N-Octane oxidation

The activity data confirm that an IR absorption band at 960 cm" is a necessary condition for titanium silicates to be active for the selective oxidation of hydrocarbons with aqueous H2O2 as suggested by Huybrechts et al. (9). However, this band is not a sufficient condition for predicting the activity of the TS-1 catalyst. Although TS-l(B) and TS-l(C) show intensities for the 960 cm- band similar to TS-1 (A), their activities are different First of all, the reaction data reveal that TS-1 (A) is much more active than TS-l(B) for phenol hydroxylation, while both samples show similar activity for n-octane oxidation and 1-hexene epoxidation. Therefore, the presence of the IR band at 960 cm-i in TS-1 catalysts may correlate with the activities for the oxidation of n-octane and the epoxidation of 1-hexene but not for phenol hydroxylation. However, note that the amorphous Ti02-Si02 also has an IR absorption band at 960 cm- and it does not activate either substrate. 

Effect of sodium and aluminum on TS-1. The catalytic activities of aluminum and/or sodium containing TS-1 are depicted in Table IV. The data show that the addition of aluminum during the synthesis of TS-1 yields a material (TAS-1(D)) that has a lower conversion of n-octane oxidation and a smaller IR peak ratio. The existence of the acid sites due to the incorporation of aluminum into the framework of TS-1 may accelerate the decomposition of H2O2 to water and oxygen during the reaction. However, reducing the number of acid sites by exchanging with sodium ions only increases the conversion by 1% (Na/TAS-1(D)). Therefore, the addition of aluminum into the synthesis mixture most likely reduce the amount of titanium present in the sample. 

Table IV. n-Octane oxidation on aluminum and/or sodium containing TS-1...

The addition of sodium during the synthesis of TS-1 completely eliminates the activity for n-octane oxidation and also the IR band at 960 cm- (this IR band is present in the amorphous precursor, Ti02-Si02). It has been shown (12) that the presence of sodium in itie synthesis gel prevents the incorporation of titanium into the zeolite framework. However, the addition of sodium ter the zeolite crystallizes does not... 

The addition of aluminum during the synthesis of TS-1 reduces its activity for n-octane oxidation. The presence of sodium in the synthesis gel of TS-1 completely eliminates the catalytic activity for alkane oxidation. However, the presence of sodium in preformed TS-1 does not have a significant effect on its catalytic activity. 

Figure 4. Selectivity of alkane oxidation with Fe/Pd/A zeolite and H2/O2. a) substrate selectivity between n-octane and cyclohexane and b) regioselectivity of n-octane oxidation.

The oxidation of various hydrocarbons such as n-octane, cyclohexane, toluene, xylenes and trimethyl benzenes over two vanadium silicate molecular sieves, one a medium pore VS-2 and the other, a novei, iarge pore V-NCL-1, in presence of aqueous HjOj has been studied. These reactions were carried out in batch reactors at 358-373 K using acetonitrile as the solvent. The activation of the primary carbon atoms in addition to the preferred secondary ones in n-octane oxidation and oxidation of the methyl substituents in addition to aromatic hydroxyiation of alkyl aromatics distinguish vanadium silicates from titanium silicates. The vanadium silicates are also very active in the secondary oxidation of alcohols to the respective carbonyl compounds. V-NCL-1 is active in the oxidation of bulkier hydrocarbons wherein the medium pore VS-2 shows negligible activity. Thus, vanadium silicate molecular sieves offer the advantage of catalysing selective oxidation reactions in a shape selective manner. 

The alkylborane is then oxidized by the addition of 150 ml of a 15% solution of hydrogen peroxide, while the pH of the reaction mixture is maintained at 7-8 by the simultaneous addition of 3 Asodium hydroxide, the process being carried out at ice-bath temperature. The reaction mixture is neutralized and subjected to steam distillation. The distillate is extracted with ether, and the extract is dried over anhydrous magnesium sulfate. After removal of the ether, distillation yields 18.0 g (70%) of n-octanal, bp 83-85733 mm. 

