Mesoporous walls

The intercept on the adsorption axis of the extrapolated linear branch gives the micropore contribution, and when converted to a liquid volume may be taken as equal to the micropore volume itself. It is sometimes convenient indeed to convert all the uptakes into liquid volumes (by use of the liquid density) before drawing the t-plots or the a,-plots. If mesopores are present (in addition to micropores) the plots will show an upward deviation at high relative pressures corresponding to the occurrence of capillary condensation (Fig. 4.12(6)). The slope of the linear branch will then be proportional to the area of the mesopore walls together with the... 

The mesoporous materials reported above are usually prepared from relatively expensive surfactants. Some of them have poor hydrothermal stability. Furthermore, the MCM-41 host structure has a one-dimensional pore system with consequent pore blockage and diffusion limitations. Shan et al. (52) reported the synthesis of a three-dimensional and randomly connected mesoporous titano-silicate (Ti-TUD-1, mesopore wall thickness = 2.5-4 nm, surface area — 700-1000 m2/g, tunable pore size —4.5-5.7 nm) from triethanolamine (TEA). Ti-TUD-1 showed higher activity (about 5.6 times) for cyclohexene epoxidation than the framework-substituted Ti-MCM-41. Its activity was similar to that of the Ti-grafted MCM-41 (52). 

Li, et al. reported ethyl-bridged PMOs with Pd(ll) complexed to 3-aminopropyl-Itrimethoxysilane grafted onto the mesoporous walls to be an efficient catalysts for Barbier reaction of benzaldehyde and allyl bromide (Figure 16) [74]. Use of water as the reaction medium combined with the presence of ethyl moiety in the framework (which increased hydrophobicity of the pores) enhanced diffusion of the organic substrates. As can be seen in Table 3 the PMO material showed superior catalytic efficiency compared to grafted SBA-15 and MCM-41 materials with values comparable to homogeneous trials. 

Our synthesis is based on the hydrolysis of a silicon alkoxide (TEOS Si(OCH2CH3)4) in a diluted solution of nonionic polyethylene oxide-based surfactants. The hydrolysis is then induced by the addition of a small amount of sodium fluoride [5], Depending on the initial mixing conditions, the size of the solubilized objects leads to either a colorless or milky emulsion. Small particles ( 300 nm) with a 3D worm-hole porous structure or small hollow spheres with mesoporous walls, are usually obtained [6]. The synthesis we report herein after exhibits an apparently slight but actually drastic change in the preparation conditions. The main feature of this approach is an intermediate step that utilizes a mild acidity (pH 2 - 4), in which, prior to the reaction, a stable colorless microemulsion containing all reactants is... 

The uncalcined and unmodified sample is denoted as MCM-U. This uncalcined sample, i.e., sample with template molecules inside mesopores, was functionalized via one-step process that includes simultaneous trialkylsilanization and extraction of the template. Two modified samples were prepared with trimethylsilyl and octyldimethylsilyl groups attached to the mesopore walls and denoted as MCM-UM and MCM-UO, respectively. A typical synthesis [17] was carried out as follows about 0.2 g of the uncalcined MCM-41 was dispersed in 10 ml of trialkylchlorosilane, and refluxed for 36 hours (modification scheme is shown on Figure 1). Subsequently, about 5 ml of anhydrous pyridine was added to the mixture and refluxed for next 18 hours. After cooling down, the mixture was filtered and washed several times with small portions of ethyl alcohol, mixture of ethyl alcohol and... 

In addition to simplicity of the proposed one-step procedure, it allows for synthesis of ordered mesoporous materials of higher hydrophobic properties than parent materials. In fact, a high coverage of the grafted alkylsilyl groups stabilizes MCM-41 with respect to various environmental factors, as it was mentioned elsewhere [4]. Also, this study shows that the proposed previously procedure for stepwise modification of uncalcined MCM-41 samples [16] requires further studies in order to carry out a selective functionalization of the external surface and mesopore walls of these materials... 

Because (Co.La)salen is immobilized on the mesopore wall of MCM-41,it is anticipated that this catalyst would exhibit high stability for catalyst recycling. Therefore, the (Co.La)salen/MCM-41 sample was recovered by filtration and used again for oxidation of styrene. The activities for (Co,La)salen/MCM-41 for two successive oxidation of styrene are listed in table 3. The catalytic activity obtained for the second run is over 90% of that for the first run. This result indicates that (Co,La)salen is firmly immobilized in MCM-41 and has high stability in oxidation styrene. 

