Selectivity in oxidation reactions

In Chapter 9 you met some chromium-based oxidants that convert primary or secondary alcohols to aldehydes or ketones. We mentioned in that chapter the possibility, when an aldehyde is the product of the oxidation, that a further oxidation will take place to make a carboxylic acid. The problem does not arise of course when a secondary alcohol is oxidized to a ketone. 

Since then you have met some other oxidizing agents too, particularly in Chapter 19  

Unlike Cr(VI), none of these reagents will oxidize a hydroxyl group they are chemoselective for C=C double bonds, but do not react with hydroxyl groups. By contrast, Cr(VI) oxidizes alcohols but not alkenes. 

Chemoselective for C=C double bonds Chemoselective for alcohols or carbonyl compounds 

In this section we will be concerned only with oxidizing agents that oxidize alcohols and carbonyl compounds, and in particular we shall be concerned with ways of choosing whether to arrest the oxidation of primary alcohols at the aldehyde stage or let it continue to the carboxylic acid. 

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Highly ordered mesoporous mono- and bi-metallic incorporated catalysts with high porosity and surfece area are very active and selective in oxidation reaction of aromatic hydrocarbons such as styrene and benzene. Their framework composition can be modified by incorporation of various transition metals in the single or associated forms. The ordered hexagonal arrangement, the morphology, the surface area, and the catalytic activity and selectivity can be modified by method of the synthesis, nature and content of the metal and the molar ratio of the metals. Very active catalysts were obtained by incorporation of chromium and nickel in the hexagonal ordered structure. 

In addition to the effect on the product selectivity, it is seen in Table 3 that, for a given synthetic method, the activity (1-hexene and H202 conversion) as well as the selectivity of H202 increases as the A1 content of the zeolite decreases. These results show that the A1 content of zeolite Ti-Beta is one of the most important factors in determining its activity and selectivity in oxidation reactions, and the benefits that the new methods of synthesizing Ti-Beta with low A1 content can provide. 

Titanium silicate (TS-1) which has a structure similar to the zeolite ZSM-5 has been shown to catalyse a number of synthetically important oxidations with hydrogen peroxide under mild conditions.34 A useful feature of the TS-1 catalyst is its enhanced product selectivity in oxidation reactions, for example, cyclohexane is selectively oxidised to cyclohexanone inside the pores of TS-1. On the external surfaces where there is little steric control cyclohexane is oxidised to the dicarboxylic acid. Spinace and co-workers have shown that these external reactions can be prevented by the addition of an antioxidant such as 2,6-dwert-butyl-4-methylphenol (BHT) but which does not interfere with the internal reactions since it is too bulky to enter the pores of the TS-1.35... 

In this section, the catalytic chemistry of selected framework metal-containing zeotype materials is reviewed, with an emphasis on commercial applications. The catalytic activities of framework metal-containing zeotype materials, especially those containing titanium, vanadium, or tin, have been investigated extensively. The enormous interest in these materials is attributed to their remarkable catalytic activities and especially their selectivities in oxidation reactions. Because hydrogen peroxide is generally used as the oxidant, water is formed as a by-product. Hence, oxidation reactions carried out with these catalysts can be considered environmentally clean processes. Several review articles have been published that summarize the catalytic reactions (2a,3b-d,89). In this section, the focus is on selected industrially relevant reactions. 

The invention of TS-1 created significant scientific interest. The material s unique properties and the resulting catalytic performance are beheved to be associated with the isolated titanium sites, which are highly active and selective in oxidation reactions with hydrogen peroxide as the oxidant. The scientists from the ENI group (3e,86,89) have since published more details about the material s synthesis and its physical and catalytic properties. However, as more than 600 papers were obtained when using TS-1 as a search keyword, we restrict our summary to only a small fraction of them, which we regard as essential papers from the early years (i.e., 1983-1995) and others that appeared in the subsequent 10-15 years. 

The selection of an efiicient catalyst and the choice of the reaction conditions are the key steps to realize an ideal oxidation procedure. Many of the heterogeneous catalysts used in liquid-phase oxidation are mixed oxides with one or more transition metak. For these catalysts, the active site is directly involved in successive redox cycles, which underlines the fundamental role of the electronic fiic-tor [30]. Transition metal oxides also exhibit surface acid-base properties. Many authors have attempted to relate these properties with the activity or selectivity in oxidation reactions [31] and photo-oxidative degradation of organic compounds. 

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]. 

In a manner analogous to classic nitrile iinines, the additions of trifluoro-methylacetonitrile phenylimine occur regiospecifically with activated terminal alkenes but less selectively with alkynes [39], The nitnle imine reacts with both dimethyl fumarate and dimethyl maleate m moderate yields to give exclusively the trans product, presumably via epimenzation of the labile H at position 4 [40] (equation 42) The nitrile imine exhibits exo selectivities in its reactions with norbornene and norbornadiene, which are similar to those seen for the nitrile oxide [37], and even greater reactivity with enolates than that of the nitnle oxide [38, 41], Reactions of trifluoroacetomtrile phenyl imine with isocyanates, isothiocyanates, and carbodiimides are also reported [42]... 

