Oxidation of diols in aqueous solution over supported- Pt, Pt-Au and Pd-Au catalysts: influence of chain length of diol and of catalyst composition

Abstract

SSCI-VIDE+CDFA+MMG:NPR:CPI:MBEInternational audienceSelective oxidation with air of -diols to hydroxyl-carboxylic acids and diacids over heterogeneous catalysts is a green attractive route for the production of polymers [1]. Noble metals are commonly used for this reaction [2] and the composition of the catalyst may affect the reaction rate, the reaction pathway and thus the final yields. Moreover, the carbon number of diol could also have an influence. The objectives of this study were to compare monometallic Pt/ZrO2 and bimetallic Pt-Au/ZrO2 and Pd-Au/ZrO2 catalysts for oxidation of C4-C6 aliphatic diols.The series of catalysts were prepared by wet-impregnation of ZrO2 (109 m2 g-1) with aqueous solution of metallic salts and NaBH4 reduction [3]. XRD patterns (Fig. 1) show Pt particles sizes of ca ~ 7 nm over monoclinic ZrO2; AuPt and AuPd alloys were formed over Pt-Au/ZrO2 and Pd-Au/ZrO2 materials with particles of ca ~ 5 nm, as confirmed by TEM analysis. The composition of the alloys by XRD were close to the nominal Au/Pt and Au/Pd ratios of 1 of the solids.Oxidation reactions of diols (1,4-butanediol BDO, 1,5-pentanediol PDO and 1,6-hexanediol HDO) were performed in a 300 mL batch reactor (0.1 M diol in water) under 40 bar air at 70°C or 90°C. Liquid samples of the reaction medium were regularly collected and analysed by HPLC and for Total Organic Carbon TOC.Conversion of the diol was very rapid and the reaction was sequential via the corresponding hydroxy-aldehyde (ALD), hydroxy-acid (HA), aldehyde-acid (AA) and diacid (DA) (Scheme 1). During oxidation of BDO, cyclization reactions took place and formed -butyrolactone (GBL), which was poorly reactive.Regardless of the nature of the catalyst, the rate of oxidation of the diol decreased withdecreasing chain length; the order of rate was Pt > Pt-Au > Pd-Au. Moreover, the productsdistribution at similar conversion (50-60%) was different according to the catalyst (Table 1).Theinitial selectivity to ALD was high over Pt and the oxidation of the aldehyde group took placesmoothly; in contrast, over Pd-Au ALD was very rapidly converted to HA, which wasconsequently formed with high initial selectivity.Under the same reaction conditions after 48 h of reaction, the yield of DA increased with thechain length. The Au-Pt catalyst yielded 66% succinic acid, 81% glutaric acid, and 96% adipicacid (Table 1). Reactions performed at 90°C improved the yield of DA over Pd-Au catalyst to97% after 48 h and greatly accelerated oxidation of BDO over Pt-Au.In conclusion, Pt/ZrO2 catalyst is active for oxidation of diols to the diacids. The use of bimetalliccatalysts allowed to improve the final yield of DA. By adjusting the operating conditions, onecan obtain very selectively the desired acid