Rate determining step and kinetics of oxygen delignification

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T33oxygen delignificationFIG. 5. Slope (1/min) vs. predicted reaction rate constant k.FIG. 6. Oxygen delignification rate of Loblolly pine kraftpulps at different kappa no. levels.FIG. 7. Phenolic group content of liquor and pulp at differentreaction times.liquor was also measured. Figure 7 showsthese values and that of the pulps at differentreaction times as well as the sumof the phenolic group content in the pulpand liquor based on original pulp. As canbe seen, the reduction in the amount ofphenolic groups in the pulp is nearly equalto the amount of phenolic groups removedwith the liquor since the sum of phenolicgroups does not change significantly duringoxygen delignification. This suggeststhat the dissolved lignin is still mostlyaromatic in nature.Finally, we found that the ratio(MeOH formation rate/32) / (delignificationrate/185), as determined from themethanol and lignin content of the liquorsamples, remains mostly constant duringdelignification at about 0.9 ± 0.2. Thismeans that methanol is released essentiallyquantitatively from lignin monomer unitswhen they are solubilized 11The delignification rate versus residuallignin of three brown stock pulps cookedto different kappa numbers from the sameLoblolly Pine (Figure 6) shows an initialfast rate. However, after oxygen delignifiashas been discussed by van Heiningenet al 17 .The first order behavior in L Ccan beinterpreted that the lignin active sites, L *– ,are uniformly distributed throughout thelignin, and have the same reactivity duringthe entire oxygen delignification process.It is generally accepted that the first stepduring oxygen delignification is the dissociationof the phenolic groups. This isthe reason why the phenolic group contentof residual lignin was measured before andafter oxygen delignification. In agreementwith literature data 7 we obtained values forthe phenolic fraction of lignin in the originalpulp of around 0.41 and after 60 min at90°C, 0.52 MPa oxygen pressure and 3.3g/l of NaOH of around 0.27. After 3 h ofoxygen delignification, the phenolic fractionof the residual lignin in pulp decreasesfurther to 0.22, i.e. to half the initial value.Based on this and on the high pKa value ofthe lignin active sites, L *– we suggests thatthe rate determining step of oxygen delignificationis not a direct attack of oxygenon the dissociated phenolic groups.The phenolic lignin content of theFIG. 8. Three experiments; (1) experiment in Berty CSTRduring 3 h; (2) 60 min batch reactor + 80 min Berty CSTR;(3) 20 min batch reactor + 75 min Berty CSTR.cation of the fresh 24.4 kappa brownstockpulp for different times (20 and 60 minutes)in a batch reactor, continued delignificationin the CSTR does not displaythe initial fast rate (Figure 8). The absenceof the initial fast rate for the latter twopulps has been explained by the absenceof hemicelluloses with reducing ends andattached lignin fragments in these pulps 17 .Figure 8 also shows that the oxygen delignifiedpulp obtained in a batch reactor hasa similar delignification rate as the originalpulp. This implies that lignin condensationis insignificant in a batch reactor since thepulps are exposed to high concentrations ofdissolved lignin at the end of the delignificationprocess.Delignification mechanismBased on the presented data we proposethat the active lignin site is not the phenolicgroup, but another less acidic sitewhich is uniformly distributed throughoutthe residual lignin to satisfy the first orderin lignin rate behavior. Because hydroperoxideshave a pKa of 12 – 13 comparedto about 10.5 for Indulin AT at room32 March 2009 Pulp & Paper Canada pulpandpapercanada.com
peer reviewedT34a)R 1OH -R 1R 1R 2 OCH 3OHpKa 10-11R 2 OCH 3 R 2OO 2OOCH3R 1O 2-R 1O-OR 1O-OHR 2 OCH 3b)OR 2 OCH 3OpKa 12-13R 1R 2 OCH 3OR 1CO 2CO 2+ CH 3 OHR 2 OCH 3OO-OR 2R 1R 2temperature 13 , a mechanism proposed by Chang and Gratzl 18was adopted to explain the present kinetics. Figure 9 shows theformation of cyclohexadienone hydroperoxide by attack of oxygenat alkaline conditions on phenolic lignin.As illustrated, a hydroperoxide anion is formed by reactionof superoxide radical anion with the phenolate radical located atthe carbon 3 position. Chemical computational modeling 19 tocalculate the enthalpies of reactions of lignin model shows thatthe reaction