Reactions nitrosyl-catalyzed

Fig. 24. Proposed mechanism for the nitrosyl-catalyzed reaction of nitric oxide with carbon monoxide.

The ability of Pd-H-ZSM-5 catalysts to form Pd(I) nitrosyl species was related to their specific behavior of selectively reducing NO to N2 (25). This statement finds support in the curve of NO conversion versus Pd content (Figure 7A). Indeed, for reaction temperatures less than 500°C, NO conversion clearly increases with Pd content, in a manner similar to the amount of Pd nitrosyl complexes versus Pd content. Above 500°C, volcano shape curves are observed and NO conversion decreases for Pd content higher than 0.5 wt.-%. This can be easily explained by the simultaneous total conversion of CH4. The absence of reductant in the feed is expected to decrease the rate of NO reduction. This implies that CH4 participates to two distinct reactions, SCR reaction and methane combustion by O2, which compete at high temperatures. This competition is confirmed by the selectivity results, which indicates that the combustion is strongly favored above 500°C. The question arises to know whether these two reactions are catalyzed by the same types of sites. 

Ketones having an a-methylene group, that is, —CH2 bound to the carbonyl group, can react with nitrous acid, nitrite esters, or nitrosyl chloride to form nitroso compounds and oximes. These reactions are catalyzed by acids or bases. 

The U.S. domestic commercial potassium nitrate of the 1990s contains 13.9% N, 44.1% I+O, 0—1.8% Cl, 0.1% acid insoluble, and 0.08% moisture. The material is manufactured by Vicksburg Chemical Co. using a process developed by Southwest Potash Division of AMAX Corp. This process uses highly concentrated nitric acid to catalyze the oxidation of by-product nitrosyl chloride and hydrogen chloride to the mote valuable chlorine (68). The much simplified overall reaction is... 

The synthetic utility of the mercuration reaction derives from subsequent transformations of the arylmercury compounds. As indicated in Section 7.3.3, these compounds are only weakly nucleophilic, but the carbon-mercury bond is reactive to various electrophiles. They are particularly useful for synthesis of nitroso compounds. The nitroso group can be introduced by reaction with nitrosyl chloride73 or nitrosonium tetrafluoroborate74 as the electrophile. Arylmercury compounds are also useful in certain palladium-catalyzed reactions, as discussed in Section 8.2. 

O-exchange studies of Ye et al. (1991) support, we believe, the catalysis by nitrite reductase of redox reversibility between nitrite and NO as depicted in the first line of Eq. (3). They observed by analyzing the 0 content of product N2O that all eight strains of denitrifying bacteria studied could catalyze the exchange of 0 between water and nitrite or NO by way of an electrophilic (nitrosyl donor) species of NO. The rates and extent of these exchange reactions depended on whether the bacterium made use of a heme- or Cu-type nitrite reductase. Contrary to the conclusions of Ye et al. (1991), we do not believe that this study otherwise informs about the pathway of denitrification or whether NO is an intermediate. 

In spite of the above speculation, the actual mechanisms used by NO-pro-ducing nitrite reductases for reduction of nitrite and its activation for nitrosyl transfer are poorly understood. The fact that both the heme and Cu types of enzyme catalyze the reaction is remarkable in view of the fact that nitrosyl compounds of Fe complexes are well known, whereas Cu-nitrosyl compounds are rare [see Garber and Hollocher (1982) and Kim and Hollocher (1984) for further discussion]. On the other hand, both Fe and Cu can coordinate O and this might be more relevant for the activation of an O atom of nitrite for N-O bond breaking. In the case of the Cu-type nitrite reductase of A. cycloclastes, the... 

Reasons for interest in the catalyzed reactions of NO, CO, and COz are many and varied. Nitric oxide, for example, is an odd electron, hetero-nuclear diatomic which is the parent member of the environmentally hazardous oxides of nitrogen. Its decomposition and reduction reactions, which occur only in the presence of catalysts, provide a stimulus to research in nitrosyl chemistry. From a different perspective, the catalyzed reactions of CO and COz have attracted attention because of the need to develop hydrocarbon sources that are alternatives to petroleum. Carbon dioxide is one of the most abundant sources of carbon available, but its utilization will require a cheap and plentiful source of hydrogen for reduction, and the development of catalysts that will permit reduction to take place under mild conditions. The use of carbon monoxide in the development of alternative hydrocarbon sources is better defined at this time, being directly linked to coal utilization. The conversion of coal to substitute natural gas (SNG), hydrocarbons, and organic chemicals is based on the hydrogen reduction of CO via methanation and the Fischer-Tropsch synthesis. Notable successes using heterogeneous catalysts have been achieved in this area, but most mechanistic proposals remain unproven, and overall efficiencies can still be improved. 

The pH dependence of the rate of formation of a nitrosyl complex shows that nitrous acid is the reactive intermediate in the reaction when the pH is in the range of 2-8. The catalysts are not deactivated during repeat cycles between their oxidized and reduced states. The catalyzed reduction appears to depend on the ability of the multiply reduced heteropolyanions to deliver electrons to the NO group bound to the iron center. 

The results are consistent with the predictions of the ligand field theory and with the observed photochemical reactions. Bond bending distortions are observed in metal-nitrosyl compounds. The photochemical reactions of CotCCO NO and the photohydrogenation catalyzed by RhCPPhjJ NO provide indirect support for the bending. 

In the solid, the square pyramidal ion has bent apical and linear basal NO groups. The linear-bent transition probably requires little energy and may well be involved in reactions of dinitrosyl intermediates in reactions such as that noted previously, leading to a hyponitrite complex, or in reactions of NO and CO, catalyzed by nitrosyl complexes, to give N20 and C02. 

Among the preparative routes to the octalin mixtures, the acid-catalyzed dehydration of 2-decalol3 and the metal-amine reduction of naphthalene4 appear most satisfactory. Apart from the purification method described in this preparation, pure A9,10-octalin has also been obtained by reaction of the octalin mixture with nitrosyl chloride. After separation of the adducts by fractional crystallization, the pure A9,10-octalin has been regenerated from its nitrosyl chloride adduct.3,5... 

An important reaction of NO gas, catalyzed by transition metal surfaces, is reduction with hydrogen to give ammonia. As a consequence, a great deal of effort has been directed into studying the reduction of coordinated nitrosyl ligands. 

This reaction is driven by the formation of the stable metal nitrosyl molecule PdNO. Pd+, as well as Cu+, Rh+, Ag+, Cd+, and Pt+ that also induce NO ionization, thus do not catalyze the reduction of nitric oxide. 

NO adds reversibly to reduced cobalamin, Cbl(II).156 It does not react directly with aquacobalamin(III), (0blni(H2O)), but it does add to Cbl,n(N02 ) and Cblm(NO).175 Acid hydrolysis of the dinitroso species releases nitrite, and binding of nitrite to Cblln(H20) generates Cbln,(. 02 ). This sequence thus affords a nitrite-catalyzed mechanism for NO substitution at Cblln(H20). The reaction of NO with Com porphyrins is quite complex.176 In the first step, NO displaces an axial water ligand to form a weakly bound mono NO complex this mono NO complex reacts with a second molecule of NO to form nitrite and a reduced Co-NO complex. This latter process is called reductive nitrosylation. Manganese(II) porphyrins bind NO very rapidly.177 Stability constants have been measured for the formation of mono and bis NO complexes of Cun(dithiocarbamate)2.157... 

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