Multi-component catalysts

The saturation coverage during chemisorption on a clean transition-metal surface is controlled by the fonnation of a chemical bond at a specific site [5] and not necessarily by the area of the molecule. In addition, in this case, the heat of chemisorption of the first monolayer is substantially higher than for the second and subsequent layers where adsorption is via weaker van der Waals interactions. Chemisorption is often usefLil for measuring the area of a specific component of a multi-component surface, for example, the area of small metal particles adsorbed onto a high-surface-area support [6], but not for measuring the total area of the sample. Surface areas measured using this method are specific to the molecule that chemisorbs on the surface. Carbon monoxide titration is therefore often used to define the number of sites available on a supported metal catalyst. In order to measure the total surface area, adsorbates must be selected that interact relatively weakly with the substrate so that the area occupied by each adsorbent is dominated by intennolecular interactions and the area occupied by each molecule is approximately defined by van der Waals radii. This... 

The Biginelli reaction involves an one-pot reaction between aldehyde 1, 1,3-dicarbonyl 2, and urea 3a or thiourea 3b in the presence of an acidic catalyst to afford 3,4-dihydropyrimidin-2(l//)-one (DHPM) 4. This reaction is also referred to as the Biginelli condensation and Biginelli dihydropyrimidine synthesis. It belongs to a class of transformations called multi-component reactions (MCRs). 

Dehydrogenation with a multi-component catalyst Catalyst development Pt, Ge, Ni, LI, Cl 89... 

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. 

Abstract Recent advances in the metal-catalyzed one-electron reduction reactions are described in this chapter. One-electron reduction induced by redox of early transition metals including titanium, vanadium, and lanthanide metals provides a variety of synthetic methods for carbon-carbon bond formation via radical species, as observed in the pinacol coupling, dehalogenation, and related radical-like reactions. The reversible catalytic cycle is achieved by a multi-component catalytic system in combination with a co-reductant and additives, which serve for the recycling, activation, and liberation of the real catalyst and the facilitation of the reaction steps. In the catalytic reductive transformations, the high stereoselectivity is attained by the design of the multi-component catalytic system. This article focuses mostly on the pinacol coupling reaction. 

It is important to select stoichiometric co-reductants or co-oxidants for the reversible cycle of a catalyst. A metallic co-reductant is ultimately converted to the corresponding metal salt in a higher oxidation state, which may work as a Lewis acid. Taking these interactions into account, the requisite catalytic system can be attained through multi-component interactions. Stereoselectivity should also be controlled, from synthetic points of view. The stereoselective and/or stereospecific transformations depend on the intermediary structure. The potential interaction and structural control permit efficient and selective methods in synthetic radical reactions. This chapter describes the construction of the catalytic system for one-electron reduction reactions represented by the pinacol coupling reaction. 

A combination of cat. Ybt and A1 is effective for the photo-induced catalytic hydrogenative debromination of alkyl bromide (Scheme 28) [69]. The ytterbium catalyst forms a reversible redox cycle in the presence of Al. In both vanadium- and ytterbium-catalyzed reactions, the multi-component redox systems are achieved by an appropriate combination of a catalyst and a co-reductant as described in the pinacol coupling, which is mostly dependent on their redox potentials. 

The multi-component procedure is also effective for the chromium-catalyzed addition of organic halides to aldehydes (the Nozaki-Hiyama-Kishi reaction) [73]. The active Cr(II) species is recycled by redox interaction with Mn powder as the stoichiometric co-reductant in the presence of MesSiCl (Scheme 34), which mainly liberates the chromium catalyst from the alkoxide adduct. The chemo- and diastereo-selective addition reaction is performed with a variety of organic halides and alkenyl triflates. In the case of crotyl bromide, the addition is highly stereoconvergent, i.e., the respective anti-... 

