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4?Production of Vanillin and Syringaldehyde by Lignin Oxidation

4?Production of Vanillin and Syringaldehyde by Lignin Oxidation

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383–427 1.1–4.5



433–453 14(ii)



433–453 ns



433



443–463 (v)



2



3



4



5



6



ns



11



Ce(IV), Cu(II),

Co(II)



Cu(OH)2



CuSO4 + FeCl3



No catalyst

CuO, Fe2O3,

CuSO4, FeCl3



-



* 9.7



16.5



-



Catalyst



4.8–10



Pt (bar)



2, 3 (iii) -



372–414 1.8–6.5



pO2

(bar)



1



T (K)



Entry Initial reaction conditions



V: 10.8



V: 10.5



Yields



V: 5.1

Sy: 9.8



CLi = 165 g/l (v)

CNaOH & 2.7 M

V: 8



V: 11.9

CLi = 117 g/l (iv)

CNaOH = 2 and 3 M

(pH = 13)



CLi * 40 g/l

CNaOH = 2 M



V: 4.7

CLi = 100 g/l

CNaOH = 1.5–3.2 M Sy: 9.5

(pH 8–13)



CLi = 30–120 g/l

CNaOH = 2 M



CLi = 30–120 g/l

CNaOH = 1–4 M



Medium



a



tmax

(min)



T = 463 K

Catalyst: Cu(II)



CNaOH = 3 M



T = 443 K ,

pO2 = 14 bar (decreasing)

CNaOH *3 M (pH 11)

Catalyst: CuSO4 + FeCl3

For V: T = 433 K

For Sy: T = 453 K



75



40



30 (V)

10 (Sy)



10



T = 406 K (i), pO2 = 2.8 bar 35

CL = 60.0 g/l



64.4

T = 414 K,

pO2 = 3.7 bar, Pt = 10 bar

CLi = 60.0 g/l, CNaOH = 2 M



Conditions



Maximum yields and conditions reported



Table 12.3 Conditions and results of lignin oxidation with molecular oxygen: literature survey

References



(continued)



[125]

O2 introduced before the

heating phase.

Decreasing pO2.

[140]

Continuous O2 supply.

(iii) Two values for

pO2 are assigned.

(iv) Values corrected

for lignin content

(65%) in dry solids.

(v) Constant and high flow [155]

of O2. Lab and pilot

trials. High yield for LS

from a low-temperature

pulping process.



Direct oxidation of kraft

[36, 118,

liquor was attempted

133]

with similar results

compared to the

isolated lignins; Total

pressure maintained by

continuous O2 supply.

[116]

Continuous O2 supply.

(i) average value(s) in

the run time.

(ii) Two runs: 1. continuous [110]

O2 supply;

2. decreasing pO2.



Notes



12

Lignin as Source of Fine Chemicals

395



373–413 2–10



393



393



443



9



10



11



12



10.8



5



5



(vi)



403



8



4.0, 6.5



390



pO2

(bar)



7



T (K)



10.8



20



-



20



10 (vi)



9.0, 9.5



Pt (bar)



Entry Initial reaction conditions



Table 12.3 (continued)



POM



perovskitetype oxides



Pd/c-Al2O3



Pd/c-Al2O3



-



-



Catalyst



80 vol%

MeOH/H2O

CLi & 8.8 g/l



CLi = 60.0 g/l

CNaOH = 2 M



CLi = 60.0 g/l

CNaOH = 2 M

CL = 30.0 g/l

CNaOH = 2 M

QL = 5 l/h



CLi = 60 g/l

CNaOH = 2 M

CL = 60 g/l

CNaOH = 2 M

(pH = 14)

QL = 1.0–2.5 l/h



Medium



V: 3.5

Methyl

vanillate:

3.5



V: 4.6

Sy: 11.5



V: 4.4

Sy: 5.8

-



V: 1.5



V: 3.3



Yields



a







Catalyst LaFe0.8Cu0.2O3



Ti = 413 K, pO2 = 5 bar

CNaOH = 0.9 M

-



QO2 = QN2 = 1.0 l/min

QL = 1.0 l/h SPBCR



pO2 = 4 bar, Pt = 9 bar



Conditions



Maximum yields and conditions reported



[121, 133]



Continuous process:

[119, 120,

1. co-current bubble

133]

column reactor;

2. bubble column

reactor (vi) QO2 = 1.0 l/

min and 2 l/min (NTP);

QN2 = 1.0 l/min (NTP).

