Noritaka Mizuno

Catalytic Chemistry of Heteropoly Compounds

Publisher Summary This chapter describes the essence of the catalytic chemistry of heteropoly compounds in solution and in the solid state. The catalytic properties of heteropoly compounds have drawn wide attention, owing to the versatility of these compounds as catalysts, which has been demonstrated both by successhl large-scale applications and by promising laboratory results. Heteropolyanions are polymeric oxoanions formed by condensation of more than two different mononuclear oxoanions. Heteropolyanions formed from one kind of polyanion are called isopolyanions. Acidic elements such as Mo, W, V, Nb and Ta, which are present as oxoanions in aqueous solution, tend to polymerize by dehydration at low pH, forming polyanions and water.Heteropoly catalysts can be applied in various ways. They are used as acid as well as oxidation catalysts. They are used in various phases, as homogeneous liquids, in two-phase liquids (in phase-transfer catalysis), and in liquid-solid and in gas-solid combinations, etc.

CU-ZSM-5 ZEOLITE AS HIGHLY ACTIVE CATALYST FOR REMOVAL OF NITROGEN MONOXIDE FROM EMISSION OF DIESEL ENGINES

Copper ion-exchanged ZSM-5 zeolite is the most active for the selective reduction of nitric oxide by ethene in the presence of oxygen at temperatures as low as 437–<Zn (873 K). The activity of copper ion-exchanged ZSM-5 zeolite did not decrease much even in a high gas hourly space velocity (GHSV) region of more than 10 000 h−1 while those of proton-exchanged zeolites and Al2O3 greatly decreased in such high GHSV regions.

Efficient stereo- and regioselective hydroxylation of alkanes catalysed by a bulky polyoxometalate

Direct functionalization of alkanes by oxidation of C–H bonds to form alcohols under mild conditions is a challenge for synthetic chemistry. Most alkanes contain a large number of C–H bonds that present difficulties for selectivity, and the oxidants employed often result in overoxidation. Here we describe a divanadium-substituted phosphotungstate that catalyses the stereo- and regioselective hydroxylation of alkanes with hydrogen peroxide as the sole oxidant. Both cyclic and acyclic alkanes were oxidized to form alcohols with greater than 96% selectivity. The bulky polyoxometalate framework of the catalyst results in an unusual selectivity that can lead to the oxidation of secondary rather than the weaker tertiary C–H bonds. The catalyst also avoids wasteful decomposition of the stoichiometric oxidant, which can result in the production of hydroxyl radicals and lead to non-selective oxidation and overoxidation of the desired products. The ability to selectively transform the C–H bonds of simple alkanes to useful functional groups such as alcohols is a key step in the move away from petrochemical feedstocks. Now, it has been shown that the oxidation of alkanes can be catalysed by a bulky polyoxometalate species using hydrogen peroxide as a stoichiometric oxidant.

INFLUENCE OF SULFUR DIOXIDE ON CATALYTIC REMOVAL OF NITRIC OXIDE OVER COPPER ION-EXCHANGED ZSM-5 ZEOLITE

Abstract The catalytic activity of copper ion-exchanged ZSM-5 zeolites, used for the selective reduction of nitric oxide by propene in the presence of oxygen was only slightly decreased by the addition of sulfur dioxide; for example, the conversion of nitric oxide into nitrogen was changed from ca. 100% in the absence of sulfur dioxide to 85% (773 K) upon the introduction of sulfur dioxide. In contrast, the catalytic activity for nitric oxide decomposition completely disappeared upon addition of sulfur dioxide. These results show that the selective reduction of nitric oxide by hydrocarbon may be practical.

Heteropolyanions in catalysis

Abstract Recent topics in the molecular catalysis of heteropoly compounds both in solid and solution states are described, regarding (a) structural characteristics, (b) acid and redox properties, and (c) catalytic properties.

Efficient Oxidative Alkyne Homocoupling Catalyzed by a Monomeric Dicopper‐Substituted Silicotungstate

The versatility and accessibility of polyoxometalates have led to various applications in the fields of analytical chemistry, medicine, electrochemistry, photochemistry, and catalysis, particularly in the field of oxidation catalysis. Interest in catalysis by metal-substituted polyoxometalates has grown significantly because of the unique reactivity that results from the composition and structure of their active sites. To date, various kinds of metal-substituted polyoxometalates have been synthesized and applied in selective oxidation reactions. 1,3-Diyne derivatives are very important materials in biological, polymer, and materials science because they can be converted into various structural entities, especially substituted heterocyclic compounds. Oxidative alkyne– alkyne coupling is a good candidate for the synthesis of 1,3diyne derivatives. Copper salts (stoichiometric amounts, Glaser conditions), copper salts with appropriate nitrogen bases and molecular oxygen (catalytic, Hay conditions), and a combination of copper and palladium salts (catalytic) have commonly been used to promote oxidative alkyne–alkyne coupling. However, most copper-catalyzed systems have shortcomings, especially their low turnover numbers, the formation of significant amounts of by-products, severe catalyst deactivation, narrow applicability to a limited number of alkynes, and/or the need for additives such as bases and co-catalysts. In 1964 Bohlmann and co-workers proposed that the copper(II)-catalyzed alkyne homocoupling reaction proceeds via the formation of the alkynyldicopper(II) intermediate {Cu2(m-C CR)2}, which would react further to give the 1,3diyne products directly (see the Supporting Information). This reaction mechanism has generally been accepted, although some detailed mechanistic work is still necessary. Thus, although it is expected that the homocoupling reaction should proceed efficiently in the presence of catalysts with a dicopper(II) core on the basis of this mechanism, an alkyne homocoupling reaction catalyzed by complexes with a dicopper(II) core is as yet unknown. Herein we report that the dicopper-substituted g-Keggin silicotungstate TBA4[g-H2SiW10O36Cu2(m-1,1-N3)2] (I, Figure 1; TBA = tetra-n-butylammonium) is an effective

