template. This change can be explained by the formation of two separate carbon networks in the bicontinuously mesoporous MCM-48 template. After the separating silica frameworks are removed, the two carbon networks join together. The joining of the two carbon networks attributes to the syirunetry change from cubic Io3d to either 74,/a or lower [12]. The new ordered mesoporous structure is indicated by the XRD pattern and transmission electron microscopic image shown in Fig. 2. 

CMK-5 is the first example of the ordered tube-type mesoporous carbons that can be characterized with well-defined Bragg diffractions by ordinary XRD instrument [6]. The XRD pattern of the CMK-5 carbon is distinguished from that of CMK-3 by the much lower intensity of the (100) diffraction. The structure of CMK-5 may be described by the substitution of the carbon nanorods in CMK-3 with nanopipes. The CMK-5 carbon is synthesized using SBA-IS, similar to CMK-3, but the carbon source and synthesis condition are somewhat different from those for CMK-3. The synthesis method for the tube-type carbon can be extended to the SBA-16 mesoporous template. The resultant CMK-7 carbon has a bicontinuous mesoporous structure [15]. 

Tamon, H., Ishizaka, H., Araki, T. and Okazaki, M., Control of mesoporous structure of organic and carbon aerogels, Carbon, 36 (1998) pp. 1257-1262. 

Yoshizawa, N., Yamada, Y., Furuta, Y., Shiraishi, M., Kojima, S., Tamai, H. and Yasuda, H., Coal-based activated carbons prepared with organometallics and their mesoporous structure. Energy Fuels 11 (1997) pp.327-330. 

This is where the synthesis of nano-sized molecular sieves is carried out in the template matrix within confined spaces. This is an ideal synthetic route if the space size and uniformity favor the crystallization, and the as-synthesized product is easily isolated from the templates. Mesoporous molecular sieves with uniform mesopore structures can be adopted as the template, such as MCM-41. In 2000, Schmidt et al.[127] first proposed such a route to synthesize ZSM-5 nanocrystals. The synthesis procedure consisted of the impregnation of mesoporous carbon black with reaction solution, followed by treatment with steam at 150 °C, and the combustion of carbon black. Compared with other methods, the advantage of this one is that the nano-sized product is easily isolated and the yield is relatively higher. However, it also has some drawbacks. First, there is a high requirement for the preparation of carbon black as the template matrix, i.e., the mesopore sizes in carbon black must be uniform. Second, the crystallization must be performed in the mesopores, not on the extra surfaces of the carbon black. Third, a large amount of carbon black will be consumed (about four-times that of the nanozeolite product). All of these factors affect the further development of this route to some degree. 

Porous carbons are commonly used as adsorbents and catalyst supports. Many porous carbons are known to exhibit periodic structures resulting from the uniform stacking of graphene sheets and periodic arrangement of atoms within these sheets. Carbons with periodic microporous or mesoporous structures have been reported only recently. 

The synthesized materials posses large pore volume and a good thermal and hydrothermal stability. Table 1 shows textural properties of the calcined materials the use of a surfactant leads to higher surface areas and pore volume than those observed for reference C-0 and C-4 samples. On the other hand, the increase in surface area correlates well with the intensity of the carbonate bands observed by DRIFT. Examination of the N2-physisorption isotherms let us assume a non-ordered mesoporous structure. This was confirmed by the lack of a small angle X-ray difliraction peak. By XRD the Ce02 cerianite phase was identified and crystal size determined by the Rietveld method showed that samples C-1 through C-3 are nanometric particles with crystal size of about 4-7 nm while reference samples C-0 and C-4 were much larger (w30 and 19 nm, respectively). 

The reduced intensity of the SAXRD pattern of the silica/carbon composite is probably due to a similar scattering power between the silica framework and the pore filling carbon [15]. On the other hand, the observed decrease of the unit cell parameter of the silica/carbon composite might be caused by condensation of the SiOH groups in the pore walls during carbonization at 800 °C. Therefore, the hexagonal mesoporous structure is probably preserved in the composite and only collapses after dissolution of the silica framework. 

With respect to this, the main objective of this contribution is to systematically investigate the controlling effects leading to the formation of mesoporous structure of ZSM-5 crystals using carbon black particles. The porous structure of such prepared zeolites was characterized by different experimental techniques and compared with ZSM-5 prepared using confined space synthesis method and carbon-lfee ZSM-5. 

Industrial applications of nanoporous carbons are based on both their porosity and surface properties, and consequently, their characterization is of great importance. The results presented here demonsfrate a great usefulness of gas adsorption measurements for the characterization of nanoporous carbons. Low-pressure measurements provide an opportunity to study the microporous structure and surface proptaties of these materials and to monitor changes in these properties that result fiom structure and surface modification. High-pressure adsorption data allow for a detailed characterization of mesoporous structures of carbonaceous porous materials, providing their surface areas and pore size distributions. 

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