Batch process.

[115]



Batch process.



References



(continued)



Continuous process (three- [115]

phase fluidized-bed).

Air bubbling: 1000 l/h;

For 2 h operation the

yield reported is higher

than the same time in

batch mode.

V: 60

Total pressure maintained [124]

Sy: 30

by continuous O2

supply. Yield is

1.4–2.5-fold the

non-catalyzed reaction.

Reaction O2 introduced before the

[126]

stopped

heating phase.

at 20

min



15 (V

and Sy)

-



Steady

state

(6 h)



75



tmax

(min)



Notes



396

P. C. Rodrigues Pinto et al.



393



9.7



-



-



No catalyst

and CuSO4



Catalyst



CL = 60.0 g/l

CNaOH = 2 M

V: 3.1



V: 1.2,

Sy: 2.5



V: 4.5

Sy: 16.1



CL * 3 g/l and

CL * 13 g/l (viii)

CNaOH = 0.75 M

(pH 13.1) and

CNaOH = 0.9 M

CL = 60.0 g/l

CNaOH = 2 M



Yields



Medium



a



-



-



Ti = 423 K, pO2 = 10 bar

CNaOH = 0.9 M, with

catalyst



Conditions



Maximum yields and conditions reported



V: 40



V: 25

Sy: 12



20



tmax

(min)



References



[113]

The author found distinct

kinetic laws for V and

Sy, but the time to

maximum is the same.

(viii) Calculated values

from available data.

Total pressure maintained [20]

by continuous O2

supply.

Total pressure maintained [20]

by continuous O2

supply.



Notes



Entries: 1 Softwood kraft lignin (MeadWestvaco Corp. and Portucel, Portugal), 2 Softwood kraft lignin (MeadWestvaco Corp.), 3 Steam-explosion lignin from aspen,

4 Precipitated lignin after hydrolysis poplar wood, 5 Fermented alkaline sulfite liquor of fir wood (used as it is), 6 Lignosulfonates obtained by different pulping conditions, 7 and

8 Indulin AT (MeadWestvaco Corp.), 9 and 10 Alkaline lignin from sugarcane bagasse, 11 Lignin from steam explosion cornstalks, 12 Indulin AT (MeadWestvaco Corp.), 13 E.

globulus lignosulfonate (purified by dialysis), 14 Organosolv beech lignin, 15 Softwood kraft lignin isolated by Lignoboost process

a

Values reported to %wt lignin; ns not specified, QL liquid-phase flow rate l/h, POM polyoxometalate, V vanillin, Sy syringaldehyde, Hy p-hydroxybenzaldehyde



3



9.7



15



3



393



14



Pt (bar)



403–433 6 and 10 ns



pO2

(bar)



13



T (K)



Entry Initial reaction conditions



Table 12.3 (continued)



12

Lignin as Source of Fine Chemicals

397



398



P. C. Rodrigues Pinto et al.

H

C



HO

MeO



H

C



O2



C CHOR



O2



R'

H

C



C CHOR

R' OH



VI



-ROH



O

MeO



CH2 OR -H 2O



H

C



V

H

C



IIa



C



MeO

OH CHOR



R'



C

R'



O

MeO



O

H



VII



H

C



O



III



H 2O



C CH 2OR

R'



-H+

C CH 2OR

R'



C

H



H

C



O

MeO



II



O

MeO



IV



H

C



OH R'



O

MeO



IIa



O

MeO



C CH 2 OR

R'



H

C



H

C



O

MeO



I



O

MeO



CH 2 OR OH



OH R'



O

MeO



H

C



C CHOR

R'



IV

H

C



C CHOR

R' OH



VI

O



MeO



O

+ R'



O

H



C

H2



H



VIII



R and R' could be H, OH, or other linked ppu



Fig. 12.7 Proposed mechanism for lignin oxidation by Tarabanko et al. [140, 141] here

represented for a typical guaiacyl type unit



patents [148, 152]. Therefore, the initial pH value in the range 13–14 has been

used in several works to maintain a high alkalinity in the entire reaction time. One

other side, in the proposed mechanism by Tarabanko et al. [140, 141] (Fig. 12.7)

the strong alkaline medium is required also for the proton detachment (step III–IV)

and nucleophilic addition of OH- to the intermediary quinonemethide (step V–VI)

and also for the final retroaldol cleavage (final step in Fig. 12.7). In fact, this step

is the main difference between the mechanism proposed by Tarabanko for vanillin

production and the mechanism via dioxetane formation established by Gierer et al.

