Mesoporous Molecular Sieve Adsorbents

This new family of mesoporous silica and aluminosilicate compounds were obtained by the introduction of supramolecular assemblies. Micellar aggregates, rather than molecular species, were used as structure-directing agents. Then, the growth of inorganic or hybrid networks templated by structured surfactant assemblies permitted the construction of novel types of nanostructured materials in the mesoscopic scale (2-100 nm) [110,113,117], 

This supramolecular directing concept has led to a family of materials whose structure, composition, and pore size can be tailored during synthesis by variation of the reactant stoichiometry, nature of the surfactant molecule, or by postsynthesis functionalization techniques [113]. 

The obtained solid phases are characterized by an ordered, not crystalline, pore wall structure, presenting sharp mesopore-size dispersions. These mesoporous materials are ordered, but not crystalline, because of the lack of precise atomic positioning in the pore wall structure as was shown by MAS-NMR and Raman spectroscopy [115]. This gave rise to inorganic solids with enormous differences in morphology and structure [113]. 

MCM-48, a cubic material, exhibits an x-ray diffraction pattern consisting of several peaks that can be assigned to the Ia3d space group [117]. The structure of MCM-48 has been proposed to be bicontinuous with a simplified representation of two, infinite, three-dimensional, mutually intertwined, unconnected network of rods [118]. 

---

Since the discovery of the mesoporous molecular sieves MCM-41,[1] these materials have been extensively investigated in many applications as catalysis, catalyst support, adsorbent, electronic and optical devices.[2-4] Especially there exists a strong motivation to use the MCM-41 materials as catalytic supports, which have large surface area( 1000 m2/g), high... 

The specific activity of Ti-sites in oxidation with aqueous H202 was shown to decrease from microporous to mesoporous molecular sieves such as MCM-41 and HMS. The surface hydrophilicity of the latter mesoporous silicates is likely one of the main reasons for the low activity of Ti sites.[47] Water molecules adsorbed on the surface silanol groups would limit the access of organophilic reagents to the Ti sites.1751 This can be limited, to a certain extent, by grafting hydrophobic groups on the surface silanols.1761... 

Nanoporous materials like zeolites and related materials, mesoporous molecular sieves, clays, pillared clays, the majority of silica, alumina, active carbons, titanium dioxides, magnesium oxides, carbon nanotubes and metal-organic frameworks are the most widely studied and applied adsorbents. In the case of crystalline and ordered nanoporous materials such as zeolites and related materials, and mesoporous molecular sieves, their categorization as nanoporous materials are not debated. However, in the case of amorphous porous materials, they possess bigger pores together with pores sized less than 100 nm. Nevertheless, in the majority of cases, the nanoporous component is the most important part of the porosity. 

It is a mass transfer between a mobile, solid, or liquid phase, and the adsorption bed packed in a reactor. To carry out adsorption, a reactor, where a dynamic adsorption process will occur, is packed with an adsorbent [2], The adsorbents normally used for these applications are active carbons, zeolites and related materials, silica, mesoporous molecular sieves, alumina, titanium dioxide, magnesium oxide, clays, and pillared clays. 

Xu, X.X., Song, C.S., Andresen, J.M., Miller, B.G., and Scaroni, A.W. Preparation and characterization of novel C02 molecular basket adsorbents based on polymer-modified mesoporous molecular sieve MCM-41. Microporous and Mesoporous Materials, 2003, 62, 29. 

Xu, X.C., Novochinskii, I., and Song, C.S. Low-temperature removal of H2S by nanoporous composite of polymer-mesoporous molecular sieve MCM-41 as adsorbent for fuel cell applications. Energy Fuels, 2004-005, 19, 2214. 

FIGURE 9.5 Amperograms at a glassy carbon electrode modified with hemoglobin adsorbed in mesoporous molecular sieves MSU/Hbjj/PDDA upon successive additions of 10 iM H2O2 to 0.2 M potassium phosphate buffer solution at pH 6.9. Applied potential -0.30 V vs. SCE. (Adapted from Sun et al., 2008. Talanta 74,1692-1698.)... 

New types of mesoporous molecular sieves (their first synthesis opened a new subfield of molecular sieve chemistry) have been prepared over the last ten years by new synthetic approaches, different from those known for zeolites. The variety of the synthetic procedures described and the differences in the textural properties due to different synthetic procedures, as well as to the high temperature treatment, give evidence that mesoporous molecular sieves of different chemical compositions are very interesting materials not only in materials science. They could be important also for the application as heterogeneous catalysis, support for immobilization of homogeneous catalysts, adsorbents or materials for synthesis of new types of inclusion compounds. 

SBA-15-type materials are novel well-ordered mesoporous molecular sieves with high surface area and narrow distribution of pores of about 60-100 A in diameter [1]. These materials have already been tested in a variety of applications, such as adsorbents, molecular sieves, catalysts and catalytic supports. However, up to now only purely siliceous SBA-15 materials were... 

Nb- and Ti-containing silica-based mesoporous molecular sieves were used as catalysts for photocatalytic oxidation of methane. It is found that methanol was formed at 323 K and water pre-adsorbed samples exhibited higher catalytic activity than their pure metal oxides. By comparison with Nb-MCM-41, Ti-MCM-41 gave better methane conversion and methanol yield although traces of formaldehyde were observed over Nb-MCM-41 catalyst. Interestingly, OH radical was detected by the in-situ ESR spectroscopy using DMPO as trapper. 

The adsorption of various amino acids from aqueous solutions using MCM-41-type mesoporous molecular sieves is discussed on the basis of their adsorption isotherms. The amounts adsorbed strongly depend on the pH and the nature of the individual amino acid Amino acids with acidic side chains are hardly adsorbed, whereas basic amino acids show very high affinities to the mesoporous adsorbent. The uptake of amino acids with non-polar side chains increases with the chain length. The adsorption complex is proposed to consist of the cationic form of the amino acid attached to the negatively charged silica surface. 

---