Because there exist a number of reviews which deals with the structural and mechanistic aspects of high-valent iron-oxo and peroxo complexes [6,7], we focus in this report on the application and catalysis of iron complexes in selected important oxidation reactions. When appropriate we will discuss the involvement and characterization of Fe-oxo intermediates in these reactions. 

The greater activity of Ti-beta (vs. TS-1) in the oxidation of the bulky cyclohexane was noted in the previous section. Table IX provides a comparison of the conversion and epoxide selectivity in the reaction catalyzed by TS-1 and three large-pore/mesoporous Ti-silicates in the epoxidation of a single, linear allyl alcohol (pentenol). 

The cylindrically chiral diphosphine neither changed, nor lost its reactivity and selectivity in hydrogenation reactions even after long exposure to air. In a 31P-NMR study, no detectable air-oxidation was observed even after a long exposure (3 weeks) to the atmosphere. The procedures for the synthesis of the chiral ligand and the asymmetric reaction described above are very simple, giving high enan-tioselectivity with many dehydroamino acids (Table 12.4). 

Adsorption of Ag on the surface of PdO is also an interesting option offered by colloidal oxide synthesis. Silver is a well-known promoter for the improvement of catalytic properties, primarily selectivity, in various reactions such as hydrogenation of polyunsaturated compounds." The more stable oxidation state of silver is -F1 Aquo soluble precursors are silver nitrate (halide precursors are aU insoluble), and some organics such as acetate or oxalate with limited solubility may also be used." Ag" " is a d ° ion and can easily form linear AgL2 type complexes according to crystal field theory. Nevertheless, even for a concentrated solution of AgNOs, Ag+ does not form aquo complexes." Although a solvation sphere surrounds the cation, no metal-water chemical bonds have been observed. 

Scheme 5. Mechanism for the NPyc membrane catalyzed selective alcohol oxidation reaction in the presence of NaOCl co-oxidant (primary alcohol is given here as an example).

Experiments conducted with mass-selected ions do not bear any direct relevance for applied catalysis, simply because the number densities of the ionic species are rather low (typically about 10 particles per cm ). Nevertheless, the advantages associated with the handling and the detection of ionic species render gas-phase studies as an ideal tool for the investigation of the elementary steps in oxidation reactions. In the same vein, this holds true for the investigation of the separate mechanistic steps, and in appropriate mass spectrometers that are able to store ions for extended timescales this can also be extended to real catalytic cycles [66]. The time-honored prototype of such a catalysis was reported by Kappes and Staley who demonstrated that bare Fe" ions initiate a catalytic conversion of CO into CO2 in the presence of N2O [67]. In the following decades, a number of other catalytic cycles involv-... 

Oxidative reactions of organic compounds with molecular oxygen take place with high efficiency and selectivity in the presence of Werner-type metal complexes used as catalysts. The catalytic effects of metal complexes have received attention also as models for metalloenzymes, which are catalysts that possess special high efficiency and high selectivity for oxidative reactions in vivo. 

The present chapter will primarily focus on oxidation reactions over supported vanadia catalysts because of the widespread applications of these interesting catalytic materials.5 6,22 24 Although this article is limited to well-defined supported vanadia catalysts, the supported vanadia catalysts are model catalyst systems that are also representative of other supported metal oxide catalysts employed in oxidation reactions (e.g., Mo, Cr, Re, etc.).25 26 The key chemical probe reaction to be employed in this chapter will be methanol oxidation to formaldehyde, but other oxidation reactions will also be discussed (methane oxidation to formaldehyde, propane oxidation to propylene, butane oxidation to maleic anhydride, CO oxidation to C02, S02 oxidation to S03 and the selective catalytic reduction of NOx with NH3 to N2 and H20). This chapter will combine the molecular structural and reactivity information of well-defined supported vanadia catalysts in order to develop the molecular structure-reactivity relationships for these oxidation catalysts. The molecular structure-reactivity relationships represent the molecular ingredients required for the molecular engineering of supported metal oxide catalysts. 

The SNCR process is characterized by a selectivity in the reaction pathways, as the injected agent (NH3) may react with NO to form N2 (the desired reaction) or be oxidized to NO by reaction with O2 (undesired). The selectivity toward NO or N2 depends mainly on the temperature and gas composition, but also the mixing of reactants is conceivably important because changes in the local conditions may favor different reaction pathways. As a continuation of the previous exercise we will use the Zwietering approach to assess qualitatively the effect of mixing on the SNCR process. 

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