Catalysts in liquid phase oxidations

Promotion and Deactivation of Platinum Catalysts in Liquid-Phase Oxidation of Secondary Alcohols... 

It has always been considered that the condition of the reactor wall is less important for liquid-phase processes than for gas-phase reactions. Now there are numerous examples of marked wall effects which induce essentially new chemical results in liquid-phase oxidations. Hence, the parts played by reactor walls, by solid surfaces, and by other solid catalysts in liquid-phase oxidations should be considered as one of the most important remaining problems. 

The incorporation of Ti into the MOR structure was confirmed by the appearance of the specific absorption band in the IR spectra [20]. Very recently, Kubota et al. have synthesized Ti-YNU-2 (MSE) [25] by the post-synthesis modification ofYNU-2 (P) that has a large number of defect sites [87]. Ti-YNU-2 has proved to be a very active catalyst in liquid-phase oxidation using H202 as oxidant [25]. 

Many other metal ions have been reported as catalysts for oxidations of paraffins or intermediates. Some of the more frequently mentioned ones include cerium, vanadium, molybdenum, nickel, titanium, and ruthenium [21, 77, 105, 106]. These are employed singly or in various combinations, including combinations with cobalt and/or manganese. Activators such as aldehydes or ketones are frequently used. The oxo forms of vanadium and molybdenum may very well have the heterolytic oxidation capability to catalyze the conversion of alcohols or hydroperoxides to carbonyl compounds (see the discussion of chromium, above). There is reported evidence that Ce can oxidize carbonyl compounds via an enol mechanism [107] (see discussion of manganese, above). Although little is reported about the effectiveness of these other catalysts for oxidation of paraffins to acetic acid, tests conducted by Hoechst Celanese have indicated that cerium salts are usable catalysts in liquid-phase oxidation of butane [108]. 

H.F.W.J. van Breukelen, M.E. Gerritsen, V.M. Ummels, J.S. Broens J.H.C. van Hooff (1997). Stud. Surf Sci. Catal., 105, Part A-C, 1029-1035. Application of CoAlPO-5 molecular sieves as heterogeneous catalysts in liquid phase oxidation of alkenes with dioxygen. 

Many of the mixed oxides used as catalysts in liquid-phase oxidation are semiconductors. A semiconductor is commonly characterized by an energy gap between its electronically populated valence band and its largely vacant conduction band [41]. This bandgap determines the wavelength required for excitation of an electron from the valence band to the conduction band. The valence band serves as the site for oxidation, whereas the conduction band promotes reduction reactions. Oxides are all semiconductors irrespective of whether they are pure, mixed, doped, or supported. The redox reactions they catalyze are all connected with their electronic properties [30]. -Type semiconductors possess anionic vacancies that are the real oxidative sites of the oxide catalyst. 

Perovskites with the general elemental composition of ABO3, where B is Fe, also attracted the attention because of their application as catalysts in liquid-phase oxidation reactions. They revealed high catalytic activities in heterogeneous Fenton-like oxidation at neutral conditions [46,47,55,56]. BiFeOs and... 

The most efficient catalysts in liquid-phase oxidation of organic compoimds were crystalline mked oxides [1]. They are ionic mixed oxides or mixed oxides containing oxides supported on oxides. In the latter case, the catalytic activity of the oxide support is increased by adding one or more metal components or is obtained by immobilization of metal oxides on inactive oxide support. Metal ions were isomorphously substituted in framework positions of molecular sieves, for example, zeolites, silicalites, silica, aluminosilicate, aluminophosphates, silico-aluminophosphates, and so on, via hydrothermal synthesis or postsynthesis modification. Among these many mixed oxides with crystalline microporous or mesoporous structure, perovskites were also used as catalysts in liquid-phase oxidation. 

