Low-temperature oxidation—reduction

Biological fuel cells have a long history in the literature,but in recent years, they have come to prominence as more conventional fuel cell technologies have approached mass-market acceptance. Driving the recent ascendance of biofuel cells are the aspects of biocatalysis that are unmatched by conventional low-temperature oxidation—reduction catalysts, namely, activity at near-room temperatures and neutral pH and, more importantly, selective catalytic activity. 

For purposes of organization, a distinction is made between oxidation-reduction processes and nonoxidative-reduction processes. The former is illustrated by summarizing briefly the work of Asinger and his associates who over the course of twenty-five years investigated the low-temperature oxidative-reductive reactions of ketones and elemental sulfur in the presence of ammonia or amines. This work deviates from hydrous conditions in nearly all cases. Nevertheless it is apropos both in relation to the temperatures employed and the variety of heterocycles produced. It points the way to reinvestigation under hydrous conditions. 

Alkanes contain only C-C and C-H bonds, both of which are relatively strong. For that reason, they have little tendency to undergo many kinds of reactions common to some other organic chemicals, such as acid-base reactions or low-temperature oxidation-reduction reactions. However, at elevated temperatures alkanes readily undergo oxidation — more specifically combustion — with molecular oxygen in air, as shown by the following reaction of propane ... 

With the fully functionalized heterocyclic core completed, synthetic attention next focused on introduction of the 3,5-dihydroxyheptanoic acid side-chain. This required initial conversion of the ethyl ester of 35 to the corresponding aldehyde through a two-step reduction/oxidation sequence. In that event, a low-temperature DIBAL reduction of 35 provided primary alcohol 36, which was then oxidized to aldehyde 37 with TRAP. Subsequent installation of the carbon backbone of the side-chain was accomplished using a Wittig olefination reaction with stabilized phosphonium ylide 38 resulting in exclusive formation of the desired -olefin 39. The synthesis of phosphonium ylide 38 will be examined in Scheme 12.5 (Konoike and Araki, 1994). 

Supported Au catalysts have been extensively studied because of their unique activities for the low temperature oxidation of CO and epoxidation of propylene (1-5). The activity and selectivity of Au catalysts have been found to be very sensitive to the methods of catalyst preparation (i.e., choice of precursors and support materials, impregnation versus precipitation, calcination temperature, and reduction conditions) as well as reaction conditions (temperature, reactant concentration, pressure). (6-8) High CO oxidation activity was observed on Au crystallites with 2-4 nm in diameter supported on oxides prepared from precipitation-deposition. (9) A number of studies have revealed that Au° and Au" play an important role in the low temperature CO oxidation. (3,10) While Au° is essential for the catalyst activity, the Au° alone is not active for the reaction. The mechanism of CO oxidation on supported Au continues to be a subject of extensive interest to the catalysis community. 

Usually oscillations of this type are described by models of the general form depicted in Fig. 6h. At a high temperature and a high reaction rate, the catalyst begins to oxidize. This causes the temperature and rate to fall, and the system eventually reaches the low-temperature oxidized state. The catalyst then goes through a reduction process that raises the temperature and the reaction rate. The process then repeats to produce oscillations. The reaction systems for which these models have been developed are CO oxidation... 

The PPR and LFR are also applied in a more recently developed dedicated process for NOx removal from off-gases. The Shell low-temperature NO reduction process is based on the reaction of nitrogen oxides with ammonia (reactions iv and v), catalyzed by a highly active and selective catalyst, consisting of vanadium and titania on a silica carrier [18]. The high activity of this catalyst allows the reaction of NO with ammonia (known as selective catalytic reduction) to be carried out not only at the usual temperatures around 300°C, but at substantially lower temperatures down to 130°C. The catalyst is commercially manufactured and applied in the form of spheres (S-995) or as granules (S-095) [19]. 

The performance of the PPR for NOx removal by the Shell low-temperature NOx reduction has been investigated extensively [20]. In the first commercial application of the Shell process with parallel-passage reactors, flue gases of six ethylene cracker furnaces at Rheinische Olefin Werke at Wesseling, Germany, are treated in a PPR system with 120-m catalyst in total to reduce the nitrogen oxide emissions to about 40 ppm v. Since its successful start-up in April 1990, the unit has performed according to expectations... 

In our laboratory it has already been established that Pt/MnOx and Pt/CoOx have promising catalytic properties for CO oxidation and NO reduction. In this paper we will compare the activity of Pt/Mn0,/Si02 and Pt/CoOx/Si02 catalysts in CO oxidation by O2 and NO reduction by CO. Special attention will be given to the low temperature oxidation of CO. 

The process of SO2 removal on activated coke followed by a simultaneous reduction of NO with ammonia has been successfully applied in industry [176]. Mitsui Mining Process [177] (Table 12) and the Sumitomo Heavy Industry Process [178] are examples of the simultaneous desulphurization and NO removal with the application of moving beds of carbon adsorbents. Apart from the two above mentioned target gases, these processes also exhibit high removal efficiency for heavy metals and dioxins. Removal of NO on activated carbons can also be carried out using two separate processes. As NO2 can be easily removed from gas streams by water, low-temperature oxidation of NO to NO2 on the porous carbon surface is considered as feasible for the removal of NO without the ammonia addition [179]. 

Reactions that are unsuited to microwave heating are transformations that are usually performed at moderate or low temperatures, oxidations, and reductions, as well as functional group additions and interconversions. However, it is freely acknowledged that examples of all these types of reactions have been reported using microwave heating. 

Many other catalytic fluidized-bed processes have been tested at various scales. These include catalytic low-temperature oxidation, catalytic gasification and pyrolysis of biomass and waste plastic, production of carbon nanotubes, dry reforming of methane, hydrogenation and dehydrogenation of hydrocarbons, methanol-to-gasoline (MTC) process, synthesis of dimethyl ether (DME), and selective catalytic reduction of nitrogen... 

Machida, M., Kurogi, D., and Kijima, T. (2000) Mn0 -Ce02 binary oxides for catalytic NO -sorption at low temperatures. Selective reduction of sorbed NO. Chem. Mater., 12,... 

The effect of the preparation method was studied by Prati and co-workers and it was fovmd that a low temperature chemical reduction of the gold-supported catalyst enhanced the activity due to the formation of gold particles in the range of 2-5 nm, in a narrower particle size distribution. The use of a higher pre-treatment temperature (400°C) resulted in lower activity due to the increase of gold particle size and also higher selectivity to glycerate due to the suppression of the over-oxidation. ... 

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