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Development of sustainable selective catalytic oxidation of alkanes

Final Report Summary - OXALKANES (Development of sustainable selective catalytic oxidation of alkanes)

Saturated hydrocarbons are chemically inert and therefore their functionalisation is challenging from a basic science viewpoint. The development of selective, energy efficient direct alkane oxidation chemistry could lead to environmentally and economically superior chemical processes and allow the vast reserves of natural gas and petrol to be used more efficiently and economically as feedstock for fuels and chemical products. Like the active centres of some enzymes, transition metal complexes can potentially activate and functionalise the non-activated carbon-hydrogen (C-H) bonds of hydrocarbons with high atom economy and low energy under mild reaction conditions. The use of a microwave technology to carry out these reactions could also lead to increases in product yield and selectivity, as well as being more energy efficient compared to conventional heating methods. There is also an urgent need to increase the sustainability of homogeneous phase reactions and to develop practical, efficient and low energy immobilisation methods for homogeneous catalysts, as well as to use cheaper and more environmentally acceptable solid supports.

Two biomimetic Schiff base ligands with dinitrogen dioxide (N2O2) and tetranitrogen (N4) coordination sphere were synthesised, according to procedures described in the literature. The structure of the ligands was confirmed by elemental analysis, Fourier transform infrared spectroscopy (FTIR), ultraviolet-visual (UV-Vis), proton nuclear magnetic resonance (1H NMR) and high-resolution electrospray ionisation (ESI) mass spectrometry. Subsequently, transition-metal complexes of the first-row (VO(IV), Mn(III), Fe(III), Co(III) and Cu(II)) were synthesised with the Schiff base ligands with N2O2 coordination sphere, as well as an Fe(II) complex with the N4 Schiff base. These transition metal complexes were characterised by elemental analysis, high resolution ESI mass spectrometry, FTIR and UV-vis. Their catalytic activity, in homogeneous phase, was studied in the room-temperature oxidation of cyclohexane, cyclooctane and n-hexane using environmentally benign reagents: hydrogen peroxide (30 % wt) as the oxygen source and acetonitrile as the solvent. Nitric acid was also used as promoter of the oxidation reaction. Their catalytic activity in the oxidation of benzene in acetonitrile at 50 degrees of Celsius was also studied using hydrogen peroxide as oxidant.

Cyclohexanone and cyclohexanol were the main products of the oxidation of cyclohexane, obtained when the following complexes were used as homogeneous catalysts in only 1 % mol based on the substrate: VO(IV), Fe(III) and Cu(II) complexes with the N2O2 Schiff base, new Fe(II) complex with the Schiff base with N4 coordination sphere and VO(IV) acetylacetonate ligand with tetraoxygen (O4) coordination sphere. The Fe(III) complex with N2O2 Schiff base ligand ((Fe(salhd)Cl)) was the homogeneous catalyst with highest activity, which could be further enhanced by the addition of methyl electron donating groups to the N2O2 Schiff base aldehyde fragment. The reaction performed using (Fe(salhd)Cl) but without the addition of 10 % mol of nitric acid (relatively to cyclohexane) clearly shows that it acts as a promoter and therefore as co-catalyst in the oxidation of cyclohexane with hydrogen peroxide, albeit with lowered selectivity towards cyclohexanol. The effect of using other acid co-catalysts, such as hydrogen chloride and acetic acid, was also tested, but they were not as effective as nitric acid. Other oxygen sources were tested, such as urea-hydrogen peroxide, an anhydrous source of hydrogen peroxide, and iodosylbenzene, but no oxygenated products were obtained using the (Fe(salhd)Cl) catalyst. Also for (Fe(salhd)Cl), the increase of temperature increases the selectivity towards cyclochexanone and decreases the yield and TON, most probably on the expenses of cyclohexanol; there is also a slight increase on other oxygenated products. The microwaves assistance increase only the yield and corresponding TON of the reaction with the VO(IV) catalysts. In fact, the yield on cyclohexanol and cyclohexanone increases as well as selectivity with increasing reaction temperature. Cyclooctane and n-hexane could also be oxidised into the corresponding ketones and alcohols with higher turnover numbers than cyclohexane by the (Fe(salhd)Cl) catalyst. Phenol was the main product of the oxidation of benzene in acetonitrile at 50 degrees of Celsius using hydrogen peroxide as oxidant. Again the Fe(II/II) and VO(IV) complexes were the best homogeneous catalysts in only 1 % mol based on the substrate. The Fe(II/III) complexes are more selective towards phenol than the VO(IV) complexes. The Fe(II/III) Schiff base complexes are more active than the Fe(II/III) acetylacetonate complexes, whereas the opposite was observed for the VO(IV) complexes. The room temperature reaction gave significantly lower phenol yield.

Due to their easy anchoring when compared with the Schiff base complexes, Fe(II), Fe(III) and VO(IV) acetylacetonate were immobilised onto economical porous supports, hexagonal mesoporous silica (synthesised according to procedures available in the literature) and activated carbon, previously functionalised with amine groups. The materials were characterised by elemental analysis, inductively coupled plasma atomic emission spectroscopy (ICP-AES), FTIR, isotherms of adsorption at 77 K and TG, showing that both porous materials were conveniently functionalised and that the metal complexes could be effectively anchored onto these materials. The immobilised iron salts were active as heterogeneous catalysts in the oxidation of cyclohexane and n-hexane at room temperature, giving the respective alcohols and ketones. They could also be recycled at least two times without loss of catalytic activity.

All the heterogeneous catalysts prepared were also active and selective in the direct oxidation of benzene to phenol with higher yield and selectivity than the original metal salts in homogeneous phase. This result showed that the modification of the metal coordination sphere improves the catalytic activity of the immobilised metal complex. The heterogeneous catalysts could also be recycled by simple filtration and re-used in three more consecutive cycles without significant loss of catalytic activity.

Hence this project comprised the synthesis and characterisation of transition metal coordination compounds with very versatile and biomimetic polydentate ligands and their application as selective homogeneous catalysts in C-H activation reactions under mild conditions and using environmentally benign hydrogen peroxide. The sustainability of the homogeneous catalysts in the catalytic oxidation reactions was improved by using also economical and environmentally acceptable solid porous supports: hexagonal mesoporous silica and activated carbon. The main products of cyclohexane oxidation, cyclohexanol and cyclohexanone, are used as precursors to the synthesis of added-value products, such Nylon-6 and Nylon-6,6. Higher yields were obtained with the economical homogeneous and heterogeneous catalysts used in this project, under mild and sustainable conditions, than in the industrial process, where 4 % conversion into oxidised products with 80 % selectivity towards cyclohexanol and cyclohexanone, are obtained at 160 degrees of Celsius and 15 bar using a cobalt(II) naphthenate catalyst.

Phenol is also an important intermediate for the synthesis of petrochemicals, agrochemicals, and plastics. The direct oxidation of benzene is an attractive and challenging route to the synthesis of phenol, because the industrial process involves three reaction steps. However, the direct one-step reaction is often affected by low benzene conversions and / or poor selectivity. With our homogeneous and heterogeneous catalysts high selectivity to phenol were achieved, with yields comparable to the ones reported in the literature, under mild and sustainable conditions.

In conclusion, the developed homogeneous and heterogeneous catalysts based on first row transition metal complexes are economical and work under mild reaction conditions and could be valuable for the improvement of the sustainability and environmental impact of oxidation processes currently used in industry.