Mesoporous organic oxidation catalyst

Metalloporphyrinosilicas as a new class of hybrid organic-inorganic materials were prepared by polymerization of 3- er -butyl-5-vinylsalicylaldehyde with styrene and divinylbenzene and used as selective biomimetic oxidation catalyst.27 Synthesis and structural characterization of rare-earth bisfdimethyl-silyl)amides and their surface organometallic chemistry on mesoporous silicate MCM-41 have been reported.28... 

Niobium and titanium incorporation in a molecular sieve can be achieved either by hydrothermal synthesis (direct synthesis) or by post-synthesis modification (secondary synthesis). The grafting method has shown promise for developing active oxidation catalyst in a simple and convenient way. Recently, the grafting of metallocene complexes onto mesoporous silica has been reported as alternate route to the synthesis of an active epoxidation catalyst [21]. Further the control of active sites, the specific removal of organic material (template or surfactant) occluded within mesoporous molecular sieves during synthesis can also be important and useful to develop an active epoxidation catalyst. Thermal method is quite often used to eliminate organic species from porous materials. However, several techniques such as supercritical fluid extraction (SFE) and plasma [22], ozone treatment [23], ion exchange [24-26] are also reported. 

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. 

Recently, Carreon and Guliants reported novel hexagonal, cubic and lamellar VPO phases, which displayed improved thermal stability, desirable chemistries (i.e. the P/V ratios and vanadium oxidation states), and pore structures for the partial oxidation of n-butane [143-145]. These novel VPO phases displayed the selectivities to maleic anhydride up to 40 mol. % at 673K at 10 % n-butane conversion [146]. A conventional organic VPO catalyst containing well-crystallized vanadyl(IV) pyrophosphate, the proposed active and selective phase for n-butane oxidation to maleic anhydride, displayed the selectivities to maleic anhydride 50 mol. % under the same reaction conditions. The low yields observed for mesoporous VPO catalysts confirmed the critical role of the vanadyl pyrophosphate phase (VO)2P207 in catalyzing the oxidation of -butane to maleic anhydride. Therefore, the amorphous nature of the mesoporous VPO... 

In 1998, Ookoshi and Onaka reported remarkable increase in activity of M0O3 when this was supported on hexagonal mesoporous silica instead of conventional one. With this catalyst (7 wt % Mo) they achieved high conversion of 1-octene into 7-tetradecene at 50°C. Similarly in 2002, Onaka and Oikawa found rhenium oxide dispersed on mesoporous alumina with uniform pore size (7 wt % Re) to be more active in 1-octene metathesis than rhenium oxide on conventional y-alumina. Although both works lacked detailed characterization of supports and prepared catalysts, they clearly showed the positive effect of organized mesoporous siqrport on catalyst activity in alkene metathesis. 

The oxide catalysts are microporous or mesoporous materials or materials containing both types of pores. In the latter case, the applicability is larger in terms of the molecular size of the reactants. Acid-base properties of these materials depend on the covalent/ionic character of the metal-oxygen bonds. These sites are involved in several steps of the catalytic oxidation reactions. The acid sites participate with the cation redox properties in determining the selective/unselective catalyst behavior [30,31]. Thus, many studies agree that partial oxidation of organic compounds almost exclusively involves redox cycles and acid-base properties of transition metal oxides and some authors have attempted to relate these properties with activity or selectivity in oxidation reactions [31,42]. The presence of both Bronsted and Lewis acid sites was evidenced, for example, in the case of the metal-modified mesoporous sihcas [30,39,43]. For the bimetallic (V-Ti, Nb-Ti) ions-modified MCM-41 mesoporous silica, the incorporation of the second metal led to the increase of the Lewis sites population [44]. This increased concentration of the acid sites was well correlated with the increased conversion in oxidation of unsaturated molecules such as cyclohexene or styrene [26,44] and functionalized compounds such as alcohols [31,42] or phenols [45]. 