Their use has, however, an important drawback that is related to the rather low surface areas and not easy preparation methods. But the recent achievements in the synthesis of metal-modified mesoporous silicas and development of new methods for the synthesis of oxides with higher surface areas diversified the number of catalysts and applications of mixed oxides as catalysts in liquid-phase oxidations. The performances recently reported opened new perspectives for the green and sustainable oxidation using such materials and extended the interest for the application of these reactions in industrial organic synthesis and water decontamination. Furthermore, coupling the photooxidation with Fenton and ozonation processes provides extremely attractive techniques in advanced oxidation processes for eliminating organic contaminants in wastewaters. 

Oxidation. Acetaldehyde is readily oxidised with oxygen or air to acetic acid, acetic anhydride, and peracetic acid (see Acetic acid and derivatives). The principal product depends on the reaction conditions. Acetic acid [64-19-7] may be produced commercially by the Hquid-phase oxidation of acetaldehyde at 65°C using cobalt or manganese acetate dissolved in acetic acid as a catalyst (34). Liquid-phase oxidation in the presence of mixed acetates of copper and cobalt yields acetic anhydride [108-24-7] (35). Peroxyacetic acid or a perester is beheved to be the precursor in both syntheses. There are two commercial processes for the production of peracetic acid [79-21 -0]. Low temperature oxidation of acetaldehyde in the presence of metal salts, ultraviolet irradiation, or osone yields acetaldehyde monoperacetate, which can be decomposed to peracetic acid and acetaldehyde (36). Peracetic acid can also be formed directiy by Hquid-phase oxidation at 5—50°C with a cobalt salt catalyst (37) (see Peroxides and peroxy compounds). Nitric acid oxidation of acetaldehyde yields glyoxal [107-22-2] (38,39). Oxidations of /)-xylene to terephthaHc acid [100-21-0] and of ethanol to acetic acid are activated by acetaldehyde (40,41). 

CoSalen Y carries oxygen as a cargo.72 The catalytic properties of the zeolite-encapsulated metal complexes depend mainly on the complexed metal atoms, which are used usually as oxidation catalysts but other applications are also beginning to emerge. The zeolite-encapsulated catalysts can be regarded as biomimetic oxidation catalysts.73 In liquid-phase oxidation reactions catalyzed... 

Table 2 reports the catalytic activities of the catalysts prepared for 2.6-DTBP oxidation. All the titanium grafted materials were active as catalysts for liquid phase oxidation of 2.6-DTBP, and catalytic activity decreased in the order of MCM-48 (24.5% conversion) > HMS (22.8%) > KIT-1 (16.0%) > MCM-41 (14.3%) > SBA-1 (5%). Apparently. 3 dimensional channel system of MCM —48, and HMS with small particle size and textual mesoporosity proved to be useful in liquid phase reaction [1,2,3], Chemical analysis of the titanium-grafted SBA-1 by EDX showed far less titanium at the surface than the others it seems surface nature of SBA-1 synthesized in acidic medium is different from the rest. All Ti-grafted samples suffered from titanium leaching during the liquid phase oxidation HMS host resulted in over 4 % loss in metal content while the rest showed 2%. 

On the oxygen tolerance of noble metal catalysts in liquid phase alcohol oxidations... 

But the situation is dramatically changed if monooxygen donors are used instead of dioxygen. The two best known examples are titanosilicalites TS-1, which proved to be excellent catalysts for liquid-phase oxidation by H202, and FeZSM-5 zeolites, which are efficient catalysts for gas-phase oxidation by N20. Numerous works with H202 are well known and discussed in several reviews [55, 56]. We shall consider the oxidation by N20. 

At the same time, N20 proved to be a poor ligand and quite an inert oxidant. It resulted in a low reaction rate and needs a high reaction temperature. The lack of catalysts that could provide an effective activation of N20 is the main factor limiting a widespread use of nitrous oxide in liquid-phase oxidations. The development of such catalysts is an important target in this field. 

As well as in the gas phase, nitrous oxide gives promising results in liquid-phase oxidations. With many reactions using both homogeneous and heterogeneous catalysts, N20 provides better selectivity than H202 or 02. However, it proved to be quite an inert molecule that allows only rather small reaction rates. The development of effective catalysts able to activate N20 at low temperature may provide a breakthrough in this field. 

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