Liquid-phase selective oxidation catalysts

The activity of elemental carbon as a metal-free catalyst is well established for a couple of reactions, however, most literature still deals with the support properties of this material. The discovery of nanostructured carbons in most cases led to an increased performance for the abovementioned reasons, thus these systems attracted remarkable research interest within the last years. The most prominent reaction is the oxidative dehydrogenation (ODH) of ethylbenzene and other hydrocarbons in the gas phase, which will be introduced in a separate chapter. The conversion of alcohols as well as the catalytic properties of graphene oxide for liquid phase selective oxidations will also be discussed in more detail. The third section reviews individually reported catalytic effects of nanocarbons in organic reactions, as well as selected inorganic reactions. 

Liquid-phase selective oxidations are normally catalysed homogeneously. A small but significant interest has recently arisen in the use of solid catalysts for liquid-phase oxidation, particularly of alkyl aromatics. Shalya et al. have compared the activity of copper, silver, and gold metals as catalysts for cumene oxidation (Table 2). Silver was found to combine good selectivity for the desired product, cumene hydroperoxide, with an activity similar to that of copper. With supported catalysts, silver is considerably more active than copper, while gold is totally inactive. 

Liquid phase selective oxidations of propylene over TS-1 have also been achieved [153, 154] but while Laufer and Hoelderich extended their TS-l/propylene work to include epoxidations of styrene and pinene over a Ti-MCM-41-based catalyst [155], in situ epoxidation reactions of more complex alkenes than propylene remain poorly documented and have yet to attain the industrially relevant success of TS-1-based catalysts in selective oxidations of even simple alkenes. 

Liquid-phase selective oxidation using hydrogen peroxide and heterogeneous catalysts is of significant interest to both the fine chemical and pharmaceutical industries. Heterogeneous catalytic oxidation is, in many situations, an advanced oxidation process (AOPs). In this case, the chemical oxidation processes occur in the presence of oxidants able to generate hydroxyl radicals ( OH) [46]. An example is the remarkable success of Fenton reagent for phenol oxidation. 

Liotta, L., Venezia, A., Deganello, G., etal (2001). Liquid Phase Selective Oxidation of Benzyl Alcohol over Pd-Ag Catalysts Supported on Pumice, Catal Today, 66, pp. 

Liquid-Phase Selective Oxidation by Multimetallic Active Sites of Polyoxometalate-Based Molecular Catalysts... 

The unconsumed reactants and products were both easily removed from the reaction mixture by extraction with K-hexane, which is not miscible with [BMIMJPFg. The brown-red ionic liquid phase containing the catalyst was reused five times with PhI(OAc)2 as the oxidant. The recovered catalyst gave catalytic activity comparable to that of the original. In epoxidation of styrene and of cyclohexene, both catalytic activity and selectivity fell slightly after five reuses. In the conversion of hept-l-ene, the reused catalyst showed the same activity and selectivity as the fresh catalyst, and the catalyst was shown to be unchanged after the reaction. 

Although V-containing zeolites have already been studied in gas-phase selective oxidations (propane oxidative dehydrogenation, for example [7]) data on the reactivity of these catalysts on the heterogeneous gas-phase selective oxidation of alkylaromatics are not available. Alky-laromatic oxidation has also been done in the liquid phase using H2O2 as oxidant [8]. The... 

Liquid phase catalytic oxidation of ethylbenzene with hydrogen peroxide over TS-1 molecular sieves is most appropriate for the production of 1-phenylethanol with high selectivity (up to 93 % of all the oxidation products in methanol) under the reaction conditions studied here. An additional increase of the 1-phenylethanol selectivity could be achieved with smaller amounts of the catalyst. The highest conversion to acetophenone is found over TS-2 zeolites but further oxidation easily takes place in this case. 

Single-step oxidation of cyclohexane to adipic acid (process 5, Figure 2.12) has been demonstrated [142]. This process involves a liquid-phase air oxidation using acetic acid as a reaction medium and cobalt acetate as an oxidation catalyst. The reaction temperatures are in the range of 70 90°C. At residence times of 6 10 h, conversions to about 80% were obtained with selectivities to adipic acid of 70-75%. Several alternate processes have been described for the oxidation of cyclohexane to form adipic acid [143 148]. 

