Bimodal mesoporous materials

Bimodal mesoporous materials can be prepared after the MCM-41 mesopo-rous materials are treated in a solution of ammonia (NH4OH) by a chemical... 

Nowadays synthesis of mesoporous materials with zeolite character has been suggested to overcome the problems of week catalytic activity and poor hydrothermal stability of highly silicious materials. So different approaches for the synthesis of this new generation of bimodal porous materials have been described in the literature like dealumination [4] or desilication [5], use of various carbon forms as templates like carbon black, carbon aerosols, mesoporous carbon or carbon replicas [6] have been applied. These mesoporous zeolites potentially improve the efficiency of zeolitic catalysis via increase in external surface area, accessibility of large molecules due to the mesoporosity and hydrothermal stability due to zeolitic crystalline walls. During past few years various research groups emphasized the importance of the synthesis of siliceous materials with micro- and mesoporosity [7-9]. Microwave synthesis had... 

N2 adsorption-desorption isotherms and pore size distribution of sample II-IV are shown in Fig. 4. Its isotherm in Fig. 4a corresponds to a reversible type IV isotherm which is typical for mesoporous solids. Two definite steps occur at p/po = 0.18, and 0.3, which indicates the filling of the bimodal mesopores. Using the BJH procedure with the desorption isotherm, the pore diameter in Fig. 4a is approximately 1.74, and 2.5 nm. Furthermore, with the increasing of synthesis time, the isotherm in Fig. 4c presents the silicalite-1 material related to a reversible type I isotherm and mesoporous solids related to type IV isotherm, simultaneously. These isotherms reveals the gradual transition from type IV to type I. In addition, with the increase of microwave irradiation time, Fig. 4c shows a hysteresis loop indicating a partial disintegration of the mesopore structure. These results seem to show a gradual transformation... 

This study demonstrated that the micro-mesoporous composite materials could be synthesized with two-step treatment by microwave using two different templates system with TPABr and MTAB. This formation was controlled by the self-assembly formation of supramolecular templates between MTA micelles and SiO /TPA gels. As varying microwave irradiation time of micro-mesoporous materials, gradually transition from the mesophase to micro-mesophase was occurred. These materials have higher dm spacing of mesoporous materials and lead to transition from mesophase to micro-microphase by an increment of synthetic time, while the calcined products is formed with bimodal and trimodal pore size distribution under microwave irradiation within 3 h. From TG-DTA and PL analysis, the self-assembly formation of supramolecular templates between MTA+ micelles and SiO /TPA+ gels were monitored. 

The textural nature of the titania and TiCex were characterised by nitrogen ad/desorption isotherms. The specific surface areas are presented in Table la. None of the materials were found to be microporous from t-plot analyses of the adsorption isotherms. From the desorption curves, the mesopore size distribution was calculated using the BJH method. All of the samples had bimodal mesopore size distributions. The volumes of the narrow and wide mesopores, presented in Table lb), were calculated from the minima between the two distributions. These results indicated, that as the amount of ceria incorporation rose, the bimodal mesopore size distribution became narrower. For the titania sample the mesopores were centred in diameters of approximately 14 and 17 nm. With ceria incorporation both of these diameters were reduced until at the highest ceria content they were approximately 9 and 15 nm. 

SBA-l pore sizes are determined to be 0.2 nm and 1.5 x 2.2 nm, resp. The smaller pore size of the SBA-l is not fully ascertained as yet, since sorption analysis suggests a somewhat bigger size for this type of pores. For SBA-16 a bcc packing of cavities with a diameter of 9.5 nm is determined, in which the cavities are connected by pores of 2.3 nm along the [111] directions. These bimodal size distributions of structural pores observed for SBA-l and SBA-6 are unique for ordered mesoporous materials. The pore size distribution of SBA-15 is probably also bimodal, in which the bigger, hexagonally ordered structural pores determined by the surfactant are connected by micropores... 

Suzuki and Sinha have prepared novel bimodal mesoporous crystalline ceria nanoparticles and evaluated their performance in VOC removal (106). The mesoporous ceria showed 92% acetaldehyde removal with 33% CO2 conversion at ambient temperature after 24 h. This acetaldehyde removal performance is nearly twice as high as that for conventional VOC removal using materials such as activated carbon or mesoporous silica. 

The preparation of mesoporous materials with bimodal pore-size distribution has also been described by Stebe and coworkers, using mixtures of F(CF2) C2H4 (OC2H4)mOH nonionic ethoxylated surfactants and conventional hydrocarbon ethoxylated surfactants [61]. 

Figure 11.13 Mesoporous materials with a bimodal pore-size distribution prepared from mixtures of fluorinated and hydrocarbon surfactants, (a) TEM image (b) Nitrogen sorption isotherms, and (c) pore-size distributions. (Reproduced from Ref [60], with permission).

FIGURE 12.17 Illustration of the preparation of a bimodal mesoporous-macroporous carbon by dual-phase separation. The macropores are formed from the spinodal decomposition of glycolic solvents (a). Bicontinuous structure, framework structure, and the large macropores left by the solvent after annealing and carbonization (b). The carbon walls display large amounts of mesopores (c) templated by the triblock copolymer. (From Liang, C. D. et al., Chemistry of Materials, 21, 2115, 2009. With permission.)... 

When neither chemical nor physical template is used in preparation of alumina, PI, the resulting material shows less ordered pore size distribution, while P4 shows bimodal pore systems with both mesopore (3.5 nm) and macropore (50 nm) after thermal treatment. 

Iron nanoparticles prepared by pyrolysis of poly(ferrocenylsilanes) inside periodic mesoporous sihca displayed the absence of room-temperature hysteresis in the magnetization curves which shows their superparamagnetic behavior [55]. However, magnetic properties cannot always be easily interpreted. For example, for this material data analysis of magnetization curves resulted in the ambiguous conclusion that either particle size distribution is bimodal, or iron particles have an oxide layer which behaves as small superparamagnetic nanoparticles. So magnetic measurements should be combined with other techniques (probably, in this case, EXAFS may be useful) to allow more accurate evaluation of particle structure. 

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