The combination of micelle-forming species used in the preparation of meso-porous materials with silicate precursors of a variety of zeolites is a promising strategy to obtain mesoporous materials with zeolite-like acidity.[69] Although some progress has been made in this field, it has yet to be proven that catalytic materials with improved performance can be obtained in this manner. Strong evidence of the presence of crystallinity in the mesopore walls, combined with an increased acidic... 

The area of catalyst immobilization has received considerable attention as can be judged from the available literature reviews.[1 30] Immobilization of oxidation catalysts shows intrinsic advantages over other catalysts as the tendency for selfoxidation will decrease. Moreover, complexes with generally low solubility, such as heme-type transition metal complexes, can be dispersed molecularly on supports. It is the aim of the present work to overview the state of knowledge on the immobilization of transition metal complexes using microporous supports, such as zeolites and laminar supports like clays. The wealth of information available for complexes immobilized on LDHs or tethered to the mesopore walls in hierarchically organized oxides will not be dealt with. 

The polymer-templated OMSs such as SBA-1545 possess pores about 8-12 nm, which are more spacious and can be decorated with more complex ligands. The SBA-15 sample studied exhibited ordered mesopores about 10 nm, and irregular micropores in the mesopore walls as shown in Figure 1. Initially chloropropyl-SBA-15 material was obtained and subsequently grafted with 2,5-dimercapto-l,3,4-thiadiazole ligand16 (OMM-3 material in Figure 1). These... 

In this case, it is the initial part of the isotherm which corresponds to monolayer-multilayer adsorption on the mesopore walls. If the corresponding section of the as-plot is linear and back-extrapolates to the origin, the slope provides a measure of a(S) which is now the total surface area. We may also conclude that there are no... 

In this particular case of a cylindrical pore shape, it seems reasonable to assume that the condensate has a meniscus of spherical form and radius rK. However, as some physisoiption has already occurred on the mesopore walls, it is evident that rE and rf are not equal. If the thickness of the adsorbed multilayer is t, and die contact angle is assumed to be zero, the radius of the cylindrical pore is simply... 

The nitrogen isotherm data on non-porous hydroxylated silica in Table 10.1 (Bharabhani et al, 1972) have been used to construct the as-plot in Figure 12.6. Since the initial linear section can be back-extrapolated to the origin, we are reasonably sure that monolayer—multilayer adsorption has occurred on the mesopore walls before the onset of pore filling at / //>° = 0.41 and therefore that there was no detectable primary micropore filling at low / // - Similar results have been obtained by Kruk et al (1997b) and Sayari et al. (1997). 

Porous solids having a regular pore structure have gathered much attention in the fields of chemistry and physics[l-7]. Those solids are expected to elucidate the interaction of gas with pores from the microscopic level. lUPAC classified pores into micropores, mesopores, and macropores using pore width w ( micropores w< 2nm, mesopores 2 nm < w< 50 nm, and macropores w> 50 nm)[8]. Physical adsorption occurs by the mechanism inherent to the pore width. Vapor is adsorbed on the mesopore wall by multilayer adsorption in the low pressure range and then vapor is condensed in the mesopore space below the saturated vapor pressure P . This is so called capillary condensation. Capillary condensation has been explained by the Kelvin equation given by eq. (1). 

Figure 2 shows the FT-IR spectra of MCM-41 and MCM-41 with the monolayer of acetonitrile adsorbed on the mesopore walls at 303 K. 

Si MAS NMR is a powerful technique for analysing the local environment of silicon in the mesoporous wall, and indicates the polymerization level of silica. 29Si MAS NMR spectra for MCM-41 are similar with those for amorphous silica. There are two main resonance peaks —100 ppm for Q3, i.e., Si(OSi)3OH, and —110 ppm for Q4, i.e., Si(OSi). In some cases, a peak at —90 ppm for Q2 can be observed. According to the different silicon species distribution, one may calculate the quantity of silanol. 

The relatively low catalytical activities of mesoporous materials such as MCM-41 and MCM-48, as compared with zeolites, can be typically attributed to the low acidity or low oxidation ability of catalytically active species, which is strongly related to the amorphous nature of the mesoporous walls. Therefore, increasing acidity, oxidation ability, and hydrothermal stability are great goals for rational syntheses of ordered mesoporous materials. 

