Benzene partial oxidation

As for the reaction of benzene partial oxidation, no one siqtported VPO system was tested before. Nevertheless, such catalysts could be quite suitable to estimate active sites nature and role of the catalyst composition, to study surface chemistry and hydrocarbons conversion mechanism. 

ZSM5 type zeolites were used as catalysts for the one-step synthesis of phenol hy benzene partial oxidation with nitrous oxide. Isomorphous substitution of Al ions by other trivalent metal ions revealed a high catalytic performance of the H-Ga-ZSM5 in a wide temperature range. Systematic variation of the partial pressures of the reactants led to satisfactory preliminary kinetic models. Deactivation could be reduced by postsynthetic catalyst silylation which is believed to block the strongest acid sites responsible for coke formation. 

Similar approaches are applicable in the chemical industry. For example, maleic anhydride is manufactured by partial oxidation of benzene in a fixed catalyst bed tubular reactor. There is a potential for extremely high temperatures due to thermal runaway if feed ratios are not maintained within safe limits. Catalyst geometry, heat capacity, and partial catalyst deactivation have been used to create a self-regulatory mechanism to prevent excessive temperature (Raghaven, 1992). 

Vaidyanathan and Doraiswamy [Chem. Eng. Sci., 23 (537), 1966] have studied the catalytic partial oxidation of benzene in a composition range where the reactions of interest all follow pseudo first-order kinetics. The pertinent stoichiometric equations are... 

Vaidyanathan and Doraiswamy (1968) studied the kinetics of the gas-phase partial oxidation of benzene (CgHg, B) to maleic anhydride (C4H2O3, M) with m in an integral PFR containing... 

Two other, more recently popular routes are shown in Figure 7—6. In the first, benzene is hydrogenated to cyclohexane, followed by a partial oxidation to cyclohexanol. The cyclohexanol is then dehydrogenated to phenol. 

Tetracyano ligands have been used to bridge between four Ru(NH3)5 moieties. The complexes [ Ru(NH3)5 4(/i-L)] + (L = tetracyanoethene, tetracyano-p-quinodimethane, 1,2,4,5-tetracyano-benzene, 2,3,5,6-tetracyanopyrazine) exhibit intense, long-wavelength electronic absorptions. Oxidation to [ Ru(NH3)5 4(yU-L)] °" " and reduction to [ Ru(NH3)5 4(//-L)] + and [ Ru(NH3)5 4-(/i-L)] + can readily be achieved. The species are fully delocalized with partially reduced ligands or partially oxidized Ru centers. Treatment of [5,10,15,20-tetrakis(4-cyanophenyl)porphyrinato] cobalt(II) or [5,10,15,20-tetrakis(4-cyano-2,6,-dimethylphenyl)porphyrinato]cobalt(II) with [Ru-(NH3)5(0S02CF3)] introduces cyano-bound pendant Ru (NH3)5 groups to the porphyrinato complexes. ... 

Hydrochloric Acid for removing basic tars Na Hydroxide for acidic tars Benzene for hydrocarbon residues Acetone for partially oxidized tars... 

Fig. 7.190. Diagram of the reactor for partial oxidation of benzene to phenol during 02-H2 fuel cell reactions. (Reprinted from K. Otsuka, M. Kunieda, and H. Yamagata, J. Electrochem. Soc. 139 2382,1992, Fig. 1. Reproduced by permission of the Electrochemical Society, Inc.)...

In another study (Watwood, White Dahm, 1991), benzene mineralization occurred in soils incubated under an inert gas for 4 weeks. No attempt was made to remove residual oxygen from these soils and the possibility exists that benzene mineralization may have been linked to the consumption of oxygen. Alternately, it may have been that benzene was partially oxidized by microorganisms and the resulting product was amenable to anaerobic decay. An earlier study (Van Beelen Van Keulen, 1990) showed an extremely rapid rate of benzene mineralization 2% mineralized in 1 h and 5% in 7 days. No samples were taken between 1 and 7 days and further benzene mineralization was not observed. 

Many substances can be partially oxidized by oxygen if selective catalysts are used. In such a way, oxygen can be introduced in hydrocarbons such as olefins and aromatics to synthesize aldehydes (e.g. acrolein and benzaldehyde) and acids (e.g. acrylic acid, phthalic acid anhydride). A selective oxidation can also result in a dehydrogenation (butene - butadiene) or a dealkylation (toluene -> benzene). Other molecules can also be selectively attacked by oxygen. Methanol is oxidized to formaldehyde and ammonia to nitrogen oxides. Olefins and aromatics can be oxidized with oxygen together with ammonia to nitriles (ammoxidation). 

