Bimodal pore system

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. 

Lu, A.H., Schmidt, W., Spliethoff, B., and Schuth, F. (2003). Synthesis of ordered mesoporous carbon with bimodal pore system and high pore volume. Advan. Mater., 15, 1602-6. 

Porous materials can also be coated with zeolite films by direct synthesis. For example, microcellular SiOC ceramic foams in the form of monoliths were coated on their cell walls with thin films of silicalite-1 and ZSM-5 using a concentrated precursor solution for in situ hydrothermal growth (Fig. 9).[62] The zeolite-coated monoliths show a bimodal pore system and are thermally stable to at least 600 °C. A related strategy is based on the conversion of macroporous Vycor borosilicate glass beads, having pores of about 100 nm, to MFI-type zeolite-containing beads retaining the same macroscopic shape.[63] This conversion was achieved by hydrothermal treatment with an aluminium source and a template such as TPABr. 

OMCs with a bimodal pore system were prepared also with the SBA-15 template by varying the concentration of carbon precursor [130-132]. Both the external and internal diameters of carbon tubes of CMK-5 can be tuned by adjusting the concentration of an FA solution. Direct carbonization of an as-synthe-sized SBA-15 also produced a porous carbon with a bimodal pore system [133], Solovyov et al. [134] performed a complete study on the properties and formation mechanisms of OMCs in the presence of the SBA-15 template. 

Fuertes and Nevskaia [139] developed a vapor deposition polymerization (VDP) method to prepare OMCs. Carbon precursor FA was infiltrated into the pores via vapor-phase adsorption at room temperature. When ordered SBA-15 silica was used as a template, the resultant carbon possessed a unimodal pore structure similar to that of CMK-3. However, when a disordered mesoporous silica was used as a template, mesoporous carbon with a well-defined bimodal pore system (mesopores centered at 3 and 12 nm) was obtained, as can be seen from Figure 2.19. A mechanism responsible for the formation of such carbons was subsequently proposed [140] based on the degree of carbon infiltration, which can be controlled with the VDP method. Kruk et al. [141] described a polymerization method for carbon infiltration, which was believed to ensure uniform filling [142] and avoid the formation of nontemplated carbon. 

Based on the control of sol-gel deposition, hierarchically porous silica-based materials with a bimodal pore system (mesopores/large meso/macropores) and a diversity of dopant elements (Al, Ti, V, and Zr) could be prepared by using a one-pot surfactant-assisted procedure [78]. Another example includes the preparation of nonionically templated [Si]-MSU-X mesoporous silicas with bimodal pore systems by adding dilute electrolytes. 

A composite micro/meso porous material has been prepared by a secondary hydrothermal treatment. The material shows bimodal pore system due to the formation of zeolite Y in mesoporous framework. When used as the support of a Pd-Pt catalyst for hydrogenation of naphthalene in the presence and absence of 4,6-DMDBT, it demonstrates that the catalyst has an enhanced activity and sulfur tolerance. When pyrene is hydrogenated, the material shows a remarkable enhancement over the USY supported eatalyst. 

For a monodisperse system this result is in good agreement with the values obtained from pore size distribution measurements, but it can be significantly in error if one is dealing with a bimodal pore size distribution (see Section 6.4.2). 

Bimodal pore size distribution in MCM-4I has been observed by several groups in the last few years [22-24], However, the relation between two types of mesopores were never fully understood. In a recent TEM study of an MCM-41-type silicate with a bimodal mesopore system, a paint-brush like morphology of the particles was observed (Figure 7) [25], It was then proposed that the two types of pores with the pore diameters of 2.5 nm and 3.5 nm respectively coexist and are parallel to each other in the particles. Due to different rates of crystal growth, the lengths of these two groups of mesopores are different, resulting in such a novel structure only on the (001) surface. 

The extra pore system in A1MCM-41 (both with Si/Al = 16 and 32) is evident due to the pore size distribution plot (Figure IB). At the higher content of aluminum, not only was a bimodal pore distribution registered, but also a main XRD peak at 2.0 nm was broadened. 

Mercury Injection data revealed a porosity of 30 % and a bimodal pore size distribution with pore size maxima at 20 and 110 nm. The capillary displacement pressure (Pd) for mercury was 2.7 MPa corresponding to an equivalent value of 0.5 MPa. For the conversion from the mercury-air to the gas-water system the following parameters were used interfacial tension values of p(Hg-air) = 480 mN/m, and p(N2-water) = 70 mN/m contact angles (Hg-air) = 141°, and 6l(N2-water) = 0°. 

The first step in the generation of hierarchical pore structured materials is the implementation of two different pore systems which build up a highly interconnected pore network in one single bead. Other morphologies, e.g. monoliths with a bimodal pore size distribution, have already been shown to have superior chromatographic performance by Nakanishi et al. [1,2,3]. 

Mesoporous carbon was obtained by sucrose carbonization in the pores of MCM-4 silica spheres with subsequently dissolution of the silica. The carbon was impregnated with the ZSM-5 synthesis gel and the crystallization was carried out under hydrothermal conditions. After burning off the carbon, ZSM-5 with a bimodal mesopore system showing mean diameters around 2 and 30 nm was obtained. Nevertheless, the hexagonal pore array of the MCM-41 was not reproduced in the ZSM-5. 

These two kinds of surfactants are mutually insoluble in aqueous systems, and they can form separated hydrogenated surfactant micelles that coexist with fluorinated surfactant micelles. Consequently, mesoporous silica with a bimodal pore-size distribution can be obtained, because of the coexistence of these two different types of aggregates that template two different pore sizes. An example of bimodal mesoporous silica is shown in Figure 11.13. 

Hierarchically ordered mesoporous carbons (HOMC) are attractive as a support for fuel cell applications because of their interconnected bimodal pore-size distribution. Both pore systems can be mesoporous or one can be mesoporous while other can be macroporous. While a mesoporous pore structure imparts high surface area and uniform distribution of catalyst particles, macropores provide efficient mass transfer. Of course, the interconnectivity between pores has a significant role in realizing the advantages of both pore stmctures. Also, a novel feature about these structures is that the two pore structures can be adjusted independently, allowing for good control over their porosity [73, 74]. Like OMC, controllable pore structure, and carbon microstracture and surface chemistry, makes them an attractive support for fuel cell catalysis. Fang et al. have shown that Pt on hollow... 

---