[146].

The energy barrier for electron transfer from the organic substrate to the oxidant

is usually high. Considering this, the temperature should be one of the most

significant factors to consider in lignin oxidation. Catalyst has also been considered to improve the yields and selectivity.

Lignin oxidation with O2 in alkaline medium has been intensively studied in

Laboratory of Engineering of Separation and Reaction (LSRE, Porto) [20, 36,

116–122, 153, 154]. The batch experiments have been performed in a jacketed

reactor Büchi with a capacity to 1 l with control and register temperature, pressure

and gas flow. The reaction mixture composed by NaOH and lignin is kept at high

stirring and the reactor is purged and pressurized with N2. At steady state temperature, the oxygen is introduced at controlled pressure and the reaction is considered to start at this point. During the reaction, the total pressure is maintained

constant thought feed of O2. Samples are collected at controlled time intervals and,

after acidification, the compounds are extracted with organic solvent and GC-FID

analysis or recovered by solid-phase extraction and analyzed by HPLC-UV using

external calibration [154].



Lignin as Source of Fine Chemicals



(a)



Vanillin



Acetovanillone



Vanillic acid



Temperature



2.0



(b)

408



2.0



404



400



1.0

396

0.5

392

0.0



Vanillin



Acetovanillone



Syringic acid



Vanillic acid



Syringaldehyde



Temperature



410



406



1.5



402



1.0



398



Temperature, K



1.5



Temperature, K



Product concentration, g/L



2.5



399



Product concentration, g/L



12



0.5

394

0.0



0



20



40

60

Reaction time, min



80



100



0



10



20

30

40

Reaction time, min



50



60



Fig. 12.8 Products concentration and temperature evolution during the reaction time for lignin

oxidation with O2 in alkaline medium (Ti = 393 K, pO2 = 3 bar, Pt = 9.7 bar, CL = 60 g/l,

CNaOH = 80 g/l) for two different lignins [20]. a Kraft lignin from softwood isolated by

Lignoboost process (supplied by Innventia AB) referred as LKBoostS in Table 12.2 and

Table 12.3. b Organosolv beech wood lignin (supplied by Fraunhofer, Germany) referred as

LOrgsB in Table 12.2



12.4.2 Evolution of Products and Temperature During Lignin

Oxidation

The typical profiles of phenolic products and temperature as function of time is

shown in Fig. 12.8 for the oxidation of a softwood kraft lignin (a) and hardwood

organosolv lignin (b) [20]. The yields of vanillin and syringaldehyde clearly

predominate over the vanillic acid and syringic acid. The concentration of the

phenolic aldehydes and their respective acids increases continuously until a

maximum value which is coincident with the maximum temperature (Tmax). In fact

the reaction is exothermic, increasing the initial temperature (Ti) of reaction

(393 K): the values of DT (Tmax-Ti) reported in these reactions were 13 and 9 K

for softwood and hardwood lignins, respectively, (Fig. 12.8). For a commercial

kraft lignin, Indulin AT, increases at the same order (10–15 K) were found [121],

although the rate of oxidation as well as the heat of reaction differs between

lignins. After that maximum, the concentration of products decreases continuously

due to the dominance of degradation over the production reactions.

The phenolic acids are formed by the cleavage of Ca–Cb in the propane chain of

ppu (as shown in Fig. 12.7), as for aldehydes, but they undergo further oxidation. The

profiles of vanillic acid and syringic acid are very close to the corresponding aldehydes, with a maximum at the same reaction time, followed by an analogous decline.

