Mesoporous molecular sieves, example

However, some characteristics of the reactant and product (and solvent) molecules used in the reactions of fine chemical synthesis have to be considered their large size, their low thermal stability, and their polarity. The small size of the micropores limits the use of zeolites to the synthesis of relatively small molecules solutions to overstep this limitation have been found with the development of larger (mesoporous) molecular sieves and of nanocrystallite zeolites, the reaction then occurring on the large external surface. The low thermal stability of the molecules leads to operation at a low ternperamre, often in the liquid phase. Last, as will be shown in the first example (Section 12.2.3.1), the differences in polarity between reactant(s), product(s), and solvent molecules have to be considered for optimizing both the zeolite catalyst and the operating conditions. 

Up to now, a variety of non-zeolite/polymer mixed-matrix membranes have been developed comprising either nonporous or porous non-zeolitic materials as the dispersed phase in the continuous polymer phase. For example, non-porous and porous silica nanoparticles, alumina, activated carbon, poly(ethylene glycol) impregnated activated carbon, carbon molecular sieves, Ti02 nanoparticles, layered materials, metal-organic frameworks and mesoporous molecular sieves have been studied as the dispersed non-zeolitic materials in the mixed-matrix membranes in the literature [23-35]. This chapter does not focus on these non-zeoUte/polymer mixed-matrix membranes. Instead we describe recent progress in molecular sieve/ polymer mixed-matrix membranes, as much of the research conducted to date on mixed-matrix membranes has focused on the combination of a dispersed zeolite phase with an easily processed continuous polymer matrix. The molecular sieve/ polymer mixed-matrix membranes covered in this chapter include zeolite/polymer and non-zeolitic molecular sieve/polymer mixed-matrix membranes, such as alu-minophosphate molecular sieve (AlPO)/polymer and silicoaluminophosphate molecular sieve (SAPO)/polymer mixed-matrix membranes. 

Grafting a modified cinchona alkaloid to hexagonal mesoporous molecular sieve SBA-15 afforded catalyst (27) with excellent activity. 1-Phenyl-1-propene was converted to the corresponding diol in 98% yield (98% ee), while trans-stilbene yielded the desired product in 97% yield (99% ee) [92]. Other examples in this field are the utilization of microencapsulated osmium tetroxide by Kobayashi [93] and the application of continuous dihydroxylation mns in chemzyme membrane reactors described by Woltinger [94]. 

In the last decade, the mesoporous molecular sieve MCM-41 has been developed (2S2) and applied as a catalyst to many acid-catalyzed reactions (2SS). However, until now, comparatively few investigations of mesoporous molecular sieves as base catalysts have been reported (169,211-214,234,235). For example, sodium- and cesium-exchanged mesoporous MCM-41 were shown to be mildly selective, water-stable, recyclable catalysts for the base-catalyzed Knoevenagel condensation, and mesoporous MCM-41 containing intraporous cesium oxide particles prepared by impregnation with aqueous cesium acetate and subsequent calcination was found to have strong-base activity for the Michael addition (211,213) and rearrangement of co-phenylalkanals to phenyl alkyl ketones (212). 

Once the multi-step reaction sequence is properly chosen, the bifunctional catalytic system has to be defined and prepared. The most widely diffused heterogeneous bifunctional catalysts are obtained by associating redox sites with acid-base sites. However, in some cases, a unique site may catalyse both redox and acid successive reaction steps. It is worth noting that the number of examples of bifunctional catalysis carried out on microporous or mesoporous molecular sieves is not so large in the open and patent literature. Indeed, whenever it is possible and mainly in industrial patents, amorphous porous inorganic oxides (e.g. j -AEOi, SiC>2 gels or mixed oxides) are preferred to zeolite or zeotype materials because of their better commercial availability, their lower cost (especially with respect to ordered mesoporous materials) and their better accessibility to bulky reactant fine chemicals (especially when zeolitic materials are used). Nevertheless, in some cases, as it will be shown, the use of ordered and well-structured molecular sieves leads to unique performances. 

Recently, important efforts have been focused on obtaining materials with higher pore sizes [62], for example, the so-called mesoporous molecular sieves (MMS) [63-66],... 

The book explores various examples of these important materials, including perovskites, zeolites, mesoporous molecular sieves, silica, alumina, active carbons, carbon nanotubes, titanium dioxide, magnesium oxide, clays, pillared clays, hydrotalcites, alkali metal titanates, titanium silicates, polymers, and coordination polymers. It shows how the materials are used in adsorption, ion conduction, ion exchange, gas separation, membrane reactors, catalysts, catalysts supports, sensors, pollution abatement, detergency, animal nourishment, agriculture, and sustainable energy applications. 