The method of transition metal catalysts modification by additives of electron-donor mono- or multidentate ligands for increase in selectivity of liquid-phase alkylarens oxidations into corresponding hydroperoxides was proposed by us for the first time. On the basis of established (Ni) and assumed (Fe) mechanisms of formation of catalytic active particles and... 

The oxidation of cyclohexanone is a reaction which has been the subject of considerable study over the years. Continued research in this area has given rise to many recent patents and papers. The product of the oxidation reaction is rather dependent on the metal complex which is used as a catalyst. When manganese(III) complexes are used the major reaction product is adipic acid [280-288]. Selectivity to adipic acid is about 70% in most cases. When copper(II) complexes are used, 5-formylvaleric acid predominates [289, 290] whereas iron complexes catalyze the formation of e-caprolactone [291,292] in up to 56% yield. In fact, liquid phase air oxidation of 2-methyl-cyclohexanone at 100 °C in the presence of copper stearate gave e-methyl- -caprolactone [292a]. Reaction scheme (190) shows the predominant reaction pathways. 

Thus, for both theoretical and practical reasons, the molecular design of active surface catalysts holds the promise of creating a family of new oxidation catalysts which are superior both to currently used homogeneous liquid-phase catalysts and to empirically derived heterogeneous systems with selectivities that could be improved. The power of this approach will ultimately be realized in the creation of selective oxidation catalysts which perform oxidations that are beyond the scope of current catalyst systems. 

The process which was developed hy DOW involves cyclodimerization of hutadiene over a proprietary copper-loaded zeolite catalyst at moderate temperature and pressure (100°C and 250 psig). To increase the yield, the cyclodimerization step takes place in a liquid phase process over the catalyst. Selectivity for vinylcyclohexene (VCH) was over 99%. In the second step VCH is oxidized with oxygen over a proprietary oxide catalyst in presence of steam. Conversion over 90% and selectivity to styrene of 92% could he achieved. ... 

Liquid phase oxidation reaction of acetaldehyde with Mn acetate catalyst can be considered as pseudo first order irreversible reaction with respect to oxygen, and the reaction occurred in liquid film. The value of kinetic constant as follow k/ = 6.64.10 exp(-12709/RT), k2 = 244.17 exp(-1.8/RT) and Lj = 3.11.10 exp(-13639/RT) m. kmor. s. The conversion can be increased by increasing gas flow rate and temperature, however the effect of impeller rotation on the conversion is not significant. The highest conversion 32.5% was obtained at the rotation speed of 900 rpm, temperature 55 C, and gas flow rate 10" m. s. The selectivity of acetic acid was affected by impeller rotation speed, gas flow rate and temperature. The highest selectivity of acetic acid was 70.5% at 500 rpm rotation speed, temperature of 55 C... 

The chemistry of vinyl acetate synthesis from the gas-phase oxidative coupling of acetic acid with ethylene has been shown to be facilitated by many co-catalysts. Since the inception of the ethylene-based homogeneous liquid-phase process by Moiseev et al. (1960), the active c ytic species in both the liquid and gas-phase process has always been seen to be some form of palladium acetate [Nakamura et al, 1971 Augustine and Blitz, 1993]. Many co-catalysts which help to enhance the productivity or selectivity of the catalyst have appeared in the literature over the years. The most notable promoters being gold (Au) [Sennewald et al., 1971 Bissot, 1977], cadmium acetate (Cd(OAc)j) [Hoechst, 1967], and potassium acetate (KOAc) [Sennewald et al., 1971 Bissot, 1977]. 

Au/C was established to be a good candidate for selective oxidation carried out in liquid phase showing a higher resistance to poisoning with respect to classical Pd-or Pt-based catalysts [40]. The reaction pathway for glycerol oxidation (Scheme 1) is complicated as consecutive or parallel reactions could take place. Moreover, in the presence of a base interconversion between different products through keto-enolic equilibria could be possible. 

---