The ratio between these two products is a measure of process selectivity for the

aldehydes. For the lignins and conditions corresponding to data presented in

Fig. 12.8, it is noteworthy that, at the maximum yield for softwood (40 min) and

hardwood (25 min), the calculated vanillin/vanillic acid ratio is 1.7 and 1.6,

respectively. This is a rather similar value, considering that the individual yields are

quite different. One other side, the ratio syringaldehyde/syringic acid for the hardwood lignin (values taken at the maximum—12 min) is much higher: 13.1. This is an



400



P. C. Rodrigues Pinto et al.



indication of higher selectivity of the process for the syringyl units of lignin. The

oxidation of vanillin is pointed out as the main route for vanillic acid production (as

well as for other secondary products) [155]. However, it is interesting to notice the

similar and parallel behavior of formation and degradation of the aldehydes and

respective acids, and also the strong decrease of vanillic acid for long reaction time in

comparison to vanillin. Gierer et al. [146] reported different routes for vanillin and

vanillic acid production from lignin oxidation, and not as subsequent reactions,

which, at least partially, would be the reason for the behavior observed.

The increase on the production of syringaldehyde in the first 10 min of reaction, and

the pronounced decrease after the maximum (Fig. 12.8b) were remarkably high

compared to the behavior of vanillin. These facts are related with the different

oxidation rates of guaiacyl and syringyl units of lignin. The syringyl units have higher

reactivity than guaiacyl counterparts in alkaline systems [156] and under conditions of

O2 oxidation in alkaline medium [110, 157]. Thus, the oxidation of syringyl units is

faster than guaiacyl units for both production and degradation of aldehydes.

Besides vanillin and syringaldehyde, and their respective acids, acetovanillone

was also found as secondary product. Acetosyringone was also found in the case of

hardwood LOrgsB, as well as p-hydroxybenzaldehyde in both lignins, but in rather

low concentration (\0.05 wt% lignin, not shown in Fig. 12.8). Vanillin, vanillic

acid, and acetovanillone have been reported as the three principal compounds

found in the reaction mixture. For example, yields of 2–8% of these three major

compounds were obtained from alkaline oxidation of lignosulfonates from Norwegian spruce [155].

The mechanism for the lignin oxidation that expresses the formation of aceto

derivatives as acetovanillone and acetosyringone [140] is based on the competing

addition of OH- to a-position and to c-position of quinonemethide: the first leads

to the acetoderivatives and the second leads to vanillin and syringaldehyde.

Acetovanillone (also known as acetoguaiacone, or apocynin, with an odor similar

to vanillin) is an interesting precursor of veratric acid (3,4-dimethoxybenzoic

acid), a building block for synthesis of pharmaceuticals. Acetovanillone is already

isolated after the lignosulfonate oxidation at Borregaard [158].

Other compounds have been also referred as secondary products from lignin

oxidation with O2 in alkaline medium as for example, guaiacol, dehydrovanillin,

5-carboxyvanillin, 5-carboxyvanillic acid, homovanillin, and the syringaldehyde

counterpart [113, 155].



12.4.3 Influence of the Parameters in Lignin Oxidation

and Vanillin Oxidation

Several authors have been studying the influence of the reaction conditions and

vanillin and syringaldehyde yield as depicted in Table 12.3. The most comprehensive work found in literature is that developed in LSRE in batch [20, 36, 116,

118, 119, 121, 133, 153] and continuous processes [119, 120, 122, 133].



Lignin as Source of Fine Chemicals



Vanillin concentration, g/L



5.0



401



(b)



7.0



(a)



6.0



pO2



4.0



2.1 bar

4.1 bar



3.0



6.6 bar

2.0



1.0



Vanillin concentration, g/L



12



5.0



1M

2M



4.0



4M



3.0

2.0

1.0

0.0



0.0

0



20



40



60



80



100



120



0



140



40



80



(c)



7.0



160



200



240



280



320



360



(d)



7.0



6.0



6.0

30 g/L



5.0



60 g/L

120 g/L



4.0

3.0

2.0

1.0



Vanillin concentration, g/L



Vanillin concentration, g/L



120



Reaction time, min



Reaction time, min



372 K

381 K

393 K

414 K



5.0

4.0

3.0

2.0

1.0



0.0



0.0

0



40



80



120



160



200



240



280



320



360



Reaction time, min



0



40



80



120



160



200



240



280



320



360



Reaction time, min



Fig. 12.9 Vanillin profile during the lignin oxidation with molecular oxygen in aqueous NaOH

at different operation conditions [36]:a Effect of pO2 (CL = 60 g/l, CNaOH = 2 M, Ti = 393 K,

