Page - 114 - in Advanced Chemical Kinetics

acetaldehydeanddilactide. In fact,LACcontinues to receive increasedattention for itspoten- tial use as amonomer in theproductionofbiodegradablepoly lactic acid. It canbeproduced byeitherbiotechnological fermentationorchemical synthesis,but the formerroute is receiving considerable interest due to environmental concerns and the limitednature of petrochemical feedstocks.However, fermentation is inherentlyaslowprocess.Analternativeroute toLACis by processing a ‘renewable’ resource such as glycerol bymeans of heterogeneous catalysis. Haruta [2]has reported thatgoldwithparticlediametersbelow10nmare surprisinglyactive formanyreactions, suchasCOoxidationandpropyleneepoxidation. Manyaspects of glycerol oxidationbyAuhavebeen studied.Ketchie et al. [3] have lookedat the effect ofAuparticle size, particularlyonsupports suchas carbon [4] and titania [5],while Wangetal. [6]haveexamined theeffectofparticle shape,andVillaetal. [7]have investigated the role of stabilizers in gold sols as catalysts in the liquid-phase oxidation of glycerol. In addition, Demirel et al. [8] have probed the promotional effect of Pt onAu/C catalysts, and Royker et al. [9] have investigated the promotional effect of Pt onAu/Al2O3 catalysts,while other authorshave studied the effect of base in the reaction. For example,Chornaja et al. [10] examined theoxidationof glycerol toglyceric acidusingPdcatalysts thatworked inalkaline media, andKetchie et al. [5] reported the promotional effect of hydroxyl ions overAu cata- lysts,whileCarretin et al. [11] haveanalysed the effect of base as a reaction initiator, proving that for Au/C catalysts, the presence of OH wasmandatory for any meaningful glycerol oxidation to occur.However, therewas seemingly very limited studyon the effect of surface acidity for this reactionusingaluminasupports, forwhich thiswork isdedicated. 2.Experimentalprocedures 2.1.Chemical reagentsandmaterials Commercialγ-Al2O3support(denotedasDegussa2010,BETspecificsurfaceareaof260m 2g 1), ammonium molybdate (NH4)6 Mo7O24 4H2O from Associated Chemical Enterprises (ACE), chloroauricacidHAuCl4 3H2OfromRandRefinery (SouthAfrica),NaOH(98%)andnitric acid (65%)fromACE,andglycerol (99.5%)fromRochelleChemicalswereused. 2.2.PreparationofMoO3/γ-Al2O3support Amassof4.6gofγ-Al2O3supportwithameasuredBETspecific surfaceareaof245m 2g 1was weighedandplacedinabeaker.About100mlsolutionof0.1Mammoniummolybdatesaltwas measured, enough to formMoO3monolayer coverage at surface concentration of 5 atoms of metalnm 2 or0.2nm2atom 1 according toStobbe-Kreemerset al. [12]andRaubenheimerand Cronje [13]. ThepHof the solutionwas thenadjusted to avaluebelow1byadditionofdilute (0.25M)HNO3acid,with agitation to ensure equaldistributionof the acid to a stablepH.The supportwas then added to the ammoniummolybdate solution and left to stand for 8 h, after which, itwas filteredand left to ovendry inair at 120 Cfor 16h. Thedried catalystprecursor was thencalcined inairat a flowrateof 300SCCMat500 Cfor4h todecomposeanyresidual ammoniumandnitrate ions fromthesupport, effectively reducing themolybdate ion toMoO3, Advanced Chemical Kinetics114