Ruthenium hydroxide on magnetite as a magnetically separable heterogeneous catalyst for liquid-phase oxidation and reduction

Three kinds of reactions, (i) aerobic oxidation of alcohols, (ii) aerobic oxidation of amines, and (iii) reduction of carbonyl compounds to alcohols using 2-propanol as a hydrogen donor, could efficiently be promoted by an easily prepared ruthenium hydroxide catalyst on magnetite (Ru(OH)x/Fe3O4). A wide variety of substrates including aromatic, aliphatic, and heterocyclic ones could be converted to the desired products in high to excellent yields without any additives such as bases and electron transfer mediators. After the reaction, the catalyst/product(s) separation could be easily achieved with a permanent magnet and more than 99% of Ru(OH)x/Fe3O4 catalyst could usually be recovered for each reaction. The catalysis for these reactions was intrinsically heterogeneous, and Ru(OH)x/Fe3O4 recovered after these reactions could be reused without appreciable loss of the catalytic performance.

A Bifunctional Tungstate Catalyst for Chemical Fixation of CO2 at Atmospheric Pressure

Chemical fixation of carbon dioxide (CO2) into useful and valuable chemicals is a key technology for sustainable lowcarbon society because CO2 is a renewable and environmentally friendly C1 source, which is in contrast to toxic CO and phosgene. However, CO2 is much less reactive than CO and phosgene, and a large energy input (e.g., highly reactive reagents, high pressures of CO2, and stoichiometric amounts of strong acids or bases) is usually required to transform CO2 into various chemicals. Therefore, the low-energy catalytic fixation of CO2 is highly desirable. Catalytic CO2 fixation at atmospheric pressure has been limited to reactive substrates (strained cyclic molecules, unsaturated compounds, etc.), and bifunctional catalysts, which allow a concerted action on both CO2 and substrates, are promising candidates for highly efficient CO2 fixation at atmospheric pressure. [1d] Catalytic C N and C O bond-formation processes with CO2 are important in both industry and academia because they offer economical and environmental advantages such as high atom efficiency and water is the only by-product. Urea derivatives, made from CO2, are important end products or intermediates for pharmaceuticals, agricultural pesticides, antioxidants in gasoline, dyes, and resin precursors. Although various base catalysts such as CsOH, Cs2CO3, and ionic liquids have been used for the synthesis of urea derivatives with CO2, these systems have disadvantages: 1) high CO2 pressures (2.5–8.0 MPa) and reaction temperatures (423–453 K), 2) narrow applicability to substrates, and 3) need of dehydrating agents or additives (see Table S1 in the Supporting Information). Recently, we have developed a series of polyoxometalates (POMs) as catalysts for various functional-group transformations. While acid and oxidation catalysis by POMs have extensively been investigated, there are no successful examples of base catalysis including chemical fixation of CO2. [4,5] The charges and sizes of POMs strongly affect the basicities of the oxygen atoms, and increase with an increase in the charge densities (i.e., the negative charge per size). On the basis of this concept, we focus on the basic property of a monomeric tungstate, [WO4] 2 , having a high charge density. First, the structures of various tungstates were optimized by the density functional theory (DFT) calculations, and the basicities of oxygen atoms in various POMs were compared with the natural bond orbital (NBO) charges (Figure 1a). The NBO charge of an oxygen atom in [WO4] 2 was 0.934, which is

1,3-Dipolar Cycloaddition of Organic Azides to Alkynes by a Dicopper-Substituted Silicotungstate

The dicopper-substituted gamma-Keggin silicotungstate TBA 4[gamma-H2SiW10O36Cu2(mu-1,1-N3)2] (I, TBA = tetra- n-butylammonium) could act as an efficient precatalyst for the regioselective 1,3-dipolar cycloaddition of organic azides to alkynes. Various combinations of substrates (four azides and eight alkynes) were efficiently converted to the corresponding 1,2,3-triazole derivatives in excellent yields without any additives. The present system was applicable to a larger-scale cycloaddition of benzyl azide to phenylacetylene under solvent-free conditions (100 mmol scale) in which 21.5 g of the analytically pure corresponding triazole could be isolated. In this case, the turnover frequency and the turnover number reached up to 14,800 h(-1) and 91,500, respectively, and these values were the highest among those reported for the copper-mediated systems so far. In addition, I could be applied to the one-pot synthesis of 1-benzyl-4-phenyl-1H-1,2,3-triazole from benzyl chloride, sodium azide, and phenylacetylene. The catalyst effect, kinetic, mechanistic, and computational studies show that the reduced dicopper core plays an important role in the present 1,3-dipolar cycloaddition.

Heterogeneously Catalyzed Synthesis of Primary Amides Directly from Primary Alcohols and Aqueous Ammonia

In the presence of a manganese oxide based octahedral molecular sieve (OMS-2), a range of primary amides could be synthesized directly from primary alcohols and ammonia. The observed catalysis was heterogeneous, and the recovered catalyst could be reused many times without an appreciable loss of its catalytic performance.