The oxidation of aromatic hydrocarbons originating from coal is one of the first organic gas phase oxidation processes carried out on an industrial scale. The development of these processes was initiated by the discovery that the V2Os catalyst used for the oxidation of sulphur dioxide was also applicable to the partial oxidation of benzene to maleic anhydride and naphthalene to phthalic anhydride. Remarkably, V2Os-based catalysts are still used in these processes today as they appear superior to any other type of catalyst. 

The gas phase oxidation of benzene with air is an important process for the production of maleic anhydride, which is the major partial oxidation product, besides products of complete oxidation. By-products are benzo-quinone, in particular at low conversion, and fumaric acid which is formed at the high conversion levels used in industrial installations [28]. Only traces of phenol and other by-products are formed. The important catalysts are based on V2Os and a maximum yield of 60—80% maleic anhydride is obtained at 350—500° C. 

The gas phase oxidation of naphthalene to phthalic anhydride over V2Os-based catalysts is one of the oldest successful partial oxidation processes and is still of industrial importance today. Common commercial catalysts are modified silica-supported V—K—S—O catalysts and catalysts similar to those used for benzene or o-xylene oxidation. Maximum phthalic anhydride yields of 80—85 mol. % (92—98 wt. %) at 350—400°C are reported. By-products are naphthoquinone (2—5%), maleic anhydride (2— 5%) and carbon oxides. 

Aromatic hydrocarbons which do not have side chains in general form p-quinones and acid anhydrides. Benzene, naphthalene and anthracene have been dealt with above. In the case of phenanthrene, no p-quinone is formed as the adjacent C—H groups of the central nucleus are the most reactive. Phthalic anhydride is the main partial oxidation product, in addition to minor products such as 9,10-phenanthraquinone. Andreikov and... 

Louis, B., Kiwi-Minsker, L., Renken, A., Synthesis of ZSM-5 coatings on stainless steel grids and their catalytic performance for partial oxidation of benzene by N20, Appl. Catal. A 2001, 210, 103-109. 

Palmisano et al. [41] in a study on the selectivity of hydroxyl radical in the partial oxidation of different benzene derivatives have investigated how the substituent group affect the distribution of the hydroxylated compounds. The reported results show that the primary photocatalytic oxidation of compounds containing an electron donor group (phenol, phenylamine, etc.) leads to a selective substitution in ortho and para positions of aromatic molecules while in the presence of an electron-withdrawing group (nitrobenzene, benzoic acid, cyanobenzene, etc.) the attack of the OH radicals is nonselective, and a mixture of all the three possible isomers is obtained. 

Maleic acid and anhydride are recovered as by-products of the oxidation of xylenes and naphthalenes to form phthalic acids, and are also made specifically by the partial oxidation of benzene over a vanadium pentoxide (V205) catalyst. This is a highly exothermic reaction, and several modifications of the basic process exist, including one using butylenes as the starting materials. 

The conversion of all compounds except pentane and benzene was close to 99% the conversion of pentane was about 98% and of benzene 90-95% during the entire time-on-stream (Figure 3). No partial oxidation products were observed COa was the sole total oxidation product and the carbon balance was within 5%. 

The kinetic experiments, activity tests, and poisoning experiments were carried out in a gas-flow isothermal fixed bed reactor [6) at the benzene partial pressure of 7.55 kPa hydro gen partial pressure 99.82 kPa thiophene partial pressure 0.032 kPa and the reaction temperatures 403, 427 and 448 K. The size of the commercial cylindrical catalyst pellet was 5x5mm (21% Ni on alumina, supplied by BASF). The nickel oxide containing precursor was activated by reduction with hydrogen at 743 K for 10 hr. 

A higher atom and mass efficiency is desirable when comparing alternative reaction choices. A clear example of this is the production of maleic anhydride (MA) starting from either benzene or n-butane.72 Benzene or w-butane is partially oxidized in the vapor phase in the presence of air and a solid catalyst at high temperature and pressure. 

In the past the most important process was based on the partial oxidation of benzene. Typical production units are 10,000-20,000 tons/y, which is smaller than those using butane. Benzene gives rise to smaller amounts of water and more maleic anhydride can be produced in the first condensation step. 

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