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3?Lignin as Source of Monomeric Compounds

3?Lignin as Source of Monomeric Compounds

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12



Lignin as Source of Fine Chemicals

Lignin



391



Process



MeO

H

HOH 2C C O



Products



End-use



Char



Combustion



Electricity



Syngas



HOHC MeO



Heat

HOH2 C



OMe

O



OH



Thermolysis



MeO



OMe

OH



O



Fuel



Aliphatic compounds:

methane, ethane, formic

acid, substituted

ciclohexanes



Chemicals



Pyrolysis



CHOH



MeO



Simple phenols: phenol,

resorcinols, cresols,

guaiacol, seringol



Gasification



CH2 OH

CH



Hydrocraking



OMe

H

HOH2 C C CH2



MeO



OMe



Oxidized lignin

monomers (vanillin,

syringaldehyde, vanillic

acid, acetovanillone)

and phenolic oligomers



Oxidation



O



Fig. 12.4 Thermochemical processes for lignin conversion, main products and end uses [38, 84,

88–90]



OH



OH



OH



O



O

O



OH



OH

O



Mass selective detector

Abundance



OO



O



O



OH

HO



O



O

O



OH



O



OH



O



OH



O



OH



OH

O



7.5e+06



O



O



OH



O



OH



O

O



O



O



O



O



O

O



OH

O



O



O

O



OH



OH

OH



1e+07



OH



O



O



OH



OH

O



O OH



O



OH

O



O

OH



OH



OH



OH

O



O



O



OH



O



OH



OH



OH



O



OH



OH

OH



OH



OH

O



5.0e+06



O



O



OH



2.5e+06



internal standard

1.0e+6

4.00



6.00



8.00



10.00



12.00



14.00



16.00



18.00



20.00



22.00



24.00



26.00



28.00



30.00



32.00



Time (min)



Fig. 12.5 Gas chromatogram with mass selective detector of monomeric products obtained from

catalyzed hydrothermal degradation of organosolv beech lignin [69]. Courtesy of Dr. Detlef

Schmiedl, Fraunhofer Institute for Chemical Technology, Germany



Today there are two commercial types of vanillin: (1) synthetic vanillin, derived

from petrochemical guaiacol and glyoxylic acid or lignosulfonates and (2) vanilla

extract obtained from the cured beans, or pods, of tropical Vanilla orchids [94, 95].

The raw material costs turn the natural vanillin more expensive than the synthetic

counterpart [94]. Hence, synthetic vanillin became competitive and widely used.



392



P. C. Rodrigues Pinto et al.



There are only few significant manufacturers of vanillin in the world. Rhodia

SA dominates the market producing vanillin by the cathecol–guaiacol route.

Borregaard (Norway) is the second largest vanillin producer and the only current

producer by oxidation of lignosulfonates. Despite the advantages of the cathecolguaiacol route over alternatives, this process is dependent of petroleum-derived

compounds, in opposition with the process by lignin oxidation.



12.3.2.2 Brief History of Vanillin Production from Lignin

In 1937 Salvo Chemical Corporation started the industrial production of vanillin

from lignin oxidation using spent liquors from pulp and paper industry by the

Howard’s [96] patented technology [91, 97]. In Canada, Howard Smith Chemicals,

Ltd. also began the industrial conversion of concentrated spent sulfite liquor to

vanillin with technology based on the process developed by Hibbert and

Tomlinson [98]. In the same country, one other industrial unit started up at about

1945 by Ontario Paper Co., Ltd. that, besides vanillin, recovered also fermentable

sugars from liquor [91]. In the beginning of 1950s, Monsanto Chemical Company

substituted the process of synthetic vanillin from eugenol by the lignosulfonates

oxidation process using fermented spent sulfite liquor [97].

Until 1980s, the major vanillin supply market share was provided by the

oxidation of lignin from the sulfite pulping [99]. After that, industrial units of

lignin-derived vanillin faced constrains that forced them to close [91]. Additionally, since the 1980s, changes introduced in the pulp and paper industry processes

led to a decrease of lignin availability: at that time, kraft process emerges as the

competing pulping process that includes the recovery boiler for burning the spent

liquor allowing the recovery of pulping chemicals and producing energy. Since

then, guaiacol-based vanillin has gained relevance [97]. Nowadays the synthesis of

vanillin from petrochemical guaiacol accounts for 85% of the world supply, with

the remaining 15% being produced from lignin [99].

The high market prices of vanilla beans and their limited supply and the

increasing concern for alternatives to the non-renewable raw materials, have

encouraged the research for alternative pathways for natural flavor production.

A detailed review on biotechnological routes of vanillin production using different

substrates and biosynthesis methods can be found elsewhere [100, 101]. Commercially, vanillin obtained by fermentation from ferulic acid is considered the

only profitable biocatalytic route [101, 102].

As future prospect, the concern for alternatives to the non-renewable raw

materials and the emerging lignocellulosic-based biorefineries could lead to a

promising future for lignin-derived vanillin; the condition is the profitability of the

production process using as raw material spent liquor lignins or lignins from new

biomass conversion technologies. Due to constant efforts of several R&D centers

around world, many challenges for the profitable large scale application of lignin

as source of aromatics, and vanillin in particular, are being progressively

overcome.



