Mesoporous silica gels

Fig. 5.8 Adsorption isotherms at 25°C of benzene and cyclohexane on a mesoporous silica gel. Curve (A), benzene curve (B), cyclohexane. Solid symbols denote desorption.

Immobilised complex ligand structures incorporated into hexagonal mesoporous silica gels can be used to bind other metals. We are currently working on other systems and we will describe these elsewhere. 

Mesoporous silica gel was reacted with APTS (1 %v/v APTS/solvent mixture), using various reaction and curing conditions. Sample-specific reaction conditions are indicated in table 9.1. 

Figure 9.9 Silane loading on dried mesoporous silica gel as a function of reaction time (M) APTS, (U)AEAPTS.

Figure 9.10 Adsorption isotherm of APTS on dehydrated (673K) mesoporous silica gel from toluene solution, by freeze sampling method.

The adsorption isotherm for AEAPTS modification of mesoporous silica gel is given in figure 9.12. 

Three mesoporous silica gels, with variable mean pore radius and specific surface area, have been studied. The substrates are named according to their approximated mean pore diameter. Measured values appeared to differ somewhat from the product names.30 The Kieselgels 40, 60 and 100 have a mean pore diameter of 4.2, 7.0 and 12.0 nm, respectively. Specific surface area increases with decreasing pore radius. Measured values, using the BET method, are given in table 9.3. 

These results were further elaborated by measuring the silane loss over short leaching times for samples cured in air and under vacuum.35 Dehydrated mesoporous silica gel was modified with APTS or APDMS in dry toluene (1 % v/v silane/toluene). Curing was performed for variable times in air at 383 K or under vacuum at 423 K. For UV tests the modified silica was stirred in a salicylic aldehyde/ethanol solution. At indicated times a Sml sample was taken, centrifuged and the supernatant was measured at 404 nm. The loss curves of APTS for variably cured samples are displayed in figure 9.17. The position and profile of the absorbance curves are indicative for the stability of the coating under study. 

In analogy to the loading step study, the effect of substrate structure has been studied by modifying mesoporous silica gels with a variable mean pore diameter.28 Sample pretreatment and curing (20 h, 423 K) were performed under vacuum. Variation of the pretreatment temperature causes a change in specific surface area and silanol number. 

Figure 9.21 Chemical loading of APTS modified mesoporous silica gel as a Junction of input ratio OH.APTS.

J. Skubiszewska-Zi ba, R. Leboda, O. Seledets, and V. M. Gun ko, Effect of Preparation Conditions of Carbon-Sihca Adsorbents Based on Mesoporous Silica Gel Si-100 and Carbonised Glucose on Their Pore Structure, Colloids Surf. A 231(1-3), 39-49 (2003). 

Abstract. A variety of pyrocarbon/silica gel adsorbents were prepared using commercial mesoporous silica gels Si-40, Si-60, and Si-100 as matrices modified by carbon deposits from pyrolysis of several organic precursors. The second type of hybrid carbon-mineral adsorbents was synthesized using spent natural palygorskite utilized in paraffin purification. The adsorbents were then heated, hydrothermally treated, or modified by additional deposition of carbon. Changes in the structural and adsorption characteristics of hybrid adsorbents before and after treatments were analyzed by microscopy, p-nitrophenol and nitrogen adsorption isotherms, and TG, TEM, XRD, and XRF methods. 

Abstract. Several series of pyrocarbon/silica adsorbents were prepared using fumed oxides of different specific surface areas, and mesoporous silica gel Si-100, as inorganic matrices. Different synthetic and natural polymers as well as glucose were used as carbon precursors. Solutions of phosphoric acid at various concentrations were utilized to prepare functionalized hybrid carbon-silica adsorbents. Nitrogen, p-nitrophenol and Cd(II) adsorption isotherms as well as AFM, XRD and XRF methods were used to estimate the structural and adsorption characteristics of the adsorbents. 

