Immobilization mesoporous supports

One of the most promising applications of enzyme-immobilized mesoporous materials is as microscopic reactors. Galameau et al. investigated the effect of mesoporous silica structures and their surface natures on the activity of immobilized lipases [199]. Too hydrophilic (pure silica) or too hydrophobic (butyl-grafted silica) supports are not appropriate for the development of high activity for lipases. An adequate hydrophobic/hydrophilic balance of the support, such as a supported-micelle, provides the best route to enhance lipase activity. They also encapsulated the lipases in sponge mesoporous silicates, a new procedure based on the addition of a mixture of lecithin and amines to a sol-gel synthesis to provide pore-size control. 

Mayoralas et al. [70] reported the aldol reaction of hydroxyacetone with different aldehydes catalyzed by immobilized L-proline on a mesoporous support. Heterogenized L-proline on MCM-41 showed higher enantioselectivity (80% ee) than its homogeneous counterpart (75% ee) in the aldol reaction of benzaldehyde with hydroxyacetone in dimethylsulfoxide (DMSO) solvent with the assistance of microwave heating. 

The metal complex was immobilized on the mesoporous support by adsorption in liquid phase. FTIR and UV/vis analyses were performed at different stages of the preparation process in order to monitor the preparation of the materials. In particular, the residual solutions obtained from the immobilization process were analyzed before and after Soxhlet extraction. 

In heterogeneous polymer-supported catalysts, binding to the activated soHd or polymer matrix is either mediated by immobilized ligands [102] or by direct incorporation of the active Pd(0) nanoparticles into the mesoporous supports [103]. The latter methodology has attracted much attention in the past few years, as the fusion between porous materials and nanoparticle technology offers the possibihty to further tune the catalyst activity and selectivity. 

Ionic liquids (ILs) have been studied either as homogeneous or heterogeneous catalysts, where in the latter ease ILs were used in the form of a supported speeies. ILs show bi-functional activity as they aet as solvent as well as eatalyst due to the eonsiderable solubility of CO on it. ILs ean easy be separated from the reaction mixture and reused. Imidazole salt, quaternary ammonium salt, quaternary phosphonium salt, etc. are some aetive ILs for this reaetion [221-226]. SiO is the most effective support for immobilizing or supporting ILs. Mesoporous materials such as SBA-15 and MCM-41 also are used to immobilize ILs [218,222]. 

Mesoporous silica functionalized by chiral primary-tertiary diamine/Bronsted acid conjugates was successfully synthesized by Xiaobing et al. Two functionalities of this material, that is, the chiral organofunctional group and the mesoporous support, provided the chiral enhancement in the asymmetric aldol reaction of acetone with various aldehydes. The catalyst exhibited good activity and enantioselectivity without loss of activity. Particularly, the catalytic activity of SBA-15 with an immobilized chiral organic group increased in enantiomeric excess value of the reaction product as compared with silica gel as the support [89]. 

Clarke and Shannon also supported copper bis(oxazoline) complexes onto the surfaces of inorganic mesoporous materials, such as MCM-41 and MCM-48, through the covalent binding of the ligand, modified by alkoxysilane functionalities [59]. The immobilized catalysts allowed the cyclopropanation of styrene with ethyldiazoacetate to be performed as for the corresponding homogeneous case, and were reused once with almost no loss of activity or selectivity. 

Although these examples show the possible immobilization on clays and mesoporous zeolites, the most widely used support for salen complexes has... 

The same type of porphyrin-Ru complex was immobilized by coordina-tive adsorption on aminopropylsilicas (Fig. 26) as either amorphous or crystalline supports [79]. Mesoporous crystalline MCM-48 was the best support, as shown by the improved results obtained in the epoxidation of styrene with 2,6-dichloropyridine N-oxide (TON > 13 000 and 74% ee). The versatility of this catalyst was demonstrated in the intramolecular cyclopropanation of frans-cinnamyl diazoacetate. TON was ten times higher than that obtained in solution and 85% ee was observed. The solid was recycled and reused, although partial loss of selectivity occurred. 

The above example outlines a general problem in immobilized molecular catalysts - multiple types of sites are often produced. To this end, we are developing techniques to prepare well-defined immobilized organometallic catalysts on silica supports with isolated catalytic sites (7). Our new strategy is demonstrated by creation of isolated titanium complexes on a mesoporous silica support. These new materials are characterized in detail and their catalytic properties in test reactions (polymerization of ethylene) indicate improved catalytic performance over supported catalysts prepared via conventional means (8). The generality of this catalyst design approach is discussed and additional immobilized metal complex catalysts are considered. 

Blanco, R.M., Terreros, P., Fernandez-Perez, M., Otero, C. and az-Gonzalez, G. (2004) Functionalization of mesoporous silica for lipase immobilization Characterization of the support and the catalysts. Journal of Molecular Catalysis B-Enzymatic, 30, 83-93. 

In addition to popular mesoporous silica materials, mesoporous silica supports with various morphologies have also been used for protein immobilization. Tang and coworkers synthesized lotus-leaf-like silica flakes with a three-dimensionally connected nanoporous structure and controllable thickness, which were used for immobilization of ribonuclease A [126]. The synthesized silica flakes have a thickness of200 nm, and a diameter of 3 mm, showing a much higher initial adsorbing rate of... 

Nanostructured silica and ordered mesoporous silicas have been envisaged as small enzyme immobilization supports [196]. The encapsulation approach is required either to further immobilize enzymes adsorbed in the channels by reducing the pore opening by further silylation or by encapsulating the enzyme directly [197]. 

In this communication a study of the catalytic behavior of the immobilized Rhizomucor miehei lipase in the transesterification reaction to biodiesel production has been reported. The main drawbacks associated to the current biodiesel production by basic homogeneous catalysis could be overcome by using immobilized lipases. Immobilization by adsorption and entrapment have been used as methods to prepare the heterogeneous biocatalyst. Zeolites and related materials have been used as inorganic lipase supports. To promote the enzyme adsorption, the surface of the supports have been functionalized by synthesis procedures or by post-treatments. While, the enzyme entrapping procedure has been carried out by sol-gel method in order to obtain the biocatalyst protected by a mesoporous matrix and to reduce its leaching after several catalytic uses. 

Dispersion of POMs onto inert solid supports with high surface areas is very important for catalytic application because the surface areas of unsupported POMs are usually very low (—10 m2g). Another advantage of dispersion of POMs onto inert supports is improvement of the stability. Therefore, immobilization of POMs on a number of supports has been extensively studied. Silica and active carbon are the representative supports [25], Basic supports such as MgO tend to decompose POMs [101-104], Certain kinds of active carbons firmly entrap POMs [105,106], The maximum loading level of POMs on active carbons is 14 wt% [107], Dispersion of POMs onto other supports such as zeolites, mesoporous molecular sieves, and apatites, is of considerable interest because of their high surface areas, unique pore systems, and possibility to modify their compositions, morphologies, and sorption properties. However, a simple impregnation of POM compounds on inert supports often results in leaching of POMs. 

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