Preparation, characterization and use of V2O5-TiO2 mixed xerogels as catalysts for sustainable oxidation with hydrogen peroxide of 2,3,6-trimethylphenol
V2O5-TiO2 mixed xerogels were prepared with different loads of TiO2 and V2O5 (5%, 10%, 15% V2O5 w/w) by the sol–gel method using titanium isopropoxide and vanadyl acetylacetonate as precursors. The solids obtained were characterized by XRD, FT-IR and SEM. Their textural properties were determined by adsorption–desorption isotherms of N2 at 77 K. Also the acidity of the catalysts was measured by means of potentiometric titration with a solution of n-butylamine in acetonitrile.The catalysts were tested in the liquid phase oxidation of 2,3,6-trimethylphenol (2,3,6-TMF) to 2,3,5-trimethyl-p-benzoquinone (2,3,5-TMBQ) at 20 °C, using ethanol as solvent and aqueous hydrogen peroxide as clean oxidizing agent. The conversion increases with time to reach at 3 h a plateau of 93% and 88% for the samples without V and with 5% of V respectively. An increase of the V concentration to 10% leads to a conversion value of only 22% at 4 h of reaction. However, the selectivity to 2,3,5-TMBQ increases with the V content in the catalyst. The stability and leaching of catalysts and the influence of the xerogel calcination temperature were also studied. By reusing the same catalyst sample with 5% of V, a difference of 5% in the selectivity values between use and reuse was observed. A decrease from 88% to 8% of 2,3,6-TMP conversion for 4 h of reaction, when the calcination temperature of xerogel increased from 200 °C to 400 °C, was observed. When t-butanol was added as radical scavenger, the oxidation was strongly suppressed. This makes it possible to propose a radical mechanism for 2,3,6-trimethylphenol oxidation.Substrates with different substitutes in phenol were also tested in the oxidation reaction. The substrates with methyl groups are the most actives and in the cases of thymol and carvacrol with isopropyl groups, the conversion is lower.
Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (175 K)Download as PowerPoint slideHighlights► V2O5-TiO2 mixed xerogels were prepared by the sol–gel method. ► They were tested in the liquid phase oxidation of 2,3,6-trimethylphenol with H2O2. ► For catalyst with 5% V, 90% conversion and 99% selectivity were observed at 4 h ► A radical mechanism for reaction was proposed ► Different substituted phenols were also tested in the oxidation reaction.
Journal: Applied Catalysis A: General - Volumes 417–418, 29 February 2012, Pages 273–280