Table 10.5. Levels of hydroxyladon and sorption of water vapour by pyrogenic and precipitated silicas and mesoporous silica gels.

Figure 10.8. Nitrogen isotherms (top) and

Figure 10.10. Argon and nitrogen isotherms at 77 K on mesoporous silica gel B (top) and microporous silica gel C (bottom) (Payne et al 1973).

As noted above, many investigations have been made of the adsorption of water on non-porous silicas. Less attention has been given to the dehydroxylation of porous silicas. An early study by Dzhigit et al. (1962) of the adsorption water vapour on a mesoporous silica gel involved both isotherm and calorimetric measurements. It was found that at very low surface coverage the adsorption enthalpy was not significantly affected by dehydroxylation, but a large difference became apparent as the surface coverage increased. A slow uptake of water vapour, which occurred after dehydroxylation, was attributed to chemisorption. 

The triethoxysilyl endgroup is a popular functional group to bind the catalyst to a polymeric support [238]. Polymeric supports include silica gel, MCM-41 (mesoporous silica gel) and ITQ-2 (delaminated zeolite) [247]. Corma et al. used this approach to synthesise gold(I) and palladium(II) NHC complexes for Suzuki cross-coupling reactions between iodobenzene and various arylboronic acids (see Figure 4.78) [247]. The results were very modest at 35-80% dependent upon the substitution pattern of the arylboronic acid. Yields with gold(I) catalysts were marginally better than those for palladium(II) complexes. 

The first results were obtained using modified mesoporous silica gels [4-8]. For such systems, water intrusion typically occurs between 10 MPa and 60 MPa, whereas extrusion occurs at relatively low pressures, from 8 MPa down to atmospheric pressure, providing a... 

The characterization of polysiloxane-modified mesoporous silica gels derived from the acid catalyzed hydrolysis of tetraethoxysilane and oligomeric silanol terminated polydimethylsiloxane using Si CPMAS NMR has been report. ... 

T. Sakamoto, C. Pac, Selective epoxidation of olefins by hydrogen peroxide in water using a polyoxometalate catalyst supported on chemically modified hydrophobic mesoporous silica gel. Tetrahedron Lett. 41 (2000) 10009. 

Carbon dioxide has been used as an alternative probe for characterizing carbonaceous materials (75) and homogeneous polymers (6,9). Carbon dioxide is about the same size as N2, but has a lower vapor pressure, allowing its isotherms to be determined at higher temperatures where the activation energy for diffusion in nanopores is overcome. Specific interactions of COj due to its quadrupole moment appear to play an unimportant role in its adsorption on carbonaceous and mineral surfaces (75 and references therein). In support of this, we have shown that adsorption of Nj and COj on a mesoporous silica gel are of similar magnitude (unpublished results). 

The low SSA-based values and fast rates of sorption exhibited by the three mesoporous silica gels suggest that the majority of the surface areas internal to gel particles are not accessible by phenanthrene under water-wet conditions because water is preferentially adsorbed on hydrophilic surfaces, forming a highly structured water phase within pores. Phenanthrene molecules can not enter such pores due to entropic effects. 

Mesoporous carbons were manufactured using mesoporous silica gel 2D hexagonal mesoporous silica gel (e.g., SBA-15) yields carbons with 2D-ordered mesopores and 3D cubic mesoporous silica gel (e.g., MCM-48) yields carbons with 3D-ordered... 

The first impregnation of mesoporous silica gel by furfuryl alcohol resulted in bimodal mesoporous carbon with the maximum pore size of 2.9 and 16 nm and further impregnation yielded unimodal carbons with the pore size of 2.8 nm. 5 was 1540-1810m /g and more than half the overall pore volume was attributed to... 

FIG. 3 Nitrogen adsorption isotherms for the unmodified mesoporous silica gel LiChrospher Si-100 and samples physically coated (CBPB-T24) and chemically bonded (CBPB-B22) with CBPB. (Data from Ref. 77.)... 

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