The multi-component systems developed quite recently have allowed the efficient metal-catalyzed stereoselective reactions with synthetic potential [75-77]. Multi-components including a catalyst, a co-reductant, and additives cooperate with each other to construct the catalytic systems for efficient reduction. It is essential that the active catalyst is effectively regenerated by redox interaction with the co-reductant. The selection of the co-reductant is important. The oxidized form of the co-reductant should not interfere with, but assist the reduction reaction or at least, be tolerant under the conditions. Additives, which are considered to contribute to the redox cycle directly, possibly facilitate the electron transfer and liberate the catalyst from the reaction adduct. Co-reductants like Al, Zn, and Mg are used in the catalytic reactions, but from the viewpoint of green chemistry, an electron source should be environmentally harmonious, such as H2. 

Richter, A., Langpape, M., Kolf, S. et al. (2002) Combinatorial preparation and high-throughput catalytic tests of multi-component deNO(x) catalysts. Appl. Catal. B Environ., 36, 261. 

Inductively coupled plasma atomic emission spectrometry (ICP-AES) is used to screen polymers, liquids and solvent extracts for residual metal atoms (catalysts, fillers, etc.). The technique can provide rapid multi-component screening of elements in solution over a wide concentration range (0.1-1,000 pg/ml). 

Dienes such as 90 can be accessed by a multi-component reaction under ruthenium catalysis involving an allene 88 and an enone (methyl vinyl ketone in this case), with cerium(m) chloride as an additive in DMF (Scheme 26).95,96 With an allene concentration of 0.25 M, yields are moderate to good. Different ruthenium catalysts and additives were tested in order to optimize this reaction. CpRu(COD)Cl 89 and CpRu(MeCN)3PF6 appeared to be more versatile ones. The mono-, di-, tri-, and tetrasubstituted allenes have been investigated with methyl vinyl... 

Although valence band spectra probe those electrons that are involved in chemical bond formation, they are rarely used in studying catalysts. One reason is that all elements have valence electrons, which makes valence band spectra of multi-component systems difficult to sort out. A second reason is that the mean free path of photoelectrons from the valence band is at its maximum, implying that the chemical effects of for example chemisorption, which are limited to the outer surface layer, can hardly be distinguished from the dominating substrate signal. In this respect UPS, discussed later in this chapter, is much more surface sensitive and therefore better suited for adsorption studies. 

The fact that LEIS provides quantitative information on the outer layer composition of multi-component materials makes this technique an extremely powerful tool for the characterization of catalysts. Figure 4.19 shows the LEIS spectrum of an alumina-supported copper catalyst, taken with an incident beam of 3 keV 4He+ ions. Peaks due to Cu, A1 and O and a fluorine impurity are readily recognized. The high intensity between about 40 and 250 eV is due to secondary (sputtered) ions. The fact that this peak starts at about 40 eV indicates that the sample has charged positively. Of course, the energy scale needs to be corrected for this charge shift before kinematic factors Ef/E-, are determined. 

In another patent, very high propane-1,2-diol yields were obtained with a multi-component oxide catalyst consisting of the metals Co, Cu, Mn and Mo, and P. At 523 K under 25 MPa, glycerol is almost quantitatively converted into propane-1,2-diol. In contrast to most studies, the reactions were performed with pure (99.5%) glycerol instead of aqueous dilutions [123],... 

When alkynes are treated with catalytic amounts of a carbene complex, polymerization instead of metathesis can occur (Figure 3.44) [565,595,597,752-754]. The use of carbene complexes to catalyze alkyne polymerization enables much better control of the reaction than with heterogeneous or multi-component catalysts. Pure acetylene oligomers (n = 3-9) with terminal fcrf-butyl groups have been prepared with the aid of a tungsten carbene complex [755]. 

The known catalyst systems for olefin metathesis can be grouped into heterogeneous catalysts, homogeneous multi-component catalysts, and homogeneous single-component catalysts. 

Table 3.14. Examples of multi-component catalysts for homogeneous-phase alkene metathesis.