Pt = 9.2-9.4 bar). b Effect of CNaOH (CL = 60 g/l, Ti = 393 K, pO2i = 3.7 bar, Pt = 9.7 bar. c

Effect of CL (CNaOH = 2 M, Ti = 393 K, pO2i = 3.7 bar, Pt = 9.6 bar). d Effect of Ti

(CL = 60 g/l, CNaOH = 2 M, pO2i = 3.7 bar, Pt = 9.7-10 bar)



Figure 12.9 shows the vanillin concentration profile during reaction of lignin in

aqueous NaOH with molecular oxygen considering variation of O2 partial pressure

(pO2), initial concentration of NaOH (CNaOH), concentration of lignin (CL) and

initial temperature (Ti) [36].

Vanillin oxidation was also studied [149] since the degradation of the produced

vanillin is a key point on the sustainability of the process. Figure 12.10 shows the

main results of experimental and simulation work in this subject [121, 149], where

the influence of pO2, pH, and Ti are shown.



12.4.3.1 Effect of O2 Partial Pressure (pO2)

The effect of pO2 in the range 2.1–6.6 bar (continuous supply) was tested for

lignin (softwood kraft lignin supplied by MeadWestvaco Corp.) at initial concentration 60 g/l and initial temperature 393 K with a total pressure (N2, O2 plus

water vapor) of about 9.3 bar [36]. The results demonstrate that the main effect of

pO2 was on the rate of vanillin formation, shortening the time to maximum, with

no influence on yield [118], as depicted in Fig. 12.9a. Other authors [110, 125]

have studied the effect of O2 using two approaches: (1) continuous supply of



402



P. C. Rodrigues Pinto et al.



(a) 3.0



p O2

2.4 bar (exp)



4.2 bar (sim)



2.0



5 bar (exp)

5 bar (sim)



1.5

1.0



9.8 (sim)



2.5



Vanillin concentration, g/L



4.2 bar (exp)



pH

9.8 (exp)



2.4 bar (sim)



2.5



Vanillin concentration, g/L



(b) 3.0



0.5



10.6 (exp)

10.6 (sim)



2.0



14 (exp)

14 (sim)



1.5

1.0

0.5



0.0



0.0

0



30



60



90



120



150



180



0



30



Reaction time, min



90



120



150



180



Reaction time, min



Ti



(c) 3.0



376 K (exp)



2.5



Vanillin concentration, g/L



60



376 K (sim)

393 K (exp)



2.0



393 K (sim)

414 K (exp)



1.5



414 K (sim)



1.0



0.5

0.0

0



30



60



90



120



150



180



Reaction time, min



Fig. 12.10 Vanillin oxidation depicted as vanillin concentration (Cv) during reaction with

molecular oxygen in aqueous NaOH for different operating conditions (exp experimental, sim

simulation) [121] (data kindly provided by Dr. Daniel Araújo, FEUP, Portugal). a Effect of pO2

(Cv = 2.5 g/l, pH 14, Ti = 414 K). b Effect of pH (Cv = 2.5 g/l, Ti = 376 K, pO2 = 4.3 bar).

c Effect of Ti (Cv = 2.5 g/l, pO2 = 4.3, pH 14)



oxygen to the reactor, maintaining the initial pO2 in the course of reaction; (2) O2

introduced at the beginning and immediately interrupted. In the second case, it was

observed a rapid decrease of pressure due to the O2 consumption in reaction.

While Xiang and Lee [125] reported lower yields on vanillin and syringaldehyde

for the first approach, Wu et al. [110] demonstrated that the yields of products did

not change, but the rate of vanillin and syringaldehyde formation was higher for

the reaction with continuous supply of O2, in accordance with Mathias’s results

[36, 118]. Furthermore, after the maximum, an accentuated decrease of aldehydes

concentration was noticed for constant supply of O2. This observation is in

accordance with the higher rate of vanillin decline after the maximum depicted in

Fig. 12.9a and in Fig. 12.10a for higher initial pO2. In the oxidation of a hardwood

lignin [110], this effect was more evident for syringaldehyde probably due to its

higher reactivity than guaiacyl counterparts in alkaline systems [156] and under

conditions of O2 oxidation in alkaline medium [157] leading to its faster

degradation.

