Hydroxylation of 2-methylnaphthalene to 2-methylnaphthoquinone over TI-substituted catalysis

[摘要] ENGLISH ABSTRACT: Partially oxygenated aromatic compounds, e.g. quinones, hydroquinones and cresols, play a vitalrole in the fine chemical industry and were initially prepared by stoichiometric oxidation processesthat produce toxic products that are hazardous towards the environment. As a result, it wasimportant to investigate environmentally friendly processes for the hydroxylation of aromaticcompounds. This resulted in newer methods using Ti-substituted microporous zeolites as catalystswith hydrogen peroxide as oxidant in the presence of a solvent.However, the methods were found to be ineffective for large, bulky substrates due to the small porestructure. This led to using Ti-mesoporous materials as catalysts but suffered from two drawbacks;the hydrophilic nature and low hydrothermal stability of the catalyst structure. Ti-microporous andTi-mesoporous materials acting as catalysts for the oxidation of bulky substrates achievedenvironmentally friendly processes but obtained low conversions and quinone yields. Therefore, thechallenge has been to develop a process that is environmentally friendly, achieves high conversions,where the catalyst acts truly heterogeneous and obtains high quinone yields for the hydroxylation ofbulky substrates. Recently, micropores/mesopores catalysts incorporating advantages of bothmicropores and mesopores materials were synthesised and seemed promising for the hydroxylationof bulky substrates.This study focuses on synthesising and evaluating the feasibility of various Ti-substituted catalysts forimproving the hydroxylation of the bulky substrate, 2-methylnaphthalene (2MN) with hydrogenperoxide as oxidant in the presence of a solvent, acetonitrile. The oxidation of 2MN produces2-methyl-1,4-naphthoquinone (2MNQ). 2MNQ is also known as menadione or Vitamin K3 and acts asa blood coagulating agent. The catalysts synthesised for this study were mesoporous catalysts, Ti-MCM-41 and Ti-MMM-2 and microporous/mesoporous catalysts, Ti-MMM-2(P123) and a highlyordered mesoporous material. The main objective of this study was to design an efficient processthat is environmentally friendly and achieves high 2MN conversions and 2MNQ yields. This wasachieved by evaluating the various catalysts synthesised, reaction conditions, testing if the catalystwas truly heterogeneous and identifying the products formed from the process.The designed process was proved to be environmentally friendly because the system did notproduce products that were harmful towards the environment. The products identified in this studywere 2MNQ, 2-methyl-1-naphthol, 2-naphthaldehyde, 3-ethoxy-4-methoxybenzaldehyde andmenadione epoxide. An investigation was conducted to determine which catalyst synthesisedfavoured this process by quantifying the effect reaction conditions have on the various catalysts. Thereaction conditions were defined in terms of the hydrogen peroxide volume, catalyst amount,solvent volume, substrate amount, reaction time and reaction temperature. The desired catalyst forthis study obtained the highest 2MN conversions in comparison with the other catalysts andfavoured the formation of 2MNQ. The catalyst achieving the highest conversions and favouring2MNQ in most cases for this investigation was the highly ordered mesoporous material. Improving operating conditions to obtain high 2MNQ yields for the oxidation of 2MN to 2MNQ overthe highly ordered mesoporous material was determined by varying the reaction conditions with theone factor at a time approach and a factorial design. The one factor at a time approach showed thatbest 2MNQ yields were obtained at 1 g substrate when investigating a change in substrate amountbetween 0.5 g and 2 g. Best 2MNQ yields were obtained at 10 ml solvent when investigating achange of solvent volume between 5 ml and 20 ml. The 2MNQ yield increased with increasing thecatalyst amount (50 mg to 200 mg), hydrogen peroxide volume (1 ml to 6 ml) and increasing thereaction times (2 hour to 6 hours) at reaction temperatures, 120°C and 150°C. The yield decreasedwith increasing the reaction time (2 hours to 6 hours) at reaction temperature, 180°C. A preliminary2 level factorial design was prepared to observe if there were any important interactions affectingthe 2MNQ yield. The results from the factorial design indicated that the hydrogen peroxide volumehad the most influence on the 2MNQ yield followed by the reaction time-reaction temperatureinteraction and reaction temperature. From the factorial design, the yield increased by increasingthe hydrogen peroxide volume and reaction temperature whilst decreasing the reactiontemperature-reaction time interaction. The highest 2MNQ yields and 2MN conversions obtained forthe hydroxylation of 2MN to 2MNQ over the highly ordered mesoporous material in this study werein the ranges 48-50 % and 97-99 %, respectively.This study indicates that the process system, reaction conditions and catalyst type have an impact onthe products formed, 2MN conversion, 2MNQ selectivity and 2MNQ yield. The highly orderedmesoporous material was found to be truly heterogeneous because no leaching occurred and thecatalyst could be recycled without losing its catalytic activity and selectivity for at least two catalystcycles. It can be concluded that the highly ordered mesoporous material is therefore a promisingcatalyst for the selective oxidation of bulky substrates with aqueous H2O2 because it produces anenvironmentally friendly process, achieves high conversions, obtains high quinone yields and thecatalyst truly acts heterogeneous.

[发布机构] Stellenbosch University