Cellulose Arrhenius plots

The TGA system was a Perkin-Elmer TGS-2 thermobalance with System 4 controller. Sample mass was 2 to 4 mgs with a N2 flow of 30 cc/min. Samples were initially held at 110°C for 10 minutes to remove moisture and residual air, then heated at a rate of 150°C/min to the desired temperature set by the controller. TGA data from the initial four minutes once the target pyrolysis temperature was reached was not used to calculate rate constants in order to avoid temperature lag complications. Reaction temperature remained steady and was within 2°C of the desired temperature. The actual observed pyrolysis temperature was used to calculate activation parameters. The dimensionless "weight/mass" Me was calculated using Equation 1. Instead of calculating Mr by extrapolation of the isothermal plot to infinity, Mr was determined by heating each sample/additive to 550°C under N2. This method was used because cellulose TGA rates have been shown to follow Arrhenius plots (4,8,10-12,15,16,19,23,26,31). Thus, Mr at infinity should be the same regardless of the isothermal pyrolysis temperature. A few duplicate runs were made to insure that the results were reproducible and not affected by sample size and/or mass. The Me values were calculated at 4-minute intervals to give 14 data points per run. These values were then used to... 

Figure 1. Arrhenius plot of the lst-order rates for the untreated cellulose samples ...

Previous studies of the decomposition of cellulose reported Ea for absorbent cotton as 54.3 kcal/mol at a high-temperature range of 270-310 °C (23). For temperatures below pyrolysis, Ea = 20 kcal/mol reflects the low-temperature degradation effects of loss of H and OH from adjacent carbon atoms in cellulose (dehydration) and the concomitant creation of C=C bonds (24). In another work Ea = 21 kcal/mol was estimated from Arrhenius plots of the degree of polymerization versus time for cellulose heated in air at 150-190 °C (25). 

The Arrhenius plot gave activation energy of 29 kJ mol for the initial decay parts. Activation energy for steady-state luminescence of cellulose was obtained to be 50 kJ mof. Those value are significantly low compared to the reported activation energies for the thermal decomposition of organic hydroperoxides which is ranging from 96 kJ mof to 116 kJ mof. ... 

CL intensities of all the saccharides were measured at elevated temperatures below 100 °C in both a nitrogen and oxygen atmosphere. The average CL intensities for different temperatures were plotted in the Arrhenius plot. Activation energies were calculated from the slope of straight lines in the plots. In order to examine temperature effect on CL emission of cellulose in an inert atmosphere, powdered cellulose was stored for two weeks in air at ca. 25 °C and ca. 10 °C. 

Figure 6. Arrhenius plot showing effect of temperature on the rate of hydrolysis of amorphous cellulose and carhoxymethylcellulose by the endo-glucanase (9)...

Figure 3 shows the reaction rate constants determined from the slopes of the lines in Figures 1 and 2 plotted against the reciprocal of absolute temperature. The slope of the line through the points allows determination of the reaction activation energy according to the Arrhenius equation k = ko e exp (-E/RT). The value of E determined in this manner is 39,500 calories/mole, which appears to apply equally well to the degradation of starch or cellulose. 

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