Small-scale preparations and the chemical modification of fine chemicals and elaborate intermediates are usually conducted in solution. For this purpose soluble metathesis catalysts of predictable and reproducible activity are generally preferred. The catalytic systems presently known can be grouped into multi-component and single-component catalysts (Tables 3.14-3.16). 

The complexity of the chemical pathways in these systems is due to the molecular complexity of the heavy feedstock rich in aromatic structures, heavy paraffins and S- and N-contairting molecules as well as the multi-component catalyst used. 

As shown in Tab. 11.5, multi-component catalyst (27) matches the activity of its corresponding monomer (4), promoting efficient RCM of (19) in just 15 minutes at 40 °C. The reaction mixture was passed through a short column in methylene chloride to isolate the desired product. Subsequent washing of the silica with diethyl ether led to quantitative recovery of the dendritic catalyst. 400 MHz NMR analysis revealed that 13% of the styrene ligands on the dendrimer were va-... 

The ample diversity of properties that the spinel compounds exhibit is derived from the fact that the possibility of synthesis of multi-component spinels by partial substitution of cations in position A and B giving rise to compounds of formula (AxA i x)(ByB 2 y)04. This accounts for the variety of reactions in which they have been used as catalyst. Moreover, partial substitution of A and B ions giving rise to complex oxides is possible, while maintaining the spinel structure. 

Notably, the use of heteronuclear surface carbonyl species can lead to the preparation of well-defined supported bimetallic entities that can be used as model catalysts to study the promoter effect of a second metal. The close intimacy achieved between the two metals in the surface carbonyl species is related to the structural characteristics and catalytic properties of the final catalyst In the preparation of supported, tailored, multi-component catalysts, the use of metal carbonyl surface species still deserves to be studied to further explore the exciting field of nano-sized entities in catalysis. 

Bahrami K, Khodaei MM, Farrokhi A (2009) Highly efficient solvent-free synthesis of dihydropyrimidinones catalyzed by zinc oxide. Synth Commun 39 1801-1808 74. Gross GA, Wurziger H, Schober A (2006) Solid-phase synthesis of 4,6-diaryl-3,4-dihydro-pyrimidine-2(lH)-one-5-carboxylic acid amide derivatives a Biginelli three-component-condensation protocol based on immobilized beta-ketoamides. J Comb Chem 8 153-155 Desai B, Dallinger D, Kappe CO (2006) Microwave-assisted solution phase synthesis of dihydropyrimidine C5 amides and esters. Tetrahedron 62 4651 664 Kumar A, Maurya RA (2007) An efficient bakers yeast catalyzed synthesis of 3,4-dihydro-pyrimidin-2-(lH)-ones. Tetrahedron Lett 48 4569-4571 77. Zalavadiya P, Tala S, Akbari J, Joshi H (2009) Multi-component synthesis of dihydropyrimidines by iodine catalyst at ambient temperature and in-vitro anti mycobacterial activity. Arch Pharm 342 469-475... 

DME hydration occurs over acid catalysts, whereas the methanol steam reforming reaction proceeds over metal catalysts. Consequently, DME steam reforming requires a multi-component catalyst. Two approaches have been proposed in the literature (a) physical mixtures of a DME hydrolysis catalyst and a methanol steam reforming catalyst (b) supported catalysts that combine the DME hydrolysis and methanol steam reforming components into a single catalyst. 

The result of all these experiments was that the idea of a particularly beneficial macro-compact porous state of the catalyst had to be abandoned, although patents had already been filed in which this had been claimed. Instead, we returned still more convinced of its correctness, to the previous more general hypothesis of the desirability of multi-component catalysts. 

Still during our experiments with ammonia catalysts, other technically important reactions were studied which promised to be accessible to the catalytic method, and particularly to the influence of multi-component catalysts. As this further development has been repeatedly described elsewhere, only a short survey will be presented here. 

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