Seite - 125 - in Advanced Chemical Kinetics

formation of LAC, as evidenced in Eq. (10) by the ratio of the rate constants of formation of LACover the twocatalysts. kAu=MoO3�Al2O3 kAu=Al2O3 ffi6 (10) Asfarasweareaware,noformationofLACfromAu-basedcatalysts (ourworkincluded)has been reported at 60�CexceptwhenPd-based catalystswere used,which also gave very low turnover frequency (TOF) values (~60 h�1) [10]. The TOF is the best measure of comparing catalyticperformanceasarguedbyBoudart [29].TOF is regardedas thenumberof times that theoverallcatalyticreactiontakesplacepercatalyticsiteperunit timeforafixedsetofreaction conditions.Boudarthasassertedthateventhoughitmaynotberigorous,aTOFmeasure leads tovalues that canbe reproduced fromone laboratory toanother, besides catalystsofdifferent characteristics being likened.Wehave therefore simplifiedTOF tomean the number of glyc- erolmoles converted per the total number ofmoles ofAuused per unit time of reaction, in hours,assumingthatall thegoldpresent in thecatalystwasactive.Wehavealsoassumedthat thereactionoccurredatconstant temperature (90�C),O2pressure (8.5bar), concentration (1.1M ofglycerol at t0), and the ratio of reactants,while the extent of reactionwas takenafter half an hour.Thus,TOFwascalculatedbytheequation: TOF¼ %conversionð Þ:ðinitialmoles glycerol 100 : 1 molesofAu : 1 time hð Þ (11) For example, the TOFof theAuroliteTM commercial catalyst (0.9-wt%Au/γ-Al2O3) after half an hour was calculated to be 4.971 h�1, compared to that of Au-MoO3/γ-Al2O3 catalyst (ca12.800h�1). This result shows thatnot onlydoes the increasedaciditydue thepresenceof Moenhance the selectivity toLAC,but it alsodrives the forward conversionof glycerol. This latterpoint couldberelated togold-support interactioneffects. Parameter* Catalyst Au/γ-Al2O3 Au/MoO3-γ-Al2O3 Value Confidence limits Value Confidence limits k1 0.019 0.018 0.02 0.035 0.032 0.038 k2 0.021 0.019 0.023 0.023 0.018 0.028 k3 0.008 0.007 0.01 0.009 0.006 0.013 k4 0.004 0.002 0.006 0.023 0.019 0.028 k5 0.001 0.000 0.003 0.005 0.002 0.008 *Parametervalueswereestimatedat95%confidence level;k=molL�1min�1. Table 2. Regression analysis of global kineticmodel assumingpseudo-zero-order kinetics. Oxidation of Glycerol to Lactic Acid by Gold on Acidified Alumina: A Kinetic and DFT Case Study http://dx.doi.org/10.5772/intechopen.70485 125