The origin of lignin and, consequently, its structure and reactivity have influence on kinetic parameters. The reaction order of vanillin production with respect

to the oxygen concentration was 1.75 [116] for Westvaco Co. kraft lignin and 1.00



12



Lignin as Source of Fine Chemicals



403



for an Eucalytus lignosulfonate (for both vanillin and syringaldehyde), showing

that the oxidation of first lignin has a higher dependence of oxygen concentration

in reaction medium.



12.4.3.2 Effect of Initial Concentration of NaOH (CNaOH)

Figure 12.9b reveals the importance of the OH- on the yield of the process of

lignin oxidation at the conditions of this study: a considerable increment of

vanillin is achieved by increasing NaOH concentration. The reasons for this

impact on yield were already stated in Sect. 4.4.1 (regarding postulate chemical

mechanism of oxidation). However, other considerations should be noted: the

concentration of vanillin at each moment is the result of formation and degradation in the reaction medium. Fargues et al. [149] studied the kinetics of

vanillin oxidation and concluded that at pH \11.5 the vanillin oxidation become

more significant being of second order in vanillin concentration and zero order in

O2 concentration. At pH [11.5, the reaction rate of vanillin oxidation is first

order for both vanillin and O2 concentration. Therefore, at least 2 M in NaOH is

required to achieve the favorable condition to preserve the produced vanillin.

Figure 12.10b demonstrates clearly the initial pH effect on vanillin oxidation. It

should be pointed out that the pH is the parameter in discussion, since the same

alkali concentration could lead to different values for aqueous solutions of

different lignins, depending of the raw material composition. Considering this,

the pH of the final solution should be measured at each case, confirming the

required value. The operational problems (incrustation in the reactor) related

with the solution NaOH 4 M had led Mathias et al. to avoid to such concentration and to adopt the 2 M.

For lignosulfonate (from E. globulus), the reaction order relative to OHconcentration is 1.9 for vanillin and 1.4 for syringaldehyde. The reason suggested

for these high values was the step involving the desulfonation reaction in the case

of lignosulfonates (carrying SO3- groups at Ca and Cc see Fig. 12.3) [113]. The

removal of sulfonic groups leads to the formation of units with double bonds in

propane lateral chain (as intermediate IIa in Fig. 12.7), species that are naturally

more reactive with O2 than the saturated counterpart.



12.4.3.3 Effect of Initial Lignin Concentration (CL)

The effect of lignin concentration on the reaction rate of vanillin production is

depicted in Fig. 12.9c. The calculated maximum yield on lignin basis (wt%)

decreases with the CL: 10, 8.3, and 3.0% for CL of 30, 60, and 120 g/l, respectively. The slope of the straight line found for initial vanillin production rate as a

function of the initial lignin concentration provides the reaction order of 1 with

respect to lignin concentration [116].



404



P. C. Rodrigues Pinto et al.



Considering the results of vanillin production from a kraft lignin in alkaline

medium, under the reported conditions, the following kinetic law was achieved:

rv ẳ kẵO2 1:75 ẵL



12:1ị



12.4.3.4 Effect of Temperature

The effect of temperature in the reaction rate and yield of vanillin production from

lignin oxidation is shown in Fig. 12.9d for the range 372–414 K: higher initial

temperatures led to higher vanillin yields in a shorter reaction time; however, the

vanillin degradation is also higher. In fact, for pH 14, the temperature has an

important effect on the rate of vanillin degradation as shown in Fig. 12.10c: at

414 K, for 40 min of reaction time about 20% of the initial vanillin was consumed

while at 393 K the decrease was only about 4%. This effect is even clearer for long

reaction times.

