Site-selective oxidation

Fig. 1. The temperature programmed desorption profiles for a-Fe203 (a) Blank desorption without adsorbates (b) ris-2-butene adsorption (c) butadiene adsorption (d) cis-2-butene adsorption from a catalyst depleted of selective oxidation sites. From ref. 5, reprinted with permission, copyright 1979 by the American Chemical Society.

The separation of the two sets of desorption products may indicate that they are from different sites. That is, branching of the selective and nonselec-tive oxidation takes place on adsorption of butene. This can be confirmed if the two sets of products can be varied independently. This is shown by two experiments. The first experiment makes use of the fact that butene and butadiene adsorb on the same sites. Butadiene is first adsorbed onto the catalyst (5). The catalyst is then heated to 210°C, desorbing all of the unreacted butadiene, but leaving on the surface the precursors of the combustion products. Since desorption of the unreacted butadiene does not involve a net chemical reaction, the adsorpton sites involved are not affected. The catalyst is then cooled to 22°C, and cis-2-butene is adsorbed. If selective oxidation and combustion take place on the same site, the adsorbed butene would undergo both reactions. If they take place on separate sites, and butene adsorbs only on the selective oxidation site (because the combustion site is covered by species from butadiene adsorption), the adsorbed butene would form only butadiene. Subsequent desorption yields a profile similar to that for a single adsorption of ds-2-butene (Fig.l, curve b). More importantly, within experimental errors, the amount of butadiene evolved is the same as in a ds-2-butene adsorption experiment, and the amount of C02 evolved is the same as in a butadiene adsorption experiment. Thus, the adsorbed butene forms only butadiene. These results show that under these experimental conditions (i.e., in the absence of gas-phase oxygen), the production of butadiene and carbon dioxide takes place on separate sites. 

The second experiment is to perform adsorption-desorption of butene on a catalyst that is depleted of selective oxidation sites. When 10 pulses of cis-2-butene are passed over a catalyst at 210°C (5), which is a temperature too low for the production of C02, the catalyst is reduced. The number of selective oxidation sites is substantially reduced as is evident by the much lower conversion to butadiene in the last pulse. Then the oxide is cooled to 22°C, and cis-2-butene is adsorbed. The resulting desorption profile is shown in Fig. 1, curve d. Clearly, there is no more butadiene production, while the combustion products are produced in a somewhat larger quantity. These results again support the conclusion that the selective oxidation and the combustion sites are independent. 

To date (ca 1996) many potentially usefiil sucrose derivatives have been synthesized. However, the economics and complexities of sucrochemical syntheses and the avadabiLity of cheaper substitutes have limited their acceptance hence, only a few of them are in commercial use. A change in the price and availability of petroleum feedstocks could reverse this trend. Additional impetus may come from regioselective, site-specific modifications of sucrose to produce derivatives to facilitate and improve the economics of sucrochemical syntheses. For example, the microbe yigwbacterium tumifaciens selectively oxidizes sucrose to a three-keto derivative, a precursor of alkylated sucroses for detergent use (50). Similarly, enzymes have been used for selective synthesis of specific sucrose derivatives (21). 

Potentiometric Titrations. If one wishes to analyze electroactive analytes that are not ions or for which ion-selective electrodes are not available, two problems arise. First, the working electrodes, such as silver, platinum, mercury, etc, are not selective. Second, metallic electrodes may exhibit mixed potentials, which may arise from a variety of causes. For example, silver may exchange electrons with redox couples in solution, sense Ag" via electron exchange with the external circuit, or tarnish to produce pH-sensitive oxide sites or Ag2S sites that are sensitive to sulfide and haUde. On the other... 

Marazano and co-workers have also applied the reactions of tryptamine with various Zincke salts, including 115 (Scheme 8.4.39), in the synthesis of pyridinium salts such as 116. This type of product is useful for further conversion to dihydropyridine or 2-pyridone derivatives. For example, in a different study, Zincke-derived chiral pyridinium salts could be oxidized site-selectively with potassium ferricyanide under basic conditions as a means of chiral 2-pyridone synthesis (117 —> 118, Scheme 8.4.40). 

Selective oxidation and ammoxldatlon of propylene over bismuth molybdate catalysts occur by a redox mechanism whereby lattice oxygen (or Isoelectronlc NH) Is Inserted Into an allyllc Intermediate, formed via or-H abstraction from the olefin. The resulting anion vacancies are eventually filled by lattice oxygen which originates from gaseous oxygen dlssoclatlvely chemisorbed at surface sites which are spatially and structurally distinct from the sites of olefin oxidation. Mechanistic details about the... 

The Holy Grail of catalysis has been to identify what Taylor described as the active site that is, that ensemble of atoms which is responsible for the surface reactions involved in catalytic turnover. With the advent of atomically resolving techniques such as scanning tunnelling microscopy it is now possible to identify reaction centres on planar surfaces. This gives a greater insight also into reaction kinetics and mechanisms in catalysis. In this paper two examples of such work are described, namely CO oxidation on a Rh(llO) crystal and methanol selective oxidation to formaldehyde on Cu(llO). 

The method outUned above was initially investigated for the introduction of isolated Ti(IV) sites onto a sihca substrate for use in selective oxidation catalysis. Since the development of a silica-supported Ti(lV) epoxida-tion catalyst by Shell in the 1970s, titania-sihca materials have attracted considerable attention [135,136]. Many other titania-sihca materials have been studied in this context including, but not hmited to, TSl and TS2 (titanium-substituted molecular sieves), Ti-/i (titanium-substituted zeolite). 