12



Lignin as Source of Fine Chemicals



Fig. 12.6 Structure of

syringaldehyde (4-hydroxy3,5-dimethoxybenzaldehyde)

and vanillin (4-hydroxy-3methoxybenzaldehyde)



393

H

C



H



O



C



OMe



O



MeO



OH

Vanillin



OMe

OH



Syringaldehyde



12.3.2.3 Recent and Future Trends: Syringaldehyde Production

Syringaldehyde differs from vanillin by a second methoxyl group at C5 position of

aromatic ring as depicted in Fig. 12.6. The oxidation of softwood lignins produces

exclusively vanillin whereas the oxidation of hardwood lignins leads to syringaldehyde plus vanillin, in a proportion that depends of the original syringyl:guaiacyl ratio in the wood.

Syringaldehyde is a valuable starting chemical for the pharmaceutical industry.

For example, as vanillin, this compound is the precursor of 3,4,5-trimethoxybenzaldehyde, which is a building block of the antibacterial agents ormetoprim

and trimethoprim, with the advantage of containing already two methoxyl groups

[103–105].

In the past, between 1930s and 1950s, the separation technologies to recover

vanillin and syringaldehyde produced by lignin oxidation were not readily

accomplished [106]. Syringaldehyde has been produced by different chemical

routes and starting materials as gallic acid, vanillin, [106] and pyrogallol [107].

New alternatives for synthesis of syringaldehyde are being investigated in order to

find environmental friendly and efficient processes to obtain higher yields [108].

However, many of these synthetic pathways involve complex procedures and/or

include expensive materials becoming not economically feasible a large scale

production.

By end of 1970s, the production process of syringaldehyde by oxidation of

hardwood spent liquor was reported [109] including a step of fractional distillation for the separation of the two aldehydes. The oxidation of hardwood lignin

to produce a syringaldehyde-rich mixture seems to be very attractive. In fact, the

production of this phenolic compound from lignin, in alkaline medium with O2,

has been emerging as research topic [20, 110–113]. However, the sustainability

of this process must be assured by the yield of products and economical

advantageous purification processes. Considering its potential applications, it is

expected an increased demand for this chemical already cited in Top ValueAdded Chemicals from Biomass [38]. As an example, syringaldehyde was

recently considered as a promising building block to dendrimers design with

high antioxidant potential: the antioxidant activity of syringaldehyde-based

dendrimer showed to be two and ten times higher than that one of quercetin and

trolox, respectively [114].



394



P. C. Rodrigues Pinto et al.



12.4 Production of Vanillin and Syringaldehyde

by Lignin Oxidation

The production of phenolic compounds, mainly vanillin, from several sources of

lignin in alkaline medium with O2 has been the subject of many publications in the

last decades [20, 36, 92, 110–113, 115–125]. Some researches have been developed also in acidic medium [126–128] or in ionic liquids [129, 130]. Biotechnological routes [100, 101, 117], electro-oxidation of lignin [56] and the use of

microwaves [131] have also been considered. Some studies reported also the direct

oxidation of wood to produce vanillin and syringaldehyde [132]. The separation of

product from the reaction medium was also a subject of intense research [133–

138], as will be pointed out in the next section.

The chemical oxidation of the spent liquors or lignin, from several sources, is

focused in operating conditions, apparatus, and catalyst. The general aim is to

achieve the maximum conversion of lignin to vanillin (in the case of softwoods) or

also to syringaldehyde (in the case of hardwoods or annual plants). Besides the

process yields on products, also the kinetics laws, and the chemical mechanisms

have been achieved. The impact of a particular lignin on its performance toward

the oxidative process have been considered more recently [20]. A summary of

some of the representative studies concerning raw material, conditions, and results

are gathered in Table 12.3.

Insights on reaction mechanism of lignin oxidation have been consistently

developed by Tarabanko et al. [19, 132, 139–142], although some are not so recent

papers [143–146] constitute also a valuable survey of information. In this chapter,

the mechanism of lignin oxidation will be briefly described.



12.4.1 Reaction Conditions

The oxidation of lignin to produce vanillin has been demonstrated at high pH

(almost 14), and high temperatures (higher than 100°C) with molecular oxygen

(oxygen pressure equal or higher than 3 bar). The main advantages of this oxidant

are its environmental friendliness, the high efficiency per weight of oxidant, and

comparatively low price (for example, air can often be used). The limitation in this

process is the low solubility of oxygen in the reaction medium of NaOH (and

lignin) in the high operational temperatures [147, 148]. Nevertheless, the oxygen

partial pressure should be controlled to avoid further oxidation of vanillin

[149, 150]. The high pH is required for the total ionization of phenolic groups and

conversion to reactive quinonemethide as presented in Fig. 12.7 (initial step I–II).

The pKa values of lignin-related phenolics are in the range of 10–11.5 at 25°C [25]

decreasing as the temperature raise [151]. However, it is expected that the phenolic

groups are even less acidic in the lignin macromolecule with the concomitant

higher pKa than the reported. A minimum of 2 M in NaOH is referred in some



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



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