The work of Mathias, Fargues and Rodrigues [36, 116, 118, 149] allowed

calculating the activation energies (Ea) for the vanillin production and oxidation:

29.1 and 46.0 kJ/mol, respectively. The kinetic constant for vanillin production

can be expressed as:





3502

7

k ¼ 1:376 Â 10 exp À

ð12:2Þ

ðl/molÞ1:75 minÀ1 :

T

For vanillin oxidation, the following kinetic law was found for pH [11.5:

rv ẳ k0 ẵO2 ẵCv



12:3ị







5530

k ẳ 4:356 10 exp

l/mol:minị:

T



12:4ị



with,

0



6



Other authors reported the kinetic laws and activation energy for vanillin and

syringaldehyde production from hardwood lignin [112, 113]. Similar Ea values

were found for vanillin and syringaldehyde, 70.5 and 62.6 kJ/mol, respectively

[113]. However, the rate constant for syringaldehyde production is higher than that

for vanillin, as depicted in the kinetic laws for syringaldehyde (Sy) and vanillin

(v) [113]:





7529

5

rsy ẳ 1:3 10 exp

ẵO2 ẵOH 1:4 ẵL

12:5ị

T





8479

ẵO2 ẵOH 1:9 ẵL

12:6ị

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12



Lignin as Source of Fine Chemicals



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12.4.4 Catalysts

The most frequent catalysts used in lignin oxidation with O2 in alkaline medium

are transition metal salts, such as CuO, CuSO4, FeCl3, and Fe2O3, which have high

oxidation potential and easily would allow electron transference from the aromatic

rings of lignin; at the same time this high oxidation potential turns the regeneration

of the metal salt in the catalytic cycle more difficult. The oxidation with catalyst

has been extensively tested on model compounds of lignin, most of them monomers. A recent and comprehensive review was recently published by Zakzeski

et al. [90] about oxidative catalysis and other perspectives of the catalytic valorization of lignin.

Oxidations experiments were performed by Mathias and Rodrigues [118] using

CuSO4 (4% of lignin weight) and comparing the reaction rate and yields to the

non-catalyzed reaction of kraft lignin at the same conditions. The yield on vanillin

was similar, as well as the time to maximum. However, a low degradation rate was

found in the case of catalyzed reaction. The same salt was tested in the oxidation

of lignosulfonates used at 20% of lignin weight [113]. The authors reported an

increment of 1.3 and 1.4 of the yields of vanillin and syringaldehyde produced in

the non-catalyzed reaction.

Tarabanko et al., reported 12–13%wt (lignin basis) of vanillin in batch oxidation of lignosulfonates using about 16 g/l of Cu(OH)2 [139]. The yield of

non-catalyzed reaction was 5.5 wt. % on vanillin at 40 min. The lignosulfonate

liquor of the same origin was the raw material for further experiences in continuous process [159]. In this case, the authors reported also the syringaldehyde yield.

Copper wire and cupric oxide wire were tested as catalyst and simultaneously as

reactor packing. In this work, the conclusions about the catalyst effect are somewhat difficult due to the simultaneous variation of parameters as, for example, the

oxygen rate in continuous process. However, for lignin from other origin, the

comparison with non-catalyzed reaction leads us to notice an increase of aldehydes

yield (from 1.2–1.8 wt% to 1.5–3.0 wt% on lignin bases) [159].

Other combinations of catalysts were tested in the oxidation of an hardwood

lignin: the mixture of CuSO4 with FeCl3 and CuO with Fe2O3 [110]. At reaction

temperature of 433 K the yields were incremented by CuSO4/FeCl3 (4.5% for

vanillin and 7.4% for syringaldehyde) in comparison with the non-catalyzed

reaction at similar reaction conditions (2.5% for vanillin and 4.5% for syringaldehyde). Based on data reported for 443 K, the additional effect of FeCl3 was

evidenced: 4.7 and 9.5% of vanillin and syringaldehyde, respectively, against 3.5

and 6.5% for the reaction at similar conditions using CuSO4 alone.

Besides the Cu(II) catalysis, Bjørsvik and Minisci [155] also tested salts of

Co(II) and Ce(IV) salts in lignin oxidation. The efficiency of the Cu(II) and Co(II)

catalysts was similar: 5.9 and 5.8%, respectively; however, in the reaction catalyzed by Cu(II), the maximum yield of vanillin was reached faster (70 min) than

with Co(II) (90 min). The salt of Ce(IV) shows a lower efficiency in the oxidation

of lignosulfonate, which was ascribed to the higher difficulty to reoxidation of the



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4?Production of Vanillin and Syringaldehyde by Lignin Oxidation

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