The micropore volume of MAS-7 sample (0.15 cm3/g) is much more than that of the SBA-15 sample prepared under the same conditions (0.05 cm3/g). Even the fact that the MAS-7 sample has thicker walls than SBA-15 was considered. The larger micropore volume in MAS-7 may be attributed to the existence of zeolite primary units in the mesoporous walls. The TEM image[203] of MAS-7 shows obvious white dots of 7 A diameter except for the mesopores (pore size of 7.4 nm), which could possibly be assigned to micropores in the mesoporous walls of MAS-7. This confirmed that the mesoporous walls are partially polycrystallized although the size is relatively small (around 2-3 nm). 

Figure 8.47 Incorporation of organic functions in mesoporous silica (a) Surface grafting of organic functions on the mesopore walls by post-synthesis direct incorporation of organic functions by co-condensation of organosilanes (b) or bridging silasesquioxanes (c). Reproduced with permission from [51]. Copyright (2002) American Chemical Society...

We will not discuss here models for pores in carbons, as this topic is treated in Chapter 5, and elsewhere in specialist [15] or general reviews [106, 107]. For similar reasons, we will not discuss porosity control [44, 108] in detail. However, porous carbons prepared by the template technique, especially the ordered ones, deserve special attention. Ordered mesoporous carbons have been known to scientists since 1989 when two Korean groups independendy reported their synthesis using mesoporous silicas as templates [109, 110]. Further achievements have been described in more recent reports [111, 112]. One might have expected that the nanotexture of these materials would merely reflect the nature of the precursor used, namely phenol-formaldehyde [109] or sucrose [110] in the two first ordered mesoporous carbon syntheses (as is well known, these two precursors would have yielded randomly oriented, isotropic carbon had they been pyrolyzed/activated under more conventional conditions). However, the mesopore walls in some ordered mesoporous carbons exhibited a graphite-like, polyaromatic character [113, 114], as described in Chapter 18. This information was obtained by nitrogen adsorption at low relative pressures, as in classical... 

The OMC structure also depends on the polymerization step. As mentioned above, the polymerization of the precursor adsorbed in the pore system of the matrix is catalyzed by acids. Different synthesis procedures were developed. For example, an acid solution can be added to the reaction mixture. In this case, the polymerization will take place throughout the entire pore system of the matrix. The resulting OMC can be described as a three-dimensional network of interconnected carbon rods. An example is CMK-3, already presented in Fig. 18.1. In this OMC, parallel-arranged carbon rods with a diameter of approximately 5 nm are connected by narrower carbon rods. The narrow carbon rods were formed in micropores that connect the mesopores of the SBA-15 silica matrix [21]. In an alternative synthesis procedure, a matrix with acid sites on the pore walls (e.g., an aluminosilicate) can be used. In this case, the polymerization of the precursor takes place on the mesopore walls and a carbon film is formed there, whereas the much narrower micropores are entirely filled with the polymerization product. Thus, after pyrolysis and removal of the matrix, the OMC consists of interconnected nanopipes, as opposed to interconnected carbon rods. An example is CMK-5. This OMC is synthesized in an acid form of the matrix used for the synthesis of CMK-3. Thus, CMK-5 consists of interconnected carbon nanopipes, arranged in the same fashion as the carbon rods of CMK-3 (Fig. 18.2) [22]. However, the pore system of these two OMCs differs. The pore system of CMK-3 consists of the voids in between the carbon rods, whereas in addition to these pores CMK-5 also has pores inside the nanopipes. 

Adsorption below a P/Pq of approximately 0.05 is due to adsorption in micropores and to adsorption on mesopore walls. It is not easy to quantify these two contributions because in OMCs the micropore volume might be small as compared to their mesopore volume. Flowever, some preliminary information on the presence of micropores can be obtained from the low-pressure portion of the adsorption isotherm. This will be illustrated by the discussion of some isotherms of nonmicroporous reference compounds. On a perfect graphene layer, adsorption at very low pressures P/Pq < 10 ) is small. An example is the... 

The FTIR spectra of the imcalcined samples between 400 and 1300 cm" are shown in Figure 2. The broad bands in the range of 1000 - 1200 cm" and that at 450 cm" are attributed to structure insensitive internal tetrahedron vibrations [10], while the band present at 800 cm" is attributed to structure insensitive internal tetrahedron or structure sensitive external tetrahedron vibrations [10]. The shoulder observed at 960 cm" indicates the presence of terminal silanol groups on the surface of the mesopore walls [11]. A structure sensitive band, which appears in the spectra around 546 cm" [10], indicates the presence of typical five ring units of the ZSM-5 structure [12]. 

---