Attempts to achieve selective oxidations of hydrocarbons or other compounds when the desired site of attack is remote from an activating functional group are faced with several difficulties. With powerful transition-metal oxidants, the initial oxidation products are almost always more susceptible to oxidation than the starting material. When a hydrocarbon is oxidized, it is likely to be oxidized to a carboxylic acid, with chain cleavage by successive oxidation of alcohol and carbonyl intermediates. There are a few circumstances under which oxidations of hydrocarbons can be synthetically useful processes. One group involves catalytic industrial processes. Much effort has been expended on the development of selective catalytic oxidation processes and several have economic importance. We focus on several reactions that are used on a laboratory scale. 

Selective oxidations are possible for certain bicyclic hydrocarbons.285 Here, the bridgehead position is the preferred site of initial attack because of the order of reactivity of C—H bonds, which is 3° > 2° > 1°. The tertiary alcohols that are the initial oxidation products are not easily further oxidized. The geometry of the bicyclic rings (Bredt s rule) prevents both dehydration of the tertiary bridgehead alcohols and further oxidation to ketones. Therefore, oxidation that begins at a bridgehead position... 

On the other hand, if the hole flow in DNA could be artificially controlled to deposit at the desired site in DNA, it may enable site-selective oxidation and strand scission of DNA, which is desirable from a therapeutical standpoint. Furthermore, understanding DNA-mediated hole transfer is expected to lead to an additional application in the development of biosensors and bioelectronic devices [9]. Therefore, the regulation of the transfer rate and direction of the hole generated in DNA is of interest from the perspective of using DNA as a building block for electronic devices. 

In summary, the total oxidation of propylene to C02 occurred at a higher rate than partial oxidation to propylene oxide and acetone total and partial oxidations occurred in parallel pathways. The existence of the parallel reaction pathways over Rh/Al203 suggest that the selective poisoning of total oxidation sites could be a promising approach to obtain high selectivity toward PO under high propylene conversion. 

One such reported example is the synthesis of polypropylene-6-polymethyl-methacrylate (PP-6-PMMA) copolymers utilizing metallocene catalysis and the borane chemistry. In the initial step, PP with chain-end olefinic unsaturations was prepared using metallocene catalysts such as Et(Ind)2ZrCl2/MAO. The unsaturation sites were then hydroborated by 9-borabicyclo[3.3.1]nonane (9-BBN) to produce borane-terminated PP (43) (Fig. 30), which was selectively oxidized and interconverted to a... 

First, selective oxidation of one of the metal sites in the cluster catalyst,... 

Cyclic imidate esters, 2-ethoxypyrrolin-5-one and 2-ethoxy-1II -indol-3-one, undergo 1,3-dipolar cycloaddition reactions with nitrile oxides, the reaction site being at the pyrroline C=N bond (317). Rigid and sterically congested pyrroline spiro compounds 148 demonstrate complete diastereofacial selection in site and regiospecific cycloaddition reactions with nitrile oxides to give products 149 (318). 

It is hoped that the better understanding of the active sites and reaction intermediates will lead to the design of superior solid titanium-containing selective oxidation catalysts. 

The multi-functionality of metal oxides1,13 is one of the key aspects which allow realizing selectively on metal oxide catalysts complex multi-step transformations, such as w-butane or n-pentane selective oxidation.14,15 This multi-functionality of metal oxides is also the key aspect to implement a new sustainable industrial chemical production.16 The challenge to realize complex multi-step reactions over solid catalysts and ideally achieve 100% selectivity requires an understanding of the surface micro-kinetic and the relationship with the multi-functionality of the catalytic surface.17 However, the control of the catalyst multi-functionality requires the ability also to control their nano-architecture, e.g. the spatial arrangement of the active sites around the first centre of chemisorption of the incoming molecule.1... 

In this chapter, we have discussed the application of metal oxides as catalysts. Metal oxides display a wide range of properties, from metallic to semiconductor to insulator. Because of the compositional variability and more localized electronic structures than metals, the presence of defects (such as comers, kinks, steps, and coordinatively unsaturated sites) play a very important role in oxide surface chemistry and hence in catalysis. As described, the catalytic reactions also depend on the surface crystallographic structure. The catalytic properties of the oxide surfaces can be explained in terms of Lewis acidity and basicity. The electronegative oxygen atoms accumulate electrons and act as Lewis bases while the metal cations act as Lewis acids. The important applications of metal oxides as catalysts are in processes such as selective oxidation, hydrogenation, oxidative dehydrogenation, and dehydrochlorination and destructive adsorption of chlorocarbons. 

A single P450 is capable of selectively oxidizing a substrate molecule at a number of different sites producing multiple metabolites. The number, identity, and relative importance of metabolites produced often reflect reaction at the energetically most easily oxidized substrate sites. 

Au-Pd alloys with compositions close to that of the bulk components and that particle sizes were ca. 25 to 50 nm in diameter. The catalysts that were effective for H2O2 synthesis were found to be wholly inactive for CO oxidation at ambient temperature, and catalysts that were effective for low temperature CO oxidation were inactive for H2O2 synthesis. This shows that selective oxidation reactions active may utilize very different sites than those for the oxidation of CO. 

---