pathway in Figure 19 9a is possible. Other phenolateradical intermediates have been proposed (McDonough 1996)where the radical is located on carbon 1 and the beta carbon inaddition to the carbon 3 shown in Figure a. All these three structuresare part of the same resonance structure in which the oddelectron formally resides at each of the three carbon positions.The explanation, why the coupling of the superoxide anion radicalwith the radical at the carbon 1 and beta carbon is not effectivefor delignification, is likely that the formation of the respectiveoxirane and carbonyl structures is much more difficult than theformation of the muconic acid and quinone structures shown inFigure 9b when the hydroperoxide anion is located at the carbon3 position.Figure 9b shows that the hydroperoxide anion forms either amuconic acid structure and MeOH or an orthoquinone structureand MeOH. Since the hydroperoxide anion is the dominant speciesat pH > 12~13, based on the present kinetics is probable thatthe carbon at the 3 position of the lignin aromatic ring is the activesite for oxygen delignification. Thus the present kinetics may beinterpreted that the rate determining step is the unimoleculardecomposition of the formed hydroperoxide anion, i.e. the rateis determined by the dissociated hydroperoxide concentration.The latter is determined by the concentration of adsorbed oxygenon methoxyl groups at the carbon 3 (the lignin active site, L*),as well as by the pKa of the hydroperoxide. Further support thatthe carbon 3 with a methoxyl group is the lignin active site, L*,O+ CH 3 OHOFigure 9 Reaction FIG. 9. mechanism Reaction of lignin mechanism with oxygen of (adopted lignin with from Chang oxygen and (adopted Gratzl, 1980 18 ). a)Formation of cyclohexadienon hydroperoxides by attack of oxygen. b) Homolytic and heterolyticfrom Chang and Gratzl, 1980 18). a) Formation of cyclo-fragmentation of para-cyclohexadienone hydroperoxidaseshexadienon hydroperoxides by attack of oxygen. b) Homolyticand heterolytic fragmentation of para-cyclohexadienonehydroperoxidases.FIG. 10. Cellulose degradation in CSTR and batch reactors;Experimental and predicted data.is that almost all residual softwood lignin monomer units containthis moiety, thus explaining the uniform reactivity and first orderkinetics in residual lignin for oxygen delignification.The pH dependence of the kinetics implies that the protonatedhydroperoxide does not fragment into muconic acid orquinone structures. Also, the fact that dissolved lignin mostlyretains its aromatic nature, and that MeOH is released essentiallyquantitatively during delignification, further support the hypothesisthat quinone formation is the dominant pathway.Kinetics of cellulose degradationThe cleavage of cellulose was modeled by Iribarne and Schroeder19 as the increase in number-average moles of cellulose per gramof pulp (m n). Similarly one can describe the cellulose degradationby the number of cellulose chain scissions during oxygen delignification.Violette and van Heiningen 20 calculate the number ofcellulose chain scissions from the average degree of depolymerizationof cellulose (DP) in the pulp at time t=0 and time t=t, as 1/DP t-1/DP 0. DP can be obtained from the intrinsic viscosity [h]by equation (15). This formula considers the actual weight ofcellulose rather than the pulp weight being responsible for theviscosity, and makes a correction for the small contribution of thehemicelluloses to the pulp intrinsic viscosity 21 .1.65[h] – 116H1.111DP of Celluose = ( ——————— ) (15)Gwhere [h] is intrinsic viscosity of the pulp in cm 3 /g, and G and Hare the mass fractions of cellulose and hemicellulose in the pulp(see Table 2). This formula considers the actual weight of celluloserather than the pulp weight being responsible for the pulpviscosity, and makes a correction for the small contribution of thehemicelluloses to the intrinsic viscosity.The number of moles of cellulose per gram of pulp, mn, canbe calculated by equation (16) as 22 :1 1 Moles162DP n+18 _ 162DP nGram Pulpm n= ––––––––– . ––––––– (–––––––––– )(16)The content of cellulose (G) and hemicellulose (H) in the pulpwas measured by high pressure anion exchange chromatography(HPAEC) on double hydrolyzed pulp samples 23 . The results forthe pulp samples are listed in Table 2.pulpandpapercanada.com Pulp & Paper Canada March 2009 33
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