Smith CA, MR Flyman (2004) Oxidation of methyl tcrt-butyl ether by alkane hydroxylase in dicyclopro-pyUcetone-induced and n-octane-grown Pseudomonas putida Gpol. Appl Environ Microbiol 70 4544-4550. 

A similar development in this direction is the synthesis of a mixed-phase material containing both micro- and mesopores (Ti-MMM-1) (223). This material was synthesized by the addition of organic templates for mesopores (cetyltrimethylammonium bromide, CTABr) and micropores (tetrapropylammo-nium bromide, TPABr) at staggered times and the variation of the temperature of a single reaction mixture. Ti-MMM-1 is more selective (for oxidation of cyclohexane and of n-octane) than either Ti-MCM-41 or TS-1. The powder X-ray diffraction pattern indicates that the material contains both MCM-41 and MFI structures. The mixed phase contains framework Ti species and more atomic order within its walls than Ti-doped MCM-41. 

In contrast to this mechanism, the one proposed in our work operates direct from the oxidation state of the alkane feedstock. The same alkyl cation intermediate can lead to both alkane isomerization (an alkyl cation is widely accepted as the reactive intermediate in these reactions) and we have shown in this paper that a mechanistically viable dehydrocyclization route is feasible starting with the identical cation. Furthermore, the relative calculated barrier for each of the above processes is in accord with the experimental finding of Davis, i.e. that isomerization of a pure alkane feedstock, n-octane, with a dual function catalyst (carbocation intermediate) leads to an equilibration with isooctanes at a faster rate than the dehydrocyclization reaction of these octane isomers (8). 

Figure 15.13 Biocatalysis for the oxidation of n-octane to 1-octanol at 10,000 tons/year.

PhI02 is rather bulky and plugs the pores, thus preventing further access of reactants to the active sites [49-50,63-64]. Therefore turn-overs are quite low when PhIO is used as oxidant. For the oxidation of methyl cyclohexane on TMPcY [49-50,63-64] and of cyclohexane on Fet.BuPcY [67] turn-overs are 5.6 and 7.6 respectively. It should be noted that the reported turn-overs for oxidations with PhIO correspond to conversions of less than 1 substrate molecule per two supercages, or to total conversions of less than 0.1 %. Therefore the observed activities and selectivities may be influenced by sorption effects. Furthermore iodosobenzene is a rather expensive oxidant and not practical to use because of its low solubility in solvents. Therefore some researchers tend to use other oxidantia such as air [65,66] and tertiary butyl hydroperoxide (t-ButOOH) [57]. In the oxidation of n-octane with t-ButOOH turn-overs as high as 6000 have been reported [57]. 

The activity of phthallocyanines is also influenced by the structure of the zeolite in which they are encaged, as shown in Figure 6 for the oxidation of n-octane with t-ButOOH. FePcY is more active than FePcVPI-5 but the latter catalyst deactivates much slower [57]. 

Figure 13. Regioselectivity in the oxidation of n-octane by t-ButOOH on FePcY and FePcVPI-5 [57].

Several preparation methods have been reported for the synthesis of TS-1. In this work, we have investigated the physicochemical properties of TS-1 samples synthesized by different preparation metiiods and tested these materials as catalysts for the oxidation of n-octane, 1-hexene and phenol using aqueous hydrogen peroxide (30 wt%) as oxidant at temperatures below 100 C. For comparison, Ti02 (anatase) and the octahedral titanium-containing silicate molecular sieve (ETS-10) (5) have been studied. The effect of the presence of aluminum and/or sodium on the catalytic activity of TS-1 is also discussed. 

