Seite - 126 - in Advanced Chemical Kinetics

3.4.ProposedroleofMon+ in lacticacidformation It isgenerallyacceptedthatLACisformedfromglycerol(GLY)viatheintermediatesasdepicted inFigure 9 [5, 30–32].Under the reaction conditions employed in this study,dihydroxyacetone (DHA)wasformedat60�Candcouldbe isolated;however,at90�C,LACwas formedinstead. It isplausible thatat the latter temperature,DHAstill formsbut reacts furtherwithLAC, thus assuming the role of an (unstable) intermediate. In thepresenceof base,DHAcan form from glycerol via the isomerisationofGLA[31].Assary et al. [33] have reported that aLewis acid- base pair catalyses this isomerisation reaction and it is conceivable that the enhanced forma- tion of LAC overAu-MoO3/γ-Al2O3 could have been a direct consequence of the increased Lewis acid-base pair sites on the catalyst. In this section, we apply transition state theory principles [34–36] viaDFTsimulations to extract thermodynamic and kinetic parameters for this isomerisationreaction.Experimentally,Rasrendraetal. [37]suggestedthat,undercatalytic conditions,GLAcanbe successfully isomerised toLAC.The rest of this chapterdiscusses the potential roleplayedbyMointheformationofLACoversupportedbi-functionalAucatalysts at lowtemperatures, assuming the reactionoccursaccording toEq. (12). GLY !Au GLA !Mo�OH LAC (12) DFTcalculations have beendone only for the secondpart of the reaction, since the calcula- tions forGLY toGLAoverAuhavebeendiscussed indetail elsewhere [31, 38]. Themecha- nism shown in Figure 10proposes that a crucial part of the isomerisation ofGLA to LAC could be an adsorbed pyruvaldehyde (PA)molecule on themolybdenum Lewis acid site forming a five membered ring C–C–O–Mo–O. The adsorbed PA is then hydrated by a surfacehydroxyl, asadvancedbyKongetal. [23].The intermediateproduct thenundergoes ahydrideshift rearrangement.Finally, aproton is then transferred fromawatermolecule to Figure9. Proposed intermediates in theconversionofglycerol toLAC. Advanced Chemical Kinetics126