Page - 128 - in Advanced Chemical Kinetics

Studio 2016wasused to automate the calculations of all theparameters. Tunnelling correc- tionswere includedinall calculations.Theprotonationstep is thermodynamicallydownhill, buthas thehighestactivationbarrier (229kJmol 1) for the forwardreactionasshownin the mentioned table. The magnitude of this barrier is in the order of the bond-dissociation energyofHO–Hbondof awatermolecule (H2O)which requires about 268 kJmol 1 at 298 K.Assuming that the reaction is controlledbykinetics, this step is the rate-limitingstep.The high energy barrier for the rate-limiting step probably explains why LAC forms at 90 C under the reaction conditions employed,butnonewasobservedat 60 C.Ahigher tempera- ture is needed to overcome the barrier ( Ea/R) in order to get appreciable rates of the formationof the finalproduct. 4.Conclusions BothAu/γ-Al2O3 andAu-MoO3/γ-Al2O3 showedzero-order kinetics under kinetic controlled glycerol oxidation. The apparentEa of glycerol oxidation under these conditionswas experi- mentallydeterminedtobeapproximately36kJmol 1.Under thesamereactionconditions, the presence ofMoO3 increased the formation of LACsixfold overAu-MoO3/γ-Al2O3 relative to Au/γ-Al2O3.AproposedDFTmodel suggested that protonationbyanadsorbedwatermole- culemightbe the rate-limiting step in the isomerisationofPAtoLACcatalysedbyaMo–OH Lewisacid-basepair. Acknowledgements Theauthorswould like to thankbothMintekandAnglo-goldAshanti throughProjectAuTek for granting the permission to publish this work and for financial support. In addition, we express gratitude to the SouthAfricanCentre forHighPerformanceComputing (CHPC) for their support inavailing tous the infrastructureused inDFTmodelling. Authordetails ThabangA.Ntho*,PumezaGqogqaandJamesL.Aluha *Addressall correspondence to: thabang.ntho@mintek.co.za Mintek,AdvancedMaterialsDivision, Johannesburg,RSA References [1] Wee Y-J, Kim J-N, RyuH-W. Biotechnological production of lactic acid and its recent applications.FoodTechnologyBiotechnology.2006;44(2):163-172 Advanced Chemical Kinetics128