The oxidation of n-octane and the epoxidation of 1-hexene were performed in a 25 ml Parr reactor using 30% aqueous H2O2 as an oxidant and acetone as solvent at 100 °C and 80 °C, respectively and stirred at 500 RPM. Prior to product analysis, the product mixtures were diluted with acetone in order to obtain a single liquid-phase. The products were analyzed on a HP 5890 Series II GC equipped with a 25 m long HP-FFAP (polar) capillary column. 

G.M. Come, V. Warth, P.A. Glaude, R. Foumet, F. Battin-Leclerc, and G. Scacchi. Computer-Aided Design of Gas-Phase Oxidation Mechanisms—Application to the Modeling of n-Heptane and Iso-Octane Oxidation. Proc. Combust. Inst., 26 755-762,1996. 

Pope, Dykstra, and Edgar (34), in 1929, published three papers on the mechanism of the vapor phase oxidation of isomeric octanes. The flow method was used. n-Octane was burned in a mixture of oxygen and nitrogen approximating the composition of air. A chemiluminescence filled the tube with occasional flashes from end to end. The principal products were aldehydes, water, and carbon monoxide. Some carbon dioxide was produced also. 

The reaction mechanism was explained on the theory that the oxidative attack was through the methyl group, n-octane being converted to n-octaldehyde and this in turn to the next lower aldehyde, with the elimination of carbon monoxide. The degradative action was believed to be repeated step by step, through a nonstop run, to formaldehyde as the final product. n-Heptaldehyde and n-butvraldehyde gave essentially the same products. It was believed the luminescence was due to the chain reaction by which each aldehyde was converted to the next lower aldehyde with the elimination of carbon monoxide. 

Fig. 8. Selectivity for ketone and alcohol formation (A) and regioselectivity (B) in the oxidation of n-octane at 5% conversion over FePcY and FePcVPI-5 catalysts. Conditions are those of Fig. 7.

It was also recently shown that VO(OBu )3 catalyzes the oxidation of n-octane by TBHP to a mixture of isomeric octanones with yields close to 80%, and reactive Vv t-butyl peroxides have been detected as intermediates.233... 

The effect of zeolite porosity on the reaction rate was also well demonstrated in liquid-phase oxidation over titanium-containing molecular sieves. Indeed, the remarkable activity in many oxidations with aqueous H2O2 of titanium silicalite (TS-1) discovered by Enichem is claimed to be due to isolation of Ti(IV) active sites in the hydrophobic micropores of silicalite.[42,47,68 69] The hydrophobicity of this molecular sieve allows for the simultaneous adsorption within the micropores of both the hydrophobic substrate and the hydrophilic oxidant. The positive role of hydrophobicity in these oxidations, first demonstrated with titanium microporous glasses,[70] has been confirmed later with a series of titanium silicalites differing by their titanium content or their synthesis procedure.[71] The hydrophobicity index determined by the competitive adsorption of water and n-octane was shown to decrease linearly with the titanium content of the molecular sieve, hence with the content in polar Si-O-Ti bridges in the framework for Si/Al > 40.[71] This index can be correlated with the activity of the TS-1 samples in phenol hydroxylation with aqueous H2C>2.[71] The specific activity of Ti sites of Ti/Al-MOR[72] and BEA[73] molecular sieves in arene hydroxylation and olefin epoxidation, respectively, was also found to increase significantly with the Si/Al ratio and hence with the hydrophobicity of the framework. 

Concurrently with the discovery and development in this country of the catalytic conversion of paraffins to aromatics (131) three different groups in the U.S.S.R. discovered this reaction independently of each other. Moldavskil and co-workers (238,239) showed that paraffins with six or more carbon atoms form aromatics by closure of a six-membered ring. For example, n-octane gives xylene and some ethylbenzene over amorphous chromia at about 470°C. Olefins also undergo this reaction. In subsequent publications, the group headed by Moldavskil demonstrated that molybdenum sulfide, titanium dioxide, and other oxides as well as activated carbon also may be used for dehydrocyclization (237,239). 

---