J. Y. Luo et al synthesized the mesoporous La-Co-Ce-O composit oxide by citric acid complexation-organic template decomposition method. The catalysts prepared by this method had a high specific surface area about 95-156.6 m. g with very uniform pore diameter (3.7-3.95 nm) (changing with the different component and calcination temperature). The catalysts were used in catalytic CO oxidation and propane oxidation, and the excellent catalytic properties and thermal stability can be observed. In the case of CO oxidation, the reaction temperatures for 50% (Tso) and 90% CO (Tgo) conversion are 140 °C and 169 °C, respectively, over mesoporous La-Co-Ce-0 eomposit oxide catalysts, which are 39 and 30 lower than those over the catalyst prepared by co-preeipitation method (LCC (0.5)-CP-500 catalyst) in the case of propane oxidation, the 301 °C and 341 °C reaction temperature are required for the Tso and Tgo, which are 29 and 64 lower than those over the LCC (0.5)-CP-500 catalyst. In addition, the ordered mesoporous strueture of catalyst was conductive to the dispersion and transmission of the reactants, and therefore the inner surface of catalyst was fully utilized, which benefited the increase in catalytic activity and efficiency by the easy accessibility, and as well as the decrease in resistance of mass-transfer. As a result, excellent catalytic properties for CO oxidation and propane oxidation can be obtained over mesoporous lanthanum-containing composite oxide catalysts [87],... 

Luo, J. Meng, M. Qian, Y. et al. Mesoporous Mixed Oxide La-Co-Ce-0 Catalysts Prepared by Citric Acid Complexation-Organic Template Decomposition Method. Chin J. Catal. 2006, 27(6), 471-473. 

The search for better catalysts has been facilitated in recent years by molecular modeling. We are seeing here a step change. This is the subject of Chapter 1 (Molecular Catalytic Kinetics Concepts). New types of catalysts appeared to be more selective and active than conventional ones. Tuned mesoporous catalysts, gold catalysts, and metal organic frameworks (MOFs) that are discussed in Chapter 2 (Hierarchical Porous Zeolites by Demetallation, 3 (Preparation of Nanosized Gold Catalysts and Oxidation at Room Temperature), and 4 (The Fascinating Structure... 

Abstract A review of the thermolytic molecular precursor (TMP) method for the generation of multi-component oxide materials is presented. Various adaptations of the TMP method that allow for the preparation of a wide range of materials are described. Further, the generation of isolated catalytic centers (via grafting techniques) and mesoporous materials (via use of organic templates) is simimarized. The implications for syntheses of new catalysts, catalyst supports, nanoparticles, mesoporous oxides, and other novel materials are discussed. 

The viabiUty of using site-isolated Ta(V) centers for cyclohexene epoxi-dation was explored by grafting ( PrO)2Ta[OSi(O Bu)3]3 onto a mesoporous silica material [83]. After calcinations, the material formed is less active and selective in the oxidation of cyclohexene than the surface-supported Ti(IV) catalysts using organic peroxides however, the site-isolated Ta(V) catalysts are more active under aqueous conditions. 

Mesoporous alumina sphere was synthesized under the catalyst of hydrochloride or ammonia in organic solvents. In a typical synthesis, 1.1 g [poly(ethylene oxide)-6-poly(propylene oxide)-6-poly(ethylene oxide) triblock copolymer (Aldrich, average molecular weight 5800, EO20PO70EO20)] was dissovled in 11.0 g (0.268 mol) acetonitrile and 1.10 g (61.1 mmol) water containing 0.1 mmol HC1 or 5 mmol NH3, the solution of 3.0 g (12.2 mmol) aluminum tri-sec-butoxide dissovled in 10 g (0.24 mol) acetonitrile was slowly dropped into with stirring. After stirring for 6 h, the product was filtered and washed with acetonitrile and dried at room temperature in air. The obtained products were calcined in air at 550 °C for 4 h to remove the templates. 

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