Alkali-exchanged mesoporous molecular sieves are suitable solid base catalysts for the conversion of bulky molecules which cannot access the pores of zeolites. For example, Na- and Cs-exchanged MCM-41 were active catalysts for the Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate (pKa=10.7) but low conversions were observed with the less acidic diethyl malonate (pKa=13.3) [123]. Similarly, Na-MCM-41 catalyzed the aldol condensation of several bulky ketones with benzaldehyde, including the example depicted in Fig. 2.38, in which a flavonone is obtained by subsequent intramolecular Michael-type addition [123]. 

As a typical example of CEDFT calculations, we present in Fig. 1 the capillary condensation isotherm of N2 in a cylindrical pore mimicking the pore channel in MCM-41 mesoporous molecular sieves. The isotherm is presented in co-ordinates adsorption N versus chemical potential p Calculations were performed at 77 K for the internal diameter of 3.3 nm up to the saturation conditions, point H. We used Tarazona s representation of the Helmholtz free energy [6] with the parameters for fluid-fluid and solid-fluid interaction potentials, which were employed in our previous papers [7]. We distinguish three regions on the isotherm. The adsorption branch OC corresponds to consecutive formation of adsorption layers. Note that the sharp transitions between the consecutive layers are not observed in experiments. They are caused by a well-known shortcoming of the model employed, which ignores intrinsic to real... 

Other specific areas of micellar catalysis in which industry has expressed interest are in micellar phase-transfer catalysis and in the synthesis of mesoporous molecular sieves [92]. In the first example of the latter application, investigators at Mobil were able to control pore size and properties by synthesizing the desired mesoporous material in the presence of appropriately sized, structured, and charged micelles [96]. The burst of research activity in this area that occurred in the next few years after this discovery has been reviewed by Huo et al. [97]. 

The key property required of the inorganic species is ability to build up (polymerize) around the template molecules into a stable framework. As is already evident in this article, the most commonly used inorganic species are silicate ions, which yield a silica framework. The silica can be doped with a wide variety of other elements (heteroatoms), which are able to occupy positions within the framework. For example, addition of an aluminium source to the synthesis gel provides aluminosilicate ions and ultimately an aluminosilicate mesoporous molecular sieve. Other nonsilica metal oxides can also be used to construct stable mesoporous materials. These include alumina, zirconia, and titania. Metal oxide mesophases, of varying stability, have also been obtained from metals such as antimony (Sb), iron (Fe), zinc (Zn), lead (Pb), tungsten (W), molybdenum (M), niobium (Nb), tantalum (Ta), and manganese (Mn). The thermal stability, after template removal, and structural ordering of these mesostructured metal oxides, is far lower, however, than that of mesoporous silica. Other compositions that are possible include mesostructured metal sulfides (though these are unstable to template removal) and mesoporous metals (e.g., platinum, Pt). 

The large uniform-shaped cavities of mesoporous molecular sieves can provide many interesting possibilities for the fabrication and hosting of quantum-sized particles. An example is the fabrication of stable carbon wires in the pore structure of mesoporous materials by polymerizing acrylonitrile within the channels. It is also possible to fill the mesopores with a semiconductor such as germaiuum. This type of molecular and quantum wires may have many... 

Most of the reported examples involve the use of MCM-41, MCM-48, hydrothermally stable mesoporous molecular sieves (SBA-15) or silica nanotube matrices in which certain selective probe molecules are covalently anchored to the pore walls. In addition, the very low detection limits observed for some of these systems suggest that the surface chelate effect also plays a role here. Following this approach, chemosensors for the signaling of Cy2+ 34 36 pg3+37 2+ 35,38 describcd. Optical pH sensors using... 

It is well known that the elements in framework of zeolite molecular sieves greatly influence the properties and behaviors of these materials [1-3], The introduction of heteroatoms into the framework has become one of most active fields in study of zeolites. The investigations were mostly focused on the methods to introduce heteroatoms into the framework (for examples, hydrothermal synthesis and post-synthesis), the mechanisms for incorporations, the effect of heteroatoms on the acid-base properties and the catalytic features of modified samples [1-10]. Relatively less attention was paid to the effect of treatment process on the porous properties of samples although the incorporation of heteroatoms, especially by the so-called post-synthesis, frequently changes the distribution of pore size. Recently, we incorporated Al, Ga and B atoms into zeolites (3 by the post-synthesis in an alkaline medium named alumination, galliation and boronation, respectively. It was found that different trivalent elements inserted into the [3 framework at quite different level. The heteroatoms with unsuitable atom size and poor stability in framework were less introduced, leading to that a considerable amount of framework silicon were dissolved under the action of base and the mesopores in zeolite crystal were developed. As a typical case, the boronation of zeolites (3 and the accompanied formation of mesopores are reported in the present paper. 

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